Infrared Resonance Enhanced Photodissociation of Au + (CO) n Complexes in the Gas Phase Joe Velasquez, III, E. Dinesh Pillai and Michael A. Duncan Department of Chemistry The University of Georgia, Athens, GA

Background Condensed phase transition metal carbonyl chemistry has been thoroughly studied experimentally and theoretically. 1 Ni(CO) 4 was first isolated over 100 years ago. Catalysis often employs transition metals (TM) where TM -,0,+ -(CO) n serves as intermediates Gas phase TM-carbonyls can serve as starting point for understanding more complicated systems –CO binding to metal surfaces –heterogeneous catalysis –organometallic synthesis 1. Lupinetti et al. Angew. Chem., Int. ed. 1998, 37, 2113

Andrews and co-workers measured infrared spectra of many TM-carbonyls frozen in rare- gas matrices. 1 Armentrout and co-workers have measured bond energies using collision induced dissociation. 2 Meijer and co-workers measured low resolution infrared spectra of carbonyl on gold cluster cations. 3 Recent Work 1. Zhou, M. et al.Chem. Rev. 2001, 101, Meyer, F. et al. J. Am. Chem. Soc. 1995, 117, Fielicke, A. et al. J. A. C. S. 2005, 127, 8416.

Laser vaporization vource coupled to time- of-flight mass spectrometer. Select ions of interest with pulsed deflection plates LaserVision tunable infrared OPO/OPA laser for IR-REPD spectroscopy. Absorption experiments not possible.

Au + (CO) n Mass Spectrum m/z n=2 Au +

Free C-O stretch is 2147 cm -1 Binding energy of first two CO is 45.8 and 45.7 kcal/mol, successively (~16,000 cm -1 ) Binding energy third CO is 3.06 kcal/mol (~1,050 cm -1 ) Calculated DFT frequency for Au + (CO) 3 is 2230 cm -1 Infrared Resonant Enhanced Photodissociation (IR-REPD) DFT calculations done with G03W using B3LYP functional with LANL2DZ basis for Au and G* basis for C and O

m/z Au + -(CO) 3 Au + -(CO) 2 Au + -(CO) 4 Au + -(CO) 3 Au + (CO) n Fragmentation Mass Spectra

Au + (CO) 3 Au + (CO) 2 + CO cm cm -1 Free CO 2147 cm -1 *calculated DFT frequency scaling factor is 0.97 Calculated * 2230 cm -1

Au + (CO) n IR-REPD cm -1 n=3 n=4 n=5 n=6 Free CO Stretch

A synergistic sharing of electron density Donation from the p-orbitals of the CO into d-orbitals of the TM (  donation) CO CO Binding in transition metal-carbonyls Back donation from TM d-orbitals into anti-bonding p orbitals of the CO (  back donation). TM  donation  back donation

Relative contributions of  donation and  back donation has a direct effect on parameters such as TM-C bond length, C-O bond length and the (CO) value. In general, increased back donation from the TM results in shorter TM-C bond lengths, longer C-O bond lengths and lower vibrational frequencies with respect to free CO. This phenomenon has been generally termed classical metal carbonyl behavior in the literature. Conversely, decreased  donation from the TM results in longer TM-C bond lengths, shorter C-O lengths and higher vibrational frequencies. This is generally known as non- classical metal carbonyl behavior. Classical versus Non-classical Carbonyl Complexes

Conclusions DFT studies indicate the Au + (CO) 3 has a C 2v structure with the third CO binding significantly weakly to the Au + than the first two CO. Both the mass spectrum and IR-REPD spectrum indicate a particular stability of the Au + (CO) 2 complex. The Au + (CO) 3-6 spectra all exhibit single peaks. Au + carbonyls display blue-shifted behavior, consistent with non-classical behavior. The blue-shift is 67 cm -1 indicating a long TM + -C bond length and decreased  back donation.