Geometrical isomerism in octahedral complexes arising from the presence of a fused ring on a triaza macrocycle

The chiral 2,5,8-triazabicyclo[7.4.01,9]tridecane (L^1), an analogue of 1,4,7-triazacyclononane, (L^2) that has a trans-cyclohexane ring fused to the tacn framework, forms bis complexes with a wide range of metal ions where two geometric forms may exist depending on the relative locations of the substituent cyclohexane units. For the crystal structure of bis(RR-2,5,8-triazabicyclo[7.4.01,9] tridecane)nickel(II) nitrate, two macrocyclic ligands each occupy a face on opposite sides of the metal-centred octahedron, with the cyclohexane rings, when viewed down an axis passing through the centre of these faces and the metal, arranged in an anti disposition. However, the crystal structure of bis(RR-2,5,8-triazabicyclo[7.4.01,9]tridecane)chromium(III) perchlorate shows the cyclohexane rings disposed in the alternate syn arrangement. Isomerism is defined simply by the spatial disposition of macrocycle substituents. Isomeric preference has been probed by a molecular mechanics analysis, including an energy profile analysis as one macrocycle is rotated relative to the other. The force field calculations predict very small differences between isomers, but a large barrier to interconversion by a twist mechanism. Complexation of L^1 with a range of labile metal(II) ions has been probed by determination of apparent stability constants via spectrophotometric titrations. In general, formation constants of L^1 complexes are similar to or even slightly higher than the unsubstituted 1,4,7-triazacyclononane (L^2) analogues, consistent with no strong steric influence of the fused cyclohexane rings in L^1 and with differences in properties reflecting minor electronic effects and ligand flexibility differences resulting from substitution.