Synthesis, fluxionality, and propene insertion reactions of zirconium boryldiene complexes with sterically undemanding Cp ligands

The reduction of (CpZrCl3)-Zr-R(dme) (Cp-R = Cp (a), C5H4SiMe3 (b), C5H4Me (c) Ind (d)) with sodium amalgam in the presence of isoprene, followed by the addition of allylmagnesiurn chloride, gives the diene complexes (CpZr)-Zr-R(eta(3)-allyl)(eta(4)-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 degrees C proceeds quantitatively to give the thermally unstable: zwitterionic complexes (CpZr)-Zr-R(eta(3)-allyl) {eta(1):eta(3)-CH2CMeCHCH2B(C6F5)(3)} (3a-d), which on warming decompose under C-H activation and propene elimination to give (CpZr)-Zr-R(C6F5){eta(4)-CH2CMeCHCHB(C6F(5))(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 (Delta G(double dagger) = 43-49 kJ mol(-1)), while there is little variation in the rotational barriers of the B-C6F5 substituents (Delta G(double dagger) = ca. 47 kJ mol(-1)). Recrystallization of 4a from diethyl ether affords the 16-electron complex 4a . OEL2, 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 (CpZr)-Zr-R(C6F5){eta(1):eta(3)-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 beta-alkyl elimination from the major chair conformation isomer.