The structure of 1,2-diaryl-1,2-bis(dimethylamino)diboron compounds and related species: boron–nitrogen analogues of buta-1,3-dienes

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Abstract
The 1,2-dihalogenodiborane(4) compound Br(Me2N)BB(NMe2)Br 1 reacts with Li(mes)(mes = C6H2Me3-2,4,6) or LiPh to form the derivatives Br(Me2N)BB(NMe2)(mes)2, (mes)(Me2N)BB(NMe2)(mes)3, and the previously reported (synthesised by a different route) species Ph(Me2N)BB(NMe2)Ph 4 in moderate yields. Compounds 2–4 are thermally stable, colourless, crystalline solids. All compounds were characterized by X-ray crystallography and NMR spectroscopy. Their structures have B–B bond lengths that vary from 1.682(16)Å in 1 to 1.717(15)Å in 3. In addition, all compounds display high angles (from 58.9 to 88.7 °) between the planes at the boron atoms and short B–N distances that are near 1.39 Å. The structures of 1–4 represent, apparently, the first such acyclic examples for diborane(4) derivatives of the type B2X2(NR2)2 where X = halogen or aryl group. Crystal data: 130 K: 1, Br(Me2N)BB(NMe2)Br, monoclinic, space group P21, a= 6.789(2), b= 7.720(2), c= 10.012(4), β= 103.06(3)°, Z= 2, R= 0.048 for 1109 [I > 2.0σ(I)] data; 2, Br(Me2N)BB(NMe2)(mes), monoclinic, space group C2/c, a= 24.890(14), b= 7.662(4), c= 18.500(12), β= 116.36(4)°, Z= 8, R= 0.063 for 1910 [I > 2.0σ(I)] data; 3(mes)(Me2N)BB(NMe2)(mes), orthorhombic, space group Pbcn, a= 37.983(14), b= 14.167(5), c= 12.282(4)Å, Z= 12, R= 0.095 for 1829 [I > 3.0σ(I)] data; 4, Ph(Me2N)BB(NMe2)Ph, orthorhombic, space group P212121, a= 8.579(2), b= 12.920(4), c= 13.925(3)Å, Z= 4, R= 0.045 for 1776 [I > 4.0σ(I)] data.
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