Valence Tautomerism in Octahedral and Square‐Planar Phenoxyl–Nickel(II) Complexes: Are Imino Nitrogen Atoms Good Friends?

Abstract

The two tetradentate ligands H2L and H2LMe afford the slightly distorted square‐planar low‐spin NiII complexes 1 and 2, which comprise two coordinated phenolate groups. Complex 1 has been electrochemically oxidized into 1+, which contains a coordinated phenoxyl radical, with a contribution from the nickel orbital. In the presence of pyridine, 1+ is converted into 1Py+, an octahedral phenolate nickel(III) complex with two pyridines axially coordinated: An intramolecular electron transfer (valence tautomerism) is promoted by the geometrical changes, from square planar to octahedral, around the metal center. The tetradentate ligand H2LMe, in the presence of pyridine, and the hexadentate ligand H2LPy in CH2Cl2 afford, respectively, the octahedral high‐spin NiII complexes 2Py and 3, which involve two equatorial phenolates and two axially coordinated pyridines. At 100 K, the one‐electron‐oxidized product 2Py+ comprises a phenoxyl radical ferromagnetically coupled to the high‐spin NiII ion, with large zero‐field splitting parameters, while 3+ involves a phenoxyl radical antiferromagnetically coupled to the high‐spin NiII ion.

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

“…The five-line superhyperfine pattern in the highfield component is assigned to two equivalent pyridine ligands axially bound to the Ni III center to form 1-py 2 + . Similarly to the findings of others, [6,7] axial coordination of pyridine to 1 + shifts the d z 2 orbital to higher energy, resulting in the transfer of an electron from the metal center to the ligand. Computational studies on 1-py 2 + ( Figure S4 in the Supporting Information) demonstrate an increase in spin density on Ni from 0.15 (for 1 + ) to 0.93, which is consistent with this shift in the locus of oxidation.…”

“…Temperature-dependent valence tautomerism has also been reported for 2 + [7,8] and 3 + , [6] whereas the analogous complex 4 + [10] was found to be a ligand radical in the range 10-295 K. Recent reports have suggested that these tautomeric shifts are associated with axial binding of water or other impurities at the solvent freezing point, rather than a shift in the locus of oxidation due to temperature alone. [8,10] In an effort to understand this process further, a solution of 1 + in the presence of a 100-fold excess of tetrabutylammonium perchlorate (TBAP) was studied by EPR spectroscopy (Figure 3 b).…”

“…The electronic spectrum of 1 ( Figure 4, Table 2) is typical of a low-spin d 8 square-planar Ni II complex [7,10] and changes substantially upon oxidation to 1 + , particularly in the lowenergy region. Low-energy absorption bands for 1 + appear at 11 300 and 9200 cm À1 , as previously reported by Shimazaki et al, [9] and similar transitions are reported for 2 + , 3 + , and 4 + .…”

Synthesis of a One-Dimensional Coordination Polymer of Nickel(II) Complex with a β-Oxodithioester Ligand

“…The solid spectrum shows a notable difference respecting precursor (Figure 3). The band LMCT at 459 nm is notably red shifted from 1 (449 nm), similar to similar systems [23]. Three more bands were observed at 571, 886, 922 and 955 nm, which were assigned to the transitions…”

Oxidation Chemistry of Metal(II) Salen-Type Complexes

“…The valence state difference dependent on solvents can be considered to be due to coordination of DMF molecules to the nickel ion. Addition of exogenous ligands such as pyridine to the CH2Cl2 solution of [Ni(Salcn)] + causes the color change from green to purple, and the solution showed the UV-vis absorption spectrum and EPR signals characteristic of Ni(III) complexes [63,64]. Further, the X-ray crystal structure of -oxo Ni III (salen) dimer has been reported to be synthesized by addition of excess O2 under basic conditions ( Figure 13) [55].…”