The Geometric and Electronic Structure of a One‐Electron‐Oxidized Nickel(II) Bis(salicylidene)diamine Complex

Abstract

The cooperativity of transition-metal ions and proradical ligands in metalloenzyme active sites is of current research interest. [1] In an effort to understand the intricacies of the interaction of metal ions with organic radicals, many transition-metal complexes with one or more organic radical ligands have been studied. [2][3][4] Depending on the relative energies of the redox-active orbitals, metal complexes with proradical ligands can exist in a limiting description as a metal-ligand-radical (M n+ (LC)) or a high-valent metal complex (M (n+1)+ (L À )). Given favorable energetics, valence tautomerism can occur through variation of the ligand field or temperature. [2,[5][6][7][8][9] In particular, much recent interest exists in nickel(II) bis(salicylidene)diamine complexes (Scheme 1), [6][7][8][9][10] ) forms.Although extensive spectroscopic and electrochemical data exists for such oxidized complexes, structural data is lacking. Herein we report the X-ray crystal structure of a Ni IIligand-radical complex (1 + ), which has a contracted coordination sphere relative to its neutral analogue 1. The difference between the first two oxidation waves in the cyclic voltammogram of 1 (DE = 500 mV) and those of the Cu (DE = 205 mV) and Zn (DE = 175 mV) analogues [11] indicates that 1

Class III Delocalization and Exciton Coupling in a Bimetallic Bis‐ligand Radical Complex

“…[9,14] The CV curve for 2 exhibits two closely-spaced reversible one-electron redox couples at E 1 1=2 = 360 mV and Table 3. [9,14] The CV curve for 2 exhibits two closely-spaced reversible one-electron redox couples at E 1 1=2 = 360 mV and Table 3.…”

“…The proradical tyrosine ligand acts as an additional redox cofactor, facilitating electron transfer during catalysis. [8][9][10] One area of recent focus are tetradentate salen ligands (salen is a common abbreviation for N 2 O 2 bis-Schiff-base bis-phenolate ligands). [3,5,6] Recent interest has centered on the design of synthetic systems that employ the cooperative effect of redox-active ligands to drive new catalytic processes.…”

“…Due to the apparent simplicity of the active site, numerous structural and functional small-molecule models of galactose oxidase have been studied. [6,[9][10][11][12][13][14][15][16][17][18][19][20] As an example, one-electron oxidized Ni salen derivatives exist in the ligand radical form Ni II A C H T U N G T R E N N U N G (LC À ) in solution and the solid state, however, the addition of exogenous ligands to Ni II -A C H T U N G T R E N N U N G (LC À ) in solution results in a shift in the oxidation locus to the Ni III A C H T U N G T R E N N U N G (L 2À ) form. [2,7] Depending on the relative energies of the redox-active orbitals, metal complexes with proradical ligands can exist as either a metal-ligand radical [M n + (LC)] or a high-valent metal complex [M (n + 1) + (L À )].…”

Proton‐Mediated Redox Control in a Nickel(II)–Bisimidazolate Complex: Spectroscopic Characterisation and Density Functional Analysis

“…with two imino and two phenolate groups in the coordination sphere of the metal ion. [2,3,40] We observed, in contrast with the phenol containing ligands, a distinct difference in the spin density distribution map. As can be seen in Figure 7, the contribution of the imidazolate rings is very weak in the description of the SOMO (a HOMO).…”

“…An important piece of evidence for the proper description of the one-electron-oxidised form of Ni II -bis(salicylidene)diamine derivatives was recently brought by the group of Stack. [3] Indeed, the X-ray crystallographic data are supportive of a nickel(II) ion bound to a phenoxyl radical. Furthermore, the authors argued that the observed valence tautomeric effect observed in this family of complexes is probably owed to an axial coordination on the metal centre at low temperature.…”