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

inorganics

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

Laurent Plasseraud †

Institut de Chimie Moléculaire de l’Université de Bourgogne (ICMUB), UMR-CNRS 6302, Université de Bourgogne Franche-Comté, 9 Avenue A. Savary, F-21078 Dijon, France; [email protected] † For Part 1, Please Refer to: CO2 Derivatives of Molecular Tin Compounds. Part 1: Hemicarbonato and Carbonato Complexes. Inorganics 2020, 8, 31; https://doi.org/10.3390/inorganics8050031.

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

Citation: Plasseraud, L. CO2 are supplemented by synthetic conditions and spectroscopic data. Derivatives of Molecular Tin Compounds. Part 2: Carbamato, Keywords: tin complexes; carbon dioxide ﬁxation; hypervalent compounds; X-ray crystallography Formato, Phosphinoformato and Metallocarboxylato Complexes . Inorganics 2021, 9, 18. https:// doi.org/10.3390/inorganics9030018 1. Introduction More than ever and everywhere in the world, the recovery and utilization of carbon Academic Editor: Daniel A. Foucher 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 Received: 16 December 2020 and chemically inert waste, but with devastating effects, carbon dioxide is now differently Accepted: 8 February 2021 Published: 24 February 2021 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

Publisher’s Note: MDPI stays neutral to transform CO2 into chemicals with higher added value, coordination and organometallic with regard to jurisdictional claims in chemistry played—and continue to play—an important and determining role in facilitating published maps and institutional affil- its activation [1]. Since the first resolution of an X-ray crystal structure highlighted the metal iations. 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(C H ) (CO )(PNP) Copyright: © 2021 by the author. 2 4 2 2 Licensee MDPI, Basel, Switzerland. compounds (M = Mo and W; PNP = 2,6-bis(diphenylphosphinomethyl)pyridine), which This article is an open access article are the first examples of stable metal complexes with coordinated ligands of ethylene and distributed under the terms and carbon dioxide [3]. In conclusion of this recent work, the authors aim, as perspectives, conditions of the Creative Commons for the conversion of C2H4 and CO2 into acrylate derivatives. Attribution (CC BY) license (https:// Since the 1990s, the chemical transformation of CO2 and its coordination on metal creativecommons.org/licenses/by/ centers have been regularly reviewed and updated [4–14]. During the last decade, the reac- 4.0/). tivity of main group elements, and in particular metals and metalloids from group 14 (Si,

Inorganics 2021, 9, 18. https://doi.org/10.3390/inorganics9030018 https://www.mdpi.com/journal/inorganics Inorganics 2021, 9, x FOR PEER REVIEW 2 of 33

Since the 1990s, the chemical transformation of CO2 and its coordination on metal Inorganics 2021, 9, 18 centers have been regularly reviewed and updated [4–14]. During the last decade, the2 of re- 32 activity 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 molecu- Ge, Sn, Pb), toward small molecules also aroused a growing interest [15,16]. In this context lar organotin compounds bearing hemi-carbonate and carbonate ligands and which result and based on our previous work in this field, we have recently reviewed molecular organ- from the reactivity of tin precursors towards carbon dioxide [1]. In this second part, we otin compounds bearing hemi-carbonate and carbonate ligands and which result from the continue to explore from a structural point of view the molecular derivatives of tin which reactivity of tin precursors towards carbon dioxide [1]. In this second part, we continue to result from the insertion of carbon dioxide into the Sn–X bonds (X = N, H, P). Thus, the explore from a structural point of view the molecular derivatives of tin which result from formation and structures of carbamato, formato and phosphinoformato tin derivatives are the insertion of carbon dioxide into the Sn–X bonds (X = N, H, P). Thus, the formation and successivelystructures of described carbamato, and formato discussed. and phosphinoformatoThe last chapter is tindevoted derivatives to metallocarboxylato are successively tindescribed complexes and exhibiting discussed. a The bridging last chapter CO2 ligand. is devoted Although to metallocarboxylato these latter compounds tin complexes are not derived from the direct reactivity of tin precursors with carbon dioxide, we found appro- exhibiting a bridging CO2 ligand. Although these latter compounds are not derived from priatethe direct to include reactivity them of in tin this precursors review. Regard with carboning the dioxide,general method we found used appropriate to carry out to this in- inventory,clude them the in thiscompounds review. Regardingdescribed below the general were methodidentified used from to the carry data out available this inventory, on the onlinethe compounds portal of the described Cambridge below Structural were identified Database from (CSD) the dataweb interface available (2017) on the [17]. online In additionportal of to the the Cambridge structural Structuraldescription, Database comparison (CSD) tables web including interface (2017)selections [17]. of In structural addition andto the spectroscopic structural description, data are also comparison displayed at tables the end including of each section selections (Tables of structural 1–8). and spectroscopicAmong the data useful are alsochemicals displayed with at higher the end added-value of each section which (Tables can be1– 8produced). by the transformationAmong the of useful CO2, the chemicals derivatives with involving higher added-value C–N bonds whichoccupy can an important be produced place, by historicallythe transformation and economically. of CO2, the Thus, derivatives from 1922, involving the German C–N bonds Bosch-Meiser occupy an process important al- lowedplace, historicallythe industrial and use economically. of carbon dioxide Thus, from(together 1922, with the Germanammonia) Bosch-Meiser as raw material process for theallowed synthesis the industrial of urea [(NH use of2)2CO]. carbon It is dioxide still today (together the most with important ammonia) industrial as raw material chemical for processthe synthesis in terms of urea of CO [(NH2 amounts2)2CO]. converted It is still today (115 theMt) most [18]. importantSince then, industrial the use of chemical carbon dioxideprocess as in C1 terms synthon of CO has2 amounts been extended converted to the (115 synthesis Mt) [18 ].of Since other then, types the of nitrogen use of carbon com- poundsdioxide such as C1 as synthon oxazolidinones, has been quinazolinediones, extended to the synthesis ureas, carbamates, of other types isocyanates of nitrogen and polyurethanescompounds such (Scheme as oxazolidinones, 1). These molecules quinazolinediones, constitute ureas,intermediates carbamates, and isocyanates derivatives and of greatpolyurethanes importance (Scheme for the1 ).chemical, These molecules agrochem constituteical and pharmaceutical intermediates and industry. derivatives The do- of maingreat importancewas firstly reviewed for the chemical, by He et agrochemical al. in 2012 [19], and pharmaceuticaland recently updated industry. by The Song domain et al. [20]was firstlyand then reviewed by Dalpazzo by He etetal. al. in [21], 2012 both [19], in and 2019. recently In most updated cases, by the Song intervention et al. [20] of and a catalyticthen by Dalpazzoprecursor—organic et al. [21], or both organometallic, in 2019. In most is required cases, the to achieve intervention the targeted of a catalytic product.precursor—organic or organometallic, is required to achieve the targeted product.

Scheme 1. Molecular representations of nitrogen-containing molecules accessible from CO2 used as a C1 building block. Scheme 1. Molecular representations of nitrogen-containing molecules accessible from CO2 used as a C1 building block.

Tin-Promoted Insertion Reaction of CO2 into C–N Bonds Tin-PromotedRegarding Insertion more specifically Reaction of COthe2 useinto of C–N main Bonds group metals as catalytic species for the insertionRegarding of CO2 moreinto the speciﬁcally C–N bond, the the use beneficial of main role group of organotin metals as compounds catalytic species was reg- for ularlythe insertion emphasized of CO in2 into the past, the C–N being bond, the subjec the beneﬁcialt of several role studies. of organotin In 2002, compounds Tominaga wasand Sasakiregularly found emphasized that di-n-butyltin in the past, oxide, being n-Bu the2 subjectSnO, was of severalan effective studies. catalyst In 2002, for Tominagathe dehy- dratingand Sasaki condensation found that of di- 1,2-aminoethanolsn-butyltin oxide, withn-Bu carbon2SnO, wasdioxide an effectiveto lead to catalyst the formation for the ofdehydrating 2-oxazolidinones condensation [22] (Scheme of 1,2-aminoethanols 2). Reactions were with carried carbon out dioxide under to5 MPa lead toof theCO2 for-, at 180mation °C, for of 2-oxazolidinones16 h, in N-methyl-2-pyrrolidone [22] (Scheme2). (NMP) Reactions as solv wereent carriedand gave out satisfactory under 5 MPa yields of ◦ inCO the2, at range 180 ofC, 53 for to 16 94%. h, in TheN-methyl-2-pyrrolidone use of NMP has been (NMP) found as to solvent be more and effective gave satisfactory than the useyields of inprotic the rangeand non-polar of 53 to 94%. solvents. The use From of NMPa mechanistic has been foundpoint of to view, be more the effective authors thanpro- posedthe use the of formation protic and of non-polar a seven-membered solvents. Fromcyclic atin mechanistic carbamate complex point of view,(Scheme the 3) authors by the proposed the formation of a seven-membered cyclic tin carbamate complex (Scheme3) 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, Inorganics 2021, 9, x FOR PEER REVIEW 3 of 33

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 intramo- Inorganics 2021, 9, x FOR PEER REVIEW 3 of 33 Inorganics 2021, 9, x FOR PEER REVIEWlecular elimination of one molecule of 2-oxazolidinone was suggested3 of to 33 explain the re-

generation of the starting complex, n-Bu2SnO. Inorganics 2021, 9, 18 3 of 32 insertion of CO2 into the Sn–N bond of a stannaoxazolidine species and whose in-situ for- 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, mation was supported by electrospray mass spectrometry. However, to our knowledge, such a species has not been yet structurally isolated. From this intermediate, the intramo- such a species has not been yet structurally isolated. From this intermediate, the intramo- thelecular intramolecular elimination elimination of one molecule of one molecule of 2-oxazolidinone of 2-oxazolidinone was suggested was suggested to explain to explain the re- lecular elimination of one molecule of 2-oxazolidinone was suggested to explain the re- thegeneration regeneration of the of starting the starting complex, complex, n-Bun2SnO.-Bu2SnO. generation of the starting complex, n-Bu2SnO.

Scheme 2. Synthesis of 2-oxazolidinones from CO2 and 1,2-aminoalcohols catalyzed by n-Bu2SnO, data from [22].

SchemeScheme 2. 2. Synthesis of 2-oxazolidinones from CO2 and 1,2-aminoalcohols catalyzed by nn-Bu2SnO, Scheme 2. SynthesisSynthesis ofof 2-oxazolidinones2-oxazolidinones from CO2 and 1,2-aminoalcohols catalyzed catalyzed by by n-Bu-Bu22SnO,SnO, data from [22]. datadata fromfrom [[22].22].

Scheme 3. Key step requiring the insertion of CO2 into the Sn–N bond of the stannaoxazolidine

species formed in situ, data from [22]. Scheme 3. Key step requiring the insertion of CO2 into the Sn–N bond of the stannaoxazolidine SchemeScheme 3.3. Key step step requiring requiring the the insertion insertion of of CO CO2 intointo the the Sn Sn–N–N bond bond of ofthe the stannaoxazolidine stannaoxazolidine species formed in situ, data from [22]. 2 speciesspeciesIn formedformed 2013, inin situ,Ghoshsitu, datadata et fromfrom al. [[22].22 revisited]. the previous reaction using chlorostannoxane deriva- In 2013, Ghosh et al. revisited the previous reaction using chlorostannoxane derivatives,InIn 2013,1,3-dichloro-1,1,3,3-tetraalkyldistannoxa2013, GhoshGhosh etet al.al. revisitedrevisited thethe previousprevious reactionreactionnes usingusing (Scheme chlorostannoxanechlorostannoxane 4), as catalytic deriva-deriva- precursor, in tives, 1,3-dichloro-1,1,3,3-tetraalkyldistannoxanes (Scheme 4), as catalytic precursor, in tives,tives,methanol 1,3-dichloro-1,1,3,3-tetraalkyldistannoxa 1,3-dichloro-1,1,3,3-tetraalkyldistannoxanes under 1.72 MPa of CO2 pressurenes (Scheme (Scheme and at 4),4 ),150 as as catalytic°C catalytic [23]. precursor, Chlorostannoxanes precursor, in were methanol under 1.72 MPa of CO2 pressure and at 150 °C ◦[23]. Chlorostannoxanes were inmethanolselected methanol underbecause under 1.72 1.72of MPa (i) MPa theof CO of presence CO2 pressure2 pressure of and multi-acand at 150 at 150°Ctive [23].C[ catalytic 23Chlorostannoxanes]. Chlorostannoxanes centers, and were (ii) the ability to selected because of (i) the presence of multi-active catalytic centers, and (ii) the ability to wereselected selected because because of (i) of the (i) presence the presence of multi-ac of multi-activetive catalytic catalytic centers, centers, and and (ii) (ii) the the ability ability to controlcontrol Lewis Lewis acidity acidity by changingby changing the substituen the substituents on the metalts on centers.the metal In comparison centers. In to comparison to tocontrol control Lewis Lewis acidity acidity by bychanging changing the thesubstituen substituentsts on the on metal the metal centers. centers. In comparison In compari- to thethe previous Tominaga’s Tominaga’s study study using usingn-Bu2SnO n-Bu [22],2SnO an improvement [22], an improvement of turnover num- of turnover num- sonthe toprevious the previous Tominaga’s Tominaga’s study studyusing usingn-Bu2SnOn-Bu [22],2SnO an [22 improvement], an improvement of turnover of turnover numbers was recorded using chlorostannoxanes, from 9 to 138. Furthermore, it has been numbersbers waswas wasrecorded recorded recorded using usingusing chlorostannoxanes, chlorostannoxanes, chlorostannoxanes, from from 9 9to tofrom 138. 138. Furthermore,9 Furthermore, to 138. Furthermore, it it has beenbeen it has been demonstrated that the activity of the catalytic precursors depends on the nature of the R demonstrateddemonstrated thatthat that thethe activityactivitythe activity ofof thethe catalyticofcatalyti the ccatalyti precursorsprecursorsc precursors dependsdepends onon depends thethe naturenature on ofof the thethe RR nature of the R and R0 ′ substituents, the n-butyl derivative (R and R’ = n-Bu) being the most efficient. andand RR R′ substituents,substituents,′ substituents, thethe n nthe-butyl-butyl n-butyl derivativederivative derivative (R(R andand R’R’ (R == nand-Bu)-Bu) R’ beingbeing = n thethe-Bu) mostmost being efﬁcient.efficient. the most efficient.

Scheme 4. Molecular representation of chlorostannoxane compounds used as tin precursors for Scheme 4. Molecular representation of chlorostannoxane compounds used as tin precursors for the synthesis of 2-oxazolidinones by direct condensation of 2-aminoalcohols with carbon dioxide the synthesis of 2-oxazolidinones by direct condensation of 2-aminoalcohols with carbon dioxide (R = n-Bu, Ph; R′ = n-Bu, Ph), data from [23]. Scheme(RScheme = n-Bu, 4. Ph;4.Molecular Molecular R′ = n-Bu, representation Ph), representation data from of [23]. chlorostannoxane of chlorostannoxane compounds compounds used as tin precursorsused as tin for precursors for thethe synthesis synthesisThe direct of 2-oxazolidinonesof synthesis 2-oxazolidinones of carbamates by direct by condensationdirect from condensationcarbon of 2-aminoalcoholsdioxide of has 2-am also withinoalcohols aroused carbon renewed dioxide with carbon dioxide The direct0 synthesis of carbamates from carbon dioxide has also aroused renewed (R(Rinterest = =n -Bu,n-Bu, in Ph; Ph;the R =pastRn′ -Bu,= ntwo-Bu, Ph), decades dataPh), from data [24]. [23 from ].The [23].state of the art was well established in 2003 by 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 Calderazzo et al. [25] and recently updated by Marchetti et al. [26]. Indeed, carbamates can Thebe viewed direct synthesisas a potential of carbamates alternative from to access carbon to dioxide isocyanates, has also which aroused are historically renewed interestcan beThe viewed in thedirect past as a twosynthesis potential decades alternative of [24 carbamates]. The to state access of from the to artisocyanates, carbon was well dioxide establishedwhich are has historically in also 2003 aroused by renewed employed for the production of polyurethanes (PU). Industrially, isocyanates are pro- Calderazzoemployedinterest in for et the al.the [past 25production] and two recently decades of polyuretha updated [24]. by nesThe Marchetti (PU). state Industrially, of et the al. [ 26art]. isocyanates Indeed,was well carbamates establishedare pro- in 2003 by duced by reacting primary amines with phosgene, but handling of this reactant is not canducedCalderazzo be viewedby reacting et as al. a primary potential [25] and amines alternative recently with tophosge updated accessne, to but isocyanates,by handlingMarchetti whichof thiset areal.reactant historically[26]. isIndeed, not carbamates employedcan be viewed for the production as a potential of polyurethanes alternative (PU). to Industrially, access to isocyanates isocyanates, are produced which are historically by reacting primary amines with phosgene, but handling of this reactant is not without

employed for the production of polyurethanes (PU). Industrially, isocyanates are produced by reacting primary amines with phosgene, but handling of this reactant is not

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without risks and requires drastic precautions. Therefore, the recent route based on the thermalwithoutrisks and decompositionrisks requires and requires drastic of precautions.carbamates drastic precautions. into Therefore, isocyanates Therefore, the recent can the be route recentconsidered based route on as based thea safer thermal on andthe greenerthermaldecomposition syntheticdecomposition of pathway carbamates of carbamates[27,28]. into isocyanates into isocyanates can be considered can be considered as a safer as and a safer greener and greenersyntheticIn the synthetic pathway past, several pathway [27,28 groups]. [27,28]. investigated the action of organotins as catalyst precursors In the past, several groups investigated the action of organotins as catalyst precursors for theIn directthe past, conversion several groups of CO investigated2 into carbamates. the ac tionIn 2001, of organotins Sakakura as et catalyst al. first precursorspublished thefor synthesis the direct of conversionconversion carbamates ofof by COCO reaction22 into carbamates. of amines and In 2001, alcohols Sakakura catalyzed et al.al. by ﬁrstfirst tin complexes publishedpublished the synthesis of carbamates by reaction of amines and alcohols catalyzed by tin complexes (then-Bu synthesis2SnO and of Mecarbamates2SnCl2) [29] by (Schemereaction of5). amines Optimal and reaction alcohols conditions catalyzed require by tin complexesa pressure n (n-Bu2SnO and Me 2SnClSnCl22))[ [29]29] (Scheme (Scheme 55).). Optimal reaction conditionsconditions requirerequire aa pressurepressure of 30 MPa (300 bar) at 200◦ °C for 24 h. Moreover, the addition of acetals (2,2-diethoxy- or 2,2-dimethoxypropane)of 30 MPaMPa (300(300 bar)bar) at at 200 200 acting C°C for for as 24 24a h. dehydrating h. Moreover, Moreover, the agent the addition addition significantly of acetalsof ac etalsimproves (2,2-diethoxy- (2,2-diethoxy- the conver- or 2,2- or dimethoxypropane) acting as a dehydrating agent signiﬁcantly improves the conversion, sion,2,2-dimethoxypropane) and the increase in actingpressure as apromotes dehydrating the urethane agent significantly selectivity. improves the conversion,and the and increase the increase in pressure in pressure promotes promotes the urethane the urethane selectivity. selectivity.

Scheme 5. Halogen-free process for the conversion of carbon dioxide to carbamates, data from [29].Scheme 5. Halogen-free process for the conversion of carbon dioxide toto carbamates,carbamates, datadata fromfrom [29]. [29]. In 2016, 2016, Choi Choi et et al. al. reported reported the the synthesis synthesis of N of-phenylcarbamatesN-phenylcarbamates from from CO2 CO and2 andani- lineaniline promotedIn 2016, promoted Choi by byetseveral al. several reported di- di-n-butyltinn the-butyltin synthesis dialkoxides: dialkoxides: of N-phenylcarbamates n-Bun-Bu2Sn(OMe)2Sn(OMe)2, fromn2-Bu, n-Bu 2COSn(OEt)22Sn(OEt) and 2ani-, n2-, Bulinen-Bu2Sn(OPr- promoted2Sn(OPr-n)2n, )andby2, and severaln-Bun-Bu2Sn(OBu- 2di-Sn(OBu-n-butyltinn)2 (Schemen)2 (Scheme dialkoxides: 6) [30].6)[30 The ].n-Bu The best2Sn(OMe) best yield yield of 2N, of-phenylcarbamaten-BuN-phenylcarba-2Sn(OEt)2, n- ◦ (41%)Bumate2Sn(OPr- (41%) was obtainedwasn)2, and obtained n under-Bu under2Sn(OBu- 5 MPa 5 MPa n(5) 2bar) (5(Scheme bar) of ofCO CO6)2 for[30].2 for 20 The 20 min min best at at 150 yield 150 °C Cof using using N-phenylcarbamate n-Bu2Sn(OMe)22 as(41%) tin precursor.was obtained The under catalytic 5 MPa activity (5 bar) chan changes of COges2 fordepending 20 min at on 150 the °C nature using ofn-Bu the2Sn(OMe) alkoxide2 substituentsas tin precursor. linked The to catalyticthe tin atom atom activity [ [n-Bu-Bu chan2Sn(OMe)Sn(OMe)ges depending22 >> nn-Bu-Bu2Sn(OPr-2 Sn(OPr-on the nnaturen)2) 2> n>-Bun of-Bu2 Sn(OEt)the2Sn(OEt) alkoxide2 >2 n>- 0 Busubstituentsn-Bu2Sn(OBu-2Sn(OBu-n linked)2n].) 2N]., NtoN′, -diphenylureaNthe-diphenylurea tin atom [n -Buis isformed2Sn(OMe) formed as as2 byproduct> byproductn-Bu2Sn(OPr- (4%). (4%).n) 2From From> n-Bu a a2 Sn(OEt) mechanisticmechanistic2 > n- pointBu2Sn(OBu- of view,n)2 the].the N hemicarbonato,hemicarbonatoN′-diphenylurea tintin is complex,complex, formed n asn-Bu-Bu byproduct22Sn(OMe)(OCOSn(OMe)(OCO (4%). 22FromMe), resultinga mechanistic from thepoint reactivity of view, of the CO hemicarbonato2 with n-Bu2Sn(OMe)Sn(OMe) tin complex,22 [1,31],[1,31], nis is-Bu claimed claimed2Sn(OMe)(OCO as as the the key key2Me), intermediate resulting offrom the reaction.the reactivity In the of case CO 2of with reactions n-Bu2Sn(OMe) involving2 [1,31],n-Bu-Bu22Sn(OBu- is claimedn)2 ,as, the the the authors authors key intermediate also also showed of that the thereaction. starting starting In thetin complex case of reactions could be involving regenerated n-Bu af after2terSn(OBu- the first ﬁrstn) 2catalytic , the authors run (by also reacting showed with that nthe-BuOH) starting and tin could complex thus could be recycled be regenerated without lossafter of the activity. first catalytic run (by reacting with n-BuOH) and could thus be recycled without loss of activity.

Scheme 6. Tin-promoted synthesis of N-phenyl carbamate from aniline and carbon dioxide (R = Me, Me, Et, Et, nn-Pr,-Pr, and n-Bu), dataScheme from 6. [[30].30Tin-promoted]. synthesis of N-phenyl carbamate from aniline and carbon dioxide (R = Me, Et, n-Pr, and n-Bu), data from [30]. Concomitantly andand supported supported by by density density functional functional theory theory (DFT) calculations,(DFT) calculations, Schaub Schaubet al.Concomitantly conﬁrmed et al. confirmed that and dialkyltin(IV) thatsupported dialkyltin(IV) dialkoxides by density dialkoxides can functional be used can asbetheory reagents used (DFT)as forreagents thecalculations, synthesis for the synthesisSchaubof aromatic et of al. carbamatesaromatic confirmed carbamates from that aromaticdialkyltin(IV) from aminesaromatic dialkoxides and amines carbon and can dioxide carbon be used [ 32dioxide ].as The reagents [32]. process The for takes pro- the cesssynthesisplace takes between ofplace aromatic 60between and carbamates 70 60 bar and of 70 pressure, from bar aromaticof pressure, at 135 amines◦C at and 135 and in°C pentanecarbonand in dioxidepentane as solvent. [32].as solvent. The A NMR pro- A NMRcessyield takes ofyield 92% place of was 92% between recorded was recorded 60 for and the 70for synthesis bar the ofsynthesis pressure, of methyl of atmethyl phenylcarbamate135 °C phenylcarbamate and in pentane from anilineasfrom solvent. aniline using A usingNMRthree molaryieldthree ofmolar equivalent 92% equivalentwas recorded of n-Bu of2 Sn(OMe)nfor-Bu the2Sn(OMe) synthesis2. Neither2. Neither of methyl methylaniline methylaniline phenylcarbamate nor diphenylurea nor diphenylurea from aniline was wasusingchromatographically chromatographically three molar equivalent detected. detected. of Extension n-Bu Extension2Sn(OMe) of the of2. reaction Neitherthe reaction conditionsmethylaniline conditions to the nor to synthesis diphenylureathe synthesis of di- 0 ofwasurethanes diurethanes chromatographically from from 4,4 -methylenedianiline 4,4′-methylenedianiline detected. Extension and 2,4-diaminotoluene and of 2,4-diaminotoluenethe reaction conditions was successful was tosuccessful the with synthesis yields with yieldsof diurethanes 60 and of 60 77%, and from respectively. 77%, 4,4 respectively.′-methylenedianiline In the perspective In the pe andrspective of 2,4-diaminotoluene an industrial of an industrial development, was development, successful the authors with the authorsyieldshave demonstrated of have 60 and demonstrated 77%, the respectively. possibility the possibility of In regenerating the ofpe rspectiveregenerating and re-usingof an and industrial re-using the dialkyltin(IV) development,the dialkyltin(IV) dialkox- the dialkoxides,authorsides, by usinghave by demonstrated an using excess an of excess dimethylcarbonate. the possibility of dimethylcarbonate. of regenerating Shortly after, Shortly and in a newre-usingafter, study, in thea new the dialkyltin(IV) same study, group the samedialkoxides,improved group the improvedby reaction using an yieldthe excess reaction (up toof 97%)dimethylcarbonate.yield by(up combining to 97%) by withShortly combining dialkyltin after, with in oxide, adialkyltin new used study, as oxide, a the tin sameprecursor, group tetraalkyl improved orthosilicates, the reaction which yield (up promote to 97%) the by generation combining of dialkyltin(IV)with dialkyltin dialkox- oxide,

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usedused asas aa tintin precursor,precursor, tetraalkyltetraalkyl orthosilorthosilicates,icates, whichwhich promotepromote thethe generationgeneration ofof dial-dialkyltin(IV) dialkoxides [33]. This positive effect had been initially shown for the tin-pro- ideskyltin(IV) [33]. This dialkoxides positive [33]. effect This had positive been initially effect shownhad been for initially the tin-promoted shown for synthesis the tin-pro- of moted synthesis of organic carbonates from carbon dioxide and alcohols [34]. organicmoted synthesis carbonates of fromorganic carbon carbonates dioxide from and alcoholscarbon di [34oxide]. and alcohols [34].

2.2.2. CarbamatoCarbamato Carbamato TinTin Tin ComplexesComplexes Complexes 2.1.2.1.2.1. EarlyEarly Early WorksWorks Works Historically,Historically,Historically, thethe the pioneering pioneering works worksworks on on carbamat carbamat carbamateseses of of of tin tin tin derivatives derivatives derivatives dates dates dates back back back to to the tothe the1960s1960s 1960s and and and are are to areto be be to credited credited be credited to to A. A. to J. J. A.Bloodworth Bloodworth J. Bloodworth and and A. andA. G. G. A.Davies Davies G. Davies who who conducted whoconducted conducted exten- exten- extensivesivesive and and fundamental fundamental and fundamental investigations investigations investigations applying applying applying a a systematic systematic a systematic approach. approach. approach. From From 1965, 1965, From they they 1965, de- de- theyscribedscribed described thethe synthesissynthesis the synthesis ofof NN-organo--organo- of N-organo-NN-trialkylstannylcarbamates-trialkylstannylcarbamatesN-trialkylstannylcarbamates resultingresulting resulting fromfrom fromthethe addi-addi- the additiontiontion of of trialkyltin trialkyltin of trialkyltin alkoxides alkoxides alkoxides to to isocyanates isocyanates to isocyanates [3 [35].5]. [ 35The The]. Thereactions reactions reactions were were were reported reported reported as as rapid rapid as rapid and and andexothermicexothermic exothermic and and and taken taken taken place place place at at atroom room room temperat temperat temperatureureure (Scheme (Scheme (Scheme 7). 77).). Alkyl Alkyl andand and arylaryl aryl isocyanatesisocyanates isocyanates exhibitexhibitexhibit aa a comparablecomparable comparable reactivity.reactivity. reactivity.

SchemeSchemeScheme 7.7. 7. FormationFormation Formation ofof of organoorgano organo ester ester of of N N-organo--organo-NN-trialkylstannylcarbamic-trialkylstannylcarbamic acid acidacid from fromfrom trialkyltin trialkyltintrialkyltin alkoxidesalkoxidesalkoxides andand and isocyanates,isocyanates, isocyanates, datadata data fromfrom from [35[35]. [35].].

AdditionallyAdditionallyAdditionally in inin 1965, 1965,1965, the thethe study studystudy was waswas then thenthen extended exextendedtended to toto bis(trialkyltin) bis(trialkyltin)bis(trialkyltin) oxides, oxides,oxides, which whichwhich reactreactreact exothermically exothermicallyexothermically with withwith isocyanates isocyanatesisocyanates to toto lead leadlead to toto trialkyltin trialkyltintrialkyltinN N-trialkylstannylcarbamatesN-trialkylstannylcarbamates-trialkylstannylcarbamates (Scheme(Scheme(Scheme8 )[8) 8) 36[36]. [36].].

SchemeSchemeScheme 8. 8. Formation FormationFormation of of trialkyltin oftrialkyltin trialkyltin ester ester esterof of N N-organo--organo- of N-organo-NN-trialkylstannylcarbamic-trialkylstannylcarbamicN-trialkylstannylcarbamic acid acid from from acid bis(trial- bis(trial- from bis(trialkyltin)kyltin)kyltin) oxides oxides and oxidesand isocyanates, isocyanates, and isocyanates, data data from from data [36]. [36]. from [36].

FromFromFrom a a coordination coordinationcoordination point point of of of view, view, view, the the the au au authorsthorsthors preferentially preferentially preferentially recommended recommended recommended the the rep- therep- representationresentationresentation ofof of thethe the carbamatocarbamato carbamato ligandligand ligand as as as N NN-coordinated-coordinated-coordinated to to tin, tin, in inin amido amidoamido form,form, but butbut diddid did not notnot excludeexcludeexclude anan an OO O-coordination,-coordination,-coordination, inin in imidoimido imido form,form, form, viavia via anan an interconversioninterconversion interconversion process.process. process. Subsequently,Subsequently, Subsequently, thisthisthis structuralstructural structural considerationconsideration consideration waswas was thethe the subjectsubject subject ofof of severalseveral several subsequentsubsequent subsequent works,works, works, involvinginvolving involving otherother other 119119 researchresearchresearch groups,groups, groups, andand and basedbased based onon on studiesstudies studies carried carried carried out out out by by by 119 SnSnSn MössbauerMössbauer Mössbauer spectroscopyspectroscopy spectroscopy [[37], 37[37],], asasas wellwell well asas as moremore more recentlyrecently recently byby by computationalcomputational computational calculationscalculations calculations [ 38[38]. [38].].

2.2.2.2.2.2. ReactivityReactivity Reactivity InInIn terms termsterms of of reactivity,of reactivity,reactivity,N-organo- NN-organo--organo-N-trialkylstannylcarbamatesNN-trialkylstannylcarbamates-trialkylstannylcarbamates have have beenhave alsobeenbeen involved alsoalso in-in- involvedvolved various inin organic variousvarious reactions. organicorganic reactions.reactions. In their initial InIn theirtheir article initialinitial [35 ], articlearticle Bloodworth [35],[35], BloodworthBloodworth and Davies and showedand DaviesDavies in particular their reactivity towards acetic anhydride, ethylamine and ethanol, which lead, ac- showedshowed inin particularparticular theirtheir reactivityreactivity towardtowardss aceticacetic anhydride,anhydride, ethylamineethylamine andand ethanol,ethanol, cording to a metathetical process, to the corresponding urethanes. In 1968, the same authors whichwhich lead, lead, according according to to a a metathetical metathetical proce process,ss, to to the the corresponding corresponding urethanes. urethanes. In In 1968, 1968, reported the decarboxylation of trialkyltin esters of N-organo-N-trialkylstannylcarbamic thethe samesame authorsauthors reportedreported thethe decarbdecarboxylationoxylation ofof trialkyltintrialkyltin estersesters ofof NN-organo--organo-NN-trial--trial- acids by isocyanates and isothiocyanates providing an alternative route to the synthesis of kylstannylcarbamickylstannylcarbamic acidsacids byby isocyanatesisocyanates andand isothiocyanatesisothiocyanates providingproviding anan alternativealternative carbodi-imides [39]. In 1992 and then in 1993, Shibata et al. underlined the positive and routeroute to to the the synthesis synthesis of of carbodi-imides carbodi-imides [39]. [39]. In In 1992 1992 and and then then in in 1993, 1993, Shibata Shibata et et al. al. un- un- efﬁcient role of methyl ester of N-ethyl-N-tributylstannylcarbamic acid as promoter for the Darzens reaction [40] and the addition of Michael [41] (Scheme9).

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Inorganics 2021, 9, 18 derlined the positive and efficient role of methyl ester of N-ethyl-N-tributylstannylcar-6 of 32 bamic acid as promoter for the Darzens reaction [40] and the addition of Michael [41] (Scheme 9).

Scheme 9. 9. Darzens-typeDarzens-type reaction reaction (top) (top) and andMich Michaelael addition addition (bottom) (bottom) promoted promoted by methyl by methylester of ester N-ethyl- of NN-ethyl--tribu-N- tributylstannylcarbamictylstannylcarbamic acid acid,, data data from from [40,41]. [40,41 ].

2.3. X-ray X-Ray Crystal Crystal Structures Structures Regarding thethe solid-statesolid-state characterizationcharacterization of of metal metal carbamates carbamates by by single-crystal single-crystal X-ray X- raydiffraction, diffraction, the areathe wasarea firstwas reviewedfirst reviewed by Calderazzo by Calderazzo et al. [25 et] andal. [25] then and recently then updatedrecently updatedby Marchetti by Marchetti et al. [26]. et To al. date, [26]. numerous To date, nu examplesmerous examples of metal carbamates of metal carbamates have thus have been thusisolated been and isolated characterized, and characterized, demonstrating demonstrating a rich coordination a rich chemistrycoordination and chemistry highlighting and a highlightinggreat diversity a ofgreat structures. diversity Herein, of structures. we will focus Herein, exclusively we will on focus X-ray exclusively structures involvingon X-ray structurestin moieties. involvingTo the best tin ofmoieties. our knowledge, To the best six CSDof our depositions knowledge, of carbamato six CSD depositions tin complexes of carbamatohave been identifiedtin complexes up to nowhave (CSD been entries identified and deposition up to now numbers (CSD entries are reported and deposition in Table1) . numbersThe firstare reported X-ray crystallographic in Table 1). investigation on a tin complex bearing a carbamato ligandThe was first reported X-ray crystallographic by Zakharov et al.investigation in 1980, as parton a oftin studies complex on organotinbearing a carbamato isocyanate ligandderivatives was reported [42]. Thus, by theZakharov Russian et group al. in 1980, characterized as part of at studies the solid-state on organotin the structure isocyanate of derivativesthe trimethyltin(IV) [42]. Thus, ester the of RussianN-trimethylstannylcarbamic group characterized at acids, the solid-state Me3SnNHC(=O)–OSnMe the structure of3 the(1), trimethyltin(IV) which describes aester polymeric of N-trimethylstannylcarbamic arrangement displaying a acids, zigzag Me one-dimensional3SnNHC(=O)–OSnMe infinite3 (chain1), which along describes the c-axis a (Figurepolymeric1). The arrangement organization displaying of 1 consists a zigzag of SnMe one-dimensional3 moieties bridged infi- niteby carbamate chain along dianions. the c-axis Two (Figure distinct 1). sites The of tinorganization atoms are distinguished:of 1 consists of (i) SnMe those3 located moieties in bridgedthe main by chain carbamate exhibiting dianions. a trigonal Two bipyramidal distinct sites geometry of tin atoms (TBP) are whosedistinguished: equatorial (i) planethose locatedis occupied in the by main three chain methyl exhibiting substituents a trigonal and thebipyramidal apical positions geometry by two(TBP) oxygen whose atoms; equatorial(ii) SnMe plane3 pendant is occupied groups, by three connected methyl to subs the maintituents chain and by the nitrogen apical positions atoms (Sn–N by two = 2.04(2) oxy- genÅ) and atoms; positioned (ii) SnMe in a3 syndiotacticpendant groups, arrangement, connected and to whose the main tin atoms chain exhibitby nitrogen a tetrahedral atoms (Sn–Ngeometry. = 2.04(2) This typeÅ) and of architecture positioned isin comparablea syndiotactic to thearrangement, chain-likestructures and whose reported tin atoms for exhibitthe organotin(IV) a tetrahedral selenite geometry. complexes, This (Metype3 Sn)of 2architectureSeO3·H2O and is comparable (Ph3Sn)2SeO to3 [the43], chain-like as well as structuresfor the organotin(IV) reported carbonatofor the organotin(IV) complexes, (Me selenite3Sn)2CO complexes,3 and (i-Bu 3(MeSn)23COSn)32SeO[44].3·H However,2O and (Phthe3 authorsSn)2SeO3 pointed [43], as outwell that, as for despite the organotin(IV) the presence carbonato of a hydrogen complexes, atom on (Me the3Sn) nitrogen2CO3 and of (carbamatoi-Bu3Sn)2CO groups,3 [44]. noHowever, hydrogen the bonds authors are observedpointed ou betweent that, despite the chains, thewhich presence is explained of a hy- drogenby the proximityatom on the of bulkynitrogen Me of3Sn carbamato groups. Moreover, groups, no we hydrogen consider bonds that compound are observed1 is thebe- N tweenonly example the chains, of an which-coordinated is explained tin by carbamate, the proximity structurally of bulky characterized Me3Sn groups. to date.Moreover, we considerThereafter, that with compound the aim of1 is applications the only example in heterogeneous of an N-coordinated catalysis, Calderazzo tin carbamate, et al. structurallyreported the characterized preparation of to two date.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.

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Inorganics 2021, 9, 18 7 of 32 Inorganics 2021, 9, x FOR PEER REVIEW 7 of 33

Figure 1. Molecular structure of 1,, adaptedadapted fromfrom [42][42] (MERCURY(MERCURY view).view). HydrogHydrogen atoms are omitted for clarity (Sn light blue, O red, N dark blue, C grey).

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 andand 3 werewere Figure 1. Molecular structuresynthesized of 1,, adaptedadapted by fromfrom treating [[42]42] (MERCURY(MERCURY anhydrous view).view). Sn(IV) HydrogenHydrog chlorideen atoms with arediethylamine omitted for clarityand di- (Sniso light-propyla- blue, O red, NN darkdark blue,blue, CC grey).grey).mine, respectively, in toluene at room temperature, under atmospheric pressure of carbon dioxide (Scheme 10). Yields higher than 90% are reported. 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. SchemeScheme 10.10. ReactionReaction schemescheme leadingleading toto CompoundsCompounds 22 andand 3 33,,, data data fromfrom[ [45].45].

RecrystallizedRecrystallized from heptane, heptane, suitable suitable single single crystals crystals of of2 werewere2 were analyzedanalyzed analyzed byby X-rayX-ray by X-ray dif-dif- diffractionfraction leading leading toto the the resolution resolution of of the the crystallographic crystallographic structure. structure. In In 2002, in the frameframe ofof metal-organicmetal-organic chemical chemical vapor vapor deposition deposition (MOCVD) (MOCVD) investigations, investigations, Molloy Molloy et al. describedet al. de- ascribed new synthetic a new synthetic route to 3 ,route starting to 3 from,, startingstarting a solution fromfrom of aa tetrakis(solutionsolutionN ofof,N -diethylamino)tin(IV)tetrakis(tetrakis(N,,N-diethyla- complexmino)tin(IV) in hexane complex that in was hexane bubbled that withwas bubbled carbon dioxide with carbon (Scheme dioxide 11)[ 46(Scheme]. 11) [46].

Scheme 10. Reaction scheme leading to Compounds 2 and 3, data from [45].

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-diethyla- SchemeScheme 11.11. AlternativeAlternative routeroute leadingleading toto compoundcompound 33,,, data data fromfrom [[46].46]. mino)tin(IV) complex in hexane that was bubbled with carbon dioxide (Scheme 11) [46]. X-rayX-ray structuresstructures of of2 and2 andand3 revealed 3 revealedrevealed comparable comparablecomparable mononuclear mononuclearmononuclear molecules. molecules.molecules. In both InIn com-bothboth pounds,compounds, the tin the atom tin isatom eight-coordinated is eight-coordinated by four by chelating four chelatingN,N-dialkylcarbamato N,,N-dialkylcarbamato ligands, whichligands, are which exclusively are exclusivelyO-donor O (Figure-donor2 (Figure). For 3 2)., the For authors 3,, thethe authorsauthors suggest suggestsuggest that the thatthat geometry thethe ge-ge- canometry be viewed can be asviewed a distorted as a distorted square antiprism. square antiprism. From a spectroscopicFrom a spectroscopic point of point view, of similar view, 119 similarSn NMR 119Sn chemical NMR chemical shifts were shifts also were recorded also recorded for 2 and for3, at 2 −andand920.8 3,, andatat −920.8−930.0 and ppm, −930.0 re- 13 spectivelyppm, respectively (Table2). (Table In C 2). NMR, In 13 theC NMR, carbamato the carbamato carbons show carbon ones resonanceshow one atresonance 164.9 ppm at 13 Schemefor164.92, andppm 11. 166.2Alternative for 2,, ppm andand route for166.2166.23 .leading ppmppm In the forfor to case compound3.. InIn of thethe2, CP/MAS casecase 3, data ofof 2from,, CP/MASCP/MASC [46]. NMR 13 measurementsC NMR measurements confirm theconfirm chemical the chemical shift recorded shift recorded in solution. in solu A thermogravimetriction. A thermogravimetric analysis analysis was also was carried also out onX-ray3 showing structures a continuous of 2 and 3 weight revealed loss comparable between room mononuclear temperature molecules. and 300 ◦InC (andboth compounds,resulting in thethe recoverytin atom ofis SnOeigh2t-coordinatedas final residue). by four Beyond chelating their N interest,N-dialkylcarbamato as precursors ligands,for the deposition which are ofexclusively tin oxide filmsO-donor [4,5 ],(Figure tin tetracarbamates 2). For 3, the authors were also suggest used asthat versatile the ge-

ometryreactants can for be various viewed organometallic as a distorted andsquare organic antiprism. syntheses From leading a spectroscopic to alkoxystannanes point of view, [47], similartetraalkynylstannanes 119Sn NMR chemical [48] (Scheme shifts were 12), and alsoO recorded-silylurethanes for 2 and [49]. 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

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carriedcarried out out on on 3 3 showing showing a a continuous continuous weight weight loss loss be betweentween room room temperature temperature and and 300 300 °C °C (and(and resulting resulting in in the the recovery recovery of of SnO SnO2 2as as final final residue). residue). Beyond Beyond their their interest interest as as precursors precursors for the deposition of tin oxide films [4,5], tin tetracarbamates were also used as versatile Inorganics 2021, 9, 18 for the deposition of tin oxide films [4,5], tin tetracarbamates were also used as versatile8 of 32 reactantsreactants for for various various organometallic organometallic and and or organicganic syntheses syntheses leadin leadingg to to alkoxystannanes alkoxystannanes [47],[47], tetraalkynylstannanes tetraalkynylstannanes [48] [48] (Scheme (Scheme 12), 12), and and O O-silylurethanes-silylurethanes [49]. [49].

FigureFigure 2. 2. Molecular MolecularMolecular structure structure structure of of 3 of 3, ,adapted 3adapted, adapted from from from [46] [46] [(MERCURY46 (MERCURY] (MERCURY view). view). view). Hydrogen Hydrogen Hydrogen atoms atoms atoms are are are omittedomitted for for clarity clarity (Sn (Sn light light blue, blue, O O red, red, N NN dark darkdark blue, blue,blue, C CC grey). grey).grey).

SchemeSchemeScheme 12. 12.12. Reaction ReactionReaction of ofof tetrakis( tetrakis(tetrakis(NNN,,N,NN-diethylcarbamato)tin(IV)-diethylcarbamato)tin(IV)-diethylcarbamato)tin(IV) with with phenylacetyl phenylacetylenephenylacetyleneene leading leading to toto tetra(phenylethynyl)tin, tetra(phenylethynyl)tin,tetra(phenylethynyl)tin, data from [47]. datadata fromfrom [[47].47].

InIn 2010, 2010, 2010, Kemp Kemp Kemp et et etal. al. al.reported reported reported the the thereactivity reactivity reactivity of of bis(2,2,5,5-tetramethyl-2,5-disila-1-aza- bis(2,2,5,5-tetramethyl-2,5-disila-1-aza- of bis(2,2,5,5-tetramethyl-2,5-disila-1- cyclopent-1-yl)tinazacyclopent-1-yl)tincyclopent-1-yl)tin toward toward toward CO CO2 2 COleading leading2 leading to to a a silyl-substituted tosilyl-substituted a silyl-substituted carbamate carbamate carbamate of of tin(II) tin(II) of tin(II) [50]. [50]. The [The50]. exposureTheexposure exposure of of {[(CH {[(CH of {[(CH2)Me2)Me2Si]2)MeSi]2N}2N}2Si]2Sn2Sn2N} in in2 Snsolution solution in solution in in pentane pentane in pentane under under under 60 60 psig psig 60 psigof of CO CO of2 CO 2(4 (4 2bar) bar)(4 bar) at at roomatroom room temperature temperature temperature causes causes causes a a rapid rapid a rapid reaction reaction reaction involving involving involving a a color color a color change change change from from from red-orange red-orange red-orange to to yellow.toyellow. yellow. In In parallel, parallel, In parallel, GC–MS GC–MS GC–MS investigations investigations investigations on on the the on reaction thereaction reaction medium medium medium have have have shown shown shown the the co- theco- formationco-formationformation of of carbon carbon of carbon monoxide monoxide monoxide and and the the and cyclic cyclic the ether cyclic ether 2,2,5,5-tetramethyl-2,5-disila-1-oxacy- 2,2,5,5-tetramethyl-2,5-disila-1-oxacy- ether 2,2,5,5-tetramethyl-2,5-disila-1- clopentane,oxacyclopentane,clopentane, [(CH [(CH2)Me2)Me [(CH2Si]2Si]2)Me2O2O (Scheme 2(SchemeSi]2O (Scheme 13). 13). Sn Sn 413(4μ(μ).4-O){4-O){ Sn4μ(μµ2-O24-O-O){2CN[SiMe2CN[SiMeµ2-O2CN[SiMe2(CH2(CH2)]2)]2}224}((CH4μ(μ2-N=C=O)2-N=C=O)2)]2}4(µ2 2- (4N=C=O)(4) )was was isolated isolated2 (4) was as as single isolatedsingle crystals crystals as single and and crystalsanalyzed analyzed and by by X-ray analyzed X-ray diffraction diffraction by X-ray analysis. analysis. diffraction The The unprec- analysis. unprec- edentedTheedented unprecedented structure structure of of 4 structure 4 can can be be described ofdescribed4 can be as as described a a tetranuclear tetranuclear as a tetranucleartin tin cluster cluster based based tin cluster on on a a [ μ based[μ4-O]Sn4-O]Sn on4 4 coreacore [µ 4and -O]Snand including including4 core and in in includingperiphery periphery intwo two periphery bridging bridging two –N=C=O –N=C=O bridging isocyanates isocyanates –N=C=O and isocyanatesand four four bridging bridging and four – – ObridgingO2CN[SiMe2CN[SiMe –O2(CH22(CHCN[SiMe2)]2)]2 2carbamato carbamato2(CH2)] 2ligands ligandscarbamato (Figure (Figure ligands 3). 3). The The (Figure presence presence3). The of of presencethe the central central of theatom atom central μ μ4-O4-O wasatomwas explained explainedµ4-O was as as explainedresulting resulting from asfrom resulting the the cleavage cleavage from of the of CO CO cleavage2 2and and not not of from COfrom2 Oand O2 2or or not H H2O.2 fromO. Furthermore, Furthermore, O2 or H2O. theFurthermore,the reaction reaction of of {[(CH the {[(CH reaction2)Me2)Me2Si]2Si] of2N}2 {[(CHN}2Sn2Sn 2was )Mewas also2 alsoSi] 2envisaged N}envisaged2Sn was with with also OCS envisagedOCS and and CS CS with2 2leading, leading, OCS andrespec- respec- CS2 tively,leading,tively, to to an respectively, an insoluble insoluble polymeric topolymeric an insoluble material material polymeric and and to to materiala a trinuclear trinuclear and cluster tocluster a trinuclear based based on on cluster the the coordi- coordi- based nationonnation the of coordinationof three three {[(CH {[(CH2 of)Me2)Me three2Si]2Si]2 {[(CHN}2N}2Sn2Sn2 )Memoieties moieties2Si]2 N}by by2 oneSn one moieties CS CS2 2molecule molecule by one acting acting CS2 molecule as as μ μ3 3ligand. ligand. acting Pre- Pre- as viously,µviously,3 ligand. during during Previously, the the 1990s, 1990s, during Sita Sita the et et 1990s,al. al. explor explor Sitaed eted al.already already explored the the alreadyreactivity reactivity the of reactivityof Ge Ge and and ofSn Sn Ge com- com- and plexesSnplexes complexes of of silyl-containing silyl-containing of silyl-containing ami amidesdes amidestoward toward towardcarbon carbon carbondioxide dioxide dioxide [51]. [51]. In In [ 51particular, particular,]. In particular, they they focused focused they focused on the reaction involving [(Me Si) N] Sn with CO , which led to the isolation of the onon the the reaction reaction involving involving [(Me [(Me3Si)3Si)2N]2N]32Sn2Sn2 with with2 CO CO2,2 ,which which2 led led to to the the isolation isolation of of the the di- di- dimeric bisalkoxide complex, [Sn(OSiMe ) ] . However, in this case and compared to the mericmeric bisalkoxide bisalkoxide complex, complex, [Sn(OSiMe [Sn(OSiMe3)32)3]22].2 .However, 2However, in in this this case case and and compared compared to to the the Kemp’s report, the tin carbamate is not stable but leads to the formation of [Sn(OSiMe3)2]2, Kemp’sKemp’s report, report, the the tin tin carbamate carbamate is is not not stab stablele but but leads leads to to the the formation formation of of [Sn(OSiMe [Sn(OSiMe3)32)]22],2 , Inorganics 2021, 9, x FOR PEER REVIEWtrimethylsilyl isocyanate, and 1,3-bis(trimethylsilyl)carbodiimide, respectively, according9 of 33 trimethylsilyltrimethylsilyl isocyanate, isocyanate, and and 1,3-bis(trimethylsilyl)carbodiimide, 1,3-bis(trimethylsilyl)carbodiimide, respectively, respectively, according according to a ligand metathesis process. toto a a ligand ligand metathesis metathesis process. process.

SchemeScheme 13.13. Reaction scheme leading toto CompoundCompound 44,, datadata fromfrom [[50].50].

Figure 3. Molecular structure of 4, adapted from [50] (MERCURY view). Hydrogen atoms are omitted for clarity (Sn light blue, O red, N dark blue, C grey, Si light yellow).

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-dime- thylcarbamato)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.

Scheme 14. Reaction scheme leading to Compound 5, data from [52].

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SchemeScheme 13. 13. Reaction Reaction scheme leading leading to to Compound Compound 4, data 4, data from from [50]. [50].

Figure 3. Molecular structure of 4, adapted from [50] (MERCURY view). Hydrogen atoms are omitted for clarity (Sn light blue, O red, N dark blue, C grey, Si light yellow). FigureFigure 3. 3. MolecularMolecular structure structure of 4, adapted adapted fromfrom [[50]50] (MERCURY(MERCURY view). view). Hydrogen Hydrogen atoms atoms are are omittedomitted for for clarity (Sn(Sn light light blue, blue, O O red, red, N darkN dark blue, blue, C grey, C grey, Si light Si yellow).light yellow). In 2011, continuing their study on the reactivity of silyl-substituted tin(II) amides to- wardsIn 2011,heteroallenes, continuing Kemp their et study al. reported on the reactivity the interaction of silyl-substituted of CO2, OCS tin(II) and amides CS2 with to- In 2011, continuing their study on the reactivity of silyl-substituted tin(II) amides towards(Me2N) heteroallenes,2Sn leading to Kempthe formation et al. reported of insertion the products interaction characterized of CO2, OCS as bis-( andN CS,N2-dime-with wards heteroallenes, Kemp et al. reported the interaction of CO2, OCS and CS2 with (Methylcarbamato)tin(II),2N)2Sn leading to bis( theN formation,N-dimethylthiocarbamato)tin(II), of insertion products characterizedand bis(N,N-dimethyldithi- as bis-(N,N- (Medimethylcarbamato)tin(II),ocarbamato)tin(II),2N)2Sn leading torespectively the formation bis( [52].N,N ofThus,-dimethylthiocarbamato)tin(II), insertion (Me2N) products2Sn in hexane characterized solution and reacts as bis-( bis( quicklyNN,N,N--dime- thylcarbamato)tin(II),dimethyldithiocarbamato)tin(II),with carbon dioxide, at bis( roomN,N temperature-dimethylthiocarbamato)tin(II), respectively and [ 52under]. Thus, atmospheric (Me2N)2 andSn pressure, in bis( hexaneN ,toN -dimethyldithi-give solution with ocarbamato)tin(II),reactsa yield quickly almost with quantitative, carbonrespectively dioxide, the new [52]. at roomcomplex Thus, temperature (Me[(Me22N)NCO2Sn and2) 2inSn] under hexane2 (5)atmospheric (Scheme solution 14). pressure, reactsThe au- quickly withtothors give carbon describe with adioxide, yield the reaction almost at room quantitative, as exothermic. temperature the newIn the an complex solid-state,d under [(Me atmospheric compound2NCO2)2Sn] 5 2 pressure,is(5 organized) (Scheme to 14asgive )a. with Thedimer authors involving describe two the types reaction of carbamato as exothermic. ligands: In the bridging solid-state, and compoundterminally5 (isη2-chelating) organized a yield almost quantitative, the new complex [(Me2NCO2)2Sn]2 (5) (Scheme 14). The au- as a dimer involving two types of carbamato ligands: bridging and terminally (η2-chelating) thorscoordinated describe to the Sn. reaction From a supr as exothermic.amolecular point In the of solid-state, view, the existence compound of intermolecular 5 is organized as a coordinatedinteractions toSn···O Sn. From[2.8499(18) a supramolecular Å] between ne pointighboring of view, dimers the existence and involving of intermolecular one oxygen dimer involving two types of carbamato ligands: bridging and terminally (η2-chelating) interactionsatom of terminal Sn···O carbamato [2.8499(18) ligands, Å] between leads neighboring to the propagation dimers of and a polymer involving chain one oxygen(Figure coordinated to Sn. From a supramolecular point of view, the existence of intermolecular atom4). The of terminalcoordination carbamato geometry ligands, of tin leads atoms to thecan propagation be viewed as of a polymerhighly distorted chain (Figure trigonal4) . interactionsThebipyramid. coordination Sn···OIn solution, geometry [2.8499(18) the of 119 tinSn Å] atoms NMR between can spectrum be ne viewedighboring exhibits as a highly dimersonly distortedone and resonance involving trigonal at bipyra-δ one –613 oxygen 119 atommid.ppm. of In In solution,terminal the 13C { the1 H}carbamato NMRSn NMR spectrum, ligands, spectrum the leads exhibitscarbamato to the only carbonpropagation one resonance also displays of at aδ polymer–613 only ppm. one chain signal, In the (Figure 13 1 4).not C{The makingH} coordination NMR it spectrum,possible geometry tothe diff carbamatoerentiate of tin carbonatomsthe bridging alsocan displaysbe and viewed terminal only as one amodes signal,highly highlighted not distorted making itintrigonal bipyramid.possiblecrystals toof differentiate 5In. solution, the the bridging 119Sn andNMR terminal spectrum modes exhibits highlighted only in one crystals resonance of 5. 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.

SchemeScheme 14.14. ReactionReaction schemescheme leadingleading toto CompoundCompound5 5,, datadata fromfrom [ [52].52]. 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-Pr C H )} ]SnNi-Pr and Scheme 14. Reaction scheme leading to Compound 5, data from [52].2 6 3 2 2 [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 (Figure5). This mode of coordination is

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unusual for carbamates and is similar to that observed in the case of hemicarbonato deriva- Figure 4. Molecular structuretives of 5 [, 1adapted]. The X-rayfrom [52] structure (MERCURY of complex view). Hydrog7 hasen not atoms been are reported omitted butfor clarity the spectroscopic (Sn light Inorganics 2021, 9, x FOR PEER REVIEW 10 of 33 blue, O red, N dark blue, C datagrey). are Intermolecular very similar hydrogen to 6 (Table bonds2) are and shown the authors by cyan dash mention lines. that for both compounds, the insertion reaction is irreversible, even under reduced pressure. 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 hemi- Figure 4. Molecular structure carbonato of 5, adapted derivatives from [[52]52] [1]. (MERCURY The X-ray view). structure Hydrog Hydrogen ofen complex atoms are 7 omittedhas not forbeen clarity reported (Sn light but the blue, O red, N dark blue, C grey).spectroscopic Intermolecular data are hydrogen very similar bondsbonds areare to shown shown6 (Table byby cyancyan2) and dashdash the lines.lines. authors mention that for both compounds, the insertion reaction is irreversible, even under reduced pressure. 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 termi- Inorganics 2021, 9, x FOR PEER REVIEWnally O-bonded to Sn [Sn−O = 2.1346(16) Å] and its orientation can be viewed almost per- 11 of 33

Schemependicular 15.15. Reaction to the scheme NCCCN leading plane toto CompoundsCompounds of the β-diketiminate 66 andand 7,7, datadata fromfrom ligand [[53].53]. (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.

Scheme 15. Reaction scheme leading to Compounds 6 and 7, data from [53].

Figure 5. Molecular structureFigure of 6, adapted 5. Molecular from structure [53] (MERCURY of 6, adapted view). from Hydrog [53] (MERCURYen atoms are view). omitted Hydrogen for clarity atoms (Sn are light blue, O red, N dark blue, C grey).omitted for clarity (Sn light blue, O red, N dark blue, C grey).

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

CSD Entry Sn−O(C) Sn−N(C) C−O(Sn) C=O N−C(O2) O−C−O N−C−O Compounds Deposition Ref. (Å) (Å) (Å) (Å) (Å) (deg) (deg) Number 2.23(2) 1.27(4) 120(2) MESNCB Me3SnNHC(=O)–OSnMe3 (1) 2.04(2) na 1.35(4) 126(3) [42] 2.24(2) 1.30(3) 114(3) 1211377 2.146(5) 1.283(7) 119.9(6) 2.217(4) 1.276(8) 122.4(6) 2.136(5) 1.279(7) 1.33(1) 117.7(6) 122.9(6) 2.195(4) 1.260(8) 1.352(9) 117.3(6) 119.7(6) MULPUE Sn(O2CNi-Pr2)4 (2) na na [45] 2.198(4) 1.268(8) 1.34(1) 116.1(6) 120.6(6) 196069

2.135(5) 1.269(7) 1.333(8) 117.2(6) 122.2(6) 2.219(4) 1.275(8) 116.1(6) 2.126(5) 1.282(7) 121.2(6) 2.156(1) 1.292(2) 121.5(2) 2.209(1) 1.277(2) 1.338(2) 117.0(1) UGOZUL Sn(O2CNEt2)4 (3) na na 121.7(1) [46] 2.163(1) 1.298(2) 1.331(2) 116.6(1) 196788 121.7(2) 2.196(1) 1.276(2) 2.331(2) 1.274(58) 2.164(3) 1.282(5) 121.3(4) 2.226(3) 1.272(5) 117.6(4) Sn4(μ4-O){μ2- 2.429(3) 1.349(5) 121.0(4) 1.291(5) 118.2(4) IFOXEH O2CN[SiMe2(CH2)]2}4 2.716(4) na na 1.345(5) 121.1(4) [50] 1.273(6) 120.3(4) 664586 (μ2-N=C=O)2 (4) 2.165(3) 1.355(5) 121.6(4) 1.289(5) 116.9(4) 2.379(2) 1.301(6) 121.5(4) 2.225(3) 1.257(5) 2.458(2) 2.332(1) 1.272(3) 120.2(2) 2.235(2) 1.280(3) 1.338(3) 118.8(2) 121.0(2) UYOBIU [(Me2NCO2)2Sn]2 (5) na na [52] 2.432(2) 1.263(3) 1.336(3) 121.8(2) 121.3(2) 806650 2.115(2) 1.295(3) 116.9(2) [HC{(Me)CN(2,6-i- 116.6(2) MIYHIN 2.1346(16) na 1.304(3) 1.238(3) 1.366(3) 121.7(2) [53] Pr2C6H3)}2]SnOC(O)Ni-Pr2 (6) 121.7(2) 791226

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

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

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

13C{1H} NMR IR 119Sn{1H} NMR Compounds -NC(O)O- ν(C=O) Ref. (δ, ppm) (δ, ppm) (cm−1) a a Sn(O2CNi-Pr2)4 (2) −920.8 164.4 na [45] b b Sn(O2CNEt2)4 (3) −930.0 166.2 1612 [46] b b Sn4(µ4-O){µ2-O2CN[SiMe2(CH2)]2}4(µ2-N=C=O)2 (4) −316.0 167.9 na [50] a a [(Me2NCO2)2Sn]2 (5) −613.0 164.5 1651 [52] b b [HC{(Me)CN(2,6-i-Pr2C6H3)}2]SnOC(O)Ni-Pr2 (6) −394.0 161.75 1595, 1575, 1552, and 1524 [53] b b [HC{(Me)CN(2,6-i-Pr2C6H3)}2]SnOC(O)NH(2,6-i-Pr2C6H3)(7) −398.0 161.59 1624, 1554, 1526, and 1517 [53] a b Measured in chloroform-d; 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 au- Inorganics 2021, 9, 18 12 of 32

thors to a ﬂattened helix. Triphenyltin moieties are bridged by bidentate formate ligands (CSD refcode: FAJZIZ, 1151980; Figure6). 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; Figure7)[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; Figure8) , [(PhCH2)3Sn(O2CH)]n (CCDC ref.: EKONAU, 783725), and [Cy3Sn(O2CH)]n (Cy = cyclo- hexyl) (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; Figure9) 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 InorganicsInorganics 2021 2021, 9, ,9 x, xFOR FOR PEER PEER REVIEW REVIEW 1313 of of 33 33 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].

FigureFigureFigure 6.6. 6.Molecular Molecular Molecular structurestructure structure ofof of [Ph[Ph [Ph33Sn(OSn(O3Sn(O22CH)]2CH)]n n(CSD (CSD refcode: refcode: FAJZIZ, FAJZIZ, 1151980), 1151980), adapted adapted from from [[58] 58[58]] (MERCURY(MERCURY (MERCURY view).view). view). HydrogenHydrogenHydrogen atomsatoms atoms areare are omittedomitted omitted forfor for clarityclarity clarity except except except that that that of of of the the the formate formate formate group group group (Sn (Sn (Sn light light light blue, blue, blue, O O O red, red, red, C C grey,C grey, grey, H H white).H white). white).

Figure 7. Inﬁnite sheets based-structure of [Me2Sn(O2CH)2]n (CCDC ref.: SIFLEY, 1258788), adapted FigureFigure 7. 7. Infinite Infinite sheets sheets based-structure based-structure of of [Me [Me2Sn(O2Sn(O2CH)2CH)2]2n] n(CCDC (CCDC ref.: ref.: SIFLEY, SIFLEY, 1258788), 1258788), adapted adapted from from [59] [59] (MERCURY (MERCURY from [59] (MERCURY view). Hydrogen atoms are omitted for clarity (Sn light blue, O red, C grey). view).view). Hydrogen Hydrogen atoms atoms are are omitted omitted for for clarity clarity (Sn (Sn light light blue, blue, O O red, red, C C grey). grey).

FigureFigure 8. 8. Polymeric Polymeric structure structure of of [ n[n-Bu-Bu2Sn(O2Sn(O2CH)2CH)2]2n] n(CCDC (CCDC ref.: ref.: EKONEY, EKONEY, 783726), 783726), adapted adapted from from [60] [60] (MERCURY (MERCURY view). view). HydrogenHydrogen atoms atoms are are omitted omitted for for clarity clarity and and for for each each n n-butyl-butyl chain; chain; only only the the α α-carbon-carbon atoms atoms bonded bonded to to tin tin are are shown shown (Sn (Sn lightlight blue, blue, O O red, red, C C grey). grey).

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Figure 6. Molecular structure of [Ph3Sn(O2CH)]n (CSD refcode: FAJZIZ, 1151980), adapted from [58] (MERCURY view). Hydrogen atoms are omitted for clarity except that of the formate group (Sn light blue, O red, C grey, H white).

Inorganics 2021, 9, 18 13 of 32 Figure 7. Infinite sheets based-structure of [Me2Sn(O2CH)2]n (CCDC ref.: SIFLEY, 1258788), adapted from [59] (MERCURY view). Hydrogen atoms are omitted for clarity (Sn light blue, O red, C grey).

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Figure 8. Polymeric structure of [n-Bu2Sn(O22CH)CH)22]n] n(CCDC(CCDC ref.: ref.: EKONEY, EKONEY, 783726), 783726), adapted adapted from from [60] [60] (MERCURY (MERCURY view). view). Hydrogen atoms are omitted for clarity and for each n-butyl chain; only the α-carbon atoms bonded to tin are shown (Sn Inorganics 2021, 9, x FOR PEER REVIEW 14 of 33 light blue, O red, C grey).

FigureFigure 9. 9.MolecularMolecular structure structure of Dmp of3SnOC(O)H Dmp SnOC(O)H (dmp = 2,6-dimethylphenyl) (dmp = 2,6-dimethylphenyl) (CCDC ref.: (CCDC ref.: EKONIC, FigureEKONIC, 9. 783727), Molecular adapted structure from [60] of (MERCURY Dmp3SnOC(O)H view). Hydrogen (dmp atoms = 2,6-dimethylphenyl) are omitted for clarity (CCDC ref.: EKONIC,except783727), that adapted of783727), the formate from adapted group [60] (Sn (MERCURYfrom light [60] blue, (MERCURY O view).red, C grey, Hydrogen Hvi white).ew). Hydrogenatoms are omittedatoms are for omitted clarity exceptfor clarity that of exceptthe formate that of group the formate (Sn light group blue, (Sn O red,light C blue, grey, O H red, white). C grey, H white).

Figure 10. Molecular structure of Sn(TPP)[(OC(O)H]2 (TPP = dianion of 5,10,15,20-tetraphenylporphyrin) (CCDC ref.: EPAZAX, 711428), adapted from [61] (MERCURY view). Hydrogen atoms are omitted for clarity except that of the formate group (Sn light blue, O red, N dark blue, C grey, H white).

3.2. X-ray Crystal Structures of Formato Tin Complexes Resulting from Reactivity with CO2 Figure 10. Molecular structure of Sn(TPP)[(OC(O)H]2 (TPP = dianion of 5,10,15,20- FigureWhile 10. theMolecular formation structure of the above of Sn(TPP)[(OC(O)H] compounds required2 the(TPP use = ofdianion formic acidof 5,10,15,20-tetraphenylpor- in their tetraphenylporphyrin) (CCDC ref.: EPAZAX, 711428), adapted from [61] (MERCURY view). Hydro- phyrin)preparation, (CCDC there ref.: are alsoEPAZAX, a few examples 711428), of adapted formato fromtin complexes [61] (MERCURY directly isolated view). un- Hydrogen atoms are omitteddergen carbon atoms for dioxide areclarity omitted atmosphere, except for that clarity resulting of the except formate from that the group of insertion the formate(Sn oflight CO group 2blue, into the (SnO red,Sn–H light N bond. blue,dark Oblue, red, C N grey, dark H blue, white).InC our grey, opinion, H white). 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 re- 3.2. X-ray Crystal Structures of Formato Tin Complexes Resulting from Reactivity with CO2 sulting from hydrostannylation reactions of carbon dioxide, ketones, aldehydes, alkynes and carbodiimidesWhile the formation with the tin(II) of thehydride above precursor, compounds [HC{(Me)CN(2,6- requiredi-Pr the2C 6useH3)} 2of]SnH formic acid in their preparation,[62]. In the case thereof the reaction are also involving a few COexamples2, bubbling of at formato room temperature tin complexes very quickly directly isolated un- causes a change in color (from yellow to colorless) of the reaction mixture (toluene solu- der carbon dioxide atmosphere, resulting from the insertion of CO2 into the Sn–H bond. tion). The treatment with n-hexane then leads to the formation of a microcrystalline pow- Inder our (at −opinion,32 °C) characterized six complexes, as being publishedthe stannylene mainly formate during complex, the [HC{(Me)CN(2,6- last decade and characterized byi-Pr X-ray2C6H3)} 2diffraction,]SnOC(O)H (8 correspond) (Scheme 16). Theto this presence catego of thery formateof compounds ligand evidenced (Table by 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

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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 (Table3). 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). InorganicsInorganics 2021 2021, ,9 9, ,x x FOR FOR PEER PEER REVIEW REVIEW 1515 ofof 3333 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 infraredinfrared was was confirmed conﬁrmedconfirmed by byby single-crystal single-crystal X-ray X-rayX-ray diffraction. diffraction. The The molecular molecular structure structure shows showsshows moreovermoreover a a distorted distorted pseudopseudo-tetrahedral-tetrahedral environment environment for for the the tin tintin atom, atom,atom, completed completedcompleted by byby one oneone electronelectron lone lone pair pair (Figure (Figure 11). 1111).).

SchemeScheme 16. 16. Reaction Reaction scheme scheme leading leading to to Compound Compound 8 8,, data datadata from fromfrom [62]. [[62].62].

Figure 11. Molecular structure of 8, adapted from [62] (MERCURY view). Hydrogen atoms are omitted FigureFigure 11. 11. Molecular Molecular structure structure of of 8 8, ,adapted adapted from from [62] [62] (MERCURY (MERCURY view). view). Hydrogen Hydrogen atoms atoms are are for clarity except that of the formate group (Sn light blue, O red, N dark blue, C grey, H white). omittedomitted for for clarity clarity except except that that of of the the formate formate group group (Sn (Sn light light blue, blue, O O red, red, N N dark dark blue, blue, C C grey, grey, H H white). white).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. DuringDuring thethe periodperiod 2007–2010, 2007–2010, thethe AlbertinAlbertin’s’s groupgroup isolatedisolated several several formato formato tintin com-com- Three of these complexes were characterized by single-crystal X-ray diffraction studies. plexesplexes by by studying studying the the reactivity reactivity of of transiti transition-metalon-metal stannyl stannyl complexes complexes with with carbon carbon diox- diox- Firstly, the reaction of Re(SnH3)(CO)2[P(OEt)3]3, in solution in benzene and at room ide.ide. Three Three of of these these complexes complexes were were characteri characterizedzed by by single-crystal single-crystal X-ray X-ray diffraction diffraction stud- stud- temperature, with CO2 (under atmospheric pressure) and during 24 h led to the binu- ies.ies. Firstly,Firstly, thethe reactionreaction ofof Re(SnHRe(SnH33)(CO))(CO)22[P(OEt)[P(OEt)33]]33, , inin solutionsolution inin benzenebenzene andand atat roomroom clear OH-bridging bis(formate) complex [Re{Sn[OC(O)H]2(µ-OH)}(CO)2{P(OEt)3}3]2 (9) temperature,temperature, with with CO CO22 (under (under atmospheric atmospheric pressure) pressure) and and during during 24 24 h h led led to to the the binuclear binuclear OH-bridgingOH-bridging bis(formate)bis(formate) complexcomplex [Re{Sn[OC(O)H][Re{Sn[OC(O)H]22((μμ-OH)}(CO)-OH)}(CO)22{P(OEt){P(OEt)33}}33]]22 ((99)) (Scheme(Scheme 17)17) [63].[63]. TheThe authorsauthors describeddescribed thethe corecore ofof 99 asas aa dimericdimeric complexcomplex inin whichwhich twotwo octahedrallyoctahedrally coordinatedcoordinated rheniumrhenium unitsunits areare joinedjoined byby aa bridgingbridging tetraformatebis(tetraformatebis(μμ-hy--hydroxo)ditindroxo)ditin moiety.moiety. TinTin atomsatoms exhibitexhibit aa highlyhighly distorteddistorted squaresquare pyramidalpyramidal coordinationcoordination 1 geometry,geometry, bearing bearing two two η η1-formate-formate ligands ligands per per metal metal atom. atom. The The central central Sn Sn22OO22 distannoxane distannoxane ringring isis planarplanar (Figure(Figure 12).12). InfraredInfrared andand 1313C{C{11H}H} NMRNMR spectraspectra attestattest alsoalso thethe presencepresence ofof −1 formatesformates byby showingshowing twotwo bandsbands atat 1674–15941674–1594 cmcm−1 ((ννOCOOCO, , asym)asym) andand oneone resonanceresonance atat δδ 166166 ppm ppm [O [OCC(O)H],(O)H], respectively respectively (Table (Table 4). 4).

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(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 1 geometry, bearing two η -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 InorganicsInorganics 2021 2021, ,9 9, ,x x FOR FOR PEER PEER REVIEW REVIEW −1 1616 ofof 3333 formates by showing two bands at 1674–1594 cm (νOCO, asym) and one resonance at δ 166 ppm [OC(O)H], respectively (Table4).

SchemeScheme 17. 17. Reaction Reaction scheme scheme leading leading to to compound compound 9 9, ,data data from from [63]. [[63].63].

FigureFigureFigure 12. 12. 12. Molecular MolecularMolecular structure structure structure of of of9 9, ,adapted 9adapted, adapted from from from [63] [63] [ 63(MERCURY (MERCURY] (MERCURY view). view). view). Hydrogen Hydrogen Hydrogen atoms atoms atoms are are are omittedomittedomitted for for for clarity clarity clarity except except except those those those of of hydroxyl ofhydroxyl hydroxyl and and and formate formate formate groups groups groups (Re (Re blue, (Reblue, blue, Sn Sn light light Sn lightblue, blue, blue,O O red, red, O P P red, orange, C grey, H white). orange,P orange, C grey, C grey, H white). H white).

InInIn 2008,2008, 2008, thethe the same same same groupgroup group reportedreported reported aa similar asimilar similar structure structure structure by by bystandingstanding standing thethe theruthenium-cy-ruthenium-cy- ruthenium- clopentadienyl-trihydridostannyl complex, Ru(SnH3)(Cp)[P(OEt)3](PPh3), under a CO2 at- clopentadienyl-trihydridostannylcyclopentadienyl-trihydridostannyl complex, complex, Ru(SnH Ru(SnH3)(Cp)[P(OEt)3)(Cp)[P(OEt)3](PPh3](PPh3), under3), under a CO a2 CO at-2 mospheremosphereatmosphere forfor for 55 h 5h (Scheme h(Scheme (Scheme 18)18) 18 [64].)[[64].64 ]. TheThe The resuresu resultingltinglting formateformate formate speciesspecies species waswas was characterizedcharacterized asas [Ru[Sn{OC(O)H}[Ru[Sn{OC(O)H}[Ru[Sn{OC(O)H}22(2(μ(μµ-OH)](Cp){P(OEt)-OH)](Cp){P(OEt)-OH)](Cp){P(OEt)33}(PPh3}(PPh}(PPh33)])]32)]2 (2(1010(10),), ), exhibitingexhibiting exhibiting aa a strustru structurecturecture basedbased based onon on twotwo μμµ-OH-OH-OH groups groups groups bridging bridging the the tin tin atoms atoms (Figure (Figure 13). 1313).). 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

SchemeScheme 18. 18. Reaction Reaction scheme scheme leading leading to to Compound Compound 10 10, ,data data from from [64]. [64].

Inorganics 2021, 9, x FOR PEER REVIEW 16 of 33

Scheme 17. Reaction scheme leading to compound 9, data from [63].

Inorganics 2021, 9, 18 Figure 12. Molecular structure of 9, adapted from [63] (MERCURY view). Hydrogen atoms are16 of 32 omitted for clarity except those of hydroxyl and formate groups (Re blue, Sn light blue, O red, P orange, C grey, H white).

unequivocallyIn 2008, the demonstrated same group byreported1H NMR a similar spectroscopy structure showing by standing a multiplet the ruthenium-cy- at δ −11 ppm. clopentadienyl-trihydridostannylThe osmium atom is hexacoordinated complex, according Ru(SnH to a distorted3)(Cp)[P(OEt) octahedral3](PPh3 geometry), under a bearingCO2 at- mospherefour phosphonite for 5 h ligands,(Scheme one 18) hybride [64]. The ligand, resu andlting one formate hydroxobis(formate) species was characterized stannyl ligand. as Inorganics 2021, 9, x FOR PEER REVIEW[Ru[Sn{OC(O)H} 2(μ-OH)](Cp){P(OEt)3}(PPh3)]2 (10), exhibiting a structure based on17 oftwo 33 The hydride and stannyl ligand are in cis positions. Regarding the coordination of the tin μatom,-OH Sngroups is tetracoordinate bridging the tin displaying atoms (Figure a tetrahedral 13). geometry (Figure 14).

Inorganics 2021, 9, x FOR PEER REVIEW 17 of 33

Scheme 18. Reaction scheme leading to Compound 10, data from [[64].64].

Figure 13. Molecular structure of 10, adapted from [64] (MERCURY view). Hydrogen atoms are omitted for clarity except those of hydroxyl and formate groups (Ru blue green, Sn light blue, O red, P orange, C grey, H white).

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. FigureFigureThe osmium 13. 13. MolecularMolecular atom structureis structure hexacoordinated of of 1010, adapted, adapted accordin from from [64]g [64 to (MERCURY] a (MERCURY distorted view). octahedral view). Hydrogen Hydrogen geometry atoms atoms are bear- are omittedomitteding four for for phosphonite clarity clarity except except ligands,those those of hydroxylone hybride and formate ligand, groups and one (Ru(Ru hydroxobis(formate) blueblue green,green, SnSn lightlight blue,blue, stannyl OO red, red,Pligand. orange, P orange, The C grey, hydride C grey, H white). H and white). stannyl ligand are in cis positions. Regarding the coordination of the tin atom, Sn is tetracoordinate displaying a tetrahedral geometry (Figure 14). 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 bear- Schemeing four 19. phosphonite Reaction scheme ligands, leading one to Compoundhybride ligand, 11, data and from one [65]. hydroxobis(formate) stannyl Scheme 19. Reaction scheme leading to Compound 11, data from [65]. 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).

Scheme 19. Reaction scheme leading to Compound 11, data from [65].

InorganicsInorganicsInorganics2021 20212021,,,9 99,,, 18 xx FORFOR PEERPEER REVIEWREVIEW 181817 of of 323333

FigureFigure 14.14.14. Molecular Molecular structurestructure of ofof 11 1111,,, adapted adaptedadapted from fromfrom [ [65]65[65]] (MERCURY (MERCURY(MERCURY view). view).view). Ph PhPh and andand OEt OEtOEt substituents substitu-substituents are omitted for clarity (Os blue, Sn light blue, O red, P orange, C grey, H white). areents omitted are omitted for clarity for clarity (Os blue, (Os Snblue, light Sn blue,light Oblue, red, O P red, orange, P orange, C grey, C Hgrey, white). H white).

InInIn 2016,2016,2016, StrohmannStrohmannStrohmann etetet al.al.al. publishedpublishedpublished thethe crystalcrystalcrystal structurestructurestructure ofofof diaquadi-diaquadi-diaquadi-µμμ-hydroxido--hydroxido--hydroxido- 3 3 tris[trimethyltin(IV)]tris[trimethyltin(IV)]tris[trimethyltin(IV)] diformatotrimethylstannate(IV),diformatotrimethylstannate(IV),diformatotrimethylstannate(IV), [(Me3Sn)3( µ-OH)2(H2O)[(Me[(Me2][Me3Sn)Sn)33Sn((μμ-- 2 2 2 3 2 {OC(O)H}OH)OH)2(H(H2O)O)2](2][Me][Me12), obtained3Sn{OC(O)H}Sn{OC(O)H} as byproduct2]] ((1212),), obtainedobtained from trapping asas byproductbyproduct reactions fromfrom with trappingtrapping trimethyltin reactionsreactions chloride withwith 2 andtrimethyltintrimethyltin formedfrom chloridechloride atmospheric andand formedformed CO2 fromfrom[66]. atmosphericatmospheric The structure COCO of 122 [66].[66].consists TheThe structurestructure of a short ofof trimethyltin 1212 consistsconsists hydroxideofof aa shortshort trimethyltin chaintrimethyltin (tristannoxane), hydroxidehydroxide positively chainchain (trist(trist charged,annoxane),annoxane), and positively apositively bisformatostannate, charged,charged, andand negatively aa bisfor-bisfor- chargedmatostannate,matostannate, (Figure negativelynegatively 15). From chargedcharged a supramolecular (Figure(Figure 15).15). point FromFrom of aa view,supramolecularsupramolecular formate ligands pointpoint of bridgeof view,view, four for-for- cationicmatemate ligandsligands tristannoxanes bridgebridge fourfour via cationiccationic hydrogen-bonding tristannoxanestristannoxanes interactions viavia hydrogen-bondinghydrogen-bonding leading to a two-dimensionalinteractionsinteractions lead-lead- network.inging toto aa two-dimensionaltwo-dimensional network.network.

Figure 15. Molecular structure of 12, adapted from [66] (MERCURY view). Hydrogen atoms of methyl groups are omitted FigureFigure 15.15. MolecularMolecular structurestructure ofof 1212,, adaptedadapted fromfrom [66][66] (MERCURY(MERCURY view).view). HydrHydrogenogen atomsatoms ofof methylmethyl groupsgroups areare omittedomitted forforfor clarity clarityclarity (Sn (Sn(Sn light lightlight blue, blue,blue, O OO red, red,red, C CC grey, grey,grey, H HH white). white).white). Lately, in 2017, Jones et al. reported the efﬁcient and highly selective use of the two- Lately,Lately, inin 2017,2017, JonesJones etet al.al. reportedreported thethe efficientefficient andand highlyhighly selectiveselective useuse ofof thethe two-two- coordinate amido-tin(II) hydride complex, LSnH (L = -N(C66H2 -i-Pr{C(H)Ph222}2-4,2,6)(Si- coordinatecoordinate amido-tin(II)amido-tin(II) hydridehydride complex,complex, LSnHLSnH (L(L == -N(C-N(C6HH2--ii-Pr{C(H)Ph-Pr{C(H)Ph2}}2-4,2,6)(Si--4,2,6)(Si-ii-- i-Pr3 3)), acting as catalytic species for the reductive hydroboration of CO22 into methanol PrPr3)),)), actingacting asas catalyticcatalytic speciesspecies forfor thethe reductivereductive hydroborationhydroboration ofof COCO2 intointo methanolmethanol equivalents, MeOBR2 (2-methoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane2 and catecholato- equivalents,equivalents, MeOBRMeOBR2 (2-methoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane(2-methoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane andand cate-cate- boryl methoxide), using boranes, HBR2, as hydrogen2 sources [67]. The tin formate complex, cholatoborylcholatoboryl methoxide),methoxide), usingusing boranes,boranes, HBRHBR2,, asas hydrogenhydrogen sourcessources [67].[67]. TheThe tintin formateformate LSn(κ2-O,O’-O CH) (13), was thus isolated during catalytic studies and also by exclusively 2 2 2 complex,complex, LSn(LSn(κκ2--OO,,OO’-O’-O2CH)CH) ((1313),), waswas thusthus isolatedisolated duringduring catalyticcatalytic studiesstudies andand alsoalso byby reacting LSnH with CO (Scheme 20) or alternatively from LSnBr with potassium for- 2 2 exclusivelyexclusively reactingreacting LSnHLSnH withwith COCO2 (Scheme(Scheme 20)20) oror alternativelyalternatively fromfrom LSnBrLSnBr withwith potas-potas- mate,sium formate, KOC(O)H. KOC(O)H. Compound Compound13 was crystallographically13 was crystallographically characterized characterized and consists and con- of asium monomeric formate, complex KOC(O)H. in whichCompound the tin 13 atom was iscrystallographically unusually chelated characterized by a formate and ligand consists of a monomeric complex in which the tin atom is unusually chelated by a formate (Figuresists of 16a ).monomeric complex in which the tin atom is unusually chelated by a formate ligandligand (Figure(Figure 16).16).

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Inorganics 2021 9 Inorganics 2021, 9, 18x FOR PEER REVIEW 1918 of of 33 32

SchemeScheme 20. 20. Reaction Reaction scheme scheme leading leading to Compound to Compound 13, data from 13, data [[67].67]. from [67].

Figure 16. Molecular structure of 13, adapted from [67] (MERCURY view). Hydrogen atoms are omitted for clarity except that of the formate group (Sn light blue, O red, N dark blue, Si orange, C grey, H white).

Table 3. Comparison of selected structural parameters found in formates of organotin complexes. Figure 16. Molecular structure of 13, adapted from [67] (MERCURY view). Hydrogen atoms are Figure 16. Molecular structure of 13, adapted from [67] (MERCURY view). Hydrogen atoms are omittedCSD Entry for clarity except omitted for clarity exceptSn that−O(C) of the formateC−O(Sn) groupC−O (Sn lightSn blue,−O−C O red,O− NC− darkO blue, Si orange, that of the formate groupCompounds (Sn light blue, O red, N dark blue, Si orange, C grey, H white). Deposition Ref. (Å) (Å) (Å) (deg) (deg) C grey, H white). Number COPLOJ [HC{(Me)CN(2,6-Table 3. Comparisoni-Pr2C6H3)}2]SnOC(O)H of selected (8) structural2.1353(15) parameters 1.299(2) found1.209(3) in formates116.40(13) of organotin126.78(18) complexes. [62] 709476 Table 3. Comparison of selected structural parameters found in formates of organotin complexes. 2.040(5) 1.29(1) 1.19(2) 123.1(6) 126(1) CIHYUO [Re{Sn[OC(O)H]2(μ-OH)}(CO)2{P(OEt)3}3]2 (9) [63]CSD Entry 2.071(7)Sn −O(C)1.26(1) C−O(Sn)1.21(1) C132.2(6)−O Sn128.8(9)−O−C CSD Entry655465O−C− O Sn−O(C) C−O(Sn) C−O Sn−O−C O−C−O CompoundsCompounds Deposition Ref. Deposition Ref. (Å)2.210(2) 1.262(4)(Å) (Å)1.216(5) (deg)128.7(2) (deg)127.9(3) KOMBUK [Ru[Sn{OC(O)H}2(μ-OH)](Cp){P(OEt)3}(PPh3)]2 (10) (Å) (Å) (Å) (deg) Number (deg) [64] 2.069(2) 1.274(5) 1.205(6) 133.5(2) 125.7(4) 708897 Number i 8 COPLOJ [HC{(Me)CN(2,6- -Pr2C6H3)}2]SnOC(O)H ( ) 2.1353(15)2.061(5) 1.299(2)1.16(1) 1.209(3)1.11(1) 116.40(13)132.8(7) 126.78(18)131(1) 709476ALAGUQ[62 ] COPLOJ [HC{(Me)CN(2,6-OsH[Sn(OH){OC(O)H}i-Pr2C6H2][PPh(OEt)3)}2]SnOC(O)H2]4 (11) (8) 2.1353(15) 1.299(2) 1.209(3) 116.40(13) 126.78(18) [65] [62] 2.040(5)2.089(5) 1.29(1)1.25(1) 1.19(2)1.21(1) 123.1(6)124.9(7) 126(1)124(1) CIHYUO813529 [Re{Sn[OC(O)H] (µ-OH)}(CO) {P(OEt) } ] (9) [63] 709476 2 2 3 3 2 2.071(7)2.2991(17) 1.26(1) 1.21(1) 132.2(6) 128.8(9) 655465EWIGIC [(Me3Sn)3(μ-OH)2(H2O)2][Me3Sn{OC(O)H}2] (12) 2.040(5)1.269(3) 1.29(1)1.224(3) 1.19(2)125.94(17) 123.1(6) 128.1(3) 126(1) [66] CIHYUO 2.210(2) 1.262(4) 1.216(5) 128.7(2) 127.9(3) KOMBUK [Re{Sn[OC(O)H][Ru[Sn{OC(O)H} 2((µμ-OH)](Cp){P(OEt)-OH)}(CO)2{P(OEt)}(PPh )] 3(}103])2 (9) 2.2990(17) 1505528[ 64] [63] 2 3 3 2 2.069(2) 1.274(5) 1.205(6) 133.5(2) 125.7(4) 708897 2.353(2)2.071(7) 1.109(4) 1.26(1) 1.21(1)91.4(2) 132.2(6) ACEQUX128.8(9) 655465 N(C6H2-i-Pr{C(H)Ph2)2-4,2,6)(Si-i-Pr3)Sn(κ2-O,O’-O2CH) (13) 2.061(5) 1.16(1) 1.11(1)na 132.8(7) 131(1)125.6(4) ALAGUQ [67] OsH[Sn(OH){OC(O)H}2][PPh(OEt)2]4 (11) 2.333(2)2.210(2) 1.310(4) 1.262(4) 1.216(5)87.5(2) 128.7(2) 1518144127.9(3)[ 65 ] KOMBUK [Ru[Sn{OC(O)H}2(μ-OH)](Cp){P(OEt)3}(PPh3)]2 (10) 2.089(5) 1.25(1) 1.21(1) 124.9(7) 124(1) 813529 [64] 2.2991(17)2.069(2) 1.274(5) 1.205(6) 133.5(2) EWIGIC125.7(4) 708897 [(Me Sn) (µ-OH) (H O) ][Me Sn{OC(O)H} ](12) 1.269(3) 1.224(3) 125.94(17) 128.1(3) [66] 3 3 2 2 2 3 2 2.2990(17)2.061(5) 1.16(1) 1.11(1) 132.8(7) 1505528 131(1) ALAGUQ OsH[Sn(OH){OC(O)H}2][PPh(OEt)2]4 (11) 2.353(2) 1.109(4) 91.4(2) ACEQUX [65] 2 na 125.6(4) [67] N(C6H2-i-Pr{C(H)Ph2)2-4,2,6)(Si-i-Pr3)Sn(κ -O,O’-O2CH) (13) 2.333(2) 2.089(5)1.310(4) 1.25(1) 87.5(2)1.21(1) 124.9(7) 1518144 124(1) 813529 2.2991(17) EWIGIC [(Me3Sn)3(μ-OH)2(H2O)2][Me3Sn{OC(O)H}2] (12) 1.269(3) 1.224(3) 125.94(17) 128.1(3) [66] 2.2990(17) 1505528 2.353(2) 1.109(4) 91.4(2) ACEQUX N(C6H2-i-Pr{C(H)Ph2)2-4,2,6)(Si-i-Pr3)Sn(κ2-O,O’-O2CH) (13) na 125.6(4) [67] 2.333(2) 1.310(4) 87.5(2) 1518144

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

1H 119Sn{1H} NMR 13C{1H} NMR IR Compounds NMR Ref. (δ, ppm) −OC(H)O (δ, ppm) (cm−1) −OC(H)O (δ, ppm) 1641 ν i 8 − a a 8.97 ( C=O) [HC{(Me)CN(2,6- -Pr2C6H3)}2]SnOC(O)H ( ) 360 166.92 3J(119Sn,H) = 52 Hz 2700 [62] (νC−H)

b b bc 1665, 1594 [Re{Sn[OC(O)H]2(µ-OH)}(CO)2{P(OEt)3}3]2 (9) −487.7 166.4 9.21 [63] (νCHO)

cd d d 1660, 1627 [Ru[Sn{OC(O)H}2(µ-OH)](Cp){P(OEt)3}(PPh3)]2 (10) −263.5 166.1 8.36 [64] (νCHO)

b e b 1673, 1634 OsH[Sn(OH){OC(O)H}2][PPh(OEt)2]4 (11) −451.0 165.5 8.93 [65] (νCHO) 2 1549 N(C6H2-i-Pr{C(H)Ph2)2-4,2,6)(Si-i-Pr3)Sn(κ -O,O’-O2CH) −134.0 a 173.80 a 8.71 a [67] (13) (νCHO) a b c ◦ d e Measured in benzene-d6; measured in toluene-d8; measured at −70 C; measured in acetone-d6; 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 (Table5), and their characterization is quite recent (last decade).

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

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

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. Inorganics 2021, 9, x FOR PEER REVIEW 21 of 33 Inorganics Inorganics 2021 2021, ,9, ,9 ,x, 18xFOR FOR PEER PEER REVIEW REVIEW 2121 20of of of 33 3233

Scheme 21. Reaction scheme leading to Compound 14, data from [69]. SchemeScheme 21. 21. Reaction Reaction scheme scheme leading leading to to Compound Compound 14, 14, data data from from [69]. [[69].69].

Figure 17. Molecular structure of 14, adapted from [69] (MERCURY view). Hydrogen atoms are FigureFigure 17. 17. MolecularMolecular structure structure of of14,14 adapted, adapted from from [69] [69 (MERCURY] (MERCURY view). view). Hydrogen Hydrogen atoms atoms are are omittedFigure 17. for Molecular clarity (Sn structure light blue, of O14 red,, adapted P orange, from N [69] dark (MERCURY blue, C grey). view). Hydrogen atoms are omittedomitted for for clarity clarity (Sn (Sn light light blue, blue, O O red, red, P P orange, orange, N N dark dark blue, blue, C C grey). grey). More recently, in 2019, Mitzel et al. reported the reactivity of the geminal frustrated MoreMore recently, recently, in in 2019, 2019,2019, Mitzel MitzelMitzel et etet al. al.al. reported reported the the reactivity reactivity of of the thethe geminal geminal frustrated frustratedfrustrated Lewis pair (F 5CC22))33SnCHSnCH22P(P(t-Bu)t-Bu)2 2withwith aa variety variety of of small small molecules molecules and, and, in in particular, particular, with LewisLewis pair pair (F (F5C5C2)23)SnCH3SnCH2P(2P(t-Bu)t-Bu)2 2with with a a variety variety of of small small molecules molecules and, and, in in particular, particular, with with CO22 [70].[70]. The The formation ofof thethe COCO22 adductadduct ofof (F (F5C5C2)2)33SnCHSnCH22P(t-Bu)2,, characterized as COCO2 2 [70].[70]. TheThe formationformation of of thethe COCO2 2adduct adduct ofof (F(F5C5C2)23)SnCH3SnCH2P(2P(t-t-Bu)Bu)2,2 , characterizedcharacterized◦ as as (F5CC22))3SnCH3SnCH2P(2P(t-Bu)t-Bu)2·CO2·CO2 (152 (),15 was), was first ﬁrst demonstrated demonstrated by NMR by NMR at −70 at °C− 70in THF-dC in THF-d8 (Table8 (F(F5C5C2)23)SnCH3SnCH2P(2P(t-Bu)t-Bu)2·CO2·CO2 2( 15(15),), was was first first demonstrated demonstrated by by NMR NMR at at − −7070 °C °C in in THF-d THF-d8 (Table8 (Table 6).(Table The6 ).authors The authors mention mention the instability the instability of the of complex the complex at room at room temperature temperature causing causing the 6).6). The The authors authors mention mention the the instability instability of of the the complex complex at at room room temperature temperature causing causing the the releasethe release of CO of2 CO(Scheme2 (Scheme 22). The22). CO The2 molecule CO2 molecule is bonded is bonded to tin and to tin phosphorus and phosphorus atoms, releaserelease of of CO CO2 2(Scheme (Scheme 22). 22). The The CO CO2 2molecule molecule is is bonded bonded to to tin tin and and phosphorus phosphorus atoms, atoms, thusatoms, leading thus leadingto the formation to the formation of a five-memb of a ﬁve-memberedered ring with ringan exocyclic with an C–O exocyclic bond. C–OThe thusthus leading leading to to the the formation formation of of a a five-memb five-memberedered ring ring with with an an exocyclic exocyclic C–O C–O bond. bond. The The PCObond.2 Theunit PCOis planar2 unitis (Figure planar (Figure18). Other 18). Otheradducts adducts resulting resulting from from the the reactivity reactivity of PCOPCO2 2 unitunit isis planarplanar (Figure(Figure 18).18). OtherOther adductsadducts resultingresulting fromfrom thethe reactivityreactivity ofof (F5CC22))33SnCHSnCH2P(2P(t-Bu)t-Bu)2,2 in, in particular particular with with SO SO22 andand CS CS2,2 ,were were also also isolated isolated in in the the solid solid state (F(F5C5C2)23)SnCH3SnCH2P(2P(t-Bu)t-Bu)2,2 ,in in particular particular with with SO SO2 2and and CS CS2,2 ,were were also also isolated isolated in in the the solid solid state state and describe comparable solid-state structures. andand describe describe comparable comparable solid-state solid-state structures. structures.

Scheme 22. ReactionReaction scheme scheme leading to Compound 15,, data data from from [70]. [70]. SchemeScheme 22. 22. Reaction Reaction scheme scheme leading leading to to Compound Compound 15 15, data, data from from [70]. [70].

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Figure 18. MolecularMolecular structure structure of of 1515, adapted, adapted from from [70] [70 (MERCURY] (MERCURY view). view). Hydrogen Hydrogen atoms atoms areare omitted for clarity (Sn light blue, O red, P orange, C grey, FF green).green).

Table 6. Selection of spectroscopic data (NMR and IR)IR) assignedassigned toto phosphinoformatophosphinoformato ofof tintin complexes.complexes.

13 1 31 1 13 1 C{ H} NMR 31 1 P{ H} NMR 119 1 C{ H} NMR P{ H} NMR 119Sn{1H} NMRSn{ H} NMR IR IR CompoundsCompounds –OC(O)P –OC(O)P –OC(O)P –OC(O)P −1 Ref.Ref. (δ, ppm) (cm ) −1 (δ, ppm) (δ, ppm) (δ, ppm) (cm ) (δ, ppm) (δ, ppm) 1629 −184 a 168.7 a 26.4 a 1629 [(i-Pr2P)2N·CO2]2Sn (14) 1 a 1 a a (νC=O) [69] (t, JSn-P = 2626− Hz)184 (d, JC-P = 95 Hz, PCO168.72) (s, broad) 26.4 [(i-Pr2P)2N·CO2]2Sn (14) 1206 (νC=O) [69] (t, 1JSn-P = 2626 Hz) (d, 1JC-P = 95 Hz, PCO2) (s, broad) −308.3 bc 162.3 bd 31.8 bd 1206 (F5C2)3SnCH2P(tBu)2·CO2 (15) 2 1 2 na [70] (hept, very broad, JSn,F−308.3= 368 bc Hz). (d, JC-P = 80 Hz, PCO162.32) bd (s, JSn,P = 56 Hz) 31.8 bd (F5C2)3SnCH2P(tBu)2·CO2 (15) na [70] a b 2 c 1 d 2 Measured in benzene-d(hept,6; verymeasured broad, inJSn,F tetrahydrofuran-d = 368 Hz). 8; (d,determined JC-P = 80 Hz, at 208PCO K;2) determined(s, JSn,P = at 56 203 Hz) K. a Measured in benzene-d6; b measured in tetrahydrofuran-d8; c determined at 208 K; d determined at 203 K. 5. Metallocarboxylato Tin Complexes 5.5.1. Metallocarboxylato Foreword Tin Complexes 5.1. ForewordThe last part of this collection is devoted to bimetallic transition-metal/tin complexes withThe bridging last part carbon of this dioxide, collection of general is devoted formula to bimetallic M(CO2)Sn, transition-metal/tin also named metallocarboxy- complexes withlato tinbridging complexes. carbon Compared dioxide, toof thegeneral categories formula previously M(CO2)Sn, described, also named their metallocarbox- synthesis does ylatonot involve tin complexes. the direct Compared use of CO2 butto the is basedcategori ones the previously reactivity betweendescribed, metallocarboxylic their synthesis doesacids not and involve halogenated the direct precursors use of of CO organotins.2 but is based However, on the we reactivity have found between it interesting metallocar- and boxyliccomplementary acids and to halogenated also include thisprecursors class of of compounds organotins. in However, this review. we Metallocarboxylato have found it in- terestingcomplexes and and complementary in particular tin to derivativesalso include beganthis class to be of studiedcompounds in the in mid-1980s,this review. often Me- tallocarboxylatoconsidered as suitable complexes models and of intermediatesin particular tin for derivatives the catalytic began conversion to be ofstudied carbon in diox- the mid-1980s,ide [4]. Most often of the considered structures as shown suitable below model weres of resolvedintermediates in the for 1990s. the catalytic The D. Gibson’s conver- siongroup of incarbon Louisville dioxide (USA) [4]. Most was one of the of thestruct mostures active shown in thisbelow field were characterizing resolved in severalthe 1990s. of Thethese D. compounds. Gibson’s group Metallocarboxylato-tin in Louisville (USA) complexes was one of have the most also been active the in subject this field of specificcharac- terizinginfrared several considerations of these [compounds.71]. Indeed, MetallocaνOCO adsorptionrboxylato-tin bands complexes constitute well-suitedhave also been probes the for orienting and determining the coordination modes of the CO ligand. subject of specific infrared considerations [71]. Indeed, νOCO adsorption2 bands constitute well-suited probes for orienting and determining the coordination modes of the CO2 lig- 5.2. X-Ray Crystal Structures and. To the best of our knowledge, in 1987, Gladysz et al. reported the first X-ray character- 5.2.ization X-ray of aCrystal transition Structures 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 To the best of our knowledge, in 1987, Gladysz et al. reported◦ the first X-ray charac- (η-C5H5)Re(NO)(PPh3)(CO2K) with 1.1 equivalent of Ph3SnCl at −78 C in THF (Scheme 23). terization of a transition metal/tin bridging CO2 complex by isolating (η- Yellow crystal prisms of 16 grew from a mixture of CH2Cl2/hexane. Compound 16 is air- and C5H5)Re(NO)(PPh3)(CO2SnPh3) (16) [72]. Compound 16 was obtained in high yield (86%) water-stable, and could also be alternatively prepared from (η-C5H5)Re(NO)(PPh3)(CO2H) by reacting (η-C5H5)Re(NO)(PPh◦ 3)(CO2K) with 1.1 equivalent of Ph3SnCl at −78 °C in THF and (Ph3Sn)2O at 25 C in THF. Moreover, the decarboxylation of 16 occurs by heating at (Scheme◦ 23). Yellow crystal prisms of 16◦ grew from a mixture of CH2Cl2/hexane. Com- 180 C for 10 min (from solid) or at 140 C, for 20 h in solution in xylene. Using Me3SnCl, pound 16 is air- and water-stable, and could also be alternatively prepared from (η- the authors claim the formation of the analogue (η-C5H5)Re(NO)(PPh3)(CO2SnMe3) with Ca5 yieldH5)Re(NO)(PPh of 95%. The3)(CO study2H) and was (Ph also3Sn) extended2O at 25 °C to thein THF. synthesis Moreover, of rhenium/germanium the decarboxylation of 16 occurs by heating at 180 °C for 10 min (from solid) or at 140 °C, for 20 h in solution

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Inorganics 2021, 9, 18 22 of 32

inin xylene.xylene. UsingUsing MeMe33SnCl,SnCl, thethe authorsauthors claimclaim thethe formationformation ofof thethe analogueanalogue ((ηη-- CC55HH55)Re(NO)(PPh)Re(NO)(PPh33)(CO)(CO22SnMeSnMe33)) withwith aa yieldyield ofof 95%.95%. TheThe studystudy waswas alsoalso extendedextended toto thethe and rhenium/lead bridging CO2 complexes using Ph3GeBr and Ph3PbCl, as precursors, synthesissynthesis ofof rhenium/germaniumrhenium/germanium andand rhenium/leadrhenium/lead bridgingbridging COCO22 complexescomplexes usingusing respectively. The crystallographic structure of 16 conﬁrms the presence of two moieties PhPh33GeBrGeBr andand PhPh33PbCl,PbCl, asas precursors,precursors, respectively.respectively. TheThe crystallographiccrystallographic structurestructure ofof 1616 based on rhenium and tin, respectively, linked by a CO bridging ligand. The tin atom confirmsconfirms thethe presencepresence ofof twotwo moietiesmoieties basedbased onon rheniumrhenium2 andand tin,tin, respectively,respectively, linkedlinked byby of 16 adopts a distorted trigonal bipyramid geometry in which the equatorial positions aa COCO22 bridgingbridging ligand.ligand. TheThe tintin atomatom ofof 1616 adoptsadopts aa distorteddistorted trigonaltrigonal bipyramidbipyramid geometrygeometry are occupied by the two oxygen atoms of the CO2 ligand and supplemented by a phenyl inin whichwhich thethe equatorialequatorial positionspositions areare ococcupiedcupied byby thethe twotwo oxygenoxygen atomsatoms ofof thethe COCO22 ligandligand group (Figure 19). The CO2 ligand can be regarded as a carboxylato fragment, which is andand supplementedsupplemented byby aa phenylphenyl groupgroup (Figure(Figure 19).19). TheThe COCO22 ligandligand cancan bebe regardedregarded asas aa corroborated by infrared analysis, bidentally bound to the tin atom and deﬁned as µ(n1-C: carboxylato fragment, which is corroborated by infrared analysis, bidentally bound to the ncarboxylato2-O,O0) ligand. fragment, which is corroborated by infrared analysis, bidentally bound to the tintin atomatom andand defineddefined asas μμ((nn11-C:-C: nn22-O,O-O,O′′)) ligand.ligand.

SchemeScheme 23.23. ReactionReaction schemescheme leadingleading toto CompoundCompound 1616,, datadata fromfrom [[72].[72].72].

FigureFigure 19.19. MolecularMolecular structurestructure structure ofof of 161616,, ,adaptedadapted adapted fromfrom from [72][72] [72 (MERCURY](MERCURY (MERCURY view).view). view). HydrogenHydrogen Hydrogen atomsatoms atoms areare are omittedomitted forfor clarityclarity (Sn(Sn lightlight blue,blue, OO red,red, PP orange,orange, NNN darkdarkdark blue,blue,blue, Re ReRe black, black,black, C CC grey). grey).grey).

InIn 1991,1991,1991, Gibson GibsonGibson et al. etet published al.al. publishpublish the solideded statethethe structuresolidsolid statestate of (η -Cstructurestructure5H5)Fe(CO)(PPh ofof ((η3η)-- (COCC55HH255)Fe(CO)(PPhSnPh)Fe(CO)(PPh3)(17) prepared33)(CO)(CO22SnPhSnPh by33)) mixing, ((1717)) preparedprepared under by nitrogenby mixing,mixing, and underunder at low nitrogennitrogen temperature, andand atat low Phlow3 SnCl tem-tem- · toperature,perature, a THF PhPh solution33SnClSnCl toto of aa CpFe(CO)(PPh THFTHF solutionsolution of3of)COOK CpFe(CO)(PPhCpFe(CO)(PPh3H2O[7333)COOK·3H)COOK·3H]. The structure22OO [73].[73]. was TheThe foundstructurestructure to bewaswas isomorphous foundfound toto bebe toisomorphousisomorphous16, with the toto tin 1616 atom,, withwith occupying thethe tintin atomatom a trigonal occupyingoccupying bipyramid aa trigonaltrigonal arrangement bipyramidbipyramid (Figurearrangementarrangement 20). Compared (Figure(Figure 20).20). to ComparedCompared the Gladysz toto thethe complex, GladyszGladysz the complex,complex, authors thethe underline authorsauthors underline forunderline17 a greater forfor 1717 differenceaa greatergreater differencedifference between thebetweenbetween lengths thethe of lengthslengths the Sn–O ofof thth bonds,ee Sn–OSn–O corresponding bonds,bonds, correspondingcorresponding to a value toto of aa valuevalue 0.219 of Åof (Table0.2190.219 Å7Å). (Table(Table However, 7).7). However,However, in the literature inin thethe literatureliterature devoted to devoteddevoted triaryltin toto organocarboxylatetriaryltintriaryltin organocarboxylateorganocarboxylate monomers mon-mon- this differenceomersomers thisthis is differencedifference generally isis much generallygenerally more muchmuch marked moremore in the markedmarked range inin of thethe 0.48 rangerange to 0.81 ofof 0.48 Å0.48 [74 toto]. 0.810.81 ÅÅ [74].[74].

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Figure 20. Molecular structure of 17, adapted from [73] (MERCURY view). Hydrogen atoms are Figure 20. Molecular structure of 17, adapted from [73] (MERCURY view). Hydrogen atoms are omittedomitted for for clarity clarity (Sn (Sn light light blue, blue, O O red, red, P Porange, orange, Fe Fe white, white, C C grey). grey). Two years later in 1993, the same group reported the solid state isolation of a new 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 Fe/Sn CO2-bridged complex, which was characterized by single crystal X-ray diffraction analysis as the indenyl derivative (η5-C H )Fe(CO)(PPh )(CO SnPh )(18). Compound 18 5 9 7 3 2 3 analysis as the indenyl derivative (η -C9H7)Fe(CO)(PPh3)(CO2SnPh5 3) (18). Compound+ 18− was prepared under nitrogen, in THF solution and at 273 K, from (η -C9H7)Fe(CO)2(PPh3) ·I was prepared under nitrogen, in THF solution and at 273 K, from (η5- and Ph SnCl [75]. As shown by the structure displayed in Figure 21, the Sn atom geometry 3 + − Cconsists9H7)Fe(CO) of a2(PPh distorted3) ·I and trigonal Ph3SnCl bipyramid. [75]. As However,shown by thethe stericstructure hindrance displayed of the in indenylFigure 21,ligand the Sn and atom the geometry resulting consists interactions of a withdistor theted phosphinetrigonal bipyramid. ligand are However, suspected the to steric cause hindrancenotable structural of the indenyl differences ligand comparedand the resulting to complexes interactions16 and with17: the (i) Sn–Ophosphine bond lengthsligand arediffer suspected from 0.467(3) to cause Å; notable (ii) the structur differenceal differences in the carboxyl compared C–O bondto complexes lengths also16 and increases 17: (i) Sn–O[0.100(4) bond Å]; lengths (iii) the differ Fe–C–O from bond 0.467(3) angles Å; exhibit (ii) the higher difference distortions in the [10.2(3)carboxyl◦] (TableC–O bond7). lengths also increases [0.100(4) Å]; (iii) the Fe–C–O bond angles exhibit higher distortions [10.2(3)°] (Table 7).

Figure 21. Molecular structure of 18, adapted from [75] (MERCURY view). Hydrogen atoms are omitted for clarity (Sn light blue, O red, P orange, Fe white, C grey). Figure 21. Molecular structure of 18, adapted from [75] (MERCURY view). Hydrogen atoms are omittedThe for Gibson’sclarity (Sn group light blue, continued O red, P to orange, be very Fe prolific white, inC grey). this field until the end of the 1990s, structurally characterizing several new iron and rhenium/tin CO2 bridged complexes. In 1994,The Gibson’s the reaction group of Cp*Re(CO)(NO)COOH continued to be very prolific with Ph in3SnCl this infield the until presence the end of Na of2 COthe 3 3 1990s,afforded structurally the formation characterizing of Cp*Re(CO)(NO)(CO several new iron2SnPh and3)( 19rhenium/tin) showing aCOµ22- ηbridgedcoordination complexes.mode forIn 1994, the CO the2 ligand reaction (Figure of Cp*Re(CO)(NO)COOH 22)[76]. As reflected by with the lengthsPh3SnCl of in the the Sn–O presence and C–O of Nabonds2CO3 (Tableafforded7), anthe asymmetricformation of bonding Cp*Re(CO)(NO)(CO mode of carboxylic2SnPh3) oxygens(19) showing to carbon a μ2-η and3 coor- tin dinationatoms was mode observed. for the CO2 ligand (Figure 22) [76]. As reflected by the lengths of the Sn–O

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Inorganics 2021, 9, 18 24 of 32 and C–O bonds (Table 7), an asymmetric bonding mode of carboxylic oxygens to carbon and tin atoms was observed.

Figure 22. MolecularMolecular structure structure of of 1919, adapted, adapted from from [76] [76 (MERCURY] (MERCURY view). view). Hydrogen Hydrogen atoms atoms areare omitted for clarity (Sn light blue, O red, P orange, N blue, Re black, C grey). omitted for clarity (Sn light blue, O red, P orange, N blue, Re black, C grey).grey).

In 1995, Gibson et al. resolved by X-ray diffraction, the structure of four new CO2- 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 bridged transitiontransition metals/tinmetals/tin complexescomplexes [77[77].]. CpFe(CO)(PPhCpFe(CO)(PPh33)(CO22SnMeSnMe33) )((2020)) andand 3 2 3 CpFe(CO)(PPh33)(CO)(CO2Sn(2Sn(n-n-Bu)Bu)3)3 )((2121) )were were synthesized synthesized by by reaction reaction in in THF betweenbetween 3 2−− + + 3 3 CpFe(CO)(PPh33)CO2−KK+ andand ClSnMe ClSnMe3 3andand ClSnPh ClSnPh3,3 respectively., respectively. In In both both cases, cases, the the bridg- bridg- 2 33 ing carboxyl ligand adopts aa µμ22-η3 coordinationcoordination mode mode in in which which the carboxyl atom is bound 2 2 3 to the the iron iron atom atom and and both both oxygens oxygens are are bound bound to tothe the tin tin atom. atom. Cp*Fe(CO) Cp*Fe(CO)2(CO2(CO2SnPh2SnPh3) (223) 3+ 4+− − 3 2 2 ◦ was(22) wasprepared prepared from from Cp*Fe(CO) Cp*Fe(CO)3+BF34−BF and4 andPh3SnCl Ph3SnCl in CH in2 CHCl2 2atCl 20 at°C 0 inC the in thepresence presence of KOH.of KOH. Compound Compound 22 displays22 displays a similar a similar structure structure to that to of that complexes of complexes 20 and20 21and in which21 in whichtin atoms tin atomsare five-coordinated are ﬁve-coordinated and adopt and adopta distorted a distorted trigonal-bipyramidal trigonal-bipyramidal geometry. geometry. The 2 2 3 COThe2-bridged CO2-bridged rhenium-tin rhenium-tin complex complex Cp*Re(CO)(NO)(CO Cp*Re(CO)(NO)(CO2SnMe32)SnMe (23) was3)(23 isolated) was isolatedas crys- 3 tallineas crystalline solid from solid Cp*Re(CO)(NO)COOH from Cp*Re(CO)(NO)COOH and ClSnMe and3 ClSnMein the presence3 in the of presence KOH. The of KOH.X-ray 2 2 2 2 structureThe X-ray of structure 23 highlighted of 23 highlighted a μ2-η2 coordination a µ2-η coordination mode of the mode bridging of the CO bridging2 ligand CO in2 ligandwhich thein which carbonyl the carbonylcarbon and carbon one andoxygen one atom oxygen are atom bonded are bondedto the rhenium to the rhenium atom and atom the and tin atom,the tin respectively. atom, respectively. As a result, As a result,the tin the atom tin adopts atom adopts a distorted a distorted tetrahedral tetrahedral geometry geometry (Fig- ure(Figure 23). 23).

Figure 23. Molecular structure ofof 2323,, adaptedadapted fromfrom [ 77[77]] (MERCURY (MERCURY view). view). Hydrogen Hydrogen atoms atoms and and one 3 onemolecule molecule of CHCl of CHCl3 are3 omittedare omitted for clarityfor clarity (Sn light(Sn light blue, blue O red,, O red, N blue, N blue, Re black, Re black, C grey). C grey).

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

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Inorganics 2021, 9, 18 25 of 32 Inorganics 2021, 9, x FORIn PEER 1997, REVIEW two new specimen of CO2-bridged rhenium/tin complexes were character-26 of 33

ized atIn the1997, solid-state two new by specimen the Gibson’s of CO group2-bridged [78]. rhenium/tinThe transmetalation complexes reaction were character-involving Cp*Re(CO)(NO)(COized at the solid-state2SnMe by the3) Gibson’s(23) and groupMe2SnCl [78].2 ledThe to transmetalation the formation reactionin good involving yield of Cp*Re(CO)(NO)(CO2SnMe3)(23) and Me2SnCl2 led to the formation in good yield of Cp*Re(CO)(NO)(COIn 221997,Sn(Cl)MeSnMe two3) ( new232) )( 24 andspecimen) jointly Me2SnCl ofwith CO2 2ledMe-bridged 3SnClto the rhenium/tin(Scheme formation 24). complexes inThe good X-ray wereyield struc- character- of Cp*Re(CO)(NO)(CO Sn(Cl)Me )(24) jointly with Me SnCl (Scheme 24). The X-ray struc- ized at 2the solid-state2 by the Gibson’s2 3 group2 3 [78]. The transmetalation reaction involving tureCp*Re(CO)(NO)(CO of complex 24 shows2Sn(Cl)Me the presence2) (24) jointly of a μ with-η 3CO Me 3ligandSnCl (Scheme bridging 24). the The two X-ray moieties struc- of ture of complex 24 shows the presence2 3 of a µ2-η CO2 2 ligand2 bridging the two moieties rheniumture of complex and Cp*Re(CO)(NO)(COtin. 24 The shows tin theatom presence exhibitsSnMe )of (23 aa )five-coordinateμ and2-η3 COMe2SnCl ligand ledgeometry bridging to the formation theadopting two moietiesin an good edge- yieldof of of rhenium and tin. The tin atom exhibits a ﬁve-coordinate geometry adopting an edge- cappedrhenium tetrahedral and Cp*Re(CO)(NO)(COtin. The arrangement tin atom 2exhibitsratherSn(Cl)Me than a2) five-coordinate( 24a distorted) jointly with trigonal Megeometry3SnCl bipyramid. (Scheme adopting 24). Thean X-rayedge- struc- capped tetrahedralture of arrangement complex 24 shows rather the than presence a distorted of a μ2- trigonalη3 CO2 ligand bipyramid. bridging the two moieties of capped tetrahedral arrangement rather than a distorted trigonal bipyramid. rhenium and tin. The tin atom exhibits a five-coordinate geometry adopting an edge- capped tetrahedral arrangement rather than a distorted trigonal bipyramid.

Scheme 24. Reaction scheme leading to compound 24, data from [78]. SchemeScheme 24.24. ReactionReaction schemescheme leadingleading toto compoundcompound 2424,, datadata fromfrom [[78].78]. An equimolarScheme 24.mixture Reaction of scheme Cp*R leadinge(CO)(NO)COOH to compound 24 ,and data compoundfrom [78]. 24 leads, in the presenceAnAn equimolar equimolarof water, mixture tomixture the formation of of Cp*Re(CO)(NO)COOH Cp*R e(CO)(NO)COOHof [Cp*Re(CO)(NO)(CO and and compound compound2)]2SnMe242 leads, (2425 ) leads,via in thethe in pres- for-the An equimolar mixture of Cp*Re(CO)(NO)COOH and compound 24 leads, in the encemationpresence of water, of water, toan the to formationhydroxyl the formation ofspecies, [Cp*Re(CO)(NO)(CO of [Cp*Re(CO)(NO)(COsuggested 2)]by2SnMe the2)]2 2 (SnMe25authors) via2 ( the25 ) formationviaas thebeing for- of presence of water, to the formation of [Cp*Re(CO)(NO)(CO2)]2SnMe2 (25) via the for- anCp*Re(CO)(NO)(COmation hydroxyl of species,an hydroxyl2 suggestedSn(OH)Me by2species,) (Scheme the authors suggested25). as Complex being Cp*Re(CO)(NO)(COby 25 wasthe also authors isolated2 Sn(OH)Me byas reactingbeing2) mation of an hydroxyl species, suggested by the authors as being (SchemeCp*Re(CO)(NO)COOHCp*Re(CO)(NO)(CO 25). Complex2Sn(OH)Me25 withwas also Me2 isolated)SnCl (Scheme2 in by the reacting25). presence Complex Cp*Re(CO)(NO)COOH of Na25 2wasCO3 .also The isolated structure with by Meof reacting 252SnCl was2 Cp*Re(CO)(NO)(CO2Sn(OH)Me2) (Scheme 25). Complex 25 was also isolated by reacting 3 indescribedCp*Re(CO)(NO)COOH the presence as two of Cp*Re(CO)(NO) Na2CO with3. The Me structure2SnCl units2 in linked of the25 presencewas to a described SnMe of Na2 moiety as2CO two3. Thethrough Cp*Re(CO)(NO) structure two μof2- 25ηunits COwas2 Cp*Re(CO)(NO)COOH with Me32SnCl2 in the presence of Na2CO3. The structure of 25 was linkedligands. to The a SnMe centralmoiety tin atom through is hexacoordi two µ -η natedCO ligands.and displayed The central a skewed tin atom trapezoidal-bi- is hexacoor-3 described as twodescribed2 Cp*Re(CO)(NO) as two Cp*Re(CO)(NO) units2 linked units 2to alinked SnMe to2 moietya SnMe2 throughmoiety through two μ2 two-η CO μ2-η2 3 CO2 dinatedpyramidalligands. andThe geometry displayedcentralligands. tin(Figure The a atom skewed central 24). is hexacoordi tin trapezoidal-bipyramidal atom is hexacoordinated andnated displayed geometryand displayed a skewed (Figure a skewed trapezoidal-bi- 24). trapezoidal-bipyramidal geometrypyramidal (Figure geometry 24). (Figure 24).

Scheme 25. Reaction scheme leading to compound 25,25, datadata fromfrom [[78].78]. Scheme 25. Reaction scheme leading to compound 25, data from [78]. Scheme 25. Reaction scheme leading to compound 25, data from [78].

Figure 24.Figure Molecular 24. structureMolecular of 25 structure, adapted of from25, adapted[78] (MERCURY from [ 78view).] (MERCURY Hydrogen atoms view). are Hydrogen omitted for atoms clarity are (Sn Figure 24. Molecularlight structure blue,omitted O red, of N25for dark, adapted clarity blue, (SnRefrom black, light [78] C blue, grey).(MERCURY O red, N darkview). blue, Hydrogen Re black, atoms C grey). are omitted for clarity (Sn light blue, O red, N dark blue, Re black, C grey). Figure 24. Molecular structure of 25, adapted fromIn 1997, [78] (MERCURYKomiya et view).al. reported Hydrogen the atomssynthesis are omittedof the CO for2 claritycomplex (Sn of iron(0), In 1997, Komiya et al. reported the synthesis of the CO2 complex of iron(0), Fe(CO2)(depe)2 light blue, O red, N dark blue, Re black, C grey).Fe(CO 2)(depe)2 [depe = 1,2-bis(diethylphosphino)ethane)], resulting from the replacement [depeIn = 1,2-bis(diethylphosphino)ethane)],1997, Komiya et al. reported resultingthe synthesis from the of replacement the CO2 ofcomplex N2 in Fe(N of 2)(depe)iron(0),2 of N2 in Fe(N2)(depe)2 with CO2. Fe(CO2)(depe)2 reacts then◦ with Ph3SnCl in Et2O at −78°C with CO2 2. Fe(CO2 2)(depe)2 reacts then with Ph3SnCl in Et2O at −78 C to lead to the forma- Fe(COIn )(depe)1997, toKomiya [depe lead to = 1,2-bis(diethylphosphino)ethantheet formational. reported of FeCl(CO the synthesis2SnPh3)(depe) e)],of resultingthe2 (26 )CO (Scheme2 fromcomplex 26)the [79]. replacement of Theiron(0), same reaction of FeCl(CO2SnPh3)(depe)2 (26) (Scheme 26)[79]. The same reaction, using Me3SnCl ofFe(CO N2 in2)(depe) Fe(N2)(depe)tion,2 [depe using2 =with 1,2-bis(diethylphosphino)ethan MeCO3SnCl2. Fe(CO as 2)(depe)tin precursor,2 reacts thene)],gives resulting with rise Ph 3to SnClfrom the inthe analogousEt replacement2O at −78°C complex as tin precursor, gives rise to the analogous complex FeCl(CO2SnMe3)(depe)2. The X-ray toof leadN2 in to Fe(N the2 )(depe)formationFeCl(CO2 2withSnMe of COFeCl(CO3)(depe)2. Fe(CO2. 2TheSnPh2)(depe) X-ray3)(depe) structure2 reacts2 (26 of)then (Scheme 26 shows with Phthat26)3SnCl [79].iron and inThe Et tin 2sameO atoms at − reac-78°C are linked structure of 26 shows that iron and tin atoms1 are2 linked by a CO2 molecule, acting as tion,to lead using to the Mebyformation a3 SnClCO2 molecule, asof FeCl(COtin actingprecursor,2SnPh as μ(n3)(depe)-C: gives n -O,O2 (26′rise) )ligand (Scheme to (Figurethe 26) analogous 25).[79]. The same complex reac- µ(n1-C: n2-O,O0) ligand (Figure 25). FeCl(COtion, using2SnMe Me3)(depe)3SnCl 2. Theas X-raytin precursor, structure ofgives 26 shows rise that to iron the and analogous tin atoms arecomplex linked 1 2 byFeCl(CO a CO2 2molecule,SnMe3)(depe) acting2. The as μX-ray(n -C: structure n -O,O′) of ligand 26 shows (Figure that 25). iron and tin atoms are linked by a CO2 molecule, acting as μ(n1-C: n2-O,O′) ligand (Figure 25).

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Scheme 26. Reaction scheme leading to Compound 26, data from [79]. Scheme 26.Reaction Reaction scheme scheme leading leading to to Compound Compound 26, data, data from from [79 [79].].

Figure 25. Molecular structure of 26, adapted from [79] (MERCURY view). Hydrogen atoms are omitted for clarity (Sn Figure 25. Molecular structureFigure of 25.26, Molecularadapted from structure [79] (MERCURY of 26, adapted view). from Hy [79drogen] (MERCURY atoms are view). omitted Hydrogen for clarity atoms (Sn are light blue, O red, P orange, Cl light green, Fe white, C grey). light blue, O red, P orange,omitted Cl lightfor green, clarity Fe (Snwhite, light C blue,grey). O red, P orange, Cl light green, Fe white, C grey). During the last decade, two new metallocarboxylato of tin complexes have been char- DuringDuring the the last last decade, decade, two two new new metallocarboxylato metallocarboxylato of of tin tin complexes complexes have have been been char- char- acterized at the solid-state. First of all, in 2011, Adams et al. investigated the reaction of acterizedacterized at at the the solid-state. solid-state. FirstFirst ofof all, all, in in 2011, 2011, Adams Adams et et al. al. investigated investigated the the reaction reaction of of Os3(CO)12 with Ph3SnOH in the presence of [Bu4N][OH] in methanol, isolating two new OsOs33(CO)(CO)1212 withwith PhPh33SnOH inin thethe presencepresence ofof [Bu[Bu44N][OH]N][OH] inin methanol,methanol, isolatingisolating two two new new 3 10 2 2 3 trinucleartrinuclear osmium osmium clusters clusters characterized characterized as asOs Os(CO)(CO)(μ-η(µ-O=COSnPh-2η -O=COSnPh)(μ-OMe))(µ-OMe) (27) and (27) trinuclear osmium clusters characterized as Os3(CO)3 10(10μ-η -O=COSnPh3)(μ3-OMe) (27) and Osand3(CO) Os (CO)10(μ-OMe)((µ-OMe)(μ-OH)µ-OH) (with (with yields yields of 18% of 18% and and 7%, 7%, respectively) respectively) [80]. [80]. The The tiphenyl- tiphenyl- Os3(CO)3 10(μ10-OMe)(μ-OH) (with yields of 18% and 7%, respectively) [80]. The tiphenyl- germylgermyl homologue of 2727,, OsOs3(CO)(CO)10((μµ-η22-O=COGePh-O=COGePh3)()(μµ-OMe),-OMe), was was also also obtained obtained from germyl homologue of 27, Os3 3(CO)1010(μ-η2-O=COGePh33)(μ-OMe), was also obtained from thethe reaction ofof PhPh3GeOHGeOH withwith Os Os(CO)3(CO)12and and using using a comparablea comparable synthetic synthetic protocol protocol (in the(in the reaction of Ph3 3GeOH with Os3 3(CO)12 12 and using a comparable synthetic protocol (in thepresence presence of [Bu of [BuN][OH]4N][OH] and and in methanol).in methanol). The The authors authors described described the the structure structure of of27 27as the presence of4 [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 beingas being a trinuclear a trinuclear cluster cluster of threeof three osmium osmium atoms atoms bearing bearing one one bridging bridging methoxy methoxy ligand ligand andand one bridging triphenlystannylcarboxylatetriphenlystannylcarboxylate ligand.ligand. Remarkably,Remarkably, thethe SnSn−−O distances and one bridging triphenlystannylcarboxylate ligand. Remarkably, the Sn−O distances showshow a a large large difference difference in in length length (2.075 (2.075 (5) (5) Å Å and 2.858 2.858 (5) (5) Å, respectively), which results show a large difference in length (2.075 (5) Å and 2.858 (5) Å, respectively), which results fromfrom an an unusual unusual mode mode of of coordination coordination showin showingg the organotin fragment in a pendant arm from an unusual mode of coordination showing the organotin fragment in a pendant arm arrangement.arrangement. The The carbon carbon atom atom and and one oxygen atom of O=COSnPh33 areare bonded to two arrangement. The carbon atom and one oxygen atom of O=COSnPh3 are bonded to two distinctdistinct osmium osmium atoms, while the tin atom is connected connected via the second oxygen atom of the distinct osmium atoms, while the tin atom is connected via the second oxygen atom of the ligandligand (Figure (Figure 26).26). ligand (Figure 26).

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FigureFigure 26. 26. MolecularMolecular structure structure of of2727, adapted, adapted from from [80] [80 (MERCURY] (MERCURY view). view). Hydrogen Hydrogen atoms atoms are are

omittedomitted for for clarity clarity (Sn (Sn light light blue, blue, O O red, red, Os Os dark dark blue, blue, C C grey,). grey,). Figure 26. Molecular structure of 27, adapted from [80] (MERCURY view). Hydrogen atoms are omitted for clarity (Sn light blue, O red, Os dark blue, C grey,). Finally,Finally, more recentlyrecently and and during during investigations investigations devoted devoted tothe to the synthesis synthesis and isolationand isola- of platinum carbonyl clusters stabilized by Sn(II)-based fragments, Zacchini et al. solved in 2016 tion ofFinally, platinum more recentlycarbonyl and clusters during stabilizedinvestigations by devotedSn(II)-based to the fragments,synthesis and Zacchini isola- et al. the X-ray structure of [NEt ] [Pt (CO) (SnCl ) (SnCl ) (Cl SnOCOSnCl )]·2.5CH COCH solvedtion of platinumin 2016 carbonylthe X-ray clusters4 4structure 9stabilized8 of by [NEt 2Sn(II)-based3 4]4[Pt3 92(CO) fragments,2 8(SnCl2) 3Zacchini(SnCl2 3)2 (Clet 2al.SnOCOS-3 3 2− (28), which contains an unusual [(Cl SnOCOSnCl )] ligand described as2– bis-stannyl- nClsolved2)]·2.5CH in 20163COCH the 3 X-ray(28), whichstructure contains of2 [NEt an4]4 [Ptunusual9(CO)2 8(SnCl [(Cl22)SnOCOSnCl3(SnCl3)2(Cl2SnOCOS-2)] ligand de- 3 0 carboxylate or carbon dioxide µ : κ -C,O,O -CO [81]. Cluster 28 was2– obtained as a side- scribednCl2)]·2.5CH as bis-stannyl-carb3COCH3 (28), whichoxylate contains3 or carbon an unusualdioxide2 [(Clμ3: κ2SnOCOSnCl3-C,O,O′-CO2)]2 [81]. ligand Cluster de- 28 was obtainedproductscribed as fromas bis-stannyl-carb a side-product the reactionoxylate atfrom room or the carbon temperature reaction dioxide at μ inroom3: κ acetone,3-C ,temperatureO,O′-CO involving2 [81]. inCluster [NEtacetone,4 28]4 [Ptwas involving15 (CO)30], [NEtobtained]Cl as and a side-product a large excess from of SnCl the reaction·2H O, andat room leading temperature to the major in acetone, product involving [NEt ] [Pt (CO) [NEt44]4[Pt15(CO)30], [NEt4]Cl and a2 large2 excess of SnCl2·2H2O, and leading to4 the4 6major6 (SnCl[NEt4]4[Pt) (SnCl15(CO))30],]. [NEt The4 authors]Cl and a described large excess the of unprecedented SnCl2·2H2O, and structure leading ofto 28theas major consisting of product2 2 [NEt4]34[Pt4 6(CO)6(SnCl2)2(SnCl3)4]. The authors described the unprecedented struc- - aproduct formally [NEt neutral4]4[Pt6(CO) Pt96core(SnCl bonded2)2(SnCl3) to4]. eightThe authors carbonyls, described three theµ 4unprecedented-SnCl2, two µ 3struc--[SnCl3] and ture of 28 as consisting of a formally neutral Pt9 core bonded to eight carbonyls, three μ4- ture of 28 as consisting of a formally neutral Pt9 core bonded to eight carbonyls,2− three μ4- the six electron donor bis-stannyl-carboxylate [(Cl2SnOCOSnCl2)] ligand (Figure 27). SnCl2, two μ3-[SnCl3]- and the six electron donor bis-stannyl-carboxylate [(Cl2SnOCOS- TheSnCl possible2, two μ3-[SnCl formation3]- and ofthe the six metallocarboxylate electron donor bis-stannyl-carboxylate group is explained [(Cl by2SnOCOS- the nucleophilic 2 2– nClnCl2)])]2– ligand (Figure (Figure 27). 27). The The possible possible formation formation of the of metallocarboxylate the metallocarboxylate group is group ex- is ex- addition of H2O to a terminal CO ligand, followed by deprotonation. Alternatively, the plainedplained by thethe nucleophilic nucleophilic addition addition of Hof2O H to2O a toterminal 2a− terminal CO ligand, CO ligand, followed followed by depro- by depro- authors also suggest that [(Cl2SnOCOSnCl2)] could be considered2– as2– a carbonite ion tonation.tonation.2− Alternatively,Alternatively, the the au authorsthors also also suggest suggest that that[(Cl2 SnOCOSnCl[(Cl2SnOCOSnCl2)] could2)] be could con- be con- [CO2] [82], stabilized by coordination2– at one Pt atom and two Sn atoms. A selection of sideredsidered as aa carbonitecarbonite ion ion [CO [CO2] 2 ][82],2– [82], stabilized stabilized by coordination by coordination at one atPt oneatom Pt and atom two and two spectroscopic data (NMR and IR) relating to metallocarboxylato-tin complexes is reported SnSn atoms. atoms. A selectionselection of of spectroscopic spectroscopic data data (NMR (NMR and IR)and relating IR) relating to metallocarboxylato- to metallocarboxylato- in Table8. tintin complexescomplexes is is reported reported in in Table Table 8. 8.

Figure 27. 27. MolecularMolecular structure structure of 28 of, adapted28, adapted from [81] from (MERCURY [81] (MERCURY view). Hydrogen view). Hydrogen atoms, two atoms, two and a a half half molecules molecules of acetone of acetone and four and tetra- fourn-butylammonium tetra-n-butylammonium cations are cations omitted are for omittedclarity for clarity (Sn light blue, O red, Cl light green, Pt light grey, C grey). Figure(Sn light 27. blue, Molecular O red, structure Cl light green, of 28, Ptadapted light grey, from C [81] grey). (MERCURY view). Hydrogen atoms, two and a half molecules of acetone and four tetra-n-butylammonium cations are omitted for clarity (Sn light blue, O red, Cl light green, Pt light grey, C grey).

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

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

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

13C{1H} NMR IR 119Sn{1H} NMR Compounds M–CO ν(OCO) Ref. (δ, ppm) 2 (δ, ppm) (cm−1) a −167.1 a (η-C5H5)Re(NO)(PPh3)(CO2SnPh3)(16) 207.6 1395, 1188 [72] J119Sn,31P = 4.6 Hz b Cp*Re(CO)(NO)(CO2SnPh3)(19) na 206.94 1429, 1174 [76] 219.63 a J 20 na d, P,C = 30.2 Hz 1433, 1132 CpFe(CO)(PPh3)(CO2SnMe3)( ) 220.42 c [77] d, JP,C = 32.5 Hz 219.56 a J n- 21 na d, P,C = 30.9 Hz 1480, 1132 CpFe(CO)(PPh3)(CO2Sn( Bu)3)( ) 220.29 c [77] d, JP,C = 31.4 Hz a Cp*Fe(CO)2(CO2SnPh3)(22) na 215.10 1470, 1146 [77] a 23 na 197.58 1512, 1162 Cp*Re(CO)(NO)(CO2SnMe3)( ) 196.10 c [77] d Cp*Re(CO)(NO)(CO2Sn(Cl)Me2)(24) na 205.66 1385, 1246 [78] d [Cp*Re(CO)(NO)(CO2)]2SnMe2 (25) na 202.14 1469, 1186 [78] 243.8 d FeCl(CO2SnPh3)(depe)2 (26) na 1306 [80] qui, JP,C = 22 Hz a b c d Measured in chloroform-d; methylenchloride-d2; measured in tetrahydrofuran-d8; measured in benzene-d6.

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 Inorganics 2021, 9, 18 29 of 32

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.

Funding: The author thanks the Centre National de la Recherche Scientifique (C.N.R.S-France) and the University of Bourgogne Franche-Comté for financial support. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Not applicable. Acknowledgments: A special thanks to three anonymous reviewers for their judicious corrections and suggestions which have contributed to improving the manuscript. Conflicts of Interest: The author declares no conflict of interest.

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