Molecule Cleft and Squares with Binicotinic Bishydrazone Ligand: Crystal Structures, Spectroscopic Properties, and Calculation

An attractive carboxamido-containing ligand 2,2′-bipyridyl-3,3′-bis(N,N′-disalicylidene)-formyhydrazone (H₄L), two binary cocrystals H₄L·H₂O and H₄L·2CH₃OH, and four bimetallic nanostructures, namely, molecule cleft [Cu₂(L)(Im)₂]·(CH₃)₂CO (1) and squares [Fe₂(H₂L)₂](ClO₄)·2H₂O (2), [Fe₂(H₂L)₂]·4H₂O (3), and [Mn₂(L)(H₂L)]·4H₂O (4) (Im = imidazole) were synthesized and characterized. H₄L with plentiful chemical information, such as allosteric interaction, ketone and enol isomers, potential axial chirality and multiple binding sites, and valent states can be recognized in assemblies. In cocrystals H₄L·H₂O and H₄L·2CH₃OH, the conformations of H₄L altered by solvent-assisted interactions in turn affect the hydrogen-bonding assembly of (R)-H₄L and (S)-H₄L molecules in solid. The architectures of bimetallic cleft 1 and squares 2−4 are mainly determined by the cooperation of the metal’s coordination modes and the conformations of H₄L. Complexes 1−4 contain discrete chiral dimeric units, further extending to multidimensional racemic hydrogen-bonding networks, in which the racemic mixture of H₄L can be partially separated through metal-induced homochiral recognition. Very interestingly, the synthesis reactions for squares 2−4 were accompanied with spontaneous redox reactions where ferric ions were half and completely reduced to Fe(II) in 2 and 3, respectively, and Mn(II) ions were half-oxidized to Mn(IV) in 4. Presumably, the electronic delocalization upon the backbone of the ligand reflected by structures and theoretical calculations triggers the redox activity. Ligand H₄L and complexes 1−4 display multiple strong absorptions in the UV−vis regions. Theoretically, the LE bond of free ligand H₄L can be attributed to a predominant phenol → carbonyl intraligand character with a minor phenol → pyridine ILCT contribution while the HE peak has a multitransition character. The spectrum of 1 features the intense ILCT and LLCT π → π* absorptions, together with intense n → π*, very similar to that for free ligand H₄L. In contrast, the spectra of 2−4 are red-shifted obviously and those electronic absorption spectra exhibit a common character of metal to ligand (MLCT) d (M) → π* (L) transitions.