Chloro- and Trifluoromethyl-Substituted Flanking-Ring m-Terphenyl Isocyanides: η6-Arene Binding to Zero-Valent Molybdenum Centers and Comparison to Alkyl-Substituted Derivatives

Presented herein are synthetic and structural studies exploring the propensity of m-terphenyl isocyanide ligands to provide flanking-ring η6-arene interactions to zerovalent molybdenum centers. The alkyl-substituted m-terphenyl isocyanides CNArMes2 and CNArDipp2 (ArMes2 = 2,6-(2,4,6-Me3C6H2)2C6H3; ArDipp2 = 2,6-(2,6-(i-Pr)2C6H3)2C6H3) react with Mo(η6-napthalene)2 in a 3:1 ratio to form tris-isocyanide η6-arene Mo complexes, in which a flanking mesityl or 2,6-diisopropylphenyl group, respectively, of one isocyanide ligand is bound to the zerovalent molybdenum center. Thermal stability and reactivity studies show that these flanking ring η6-arene interactions are particularly robust. To weaken or prevent formation of a flanking-ring η6-arene interaction, and to potentially provide access to the coordinatively unsaturated [Mo(CNArR)3] fragment, the new halo-substituted m-terphenyl isocyanides CNArClips2 and CNArDArF2 (ArClips = 2,6-(2,6-Cl2C6H3)2(4-t-Bu)C6H2; ArDArF2 = 2,6-(3,5-(CF3)2C6H3)2C6H3) have been prepared. Relative to their alkyl-substituted counterparts, synthetic and structural studies demonstrate that the flanking aryl rings of CNArClips2 and CNArDArF2 display a lower tendency toward η6-binding. In the case of CNArDArF2, it is shown that an η6-bound 3,5-bis(trifluoromethyl)phenyl group can be displaced from a zerovalent molybdenum center by three molecules of acetonitrile. This observation suggests that the CNArDArF2 ligand effectively masks low-valent metal centers in a fashion that provides access to low-coordinate isocyano targets such as [Mo(CNArR)3]. A series of Mo(CO)3(CNArR)3 complexes were also prepared to compare the relative π-acidities of CNArMes2, CNArClips2, and CNArDArF2. It is found that CNArDArF2 shows increased π-acidity relative to CNArMes2 and CNArClips2, despite the fact that its electron-withdrawing CF3 groups are fairly distal to the terminal isocyano unit.