Chloro- and Trifluoromethyl-Substituted Flanking-Ring <i>m</i>‑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 <i>m</i>-terphenyl isocyanide ligands to provide flanking-ring η⁶-arene interactions to zerovalent molybdenum centers. The alkyl-substituted <i>m</i>-terphenyl isocyanides CNAr^Mes2 and CNAr^Dipp2 (Ar^Mes2 = 2,6-(2,4,6-Me₃C₆H₂)₂C₆H₃; Ar^Dipp2 = 2,6-(2,6-(<i>i</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(CNAr^R)₃] fragment, the new halo-substituted <i>m</i>-terphenyl isocyanides CNAr^Clips2 and CNAr^DArF2 (Ar^Clips = 2,6-(2,6-Cl₂C₆H₃)₂(4-<i>t</i>-Bu)­C₆H₂; Ar^DArF2 = 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 CNAr^Clips2 and CNAr^DArF2 display a lower tendency toward η⁶-binding. In the case of CNAr^DArF2, 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 CNAr^DArF2 ligand effectively masks low-valent metal centers in a fashion that provides access to low-coordinate isocyano targets such as [Mo­(CNAr^R)₃]. A series of Mo­(CO)₃(CNAr^R)₃ complexes were also prepared to compare the relative π-acidities of CNAr^Mes2, CNAr^Clips2, and CNAr^DArF2. It is found that CNAr^DArF2 shows increased π-acidity relative to CNAr^Mes2 and CNAr^Clips2, despite the fact that its electron-withdrawing CF₃ groups are fairly distal to the terminal isocyano unit

Chloro- and Trifluoromethyl-Substituted Flanking-Ring <i>m</i>‑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 <i>m</i>-terphenyl isocyanide ligands to provide flanking-ring η⁶-arene interactions to zerovalent molybdenum centers. The alkyl-substituted <i>m</i>-terphenyl isocyanides CNAr^Mes2 and CNAr^Dipp2 (Ar^Mes2 = 2,6-(2,4,6-Me₃C₆H₂)₂C₆H₃; Ar^Dipp2 = 2,6-(2,6-(<i>i</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(CNAr^R)₃] fragment, the new halo-substituted <i>m</i>-terphenyl isocyanides CNAr^Clips2 and CNAr^DArF2 (Ar^Clips = 2,6-(2,6-Cl₂C₆H₃)₂(4-<i>t</i>-Bu)­C₆H₂; Ar^DArF2 = 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 CNAr^Clips2 and CNAr^DArF2 display a lower tendency toward η⁶-binding. In the case of CNAr^DArF2, 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 CNAr^DArF2 ligand effectively masks low-valent metal centers in a fashion that provides access to low-coordinate isocyano targets such as [Mo­(CNAr^R)₃]. A series of Mo­(CO)₃(CNAr^R)₃ complexes were also prepared to compare the relative π-acidities of CNAr^Mes2, CNAr^Clips2, and CNAr^DArF2. It is found that CNAr^DArF2 shows increased π-acidity relative to CNAr^Mes2 and CNAr^Clips2, despite the fact that its electron-withdrawing CF₃ groups are fairly distal to the terminal isocyano unit