B-B single bond activation at the low-valent single nickel center supported by a PPP ligand

Abstract

Diborane(4) compounds have been extensively utilized as powerful borylating reagents to synthesize organoboron compounds such as arylboronates commonly used as nucleophilic partners in Suzuki–Miyaura cross–coupling reactions. In recent years, these diborane(4) compounds are further applied to a variety of intriguing reactions including metal-catalyzed C–H borylation and CO$_2$ reduction by virtue of their ease of handling, synthetic accessibility, and reliability. One of the crucial steps in these reactions is a B–B single bond activation, which is generally proposed either oxidative addition or σ-bond metathesis. Depending on the strength of the π-backdonation, σ-diborane(4) complexes can be formed known as a transition state or B–B bond scission can occur to form diboryl complexes resulted from oxidative addition. Although several boryl complexes have been reported, σ-diborane(4) complexes have not been synthesized and characterized until now. Only a theoretical study was reported with a unique Y-shaped three-coordinate Pt complex by Sakaki and coworkers. In this study, the first example of an unusual σ-diborane(4) nickel complex, (PP$^{Me}$P)Ni(η$^2$-B$_2$cat$_2$), was synthesized by the reaction of {(PP$^{Me}$P)Ni}$_2$(μ-N$_2$) with B$_2$cat$_2$. The σ-diborane(4) complex of nickel was successfully characterized by various spectroscopic techniques, single-crystal X-ray diffraction analyses, and DFT studies. The reactivity of the σ-diborane(4) complex was investigated by the reaction with small molecules such as H$_2$, CO, and CO$_2$.