Early-stage formation of (hydr)oxo bridges in transition-metal catalysts for photosynthetic processes

Water-oxidizing metal–(hydr)oxo catalyst films can be generally deposited and activated by applying a positive electrochemical potential to suitable starting aqueous solutions. Here, we used ab initio simulations based on density functional theory to investigate the initial formation process of hydroxo and oxo bridges between divalent transition metals [namely Co(II), Mn(II), and Ni(II)] in aqueous solution, leading to the growth of extended structures. Our simplified yet realistic model, rooted in the computational hydrogen electrode approximation, has been able to provide estimates in agreement with experimental measurements of the positive potential U required for deposition of the active amorphous metal–(hydr)oxo catalyst, taking into account variations in the solution composition. Our results reveal that: (i) Co, Mn, and Ni exposed to a positive potential form dinuclear building blocks through different reaction pathways, indicating structural features consistent with those previously reported for extended systems; (ii) key steps in the formation of stable hydroxo and oxo bonds are preceded by structural rearrangements of M(II)[H2O]6 complexes, which are stabilized by H-bond formation among the hydration shells upon the approach of two units. This arrangement yields suitable dinuclear precursors with one or two water molecules holding a bridging position between metals; (iii) anionic phosphate (for Co) and acetate (for Mn) ligands favor the formation of stable dinuclear structures, lowering the electrochemical potentials required to oxidize metals; (iv) in the case of manganese, acetate facilitates the formation of a Mn–(hydr)oxo dinuclear complex by lowering the required applied potential; this behavior parallels the initial stage in the formation of the Mn4Ca cluster, the active site of the photosynthetic water-oxidizing catalyst in living organisms.