which has elsewhere['41 manifested a special coordination ability. Irradiation (d = 289 nm) of [Cr(C0),(q4-nbd)] and ECO in 2-methylpentane at -40°C affords mer-and juc-[Cr(C0),(q4-nbd) (q2-eco)], which are easily distinguished on the basis of their CO stretching bands (rner complex, f [cm-'] = 1997(m), 1925 (sst); fac complex < [cm-'] = 1968, 1904, 1876). The mer complex, which is more stable, is the only observed product at room temperature. In order to determine the quantum yield (@ = 0.18 at 300 nm, 0.1 1 at 365 nm), its formation was followed quantitatively by IR spectroscopy. If this reaction is assumed to serve as a model for the generation of the catalytically active [Cr(CO),] complex (Qind), then the relation eta, = Gind x (turnovers) gives a rough estimate of 10 to 20 for the quantum yield of the photocatalytic hydrogenation of norbornadiene (Qca,). The experimentally determined values (1.9,['] ca. 0.3[,') are considerably lower, probably because the active [Cr(CO),] complex generated in situ is rapidly transformed into inactive [Cr(CO),(q4-nbd)] by the CO still present in the system. The stability of [M(C0),(q4-nbd)(q2-olefin)] complexes increases markedly in the series Cr < Mo < W. As a result, the catalytically active species is trapped by excess substrate (NBD) as [M(CO), (q4-nbd)(q2-nbd)] in the cases of tungsten and molybdenum.['0] The analogous chromium complex, by contrast, has not yet been detected. Thus, the drastic decrease in the catalytic activity in the series Cr $ Mo > WI3] is readily explained.In analogy to the hydrogenation, norbornadiene can also be hydrosilylated with silanes (R,SiH), both photocatalytically with [Cr(CO), (q4-nbd)] and in the dark with 1. As we will report in detail elsewhere, both variants afford endo 1,2-, exo 1,2-, and homo 1,4-adducts with comparable product distributions.
Experimental ProcedurePreparation of 1: [Cr(C0),(q4-nbd)] (1.00 g, 3.9 mmol) was irradiated in 200 mL of ethene-saturated pentane at -50°C in an immersion lamp apparatus (Solidex glass, i . 2 280 nm; Philips HPK 125-W Hg lamp) for about 3 h, until the starting material (Cco [cm-'1 = 2043, 1961, 1946, 1917) had largely reacted. The temperature was maintained at I -30°C during workup. After addition of about 2 g of silica gel (to absorb readily decomposable by-products), the solution was stirred for several minutes and then evaporated to dryness under vacuum. The residue was extracted into 3 x 10 mL of ethene-saturated pentane, transferred to a column (silica gel 60, Merck; 2 x 15 cm), and eluted with additional solvent. The first of the two yellow fractions was concentrated under vacuum to about 50mL and cooled to -78°C whereupon pure mer-[Cr(C0),(q4-nbd)(q2-ethene)] (0.46 g, 46 %) crystallized out. M.p. = 46°C; correct elemental analysis (C, H, Cr); IR (n-hexane): F[cm-'] = 2005(m), 1933 (sst) (CO). 'H NMR (400 MHz, C,D,, 243 K): 6 = 0.85 (m, 2 H), 1.85 (s, br, 4 H, ethene), 3.00 (s, br, 2 H), 3.16 (s, br, 2 H), 3.89 (s, br, 2 H). "C NMR (100.6 MHz, C,D,, 243 K): 6 = 47.41 (CH), 52.78 (ethene)...
