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(l) carbonyl compounds possessing tripodal tetradentate ligands, [Cu-l(L)(CO)]B(C6F5)(4) (L = Me2N-TMPA (1(ME2N)), MeO-TMPA (1(MeO)), H-TMPA (1(H)), PMEA (2(pmea)), PMAP (2(pmap)), BQPA (3(bqpa)). Detailed structural, electrochemical, and infrared characterization has been accomplished. In addition, various experimental techniques were utilized to determine equilibrium binding constants (K-CO), association (k(CO)), and dissociation (k-(CO)) rate constants, as well as the thermodynamic (Delta H degrees, Delta S degrees) and activation parameters (Delta(double dagger), Delta St(double dagger)) that regulate these processes. With increased ligand-electron-donating ability, greater pi back-bonding results in stronger Cu-CO bonds, leading to K-CO values on the order 1(Me2N)-CO > 1(MeO)-CO > 1(H)-CO. With systematic synthetic expansion of the five-membered chelate rings like 1(R) to six-membered chelate rings like 2(R), the stability of the CO adduci decreases, 1(H)-CO > 2(pmea)-CO > 2(pmap)-CO. The CO-binding properties of 3(bqpa) 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 1(Me2N)-CO, 1(MeO)-CO, 1(H)-CO, and 3(bqpa)-CO in coordinating (CH3CN) and weakly coordinating (THF) solvent but not from 2(pmea)-CO and 2(pmap)-CO. Comparisons to O-2-binding data available for these copper complexes as well as to small molecule (O-2, CO, NO) reactions with hemes and copper proteins are discussed.