Stable Cu(I) complexes with thioamidoguanidine possessing halide-bridge structure

Article abstract of DOI:10.1021/ic301429g

Treatment of an aryl-sec-alkyl unsymmetrical thiourea (Tu) with Cu ^IIX₂ (X = I, Br) transform the thiourea (Tu) into a thioamidoguanidino (Tag) moiety with concomitant reduction of Cu^II to Cu^I, which forms a [Cu₂^I(μ₂-X) ₂Tag₂] (X = I (for A) and X = Br (for B)) complex. Meanwhile, the treatment of same unsymmetrical thiourea (Tu) with Cu ^IX (X = Br) forms a stable cluster with a [Cu^I ₃(μ₂-S)₄Tu₄X₃] core (C). Single crystal X-ray diffraction revealed that compounds A and B exhibit 1D chain with a Cu₂(μ₂-X)₂ core, whereas compound C is a Cu^I₃S₄Br₃ cluster. Compound A is centrosymmetric due to the trans orientation of two Tag units whereas compound B is ascentric due to the cis orientation of two Tag units. In compound A, the Cu₂I₂ core is perfectly rhomboidal where the iodine atoms are trans oriented. However in compound B, the Cu ₂Br₂ core is not perfectly rhomboidal (bowl shaped) and the bromine atoms are cis oriented. It is interesting to note that although the Tag moiety in compounds A and B contain two morpholine-O atoms; only one of the morpholine-O atoms (O2) is involved in the generation of three-dimensional network. The Cu^I₃S₄Br₃ cluster in compound C contains one tri- and two tetra-coordinated Cu^I centers. The Cu^I cluster in C contains a Cu₂(μ₂-S) ₂ rhomboidal plane exhibiting a chair and a boat form containing the Cu^I₃S₃ unit. In compounds A and B, the two Cu^I centers are μ₂-X bridged and have a μ₁-S linkage, whereas in compound C the linkages are opposite having four μ₂-S bridges and three μ₁-Br linkages.

Full text of DOI:10.1021/ic301429g

Article
pubs.acs.org/IC
Stable Cu(I) Complexes with Thioamidoguanidine Possessing Halide-
Bridge Structure
Santosh K. Sahoo, Nilufa Khatun, Himanshu Sekhar Jena, and Bhisma K. Patel*
Department of Chemistry, Indian Institute of Technology Guwahati, Guwahati 781 039, Assam, India
S
* Supporting Information
ABSTRACT: Treatment of an aryl-sec-alkyl unsymmetrical
thiourea (Tu) with CuIIX2 (X = I, Br) transform the thiourea
(Tu) into a thioamidoguanidino (Tag) moiety with con-
comitant reduction of CuII to CuI, which forms a [Cu2 (μ2-
I
X)2Tag2] (X = I (for A) and X = Br (for B)) complex.
Meanwhile, the treatment of same unsymmetrical thiourea
(Tu) with CuIX (X = Br) forms a stable cluster with a
[CuI3(μ2-S)4Tu4X3] core (C). Single crystal X-ray diffraction
revealed that compounds A and B exhibit 1D chain with a
Cu2(μ2-X)2 core, whereas compound C is a CuI3S4Br3 cluster.
Compound A is centrosymmetric due to the trans orientation
of two Tag units whereas compound B is ascentric due to the cis orientation of two Tag units. In compound A, the Cu2I2 core is
perfectly rhomboidal where the iodine atoms are trans oriented. However in compound B, the Cu2Br2 core is not perfectly
rhomboidal (bowl shaped) and the bromine atoms are cis oriented. It is interesting to note that although the Tag moiety in
compounds A and B contain two morpholine-O atoms; only one of the morpholine-O atoms (O2) is involved in the generation
of three-dimensional network. The CuI3S4Br3 cluster in compound C contains one tri- and two tetra-coordinated CuI centers.
The CuI cluster in C contains a Cu2(μ2-S)2 rhomboidal plane exhibiting a chair and a boat form containing the CuI3S3 unit. In
compounds A and B, the two CuI centers are μ2-X bridged and have a μ1-S linkage, whereas in compound C the linkages are
opposite having four μ2-S bridges and three μ1-Br linkages.
this strategy works, a competitive formation of Hugerschoff
product could be avoided as was the case using bromine3a and
iodine3b and only anti-Hugerschoff product could be achieved.
Ideally salts of CuII suit the above envisaged strategy.
Treatment of thiourea (Tu) with CuII salt gave our anticipated
thioamidoguanidine (Tag) product along with the formation of
an unprecedented CuI complex with Tag 1a. The reduction of
CuII to CuI is at the expense of thiourea getting oxidized to
corresponding disulfide. Herein, we report the synthesis of a
INTRODUCTION
■
The classical Hugerschoff reaction known since 1901 involves
the reaction of molecular bromine (Br2) with 1,3-diarylthiourea
in chloroform to produce 2-amino benzothiazole.1 However,
the products obtained by the reaction of aryl-sec-alkyl
unsymmetrical thioureas 1 with bromine or its equivalents are
different ones. While Jordan et al. and Le et al.2 have reported
the formation of 2-amino benzothiazole (Hugerschoff) as the
exclusive product for certain substrates, we have reported the
formation of thioamidoguanidine (Tag) (anti-Hugerschoff
product) 1a as the exclusive or major product.3a The use of
molecular iodine instead of bromine also gave an identical
result, thus further supporting our proposition.3b The
formation of thioamidoguanidine (Tag) 1a essentially involved
an oxidative dimerization (S−S bond formation) of an
unsymmetrical thiourea followed by an intramolecular imine-
disulfide rearrangement.3a During the formation of the
Hugerschoff product the thiophilic bromine activates the sulfur
of a thiourea toward an intramolecular aromatic electrophilic
substitution reaction while it acts as a mere oxidizing (S−S
bond forming) agent during the formation of a thioamidogua-
nidine (Tag) moiety. Taking clues from the above observations
we thought of using another mild thiophilic environmentally
benign redox-active metal which would only promote oxidative
dimerization of thiourea leading to the exclusive formation of
the thioamidoguanidine (Tag)/anti-Hugerschoff product 1a. If
I
[Cu2 (μ-X)2Tag2] (A and B) complex and an air stable cluster
[CuI3(μ2-S)4Tu4X3] (C) from thiourea (Tu) using CuIIX2 and
CuIBr salts, respectively.
EXPERIMENTAL PROCEDURES
■
General Procedures. All reagents and solvents were purchased
from commercial sources which were of reagent grade. Phenyl
isothiocyanate was synthesized following the literature procedure.4
UV−visible spectra were recorded on a Perkin-Elmer lambda 25 UV−
visible spectrophotometer. FT-IR spectra were taken on a Perkin-
1
Elmer spectrophotometer with sample prepared as KBr pellets. H,
13C NMR spectra were recorded with a 400 MHz Varian FT-
spectrometer. Chemical shifts (ppm) were referenced either with an
internal standard (Me4Si) for organic compounds and complexes or to
the residual solvent peaks in CDCl3. The X-band electron para-
Received: June 5, 2012
Published: September 24, 2012
© 2012 American Chemical Society
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Article
Table 1. Crystallographic Data and Refinement Parameters for Compounds A, B, and C
A
B
C
formula
C22H26CuIN4O2S
8757778
600.99
C22H26BrCuN4O2S
876074
C44H56Br3Cu3N8O4S4
877794
CCDC number
formula weight
T (K)
553.99
1319.60
296(2)
296(2)
296(2)
wavelength (Å)
crystal system
space group
a (Å)
0.71073
0.71073
orthorhombic
Pccn
0.71073
monoclinic
C2/c
triclinic
P1
̅
22.3537(12)
15.7364(10)
14.7465(10)
90.00
15.4710(8)
20.2116(10)
14.6730(8)
90.00
12.8009(5)
14.6565(6)
14.8317(6)
81.033(2)
75.174(2)
77.727(2)
2613.34(18)
2
b (Å)
c (Å)
α (deg)
β (deg)
114.048(3)
90.00
90.00
γ (deg)
V (Å3)
90.00
4737.1(5)
8
4588.2(4)
8
Z
θmin, θmax
1.63, 28.42 2
1.685
1.66, 25.25
1.604
1.91, 28.36
1.677
Dcalcd (g·m−3
)
μ (mm−1
F(0 0 0)
)
2.340
2.811
3.711
2400.0
2256.0
1328.0
reflection collected
5953
4164
13060
unique reflections
3286
3316
7046
goodness of fit (GOF) on F2
R1, wR2 (I ≥ 2σ(I))
R1, wR2 (all data)
1.095
1.071
1.020
0.0799, 0.2332
0.1177, 0.2485
0.0453, 0.1315
0.0557, 0.1354
0.0366, 0.0756
0.0896, 0.0898
magnetic resonance (EPR) spectra of the complexes were recorded on
a JES-FA200 ESR spectrometer. Elemental analyses were obtained
from a Perkin-Elmer Series II Analyzer. Mass spectra of the
compounds in CH3CN were recorded in a Waters Q-Tof Premier
and Aquity instrument.
Table 2. Selected Bond Distances (Å) and Bond Angles
(deg) in Compounds A and B
complex A (X = I)
complex B (X = Br)
Cu(1)−X(1)
Cu(1)−X(1A)
Cu(1)−O(2)
Cu(1)−S(1)
Cu(1)−Cu(1A)
S(1)−Cu(1)−O(2)
S(1)−Cu(1)−X(1)
S(1)−Cu(1)−X(1A)
O(2)−Cu(1)−X(1)
O(2)−Cu(1)−X(1A)
X(1)−Cu(1)−X(1)
2.592(1)
2.560(1)
2.635(1)
2.259(2)
2.799(1)
79.2(1)
2.457(1)
2.407(1)
2.724(4)
2.224(2)
2.945(1)
79.8(1)
General Procedures for X-ray Structure Determinations. The
crystals were mounted on a glass fiber and the intensity data were
collected using a Bruker SMART APEX II CCD diffractometer,
equipped with a fine focus 1.75 kW sealed tube Mo Kα radiation (λ =
0.71073 Å) at 298(2) K, with increasing ω (width of 0.3° per frame) at
a scan speed of 3 s per frame. The SMART software was used for data
acquisition. Data integration and reduction were under taken with
SAINT and XPREP5 software. Multiscan empirical absorption
corrections were applied to the data using the program SADABS.6
Structures were solved by direct methods using SHELXS-977a and
were refined by full-matrix least-squares on F2 using the SHELXL-977b
program package. In all the three, non-hydrogen atoms were refined
anisotropically. Hydrogen atoms attached to all carbon atoms are
geometrically fixed. Structural illustrations have been generated using
ORTEP-38a and MERCURY1.38b for Windows. The molecular
structures of compounds A, B, and C were determined, and the
crystallographic and refinement parameters are listed in Table 1. The
selected bond distances and angles found in compounds A and B are
listed in Table 2, and those in compound C are listed in Table 3.
Experimental Procedure for the Synthesis of Thiourea (1). To a
stirred solution of phenyl isothiocyanate (405 mg, 3 mmol) in ethanol
(10 mL) was added dropwise morpholine (261 mg, 3 mmol).
Formation of thiourea 1 was observed within 15 min as judged from
TLC. The crude thiourea 1 was used as such for the next step. For
structural confirmation, ethanol was evaporated under a reduced
pressure and dried in a vacuum drier, a white solid compound was
obtained in nearly quantitative yield. Mp 124−126 °C. 1H NMR (400
MHz, CDCl3): δ 3.65 (t, 4H, J = 4.8 Hz), 3.73 (t, 4H, J = 4.4 Hz), 7.13
(m, 3H), 7.30 (t, 2H, J = 8.0 Hz), 7.66 (brs, 1H). 13C NMR (100
MHz, CDCl3) δ 49.4, 66.1, 123.7, 125.4, 129.0, 139.9, 183.2. IR (KBr):
3436, 3170, 3027, 2917, 2851, 1651, 1594, 1532, 1495, 1409, 1321,
1264, 1205, 1111, 1026, 935, 853 cm−1. Anal. calcd for C11H14N2OS:
C, 59.43; H, 6.35; N, 12.60; S, 14.42. Found: C, 59.48; H, 6.42; N,
12.53; S, 14.48; MS (ESI): 223.2543 (MH+).
124.12(6)
120.54(6)
93.6(1)
128.45(5)
126.69(5)
89.3(1)
109.2(1)
114.18(4)
117.4(1)
103.14(4)
Table 3. Selected Bond Distances (Å) and Bond Angles
(deg) in Compound C
complex C
Cu1−S1
Cu1−Br1
Cu1−S3
Cu1−S4
Cu2−S1
Cu2−Br2
Cu2−S2
S1−Cu1−S3
S1−Cu1−S4
S1−Cu1−Br1
S3−Cu1−S4
S3−Cu1−Br1
S4−Cu1−Br1
2.3637(8)
2.3603(7)
2.392(1)
2.487(1)
2.245(1)
2.3968(6)
2.261(1)
97.96(3)
102.52(3)
118.83(3)
106.61(3)
117.61(3)
111.38(3)
Cu3−S2
Cu3−Br3
Cu3−S3
Cu3−S4
Cu1−Cu2
Cu3−Cu1
Cu3−Cu2
S2−Cu3−S3
S2−Cu3−S4
S2−Cu3−Br3
S3−Cu3−S4
S3−Cu3−Br3
S4−Cu3−Br4
2.4213(9)
2.4588(6)
2.322(1)
2.350(1)
3.2197(7)
2.7105(8)
3.1196(6)
104.61(3)
103.15(3)
113.40(3)
113.72(4)
111.18(3)
110.47(3)
Experimental Procedure for the Synthesis of Thioaminoguani-
dine (Tag) [anti-Hugerschoff Product] (1a). To a solution of thiourea
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Figure 1. ORTEP view (30% probability ellipsoids) of compound A.
1 (3 mmol, 666 mg) dissolved in ethanol (30 mL) was added a
solution of CuBr2 (3 mmol, 672 mg) dissolved in ethanol (5 mL), and
the resultant reaction mixture was stirred at room temperature for 20
min. During this period, a pale yellow solution along with yellow
precipitate was obtained. After completion of the reaction, ethanol was
evaporated completely and ethyl acetate (30 mL) was admixed to the
reaction mixture. Aqueous ammonia (20%, 10 mL) was added to the
above ethyl acetate suspended reaction mixture and the heterogeneous
mixture was stirred at room temperature. During this time (10 min)
the suspended insoluble yellow solid got dissolved into the ethyl
acetate layer leaving the aqueous ammoniacal layer blue in color. The
ethyl acetate layer was separated and dried over anhydrous Na2SO4
and concentrated under a reduced pressure. The product so isolated is
sufficiently pure to be used for the next step. However, if desired it can
be further purified by recrystallization using ethyl acetate and hexane
prepared was found to be identical in all respect to that prepared
following method 1.
Preparation of [Cu2 (μ-Br)2Tag2] (B). B was prepared following
I
method 1, and CuBr2 was used instead of CuI2. Yield 440 mg (80%).
1
Pale yellow solid. Mp 206−207 °C. H NMR (400 MHz, CDCl3): δ
3.46 (m, 16H), 6.97 (d, 4H, J = 8.0 Hz), 7.11 (t, 2H, J = 7.2 Hz), 7.18
(t, 4H, J = 8.0 Hz). IR (KBr): 2961, 2921, 2855, 1637, 1586, 1485,
1421, 1302, 1239, 1157, 1109, 1062, 994, 935, 765, 753, 691 cm−1.
Anal. calcd for C44H52Cu2Br2N8O4S2: C, 47.70; H, 4.73; N, 10.11; S,
5.79. Found: C, 47.76; H, 4.78; N, 10.05; S, 5.72.
Preparation of [CuI3(μ2-S)4Tu4X3] (C). To a solution of thiourea 1
(1 mmol, 222 mg) dissolved in ethanol (15 mL) was added a solution
of CuBr (1 mmol, 143.5 mg) dissolved in ethanol (5 mL), and the
resultant reaction mixture was stirred for 20 min. A yellow solution
was formed which was filtered and kept for crystallization, which
deposited a pale yellow crystalline solid after few days. Yield of first
1
(9:1) (550 mg, 88%). White solid. Mp 163−165 °C. H NMR (400
1
crops 250 mg (76%). Pale yellow solid. Mp 138−139 °C. H NMR
MHz, CDCl3): δ 2.90−3.52 (m, 16H), 6.95 (d, 2H, J = 8.0 Hz), 6.98
(d, 2H, J = 7.6 Hz), 7.09 (t, 2H, J = 7.6 Hz), 7.17 (t, 2H, J = 8.0 Hz),
7.30 (d, 2H, J = 7.6 Hz). 13C NMR (100 MHz, CDCl3) δ 46.8, 50.6,
65.4, 66.2, 121.2, 122.1, 122.8, 122.9, 124.7, 128.8, 129.7, 142.8, 149.3,
185.2. IR (KBr): 2963, 2920, 2856, 1637, 1587, 1485, 1421, 1302,
1277, 1239, 1109, 1062, 994, 765, 753, 691 cm−1. Anal. calcd for
C22H26N4O2S: C, 64.36; H, 6.38; N, 13.65; S, 7.81. Found: C, 64.43;
H, 6.41; N, 13.60; S, 7.89. MS (ESI): 411.2011 (MH+).
(400 MHz, CDCl3): δ 3.53 (s, 8H), 7.00 (t, 1H, J = 7.2 Hz), 7.19 (m,
4H), 10.22 (brs, 1H). IR (KBr): 3164, 3129, 2959, 2846, 1591, 1536,
1515, 1446, 1360, 1322, 1267, 1228, 1196, 1115, 1024, 951, 764, 689
cm−1. Anal. calcd for C44H56Cu3Br3N8O4S4: C, 40.05; H, 4.28; N, 8.49;
S, 9.72. Found: C, 40.12; H, 4.34; N, 8.43; S, 9.67.
RESULTS AND DISCUSSION
■
General Procedure for Preparation of Complexes. Method 1:
The in situ generated unsymmetrical thiourea (Tu) 1 obtained
by reacting phenyl isothiocyanate and morpholine in EtOH was
treated with CuI2. The greenish color of CuI2 disappeared
giving a pale yellow precipitate and leaving behind a pale yellow
solution. A portion of this precipitate was dissolved in ethanol
and was left aside for crystallization which gave a rod shaped
pale yellow crystal A. The molecular structure of A was
determined by X-ray crystallographic analysis (Figure 1). As
revealed from the structure, the ligand bound to the Cu centers
is nothing but the thioamidoguanidine (Tag) product 1a. In
keeping with our earlier observations,3 it can be speculated that
CuII being oxidizing in nature oxidizes thiourea in to a disulfide
intermediately which then undergoes an imine disulfide
rearrangement to give the Tag moiety; during the process,
CuII gets reduced to CuI. The ligand (Tag) 1a was isolated
from the complex A by removing the Cu either using a
disodium salt of ethylenediaminetetraacetic acid (EDTA) or an
aqueous ammoniacal solution, and the product was charac-
terized by spectroscopic analysis. No Hugerschoff product (2-
amino benzothiazole) was isolated from the reaction mixture
(Scheme 1). The ligand 1a having a soft sulfur atom reacts with
I
[Cu2 (μ-I)2Tag2] (A). To a solution of thiourea 1 (1 mmol, 222 mg)
dissolved in ethanol (15 mL) was added an ethanolic solution of CuI2
(1 mmol, 317.5 mg), and the resultant reaction mixture was stirred for
20 min. During this period a yellow solution along with yellow
precipitate was formed. The precipitate was dissolved by warming the
reaction mixture. The resultant solution was filtered under hot
condition and kept for crystallization. A pale yellow crystalline solid
were obtained upon standing. Yield of first crops 204 mg (68%). Mp
212−213 °C. 1H NMR (400 MHz, CDCl3): δ 3.45 (m, 16H), 6.97 (m,
4H), 7.11 (t, 2H, J = 7.2 Hz), 7.18 (t, 4H, J = 7.6 Hz). IR (KBr): 2964,
2920, 2853, 1640, 1587, 1486, 1422, 1356, 1299, 1276, 1239, 1108,
1063, 993, 850, 765, 753 cm−1. Anal. calcd for C44H52Cu2I2N8O4S2: C,
43.97; H, 4.36; N, 9.32; S, 5.34. Found: C, 44.01; H, 4.41; N, 9.27; S,
5.29.
I
Method 2: [Cu2 (μ-I)2Tag2] (A). To a solution of thioamidoguani-
dino ligand 1a (1 mmol, 410 mg) in acetonitrile (15 mL) was added
CuI (1 mmol, 190 mg) dissolved in acetonitrile (15 mL), and the
resultant reaction mixture was stirred for 20 min. A yellow solution
was formed along with a yellow precipitate. The entire precipitate was
dissolved by warming the reaction mixture in a water bath. The clear
solution was kept for crystallization, which deposited a pale yellow
crystalline solid. Yield of first crops 535 mg (89%). The compound so
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Scheme 1. Formation of Tag, [Cu2 (μ-X)2Tag2], Complexes A and B and CuI3S4Br3 Cluster (C)
I
Figure 2. ORTEP view (30% probability ellipsoids) of compound B.
Figure 3. 1D coordination polymer of compound A.
the soft CuI center to form complex A (Scheme 1). The two
CuI centers in the bimetallic complex A are held by two iodide
ions. The EPR silence (see the Supporting Information, Figure
S2), diamagnetic nature (μeff = 0), and the absence of d−d
transition in the UV−vis spectrum (see the Supporting
Information, Figure S1), support the existence of the +1
oxidation state for copper in complex A.9
Further, the reduction of CuII to CuI by the thiourea (Tu)
has been independently confirmed by UV−vis titration. The d−
d transition exhibited by green colored CuI2 at λmax = 804 nm
in EtOH:H2O (9:1) disappeared upon addition of an
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Figure 4. 1D coordination polymer of compound B.
equivalent of thiourea 1 (see Supporting Information Figure
S1). Encouraged by the presence of an interesting structural
motif in complex A, the in situ generated thiourea 1 was then
treated with CuBr2 instead of CuI2. Here, also similar
observations were noticed. The resultant compound B was
crystallized from ethanol giving a rod shaped pale yellow
crystal. X-ray crystallographic analysis of complex B showed an
equally interesting structural motif to that of A as shown in
Figure 2.
respectively contains a dimeric core of [Cu2(μ-X)2] in their
crystal structure. As shown in Figures 1 and 2, the asymmetric
units of compounds A and B contain a distorted tetrahedral CuI
center bridged by two halogen atoms via a Cu2(μ-X)2 bridge. A
monodentate sulfur atom of a Tag moiety is in the basal plane
and one of the morpholine-O atoms of another Tag moiety
directed axially in their crystal structures. The axially
coordinating morpholine-O atom is nearly perpendicular to
the virtual [Cu2(μ-X)2Tag2] plane. In compounds A and B, the
distortion from regular tetrahedron geometry is evident from
the bond angles around CuI center (Table 2). Compound A is
centrosymmetric due to the trans orientation of two Tag units
whereas the compound B is ascentric due to its cis orientation.
As can be seen from the crystal structures of A and B despite
the presence of several hard nitrogen atoms in the Tag ligand
1a, only the soft sulfur and halogen atoms are coordinating to
the CuI centers. Such monodentate sulfur coordination modes
are rarely found in the literature.13 Due to the presence of two
or more lone pairs in a single atom, usually halogens (Cl, Br,
and I) and sulfur atoms are ideal candidates for single-atom-
bridging (SAB), and thus, an efficient contributor to metal−
organic networks (MONs). Similar types of four-coordinated
CuI bridges by a pair of iodide [Cu2(μ-I)2] and thiocarbonyl
sulfur [Cu2(μ-S2)] atoms each acting as SAB’s is reported.14 In
compounds A and B, comparison of bond angles around the
CuI centers (Table 2) indicates that the latter is more distorted
than the former. In both the complexes A and B, the central
Cu2(μ-X)2 core is rhomboidal with a bridging bond distances of
Cu−I (2.59 Å) and Cu−Br (2.45 Å) very close to their terminal
Cu−I (2.58 Å) and Cu−Br (2.43 Å) bond distances. The
angles in the core of A are 65.82(3)° for Cu1−I1−Cu1A and
114.18(4)° for I1−Cu1−I1A. The similar Cu2(μ-I)2 motif
exists where the copper is tetra-coordinated; the bond angles
are reported to be 56.3° for I1−Cu1−I1(A) and 123.7° for
Cu1−I1−Cu1A.10b The angles in the core of B are 103.14(4)°
for Br1−Cu1−Br1A and 74.50(3)° for Cu1−Br1−Cu1A. The
similar Cu2(μ-Br)2 core exists where the copper is tetra-
coordinated, and the bond angles follow the opposite trend
50.6° for Br1−Cu1−Br1A and 120.4° for Cu1−Br1−Cu1A.10b
The Cu−S bond distance is 2.259 Å and S1−Cu1−Cu1A
A plethora of CuI complexes prepared in the literature is by
the treatment of various ligands containing soft centers with
salts of CuI, and there are only few examples where it is
prepared from CuII salts. In the latter case CuII is reduced in
situ to CuI in the presence of an external reducing agent.10 In
the present case, the complexes of CuI, A and B are prepared
from their respective CuIIX2 (X = I, Br) salts which act as the
source for CuI generated in situ upon reduction by thiourea
(Tu) 1. The thiourea get transformed to the Tag unit 1a by an
oxidative dimerization (S−S bond formation) followed by an
imine-disulfide rearrangement. The Tag unit binds to the in situ
generated CuI giving complexes A and B, a process not
documented in the literature so far. Due to the presence of soft
sulfur atom in thioureas and substituted thioureas, they are
effective ligands for CuI salts.11a Unlike CuII, generally CuI
centers can have variable coordination numbers ranging from 2
to 4.11b
The isolated thioamidoguanidino (Tag) 1a ligand has no
affinity for CuIIX2 (X = I, Br) salts whereas it has strong affinity
toward CuI and forms complexes A and B when treated with
CuI and CuBr, respectively. With the reduced form of Cu, i.e
CuI, no transformation of thiourea (Tu) 1 to thioamidogua-
nidino (Tag) 1a was observed thus supporting the redox
mechanism. Interestingly, the in situ generated thiourea 1 forms
a cluster with a CuI3S4Br3 core C as shown in Figure 3 when
treated with CuIBr. A vast array of CuI clusters are known
where sulfur atom of thioureas behaves as SAB,11a,12 but it is
often difficult to predict as to when sulfur and halides behave as
the SAB and when as monodentate.
I
Crystal Structure of [Cu2 (μ-X)2Tag2]n (X = I (A), Br (B)).
Compounds A and B with space group C2/c and Pccn
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Scheme 2. Schematic Diagram of Coordination Polymer of Complex A and B and Cluster C
atoms are not linear with an angle of 169.80° in A, and the
Cu2I2 core is perfectly rhomboidal in shape (Figure 1). The
corresponding Cu2Br2 core in B is not perfectly rhomboidal;
rather, it is bowl shaped. The bromine atoms (μ-Br)2 are out of
the plane by 20.45° and S1−Cu1−Cu1A atoms are nearly
linear with an angle of 178.10° (Figure 2). This may be due to
the trans orientation of iodine atoms in A and cis orientation of
bromine atoms in B.
In compounds A and B, the axially coordinating morpholine-
O atoms of Tag extends the dimeric [Cu2(μ-X)2] core to a 1D
coordination polymer as shown in Figures 3 and 4. In the
polymeric unit the centroid−centroid distances between two
nearby Cu2(μ-X)2 core are 7.37 Å (for compound A) and 7.33
Å (for compound B), respectively. It is interesting to note that
although the Tag moiety contains two morpholine-O atoms,
only one of the morpholine-O atoms (O2) extends the network
whereas another morpholine-O atom (O1) exhibits C−H···O
interactions (Scheme 2). In compound A, the hydrogen
bonding interactions C4−H4···O1 = 3.45(1) and C6−H6···I1
= 3.82(1) Å between the Tag moieties extending the 1D chain
Figure 5. ORTEP view (30% probability ellipsoids) of compound C.
to a 2D ladder along the bc plane (see Supporting Information
Figure S5). Similarly, in compound B, noncovalent interactions
such as C15−H15···O1 = 3.37(1), C8−H8···N2 = 3.61(1), and
C6−H6···Br1 = 3.65(1) Å extend the 1D chain to a 2D layer
along the axis (see Supporting Information Figure S6).
centers with four (μ2-S) bridging of Tu ligands and three axially
coordinating bromine atoms. Unlike in compounds A and B
where the two CuI centers are μ2-X bridged and have a μ1-S
linkage, in compound C, the linkages are opposite possessing
four (μ2-S) bridges and three μ1-Br linkages (Scheme 2). The
tri-coordinated CuI center (Cu2) form bonds with two S atoms
of Tu (Cu2−S1 = 2.245(1) Å, Cu2−S2 = 2.261(1) Å) and one
I
Crystal Structure of [Cu3 Br3(Tu)4] (C). Complex C
crystallized in the P1 space group possessing a CuI S Br
̅
3
4
3
cluster. As shown in Figure 5, the asymmetric unit of
compound C consists of a tri- and two tetra-coordinated CuI
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Figure 6. Ball−stick model of CuI3S4Br3 cluster core (left) having both chair (middle) and boat form (right) of CuI3S4 cluster.
Br atom (Cu2−Br2 = 2.397(1) Å). The mean angle at Cu2
center is 119.85(3)° which suggests almost a trigonal planar
geometry at the Cu2 center. The coordination environment
around the tetra-coordinated Cu1 and Cu3 centers are satisfied
by three S atoms of the Tu and one Br atom with a quasi-
tetrahedral geometry. The Cu−S bond distances are in the
range of (2.322(1)−2.487(1) Å) with an average bond distance
of 2.414(1) Å around the Cu1 center and of 2.364(1) Å around
Cu3 center. This may be compared with the Cu−S−Cys bonds
in the range from 2.145−2.395 Å found in structurally
characterized Cu8(CysS)10 and (Cu8−yeastMT) clusters.15
The Cu−Br distances are of 2.361(1) and 2.459(1) Å around
Cu1 and Cu3 centers respectively. The axially coordinating Br1
atom is nearly perpendicular to the virtual plane containing
S1S3S4 atoms, while the Br3 atom is nearly perpendicular to
the plane containing S2S3S4 atoms. In compound C,
comparison of bond angles (Table 3) around Cu1 and Cu3
centers indicates that the former Cu center is more distorted
than the latter.
Figure 7. Mercury drawing representing intramolecular H−bonding
interactions in compound C.
The central plane with Cu2(μ2-S)2 core contains two
tetrahedral copper center which is rhomboidal in shape. The
bridging bond distances of Cu1−S3; 2.392(1), Cu1−S4;
2.487(1), Cu2−S3; Cu2−S4; 2.350(1) Å, respectively, are
shorter in comparison to those of complexes A and B. The
angles in the Cu2(μ2-S)2 core respectively are 106.61(4)° for
S3−Cu1−S4; 113.72(4)° for S3−Cu3−S4; 70.18(3)° for
Cu1−S3−Cu3; and 68.11(3)° for Cu1−S4−Cu3. A closer
look at the CuI3S4Br3 cluster revealed the existence (Figure 6,
left), of both chair (S3Cu3S2Cu2S1Cu1S3, middle) and boat
forms (S4Cu3S2Cu2S1Cu1S4, right) within the cluster (Figure
6). The axially bridged Br atoms (Br2 and Br3) exhibit
intramolecular H−bonding interactions with NH group of the
Tu (Figure 7).10b The H−bonding distances are N1−H1···Br2
= 3.34(1), N3−H3···Br2 = 3.36(1), and N7−H7···Br3 =
3.24(1) Å, respectively.
It is interesting to note that despite the presence of four
morpholine-O atoms from four Tu, none of them extend the
cluster to either 1D, 2D, or 3D structures as was found in
compounds A and B. Compound C exhibits different
noncovalent interactions such as C6−H6···O1 = 3.18(1),
C43−H43···O2 = 3.44(1), C19−H19···O3 = 3.23(1), C8−
H8···S3 = 3.70(1), C27−H27···Br1 = 3.76(1), and C31−
H31···Br3 = 3.53(1) Å extending the CuI cluster to a 2D sheet
structure along the bc plane (see Supporting Information
Figure S7).
resultant transformed ligand (Tag) and halide ions to form a
[Cu2 (μ2-X)2Tag2] complex. However in the presence of CuI,
I
the thiourea forms a [CuI3(μ2-S)4Tu4Br3] cluster. Compounds
A and B contain only tetra-coordinated CuI centers whereas
compound C contains one tri- and two tetra-coordinated CuI
centers. Compounds A and B exhibit 1D chain structures with a
Cu2(μ2-X)2 core whereas compound C is a CuI3S4Br3 cluster.
Compound A is centrosymmetric, whereas the compound B is
ascentric. In compound A, the Cu2I2 core is perfectly
rhomboidal, whereas in compound B, the Cu2Br2 core is
bowl shaped. Thus the synergism of the potentially bridging
ligands, the effectiveness of the halide ions as SAB and
directional nature of noncovalent interactions provide complex
connectivity patterns and a remarkable structural diversity. The
occurrence of different noncovalent interactions between
ligands and halide ions extend the networks to a 2D structure.
These complexes are air stable, non−hygroscopic thus might
find potential application as catalyst, in the construction of
metal organic framework, in light-emitting diode (LED)
technology and also understanding of metallothionenes.
ASSOCIATED CONTENT
■
S
* Supporting Information
Crystallographic data in CIF, 1H, 13C NMR spectra, and HRMS
and EPR spectra. This material is available free of charge via the
Internet at http://pubs.acs.org.
CONCLUSION
■
In conclusion we have developed a method where an aryl-sec-
alkyl thiourea (Tu) undergoes an oxidative rearrangement in
the presence of a redox active metal salt CuIIX2 to give a
thioaminoguanidino moiety (Tag) and itself gets reduced to
CuI. An interesting bimetallic CuI complex is formed from the
AUTHOR INFORMATION
Corresponding Author
*Fax: +91-3612690762. E-mail: patel@iitg.ernet.in.
■
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Article
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
B.K.P. acknowledges the support of this research by the
Department of Science and Technology (DST) (SR/S1/OC-
79/2009), New Delhi, and the Council of Scientific and
Industrial Research (CSIR) (01(2270)/08/EMR-II). S.K.S. and
N.K. thank CSIR for fellowships. Thanks are due to Central
Instruments Facility (CIF) IIT Guwahati for NMR spectra and
DST-FIST for XRD facility.
DEDICATION
■
We dedicate this paper to Prof. Mihir K. Chaudhuri on the eve
of his 65th birthday.
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