THE CARBENOID NATURE OF HALOPHOSPHINES - CARBENE-LIKE INSERTION MECHANISM IN THE REACTION OF CHLOROPHOSPHINES WITH CP2NBH3

We have studied the reactivity of Cp2NbH3 (1) with different chloropho sphines 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 tr ansfer mechanism. Reaction of I with the unhindered dialkyl-and diaryl -chlorophosphines ClPR2 (R = Et, Bu, Pr-i, Ph) gave the insertion prod uct - the ionic complexes [Cp2NbH2(PHR2)]Cl (2) - in high yields. In c ontrast, reaction of 1 with the bulkier CIPBu2t proceeded much slower and resulted in the steady formation of Cp2NbCl2 and HPBu2t. Cp2NbCl2 was also the product of the reactions of 1 with excess of ClP(OEt)(2) and catecholechlorophosphite 7. The reaction of 1 with one equivalent of cyclic chlorodiaminophosphine ClP(N(CH3)(2)CH2-)(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 react ion of 1 with two equivalents of cyclic 1-chloro-5,5-dimethyl-1-phosph a-2,6-dioxahexane gave cationic diphosphite complex [Cp2Nb(PHR2)(2)]Cl (8) in high yield. This reactivity was explained in terms of the carb enoid 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 t o 6 and eventually to Cp2NbCl2, could be operative. This concept also allowed us to explain the reactivity of chlorophosphines toward differ ent unsaturated organic products. The facile syntheses of complexes Cp 2Nb(PHR2)Cl (6) and Cp2Nb(PHR2)H (11) by the thermolysis and deprotona tion of 2, respectively, was performed and the comparison with other m ethods was undertaken. Crystallographic studies of complexes 8 and Cp2 Nb(PHPr2i)Br are reported. (C) 1997 Elsevier Science S.A.