Zirconium and hafnium diene and dienyl half-sandwich complexes:
synthesis, polymerization catalysis and deactivation pathways. The
molecular structures of
[M(η3-C3H5)(2,3-Me2
C4H4){η-C5H
3(SiMe3)2-1,3}]
(M = Zr or Hf
) and
[Hf(η3-C3H5){
η3-CH2CMeCMeCH2B(C6
F5)3}{η-C5
H3(SiMe2)-1,3}]
Abstract
The reduction of [MCl
3
Cp″] [M = Zr or
Hf;
Cp″ = η-C
5
H
3
(SiMe
3
)
2
-1,3] with sodium amalgam in the presence of dienes
gave the compounds [MCl(diene)Cp″] which are alkylated with MeMgBr
or RMgCl (R = allyl) to give [MX(diene)Cp″]
(X = CH
3
or
η
3
-C
3
H
5
;
diene = 2,3-dimethylbuta-1,3-diene or isoprene). The
reduction of [ZrCl(CHCMeCMeCH)Cp″] with an excess of Na–Hg
leads to the binuclear η
4
(5e)-butadienyl complex
[Zr(µ-η
1
∶η
4
-C
4
H
3
Me
2
-2,3)Cp″]
2
, also
formed from [ZrMe(C
4
H
4
Me
2
)Cp″] by
methane elimination. The butadiene complex
[Zr(η
3
-CH
2
CMeCHCH
2
)(η
4
-C
4
H
6
)Cp″] is obtained directly from
[ZrCl
3
Cp″] and MeCHCHCH
2
MgCl. The complexes
[M(allyl)(diene)Cp″] react with
B(C
6
F
5
)
3
to give the zwitterionic
complexes
[Cp″M
+
(η
3
-C
3
H
5
)
{η
3
-C
4
H
4
R
1
R
2
B
-
(C
6
F
5
)
3
}]
which contain a 14-electron [CpM(allyl)
2
]
+
core
stabilised by agostic bonding of the B–CH
2
methylene
hydrogens. These zwitterions catalyse the polymerisation of ethene to
high molecular weight polyethene. Catalysts with similar activities are
obtained by the activation of [M(allyl)(diene)Cp″] with
[CPh
3
][B(C
6
F
5
)
4
]. The
thermal stability of the zwitterionic active species depends strongly on
the steric requirements of the dienyl ligands and decreases sharply in
the order
R
1
= R
2
= Me
> R
1
= Me,
R
2
= H ![[double greater-than, compressed]](https://www.rsc.org/images/entities/char_2aa2.gif)
R
1
= H, R
2
= H; i.e. the
dimethylbutadiene derivatives are stable at room temperature, while in
the latter case decomposition is significant even at
-60 °C. The complexes
[Zr(η
3
-CH
2
CHCHR
1
){η
3
-CH
2
CR
2
MeCHCH
2
B(C
6
F
5
)
3
}Cp″]
(R
1
= H, R
2
= Me, 9a;
R
1
= Me, R
2
= H, 10)
decompose via an unusual C–H activation pathway, with
alkene elimination and concomitant migration of a
C
6
F
5
substituent from boron to zirconium, to give
the catalytically inactive boryldiene complexes
[Zr(C
6
F
5
){η
4
-CH
2
CR
1
CHCHB(C
6
F
5
)
2
}Cp
″]. The crystal structures of
[M(C
3
H
5
)(Me
2
C
4
H
4
)Cp″] (M = Zr or Hf) and
[Hf(η
3
-C
3
H
5
){η
3
-CH
2
CMeCMeCH
2
B(C
6
F
5
)
3
}Cp″] are reported.
R
1
= H, R
2
= H; i.e. the
dimethylbutadiene derivatives are stable at room temperature, while in
the latter case decomposition is significant even at
-60 °C. The complexes
[Zr(η
3
-CH
2
CHCHR
1
){η
3
-CH
2
CR
2
MeCHCH
2
B(C
6
F
5
)
3
}Cp″]
(R
1
= H, R
2
= Me, 9a;
R
1
= Me, R
2
= H, 10)
decompose via an unusual C–H activation pathway, with
alkene elimination and concomitant migration of a
C
6
F
5
substituent from boron to zirconium, to give
the catalytically inactive boryldiene complexes
[Zr(C
6
F
5
){η
4
-CH
2
CR
1
CHCHB(C
6
F
5
)
2
}Cp
″]. The crystal structures of
[M(C
3
H
5
)(Me
2
C
4
H
4
)Cp″] (M = Zr or Hf) and
[Hf(η
3
-C
3
H
5
){η
3
-CH
2
CMeCMeCH
2
B(C
6
F
5
)
3
}Cp″] are reported.
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) and
[Hf(η3-C3H5){
η3-CH2CMeCMeCH2B(C6
F5)3}{η-C5
H3(SiMe2)-1,3}]