Photoacoustic calorimetry studies of ligand-exchange of aliphatic amines with Cr(CO) 6: Steric effects and hydrogen bonding

Abstract

The exchange of aliphatic amines with Cr(CO) 6 in heptane occurs in two steps: a CO is lost forming a solvent-coordinated complex, Cr(CO) 5(heptane), and then heptane is lost to form Cr(CO) 5(amine). The enthalpies of exchange of aliphatic amines (piperidine, n-butylamine, di- n-butylamine and tri- n-butylamine) with Cr(CO) 6 have been determined in heptane at ambient temperatures. The enthalpies of reaction determined by photoacoustic calorimetry (PAC) include contributions from hydrogen-bond formation between free amine and the pentacarbonylamine chromium, 1. The enthalpy of hydrogen-bond formation was found to be about 4 kcal mol −1 and was determined from the temperature and concentration dependence of the amine NH chemical shifts of 1. The PAC values were corrected for hydrogen-bond formation contributions to obtain only the enthalpy of ligand-exchange (Δ H 12). The Δ H 12 was more endothermic for bulky amines, and this steric effect can be attributed to inhibition of CRN bond formation. These results, along with those in the literature, provide a detailed description of the energetics of each step of the ligand-exchange in both directions. The results suggest there is a continuum from an associative to a dissociative process for the displacement of heptane by amine that depends on the steric bulk of the amine.