Cationic and charge-neutral calcium tetrahydroborate complexes and their use in the controlled ring-opening polymerisation of rac-lactide

Article abstract of DOI:10.1039/c0cc04348f

The first cationic main group tetrahydroborate complexes are reported. [Ca(BH₄)(THF)₅][BPh₄] and the charge neutral (Tp^tBu,Me)Ca(BH₄)(THF) are initiators for the living ring opening polymerization of

Full text of DOI:10.1039/c0cc04348f

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COMMUNICATION
Cationic and charge-neutral calcium tetrahydroborate complexes and
their use in the controlled ring-opening polymerisation of rac-lactidew
Michael G. Cushion and Philip Mountford*
Received 11th October 2010, Accepted 26th November 2010
DOI: 10.1039/c0cc04348f
The first cationic main group tetrahydroborate complexes
Initial studies using commercially available Ca(BH4)2(THF)2
and e-CL or rac-LA in THF gave disappointing performances,
in contrast to the well-controlled trivalent Ln(BH4)3(THF)3
systems (La = Ln, Sm, Nd).6 However, bearing in mind very
recent reports of mono- and di-cationic ROP initiators,4a,8,9 we
carried out the reaction of Ca(BH4)2(THF)2 with [Et3NH][BPh4]
in THF at À78 1C. This gave [Ca(BH4)(THF)5][BPh4] (1–BPh4)
in 51% recrystallized yield (Scheme 1).w As discussed below,
1–BPh4 is an effective initiator for the ROP of rac-LA. The
1H NMR spectrum of 1–BPh4 shows a 1 : 1 : 1 : 1 quartet at
are reported. [Ca(BH4)(THF)5][BPh4] and the charge neutral
t
(Tp Bu,Me)Ca(BH4)(THF) are initiators for the living ring opening
polymerization of rac-lactide, the latter producing PLA with high
levels of heterotactic enrichment. These represent a new class of
ROP initiators for main group metals.
The last decade has witnessed an exponential increase in
activity in the synthesis, structures, reactivity and catalytic
applications of compounds of the heavier alkaline earth (Ae)
metals.1 Most of this research has centered on calcium, the
ionic radius of which is very similar to that of ytterbium, and
which is abundant, non-toxic and inexpensive. Nonetheless,
both the general solution chemistry and catalytic applications
of this element remain considerably underdeveloped.
À0.19 ppm for the BH4 ligand (1J
B = 82 Hz) which appears
1H11
at À36.1 ppm in the 11B NMR spectrum. The IR spectrum is
consistent with a k3 bound BH4 ligand.5f Recalling the recently
reported successful dicationic yttrium ROP initiator
[(Tpm)Y(OiPr)(THF)3]2+ 8b
we treated 1–BPh4 with HC(3,5-
,
Me2pz)3 (Tpm) in dichloromethane at À78 1C which gave
2–BPh4 in 74% yield. The Tpm-supported half-sandwich
cation [(Tpm)Ca(BH4)(THF)2]+ (2+) also possesses a k3
The ring-opening polymerization (ROP) of cyclic esters such
as e-caprolactone (e-CL) and lactide (LA) towards biocompa-
tible and biodegradable polymers is an area of considerable
current interest.2 Although calcium’s lighter congener, Mg, and
the lanthanides are highly developed for ROP, only a handful
of well-defined, heteroleptic initiators of the type (L)Ca–X
(X = amide or aryloxide) have been reported.3,4
bound BH4 ligand (d(1H) = 0.15 ppm (1J1H11 = 82 Hz);
B
1
d(11B) = À35.6 ppm in CD2Cl2). H and 11B NMR showed
that 2+ does not undergo redistribution (e.g. to [(Tpm)2Ca]2+
and Ca(BH4)2(THF)2) in THF-d8 at RT or 70 1C.
In the last 5–8 years lanthanide tetrahydroborates,5 either as
The solid state structures of the tetrahydroborate cations
[Ca(BH4)(THF)5]+ (1+) and [(Tpm)Ca(BH4)(THF)2]+ (2+)
6
‘‘ligand-free’’ Ln(BH4)3(THF)x or heteroleptic (L)Ln(BH4)
systems,7 have become well-established as effective initiators
for the polymerisation of a range of cyclic esters and other
monomers. However, no main group borohydride has been
established in this regard.5 Furthermore, in terms of both
initiating efficiency and molecular weight control, (L)Ln–BH4
systems are better controlled than the corresponding amides,
(L)Ln–NR2,7a and comparable to alkoxides, (L)Ln–OR.6c
Drawing upon the potential parallels with the lanthanides, we
set out to develop calcium tetrahydroborate complexes as a new
class of main group initiators for the ROP of rac-LA.
Chemistry Research Laboratory, University of Oxford,
Mansfield Road, Oxford OX1 3TA, UK.
E-mail: philip.mountford@chem.ox.ac.uk
w Electronic supplementary information (ESI) available: 1st order plot
for consumption of rac-LA using 1–BPh4; additional plots of Mn vs.
rac-LA converted using 1–BPh4, 2–BPh4 and 3; experimental and
characterising data; CIF data for 1–BPh4 and 2–BPh4. CCDC
796412–796413. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c0cc04348f
Scheme 1 Synthesis of new calcium tetrahydroborate complexes.
[BPh4]À anions are omitted for 1+ and 2+
.
c
2276 Chem. Commun., 2011, 47, 2276–2278
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The slow ROP of rac-LA with 1–BPh4 was alleviated by
using 2–BPh4, the Tpm ligand providing a better-defined
coordination environment, and perhaps guarding against
binuclear or extensively LA chain chelated resting states.
Thus up to 250 equivs rac-LA could be converted at RT
within 2 hours with Mw/Mn values between 1.2 and 1.4. The
gradient of the best-fit line was 157(11) g molÀ1 (equivs
converted)À1) in excellent agreement with that predicted
(144.1) for one PLA chain (see the ESIw), but in general the
Mn values were a little higher than expected (e.g. 43 200 g molÀ1
(GPC) vs. 32 800
g
molÀ1 (predicted) for [rac-LA]0 :
[2–BPh4]0 = 250). Nonetheless, it is clear that the slow
rac-LA ROP rates typically associated with cationic
initiators4a,8b can be alleviated with the appropriate
supporting ligand, thus offering considerable potential for
further development.14
Fig. 1 Molecular structures of [Ca(BH4)(THF)5]+ (1+, left) and
[(Tpm)Ca(BH4)(THF)2]+ (2+, right). The H atoms of the k3-BH4
ligands were located from Fourier difference maps and positionally
and isotropically refined. C-bound H atoms omitted.
The PLAs formed with 1–BPh4 and 2–BPh4 were atactic.
However, it is known that use of bulkier ancillary ligands can
are shown in Fig. 1. The only previously structurally
characterized calcium tetrahydroborate compounds are
Ca(BH4)2(dme)2 and Ca2(BH4)2(diglyme)2.10 A number of
cationic tetrahydroborate compounds have been reported for
the d- and f-block elements.8b,9,11 However, although the
anionic complexes [Zn(BH4)3]À and [Al(BH4)4]À are known
(having k2-BH4 ligands),12 no cationic tetrahydroborate
complexes have yet been described for any main group
element. Fully-authenticated cationic main group hydrides of
the type [(L)MH2]+ are also few in number, and are established
only for M = Al and Ga.13
induce tacticity through chain-end control. Drawing upon
t
Chisholm’s reports on (Tp Bu)Ca(X)(THF) (X = N(SiMe3)2 or
O-2,6-C6H3 Pr2)3c we turned to the bulky tris(pyrazolyl)-
i
t
hydroborate complex (Tp Bu,Me)Ca(BH4)(THF) (3, Scheme 1)
which is readily prepared at RT directly from Ca(BH4)2(THF)2
t
and KTp Bu,Me in 65% recrystallized yield. The IR and NMR
data for formally zwitterionic 3 are consistent with the
structure proposed in Scheme 1 which is analogous to that
of cationic 2+ and Sella and Takats’ recently reported
t
(Tp Bu,Me)Yb(BH4)(THF).15
1–BPh4 rapidly polymerized 200 equivs e-CL to 100%
conversion within 2 min at RT in THF giving a,o-dihydroxy
telechelic PCL of the form HO(CH2)5C(O)–[PCL]–O(CH2)6OH
as determined by 1H NMR spectroscopy and MALDI-ToF-MS
analysis. The formation of a,o-dihydroxy telechelic PCL is a
particular characteristic of (L)Ln(BH4)-initiated e-CL ROP and
the mechanism of formation has been studied in detail.7a,b,d The
formation of dihydroxy PCL with 1–BPh4 is consistent with an
analogous mechanism.
Very good agreement between measured and predicted
Mn was found for the ROP of rac-LA with 1–BPh4.
Polymerization at 70 1C in THF over 3–5 hours gave PLA
with number-average molecular weights up to 35 000 g molÀ1
.
Consumption of rac-LA followed well-behaved 1st order
kinetics and gave linear Mn (GPC) vs. % conversion plots
(see the ESIw). Fig. 2 (top) shows a plot of Mn vs. equivs rac-
LA converted for [rac-LA]0 : [1–BPh4]0 ratios between 50 and
300. The gradient of the best-fit line was 152(3) g molÀ1 (equivs
converted)À1 (R2 = 0.998) in excellent agreement with that
predicted (144.1) for one PLA chain growing per Ca–BH4
group of 1+ in a living-type fashion. MALDI-ToF-MS
analysis indicated the formation of dihydroxy-terminated
HOCH(Me)C(O)–[PLA]–OCH(Me)CH2OH, again consistent
with the previously studied coordination–insertion mechan-
isms with (L)Ln(BH4) systems.6,7 When L-LA was used as
a monomer ([L-LA]0 : [1–BPh4]0 = 50) no epimerization
occurred according to NMR analysis, and only SSSS (iii)
Fig. 2 Mn (as measured by GPC) vs. equivs rac-LA converted using
t
[Ca(BH4)(THF)5][BPh4] (1–BPh4, top) and (Tp Bu,Me)Ca(BH4)(THF)
(3, bottom). The gradient of the fitted lines (R2 = 0.998 in each case)
are 152(3) and 162(4) g molÀ1 for 1–BPh4 and 3, respectively
(expected 144.1 g molÀ1), with an intercept of Mn = 0. Conditions
for 1–BPh4: THF, 70 1C, 3 or 5 h, [rac-LA]0 : [1–BPh4]0 = 50, 100,
150, 200, 250, 300; for 3: THF, À20 1C, 5 min, [rac-LA]0 : [3]0 = 40,
100, 120, 160, 200.
tetrad sequences were detected. While Fig.
2 (top) is
consistent with living-type behavior, the Mw/Mn values for
the PLAs were ca. 1.4. This is attributed to concomitant
intermolecular transesterification processes.
c
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Chem. Commun., 2011, 47, 2276–2278 2277

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A benchmarking experiment of 3 with e-CL again gave
efficient conversion of 200 equivs to a,o-dihydroxy telechelic
PCL within 10 minutes. The molecular weight control achieved
with 3 (Mn (GPC) = 17 600 g molÀ1; Mw/Mn = 1.3; Mn
(calcd) = 22 820 g molÀ1) was better than with 1–BPh4 under
Macromolecules, 1996, 29, 50; S. M. Li, I. Rashkov, J. L. Espartero,
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and M. Visseaux, Coord. Chem. Rev., 2010, in press
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¨
analogous conditions (Mn (GPC)
Mw/Mn = 1.3; Mn (calcd) = 22 820 g molÀ1). Gratifyingly,
= 39 200 g ;
molÀ1
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3 is also a very efficient initiator for the ROP of rac-LA either
6 Selected examples: (a) Y. Nakayama, K. Sasaki, N. Watanabe,
Z. Cai and T. Shiono, Polymer, 2009, 50, 4788; (b) I. Palard,
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7 Selected examples: (a) H. D. Dyer, S. Huijser, N. Susperregui,
F. Bonnet, A. D. Schwarz, R. Duchateau, L. Maron and
P. Mountford, Organometallics, 2010, 29, 3602; (b) J. Jenter,
P. W. Roesky, N. Ajellal, S. G. Guillaume, N. Susperregui and
L. Maron, Chem.–Eur. J., 2010, 16, 4629; (c) T. V. Mahrova,
G. K. Fukin, A. V. Cherkasov, A. A. Trifonov, N. Ajellal and
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at RT or À20 1C. For example, with [rac-LA]0 : [3]0
=
200, >90% conversion was achieved within 5 min at both
temperatures. Fig. 2 (bottom) shows a plot of experimental
Mn vs. equivs rac-LA converted for [rac-LA]0 : [3]0 ratios
between 50 and 200. The gradient of the best-fit line is
162(4) g molÀ1 (equivs converted)À1 (R2 = 0.998) in very
good agreement with that predicted (144.1 g molÀ1 (equiv.
converted)À1) for one PLA chain growing per Ca–BH4 group
of 3 in a living-type fashion. The corresponding plot for ROP
at RT was also linear (R2 = 0.996, see the ESIw) but the
gradient of the best-fit line was 239(7) g molÀ1 (equivs
converted)À1 indicating less optimal control of the relative
rates of initiation and propagation for this very active
system. Tetrad analysis of the CH(Me)O region of the
selectively homonuclear decoupled 1H NMR spectra of the
PLA samples formed with 3 revealed heterotactically-enriched
polymer with high Pr values of 0.88–0.90 at À20 1C
(ca. 0.80 at RT).
8 (a) C. A. Wheaton and P. G. Hayes, Chem. Commun., 2010, 46,
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R. Duchateau and P. Mountford, Chem. Commun., 2010, 46, 273;
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2667.
We thank the EPSRC for a scholarship to M. G. C.
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c
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This journal is The Royal Society of Chemistry 2011