Activation of Dihydrogen by Masked Doubly Bonded Aluminum Species

Abstract

Activation of dihydrogen by masked dialumenes (Al=Al doubly bonded species) is reported. Reactions of barrelene-type dialumanes, which have the reactivity as masked equivalents of 1,2-diaryldialumenes ArAl=AlAr, with H2 afforded dihydroalumanes ArAlH2 at room temperature (Ar: bulky aryl groups). These dihydroalumanes form hydrogen-bridged dimers [ArHAl(μ-H)]2 in the crystalline state, while a monomer-dimer equilibrium was suggested in solution. The 1,2-diaryldialumenes generated from the barrelene-type dialumanes are the putative active species in the cleavage of H2 .

Activation of Dihydrogen by Masked Doubly Bonded Aluminum Species

“…X-Ray diffraction analysis on single crystals of 3b revealed its molecular structure (Figure 1), [13] thereby showing ac entrosymmetric dimeric structure with the two aluminum atoms bridged by the hydrogen atoms.T he aluminum-bound hydrogen atoms (H1 and H2) were found in the Fourier map and refined isotropically.The bridging AlÀ Hbond lengths (1.72(2) and 1.68(2) )and the terminal AlÀ Hb ond length (1.60 (2) )a re comparable to those of the dihydroalumane dimer [Mes*HAl(m-H)] 2 (Al-H bridging 1.69(4) and 1.70(4) ,A l-H terminal 1.50(4) ,M es* = 2,4,6-(t-Bu) 3 C 6 H 2 ). [14] TheA l-Al distance (2.632(1) )i ss imilar to that of the reported dihydroalumane dimer [Mes*HAl(m-H)] 2 (2.652 (2) ).…”

Reaction of Dialumane Incorporating Bulky Eind Groups with Pyridines

“…The bond angles around the aluminum atom (C1-Al1-N1 = 112.75(8), C1-Al1-N2 = 117.05 (8), and N1-Al1-N2 = 104.44(8) • ) imply some steric hindrance between these coordinating ligands. The Al-C bond length in 3 (2.031(2) Å) is elongated compared to those in 1 (1.970(2) Å) [16] and III (1.957(4) Å) [17], which may be associated with the increasing electron density at the aluminum center due to the coordination of the 1,4-dihydropyrid-1-yl and Py ligands. The lengths of the two Al-N bonds in 3 are distinctly different from each other; thus, the Al-N distance for the 1,4-dihydropyrid-1-yl group (Al1-N1 = 1.8568(19) Å) is much shorter than that for the coordinated Py (Al1-N2 = 2.0050(19) Å), each of which corresponds to the covalent and dative Al-N bonds [13].…”

“…We have recently synthesized some new discrete aluminum hydride compounds by introducing bulky aryl groups [16,17]. As shown in Scheme 1a, the aryldihydroalumane dimer [dialumane (6)], (Eind)HAl(µ-H) 2 AlH(Eind) (1), incorporating the fused-ring bulky Eind groups (Eind = 1,1,3,3,5,5,7,7-octaethyl-s-hydrindacen-4-yl), was obtained by the treatment of the lithium aryltrihydroaluminate dimer, [Li(OEt 2 )] 2 [(Eind)AlH 3 ] 2 (5), with chlorotrimethylsilane (Me 3 SiCl) [16].…”

“…As shown in Scheme 1a, the aryldihydroalumane dimer [dialumane (6)], (Eind)HAl(µ-H) 2 AlH(Eind) (1), incorporating the fused-ring bulky Eind groups (Eind = 1,1,3,3,5,5,7,7-octaethyl-s-hydrindacen-4-yl), was obtained by the treatment of the lithium aryltrihydroaluminate dimer, [Li(OEt 2 )] 2 [(Eind)AlH 3 ] 2 (5), with chlorotrimethylsilane (Me 3 SiCl) [16]. Scheme 1b shows the facile activation of the dihydrogen (H 2 ) molecule by the barrelene-type of dialuminum compounds supported by the bulky aryl (Ar) groups [Ar = Tbb (I) and Bpb (II); Tbb = 2,6-{CH(SiMe 3 ) 2 } 2 -4-t Bu-C 6 H 2 , Bpb = 2,6-{CH(SiMe 3 ) 2 } 2 -C 6 H 3 ], thus also producing the corresponding dialuminum hydrides, (Ar)HAl(µ-H) 2 AlH(Ar) [Ar = Tbb (III) and Bpb (IV)], concurrent with the release of benzene [17]. The molecular structures of the doubly hydrogen-bridged dimers of the aryldihydroalumanes 1 and III were determined by X-ray crystallography [16,17], and their dynamic monomer-dimer equilibriums in solution were examined by the crossover experiments, as shown in Scheme 1c, resulting in the formation of equilibrium mixtures containing asymmetrical dialumane, (Tbb)HAl(µ-H) 2 AlH(Bpb) (V), and (Tbb)HAl(µ-H) 2 AlH(Eind) (VI), as judged by the 1 H NMR spectra [17].…”

“…Scheme 1b shows the facile activation of the dihydrogen (H 2 ) molecule by the barrelene-type of dialuminum compounds supported by the bulky aryl (Ar) groups [Ar = Tbb (I) and Bpb (II); Tbb = 2,6-{CH(SiMe 3 ) 2 } 2 -4-t Bu-C 6 H 2 , Bpb = 2,6-{CH(SiMe 3 ) 2 } 2 -C 6 H 3 ], thus also producing the corresponding dialuminum hydrides, (Ar)HAl(µ-H) 2 AlH(Ar) [Ar = Tbb (III) and Bpb (IV)], concurrent with the release of benzene [17]. The molecular structures of the doubly hydrogen-bridged dimers of the aryldihydroalumanes 1 and III were determined by X-ray crystallography [16,17], and their dynamic monomer-dimer equilibriums in solution were examined by the crossover experiments, as shown in Scheme 1c, resulting in the formation of equilibrium mixtures containing asymmetrical dialumane, (Tbb)HAl(µ-H) 2 AlH(Bpb) (V), and (Tbb)HAl(µ-H) 2 AlH(Eind) (VI), as judged by the 1 H NMR spectra [17]. It is noteworthy that the dialuminum hydrides V and VI are rare examples of the group 13 element compounds containing asymmetrical three-center two-electron (3c-2e) bonds.…”