Bis(pyridino-o-carboranyl)phosphenium Cation: A Platform for Dual-Site Reactivity

Phosphenium salt, [L₂P⁺][B(C₆F₅)₄], substituted by two pyridino-ortho-carboranyl groups (L), was prepared, fully characterized, and its preliminary reactivity with small molecules was studied. Surprisingly, cation [L₂P⁺][B(C₆F₅)₄] reacts differently with O-H and N-H bonds. With RO-H (R = H, Me, Ph), the reaction takes place at P center in [L₂P⁺][B(C₆F₅)₄], via hydrolysis or alcoholysis of the P-C bonds, leading to the carboranopyridinium salt ([LH₂⁺][B(C₆F₅)₄]) and the complementary (RO)₂P(O)H. In contrast, the reaction of [L₂P⁺][B(C₆F₅)₄] with R₂N-H (Et₂NH, Ph(H)NH, ⁱPr(H)NH) does not proceed at the P center, but takes place at the ortho-carboranyl (oCb) moiety by deboronation, producing zwitterionic phosphenium (L(L^nido)P⁺), with significant reduction in the reactivity of the P center compared to L₂P⁺. Remarkably, [L₂P⁺][B(C₆F₅)₄] reacts with SO₂ in MeCN at both sites, the P and oCb, stripping both oxygens from sulfur, leading to zwitterionic phosphonium sulfide (L(L^nido)P⁺═S).