Synthesis of the tridentate pyridine donor 2,6-bis[1-phenyl-1-(pyridin-2-yl)ethyl]pyridine (L), including separation of meso and rac diastereoisomers via methylmercury(II) derivatives, and an X-ray structural study of [HgMe(meso-L)]NO3·2H2O

Abstract

A synthesis of the new tridentate pyridine donor ligand 2,6-bis[1-phenyl-1-(pyridin-2-yl)ethyl]pyridine, L, by reaction of 2,6-dichloropyridine with lithiated 2-(1-phenylethyl)pyridine is described. The ligand reacts with methylmercury(II) nitrate to form complexes of the meso and rac diastereoisomers of L, [HgMe(L)]NO₃·2H₂O, which may be readily separated by fractional crystallization. The ligands meso-L and rac-L may be liberated from the complexes by reaction with cyanide ion in water. The crystal structure of the meso complex has been determined by single-crystal X-ray diffraction at 295 K and refined by least-squares methods to R= 0.046 for 2 600 ‘observed’ reflections [monoclinic, space group P2₁/n, with a= 20.946(9), b= 14.767(4), c= 9.817(3)Å, β= 91.12(2)°, and Z= 4]. The [HgMe(meso-L)]⁺ cations have irregular co-ordination geometry for the mercury atom ‘HgCN₃’ with the ligand present as a tripodal tridentate. The central nitrogen forms the strongest bond to mercury [2.283(9)Å] with C–Hg–N 166.3(5)°, with the other nitrogen donors 2.546(11) and 2.595(10)Å from mercury and forming C–Hg–N angles of 113.3(5) and 113.7(5)°, respectively. ¹H N.m.r. Spectra for the complexes suggest that the ligand is at least bidentate in methanol.