Mechanistic Study of Rhodium/xantphos-Catalyzed Methanol Carbonylation

Abstract

Rhodium/iodide catalysts modified with the xantphos ligand are active for the homogeneous carbonylation of methanol to acetic acid using either pure CO or CO/H2. Residues from catalytic reactions contain a Rh(III) acetyl complex, [Rh(xantphos)(COMe)I2] (1), which was isolated and crystallographically characterized. The xantphos ligand in 1 adopts a “pincer” κ3-P,O,P coordination mode with the xanthene oxygen donor trans to the acetyl ligand. The same product was also synthesized under mild conditions from [Rh(CO)2I]2. Iodide abstraction from 1 in the presence of donor ligands (L = MeCN, CO) gives the cationic acetyl species [Rh(xantphos)(COMe)I(L)]+, whereas in CH2Cl2 migratory CO deinsertion gives [Rh(xantphos)(Me)I(CO)]+ (4), which reacts with H2 to liberate methane, as observed in catalytic reactions using syngas. A number of Rh(I) xantphos complexes have been synthesized and characterized. Oxidative addition of methyl iodide to the cation [Rh(xantphos)(CO)]+ is very slow but can be catalyzed by addition of an iodide salt, via a mechanism involving neutral [Rh(xantphos)(CO)I] (6). IR spectroscopic data and DFT calculations for 6 suggest the existence in solution of conformers with different Rh–O distances. Kinetic data and activation parameters are reported for the reaction of 6 with MeI, which proceeds by methylation of the Rh center and subsequent migratory insertion to give 1. The enhancement of nucleophilicity arising from a Rh- - -O interaction is supported by DFT calculations for the SN2 transition state. A mechanism for catalytic methanol carbonylation based on the observed stoichiometric reaction steps is proposed. A survey of ligand conformations in xantphos complexes reveals a correlation between P–M–P bite angle and M–O distance and division into two broad categories with bite angle <120° (cis) or >143° (trans).

PC‐Activated Bimetallic Rhodium Xantphos Complexes: Formation and Catalytic Dehydrocoupling of Amine–Boranes

“…All these data are consistent with η 1 ‐coordination of the amine–borane being retained in solution 17. 19 Complex 4 can also be formed by reaction of 5 with Na[H 3 BNMe 2 BH 3 ], revealing that the xantphos′ ligand can adopt a flexible coordination mode20 in response to the requirements of the amine–borane ligands.…”

Dehydrogenative Boron Homocoupling of an Amine‐Borane

“…However, the Ir pincer system Ir(tBuPOCOPtBu)(H) 2 [tBuPO-COPtBu = k 3 P,C,P -1,3-(OPtBu 2 ) 2 C 6 H 3 ] catalyzes the dehydropolymerization of H 3 B·NMeH 2 , for which polymer growth kinetics suggest a coordination insertion mechanism consistent with the activation of two amine-boranes at the metal center before B À N bond formation. [22] Complex 2 is closely related to acetone and NCMe adducts such as [Rh(k 3 P,O,P -Xantphos)(H) 2 (NCMe)]-[BAr F 4 ]. [20] Addition of H 3 B·NMe 3 to the precursor [Rh(k 2 P,P -Xantphos)(NBD)][BAr F 4 ] [21] under a H 2 atmosphere resulted in rapid hydrogenation of the diene and coordination of the amine-borane to the resulting Rh III dihydride to give [Rh(k 3 P,O,P -Xantphos)(H) 2 (h 1 -H 3 B·NMe 3 )][BAr F 4 ], 2, in essentially quantitative yield as measured by NMR spectroscopy (Scheme 2).…”

“…Complex 2 was also characterized by singlecrystal X-ray diffraction (Figure 1), which demonstrates a pseudo octahedral Rh III center with an H 3 B·NMe 3 ligand coordinated through a single Rh-H-B interaction (Rh1ÀB1, 2.759(10) ), and a mer-k 3 Xantphos ligand (Rh1ÀO1, 2.199(3) ). [22] Complex 2 is closely related to acetone and NCMe adducts such as [Rh(k 3 P,O,P -Xantphos)(H) 2 (NCMe)]-[BAr F 4 ]. [21] The solution NMR data (Supporting Information) are in full accord with the solid-state structure and are consistent with the h 1 -coordination mode of the borane.…”

Influence of Xantphos Derivative Ligands on the Coordination in Their Copper(I) and Silver(I) Complexes

“…Although such coordination is not common in copper complexes with xanthene ligands, few examples have been reported with t BuXantphos ligand with Cu–O distances in the range of 2.25 and 2.75 Å . Actually, an inverse relationship between the P–M–P bite angle and the M–O distance of the Xantphos ligand has been stablished . This correlation is observed in 1 and 3 .…”