Synthesis and Characterization of a Masked Terminal Nickel‐Oxide Complex

Abstract

In exploring terminal nickel‐oxo complexes, postulated to be the active oxidant in natural and non‐natural oxidation reactions, we report the synthesis of the pseudo‐trigonal bipyramidal NiII complexes (K)[NiII(LPh)(DMF)] (1[DMF]) and (NMe4)2[NiII(LPh)(OAc)] (1[OAc]) (LPh=2,2’,2’’‐nitrilo‐tris‐(N‐phenylacetamide); DMF=N,N‐dimethylformamide; −OAc=acetate). Both complexes were characterized using NMR, FTIR, ESI‐MS, and X‐ray crystallography, showing the LPh ligand to bind in a tetradentate fashion, together with an ancillary donor. The reaction of 1[OAc] with peroxyphenyl acetic acid (PPAA) resulted in the formation of [(LPh)NiIII−O−H⋅⋅⋅OAc]2−, 2, that displays many of the characteristics of a terminal Ni=O species. 2 was characterized by UV‐Vis, EPR, and XAS spectroscopies and ESI‐MS. 2 decayed to yield a NiII‐phenolate complex 3 (through aromatic electrophilic substitution) that was characterized by NMR, FTIR, ESI‐MS, and X‐ray crystallography. 2 was capable of hydroxylation of hydrocarbons and epoxidation of olefins, as well as oxygen atom transfer oxidation of phosphines at exceptional rates. While the oxo‐wall remains standing, this complex represents an excellent example of a masked metal‐oxide that displays all of the properties expected of the ever elusive terminal M=O beyond the oxo‐wall.

Aromatic and aliphatic hydrocarbon hydroxylation via a formally NiIVO oxidant

“…The overlay of these spectra revealed a different speciation. In fact, the EPR spectrum of [Ni III (m-CBA)(L Ph )] − displayed similarities with that obtained for the Ni III -(hydr)oxide complex (isotropic signal, no 14 N hyperfine), 49 consistent with a highly symmetri- Importantly, we were also able to generate 2 using peroxybenzoic acid (PBA) as an alternative to m-CPBA. The obtained UV-Vis and EPR features assigned to 2 prepared with PBA matched those of 2 that had been generated using m-CPBA (Fig.…”

“…1[OAc] (Scheme 1) was prepared according to a previously reported method. 49 Addition of m-CPBA (1.0 equiv., CH 2 Cl 2 ) to A colour change from a pale green to a dark green colour was noted and a maximum yield was achieved within 60 s. The new absorption spectrum was markedly different to that obtained when 1[OAc] was reacted with aliphatic peracids, 49 indicating a different species had formed. Exactly one equivalent of m-CPBA was required to yield the maximum yield of 2 (Fig.…”

“…3, S5 and Table S1 †). Analysis of a sample of 1[OAc] showed a K-edge energy of 8343.4 eV, 49 with a distinct pre-edge feature at 8332.7 eV corresponding to an electronic-dipole forbidden 1s-to-3d transition, which gains intensity in non-centrosymmetric geometries due to p-d mixing. 52,53 The Ni-K-edge for 2 exhibited an edge energy of 8344.4 eV, with a +1.0 eV blue shift compared to 1[OAc] (Table 1).…”

“…In order to probe this mechanism, we attempted to identify the faith of the hydrogen atom, using the isotopically labelled 1[OAc]-D 15 . 49…”

“…2 displayed k 2 values comparable to previous Ni III -OX examples. [63][64][65] The metastable formally Ni IV vO entity was a superior oxidant, while [Ni III (O-H⋯OAc)(L Ph )] 49 was also a capable hydrocarbon oxidant.…”