2-Halo- and/or 4-ethoxycarbonyl-substituted asymmetric 1,3-diaryltriazenes and 1,3-diarylamidines as well as N-methylated congeners

1,3-Diaryltriazenes Ar-NN-N(R)-R' [R = H; Ar/R' = C6H4-4-COOEt/Ph (1), C6H3-2-Br-4-COOEt/Ph (2), CH2Ph/C6H4-4-COOEt (3)] and the N-methylated congeners [R = Me; Ar/R' = C6H4-4- COOEt/Ph (9), C6H2-2,5-Br-2-4-NH2/Ph (10), C6H4-4-Br/C6H4-4-COOEt (11), C6H2-2-Br-4- COOEt/Ph (12), C6H2-2-Br-4-COOEt-6-Me/C6H4-2-Me (13), C6H3-2-Br-4-F/Ph (14), C6H3-2-Br-6- F/Ph (15), C6H3-2-Br-4-Cl/Ph (16), C6H3-2,4-Br-2/Ph (17)] as well as 1,3-diarylformamidines of the type Ar-NCH-N(R)-R' [R = H; Ar/R' = C6H3-4-COOEt/Ph (4), C6H3-2-Br-4-COOEt/Ph (5), C6H3- 2-Br-4-F/Ph (6)] and 1,3-diaryl-3-methylamidines [R = Me; Ar/R' = C6H3-4-COOEt/Ph (18), C6H3-2- Br-4-COOEt/Ph (19), C6H3-2-Br-4-Cl/Ph (20), C6H3-2,4-Br-2/Ph (21)] crystallize preferably with (syn- E) configuration. For comparison reasons sterically crowded Ph-N(R)-C (R') = N-C6H3-2-Br-4- COOEt [R = H, R' = Ph (7), tBu (8); R = Me, R' = Ph (22), tBu (23)] are included. Large substituents destabilize this isomeric form due to intramolecular steric repulsion. 1,3-Diarylformamidines dimerize via two nearly parallel N-H...N hydrogen bridges. This kind of aggregation is very beneficial because the N=C-N bond angles are slightly larger than 120 degrees. Narrower N=N-N bond angles are found for the 1,3-diaryltriazenes and therefore, aggregation occurs via intermolecular N-H...O hydrogen bridges to O-Lewis bases. The 1,3-diaryl-3-methylformamidines and -triazenes show very similar bonding parameters. Thus, the bond lengths of the N=C/N=N double bonds are approx. 4-9 pm shorter than the N-C/N-N single bonds, supporting charge delocalization within the diazaallyl- and triazaallyl systems regardless of N-methylation. (C) 2019 Elsevier B.V. All rights reserved.