Ligand Transfer Reactions of Mixed-Metal Lanthanide/Magnesium
Allyl Complexes with -Diketimines: Synthesis, Structures, and
Ring-Opening Polymerization Catalysis
Luis F. Sa ´nchez-Barba,
²,§
David L. Hughes,
²
Simon M. Humphrey,
‡
and
Manfred Bochmann*
,²
Wolfson Materials and Catalysis Center, School of Chemical Sciences and Pharmacy, UniVersity of East
Anglia, Norwich, NR4 7TJ, U.K., and UniVersity Chemical Laboratory, UniVersity of Cambridge, Lensfield
Road, Cambridge, CB2 1EW, U.K.
ReceiVed October 17, 2005
The reaction of the mixed-metal lanthanide/magnesium allyl complexes [Ln(η
3
-C
3
H
5
)
3
(µ-C
4
H
8
O
2
)‚
Mg(η
1
-C
3
H
5
)
2
(µ-C
4
H
8
O
2
)
1.5
]
n
(Ln ) La, 1; Y, 2) with the diketimine 2-(2,6-diisopropylphenyl)amino-
4-(2,6-diisopropylphenyl)imino-2-pentene (BDI-H) in THF at 60 °C leads to the isolation of the magnesium
-diketiminato complex Mg{HC(MeCNC
6
H
3
Pr
i
2
-2,6)
2
}(η
1
-C
3
H
5
)(THF) (3). The same product was obtained
in the reaction of BDI-H with the ionic allyl lanthanate compounds [Mg(THF)
6
][Nd(η
3
-C
3
H
5
)
4
]
2
‚2THF
(4) and [Mg(THF)
6
][Sm(η
3
-C
3
H
5
)
4
]
2
‚2THF (5). No lanthanide-containing product could be isolated. The
X-ray structure of 3 shows that the complex is a monomeric neutral species with the magnesium center
in a distorted tetrahedral arrangement. Complex 3 was found to hydrolyze after repeated crystallization
attempts at -26 °C to give the hydroxy complex [Mg{HC(MeCNC
6
H
3
Pr
i
2
-2,6)
2
}(µ-OH)(THF)]
2
‚4THF
(6). The X-ray structure of 6 consists of a hydroxide-bridged dimer with the magnesium center in a
distorted square pyramid. Lanthanide BDI compounds were, however, obtained by reacting LnCl
3
(THF)
x
(Ln ) La, x ) 4; Ln ) Y, x ) 3) with the lithium salt [Li(BDI)(THF)]
2
to give the dimeric complexes
[LnCl
2
{HC(MeCNC
6
H
3
Pr
i
2
-2,6)
2
}]
2
(THF)‚2toluene (Ln ) La, 7; Y, 8). According to its X-ray structure,
8 possesses distorted octahedral yttrium centers, one terminal chloride, and three bridging chloride ligands.
The reaction of 7 or 8 using 4 equiv of allylMgCl did not give the expected mixed-ligand allyl lanthanide
complex but led again to the magnesium complex 3 via BDI ligand exchange. Complexes 1-5 are highly
effective single-component catalysts for the ring-opening polymerization of ǫ-caprolactone. Complex 3
also polymerizes rac-lactide efficiently and under mild conditions but with low stereospecificity.
ǫ-Caprolactone is polymerized in minutes to high molecular weight materials by an allyl end-group transfer
mechanism. The catalysts are active over a wide temperature range.
Introduction
Organolanthanide compounds have found widespread ap-
plications in organic synthesis and catalysis, as shown in recent
excellent reviews on the synthesis, structural chemistry, and
ligand design in “non-cyclopentadienyl” organolanthanides.
1
Allyl lanthanide complexes in particular have been shown to
be excellent precatalysts.
2
For instance, Taube et al.
2g
reported
the synthesis of the first neutral tris(η
3
-allyl)lanthanide com-
plexes [{La(η
3
-C
3
H
5
)
3
(η
1
-C
4
H
8
O
2
)}
2
(µ-C
4
H
8
O
2
)] and [Nd(η
3
-
C
3
H
5
)
3
(µ-C
4
H
8
O
2
)]
n
, which catalyze the 1,4-trans-polymeriza-
tion of butadiene in toluene with high 1,4-trans-stereoselectivity.
Other allyl complexes have been used in the polymerization of
polar monomers; for example, we showed recently that anionic
ansa-bis(allyl) lanthanide complexes
3b
and allyl-bridged oligo-
nuclear complexes
3a,c
are highly active single-component cata-
lysts for the polymerization of methyl methacrylate, butadiene,
4
and cyclic esters. We have also recently reported that the
reaction between LnCl
3
(THF)
n
(Ln ) Y, Sm; n ) 3) and 3
equiv of allylmagnesium chloride affords the new homoleptic
tris(η
3
-allyl) complexes [Ln(η
3
-C
3
H
5
)
3
(µ-C
4
H
8
O
2
)]
n
(Ln ) Y,
Sm) in very good yields in an improved one-pot procedure.
5
By contrast, the reaction of iodide complexes LnI
3
(THF)
n
(Ln
) La, n ) 4; Ln ) Y, Sm, Nd, n ) 3.5) with allylMgI under
the same reaction conditions yielded unexpectedly either neutral
mixed-metal compounds [Ln(η
3
-C
3
H
5
)
3
(µ-C
4
H
8
O
2
)‚Mg(η
1
-
C
3
H
5
)
2
(µ-C
4
H
8
O
2
)
1.5
]
n
(Ln ) La, 1; Y, 2) or the salts [Mg-
(THF)
6
][Sm
2
(η
3
-C
3
H
5
)
6
(µ-η
3
:η
3
-C
3
H
5
)]
2
‚toluene and [Mg(THF)
6
]-
* Corresponding author. E-mail: M.Bochmann@uea.ac.uk.
²
University of East Anglia.
‡
University of Cambridge.
§
New address: Universidad Rey Juan Carlos, Escuela Superior de
Ciencias Experimentales y Tecnologı ´a, Mo ´stotes 28933, Madrid, Spain.
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10.1021/om050892h CCC: $33.50 © 2006 American Chemical Society
Publication on Web 01/12/2006