New electron-rich diketiminate ligands : an experimental and computational investigation on the isolation of reactive species

Abstract

Boron analogues of NHCs have long eluded detection, likely due to their predicted low singlet-to-triplet energy gap. Two new N,N’-chelating ligands have been prepared and investigations into their reactivity and coordinative properties have started. A boron carbenoid that incorporates one of the new ligands has been probed computationally using both DFT and second-order Møller-Plesset methods. It has been found that the singlet state of this carbenoid is between 13.98 and 36.85 kJ mol[superscript -1] more stable than its triplet state. To date, this is the only computationally investigated ligand system that stabilizes a neutral, six-membered heterocyclic, ground state singlet boron carbenoid. Attempts to prepare, and isolate, this compound have been unsuccessful, however, experimental evidence suggests further investigations should be made.

Organometallic complexes of the two new ligands have also been prepared and present interesting structural features. Rhodium carbonyl compounds have been synthesized with each ligand and these were studied to determine the ligands’ electronic features, in comparison to each other and to the common 1,3-diketiminate (nacnac) ligand. It was found that one was experimentally identical to nacnac and the other had increased donor capabilities. A dimethylaluminum complex has also been prepared and both X-ray and DSC studies suggest that it assumes different solid state morphologies at various temperatures. Additionally, a long, Zn⋯C [pi]type interaction was observed in the corresponding ethylzinc complex.