The reduction of Cp
RZrCl
3(dme) (Cp
R = Cp (
a), C
5H
4SiMe
3 (
b), C
5H
4Me (
c), Ind (
d)) withsodium amalgam in the presence of isoprene, followed by the addition of allylmagnesiumchloride, gives the diene complexes Cp
RZr(
3-allyl)(
4-isoprene) (
2a-
d). The preparationsare conveniently carried out as one-pot reactions. The reaction of
2a-
d with B(C
6F
5)
3 intoluene solution at -78
C proceeds quantitatively to give the thermally unstable zwitterioniccomplexes Cp
RZr(
3-allyl){
1:
3-CH
2CMeCHCH
2B(C
6F
5)
3} (
3a-
d), which on warming decompose under C-H activation and propene elimination to give Cp
RZr(C
6F
5){
4-CH
2CMeCHCHB(C
6F
5)
2} (
4a-
d). The complexes are stabilized by the coordination of one
o-Fatom of a boryl-C
6F
5 ring to the metal center. Compounds
4 are fluxional. The rotation ofthe Zr-C
6F
5 ligand is influenced by the steric demand of the Cp ligands (
G = 43-49 kJmol
-1), while there is little variation in the rotational barriers of the B-C
6F
5 substituents(
G = ca. 47 kJ mol
-1). Recrystallization of
4a from diethyl ether affords the 16-electroncomplex
4a·OEt
2, in which fluorine coordination is replaced by an ether ligand. The structureof this complex has been determined; unlike its C
5Me
5 and C
5H
3(SiMe
3)
2 congeners, it showsthe boryldiene moiety to occupy a prone (endo) conformation. Propene inserts into the CH
2terminus of the boryldiene ligand under ambient conditions to give the metallacycles Cp
RZr(C
6F
5){
1:
3-CH
2CH(Me)-CH
2C(Me)CHCHB(C
6F
5)
2} (
5a-
d), with complete regioselectivityand very high stereoselectivity. The insertion process is reversible; propene extrusion occursvia
-alkyl elimination from the major chair conformation isomer.