NiII molecular complex with a tetradentate aminoguanidine-derived Schiff base ligand: structural, spectroscopic and electrochemical studies and photoelectric response

NiII ions templated the condensation of aminoguanidine with two different aldehyde molecules with the formation of a new molecular nickel(II) complex with a tetradentate chelating ligand.


Chemical context
Guanidine, the functional group on the side chain of arginine, has attracted much attention in the fields of drug development (Santos et al., 2015;Hirsh et al., 2008) and natural product synthesis (Berlinck & Romminger, 2016;Kudo et al., 2016). Guanidine derivatives have also been explored as catalysts and superbases (Selig, 2013;Ishikawa, 2009). Aminoguanidine (AG) is an antioxidant and nucleophilic agent with strong scavenging activities against reactive carbonyl species (RCS)a class of byproducts originating from exogenous and endogenous oxidation. RCS react with nucleophilic targets such as nucleic acids, phospholipids and proteins to form damaging adducts (Colzani et al., 2016;Ramis et al., 2019). Diabetic and Alzheimer's disease patients were both found to have increased RCS levels in their circulatory systems (Kalousova et al., 2002;Picklo et al., 2002). Blocking RCS by carbonyl quenchers is an encouraging therapeutic strategy and the investigation of conjugates of AG and arylaldehydes as well as ISSN 2056-9890 their metal complexes has been at the focus of research interest for several decades (Fukumoto et al., 2002;Qian et al., 2010;Vojinović-Ješić et al., 2014).
In our previous study, the condensation reactions of aminoguanidine freshly liberated from AGÁHCl or AGÁHNO 3 and arylaldehydes (salicylaldehyde, 5-bromosalicylaldehyde, pyridine-2-carbaldehyde) produced the expected 1:1 Schiff base ligands isolated as protonated cations of nitrate or chloride salts as well as Cu II and Co III mononuclear complexes (Buvaylo et al., 2013(Buvaylo et al., , 2016(Buvaylo et al., , 2017. The dichloridocopper(II) complex bearing a pyridine-2-carbaldehyde aminoguanidine Schiff base ligand revealed prominent catalytic activity towards the oxidation of cyclohexane with hydrogen peroxide in the presence of various promoters (Buvaylo et al., 2017). In contrast, the interaction of AG with formaldehyde yielded a completely different compound with a high nitrogen content that had not been reported before (Buvaylo et al., 2018). 2,20-Methylenedihydrazinecarboximidamide, which was isolated in its protonated form as the dinitrate salt, resulted from the condensation between two AG molecules and one molecule of formaldehyde.
In the present work, we attempted to synthesize an Ni complex with the Schiff base ligand derived from AG and salicylaldehyde. However, 5-bromosalicylaldehyde was also mistakenly introduced into the flask. As a result, the new tetradentate ligand (2-hydroxybenzylidene)(5-bromo-2-hydroxybenzylidene)aminoguanidine, H 2 L, was formed from the in situ condensation of one AG molecule and two different molecules of the aldehydes in the presence of Ni 2+ ions. Herein, the crystal structure of [NiL]ÁDMFÁH 2 O (DMF = N,N-dimethylformamide), (I), is presented along with the elemental analyses, IR, NMR and cyclic voltammetry results as well as photoelectric response characteristics.

Structural commentary
Compound (I), [Ni(C 15 H 11 BrN 4 O 2 )]ÁC 3 H 7 NOÁH 2 O, crystallizes in the triclinic space group P1 and is assembled from discrete NiL molecules and solvent molecules of crystallization. The chelating ligand L 2is deprotonated at the phenol O atoms and coordinates the Ni II ion through the two azomethine N and two phenolate O atoms in a cis-NiN 2 O 2 square-planar configuration (Fig. 1). The Ni-N/O distances fall in the range 1.8383 (11)-1.8562 (10) Å , the cis angles at the metal atom vary from 83.08 (5) to 95.35 (5) and the trans angles are equal to 177.80 (5) and 178.29 (5) ( Table 1). The molecule is quite planar, the atoms with the largest deviations being C15 [ = 0.059 (2) Å ] and C23 [ = 0.057 (2) Å ] although there is very slight 'bowing' at the Ni atom. The dihedral angle between the two phenyl rings is 3.37 (5) .

Supramolecular features
In the crystal, the NiL molecules form centrosymmetrically related pairs with an interplanar distance of approximately 3.32 Å and the NiÁ Á ÁNi separation being 3.4191 (3) Å (Fig. 2). There are no hydrogen bonds between the NiL molecules and nostacking is observed owing to the trans-orientation of the two paired molecules. Instead, the NiL molecule creates centrosymmetric hydrogen-bonded pairs through one H atom on the amine nitrogen N4, its other hydrogen forming a hydrogen bond to a centrosymmetrically related water molecule as shown by the N4Á Á ÁN3 {Àx + 2, Ày + 2, Àz + 1} and

Figure 2
View of a pair of centrosymmetically related trans-oriented NiL molecules showing the absence ofstacking.

Figure 1
Molecular structure and atom labelling of [ To our knowledge, only one example of a Schiff base metal complex structurally similar to (I) has been reported. The reaction between (salicylideneamino)nitroguanidine and salicylaldehyde in the presence of Ni 2+ ions used as templating agents and K + cations produced potassium (N,N 0 -bis(salicylideneamino)-N 00 -nitroguanidinato-N,N 0 ,O,O 0 )nickel(II) with a cis-NiN 2 O 2 square-planar chromophore (TUFDAZ; Starikova et al., 1996). Obviously, the Ni II -assisted condensation of AG or its NO 2 -substituted analogue with two aldehyde molecules in the case of (I) and TUFDAZ occurred due to a combination of structural and electronic factors unique to the nickel(II) cation, which is prone to adopt a tetradentate square-planar geometry, and the favourable stoichiometry of the condensation reaction.

IR and 1 H NMR spectroscopy measurements
The infrared spectrum of complex (I) in the 4000-400 cm À1 range is very rich and shows all characteristic functional group peaks. A broad absorption near 3500 cm À1 and multiple overlapping bands in the range 3358-3134 cm À1 are attributed to (OH) and (NH) stretching vibrations, respectively. Bands arising above 3000 cm À1 are due to aromatic CH stretching of the ligand; alkyl CH stretching vibrations of L 2and DMF solvent are seen from 2958 to 2808 cm À1 . Very intense overlapping signals in the 1668-1584 cm À1 region represent (C O) stretching of the DMF molecule, deformation vibrations of the amino group, a group mode of the CN 3 unit of the ligand, as (CN 3 ), and (C N) peaks of L 2that cannot be distinguished from each other. The symmetric stretching mode s (CN 3 ) of the CN 3 unit falls in the 1600-1400 cm À1 range of the aromatic ring vibrations. Several sharp bands of medium intensity are observed in the out-of-plane CH bending region (800-700 cm À1 ).

Figure 3
Fragment of the crystal packing of (I), viewed along the b-axis direction, showing intermolecular N-HÁ Á ÁN/O and O-HÁ Á ÁO interactions (CH hydrogen atoms were omitted for clarity; hydrogen bonds are shown as blue dashed lines; green lines joining Ni centres do not represent bonds).
[Symmetry codes: (i) Àx + 2, Ày + 2, Àz + 1; (ii) Àx + 2, Ày + 1, Àz + 1.] of seven aromatic protons in the range 7.57-6.58 ppm observed as one singlet, four doublets and two triplets evidence the presence of two chemically inequivalent rings. A broad singlet at 7.25 ppm is due to the NH 2 group adjacent to the carbon atom of the guanidine moiety. The absence of the phenolic OH singlets detected at 11.55 ppm in the 1 H NMR spectrum of (5-bromosalicylidene)aminoguanidineÁHNO 3 (Buvaylo et al., 2016) points out the deprotonation of H 2 L upon coordination to the Ni II centre in (I). Three sharp singlets in a 1:3:3 ratio at 7.94, 2.88 and 2.72 ppm were attributed to the CH and two CH 3 groups of DMF, respectively.

Cyclic voltammetry
The electrochemical features of complex (I) were studied in methanol in the presence of 0.1 M acetate buffer (pH 4) and NaClO 4 (70:28:2) as supporting electrolyte by using a threeelectrode setup (glassy carbon working electrode, platinum auxiliary electrode and Ag/AgCl reference electrode) in the potential range +1.0 to À1.0 V at a scan rate of 100 mV s À1 . The anodic scan, starting from the open circuit potential (0.24 V vs Ag/AgCl), displays an oxidation wave at E pa = +0.42 V coupled with a corresponding reduction wave at E pc = +0.17 V (Fig. 5). A large separation between the cathodic and anodic peak potentials (250 mV) indicates a quasi-reversible redox process which can be assigned to Ni +2 /Ni +3 couple with E 1/2 = +0.295 V (vs Ag/AgCl). The non-equivalent current intensity of cathodic and anodic peaks (i c /i a = 0.551) suggests that the Ni III complex generated by oxidation of Ni II is not stable.

Electro-optical measurements
The ability of (I) to form thin films on its own when cast from methanol solution prompted us to examine its photoelectric response under illumination with visible light. The thin film of the complex with estimated thickness of about 1.5 mm was obtained by drop casting of a methanol solution of (I) on an electroconducting ITO (SnO 2 : In 2 O 3 ) layer of a standard glass slide and subsequent drying. A Kelvin probe technique was employed to track the contact potential difference between the free surface of the film and the probe with a BM8020 USB oscilloscope according to Davidenko et al. (2016). A 4 mm diameter aluminium plate placed $50 mm above the surface with a vibration frequency of 4 kHz was used as the reference probe. A white-light-emitting diode (LED) with power density I ' 40 W m À2 was used to illuminate the film from the ITO substrate side.
The thin-film sample of (I) showed a rather fast photoelectric response upon exposure to visible light with the surface potential V PH reaching its maximum value of $178 mV within 6 s. Then the potential diminished slightly to stay nearly constant until the light was turned off at t = 100 s (Fig. 6). The V PH relaxation in the film occurred almost as fast as its growth. The free surface of the film acquired a positive charge under illumination meaning the photogenerated electrons transfer to the ITO substrate. The fast kinetics of the surface photovoltage growth and decay indicates a high mobility of the photogenerated charge carriers in (I). Cyclic voltammogram of (I), 0.1 mM in methanol mixed with 0.1 M acetate buffer (pH 4) and NaClO 4 (70:28:2) as supporting electrolyte at a glassy carbon electrode and Ag/AgCl as a reference electrode (scan rate: 100 mV s À1 ; T = 293 K).

Figure 6
Time dependence of V PH of a thin film sample of (I) with a free film surface upon illumination with a white LED (I = 40 W m À2 ) from the side of a transparent ITO electrode; illumination stopped at the point shown by the vertical arrow. 400 MHz 1 H NMR spectrum of (I) in DMSO-d 6 at 293 K in the range 8.5-6.5 ppm.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. All hydrogen atoms bound to carbon were included in calculated positions and refined using a riding model with isotropic displacement parameters based on those of the parent atom (C-H = 0.95 Å , U iso (H) = 1.2U eq C for CH, C-H = 0.98 Å , U iso (H) = 1.5U eq C for CH 3 ). Water and NH 2 hydrogen atoms were refined without restraints. Anisotropic displacement parameters were employed for the non-hydrogen atoms.

Funding information
Funding for this research was provided by: Ministry of Education and Science of Ukraine (grant for the perspective development of the scientific direction 'Mathematical sciences and natural sciences' at the Taras Shevchenko National University of Kyiv).