Organo-tin compounds have wide applications as pesticides and as intermediates for organic synthesis.¹ They are invariably Sn(IV) derivatives and are generally four-coordinate.² The mixed organo/chioro compounds of the type RnSnCI4-n do however have the ability to expand their coordination numbers to five or six. This depends critically on the substituents - with four organic groups, R₄Sn, there is no tendency at all to coordinate extra ligands, while at the other extreme SnCl₄ readily forms six-coordinate [SnC1₄L₂] complexes since the electronegative halo groups increase the Lewis acidity of the tin centre

Lloyd, Nicholas C.

Nicholson, Brian K.

Thomson, Ralph A.

Wilkins, Alistair L.

English

Research Commons@Waikato

Novel Six-coordinate Aryl- and Alkyltin ComplexesNicholas C. Lloyd*, Brian K Nicholson , Alistair L Wilkins and Ralph ThomsonDepartment of Chemistry, University of Waikato, Private Bag 3105, Hamilton.IntroductionOrgano-tin compounds have wide applications aspesticides and as intermediates for organic synthesis.1 Theyare invariably SnIV derivatives and are generally four-coordinate 2 The mixed organo/chioro compounds of thetype R,,SnCI4 do however have the ability to expand theircoordination numbers to five or six. This depends criticallyon the substituents - with four organic groups, R4Sn, thereis no tendency at all to coordinate extra ligands, while atthe other extreme SnCl4 readily forms six-coordinate[SnC14L2] complexes since the electronegative halo groupsincrease the Lewis acidity of the tin centre.3a X3 MeCI2 4a X3 Cl3 53b X3 Mo2CI 4b X3 PhD24c X3= Ph2CI4d X3 = Ph3In a previous study3 it was shown that the complete seriesof six-coordinate methyl-tin-chloride complexes could beprepared using Trofiinenko's trispyrazolylborate ligand,4including the first example of a six-coordinate trirnethyltincentre in Me3Sn[pz3BH], as in 1. The strong tendencyof the trispyrazolylborate ligand to act as a tridentatechelating ligand overcomes the reluctance of the Me3Sncentre to form three extra bonds,Recently Klaui5 has introduced a new tridentate ligand,[CpCo{P03R2}3]-, 2, R = Me, Et which is analogous tothe Trofimenko one, but coordinates through three oxygen,rather than nitrogen, atoms. Although this ligand has beenquite widely used in transition metal chemistry, there havebeen no previous reports in organo-tin chemistry, so weare currently investigating this aspect.ResultsThe reactions between the Klaui ligand as the sodium saltand the organotin chlorides Me SnCl or Ph SnCl n = 0-ii 4 fl 4n3 generally proceeded smoothly in dichioromethanesolution, with elimination ofNaCl, to give the correspondingnew complexes 3a-b, 4a-d. The only example for whichno product has yet been isolated isolated is with Me3SnCI.To prove that the tin atom was six-coordinate the single-crystal X-ray structures ofthe Ph3Sn and the CI3Sn derivativeswere determined. The structures are illustrated in Figures 1and 2 and show that the tin is indeed fully complexed - thefirst example to be established for a Ph3Sn centre, with aC303 coordination sphere. As expected the tridentate ligandis more tightly bound for the trichloro example Sn-O 2.08A than for the triphenyl one Sn-O 2.21 A, reflecting themore acidic centre in the forn-mer. Other differences areillustrated by the Cl-Sn-Cl and O-Sn-O angles of 87° and95° respectively for 4a and corresponding C-Sn-C and 0-Sn-O angles of 104° and 79° for 4d. While the X-raystructural data is for the solid state only, NIvIR studies seebelow confirm that the six-coordination is maintained insolution.Figure 1. The structure of[CpCo{PO3Me2}3]SnPh3Figure 2. The structure of[CpCo{PO3Me2}3]SnCI3ll40SnM/ MeMeOMeMeOOMeMeOP PIj MeO''OMNVSn/xRO ORPOR0,,2HNs/S1PhPhChemistry In New Zealand March 2002 Page 53It is interesting that the Ph3Sn complex of the Klaui ligandformed straightforwardly, while the corresponding Me3Snone did not, since for the trispyrazolylborate system thereverse was found - Me1Sn{pz3BH} was readily isolated3whereas Ph3Sn{pz3BH} has been reported to be unstable.6Reasons for this difference are not yet apparent.NMR StudiesThe new complexes are rich in potential NMR infonnation,with `H, `3C, 31P and `19Sn nuclei amenable to study. The`H and `3C spectra are complicated by virtual couplingarising from the high symmetry so that, for example, thesignals for the Me groups in the Ph3Sn[CpCo{PO3Me2}3]example show a complex splitting pattern to the threeequivalent P nuclei.The "Sn spectra are the most interesting, since thechemical shifts are characteristic of coordination numberin solution, and provide information about the structuralrigidity of the complexes. For the symmetricalX,Sn[CpCo{PO3Me2}3] the "9Sn signal is a quartet, arisingfrom coupling to three equivalent phosphorus atoms, whilefor the unsymmetrical RC12Sn or R2ClSn complexes thesignals are more complicated, indicating some fluxionality.For all of the complexes the "9Sn shifts lie in the -300 to -700 ppm region, where six-coordinate tin is expected, andfurthemore they lie on a straight line when plotted againstnumber of Cl groups on the tin Figure 3. This shows thefull series maintains in solution the six-coordination foundcrystallographically in the solid state.0 1 2 3-700Figure 3. The Sn chemical shift versus degree of phenylsubsitution for [CpCo{POMe2}3JSnX3Complexes With Trismethimazolylborate LigandAnother tridentate ligand related to the pyrazolyl-borate andKlaui ligands has been iiitroduced very recently by Reglinski.7This is the trismnethimazolylborate anion 5 whichcoordinates through three sulfur atoms. This is therefore asofter Lewis base than the N- or 0-donor analogues, so mightbe expected to form stable complexes with the relativelysoft organotin centres. However our preliminary results showthat with Ph3SnC1, the complex that forms is four-coordinatewith only one of the sulfur atoms attached to tin and theother two dangling. This is illustrated in Figure 4. "9SnNMRfor this compound gives a signal at -58 ppm, which showsNumber of phenyl groups on tinthat four-coordination is maintained in solution. This is aunique example for this ligand since all other complexesreported so far involve tridentate coordination. Further workis needed to understand the reasons for this behaviour.Figure 4. The structure of [HB{C3N,S3]SnPh3ConclusionsThe Klaui ligand is clearly useful for enforcing highcoordination numbers on alkyl- and aiyltin centres. Itbehaves differently from the pyrazolylborate ligands andso extends the range of compounds known. The relatedtrisrnethimazolylborate ligands however seem lesspowerful in encouraging higher coordination in tinchemistry. The three mono-negative tridentate ligands havecomplementary roles in their coordination chemistry ofmain group elements.AcknowledgementsWe thank Professor Ward Robinson and Dr Jan WikairaUniversity of Canterbuty, and Associate Professor CliffRickard The University of Aucldand for X-ray data sets.References1. Blunden, S. J.; Cusack, P. A. and Hill, R., TheIndustrial Uses of Tin Chemicals, Royal SocietyChemistiy, London, 1985.2. Harrison, P. 0., The Chemistry of Tin, Chapman andHall: New York, 1989; Davies, A.G., OrganotinChemistry, VCH: Weinheim, 1997.3. Nicholson, B. K.,J. Organornetal. Chein., 1984, 265,153; Lee, S. K. and Nicholson, B. K., J. Organometal.Chein., 1986, 309, 257.4. Trofimenko, S., Chein. Rev., 1993, 93, 943.5. Klaui, W., Angew. Chern., Jut. Ed. Engl. 1990,29, 627.6 Lobbia, G. G.; Valle, 0.; Calogero, S.; Cecchi, P.;Santini, C. and Marchetti, F., J. Chem. Soc., DaltonTrans., 1996, 2475; Calogero, S.; Valle, G.; Lobbia,0. G.; Cecchi, P.; Santini, C. and Stievano, L., J.Qrganometal. Chein., 1996, 526, 269.7. Reglinski, J.; Gamer, M.; Cassidy, 1. D.; Slavin, P.A.; Spicer, M. D. and Annstrong, D. R., J Chain,Soc., Dalton Trans., 1999, 2119.Page 54 Chemistry In New Zealand March 2002

Novel six-coordinate Aryl- and Alkyltin complexes

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