Complexes with phosphorus analogues of imidazoyl carbenes: unprecedented formation of phosphenium complexes by coordination induced PCl bond heterolysis

Diazaphospholenium complexes are readily accessible from reactions of complexes [M(bipy)(CO)3(L)] (L=CO, MeCN, M=Mo, W) with both the 1,3-dimesityl-4-chloro-1,3,2-diazaphospholenium triflate (1) [OTf] and the corresponding p-chloro-diazaphospholene (8). The latter reaction proceeds via an unprecedented coordination induced ionisation of PCl bonds, which requires no further assistance by an external electrophile. The complexes were found to be configurationally stable, but may undergo selective substitution of trans-ligands with retention of the phosphenium moiety. All compounds were characterised by analytical and spectroscopic techniques, and two of the complexes were investigated by single-crystal X-ray diffraction. The spectroscopic and structural data provide evidence for considerable MP double bond character which, leads to a marked reduction of π-delocalization in the diazaphospholenium unit. Studies of metal NMR spectra of tungsten complexes revealed further, a linear correlation between δ183W and 1JWP which allows monitoring of trends in metalphosphorus multiple bonding. Surprisingly, spectroscopic and structural data suggest that the cation 1 displays a higher π-acceptor ability than an analogous CC-saturated heterocyclic phosphenium ligand, which contrasts with the lower electrophilicity of 1 with respect to unconjugated diaminophosphenium species.