Sigma-bond metathesis reactions of Sc(OCD 3) 2+ with water, ethanol, and 1-propanol: measurements of equilibrium constants, relative bond strengths, and absolute bond strengths

Abstract

Fourier transform ion cyclotron resonance (FTICR) mass spectrometry has been used to examine the reactions of Sc(OCD 3) 2 + with water, ethanol, and 1-propanol. Sigma-bond metathesis resulting in the elimination of CD 3OH is the initial reaction observed, with further solvation of the metal center and subsequent elimination of hydrogen occurring as additional reaction channels. These processes are facile at room temperature and involve little or no activation energy. Measured equilibrium constants for the reaction Sc(OCD 3) 2 + +ROH ⇌ CD 3OScOR + +CD 3OH with R =H, ethyl, and n-propyl are 0.013 ±0.004, 0.5 ±0.15, and 0.7 ±0.2, respectively. For the reaction ROScOCD 3 + +ROH ⇌ Sc(OR) 2 + +CD 3OH with R =H and ethyl the measured equilibrium constants are 0.013 ±0.004 and 0.3 ±0.1, respectively. Δ S is estimated for these processes using theoretical calculations and statistical thermodynamics, and in conjunction with the measured equilibrium constants we have evaluated Δ H for these reactions and the relative and absolute bond strengths of the Sc +–OR bonds, R =H, methyl, ethyl, and n-propyl. The relative bond strengths, D 298 o (CD 3OSc +–OR)– D 298 o (CD 3OSc +–OCD 3), for R =H, methyl, ethyl, and n-propyl are +11.9, 0, −0.1, and −1.4 kcal mol −1, respectively. The absolute bond strengths for HOSc +–OCD 3, CD 3OSc +–OCD 3, CD 3OSc +–OC 2H 5, CD 3OSc +–OCH 2CH 2CH 3, and H 5C 2OSc +–OC 2H 5 are 115.0, 115.0, 114.9, 113.6, and 114.7 kcal mol −1, respectively. Theoretical calculations with an LAV3P∗ ECP basis set at the level of localized second-order Møller–Plesset perturbation theory were performed to evaluate Δ S and Δ G for the specific equilibria Sc(OH) 2 + +CD 3OH ⇌ CD 3OScOH +H 2O, CD 3OScOH +CD 3OH ⇌ Sc(OCD 3) 2 + +H 2O, and Sc(OCD 3) 2 + +C 2H 5OH ⇌ CD 3OScOC 2H 5 + +CD 3OH. The theoretically determined Δ G values agree reasonably well with the experimentally determined Δ G values. In accordance with earlier theoretical predictions, these metathesis reactions are consistent with an allowed four-center mechanism similar to that of a 2 σ +2 σ cycloaddition.