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

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John J Meyers
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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<sub>2</sub>-azulene ligands (X = H, Me, CO<sub>2</sub>Et, Br, and CN) and the corresponding zero-valent complexes thereof, [(OC)<sub>5</sub>Cr(2-isocyano-1,3-X<sub>2</sub>-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<sub>1/2</sub>(Cr<sup>0/1+</sup>)}, <sup>13</sup>C NMR {δ(<sup>13</sup>CN), δ(<sup>13</sup>CO)}, UV-vis {dπ(Cr) → pπ*(CNAz) Metal-to-Ligand Charge Transfer}, and FTIR {ν<sub>N</sub><sub>≡</sub><sub>C</sub>, ν<sub>C</sub><sub>≡</sub><sub>O</sub>, k<sub>C</sub><sub>≡</sub><sub>O</sub>} analyses of the [(OC)<sub>5</sub>Cr(2-isocyano-1,3-X<sub>2</sub>-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(<sup>13</sup>CO<sub>trans</sub>) vs. δ(<sup>13</sup>CN), δ(<sup>13</sup>CO<sub>cis</sub>) vs. δ(<sup>13</sup>CN), and δ(<sup>13</sup>CO<sub>trans</sub>) vs. k<sub>C</sub><sub>≡</sub><sub>O,trans</sub> force constant. Remarkably, the net electron withdrawing capability of the 2-isocyano-1,3-dicyanoazulene ligand rivals those of perfluorinated isocyanides CNC<sub>6</sub>F<sub>5</sub> and CNC<sub>2</sub>F<sub>3</sub>.
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