Spectroscopic detection and theoretical confirmation of the role of Cr 2(CO)5(C5R5)2 and ·Cr(CO)2(ketene)(C5R5) as intermediates in carbonylation of N=N=CHSiMe3 to O=C=CHSiMe3 by ·Cr(CO)3(C5R5) (R = H, CH3)

George C. Fortman, Tamás Kégl, Qian Shu Li, Xiuhui Zhang, Henry F. Schaefer, Yaoming Xie, R. Bruce King, Joshua Telser, Carl D. Hoff

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Abstract

Conversion of N=N=CHSiMe3 to O=C=CHSiMe3 by the radical complexes ·Cr(CO)3C5R5 (R = H, CH3) derived from dissociation of [Cr(CO)3(C 5R5)]2 have been investigated under CO, Ar, and N2 atmospheres. Under an Ar or N2 atmosphere the reaction is stoichiometric and produces the Cr≡Cr triply bonded complex [Cr(CO)2(C5R5)]2. Under a CO atmosphere regeneration of [Cr(CO)3(C5R5)] 2 (R = H, CH3) occurs competitively and conversion of diazo to ketene occurs catalytically as well as stoichiometrically. Two key intermediates in the reaction, ·Cr(CO)2(ketene)(C 5R5) and Cr2(CO)5(C 5R5)2 have been detected spectroscopically. The complex ·Cr(13CO)2(O=13C=CHSiMe 3)(C5Me5) has been studied by electron spin resonance spectroscopy in toluene solution: g(iso) = 2.007; A(53Cr) = 125 MHz; A(13CO) = 22.5 MHz; A(O=13C=CHSiMe3) = 12.0 MHz. The complex Cr2(CO)5(C5H 5)2, generated in situ, does not show a signal in its 1H NMR and reacts relatively slowly with CO. It is proposed to be a ground-state triplet in keeping with predictions based on high level density functional theory (DFT) studies. Computed vibrational frequencies are also in good agreement with experimental data. The rates of CO loss from 3Cr2(CO)5-(C5H5) 2 producing 1[Cr(CO)2(C5H 5)]2 and CO addition to 3Cr2(CO) 5(C5H5)2 producing 1[Cr(CO)3(C5H5)]2 have been measured by kinetics and show ΔH ≅ 23 kcal mol-1 for both processes. Enthalpies of reduction by Na/Hg under CO atmosphere of [Cr(CO)n(C5H5)]2 (n = 2,3) have been measured by solution calorimetry and provide data for estimation of the Cr≡Cr bond strength in [Cr(CO)2(C5H 5)]2 as 72 kcal mol-1. The complex [Cr(CO) 2(C5H5)]2 does not readily undergo 13CO exchange at room temperature or 50°C implying that 3Cr2(CO)5(C5H5) 2 is not readily accessed from the thermodynamically stable complex [Cr(CO)2(C5H5)]2. A detailed mechanism for metalloradical based conversion of diazo and CO to ketene and N2 is proposed on the basis of a combination of experimental and theoretical data.

Original languageEnglish
Pages (from-to)14388-14400
Number of pages13
JournalJournal of the American Chemical Society
Volume129
Issue number46
DOIs
Publication statusPublished - Nov 21 2007

ASJC Scopus subject areas

  • Catalysis
  • Chemistry(all)
  • Biochemistry
  • Colloid and Surface Chemistry

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