The Development of Transition Metal Silylene and Germylene Complexes for Small Molecule Activation
- Author(s): Barrientos, Marissa
- Advisor(s): Harman, Hill
- et al.
While N-heterocyclic carbenes (NHC) are widely recognized for their utility as catalysts and ligands for catalysts, their heavier analogues, namely N-heterocyclic silylenes and germylenes have not been as extensively explored. These divalent group-14 compounds are intriguing as they have a more accessible empty p-orbital due to decreased overlap with the nitrogen centered lone pairs. This fact results in both nucleophilic and electrophilic character, or ambiphilicity, in silylenes and germylenes. We are interested in utilizing this property to design transition metal complexes of N-heterocyclic silylenes (NHSi) and germylenes (NHGe) Herein we report the synthesis of a several diphosphine pincer frameworks anchored by silicon and germanium atoms. In the case of the silicon-based ligands, dichlorosilane, hydrochlorosilane derivatives function as proligands whereas the germanium derivative is accessible as the free germylene.
Metallation of the Si-anchored pincers with the Pt group metals (Ni, Pd, and Pt) proceeds by Si–Cl or Si–H activation of the proligands to give divalent square planar complexes featuring a chlorosilyl anchoring ligand. Two-electron reduction of the Pd derivative affords a bimetallic dipalladium(0) disilylene species with inequivalent metal sites that are in dynamic exchange in solution. This bimetallic disilylene activates the O–H bonds of both water and phenol to give the corresponding disilyl dipalladium(I) species. In contrast to the analogous dinickel system explored in our lab, it does not react with H2. CO2 reacts with the disilylene species to give CO and a bridging disilylcarbonate complex. These results highlight the viability of cooperative small molecule activation at late-metal silylene complexes.
We also report a large family of bimetallic complexes of first row metals (Mn, Fe, Co, and Ni) supported by the germylene-anchored pincer. Unlike the silicon-based ligands discussed above, which bind metals in a 1:1 ratio, the germylene ligand variant typically binds two metals. These bridging germylene complexes exhibit a range of metal-metal distances and electronic interactions resulting in both diamagnetic and paramagnetic complexes. Unlike the silicon anchored species, the bimetallic germylene complexes were generally electronically and/or coordinatively saturated, and thus typically unreactive.These results highlight the substantial differences in germylene and silylene ligand owing to the larger size of Ge versus Si.