Stable monovalent aluminum(i) 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.