Actinide complexes possessing six-membered N-heterocyclic iminato moieties: Synthesis and reactivity

Article abstract of DOI:10.1021/acs.organomet.7b00037

A novel class of ligand systems possessing a sixmembered N-heterocyclic iminato [perimidin-2-iminato (Pr^RN, where R = isopropyl, cycloheptyl)] moiety is introduced. The complexation of these ligands with early actinides (An = Th and U) results in powerful catalysts [(Pr^RN)An(N{SiMe₃)₂}₃] (3-6) for exigent insertion of alcohols into carbodiimides to produce the corresponding isoureas in short reaction times with excellent yields. Experimental, thermodynamic, and kinetic data as well as the results of stoichiometric reactions provide cumulative evidence that supports a plausible mechanism for the reaction.

Full text of DOI:10.1021/acs.organomet.7b00037

Article
pubs.acs.org/Organometallics
Actinide Complexes Possessing Six-Membered N‑Heterocyclic
Iminato Moieties: Synthesis and Reactivity
Tapas Ghatak, Natalia Fridman, and Moris S. Eisen*
Schulich Faculty of Chemistry, Technion − Israel Institute of Technology, Haifa City, 32000, Israel
S
* Supporting Information
ABSTRACT: A novel class of ligand systems possessing a six-
membered N-heterocyclic iminato [perimidin-2-iminato
(PrRN, where R = isopropyl, cycloheptyl)] moiety is
introduced. The complexation of these ligands with early
actinides (An = Th and U) results in powerful catalysts
[(PrRN)An(N{SiMe3)2}3] (3−6) for exigent insertion of
alcohols into carbodiimides to produce the corresponding
isoureas in short reaction times with excellent yields.
Experimental, thermodynamic, and kinetic data as well as the
results of stoichiometric reactions provide cumulative evidence
that supports a plausible mechanism for the reaction.
he rapid growth in the field of organometallic and
coordination chemistry concerning early actinide com-
An alternative method for the formation of the carbon−
T
heteroatom bonds is the insertion of E−H (E = N, O, P, S) into
various heterocumulenes, affording guanidine, isoureas, phos-
phaguanidine, and thiourea products, with great potential
applications in coordination,12 and medicinal chemistry.13
In a recent report, we disclose that, although the imidazolin-
2-iminato complexes, [(ImRN)An{(N(SiMe3)2}3] (An = Th,
U), are excellent catalysts for the insertion of nonoxygenated
species into various heterocumulenes, the challenging insertion
of oxygenated substances into carbodiimides remained
infeasible.14,15 We have recently disclosed that highly con-
strained and sterically opened actinide metallacycles
[{(Me3Si)2N}2An{κ2-C,N-CH2Si(CH3)2N(SiMe3)}] (An =
Th (1), U (2)) can be utilized as efficient precatalysts for the
exigent insertion, for actinides, of alcohols into carbodiimides.16
The difference of the aforementioned two actinide systems is
the presence of a highly steric encumbered imidazolin-2-
iminato ligand, which hindered the substrates to come in close
proximity with the active center. Inspired by this finding, we
sought to design a new family of highly nucleophilic ligands,
which are isolobal with imidazolin-2-iminato or cyclopenta-
dienyl moieties.17 The N-heterocyclic iminato species have
been restricted to the five-membered heterocyclic rings,
whereas A and B represent the leading architectures of this
type of ligands (Scheme 1).
pounds (thorium and uranium) has been the subject of intense
investigation over the past three decades that has achieved a
high level of research elegance.1 These compounds have drawn
special attention due to their remarkable performances in
catalytic transformations because of their unique structure−
reactivity relationships.2 The state of the art in particular
includes a large variety of actinide-mediated carbon heteroatom
bond formations, which are generally achieved by the
hydroelementation of unsaturated bonds such as the hydro-
amination,3 hydrothiolation,4 hydrosilylation,5 etc. These types
of catalytic processes are of interest since they are involved in
an atom-economical route for the synthesis of various families
of organic molecules.6 However, taking into consideration the
high oxophilicity of the actinides, which results in strong An−O
bond strength (Th−O = 208.0 kcal/mol, U−O = 181.0 kcal/
mol),7 catalytic transformations with oxygen containing
substrates remain a major challenge. A limited number of
catalytic transformations involving oxygen-containing substrates
have been reported, such as the catalytic dimerization of
aldehydes to esters promoted by actinide complexes bearing the
Cp* (Cp* = C5Me5) or the isolobal imidazolin-2-iminato
8
t
ligands (ImRN, where R = Bu, Dipp, Mes), the ring-opening
polymerization of lactones catalyzed by Cp*2Th(ImRN)(Me)9
and [UO2(OAr)2(THF)2],10 and the ring-opening polymer-
ization of propylene oxide and cyclohexene oxide mediated by
uranyl aryloxide, [UO2(OAr)2(THF)2], uranyl chloride
[UO2Cl2(THF)3] or [UO2Cl2(THF)2]2 as precatalyst.11
However, the most remarkable example has been the
intramolecular hydroalkoxylation/cyclization of alkynyl alco-
hols affording cyclic ethers mediated by the constrained
geometry catalysts (CGC)Th(NMe2)2.4a This latter reaction
was long thought to be unrealistic due to the potential
formation of intractable actinide-oxo species.
Strategic modification of these core structures can be
achieved by three methods: (1) change in wingtip substitutions,
(2) modification of backbone, and (3) expansion of ring size.
Hence, we sought to design N-heterocyclic imine moieties
around a unique perimidine core, in order that all
Received: January 15, 2017
© XXXX American Chemical Society
A
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The selected bond distances and angles of complexes 3−6 are
presented in Table 1.
Scheme 1. Schematic Presentation of the Imidazolin-2-
iminato (A), the Imidazolidin-2-iminato (B), and the
Perimidin-2-iminato Core (Bottom) Systems
Table 1. Selected Bond Lengths (Å) and Angles (deg) for
Complexes 3−6
complex 3
complex 4
complex 5
complex 6
An−N1
An−N4
An−N5
An−N6
2.190(5)
2.380(4)
2.370(5)
2.361(5)
1.289(7)
162.7(4)
108.51(16)
96.38(16)
103.61(16)
118.56(15)
108.25(15)
119.16(16)
113.2
2.225(7)
2.335(6)
2.312(5)
2.385(6)
1.288(11)
156.1(5)
94.5(2)
2.128(4)
2.308(4)
2.313(4)
2.294(4)
1.295(6)
163.4(4)
109.91(15)
95.68(15)
102.67(14)
119.76(14)
107.78(14)
118.60(13)
114.0
2.156(7)
2.287(6)
2.328(7)
2.280(6)
1.294(12)
157.6(6)
93.4(2)
N1−C1
An−N1−C1
N1−An−N4
N1−An−N5
N1−An−N6
N4−An−N5
N4−An−N6
N5−An−N6
cone angle
121.0(2)
104.8(3)
114.1(2)
117.8(2)
104.9(2)
124.4
124.6(2)
103.9(3)
113.9(2)
116.8(2)
104.6(3)
123.5
aforementioned alterations would be achieved on a single
ligand scaffold (Scheme 1 - bottom).
These structural adaptations influence the donor properties
of the imine nitrogen as well as impose geometric constraints
on the N-substituents compared to well-known imidazolin-2-
imine motifs.
The mono(perimidin-2-iminato) Th(IV) complexes (3 and
4) were isolated as white crystalline compounds in high yields.
The solid state structures of 3 and 4 are depicted in Figure 1a,
b, respectively.
In the present study, we introduce new highly electron-rich
six-membered N-heterocyclic iminato ligands (perimidin-2-
iminato) and demonstrate their general synthesis containing
alkyl N-substituents. The synthesis of the corresponding
actinide(IV) complexes and their detailed structural examina-
tion bearing the perimidin-2-iminato moiety, as well as their
applications toward the challenging addition of alcohol to
diisopropylcarbodiimide (DIC) and 1,3-di-p-tolylcarbodiimide
(DTC) to form the corresponding isoureas from moderate to
excellent yields, under mild reaction condition, and short
reaction times, are addressed. The presented study presents the
scope of substrates, the effect of ancillary ligands, kinetics and
thermodynamics studies of this challenging insertion reaction.
To the best of our knowledge, this is the first disclosure of any
organometallic complex containing the six-membered N-
heterocyclic iminato system.
The synthesis of actinide(IV) complexes 3−6 was accom-
plished by a one-pot reaction between the actinide metalla-
cycles 1 or 2 with 1 equiv of respective perimidin-2-imine at
room temperature in toluene (Scheme 2). The reaction mixture
was stirred for 12 h, and recrystallization from concentrated
toluene solutions at −35 °C afforded X-ray quality crystals. The
molecular structures of [(L1)An{N(SiMe3)2}3] and [(L2)An-
{N(SiMe3)2}3] (where L1 = (1,3-diisopropyl-1H-perimidine-
2(3H)-imine), L2 = (1,3-dicycloheptyl-1H-perimidine-2(3H)-
imine), and An = Th, U) were determined by X-ray diffraction.
Figure 1. Molecular structure of complexes 3 (a) and 4 (b), with
thermal ellipsoids set at the 50% probability levels. All hydrogen atoms
are omitted for the clarity.
The X-ray structural analysis revealed that the central Th(IV)
is in a distorted tetrahedral geometry consisting of one
perimidin-2-iminato ligand and three N-silylated amido (N{Si-
(CH3)3}2) groups. The perimidin-2-iminato ligand binds with
the metal in a monodentate fashion, and the remaining three
positions are occupied by amido ligands, completing the
tetrahedral environment. The N1−C1ipso bond distances are
1.289(7) and 1.288(11) Å for complexes 3 and 4, respectively,
which are analogous to the corresponding bond lengths in
related imidazolin-2-iminato complexes.8c The Th−N1 bond
distances in complexes 3 and 4 are 2.190(5) and 2.225(7) Å,
respectively, which are found to be on average 0.19 Å shorter
than the Th−Namido bond length in complex 3 and 0.12 Å
shorter in complex 4. The small bent Th−N1−C1 angles
162.7(4)° and 156.1(5)° for complexes 3 and 4, respectively,
allow a substantial π donation from the ligand to the metal,
indicating a double bond between the metal and the imine
nitrogen.
Scheme 2. Synthesis of Mono(perimidin-2-iminato)
Actinide(IV) Complexes 3−6
B
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The mono(perimidin-2-iminato) U(IV) complexes were
isolated as brown crystalline compounds also in very high
yields. The molecular structure of the [(L1)U{N(SiMe3)2}3]
(5) and [(L2)U{N(SiMe3)2}3] (6) is depicted in Figure 2a,b,
Table 2. Complex 3 Catalyzed Insertion of Alcohols to
Carbodiimides
a
Figure 2. Molecular structure of complexes 5 (a) and 6 (b), with
thermal ellipsoids set at the 50% probability levels. All hydrogen atoms
are omitted for the clarity.
respectively. The distorted tetrahedral coordination environ-
ment around the central metal atom is approximately
equivalent to that of its Th(IV) analogue. The U1−N1−C1
angles also depart from linearity and exhibit values of 163.4(4)°
and 157.6(6)° for complexes 5 and 6, respectively. Marginally
shorter U−N1 bond lengths are observed as expected in
comparison to the corresponding Th−N1 distances, displaying
the values of 2.128(4) and 2.156(7) Å for compounds 5 and 6,
respectively. The short U−N1 bond lengths and the large U−
N1−C1 angles indicate a significant π-character of the U−N1
bond, comparable to the respective Th−N1 bond (vide supra).
The N1−Cipso bond lengths in the mono(perimidin-2-iminato)
uranium complexes 5−6 are almost identical to those in the
thorium analogues, indicating that the bonding of the ligand to
the metal can be described as a metalla-heterocumulene (An
NC). Unexpectedly, as compared to the corresponding
imidazolin-2-iminato thorium and uranium complexes, almost
identical metal−amido and metal−imido bond distances are
observed, with the only difference being the proximity of the N-
alkyl substituents to the metal center.
In order to quantify the proximity of the wingtip
substitutions to the coordinated metal, cone angles were
measured for the present systems (113.2°, 124.4°, 114.0°, and
123.5° for complexes 3, 4, 5, and 6, respectively). These values
are much smaller than in the analogous complex [(ImRN)An-
{(N(SiMe3)2}3] (where R = Dipp), which exhibits cone angle
values 210° and 212° for Th and U, respectively. The smaller
cone angle of the present systems implies significantly greater
space available for the incoming substances to approach the
metal center.
a
Reaction conditions: 4 mg of catalyst (0.004 mmol, ∼0.4 mol %);
cat./alcohol: 1/100; cat./carbodiimide: 1/100; 550 μL of C6D6; 75 °C;
1
yield was determined by H NMR spectroscopy of the crude reaction
mixture.
Encouraged by these observations, we investigate the
challenging addition of alcohols to carbodiimides. In order to
determine the catalyst with the highest possible efficiency, our
initial exploratory findings showed that all complexes (3−6) are
potentially capable to catalyze the addition of alcohols to
carbodiimides, and all of them exhibit comparable activities
particular substrates combination generated 93%, 86%, 89%,
and 86% of the isourea catalyzed by complexes 3, 4, 5, and 6,
respectively, within 2 h. It is important to point out that the
ligand itself does not catalyze the reaction and freshly
recrystallized complexes were used to run the catalytic
reactions. Hence, further catalytic studies were carried out
with complex 3. The increasing trends of reactivity observed
t
(Table S2). The catalytic reaction of BuOH with DTC is
particularly impressive as it produces the corresponding isourea
in excellent yield in short reaction time. For example, this
C
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from the aryl substituted carbodiimide (DTC) as compared to
the alkyl substituted carbodiimide (DIC) are attributed to the
higher electrophilic character (sp hybridized carbon) of DTC.
The composite effect of acidity and steric congestion of the
alcohol played a crucial role in determining the product
formation. With the increase of the steric hindrance from
t
methanol to BuOH, the amount of product formation with
DIC markedly dropped significantly (entries [a]−[d], Table 2).
Complex 3 was found to be superior considering our previous
report, which showed no catalytic turnover in the DIC-tBuOH
substrate combination.16 The combination of DIC and 1-
adamatanol did not lead to the product formation presumably
to the steric congestion imposed by the adamantyl group.
Sterically less crowded alcohols, viz., diphenylmethanol and
t
Figure 3. Plot of the initial reaction rate as a function of DTC
concentration in the reaction of Bu2PhOH with DTC.
cyclohexanol, afforded higher conversion compared to BuOH
t
(entries [e] and [f], Table 2). However, the reaction of highly
acidic phenol with DIC (not DTC) can be performed without a
catalyst. The combination of DTC and alcohols (entries [g]−
[s], Table 2) afforded excellent yields (80−99%) in 2 h.
Identical trends as presented above are operative for DTC,
when the amount of the product decreases with an increase of
the steric encumbrance of the alcohol (entries [k]−[m], Table
2).
Di- and trihydroxy substrates provided almost quantitative
formation of the corresponding di- and tri-substituted isoureas
in 6 h (entries [o]−[p], Table 2). In order to determine the
competing reactivity of aliphatic and aromatic alcohols, the
insertion reaction was attempted with salicylic alcohol (entry
[r], Table 2). The aromatic alcohol selectively inserts into
DTC, which can be attributed to its higher acidity as compared
to its aliphatic analogue.
t
Figure 4. Plot of the initial reaction rate as a function of Bu2PhOH
concentration in the reaction of Bu2PhOH with DTC.
Diphenylmethanol was found to be extremely reactive, and
95% product formation was obtained within 15 min (entry [s],
Table 2). In order to investigate the effect of the substituents
present in the phenyl group of DTC, we carried out the
t
dp
dt
1
1
= kobs × [DTC]1 × [tBu2PhOH] × [3]
(1)
t
reaction with di-o-tolylcarbodiimimde (o-DTC) and BuOH or
Equation 1: Kinetic rate equation of the insertion of tBu2PhOH
to DTC.
In addition, the kinetic isotope effect was studied by
performing the reaction with the deuterated alcohol
(tBu2PhOD), resulting in a KIE value of 1.74, indicating that
the protonolysis step is the turn over limiting step (Scheme
3).18
isopropanol. In both cases, o-DTC gave lower yields (entries [t]
and [u], Table 2) as compared to its para analogue (DTC),
presumably due to its steric effects.
In order to gain an insight into the reaction mechanism,
stoichiometric reactions of complex 3 with 2,4-di-tert-
butylphenol (tBu2PhOH) and DTC were carried out
independently. The addition of stoichiometric amounts or
even an excess of DTC to complex 3 did not led to the
displacement of the coordinating iminato ligand to form the
corresponding guanidinate species, suggesting the retention of
the ligand coordinated to the metal during the catalytic cycle.
The addition of 3 equiv of alcohol led to dislodgement of 1
equiv of [HN(SiMe3)2] in 15 min and complete replacement of
the other two amido bonds up to 45 min (Figure S9), without
affecting the iminato ligand coordination, indicating that the
tris-alkoxo species [(PrRN)An(OR)3] (I) can be proposed as
an operative active intermediate for the reaction.
The initial rate of reaction was measured to determine the
order of the substrates and catalyst. Reactions were performed
t
with variable concentrations of catalyst, Bu2PhOH, and DTC,
while keeping the other reagents concentration unchanged. The
initial rate of the reaction varies linearly with the slope,
revealing that the reactions displayed first-order dependence on
t
DTC (Figure 3), Bu2PhOH (Figure 4) and as well as on
Figure 5. Plot of reaction rate as a function of complex 3
concentration in the reaction of Bu2PhOH with DTC.
t
complex 3 (Figure 5), giving rise to the kinetic rate eq 1.
D
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equiv of complex I, and the intermediate was characterized by
Scheme 3. Plausible Mechanism of the Actinide-Mediated
Catalytic Insertion of Alcohol into Carbodiimide
1H NMR (Figure 7). We decided to perform the reaction with
The activation parameters were determined from the Eyring
(Figure 6) and Arrhenius plots, with ΔH⧧, ΔS⧧, and Ea, values
Figure 7. NMR spectrum of complex II (S = solvent).
1 equiv of DTC to avoid overlapping signals when more
equivalents of DTC insert into the additional alkoxo moieties.
In addition, theoretically there are two fashions in which we can
depict the migratory inserted complex II: One in which the two
nitrogen atoms of the inserted DTC are attached to the metal
or a complex in which one nitrogen and one oxygen atom are
attached to the metal as shown in Scheme 3 and corroborated
by the NMR of this complex.
CONCLUSION
■
Here, we disclose for the first time the synthesis of a new family
of six-membered N-heterocyclic iminato ligands and their
incorporation with actinides to obtain active catalysts for the
insertion of primary, secondary, tertiary, and aromatic alcohols
into carbodiimides, affording the corresponding isoureas. Diol
and triol substrates were also found to be highly active,
producing the corresponding bis- and tris-isoureas, in
quantitative yields. The intermediate complexes I and II were
trapped spectroscopically. Here, we have shown that the steric
effects of the perimidin-2-iminato moiety, as opposed to the
electronic effects, are the major operative factor for the product
formation, as compared to the imidazolin-2-iminato moiety in
actinide complexes.
t
Figure 6. Eyring plot of Bu2PhOH addition to DTC mediated by
complex 3.
of 4.8 0.4 kcal mol−1, −51.48 0.4 e.u, and 5.3 0.4 kcal
mol−1, respectively. The large negative entropy value
corroborates a highly order transition state at the rate-
determining step.18 On the basis of the kinetics and the
thermodynamic parameters in combination with the stoichio-
metric reactions, a plausible mechanism is presented in Scheme
3.
The first step of the proposed mechanism is the rapid
protonolysis of the starting actinide complex 3 by the alcohol
with the displacement of 3 equiv of hexamethyldisilazane
producing complex I. This process is thermodynamically
favorable due to the oxophilic nature of the thorium (ΔHcalcd
= −68 kcal/mol).19 Migratory insertion of DTC to the
corresponding Th−O is in a rapid equilibrium (step 2),
producing complex II that follows a protonolytic cleavage
(RDS) with another equivalent of an alcohol (step 3), yielding
the isourea product and regenerating the active catalyst I.
Interestingly, starting the catalytic reaction with the trisalkoxo
complex I was found to yield similar results as compared with
when the reaction was performed with complex 3. For example,
EXPERIMENTAL SECTION
■
All manipulations of air-sensitive materials were performed with the
rigorous exclusion of oxygen and moisture in flamed Schlenk-type
glassware or J-Young Teflon valve-sealed NMR tubes on a dual
manifold Schlenk line interfaced to a high vacuum (10−5 Torr) line, or
in a nitrogen-filled Innovative Technologies glovebox with a medium-
capacity recirculator (1−2 ppm of O2). Argon and nitrogen were
purified by passage through a MnO oxygen-removal column and a
Davison 4 Å molecular sieve column. Hydrocarbon solvents benzene-
d6 (Cambridge Isotopes), toluene (Bio-Lab), and diethyl ether (Bio-
Lab) were distilled under vacuum from Na/K alloy. 1,3-Diisopro-
pylcarbodiimide was distilled from sodium bicarbonate under a
nitrogen atmosphere, and 1,3-di-p-tolylcarbodiimide was dried under
vacuum (10−6) for 12 h on a high vacuum line. Methanol and ethanol
were dried using sodium (Na) metal, distilled, and stored over 4 Å
molecular sieves. Isopropanol and tert-butanol were refluxed over
CaH2, distilled, and stored over 4 Å molecular sieves. The actinide
complexes [(Me3Si)2N]2An[κ2-(N,C)CH2Si(CH3)2N(SiMe3)] (An =
Th (1), U (2)),20 N1,N8-diisopropylnaphthalene-1,8-diamine, and
N1,N8-dicycloheptylnaphthalene-1,8-diamine were prepared according
t
in the reaction of Bu2PhOH with DTC, 80% yield of the
corresponding isourea was obtained after 2 h (Figure S11) as
compared to 87% using complex 3 (entry [j] in Table 2).
To trap the migratory inserted intermediate II of the catalytic
cycle shown in Scheme 3, 1 equiv of DTC was reacted with 1
E
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to published procedures.21 All the aforementioned reagents were
stored in an inert atmosphere glovebox prior to use.
NMR spectra were recorded on Bruker Avance 300, Bruker Avance
III 400 spectrometers. Chemical shifts for 1H and 13C NMR are
referenced to internal protio solvent and reported relative to
tetramethylsilane. J-values are reported for 1H NMR coupling
constants in the unit of hertz (Hz).
(92%). H NMR (400 MHz, CDCl3): δ = 7.22−7.18 (m, 2H, Ar−
CH), 7.09 (d, 2H, J = 6.0 Hz, ArCH), 6.59 (d, 2H, J = 6.0 Hz, ArCH),
4.58−4.46 (m, 2H, CHcycloheptyl), 2.38−2.28 (m, 4H, CH2), 1.89−1.79
(m, 4H, CH2), 1.79−1.70 (m, 4H, CH2), 160−1.51 (m, 12H, CH2);
13C NMR (100 MHz, CDCl3): δ = 150.6 (NCN), 134.3 (ArC), 127.2
1
(ArC), 118.5 (ArC), 116.5 (ArC), 64.7, 31.3, 28.7, 27.0 ppm. Calcd for
C25H33N3: C, 79.95; H, 8.86; N, 11.19. Found: C, 79.88; H, 8.81; N,
11.12.
The single-crystal material was immersed in perfluoropolyalkylether
and was quickly fished with a glass rod and mounted on a Kappa CCD
diffractometer 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.22 Accurate cell parameters were
obtained with the amount of indicated reflections.23 The structure was
solved by SHELXS-97 direct methods24and refined by the SHELXL-
97 program package.25 The atoms were refined anisotropically.
Hydrogen atoms were included using the riding model. Figures were
drawn (50% probability thermal ellipsoids) using Diamond V3.1.26
Synthesis of L1·HBr (2-Amino-1,3-diisopropyl-1H-perimidi-
nium Bromide). A solution of cyanogen bromide (0.61 g, 5.76
mmol) in toluene (20 mL) was added dropwise to a stirred solution of
diamine N,N′-diisopropyl-1,8-diaminonaphthalene (1.1634 g, 4.8
mmol) in toluene at 110 °C. A dark solid started to precipitate
during the course of the addition. After complete addition, the mixture
was stirred at 110 °C for 12 h. The mixture was allowed to cool to
room temperature, and the precipitate was filtrated, followed by
washed with diethyl ether (3 × 30 mL) and dried in vacuum, to afford
General Procedure for the Synthesis of Mono(perimidin-2-
iminato) Actinide (IV) Complexes. A toluene (10 mL) solution of
actinide metallacycle 1 or 2 (200 mg) was reacted with the respective
perimidin-2-imine (1 equiv in 10 mL) at room temperature, and the
reaction was stirred for an additional 12 h at room temperate. The
solvent was removed under reduced pressure to afford the crude solid
3−6. X-ray quality crystals were grown from concentrated toluene
solution at −35 °C.
[(L1)Th{N(SiMe3)2}3] (3). Yield 94% (260 mg, 0.265 mmol); 1H
NMR (300.0 MHz, C6D6): δ = 7.16−7.12 (m, 4H, ArCH), 6.81−6.78
(m, 2H, ArCH), 5.93−5.83 (m, 2H, CHMe2), 1.52 (d, J = 8.0 Hz,
12H, CHMe2), 0.45 (s, 54H, Si(CH3)3); 13C NMR (75.5 MHz, C6D6):
δ = 143.1 (Cipso=N), 135.8 (ArC), 135.1 (ArC), 129.0 (ArC), 126.4
(ArC), 118.7 (ArC), 117.4 (ArC), 106.0 (ArC), 49.7 (CHMe2), 19.7
(CH3), 4.75 (Si(CH3)3) ppm. Calcd for C35H74N6Si6Th: C, 42.91; H,
7.61; N, 8.58. Found: C, 42.98; H, 7.67; N, 8.65.
[(L2)Th{N(SiMe3)2}3] (4). Yield 92% (280 mg, 0.257 mmol); 1H
NMR (300.0 MHz, C6D6): δ = 7.22−7.11 (m, 4H, ArCH), 6.83−6.80
(m, 2H, ArCH), 5.39−5.28 (m, 2H, CH), 2.39−2.28 (m, 2H, CH2),
2.01−1.92 (m, 2H, CH2), 1.69−1.57 (m, 6H, CH2), 1.46−1.37 (m,
2H, CH2), 0.45 (s, 54H, Si(CH3)3); 13C NMR (75.5 MHz, C6D6): δ =
144.5 (Cipso=N), 136.7 (ArC), 134.9 (ArC), 126.3 (ArC), 118.6 (ArC),
118.7 (ArC), 107.9 (ArC), 59.7 (ArC), 31.5 (CH2), 29.4 (CH2), 26.3
(CH2), 5.26 (Si(CH3)3) ppm. Calcd for C43H86N6Si6Th: C, 47.48; H,
7.96; N, 7.72. Found: C, 47.54; H, 8.01; N, 7.79.
1
a dark solid. Yield: 1.55 g (93%). H NMR (400 MHz, CDCl3): δ =
8.35 (s, 2H, NH2), 7.50−7.38 (m, 4H, ArCH), 7.10 (d, J = 8.0 Hz,
2H), 5.06 (m, 2H), 1.70 (d, J = 6.0 Hz, 12H); 13C NMR (100 MHz,
CDCl3): δ = 153.0 (NCN), 133.8 (ArC), 132.2 (ArC), 129.1 (ArC),
128.4 (ArC), 127.0 (ArC), 125.2 (ArC), 123.1 (Ar C), 110.4 (ArC),
55.4 (CHMe2), 21.2 (CH3) ppm. ESI-MS, m/z: 348.0889, Anal. Calcd
for C18H21N3Br: C, 60.17; H, 5.89; N, 11.70. Found: C, 60.80; H,
5.92; N, 11.79.
[(L1)U{N(SiMe3)2}3] (5). Yield 93% (255 mg, 0.258 mmol); 1H NMR
(300.0 MHz, C6D6, 298 K): δ = 13.02 (br, 12H, CHMe2), 11.52−
11.22 (m, 4H, ArCH), 7.28−6.99 (m, 2H, ArCH), 0.22−0.06 (m, 2H,
CHMe2), −11.48 (s, 54H, Si(CH3)3); 13C NMR (75.5 MHz, C6D6,298
K): δ = 168.86 (Cipso=N), 146.03 (ArC), 135.29 (ArC), 126.39 (ArC),
122.89 (ArC), 118.69 (ArC), 48.07 (CHMe2), 20.30 (CH3), 2.13
(Si(CH3)3) ppm. C35H74N6Si6U: C, 42.65; H, 7.57; N, 8.53. Found: C,
42.71; H, 7.61; N, 8.59.
Synthesis of L2·HBr (2-Amino-1,3-dicycloheptyl-1H-perimi-
dinium Bromide). This was synthesized following the similar
procedure described for the synthesis of L1·HBr by reaction between
cyanogen bromide (0.9 g, 5.66 mmol) and N,N′-diicycloheptyl-1,8-
diaminonaphthalene. The resulting precipitate was washed with diethyl
ether (3 × 30 mL) and dried in vacuum to afford an off-white solid.
1
Yield: 1.1 g (94%). H NMR (400 MHz, CDCl3): δ = 7.93 (s, 2H,
[(L2)U{N(SiMe3)2}3] (6). Yield 90% (275 mg, 0.251 mmol); 1H NMR
(300.0 MHz, C6D6, 298 K): δ = 19.0 (br, 3H, CHcycloheptyl), 14.9 (br, s,
2H, ArCH), 14.3 (br, s, 2H, CHcycloheptyl), 11.1 (m, 2H, ArCH). 11.0
(m, 2H, ArCH), 4.95 (br, s, 4H, CHcycloheptyl), 3.55 (br, s, 9H,
CHcycloheptyl), −12.33 (s, 54H, Si(CH3)3); 13C NMR (75.5 MHz, C6D6,
298 K): δ = 169.1 (Cipso=N), 135.8 (ArC), 143.5 (ArC), 134.0 (ArC),
130.0 (Ar C), 123.9 (ArC), 121.0 (ArC), 42.0 (CHcycloheptyl), 31.4
(CH2cycloheptyl), 30.7 (CHcycloheptyl), 5.51 (Si(CH3)3) ppm. Calcd for
C43H86N6Si6U: C, 47.22; H, 7.92; N, 7.68. Found: C, 47.30; H, 7.98;
N, 7.73.
NH2), 7.47−7.37 (m, 4H, ArCH), 6.97 (d, 2H, J = 8.0 Hz, ArCH),
4.67−4.57 (m, 2H), 2.43−2.32 (m, 4H, CH2), 2.18−2.08 (m, 4H,
CH2), 1.80−1.70 (m, 12H, CH2), 163−1.57 (m, 4H, CH2); 13C NMR
(75.5 MHz, CDCl3): δ = 151.0 (NCN), 133.8 (ArC), 127.6 (ArC),
122.9 (ArC), 120.6 (ArC), 110.5 (ArC), 64.8, 31.8, 27.8, 26.4 ppm;
Anal. Calcd for C25H34N3Br: C, 65.78; H, 7.51; N, 9.21. Found: C,
65.87; H, 7.57; N, 9.29.
Synthesis of L1 (1,3-Diisopropyl-1H-perimidine-2(3H)-
imine). Aqueous KOH (1.00 g, 17.82 mmol) was added to the
diethyl ether (40 mL) suspension of L1·HBr (2.50 g, 7.18 mmol), and
the mixture was vigorously stirred for 30 min at ambient temperature.
In a separatory funnel, the two layers were separated and the aqueous
layer was extracted with diethyl ether (3 × 30 mL). The combined
organic phases were dried over anhydrous Na2SO4. After filtration, the
solvent was removed in vacuo to afford an off-white solid. Yield: 1.75 g
(92%). 1H NMR (400 MHz, CDCl3): δ = 7.24−7.19 (m, 2H, ArCH),
7.12 (d, J = 8.0 Hz, ArCH, 1H), 6.66 (d, J = 8.0 Hz, ArCH, 2H), 4.93
(br, 2H, CHMe2), 1.52 (d, 12H, CHMe2); 13C NMR (100 MHz,
CDCl3): δ = 151.6 (NCN), 137.4 (ArC), 134.4 (ArC), 127.2 (ArC),
118.1 (ArC), 116.5 (ArC), 105.2 (ArC), 49.6 (CHMe2), 19.6 (CH3)
ppm. ESI-MS, m/z: 268.1827, Anal. Calcd for C18H20N3: C, 77.66; H,
7.24; N, 15.09. Found: C, 77.58; H, 7.31; N, 15.15.
General Procedure for the Catalytic Addition of Alcohols to
Carbodiimides. In a typical experiment, approximately 4 mg of the
desired catalyst in 600 μL of C6D6 was transferred to a J-Young Teflon
sealed NMR tube, followed by the addition of carbodiimide (100
equiv) and alcohol (100 equiv). Samples were then placed in an oil
bath preheated to 75 °C, and the reaction progress was monitored at
1
regular intervals using H NMR spectroscopy for up to 24 h. After
1
completion of the reaction, crude mixtures were analyzed using H,
13C spectroscopy as well as mass spectrometry. Known compounds
were compared to those previous reported in the literature.
ASSOCIATED CONTENT
* Supporting Information
■
Synthesis of L2: (1,3-Dicycloheptyl-1H-perimidine-2(3H)-
imine). This was synthesized following the similar procedure
described for the synthesis of L1 by reaction between L2·HBr (0.8 g,
5.66 mmol) and aqueous KOH in diethyl ether. The aqueous layer was
extracted with diethyl ether (3 × 30 mL). The combined organic
phases were dried over anhydrous Na2SO4. After filtration, the solvent
was removed under vacuo to afford a light brown solid. Yield: 0.6 g
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.7b00037.
Schematic synthesis of L1 and L2; characterization of
isoureas; characterization of L1·HBr, L2·HBr, L1, L2, and
F
DOI: 10.1021/acs.organomet.7b00037
Organometallics XXXX, XXX, XXX−XXX

Organometallics
complexes 3−6; catalytic addition of alcohols to
Article
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ed.; CRC Press: Boca Raton, FL, 2015; pp 2015−2016.
(8) (a) Sharma, M.; Andrea, T.; Brookes, N. J.; Yates, B. F.; Eisen, M.
S. J. Am. Chem. Soc. 2011, 133, 1341−1356. (b) Andrea, T.; Barnea, E.;
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carbodiimides mediated by complexes 3−6 as a function
of time; stoichiometric reaction of complex 3 and
tBu2PhOH; stoichiometric reaction of complex 3,
tBu2PhOH, and DTC; and crystallographic table (PDF)
Crystallographic data for 5 (CIF)
Crystallographic data for 6 (CIF)
Crystallographic data for 3 (CIF)
Crystallographic data for 4 (CIF)
(9) Karmel, S. R. I.; Fridman, N.; Tamm, M.; Eisen, M. S.
Organometallics 2015, 34, 2933−2942.
(10) Walshe, A.; Fang, J.; Maron, L.; Baker, R. J. Inorg. Chem. 2013,
52, 9077−9086.
(11) Baker, R. J.; Walshe, A. Chem. Commun. 2012, 48, 985−987.
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AUTHOR INFORMATION
Corresponding Author
*E-mail: chmoris@tx.technion.ac.il.
■
(13) Zarate, S. G.; Santana, A. G.; Bastida, A.; Revuelta, J. Curr. Org.
Chem. 2014, 18, 2711−2749.
(14) Karmel, S. R. I.; Eisen, M. S.; Tamm, M. Angew. Chem., Int. Ed.
2015, 54, 12422−12425.
ORCID
Moris S. Eisen: 0000-0001-8915-0256
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
This work was supported by the Israel Science Foundation
administered by the Israel Academy of Science and Humanities
under Contract No. 78/14; and by the PAZY Foundation Fund
(2015) administered by the Israel Atomic Energy Commission.
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