Tetrel bonding interactions at work: Impact on tin and lead coordination compounds

Coordination Chemistry Reviews 384 (2019) 107–125

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

Review

Tetrel bonding interactions at work: Impact on tin and lead coordination compounds Antonio Bauzá a, Saikat Kumar Seth b, Antonio Frontera a,⇑ a b

Universitat de les Illes Balears, Crta de Valldemossa km 7.7, 07122 Palma de Mallorca (Baleares), Spain Department of Physics, Jadavpur University, Kolkata 700032, India

a r t i c l e

i n f o

Article history: Received 25 July 2018 Received in revised form 30 December 2018 Accepted 4 January 2019

Keywords: Tetrel bonding interactions Main group chemistry r-Hole interactions Crystal engineering Supramolecular chemistry

a b s t r a c t A tetrel bond can be defined as an interaction between any electron donating moiety and a group 14 element acting as Lewis acid. Experimental and theoretical research on this interaction has rapidly grown in recent years. A reason for that is related to the fact that it can be easily exploited to design supramolecular structures. This review is devoted to highlight the role of heavier tetrel atoms (tin and lead) as promising building blocks for the construction of supramolecular assemblies and supramolecular MOFs based on tetrel bonding interactions. In the case of tin, the number of investigations on this topic is limited, therefore we also provide a survey of crystal structures retrieved from the Cambridge Structural Database where this interaction is crucial for the crystal packing stability. We have subdivided this part taking into consideration the atom or group of atoms acting as lone pair donor atom (O, S, N, halogen) and also the intra- or intermolecular nature of the interaction. This survey evidences that close contacts between Sn and lone-pair-possessing atoms are quite common and oriented along the extension of the covalent bond formed by the Sn and the most electron-withdrawing substituent. Moreover, it provides experimental evidence of the ability to act as electrophilic site (tetrel bond donor). This ability has a prominent role upon the conformations and the packing of Tin organic derivatives in the solid state. For Pb(IV) organic compounds, where the geometry around the metal center is tetrahedral we also provide a survey of crystal structures retrieved from the Cambridge Structural Database. To our knowledge, this type of analysis in Pb(IV) is unprecedented in the literature. For Pb(II), we have selected several works where the tetrel bonding interaction is used as a robust tool in crystal engineering and supramolecular chemistry. Ó 2019 Elsevier B.V. All rights reserved.

Contents 1. 2. 3. 4.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Origin of tetrel bonding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . History . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Tetrel bonding interactions involving tin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1. Intramolecular Sn tetrel bonds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1.1. Sn


O tetrel bonds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1.2. Sn


S tetrel bonds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1.3. Sn


N tetrel bonds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1.4. Sn


Hlg tetrel bonds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2. Intermolecular Sn tetrel bonds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.1. Sn


O tetrel bonds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.2. Sn


S tetrel bonds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.3. Sn


N tetrel bonds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.4. Sn


Hlg tetrel bonds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

⇑ Corresponding author. E-mail address: [email protected] (A. Frontera). https://doi.org/10.1016/j.ccr.2019.01.003 0010-8545/Ó 2019 Elsevier B.V. All rights reserved.

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5.

Tetrel bonding interactions involving lead . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1. Tetrel bonding interactions involving Pb(IV) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2. Tetrel bonding interactions involving Pb(II) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6. Conclusions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Appendix A. Supplementary data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1. Introduction Supramolecular chemistry is devoted to the study of assemblies of greater complexity than individual molecules [1,2]. Undoubtedly it is the multidisciplinary field that grows faster in Chemistry, in a parallel way to the field of synthetic chemistry, where complicated structures using covalent chemistry are built presenting unique sizes, shapes and functionalities. Supramolecular assembly is intimately related to the concept of molecular recognition [3]. That is, the molecules interact through intermolecular forces (non-covalent interactions), in a controlled and spontaneously way leading to the formation of assemblies. The theory and principles of recognition and the physical nature of the interaction are basically the same in gas phase, solution and in the solid state [4,5]. However, the study of the supramolecular processes in the gas-phase and solution is usually understood as molecular recognition while in solid state it is often known as crystal engineering [6]. The deep understanding of non-covalent interactions is necessary to control chemical and biological processes, which are often orchestrated by an intricate combination of non-covalent interactions [7,8]. Indeed, these interactions are the foundation of the life process itself, the definitive expression of function. Therefore, both the understanding and accurate description of the non-covalent forces that are established between molecules are essential for the continuous development of the supramolecular chemistry. Chemists working in this field frequently rely on strong, directional interactions like hydrogen bonding [9] and halogen bonding [10], and less directional forces like ion pairing. In addition, noncovalent interactions involving aromatic rings are also extremely important in this field [11–14]. Apart from the noncovalent interactions mentioned above, other types of interactions can be exploited to design supramolecular structures and catalysts [15,16]. Nowadays, there is a growing recognition that r-hole [17–20] and p-hole interactions [21–26] can also be utilized in supramolecular chemistry and crystal engineering. In particular tetrel [27–31], pnictogen [32–43], chalcogen [44–56], aerogen bonding [57–59] and even regium bonding [60–62] are becoming new players in this field [63]. All follow the same bonding scheme that is indeed comparable to hydrogen bonding: X–D


A, where X is any atom, D is the donor atom (Lewis acid) from the main group of elements, and A is any electron rich atom or moiety (Lewis base). The r-hole can be represented as a region of positive electrostatic potential found on an empty r* orbital. The magnitude of the r-hole depends on two factors: (i) it becomes more positive (stronger interaction) when D is more polarizable and (ii) it becomes more positive when the X atom is more electron withdrawing. The atomic polarizability increases in a given group on going from lighter to heavier elements, so that for tetrels (group 14): C = 11.5 au, Si = 38.1 au, Ge = 40.3 au, and Sn = 55.6 au (au = atomic units) [18]; thus a stronger interaction should occur when descending the group, and, consequently, tin and lead are expected to form the strongest r-hole interactions [64–66]. There are many examples in the literature of experimental [67] and theoretical [68,69] investigations devoted to tetrel bonding, which was named as such in 2013 [70–73]. A differential feature of tetrel bonding compared to the rest of r-hole interactions is that

119 119 120 124 124 124 124

the electrostatic potential on the tetrel atom is not anisotropic (absence of negative belts or lone pairs) and also that the r-holes are located in the middle of three sp3-hybridized bonds, reducing the accessibility to the r-hole. Therefore, all tetrel atoms are not equally suited for noncovalent r-hole bonding because some easily expand their valence, thus engaging in more traditional chemistry. Indeed, tin and lead have rich coordination chemistry [74–77], and are commonly seen as metals. Moreover, hypervalent species with Ge and Si (also considered metalloids) are also well-known [78–88]. Nevertheless, the heavier tetrel atoms (Ge–Pb) can engage in noncovalent interactions when placed in a chemical context that prevents hypervalency [89]. For instance, it has been recently reported the importance of S


Sn tetrel bonding interactions in the activation of peroxisome proliferator-activated receptors (PPARs) by organotin molecules [90]. The aim of this review is to highlight the recent research on tetrel bonding interactions where covalently bonded tin and lead are the r-hole donors. In the case of tin, explicit works where tetrel bonds have been used in crystal engineering are very scarce. Therefore, we also include a survey of crystal structures retrieved from the Cambridge Structural Database (CSD). This survey evidences that close contacts between Sn and lone-pair-possessing atoms are very common. We have divided this part of the review into two different sections, depending on the intra- or intermolecular nature of the contacts. The tetrel bonding interactions are usually oriented along the extension of the covalent bond formed by the tetrel atom with the most electron-withdrawing substituent. To this respect several structures are used as examples to illustrate the effect of replacing Sn–C bonds by Sn–Y bonds where Y = halogen, oxygen, sulfur, or nitrogen. In the case of Pb, we have differentiated organolead compounds where the oxidation state is +4 from those structures where Lead has the common one +2. For the former, no previous work has been found in the literature devoted to study tetrel bond in four-coordinated Pb(IV) structures. Consequently, we provide a survey of crystal structures retrieved from the CSD to evidence the ability of tetrahedral Pb(IV) to establish tetrel bonding interactions with lone pair donor atoms (O, S, N and Br). For the latter, significant work has been published related to crystal engineering and supramolecular chemistry taking advantage of tetrel bonding interactions involving Pb(II). The final aim of this review is to convince scientist working in the fields of supramolecular chemistry and crystal engineering that the tetrel bond encompassing Tin and Lead is a consolidated and robust interaction.

2. Origin of tetrel bonding In 2013, some of us and others devised the term ‘‘tetrel bonding” to describe the tendency of heavier tetrel atoms to interact with anions or electron rich atoms [31]. The elements of group IV (or 14) are also known as tetrels coming from the Greek ‘‘tetra” that simply means four. Our initial theoretical investigations were inspired by the disiloxane derivative highlighted in Fig. 1a. The molecular electrostatic potential calculations showed that the

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Fig. 1. Ball and stick representations of refcodes QOMBID (a), OYOMOF (b) and OYOMIZ (c). Tetrel bonds are represented as black dashed lines; hydrogens have been omitted for clarity. Distances in Å are shown close to the respective interactions. Color code: gray carbon, red oxygen, pale yellow silicon, yellowish green fluorine.

most positive part of the surface is located along the bisectrix of the Si–O–Si angle (obtuse part) as a consequence of the superposition of both r-holes at the Si atoms on the extension of the Si–O bonds. The X-ray structures of two binuclear pentacoordinate silicon complexes of diketopiperazine are also shown in Fig. 1. These compounds were used by Muhammad et al. to analyze the SN2 reaction mechanism in five differently substituted analogous structures (leaving group F, Cl, OTf, Br, and I) [91]. Two extreme cases that were characterized by X-ray analysis are shown in Fig. 1b,c. When fluoride is used, a covalent bond (F–Si, Fig. 1c) is observed, since it is a very poor leaving group, and an intramolecular O


Si tetrel bond is formed. In sharp contrast, when an excellent leaving group (I) is used in the substitution reaction, an intermolecular tetrel bond (I


Si, Fig. 1b) is clearly observed, and a covalent O–Si bond is formed. The initial theoretical investigations were also inspired by relevant experimental work, which demonstrated that tetrel bonding interactions induce planarity in five- and six-membered silicon rings. The formation of these interesting ternary assemblies in perhalogenated cyclohexasilanes [92,93] (Si6Cl12 X-ray structure is shown in Fig. 2a) interacting with halides (see Fig. 2b-e) is due

to the formation of six concurrent X


Si (X = Cl, Br, I) bonds tetrel bonds. The same behavior has been observed in perchlorocyclopentasilane [94]. 3. History The lighter tetrel atom (carbon) was the protagonist four decades ago of the first work describing noncovalent interactions involving the carbon acting as Lewis acid. That is, Johnson et al. showed the ability of carbon to establish attractive interactions with lone-pair possessing atoms (Lewis bases) [95]. They demonstrated that the arrangement of the [H2O–CO2] dimer is governed by a C


O contact that is stronger than that the dimer formed by a H


O hydrogen bond. As a matter of fact, several years later, Klemperer et al. corroborated this finding via microwave spectral analysis [96]. They showed that the equilibrium geometry of the supramolecular dimer features a O2C


OH2 tetrel bond instead of a hydrogen bonded HO–H


O@CO geometry. Simple calculations at the B97D/6-31 + G* (see Fig. 3) evidences that the tetrel bonded complex is 1.8 kcal/mol more stable than the H-bonded one. Interestingly, it was also demonstrated during the 1980s that other

Fig. 2. Ball and stick representations of refcodes AZAZUX (a), AHASEJ (b), AZEBAJ (c), AZEBEN (d) and AZEBIR (e). Color code: green chloride, brown bromine, pale yellow silicon, purple iodine.

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plexes [99]. In another related work, Alkorta studied the conformational behavior of aminopropylsilanes (see Fig. 4) [100]. Experimental data from NMR experiments (15N and 29Si chemical shifts and JN–Si coupling constants) in combination with theoretical results supported the existence of an equilibrium between the open chain structure (entropy favored) and the supramolecular cycle (enthalpy favored).

4. Tetrel bonding interactions involving tin Fig. 3. Ball and stick representations of two optimized complexes: (a) tetrel bond and (b) hydrogen bond between water and CO2. Their relative interaction energies are also indicated. Distances in Å are shown close to the respective interactions. Color code: gray carbon, red oxygen, light grey hydrogen.

H-bonding donor molecules like HBr [97] and HCN [98] form stronger tetrel bonding complexes with carbon dioxide. In 2001, Alkorta et al. investigated molecular complexes between silicon derivatives (SiX4, X = halogen) and lone pair donors (Z = NH3, H2O, etc.). The Si


Z interaction distances obtained from the calculations range between 2.1 and 4.1 Å, and the interaction energies up to 10 kcal/mol for the stronger com-

Fig. 4. Open chain and supramolecular cycle conformations of an aminopropyl silane derivative. Distance in Å is shown close to the tetrel bond interaction. Color code: gray carbon, blue nitrogen, light grey hydrogen. Light blue, fluorine.

It has been recently reported the first work where supramolecular assemblies are constructed via the rational utilization of noncovalent tetrel bonding interactions involving tin [101]. Baker’s group has used a series of R2Sn (R = Me, Ph) complexes of the Schiff’s base salicylaldehyde acyldihydrazone with a methylene spacer of variable length to investigate the prevalence of Sn


O noncovalent interactions. Interestingly, the X-ray solid state structures show that steric effects are important, as anticipated by the low accessibility of the r-hole in tetrel atoms. As shown in Fig. 5, the Sn


O tetrel bonding interactions are seen only for compounds with Me2Sn moieties (refcode PAXVAO) while they are not observed for Ph2Sn (refcode PAXVIW) ones. Moreover, they also evidence that apart from steric effects, the C–H


O interactions compete with the Sn


O interactions. These interactions are mostly electrostatic in origin with little evidence of covalency, as shown by DFT calculations [101], in combination with the quantum theory of atoms in molecules (AIM). To the best of our knowledge, the work commented above is distinctive in the sense that it is the only one that specifically mentions the crucial role of r-hole tetrel bonding interactions in the solid state architecture of tin coordination compounds. Below, we discuss a selected number of crystalline structures of organic derivatives of tin in which this element participates in tetrel bonding interactions with different lone pair possessing atoms. We have selected some illustrating examples (a comprehensive treatment is not the purpose of this review) which are structurally simple and

Fig. 5. Ball and stick representations of refcodes PAXVAO (a) and PAXVIW (b). Tetrel bonds are represented as black dashed lines; hydrogens have been omitted for clarity. Distances in Å are shown close to the respective interactions. Color code: gray carbon, blue nitrogen, red oxygen, teal tin.

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also have insignificant contributions from other noncovalent forces. 4.1. Intramolecular Sn tetrel bonds In this section we provide some examples where intramolecular tetrel bonding interactions determine the conformation adopted by several organic derivatives of tin. 4.1.1. Sn


O tetrel bonds In Fig. 6 we gather some X-ray structures exhibiting tetrel bonding contacts where the O atom acts as electron donor and tin as Lewis acid. The sum of van der Waals radii (RRvdW) of tin and oxygen atoms is 3.69 Å whilst the sum of their covalent radii (RRcov) is 2.05 Å. The Sn


O distances in the examples shown in Fig. 6 are significantly shorter than the RRvdW. However, they are only moderately longer than the RRcov. Therefore, a strong covalent character of the interaction can be anticipated. In case the Sn


O distance is only slightly longer than the RRcov it could be difficult to differentiate between an elongated covalent bond, a coordination bond or a tetrel bonding interaction. Herein we have used a simple criterion, that is we have considered a given contact as noncovalent tetrel bonding if the donor-acceptor distance is at least 0.5 Å longer than the RRcov. We believe that this issue should be further discussed in the literature in the near future and it is beyond the scope of the present review. The first structure shown in Fig. 6a is a organotin(IV) derivative (LSnPhCl2) where L is a O,C,O-chelating ligand [2,6C6H3(CH2OEt)2] (refcode LIVHOO) [102]. One O atom belonging to one of the –CH2OEt arms is coordinated to Sn (2.44 Å) thus the Sn is penta-coordinated. The other –CH2OEt arm establishes an intramolecular tetrel bond that governs the conformation of an organotin complex in the solid state. In Fig. 6b we have represented the structure of a trimethylstannylcarbomethoxy derivative (refcode KASYOS) [103], where a carbonyl oxygen atom acts as an

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effective tetrel bond acceptor. Similar tetrel bonding interactions are also present in carbamates (refcode EABFES) [104], aldehydes (refcode ACUKOY) [105], and esters (refcode ACUKIS01) [106]. Finally, in Fig. 6c we show the X-ray structure of iodo-(2,6-bis(m ethoxymethyl)phenyl)-diphenyl-tin (refcode RAKBOV) [107]. As discussed above, a common feature of r-hole interactions is that the more electron-withdrawing the atom covalently bonded to the r-hole donor atom is, the more positive the r-hole is observed. Interestingly, the Sn


O intramolecular distance in RAKBOV is longer than that in LIVHOO where the phenyl ring opposite to the lone pair donor has been replaced by a more electronegative chlorine atom. In the KASYOS structure where the tin atom is tetra-coordinated and the Sn


O distance is the shortest one, the tetrahedral geometry around the metal center is preserved, thus suggesting the non-covalent nature of the interaction. 4.1.2. Sn


S tetrel bonds In Fig. 7 we show some examples of X-ray structures retrieved from the CSD where the S atom acts as electron donor. The RRvdW of tin and sulfur atoms is 3.97 Å and the RRcov is 2.44 Å. Similarly to the aforementioned Sn


O contacts, the examples shown in Fig. 7 present Sn


S significantly shorter than the RRvdW and moderately longer than the RRcov. The BOMCOW [108] structure is a macrocyle composed by two bridging l2-(benzene-1,3-diyldime thanediyl)-bis(isobutylcarbamodithioato) ligands. The tin atoms are embedded in skewed-trapezoidal-bipyramidal coordination polyhedra with asymmetrically coordinating trans-oriented bisdithiocarbamate groups, exhibiting two Sn


S tetrel bonding interactions. Martinez-Otero et al. have studied intramolecular transannular chalcogen–tin interactions in dithiastannecine compounds [109]. A selected structure is represented in Fig. 7b (refcode FAMMOX) where the conformation adopted in the solid state is clearly determined by the intramolecular tetrel bonding (3.080 Å). The 119Sn NMR data of this compound suggest a four coordinated structure

Fig. 6. Ball and stick representations of refcodes LIVHOO (a), KASYOS (b) and RAKBOV (c). Tetrel bonds are represented as black dashed lines; hydrogens have been omitted for clarity. Distances in Å are shown close to the respective interactions. Color code: gray carbon, green chlorine, purple iodine, red oxygen, teal tin.

Fig. 7. Ball and stick representations of refcodes BOMCOW (a), FAMMOX (b) and GESNEX (c). Tetrel bonds are represented as black dashed lines; hydrogens have been omitted for clarity. Distances in Å are shown close to the respective interactions. Color code: gray carbon, green chlorine, yellow sulfur, teal tin.

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Fig. 8. Ball and stick representations of refcodes FEWXOU (a), ZANKEE (b) and IHOZAH (c). Tetrel bonds are represented as black dashed lines; hydrogens have been omitted for clarity. Distances in Å are shown close to the respective interactions. Color code: gray carbon, blue nitrogen, yellowish-green fluorine, teal tin.

in solution, thus supporting the noncovalent nature of the Sn


S interaction. It is also interesting to highlight the conformation of 2,10-dichloro-2,10-dimethyl-2,10-distanna-6-thiaundecane (ref code GESNEX) [110] that is basically determined by two tetrel bonding interactions. They are very directional (171°) and opposite to the electronegative chlorine atoms. In this structure the geometry around the tin atoms approximates to a trigonal bipyramid thus indicating a strong interaction. This conformation is also facilitated by the presence of the chlorine atoms that are good leaving groups. 4.1.3. Sn


N tetrel bonds In Fig. 8 we represent three selected examples of X-ray structures exhibiting Sn


N contacts. The geometry of 5-methyl-1-az a-5-stanna-bicyclo(3.3.3)undecane (refcode FEWXOU) [111] shows a short Sn


N contact (2.623 Å) that is only 0.523 Å longer than RRcov = 2.10 Å. Remarkably, if the methyl group opposite to the interacting N atom is replaced by a strong electron withdrawing group like fluorine (refcode ZANKEE, see Fig. 8b) [112], the Sn


N interaction is reinforced and the distance shortens to 2.394 Å, which is only 0.294 Å loner than RRcov. Therefore, ZANKEE can be considered as an hypervalent structure in agreement with the geometry of Sn that changes from tetrahedral in FEWXOU (C–Sn–C angle 113°) to trigonal bipyramid in ZANKEE (C–Sn–C angle 118°). In the solid state crystal structure of (cyclopenta-2,4 -dien-1-yl)-(2-(dimethylaminomethyl)phenyl)-diphenyl-tin (refTable 1 Data of RRvdW and RRcov values in Å for tin combined with the halogen series. The threshold for considering a contact as noncovalent bond is also given (RRcov + 0.5). Sn


Hlg

RRvdW

RRcov

RRcov + 0.5

Hlg = F Hlg = Cl Hlg = Br Hlg = I

3.64 3.92 4.02 4.15

1.96 2.41 2.59 2.78

2.46 2.91 3.09 3.28

code IHOZAH) [113] a clear tetrel bonding interaction determines the conformation of the 2-(dimethylaminomethyl)phenyl arm. In this structure the Sn


N distance (2.729 Å) is longer than those observed in the 1-aza-5-stanna-bicyclo(3.3.3)undecane derivatives likely due to the presence of a more rigid arm and the electronic nature of the cyclopenta-2,4-dien-1-yl group opposite to the lone pair donor amino group.

4.1.4. Sn


Hlg tetrel bonds In Table 1 we summarize the RRvdW and RRcov values for tin combined with the series of halogen (Hlg) atoms. We also indicate the threshold values for considering a contact as noncovalent tetrel bond. In Fig. 9 we represent the X-ray structures of four selected examples to illustrate intramolecular tetrel bonding interactions between tin and halogen atoms as electron donors. It can be observed than in most cases the tetrel bonding distances are considerably longer than the RRcov values and slightly shorter than RRvdW. This behavior is opposite to that found for nitrogen and oxygen lone pair donors (see Figs. 6 and 8), in line with the lower basicity of the halogen atoms. The 1,2,3,3,3-pentafluoroprop-1-en1-yl)-triphenyl-tin (refcode ADUKOB, see Fig. 9a) [114] structure exhibits a strong and linear tetrel bond oriented along the extension of the C–Sn bond, thus influencing the conformation of the fluorinated arm and governing the formation of a tetrelbonded five-membered ring. Similarly, the structure of the tetrakis [(2-chlorophenyl)methyl]-stannane (refcode LAPJUK, see Fig. 9b) [115] also forms a supramolecular five membered ring by means of the formation of a Sn


Cl tetrel bonding interaction. As examples of Sn


Br and Sn


I intramolecular tetrel bonds we have selected CALDUP [116] and SICSOM [117] structures (Fig. 9c,d). In both structures the halogen atom acts as Lewis base and approaches the Sn atom along the extension of the Br–Sn bond consistently with the fact that the most positive r-hole on tin is located at this

Fig. 9. Ball and stick representations of refcodes ADUKOB (a), LAPJUK (b), CALDUP (c) and SICSOM (d). Tetrel bonds are represented as black dashed lines; hydrogens have been omitted for clarity. Distances in Å are shown close to the respective interactions. Color code: gray carbon, yellowish-green fluorine, green chlorine, brown bromine, purple iodine, teal tin.

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Fig. 10. Ball and stick representations of 1D assemblies in refcodes SIWRAR (a), TMSNTZ10 (b) and IMIGUH (c). Tetrel bonds are represented as black dashed lines; hydrogens have been omitted for clarity. Distances in Å are shown close to the respective interactions. Color code: gray carbon, red oxygen, blue nitrogen, yellow sulfur, yellowish-green fluorine, teal tin.

Fig. 11. Ball and stick representation of a 2D assembly in refcode TMSNSN. Tetrel bonds are represented as black dashed lines; hydrogens have been omitted for clarity. Distances in Å are shown close to the respective interactions. Color code: gray carbon, red oxygen, blue nitrogen, yellow sulfur, teal tin.

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position since bromine is slightly more electron-withdrawing than the carbon atom. 4.2. Intermolecular Sn tetrel bonds In this section of the review we have selected remarkable examples to illustrate the importance of intermolecular tetrel bonding interactions governing the formation of one, two and threedimensional supramolecular assemblies in the solid state. 4.2.1. Sn


O tetrel bonds In Fig. 10 we represent several X-ray structures exhibiting 1D supramolecular assemblies in the solid state formed by tetrel bonding interactions. In trimethyl-diazo(ethoxycarbonyl)methyl-t in (refcode SIWRAR) [118] and 2-(trimethyl-tin)-1,3,5,2,4,6-trithia triazine-1,1-dioxide (refcode TMSNTZ10) compounds [119], infinite 1D supramolecular chains are formed with very directional C/N–Sn


O interactions (176° and 174° for SIWRAR and TMSNTZ10, respectively), as expected for r-hole interactions. Moreover, the tetrel bonding distance is shorter for TMSNTZ10 due to the electron withdrawing nature of the trithiatriazine-1,1dioxide ring bonded to tin. Concomitantly, the flattening of the three methyl substituents of the Sn atom is more pronounced in this structure. In Fig. 10c we show the solid state X-ray structure of 4,5,6,7-tetra fluoro-2-(trimethylstannyl)-1H-isoindole-1,3(2H)-dione (refcode

IMIGUH) [120] that is readily accessible by treatment of tetrafluorophthalimide with a mixture of NEt3 and Me3SnCl. Its solid state architecture reveals the formation of 1D supramolecular chains where two geometrically different tetrel bonds are established. Both are highly directional (174° and 176°) but with quite different Sn


O distances (3.063 and 2.882 Å). As example of 2D assembly in the solid state governed by Sn


O tetrel bonding interactions we have selected the 1,3-bis(tri methyl-tin)-1,3,5,7-tetra-aza-2,4,6,8-tetrathiocin-2,2-dioxide molecule (refcode TMSNNS, see Fig. 11) [121]. It presents two trimethyltin centers and several lone pair donors (O, N and S). In the X-ray structure, 2D sheets are formed where two different noncovalent tetrel bonding interactions are established, one where the lone pair donor is one O atom from the sulphoxide group (shortest interaction, 3.091 Å) and the other one with the N atom of the tetraaza-tetrathiocin ring (3.380 Å). Taking into consideration the tetrel bond distances, the tetrahedral geometry of the Sn atom and the almost linearity of the interactions, it can be concluded that both contacts are archetypal tetrel bonding r-hole interactions. In 1996, Eaton’s group discovered that 1,3,5,7-tetranitrocubane was able to react with an excess of trialkyl(amino)stannanes to yield 1,3,5,7-tetranitro-2,4,6,8-tetrakis(trimethylstannyl)cubane (refcode NACXOE) [122], as a stable white solid and easy to purify by column chromatography. The single-crystal X-ray structural analysis (see Fig. 12) confirmed the presence of the four

Fig. 12. Ball and stick representation of a 3D assembly in refcode NACXOE. Tetrel bonds are represented as black dashed lines; hydrogens have been omitted for clarity. Distances in Å are shown close to the respective interactions. Color code: gray carbon, red oxygen, blue nitrogen, teal tin.

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trimethylstannyl groups. The solid state architecture of this compound reveals the existence of an intricate network of Sn


O noncovalent tetrel bonding interactions. The oxygen atoms belonging to the four nitro groups interact with the Sn atoms of the adjacent molecules in the crystal structure, thus having a crucial role in the 3D architecture and crystal packing of this compound. 4.2.2. Sn


S tetrel bonds In Fig. 13 we represent the infinite 1D supramolecular assemblies observed in the solid state of bis(dibromo-dimethyl-tin) 1,4-dithiane cocrystal (refcode FECWUF) [123] and (N,N-dime thyl-dithiocarbamato)-trimethyl-tin (refcode MCARSN01) [124]. In the FECWUF structure the 1D supramolecular polymer is assembled by means of two different tetrel bonding interactions. One is established between the tin and the axial lone pairs of the 1,4dithiane molecule (Sn


S, 3.129 Å). Another one is formed between the bromine and the tin that generates self-assembled dimers (Sn


Br, 3.606 Å). These self-assembled dimers are connected via the bridging 1,4-dithiane molecules, giving rise to the 1D infinite chain. It is worthy to mention the high directionality of both interactions (177° for the S


Sn–Br and 172° for the Br


Sn–Br). Moreover, both interactions occur opposite to the electronegative Br atoms instead of the C atoms, as common in r-hole contacts.

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In MCARSN01 structure, the infinite 1D supramolecular assembly is governed by the formation of highly directional Sn


S interactions (172° for the S


Sn–S tetrel bond) that are established between the dithiocarbamato group. These quite long tetrel bonding distances suggest a noncovalent nature of the interactions. In Fig. 14 we show the X-ray structure of bis(l2-4,40 -thiodiben zene-1,10 -dithiolato)-bis(dimethyl-tin), refcode CITMEY [125], which is a centrosymmetric dinuclear macrocyclic compound with 4,40 -thiodibenzenethiol bridging the adjacent tin atoms generating a twenty-four membered ring. The supramolecular structure of this compound is a two-dimensional network in the bc plane, as shown in Fig. 14, in which the discrete molecules are connected through non-bonded and directional Sn


S interactions (3.700 Å) in one direction. Moreover, weaker Sn


S interactions (slightly longer than the RRvdw) are also formed in a perpendicular direction. Interestingly, the solid state X-ray structure of the analogous bis(l2-4,40 -thiodibenzene-1,10 -dithiolato)-bis(diphenyl-tin) where the methyl groups are substituted by phenyl groups (refcode CITMIC) does not exhibit tetrel bonding interactions [125]. The presence of the bulky phenyl groups instead of methyl bonded hinders the approximation of the lone pair donor S atoms. As an example of 3D assembly governed by Sn


S tetrel bonding interactions we have selected (l4-meso-2,3-dimercaptosucci nate-O,O0 ,S,S0 )-dodecamethyl-tetra-tin (refcode MEQWUB) [126].

Fig. 13. Ball and stick representations of 1D assemblies in refcodes FECWUF (a) and MCARSN01 (b). Tetrel bonds are represented as black dashed lines; hydrogens have been omitted for clarity. Distances in Å are shown close to the respective interactions. Color code: gray carbon, blue nitrogen, yellow sulfur, brown bromine, teal tin.

Fig. 14. Ball and stick representation of a 2D assembly in refcode CITMEY. Tetrel bonds are represented as black dashed lines; hydrogens have been omitted for clarity. Distances in Å are shown close to the respective interactions. Color code: gray carbon, yellow sulfur, teal tin.

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It is a tetranuclear tin complex containing two symmetrical (Me3Sn)2(SCHCO2) moieties. The supramolecular structure of this complex (see Fig. 15) is a 3D network linked by intermolecular Sn


S interactions, where each molecule forms two tetrel bonds as donor and two as acceptor. All four interactions are symmetrically related (3.286 Å) and are highly directional (S


Sn–O: 172°) and opposite to the most electronegative atom. Quite remarkably, the Sn atoms that are bonded to the less electronegative S atom instead of O (see Fig. 15), do not participate in tetrel bonding interactions. 4.2.3. Sn


N tetrel bonds In Fig. 16 we have represented the X-ray structures of tris(trimethylstannyl)acetonitrile (refcode TUQMIB) [127] and 4(2-methyl-3-oxo-3-((trimethylstannyl)oxy)prop-1-en-1-yl)benzo nitrile (refcode FIKTIE) [128]. In both structures the nitrogen

atom of nitrile group acts as lone pair donor in the tetrel bonds that control the formation of infinite 1D polymeric chains in the solid state. In both structures the Sn


N tetrel bonds are highly directional, i.e. the N


Sn–C angle is 176° in TUQMIB and the N


Sn–O angle is 173° in FIKTIE. Interestingly, the tetrel bonding distance is shorter in FIKTIE since the Sn tetrel atom is bonded to the more electronegative O atom. Moreover, it has been demonstrated that FIKTIE compound is able to catalyze the transesterification of sunflower oil using methanol, oil, using a catalyst molar ratio of 400:100:1. The good catalytic activity of this particular molecule is attributed to presence of the small methyl groups which cause less hindrance when the bulky triglyceride approximates to the catalytic tin center, thus forming a pre-reactive complex stabilized by an Sn


O tetrel bonding interaction.

Fig. 15. Ball and stick representation of a 3D assembly in refcode MEQWUB. Tetrel bonds are represented as black dashed lines; hydrogens have been omitted for clarity. Distances in Å are shown close to the respective interactions. Color code: gray carbon, red oxygen, yellow sulfur, teal tin.

Fig. 16. Ball and stick representations of 1D assemblies in refcodes TUQMIB (a) and FIKTIE (b). Tetrel bonds are represented as black dashed lines; hydrogens have been omitted for clarity. Distances in Å are shown close to the respective interactions. Color code: gray carbon, blue nitrogen, red oxygen, teal tin.

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As illustrative examples of 2D supramolecular assemblies controlled by Sn


N interactions, we have selected two simple molecules, which are dimethyltin dicyanide (refcode DMCYSN, Fig. 17a) [129] and (dicyanoethene-1,2-dithiolate)-dimethyl-tin (refcode AYADUZ, Fig. 17b) [130]. In the former structure, the Sn


N interactions lead to the formation of infinite polymeric sheets of molecules perpendicular to the b direction. Intermolecular distances (bifurcated tetrel bonds) within the polymeric sheets are quite short (2.680 Å) due to the strong electron withdrawing nature of the CN group. Consequently, the arrangement of the substituents and lone pair donors around the Sn is approximately octahedral instead of tetrahedral. In solution, the (dicyanoethene-1,2-dithiolate)-dimethyl-tin compound shows a NMR 2J(119Sn–1H) coupling constant value of 74.7 Hz that is typical of tetra-coordinated tin species. Moreover, on the basis of Lockarts’s equation [131], the Me–Sn–Me angle is approximately 124.6° in agreement with the values found in the crystal structure. In the solid state, it forms 2D supramolecular sheets by means of cooperative tetrel Sn


N and chalcogenide

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S


N bonding interactions. The tetrel bond (3.068 Å) exists between the Sn atom and two nitrogen atoms from two adjacent molecules and the chalcogen bond (3.173 Å) exists between two sulfur atoms of the same molecule and two nitrogen atoms from two adjacent molecules. 4.2.4. Sn


Hlg tetrel bonds In Fig. 18a we represent the X-ray structure of tricyclohexyl-tin fluoride (refcode BAJWOY) [132]. For this structure and its Cl, Br and I analogs (tricyclohexyltin halides) both the X-ray and Mössbauer spectroscopic data indicate that they are characterized by discrete tetrahedral units in their crystal structures, thus supporting the noncovalent nature of the interaction (valence expansion of Sn not observed). In Fig. 18b we represent the polymeric 1D assembly found in the solid state of 1-(t-butyl)-4-(dimethyl(chloro)stan nyl)-benzene (refcode CALGOM) [133]. It exhibits a herringbone shaped architecture because the methyl groups bonded to Sn establish secondary C–H


p interactions with the phenyl ring. The Sn


Cl noncovalent tetrel bonds (3.669 Å) are very directional

Fig. 17. Ball and stick representations of 2D assemblies in refcodes DMCYSN (a) and AYADUZ (b). Tetrel and chalcogen bonds are represented as black dashed lines; hydrogens have been omitted for clarity. Distances in Å are shown close to the respective interactions. Color code: gray carbon, blue nitrogen, yellow sulfur, teal tin.

Fig. 18. Ball and stick representations of 1D assemblies in refcodes BAJWOY (a) and CALGOM (b). Tetrel bonds are represented as black dashed lines; hydrogens have been omitted for clarity. Distances in Å are shown close to the respective interactions. Color code: gray carbon, yellowish green fluorine, green chlorine, teal tin.

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with a Cl–Sn


Cl angle of 175°. In both structures shown in Fig. 18, the electron rich halogen atom approaches the Sn atom along the extension of the X–Sn bond (X = F, Cl) consistently with the fact that the most positive r-hole on tin is located at this position. As examples for Sn


X tetrel bonds for X = Br and I, we have selected dibromo-diethyl-tin (refcode DESNBR) [134] and iodo-tris(trimethylsilylmethyl)-tin (refcode ISIMSN) [135] X-ray structures. In DESNBR compound, the individual molecules interact to form infinite 1D chains. Each molecule establishes two Sn


Br tetrel bonding interactions as donor and two as acceptor. The interaction is directional and, interestingly, it is the negative belt at the Br atom that interacts with the r-hole opposite to the Br–Sn bond. As a result of these interactions, there is a significant angular distortion from tetrahedral values in the individual molecule (C–Sn–C angle 134.0°). The 1D structure of ISIMSN (Fig. 19b) is formed by molecules that are piled along the c axis with non-covalent intermolecular Sn


I distances of 4.107 Å (slightly shorter than RRvdW = 4.15 Å) Similarly to BAJWOY structure (see Fig. 18a), it presents a high directionality with a I–Sn


I angle of 179°. As examples of 2D supramolecular assemblies controlled by Sn


F and Sn


Cl interactions, we have selected 2,6-difluoro-4-bi s(trimethylstannyl)amine-1,3,5-triazine (refcode ZARBAV,

Fig. 20a) [136] and tetrakis(chloromethyl)stannane (refcode UGATEB, Fig. 20b) [137]. In the former structure, a combination of Sn


N and Sn


F tetrel bonding interactions lead to the formation of a 2D supramolecular assembly perpendicular to the c direction. The Sn


F tetrel bonding interaction (3.396 Å) is shorter than the Sn


N one (3.597 Å) likely due to the low basicity of the sp2 hybridized N atom of triazine due to the presence of strong electron withdrawing F atoms. Both tetrel bonds are very directional with N–Sn


F and N–Sn


N angles of 175° and 176°, respectively. In tetrakis(chloromethyl)stannane, the 2D sheets observed in its solid state architecture are dominated by two symmetrically equivalent Sn


Cl tetrel bonding interactions which are slightly shorter than RRvdW = 3.92 Å. The geometry around the tin atom is perfectly tetrahedral in agreement with the weak Sn


Cl as expected by the low electronegativity of the C atom opposite to the r-hole. Finally, we highlight in Fig. 21 the X-ray structure of tris(trimethylstannyl)-ammonium iodide (refcode RONDAZ) [138]. It can be observed that the iodide anion is stabilized by three symmetrically equivalent Sn


I interactions. Due to the anionic nature of iodide the Sn


I distance is very short (2.875 Å), only slightly longer than the RRcov = 2.78 Å thus indicating a strong covalent character.

Fig. 19. Ball and stick representations of 1D assemblies in refcodes DESNBR (a) and ISIMSN (b). Tetrel bonds are represented as black dashed lines; hydrogens have been omitted for clarity. Distances in Å are shown close to the respective interactions. Color code: gray carbon, pale yellow silicon, brown bromine, purple iodine, teal tin.

Fig. 20. Ball and stick representations of 2D assemblies in refcodes ZARBAV (a) and UGATEB (b). Tetrel bonds are represented as black dashed lines; hydrogens have been omitted for clarity. Distances in Å are shown close to the respective interactions. Color code: gray carbon, blue nitrogen, yellowish green fluorine, green chlorine, teal tin.

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Fig. 21. Ball and stick representation of a 3D assembly in refcode RONDAZ. Tetrel bonds are represented as black dashed lines; hydrogens have been omitted for clarity. Distances in Å are shown close to the respective interactions. Color code: gray carbon, blue nitrogen, purple iodine, teal tin.

Table 2 Data of RRvdW and RRcov values in Å for Pb combined with lone pair donor atoms (Y). The threshold for considering a contact as noncovalent bond is also given (RRcov + 0.5). Sn


Y

RRvdW

RRcov

RRcov + 0.5

Y=O Y=N Y=S Y=F Y = Cl Y = Br Y=I

3.54 3.57 3.82 3.49 3.77 3.87 4.00

2.12 2.17 2.51 2.03 2.48 2.66 2.85

2.62 2.67 3.01 2.53 2.98 3.16 3.35

5. Tetrel bonding interactions involving lead 5.1. Tetrel bonding interactions involving Pb(IV) In this section of the review we present a survey of X-ray structures where tetrahedral Pb(IV) organic derivatives participates in

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relevant tetrel bonding interactions. To our knowledge, this particular tetrel bonding interaction where tetracoordinated Pb(IV) atom is acting as Lewis acid have not been previously analyzed using the CSD. The sum of van der Waals and covalent radii of Pb combined with several elements are given in Table 2. In Fig. 22 we show two 0D assemblies exhibiting tetrel bonding interactions in tetravalent organo-lead compounds. In the (l3-sulfido)-(l2-sulfido)-trimethyl-tetrakis(triphenylphosphine)-lea d-di-platinum hexafluorophosphate salt (refcode CIMDUY) [139], the trimethyl-lead group is bonded to the l3-sulfido S-atom. The r-hole opposite to the Pb–S bond interacts with the [PF6] anion establishing a bifurcated tetrel bonding with two fluorine atoms (see Fig. 22a). One of the tetrel bond distances is shorter than RRvdW and the other is slightly longer (0.2 Å). Interestingly, the 31 P–{1H}NMR spectrum of this complex shows a single sharp resonance at room temperature, evidencing that the complex is fluxional in solution, with the PbMe3 group is hopping between S centers. In Fig. 22b the X-ray structure of diphenyl-(2,20 -sulfane diyldibenzenethiolato-S,S0 )-lead (refcode QORWUQ) [140] forms self-assembled dimers in the solid state. They are strongly influenced by the formation of two symmetrically equivalent Pb


S interactions that are slightly shorter than RRvdW (see Table 2). In both compounds (CIMDUY and QORWUQ) the tetrel bond is established opposite to the most polarized Pb–S bond, as common in r-hole interactions. As examples of 1D supramolecular assemblies in the solid state exhibiting Pb


O and Pb


S tetrel bonding interactions in organolead(IV) compounds we have selected the X-ray structures of trimethyl-lead-diazo-ethylacetate (refcode MEPBAZ) [141] and trimethyl-methylthiolato-lead (refcode QASGOH) [142], respectively. In MEPBAZ structure (Fig. 23a) the lead atom of a molecule and the carbonyl oxygen atom of the neighbouring molecule are linked by a directional intermolecular tetrel bonding interaction (C–Pb


O angle 175°) thus explaining the formation of the 1D supramolecular chain. The Pb


O distance is significantly longer than RRcov and 0.5 A shorter than RRvdW suggesting a strong noncovalent interaction. In the X-ray solid state structure of QASGOH (see Fig. 23b) the infinite 1D assembly is clearly dominated by the directional Pb


S noncovalent interactions (C–Pb


S angle 174°). The lone pair of the S atom approaches the Pb(IV) atom along the extension of the C–Pb bond consistently with the fact that the most positive r-hole on lead is located at this position. In Fig. 24 we show additional examples of 1D assemblies involving tetrahedral Pb(IV) and N and Br as electron donors. In the triphenyl-(1-phenyl-5-mercapto-1H-1,2,3,4-tetrazolato-S)lead structure (refcode KEPCIR) [143] the molecules pile along b

Fig. 22. Ball and stick representations of refcodes CIMDUY (a) and QORWUQ. Tetrel bonds are represented as black dashed lines; hydrogens have been omitted for clarity. Distances in Å are shown close to the respective interactions. Color code: gray carbon, orange phosphorous, yellow sulfur, pale grey platinum, dark gray lead.

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Fig. 23. Ball and stick representations of 1D assemblies in refcodes MEPBAZ (a) and QASGOH (b). Tetrel bonds are represented as black dashed lines; hydrogens have been omitted for clarity. Distances in Å are shown close to the respective interactions. Color code: gray carbon, red oxygen, blue nitrogen, yellow sulfur, dark gray lead.

Fig. 24. Ball and stick representations of 1D assemblies in refcodes KEPCIR (a) and BRPHPB (b). Tetrel bonds are represented as black dashed lines; hydrogens have been omitted for clarity. Distances in Å are shown close to the respective interactions. Color code: gray carbon, blue nitrogen, yellow sulfur, dark gray lead.

direction such that a N atom of the tetrazole ring approaches an adjacent metal center (3.339 Å) forming a Pb


N tetrel bonding interaction opposite to the polarizable S–Pb bond. In solution, 207 Pb-NMR spectrum of this compound indicates a tetrahedral geometry of lead. In Fig. 24b we represent the X-ray structure of triphenyl-(2-bromophenyl-thiolato)-lead (refcode BRPHPB) [144] where Pb


Br tetrel bonding interactions can be observed. Interestingly, the arrangement of the Br atom facilitates the interaction

of the r-hole opposite to the S–Pb bond with the negative belt of the halogen atom. 5.2. Tetrel bonding interactions involving Pb(II) It is well-known that compounds of lead are usually found in the +2 oxidation state instead of the +4 oxidation state that is common in the lighter members of tetrel group. Some exceptions

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are basically limited to organolead compounds as those described in the previous section. Lead is a deadly toxic element exhibiting a biological poison similar to mercury [145–149]. Pb(II) presents a highly versatile coordination chemistry because of its large radius and ability to adopt different coordination numbers. The interest in the study of lead coordination compounds comes from the necessity to develop new ligands to trap lead(II), either from human body or from drinking water or paints [150–155]. The 6s2 electron pair of Pb(II) usually remains as an inert pair [156–158], and the extent to which the lone pair is stereochemically active [159– 161] has been studied and discussed before in the literature [162–167]. Tetrel bonds with hemi-directionally coordinated lead have been reported in the literature and play an important role in the solid-state chemistry of lead. As a matter of fact, the divalent form of lead (Pb2+) has attracted the interest of physical chemists due to its coordination and organization properties, namely holodirected and hemidirected (Fig. 25) [168]. Lead(II) with a variety of N/O/S/X-donor ligands (X = halide or pseudohalide) are hemidirected for coordination number up to five (n  5), and holodirected for coordination number six and higher (n  6) [169–170]. In this section of the review we summarize the most relevant works devoted to the utilization of tetrel bonding interactions involving Pb(II) in the construction of supramolecular assemblies and supramolecular MOFs. In 2015, Servati-Gargari et al. [66] reported the synthesis and X-ray characterization of three solid materials obtained from the reaction of Pb(SCN)2 and an unsymmetrical bis-pyridyl hydrazone ligand that acts both as a bridging and as a chelating ligand (see Fig. 26 for two examples). In all three compounds the lead center is hemidirectionally coordinated and is thus sterically optimal for participation in tetrel bonding. In the crystal structures of all three compounds, the lead atoms partici-

Fig. 25. Graphic representation of the holodirected (left) and hemidirected (right) coordination modes of lead(II). The location of the inert lone pair is also indicated.

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pate in short contacts with thiocyanate sulfur or nitrogen atoms interconnecting the covalently bonded units (two of them are shown in Fig. 26). Moreover, DFT calculations showed that the tetrel bonds are moderately strong (6.5–10.5 kcal/mol for Pb


S and 16.5 kcal/mol for Pb


N). In this work Servati-Gargari et al. [66] also reported a survey of structures in the CSD showing similar contacts and they conclude that tetrel bonding interactions are common in crystal structures of Pb(II) complexes. Therefore, the tetrel bonding plays a significant role in the supramolecular chemistry of Pb(II). After the aforementioned pioneering work, one year later Frontera and co-workers [171] reported an interesting study where they synthesized and X-ray characterized four Pb(II) complexes with N0 -(phenyl(pyridin-2-yl)methylene)isonicotinohydrazide ligand and different anions as coligands (see Fig. 27 for the struc tures with I, NO 2 and SCN anions). Remarkably, in all complexes the Pb(II) is hemidirectionally coordinated and participates in concurrent noncovalent tetrel bonding and agostic interactions (the latter refers to the interaction of a coordinatively-unsaturated transition metal with a C–H bond). Both forces along with unconventional chelate ring–chelate ring stacking interactions (not shown in Fig. 27) control the supramolecular architectures observed in their solid state architecture. This is the first work reported in the literature where the formation of supramolecular MOFs is controlled by agostic and tetrel bonding interactions. Mahmoudi et al. have published several works where Pb(II) exhibits hemidirectional coordination in complexes with (iso)nicotinohydrazide [65,172] and nicotinoylhydrazones-based [173] ligands and a variety of anions (halides, pseudohalides, nitrate and acetate). In these complexes the Pb(II) participates in noncovalent tetrel bonding interactions. In line with previous examples, the Pb(II) shows a marked tendency for the hemidirectional coordination mode in which the tetrel atom has a clear void in the distribution of bonds to the ligands. Such void in the coordination sphere facilitates the approach of electron donors to the r-hole of the Pb(II) enabling the formation of a strong tetrel bond with a very predictable geometry. Some examples are shown in Fig. 28 where the Pb is coordinated to nitrate (refcode IPAHEO) and acetate (refcode IPAHAK) [173]. In 2017, Bryce and collaborators explored the feasibility and value of 207Pb solid-state NMR experiments on compounds featuring lead tetrel bonds [64]. They defined lead tetrel bonding as an attractive interaction between an electrophilic region associated with lead in a molecular entity and a nucleophilic region in another, or the same, molecular entity. In fact, unambiguous identification of lead tetrel bonds can be challenging due to the hypervalent tendency of lead. They reported a series of 207Pb solid-state NMR experiments on five metal–organic frameworks featuring

Fig. 26. Ball and stick representations of refcodes QUQBIP (a) and QUQBOV (b). Tetrel bonds are represented as black dashed lines; hydrogens have been omitted for clarity. Distances in Å are shown close to the respective interactions. Color code: gray carbon, red oxygen, blue nitrogen, yellow sulfur, dark gray lead.

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Fig. 27. Ball and stick representations of refcodes TAGMUM (a), TAGNEX and TAGNAT (b). Tetrel bonds and agostic interactions are represented as black dashed lines. Distances in Å are shown close to the respective interactions. Color code: gray carbon, red oxygen, blue nitrogen, yellow sulfur, dark gray lead.

Fig. 28. Ball and stick representations of refcodes IPAHEO (a) and IPAHAK (b). Tetrel bonds are represented as black dashed lines; hydrogens have been omitted for clarity. Distances in Å are shown close to the respective interactions. Color code: gray carbon, red oxygen, blue nitrogen, yellow sulfur, dark gray lead.

lead coordinated to hydrazone-based ligands. Such frameworks are partially held together via lead tetrel bonds. The lead centers were characterized by 207Pb isotropic chemical shifts ranging from 426 to 2591 ppm. From the inspection of the structures of the compounds and the available 207Pb NMR data from the literature, it is suggested that a tetrel bond to lead results in chemical shift parameters which are intermediate between holodirected and hemidirected lead coordination geometries. Therefore, 207Pb NMR spectroscopy is a valuable tool in studying compounds featuring lead tetrel bonds to differentiate the various classes of weak interactions that may occur between a Lead atom and an electron rich entity. Mahmoudi et al. [174] reported the design and structural characterization of six coordination polymers, which were fabricated from PbCl2 and a series of bis-pyridyl ligands. Interestingly, these compounds present blue, bluish-green and white luminescence, depending on the ligand used. They are emissive in the solid state at ambient temperature. The topology of the obtained networks (from 1D to 3D) is dictated by the geometry of the organic ligand and tetrel bonding interactions. In Fig. 29 we represent one selected example, which is a 2D layered coordination polymer (refcode KEHLIV). The presence of two 3-pyridyl functions in the structure makes this ligand incapable of chelating to the Pb(II) center

giving rise to the formation of a 2D structure (Fig. 29). The Pb(II) atom is coordinated to two pyridine nitrogen atoms, one carbonyl oxygen atom from three different ligands as well as three chloride ligands. The coordination sphere around the metal center is a significantly distorted octahedron that is indicative of a stereochemically active lone pair that appears to cap one of the N–O edges. This facilitates the formation of Pb


N tetrel bonds of 3.391 Å involving the imine nitrogen atom (Fig. 29). This work and those commented above demonstrate that tetrel bonding can play a key role for the supramolecular aggregation of building units in solid state and can be considered as a powerful tool to design metal–organic frameworks with different dimensionalities and topologies. Nicotinohydrazide and carbazide derivatives have been recently used [175] as linker precursors in the synthesis of novel Pb(II) extended structures by reacting those ligands with PbX2  (X = NO 3 , CH3COO ) salts. In the resulting complexes, the Pb(II) center exhibited a hemidirected coordination geometry with all the covalent bonds being concentrated on one hemisphere of the coordination sphere. The sterically available Pb(II) ion participates in Pb


O/N tetrel bonding extending the complexes into higher dimensional frameworks. In Fig. 30 we represent two Pb(II) complexes with the carbazide ligand (refcodes JERPAA and JERPEE)

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Fig. 29. Ball and stick representation of a 2D assembly in refcode KEHLIV. Tetrel bonds are represented as black dashed lines; hydrogens have been omitted for clarity. Distances in Å are shown close to the respective interactions. Color code: gray carbon, red oxygen, blue nitrogen, green chlorine, dark gray lead.

Fig. 30. Ball and stick representations of refcodes JERPAA (a) and JERPEE (b). Tetrel bonds are represented as black dashed lines; hydrogen atoms have been omitted for clarity. Distances in Å are shown close to the respective interactions. Color code: gray carbon, red oxygen, blue nitrogen, dark gray lead.

Fig. 31. Ball and stick representations of refcodes REZFIO (a) and ZETLES (b). Tetrel bonds are represented as black dashed lines; hydrogen atoms have been omitted for clarity. Distances in Å are shown close to the respective interactions. Color code: gray carbon, red oxygen, blue nitrogen, yellow sulfur, green nickel, dark gray lead.

[175]. In the first one, a self-assembled dimer is shown that is formed by the association of two Pb (II) dinuclear complexes (see Fig. 30a). In this assembly, two equivalent Pb


O tetrel bonds are

stablished (3.061 Å). The JERPEE is a 1D polymeric structure where Pb


O contacts (3.518) influence the final architecture observed in its solid state X-ray structure (see Fig. 30b). The tetrel bonding

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contacts were also characterized by using Hirshfeld surface analysis, which further confirmed the importance of this interaction in the solid state. Non-covalent tetrel bonding interactions have been also reported recently in hemidirectional lead(II) complexes with nickel(II)-salen type metalloligands [176]. Both complexes represented in Fig. 31 exhibit relevant tetrel bonding interactions in the solid state that generate different supramolecular assemblies: infinite 1D chain for REZFIO and a self-assembled dimer in ZETLES. DFT calculations combined with molecular electrostatic potential calculations show that these non-covalent Pb


N/S interactions are strong due to electrostatic effects. In summary, the examples gathered in this section of the review demonstrate that the tetrel bonding interaction is common and relevant in the crystal engineering of organic–inorganic material that contain hemidirectionally coordinated lead(II) centers. 6. Conclusions In conclusion, the crystal structures discussed in the first part of this review provide substantial experimental evidence that organotin derivatives have a strong tendency to participate in tetrel bonding interactions (r-hole donor) with lone-pair possessing atoms. Moreover, this interaction determines the structure in crystalline solids of organotin derivatives. We have also provided evidence that intramolecular Sn


O/S/N/F/Cl/Br/I tetrel bonds can be found in crystals influencing the preferred conformation of the molecule. Moreover, the same set of intermolecular tetrel bonding interactions can be found in crystal structures determining the 1D/2D/3D supramolecular assemblies. Similarly, in the case of the less common Pb(IV) oxidation state, we have found a sufficient number of crystal structures where a network of Pb(IV)


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