Mixed Imidazolin-2-iminato-Cp? Thorium(IV) Complexes: Synthesis and Reactivity Toward Oxygen-Containing Substrates

Article abstract of DOI:10.1021/acs.organomet.5b00273

The mixed pentamethylcyclopentadienyl thorium(IV) imidazolin-2-iminato complexes Cp?₂Th(Im^DippN)(Me) (2) and Cp?₂Th(Im^MesN)(Me) (3) were synthesized in quantitative yields via rapid protonolysis of Cp?₂Th(Me)₂ (1) with the respective neutral imidazolin-2-iminato ligand Im^RNH. Cp?₂Th(Im^DippN)(Me) (2) and Cp?₂Th(Im^MesN)(Me) (3) display short Th-N bond lengths and large Th-N-C angles. The reactivity of complex 2 and 3 toward oxygen-containing substrates was studied, and the catalytic activity of 2 was compared to the dimethyl (bispentamethyl-cyclopentadienyl) thorium complex 1. Complex 2 was applied in the catalytic Tishchenko reaction with aromatic, heteroaromatic, and branched aliphatic aldehydes, displaying a higher catalytic activity than Cp?₂Th(Me)₂ and Cp?₂Th(Im^MesN)(Me). Furthermore, 2 was applied as a catalyst in the crossed Tishchenko reaction and in the oligomerization of bis(aldehydes), as well as in the ring-opening polymerization of ε-caprolactone. In all reactions, the activity of Cp?₂Th(Im^DippN)(Me) (2) and Cp?₂Th(Im^MesN)(Me) (3) was higher than that observed for Cp?₂Th(Me)₂ (1), which can be attributed to the increased electron density introduced by the coordination of the imidazolin-2-iminato ligand. (Chemical Equation Presented).

Full text of DOI:10.1021/acs.organomet.5b00273

Article
pubs.acs.org/Organometallics
Mixed Imidazolin-2-iminato−Cp* Thorium(IV) Complexes: Synthesis
and Reactivity Toward Oxygen-Containing Substrates
Isabell S. R. Karmel,† Natalia Fridman,† Matthias Tamm,*,‡ and Moris S. Eisen*,†
†Schulich Faculty of Chemistry, Institute of Catalysis Science and Technology, Technion − Israel Institute of Technology, Technion
City, Haifa, 32000 Israel
‡Institut fur Anorganische und Analytische Chemie, Technische Universitat Braunschweig, Hagenring 30, 38106 Braunschweig,
̈
̈
Germany
S
* Supporting Information
ABSTRACT: The mixed pentamethylcyclopentadienyl thorium-
(IV) imidazolin-2-iminato complexes Cp*2Th(ImDippN)(Me) (2)
and Cp*2Th(ImMesN)(Me) (3) were synthesized in quantitative
yields via rapid protonolysis of Cp*2Th(Me)2 (1) with the
respective neutral imidazolin-2-iminato ligand ImRNH. Cp*2Th-
(ImDippN)(Me) (2) and Cp*2Th(ImMesN)(Me) (3) display short
Th−N bond lengths and large Th−N−C angles. The reactivity of
complex 2 and 3 toward oxygen-containing substrates was studied,
and the catalytic activity of 2 was compared to the dimethyl
(bispentamethyl−cyclopentadienyl) thorium complex 1. Complex
2 was applied in the catalytic Tishchenko reaction with aromatic,
heteroaromatic, and branched aliphatic aldehydes, displaying a
higher catalytic activity than Cp*2Th(Me)2 and Cp*2Th(ImMesN)-
(Me). Furthermore, 2 was applied as a catalyst in the crossed Tishchenko reaction and in the oligomerization of bis(aldehydes),
as well as in the ring-opening polymerization of ε-caprolactone. In all reactions, the activity of Cp*2Th(ImDippN)(Me) (2) and
Cp*2Th(ImMesN)(Me) (3) was higher than that observed for Cp*2Th(Me)2 (1), which can be attributed to the increased
electron density introduced by the coordination of the imidazolin-2-iminato ligand.
bridged cyclopentadienyl ligands, which leads to an opening of
the coordination sphere in the respective actinide complexes.12
While some of the aforementioned catalytic transformations
display similar product distributions, chemo- and regioselectiv-
ities as early transition metals and lanthanide complexes, some
of these reactions exhibit unique reactivity patterns. An example
for this distinguished reactivity is displayed in the catalytic
hydrothiolation reaction of terminal acetylenes, which proceeds
with a high selectivity toward the formation of the Markovnikov
addition products with aliphatic, aromatic, and benzylic thiols.7
This reaction states a clear contrast to the anti-Markovnikov
selectivities displayed by transition-metal catalysts. Similary, the
distinctive reactivity of organoactinides is observed in the
catalytic dehydrocoupling of silanes with a variety of amines,
yielding unique cyclic oligomers.13 However, examples of
catalytic transformations involving oxygen-containing substrates
are very limited and remain a challenge in the field, which can
be attributed to the high oxophilicity of the early actinide
elements.10,11,14 Recently, we reported an extraordinarily high
catalytic activity in the ROP of ε-caprolactone with a
uranium(IV) bis(imidazolin-2-iminato) catalyst.10e The
strongly basic, highly nucleophilic imidazolin-2-iminato ligands
INTRODUCTION
■
The application of organometallic actinide complexes in
homogeneous catalysis can be traced back to the early 1980s,
and the number of studies in this field has shown a steady
growth ever since.1 This can be mainly attributed to the ability
of actinide complexes to display unusual reactivities toward
organic substrates. Therefore, the products obtained are, in
many cases, complementary to the product distribution
achieved with late-transition-metal, lanthanide, and main
group complexes.2 The distinguished reactivity of the early
actinides, uranium and thorium, is often ascribed to their large
ionic radii, allowing high coordination numbers, and to the
p r e s e n c e o f
f
o r b i t a l s . 3 T h e u s e o f b i s -
(pentamethylcyclopentadienyl) actinide complexes of the type
Cp*2AnX2 (An = Th, U; X = alkyl, benzyl, Cl) is considered
pioneering in the field of homogeneous organoactinide
catalysis, and the reactivity of these metallocenes was
investigated in a broad array of organic transformations, such
as the polymerization of α-olefins,4,1a hydrosilylation,5 hydro-
amination,6 hydrothiolation, and hydroalkoxylation of alkynes,7
isonitrile−alkyne coupling,8 coupling of terminal alkynes,9 as
well as in the ring-opening polymerization (ROP) of cyclic
esters10 and the Tishchenko reaction with aromatic alde-
hydes.11 An improvement of the catalytic activity was achieved
by employing a constrained-geometry catalyst (CGC) with
Received: March 31, 2015
Published: June 8, 2015
© 2015 American Chemical Society
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(ImRNH),15 which can be considered as 2σ, 4π electron donors
selected bond lengths and angles are presented in Tables 1 and
2. Considering previous attempts to synthesize the respective
(Scheme 1) in transition-metal16 and lanthanide complexes,17
Scheme 1. Resonance Structures of the Imidazolin-2-iminato
Ligand
Table 1. Selected Bond Lengths and Angles for Complex 2
a
a
bond lengths (Å)
bond angles (deg)
Th−N3−C22
Th−N3
2.235(9)
2.508(12)
2.149
171.7(9)
129.5
Th−C21
Th−Ccent1
Th−Ccent2
Th−Cav1
Th−Cav2
N3−C22
Ccent1−Th−Ccent2
Ccent1−Th−C21
Ccent2−Th−C21
Ccent1−Th−N3
Ccent2−Th−C21
N3−Th−C21
98.84
2.109
97.77
2.861
112.55
110.80
98.70(4)
2.816
1.278(14)
are expected to slightly increase the electron density of the
respective actinide complex. The increased electron density will
likely decrease the oxophilicity of the actinide center, facilitating
the insertion of oxygen-containing molecules. This, in turn, will
render the formation of a thermodynamically stable actinide-
oxo species less favorable, leading to an enhanced catalytic
activity with oxygen-containing molecules. The selectivity for a
specific substrate is a major challenge in the field of catalysis
and can be achieved by tuning the coordination sphere of the
metal complex, e.g., by a modification of the ligand system.
In this study, we address the fundamental question of
synthesizing an actinide catalyst with an enhanced catalytic
activity toward oxygen-containing molecules, yet displaying a
structural similarity to Cp*2AnX2 complexes. In view of the
basicity of the imidazolin-2-iminato system, we attempted the
synthesis of a mixed pentamethylcyclopentadienyl imidazolin-2-
iminato actinide complex Cp*2An(ImRN)(X), bearing one
labile ligand (X).10a,11
The Tishchenko reaction, which is known as the
disproportionation of two aldehyde monomers, is a waste-free
atom economic route to obtain esters, a widely applied class of
synthons in organic synthesis.18 Recently, a modification of the
classical Tishchenko reaction, known as the Aldol-Tishchenko
reaction, was rendered catalytically enantioselective using
lanthanide triflates in combination with BINOL-type ligands
by the group of Shibasaki et al.19 However, the lack of
selectivity and the limited scope of aldehydes tolerated by the
metal catalysts still remain major impediments for the use of
the Tishchenko reaction in organic synthesis.20 Previously, we
reported the activity of organoactinide catalysts toward
aromatic aldehydes,11 as well as the selective dimerization
between an aromatic and a cyclic or branched aliphatic
aldehyde to yield selectively the asymmetrically substituted
ester.21 In the present study, we investigated the inter- and
intra-molecular Tishchenko reaction with aromatic, hetero-
aromatic, cyclic, and aliphatic aldehydes. Moreover, we present
the homocoupling of bis(aldehydes). Furthermore, we compare
the activity of the mixed complex Cp*2Th(ImDippN)(Me) (2)
and Cp*2Th(ImMesN)(Me) (3) to Cp*2Th(Me)2 (1) in the
Tishchenko reaction and the catalytic ROP of ε-caprolactone.
a
Centroid 1 was defined as the center of C1-C5-C4-C3-C2. Centroid
2 was defined as the center of C14-C15-C11-C12-C13.
Table 2. Selected Bond Lengths and Angles for Complex 3
a
a
bond lengths (Å)
bond angles (deg)
Th−N1
2.219(4)
2.518(6)
2.143
Th−N1−C22
173.7(4)
130.56
Th−C21
Th−Ccent1
Th−Ccent2
Th−Cav1
Th−Cav2
N1−C22
Ccent1−Th−Ccent2
Ccent1−Th−C21
Ccent2−Th−C21
Ccent1−Th−N3
Ccent2−Th−C21
N1−Th−C21
100.06
2.147
103.72
2.844
108.45
2.834
112.34
1.275(6)
99.65(11)
a
Centroid 1 was defined as C2-C3-C4-C5-C1. Centroid 2 was defined
as C13-C12-C11-C15-C14.
Cp*2U(ImRN)(Cl) complexes from Cp*2UCl2 and the N-
silylated imidazolin (ImRNSiMe3), which resulted in a ring-
opening of the imidazolin ring and an oxidation to uranium-
(V),22 we attempted the metathesis elimination of methane
instead of trimethylsilyl chloride, which afforded complexes 2
and 3.
Complex 2 (Figure 1) crystallizes as colorless plates in the
crystallographic monoclinic P21/c space group. The Th−
N3imine bond distance (2.235(9) Å) is shorter than the Th−
Namido bonds (2.308(7)−2.32(2) Å) in Cp*2Th(NHR)(X)
complexes23 and the Th−N bond distances (2.272(5)−
2.268(5) Å) in the ketimido complexes of the type Cp*2Th-
(NCRAr)2.24 The thorium−carbon distances, i.e., Th−Ccentroid
= 2.149 and 2.109 Å, and the Th−C21methyl = 2.508(12) Å, are
slightly shorter than those observed for Cp*2Th(CH3)2.24a The
Ccentroid−Th−Ccentroid angle of 129.5° falls in the range observed
for other Cp*2Th(X)2 systems.24a
RESULTS AND DISCUSSION
■
Synthesis and Molecular Structure of Cp*2Th(ImRN)-
(CH3). The thorium complexes 2 and 3 were synthesized by a
slow addition of a toluene solution of the imidazolin-2-imine to
a toluene solution of Cp*2Th(CH3)2 (1) at room temperature,
which resulted in the immediate evolution of gas (CH4) (eq 1).
Subsequent stirring for 12 h at room temperature and removal
of the solvent afforded complexes 2 and 3 as pale yellow
powders in quantitative yields. Single crystals for X-ray analysis
were obtained from a concentrated toluene solution at −35 °C;
Complex 3 (Figure 2) crystallizes as colorless prisms in the
triclinic P-1 space group with one molecule of toluene per unit
̅
cell. The Th−N1imine and Th−C21methyl bond distances display
values of 2.219(4) and 2.518(6) Å, respectively, which are very
similar to the distances found in complex 2; in addition, the
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is further sustained by a comparison to the bond distances
displayed in Th(IV) ketimido complexes (Th−Nketimido
=
2.272(5)−2.268(5) Å), which are notorious for a higher Th−N
bond order.24c However, further theoretical studies (i.e., DFT
calculations) are required in order to determine the nature of
the Th−N bond in the imidazolin-2-iminato thorium
complexes 2 and 3.
Catalytic Tishchenko Reaction. The Tishchenko reaction
is known as a dimerization reaction of two aldehydes, which
proceeds via a hydride shift from one of the aldehydes to the
other aldehyde unit and can, therefore, be considered as a
redox-reaction (disproportionation), yielding the respective
ester.26,18 Since the first report of a transition-metal-catalyzed
Tishchenko reaction in 1978,27 the catalytic activity of many
main group,28 late-transition-metal,29 and lanthanide30 com-
plexes toward aldehydes has been investigated. Despite the
progress made in this area, many of the investigated transition-
metal and lanthanide catalysts do not show a tolerance toward
functional groups, or protons in the α-position, which reduces
the scope of accessible esters.26−30 We have previously reported
the reactivity of Cp*2Th(CH3)2 (1) toward aromatic aldehydes
and revealed the tolerance of 1 toward several functional
groups, such as nitro (NO2), chloro (Cl), methoxy (OCH3),
and nitrile (CN) substituents, as well as toward the heterocycle
furfural.11b In the present study, we expand the scope of
accessible products toward heterocyclic, cyclic, and aliphatic
aldehydes. Moreover, the synthesis of mixed cyclopentadienyl
imidazolin-2-iminato thorium complexes allows the evaluation
of the catalytic activity of a single methyl group in complex 2 as
compared to the dimethyl complex 1. Furthermore, Shvo et al.
reported that, by increasing the electron density of a transition-
metal complex, it increases the activity of the respective
complex toward aldehydes in the catalytic Tishchenko
reaction.31Therefore, it was important to compare the activity
of complexes Cp*2Th(ImDipp)(Me) (2) and Cp*2Th(ImMes)-
(Me) (3), bearing one imidazolin-2-iminato moiety, which
increases the electron density at the metal center to the activity
of complex 1.
Figure 1. ORTEP drawing of complex 2 with thermal displacement
parameters at 50% probability. Hydrogen atoms are omitted for clarity.
Figure 2. ORTEP drawing of complex 3 with thermal displacement
parameters at 50% probability. Hydrogen atoms are omitted for clarity.
Th−Ccentroid distances and the Ccentroid−Th−Ccentroid angle are
comparable to those in complex 2 with values of 2.143 Å, 2.147
Å, and 130.56°, respectively. Interestingly, the thorium
Symmetric Intermolecular and Intramolecular Tish-
chenko Reaction. The symmetric Tishchenko reaction to give
the symmetrically substituted esters is presented in Scheme 2.
1
complexes 2 and 3 display similar H NMR spectra, exhibiting
a high field shift for the Th-CH3 protons of −0.30 and −0.31
ppm for complexes 2 and 3, respectively. Also the signals
corresponding to the C5(CH3)5 protons appear in both cases as
a singlet at 1.78 and 2.13 ppm for complexes 2 and 3,
respectively. This trend is also observed for the 13C NMR
spectra of compounds 2 and 3, displaying a chemical shift of
11.59 and 11.47 for the C5(CH3)5 carbons and 56.08 and 60.04
ppm for the Th-CH3 carbons for complexes 2 and 3,
respectively. The similarity of the bond distances, bond angles,
and NMR spectra of the organometallic thorium species 2 and
3 indicates that these compounds display a structural similarity
in the solid state as well as in solution.
Scheme 2. Symmetric Tishchenko Reaction
Similar to transition-metal and lanthanide complexes,25 the
thorium(IV) imidazolin-2-iminato complexes 2 and 3 display
short Th−N bonds and large, almost linear Th−N−C angles.
The Th−Nimine bond distances in the imidazolin-2-iminato
complexes 2 and 3 are shorter than the average Th−Namido
bond single bond, displaying an average value of 2.308 Å, but
longer in comparison to Th−Nimido systems (ThN =
2.045(8) Å).6a The Th−N−C bond angles in complexes 2
and 3 display large, almost linear values, comparable to that in
thorium imido compounds (ThN−C = 171.5(7)°),6a which
suggests a higher bond order than 1 for the Th−N bond. This
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The reaction was carried out with aromatic, heteroaromatic,
cyclic, and aliphatic aldehydes (Table 3) by the thorium
imidazolin-2-iminato moiety (vide supra). A substantial
question in this context concerns the number of moles of
precatalyst that are active in the catalytic transformation, as well
as the number of active sites per catalyst. In order to determine
the percentage of active catalytic active species in the reaction
mixture, as well as the number of methyl groups which are
catalytically active, poisoning experiments with iso-propanol
(iPrOH) were performed. When the reaction was carried out
with a catalyst 2 to iso-propanol ratio of 1 to 0.25, a decrease in
the catalytic activity of 25% was observed, while a catalyst to iso-
propanol ratio of 1 to 0.50 reduced the catalytic activity to 50%
of the originally observed value. Similarly, when poisoning
experiments were carried out with catalyst 1 (two methyl active
groups), and a catalyst to iso-propanol ratio of 1 to 0.25, the
catalytic activity decreased by 12.5%, while a catalyst to iso-
propanol ratio of 1 to 0.50 led to a decrease of 25% of the
initially observed value without iso-propanol. These results
corroborate that both methyl groups in complex 1 are active in
the catalytic process. Furthermore, the poisoning experiments
suggest that all moles of complexes 1 and 2 are catalytically
Table 3. Homocoupling of Aldehydes Mediated by
Complexes 1 and 2
b
NMR conversion (%)
(24 h)
turnover frequency
d
per methyl group (h−1
)
a
entry
RCHO
Ph
complex 1 complex 2 complex 1 complex 2
1
2
3
4
5
6
7
45
95
60
97
1.41
2.97
2.16
1.22
2.13
1.34
0.67
3.75
6.06
4.81
2.81
5.19
2.69
1.38
4-NO2Ph
1-naphthyl
2-naphthyl
2-pyridyl
69
77
39
45
68
83
2-furyl
43
43
2-thiophen
cyclohexyl
cyclopentyl
iso-propyl
22
22
c
8
100
100
100
100
100
100
25
50
c
9
25
25
50
50
c
10
a
b
c
Reaction conditions: catalyst/RCHO 1:150, rt, 0.5 mL of C6D6.
NMR conversion determined from 1H NMR spectroscopy after 24 h.
1
d
active. H NMR experiments confirmed that the imidazolin-2-
NMR conversion determined after 3 h. Activity determined as
mol(ester)·mol(catalyst)−1·time(h)−1·(nr. of methyl groups in cata-
lyst)−1.
iminato ligand in complex 2 stays attached to the metal center
throughout the whole catalytic cycle, while the aldehyde inserts
into the Th−CH3 bond. Therefore, we defined the “activity per
methyl”, which normalizes the activity of the catalyst per labile
ligand (Table 3). The activity, normalized per methyl group,
displays considerably higher values for complex 2, reinforcing
our initial hypothesis that a higher electron density increases
the activity of the electrophilic thorium complexes toward
oxygen-containing substrates (vide supra). Interestingly, when
cyclic and branched aliphatic aldehydes were used as substrates,
the reaction proceeded to completion within less than 3 h,
complexes 1 and 2 and 3 as precatalysts. The activity of
complex 2 (Table 3) was in all cases slightly higher (10−15%)
as compared to the activity displayed by the thorium complex
3; therefore, Cp*2Th(ImDipp)(Me) (2) was applied for all
further catalytic studies. The activity displayed by complex 2 is
slightly higher than that for complex 1, which can be attributed
to the increased electron density of 2 in comparison to that of
1, resulting from the additional electron donation of the
Scheme 3. Proposed Mechanism for the Tishchenko Reaction Mediated by Complex 2.11
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Scheme 4. Crossed Tishchenko Reaction Mediated by Complex 2
a
Table 4. Crossed Tishchenko Reaction
entry
RCHO
R′CHO
RCHO/R′CHO
RCH2OCOR
R′CH2OCOR′
R′CH2OCOR
RCH2OCOR′
1
2
3
4
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
2-naphthyl
2-naphthyl
2-pyridyl
50/50
25/75
50/50
25/75
25/75
50/50
50/50
75/25
25/75
50/50
25/75
75/25
50/50
25/75
75/25
50/50
25/75
75/25
50/50
22
9
28
38
26
3
25
20
7
25
17
3
2-pyridyl
25
25
b
5
2-pyridyl
48
13
44
13
71
35
55
14
26
41
7
6
7
2-furyl
8
5
3
12
10
10
18
9
cyclohexyl
cyclohexyl
cyclohexyl
iso-propyl
iso-propyl
iso-propyl
cyclopentyl
cyclopentyl
cyclopentyl
iso-propyl
iso-propyl
iso-propyl
cyclohexyl
c
8
30
3
5
c
9
c
10
9
21
32
10
24
8
c
11
9
c
12
28
24
8
14
26
8
c
13
cyclohexyl
cyclohexyl
cyclohexyl
cyclohexyl
cyclohexyl
cyclohexyl
phthal(CHO)2
c
14
c
15
c
43
25
10
56
46
9
9
16
26
53
8
23
8
24
7
c
17
c
18
8
8
d
19
37
a
b
Reaction conditions: catalyst/RCHO 1:100, rt, 0.5 mL of C6D6. NMR conversion was determined after 24 h. NMR conversion was determined
c
d
after 96 h. NMR conversion was determined after 12 h. Yield was determined after 8 h.
whereas the use of aromatic and heteroaromatic aldehydes
required longer reaction times. Furthermore, the electronic and
steric nature of the R substituent (Scheme 2) defines the
reaction rate, leading to higher conversion when using electron-
rich aldehydes. When the reaction is carried out with ortho-
bis(aldehydes), the intramolecular Tishchenko reaction pro-
ceeds rapidly, to yield exclusively the respective lactone as the
single product (Scheme 2). Mechanistic studies with
stoichiometric amounts of precatalyst 2 and aldehydes support
the coordination insertion mechanism presented in Scheme 3.
In the first step of the catalytic cycle, the incoming aldehyde
inserts into the Th−C bond to give the thorium alkoxo
intermediate A. A subsequent insertion of a second aldehyde
monomer into the Th−O bond gives intermediate B, which,
upon its hydride transfer with an additional aldehyde via the
six-membered transition state C, will yield the catalytically
active thorium alkoxo complex D and 1 equiv of the α-
methylated ester. The incoming aldehyde inserts into the Th−
O bond of D, forming intermediate E, which reacts with an
additional equivalent of aldehyde, via a hydride transfer in a six-
membered transition state (F) to give the respective ester
under regeneration of the catalytic active thorium alkoxo
species D.
4). However, selectivity toward the asymmetrically substituted
ester still remains a major challenge.20a We investigated the
reactivity of the thorium(IV) complex 2 in the crossed
Tishchenko reaction (Table 4).
When the crossed Tishchenko reaction is carried out with an
equimolar ratio of two aromatic aldehydes (Table 4, entry 1),
the mixture of esters obtained displays a nearly equal
distribution of all four products (16−19), indicating a
competition of the aldehydes in the reaction mixture. If an
excess of R′CHO is used, the amount of symmetrically
substituted ester R′CH2OCOR′ (17a) increases, while the
amount of the ester RCH2OCOR (16a) decreases; however,
the selectivity toward the asymmetrically substituted esters
R′CH2OCOR (18a) and RCH2OCOR′ (19a) remains almost
the same, indicating an unselective reaction. When R′CHO is a
heteroatom-containing aldehyde, such as 2-pyridylcarboxalde-
hyde (vide supra), the major product obtained is the
symmetrically substituted ester R′CH2OCOR′ (17b), if the
ratio of RCHO and R′CHO is equimolar. If the reaction is
carried out with an excess of 2-pyridylcarboxaldehyde, the
asymmetrically substituted ester R′CH2OCOR (18b) is
obtained as the major product (entry 4). After complete
consumption of benzaldehyde (entry 5), the symmetrically
substituted ester R′CH2OCOR′ (17b) is obtained (Chart 1).
The selectivity observed toward ester 18b can be attributed to
additional coordination of the pyridine nitrogen atom to the
Crossed Tishchenko Reaction. In the crossed Tishchenko
reaction, two different esters are reacted in the presence of a
catalyst to yield either a mono- or a bifunctional ester (Scheme
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Chart 1. Crossed Tishchenko Reaction with 2-
Pyridylaldehyde and Benzaldehyde (75/25 Ratio)
Scheme 5. Tishchenko Reaction with Bis(aldehydes)
thorium center, facilitating its insertion into the Th−C bond.
The second aldehyde unit can now be either PhCHO or 2-
PyCHO to yield either the asymmetrically or the symmetrically
substituted ester, respectively. Because of the higher electron
density of PhCHO, it will act as hydride donor,32 and the
asymmetrically substituted ester is obtained almost exclusively
(entry 4). When all PhCHO is consumed, 2-pyridylcarbox-
aldehyde acts as hydride donor to give the symmetrically
substituted ester 17b. The reaction with other electron-rich
aldehydes, such as cyclohexanecarboxaldehyde and isobutyr-
aldehyde (entries 7−15), did not afford the same selectivity as
observed for 2-pyridylcarboxaldehyde, which can be attributed
to the lack of additional coordination through the pyridyl
nitrogen atom, leading to a mixture of products (16−19).
Similarly, the reaction between two cyclic aldehydes (entries
13−15), or cyclic and branched aliphatic aldehydes (entries
16−18) yields a mixture of the products 16−19, depending on
the ratio of the respective aldehydes. When the reaction is
carried out with phthalaldehyde (entry 19), the major product
corresponds to the intramolecular lactonization, and the
bifunctional bis(ester) is not observed, which can be attributed
to the higher rate of the intramolecular lactonization as
compared to the intermolecular dimerization of aldehydes.
Reactivity toward Bis(aldehydes). The reactivity of 2
toward phthalaldehyde, and 2,3-naphthylbisaldehyde afforded
the lactones 14 and 15 within 10 min in quantitative yields
(Scheme 2). Therefore, we decided to also investigate the
reactivity of complex 2 toward terephthalaldehyde and iso-
phthalaldehyde. Previous studies with cyclopentadienyl-based
lanthanide complexes have shown to oligomerize and polymer-
ize bis(aldehydes) to yield the respective polyesters.30a When
the thorium(IV) complex 2 is reacted with terephthalaldehyde
at room temperature (Scheme 5), the starting material is
consumed within 24 h, yielding a mixture of oligomers (n + m =
2−4). However, if the reaction is carried out at elevated
temperatures, the respective pentamer (20) is obtained in 80%
yield after 24 h as a colorless precipitate, while dimers and
trimers remain in solution. Similarly, the reaction of iso-
phthalaldehyde with 2 leads to the formation of a mixture of
oligomers at room temperature, whereas elevated temperatures
lead to the formation of higher oligomers (Scheme 5). The
oligomers were isolated by filtration and characterized by NMR
spectroscopy and MS-MS analysis. However, regardless of the
reaction conditions, no polymerization or cyclooligomerization
of the respective bis(aldehyde) was observed.
early 1980s can be ascribed to its mechanical strength, low
melting point (59−64 °C), high crystallinity, good solubility in
many organic solvents, outstanding mechanical compatibility
and miscibility with a large range of polymers and thoroughly
studied degradation kinetics.33 The well investigated properties
of the polyester lead to a various applications, e.g., in the field of
tissue engineering,34 long-term drug delivery systems,35 pack-
aging materials,36 microelectronics,37 and adhesives.38 The ring-
opening polymerization (ROP) of the cyclic ester, ε-
caprolactone, has been investigated with numerous and diverse
initiators, such as main group metals, transition metals,
lanthanides, and several actinide complexes.39 For example,
we have recently reported the catalytic activity of a uranium-
(IV) bis(imidazolin-2-iminato) complex toward ε-caprolactone,
which exhibited an extraordinarily high activity of 7 × 103
kg(PCL)·mol(catalyst)−1·h−1.10e In this study, we investigated
the catalytic activity of the Cp*2Th(ImDippN)(Me) (2) and
Cp*2Th(ImMesN)(Me) (3) in the ROP of ε-caprolactone, in
which complex 2 displayed a slightly higher catalytic activity
(vide supra) and was, therefore, applied for all further catalytic
studies. In order to compare the activity of complex 2 to the
catalytic activity and mechanism we reported for complex 1,10e
we carried out kinetic and thermodynamic studies with
complex 2 (vide infra). The polymerization results are displayed
in Table 5.
The ROP of ε-caprolactone, mediated by complex 2, displays
high values for the initial rate of the reaction, which decrease as
a function of time until full conversion of the monomer is
achieved after 5 h. This is further reinforced by the high values
calculated for the rate of monomer insertion Ri (eq 1), which
decrease as the reaction progresses. In addition, the molecular
weight of the polymer obtained increases while the rate of the
reaction decreases. The increase in the molecular weight of the
polymer in combination with a decrease in the reaction rate can
be attributed to the increased viscosity of the solution, leading
to a mass transport limited reaction (Trommsdorff−Norrish
effect).
Monomer insertion rate
m(PCL) [g]
Mw(PCL) [g mol−1]·time [h]
(Ri) =
(1)
Monomer termination rate
m(PCL) [g]
Mn(PCL) [g mol−1]·time [h]
(Rt) =
Ring-Opening Polymerization of ε-Caprolactone. The
growing attention that polycaprolactone has received since the
(2)
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Article
a
Table 5. Polymerization Results for the ROP of ε-Caprolactone Mediated by 2
entry
2:εCL
time (min)
activity (g(PCL)·mol−1·h−1
)
Mw (Dalton)
Mn (Dalton)
PD
Ri (mol·h−1
)
Rt (mol·h−1)
1
2
3
4
5
1:500
1:500
1:500
1:500
1:500
1:500
1:500
1:1000
30
60
77 160
43 060
21 730
15 460
11 050
6370
150 000
173 200
174 300
180 200
200 100
147 600
236 700
93 000
115 000
120 000
130 000
143 000
92 500
1.50
1.51
1.45
1.38
1.39
1.59
1.20
1.52
2.79 × 10−6
1.35 × 10−6
6.77 × 10−7
4.66 × 10−7
2.99 × 10−7
2.34 × 10−7
1.09 × 10−7
4.13 × 10−7
4.49 × 10−6
2.03 × 10−6
9.83 × 10−7
6.46 × 10−7
4.19 × 10−7
3.74 × 10−7
1.31 × 10−7
6.29 × 10−7
120
180
300
300
720
300
b
6
7
8
4760
197 000
143 000
16 570
218 000
a
b
Polymerization conditions: 5 mL of toluene, 5.43 μmol of catalyst, 70 °C. Room temperature. Ri = rate of insertion. Rt = rate of termination.
Scheme 6. Coordination−Insertion Mechanism for the ROP of ε-Caprolactone Mediated by 2
In order to shed light on the mechanism of the ROP of ε-
caprolactone mediated by complex 2, we carried out H NMR
hydrolysis of the growing polymer chain E with an additional
monomer.
1
experiments with low amounts of ε-caprolactone, leading to the
observation of 1 equiv of free methane, while the imidazolin-2-
iminato ligand stayed coordinated to the metal center
throughout the whole polymerization process, suggesting a
coordination−insertion mechanism for the polymerization.
CONCLUSIONS
■
In this study, we reported the syntheses of the mixed
pentamethylcyclopentadienyl thorium(IV) imidazolin-2-imina-
to complexes Cp*2Th(ImDippN)(Me) (2) and Cp*2Th-
(ImMesN)(Me) (3) via protonolysis reactions of Cp*2Th(Me)2
(1) with the respective neutral imidazolin-2-imine ligand
precursors. The thorium(IV) complexes 2 and 3 were
characterized by single-crystal X-ray diffraction, revealing
short Th−N bonds and large Th−N−C angles, which are
comparable to the respective bis(pentamethylcyclopentadienyl)
thorium(IV) ketimido complexes. As a result of the highly
nucleophilic nature of the imidazolin-2-iminato moiety, the
electron density of the highly electrophilic thorium complexes
is increased, rendering more reactive toward oxygen-containing
substrates. Despite the steric crowding around the metal center
in complexes 2 and 3, the methyl group can be activated by
aldehydes and the cyclic ester ε-caprolactone. The reactivity of
the methyl group can be attributed to the increased electron
density of 2, as compared to other thorium complexes with a
mixed ligand system.40 Because of the slightly increased
catalytic activity of complex 2 as compared to that of complex
3, complex 2 was used for all further catalytic studies. The
reactivity of complex 2 in the catalytic Tishchenko reaction was
investigated, displaying a much higher catalytic activity than
Cp*2Th(Me)2 (1), when normalized per methyl group.
Poisoning experiments confirmed that all moles of the complex
1
Furthermore, we performed kinetic H NMR experiments, in
order to obtain the kinetic dependence of the reaction on ε-
caprolactone and complex 2, displaying a first-order depend-
ence on monomer and precatalyst (eq 3).
∂p
∂t
= kobs·[complex 2]·[ε‐caprolactone]
(3)
The thermodynamic parameters were determined from the
Eyring (ΔH⧧ = 18.6(5) kcal·mol−1, ΔS⧧ = −15.5(5) cal·mol−1·
K−1) and Arrhenius (Supporting Information) plots (Ea =
1
19.3(5) kcal·mol−1). The H NMR experiments performed
with a slight excess of ε-caprolactone, as well as the
thermodynamic parameters determined, corroborate a coordi-
nation−insertion mechanism (Scheme 6). In the first step of
the catalytic cycle, a protonolysis takes place, yielding 1 equiv of
methane. The intermediate A undergoes a rapid isomerization
to give the Th-alkoxocaprolate complex B, which reacts with an
incoming monomer unit in a four-membered transition state
(C), leading to the open polymer chain D. The growing
polymer chain E is obtained after further insertion of substrate
into the Th−O bond of D. The product F is obtained after
2939
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Article
Synthesis of Cp*2Th(ImDippN)(Me) (2). A solution of Cp*2Th-
(Me)2 (1) (500 mg, 0.94 mmol) in toluene (5.0 mL) was treated
dropwise with a toluene (10 mL) solution of ImDippNH (380 mg, 0.94
mmol) inside the glovebox. The pale yellow solution was stirred for 12
h at room temperature. Subsequent removal of the solvent and
recrystallization from a concentrated toluene solution afforded 2 as
are catalytically active. Furthermore, we have shown that the
Tishchenko reaction proceeds with a variety of aromatic and
heteroaromatic aldehydes, as well as with cyclic and branched
aliphatic aldehydes, expanding the scope of substrates for
actinide catalysis. Surprisingly, the intermolecular reaction with
cyclic and aliphatic aldehydes displayed full conversion in less
than 3 h. When the Tishchenko reaction catalyzed by complex
2 was carried out with two different aldehydes, only the
reaction between 2-pyridylcarboxaldehyde and benzaldehyde
displayed the desired selectivity toward the asymmetrically
substituted ester, which can be attributed to the additional
coordination of the pyridyl nitrogen to the thorium center.
Moreover, we carried out catalytic studies with bis(aldehydes),
which led to the rapid and quantitative formation of lactones
for ortho-bis(aldehydes) or of oligomeric esters for meta- and
para-bis(aldehydes). The reactivity of complex 2 in the ROP of
ε-caprolactone was studied, displaying a higher activity than we
have previously reported for the dimethyl complex 1.10a Kinetic
and mechanistic studies corroborate a coordination−insertion
mechanism with a first-order dependence on complex 2 and ε-
caprolactone.
1
colorless crystals. Yield: 98% (848 mg, 0.92 mmol). H NMR (C7D8,
300.00 MHz): δ −0.31 (s, 3 H, CH3), 1.03 (d, 12 H, J = 6.64 Hz,
CH(CH3)2), 1.44 (d, 12 H, J = 6.69 Hz, CH(CH3)2), 1.78, (s, 30 H,
C5(CH3)5), 3.28−3.31 (m, 4 H, CH(CH3)2), 5.91 (s, 2 H, CH), 7.10−
7.15 (m, 6 H, Har). 13C NMR (C7D8, 75.50 MHz): δ 11.59
(C5(CH3)5), 23.01 (CH(CH3)2), 26.1 (CH(CH3)2, 28.75 (CH-
(CH3)2), 56.08 (CH3), 114.53 (CH), 121.28−129.14 (Car-H),
136.5−137.43 (C5(CH3)5), 142.15−145.32 (Car-C), 147.31 (Cipso

N). Anal. Calcd for C48H69N3Th: C, 62.66; H, 7.56; N, 4.57. Found:
C, 63.01; H, 7.53; N, 4.61.
Synthesis of Cp*2Th(ImMesN)(Me) (3). A solution of Cp*2Th-
(Me)2 (1) (500 mg, 0.94 mmol) in toluene (5.0 mL) was treated
dropwise with a toluene (10 mL) solution of ImMesNH (300 mg, 0.94
mmol) inside the glovebox. The dark yellow solution was stirred for 12
h at room temperature. Subsequent removal of the solvent and
recrystallization from a concentrated toluene solution afforded 3 as
1
yellow crystals. Yield: 96% (754 mg, 0.90 mmol). H NMR (C7D8,
300.00 MHz): δ −0.03 (s, 3 H, CH3), 2.13 (s, 30 H, C5(CH3)5), 2.46
(s, 6 H. para-CH3), 2.52 (s, 12, ortho-CH3), 5.90 (s, 2 H, CH), 7.11 (s,
4 H, Har). 13C NMR (C7D8, 75.50 MHz): δ 11.47 (C5(CH3)5), 19.22
(C5(CH3)5), 23.15 (C6H2(CH3)3), 60.04 (CH3), 112.36 (CH),
121.14−128.06 (Car-H), 136.98−137.66 (C5(CH3)5), 143.67−145.23
(Car-C), 148.23 (CipsoN). Anal. Calcd for C42H57N3Th: C, 60.34; H,
6.87; N, 5.03. Found: C, 60.73; H, 6.91; N, 5.06.
General Procedure for the Catalytic Tishchenko Reaction
with Monoaldehydes. A sealable J. Young NMR tube was loaded
with 5.43 μmol of complex 1 or 2 from a stock solution in C6D6 inside
the glovebox. The respective aldehyde (0.815 mmol, 150 equiv) was
added, and the reaction was immediately diluted to 550 μL with C6D6.
Solid aldehydes were dissolved in 300 μL of C6D6 before being added
to the solution of catalyst. The progress of the reaction was monitored
by 1H NMR spectroscopy. After 24 h, the tube was opened to air and
the reaction was quenched with methanol. The products were
identified by 1H NMR spectroscopy and MS analysis; the values
were compared to the previous literature, if possible.11b,47
General Procedure for the Crossed Tishchenko Reaction
with Monoaldehydes. A sealable J. Young NMR tube was loaded
with 5.43 μmol of complex 2 from a stock solution in C6D6 inside the
glovebox. The respective aldehydes (0.543 mmol, 100 equiv) were
added, and the reaction mixture was immediately diluted to 550 μL
with C6D6. Solid aldehydes were dissolved in 300 μL of C6D6 before
being added to the solution of catalyst. The progress of the reaction
was monitored by 1H NMR spectroscopy for 24 h. The ratio between
the products was identified by integration of the methylene signals of
the respective esters of the crude reaction mixture. Then, the tube was
opened to air, and the reaction mixture was quenched with methanol
and submitted to MS analysis.
General Procedure for the Oligomerization of Bis-
(aldehydes). A flame-dried round-bottom flask, equipped with a
magnetic stirring bar, was loaded with the respective bis-aldehyde
(0.815 mmol, 150 equiv), which was dissolved in 20 mL of dry toluene
inside the glovebox. A toluene solution of complex 2 (5.00 mg, 5.43
μmol; from a toluene stock solution) was added to the aldehyde
solution, and the reaction flask was sealed and heated to 90 °C for 24
h. Then, the reaction mixture was quenched with cold methanol and
filtered, and the precipitate was washed with toluene (3 × 10 mL) and
dried under vacuum.
EXPERIMENTAL SECTION
■
All manipulations of air-sensitive materials were performed with the
rigorous exclusion of oxygen and moisture in flamed Schlenk-type
glassware on a high-vacuum line (10−5 Torr), or in nitrogen-filled
MBraun and Vacuum Atmospheres gloveboxes with a medium
capacity recirculator (1−2 ppm oxygen). Argon and nitrogen were
purified by passage through a MnO oxygen removal column and a
Davison 4 Å molecular sieve column. Analytically pure solvents were
dried and stored with Na/K alloy and degassed by three freeze−
pump−thaw cycles prior to use (hexane, toluene, benzene-d6, toluene-
d8). ImDippNH, ImMesNH,15 and Cp*2Th(Me)2 were synthesized
41
according to published literature procedures. ε-Caprolactone (Sigma-
Aldrich) was distilled under reduced pressure from CaH2 and stored in
the glovebox prior to use. Benzaldehyde, 2-pyridinylcarboxaldehyde,
furfural, 2-thiophencarboxaldehyde, cyclohexylcarboxaldehyde, cyclo-
pentylcarbaldehyde, isobutyraldehyde, and 1-naphthaldehyde (Sigma-
Aldrich) were distilled over sodium bicarbonate and stored in a
glovebox prior to use. 2-Naphthaldehyde, 2,3-naphthylbisaldehyde,
phthaladehyde, isophthalaldehyde, and terephthalaldehyde (Sigma-
Aldrich) were dried for 12 h on a high-vacuum line (10−5 Torr) and
stored in a glovebox prior to use. NMR spectra were recorded on
DPX200, Avance 300, and Avance 400 Bruker spectrometers.
Chemical shifts for 1H NMR and 13C NMR are reported in ppm
and referenced using residual proton or carbon signals of the
deuterated solvent relative to tetramethylsilane. Elemental analysis
was carried out by the microanalysis laboratory at the Hebrew
University of Jerusalem. GPC measurements were carried out on a
Waters Breeze system with a styrogel RT column and with THF
(HPLC grade, T.G. Baker) as mobile phase at 30 °C. Relative
calibration was done with polystyrene standards (Aldrich, 2000−
1 800 000 range). Mn values were multiplied by a factor of 0.58 and
correlated to actual PCL values. MS experiments were performed at
200 °C (source temperature) in Maxis Impact (Bruker) mass
spectrometer with an APCI solid probe method.
The single-crystal material was immersed in Paratone-N oil and was
quickly fished with a glass rod and mounted on a Kappa CCD
difractometer under a cold stream of nitrogen. Data collection was
performed using monochromated Mo Kα radiation using φ and ω
scans to cover the Ewald sphere.42 Accurate cell parameters were
obtained with the amount of indicated reflections.43 The structure was
solved by SHELXS-97 direct methods44 and refined by the SHELXL-
97 program package.45 The atoms were refined anisotropically.
Hydrogen atoms were included using the riding model. Software
used for molecular graphics: Mercury 3.1.46
General Procedure for the ROP of ε-Caprolactone Mediated
by Complex 2. A sealable J. Young glass tube, equipped with a
magnetic stirring 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
2940
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Organometallics 2015, 34, 2933−2942

Organometallics
Article
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)·time
(h). A sample of the obtained PCL (40 mg) was dissolved in THF and
used for determination of the molecular weight.
(7) (a) Weiss, C. J.; Wobser, S. D.; Marks, T. J. Organometallics 2010,
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Eisen, M. S. Organometallics 2006, 25, 320−322. (b) Rabinovich, E.;
Aharonovich, S.; Botoshansky, M.; Eisen, M. S. Dalton Trans. 2010, 39,
6667−6676. (c) Walshe, A.; Fang; Maron, L.; Baker, R. J. Inorg, Chem.
2013, 53, 9077−9086. (d) Hayes, C. E.; Sarazin, Y.; Katz, M. J.;
Carpentier, J.-F.; Leznoff, D. B. Organometallics 2013, 32, 1183−1192.
(e) Karmel, I. S. R.; Botoshansky, M.; Tamm, M.; Eisen, M. S. Inorg.
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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 glovebox, and the
tube was sealed. The reaction mixture was frozen at liquid nitrogen
1
temperatures, until starting the H NMR measurements. The sample
was heated (if required) inside the NMR spectrometer. Similar
experiments were performed for the thermodynamic studies.
ASSOCIATED CONTENT
* Supporting Information
■
S
CIF files and crystallographic information for complexes 2 and
3. The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.5b00273.
(11) (a) Andrea, T.; Barnea, E.; Eisen, M. S. J. Am. Chem. Soc. 2008,
130, 2454−2455. (b) Sharma, M.; Andrea, T.; Brookes, N. J.; Yates, B.
F.; Eisen, M. S. J. Am. Chem. Soc. 2011, 133, 1341−1359.
(12) (a) Schnabel, R. C.; Scott, B. L.; Smith, W. H.; Burns, C. J. J.
Organomet. Chem. 1999, 591, 14−23. (b) Stubbert, B. D.; Stern, C. L.;
Marks, T. J. Organometallics 2003, 22, 4836−4838.
AUTHOR INFORMATION
Corresponding Authors
*E-mail: chmoris@tx.technion.ac.il. Phone: +972-4-8292680
(M.S.E.).
*E-mail: m.tamm@tu-bs.de (M.T.).
Author Contributions
The manuscript was written through contributions of all
authors. All authors have given approval to the final version of
the manuscript.
■
(13) Wang, J. X.; Dash, A. K.; Berthet, J. C.; Ephritikhine, M.; Eisen,
M. S. J. Organomet. Chem. 2000, 610, 49−57.
(14) (a) Fang, J.; Walshe, A.; Maron, L.; Baker, R. J. Inorg. Chem.
2012, 51, 9132−40. (b) Baker, R. J.; Walshe, A. Chem. Commun. 2012,
48, 985−987.
Notes
(15) Tamm, M.; Petrovic, D.; Randoll, S.; Beer, S.; Bannenberg, T.;
Jones, P. G.; Grunenberg. J. Org. Biomol. Chem. 2007, 5, 523−530.
(16) (a) Tamm, M.; Randoll, S.; Bannenberg, T.; Herdtweck, E.
Chem. Commun. 2004, 876−877. (b) Wu, X.; Tamm, M. Coord. Chem.
Rev. 2008, 37, 550−567. (c) Haberlag, B.; Wu, X.; Brandhorst, K.;
Grunenberg, J.; Daniliuc, C. G.; Jones, P. G.; Tamm, M. Chem.Eur.
J. 2010, 16, 8868−8877. (d) Sharma, M.; Yameen, H. S.; Tumanskii,
B.; Filimon, S.-A.; Tamm, M. J. Am. Chem. Soc. 2012, 134, 17234−
17244. (e) Shoken, D.; Sharma, M.; Botoshansky, M.; Tamm, M.;
Eisen, M. S. J. Am. Chem. Soc. 2013, 135, 12592−12595. (f) Lysenko,
S.; Daniliuc, C. G.; Jones, P. G.; Tamm, M. J. Organomet. Chem. 2013,
744, 7−14.
The authors declare no competing financial interests
ACKNOWLEDGMENTS
This work was supported by the German Israel Foundation
GIF under Contract I-1264-302.5/2014.
■
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