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

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.