Abstract
Fluoroarene complexes of the small bite angle bisphosphine Cy2PCH2PCy2 (dcpm) have been prepared: [Rh(dcpm)(η6‐1,2‐F2C6H4)][Al{OC(CF3)3}4] and [Rh(dcpm)(η6‐1,2,3‐F3C6H3)][Al{OC(CF3)3}4]. These complexes act as precursors to a previously inaccessible σ‐amine–borane complex [Rh(dcpm)(η2‐H3B·NMe3)][Al{OC(CF3)3}4] of a small bite‐angle phosphine. This complex is a poor catalyst for the dehydrocoupling of H3B·NMe2H. Instead, formation of the bridging borylene complex [{RhH(µ‐dcpm)}2(µ‐H)(µ‐BNMe2)][Al{OC(CF3)3}4] occurs, which has been studied by NMR, mass spectrometry, crystallographic and DFT techniques. This represents a new route to bridging borylene complexes.
Highlights
The transition metal catalysed dehydrocoupling and dehydropolymerisation of amine–boranes has been the subject of considerable recent attention due to both fundamental interest in BH/NH activation processes and as routes to new BN-based materials.[1,2,3] Amine–borane σ-complexes[4] are often implicated as intermediates in such processes
The P–Rh–P bite angle has an inverse effect on arene binding in [Rh(bisphosphine)(arene)]+ cations, with η6arene binding more favourable for smaller bite angle ligands,[7,12,13] for which we suggest amine–borane σ-complexation is weaker
Similar π-coordination of [BArF4]– is well established.[16,20,21,22,23]. With these observations in hand, we speculated that in order to synthesise an amine–borane complex with small bite angle bisphosphine supporting co-ligand a very weakly ligating fluoroarene would be needed to be coupled with manipulation of the anion to avoid zwitterion formation. In this contribution we demonstrate that Rh-complexes with the exceptionally small bite angle Cy2PCH2PCy2 ligand[13,24] combined with moderately fluorinated arenes can be accessed using the [Al{OC(CF3)3}4]– anion, providing a route to synthetically useful quantities of a trifluorobenzene complex (Scheme 1c)
Summary
The transition metal catalysed dehydrocoupling and dehydropolymerisation of amine–boranes has been the subject of considerable recent attention due to both fundamental interest in BH/NH activation processes and as routes to new BN-based materials.[1,2,3] Amine–borane σ-complexes[4] are often implicated as intermediates in such processes. To avoid competition from zwitterion formation through coordination of [BArF4]–, the very weakly coordinating anion [Al{OC(CF3)3}4]– was employed, the use of which has been pioneered and widely applied by Krossing.[25,26] Hydrogenation of [Rh(dcpm)(COD)][Al{OC(CF3)3}4] (COD = cyclooctadiene) in 1,2F2C6H4 or 1,2,3-F3C6H3 solution gave the corresponding fluoroarene complexes [Rh(dcpm)(FnC6H6–n)][Al{OC(CF3)3}4] [n = 2 (1), 3 (2)], Scheme 2. Both species were observed in situ, and [Rh(dcpm)(1,2-F2C6H4)][BArF4] (3) could be isolated in 82 % yield as the only product.
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