Borane reaction chemistry. Alkyne insertion reactions into boron-containing clusters. Products from the thermolysis of [6,9-(2-HC C-C5H4N)(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-C5H4N)-1,2-closo-C2B10H11] 2 (13%), together with new [mu-5(N),6(C)-(NC5H4-ortho-CH2)-nido-6-CB9H10] 3 (0.4%), [mu-7(C),8(N)-(NC5H4-ortho-CH2)-nido-7-CB10H11] (0.4%) 4, binuclear [endo-6'-(closo-1,2-C2B10H10)-mu-(1(C),exo-6'(N))-(ortho-C5H4N)-mu-(exo-8'(C),exo-9'(N))-(ortho-(CH2CH2)-C5H4N)-arachno-B10H10] (0.5%) 5, and [exo-6(C)-endo-6(N)-(ortho-(CH=CH)-C5H4N)- exo-9(N)-(ortho-(HC=C)-C5H4N)-arachno-B10H11] 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-B11H11][(NBu4)-Bu-n](2) with CF3COOH and [ortho-(HC=C)-C5H4N], which gives [mu-1(C),2(B)-[ortho-C5H4N-CH2]-closo-1-CB11H10] 7 (40%) in which one carbon atom is incorporated into the cluster; a similar reaction with [ortho-(N=C)-C5H4N] affords [(NBu4)-Bu-n][7-(ortho-N=C-C5H4N)-nido-B11H12], 8 (68%) and stirring [ortho-(N=C)C5H4N] with [nido-B10H14] quantitatively affords the cyano analogue of 1, [6,9-{ortho-(N=C)-C5H4N}(2)-arachno- B10H12] 9. All compounds were characterised by single-crystal X-ray diffraction analysis and NMR spectroscopy.