Organometallic chemistry in aqueous solution: Reactions catalyzed by water-soluble molybdocenes

The aqueous chemistry of molecules containing the Cp2Mo2+ unit (referred to as “molybdocenes” in this paper) is reviewed. Aqueous molybdocenes are generated by hydrolysis of the Mo–X bonds in Cp2MoX2 complexes (X = halide or pseudo-halide) or by dissolving isolable dimers of the form [Cp2Mo(μ-OH)2MoCp22+][OTs−]2 in water. The nature of the molybdocene species in solution is pH dependent: Cp2Mo(H2O)22+ has pKa1 = 5.5 and pKa2 = 8.5; thus, at neutral and physiological pH, Cp2Mo(OH)(OH2)+ is the dominant monomer in aqueous solution. This monomer is in equilibrium with [Cp2Mo(μ-OH)]22+ (Keq = 3.5 × 10−2 M ± 1.3 × 10−3 M at pD 3.5). Cp2Mo(OH)(OH2)+ and Cp2Mo(H)(OH2)+ (and the Cp′ analogs) are catalysts for a variety of reactions in aqueous solution, including H/D exchange reactions that proceed through C–H bond activation pathways, transfer hydrogenation reactions of ketones and aldehydes, nitrile hydration, and the hydrolysis of ethers, carboxylic esters, phosphate esters, and thiophosphinates. In these reactions, the molybdenum center acts as a Lewis acid, activating substrates toward intra- or intermolecular nucleophilic attack by a bound hydroxo ligand or a free water molecule. Mechanistic evidence suggests that the intramolecular hydration and hydrolysis reactions proceed via strained, four-membered ring intermediates.