Piano-stool d6-rhodium(III) complexes of chelating pyridine-based ligands and their papain bioconjugates for the catalysis of transfer hydrogenation of aryl ketones in aqueous medium

Two half-sandwich d6-rhodium(III) complexes of the general formula [(η5-Cp∗)Rh(N^N)Cl]Cl where N^N is a phenanthroline or a bispyridine methane derivative carrying a thiol-targeting maleimide or chloroacetamide function were synthesized and characterized. Both complexes were able to catalyse the transfer hydrogenation of 2,2,2-trifluoroacetophenone in aqueous medium using formate or phosphite as hydrogen donor. Covalent anchoring of these complexes to the cysteine endoproteinase papain yielded hybrid metalloproteins with transfer hydrogenase properties. Under optimized conditions of pH, hydrogen donor concentration and catalyst load, conversion of substrate was nearly quantitative within 24 h at 40 °C and the (S)-enantiomer was obtained preferably albeit with a modest enantiomeric excess of 7-10%. Covalent docking simulations complemented the experimental findings suggesting a molecular rationale for the observed low enantioselectivity. The harmonious use of experimental and theoretical approaches represents an unprecedented starting point for driving the rational design of artificial metalloenzymes built up from papain with higher catalytic efficiency.