A well-defined iron(II) alkoxide initiator for the controlled polymerisation of lactide

Article abstract of DOI:10.1039/b209703f

The three-coordinate iron(II) complex, (Bu^t-BDI)FeOBu^t [Bu^t-BDI = HC(C(Bu^t)N-2,6-ⁱPr₂C₆H ₃)₂] is found to be a highly active initiator for the polymerisation of lactide and caprolactone at room temperature.

Full text of DOI:10.1039/b209703f

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A well-defined iron(II) alkoxide initiator for the controlled
polymerisation of lactide
Vernon C. Gibson,* Edward L. Marshall, Diana Navarro-Llobet, Andrew J. P. White and
David J. Williams
Department of Chemistry, Imperial College of Science, Technology and Medicine,
Exhibition Road, South Kensington, London, UK SW7 2AY. E-mail: v.gibson@ic.ac.uk
Received 3rd October 2002, Accepted 17th October 2002
First published as an Advance Article on the web 4th November 2002
The three-coordinate iron(II) complex, (But-BDI)FeOBut
[But-BDI ؍ HC(C(But)N-2,6-iPr2C6H3)2] is found to be a
highly active initiator for the polymerisation of lactide and
caprolactone at room temperature.
There is currently much interest in the development of well-
defined initiators for the controlled ring-opening polymeris-
ation (ROP) of cyclic esters due to their use in the tailored
synthesis of biodegradable amd bioresorbable materials. While
β-diketimate complexes1 based on zinc,2 magnesium3 and tin4
have been shown to afford living systems for the ROP of lactide,
there is considerable interest in the development of iron based
initiators due to their attractively low bio-toxicity. A recent
report5 described the use of iron() initiators for lactide
and caprolactone polymerisation, including binuclear Fe2-
(OCHPh2)6 and monomeric L2FeOR (R = Et, CHPh2; L =
N,NЈ-bis(trimethylsilyl)benzamidinate). To our knowledge,
there have been no reports to date on divalent iron initiator
Fig. 1 The molecular structure of 2. Selected bond lengths (Å) and
systems. We were attracted to a report by Holland and co-
workers6 of a remarkably robust three-coordinate iron()
chloride complex stabilised by But-BDI (1, Scheme 1) which we
angles (Њ): Fe–N(1) 1.965(3), Fe–N(3) 1.993(3), Fe–O(36) 1.786(3),
N(1)–C(1) 1.340(5), N(3)–C(3) 1.330(5); N(1)–Fe–N(3) 95.79(13),
N(1)–Fe–O(36) 143.8(2), N(3)–Fe–O(36) 120.2(2), Fe–O(36)–C(37)
150.3(3).
2,6-diisopropylphenyl arms, causes the oxygen atom to be sited
away from the exterior bisector of the N–Fe–N angle, the two
N–Fe–O angles differing by ca. 23Њ. In the chloro analogue,6 the
two N–Fe–Cl angles are identical [131.83(5)Њ], a consequence of
the mirror symmetry. The positioning of this oxygen atom
closer to N(3) is reflected in the Fe–N bond distances, with that
to N(3) being noticeably longer [at 1.993(3) Å] than that to N(1)
[1.965(3) Å]. The geometry at iron is distorted trigonal planar,
the iron atom lying 0.05 Å out of the N2O plane. The six-
membered chelate ring is coplanar to within 0.02 Å, but
the tert-butoxide group is bent slightly out of this plane, the
Scheme 1 Reagents and conditions: (i) NaOBut, Et2O, rt, 12 h,
extraction into toluene, recrystallisation from Et2O, 80% yield.
oxygen atom lying 0.13 Å out of plane. The Fe–O and C᎐N
᎐
envisaged could act as a potentially suitable precursor to a well-
defined iron() initiator.6 Here, we describe the synthesis of the
novel three-coordinate alkoxide derivative 2 (Scheme 1) and its
use to polymerise lactide with high efficiency and in a controlled
manner.
bond lengths are unexceptional. The electronic properties of
planar three-coordinate iron() complexes have recently been
described.7
2 has been investigated as an initiator for the polymerisation
of the cyclic esters rac-lactide (LA) and ε-caprolactone (CL). It
is found to be highly active for both monomers at room tem-
perature. Polymerisation of 100 equivalents of rac-LA in tolu-
ene gives 94% conversion within 20 min.§ The resultant PLA
has a polydispersity (PDI) of 1.12 and Mn 37500 Da (based on
polystyrene standards). NMR analyses (1H homodecoupled
and 13C{1H} experiments) show that atactic PLA is formed.
The linear relationship between the number average molecu-
lar weight (Mn) and [M]o/[I]o (Fig. 2), and Mn versus conversion
(Fig. 3) for the polymerisation of rac-LA are indicative of a
well-controlled polymerisation. The slightly broadened PDI’s
evident in Fig. 2, where conversions are typically >95%, are
attributable to transesterification side reactions (evident by
13C NMR) which become more prevalent as the monomer
concentration falls.
Reaction of equimolar quantities of (But-BDI)FeCl and
NaOBut in diethyl ether at room temperature, followed by
extraction with toluene, gives an orange solid in good yield.
MS(CI) and elemental analyses are consistent with its formul-
ation as a mono-alkoxide species, and a solution magnetic
moment of 4.8 µB confirms the presence of high-spin d6 iron()
1
centres. The H NMR spectrum of 2† in C6D6 reveals eight
broadened paramagnetic contact-shifted resonances. Crystals
suitable for X-ray diffraction‡ were obtained by recrystallis-
ation from diethyl ether at Ϫ10 ЊC. The structure (Fig. 1) is
closely related to that of its chloro analogue,6 but the preference
for the oxygen atom of the tert-butoxide group in 2 to adopt a
bent geometry precludes the presence of the crystallographic
mirror plane that was observed in the chloro species. This bend
[Fe–O(36)–C(37) 150.3(3)Њ], combined with the steric bulk
of the tert-butyl moiety and the cavity formed by the two
The activity of this iron() system for lactide polymerisation
is comparable to its zinc() relative, and occupies a position
DOI: 10.1039/b209703f
J. Chem. Soc., Dalton Trans., 2002, 4321–4322
This journal is © The Royal Society of Chemistry 2002
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and CL. For rac-LA, the system is well controlled though
transesterification side reactions become competitive at low
monomer concentrations.
Notes and references
† Selected spectroscopic data for 2: δH (250 MHz, C6D6, 25 ЊC): 44.0,
8.5, 3.2, 1.3, Ϫ0.5, Ϫ13.7, Ϫ32.1, Ϫ98.0, µeff(Evans) = 4.8 µB.
Anal.Calcd for FeN2OC39H62: C, 74.26; H, 9.91; N, 4.44. Found: C,
74.34; H, 10.01; N, 4.38%. MS (CI): m/z = 631 (M ϩ H).
‡ Crystal data for 2: C39H62N2OFe, M = 630.8, monoclinic, P21/n
(no. 14), a = 9.783(1), b = 17.927(2), c = 21.781(2) Å, β = 96.91(1)Њ,
V = 3792.2(5) Å3, Z = 4, Dc = 1.105 g cmϪ3, µ(Cu-Kα) = 3.40 mmϪ1
,
T = 203 K, orange plates; 5553 independent measured reflections,
F 2 refinement, R1 = 0.056, wR2 = 0.119, 3501 independent observed
absorption corrected reflections [|Fo| > 4σ(|Fo|), 2θ ≤ 120Њ], 407 param-
eters. One of the isopropyl groups was found to be disordered, and two
50% occupancy orientations were identified. CCDC reference number
191757. See http://www.rsc.org/suppdata/dt/b2/b209703f/ for crystallo-
graphic data in CIF or other electronic format.
Fig. 2 Plot of Mn (determined by GPC) versus the number of
equivalents of LA; rt, toluene.
§ Typical polymerisation procedure: polymerisation reactions were
performed under a nitrogen atmosphere. The initiator was added to the
monomer in toluene. After stirring at rt for a period of time, the solvent
was evaporated under reduced pressure to give the polymer, which was
analysed by NMR. Conversions were calculated by integration of the
CH signals of LA relative to those of PLA and by integration of the
CH2 signals of ε-CL relative to those of PCL. GPC analyses were
performed in chloroform using a Gynkotek 300 HPLC pump con-
nected to two 5 micron mixed bed columns (Polymer Laboratories) and
a Knauer differential refractometer. Columns were calibrated using
polystyrene standards (103–106 Da) and the chromatograms were
analysed using Viscotek Trisec software (conventional calibration).
¶ Zn and Fe have similar electronegativities on the Allred–Rochow
scale: 1.66 and 1.64, respectively. Mg = 1.23, Sn = 1.72.8
Fig. 3 Relationship between Mn (determined by GPC) and monomer
conversion (determined by 1H NMR); LA : 2 = 100 : 1, rt, toluene.
1 For a review of metal β-diketiminate complexes, see: L. Bourget-
Merle, M. F. Lappert and J. R. Severn, Chem. Rev., 2002, 102, 3031.
2 M. Cheng, A. B. Attygalle, E. B. Lobkovsky and G. W. Coates,
J. Am. Chem. Soc., 1999, 121, 11583.
3 M. H. Chisholm, J. C. Huffman and K. Phomphrai, J. Chem. Soc.,
Dalton Trans., 2001, 222.
4 A. P. Dove, V. C. Gibson, E. L. Marshall, A. J. P. White and
D. J. Williams, Chem. Commun., 2001, 283.
5 B. J. O’Keefe, L. E. Breyfogle, M. A. Hillmyer and W. B. Tolman,
J. Am. Chem. Soc., 2002, 124, 4384.
6 J. M. Smith, R. J. Lachicotte and P. L. Holland, Chem. Commun.,
2001, 1542.
7 H. Andres, E. L. Bominaar, J. M. Smith, N. A. Eckert, P. L. Holland
and E. Munck, J. Am. Chem. Soc., 2002, 124, 3012–3025.
8 (a) A. L. Allred and E. G. Rochow, J. Inorg. Nucl. Chem., 1958, 5, 264;
(b) E. J. Little and M. M. Jones, J. Chem. Educ., 1960, 37, 231.
amongst the most active systems for controlled lactide polymer-
isation. For divalent centres stabilised by β-diketiminate
ligands, activities follow the trend Mg > Zn ≈ Fe > Sn and
correlate reasonably well with the electrophilicities of the metal
centres,¶ although the Sn derivative is undoubtedly also
strongly influenced by the presence of its stereochemically
active lone pair of electrons.
2 is also highly active, though less controlled, for the ring-
opening polymerisation of ε-caprolactone. The polymerization
of 100 equivalents of ε-CL in toluene yielded 95% conversion
within 5 min, affording polycaprolactone with Mn = 86,200 Da
and PDI = 1.38.
In summary, (But-BDI)Fe(OBut) is amongst the most active
initiators for the ROP of cyclic ester monomers such as rac-LA
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J. Chem. Soc., Dalton Trans., 2002, 4321–4322