Formation, Characterization, and Reactivity of Bis(μ-oxo)dinickel(III) Complexes Supported by A Series of Bis[2-(2-pyridyl)ethyl]amine Ligands

Abstract

Bis(mu-oxo)dinickel(III) complexes supported by a series of bis[2-(2-pyridyl)ethyl]amine ligands have been successfully generated by treating the corresponding bis(mu-hydroxo)dinickel(II) complexes or bis(mu-methoxo)dinickel(II) complex with an equimolar amount of H(2)O(2) in acetone at low temperature. The bis(mu-oxo)dinickel(III) complexes exhibit a characteristic UV-vis absorption band at approximately 410 nm and a resonance Raman band at 600-610 cm(-1) that shifted to 570-580 cm(-1) upon (18)O-substitution. Kinetic studies and isotope labeling experiments using (18)O(2) imply the existence of intermediate(s) such as peroxo dinickel(II) in the course of formation of the bis(mu-oxo)dinickel(III) complex. The bis(mu-oxo)dinickel(III) complexes supported by the mononucleating ligands (L1(X) = para-substituted N,N-bis[2-(2-pyridyl)ethyl]-2-phenylethylamine; X = OMe, Me, H, Cl) gradually decompose, leading to benzylic hydroxylation of the ligand side arm (phenethyl group). The kinetics of the ligand hydroxylation process including kinetic deuterium isotope effects (KIE), p-substituent effects (Hammett plot), and activation parameters (Delta H(H)(*) and Delta S(H)(*)) indicate that the bis(muxo)dinickel(III) complex exhibits an ability of hydrogen atom abstraction from the substrate moiety as in the case of the bis(mu-oxo)dicopper(III) complex. Such a reactivity of bis(mu-oxo)dinickel(III) complexes has also been suggested by the observed reactivity toward external substrates such as phenol derivatives and 1,4-cyclohexadiene. The thermal stability of the bis(mu-oxo)dinickel(III) complex is significantly enhanced when the dinucleating ligand with a longer alkyl strap is adopted instead of the mononucleating ligand. In the m-xylyl ligand system, no aromatic ligand hydroxylation occurred, showing a sharp contrast with the reactivity of the (mu-eta(2):eta(2)-peroxo)dicopper(II) complex with the same ligand which induces aromatic ligand hydroxylation via an electrophilic aromatic substitution mechanism. Differences in the structure and reactivity of the active oxygen complexes between the nickel and the copper systems are discussed on the basis of the detailed comparison of these two systems with the same ligand.

Dioxygenase‐Like Reactivity of an Isolable Superoxo–Nickel(II) Complex

“…It seems clear, though, that the bulkiness of the b-diketiminato ligand may be understood as playing two roles: on one hand it stabilizes the Ni-superoxo unit by sterically protecting it, but on the other hand, it constitutes a potential nearby substrate susceptible to be attacked that may partially hamper the interaction with exogenous substrates. Intramolecular ligand oxidation by well-defined dinuclear nickel-dioxygen species has been reported in a few cases [14][15][16] and it has also been observed for putative mononuclear nickel-dioxygen intermediates. [24,54] …”

“…[24,25] This phenomenon has been observed for selected dinuclear systems described above. For example, the well-defined bis-A C H T U N G T R E N N U N G (m-oxo)dinickelA C H T U N G T R E N N U N G (III) configuration can decompose through aromatic hydroxylation, [15] aliphatic hydroxylation [16] or Ndealkylation [14] of the supporting ligand. In the case of the mononuclear (peroxo)nickelA C H T U N G T R E N N U N G (III) configuration reactivity studies with exogenous organic substrates indicate that it behaves as a nucleophile in the deformylation of aldehydes [20] while for (alkylperoxo)nickel(II) some electrophilic character (oxidation of triphenylphosphine and CO) has also been observed.…”

Ni K-edge XAS suggests that coordination of NiII to the unstructured amyloidogenic region of the human prion protein produces a Ni2 bis-μ-hydroxo dimer

“…We therefore compared the edge features of [(L1 H Ni II ) 2 (l-OH) 2 ](ClO 4 ) 2 (a Ni II 2 bis-lhydroxo dimer originally prepared by Itoh and coworkers [13]; Scheme 1) 3 with ½Ni II 2 ðHO À Þ 2 ðPrPð93-114ÞÞ. The XANES of [(L1 H Ni II ) 2 (l-OH) 2 ](ClO 4 ) 2 displays a Ni 1s !…”

“…Although it is impossible to make any definitive conclusions concerning the identity of the N/O ligands on the basis of these data we suspect that the shorter N/O ligands are anionic amide nitrogens (from the peptide backbone) while the longer N/O ligands are a mixture of nitrogen and oxygen (from hydroxide) ligands. These assignments are based on a comparison of the EXAFS data with synthetic compounds; [(L1 H Ni II ) 2 -(l-OH) 2 ](ClO 4 ) 2 contains bridging Ni-hydroxo ligands at an average distance of 1.99 Å [13], while amide bound Ni II -complexes have Ni-N distance at $1.85 Å [15][16][17]. Although amide-nitrogens are unusual biological ligands [18] amide-coordination is observed in Ni-containing metallopeptides [15,19] and amides are thought to be involved in Cu II coordination to the PrP [20].…”

Oxidation Reactivity of Bis(μ‐oxo) Dinickel(III) Complexes: Arene Hydroxylation of the Supporting Ligand

“…The band can be assigned as a Ni 2 O 2 core vibration typical of bis(m-oxo) dinickel(III) complexes bearing tridentate nitrogen donors (599-612 cm À1 ). [7,8] Thus the brown species can be assigned as the bis…”