Energy production from renewable energy sources is gradually becoming the global norm. Regarded as one of the cleanest renewable sources of energy, hydrogen (H2) can be obtained from water, the most promising and abundant source. However, due to the challenging nature of the water splitting reaction given by 2H2O → O2 +2H2 (which comprises a water oxidation half reaction (2H2O → O2 + 4H+ + 4e-) and a water reduction half reaction (4H2O+ 4e- → 2H2+ 4OH-), it is necessary that robust and efficient water reduction catalysts be developed. In some binuclear analogues of certain monometallic catalysts, activity is expected to be enhanced when two metal centers are in close proximity. This is because the close distance between the two metal centers enhances cooperativity (two metal centers working in tandem). There are two main forms of cooperativity, namely; structural cooperativity (only one metal center is involved in electron transfer while the other metal center acts as a structural reinforcer) and electronic cooperativity (both metal centers are involved in electron transfer). We hypothesize that bimetallic species that display cooperativity among metals will lead to efficient catalysis. In this research, we study the development of binuclear cobalt and nickel catalysts with a 2,6-bis [(bis(2-pyridylmethyl) amino) methyl]-4-methylphenol ligand (HL1), to assess the presence and mechanisms of cooperation for water reduction. With the results obtained, we have theoretically and experimentally proven that the bimetallic nickel catalyst exhibits no cooperativity whereas the bimetallic cobalt catalyst does show structural cooperativity. This is because the second cobalt is not engaged in electron transfer. However, it has additional electronic characteristics because the HS/LS flip prevents dissociation of pyrazole.

Mechanistic cooperativity for water reduction with homobimetallic cobalt and nickel catalysts

ACS Spring Meeting 2021

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