Rhenium Oxo Complexes of a Chelating Diyne Ligand. Synthesis and Study of the Kinetics of Protonation

A series of oxo complexes, Re(O)X(diyne) (X = I, Me, Et), have been prepared from 2,7-nonadiyne and Re(O)I3(PPh3)2. Addition of B(C6F5)3 to Re(O)I(2,7-nonadiyne) (5) results in coordination of the oxo ligand to the boron.
The protonation of Re(O)(X)(2-butyne)2 and Re(O)(X)(2,7-nonadiyne)2 with a variety of acids has been examined.
With 5 and HBF4/Et2O, the ultimate product was [Re(CH3CN)3(I)(2,7-nonadiyne)]2+ (7). The conversion of 5 to
7 changes the conformation of the diyne ligand from a “chair” to a “boat” and shifts its propargylic protons
considerably downfield in the 1H NMR. The kinetics of the protonation of Re(O)I(2,7-nonadiyne) (5) by CF3SO3H
in CH3CN have been monitored by visible spectroscopy, in a stopped-flow apparatus, and by low temperature 1H
NMR. Two second-order rate constants, presumably successive protonations, were observed in the stopped-flow,
k1 = 11.9 M-1 s-1 and k2 = 3.8 M-1 s-1. Low temperature 1H NMR spectroscopy indicated that the resulting
solution contained a mixture of two doubly protonated intermediates X and Y, each of which slowly formed the
product 7 via an acid-independent process.