The structure and properties of Unsymmetrical-Facial 2,3-Bis[(2-aminoethyl)(methyl)-aminoethyliminol-1-methylpiperazinecobalt(III) perchlorate monohydrate, u-fac-[Co(amam)] (ClO₄)₃,H₂O

Abstract

X-ray crystallographic analysis of the title complex, which was formed spontaneously from aqueous s-fac-[Co(medien)₂]³⁺ [medien = 2,2?- methyliminodi(ethylamine)] in the presence of charcoal and oxygen, shows that it has the u-fac geometry of the [Co(medien)₂]³⁺ moiety. Two coplanar en-type rings of this moiety are bridged by a new planar ring which is fused through a di-amidine grouping to a piperazine ring. The close planarity of this linked ring system, which results from the conjugated unsaturation and resonance in the di-amidine grouping, provides a powerful force in stabilizing the u-fac geometry. The crystals are monoclinic, spacegroup P2_1/c with a 10.700(6), b 13.38(6), c 20.076(5) Ǻ, β 98.58(13)°. The structure was refined to an R-factor (on F) of 0.077 by blocked-matrix least-squares techniques. The complex has an acidic amidine proton (pKa 10.3 at 20°) located on the piperazine nitrogen. The ¹³C n.m.r, spectra of the complex and its deprotonated form are interpreted and indicate that the predominant solution tautomer is the same as that in the solid state. The existence of this complex indicates that it might be possible to prepare the unstable u- fac-[Co(medien)₂]³⁺ isomer.