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

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)-N=CHN(CH3)(2)(-) (4) and C5N3-C4N(CN)(2)-NHCONHCH2CH2CH3- (5) that have been characterized by elemental analysis and IR, UV-Vis and H-1-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)-N=CHC6H4OH-, 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.