Ultrafast Infrared Studies of Bond Activation in Organometallic Complexes

Abstract

The ability to directly monitor the progress of chemical reactions as they take place is one of the major recent developments in physical chemistry. Many of the advances in our understanding of chemical processes have come about as a result of time-resolved spectroscopic techniques. Conventional methods of time resolution, however, are limited to microsecond and slower regimes, curtailing the applicability of such techniques in many cases of interest. Ultrafast spectroscopy, made possible by the invention and subsequent refinement of pulsed laser technology, has become one of the most powerful tools available to chemists by providing insight into those physical and chemical phenomena which occur too rapidly to be studied by traditional methods. Processes amenable to investigation by ultrafast techniques include electron transfer reactions, vibrational and electronic relaxation in molecules large and small, reaction dynamics in natural and artificial photosynthetic systems, and surface phenomena in both bulk and microscopic systems, to name only a few. Predominate in the field are UV-pump visible-probe techniques, a fact which derives largely from the early development of practical UV and visible sources. UV-vis experiments, widely applied to the study of physical phenomena, convey information about those (primarily electronic) transitions which involve the absorption of visible probe photons. Chemically important information, including structures, can be obtained by examining the IR region of the spectrum, which offers better spectral resolution, a greater number of spectral features, and increased sensitivity to the chemical environment. One problem which is amenable to study using ultrafast vibrational spectroscopy is bond activation, the weakening of otherwise stable chemical bonds so as to increase their reactivity. This is an area of particular interest to organic chemists since many organic reactions involve breaking the extremely stable (>90 kcal/mol) carbon-hydrogen (C-H) and silicon-hydrogen (Si-H) bonds of alkanes and silanes, respectively. A mild (i.e., room temperature and ambient pressure) path to C-H bond activation is also of obvious interest to the petrochemical industry. Si-H bond activation by certain transition-metal-containing compounds was discovered by Jetz and Graham in 1971.1 This was followed in 1982 by the discovery of a related activation reaction of alkane C-H bonds involving Cp*(L)IrH2 (Cp* ) (CH3)5C5) and Cp*Ir(CO)2. Efforts to understand the dynamics of these activation reactions have been made by several groups using gas-phase,5 lowtemperature/high-pressure liquid noble gas,6,7 or matrix isolation8-10 experiments. These techniques slow the reactions, thereby rendering the intermediates observable using conventional Fourier transform IR (FTIR), UV-vis, and fluorescence spectroscopies. Though fruitful, these methods share the disadvantage of observing the activation reactions under conditions quite different from those under which they are normally carried out. Femtosecond infrared spectroscopy provides an ideal tool for investigating the real-time dynamics of these reactions because it provides sufficient temporal resolution to observe all early intermediate steps under ambient reaction conditions. Carbonyl stretches in the 1850-2050 cm-1 range have large absorption cross sections and display significant shifts (>10 cm-1) with changing chemi-