Comparative Measure of the Electronic Influence of Highly Substituted Aryl Isocyanides

To assess the relative electronic
influence of highly substituted aryl isocyanides on transition metal
centers, a series of C4v-symmetric Cr­(CNR)­(CO)5 complexes featuring various alkyl,
aryl, and m-terphenyl substituents have been prepared.
A correlation between carbonyl-ligand 13C­{1H}
NMR chemical shift (δCO) and calculated Cotton–Kraihanzel
(C–K) force constant (kCO) is presented
for these complexes to determine the relative changes in isocyanide
σ-donor/π-acid ratio as a function of substituent identity
and pattern. For nonfluorinated aryl isocyanides possessing alkyl
or aryl substitution, minimal variation in effective σ-donor/π-acid
ratio is observed over the series. In addition, aryl isocyanides featuring
strongly electron-releasing substituents display an electronic influence
that nearly matches that of nonfluorinated alkyl isocyanides. Lower
σ-donor/π-acid ratios are displayed by polyfluorinated
aryl isocyanide ligands. However, the degree of this attenuation relative
to nonfluorinated aryl isocyanides is not substantial and significantly
higher σ-donor/π-acid ratios than CO are observed in all
cases. Substituent patterns for polyfluorinated aryl isocyanides are
identified that give rise to low relative σ-donor/π-acid
ratios but offer synthetic convenience for coordination chemistry
applications. In order to expand the range of available substitution
patterns for comparison, the syntheses of the new m-terphenyl isocyanides CNArTripp2, CNp-MeArMes2, CNp-MeArDArF2,
and CNp-FArDArF2 are also reported (ArTripp2 = 2,6-(2,4,6-(i-Pr)3C6H2)2C6H3); p-MeArMes2 = 2,6-(2,4,6-Me3C6H2)2-4-Me-C6H2); p-MeArDArF2 = 2,6-(3,5-(CF3)2C6H3)2-4-Me-C6H2); p-FArDArF2 = 2,6-(3,5-(CF3)2C6H3)2-4-F-C6H2).