Synthetic mimics of protein active sites enable the study of elusive thermodynamic properties. Cobalt cubanes have been studied extensively and are models for the Oxygen Evolving Complex of Photosystem II. However, little attention has been paid to related dimeric forms, such as [Co2(µ-OH)2(OAc)3(py)4]PF6, which was prepared as part of a mixture by Sumner in 1988. We have demonstrated how to selectively synthesize this complex by using a low pH and strong oxidant. This cobalt dimer features bridging hydroxido groups and mono- and bidentate acetate ligands that participate in an intriguing network of hydrogen-bonds (H-bonds). We reported our ability to tune the relative position of the proton in the H-bond and its resulting acidity by modifying the para-substituents on the pyridine ligands (methyl, methoxy, tbutyl, CN, acetyl, CF3, and DMAP) and verified a linear relationship through 1H-NMR and stopped-flow UV-Vis kinetics. The presence of electron-donating groups on the pyridine ligands drew the protons closer to the bridging hydroxido moiety, while electron-withdrawing groups pushed the protons further toward the bridging acetate. Our focus has shifted to determining the ability of these complexes to undergo proton coupled electron transfer (PCET). Bulk electrolysis results in decomposition of complex while for reaction with chemical oxidants, different products are obtained whether the oxidation is performed in coordinating or non-coordinating solvents. In MeCN, [Co2(µ-OH)2(µ-OAc)(OAc)(CH3CN)(py)4]2+ was isolated as the reaction product and further oxidation results in formation of [Co2(µ-OH)2(µ-OAc)(CH3CN)2(py)4]3+; both structures have been fully characterized by XRD and NMR analysis. While in chloroform, a powder formed that by solid-FTIR and 13C-NMR analyses suggested formation of a new complex in which only bridging acetates are observed. The powder is unable to be characterized by solution methods as overtime in solution it reforms [Co2(µ-OH)2(OAc)3(py)4]+ and other solvent adducts. When the reaction was performed with [Co2(µ-OH)2(OAc)3(py)4]BArF20, an XRD quality crystal was obtained and structure solution was [Co2(µ-OH)2(µ-OAc)2(py)4][BArF20]2. Further studies are underway to confirm if the same di-bridged intermediate is forming in coordinating solvents. Methods are being explored on how to trap that reactive intermediate in solution and DFT studies are underway in helping with proposed reactivity pathways.

Modeling oxidase metalloenzymes with dicobalt bis(µ-hydroxo) complexes

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