Synthesis, Structure, Spectral and Electrochemical Properties, and Catalytic Use of Cobalt(III)−Oxo Cubane Clusters

Abstract

Olive-green cobalt(III) complexes having the general formula Co4O4(O2CMe)4L4 (1) where L = py (1a), 4-Mepy (1b), 4-Etpy (1c), and 4-CNpy (1d) have been prepared by the H2O2 oxidation of a mixture of Co2+, MeCO2-, and pyridine or 4-substituted pyridines in a 1:2:1 molar ratio in methanol. Spectroscopic and X-ray crystallographic studies show that these complexes contain a tetrameric cubane-like core [Co4(mu3-O)4]4+ where the four cobalt atoms form an approximate tetrahedron with edge lengths of approximately 2.75 A. Each cobalt in the crystallographically determined structure of Co4(mu3-O)4(mu-O2CMe)4L4 in 1a.NaClO4.3.5H2O and 1b.3H2O is hexacoordinate. Infrared spectra of the complexes show characteristic bands near 700, 635, and 580 cm-1 due to the central cubane-like core. 1H NMR spectra of the complexes show that the dissolved species are essentially diamagnetic and also that the complexes maintain their integrity in solution. UV-vis spectra of the green solutions have been interpreted in terms of ligand-field and charge-transfer bands. The electrochemical behavior of the complexes studied by cyclic and differential pulse voltammetric techniques indicates that the [(CoIII)4(mu3-O)4]4+ core present in the complexes undergoes a reversible one-electron oxidation to the [(CoIII)3CoIV(mu3-O)4]5+ core with an E1/2 value below 1 V. This suggests that these complexes of cobalt may be suitable as catalysts for the oxidation of organic compounds. Preliminary investigations indicate that 1a has a role to play in the cobalt-catalyzed aerobic oxidation of neat ethylbenzene and p-xylene.

Mechanistic Investigations of Water Oxidation by a Molecular Cobalt Oxide Analogue: Evidence for a Highly Oxidized Intermediate and Exclusive Terminal Oxo Participation

“…With respect to 1, the Co−OH bonds of [1H]OTs are elongated to an average length of 1.897(2) Å, while the remaining Co−O bond lengths remain unchanged with an average of 1.865(2) Å (1 has a Co−O bond length range of 1.860−1.876 Å). 34 A close contact of 2.586(2) Å between an oxygen atom of the cubane (O(4)) and the tosylate oxygen atom O(13) is consistent with O···H···O hydrogen bonding. The 1 H NMR spectrum of [1H]OTs in CD 3 CN at room temperature suggests that the molecule is C s symmetric with broadened resonances, suggesting slow proton exchange on the NMR time scale.…”

Catalytic Oxygen Evolution by Cobalt Oxido Thin Films

“…The half-wave potential of the pre-feature is observed at~+0. 95 [44] respectively. For the series Na (Fig.…”

“…The aim is not to provide a review of the molecular water oxidation catalysts, cobalt-containing or otherwise, but to highlight the results obtained from studying molecules that have refined our thinking with regard to the mechanisms germane to the activity of Co-OEC. [95], were not only shown to be useful for EPR studies, but have also been explored as discrete water oxidation catalysts [101][102][103]. Though some evidence of electrochemical water oxidation was presented in these studies, the primary mode of characterization of the water oxidation activity was through the use of a photochemical assay, utilizing Ru(bpy) 3 2+ /persulfate as the photosensitizer/sacrificial oxidant.…”

Artificial photosynthesis: a molecular approach to photo-induced water oxidation

“…3) deserved particular attention since (i) cobalt is a low-cost and abundant metal, (ii) its tetranuclear oxo core mimics the natural oxygen-evolving complex of Photosystem II [38][39][40], and (iii) it stands as the homogeneous analogue of the amorphous cobalt oxide/hydroxide film (Nocera's catalyst, sometimes referred to as Co-Pi) [41], whose electrocatalytic potential has been recently exploited within light activated devices [42][43][44]. Photoinduced water oxidation to molecular oxygen takes place in system where [Ru(bpy) 3 ] 2+ is the photosensitizer, Co 4 O 4 -H the molecular catalyst and Na 2 S 2 O 8 the sacrificial electron acceptor.…”

“…The isostructural complexes are obtained according to literature protocols [38], and their solution identity and stability has been confirmed by NMR and ESI-MS. Inspection of the Co 4 O 4 -X catalyst properties has been initially addressed under dark conditions, with cyclic voltammetry (CV), by evaluating the water discharge overpotentials (η) and the onset of a catalytic current.…”