Synthesis and crystal structure of a disubstituted nickel(II) bis[(dimethylaminophenylimino)ethyl]pyridine chloride complex

The synthesis and structural determination of a nickel(II) complex where the two nickel cations in the asymmetric unit each are coordinated by two tridentate, potentially redox non-innocent bis-iminopyridyl ligands are reported.


Chemical context
Non-innocent ligand systems in organometallics can produce secondary reactivity and allow for unique mechanistic and redox properties (Babbini & Iluc, 2015;Praneeth et al., 2012). Redox non-innocence is usually observed with chelate ligands which possess low-lying -systems that can allow for electron transfer (Lyaskovskyy & de Bruin, 2012). These ligand systems can also allow for multiple-electron redox events to take place on metal cations which are usually relegated to single-electron events (Haneline & Heyduk, 2006). This can allow for the utilization of benign and economically viable base metal catalysts in lieu of traditional noble-metal catalysts (Chirik & Wieghardt, 2010). The development of new and varied organometallic complexes is essential for understanding the structure-property relationships, which give rise to redox non-innocence properties. With expanding interest in redox-active organometallic systems, we report here the synthesis and structural determination of a potentially redoxactive nickel(II) complex possessing two pyridine diimine ligands containing electron-donating substituents.

Structural commentary
The title compounds crystallizes with two complex Ni II cations, associated chloride anions, adventitious water and dichloromethane molecules of solvation in the asymmetric unit (Fig. 1). Although the two cations are crystallographically independent they are chemically identical and the general discussion for ISSN 2056-9890 one, holds for the second molecule. We inspected the structure for higher and missed symmetry (Spek, 2009), but there is none.
Each nickel(II) cation is coordinated in a distorted octahedral geometry by the imine and pyridine nitrogen atoms of the two tridentate 2,6-bis[1-(4-dimethylaminophenylimino)ethyl]pyridine (PDI-DMA) ligands ( Fig. 1, Table 1 for numerical details). The derived metrics for the molecules are as expected. It should be noted that the Ni-N py bond lengths are all considerably shorter than the Ni-N imine bond lengths. However, both interactions are typical for these types of bonds/moieties. An interesting feature of the cations is the orientation of the pendant dimethylaminophenyl rings with respect to the pyridine ring of each ligand. In all cases, one pendant phenyl group is oriented close to perpendicular to the plane of the parent pyridine ring while the other is canted at an angle of around 60 (Fig. 2); numerical details of these features are collated in Table 2. Inspection of the molecules shows that a combination of steric andstacking interactions are the cause of these orientations. The phenyl rings that are close to perpendicular to the parent pyridine are sterically constrained by the pyridine rings of the second ligand and include weak intramolecularinteractions (  Selective labelling scheme for [Ni(C 25 H 29 N 5 ) 2 ]Cl 2 Á2CH 2 Cl 2 Á2H 2 O. Atomic displacement ellipsoids are depicted at the 50% probability level and H atoms shown as spheres of arbitrary radius. Hydrogen-bonding interactions are denoted as blue, dashed lines. Table 1 Selected geometric parameters (Å , ).  (7) N13-Ni2-N16 93.21 (7) methylaminophenyl group is less hindered and adopts a typical tilted orientation.

Supramolecular features
Within the intermolecular packing of the cationic molecules pairs of solvent water molecules form a hydrogen-bonded dimer with pairs of chloride anions (Fig. 1, Table 3). Each dichloromethane solvent molecule also forms a weak, but directional, hydrogen bond with a chloride anion (Table 3). Surprisingly, the cation does not have any particular directional interactions with other species in the structure, aside from a weak C-HÁ Á ÁN interaction with one of the dichloromethane solvents (Table 3) and typical van der Waals contacts (Fig. 3).

Database survey
A search of the Cambridge Structure Database (Version 5.38 + three updates; Groom et al., 2016) reveals 13 structures that incorporate Ni II coordinated by two bis-iminoarylpyridyl ligands. Of these, there are only three related structures with the imine carbon atoms methylated (FADFUN, Patel et al., 2010;MEGDUX, de Bruin et al., 2000;QEZJOV, Trivedi et al., 2007).

Synthesis and crystallization
The reagent 2,6-diacetylpyridine was synthesized according to a previously reported method (Su & Feng, 2010). The ligand was prepared by a modification of previously reported Schiffbase condensation methods (Small & Brookhart, 1999;Chen et al., 2003). All other reagents and solvents were purchased commercially and used without further purification. 1 H NMR data were collected on a Varian 60 MHz NMR. Mass spectra were collected using direct injection on a ThermoScientific TSQ-ESI Mass spectrometer. Synthesis of 2,6-bis(1-(4-dimethylaminophenylimino)ethyl)pyridine (PDI-DMA). A solution of 2,6-diacetylpyridine (1.0 g, 6.10 mmol), 4-(dimethylamino)aniline (1.7 g, 12.5 mmol) and formic acid (1 ml) was prepared in toluene (100 ml) under nitrogen atmosphere and then stirred for 12 h on molecular sieves. The reaction mixture was filtered and extracted with excess dichloromethane, then the amount of solvent was reduced in vacuo. The crude yellow product was then washed with cold methanol, followed by diethyl ether and filtered producing a pure bright-yellow solid (   Cg1 and Cg2 are the centroids for the named phenyl and pyridine rings.

Figure 2
A single cationic unit (Ni1) displaying intramolecularinteractions between phenyl and pyridyl rings.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 4. H atoms bonded to carbon were placed in geometric positions with C-H = 0.95, 0.99 and 0.98 Å and U iso (H) = 1.2 Â, 1.2 Â or 1.5 Â U eq (C) for aromatic, methylene and methyl H atoms, successively. Water H atoms were initially located from a difference Fourier map and included in their initially observed positions and allowed to ride with the position of the parent oxygen atom. Displacement parameters of the hydrogen atom were freely refined. Two reflections (341 and 0,10,2) were omitted from the refinement for poorly agreeing statistics. It is not clear from the diffraction data why these two reflections agree poorly. Packing diagram of [Ni(C 25 H 29 N 5 ) 2 ]Cl 2 Á2CH 2 Cl 2 Á2H 2 O, viewed along the a axis.

Bis[2,6-bis(1-{[4-(dimethylamino)phenyl]imino-κN}ethyl)pyridine-κN]nickel dichloride-dichloromethane-water
(1/2/2) Crystal data [Ni(C 25   Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.