NHI- and NHC-Supported Al(III) Hydrides for Amine–Borane Dehydrocoupling Catalysis

Abstract

The catalytic dehydrocoupling of amine-boranes has recently received a great deal of attention due to its potential in hydrogen storage applications. The use of aluminum catalysts for this transformation would provide an additional cost-effective and sustainable approach towards the hydrogen economy. Herein, we report the use of both N-heterocyclic imine (NHI)-and carbene (NHC)-supported Al(III) hydrides and their role in the catalytic dehydrocoupling of Me 2 NHBH 3 . Differences in the σ-donating ability of the ligand class resulted in a more stable catalyst for NHI-Al(III) hydrides, whereas a deactivation pathway was found in the case of NHC-Al(III) hydrides.

Molecular Main Group Metal Hydrides

“…More recently, Inoue and coworkers have investigated the use of a range of carbene and N -heterocyclic imine (NHI)-supported aluminum hydrides for the dehydrocoupling of Me 2 NH·BH 3 . While the NHI-alanes ( 553 and 703 ) required greater thermal activation (80 °C rather than 50 °C), they were shown to be significantly more robust, with the corresponding NHC-alane supported catalysts being prone to deactivation via transfer of the carbene donor from aluminum to boron.…”

“…While the NHI-alanes ( 553 and 703 ) required greater thermal activation (80 °C rather than 50 °C), they were shown to be significantly more robust, with the corresponding NHC-alane supported catalysts being prone to deactivation via transfer of the carbene donor from aluminum to boron. Mechanistically, a proposal akin to that advanced by Wright and coworkers (Scheme a) is thought to operate, with turnover proceeding via: (1) protonolysis of the Al–H bond of the catalyst by the N–H bond of Me 2 NH·BH 3 to give an Al–NMe 2 BH 3 functional group, (2) insertion of the [Me 2 NBH 2 ] monomer unit to give a Al–NMe 2 BH 2 NMe 2 BH 3 unit, and (3) γ-H elimination to yield cyclic [H 2 B­(μ-NMe 2 ) 2 BH 2 ] and restoration of Al–H functionality …”

“…18 The relatively slow nature of the catalytic process in this case has been attributed to the poor solubility of Li[AlH 4 ] in solvents such as THF or toluene. In related work, the tert-butoxy-derivatized aluminates 913 5.4.1.4. Frustrated Lewis Pairs (FLPs).…”

Dialumenes – arylvs.silyl stabilisation for small molecule activation and catalysis

“…It was also possible to use 1 mol% of 3; again, fast consumption of Me 2 NHBH 3 was observed within the rst few hours ( Table 2, entry 4) with the reaction rate slowing with increased Me 2 NHBH 3 consumption (Table 2, entry 5). As aluminium-hydrides have been used in amine borane catalysis previously, 73 and to rule out complete hydrogenation and Al-Al bond cleavage during the reaction, compound 1 was tested for dehydrocoupling activity. Aer 24 h at room temperature (Table 2, entry 6) no conversion of Me 2 NHBH 3 was observed; on increasing the temperature to 60 C (Table 2, entry 7), some formation of H 2 and products A-D was observed.…”

(Benz)Imidazolin-2-iminato Aluminum, Zinc, and Magnesium Complexes and Their Applications in Ring Opening Polymerization of ε-Caprolactone

“…Although imidazolin-2-iminato ligands have been widely used as a supporting scaffold for early transition, late-transition, lanthanide, and actinide metals, only a very limited number of corresponding aluminum, zinc, and magnesium complexes have been reported. Inoue et al synthesized a series of binuclear imidazolin-2-iminato aluminum hydride complexes and investigated their catalytic performance in small molecule transformations; − Kuhn et al and Masuda et al explored the reactivity of imidazolin-2-imine ligands with trimethylaluminum, but the catalytic performance of the corresponding organoaluminum compounds has not been studied. , Herein, we reported the preparation and characterization of a new family of monomeric and dimeric imidazolin-2-iminato Al, Mg, and Zn complexes; furthermore, to investigate the influence of the ligand skeletons on the structure of the corresponding complexes, modification on the backbone of the imidazolin-2-iminato ligands by benzannulation was accomplished, , and accordingly, we prepared a series of analogous benzimidazolin-2-iminato Al, Mg, and Zn complexes.…”