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A set of diboryldiborenes are prepared by the mild, catalyst-free, room-temperature diboration of the B-B triple bonds of doubly base-stabilized diborynes. Two of the product diboryldiborenes are found to be air- and water-stable in the solid state, an effect that is attributed to their high crystallinity and extreme insolubility in a wide range of solvents.

Molecules containing multiple bonds between atoms-most often in the form of olefins-are ubiquitous in nature, commerce, and science, and as such have a huge impact on everyday life. Given their prominence, over the last few decades, frequent attempts have been made to perturb the structure and reactivity of multiply-bound species through bending and twisting. However, only modest success has been achieved in the quest to completely twist double bonds in order to homolytically cleave the associated π bond. Here, we present the isolation of double-bond-containing species based on boron, as well as their fully twisted diradical congeners, by the incorporation of attached groups with different electronic properties. The compounds comprise a structurally authenticated set of diamagnetic multiply-bound and diradical singly-bound congeners of the same class of compound.

Precatalysts active for the dehydropolymerisation of primary amine-boranes are generally based on mid or late transition metal. We have found that the activity of the precatalyst system formed from Cp R 2 TiCl 2 and 2 n BuLi towards the dehydrogenation of the secondary amine-borane Me 2 NH·BH 3 , to yield the cyclic diborazane [Me 2 N-BH 2 ] 2 , increases dramatically with increasing electron-donating character of the cyclopentadienyl rings (Cp R ). Application of the most active precatalyst system (Cp R = η-C 5 Me 5 ) to the primary amine-borane MeNH 2 ·BH 3 enabled the first synthesis of high molar mass poly( N -methylaminoborane), [MeNH-BH 2 ] n , the BN analogue of polypropylene, by an early transition metal such as catalyst. Significantly, unlike other dehydropolymerization precatalysts for MeNH 2 ·BH 3 such as [Ir(POCOP)H 2 ], skeletal nickel, and [Rh(COD)Cl] 2 , the Ti precatalyst system was also active towards a range of substrates including BzNH 2 ·BH 3 (Bz = benzyl) yielding high molar mass polymer. Moreover, in contrast to the late transition metal catalysed dehydropolymerisation of MeNH 2 ·BH 3 and also the Ziegler-Natta polymerisation of olefins, studies indicate that the Ti-catalyzed dehydropolymerization reactions proceed by a step-growth rather than a chain-growth mechanism.

There is currently tremendous interest in the previously documented example of a stable species exhibiting a boron-boron triple bond ( Science , 2012, 336 , 1420). Notably, it has recently been stated using arguments based on force constants that this diboryne may not, in reality, feature a boron-boron triple bond. Here, we use advanced solid-state NMR and computational methodology in order to directly probe the orbitals involved in multiple boron-boron bonds experimentally via analysis of 11 B- 11 B spin-spin ( J ) coupling constants. Computationally, the mechanism responsible for the boron-boron spin-spin coupling in these species is found to be analogous to that for the case of multiply-bonded carbon atoms. The trend in reduced J coupling constants for diborenes and a diboryne, measured experimentally, is in agreement with that known for alkenes and alkynes. This experimental probe of the electronic structure of the boron-boron multiple bond provides strong evidence supporting the originally proposed nature of the bonds in the diboryne and diborenes, and demonstrates that the orbitals involved in boron-boron bonding are equivalent to those well known to construct the multiple bonds between other second-row elements such as carbon and nitrogen.