Synthesis, Fluxionality, and Propene Insertion Reactions of Zirconium Boryldiene Complexes with Sterically Undemanding Cp Ligands

The reduction of CpRZrCl3(dme) (CpR = Cp (a), C5H4SiMe3 (b), C5H4Me (c), Ind (d)) with
sodium amalgam in the presence of isoprene, followed by the addition of allylmagnesium
chloride, gives the diene complexes CpRZr(η3-allyl)(η4-isoprene) (2a−d). The preparations
are conveniently carried out as one-pot reactions. The reaction of 2a−d with B(C6F5)3 in
toluene solution at −78 °C proceeds quantitatively to give the thermally unstable zwitterionic
complexes CpRZr(η3-allyl){η1:η3-CH2CMeCHCH2B(C6F5)3} (3a−d), which on warming decompose under C−H activation and propene elimination to give CpRZr(C6F5){η4-CH2CMeCHCHB(C6F5)2} (4a−d). The complexes are stabilized by the coordination of one o-F
atom of a boryl−C6F5 ring to the metal center. Compounds 4 are fluxional. The rotation of
the Zr−C6F5 ligand is influenced by the steric demand of the Cp ligands (ΔG⧧ = 43−49 kJ
mol-1), while there is little variation in the rotational barriers of the B−C6F5 substituents
(ΔG⧧ = ca. 47 kJ mol-1). Recrystallization of 4a from diethyl ether affords the 16-electron
complex 4a·OEt2, in which fluorine coordination is replaced by an ether ligand. The structure
of this complex has been determined; unlike its C5Me5 and C5H3(SiMe3)2 congeners, it shows
the boryldiene moiety to occupy a prone (endo) conformation. Propene inserts into the CH2
terminus of the boryldiene ligand under ambient conditions to give the metallacycles CpRZr(C6F5){η1:η3-CH2CH(Me)−CH2C(Me)CHCHB(C6F5)2} (5a−d), with complete regioselectivity
and very high stereoselectivity. The insertion process is reversible; propene extrusion occurs
via β-alkyl elimination from the major chair conformation isomer.