1,3,2-Dioxaphospholanes with an Annelated 1,2-Dicarba-closo-dodecaborane(12) Unit: Formation and Dimerization

By using 1,2-hydroxy-1,2-dicarba-closo-dodecaborane(12) and the corresponding dilithium salt [1,2-(LiO)(2)-1,2-C2B10H10] for the synthesis of 1,3,2-dioxaphospholanes, it was shown that these species are short-lived with few exceptions. The phosphorus halides undergo disproportionation reactions towards phosphite derivatives, whereas the P-organo-substituted five-membered rings dimerize to give ten-membered rings. Even the P-diethylamino and the P-ethoxy compounds dimerize slowly. Dimerization is the favored process when using the dilithium salt together with ether. The reactions were monitored by multinuclear magnetic resonance spectroscopy (H-1, B-11, C-13, P-31 NMR). The gas-phase structures were optimized by DFT methods [B3LYP/6-311+G(d,p) level of theory], and NMR spectroscopic parameters were calculated. One dimer was characterized by X-ray analysis. All products were found to be sensitive to hydrolysis and oxidation. Thus, crystals of 1-[(1,1-dimethylethyl)hydroxyphosphinyl]oxy-2-hydroxy-1,2-dicarba-closo-dodecaborane(12) could be isolated and characterized by X-ray analysis, as well as the starting material 1,2-hydroxy-1,2-dicarba-closo-dodecaborane(12) (with HNEt2/HCl). In all three solid-state structures, the C-C(carborane) bond lengths are elongated with respect to all other neutral ortho-carborane derivatives bearing second-row substituents at one or both carbon atoms.