Cora E. MacBeth, PhD , Emory University
Ligand systems that incorporate amidate donors were selected as good candidates for several reasons: (1) The amide functional group ([RNHC(O)R’]) can be readily synthesized in high yields from simple amine precursors, making ligands that incorporate amidates highly modular. (2) The acyl substituents on amide-based ligands can be varied to regulate both the electronic and steric features of the resulting transition metal fragments. (3) Amide functional groups are chemically robust; resistant to both oxidation and hydrolysis. However, a significant challenge in the application of these ligands to catalysis is controlling the amidate coordination mode to afford predictable coordination geometries. We used the PRF-DNI funds specifically to begin exploring tris(amidate) ligand systems for C-H oxidation reactions. We have accomplished the following:
1. We have demonstrated the coordination diversity of deprotonated organic amides (amidates) donors. Amidate have recently been recognized as potent ligand scaffolds for metal ion mediated catalytic and stoichiometric transformations. Amidate ligands can interact with metal centers through a variety of coordination modes, including: (1) monodentate (coordinating through either the N- or O-amidato donor), (2) bridging (where the N- and O-donors coordinate to two separate metal ions), and (3) chelating. Using these ligands we were able to isolate a rare example of an A(III) tris(κ2-amidato) species that are proposed intermediates in trans(amidation) chemistry.
2. We’ve explored the synthesis and characterization of nickel complexes supported by a series of tris(amidate) ligands([N(o-PhNC(O)R)3]3-, R = i-Pr, t-Bu, and Ph) and have shown that the ligands’ amidate acyl substituents can be used to control both the coordination number of the nickel ion and the coordination modeof the amidate donors in the resulting metal complexes. In addition, we have studied the ability of these nickel complexes to bind cyanide and have shown that in some cases, complexes supported by these ligand systems can be used to assemble a heterobimetallic complexes (Ni/Co).
3. We shown that Fe(II) complexes supported by tris(amidate) amidate ligands are capable of dioxygen activation and subsequent C-H and C-F bond oxidation. Mechanistic studies of these processes are ongoing in our laboratories.
4. Work in our laboratory has demonstrated that bimetallic Fe(II) complexes that incorporate bridging amidate donors can be used for the aerobic oxidation of C-H bonds. These processes appear to require the formation of bridged, high-valent oxo species.
5. We have also demonstrated that amidate donors can be coupled to redox-active tridentate backbones to afford systems capable of catalytic dioxygen activation. These systems do not affect C-H oxidation but are competent in oxygen atom transfer reactions.