An alternative strategy to an electron rich phosphine based carbonylation catalyst

摘要

The complexes [Rh(CO)Cl(2-Ph_2PC_6H_4COOMe)], 1, and trans-[Rh(CO)Cl(2-Ph_2PC_6H_4COOMe)_2], 2, have been synthesized by the reaction of the dimer [Rh(CO)_2Cl]_2 with 2 and 4 molar equivalents of 2-(diphenylphosphino)-methyl benzoate. The complexes 1 and 2 show terminal v(CO) bands at 1979 and 1949 cm~(-1) respectively indicating high electron density at the metal centre. The molecular structure of the complex 2 has been determined by single crystal X-ray diffraction. The rhodium atom is in a square planar coordination environment with the two phosphorus atoms trans to each other; the ester carbonyl oxygen atom of the two phosphine ligands points towards the rhodium centre above and below the vacant axial sites of the planar complex. The rhodium-oxygen distances (Rh …O(49) 3.18 A; Rh … O(19)3.08 A) and the angle O(19) … Rh … O(49) 179°indicate long range intramolecular secondary Rh … O interactions leading to a pseudo-hexacoordinated complex. The complexes 1 and 2 undergo oxidative addition (OA) reactions with CH_3I to produce acyl complexes [Rh(COCH_3)ClI(2-Ph_2PC_6H_4COOMe)], 4, and trans-[Rh(COCH_3)ClI(2-Ph_2PC_6H_4COO-Me)(2-Ph_2PC_6H_4COOMe)], 5, and the kinetics of the reactions reveal that the complex 1 undergoes faster OA reaction than that of the complex 2. The catalytic activity of the complexes 1 and 2 in the carbonylation of methanol were higher than that of the well known species [Rh(CO)_2I_2]~- and the complex 1 shows higher activity than 2.