Iron-Catalyzed Dehydrocoupling/Dehydrogenation of Amine-Boranes

The readily available iron carbonyl complexes, [CpFe(CO)(2)](2) (1) and CpFe(CO)(2)I (2) (Cp = eta-C5H5), were found to be efficient precatalysts for the dehydrocoupling/ dehydrogenation of the amine-borane Me2NH center dot BH3 (3) to afford the cyclodiborazane [Me2N-BH2](2) (4), upon UV photoirradiation at ambient temperature. In situ analysis of the reaction mixtures by B-11 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.