Unprecedented rearrangement during the formation of P–P homoatomic N-phosphino formamidine complexes

Abstract

A variety of homoatomic P–P donor–acceptor homoleptic (R = R′) and heteroleptic (R ≠ R′) N-phosphino formamidine complexes [ iPr 2N–C(H) N–PR 2–PR′ 2]Cl were synthesized from the addition of N-phosphino formamidine (phosfam) donor reagent iPr 2N–C(H) N–PR 2 on halogenophosphane compounds R′ 2PCl which are synthetic sources for the corresponding phosphenium derivatives R 2P +. We have demonstrated that the dynamic equilibrium observed between the different species is shifted either completely to the side of the free species or to the side of the donor–acceptor adduct [ iPr 2N–C(H) N–PPh 2–PPh 2]Cl by changing the solvent or by varying the temperature. Activation parameters of Δ S ≠ = (−130 ± 7.2) J mol −1 K −1, Δ H ≠ = (8.4 ± 0.6) kJ mol −1 and Δ G ≠ (298.15 K) = (53.6 ± 2.3) kJ mol −1 were determined by an Eyring analysis over the temperature range of 193–293 K. The negative entropy of activation is consistent with an associative pathway and the low value of Δ H ≠ suggests that the energy barrier for this reaction is entropically controlled. Phosphine–phosphenium adducts is the most appropriate term to describe the dynamic process observed at variable temperature for complexes [ iPr 2N–C(H) N–PR 2 → PR′ 2] +, but the 31P NMR chemical shift and the calculated electronic charges are more in favor of a phosphinophosphonium Lewis drawing [ iPr 2N–C(H) N–PR 2–PR′ 2] +. Formation of the homoatomic P–P heteroleptic formamidine complexes [ iPr 2N–C(H) NPR′ 2PR 2]Cl (R = Ph, R′ = Et, iPr) results in the formal insertion of the phosphino group of the corresponding alkyl chlorophosphanes R′ 2PCl into the N–P bond of the starting phosfam ligand iPr 2N–C(H) N–PR 2. Computed data are in agreement with the transient formation of a heteroatomic N–P intermediate [ iPr 2N–C(H) N(PR 2)PR′ 2]Cl, which then rearranges to the more thermodynamically favored homoatomic P–P compound [ iPr 2N–C(H) N–PR 2–PR′ 2]Cl.