Amine boranes, polyboranes and boron -based polymers: Syntheses and reactivities

Iodine oxidation of B3H8– in glyme solution to produce (glyme)B3H7, followed by displacement of the coordinated glyme by reaction with anhydrous ammonia provides a safe and convenient preparation of ammonia triborane, NH3B 3H7. X-ray crystallographic determinations and DFT computational studies of both NH3B3H7 and the NH3 B3H7·18-crown-6 adduct demonstrate that while computations predict a symmetric single bridging-hydrogen conformation, NH3B3H7 has a highly asymmetric structure in the solid-state that results from intermolecular N-H+-H –-B dihydrogen bonding interactions. Comparisons of the experimentally-observed solution and DFT/GIAO-calculated 11B NMR chemical shifts for NH3B3H7 indicate that these intermolecular interactions also lead to aggregate formation in solution. Studies of its hydrolytic reactions have shown that upon the addition of acid or an appropriate transition metal catalyst, aqueous solutions of NH3B3H 7 rapidly release hydrogen, with 6.1 materials-wt% H2-release being achieved from a 22.7-wt% aqueous solution of NH3B3H 7 at room temperature in the presence of 5wt%-Rh/Al2O 3 (1.1 mol%). The rate of H2-release can be controlled by both the catalyst loadings and temperature. Likewise, NH3B 3H7 was shown to act as an efficient promotor for thermolytic H2 release from NH3B3 when used with proton sponge. Model studies suggested that the [B3H7NH 2]– anion may be a key intermediate for the anionic polymerization of NH3B3. Quantum mechanical computational studies of possible mechanistic pathways for B10H13– dehydrogenative alkyne-insertion and olefin-hydroboration reactions demonstrate that, depending on the reactant and reaction conditions, B10H13– can function as either an electrophile or nucleophile. For reactions with nucleophilic alkynes, such as propyne, the calculations indicate that at the temperatures (∼110-120°C) required for these reactions, the ground state B10H13– (1) structure can rearrange to an electrophilic-type cage structure 3 having a LUMO orbital strongly localized on the B6 cage-boron. Alkyne binding at this site followed by subsequent steps involving the formation of additional boron-carbon bonds, hydrogen-elimination, protonation and further hydrogen-elimination then lead in a straightforward manner to the experimentally observed ortho-carborane products resulting from alkyne-insertion into the decaborane framework. A similar mechanistic sequence was identified for the reaction of propyne with 6-R-B10H12– leading to the formation of 1-Me-3-R-1,2-C2B10H 11 carboranes. On the other hand, both B10H13 – and 4,6-C2B7H12– have previously been shown to react at much lower temperatures with strongly polarized alkynes and the DFT and IRC calculations support an alternative mechanism involving initial nucleophilic attack by these polyborane anions at the positive terminal acetylenic carbon to produce terminally substituted olefinic anions. In the case of the B10H13– reaction, subsequent cyclization steps were identified that provide a pathway to the experimentally observed arachno-8-(NC)-7,8-C 2B10H14– carborane. The computational study of B10H13– propylene-hydroboration also supports a mechanistic pathway involving a cage rearrangement to the electrophilic 3 structure. Olefin-binding at the LUMO orbital localized on the B6 cage-boron, followed by addition of the B6-H group across the olefinic double bond and protonation then leads to the experimentally observed 6-R-B10H13 products. Poly(6-hexenyldecaborane) polymers (PHDB\u27s) were prepared from 6-(CH2=CH(CH2)4)-B10H14 (6-hexenyldecaborane, HDB) using Brookhart catalysts. Molecular weight measurements with GPC using both multiangle laser light scattering (MALLS) and differential refractive index (DRI) detectors showed that the molecular weight distributions of the resulting PHDB\u27s were multimodal, while the resulting PHOC\u27s showed well-defined molecular weight distributions that were 4-5 times higher than those obtained by using the previously reported early transition-based catalyst system, Cp2ZrMe2/B(C 6F5)3