Borane reaction chemistry. Alkyne insertion reactions into boron-containing clusters. Products from the thermolysis of [6,9-(2-HC≡C-C 5H4N)2-arachno-B10H12]

The stirring of [ortho-(HC≡C)-C5H4N] with [nido-B10H14] in benzene affords [6,9-{ortho-(HC≡C)- C5H4N}2-arachno-B10H12] 1 in 93% yield. In the solid state, 1 has an extended complex three-dimensional structure involving intramolecular dihydrogen bonding, which accounts for its low solubility. Thermolysis of 1 gives the known [1-(ortho-C5H 4N)-1,2-closo-C2B10H11] 2 (13%), together with new [μ-5N,6C-(NC5H 4-ortho-CH2)-nido-6-CB9H10] 3 (0.4%), [μ-7C,8N-(NC5H4-ortho- CH2)-nido-7-CB10H11] (0.4%) 4, binuclear [endo-6′-(closo-1,2-C2B10H10)-μ- (1C,exo-6′N)-(ortho-C5H 4N)-μ-(exo-8′C,exo-9′N)-(ortho- (CH2CH2)-C5H4N)-arachno-B 10H10] (0.5%) 5, and [exo-6C-endo-6 N-(ortho-(CH≡CH)-C5H4N)-exo-9 N-(ortho-(HC≡C)-C5H4N)-arachno-B 10H11] 6. An improved solvent-free route to 2 is also presented. This set of compounds features an increasing cluster incorporation of the ethynyl moiety, initially by an effective internal hydroboration, affording an arachno to nido and then a nido to arachno:closo sequence of cluster geometry. An alternative low-temperature route to internal hydroboration is demonstrated in the room temperature reaction of [closo-B11H 11][NnBu4]2 with CF3COOH and [ortho-(HC≡C)-C5H4N], which gives [μ-1 C,2B-[ortho-C5H4N-CH 2]-closo-1-CB11H10] 7 (40%) in which one carbon atom is incorporated into the cluster; a similar reaction with [ortho-(N≡C)-C5H4N] affords [NnBu 4][7-(ortho-N≡C-C5H4N)-nido-B 11H12], 8 (68%) and stirring [ortho-(N≡C)-C 5H4N] with [nido-B10H14] quantitatively affords the cyano analogue of 1, [6,9-{ortho-(N≡C)-C 5H4N}2-arachno-B10H12] 9. All compounds were characterised by single-crystal X-ray diffraction analysis and NMR spectroscopy. © The Royal Society of Chemistry.