Synthesis and Reactivity of o‑Phosphane Oxide Substituted Aryl(hydro)borates and Aryl(hydro)boranes

The reaction of 1,2-C₆H₄(P­(O)-t-Bu₂)­(Li) (Ph*Li) with B­(OMe)₃ furnishes a mixture of Li­[Ph*B­(OMe)₃] (Li­[2a]) and Ph*B­(OMe)₂ (2b). Further treatment with Li­[AlH₄] provides the trihydroborate Li­[Ph*BH₃] (Li­[3]), which can subsequently be converted into the intramolecular PO–B adduct Ph*BH₂ (4) through hydride abstraction with Me₃SiCl. Addition of C₆F₅MgBr yields Mg­[Ph*BH₂(C₆F₅)]₂ (Mg­[5]₂), which is inert toward Me₃SiCl but reacts with water to give Ph*B­(H)­C₆F₅ (6). Upon addition of further C₆F₅MgBr, a mixture is formed, from which crystals of Mg­[Ph*BH­(C₆F₅)₂]₂ (Mg­[7]₂) were obtained. The reaction of Ph*Li with (C₆F₅)₂BH·SMe₂ provides access to Li­[7], but again with limited product selectivity. The targeted acidic hydrolysis of Li­[7] furnishes Ph*B­(C₆F₅)₂ (8), while Mg­[7]₂ reacts back to 6. The anions of the hydroborates Li­[3] and Mg­[5]₂ act as BH₂,O-chelating ligands toward their metal ions. Therefore, Li­[Ph*₂BH₂] (Li­[13]) was also synthesized to obtain the corresponding pincer-type species. 4, 6, and 8 exist as water-stable intramolecular PO–B adducts both in solution and in the solid state. X-ray crystallography and ³¹P NMR spectroscopy indicate an increase in the Lewis acidity of the boryl groups in the order 4 < 6 < 8.