Probing the Solvation Properties of Liquid versus Supercritical Fluids with Laser Flash Photolysis of W(CO)₆ in the Presence of 2,2‘-Bipyridine

Abstract

The ring-closure reaction of M(CO)5L−L (M = W, L−L = bipyridine) to produce M(CO)4(L−L) in nonpolar supercritical (sc) CO2/benzene mixtures was selected to investigate the repulsive (or intrinsic) part of the activation volume associated with this reaction. The activation volumes over the range between 7.5 and 24 MPa at different temperatures were higher than those in liquid benzene and liquid CO2/benzene, but much lower than those in pure sc CO2. The UV/vis spectral studies of this reaction showed that the addition of small amounts of cosolvent can increase the solubility of the ring-closure species. The kinetic traces recorded for the reaction also demonstrated this solubility enhancement.

New Directions in Inorganic and Metal-Organic Coordination Chemistry in Supercritical Fluids

“…Indeed, we show later how the low solubility of W(CO) 4 (pyridine) 2 in scCO 2 can be exploited for synthetic chemistry. Therefore, reactions involving 2,2‘-bipyridine were carried out in scCO 2 /benzene mixtures . The ring closure is really quite slow under these conditions, taking nearly 200 s to reach completion.…”

Homogeneous Catalysis in Supercritical Carbon Dioxide

“…Interestingly, the same type of rate enhancement in the vicinity of the critical point at 30 • C and 1000 psi was not observed with Mo(CO) 5 (1,10-phenanthroline) even though the compressibility of CO 2 is also very high at these conditions. Subsequently, the same group investigated the effect of using cosolvents in scCO 2 on activation volumes for the ring closure reaction of W(CO) 5 (2,2 -bipyridene) (65). With benzene as the cosolvent, the activation volumes were found to be significantly smaller than the phenanthroline system in pure scCO 2 and decreased with increasing concentration of cosolvent in the mixture ranging from 66.2 to 6.5 cm 3 /mol.…”

Spectroscopic Investigations of Intermolecular Interactions in Supercritical Fluids

“…Laser flash photolysis (LFP) and transient absorption measurements have been used for the quantification of energy transfer and diffusion-controlled reactions in supercritical fluids [24][25][26][27][28][29][30][31]. For example, Worrall and Wilkinson [24) have studied the rate of triplet -triplet energy transfer for the anthracene/ azulene and benzophenone/naphthalene donor/acceptor systems and determined how the reaction rate is affected by cosolvent concentration in scC02 • By first studying the effect of changing cosolvent (CH3CN or n-hexane) on the anthracene/azulene system, the authors determined that the rate of energy transfer for this system was not affected by the cosolvent used.…”