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

Abstract 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 CIPR2 (R = Et, Bu, Pr1, Ph) gave the insertion product — the ionic complexes [Cp2NbH2(PHR2)]Cl (2) — in high yields. In contrast, reaction of 1 with the bulkier ClPBu21 proceeded much slower and resulted in the steady formation of Cp2NbCl2 and HPBu21. Cp2NbCl2 was also the product of the reactions of 1 with excess of ClP(OEt)2 and catecholechlorophosphine 7. The reaction of 1 with one equivalent of cyclic chlorodiaminophosphine ClP(N(CH3)2CH2−)2 (5) resulted in two competitive routes that led to the ionic complex [Cp2NbH2(HP(NR2)2)]Cl (2f) and molecular complex Cp2NbCl(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]Cl (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 ClPR2 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 Cp2NbCl2, 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)Cl (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(PHPr21)Br are reported.