The conformational space of diphenylamine-2,2'-dicarboxaldehyde. “Whoever reports a saying in the name of the one who said it brings redemption to the world.”

Diphenylamine-2,2'-dicarboxaldehyde (DPADA), first reported in 1961, has served as a key building block (head unit) in dicarbonyl-diamine condensations, providing a variety of pincer-style Schiff-base ligands, leading to macrocyclic metallic complexes. The present article reports a DFT study at B3LYP/6-311G++(d,p)/PCM(methanol) of the conformational spaces of DPADA and DPADA monohydrazone. All ten potential conformers (excluding enantiomers) of DPADA were revealed. They were characterized by four stereodescriptors: anti/syn for the torsion angles around C ¹ -N ¹ and C ^1' -N ¹ and E/Z for the fractional bond orders of the bonds C ² -C ⁷ and C ^2' -C ^7' . The global minimum of DPADA is the C ₂ -1a1'a2Z2'Z conformer, with two intramolecular bifurcated N ¹ -H ^… O ⁸ hydrogen bonds and a dihedral angle of 44° between the two phenyl groups, in good agreement with the reported X-ray structure. Two relevant (energy-wise) local minima were found: C ₁ -1a1'a2E2'Z and C ₁ -1a1's2Z2'E, 0.4 and 2.9 kJ/mol less stable than the global minimum, each exhibiting one intramolecular N-H ^… O hydrogen bond. These three conformers are instrumental for the pathways of DPADA-diamine condensations, resulting in preferred 2+2 cyclizations, leading to macrocycles. The conformers of DPADA monohydrazone are also characterized by the above stereodescriptors and the fifth stereodescriptor 7'E/7'Z, referring to the diastereoisomerism in C ^7' =N ^8' -N ^9' H ₂ . Thirty (out of potential 32) conformers were characterized. The global minimum is the C ₁ -1a1'a2Z2'Z7'E conformer. The four most stable conformers are characterized by 7'E and by intramolecular N ¹ -H ^… O ⁸ and /or N ¹ -H ^… N ^8' hydrogen bonds. Due to their spatial arrangements, the 2+2 "intermolecular" carbonyl-hydrazone condensation pathway of DPADA monohydrazone is preferred over the corresponding 1+1 "intramolecular" pathway.