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

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, H-1, F-19{H-1}, P-31{H-1} and Sn-119 NMR spectroscopy, with the expected P-31-Sn-119 and P-31-Pb-207 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. F-19{H-1} 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)(2)C6H3}(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 p(z) 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.