Coordination Chemistry of a Model for the GP Cofactor in the Hmd Hydrogenase:  Hydrogen-Bonding and Hydrogen-Transfer Catalysis

Cp*M²⁺ complexes (M = Rh, Ir; Cp* = C₅Me₅) are described for 6-(carboxymethyl)-4-methyl-2-hydroxypyridine (cmhpH₂), an analogue of the guanylylpyridone cofactor in the hydrogenase Hmd. Three findings indicate that Cp*M(Hcmhp)⁺ stabilizes the binding of hydrogen-bond acceptors to the sixth coordination site:  (i) water binds in preference to Cl^-, (ii) the adduct Cp*Rh(cmhp)(2-hydroxypyridine) exhibits a very short intramolecular hydrogen bond (r_O-O = 2.38 Å; ¹H NMR δ_H 17.2), and (iii) Cp*Ir(cmhpH)Cl efficiently catalyzes the dehydrogenation of PhCH(OH)Me to PhC(O)Me.