Binding energies of gas-phase metal ions with pyrrole: Experimental and quantum chemical results

Binding energies to pyrrole were determined for a number of main-group and transition-metal, nations (both monomer complexes with one pyrrole ligand and dimer complexes with two ligands). Experimental data were obtained by radiative association kinetics measurements in the Fourier transform ion cyclotron resonance ion trapping mass spectrometer, along with ligand exchange equilibrium determinations (for the Mg+ and Al+ cases) using benzene as the reference ligand. Density functional calculations using the B3LYP hybrid functional were carried out on all complexes. The calculations indicated binding only to the,pi site of pyrrole, with no significantly stable binding site being found for binding of any metal ion in the vicinity of the nitrogen. Experimental binding energies for the transition-metal monomer complexes were parallel to previously reported benzene values. Mg+ and Al+ were more strongly bound to pyrrole than benzene, presumably due to the dipole moment of pyrrole. The quantum chemical binding energy values for the monomers were reasonably parallel to the experimental values, but were generally lower by a few kcal/mol. For the dimer complexes, the experimental and quantum chemical values were in satisfactory agreement. The pyrrole transition-metal dimers contrasted strongly with the trend previously reported for the corresponding benzene dimers, showing relatively weaker binding for the early transition metals falling to a minimum at Mn+, rising sharply for the later transition metals, and dipping again for Cu+.