Sequential Photosubstitution of Carbon Monoxide by (E)-Cyclooctene in Hexacarbonyltungsten:  Structural Aspects, Multistep Photokinetics, and Quantum Yields

The photochemical conversion of W(CO)6 (1) into a trans-W(CO)4(η2-olefin)2 complex has been investigated using (E)-cyclooctene (eco) as a model olefin possessing extraordinary coordination properties. trans-W(CO)4(η2-eco)2 (4) is generated as an equimolar mixture of two diastereoisomers (4a, S 4 symmetry; 4b, D 2 symmetry) which can be separated by fractional crystallization. The entire reaction sequence involves the intermediate formation of W(CO)5(η2-eco) (2) and cis-W(CO)4(η2-eco)2 (3: two diastereoisomers, 3a and 3b, with apparent C s and C 2 symmetry, respectively). Complexes 2 and 3, although difficult to isolate from the photochemical reaction mixture, are conveniently accessible via alternative thermal ligand exchange routes. The molecular structures of 2 and 4a in the crystal were determined by X-ray diffraction techniques. The olefin double bonds, with trans-orthogonal arrangement in 4a, are eclipsed to a OC−W−CO axis in either case. The course of the conversion of 1 into the olefin-substituted products was monitored by quantitative IR spectroscopy. Photokinetic equations developed for this study describe the concentrations of all four components as implicit functions of the amount of light absorbed by the system, of the quantum yields of the individual photoprocesses, and of the UV−vis absorbance coefficients of the compounds involved. Based on these functional relationships, the individual quantum yields at λexc = 365 nm (Φ12 = 0.73, Φ23 = 0.34, Φ24 = 0.16, Φ34 = 0.15) were evaluated from a series of experimental data sets by an iterative procedure which involves variation of the quantum yield input data until the best fit of the computed to the measured concentrations is achieved. Low-temperature matrix isolation techniques were employed to characterize the W(CO)4(η2-eco) fragment (5) as a key intermediate in the photolysis of W(CO)5(η2-eco) (2).