Matrix isolation and transient absorption studies of (bis(bis(pentafluoroethyl)phosphino)ethane)tetracarbonyl-chromium: Intermolecular alkane complexes and intramolecular F-coordination

Article abstract of DOI:10.1021/ja00036a032

Photolysis of (η²-dfepe)Cr(CO)₄ [dfepe = (C₂F₅)₂PCH₂CH₂P(C ₂F₅)₂] has been conducted in rigid matrices at 12 K (Ar, CH₄, N₂, CO) and 30 K (Xe) and in alkane and perfluoroalkane solutions. In Ar, CH₄, and Xe matrices a combination of IR and UV-vis spectroscopic data demonstrate photodissociation of the cis carbonyl ligand to generate fac-(dfepe)Cr(CO)₃L, where L is either the matrix host or an intramolecularly coordinated F atom donated by one of the pendant CF₃CF₂- groups of the dfepe ligand. Only traces of the mer isomer were observed. In nitrogen matrices there is evidence for formation of both fac- and mer-(dfepe)Cr(CO)₃N₂. Carbonyl stretching frequencies of the matrix-isolated photoproducts were measured and bonding force constants and ligand effect constants were calculated. In solution both the Cr-CO and Cr-P bonds are photolabile. Results of nanosecond and microsecond transient absorption spectroscopic experiments in hexane and cyclohexane solvents indicate formation of alkane complexes of the type (η²-dfepe)Cr(CO)₃(alkane) and (η¹-dfepe)Cr(CO)₄(alkane), as well as formation of intramolecularly F-coordinated species Cr(CO)₄(η¹-dfepe) and Cr(CO)₃(η²-dfepe). By use of transient absorption techniques, rates of reaction of these species in alkane solvents with and without added trapping ligands (CO, water, triethylsilane) have been measured. The F-coordinated species react with CO approximately 50 times faster than the alkane complexes. In experiments using (η¹-dfepe)Cr(CO)₅, similar results have been obtained which support the interpretation of the behavior of (η²-dfepe)Cr(CO)₄. Products from ¹³CO trapping of the transients generated in hexane show that all four possible isomers, fac- and mer-(η²-dfepe)Cr(CO)₃*CO and cis- and trans-(η¹-dfepe)Cr(CO)₄*CO, are formed in significant quantities. Experiments carried out at a different ¹³CO pressures show that ¹³CO trapping of cis-(η¹-dfepe)Cr(CO)₄(L) is competitive with ring closure back to (η²-dfepe)Cr(CO)₄.