A Dinuclear Copper Complex Featuring a Flexible Linker as Water Oxidation Catalyst with an Activity Far Superior to Its Mononuclear Counterpart

The development of highly active, stable, and inexpensive molecular water oxidation catalysts (WOCs) is important for eventually realizing artificial photosynthesis. The dinuclear earth-abundant molecular WOCs are of great interest as the latent synergy of two adjacent metals of the molecule could enhance the activity of the catalyst for water oxidation, just as the synergistic catalysis effect of metal ions in many metalloenzymes. Herein we report the dinuclear copper complex [L1Cu(2)(mu-OH)](BF4)(3) (1, L1 = N,N'-dimethyl-N,N'bis{2-[bis(2-pyridinylmethyl)amino]ethyllethane-1,2-diamine) with a flexible linker and its mononuclear counterpart [L2Cu(OH2)]-(BF4)(2)(2, L2 = N,N-dimethyl-N',N'-bis(2-pyridylmethyl)ethane-1,2-diamine) as WOCs. X-ray diffraction analysis showed that in the crystal structure of 1 there is an extraneous water molecule located very close to the bridged O atom, resembling the proposed structure of the transition state for the O-O bond formation. Comparative studies on the performances of 1 and 2 for electrochemical water oxidation at pH 12 manifested that 1 displayed a much higher activity and better stability than that of 2. The k(cat1) of 144 s(-1) for 1 is on a par with those of the state-of-the-art earth-abundant molecular WOCs reported to date under similar test conditions. Experimental studies and DFT calculations suggest that the water oxidation catalyzed by 1 proceeds via a unimolecular two-site mechanism with a much lower energy barrier for the O-O bond formation step compared to that for 2.