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 {E1/2(Cr0/1+)}, 13C NMR {δ(13CN), δ(13CO)}, UV-vis {dπ(Cr) ® pπ*(CNAz) Metal-to-Ligand Charge Transfer}, and FTIR {nNºC, nCºO, kCºO} 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(13COtrans) vs. δ(13CN), δ(13COcis) vs. δ(13CN), and δ(13COtrans) vs. kCºO,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.

Two isomeric ruthenium(II) 5,10,15,20-tetraphenylporphyrins axially coordinated to the redoxactive, low-optical gap-containing 2-or 6-isocyanoazulene ligands have been prepared and characterized by NMR, UV-vis, and MCD spectroscopy methods, high-resolution mass spectrometry, and X-ray crystallography. The UV-vis and MCD spectra are suggestive of the presence of the low-energy, azulene-centered transitions in the Q-band region of the porphyrin chromophore. The first coordination sphere in new L2RuTPP complexes is reflective of compressed tetragonal geometry. The redox properties of the new compounds were studied by electrochemical and spectroelectrochemical methods and correlated with the electronic structures predicted by the Density Functional Theory calculations. The experimental and theoretical data are suggestive of the low-potential reduction processes centred at the axial azulene ligands and oxidation processes centred at the ruthenium ion or porphyrin core.