Several new iridium(I) and iridium(III) carbonyl complexes supported by aminophosphinite pincer ligands have been prepared and characterized. A surprising diversity of reaction pathways was encountered upon treatment of Ir carbonyl complexes with Li + , Na + , Ca 2+ , and La 3+ salts. Iridium(III) hydridocarbonyl chloride complexes underwent either halide abstraction or halide substitution reactions, whereas iridium(I) carbonyl complexes underwent protonative oxidative addition reactions. When the nitrogen donor of the pincer ligand is an aza-crown ether macrocycle, cation−macrocycle interactions could be supported, leading to divergent reactivity in some cases.

The (PNP)PdOTf complex is a suitable synthetic equivalent of the [(PNP)Pd](+) fragment in reactions with various HX substrates. The [(PNP)Pd](+) fragment either simply binds HX molecules as L-type ligands (X = NH(2), PCy(2), imidazolyl) or heterolytically splits the H-X bond to produce [(PN(H)P)Pd-X](+) (X = H, CCR, SR). DFT calculations analyze the relative energetics of the two outcomes and agree with the experimental data. Calculations also allow to assess the unobserved Pd(IV) isomer [(PNP)Pd(H)(2)](+) and validate its unfavourability with respect to the Pd(II) isomer [(PN(H)P)PdH](+).

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.

Iridium complexes of pincer ligands containing aza-crown ether macrocycles are precatalysts for methanol carbonylation. Turnover numbers for all acetyl-containing products could be tuned from 265 to 1950 using metal and tetrabutylammonium salts.