Modulating the Elementary Steps of Methanol Carbonylation by Bridging the Primary and Secondary Coordination Spheres

Abstract

The rate of catalytic methanol carbonylation to acetic acid is typically limited by either the oxidative addition of methyl iodide or the subsequent C–C bond-forming migratory insertion step. These elementary steps have been studied independently in acetonitrile solution for iridium amino­phenyl­phosphinite (NCOP) complexes. The modular synthesis of NCOP ligands containing a macrocyclic aza-crown ether arm enables a direct comparison of two complementary catalyst optimization strategies: synthetic modification of the phenyl backbone and noncovalent modification through cation–crown interactions with Lewis acids in the surrounding environment. The oxidative addition of methyl iodide to iridium­(I) carbonyl complexes proceeds readily at room temperature to form iridium­(III) methyl­carbonyl­iodide complexes. The methyl complexes undergo migratory insertion under 1 atm CO at 70 °C to produce iridium­(III) acetyl species. Synthetic tuning, by incorporation of a methoxy group into the ligand backbone, had little influence on the rate. The addition of lithium and lanthanum salts, in contrast, enhanced the rate of C–C bond formation up to 25-fold. In the case of neutral iodide complexes, mechanistic studies suggest that Lewis acidic cations act as halide abstractors. In halide-free, cationic iridium complexes, the cations bind the macrocyclic ligand arm, further activating the iridium­(III) center. The macrocyclic ligand is essential to the observed reactivity: complexes supported by acyclic diethylamine-containing ligands underwent migratory insertion slowly, Lewis acid effects were negligible, and the acetyl products decomposed over time.

An Ion‐Responsive Pincer‐Crown Ether Catalyst System for Rapid and Switchable Olefin Isomerization

“…Considering the lack of methods for controlling substrate binding in small organometallic catalysts, we have initiated a program to develop ligands with ion‐tunable hemilability . With a macrocycle as hemilabile ligand, dissociation of the chelate reveals a receptor for cation binding.…”

Decarbonylative ether dissection by iridium pincer complexes

“…30 ppm upeld shi of the phosphinite resonance relative to 1 18c6 (from d 143.67 to d 171.32) in the 31 P NMR spectrum provided an early indication that the product was an iridium(I) complex. 13,18 We initially vetted the spectroscopic data against the possibility of iridium(I) complexes with either (a) a tetradentate pincer-crown ether ligand containing one crown ether oxygen bound, (k 4 -MeO-18c6 NCOP)Ir, or (b) a tridentate pincer-crown ether ligand with N 2 completing the square planar coordination sphere, (k 3 -MeO-18c6 NCOP)Ir(N 2 ). However, neither of these structures could be explained by the NMR data.…”

Study on Rh(I)/Ru(III) Bimetallic Catalyst Catalyzed Carbonylation of Methanol to Acetic Acid

“…However, Rh(I) is unstable during the reaction of the Monsanto rhodium/iodine catalyst. When the CO partial pressure is low, (Rh(CO) 2 I 2 ) − interacts with hydroiodic acid to form dimers to form Rh(III) precipitates [ 10 , 11 , 12 , 13 ], causing the deactivation of the precious metal catalyst. Therefore, a large number of polar solvents such as H 2 O and acetic acid are added to the system to increase the solubility of the catalyst, thereby improving the stability of the catalyst.…”