N , N (cid:48) -4,5-Dimethoxy-1,2-phenylenebis(salicylideneiminato)nickel(II)

: 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.


Introduction
Nickel complexes with tetradentate N 2 O 2 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(CH 3 O) 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.

X-ray Structural Analysis
The crystal structures of the [Ni(Salphen(CH 3 O) 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).  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(CH 3 O) 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.

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).

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. 1 H, 13 C-{ 1 H} NMR spectra ( Figures S2 and S3), 1 H-13 C HMQC ( Figure S4), 1 H-13 C HMBC ( Figure S5), 1 H-1 H dqf-COSY ( Figure S6) as well as 1 H-1 H NOESY (mixing time from 0.5 to 2 s) ( Figure S7) Figure 3). Table 2. IR band assignment according to [20].   Figure 3). 13 [20].  Analysis of the 1 H-1 H NOESY spectrum (mixing time variation) indicates the planar structure of the complex. Thus, the cross-peaks H 2 /CH=N, CH=N/H 3 , 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.