Monomeric and Oligomeric Amine−Borane σ-Complexes of Rhodium. Intermediates in the Catalytic Dehydrogenation of Amine−Boranes

Abstract

A combined experimental/quantum chemical investigation of the transition metal-mediated dehydrocoupling reaction of H(3)B.NMe(2)H to ultimately give the cyclic dimer [H(2)BNMe(2)](2) is reported. Intermediates and model complexes have been isolated, including examples of amine-borane sigma-complexes of Rh(I) and Rh(III). These come from addition of a suitable amine-borane to the crystallographically characterized precursor [Rh(eta(6)-1,2-F(2)C(6)H(4))(P(i)Bu(3))(2)][BAr(F)(4)] [Ar(F) = 3,5-(CF(3))(2)C(6)H(3)]. The complexes [Rh(eta(2)-H(3)B.NMe(3))(P(i)Bu(3))(2)][BAr(F)(4)] and [Rh(H)(2)(eta(2)-H(3)B.NHMe(2))(P(i)Bu(3))(2)][BAr(F)(4)] have also been crystallographically characterized. Other intermediates that stem from either H(2) loss or gain have been characterized in solution by NMR spectroscopy and ESI-MS. These complexes are competent in the catalytic dehydrocoupling (5 mol %) of H(3)B.NMe(2)H. During catalysis the linear dimer amine-borane H(3)B.NMe(2)BH(2).NHMe(2) is observed which follows a characteristic intermediate time/concentration profile. The corresponding amine-borane sigma-complex, [Rh(P(i)Bu(3))(2)(eta(2)-H(3)B.NMe(2)BH(2).NHMe(2))][BAr(F)(4)], has been isolated and crystallographically characterized. A Rh(I) complex of the final product, [Rh(P(i)Bu(3))(2){eta(2)-(H(2)BNMe(2))(2)}][BAr(F)(4)], is also reported, although this complex lies outside the proposed catalytic cycle. DFT calculations show that the first proposed dehydrogenation step, to give H(2)B horizontal lineNMe(2), proceeds via two possible routes of essentially the same energy barrier: BH or NH activation followed by NH or BH activation, respectively. Subsequent to this, two possible low energy routes that invoke either H(2)/H(2)B horizontal lineNMe(2) loss or H(2)B horizontal lineNMe(2)/H(2) loss are suggested. For the second dehydrogenation step, which ultimately affords [H(2)BNMe(2)](2), a number of experimental observations suggest that a simple intramolecular route is not operating: (i) the isolated complex [Rh(P(i)Bu(3))(2)(eta(2)-H(3)B.NMe(2)BH(2).NHMe(2))][BAr(F)(4)] is stable in the absence of amine-boranes; (ii) addition of H(3)B.NMe(2)BH(2).NHMe(2) to [Rh(P(i)Bu(3))(2)(eta(2)-H(3)B.NMe(2)BH(2).NHMe(2))][BAr(F)(4)] initiates dehydrocoupling; and (iii) H(2)B horizontal lineNMe(2) is also observed during this process.

Boryl-Mediated Reversible H2 Activation at Cobalt: Catalytic Hydrogenation, Dehydrogenation, and Transfer Hydrogenation

“…The resulting solution was monitored by 11 B NMR spectroscopy, and the identities of the products were assigned according to the literature. 2 …”

[Ir(PCy3)2(H)2(H2BNMe2)]+ as a Latent Source of Aminoborane: Probing the Role of Metal in the Dehydrocoupling of H3B⋅NMe2H and Retrodimerisation of [H2BNMe2]2

“…The relative energy difference of 2 a' and 3 a' is larger than those reported for their Rh analogues (0.5 kJ mol À1 ). [15] However, the calculated overall barrier to dehydrogenation is much lower for the rhodium complexes compared with iridium (87.0 kJ mol À1 vs. 126.3 kJ mol…”

Ruthenium Bis(σ‐BH) Aminoborane Complexes from Dehydrogenation of Amine–Boranes: Trapping of H2BNH2

“…[6] Starting from precursors I that are likely to undergo dehydrogenation, known complexes retaining a B À N unit are restricted to two cationic rhodium examples, as reported by Weller et al They are amineborane adducts with h 1 -and/or h 2 -H 3 BÀNRR'H bonding modes (R,R' = H, Me; Scheme 1). [7,8] With this work in mind, and following our recent findings on the isolation of bis(s-B À H) complexes from the reaction of dihydrogenoboranes with the bis(dihydrogen) complex [RuH 2 (h 2 -H 2 ) 2 (PCy 3 ) 2 ], [9,10] we herein report the synthesis of the first "true" bis (s-BÀH) aminoborane ruthenium complexes. [11] Stabilization of the simplest aminoborane units H 2 BÀNMe nÀ2 H n (n = 1-2) results from the stoichiometric dehydrogenation of the amine-borane precursors by the ruthenium complex.…”