Organometallic calcium and strontium borohydrides as initiators for the polymerization of ε-caprolactone and l-lactide: Combined experimental and computational investigations

Article abstract of DOI:10.1039/c3dt00037k

[Ca(BH₄)₂(THF)₂] (1a), a known compound, was easily prepared following a convenient new procedure from [Ca(OMe) ₂] and BH₃·THF in THF. Reaction of 1a with KCp* (Cp* = (η⁵-C₅Me₅)) and K{(Me₃SiNPPh₂)₂CH} in a 1:1 ratio in THF resulted in the corresponding dimeric heteroleptic mono-borohydride derivatives [Cp*Ca(BH₄)(THF)_n]₂ (2a) and [{(Me ₃SiNPPh₂)₂CH}Ca(BH₄)(THF) ₂] (3a), respectively. Both compounds were fully characterized and the solid-state structure of 3a was established by single crystal X-ray diffraction. Compounds 1a, 2a, and 3a, together with the earlier reported compounds [Sr(BH₄)₂(THF)₂] (1b), [Cp*Sr(BH₄)(THF)₂]₂ (2b), and [{(Me ₃SiNPPh₂)₂CH}Sr(BH₄)(THF) ₂] (3b), were used as initiators for the ROP of polar monomers. The general performances of the complexes in the ROP of ε-caprolactone and l-lactide demonstrate a relatively good control of the polymerization under the operating conditions established. α,ω-Dihydroxytelechelic poly(ε-caprolactone)s (PCLs) and poly(lactide)s (PLAs) were thus synthesized. DFT calculations on the initiation step of the ROP of ε-CL were carried out. Gibbs free energy profiles were determined for the three calcium complexes highlighting slightly more active calcium complexes as compared to strontium analogues, in agreement with experimental findings.

Full text of DOI:10.1039/c3dt00037k

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Organometallic calcium and strontium borohydrides as
initiators for the polymerization of ε-caprolactone and
L-lactide: combined experimental and computational
investigations†
Cite this: DOI: 10.1039/c3dt00037k
Magdalena Kuzdrowska,a Liana Annunziata,b Sebastian Marks,a Matthias Schmid,a,b
Cedric G. Jaffredo,b Peter W. Roesky,*a Sophie M. Guillaume*b and Laurent Maron*c
[Ca(BH4)2(THF)2] (1a), a known compound, was easily prepared following a convenient new procedure
from [Ca(OMe)2] and BH3·THF in THF. Reaction of 1a with KCp* (Cp* = (η5-C5Me5)) and K{(Me3SiNPPh2)2CH}
in a 1 : 1 ratio in THF resulted in the corresponding dimeric heteroleptic mono-borohydride derivatives
[Cp*Ca(BH4)(THF)n]2 (2a) and [{(Me3SiNPPh2)2CH}Ca(BH4)(THF)2] (3a), respectively. Both compounds were
fully characterized and the solid-state structure of 3a was established by single crystal X-ray diffraction.
Compounds 1a, 2a, and 3a, together with the earlier reported compounds [Sr(BH4)2(THF)2] (1b),
[Cp*Sr(BH4)(THF)2]2 (2b), and [{(Me3SiNPPh2)2CH}Sr(BH4)(THF)2] (3b), were used as initiators for the ROP
of polar monomers. The general performances of the complexes in the ROP of ε-caprolactone and
L-lactide demonstrate a relatively good control of the polymerization under the operating conditions
established. α,ω-Dihydroxytelechelic poly(ε-caprolactone)s (PCLs) and poly(lactide)s (PLAs) were thus syn-
thesized. DFT calculations on the initiation step of the ROP of ε-CL were carried out. Gibbs free energy
profiles were determined for the three calcium complexes highlighting slightly more active calcium com-
plexes as compared to strontium analogues, in agreement with experimental findings.
Received 4th January 2013,
Accepted 26th February 2013
DOI: 10.1039/c3dt00037k
www.rsc.org/dalton
synthesis of new compounds,10–12 their reactivity,13–16 and
some catalytic applications.17–20
Introduction
There is significant increasing interest in the coordination and
Among others, the ring-opening polymerization (ROP) of
organometallic chemistry of the heavier alkaline earth metals cyclic esters such as ε-caprolactone (ε-CL) and lactide (LA)
(Ca, Sr, Ba).1–6 Within these series, calcium compounds using well-characterized calcium, and some strontium and
remain the most largely investigated.4,5,7 Although calcium is barium compounds, has been reported lately.21–31 By using
an inexpensive and non-toxic element present in significant well-defined alkaline earth metal compounds as initiators, the
amount in the human body, access to its organometallic and access to polymers with controlled molecular features – in par-
coordination compounds is often limited by their kinetic labi- ticular molar mass and dispersity (ÐM = Mw/Mn) – end-group
lity with detrimental Schlenk-type equilibria.8,9 Modern fidelity and, in some cases, controlled stereochemistry, is
research of organometallic and coordination compounds of possible.4,32,33
the heavier alkaline earth metals essentially deals with the
Mainly alkoxides/phenoxide26–30,34 and amido com-
plexes22,25,35 of the alkaline earth metals have been reported as
initiators for the ROP of cyclic esters. However, these remain
underdeveloped in comparison to the related rare-earth metal
analogues. On the other hand, borohydride derivatives, which
nowadays are well-established ROP initiators in rare earth
chemistry,36 have been so far nearly neglected in the polymeri-
zation chemistry of alkaline earth elements. The first reports
in this area were only published recently by Mountford et al.
on calcium,31,37 followed by a report from our groups on stron-
tium.24 This is rather surprising given that [Ca(BH4)2(THF)2]
(1a) is commercially available for applications in material
aInstitut für Anorganische Chemie and Helmholtz Research School: Energy-Related
Catalysis, Karlsruher Institut für Technologie (KIT), Engesserstr. 15, Geb. 30.45,
76131 Karlsruhe, Germany. E-mail: roesky@kit.edu
bInstitut des Sciences Chimiques de Rennes, Organometallics, Materials and
Catalysis, UMR 6226, CNRS – Université de Rennes 1, Campus de Beaulieu,
35042 Rennes Cedex, France. E-mail: sophie.guillaume@univ-rennes1.fr
cUniversité de Toulouse, INSA, UPS, CNRS-UMR5215, LPCNO, 135 Avenue de
Rangueil, 31077 Toulouse, France. E-mail: laurent.maron@irsamc.ups-tlse.fr
†Electronic supplementary information (ESI) available: CIF file of 3a. CCDC
915058. For ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/c3dt00037k
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synthesis of [Sr(BH4)2(THF)2] (1b),39 from [Ca(OMe)2]50 and
BH3·THF in THF in a 1 : 3 ratio to get a pure product
(Scheme 2). In comparison to the literature,49 no pyridine is
needed for the synthesis. Reaction of 1a with KCp* and
K{(Me3SiNPPh2)2CH}51 in a 1 : 1 ratio in THF resulted in the
corresponding heteroleptic mono-borohydride derivatives
[Cp*Ca(BH4)(THF)n]2 (2a) and [{(Me3SiNPPh2)2CH}Ca(BH4)-
(THF)2] (3a), respectively (Scheme 3).
Scheme 1 Investigated alkaline earth metal borohydride compounds.
The new complexes 2a and 3a have been characterized by
standard analytical/spectroscopic techniques. The 1H NMR
spectrum of 2a shows the expected resonance for the Cp*
science38 and that [Sr(BH4)2(THF)2] (1b) was reported as early
as in the mid-nineties.39
−
ligand at δ 2.00 ppm and a broad quartet for the BH4 group
As part of our ongoing efforts aimed both at implementing
alkaline earth complexes in fine organic catalysis40–45 and at
valorizing original organometallic borohydride complexes in
polymerization catalysis,46–48 we communicated in our pre-
vious study the application of 1b in the ROP of ε-CL as well as
the synthesis and structural characterization of the boro-
hydride half-sandwich [Cp*Sr(BH4)(THF)2]2 (Cp* = (η5-C5Me5))
(2b) and the borohydride bis(phosphinimino)methanide
at δ −0.44 ppm (JBH = 83 Hz). For 3a the expected signals of the
1
bis(phosphinimino)methanide ligand were observed in the H
NMR spectrum. The characteristic signal of the methine group
was not well resolved, resulting in a broad peak at δ 1.85 ppm,
as recorded at room temperature. As for 2a, the expected broad
−
quartet for the BH4 group was observed at δ 0.10 ppm (JBH
=
82 Hz) for 3a. In the 1H NMR spectra of compounds 2a and 3a,
the different coordination modes of the B–H protons observed
in the solid state (see below) do not give rise to resolved
signals. This may result from a flexible coordination mode of
[{(Me3SiNPPh2)2CH}Sr(BH4)(THF)2]
(3b)
complexes
(Scheme 1).24 These compounds 1b, 2b, and 3b subsequently
proved successful in the ROP of ε-CL.24 Noteworthy, the
strontocene borohydride complex 2b afforded the highest
molar mass poly(ε-caprolactone) (PCL) prepared from a mole-
cular alkaline earth metal initiator in a quite controlled ROP
approach. α,ω-Dihydroxytelechelic PCLs were thus obtained
under mild operating conditions (23 °C, THF) as evidenced
from NMR analyses, and the mechanism was investigated by
DFT calculations.24 Whereas the computed initiation step was
found to be kinetically and thermodynamically possible for all
three derivatives 1b, 2b and 3b, the reaction appeared more
kinetically facile with the more electron-enriched Cp* species
2b, while it was calculated to be almost athermic in the case of
the homoleptic bisborohydride compound 1b.
−
the BH4 group in solution. The 11B NMR spectra of 3a
−
showed the awaited quintets (JBH = 82 Hz) of the BH4 group
for both compounds at δ −36.0 (2a) and −34.8 (3a) ppm,
which is in the range of that recorded with 1a (δ −36.6 ppm).
In the 31P{1H} spectrum of 3a, a singlet was observed for
the two magnetically equivalent phosphorus atoms at
δ 16.4 ppm, in good agreement with the related complexes
[{(Me3SiNPPh2)2CH}CaI(THF)2] (δ 18.7 ppm)43 and 3b
(δ 15.2 ppm).24 Noteworthy, the NMR data did not show any
sign of a ligand redistribution in solution. The IR bands of the
−
BH4 group are broad precluding unambiguous identification
of its coordination mode which could, however, in the case of
3a be determined by X-ray analysis.52,53 The poor quality of the
With the aim to evaluate the potentially different polymeri-
zation abilities of strontium and calcium analogues, we then
investigated, both experimentally and computationally, the
ROP of ε-CL promoted by the corresponding calcium com-
plexes [Ca(BH4)2(THF)2] (1a), [Cp*Ca(BH4)(THF)n]2 (2a) and
[{(Me3SiNPPh2)2CH}Ca(BH4)(THF)2] (3a) (Scheme 1), and also
extended the work to the ROP of L-lactide (LLA). Thus, we now
report herein the synthesis and the structural studies of the
analogous calcium borohydride complexes 2a and 3a
(Scheme 1). We also present a full account of the experimental
and computational evaluation of the catalytic efficiency of 1a,
b, 2a,b, and 3a,b as initiators for the ROP of ε-CL and LLA,
including DFT calculations on the mechanism of the ROP of
ε-CL.
Scheme 2 Synthesis of [Ca(BH4)2(THF)2] (1a).
Results and discussion
Synthesis and characterization of calcium borohydride
complexes 1a, 2a and 3a
Although [Ca(BH4)2(THF)2] (1a)49 is commercially available, we
Scheme 3 Synthesis of [Cp*Ca(BH4)(THF)n]2 (2a) and [{(Me3SiNPPh2)2CH}Ca-
prepared it following a new procedure which is similar to the (BH4)(THF)2] (3a).
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Scheme 4 ROP of ε-CL and LLA in THF with calcium and strontium boro-
hydride complexes 1a,b, 2a,b, 3a,b.
Fig. 1 Solid-state structure of 3a, omitting some hydrogen atoms. Selected dis-
tances [Å] and angles [°]: Ca–N1 2.493(3), Ca–N2 2.488(3), Ca–O1 2.461(3), Ca–
O2 2.452(3), Ca–C1 2.719(4), Ca–H40 2.37(5), Ca–H41 2.40(7), Ca–H42 2.61(6),
Ca–B 2.633(5), N1–P1 1.584(4), N1–Si1 1.708(4), N2–P2 1.590(4), N2–Si2
1.706(3), C1–P1 1.726(4), C1–P2 1.741(4), B–H40 1.10(4), B–H41 1.04(7), B–H42
1.04(6), B–H43 1.09(5); N1–Ca–N2 94.09(11), N1–Ca–O1 88.36(11), N1–Ca–C1
62.88(12), N1–Ca–B 110.54(15), N2–Ca–B 107.04(14), N2–Ca–C1 63.41(11), O1–
Ca–B 95.79(15), O2–Ca–B 95.24(15), P1–C1–P2 125.0(2).
group as monodentate, the calcium atom is six-fold coordi-
nated. The coordination polyhedron can be considered as a
distorted octahedron. Compound 3a has a non-crystallo-
graphic mirror plane through B–Ca–C1.
Ring-opening polymerization studies
The catalytic activities of the calcium and strontium series of
crystals grown from 2a precluded its characterization by X-ray borohydride complexes 1a,b, 2a,b, and 3a,b were then assessed
diffraction. Therefore, in order to establish the dimeric struc- (Scheme 4). Polymerization studies were typically conducted in
ture of 2a, both compounds 2a and 3a were analyzed by EI-MS. THF, with an initial monomer concentration of 1.0 M at either
For 2a, the molecular peak fitted with the formulation of 2a as 23 °C or 50 °C. Selected data obtained with complexes 1a,b,
a dimeric compound. This formulation is also consistent with 2a,b, and 3a,b are collected in Tables 1 and 2.
the analogous dimeric strontium compound 2b.24 Also, the
The ability of the newly synthesized homoleptic bisboro-
molecular peak observed in the EI-MS of 3a supported the hydride calcium complex 1a to promote the ROP of ε-CL and
data acquired by other methods allowing the formulation of 3a LLA was first evaluated (Table 1, entries 1–4, 13–16). Indeed,
as a monomeric species. Based on the analytical data the although some previously reported studies using the commer-
number of coordinated THF molecules in 2a cannot be exactly cially available compound 1a gave “disappointing perform-
determined. In analogy to 2b we suggest that two molecules of ances”,37 our preliminary investigations24 with the strontium
THF are coordinated to the Ca atom in 2a. This analogy is jus- analogue 1b showed that it was active in the ROP of ε-CL at
tified because 1a and 1b as well as 3a and 3b have the same 23 °C in THF (Table 2, entries 1–4).
amount of THF molecules coordinated to the center metal.
Even though the larger strontium derivatives have been
The solid-state structure of 3a was established by single reported to be more active than the calcium congeners in
crystal X-ray diffraction (Fig. 1). Complex 3a crystallizes in the ROP,26,30 we anticipated that 1a would, at least to some reason-
ˉ
triclinic space group P1 as a monomeric compound. The hydro- able extent and possibly under more severe operating con-
−
gen atoms of the BH4 group were localized in the difference ditions than those used with 1b, allow the controlled synthesis
−
Fourier map and refined isotropically showing that the BH4
of PCL. Rewardingly, under similar working conditions as
group is κ3(H)-coordinated to the calcium atom. The Ca–B dis- those used with 1b, up to 500 ε-CL units were successfully con-
tance is 2.633(5) Å and the Ca–H distances range from 2.37(5) Å verted in high yields (80–99%) within two hours at 23 °C
to 2.61(6) Å. As expected, the bis(phosphinimino)methanide (Table 1, entries 1–4).
ligand forms a six-membered metallacycle (N1–P1–C1–P2–
The control over the ROP process was rather good,
N2–Ca) upon chelation of the two trimethylsilylimine groups affording PCLs featuring a good match between the observed
to the metal atom. The Ca–N distances (Ca–N1 2.493(3) Å and (as determined by NMR and SEC) and calculated (based on
−
Ca–N2 2.488(3) Å) show an almost symmetric bonding of the two active BH4 functions) molar mass values, as well as
ligand onto the metal center. As usually observed for bis(phos- moderate dispersity data (ÐM = Mw/Mn < 1.40). The overall
phinimino)methanide complexes,43,54 the metallacycle adopts efficiency of the calcium initiator 1a toward the ROP of ε-CL
a twist boat conformation in which both the central carbon was thus similar to that of the strontium analogue 1b (Table 1,
(C1) and calcium atoms are displaced from the N2P2 least- entries 1–4 vs. Table 2, entries 1–2). The bisborohydride
squares plane. The interaction between the methine carbon calcium complex 1a was also found to be active in the ROP of
atom (C1) and the calcium atom (2.719(4) Å), which is longer LLA (Table 1, entries 13–16). However, rather prolonged reac-
than common Ca–C distances, is in the range of the related tion times were required with this monomer as compared to
bonds in [{(Me3SiNPPh2)2CH}2Ca] (2.765(1) Å)55 and those typically used for the ROP of ε-CL, reaching only quite
[{(Me3SiNPPh2)2CH}CaI(THF)2] (2.735(6) Å).43 Upon consider- low LLA conversions and thus affording oligomers with, corres-
−
ing the {(Me3SiNPPh2)2CH}− ligand as tridentate and the BH4
pondingly, a rather good control (MnSEC up to 6400 g mol−1
;
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Table 1 ROP of ε-CL and LLA initiated by [Ca(BH4)(THF)2], 1a, [Cp*Ca(BH4)(THF)n]2, 2a, and [{(Me3SiNPPh2)2CH}Ca(BH4)(THF)2], 3a
[Monomer]0/
[BH4]0
Temp.
(°C)
Reaction
timeb
Conv.c
(%)
Mntheo
MnNMR
MnSEC
d
e
f
Monomera
(g mol−1
)
(g mol−1
)
(g mol−1
)
ÐM
a
g
Entry
Complex
1
2
3
4
5
6
7
8
1a
1a
1a
1a
2a
2a
2a
3a
3a
3a
3a
3a
1a
1a
1a
1a
2a
2a
2a
3a
3a
3a
3a
3a
ε-CL
ε-CL
ε-CL
ε-CL
ε-CL
ε-CL
ε-CL
ε-CL
ε-CL
ε-CL
ε-CL
ε-CL
LLA
LLA
LLA
LLA
LLA
LLA
LLA
LLA
LLA
LLA
LLA
LLA
50/1
23
23
23
23
23
23
23
23
23
23
23
50
23
23
23
23
23
23
23
23
23
23
50
23
10 min
20 min
60 min
110 min
10 min
15 min
20 min
20 min
60 min
180 min
360 min
360 min
65 h
65 h
66 h
67 h
20 min
40 min
60 min
15 h
18 h
24 h
99
99
96
80
99
100
100
100
98
75
45
60
53
42
24
6
50
31
26
80
67
33
48
0
5600
11 300
32 800
45 600
11 300
34 200
57 000
11 400
33 500
42 750
51 300
68 400
3800
6000
8600
4300
7200
13 400
18 700
11 500
28 950
23 800
34 600
—
4300
8100
22 100
—
14 800
33 000
41 400
9300
20 700
33 400
38 000
—
3900
2800
6200
4700
4250
6800
9500
5100
9900
1.38
1.35
1.38
1.40
1.41
1.49
1.47
1.48
1.31
1.28
1.35
1.33
1.27
1.22
1.25
1.12
1.25
1.22
1.19
1.35
1.25
1.30
1.36
—
100/1
300/1
500/1
100/1
300/1
500/1
100/1
300/1
500/1
1000/1
1000/1
50/1
100/1
250/1
500/1
100/1
300/1
500/1
100/1
300/1
500/1
500/1
1000/1
24 800
39 800
16 100
26 900
33 200
11 200
28 500
35 150
43 600
57 600
4100
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
3920
6400
5500
5900
7600
10 100
13 800
25 300
21 900
26 900
—
10 200
24 600
20 800
27 800
—
18 h
24 h
a All reactions were performed in THF with [monomer]0 = 1.0 M; results are representative of at least duplicated experiments. The molar mass of
2b was calculated with two molecules of THF. b Reaction times were not necessarily optimized. c Monomer conversion determined by 1H NMR of
the crude reaction mixture. d Theoretical molar mass values calculated from the relation: [monomer]0/[BH4]0 × monomer conversion × MMonomer
with [BH4]0 = 2 [Ca(BH4)2(THF)2]0, [[Cp*Ca(BH4)(THF)2]2]0 or [{(Me3SiNPPh2)2CH}Ca(BH4)(THF)2]0, Mε-CL = 114 g mol−1, and MLLA = 144 g mol−1
,
.
e Molar mass values determined by NMR analysis of the isolated polymer (refer to the Experimental section). f Number of average molar mass
values determined by SEC in THF at 20 °C vs. polystyrene standards and corrected (refer to the Experimental section). g Dispersity calculated
from SEC traces.
ÐM ≤ 1.27). Comparatively, the strontium analogue 1b failed to the activity of 2a was dramatically higher than that reached
polymerize LLA even at 50 °C (Table 2, entries 14–15). Our with the homoleptic bisborohydride congener 1a (Table 1,
results on the successful behavior of our home-made discrete entry 19 vs. 16). The recovered PCLs and PLLAs were all mono-
sample of 1a – which was fully characterized as a pure sample, dispersed with relatively narrow dispersity values (ÐM
=
as discussed above – in the ROP of ε-CL and LLA are in con- 1.19–1.49), and the experimental molar mass values matched
trast to the previous literature claims mentioned above from well the ones calculated on the basis of monomer-to-BH4
the commercial 1a sample.37 Besides, in comparison to the ratios.
calcium amide [Ca{N(SiMe3)2}(THF)2] or alkoxide initiating
Our preliminary investigations on the ROP of ε-CL pro-
systems derived from either binary catalytic systems moted by 2b showed a quite good activity at 50 °C.24 Closer
[Ca{N(SiMe3)2}(THF)2]/ROH (R
=
Me, iPr) or discrete examination of the behavior of 2b at ambient temperature
[{Ca(OR)2}n] compounds,21,22,56,57 the bisborohydride complex revealed some activity, although lower than at higher temp-
1a thus represents, to our knowledge, the homoleptic calcium erature (Table 2, entries 3–11). In [ε-CL]0/[BH4]0 = 300 : 1 experi-
initiator providing the highest molar mass polyester (MnPCL
≤
ments, the strontium complex 2b led to 86% conversion of the
monomer at 23 °C within 120 min, while the full conversion
39 800 g mol−1).
By analogy with the strontium analogues 1b, 2b, and 3b, for was achieved within 90 min at 50 °C (Table 2, entries 5 and 6).
which the experimental and DFT investigations showed a As many as 560 ε-CL units were successfully converted within
favorable activity for 2b, we next considered the ROP of ε-CL 480 min at 50 °C. These polymerizations showed good control
and LLA initiated by the calcium congener 2a (Table 1, entries in terms of theoretical/experimental molar mass agreement as
5–7 and 17–19).58 In THF solution at 23 °C, the activity of 2a well as dispersity values below 1.38, allowing the preparation
was about one half order of magnitude higher than that of a PCL with a molar mass up to MnSEC = 59 200 g mol−1
.
observed with 1a, suggesting a clear beneficial effect of the Also, 2b was less efficient in the ROP of LLA than compared
ancillary Cp* ligand. Indeed, whereas only 20 min were to that of ε-CL with at best 140 turnovers in 48 h at 23 °C
required to quantitatively convert 500 equiv. of ε-CL from 2a, (Table 2, entries 17–20). PLLAs of a molar mass up to MnSEC
=
only 80% were consumed in 110 min using 1a (Table 1, entry 7 20 500 g mol−1, corresponding to 140 LLA units being
vs. 4). The same favorable influence of the Cp* surroundings converted, were thus formed. Overall, complex 2b displayed
was gained in the ROP of LLA, to an even larger extent. Indeed, better activity compared to the homoleptic analogue 1b. Also,
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Table 2 ROP of ε-CL and LLA initiated by [Sr(BH4)(THF)2], 1b, [Cp*Sr (BH4)(THF)2]2, 2b, and [{(Me3SiNPPh2)2CH}Sr(BH4)(THF)2], 3b
[Monomer]0/
[BH4]0
Temp.
(°C)
Reaction
timeb
Conv.c
(%)
Mntheo
MnNMR
MnSEC
d
e
f
Monomera
(g mol−1
)
(g mol−1
)
(g mol−1
)
ÐM
a
g
Entry
Complex
1
2
3
4
5
6
7
8
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
1b
1b
2b
2b
2b
2b
2b
2b
2b
2b
3b
3b
3b
1b
1b
2b
2b
2b
2b
3b
3b
3b
3b
ε-CL
ε-CL
ε-CL
ε-CL
ε-CL
ε-CL
ε-CL
ε-CL
ε-CL
ε-CL
ε-CL
ε-CL
ε-CL
LLA
LLA
LLA
LLA
LLA
LLA
LLA
LLA
LLA
LLA
300/1
500/1
100/1
100/1
300/1
300/1
500/1
500/1
1000/1
1000/1
100/1
300/1
500/1
100/1
100/1
100/1
300/1
500/1
1000/1
100/1
300/1
500/1
1000/1
23
23
23
50
23
50
23
50
23
50
23
23
23
23
50
23
23
23
23
23
23
23
23
60 min
135 min
40 min
30 min
120 min
90 min
240 min
300 min
480 min
480 min
30 min
80 min
150 min
65 h
65 h
18 h
29 h
48 h
65 h
12.5 h
14 h
13.5 h
14 h
90
75
95
100
86
100
75
100
42
56
94
97
95
0
15 390
21 380
10 800
11 400
29 400
34 200
42 700
57 000
47 900
63 800
10 700
33 170
53 600
—
15 200
27 800
8500
7300
22 300
20 700
—
—
—
—
7600
31 700
—
—
—
6900
12 800
17 900
—
10 200
20 000
18 300
21 000
19 100
35 000
9600
1.57
1.31
1.23
1.38
1.28
1.27
1.30
1.29
1.40
1.38
1.40
1.31
1.85
—
8100
24 800
18 200
38 700
43 500
45 100
59 200
8300
28 400
41 300
—
0
—
8600
—
7200
—
60
35
28
0
100
52
54
22
1.30
1.27
1.62
—
1.64
1.56
1.60
1.38
15 100
20 160
—
14 400
22 400
38 900
31 680
13 600
20 500
—
10 600
18 800
20 700
19 800
a All reactions were performed in THF with [monomer]0 = 1.0 M; results are representative of at least duplicated experiments. b Reaction times
were not necessarily optimized. c Monomer conversion determined by 1H NMR of the crude reaction mixture. d Theoretical molar mass value
calculated from the relation: [monomer]0/[BH4]0 × monomer conversion × MMonomer, with [BH4]0 = 2 [Sr(BH4)2(THF)2]0, [[Cp*Sr(BH4)(THF)2]2]0 or
[{(Me3SiNPPh2)2CH}Sr(BH4)(THF)2]0, Mε-CL = 114 g mol−1, and MLLA = 144 g mol−1 e Molar mass values determined by NMR analysis of the
.
isolated polymer (refer to the Experimental section). f Number of average molar mass values determined by SEC in THF at 20 °C vs. polystyrene
standards and corrected (refer to the Experimental section). g Dispersity calculated from SEC traces.
the calcium complexes 1a and 2a were generally more effective LLA in the presence of 1000 equiv. at 23 °C within 24 h
in the ROP of ε-CL and LLA than the strontium analogues 1b (Table 1, entry 24). Unlike in our previous attempts,24 the
and 2b.
strontium analogue 3b was found to be active in the ROP of
The bis(phosphiniminomethanide) borohydride calcium ε-CL at 23 °C. A turnover of 475 was reached within 2.5 h with
complex 3a was also found to be active in the ROP of ε-CL and a fair control of molar masses (Table 2, entries 12–14). Under
LLA. At room temperature, 3a smoothly afforded PCL with the same operating conditions, the ROP of LLA promoted from
quite high molar masses (MnSEC ≤ 57 600 g mol−1) (Table 1, 3b did afford PLLA, yet with a lower activity as compared to the
entries 8–12). However, the complex showed lower efficiency ROP of ε-CL (Table 2, entries 21–24). While 100 LLA units were
with increased loading of ε-CL. Indeed, the calculated molar successfully polymerized within 12.5 h, only 270 turnovers
mass and the Mn values determined experimentally from NMR were reached at best within 14 h under such operating con-
and SEC analyses agreed relatively well, while the dispersity ditions (Table 2, entries 21 and 23).
remained in the range ÐM = 1.28–1.48. However, when the
The general comparison of the data for the ROP of 300
initial monomer charge overcame 300 equiv., reaction times equivalents of monomer per borohydride group initiated by
proportionally longer than hinted from lower ε-CL feed experi- complexes 1a,b, 2a,b, and 3a,b is informative. It shows that,
ments were required to reach high ε-CL conversion. This lower among all evaluated complexes, calcium borohydride 2a pro-
activity could be compensated to some extent upon raising the vides the best compromise in terms of activity and control.
temperature from 23 °C to 50 °C (Table 1, entries 11–12). As However, the activity as well as the control imparted by these
many as 600 ε-CL units were thus polymerized, yet quite slowly large calcium and strontium borohydride series of complexes
(within 6 h). The same behavior was observed in the ROP of in the ROP of ε-CL or LLA, remained – similarly to literature
LLA (Table 1, entries 20–24). However, the ROP of LLA was sig- data on related alkaline earth metal borohydride initiating
nificantly slower than that of ε-CL, in agreement with the complexes31,37 – lower compared to those achieved with
typical difference in both monomers’ reactivity generally related rare earth element borohydride complexes or with
observed in the literature.21 Similarly, a higher polymerization other post-metallocene non-borohydride alkaline earth metal
temperature favored the conversion of an initial loading of 500 precursors used in an “immortal” ROP procedure.21
equiv. of LLA, affording at best 240 turnovers in 18 h at 50 °C
The nature of the end-groups capping the PCL or PLLA
(Table 1, entry 23). Complex 3a was also similarly less active in chains was examined by NMR analyses of the polymer samples
the presence of larger LLA loadings, failing to polymerize any recovered after precipitation. In particular, 1H NMR
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spectroscopy attested the presence of –CH2OH or –OCH(Me)- leads to the best compromise since both barriers are equival-
CH2OH and –C(O)CH(Me)OH termini on PCL or PLLA, ent, around 32–36 kcal mol−1. Indeed, for complexes 1a and
respectively. Such α,ω-dihydroxytelechelic polymers were 3a, the second barrier is around 36–38 kcal mol−1 whereas the
expected based on previous literature reports on the ROP of first one is lower than 30 kcal mol−1. This is in harmony with
such lactone or diesters initiated by rare earth element boro- the above experimental observations. Moreover, comparing the
hydride derivatives.46–48,59–65 Indeed, detailed insights, calcium complexes activities with those of their strontium ana-
especially based on NMR, IR, MALDI-ToF and DFT investi- logues revealed that the calcium complexes lead to energetic
gations, have established that the reduction of the oxygen of parameters slightly lower than those calculated for the related
the carbonyl adjacent to the borohydride group in the first strontium complexes.24 Therefore, the calcium complexes are
inserted monomer unit occurred, thereby inducing the for- slightly more active than their strontium analogues. These
mation of a second hydroxyl end-function in place of the usual conclusions are in good agreement with the experimental
C(O)X terminus generally generated upon ROP forming a non- remarks.
borohydride precursor [LMX] with L = ancillary ligand(s), M =
metal center and X = nucleophile such as OR− or NR2−. Note
that the other hydroxyl end-capping group is formed upon
Summary
deactivation of the active metal–oxygen bond.45–48,58–63
In light of our already published studies on strontium [Ca(BH4)2(THF)2] (1a), which is also commercially available,
analogues,24 DFT calculations on the initiation step of the was prepared following a new procedure from [Ca(OMe)2]
ROP of ε-CL were carried out. Gibbs free energy profiles were and BH3·THF in THF. Reaction of 1a with KCp* and
determined for the three calcium complexes 1a, 2a, and 3a K{(Me3SiNPPh2)2CH} in a 1 : 1 ratio in THF resulted in the corre-
(Fig. 2). The computed reaction profiles are very similar to sponding heteroleptic mono-borohydride derivatives [Cp*Ca-
those obtained for the parent strontium compounds, as it pro- (BH4)(THF)n]2 (2a) and [{(Me3SiNPPh2)2CH}Ca(BH4)(THF)2] (3a),
ceeds in two steps. Indeed, the first B–H activation is concomi- respectively. Compounds 1a, 2a, and 3a were together with the
tant with the BH3 trapping by the endocyclic oxygen of the earlier reported compounds [Sr(BH4)2(THF)2] (1b), [Cp*Sr(BH4)-
ε-CL. Moreover, the ring-opening is determined as the rate (THF)2]2 (2b), and [{(Me3SiNPPh2)2CH}Sr(BH4)(THF)2] (3b) used
determining step of the reaction, as the barrier is the highest as initiators for the ROP of polar monomers.
of the two. The relative instability of the ε-CL adduct of
The general performances of the complexes 1a,b, 2a,b, and
complex 3a is due to the fact that this is not a simple adduct. 3a,b in the ROP of ε-CL and LLA demonstrate relatively good
Indeed, this complex was obtained by following the intrinsic control of the polymerization under the operating conditions
reaction coordinate from the nucleophilic attack TS. The inter- described, in terms of both theoretical vs. experimental molar
mediate obtained for 3a is distorted with a short B–H⋯C dis- mass values agreement and of moderate dispersity data.
tance of 1.27 Å.
Overall, significantly lower catalytic activity and control were
Detailed analysis of the energetics of the reaction profiles observed with these alkaline earth metal complexes in com-
showed that although the three complexes are quite similar parison to the corresponding rare earth element catalysts.
within the precision of the computational method, complex 2a Although active, these calcium and strontium catalytic systems
may exhibit a somewhat limited scope regarding the living
ROP of larger monomer loadings, especially in the case of the
larger strontium complexes. Interestingly, the calcium and
strontium borohydride complexes afford α,ω-dihydroxyteleche-
lic polymers, with the former being slightly more efficient
than the latter ones, as further supported by DFT insights.
Experimental
General considerations
All manipulations of air-sensitive materials were performed
with the rigorous exclusion of oxygen and moisture in flame-
dried Schlenk-type glassware either on
a dual-manifold
Schlenk line, interfaced to a high vacuum (10−3 Torr) line, or
in an argon-filled MBraun or Jacomex glovebox. Hydrocarbon
solvents (n-pentane) were dried using an MBraun solvent puri-
fication system (SPS-800). Tetrahydrofuran was distilled under
nitrogen from K benzophenone ketyl prior to use. All solvents
were stored in vacuo over LiAlH4 in a re-sealable flask.
Deuterated tetrahydrofuran was obtained from Aldrich Inc.
Fig. 2 Gibbs free energy profiles computed at room temperature for the
initiation step of the ROP of ε-CL initiated by calcium borohydride complexes 1a,
2a, and 3a. The energies are given in kcal mol−1
.
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Paper
(all 99 atom% D) and was degassed, dried, and stored in vacuo residue was washed with 15 mL of n-pentane and dried in
over Na/K alloy in a re-sealable flask. [Ca(OMe)2],50 [Sr(BH4)2(THF)2] vacuum to obtain a pale yellow solid. Yield: 188 mg (56%).
(1b),39 [Cp*Sr(BH4)(THF)2]2 (2b),24 [{(Me3SiNPPh2)2CH}Sr(BH4)-
1H NMR (d8-THF, 300.13 MHz): δ 2.00 (s, 15 H, CH3), −0.44
1
(THF)2] (3b),24 KCp*,66 and K{(Me3SiNPPh2)2CH}51 were pre- (q, 4 H, BH4, JH-B = 83 Hz) ppm. 1H{11B} NMR (d8-THF,
pared according to literature procedures.
300.13 MHz): δ 2.00 (s, 15 H, CH3), −0.44 (s, 4 H, BH4) ppm.
NMR spectra were recorded on a Bruker Avance 400 MHz, 11B NMR (d8-THF, 96.29 MHz): δ −36.0 (qt, 1JH-B = 83 Hz) ppm.
Avance II NMR 300 MHz or Bruker AC-500 500 MHz spec- 13C{1H} NMR (d8-THF, 75.47 MHz): δ 111.4 (C-CH3), 10.5
trometer. Chemical shifts were referenced to internal solvent (C-CH3) ppm. IR (ATR, cm−1): 2962 (w), 2913 (w), 2870 (w),
resonances and were reported relative to tetramethylsilane (1H 2279 (m, B-H), 1627 (w), 1439 (m), 1378 (m), 1259 (s), 1226 (s),
and 13C NMR), 15% BF3·Et2O (11B NMR), and 85% phosphoric 1084 (vs), 1034 (vs), 956 (s), 914 (s), 779 (vs), 759 (m), 722 (m),
acid (31P NMR), respectively. IR spectra were obtained on an 668 (m), 523 (s). EI-MS (70 eV, 170 °C): m/z (%) = 380
FTIR spectrometer Bruker Tensor 37. Mass spectra were ([M − (4THF)]+, <1), 365 ([M − (4THF, BH4)]+, <1), 310
recorded at 70 eV on a DFS Thermo Scientific. Elemental ana- ([Cp*2Ca]+, 1), 245 ([M − (4THF, Cp*)]+, 1), 190 ([M − (4THF)/
lyses were carried out with an Elementar vario Micro Cube.
2]+, 1), 175 ([Cp*Ca]+, 3), 136 ([Cp*]+, 95), 110 ([M − (4THF,
Monomer conversions were determined from 1H NMR 2Cp*)]+, 29), 70 ([Ca(BH4)2]+, 23), 55 ([Ca(BH4)]+, 60).
spectra of the crude polymer sample, from the integration C18H35BO2Ca (334.36): calc. C 64.66, H 10.55; found C 59.92,
(Int.) ratio Int.PCL/[Int.PCL + Int.ε-CL] or Int.PLLA/[Int.PLLA
Int.LLA], using the –CH2OC(O) methylene triplet (δPCL remains constantly low, maybe due to carbide formation).
4.04 ppm, δε-CL 4.19 ppm for) for ε-CL, or the methine hydro- [{(Me3SiNPPh2)2CH}Ca(BH4)(THF)2] (3a). THF (10 mL) was
+
H 10.28 (despite several attempts, the carbon value of 2a
gen in –OCHCH3C(O) (δLLA 5.05 ppm, δPLLA 5.18 ppm) for LLA. condensed onto a mixture of [Ca(BH4)2(THF)2] (107 mg,
The number of average molar mass (MnSEC) and dispersity 0.5 mmol) and K{(Me3SiNPPh2)2CH} (298 mg, 0.5 mmol) at
(ÐM = Mw/Mn) values were determined by size exclusion chrom- −78 °C. The colorless suspension was stirred for 18 h at
atography (SEC) in THF at 30 °C (flow rate = 1.0 mL min−1) on ambient temperature. The mixture was filtrated and the
a Polymer Laboratories PL50 apparatus equipped with a refrac- solvent was concentrated in vacuum to 2 mL and layered with
tive index detector and a set of two ResiPore 300 × 7.5 mm 10 mL of n-pentane to obtain colorless single crystals suitable
columns. The recovered polymer samples were dissolved in for X-ray analysis. Yield: 170 mg (45%).
THF (2 mg mL−1) and filtered; only the soluble fraction was
1H NMR (d8-THF, 300.13 MHz): δ 7.58–7.52 (m, 8 H, m-PPh),
analyzed. All elution curves were calibrated with polystyrene 7.26–7.13 (m, 12 H, o-, p-PPh), 1.85 (br, 1 H, P-CH-P), 0.10 (q,
1
standards. MnSEC values of PCLs and PLLAs were calculated br, 4 H, BH4, JH-B = 82.2 Hz), 0.13 (s, 18 H, SiCH3) ppm. 11B
1
using the correcting factors previously reported (PCL: MnSEC
=
NMR (d8-THF, 96.29 MHz): δ −34.8 (qt, JH-B = 83 Hz) ppm.
MnSEC raw data × 0.56; PLLA: MnSEC = MnSEC raw data × 13C{1H} NMR (d8-THF, 75.47 MHz): δ 140.7 (dd, 1JC-P = 91.0 Hz,
0.58),59,67,68 thus taking into account the difference in hydro- 3JC-P = 4.2 Hz, i-PPh), 132.3 (t, JC-P = 5.2 Hz, m-PPh), 130.5
3
4
2
dynamic radii of PCL and PLLA vs. polystyrene. SEC traces of (t, JC-P = 1.2 Hz, p-PPh), 128.5 (t, JC-P = 5.8 Hz, o-PPh), 23.3 (t,
the polymers all exhibited a unimodal and symmetrical peak. 1JC-P = 107.1 Hz, P-CH-P), 5.5 (t, JC-P = 1.9 Hz, SiCH3) ppm.
3
The molar masses of short-chain polymer samples were deter- 31P{1H} NMR (d8-THF, 121.49 MHz): δ 16.4 ppm. IR (ATR,
mined by H NMR analysis in CDCl3 from the relative intensi- cm−1): 3054 (w), 2948 (w), 2886 (w), 2218 (m), 1976 (m),
1
ties of the signals of the PCL or PLLA main-chain methylene 1590 (w), 1483 (w), 1435 (m), 1269 (s), 1239 (s), 1166 (w),
protons for PCL (–CH2OC(O), δ 4.04 ppm) or methine protons 1102 (m), 1069 (s), 1037 (w), 999 (w), 954 (w), 827 (vs), 802 (s),
for PLLA (–CHCH3C(O), δ 5.15 ppm), and those of the chain- 777 (s), 739 (s), 708 (vs), 694 (vs), 653 (s), 634 (s), 605 (m),
end methylene protons (–CH2OH, δ 3.65 ppm).
587 (m), 555 (m), 532 (m). EI-MS (70 eV, 190 °C): m/z (%) =
suspension of [Ca(OMe)2] 613 ([M]+ − (2THF), 1), 598 ([M]+ − (2THF,BH4), 6), 558 ([CH-
[Ca(BH4)2(THF)2] (1a). To
a
(511 mg, 5.0 mmol) in 25 mL of THF was added 15 mL of 1.0 (PPh2NSiMe3)2]+, 39), 543 ([CH(PPh2NSiMe3)2 − Me]+, 100), 471
M BH3·THF in THF (15.0 mmol). The mixture was heated to ([CH(PPh2)(PPh2NSiMe3)]+, 16), 455 ([CH(PPh2)(PPh2NSiMe2)]+,
reflux for 3 h to obtain an almost clear solution. The solution 21). C39H59BN2O2P2Si2Ca (756.91): calc. C 61.89, H 7.86,
was allowed to cool to ambient temperature and filtrated. The N 3.70; found C 61.22, H 7.92, N 3.69.
volatile components were evaporated in vacuum and the
Details of the X-ray diffraction measurement, structure
solution and refinement
product was obtained as a colorless powder. Yield: 1.03 g
(96%).
1H NMR (d8-THF, 300.13 MHz): δ 0.41 (q, br, 8 H, BH4,
General. Data were collected on a diffractometer equipped
1JH-B = 82 Hz) ppm. 11B NMR (d8-THF, 96.29 MHz): δ −36.6 (qt, with a STOE imaging plate detector system IPDS2 using MoKα
1JH-B = 82 Hz) ppm.
radiation with graphite monochromatization (λ = 0.71073 Å) at
[Cp*Ca(BH4)(THF)n]2 (2a). THF (15 mL) was condensed 153 K. The structure solution was performed by direct
onto a mixture of [Ca(BH4)2(THF)2] (290 mg, 1.1 mmol) and methods, full-matrix-least squares refinement against F2 using
KCp* (193 mg, 1.1 mmol) at −78 °C. The colorless suspension SHELXS-97 and SHELXL-97 software.69
was stirred for 18 h at ambient temperature. The mixture was
Crystallographic data (excluding structure factors) for the
filtrated and the solvent was evaporated in vacuum. The structure reported in this paper have been deposited with the
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Cambridge Crystallographic Data Centre as a supplementary
publication no. CCDC 915058.
2 A. Torvisco and K. Ruhlandt-Senge, Inorg. Chem., 2011,
12223–12240.
Crystal data for 3a: C39H59BN2O2Si2P2Ca, M = 756.89, triclinic,
a = 10.107(2) Å, b = 11.994(2) Å, c = 18.948(4) Å, α = 79.98(3)°, β =
89.67(3)°, γ = 70.24(3)°, V = 2125.2(7) Å3, T = 153.15 K, space
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5 M. Westerhausen, Coord. Chem. Rev., 2008, 252,
1516–1531.
group P1, Z = 2, μ(MoKα) = 0.313 mm−1, 19 390 reflections
ˉ
measured, 10 943 independent reflections (Rint = 0.1290). The
final R1 value was 0.0769 (I > 2σ(I)). The final wR(F2) value was
0.1963 (all data). The goodness of fit on F2 was 0.860.
6 W. D. Buchanan, D. G. Allis and K. Ruhlandt-Senge, Chem.
Commun., 2010, 46, 4449–4465.
7 P. Jochmann, T. S. Dols, T. P. Spaniol, L. Perrin, L. Maron
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Typical polymerization experiment
In a typical experiment (Table 1, entry 5), compound 2a
(10 mg, 15 μmol, 1 equiv.) was dissolved in dry THF and then
ε-CL (341 mg, 2.99 mmol, 200 equiv.) was added to produce an
initial monomer concentration of 1.0 M. The mixture was then
stirred at the desired temperature over the appropriate reaction
time (reaction times were not systematically optimized). The
polymerization was quenched with excess acetic acid (ca.
0.1 mL of a 1.6 mM solution in toluene). The resulting mixture
was concentrated to dryness under vacuum and the conversion
was determined from 1H NMR analysis of the residue in
CDCl3. The crude polymer was then dissolved in CH2Cl2
(2 mL) and precipitated in cold pentane (10 mL), filtered and
dried under vacuum. The final polymer was then analyzed by
NMR and SEC (Tables 1 and 2).
Computational details
Calculations were carried out at the DFT level using the hybrid
functional B3PW9170,71 with the Gaussian 0372 suite of pro-
grams. Calcium, phosphorus and silicon were represented by a
relativistic effective-core potential (RECP)73,74 from the Stutt-
gart group and their corresponding optimized basis set. Polar-
ized all-electron double-ζ 6-31G(d,p)75 basis sets were used for
B, C, H, O and N. Geometry optimizations were carried out
without any symmetry restrictions. The nature of the extrema
(minimum or transition state) was verified with analytical fre-
quency calculations.
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Acknowledgements
We are grateful to the CNRS, the University of Rennes 1 and
UPS and the Helmholtz Research School: Energy-Related Cata-
lysis, and the DFG-funded trans-regional collaborative research
center SFB/TRR 88 “3MET” for financial support of this work.
L.M. is a member of the Institut Universitaire de France.
CINES and CalMip are acknowledged for a generous grant of
computing time. L.M. also thanks the Humboldt Foundation
for financial support. M.S. thanks the Cusanuswerk for a doc-
toral fellowship.
25 T. K. Panda, K. Yamamoto, K. Yamamoto, H. Kaneko,
Y. Yang, H. Tsurugi and K. Mashima, Organometallics,
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Notes and references
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Dalton Trans.
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