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

Abstract

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-X₂-azulene ligands (X = H, Me, CO₂Et, Br, and CN) and the corresponding zero-valent complexes thereof, [(OC)₅Cr(2-isocyano-1,3-X₂-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+)}, ¹³C NMR {δ(¹³CN), δ(¹³CO)}, 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)₅Cr(2-isocyano-1,3-X₂-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(¹³CO_trans) vs. δ(¹³CN), δ(¹³CO_cis) vs. δ(¹³CN), and δ(¹³CO_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 CNC₆F₅ and CNC₂F₃.

Keywords: azulene; back-bonding; chromium pentacarbonyl; isocyanide; metal-to-ligand charge transfer; voltammetry.