A nickel(II) complex with an unsymmetrical tetradentate chelating ligand derived from pyridine-2,6-dicarbaldehyde and 2-aminothiophenol

[(2-{[6-(1,3-Benzothiazol-2-yl)pyridin-2-yl]carbonylazanidyl}phenyl)sulfanido]nickel(II) crystallizes in the centrosymmetric monoclinic space group P21/n. Both arms of the expected bis-Schiff base ligand based on pyridine-2,6-dicarbaldehyde and 2-aminothiophenol had oxidized; one by cyclization to a benzothiazole, the other by oxidation of its imine linkage to the corresponding amide.

In this context, we were interested in developing such double Schiff base ligands that are redox-active, and targeted ligand 2, with the intention that formation or cleavage of a disulfide bond would give the necessary redox activity.However, in situ formation of the ligand through condensation of pyridine-2,6-dicarbaldehyde with two equivalents of 2-aminothiophenol, followed by reaction with Ni(NO 3 ) 2 Á3H 2 O in refluxing methanol, did not yield the expected Ni II complex of ligand 2, but instead gave the title complex 1 in good yield, in which the two ligand arms have both been oxidized, but in very different manners.

Structural commentary
Compound 1 crystallizes in the monoclinic space group P2 1 /n with one molecule in the asymmetric unit (Fig. 1).Selected bond lengths and angles are listed in Table 1.The central Ni1 atom has a distorted square-planar geometry with an N 3 S donor set, in which the X-Ni1-Y angles (Table 1) differ by up to 15 from either 90 or 180 .The whole molecule can be considered as planar, with the r.m.s.deviation of the atoms from their mean plane being 0.0867 A ˚, and the oxygen atom O1 showing the largest deviation from the plane of 0.210 (3) A ˚.It is immediately clear from the structure that the expected nickel complex of ligand 2 had not formed.Instead, the two ligand arms have each been differently oxidized in such a way as to yield a very unsymmetrical ligand.
The expected imine linkage of arm 1 (that including atom S1) has been oxidized to an amido functionality, as is clear from the short C7-O1 bond length of 1.229 (4) A ˚.There are two possible scenarios here.One is that one of the aldehyde groups oxidized to the corresponding carboxylic acid, followed by reaction with the aminothiophenol to form the amide.The other is that the Schiff base arm formed as expected, but with subsequent nucleophilic attack by water on the imino carbon atom, followed by oxidation to yield the amide.No significant electron density corresponding to a possible H atom could be found near S1, so this can be assigned as a deprotonated thiophenolato group.C15 forms an intramolecular C-HÁ Á ÁS hydrogen bond to S1 (Table 2), while any H atom bonded to S1 would lead to an unrealistic short contact to H15.Ni II complexes of ligands containing such amidobenzenethiolate units have previously been reported (Seratne et al., 2018), and their Ni-N and Ni-S distances [1.874 (3)-1.896(9) A ˚and 2.126 (4)-2.1343(9) A ˚] are similar to the corresponding bond lengths in 1, 1.871 (2) and 2.1508 (9) A ˚, respectively, although Ni1-N1 in 1 is slightly shorter, and Ni1-S1 slightly longer, than in these literature values.
The other arm of the ligand is also oxidized relative to the expected structure of 2, but here this has involved an oxidative cyclization, in which the sulfur atom S2 has initially attacked the imine carbon C13 to give a benzo[d]thiazol-2-yl functional group.Such oxidative cyclization has been previously observed in a related ligand system in which a 2,6-phenoldicarbaldehyde was condensed with two equivalents of 2-aminothiophenol (Gulcan et al., 2014).An Ni II complex with a chelating 2-(2 0 -pyridyl)-benzothiazole ligand has previously been structurally characterized (Patel et al., 2010), in which the Ni-N(thiazole) distance was 2.116 (2) A ˚, thus significantly longer than Ni1-N3 in 1 [1.952 (2) A ˚].However, the reported complex was octahedral rather than square planar, and the benzothiazole N atom was trans to an aqua ligand rather than the negatively charged deprotonated N atom in 1.The benzothiazolyl arm is clearly neutral, while the other formally carries negative charges on S1 and N1, with the ligand as a whole thus a dianion.This is consistent with the calculated valency for Ni1 of 2.14 obtained from bond-valence-sum analysis (Brese & O'Keeffe, 1991;Liu & Thorp, 1993).

Figure 1
Molecular structure of 1 with atom labelling; displacement ellipsoids represent 50% probability levels

Database survey
A survey of the Cambridge Structural Database (CSD, v5.44, including updates to June 2023; Groom et al. 2016) showed that no crystal structure of 1, nor any other complex of the same or related unsymmetrical ligand, nor the free ligand itself, has previously been reported.Two complexes of the bisdeprotonated target ligand 2 have been reported: the Zn 2+ complex BTAQZN10 (Goedken & Christoph, 1973) and the methylthallium complex TPAMTL (Henrick et al., 1977).In a further 13 structures, the two S atoms are bonded to an organic functional group (usually methyl, but in some cases the sulfur atoms are linked via di-or trimethylene chains to form a macrocycle); in these ligands the S atoms are unable to carry a negative charge.The structures of six complexes of the symmetrical ligand 2,6-bis-(benzo[d]thiazol-2-yl)pyridine were found, but all with metals other than nickel.11 structures were found for complexes with ligands in which a pyridine ring carried either one or two doubly deprotonated 2-thiophenolatoamido groups, but again no nickel complexes were among these.The structures of 15 further complexes, in which the S atom(s) of these ligands carry an organic functional group, were found.Six of these were nickel complexes, but were all octahedral hexacoordinate, in contrast to the square-planar 1.

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.
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )

Table 3
Experimental details.