Imidazolin-2-iminato Ligand-Supported Titanium Complexes as Catalysts for the Synthesis of Urea Derivatives

Article abstract of DOI:10.1021/acs.inorgchem.5b02302

The reactions of tetrakis(dimethylamido)titanium(IV) [Ti(NMe₂)₄] with three different imidazolin-2-imines (Im^RNH; R = tert-butyl (tBu), mesityl (Mes), and 2,6-diisopropylphenyl (Dipp)) afforded the corresponding titanium i

Full text of DOI:10.1021/acs.inorgchem.5b02302

Article
pubs.acs.org/IC
Imidazolin-2-iminato Ligand-Supported Titanium Complexes as
Catalysts for the Synthesis of Urea Derivatives
†
†
†
§
,†
Kishor Naktode, Suman Das, Jayeeta Bhattacharjee, Hari Pada Nayek, and Tarun K. Panda*
†Department of Chemistry, Indian Institute of Technology Hyderabad, Kandi Sangareddy, 502 285 Telangana, India
§Department of Applied Chemistry, Indian School of Mines, Dhanbad, 826004 Jharkhand, India
*
S Supporting Information
ABSTRACT: The reactions of tetrakis(dimethylamido)titanium(IV) [Ti-
R
(
NMe ) ] with three different imidazolin-2-imines (Im NH; R = tert-butyl
2 4
(
tBu), mesityl (Mes), and 2,6-diisopropylphenyl (Dipp)) afforded the
R
corresponding titanium imidazolin-2-iminato complexes [(Im N)Ti(NMe ) ]
2
3
(
R = tBu, 1a; R = Mes, 1b; R = Dipp, 1c). Treatment of complex 1a with two
different carbodiimides [R′NCNR′; R′ = cyclohexyl (Cy) and isopropyl
(
(
(
iPr)] resulted in the formation of imidazolin-2-iminato titanium mono-
R
guanidinate) complex of the type [(Im N)Ti(R′NC(NMe )NR′) (NMe )
2
2 2
R′ = iPr; R = tBu (2a), R = Dipp (2c); R′ = Cy, R = tBu (3a)], as yellow
solid in 94% yield. However, a similar reaction of 1b and 1c with 2 equiv of
phenyl isocyanates at ambient temperature resulted in the formation of
R
2
corresponding titanium bis(ureate) complexes [(Im N)Ti{κ -OC(NMe )-
2
NPh} (NMe )] (R = Mes, 4b and R = Dipp, 4c). Three equivalents of phenyl
2
2
isothiocyanate reacted with complex 1c to afford respective titanium
Dipp
2
1
tris(thioureate) complex [(Im N)Ti{κ -SC(NMe )NPh} {κ -SC(NMe )NPh}] (6c). The molecular structures of 1a−c, 2a,
2
2
2
2
c, 3a, 4c, and 6c were established by X-ray diffraction analyses, and, from the solid-state structures of 1a−c, 2a, 2c, 3a, 4c, and
c, it was confirmed that the imidazolin-2-iminato titanium bond in each case is very short and possesses a multiple-bonding
6
character. The imidazolin-2-iminato titanium complex 1c was utilized as a precatalyst for the addition of amine N−H bond to
phenyl isocyanate. High yields of the corresponding urea derivatives were achieved under mild conditions. The mechanistic study
of the aforementioned catalytic reaction was performed, and the active catalyst complex 7b was isolated using 2 equiv of
iminopyrrole [2-(2,6-iPr C H NCH)C H NH] and the complex 4b. The molecular structure of 7b was thereafter established.
2
6
3
4
3
1
4
INTRODUCTION
carbonylation of amines have been developed, the catalytic
addition of amine N−H to an isocyanate is limited to very few
metal complexes. This is in sharp contrast to the preparation of
guanidine where a wide range of metal complexes are known to
■
Catalytic hydroamination reactions of alkenes and alkynes as a
powerful tool for the formation of C−N bonds have been
studied with a wide range of transition metal and lanthanide
1
5
1
−5
act as efficient catalysts. Very recently, Tamm and Eisen have
described the thorium complex-mediated catalytic addition of
E−H bonds (E = N, P, S) to carbodiimides, isocyanates, and
isothiocyanates leading to the formation of corresponding
metal complexes.
The addition of amine N−H bond to
carbodiimdes using an efficient metal catalyst can provide the
atom efficient route to the formation of multisubstituted
guanidine. In an analogous method, the addition of amine
6
,7
16
inserted products.
N−H bond to the isocyanates, using a suitable metal catalyst,
produces corresponding urea derivatives. Guanidine, and the
substituted guanidine derivatives, are an important class of
compounds present in biologically and pharmaceutically active
molecules. They have received considerable attention due to
8
In the reported imidazolin-2-iminato thorium(IV) alkyl
complex, the role of imidazolin-2-iminato ligand was found to
be very important for stabilizing the precatalyst. Monoanionic
imidazolin-2-iminato ligands such as Im N are often described
by the two limiting resonance structures, thus indicating that
the ability of the imidazolium ring to stabilize a positive charge
R
−
9
their electronic and variable static effects. Today, guanidine
derivatives are utilized for many purposes, as they can serve as
building blocks in various pharmaceutical and natural
17
leads to highly basic ligands with a strong electron-donating
18
1
0
capacity toward early transition metals. Because of their
ability to act as 2σ, 4π-electron donors, these ligands can be
regarded as monodentate analogues of cyclopentadienyls, C R ,
products. These molecules can also act as organic bases and
11
catalyze various organic transformations. Guanidines are also
used as ancillary ligands in the preparation of a variety of metal
complexes including those of main, transition, and lanthanides
5
5
and also as monoanionic imido ligands similar to those
1
2
metals. Urea functional groups play important roles in
13
organic, medicinal, supramolecular, and material chemistry.
Although a number of methodologies including oxidative
Received: October 10, 2015
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.5b02302
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Inorganic Chemistry
Article
19
described for related phosphoraneiminato ligands. This
concept was successfully utilized by Tamm and co-workers to
introduce imidazolin-2-iminato ligands into transition metals,
lanthanide, and more recently, actinide metals to achieve very
Scheme 1. Syntheses of Titanium Complexes 1a−c
2
0
19
short M−N bonds. We and others have already established
that monoanionic imidazolin-2-iminato ligands are efficient
systems for the preparation of catalytically active transition
21
metal and rare earth metal complexes.
The structural characterization of rare earth metal complexes
three dimethyl amide groups appeared as a sharp singlet for
R
8
R
of the types [(Im N)MCl (THF) ] and [(η -C H )M(Im N)-
2
3
8
8
each case (δ 3.37 for 1a, 3.01 for 1b, and 2.91 ppm for 1c) in
(
THF) ] (THF = tetrahydrofuran) revealed, in all cases, the
1
n
the H NMR spectra. Additionally, complexes 1a−c showed
presence of terminal imidazolin-2-iminato ligands with excep-
another singlet (δ 5.95 for 1a, 5.68 for 1b, and 5.92 ppm for 1c)
for the resonances of two olefinic protons present in the
imidazole backbone. These chemical shift values were in the
20c,h
tionally short metal−nitrogen bonds.
This led to the
consideration of a multiple-bonding character between M−N
2
2
bonds. Today, various metal complexes supported by
imidazolin-2-iminato ligands display high activity in ethylene
R
range (5.93 ppm) previously reported for [(Im N) TiCl ],
2
2
R
5
18a,c
R
[
(Im N)Ti(η -Cp)Cl ]
and [Im NLnCl (THF) ] (Ln =
2
2 3
23
(
co)polymerization and alkyne metathesis.
20c,h
Sc, Y, Lu).
The tert-butyl protons in complex 1a were
Recently, we reported that a group 4 metal−nitrogen bond,
observed as a sharp singlet at δ 1.58 ppm whereas the mesityl
methyl protons for complex 1b appeared as two singlets at δ
inserted into the carbon−nitrogen double bond of carbodii-
mides and α-diimines to afford guanidinate and amido−imino
2
.25 and 2.13 ppm, respectively. In complex 1c having 2,6-
24
ligand, supported group 4 metal complexes. With such
knowledge, we were specifically interested about the use of
early transition metal complexes stabilized by monoanionic
imidazolin-2-iminato ligand. We were curious to explore their
catalytic activity to prepare urea derivatives by the reaction of
isocyanate and amines. While some of the titanium imidazolin-
diisopropylphenyl (Dipp) groups attached to the imidazol-2-
imine moiety, the H NMR spectrum exhibited two doublet
resonances (1.41 and 1.20 ppm) having a coupling constant of
1
6
.85 Hz each for the isopropyl CH groups, thus indicating that
3
rotation around the C−N−Ti axis is restricted. The single
crystals for complexes 1a−c were grown at −35 °C from n-
pentane. The solid-state structures of complexes 1a−c were
established by single-crystal X-ray analysis.
R
R
2
-iminato complexes such as [(Im N)TiCl ], [(Im N)-
3
R
TiCpCl ], and [(Im N)TiCpMe ] have already been repor-
2
2
18c
ted by Tamm et al., their exploitation as a catalyst toward
catalytic addition of the amine N−H bond to heterocumelens
has not been reported until date.
Complex 1a crystallizes in monoclinic space group C2/c
having one molecule of 1a along with half molecule of neutral
tBu
imidazolin-2-imine (Im NH) in the asymmetric unit.
We report here the synthesis and structural details of various
However, complex 1b crystallizes in monoclinic space group
imidazolin-2-iminato titanium amido complexes of general
P2 /c having four molecules and complex 1c in triclinic space
1
R
formula [(Im N)Ti(NMe ) ] (R = tBu, 1a; R = mesityl (Mes),
2
3
group P1 having two molecules in their respective unit cells.
̅
1
b; R = 2,6-diisopropylphenyl (Dipp), 1c). We also describe
Thus, the differences in space groups observed for complexes
the synthetic and structural aspects of various complexes like
1a−c could be due to the different substituents in the imidazole
R
[
(
(Im N)Ti-(R′NC(NMe )NR′)(NMe ) (R′ = iPr; R = tBu
2
2 2
rings. The molecular structures of complexes 1a−c confirmed,
in each case, the coordination of imidazolin-2-iminato ligand
with the titanium ion. The solid-state structure of complex 1a is
shown in Figure 1, whereas Figures FS1 and FS2 in the
Supporting Information display the molecular structures of
complexes 1b and 1c. Detailed structural and refinement
parameters for complexes 1a−c are given in Table TS1 in the
Supporting Information, and selected bond lengths and angles
of complexes 1a−c are given in Table TS2 in Supporting
Information. In all the complexes 1a−c, the titanium ion is
tetracoordinated with the three amido nitrogen atoms from
dimethyl amides and one imido nitrogen atom from imidazolin-
2-iminato moiety to adopt a distorted tetrahedral geometry
R 2
2a), R = Dipp (2c); R′ = Cy, R = tBu (3a)], [(Im N)Ti{κ -
Dipp
2
OC(NMe )NPh} (NMe )] (4c) and [(Im N)Ti{κ -SC-
2
2
2
1
(
NMe )NPh} {κ -SC(NMe )NPh}] (6c) obtained by the
2 2 2
stoichiometric reaction of 1 with different heterocumelenes.
The results of catalytic additions of different amines to the
phenyl isocyanate using 1c as the catalyst are also reported here
along with the mechanistic details.
RESULTS AND DISCUSSION
■
Preparation and Structural Characterization of Com-
R
plexes [(Im N)Ti(NMe ) ] (R = tBu, 1a; R = Mes, 1b; R =
2
3
Dipp, 1c). Tetrakis(dimethylamido)titanium(IV) [Ti(NMe ) ]
2
4
R
was reacted with 1 equiv of imidazolin-2-imines (Im NH; R =
around the titanium ion. The Ti−N
distances in 1b and 1c
amide
tBu, Mes, and Dipp) to afford the corresponding titanium
are very similar (1.916−1.925 Å for 1b and 1.910−1.924 Å for
1c), while varying slightly for 1a (1.905−1.951 Å), presumably
due to the bulky tert-butyl substituent over imidazolium
nitrogen atoms of imidazolin-2-iminato ligand.
R
imidazolin-2-iminato complexes [Im NTi(NMe ) ] (R = tBu
2
3
(
tert-butyl), 1a; R = Mes, 1b; R = Dipp, 1c) as a yellow solid
and in good yield. All the complexes were obtained as pure
forms in good yield through recrystallization from n-pentane at
In complex 1a, the imidazolin-2-iminato moiety coordinates
with the titanium ion in an essentially linear fashion [Ti−N3−
−
35 °C. All the titanium complexes showed good solubility in
common organic solvents such as THF, pentane, and toluene.
C1 = 178.4(1)°]. Further, the Ti−N
bond length of
iminato
1
13
1
Complexes 1a−c were characterized by H and C{ H}
NMR spectral data, combustion analysis, and single-crystal X-
ray crystallography. Well-resolved NMR spectra measured in
C D were obtained for complexes 1a−c, and each complex
1.824(1) Å is in the range of observed titanium imidazolin-2-
tBu
5
iminato complexes [(Im N)Ti(η -Cp)Cl ] (170.9° and 1.765
2
tBu
18a,c
Å) and [(Im N) TiCl ] (170.9° and 1.788 Å).
However,
2
2
for complexes 1b and 1c, deviations from linearity are observed
6
6
1
13
1
R
showed one set of H and C{ H} NMR resonances for the
NMe moieties. The resonances for 18 protons attached to
in coordination of Im N moiety [Ti−N1−C1 = 164.5(1)° and
169.2(1)°, respectively]. The Ti−N
-
bond lengths
iminato
2
B
DOI: 10.1021/acs.inorgchem.5b02302
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Inorganic Chemistry
Article
of monomeric imidazolin-2-iminato titanium(IV) bis-
(
dimethylamido) (guanidinate) complexes. Complexes 2a, 2c,
and 3a were characterized by standard spectroscopic analysis,
and suitable crystals for single-crystal X-ray diffraction analysis
were grown from concentrated toluene solution at −35 °C. H
NMR spectra for complexes 2a, 2c, and 3a was recorded at 25
1
°
C in benzene-d . The resonance signals, at δ 5.82 ppm (for
6
2
a), 5.92 ppm (for 2c), and 5.90 ppm (for 3a), could be
R
−
assigned to the two olefinic backbone protons of Im N moiety
in each complex. In addition, multiplets at δ 3.51−3.58 ppm for
complex 2a and 3.71−3.76 ppm for 2c were observed for
methine protonsCH of isopropyl groups from guanidine
moiety. Doublet signals appeared at δ 1.12 ppm (for 2a) and δ
1
.14 ppm (for 2c) for the methyl protons of isopropyl groups.
This was well within the agreement with relative [{(iPrN) -
CNMe } Ti(NMe )] complex as reported by us earlier. For
2
24
2
3
2
Figure 1. Molecular structure of complex 1a showing a distorted
tetrahedron polyhedron around a titanium metal ion. Hydrogen atoms
are omitted for clarity. Selected bond lengths [Å] and bond angles
the complex 3a, multiplets were also found at δ 3.22−3.19,
1
.88−1.85, and 1.28−1.24 ppm for the resonance of
corresponding methylene −CH protons of the cyclohexyl
2
[
deg]: 1a: Ti1−N3 1.824(2), Ti1−N5 1.904(2), N3−C1 1.295(2),
groups of guanidinate moiety. Singlet resonances also appeared
at δ 1.58 (for 2a) and 1.60 ppm (for 3a) for respective tBu-
protons present in the ligand fragment of respective
compounds, and they were in the range similar to that of
complex 1a (1.58 ppm). However, complex 2c displayed a
doublet signal centered at δ 1.14 ppm and a multiplet centered
C1−N3−Ti1 178.4, N3−Ti1−N5 105.5(8), N5−Ti1−N6 112.6(8).
1b: Ti1−N1 1.850(1), Ti1N5 1.916(1), N1C1 1.281(2), C1N1Ti1
164.5(1), N5−Ti1−N6 111.6(6). 1c: Ti1−N1 1.853(1), Ti−N5
1.920(2), N1−C1 1.274(2), C1−N1−Ti1 169.2(1), N5−Ti1−N6
104.9(8). More bond lengths and angles are given in Supporting
Information.
Dipp
−
at δ 3.46 ppm for the isopropyl groups present in the Im
N
[
1.851(1) Å for 1b and 1.853(1) Å for 1c] are also slightly
moiety.
larger than that in 1a. Thus, it is evident that the tert-butyl
groups, which have positive inductive effects, reasonably
enhance electron donation from imidazolin-2-iminato moiety
to titanium ion as compared to the effect of electron-
withdrawing aromatic substitution present in imidazolium
nitrogen atoms for 1b and 1c. Similar observations have been
The solid-state structures of complexes 2a and 3a are
presented in Figures 2 and 3, respectively, and the molecular
18a,c
made in previous reports.
Preparation and Structural Characterization of Tita-
R
nium Mono(guanidinate) Complex [(Im N)Ti(R′NC-
(
NMe )-NR′)(NMe ) ] (R = tBu; R′ = iPr (2a), R = Dipp;
2
2 2
R′ = iPr (2c); R = tBu; R′ = Cy (3a)). As a part of our interest
in the coordination chemistry of Ti−N
complexes, we
iminato
attempted the reaction of equivalent amount of N,N′-
R
diisopropylcarbodiimide with [(Im N)Ti(NMe ) ] (where R
2
3
=
tBu (1a) and Dipp (1c)). Both the reactions were performed
in toluene at 80 °C. The corresponding titanium guanidinate
complexes 2a and 2c were isolated in good yield (Scheme 2).
Figure 2. Solid-state structure of complex 2a showing distorted
trigonal bipyramidal polyhedron around titanium ion. Hydrogen atoms
are omitted for clarity. Selected bond lengths [Å] and bond angles
Scheme 2. Synthesis of Complexes 2a, 2c, and 3a
[
deg]: Ti1−N3 1.836(4), Ti1−N4 1.952(4), Ti1−N6 2.114(4), Ti1−
N7 2.192(4), C1−N3−Ti1 173.1(3), N3−Ti1−N4 98.3(2), N6−
Ti1−N7 61.7(1), N6−C19−N7 111.6(5). More bond lengths and
angles are given in Supporting Information.
structure of complex 2c is given in Figure FS3 in the
Supporting Information. Selected bond lengths and bond
angles of complexes 2a, 2c, and 3a are given in Table TS2 in
Supporting Information. Detailed structural parameters for 2a,
2c, and 3a are given in Table TS1. Complexes 2a, 2c, and 3a
carry similar structural features. Complex 2a crystallizes in
orthorhombic space group P2 2 2 having four molecules in its
Similarly, the reaction of N,N′-dicyclohexylcarbodiimide with
1
a afforded, in good yield, the monoinserted guanidinate
tBu
product [(Im N)Ti(Me NC-(NCy) )(NMe ) ] (3a; Scheme
2
2
2 2
2
).
1
1 1
The syntheses of 2a, 2c, and 3a followed a relatively easy
unit cell, whereas both the complexes 2c and 3a crystallize in
insertion reaction, wherein one of the labile amide moieties
replaced from the primary ligand sphere of the metal were
inserted on the central carbon atoms of the N,N′-
diisopropylcarbodiimide ligands, thus leading to the formation
monoclinic space group P2 /c having four molecules in their
respective unit cells.
In each complex 2a, 2c, and 3a, the central metal
titanium(IV) ion is bonded with one nitrogen atom of
1
C
DOI: 10.1021/acs.inorgchem.5b02302
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Inorganic Chemistry
Article
since the interaction of the second carbodiimide molecule
with titanium(IV) ion is prevented by the absence of a free
coordination site on complex 3a and the lower electrophiliity of
the carbon center of carbodiimide molecules. In contrast, for
isocyanates, a more electronegative oxygen atom can make the
CN bond more polar, thus enabling it to undergo insertion
reactions. For a comprehensive study, we reacted titanium
complexes 1b and 1c with phenyl isocyante (PhNCO) in
toluene at room temperature to obtain the corresponding
insertion products. A bis-insertion product of composition
R
2
[
(Im N)Ti[κ -OC(NMe )-NPh] (NMe )] (R = Mes, 4b and
2 2 2
R = Dipp, 4c) were isolated from both 1:1 or 1:2 molar ratio of
reactions indicating the higher reactivity of isocyanate toward
titanium complexes 1b and 1c. The CO double bond of the
Figure 3. Solid-state structure of complex 3a showing a distorted
trigonal bipyramidal polyhedron around a titanium ion. Hydrogen
atoms are omitted for clarity. Selected bond lengths [Å] and bond
angles [deg]: Ti1−N3 1.838(1), Ti1−N5 1.953(1), Ti1−N6 2.276(1),
Ti1−N7 2.075(1), N3−C1 1.298(2), C1−N3−Ti1 173.8(1), N3−
Ti1−N5 98.0(6), N6−Ti1−N7 61.2(6), N6−C16−N7 112.7(1).
More bond lengths and angles are given in Supporting Information.
PhNCO molecule was inserted into the Ti−N bond to
2
form the κ -OC(NMe )NPh moiety with electronic delocaliza-
2
tion over the O−C−N unit (Scheme 3). Similar type of
Scheme 3. Syntheses of Titanium Complexes 4b and 4c
R
−
Im N ligand, two nitrogen atoms from the guanidinate group,
and two nitrogen atoms from two dimethyl amido moieties.
The coordination geometry around the titanium(IV) ion in
each case can therefore be described as a distorted trigonal
bipyramidal. In the distorted trigonal bipyramidal geometry, a
nitrogen atom each from the −NMe and the guanidinate
2
groups occupied both the apical positions, while the remaining
three nitrogen atoms were located in the equatorial position
insertion reactions were reported by Xie et al. in complexes
1
5
3
(
Figures 2 and 3). The Ti1−Niminato bond lengths in 2a
[σ:η :η -(OCH )(Me NCH )C B H ]Ti[η -OC(NMe )-
2 2 2 2 9 9 2
26
5
2
[
(1.836(4) Å)], 2c [(1.868(1) Å)], and 3a [1.838(1) Å] were
NPh]
(NMe )NPh] .
and [η :σ-Me C(C H )(C B H )]-Zr[η -OC-
2 9 6 2 10 10
27
in a similar range to that of the complexes 1a (1.8243(16) Å)
and 1c (1.853(1) Å). The bond distances between titanium and
guanidinate nitrogen atoms were Ti1−N6 (2.114(4) Å) and
Ti1−N7 (2.192(4) Å) for complex 2a, whereas for complex 2c
it was Ti1−N5 (2.065(1) Å) and Ti1−N6 (2.305(2) Å) and
was comparable to the corresponding distances in reported
complex [{(iPrN) CNMe } Ti(NMe )] (2.2080(6)−2.158(6)
2
2
In Fourier transform infrared (FT-IR) spectroscopy, the acyl
CO stretching frequency of 4b and 4c were observed at 1643
−1
and 1640 cm , which is substantially lower than that of the
−1
starting material phenyl isocyanate CO (2274 cm ), thus
indicating a marked decrease in CO bond strength upon
1
formation of the complexes 4b and 4c, respectively. The H
2
2
3
2
24
Å). In complex 3a, the delocalized π interaction within the
NCN moiety (N6−C1−N7) led to partial double bonds in C−
N with an average distance of 1.332 Å. Ti1−N6 and Ti1−N7
bond distances of 2.276(1) and 2.075(1) Å were comparable to
NMR spectrum for complexes 4b (Figure FS3 in Supporting
Information) and 4c was recorded in benzene-d . The spectral
6
data revealed that bis-insertion of phenyl isocyanate had taken
place. The dimethyl amido protons shifted downfield from δ
3.01 and 2.91 ppm obtained in complexes 1b and 1c to δ 3.33
and 3.31 ppm obtained in complexes 4b and 4c, respectively;
this was presumably due to coordination of two ureate moieties
formed after insertion of dimethyl amide into phenyl
isocyanate. Further, the migrated amido protons appeared at
δ 2.10 ppm in case of complex 4b, whereas at δ 1.56 ppm in
case of 4c as a sharp singlet. The olefinic protons of the
imidazolin-2-iminato ligand backbone resonated at δ 5.65 and
5.87 ppm for complexes 4b and 4c, respectively, which was
shifted slightly upfield from that observed in 1b (5.68 ppm)
and 1c (5.92 ppm). In case of complex 4b, the methyl protons
of mesityl group resonate at δ 2.37 and 2.33 ppm. Whereas in
case of complex 4c, the methyl protons of isopropyl groups
displayed the resonance signals as doublets centered at δ 1.20
and 1.25 ppm, respectively, with a coupling constant of 6.86 Hz
each along with the methine proton of the isopropyl group
the bond distances in the closely related complex [{(CyN) -
2
24
CNMe }Ti(NMe ) ].
2
2 3
The C−N bond lengths in the NCN fragment [N6−C1
1.355(6), N7−C19 1.315(6) Å] of complex 2a and complex
c [N6−C32 1.318(2), N5−C32 1.347(3) Å] are indicative of
9
2
the electron density delocalization within the guanidinate
ligand. The considerably longer distances of C19−N8
[
2
1.384(7) Å] in 2a and C32−N8 (1.397(2) Å) in complex
c indicate that -NMe moiety takes part in the conjugation in
2
the guanidinate building block in the respective complexes.
Preparation and Structural Characterization of Tita-
R
2
nium bis(ureate) Complex [(Im N)Ti[κ -OC(NMe )-
2
NPh] (NMe )] [R = Mes (4b) and Dipp (4c)]. To study
2
2
the reactivity of 1a−c toward heterocumulenes, we performed
stoichiometric reactions of complex 1 with carbodiimide,
phenyl isocyanate, and phenyl isothiocyanate and isolated the
stable product in each case. We found that carbodiimides could
13
1
resonating at δ 3.39 ppm as a broad septet. The C{ H} NMR
spectrum displayed significant changes as compared to phenyl
isocyanate (δ 134 ppm) for ipso-carbon of phenyl isocyanate,
which appeared at δ 150.5 (4b) and 147.9 (4c) ppm,
respectively. The carbonyl carbon atom resonated at 149.2
ppm, which was shifted slightly upfield to that of a similar
undergo monoinsertion into only the Ti−N
bond. Further,
amide
despite the presence of excess carbodiimides, complexes 2a, 2c,
and 3a did not undergo further reactions even at higher
reaction temperatures and longer reaction times. This can be
attributed to steric crowding [{(Cy) CNMe } Ti(NMe )],
2
2
3
2
D
DOI: 10.1021/acs.inorgchem.5b02302
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
5
2
complex [η :σ-Me Si(C H )(C B H )]Zr[η -OC(NMe )-
and similar for both the ligands. The Ti−N
bond distance
iminato
2
9
6
2
10 10
2
2
7
NPh] reported in literature.
[1.836(3) Å] is similar with that in 1c (1.853(2) Å), and the
bond angle of iminato nitrogen at 160.6(2)° (C1−N3−Ti1) is
narrower than that of starting material 1c (169.2(1)°), could be
due to higher steric congestion around the titanium ion in 4c as
compared to 1c.
2
To confirm the molecular structure of complex 4c, single-
crystal X-ray diffraction analysis was performed with a suitable
crystal, which was grown at −35 °C from concentrated n-
pentane. Complex 4c crystallized in the orthorhombic space
group Pna2 with eight molecules in the unit cell. Figure 4
We observed that, even in the presence of excess
carbidiimides and higher reaction temperature, carbodiimides
underwent monoinsertion only into the Ti−N σ−bond. The
formation of bis-inserted product 4c with the reaction of
isocyanate clearly indicates that the enhanced reactivity of
PhNCO is because of the greater electrophilic character of
allenic carbon. Further, reaction of excess isocyanate with
complex 1c resulted in the stable bis-inserted product 4c along
with trimer product 1,3,5-trisphenyl-1,3,5-triazinane-2,4,6-tri-
1
one (PhNCO) (5), which was isolated and characterized by
3
NMR spectroscopy. Spectra of product 5 were compared to the
28
reported values. Thus, the formation of compound 5 suggests
that the third equivalent of isocyanate was unable to undergo
further insertion into the Ti−N σ-bond, and it instead reacted
Dipp
2
with [(Im N)Ti[κ -OC(NMe )NPh] (NMe )] moiety to
2
2
2
result in the formation of cyclic trimer (PhNCO)3 (5),
presumably due to steric crowding around the titanium ion
Figure 4. Solid-state structure of complex 4c showing a distorted
octahedral polyhedron around a titanium metal ion. Hydrogen atoms
are omitted for clarity. Selected bond lengths [Å] and bond angles
27
(
Scheme 4). Xie and co-workers have already reported the
[
deg]: Ti1−N1 1.836(3), Ti1−N4 2.111(3), Ti1−N6 2.302(3), Ti1−
N8 1.926(3), Ti1−O1 2.070(2), Ti1−O2 2.168(2), C28−O1
.288(4), C28−N6 1.337(4), C28−N7 1.352(5), C31−O2 1.288(4),
Scheme 4. Reaction of Complex 1c with Excess Phenyl
Isocyanate
1
C31−N4 1.353(4), N1−C1 1.288(4), C1−N1−Ti1 160.6(2), N1−
Ti1−N4 106.4(1), N1−Ti1−N8 100.8(1), N1−Ti1−N6 155.5(1),
O2−Ti1−N4 62.0(1), N1−Ti1−O1 95.4(1), N6−Ti1−N4 92.8(1),
N6−Ti1−O1 60.5(9), N6−Ti1−O2 78.3(9), N8−Ti1−O1 107.7(1),
N8−Ti1−N6 91.5(1), N8−Ti1−O2 155.7(1).
5
shows the molecular structure of complex 4c along with
reaction of [η :σ-Me C(C H )(C B H )]Zr(NMe ) with
2
9
6
2
10 10
2 2
selected bond lengths and bond angles. The solid-state
excess of phenyl isocyanate thus affording formation of the
same cyclic trimer 5. Similar trimarization processes are also
Dipp
−
structure of 4c confirms the ligation of Im N , [OC(
−
−
25,32,33
29
NMe )NPh] , and Me N groups onto the titanium ion.
reported with lanthanide complexes and titanium com-
2
2
30
The geometry around the Ti(IV) center can be best described
as distorted octahedral. The coordination sphere of titanium
ion consists of one N atom of imidazolin-2-iminato ligand, two
plexes.
Preparation and Structural Characterization of Tita-
Dipp
2
nium Tris(thioureate) Complex [(Im N)Ti{κ -SC(NMe )-
2
2
1
N and O atoms coordinated κ fashion from ureate ligands
NPh} {κ -SC(NMe )NPh}] (6c). We observed the diverse
2
2
[
(OC(NMe )NPh)] derived from the insertion of phenyl
reactivity of imidazolin-2-iminato titanium complex with
2
isocyanates into two of the Ti−NMe bonds of the starting
various heteroallene molecules. In those reactions, Ti−N
2
amide
material, along with one remaining −NMe ligand. Coordina-
bond was monoinserted into RNCNR and bis-inserted
into PhNCO framework leading to the corresponding
guanidinate and ureate complexes, respectively. It was further
observed that excess equivalent of hetero allene moiety did not
2
tion of two ureate ligands resulted in the transformation of
coordination geometry of titanium from a distorted tetrahedral
(
in 1c) to a distorted octahedral (in 4c). Steric factors may also
have dominated migration of the −NMe unit, leading to the
react with the Ti−N
bond. To get a better insight into the
2
iminato
trans arrangement of two ureate groups in the complex 4c.
insertion reactions with polar allene, we performed a reaction of
1c with 3 equiv of phenyl isothiocyanate (PhNCS) in toluene
solution and at ambient temperature. The tris-insertion product
2
Both the κ -[(OC(NMe )NPh)] ureate ligands bonded to the
2
titanium ion through one oxygen and one nitrogen atom to
2
Dipp
2
1
yield a planar four-membered ring of sp -hybridized N and C
[(Im N)Ti{κ -SC(NMe )NPh} {κ -SC(NMe )NPh}] (6c)
2 2 2
centers with an average bite angle of 61.25° (O−Ti−N). The
was obtained in 54% yield (Scheme 5). It was noted that the
controlled reaction to isolate mono- or bis-inserted product led
to the formation of the complex 6c in each case.
Ti−O bond lengths are different and found to be 2.070(2) Å
(
Ti1−O1) and 2.168(2) Å (Ti1−O2)well in agreement with
1
5
that of reported complexes2.041(2) Å for [σ:η :η -(OCH )-
The complex 6c was characterized by standard NMR
2
3
26
(
Me NCH )C B H ]Ti[η -OC-(NMe )NPh] and 2.165(3)
spectroscopy and combustion analysis, and its solid-state
2 2 2 9 9 2
5
2
1
Å for [η :σ-Me C(C H )(C B H )]Zr[η -OC-(NMe )-
structure was established by single-crystal X-ray analysis. H
2
9
6
2
10 10
2
2
7
NPh]2. The delocalized π interactions within the OCN
moiety (O2−C31−N4 and O1−C28−N6) lead to partial
double bond character upon the C−N frame with an average
distance of 1.345 Å, whereas O−C bond lengths were 1.288 Å
NMR spectra measured in C D showed characteristic peaks at
6
6
δ 1.21 and 1.18 ppm for diastereotopic methyl protons of
isopropyl groups with coupling constant of 6.86 Hz each,
whereas methine protons of isopropyl groups appeared at δ
E
DOI: 10.1021/acs.inorgchem.5b02302
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Inorganic Chemistry
Article
Scheme 5. Synthesis of Complex 6c from 1c
third thioureate ligand coordinated through the sulfur atom in a
κ -mode. The reason why sulfur was preferred over nitrogen to
1
coordinate to the titanium ion in the latter case was that steric
hindrance around the titanium metal center did not allow the
2
third thioureate ligand to coordinate in κ -mode. Two four-
membered metallacycles (S3−C46−N8−Ti1 and S2−C37−
N6−Ti1) were formed by the chelating of two thioureate
building blocks having an average bite angle of (S−Ti−N)
1
6
5.68°. The Ti−S bond distance observed for κ -(SC(NMe )-
2
NPh) moiety [Ti1−S1 2.393(7) Å] was much shorter than that
2
present in κ -(SC(NMe )NPh) moieties (Ti1−S2) 2.464(7)
2
3.29 ppm as broad septets. The olefinic protons of the
and (Ti1−S3) 2.516(7) Å due to greater accumulation of
1
imidazolin-2-iminato ligand backbone resonated at δ 5.83 ppm,
which was slightly shifted upfield to that of complex 1c (5.92
ppm). Aromatic protons of three phenyl rings resonated at δ
negative charge over sulfur atom in κ -(SC(NMe
2
)NPh)
moiety, thus indicating that S1 is bonded to titanium ion by
2
σ-bonding, whereas S2 and S3 present in κ -(SC(NMe )NPh)
2
1
3
1
7.37−7.20 ppm. The C{ H} NMR spectra gave crucial
moieties were participating in delocalization of electronic
charge through SCN skeletons. The delocalized π interactions
within the SCN moiety (S2−C37−N6 and S3−C46−N8) led
to partial double bonded C−N distances [C37−N7 1.362(3) Å
and C46−N9 1.357(3) Å], whereas the S−C bond lengths were
similar for both the ligands. The S2−C37 and S3−C46 bond
lengths [1.748(3) and 1.728(3) Å] are slightly shorter than that
of S1−C28 1.790(2) Å, thereby indicating that S1−C28 is
purely of single-bond character.
informationtwo resonance peaks at δ 151.2 and 150.8 ppm
were observed by usthese can be assigned to the two
thioureate carbons (NCS), and the values are slightly upfield
shifted to that (174.91 ppm) of complex reported by Xie and
5
2
co-workers [η :σ-Me Si(C H )(C B H )]Zr(NMe )[η -SC-
2
9
6
2
10 10
2
n
27
(
NMe )N Bu]. Two distinct resonating signals for SCN
2
carbon appeared due to the different electronic environments
2
1
of ambidentate thioureate ligand, which showed both κ and κ
coordination modes. The ipso-carbon of phenyl ring appeared
at δ 147.5, 147.0, and 134.8 ppm.
The complex 6c is a rare example in which the hard titanium
metal center is preferred to coordinate a soft sulfur atom
instead of a hard nitrogen donor from the thioureate scaffold to
achieve a lesser crowding arrangement around the metal center,
as the coordination through nitrogen could lead to much more
steric crowding onto the metal ion.
Catalytic Addition of N−H Bond to Isocyanates in the
Presence of Precatalyst Complex 1c. In the stoichiometric
reaction of 1 with various heterocumelenes like carbodiimide,
phenyl isocyanate, and phenyl isothiocyanate, we observed that
carbodiimides underwent monoinsertion, whereas isocyanate
yielded bis-insertion, while isothiocyanate afforded tris-
The solid-state structure of complex 6c (shown in Figure 5)
confirmed the attachment of thioureate fragment to titanium
metal through two different coordination modes. Two
thioureate ligand moieties coordinated to the titanium metal
2
ion in κ -mode through sulfur and nitrogen atoms, while the
insertion into the Ti−N
bond of complex 1. From these
amido
results we can envisage that titanium complex 1 can be used as
a precatalyst for the preparation of guanidine and urea
derivative by the reaction of either carbodiimides or isocyanates
with various amines through insertion reactions. Using a similar
approach, Tamm and Eisen recently demonstrated the catalytic
addition of E−H bonds (E = N, P, S) to carbodiimides,
isocyanates, and isothiocyanates using thorium complex
Dipp
16
[
Th(Im N){N(SiMe ) } ].
3 2 3
In this study, we described the catalytic addition of N−H
bonds from various amines to phenyl isocyanate and
carbodiimides with complex 1c as the precatalyst. Catalytic
experiments were performed using 5 mol % of titanium
Dipp
complex [Im NTi(NMe ) ] (1c) and equimolar amounts of
2
3
Figure 5. Solid-state structure of complex 6c. Hydrogen atoms are
omitted for clarity. Selected bond lengths [Å] and bond angles [deg]:
Ti1−N1 1.800(2), N1¬−C1 1.304(3), Ti1−S1 2.393(7), Ti1−S2
either isocyanate or carbodiimides and amines, which were
added to a toluene solution of the catalyst under an inert
atmosphere (Scheme 6).
2
.464(7), Ti1−S3 2.516(7), Ti1−N8 2.180(2), Ti1−N6 2.250(2),
S3−C46 1.728(3), N8−C46 1.325(3), C46−N9 1.357(3), S1−C28
.790(2), N5−C28 1.290(3), N4−C28 1.368(3), S2−C37 1.748(3),
The reaction mixture in each case was kept under stirring
either at room temperature or elevated temperature (60 °C for
entry 2) for 12 h, and the respective urea or guanidine products
1
C37−N6 1.318(3), N7−C37 1.362(3), Ti1−N1−C1 170.3(2), N1−
Ti1−S1 90.5(6), N1−Ti1−N6 157.2(8), N1−Ti1−N8 102.7(8), N1−
Ti1−S2 91.8(6), N1−Ti1−S3 100.3(6), S3−Ti1−S2 158.9(3), S1−
Ti1−S3 92.4(3), S3−Ti1−N8 65.7(5), S2−Ti1−N6 92.3(5), S2−
Ti1−N8 94.8(5), S3−Ti1−N6 102.2(5), S1−Ti1−S2 104.9(3), S2−
C37−N6 112.5(2), S2−C37−N7 120.2(2), N6−C37−N7 127.2(2),
S3−C46−N8 113.5(2), S3−C46−N9 121.6(2), N8−C46−N9
1
were isolated. The isolated products were analyzed through H
13
and C NMR spectroscopy, and yields were calculated after
isolation of pure products (Table 1). The reaction of phenyl
isocyanate with aromatic amines like 2,6-diisopropylaniline
mesityl amine and 2,6-dimethylamine afforded the respective
urea derivative in high yield (entries 1−3). However, the
reactions with 1-fluoroaniline were sluggish, as only low yield of
1
24.9(2), S1−C28−N5 125.2(2), S1−C28−N4 116.8(2), N4−C28−
N5 117.8(2).
F
DOI: 10.1021/acs.inorgchem.5b02302
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Inorganic Chemistry
Article
Scheme 6. Catalytic Formation of Urea and Guanidine
Derivatives Using 1c as Precatalyst
mides (N,N′-dicyclohexyl or N,N′-diisopropyl) to yield the
corresponding guanidine were not observed (entries 5 and 6)
even after prolonged reaction times and heating to 80 °C. Thus,
the scope of catalytic addition of the N−H bonds by using
imidazolin-2-iminato titanium complex 1c as catalyst was
limited to the phenylisocyanate.
Scheme 7. Most Plausible Mechanism for the Catalytic
Addition of N−H Bond to Isocyanate by Titanium
Precatalyst 1c
Table 1. Catalytic Reactions of Isocyanates with ArNH
Using 1c as Precatalyst
2
a
The most plausible mechanism for the catalytic addition of
N−H bond to isocyanate by the titanium precatalyst 1c is
shown in Scheme 6. Similar mechanistic cycles were also
demonstrated recently by Tamm and Eisen using [Th-
Dipp
16
(
Im N){N(SiMe ) } ] complex as precatalyst. The stoi-
3 2 3
chiometric reaction between PhNCO and the complex 1c
led to the formation of titanium complex 4c whose molecular
structure was already established. This indicated that
nucleophilic attack by the titanium bound -NMe group to
2
generate complex 4c occurred at the phenyl isocyanate carbon
center in the initiation step. In the next step, complex 4c
reacted with 2 equiv of amines to give the corresponding
titanium amido complex 7, which was converted to ureate
complex 4c′, analogous to 4c, by the reaction of 2 equiv of
phenyl isocyanate. In the final step to propagate the catalytic
cycle (Scheme 6), complex 4c′ reacted with another 2 equiv of
amine to yield the product urea and the active catalyst 7.
To get more insight about the mechanistic cycle, we
performed a stoichiometric reaction of 4b and freshly distilled
2
,6-diisopropylaniline in toluene at ambient temperature to
isolate the titanium amido complex 7a. The resulting
compound was dried under vacuum. Several attempts to
crystallize the compound were not successful, and the NMR
spectrum of the crude compound is presented in Supporting
Information (FS 13). The spectral data of the crude compound
indicated the formation of complex 7a along with the
corresponding urea derivative.
However, the formation of the active catalyst was confirmed
by isolating the titanium complex 7b prepared by the
stoichiometric reaction of complex 4b in toluene solution at
ambient temperature (Scheme 8). We chose the iminopyrole
ligand as an amine source due to its bulky nature and potential
bidentate coordination mode, which could stabilize the
corresponding intermediate titanium complex.
a
All reactions were performed in toluene solvent; reaction time 12 h at
room temperature (60 °C for entry 2); compounds were isolated and
purified by washing with n-pentane. All compounds were characterized
by NMR spectroscopy using C D6 for compounds a−d, whereas
6
b
CDCl for compounds e−h as solvent. Yields are calculated from
3
isolated pure products.
Scheme 8. Synthesis of Active Catalyst 7b from Complex 4b
the respective urea derivatives as products were observed (entry
4) under similar conditions. While the conversion of the
aliphatic primary amines (entry 5) was quite good, the
secondary amines (entry 6 and 7) were converted to their
respective inserted product in much higher yield presumably
due to positive inductive effect of the alkyl groups attached to
the nitrogen atom. In contrast, the conversions of carbodii-
G
DOI: 10.1021/acs.inorgchem.5b02302
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Inorganic Chemistry
Article
Complex 7b was characterized using spectroscopic and
short Ti−N bond in all the complexes was observed to manifest
the strong electron donation from the imidazolin-2-iminato
fragment to the titanium ion. In the stoichiometric reaction of 1
with carbodiimides, monoinsertion of the RNCNR to the Ti−
1
13
1
analytical methods. H and C{ H} NMR spectra were
1
recorded in benzene-d . H NMR spectra showed two sharp
6
resonance peaks at 7.92 and 7.75 ppm, which could be assigned
to the imine proton of two different iminopyrrolyl ligands
Namide occurred. However, the reactions with isocyanate and
1
2
coordinated in κ and κ mode. The molecular structure of
complex 7b, as shown in Figure 6, was confirmed by single-
crystal X-ray diffraction analysis.
isothiocyanate with 1 under a similar condition yielded bis- and
tris-inserted products, respectively, indicating the enhanced
electrophilicity of the heterocumelene carbon center in
isocyanate and isothiocyanate rather than that in carbodiimides.
We have also demonstrated the scope of the catalytic addition
of the N−H bonds of various amines with phenyl isocyanate
and carbodiimides (DCC or DIC) by using imidazolin-2-
iminato titanium complex 1c, and observed that while a smooth
conversion of the amines with isocyanate occurred, no
conversion was observed with carbodiimides under similar
reaction conditions. In addition, we have established the
mechanism of the catalytic reaction confirming the molecular
structure of the active catalyst 7b.
EXPERIMENTAL SECTION
■
General Consideration. All manipulations of air-sensitive
materials were performed with the rigorous exclusion of oxygen and
moisture in flame-dried Schlenk-type glassware either on a dual-
manifold Schlenk line interfaced with a high-vacuum (1 × 10− Torr)
line or in an argon-filled M. Braun glovebox. Hydrocarbon solvents
(toluene and n-pentane) were distilled under nitrogen from LiAlH4
4
Figure 6. Molecular structure of complex 7b. Hydrogen atoms
omitted for clarity. Selected bond lengths [Å] and bond angles [deg]:
Ti1−N1 1.806(2), Ti1−N4 1.916(2), Ti1−N5 2.139 (2), Ti1−N6
and stored in the glovebox. Dichloromethane was freshly distilled
1
2
.266(2), Ti1−N7 2.119(2), N1−C1 1.309(3), C42−N7 1.386(3),
under CaH and stored under molecular sieves. H NMR (400 MHz)
2
and 13C{ H} NMR (100 MHz) spectra were recorded on a Bruker
Avance III-400 spectrometer. A Bruker Alpha FT-IR was used for the
FT-IR measurements. Elemental analyses were performed on a Bruker
1
C43−N8 1.283(3), N5−C25 1.386(3), C26−N6 1.309(3), C1−N1−
Ti1 160.0(1), N1−Ti1−N4 103.4(8), N1−Ti1−N5 103.6(7), N1−
Ti1−N6 107.0(7), N1−Ti1−N7 94.9(7), N6¬−Ti1−N7 152.1(7),
N5¬−Ti1−N7 84.8(7), N4−Ti1−N5 150.3(7).
Euro EA at the Indian Institute of Technology Hyderabad. Imidazolin-
R
2
(
-imines Im NH (R = tBu, Mes, Dipp) and N-aryliminopyrrolyl
17
ImpDipp) were prepared according to the published procedures.
̅
Complex 7b crystallized in the triclinic space group P1, with
Ti(NMe ) , carbodiimides, tert-butylamine, 2,6-diisopropylaniline, and
2
4
two molecules in the unit cell along with two molecules of
toluene as solvent. The coordination polyhedron in complex 7b
was formed by the coordination with one imidazolin-2-iminato
nitrogen atom, two pyrrolide, and one imine nitrogen atom
from two iminopyrrolyl ligands, and one nitrogen atom from
the dimethylamide group. The overall geometry of the central
titanium ion can be described as a distorted square pyramidal
with the imido nitrogen N1 atom positioned at the axial
position and the other four nitrogen atoms, N4, N5, N6, and
N7, forming the basal plane of the square pyramid. The Ti−
mesityl amine were purchased from Alfa Aesar and used as received.
The NMR solvents CDCl and C D were purchased from Sigma-
3
6
6
Aldrich and dried by either molecular sieve (CDCl ) or a Na/K alloy
3
(C D ) prior to use. Caution! Phenyl isocyanate and phenyl
6 6
isothiocyanate are very hazardous substances.
General Procedure for the Preparation of Imidazolin-2-
R
iminato Titanium Amido Complexes [(Im N)Ti(NMe ) ] [R =
2
3
tBu, (1a); Mes, (1b) and Dipp, (1c)]. In a 25 mL dry Schlenk flak,
respective imidazoline-2-imine [Im NH; R = tBu (196 mg), R = Mes
R
(320 mg), and R = Dipp (404 mg)] was placed, and 5 mL of dry
toluene was added to it. To this solution, a mixture of tetrakis-
(dimethylamino)titanium (225 mg) and 5 mL of toluene was also
added. The solution immediately turned red, and resulting mixture was
heated to 90 °C for 12 h and kept under stirring condition. The
solvent was evaporated under vacuo to obtain a red colored solid,
which was recrystallized as yellow crystals from n-pentane at −35 °C.
Compounds 1a−c are soluble in THF, n-pentane, n-hexane, and
toluene.
N
bond distance [1.806(2) Å] for 7b was within the
iminato
values obtained for 4c [1.836(3) Å]. Further, the Ti1−N1−C1
angles [160.0(1)° for 7b vs 160.6(2) for 4c] indicate a stronger
coordination of the imidazolin-2-iminato nitrogen to the
titanium ion. From the molecular structure we could observe
2
that one iminopyrrolide ligand was bonded in κ mode, whereas
the second iminopyrrolide ligand was coordinated to the
tBu
Analytical Data for [(Im N)Ti(NMe ) ]) (1a). Yield: 448 mg
2
3
1
titanium atom through κ mode. However, such differentiation
1
(
(
95%). H NMR (400 MHz, C D , 25 °C): δ 5.95 (s, 2H, NCH), 3.37
6 6
13 1
in coordination modes can presumably be due to the smaller
ionic radius of the titanium ion and sterically larger size of the
s, 18H, N(CH ) ), 1.58 (s, 18H, C(CH ) ) ppm. C{ H} NMR
3
2
3 3
(100 MHz, C D , 25 °C): 141.2 (NCN), 106.9 (NCHCHN) 55.6
6
6
ligands. The bond distances of Ti−N [Ti1−N5 2.139(2) Å;
(C(CH
calculated (%) for C45
3
)
3
), 45.8 (N(CH
3
)
2
), 28.0 (C(CH
2
3
)
3
). Elemental analysis
pyr
tBu
Ti1−N7, 2.119(2) Å] were shorter than those of Ti1−N
H
97
N
15Ti (944.1; 2x1a.Im NH): C 57.25, H
imine
1
0.36, N 22.25; found C 56.89, H 9.93, N 21.83.
[
Ti1−N6, 2.266(2) Å] and are in the range reported in the
Mes
31
Analytical Data for [(Im N)Ti(NMe ) ]) (1b). Yield: 437 mg
2
3
literature.
1
(
(
(
1
88%). H NMR (400 MHz, C D , 25 °C): δ 6.80 (s, 4H, m-Ph), 5.68
6 6
s, 2H, NCH), 3.01 (s, 18H, N(CH ) ), 2.25 (o-attached CH ), 2.13
3
2
3
SUMMARY
■
13
1
p-attached CH ) ppm. C{ H} NMR (100 MHz, C D , 25 °C):
3
6
6
With this contribution, we have demonstrated the synthesis and
molecular structures of three different (mono) imidazolin-2-
iminato titanium dimethylamide complexes (1a−c). A very
52.0 (ipso-phenyl), 138.6 (NCN), 137.0 (o-phenyl), 134.9 (p-
phenyl), 128.9 (m-phenyl), 111.7 (NCHCHN), 38.9 (NCH ),
3
21.2 (p-attached CH ), 18.6 (o-attached CH ). Elemental analysis
3
3
H
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Inorganic Chemistry
Article
calculated (%) for C H N Ti (498.5): C 65.05, H 8.49, N 16.86;
N(CH ) ), 2.37 (s, 6H, CH ), 2.33 (s, 12H, CH ), 2.10 (s, N(CH ) )
2
7
42
6
3
2
3
3
3 2
ppm. 1 C{ H} NMR (100 MHz, C D , 25 °C): 150.5 (OCN), 148.3
3
1
found C 64.72, H 8.19, N 16.41.
6 6
Dipp
Analytical Data for [(Im N)Ti(NMe ) ]) (1c). Yield: 525 mg
(ipso-Ar), 141.5 (NCN), 137.8 (o-phenyl), 137.2 (p-Ar), 134.2 (ipso-
Ar), 129.0 (m-Ar), 127.8 (o-Ph), 127.4 (m-Ph), 120.7 (p-Ph), 112.5
(NCHCHN), 51.3 (N(CH ) ), 37.5 (N(CH ) ), 21.2 (o-attached
CH ), 18.3 (p-attached CH ) ppm. Elemental analysis: C H N O Ti
3 3 41 51 8 2
2
3
1
(
7
90%). H NMR (400 MHz, C D , 25 °C): δ 7.34−7.17 (m, 4H, Ar),
6
6
.15−7.10 (m, 2H, Ar), 5.92 (s, 2H, NCH), 3.24 (sept, 4H,
3
2
3 2
CH(CH ) , 2.91 (s, 18H, N(CH ) ), 1.41 (d, 12H, J = 6.85 Hz,
3
2
3
2
13
1
CH(CH ) ), 1.20 (d, 12H, J = 6.85 Hz, CH(CH ) ) ppm. C{ H}
(736.8): Calcd (%) C 66.84, H 7.11, N 15.21. Found C 66.59, H 6.93,
N 14.88.
3
2
3 2
NMR (100 MHz, C D , 25 °C): 147.9 (ipso-phenyl), 141.5 (NCN),
6
6
Dipp
2
1
35.3 (o-phenyl), 129.3 (p-phenyl), 124.1 (m-phenyl), 113.8 (NCH
Analytical Data for [(Im N)Ti(κ -OC(NMe )-NPh) (NMe )]
2 2 2
1
CHN), 45.1 (NCH ), 29.0 (CHCH ), 24.6 (CHCH ), 23.4 (CHCH ).
(4c). Yield: 197 mg, 70%. H NMR (400 MHz, C D ): δ 7.23−7.17
3
3
3
3
6
6
Elemental analysis calculated (%) for C H N Ti (582.7): C 68.02, H
(m, 2H, Ar), 7.15−7.10 (m, 10H, Ph), 6.86−6.80 (m, 4H, Ar), 5.87 (s,
33
54
6
3
9
.34, N 14.42; found C 67.79, H 8.87, N 14.07.
General Procedure for the Preparation of Imidazolin-2-
2H, NCH), 3.39 (sept, 4H, J = 6.80 Hz CH(CH ) , 3.13 (s, 6H,
HH
3 2
3
N(CH ) ), 1.56 (s, 12H, N(CH ) ), 1.25 (d, 24H, J = 6.85 Hz,
3 2 3 2 HH
R
3
13
1
iminato Titanium Mono(guanidinate) Complexes [(Im N)Ti-
CH(CH ) ), 1.20 (d, 12H, J = 6.88 Hz, CH(CH ) ) ppm. C{ H}
3 2 HH 3 2
(
R′NC(NMe )NR′)(NMe ) ] (R = tBu; R′ = iPr (2a), R = Dipp; R′ =
2
2 2
NMR (100 MHz, C D , 25 °C): 149.2 (OCN), 147.9 (ipso-phenyl),
6 6
iPr (2c); R = tBu; R′ = Cy (3a)). In a 25 mL dry Schlenk flask,
compound 1 (0.267 mmol) was placed, and a solution of respective
carbodiimides (0.267 mmol) and 5 mL of toluene was added on to it.
The reaction mixture was kept heated to 90 °C and kept under stirring
condition for 6 h. The solvent was evaporated under vacuo to obtain a
red residue, which was washed with n-pentane (2 × 5 mL). The title
compound was recrystallized from toluene at −35 °C.
1
1
3
41.5 (NCN), 138.6 (o-phenyl), 134.9 (ipso-Ph), 129.3 (m-phenyl),
27.9 (Ph), 123.7 (p-phenyl), 114.3 (NCHCHN), 51.6 (N(CH ) ),
3
2
7.4 (N(CH ) ), 29.0 (CHCH ), 24.8 (CH(CH ) ), 23.7 (CH-
3
2
3
3 2
(
CH ) ) ppm. FT-IR (selected frequencies): ν = 3298 (s), 2957 (s),
3 2
2
8
865 (s), 1640 (s), 1530 (s), 1471 (m), 1370 (m), 1247 (s), 1059 (w),
−1
05 (s), 753 (s), 603 (m) cm . Elemental analysis: C H N O Ti
47
64
8
2
(820.46): Calcd (%) C 68.76, H 7.86, N 13.65. Found C 67.92, H
tBu
Analytical Data for [(Im N)Ti(iPrNC(NMe )-NiPr) (NMe ) ]
2
2 2
7
.57, N 13.23.
1
2a). Yield: 113 mg, 85%. H NMR (400 MHz, C D , 25 °C): δ 5.85
s, 2H, NCH), 3.51−3.58 (m, 2H, CH(CH ) ), 3.39 (s, 12H,
6 6
Preparation of of 1,3,5-trisphenyl-1,3,5-triazinane-2,4,6-
3
2
trione (5). A solution of 1c (200 mg, 0.343 mmol) in toluene (6
mL) was added to a solution of phenyl isocyanate (245.13 mg, 2.058
mmol) in toluene (15 mL). The reaction mixture was kept under
stirring at room temperature for 6 h. Solvent was evaporated under
vacuo, and the compound was extracted in n-pentane (20 mL) and
kept for crystallization after reducing the solvent volume to 4 mL. Red
colored crystals were obtained after 12 h, and the crystals that were
separated from mother liquor and characterized by X-ray crystallog-
raphy were identified as complex 4c. From same mother liquor
colorless crystals obtained after 24 h were characterized by NMR
3
2
3
2
3 3
3
13
1
1
2
2H, J = 6.4 Hz, CH(CH ) ppm. C{ H} NMR (100 MHz, C D ,
HH 3 6 6
5 °C): 153.2 (NCN), 141.9 (NCN), 107.3 (NCHCHN) 57.4
(
N(CH ) ), 55.6 (C(CH ) ), 45.8 (N(CH ) ), 28.0 (C(CH ) ) ppm.
3
2
3
3
3
2
3 3
FT-IR (selected frequencies): ν = 2922 (s), 2843 (s), 2753 (s), 1636
(
(
5
m), 1512 (s), 1442 (m), 1389 (m), 1164 (s), 943 (s), 801 (s), 550
−1
s) cm . Elemental analysis: C H N Ti (500.38): Calcd (%) C
24
52
8
7.58, H 10.47, N 22.38. Found C 57.31, H 10.24, N 21.98.
Analytical Data for [(ImDippN)Ti(iPrNC(NMe )-NiPr) (NMe ) ]
2
2 2
1
(2c). Red crystals were obtained after 2 d. Yield: 151 mg, 80%. H
1
spectroscopy and identified as compound 5. Yield: 0.105 g, 42%. H
NMR (400 MHz, C D , 25 °C): δ 7.20−7.15 (m, 6H, Ar), 5.95 (s, 2H,
6
6
13
1
NMR (400 MHz, CDCl ): δ 7.53−7.40 ppm. (m, 15H, Ar); C{ H}
3
NCH), 3.71−3.76 (m, 2H, CH(CH ) ), 3.46 (m, 4H, CH(CH ) ),
3
2
3 2
3
NMR (100 MHz, CDCl
) δ 148.4, 133.2, 129.1, 128.0 ppm. Elemental
3
2
6
=
.88 (s, 12H, N(CH ) ), 2.50 (s, 6H, N(CH ) ), 1.14 (d, 12H, J
.80 Hz, CH(CH ) ), 1.12 (br, 12H, CH(CH ) ), 1.16 (d, 12H, J
6.84 Hz, CH(CH ) ) ppm. C{ H} NMR (100 MHz, C D , 25
=
3
2
3
2
HH
3
analysis: C H N O (357.36): Calcd (%) C 70.58, H 4.23, N 11.76.
21
15
3
3
3
2
3
2
HH
13
1
Found C 70.24, H 4.65, N 11.12.
3
2
6
6
Preparation of Imidazolin-2-iminato Titanium Tris-
°
C): 153.2 (NCN), 150.1 (ipso-phenyl), 138.2 (NCN), 135.3 (o-
phenyl), 130.2 (m-phenyl) 126.0 (p-phenyl), 116.3 (NCHCHN)
9.4 (N(CH ) ), 45.8 (N(CH ) ), 29.0 (CH(CH ) ), 24.6 (CH-
Dipp
2
1
(
(
thioureate) Complex [(Im N)Ti{κ -SC(NMe )NPh} {κ -SC-
2 2
NMe )NPh}] (6c). A solution of 2c (200 mg, 0.343 mmol) in
2
4
3
2
3
2
3 2
toluene (6 mL) was added to a solution of phenyl isothiocyanate
139.11 mg, 1.029 mmol) in toluene (8 mL), which resulted an
(CH ) ), 23.4 (CH(CH ) ) ppm. Elemental analysis: C H N Ti
3 2 3 2 40 68 8
(
(
9
706.85): Calcd (%) C 67.77, H 9.67, N 15.81; Found C 67.34, H
immediate formation of dark red solution. The resulting reaction
mixture was stirred for another 6 h at ambient temperature. The
solvent was evaporated under vacuo, and the compound was extracted
.26, N 15.21.
Analytical Data for [(ImtBuN)Ti(CyNC(NMe )-NCy)(NMe ) ]
2
2 2
1
(
5
3a). Yield 106 mg, 68%. H NMR (400 MHz, C D , 25 °C): δ
6 6
from 15 mL of n-pentane. The title compound was recrystallized from
.90 (s, 2H, NCH), 3.58 (s, 12H, N(CH ) ), 3.22−3.19 (m, 2H, Cy),
3
2
1
3
mL of n-pentane at −35 °C. Yield: 0.183 g, 54%. H NMR (400
2
.64 (s, 6H, N(CH ) ), 1.88−1.85 (m, 10H, Cy), 1.60 (s, 18H,
3
2
13
1
MHz, C D ): δ 7.37−7.20 (m, 15H, Ph), 7.03−6.98 (4H, Ar), 6.82−
6
6
C(CH ) ), 1.28−1.24 (m, 10H, Cy) ppm. C{ H} NMR (100 MHz,
3
3
3
6
.76 (m, 2H, Ar), 5.83 (s, 2H, NCH), 3.28 (sept, 4H, J = 6.84 Hz
HH
C D , 25 °C): 169.6 (NCN), 138.4 (NCN), 106.6 (NCHCHN)
6
6
CH(CH ) ), 3.4 (s, 6H, N(CH ) ), 2.38 (s, 12H, N(CH ) ), 1.21 (d,
3
2
3
2
3 2
5
7.4 (N(CH ) ), 55.6 (C(CH ) ), 50.0 (N(CH ) ), 40.4 (Cy), 35.9
3 2 3 3 3 2
3
3
2
4H, J = 6.85 Hz, CH(CH ) ), 1.18 (d, 12H, J = 6.84 Hz,
HH 3 2 HH
(
Cy), 29.0 (C(CH ) ), 27.0 (Cy), 26.5 (Cy) ppm. FT-IR (selected
3 3
13
1
CH(CH ) ) ppm. C{ H} NMR (100 MHz, C D , 25 °C): 151.2
3
2
6
6
frequencies): ν = 3234 (w), 2929 (w), 2843 (w), 2279 (s), 1619 (m),
1
−1
(SCN), 150.8 (SCN), 147.8 (ipso-phenyl), 147.5 (ipso-phenyl), 147.0
585 (s), 1452 (m), 1329 (m), 1112 (s), 811 (s) cm . Elemental
(
ipso-Ph), 137.8 (o-phenyl), 134.8 (ipso-Ph), 130.1 (m-phenyl), 129.4
analysis: C H N Ti (580.73): Calcd (%) C 62.05, H 10.41, N 19.30.
30
60
8
(Ar), 127.9 (Ph), 127.1 (Ph), 124.7 (p-phenyl), 124.5 (p-phenyl),
Found C 61.88, H 10.37, N 19.21.
1
23.4 (Ar), 121.8 (Ar), 120 (Ar), 114.6 (NCHCHN), 51.6
Preparation of Imidazolin-2-iminato Titanium Bis(ureate)
R
2
(N(CH ) ), 40.6 (N(CH ) ), 40.5 (N(CH ) ), 25.4 (CH(CH ) ),
Complex [(Im N)Ti{κ -OC(NMe )NPh} (NMe )] (4b, R = Mes, 4c,
3
2
3
2
3
2
3 2
2
2
2
R = Dipp). A solution of 1b (200 mg, 0.401 mmol) or 1c (200 mg,
.343 mmol) in toluene (6 mL) was added to a solution of phenyl
25.1 (CH(CH
C H N S Ti (988.25): Calcd (%) C 65.63, H 7.04, N 12.76.
54 69 9 3
3 2 3 2
) ), 23.3 (CH(CH ) ) ppm. Elemental analysis:
0
isocyanate (95.53 mg, 0.802 mmol for 4b) and (81.71 mg, 0.686 mmol
for 4c) in toluene (8 mL). An immediate dark red colored solution
was formed. Resulting reaction mixture was stirred for 6 h at ambient
temperature. The solvent was evaporated under vacuo, and the
compound was extracted from 15 mL of n-pentane. The compound 4c
was recrystallized from 3 mL of n-pentane.
Found C 65.26, H 6.84, N 12.69.
Mes
Preparation of [(Im N)Ti{2-(2,6-iPr
C
H
NCH)-C
NMe ) ] (7b). In a flame-dried 25 mL Schlenk flask compound 4b
2 2
H N} -
4 3 2
2
6
3
(
[prepared from 200 mg, 0.401 mmol 1b and phenyl isocyanate (96
mg, 0.806 mmol) analogous to 4c] and 5 mL of toluene was taken. To
this solution, (2,6-diisopropylphenyl)[(1H-pyrrol-2-yl)methylene]-
amine (204.1 mg, 0.804 mmol) was added along with 5 mL of
toluene. The resulting reaction mixture was kept under stirring for 12
h at room temperature. The solvent was evaporated under vacuo, and
Mes
2
Analytical Data for [(Im N)Ti(κ -OC(NMe )-NPh) (NMe )]
2
2
2
1
(
(
4b). Yield: 222 mg, 75%. H NMR (400 MHz, C D ): δ 7.09−7.15
m, 10H, Ph), 6.81 (s, 4H, Ar), 5.65 (s, 2H, NCH), 3.33 (s, 6H,
6
6
I
DOI: 10.1021/acs.inorgchem.5b02302
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
red color solid was obtained, which was dissolved in toluene (10 mL)
and filtered off. The filtrate was concentrated to 3 mL and was stored
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6
4
80
8
(
(
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3
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X-ray crystallographic information for 1b. (CIF)
X-ray crystallographic information for 1c. (CIF)
X-ray crystallographic information for 2a. (CIF)
X-ray crystallographic information for 2c. (CIF)
X-ray crystallographic information for 3a. (CIF)
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Corresponding Author
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*
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The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
(i) Riegert, D.; Collin, J.; Meddour, A.; Schulz, E.; Trifonov, A. J. Org.
This work was supported by Science and Engineering Research
Board (SERB), Department of Science and Technology (DST),
India, under Project No. SB/S1/IC/045/2013. The instru-
mental facilities were provided by Indian Institute of
Technology Hyderabad (IITH). K.N. and J.B. thank the
Univ. Grants Commission, India, for their Ph.D. fellowships.
We thank too the reviewers for their valuable comments to
improve the manuscript.
Chem. 2006, 71, 2514−2517. (j) Bu
̈
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