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Synthesis and Reactivity of Group 6 m--Terphenyl Isocyanides


A series of isocyanide ligands supported by m-terphenyls were synthesized and their utility for the isolation of isocyanide analogues to the unsaturated group 6 metal carbonyls was explored. The m-terphenyl isocyanide CNArDipp² (ArDipp² = 2,6-(2,6-(i-Pr)₂C₆H₃)₂C₆H₃) was prepared. The steric attributes of CNArDipp² and the less encumbering m-terphenyl isocyanide CNArMes² (ArMes² = 2,6- (2,4,-Me₃C₆H₂)₂C₆H₃) are compared through the extent of isocyanide ligation to Cu(I), Ag(I), and Mo(0) centers. Structural comparisons of the resulting complexes revealed that while the aforementioned metals could bind three equivalents of CNArMes², maximal ligation was limited to two isocyanides with CNArDipp² . Additionally, an oxidative decarbonylation/reduction synthetic strategy was used in efforts to generate coordinatively unsaturated group 6 isocyanides [M(CNArR²)₂₋₃] from the mixed carbonyl /isocyanide precursors. Accordingly, the zerovalent, bisisocyanide ([Eta]⁶-C₆H₆)Mo(N₂)(CNArDipp²)₂, and trisisocyanide M(([Eta]⁶-(Mes)-[kappa]¹-C- CNArMes)(CNArMes²)₂ (M = Cr and Mo) complexes were prepared. However, coordinative unsaturation in these species was precluded by ([Eta]⁶-binding of benzene or the mesityl ring of the m-terphenyl group to the metal center. Further, in an effort to prevent or weaken the formation of flanking ring ([Eta]⁶-arene interactions observed with the trisisocyanide Mo([Eta]⁶-(R)-)-[kappa]¹-C- CNArR)(CNArR²)₂ (ArR² = ArMes² and ArDipp²) complexes, the halo-substituted m-terphenyl isocyanide ligands CNArClips² (ArClips² = 2,6-(2,6-Cl-₂C₆H₃)₂(4-t-Bu)C₆H₂) and CNArDArF² (ArDArF² = 2,6-(3,5-(CF₃)₂C₆H₃)₂C₆H₃) were prepared. Although Mo([Eta]⁶-(R)-)-[kappa]¹-C-CNArR)(CNArR²)₂ (ArR² = CNArClips² and CNArDArF²) complexes were obtained, in contrast to their alkyl-substituted counterparts, [Eta]⁶- coordination of the tethered isocyanide ligand could be disrupted by addition of benzene or acetonitrile. Following, the reactivity of the Mo([Eta]⁶-(R)-)-[kappa]¹- C-CNArMes)(CNArMes²)₂ (M = Cr and Mo) complexes towards electrophilic substrates was investigated. Electrophilic addition of [H⁺] and [CH₃⁺] was shown to occur exclusively at the nitrogen atom of the geometrically constrained, arene-tethered isocyanide ligands of the M(([Eta]⁶-(Mes)- [kappa]¹-C-CNArMes)(CNArMes²)₂ (M = Cr and Mo) complexes. Lastly, the four-coordinate tetrakisisocyanide complex [Mo(CNArMes²)₄] was targeted. However, reduction of the tetraisocyanide salt [MoI₂(CNArMes²)₄](OTf) proceeded with loss of CNArMes² and formation of Mo([Eta]⁶-(R)-)-[kappa]¹ -C-CNArMes)(CNArMes²)₂

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