Synthesis and Characterization of 1,3-Diphosphapropene and Alkali-Metal 1,3-Diphosphaallyl Complexes and Unexpected 1,3-Rearrangement of a Cesium 1,3-Diphosphaallyl Complex to a Cesium Secondary Phosphanide

Protonation of [{t-BuC(PMes)2}Li(THF)3] (2; Mes = 2,4,6-(CH3)3C6H2) with either t-BuCl
or H2O yields the 1,3-diphosphapropene (Z)-MesPC(t-Bu)P(H)Mes (3) after recrystallization;
treatment of 3 with n-butyllithium at −78 °C in THF regenerates 2. Metathesis of 2 with
MOR (M = Na, K, R = t-Bu; M = Rb, Cs, R = CH2CH(Et)CH2CH2CH2CH3) at −78 °C, followed
by recrystallization in the presence of crown ethers, allows access to the 1,3-diphosphaallyl
complexes [t-BuC(PMes)2][M(15-crown-5)2] (M = Na (4), K (5), Cs (7)) and [t-BuC(PMes)2][Rb(12-crown-4)2] (6), the latter of which exhibits a tert-butyl CH3···Rb contact in the solid
state. An attempt to prepare [t-BuC(PMes)2][Cs(12-crown-4)2] (8) by metathesis resulted in
ligand degradation to give, after recrystallization, the secondary phosphanide complex
[Mes2P][Cs(12-crown-4)2] (9) in low yield. Studies reveal that the heavier alkali-metal
complexes (5−7) are stable indefinitely in crystalline form under a dry nitrogen atmosphere,
but are, as the group is descended, increasingly unstable in solution in the presence of the
lithium alkoxide byproduct and may degrade to give mixtures containing secondary
phosphanides, the known ditertiary diphosphane (Mes2P)2 (10), t-BuC⋮P (11), and other
minor unidentified phosphorus-containing compounds. DFT studies at the BL3YP/6-31G*
level on the precursor compounds (Z)- and (E)-MesPC(t-Bu)P(Cl)Mes (1a,c) indicate that
the Z isomer is more stable than the E isomer by 47.4 kJ mol-1, confirming the experimentally
observed preference for the Z configuration of the PC bond.