An unusual high-pressure (<5000 psi) and low-temperature (>30 K) cell is used to study the photochemistry of d 6 metal carbonyl complexes and related compounds in a polyethylene (PE) matrix. This approach combines some of the advantages of traditional matrix isolation with those of low temperature solvents (e.g. liquid Xe). UV photolysis of M(CO) 6 under pressures of N 2 lead initially to the formation of M(CO) 5 N 2 and, on longer photolysis, to more highly substituted M(CO) 6-n (N 2 ) n (n e 4) species. Photolysis under a pressure of H 2 leads to M(CO) 5 (η 2 -H 2 ) and cis-M(CO) 4 (η 2 -H 2 ) 2 , disubstituted compounds which were previously unknown for Mo and W. The thermal reactivities of all of these compounds were qualitatively established by raising the temperature of the PE matrix and monitoring the reaction with CO. The UV photolysis of W(CO) 5 CS and (η 6 -C 6 H 3 (CH 3 ) 3 )M(CO) 3 (M ) Cr and Mo) also leads to previously uncharacterized H 2 and N 2 complexes. The low-temperature, high-pressure (LT-HP) cell allows gases to be exchanged during the course of a single experiment. The conditions needed for H 2 and N 2 to penetrate the PE and to be removed from it have been investigated, and this information has been used to study thermal exchange reactions between coordinated N 2 and η 2 -H 2 ligands in M(CO) 5 L compounds. It was found that the reactivity followed the order Mo > Cr > W.

This paper presents a new method for investigating the mechanisms of homogeneously catalyzed reactions involving gases, particularly H 2 . We show how the combination of polyethylene (PE) matrices and high pressure-low temperature (HPLT) experiments can be used to provide new mechanistic information on hydrogenation processes. In particular, we show how we are able to generate reaction intermediates at low temperature, and then to extract the contents of the PE film at room temperature to characterize the organic products using GC-MS. We have used our new technique to probe both the hydrogenation of dimethyl fumarate (DF), using Fe(CO) 4 (η 2 -DF) as the catalytic species, and the hydrogenation of norbornadiene (NBD), using (NBD)M(CO) 4 (M ) Cr or Mo) as the catalytic species. Irradiation of Fe(CO) 4 (η 2 -DF) in a PE matrix at 150 K resulted in the formation of an intermediate complex tentatively assigned Fe(CO) 3 (η 4 -DF). Warming this complex to 260 K under H 2 leads to the formation of Fe(CO) 3 (η 2 -DF)(η 2 -H 2 ). Further warming of the reaction system results in the hydrogenation of the coordinated DF, to generate dimethyl succinate (DS). Characterization of the intermediate species was obtained using FTIR spectroscopy. Formation of DS was confirmed using both FTIR spectroscopy and GC-MS analysis. UV photolysis of (NBD)M(CO) 4 in PE under H 2 in the presence of excess NBD results in the formation of the hydrogenated products norbornene (NBN) and nortricyclene (NTC), with trace amounts of norbornane (NBA) being observed. These products were in similar ratios to those observed in fluid solution. However, for (NBD)Mo(CO) 4 , the relative amounts of the organic products change considerably when the reaction is repeated in PE under H 2 in the absence of free NBD, with NBA being the major product. The use of our HPLT cell allows us to vent and exchange high pressures of gases with ease, and as such we have performed gas exchange reactions with H 2 and D 2 . Analysis of the reaction products from these exchange reactions with GC-MS provides evidence for the mechanism of formation of NBA, in both the presence and absence of excess NBD, a reaction which has been largely ignored in previous studies.

Employing time-resolved IR spectroscopy, we have demonstrated that, following photolysis of [Cp*Cr(CO)2]2 (1) in n-heptane, the only initial photoproduct is the triply bridged CO-loss intermediate Cp*Cr(μ-CO)3CrCp* (4). Two bridging ν(CO) bands are observed, resulting from the splitting of the e‘ mode of 4, which leads to the conclusion that the CO bridging groups are asymmetric. The rate of the back reaction of 4 with CO has been measured (k 2 = 5.2 (±0.5) × 107 M-1 s-1), and this is 1000 times faster than the rate of the analogous back reaction of CO with CpFe(μ-CO)3FeCp. Moreover, unlike many CO-loss intermediates, there is no reaction of 4 with PPh3. The rate of reaction of 4 with THF, to produce Cp*2Cr2(CO)3(THF) (6), has been estimated (k 2 = 1.6 (±0.2) × 108 M-1 s-1).