Metal carbonyls time-resolved infrared

Coordinatively Unsaturated Metal Carbonyls in the Gas Phase via Time-Resolved Infrared Spectroscopy... 

Despite the considerable amount of information that has been garnered from more traditional methods of study it is clearly desirable to be able to generate, spectroscopically characterize and follow the reaction kinetics of coordinatively unsaturated species in real time. Since desired timescales for reaction will typically be in the microsecond to sub-microsecond range, a system with a rapid time response will be required. Transient absorption systems employing a visible or UV probe which meet this criterion have been developed and have provided valuable information for metal carbonyl systems [14,15,27]. However, since metal carbonyls are extremely photolabile and their UV-visible absorption spectra are not very structure sensitive, the preferred choice for a spectroscopic probe is time resolved infrared spectroscopy. Unfortunately, infrared detectors are enormously less sensitive and significantly slower... 

A kinetic trace for a particular metal carbonyl intermediate is recorded at a specific wavelength obtained from the time-resolved infrared absorption spectrum. Suitable data analysis allows determination of the kinetics of the decay of the intermediate. 

Technological advances have allowed the development of time-resolved infrared (TRIR) techniques in recent years and they too have exploited the intense, sharp spectra of the transition metal carbonyls. Some TRIR work has now been carried out, which confirms earlier matrix work " and a review was published some time ago. TRIR involving metal carbonyl species is now a maturing technique, with a considerable number of publications logged over the past 10 years. 

Time-resolved Infrared spectroscopy (TRIR), a combination of UV flash photolysis and fast IR spectroscopy (ns), has been outstandingly successful in identifying reactive intermediates [5] and excited states [6] of metal carbonyl complexes in solution at room temperature. We have used infrared spectroscopy to probe the mechanism of photo-17] and electrochemical [8] catalytic reduction of COj. We have used TRIR to study organometallic reactions in supercritical fluids on a nanosecond time-scale [9-10]. 

Another method found to be useful for the direct observation of alkane metal complexes is fast time-resolved infrared (TRIR) spectroscopy.- - - Upon UV irradiation of a metal-carbonyl complex, such as Cp Rh(CO)2 in liquid rare gases, an intermediate is formed, which, in the presence of an alkane, forms a weakly bound alkane complex [Eq. (6.93)]. Finally, the latter complex rapidly transforms through the transition state 112 into the alkyl hydride [Eq. (6.94)]. 

Time-resolved infrared spectroscopy (TRIR) has been outstandingly successful in identifying reactive intermediates and excited states of both metal carbonyl [68,69] and organic complexes in solution [70-72]. Some time ago, the potential of TRIR for the elucidation of photochemical reactions in SCFs was demonstrated [73]. TRIR is particularly suited to probe metal carbonyl reactions in SCFs because v(CO) IR bands are relatively narrow so that several different species can be easily detected. Until now, TRIR measurements have largely been performed using tunable IR lasers as the IR source and this has restricted the application of TRIR to the specialist laboratory [68]. However, recent developments in step-scan FTIR spectroscopy promise to open up TRIR to the wider scientific community [74]. 

Infrared spectroscopy has been used for many years in order to investigate metal carbonyl structure, and more importantly for this review, photochemical intermediates. For a description of the typical apparatus used for the study of photo-intermediates of transition metal carbonyls using time-resolved IR spectroscopy, see the paper by Dixon et al. [14] and the recent review by Leadbeater [15],... 

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