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


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 (ΔH12). The ΔH12 was more endothermic for bulky amines, and this steric effect can be attributed to inhibition of CRN 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. © 1989.

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