Rhodium(I) carbonyl complexes of tetradentate chalcogen functionalized phosphines, [P′(X)(CH2CH2P(X)Ph2)3] {X = O, S, Se}: Synthesis, reactivity and catalytic carbonylation reaction

The reaction of [Rh(CO)2Cl]2 with 0.5 mol equivalent of the ligands [P′(X)(CH2–CH2P(X)Ph2)3](P′P3X4) {where X = O(a), S(b) and Se(c)} affords tetranuclear complexes of the type [Rh4(CO)8Cl4(P′P3X4)] (1a–1c). The complexes 1a–1c have been characterized by elemental analyses, mass spectrometry, IR and multinuclear NMR spectroscopy, and the ligands b and c are structurally determined by single crystal X-ray diffraction. 1a–1c undergo oxidative addition (OA) reactions with CH3I to generate Rh(III) oxidised products. Kinetic data for the reaction of 1a and 1b with excess CH3I indicate a pseudo first order reaction. The catalytic activity of 1a–1c for the carbonylation of methanol to acetic acid and its ester show a higher Turn Over Frequency (TOF = 1349–1748 h−1) compared to the well-known species [Rh(CO)2I2]− (TOF = 1000 h−1) under the similar experimental conditions. However, 1b and 1c exhibit lower TOF than 1a, which may be due to the desulfurization and deselinization of the ligands in the respective complexes under the reaction conditions.

Three new tetranuclear rhodium carbonyl complexes of chalcogen donor ligands, [P′(X)(CH2CH2P(X)Ph2)3] (X = O, S and Se), have been synthesized. The complexes exhibit higher catalytic activity (TOF = 1349–1748 h−1) in the carbonylation of methanol to acetic acid and methyl acetate than well-known Monsanto’s species [Rh(CO)2I2]−.Figure optionsDownload as PowerPoint slideHighlights
► Synthesis of rhodium carbonyl complexes of tetradentate functionalized phosphine.
► Oxidative addition of CH3I to square planar Rh(I) complexes.
► Kinetic studies for the oxidative addition reactions of CH3I.
► Carbonylation of methanol to acetic acid and methyl acetate.