A stable anionic N-heterocyclic carbene and its zwitterionic complexes.

Area of Science:

• Organometallic Chemistry
• Carbene Chemistry
• Coordination Chemistry

Background:

• N-heterocyclic carbenes (NHCs) are versatile ligands in organometallic chemistry.
• Developing NHCs with unique functional groups can lead to novel complex structures and reactivity.
• Pyrimidinium betaines offer a modular synthetic route to new NHC precursors.

Purpose of the Study:

• To synthesize and characterize a new class of NHCs derived from pyrimidinium betaines, termed maloNHCs.
• To explore the utility of maloNHCs as ligands for forming zwitterionic metal complexes.
• To investigate the coordination behavior and electronic properties of metal complexes featuring maloNHCs.

Main Methods:

• Modular synthesis of pyrimidinium betaines from formamidine and malonic acid derivatives.
• Deprotonation of pyrimidinium betaines using strong bases (nBuLi or KHMDS) to generate maloNHC species.
• Synthesis and characterization of metal complexes (Rh, Fe, Ag) with maloNHC ligands using spectroscopic and crystallographic techniques.

Main Results:

• Stable pyrimidinium betaines were synthesized and readily deprotonated to form maloNHC lithium salts.
• MaloNHCs act as both a 2 e- donor ligand and a noncoordinating counterion, facilitating the formation of zwitterionic complexes.
• A neutral 14-electron rhodium complex, Rh(maloNHC)(COD), was synthesized, featuring an unusual labile interaction between the metal and a mesityl ipso carbon.

Conclusions:

• MaloNHCs represent a novel class of NHCs with a built-in anionic functional group.
• These ligands provide a rational approach to constructing zwitterionic organometallic complexes.
• The unique coordination mode observed in the rhodium complex highlights the potential for developing new reactivity and catalytic applications.