Cooperative Bond Activation and Catalytic CO<sub>2</sub> Functionalization with a Geometrically Constrained Bis(silylene)-Stabilized Borylene
Xi Chen, Yin Yang, Hao Wang, Zhenbo Mo
Abstract
Metal–ligand cooperativity has emerged as an important strategy to tune the reactivity of transition-metal complexes for the catalysis and activation of small molecules. Studies of main-group compounds, however, are scarce. Here, we report the synthesis, structural characterization, and reactivity of a geometrically constrained bis (silylene)-stabilized borylene. The one-pot reaction of [(SiNSi)Li(OEt 2 )] (SiNSi = 4,5- bis (silylene)-2,7,9,9-tetramethyl-9 H -acridin-10-ide) with 1 equiv of [BBr 3 (SMe 2 )] in toluene at room temperature followed by reduction with 2 equiv of potassium graphite (KC 8 ) leads to borylene [(SiNSi)B] ( 1 ), isolated as blue crystals in 45% yield. X-ray crystallography shows that borylene ( 1 ) has a tricoordinate boron center with a distorted T-shaped geometry. Computational studies reveal that the HOMO of 1 represents the lone pair orbital on the boron center and is delocalized over the Si–B–Si unit, while the geometric perturbation significantly increases its energy. Borylene ( 1 ) shows single electron transfer reactivity toward tris(pentafluorophenyl)borane (B(C 6 F 5 ) 3 ), forming a frustrated radical pair [(SiNSi)B] •+ [B(C 6 F 5 ) 3 ] •–, which can be trapped by its reaction with PhSSPh, affording an ion pair [(SiNSi)BSPh][PhSB(C 6 F 5 ) 3 ] ( 3 ). Remarkably, the cooperation between borylene and silylene allows the facile cleavage of the N–H bond of aniline, the P–P bond in white phosphorus, and the C═O bond in ketones and carbon dioxide, thus representing a new type of main-group element-ligand cooperativity for the activation of small molecules. In addition, 1 is a strikingly effective catalyst for carbon dioxide reduction. Computational studies reveal that the cooperation between borylene and silylene plays a key role in the catalytic chemical bond activation process.