Developing 16- and 18-Atom Ringed Macrocyclic Tetracarbene Complexes

Stabilizing metal-ligand multiple bonds is important for the understanding of biological intermediates, as well as, for their use in group transfer reactions. Two considerations to make when developing a ligand system to support metal-ligand multiple bonds are the strength of the ligand and the symmetry of the ligand. Herein, our group has proposed a class of four-coordinate, strong σ-donor ligands by synthesizing macrocyclic tetraimidazoliums that form tetracarbenes upon deprotonation. By examining group theory, these systems should allow for the formation of complexes with metal-ligand multiple bonds in a bent square pyramidal geometry that exhibit high oxidation states and novel spin states.

The first step in accomplishing this goal was the synthesis of a neutral, 18-atom ringed macrocyclic tetraimidazolium system. By employing a weak base deprotonation strategy, we were able to form a platinum tetracarbene complex. Unlike previous macrocyclic tetracarbene systems, our complex was rigid in solution, allowing for accessible apical positions. A major drawback to this first neutral, 18-atom ringed macrocyclic tetracarbene system was its inherent insolubility in nonpolar solvents. In order to compensate for this shortcoming, we added two borate moieties to second generation macrocyclic tetraimidazoliums, leading to dianionic ligands upon deprotonation. These three additional systems included two 16-atom ringed tetraimidazoliums and one 18-atom ringed tetraimidazolium. While results of forming metal complexes with the 16-atom ringed variants are incomplete, we were able to form metal complexes with the 18-atom ringed, borate-based tetracarbene ligand. Not only did the complexes formed by the 18-atom ringed, borate-based tetracarbene ligand have enhanced solubility in solvents such as toluene, but preliminary results suggest they may stabilize metal-ligand multiple bonds.