Formation of a Cobalt(III)−Phenoxyl Radical Complex by Acetic Acid Promoted Aerobic Oxidation of a Co(II)salen Complex

Abstract

The activation of N,N'-bis(3,5-di-tert-butylsalicylidene)-1,2-cyclohexane-diamino Co(II), [Co(II)(1)], by the addition of acetic acid under aerobic conditions has been investigated by a range of spectroscopic techniques including continuous-wave EPR, HYSCORE, pulsed ENDOR, and resonance Raman. These measurements have revealed for the first time the formation of a coordinated cobalt(III)-bound phenoxyl radical labeled [Co(III)(1(*))(OAc)(n)](OAc)(m) (n = m = 1 or n = 2, m = 0). This cobalt(III)-bound phenoxyl radical is characterized by the following spin Hamiltonian parameters: g(x) = 2.0060, g(y) = 2.0031, g(z) = 1.9943, A(x) = 17 MHz, A(y) = 55 MHz, and A(z) = 14 MHz. Although the radical contains coordinated acetate(s), the experiments unambiguously proved that the phenoxyl radical is situated on ligand (1) as opposed to a phenoxyl radical ligated to cobalt in the axial position. Density functional theory computations on different models corroborate the stability of such a phenoxyl radical species and suggest the ligation of one or two acetate molecules to the complex. A mechanism is proposed, which accounts for the formation of this unusual and extremely robust phenoxyl radical, never previously observed for [Co(1)].

Syntheses, Electronic Structures, and EPR/UV−Vis−NIR Spectroelectrochemistry of Nickel(II), Copper(II), and Zinc(II) Complexes with a Tetradentate Ligand Based on S-Methylisothiosemicarbazide

“…11,12 The use of other than copper(II) metal ions, e.g. nickel(II), cobalt(II) or zinc(II),13,14,15 is another dimension emerging in this research area, which can lead to creation of novel synthetic catalysts for selective oxidation reactions. Some of these complexes can be chemically or electrochemically oxidized by one electron to give products which exhibit temperature- or ligand field geometry-dependent valence tautomerism between a higher-valent metal-phenolate and a metal-phenoxyl radical (i.e., [M n + −(PhO − )] ( n −1)+ vs [M ( n −1)+ −(PhO • )] ( n −1)+ ) 12,16,17,18.…”

Chemical-promoted oxidative polymerization of modified-cobalt salen complexes, efficient catalysts for the dynamic HKR

“…26 The polymerization of complex 3 in air has thus been tested, with Poly-3b being recovered with quantitative yield (Table 1, entry 2). 28 The EPR spectrum of Poly-3b showed no noticeable difference in comparison to that of Poly-Co(III), while a strong signal of Co(II) and a totally negligible signal of radical were observed for Poly-3a. 27 The EPR spectrum of electrogenerated Poly-Co(III) showed a strong signal for the radical (g = 2.002) at 3500 G and also a strong signal for paramagnetic Co(II) at 1500 G, which are in accordance with the reported EPR signals of Jacobsen's Co(III)-OAc complex, as the salen Co(III) complex is in equilibrium with a Co(II) phenoxyl radical species.…”

Cobalt supported on dendronized magnetic nanoparticles: A new highly efficient and recyclable catalyst for the Mizoroki–Heck cross‐coupling reaction

“…It is apparent from the previous study [55] that the formation of the active Co I species would advance the cross-coupling reaction. [56] 3.4 | Comparison of MNP@PAMAM-Co results with those obtained by other workers for the Mizoroki-Heck reaction In the next steps, olefin coordination with (B) and migratory insertion lead to the formation of adducts (C) and (D), respectively.…”