Kinetics of Substitution of Weakly Coordinating Nitrate by Chloride in (η 5 -Cp)Ru(CO)(ER 3 )ONO 2 (ER 3 = AsPh 3 , PPh 3 , P( p -anisyl) 3 , PPh 2 ( o -anisyl), P(OPh) 3 ) in Dichloromethane
2001; American Chemical Society; Volume: 20; Issue: 11 Linguagem: Inglês
10.1021/om000670d
ISSN1520-6041
AutoresMinhton Cao, Võ Thành Liêm, Norris W. Hoffman, M.-L. Kwan, Julie K. Little, J. Michael McGilvray, Christopher B. Morris, Björn C.G. Söderberg, Anthony S. Wierzbicki, Thomas R. Cundari, Charles H. Lake, E. Valente,
Tópico(s)Asymmetric Hydrogenation and Catalysis
ResumoThe d6 complexes of formula (η5-Cp)Ru(CO)(ER3)ONO2, where ER3 = AsPh3, PPh3, P(p-anisyl)3, PPh2(o-anisyl), and P(OPh)3, were prepared by reaction of their chloro analogues with AgNO3 in CH2Cl2. They underwent moderately slow substitution of the relatively weakly coordinating nitrate by chloride in dry CH2Cl2 in the presence of excess [N(PPh3)2+]Cl-. A kinetics study showed the reaction to be first order in nitrato complex and independent of chloride salt concentration under pseudo-first-order conditions. First-order rate constants for nitrate metathesis follow the trend P(OPh)3 < PPh3 < P(p-anisyl)3 < AsPh3 ≪ PPh2(o-anisyl), with k1 at 25 °C for the first four in the series from 1.4 to 4.4 × 10-5 s-1. Activation parameters for conversion of (η5-Cp)Ru(CO)(AsPh3)ONO2 to its chloride are ΔH‡ = 17(2) kcal/mol and ΔS‡ = −21(5) eu. Even use of a slightly moist solvent increased the rate of nitrate metathesis by 20−30% without changing the form of the rate law. The complex containing PPh2(o-anisyl) was approximately 2 orders of magnitude more reactive (k1 = 4.9 × 10-3 s-1). A likely explanation is stabilization of a coordinatively unsaturated intermediate by weak coordination of the potentially chelating o-OMe group. A mechanism entailing rate-limiting conversion of the neutral nitrato complex into a coordinatively unsaturated ion-paired species is consistent with this set of data. Semiempirical calculations (PM3(tm)), which model the structures of complexes in these systems quite well, supported such behavior. X-ray crystal structures were determined for the AsPh3 nitrato and chloro complexes and for the PPh3 chloro complex.
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