Addition of E-H (E = N, P, C, O, S) Bonds to Heterocumulenes Catalyzed by Benzimidazolin-2-iminato Actinide Complexes

Article abstract of DOI:10.1021/acs.organomet.7b00502

The synthesis and characterization of benzimidazolin-2-iminato actinide(IV) complexes [(Bim^R1/R2N)An(N{SiMe₃}₂)₃] (An = U, Th) (1-6) is reported. All complexes were obtained in high yields, and their solid state structures were established through single-crystal X-ray diffraction analysis. Using 1-6 as precatalysts, the addition of mono- and bifunctional E-H (E = N, P, C, O, S) substrates to various heterocumulenes, including carbodiimides, isocyanates, and isothiocyanates, was investigated, affording the respective addition products in high yields under very mild reaction conditions. Various amines were applicable to this reaction, indicating a large scope capability of amine nucleophiles for the insertion process.

Full text of DOI:10.1021/acs.organomet.7b00502

Article
pubs.acs.org/Organometallics
Addition of E−H (E = N, P, C, O, S) Bonds to Heterocumulenes
Catalyzed 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 and characterization of benzimi-
dazolin-2-iminato actinide(IV) complexes [(BimR1/R2N)An(N-
{SiMe3}2)3] (An = U, Th) (1−6) is reported. All complexes
were obtained in high yields, and their solid state structures
were established through single-crystal X-ray diffraction
analysis. Using 1−6 as precatalysts, the addition of mono-
and bifunctional E−H (E = N, P, C, O, S) substrates to various
heterocumulenes, including carbodiimides, isocyanates, and
isothiocyanates, was investigated, affording the respective
addition products in high yields under very mild reaction
conditions. Various amines were applicable to this reaction, indicating a large scope capability of amine nucleophiles for the
insertion process.
diversified substrates. Previous studies had shown that actinide
complexes displayed good tolerance toward nitrogen and
silicon heteroatoms.5,10,43−46 When it comes to oxygen-
containing substrates, however, catalytic transformations are
extremely challenging because of the formation of strong
oxygen-actinide bonds due to the high oxophilicities of the
metal centers (bond strength of 208.0 kcal mol−1 for Th−O
and 181.0 kcal mol−1 for U−O). So far, only a very limited
number of catalytic examples are available for organoactinide
mediated reactions involving oxygen-containing substrates,
such as the Tishchenko reaction,47−49 hydroalkoxylation,50,51
small molecule activation,35,52 and polymerization of cyclic
esters.53−55 On the basis of this consideration, a series of
oxygen-containing nucleophiles and heterocumulenes were also
investigated in the present work. Moreover, it has been well-
recognized that steric bulkiness and electronic properties are
powerful methods to manipulate the reactivity of organo-
actinide catalysts. Previous studies in our group proved that the
reactivity of coordinative unsaturated actinide complexes could
be increased by regulating the electronic properties of the
ancillary ligands.54,56 Therefore, in the present work, we
combine these steric and electronic factors together into a
new family of actinide complexes, and investigate their impact
on the hydroelementation process.
INTRODUCTION
■
Organoactinide complex-mediated hydroelementation pro-
cesses, usually realized by the addition of E−H (E = N, S, P,
Si, O) moieties to unsaturated bonds, have experienced a
phenomenal acceleration in research activity over the past two
decades. Some representative results that were made in this
field comprise hydrosilylation,1−4 hydroamination,5−10 hydro-
thiolation11−13 reactions, etc., revealing facile and atom-efficient
approaches for the construction of carbon-heteroatom bonds.
However, hydroelementation of heterocumulenes,14−27 which
straightforwardly affords guanidine-type compounds that have
considerable application in coordination chemistry and
biorelevant fields, has been scarcely investigated using actinide
precursors. Compared with transition metals and lanthanide
complexes, organoactinides exhibit metal centers bearing 5f
valence orbitals and very sizable ionic radii, which in turn
enable large coordination numbers and unusual coordination
geometries of the corresponding complexes.28−39 These
properties usually allow them to display unique catalytic
behaviors in organic transformations, such as enhanced catalytic
activities as well as regio- and chemoselectivities.1,5,40−42 These
considerations prompted us to thoroughly investigate the
aforementioned hydrofunctionalization of heterocumulenes
with the aid of various organoactinide systems. In addition,
previous reports on the hydrofunctionalization reactions by
using transition metals and lanthanide complexes usually
involve only one or two certain kinds of E−H nucleophiles
or heterocumulene substrates, and thus provide only a small
substrate scope. Therefore, scientifically, it is highly challenging
and demanding to develop some catalytic systems that exhibit
high tolerance toward heteroatoms and are able to catalyze
Our interest was focused on imidazolin-2-iminato-type
ligands, due to their highly nucleophilic and strongly basic
properties.57−60 In addition, through efficient stabilization of a
positive charge on the imidazolium ring, such ligands can be
Received: July 3, 2017
© XXXX American Chemical Society
A
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regarded as 2σ,4π-electron donors and thus as cyclopentadienyl
analogues. Previous work has shown that the nature of the
backbone has a great influence on the zwitterion structure of
the ligand and thereby on the properties of the M−N(CN)
bond.61 Inspired by this, we decided to incorporate a phenyl
ring in the backbone of the imidazolin-2-iminato ligand
(Scheme 1), based on the consideration that the presence of
Scheme 2. Synthetic Procedures for Benzimidazolin-2-
iminato Actinide Complexes [(BimR /R2N)AnN″3] (An = U,
1
Th; N″ = N(SiMe3)2)
Scheme 1. Mesomeric Structures of Imidazolin-2-iminato
(Top) and Benzimidazolin-2-iminato (Bottom) Ligands
the phenyl ring will impact the nature of the CN double
bond and thus influence the bonding properties of An−N
linkage (An = Th, U).62 Moreover, asymmetric ligands with
one small group on one side and a sterically demanding group
on the other side are preferred here, which will potentially
provide more space for the incoming substrates for accessing
the actinide centers.
RESULTS AND DISCUSSION
■
Synthesis and Characterization of Complexes. A series
of asymmetric benzimidazolin-2-iminato ligands bearing
substituents of different steric demand were prepared (see
the Supporting Information). Subsequent treatment with 1
equiv of the actinide metallacycles [(Me3Si)2N]2An[κ2-(N,C)−
CH2Si(CH3)2N(SiMe3)] (An = Th, U) at room temperature
for 12 h and recrystallization from toluene at −35 °C afforded
the target mono(benzimidazolin-2-iminato) actinide(IV) com-
plexes [(BimR1/R2N)AnN″3] (An = U, Th; N″ = N(SiMe3)2)
(1−6) in high yields (Scheme 2).63 Detailed crystallographic
data and selected bond lengths and angles are presented in
Table S1−S2 (Supporting Information).
Figure 1. Solid state structures of complexes 3 and 4 (thermal
ellipsoids are drawn at 50% probability). Hydrogen atoms are omitted
for clarity. (C1−N1:1.283(4) Å (3), 1.248(7) Å (4); N1−
An1:2.131(3) Å (3), 2.155(5) Å (4); C1−N1−An1:166.0(2)° (3),
164.6(4)° (4)).
X-ray analysis revealed that all six complexes are isostructural
(Figures 1−2), displaying distorted tetrahedral geometries
around the metal atoms. From the crystallographic data, a clear
trend can be found, i.e., the An−N(1) bond distances are
shorter, with values of 2.133(3), 2.200(4), 2.131(3), 2.155(5),
2.139(7) and 2.184(3) Å for 1, 2, 3, 4, 5 and 6, respectively,
compared to the An−Namino bond distances. The differences in
the U−N(1) bond distances for the uranium complexes, 1, 3, 5,
or among the thorium complexes 2, 4, 6 are neglectable.
Moreover, the An−N(1)−C(1) angles, with values of 161.0(2),
159.6(3), 166.0(2), 164.6(4), 176.0(5) and 175.2(3)° for 1, 2,
3, 4, 5 and 6, respectively, are more or less close to linearity.
These data indicate a substantial π-character and thus a higher
bond order of the An−N(1) bonds. Similar results were also
observed in other imidazolin-2-iminato-supported transition
metal, lanthanide and actinide complexes.57,59,64−66 In order to
make clear whether the presence of the benzene ring on the
imidazolin-2-imine backbone will influence the electronic
structure of the resulting complexes, a comparison between
the present six complexes and previously reported imidazolin-2-
Figure 2. Solid state structures of complexes 5 and 6 (thermal
ellipsoids are drawn at 50% probability). Hydrogen atoms are omitted
for clarity. (C1−N1:1.302(10) Å (5), 1.280(5) Å (6); N1−
An1:2.139(7) Å (5), 2.184(3) Å (6); C1−N1−An1:176.0(5)° (5),
175.2(3)° (6)).
iminato actinide [(ImRN)AnN″3] counterparts was per-
formed.67 Unexpectedly, no obvious changes of the structural
parameters were observed. For instance, for the [BimMe/MesN]
ligated U (3) and Th (4) complexes, the An−N(1) bond
distances are 2.131(3) Å (3) and 2.155(5) Å (4), which are
quite similar to the reported [ImMesN)AnN″3] counterparts
with values of 2.143(4) Å (U) and 2.189(7) Å (Th),
respectively. Similar phenomena were also observed for the
B
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[BimMe/tBuN] ligated U and Th complexes, displaying values of
2.139(7) Å (5) and 2.184(3) Å (6), vs 2.118(8) Å (U) and
2.176(8) Å (Th) in [ImtBuN)AnN″3] analogues. From these
results, it can be seen that the incorporation of the benzene ring
in the ligand backbone shows little influence on the electronic
properties of the corresponding complexes.
Table 1. Catalytic Reaction of Carbodiimide with N−H
a
Species Mediated by Complexes 1−6
b
c
run cat. RNCNR
HER1R2 /HER3 time (h) conv. (%)
prod.
1
1
2
3
4
5
6
1
2
1
2
1
2
1
2
1
2
1
2
1
2
1
2
1
2
1
2
1
2
DIC
PhNH2
6
96
8af
2
6
>99
98
3
6
In contrast to the little changes in the electronic properties,
the steric properties differ significantly as indicated by
comparison of the cone angles, which are defined as the
angle between the metal at the vertex of the cone, formed by
the coordinating ligands and the metal center, and the
hydrogen atoms at its perimeter. The present six complexes
display cone angles of 138° (1), 140° (2), 114° (3), 111° (4),
99° (5), 98° (6) respectively, which are much smaller as
compared to the corresponding values in all imidazolin-2-
iminato actinide [(ImRN)AnN″3] systems (with cone angles of
204° (R = Dipp, An = Th), 205° (R = Dipp, An = U), 139° (R
= Mes, An = Th), 138° (R = Mes, An = U), 117°(R = tBu, An
= Th), 118°(R = tBu, An = U) respectively), implying that the
new actinide complexes exhibit much more open space around
the metal for the incoming substrates. Besides, for the present
4
6
>99
95
5
6
6
6
96
7
DIC
DIC
DIC
DIC
DIC
DIC
DTC
DTC
DTC
DTC
DCC
pCl-PhNH2
pF-PhNH2
pNO2−PhNH2
pMe-PhNH2
oMeO-PhNH2
Morpholine
PhNH2
6
97
8ag
8ah
8ai
8
6
97
9
6
96
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
6
98
24
24
6
>99
95
>99
96
8aj
6
6
96
8ak
7al
6
>99
36
12
12
4
39
t
six complexes, changing the substituents from Dipp, Mes, Bu
>99
>99
>99
>99
>99
>99
77
8bf
7bl
7bm
7bn
8cf
affords a decrease of the cone angle, suggesting that catalytically
active species generated from 1 and 2 can be expected to be the
most sterically encumbered.
3
Morpholine
Et2NH
12
12
12
12
24
24
6
Catalytic Addition Reactions. The results of the catalytic
addition reactions of a series of REH (E = N, P, C, O, S)
moieties to N,N′-diisopropylcarbodiimide (DIC), N,N′-dicy-
clohexylcarbodiimide (DCC), 1,3-di-p-tolylcarbodiimide
(DTC), phenylisocyanate (PhNCO), and phenylisothiocyanate
(PhNSC) are presented below. The reaction of aniline PhNH2
with DIC was first investigated by using the different
precatalysts (1−6). As a control experiment, a blank reaction,
i.e., only DIC and PhNH2, was carried out at 75 °C for 24h, and
no formation of product was observed. In contrast, with the aid
of a catalytic amount (2 mol %) of the benzimidazolin-2-
iminato actinide complexes 1−6, fast guanylation reactions
took place, affording 7af in high yields, which then underwent a
1,3-hydride shift and gave the final product 8af (Table 1, runs
1−6). A representative preparative scale-up reaction using
complex 2 was performed, giving 8af in high yield (97.8%).
Additionally, no significant differences were observed when
varying the steric structure of the ligand and the type of actinide
center, indicating the high efficiency of all six complexes.
Compared to previous guanylation processes promoted by
[ImDippN)ThN″3],45 higher conversions, even in a shorter
amount of time, were observed herein, suggesting a higher
iPr2NH
69
PhNH2
>99
>99
6
a
Reaction conditions: 2 mol % (7 μmol) precatalyst, 50 equiv of
b
substrates, 600 μL C6D6, 75 °C. DIC: iPrNCNiPr; DTC: (p-
tol)NCN(p-tol); DCC: CyNCNCy. Determined by H NMR of the
c
1
crude reaction mixture.
Scheme 3. Catalytic Insertion of Amines N−H into
Carbodiimide
reactivity of the [(BimR /R2N)AnN″3] complexes. These
1
superior results can be explained by the decreased cone angles
around the catalytically active actinide species, which provides
better accessibility of substrate to the metal center and thus
results in higher catalytic activities.
runs 15−16, Table 1) allows the reaction to go to completion
in 6 h. However, using p-nitroaniline (p-NO2−PhNH2, runs
11−12, Table 1), a much longer reaction time (24 h) was
needed in order to achieve full consumption of the starting
materials, indicating the decreased nucleophilicity due to the
presence of the strong electron-withdrawing nitro group.
Besides varying electronic effects, substituted anilines with
bulkier groups in the ortho-position, e.g., 2,6-Me2PhNH2,
2,6-iPr2PhNH2, etc., were also tried for this reaction,
unfortunately, no reactions were observed, suggesting the
inaccessibility of the actinide center for sterically encumbered
anilines.
The uranium (1) and thorium (2) complexes were then
chosen as precatalysts for the addition of various amines to
carbodiimides (Scheme 3). Representative results are summar-
ized in Table 1. It can be observed that a wide range of primary
and secondary amines can be used for this reaction. C−X (X =
Cl, F, O, NO2) bonds survived during the reaction, indicating a
good functional group tolerance of the active species. The
presence of either electron-withdrawing or electron-donating
substituents on the aniline showed big impact on the reactivity
of the starting materials in the allotted time. For instance,
reacting aniline with electron-donating groups (o-MeO-PhNH2,
C
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Similar reactions were performed using DTC and DCC as
the heterocumulenes (runs 19−20, 27−28, Table 1), from
which, in comparison with DIC, no different catalytic behavior
was revealed for the substrate DCC; in contrast, it took only
half of the reaction time for the full consumption of DTC,
implying an accelerated reaction rate for this substrate. This
superior reactivity is an expected corollary of the increased
electrophilic property of the NCN moiety, due to the presence
of the phenyl ring in DTC, which resulted in a rapid insertion
reaction and more labile An−N bonds afterward. Moreover, it
should be noted that DTC can react with the aliphatic
Table 2. Catalytic Reaction of Carbodiimide,
Phenylisocyanate and Phenylisothiocyanate with E−H
a
Species Mediated by Complexes 1−6
RNCNR/
HER1R2
time
(h)
conv.
c
b
run cat.
PhNCX
/HER3
(%)
prod.
1
1
2
3
4
5
6
1
2
1
2
1
2
1
2
1
2
1
2
1
2
1
2
DTC
Ph2PH
6
>99
>99
>99
>99
>99
>99
69
7bo
2
9
3
9
4
9
5
9
6
9
i
secondary amines morpholine, Et2NH, and Pr2NH, albeit in
longer times, affording the target products 7bl, 7bm, 7bm, in
7
DTC
DTC
DTC
DTC
PhCCH
6
9bs
8
6
>99
>99
>99
>99
>99
>99
>99
69
high yields, whereas similar reactions proved to be inaccessible
9
PhCH2OH
PyCH2OH
PhCH2SH
PhCH2SH
Ph2PH
1
9bp
i
for DIC (for Et2NH, Pr2NH) or led to much lower yields of
10
11
12
13
14
15
16
17
18
19
20
21
22
1
the corresponding products (for morpholine), which reflects
once again the higher reactivity of DTC.
Encouraged by the extraordinarily high catalytic activities and
the unusual functional group tolerance in the aforementioned
guanylation reactions, our attention was turned to the
possibility of addition of other E−H moieties (PH, CH, OH,
SH) to carbodiimides (Scheme 4, Table 2). DTC was selected
1
9bq
1
6
9br
6
PhNCO
12
12
12
12
12
12
12
12
11dr
10do
11er
10eo
91
58
32
Scheme 4. Catalytic Insertion of E−H into Different
Heterocumulenes
PhNCS
PhCH2SH
Ph2PH
73
58
70
63
a
Reaction conditions: 2 mol % (7 μmol) precatalyst, 50 equiv of
substrates, 600 μL C6D6, 75 °C, runs 5−10 were studied at room
b
c
temperature. DTC: (p-tol)NCN(p-tol). Determined by 1H NMR of
the crude reaction mixture.
insertion of DTC into the E−H bonds of PhCH2OH,
PyCH2OH, PhCH2SH, affording the corresponding isourea
and isothiourea products quantitatively at room temperature.
The presence of a coordinating pyridyl group showed negligible
influence on the activity, pointing at a potentially good
tolerance of heterocyclic functionalities. The substrate scope
of the insertion reaction was further expanded to isocyanates
and isothiocyanates. Using phenylisocyanate and phenyl-
isothiocyanate, as the heterocumulene source, the addition of
Ph2PH and PhCH2SH was studied, affording products 10 and
11 in moderate to high yields. These results demonstrate the
generation of a quite robust catalytically active species from
complexes 1 and 2, which paves the way for further
transformation of oxygen- and sulfur-containing substrates by
employing organoactinide precursors.
To further increase the scope of the precatalysts and to
achieve the synthesis of bis(guanidine) compounds, that
potentially serve as templates for the constructing of larger
molecules, bifunctional substrates were also applied in this
catalytic addition reaction (Scheme 5, Table 3). With 2 mol %
loadings of the precatalysts 1 and 2, the reaction between
homobifunctional compounds, the diamines (m-NH2PhNH2, p-
NH2PhNH2) and the diol (p-HOCH2−PhCH2OH), with DTC
in a 1:2 molar ratio produced the bis(guanidine) 13at, 13au
and the bis(isourea) 12bv in almost quantitative yields.
Reacting the heterobifunctional substrate 2-propyn-1-ol,
which contains hydroxyl (OH) and terminal alkyne (C
CH) as different nucleophiles, with an equimolar amount of
DTC afforded compound 14bw with an isourea structure in
high yields, indicating the superior reactivity of the hydroxyl
group as compared to the terminal alkyne toward the metal
here due to its higher reactivity as revealed above. Reactions
between diphenylphosphine (Ph2PH) and DTC were first
carried out by using complexes 1−6 as catalyst precursors, and
quantitative conversions to phosphaguanidine (7bo) were
observed. Subsequent experiments using phenylacetylene
(PhCCH) as the nucleophile for the addition to DTC, and
the thorium precatalyst 2 was found to be more active than its
uranium congener 1 in these reactions, in which product yields
of 99% and 69% were furnished, respectively. The superior
activity of complex 2 can probably be ascribed to different
electronic properties of the actinide centers. In contrast to the
catalytic addition of phosphine and terminal alkynes to
carbodiimides, which have been studied using organolanthanide
precursors, alcohol and thiol moieties containing substrates
have long been neglected. This is due to the catalyst poisoning,
emanating from the strong oxygen and sulfur bonds with the
actinide metals (Th−S 145.2, and U−S 121.9 kcal mol−1).68
Pioneering work was accomplished recently by our group
when using sterically less congested actinide amido complexes.
An unprecedented actinide-catalyzed addition of alcohols to
carbodiimides was achieved,51 implying an unusually high
activity of An−O bonds toward carbodiimides. Therefore, the
use of OH and SH containing nucleophiles was also attempted
herein, and it was exciting to find that complexes 1 and 2 are
also capable of serving as highly efficient catalysts for the
D
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heating at 50 °C for 24 h (specific NMR spectra are presented
in the Supporting Information), indicating that the sterically
encumbered bis(trimethylsilyl)amido moieties, N(SiMe3)2,
cannot add to the carbodiimide. In contrast, reactions between
catalyst 2 and 3 equiv of PhNH2 gave instantly the
corresponding thorium anilide compounds, and the concom-
itant appearance of the peak of free amine HN(SiMe3)2. After
heating at 50 °C for 24 h, all Th−N(SiMe3)2 moieties were
replaced by Th-NHPh, with simultaneous release of 3 equiv of
HN(SiMe3)2. To this reaction mixture another 3 equiv of DIC
were added and immediate NMR spectroscopic changes were
observed for the iPr group (from DIC), indicating an effective
insertion of the carbodiimide into the thorium-anilide bonds.
On the basis of these results, it can be concluded that the Th-
NHPh moiety, originating from reaction between catalyst 2 and
aniline, is the operative active species during this catalytic cycle.
Kinetic studies of catalytic addition of PhNH2 to DIC by
using precatalyst 2 revealed a first-order dependence on DIC,
PhNH2 and 2, giving rise to the rate eq 1:69
Scheme 5. Catalytic Insertion of Bifunctional Substrates into
Carbodiimide
∂p
∂t
= kobs·[2][DIC][PhNH2]
(1)
Deuterium labeling studies with aniline-N,N-d2 afforded a
kinetic isotopic effect (KIE) of KH/KD = 1.37, corroborating
that the rate-determining step (r.d.s.) of the catalytic cycle is
the protonolysis of the guanidinate. The thermodynamic
activation parameters were experimentally calculated from the
Erying and Arrhenius plots (Supporting Information), affording
a rather low activation barrier (Ea) of 11.3(6) kcal mol−1, and
the enthalpy (ΔH‡) and entropy (ΔS‡) of activation of 10.7(3)
kcal mol−1 and −45.0(6) e.u. respectively. The latter value
indicates as expected for actinides, a four-centered ordered
transition state.
On the basis of the above analysis, a plausible catalytic
mechanism for the addition of RE−H moieties into
carbodiimides could be proposed as shown in Scheme 6. In
the initial step, the actinide amido compound An−N(SiMe3)2
undergoes protonolysis with protonated RE−H, affording the
active species A, which then undergoes a rapid equilibrium for
the nucleophilic addition reaction to carbodiimide, giving rise
to the actinide guanidinate species B. A subsequent slow
Table 3. Catalytic Reaction of Carbodiimide and
Bifunctional Substrates HE−R′−E′H by Complexes 1−2
a
time conv.
b
run
cat.
HE−R′−E′H
(h)
(%)
prod.
1
1
2
1
2
1
2
1
E = E′ m-NH2−PhNH2
6
>99
>99
>99
>99
>99
>99
>99
13at
2
3
4
5
6
7
6
p-NH2−PhNH2
6
13au
12bv
14bw
6
p-HOCH2−PhCH2OH
12
12
3
E ≠ E′ HO−CH2CCH (E =
O, E′ = C)
8
9
2
1
3
>99
>99
d
5-NH2−Indole (E =
NH, E′ = N)
24
15ax
16ax
cd
,
10
1
24
>99
a
Reaction conditions: 2 mol % (7 μmol) precatalyst, 50 equiv of
b
1
substrates, 600 μL C6D6, 75 °C. Determined by H NMR of the
c
d
Scheme 6. Plausible Mechanism for Catalytic Insertion of
REH into Carbodiimide
crude reaction mixture. Another 1.0 equiv of DTC was added. 110
°C in toluene-d was applied; molar ratio of [DIC]/[5-NH2−Indole] =
1/1 and 2/1 was used for runs 9 and 10, respectively.
atom. Further addition of 1 equiv of DTC to this reaction
mixture led to unidentifiable polymeric products, which could
not be isolated. The reaction of 5-aminoindole with 1.0 equiv of
DIC proceeded selectively at the primary amino group to afford
the monoguanidine compound 15ax, demonstrating that
primary amino groups can be distinguished from secondary
amino groups under the present reaction conditions. Heating
this reaction mixture with one more equivalent of DIC gave rise
to the bis(guanidine) 16ax in an almost quantitative yield.
Compound 16ax can be also obtained by reaction of 5-
amidoindole with 2.0 equiv of DIC in a one-pot reaction.
To shed light on the active catalyst species, stoichiometric
experiments using the system composed of DIC, PhNH2 and
catalyst 2 were performed. The initial reaction between catalyst
2 and 3 equiv of DIC was studied by in situ NMR spectroscopy
in C6D6; unfortunately, no reaction was observed even after
E
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the solvent was removed partially, recrystallization from concentrated
protonolytic cleavage by an additional molecule of RE−H
completes the catalytic cycle with the concomitant releasing of
the target product and the regeneration of the active species A.
toluene solution afforded the target complex in high yield. Yield: 0.13
1
g, (0.14 mmol, 93%). H NMR (300 MHz, C6D6) δ 96.84 (s, 3H,
CH3), 26.33 (d, J = 7.8 Hz, 1H, HAr), 14.88 (d, J = 7.8 Hz, 1H, HAr),
11.80 (d, J = 7.1 Hz, 1H, HAr), 8.12 (d, J = 7.1 Hz, 1H, HAr), 0.24 (s,
1H, HAr), 0.06 (br, 3H, CH3), −4.33 (s, 1H, HAr), −5.07 (s, 3H, CH3),
−5.61 (s, 3H, CH3), −8.26 (s, 9H, Si(CH3)3), −11.47 to −12.19 (br,
45H, Si(CH3)3). 13C NMR (75 MHz, C6D6) δ 173.29 (CN), 170.16
(CAr), 157.21(CAr), 132.37(CAr), 129.24(CAr), 128.44(CAr), 125.56-
(CAr), 124.67(CAr), 124.17(CAr), 115.93(CAr), 113.52(CAr), 112.78-
(CAr), 31.64(CH3), 22.49(CH3), 19.99(CH3), 5.88(Si(CH3)3). Anal.
Calcd For C35H72N6Si6U: C, 42.74; H, 7.38; N, 8.54. Found: C, 42.89;
H, 7.30; N, 8.67.
CONCLUSION
■
The synthesis of mono(benzimidazolin-2-iminato) actinide(IV)
complexes [(BimR /R2N)An(N{SiMe3}2)3] was performed by
1
treating actinide metallacycles with benzimidazolin-2-imine
ligands to afford 1−6 in high yields. These catalysts were
found to be highly active in catalyzing the addition of E−H (E
= N, P, C, O, S) moieties to different heterocumulenes,
including carbodiimides, isocyanates and thioisocyanates,
affording the corresponding products in high yields under
mild reaction conditions. Bifunctional substrates were found to
be suitable substrates, and a series of bis(guanidine) and
bis(isourea) compounds were obtained. For the present system,
diversified substrates bearing various functionalities can be
employed, indicating a high tolerance of the catalytically active
species toward heteroatoms. It can be deduced from the kinetic
studies that the insertion reactions follow first order depend-
ence on both of the two substrates and the precatalyst, and
deuterium-labeling studies showed that the protonolytic
cleavage of the product is the rate-determining step of the
catalytic cycle.
Synthesis of Complex 4. Preparation of complex 4 was carried out
using similar method as (3) using thorium metallacycle (0.11 g, 0.15
mmol) and benzimidazolin-2-imine ligand BimMes/MeNH (0.04 g, 0.15
mmol). Yield: 0.14 g, (0.14 mmol, 97%). 1H NMR (300 MHz, C6D6)
δ 6.93−6.84 (m, 1H, HAr), 6.82−6.73 (m, 1H, HAr), 6.72 (s, 2H, HAr),
6.63−6.53 (m, 1H, HAr), 6.19 (d, J = 7.7 Hz, 1H, HAr), 3.34 (s, 3H,
CH3), 2.12 (s, 3H, CH3), 1.98 (s, 6H, CH3), 0.36 (s, 54H, Si(CH3)3).
13C NMR (75 MHz, C6D6) δ 143.73(CN), 138.38(CAr), 137.10-
(CAr), 131.24(CAr), 130.73(CAr), 130.32(CAr), 129.33(CAr), 128.85-
(CAr), 121.20(CAr), 121.09(CAr), 107.40(CAr), 106.49(CAr), 28.54-
(CH3), 20.64(CH3), 17.90(CH3), 4.37(Si(CH3)3). Anal. Calcd For
C35H72N6Si6Th: C, 43.00; H, 7.42; N, 8.60. Found: C, 43.12; H, 7.48;
N, 8.69.
Synthesis of Complex 5. Preparation of complex 5 was carried out
using similar method as (3) using uranium metallacycle (0.11 g, 0.15
mmol) and benzimidazolin-2-imine ligand BimtBu/MeNH (0.03 g, 0.15
mmol). Yield: 0.12g, (0.13 mmol, 89%). 1H NMR (300 MHz, C6D6) δ
44.50−40.37 (b, 3H, CH3), 18.16 (b, 9H, C(CH3)3), 17.12 (m, 1H,
HAr), 13.49 (m, 1H, HAr), 13.30 (m, 1H, HAr), 0.27 to −0.22 (m, 1H,
HAr), −11.10 (b, 54H, Si(CH3)3). 13C NMR (75 MHz, C6D6) δ
166.75(CN), 161.14(CAr), 130.94(CAr), 129.28(CAr), 128.84(CAr),
125.26(CAr), 120.98(CAr), 81.82(C(CH3)3), 39.23(CH3), 2.14(Si-
(CH3)3). Anal. Calcd For C30H70N6Si6U: C, 39.10; H, 7.66; N, 9.12.
Found: C, 39.00; H, 7.57; N, 9.03.
Synthesis of Complex 6. Preparation of complex 6 was carried out
using similar method as (3) using thorium metallacycle (0.11 g, 0.15
mmol) and benzimidazolin-2-imine ligand BimtBu/MeNH (0.03 g, 0.15
mmol). Yield: 0.11 g, (0.12 mmol, 82%). 1H NMR (300 MHz, C6D6)
δ 7.31−6.76 (m, 3H, HAr), 6.67−6.37 (m, 1H, HAr), 3.30 (s, 3H, CH3),
1.87 (s, 9H, C(CH3)3), 0.68−0.48 (br, 54H, Si(CH3)3). 13C NMR (75
MHz, C6D6) δ 144.48(CN), 131.68(CAr), 129.80(CAr), 125.43(CAr),
120.60(CAr), 112.45(CAr), 106.87(CAr), 57.54(C(CH3)3), 30.39(CH3),
28.20(CH3), 4.92(Si(CH3)3), 3.77(Si(CH3)3). Anal. Calcd For
C30H70N6Si6Th: C, 39.36; H, 7.71; N, 9.18. Found: C, 39.45; H,
7.88; N, 9.25.
EXPERIMENTAL SECTION
■
General Considerations. 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 an 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). Aniline was refluxed over stannous chloride and
distilled under vacuum, followed by refluxing in calcium hydride under
nitrogenous atmosphere and distilling under vacuum. 1,3-di-
isopropylcarbodiimide (DIC), phenylisocyanate, phenylisothiocyanate,
ortho-anisidine, 4-fluoroaniline, diethylamine, benzyl alcohol, 2-
pyridinemethanol and benzylmercaptan were distilled from sodium
bicarbonate or CaH2 under nitrogen atmosphere. 1,3-Di-p-tolylcarbo-
diimide (DTC), 4-chloroaniline, 4-nitroanline, 4-methylaniline were
dried under vacuum (10−6 atm) for 12 h on a high vacuum line.
Phenylacetylene (ABCR) was distilled under vacuum and degassed by
three freeze−pump−thaw cycles. Diphenylphosphine (Sigma-Aldrich)
was used as received and stored in the glovebox. All the
aforementioned reagents were stored in an inert atmosphere glovebox
prior to use. The benzimidazolin-2-imine ligand BimDipp/MeNH was
prepared according to previous reports,62 and the other two ligands
BimMes/MeNH and BimtBu/MeNH were prepared using similar methods,
which can be found in the Supporting Information. The actinide
metallacycles were prepared according to previous reports.70
Benzimidazolin-2-iminato actinide complexes 1 and 2 were prepared
according to our previous work.62 PhND2 was synthesized according
to previous reports.46 NMR spectra were recorded on Bruker Avance
General Procedure for the Catalytic Addition of Nucleo-
philes into Heterocumulenes. A sealable J. Young NMR tube was
loaded with approximate 5 mg of the desired catalyst from a stock
solution in C6D6 inside the glovebox, followed by the addition of
heterocumulenes (50 equiv) and nucleophiles (50 equiv), and the
reaction was immediately diluted to 600 μL with C6D6. Samples were
taken out of the glovebox and the reaction progress was monitored by
1
1H NMR spectroscopy. The crude mixtures were analyzed using H
NMR, 13C NMR and mass spectrometry, the values were compared to
previous literature.
Kinetic Studies of PhNH2 into DIC Using Complex 2. All the
kinetic experiments were done in a similar method. In a J. Young
NMR tube, typical amount of precatalyst 2, DIC, aniline and C6D6 was
added in the glovebox and then the tube was sealed. Take the tube out
of the glovebox and freeze it in 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 data was
collected every 2 min up to one and a half hours. The product
concentrations were measured by the area ratio of methyl group at
0.99 and 0.84 ppm, which were assigned to the starting material and
product respectively normalized with an internal standard. Initial
reaction rates were determined by least-squares fit of product
1
300 and Avance 400 Bruker spectrometers. Chemical shifts for H
NMR and 13C NMR measurements are reported in ppm and
referenced using residual proton or carbon signals of the deuterated
solvent relative to tetramethylsilane. (Warning: uranium (primarily
isotope 238U) and natural thorium (232Th) are weakly radioactive with
a half-life of 4.47 × 109 years and 1.41 × 1010 years, respectively).
Synthesis of Complex 3. Complex 3 was prepared using a similar
method as our previous work.62 A solution of uranium metallacycle
(0.11g, 0.15 mmol) in 2 mL toluene was treated with 1.0 equiv of
benzimdiazol-2-imine ligand (0.040 g, 0.15 mmol) in 3 mL toluene.
The reaction mixture was stirred at room temperature for 12 h, and
F
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Article
concentration versus time, and the plots were shown in Figures S1−
S6.
Notes
The authors declare no competing financial interest.
Activation parameters including enthalpy (ΔH‡), entropy (ΔS‡)
and activation energy (Ea) were calculated from kinetic data using
Eyring and Arrhenius plots. In a typical sample, the J. Young tube was
loaded with desired amount of catalyst 2, DIC, aniline and sealed.
Then the sample was inserted into Bruker Avance 300 spectrometer,
which had been previously set to the desired temperature. The data
was collected every 1 min up to one and a half hours. Initial reaction
rates were determined by the least-square fit of product concentration
versus time, and Eyring and Arrhenius plots were shown in Figures
S7−S9. Enthalpy (ΔH‡), entropy (ΔS‡) and activation energy (Ea)
were calculated from the slope and intercept of the least-squares fit.
The Rate Kinetic Law. According to the proposed mechanism
(Scheme 6), the protonation step is the rate-determining step (k2),
based on the kinetic isotope effect, which is well-known to be in this
order of magnitude for RDS in four centered transition states with
actinides and lanthanides complexes.71 Therefore, we can assume a
steady-state approximation giving the following rate equations:
ACKNOWLEDGMENTS
■
This work was supported by the German Israel Foundation
GIF under Contract I-1264-302.5/2014, and H.L. thanks to the
Technion-Guangdong Fellowship.
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AUTHOR INFORMATION
Corresponding Authors
*E-mail: m.tamm@tu-bs.de.
*E-mail: chmoris@tx.technion.ac.il. Phone: +972-4-8292680.
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ORCID
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Matthias Tamm: 0000-0002-5364-0357
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