Magnesocenophane‐Catalyzed Amine Borane Dehydrocoupling

Abstract

The Lewis acidities of as eries of [n]magnesocenophanes (1a-d)h ave been investigatedc omputationally and found to be af unctiono ft he tilt of the cyclopentadienyl moieties. Their catalytic abilities in amine borane dehydrogenation/dehydrocoupling reactions have been probed, and C[1]magnesocenophane (1a)h as been shown to effectively catalyze the dehydrogenation/dehydrocoupling of dimethyl-amine borane (2a)a nd diisopropylamine borane (2b)u nder ambient conditions. Furthermore, the mechanism of the reaction with 2a has been investigated experimentally and computationally,a nd the results imply al igand-assisted mechanism involving stepwise protona nd hydride transfer, with dimethylaminoborane as the key intermediate. [a] E = bridginga tom(s) of ansa bridge;[ b] n = number of bridging atom(s) in ansa bridge.Scheme6.Reactionofm agnesium complex 5 with 2,2-dicyclopentadienylpropanetog ive magnesocenophane 1a.Scheme8.Simplified proposed catalytic cycle (coordinating solventm olecules omitted) for the dehydrogenation of Me 2 NH·BH 3 (2a)and Me 2 NHÀBH 2 ÀNMe 2 À BH 3 (3b)t oc yclic diborazane 3a,catalyzedbym agnesocenophane 1a.Scheme7.Dehydrocoupling of Me 2 NHÀBH 2 ÀNMe 2 ÀBH 3 (3b)catalyzed by 1a. Figure 10. Calculated reaction pathwaysfor the dehydrogenation of dimethylamine borane (2a)v ia dimethylaminoborane (4a), catalyzed by 1a·dme (calculated at the B3LYP-D3/def2-TZVP leveloft heory [11] ;hydrogena toms of CH 2 and CH 3 groupso mittedf or clarity).

Main‐Group Metallocenophanes

“…reported the use of magnesocenophanes as catalysts in dehydrocoupling reactions of amine boranes. In particular [1]magnesocenophanes were found to be very potent catalysts in the dehydrocoupling of dimethylamine borane (Scheme 4), outperforming many other group 2 compounds [28] …”

“…In case of magnesium, sila-and carba-bridged [1]-and [2]magnesocenophanes, 1,w ith av ariety of substitution patterns have been reported (Figure 3). [26][27][28][29][30][31][32][33][34][35] The synthesis usually follows the same route, which is the synthesis of the corresponding neutral ansa-ligand system, followed by treatment of this protonated ligand with as uitable magnesium base like dibutyl magnesium (Scheme 1). This synthetic route was first realized in 1997b yB urger et al in the preparation of Me 2 C[1]magnesocenophane 1a, [26] and has since been adopted for severale xamples of [1]-and [2]magnesocenophanes ( Figure 3).…”

“…It mustb et aken into consideration that the solvent coordination can have as trong influence on the overall geometry.F or instance, in case of the aforementioned C[1]magnesocenophane 1a,d me coordination to the magnesium atom resultsi na ni ncreased dihedrala ngle of the Cp planes, a,c ompared to what is predicted for the solvent-free form, by DFT calculations. [28] Furthermore, thf and acetonitrile coordination to C [1]magnesocenophane 1a result in ring slippage of one of the Cp ligandsi nthe solid state structure, as h 1 /h 5 coordination is found in 1a•(thf) 2 and h 2 /h 5 coordination is found in 1a•(NCMe)(thf). [26] Structural features like this are a result of the highly ionic MgÀCp bonds and are found throughout many crystal structures of magnesocenophanes, 1 (Table 1), buta re usually not reflected by NMR spectroscopy in solution.F or convenience, all structures of magnesocenophanes, 1,i nF igure 3a re presented in ag eneral form without solventm olecules and with h 5 /h 5 coordination mode.…”

“…In particular [1]magnesocenophaneswere found to be very potent catalysts in the dehydrocoupling of dimethylamine borane (Scheme 4), outperforming many other group 2c ompounds. [28] Scheme3.Synthesis of C[1]magnesocenophane 1i,j (i:R= CH 2 CH=CH 2 ; j: R = CH 2 CMe = CH 2 ). Table 1.…”

“…M-Cp [a] [pm] Cp hapticity [28] 223.3;2 25.4;230.4 h 5 /h 5 77.9;8 0.1 119.9;1 21.7 1b•(thf) 2 [29] 221.0;2 36.2…”