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

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.