Characterization of Manganese Oxidation States in Diguanidylpyridine Complexes by Vibrational Dissociation Spectroscopy and DFT Calculations.

Infrared multiple-photon dissociation (IRMPD) spectra in the 800-1750 cm-1 range of a set of pseudo-octahedrally coordinated manganese complexes {[Mn(dgpy)2][PF6]x}n+ (x = 0, 2, 3; n = 1, 2, 3; dgpy = 2,6-diguanidylpyridine) isolated in an ion trap are presented. The complexes contain two neutral tridendate dgpy ligands and in some cases [PF6]- ions form ion clusters with the positive manganese species, exhibiting different manganese oxidation states within the same ligand environment. While IR spectra assigned to Mn(II, III, IV) species show similar vibrational patterns, confirming largely metal-centered redox processes, a clearly different spectrum was observed for the in situ generated monocation [Mn(dgpy)2]+, raising the question of whether this complex contains a MnI central ion or a reduced ligand (dgpy)•-. Comparison of the spectroscopic data to the results of density functional theory (DFT) calculations using the CAM-B3LYP functional assigns its structure to a [MnII(dgpy)(dgpy)•- ]+ configuration. This "non-innocent" electronic behavior of the dgpy ligand in the gas phase is elucidated by electronic structure computations and attributed to the asymmetric, localized charge distribution of the extra electron on the pyridine moiety of only one of the dgpy ligands. The results demonstrate the novel perspectives offered by mass spectrometrically controlled gas phase spectroscopy for the generation, isolation, and characterization of unusual charge states of ligand-stabilized metal complexes.