Matrix isolation and transient absorption studies of [bis[bis(pentafluoroethyl)phosphino]ethane]tetracarbonylchromium: intermolecular alkane complexes and intramolecular F-coordination

Abstract

Photolysis of (t;2-dfepe)Cr(CO)4 [dfepe = (C2F5)2PCH2CH2P(C2F5)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/ac-(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 wer-(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 (tj2-dfepe)Cr(CO)3(alkane) and (V-dfepe)Cr(CO)4(alkane), as well as formation of intramolecularly F-coordinated species Cr(CO)4(V-dfepe) and Cr(CO)3(r;2-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 (j)'-dfepe)Cr(CO)5, similar results have been obtained which support the interpretation of the behavior of (?i2-dfepe)Cr(CO)4. Products from 13CO trapping of the transients generated in hexane show that all four possible isomers, fac- and mer-(jj2-dfepe)Cr(CO)3*CO and cis- and tr<znj-(ijl-dfepe)Cr(CO)4*CO, are formed in significant quantities. Experiments carried out at a different l3CO pressures show that l3CO trapping of ci>(V-dfepe)Cr(CO)4(L) is competitive with ring closure back to (rj2-dfepe)Cr(CO)4.