New platinum(II) complexes with benzothiazole ligands

Four new platinum(II) complexes, [NEt4][PtBr3(L)], containing benzothiazole ligands have been structurally characterized by single-crystal X-ray diffraction techniques. All complexes adopt the expected square-planar coordination geometry, and the benzothiazole is engaged in bonding to the metal atom through the imine N atom (Pt—N).


Chemical context
The synthesis of new platinum complexes as potential drugs for cancer is still of interest for medicinal chemists. The structural details of these complexes provide the opportunity to predict, to a certain extent, the potential biological activity of these species. In this regard, four new platinum(II) complexes with benzothiazole ligands of general formula [PtBr 3 L] À have been synthesized according to the equation below and their structures characterized.

Structural commentary
To elucidate with certainty and accurately the platinum coordination patterns, the structural determination of the complexes was performed by single crystal X-ray diffraction technique. Table 1 contains selected bond lengths, dihedral angles and torsion angles. All of the title complexes adopt a square-planar coordination geometry about the Pt II atom with a deviation of no more than 4 from ideal 180 and 90 angles. As reported previously, although not predicted using Pearson's hard-soft acid base theory, the benzothiazole is engaged in bonding to the metal through the imine nitrogen (Pt-N) instead of Pt-S coordination (Muir et al., 1987(Muir et al., , 1988a(Muir et al., ,b, 1990Gomez et al., 1988;Lozano et al., 1994). Also the benzothia-zole ligand is positioned out of the square plane as discussed below.
Figs. 1-4 show the molecular structures of the four new complexes. [NEt 4 ][PtBr 3 (2-Me-benzothiazole)] (1) crystallizes in an orthorhombic unit cell with eight formula units. It is a square-planar complex with Pt-N and average Pt-Br bond lengths of 2.035 (5) and 2.433 (6) Å , respectively, which are within the expected range for Pt II complexes. There is no trans-influence observed in the Pt-Br bond trans to the Pt-N bond. The benzothiazole ligand is planar and the methyl group resides in the ligand plane. The dihedral angle between the PtBr 3 N unit and the benzothiazole ring is 88.1 (4) , similar to those observed in other Pt II -benzothiazole complexes, as a result of reducing the steric strain between PtBr 3 and the benzothiazole ligand (Muir et al., 1987(Muir et al., , 1988a(Muir et al., ,b, 1990Gomez et al., 1988;Lozano et al., 1994). Two types of N-C bonds are present, one long [N-C2 1.408 (7) Table 1 Selected bond distances and angles (Å , ).
The dihedral angle is between the Pt-Br 3 N unit and the benzothiazole ring. The torsion angle is between the benzothiazole ring and the R group.
1.309 (7) Å ], indicating the presence of single-and doublebond character in the thiazole ring. The angle at the S atom in the thiazole ring is 90.3 (3) suggesting it is using unhybridized p orbitals for bonding.

Synthesis and crystallization
The parent complex [NEt 4 ] 2 [Pt 2 Br 6 ] was prepared as reported in the literature (Livingstone & Whitley, 1962). Ligands were purchased from Sigma-Aldrich and were used without further purification.

Special details
Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq Occ. (