The 2,6-Diisocyanoazulene Motif:  Synthesis and Efficient Mono- and Heterobimetallic Complexation with Controlled Orientation of the Azulenic Dipole

• Published in: Journal of the American Chemical Society Vol. 128; no. 7; pp. 2300 - 2309
• Main Authors: Holovics, Thomas C, Robinson, Randall E, Weintrob, Edward C, Toriyama, Masaharu, Lushington, Gerald H, Barybin, Mikhail V
• Format: Journal Article
• Language: English
• Published: Washington, DC American Chemical Society 22-02-2006
• Subjects: Carbonyl and nitrosyl compounds, and their complexes; Carbonyl and nitrosyl metals; Chemistry; Condensed matter: structure, mechanical and thermal properties; Exact sciences and technology; Inorganic chemistry and origins of life; Inorganic compounds; Physics; Preparations and properties; Structure of solids and liquids; crystallography; Structure of specific crystalline solids; Molecular structure; Chromium Carbonyl Complexes; Organic ligand; Transition metal Carbonyl Complexes; Inorganic synthesis; X ray diffraction; Regiospecificity; Electronic structure; Heteronuclear complex; Dinuclear complex; Bicyclic compound; Aromatic compound; Charge transfer compound; Tungsten Carbonyl Complexes; Ultraviolet visible spectrometry; Crystalline structure
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/ja053933+

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 −CO2Et 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 POCl3. 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.