A Copper(I) Complex with Two Unpaired Electrons, Synthesised by Oxidation of a Copper(II) Complex with Two Redox‐Active Ligands

Abstract

Two homoleptic copper(II) complexes [Cu(L1)2] and [Cu(L2)2] with anionic redox‐active ligands were synthesised, one with urea azine (L1) and the other with thio‐urea azine (L2) ligands. One‐electron oxidation of the complexes initiates an unprecedented redox‐induced electron transfer process, leading to monocationic copper(I) complexes [Cu(L1)2]+ and [Cu(L2)2]+ with two oxidised ligands. While [Cu(L1)2]+ is best described as a CuI complex with two neutral radical ligands that couple antiferromagnetically, [Cu(L2)2]+ is a CuI complex with two clearly different ligand units in the solid state and with a magnetic susceptibility close to a diamagnetic compound. Further one‐electron oxidation of the complex with L1 ligands results in a dication [Cu(L1)2]2+, best described as a CuI complex with a twofold oxidised, monocationic ligand and a neutral radical ligand. The stability in at least three redox states, the accumulation of spin density at the ligands and the facile ligand‐metal electron transfer make these complexes highly attractive for a variety of applications; here the catalytic aerobic oxidation of alcohols to aldehydes is tested.

Peculiar Differences between Two Copper Complexes Containing Similar Redox-Active Ligands: Density Functional and Multiconfigurational Calculations

“…In addition, one of the Cu−N 1,3 bond lengths increases significantly upon oxidation, leading to a larger average Cu−N bond length compared to the neutral complex. The dihedral angle between the N−Cu−N planes of each ligand is also increased (from 51.5 to 65.4°), 27 arguing for a higher occupation of the Cu 3d orbitals and supporting a description rather in terms of a Cu(I) atom and two oxidized, neutral radical ligands. On the other hand, in the solid-state structure of [Cu(L2) 2 ]SbF 6 , the copper atom is coordinated by two clearly different L2 ligands.…”

“…In addition, one of the Cu−N 1,3 bond lengths increases significantly upon oxidation, leading to a larger average Cu−N bond length compared to the neutral complex. The dihedral angle between the N−Cu−N planes of each ligand is also increased (from 51.5 to 65.4°), 27 both of the two L2 ligands, the bond lengths within the central C 1 −N 2 −N 3 −C 2 units of the ligands harmonize (see Table 1), but the change is clearly smaller for one of the ligands. Furthermore, the two Cu−N 1,3 bonds in [Cu(L2) 2 ] + are shortened on average compared to the neutral [Cu(L2) 2 ] complex, in contrast to the [Cu(L1) 2 ] + complex, where one Cu−N 1,3 distance is strongly elongated.…”

“…Moreover, for some of these complexes, a RIET process was observed upon oxidation. 27 For the similar neutral mono-nuclear homoleptic tetrahedral copper complexes [Cu(L1) 2 ] and [Cu(L2) 2 ] sketched in Figure 1a, with the two different This figure is divided into four main sections: (a) cyclic voltammograms: The anodic scan is performed at 100 mV s −1 in CH 2 Cl 2 , and the results are referenced externally to the redox couple ferrocenium/ferrocene (Fc + /Fc). The redox waves, which signal the stabilization of the oxidized, monocationic form of the copper complexes, are highlighted in pink.…”

“…A stronger deviation from the free-electron g factor is evident in [Cu(L2) 2 ]PF 6 . For additional details on the experimental data, refer to the cited literature 27.…”

Asymmetric Sonogashira C(sp3)–C(sp) bond coupling enabled by a copper(i) complex of a new guanidine-hybrid ligand

“…7 Chiral guanidines and their derivatives have emerged as useful organocatalysts owing to their basicity and hydrogen bond donor ability. 8 Although the ligating properties of guanidines have been characterized, 9 the application of chirally neutral guanidine–metal complexes in asymmetric catalysis remains elusive. Earlier, Anders and coworkers tested the zinc( ii ) complex of guanidine–amine for the asymmetric Henry reaction.…”