The divalent carbene carbon centre in cyclic (alkyl)(amino)carbenes (CAACs) is known to exhibit transition-metal-like insertion into E–H σ-bonds (E = H, N, Si, B, P, C, O) with formation of new, strong C–E and C–H bonds. Although subsequent transformations of the products represent an attractive strategy for metal-free synthesis, few examples have been reported. Herein we describe the dehydrogenation of phosphine-boranes, RR’PH·BH3, using a CAAC, which behaves as a stoichiometric hydrogen acceptor to release monomeric phosphinoboranes, [RR’PBH2], under mild conditions. The latter species are transient intermediates that either polymerise to the corresponding polyphosphinoboranes, [RR’PBH2]n (R = Ph; R’ = H, Ph or Et), or are trapped in the form of CAAC-phosphinoborane adducts, CAAC·H2BPRR’ (R = R’ = tBu; R = R’ = Mes). In contrast to previously established methods such as transition metal-catalysed dehydrocoupling, which only yield P-monosubstituted polymers, [RHPBH2]n, the CAAC-mediated route also provides access to P-disubstituted polymers, [RR’PBH2]n (R = Ph; R’ = Ph or Et).

The rhodium complex [Rh(Ph PCH CH CH PPh )(η -FC H )][BAr ], 2, catalyzes BH/BD exchange between D and the boranes H B⋅NMe , H B⋅SMe and HBpin, facilitating the expedient isolation of a variety of deuterated analogues in high isotopic purities, and in particular the isotopologues of N-methylamine-borane: R B⋅NMeR 1-d (R=H, D; x=0, 2, 3 or 5). It also acts to catalyze the dehydropolymerization of 1-d to give deuterated polyaminoboranes. Mechanistic studies suggest a metal-based polymerization involving an unusual hybrid coordination insertion chain-growth/step-growth mechanism.

N-Heterocyclic germylenes/stannylenes featuring diazaborolyl groups as the N-substituents have been investigated.

The reactions of the N-heterocyclic carbenes (NHCs) IDipp and ItBu and the cyclic(alkyl)amino carbene (CAAC) CAAC Me with polyaminoborane [MeNH–BH2] n were investigated. Stoichiometric quantities of each carbene were found to cause rapid and complete depolymerization, with the major B–N-containing product identified as the NHC-aminoborane adduct, IDipp–BH2NMeH (1), cyclic borazane [MeNH–BH2]3, or borazine [MeNBH]3 with IDipp, ItBu, and CAAC Me , respectively. With substoichiometric quantities of IDipp and ItBu (down to 10 and 2.5 mol %, respectively), complete loss of high molar mass material was also detected, indicating that the depolymerization is catalytic. The main products of the reaction with substoichiometric IDipp were IDipp–BH2NMeH (1) and [MeNH–BH2]3 and with substoichiometric ItBu, [MeNH–BH2]3, and [MeNBH]3 with product ratios dependent on the quantity of NHC used. Under analogous conditions with CAAC Me , high molar mass material persisted alongside the formation of [MeNBH]3. Further reactivity studies with cyclic borazane [MeNH–BH2]3 and MeNH2·BH3 provided insights into depolymerization pathways. IDipp showed no reactivity toward [MeNH–BH2]3, whereas with 3 equiv of ItBu and CAAC Me , the dehydrogenation product [MeNBH]3, was formed. With MeNH2·BH3, 2 equiv of carbene were used as the first acts to accept dihydrogen; the major products with IDipp, ItBu, and CAAC Me were IDipp–BH2NMeH (1), [MeNBH]3, and (CAAC Me H)HBNMeH (2), respectively. The double E–H (E = B, N) bond activation product (CAAC Me H)HBNMe(HCAAC Me ) (3) was isolated from the reaction between 3 equiv of CAAC Me and MeNH2·BH3. A unified mechanism for donor-mediated depolymerization of [MeHN–BH2] n is proposed.