Stable Monovalent Aluminum(I) in Reduced Phosphomolybdate Cluster as 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, hourglass phosphomolybdate cluster with a metal-center sandwiched by two benzene-like planar subunits and large steric-hindrance is used as scaffold to stabilize low-valent Al(I) species. Two hybrid structures, (H3O)2(H2bpe)11[AlIII(H2O)2]3{[AlI(P4MoV6O31H6)2]3∙7H2O (abbr. Al6{P4Mo6}6) and (H3O)3(H2bpe)3[AlI(P4MoV6O31H7)2]∙3.5H2O (abbr. Al{P4Mo6}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 monovalent group 13 element with dual Lewis and Brǿnsted acid sites. As dual-acid catalyst, these hourglass structures efficiently catalyze the 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 polyoxometalate scaffold.