Five Coordinate M(II)-Diphenolate [M = Zn(II), Ni(II), and Cu(II)] Schiff Base Complexes Exhibiting Metal- and Ligand-Based Redox Chemistry

Abstract

Five-coordinate Zn(II), Ni(II), and Cu(II) complexes containing pentadentate N(3)O(2) Schiff base ligands [1A](2-) and [1B](2-) have been synthesized and characterized. X-ray crystallographic studies reveal five coordinate structures in which each metal ion is bound by two imine N-donors, two phenolate O-donors, and a single amine N-donor. Electron paramagnetic resonance (EPR) spectroscopic studies suggest that the N(3)O(2) coordination spheres of [Cu(1A)] and [Cu(1B)] are retained in CH(2)Cl(2) solution and solid-state superconducting quantum interference device (SQUID) magnetometric studies confirm that [Ni(1A)] and [Ni(1B)] adopt high spin (S = 1) configurations. Each complex exhibits two reversible oxidation processes between +0.05 and +0.64 V vs [Fc](+)/[Fc]. The products of one- and two-electron oxidations have been studied by UV/vis spectroelectrochemistry and by EPR spectroscopy which confirm that each oxidation process for the Zn(II) and Cu(II) complexes is ligand-based with sequential formation of mono- and bis-phenoxyl radical species. In contrast, the one-electron oxidation of the Ni(II) complexes generates Ni(III) products. This assignment is supported by spectroelectrochemical and EPR spectroscopic studies, density functional theory (DFT) calculations, and the single crystal X-ray structure of [Ni(1A)][BF(4)] which contains Ni in a five-coordinate distorted trigonal bipyramidal geometry.

Structural and Biochemical Insights into Dimethylsulfoniopropionate Cleavage by Cofactor-Bound DddK from the Prolific Marine Bacterium Pelagibacter

“…19 The results Figure S6). A shoulder near nm (~ 860 M -1 cm -1 ) was observed that can be attributed to tyrosinate-Ni(II) interaction, given a Ni(II)-phenolate chromophore has been shown to exhibit bands between 200-400 nm 53 and a Ni(II)-thiolate species has an absorption feature at 315 nm. 54,55 In contrast, intense LMCT transitions due to Ni(II)-phenolate that appear between 400-600 nm (ε >900 M -1 cm -1 ) center (3.9 Å), the pKa is most likely lowered producing the deprotonated form ready to initiate catalysis.…”

Five-Coordinate Zinc(II) Complex: Synthesis, Characterization, Molecular Structure, and Antibacterial Activities of Bis-[(E)-2-hydroxy-N′-{1-(4-methoxyphenyl)ethylidene}benzohydrazido]dimethylsulfoxidezinc(II) Complex

“…Due to this reason, 5-coordinate Zn complex is considered unusual and expected to be unstable. Nevertheless, 5-coordinate Zn complexes were previously reported [10][11][12][13][14]. Similar to 4-coordinate Zn complexes, the common oxidation state for Zn ion in 5-coordinate Zn complexes is +2; however, the one reported by Song et al is in 0 oxidation state [13].…”

Electrocatalytic and Photocatalytic Hydrogen Evolution by Ni(II) and Cu(II) Schiff Base Complexes

“…These complexes possess a pseudo‐square pyramidal geometry involving one unsaturated coordination site. Previous study found that oxidation of these complexes primarily occurs in the Schiff‐base ligand, yielding mono‐ and bis‐ phenoxy radical species [8,23–26] . The redox properties of these phenolate‐rich complexes are reminiscent of the active metal site of GAO, whose catalytic pathway also includes Cu‐phenolate radical intermediates.…”

“…The N 3 O 2 type of Schiff‐base (Scheme 1, H 2 L =N,N′‐bis(3,5‐di‐tert‐butyl‐2‐hydroxybenzyliden)‐1,7‐diamino‐4‐azaheptane) forms five‐coordinate complexes with first‐row transition metals [23] . These complexes possess a pseudo‐square pyramidal geometry involving one unsaturated coordination site.…”