Novel tetraalkyltetraboranes of the type B4R4, B4H2R4 and B4H4R4

The reduction of RBHal(2) (Hal = F, Cl, Br) with Na/K alloy gives the tetra boranes B4R4, B4H2R4 or their mixtures, depending on temperature, reaction time and solvent. We describe here (B4Bu4)-Bu-t, B-4((CH2Bu)-Bu-t)(4) (B4H2 Bu4)-Bu-t, B4H2((CH2Bu)-Bu-t)(4) and B4H2(CMe2Et)(4). (B4Bu2)-Bu-t((CH2Bu)- Bu-t)(2) is formed by reducing the diborane Cl-B(Bu-t)-B((CH2Bu)-Bu-t)-Cl w ith Li. The conversion of (B4H2Bu4)-Bu-t into (B4H4Bu4)-Bu-t proceeds in th ree steps: (i) reduction (-->[(B4H2Bu4)-Bu-t](2-)); (ii)first protonation ( -->[(B4H3Bu4)-Bu-t](-)), and (iii) second protonation (-->(B4H4Bu4)-Bu-t). The hypercloso-species B4R4 have: tetrahedral structures. The two opposite edges of a B-4 tetrahedron of the closo-B4H2R4 are bridged by H atoms (D-2d ). The nido-species have a bicyclobutane-type B-4 skeleton; three of the fo ur outside edges are bridged by H atoms; two terminal ligands are bound to one of the B atoms: H in exo- and Bu-t in endo-position (C-1). Structural e vidence is based on low-temperature NMR spectra, ab initio geometries, and IGLO(DZ) calculations of the B-11 NMR shifts. Dynamical behavior of bridgin g H atoms is indicated by the NMR spectra at higher temperature. (C) 1999 E lsevier Science S.A. Pill rights reserved.