Catalytic Addition of Alcohols to Carbodiimides Mediated by Benzimidazolin-2-iminato Actinide Complexes

Article abstract of DOI:10.1021/acs.organomet.7b00432

The synthesis of methyl and methoxy substituted benzimidazolin-2-iminato actinide (IV) complexes (1-4), [(Bim^2-MeOPh/MeN)AnN″₃] and [(Bim_5-Me^Dipp/MeN)AnN″₃] (An = U, Th; N″ = N(SiMe₃)₂

Full text of DOI:10.1021/acs.organomet.7b00432

Article
pubs.acs.org/Organometallics
Catalytic Addition of Alcohols to Carbodiimides Mediated by
Benzimidazolin-2-iminato Actinide Complexes
Heng Liu,† Natalia Fridman,† Matthias Tamm,*,‡ and Moris S. Eisen*,†
†Schulich Faculty of Chemistry, Technion − Israel Institute of Technology, Haifa City 32000, Israel
‡Institut fur Anorganische und Analytische Chemie, Technische Universitat Braunschweig, Hagenring 30, 38106 Braunschweig,
̈
̈
Germany
S
* Supporting Information
ABSTRACT: The synthesis of methyl and methoxy sub-
stituted benzimidazolin-2-iminato actinide (IV) complexes
(1−4), [(Bim2‑MeOPh/MeN)AnN″3] and [(Bim5‑MeDipp/MeN)-
AnN″3] (An = U, Th; N″ = N(SiMe3)2), was performed by
the protonolysis of the actinide metallacycles with the
respective neutral benzimidazolin-2-imine ligand precursors.
Full characterization, including X-ray diffraction studies for all
the complexes, is reported. Despite the high oxophilicity of the
actinide metal centers, these complexes displayed extremely
high activities in the catalytic addition of aliphatic and aromatic
alcohols to carbodiimides, under very mild conditions,
providing a facile and highly efficient strategy for the
construction of carbon−oxygen bonds. Various kinds of diols and triols can also be used in this intermolecular insertion,
representing a large substrate scope for the application of these organoactinide precatalysts.
Scheme 1. (i) Catalytic Addition of Alcohols to
Carbodiimides and (ii) Mesomeric Structures of
Benzimidazolin-2-iminato Ligands and the Corresponding
Complexes
INTRODUCTION
■
Metal-catalyzed addition of O−H functionalities across
carbon−carbon unsaturated bonds is a promising method for
the construction of carbon−oxygen linkages. Some representa-
tive examples are the transition metal and lanthanide catalyzed
hydroalkoxylation across alkenes and alkynes;1−16 however, for
these transformations, the catalytic reactivities and selectivities
are highly dependent on the structural and electronic nature of
the substrates, and sometimes the formation of byproducts is
inevitable. Therefore, exploring novel selective and atom-
economical catalytic systems, which can be also performed
under relatively mild reaction conditions and generate
negligible amounts of side products, is highly desirable. From
the synthetic point of view, besides hydroalkoxylation of
carbon−carbon unsaturated bonds, hydroalkoxylation of
carbon−heteroatom multiple bonds, as present for instance in
carbodiimides, can be treated as an alternative approach for the
formation of carbon−oxygen bonds (Scheme 1(i)). Moreover,
the produced isoureas are very important synthons in organic
transformations and have been widely used as selective
alkylating reagents.17 Intriguingly, despite the intensive research
on the addition of different nucleophiles E−H (E = N, P, C)
to carbodiimides,18−23 a very limited number of systems are
available to catalyze the addition of alcohols to carbodiimides,24
which raised our curiosity in exploring such a catalytic process
using organoactinide complexes.
Special Issue: Organometallic Actinide and Lanthanide Chemistry
Received: June 11, 2017
In the past three decades, we have witnessed remarkable
advances in the field of organoactinide catalysis, impelled
initially by the distinctive characteristics of the actinide metal
© XXXX American Chemical Society
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centers, such as sizable ionic radii and unusual coordination
numbers bringing unique geometries, and more recently by
their capabilities in constructing carbon−carbon or carbon−
heteroatom bonds. Hydroelementations,25−32 coupling reac-
tions,33−37 olefin/diene polymerizations,38−40 small-molecule
activations,41−48 etc. embrace some of these advances.
However, most of these reactions excluded oxygenated
substrates, due to the high oxophilicity of the actinide metals,
which will usually form strong metal−oxygen bonds leading to
partial deactivation of the active species.14,49−53 Therefore,
developing new generations of catalytic systems, which are
capable of transforming oxygenated substrates, has been a long-
standing subject. Some exciting breakthroughs have been made
in very recent years. Being supported by highly nucleophilic,
strongly basic donor ligands, such as cyclopentadienyl,
amidinate, and imidazolin-2-iminato ligands, organoactinide
complexes can be successfully employed in aldehyde dimeriza-
tion (Tishchenko reaction),54−57 hydroalkoxylation/cyclization
of alkynyl alcohols,58 and cyclic ester/epoxide ring-opening
polymerizations.59−64 These impressive results innovated the
understanding of actinide catalysis and revealed that actinide−
oxygen (An−O) bonds can be catalytically active to some
extent, despite of their high bond energies (Th−O = 208.0, U−
O = 181.0 kcal mol−1).65 Inspired by these findings, uranium-
and thorium-based complexes were applied toward the
intermolecular addition of alcohols to carbodiimides. Explor-
atory investigation employing sterically accessible actinide−
amido complexes shows that the addition products, isoureas,
can be obtained in high yields under very mild conditions;66
however, the exact structure of the active species was
inconclusive due to the formation of a mixture of actinide
alkoxides, which usually had poor solubilities in hydrocarbon
solvents.67,68 A great improvement has been achieved by using
benzimidazolin-2-iminato-supported actinide complexes, which
not only displayed high activities in the hydroalkoxylation
because of the sterically accessible nature of the actinide center
but also allowed us to elucidate the structure of active species
due to the enhanced solubility provided by the benzimidazolin-
2-iminato complexes.69 Nevertheless, when we exposed these
benzimidaozlin-2-iminato actinide complexes to an excess of
alcohol at high temperature (50 °C), cleavage of the actinide−
nitrogen (An−N) bonds was observed, despite of the high
bond order resulting from the ability of the imidazolin-2-
iminato ligand to act as a strong 2σ,4π-electron donor (Scheme
1(ii)).70 In order to enhance the thermal stability of the
benzimidazolin-2-iminato complexes, various electron-donating
groups were introduced to the backbone, based on the
consideration that these groups might further increase the
electron-donating ability of the ligands and therefore the
stability of the resulting An−N bonds. A detailed study of the
catalytic performance of the corresponding complexes is also
presented herein.
Scheme 2. Synthetic Procedures for Ligands a and b and
Complexes 1−4
amount of the actinide metallacycles, [(Me3Si)2N]2An[κ2-
(N,C)-CH2Si(CH3)2N(SiMe3)] (An = Th, U), at room
temperature for 24 h and recrystallization from a concentrated
toluene solution afforded the benzimidazolin-2-iminato actinide
1
complexes (1−4), in high yields. When compared with H
NMR spectra of the free ligands, complete disappearance of
characteristic NH broad singlets at δ = 4.51 ppm (a) and 4.33
ppm (b) can be easily observed, indicating the formation of the
corresponding actinide complexes.
Complexes 1−4 showed high instability and decomposed
immediately at room temperature when exposed to air and
moisture. To determine the crystal structures of these
complexes, the single crystalline materials were submerged in
cold (−35 °C) perfluoropolyalkylether oil in a vial, and
subsequently submerged in liquid nitrogen, and the single
crystals were fished from the vial at −78 °C and rapidly
mounted on the diffractometer, receiving a cold jet of liquid
nitrogen. The solid structures of the benzimidazolin-2-iminato
uranium(IV) and thorium(IV) complexes, [(Bim2‑MeOPh/MeN)-
UN″3] (1) and [(Bim2‑MeOPh/MeN)ThN″3] (2), are shown in
Figure 1; both complexes display a distorted tetrahedral
geometry around the metal center, which is built by four
nitrogen atoms. Despite of the highly oxophilic nature of the
actinide centers, neither intermolecular nor intramolecular
interactions between the methoxy group and the U/Th atom
was observed. Like in other imidazolin-2-iminato actinide
complexes, the An−NCN bond distances (2.105(5), 2.171(6)
Å, for complexes 1 and 2, respectively) are on average 0.2 Å
shorter than the An−Namido bonds in the same complexes,
indicative of the higher bond order of the benzimidazolin-2-
iminato ligand to the metal center. The An−N(1)−C(1) angles
are close to linearity, displaying values of 165.7(4)° and
164.0(4)° for complexes 1 and 2, respectively. It is worthy of
RESULTS AND DISCUSSION
■
Synthesis and Structure Characterization. Benzimida-
zolin-2-imines with electron-donating groups (OMe (a), Me
(b)) were synthesized according to Scheme 2. Coupling
reaction between nitro-substituted aryl halides and substituted
anilines afforded a1 and b1, which were then reduced, followed
by ring closure to give rise to the ligand precursor a2 and b2 in
high yields. Methylation of a2 and b2 using methyl triflate and
subsequent deprotonation with KH furnished the target ligands,
a and b. The reaction of these neutral ligands with an equimolar
B
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Figure 1. ORTEP drawings of complexes 1 (left) and 2 (right) with
thermal displacement parameters at 50% probability. Hydrogen atoms
are omitted for clarity. Bond lengths (Å): An−N1 2.105(5), 2.171(6);
An−N4 2.317(4), 2.383(6); An−N5 2.286(4), 2.344(10); An−N6
2.312(5), 2.359(6); C1−N1 1.297(7), 1.275(8). Bond angles (deg):
An−N1−C1 165.7(4), 164.0(4); N1−An−N4 118.70(17), 115.9(2);
N1−U−N5 92.30(16), 92.95(18); N1−An−N6 100.17(17), 101.1(2);
N4−An−N5 116.87(16), 101.1(2); N4−An−N6 108.81(16),
108.7(2); N5−An−N6 118.34(17), 118.6(2). All the values are first
for complex 1 and then for complex 2.
Figure 2. ORTEP drawings of complexes 3 (left) and 4 (right) with
thermal displacement parameters at 50% probability. Hydrogen atoms
are omitted for clarity. Bond lengths (Å): An−N1 2.131(3), 2.206(3);
An−N4 2.313(3), 2.389(3); An−N5 2.290(3), 2.360(5); An−N6
2.293(3), 2.355(4); C1−N1 1.279(5), 1.279(4). Bond angles (deg):
An−N1−C1 163.2(3), 161.5(2); N1−An−N4 117.74(13),
115.32(10); N1−U−N5 95.11(13), 95.87(12); N1−An−N6
104.61(13), 105.52(13); N4−An−N5 116.50(12), 116.43(11); N4−
An−N6 107.58(12), 107.90(11); N5−An−N6 114.61(13),
115.08(15). All the values are first for complex 3 and then for
complex 4.
note that these values are on average slightly smaller than those
in the corresponding imidazolin-2-iminato actinide complexes
(169.5(5)° and 170.7(7)°) decorated with symmetrical
ligands.57 This difference can be ascribed to the asymmetric
nature of the benzimidazolin-2-iminato ligand, with the bulkier
diisopropylphenyl group repelling the adjacent N(SiMe3)2
substituents and subsequently pushing the An−N(1) bond
toward the sterically more open side of the smaller methyl
group. To describe the proximity of the ligand to the metal
center, cone angles were determined.71 The resulting cone
angles for complexes 1 and 2 are 136.5° and 132.7°,
respectively, which are much smaller than the cone angles in
imidazolin-2-iminato actinide complexes (204°, 206°),57
indicating a more open space around the metal center herein.
For all the complexes, no agostic interaction between the
−N(SiMe3)2 or the isopropyl groups and the actinide metal was
observed.
Single crystal structures of the methyl substituted benzimi-
dazolin-2-iminato uranium(IV) and thorium(IV) complexes,
[(Bim5‑MeDipp/MeN)UN″3] (3) and [(Bim5‑MeDipp/MeN)UN″3]
(4), are shown in Figure 2, and similarly to complexes 1 and 2,
distorted tetrahedral geometries around the metal center were
observed. Moreover, in both complexes, shorter An−NCN
bond as well as approximate linearity of the An−N(1)−C(1)
bond angle are displayed, with values of 2.131(3) Å, 163.2(3)°
for complex 3 and 2.206(3) Å, 161.5(2)° for complex 4,
respectively. Due to the bigger size of Dipp than 2-MeOPh (in
complexes 1 and 2), larger An−N−C bend angles are found in
complexes 3 and 4.
a
Table 1. Catalytic Addition of Various Alcohols to DTC
entry
cat.
R′−OH
MeOH
time (h)
yield (%)
product
1
1
2
3
4
1
2
1
2
1
2
1
2
1
2
1
2
1
2
1
2
1
2
1
2
1
2
1
2
1
2
1
1
>99
>99
>99
>99
>99
>99
96
5a
2
3
1
4
1
5
EtOH
1
5b
5c
5d
5e
5f
6
1
7
iPrOH
1
8
1
97
9
tBuOH
12
12
1
70
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
86
BnOH
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
84
1
Py-2-CH2OH
Thio-2-CH2OH
Furan-2-CH2OH
CyOH
1
1
1
5g
5h
5i
1
1
1
4
4
Ph2CHOH
1-Adamantanol
PhOH
12
12
6
5j
86
>99
>99
>99
>99
92
5k
5l
6
1
Catalytic Insertion of Alcohols into Carbodiimides.
The results for catalytic addition of various kinds of alcohols to
1,3-di-p-tolylcarbodiimide (DTC) are presented in Table 1.
The reaction of methanol and DTC was first investigated by
using the different catalysts (1−4) at room temperature, and all
the complexes exhibited high activities during the catalytic
process, affording the isourea products almost quantitatively
within 1 h, indicating the high efficiencies of the four complexes
(Scheme 3). It is important to note that no detectable products
were observed in the corresponding blank reactions without the
actinide complexes.
1
2,6-Me2PhOH
2,4-tBuPhOH
1
5m
5n
1
95
12
12
77
83
a
Reaction conditions: 2 mol % precatalyst, 50 equiv of substrates (0.35
mmol), 500 μL of C6D6, room temperature. The yields were
1
determined by H NMR of the crude reaction mixture.
C
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Scheme 3. Catalytic Addition of Alcohols to DTC
Complexes 1 and 2 were then chosen as precatalysts to study
the scope of this intermolecular coupling. Table 1 shows that a
wide range of primary, secondary, and tertiary alcohols can be
used for this reaction. Increasing the size of the alcohol from
Figure 3. Reaction progress in the coupling of 2,4-di-tert-butylphenol
and DTC catalyzed by complex 2.
i
MeOH to EtOH or to PrOH showed a modest influence on
the yield of the insertion products, and all of them were almost
quantitatively converted to the corresponding isoureas in the
same period of time at room temperature. However, further
increasing the size of the alcohol to tBuOH resulted in a drastic
slowdown of the reaction rate, and only 70% and 86%
conversion was observed after 12 h for complexes 1 and 2,
respectively. This result indicates the reduced accessibility of
the carbodiimide to the actinide center due to the sterically
demanding tert-butyl group. Similarly, when other tertiary
alcohols such as diphenylmethanol or 1-adamantanol as the
nucleophiles were used, much longer reaction times were
required to completely consume the starting material (runs
21−24, Table 1).66 Interestingly, the use of heteroatomic
alcohols (runs 13−18), that is, 2-pyridinemethanol, 2-
thiophenemethanol, or 2-furanmethanol, did not reduce the
activity of the complexes, which proceeded to completion
within an hour. Compared with actinide amido mediated
systems, the present two complexes performed with higher
activities due to the presence of the benzimidazolin-2-iminato
ligand. In this sense, the ligand will lead to a reduced
electrophilicity on the metal center and therefore an expected
increase in its catalytic reactivity.66 In addition, the presence of
the ligand improves the solubility of the alkoxo active species in
benzene. Interestingly, in the catalytic addition of alcohols to
1,3-di-p-tolylcarbodiimide, a lower reactivity is obtained when
using the metallacycle thorium amido complex
[(Me3Si)2N]2Th[κ2-(N,C)-CH2Si(CH3)2N(SiMe3)], due to
the formation of insoluble actinide alkoxides moieties.66
Phenols of different size were also investigated as substrates
for the intermolecular coupling (entries 25−30, Table 1).
Similar to the aliphatic alcohols, less bulky phenol (PhOH) and
2,6-dimethylphenol (2,6-Me2PhOH) react with DTC furnish-
ing the insertion products in high yields within an hour. For the
more sterically encumbered 2,4-di-tert-butylphenol
(2,4-tBu2PhOH), a sluggish reactivity with moderate yields of
77% and 83% was observed after 12 h for complexes 1 and 2,
respectively. Figure 3 shows the progress of the reaction as a
function of DTC/2,4-tBu2PhOH consumption and isourea
formation using complex 2. The signals at 1.24 and 1.50 ppm
were assigned to the ortho- and para-tert-butyl groups of the
starting material 2,4-tBu2PhOH, whereas the signals at 1.13 and
1.41 ppm were assigned to the corresponding tert-butyl groups
of the addition product.
Scheme 4. Catalytic Addition of Methanol to Various Kinds
of Carbodiimides
Table 2. Catalytic Addition of Methanol to Various Kinds of
a
Carbodiimides (R″NCNR″)
entry
cat.
R″
Mes
time (h)
yield (%)
product
1
2
3
4
5
6
1
2
1
2
1
2
1
1
1
1
1
1
>99
>99
>99
>99
>99
>99
5o
4-ClPh
5p
5q
4-MeOPh
a
Reaction conditions: 2 mol % precatalyst, 50 equiv of substrates (0.35
mmol), 500 μL of C6D6, room temperature. The yields were
1
determined by H NMR of the crude reaction mixture.
(MesNCNMes) or strongly electron-withdrawing groups ((4-
ClPh)NCN(4-ClPh)) or strongly electron-donating groups
((4-MeOPh)NCN(4-MeOPh)) are able to undergo efficient
insertion to afford the corresponding isoureas in high yields
within 1 h. Note that changing the metal center showed little
influence on the catalytic behaviors, and both uranium (1) and
thorium (2) complexes displayed high activities in this process.
In order to expand the scope of the precatalysts 1 and 2 and
to synthesize polyfunctionalized compounds with the potential
to serve as templates for the construction of larger molecules,
diols and triols were also investigated. In the presence of
catalytic amounts of the complexes 1 and 2, symmetric diols or
triols reacted with DTC in a 1:2 or 1:3 molar ratio furnishing,
in high yields, the corresponding diisourea and triisourea,
respectively (Scheme 5). Reacting the nonsymmetric diol 1,3-
butanediol in a 1:1 ratio with DTC with catalytic amounts of
complex 2 afforded the monoisourea products 8a and 8b in a
The reaction scope was next explored with a series of
diarylcarbodiimides using 2 mol % of the actinide complexes 1
and 2 (Scheme 4 and Table 2). It was found that the
carbodiimide substrates with higher steric encumbrance
D
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complex starts at 80 °C, indicating that the introduction of the
methoxy group can only partially increase the stability of the
active species.
Scheme 5. Catalytic Addition of Diols and Triols to DTC
Kinetic studies for the catalytic addition of 2,4-di-tert-
butylphenol to DTC employing precatalyst 2 were next
conducted by changing one substrate or catalyst while keeping
the other reagents constant, and the progress of the reaction
1
was monitored by in situ H NMR spectroscopy. As shown in
Figures 4−6, the rate of product formation displays a first-order
ratio of 1:1.7, respectively, revealing a higher reactivity for the
primary alcohol moiety. Addition of another equivalent of DTC
to this reaction mixture gave the diisourea 9 quantitatively
(Scheme 6).
Figure 4. Plot of initial reaction rate against the inverse concentration
of 2,4-di-tert-butylphenol (2,4-tBu2PhOH).
Scheme 6. Catalytic Addition of 1,3-Butanediol to DTC
Catalyzed by Complex 2
Figure 5. Plot of initial reaction rate against the concentration of
DTC.
dependence on DTC and complex 2 and an inverse first-order
dependence on 2,4-di-tert-butylphenol, giving rise to the kinetic
eq 1 (detailed analysis of the kinetic equation can be found in
the experimental section). The inverse first order indicates that
an excess of alcohol coordinates to the active species CatA
To elucidate the catalytically active species and to investigate
their thermal stabilities, stoichiometric reactions of complex 4
with 10 equiv of 2,4-di-tert-butylphenol were carried out in situ
in benzene-d6. Upon addition, 3 equiv of hexamethyldisilazane
were released immediately, affording the corresponding
benzimidazolin-2-iminato thorium(IV) trialkoxide complex,
[(Bim5‑MeDipp/MeN)Th(O-2,4-tBu2Ph)3]. To this reaction mix-
ture, 10 equiv of DTC were added, and the rapid appearance of
the isourea product was observed. In contrast, leaving the
mixture of complex 4 with 10 equiv of DTC at room
temperature for 24 h did not afford any addition of the amido
groups to the carbodiimide, indicating that the actinide
trialkoxide complexes serve as a truly active species during
the catalytic cycle. Exposing the methylated benzimidazolin-2-
iminato thorium(IV) trialkoxide complex [(Bim5‑MeDipp/MeN)-
Th(O-2,4-tBu2Ph)3] to an excess of 2,4-di-tert-butylphenol at
50 °C induced the protonolysis of the ligand.69 However, for
the methoxylated benzimidazolin-2-iminato thorium(IV) trialk-
oxide species [(Bim2‑MeOPh/MeN)Th(O-2,4-tBu2Ph)3], the pres-
ence of an excess amount of 2,4-di-tert-butylphenol at 50 °C for
12 h, did not induce any decomposition. Protonolysis of the
Figure 6. Plot of initial reaction rate against the concentration of
precatalyst 2.
E
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(Scheme 6) taking it out of the catalytic cycle with the
complex 2 revealed a KIE value of KH/KD = 1.04, indicating
that the protonolysis is a fast step and that the insertion of
DTC into the actinide alkoxide moiety is the rate-determining
step.
formation of the complex C.72
∂p
∂t
= Kobs[2]1[DTC]1[tBu2PhOH]−1
Regarding the yields of the reaction (Tables 1 and 2), it is
important to note that when 2 mol % of the precatalyst is used,
6% of [LAn]−OR′ (intermediate CatA, Scheme 7) will be
formed. At the end of the reaction, the precatalyst will finally
remain as the actinide pre-isourea intermediate (intermediate
B), rather than in the CatA form. For intermediate B, which
only needs external protons to release the final product, the
characteristic 1H NMR always overlapped with the 1H NMR of
the final isourea products, which prevents us from differ-
entiating them. Based on these considerations, we have
calculated the ratio of the products (including intermediate B
and the isourea products) and the starting material. In addition,
quenching every reaction mixture with external proton
resources released the 6% of the isourea from complex B
affording yields of >99%.
(1)
Thermodynamic activation parameters were determined from
the Eyring (Figure 7) and Arrhenius plots, displaying a low
CONCLUSION
■
Figure 7. Eyring plot of 2,4-di-tert-butylphenol (2,4-tBu2PhOH),
DTC, and 2 system.
The synthesis and characterization of methyl and methoxy
substituted benzimidazolin-2-iminato actinide complexes (1−
4) are reported herein. X-ray single crystal analysis revealed
short An−NCN bonds, indicative of a higher An−NCN bond
order. At room temperature, all four complexes showed high
activities in catalyzing the addition of aliphatic and aromatic
alcohols to carbodiimides. In addition, various diols and triols
were also employed in this reaction. Kinetic studies revealed
that the reaction follows a first-order dependence on precatalyst
and carbodiimide and an inverse first-order on the alcohol. A
plausible mechanism is proposed, in which the insertion of the
carbodiimide into the actinide alkoxide species was found to be
the rate-determining step.
activation barrier (Ea) of 11.2(9) kcal mol−1, and enthalpy
(ΔH⧧) and entropy (ΔS⧧) of activation of 10.6(7) kcal mol−1
and −44.8(8) eu, respectively, indicating an ordered transition
state during the rate-determining insertion step. Based on the
above analysis, a plausible mechanism for the catalytic addition
of alcohols to DTC is proposed in Scheme 7. In the first step,
Scheme 7. Plausible Mechanism for Catalytic Insertion of
Alcohols into DTC
EXPERIMENTAL SECTION
■
All manipulations of air sensitive materials were performed with
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). MeOH,
i
t
EtOH, PrOH, BuOH, BnOH, 2-pyridinemethanol, 2-thiopheneme-
thanol, 2-furanmethanol, CyOH, HO(CH2)3OH, and N-
(CH2CH2OH)3 were distilled under CaH2 and stored over 4 Å
molecular sieves. PhOH, 2,6-dimethylphenol (2,6-Me2PhOH), 2,4-di-
tert-butylphenol, Ph2CHOH, 1-adamantanol, and 1,3-di-p-tolylcarbo-
diimide (Sigma-Aldrich) were dried for 12 h on a high vacuum line
(10−5 Torr) and stored in a glovebox prior to use. The actinide
metallacycles were prepared according to previous reports.73
Deuterated 2,4-di-tert-butylphenol (2,4-tBu2PhOD) was prepared
using a similar method according to published procedures.74 1,3-Di-
(2,4,6-trimethylphenyl)carbodiimide,75 1,3-di-p-chlorophenylcarbodii-
mide,76 and 1,3-di-p-methoxylphenylcarbodiimide,76 were prepared
according to previous reports. 1-Fluoro-2-nitrobezene, o-anisidine, 4-
bromo-3-nitrotoluene, 2,6-diisopropylaniline (97%), tris-
(dibenzylideneacetone)dipalladium(0) (Pd2(dpa)3), ( )-2,2′-bis-
(diphenylphosphino)-1,1′-binaphthalene ( BINAP), cesium carbo-
nate (Cs2CO3), palladium on activated charcoal (Pd/C; 5% Pd), and
methyl trifluoromethanesulfonate (CF3SO3CH3) were purchased from
Sigma-Aldrich and used as received.
the active species, the benzimidazolin-2-iminato actinide
alkoxide (CatA), is formed by protonolysis of the amido
groups with the alcohol with the concomitant release of three
equivalents of HN(SiMe3)2. CatA was characterized by NMR
spectroscopy obtained from the stoichiometric reaction
between complex 4 with 3 equiv of 2,4-tBu2PhOH. Then
CatA reacts with DTC, affording the actinide−isourea species
B, which is subsequently protonated by another molecule of
alcohol, furnishing the final isourea with regeneration of CatA.
Deuterium labeling studies using 2,4-tBu2PhOD, DTC, and
F
DOI: 10.1021/acs.organomet.7b00432
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
NMR spectra were recorded on Bruker Avance 300 and Avance 400
immediately), then the mixture was stirred at room temperature for
30 min and subsequently refluxed for 2 h. The solvent was completely
removed, and 30 mL of diethyl ether was added to extract the target
ligand. Filtration and evaporation afforded the final ligand (a) as a
1
spectrometers. Chemical shifts for H and 13C NMR measurements
are reported in ppm and referenced to the residual proton or carbon
signals of the deuterated solvents relative to tetramethylsilane.
Elemental analyses were carried out by the microanalysis laboratory
at the Hebrew University of Jerusalem. MS experiments were
performed at 200 °C (source temperature) on a Maxis Impact
(Bruker) mass spectrometer with an APCI solid probe method. For X-
ray crystallographic measurements, the single crystals were immersed
in perfluoropolyalkylether oil and quickly mounted on a Kappa CCD
diffractometer under flow of liquid nitrogen. Data collection was
performed using monochromated Mo Kα radiation using φ and ω
scans to cover the Ewald sphere.77 Accurate cell parameters were
obtained with the amount of indicated reflections (Table S1).78 The
structure was solved by SHELXS-97 direct methods79 and refined by
the SHELXL-97 program package.80 The atoms were refined
anisotropically. Hydrogen atoms were included using the riding
model. CCDC 1544404−1544407 for 1−4.
Synthesis of 2-Methoxy-N-(2-nitrophenyl)aniline (a1). To a
100 mL round-bottom flask, 1-fluoro-2-nitrobezene (8.46 g, 60 mmol),
o-anisidine (12.6 g, 100 mmol), and potassium fluoride (6.96 g, 120
mmol) were added. The reaction mixture was refluxed at 180 °C for
24 h. After cooling to room temperature, the mixture was washed with
water (3 × 50 mL) and extracted with dichloromethane (3 × 50 mL),
dried over MgSO4, and concentrated under vacuum. Excess o-anisidine
was removed by vacuum distillation, and then the crude product was
purified by column chromatography on silica with the eluent of n-
hexane/ethyl acetate (10/1), affording the target product a1, after
evaporation of the solvent, as a yellow powder. Yield: 10.3 g (77%). 1H
NMR (400 MHz, CDCl3) δ 9.36 (s, 1H, NH), 8.12 (m, 1H, HAr),
7.31−7.24 (m, 2H, HAr), 7.22−7.15 (m, 1H, HAr), 7.15−7.03 (m, 1H,
HAr), 6.95−6.84 (m, 2H, HAr), 6.72−6.63 (m, 1H, HAr), 3.79 (s, 3H,
CH3). 13C{1H} NMR (101 MHz, CDCl3) δ 152.56, 142.51, 135.44,
133.80, 127.88, 126.67, 125.77, 123.29, 120.68, 117.43, 116.22, 111.64,
55.72. MS (APCI): m/z 245.0310 (M + H)+.
1
white powder. Yield: 1.93 g (91%). H NMR (400 MHz, CDCl3) δ
7.45−7.35 (m, 1H, HAr), 7.32−7.22 (m, 1H, HAr), 7.11−6.98 (m, 2H,
HAr), 6.98−6.85 (m, 1H, HAr), 6.89−6.74 (m, 2H, HAr), 6.44 (d, J =
7.6 Hz, 1H, HAr), 4.51 (br, 1H, NH), 3.72 (s, 3H, CH3), 3.39 (s, 3H,
CH3). 13C{1H} NMR (101 MHz, CDCl3) δ 156.30, 155.13, 132.76,
132.37, 130.52, 130.38, 122.72, 121.43, 120.91, 120.13, 112.80, 107.17,
106.12, 55.76, 28.00. MS (APCI): m/z 254.1138 (M + H)+.
Synthesis of (2,6-Diisopropylphenyl)-(4-methyl-6-nitro-
phenyl)amine (b1). An oven-dried 250 mL Schlenk flask was
charged with 4-bromo-3-nitrotoluene (4.32 g, 20 mmol) and 50 mL of
toluene. 2,6-Diisopropylaniline (5.32 g, 30 mmol), Pd2(dpa)3 (0.45 g,
0.5 mmol), rac-2,2′-bis(diphenylphosphino)-1,1′-binaphthyl
( BINAP) (0.46 g, 0.75 mmol), and Cs2CO3 (3.26 g, 10 mmol)
were added to the solution. The reaction mixture was degassed and
refilled with nitrogen gas three times and sealed afterward. The flask
was heated to 110 °C for 24 h and then cooled to room temperature.
The resulting slurry was filtered through a pad of Celite followed by
removal of the solvent on a rotatory evaporator, and the final product
was purified by chromatography on silica gel with the eluent of n-
hexane/ethyl acetate (15/1), affording (2,6-diisopropylphenyl)-(2-
methyl-6-nitro-phenyl)amine as a yellow powder after the removal of
1
the solvent. Yield: 5.0 g (81%). H NMR (400 MHz, CDCl3) δ 9.04
(s, 1H, NH), 7.96 (s, 1H, HAr), 7.33−7.25 (m, 1H, HAr), 7.21−7.14
(m, 2H, HAr), 7.02 (d, J = 8.7 Hz, 1H, HAr), 6.23 (d, J = 8.7 Hz, 1H,
HAr), 2.96 (hept, J = 6.8 Hz, 2H, CH(CH3)2), 2.20 (s, 3H, CH3), 1.10
(d, J = 6.8 Hz, 6H, CH3), 1.04 (d, J = 6.8 Hz, 6H, CH3). 13C{1H}
NMR (101 MHz, CDCl3) δ 147.33, 144.01, 137.53, 132.70, 131.29,
128.52, 125.96, 125.73, 124.18, 115.49, 28.22, 24.63, 22.98, 19.65. MS
(APCI): m/z 313.1809 (M + H)+.
Compound b2 was prepared using a similar method as that for a2.
Yield: 1.4 g (72%). 1H NMR (400 MHz, CDCl3) δ 7.44 (d, J = 7.9 Hz,
1H, HAr), 7.32−7.24 (m, 2H, HAr), 7.23−7.17 (m, 1H, HAr), 6.77−
6.70 (m, 1H, HAr), 6.52 (d, J = 7.9 Hz, 1H, HAr), 4.51 (br, 2H, NH2),
2.44 (hept, J = 6.8 Hz, 2H, CH(CH3)2), 2.36 (s, 3H, CH3), 1.08 (d, J
= 6.8 Hz, 5H, CH(CH3)2), 0.95 (d, J = 6.8 Hz, 6H, CH(CH3)2).
13C{1H} NMR (101 MHz, CDCl3) δ 153.63, 148.57, 142.44, 133.79,
Synthesis of 1-(2-Methoxyphenyl)-3-methylbenzimidazolin-
2-imine (a). To a 100 mL round-bottom flask, 2-methoxy-N-(2-
nitrophenyl)aniline (2.0 g, 8.2 mmol), THF (30 mL), and 5% Pd/C
(0.4 g) were added, and the flask was carefully placed under 1 atm H2.
The mixture was stirred vigorously overnight at room temperature
(TLC was used to monitor the reduction process) and then filtered
through a pad of Celite and concentrated, affording the diamine
quantitatively. Then the flask was charged with cyanogen bromide
(1.06 g, 10 mmol) and 30 mL of absolute ethanol, and the mixture was
stirred at room temperature for 30 min and then refluxed for 2 h. To
complete the reaction, the temperature was then increased to 150 °C
for 1 h. The solvent was then removed completely under vacuum, and
50 mL of diethyl ether was added. The mixture was stirred for 30 min,
and the resulting precipitate was filtered off, washed with diethyl ether,
and then dissolved in 50 mL of CHCl3. To this solution, 50 mL of 1 M
NaOH aqueous solution was added, and the mixture was stirred for 2
h. The organic layer was collected, and the remaining aqueous phase
was extracted with CHCl3 (3 × 30 mL). The combined organic
fraction was dried over MgSO4 and concentrated in vacuo to afford 1-
(2-methoxyphenyl)-1H-benzimidazol-2-amine (a2) as the final prod-
131.33, 130.51, 128.88, 124.78, 121.11, 116.73, 108.00, 28.30, 24.38,
23.88, 21.58. MS (APCI): m/z 308.2060 (M + H)+.
The ligand b was prepared using a similar method as that for ligand
a. Yield: 0.74 g (94%). 1H NMR (400 MHz, CDCl3) δ 7.56−7.46 (m,
1H, HAr), 7.36 (d, J = 7.8 Hz, 2H, HAr), 6.79 (s, 1H, HAr), 6.70 (d, J =
7.8 Hz, 1H, HAr), 6.29 (d, J = 7.8 Hz, 1H, HAr), 4.33 (br, 1H, NH),
3.52 (s, 3H, CH3), 2.67 (hept, J = 6.9 Hz, 2H, CH(CH3)2), 2.40 (s,
3H, CH3), 1.18 (d, J = 6.9 Hz, 6H, CH(CH3)2), 1.05 (d, J = 6.9 Hz,
6H, CH(CH3)2). 13C{1H} NMR (101 MHz, CDCl3) δ 155.90, 149.36,
132.66, 130.87, 130.69, 130.25, 128.66, 124.74, 120.77, 107.20, 106.61,
28.45, 28.08, 24.19, 24.09, 21.49. MS (APCI): m/z 322.2706 (M +
H)+.
Synthesis of 1-(2-Methoxyphenyl)-3-methylbenzimidazolin-
2-iminato Uranium(IV) (Complex 1) [(Bim2‑MeOPh/MeN)UN″3]. A
solution of the uranium metallacycle (0.11 g, 1.5 mmol) in 2 mL of
toluene was reacted with 1.0 equiv of ligand a in 3 mL of toluene. The
reaction mixture was stirred at room temperature for 12 h, and after
partial removal of the solvent, recrystallization from the concentrated
toluene solution afforded the target complex as deep-brown crystals in
1
uct. Yield: 1.67 g (85%). H NMR (400 MHz, CDCl3) δ 7.45−7.37
(m, 2H, HAr), 7.34−7.30 (m, 1H, HAr), 7.11−7.03 (m, 3H, HAr),
6.97−6.89 (m, 1H, HAr), 6.82−6.76 (m, 1H, HAr), 4.64 (s, 2H, NH),
3.74 (s, 3H, CH3). 13C{1H} NMR (101 MHz, CDCl3) δ 176.76,
154.93, 153.12, 149.21, 141.71, 135.26, 130.51, 128.95, 123.04, 121.78,
120.12, 117.47, 116.25, 112.82, 108.50, 55.35. MS (APCI): m/z
240.0994 (M + H)+.
1
high yield. Yield: 0.13 g (90%). H NMR (300 MHz, C6D6) δ 26.36
(d, J = 7.6 Hz, 1H, HAr), 15.19 (d, J = 7.6 Hz, 1H, HAr), 12.18 (d, J =
7.1 Hz, 1H, HAr), 9.22−9.10 (m, 1H, HAr), 7.13−7.04 (m, 1H, HAr),
2.06 (s, 1H, HAr), 0.13 to −0.07 (m, 1H, HAr), −2.76 to −2.93 (m, 1H,
HAr), −5.02 to −5.91 (t, 3H, CH3), −6.88 (s, 3H, CH3), −10.48 to
−12.28 (br, 54H, N(SiMe3)2). 13C{1H} NMR (75 MHz, C6D6) δ
160.28, 136.33, 132.83, 129.87, 128.83, 128.06, 125.19, 116.30, 116.28,
112.18, 108.63, 43.41, 20.94, 6.47. Anal. Calcd for C33H68N6OSi6U: C,
40.80, H, 7.06, N, 8.65. Found: C, 40.41, H, 6.98, N, 8.61.
The precursor obtained above (2 g, 8.35 mmol) was redissolved in
CH2Cl2 (30 mL), and the solution was cooled to −40 °C. CF3SO3CH3
(1.1 equiv) was added dropwise, and the reaction mixture was stirred
at this temperature for 1 h. The mixture was allowed to warm to room
temperature and stirred overnight. After removal of the solvent in
vacuo, the remaining solid was washed with diethyl ether (2 × 20 mL),
dried in vacuo, and redissolved in 30 mL of THF. KH (0.38 g, 9.5
mmol) was added to the solution slowly (gas was evolved
Synthesis of 1-(2-Methoxyphenyl)-3-methylbenzimidazolin-
2-iminato Thorium(IV) (Complex 2) [(Bim2‑MeOPh/MeN)ThN″3]. A
G
DOI: 10.1021/acs.organomet.7b00432
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
solution of the thorium metallacycle (0.11 g, 1.5 mmol) in 2 mL of
toluene was reacted with 1.0 equiv of ligand a in 3 mL of toluene. The
reaction mixture was stirred at room temperature for 12 h, and after
partial removal of the solvent, recrystallization from the concentrated
toluene solution afforded the target complex as colorless crystals in
high yield. Yield: 0.13 g (92%). 1H NMR (300 MHz, C6D6) δ 7.30 (d,
J = 7.7 Hz, 1H, HAr), 7.12−7.03 (m, 1H, HAr), 6.91−6.80 (m, 2H,
HAr), 6.76 (d, J = 7.7 Hz, 1H, HAr), 6.55−6.47 (m, 2H, HAr), 6.35 (d, J
= 7.7 Hz, 1H, HAr), 3.30 (s, 3H, CH3), 3.09 (s, 3H, CH3), 0.37 (s,
54H, N(SiMe3)2). 13C{1H} NMR (75 MHz, C6D6) δ 156.14, 143.63,
131.56, 131.27, 131.13, 129.90, 128.84, 124.41, 120.99, 120.86, 111.61,
108.01, 106.48, 54.33, 28.58, 4.37. Anal. Calcd for C33H68N6OSi6Th:
C, 41.05, H, 7.10, N, 8.70. Found: C, 40.62, H, 7.04, N, 8.68.
extracted with toluene and filtered to give a clean solution. After
removal of part of the solvent, subsequent recrystallization from
toluene gave the final isourea products in 95% (1.2 g) yield. This
method is also general for isolating other insertion products.
Insertion of Benzyl Alcohol (BnOH) into DTC. The insertion of
benzyl alcohol into DTC was carried out following the general
1
procedure described above. H NMR (300 MHz, C6D6) δ 7.55−7.31
(m, 2H, HAr), 7.31−7.06 (m, 7H, HAr), 7.01−6.81 (m, 4H, HAr), 6.03
(s, 1H, NH), 5.55 (s, 2H, CH2), 2.26 (s, 3H, CH3), 2.11 (s, 3H, CH3).
13C{1H} NMR (75 MHz, C6D6) δ 149.77, 145.96, 137.12, 136.29,
132.25, 131.90, 130.48, 129.34, 128.41, 128.10, 122.69, 120.84, 68.34,
20.75, 20.44. MS (APCI): 331.1897 (M + H)+.
Insertion of 2-Pyridinemethanol (Py-2-CH2OH) into DTC. The
insertion of 2-pyridinemethanol into DTC was carried out following
the general procedure described above. 1H NMR (300 MHz, C6D6) δ
8.61−8.34 (m, 1H, HAr), 7.45−6.83 (m, 10H, HAr), 6.75−6.58 (m, 1H,
HAr), 6.25 (s, 1H, NH), 5.79 (s, 2H, CH2), 2.23 (s, 3H, CH3), 2.14 (s,
3H, CH3). 13C{1H} NMR (50 MHz, C6D6) δ 157.32, 149.56, 149.25,
145.79, 132.37, 131.82, 130.33, 129.35, 128.10, 122.64, 122.03, 121.17,
121.10, 69.00, 20.58, 20.49. MS (APCI): 331.2009 (M)+.
Synthesis of 1-(2,6-Diisopropylphenyl)-3,5-dimethyl-1,3-di-
hydro-benzimidazolin-2-imine Uranium(IV) (Complex 3)
[(Bim5‑MeDipp/MeN)UN″3]. Complex 3 was prepared using a similar
method as for the synthesis of 1 and isolated as deep-brown crystals.
1
Yield: 0.15 g (94%). H NMR (300 MHz, C6D6) δ 97.04 (s, 2H,
CH(CH3)2), 25.95 (s, 1H, HAr), 14.32 (s, 9H, CH(CH3)2 and CH3),
12.00 (d, J = 6.6 Hz, 1H, HAr), 9.41 (d, J = 6.6 Hz, 1H, HAr), 8.35 (s,
3H, CH3), −0.17 (s, 6H, CH(CH3)2), −0.54 to −0.73 (m, 1H, HAr),
−2.75 (b, 2H, HAr), −12.41 (b, 42H, N(Si(CH3)3)2), −20.27 (s, 12H,
N(Si(CH3)3)2). 13C{1H} NMR (75 MHz, C6D6) δ 169.82, 157.27,
141.96, 131.35, 129.21, 126.59, 126.53, 120.08, 118.61, 116.76, 111.40,
34.12, 28.76, 23.01, 18.53, 8.43. Anal. Calcd for C39H80N6Si6U: C,
45.06, H, 7.76, N, 8.08, Found: C, 45.20, H, 7.86; N, 8.00.
Insertion of 2-Thiophenemethanol (Thio-2-CH2OH) into
DTC. The insertion of 2-thiophenemethanol into DTC was carried
1
out following the general procedure described above. H NMR (300
MHz, C6D6) δ 7.38−7.06 (m, 5H, HAr), 7.04−6.65 (m, 7H, HAr), 5.96
(br, 1H, NH), 5.60 (s, 2H, CH2), 2.27 (s, 3H, CH3), 2.10 (s, 3H,
CH3). 13C{1H} NMR (75 MHz, C6D6) δ 149.43, 145.76, 139.12,
136.11, 132.32, 132.00, 130.47, 130.18, 129.31, 126.80, 126.30, 124.15,
122.72, 120.91, 62.58, 20.64, 20.41. MS (APCI): 337.1412 (M + H)+.
Insertion of 2-Furanmethanol (Furan-2-CH2OH) into DTC.
The insertion of 2-furanmethanol into DTC was carried out following
the general procedure described above. 1H NMR (300 MHz, C6D6) δ
7.30−7.20 (m, 1H, HAr), 7.17−7.01 (m, 4H, HAr), 6.97−6.77 (m, 4H,
HAr), 6.37−6.24 (m, 1H, HAr), 6.15−6.02 (m, 1H, HAr), 5.95 (br, 1H,
NH), 5.49 (s, 2H, CH2), 2.26 (s, 3H, CH3), 2.09 (s, 3H, CH3).
13C{1H} NMR (75 MHz, C6D6) δ 150.67, 149.46, 145.81, 142.76,
136.20, 132.19, 131.93, 130.45, 129.32, 122.67, 120.61, 110.47, 99.88,
60.14, 20.66, 20.13. MS (APCI): 321.1659 (M + H)+.
Synthesis of 1-(2,6-Diisopropylphenyl)-3,5-dimethyl-1,3-di-
hydro-benzimidazolin-2-imine Thorium(IV) (Complex 4)
[(Bim5‑MeDipp/MeN) ThN″3]. Complex 4 was prepared using a similar
method as for the synthesis of 2 and isolated as colorless crystals.
1
Yield: 0.14 g 92%. H NMR (300 MHz, C6D6) δ 7.34−7.04 (m, 3H,
HAr), 6.52 (d, J = 7.8 Hz, 1H, HAr), 6.36 (s, 1H, HAr), 6.12 (d, J = 7.8
Hz, 1H, HAr), 3.38 (s, 3H, CH3), 2.88 (hept, J = 6.7 Hz, 2H,
CH(CH3)2), 2.16 (s, 3H, CH3), 1.26 (d, J = 6.7 Hz, 6H, CH(CH3)2),
0.87 (d, J = 6.7 Hz, 6H, CH(CH3)2), 0.52−0.14 (m, 54H,
N(Si(CH3)3)2). 13C{1H} NMR (75 MHz, C6D6) δ 148.02, 144.42,
131.58, 131.16, 130.45, 129.97, 129.35, 128.84, 125.20, 124.34, 121.18,
109.11, 107.35, 29.33, 28.20, 25.16, 23.75, 21.02, 4.74, 3.53. Anal.
Calcd for C39H80N6Si6U: C, 45.32, H, 7.80, N, 8.13. Found: C, 45.26,
H, 7.76; N, 8.09.
Insertion of Cyclohexanol (CyOH) into DTC. The insertion of
cyclohexanol into DTC was carried out following the general
procedure described above. 1H NMR (300 MHz, C6D6) δ 7.25−
6.69 (m, 8H, HAr), 6.01 (br, 1H, NH), 5.70−5.30 (m, 1H, CH), 2.44−
1.86 (m, 8H, CH3 and CH2), 1.83−1.53 (m, 4H, CH2), 1.33 (m, 4H,
CH2). 13C{1H} NMR (75 MHz, C6D6) δ 149.18, 146.31, 136.74,
131.90, 131.60, 130.49, 129.26, 122.70, 120.57, 74.09, 31.66, 25.65,
General Procedure for the Catalytic Addition of Alcohols
into Carbodiimides. For Th(IV) complexes, a sealable J. Young
NMR tube was loaded with approximately 2 mol % of the desired
catalyst from a stock solution in C6D6 inside the glovebox, followed by
the addition of carbodiimide (0.35 mmol, 50 equiv) and alcohol (0.35
mmol, 50 equiv). The reaction was immediately diluted to 500 μL with
C6D6. Samples were taken out of the glovebox, and the reaction
progress was monitored by 1H NMR spectroscopy. The crude
+
23.67, 20.65, 20.42. MS (APCI): 323.2176 (M + H) .
Insertion of Diphenylmethanol (Ph2CHOH) into DTC. The
insertion of diphenylmethanol into DTC was carried out following the
general procedure described above. 1H NMR (300 MHz, C6D6) δ 7.75
(s, 1H, CH), 7.63−7.43 (m, 4H, HAr), 7.31−6.77 (m, 14H, HAr), 5.98
(br, 1H, NH), 2.22−2.14 (s, 6H, CH3). 13C{1H} NMR (75 MHz,
C6D6) δ 148.96, 145.73, 141.51, 136.28, 136.22, 132.57, 131.89,
130.38, 129.35, 124.15, 122.59, 121.39, 79.08, 20.64, 20.54. MS
1
mixtures were analyzed using H and 13C NMR spectroscopy and
mass spectrometry; the values were compared to previously published
data.69
For U(IV) complexes, due to their paramagnetic properties, a
different method was applied. A 10 mL vial was loaded with 2 mol %
of the desired catalyst from a stock solution in C6D6, followed by the
addition of carbodiimide (0.35 mmol, 50 equiv) and alcohol (0.35
mmol, 50 equiv). The reaction was immediately diluted to 500 μL with
C6D6. A 40 μL sample of the reaction mixture was taken every hour
+
(APCI): 407.2197 (M + H) .
Insertion of 1-adamantanol into DTC. The insertion of 1-
adamantanol into DTC was carried out following the general
procedure described above. 1H NMR (300 MHz, C6D6) δ 7.26−
6.71 (m, 8H, HAr), 5.93 (br, 1H, NH), 2.46 (br, 6H, CH2), 2.36−1.98
(m, 9H, CH3 and CH), 1.78−1.41 (m, 6H, CH2). 13C{1H} NMR (75
MHz, C6D6) δ 147.92, 146.20, 131.55, 131.41, 130.16, 129.26, 124.12,
122.60, 120.44, 81.01, 41.80, 36.33, 31.17, 20.63, 20.56. MS (APCI):
1
and diluted with C6D6. H NMR spectroscopy was performed on the
sample immediately to monitor the process.
The NMR spectra for the catalytic insertion of MeOH, EtOH,
iPrOH, tBuOH, PhOH, 2,6-Me2PhOH, 2,4-tBuPhOH, HO(CH2)3OH,
and N(CH2CH2OH)3 into DTC were compared with previous
reports.69
Preparative Scale-up Reaction. In the glovebox, a solution of
methanol (0.2 mL, 5 mmol) in toluene (1 mL) was added to a
solution of complex 2 (96 mg, 0.1 mmol) in toluene (1 mL) in a
Schlenk tube. DTC (1.11 g, 5 mmol) was then added to the above
reaction mixture. The Schlenk tube was taken outside the glovebox,
and the mixture was stirred at room temperature for 3 h. After the
solvent was removed under reduced pressure, the residue was
+
375.2452(M + H) .
Insertion of Methanol into 1,3-Dimesitylcarbodiimide
((Mes)NCN(Mes)). The insertion of MeOH into (Mes)NCN(Mes)
1
was carried out following the general procedure described above. H
NMR (300 MHz, C6D6) δ 6.90 (s, 2H, HAr), 6.58 (s, 2H HAr), 4.44 (s,
1H, NH), 3.60 (s, 3H, CH3), 2.30 (s, 6H, CH3), 2.24 (s, 3H, CH3),
2.04 (s, 3H, CH3), 1.98 (s, 6H, CH3). 13C{1H} NMR (75 MHz, C6D6)
δ 149.37, 142.89, 135.62, 132.97, 132.09, 130.85, 129.28, 129.05,
128.85, 128.38, 53.05, 20.47, 20.40, 18.11, 17.89. MS (APCI):
311.2160 (M + H)+.
H
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Insertion of Methanol into 1,3-Di-p-chlorophenylcarbodii-
mide ((4-ClPh)NCN(4-ClPh)). The insertion of MeOH into (4-
ClPh)NCN(4-ClPh) was carried out following the general procedure
described above. 1H NMR (300 MHz, C6D6) δ 7.07 (s, 2H, HAr), 6.92
(s, 2H, HAr), 6.62 (s, 2H, HAr), 6.40 (s, 2H, HAr), 5.45 (s, 1H, NH),
3.55 (s, 3H, CH3).13C{1H} NMR (75 MHz, C6D6) δ 149.57, 129.59,
128.54, 123.72, 121.72, 53.42. MS (APCI): 295.0438 (M + H)+.
Insertion of Methanol into 1,3-Di-p-methoxylphenylcarbo-
diimide ((4-MeOPh)NCN(4-MeOPh)). The insertion of MeOH into
(4-MeOPh)NCN(4-MeOPh) was carried out following the general
tube was taken out of the glovebox and frozen in an ice bath until the
1H NMR experiment began. All the experiments were done by
changing one substrate or catalyst while keeping the other reagents
constant, and the progress of the reaction was initially monitored every
2 min and then in longer intervals up to 1.5 h. The product
concentrations were measured by the area ratio of the tert-butyl group
at 1.11 and 1.23 ppm, which were assigned to the starting material and
reaction product, respectively. Reaction rates were determined by
least-squares fit of the initial product concentration versus time, and
the plots of the initial reaction rates against precatalyst 2, DTC, and
2,4-tBu2PhOH are shown in Figures 4−6.
1
procedure described above. H NMR (300 MHz, C6D6) δ 7.08−6.85
Activation parameters including enthalpy (ΔH⧧), entropy (ΔS⧧),
and activation energy (Ea) were calculated from the kinetic data using
Eyring and Arrhenius plots. In a typical sample, the J. Young tube was
loaded with the desired amount of catalyst 2, DTC, 2,4-di-tert-
butylphenol, and solvent and then was sealed and frozen. Then the
sample was inserted into a Bruker Avance 300 spectrometer, which
had been previously set to the desired temperature. The progress was
initially monitored every minute and then in longer intervals up to one
and a half hours. Reaction rates were determined by least-squares fit of
the initial product concentration versus time, and Eyring and
Arrhenius plots are shown in Figure 7. Enthalpy (ΔH⧧), entropy
(ΔS⧧), and activation energy (Ea) were calculated from the slope and
intercept of the least-squares fit.
(m, 2H, HAr), 6.85−6.66 (m, 4H, HAr), 6.66−6.50 (m, 2H, HAr), 5.78
(s, 1H, NH), 3.70 (s, 3H, OCH3), 3.32 (s, 3H, CH3), 3.24 (s, 3H,
CH3). 13C{1H} NMR (75 MHz, C6D6) δ 156.19, 155.60, 150.92,
141.40, 131.58, 124.88, 123.07, 114.97, 113.77, 54.54, 53.15. MS
(APCI): 287.1434 (M + H)+.
Insertion of 1,4-Benzenedimethanol into DTC. The insertion
of 1,4-benzenedimethanol into DTC was carried out following the
general procedure described above, except at 50 °C due to the poor
1
solubilities of substrates in C6D6. H NMR (300 MHz, C6D6) δ 7.21
(s, 4H, HAr), 7.05−6.89 (m, 8H, HAr), 6.81−6.64 (m, 8H, HAr), 5.87
(s, 8H, NH), 5.37 (s, 4H, CH2), 2.12 (s, 6H, CH3), 1.97 (s, 6H, CH3).
13C{1H} NMR (75 MHz, C6D6) δ 149.50, 145.72, 136.33, 136.07,
132.03, 131.69, 130.24, 129.11, 127.95, 122.46, 120.60, 67.85, 20.40,
20.21. MS (APCI): m/z 583.02 (M + H)+.
Insertion of 1,4-Butynediol into DTC. The insertion of 1,4-
butynediol into DTC was carried out following the general procedure
described above, except at 50 °C due to the poor solubilities of
substrates in C6D6. 1H NMR (300 MHz, C6D6) δ 7.25−6.61 (m, 16H,
HAr), 5.94 (br, 2H, NH), 5.05 (br, 4H, CH2), 2.11 (br, 12H, CH3).
13C{1H} NMR (75 MHz, C6D6) δ148.71, 145.20, 135.78, 132.17,
131.71, 129.16, 123.78, 122.36, 120.73, 81.54, 53.99, 20.47, 20.30. MS
(APCI): 531.2822 (M + H)+.
Deuterium labeling studies using 2,4-tBu2PhOD and DTC indicated
that the insertion of DTC into CatA is the turnover limiting step
during the catalytic cycle, and the remaining steps of the cycle are
rapid, so the following rate equation can be obtained based on the
assumption of a steady-state:
∂p
= k1[CatA][DTC]
∂t
Insertion of 1,3-Butanediol into DTC. The insertion of 1,3-
butanediol into DTC was carried out following the general procedure
described above. For the monoisourea compounds 8a and 8b, molar
ratio of 1/1 between 1,3-butanediol and DTC was applied, and the
product ratio between 8a and 8b was calculated from CH assignment;
to this reaction mixture was added another equivalent of DTC
In the beginning step, formation of active CatA, that is, alcoholysis
of [(BimR /R N)AnN″3], is rapid and irreversible; however, the
equilibrium of alcohol coordination/decoordination to the actinide
center must be considered (this step has been verified in similar
benzimidazolin-2-iminato actinide systems in our previous report),69
giving the following expression:
1
2
1
affording the final diisourea products. H NMR (300 MHz, C6D6) δ
7.06−6.86 (m, 8H, HAr), 6.91−6.67 (m, 8H, HAr), 5.82 (d, 2H, NH),
5.69−5.55 (m, 1H, CH), 4.55 (t, J = 5.9 Hz, 2H, CH2), 2.08 (br, 6H,
CH3), 1.97 (br, 6H, CH3), 1.88−1.70 (m, 2H, CH2), 1.23 (d, J = 6.0
Hz, 1H, CH2). 13C{1H} NMR (75 MHz, C6D6) δ 149.72, 149.26,
145.99, 136.28, 136.18, 136.07, 134.79, 131.99, 131.46, 131.41, 130.15,
129.92, 129.16, 129.10, 123.88, 122.51, 120.85, 69.88, 63.24, 35.14,
20.36, 20.18, 19.68. MS (APCI: 557.2941 (M + Na)+.
[sat CatA]
[R′OH][CatA]
Keq
=
From the reaction cycle, it can be found that the sum of active
catalysts (CatA) and alcohol-saturated complex (sat CatA) is the total
amount of catalyst loading [An] used in the reaction: [CatA] + [sat
CatA] = [An]. After substitution of [CatA] from the equilibrium
expression into the rate equation, the following kinetic rate law is
obtained:
Stoichiometric Reactions between Complex 4 and Three
Equivalents of 2,4-tBu2PhOH To Prepare Active Species CatA.
A J. Young NMR tube was loaded with 20.0 mg (19.3 mmol) of
thorium complex 4 [(Bim5‑MeDipp/MeN)ThN″3], 3.0 equiv of
2,4-tBu2PhOH (12.0 mg, 58 mmol), and 500 μL of C6D6. Then the
tube was taken out of the glovebox and monitored by 1H NMR
spectroscopy. The release of HN(SiMe3)2 was immediately observed,
as evidenced from the appearance of the characteristic peak of the
methyl group at δ = 0.05 ppm, and the reaction can be completed in 1
h, affording the [(Bim5‑MeDipp/MeN)Th(O-2,4-tBu2Ph)3] intermediate
(Figure S44 and S45). [(Bim5‑MeDipp/MeN)Th(O-2,4-tBu2Ph)3]: 1H
NMR (300 MHz, C6D6) δ 7.50−7.35 (m, 4H, HAr), 7.03−6.85 (m,
9H, HAr), 6.62−6.39 (m, 2H, HAr), 3.52 (s, 3H, CH3), 2.62 (s, 2H,
CH), 1.56 (s, 27H, C(CH3)3), 1.49−1.38 (m, 12H, CH(CH3)2), 1.28
(s, 30H,C(CH3)3 and CH3), 1.03−0.82 (m, 12H, CH(CH3)2), 0.05 (s,
54H, Si(CH3)2). 13C NMR (75 MHz, C6D6) δ 162.24, 157.74, 148.51,
139.47, 136.06, 125.27, 124.96, 124.64, 123.77, 123.35, 122.89, 121.61,
34.70, 33.83, 31.58, 30.57, 30.35, 28.38, 22.67, 15.78.
∂p
∂t
[An][DTC]
= k
1(1 + Keq[R′OH])
From this equation, inverse-first order behavior can be concluded if
Keq[R′OH] ≫ 1.
Deuterium Labeling Studies. All the deuterium labeling
experiments were done with a similar method. In a J. Young NMR
tube, 2 mol % precatalyst 2, DTC (0.35 mmol, 50 equiv), and
2,4-tBu2PhOH (or 2,4-tBu2PhOD) (0.35 mmol, 50 equiv) were added
in the glovebox and then diluted to 0.5 mL using C6D6. The tube was
sealed and taken out of the glovebox and cooled in an ice bath until
1
the H NMR experiment began at 25 °C. The progress was initially
monitored every minute and then in longer intervals up to one and a
half hours. The product concentration was measured by the integrated
area ratio of tert-butyl group at 1.11 and 1.23 ppm, which were
assigned to the starting material and reaction product, respectively.
Reaction rates were determined by least-squares fit of the initial
product concentration versus time. The KIE value was calculated from
the ratio of the two slopes.
Kinetic Studies of 2,4-tBu2PhOH insertion into DTC Using
Complex 2. All the kinetic experiments were performed following a
similar method. In a J. Young NMR tube, 2 mol % precatalyst 2, DTC
(0.35 mmol, 50 equiv), 2,4-tBu2PhOH (0.35 mmol, 50 equiv), and
C6D6 were mixed in the glovebox, and then the tube was sealed. The
I
DOI: 10.1021/acs.organomet.7b00432
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Organometallics
Article
(11) Choy, S. W. S.; Page, M. J.; Bhadbhade, M.; Messerle, B. A.
Organometallics 2013, 32, 4726−4729.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.7b00432.
■
S
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Crystallographic data for complexes 1−4 and H NMR
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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.).
■
ORCID
Heng Liu: 0000-0002-6035-3428
Matthias Tamm: 0000-0002-5364-0357
Moris S. Eisen: 0000-0001-8915-0256
Author Contributions
(29) Broderick, E. M.; Gutzwiller, N. P.; Diaconescu, P. L.
The manuscript was written through contributions of all
authors. All authors have given approval to the final version of
the manuscript
Organometallics 2010, 29, 3242−3251.
(30) Hayes, C. E.; Platel, R. H.; Schafer, L. L.; Leznoff, D. B.
Organometallics 2012, 31, 6732−6740.
Notes
(31) Dash, A. K.; Wang, J. X.; Berthet, J. C.; Ephritikhine, M.; Eisen,
M. S. J. Organomet. Chem. 2000, 604, 83−98.
The authors declare no competing financial interest.
(32) Karmel, I. S. R.; Tamm, M.; Eisen, M. S. Angew. Chem. Int. Ed.
2015, 54, 12422−12425.
ACKNOWLEDGMENTS
■
(33) Haskel, A.; Straub, T.; Dash, A. K.; Eisen, M. S. J. Am. Chem. Soc.
1999, 121, 3014−3024.
This work was supported by the German Israel Foundation
(GIF) under Contract I-1264-302.5/2014 and by the PAZY
Foundation Fund (2015) administered by the Israel Atomic
Energy Commission, and H.L. was supported by the Technion-
Guangdong Fellowship.
(34) Haskel, A.; Wang, J. Q.; Straub, T.; Neyroud, T. G.; Eisen, M. S.
J. Am. Chem. Soc. 1999, 121, 3025−3034.
(35) Wang, J. Q.; Dash, A. K.; Berthet, J. C.; Ephritikhine, M.; Eisen,
M. S. Organometallics 1999, 18, 2407−2409.
(36) Wang, J. X.; Dash, A. K.; Kapon, M.; Berthet, J. C.; Ephritikhine,
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(37) Kosog, B.; Kefalidis, C. E.; Heinemann, F. W.; Maron, L.;
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