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

A variety of homoatomic P–P donor–acceptor homoleptic (R = R′) and heteroleptic (R ≠ R′) N-phosphino formamidine complexes [iPr2N–C(H)double bond; length as m-dashN–PR2–PR′2]Cl were synthesized from the addition of N-phosphino formamidine (phosfam) donor reagent iPr2N–C(H)double bond; length as m-dashN–PR2 on halogenophosphane compounds R′2PCl which are synthetic sources for the corresponding phosphenium derivatives R2P+. 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 [iPr2N–C(H)double bond; length as m-dashN–PPh2–PPh2]Cl by changing the solvent or by varying the temperature. Activation parameters of ΔS≠ = (−130 ± 7.2) J mol−1 K−1, ΔH≠ = (8.4 ± 0.6) kJ mol−1 and ΔG≠ (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 ΔH≠ 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 [iPr2N–C(H)double bond; length as m-dashN–PR2 → PR′2]+, but the 31P NMR chemical shift and the calculated electronic charges are more in favor of a phosphinophosphonium Lewis drawing [iPr2N–C(H)double bond; length as m-dashN–PR2–PR′2]+. Formation of the homoatomic P–P heteroleptic formamidine complexes [iPr2N–C(H)double bond; length as m-dashNPR′2PR2]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 iPr2N–C(H)double bond; length as m-dashN–PR2. Computed data are in agreement with the transient formation of a heteroatomic N–P intermediate [iPr2N–C(H)double bond; length as m-dashN(PR2)PR′2]Cl, which then rearranges to the more thermodynamically favored homoatomic P–P compound [iPr2N–C(H)double bond; length as m-dashN–PR2–PR′2]Cl.