Cobalt(III)-oxo cubane clusters as catalysts for oxidation of organic substrates

Abstract

Transition metal coordination complexes play a vital role as catalysts in the oxidation of organic substrates including renewable chemicals in an economically viable and environmentally friendly way. Here we highlight the preparation, characterization and application of oxo-cubane complexes of cobalt(III) as oxidation catalysts using air and water as oxidants. Cobalt(III)-oxo complexes of the type Co 4 O 4 (O 2 CR) 4 L 4 have been prepared by a general method and these have been characterized by analytical, spectroscopic, electrochemical and crystallographic methods. These soluble complexes have shown promising utility as catalysts in the aerobic oxidation of side chains of alkylaromatic hydrocarbon compounds. Oxidation of neat ethylbenzene has shown very high conversion and selectivity for acetophenone formation. On the other hand, oxidation of p-xylene has been found to yield both p-toluic acid and terephthalic acid. It is also possible to oxidize p-xylene in an aqueous medium under moderate applied O 2 pressure. Selective epoxidation of α-pinene with air as the oxidant also takes place with the cobalt(III)-based homogeneous catalysts.

Synthesis, crystal structures and characterization of late first row transition metal complexes derived from thiosemicarbazone hub: DNA binding/cleavage studies

“…The bonding of the ligand, with CoN3, CoS1 and CoO1 distances of 1.880, 2.175 and 1.993 Å forming two five‐membered C 2 NOCo and CN 2 SCo chelate rings with bite angles of 82.6° and 86.2°, respectively, represents distortion from an ideal octahedral geometry . The CoO1 distance is found to be slightly longer than those reported in the literature for Co(III) complexes . The bond lengths in the complex C1 are: N2N3 = 1.39(1), C10O1 = 1.43(1) and C7S1 = 1.75(9) Å, which are longer or shorter than those of the corresponding distances in the free ligand LH: N2N3 = 1.380(3), C10O1 = 1.392(4), C7S1 = 1.677(3) Å due to the bonding of the Co(III) centre with the ligand.…”

X-ray Spectroscopic Characterization of Co(IV) and Metal–Metal Interactions in Co4O4: Electronic Structure Contributions to the Formation of High-Valent States Relevant to the Oxygen Evolution Reaction

“…Nonetheless, the design of homogeneous inorganic complexes as models of heterogeneous OEC catalysts is difficult to achieve, and few bona fide examples exist where both structure and activity are replicated. In recent years, inorganic molecular clusters have been developed as potential water oxidation catalysts, inspired by the Mn 4 CaO 5 cluster of photosystem II (PSII) OEC. − An intensively studied heterogeneous/homogeneous pair with regard to OEC is the cobalt phosphate (CoP i )/Co 4 O 4 cubane pair, respectively, as CoP i comprises Co 4 O 4 cubanes as its basic structural element. , Co 4 O 4 structures with different ligand sets (Figure shows the Co 4 O 4 (OAc) 4 (py) 4 primary ligation sphere) have been synthesized and characterized − and they have been intensively researched as molecular mimics of CoP i . − The active sites of CoP i are Co­(IV) centers, and indeed, Co 4 O 4 cubanes can house S = 1 / 2 Co­(IV) centers that exhibit electron paramagnetic resonance (EPR) signals comparable to those observed for Co­(IV) in activated catalyst films. , The multifrequency EPR results (at <10 K) indicate that the Co­(IV) spin density is symmetrically delocalized over the Co 4 O 4 core, and the X-ray crystal structure of the oxidized cubane has also been interpreted as fully delocalized . These observations are related to the degree of electron–hole delocalization over the Co 4 O 4 core, which can be described in terms of mixed-valency [i.e., effectively Co(3.25) 4 vs Co­(IV)­Co­(III) 3 ].…”

Bioinspired cobalt cubanes with tunable redox potentials for photocatalytic water oxidation and CO2 reduction

“…2 , three absorption bands are observed in the UV–vis spectra. The lowest energy absorption appearing as a shoulder at 645 to 660 nm, is associated with the d–d transitions involving 1 A 1 → 1 T 1 and 1 A 1 → 1 T 2 for the approximately octahedral Co complex [ 53 – 55 ]. As judged by the observed intensities, the other two bands are attributable to absorptions rather than d–d transitions.…”

“…Thus, the control of catalytic properties via molecular design by tunable ligand substitution is essential in the development of Co 4 O 4 -based cubane catalysts. However, most of the researches focused on the oxidative properties of the Co 4 O 4 core [ 53 ], and its use for reduction reactions is rarely covered. Theoretically, the redox potential of Co 4 O 4 cubane clusters should be tuned by virtue of different ligand substitutions, thus it is highly possible to develop a Co 4 O 4 -based catalyst for reduction applications, such as H 2 evolution and CO 2 fixation.…”