The preparation of polyphosphinoboranes initially attracted interest in the 1950s based on their potential as flame-retardant materials with high thermal stability, however soluble, high molar mass polymeric materials were not fully characterised by modern standards until the late 1990s when our group began research in this field. Since then a range of transition metal catalysed routes, and more recently metal-free routes, have been reported for the synthesis of high molar mass P-monosubstituted polyphosphinoboranes. The challenge of synthesising high molar mass P-disubstituted polyphosphinoboranes remained open – until now.
This primary motivation of this project was to undertake a comparative study of the reactivity between phenyl phosphine-borane with an N-heterocyclic carbene (NHC) and a cyclic (alkyl)(amino)carbene (CAAC). With the NHC deprotonation of a protic P–H bond took place to yield the imidazolium phosphidoborane salt, whereas with a CAAC activation of the P–H bond by the carbene centre was the initial product.
This work stems from the unexpected observation that the aforementioned P–H activation compound was observed to further react to form (CAAC)H2 and polymeric material. The conditions were optimised to produce high molar mass material and mechanistic studies were carried out. In depth DFT studies showed that reversible dissociation of the P–C bond in the initial activation compound occurs followed by B–H hydride abstraction to give (CAAC)H2 and transient phosphinoborane monomer which undergoes a head-to-tail polymerisation.
We were particularly excited to discover that this relatively simple method could be successfully extended to disubstituted phosphine-boranes to give the first examples of high molar mass P-disubstituted polyphosphinoboranes [PhRPBH2]n (R = Ph, Me). Ultimately in this novel metal-free polymerisation route the CAAC acts as a stoichiometric hydrogen acceptor to dehydrogenate phosphine-boranes to yield highly reactive phosphinoborane monomers which subsequently polymerise.
We believe that this study has established a potentially broad utility for CAACs to be used to access new inorganic polymers and materials. Moreover, we now know that the previously elusive P-disubstituted polyphosphinoboranes can be accessed and we are further driven to develop more efficient, and importantly catalytic, synthetic routes towards these novel polymers.
For more details, please see our paper: “Metal-free dehydropolymerisation of phosphine-boranes using cyclic (alkyl)(amino)carbenes as hydrogen acceptors” published in Nature Communications. (https://doi.org/10.1038/s41467-019-08967-8)