Fully Delocalized Mixed‐Valent Cu^1.5Cu^1.5 Complex: Strong Cu‐Cu interaction and Fast Electron Self‐Exchange Rate Despite Large Structural Changes**

Abstract

A flexible macrocyclic ligand with two tridentate {CNC} compartments can host two Cu ions in reversibly interconvertible states, Cu I Cu I (1) and mixed-valent Cu 1.5 Cu 1.5 (2). They were characterized by XRD and multiple spectroscopic methods, including EPR, UV/Vis absorption and MCD, in combination with TD-DFT and CASSCF calculations. 2 features a short Cu•••Cu distance ( � 2.5 Å; compared to � 4.0 Å in 1) and a very high delocalization energy of 13 000 cm À 1 , comparable to the mixed-valent state of the biological Cu A site. Electron self-exchange between 1 and 2 is rapid despite large structural reorganization, and is proposed to proceed via a sequential mechanism involving an active conformer of 1, viz. 1'; the latter has been characterized by XRD. Such electron transfer (ET) process is reminiscent of the conformationally gated ET proposed for biological systems. This redox couple is a unique pair of flexible dicopper complexes, achieving fast electron self-exchange closely related to the function of the Cu A site.

Cu4S Cluster in “0-Hole” and “1-Hole” States: Geometric and Electronic Structure Variations for the Active CuZ* Site of N2O Reductase

“…This redox potential is significantly cathodically shifted compared to the 0-hole model complexes [(μ 2 -dppm) 4 Cu 4 (μ 4 -S)]­(PF 6 ) 2 and [(μ 2 -dppa) 4 Cu 4 (μ 4 -S)]­(PF 6 ) 2 (Figure b, A and B ; E pa = +0.26 and −0.05 V, vs Fc +/0 , respectively) for which only irreversible oxidation events were observed. ,,, This difference can likely be attributed to the strong σ-donating ability of the NHC ligands; at the same time, the NHC-based scaffold L is capable of stabilizing the {Cu 4 (μ 4 -S)} core in both relevant oxidation states. However, the redox event at E 1/2 = −0.65 V becomes irreversible when the scan range is extended anodically to +0.42 V (vs Fc +/0 ), and a quasi-reversible redox event at E 1/2 = −0.06 V appears (Figure a) that is strikingly similar to the redox couple 1 ox / 1 ( E 1/2 = −0.09 V) . This can be attributed to the rapid decomposition of the species resulting after 2-fold oxidation of 2 , giving two equiv of 1 .…”

“…The continuous-wave X-band (9.63 GHz) EPR spectrum of 3 in acetone recorded at 50 K is pseudoaxial with a hyperfine pattern on the low field side attributed to the 63/65 Cu ( I = 3/2) nuclei, with relatively small splittings and an intensity pattern that suggests the unpaired electron is delocalized over more than one Cu ion (Figure a). The spectrum features a seven-line hyperfine splitting (2 nI +1; where n is the number of equivalent nuclei) and intensity pattern resulting from two equivalent copper centers . To further resolve the EPR parameters, a two-pulse echo-detected Q-band (34.0 GHz) EPR spectrum of 3 in acetone was collected at 15 K. This higher-frequency spectrum resolved the rhombic splitting of the g-tensor, particularly the narrow g 3 feature (Figure S23).…”

“…In particular, a system that allows for the isolation and investigation of a pair of {Cu 4 (μ 4 -S)} complexes in the biologically relevant [3Cu I :Cu II ] (1-hole, S = 1/2) and [4Cu I ] (0-hole, S = 0) states is still lacking. N -Heterocycle carbenes (NHCs) are increasingly employed as alternative ligands and as surrogates of histidine residues in bioinspired model chemistry, facilitating the stabilization of reactive intermediates and unusual oxidation states. − In our recent work, we found that the macrocycles {py 2 NHC 2 } (L’, py = pyridine) and {py 2 NHC 4 } (L), featuring combinations of pyridine and NHC donors, show great flexibility and can support mononuclear complexes [L’Cu] 1/2/3+ in various Cu oxidation states (+I, +II, and +III) , as well as binuclear complexes [LCu 2 ] 2/3+ in the Cu I Cu I and mixed-valent Cu 1.5 Cu 1.5 states, respectively. The mixed-valent complex [LCu 1.5 Cu 1.5 ] 3+ features a large spin delocalization energy and fast electron self-exchange rate, which structurally and functionally mimics the Cu A site.…”

Synthesis, Characterization, and Spectroscopic Studies of 2,6-Dimethylpyridyl-Linked Cu(I)–CNC Complexes: The Electronic Influence of Aryl Wingtips on Copper Centers

“…While the inequivalent halves of the CNC system are found in the 1 H NMR spectra of many transition metal complexes with lutidine-based bis-NHC ligands, group 11 metals acted differently. In all the 2,6-dimethylpyridyl-linked Cu­(I)–CNC complexes reported thus far, the CH 2 -linkers appeared as a sharp singlet regardless of the nature of the wingtips or the deuterated NMR solvent. ,,– The different behavior of the Cu–CNC complexes is likely due to their less constrained linear or T -shape geometry, which allows fluxional processes relating both arms of the pincer ligand under ambient conditions. The conformational exchanges of the pincer CNC ligands were reported in Rh­(I)- R CNC complexes (R = Me, Mes), wherein sharp singlets for the pyridylic protons were found in the presence of a labile coligand like CH 3 CN.…”

Photocatalytic C(sp3)–H bond functionalization by Cu(i) halide cluster-mediated O2 activation

“…17,18 Compared with a mononuclear copper complex, Cu clusters formed by coordination interlocking might provide catalytic synergy and stability. 19,20 Moreover, it was considered that the use of Cu( i )–X clusters would be competent as antennas to absorb external light energy and generate a high energy state, which could conveniently drive the reduction activation of O 2 through electron transfer events.…”