UC Riverside

N-heterocyclic carbenes (NHC) are strong Lewis bases that have demonstrated broad utility as ligands for transition metal (TM) catalysts. Silicon analogues, N-heterocyclic silylenes (NHSi), are comparatively less well studied. The decreased π–π overlap between the Si 3p orbital and adjacent nitrogen lone pairs creates a more accessible vacant orbital on Si, resulting in increased ambiphilic character. When bound to a transition metal, this empty orbital presents an opportunity to cooperatively bind and activate substrate molecules. To increase the stability of a potential metal–silylene complex we targeted the diphosphine ligand precursor 1,2-bis(R2PCH2NCH2)C6H4 (R = alkyl or aryl), first reported by Yamashita and Nozaki in the construction of a boron-anchored pincer. Reported here are (PCy)2Si pincer ligands and their TM–silylene complexes for small molecule activation.

The bimetallic dinickel disilylene complex, (PCy)4Ni2Si2, is reactive with dihydrogen, phenol, thiophenol and diphenyl disulfide, resulting in addition of the E–H or E–E bond (E = H, O, or S) across the Ni2Si2 core and the conversion of the silylene ligands to bridging silyls. We also investigated the reactivity of (PCy)4Ni2Si2 with CO2. Addition of excess CO2 to (PCy)4Ni2Si2 revealed a new product, (PCy)4Ni2(µ-CO)Si2O, featuring a bridging CO across the two Ni centers and a Si–O–Si linkage. This product corresponds to the direct deoxygenation of CO2 and the transfer of the oxygen atom to the silylene ligands. CO dissociation is observed under certain conditions to give an oxo-bridged 16 e– (PCy)4Ni2(Si2O) complex that can also be synthesized via direct O-atom transfer to (PCy)4Ni2Si2. A transient side-bound CO2 intermediate was hypothesized to initially form across one of the two Ni0–silylene interactions prior to C=O bond cleavage and the formation of (PCy)4Ni2(µ-CO)Si2O. Trapping experiments with TMSCl support this hypothesis, as an intermediate can be isolated, (PCy)4NiClNi(CO)Si2O-TMS, featuring a terminal Ni–CO and a trimethylsiloxyl moiety on one of the two Si centers, suggesting the cooperativity of the Ni–Si bond in activating CO2. This work highlights the promise of using TM–silylene interactions for the cooperative activation of small molecules.