BP double bonds relieved from steric encumbrance: matrix-isolation infrared spectroscopy of the phosphaborene F2B–PBF and the triradical BPF3

Abstract

Free phosphaborenes have a labile boron–phosphorus double bond and therefore require extensive steric shielding by bulky substituents to prevent isomerisation and oligomerisation. In the present work, the small free phosphaborene F₂B–PBF was isolated by matrix-isolation techniques and was characterised by infrared spectroscopy in conjunction with quantum-chemical methods. In contrast to its sterically hindered relatives, this small phosphaborene exhibits an acute BPB angle of 83° at the CCSD(T) level. An alternative orbital structure for the BP double bond is found in the triradical BPF₃, the direct adduct of laser-ablated atomic B and PF₃. The single-bonded isomer F₂B–PF and the dimer F₃P–BB–PF₃ are also tentatively assigned.