Structure and photochemistry of di--butyldiphosphatetrahedrane

Di- tert -butyldiphosphatetrahedrane ( t BuCP) 2 ( 1 ) is a mixed carbon- and phosphorus-based tetrahedral molecule, isolobal to white phosphorus (P 4 ). However, despite the fundamental significance and well-explored reactivity of the latter molecule, the precise structure of the free ( t BuCP) 2 molecule ( 1 ) and a detailed analysis of its electronic properties have remained elusive. Here, single-crystal X-ray structure determination of 1 at low temperature confirms the tetrahedral structure. Furthermore, quantum chemical calculations confirm that 1 is isolobal to P 4 and shows a strong largely isotropic diamagnetic response in the magnetic field and thus pronounced spherical aromaticity. A spectroscopic and computational study on the photochemical reactivity reveals that diphosphatetrahedrane 1 readily dimerises to the ladderane-type phosphaalkyne tetramer ( t BuCP) 4 ( 2 ) under irradiation with UV light. With sufficient thermal activation energy, the dimerisation proceeds also in the dark. In both cases, an isomerisation to a 1,2-diphosphacyclobutadiene 1′ is the first step. This intermediate subsequently undergoes a [2 + 2] cycloaddition with a second 1,2-diphosphacyclobutadiene molecule to form 2 . The 1,2-diphosphacyclobutadiene intermediate 1′ can be trapped chemically by N -methylmaleimide as an alternative [2 + 2] cycloaddition partner. Crystallographic and quantum chemical studies reveal a close relationship of ( t BuCP) 2 with P 4 . Photochemical transformations proceed via the 1,2-diphosphacyclobutadiene isomer, which dimerises to give ( t BuCP) 4 in the absence of a trap.