Polymorphism in [(polyamine)Co(NO 2) x]Y crystals caused by changes in torsional angles of the (amine)N–Co–N–O fragments ( x = 2, Y = NO 3−: x = 3, Y = 0)

Abstract

Herein, we report the synthesis and characterization of a third polymorph of trans-[Co(2,3,2-tet)(NO 2) 2]NO 3, III, crystallizing in space group P 1 ¯ (No. 2) obtained during an attempt to reproduce the synthesis of a previously reported polymorph, I (for more details of polymorphs I and II, see Introduction and references cited therein). The cations of polymorphs I and II differ primarily by the angles that the planes of the two –NO 2 ligands make with one another; the former being considerably larger than that in II. Polymorph III resembles II in that the torsional angular differences between the trans-nitro ligands are also small, but differ notably from that in I. The structure of the compound [(5-Me-(dpt)Co(NO 2) 3], was determined also. The space group is P2 1/ n, with two molecules in the asymmetric unit, whose occupancies are 65% and 35% for molecules IVa and IVb, respectively. Again, the two differ by the torsional angles of the nitro ligands, specially two of them whose angular orientations are vastly different. Molecules IVa and IVb are compared with a previously obtained polymorph V of this same compound reported earlier. Once more, V is closely related in stereochemistry to IVb, but differs markedly from IVa in nitro torsional angles. In all cases, the Co(amine) fragments are closely super-imposable and the differences in nitro torsional angles are the result of the availability of several amine hydrogens of the basal plane with which to make intra-molecular hydrogen bonds. Clearly, these hydrogen bonds must be of very similar strength and the barriers to rotation of the –NO 2 ligands must have energies similar to the energetics of the hydrogen bonds causing the torsional motions.