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 ZnII 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 ZnII in complex 1. On the other hand,
the morpholine nitrogen of L1 is coordinated to ZnII 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.