Dimethylsilyl bis(amidinate)actinide complexes: Synthesis and reactivity towards oxygen containing substrates

Article abstract of DOI:10.1039/c4dt01361a

The reactivity of the monoanionic amidinate ligand [(CH₃) ₃CNC(Ph)NSiMe₂NC(Ph)-NHC(CH₃)₃]Li (1) with a silyl amido side arm towards the early actinides, uranium and thorium, was investigated. While the salt metathesis reaction with ThCl ₄(thf)₃ afforded the bis(amidinate)thorium(iv) dichloride complex [(CH₃)₃CNC(Ph)NSi(CH₃) ₂NC(Ph)-NHC(CH₃)₃]ThCl₂ (2) in high yield, the reaction of ligand 1 with UCl₄ leads to a Lewis acid supported nucleophilic attack of an incoming ligand unit, yielding the trichloro uranium complex [(CH₃)₃CNC(Ph)Si(CH₃) ₂-N(C(CH₃)₃)C(Ph)NSi(CH₃) ₂NC(Ph)N-(C(CH₃)₃]UCl₃ (4). The exposure of in situ formed complex 2 to wet THF solutions (<1% w of water), gave the mono(amidinate)Th(iv)(chloro)(bis-hydroxo) dimeric complex [(CH ₃)₃CNC(Ph)NSiMe₂NC(Ph)NHC(CH₃) ₃Th(OH)₂(Cl)]₂·(3) as bright red needles, exhibiting extremely short Th-OH bond distances (1.741(5) A and 1.737(5) A). The reactivity of the thorium complex 2 in the ring opening polymerization (ROP) was studied, showing high activity. Thermodynamic and kinetic measurements were performed to shed light on the mechanism for the ROP. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4dt01361a

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Dimethylsilyl bis(amidinate)actinide complexes:
synthesis and reactivity towards oxygen containing
substrates†
Cite this: Dalton Trans., 2014, 43,
11376
Isabell S. R. Karmel, Tatyana Elkin, Natalia Fridman and Moris S. Eisen*
The reactivity of the monoanionic amidinate ligand [(CH3)3CNC(Ph)NSiMe2NC(Ph)–NHC(CH3)3]Li (1) with
a silyl amido side arm towards the early actinides, uranium and thorium, was investigated. While the salt
metathesis reaction with ThCl4(thf)3 afforded the bis(amidinate)thorium(IV) dichloride complex
[(CH3)3CNC(Ph)NSi(CH3)2NC(Ph)–NHC(CH3)3]ThCl2 (2) in high yield, the reaction of ligand 1 with UCl4 leads
to a Lewis acid supported nucleophilic attack of an incoming ligand unit, yielding the trichloro uranium
complex [(CH3)3CNC(Ph)Si(CH3)2–N(C(CH3)3)C(Ph)NSi(CH3)2NC(Ph)N–(C(CH3)3]UCl3 (4). The exposure of
in situ formed complex 2 to wet THF solutions (<1% w of water), gave the mono(amidinate)Th(IV)(chloro)-
(bis-hydroxo) dimeric complex [(CH3)3CNC(Ph)NSiMe2NC(Ph)NHC(CH3)3Th(OH)2(Cl)]2·(3) as bright red
needles, exhibiting extremely short Th–OH bond distances (1.741(5) Å and 1.737(5) Å). The reactivity of the
thorium complex 2 in the ring opening polymerization (ROP) was studied, showing high activity. Thermo-
dynamic and kinetic measurements were performed to shed light on the mechanism for the ROP.
Received 7th May 2014,
Accepted 4th June 2014
DOI: 10.1039/c4dt01361a
www.rsc.org/dalton
tronic properties of the metal complex formed, and therefore
allowing a designed manipulation of its reactivity.7 Despite the
Introduction
The chemistry of the early actinide elements, thorium and steric similarity between cyclopentadienyl and amidinate
uranium, has undergone a long journey, since the discovery of ligands,6a,b their electronic properties are very different. While
the first organoactinide complex, Cp3UCl, reported by Wilkin- the cyclopentadienyl moiety is a six electron donor, the amidi-
son et al. in 1956.1 Over the past three decades, the organome- nate group is only a four electron donor, rendering the respect-
tallic chemistry of these elements has been mainly dominated ive metal center more electron deficient. The higher
by cyclopentadienyl derivatives,2 and has only recently started electrophilicity of the metal center should in turn increase its
to be developed towards heteroatom containing ligand oxophilicity and therefore also the reactivity towards oxygen
systems.3–5 Amidinate ligands, containing the NCN hetero- containing molecules, such as water, alcohols, or esters,
allylic core, are considered to be steric analogues of the cyclo- impeding in most cases their use as catalytic precursors. Ami-
pentadienyl moiety, displaying very similar cone angles of 137° dinate complexes of the lanthanide metals have indeed shown
for the cyclopentadienyl, and 136° for the benzamidinate an extraordinary high activity in the ring opening polymeri-
moiety, respectively.6a,b Amidinates have found a wide appli- zation (ROP) of ε-caprolactone and L-lactide,8 as well as in the
cation as ancillary ligands for main group elements, transition copolymerization of these monomers, yielding biodegradable
metals, lanthanides and actinides.6c,d This can be attributed and biocompatible polymers with a wide range of applications
mainly to the availability of the starting materials and ease of in biomedicine,9 environmentally friendly packaging
synthesis, as well as the easy modification of the steric and materials,10 microelectronics11 and adhesives.12 Despite the
electronic properties of the ancillary ligands by variation of the fact that the coordination chemistry of the amidinate ligands
substituents on the nitrogen, and the ipso-carbon atoms, with the early actinides, uranium and thorium, has been
allowing for a delicate control of the steric hindrance and elec- studied previously,13 only few reports are available regarding
the reactivity of these complexes with oxygen containing sub-
strates.13f,k,l Recently, we have reported that the activity of co-
Schulich Faculty of Chemistry, Institute of Catalysis Science and Technology,
ordinative unsaturated uranium(IV) complexes can be modified
by adjusting the electronic properties of the ligands. By using
highly nucleophilic, strongly basic imidazolin-2-iminato
ligands, and therefore increasing the electron density on the
Technion – Israel Institute of Technology, Technion City, 32000 Israel.
E-mail: chmoris@tx.technion.ac.il
†Electronic supplementary information (ESI) available. CCDC 1001486–1001489.
For ESI and crystallographic data in CIF or other electronic format see DOI:
highly electrophilic uranium center, the activity in the catalytic
10.1039/c4dt01361a
11376 | Dalton Trans., 2014, 43, 11376–11387
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ROP of ε-caprolactone reached extremely high values up to tion was worked-out in wet THF, containing <1% wt of water,
7 × 103 kg mol−1 h−1 20g
A conceptual question regards the complex was obtained in 67% yield as red crystals
.
3
ability to increase the activity of actinide complexes by tuning (Scheme 1). Crystallographic data for complexes 2 and 3 are
the steric hindrance around the metal center, forming partial presented in Table 1, selected bond lengths and angles are
coordinative saturated complexes. By using the slightly basic presented in Tables 2 and 3, respectively. The bis(amidinate)
amidinate ligands with a side-arm functionality, the steric hin- thorium(IV) complex 2 crystallizes in the orthorhombic space
drance around the metal center is increased, yielding sterically group Pccn with one molecule of hexane per unit cell (Fig. 1).
encumbered, coordinatively saturated metal complexes. More- The thorium center is chelated by two amidinate ligands, two
over, the amidine functionality of the side-arm will increase chloro ligands, and additional coordination of the amidine
the electron density on the metal center, by coordination of side arms, which completes the coordination number to eight.
the nitrogen lone-pair to the actinide center, leading to a Both amidinate moieties are equivalent, displaying the same
reduced electrophilicity and therefore an expected increased values for bond lengths and angles, leading towards a C2 sym-
catalytic reactivity of the actinide complex towards oxygen con- metric complex. The Th–N bond distances display different
taining molecules.
values for all three coordinated N-atoms, with 2.611(3) Å, 2.374(3)
In this study we report the synthesis of the first thorium Å and 2.621(3) Å for Th–N1, Th–N3 and Th–N4, respectively,
amidinate complexes [(CH3)3CNC(Ph)NSi(CH3)2NC(Ph)NHC- corroborating a non-equal electron distribution in the amidi-
(CH3)3]ThCl2 (2) and [(CH3)3CNC(Ph)NSiMe2NC(Ph)NHC- nate core, a highly localized Th–N bond, which is further sus-
(CH3)3Th(OH)2(Cl)]2 (3) with a free side arm on the ligand and tained by the N–C bond distances of the amidinate ligand.
the catalytic activity of the former in the ROP of ε-caprolactone. While N4–C12 and N1–C1 bond are rather short with values of
Moreover, a comparison between the reactivity of thorium and 1.300(5) Å and 1.303(5) Å respectively, the N3–C12 bond is
uranium towards the silicon containing amidinate monoanio- slightly elongated with a distance of 1.362(5) Å. Häfelinger and
nic ligand [(CH3)3CNC(Ph)NSiMe2NC(Ph)NHC(CH3)3]Li (1) is Kuske have defined the parameter ΔCN = d(C–N) − d(CvN)
presented. While the synthesis of the thorium complexes 2 (where d is the bond length in Å) for the central N–C–N
and 3 can readily be achieved via a salt metathesis reaction linkage of amidines. This parameter ranges from 0 to 0.178 Å
between the lithiated amidinate ligand 1 and ThCl4(thf)3, the for highly to non-conjugated systems, respectively. In our case,
analogue reaction with UCl4 leads to a Lewis acid supported the parameter ΔCN values are found to be 0.062 and 0.017 Å
nucleophilic attack of an incoming ligand unit on the for the unsymmetrical coordinated and almost symmetrical
dimethylsilyl moiety of the uranium intermediate, leading to non-coordinated ligand, respectively in complex 2.14
the formation of the uranium complex [(CH3)3CNC(Ph)Si-
Hence, the higher electron density of the N1–C1 and N4–
(CH3)2–N(C(CH3)3)C(Ph)NSi(CH3)2NC(Ph)N(C(CH3)3]UCl3 (4) C12 bonds is a result of their weaker electron donation to the
under the elimination of a lithium amidinate salt. When the thorium center, and therefore weaker Th–N bonds. The higher
neutral amidine [(CH3)3CNHC(Ph)NSiMe2NC(Ph)NHC(CH3)3] electron density of the Th–N3 bond can be attributed to the
(5) is reacted with UCl4 under the addition of a pyridine, as a stabilization of the negative charge in the α-position to the
mild base, the uranium(IV) complex [C(CH3)3NHC(Ph)NSi- dimethyl silyl group (α-carbanion stabilization by silicon). The
(CH3)2NC(Ph)NHC(CH3)3]UCl4–(C5H5N) (6) is obtained in mod- average Th–N and N–C bond lengths are yet comparable to
erate yields.
other thorium(IV) amidinate complexes.13l The N3–Th–N1, and
N3–Th–N4 angles are comparable with values of 60.41(10)°
and 52.24(11)°, respectively. The N1–Th–Cl linkage displays a
value of 90.04(8)°, which is slightly larger than the N3–Th–Cl
angle of 81.10(8)°. The values of the dihedral angles depend
Results and discussion
The chemistry of thorium and uranium amidinates systems on the spacer group, while the Th–N3–C12–N5 angle shows a
has been limited to simple benzamidinate and pyridylamidi- value of 32.20°, the Th–N1–Si–N3 angle, with a SiMe2 spacer,
nate complexes.13 Herein, we report the synthesis and struc- displays a value of 15.10°, indicating that the C-atom and Si-
ture of the thorium amidinate complexes 2 and 3, containing atom are not in the same plane. The Th–N1–N3–N4 torsion
an amidine side arm and its structural effect on the metal angle displays a value of 0.80°, showing the formation of a
center, as well as the reactivity of 2 towards ε-caprolactone. plane by the N1, N3, and N4 atoms with the metal center.
Complex 2 was synthesized by a slow addition of a THF solu-
The dinuclear bridged μ-dichloro thorium(IV) bis(hydroxo)-
tion of the ligand 1 to a THF solution of ThCl4(thf)3 at −78 °C. amidinate complex 3 (Fig. 2) is obtained in 67% yield when
Subsequent warming of the solution to room temperature, stir- the salt metathesis is carried in dry THF and the work-up of
ring of the solution for 48 hours and extraction with toluene the reaction is performed in wet THF, containing less than 1%
ˉ
gave complex 2 in 80% yield as a slightly yellow powder. Crys- wt water. Complex 3 crystallizes in the triclinic P1 space group
tallographic measurements were performed on single crystals as bright red needles with one molecule of toluene per unit
grown from a concentrated toluene solution, layered with cell. The seven coordinated thorium(IV) center is surrounded
hexane at −6 °C. Since these compounds are very oxophilic, we by one chelating amidinate ligand, with an amidine side arm
decided to study their reactivity towards small amounts of coordinated to the metal center, two hydroxo groups, and two
water as present in regular THF. Hence, when the same reac- µ-chloro ligands bridging between the two thorium centers.
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Scheme 1 Synthesis of the thorium amidinate complexes 2 and 3.
Table 1 Crystallographic data for complexes 2, 3, 4 and 6
Complex
2
3
4
6
Empirical formula
C52H84Si2Cl2Th
1068.34
C56H82N8Si2Cl2O4Th2
1522.45
C37H55Cl3N6Si2U
984.43
C29H41N5SiCl4U
867.60
Formula weight/g mol−1
T/K
λ/Å
250(2)
0.71073
250(2)
0.71073
240(2)
0.71073
250(2)
0.71073
Crystal system
Space group
Orthorhombic
Pccn
Triclinic
P1
Monoclinic
P21/C
Monoclinic
C2/C
ˉ
a/Å
b/Å
c/Å
α/°
15.7880(5)
18.7850(6)
20.1863(7)
90
10.9649(6)
12.6366(7)
12.9693(7)
97.368
19.5150(4)
10.5640(2)
22.4540(4)
90
12.2835(4)
14.8751(5)
23.0777(8)
90
β/°
90
90
5986.8(3)
4
1.336
2.660
2464
97.368
98.750
1696.07(16)
1
1.563
4.542
786
109.1960(9)
90
4371.66(14)
4
1.496
3.983
93.3560(10)
90
4209.5(2)
4
1.479
4.166
γ/°
V/Å3
Z
ρ/g cm−3
µ(Mo-Kα)/mm−1
F(000)
1960
1832
θ Range for data collection/°
Limiting indices
1.68 to 25.68
−19 ≤ h ≤ 19
−22 ≤ k ≤ 22
−24 ≤ l ≤ 15
23 969/5626 (0.0195)
98.8%
1.64 to 25.02
−13 ≤ h ≤ 13
−15 ≤ k ≤ 15
−15 ≤ l ≤ 15
62 495/5981 (0.0392)
99.7%
1.10 to 25.05
0 ≤ h ≤ 23
0 ≤ k ≤ 12
−26 ≤ l ≤ 25
7747/7747 (0.0000)
99.9%
1.77 to 26.42
−15 ≤ h ≤ 15
−18 ≤ k ≤ 18
−28 ≤ l ≤ 28
27 898/4304 (0.0339)
99.7%
Reflections collected/unique (Rint
)
)
Completeness to θ
GOF on F2
1.005
1.060
1.028
1.163
R1, wR2 [I > 2σ(I)]
0.0305, 0.0884
0.0499, 0.0989
1.188 and −0.765
0.0354, 0.1074
0.0384, 0.1092
3.228 and −0.960
0.0317, 0.0692
0.0538, 0.0759
0.604, −0.788
0.0427, 0.1112
0.0466, 0.1128
1.482, −0.895
R1, wR2 (all data)
Largest diff. peak and hole (e Å−3
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Table 2 Selected bond lengths and angles for complex 2
Bond lengths (Å)
Bond angles (°)
Th–N1
Th–N3
Th–N4
Th–Cl
N1–C1
N2–C1
N3–C12
N4–C12
2.611(3)
2.374(3)
2.621(3)
2.6939(11)
1.303(5)
1.320(5)
1.362(5)
1.300(5)
N3–Th–N1
N3–Th–N4
N4–Th–N1
N1–Th–Cl
N3–Th–Cl
N4–Th–Cl
Th–N3–C12–N4
Th–N1–Si–N3
Th–N1–N3–N4
Cl–Th–Cl
60.41(10)
52.24(11)
112.65(9)
90.04(8)
82.99(8)
81.10(8)
32.20
15.10
0.80
89.9
Fig. 2 Molecular structure of complex 3·(C7H8) (50% probability ellip-
soids). Color code: Th, blue, Cl, green, Si, yellow, N, purple, O, red, C,
grey. Hydrogen atoms are omitted for clarity.
Table 3 Selected bond lengths and angles for complex 3
Bond lengths (Å)
Bond angles (°)
can be attributed to the ability of the dimethyl silyl group to
stabilize a carbanion in α-position, which is further reflected
in the slightly shorter N3–C14 bond distance. The values for
the N–Th–N angles depend on the linkage between the nitro-
gen atoms, while the N1–Th–N2 is 54.22(19), the SiMe2 linkage
slightly increases the value of the N2–Th–N3 angle to
60.26(18)°. The dihedral angles display values of 21.70° for
Th–N2–C1–N1, and 19.84° Th–N4–Si–N3, indicating different
planes for the C-, and Si-atoms. However the N1, N2 and N3
atoms are in the same plane, as shown by the small value of
the Th–N1–N2–N3 torsion angle (1.83°). The hydroxo ligands
are equivalent, displaying extremely short Th–O distances of
1.741(5) Å and 1.737(5) Å, which are shorter than the average
Th–O single bond (Th–O: 1.92 Å–2.42 Å),15a,b and the ThvO
double bonds (1.929 Å in a solvated metal complex, and
1.840 Å, in the gas phase15c,d). The O1–Th–O2 angle, is close to
linearity with a value of 176.4(2)°, and the short Th–O bond
length suggest a large electron-donation from the oxygens
lone-pairs into the empty orbitals of the metal, leading to a
thorium bis(hydroxo) analogue of the uranyl moiety, with
similar bond lengths (UvO: 1.725 Å–2.350 Å, and 1.800 Å in
average) and O–M–O angles (O–UVI–O: 169°–180°),16 introdu-
cing a new structural motif in the coordination chemistry of
actinides. The large electron donation from the oxygen lone
pairs, is further reflected in the increasing acidity of the
hydroxyl protons, observed in the high downfield shift
(10.02 ppm) in 1H-NMR spectroscopy. In contrast, the signal of
the NH protons (1.42 ppm), displays a slight high field shift as
compared to the NH protons (2.21 ppm) in complex 2. The NH
and OH protons can be distinguished by integration of the
Th–O1
Th–O2
Th–N1
Th–N2
Th–N3
Th–Cl
N2–C1
N3–C14
N4–C14
N1–C1
1.741(5)
1.737(5)
2.520(5)
2.327(5)
2.623(5)
2.85536(16)
1.351(8)
1.300(9)
1.328(9)
1.304(9)
O1–Th–O2
O1–Th–N1
O1–Th–N2
O2–Th–N1
O2–Th–N2
N1–Th–N2
N2–Th–N3
O1–Th–Cl1
O2–Th–Cl1
Th–N2–C1–N1
Th–N4–Si–N3
Th–N1–N2–N3
176.4(2)
94.5(2)
93.7(2)
88.4(2)
89.7(2)
54.22(19)
60.26(18)
89.94(16)
86.68(15)
21.70
19.84
1.83
Fig. 1 Molecular structure of complex 2·(C6H14) (50% probability ellip-
soids). Color code: Th, blue, Cl, green, Si, yellow, N, purple, C, grey.
Hydrogen atoms are omitted for clarity.
The Th–N bond distances display different values for all three respective signals in 1H-NMR spectroscopy. The increased
Th–N bonds, with 2.520(5) Å, 2.327(5) Å and 2.623(5) Å for acidity of the hydroxyl proton in complex 3 is further sustained
Th–N1, Th–N2, and Th–N3, respectively. Similar to complex 2, by the O–H stretching (3340–3234 cm−1), and the Th–O–H
the C–N bond distances in complex 3, also exhibit a slightly stretch (502–416 cm−1), which are shifted to lower energies, as
uneven electron distribution throughout the amidinate core compared to previously reported values.17 The O–Th–N angles
with values of 1.304(9) Å, 1.351(8) Å, 1.300(9) Å, and 1.328 Å are close to 90° with 89.7(2)°, 93.7(2)°, 88.2(2)°, 94.5(2)°, 90.2°,
for N1–C1, N2–C1, N3–C14, and N4–C14, respectively. The and 90.6(2)° for O2–Th–N2, O1–Th–N2, O2–Th–N1, O1–Th–N1,
Häfelinger and Kuske parameter ΔCN was calculated to be O2–Th–N3, and O1–Th–N3, respectively. The bridging chlor-
0.053 and 0.028 Å for the unsymmetrical coordinated and ides display a Th–Cl bond length of 2.8536(16) Å, and a Cl–Th–
almost symmetrical non-coordinated ligand, respectively in Cl bond angle of 72.53(5)°. The O–Th–Cl linkages exhibit
complex 3.14 The higher electron density of the Th–N2 bond values close to 90° with 89.94(16)° for O1–Th–Cl1 and
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86.68(15)° for O2–Th–Cl1, corroborating a O–Th–O plane,
which is perpendicular to the thorium amidinate and thorium
chloride plane.
Table 4 Selected bond lengths and angles for complex 4
Bond lengths (Å)
Bond angles (°)
Interestingly, similar to the recently reported µ-chloro
U–N1
U–N2
U–N4
U–N5
U–Cl1
U–Cl2
U–Cl3
N1–C5
N2–C5
Si1–N2
Si1–N3
Si2–N5
Si2–N4
2.486(4)
2.349(4)
2.533(3)
2.525(4)
2.6233(12)
2.6608(13)
2.6372(12)
1.289(5)
1.327(5)
1.691(4)
1.808(4)
1.747(4)
1.762(4)
N1–U–N2
N2–U–N4
N4–U–N5
54.95(13)
75.53(12)
60.96(11)
115.70(9)
102.00(9)
78.86(9)
94.51(10)
156.94(10)
84.30(9)
8.62
thorium(IV) corrole complex,18 complex
3 crystallizes as
unusual bright red crystals. The UV-vis spectrum of complex 3
displays similar absorption bands in the region between
550 nm–300 nm to the reported µ-chloro thorium(IV) corrole
compound (see ESI†). This coloring can probably be attributed
to a ligand to metal charge transfer.
N1–U–Cl1
N1–U–Cl2
N1–U–Cl3
N2–U–Cl1
N2–U–Cl2
N2–U–Cl3
U–N1–C5–N3
U–N2–N3–N4
U–N2–Si1–N3
U–N5–Si2–N4
The reactivity of ligand 1 towards the uranium(IV) leads to
an unexpected nucleophilic attack of an incoming lithiated
amidinate moiety on the dimethylsilyl group of the monoami-
dinate uranium(IV) complex, obtained as an intermediate, after
the salt-metathesis with UCl4 (Scheme 2). The nucleophilic
attack is assisted by the coordination to the Lewis acidic
uranium(IV) center, and proceeds under the elimination of a
lithiated amidine moiety to yield complex 419 (Scheme 3). The
absence of free benzonitrile in the reaction mixture indicates
that a retro Brook mechanism is not a major operative
pathway. The crystallographic data for complex 4 is presented
in Table 1, and selected bond length and angles in Table 4,
respectively. The uranium complex 4 crystallizes as dark red-
brown prisms in the monoclinic P21/c space group. The geo-
metry around the uranium center can be described as capped
trigonal prismatic, with four nitrogen atoms and three chlor-
22.22
58.41
16.59
Fig. 3 Molecular structure of complex 4 (50% probability ellipsoids).
Color code: U, blue, Cl, green, Si, yellow, N, purple, C, grey. Hydrogen
atoms are omitted for clarity.
ides coordinated to the uranium(IV) center (Fig. 3). The length
of the U–N bond distances varies from 2.533(3) Å for U–N4 to
2.349(4) Å for U–N2, which can be attributed to the nature of
bonding of each nitrogen atom in the amidinate ligand.
The different lengths of the C–N bonds in the amidine
moiety, are 1.289(5) Å and 1.327(5) Å for N1–C5 and N2–C5,
respectively. The Häfelinger and Kuske parameter ΔCN =
0.037 Å,14 suggest a slightly uneven electron distribution along
the amidine core, which is further reflected in the unequal
length of the U–N bonds. This is further sustained by different
lengths of the N–Si bonds, with values of 1.691(4) Å for Si1–N2
and 1.808(4) Å for Si1–N3, displaying a shorter N–Si bond
closer to the amidine core, which can be accredited to the
ability of the dimethyl silyl moiety to stabilize a negative
charge in α-position. The U–N4 and U–N5 exhibit similar bond
distances with values of 2.533(3) Å and 2.523(4) Å, respectively,
which are slightly longer than the distances found for U–N1
and U–N2, indicating an interaction between the a lone pair of
the N4 and N5 atoms and the uranium(IV) center. The U–Cl
bond distances display similar values of 2.6233(12) Å,
2.6608(13) Å, and 2.6372(12) Å, for U–Cl1, U–Cl2, and U–Cl3,
respectively. The N–U–N angles depend on the spacer group,
similar to the complexes 2 and 3. While the N1–U–N2 angles
with a C–Ph spacer, shows a value of 54.95(13)°, the N4–U–N5
angles with a SiMe2 spacer group is slightly larger (60.96(11)°).
The largest N–U–N angles was found for N2–U–N4 with a
Scheme 2 Synthesis of the uranium complex 4.
Scheme 3 Plausible mechanism for the synthesis of complex 4.
11380 | Dalton Trans., 2014, 43, 11376–11387
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value of 75.53(12)°, with a silylamido spacer, forming a six value of 62.2(2)°. The U–Cl bond distances are similar to those
membered ring with the uranium center. The dihedral angles found for complex 4 with values 2.6277(17) Å and 2.6576(16) Å
were determined for U–N1–C5–N3 (8.62°), U–N2–N3–N4 for U–Cl1 and U–Cl2, respectively. The U–N3 bond distance to
(22.22°), U–N2–Si1–N3 (58.41°), U–N5–Si2–N4 (16.59°). The the coordinated pyridine moiety displays a value of 2.727(7) Å,
angle formed between the U–N1–C5–N2 and U–N5–Si2–N4 which is longer than the other U–N distances, as expected.
planes, displays a value of 130°.
Catalytic ring opening polymerization of ε-caprolactone
In order to avoid the nucleophilic attack of the lithium ami-
dinate assisted by the uranium complex, we attempted to
deprotonate the neutral amidine ligand 5 with pyridine as a
mild base, which induced the formation of complex 6
(Scheme 4). Crystallographic data for complex 6 is presented in
Table 1, and selected bond lengths and angles in Table 5,
respectively.
The uranium(IV) complex 6 crystallizes as green crystals in
the monoclinic C2/c space group with one molecule of
benzene per unit cell (Fig. 4). The U–N1 bond distance is
slightly longer than the amidinate U–N1 bond distance in
complex 4 with 2.474(5) Å, yet it still lies within the same
range of the U–N amidinate bond. The N1–U–N1 linkage is
comparable to the N4–U–N5 angle in complex 4 displaying a
The early actinide elements thorium and uranium were long
thought to be inactive towards oxygen containing molecules,
such as alcohols, aldehydes, esters and cyclic lactones, due to
their high electrophilicity and resulting high oxophilicity. Con-
sequently the examples for catalytic conversions of oxygen con-
taining substrates by thorium and uranium are rare, and still
remain a challenge in the field of catalysis.13l,20 Herein, we
study the reactivity of the thorium complexes 2 and 3 towards
the opening of the cyclic ester, ε-caprolactone, leading towards
the biodegradable polymer polycaprolactone. The amidinate
complexes 2 and 3 containing a pendant side-arm, which
donates electron density to the metal-center seem to be good
candidates as catalysts for the ROP of ε-caprolactone. While
this assumption has been proven to be true for complex 2,
complex 3 doesn’t react with ε-caprolactone, which can be
attributed to the saturated coordination sphere of 3 in com-
parison to complex 2, due to the electron donation of the
corresponding hydroxo ligands. The polymerization results for
the ROP of ε-caprolactone by complex 2 are presented in
Table 6. Since, the uranium(IV) complexes 4 and 6 only display
a slight activity towards ε-caprolactone, at elevated tempera-
tures, and no activity at room temperature, these results will
not be further discussed.
The polymerization of ε-caprolactone mediated by complex
2 shows an increase in activity and molecular weight of the
polymers obtained as a function of time, until all the
monomer is consumed after 120 minutes. When the reaction
is carried out at elevated temperatures, higher catalytic activi-
ties and higher molecular weights of the polymers can be
achieved, as expected. The low polydispersity of the polymers
together with an increase of the molecular weight as a function
of time indicates that the polymerization is performed via a
single site catalyst in a quasi-living polymerization fashion.
These quasi living situations are expected to be operative due
to the different coordination of the growing polymer chain to
the metal center. The rates of monomer insertion and chain
termination were calculated using eqn (1) and (2), respectively.
Scheme 4 Synthesis of complex 6 with the neutral amidine 5.
Table 5 Selected bond lengths and angles for complex 6
Bond lengths (Å)
Bond angles (°)
U–N1
U–N3
U–Cl1
Si–N1
N1–C1
N2–C1
2.474(5)
2.727(7)
2.6576(16)
2.6277(16)
1.315(7)
1.333(7)
N1–U–N1#1
N1–U–Cl1
N1–U–Cl2
N3–U–Cl1
N3–U–Cl2
U–N1–Si–N1
62.2(2)
79.57(11)
127.43(12)
82.97(4)
75.11(5)
0.00
mðpolymerÞ½gꢀ
Monomer insertion rate ðRiÞ ¼
MwðmonomerÞ½g moleꢁ1ꢀ ꢂ time ½hꢀ
ð1Þ
mðpolymerÞ½gꢀ
Chain termination rate ðRtÞ ¼
ð2Þ
MnðpolymerÞ ꢂ time ½hꢀ
As the monomer is being consumed, the rates of insertion
is reduced, however there is a continuous increase in the mole-
cular weight. In order to obtain the kinetic dependence of the
reaction on ε-caprolactone and complex 2, kinetic NMR
measurements were performed, displaying a first-order depen-
Fig. 4 Molecular structure of complex 6·(C6H6) (50% probability ellip-
soids). Color code: U, blue, Cl, green, Si, yellow, N, purple, C, grey.
Hydrogen atoms are omitted for clarity.
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Table 6 Polymerization results for the ROP of ε-caprolactone mediated by 2
Entrya
2 : ε-CL
Time/min
Activity/g mol−1 h−1
Mw/Dalton
Mn/Dalton
PD
Ri/mol h−1
Rt/mol h−1
1
2
3
4
5
6b
7
8
1 : 1000
1 : 1000
1 : 1000
1 : 1000
1 : 1000
1 : 1000
1 : 500
10
30
60
120
300
120
120
120
24 900
33 680
51 590
78 210
24 480
102 510
22 430
230 550
70 800
106 800
108 100
357 400
481 200
634 500
226 600
496 900
48 900
82 800
69 700
266 700
422 100
425 800
155 200
417 600
1.45
1.29
1.55
1.34
1.14
1.49
1.46
1.19
2.03 × 10−3
1.32 × 10−3
1.00 × 10−3
7.64 × 10−4
4.40 × 10−4
1.00 × 10−3
5.21 × 10−4
2.12 × 10−3
4.75 × 10−6
1.81 × 10−6
1.25 × 10−6
4.31 × 10−7
1.18 × 10−7
2.94 × 10−7
3.83 × 10−7
5.82 × 10−7
1 : 2000
a Polymerization conditions: 5 mL toluene, 2.23 µmol catalyst, rt. b 90 °C, Ri = Rate of insertion, Rt = Rate of termination.
Fig. 5 Plot of the rate of polymerization (∂p/∂t) versus concentration of
catalyst 2.
dence on the substrate and catalyst 2 (Fig. 5) giving raise to the
kinetic eqn (3).
Scheme 5 Plausible mechanism for the ROP of ε-caprolactone
mediated by complex 2.
@p
@t
¼ kobs½complex 2ꢀ½ε-caprolactoneꢀ
ð3Þ
Insertion of an incoming monomer unit as the rate deter-
mining step, leads towards the open chain complex (B) that
upon additional insertion induces the growing polymer chain
(C). After hydrolysis with methanol, polycaprolactone with a
caprolactonyl end-group can be observed by 1H-NMR spec-
troscopy, supporting the proposed cationic mechanism.
1H-NMR experiments with stoichiometric amounts of ε-capro-
lactone and complex 2, showed no hydrolysis of the ligand
unit upon addition of ε-caprolactone and the formation of
complex 2–caprolactone adduct (see ESI†), reinforcing the
coordination of the ligand throughout the polymerization
process. Interestingly, the data at hand indicates that the
possibility to induce the elimination of the polymer chain with
the acidic proton of the ε-caprolactone or the cationic ring is
not a major operative pathway, as compared to the insertion of
additional monomers (the molecular weight increased as a
function of time and the mole number of chains is always
lower than the mole of catalyst used) indicating the interaction
of the cationic close ring with the metal center.
The thermodynamic parameters were determined from the
Arrhenius (Fig. 6) (Ea = 20.08 kcal mol−1) and Eyring (ΔH‡
=
20.02 kcal mol−1, ΔS‡ = −12.72 cal mol−1 K) plots. A plausible
general mechanism for the ROP of ε-caprolactone mediated by
the thorium amidinate complex 2 is presented in Scheme 5. At
the first step of the mechanism, the substrate ε-caprolactone is
rapidly activated by the Lewis acidic thorium complex, to form
the Th–alkoxocaprolate complex (A).
Conclusions
The reactivity of the amidinate ligand 1, containing a silylami-
dine side chain towards the early actinide elements thorium
Fig. 6 Arrhenius plot for the polymerization of ε-caprolactone
mediated by complex 2.
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and uranium was investigated. When 1 was reacted with half silane. Elemental analysis was carried out by the microanalysis
an equivalent of ThCl4(thf)3, complex 2 was obtained in 80% laboratory at the Hebrew University of Jerusalem. GPC
yield. The octahedral amidinate complex
2 displays an measurements were carried out on a Waters Breeze system
additional coordination of the amidine nitrogen to the metal with a styrogel RT column and with THF (HPLC grade, T.G.
center, completing a coordination number of eight. When the Baker) as mobile phase at 30 °C. Relative calibration was done
work-up of the salt metathesis reaction of 1 with ThCl4(thf)3 with polystyrene standards (Aldrich, 2000–1 800 000 range). Mn
was carried out in wet THF, complex 3 was obtained as an values were multiplied by a factor of 0.58 and correlated to
unusual bright red needle shaped crystals. The thorium actual PCL values.
centers in complex 3 are bridged by two µ-chloro ligands,
leading to a symmetric thorium(IV) dimer. Each of the thorium
X-ray crystallographic measurements
centers is coordinated by one amidinate moiety, with an The single-crystal material was immersed in Paratone–N oil
additional coordination of the amidine nitrogen, two µ-chloro and was quickly fished with a glass rod and mounted on a
ligands, and two hydroxo ligands, which are perpendicular to Kappa CCD diffractometer under a cold stream of nitrogen.
the amidinate-chloro plane. The observed Th–OH bond dis- Data collection was performed using monochromated Mo Kα
tances are very short indicating multiple bond order. While radiation using φ and ω scans to cover the Ewald sphere.24
complex 2 catalyses the ROP of ε-caprolactone via a cationic Accurate cell parameters were obtained with the amount of
“quasi living” mechanism, based on the Lewis acidity of the indicated reflections (Table 1).25 The structure was solved by
metal center, complex 3 shows no reactivity towards cyclic lac- SHELXS-97 direct methods26 and refined by the SHELXL-97
tones and aldehydes. When ligand 1 is reacted with UCl4 program package.27 The atoms were refined anisotropically.
complex 4 is obtained by a Lewis acid assisted nucleophilic Hydrogen atoms were included using the riding model. Soft-
attack of the same ligand 1 on the dimethylsilyl group of a ware used for molecular graphics: Mercury 3.1.28
metal coordinated ligand, as brown-red crystals in a moderate
Procedure for the synthesis of [(CH3)3CNC(Ph)NSiMe2NC(Ph)-
NHC(CH3)3]Li (1)
yield. Complex 4 shows only a low reactivity at elevated temp-
eratures towards several oxygen containing substrates, such as
ε-caprolactone, and no reactivity towards aromatic aldehydes A flame dried Schlenk flask, equipped with a magnetic stirring
and alcohols. When the reaction to form complex 4 is worked bar, was charged with [(CH3)3CNHC(Ph)NSiMe2NC(Ph)NHC-
out in wet THF, the reaction mixture turns instantaneously (CH3)3] (5) (2.0 g, 4.89 mmol), 50 mL of hexane were added
yellow, suggesting the oxidation to uranium(VI). However, under a constant stream of nitrogen and the colorless solution
when the reaction of UCl4 with the neutral amidine 5 is was cooled to −78 °C (acetone/dry ice bath). n-BuLi (3.40 mL,
carried out under the addition of a mild external base, such as 1.1 equiv., 1.6 M in hexane) was added dropwise via syringe to
pyridine, complex 6 is isolated, showing a coordination of the
[(CH3)3CNHC(Ph)NSiMe2NC(Ph)NHC(CH3)3]
solution
both amidine nitrogens and an additional pyridine moiety to under a constant stream of nitrogen. The yellow reaction
the UCl4 center. Complex 5 exhibits no activity in the ROP of mixture was warmed to room temperature and stirred for
ε-caprolactone.
10 hours. Subsequent removal of the solvent and recrystalliza-
tion from a concentrated hexane solution at −6 °C provided
the lithium amidinate 1 as a yellow microcrystalline powder
(1.90 g, 94%).
Experimental
1H-NMR (C6D6, 300.00 MHz) δ −0.27 (s, 6 H, Si(CH3)2), 1.21
All manipulations of air sensitive materials were performed (s, 9 H, C(CH3)3), 1.14 (s, 9 H, C(CH3)3), 2.45 (brs, 1 H, NH),
with the rigorous exclusion of oxygen and moisture in flamed 6.96–7.18 (m, 10 H, Har). 13C-NMR (C6D6, 75.00 MHz) δ 3.63
Schlenk-type glassware on a high vacuum line (10−5 torr), or in (Si(CH3)2), 33.54 (C(CH3)3), 33.76 (C(CH3)3), 51.31 (C(CH3)3),
nitrogen filled MBraun and Vacuum Atmospheres gloveboxes 51.54 (C(CH3)3), 132.01 (Car-H), 132.41 (Car-H), 143.54 (Car-C),
with a medium capacity recirculator (1–2 ppm oxygen). Argon 175.51 (NC(Ph)N). 29Si-NMR (C6D6, 60 MHz)
δ −16.60
and nitrogen were purified by passage through a MnO oxygen (Si(CH3)2). Elemental analysis calculated: C: 69.53, H: 8.51,
removal column and a Davison 4 Å molecular sieve column. N: 13.51. Found: 69.98, H: 8.59, N: 13.65.
Analytically pure solvents were dried and stored with Na/K
Procedure for the synthesis of [(CH3)3CNC(Ph)NSi(CH3)2-
NC(Ph)NHC(CH3)3]ThCl2 (2)
alloy and degassed by 3 freeze–pump–though cycles prior to
use (THF, hexane, toluene, benzene-d6, toluene-d8). Amidine
5,21 UCl4 and ThCl4(thf)3 were synthesized according to A flame dried Schlenk flask, equipped with a magnetic stirring
published literature procedures. ε-Caprolactone and pyridine bar and a frit was charged with ThCl4(thf)3 (200 mg,
(Sigma Aldrich) were distilled under reduced pressure from 0.338 mmol) inside the glovebox. A second Schlenk flask was
CaH2 and stored in the glovebox prior to use. NMR spectra charged with [(CH3)3CNC(Ph)NSiMe2NC(Ph)NHC(CH3)3]Li (1)
were recorded on DPX200, Avance 300 and Avance 500 Bruker (281 mg, 0.677 mmol) inside the glovebox. THF (ca. 40 mL)
22
23
spectrometers. Chemical shifts for H-NMR and 13C-NMR are was condensed into both flaks using vacuum transfer. The
1
reported in ppm and referenced using residual proton or reaction flask, containing ThCl4(thf)3 was cooled to −78 °C
carbon signals of the deuterated solvent relative to tetramethyl- (acetone/dry ice bath) and the THF solution of the amidinate
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ligand was added slowly via a syringe to the ThCl4(thf)3 solu- [(CH3)3CNC(Ph)NSiMe2NC(Ph)NHC(CH3)3]Li (1) (437 mg,
tion under a constant stream of argon. Then, the reaction 1.054 mmol) inside the glovebox and THF (ca. 40 mL) was con-
mixture was warmed slowly to room temperature and stirred densed into both flaks using vacuum transfer. The reaction
for 48 hours. The solvent was removed, the solid residue flask, containing UCl4 was cooled to −78 °C (acetone/dry ice
washed with hexane (3 × 15 mL) and the product 2 was isolated bath) and the THF solution of the amidinate ligand was added
as a slightly yellow powder (303 mg, 80%). Crystals suitable for slowly via a syringe to the UCl4 suspension under a constant
X-ray crystallography were obtained from a hexane layered stream of argon. The reaction mixture was warmed slowly to
toluene solution at −6 °C.
room temperature and stirred for 48 hours. The solvent was
1H-NMR (C6D6, 300.00 MHz) δ 0.02 (s, 6H, Si(CH3)2), 1.49 removed and the solid residue washed with hexane (3 × 15 mL)
(s, 9H, C(CH3)3 bound), 1.65 (s, 9H, C(CH3)3 side-arm), 2.12 (br, and the product (4) was isolated as a brown powder (338 mg,
1H, NH), 7.08–7.17 (m, 5H, Har), 7.51–7.53 (m, 5H, Har). 65%). Crystals suitable for X-ray crystallography were obtained
13C-NMR (C6D6, 75.00 MHz)
(C(CH3)3 bound), 34.6 (C(CH3)3 side-arm), 53.4 (C(CH3)3 bound),
54.6 (C(CH3)3 side-arm), 128.0–131.0 (Car-H), 135.1 (Car-C) 145.1 FWHM 2.93 Hz), −17.62 (s, 3 H, Si(CH3)3, FWHM 3.59 Hz),
(NCN), 171.6 (NCNH). 29Si-NMR (C6D6, 60 MHz) δ −22.89 −17.00 (s, Si(CH3)3, FWHM 3.08 Hz), 0.30 (s, 9H, C(CH3)3 bound
(Si(C(CH3)2)). Elemental analysis calculated: C: 41.53, H: 5.69, FWHM 2.59 Hz), 1.36 (s, 9 H, NC(CH3)3, FWHM 9.03 Hz),
δ
3.5 (Si(CH3)2), 34.0 from a concentrated toluene solution at −6 °C.
1H-NMR (C6D6, 300.00 MHz) δ −24.14 (s, 6 H, Si(CH3)3,
,
N: 6.46, Cl: 16.34; Found: C: 41.97, H: 5.71, N: 6.39, Cl: 16.41.
1.65 (s, 9 H, C(CH3)3 side-arm, FWHM 9.12 Hz), 5.57 (m, 5 H,
Har, FWHM 2.77 Hz), 6.49 (m, 5 H, Har, FWHM 3.78 Hz), 7.28
(m, 5 H, Har, FWHM 5.06 Hz), 14.73 (brs, 1 H, NH, FWHM 4.63
Hz). 13C-NMR (C6D6, 125.00 MHz) δ 0.9 (Si(CH3)), 29.9
Procedure for the synthesis of [(CH3)3CNC(Ph)NSiMe2NC(Ph)-
NHC(CH3)3Th(OH)2(Cl)]2·C7H8 (3)
A flame dried Schlenk flask, equipped with a magnetic stirring (C(CH3)3 bound), 31.2 (NC(CH3)3), 38.1 (C(CH3)3 side-arm) 52.9
bar and a frit was charged with ThCl4(thf)3 (200 mg, (C(CH3)3 bound), 53.3 (NC(CH3)3), 62.9 (C(CH3)3 side-arm),
0.338 mmol) inside the glovebox. A second Schlenk flask was 121.5–133.7 (Car-H), 141.5–144.4 (Car-C), 185.5 (NCN),
charged with [(CH3)3CNC(Ph)NSiMe2NC(Ph)NHC(CH3)3]Li (1) 184.2 (NHCN), 197.1 (NCNH). 29Si-NMR (C6D6, 60.00 MHz) δ
(281 mg, 0.677 mmol) inside the glovebox. THF (ca. 40 mL) −21.5 (Si(CH3)2). Elemental analysis calculated: C: 45.14, H:
was condensed into the flask with the ligand using vacuum 5.63, N: 8.54, Cl: 10.80. Found: C: 45.55, H: 5.71, N: 8.46, Cl:
transfer and THF (ca. 40 mL) was added to the flask containing 10.76.
ThCl4(thf)3. The reaction flask, containing ThCl4(thf)3 was
cooled to −78 °C (acetone/dry ice bath) and the THF solution
of the amidinate ligand was added slowly via a syringe to the
ThCl4(thf)3 solution under a constant stream of argon. The
Procedure for the synthesis of [C(CH3)3NHC(Ph)NSi(CH3)2-
NC(Ph)NHC(CH3)3]UCl4(C5H5N)·C6H6 (6)
reaction mixture was warmed slowly to room temperature, dis- A flame dried Schlenk flask, equipped with a magnetic stirring
tilled water (0.50 mL) was added via a syringe under a constant bar and a frit was charged with UCl4 (200 mg, 0.527 mmol)
stream of argon to the colourless solution, which instan- inside the glovebox. A second Schlenk flask was charged with
taneously turned bright red upon the addition of water and [(CH3)3CNC(Ph)NHSiMe2NC(Ph)NHC(CH3)3] (5) (437 mg,
the solution was stirred for 48 hours at room temperature. The 1.054 mmol) inside the glovebox and THF (ca. 40 mL) was con-
solvent was removed, the solid residue washed with hexane densed into both flaks via vacuum transfer. The reaction flask,
(3 × 15 mL) and the product (3) was isolated as dark red containing UCl4 was cooled to −78 °C (acetone/dry ice bath)
powder (345 mg, 67%). Crystals suitable for X-ray crystallogra- and the THF solution of the amidinate ligand was added
phy were obtained from toluene solution at −6 °C.
slowly via syringe to the UCl4 suspension under a constant
1H-NMR (THF-d8, 300.00 MHz) δ 1.25 (s, 18 H, C(CH3)side-arm), stream of argon. The reaction mixture was warmed slowly to
1.42 (br, 2 H, NH), 1.50 (s, 18 H, C(CH3)bound), 2.30 (s, 12 room temperature and pyridine (5 mL) was added via syringe
H. Si(CH3)2), 7.00–7.46 (m, 20 H, Har), 10.02 (br, 4 H, OH). under a constant stream of argon. The brown reaction mixture
13C-NMR (THF-d8, 125.00 MHz) δ = 3.6 (Si(CH3)2), 32.1 was stirred for 48 hours at room temperature. The solvent was
(C(CH3)3 side-arm), 34.5 (C(CH3)3 bound), 54.3 (C(CH3)3 side-arm), removed and the solid residue washed with hexane (3 15 mL)
55.9 (C(CH3)3 bound), 126.2–130.6 (Car-H), 139.2 (Car-C), and the product (5) was isolated as a brown powder (206 mg,
145.2 (Car-C), 169.0 (NCNH), 172.0 (NCN). 29Si-NMR (THF-d8) 45%). Crystals suitable for X-ray crystallography were obtained
δ −19.0 (Si(CH3)2). Elemental analysis calculated: C: 44.74, from a concentrated benzene solution at 6 °C.
H: 5.46, N: 7.59, Cl: 4.80. Found: C: 44.68, H: 5.55, N: 7.58,
Cl: 4.45.
1H-NMR (THF-d8, 200.00 MHz) δ 0.87 (s, 6 H, Si(CH3)2,
FWHM 20.97), 1.14 (s, 18 H, C(CH3)3, FWHM 5.69), 2.26 (br,
2 H, NH, FWHM 6.54), 6.93–7.40 (m, 10 H, Har, FWHM 2.13).
13C-NMR (THF-d8, 50.00 MHz) δ 3.0 (Si(CH3)2), 32.7 (C(CH3)3),
51.5 (C(CH3)3), 131.1–132.4 (Car-H), 143.5 (Car-C), 175.5
Procedure for the synthesis of [(CH3)3CNC(Ph)Si(CH3)2–
N(C(CH3)3)C(Ph)NSi(CH3)2NC(Ph)N(C(CH3)3)]UCl3·C7H8 (4)
A flame dried Schlenk flask, equipped with a magnetic stirring (NHCN). 29Si-NMR (THF-d8, 60 MHz) −18.7 (Si(CH3)2). Elemen-
bar and a frit was charged with UCl4 (200 mg, 0.527 mmol) tal analysis calculated: C: 44.45, H: 5.01, N: 7.41, Cl: 15.00.
inside the glovebox. A second Schlenk flask was charged with Found: C: 44.96, H: 5.01, N: 7.45, Cl: 15.04.
11384 | Dalton Trans., 2014, 43, 11376–11387
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General procedure for the catalytic polymerization of
ε-caprolactone mediated by complex 2
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A sealable J-Young glass tube, equipped with a magnetic stir-
ring bar, was loaded with 2.5 mg of complex 2 from a stock
solution, the required amount of ε-caprolactone and 5 mL of
dry toluene inside the glovebox. The polymerization was
carried out under strong stirring for the required amount of
time and temperature. Then, the reaction was quenched by the
addition of methanol. After removing the solvent under
reduced pressure, the polymer was precipitated from cold
methanol, isolated by filtration, washed with three portions of
cold methanol (3 × 20 mL) and dried overnight under vacuum.
The activity was determined as PCL (g) mol (cat)−1 time (h). A
sample of the obtained PCL (40 mg) was dissolved in THF and
used for determination of the molecular weight.
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For the kinetic 1H-NMR studies a J-Young NMR tube was
loaded with the respective amount of complex 2 from a stock
solution, ε-caprolactone and toluene-d8 were added inside the
glove box and the tube was sealed. The reaction mixture was
frozen at liquid nitrogen temperatures, until staring the
1H-NMR measurements. The sample was heated (if required)
inside the NMR spectrometer. Similar experiments were per-
formed for the thermodynamic studies.
Acknowledgements
This research was supported by the USA–Israel Binational
Science Foundation under Contract 2010109.
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