Low coordinate magnesium chemistry supported by a bulky
-diketiminate ligand
Andrew P. Dove, Vernon C. Gibson,* Pimpa Hormnirun, Edward L. Marshall, John A. Segal,
Andrew J. P. White and David J. Williams
Department of Chemistry, Imperial College London, South Kensington, London, UK SW7 2AY.
E-mail: v.gibson@imperial.ac.uk
Received 31st March 2003, Accepted 21st May 2003
First published as an Advance Article on the web 1st July 2003
A series of magnesium() alkyl, alkoxide, carboxylate, amide and halide complexes stabilised by the bulky
β-diketiminate ligand, HC(C(Me)N-2,6-
i
Pr
2
C
6
H
3
)
2
(BDI), have been synthesised and structurally characterised.
(BDI)H reacts with MgMe
2
in Et
2
O to give the four-coordinate complex (BDI)MgCH
3
(Et
2
O), 1, and in toluene to
afford [(BDI)Mg(μ-CH
3
)]
2
, 2. Three coordinate complexes may be accessed by increasing the size of the alkyl ligand;
hence, the reaction of (BDI)H with
t
Bu
2
Mg yields (BDI)Mg
t
Bu, 3, while Li(BDI) reacts with
i
PrMgCl to afford
(BDI)Mg
i
Pr, 4; a similar reaction with PhMgCl affords the diethyl ether adduct (BDI)MgPh(Et
2
O), 5. The etherates
1 and 5 may be converted into the base-free complexes, 2 and (BDI)MgPh, 6, respectively, upon heating in vacuo.
The direct reaction of (BDI)H with RMgX (X = Cl or Br) results in relatively inert halide-bridged dimers of formula
[(BDI)Mg(μ-X)]
2
, (X = Cl, 7; X = Br, 8). The alkylmagnesium derivatives react readily with alcohols, amines or
carboxylic acids to yield alkoxide, amide and carboxylate complexes, respectively. For example, 4 reacts with
i
PrOH
(or O
2
) to form [(BDI)Mg(μ-O
i
Pr)]
2
, 9. Convenient one-pot synthetic procedures have been developed using
commercially available Bu
2
Mg. Treatment of Bu
2
Mg with (BDI)H, followed by its reaction with MeOH,
t
BuOH,
i
Pr
2
NH, (Me
3
Si)
2
NH, MeCO
2
H or PhCO
2
H affords [(BDI)Mg(μ-OMe)]
2
, 10, [(BDI)Mg(μ-O
t
Bu)]
2
, 11,
(BDI)Mg(N
i
Pr
2
), 12, (BDI)Mg(NTMS
2
), 13, [(BDI)Mg(μ-O
2
CMe])]
2
, 14, and [(BDI)Mg(μ-O
2
CPh)]
2
, 15,
respectively. The molecular structures of complexes 4–8 and 12–15 are reported.
Introduction
In recent years there has been great interest in the use of
sterically demanding β-diketiminate ligands to stabilise unusual
main group metal species, especially derivatives with low co-
ordination numbers and/or low metal oxidation states.
1
In a
preliminary report we described the synthesis of a series of low
co-ordinate alkylmagnesium complexes stabilised by HC-
(C(Me)N-2,6-
i
Pr
2
C
6
H
3
)
2
(BDI).
2
In an independent study,
closely related chemistry using a tert-butyl ketiminato analogue
of BDI was described by Bailey and co-workers.
3,4
Low coordinate (BDI)Mg compounds have found important
applications as single-site initiators for controlled poly-
merisation: alkoxide derivatives have been shown to be highly
active for the polymerisation of lactide,
5,6
enolate species medi-
ate the living polymerisation of methyl methacrylate to highly
syndiotactic poly(methyl methacrylate),
7
while the amide com-
plex (BDI)Mg(NTMS
2
) has been employed to initiate the ring-
opening polymerisation of a β-lactam derived from α-benzyl--
aspartic acid.
8
Here we report the synthesis and characterisation of a family
of (BDI)Mg compounds containing alkyl, halide, alkoxide,
amide and carboxylate ligands, and the use of commercially
available dibutylmagnesium as a convenient precursor for ‘one-
pot’ syntheses of some of these products.
Results and discussion
(i) Alkyl complexes
In an earlier report
2
we described how the alkyl derivatives 1–3
can be prepared from appropriate dialkylmagnesium precursors
according to Scheme 1.
However, relatively few magnesium dialkyls are commercially
available and their syntheses, although straightforward, can be
laborious. We have therefore developed an alternative synthetic
approach that employs more readily available Grignard
reagents. In situ lithiation of (BDI)H and its subsequent treat-
ment with RMgX affords the appropriate alkyl complexes in
high yield (Scheme 2). For example, the methyl-bridged binu-
clear complex 2 is obtained in 57% yield upon treatment of
Me
2
Mg with (BDI)H, while the reaction of MeMgBr with
(BDI)Li affords 2 in 84% isolated yield. Similarly, addition
of an ethereal solution of
i
PrMgCl to a toluene solution of
(BDI)Li allows the base-free iso-propyl complex, 4, to be
isolated in > 90% yield. By analogy to the synthesis of 1, when
this reaction is performed in Et
2
O, the product is the four-
coordinate base adduct (BDI)Mg
i
Pr(Et
2
O).
9
X-Ray diffraction quality crystals of 4 were grown from tolu-
ene and the molecular structure is shown in Fig. 1; the complex
exhibits a three-coordinate trigonal structure closely related to
its tert-butyl analogue 3.
2
The molecule displays non-crystallo-
graphic C
s
symmetry about a plane containing the magnesium
centre, C(2) and C(30). The iso-propyl ligand exhibits mirror
disorder (55 : 45) relative to the chelate ring plane; the param-
eters described in the following discussion refer to the major
occupancy conformer. The geometry at magnesium, like that
already observed for 3, is essentially trigonal planar, the metal
atom lying ca. 0.1 Å out of the plane of its substituents. The
Scheme 1
DOI: 10.1039/ b303550f
3088
Dalton Trans. , 2003, 3088–3097 This journal is © The Royal Society of Chemistry 2003