Overview of Results:
We successfully developed the high yielding synthesis of a family of sp2-sp3 diboron(5) species, e.g. CatB-BCl2(NHC) (1, Figure 1, NHC = N-heterocyclic carbene), in two steps from commercial materials. The synthesis consists of combing a NHC adduct of B2(cat)4 (cat=catechol) with BCl3 to form 1. This synthesis works with an array of different NHCs and with different boron trihalides (BCl3 and BBr3).
Our next objective was to access and assess the reactivity of borocations derived from 1 by halide abstraction. A range of halide abstracting agents were reacted with 1 including neutral halophilic Lewis acids (e.g. AlCl3) and Na salts of weakly coordinating borate anions. However, only decomposition products were observed (e.g. CatBCl). Attempts to trap the putative cation [CatB-B(X)(NHC)]+ by using pi nucleophiles or small molecules (e.g. H2) to react with it led to complex mixtures or again formation of CatBCl and other unidentified boron products. To overcome this, we performed the halide abstraction in the presence of Lewis bases (LB = 2-DMAP and PPh3). This strategy resulted in the formation of new Lewis base-stabilised diboron(5) monocations, e.g. CatB-B(Cl)(NHC)(LB) (2, Figure 1). However, these stabilised diboron(5) cations proved insufficiently electrophilic to activate pi nucleophiles.
Due to the instability of the diboron(4) cation derived from halide abstraction from 1, our next target was exchanging halide for an alternative substituent, specifically CatB-BH2(NHC), 3. The reaction of 1 with tin hydrides and a catalytic amount of halide abstractor resulted in the formation of the aforementioned dihydro-diboron compound, (3, Figure 1). Again in our hands it was not possible to achieve the synthesis and isolation of the putative 1-hydrodiboron(4) monocation, [CatB-B(H)(NHC)]+ derived from hydride abstraction due to its instability, with CatBH observed to form rapidly on hydride abstraction from 3. Again attempts to assess the reactivity of the cation derived from 3 by synthesising it in the presence of small molecules or pi nucleophiles led to complex intractable mixtures or the same outcome as in their absence (formation of CatBH).
The other key objective was using CatB-BX2(NHC) precursors to form B=B containing systems. Thus reaction of the dibromo congener, CatBBBr2(NHC) 4, with reducing agents (e.g. KC8) resulted in the reductive coupling of two diboron units forming the desired coplanar B4 chains (Figure 2). DFT calculations showed that the HOMO is strongly stabilized by pi delocalisation from the B=B bond to the outer boron atoms. These are the first boryl functionalized diborenes, thus are the first boron chain compounds showing extended pi conjugation along multiple (>2) boron centres.
Dissemination Summary:
This work has been presented by the Marie Curie Fellow, Dr. Jessica Cid, at the Summer School on Molecular Boron Chemistry (Wuerzburg, 2016), the 12th International Conference of Heteroatom Chemistry (Vancouver, 2017) and in the Annual Main Group Interest Group Meeting and AGM (London, 2017). Furthermore, this work has been published in two separate publications . In addition other team members (e.g. Dr Michael Ingleson and Prof. Holger Braunschweig) have presented elements of this work multiple times in their home countries and overseas).
