Rare-Earth Metal Complexes-Catalyzed Dehydropolymerization of Methylamine-Boranes

doi:10.6023/cjoc202301026

Abstract

Rare-earth metal complexes-catalyzed dehydropolymerization of amine-borane was achieved. Rare-earth metallo- cene alkyl complexes [(C₅Me₄R)₂LnR', R=Me, H; Ln=Sc, Y; R'=CH₂SiMe₃, CH(SiMe₃)₂] were found to be active catalysts for the dehydropolymerization of methylamine-borane (MAB), generating corresponding boron-nitrogen polymer [(MeNHBH₂)_n, PMAB]. The results showed that the polymerization activity would decrease with the increase of ion radius of the metal center or the decrease of the steric hindrance of the supporting ligands. In the mechanistic study, a β-B-agostic-type scandium amidoborane complex was successfully isolated through the stoichiometric reaction of Sc complex with MAB. Moreover, the control experiment confirmed the existence of scandium hydride species, which may be the catalytic active intermediate.

Key words: rare-earth, amine-borane, dehydrogenation, polymerization, β-hydride elimination

Cite this article

Shengjie Jiang, Yang Wang, Xin Xu. Rare-Earth Metal Complexes-Catalyzed Dehydropolymerization of Methylamine-Boranes[J]. Chinese Journal of Organic Chemistry, 2023, 43(5): 1786-1791.

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Fig. & Tab. 7

Scheme 1. Dehydropolymerization of amine-boranes and the representative examples of pre-catalysts

Entry	Cat.	Catalyst/ mol%	T/℃	Time/h	Conv./%	M_n/	Ð (M_w/M_n)
1	1	5	25	5	>99	3010	1.4
2	2	5	25	6	>99	5070	1.5
3	3	5	25	24	>99	3050	1.3
4	4	5	25	12	>99	2550	1.3
5	1	3	25	7.5	>99	2440	1.3
6	1	5	0	24	44	2670	1.3
7	1	5	60	24	>99^b	—	—

Table 1. MAB dehydropolymerization catalyzed by rare-earth metallocene alkyls

Scheme 2. Stoichiometric reaction of scandocene alkyl complex 1 with MAB

Figure 1. Molecular structure of complex 5 Hydrogen atoms (except for N—H and B—H) are omitted for clarity, and displacement ellipsoids are drawn at the 30% probability level. Selected bond lengths (nm) and angles (°): Sc(1)—N(1), 0.2206(3); Sc(1)—H(1A), 0.205(3); N(1)—B(1), 0.1568(5); N(1)—H(2), 0.100(4); B(1)—H(1A), 0.112(3); B(1)—H(1B), 0.109(4); B(1)—H(1C), 0.114(4); N(1)—Sc(1)—H(1A), 60.6(8); Sc(1)—N(1)—B(1), 86.0(2); H(1A)—B(1)—N(1), 105.1 (17)

Scheme 3. Stoichiometric reaction of complex 5 with DMAP

Figure 2. Molecular structure of complex 6 Hydrogen atoms (except for N—H and B—H) are omitted for clarity, and displacement ellipsoids are drawn at the 30% probability level. Selected bond lengths (nm) and angles (°): Sc(1)—N(1), 0.2245(4); Sc(1)—N(2), 0.2205(17); N(2)—B(1), 0.1574(11); B(1)—C(21), 0.1611(9); N(1)—C(21), 0.1364(7); B(1)—H(1A), 0.111(8); B(1)—H(1B), 0.120(8); N(1)—Sc(1)—N(2), 78.5(4); Sc(1)—N(2)—B(1), 114.8(10); N(2)—B(1)—C(21), 109.6(8); N(1)—C(21)—B(1), 118.3(4); Sc(1)—N(1)—C(21), 116.9(3)

Scheme 4. A plausible mechanism for MAB dehydropolymerization catalyzed by rare-earth metal alkyl complexes

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