121354-22-1Relevant articles and documents
Spectroscopy and linkage isomerization kinetics of (chloranilato)bis(phosphine)palladium(II) compounds
Jeong, Woo-Yeong,Holwerda, Robert A.
, p. 2674 - 2679 (2008/10/08)
In order to better understand the electronic and steric factors that influence the transformation of sp3- to sp2-hybridized carbon in the first coordination sphere of palladium(II), linkage isomerization induced by the reactions of 24 triarylphosphine (or triarylarsine), trialkylphosphine, and diphosphine ligands with [Pd(C-CA)(CH3CN)2] (C-CA2- = chloranilate ligated as a dicarbanion through C-Cl carbon atoms) has been investigated. Infrared and UV-visible spectra of the [Pd(CA)(PR3)2] products reveal that π-CA2- (chloranilate ligated as a p-quinone) is present in all cases. A linear correlation between 31P coordination chemical shifts and Hammett σp substituent constants for seven compounds of the type [Pd(π-CA)(P{Ph-p-X}3)2] (X = H, F, Cl, CH3, CF3, OCH3, NMe2) suggests that polarization of phosphorus 3p electron density toward the electrophilic Pd(II) center contributes significantly to Δ(31P). Stopped-flow kinetic studies of [Pd(π-CA)(P{Ph-X)3)2] formation (X = m- and p-CH3, m-and p-OCH3, m- and p-Cl, and p-F) from [Pd(C-CA)(CH3CN)2] in acetonitrile solution (25°C) support the mechanism previously suggested for X = H. Rate-limiting linkage isomerization (rate constant ki) follows preequilibrium formation of [Pd(C-CA)-(P{Ph-p-X}3)2] (stepwise formation constants K1 and K2). Considering only those phosphines with p-X substituents, Hammett plots of log K2 and log ki vs σp are reasonably linear, yielding ρ constants of -3.12 and -1.32, respectively. Enhancements in ki with increasing electron-donating power of X suggest that the polarization of phosphine electron density toward Pd(II) destabilizes the trans Pd-carbanion C bonds, forcing chloranilate to function as a weaker σ donor in the π-CA linkage isomer.
