Organic–inorganic hybrids: From magnetic perovskite metal(II) halides to multifunctional metal(II) phosphonates

Accepted Manuscript Title: Organic-Inorganic Hybrids: From Magnetic Perovskite Metal(II) Halides to Multifunctional Metal(II) phosphonates Author: Carlo Bellitto Elvira M. Bauer Guido Righini PII: DOI: Reference:

S0010-8545(14)00278-1 http://dx.doi.org/doi:10.1016/j.ccr.2014.10.005 CCR 111943

To appear in:

Coordination Chemistry Reviews

Received date: Revised date: Accepted date:

23-7-2014 13-10-2014 14-10-2014

Please cite this article as: C. Bellitto, E.M. Bauer, G. Righini, OrganicInorganic Hybrids: From Magnetic Perovskite Metal(II) Halides to Multifunctional Metal(II) phosphonates, Coordination Chemistry Reviews (2014), http://dx.doi.org/10.1016/j.ccr.2014.10.005 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Organic-Inorganic Hybrids: from Magnetic Perovskite Metal(II) Halides to Multifunctional Metal(II) phosphonates Carlo Bellitto, Elvira M. Bauer and Guido Righini

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ISM-CNR, Via Salaria km 29.3, I-00015 Monterotondo Staz. Italy.

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e-mail: [email protected]

Abstract

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The synthesis, crystal structure and solid state properties of three different groups of magnetic organic-inorganic hybrid compounds are described and discussed. The reported examples are crystalline solids built from metal ions or metal clusters bonded to the organic moieties to form a single phase. The three families are a) perovskite metal halides, b) electrical conductive radical-cation salts and c) transition metal phosphonates and they differ by the type of chemical bonding between the organic and inorganic subnetworks. Organic-inorganic hybrid materials gained interest due to their potential applications in several fields and for their unusual collective electronic properties, such as magnetism, optical and electrical conductivity properties. Today they are experiencing a revival due to the discovery in some hybrids of coexistence of two physical properties such as ferroelectricity and magnetic order or ferromagnetism and metal-like electrical conductivity (dualfunctional materials). The aim of this review is a survey of the crystal structure and properties of magnetic organic-inorganic hybrid compounds selected on the basis of their interesting electrical properties and for the potential applications.

Keywords: Organic-Inorganic Hybrids, Perovskite Metal Halides, Radical-Cation Salts, Metal(II) Phosphonates, Crystal Structure, Ferromagnets, Canted Antiferromagnets, Multifunctional Materials. 1

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Contents

1. Introduction

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3. Electrical Conductive Radical-Cation Salts 3.1. TTF Polyoxometalate Systems 3.2. BEDT-TTF Polyoxometalate Systems 3.3. BEDT-TTF Spin-Peierls Systems

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2. Perovskite Metal (II) Halides 2.1. Ferromagnetic and Antiferromagnetic Metal(II) halides 2.2. Electrical Conductive Perovskite Metal(II) halides

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4. Magnetic Order in Hybrids based on Transition Metal(II) Phosphonates 4.1. Ferromagnetic Metal(II) Phosphonates 4.1.1. Magnetic Order in Layered Ni[(CH3(CH2)17PO3)(H2O)], Ni[(CH3PO3)(H2O)] and Ni[(C6H5PO3)(H2O)] 4.1.2. Ni2[(PDI-BP)(H2O)2]3H2O, a Ferromagnetic Dye 4.2. Non-Centrosymmetric Metal(II) Phosphonates 4.2.1. Cr[NH3(CH2)2PO3(Cl)(H2O)], a Polar Magnet 4.2.2. Fe[(CH3(CH2)2PO3)(H2O)], a Weak Ferromagnet 5. Conclusions

6. Acknowledgments

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7. References

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1. Introduction In the last century there was an intense research activity on crystalline moleculebased materials and coordination polymers because their properties have become attractive for solid state studies and technological applications [1]. A onedimensional (1D) transition metal compound exhibiting metallic electrical

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conductivity at room temperature was reported for the first time in early 1970 [2]. This coordination compound of formula K2[Pt(CN)4]Br0.3•H2O, known also as a

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Kroogman‟s salt, is metallic in a limited range of temperature and displays highly

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anisotropic electrical and optical properties [3a]. It was also the first mixed-valence platinum chain to undergo Peierls distortion due to pairing of unpaired electrons

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during lattice dimerization [3b]. After that a vast range of coordination polymers with low dimensional structures were prepared and studied. The appearance in literature of novel compounds classified as “synthetic” metals [4], single-molecule magnets [5],

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organic superconductors [6], or ferromagnetic organic metals [7] opened the field to the synthetic chemists drawing the attention to the possibility to design and

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synthesize more complex solids with interesting physical properties. Among the latter a class of intercalation compounds emerged i.e. the so-called “organic–inorganic

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hybrids”. They are characterized by the coexistence of inorganic and organic components within a single phase and feature enhanced properties, such as greater mechanical and high thermal stability compared to the organic solids. An interesting

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example is represented by [BEDT-TTF]3[MnCr(C2O4)3], a ferromagnetic hybrid, which exhibits metal-like electrical conductivity [7]. The preparation of these materials is based on methods typical of “soft” chemistry, which permits the assembly of metal ions donor with multifunctional organic ligands. The crystals are mainly obtained by diffusion crystallization, electrocrystallization and by solvo/hydrothermal reactions. These methods can guarantee the advantage of 3

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isolation of single phase composites at low temperatures. Organic-inorganic hybrids are also interesting because they can be used as precursors for the synthesis of nanophasic inorganic materials [8]. Now, organic-inorganic hybrids show several interesting chemical and physical properties but among them electrical, optical and magnetic properties result to be the most attractive also due to the possibility of

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technological application. In addition, materials belonging to the family of organicinorganic hybrids could also possess new interesting properties which originate from

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the coupling between the organic and inorganic components. Some hybrids are multi-

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functional, that is they exhibit two or more different physical properties as in the case of [Mn3(HCOO)6](EtOH), where recently co-existence of ferrimagnetism and

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ferroelectricity has been found [9]. The latter compound shows a paraelectricferroelectric transition at 165 K and a ferromagnetic transition at 8.5 K. This is a rare example of a multiferroic system, in which the lattice exhibits magnetic order and the

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guest molecule can induce ferroelectric behavior. In fact, multiferroic materials are extremely interesting for constructing innovative devices, since charge can be

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controlled by an external magnetic field and spin by an electric field. In general chemical engineering of organic-inorganic hybrids allows to design compounds that

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combine in the same lattice two or more physical properties which are difficult to find in continuous inorganic structures. A possible approach can be the chemical assembling of one molecular network to an inorganic one, such as cation/anion salts.

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In the composite lattice each component carries one functionality providing so possible multifunctional materials. The aim of this review is a survey of the crystal structures and the physical properties (mainly magnetism and electrical conductivity) in some selected organic-inorganic hybrids belonging to the following families of compounds: a) Perovskite Metal Halides, b) Electrical Conductive Radical-Cation Salts and c) Hybrids based on Metal (II) Phosphonates and to present our 4

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contribution to this field. A relationship between the three classes of organicinorganic hybrids on the basis of the bonding between the two components and on the type of molecular brick used in the assembly of the lattice will be examined.

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2. Perovskite Metal (II) Halides

In this first section organic-inorganic hybrids based on perovskite structures will be

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presented and discussed. The family includes 3D cubic perovskite AMX3 and layered

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A2MX4 perovskite structures. The crystal structure of the AMX3 compounds consists of a three-dimensional network of corner sharing [MX6] octahedra, where M is a

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metal cation, X is a halogen and A is a unipositive cation. The inorganic A cation can be replaced by an organic cation small in size to fit in the structure such as the mono-

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methyl ammonium cation, CH3NH3. Interesting examples are CH3NH3MX3, where M = Sn, Pb; X = Cl, Br, I. The layered A2MX4 perovskite structure consists of inorganic layers comprising divalent transition metal ions bridged by single halogen atoms. The

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inorganic layers are charged -2 per formula unit and are balanced by unipositive ions; the latter are located in between the inorganic layers to form perovskite metal halides

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of general formula (R–NH3)2MX4 and (H3N-R–NH3)MX4 (R is an aliphatic or aromatic group, M is a divalent metal ion, X a halide). They have played an important role in the study of structural phase transition and in low-dimensional

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magnetism in the period around 1970 – 1990 [10,11]. Subsequently, in the case of non-magnetic metal halide hybrids (lead and tin) they found interest for their semiconducting behavior [12], and in the case of europium derivatives for their luminescence properties. A review on this topic has been published in 1999 by D.B. Mitzi [12].

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The synthesis of these solids in general is made by self assembly of inorganic and organic components using “soft chemistry” methods instead of the usual high temperature solid state reactions (solution or hydrothermal reactions, Schlenk techniques etc.) according to the following reaction scheme:

(H3

3)X2

+ MX2

solv. solv. solv.

(RNH3)2MX4 (H3

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2 RNH3X + MX2

3)MX4

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M = divalent metal-ion, X = halogen, R = alkyl or alkylene group

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Both reagents, the alkylammonium halide and the metal halide are dissolved in a suitable solvent and the respective solutions are mixed together and brought to

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reaction under mild conditions. The perovskite metal halides can be isolated by evaporation of the solvent, the driving force for the reaction being the formation of the M-X bonds. The resulting ionic compounds, i.e. (R-NH3)2MX4 or (H3N-R–

M

NH3)MX4, crystallize in two-dimensional layer perovskite structures, schematically reported in Figure 1. The overall crystal structure consists of an alternation of

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inorganic and organic layers along the direction perpendicular to the sheets. The inorganic layer is made of oriented [MX6] octahedra, linked by sharing equatorial

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vertices in a squared array. These inorganic sheets are separated by layers of the organic ammonium cations. The structure is related to the compound K2NiF4 (see Figure 2), where the K2F2 rock salt layer has been replaced by the organic bi-layer.

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In (RNH3)2MX4 perovskites the cationic R–NH3+ moieties alternate on both sites of the inorganic layers made by the [MX4]2- anions. The ammonium group of these organic cations are bonded by electrostatic as well as hydrogen bonds to the non bridging halogens located in the inorganic sheets. The R-group extends into the space between layers, and van der Waals interactions between the two adjacent organic groups R are established. These structural characteristics allow the exfoliation of the 6

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crystals as well as the preparation of thin films. In the case of mono-alkyl ammonium salts the metal atoms in adjacent layers are shifted resulting in a staggered configuration. In contrast the structure is eclipsed in the case of dialkylene ammonium salts [H3N-R-NH3]MX4. In the latter both NH3+ groups form ionic and hydrogen bonds; van der Waals forces are absent and the lattice results to be more

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rigid. The length of the organic group can be changed by increasing the number of carbon atoms in the aliphatic chain and, in the case of mono-alkylammonium metal

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halide salts, one can vary the interlayer distances by a factor of two by simply

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increasing the chain length R (R = (CnH2n+1)). In addition some chemistry can be done on the R-group by inserting further functional groups. The unusual structural and

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chemical characteristics of these organic-inorganic perovskites provide for interesting physical properties depending on the metal ion contained in the lattice. The 3d-block elements (Cr(II), Mn(II), Fe(II), Cu(II)) compounds show long range magnetic order

M

at low temperatures and where the magnetic exchange interactions are based on the mechanism of superexchange [13]. Two classes of compounds have been identified,

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i.e. ferromagnetic and antiferromagnetic metal halides. An early review on the magnetic properties of Mn(II) and Cu(II) halides appeared in 1974 by deJongh and

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Miedema [11].

2.1 Ferromagnetic and Antiferromagnetic Metal(II) Halide Salts

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Until the end of the „70, magnetic materials were synonymous of metals, alloys or transition metal oxides. Only a few examples of molecule-based ferromagnets were known as for example diethyldithiocarbamato-iron(III) chloride [14], or those characterized by having a continuous lattice such as the A2CuX4 salts (A = unipositive cation, X = F, Cl) [15] and the perovskite salts (RNH3)2CrX4 (R =

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(CnH2n+1, X = Cl, Br) [16]. The chromium derivatives are ferromagnetic showing a long-range ferromagnetic ordering below T = 50 K. The Magnetization vs. T plot for (CH3NH3)2CrCl4

and

a

hysteresis

loop

registered

at

T

=

6

K

for

(C6H5CH2NH3)CrBr3.5I0.5 are shown in Figure 3. The synthesis and crystal growth of the air-sensitive Cr(II) derivatives are described in ref. [16d]. The Cr(II) and Cu(II)

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salts exhibit a strong Jahn-Teller distortion (JT) in [MX6] octahedra, with antiferrodistortive arrangement of the molecular axes in the plane of the perovskite

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sheet which leads to ferromagnetic interactions (see Figure 4). The JT theorem states

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that for non-linear polyatomic molecules any orbitally degenerate electronic state is inherently unstable toward a distortion that removes the degeneracy [17, 18]. In the case of transition metal ion in octahedral coordination the electronic configurations d4

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and d9 are those which exhibit the JT distortion. The antiferrodistortive configuration of the [CrIIX6] units means that the tetragonal axes of adjacent octahedra are

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perpendicular to each others and this leads to the superexchange pathway Cr–X…Cr between half-filled dz² orbitals of one Cr(II) ion and the empty dx²-y² orbital of the

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other adjacent one, giving rise to a ferromagnetic exchange within the inorganic sheets of the perovskite. The magnetic ordering along the Cr(II) series is

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ferromagnetic and the critical (Curie) temperature, Tc, ranges between 37-52 K, depending on the type of halogen X in the lattice. Quite interesting, the Curie temperature is nearly independent from the distance between inorganic layers as

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reported in Table 1. These materials are insulators and nearly transparent in the visible region. The optical absorption spectra of these Cr(II) salts are dominated by two very sharp spin-forbidden ligand-field (quintet triplet) transitions, which disappear at liquid helium temperature, making them transparent ferromagnets at low temperatures. The reason for this behavior lies in the mechanism which give intensity to the transitions [19]. Figure 5 shows as an example the visible crystal absorption 8

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spectra of (C2H5NH3)2CrCl4 above and below TC. It is worth noticing that the research field on ferromagnetic insulators [20] and molecular based magnets [21] developed a few years before the discovery of the “Single-Molecule Magnets” [5a]. Layered Mn(II) and Fe(II) halide analogues exhibit a different magnetic behavior. For the Mn(II) (S = 5/2) compounds (CnH2n+1NH3)2MnCl4 (n = 1, 2, 3,…) the

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transition to 3D antiferromagnetic ordering occurs at the Néel temperature TN = 43.8 K, 42.1 K and 39.3 K for the first three members of the series. The antiferromagnetic

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interaction is a result of nearly linear and symmetrically Mn–X–Mn bonds between the magnetic ions [22a]. The magnetic orbitals of adjacent metal ions overlap through

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the common halide orbital. (Prn–NH3)2MnCl4 (Prn = n-propyl group) is

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antiferromagnetic and it shows a sharp peak in the χ vs. T plot at TN = 39.2 K as reported in Figure 6. The compound however order antiferromagnetically with canting in the magnetic sublattices, which results in a weak ferromagnetic moment

M

(weak-ferromagnetism) [22b]. (CnH2n+1NH3)2MnCl4 (n = 1, 2, 3) show room temperature photoluminescence peaks at 597, 588 and 602 nm, respectively; and the

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intrinsic red emission results from the 4T1→ 6A1 electronic transition, where A and T refer to non-degenerate and triple degenerate crystal field levels of Mn2+ (3d5) ion in

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octahedral field. Quite recently a structural and magnetic study on (CH3NH3)2FeCl4 reported for the compound a magnetic phase transition at 95 K. The structures consists of square grid layers of FeCl6 octahedra and changes from the high

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symmetry I4/mmm to low symmetry Pccn are observed. In the former the iron and the bridging halogen atoms are within the layer while the organic cations are located in the middle of each square grid . in the latter the octahedral are tilted in pairs. The zfc and fc Magnetization vs. T plots reveal a fully compensated antiferromagnetic ground state consisting of four sublattices with a hidden canting is transformed in canted antiferromagnet in field exceeding the metamagnetic critical, Hc, of 200 Oe 9

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[23]. Moreover quite recently, the layered perovskite salts have attracted interest as multiferroic materials and the group led by T.T.M. Palstra of the University of Groningen (The Netherlands), reported the coexistence of ferroelectricity, which sets below T = 340 K and ferromagnetism, which was observed below T= 13 K in [(C6H5(CH2)2NH3)2]CuCl4. The compound shows a novel type of multiferroism,

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which originates from the organic sublattice for the ferroelectricity and from the inorganic one for ferromagnetism with a mechanism described above [24]. The

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electrical polarization results from the spatial ordering of hydrogen bonding that link

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the organic sublattice to the inorganic one made by the copper chloride [CuCl 6] units. It should be interesting to check what happens in the Cr(II) analogues: [(C6H5(CH2)2NH3)2]CrCl4. The latter compound is expected to show the critical

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temperature for paramagnetic-ferromagnetic transition around 40 – 50 K, in other

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words five times higher than the Cu(II) analogues.

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2.2 Electrical Conductive Perovskite Metal Halides The AMX3 perovskite salts of metal ions belonging to the group 14 (IVA) are

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interesting materials due to their luminescent [25] and semiconducting properties [12, page 90-91]. They show a strong room temperature photoluminescence, the wavelength of which depends on an appropriate choice of the metal ion, i.e. Ge, Sn, Pb, the halogen, i.e. X = Cl, Br, I or the interlayer distance of the perovskite structure

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[26]. Electrical semiconductivity was first observed in the series (CH3NH3)SnX3, (X=Br, I,) where dark red to black color were observed, depending on the type of halogen. The latter series crystallizes in a three-dimensional perovskite structure; (CH3NH3)SnI3 is metallic in the temperature range from 1.8 - 325 K with room resistivity of ~ 7 mΩcm, depending on preparation conditions. A recent application of

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the Pb(II) analogue (CH3NH3)PbI3 has been found in the field of photovoltaic cells. In this regards the perovskite-based material achieved more than 15% of conversion efficiency [27]. Organic-inorganic hybrid semiconductors could be the basis for the

3. Electrical Conductive Charge-Transfer Complexes

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next generation solar cells.

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In this section we will review some electrical conductive organic-inorganic hybrids based on tetrathiafulvalene, TTF, charge-transfer complexes. The magnetic properties

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originate from both the components of the hybrid, while the electrical conductivity arises from the organic part.

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This class of materials comprises charge-transfer compounds made by a reaction of an electron donor (D) molecule with an acceptor (A) molecule to give [D ][A], the

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donor having a low ionization potential and the acceptor high electron affinity. A typical example of an electron donor molecule is given by tetrathiafulvalene, TTF,

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prepared for the first time by Wudl in 1970 [28], while the tetracyano-pquinodimethane (TCNQ) molecule represents an example of electron acceptor

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molecule. The first conducting organic compound was discovered at Dupont and is based on the acceptor TCNQ: it shows a room temperature conductivity of ~ 100 Ω1

cm-1 and a transition to an insulating state at 200 K [29]. It was followed by the

synthesis in 1973 of the charge-transfer complex [TTF][TCNQ] [30] that led to a

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tremendous increase of activity in the field of “synthetic or organic metals”. These conducting Charge-Transfer (CT) compounds have provided up to now a thousand of organic metals and organic superconductors and among them, the most studied ones are derivatives of the electron donor molecule TTF, which easily oxidizes according to the following Scheme:

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Oxidation of TTF gives sequentially and reversibly the radical-cation and dication

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states at relatively low oxidation potential values (E1/21 = 0.37 V and E1/22 = 0.67 V in dichloromethane vs. the saturated calomel electrode). Reaction of TTF with TCNQ

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produces the charge-transfer complex [TTF][TCNQ], where the charge distribution δ = 0.59 [30a]. This CT compound, synthesized by Cowan et al., is

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metallic in a wide range of temperatures and, below T = 54 K, the compound undergoes to a sharp metal-insulator transition (Peierls transition). The electrical

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behavior is highly anisotropic and it can be explained by looking at its crystal structure, which consists of uniform segregated chains of donors and acceptor

M

molecules as reported in Figure 7. The crystal packing gives arise to delocalized electron energy bands due to the overlap between the -orbitals of adjacent TTF

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molecules along the chains. The partially filling with electrons by charge transfer from donors to acceptors of this band is responsible for the metallic conductivity. In

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1980 the first organic superconductor was discovered, i.e. the radical-cation salt [TMTSF]2X, where (TMTSF = tetramethyl-tetra–seleno-fulvalene, X = [PF6]-, [AsF6]-, [ClO4]-) [6]. Superconductivity in [TMTSF]2[PF6] was found below Tc = 0.9 K under a pressure of 12 kbar. The crystal structure of these radical-cation salts

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consists of segregated slightly dimerized stacks of planar radical–cations (TMTSF)0.5+, which are interleaved by diamagnetic inorganic anions charged -1. The electrical conductivity is ascribed to the overlap of selenium -orbitals of TMTSF molecules along the a direction of the unit cell which originates the formation of a conduction band (see in ref. [6a]). Since then a great number of electrically

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conducting and superconducting materials have been synthesized, the most interesting

being

the

(BEDT-TTF)2X

salts

(where

BEDT-TTF

is

bis(ethylenedithio)tetrathiafulvalene, X = monovalent anion [4]. In particular the combination of (BEDT-TTF) electron donor with the anion Cu[N(CN)2]Br- was found to be superconducting below 12 K [4c]. After that, three decades later,

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[TTF][TCNQ] and [TMTSF][TCNQ] CT compounds have provided an unexpected phenomenon at the interface when a single-crystal of neutral TTF or TMTSF was

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placed next to a crystal of TCNQ in a way to form an organic heterostructure. Despite

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the single crystals are insulating they form in both cases a conductive strip at the interface. This is due to the large charge-transfer between the two crystals from the

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donor to the acceptor molecule which generates an interfacial 2D metallic conductor different from that of the bulk. The different behavior at the interface opens the way to a new class of electronic systems [30b,c]. Palstra et. al. demonstrated that this

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method can be also used to increase the electrical conductivity when applied to the inorganic-organic perovskite halide [(C6H5(CH2)2NH3)2]CuCl4. This copper based

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hybrid, also described in chapter 2, is a ferromagnetic insulator with a Curie temperature of 13 K. Evaporation of a TTF layer on the top of the perovskite halide

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single crystals enhances the surface conductivity by five orders of magnitude [30d].

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3.1 TTF Polyoxometalate Systems

One of the developments in the field of hybrid charge-transfer compounds was the replacement in the lattice of the diamagnetic anions with magnetic anions. For this purpose several types of inorganic clusters were used and among these the attention was focused on polyoxometalates (hereafter POM). This class of materials attracted

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Page 13 of 64

the attention not only for the huge variety of shapes, sizes and composition but added also significant results in the field of analytical chemistry, catalysis, medicine and material science [31a]. In the past several reviews have summarized different aspects of POM chemistry and in particular an important contribution to the field of electrical conductive radical-anion salts have been given by the research groups of P. Batail

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[31b] and E. Coronado [31c]. The first example of an tetramethyltetrathiafulvalene (TMTTF) based organic charge transfer salt with an hexanuclear metal halide cluster

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as acceptor anion was reported by P. Batail et. al in 1986 [32a]. Single crystals of

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(TMTTF)2Mo6Cl14 have been obtained by electrocrystallization technique but the compound was found to be an insulator. Later also metal-oxygen clusters of the

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Lindquist or -Keggin salts have been used in the synthesis of new [D] m[A]n materials [32-34]. In order to investigate the role of the size, shape, charge and

M

magnetic moment of the acceptor anion on the crystal structure and on the physical properties of these CT solids the reaction between the donor molecules TTF and polyoxometalate derivatives such as [M6O19]2- (M = MoVI, WVI) have been studied.

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The aim was to isolate organic–inorganic hybrids in which localized magnetic moments (inorganic component) and delocalized electrons (organic component)

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could coexist within the same lattice. Reaction between TTF and the inorganic diamagnetic polyoxoanion [Mo6O19]2- resulted in isolation of two different TTFradical cation salts: (TTF)3[Mo6O19] (1) and (TTF)2[Mo6O19] (2) [32b-d].

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(TTF)3[Mo6O19] has been obtained by electrocrystallization technique of TTF in presence of Lindquist anion, while (TTF)2[Mo6O19] has been prepared by methathesis reaction of (TTF)3[BF4]2 with ((C2H5)4N)2[Mo6O19] in a inert atmosphere. These two TTF polyoxometallate systems differ in the stoichiometry, crystal structure and physical properties. The crystal structure of (1) is segregated (see Figure 8) and it consists of 1D stacks of donor TTF trimers, which fit in channels formed by the 14

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diamagnetic anions. Within the stack the chain of TTF trimers show an unusual crisscross arrangement with two different degrees of oxidation, the external ones being +0.5 and the internal one +1. The single-crystal electrical conductivity at room temperature of (TTF)3[Mo6O19] (1) (σ = 10-4 Ω-1cm-1) indicates a semiconducting behavior and is ascribed to the mixed-valence structure of the TTFρ+ chain. The

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unusual packing arrangement of TTFcations observed in compound (1) is probably due to the localization of charge induced by the charge (-2) of the inorganic anions.

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The crystal structure of (TTF)2[Mo6O19] (2), shown in Figure 9, consists of dimeric

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(TTF•+)2 molecules interspersed with the diamagnetic polyoxoanions. Also the tungsten polyoxoanion [W6O19]2- reacts with TTF organic donor molecules to give

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(TTF)3[[W6O19] (3) crystalline materials [32d]. The crystal structure of the tungsten derivative (3) recalls the criss-cross overlap between TTF trimers previously

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observed in the corresponding molybdenium compound (TTF) 3[Mo6O19] (1).

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Compound (1) and (3) resulted to be diamagnetic semiconductors.

3.2 BEDT-TTF Polyoxometalate Systems

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An interesting inorganic molecular brick for the construction of radical-cation systems is represented by the electron acceptor α-Keggin anion [XM12O40](8-n)- (where Xn+ = PV, SiIV and M = MoVI, WVI) [33, 34]. The diamagnetic phosphomolybdate cluster [PMo12O40]3- can be easily reduced to a paramagnetic mixed-valence cluster

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[PMo12O40]4-, and to a less stable diamagnetic two-electron-reduced [PMo12O40]5cluster. On the other hand [SiW12O40]4- anion is similar in size but diamagnetic and therefore both clusters were good candidates for incorporating them into the hybrid lattice of conductive BEDT-TTF derivatives. The first reduced anion [PMo12O40]4could give an insight of the interaction of localized magnetic moments of the

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Page 15 of 64

paramagnetic anions with the electron gas in the organic donors when compared to the silicotungstate cluster [SiW12O40]4-. Furthermore, the less stable diamagnetic reduced [PMo12O40]5- cluster could give information on the effect of the high value of the negative charge on the formation, stoichiometry and crystal structure of the radical ion salt. The electrocrystallization of BEDT-TTF in presence of the anions

produces

TTF)8[PMo12O40]•(2CH3CN•2H2O)

(4)

two [34]

phases:

(BEDT-

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[PMo12O40]3-

and

(BEDT-TTF)6

cr

[PMo12O40]•(4CH3CN•6H2O) (5) [35]. The reaction of BEDT-TTF with the

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diamagnetic silicotungstate anion [SiW12O40]4- under the same conditions, leads to formation of (BEDT-TTF)8[SiW12O40] (6) [33]. The structure of compound (4) consists of alternate layers of BEDT-TTF, organized as a pseudo-α-phase, and of the

an

α-Keggin inorganic anions and solvents of crystallization, along the b axis. An important feature of the lattice is represented by the presence of two different types of

M

BEDT-TTF+0.5 chains in the organic layers: one uniform and one dimerized (see Figure 10). The electrical conductivity of the [PMo12O40] salt (4) at room

ed

temperature is σRT = 10-1 S cm-1, EA = 0.088 eV for T < 150 K and 0.177 eV for T > 200 K and for the [SiW12O40]4- salt (6) σRT = 0.4 S cm-l and EA = 0.098 eV. In both

ce pt

cases the electrical conductivity is anisotropic and the magnetic properties of these

Keggin POM conductors are typical of a localized S = ½ spin system. In Figure 11 the temperature dependence of the molar magnetic susceptibility of (BEDT-

Ac

TTF)8[PMo12O40]•(2CH3CN•2H2O) (4) is reported. The contribution of the organic [BEDT-TTF] component to the molar magnetic susceptibility arises from two types of chains: a uniform 1D linear chain antiferromagnet [36] and a dimerized chain antiferromagnet (Bleaney-Bowers model [37]. The observed Curie paramagnetism due to the anion contribution indicates a 4- charge for the polyoxometalate cluster. These magnetic properties correlate well with the solved crystal structures. The 16

Page 16 of 64

optical data show a polarized intra-band transition at ~2000 cm-l (0.25 eV) consistent with

the

electrical

measurements.

Compound

(5),

(BEDT-TTF)6

[PMo12O40]•(4CH3CN•6H2O), has also been prepared by electro-crystallization technique and the donor molecule is in the 6:1 stoichiometry. The crystal structure is triclinic Pī and it consists of two-dimensional organic and inorganic sublattices

ip t

alternating along the [001] axis, while the water and acetonitrile molecules occupy the interstitial space between the cluster anions in the inorganic layers. The radical

cr

cations are arranged in two almost identical chains and the sequence of three

us

independent BEDT-TTF molecules within the chain is homogeneous. This packing is different from that observed in (BEDT-TTF)8[PMo12O40]•(2CH3CN•2H2O) (4). The

an

most striking difference between the two compounds (4) and (5) is the number of donor molecules and of solvent molecule per cluster, while maintaining the structure the

charge

of

the

cluster.

As

observed

in

(BEDT-

M

and

TTF)8[PMo12O40]•(2CH3CN•2H2O) (4), the molecules in the organic layers carry variable charges (the higher charges are carried by molecules in close proximity to

ed

the anion and the molecules with less charge are next to the solvent molecules). The organic layers as well as the inorganic layers are more densely packed in the hexa-

ce pt

(BEDT-TTF) system (5) than in the octa-(BEDT-TTF) hybrid (4) due to the more efficient arrangement of the solvent molecules. The electrical conductivity measurements performed on compound (5) show semiconducting behavior due to the

Ac

localization of charges in the organic layers. Investigations made on these three polyoxometalate systems can be summarized as following: The high negative charge and the large volume of the POM anion both generate irregular BEDT-TTF stacks and charge localization in the organic sublattice. As a consequence the POM‟s

17

Page 17 of 64

compounds show poor electrical conductivity and magnetism typical of 1D AF magnetic systems [38].

3.3 BEDT-TTF Spin-Peierls Systems In this section two examples of BEDT-TTF radical ion salts showing Spin-Peierls

derivative,

namely

(BEDT-TTF)8[PMo3NbW8O40]

ip t

transition are described. The first one is given by the -Keggin polyoxometalate [39].

The

diamagnetic

cr

polyoxometalate cluster [PMo3NbW8O40] has been reacted with the organic donor

us

BEDT-TTF in order to investigate its influence on the magnetic properties of the resulting radical ion salts. The observed magnetic susceptibility vs. T plot for (BEDT-

an

TTF)8[PMo3NbW8O40] (see Figure 12) has been fitted according to uniform (S=1/2) 1D Heisenberg model of nearly ionized BEDT-TTF molecules. A sharp fall in the

M

susceptibility vs. T plot has been found at T= 7 K which can be associated to the Spin-Peierls (SP) system [39a]. In S = ½ 1D Heisenberg AF uniform chains the

ed

magnetic susceptibility has a finite value as T goes to zero suggesting a gapless spin system. In Spin-Peierls systems the 1D antiferromagnetic chain of localized spins couple with 3D phonons and as a result the lattice dimerizes into alternating linear

ce pt

chains. As a consequence the magnetic susceptibility falls to zero as T goes to zero. The dimerization process depends on the temperature. If the exchange energy exceeds the lattice distortion energy then the SP transition generates an energy gap

Ac

between the singlet ground state and the triplet exited states. The second example is given by '-(BEDT-TTF)2Ag(CN)2 [39b]. Figure 13 shows the precise measurements of the susceptibility of '-(BEDT-TTF)2Ag(CN)2 as a function of temperature in applied fields from 0.05 – 5 T. These measurements confirm that for 20 – 300 K it behaves as a one-dimensional antiferromagnet. Below 6 K the abrupt

18

Page 18 of 64

drop in susceptibility is interpreted as the onset of a spin-Peierls transition. The data at temperatures above 20 K are found to be best fitted to the Bonner-Fisher model [36] for a linear chain antiferromagnet with J/kB = 59 K. Below 6 K the susceptibility is fitted with a Bulaevskii model for a temperature dependent dimerising chain with = 0.75 at 0.05 T increasing to 0.80 at 3 T. The electron-phonon coupling parameter

ip t

 is 0.27, the BCS gap (0) = 13.8 K is higher than the theoretical value (10.2 K) and the effective phonon frequency (7.5 cm-1).

cr

Radical-cation salts presented in this section are ionic hybrids where electrostatic

us

bonds are established between the cationic organic component and the anionic inorganic component. The magnetic properties result from the AF exchange

an

interaction between the radical spins localized on the chains of (BEDT-TTF+0.5)2 molecules within the organic layer. The semiconducting behavior is ascribed to the

M

poor -electron delocalization between (BEDT-TTF+0.5)2 along the chain.

ed

4. Magnetic Order in Hybrids based on Transition Metal(II) Phosphonates The third class of magnetic multifunctional organic-inorganic hybrids we are dealing with here, is represented by metal phosphonates. In this widespread group of solids

ce pt

the metal ions are bound covalently by organic linkers, and thus forming a lattice with high thermal and mechanical stability. These materials can find applications in the following fields: catalysis, ionic exchangers, protonic conductors, uptake and

Ac

separation of gases (H2 gas, methane, or carbon dioxide). Soft chemistry methods are used for preparation and in some cases only scarcely crystalline materials can be obtained. Generally transition metal(II) phosphonates are synthesized by heating an aqueous solution containing phosphonate (hereafter RPO3H2, R = CnH2n+1) or diphosphonate ligands (R(PO3H2)2, R = CnH2n) and metal ion salts in the presence of

19

Page 19 of 64

a base [40]. Under these conditions the phosphonate ligand deprotonates according to the following scheme: RPO3H2  [RPO2(OH)]- (RPO3)2The three oxygens of the phosphonate ligands bind easily metal ions and thus several

ip t

different hybrid structures (1D, layered, MOF, etc.) can be obtained depending on the charge of the metal ion, the steric hindrance of the R group and the pH of the

cr

solution. A comprehensive review dedicated to the synthesis, structural and chemical characterization of metal phosphonates has been published recently by A. Clearfield

us

[40a,b]. The phosphonate ligands are also interesting for another reason. In fact, the organic part R can be chemically functionalized with a variety of substituents, such as

reaction

in

presence

of

ancillary

an

carboxylic, amine, thiol and even additional phosphonic groups. Furthermore, ligands

(carboxylates,

pyridine,

3,5-

M

dimethylpyrazole, collidine) can lead to the formation of polynuclear transition metal cages [40c]. In addition, the coordination ability of dialkyl and diarylphosphonic

ed

ligands have been used either for the preparation of monolayers or multilayers on several substrates or for surface modification of nanoparticles [41].

ce pt

The synthesis of divalent transition metal phosphonates of formula MII(RPO3)(H2O) where R= C6H5 and M = Mn, Fe, Co, Ni, was first reported by Cunningham in 1979 [42] and the crystal structure of Mn(C6H5PO3)(H2O) in 1988 by Mallouk et al. [43]. The Mn(II) phenylphosphonate crystallizes in a layered structure, which comprises

Ac

zig-zag inorganic layers alternating along one direction of the unit cell. The pendent organic group R of the phosphonate ligand lies in the interlayer space and two organic layers having van der Waals contacts are interspersed to the inorganic ones. A schematic view of the structure of metal(II) phosphonates is shown in Figure 14. The inorganic layer is made of metal ions coordinated by six oxygen atoms, five of

20

Page 20 of 64

which belong to the phosphonate ligands and one to a coordinated water molecule. It is worth of noticing the similarity of the layered structure of these metal phosphonates to that of the perovskite metal halides. The difference resides in the type of bonding which held together the organic and the inorganic layers, that is

ip t

covalent in the former and ionic in the latter.

cr

4.1 Ferromagnetic Metal(II) Phosphonates

us

Divalent transition metal phosphonates are of particular interest due to their magnetic properties typical of an extended lattice. In this regards Mn(II) and Fe(II) alkylphosphonates are the most studied ones [44,45]. The 2D character of the

an

inorganic sub-network in transition metal phosphonates favors the exchange interactions between nearest neighbor magnetic ions and hence a long range magnetic

M

ordering at low temperature [46]. This kind of structure is often associated to weak ferromagnetism or canted antiferromagnetism also because these structures are

ed

related to the two sources of weak ferromagnetism, i.e. single-ion anisotropy and/or antisymmetric exchange.[47] In fact a common characteristic in the structure of weak

ce pt

ferromagnets is the requirement of two non-symmetry related nearest neighbour magnetic ions, which is consistent with an acentric structure. Additionally, if a molecule-based weak ferromagnet crystallizes in an acentric structure there is also

Ac

the possibility to get multifunctional magnets with interesting non-linear optical properties, ferroelectricity or dielectric properties [48]. In the case of multiferroic systems materials where the magnetic order is independent from the ferroelectric one and those where some magneto-electric coupling exists can be identified, the latter being more interesting from the application point of view. Examples of ferroelectric weak ferromagnets have been observed in transition metal oxides [49].

21

Page 21 of 64

4.1.1 Magnetic Order in Layered Ni(II) Organophosphonates In this section the magnetic properties of interesting Ni(II) monoalkyl phosphonates will be presented and discussed. Unlike most metal phosphonates, they were isolated as single crystals of sufficient size and quality for single-crystal structure determinations by X-ray diffraction. The compounds were found to show a different behavior

nevertheless

they

exhibit

similar

crystal

structures.

ip t

magnetic

Ni[(C6H5PO3)(H2O)] crystallizes in the orthorhombic space group Pmn21 [50]. and is

cr

isomorphous and isostructural with Mn(II), Fe(II) and Co(II) analogues. It presents

us

the typical features of hybrid 2D structures, consisting of alternating inorganic and organic layers. The inorganic layers are capped by the phenyl phosphonate groups,

an

with phenyl groups of two adjacent ligands forming a hydrophobic bi-layer region, and van der Waals contacts are established between them. The phenyl group is

M

disordered between two orientations perpendicular to each other within a single layer. Ni[(C6H5PO3)(H2O)] is a paramagnet from r.t. down to 10 K, with the following

ed

parameters: C = 1.26 cm3 K mol-1 and θ= -22.4 K. This implies that Ni(II) ions with S = 1 spin configuration are present and that adjacent spin carriers exhibit AF exchange

interactions.

At

low

temperatures

the

magnetic

behavior

of

ce pt

Ni[(C6H5PO3)(H2O)] is typical of a “canted antiferromagnet”. In fact the observed decrease of m*T vs. T plot down to 5 K and then a steep upturn is a signature of canted AF long range order. The occurrence of a weak ferromagnetic state is due to

Ac

the spin canting. The local spins in this configuration are not perfectly antiparallel leading to a net spontaneous magnetization which saturates in a small field. The latter results can be clearly observed from the Zero field cooled (zfc) and field cooled (fc) χ vs. T plots reported in Figure 15. The other two, i.e. Ni[(CH3PO3)(H2O)] and Ni[(C18H37PO3)(H2O)] are both layered and the variation of the number of the carbon

22

Page 22 of 64

atoms in the alkyl chains allows a tuning of the spacing between the inorganic layers from 8.7 Å to 42.31 Å. [50]. Ni(II) methylphosphonate is a paramagnet down to 1.8 K. Surprisingly, in spite Ni[(C18H37PO3)(H2O)], which has a similar layered structure shows a different magnetic behavior. In fact, the latter compound exhibit ferromagnetic-like behavior characterized by a) a divergence between fc and zfc

ip t

magnetic susceptibility below Tc = 20 K, b) the onset of χ‟‟ of AC magnetic susceptibility measurements associated to the peak of the in-phase χ‟ susceptibility,

cr

as shown in Figure 16. The M = M(H) plot at T = 2 K shows an hysteretic behavior with a large coercive field Hc = 2500 Oe, the largest observed in the series. The value

us

of the magnetization at 5 T (0.8 μB) is 40 % of that expected for fully aligned S = 1

an

spins. In fact the latter compound is ferromagnetic. The ferromagnetic exchange is related most likely to the degree of corrugation of the inorganic layers. In general the three Ni(II) phosphonates described here contain all Ni(II) ions in octahedral

M

symmetry. Spin canting, where present, should originate from the lack of a center of symmetry between adjacent spins. The absence of canting in Ni[(CH3PO3)(H2O)]

ed

down to T = 1.8 K can be related to a different degree of symmetry of the coordinating [PO3]2- group. The antisymmetric exchange arises from the synergic

ce pt

effect of local spin-orbit coupling and exchange interactions between magnetic centers and vanishes when the dinuclear unit is centrosymmetric. Canting can occur either between the layers or within the layers. In fact, in spite of having similar

Ac

layered structures, the three Ni(II) compounds exhibit different magnetic behavior, depending on the degree of corrugation of the layers, on the interlayer distance and on the nature of the spacers R. Ni[(CH3PO3)(H2O)] and Ni[(C6H5PO3)(H2O)] show basically antiferromagnetic behaviour, whereas the long alkyl chain Ni(II) compound Ni[(C18H37PO3)(H2O)] is the only one showing a ferromagnetic–like behaviour in the whole range of temperature. 23

Page 23 of 64

4.1.2 Ni2[PDI-BP](H2O)2]3(H2O), a Ferromagnetic Dye Aromatic molecules, such as 3,4;9,10-Perylenediimide (PDI) show high thermal stability, well-defined redox chemistry and remarkable optical properties. Chemical insertion on the PDI molecule of donor groups through the diimide nitrogen atoms or via substitution on the aromatic core, provides derivatives whose absorption or

ip t

emission may be modulated and the resulting ligands can be used in the construction of hybrid organic-inorganic lattices. An interesting chemical modification is the

cr

insertion of one or two phosphonate groups, since they have a well established

us

bonding ability towards metal ions. Thus, phosphonate-substituted aromatic diimides are suitable molecules for the design of multifunctional organic-inorganic hybrids,

an

either as bulk or as thin film, by chemical self-assembling, when left to react with metal ions in water. In the search of new multifunctional magnetic hybrids we chose to couple the optical properties of perylenediimide (PDI) with long range magnetic

M

ordering provided by Ni(II) ions in a suitable layered structure. In order to attain this target we designed and synthesized Ni(II) 3,4;9,10-

ed

Perylenediimide bis-phosphonate pentahydrate, i.e. Ni2[(PDI-BP)(H2O)2]∙3H2O, which can be obtained by reaction of perylenediimide bis-phosphonate (PDI-BP) and

ce pt

nickel chloride in water solution [51]. The poorly crystalline red powder has been studied by Energy Dispersive X-ray Diffraction analysis, EDXD, in the attempt to get more insight in the structure. The fitting procedure of the resulting radial function

Ac

showed a stacked layered structure containing perylene planes shifted in the direction perpendicular to the stacking axes, in a such a way that only the outer rings overlap. The edges of the perylene planes bind to the phosphonate groups which coordinate the nickel ions. Two bonded water molecules ensure an overall distorted octahedral Ni coordination. The Ni-O-Ni angles are in the range where ferromagnetic exchange interactions are expected and, accordingly, Ni2[(PDI-BP)(H2O)2]∙3H2O presents a 24

Page 24 of 64

ferromagnetic intra-chain coupling and weak antiferromagnetic inter-chain coupling through phosphonate groups below T ~ 20 K. Hysteresis loops at T = 10, 20 and 22 K are reported in Figure 17. Below the ordering temperature the nickel compound presents long range ferromagnetic ordering with a small canting angle and with a spin glass-like behavior due to some disorder in the inorganic layer. Hysteresis cycles increased and vanishes at ca. 20 K.

us

4.2 Non-Centrosymmetric Metal(II) Phosphonates

cr

ip t

show a coercive field of ca. 272 mT at 2 K that decreases as the temperature is

Surprisingly, amongst the reported Cr(II) and Fe(II) organophosphonates the

an

Cr[NH3(CH2)2PO3(Cl)(H2O)] [52] and Fe[(CH3(CH2)2PO3)(H2O)] [53] derivatives deserve attention because they crystallize in a non-centrosymmetric space group and

M

are magnetic at low temperatures. While both organic and inorganic solids can be polar or magnetic, the combined properties rarely are observed in either class of

ed

materials.

ce pt

4.2.1 Cr[NH3(CH2)2PO3(Cl)(H2O)], a Polar Magnet Cr(II) ammoniumethylphosphonate chloride monohydrate, crystallizes in the noncentrosymmetric space group P21, with all the molecules almost parallel to the b-axis (polar crystal axis) of the unit-cell, and all the chlorine atoms pointing in the same

Ac

direction, as reported in Figure 18. On the whole, the layered hybrid system is composed of covalent inorganic [CrO4+1Cl] layers, alternating with the organic ones, which are ionically bonded to the adjacent layers and oriented towards the (-NH3+) groups. The inorganic layers are held together by ethylammonium groups, i.e. the terminal ammonium group is located in the cavity formed by adjacent apical chloride

25

Page 25 of 64

ligands and hydrogen bonds are also established between them (see below). Another remarkable feature of this structure is represented by the hydrogen bonds established between the coordinated water molecule and the oxygens of adjacent ligands. All these types of bonds stabilize the crystal lattice and are responsible for the high thermal stability and the slow decomposition to the air of the Cr(II) derivative. The

ip t

positive charge located exclusively on the ethylammonium group and the negative counterpart on the chloride ion confers a net charge separation in the lattice

cr

generating an electric dipole responsible of the polar character of the Cr(II)

us

phosphonate. Remarkably, the compound was found to be a canted antiferromagnet below TN = 5.5 K [52a]. The unusual coexistence of a polar structure and magnetic

an

order observed in the compound suggested the possible occurrence of ferroelectricity or non-linear optical effects associated to weak ferromagnetism in the same material, but

it

resulted

not

being

ferroelectric.

The

Cr(II)

compound

showed

M

magnetodielectric coupling below the Néel temperature by investigation of the temperature dependence of the dielectric constant ε [52b]. The magnetocapacitance

ed

was enhanced by an order of magnitude and due to the appearance of long-range magnetic ordering (see Figure 19). To gain more insight into the magnetic behavior

ce pt

of this novel compound below TN, a high resolution neutron powder diffraction analysis at low temperatures (above and below TN) and Magnetization vs. T and H measurements were carried out on a partially deuterated sample. The results of the

Ac

latter investigations are shown in Figure 20 [52c]. The neutron powder diffraction pattern recorded at T = 10 K (above TN) showed that the partially deuterated material crystallizes in the same non-centrosymmetric monoclinic space group P21 as the undeuterated compound. Moreover, the neutron diffraction enabled to locate the coordinated water molecule. Below TN = 5.5 K a commensurate canted antiferromagnetic state was observed. The determined magnetoelectric symmetry 26

Page 26 of 64

P21‟ can explain the strong magnetodielectric coupling present in this material and the weak ferromagnetic moment of 0.3 B. The transition from paramagnetic to weak-ferromagnetic state occurs without any intermediate antiferromagnetic state.

4.2.2 Fe[(CH3(CH2)2PO3)(H2O)], a Weak Ferromagnet

ip t

Among the reported layered iron(II) phosphonates the organic-inorganic hybrid Fe(II) propylphosphonate has attracted our attention because it crystallizes in the

cr

non-centrosymmetric monoclinic space group P21 and exhibits weak ferromagnetism

us

with a reasonable high critical temperature TN of 22 K as shown in Figure 21 [53]. The structure of the hybrid compound is lamellar made of alternating inorganic and

an

organic layers along the c-direction (see Figure 22). The inorganic layers consist of Fe(II) ions octahedrally coordinated by five phosphonate oxygen atoms and one from

M

the water molecule, separated by bi-layers of propyl groups. The C3H7-groups interpenetrate as in a double comb along the c-axis of the unit-cell. At each end of the organic group a large vacant site has been observed and so the slabs of neutral

ed

Fe[(CH3(CH2)2PO3)(H2O)] are translationally related along the c-axis with only van der Waals contacts between them. A quite similar structure is also found in one form

ce pt

of the polymorphic methylphosphonate iron(II) derivative [54]. Quite recently the family of polar metal(II) phosphonates has been enlarged by several other examples. Synthesis in presence of asymmetric phosphonate ligands

Ac

have provided in several cases formation of polar compounds. For example the hydrothermal reaction of cobalt(II) and zinc(II) nitride with an unsymmetric phosphonic acid, such as 4-[(phosphonomethylamino)methyl]benzoic acid afforded the formation of two new polar metal(II) phosphonates [55] both having a 3D pillared structure. The cobalt(II) derivative is a weak ferromagnet with a magnetic ordering temperature of ~ 2 K. Zheng et al. used a less complex unsymmetric phosphonate 27

Page 27 of 64

ligand, 2-(phosphonomethyl)benzoic acid (2-pmbH3), in combination with copper(II) and cobalt(II) salts to obtain polar metal(II) phosphonates [56]. Their recent paper reports on the synthesis and magnetic properties of another two new polar materials, i.e. Co2(μ4-OH)(2-pmb) and Cu4(μ3-OH)2(2-pmb)2. The cobalt(II) phosphonate is monoclinic (space group Cc) while the copper(II) compound crystallizes in the

ip t

orthorhombic polar space group Pca21. It is worth to notice that Co2(μ4-OH)(2-pmb)

cr

behaves as a canted antiferromagnet below 31 K.

us

5. Conclusions

Three types of magnetic organic-inorganic hybrid compounds have been reviewed

an

and the structural and physical properties have been examined. All the examples reported show a 1D or 2D hybrid structure and are characterized by the presence of unpaired electrons either as radicals and/or transition metal ions in the lattice. The

M

hybrid materials chosen as examples in this review have been selected due to their electrical conductivity, magnetism and dielectric properties. They demonstrate the

ed

possibility to design and synthesize by “soft chemistry” methods multifunctional materials with the desired properties. This has been achieved by using suitable

ce pt

molecular bricks either organic or inorganic. The first group deals with perovskite metal(II) halides. The overall crystal structure consist of an alternation between organic and inorganic layers which are held together by an ionic bond. The

Ac

perovskite metal halides A2M(II)X4, where M = Cr(II) and Cu(II) salts, have been of interest since 1970 because they show 2D ferromagnetism at low temperatures. Today the perovskite metal halides are receiving an unexpected renewed interest for two reasons. The first one is the discovery of light-harvesting properties of the cubic perovskite (CH3NH3)PbI3. Perovskite metal halides have been proposed as photoactive materials replacing silicon in solar cells. Power conversion efficiency 28

Page 28 of 64

exceeding 15% seems to encourage the use of perovskite-based materials in thin-film PV technology. In the light of this finding organic-inorganic hybrid semiconductors of group 14 (IVA) elements could be the basis for the next generation solar cells. The second reason is related to the dual-function properties showed by several layered perovskite salts. They have attracted interest as multiferroic materials after the recent

ip t

discovery of coexistence of weak ferromagnetism and antiferroelectric order by Jain et al. in [(CH3)2NH2]Mn(HCOO)3 [49].

cr

The second group reported here, i.e. the radical-cation salts, belongs to the family of

us

molecular conductors and semiconductors. Electron donor molecules such as tetrathiafulvalene, TTF, in combination with many electron acceptor molecules or

an

simple inorganic anions have produced a variety of organic semiconductors, metals and superconductors. Electro-crystallization technique was often helpful to obtain single crystals especially in the case of the reported BEDT-TTF salts of α-Keggin and

M

Lindquist anions. The α-Keggin derivatives mainly crystallize in a layered structure, where the two subunits are held together by ionic bonds generated by the redox

ed

process during synthesis. Combination of α-Keggin anion [PMo12O40]4- with BEDTTTF donor molecules leads to formation of an antiferromagnetic hybrid

ce pt

semiconductor, (BEDT-TTF)8[PMo12O40]•(2CH3CN•2H2O). Moreover, the magnetic properties of two examples of Spin Peierls systems, (BEDT-TTF)8[PMo3NbW8O40] and '-(BEDT-TTF)2Ag(CN)2 were presented. In these solids the magnetic and

Ac

electrical properties arise from the organic component. The electron donor properties of the TTF molecule presented in this section have been used recently also to cover the surface of hybrid organic-inorganic perovskite (C6H5(CH2)2NH3)2CuCl4 crystals. Doping of a crystal of perovskite copper (II) halide with a organic neutral TTF form a conductive strip at their interface which increases the electrical conductivity by five orders of magnitude [30d]. This method offers the opportunity to combine the robust 29

Page 29 of 64

mechanical properties of inorganic materials with the versatility of organic compounds. Further investigation of the optical and magnetic properties of this new multilayer systems would be of interest. The third part of the review describes the chemical and physical properties (mainly magnetism) and the crystal and molecular structure of selected magnetic Cr(II), Ni(II)

ip t

and Fe(II) hybrid compounds based on phosphonates. In simple layered monoalkyl or monoaryl metal phosphonates only covalent and Van der Waals bonds hold together

cr

the organic and inorganic part of the materials. The R group can be chemically

us

modified or by increasing the number of carbon atoms or by introducing another functional group such as a second phosphonate or an amino group. Chemical modification of the phosphonate ligands opens the opportunity to use methods of

an

“crystal engineering” and to obtain multifunctional materials. The insertion of an amino group has made it possible to isolate a novel Cr(II) ammonium

M

ethylphosphonate chloride of formula Cr[NH3(CH2)2PO3(Cl)(H2O)], which resulted in being a polar canted antiferromagnet. Interestingly this compound represents the

ed

first example of a polar hybrid which shows a magnetodielectric coupling. The subsequence of positive (NH3)+ and negative charged (Cl)- counter parts generates

ce pt

also ionic and hydrogen bonds in this layered hybrid material. It is worth to notice that alternation between inorganic and organic layers and establishment of ionic and hydrogen bonds have been observed also in perovskite metal halides presented in the

Ac

first part of this review. Furthermore layered metal di-phosphonates can be obtained from di-phosphonic acid precursors bearing two final phosphonate groups. The ferromagnetic properties of Ni2[(PDI-BP)(H2O)2]∙3H2O, based on an extended aromatic perylenediimide bis-phosphonate dye has been discussed as an example of a multifunctional hybrid material. By following this method it is possible to design new robust hybrid compounds characterized by having more than one physical properties 30

Page 30 of 64

in the same lattice. Moreover the development in the field of metal phosphonates is directed towards the synthesis of new functional materials or precursors of interesting metal phosphates such as LiMIIPO4 and amorphous MIIPO4 where M is a first raw transition metal.

ip t

6. Acknowledgements

Financial support from the Italian Consiglio Nazionale delle Ricerche is gratefully

cr

acknowledged. Thanks are due to all the coworkers whose names appear in the

us

references and without of whom this work would not have been possible. One of us (C.B.) thanks Peter Day for opportunity to work in his group as a post-doc at Oxford

an

(1975-1977) and for many years of collaboration and discussions. E.M. B. is grateful

Ac

ce pt

ed

M

to G. Nénert and C.J. Gómez-García for the fruitful collaboration.

31

Page 31 of 64

7. References [1] C.N.R. Rao, A.K. Cheetham, A. Thirumurugan, J. Phys. Condens. Matter 20 (2008) 083202 (21pp). [2] L.V. Interrante, K.W. Browal,l F.P. Bundy, Inorg. Chem. 13 (1974) 1158.

Lambert, H.R. Zeller, Phys.Status Solidi B 58 (1973) 587.

ip t

[3] a) K. Krogman, Angew. Chem. Inter. Ed. Engl. 8 (1969) 35; b) R. Comes, M.

[4] See for example a) P.M. Chaikin, R.L. Greene, Physics Today 39(5) (1986) 24; b)

cr

G. Saito, Y. Yoshida, The Chemical Record, 11 (2011) 124; A.M. Kini, U.

us

Geiser, H.H. Wang, K.D. Carlson, J.M. Williams, W.K. Kwok, K.G. Vandervoort, J.E. Thompson, D.L. Stupka, D. Jung, M.-H. Whangbo, Inorg.

an

Chem. 29 (1990) 2555.

[5] a) R. Sessoli, D. Gatteschi, A. Caneschi, M. A. Novak, Nature 365 (1993) 141; b)

State Scien. 10 (2008) 1701.

M

D. Gatteschi, L. Bogani, A. Cornia, M. Mannini, L. Sorace , R. Sessoli, Solid

[6] a) D. Jérome, A. Mazaud, M. Ribault, K. Bechgaard, J. Phys. Lett. (Paris) 41/4

ed

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Captions to Figures

Figure 1. Schematic layered perovskite structures of (a) (R-NH3)2MX4 and (b) (NH3R-NH3)MX4 , M = divalent metal ion. Figure 2. K2NiF4 crystal structure.

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Figure 3. Reduced Magnetization vs. T plots for a) (C6H5CH2NH3)2CrBr4 [16] and b) Magnetic hysteresis loop for (C6H5CH2NH3)CrBr3.3I0.7 [16c,d]. Reproduced, slightly modified, with permission © 1986 Elsevier, 1987 American Chemical Society, 1991 and 1992 Royal Society of Chemistry.

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Figure 4. Antiferrodistortive arrangement of [CrBr6] units in [CrBr4]2- due to JahnTeller effect.

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Figure 5. Section of the visible crystal absorption spectra of (C2H5NH3)2CrCl4 at r.t. and T = 7 K [16d,e]. Reproduced, slightly modified, with permission © 1987 American Chemical Society and 1992 Royal Society of Chemistry.

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M

Figure 6. m vs. T plot in the temperature range from r.t. down to liq. He temperature for (C3H7NH3)2MnCl4 [22b]. Reproduced, with permission © 1979 Elsevier. Figure 7. Crystal Structure of [TTF][TCNQ] which consists of uniform segregated chains of donors and acceptor molecules.

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Figure 8. View of the crystal structure of [TTF] 3[Mo6O19] along the b axis. Figure 9. The crystal structure of[TTF] 2[Mo6O19]. Figure 10. The crystal structure of [BEDT-TTF] 8[PMo12O40]{2CH3CN.H2O}.

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Figure 11. Temperature dependence of the a) molar magnetic susceptibility and b) after subtracting the cluster and TIP for [BEDTTTF]8[PMo12O40]{2CH3CN.2H2O} [34]. Reproduced, with permission © 1995American Chemical Society. Figure 12. Molar magnetic susceptibility vs. T plot of a) molar magnetic susceptibility and b) after subtracting the Curie tail for [BEDT-

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TTF]8[PMo3NbW8O40] [39a]. Reproduced, with permission © 1997 Elsevier. Figure 13. Molar magnetic susceptibility vs. T plots of ’-[BEDT-TTF]2[Ag(CN)2] [39b]. Reproduced, with permission © 1993 Elsevier. Figure 14. Unit-cell structure of M[CH3CH2PO3](H2O) viewed along the a-axis.

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Figure 15. Zfc and fc  vs.T plots of Ni[(C6H5PO3)(H2O)] in the temperature range 5 to 10 K.

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Figure 16. Thermal variation plot of the χ’-χ’’ AC magnetic susceptibility of Ni[(C18H37PO3)(H2O)] [50]. Reproduced, with permission © 2008 American Chemical Society.

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Figure 17. Isothermal magnetization of Ni2[(PDI-BP)(H2O)2]∙3H2O at T = 10, 20 and 22 K.

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Figure 18. Unit-cell structure of Cr[H3N-(CH2)2PO3](Cl)(H2O)viewed along the c– axis [52a].

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Figure 19. Dielectric constant  vs. T plot of Cr[H3N-(CH2)2PO3](Cl)(H2O) showing an anomaly below TN [52b]. Reproduced, with permission © 2008 American Physical Society.

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Figure 20. Magnetization of Cr[D3N-(CH2)2PO3](Cl)(D2O) as a function of the magnetic field in the temperature range 6 to 2K [52c]. Reproduced, with permission © 2013 American Chemical Society. Figure 21. Zfc and fc M vs.T plots of Fe[CH3-(CH2)2PO3](H2O) in the temperature range 2 to 30 K [53]. Reproduced, with permission © 2006 Elsevier.

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Figure 22. Unit-cell packing of Fe[CH3-(CH2)2PO3](H2O) viewed along the b – axis.

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Tables

θ/K

TC/θ

(CH3NH3)2CrCl4

42.0

13.0

59

0.71

(C2H5NH3)2CrCl4

41.0

10.1

58

(C3H7NH3)2CrCl4

39.5

9.3

57

(C6H5CH2NH3)2CrCl4

37.0

10.6

(C6H5CH2NH3)2CrBr4

52.0

13.1

KBTC/J

cr

(J/kB)/K

3.23

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TC/Ka

Interlayer spacing/Å 9.44

4.06

10.71

0.69

4.25

12.35

58

0.64

3.49

15.71

77

0.68

3.97

16.24

M

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0.71

From SQUID and ac mutual inductance technique

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a

Compound

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Table 1: Magnetic Properties and Interlayer Spacings of (RNH3)2CrX4 (R = Alkyl Group; X = Cl, Br)[16]. Reproduced, slightly modified, with permission © 1987 American Chemical Society, 1991 and 1992 Royal Society of Chemistry.

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Graphical Abstract (for review)

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*Highlights (for review)

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We present selected examples of magnetic organic-inorganic hybrids. Structural and magnetic properties are correlated. Recent discovery of dual-functional hybrids renewed the interest in this field. New multifunctional materials are presented and discussed.

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Figure 22

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