Synthesis, spectroscopic characterisation and transmetalation of lithium and potassium diaminophosphanide-boranes

Abstract

A secondary diaminophosphane-borane (Et₂N)₂PH(BH₃) was prepared from a chlorophosphane precursor and LiBH₄ and metalated by reaction with anion bases (n-BuLi, KN(SiMe₃)₂) to yield the corresponding metal diaminophosphanide-boranes [(Et₂N)₂P(BH₃)]M (M = Li, K). Multinuclear NMR studies permitted the first spectroscopic characterisation of the metalation products and revealed the presence of monomeric (for M = Li) contact ion pairs in solution. NMR spectroscopic evidence that the ions in each pair interact via Li⋯P- rather than Li⋯H₃B-interactions as had been inferred for a Ph-substituted analogue was confirmed by DFT studies, which revealed also that the borane coordination plays a decisive role in boosting the PH-acidity of the original secondary diaminophosphane precursor. Transmetalation of the potassium and lithium diaminophosphanide-boranes with Cu(I) and Zn(II) chlorides afforded the first functional transition metal complexes of a P-heteroatom-functionalised phosphanide-borane ligand. Both products were fully characterised. Thermolysis of the Cu-complex induced a reaction which involved transfer of an NHC ligand from the metal to the phosphorus atom and yielded a phosphaalkene NHCPH (NHC = N-heterocyclic carbene) as the major phosphorus-containing product.