The Electronic Nature of Terminal Oxo Ligands in Transition-Metal Complexes: Ambiphilic Reactivity of Oxorhenium Species

The synthesis of the Lewis acid–base adducts of B­(C₆F₅)₃ and BF₃ with [DAAmRe­(O)­(X)] DAAm = <i>N</i>,<i>N</i>-bis­(2-arylaminoethyl)­methylamine; aryl = C₆F₅ (X = Me, <b>1</b>, COCH₃, <b>2</b>, Cl, <b>3</b>) as well as their diamidopyridine (DAP) (DAP=(2,6-bis­((mesitylamino)­methyl)­pyridine) analogues, [DAPRe­(O)­(X)] (X = Me, <b>4</b>, Cl, <b>5</b>, I, <b>6</b>, and COCH_3, <b>7</b>), are described. In these complexes the terminal oxo ligands act as nucleophiles. In addition we also show that stoichiometric reactions between <b>3</b> and triarylphosphine (PAr₃) result in the formation of triarylphosphine oxide (OPAr₃). The electronic dependence of this reaction was studied by comparing the rates of oxygen atom transfer for various para-substituted triaryl phosphines in the presence of CO. From these experiments a reaction constant ρ = −0.29 was obtained from the Hammett plot. This suggests that the oxygen atom transfer reaction is consistent with nucleophilic attack of phosphorus on an electrophilic metal oxo. To the best of our knowledge, these are the first examples of mono-oxo d² metal complexes in which the oxo ligand exhibits ambiphilic reactivity