No abstract

A series of Group 7 organometallic noble gas complexes, (η5-C5R5)M(CO)2L (M = Mn and Re; R = H, Me. and Et (Mn only); L = Kr and Xe) have been characterized in supercritical noble gas solution at room temperature using fast time-resolved infrared spectroscopy. The kinetics and activation parameters for the reaction of these complexes with CO were measured and compared to the analogous alkane complexes, (η5-C5H5)M(CO)2(n-heptane) (M = Mn and Re). Evidence obtained from values of the activation enthalpies and experiments, in which the lifetimes of (η5-C5R5)Mn(CO)2Xe (R = H, Me, and Et) were measured as a function of [CO] at a constant [CO]/[Xe] ratio, suggests that the reaction of the noble gas complexes with CO occurs mainly via a dissociative or dissociative interchange mechanism.

Fast time-resolved infrared (TRIR) spectroscopy has been used to study a wide range of organometallic alkane and noble-gas complexes at ambient temperature. We have shown that the reactivity of the n-heptane complexes decreases both across and down Groups V, VI, and VII, and that the corresponding xenon complexes have similar reactivities.

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 fast time-resolved infrared (TRIR) spectroscopy we have characterised CpM(CO) 3 (Xe) (M = Nb or Ta) at room temperature in supercritical Xe solution; CpM(CO) 3 (Xe) were found to exhibit a similar reactivity with CO to the corresponding alkane complexes, CpM(CO) 3 (n-heptane) and we report a trend in reactivity of the early transition metal Xe complexes.