Tetraphosphacubane: An unexpectedly strong base in the gas phase

The experimental determination of physical properties of tetra-tert-butylphosphocubane (TtBuPC) is of paramount importance for understanding the reactivity of this fascinating molecule. The gasphase basicity of TtBuPC measured by FT-ICR spectroscopy (GB = 221.8 kcal mol(-1), PA = 230.5 kcal mol(-1)) is surprisingly high although protonation in solution is only achieved under drastic conditions. A molecular orbital treatment, including electron correlation effects, predicts the unsubstituted parent compound phosphacubane (PC) to be a carbon base. Carbon protonation implies a P-C bond fission which alleviates the strain of the system. A similar behavior is also predicted for the tetramethyl derivative. However, TtBuPC is found to be a phosphorus base, because the strong repulsive interactions which appear between the tert-butyl substituents destabilize significantly the C-protonated form. These effects decrease dramatically when just one tert-butyl group is removed and both P- and C-protonated species become almost degenerate. As for other strained systems, the PAs of PC and TtBuPC are only adequately reproduced when G2-type [6-311+(3df,2p)] basis sets are used.