Probing the mechanism of carbon-hydrogen bond activation by photochemically generated hydridotris(pyrazolyl)borato carbonyl rhodium complexes: New experimental and theoretical investigations

Alexander J. Blake, Michael W. George, Michael B. Hall, Jonathan McMaster, Peter Portius, Xue Z. Sun, Michael Towrie, Charles Edwin Webster, Claire Wilson, Snežana D. Zarić

Research output: Journal PublicationArticlepeer-review

26 Citations (Scopus)

Abstract

Fast time-resolved infrared (TRIR) experiments and density functional (DFT) calculations have been used to elucidate the complete reaction mechanism between alkanes and photolytically activated hydridotris(pyrazoly-1-yl) boratodicarbonylrhodium. TRIR spectra were obtained after photolysis of Rh(Tp4-tBu-3,5-Me)(CO)2 in n-heptane, n-decane, and cyclohexane and of Rh(Tp3,5-Me)(CO)2 in n-heptane and cyclohexane. Initial photolysis produces a coordinatively unsaturated, 16-electron monocarbonyl species that vibrationally relaxes to an intermediate with vco of 1971 cm-1 in n-heptane solution (species A). DFT calculations on Rh(Tp3,5Me)(CO)-RH (RH = C2H 6, C6H12) suggest that A is the triplet state of a five-coordinate, square-pyramidal Rh(κ3-Tp 3,5-Me)(CO)-RH, in which the alkane is weakly bound. Within the first 2 ns, a new transient grew in at 1993 cm-1 (species B). The calculations show that the observed species B is the singlet state of a four-coordinate Rh(κ2-Tp3,5-Me)(CO)(RH), in which the alkane is strongly bound and one pyrazolyl ring is rotated, decoordinating one N. The transient due to B grew at the same rate as A partially decayed. However, A did not decay completely, but persisted in equilibrium with B throughout the time up to 2500 ps. The v(CO) bands due to A and B decayed at the same rate as a band at 2026 cm-1 grew in (τ ca. 29 ns, n-heptane). The latter band can be readily assigned to the final alkyl hydride products, Rh(κ3-Tp4-tBu-3,5-Me)(CO)R(H) and Rh(κ3-Tp3,5-Me)(CO)R(H) (species D). The experimental data do not allow the elucidation of which of the two alkane complexes, A or B, is C-H activating, or whether both of the complexes react to form the final product. The calculations suggest that a third intermediate (species C) is the C-H activating species, that is, the final product D is formed from C and not directly from either A or B. Species C is nominally a five-coordinate, square-pyramidal Rh(κ21/2-Tp 3,5-Me)(CO)(RH) complex with a strongly bound alkane and one pyrazolyl partially decoordinated, but occupying the apical position of the square pyramid. Intermediate C is unobserved, as the calculations predict it possesses the same CO stretching frequency as the parent dicarbonyl. The unobserved species is predicted to lie on the reaction path between A and B and to be in rapid equilibrium with the four-coordinate species B.

Original languageEnglish
Pages (from-to)189-201
Number of pages13
JournalOrganometallics
Volume27
Issue number2
DOIs
Publication statusPublished - 28 Jan 2008
Externally publishedYes

ASJC Scopus subject areas

  • Physical and Theoretical Chemistry
  • Organic Chemistry
  • Inorganic Chemistry

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