Infrared Photodissociation Spectroscopy of Silicon Carbonyl Cations Antonio Brathwaite and Michael Duncan maduncan.myweb.uga.edu/ Department of Chemistry, University of Georgia Athens, GA 30602

Transition Metal-Carbonyls Stable transition metal-carbonyls have been studied for over a century Cr(CO)6 Fe(CO)5 Ni(CO)4 Stability determined by 18 electron rule We studied cations for comparison Mn(CO)6+ Co(CO)5+ Ni(CO)4+ Free molecular C-O vibration - 2143 cm-1 C-O stretching frequency shifts systematically depending on the metal atom, charge and electronic structure There are limited studies on main group element-carbonyl systems We use infrared photodissociation spectroscopy to study silicon carbonyls by probing the carbonyl stretching region

Classical Metal Carbonyl Bonding Dewar-Chatt-Duncanson complexation model can be used to explain CO shifts σ donation occurs along the CO axis into empty metal d orbitals Results in a blue-shifted CO frequency Filled d orbitals back donate into the antibonding * orbital on CO This causes a red-shift in the CO stretching frequency Combined effect produces a red shifted CO frequency

Experiment

Photodissociation by elimination of excess ligands Mass selected ion with excess CO h (tunable IR) Fragment ion after CO elimination External CO Weakly bound (CO-CO dimer, D0 ~100 cm-1)

Rare Gas Tagging Si+(CO)2Ar Si+(CO)2 h (tunable IR) Bonds are sometimes too strong (D0 > 23 kcal/mol) to break with infrared light (e.g., C-O stretch 2143 cm-1 = 5.7 kcal/mol) We attach a weakly bound “tag” atom to enhance fragmentation. They are eliminated when light is absorbed providing indirect evidence of absorption Computations on tagged and untagged ions are done Si+(CO)2Ar Si+(CO)2 h (tunable IR) Mass selected ion, argon tagged Fragment ion after argon elimination

Ions vs. Neutrals Molecule IR frequency Fe(CO)5 2013, 2034 cm-1  backbonding is the most important interaction in neutral metal-carbonyls Cations have less electron density to disperse as the charge on the metal atom will contract the valence electrons. Cations have reduced  backbonding and less red shifted frequencies than their isoelectronic neutrals. How does Si(CO)2+ compare to its isoelectronic neutral Al(CO)2. Molecule IR frequency Fe(CO)5 2013, 2034 cm-1 Co(CO)5+ 2140, 2150 cm-1 Cr(CO)6 2003 cm-1 Mn(CO)6+ 2114 cm-1 Compare relative shifts of transition metal neutrals and ions vs main group carbonyl J. Phys. Chem. A 2009, 113, 4701. J. Am. Soc. Mass Spectrum. 2010, 21, 5.

Asymmetric Carbonyl Coordination A. J. Lupinetti, S. Fau, G. Frenking, S. H. Strauss, "Theoretical Analysis of the Bonding between CO and Positively Charged Atoms," J. Phys. Chem. 101 (1997) 9551 A. J. Lupinetti, S. Fau, G. Frenking, S. H. Strauss, "Theoretical Analysis of the Bonding between CO and Positively Charged Atoms," J. Phys. Chem. 101 (1997) 9551 Asymmetric Carbonyl Coordination Asymmetric ligand clustering has been observed for Mg+ and Al+ Initial ion-ligand interactions cause polarization of the occupied 3 s orbital. Subsequent ligands tend to bind on the same side as the first Si+ has similar 3s orbital occupation with and an additional electron in the 3p orbital What kind of bonding is present in Si(CO)n + complexes? Another interesting comparison will be the coordination of these systems G. Gregoire, N. R. Brinkman, D. van Heijnsbergen, H. F. Shaefer, M. A. Duncan, J. Phys. Chem. A 2003, 107, 218 Walters, R. S. Jaeger, T. D. Gregoire, N. R. Brinkman, H. F. Shaefer, M. A. Duncan, J. Phys. Chem. A 2003, 107, 7396

Mass Spectrum of Si(CO)n+

Infrared photofragmentation mass spectra All complexes larger than n = 2, fragment by sequential ligand termination ending at n= 2 Weakly bound ligands are easily eliminated by IR photons These results are consistent with a coordination of two

Infrared photodissociation Spectra of Si(CO)n+ Ar clusters

Infrared photodissociation Spectra of Si(CO)n+ clusters Spectra detected by elimination of CO The bands at 2123 cm-1 represent the asymmetric stretch and the bands at 2152 cm-1 represent the symmetric stretch The blue-shifted band at approximately 2174 cm-1 observed. This band is attributed to the weakly bound “external” CO ligands

Structures of neutrals and ions Experiment and theory compared to Al(CO)n measured by Douberly and co- workers Though greater in magnitude, the frequencies observed here are qualitatively consistent with transition metal-carbonyl trends Molecule IR frequency Asymmetric Symmetric Si(CO)2+ 2123 cm-1 2154 cm-1 Al(CO)2 1920 cm-1 1960 cm-1 Note basis set used. Liang, T. Flynn, S. D. Morrison, A. M. Douberly, G. J. Phys. Chem. A 2009, 113, 4701

Conclusions Si(CO)2+ is the fully coordinated silicon carbonyl cation Si(CO)2+ has a V-shaped structure analogous to that of isoelectronic Al(CO)2 Si(CO)n+ and Al(CO)n have the same qualitative trend as transition metal- carbonyl isoelectronic analogues

Future Work Early transition metal-carbonyls Metal cluster-carbonyls Test the limits of metal carbonyl coordination Metal cluster-carbonyls Different binding sites have characteristic frequencies Metal oxide-carbonyls Pure metal vs. metal oxides