Solvent Study of the Kinetics of Molybdenum Radical Self-Termination

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• 作者：John C. Linehan ; Clement R. Yonker ; R. Shane Addleman ; S. Thomas Autrey ; J. Timothy Bays ; Thomas E. Bitterwolf ; and John L. Daschbach
• 刊名：Organometallics
• 出版年：2001
• 出版时间：February 5, 2001
• 年：2001
• 卷：20
• 期：3
• 页码：401 - 407
• 全文大小：90K
• 年卷期：v.20,no.3(February 5, 2001)
• ISSN：1520-6041

文摘

The kinetics of (n-butylCp)Mo(CO)₃ (n-butylCp is n-butyl-RC="/images/gifchars/eta.gif" BORDER=0 >⁵-cyclopentadienyl) radical self-termination to form a nonequilibrium mixture of trans- and gauche-[(n-butylCp)Mo(CO)₃]₂and the kinetics of the gauche-to-trans isomerization have been determined in the liquidsolvents n-heptane, tetrahydrofuran, xenon (350 bar), and CO₂ (350 bar) at 283 K by step-scan FTIR spectroscopy. The overall rate constant for the disappearance, 2k_R, of the (n-butylCp)Mo(CO)₃ radical increases with decreasing solvent viscosity as expected, except inCO₂, which is anomalously slower. The slower overall termination rate in liquid CO₂ isconsistent with the formation of a transient molybdenum radical-CO₂ complex. The observedoverall rate constants for (n-butylCp)Mo(CO)₃ self-termination, 2k_R, are (7.9 ± 0.5) × 10⁹M^-1 s^-1 in xenon; (3.2 ± 0.5) × 10⁹ M^-1 s^-1 in heptane; (2.2 ± 0.8) × 10⁹ M^-1 s^-1 in THF; and(1.7 ± 0.5) × 10⁹ M^-1 s^-1 in CO₂. The first determinations of the radical self-termination-to-gauche rate constants, k_G, are presented. The values of k_G are much slower than thecorresponding recombination to trans, k_T, reflecting a steric contribution to the rate. Therate of isomerization (rotation about the molydenum-molybdenum bond) from gauche totrans is unaffected by the solvent and is 3 times faster than the reported isomerization ratefor the nonsubstituted [CpMo(CO)₃]₂ molecule.