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).