Spectroscopic Detection and Theoretical Confirmation of the Role of Cr2(CO)5(C5R5)2 and ·Cr(CO)2(ketene)(C5R5
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- 作者：George C. Fortman ; Tamá ; s Ké ; gl ; Qian-Shu Li ; Xiuhui Zhang ; Henry F. Schaefer III ; Yaoming Xie ; R. Bruce King ; Joshua Telser ; Carl D. Hoff
- 刊名：Journal of the American Chemical Society
- 出版年：2007
- 出版时间：November 21, 2007
- 年：2007
- 卷：129
- 期：46
- 页码：14388 - 14400
- 全文大小：562K
- 年卷期：v.129,no.46(November 21, 2007)
- ISSN：1520-5126
文摘
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(C5R5)]2 have been investigated under CO, Ar, and N2atmospheres. Under an Ar or N2 atmosphere the reaction is stoichiometric and produces the CrCr triplybonded 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)(C5R5) and Cr2(CO)5(C5R5)2 have been detectedspectroscopically. The complex ·Cr(13CO)2(O=13C=CHSiMe3)(C5Me5) has been studied by electron spinresonance 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(C5H5)2, generated in situ, does not show a signalin its 1H NMR and reacts relatively slowly with CO. It is proposed to be a ground-state triplet in keepingwith predictions based on high level density functional theory (DFT) studies. Computed vibrationalfrequencies are also in good agreement with experimental data. The rates of CO loss from 3Cr2(CO)5(C5H5)2 producing 1[Cr(CO)2(C5H5)]2 and CO addition to 3Cr2(CO)5(C5H5)2 producing 1[Cr(CO)3(C5H5)]2 havebeen measured by kinetics and show H 23 kcal mol-1 for both processes. Enthalpies of reduction byNa/Hg under CO atmosphere of [Cr(CO)n(C5H5)]2 (n = 2,3) have been measured by solution calorimetryand provide data for estimation of the CrCr bond strength in [Cr(CO)2(C5H5)]2 as 72 kcal mol-1. Thecomplex [Cr(CO)2(C5H5)]2 does not readily undergo 13CO exchange at room temperature or 50 C implyingthat 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 proposedon the basis of a combination of experimental and theoretical data.