Rate constant measurement and mechanistic probe of protonation of rhenium(III) oxo alkyne complexes

Abstract

Separated and somewhat broadened peaks were found for their protonated derivatives when Re(O)(Et)(MeC=CMe)2 and Re(O)(Me)(MeC=CMe)2 were treated with acids below room temperature. The upper limit of the rate constant for the protonation of Re(O)(Et)(MeC=CMe)2 by CF3SO3H was estimated to be 120 M-1s-1 at -15 °C, much slower than a diffusion-controlled one. The protonation of oxo (iodo)(η2-2,7-nonadiyne) rhenium(III), (1), was studied thoroughly in order to determine the rate constants. Strong acids were needed to protonate the terminal oxo ligand. Crystals of the final product, Re(I)(CH3CN)3(CH3C=C(CH2)3CC=CH3)(BF4)2, (3), were obtained from protonation with HBF4·(CH3)2O in CH3CN/CH2CI2 mixed solution, so that the X-ray structure of 3 could be determined. A two-step consecutive protonation of the terminal oxo ligand, A→B→C, was observed when the reaction of 1 with CF3SO3H in acetonitrile was monitored by a rapid scan stopped-flow apparatus at low temperature. Both steps proved to be second order, first in [1] and in [CF3SO3H]. The second order rate constants were of the order of 101 M-1s-1 at -40 °C, much slower than the diffusion-controlled one. Rearrangements involved in the protonation were modeled by the reaction of 1 with B(C6F5)3, a model for protonation at oxo ligand. Two intermediates, X and Y, were found when complex 1 was treated with CF3SO3H. In low temperature 1H NMR spectra the observed first-order rate constants for the conversion of X and Y to 3, obtained from exponential fitting of the experimental data, showed no correlation with acid concentration. A mechanistic scheme was proposed to account for all the experimental findings. A seemingly simple proton transfer can be very complicated.