On the coordination chemistry of organochalcogenolates RNMe2^E− and RNMe2^E^O− (E = S, Se) onto lead(ii) and lighter divalent tetrel elements

Abstract

Several families of heteroleptic tetrelenes of general formulae M(E^R^NMe2)[N(SiMe₃)₂] and M(O^E^R^NMe2)[N(SiMe₃)₂] (where E = S, Se; M = Ge, Sn, Pb; R^NMe2 = 2-(Me₂NCH₂)C₆H₄] supported by organochalcogenolato ligands have been prepared and fully characterised. The coordination chemistry of these ligands containing both hard (N, O) and soft (S, Se) atoms onto metals of varying size, polarisability, electropositivity and electrostatic surface potential has been explored. In the molecular solid-state, the complexes M(E^R^NMe2)[N(SiMe₃)₂] are monomeric, although an occurrence of weak Pb⋯Se intermolecular interactions yielding a bimolecular species has been identified in the case of the plumbylene Pb[SeC₆H₄(CH₂NMe₂)-2][N(SiMe₃)₂]. On the other hand, all complexes M(O^E^R^NMe2)[N(SiMe₃)₂] form centro-symmetric bimetallic dimers with O-bridging atoms. Multinuclear (²⁹Si, ⁷⁷Se, ¹¹⁹Sn, ²⁰⁷Pb) NMR spectroscopy and crystallographic studies reveal that the metal preferably remains 3-coordinated in all these heteroleptic complexes with absence of coordination of N and S/Se atoms, unless severe depletion of electronic density onto the metal is enforced. Coordination of these heteroelements can thus be achieved either through replacement of α-CH₃ substituents (as in the ligand 2-(Me₂NCH₂)C₆H₄SeCH₂C(Me)₂O⁻) by electron-withdrawing α-CF₃ moieties (as in the ligand 2-(Me₂NCH₂)C₆H₄SeCH₂C(CF₃)₂O⁻), or else with recourse to the use of a cationizing agent leading to the formation of the ion pair [{2-(Me₂NCH₂)C₆H₄SeCH₂C(Me)₂O}Pb]⁺·[H₂N{B(C₆F₅)₃}₂]⁻ where the cationic metal complex is associated to a weakly-coordinating anion. The data collated herein provide compelling evidence that the coordination chemistry of divalent tetrel elements with ligands featuring both hard and soft donors cannot be reliably anticipated by sole use of general concepts such as the HSAB theory. The related metal complexes containing the rigid 8-(NMe₂)naphthalen-1-yl group are also discussed.