Coordination chemistry and hydroformylation activity of platinum complexes containing 1-aryl-phospholes

Abstract

Abstract The reaction of the sterically crowded 1-aryl-phospholes (1-(2′,4′,6′-tri-isopropylphenyl)-3-methylphosphole (1), 1-(2′,4′,6′-tri-tert-butylphenyl)-3-methyl-phosphole (2) and 1-(2′,4′-di-tert-butyl-6′-methylphenyl)-3-methylphosphole (3)) with PtCl2(PhCN)2 was investigated by NMR spectroscopy. Significant differences in their reactivity towards PtCl2(PhCN)2 have been observed. Both cis-PtCl2(phosphole)(PhCN) and trans-PtCl2(phosphole)2 complexes were formed under normal reaction conditions. The corresponding reaction with the more basic 1-phenyl-3,4-dimethyl-phosphole (4) resulted in the immediate formation of cis-PtCl2(4)2 as the major product accompanied by ≈ 3% trans-PtCl2(4)2. The structure of the two types of platinum complexes in solution has been confirmed by 1J(195Pt,31P) coupling constants in 31P-NMR and several shielding effects in 1H-NMR spectrometry. The reduced aromatic character of the coordinated phosphole ligands has been confirmed by the pyramidalization of the phosphorus and the loss of the perpendicular arrangement of the phosphole and aryl rings. While the reaction of the sterically congested phospholes (1, 2) with [Rh(nbd)Cl]2 resulted in the formation of Rh(nbd)(phosphole)Cl complexes, the most basic 4 brought about the [Rh(nbd)(4)2]+ cation. Both the conversion and the regioselectivity of platinum- and rhodium-catalysed hydroformylation of styrene show strong dependence on the basicity of the phosphole ligand.