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 = N,N-bis­(2-arylaminoethyl)­methylamine;
aryl = C6F5 (X = Me, 1, COCH3, 2, Cl, 3) as well as their diamidopyridine
(DAP) (DAP=(2,6-bis­((mesitylamino)­methyl)­pyridine) analogues, [DAPRe­(O)­(X)]
(X = Me, 4, Cl, 5, I, 6, and
COCH3, 7), are described. In these complexes
the terminal oxo ligands act as nucleophiles. In addition we also
show that stoichiometric reactions between 3 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.