Research Interests

The group's research interests are in the general areas of physical-organic and physical-organometallic chemistry, and mechanistic organic and organometallic photochemistry. Our work focuses largely on mechanistic studies of the reactions of highly reactive molecules and reaction intermediates in organosilicon, -germanium, and -tin chemistry, using fast time-resolved spectroscopic methods to enable their direct detection and detailed studies of their reactivities. Students gain experience in a wide variety of synthetic, photochemical, analytical, and spectroscopic techniques. These include the common ones - such as high field NMR spectroscopy, FT-IR spectroscopy, GC and HPLC, GC/MS, and single crystal x-ray crystallography - as well as specialized techniques such as laser flash photolysis (LFP) and stopped-flow kinetics (SFK) methods. The LFP technique allows the study of intrinsically short-lived (50 ns to ~100 ms) reactive molecules in solution, which are generated by photochemical reactions. The SFK method allows the study of fast reactions of isolable but nevertheless highly reactive molecules on timescales of ~10 ms to minutes.  Both methods enable the measurement of the uv-vis absorption spectra, lifetimes, and rate constants for their reactions with various added substrates, and of the intermediates that are formed during the course of these reactions.  Fast kinetic studies by LFP are supported with preparative photochemical experiments to enable the isolation and identification of the ultimate products of the reactions of interest.

Group 14 Reactive Intermediates

To organic chemists, the term "reactive intermediate" brings to mind such things as carbenes, free radicals, carbenium ions, strained double bonds, and many others. How does the extraordinary - and often fascinating - reactivity of such molecules change when we replace one or more of the carbon atoms with another Group 14 element, like silicon for example? What happens when the "active" carbon is replaced, as for example in going from R2C: (carbene) to R2Si: (silylene)? What happens when the switch is made at a neighbouring or even more remote site? How do reaction mechanisms change? Does this knowledge help us predict what might happen when we go one or two steps further, as in replacing silicon with germanium or tin? These are just some of the questions that our research in Group 14 Reactive Intermediates is directed at answering.

One might expect that the silicon, germanium, and tin analogues of all the species listed above should be quite reactive indeed, and they'd be right. What you might not expect is that some of the relatively stable organic functional groups, such as olefinic C=C bonds and carbonyl groups, become extremely "hot" when this simple atomic substitution is made. In fact, multiple bonds involving the "heavier" Group 14 elements are so thermodynamically unstable that up until about 40 years ago, there were no known examples of molecules containing them. We now know a few hundred examples of molecules which contain Si=C (silene), Ge=C (germene), Si=Si (disilene), and Ge=Ge (digermene) bonds. Many are stable enough that they can be synthesized and placed in a bottle. Most, however, are highly reactive species with lifetimes on the order of microseconds or less in the gas phase or in solution. Understanding their chemistry is important from the fundamental perspective of bonding and reactivity in main group chemistry, and also because they are involved as reactive intermediates in a wide variety of thermal and photochemical reactions in organosilicon, -germanium, and -tin chemistry, many of which are important technologically. Our group employs laser flash photolysis techniques to generate various R2M: ("tetrellylene"), M=C ("tetrellene"), and M=M ("ditetrellene") species photochemically, detect them directly, and study their rich reactivity in solution. The main thrust of our work so far has been the elucidation of the mechanisms of "classic" reactions of silenes and germenes (such as the addition of alcohols, amines, carbonyl compounds, alkenes, etc.) and of silylenes and germylenes (such as O-H insertions, addition to alkenes and dienes, reaction with halocarbons, and O- and S-abstractions from oxiranes and thiiranes, etc.) through product- and kinetic-studies of photochemically-generated, transient derivatives.  One of the crucial challenges of the work is the design and synthesis of new "precursor" molecules which liberate the desired species selectively and efficiently upon photochemical activation, so that the chemistry of the desired molecule can be studied without interference from other potential side-products of the photochemistry.  Some of the different classes of molecules used in our studies are featured in the list of selected publications given below.  

Representative Publications (selected from the master publications list).

A.G. Moiseev and W.J. Leigh,* "The Direct Detection of Diphenylsilylene and Tetraphenyldisilene in Solution", Organometallics 26, 6268-6276 (2007). [Get Reprint]

W. J. Leigh*, S. S. Kostina, A. Bhattacharya, and A. G. Moiseev, "A Fast Kinetics Study of the Reactions of Transient Silylenes with Alcohols. Direct Detection of Silylene-Alcohol Complexes in Solution", Organometallics, 29, 662-670 (2010). |Get Reprint

S. S. Kostina and W. J. Leigh*, "Silanones and Silanethiones from the Reactions of Transient Silylenes with Oxiranes and Thiiranes in Solution. The Direct Detection of Diphenylsilanethione", J. Am. Chem. Soc., 133, 4377-4388 (2011). | Get Reprint

W.J. Leigh,* C.R. Harrington, and I. Vargas-Baca, "Organogermanium Reactive Intermediates. The Direct Detection and Characterization of Transient Germylenes and Digermenes in Solution", J. Am. Chem. Soc., 126, 16105-16116 (2004).  [Get Reprint]

L. A. Huck and W. J. Leigh*, "Kinetic and Mechanistic Studies of the Formal (1+2)- and (1+4)-Cycloadditions of Germylenes to Conjugated Dienes." Organometallics, 28, 6777-6788 (2009). [Get Reprint]

P. S. Billone, K. Beleznay, C. R. Harrington, L. A. Huck, and W. J. Leigh*, "A Glimpse at the Chemistry of GeH2 in Solution. Direct Detection of an Intramolecular Germylene-Alkene π-Complex", J. Am. Chem. Soc., 133, 10523-10534 (2011) . | Get Reprint

R. Becerra, P.P. Gaspar,* C.R. Harrington, W.J. Leigh,* I. Vargas-Baca, R. Walsh,* and D. Zhou, "Direct Detection of Dimethylstannylene and Tetramethyldistannene in Solution and the Gas Phase by Laser Flash Photolysis of 1,1-Dimethylstannacyclopent-3-enes", J. Am. Chem. Soc., 127, 17469-17478 (2005)[Get Reprint]

W. J. Leigh*, A. G. Moiseev, E. Coulais, F. Lollmahomed, and M. S. Askari, “Substituent Effects on Silene Reactivity. Reactive Silenes from Photolysis of Phenylated Tri- and Tetrasilanes”, Can. J. Chem. 86, 1105-1117 (2008). [Get Reprint]

T.L. Morkin and W.J. Leigh,* "Substituent Effects on the Reactivity of the Silicon-Carbon Double Bond", Acc. Chem. Res. 34, 129-136 (2001). [Get Reprint]

T.L. Morkin, W.J. Leigh,* T.T. Tidwell, and A.D. Allen, "Direct Detection of Wiberg’s Silene (1,1-Dimethyl-2,2-bis(trimethylsilyl)silene) and Absolute Rate Constants for its Reactions in Solution", Organometallics, 20, 5707-5716 (2001). [Get Reprint]

W.J. Leigh,* G.D. Potter, L.A. Huck, and A. Bhattacharya, “Competing Germene and Germylene Extrusion from Photolysis of 1,1-Diarylgermacyclobutanes. Substituent Effects on Germene Reactivity”, Organometallics 27, 5948-5959 (2008). [Get Reprint]  

01Nov11; wjl