Synthesis and Basic Coordination Properties of Side-chain Functionalized Bis-phosphonio-benzophospholides

Abstract

Salts containing bis-phosphonio-benzophospholide cations 2 a±d with an additional donor site in one of the phosphonio-moieties were synthesized either via quaternisation of the Ph 2 P moiety in the neutral phosphonio-benzophospholide 3, or via ring-closure of the functionalized bisphosphonium ion 6. The Ph 2 P-substituted cation 2 d formed chelate complexes [M(k 2 P,P'-2 d)(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(kP-2 d) 2 (CO) 4 ] 2+ was observed, and interpreted as a consequence of antagonism between the stabilizing chelate effect and destabilizing ligand±ligand repulsions. The formation of stable Pd II and Pt II complexes of 2 d suggests that the chelate effect may also overcome the kinetic inhibition which so far prevented isolation of complexes of these metals with bis-phosphonio-benzophospholides. The newly synthesized ligands and complexes were characterized by spectroscopic data, and an X-ray crystal structure analysis of 2 a [Br]. The reactivity of chelate complexes towards Ph 3 P indicates that the ring phosphorus atom is a weaker donor than the pendant Ph 2 P-group.