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)N–PR2–PR′2]Cl were synthesized from the addition of N-phosphino formamidine (phosfam) donor reagent iPr2N–C(H)N–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)N–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)N–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)N–PR2–PR′2]+. Formation of the homoatomic P–P heteroleptic formamidine complexes [iPr2N–C(H)NPR′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)N–PR2. Computed data are in agreement with the transient formation of a heteroatomic N–P intermediate [iPr2N–C(H)N(PR2)PR′2]Cl, which then rearranges to the more thermodynamically favored homoatomic P–P compound [iPr2N–C(H)N–PR2–PR′2]Cl.

Formation of the homoatomic P–P heteroleptic (R ≠ R′) N-phosphino formamidine (phosfam) complexes [iPr2N–C(H)NPR′2PR2]Cl 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)N–PR2. Computed data are in agreement with the transient formation of a heteroatomic N–P intermediate [iPr2NC(H)N(PR2)PR′2]Cl illustrated by 4.Figure optionsDownload as PowerPoint slide