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 prepare d from 2,7-nonadiyne and Re(O)I-3(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) wi th 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 downfieId in the H-1 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 appar atus, and by low temperature H-1 NMR. Two second-order rate constants, pres umably successive protonations, were observed in the stopped-flow, k(1) = 1 1.9(-)M(-1) s(-1) and k(2) = 3.8 M-1 s(-1). Low temperature H-1 NMR spectro scopy indicated that the resulting solution contained a mixture of two doub ly protonated intermediates X and Y, each of which slowly formed the produc t 7 via an acid-independent process.