Supraicosahedral Polyhedra in Metallaboranes: Synthesis and Structural Characterization of 12‑, 15‑, and 16-Vertex Rhodaboranes

Syntheses and structural characterization of supraicosahedral rhodaborane clusters are reported. Reaction of [(Cp*RhCl₂)₂], (Cp* = η⁵-C₅Me₅) with [LiBH₄·thf] followed by thermolysis with excess of [BH₃·thf] afforded 16-vertex <i>closo</i>-[(Cp*Rh)₃B₁₂H₁₂Rh­{Cp*RhB₄H₉}], <b>1</b>, 15-vertex [(Cp*Rh)₂B₁₃H₁₃], <b>2</b>, 12-vertex [(Cp*Rh)₂B₁₀H_<i>n</i>(OH)_<i>m</i>], (<b>3a</b>: <i>n</i> = 12, <i>m</i> = 0; <b>3b</b>: <i>n</i> = 9, <i>m</i> = 1; <b>3c</b>: <i>n</i> = 8, <i>m</i> = 2) and 10-vertex [(Cp*Rh)₃B₇H₇], <b>4</b>, and [(Cp*Rh)₄B₆H₆], <b>5</b>. Cluster <b>1</b> is the unprecedented 16-vertex cluster, consists of a sixteen-vertex {Rh₄B₁₂} with an <i>exo</i>-polyhedral {RhB₄} moiety. Cluster <b>2</b> is the first example of a carbon free 15-vertex supraicosahedral metallaborane, exhibits icosihexahedron geometry (26 triangular faces) with three degree-six vertices. Clusters <b>3a</b>-<b>c</b> have 12-vertex <i>isocloso</i> geometry, different from that of icosahedral one. Clusters <b>4</b> and <b>5</b> are attributed to the 10-vertex <i>isocloso</i> geometry based on 10-vertex bicapped square antiprism structure. In addition, quantum-chemical calculations with DFT methods at the BP86 level of theory have been used to provide further insight into the electronic structure and stability of the optimized structures which are in satisfactory agreement with the structure determinations. All the compounds have been characterized by IR, ¹H, ¹¹B, ¹³C NMR spectroscopy in solution, and the solid state structures were established by crystallographic analysis of compounds <b>1</b>–<b>5</b>.