Mechanistic Insight into the NN Bond‐Cleavage of Azo‐Compounds that was Induced by an AlAl‐bonded Compound [L²⁻Al^IIAl^IIL²⁻]

Abstract

An α-diimine-stabilized Al-Al-bonded compound [L(2-)Al(II)-Al(II)L(2-)] (L = [{(2,6-iPr(2)C(6)H(3))NC(Me)}(2)]; 1) consists of dianionic α-diimine ligands and sub-valent Al(2+) ions and thus could potentially behave as a multielectron reductant. The reactions of compound 1 with azo-compounds afforded phenylimido-bridged products [L(-)Al(III)(μ(2)-NPh)(μ(2)-NAr)Al(III)L(-)] (2-4). During the reaction, the dianionic ligands and Al(2+) ions were oxidized into monoanions and Al(3+), respectively, whilst the [NAr](2-) imides were produced by the four-electron reductive cleavage of the N=N double bond. Upon further reduction by Na, the monoanionic ligands in compound 2 were reduced to the dianion to give [(L(2-))(2)Al(III)(2)(μ(2)-NPh)(2)Na(2)(thf)(4)] (5). Interestingly, when asymmetric azo-compounds were used, the asymmetric adducts were isolated as the only products (compounds 3 and 4). DFT calculations indicated that the reaction was quite feasible in the singlet electronic state, but the final product with the triplet-state monoanionic ligands could result from an exothermic singlet-to-triplet conversion during the reaction process.

Reactivity of Dialumane and “Dialumene” Compounds toward Alkenes

“…These results confirm that the charge transfer is from aluminum onto the alkene molecule in compound 2 , thereby showing almost no change in the α‐diimine ligands (see the Supporting Information, Table S2). This result is different from the reaction of compound 1 with azo‐compounds, wherein the 4 e − reduction involved both the metal centers and the dianionic α‐diimine ligands 7. However, a similar metal‐centered‐reduction process was observed in the reduction of the alkyne into the alkenediyl compound by the quintuply bonded [( H LCr) 2 ] compound ( H L={(2,6‐ i Pr 2 C 6 H 3 )NCH} 2 ) 13…”

“…Synthesis of [L(thf)Al(CH(Ph)CH(Ph))AlL] (2) : To a solution of [L(thf)AlAl(thf)L] ( 1 ,7 0.5 mmol) in toluene/THF (60 mL, 9:1) was added a slight excess of trans ‐stilbene (0.108 g, 0.6 mmol). The resultant brown‐green solution was stirred at ambient temperature for 5 days.…”

“…Recently, we synthesized a new AlAl singly bonded compound with an α‐diimine ligand, [L(thf)AlAl(thf)L] ( 1 ; here L is the dianion of the diimine ligand, L=[{(2,6‐ i Pr 2 C 6 H 3 )NC(Me)} 2 ] 2− ), which contained subvalent Al II centers and electron‐rich dianionic α‐diimine ligands (L 2− ). The reaction of compound 1 with azobenzene derivatives proceeded through a four‐electron‐reduction pathway that involved both the Al II centers and the L 2− ligands 7. Encouraged by this multi‐electron‐reduction property of dialumane 1 , we explored the reactivity of compound 1 toward CC double bonds, including some industrially important internal and terminal alkenes (stilbene and styrene) and conjugated dienes (2,3‐dimethyl‐1,3‐butadiene and 2‐methyl‐1,3‐butadiene, Scheme ).…”

“…The reaction of compound 1 with azobenzene derivatives proceeded through a four-electron-reduction pathway that involved both the Al II centers and the L 2À ligands. [7] Encouraged by this multi-electron-reduction property of dialumane 1, we explored the reactivity of compound 1 toward C=C double bonds, including some industrially important internal and terminal alkenes (stilbene and styrene) and conjugated dienes (2,3-dimethyl-1,3-butadiene and 2-methyl-1,3-butadiene, Scheme 1). These reactions represent the first examples of reactions of Al À Al-bonded compounds with alkenes.…”

Cycloaddition versus Cleavage of the C=S Bond of Isothiocyanates Promoted by Digallane Compounds with Noninnocent α‐Diimine Ligands

“…α‐Diimine ligands proved to be successful in stabilizing a variety of low‐valent metal complexes, including group 13 metal–metal‐bonded compounds such as digallanes and dialumanes –. These complexes have shown promising reactivity towards various small molecules, such as alkynes, alkenes, disulfides, azobenzenes, phenazine, sulfur dioxide, and ketone derivatives . Further, the α‐diimines are redox‐active ligands that can be easily oxidized or reduced, can serve as electron reservoirs, and allow significant extension of the reactivity of metal complexes.…”