The carbenoid nature of halophosphines. Carbene-like insertion mechanism in the reaction of chlorophosphines with Cp2NbH3

Article abstract of DOI:10.1016/S0022-328X(97)00278-7

We have studied the reactivity of Cp₂NbH₃ (1) with different chlorophosphines and showed that all the reactions occur at the niobium-hydride bond and proceed by two competitive routes, via the direct insertion of a chlorophosphine into the Nb-H bond and electron or/and hydride transfer mechanism. Reaction of 1 with the unhindered dialkyl-and diaryl-chlorophosphines C1PR₂ (R = Et, Bu, Prⁱ, Ph) gave the insertion product - the ionic complexes [Cp₂NbH₂(PHR₂)]C1 (2) - in high yields. In contrast, reaction of 1 with the bulkier C1PBu¹₂ proceeded much slower and resulted in the steady formation of Cp₂NbC1₂ and HPBu¹₂. Cp₂NbC1₂ was also the product of the reactions of 1 with excess of C1P(OEt)₂ and catecholechlorophosphite 7. The reaction of 1 with one equivalent of cyclic chlorodiaminophosphine C1P(N(CH₃)₂CH₂-)₂ (5) resulted in two competitive routes that led to the ionic complex [Cp₂NbH₂(HP(NR₂)₂)]C1 (2f) and molecular complex Cp₂NbC1(HP(NR₂)₂) (6j). The reaction of 1 with two equivalents of cyclic 1-chloro-5,5-dimethyl-1-phospha-2,6-dioxahexane gave cationic diphosphite complex [Cp₂Nb(PHR₂)₂]C1 (8) in high yield. This reactivity was explained in terms of the carbenoid nature of halophosphines due to the proposed relationship in the electronic structures of halophosphines and singlet methylene. Thus, reaction of 1 with the unhindered dialkyl-and diaryl-chlorophosphines proceeded by the direct insertion of a chlorophosphine C1PR₂ into the Nb-H bond whereas with the more electron withdrawing substituents R at phosphorus the electron or/and hydride transfer mechanisms, leading to 6 and eventually to Cp₂NbC1₂, could be operative. This concept also allowed us to explain the reactivity of chlorophosphines toward different unsaturated organic products. The facile syntheses of complexes Cp₂Nb(PHR₂)C1 (6) and Cp₂Nb(PHR₂)H (11) by the thermolysis and deprotonation of 2, respectively, was performed and the comparison with other methods was undertaken. Crystallographic studies of complexes 8 and Cp₂Nb(PHPrⁱ₂)Br are reported.