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-organic radicals and ions. The metal complexes were produced in pulsed laser vaporization molecular beams and studied by pulsed field ionization-zero electron kinetic energy (ZEKE) and infrared-ultraviolet (IR-UV) ionization 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.
Metallocenes. Metallocenes play a central role in organic and organometallic synthesis and chemical catalysis. Compared to transition metal cyclopentadienyl (Cp) complexes where metal atoms are strongly bound in a fivefold mode, the binding between main group elements and Cp is generally weaker and more diverse. It ranges from single to fivefold binding mode and from highly ionic to largely covalent character across the periodic table. We have studied several Cp complexes of group 13 metal atoms and clusters and found very interesting binding schemes in these main group metallocenes. For example, four isomeric structures of cyclopentadienyl dialuminum, with two Al atoms residing on the same or opposite sites of the Cp plane, are predicted by second-order Møller-Plesset perturbation theory. A mixed-valent half-sandwich structure of this complex is identified by the experiment. In this structure, an Al atom is directly bound to the Cp ligand in the fivefold mode, whereas the other Al atom acts like a spectator. The ground electronic states of the neutral and ionized complexes are 2A" in Cs symmetry and 1A1 in C5v symmetry, respectively. The metal-ligand bonding consists of orbital and electrostatic contributions. Ionization of the 2A" neutral state enhances the metal-ligand bonding, but weakens the metal-metal interaction.
Metal-Nucleobase Complexes. Metal coordination with nucleobases may affect the formation, replication, and cleavage of DNA and RNA. These effects include the formation of unusual tautomeric forms of the nucleobases by differential metal binding to specific base sites and the modification of the genetic information transfer by disrupting hydrogen-bond base pairs. On the other hand, introduction of metal-coordinated base pairs into DNA can create self-assembled nanoscale arrays with distinct structures and properties. In these and other applications, fundamental interactions between metals and nucleobases play an important role.
We have completed studies on Al-uracil and Al-thymine. The low-energy tautomers of isolated uracil include the diketo, keto-enol, and dienol forms, with the keto tautomers having the lowest energy. Al binding to these tautomers forms several low-energy isomers, and their relative energies depend on the tautomeric forms of uracil and the binding sites of Al. Among all three forms of uracil, Al binding to the diketo tautomer yields the most stable Al-uracil structure, while Al binding to the dienol tautomer forms the least stable structure. The relatively stability of these Al-uracil isomers is different from that of the free uracil tautomers, and metal coordination stabilizes significantly the keto-enol tautomers and destabilizes the dienol forms. For a specific tautomer, oxygen is the preferred site for Al binding. The observed Al-uracil isomer is formed by Al binding to the diketo form of uracil and corresponds to the most stable structure predicted by theory. In this structure, Al binds to the O4 atom and is in the same plane as that of the uracil molecule. Although the Al binding is largely s character, p orbital interaction between Al and O4 also contributes to the metal-uracil binding. The ionization energy of this Al-O4 structure is lower than that of the bare Al atom, and the Al+-uracil ion binding is stronger compared to Al-uracil neutral binding due to the additional charge-dipole interaction.
Like uracil, thymine is a derivative of pyrimidine, and its most stable tautomer is in the diketo form. However, it has a hydrogen atom replaced by a methyl group. This methyl substitution reduces the effective molecular symmetry and increases the electronic-vibrational transition activity. As a result, 22 vibrational modes are observed in the ZEKE spectrum of Al-thymine, compared to four modes observed for Al-uracil. In addition, two N-H and three C-H (methyl group) stretches are measured from the IR-UV resonance-enhanced two-photon ionization spectrum of Al-thymine. The observation of these N-H and C-H stretches shows that the thymine molecule remains in the canonical form upon Al coordination. The theoretical calculations predict a number of low-energy isomers with Al binding to single oxygen or adjacent oxygen and nitrogen atoms of thymine. Among these isomers, the structure with Al binding to the O4 atom of the diketo tautomer is predicted to be the most stable one by the theory and is probed by both ZEKE and IR-UV measurements. This work presents the first application of the IR-UV resonant ionization to metal-organic molecule systems. Like ZEKE spectroscopy, the IR-UV photoionization technique is sensitive for identifying isomeric structures of metal association complexes.
