Dissociative phosphine exchange for cyclopentadienylmolybdenum(III) systems. Bridging the gap between Werner-like coordination chemistry and low-valent organometallic chemistry

Abstract

Several phosphine exchange processes on 17-electron CpMoCl2(PR3)2 systems have been investigated. The exchange of two PPh3 ligands with either two PMe3 ligands or with Ph2PCH2PPh2 (dppe) is complete within a few minutes at −80 °C. Equally fast is the exchange of two PEt3 ligands with two PMe3 ligands. On the other hand, the exchange of two PEt3 ligands with dppe is much slower (t12 ≈ 15 min to a few hours at r.t.), with excess dppe accelerating the exchange and free PEt3 retarding it. The self-exchange reaction of PMe3 is extremely slow (less than 25% exchange at r.t. in 6 h at r.t.) and an analysis of the initial rate of this reaction shows a two-term rate law with one [PMe3]-dependent and one independent term. Finally, PMe3 self-exchange on Cp∗MoCl2(PMe3)2 proceeds over one order of magnitude faster than for the corresponding Cp system, with a substantially [PMe3]-independent rate law. All these data are indicative of a dominant dissociative exchange mechanism involving rupture of the MoPR3 bond in the slow step and formation of a 15-electron intermediate. The rate of phosphine dissociation qualitatively correlates with the MoP distance in the 17-electron starting complex. Only for the CpMoCl2(PMe3)2 system is phosphine dissociation sufficiently slowed down so that the alternative associative exchange pathway becomes competitive. Possible reasons for a low activation barrier in these dissociative exchanges are discussed.