Structure of Iminoboranes

The BN bonding energy was found to be 88 kcal/mol. When compared with 94 kcal/mol for the isoelectronic ethyne, the B—N bond in HBNH seems to be somewhat weaker than a C=C triple bond. (Both values were calculated without taking electron correlation into account and would be lower, therefore, than values from experiments this does not limit a qualitative comparison.) The total BN overlap population was found to be 0.765, while the corresponding values for a B—N bond in the cyclooligomers (HBNH)2 and (HBNH)j are 0.392 and 0.425, respectively. 

Apparently there is a substantial contribution to the B—N bond of HBNH from one a- and two orthogonal 7i-bonds. Expressed in simple terms, there is a B=N triple bond in iminoboranes. Concerning a structural formula for HBNH, the real situation is represented best by H—B=N—H. 

The compound HBNH is expected to be thermodynamically unstable toward oligomerization. The cyclodimer, (HBNH)2, isoelectronic with cyclobutadiene, is found to be more stable by 63.3 kcal/mol, and the cyclotrimer, (HBNHlj, by 193.8 kcal/mol than 2 or 3 mol of HBNH, respectively (23). The oligomerization energy is due to an increase in the number of (7-bonds at the expense of the relatively weak Tt-bonds. Note that the situation is the same for acetylene. Oligomers like cyclobutadiene, benzene, benzvalene, and cyclooctatetraene are 

From gross electronic populations, the excess atomic charges can be derived as follows  

A striking difference between alkynes and iminoboranes appears to be their kinetic stability. As was pointed out in Section II, iminoboranes are metastable, in general, at temperatures far below room temperature. Alkynes are also metastable, but their stabilization requires either high temperature or effective catalysts. We assume the polarity of the B—N bond to be a chief reason for these differences. This idea is supported by the observation that strongly polar alkynes (e.g., FC=CH, FC=CfBu) do oligomerize or polymerize at room temperature quite rapidly (25). Polar additions will generally be the predominant reaction for iminoboranes (Sections V,VI). 

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Infrared spectroscopy is an excellent tool in iminoborane chemistry, which readily permits, to distinguish between iminoboranes and nitrile-borane adducts and to identify monomeric and dimeric forms of iminoboranes. This event is due to the fact that the i>CN of CN multiple bonds absorbs outside the fingerprint region and can be considered to be a valuable group frequency even when mixed with other vibrational modes. In some cases other vibrations like NH, BH, B-halogen or B-S stretching modes are helpful for determining the structure of iminoboranes. 

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