Synthesis, characterization and application of new rhodium complexes for indirect electrochemical cofactor regeneration

Electroenzymatic synthesis often suffers from electrochemical reaction steps which proceed slower than the coupled enzyme reaction. For indirect electrochemical cofactor regeneration, we here report two new mediators with superior properties compared to the established rhodium complex (2,2'-bipyridyl)(pentamethylcyclopentadienyl)rhodium [Cp*Rh(2,2'-bipyridine)]. After constructing a robotic system for fast and reliable cyclic voltammetry measurements, we screened twelve rhodium complexes with substituted 2,2'-bipyridine ligands for their reduction potentials and catalytic activity towards the reduction of NADP. Promising complexes were investigated in more detail by cyclic voltammetry and under batch electrolysis conditions. The new complexes Cp*Rh(5,5'-methyl-2,2'-bipyridine) and Cp*Rh(4,4'-methoxy-2,2'-bipyridine) reduced NADP to NADPH three times faster than the established mediator, resulting in volumetric productivities of up to 136 mmol L(-1)d(-1) and turnover frequencies of up to 113 h(-1). This increased reaction rate of these new mediators makes indirect electrochemical approach significantly more competitive to other methods of cofactor regeneration.