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 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 are reported. 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 ‡ , ΔS ‡ ) were measured through variable temperature (193 K -293 K) transient absorbance laser flash photolysis experiments, λ ex = 355 nm. Carbon monoxide dissociation rate constants (k −CO ) and corresponding activation parameters (ΔH ‡ , ΔS ‡ ) 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 that seen in previous work, the K CO values increased as the k CO and k −CO values declines; the latter decreased at a faster rate. By using the 'flash-and-trap' method (λ ex = 355 nm ; 188 K -218 K), the kinetics and thermodynamics (k O2 ; ΔH ‡ , ΔS ‡ ) for O 2 binding to NMe 2 LCu I and Im LCu I were measured and compared to Py LCu I . A surprising change in the O 2 binding mechanism was deduced from the thermodynamic ΔS ‡ 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 that O 2 / THF coordinate. The equilibrium O 2 binding constant (K O2 ; ΔH°, ΔS°) and O 2 dissociation rate constants (k −O2 ; ΔH ‡ , ΔS ‡ ) were also determined. Overall the results demonstrate that subtle changes in the coordination environment, as occurs 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 k...

Homogeneous catalytic O 2 reduction to water by a cytochrome c oxidase model with trapping of intermediates and mechanistic insights

“…5B) also starts via fast electron transfer from Fc* to the heme, forming 6 LFe II . In the absence of the Cu, however, as discussed, the subsequent O 2 -binding is slower than that observed for ½ In fact, this conclusion about the role of Cu agrees with other notable findings: (i) O 2 -binding to copper complexes can occur at near diffusion-controlled rates (26,27), (ii) CcO enzyme studies in fact implicate Cu B as the entry point for O 2 during catalysis (28)(29)(30).…”

Asymmetric cyclopropanation of olefins catalysed by Cu(i) complexes of chiral pyridine-containing macrocyclic ligands (Pc-L*)

“…As revealed by IR in solution, a sharp band at 2111 cm −1 was present immediately after (5 min) exposure to CO at atmospheric pressure. The observed frequency of coordinated CO in complex 7d is higher than those reported in the case of tetracoordinated copper(I) CO complexes of [bis(2-pyridylmethyl)amine], 24 probably due to the lower efficiency of aliphatic amines in transferring electron density to the metal centre compared to pyridine.…”

Characterization of Metastable Intermediates Formed in the Reaction between a Mn(II) Complex and Dioxygen, Including a Crystallographic Structure of a Binuclear Mn(III)–Peroxo Species

“…51,86 Oxygen binding to six-coordinate iron(II) further decrease upon increasing the steric bulk of the ligand (e.g., [Fe 2 (OH) 2 (6-Me 3 TPA) 2 ] 2+ ). 75,86 Oxygen binding to coordinatively unsaturated Co(II) or Cu(I) is even more rapid (Table 5), 76,87-89 but it can be also retarded by steric hindrance (e.g. [Cu(HIPT 3 )tren] + ).…”

“…Although oxygen binding to biologically relevant Fe(II), Cu(I), and Co(II) has been extensively studied, 62,91,92 the observation of more than one intermediate in these processes is relatively rare. 76,87,88,93 Manganese dioxygen chemistry, on the other hand, remains largely unexplored. 32,35,40 Low-temperature stopped-flow experiments reported herein show that a dioxygen intermediate, [Mn III (S Me2 N 4 (6-MeDPEN))(O 2 )] + ( 4 ), rapidly forms upon the addition of O 2 to coordinatively unsaturated 1 consistent with O 2 binding to an open coordination site.…”