Cationic borohydrido-neodymium complex: Synthesis, characterization and its application as an efficient pre-catalyst for isoprene polymerisation

Article abstract of DOI:10.1039/b806669h

The cationic borohydrido lanthanide complex [Nd(BH₄) ₂(THF)₅][B(C₆F₅)₄] 1, prepared from the reaction of Nd(BH₄)₃(THF)₃ with [HNMe₂Ph][B(C_6<

Full text of DOI:10.1039/b806669h

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Cationic borohydrido–neodymium complex: Synthesis, characterization and
its application as an efficient pre-catalyst for isoprene polymerisation†
Marc Visseaux,*a Michael Mainil,a Michael Terrier,a Andre´ Mortreux,a Pascal Roussel,a Thomas Mathivetb
and Mathias Destaracb,c
Received 21st April 2008, Accepted 19th June 2008
First published as an Advance Article on the web 11th July 2008
DOI: 10.1039/b806669h
The cationic borohydrido lanthanide complex [Nd-
(BH4)2(THF)5][B(C6F5)4] 1, prepared from the reaction of
Nd(BH4)3(THF)3 with [HNMe2Ph][B(C6F5)4], is highly active
towards isoprene polymerisation upon activation with Al(i-
Bu)3, whereas the in situ prepared ternary catalytic com-
bination Nd(BH4)3(THF)3/[HNMe2Ph][B(C6F5)4]/Al(i-Bu)3,
albeit less active, displays higher cis-selectivity and better
control in terms of macromolecular data.
compound was revealed to be highly active towards the
polymerisation of isoprene. The ternary catalytic system
Nd(BH4)3(THF)3/[HNMe2Ph][B(C6F5)4]/Al(i-Bu)3, prepared in
situ without isolation of 1, behaves very similarly but with better
control in terms of macromolecular data, together with higher
levels of cis-selectivity.
Ephritikhine and co-workers reported a decade ago that
the protonation of the monoborohydrido (COT)Nd(BH4)(THF)2
derivative (COT = C8H8) with HNEt3BPh4 afforded the cationic
(COT)Nd(THF)2 compound.9a Following this idea, we investi-
+
Catalysts based on rare-earth metals are known to be highly
effective towards conjugated diene polymerisation.1 Among these
catalysts, well-defined cationic species are of particular interest,
due to their excellent activity along with cis-stereocontrol.2
Such compounds are generally prepared by the straightforward
reaction of an alkylanilinium borate with a lanthanide alkyl,3
allyl,4 hydride,5 amido,6 or a chlorolanthanide/alkylaluminium
intermediate.7 However, the synthesis and isolation of these lan-
thanide initiators require difficult experimental work-up, owing to
their great sensitivity towards moisture and oxygen, together with
their thermal stability.8 It is thus of interest to discover more simple
precursors and procedures that could be utilized successfully in
catalytic combinations, enabling controlled polymerisations.
We and other groups have demonstrated that the lanthanide
trisborohydrides Ln(BH4)3(THF)3, considered as a true alternative
to trichlorides for organometallic syntheses only recently,9 could
also initiate the polymerisation of a wide range of polar10 and non-
polar11 monomers. In particular, we established that combined
with MgR2 co-catalysts, Nd(BH4)3(THF)3 affords the controlled
polymerisation of isoprene in a trans-selective manner.12
1
gated by H NMR monitoring the reaction of Nd(BH4)3(THF)3
with HNEt3BPh4 in THF-D8. At room temperature, the con-
sumption of the starting material was quite low but traces of
released H2 (d = 4.4 ppm) were an indication of the expected
protonation reaction.13,14 In the presence of one equivalent of
[HNMe2Ph][B(C6F5)4] in THF-D8, Nd(BH4)3(THF)3 was this time
immediately and completely consumed, along with unambiguous
formation of H2. The experiment was then conducted on a bulk
scale (Scheme 1), and after experimental work-up (see ESI†),
purple crystals of 1 could be isolated in good yield (68%).
Elemental analysis confirmed the chemical composition of a
bisborohydrido compound, and corresponded to the formula
[Nd(BH4)2(THF)5][B(C6F5)4].
Scheme 1 Synthesis of complex 1.
Crystals of 1 were grown from THF–pentane at −20 ◦C. An
X-ray structure determination established the expected ionic pair
structure, without any interaction between Nd(BH4)2(THF)5+ and
its borate counteranion (Fig. 1).‡
In this communication, we report that upon straightfor-
ward reaction between Nd(BH4)3(THF)3 and [HNMe2Ph]-
[B(C6F5)4], the first borohydrido cationic compound, [Nd(BH4)2-
(THF)5][B(C6F5)4], 1, could be isolated and fully characterized,
including its X-ray structure. Combined with Al(i-Bu)3, this
The cationic moiety adopts a distorted heptahedral ar-
3
rangement and BH4 ligands both exhibit an g -H3BH termi-
nal bonding mode. The molecular structure of the cationic
+
Nd(BH4)2(THF)5 is fully comparable to the samarium
aUnite´ de Catalyse et de Chimie du Solide (UCCS, UMR 8181 CNRS),
ENSCL, Baˆt. C7, Cite´ Scientifique, B.P. 90108, 59652, Villeneuve d’Ascq
ce´dex, France. E-mail: marc.visseaux@ensc-lille.fr; Fax: +33 (0)320 6585;
Tel: +33 (0)320 6483
one found in the recently described bimetallic compound
[Sm(BH4)2(THF)5]+[Cp*ꢀSm(BH4)3]− (Cp*ꢀ = C5Me4nPr), which
exhibited an ionic structure in the solid state only.15 The Nd–
bRhodia Ope´rations, Centre de Recherche et Technologies d’Aubervilliers, 52
rue de la Haie Coq, 93308, Aubervilliers, France
˚
B (BH4) distances in 1 (average 2.618 A) are comparable or
slightly shorter than those reported for monocyclopentadienyl
bisborohydrido neodymium compounds.9
cLaboratoire He´te´rochimie Fondamentale et Applique´e (UMR 5069),
Universite´ Paul Sabatier, 118 route de Narbonne, 31062 Toulouse Cedex
9, France
† Electronic supplementary information (ESI) available: Experimental
details of the synthesis and analytical data for complex 1, and for the
polymerisations reactions. CCDC reference number 660039. For ESI
and crystallographic data in CIF or other electronic format see DOI:
10.1039/b806669h
The ability of 1 to polymerise isoprene was studied in the
presence of Al(i-Bu)3, as described for typical polymerisation
experiments involving a cationic lanthanide precursor.16 At 20 ◦C
and in the presence of 10 equivalents of Al(i-Bu)3, the monomer
conversion was nearly complete (82% recovered polymer), in only
4558 | Dalton Trans., 2008, 4558–4561
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(during ca. 10 min), and for high [I]/[Nd] ratios (3000), the cis
percentage can reach a value of 92% while the activity remains
high, whatever the quantity of toluene added subsequently (entry
3). In diluted conditions (entry 4: monomer/catalyst ratio = 1000,
1 mL toluene) the activity decreases to less than 2 kg mol−1 h−1.
Besides, the amount of co-catalyst seems to be directly related
to the selectivity: in the presence of 3.5 equiv. Al(i-Bu)3 the cis
rate decreases below 80% (entry 5). It is however possible to
increase the cis selectivity up to 90% for low monomer/catalyst
ratios, by varying the quantity of borate activator (entries 6, 7);
the highest activity (ca. 120 kg(PI) mol(Nd)−1 h−1) is observed
using 1.5 borate per Nd (entry 6). In heptane (entry 8): the
system is less active and the cis selectivity is lowered, in sharp
contrast to what is generally observed in hydrocarbon solvents.17
Raising the temperature allows an improvement in the monomer
conversion, but to the detriment of the selectivity and the control of
macromolecular data (entries 9–12). Nevertheless, it is noteworthy
that under such conditions, and in the presence of large excesses of
co-catalyst, low molecular weight polyisoprene can be obtained,
involving probably some transfer between Nd and Al (see further,
transfer reactions section).
Fig. 1 ORTEP drawing of 1 (ellipsoid probability level 30%). Hydrogen
˚
atoms except those on BH4 omitted for clarity. Selected bond lengths [A]
and angles [◦]: Nd1–B1 2.596(4); Nd1–B2 2.641(4); B1–Nd–B2 177.67 (13).
30 min (entry 1; Table 1). The activity was high and the stereospeci-
ficity observed was fairly good (84% cis) but the SEC (size exclu-
sion chromatography) curve, albeit monomodal, was found to be
somewhat broad, characteristic of a rather low initiation process
(PDI 2.31). This was attributed to the poor solubility of crystalline
1 in toluene. These observations prompted us to study the poly-
merisation of isoprene with a catalytic system in which the cationic
In all cases, the chromatograms show a monomodal character,
typical of a single-site catalysis. The PDI values remain in a range
typical of a rather controlled process (< 1.9), at temperatures
not exceeding 75 ◦C. Fig. 2 shows two typical SEC traces, and
one can observe the much narrower molecular weight distribution
provided by the ternary catalytic system (right, entry 3) vs. isolated
1 (left, entry 1).
+
Nd(BH4)2 , formed in situ, would not be isolated. Preliminary
polymerisation experiments were then conducted with the ternary
catalytic system Nd(BH4)3(THF)3/[HNMe2Ph][B(C6F5)4]/Al(i-
Bu)3, under the same conditions as for 1 (the mixture was allowed
to react at a given temperature for a given time. Representative
results are reported Table 1, entries 2–12).
As expected, a better control of macromolecular data is
observed at 20 ◦C (monomodal curves, PDI 1.34–1.91). In the
presence of 1 equiv. [HNMe2Ph][B(C6F5)4] per Nd, the results
are comparable to those obtained from isolated 1. The activity
is sensibly lower (184 vs. 350 kg mol−1 h−1) but this result is
balanced by a narrower PDI value, indicating that the process
is more controlled (entry 2). The isoprene concentration has a
strong effect on the course of the polymerisation: when the reaction
between the pre-catalyst and the ionic activator is carried out
in a minimum of toluene (25 to 100 lL) in a preliminary stage
Fig. 2 SEC curve of entries 1 (cationic precatalyst 1, left) and 3 (ternary
catalytic system, right) showing the monomodal character.
Table 1 Polymerisation of isoprene with Nd(BH4)3(THF)3/[HNMe2Ph][B(C6F5)4]/Al(i-Bu)3
Activator Al(i-Bu)3
Average activity/
[I]/[Nd] T/◦C Time/h Yield (%) kg mol−1 h−1
% trans;
Entry a (equiv.)
(equiv.)
% cis h % 3,4h
Mn,exp
Mw/Mn (PDI) Eff.i (%)
1b
2c
3d
4
—
1
1
1
1
1.5
2
1
1
1
10
7
3125
3000
3000
1000
1000
1000
1500
3000
1000
1000
1000
1000
20
20
20
20
20
20
20
20
50
50
75
90
0.5
0.5
0.75
24
1.5
0.5
48
3.5
24
24
24
4
82
46
25
66
84
90
50
30
78
29
87
75
350
184
68
84
86
92
81
78
80
90
76
76
66
46
38
10; 6
10; 4
3.4; 4.6
15; 4
18; 4
17; 3
6; 4
20; 4
19; 5
29; 5
51; 4
58; 4
85 000 2.31
121 200 1.63
55 700 1.34
47 100 1.71
109 400 1.91
62 900 1.70
56 400 1.74
99 500 1.62
37 000 1.87
10 200 1.65
9500 1.90
51
76
61
100
52
100
91
10
10
3.5
10
20
8
10
50
50
10
1.9
5
38
122
1
17.5
3.25
8
2.5
12.7
6e
7f
8g
9
60
143
196
630
140
10
11
12
1
1
36 700 2.21
a 10 lmol Nd, 1 mL toluene. b Pre-catalyst 1 (9.6 lmol), 0.05 mL toluene. c (100 lL + 0.4 mL) toluene. d (50 lL + 0.4 mL) toluene. e 5 lmol Nd, 0.025 mL
toluene. f 0.1 mL toluene. g Solvent = heptane 0.5 mL. h Determined by 1H and 13C NMR. i Eff = 100 × Mn,theor/Mn,exp, Mn,theor = ([I]/[Nd]) × yield% ×
68; efficiency factor calculated for one growing chain per metal.
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Table 2 Effect of the [Al]/[Nd] ratio
Entry a
Al(i-Bu)3 (equiv.)
Yield (%)
Time/h
% cis
Mn,exp
Mw/Mn (PDI)
Eff.c (%)
13
14
15
30
40
30
40
70
2
6
20
78
75
67
29 800
22 300
14 400
1.68
1.74
1.55
200
360
970
100 b
a 1 Nd(BH4)3(THF)3/1 [HNMe2Ph][B(C6F5)4]/1 mL toluene, [I]/[Nd] = 3000, 50 ◦C. b Addition of Al co-catalyst in two stages: Nd(BH4)3(THF)3
+
[HNMe2Ph][B(C6F5)4] + 1 mL toluene + 40 equiv. Al + isoprene + 60 equiv. Al. c Same as in Table 1.
As reported in general in the literature for Nd-based catalysts,
a high degree of control of macromolecular data (including
narrow PDI values), together with full initiation efficiency of the
lanthanide metal (i.e. calculated for one growing chain per metal;
Eff. = 100 × Mn,theor/Mn,exp, Mn,theor = ([I]/[Nd]) × yield% × 68),
is still hard to obtain, particularly in the specific case of isoprene.
In our case, the efficiency of this new catalytic system, considering
the protonation of one BH4 group per metal, is much higher than
previously described: from ca. 52% (entry 4), to 100% at 20 ◦C
(entry 3).§
The authors are grateful for financial support from The Re´gion
Nord-Pas de Calais (NanoCat ARCIR Project), the “Fonds Eu-
rope´en de De´veloppement Re´gional” (FEDER), and the CNRS,
and wish to thank A. M. Caze for SEC analyses, M. Bria for 11
B
NMR and Dr P. Zinck and Dr F. Bonnet for helpful discussions.
Notes and references
‡ Complex 1 (C44H48B3F20O5Nd, M = 1213.5 g mol−1) crystallizes in the
monoclinic space group P21/c◦with a = 8.6142(2), b = 34.3484(8), c =
3
˚
˚
16.4956(4) A, b = 96.9390(10) , V = 4845.0(2) A , T = 100(2) K, q =
1.663 g cm−3 for Z = 4. 59 711 total reflections, 10 233 independent
reflections, Rint = 0.0362. final R values = 4.37%, wR = 4.99% [I>3.0r(I)];
R = 6.28%, wR = 5.32% for (all reflections). CCDC 660039.
The mastering of transfer reactions in polymerisation catalysis,
so called “catalysed chain growth polymerisation”, is of interest
because it allows the preparation of functionalised polymers
and/or oligomers.18 As far as we know, the very few reports
concerning conjugated dienes and involving lanthanide catalysts
all refer specifically to butadiene.1,19 In the presence of large
excesses of Al co-catalyst at 50 ◦C (Table 2), we observed that
Mn values strongly decrease when the Al(i-Bu)3 amount varies
from 30 to 100 equivalents. We assume that a non-negligible
part of the Al co-catalyst acts as a transfer agent, allowing a
number of growing chains per metal up to ca. 10 (Eff. = 970%,
entry 15). Interestingly, the PDI values remain quite narrow (1.5–
1.7), indicating a rapid exchange between Al and Nd, but to
the detriment of both the selectivity, as already reported with
butadiene,20 and the activity (entry 10, Al/Nd = 50, vs. entry 9,
Al/Nd = 10, Table 1). Nevertheless, increasing the temperature
to 75 ◦C allows enhancement of the conversion, as well as the
transfer process (Eff. = 630%, entry 11, Table 1).
§ Efficiency of the metal is here expressed as commonly reported in the
literature for Nd-based catalysts, even in the presence of Al co-catalyst
excess,1,19,20 i.e. by reference to Nd metal, despite the fact that Al may
contribute via Al–Nd transfer (see next paragraph).
1 L. Friebe, O. Nuyken and W. Obrecht, Adv. Polym. Sci., 2006, 204, 1
and references therein.
2 P. M. Zeimentz, S. Arndt, B. R. Elvidge and J. Okuda, Chem. Rev.,
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In conclusion, the new borohydrido ionic [Nd(BH4)2-
(THF)5][B(C6F5)4] can be prepared straightforwardly from the
simple precursor Nd(BH4)3(THF)3 and [HNMe2Ph][B(C6F5)4].
Combined with Al(i-Bu)3 this cationic compound polymerises iso-
prene with good activity but a more controlled process is obtained
when the cation is prepared in situ by using the ternary catalytic
system Nd(BH4)3(THF)3/[HNMe2Ph][B(C6F5)4]/Al(i-Bu)3. This
strongly supports the formation of a cationic active species during
the in situ polymerisation process. Depending on the experimental
conditions, it is possible to reach up to 92% cis selectivity, together
with good control of Mn and PDI, and high initiation efficiency
of the catalyst. In the presence of large excesses of Al co-catalyst,
transfer to aluminium is evidenced, which opens the possibility
of regulation of the molecular weight of the polymer, as well as a
convenient route to functionalized polymers, without the necessity
of expanding the quantity of Nd-based initiator. This study clearly
confirms the high potential of borohydrido Ln-based catalysts
toward polymerisation. Further studies are aimed at the synthesis
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14 The preparation of cationic 1 was preliminarily mentioned in: M.
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