Cyclen-Based Octaamine Ligand Supporting the Formation of Dinuclear Metal Compounds

A series of divalent first-row dinuclear transition metal complexes─LCr, LMn, LFe, LCo, and LZn─are synthesized and characterized using a 1,4,7,10-tetraazacyclododecane (cyclen)-derived octaamine ligand (LH) as a dinucleating platform. The ligand scaffold stabilizes these complexes without the need of exogenous ligands to complete the coordination sphere, giving rise to coordinatively unsaturated complexes. Crystallographic analysis reveals that the Mn, Fe, Co, and Zn complexes are isostructural, adopting coordination environments with the metal atoms situated in pseudotetrahedral and square pyramidal environments. In contrast, the Cr complex exhibits a structure where the two metal atoms reside in identical and cofacial pseudo-square planar geometries. DFT calculations, electron localization function analysis, and Wiberg bond indices suggest varying degrees of metal-metal bonding interactions across all complexes described here. In LCr, the short Cr-Cr distance of 1.9609(7) Å is consistent with a quadrupole bond, which is supported by DFT calculations. These results demonstrate the utility of this cyclen-based ligand scaffold in templating the synthesis of dinuclear complexes establishing a range of weak, in the case of the isostructural LMn, LFe, LCo, and LZn, to strong metal-metal interactions in LCr. The dinuclear complexes supported by weak field amido and amine donors in LH represent a promising platform to investigate biomimetic cooperative small molecule activation.