Air-Tolerant 1-Amino-1,3-diphosphacyclobutane-2,4-diyls Featuring Strong Electron-Donating Properties and Small HOMO-LUMO Gaps

The sterically encumbered 1-amino-1,3-diphospha-cyclobuten-4-yl anion, prepared from 1-(2,4,6-tri-tert-butylphenyl)-2-phosphaethyne and lithium diisopropylamide (0.5 equiv.), was characterized spectroscopically and afforded air-tolerant amino-substituted 1,3-diphosphacyclobutane-2,4-diyls by S(N)2-type alkylation and arylation reactions with benzyne. The photoabsorption and electrochemical parameters of the P-heterocyclic singlet open-shell compounds revealed that they had higher electron-donating character than the relevant hydrocarbon-substituted P-heterocycles in addition to a reduction between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) levels. The structures of the amino-substituted 1,3-diphosphacyclobutane-2,4-diyls were unambiguously determined by X-ray crystallography, and the metric parameters were compatible with the physical properties. The successful synthesis and full characterization of the amino-substituted phosphorus congener of cyclobutane-1,3-diyl will be helpful to develop functional materials based on open-shell neutral and ionized phosphorus heterocycles.