Group 2 Promoted Hydrogen Release from NMe₂H⋅BH₃: Intermediates and Catalysis

Abstract

Both homo- and heteroleptic alkyl and amide complexes of the Group 2 elements Mg and Ca are shown to be active for the catalytic dehydrocoupling of Me(2)NH.BH(3). Reactions of either magnesium dialkyls or the beta-diketiminate complex [HC{(Me)CN(Dipp)}(2)MgnBu] with four or two equivalents of Me(2)NHBH(3), respectively, produce compounds containing the [H(3)BNMe(2)BH(2)Me(2)N](-) ion, which coordinates to the magnesium centers through Mg-N and Mg...HB interactions in both the solution and solid states. Thermolysis of these compounds at 60 degrees C produces the cyclic product [(H(2)BNMe(2))(2)] and, it is proposed, magnesium hydrido species by an unprecedented delta-hydride elimination process. Calcium-based species, although less reactive than their magnesium-based counterparts, are found to engage in similar dehydrocoupling reactivity and to produce a similar distribution of products under thermally promoted catalytic conditions. A mechanism for these observations is presented that involves initial production and insertion of H(2)B=NMe(2) into polarized M-N bonds as the major B-N bond-forming step. The efficacy of this insertion and subsequent beta- or delta-hydride elimination steps is proposed to be dependent upon the charge density and polarizing capability of the participating Group 2 center, providing a rationale for the observed differences in reactivity between magnesium and calcium.

Lithium Dihydropyridine Dehydrogenation Catalysis: A Group 1 Approach to the Cyclization of Diamine Boranes

“…The major resonance, a quartet at −19.2 ppm, corresponds to Ia and partially obscures a second lower‐intensity quartet at −21.7 ppm, which we assign to a species IIa resulting from initial deprotonation of an acidic N−H group. This step is in agreement with the accepted mechanism for the dehydrocoupling of HNMe 2 ⋅BH 3 9a. Finally, a low‐intensity doublet at 26.2 ppm is attributed to the B−H group of the cyclization product IVa .…”

The Catalytic Dehydrocoupling of Amine–Boranes and Phosphine–Boranes

“…The mechanism of formation of these species is proposed to proceed via the formation of an amidoborane complex which undergoes β-hydride elimination to give an aminoborane and a metal hydride. The free amine then reacts with the The first example of a main-group catalyst which formed a product with an equal B:N ratio was also from the group of Hill [132] BuH] 2 (45%) although 14% of the starting substrate remained unreacted [133].…”

Experimental and Theoretical Studies of the Potential Interconversion of the Amine–Borane iPr2NH·BH(C6F5)2 and the Aminoborane iPr2N=B(C6F5)2 Involving Hydrogen Loss and Uptake

“…[4][5][6][7][8][9][10] More recently, the range of metal complexes reported to be capable of catalytically dehydrogenating amine-boranes has been significantly expanded. For example, homogeneous systems based on Rh, [11] Ru, [12,13] Re, [14] Ni [15,16] and Ir [17][18][19] are now known, along with those based on Group 2 and 4 metals, [20][21][22][23][24][25] and systems based on p-block metals. [26] In addition, photoactivated catalysts of the first-row transition metals have also recently been reported.…”