INFRARED SPECTROSCOPIC STUDIES ON THE PHOTOCATALYTIC HYDROGENATION OF NORBORNADIENE BY GROUP-6 METAL-CARBONYLS .1. THE ROLE OF H-2 AND THE CHARACTERIZATION OF NONCLASSICAL DIHYDROGEN COMPLEXES, (ETA-4-NORBORNADIENE)M(CO)3(ETA-2-H2)

This paper1 describes the IR characterization of nonclassical H2 complexes where ethene or norbornadiene (NBD) is coordinated to the same d6 metal center as the η2-H6 metal carbonyl compounds. The dihydrogen complexes are generated in solution by the photolysis of alkene and diene carbonyl complexes in the presence of an overpressure of H2 or D2. Photolysis of trans-(C2H4)2M(CO)4 (M = Cr, Mo, and W) (1) in liquefied xenon (LXe) doped with H2 or D2 at -90 °C leads to the formation of mer-(C2H4)2M(CO)3(H2) (2) (with trans C2H4 groups) and cis-(C2H4)M(CO)4(H2) (3). These species have been characterized by FTIR, and the v(H-H) IR band of the coordinated η2-H2 has been detected for the W complexes. All of these compounds react thermally with N2 to yield the corresponding dinitrogen complexes. Fast (microsecond) time-resolved IR (TRIR) is used to identify fac- and mer-(NBD)M(CO)3(n-hept) as the primary photoproducts of the photolysis of (NBD)M(CO)4 [M = Cr, Mo, and W] in n-heptane at room temperature. Detailed studies with Mo show that these intermediates react stereospecifically with N2 and H2 to give fac- and mer-(NBD)Mo(CO)3(X2) (X2 = N2 and H2). These species can also be generated in LXe by photolysis of either (NBD)Mo(CO)4 or mer-(NBD)Mo(CO)3(C2H4). In addition, photolysis of (NBD)M(CO)4 [M = Mo or W] in LXe yields products apparently containing η2-NBD, which are not observed in the photolysis at room temperature, probably as a result of differences in temperature and photolysis sources. fac-(NBD)Mo(CO)3(H2) appears to decay largely by loss of H2, while fac-(NBD)Cr(CO)3(H2) and mer-(NBD)M(CO)3(H2) (M = Cr and Mo) do not decay by this pathway. A mechanism is proposed whereby transfer of H2 to the diene in mer-(NBD)M(CO)3(H2) leads to formation of norbornene (NBN) and in fac(NBD)M(CO)3(H2) leads to nortricyclene (NTC). The geometries of these two dihydrogen complexes provide a simple rationalization for the observed stereospecificity of the catalytic hydrogenation. © 1990, American Chemical Society. All rights reserved.