Synthesis, Structure and Redox Properties of Asymmetric (Cyclopentadienyl)(ene-1,2-dithiolate)cobalt(III) Complexes Containing Phenyl, Pyridyl and Pyrazinyl Units

• Published in: European journal of inorganic chemistry 2015-07, Vol.2015 (21), p.3550-3561
• Main Authors: Dicks, James P., Zubair, Muhammad, Davies, E. Stephen, Garner, C. David, Schulzke, Carola, Wilson, Claire, McMaster, Jonathan
• Format: Article
• Language: English
• Subjects: Catalysis; Catalysts; Cobalt; Cofactors; Enzymes; Metalloenzymes; Molybdenum; Molybdopterins; Orbitals; Oxygen atom transfer; Phenyls; Redox chemistry; Redox reactions; Reduction
• ISSN: 1434-1948; 1099-0682
• DOI: 10.1002/ejic.201500138

The compounds [(η5‐C5H5)Co{SC(H)CRS}] [R = phenyl (1), pyridin‐3‐yl (2) or pyrazin‐2‐yl (3)] have been synthesized and characterized by elemental analysis, 1H NMR, mass spectrometry and X‐ray crystallography. The variation in the UV/Vis and redox properties of these compounds alone and upon acidification has been investigated. In CH2Cl2 solution each compound undergoes a reversible one‐electron reduction, and the EPR spectrum of each monoanion has been recorded. In the presence of a 5:1 excess of trifluoroacetic acid the one‐electron reduction of both 2 and 3 occurs at a less negative potential. The information obtained has been interpreted with the aid of DFT calculations for [1]y, [2]y and [3]y (y = 0 or –1) and the monoprotonated forms [2H]z and [3H]z (z = +1 or 0), and this has provided insight into the nature of the redox‐active orbitals in these complexes. The HOMOs and LUMOs of these species are delocalized and each involves contributions from cobalt, ene‐1,2‐dithiolate and R orbitals. The information from the experimental and theoretical investigations is used to suggest that, for the pyran ring‐opened form of the molybdenum cofactor of oxygen‐atom‐transfer enzymes, the pterin may participate in the redox reactions involved in the catalytic cycle. Structural, electrochemical, spectroscopic and theoretical studies of [(η5‐C5H5)Co(dithiolene)] complexes suggest that the pyran ring‐opened form of the molybdenum cofactor of Mo‐ and W‐containing oxygen‐atom‐transfer enzymes may participate in the redox reactions involved in the catalytic cycle.