Polyhedral Interconversion Coupled with Proton Transfer between an Ammonium Cation and the [Co(CO)4]- Ion

摘要

Proton transfer between ammonium cations and [CoL(CO)3]- ions (L=CO, PR3) has been illustrated by Brammer et al. through a series of crystal structures according to the structure-correlation principle, which states that if a correlation can be found between two or more independent parameters describing the structure of a given fragment in a variety of environments, then the correlation function maps a minimum energy path in the corresponding parameter space. The two ends of the proton transfer reaction correspond to an ionic pair formed by an ammonium cation and the [CoL(CO)3]- ion (1 a) and independent amine and hydrido carbonyl complexes 1 b. The structures analyzed by Brammer et al. correspond to compounds with NHCo hydrogen bonds, a behavior consistent with the acidity of the hydrido carbonyl complex in water. According to the structure-correlation principle, the variety of HCo distances found (between 2.63 ? for a NHCo hydrogen-bonded species and 1.43 ? for the hydrido complex) should correspond to snapshots along the proton-transfer reaction coordinate. Since the coordination spheres of both the N and Co atoms are significantly affected by the progress of the proton-transfer reaction, these systems provide an excellent opportunity to test the applicability of the polyhedral-shape measures as reaction coordinates and to correlate the potential-energy surface with the structural evolution. Therefore, the aim of this communication is to show how DFT calculations provide an energy profile for such a reaction that accounts for the fine details of the distribution of structures along the path and how the proton transfer is coupled to changes in the coordination polyhedra of the N and Co atoms. An outcome of this study is that the energy and the stereochemical changes that accompany the proton transfer can be described as a function of a single parameter, the generalized polyhedral-interconversion coordinate for the transformation of a [CoL(CO)3]- (L=ligand) tetrahedron into a vacant trigonal bipyramid.