Protic Conversion of Nitrile into Azavinylidene Complexes of Rhenium, a Mechanistic Theoretical Study

The mechanism of the protonation of the rhenium nitrile chloro-complexes [ReCl(NCCH3)(PH3)4] (2), taken
as models of the real systems [ReCl(NCR)(dppe)2] (dppe = Ph2PCH2CH2PPh2), leading to the azavinylidene
products [ReCl(NC(H)CH3)(PH3)4]+ (3) was investigated by theoretical methods at the B3LYP level of theory.
Electrostatic and molecular orbital arguments and thermodynamic, kinetic, and steric factors are analyzed
and indicate that the chlorine atom is the most probable site of the initial proton attack, although the direct
protonation of the nitrile carbon atom is also possible as a concurrent process. For the cis-isomer of 2, the
initially formed chloro-protonated species cis-[Re(ClH)(NCCH3)(PH3)4]+ further converts to the azavinylidene
cis-3 via either an acid-independent 1,4-proton shift or an acid−base catalyzed pathway involving a second
protonation of the nitrile carbon atom to give cis-[Re(ClH)(NC(H)CH3)(PH3)4]2+ followed by elimination of
the proton from the chlorine atom.