Superbasic Bridgehead Diphosphines: the Effects of Strain and Intrabridgehead P...P Bonding on Phosphine Basicity

The basicity order in acetonitrile for a series of phosphines is: 1,5-diphosphabicyclo[3.3.3]undecane 1 (pKa for the protonated ion ∼ 17.9) < 1,6-diphosphabicyclo[4.3.3]dodecane 2 (pKa ∼ 22.5) < 1,6-diphosphabicyclo[4.4.3]tridecane 3 (pKa 27.8). The latter is therefore comparable to Schwesinger’s P1–t-Bu base but still significantly weaker than Verkade’s proazaphosphatrane bases. The pKa for the protonated ion of 1,6-diphosphabicyclo[4.4.4]tetradecane 4 could not be determined due to rearrangement probably caused by preferred deprotonation at α-carbon. However, the X-ray structure of 4H·PF6 (1JPP 178 Hz) shows an in,out structure with a P⋯P distance of 2.58 A, indicative of significant bonding. Diphosphines 2 and 3 protonate to give in,out-2H+ (1JPP 251 Hz) and in,out-3H+ (1JPP 253 Hz), but 1H+ shows no P–P coupling and is believed to be out,out. Proton affinities (PA) for the bridgehead diphosphines have been calculated at the B3LYP/6-31G* level: 1, 1001, 2, 1040, 3, 1085, and 4, 1105 kJ mol−1 and are compared with PA[(t-Bu)3P] = 1028 kJ mol−1. All the bridgehead diphosphines are strongly flattened and the question of how much this may contribute to their enhanced basicity, in addition to the effect of intrabridgehead bonding, is discussed.