Synthesis, spectroscopic and structural properties of Sn(ii) and Pb(ii) triflate complexes with soft phosphine and arsine coordination

Abstract

Reaction of the divalent M(OTf)2 (M = Sn, Pb; OTf = CF3SO3) with soft phosphine and arsine ligands, L, where L = o-C6H4(ER2)2 (E = P, R = Me or Ph; E = As, R = Me), MeC(CH2ER2)3 (E = P, R = Ph; E = As, R = Me), PhP(CH2CH2PPh2)2 or P(CH2CH2PPh2)3, affords complexes of stoichiometry M(L)(OTf)2 as white powders, which have been characterised via elemental analysis, 1H, 19F{1H}, 31P{1H} and 119Sn NMR spectroscopy, with the expected 31P-119Sn and 31P-207Pb couplings clearly evident. The crystal structures of nine of these pnictine complexes are reported, in each case revealing retention of one or both OTf anions, which gives rise to a diverse range of coordination environments including monomers, as well as varying degrees of oligomerisation to form weakly associated (OTf-bridged) dimers, trimers and polymers. 19F{1H} NMR spectra indicate that the OTf is essentially anionic (dissociated) in solution. Anion metathesis of [M(OTf)2{MeC(CH2PPh2)3}] with Na[BArF] (BArF = B{3,5-(CF3)2C6H3}4) yields the corresponding [M{MeC(CH2PPh2)3}][BArF]2 salts, the crystal structures of all three (M = Ge, Sn, Pb) reveal pyramidal dications with discrete [BArF]− anions providing charge balance. Density functional theory (DFT) calculations on these [M{MeC(CH2PPh2)3}]2+ (M = Ge, Sn, Pb) dications using the B3LYP-D3 functional show the presence of a directional lone pair, which is a mixture of valence s and pz character, with the valence p-orbital character decreasing down group 14. Natural bond orbital (NBO) analysis also shows that the natural charge at the metal centre increases and the charge on the P centre decreases upon going down group 14.