Multinuclear rhodium complexes supported by tetra-tert-butoxy disiloxide ligand: synthesis, structure, and reactivity

Abstract

Rhodium siloxide complexes have been used as homogeneous catalysts and model compounds for silica-supported metal catalysts. However, rhodium complexes with multidentate siloxide ligands have rarely been prepared. In this study, a dinuclear rhodium complex supported by a bidentate tetra-tert-butoxy disiloxide ligand, {Rh(COD)}₂{μ-(OSi(O^tBu)₂)₂O} (3) (COD = cyclooctadiene), was synthesized and its basic reactivity was elucidated. When 3 reacted with carbon monoxide, a dinuclear rhodium carbonyl complex, {Rh(CO)₂}₂{μ-(OSi(O^tBu)₂)₂O} (A), formed as an intermediate, which produced a bis-dinuclear rhodium carbonyl complex, [{Rh(CO)₂}₂{μ-(OSi(O^tBu)₂)₂O}]₂ (4), by spontaneous dimerization. The dimerization of A could be attributed to the relatively weak electron-donating ability of the tetra-tert-butoxy disiloxide ligand, suggesting that substituents on siloxide ligands exert a significantly large effect on the formation of rhodium siloxide complexes. In addition, the reaction of 3 with trimethylphosphine (PMe₃) afforded different dinuclear rhodium complexes, {(PMe₃)Rh(COD)}₂{(OSi(O^tBu)₂)₂O} (5), {(PMe₃)₂Rh}{Rh(COD)}{μ-(OSi(O^tBu)₂)₂O} (6), and {Rh(PMe₃)₂}₂{μ-(OSi(O^tBu)₂)₂O} (7), depending on the reaction temperature and the equivalent of PMe₃ used, indicating that the substitution reaction of COD by PMe₃ proceeds in a stepwise manner. Understanding the basic reactivity of these rhodium siloxide complexes may contribute to the development of silica-supported metal catalysts.