Tuning π-Acceptor/σ-Donor Ratio of the 2-Isocyanoazulene Ligand: Non-Fluorinated Rival of Pentafluorophenyl Isocyanide and Trifluorovinyl Isocyanide Discovered

Isocyanoazulenes (CNAz) constitute a relatively new class of isocyanoarenes that offers rich structural and electronic diversification of the organic isocyanide ligand platform. This article considers a series of 2-isocyano-1,3-X(2)-azulene ligands (X = H, Me, CO(2)Et, Br, and CN) and the corresponding zero-valent complexes thereof, [(OC)(5)Cr(2-isocyano-1,3-X(2)-azulene)]. Air- and thermally stable, X-ray structurally characterized 2-isocyano-1,3-dimethylazulene may be viewed as a non-benzenoid aromatic congener of 2,6-dimethyphenyl isocyanide (2,6-xylyl isocyanide), a longtime “workhorse” aryl isocyanide ligand in coordination chemistry. Single crystal X-ray crystallographic {Cr–CNAz bond distances}, cyclic voltametric {E(1/2)(Cr(0/1+))}, (13)C NMR {δ((13)CN), δ((13)CO)}, UV-vis {dπ(Cr) → pπ*(CNAz) Metal-to-Ligand Charge Transfer}, and FTIR {ν(N)(≡)(C), ν(C)(≡)(O), k(C)(≡)(O)} analyses of the [(OC)(5)Cr(2-isocyano-1,3-X(2)-azulene)] complexes provided a multifaceted, quantitative assessment of the π-acceptor/σ-donor characteristics of the above five 2-isocyanoazulenes. In particular, the following inverse linear relationships were documented: δ((13)CO(trans)) vs. δ((13)CN), δ((13)CO(cis)) vs. δ((13)CN), and δ((13)CO(trans)) vs. k(C)(≡)(O,trans) force constant. Remarkably, the net electron withdrawing capability of the 2-isocyano-1,3-dicyanoazulene ligand rivals those of perfluorinated isocyanides CNC(6)F(5) and CNC(2)F(3).