Square-Planar Carbonylchlororhodium(I) Complexes Containing <i>trans</i>-Spanning Diphosphine Ligands as Catalysts for the Carbonylation of Methanol

The rhodium(I) complexes <i>trans</i>-[Rh(diphos)(CO)Cl] <b>7</b> (diphos=pbpb), <b>8</b> (diphos=nbpb), and <b>9</b> (diphos=cbpb) were synthesized (<i>Scheme 4</i>) and used as catalysts for the carbonylation of MeOH to AcOH (<i>Scheme 1</i>). The <i>trans</i> coordination imposed by the rigid C-spacer framework of the diphos ligands pbpb, nbpb, and cbpb, demonstrated by ³¹P-NMR and IR spectroscopy of <b>7</b>-<b>9</b> and unambiguously confirmed by single-crystal X-ray structure analysis of <b>7</b>, improved the thermal stability of the rhodium(I) system under carbonylation conditions and, hence, the catalytic performance of the complexes. For the catalytic carbonylation of MeOH, the active catalyst could be prepared <i>in situ</i> from the mixture of [Rh(CO)₂Cl]₂ and the corresponding diphos ligand pbpb, nbpb, or cbpb, giving the same results as carbonylation in the presence of the isolated complexes <b>7</b>, <b>8</b> or <b>9</b> (see <i>Table</i>). The highest activity was observed for complex <b>7</b> (or the mixture [Rh(CO)₂Cl]₂/pbpb, the catalytic turnover number (TON) being 950 after 15 min (170°, 22 bar).