Activation of Homolytic SiZn and SiHg Bond Cleavage, Mediated by a Pt⁰ Complex, via Novel PtZn and PtHg Compounds

Abstract

The thermally stable [(tBuMe 2 Si) 2 M] (M = Zn, Hg) generate R 3 SiC radicals in the presence of [(dmpe)Pt(PEt 3 ) 2 ]at 60-80 8 8C. The reaction proceeds via hexacoordinate Pt complexes,( M= Zn (2a and 2b), M = Hg (3a and 3b)) which were isolated and characterized. Mild warming or photolysis of 2 or 3 lead to homolytic dissociation of the PtÀMSiR 3 bond generating silyl radicals and novel unstable pentacoordinate platinum paramagnetic complexes (M = Zn (5), Hg (6)) whose structures were determined by EPR spectroscopya nd DFT calculations.The vigorously developing chemistry of heterometallic complexes containing two or more different metals is of considerable contemporary interest with avariety of applications in synthesis,c atalysis,a nd materials chemistry. [1] More specifically,r ecently heterometallic complexes containing Pt and Zn atoms attracted attention and several were isolated. [2][3][4] Several methodologies are available for their synthesis.O ne involves the formation of PtÀZn heteronuclear dative linkages from Lewis basic and Lewis acidic metals (termed metal-only Lewis pairs (MOLPs)). [1a] Fore xample, recently aM OLP Pt À Zn complex was isolated from the reaction of [Pt(PCy 3 ) 2 ]( Cy = cyclohexyl) and ZnBr 2 . [2] Fischer et al. reported an umber of novel {PtÀZnCp*} (Cp* = pentamethylcyclopentadienyl) complexes in which ad ivalent zinc is bonded directly to platinum. [3] Complexes in which Pt and Zn are bridged by ligands were also isolated and characterized. [4] Herein we report that thermally stable SiÀZn and SiÀHg bonds are activated towards homolytic cleavage by aP t 0 complex, [(dmpe)Pt(PEt 3 ) 2 ]( 1)d mpe = 1,2-bis(dimethylphosphanyl)ethane,y ielding hexacoordinate Pt isomers 2a and 2b (M = Zn) or 3a, 3b (M = Hg), respectively,h aving aP t ÀMSiR 3 moiety [Eq. (1)].T hese novel hexacoordinate PtÀMSiR 3 compounds were isolated and characterized by Xray crystallography and their photolysis and thermolysis is reported. Photolysis of 2 (i.e., the obtained 63:37 mixture of 2a+ 2b,respectively) or of 3 (i.e., the obtained 26:74 mixture of 3a+ 3b,r espectively) yields quantitatively the tetracoordinate platinum complex 4 and the EPR spectra of the intermediate pentacoordinate platinum paramagnetic species, 5 (M = Zn) or 6 (M = Hg), respectively,a re observed [Eq. (2)].Bis(silyl)zincs and bis(silyl)mercuries are good precursors for the photochemical generation of R 3 Si radicals. [5] For example,upon day-light exposure for 30 min [(tBuMe 2 Si) 2 Zn] (7)o r[ ( tBuMe 2 Si) 2 Hg] (8)u ndergo quantitative demetallation forming short-lived [tBuMe 2 Si]C radicals which dimerize to disilane 9 (95 %) or abstract an hydrogen (5 %) to form silane 10 [Eq. (3)].In contrast to their facile photolysis,b is(silyl)zincs and bis(silyl)mercuries are generally thermally stable;f or example no decomposition of [(tBuMe 2 Si) 2 Zn] (7), [(tBu-Me 2 Si) 2 Hg] (8), [(tBu 2 MeSi) 2 Zn] (11), or [(tBu 2 MeSi) 2 Hg] (12)isobserved after their heating at 120 8 8Cfor several days. However,i nt he presence of as toichiometri...