CO₂ Derivatives of Molecular Tin Compounds. Part 2: Carbamato, Formato, Phosphinoformato and Metallocarboxylato Complexes ^†

Abstract

Single-crystal X-ray diffraction structures of organotin compounds bearing hemicarbonate and carbonate ligands were recently reviewed by us—“CO₂ Derivatives of Molecular Tin Compounds. Part 1: Hemicarbonato and Carbonato Complexes”, Inorganics 2020, 8, 31—based on crystallographic data available from the Cambridge Structural Database. Interestingly, this first collection revealed that most of the compounds listed were isolated in the context of studies devoted to the reactivity of tin precursors towards carbon dioxide, at atmospheric pressure or under pressure, thus highlighting the suitable disposition of Sn to fix CO₂. In the frame of a second part, the present review carries on to explore CO₂ derivatives of molecular tin compounds by describing successively the complexes with carbamato, formato, and phosphinoformato ligands, and obtained from insertion reactions of carbon dioxide into Sn–X bonds (X = N, H, P, respectively). The last chapter is devoted to X-ray structures of transition metal/tin CO₂ complexes exhibiting metallocarboxylato ligands. As in Part 1, for each tin compound reported and when described in the original study, the structural descriptions are supplemented by synthetic conditions and spectroscopic data.

1. Introduction

More than ever and everywhere in the world, the recovery and utilization of carbon dioxide have become a real priority, mobilizing many actors, from academic research to industry, involved in various domains and specialties. Long considered simply as abundant and chemically inert waste, but with devastating effects, carbon dioxide is now differently viewed by chemists, considered and accepted as a promising C1 building block, useful for a wide range of reactions and leading to a wide diversity of organic compounds. In this quest to transform CO₂ into chemicals with higher added value, coordination and organometallic chemistry played—and continue to play—an important and determining role in facilitating its activation [1]. Since the first resolution of an X-ray crystal structure highlighted the metal coordination of a CO₂ molecule—it was in 1975 that M. Aresta’s group published the characterization of a (carbon dioxide)bis(tricyclohexylphosphine) nickel complex [2]—advances and knowledges relative to carbon dioxide binding modes have continued to progress, and have proved particularly beneficial in catalysis and organic synthesis. Much later in 2019, for instance, Galindo et al. reported the X-ray structures of cis-M(C₂H₄)₂(CO₂)(PNP) compounds (M = Mo and W; PNP = 2,6-bis(diphenylphosphinomethyl)pyridine), which are the first examples of stable metal complexes with coordinated ligands of ethylene and carbon dioxide [3]. In conclusion of this recent work, the authors aim, as perspectives, for the conversion of C₂H₄ and CO₂ into acrylate derivatives.

Since the 1990s, the chemical transformation of CO₂ and its coordination on metal centers have been regularly reviewed and updated [4,5,6,7,8,9,10,11,12,13,14]. During the last decade, the reactivity of main group elements, and in particular metals and metalloids from group 14 (Si, Ge, Sn, Pb), toward small molecules also aroused a growing interest [15,16]. In this context and based on our previous work in this field, we have recently reviewed molecular organotin compounds bearing hemi-carbonate and carbonate ligands and which result from the reactivity of tin precursors towards carbon dioxide [1]. In this second part, we continue to explore from a structural point of view the molecular derivatives of tin which result from the insertion of carbon dioxide into the Sn–X bonds (X = N, H, P). Thus, the formation and structures of carbamato, formato and phosphinoformato tin derivatives are successively described and discussed. The last chapter is devoted to metallocarboxylato tin complexes exhibiting a bridging CO₂ ligand. Although these latter compounds are not derived from the direct reactivity of tin precursors with carbon dioxide, we found appropriate to include them in this review. Regarding the general method used to carry out this inventory, the compounds described below were identified from the data available on the online portal of the Cambridge Structural Database (CSD) web interface (2017) [17]. In addition to the structural description, comparison tables including selections of structural and spectroscopic data are also displayed at the end of each section (Table 1, Table 2, Table 3, Table 4, Table 5, Table 6, Table 7 and Table 8).

Among the useful chemicals with higher added-value which can be produced by the transformation of CO₂, the derivatives involving C–N bonds occupy an important place, historically and economically. Thus, from 1922, the German Bosch-Meiser process allowed the industrial use of carbon dioxide (together with ammonia) as raw material for the synthesis of urea [(NH₂)₂CO]. It is still today the most important industrial chemical process in terms of CO₂ amounts converted (115 Mt) [18]. Since then, the use of carbon dioxide as C1 synthon has been extended to the synthesis of other types of nitrogen compounds such as oxazolidinones, quinazolinediones, ureas, carbamates, isocyanates and polyurethanes (Scheme 1). These molecules constitute intermediates and derivatives of great importance for the chemical, agrochemical and pharmaceutical industry. The domain was firstly reviewed by He et al. in 2012 [19], and recently updated by Song et al. [20] and then by Dalpazzo et al. [21], both in 2019. In most cases, the intervention of a catalytic precursor—organic or organometallic, is required to achieve the targeted product.

Tin-Promoted Insertion Reaction of CO₂ into C–N Bonds

Regarding more specifically the use of main group metals as catalytic species for the insertion of CO₂ into the C–N bond, the beneficial role of organotin compounds was regularly emphasized in the past, being the subject of several studies. In 2002, Tominaga and Sasaki found that di-n-butyltin oxide, n-Bu₂SnO, was an effective catalyst for the dehydrating condensation of 1,2-aminoethanols with carbon dioxide to lead to the formation of 2-oxazolidinones [22] (Scheme 2). Reactions were carried out under 5 MPa of CO₂, at 180 °C, for 16 h, in N-methyl-2-pyrrolidone (NMP) as solvent and gave satisfactory yields in the range of 53 to 94%. The use of NMP has been found to be more effective than the use of protic and non-polar solvents. From a mechanistic point of view, the authors proposed the formation of a seven-membered cyclic tin carbamate complex (Scheme 3) by the insertion of CO₂ into the Sn–N bond of a stannaoxazolidine species and whose in-situ formation was supported by electrospray mass spectrometry. However, to our knowledge, such a species has not been yet structurally isolated. From this intermediate, the intramolecular elimination of one molecule of 2-oxazolidinone was suggested to explain the regeneration of the starting complex, n-Bu₂SnO.

In 2013, Ghosh et al. revisited the previous reaction using chlorostannoxane derivatives, 1,3-dichloro-1,1,3,3-tetraalkyldistannoxanes (Scheme 4), as catalytic precursor, in methanol under 1.72 MPa of CO₂ pressure and at 150 °C [23]. Chlorostannoxanes were selected because of (i) the presence of multi-active catalytic centers, and (ii) the ability to control Lewis acidity by changing the substituents on the metal centers. In comparison to the previous Tominaga’s study using n-Bu₂SnO [22], an improvement of turnover numbers was recorded using chlorostannoxanes, from 9 to 138. Furthermore, it has been demonstrated that the activity of the catalytic precursors depends on the nature of the R and R′ substituents, the n-butyl derivative (R and R’ = n-Bu) being the most efficient.

The direct synthesis of carbamates from carbon dioxide has also aroused renewed interest in the past two decades [24]. The state of the art was well established in 2003 by Calderazzo et al. [25] and recently updated by Marchetti et al. [26]. Indeed, carbamates can be viewed as a potential alternative to access to isocyanates, which are historically employed for the production of polyurethanes (PU). Industrially, isocyanates are produced by reacting primary amines with phosgene, but handling of this reactant is not without risks and requires drastic precautions. Therefore, the recent route based on the thermal decomposition of carbamates into isocyanates can be considered as a safer and greener synthetic pathway [27,28].

In the past, several groups investigated the action of organotins as catalyst precursors for the direct conversion of CO₂ into carbamates. In 2001, Sakakura et al. first published the synthesis of carbamates by reaction of amines and alcohols catalyzed by tin complexes (n-Bu₂SnO and Me₂SnCl₂) [29] (Scheme 5). Optimal reaction conditions require a pressure of 30 MPa (300 bar) at 200 °C for 24 h. Moreover, the addition of acetals (2,2-diethoxy- or 2,2-dimethoxypropane) acting as a dehydrating agent significantly improves the conversion, and the increase in pressure promotes the urethane selectivity.

In 2016, Choi et al. reported the synthesis of N-phenylcarbamates from CO₂ and aniline promoted by several di-n-butyltin dialkoxides: n-Bu₂Sn(OMe)₂, n-Bu₂Sn(OEt)₂, n-Bu₂Sn(OPr-n)₂, and n-Bu₂Sn(OBu-n)₂ (Scheme 6) [30]. The best yield of N-phenylcarbamate (41%) was obtained under 5 MPa (5 bar) of CO₂ for 20 min at 150 °C using n-Bu₂Sn(OMe)₂ as tin precursor. The catalytic activity changes depending on the nature of the alkoxide substituents linked to the tin atom [n-Bu₂Sn(OMe)₂ > n-Bu₂Sn(OPr-n)₂ > n-Bu₂Sn(OEt)₂ > n-Bu₂Sn(OBu-n)₂]. N,N′-diphenylurea is formed as byproduct (4%). From a mechanistic point of view, the hemicarbonato tin complex, n-Bu₂Sn(OMe)(OCO₂Me), resulting from the reactivity of CO₂ with n-Bu₂Sn(OMe)₂ [1,31], is claimed as the key intermediate of the reaction. In the case of reactions involving n-Bu₂Sn(OBu-n)₂, the authors also showed that the starting tin complex could be regenerated after the first catalytic run (by reacting with n-BuOH) and could thus be recycled without loss of activity.

Concomitantly and supported by density functional theory (DFT) calculations, Schaub et al. confirmed that dialkyltin(IV) dialkoxides can be used as reagents for the synthesis of aromatic carbamates from aromatic amines and carbon dioxide [32]. The process takes place between 60 and 70 bar of pressure, at 135 °C and in pentane as solvent. A NMR yield of 92% was recorded for the synthesis of methyl phenylcarbamate from aniline using three molar equivalent of n-Bu₂Sn(OMe)₂. Neither methylaniline nor diphenylurea was chromatographically detected. Extension of the reaction conditions to the synthesis of diurethanes from 4,4′-methylenedianiline and 2,4-diaminotoluene was successful with yields of 60 and 77%, respectively. In the perspective of an industrial development, the authors have demonstrated the possibility of regenerating and re-using the dialkyltin(IV) dialkoxides, by using an excess of dimethylcarbonate. Shortly after, in a new study, the same group improved the reaction yield (up to 97%) by combining with dialkyltin oxide, used as a tin precursor, tetraalkyl orthosilicates, which promote the generation of dialkyltin(IV) dialkoxides [33]. This positive effect had been initially shown for the tin-promoted synthesis of organic carbonates from carbon dioxide and alcohols [34].

2. Carbamato Tin Complexes

2.1. Early Works

Historically, the pioneering works on carbamates of tin derivatives dates back to the 1960s and are to be credited to A. J. Bloodworth and A. G. Davies who conducted extensive and fundamental investigations applying a systematic approach. From 1965, they described the synthesis of N-organo-N-trialkylstannylcarbamates resulting from the addition of trialkyltin alkoxides to isocyanates [35]. The reactions were reported as rapid and exothermic and taken place at room temperature (Scheme 7). Alkyl and aryl isocyanates exhibit a comparable reactivity.

Additionally in 1965, the study was then extended to bis(trialkyltin) oxides, which react exothermically with isocyanates to lead to trialkyltin N-trialkylstannylcarbamates (Scheme 8) [36].

From a coordination point of view, the authors preferentially recommended the representation of the carbamato ligand as N-coordinated to tin, in amido form, but did not exclude an O-coordination, in imido form, via an interconversion process. Subsequently, this structural consideration was the subject of several subsequent works, involving other research groups, and based on studies carried out by ¹¹⁹Sn Mössbauer spectroscopy [37], as well as more recently by computational calculations [38].

2.2. Reactivity

In terms of reactivity, N-organo-N-trialkylstannylcarbamates have been also involved in various organic reactions. In their initial article [35], Bloodworth and Davies showed in particular their reactivity towards acetic anhydride, ethylamine and ethanol, which lead, according to a metathetical process, to the corresponding urethanes. In 1968, the same authors reported the decarboxylation of trialkyltin esters of N-organo-N-trialkylstannylcarbamic acids by isocyanates and isothiocyanates providing an alternative route to the synthesis of carbodi-imides [39]. In 1992 and then in 1993, Shibata et al. underlined the positive and efficient role of methyl ester of N-ethyl-N-tributylstannylcarbamic acid as promoter for the Darzens reaction [40] and the addition of Michael [41] (Scheme 9).

2.3. X-Ray Crystal Structures

Regarding the solid-state characterization of metal carbamates by single-crystal X-ray diffraction, the area was first reviewed by Calderazzo et al. [25] and then recently updated by Marchetti et al. [26]. To date, numerous examples of metal carbamates have thus been isolated and characterized, demonstrating a rich coordination chemistry and highlighting a great diversity of structures. Herein, we will focus exclusively on X-ray structures involving tin moieties. To the best of our knowledge, six CSD depositions of carbamato tin complexes have been identified up to now (CSD entries and deposition numbers are reported in Table 1).

The first X-ray crystallographic investigation on a tin complex bearing a carbamato ligand was reported by Zakharov et al. in 1980, as part of studies on organotin isocyanate derivatives [42]. Thus, the Russian group characterized at the solid-state the structure of the trimethyltin(IV) ester of N-trimethylstannylcarbamic acids, Me₃SnNHC(=O)–OSnMe₃ (1), which describes a polymeric arrangement displaying a zigzag one-dimensional infinite chain along the c-axis (Figure 1). The organization of 1 consists of SnMe₃ moieties bridged by carbamate dianions. Two distinct sites of tin atoms are distinguished: (i) those located in the main chain exhibiting a trigonal bipyramidal geometry (TBP) whose equatorial plane is occupied by three methyl substituents and the apical positions by two oxygen atoms; (ii) SnMe₃ pendant groups, connected to the main chain by nitrogen atoms (Sn–N = 2.04(2) Å) and positioned in a syndiotactic arrangement, and whose tin atoms exhibit a tetrahedral geometry. This type of architecture is comparable to the chain-like structures reported for the organotin(IV) selenite complexes, (Me₃Sn)₂SeO₃·H₂O and (Ph₃Sn)₂SeO₃ [43], as well as for the organotin(IV) carbonato complexes, (Me₃Sn)₂CO₃ and (i-Bu₃Sn)₂CO₃ [44]. However, the authors pointed out that, despite the presence of a hydrogen atom on the nitrogen of carbamato groups, no hydrogen bonds are observed between the chains, which is explained by the proximity of bulky Me₃Sn groups. Moreover, we consider that compound 1 is the only example of an N-coordinated tin carbamate, structurally characterized to date.

Thereafter, with the aim of applications in heterogeneous catalysis, Calderazzo et al. reported the preparation of two N,N-dialkylcarbamato tin(IV) complexes, Sn(O₂CNi-Pr₂)₄ (2) and Sn(O₂CNEt₂)₄ (3), which were used as precursors for the chemical implantation of metal cations on a silica support [45]. The grafting method consisted of promoting reactivity of N,N-dialkylcarbamates with silanol groups of silica. Compounds 2 and 3 were synthesized by treating anhydrous Sn(IV) chloride with diethylamine and di-iso-propylamine, respectively, in toluene at room temperature, under atmospheric pressure of carbon dioxide (Scheme 10). Yields higher than 90% are reported.

Recrystallized from heptane, suitable single crystals of 2 were analyzed by X-ray diffraction leading to the resolution of the crystallographic structure. In 2002, in the frame of metal-organic chemical vapor deposition (MOCVD) investigations, Molloy et al. described a new synthetic route to 3, starting from a solution of tetrakis(N,N-diethylamino)tin(IV) complex in hexane that was bubbled with carbon dioxide (Scheme 11) [46].

X-ray structures of 2 and 3 revealed comparable mononuclear molecules. In both compounds, the tin atom is eight-coordinated by four chelating N,N-dialkylcarbamato ligands, which are exclusively O-donor (Figure 2). For 3, the authors suggest that the geometry can be viewed as a distorted square antiprism. From a spectroscopic point of view, similar ¹¹⁹Sn NMR chemical shifts were also recorded for 2 and 3, at −920.8 and −930.0 ppm, respectively (Table 2). In ¹³C NMR, the carbamato carbons show one resonance at 164.9 ppm for 2, and 166.2 ppm for 3. In the case of 2, CP/MAS ¹³C NMR measurements confirm the chemical shift recorded in solution. A thermogravimetric analysis was also carried out on 3 showing a continuous weight loss between room temperature and 300 °C (and resulting in the recovery of SnO₂ as final residue). Beyond their interest as precursors for the deposition of tin oxide films [4,5], tin tetracarbamates were also used as versatile reactants for various organometallic and organic syntheses leading to alkoxystannanes [47], tetraalkynylstannanes [48] (Scheme 12), and O-silylurethanes [49].

In 2010, Kemp et al. reported the reactivity of bis(2,2,5,5-tetramethyl-2,5-disila-1-azacyclopent-1-yl)tin toward CO₂ leading to a silyl-substituted carbamate of tin(II) [50]. The exposure of {[(CH₂)Me₂Si]₂N}₂Sn in solution in pentane under 60 psig of CO₂ (4 bar) at room temperature causes a rapid reaction involving a color change from red-orange to yellow. In parallel, GC–MS investigations on the reaction medium have shown the co-formation of carbon monoxide and the cyclic ether 2,2,5,5-tetramethyl-2,5-disila-1-oxacyclopentane, [(CH₂)Me₂Si]₂O (Scheme 13). Sn₄(μ₄-O){μ₂-O₂CN[SiMe₂(CH₂)]₂}₄(μ₂-N=C=O)₂ (4) was isolated as single crystals and analyzed by X-ray diffraction analysis. The unprecedented structure of 4 can be described as a tetranuclear tin cluster based on a [μ₄-O]Sn₄ core and including in periphery two bridging –N=C=O isocyanates and four bridging –O₂CN[SiMe₂(CH₂)]₂ carbamato ligands (Figure 3). The presence of the central atom μ₄-O was explained as resulting from the cleavage of CO₂ and not from O₂ or H₂O. Furthermore, the reaction of {[(CH₂)Me₂Si]₂N}₂Sn was also envisaged with OCS and CS₂ leading, respectively, to an insoluble polymeric material and to a trinuclear cluster based on the coordination of three {[(CH₂)Me₂Si]₂N}₂Sn moieties by one CS₂ molecule acting as μ₃ ligand. Previously, during the 1990s, Sita et al. explored already the reactivity of Ge and Sn complexes of silyl-containing amides toward carbon dioxide [51]. In particular, they focused on the reaction involving [(Me₃Si)₂N]₂Sn with CO₂, which led to the isolation of the dimeric bisalkoxide complex, [Sn(OSiMe₃)₂]₂. However, in this case and compared to the Kemp’s report, the tin carbamate is not stable but leads to the formation of [Sn(OSiMe₃)₂]₂, trimethylsilyl isocyanate, and 1,3-bis(trimethylsilyl)carbodiimide, respectively, according to a ligand metathesis process.

In 2011, continuing their study on the reactivity of silyl-substituted tin(II) amides towards heteroallenes, Kemp et al. reported the interaction of CO₂, OCS and CS₂ with (Me₂N)₂Sn leading to the formation of insertion products characterized as bis-(N,N-dimethylcarbamato)tin(II), bis(N,N-dimethylthiocarbamato)tin(II), and bis(N,N-dimethyldithiocarbamato)tin(II), respectively [52]. Thus, (Me₂N)₂Sn in hexane solution reacts quickly with carbon dioxide, at room temperature and under atmospheric pressure, to give with a yield almost quantitative, the new complex [(Me₂NCO₂)₂Sn]₂ (5) (Scheme 14). The authors describe the reaction as exothermic. In the solid-state, compound 5 is organized as a dimer involving two types of carbamato ligands: bridging and terminally (η²-chelating) coordinated to Sn. From a supramolecular point of view, the existence of intermolecular interactions Sn···O [2.8499(18) Å] between neighboring dimers and involving one oxygen atom of terminal carbamato ligands, leads to the propagation of a polymer chain (Figure 4). The coordination geometry of tin atoms can be viewed as a highly distorted trigonal bipyramid. In solution, the ¹¹⁹Sn NMR spectrum exhibits only one resonance at δ –613 ppm. In the ¹³C {¹H} NMR spectrum, the carbamato carbon also displays only one signal, not making it possible to differentiate the bridging and terminal modes highlighted in crystals of 5.

To our knowledge, the latest tin carbamate structure resolved to date by X-ray diffraction is to be credited to Fulton et al., who published in 2011, the reactivity of β-diketiminate tin derivatives with carbon dioxide [53]. [HC{(Me)CN(2,6-i-Pr₂C₆H₃)}₂]SnNi-Pr₂ and [HC{(Me)CN(2,6-i-Pr₂C₆H₃)}₂]SnNH(2,6-i-Pr₂C₆H₃), dissolved in toluene, react with CO₂ (1 atm) at room temperature yielding to the corresponding carbamato tin(II) complexes, [HC{(Me)CN(2,6-i-Pr₂C₆H₃)}₂]SnOC(O)Ni-Pr₂ (6) and [HC{(Me)CN(2,6-i-Pr₂C₆H₃)}₂]SnOC(O)NH(2,6-i-Pr₂C₆H₃) (7), respectively (Scheme 15). In the solid-state, 6 consists of a mononuclear complex describing a trigonal pyramidal arrangement of the ligands around the tin atom and in an endo configuration. The carbamate moiety is terminally O-bonded to Sn [Sn−O = 2.1346(16) Å] and its orientation can be viewed almost perpendicular to the NCCCN plane of the β-diketiminate ligand (Figure 5). This mode of coordination is unusual for carbamates and is similar to that observed in the case of hemicarbonato derivatives [1]. The X-ray structure of complex 7 has not been reported but the spectroscopic data are very similar to 6 (Table 2) and the authors mention that for both compounds, the insertion reaction is irreversible, even under reduced pressure.

Table 1. Comparison of selected structural parameters found in carbamates of organotin complexes.

Compounds	Sn−O(C) (Å)	Sn−N(C) (Å)	C−O(Sn) (Å)	C=O (Å)	N−C(O2) (Å)	O−C−O (deg)	N−C−O (deg)	CSD Entry Deposition Number	Ref.
Me3SnNHC(=O)–OSnMe3 (1)	2.23(2) 2.24(2)	2.04(2)	1.27(4) 1.30(3)	na	1.35(4)	126(3)	120(2) 114(3)	MESNCB1211377	[42]
Sn(O2CNi-Pr2)4 (2)	2.146(5) 2.217(4) 2.136(5) 2.195(4) 2.198(4) 2.135(5) 2.219(4) 2.126(5)	na	1.283(7) 1.276(8) 1.279(7) 1.260(8) 1.268(8) 1.269(7) 1.275(8) 1.282(7)	na	1.33(1) 1.352(9) 1.34(1) 1.333(8)	117.7(6) 117.3(6) 116.1(6) 117.2(6)	119.9(6) 122.4(6) 122.9(6) 119.7(6) 120.6(6) 122.2(6) 116.1(6) 121.2(6)	MULPUE 196069	[45]
Sn(O2CNEt2)4 (3)	2.156(1) 2.209(1) 2.163(1) 2.196(1)	na	1.292(2) 1.277(2) 1.298(2) 1.276(2)	na	1.338(2) 1.331(2)	117.0(1) 116.6(1)	121.5(2) 121.7(1) 121.7(2)	UGOZUL 196788	[46]
Sn4(μ4-O){μ2-O2CN[SiMe2(CH2)]2}4(μ2-N=C=O)2 (4)	2.331(2) 2.164(3) 2.226(3) 2.429(3) 2.716(4) 2.165(3) 2.379(2) 2.225(3) 2.458(2)	na	1.274(58) 1.282(5) 1.272(5) 1.291(5) 1.273(6) 1.289(5) 1.301(6) 1.257(5)	na	1.349(5) 1.345(5) 1.355(5)	121.0(4) 121.1(4) 121.6(4)	121.3(4) 117.6(4) 118.2(4) 120.3(4) 116.9(4) 121.5(4)	IFOXEH 664586	[50]
[(Me2NCO2)2Sn]2 (5)	2.332(1) 2.235(2) 2.432(2) 2.115(2)	na	1.272(3) 1.280(3) 1.263(3) 1.295(3)	na	1.338(3) 1.336(3)	118.8(2) 121.8(2)	120.2(2) 121.0(2) 121.3(2) 116.9(2)	UYOBIU 806650	[52]
[HC{(Me)CN(2,6-i-Pr2C6H3)}2]SnOC(O)Ni-Pr2 (6)	2.1346(16)	na	1.304(3)	1.238(3)	1.366(3)	121.7(2)	116.6(2) 121.7(2)	MIYHIN 791226	[53]

Table 1. Comparison of selected structural parameters found in carbamates of organotin complexes.

Compounds	Sn−O(C) (Å)	Sn−N(C) (Å)	C−O(Sn) (Å)	C=O (Å)	N−C(O2) (Å)	O−C−O (deg)	N−C−O (deg)	CSD Entry Deposition Number	Ref.
Me3SnNHC(=O)–OSnMe3 (1)	2.23(2) 2.24(2)	2.04(2)	1.27(4) 1.30(3)	na	1.35(4)	126(3)	120(2) 114(3)	MESNCB1211377	[42]
Sn(O2CNi-Pr2)4 (2)	2.146(5) 2.217(4) 2.136(5) 2.195(4) 2.198(4) 2.135(5) 2.219(4) 2.126(5)	na	1.283(7) 1.276(8) 1.279(7) 1.260(8) 1.268(8) 1.269(7) 1.275(8) 1.282(7)	na	1.33(1) 1.352(9) 1.34(1) 1.333(8)	117.7(6) 117.3(6) 116.1(6) 117.2(6)	119.9(6) 122.4(6) 122.9(6) 119.7(6) 120.6(6) 122.2(6) 116.1(6) 121.2(6)	MULPUE 196069	[45]
Sn(O2CNEt2)4 (3)	2.156(1) 2.209(1) 2.163(1) 2.196(1)	na	1.292(2) 1.277(2) 1.298(2) 1.276(2)	na	1.338(2) 1.331(2)	117.0(1) 116.6(1)	121.5(2) 121.7(1) 121.7(2)	UGOZUL 196788	[46]
Sn4(μ4-O){μ2-O2CN[SiMe2(CH2)]2}4(μ2-N=C=O)2 (4)	2.331(2) 2.164(3) 2.226(3) 2.429(3) 2.716(4) 2.165(3) 2.379(2) 2.225(3) 2.458(2)	na	1.274(58) 1.282(5) 1.272(5) 1.291(5) 1.273(6) 1.289(5) 1.301(6) 1.257(5)	na	1.349(5) 1.345(5) 1.355(5)	121.0(4) 121.1(4) 121.6(4)	121.3(4) 117.6(4) 118.2(4) 120.3(4) 116.9(4) 121.5(4)	IFOXEH 664586	[50]
[(Me2NCO2)2Sn]2 (5)	2.332(1) 2.235(2) 2.432(2) 2.115(2)	na	1.272(3) 1.280(3) 1.263(3) 1.295(3)	na	1.338(3) 1.336(3)	118.8(2) 121.8(2)	120.2(2) 121.0(2) 121.3(2) 116.9(2)	UYOBIU 806650	[52]
[HC{(Me)CN(2,6-i-Pr2C6H3)}2]SnOC(O)Ni-Pr2 (6)	2.1346(16)	na	1.304(3)	1.238(3)	1.366(3)	121.7(2)	116.6(2) 121.7(2)	MIYHIN 791226	[53]

Table 2. Selection of spectroscopic data (NMR and IR) assigned to carbamates of tin complexes.

Compounds	119Sn{1H} NMR (δ, ppm)	13C{1H} NMR -NC(O)O- (δ, ppm)	IR ν(C=O) (cm−1)	Ref.
Sn(O2CNi-Pr2)4 (2)	−920.8 a	164.4 a	na	[45]
Sn(O2CNEt2)4 (3)	−930.0 b	166.2 b	1612	[46]
Sn4(μ4-O){μ2-O2CN[SiMe2(CH2)]2}4(μ2-N=C=O)2 (4)	−316.0 b	167.9b	na	[50]
[(Me2NCO2)2Sn]2 (5)	−613.0 a	164.5a	1651	[52]
[HC{(Me)CN(2,6-i-Pr2C6H3)}2]SnOC(O)Ni-Pr2 (6)	−394.0 b	161.75 b	1595, 1575, 1552, and 1524	[53]
[HC{(Me)CN(2,6-i-Pr2C6H3)}2]SnOC(O)NH(2,6-i-Pr2C6H3) (7)	−398.0 b	161.59 b	1624, 1554, 1526, and 1517	[53]

^a Measured in chloroform-d; ^b measured in benzene-d₆.

Table 2. Selection of spectroscopic data (NMR and IR) assigned to carbamates of tin complexes.

Compounds	119Sn{1H} NMR (δ, ppm)	13C{1H} NMR -NC(O)O- (δ, ppm)	IR ν(C=O) (cm−1)	Ref.
Sn(O2CNi-Pr2)4 (2)	−920.8 a	164.4 a	na	[45]
Sn(O2CNEt2)4 (3)	−930.0 b	166.2 b	1612	[46]
Sn4(μ4-O){μ2-O2CN[SiMe2(CH2)]2}4(μ2-N=C=O)2 (4)	−316.0 b	167.9b	na	[50]
[(Me2NCO2)2Sn]2 (5)	−613.0 a	164.5a	1651	[52]
[HC{(Me)CN(2,6-i-Pr2C6H3)}2]SnOC(O)Ni-Pr2 (6)	−394.0 b	161.75 b	1595, 1575, 1552, and 1524	[53]
[HC{(Me)CN(2,6-i-Pr2C6H3)}2]SnOC(O)NH(2,6-i-Pr2C6H3) (7)	−398.0 b	161.59 b	1624, 1554, 1526, and 1517	[53]

^a Measured in chloroform-d; ^b measured in benzene-d₆.

3. Formato Tin Complexes

3.1. Early Works

To our knowledge, pioneering work on formato tin complexes dates back to 1927. Elöd and Kolbach described a general method to prepare a series of Sn(II) compounds of formula M₂Sn(O₂CH)₄·5H₂O (M = Na, K, and NH₄) by dissolving tin(II) chloride into solutions of alkali metal or ammonium formate, in formic acid (35% w/w) [54]. In 1964, Donaldson and Knifton revisited the synthetic protocol by reporting the preparation and analysis of alkali metal (K, Rb, and Cs) and ammonium derivatives of the triformatostannate(II) anion [Sn(O₂CH)₃]⁻ [55]. In 1969, Jelen and Lindqvist solved the crystal structure of potassium triformatostannate(II), KSn(HCO₂)₃ in which the tin atom is coordinated pyramidally by three oxygen atoms of three formate groups (CSD refcode: KTFORM, 1200243) [56]. Thereafter in 1974, Harrison and Thornton published an X-ray diffraction study devoted to the structure determination of Sn(O₂CH)₂ arranged in infinite two-dimensional sheets in which tin(II) atomes are bridged by formate groups (CSD refcode: SNFORM, 1260779) [57].

Solid-state structures of tin(IV) formate complexes were established from the 1980s. In 1987, Molloy et al. reported the crystal structure of triphenyltin formate prepared from a mixture of triphenyltin hydroxide and formic acid in refluxing toluene [58]. [Ph₃Sn(O₂CH)]_n is organized in a polymeric chain propagating along c axis and compared by the authors to a flattened helix. Triphenyltin moieties are bridged by bidentate formate ligands (CSD refcode: FAJZIZ, 1151980; Figure 6). In 1990, Mistry et al. resolved the structure of bis(formato)dimethyltin(IV), [Me₂Sn(O₂CH)₂]_n, which was described as a sheet polymer with linear Me₂Sn moieties nearly symmetrically bridged by formate anions. Sn atoms exhibit an octahedral geometry (CCDC ref.: SIFLEY, 1258788; Figure 7) [59]. More recently, Power et al. published the synthesis and characterization of several organotin(IV) formates prepared from hydrides and hydroxides of organotins in the presence of formic acid [60]. X-ray crystallographic structures investigations were conducted on five compounds. [n-Bu₂Sn(O₂CH)₂]_n (CCDC ref.: EKONEY, 783726; Figure 8), [(PhCH₂)₃Sn(O₂CH)]_n (CCDC ref.: EKONAU, 783725), and [Cy₃Sn(O₂CH)]_n (Cy = cyclohexyl) (crude structure owing to the low quality of crystals) exhibit polymeric structures, while Mes₃SnOC(O)H (Mes = 2,4,6-trimethylphenyl) (CCDC ref.: EKONOI, 783728) and Dmp₃SnOC(O)H (dmp = 2,6-dimethylphenyl) (CCDC ref.: EKONIC, 783727; Figure 9) are monomeric tin formates. In 2010, McMurtie and Arnold reported the crystal structure of meso-tetraphenylporphyrinatotin(IV) diformate, prepared from Sn(TPP)(OH)₂ (TPP = dianion of 5,10,15,20-tetraphenylporphyrin) and formic acid, and in which the central tin atom exhibits an octahedral geometry with two axially O-coordinated formato ligands (Figure 10) [61].

3.2. X-Ray Crystal Structures of Formato Tin Complexes Resulting from Reactivity with CO₂

While the formation of the above compounds required the use of formic acid in their preparation, there are also a few examples of formato tin complexes directly isolated under carbon dioxide atmosphere, resulting from the insertion of CO₂ into the Sn–H bond. In our opinion, six complexes, published mainly during the last decade and characterized by X-ray diffraction, correspond to this category of compounds (Table 3). In 2009, Roesky et al. reported the structural and spectroscopic characterization of several complexes resulting from hydrostannylation reactions of carbon dioxide, ketones, aldehydes, alkynes and carbodiimides with the tin(II) hydride precursor, [HC{(Me)CN(2,6-i-Pr₂C₆H₃)}₂]SnH [62]. In the case of the reaction involving CO₂, bubbling at room temperature very quickly causes a change in color (from yellow to colorless) of the reaction mixture (toluene solution). The treatment with n-hexane then leads to the formation of a microcrystalline powder (at −32 °C) characterized as being the stannylene formate complex, [HC{(Me)CN(2,6-i-Pr₂C₆H₃)}₂]SnOC(O)H (8) (Scheme 16). The presence of the formate ligand evidenced by infrared was confirmed by single-crystal X-ray diffraction. The molecular structure shows moreover a distorted pseudo-tetrahedral environment for the tin atom, completed by one electron lone pair (Figure 11).

During the period 2007–2010, the Albertin’s group isolated several formato tin complexes by studying the reactivity of transition-metal stannyl complexes with carbon dioxide. Three of these complexes were characterized by single-crystal X-ray diffraction studies. Firstly, the reaction of Re(SnH₃)(CO)₂[P(OEt)₃]₃, in solution in benzene and at room temperature, with CO₂ (under atmospheric pressure) and during 24 h led to the binuclear OH-bridging bis(formate) complex [Re{Sn[OC(O)H]₂(μ-OH)}(CO)₂{P(OEt)₃}₃]₂ (9) (Scheme 17) [63]. The authors described the core of 9 as a dimeric complex in which two octahedrally coordinated rhenium units are joined by a bridging tetraformatebis(μ-hydroxo)ditin moiety. Tin atoms exhibit a highly distorted square pyramidal coordination geometry, bearing two η¹-formate ligands per metal atom. The central Sn₂O₂ distannoxane ring is planar (Figure 12). Infrared and ¹³C{¹H} NMR spectra attest also the presence of formates by showing two bands at 1674–1594 cm⁻¹ (ν_OCO, asym) and one resonance at δ 166 ppm [OC(O)H], respectively (Table 4).

In 2008, the same group reported a similar structure by standing the ruthenium-cyclopentadienyl-trihydridostannyl complex, Ru(SnH₃)(Cp)[P(OEt)₃](PPh₃), under a CO₂ atmosphere for 5 h (Scheme 18) [64]. The resulting formate species was characterized as [Ru[Sn{OC(O)H}₂(μ-OH)](Cp){P(OEt)₃}(PPh₃)]₂ (10), exhibiting a structure based on two μ-OH groups bridging the tin atoms (Figure 13).

In 2010, Albertin et al. studied the reactivity of hydride-trihydridestannyl complexes MH(SnH₃)P₄ [M = Fe, Os; P = P(OEt)₃, PPh(OEt)₂] toward carbon dioxide leading to solids characterized as hydroxobis(formate)stannyl derivatives, MH[Sn(OH){OC(O)H}₂]P₄ [65]. Reactions took place at room temperature, in solution in toluene and under atmospheric pressure of CO₂ (Scheme 19). Starting from OsH(SnH₃)(PPh(OEt)₂)₄, the compound OsH[Sn(OH){OC(O)H}₂][PPh(OEt)₂]₄ (11) was obtained and its molecular structure was established by single-crystal X-ray diffraction showing the insertion of two CO₂ molecules into two Sn–H bonds (Scheme 19). The Os–H bond remains intact. The hydride ligand is unequivocally demonstrated by ¹H NMR spectroscopy showing a multiplet at δ −11 ppm. The osmium atom is hexacoordinated according to a distorted octahedral geometry bearing four phosphonite ligands, one hybride ligand, and one hydroxobis(formate) stannyl ligand. The hydride and stannyl ligand are in cis positions. Regarding the coordination of the tin atom, Sn is tetracoordinate displaying a tetrahedral geometry (Figure 14).

In 2016, Strohmann et al. published the crystal structure of diaquadi-μ-hydroxido-tris[trimethyltin(IV)] diformatotrimethylstannate(IV), [(Me₃Sn)₃(μ-OH)₂(H₂O)₂][Me₃Sn{OC(O)H}₂] (12), obtained as byproduct from trapping reactions with trimethyltin chloride and formed from atmospheric CO₂ [66]. The structure of 12 consists of a short trimethyltin hydroxide chain (tristannoxane), positively charged, and a bisformatostannate, negatively charged (Figure 15). From a supramolecular point of view, formate ligands bridge four cationic tristannoxanes via hydrogen-bonding interactions leading to a two-dimensional network.

Lately, in 2017, Jones et al. reported the efficient and highly selective use of the two-coordinate amido-tin(II) hydride complex, LSnH (L = -N(C₆H₂-i-Pr{C(H)Ph₂}₂-4,2,6)(Si-i-Pr₃)), acting as catalytic species for the reductive hydroboration of CO₂ into methanol equivalents, MeOBR₂ (2-methoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane and catecholatoboryl methoxide), using boranes, HBR₂, as hydrogen sources [67]. The tin formate complex, LSn(κ²-O,O’-O₂CH) (13), was thus isolated during catalytic studies and also by exclusively reacting LSnH with CO₂ (Scheme 20) or alternatively from LSnBr with potassium formate, KOC(O)H. Compound 13 was crystallographically characterized and consists of a monomeric complex in which the tin atom is unusually chelated by a formate ligand (Figure 16).

Table 3. Comparison of selected structural parameters found in formates of organotin complexes.

Compounds	Sn−O(C) (Å)	C−O(Sn) (Å)	C−O (Å)	Sn−O−C (deg)	O−C−O (deg)	CSD Entry Deposition Number	Ref.
[HC{(Me)CN(2,6-i-Pr2C6H3)}2]SnOC(O)H (8)	2.1353(15)	1.299(2)	1.209(3)	116.40(13)	126.78(18)	COPLOJ 709476	[62]
[Re{Sn[OC(O)H]2(μ-OH)}(CO)2{P(OEt)3}3]2 (9)	2.040(5) 2.071(7)	1.29(1) 1.26(1)	1.19(2) 1.21(1)	123.1(6) 132.2(6)	126(1) 128.8(9)	CIHYUO 655465	[63]
[Ru[Sn{OC(O)H}2(μ-OH)](Cp){P(OEt)3}(PPh3)]2 (10)	2.210(2) 2.069(2)	1.262(4) 1.274(5)	1.216(5) 1.205(6)	128.7(2) 133.5(2)	127.9(3) 125.7(4)	KOMBUK 708897	[64]
OsH[Sn(OH){OC(O)H}2][PPh(OEt)2]4 (11)	2.061(5) 2.089(5)	1.16(1) 1.25(1)	1.11(1) 1.21(1)	132.8(7) 124.9(7)	131(1) 124(1)	ALAGUQ 813529	[65]
[(Me3Sn)3(μ-OH)2(H2O)2][Me3Sn{OC(O)H}2] (12)	2.2991(17) 2.2990(17)	1.269(3)	1.224(3)	125.94(17)	128.1(3)	EWIGIC 1505528	[66]
N(C6H2-i-Pr{C(H)Ph2)2-4,2,6)(Si-i-Pr3)Sn(κ2-O,O’-O2CH) (13)	2.353(2) 2.333(2)	1.109(4) 1.310(4)	na	91.4(2) 87.5(2)	125.6(4)	ACEQUX 1518144	[67]

Table 3. Comparison of selected structural parameters found in formates of organotin complexes.

Compounds	Sn−O(C) (Å)	C−O(Sn) (Å)	C−O (Å)	Sn−O−C (deg)	O−C−O (deg)	CSD Entry Deposition Number	Ref.
[HC{(Me)CN(2,6-i-Pr2C6H3)}2]SnOC(O)H (8)	2.1353(15)	1.299(2)	1.209(3)	116.40(13)	126.78(18)	COPLOJ 709476	[62]
[Re{Sn[OC(O)H]2(μ-OH)}(CO)2{P(OEt)3}3]2 (9)	2.040(5) 2.071(7)	1.29(1) 1.26(1)	1.19(2) 1.21(1)	123.1(6) 132.2(6)	126(1) 128.8(9)	CIHYUO 655465	[63]
[Ru[Sn{OC(O)H}2(μ-OH)](Cp){P(OEt)3}(PPh3)]2 (10)	2.210(2) 2.069(2)	1.262(4) 1.274(5)	1.216(5) 1.205(6)	128.7(2) 133.5(2)	127.9(3) 125.7(4)	KOMBUK 708897	[64]
OsH[Sn(OH){OC(O)H}2][PPh(OEt)2]4 (11)	2.061(5) 2.089(5)	1.16(1) 1.25(1)	1.11(1) 1.21(1)	132.8(7) 124.9(7)	131(1) 124(1)	ALAGUQ 813529	[65]
[(Me3Sn)3(μ-OH)2(H2O)2][Me3Sn{OC(O)H}2] (12)	2.2991(17) 2.2990(17)	1.269(3)	1.224(3)	125.94(17)	128.1(3)	EWIGIC 1505528	[66]
N(C6H2-i-Pr{C(H)Ph2)2-4,2,6)(Si-i-Pr3)Sn(κ2-O,O’-O2CH) (13)	2.353(2) 2.333(2)	1.109(4) 1.310(4)	na	91.4(2) 87.5(2)	125.6(4)	ACEQUX 1518144	[67]

Table 4. Selection of spectroscopic data (NMR and IR) assigned to formates of tin complexes.

Compounds	119Sn{1H} NMR (δ, ppm)	13C{1H} NMR −OC(H)O (δ, ppm)	1H NMR −OC(H)O (δ, ppm)	IR (cm−1)	Ref.
[HC{(Me)CN(2,6-i-Pr2C6H3)}2]SnOC(O)H (8)	−360 a	166.92 a	8.97 3J(119Sn,H) = 52 Hz	1641 (νC=O) 2700 (νC−H)	[62]
[Re{Sn[OC(O)H]2(μ-OH)}(CO)2{P(OEt)3}3]2 (9)	−487.7 b	166.4 b	9.21 bc	1665, 1594 (νCHO)	[63]
[Ru[Sn{OC(O)H}2(μ-OH)](Cp){P(OEt)3}(PPh3)]2 (10)	−263.5 cd	166.1 d	8.36 d	1660, 1627 (νCHO)	[64]
OsH[Sn(OH){OC(O)H}2][PPh(OEt)2]4 (11)	−451.0 b	165.5 e	8.93 b	1673, 1634 (νCHO)	[65]
N(C6H2-i-Pr{C(H)Ph2)2-4,2,6)(Si-i-Pr3)Sn(κ2-O,O’-O2CH) (13)	−134.0 a	173.80 a	8.71 a	1549 (νCHO)	[67]

^a Measured in benzene-d₆; ^b measured in toluene-d₈; ^c measured at −70 °C; ^d measured in acetone-d₆; ^e determined for OsH[Sn(OH){OC(O)H}₂][P(OEt)₃]₄.

Table 4. Selection of spectroscopic data (NMR and IR) assigned to formates of tin complexes.

Compounds	119Sn{1H} NMR (δ, ppm)	13C{1H} NMR −OC(H)O (δ, ppm)	1H NMR −OC(H)O (δ, ppm)	IR (cm−1)	Ref.
[HC{(Me)CN(2,6-i-Pr2C6H3)}2]SnOC(O)H (8)	−360 a	166.92 a	8.97 3J(119Sn,H) = 52 Hz	1641 (νC=O) 2700 (νC−H)	[62]
[Re{Sn[OC(O)H]2(μ-OH)}(CO)2{P(OEt)3}3]2 (9)	−487.7 b	166.4 b	9.21 bc	1665, 1594 (νCHO)	[63]
[Ru[Sn{OC(O)H}2(μ-OH)](Cp){P(OEt)3}(PPh3)]2 (10)	−263.5 cd	166.1 d	8.36 d	1660, 1627 (νCHO)	[64]
OsH[Sn(OH){OC(O)H}2][PPh(OEt)2]4 (11)	−451.0 b	165.5 e	8.93 b	1673, 1634 (νCHO)	[65]
N(C6H2-i-Pr{C(H)Ph2)2-4,2,6)(Si-i-Pr3)Sn(κ2-O,O’-O2CH) (13)	−134.0 a	173.80 a	8.71 a	1549 (νCHO)	[67]

^a Measured in benzene-d₆; ^b measured in toluene-d₈; ^c measured at −70 °C; ^d measured in acetone-d₆; ^e determined for OsH[Sn(OH){OC(O)H}₂][P(OEt)₃]₄.

4. Phosphinoformato Tin Complexes

4.1. Foreword

This type of compound, rarely described so far, results from the insertion of CO₂ into the Sn–P bond then leads to the formation of the Sn–OC(O)P moiety which can be qualified as a phosphinoformato derivative, by analogy with phosphino formic acid (H₂PCOOH) according to the recent work published by Kaiser et al. [68]. To the best of our knowledge, only two examples of this type of complex have been described so far (Table 5), and their characterization is quite recent (last decade).

Table 5. Selection of structural parameters found in complexes 14 and 15.

Compounds	Sn–O(C) (Å)	P–C(O2) (Å)	C–O(Sn) (Å)	C–Oexo (Å)	Sn–O–C (deg)	O–C–O (deg)	O–C–P (deg)	CSD Entry Deposition Number	Ref.
[(i-Pr2P)2N·CO2]2Sn (14)	2.344(4) 2.344(5)	1.875(7) 1.867(6)	1.272(8) 1.273(8)	1.210(8) 1.231(7)	128.2(5) 132.2(4)	126.8(6) 129.5(6)	114.5(5) 116.0(5) 117.8(5) 115.4(5)	RANBUF 831553	[69]
(F5C2)3SnCH2P(t-Bu)2·CO2 (15)	2.233(2)	1.891(6)	1.268(4)	1.216(4)	122.9(2)	128.1(3)	114.9(2) 117.0(3)	FONREI 1892689	[70]

Table 5. Selection of structural parameters found in complexes 14 and 15.

Compounds	Sn–O(C) (Å)	P–C(O2) (Å)	C–O(Sn) (Å)	C–Oexo (Å)	Sn–O–C (deg)	O–C–O (deg)	O–C–P (deg)	CSD Entry Deposition Number	Ref.
[(i-Pr2P)2N·CO2]2Sn (14)	2.344(4) 2.344(5)	1.875(7) 1.867(6)	1.272(8) 1.273(8)	1.210(8) 1.231(7)	128.2(5) 132.2(4)	126.8(6) 129.5(6)	114.5(5) 116.0(5) 117.8(5) 115.4(5)	RANBUF 831553	[69]
(F5C2)3SnCH2P(t-Bu)2·CO2 (15)	2.233(2)	1.891(6)	1.268(4)	1.216(4)	122.9(2)	128.1(3)	114.9(2) 117.0(3)	FONREI 1892689	[70]

4.2. X-Ray Crystal Structures

The first example of the phosphinoformato tin complex, characterized by X-ray diffraction as [(i-Pr₂P)₂N·CO₂]₂Sn (14), was resolved in 2011 by Kemp et al. [69]. Complex 14 was isolated as a white precipitate by bubbling CO₂ at room temperature (for 10 min) through a pentane solution of [(i-Pr₂P)₂N]₂Sn, which caused a color change of the solution from orange to yellow (Scheme 21). Structural analysis of single-crystals confirmed the capture of two molecules of CO₂ by [(i-Pr₂P)₂N]₂Sn forming a six-membered ring complex in which CO₂ is inserted into one Sn–P bond of each ligand (Figure 17). Moreover, the authors studied the stability of 14 in particular by thermogravimetric analysis and showed the easy release of CO₂ from 90 °C. They showed also the possibility of recovering the starting complex by simply heating a solid sample of 14 under argon. Compound 14 is stable for several months stored under CO₂ at room temperature or under argon at −25 °C. On the basis of these observations, the incorporation of CO₂ as an adduct is preferentially considered rather than as a real insertion.

More recently, in 2019, Mitzel et al. reported the reactivity of the geminal frustrated Lewis pair (F₅C₂)₃SnCH₂P(t-Bu)₂ with a variety of small molecules and, in particular, with CO₂ [70]. The formation of the CO₂ adduct of (F₅C₂)₃SnCH₂P(t-Bu)₂, characterized as (F₅C₂)₃SnCH₂P(t-Bu)₂·CO₂ (15), was first demonstrated by NMR at −70 °C in THF-d₈ (Table 6). The authors mention the instability of the complex at room temperature causing the release of CO₂ (Scheme 22). The CO₂ molecule is bonded to tin and phosphorus atoms, thus leading to the formation of a five-membered ring with an exocyclic C–O bond. The PCO₂ unit is planar (Figure 18). Other adducts resulting from the reactivity of (F₅C₂)₃SnCH₂P(t-Bu)₂, in particular with SO₂ and CS₂, were also isolated in the solid state and describe comparable solid-state structures.

Table 6. Selection of spectroscopic data (NMR and IR) assigned to phosphinoformato of tin complexes.

Compounds	119Sn{1H} NMR (δ, ppm)	13C{1H} NMR –OC(O)P (δ, ppm)	31P{1H} NMR –OC(O)P (δ, ppm)	IR (cm−1)	Ref.
[(i-Pr2P)2N·CO2]2Sn (14)	−184 a (t, 1JSn-P = 2626 Hz)	168.7 a (d, 1JC-P = 95 Hz, PCO2)	26.4 a (s, broad)	1629 (νC=O) 1206	[69]
(F5C2)3SnCH2P(tBu)2·CO2 (15)	−308.3 bc (hept, very broad, 2JSn,F = 368 Hz).	162.3 bd (d, 1JC-P = 80 Hz, PCO2)	31.8 bd (s, 2JSn,P = 56 Hz)	na	[70]

^a Measured in benzene-d₆; ^b measured in tetrahydrofuran-d₈; ^c determined at 208 K; ^d determined at 203 K.

Table 6. Selection of spectroscopic data (NMR and IR) assigned to phosphinoformato of tin complexes.

Compounds	119Sn{1H} NMR (δ, ppm)	13C{1H} NMR –OC(O)P (δ, ppm)	31P{1H} NMR –OC(O)P (δ, ppm)	IR (cm−1)	Ref.
[(i-Pr2P)2N·CO2]2Sn (14)	−184 a (t, 1JSn-P = 2626 Hz)	168.7 a (d, 1JC-P = 95 Hz, PCO2)	26.4 a (s, broad)	1629 (νC=O) 1206	[69]
(F5C2)3SnCH2P(tBu)2·CO2 (15)	−308.3 bc (hept, very broad, 2JSn,F = 368 Hz).	162.3 bd (d, 1JC-P = 80 Hz, PCO2)	31.8 bd (s, 2JSn,P = 56 Hz)	na	[70]

^a Measured in benzene-d₆; ^b measured in tetrahydrofuran-d₈; ^c determined at 208 K; ^d determined at 203 K.

5. Metallocarboxylato Tin Complexes

5.1. Foreword

The last part of this collection is devoted to bimetallic transition-metal/tin complexes with bridging carbon dioxide, of general formula M(CO₂)Sn, also named metallocarboxylato tin complexes. Compared to the categories previously described, their synthesis does not involve the direct use of CO₂ but is based on the reactivity between metallocarboxylic acids and halogenated precursors of organotins. However, we have found it interesting and complementary to also include this class of compounds in this review. Metallocarboxylato complexes and in particular tin derivatives began to be studied in the mid-1980s, often considered as suitable models of intermediates for the catalytic conversion of carbon dioxide [4]. Most of the structures shown below were resolved in the 1990s. The D. Gibson’s group in Louisville (USA) was one of the most active in this field characterizing several of these compounds. Metallocarboxylato-tin complexes have also been the subject of specific infrared considerations [71]. Indeed, ν_OCO adsorption bands constitute well-suited probes for orienting and determining the coordination modes of the CO₂ ligand.

5.2. X-Ray Crystal Structures

To the best of our knowledge, in 1987, Gladysz et al. reported the first X-ray characterization of a transition metal/tin bridging CO₂ complex by isolating (η-C₅H₅)Re(NO)(PPh₃)(CO₂SnPh₃) (16) [72]. Compound 16 was obtained in high yield (86%) by reacting (η-C₅H₅)Re(NO)(PPh₃)(CO₂K) with 1.1 equivalent of Ph₃SnCl at −78 °C in THF (Scheme 23). Yellow crystal prisms of 16 grew from a mixture of CH₂Cl₂/hexane. Compound 16 is air- and water-stable, and could also be alternatively prepared from (η-C₅H₅)Re(NO)(PPh₃)(CO₂H) and (Ph₃Sn)₂O at 25 °C in THF. Moreover, the decarboxylation of 16 occurs by heating at 180 °C for 10 min (from solid) or at 140 °C, for 20 h in solution in xylene. Using Me₃SnCl, the authors claim the formation of the analogue (η-C₅H₅)Re(NO)(PPh₃)(CO₂SnMe₃) with a yield of 95%. The study was also extended to the synthesis of rhenium/germanium and rhenium/lead bridging CO₂ complexes using Ph₃GeBr and Ph₃PbCl, as precursors, respectively. The crystallographic structure of 16 confirms the presence of two moieties based on rhenium and tin, respectively, linked by a CO₂ bridging ligand. The tin atom of 16 adopts a distorted trigonal bipyramid geometry in which the equatorial positions are occupied by the two oxygen atoms of the CO₂ ligand and supplemented by a phenyl group (Figure 19). The CO₂ ligand can be regarded as a carboxylato fragment, which is corroborated by infrared analysis, bidentally bound to the tin atom and defined as μ(n¹-C: n²-O,O′) ligand.

In 1991, Gibson et al. published the solid state structure of (η-C₅H₅)Fe(CO)(PPh₃)(CO₂SnPh₃) (17) prepared by mixing, under nitrogen and at low temperature, Ph₃SnCl to a THF solution of CpFe(CO)(PPh₃)COOK·3H₂O [73]. The structure was found to be isomorphous to 16, with the tin atom occupying a trigonal bipyramid arrangement (Figure 20). Compared to the Gladysz complex, the authors underline for 17 a greater difference between the lengths of the Sn–O bonds, corresponding to a value of 0.219 Å (Table 7). However, in the literature devoted to triaryltin organocarboxylate monomers this difference is generally much more marked in the range of 0.48 to 0.81 Å [74].

Two years later in 1993, the same group reported the solid state isolation of a new Fe/Sn CO₂-bridged complex, which was characterized by single crystal X-ray diffraction analysis as the indenyl derivative (η⁵-C₉H₇)Fe(CO)(PPh₃)(CO₂SnPh₃) (18). Compound 18 was prepared under nitrogen, in THF solution and at 273 K, from (η⁵-C₉H₇)Fe(CO)₂(PPh₃)⁺·I⁻ and Ph₃SnCl [75]. As shown by the structure displayed in Figure 21, the Sn atom geometry consists of a distorted trigonal bipyramid. However, the steric hindrance of the indenyl ligand and the resulting interactions with the phosphine ligand are suspected to cause notable structural differences compared to complexes 16 and 17: (i) Sn–O bond lengths differ from 0.467(3) Å; (ii) the difference in the carboxyl C–O bond lengths also increases [0.100(4) Å]; (iii) the Fe–C–O bond angles exhibit higher distortions [10.2(3)°] (Table 7).

The Gibson’s group continued to be very prolific in this field until the end of the 1990s, structurally characterizing several new iron and rhenium/tin CO₂ bridged complexes. In 1994, the reaction of Cp*Re(CO)(NO)COOH with Ph₃SnCl in the presence of Na₂CO₃ afforded the formation of Cp*Re(CO)(NO)(CO₂SnPh₃) (19) showing a μ₂-η³ coordination mode for the CO₂ ligand (Figure 22) [76]. As reflected by the lengths of the Sn–O and C–O bonds (Table 7), an asymmetric bonding mode of carboxylic oxygens to carbon and tin atoms was observed.

In 1995, Gibson et al. resolved by X-ray diffraction, the structure of four new CO₂-bridged transition metals/tin complexes [77]. CpFe(CO)(PPh₃)(CO₂SnMe₃) (20) and CpFe(CO)(PPh₃)(CO₂Sn(n-Bu)₃) (21) were synthesized by reaction in THF between CpFe(CO)(PPh₃)CO₂⁻K⁺ and ClSnMe₃ and ClSnPh₃, respectively. In both cases, the bridging carboxyl ligand adopts a μ₂-η³ coordination mode in which the carboxyl atom is bound to the iron atom and both oxygens are bound to the tin atom. Cp*Fe(CO)₂(CO₂SnPh₃) (22) was prepared from Cp*Fe(CO)₃⁺BF₄⁻ and Ph₃SnCl in CH₂Cl₂ at 0 °C in the presence of KOH. Compound 22 displays a similar structure to that of complexes 20 and 21 in which tin atoms are five-coordinated and adopt a distorted trigonal-bipyramidal geometry. The CO₂-bridged rhenium-tin complex Cp*Re(CO)(NO)(CO₂SnMe₃) (23) was isolated as crystalline solid from Cp*Re(CO)(NO)COOH and ClSnMe₃ in the presence of KOH. The X-ray structure of 23 highlighted a μ₂-η² coordination mode of the bridging CO₂ ligand in which the carbonyl carbon and one oxygen atom are bonded to the rhenium atom and the tin atom, respectively. As a result, the tin atom adopts a distorted tetrahedral geometry (Figure 23).

In 1997, two new specimen of CO₂-bridged rhenium/tin complexes were characterized at the solid-state by the Gibson’s group [78]. The transmetalation reaction involving Cp*Re(CO)(NO)(CO₂SnMe₃) (23) and Me₂SnCl₂ led to the formation in good yield of Cp*Re(CO)(NO)(CO₂Sn(Cl)Me₂) (24) jointly with Me₃SnCl (Scheme 24). The X-ray structure of complex 24 shows the presence of a μ₂-η³ CO₂ ligand bridging the two moieties of rhenium and tin. The tin atom exhibits a five-coordinate geometry adopting an edge-capped tetrahedral arrangement rather than a distorted trigonal bipyramid.

An equimolar mixture of Cp*Re(CO)(NO)COOH and compound 24 leads, in the presence of water, to the formation of [Cp*Re(CO)(NO)(CO₂)]₂SnMe₂ (25) via the formation of an hydroxyl species, suggested by the authors as being Cp*Re(CO)(NO)(CO₂Sn(OH)Me₂) (Scheme 25). Complex 25 was also isolated by reacting Cp*Re(CO)(NO)COOH with Me₂SnCl₂ in the presence of Na₂CO₃. The structure of 25 was described as two Cp*Re(CO)(NO) units linked to a SnMe₂ moiety through two μ₂-η³ CO₂ ligands. The central tin atom is hexacoordinated and displayed a skewed trapezoidal-bipyramidal geometry (Figure 24).

In 1997, Komiya et al. reported the synthesis of the CO₂ complex of iron(0), Fe(CO₂)(depe)₂ [depe = 1,2-bis(diethylphosphino)ethane)], resulting from the replacement of N₂ in Fe(N₂)(depe)₂ with CO₂. Fe(CO₂)(depe)₂ reacts then with Ph₃SnCl in Et₂O at −78°C to lead to the formation of FeCl(CO₂SnPh₃)(depe)₂ (26) (Scheme 26) [79]. The same reaction, using Me₃SnCl as tin precursor, gives rise to the analogous complex FeCl(CO₂SnMe₃)(depe)₂. The X-ray structure of 26 shows that iron and tin atoms are linked by a CO₂ molecule, acting as μ(n¹-C: n²-O,O′) ligand (Figure 25).

During the last decade, two new metallocarboxylato of tin complexes have been characterized at the solid-state. First of all, in 2011, Adams et al. investigated the reaction of Os₃(CO)₁₂ with Ph₃SnOH in the presence of [Bu₄N][OH] in methanol, isolating two new trinuclear osmium clusters characterized as Os₃(CO)₁₀(μ-η²-O=COSnPh₃)(μ-OMe) (27) and Os₃(CO)₁₀(μ-OMe)(μ-OH) (with yields of 18% and 7%, respectively) [80]. The tiphenylgermyl homologue of 27, Os₃(CO)₁₀(μ-η²-O=COGePh₃)(μ-OMe), was also obtained from the reaction of Ph₃GeOH with Os₃(CO)₁₂ and using a comparable synthetic protocol (in the presence of [Bu₄N][OH] and in methanol). The authors described the structure of 27 as being a trinuclear cluster of three osmium atoms bearing one bridging methoxy ligand and one bridging triphenlystannylcarboxylate ligand. Remarkably, the Sn−O distances show a large difference in length (2.075 (5) Å and 2.858 (5) Å, respectively), which results from an unusual mode of coordination showing the organotin fragment in a pendant arm arrangement. The carbon atom and one oxygen atom of O=COSnPh₃ are bonded to two distinct osmium atoms, while the tin atom is connected via the second oxygen atom of the ligand (Figure 26).

Finally, more recently and during investigations devoted to the synthesis and isolation of platinum carbonyl clusters stabilized by Sn(II)-based fragments, Zacchini et al. solved in 2016 the X-ray structure of [NEt₄]₄[Pt₉(CO)₈(SnCl₂)₃(SnCl₃)₂(Cl₂SnOCOSnCl₂)]·2.5CH₃COCH₃ (28), which contains an unusual [(Cl₂SnOCOSnCl₂)]²⁻ ligand described as bis-stannyl-carboxylate or carbon dioxide μ₃: κ³-C,O,O′-CO₂ [81]. Cluster 28 was obtained as a side-product from the reaction at room temperature in acetone, involving [NEt₄]₄[Pt₁₅(CO)₃₀], [NEt₄]Cl and a large excess of SnCl₂·2H₂O, and leading to the major product [NEt₄]₄[Pt₆(CO)₆(SnCl₂)₂(SnCl₃)₄]. The authors described the unprecedented structure of 28 as consisting of a formally neutral Pt₉ core bonded to eight carbonyls, three μ₄-SnCl₂, two μ₃-[SnCl₃]^- and the six electron donor bis-stannyl-carboxylate [(Cl₂SnOCOSnCl₂)]²⁻ ligand (Figure 27). The possible formation of the metallocarboxylate group is explained by the nucleophilic addition of H₂O to a terminal CO ligand, followed by deprotonation. Alternatively, the authors also suggest that [(Cl₂SnOCOSnCl₂)]²⁻ could be considered as a carbonite ion [CO₂]²⁻ [82], stabilized by coordination at one Pt atom and two Sn atoms. A selection of spectroscopic data (NMR and IR) relating to metallocarboxylato-tin complexes is reported in Table 8.

Table 7. Selection of structural parameters found in metallocarboxylato tin complexes.

Compounds	Sn−O(C) (Å)	M−C(O2) (Å)	C−O (Å)	O−C−O (deg)	O−Sn−O (deg)	CSD Entry Deposition Number	Ref.
(η5-C5H5)Re(NO)(PPh3)(CO2SnPh3) (16)	2.257(7) 2.175(7)	2.058(9)	1.269(11) 1.313(11)	112.2(8)	57.8(2)	FODLAL 1158240	[72]
(η5-C5H5)Fe(CO)(PPh3)(CO2SnPh3) (17)	2.123(4) 2.342(4)	1.931(5)	1.270(6) 1.305(6)	113.4(4)	57.4(1)	JIDLUD 1185927	[73]
(η5-C9H7)Fe(CO)(PPh3)(CO2SnPh3) (18)	2.069(2) 2.536(2)	1.933(3)	1.236(3) 1.336(3)	115.9(3)	55.42(7)	HADNUV 1171252	[75]
Cp*Re(CO)(NO)(CO2SnPh3) (19)	2.092(3) 2.399(1)	2.100(9)	1.24(1) 1.322(9)	114.6(7)	56.79(5)	YEGCUI 1300837	[76]
CpFe(CO)(PPh3)(CO2SnMe3) (20)	2.476(3) 2.089(3)	1.936(4)	1.260(6) 1.321(5)	114.3(3)	55.94(9)	YOTBEO 1305570	[77]
CpFe(CO)(PPh3)(CO2Sn(n-Bu)3) (21)	2.105(4) 2.432(4)	1.934(6)	1.266(7) 1.316(7)	114.2(5)	56.5(1)	YOTBIS 1305571	[77]
Cp*Fe(CO)2(CO2SnPh3) (22)	2.102(2) 2.394(2)	1.956(3)	1.252(3) 1.312(3)	114.5(2)	56.90(6)	YOTBOY 1305572	[77]
Cp*Re(CO)(NO)(CO2SnMe3 (23)	2.054(4) 2.806(4)	2.103(5)	1.237(6) 1.311(6)	117.0(5)	50.3(1)	YOTBUE 1305573	[77]
[Cp*Re(CO)(NO)(CO2)]2SnMe2 (24)	2.079(6) 2.524(8)	2.11(1)	1.26(1) 1.31(1)	116.8(9)	55.7(2)	NOXRUN 1223123	[78]
Cp*Re(CO)(NO)(CO2Sn(Cl)Me3) (25)	2.109(8) 2.287(8)	2.11(1)	1.24(1) 1.32(1)	115.4(9)	58.8(3)	NOXROH	[78]
FeCl(CO2SnPh3)(depe)2 (26)	2.097(6) 2.312(7)	1.87(1)	1.28(1) 1.32(1)	108.4(9)	57.0	NOMQIP 1222229	[79]
Os3(CO)10(μ-η2-O=COSnPh3)(μ-OMe) (27)	2.076(5)	2.080(6)	1.277(8) 1.295(8)	115.6(8)	116.0(4)	IXODAB 807088	[80]
[NEt4]4[Pt9(CO)8(SnCl2)3(SnCl3)2-(Cl2SnOCOSnCl2)]·2.5CH3COCH3 (28)	2.088(9) 2.122(9)	2.03(1)	1.28(1) 1.31(2)	116(1)	115.5(7) 117.8(8)	KAKLOA 1439435	[81]

Table 7. Selection of structural parameters found in metallocarboxylato tin complexes.

Compounds	Sn−O(C) (Å)	M−C(O2) (Å)	C−O (Å)	O−C−O (deg)	O−Sn−O (deg)	CSD Entry Deposition Number	Ref.
(η5-C5H5)Re(NO)(PPh3)(CO2SnPh3) (16)	2.257(7) 2.175(7)	2.058(9)	1.269(11) 1.313(11)	112.2(8)	57.8(2)	FODLAL 1158240	[72]
(η5-C5H5)Fe(CO)(PPh3)(CO2SnPh3) (17)	2.123(4) 2.342(4)	1.931(5)	1.270(6) 1.305(6)	113.4(4)	57.4(1)	JIDLUD 1185927	[73]
(η5-C9H7)Fe(CO)(PPh3)(CO2SnPh3) (18)	2.069(2) 2.536(2)	1.933(3)	1.236(3) 1.336(3)	115.9(3)	55.42(7)	HADNUV 1171252	[75]
Cp*Re(CO)(NO)(CO2SnPh3) (19)	2.092(3) 2.399(1)	2.100(9)	1.24(1) 1.322(9)	114.6(7)	56.79(5)	YEGCUI 1300837	[76]
CpFe(CO)(PPh3)(CO2SnMe3) (20)	2.476(3) 2.089(3)	1.936(4)	1.260(6) 1.321(5)	114.3(3)	55.94(9)	YOTBEO 1305570	[77]
CpFe(CO)(PPh3)(CO2Sn(n-Bu)3) (21)	2.105(4) 2.432(4)	1.934(6)	1.266(7) 1.316(7)	114.2(5)	56.5(1)	YOTBIS 1305571	[77]
Cp*Fe(CO)2(CO2SnPh3) (22)	2.102(2) 2.394(2)	1.956(3)	1.252(3) 1.312(3)	114.5(2)	56.90(6)	YOTBOY 1305572	[77]
Cp*Re(CO)(NO)(CO2SnMe3 (23)	2.054(4) 2.806(4)	2.103(5)	1.237(6) 1.311(6)	117.0(5)	50.3(1)	YOTBUE 1305573	[77]
[Cp*Re(CO)(NO)(CO2)]2SnMe2 (24)	2.079(6) 2.524(8)	2.11(1)	1.26(1) 1.31(1)	116.8(9)	55.7(2)	NOXRUN 1223123	[78]
Cp*Re(CO)(NO)(CO2Sn(Cl)Me3) (25)	2.109(8) 2.287(8)	2.11(1)	1.24(1) 1.32(1)	115.4(9)	58.8(3)	NOXROH	[78]
FeCl(CO2SnPh3)(depe)2 (26)	2.097(6) 2.312(7)	1.87(1)	1.28(1) 1.32(1)	108.4(9)	57.0	NOMQIP 1222229	[79]
Os3(CO)10(μ-η2-O=COSnPh3)(μ-OMe) (27)	2.076(5)	2.080(6)	1.277(8) 1.295(8)	115.6(8)	116.0(4)	IXODAB 807088	[80]
[NEt4]4[Pt9(CO)8(SnCl2)3(SnCl3)2-(Cl2SnOCOSnCl2)]·2.5CH3COCH3 (28)	2.088(9) 2.122(9)	2.03(1)	1.28(1) 1.31(2)	116(1)	115.5(7) 117.8(8)	KAKLOA 1439435	[81]

Table 8. Selection of spectroscopic data (NMR and IR) assigned to metallocarboxylato tin complexes.

Compounds	119Sn{1H} NMR (δ, ppm)	13C{1H} NMR M–CO2 (δ, ppm)	IR ν(OCO) (cm−1)	Ref.
(η-C5H5)Re(NO)(PPh3)(CO2SnPh3) (16)	−167.1 a J119Sn,31P = 4.6 Hz	207.6 a	1395, 1188	[72]
Cp*Re(CO)(NO)(CO2SnPh3) (19)	na	206.94 b	1429, 1174	[76]
CpFe(CO)(PPh3)(CO2SnMe3) (20)	na	219.63 a d, JP,C = 30.2 Hz 220.42 c d, JP,C = 32.5 Hz	1433, 1132	[77]
CpFe(CO)(PPh3)(CO2Sn(n-Bu)3) (21)	na	219.56 a d, JP,C = 30.9 Hz 220.29 c d, JP,C = 31.4 Hz	1480, 1132	[77]
Cp*Fe(CO)2(CO2SnPh3) (22)	na	215.10 a	1470, 1146	[77]
Cp*Re(CO)(NO)(CO2SnMe3) (23)	na	197.58 a 196.10 c	1512, 1162	[77]
Cp*Re(CO)(NO)(CO2Sn(Cl)Me2) (24)	na	205.66 d	1385, 1246	[78]
[Cp*Re(CO)(NO)(CO2)]2SnMe2 (25)	na	202.14 d	1469, 1186	[78]
FeCl(CO2SnPh3)(depe)2 (26)	na	243.8 d qui, JP,C = 22 Hz	1306	[80]

^a Measured in chloroform-d; ^b methylenchloride-d₂; ^c measured in tetrahydrofuran-d₈; ^d measured in benzene-d_6.

Table 8. Selection of spectroscopic data (NMR and IR) assigned to metallocarboxylato tin complexes.

Compounds	119Sn{1H} NMR (δ, ppm)	13C{1H} NMR M–CO2 (δ, ppm)	IR ν(OCO) (cm−1)	Ref.
(η-C5H5)Re(NO)(PPh3)(CO2SnPh3) (16)	−167.1 a J119Sn,31P = 4.6 Hz	207.6 a	1395, 1188	[72]
Cp*Re(CO)(NO)(CO2SnPh3) (19)	na	206.94 b	1429, 1174	[76]
CpFe(CO)(PPh3)(CO2SnMe3) (20)	na	219.63 a d, JP,C = 30.2 Hz 220.42 c d, JP,C = 32.5 Hz	1433, 1132	[77]
CpFe(CO)(PPh3)(CO2Sn(n-Bu)3) (21)	na	219.56 a d, JP,C = 30.9 Hz 220.29 c d, JP,C = 31.4 Hz	1480, 1132	[77]
Cp*Fe(CO)2(CO2SnPh3) (22)	na	215.10 a	1470, 1146	[77]
Cp*Re(CO)(NO)(CO2SnMe3) (23)	na	197.58 a 196.10 c	1512, 1162	[77]
Cp*Re(CO)(NO)(CO2Sn(Cl)Me2) (24)	na	205.66 d	1385, 1246	[78]
[Cp*Re(CO)(NO)(CO2)]2SnMe2 (25)	na	202.14 d	1469, 1186	[78]
FeCl(CO2SnPh3)(depe)2 (26)	na	243.8 d qui, JP,C = 22 Hz	1306	[80]

^a Measured in chloroform-d; ^b methylenchloride-d₂; ^c measured in tetrahydrofuran-d₈; ^d measured in benzene-d_6.

6. Conclusions

In the conclusion of the first part of this inventory, “CO₂ Derivatives of Molecular Tin Compounds. Part 1: Hemicarbonato and Carbonato Complexes”, we previously claimed that the CO₂ derivatives of molecular tin compounds could truly be considered as a class of compounds in their own right. The additional solid-state structures described in this second part and focusing more specifically on carbamato, formato, phosphinoformato and metallocarboxylato complexes, contribute assuredly to reinforce this point of view. Once again, a rich diversity of architectures, including discrete, dimeric, polynuclear and polymeric structures, and exhibiting various modes of coordination, sometimes totally unusual, were highlighted. Thus, this collection of compounds supports again the great reactivity of molecular tin complexes toward carbon dioxide and underlines the facile insertion of CO₂ into Sn–X bonds (X = OR, N, H, P). This reactivity, which has been known for a long time, has already been efficiently used for catalytic applications involving CO₂ and organotins, in particular as has been shown, with the aim of accessing carbamate compounds and their derivatives. However, the latest advances recently published are very interesting and promising, suggesting new perspectives in terms of reactivity of organotins with respect to small molecules and in particular for the activation of carbon dioxide. This is especially the case of studies relating to the reductive hydroboration of CO₂ or those involving the formation of CO₂-Sn/phosphorus frustrated Lewis pairs (FLP) adducts, which start new insights. In the field of inorganic chemistry, the isolation of a new cluster incorporating a bis-stannyl-carboxylate group, also viewed as the coordination of a carbonite fragment, constitutes an innovative result which can be considered as unprecedented. Thus, the second half of the last century was incontestably a period of strong developments and fundamental advances in organotin chemistry, but the coming decades promise to be just as fruitful and exciting in terms of progress and inventiveness, and in particular, in the quest to activate carbon dioxide.