Metal-catalyzed dehydrocoupling/dehydrogenation of amine-borane adducts has attracted growing attention from the perspectives of hydrogen storage, [1] hydrogen transfer to organic substrates, [2] and the preparation of new inorganic polymeric materials. [3] Mechanistic interest in the associated transformations has also recently led to the rapid emergence of a new area of coordination chemistry associated with amine-borane ligands with metals of the d and s block. [4, 5] Although many important advances have been made over the last decade, [1][2][3][4][5][6][7][8][9][10][11] the development of convenient to handle, cheap, and efficient catalysts of low toxicity that are effective for a broad range of boron-nitrogen substrates is of considerable interest. Herein we report on recent work concerning an interesting new photogenerated iron-based catalytic system that operates on a series of different adducts.Treatment of the secondary amine-borane adduct Me 2 NH·BH 3 (2) with a catalytic amount (5 mol %) of [{CpFe(CO) 2 } 2 ] (1) in THF at 20 8C led to no detectable reaction, as indicated by 11 B NMR spectroscopy. However, a rapid (4 h), virtually quantitative conversion to the cyclic diborazane, [Me 2 N-BH 2 ] 2 , (3) was observed under photoirradiation conditions that allowed the escape of gaseous byproducts (Scheme 1). A significantly slower conversion (80 % after 6 h) was detected when the reaction was performed in a closed system, consistent with evolved CO and/ or H 2 , thus competitively consuming the photogenerated, coordinatively unsaturated active catalytic species.Treatment of the more sterically encumbered adduct iPr 2 NH·BH 3 (4) with 1 (5 mol %) under photoirradiation conditions that allowed the escape of gaseous byproducts also led to dehydrogenation but the process was significantly slower than for 2 (15 h for almost quantitative conversion under the same conditions; Scheme 2). In this respect, the iron system shows a much more similar behavior to that of late-transition-metal precatalysts such as [{RhA C H T U N G T R E N N U N G (1,5-cod)Cl} 2 ], [6] rather than one of the systems containing early transition metals such as [Cp 2 Ti], [9] which show higher activity towards this particularly bulky substrate than towards 2.The activity of the photoirradiated [{CpFe(CO) 2 } 2 ] (1) precatalyst towards the primary amine-borane adduct MeNH 2 ·BH 3 (6) was also investigated. Analysis of the products by 11 B NMR spectroscopy after three hours of photo-[3c] was formed in approximately 90 % yield. Subsequent characterization of the polymeric product by GPC showed that the material has a high molecular weight (M w = 117 700, PDI = 1.83). However, upon prolonged irradiation in the presence of 1 we found that intermediate 7 lost a further equivalent of hydrogen to form mainly N-methylborazine 8. Indeed, after 16 hours, no residual polyaminoborane 7 was detected and 8 was formed in 60 % yield as the only identifiable product (Scheme 3). Scheme 2. The dehydrogenation of 4 with precatalyst 1 (5 mol %) result...

The readily available iron carbonyl complexes, [CpFe(CO)2]2 (1) and CpFe(CO)2I (2) (Cp = η-C5H5), were found to be efficient precatalysts for the dehydrocoupling/dehydrogenation of the amine-borane Me2NH·BH3 (3) to afford the cyclodiborazane [Me2N-BH2]2 (4), upon UV photoirradiation at ambient temperature. In situ analysis of the reaction mixtures by (11)B NMR spectroscopy indicated that different two-step mechanisms operate in each case. Thus, precatalyst 1 dehydrocoupled 3 via the aminoborane Me2N═BH2 (5) which then cyclodimerized to give 4 via an off-metal process. In contrast, the reaction with precatalyst 2 proceeded via Me2NH-BH2-NMe2-BH3 (6) as the key intermediate, affording 4 as the final product after a second metal-mediated step. The related complex Cp2Fe2(CO)3(MeCN) (7), formed by photoirradiation of 1 in MeCN, was found to be a substantially more active dehydrocoupling catalyst and not to require photoactivation, but otherwise operated via a two-step mechanism analogous to that for 1. Significantly, detailed mechanistic studies indicated that the active catalyst generated from precatalyst 7 was heterogeneous in nature and consisted of small iron nanoparticles (≤10 nm). Although more difficult to study, a similar process is highly likely to operate for precatalyst 1 under photoirradiation conditions. In contrast to the cases of 7 and 1, analogous experimental studies for the case of photoactivated Fe precatalyst 2 suggested that the active catalyst formed in this case was homogeneous. Experimental and computational DFT studies were used to explore the catalytic cycle which appears to involve amine-borane ligated [CpFe(CO)](+) as a key intermediate.

Complexes of Group 4 metallocenes in the +3 oxidation state and amidoborane or phosphidoborane function as efficient precatalysts for the dehydrocoupling/dehydrogenation of amine-boranes, such as Me(2) NH⋅BH(3). Such Ti(III) -amidoborane complexes are generated in [Cp(2)Ti]-catalyzed amine-borane dehydrocoupling reactions, for which diamagnetic M(II) and M(IV) species have been previously postulated as precatalysts and intermediates.

The catalyst loading is the key to control the molecular weight of the polymer in the iron-catalyzed dehydropolymerization of phosphine-borane adducts. Studies showed that the reaction proceeds through a chain-growth coordination-insertion mechanism.

The dehydropolymerization of the primary phosphine–boranes, RPH2•BH3 (1a–f) (R = 3,4‐(OCH2O)C6H3 (a), Ph (b), p‐(CF3O)C6H4 (c), 3,5‐(CF3)2C6H3 (d), 2,4,6‐(CH3)3C6H2 (e), 2,4,6‐tBu3C6H2 (f)) is explored using the precatalyst [CpFe(CO)2OTf] (I) (OTf = OS(O)2CF3), based on the earth abundant element Fe. Formation of polyphosphinoboranes [RPH‐BH2]n (2a–e) is confirmed by multinuclear NMR spectroscopy, but no conversion of 1f to 2f is detected. Analysis by electrospray ionization mass spectrometry confirms the presence of the anticipated polymer repeat units for 2a–e. Gel permeation chromatography (GPC) confirms the polymeric nature of 2a–e and indicates number‐average molecular weights (Mn) of 12 000–209 000 Da and polydispersity indices between 1.14 and 2.17. By contrast, thermal dehydropolymerization of 1a–e in the absence of added precatalyst leads to formation of oligomeric material. Interestingly, polyphosphinoboranes 2c and 2d display GPC behavior typical of polyelectrolytes, with a hydrodynamic radius dependent on concentration. The thermal transition behavior, thermal stability, and surface properties of thin films are also studied.