New linear tricobalt complex of di(2-pyridyl)amide (dpa), [Co3(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 the 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 both crystals are essentially symmetrical, though this is not required by crystal 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 with Co-N distances in the range 2.068-2.111 Angstrom at 213 K. The spiral arrangement of ligands gives an overall idealized D-4 point group symmetry for the cation [Co-3(dpa)(4)(CH3CN)(2)](2+). Chiral crystals of both Delta and Lambda configurations in the P2(1) form have been isolated. The absolute configurations were determined by X-ray crystallography and their mirror-image circular dichroism spectra measured. The D-4 symmetry of the cation appears to be preserved in solution as judged by the presence of only five proton resonance signals in the H-1 NMR spectrum. Magnetic susceptibility measurements 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.