Kinetic and thermodynamic studies of reaction of •Cr(CO)3C5Me5, HCr(CO)3C5Me5, and PhSCr(CO)3C5Me5 with •NO. Reductive elimination of thermodynamically unstable molecules HNO and RSNO driven by formation of the strong CR-NO bond

Kenneth B. Capps, Andreas Bauer, Kengkaj Sukcharoenphon, Carl D. Hoff

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12 Scopus citations

Abstract

Reaction of H-Cr(CO)3C5Me5 with •NO at 1-2 atm pressure in toluene solution yields Cr(NO)(CO)2C5Me5 as the sole metal-containing product in addition to N2O and HNO2 as the principle nitrogen-containing products. N2O and HNO2 are attributed to decomposition of the initial product HNO. Kinetic studies yield the rate law d[P]/dt = -k2nd order[HCr(CO)3C5Me5][•NO]; k2nd order = 0.14 M-1 s-1 at 10 °C, with ΔH = 11.7 ± 1.5 kcal/mol and ΔS = -16.3 ± 3.5 cal/(mol deg). The rate of reaction is not inhibited by CO. The kinetic isotope effect for. reaction of D-Cr(CO)3C5Me5 is kH/kD = 1.7. These observations are consistent with a first step involving direct H (D) atom transfer from the metal hydride to •NO, forming HNO. Also supporting this mechanism is the ∼150 times slower reaction of H-Mo(CO)3C5Me5 and failure to observe reaction for H-W(CO)3C5Me5 in keeping with metal-hydrogen bond strengths Cr < Mo < W. Reaction of PhS-Cr(CO)3C5Me5 with NO at 1-2 atm pressure in toluene solution also forms Cr(NO)(CO)2C5Me5 as the sole metal-containing product. The initial product is the unstable nitrosothiol PhS-NO. Kinetic studies yield the rate law d[P]/df = -k1st order [PhS-Cr(CO)3C5Me5]; k1st order = 3.1 ± 0.3 × 10-3 s-1 at 10 °C, with ΔH = 21.6 ± 1.2 kcal/mol, ΔS = + 3.9 ± 1.5 cal/(mol deg). The rate of reaction is independent of both NO and CO pressure. The transition state in the first-order process is proposed to involve migration of bound thiolate to coordinated CO, forming Cr(CO)2 (n2-C(=O)SPh)C5Me5. The enthalpy of reaction of •Cr(CO)3C5Me5 and NO yielding Cr(NO)(CO)2C5Me5 and CO has been measured by solution calorimetry: Δ° = -33.2 ± 1.8 kcal/mol. The Cr-NO bond strength is estimated as ∼70 kcal/mol and provides the net thermodynamic driving force for the proposed elimination of the unstable molecules HNO and PhSNO.

Original languageEnglish (US)
Pages (from-to)6206-6211
Number of pages6
JournalInorganic Chemistry
Volume38
Issue number26
StatePublished - Dec 1 1999

ASJC Scopus subject areas

  • Physical and Theoretical Chemistry
  • Inorganic Chemistry

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