CO and O2 Binding to Pseudo-tetradentate Ligand-Copper(I) Complexes with a Variable N-Donor Moiety: Kinetic/Thermodynamic Investigation Reveals Ligand-Induced Changes in Reaction Mechanism

The kinetics, thermodynamics, and coordination dynamics are reported for O-2 and CO 1:1 binding to a series of pseudo-tetradentate ligand copper(I) complexes ((LCuI)-L-D) to give Cu-I/O-2 and Cu-I/CO product species. Members of the (LCuI)-L-D series possess an identical tridentate core structure where the cuprous ion binds to the bispicolylamine (L) fragment. L-D also contains a fourth variable N-donor moiety {D = benzyl (Bz); pyridyl (Py); imidazolyl (Im); dimethylamino (NMe2); (tert-butylphenyl)pyridyl (TBP); quinolyl (Q)}. The structural characteristics of (LCuI)-L-D-CO and (LCuI)-L-D are detailed, with X-ray crystal structures reported for (LCuI)-L-TBP-CO, (LCuI)-L-Bz-CO, and (LCuI)-L-Q. Infrared studies (solution and solid-state) confirm that (LCuI)-L-D-CO possess the same four-coordinate core structure in solution with the variable D moiety dangling, i.e., not coordinated to the copper(I) ion. Other trends observed for the present series appear to derive from the degree to which the D-group interacts with the cuprous ion center. Electrochemical studies reveal close similarities of behavior for (LCuI)-L-Im and (LCuI)-L-NMe2 (as well as for (LCuI)-L-TBP and (LCuI)-L-Q), which relate to the O-2 binding kinetics and thermodynamics. Equilibrium CO binding data (K-CO, Delta H degrees, Delta S degrees) were obtained by conducting UV-visible spectrophotometric CO titrations, while CO binding kinetics and thermodynamics (k(CO), Delta H-not equal, Delta S-not equal) were measured through variable-temperature (193-293 K) transient absorbance laser flash photolysis experiments, lambda(ex) = 355 nm. Carbon monoxide dissociation rate constants (k(-CO)) and corresponding activation parameters (Delta H-not equal, Delta S-not equal) have also been obtained. CO binding to (LCuI)-L-D follows an associative mechanism, with the increased donation from D leading to higher k(CO) values. Unlike observations from previous work, the K-CO values increased as the k(CO) and k(-CO) values declined; the latter decreased at a faster rate. By using the flash-and-trap method (lambda(ex) = 355 nm, 188-218 K), the kinetics and thermodynamics (k(O2), Delta H-not equal, Delta S-not equal) for O-2 binding to (LCuI)-L-Nme2 and (LCuI)-L-Im were measured and compared to those for (LCuI)-L-Py. A surprising change in the O-2 binding mechanism was deduced from the thermodynamic Delta S-not equal values observed, associative for (LCuI)-L-Py but dissociative for (LCuI)-L-Nme2 and (LCuI)-L-Im; these results are interpreted as arising from a difference in the timing of electron transfer from copper(I) to O-2 as this molecule coordinates and a tetrahydrofuran (THF) solvent molecule dissociates. The change in mechanism was not simply related to alterations in (LCuII/I)-L-D geometries or the order in which O-2 and THF coordinate. The equilibrium O-2 binding constant (K-O2, Delta H degrees, Delta S degrees) and O-2 dissociatin rate constants (k(-O2), Delta H-not equal, Delta S-not equal) were also determined. Overall the results demonstrate that subtle changes in the coordination environment, as occur over time through evolution in nature or through controlled ligand design in synthetic systems, dictate to a critically detailed level the observed chemistry in terms of reaction kinetics, structure, and reactivity, and thus function. Results reported here are also compared to relevant copper and/or iron biological systems and analogous synthetic ligand copper systems.