New linear tricobalt complex of di(2-pyridyl)amide (dpa), [Co-3(dpa)(4)(CH3CN)(2)][PF6](2)

Reaction of the linear tricobalt compound Co-3(dpa)(4)Cl-2 (1) (dpa = di(2- pyridyl)amide) with silver hexafluorophosphate in acetonitrile yields [Co-3 (dpa)(4)(CH3CN)(2)] [PF6](2) (2). Two crystalline forms are obtained from t he same solution, namely, a monoclinic (P2(1)) form 2 . CH3CN . 2Et(2)O and a triclinic (P (1) over bar) form, 2 . 3CH(3)CN. The tricobalt units in bo th crystals are essentially symmetrical, though this is not required by cry stal symmetry, with Co-Co distances in the range 2.298-2.304 Angstrom. Each of the two terminal Co atoms is coordinated to an acetonitrile molecule wi th Co-N distances in the range 2.068-2.111 Angstrom at 213 K. The spiral ar rangement of ligands gives an overall idealized D-4 point group symmetry fo r the cation [Co-3(dpa)(4)(CH3CN)(2)](2+). Chiral crystals of both Delta an d Lambda configurations in the P2(1) form have been isolated. The absolute configurations were determined by X-ray crystallography and their mirror-im age circular dichroism spectra measured. The D-4 symmetry of the cation app ears to be preserved in solution as judged by the presence of only five pro ton resonance signals in the H-1 NMR spectrum. Magnetic susceptibility meas urements in the solid state indicates that 2 has a doubler ground state and exhibits an increase of the effective moment at high temperature (similar to 160 K) due to a spin crossover process.