N,N′-4,5-Dimethoxy-1,2-phenylenebis(salicylideneiminato)nickel(II)

Abstract

The title compound, which is potentially interesting as a building block for electrochemically active metallopolymers, was synthesized and characterized by single-crystal X-ray diffraction, IR and NMR spectroscopies.

1. Introduction

Nickel complexes with tetradentate N₂O₂ Schiff bases are used as monomers to prepare polymeric functional materials for advanced electrochemical energy storage devices, electrocatalytic systems, sensors, and electrochromic devices [1,2,3,4,5,6]. The properties of these materials depend on the presence of electron-donating or electron-withdrawing substituents in the Schiff base [7,8,9,10] and on the structure of the metal complex [7,8,11]. The 1,2-phenylenediimine bridging unit of the Salphen-type Schiff base complexes is a prospective structural site for modulating properties that are insufficiently explored in the literature.

Introducing vicinal hydroxyl groups in para positions to imine units of 1,2-phenylenediimine provide additional metal binding sites in Salphen-type complexes [12]. Crown-etherification of these sites further tunes the coordination ability of complexes [13,14]. Etherification of the hydroxyls has been reported with the intent of solubility enhancement for supramolecular [15] and polymeric [16] systems and for control of the supramolecular assembly of tris(salphen)-type trinickel(II) metallocryptands encapsulated a guanidinium ion [17] or alkali metal cations [18]. Oxidative chemistry of protected (unpolymerizable) nickel complex with a doubly methoxylated bridging unit of the Salphen complex has also been reported [19].

The structure of the simplest dimetoxylated Salphen nickel complex has not been reported so far. Thus, we perform a synthesis and investigations of structural and spectral properties of the novel complex N,N′-(4,5-dimethoxy-1,2-phenylene)bis(salicylideneaiminato)nickel(II) ([Ni(Salphen(CH₃O)₂)]) bearing two strongly donating methoxy substituents in the bridging phenylenediimine unit as a first step in exploring the influence of such monomer structural modification on metallopolymers electrochemistry.

2. Results and Discussion

2.1. X-ray Structural Analysis

The crystal structures of the [Ni(Salphen(CH₃O)₂)] was determined by the X-ray structural analysis. Samples of the complex suitable for studying were isolated by crystallization from a saturated acetonitrile solution (Avantor Performance Materials).

According to the X-ray diffraction data, the compound under study crystallizes in two forms (a and b) that differ in the way of the complex molecule solvation by acetonitrile (Figure 1) (Table 1).

Figure 1. Molecular view of [Ni(Salphen(CH₃O)₂)]·CH₃CN in representation of atoms with thermal ellipsoids (p = 50%), structures (a,b).

Table 1. Crystallographic data, experimental parameters, and refinement of the [NiSalphen(CH₃O)₂)]·CH₃CN complex structure.

Parameter	Value
Structure	a	b
Empirical formula	C24H21NiN3O4	C24H21NiN3O4
M	474.15	474.15
Temperature/K	100(2)	100(2)
Crystal system	monoclinic	trigonal
Space group	P21/n	R3c
a/Å	7.95760(10)	38.7229(7)
b/Å	12.98650(10)	38.7229(7)
c/Å	20.1006(2)	7.2027(2)
α/°	90	90
β/°	99.9830(10)	90
γ/°	90	120
Volume/Å3	2045.77(4)	9353.2(4)
Z	4	18
ρ calc/g/cm3	1.539	1.515
μ/mm−1	1.694	1.668
F(000)	984.0	4428.0
Crystal size/mm	0.1 × 0.08 × 0.05	0.16 × 0.04 × 0.02
Radiation/λ/Å	CuKα1.54184)	CuKα (1.54184)
2Θ range for data collection/°	8.142–138.212	4.564 to 139.994
Index ranges	−9 ≤ h ≤ 9, −10 ≤ k ≤ 15, −24 ≤ l ≤ 24	−46 ≤ h ≤ 32, −47 ≤ k ≤ 41, −8 ≤ l ≤ 8
Reflections collected	11517	18976
Independent reflections	3812 [Rint = 0.0261, Rsigma = 0.0245]	3872 [Rint = 0.0453, Rsigma = 0.0347]
GOOF by F2	1.074	1.039
R factors [I > = 2σ (I)]	R1 = 0.0278, wR2 = 0.0721	R1 = 0.0310, wR2 = 0.0773
R factors [all reflections]	R1 = 0.0304, wR2 = 0.0737	R1 = 0.0325, wR2 = 0.0783
Largest diff. peak/hole, e Å−3	0.23/−0.29	0.29/−0.28

Figure 1 and Figure 2 represent the molecular structures of the complex structures a and b. The Ni atom in the complex is bound to two phenolate oxygen atoms and two nitrogen atoms located at the vertices of a distorted square. The distortion degree can be characterized with the values of the torsion angles Ni¹O¹C¹C² [3.1(2)°] (structure a), [12.5(5)°] (b) and Ni¹N²C¹⁴C¹² [0.6(2)°] (a), [5.5(5)°] (b). The values of the torsion angles show that the distortion of structure (b) (Figure 2b) is higher than structure (a) (Figure 2a).

Figure 2. Comparison of the distortion of structures (a,b) with thermal ellipsoids at a 50% probability.

At the same time, the bond angles characterizing nickel bonds with donor atoms are approximately the same for both structures: O¹Ni¹N² [179.47(5)°] (a) and [179.17(12)°] (b), N¹Ni¹O² [178.57(5)°] (a) and [179.03(11)°] (b).

The different nature of the solvation of structures (a) and (b), as well as a higher degree of distortion of the structure (b), lead to the following differences in crystal lattices: monoclinic (a) and trigonal (b), and a much larger unit cell volume of (b) (see Table 1).

In general, the [Ni(Salphen(CH₃O)₂)] complex, as well as most nickel complexes of the salen- and salphen types, has a distorted square-planar geometry, which suggests the possibility of obtaining on its basis functional polymeric materials for electrochemical devices.

2.2. Infrared Spectroscopic and Nuclear Magnetic Resonance Studies

IR spectra (Figure S1) were registered on Shimadzu IRPrestige-21 spectrometers with samples in KBr pellets. The main experimental IR bands and their assignment are shown in Table 2. ¹H, ¹³C-{¹H} NMR spectra (Figures S2 and S3), ¹H–¹³C HMQC (Figure S4), ¹H–¹³C HMBC (Figure S5), ¹H-¹H dqf-COSY (Figure S6) as well as ¹H-¹H NOESY (mixing time from 0.5 to 2 s) (Figure S7) experiments were acquired on a Jeol ECX400A spectrometer (400 MHz for ¹H nuclei and 100 MHz for ¹³C nuclei) in DMSO-d₆. The residual signals of the solvent (DMSO-d₆: 2.50 ppm for ¹H nuclei and 39.6 ppm for ¹³C nuclei) were used as internal standard.

Table 2. IR band assignment according to [20].

IR band, cm−1	Assignment
ligand core molecule
1585–1608	C = N str in azomethine and C –C str in phenolic ring
1518	C–C str in phenolic ring
1450–1465	C–O, C=N and C–C str in phenolic and six-membered chelate ring
1367	C = N in six-membered chelate ring and C –C str in phenolic ring
1333	C –O and C –C str in phenolic ring
1244	C –C str in phenolic and six-membered chelate ring
1109–1189	C –N str in five-membered chelate ring
729–753	ring vib and C –H out-of-plane def in phenolic ring
metal chelate
594	asym O –Ni –O str
459	asym N –Ni –N str
substituents in phenolate moieties
2831	sym and asym C –H str in –O–CH3
1090–1100	sym C–O–C str in phenyl–O–CH3
diimine backbone
3042–3012	sym and asym C–H str in o –phenylene
1146	C–H in-plane def in o –phenylene
753	ring vib and C –H out-of-plane def in o -phenylene

¹H NMR: 3.83 (3H, OCH₃), 6.61 (1H, ddd, H⁴, ³J 7.9, 6.8, ⁴J 0.9 Hz), 6.82 (1H, br.d, H⁶, ³J 8.5 Hz), 7.23 (1H, ddd, H⁵, ³J 8.5, 6.8, ⁴J 1.7 Hz), 7.52 (1H, dd, H³, ³J 7.9, ⁴J 1.7 Hz), 7.59 (1H, s, H^2′), 8.64 (1H, s, CH=N) (Atom labeling shown on Figure 3).

Figure 3. Key correlations in ¹H-¹³C HMBC (blue arrows) and ¹H-¹H NOESY (double-edged red arrows) spectra.

¹³C{¹H} NMR: 56.73 (OCH₃), 99.05 (C^2′), 115.58 (C⁴), 120.57 (C⁶), 120.91 (C²), 134.29 (C³), 135.10 (C⁵), 136.12 (C^1′), 149.75 (C^3′), 155.20 (C=N), 164.95 (C¹).

Assignment of signals of protons and carbon atoms in ¹H and ¹³C NMR spectra was carried out using homo- (¹H-¹H COSY, ¹H-¹H NOESY) and heteronuclear (¹H-¹³C HMQC, ¹H-¹³C HMBC) experiments. Notably, the presence of cross peaks in the ¹H-¹H COSY spectrum due to long-range spin–spin interactions through 5 bonds (CH₃/H^2′, H^2′/CH=N, CH=N/H⁶), as well as typical for ortho-substituted benzene ring J-coupling constant through 4 bonds (H³/H⁵, H⁴/H⁶). The key cross peaks used for interpretation in the ¹H-¹³C HMBC spectrum are 3.83 (OCH₃)/149.75 (C^3′); 7.59 (H^2′)/136.12 (C^1′); 8.64 (CH=N)/120.91 (C²); 7.52 (H³)/155.20 (C=N); 7.23 (H⁵)/164.95 (C¹) (Figure 3).

Analysis of the ¹H-¹H NOESY spectrum (mixing time variation) indicates the planar structure of the complex. Thus, the cross-peaks H^2′/CH=N, CH=N/H³, due to the nuclear Overhauser effect, indicate the spatial proximity of the azomethine proton simultaneously with two protons of different aromatic rings, which is possible with the coplanar organization of the (E)-azomethine block.

3. Materials and Methods

All chemicals used in the synthesis were of “reagent-grade” purity and were purchased from local suppliers.

4,5-Dimethoxy-1,2-phenylenediamine was obtained in two steps, as reported in [21]. 1,2-Dimethoxybenzene was converted into 4,5-dimethoxy-1,2-dinitrobenzene (86% yield) by the reaction with concentrated nitric acid (65%) followed by the reduction with hydrazine monohydrate and Pd/C catalyst yielding 4,5-dimethoxy-1,2-phenylenediamine (55% yield).

The ligand was prepared by the standard method of refluxing an ethanolic solution containing salicylaldehyde (Aldrich) and a 4,5-dimethoxy-1,2-phenylenediamine in stoichiometric amounts (95% yield). Nickel(II) complex [Ni(Salphen(CH₃O)₂)] was prepared by refluxing ethanolic solutions of nickel(II) acetate (Aldrich) with the Schiff base ligand, as described in [22]. Obtained complexes were recrystallized from acetonitrile and dried at 60 °C for several hours (70% yield).

X-ray diffraction analysis was performed at 100 K on a XtaLAB Synergy-S diffractometer (Rigaku, Japan) equipped with a HyPix-6000HE CCD detector (Rigaku, Tokio, Japan), CuKα radiation (λ 1.54184 Å). The structure was solved using the ShelXT-2013 software package [23] and refined using the ShelXL-2013 package [24] included in the OLEX2 interface [25]. The crystallographic parameters have been deposited in the Cambridge X-Ray Database (CCDC 2189692-2189693).