The Development of Flexible Syntheses of Cobalt(II) and Iron(II) Complexes of the Tripodal Mesoionic Carbene Ligand Tris[4-(1-mesityl-3-methyl-1,2,3-triazol-5-ylidene)]amine, Revolving Around Initial Generation of a 'Free Carbene'.

It has been reported that cobalt(II) complexes of the tris[4-(1-mesityl-3-methyl-1,2,3-triazol-5- ylidene)methyl]amine mesoionic carbene ligand, mesTTzA, can be prepared by transmetalation of silver(I) complexes, [Ag3( mesTTzA)2](X)3, with the ligand binding in a tridentate fashion with an additional acetonitrile co-ligand ligand to make a four coordinate, tetrahedral complex. This approach is a multistep synthesis and has been proven to be difficult to apply to alternative mid-to late transition metals like iron. Additionally, the strong binding between cobalt and the acetonitrile co-ligand ligand, limits further expansions of the chemistry by ligand substitution. Herein, a new synthesis method revolving around the generation of a free mesoionic carbene has been successfully developed and used to synthesise [CoII( mesTTzA)Cl](OTf) and [FeII( mesTTzA)Cl](OTf) complexes. This one-step reaction can be conducted easily at room temperature, inside an inert atmosphere glovebox due to the surprising stability of the mesoionic carbene for short periods of time. Slow addition of the base to the reaction mixture allows the mesoionic carbene generated to react instantaneously with the metal precursors to form the complexes, which are obtained cleanly and in high yields (≥80%). This led to a significant reduction in impurities and greater overall atom economy compared to silver transmetalation techniques. Complete displacement of the chloride co-ligand in favour of an azide ligand, to form [FeII( mesTTzA)(N3)](OTf) and [CoII( mesTTzA)(N3)](OTf), was also successful. These complexes have significant potential to act as precursors to high and low oxidation state, mid- to late transition metal complexes in future research endeavours.