Carbon Monoxide Coordination and Reversible Photodissociation in Copper(I) Pyridylalkylamine Compounds

Systematic studies of CO coordination and photodissociation have been carried out for a series of copper(I) carbonyl
compounds possessing tripodal tetradentate ligands, [CuI(L)(CO)]B(C6F5)4 (L = Me2N-TMPA (1Me2N), MeO-TMPA
(1MeO), H-TMPA (1H), PMEA (2pmea), PMAP (2pmap), BQPA (3bqpa). Detailed structural, electrochemical, and infrared
characterization has been accomplished. In addition, various experimental techniques were utilized to determine
equilibrium binding constants (KCO), association (kCO), and dissociation (k-CO) rate constants, as well as the
thermodynamic (ΔH°, ΔS°) and activation parameters (ΔH⧧, ΔS⧧) that regulate these processes. With increased
ligand-electron-donating ability, greater π back-bonding results in stronger Cu−CO bonds, leading to KCO values
on the order 1Me2N−CO > 1MeO−CO > 1H−CO. With systematic synthetic expansion of the five-membered chelate
rings like 1R to six-membered chelate rings like 2R, the stability of the CO adduct decreases, 1H−CO > 2pmea−CO
> 2pmap−CO. The CO-binding properties of 3bqpa did not follow trends observed for the other compounds, presumably
because of its bulkier ligand framework. Through solid- and solution-state analyses, we concluded that the photolabile
carbonyl species in solution possess a tridentate coordination mode, forming strictly five-membered chelate rings
to the copper ion with one dangling arm of the tripodal ligand. Carbon monoxide reversibly photodissociated from
complexes 1Me2N−CO, 1MeO−CO, 1H−CO, and 3bqpa−CO in coordinating (CH3CN) and weakly coordinating (THF)
solvent but not from 2pmea−CO and 2pmap−CO. Comparisons to O2-binding data available for these copper complexes
as well as to small molecule (O2, CO, NO) reactions with hemes and copper proteins are discussed.