Mixed donor-functionalised phosphinomethanide complexes of the alkali metals; synthesis, structures, and solution dynamics

Abstract

The mixed donor tertiary phosphine {(Me₃Si)₂CH}P(C₆H₄-2-CH₂NMe₂)(C₆H₄-2-CH₂OMe) (11) is accessible via the stepwise addition of [Li(C₆H₄-2-CH₂NMe₂)] followed by [Li(C₆H₄-2-CH₂OMe)] to the dichlorophosphine {(Me₃Si)₂CH}PCl₂. Phosphine 11 is readily deprotonated by BuⁿLi to give the lithium phosphinomethanide [[{(Me₃Si)₂C}P(C₆H₄-2-CH₂NMe₂)(C₆H₄-2-CH₂OMe)]Li] (15), which undergoes metathesis reactions with the alkoxides MOBu^t [M = Na, K] to give the heavier alkali metal phosphinomethanides [[{(Me₃Si)₂C}P(C₆H₄-2-CH₂NMe₂)(C₆H₄-2-CH₂OMe)]M(L)_n]_x in good yield [M = Na (12), (L)_n = Et₂O, x = 1; M = K (13), n = 0, x = 2]. Compounds 12 and 13 adopt monomeric and dimeric structures, respectively, in the solid state. Variable-temperature NMR studies indicate that compounds 12 and 13 are highly fluxional in solution; this fluxionality arises from a dynamic equilibrium between two conformers, which differ in the orientation of the aromatic rings of the ligand. The nature of these conformers and their relative energies have been probed by DFT calculations, which show the two principal conformers to differ in energy by just 1.6 kcal mol⁻¹. Calculation of the ¹H NMR shielding tensors using the GIAO method reveals that the low field chemical shifts of one benzylic and one aromatic proton in the ground state conformer are due to their close proximity to the carbanion centre.