Thermally Induced Dehydrogenation of Amine–Borane Adducts and Ammonia–Borane by Group 6 Cyclopentadienyl Complexes Having Single and Triple ­Metal–Metal Bonds

Abstract

Treatment of solutions of ammonia–borane (NH3·BH3, AB) with catalytic amounts (5 mol‐%) of the singly bonded dimers [M2Cp2(CO)6] [M = Cr (1a), Mo (1b), W (1c); Cp = cyclopentadienyl] under mild thermal activation (333 K) led to the progressive dehydrogenation of the adduct and quantitative conversions were achieved after 12, 24, and >34 h, respectively. At the initial stages of these reactions (low conversions), the major products were cyclic and branched oligomers of aminoborane (NH2=BH2). However, at longer reaction times (high conversions), the major products were, in all cases, borazine, [HNBH]3, and polyborazylene, [NBHx] (x < 1), whereas other minor products were derived from B–N bond‐cleavage processes. Over the course of these reactions, complexes 1a–c were transformed into the corresponding mononuclear hydrides [MCpH(CO)3] [M = Cr (2a), Mo (2b), W (2c)], which are supposed to be the catalytically active species in these processes, as also supported by similar catalytic activity exhibited by pure samples of the dihydride [Mo2Cp2(H)2(μ‐Ph2PCH2PPh2)(CO)2] (2b′). Under similar conditions, 1a–c were also active catalysts for the dehydrogenation of adducts derived from substituted amines (tBuH2N·BH3 and Me2HN·BH3), although the rate of dehydrogenation was significantly lower than that of AB. This lower activity follows from deprotonation of hydrides 2 by the free amines, which are in turn generated through B–N bond‐cleavage processes. The dehydrogenation products of tBuH2N·BH3 are also derived from oligomerization processes of the corresponding aminoborane (tBuHN=BH2), which in this case was identified in the reaction mixtures, but even at long reaction times, the formation of the borazine‐like product was not complete, and the reaction mixture contained significant amounts of (poorly defined) soluble polymeric materials. For Me2HN·BH3, the major product obtained in all of the reactions was cyclic dimer [Me2N=BH2]2. Similar studies were performed with triply bonded complexes [Mo2Cp2(CO)4] (3b) and [Mo2Cp2(μ‐Ph2PCH2PPh2)(CO)2] (3b′), which displayed similar catalytic activity while remaining essentially unperturbed along the reactions, and these complexes yielded product distributions that were similar to those observed for singly bonded dimers 1a–c.

Catalytic Dehydrogenation of Amine‐Boranes using Geminal Phosphino‐Boranes

“…For comparison, a range of Ru pincer complexes performed the same reaction with TONs of between 2 and 99 after 24 h. [20] The turnover frequency (TOF) after approximately 10 % conversion of the dimethylamine-borane substrate at 50°C was found to be 0.62 h -1 , which is not high, but is comparable with several transition metal complexes reported by Travieso-Puente and co-workers. [21] As expected, higher TOFs were obtained at higher temperatures and after approximately 10 % conversion at 60, 70, and 80°C TOFs of 1.89, 2.65, and 4.36 h -1 were obtained, respectively. At these elevated temperatures faster catalyst decomposition was observed resulting in lower turnover frequencies as the conversion increases.…”

The Catalytic Dehydrocoupling of Amine–Boranes and Phosphine–Boranes

“…The dehydrocoupling of amine-boranes is often proposed to proceed via formation of aminoboranes H 2 B¼NRR 0 that arise from initial dehydrogenation, and aminoboranes such as H 2 B¼N t BuH and H 2 B¼NMe 2 have been directly observed as intermediates in catalytic dehydrocoupling of their respective amine-boranes [17,18]. The kinetics for the "off-metal" dimerisation of H 2 B¼NMe 2 to form [H 2 BNMe 2 ] 2 have been explored and found to be a second-order process with a large negative entropy of activation [19].…”

Boron–nitrogen doped carbon scaffolding: organic chemistry, self-assembly and materials applications of borazine and its derivatives

“…Several Cr( iv ), Mo( iii ), and W( iv ) complexes were studied as catalysts, with the Cr( iv ) complexes displaying the highest dehydrogenation activities with NH 3 ·BH 3 or t BuNH 2 ·BH 3 . 32 Hexacarbonyl complexes of such metals (Cr(0), Mo(0), W(0)) were also employed for the synthesis of N -alkylborazines from alkylamine–borane complexes. 32 The catalyst and the boron-nitrogen precursors were dissolved in benzene and irradiated for 8 h with a Hg lamp, then left in the dark at room temperature for further 24 h. Similarly to the previously discussed studies, the best results were obtained using Cr(0) complexes.…”