Inorganic Chemistry, Vol.52, No.22, 13216-13229, 2013
Chloro- and Trifluoromethyl-Substituted Flanking-Ring m-Terphenyl Isocyanides: eta(6)-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 eta(6)-arene interactions to zerovalent molybdenum centers. The alkyl-substituted m-terphenyl isocyanides CNArMes2 and CNArDipp2 (Ar-Mes2 = 2,6-(2,4,6-Me3C6H2)(2)C6H3; Ar-Dipp2 = 2,6-(2,6-(i-Pr)(2)C6H3)(2)C6H3) react with Mo(eta(6)-napthalene)(2) in a 3:1 ratio to form tris-isocyanide eta(6)-arene Mo complexes, in which a flanking mesityl or 2,6-diisopropylphenyl group, respectively, of one iso cyanide ligand is bound to the zerovalent molybdenum center. Thermal stability and reactivity studies show that these flanking ring eta(6)-arene interactions are particularly robust. To weaken or prevent formation of a flanking-ring eta(6)-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 (Ar-Clips = 2,6-(2,6-Cl2C6H3)(2)(4-t-Bu)C6H2; Ar-DArF2 = 2,6-(3,5-(CF3)(2)C6H3)(2)C6H3) 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 eta(6)-binding. In the case of CNArDArF2, it is shown that an eta(6)-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 pi-acidities of CNArMes2, CNArClips2, and CNArDArF2. It is found that CNArDArF2 shows increased pi-acidity relative to CNArMes2 and CNArClips2, despite the fact that its electron-withdrawing CF3 groups are fairly distal to the terminal isocyano unit.