Synthesis, structure, and catalytic activity of benzyl thorium metallocenes

Article abstract of DOI:10.1016/j.inoche.2013.01.014

Treatment of the chloride metallocene [η⁵-1,3-(Me ₃C)₂C₅H₃]₂ThCl ₂ or [η⁵-1,2,4-(Me₃C)₃C ₅H₂]₂ThCl₂ w

Full text of DOI:10.1016/j.inoche.2013.01.014

Inorganic Chemistry Communications 30 (2013) 26–28
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Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Synthesis, structure, and catalytic activity of benzyl thorium metallocenes
Wenshan Ren a,b, Ning Zhao b, Liang Chen b, Guofu Zi b,
⁎
a
College of Chemistry and Chemical Engineering, Southwest University, Chongqing 400715, China
b
Department of Chemistry, Beijing Normal University, Beijing 100875, China
a r t i c l e i n f o
a b s t r a c t
Treatment of the chloride metallocene [η5-1,3-(Me3C)2C5H3]2ThCl2 or [η5-1,2,4-(Me3C)3C5H2]2ThCl2 with 2
equiv of PhCH2K in diethyl ether at room temperature gives, after recrystallization from a benzene solution,
the benzyl metallocenes [η5-1,3-(Me3C)2C5H3]2Th(CH2Ph)2 (1) and [η5-1,2,4-(Me3C)3C5H2]2Th(CH2Ph)2 (2),
respectively, in good yields. Complexes 1 and 2 have been characterized by various spectroscopic techniques,
elemental analyses, and X-ray diffraction analyses. They are active catalysts for the polymerization of rac-lactide,
leading to the atactic polylactides with high molecular weights and narrow molecular weight distributions.
© 2013 Elsevier B.V. All rights reserved.
Article history:
Received 26 December 2012
Accepted 16 January 2013
Available online 1 February 2013
Keywords:
Thorium
Benzyl complex
Polymerization of lactide
Over the past decades organoactinide complexes have received
widespread attention due to their fundamental importance, their
potential applications in organic synthesis and catalytic processes
[1–5]. For instance, the reactivity of the thorium-carbene, thorium-oxo,
thorium-sulfido or thorium-imido bonds of Th=CR2, Th=O, Th=
S and Th=NR complexes was reported recently [6–10]. Although
many organoactinide complexes have been studied extensively,
only a small number of alkyl organoactinide complexes, which
are potential catalysts for various chemical transformations such
as activation of small molecules and initiation of the polymeriza-
tion of cyclic esters, have been reported [11–18]. Even within this
class, few of them show significant reactivity [11–18]. Thus, the de-
velopment of new alkyl organoactinide complexes is still a desirable
goal. In this paper, we report on some observations concerning the
chemistry of benzyl thorium metallocenes 1 and 2 with the sterical-
ly encumbered cyclopentadienyl ligands, [1,3-(Me3C)2C5H3] and
[1,2,4-(Me3C)3C5H2], respectively, because of their good solubility
in various solvents [6–10,19–21].
Benzyl complexes are readily prepared by salt metathesis reaction.
For example, treatment of the chloride metallocene [η5-1,3-
(Me3C)2C5H3]2ThCl2 with 2 equiv of PhCH2K in diethyl ether at room tem-
perature gives, after recrystallization from a benzene solution, the benzyl
metallocene [η5-1,3-(Me3C)2C5H3]2Th(CH2Ph)2 (1) in 92% yield
(Scheme 1) [22]. Under similar reaction conditions, reaction of
[η5-1,2,4-(Me3C)3C5H2]2ThCl2 with 2 equiv of PhCH2K also affords the
desired complex [η5-1,2,4-(Me3C)3C5H2]2Th(CH2Ph)2 (2) in 85% yield
(Scheme 2) [22]. Complexes 1 and 2 are stable in dry nitrogen atmo-
sphere, while they are very sensitive to moisture. They are soluble and
stable in most common organic solvents such as THF, DME, pyridine,
toluene, and benzene, but slightly soluble in n-hexane. Complexes 1
and 2 have been characterized by various spectroscopic techniques, el-
emental analyses, and X-ray diffraction analyses [23]. The 1H NMR spec-
trum of 1 shows the CH2Ph resonance at 2.00 ppm, which is slightly
upfield relative to 2 (δ 2.28 ppm).
The single-crystal X-ray diffraction analyses show that in [η5-1,3-
(Me3C)2C5H3]2Th(CH2Ph)2 (1) or [η5-1,2,4-(Me3C)3C5H2]2Th(CH2Ph)2
(2) the Th4+ ions are η5-bound to two Cp-rings and σ-bound to two car-
bon atoms from two benzyl groups to form a distorted tetrahedron
(Figs. 1 and 2). The average Th-C(ring) distance is 2.814(17) Å and
2.874(3) Å for 1 and 2, respectively. In the solid state, 1 has crystallo-
graphic C2 symmetry with staggered cyclopentadienyl ligands. The
bulky Me3C-groups are nearly eclipsed, and the Cp-ligands point to-
ward the front of the metallocene wedge like those in [η5-1,3-
(Me3C)2C5H3]2ThMe2 [7]. The orientation of the cyclopentadienyl
rings in 2 is nearly also eclipsed, with the Me3C-groups on each
ring at the back of the wedge located as far from each other as possible.
This orientation sets the disposition of the other four Me3C-groups such
that two of them pointing towards the open wedge are nearly
eclipsed. The two benzyl groups are oriented on either side of the
eclipsed Me3C-groups such that the molecule has approximate C2
symmetry. Complexes 1 and 2 have similar (PhCH2)C-Th-C(CH2Ph)
angles, and have the average Th-C(CH2Ph) distances of 2.52(2) and
2.524(3) Å for 1 and 2, respectively, comparable to that (2.552(7) Å)
found in (η5-Me5C5)2Th(CH2Ph)2 [24].
From the polymerization data it is evident that 1 and 2 can initiate the
ring opening polymerization (ROP) of racemic-lactide (rac-LA) [25] under
mild conditions (Table 1) [26]. Complete conversion of 500 equiv of
lactide occurs within 5 h at 40 °C in CH2Cl2 at [rac-LA]=1.0 mol L−1
,
and the molecular weight distribution is very narrow (ca. 1.15) over the
entire monomer-to-initiator ratio range, indicating a single-site catalyst
system (Table 1, entries 1 and 2). However, polymerizations with these
thorium initiators/catalysts proceed much more slowly in THF (Table 1,
entries 5 and 6), presumably because of competitive monomer-solvent
⁎
Corresponding author. Tel.: +86 10 5880 6051; fax: +86 10 5880 2075.
E-mail address: gzi@bnu.edu.cn (G. Zi).
1387-7003/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.inoche.2013.01.014

W. Ren et al. / Inorganic Chemistry Communications 30 (2013) 26–28
27
Scheme 1. Synthesis of complex 1.
Scheme 2. Synthesis of complex 2.
coordination to the thorium atom [27]. This activity difference between
CH2Cl2 and THF solvent is more pronounced at 20 °C (Table 1, entries 3,
4, 7 and 8) than at 40 °C (Table 1, entries 1, 2, 5 and 6). The resulting
polylactides are atactic under the conditions examined, and the molecu-
lar weights range from 21.3 to 71.2 kg mol−1. The catalytic activities of 1
and 2 are slightly less active than that of thorium hydride complex
[η5-1,3-(Me3C)2C5H3]3ThH [27], presumably because of the electronic
effect between the thorium ion and benzyl groups, while the micro-
structure of the resulting polylactides is similar to those initiated by
[η5-1,3-(Me3C)2C5H3]3ThH [27]. Unlike thorium hydride complex
[η5-1,3-(Me3C)2C5H3]3ThH [27], there are two active benzyl groups
in the complexes 1 and 2, whether both the benzyl groups promoted
the reaction or not is not clear at this time, which makes the proposal
Fig. 2. Molecular structure of 2 (thermal ellipsoids drawn at the 35% probability level).
Selected bond lengths (Å) and bond angles (deg): C(Cp)-Th (av.) 2.74(3), Cp(cent)-Th
2.611(3), 2.606(3), Th-C(CH2Ph) 2.521(3), 2.527(3), Cp(cent)-Th-Cp(cent) 141.3(1),
(PhCH2)C-Th-C(CH2Ph) 97.9(1).
of a transition state for the complexes 1 and 2 in this transformation in-
feasible. Further investigation of the reaction is still underway.
In conclusion, two new benzyl metallocenes 1 and 2, have been
readily prepared via salt metathesis reaction between PhCH2K and
the chloride metallocene [η5-1,3-(Me3C)2C5H3]2ThCl2 or [η5-1,2,4-
(Me3C)3C5H2]2ThCl2, respectively. Both complexes are active catalysts
for the polymerization of rac-lactide, leading to atactic polylactides with
high molecular weights and narrow molecular weight distributions.
Acknowledgments
This work was supported by the National Natural Science Foundation
of China (Grant Nos. 21074013 and 21172022), the Program for New
Century Excellent Talents in University (NCET-10-0253), the Fundamen-
tal Research Funds for the Central Universities (XDJK2013C032) and
Doctoral Foundation of Southwest University.
Table 1
Polymerization of rac-lactide catalyzed by benzyl thorium complexes.a
b
c
c
Entry Precatalyst
T
Solvent Conv. Mcal
Mn
Mw/Mn
(°C)
(%)
(kg/mol) (kg/mol)
1
2
3
4
5
6
7
8
1
2
1
2
1
2
1
2
40
40
20
20
40
40
20
20
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
THF
THF
THF
THF
100
100
72
67
86
82
34
28
72.1
72.1
51.8
48.2
61.9
59.0
24.5
20.2
71.2
69.4
52.4
49.1
60.6
58.7
25.6
21.3
1.13
1.16
1.14
1.20
1.35
1.32
1.44
1.40
a
Conditions: precat./LA (mol/mol)=1/500; polymerization time, 5 h; solvent,
Fig. 1. Molecular structure of 1 (thermal ellipsoids drawn at the 35% probability
level). Selected bond lengths (Å) and bond angles (deg): C(Cp)-Th (av.) 2.814(17),
Cp(cent)-Th 2.542(9), 2.546(9), Th-C(CH2Ph) 2.50(2), 2.53(2), Cp(cent)-Th-Cp(cent)
146.6(9), (PhCH2)C-Th-C(CH2Ph) 97.5(9).
5 mL; [LA]=1.0 mol/L.
b
Mcal=([LA]/[Th])×144.13×X (X=conversion).
Measured by GPC (using polystyrene standards in THF).
c

28
W. Ren et al. / Inorganic Chemistry Communications 30 (2013) 26–28
[18] E. Barnea, D. Moradove, J.C. Berthet, M. Ephritikhine, M.S. Eisen, Surprising activ-
Appendix A. Supplementary material
ity of organoactinide complexes in the polymerization of cyclic mono- and dies-
ters, Organometallics 25 (2006) 320–322.
CCDC 914553 and 914554 contain the supplementary crystallograph-
ic data for 1 and 2. These data can be obtained free of charge via http://
www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge Crys-
tallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK;
fax: (+44) 1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk. Sup-
plementary data to this article can be found online at http://dx.doi.
org/10.1016/j.inoche.2013.01.014.
[19] G. Zi, L. Jia, E.L. Werkema, M.D. Walter, J.P. Gottfriedsen, R.A. Andersen, Prepara-
tion and reactions of base-free bis(1,2,4-tri-tert-butylcyclopentadienyl)uranium
oxide, Cp’2UO, Organometallics 24 (2005) 4251–4264.
[20] G. Zi, L.L. Blosch, L. Jia, R.A. Andersen, Preparation and reactions of base-free
bis(1,2,4-tri-tert-butylcyclopentadienyl)uranium methylimide, Cp′2U=NMe, and
related compounds, Organometallics 24 (2005) 4602–4612.
[21] W. Ren, W.W. Lukens, G. Zi, L. Maron, M.D. Walter, Is the bipyridyl thorium
metallocene a low-valent thorium complex? A combined experimental and com-
putational study, Chem. Sci. (2013), http://dx.doi.org/10.1039/c2sc22013j.
[22] Preparation of 1. Under nitrogen gas, after a diethyl ether (50 mL) slurry of
[η5-1,3-(Me3C)2C5H3]2ThCl2 (2.00 g, 3.0 mmol) and PhCH2K (0.78 g, 6.0 mmol)
was stirred at room temperature overnight, the mixture was filtered and the res-
idue was washed with diethyl ether (5 mL×3). The solvent was removed, and
the resulting white solid was recrystallized from a benzene solution to give 1 as
colorless crystals. Yield: 2.12 g (92%). Mp: 132-134 °C (dec.). 1H NMR (C6D6): δ
7.28 (t, J=7.6 Hz, 4H, Ph), 7.10 (d, J=7.5 Hz, 4H, Ph), 6.83 (t, J=7.2 Hz, 2H,
Ph), 6.37 (t, J=2.5 Hz, 2H, ring CH), 6.01 (d, J=2.5 Hz, 4H, ring CH), 2.00 (s,
4H, CH2), 1.20 (s, 36H, (CH3)3C). 13C NMR (C6D6): δ 151.3, 149.5, 128.5, 125.6,
120.8, 115.7, 110.8, 89.0, 33.3, 32.0. IR (KBr, cm-1): ν 2962 (s), 1598 (w), 1444
(s), 1384 (s), 1361 (s), 1260 (s), 1093 (s), 1018 (s), 799 (s). Anal. Calcd for
References
[1] E. Barnea, M.S. Eisen, Organoactinides in catalysis, Coord. Chem. Rev. 250 (2006)
855–899.
[2] A.R. Fox, S.C. Bart, K. Meyer, C.C. Cummins, Towards uranium catalysts, Nature
455 (2008) 341–349.
[3] T. Andrea, M.S. Eisen, Recent advances in organothorium and organouranium ca-
talysis, Chem. Soc. Rev. 37 (2008) 550–567.
[4] T.W. Hayton, Metal–ligand multiple bonding in uranium: structure and reactivity,
Dalton Trans. 39 (2010) 1145–1158.
[5] P.L. Arnold, Uranium-mediated activation of small molecules, Chem. Commun. 47
(2011) 9005–9010.
[6] W. Ren, X. Deng, G. Zi, D.C. Fang, The Th=C double bond: an experimental and com-
putational study of thorium poly-carbene complexes, Dalton Trans. 40 (2011)
9662–9664.
[7] W. Ren, G. Zi, D.C. Fang, M.D. Walter, A base-free thorium terminal imido metallocene:
synthesis, structure and reactivity, Chem. Eur. J. 17 (2011) 12669–12682.
[8] W. Ren, G. Zi, D.C. Fang, M.D. Walter, Thorium oxo and sulfido metallocenes: synthe-
sis, structure, reactivity and computational studies, J. Am. Chem. Soc. 133 (2011)
13183–13196.
C
40H56Th: C, 62.48; H, 7.34. Found: C, 62.55; H, 7.33%. Preparation of 2. This com-
pound was prepared as colorless crystals from the reaction of [η5-1,2,4-(Me3C)3-
C5H2]2ThCl2 (2.00 g, 2.6 mmol) with PhCH2K (0.68 g, 5.2 mmol) in diethyl ether
(50 mL) and recrystallization from a benzene solution by a similar procedure as
in the synthesis of 1. Yield: 1.95 g (85%). Mp: 170-172 °C. 1H NMR (C6D6): δ
7.29 (t, J=7.4 Hz, 4H, Ph), 7.18 (d, J=7.4 Hz, 4H, Ph), 6.85 (t, J=7.2 Hz, 2H,
Ph), 6.43 (s, 4H, ring CH), 2.28 (s, 4H, CH2), 1.37 (s, 36H, (CH3)3C), 1.35 (s, 18H,
(CH3)3C). 13C NMR (C6D6): δ 153.1, 145.7, 145.2, 129.1, 126.7, 121.3, 115.2,
91.4, 35.0, 34.0, 33.7, 32.7. IR (KBr, cm-1): ν 2960 (s), 2868 (m), 1595 (s), 1487
(s), 1459 (s), 1361 (s), 1260 (s), 1092 (s), 1026 (s), 799 (s). Anal. Calcd for
C
48H72Th: C, 65.43; H, 8.24. Found: C, 65.45; H, 8.18%.
[23] Crystal data for 1: C40H56Th, fw=768.89, monoclinic, Cc, a=10.362(1) Å,
b=19.563(2) Å, c=17.299(2) Å, β=96.52(1)° V=3483.8(7) Å3, Z=4,
R1=0.056 for 3982 (I>2σ(I)), wR2=0.148 (all data), GOF=1.08. Crystal
data for 2: C48H72Th, fw=881.10, monoclinic, P21/c, a=11.856(1) Å,
b=18.978(2) Å, c=20.349(1) Å, β=112.41(1)° V=4232.7(6) Å3, Z=4,
R1=0.028 for 9743 (I>2σ(I)), wR2=0.063 (all data), GOF=1.03.
[24] K.C. Jantunen, C.J. Burns, I. Castro-Rodriguez, R.E. Da Re, J.T. Golden, D.E. Morris,
B.L. Scott, F.L. Taw, J.L. Kiplinger, Thorium(IV) and uranium(IV) ketimide com-
plexes prepared by nitrile insertion into actinide-alkyl and -aryl bonds, Organo-
metallics 23 (2004) 4682–4692.
[25] General procedure for polymerization of rac-lactide. In a glovebox filled with nitro-
gen gas, a Schlenk flask was charged with a solution of thorium complex (typical-
ly 0.01 mmol) in CH2Cl2 (0.2 mL) or THF (0.2 mL). To this solution was added
rapidly a CH2Cl2 or THF solution (5.0 mL) of rac-lactide (5.0 mmol), and the reac-
tion mixture was vigorously stirred for 5 h at 20 or 40 °C. The polymerization was
quenched by the addition of acidified methanol. The resulting precipitated
polylactide was collected, washed with methanol several times, and dried in vacu-
um at 50 °C overnight. The molecular weight and the molecular weight distribution
[9] W. Ren, G. Zi, M.D. Walter, Synthesis, structure and reactivity of
a thorium
metallocene containing a 2,2′-bipyridyl ligand, Organometallics 31 (2012) 672–679.
[10] W. Ren, H. Song, G. Zi, M.D. Walter, A bipyridyl thorium metallocene: synthesis,
structure and reactivity, Dalton Trans. 41 (2012) 5965–5973.
[11] T. Straub, A. Haskel, T.G. Neyroud, M. Kapon, M. Botoshansky, M.S. Eisen,
Intermolecular hydroamination of terminal alkynes catalyzed by organoactinide
complexes. Scope and mechanistic studies, Organometallics 20 (2001) 5017–5035.
[12] E. Barnea, T. Andrea, M. Kapon, J.C. Berthet, M. Ephritikhine, M.S. Eisen, Catalytic
coupling of terminal alkynes with isonitriles promoted by organoactinide com-
plexes, J. Am. Chem. Soc. 126 (2004) 10860–10861.
[13] J.A. Pool, B.L. Scott, J.L. Kiplinger, A new mode of reactivity for pyridine N-oxide:
C-H activation with uranium(IV) and thorium(IV) bis(alkyl) complexes, J. Am.
Chem. Soc. 127 (2005) 1338–1339.
[14] E. Barnea, T. Andrea, J.C. Berthet, M. Ephritikhine, M.S. Eisen, Coupling of terminal
alkynes and isonitriles by organoactinide complexes: scope and mechanistic in-
sights, Organometallics 27 (2008) 3103–3112.
[15] C.J. Weiss, S.D. Wobser, T.J. Marks, Organoactinide-mediated hydrothiolation of
terminal alkynes with aliphatic, aromatic, and benzylic thiols, J. Am. Chem. Soc.
131 (2009) 2062–2063.
[16] C.J. Weiss, S.D. Wobser, T.J. Marks, Lanthanide- and actinide-mediated terminal
alkyne hydrothiolation for the catalytic synthesis of Markovnikov vinyl sulfides,
Organometallics 29 (2010) 6308–6320.
[17] M. Sharma, T. Andrea, N.J. Brookes, B.F. Yates, M.S. Eisen, Organoactinides pro-
mote the dimerization of aldehydes: scope, kinetics, thermodynamics, and calcu-
lation studies, J. Am. Chem. Soc. 133 (2011) 1341–1356.
of the resulting polymer were determined by GPC. The tacticity of the polylactide
1
was determined according to the methine region homonuclear decoupling
NMR spectrum [27].
H
[26] The polymerization of rac-lactide also occurs in other organic solvents such as
toluene, but requires a high temperature (over 70 °C).
[27] W. Ren, N. Zhao, L. Chen, H. Song, G. Zi, Synthesis, structure, and catalytic activity of
an organothorium hydride complex, Inorg. Chem. Commun. 14 (2011) 1838–1841.