Two Forgotten Ten-Vertex arachno Triheteroboranes: arachno-5,6,9-C2SB7H11 and arachno-5,6,9-C3B7H13, Their Molecular Structure Determination by ab initio/NMR Approach and Synthesis of the Thiacarbaborane

A solution of nido-5,6-C2B8H12 reacted in the presence of Et3N with sulfur to yield the first mixed triheteroborane based on the [arachno-B10H14]2- framework (C2v symmetry), i.e. arachno-5,6,9-C2SB7H11. This compound is isoelectrolobal with known arachno-5,6,9-C3B7H13. The structures of these two ten-vertex arachno triheteroboranes have been established by the ab initio/IGLO (and GIAO-SCF)/NMR method. The presence of three heteroatoms and the absence of the hydrogen atom bridging the 5 and 10 atoms are responsible for a symmetry reduction from C2v to C1 in both compounds. The carbon atoms are compressed toward the center and the sulfur atom is pushed away from the center of the respective cluster relative to the positions they would have in [arachno-B10H14]2-. These distortions from the parent compound are well described by the MP2/6-31* geometrical parameters. A good fit between the computed and measured 11B chemical shifts revealed that the MP2/6-31G* internal coordinates are good representations of the molecular geometries of these two ten-vertex arachno heteroboranes.