Helical racemate architecture based on osmium(II)-polypyridyl complexes: Synthesis and structural characterisation

New polypyridyl osmium(II) complexes [Os(κ3-tptz)(EPh3)2Cl]BF4 (E = P, 1; As, 2) with group 15 donor ligands are reported. Structural studies on the representative complex [Os(κ3-tptz)(PPh3)2Cl]BF4 revealed formation of helical racemates with sidewise stacking of right and left-handed anti-parallel helical strands. Salient structural features and DNA binding studies along with binding constant [6.6 × 103 M−1] and site size [0.12] of the complex 1 with calf thymus (ct) DNA by absorption spectroscopy are described.