Stephen G. Kukolich

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Research Interests

Our primary research area is the determination of the molecular structures and electronic charge distributions of transition metal complexes, hydrogen-bonded complexes and small molecules using microwave spectroscopy. The complete, accurate, 3-D structures obtained are very helpful in understanding reaction mechanisms and in understanding the bonding and electronic structures of molecules. There are many important examples in chemistry and biochemistry where a detailed knowledge of molecular structures has provided the key to understand the details of how reactions proceed. Microwave spectroscopy has been the most accurate and precise method for measuring the bond lengths and angles in free molecules for many years, and these gas-phase structures can be directly compared with quantum theory results.

Transition metals and transition metal complexes function as catalysts in a wide variety of chemical reactions which are important in biology and the chemical industry  To better understand how the bonding and reactivity of compounds are modified by forming complexes with metal atoms, it is important to determine the structures and electronic properties of the complexes, or related model compounds. Comparisons of the measured and calculated structures and electronic properties is very useful in testing and improved the theoretical models so they can be used with more confidence in predicting properties of larger and more complex molecules. When a complex is formed involving a transition metal and a small organic molecule, both th  structure and reactivity of the organic ligand are modified by this interaction. We have recently directly measured the changes in the structure of acetylene, ethylene and benzene due to interactions with a transition metal.

The molecular structure for a rhenium metallacyclopropene, acetylenemethyl-dioxorhenium was obtained by measuring and analyzing the rotational spectra for 14 isotopomers. These measurements on this complex were motivated by the importance of metallacycles as reaction intermediates in OsO4 and methyltrioxorhenium(MTO) catalysed oxidation and epoxidation reactions. For the acetylenemethyl-dioxorhenium complex, the structure of the acetylene ligand is modified through interaction with the metal atom, and exhibits partial sp2 hybridization in the complex. The C-C bond length is increased by 0.08 Å to 1.29 Å. The H-C-C interbond angles are reduced from 180° to 146°, and 147°.

Theoretical BPW91-DFT calculations provided structural parameters in remarkably good agreement with measured parameters. The Measured structural parameters fo  the acetylene-methyldioxorhenium complex are shown in the figure below.

Figure 1. Measured structural parameters for the acetylene-methyldioxorhenium complex.

The complete, 3-D structure was obtained for the simplest stable olefin-iron complex, iron tetracarbonyl ethylene. The modifications of the structure and reactivity of olefins on metal surfaces are of considerable interest, and this is a good example of detailed structural measurements of the "one-on-one" complex of ethylene with iron. The ethylene ligand exhibits significant structural changes upon complexation to iron. There is an an increase in the C-C bond length to ro = 1.419(7) Å, compared to 1.339(1) Å for free gaseous ethylene. The plane of the hydrogen atoms is displaced 0.217(2) Å above the ethylene carbon atoms. The hydrogen atom locations were not obtained from previous electron diffraction, or solid-state x-ray work. Accurate hydrogen coordinates for this molecule are very important because they are an excellent indicator of the hybridization of the ethylene carbon atoms. It is important to have detailed and precise data on these simpler complexes in order to evaluate the accuracy of the various theoretical calculations.

Figure 2. Structural Changes in Ethylene upon Complexation to Iron.

Two distinct structural isomers were observed, in the gas-phase for the olefin-metal complex allyltricarbonylironbromide. There has been much continued interest in the allyl ligand in organometallic chemistry due to its special property of being a small resonance stabilized ligand. This ligand, formally a three-electron donor, has been shown to form stable compounds with a wide variety of metal centers. The molecular structures for both the anti and syn isomers of allyltricarbonylironbromide were characterized using high-resolution, Fourier-transform microwave spectroscopy and DFT calculations. Rotational transitions for two structural isomers of allylirontricarbonylbromide have been clearly observed in the cold molecular beam of a pulsed-beam Fourier transform microwave spectrometer.

Figure 3. Two observed structural isomers for allylirontricarbonylbromide

Microwave measurements of the complete three-dimensional structures were made for six transition metal hydrides. Since the earlier work of Kubas, et al., there has been much recent interest in examining transition metal complexes with two or more H atoms. It is important to determine if they are the classical dihydride type of complexes or the more interesting, and more reactive, dihydrogen complexes. For the classical dihydrides, the hydrogen atoms are individually bound to the metal center and H-H distances are usually greater than 1.5 Å. For the dihydrogen complexes, a nearly intact hydrogen molecule is 2 bound to the metal center and the H-H distance is usually 1.0 Å, or less.

Figure 4. The basic structure of the dihydrogen complexes, and the potential energy as a function of H-H distance for the Os and Fe complexes.

Metal-metal bonding interactions have continued to be an important area of theoretical and experimental work in inorganic chemistry. Some of the most effective catalysts for chiral syntheses have been dirhodium transition metal complexes. The extremely high sensitivity and high resolution of the pulsed-beam, Fourier-transform microwave spectrometer (PBFT microwave spectrometer) systems has opened the door for numerous new structural measurements on larger and less stable molecules and complexes. One of the most recent projects is the dinuclear complex, MnRe(CO)10. This appears to be the first microwave measurement of the rotational spectrum of a transition metal - dinuclear complex.

Figure 5. Dinuclear Complex - (CO)5MnRe(CO)5

During the course of our research, graduate students would receive training and experience in a number of different techniques and areas. They would obtain "hands-on" experience in at least a few of the following areas: computer interfacing, programing for data analyis and/or and DFT calculations, apparatus development and construction, electronic, microwave and vacuum systems, molecular spectroscopy, molecular structures and inorganic synthesis. There has been a continued effort to develop and improve the spectrometer systems and to extend the work into other areas.

Selected Publications

Subramanian R, Karunatilaka C, Schock RO, Drouin BJ, Cassak PA, Kukolich SG. Aug 2005. Determination of structural parameters for ferrocenecarboxaldehyde using Fourier transform microwave spectroscopy. J Chem Phys, 123:54317

Subramanian R, Karunatilaka C, Keck KS, Kukolich SG. May 2005. The gas phase structure of ethynylferrocene using microwave spectroscopy. Inorg Chem, 44:3137-45

Tanjaroon C, Keck KS, Kukolich SG. Jan 2004. Microwave spectroscopy measurements of rotational spectra and DFT calculations for two distinct structural isomers of 1,1'-dimethylferrocene. J Am Chem Soc, 126:844-50

Tanjaroon C, Keck KS, Kukolich SG, Palmer MH, Guest MF. Mar 2004. The rotational spectrum and theoretical study of a dinuclear complex, MnRe(CO)(10). J Chem Phys, 120:4715-25