American Chemical Society
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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

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journal contribution
posted on 2010-09-22, 00:00 authored by Heather R. Lucas, Gerald J. Meyer, Kenneth D. Karlin
The kinetics, thermodynamics, and coordination dynamics are reported for O2 and CO 1:1 binding to a series of pseudo-tetradentate ligand−copper(I) complexes (DLCuI) to give CuI/O2 and CuI/CO product species. Members of the DLCuI series possess an identical tridentate core structure where the cuprous ion binds to the bispicolylamine (L) fragment. DL 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 DLCuI−CO and DLCuI are detailed, with X-ray crystal structures reported for TBPLCuI−CO, BzLCuI−CO, and QLCuI. Infrared studies (solution and solid-state) confirm that DLCuI−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 ImLCuI and NMe2LCuI (as well as for TBPLCuI and QLCuI), which relate to the O2 binding kinetics and thermodynamics. Equilibrium CO binding data (KCO, ΔH°, ΔS°) were obtained by conducting UV−visible spectrophotometric CO titrations, while CO binding kinetics and thermodynamics (kCO, ΔH, ΔS) were measured through variable-temperature (193−293 K) transient absorbance laser flash photolysis experiments, λex = 355 nm. Carbon monoxide dissociation rate constants (k−CO) and corresponding activation parameters (ΔH, ΔS) have also been obtained. CO binding to DLCuI follows an associative mechanism, with the increased donation from D leading to higher kCO values. Unlike observations from previous work, the KCO values increased as the kCO and k−CO values declined; the latter decreased at a faster rate. By using the “flash-and-trap” method (λex = 355 nm, 188−218 K), the kinetics and thermodynamics (kO2, ΔH, ΔS) for O2 binding to NMe2LCuI and ImLCuI were measured and compared to those for PyLCuI. A surprising change in the O2 binding mechanism was deduced from the thermodynamic ΔS values observed, associative for PyLCuI but dissociative for NMe2LCuI and ImLCuI; these results are interpreted as arising from a difference in the timing of electron transfer from copper(I) to O2 as this molecule coordinates and a tetrahydrofuran (THF) solvent molecule dissociates. The change in mechanism was not simply related to alterations in DLCuII/I geometries or the order in which O2 and THF coordinate. The equilibrium O2 binding constant (KO2, ΔH°, ΔS°) and O2 dissociation rate constants (k−O2, ΔH, ΔS) 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.