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).
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