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[BAr^F] (BAr^F = B{3,5-(CF₃)₂C₆H₃}₄) yields the corresponding [M{MeC(CH₂PPh₂)₃}][BAr^F]₂ salts, the crystal structures of all three (M = Ge, Sn, Pb) reveal pyramidal dications with discrete [BAr^F]⁻ 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.