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45367-AC6
Spectroscopy and Structures of Group 3 and Group 4 Transition Metal Complexes of Polycyclic Aromatic Hydrocarbons
In this reporting period, we published several peer-reviewed papers on the spectroscopy, structures and energetics of metal complexes with aromatic molecules. The metal complexes were produced in pulsed laser vaporization molecular beams and studied by threshold photoionization and pulsed field ionization-zero electron kinetic energy (ZEKE) spectroscopy and electronic structure calculations. These studies determined electronic states, molecular conformations, adiabatic ionization energies, metal-ligand and ligand-based vibrational frequencies, and metal-ligand bond dissociation energies.
M-(Benzene)2 (M = Ti and V). Recently, infrared photodissociation spectroscopic studies of cationic bis(benzene) titanium and vanadium complexes showed that the observed spectra were not consistent with DFT predictions for their ground electronic states and raised a serious question about the ability of DFT to handle the correct spin states for these transition metal ions. In addition to the issue of the ground electronic states of these ions, previous measurements have shown discrepancies about the ionization energies (IE) of the corresponding neutral complexes. Thus, our motivation was to investigate the ground electronic states and improve the IE values of these two complexes. From our studies, the ground electronic states of the neutral Ti- and V-(C6H6)2 complexes are determined to be 1A1g and 2A1g, and their adiabatic IEs are measured to be 5.732 ± 0.001 and 5.784 ± 0.002 eV, respectively. These neutral complexes have six-fold binding and are in an eclipsed D6h configuration with flat benzene rings. Ionization of the 1A1g and 2A1g neutral states of Ti- and V-bz2 yields the 2B1g and 3B1g ions states, respectively, in a D2h point group with slightly puckered benzene rings. The 1A1g state of Ti-(C6H6)2 and the 2A1g and 3B1g states of V- and V+-(C6H6)2 are the lowest energy states predicted by various DFT and ab initio calculations, whereas the 2B1g state of Ti+-(C6H6)2 is calculated to have higher energy than the 4A1g state.
Cu-(Pyridine)n (n = 1, 2). The ZEKE spectrum of the mono-pyridine complex exhibits a narrower linewidth and additional vibrational transitions compared to a previously reported ZEKE spectrum, whereas the ZEKE spectrum of the di-ligand complex is first studied in this work. The adiabatic IE of the mono-ligand complex is much lower than that of Cu atom, and the adiabatic IE of the di-ligand complex is further reduced compared to that of the mono-ligand species. These IE shifts are the result of stronger metal-ligand bonding in the ion than in the neutral complex, and the increased ion binding is due to the additional charge-dipole interaction and reduced electron repulsion in the ion. The ion bond energy of the di-ligand complex is about twice stronger than that of the mono-ligand species, whereas the difference of the corresponding neutral bond energies is increased to five times. Copper binds to a single pyridine molecule in a planar C2v structure and to two pyridine molecules in an eclipsed configuration, both through the nitrogen atoms. In addition to the Cu-N s bonding in the mono- and di-ligand species, important p binding between the Cu and N pp orbitals greatly enhances the metal-ligand bonding in the di-ligand complex. In comparing the experimental measurements with the theoretical calculations, we find that the geometries and vibrational frequencies predicted by the MP2 calculations are very good for both the mono- and di-ligand species, but the ionization and dissociation energies of the di-ligand neutral complex are underestimated due to the spin contamination in the 2B3u state. The CCSD(T) calculations improve the ionization energies for both the mono- and di-ligand complexes, but overestimate the bond energies, except for the neutral Cu-(pyridine)2 complex.
Al- and Cu-Imidazole. The s and p structures of these two complexes are predicted by MP2 calculations, but only the s structures are identified by the experimental measurements. For these s structures, IEs and several vibrational frequencies are measured from the ZEKE spectra, the ground electronic states of the neutral and ionized complexes are determined by comparing the observed and calculated spectra, and the metal-ligand bond dissociation energies of the neutral states are derived by using a thermochemical relation. The measured vibrational modes include the metal-ligand stretch and bend and ligand ring distortions. The metal-ligand stretch frequencies of these transient complexes are compared with those of coordinately saturated, stable metal compounds, and the ligand-based distortion frequencies are compared with those of the free ligand. Al-imidazole has a larger bond dissociation energy than Cu-imidazole, although the opposite order was previously found for the corresponding ions. The weaker bonding of the Cu complex is attributed to the antibonding interaction and the electron repulsion between the Cu 4s and N lone-pair electrons.
