Novel tetraalkyltetraboranes of the type B4R4, B4H2R4 and B4H4R4

Article abstract of DOI:10.1016/s0020-1693(99)00058-4

The reduction of RBHal₂ (Hal=F, Cl, Br) with Na/K alloy gives the tetraboranes B₄R₄, B₄H₂R₄ or their mixtures, depending on temperature, reaction time and solvent. We describe here B₄^tBu₄, B₄(CH₂^tBu)₄, B₄H₂^tBu₄, B₄H₂(CH₂^tBu)₄ and B₄H₂(CMe₂Et)₄. B₄^tBu₂(CH₂^tBu) ₂ is formed by reducing the diborane Cl-B(^tBu)B(CH₂^tBu)-Cl with Li. The conversion of B₄H₂^tBu₄ into B₄H₄^tBu₄ proceeds in three steps: (i) reduction (→[B₄H₂^tBu₄]²);(ii) first protonation (→[B₄H₃^tBu₄]^-), and (iii) second protonation (→B₄H₄^tBu₄). The hypercloso-species B₄R₄ have tetrahedral structures. The two opposite edges of a B₄ tetrahedron of the closo-B₄H₂R₄ are bridged by H atoms (D_2d). The nido-species have a bicyclobutane-type B₄ skeleton; three of the four outside edges are bridged by H atoms; two terminal ligands are bound to one of the B atoms: H in exo- and^tBu in endo-position (C₁). Structural evidence is based on low-temperature NMR spectra, ab initio geometries, and IGLO(DZ) calculations of the¹¹B NMR shifts. Dynamical behavior of bridging H atoms is indicated by the NMR spectra at higher temperature.