Synthesis and basic coordination properties of side-chain functionalized bis-phosphonio-benzophospholides

Salts containing bis-phosphonio-benzophospholide cations 2a-d with an addit ional donor site in one of the phosphonio-moieties were synthesized either via quaternisation of the Ph2P moiety in the neutral phosphonio-benzophosph olide 3, or via ring-closure of the functionalized bis-phosphonium ion 6. T he Ph2P-substituted cation 2d formed chelate complexes [M(kappa P-2,P ' -2d )(CO)(n)](+) with M(CO)(n) = Ni(CO)(2), Fe(CO)(3), Cr(CO)(4). In the latter case, competition between formation of the chelate and a complex [Cr(kappa P-2d)(2)(CO)(4)](2+) was observed, and interpreted as a consequence of anta gonism between the stabilizing chelate effect and destabilizing ligand-liga nd repulsions. The formation of stable Pd-II and Pt-II complexes of 2d sugg ests that the chelate effect may also overcome the kinetic inhibition which so far prevented isolation of complexes of these metals with bis-phosphoni o-benzophospholides. The newly synthesized ligands and complexes were chara cterized by spectroscopic data, and an X-ray crystal structure analysis of 2a[Br]. The reactivity of chelate complexes towards Ph3P indicates that the ring phosphorus atom is a weaker donor than the pendant Ph2P-group.