Alkylation and addition to isocyanate of 2-(5-amino-3,4-dicyano-2H-pyrrol-2-ylidene)-1,1,2-tricyanoethanide. X-Ray crystal and molecular structures of methyl-, dimethyl-, dimethylaminoethylene- and propylaminocarbonyl-amino derivatives

Abstract

Several substituents have been introduced on the amino group of 2-(5-amino-3,4-dicyano-2H-pyrrol-2-ylidene)-1,1,2-tricyanoethanide (C₅N₃–C₄N(CN)₂–NH₂⁻, L′) to give the derivatives C₅N₃–C₄N(CN)₂–NHCH₃⁻ (1), C₅N₃–C₄N(CN)₂–N(CH₃)₂⁻ (2), C₅N₃–C₄N(CN)₂–NHCH₂C₆H₅⁻ (3), C₅N₃–C₄N(CN)₂–NCHN(CH₃)₂⁻ (4) and C₅N₃–C₄N(CN)₂–NHCONHCH₂CH₂CH₃⁻ (5) that have been characterized by elemental analysis and IR, UV–Vis and ¹H-NMR spectroscopy. The reactions carried out were nucleophilic substitution for 1 and 2, condensation with aromatic aldehydes and subsequent reduction for 3, condensation with amido-acetals for 4 and addition to isocyanates for 5. The main optical band of these compounds is shifted to lower or higher energy in comparison to L′. These shifts are easily accounted for by the inductive or mesomeric substituent effect on the amino group of L′. The X-ray crystal and molecular structures of 1, 2, 4 and 5 have been determined from diffraction studies on single crystals of their tetraphenylarsonium salts. The geometrical features of the four anions, together with those of the previously reported L′ and salicylideneamino derivative C₅N₃–C₄N(CN)₂–NCHC₆H₄OH⁻, allow us to see the effects of the substituents and of the crystal packing interactions on the electronic charge distribution within the tricyanovinyl-2H-pyrrole conjugated system. While 1 and 3 behave as chelating ligands towards transition metal cations, as does L′, the other anions do not. The absence of hydrogen atoms on the amino group (in the case of 2 and 4) and the steric hindrance due to strong intramolecular hydrogen bonds (in the case of 5) prevent chelation.