Synthesis, crystal structures, and fluorescent properties of zinc(II) complexes with benzazino-2-carboxalidin-2-aminophenols

Abstract Complexes ZnL2 with novel fluorinated benzazines as tridentate ligands (HL = 6,7-difluoroquinoxalinand 6,7-difluoroquinolincarboxalidin-2-aminophenol) have been prepared. The photophisical properties of the ligands and the complexes has been studied. Graphical Abstract


Introduction
Benzazines containing the azomethine fragment in position 2 could be considered as aza-analogs of 2-styrylbenzazines, which garner attention because of their application in materials for electronic devices [1][2][3][4]. On the other hand, the presence of an azometine group allows for the production of metalochelates, which have photoluminescent properties and applications for electroluminescent materials [5][6][7][8]. Recently special attention has been paid to N,N,O-ligands, as they are capable to form individual lanthanide complexes with intense luminescence [9].
The presence of a fluorine atom in a ligand leads to an increase of thermal and chemical stability, and solubility in organic solvents, and as a result, expands possibilities for application in equipment; in particular, it improves the quality of films based on such materials possessing unique physical and chemical properties [10]. Recently we have managed to obtain new fluorinecontaining N,N,O-ligands such as (2-phenylquinazolin-4-yl)hydrazones of 2-hydroxybenzaldehydes and have studied their photophysical properties [11]. Quinoxalino-2carboxalidin-2-aminophenols-which represent attractive tridentate ligands for Mn(II), Fe(III), Ni(II), Cu(II) and Co(II) complexes-have been investigated [12], but data on Zn(II) complexes are extremely limited though. For instance, work [13] reports about the octahedral structure of quinoxalino-2-carboxalidin-2-amino-5-methylphenol zinc complex, and the geometry of the Zn(II) complex is supposedly similar to the structure of the Ni(II) complex, but X-ray data in this publication are absent. 6,7-Difluoro-analogs of such Schiff bases are not described. Even 6,7-difluoroquinoxalin-2carbaldehyde, the major intermediate for their synthesis, is not mentioned in the literature. In the present paper, new Zn(II) complexes 5a-d (Scheme 1) have been synthesized and their fluorescent properties investigated. spectra were recorded on a Bruker DRX 400 instrument (400.13 МHz). Chemical shifts are given in ppm (δ) from the internal TMS standard. Mass spectra were obtained from a MicrOTOF-Q II mass spectral instrument (Bruker Daltonics, positive or negative APCI ion source, electrospray ionization). Acetonitrile (99% purity) and 2-methyltetrahydrofurane (99%, stab. with ca 150-400 ppm BHT) (Alfa Aesar, UK) were used at UV-spectroscopy grade. Absorption spectra were scanned on a UV-2401 PC absorption spectrophotometer (Shimadzu, Japan) using a 1 cm quartz cell, concentration: 0.0001 mmol mL -1 (acetonitrile). Emission spectra were recorded on a Cary Eclipse spectrofluorimeter (Varian, USA). The fluorescence of the acetonitrile solution at T = 293 K was measured in a 1-cm cuvette. Low-temperature measurements were carried out in a liquid nitrogen cryostat OptistatDN (Oxford Instrument) in 2-methyltetrahydrofurane solution at T = 293 K and T = 77 K.
All reactions were monitored by thin layer chromatography (TLC) on 0.2 mm silica gel F-254 (Merck) plates using light (254 and 365 nm) for detection. Elemental analyses were performed at the Microanalytical Laboratory of the Postovsky Institute of Organic Synthesis.

X-ray crystallography
X-Ray analysis, including data collection, cell refinement and data reduction, was carried out with an Oxford Diffraction Xcalibur S CCD diffractometer using CrysAlisPro software package on standard procedure (graphite monochromatized radiation with λ(MoK α )= 0.71073 Å, ω-scanning with a step 1 o ); the empirical absorption correction [16] was applied. The structure was solved by direct method and refined against F 2 by full-matrix leastsquares using the SHELXTL package [17]. All non-hydrogen atoms were refined anisotropically; hydrogen atoms were placed in idealized positions and were constrained to ride on their parent atoms. The crystallographic data for the complex 5a are summarized in Table 1. Selected bond lengths and angles are summarized in Table 2.   There are no signals of OH-groups in 1 H NMR spectra of 5a-d. The downfield shift of H-8 proton signals for difluoroderivatives 5b,d and the upfield shift of such protons for non-fluorinated derivatives 5a,c are observed in comparison with signals of the corresponding ligands 4. Shift of CH=N proton signal of complexes 5a-d in the weak field was noted, as was shift of H-6' protons signals for 5a-d and H-3' protons signals for 5a,b,d in the strong field, in comparison with signals of 4.

Crystal structure
X-ray crystal structure determination was performed to confirm the structure of the complex 5а (Fig. 1, the numbering of atoms accepted in structural experiment is shown). In x-ray data, the compound grown up from DMF is crystallized as solvate 5а:DMF 1:2.
The complex is crystallized in the centrosymmetric space group. The molecule of the complex is placed in a private position on a rotation axis, while the Zn atom coordinates around itself two tridentate ligands. The ligand molecule closes two five-membered helate cycles. Coordination around the central atom is a distorted octahedron, thus bond lengths Zn(1)-O(1) and Zn(1)-N(2) are comparable (2.074(2) and 2.087(2) Å respectively), bond length Zn(1)-N(1) is much greater (2.346(2) Å), which indicates a mutual pushing away of a proton at C(9) from the atoms of the second ligand, and a general removal of the quinoline fragment from its plane. Fragments of the ligands are planar; the maximal deviations of non-hydrogen atoms from least-squared plane is 0.077 Å. The planes of the ligands are situated almost perpendicularly (with a deviation less 1 0 ). This stereometry of the complex is typical and, in particular, is comparable with a stereometry of Ni-complex of quinoxaline derivative from paper [13]. However, in the Ni-complex [13], the deviations of the atoms of ligands from least-squared plane are more significant (appox. 0.16 Å), and the ligand is probably characterized better by conformational flexibility.

Fluorescent character of the ligands and the complexes
The Schiff bases 4a-d and the Zn(II) complexes 5a-d display low photoluminescence in acetonitrile solution at room temperature. Complex formations are accompanied by chelatation of o-hydroxyazomethine, increasing the rigidity of the ligand and preventing photoinduced transfer of an electron, which leads to stronger emission [22][23][24]. The 5 complexes possess low red photoluminescence (for example, compound 5a, Fig. 2).
Fluorescence of complex 5a in 2-methyltetrahydrofurane solution at room temperature is also low, nevertheless at 77 К it increases considerably (Fig. 3). Complexes 5b-d demonstrate the same increase in fluorescent intensity. Photophysical data for UV and FL spectra at 77 K are shown in Table 3. Schiff bases bearing quinoxaline fragments (4c,d) show long-wave shift of absorption bands compared with analogs bearing quinoline fragments (4a,b). The 5 complexes 5 show considerable long-wave shift of absorption maximum compared with the corresponding ligands 4.
The 5 complexes exhibit intense red photoluminescence with maxima at 616-715 nm. Emission spectra of difluoroderivatives 5b, 5d are characterized by blue shifts in comparison with nonfluorinated analogs 5а, 5с (Table 3). It should be noted that in such conditions, ligand 4d demonstrates a weak orange luminescence (Table 3).

Conclusion
New Schiff bases as types of N,N,O-ligands were prepared, and their complex formations with zinc cations were studied. The complexes were characterized by elemental analysis, 1 Н NMR spectra, and single crystal X-ray determination. Influence of structural factors on photophysical properties of ligands and complexes was shown; the fluorescent properties of the complexes indicate that they may have interesting applications.  (3) and fluorescence emission (λ ex =530); (4) in 2-Me THF at 77 K.