Metal-ligand cooperation in bis(phosphinimine)-supported rhodium pincer complexes: from dehydrogenation to value-added products

Abstract

Expansion of methodology towards the synthesis of pincer ligand (L = κ3-NNN’ = 2,5-[iPr2P=N(4-iPrC6H4)]2-N’(C4H2)-) supported rhodium tetrylene (κ2-L(CO)Rh(ER2); E = Si, Ge; κ2-L = κ2-NN’-Rh, κ1-N-E) and borylene (κ2-L(CO)Rh(BMes); Mes = mesityl) species is described, followed by investigations into the reactivity of these new complexes.

A metal-ligand cooperative approach was used to dehydrogenate group 14 starting materials of the form RR’EH2 (E = Si, Ge; R = aryl or alkyl; R’ = H, aryl or alkyl) and MesBH2 (Mes = mesityl), and the resulting complexes were characterized by a variety of instrumentational techniques (NMR, IR, EA). X-ray crystallography was used to unambiguously confirm the solid-state structure within these species, establishing the hemilability of the R3P=NAr “phosphinimine” nitrogen-donor of the pincer ligand as well as its base-stabilization capabilities.

Diverse reaction chemistry was demonstrated for these species, starting with dehydrocoupling reactions with pinacol, which provided proof that the activated main group functionalities could be used as {BMes}, {SiRR’}, and {GeRR’} synthons and transferred to organic substrates. Additionally, reaction of a base-stabilized borylene complex with a variety of alkynes and CO gas yielded highly functionalized boron-containing heterocycles ((PhC=CRBMesO)C=NPipp) that would be difficult to synthesize by alternative routes, showcasing the potential these systems have for applications. Finally, the H-substituted silylene complex, κ2-L(CO)Rh(Si(H)Mes), was demonstrated to react with tris(pentafluorophenyl)borane to yield an elusive silylyne complex, the first of its kind with rhodium.

Altogether, a cohesive body of work encompassing the development of new methodologies, diverse reaction chemistry, and the presentation of future areas for pursuit are described herein.