Isomerization pathway of a C-C sigma bond dinickel(II) bis(octaazamacrocyclic) complex activated by deprotonation - a DFT study

The anti ( a ) to syn ( s ) isomerization pathway of the deprotonated form of the carbon-carbon (C-C) σ dimer complex with two nickel(II) 15-membered octaazamacrocyclic units was investigated. For the initial anti ( a ) structure, a deprotonation of one of the bridging (sp 3 hybridized) carbon atoms is suggested to allow for an a to s geometry twist. A 360° scan around the bridging C-C dihedral angle was performed first to find an intermediate geometry. Subsequently, the isomerization pathway was explored via individual steps using a series of mode redundant geometry optimizations and geometry relaxations leading to the s structure. The prominent geometries (intermediates) of the isomerization pathway are chosen and compared to the a and s structures. Finally, geometry relaxations of the protonated forms of selected intermediates are considered. It is shown that albeit the reaction barrier appears to be higher than that obtained in the experiment, the DFT series of mode redundant geometry optimizations and geometry relaxations proves itself useful in the exploration of the isomerization pathway of the chosen diastereomeric dinickel(II) bis(octaazamacrocyclic) complex.