Transformation of 3, 4-Bis(isopropylidene)-2, 5-dichloro-1, 2, 5-thiadiborolane into nido-2, 4, 5-Thiadicarbahexaborane(5) Derivatives - Formation and Computational Elucidation of a Thiadicarbanonaborane(8) with a ni-9 Configuration

摘要

The 3, 4-bis(isopropylidene)-2, 5-dichloro-1, 2, 5-thiadiborolane (5b), obtained from 3, 4-bis(dichloroboryl)-2, 5-dimethyl-2, 4-hexadiene (4) and (Me_3Si)_2S, reacts with Li[RBH_3] (R = H, C_6H_5, C_6Me_4H) to yield the corresponding derivatives of the nido-4, 5-diisopropyl-2, 4, 5-thiadicarbahexaboranes 2. Replacements of the chlorine atoms in 5b with two hydrido, or with one hydrido and one aryl (phenyl or duryl) group, followed by the hydroboration of the isopropylidene substituents with RBH_2 (R = H, C_6H_5, C_6Me_4H) lead to four nido-2, 4, 5-thiadicarbahexaboranes (2a-d) in low yields. Their composition follows from MS and NMR data; not refined X-ray structural data of 2d support the proposed skeletal structure. In addition to 2a, larger thiacarboranes were detected by GC/MS and ~(11)B NMR. The structure of the nine-vertex cluster 6a could be identified by applying the ab initio/IGLO/NMR method. Geometry optimizations at the MP2(fc)/6-31 G~* level rule out the C_s nido-SC_2B_6H_8 isomer 7. The C_(2v) structure of 6 consists of a nine-vertex cluster with two elongated B-B distances. As this 22e cluster 6 represents an exception to the Wade rules, the nature of the bonding of nine-vertex clusters with different electron counts is discussed.