The crystal structures of two isomers of a macrocyclic binuclear compound each containing both divalent and trivalent cobalt

Abstract

The compound Lco₂Br₃,2H₂O (where L is a binucleating ligand derived from the macrocycle LH₂ formed by condensation of two molecules each of propane-1,3-diamine and 2-hydroxy-5- methylisophthalaldehyde) was found to occur in two crystalline forms. The structures of both have been determined by single-crystal X-ray diffraction techniques, revealing that each corresponds to a different geometrical isomer of composition Lco₂Br₃,2H₂O. Each structure was determined by the conventional Patterson-Fourier method and refined by a least-squares procedure, with only the Co, Br, N and O atoms being refined anisotropically. The first form, designated the A-isomer, crystallizes in the orthorhombic space group Pn2₁a with four formula units in a cell of dimensions a 15.173(1), b 17.370(2) and c 10.493(1) Ǻ. The structure refined to a conventional R-factor of 0.074 by means of 1883 unique reflections collected by counter methods. The second form, or B-isomer, crystallizes in the monoclinic space group P2₁ with two formula units in a cell of dimen- sions a 10.676(1), b 15.235(2), c 9.304(1) Ǻ and β 111.83(2)°. The structure refined to a conventional R-factor of 0.101 by means of 2006 independent reflections collected by counter methods. Both isomers are composed of discrete bromide anions together with discrete binuclear cations of com- position [Lco₂Br₂,2H₂O]⁺, each containing one cobalt(II) and one cobalt(III) atom with a Co...Co distance of 3.13 Ǻ. In both isomers the cobalt atoms are in octahedral environments. In the A-isomer two bromine atoms are bonded to the cobalt(III) atom in positions perpendicular to the ligand N₂O₂ donor plane, with the two water molecules occupying similar positions about the cobalt((II). But in the B-isomer, one water molecule and one bromine atom are bonded to each of the cobalt(III) and the cobalt((II) atoms in the comparable positions. In both structures the ionic bromide is involved in a hydrogen-bonding scheme with the coordinated water molecules.