Borane complexes energies

Weibel N, Charbonniere LJ, Guardigli M, Roda A, Ziessel R. Pyrromethene dialkynyl borane complexes for "cascatelle" energy transfer and protein labeling. Angew Chem Int Ed Engl 2005 44 3694-8. 

If similar processes could be developed at energy conversion efficiency levels that are comparable to the present day SMR-based NH3 synthesis plants, then it would be possible to realize a major reduction in the production costs of ammonia-borane complex. We note that a concept similar to that discussed above has already been developed for nitric acid synthesis process based on boron nitride analogous to the Haber-Bosch route for nitric acid production from NH3. Finally, recent results have shown that unusual parallel behavior exists between hydrocarbons and their corresponding B-N analogues. Thus, hydrogenation of benzene to cyclohexane may also provide a model for the reformation of borazine to other amine-boranes. 

A. T-Raissi, "Ammonia and Ammonia-Borane Complexes as Hydrogen Energy Storers for Fuel Cell Applications," at Session A2.8 Hydrides II, 14th WHEC, Montreal, Canada, June 10 (2002). 

N- -B bond distances and dissociation energies in substituted amine borane complexes... 

Table 16.1. Dissociation energies (at 298 K) and N — B bond distances in gaseous substituted amine borane complexes.

Like the borane complexes described previously, trimethylalane complexes are sensitive both the change of state from gas to crystal and to substitution effects The N->A1 bond distance in (CH3 )3N A1(CH3 )a decreases to from about 210 pm in the gas phase to 205 pm in the solid phase. Exchange of the methyl groups on A1 by more electronegative Cl atoms to give (CH3)3NA1C13 increases the dative bond energy from about 126 to about 196 kJ moP and decreases the bond distance in the solid phase to 195 pm. 

In 2008, Ueng et al. [73] hypothesized that complexes of NHCs and borane would have weaker B—H bonds than borane alone and would thus be candidates for hydrogen donation reactions. They synthesized two NHC-borane complexes (Figure 15.18) from the respective NHCs and BH3.THF. Bond dissociation energies (BDE) were calculated for the synthesized complexes as well as for BH3 and BH3-THF. It was found that THF lowers the BDE by about 2kcalmol, while the NHCs lower the BDE by a substantial 74-80 kcal mol . ... 

Two modifications of crystalline boron nitride are known. In the cubic modification, each B atom is surrounded by four N atoms at the comers of a tetrahedron and vice versa. The mean bond energy calculated from the energy of atomization is 321 kJ mol and about 2.5 times larger than the N-B bond dissociation energy of the amine borane complex. 

Both experimental [7] and theoretical [8] investigations have shown that the anti complexes of acrolein and boranes are the most stable and the transition states were located only for these four anti complexes. The most stable transition-state structure was calculated (RHF/3-21G) to be NC, while XT is the least stable of the four located. The activation energy has been calculated to be 21.6 kcal mol for the catalyzed reaction, which is substantially above the experimental value of 10.4 1.9 kcal mol for the AlCl3-catalyzed addition of methyl acrylate to butadiene [4a]. The transition-state structure NC is shown in Fig. 8.5. 

In order to estimate and compare the magnitude of the M-B interactions in these isoelectronic complexes, a whole set of structural and spectroscopic parameters determined experimentally and/or computed theoretically were considered. This includes the M - B distance the ratio r between the M -B distance and the sum of covalent radii (to take into account the different sizes of the metals involved), the pyramidaliza-tion of the boron environment XB, the rlB NMR chemical shift <5 11B, the difference AqB between the charge at boron in the metal boratrane and the free ligand TPB, the difference A M between the charge at the metal in the metal boratrane and that in the related borane-free complex [M(i-Pr2PPh)3], and the NBO delocalization energy A NBo associated with the main donor-acceptor M-B interaction found at the second order in the NBO analysis (Table 2). Only the conclusions of this detailed analysis will be recalled here ... 

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