INFRARED SPECTROSCOPIC STUDIES ON THE PHOTOCATALYTIC HYDROGENATION OF NORBORNADIENE BY GROUP-6 METAL-CARBONYLS .2. THE ROLE OF THE DIENE AND THE CHARACTERIZATION OF (ETA-4-NORBORNADIENE)(ETA-2-NORBORNADIENE)M(CO)3 COMPLEXES

UV laser flash photolysis of M(CO)6 and (NBD)M(CO)4 (NBD = norbornadiene) is used to generate, respectively, the intermediates M(CO)5(s) (s = η-heptane; M = Cr, Mo, and W) and (η4-NBD)M(CO)3(s) (fac and mer isomers; = Mo and W). Fast (ηs) time-resolved IR spectroscopy is then used to measure the rates of reaction of these intermediates with species likely to be components of the catalytic hydrogenation mixture (e.g., H2, NBD, CO, etc.). The measurements show that, for a particular intermediate, the rates of reaction with H2 and NBD are quite similar. Thus, in a typical catalytic reaction mixture, intermediates are more likely to react with NBD than H2, because the concentration of NBD is considerably higher than that of H2. Fourier-transform IR is used to monitor the carbonyl species present in solution during photocatalytic deuteration at room temperature. These include the previously unknown complexes fac- and mer-(η4-NBD)(η2-NBD)M(CO)3. In the case of W, it is shown that accumulation of mer-(η4-NBD)(η2-NBD)W(CO)3 in the solution frustrates the catalytic deuteration [as monitored by the p(C-D) IR bands of the deuterated products]. Furthermore, there is a striking wavelength dependence in the catalytic efficiency of the W system. mer-(η4-NBD)(η2-NBD)W(CO)3 is sufficiently stable for us to have isolated a sample (ca. 90% purity). 13C NMR data support the IR identification of the complex. Liquid Xe is used to generate samples of fac-(η4-NBD)(C2H4)Mo(CO)3, which is used as a model for the photochemical behavior of fac-(η4NBD)(η2-NBD)M(CO)3. The results of this paper are combined with those of part 1 (preceding paper) to propose an overall scheme for the catalytic hydrogenation. The suggested role of the (η4-NBD)(η2-NBD)M(CO)3 compounds in this scheme is to facilitate the fac → mer isomerization of the (η4-NBD)M(CO)3 moiety. © 1990, American Chemical Society. All rights reserved.