Thermally induced dehydrogenation of amine-borane adducts and ammonia-borane by group 6 cyclopentadienyl complexes having single and triple metal-metal bonds

Article abstract of DOI:10.1002/ejic.201300629

Treatment of solutions of ammonia-borane (NH₃·BH ₃, 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, [NBH_x] (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-Ph ₂PCH₂PPh₂)(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=BH ₂]₂. Similar studies were performed with triply bonded complexes [Mo₂Cp₂(CO)₄] (3b) and [Mo ₂Cp₂(μ-Ph₂PCH₂PPh ₂)(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. Readily accessible group 6 binuclear cyclopentadienyl complexes having single and triple M-M bonds are efficient catalysts for the dehydrogenation of a range of amine-borane adducts, including ammonia-borane, under mild thermal activation (333 K). Copyright