Lanthanide mono(borohydride) complexes of diamide-diamine donor ligands: Novel single site catalysts for the polymerisation of methyl methacrylate

Article abstract of DOI:10.1039/b417279p

Samarium chloride and borohydride complexes of the diamide-diamine ligands (2-C₅H₄N)CH₂N(CH₂CH ₂NR)₂ (R = SiMe₃ or mcsityl) are described; the borohydride compounds are the first

Full text of DOI:10.1039/b417279p

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C O M M U N I C A T I O N
Lanthanide mono(borohydride) complexes of diamide-diamine donor
ligands: novel single site catalysts for the polymerisation of methyl
methacrylate†
Fanny Bonnet, Anna C. Hillier, Anna Collins, Stuart R. Dubberley and Philip Mountford*
Chemistry Research Laboratory, University of Oxford, Mansfield Road, Oxford, UK OX1 3TA.
E-mail: philip.mountford@chem.ox.ac.uk
Received 11th November 2004, Accepted 16th December 2004
First published as an Advance Article on the web 10th January 2005
Samarium chloride and borohydride complexes of the
diamide-diamine ligands (2-C5H4N)CH2N(CH2CH2NR)2
(R = SiMe3 or mesityl) are described; the borohydride com-
pounds are the first polydentate amide-supported single
component lanthanide catalysts for the controlled poly-
merisation of polar monomers, and also represent the first
lanthanide borohydride complex for the polymerisation of
methyl methacrylate.
Chart 1
type [M(N2NNTMS)(X)n] (no f-element derivatives or catalytic
chemistry for these ligands has yet been described).6 The
dianionic tetradentate ligands N2NNTMS are clearly relatives of
the bis(phenoxide) ligands successfully exploited in complexes I,
but have the added advantage that the steric and electronic effects
of the amide N-substituents are well-expressed, since they are
attached to the atom directly bonded to the metal. Since amide
N-substituents are well-known3 to profoundly influence cata-
lyst performance we have also prepared† a new amide N-mesityl
protio ligand H2N2NNMes and its lithiated derivate Li2N2NNMes
2 (Chart 1). H2N2NNMes was prepared in 97% yield from (2-
The search for non-cyclopentadienyl ligand environments for the
lanthanide metals is mainly driven by the potentially superior
control over metal-centred reactivity that may be gained.1 This
is especially relevant for the design of new polymerisation
1a,b
catalysts.
The lanthanides are large, “hard” ions and thus
the quest for non-metallocene, activatorless catalysts of the
type [(L)nM(X)] (M = trivalent lanthanide, (L)n is a dianionic
supporting ligand or ligand set; X = “active group” for
initiation) had led to an increased focus on N- and O-donor
ligands. Sterically tuneable, polydentate ligands are deemed to
be important so as to restrain the coordination sphere with
regard to ligand redistribution reactions and protect against
aggregation of low-coordinate complexes or intermediates.
Very recently, several reports2 of the controlled polymeri-
sation of polar a-olefin monomers by catalysts supported by
6a
C5H4N)CH2N(CH2CH2NH2)2 and mesityl bromide, employ-
ing the catalytic system 1.5 mol% Pd2(dba)3, 3.8 mol% rac-
BINAP and NaOtBu (3 equivs.). Treatment of H2N2NNMes with
BunLi (2 equivs.) gave 2 in 82% yield.
2a
tetradentate, bis(phenoxide) ligands have appeared (e.g., I,
Preliminary studies indicate that chloride and borohydride
lanthanide complexes of N2NNTMS or/and N2NNMes can be
prepared for a range of lanthanides from La to Lu. We focus
here on the samarium complexes since this larger metal is well
known to afford efficient polymerisation catalysts. The new
complexes were synthesised by addition of a THF solution
of the dilithium salt to a cold slurry of the metal trichlo-
ride or tris(borohydride) salts in THF. The resultant dimeric
Chart 1). Dianionic, chelating diamide-based ligands have been
remarkably successful in controlling polymerisation activity and
polymer structure for the comprehensively studied Group 4
metals.3 However, the track record of tri- and tetra-dentate
diamide ligands in the polymerisation of a-olefins by lanthanide
complexes [(L)nM(X)] has been undistinguished.1 For example,
compounds II (Chart 1) provide poor activity and control
of methyl methacrylate (MMA) polymerisation for all but
the smallest Group 3 metal,4 possibly indicating that the
tridentate diamide-pyridine ligand in II does not offer sufficient
stabilisation for the larger metals. Lanthanide borohydrides
have only very recently been introduced as effective lanthanide
(pre)catalysts.5 For example, they are able to initiate the ring
opening polymerisation of e-caprolactone (CL) with good
complexes [Sm(N2NNTMS)Cl]2 3 and [Sm(N2NNTMS)BH4]2
4
[Sm(N2NNMes)Cl]2 5, [Sm(N2NNMes)(BH4)2Li]2 6 were obtained
in good yields (Scheme 1).† Clearly complexes 3 and 5 are
potentially useful synthons through halide metathesis reactions.
5a
control of molecular weights. They also afford trans-specific
5b
diene polymerisation catalysts in the presence of MgBu2.
However, lanthanide borohydrides have not yet been shown to
initiate the controlled polymerisation of methyl methacrylate,
nor have any non-metallocene derivatives been exploited in
any a-olefin polymerisation. Here we report the first non-
hydrocarbyl lanthanide complexes of the type [(L)nM(X)] (X =
BH4). They are also the first BH4-derived single site and single
component catalysts for the controlled polymerisation of MMA.
We recently showed how the lithiated diamide-diamine salt
Li2N2NNTMS (1, Chart 1) and its protio analogue H2N2NNTMS
allow entry to a range of early metal complexes of the
Scheme 1 Reagents and yields: (i) SmCl3, 34% (3) and 81% (5); (ii)
[Sm(BH4)3(THF)2], 76% (4) and 79% (6).
The structures of 3, 5 and 6 were confirmed by X-ray
crystallography,‡ and those of 5 and 6 are shown in Figs. 1 and
2, respectively. All three compounds possess dimeric structures
in the solid state with chloride or lithium borohydride bridges.
† Electronic supplementary information (ESI) available: Selected char-
acterising data. See http://www.rsc.org/suppdata/dt/b4/b417279p/
T h i s j o u r n a l i s
T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 5
D a l t o n T r a n s . , 2 0 0 5 , 4 2 1 – 4 2 3
4 2 1
©

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1
features one BH4 ligand per N2NNTMS ligand in its H NMR
7
spectrum and no evidence for bound lithium in its Li NMR
spectrum. The IR data for 4 are not consistent with a terminal
3
g -bound BH4 group,8 but are consistent with a BH4-bridged
dimer.8
Polymerisation reactions of e-caprolactone were carried out
with the borohydride complexes 4 and 6. The reactions were
performed in toluene with added THF in order to dissolve the
catalyst and, we believe based on NMR experiments, cleave the
dimeric structures. Upon addition of the monomer the solution
rapidly became viscous. The polymerisation of 250 equivs. of e-
caparolcatone by complex 4 was complete within l min, affording
poly(e-caprolactone) in 96% yield with Mn(exp) = 24000 and
Mn/Mw = 1.17.§ The molecular weight is in accordance with
the expected value (Mn(theory) = 26700), calculated on the basis
of a single chain growing per samarium centre. In contrast,
the otherwise identical chloride analogue 3 showed very low
activity (only 32% yield after 2 h) with very poor control of
the molecular weight (Mn(exp) = 26900 vs. Mn(theory) = 14800;
Mn/Mw = 1.80). This shows that while the Sm–Namide linkages
are slightly reactive, this is negligeable in comparison to the
BH4 centered initiation which affords quantitative (100%) e-
caprolactone conversion within 1 min with a good molecular
weight control. The borohydride compound 6 was half as active
as 4 under the same conditions. This may be due to the greater
steric encumbrance of the mesityl groups.
Fig. 1 Displacement ellipsoid plot of [Sm(N2NNMes)Cl]2 5.
We also investigated the polymerisation of methyl
methacrylate9 with the lanthanide borohydride complexes 4 and
6. The results of MMA polymerisation experiments are sum-
marised in Table 1. Both complexes 4 and 6 afford poly(methyl
methacrylate) under these conditions. Compound 4 is the most
active, leading to very narrow polydispersity indices of ca.
1.2. The polymerisation◦ with 4 occurs at a wide range of
◦
temperatures, from +25 C to −78 C. The microstructure of
the poly(methyl methacrylate) is dependant on the temperature,
and the syndiotactic ratio increases from 34.8% at 25 ◦C to
Fig. 2 Displacement ellipsoid plot of [Sm(N2NNMes)(BH4)2Li]2 6.
◦
64.5% at −78 C. Compound 6 has not been evaluated at low
temperature, its activity being already low at room temperature.
This is the first example of the polymerisation of MMA by a
lanthanide borohydride compound. The chloride compound 3
was also tested for the polymerisation of MMA at 25 ◦C and was
found to be completely inactive even after 24 h, which is again
consistent with the BH4 group (or a hydride derived therefrom)
initiating the polymerisation.
Compounds 3 and 5 feature 6-coordinate, distorted octahedral
metal centres. Compound 6 was obtained as an “ate” complex
with one residual LiBH4 per Sm (confirmed by 1H and 7Li
NMR spectroscopy); the coordination number of each Sm in
6 is 9 including 5 Sm · · · H(B) contacts [one BH4 is tridentate;
the other is bidentate with Sm–B distances of Sm(1)–B(1) 2.927
˚
and Sm(1)–B(2) 2.764 A]. In this structure the lithium atoms are
In conclusion, we have described the first polydentate amide-
supported single component and single site lanthanide catalysts
for the controlled polymerisation of polar monomers, and also
the first lanthanide borohydride complex for the polymerisation
of methyl methacrylate. These systems clearly have considerable
scope for development (metal, diamide-donor ligand set and N-
substituents). Further studies on other lanthanide borohydride
complexes of N2NNTMS, N2NNMes and related ligands are in
progress and will be reported in due course.
the bridges for the two samarium amido borohydride fragments
and both are linked to three hydrogen atoms of three different
BH4 groups. The lithium atoms are each further supported by
coordination to a mesityl aromatic ring. There is no donor
solvent in the coordination sphere. This is a new structure of
this type, and the most similar one was for an ytterbocene
borohydride “ate” complex reported by Khvostov et al. which
was polymeric and contained two THF molecules per metal
centre.7 The additional interaction between the mesityl rings
and the lithiums appears to be important in the formation of
this “ate” complex. Compound 4 is not an “ate” complex and
We thank the Leverhulme Trust for support, Dr P. L. Burn
for use of his GPC apparatus and Dr S. Guillaume for a gift of
lanthanide borohydrides and helpful discussions.
Table 1 Methyl methacrylate polymerisation experiments with borohydride complexes 4 and 6a
Tacticity (%)c
Complex
T/◦C
t/h
Yield (%)
Mn(exp)b/g mol−1
PDIb
mm
mr
rr
4
4
4
6
25
0
3
3
1.5
12
50
50
7.6
26
35,660
52,800
—
1.21
1.25
—
27
7
11
25
38
35
24.5
40
35
58
64.5
35
d
d
−78
25
12,400
1.64
a Conditions: solvent = 1 mL of toluene and 0.1 mL of THF, [MMA]/[catalyst] = 400. b Measured by gel permeation chromatography at 30 ◦C in
THF relative to PS standards with Mark–Houwink corrections for Mn. c Measured by 1H NMR in CDCl3. d Polymer not soluble in THF
4 2 2
D a l t o n T r a n s . , 2 0 0 5 , 4 2 1 – 4 2 3

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‡ Crystal Data: for C62H78Cl2N8Sm2 (5), M = 1307.06, monoclinic,
˚
space gr◦oup P21/n, a = 16.165(3), b = 21.332(4), c = 17.402(4) A, b =
97.46(3) , V = 5949.8(21) A , T = 150 K, Z = 4, l = 2.089 mm−1, 26058
3
˚
reflections measured, 13576 unique, Rint = 0.03. Final R values were R1 =
0.0346 and Rw = 0.0387 (for I > 3r(I)). For C28H44B2LiN4Sm (6), M =
¯
615.65, triclinic, space group P1, a = 10.5720(1), b = 11.6396(2), c =
◦
˚
˚
12.6413(2) A, a = 101.4560(6), b = 105.0218(7), c = 93.5125(10) , V =
3
1461.99(4) A , T = 150 K, Z = 2, l = 2.031mm−1, 15945 reflections
measured, 8786 unique, Rint = 0.018. Final R values were R1 = 0.0293
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I > 2r(I)). (Data for 3 have been made available to referees and will
be disclosed in a future publication.) CCDC reference numbers 251360
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crystallographic data in .cif or other electronic format.
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D a l t o n T r a n s . , 2 0 0 5 , 4 2 1 – 4 2 3
4 2 3