E-H (E = B, Si, C) Bond Activation by Tuning Structural and Electronic Properties of Phosphenium Cations

In this work, strategic enhancement of electrophilicity of phosphenium cations for the purpose of small-molecule activation was described. Our synthetic methodology for generation of novel two-coordinate phosphorus(III)-based compounds [{C₆H₄(MeN)₂C}₂C·PR]²⁺ ([2a]²⁺, R = NⁱPr₂; [2b]²⁺, R = Ph) was based on the exceptional electron-donating properties of the carbodicarbene ligand (CDC). The effects of P-centered substituent exchange and increase in the overall positive charge on small substrate activation were comparatively determined by incorporating the bis(amino)phosphenium ion [(ⁱPr₂N)₂P]⁺ ([1]⁺) in this study. Implemented structural and electronic modifications of phosphenium salts were computationally verified and subsequently confirmed by isolation and characterization of the corresponding E-H (E = B, Si, C) bond activation products. While both phosphenium mono- and dications oxidatively inserted/cleaved the B-H bond of Lewis base stabilized boranes, the increased electrophilicity of doubly charged species also afforded the activation of significantly less hydridic Si-H and C-H bonds. The preference of [2a]²⁺ and [2b]²⁺ to abstract the hydride rather than to insert into the corresponding bond of silanes, as well as the formation of the carbodicarbene-stabilized parent phosphenium ion [{C₆H₄(MeN)₂C}₂C·PH₂]⁺ ([2·PH₂]⁺) were experimentally validated.