Oxygen Atom Transfer Chemistry of [Mo(VI)O2]2+ Cores and Geometric Rearrangement in [Mo(V)O]3+ Cores: Reactivity, Mechanisms and Electronic Structure of Functional Molybdoprotein Model Systems

The oxygen atom transfer reactivity of a discrete dioxo-molybdenum(VI) complexes, TpiPrMoO2(OPh) (where TpiPr= hydrotris(3-isopropylpyrazol-1-yl)borate), TpMe2MoO2(OPh), TpMe2MoO2(SPh), TpMe2MoO2(Cl), (where TpMe2= hydrotris(3,5-dimethylpyrazol-1-yl)borate), with seven tertiary phosphines (PMe3, PMe2Ph, PEt3, P(n-Bu)3, PEt2Ph, PPh2Et and PPh2Me) have been investigated. The first nucleophilic step follows a second-order rate with an associative transition state in all cases. The second step of the reaction, i.e., the exchange of the coordinated phosphine oxide with acetonitrile, follows a first-order process. The reaction follows a dissociative interchange (Id) or associative interchange (Ia) type mechanism, as it is substrate and compound dependent. It has been established that there are three main physico-chemical parameters that contribute to the reactivity of phosphorous (III) compounds, two of which are electronic and the third is of steric origin. It is commonly accepted that these reactions involve a σ-basicity component, a π-acidity component and a steric/size component. However, there has been little investigation into the reactivity of the analogue oxo-phosphorous (V) compounds which are typically generated during oxygen atom transfer reactions (OAT) when the parent phosphorous (III) compounds act as nucleophiles toward oxygen. Here, we explore the current concepts associated with reactivity, the origin of reaction parameters and the general applicability of phosphorous (III) parameters toward reactions that evolve phosphorous (V) products. Lastly, the regeneration of a catalytically active enzyme, after formal OAT has occurred, is believed to involve two, one electron/ one proton transfer steps that result in a formal Mo(V) intermediate. However, no crystal structure exists for the enzyme in a transient 5+ state. To this end we have synthesized, and fully characterized a series of monooxo-Mo(V) complexes in attempt to model the transient Mo(V) state of the enzyme. Furthermore we show that these complexes can be isolated as two discrete isomers with respect to the position of a heteroatom donor relative to the oxo-group and have detailed the kinetics of isomerization and electronic structure of these complexes. Based upon our findings we have postulated a serine gated electron transfer hypothesis (SGET) to provide a possible explanation for the role of isomerism in the regeneration step of a catalytically active protein