Synthesizing Strong Donor Macrocyclic Tetracarbene Metal Complexes for Catalytic Aziridination

A small ringed macrocyclic tetracarbene ligand was developed due to the inherent ability of N-heterocyclic carbenes (NHCs) to stabilize high oxidation states of transition metals. This new strong donor ligand was prepared by first synthesizing an 18-atom ringed macrocyclic tetraimidazolium ligand precursor. The tetraimidazolium can be prepared by a two-step procedure. This ligand precursor was deprotonated to prepare a monomeric platinum tetracarbene complex.

A new iron macrocyclic tetra-carbene complex was synthesized by an in situ strong base deprotonation strategy of the ligand precursor. The iron tetracarbene complex was found to catalyze the aziridination of a wide array of functionalized aryl azides and a variety of substituted aliphatic alkenes, including tetra-substituted.

The aziridination intermediate was probed by mass spectrometry and found to likely be and iron(IV) imido. Further investigation of this intermediate discovered that the an iron(IV) tetrazene forms when excess aryl azide was added, probably by a 1,3-cycloaddition of an additional equivalent of azide to an imido. Utilizing single crystal X-ray diffraction, NMR spectroscopy, and Mossbauer spectroscopy the metal center was formally assigned as a low spin (S = 0) iron(IV). Additional reactivity studies indicates this tetrazene is capable of performing aziridination and therefore is an additional reaction pathway in the catalytic cycle.

A large disadvantage of the aforementioned iron tetracarbene catalyst is poor yield. To overcome low yields and to prepare several transition metal tetracarbene complexes, a dimeric macrocyclic tetracarbene silver complex was synthesized. This complex was shown to successfully extend transmetallation of polydentate NHCs beyond bidentate NHCs. The silver complex was utilized in the preparation of a variety of mononuclear tetracarbene complexes ranging from early first row to late third row transition metals in moderate to high.

In an attempt to move toward improving solubility of the tetracarbene catalysts, a second generation variant with two borate moieties in the ligand backbone was utilized. With this dianionic 18-atom macrocyclic tetracarbene ligand, the first tetracarbene complexes of Group 13 and 14 metals were synthesized. The tin, indium, and aluminium tetracarbene complexes are structurally analogous to their catalytically active porphyrin or salen analogues.