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.