The chemistry of amine boranes has received wide attraction in recent years due to the possibility to use these compounds as hydrogen storage media or as molecular precursors for boron-nitrogen polymers such as poly(aminoborane)s. These materials can formally be regarded as valence isoelectronic and isosteric inorganic analogues of polyolefins. Transition metal complex catalysed dehydropolymerisation of primary amine boranes such as methylamine borane, H3BNMeH2, is an atom-efficient route to prepare such polymers, however, only a handful of selective catalyst systems that yieldproducing the desired product in high yields are known. Rhodium complexes of the type [(diphosphine)Rh]+, pioneered by Weller and co-workers for this purpose, were at the forefront of the development of dehydropolymerisation of H3BNMeH2. Using these systems, it was possible to identify different mechanisms for polymer formation. Also, pincer complexes of late transition metals were widely studied in this context, recent examples including mostly Fe, Co, Rh, and Ir complexes. The hitherto unknown bis(thiophosphinite) pincer complexes [(RPSCSPR)Rh(py)(H)(Cl)] (RPSCSPR = C6H4-2,6-(SPR2)2 with R = iPr, 2a and R = Ph, 2b) are prepared by metalation of the RPSCSPR ligand precursor with [Rh(cod)Cl]2 in the presence of pyridine. Both complexes are highly active for the dehydropolymerisation of H3BNMeH2, producing poly(aminoborane)s (H2BNMeH)n (e.g. 1 mol% 2a, Mn = 20.200 g/mol) in very short reaction times at room temperature (Figure 1). Detailed NMR studies on the iPr substituted complex 2a reveal B-N bond cleavage as a dominant process, resulting in initial formation of a BH3 adduct, which inhibits catalysis, as evidenced by pronounced induction periods. These can be removed by addition of amine to bringing the catalyst on-cycle for dehydrogenation and polymer formation. A mechanistic scenario is discussed based on the volumetric, spectroscopic and size-exclusion chromatography data as well as M06L-SCRF density functional theory computations.

Dehydropolymerisation of methylamine borane using rhodium(III) bis(thiophosphinite) pincer complexes: catalytic and mechanistic insights

ACS Spring Meeting 2021

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