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

Abstract

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 ((D)LCu(I)) to give Cu(I)/O(2) and Cu(I)/CO product species. Members of the (D)LCu(I) series possess an identical tridentate core structure where the cuprous ion binds to the bispicolylamine (L) fragment. (D)L also contains a fourth variable N-donor moiety {D = benzyl (Bz); pyridyl (Py); imidazolyl (Im); dimethylamino (NMe(2)); (tert-butylphenyl)pyridyl (TBP); quinolyl (Q)}. The structural characteristics of (D)LCu(I)-CO and (D)LCu(I) are detailed, with X-ray crystal structures reported for (TBP)LCu(I)-CO, (Bz)LCu(I)-CO, and (Q)LCu(I). Infrared studies (solution and solid-state) confirm that (D)LCu(I)-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 (Im)LCu(I) and (NMe(2))LCu(I) (as well as for (TBP)LCu(I) and (Q)LCu(I)), which relate to the O(2) binding kinetics and thermodynamics. Equilibrium CO binding data (K(CO), ΔH°, ΔS°) were obtained by conducting UV-visible spectrophotometric CO titrations, while CO binding kinetics and thermodynamics (k(CO), ΔH(double dagger), ΔS(double dagger)) were measured through variable-temperature (193-293 K) transient absorbance laser flash photolysis experiments, λ(ex) = 355 nm. Carbon monoxide dissociation rate constants (k(-CO)) and corresponding activation parameters (ΔH(double dagger), ΔS(double dagger)) have also been obtained. CO binding to (D)LCu(I) 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 (λ(ex) = 355 nm, 188-218 K), the kinetics and thermodynamics (k(O(2)), ΔH(double dagger), ΔS(double dagger)) for O(2) binding to (NMe(2))LCu(I) and (Im)LCu(I) were measured and compared to those for (Py)LCu(I). A surprising change in the O(2) binding mechanism was deduced from the thermodynamic ΔS(double dagger) values observed, associative for (Py)LCu(I) but dissociative for (NMe(2))LCu(I) and (Im)LCu(I); 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 (D)LCu(II/I) geometries or the order in which O(2) and THF coordinate. The equilibrium O(2) binding constant (K(O(2)), ΔH°, ΔS°) and O(2) dissociation rate constants (k(-O(2)), ΔH(double dagger), ΔS(double dagger)) 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.

Figure 1

Overview of reduced copper containing protein active-site O₂ and CO interactions, highlighting the heme_a3-Cu_B active-site for O₂ binding/reduction of mitochondrial cytochrome c oxidase (CcO), the arthropodal and molluscan hemolymph O₂-carrier Hc, and the mammalian neuropeptide hormone producing monooxygenase PHM.

Figure 2

ORTEP diagrams of A. ^TBPLCu^I-CO and B. ^BzLCu^I-CO; hydrogen atoms and the B(C₆F₅)₄ counteranion have been omitted for clarity. Selected bond lengths are: A. Cu–C 1.801(6) Å, C–O 1.124(6) Å, Cu–N_alkylamine 2.147(4) Å, Cu–N_{pyridine(avg)} 2.051(5) Å; and B. Cu–C 1.815(1) Å, C–O 1.123(2) Å, Cu–N_alkylamine 2.158(6) Å, and Cu–N_{pyridine(avg)} 2.048(3) Å. Selected bond angles are: A. C–Cu–N_{pyridine(avg)} 121.4(2)°, C–Cu–N_alkylamine 131.0(0)°, N_{pyridine(avg)}–Cu–N_alkylamine 81.0(9)°, N_pyridyl–Cu–N_pyridyl 109.6(0)°; and B. C–Cu–N_{pyridine(avg)} 121.95(7)°, C–Cu–N_alkylamine 129.94(7)°, N_{pyridine(avg)}–Cu–N_alkylamine 80.80(6)°, and N_pyridyl–Cu–N_pyridyl 108.94(6)°.

Figure 3

ORTEP diagram of ^QLCu^I with hydrogen atoms and the B(C₆F₅)₄ counteranion omitted for clarity. Selected bond lengths are: Cu–N_pyridine(1) 1.988(2) Å, Cu–N_pyridine(3) 2.006(2) Å, Cu–N_{alkylamine(2)} 2.185(2) Å, and Cu–N_quinoline(4) 2.012(2) Å. Selected bond angles are: N_pyridyl(avg)–Cu–N_{alkylamine(2)} 82.84(8)°, N_pyridyl(1)–Cu–N_pyridyl(3) 123.63(8)°, N_pyridyl(1)–Cu–N_quinolyl(4) 119.95(8)°, N_amine(2)–Cu–N_quinolyl(4) 82.58(7)°, and N_pyridyl(3)–Cu–N_quinolyl(4) 111.70(8)°.

Figure 4

Spectrophotometric titration of CO to ^TBPLCu^I in THF at room temperature. The inset is a Van't Hoff plot of the variable temperature K_CO data collected from −30 °C to +20 °C.

Figure 5

Transient absorption difference spectra observed after pulsed 355 nm excitation of ^TBPLCu^I-CO in THF at room-temperature under 1 atm of CO. CO photodissociation and the formation of ^TBPLCu^I followed by subsequent CO recombination is measured and shown at various delay times: 0 ns (black); 100 ns (red); 250 ns (blue); 500 ns (green); 1000 ns (purple). The inset is an Eyring analysis plot of the k_CO data collected at variable temperature (−70 °C to 10 °C).

Figure 6

Absorption difference spectra [^NMe₂LCu^I/II-X − ^NMe₂LCu^I-CO] recorded after pulsed λ_ex =355 nm excitation of ^NMe₂LCu^I-CO in THF at 193 K under 1 atm O₂:CO (1:1) mixture. The spectra were recorded at various delay times: (A) 0 to 5 μs representing the conversion of ^NMe₂LCu^I to a mixture of ^NMe₂LCu^I-CO and ^NMe₂LCu^II-O₂⁻: black squares, 0 μs; red circles, 0.1 μs; green triangles, 0.25 μs; blue diamonds, 1 μs; magenta stars, 2.5 μs; orange circles, 5.0 μs. The inset is an absorption transient monitored at 425 nm with a superimposed first-order fit (in red), k_obs = 2.92 × 10⁵ s^-1 and (B) 0 ms to 20 ms representing the conversion from ^NMe₂LCu^II-O₂⁻ to ^NMe₂LCu^I-CO: black squares, 0 ms; red circles, 2.5 ms; green triangles, 5.0 ms; blue diamonds, 10 ms; orange circles, 20 ms. The inset is an absorption transient monitored at 425 nm with a superimposed first-order fit (in red), k_obs = 236 s^-1.

Figure 7

UV-visible spectra in MeTHF at −128 °C representing O₂ + ^DLCu^I (dark blue) to yield: A, [{(^ImL)Cu^II}₂(O₂²⁻)]²⁺ (purple); B, ^PyLCu^II-O₂⁻ (green) and [{(^PyL)Cu^II}₂(O₂²⁻)]²⁺ (purple); C, [{(^NMe₂L)Cu^II}₂(O₂²⁻)]²⁺ (purple) and a new copper-dioxygen adduct of ^NMe₂L (brown).

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