Stable monovalent aluminum() in a reduced phosphomolybdate cluster as an active acid catalyst

Low-valent aluminum Al( i ) chemistry has attracted extensive research interest due to its unique chemical and catalytic properties but is limited by its low stability. Herein, a hourglass phosphomolybdate cluster with a metal-center sandwiched by two benzene-like planar subunits and large steric-hindrance is used as a scaffold to stabilize low-valent Al( i ) species. Two hybrid structures, (H 3 O) 2 (H 2 bpe) 11 [Al III (H 2 O) 2 ] 3 {[Al I (P 4 Mo V 6 O 31 H 6 ) 2 ] 3 ·7H 2 O (abbr. Al 6 {P 4 Mo 6 } 6 ) and (H 3 O) 3 (H 2 bpe) 3 [Al I (P 4 Mo V 6 O 31 H 7 ) 2 ]·3.5H 2 O (abbr. Al{P 4 Mo 6 } 2 ) (bpe = trans -1,2-di-(4-pyridyl)-ethylene) were successfully synthesized with Al( i )-sandwiched polyoxoanionic clusters as the first inorganic-ferrocene analogues of a monovalent group 13 element with dual Lewis and Brønsted acid sites. As dual-acid catalysts, these hourglass structures efficiently catalyze a solvent-free four-component domino reaction to synthesize 1,5-benzodiazepines. This work provides a new strategy to stabilize low-valent Al( i ) species using a polyoxometalate scaffold. Monovalent aluminum( i ) species was successfully stabilized using a reduced phosphomolybdate scaffold as a dual-acid catalyst for a four-component domino reaction.