Chemical fixation and conversion of CO2 into cyclic and cage-type metal carbonates

Coordination Chemistry Reviews 334 (2017) 199–231

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Coordination Chemistry Reviews journal homepage: www.elsevier.com/locate/ccr

Review

Chemical fixation and conversion of CO2 into cyclic and cage-type metal carbonates Katarzyna Sołtys-Brzostek a,1, Michał Terlecki b,1, Kamil Sokołowski a,1, Janusz Lewin´ski a,b,⇑ a b

Institute of Physical Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, 01-224 Warsaw, Poland Faculty of Chemistry, Warsaw University of Technology, Noakowskiego 3, 00-664 Warsaw, Poland

a r t i c l e

i n f o

Article history: Received 1 June 2016 Received in revised form 21 October 2016 Accepted 24 October 2016 Available online 26 October 2016 Keywords: Carbon dioxide Fixation Functional materials Metal carbonates Metal organic cages Synthesis Structure

a b s t r a c t Analysis of the Cambridge Structural Database (CSD) revealed over 1200 metal carbonate compounds. At least 130 of them can be described as molecular mono- or polycarbonate metal cage complexes. The formation of most of these structurally diverse compounds is a classic example of serendipitous fixation of atmospheric CO2. Only a small number of compounds of this type were obtained by the intentional addition of Na2CO3, NaHCO3, or other simple carbonates as precursors of carbonate anion. This review article outlines a great potential of CO2 fixation in various inorganic reaction systems as an important pathway to novel entities based on mono- and polycarbonate metal cores. Particularly we focus on: (i) structure and chemistry of carbonate anions and common pathways leading to their generation in inorganic systems, (ii) multinuclear cages with ability to encapsulate CO2
3 ions, (iii) metal clusters templated by carbonate anions and (iv) carbonate anions as building units of macrocyclic and closed polyhedron systems. This review can be regarded as a flexible guide to the classification of these systems. We present an unusual diversity of molecular structures of metal carbonates derived from the chemical fixation of CO2, and describe composition and geometry of a variety of metal carbonate cores. Unique features of the carbonate anion, i.e. the geometry, low steric hindrance and ability to form a plethora of coordination modes, make it a universal templating agent mediating the formation of a variety of multinuclear aggregates ranging from macrocyclic systems to closed polyhedra. There are also provided some insights into interesting reaction systems, in which the reaction outcome strongly depended on the carbonate’s source. The benefits of usage of CO2 as a precursor of carbonate anions are also delineated. When relevant, aspects related to materials chemistry are presented. We believe that the collected results offer a perspective and guidelines for the future research and development of reaction systems leading to metal carbonates of high scientific as well as practical value. Ó 2016 Elsevier B.V. All rights reserved.

Contents 1. 2.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Carbonate ions in metal complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. Coordination modes of carbonate anion in metal complexes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2. Generation of carbonate anions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.1. Carbon dioxide hydration. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.2. Nucleophilic CO2 fixation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.3. Reductive disproportionation of CO2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Abbreviations: Ac, acetate; CA, carbonic anhydrase; DEF, N,N0 -diethylformamide; DME, dimethyl ether; DMF, N,N0 -dimethylformamide; DMSO, dimethyl sulfoxide; Et, Ethyl; i-Pr, iso-propyl; Ln, lanthanide; Me, methyl; n-Bu, n-butyl; OTf, trifluoromethanesulfonate; Ph, phenyl; Piv, pivalate; RE, rare earth; t-Bu, tert-butyl; THF, tetrahydrofuran. ⇑ Corresponding author at: Faculty of Chemistry, Warsaw University of Technology, Noakowskiego 3, 00-664 Warsaw, Poland and Institute of Physical Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, 01-224 Warsaw, Poland. E-mail address: [email protected] (J. Lewin´ski). 1 These authors contributed equally to this work. http://dx.doi.org/10.1016/j.ccr.2016.10.008 0010-8545/Ó 2016 Elsevier B.V. All rights reserved.

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3.

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Carbonate anion as templating ligand or building block . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Molecular metal-organic frameworks with encapsulated carbonate ion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1.1. Carbonate ion encapsulated in multinuclear cages supported by mono- or bidentate ligands . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1.2. Carbonate ion encapsulated in multinuclear cages supported by polydentate ligands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1.3. Two carbonate ions encapsulated in multinuclear cages supported by polydentate ligands . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. Open molecular metal-organic frameworks templated by carbonate anion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.1. Molecular metal-organic frameworks templated by one carbonate anion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.2. Self-assembly of metal clusters mediated by one carbonate anion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.3. Supramolecular architectures templated by two carbonate anions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3. Carbonates as a building motif of macrocyclic and closed polyhedral systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.1. Macrocyclic metal polycarbonate assemblies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.2. Macrocyclic metal polycarbonate assemblies supported by multidentate ligands. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.3. Metal polycarbonate assemblies with prismatic core . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.4. Metal polycarbonate assemblies with octahedral core . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.5. High-nuclearity polycarbonate metal assemblies with ball-like core . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.6. High-nuclearity distorted polycarbonate lanthanide assemblies. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusion and perspectives of the conversion of CO2 into metal carbonate-based functional materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1. Introduction Over the last two decades, fixation approaches for CO2 sequestration have received much attention on account of their great environmental significance in global warming mitigation and various promising applications in synthetic and material chemistry [1,2]. CO2 is the most abundant and important C-1 building block in Nature. It is converted into carbohydrates by photosynthetic organisms and cumulated in the shells of marine organisms as metal carbonates as a result of biomineralization processes (Fig. 1). The scale of these transformations and relative stability of metal carbonates in environmental conditions make them the most important minerals building Earth’s lithosphere, and critical players in the carbon cycle [3]. Due to great abundance, accessibility and processability, metal carbonates are also one of the most widely used groups of materials across many important areas of human activity, ranging from housing industry (CaCO3), metallurgy, ceramics, glassmaking and optics to medicine [1]. Recently, the emerging need of new materials for nanotechnology and more-in-depth understanding of biomineralization processes [4], turned the attention of researchers towards usage of molecular or nanoparticulate metal carbonates in materials chemistry, and provided intriguing pathways to inorganic nanomaterials of complex architectures. Indeed, carbonate anions are very attractive templating factors, which can bridge

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two or more metal centers, which leads to a variety of molecular structures of interesting geometry and functionality. However, while CO2 is widely used as C1-chemical feedstock for the preparation of useful organic compounds in metal-catalyzed systems [1], the rational construction of functional materials based on CO2-tometal carbonate conversion is much less explored and such materials have mostly been isolated accidentally [5]. All of the mentioned aspects have prompted extremely high investments outlays dedicated to projects concerning capture, storage [2] and conversion of CO2 into value-added products [1], as well as investigations on CO2 as the basic metabolite of aerobic respiration and the modelling of the mode of action of carbonic anhydrase (CA) [6]. Particularly the last aspect has appeared as one of the most intriguing issues of frontier area of bioinorganic chemistry. In this review, we focused on the great potential of CO2 fixation in inorganic systems as a fascinating pathway to novel entities based on metal carbonate cores. The gathered literature data comprise information about: (i) carbonate anion coordination modes and common pathways leading to generation of carbonate anion in inorganic systems (Chapter 2), (ii) molecular systems encapsulating CO2
ions in multinuclear cages (Chapter 3.1), (iii) open 3 molecular metal-organic frameworks templated by carbonate anions (Chapter 3.2), and (iv) carbonate anions as building units of macrocyclic and closed polyhedron metal systems (Chapter 3.3). This review can be regarded as a flexible guide to the classification of these systems. Hopefully, the collected data will offer a perspective and guidelines for the future research and development of metal carbonates. 2. Carbonate ions in metal complexes 2.1. Coordination modes of carbonate anion in metal complexes

Fig. 1. The importance of CO2 transformations: photosynthesis and biomineralization processes.

Carbonate anion is a versatile bridging ligand with low steric hindrance and three oxygen atoms readily available for metal centers in coordination complexes. The anion possesses flat D3h molecular symmetry with sp2 hybridization of the central carbon atom (Fig. 2). The double bond is symmetrically delocalized between all three CAO bonds, which are of equal length of 1.28 Å. As bridging ligand, the carbonate anion may link together from two to nine metal centers in a variety of ways. The most commonly observed coordination modes of CO2
3 anion are presented in Fig. 3. In the literature, there exist inconsistencies in nomenclature describing the coordination modes of CO2
3 , in this work we apply the description according to the IUPAC recommendations [7]. In

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Fig. 2. Structure of the carbonate anion.

Fig. 3. The most common coordination modes of the carbonate anion in metal complexes.

Fig. 4. Mechanism of the CO2 hydration process [8].

the applied nomenclature we specify (i) how many metal centers are brought together by the carbonate anion (lx; where x – integer, x P 2) and (ii) in which way each metal center is coordinated to

the carbonate anion (j for metals involved in chelating species; g1 for metal bound to the carbonate ion as monodentate). When the detailed description is omitted (only lx is applied) metal cen-

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zinc-based metalloenzyme that catalyzes the reversible hydration of CO2 and is responsible for its fast metabolism [6]. This process is based on a Zn-OH species located in the active site of CA, and consists of the following stages: (i) proton release from bound water molecule, (ii) nucleophilic CO2 fixation by the hydroxo zinc moiety, (iii) replacement of the bicarbonate group by a water molecule (Fig. 5). Hence, a number of mono- or dinuclear zinc complexes supported by multidentate ligands and featuring terminal or bridging hydroxide ligation has been widely investigated as synthetic analogues of CA, which was summarized in excellent reviews [6,9]. The kinetics of this process and structure of the active center of CA has also been the subject of a plethora of interesting studies. There are several factors affecting the nucleophilicity of metalhydroxide species. These involve the character of a metal center and its coordination sphere, as well as local environment around the hydroxide moiety. For example, the reactivity of a series of model isostructural hydroxo metal complexes towards CO2 decreased in order Zn > Cu > Ni  Co > Mn > Fe [10]. In turn, the catalytic activity of active centers of metal-substituted carbonic anhydrases decreased in order: Zn > Co  Ni  Mn > Cu  0. Despite some discrepancies, these data showed that Zn-OH is the most nucleophilic moiety, with unusually high reactivity towards CO2. Fig. 5. The mode of action of CA.

ters are located in the most simple, regular and symmetrical way. In a few cases also mutual conformation of MAO bonds may be specified, which can be important for both structure and properties of the complex. The structural diversity of carbonate metal complexes is widely discussed in Section 3. 2.2. Generation of carbonate anions There is a variety of possible sources of carbonate anions in synthetic chemistry: (i) inorganic carbonate and bicarbonate salts (e.g. Na2CO3, NaHCO3), (ii) decomposition of carbonyl compounds (e.g., DMF, carboxylic acids) mostly in high temperature and/or high pressure and (iii) fixation and hydration of CO2. Carbonate salts are mostly insoluble neither in water, nor in organic solvents. Therefore, complicating the reaction system by providing additional cationic species may influence the structure of the final product. Carbon dioxide is a particularly convenient reagent that can be easily applied both from gaseous and solid-state phase avoiding use of carbonate salts. Furthermore, the fixation of CO2 can occur in ambient temperature and pressure in water environment as well as organic solvents. Carbon dioxide can be converted into carbonate species in three ways: (i) hydration in solution and dissociation of carbonic acid to bicarbonate and carbonate anions, (ii) fixation by hydroxo- or oxo-metal complexes or (iii) reductive disproportionation. 2.2.1. Carbon dioxide hydration CO2 dissolved in water exists in equilibrium with its hydrated form (carbonic acid, H2CO3). H2CO3 is a weak diprotic acid which may dissociate forming bicarbonate and carbonate anions (Fig. 4) [8]. The concentration of each form strongly depends on pH: addition of base to a solution of carbonic acid gives bicarbonate ion, HCO
3 (hydrogen carbonate), which can be further deprotonated at higher pH providing carbonate ions. 2.2.2. Nucleophilic CO2 fixation CO2 fixation by metal complexes is most significally represented by carbonic anhydrase (CA)-like reaction systems. CA is a

2.2.3. Reductive disproportionation of CO2 Another intriguing pathway to in situ generation of carbonate anion is the reductive disproportionation of CO2, which proceeds according to the equation:

2CO2 þ 2e
! CO2
3 þ CO

ð1Þ

This process occurs at low valence metallic centers and is known for various metal systems including Mg(I) [11], Ti(II) [12], Fe(I) [13,14], Mo(II) [15], Sm(II) [16] and U(III) [17–20]. Mechanism of these processes was widely investigated by Meyer et al. (Fig. 6) [17,18]. First, a CO2 molecule is reductively coupled to the active metal center and then dissociates giving bridging oxo-metal species and CO. The formed oxo-metal complex fixates another carbon dioxide molecule with the formation of a carbonate anion. Processes of this type may be affected by the presence of alkali metal ions. For example, the reduction of [U((t-BuO)3SiO)4] by KC8 resulted in the formation of K[U((t-BuO)3SiO)4], which upon reaction with CO2 yielded mononuclear uranium (V) complex K[UO ((t-BuO)3SiO)4] [20]. To conclude this chapter, the fixation and conversion of CO2 to metal carbonate-based products is enabled by the utilization of hydroxo- or oxo-metal complexes as well as redox active metal centers. Thus, it appeared recently as a very attractive path to multinuclear compounds with emerging prospects in material chemistry. In Chapter 3 we provide a general overview of reaction systems and structural diversity of cage-type and macrocyclic metal carbonate systems obtained in this way. 3. Carbonate anion as templating ligand or building block On account of their geometry and a variety of binding modes, carbonate anions have appeared as useful ligands for the construction of polymetallic clusters of emerging importance in material chemistry. An analysis of the Cambridge Structural Database (CSD) (v. 5.37, November 2015 release) revealed over 1200 metal-carbonate compounds. At least 130 of them can be described as molecular mono- or polycarbonate metal cage complexes. The rigidity and geometry of CO2
3 anion strongly influence the architecture of the resultant clusters. Depending on metal centers and auxiliary organic ligands, those clusters can act as luminescent

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Fig. 6. Reductive disproportionation of CO2 by U(III) complexes.

Fig. 7. The molecular structures of silver cage clusters and representation of their multimetalic cores: (a) 1, (b) 2, (c) 3; (d) view of the 1D chain formed by clusters 4 paired by OAc ligands through the l2-bridges.

compounds, molecular magnets or molecular building units for supramolecular systems. Formation many of these versatile compounds is a classic example of serendipitous fixation of atmospheric CO2. On the other hand, only a few metal carbonates were obtained by the intentional addition of Na2CO3, NaHCO3 or other related salts. Taking into account limitations of the latter approach (i.e. applicability essentially limited to water environment and possible production of insoluble and amorphous products), it is surprising that there are only a few examples of high nuclearity metal carbonate clusters obtained by deliberate employment of CO2 as a reaction ingredient. The following chapter provides an overview on molecular metal clusters based on car-

bonate ligand(s) obtained as a result of CO2 fixation mediated by the corresponding metal precursor, which can be viewed as either encapsulated chemical species or constituent template of assembled carbonate cages. 3.1. Molecular metal-organic frameworks with encapsulated carbonate ion The design of new frameworks that promote the formation of encapsulating environments is a burgeoning area of research. It covers a wide range of molecular host systems enabling structural investigations on a variety of chemical species. For example, the

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encapsulation of carbonate anion in multimetallic polyhedral structures with defined inner cavity led to the identification of new and unusual coordination modes of carbonate anion (Chapter 3.1.1). Moreover, an encapsulated central carbonate ion can contribute to the geometry of the encapsulating metal-organic framework, facilitating the proper arrangement of organic ligands and therefore providing unusual molecular structures of the resultant coordination complexes (Chapter 3.1.1 and 3.1.2). In some cases, the encapsulating framework can be also regarded as a molecular system for selective separation of carbonate ions. Herein, we describe systems in which one or two carbonate ion(s) are encapsulated within multinuclear metal cages supported by mono- or bidentate organic ligands (Chapter 3.1.1), or polydentate ligands (Chapter 3.1.2). Strikingly, most of these assemblies have been reported very recently.

3.1.1. Carbonate ion encapsulated in multinuclear cages supported by mono- or bidentate ligands One of the most spectacular series of systems with reported separation of carbonate ions was based on silver monocarbonate cage clusters: a heptadecanuclear, [Ag17(t-BuC„C)14(l5-CO3)]OTf (1), a nonadecanuclear, [Ag19(t-BuC„C)16(l9-CO3)]BF4 (2) [21], as well as two icosanuclear, [Ag20(t-BuS)10(l9-CO3) (OAc)8(DMF)4] (3) [22] and [Ag20(t-BuS)10(l9-CO3)(OAc)8(DMF)2] (4) [23]. The reaction of monodentate silver alkynyl t-BuC„CAg with AgOTf under alkaline conditions of tetramethylethylenediamine (TMEDA) led to the isolation of 1. Interestingly, when AgBF4 was used as the metal center precursor in the same preparative procedure, a higher nuclearity silver cluster 2 was isolated. This observation indicated the influence of the precursor’s counterion on the nuclearity of the resultant clusters. Cluster 1 consists of seventeen silver atoms bridged by fourteen l3-alkynyl ligands, which altogether form a skeleton resembling the shape of an antique clock (Fig. 7a). 1 possesses m symmetry with the mirror passing through enclosed CO2
ion of l5-g1(O):g1(O):g1(O):g1(O0 ):g1(O0 )-coordination 3 mode, which acts as a template for the formation of this silver cluster. The AgAO bond distances vary from 2.39 to 2.75 Å for two O atoms of CO2
anion, while the third one is only weakly bound 3 to silver centers with the shortest AgAO distance of 2.84 Å. Cluster 2 is constructed by nineteen Ag atoms, which are templated by sixteen peripheral alkynyl ligands adopting the l3-bridging mode (Fig. 7b). The shape of this silver cluster is less regular that of 1. It can be described as a flying saucer in which nine silver atoms and a carbonate ion construct the equatorial rim, and two groups of five silver atoms act as the upper and lower rim. This [Ag19] cluster is templated by l9-j2(O0 ,O00 ):g1(O):g1(O):g1(O):g1(O0 ):g1(O0 ): g1(O0 ):g1(O00 ):g1(O00 )-carbonate anion with AgAO distances falling in the range 2.45–2.86 Å [21]. Application of another precursor, Ag(t-BuS), led to the formation of disc-like AgAS nanoclusters 3 and 4, which are almost isostructural [22]. Cluster 3 was synthesized by a one-pot synthesis from Ag(t-BuS) precursor and silver acetate, AgOAc, in methanol/ ethanol/DMF solution under the ultrasonic conditions. Compound 4 was formed from a mixture of Ag(t-BuS), (NH4)3[CrMo6O24H6] 7H2O, Ni(OAc)24H2O, Ag(O2CCF3) and AgBF4 in a methanol/acetonitrile/DMF solvent system. Both resulting centrosymmetric monocarbonate clusters contain [Ag20(t-BuS)10(l9-CO3)] core encapsulating a l9-CO2
anions (the AgAO distances fall in the 3 range 2.405–2.743 Å), which are derived from the fixation of atmospheric carbon dioxide (Fig. 7c). Supramolecular structure of 3 is determined by weak interactions between DMF and OAc ligands of neighboring clusters, while adjacent clusters of 4 are connected by two inverted OAc ligands through l2-bridges. As a result, 4 selfassembles to form a 1D chain (Fig. 7d). Interestingly, 4 exhibits potential wide-gap semiconductor properties, reversible ther-

mochromic luminescence behavior and excellent electrochemical properties [23]. A family of highly-symmetrical heteronuclear Fe(III)/Ln(III) phosphonate clusters of the type [Fe6Ln6P6] with isostructural framework related to the Wells
Dawson polyoxometalates, were reported by Winpenny and McInnes [24]. The fixation of atmospheric CO2 in solvothermal reactions involving an iron(III) carboxylate, [Fe3(l3-O)(O2C(t-Bu))6(HO2C(t-Bu))3](O2C(t-Bu)), a lanthanum(III) carboxylate dimer [Ln2(Piv)6(HPiv)6], and RPO3H2 (where R = methyl, phenyl, or n-hexyl) in alkaline conditions resulted in the formation of oxo-carbonate clusters [Fe6Gd6(l3O)2(l6-CO3)(O3PR)6(O2C(t-Bu))18] (where R = methyl (5), phenyl (6), or n-hexyl (7)) and [Fe6Ln6(l3-O)2(l6-CO3)(O3PR)6(O2C(tBu))18] (where R = methyl [Ln = Dy (5a), Ho (5b)] or phenyl [Ln = Tb (6b)]). Molecular structures of all compounds are analogous and possess D3d (6, 6b and 7) or C2h point symmetry (5). The representative molecular structure of [Fe6Gd6(l3-O)2(l6-CO3)(O3PPh)6(O2C (t-Bu))18] (6) is shown in Fig. 8a. In these assemblies, the metalcarbonate core forms a layered structure capped from both sides by two [Fe3] triangles and the supporting organic ligands. The l6-carbonate anion is encapsulated inside the cage and bridging all six Ln ions. The organic shell of the cages consists of phosphonate and pivalate ligands, which wraps up the inorganic core giving an overall rugby ball-like shape (Fig. 8b). All the described compounds 5–7 exhibited strong antiferromagnetic FeAFe interactions and very weak LnALn and LnAFe exchange interactions. 3.1.2. Carbonate ion encapsulated in multinuclear cages supported by polydentate ligands Another group of molecular capsules consists of multinuclear metal cages supported by polydentate organic ligands which bind together one or two types of metal centers. Application of multifunctional ligands provided complex polyhedral structures with unique intramolecular environment. In these polyhedral structures, one or two carbonate anions can be encapsulated. Within this family of compounds, there have been described four examples of molecular capsules containing one carbonate anion and only three of those with two carbonates encapsulated within a molecular metal-organic framework. Interestingly, these exceptional assemblies were derived by fixation of CO2. One of the simplest molecular capsule containing in the inner cavity carbonate anion is a decanuclear Cd(II) cluster, [Cd10(imap)4(Himap)6(CO3)(ClO4)2]2+ (8) (H2imap = (E)-2-((1H-imi dazol-2-yl)methyleneamino)phenol) [25]. The monocarbonate cluster 8 was isolated as by-product from the reaction system involving Cd(AcO)22H2O, Cd(H2O)6(ClO4)2 and H2imap in methanol as a result of fixation of atmospheric CO2 by an anticipated (hydroxo)cadmium intermediate [Cd2(imap)(Himap)(OH)]. In the crystal structure of 8, two isomeric entities are present. They possess essentially the same overall shape (Fig. 9), but significantly differ in the coordination mode of the encapsulated carbonate molecule. In isomers 80 and 800 , the carbonate anion adopts the l8- and l9-coordination mode, respectively (Fig. 9c). The molecular structures of these monocarbonate decanuclear Cd(II) clusters consist of a twenty-eight-vertex polyhedral capsule of the [Cd10Cl2O16] core in which the metal ions are either penta-, hexa- or heptacoordinated (Fig. 9b). The metallic skeleton of the cage forms a pentagonal antiprism resembling a wheel or a double crown. Another type of carbonate-encapsulated systems is a series of zeppelin-like supramolecules, in which the entrapped carbonate anion is located inside a complicated metal-organic framework supported by chiral polydentate ligands [26,27]. For example, carbonate anion supports the self-assembly of a lanthanide salt and enantiomorphous chiral amino acids: donated (S)-Lig1 ligand and (R)-Lig1 ligand in the presence of CO2 into olive-shaped chiral

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Fig. 8. The side and top (along the C3 axis) view of: (a) molecular structure of heterometallic Fe(III)/Gd(III) cluster 6 and (b) representation of its [Fe6Gd6P6(l6-CO3)] core with the presentation of the centered l6-carbonate anion.

Fig. 9. (a) The molecular structure of decanuclear monocarbonate cluster 8 (side and top view); (b) representation of its cage-type core, [Cd10Cl2O16] and (c) presentation of the centered l8- and l9-carbonate anions of two isomers of 8 (from the left 80 and 800 , respectively).

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Fig. 10. (a) The molecular structure (side and top view) of the chiral assembly 9; (b) representation of the [La7(l3-CO3)] core of 9 with the centered l3-carbonate anion; (c) the space-filling representation of 9 with a chiral helical arrangement of six (S)-Lig1 ligands.

Fig. 11. (a) The molecular structure (side and top view) of chiral supramolecule 11; representation of the core of 11; (b) side and (c) top view with presentation of the centered l3-carbonate anion (OTf
anions were omitted for clarity).

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Fig. 12. (a) The molecular structure of tetradecanuclear aggregate 13 (side and top view) and (b) representation of its core (side and top view) with presentation of the central [Mn6(l6-CO3)] and the outer [Mn4] unit.

heptanuclear lanthanide assemblies [La7((S)-Lig1)6(l3-CO3) (NO3)6(OCH3)(CH3OH)7]2CH3OH5H2O (9) and [La7((R)Lig1)6(l3CO3)(NO3)6(OCH3)(CH3OH)7]2CH3OH5H2O (10) [26]. The pentanuclear core of 9 and 10 is represented by an elongated trigonal bipyramid with additional two La(III) ions symmetrically located on the axis at both sides of the core (Fig. 10a). One of the most notable structural features of these molecules is that three equatorial La(III) ions in the cluster core are trigonally-arranged and bonded by the central CO2
3 ion, which adopts a rare, nearly symmetrical l3-j2:j2:j2-bridging mode. All LaAO bonds are almost equal in length (2.544 Å in average). Interestingly, the whole framework of the cluster exists as a homochiral helix formed via self-assembly of three equatorial [((S)-Lig1)–La–((S)-Lig1)] units, which are related by a quasi-C3 molecular symmetry (Fig. 10c). Notably, the described chiral supramolecular structures have an excellent ability to selectively recognize CO2
3 anions in water. An intriguing product of encapsulation of the carbonate anion by a Cd(II)-based chiral supramolecule with a trigonal bipyramidal arrangement was reported by Nitschke and coworkers [27]. The self-assembly of sub-components: dialdehyde, p-anisidine and Cd (OTf)2 (OTf
– trifluoromethanesulfonate), in mild reaction conditions led to the formation of monocarbonate assembly [Cd5(Lig2)6 (l3-CO3)(OTf)2] (11). Compound 11 possesses a [Cd5(Lig2)6] framework, which displays C1 symmetry (Fig. 11). Multidentate ligands Lig2 forms the edges of the bipyramid, which leads to generating two interior cavities separated by an equatorial ring of the three metal centers. These cavities are occupied by two encapsulated OTf ions, while the encapsulated l3-carbonate anion templates three equatorial Cd(II) ions with the CdAO bond lengths ranging from 2.147 to 2.222 Å. Interestingly, the analogous [Zn5(Lig2)6] framework (12) was capable of binding CO2 only after the addition


of an excess of BF
4 or OTf anions. These observations suggest that the cavity of 12 must first be opened in the presence of through BF
4 or OTf
templates before CO2 fixation may occur. The monocarbonate metal-organic framework of the highest nuclearity reported is a tetradecanuclear Mn(II) aggregate [Mn14(l6-CO3)(l3-OH)6(HLig-I)6(HLig-II)3(OH2)3]3(H3O)4 (13) [28]. Cluster 13 was derived from the reaction system involving 2-(N0 -d icyanomethylene-hydrazino)-benzoic acid (DHB), MnCl2 and Et3N in MeCN. Non-planar topology of the core is characteristic of the aggregate 13, which is composed of three subunits: two outer tetranuclear units of the [Mn4] type and one central hexanuclear, [Mn6(l6-CO3)], both of which are supported by in situ generated ligands, (HLig-I)3
and (HLig-II)2
(Fig. 12). In the central [Mn6(l6-CO3)] unit, the l6-carbonate (formed by the fixation of atmospheric CO2) connects six Mn(II) ions to give a trigonal antiprismatic arrangement. Cluster 13 is further stabilized by (HLig-I)3
and (HLig-II)2
ligands as well as a net of intramolecular hydrogen bonds. Moreover, intermolecular hydrogen bonding between neighboring aggregates of 13 led to the formation of a 3D network with an open void space of about 57.6% of the unit cell volume. The magnetic measurements of 13 indicated the presence of dominating antiferromagnetic interactions between the metal centers.

3.1.3. Two carbonate ions encapsulated in multinuclear cages supported by polydentate ligands The described family of supramolecular systems includes also those with two carbonate anions encapsulated in the complex molecule. The spontaneous fixation of atmospheric CO2 and encapsulation of two nascent CO2
3 anions appear as an attractive way to synthetize novel high-nuclearity metal clusters. However, assem-

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Fig. 13. (a) The molecular structure of 14 encapsulating two carbonate ions and (b) representation of its [La12(l6-CO3)2] core with the central l6-carbonate anions; (c) the molecular structure of aggregate 15 with two encapsulated carbonate ions and (d) representation of its [K4Ni12(l6-CO3)2] core with central l6-carbonate anions.

blies of this type derived by the CO2 fixation are very rare – only three examples were reported up to now. The first one was described by Roesky and coworkers [29]. Dodecanuclear La(III) hydroxido cluster [La12(OH)12(H2O)4(Ph2acac)18(Phgly)2(l6-CO3)2] (14) (Ph2acac = dibenzoylmethane, Phgly = phenylglyoxylate) was isolated from the reaction of LaCl3 and HPh2acac in methanol with an excess of triethylamine and in the presence of atmospheric CO2. Interestingly, it seems that slow absorption of atmospheric CO2 is crucial for the formation of 14; any attempts to isolate the desired compound from other sources of carbonates (i.e., K2CO3, (NH4)2CO3) failed. The molecular structure of 14 is shown in Fig. 13a. In this cluster, each l6-j2:g1:g1:g1:g1:g1-carbonate anion binds six metal centers in a chelating and bridging configuration (Fig. 13b). Both CO2
3 anions have a disordered geometry with the CAO bond lengths in the range 1.226–1.431 Å. A core structure related to that observed in 15 was found in a polynuclear Ni(II) aggregate, [K4Ni12(O2C(t-Bu))16(OH)8(l6CO3)2(H2O)2(EtOAc)4] (15) (Fig. 13c). Compound 15 was obtained by recrystallization of a Ni(II) pivalate precursor, KNi4(O2C(t-

Bu))7(OH)2(EtOH)6, under atmospheric conditions [30]. It comprises a dodecanuclear non-regular Ni(II) core surrounded by four K+ ions, which form an elongated ball-shaped cluster encapsulating two carbonate ions (Fig. 13d). Aromi and co-workers reported an unusual centrosymmetric bimetallic cation, [Co8Na4(ohhp)4(OH)2(CO3)2(py)10](BF4)2+ (16), stabilized by a multidentate ligand, i.e. deprotonated 2,6-bis-(3-o xo-3-(2-hydroxyphenyl)-propionyl)-pyridine (H4ohhp) [31]. Aggregate 16 was isolated from the reaction mixture involving Co(BF4)26H2O, NaH, H4ohhp in pyridine under strongly alkaline conditions as a result of fixation of atmospheric CO2. The [Co8Na4] metallic skeleton of 16 consists of two rhombic tetranuclear [Co (II)2Co(III)2] units supported by the N,O2,O0 2,O00 2-heptadentate ohhp ligands in a combination with Na+ cations (Fig. 14a). Interestingly, the geometry of the cage allowed for the encapsulation of two carbonate anions simultaneously, constraining their extremely close proximity with the shortest intermolecular distance between carbonate species ever reported. Two disordered positions of the cage affect both the value of the intermolecular O  O distances

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Fig. 14. (a) The molecular structure of bimetallic dicarbonate cluster 16 and (b) representation of its [Co8Na4(l6-CO3)2] core with the central l6-carbonate anions (O  O distance of 1.946 Å).

Fig. 15. The molecular structures of trinuclear complexes with triangle cores determined by central templating l3-carbonate anion: (a) 17, (b) 18, (c) 19.

(as short as 1.946 and 1.971 Å) as well as the l6- or l5coordination modes of carbonate anions (Fig. 14b). Compound 16 possesses an inversion center lying between two CO2
3 anions with unsymmetrical CAO bond distances ranging from 1.240 to 1.340 Å. 3.2. Open molecular metal-organic frameworks templated by carbonate anion The symmetry of carbonate anion appears as an intriguing feature, which can influence the geometry of multinuclear clusters formed in self-assembly processes of mono- and multinuclear coordination species, where carbonates acts as templating agents. Within the described family of complexes, a variety of core structures with one or two carbonate anion(s) were reported. These include tri- (Ni(II), Cu(II), Cd(II), Ln(III) = La, Nd, Sm, Eu, Gd, Tb,

Lu), tetra- (Tb(III)), penta- (Mg(II), Co(II)), hexa- (Zn(II), Ni(II), Ln (III) = Gd, Tb, Dy), octa-(Na(I)/Ln(III) = Eu, Tb), nona- (Mg(II), Ni (II)), deca- (Ln(III) = Pr, Nd), dodeca- (Ni(II), Cu(II)), and octadecanuclear (La(III)) clusters with polygon or approximately cyclic core topology. The unique ability of carbonate anions to bind metal centers was also manifested by their propensity to template several smaller metal-organic aggregates, which results in forming larger supramolecular structures. 3.2.1. Molecular metal-organic frameworks templated by one carbonate anion The simplest examples of this class of compounds are trinuclear metal complexes consisting in an approximately equilateral triangle core templated by central carbonate anion with l3-j2:j2:j2-coordination mode. A variety of trinuclear metal

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Fig. 16. The molecular structures of the cyclic monocarbonate clusters: (a) tetranuclear Tb(III) 22 and (b) pentanuclear Mg(II) 25; representation of their cores: (c) 22 with central l4-carbonate anion and (d) 25 with central l5-carbonate anion.

Fig. 17. The molecular structures of hexanuclear Ni(II) complexes: (a) 26 and (b) 27; representation of their cores with central l6-carbonate anions of chair conformation: (c) 26 and (d) 27.

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carbonates supported by di- [32], tri- [33,34], tetra- [35], and hexadentate [36] ligands were reported, all of them formed as a result of CO2 fixation. For example, a monocarbonate nickel cluster [Ni (H2Nsalme)2(l3-CO3)(Ni2(HNsalme)2(NCS)2)] (17) (H2Nsalme = 2-[(3-methylamino-propylimino)-methyl]-phenol) was obtained by spontaneous fixation of atmospheric CO2 by a mixture of [Ni (HNsalme)2] and Ni(NCS)24H2O in methanol [34]. The [Ni3(l3-CO3)]4+ core of quasi-symmetrical compound 17 possesses a slightly distorted trigonal planar geometry with approximately equal NiAO distances (Fig. 15a). The carbonate oxygen atoms have T-shaped coordination environment including an almost linear CAOANi arrangement. The coordination environments of the two metal centers are supported by the deprotonated N,N0 ,O-tridentate HNsalme ligand and the third one is chelated by two protonated H2Nsalme ligands. A trinuclear metal carbonate complex, [Cd3(cyclam)3(l3-CO3)] (ClO4)4 (18) stabilized by a N4-tetradentate macrocyclic ligand, 1, 4,8,11-tetraazacyclotetradecane (cyclam), was obtained via fixation of atmospheric CO2 by a cadmium precursor, Cd(cyclam) (ClO4)2 [35]. The molecular structure of 18 consist of the central planar carbonate anion templating three [Cd(cyclam)]2+ units of C3 symmetry (Fig. 15b). Similar examples of trinuclear metal carbonates are described include a series of isostructural lanthanide(III) complexes of the type [Ln(H2tidp)(H2O)Cl]3(l3-CO3)](ClO4)4 (Ln(III) = Nd (19), Gd (20), Tb (21)) supported by a N2,N0 2,O2-hexadentate tetraiminodiphenolate macrocyclic ligand, (H2tidp) [36]. The reaction of hexameric Pb(II) compound [(Pb(H2tidp)(ClO4))6](ClO4)6 with LnCl3nH2O in methanol/water solution gave [Ln(H2tidp)(H2O) Cl]2+ species, which self-assembled in the presence of atmospheric CO2 to give trinuclear compounds 19–21. The molecular structure of the representative carbonate 19 is shown in Fig. 15c. Compounds 19–21 possess pseudo C3 symmetry with average carbonate CAO distance of 1.281 Å. Magnetic susceptibility data of 20 indicated very weak antiferromagnetic exchange interactions between the Gd(III) centers within the metal triangle through the carbonate bridge. Among reported examples of tetranuclear metal carbonates, a series of isostructural N,N-di-butylcarbamato lanthanide complexes [NH2(n-Bu)2]2[Ln4(l5-CO3)(O2CN(n-Bu)2)12] (Ln = Tb (22), Sm (23), Eu (24)) was derived by fixation of CO2. Clusters 22–24 were obtained in the reaction of the corresponding metal chlorides with CO2 in the presence of dibutylamine, NH(n-Bu)2 [37]. The anionic part of compounds 22–24, [Ln4(l4-CO3)(O2CN(nBu)2)12]2
, consists of a planar trapezoid cage of four metal atoms gathered around the central l4-j2:j2:g1:g1-carbonate ligand. All metal centers are surrounded by oxygen atoms from both terminal and bridging carbamates (Fig. 16a). Two dibutylammonium cations, [NH2(n-Bu)2]+, placed on the opposite grooves of the described anion, are stabilized by hydrogen bonds. The only example of cyclic pentanuclear cluster templated by one carbonate anion obtained by the CO2 fixation is a magnesium cage compound [Mg5(l5-CO3)(O2CN(i-Pr)2)8(HMPA)2] (25) (where HMPA = hexamethylphosphoramide). The reaction of CO2 with magnesium di-iso-propylamide, Mg(N(i-Pr)2)2 in a THF solution and in the presence of traces of H2O followed by recrystallization of the product in a mixture of HMPDA/toluene led to the isolation of 25 [38]. Compound 25 consists of a pentanuclear assembly of magnesium atoms bridged by di-iso-propylcarbamato ligands and the encapsulated l5-j2:g1:g1:g1:g1-carbonate anion (Fig. 16b). Due to the bridging and chelating character of the CO2
3 anion its ideal trigonal geometry is here slightly distorted. Fixation of CO2 also opened the way to the synthesis of hexanuclear complexes with encapsulated carbonate anion. For example, cationic hexanuclear complexes of the type [Ni6(l6-CO3)(N3)6(dpcp(OMe))3(MeOH)x(H2O)y]+ (x = 3, y = 0 (26a); x = 2, y = 1

211

(26b) (dpcp = di-2,6-(2-pyridylcarbonyl)pyridine) are templated by the symmetric l6-carbonate anion and supported by a N3,O2multidentate ligand (Fig. 17a) [39]. The six Ni(II) ions are arranged in a heterocycle adopting chair conformation. In this case, atmospheric CO2 was considered as the carbonate anion source. A similar central carbonate support for the hexanuclear chair arrangement of Ni(II) centers was found in [((bhimmp)Ni2)3(l6CO3)(H2O)8]+ (27) cluster supported by a N2,O2,O0 -pentadentate ligand (bhimmp = deprotonated 2,6-bis-[[2-hydroxy-(1,1-dimethy lethyl)imino]methyl]-4-methyl-phenol) [40]. In 27 the l6carbonate anion is located near the center of the [Ni6] core and brings together three binuclear fragments, which results in a hexanuclear assembly (Fig. 17b). The magnetic measurements demonstrated that compounds 26 and 27 exhibit antiferro- and ferromagnetic intramolecular interactions, respectively. An unusual arrangement of metal centers around l6-carbonate anion was found for a series of isostructural hexanuclear lanthanide clusters of the general formula [Ln6(Htea)2(H2tea)2(l6CO3)(NO3)2(chp)7(H2O)](NO3)4.5MeOH1.5H2O (Ln = Gd (28), Tb (29), and Dy (30)) (H3tea = triethanolamine; Hchp = 6-chloro-2hydroxypyridine) [41]. The metallic core of 28–30 is best described as four co-planar Ln ions arranged in a trapezoid, with the two others ions lying above and below the central plane (Fig. 18a). The central metal core is supported by partly deprotonated N,O3multidentate H3tea ligands and monoanionic O,N-bidentate chp ligands. Compounds 29 and 30 exhibited single-molecule-magnet behavior. Higher nuclear metal clusters with an encapsulated carbonate anion have also been reported. For example, a nonanuclear Ni(II) cation of the formula [Ni9(l6-CO3)(OH)6(chp)3(Hchp)3(O2CCH2NMe3)9Cl]6+ (31) (chp = 6-chloro-2-pyridonate) was derived from the reaction between a hydrated nickel chloride, Na(chp) and betaine in MeOH [42]. The structure of 31 is based on a planar hexagon of six Ni centers with the central l6-carbonate ion lying on a crystallographic threefold axis (Fig. 18b). The other three Ni (II) ions are symmetrically arranged below the hexagon. This open structure is supported by three monoanionic N,O-chelating chp ligands, three bridging Hchp ligands, as well as nine betaine ligands. Cluster 31 is further surrounded by [Ni(chp)3]
and chloride anions, which block the access to the central carbonate anion. 3.2.2. Self-assembly of metal clusters mediated by one carbonate anion Supramolecular systems derived from carbonate ligand-driven self-assembly of metal-organic aggregates are very rare. For example, the Schiff-base condensation reaction of 1,3-diaminopropane and (S)-(2,6-diformyl-4-tert-butylphenyl)dimethylthiocarbamate (H2L) in the presence of Zn(ClO4)2 and atmospheric CO2 afforded a hexanuclear [Zn6(L2)3(l3-CO3)](ClO4)4] (32) cluster stabilized by ditopic macrocyclic ligand L2 [43]. Compound 32 displays pseudo-C3 point symmetry derived from three bimetallic [Zn2(L2)]2+ units hold together via l3-bridging carbonate lying approximately in the [Zn3] plane (Fig. 19). Another intriguing example showing unique templating properties of carbonate anion concerns molecular assemblies of cubic oxo-metal clusters. The reaction system consisting of Ni(SO4) 6H2O, Ni(ClO4)26H2O, 2-methylbenzoic acid (MeC6H4CO2H), 3dimethylamino-1-propanol with Et3N in a methanol solution allowed for the isolation of a dodecanuclear cationic cluster of the formula [Ni12(l6-CO3)(MeO)12(MeC6H4CO2)9(MeOH)10(H2O)2]+ (33) [44]. The highly symmetrical structure of 33 consists of three distorted [Ni4O4]-type cubanes, which are templated by central l6CO2
3 anion (Fig. 20a). The [Ni4O4] cubanes are additionally bridged by two syn,syn-carboxylate ligands. From another reaction system: magnesium methoxide and CO2 in a methanol solution containing traces of H2O, a nonanuclear monocarbonate cluster of the formula [Mg9(l5-CO3)(O2COMe)8(OMe)8(MeOH)13] (34) was obtained [45].

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Fig. 18. The molecular structures of hexanuclear clusters: (a) Tb(III) 29 and (b) Ni(II) 31; representation (side and top view) of their cores with central l6-carbonate anion of chair conformation: (c) 29 and (d) 31.

Fig. 19. (a), (b) The schematic representation of molecular structure of complex 32 with central l3-carbonate anion; (c) side view of its core.

The molecular structure of 34 consists of two distorted [Mg4O4] cubes, which are linked by the central l5-carbonate ligand and two bridging methylcarbonate groups (Fig. 20b). In this structure, a peripheral magnesium center is present instead of the third [Mg4O4] cluster in 33. The carbonate ligand exhibits planar geometry with near equal CAO bond lengths (1.295 Å) and angles, which indicates a fully delocalized system (the MgAO distances range from 2.031 to 2.148 Å). 3.2.3. Supramolecular architectures templated by two carbonate anions The described family of metal complexes with a carbonate ion as the templating agent also contains systems with two carbonate anions encapsulated simultaneously. An interesting doublecarbonate centered wheel-like arrangement of metal centers was found for a Co(II) carbonate cation, [Co5(CO3)2(bpp)5]+ (35) (Hbpp = 2,6-bis(phenyliminomethyl)-4-methylphenol). 35 was derived from the reaction of Hbpp and Co(ClO4)2 in alkaline conditions (NaOMe) followed by atmospheric CO2 uptake [46]. Cluster

35 comprises quasi-ideal planar pentagonal arrangement of Co ions connected pairwise by the phenolate donors of five N2,O-tridentate bpp ligands (Fig. 21). The approximate D5 symmetry of this assembly is broken by the presence of two central carbonate ligands, capping each face of the pentagon in l3- and l4-coordination modes. These two carbonate anions are located above and below the [Co5] plane defined by the surrounding metals. Interestingly, the irregularity of the carbonate ions coordination to the metal centers resulted in an architecture containing a combination of high-spin of the five- and sixcoordinated Co(II) ions, which strongly influenced magnetic properties of 35. Roughly planar metal-carbonate cores also form structures consisting of two adjacent polygon units. For example, decanuclear [Ln10(l5-CO3)2(OAc)18(HL)2(H3L)2(CH3OH)2]-type clusters (Ln = Pr (III) (36), Nd(III) (37)) stabilized by polydentate Schiff-base H4L, (H4L = 2-(((2-hydroxy-3-methoxyphenyl)methylene)amino)-2-(hy droxymethyl)-1,3-propanediol), were obtained in the reactions of Ln(III) acetate (Ln = Pr and Nd, respectively) and a polydentate

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Fig. 20. The molecular structures of metal-organic aggregates: (a) Ni(II) (33) and (b) Mg(II) (34) with representation of central carbonate anion templating metal-oxo cubanes of the type [M4O4].

Fig. 21. (a) The molecular structure of Co(II) cluster 35 templated by two l3- and l4-carbonate anions and (b) representation of its core (side view).

Schiff-base H4L in a mixture of methanol and acetonitrile, followed by spontaneous fixation of atmospheric carbon dioxide [26,29]. Single-crystal X-ray analysis revealed that both 36 and 37 complexes are isostructural and their decanuclear core is constituted by two sets of l5-carbonate bridged Ln5 pentagons held together through acetate and alkoxide bridges of the multidentate Schiffbase ligand (Fig. 22a). A salient feature of the crystal structures

of 36 and 37 is the presence of the pentanuclear aggregate [Nd5(l5-CO3)(OAc)9(HL)(H3L)(CH3OH)], in which the CO2
anion 3 adopts the l5-j2:g1:g1:g1:g1-coordination mode bridging five metal centers located in the pentagon’s vertices. Thus, these compounds can be formally considered as dimers of pentanuclear aggregates. Interestingly, compound 37 exhibits weak antiferromagnetic coupling between the metal atoms.

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Fig. 22. The molecular structures of metal-organic aggregates with two templating carbonate ions: (a) Nd(III) 37 and (b) Na(I)/Tb(III) 39; schematic representation (side and top view) of their cores: (c) [Nd10(l5-CO3)2] of 37 and [Tb4Na4(l5-CO3)2] of 39.

Fig. 23. (a) The molecular structure of 40 templated by two carbonate ions and (b) representation of its quasi-planar hexanuclear subunit with central l6-carbonate anion and encapsulated ClO
4 anion.

Another example of a supramolecular architecture templated by two carbonate anions is a series of isostructural [((dhop3Ln)2(l5CO3))2Na4(DMSO)4] heterometallic compounds (Ln = Eu (38), Tb (39)) (dhop = 2-(4,5-dihydro-1,3-oxazol-2-yl)phenol). These compounds were obtained by crystallization from a solution containing the corresponding lanthanide salt and NaOH under ambient conditions [47]. Centrosymmetric compounds 38 and 39 consist of two [Na2(dhop3Ln)2(l5-CO3)] moieties forming l5-carbonate bridged pentagons connected by the common edge formed by two Na atoms (Fig. 22b). The carbonate anion bridging mode can be described as l5-j2:j2:j2:g1:g1 with various NaAO distances in the range of 2.278–2.792 Å and similar LnAO lengths of 2.518 Å. A unique ability of carbonate anions to provide symmetrical arrangement of metal centers can be further exemplified by carbonate-mediated hierarchical assembly of metal complexes supported by chiral organic ligands resulting in chiral supramolecular structures. For example, Munno and coworkers showed that

self-organization of ligand-metal species on carbonate cores leads to interesting capsule-like structures [14]. The authors isolated a chiral dodecanuclear Cu(II) complex of the formula [Cu12(Hcyd)12(l6-CO3)2](ClO4)811H2O (40) (H2cyd = cytidine) resulting from spontaneous self-organization of partly deprotonated cytidinate ligands and Cu ions upon atmospheric CO2 uptake in aqueous solution (Fig. 23). Compound 40 exhibits an aesthetic globular structure in which twelve Cu(II) ions are connected by two templating carbonate ions and six monodeprotonated cytidine molecules, to form an ellipsoidal capsule supplied by further six hanging Hcyd
ligands. The l6-carbonate ions are perfectly staggered by two quasi-planar hexanuclear subunits held together by six Hcyd ligands. Six terminal Hcyd
ligands on the opposite sides of the cluster interact through intramolecular hydrogen bonds to build two basketlike cavities capable of anion hosting. An unusual chiral spherical supramolecule [La18((S,S)pcmdo)24(l3-CO3)2(H2O)32]2+ (41) ((S,S)-pcmdo = (S,S)-4,5-di(4-car

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Fig. 24. (a) The molecular structure of aggregate 41 with two templating carbonate ions; (b) representation of its core with two [La9] units connected by (S,S)-H2pcmdo ligand; (c) the right-handed double helices formed in the self-assembly processes involving molecules of 41 and (d) schematic representation of the core with the central l3carbonate anions.

Fig. 25. (a) Schematic representation of the reversible fixation of CO2 by [(2,6-Mes2C6H3)In(l2-OH)2]4 and (b) the molecular structure of indium dicarbonate 42.

boxyphenyl)-2,2-dimethyl-[1,3]dioxolane) was formed in a reaction system involving LaCl36H2O and a chiral ditopic carboxylate proligand (S,S)-H2pcmdo in DEF at 60 °C [48]. The origin of carbonate ions was not clear, however, it was suggested that they might be formed by fixation of atmospheric CO2. The ferritin-like framework of 41 exhibits a trigonal antiprismatic arrangement, and consists of two symmetrical [La9] units (Fig. 24). The nine La ions of each unit possess pyramidal geometry with carbonate anion located at the apex. The edges of the pyramids are defined by three sets of La3[(S,S)-pcmdo]4 units in which La ions are arranged linearly. Every unit is connected with four other equatorial units by

four organic ligands to form a completely integrated assembly. The l3-j2:j2:j2-carbonate anions are located in the center of each [La9] unit, one above another and with slightly disordered oxygen atoms. Single molecules of 41 self-assemble into right-handed double helices stabilized by van der Waals interactions. 3.3. Carbonates as a building motif of macrocyclic and closed polyhedral systems Macrocyclic and polyhedral metal clusters have recently emerged as very attractive building blocks of a variety of inor-

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Fig. 26. (a) The molecular structure of macrocyclic compound 43 and (b) the central [Rh2(l4-CO3)]3 core structure of 43.

ganic–organic supramolecular materials. Interest in coordinationdriven formation of such molecular complexes stems from diversity of their architectures and interesting physicochemical properties. Therefore, the design of self-assembling systems which can produce architectures of this type attract a great attention of the scientific community. Unique features of carbonate anions, i.e. geometry, low steric hindrance and diversity of coordination modes make them a universal templating agent, which can bind metal centers not only as a central ion (Chapter 3.2), but also can form a variety of multinuclear aggregates ranging from macrocyclic systems to closed polyhedrons in which CO2
3 anions act as building units. This carbonate-mediated formation of multinuclear systems is often supported by additional auxiliary multidentate organic ligands leading to specific molecular architectures of unusual geometry and shape.

3.3.1. Macrocyclic metal polycarbonate assemblies Carbonate anions may act as templating moieties taking part in the assembly of smaller metal entities into larger supramacrocyclic architectures. In the following sub-chapter, we describe systems in which simple mono- or bimetallic moieties supported by quite simple ligands are linked by two, three, four, or six carbonate anions to form ring-type core-based structures. Within this group of compounds were found both homo- (tetra-, hexa-, octa-, and dodecanuclear) as well as heterometallic architectures. One of the simplest metal carbonate cylindrical systems is a dimeric In(III) tetranuclear complex, [(2,6-Mes2C6H3)4In4(l2CO3)2(l2-OH)4(H2O)2] (42), which was formed after a few days of exposure of a THF solution of the hydroxo precursor [(2,6Mes2C6H3)In(l2-OH)2]4 to air (Fig. 25) [49,50]. This process can

be accelerated by bubbling CO2 through the reaction mixture. CO2 fixation in this system is reversible: 42 releases CO2 in the temperature range 100–200 °C. The centrosymmetric molecular structure of 42 comprises of two dinuclear indium hydroxide species connected by two CO2
anions. In addition, the carbonate 3 anions form hydrogen bonds between oxygen atom and water molecule coordinated to the metallic center. An original trimeric cyclic metal carbonate structure was found in a simple hexanuclear rhodium carbonate, [(COD)2Rh2(l4-CO3)]3 (43) (COD = cyclooctadiene), which was obtained in the CO2 fixation process mediated by a ruthenium hydroxide precursor, [(COD)2Rh2(OH)2] [51]. Compound 43 consists of three l4carbonate anions, which bind six [Rh(COD)]+ moieties. The resulting macrocycle possesses pseudo C3 symmetry with carbon atoms of CO2
3 anions located in the corners of an almost equilateral triangle (Fig. 26). A larger bimetallic polycarbonate system, (K2(DME)1,5)4[(Cu ((CF3)2(CH3)O)2(l2-CO3))4] (44), was described by Doerrer and coworkers [52]. The Cu(II) carbonate cluster 44 was obtained by CO2-purging of a solution of mixed Cu(III) oxo species, formed in situ by the reaction of K[Cu((CF3)2(CH3)O)2] with molecular oxygen. The central anionic polycarbonate core [Cu((CF3)2(CH3)O)2(l2CO3)]8
4 exhibits S4 molecular symmetry, as it is composed of four [Cu((CF3)2(CH3)O)2] moieties connected by l2-j2:g1-carbonate anions (Fig. 27). The negative charge of copper cluster 44 is compensated by eight potassium cations involved in the construction of the central core. The potassium atoms are additionally caped by the (CF3)2(CH3)O anions with numerous KAF and KAO weak interactions. A very interesting ruthenium-based macrocyclic arrangement of carbonate anions was found in an aggregate [NEt4]2Na2[((Ru (CO)2)2(l4-CO3)Cl)4] (45), which was formed under atmospheric conditions in an alkaline methanolic solution of [Ru(CO)3Cl2(THF)] in the presence of sodium cations [53]. Compound 45 may also be obtained with higher yield when NaHCO3 is used as a carbonate anion source. The tetraanionic ruthenium cluster 45 possesses the S4 symmetry and consists of four dinuclear [Ru(CO)2]2+ 2 moieties bridged by four l4-CO2
and four l2-Cl
anions (Fig. 28). 3 The carbonate anions act like a double crown capping two sodium cations. As a result, a 1D supramolecular structure is formed with [Na(H2O)(EtOH)]2+ 2 linking species. A family of suprametallomacrocycles based on six carbonate units is represented by three following compounds, every of which exhibits an entirely different molecular architecture. Two of them consist of a hexagonal core, whereas the third one can be regarded as built from two coaxially assembled trimeric metal-carbonate cycles (Fig. 29–31, respectively). Compound Li6[(t-Bu)ArN)3Ti(l4-

Fig. 27. (a) The molecular structure of 44; (b) the anionic Cu(II) polycarbonate [Cu((CF3)2(CH3)O)2(l2-CO3)]8
4 unit and (c) top view of the [Cu(l2-CO3)]4 core along the S4 symmetry axis.

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4
Fig. 28. (a) The molecular structure of 45; (b) an anionic polycarbonate [(Ru(CO)2)2(l4-CO3)Cl]4
4 unit; (c) top view of the [Ru2(l4-CO3)Cl]4 core along the S4 symmetry axis and (d) supramolecular 1D structure of 45.

Fig. 29. The molecular structure of (a) monomeric Li[(t-Bu)ArN)3Ti(l2-CO3)] unit and (b) heterometallic assembly 46; (c) side and top (along the S6 symmetry axis) view of the hexagonal [Li(CO3)]6
6 core.

CO3)]6 (46) (Ar = 3,5-Me2C6H3), reported by Cummins and coworkers, was obtained in the reaction of CO2 with a lithium salt of a titanium oxo complex, Li[(t-Bu)ArN)3TiO] [54]. It consists of a hexameric structure based on a prismatic Li6O6 core with the S6 symmetry (Fig. 29). The monomeric unit Li[(t-Bu)ArN)3Ti(l2CO3)] contains Li(I) ion coordinated in an anisobidentate manner. Within the macrocycle 46, l4-j2(O,O0 ):g1(O):g1(O):g1(O00 )carbonate anions are located in vertices of a triangular antiprism, each one bridging one Ti and three Li centers. Interestingly, compound 46 released CO2 after dissolution in diethylether followed by addition of 12-crown-4. A larger hexagonal polycarbonate metal macrocycle was found in an uranium(IV) compound [(Me5Cp)2U(l2-CO3)]6 (47). This compound was obtained by reductive disproportionation of carbon dioxide on a variety of U(III) complexes [55]. In this case, all the l2j2:g1-CO2
3 anions are located in vertices of a hexagon and connect two uranium atoms forming a large macrocycle with S6 molecular symmetry (Fig. 30).

A novel type of metal carbonate cluster was reported very recently by Lewin´ski and coworkers [56]. The CO2 fixation by a well-defined alkylzinc hydroxide precursor, [t-BuZnOH]6, in the presence of t-Bu2Zn led to the formation of a dodecanuclear alkylzinc polycarbonate cluster, [(t-BuZn)2(l5-CO3)]6 (48), the first reported alkylzinc carbonate aggregate. Architecture of 48 is a result of self-assembly of six [(t-BuZn)2(CO3)] units into a nanometer-sized barrel-like structure (ca. 14.0–17.0 Å). The molecular structure of 48 consists of two coaxially capped trimeric zinc carbonate, [(t-BuZn)2(l4-CO3)]3, rings (Fig. 31). The molecule possesses the S6 symmetry and all six l5-j2:g1:g1:g1:g1carbonate anion bind five Zn(II) centers. 3.3.2. Macrocyclic metal polycarbonate assemblies supported by multidentate ligands Macrocyclic metal-carbonate cores may be additionally stabilized by multidentate ligands, which can form extra connections between metal centers. This enables self-assembly of more com-

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Fig. 30. (a) The molecular structure of suprametallomacrocycle 47 and (b) top (along the S6 symmetry axis) view of the hexagonal [U(l2-CO3)]12+ core. 6

Fig. 31. (a) The molecular structure of barrel-like organozinc polycarbonate 48 and (b) schematic representation of the cyclic [(t-BuZn)2(l4-CO3)]3 subunit; (c) view along S6 symmetry axis of the zinc carbonate core.

plex di- and trimetallic subunits into macrocyclic structures. In this sub-chapter, there are described metal-carbonate compounds supported by multidentate ligands forming suprametallomacrocycles of a square or hexagonal geometry. Up to now, four homometallic [57–59] and four heterometallic [60] systems of this type obtained by the CO2 fixation are known. One of the simplest macrocyclic metal polycarbonate compound belonging to this family is an octanuclear Ni(II) cluster of the formula [Ni8(H2bpmp)4(l4-CO3)4(HIm)8](NO3)4 (49) [57]. Compound 49 was isolated form the reaction mixture involving Ni (NO3)26H2O, a 2,6-bis-[(3-hydroxy-propylimino)-methyl]-4-meth yl-phenol proligand (H3bpmp), imidazole (HIm), and Et3N. The cationic part of 49 is built by four dinuclear [Ni2(H2bpmp)]3+ units linked by four l4-carbonates generated in situ through the fixation of atmospheric CO2 (Fig. 32). The CO2
3 anions are located in vertices of an slightly distorted square and adopt l4-coordination mode. The eight Ni(II) ions are positioned in vertices of a distorted cube making the whole molecule possess S4 molecular symmetry. The macrocyclic core is stabilized by a partly deprotonated N2, O3-pentadentate ligand H2bpmp.

A more distorted-square polycarbonate structure was revealed in an octanuclear Dy(III) cluster, [Dy8(l4-CO3)4(ovpyh)8(H2O)8] (50). This compound was obtained under atmospheric conditions in the reaction of DyCl36H2O with (E)-N0 -(2-hyborxy-3-methoxy benzylidene)pyrazine-2-carbohydrazide (H2ovpyh) in MeOH/CH2Cl2 solution in the presence of triethylamine [58]. The molecular structure of 50 consists of the metal-carbonate core with the pseudo S4 symmetry. The core is built by eight Dy(III) centers connected by four l4-j2:g1:g1:g1-CO2
3 anions and eight l2-O bridges of carbonyl groups from supporting hydrazone ligands (Fig. 33). The resulting macrocyclic core is stabilized by eight anionic multidentate ovpyh ligands. Additionally, the metal centers coordinate two water molecules, which in turn form hydrogen bonds with one of the carbonate g1-oxygen atoms and the ligand-appended methoxy groups. Additional intermolecular double hydrogen bonds between dysprosium-coordinated water molecules and the pyrazine groups of adjacent molecules in 50 stabilized the formed 1D zig-zag supramolecular polymeric structure (Fig. 33c). A novel family of macrocyclic heterometallic polycarbonates, [Ni8RE4(hmbp)8(l5-CO3)4(CH3OH)4(H2O)8](ClO4)4 (RE = Y (51), Dy

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Fig. 32. (a) The molecular structure of square-like suprametallomacrocycle 49 and (b) top (along the S4 symmetry axis) view of its [Ni(l4-CO3)]4 core.

Fig. 33. (a) The molecular structure of square-like suprametallomacrocycle 50 and (b) side and top (along C2 symmetry axis) view of its [Dy(l4-CO3)(H2O)]4+ 4 core; (c) the supramolecular structure of the 1D zig-zag polymer.

(52)) and [Ni8RE4(hmbp)8(l5-CO3)4(j2-CO3)2(H2O)14] (RE = Gd (53), Dy (54)), was obtained upon crystallization from a methanolic or ethanolic solution of a mixture of RE(NO3)3 (RE = Y, Dy or Gd), Ni (ClO4), 2-(2-hydroxy-3-methoxybenzylideneamino)phenol) (H2hmbp) and Et3N under atmospheric conditions [60]. The resulting complexes possess analogous core structures with four l5carbonate anions binding two RE(III) and three Ni(II) atoms

(Fig. 34). The Ni centers are paired by double l2-O aryloxide bridges forming dinuclear [Ni2(hmbp)2] moieties supported by the fully deprotonated N,O,O0 ,O00 -tetradentate hmbp2
ligand molecules. These moieties act as metalloligands capping both sides of the [RE(l2-CO3)]4
macrocycle. The RE(III) atoms additionally 4 coordinate two water molecules, which form hydrogen bonds with the carbonate g1-oxygen atoms and the ligands’ methoxy groups.

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Fig. 34. (a) The molecular structure of heterometallic polycarbonate 52 and (b) top view of [Ni8Dy4(l5-CO3)4]20+ core along S4 symmetry axis.

The coordination sphere of the metal centers in 51 and 52 is completed by methanol molecules or, in the case of 53 and 54, by methanol and terminal j2-carbonate anions. An original polycarbonate-metal hexagonal system was revealed in dodecanuclear lanthanide clusters, [Ln12(msta)6(OH)4O2(l4-CO3)6][Ln12(msta)6(OH)4O4(l4-CO3)4(l2-CO3)2](ClO4)4 (Ln = Dy (55), Ho (56)) [59]. These compounds were obtained in the reaction of Ln(ClO4) and N1,N3-bis(3-methoxysalicylidene)diethyle netriamine (H2msta) in the presence of Et3N and atmospheric CO2. Compounds 55 and 56 are isostructural and, in the crystal structure, they contain two slightly different entities, [Ln12(msta)6(OH)4O2(l4-CO3)6]4+ and [Ln12(msta)6(OH)4O4(l4-CO3)4(l2-CO3)2] (Fig. 35). The cores of both clusters consist of six trinuclear [Ln3(msta)]7+ moieties bridged by six alternately located carbonate anions. Every trimetallic moiety includes one metal center located inside the ligand’s N3O2-cavity, and two other metal atoms are flanked by four oxygen atoms from two msta2
ligands (Fig. 35e). The Ln(III) atoms are additionally connected by hydroxo l3-OH or oxo l3-O species. In the cationic unit, [Ln12(msta)6(OH)4O2(l4CO3)6]4+, all CO2
ions are bound to the four metal centers. For 3 comparison, the second neutral unit [Ln12(msta)6(OH)4O4(l4CO3)4(l2-CO3)2] additionally possesses a l2-O2
bridge and two of carbonate anions that are coordinated only to two metal centers. Moreover, in the latter case, the third oxygen atom of CO2
3 is directed towards the center of the molecule and is stabilized by hydrogen bonds formed with the OH groups. 3.3.3. Metal polycarbonate assemblies with prismatic core A triangular prism is one of the simplest polyhedral geometries that may be templated by carbonates as building blocks. Interestingly, the prismatic metal carbonate complexes are formed only in the CO2 fixation processes mediated by the corresponding metal precursor. Two families of metal-carbonate assemblies with a prismatic metal-carbonate core were identified: (i) a group of five lanthanide compounds with a trigonal prismatic and a space-centered trigonal prismatic core, and (ii) a group of three nickel clusters which possess essentially the same rectangular face-centered triangular prism metal-carbonate core. There is also one example of Cd(II) compound with a triangular antiprism geometry. An ideally prismatic metal-carbonate core was found in two similar hexanuclear Dy(III) compounds stabilized by two similar multi-

dentate ligands: [Dy6(ovpich)4(Hovpich)2(l3-CO3)2Cl4(H2O)2] (57) (H2ovpich = (E)-N0 -(2-hydroxy-3-methoxybenzylidene)picolinohy drazide) and [Dy6(ovpyh)5(Hovpyh)(l3-CO3)2(OAc)3(MeOH)2] (58) (H2ovpyh = (E)-N0 -(2-hyborxy-3-methoxybenzylidene)pyrazine-2carbohydrazide) [61,62]. Both complexes consist the same [Dy6(l3CO3)2]14+ core comprising two triangular structures built by three Dy(III) ions templated by a l3-j2:j2:j2-carbonate anion. The carbonate anions constitute two base plains of the triangular prism (Fig. 36). Cluster 57 was obtained by bubbling CO2 through the reaction mixture, which involved DyCl36H2O, H2ovpich and Et3N. Interestingly, the use of Na2CO3 instead CO2 led to formation of macrocyclic assembly [Dy8(ovpich)8(l4-CO3)4(H2O)8], which is isostructural with the compound 50 (Fig. 33). In turn, compound 58 was obtained in the reaction of Dy(OAc)36H2O and H2ovpyh in the presence of Et3N under atmospheric conditions. Notably, a similar synthetic procedure involving DyCl36H2O (instead of Dy(OAc)3) yielded the earlier-described macrocyclic compound 50 [58]. The described results nicely show how the source of carbonate anions can influence transformations in the reaction systems and, consequently, the structure of the final product (Fig. 36c). A group of isostructural Ln(III) complexes, [Ln7(l3-OH)6(l3-CO3)3(sach)3(Hsach)3(MeOH)6] (Ln = Gd (59), Tb (60), Dy (61)) (H2sach – N-salicylidene-2-aminocyclo-hexanol), stabilized by a partially deprotonated N,O,O0 -tridentate ligand comprise a metal-carbonate core of space-centered trigonal prism geometry (Fig. 37) [63]. The base plains of the prisms are built by [Ln(j3-sach)] moieties connected by l3-OH bridges forming [Ln(l3-OH)]3 hydroxo lanthanide triangles. The [Ln(l3-OH)]3 units are further bound by three l3-j2:j2:g1-carbonate anions with the central Ln(III) ion. A different type of centered prismatic geometry occurs in three diverse Ni(II) hydroxo dicarbonate compounds, [Ni9(l3-OH)6(l6CO3)2(l2-pz)6(Hpz)12](ClO4)0.5(OH)1.5(H2O)7.5 (62), K4[Ni9(tu)6(OAc)6(H2O)6(l3-OH)6(l6-CO3)2] (63) and [Ni9(Hpspa)6(l3OH)6(l6-CO3)2(H2O)6]3H2O(OH)2 (64), all obtained by the fixation of atmospheric carbon dioxide [64–66]. The prismatic assembly 62 is stabilized by protonated (Hpz) and deprotonated (pz) pyrazole moieties acting as mono- and bidentate ligands, respectively. In contrast, compound 63 consists of two types of stabilizing ligands, namely acetate and monodeprotonated taurine (H2tu
). The latter is coordinated only by the amine group. For comparison,

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Fig. 35. The molecular structure of two types of entities present in the crystal structure of 55: (a) [Dy12(msta)6(OH)4O2(l4-CO3)6]4+ and (b) [Dy12(msta)6(OH)4O4(l4-CO3)4(l2CO3)2]; (c) side and top view of [Dy12(OH)4O2(l4-CO3)6]16+ and (d) [Dy12(OH)4O4(l4-CO3)4(l2-CO3)2]12+ core; (e) schematic representation of the [Dy3(msta)(O)]5+ building unit.

cluster 64 is stabilized only by N,O2-tridentate 3-(Pyridine-3-sulfo nylamino)-propionic anion (Hpspa). Despite different stabilizing ligands, all these compounds are based on analogous [Ni9(l3OH)6(l6-CO3)2]8+ core with slightly twisted rectangular facecentered triangular prism geometry (Fig. 38). Two l6 carbonate anions are located in the bases of the prism, each bonded with three prism’s vertices and three face-centered Ni atoms. CO2
3 anions are mutually rotated for about 44°–48°. A unique cadmium carbonate complex, [Cd6(mpat)6(l3-CO3)2] (ClO4)2 (65), possesses an ideal triangular antiprism core geometry. This hexanuclear cluster was obtained from the mixture of Cd (ClO4)26H2O and 2-((2-(methylthio)ethyl)(pyridin-2-ylmethyl)am ino)ethanethiol (Hmpat) in the presence of Et3N and atmospheric CO2 (Fig. 39) [67]. Compound 65 is stabilized by the N,N0 ,S,S0 -tetra dentate thiolate ligand and consists of two metal-carbonate triangles, which are analogous to those observed in 57 and 58. However, in 65 these triangles are mutually rotated by 60°, which is consistent with the S6 molecular symmetry of the assembly.

3.3.4. Metal polycarbonate assemblies with octahedral core On account of the D3h symmetry of the carbonate anion, this ligand is expected to template regular metallic systems. Indeed, there are several metal polycarbonates with a highly symmetric metal core. This emerging family of octahedral cages is of arousing interest owing to their intriguing molecular and supramolecular structural diversity and unique physicochemical properties. Two isostructural Ni(II) compounds, [Ni6(l3CO3)4(TMEDA)6(H2O)12]Br4 (66) and [Ni6(l3-CO3)4(TMEDA)6 (H2O)12]Cl4 (67), were reported by Reglinski and Anderson [68,69]. These assemblies consist of an octahedral [Ni6(l3-CO3)4]4+ core stabilized by simple N,N0 -bidentate tetramethylethylenediamine (TMEDA) ligand. Compound 66 was obtained in the reaction of nickel bromide with TMEDA in the presence of tris-[(2-hydroxyben zylidene)aminoethyl]amine (TrenSal) and atmospheric carbon dioxide. In contrast, compound 67 was isolated from a mixture of bis-(cycloocta-1,5-diene)nickel and TMEDA in a CO2-saturated THF solution. The cationic unit of these complexes consist of six

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Fig. 36. (a) The molecular structure of dicarbonate cluster 57 and (b) schematic representation of prismatic [Dy6(l3-CO3)2]14+ core; (c) synthetic pathways leading to a variety of Dy(III) compounds stabilized by ligands derived from H2ovpich and H2ovpych.

Fig. 37. (a) The molecular structure of polycarbonate Dy(III) assembly 61 and (b) schematic representation of its prismatic [Dy7(l3-CO3)3]15+ core.

[Ni(TMEDA)(H2O)]2+ units bound together by four l3-carbonate anions which are located on the sides of the metal octahedron (Fig. 40). The central core is additionally stabilized by hydrogen bonds formed between carbonate oxygen atoms and the coordinated water molecules. The corresponding halogen anions also reside in the second coordination sphere. Both complexes crystalized in regular crystallographic systems. Compound 66 possesses two polymorphic forms, one of which assembles into a porous supramolecular structure with remarkably large pore space

(4781 Å3 per unit cell which is 25.6% of the unit cell volume) filled by water or solvent molecules (Fig. 40d). One of the most spectacular groups of multinuclear metalcarbonate clusters is this of decanuclear complexes with the general formula [M10(l6-CO3)4(q)12] (M = Zn(II) (68) [70], Co(II) (69) [71,72], Mn(II) (70) [71] (Hq = 8-hydroxyquinoline). These clusters are also unique as metal carbonate aggregates of fused networks of four- and six-membered heterocyclic rings. As an example, Lewin´ski and coworkers successfully employed an original

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Fig. 38. (a) The molecular structure of dicarbonate Ni(II) assembly 62 and (b) side and top view (along C3 symmetry axis) of the [Ni9(l6-CO3)2]14+ core.

Fig. 39. (a) The molecular structure of dicarbonate Cd(II) assembly 65 and (b) side and top (along S6 symmetry axis) view of antiprismatic [Cd6(l6-CO3)2]8+ core.

organozinc hydroxide [Zn4(l3-OH)(q)4(t-Bu)2] supported by 8hydroxyquinolinate ligands to react with CO2 at room temperature. As a result, a highly luminescent decanuclear zinc carbonate cluster 68 (denoted also as WUT-1) is formed (Fig. 41) [70]. The molecular structure of WUT-1 can be viewed as a tetrahedral zinc carbonate core [Zn(l6-CO3)]4 encapsulated in an octahedral hexazinc quinolinate shell [Zn(q)2]6. Each carbonate anion acts as a l3bridging ligand located between six zinc centers. Four carbonate ions and four Zn centers occupy alternate positions above the facets of the octahedral core. Interestingly, all three quinolinate ligands are oriented perpendicularly to the plane of carbonate ions, which implies the presence of four triangular pockets (ca. 3.5 Å in diameter) behaving as intrinsic pores in the solid-state structure of WUT-1. The spherical nanoclusters of WUT-1 self-assemble

through CHAr-p cooperative interactions to produce an extended 3D network with two types of interconnected voids. Molecules of WUT-1 pack to form a diamondoid lattice (Fig. 41d). WUT-1 exhibited excellent gas sorption and luminescent properties. As opposite to WUT-1, the analogous clusters 69 and 70 were obtained under solvothermal conditions from the corresponding inorganic salts, cobalt (II) acetate [72], or cobalt (II) and manganese (II) chloride, respectively, in a DMF solution. In both complexes, the origin of carbonate anions was unclear. The authors stated that CO2
could be derived from the decomposition of the acetate 3 ligand or DMF under the applied reaction conditions. Interestingly, while WUT-1 exhibits diamondoid crystal lattice with high surface area, the supramolecular structures of 69 and 70 are based on close packing of molecules and are essentially nonporous.

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Fig. 40. (a) The molecular structure of cluster 66 and (b) schematic representation of octahedral [Ni6(l3-CO3)4]4+ core; (c) the network of hydrogen bonds (green dashed lines) in structure 66 (ligands has been omitted for clarity); (d) the porous supramolecular structure of compound 66 (view along the crystallographic a axis).

Fig. 41. (a) Molecular structure of cluster 68; (b) schematic representation of the [Zn10(l6-CO3)4]12+ core; (c) space-filled view of the molecule of 68 (yellow spheres represent 3.5 Å diameter ultramicropockets resulting from the arrangement of quinolinate ligands); (d) projection of the supramolecular porous structure of WUT-1 along (1 1 0) with yellow spheres representing main cavities.

Among zinc carbonate octahedral clusters, another particularly interesting result was reported by Cooper and coworkers [73]. Using an intrinsically porous dodecaamine cage as the organic lin-

ker donor (Fig. 42), and Zn(NO3)26H2O as metal precursor, the authors received an interesting supramolecular system denoted as [(Zn6(l3-CO3)4(C48H72N12)](CO3)(NO3)2(H2O)19 (71). Here, the

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Fig. 42. Schematic representation of (a) the organic cage linker and (b) inorganic octahedral [Zn6(l3-CO3)4]4+ core; (c) the 3D supramolecular structure of compound 71; coordination environment of the (d) organic ligand and (e) Zn(II) cluster.

Fig. 43. (a) The molecular structure of the Ce(III) polycarbonate assembly 73 and (b) schematic representation of the [Ce13(CO3)14]11+ core with the presentation of internal and external carbonate anions.

uptake of atmospheric CO2 resulted in the forming of octahedral Zn (II) carbonate [Zn6(l3-CO3)4]4+ cores stabilized by the N-donor centers of the dodecaamine cage. The multinuclear zinc clusters and the macrocyclic organic linkers self-assembled into a 3D cage metal-organic framework, which exhibited unique gas sorption properties. 3.3.5. High-nuclearity polycarbonate metal assemblies with ball-like core The CSD contains many high nuclearity metal carbonate clusters with a core of approximately spherical, distorted polyhedron geometry. Most of them were obtained with use of inorganic salts as a source of carbonate anions, only four compounds were obtained by the CO2 fixation. Three of them form a group of

isostructural ‘‘lanthaballs”, and the fourth is a Co15 cluster with distorted polyhedral core. A series of isostructural tridecanuclear Ln(III) complexes, [Ln13(ccnm)6(l3-CO3)14(H2O)6(phen)18]5+ (Ln = La (72), Ce (73), Pr (74)), was isolated from the reaction mixtures involving LnCl3xH2O salt, (Et4N)(ccnm) (where ccnm = carbamoylcyanonitrosometha nide), 1,10-phenanthroline (phen) and CO2 [74]. The core of these complexes consists of twelve Ln(III) centers arranged in distorted icosahedron built around the central Ln(III) center surrounded by six carbonates (Fig. 43). The metal atoms located on the polyhedron surface are bound together by six inwardly facing l3-j2:j2: j2-carbonate ligands. The central Ln atom possesses a unique twelve-membered coordination sphere. Next, eight l3-j2:j2:j2and l3-j2:j2:g1-carbonate anions are located on the polyhedron

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Fig. 44. (a) The molecular structure of the Co(II) polycarbonate assembly 75 and (b) schematic representation of the [Co15(l6-CO3)6]18+core.

Fig. 45. (a) The molecular structure of the La(III) polycarbonate assembly 76 and (b) schematic representation of its [La6(l3-CO3)6]6+ core.

surface. Interestingly, molecules of both types of supporting organic ligands, phen and ccnm
, are bound to single lanthanide center, and adopt the j2- or g2(N,O)-coordination mode, respectively. Analogous high-nuclearity polycarbonate assemblies were obtained by the use of NaHCO3 and Na2CO3 instead of CO2 as a carbonate ion source. However, in this case, the polycarbonates possess a different arrangement of molecules in the crystal lattice [74]. This difference shows that the source of carbonate ion can affect not only the molecular structure of the resulting compounds, but can also influence self-assembly processes, and lead to different supramolecular systems. Dalgarno and co-workers reported a higher nuclearity polycarbonate metal cluster [Co15(H1.33mbhph)6(l6-CO3)6(l3-

OMe)2(l-dmf)3(dmf)5(MeOH)0.5(H2O)2.5](MeOH)(dmf)5 (75) (H4mbhph = 6,60 -methylenebis(4-(tert-butyl)-2-(hydroxymethyl)-phe nol), which possesses a distorted polyhedral core containing fifteen Co(II) centers [75]. Compound 75 was isolated from the reaction of Co(NO3)26H2O and H4mbhph in the presence of Et3N and atmospheric CO2. The core of 75 is built by fifteen Co(II) ions located in vertices of highly distorted polyhedron, which may be envisaged as a loosely equatorial enneanuclear belt, bi-capped by triangular clusters at each pole (Fig. 44). The metal centers are connected by six l6-CO2
3 anions located on the surface of the polyhedron. This system is stabilized by six partially deprotonated O2,O02 tetradentate mbhph ligands and two l3-OMe groups. The coordination sphere of Co(II) centers is completed by solvents molecules.

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Fig. 46. (a) The molecular structure of the heterometallic Gd(III)/Cu(II) cluster 77 and (b) schematic representation of its [Gd6Zn2(CO3)5]12+ core.

Fig. 47. (a) The molecular structure of the heterometallic Gd(III)/Cu(II) assembly 82 and (b) schematic representation of the [Gd6Cu3(CO3)4]16+ core; (c) the supramolecular structure of a 2D honeycomb-like framework of 82 (view along crystallographic c axis).

3.3.6. High-nuclearity distorted polycarbonate lanthanide assemblies Numerous high-nuclearity polycarbonate lanthanide complexes were obtained using various sources of CO2
3 anions. In this subchapter, we focus only on those obtained by the CO2 fixation. This group of lanthanide polycarbonates exhibits significantly lower symmetry in comparison to metal carbonates described above (cf. Chapter 3.3.3–3.3.5). A typical example is a hexameric carbonate lanthanum cluster, [La6(tpen)4(l3-CO3)6(CH3CN)2(H2O)2][OTf]66CH3CN (76), which is stabilized by tetrapodal N,N,N0 ,N0 -tetrakis-

(2-pyridyl-methyl)ethylenediamine (tpen) ligand. Cluster 76 was obtained by recrystallization of a hydroxo precursor [La(tpen)(l-OH)]2(l-OTf)[OTf]33MeCN under atmospheric conditions [76]. The core of 76 can be viewed as a highly distorted octahedron with six La(III) ions located at the vertices, and spanned by six bridging carbonate ligands: two l4 -carbonates facing inwards and four l3-j2:j2:g1-carbonates in sides of the polyhedron (Fig. 45). In many cases lower of symmetry of a metal-carbonate core is caused by the presence of different metal cluster units. The

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Fig. 48. (a) The molecular structure of the heterometallic Gd(III)/Cu(II) assembly 83 and (b) schematic representation of the [Cu15Gd7(CO3)4]43+ core.

carbonate-templated high-nuclearity 3d–4f heterometallic aggregates received considerable attention because of their fascinating self-assembled structures, as well as interesting magnetic, optical and electronic properties and numerous potential applications. Here, we focus only on those derived for the CO2 fixation. A series of isostructural Ln(III)/Zn(II) compounds, [Ln6Zn2(l4-CO3)2(l3CO3)3(OH)(H2apeo)4(H3apeo)2(H4apeo)]+ (Ln = Gd (77), Dy (78), Sm (79), Eu (80), Tb (81)) (H4apeo = (Z)-1-(3-((bis(2-hydroxye thyl)aminomethyl)-2-hydroxy-5-methylphenyl)ethan-1-one oxime)), was obtained via serendipitous fixation of atmospheric CO2 in the one-pot synthesis involving Zn(NO3)2, Ln(NO3)3, H4apeo and a various ratio of t-BuONa and Et3N in MeOH [77]. The metallic skeleton of the resulting complexes is based on a highly distorted [Ln6] octahedron with two Zn(II) ions, both capping a different triangular face (Fig. 46). The metallic centers are linked by five l3-j2:j2:j2-, l3-j2(O, 0 O ):g1(O):g1(O00 )- and l4-j2:j2:g1:g1-carbonate anions. The [Ln6Zn2(CO3)5]10+ core of the molecule is stabilized by partially deprotonated multidentate ligands which are coordinated to the metal centers in a few different ways. Two types of carbonate-panelled heterometallic Gd(III)/Cu(II) cages with different nuclearity were obtained in reactions between a N,O2-tridentate proligand, pyridine-2,6-dimethanol (H2pdm) and different copper salts as Cu(II) sources [78,79]. Compound [Gd6Cu3(OH)(pdm)3(CO3)4(Piv)9(MeOH)3] (82) was formed by CO2 bubbling through a methanol solution of Cu(NO3)2, Gd(NO3)3, H2pdm and sodium pivolate. In contrast, [Cu15Gd7(OH)6(pdm)9(H2pdm)3(CO3)4(O2CPh)19(H2O)2] (83) is derived from a MeCN solution of Cu(O2CPh)2, Gd(NO3)3, H2pdm and Et3N and formed under atmospheric conditions. Cluster 82 contains a metallic cage described as a distorted tridiminished icosahedron [79]. The metal ions in the central core are bridged by three l5:j2:g1:g1:g1:g1carbonate anions paneling the pentagonal [Gd4Cu] faces of the prism (Fig. 47). Three Cu(II) and three Gd(III) anions in the base of the polyhedron are bridged by fourth l6-carbonates. In the crystal structure, adjacent molecules of 82 self-assemble by intermolecular p–p and CAH  O interactions into a 2D honeycomblike framework with 1D open channels along the crystallographic c-axis (Fig. 47c). The core of compound 83 consists of seven Gd(III) ions located in the vertices of an irregular polyhedron, which may be described as a capped triangular antiprism. The metal centers are connected

by three l5-j2:g1:g1:g1:g1- and one l6-carbonate bridges (Fig. 48). The resulting inner [Cu3Gd7(CO3)4]19+ sub-core is further bridged through l3-hydroxo and alkoxo groups to three extrinsic [Cu4] subunits. The peripheral ligation of the core is provided by chelating pdm and H2pdm ligands, two terminal H2O molecules, and nineteen PhCO2 ligands. A series of unusual multinuclear heterometallic clusters was recently reported by Zheng and coworkers. The authors isolated clusters, whose structure was based on a triple bowl-like fiftytwo-metal-cation core [Ln42M10(l3-OH)68(l5-CO3)12]55+ (Ln = Gd, Dy; M = Co2+/3+, Ni2+) templated by twelve carbonate ions [80]. Particularly, compound [Gd42Co10(l3-OH)68(l5-CO3)12]55+ (84) draws attention as it is the only one within this group containing metal centers on different, Co(II)/Co(III), oxidation states. Moreover, this aggregate exhibits impressively large magnetocaloric effect. The polycarbonate 84 is constructed from three different types of subunits: [Gd8(l3-OH)9(l5-CO3)2]11+ (I), [Co(III)Co(II)3(l3-OH)(l5CO3)3]2+ (II) and [Gd6Co2(l3-OH)12(l5-CO3)]8+ (III) (Fig. 49). Subunit I is made of one [Gd5(l3-OH)5]10+ square pyramid and one cubane-like [Gd4(l3-OH)4]8+ cluster, which share one Gd(III) vertex. Subunit II is a distorted tetrahedron with three Co(II) centers joined by (l3-OH) bridges in a basal plane and one Co(III) in the fourth vertex capped with three bridging carbonate anions. Subunit III possesses two distorted cubane-like [Gd3Co(l3-OH)4]7+ clusters and one cuboidal [Gd3(l3-OH)4]5+ cluster sharing three Gd(III) vertices. Compound 84 consists of three type I, one central type II, and three type III moieties connected by l5:j2:g1:g1:g1:g1-CO2
3 and l3-OH
bridges, which form three bowl-like cavities with encapsulated ClO4 anions (see Fig. 49). 4. Conclusion and perspectives of the conversion of CO2 into metal carbonate-based functional materials An intentional usage of CO2 as freely available C1 building block of value-added compounds has emerged as one of the most important issues in the development of environment friendly synthetic approaches. In this regard, CO2 fixation in inorganic systems is a pathway of great potential in the synthesis of novel molecular and higher organized entities of desired functionalities. Unique features of the carbonate anion, i.e. the geometry, low steric hindrance and ability to form a plethora of coordination modes, make it a universal templating agent and building unit. Nevertheless,

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Fig. 49. (a) The molecular structure of the heterometallic Gd(III)/Co(III)/Co(II) assembly 84 and (b) schematic representation of the tribowl-like metal polycarbonate core of 84; (c) schematic representation of complex structure of cluster 84.

Fig. 50. (a) Schematic representation of reactive species in a RZnOH moiety and (b) the transformations of RZnOH/CO2 reaction systems (from the left) to mesoporous solid based on ZnCO3 nanoparticles, organozinc carbonate nanocluster 48 and 68 as nanometric molecular building block of microporous material WUT-1.

while CO2 is widely used for the preparation of organic compounds in metal-catalyzed systems [6], the rational construction of functional materials formed as a result of CO2 fixation is a much less explored and highly undeveloped issue. Strikingly, materials based on metal carbonates were in most cases isolated accidentally [9].

We provided a comprehensive overview on the structural diversity of multinuclear metal monocarbonate clusters and polycarbonate macrocyclic assemblies derived from the chemical fixation of CO2. Their structural analysis shows the potential of carbonate ions as a versatile building motif for the construction of polymetallic clusters of emerging importance in materials chemistry. The carbonate anion appears as a universal templating agent, which can mediate the formation of a variety of multinuclear aggregates ranging from macrocyclic systems to closed polyhedra. Moreover, the encapsulation of CO2
3 within multimetallic polyhedral structures with defined inner cavity led to the identification of new and unusual coordination modes of carbonate anion. A significant number of metal-carbonate cages have been obtained serendipitously, usually in low yields, from the reaction systems involving moist solvents in the presence of atmospheric CO2. Only a few examples of high-nuclear metal clusters were obtained by deliberate employment of CO2 as a reaction ingredient. It seems particularly surprising taking into account the advantages of CO2 as a reagent, i.e. (i) applicability of both gaseous and solid-state forms, (ii) processing in ambient temperature and pressure as well as in organic solvents and aqueous environment. Interestingly, some types of particularly interesting polycarbonate metal assemblies are formed only by CO2 fixation. The presented literature survey and in-depth analysis of reaction environments leading to metal carbonates, provide useful hints about how to design efficient systems for CO2 conversion into interesting and valuable inorganic products [81]. It is reasonable to assume that, in most of the described cases, one of the prerequi-

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sites of the fixation of CO2 on metal centers is the presence of water or/and hydroxide units in the reaction environment. Furthermore, taking into consideration mode of action of carbonic anhydrase and reversibility of the CO2-hydration as well as the equilibrium between bicarbonate and carbonate ions in solution (Chapter 2.2), it seems desirable for precursor system for CO2 conversion to consist not only of metal hydroxide moieties but also the species capable of efficient proton accepting. This process can be identified as a force driving the reaction toward metal carbonate products. This view is supported by very recent studies demonstrating that RZnOH-type moieties featuring a CO2-reactive Zn-OH group and a proton-reactive ZnAC bond (Fig. 50a) possess unique ability to fixate CO2 and afford various zinc carbonate products (Fig. 50b). These preliminary investigations provided the first example of structurally-characterized alkylzinc hydroxide, [t-BuZnOH]6 [82]. In the presence of CO2, this cluster can form different carbonatetemplated nanomaterials, e.g. ZnCO3 nanoparticles or a discrete alkylzinc carbonate cluster [(t-BuZn)2(l5-CO3)]6 (48) (Chapter 3.3.1) [56]. Moreover, the introduction of an auxiliary ligand L to the RZnOH system followed by the reaction with CO2 led to the formation of a nanometric cluster [Zn10(l6-CO3)4(q)12] (Hq = 8hydroxyquinoline) (Fig. 50b), which in turn self-assembles into a non-covalent porous material WUT-1 (68) (Chapter 3.3.4) [70,83]. These studies clearly underline the increasing potential of simple organozinc precursors featuring CO2-reactive Zn-OH groups and proton-reactive ZnAC bonds for the design of new functional materials based on zinc carbonate components. To conclude, the carbonate anion can serve as a very efficient templating agent for the construction of a variety of inorganic building blocks of functional materials. We envision that research in this area will expand greatly in the near future, which will contribute to advances in chemistry and materials science. Further efforts should concentrate on the search for novel carbonatebased metal systems and exploring their properties in the applications-oriented research. The combination of carbonate anions with metal cations can pave the way towards many classes of novel useful materials. Finally, the attention in the future engagement should be paid to new ways of increasing the role of natural components such as CO2 in the development of valueadded compounds. We hope that this review will increase the awareness of CO2-derived metal systems and accelerate the development of functional systems based on these eco-friendly carbonate building units.

Acknowledgements The authors gratefully acknowledge the support of the National Science Centre – Poland (Grant Maestro DEC-2012/04/A/ST5/00595 and Grant Preludium 7 2014/13/N/ST5/03427) and the Ministry of Science and Higher Education – Poland within the Iuventus Plus Programme (IP2014 043573; K.S.).

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