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.

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]+.

Interaction of aminoferrocene with a 65/35 mol % mixture of phenyl formate/phenol followed by subsequent dehydration of the resulting ferrocenylformamide with POCl3 produced a high yield of air- and thermally stable, peach-colored isocyanoferrocene (CNFc). Combining 6 equiv of CNFc with bis(naphthalene)chromium(0) afforded orange-red Cr(CNFc)6, the first homoleptic complex of CNFc. Successive one-electron oxidations of the latter with Ag+ quantitatively produced saddle-brown [Cr(CNFc)6]+ and forest-green [Cr(CNFc)6]2+. The compounds [Cr(CNFc)6]0,1+,2+ are air-, light-, and thermally robust and are remarkable due to the incorporation of seven potentially electroactive transition metal ions within a relatively compact ML6 motif. Their properties demonstrate that the pi-accepting potential of the ferrocenyl substituent, customarily compared to an alkyl group, may have been underestimated. Indeed, the donor/acceptor ratio of the CNFc ligand appears to be similar to those of aryl isocyanides but quite different from those of alkyl isocyanides. Spectroscopic (including multinuclear paramagnetic NMR), magnetic, structural, and electrochemical characteristics of the above substances are discussed. In addition, a DFT analysis of the Frontier molecular orbitals of CNFc is presented.

This article reports on the chemistry of two organometallic isocyanocyclopentadienides, which represent an emerging new class of aromatic isocyanides incorporating nonbenzenoid π-systems. Interaction of aminoferrocene with a mixture of phenyl formate/phenol followed by subsequent dehydration of the resulting ferrocenylformamide with POCl 3 produced air and thermally stable isocyanoferrocene (CNFc, Fc ) ferrocenyl) in a high yield. Treating lithiocymantrene, LiCm (Cm ) (η 5 -C 5 H 4 )Mn(CO) 3 ), with tosyl azide afforded thermally sensitive cymantrenyl azide. Without isolation, CmN 3 was reduced by NaBH 4 to form aminocymantrene, which was converted into air stable but thermally and light sensitive isocyanocymantrene, CNCm. Combining 6 equiv of CNR (R ) Fc, Cm) with bis(naphthalene)chromium(0) afforded Cr(CNR) 6 . Successive one-electron oxidations of Cr(CNR) 6 with Ag + produced the corresponding paramagnetic [Cr(CNR) 6 ] + and [Cr(CNR) 6 ] 2+ . The compounds [Cr(CNR) 6 ] 0,1+,2+ (R ) Fc, Cm) are remarkable due to the incorporation of seven transition metal atoms within relatively compact ML 6 motifs. The physical, chemical, electrochemical, and spectroscopic properties of the structurally characterized series [Cr(CNFc) 6 ] 0,1+,2+ indicate that the electronic influence of the ferrocenyl moiety, often compared to an alkyl group, is in fact more similar to that of aryl substituents. Electrochemical properties of [Cr(CNR) 6 ] 0,1+,2+ (R ) Fc, Cm) are consistent with isocyanocymantrene being a substantially stronger π-acid than isocyanoferrocene. This conclusion was unambiguously corroborated by a DFT analysis of the Frontier molecular orbitals of CNFc and CNCm. Unpaired spin delocalization within odd-atom, nonbenzenoid aromatic π-systems of [Cr(CNR) 6 ] 1+,2+ (R ) Fc, Cm) was studied by multinuclear paramagnetic NMR and contrasted with patterns observed for similar complexes incorporating benzenoid and even-atom, nonbenzenoid aromatic moieties.