Deprotonation Induced Ligand Oxidation in a Ni^II Complex of a Redox Noninnocent N¹-(2-Aminophenyl)benzene-1,2-diamine and Its Use in Catalytic Alcohol Oxidation

Abstract

Two nickel(II)-complexes, [Ni(II)(H3L)2](ClO4)2 ([1](ClO4)2) and [Ni(II)(HL)2] (2), containing the redox-active tridentate ligand N(1)-(2-aminophenyl)benzene-1,2-diamine (H3L) have been synthesized. Complex [1](ClO4)2 is octahedral containing two neutral H3L ligands in a facial coordination mode, whereas complex 2 is a singlet diradical species with approximately planar configuration at the tetracoordinate metal atom with two pendant NH2 side arms from each of the coordinated ligands. Both complexes are found to be chemically interconvertible; complex [1](2+) gets converted to complex 2 when exposed to base and oxygen via simultaneous deprotonation and oxidation of the coordinated ligands. Molecular and electronic structures of the isolated complexes are scrutinized thoroughly by various spectroscopic techniques, single crystal X-ray crystallography, and density functional theory. The observed dissociation of a ligand arm upon oxidation of the ligand was exploited to bring about catalytic alcohol oxidation using coordinatively saturated complex [1](ClO4)2 as a catalyst precursor. Both the complexes [1](ClO4)2and 2 were tested for catalytic oxidation of both primary and secondary alcohols.

Coordination Chemistry of the Redox Non‐Innocent Ligand Bis(2‐amino‐3,5‐di‐tert‐butylphenyl)amine with Group 10 Metal Ions (Ni, Pd, Pt)

“…The ligands are orientated in a trans ‐configuration, and the metal ion lies in a strictly square‐planar geometry. This geometry is in line with that recently reported for the nickel complex of an unsubstituted related pincer ligand . The most noticeable difference between 1 , 2 , and 3 concerns the coordination bond lengths: The mean M–N bond lengths are 1.827, 1.964, and 1.958 Å for 1 , 2 , and 3 , respectively (Table ).…”

“…It is worth noting that the reaction between H 3 L 3N and a single equivalent of nickel always led to the isolation of a complex identified as 1 , regardless of the nature and amount of the base (Et 3 N, NaOH, n BuLi) and the solvent used (CH 3 CN, MeOH, THF). Paul et al investigated the coordination chemistry of the non‐ tert ‐butylated derivative of H 3 L 3N with nickel . They observed the formation of an octahedral triplet Ni II complex wherein the ligand remains protonated and coordinates in a tridentate fashion.…”

Generation and Oxidative Reactivity of a Ni(II) Superoxo Complex via Ligand-Based Redox Non-Innocence

“…Ligand-based storage of electrons is an important motif in transition-metal reactivity and catalysis. − An archetypal example is found in nature, where oxidation of the porphyrin ligand in heme systems enables the activation of O 2 for further oxidative reactivity. − Despite the utility of ligand-based redox chemistry, most aerobic oxidations in biological and synthetic systems still require metals such as Fe and Cu with accessible redox couples such as Fe­(II)/(III)/(IV) and Cu­(I)/(II). − In contrast to these cases, accessing Ni (II/III) or (II/IV) redox couples is less facile, and Ni-mediated aerobic oxidations are comparatively rare. − In biological systems such as Ni superoxide dismutase, the Ni­(II)/(III) redox potential must be specifically tuned by the ligand environment, mainly via strongly donating thiolate ligands. , Recently, however, an alternative mechanism involving electron transfer from an ancillary ligand to O 2 to generate a Ni­(II) superoxo complex has been invoked in the Ni-containing enzyme quercetin dioxygenase. , Nickel-superoxo species are generally rare even in synthetic systems, and Ni-mediated oxygen activation without accessing Ni­(I) or Ni­(III) oxidation states has little synthetic precedent. − Therefore, the proposed enzymatic mechanism for quercetin dioxygenase motivates studies to examine whether ligand cooperativity is a viable strategy for Ni systems to activate O 2 and mediate oxidative transformations.…”

X‐ray structures, spectroscopic, electrochemical, thermal, antibacterial, and DFT studies of two nickel(II) and cobalt(III) complexes constructed from a new quinazoline‐type ligand

“…The oxidation–reduction peaks correspond to Ni(II)/Ni(I), E pa = 0.046 V, E pc = −1.882 V, with the average formal potential [ E 1/2 = ( E pa + E pc)/2] is −0.918 V. The peak‐to‐peak separation between the anodic and cathodic (Δ E p) is 1.928 V. The ratio of anodic peak current over cathodic peak current ( i pa/ i pc) is 0.60 ( i pa = 18.06 μA, i pc = −29.89 μA). These features are indicative of a quasi‐reversible electrode process …”