Synthesis and crystal structure of a new chiral α-aminooxime nickel(II) complex

The reaction of a nickel precursor with an enantiomerically pure amino-oxime issued from (R)-limonene led to the formation of bis[κ3 N,N,N-(aminooxime)-μ-chlorido]dichlorodinickel as a new dinuclear nickel complex.


Chemical context
Asymmetric synthesis allows the preparation of enantiomerically enriched compounds either by using a chiral auxiliary, which will be temporarily introduced, or by using catalytic procedures (Gawley & Aubé, 2012). This latter method is particularly attractive as it contributes to the development of green chemistry, which maximizes efficiency and minimizes hazardous effects on human health and the environment (Anastas & Zimmerman, 2013). Thus, asymmetric catalysis avoids synthetic steps and only catalytic amounts of the optically pure auxiliary are needed (Ojima, 2010). As part of the development of this chemistry, the synthesis of new chiral organometallic complexes is always challenging. The pivotal point is then the synthesis of optically pure ligands, which will be coordinated to the metal center. In terms of sustainable chemistry, using the chiral pool to develop new ligands is most interesting (Elalami et al., 2015). Coordination metal complexes containing terpenoid fragments are widely used in the pharmaceutical field and in catalysis. We have therefore developed ligands based on terpenes such as pinene and limonene (El Alami et al., 2009Chahboun et al., 2012). In particular, the synthesis of optically pure aminooxime ligands has been performed successfully from (R)limonene (El Alami et al., 2012). These compounds possess structures with two or three nitrogen atoms as donor heteroatoms that could coordinate to the metal center. They have advantageously replaced phosphine ligands, which are generally unstable under air. Ruthenium (Benabdelouahab et al., 2015) and palladium (de la Cueva-Alique et al., 2019) ISSN 2056-9890 complexes have already been synthezised with these ligands. Here we report the first synthesis of a limonene-basedaminooxime nickel complex and its crystal structure. In the dinuclear title complex, each nickel ion is coordinated by (1S,4R)-1-picolylamino-p-menth-8-en-2-one oxime. The ligand was first synthesized from (R)-limonene through the addition of nitrosyl chloride, NOCl, to a picolylamine moiety, allowing the formation of the oxime moiety.

Figure 2
Intermolecular and intramolecular hydrogen bonds in the structure, shown as dashed lines.
In particular, the two {Ni(aminoxime)-Cl}Cl units are slightly asymmetrical with the existence of a hydrogenbonding interaction between the amine N2-H2 linked to Ni1 and the chlorine atom Cl4 linked to Ni2. In addition, the two oxygen atoms O1 and O2 of the oxime groups are involved in intramolecular O1-H1Á Á ÁCl1 and O2-H2AÁ Á ÁCl4 hydrogen bonds and in intermolecular C3-H3Á Á ÁO1 and C26-H26Á Á ÁO2 interactions.

Database survey
The aminooxime ligand used in this study was previously reacted with palladium and platinum precursors, generating three N-coordinated cationic complexes as enantiopure compounds (de la Cueva-Alique et al., 2019). A heteronuclear Ti IV /Pd II complex has also been described. The compounds were studied to assess their potential biological activity, a high anticancer activity (de la Cueva-Alique et al., 2019).

Synthesis and crystallization
To a solution of Ni II chloride ethylene glycol dimethyl ether (0.15 g, 1.48 mmol) in MeOH (5 mL) was added (1S,4R)-1picolylamino-p-menth-8-en-2-one-oxime (0.101 g, 0.36 mmol) dissolved in MeOH (3 mL). The solution turned green. The mixture was stirred overnight at room temperature during which time the mixture changed color to blue-green. The solvent was then evaporated to produce a crude solid that was washed with diethyl ether before crystallization. Single crystals were grown by slow diffusion at room temperature of diethyl ether into a dichloromethane solution.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. N-and O-bound atoms were refined with the restraint U iso (H) = 1.2U eq (N) or 1.5U eq (O). H atoms were positioned geometrically(C-H = 0.95-1.00 Å ) and refined as riding with U iso (H) = 1.2U eq (C) or 1.5U eq (Cmethyl) Packing diagram.

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.