Combined Experimental and Theoretical Investigation of Ligand and Anion Controlled Complex Formation with Unprecedented Structural Features and Photoluminescence Properties of Zinc(II) Complexes

By using two potential tridentate ligands, HL¹ [4-chloro-2-[(2-morpholin-4-yl-ethylimino)-methyl]-phenol] and HL² [4-chloro-2-[(3-morpholin-4-yl-propylimino)-methyl]-phenol], which differ by one methylene group in the alkyl chain, four new Zn^II complexes, namely, [Zn(L²H)₂](ClO₄)₂ (1), [Zn(L¹)(H₂O)₂][Zn(L¹)(SCN)₂] (2), [Zn(L¹)(dca)]n (3), and [Zn₂(L¹)₂(N₃)₂(H₂O)₂] (4) [where dca = dicyanamide anion] were synthesized and structurally characterized. The results indicate that the slight structural difference between the ligands, HL¹ and HL², because of the one methylene group connecting the nitrogen atoms provokes a chemical behavior completely different from what was expected. Any attempt to isolate the Zn(L²) complexes with thiocyanato, dicyanamido, and azide was unsuccessful, and perchlorate complex 1 was always obtained. In contrast, with HL¹ we obtained structural diversity on varying the anions, but we failed to isolate the analogous perchlorate complex of HL¹. Single-crystal X-ray analyses revealed that the morpholine nitrogen of ligand L² is protonated and thus does not take part in coordination with Zn^II in complex 1. On the other hand, the morpholine nitrogen of L¹ is coordinated to Zn^II in 2–4. Of these, 2 and 4 are rare examples of a cocrystallized cationic/anionic complex and of a dinuclear complex bridged by a single azide, respectively. Some of these unexpected findings and some interesting noncovalent interactions leading to the formation of dimeric entities in solid-state compound 4 were rationalized by a DFT approach. Photoluminescence properties of the complexes as well as the ligands were investigated in solution at ambient temperature and at 77 K. The very fast photoinduced electron transfer (PET) from the nitrogen lone pair to the conjugated phenolic moiety is responsible for very low quantum yield (Φ) exhibited by the ligands, whereas complexation prevents PET, thus enhancing the Φ in the complexes. The origin of the electronic and photoluminescence properties of the ligands and complexes was assessed in light of theoretical calculations