4138
J. Phys. Chem. A 1998, 102, 4138-4147
Ultrafast Dynamics of Transition Metal Carbonyls: Photodissociation of Cr(CO)6 and Cr(CO)6‚(CH3OH)n Heteroclusters at 280 nm Michael Gutmann,*,† Jo1 rg M. Janello, Markus S. Dickebohm, Markus Grossekatho1 fer, and Ju1 rgen Lindener-Roenneke Institut fu¨ r Physikalische Chemie, Luxemburger Strasse 116, D-50939 Ko¨ ln, Germany ReceiVed: NoVember 24, 1997; In Final Form: February 6, 1998
We report on first results of real-time studies on the photodissociation dynamics of electronically excited Cr(CO)6 and Cr(CO)6‚(CH3OH)n heteroclusters generated in a molecular beam. The red part of the 1T1u metal to ligand charge-transfer state at 280 nm is excited, and the resulting photofragments are detected by multiphoton ionization. The decarbonylation dynamics is found to be much faster than the cross correlation width (230 fs) of our pump and probe pulses. The observed fragmentation pattern and its dynamics can tentatively be attributed to two competing dissociation channels, both starting from the primary photofragment Cr(CO)5, a simultaneous and a sequential channel. For the clusters we find that both these channels appear to be operative depending on pump laser intensity and that decarbonylation precedes evaporative loss of solvent molecules, which takes place on the picosecond time scale. Solvated cluster species as well as coordinatively unsaturated carbonyls are stabilized by evaporation.
1. Introduction Transition metal carbonyls are model systems for organometallic compounds, which are important for, for example, producing metal films by vapor deposition1 and catalysis of olefin isomerization and oligomerization.2 Particularly the metastable coordinatively unsaturated species are of relevance in this respect. Therefore the spectroscopy and dynamics of coordinatively unsaturated transition metal carbonyls have received considerable attention throughout the literature, and a large number of experimental studies on the group VIb hexacarbonyls Cr(CO)6, Mo(CO)6, and W(CO)6 and on Fe(CO)5 in condensed phase and in gas phase have been performed during the last two decades. As quite a large body of important results on the photofragmentation of those systems has been accumulated, it appears to be in order to summarize some of the work relevant to our study. Condensed-Phase Studies. From IR spectroscopy after UV excitation of group VIb hexacarbonyls ML6 (M ) metal, L ) CO) in inert gas matrixes and by using isotopic labeling, the metastable coordinatively unsaturated photofragments ML5, ML4, and ML3 could be characterized and their most stable structures were deduced to be a square pyramid (C4V) for ML5, octahedral cis-divacant (C2V) for ML4, and octahedral cistrivacant (C3V) for ML3.3-9 By investigating the matrix photochemistry of the primary photofragment ML5 with polarized light, dichroic photodepletion caused by reorientation of ML5 was observed.8,9 These findings were explained by first formation of naked ML5 in an excited electronic 1E state (square pyramid, C4V) after UV irradiation of the parent molecule ML6, followed by second reorientation via a trigonal bipyramid D3h structure (equilibrium structure of the initially formed 1E state), and third return to a rotated square pyramid C4V structure in the 1A electronic ground state. Thus, mixing of the 1A ground 1 1 state and the 1E excited state potential surfaces at the D3h structure leading to internal conversion had to be invoked. From †
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these matrix studies and earlier flash photolysis experiments10,11 it was concluded that the primary coordinatively unsaturated photofragment following UV excitation in condensed phase is ML5, formed by ejection of a single CO ligand. After formation the naked ML5 photofragment coordinates a solvent molecule or atom (S) or reformation of the parent via cage recombination occurs.12 Further absorption of photons by ML5 can lead to the higher coordinatively unsaturated species down to ML25 (sequential model). This interpretation has been confirmed after a series of timeresolved studies on the femtosecond (fs) and the picosecond (ps) timescale.12-23 Simon and co-workers observed in their transient absorption measurements in the visible after UV photolysis of CrL6 in hexane a pulse width limited (
