Bioinspired functional analogs of the active site of molybdenum enzymes: Intermediates and mechanisms

Abstract

Molybdenum(VI) complexes of the general type MoVI(L)2(E)O (L=2-(arylNCH)-pyrrolylato, E=O, NtBu) and their relevance as bioinspired functional analogs for molybdenum-containing oxotransferases are reviewed. All complexes are capable of transfering oxygen atoms to PR3 (forward oxygen atom transfer) giving OPR3 and phosphane molybdenum(IV) complexes MoIV(L)2(E)(PR3) (with a second equivalent PR3) via the transient phosphoryl complex MoIV(L)2(E)(OPR3) and the five-coordinate intermediate MoIV(L)2(E). Reactivity of MoIV(L)2(E) and the favored stereochemistry of products from excess PR3MoIV(L)2(E)(PR3) depend on the steric demand of the chelate ligands (L)−, the π donor ligand E and the substrate. The large phosphane PPh3 is unable to coordinate to MoIV and (abiological) dinuclear oxido-bridged molybdenum complexes [Mo(L)2O]2(μ-O) or [Mo(L)O]2(μ-O)2 are formed via interception of MoIV(L)2(O) by MoVI(L)2(O)2. Dioxido complexes (E=O) catalyze the transfer of oxygen from dimethyl sulfoxide to phosphanes (forward/backward oxygen atom transfer) with the [Mo(L)2O]2(μ-O) dimer as off-loop species. The mixed imido/oxido complex (E=NtBu) does not form dimers but the active site is poisoned by the phosphane. Propylene sulfide transfers sulfur to MoIV(L)2O yielding MoVI(L)2O(η2-S2). With respect to biologically relevant oxygen atom transfer and subsequent electron transfer mononuclear molybdenum(V) intermediates are accessible by oxidation of MoIV(L)2(E)(PR3) to [MoV(L)2(E)(PR3)]+ and by reduction of MoVI(L)2(O)2 to [MoV(L)2(O)2]−. The latter one can be intercepted by silylation or protonation at an oxygen atom to give MoV(L)2O(OSiMe3) or MoV(L)2O(OH), respectively, closing the catalytic cycle. In second generation model systems oxygen atom transfer and one-electron oxidation of MoIV to MoV can be intimately coupled using built-in ferrocenium/ferrocene redox centers as functional analogs of natural redox cofactors such as cytochromes or iron-sulfur clusters. In the limiting case of rapid intramolecular electron transfer from MoIV to FeIII the phosphoryl ligand appears to remain bound to the transient MoIV/FeIII intermediate and only dissociates from the MoV/FeII complex after electron transfer. Finally, a full biomimetic catalytic cycle involving oxygen atom transfer from water and two subsequent electron/proton transfer steps is established with a polymer-immobilized MoVI(L)2(O)2 catalyst and soluble ferrocenium salts as redox cofactors. Future challenges and developments in this bioinspired research field are envisaged.