Preparation and reactivity of zirconium(III), niobium(III), and molybdenum(III) complexes stabilized by a cyclopentadienyl unit with pendant phosphine donors

This paper deals with the organometallic chemistry of the trivalent second-row transition metals of groups 4-6, namely, Zr, Nb, and Mo, with the cyclopentadienyldiphosphine ligand [eta(5)-C5H3-1,3-(SiMe2CH2PR2)(2)], abbreviated as [(P2Cp)-P-R] (where R = i-Pr and Ph). The Zr(IV) complex [i-PrP2Cp]ZrCl3 undergoes reduction with Na/Hg to form the trivalent Zr derivative, [i-PrP2Cp]ZrCl2, which undergoes a metathetical reaction with MeMgBr to yield the monomethyl derivative [i-PrP2Cp]Zr(CH3)Cl. The reaction of the Zr(III) complex [i-PrP2Cp]ZrCl2 with excess carbon monoxide results in disproportionation to the Zr(IV) complex, [i-PrP2Cp]ZrCl3, and the Zr(II) compound [i-PrP2Cp]Zr(CO)(2)Cl. This reaction is reversible, and upon removal of CO the starting material, [i-PrP2Cp]ZrCl2 is formed. The preparation of the diamagnetic Nb(III) complex [(P2Cp)-P-R]NbCl2 is achieved by the reaction of NbCl3(DME) with [(P2Cp)-P-R]Li. [(P2Cp)-P-R]NbCl2 complexes react with excess CO to form the CO adducts whose solid-state structures have been determined. The Nb(IV) derivatives [(P2Cp)-P-R]NbCl3 are formed via the reaction of [(P2Cp)-P-R]NbCl2 with PbCl2. These complexes can also be produced when Nb(O)Cl-3(THF)(2) is allowed to react with excess [(P2Cp)-P-R]Li. These Nb(IV) derivatives are ESR active, and their solid-state molecular structures show distorted octahedral geometries around the Nb center. MoCl3(THF)(3) reacts with [(P2Cp)-P-R]Li to generate the corresponding Mo(III) complexes [(P2Cp)-P-R]MoCl2. These compounds are low-spin, paramagnetic complexes as evidenced by their ESR spectra.