Air-Tolerant 1-Amino-1,3-diphosphacyclobutane-2,4-diyls Featuring High Electron-Donating Property and Low HOMO-LUMO Gap

The sterically encumbered 1-amino-1,3-diphosphacyclobuten-4-yl anion, prepared from 1-(2,4,6-tri-t-butylphenyl)-2-phosphaethyne and 0.5 equiv of lithium diisopropylamide (LDA), was characterized spectroscopically, and afforded the corresponding air-tolerant amino-substituted 1,3-diphosphacyclobutane-2,4-diyls via SN2-type alkylation and arylation with benzyne. The photo-absorption and electrochemical parameters of the P-heterocyclic singlet open-shell compounds revealed higher electron-donating character and reduction between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) levels compared with the relevant hydrocarbon-substituted P-heterocycles. 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 fully 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.

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