Synthesis of the remarkably air- and thermally stable 2,6-diisocyano-1,3-diethoxycarbonylazulene linker from 2-amino-1,3-diethoxycarbonylazulene in 57% cumulative yield was developed. Incorporation of the ester "arms" in the design of this first diisocyanoazulene bridge permitted fully controlled stepwise installation and complexation of its isocyano junction groups. The -CO(2)Et arms in 2,6-diformamido-1,3-diethoxycarbonylazulene effectively suppress the rate of dehydration of its 2-NHCHO end relative to that of the 6-NHCHO end leading to practically exclusive formation of 6-isocyano-2-formamido-1,3-diethoxycarbonylazulene upon treatment of the above diformamide with an equimolar amount of POCl(3). This crystallographically characterized 6-isocyano-2-formamidoazulene derivative was employed to access mono- and heterobimetallic complexes of the 2,6-diisocyanoazulene scaffold with controlled orientation of the azulenic dipole. A complete series of monometallic, homobimetallic, and isomeric heterobimetallic ([M] = M(CO)(5), M = Cr and/or W) complexes of the 2,6-diisocyanoazulene motif was isolated and studied by a variety of techniques, including X-ray crystallography. The metal-to-bridge charge transfer in mono- and dinuclear adducts of 2,6-diisocyanoazulene, the assignment of which was corroborated by time-dependent density functional theory calculations, occurs at a dramatically lower energy as compared to the analogous systems featuring the 1,4-diisocyanobenzene scaffold. Moreover, the metal-to-diisocyanide charge transfer exhibits a substantially greater red shift upon binucleation of the mononuclear [M(CO)(5)] adducts of the nonbenzenoid 2,6-diisocyanoazulene linker versus the 1,4-diisocyanobenzene bridge.

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Efficient syntheses of all five possible isocyanoazulenes, the four isomeric archetypal compounds CN1Az, CN2Az, CN4Az, and CN6Az, as well as the 1,3-di-tert-butyl derivative of CN5Az (Az = azulenyl), are described. Compounds CN1Az and CN2Az show unexpected shifts of the S0 → S1 transition in their electronic spectra relative to azulene. The origins of these “anomalous” shifts have been addressed by DFT calculations, cyclic voltammetry, and comparison of the electronic spectra of isocyanoazulenes with those of the corresponding isomeric cyanoazulenes. Despite the high propensity of the azulenic nucleus to undergo multihapto coordination and C−C coupling in the presence of low-valent metals, the isocyanoazulenes react with 1/6 equiv of Cr(η6-naphthalene)2 to afford thermally stable Cr(CN x Az)6 (x = 1, 2, 4, 6), which contain six discrete azulenyl groups separated from the Cr center by isocyanide linkers. All Cr(CN x Az)6 species undergo oxidation to form the corresponding paramagnetic cations [Cr(CN x Az)6]+, which have been crystallographically characterized. Changing the atom of attachment of the azulenyl groups to the “Cr(CN)6” core substantially alters the donor/acceptor properties of the isocyanoazulene ligands. The half-wave Cr0/+ and Cr+/2+ redox potentials for [Cr(CN x Az)6] z form the “electrochemical series” that constitutes a quantitative measure of electronic inhomogeneity of the azulenic framework. Unpaired spin delocalization within the azulenic moieties of [Cr(CN x Az)6]+ has been observed by multinuclear NMR. The CrI(dπ)→CN x Az(pπ*) interaction has been shown to be an important contributor to the mechanism of unpaired electron delocalization in [Cr(CN x Az)6]+.

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