TIME-RESOLVED RAMAN-SPECTROSCOPY AND MATRIX-ISOLATION STUDIES OF ANTI-SYN PHOTOISOMERIZATION IN METAL-CARBONYL CARBENES

Photoinduced anti-syn isomerization in tungsten carbonyl carbenes, (CO )(5)W=C(OR)R', [1 with R = Me, R' = Me; 2 with R = Me, R' = p-Tolyl; 3 with R = Et, R' = Ph] brought about by irradiation in the MLCT absorp tion region has been investigated by laser flash photolysis coupled wi th transient absorbance and time-resolved resonance Raman spectroscopy (TR(3)S). The studies, carried out in solution in solvents of varying polarity, have been supplemented by matrix isolation studies using UV -vis and IR detection. Pulsed irradiation in the MLCT absorption regio n of 1-3 results in the formation within the pulse risetime of the syn isomer from the ground state (anti) form, followed by thermal relaxat ion/isomerization back to the ground state. In all three complexes the transient lifetimes increase with increasing solvent polarity falling in the range 120 mu s-3 ms for 1 and 1-2 orders of magnitude shorter for 2 and 3. Growth and decay of the syn isomer were also monitored by TR(3)S. For 1, bands in the Raman attributable to modes of the carben e group showed little change in frequency between the anti and syn. is omers. In the case of the anti forms of 2 and 3, a band near 1235 cm-l attributable to upsilon(C-carbene-OR) shifts to ca. 1270 cm(-1) in th e spectra of the syn isomer. The differences beween 1,2 and 3 in respe ct of both vibrational spectra and rates of isomerization are discusse d in relation to the possible influence on electronic stabilization of reorientation of the R' group which accompanies anti-syn transformati on. Comparison of the anti-syn photoisomerization of 1 in an argon mat rix with that of 4 (R = Me, R' = Ph) shows that the process is photore versible only in the case of 1.