2-Benzoylpyridine thiosemicarbazone as a novel reagent for the single pot synthesis of dinuclear CuI-CuII complexes: Formation of stable copper(ii)-iodide bonds

Article abstract of DOI:10.1039/c2dt30068k

2-Benzoylpyridine thiosemicarbazone {R¹R²C ²N²·N³H-C¹(S)-N ⁴H₂, R¹ = py-N¹, R² = Ph; Hbpytsc} with copper(i) iodide in acetonitrile-dichloromethane mixture has formed stable Cu^II-I bonds in a dark green Cu^II iodo-bridged dimer, [Cu₂^II(μ-I)₂(η ³-N¹,N²,S-bpytsc)₂] 1. Copper(i) bromide also formed similar Cu^II-Br bonds in a dark green Cu ^II bromo-bridged dimer, [Cu₂^II(μ-Br) ₂(η³-N¹,N²,S-bpytsc) ₂] 3. The formation of dimers 1 and 3 appears to be due to a proton coupled electron transfer (PCET) process wherein copper(i) loses an electron to form copper(ii), and this is accompanied by a loss of -N³H proton of Hbpytsc ligand resulting in the formation of anionic bpytsc^-. When copper(i) iodide was reacted with triphenylphosphine (PPh₃) in acetonitrile followed by the addition of 2-benzoylpyridine thiosemicarbazone in dichloromethane (Cu:PPh₃:Hbpytsc in the molar ratio 1:1:1), both Cu^II dimer 1 and an orange Cu^I sulfur-bridged dimer, [Cu₂^II₂(μ-S-Hbpytsc)₂(PPh ₃)₂] 2 were formed. Copper(i) bromide with PPh₃ and Hbpytsc also formed Cu^II dimer 3 and an orange Cu^I sulfur-bridged dimer, [Cu₂^IBr₂(μ-S-Hbpytsc) ₂(PPh₃)₂] 4. While complexes 2 and 4 exist as sulfur-bridged Cu^I dimers, 1 and 3 are halogen-bridged. The central Cu₂S₂ cores of 2 and 4 as well as Cu₂X ₂ of 1 (X = I) and 3 (X = Br) are parallelograms. One set of Cu ^II-I and Cu^II-Br bonds are short, while the second set is very long {1, Cu-I, 2.565(1), 3.313(1) A; 3, Cu-Br, 2.391(1), 3.111(1) A}. The Cu...Cu separations are long in all four complexes {1, 4.126(1); 2, 3.857(1); 3, 3.227(1); 4, 3.285(1) A}, more than twice the van der Waals radius of a Cu atom, 2.80 A. The pyridyl group appears to be necessary for stabilizing the Cu^II-I bond, as this group can accept π-electrons from the metal. The Royal Society of Chemistry 2012.

Full text of DOI:10.1039/c2dt30068k

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PAPER
2
-Benzoylpyridine thiosemicarbazone as a novel reagent for the single pot
I
II
synthesis of dinuclear Cu –Cu complexes: formation of stable copper(II)–
iodide bonds†
a
a
b
Tarlok S. Lobana,* Sonia Khanna and Ray J. Butcher
Received 11th January 2012, Accepted 6th February 2012
DOI: 10.1039/c2dt30068k
1
2
2
2
3
1
4
1
1
2
2
-Benzoylpyridine thiosemicarbazone {R R C vN ·N H–C (vS)–N H , R = py–N , R = Ph;
2
II
Hbpytsc} with copper(I) iodide in acetonitrile–dichloromethane mixture has formed stable Cu –I bonds
in a dark green Cu iodo-bridged dimer, [Cu (μ-I) (η -N ,N ,S-bpytsc) ] 1. Copper(I) bromide also
formed similar Cu –Br bonds in a dark green Cu bromo-bridged dimer, [Cu (μ-Br) (η -N ,N ,S-
II
II
3
1
2
2
2
2
II
II
II
3
1
2
2
2
bpytsc) ] 3. The formation of dimers 1 and 3 appears to be due to a proton coupled electron transfer
2
(
PCET) process wherein copper(I) loses an electron to form copper(II), and this is accompanied by a loss
3
−
of –N H proton of Hbpytsc ligand resulting in the formation of anionic bpytsc . When copper(I) iodide
was reacted with triphenylphosphine (PPh ) in acetonitrile followed by the addition of 2-benzoylpyridine
3
II
thiosemicarbazone in dichloromethane (Cu : PPh : Hbpytsc in the molar ratio 1 : 1 : 1), both Cu dimer 1
and an orange Cu sulfur-bridged dimer, [Cu I (μ-S-Hbpytsc) (PPh ) ] 2 were formed. Copper(I)
bromide with PPh and Hbpytsc also formed Cu dimer 3 and an orange Cu sulfur-bridged dimer,
Cu Br (μ-S-Hbpytsc) (PPh ) ] 4. While complexes 2 and 4 exist as sulfur-bridged Cu dimers, 1 and 3
3
I
I
2
2
2
3 2
II
I
3
I
2 2 2 3 2
I
[
are halogen-bridged. The central Cu S cores of 2 and 4 as well as Cu X of 1 (X = I) and 3 (X = Br) are
2
2
2 2
II
II
parallelograms. One set of Cu –I and Cu –Br bonds are short, while the second set is very long {1, Cu–I,
2
.565(1), 3.313(1) Å; 3, Cu–Br, 2.391(1), 3.111(1) Å}. The Cu⋯Cu separations are long in all four
complexes {1, 4.126(1); 2, 3.857(1); 3, 3.227(1); 4, 3.285(1) Å}, more than twice the van der Waals
II
radius of a Cu atom, 2.80 Å. The pyridyl group appears to be necessary for stabilizing the Cu –I bond, as
this group can accept π-electrons from the metal.
Introduction
the nuclearity and nature of bonding of complexes of copper(I)
halides, forming mono-, di- and poly-nuclear copper(I) com-
7
The stabilization of copper(II)–iodide bonding has been enig-
matic owing to the reducing nature of the bulky iodide anion. It
generally appears outside the coordination sphere, as exemplified
by copper(II) complexes, [CuL]I {L = 2(2-aminoethyl)pyri-
plexes. These complexes of copper(I) halides with neutral thio-
semicarbazones could usually be crystallized in the presence of
7
b–i,k,l
triphenyl phosphine as a co-ligand,
crystalline products were formed in the absence of triphenyl
and only in a few cases
1
2
7
j,m
dine}, [CuClL ]I {L = 1,10-phenanthroline}, and [CuLL′ ]I
2
2
phosphine.
Further, in all these cases, the thio-ligands coordi-
nated to Cu as neutral ligands either in η -S or μ-S modes.
It was noted in the reactions of copper(I) iodide with thiosemi-
carbazones (Htsc) that the variation of R and R substituents at
the C carbon gave different products (Chart 1). For the sub-
3
I
1
7b–m
{L = 4-methylbenzoic acid, L′ = 2,2′-bipyridine}. However,
in the literature, polydentate N-donor ligands have been used
1
2
for the formation of copper(II)–iodide bonds with distances of
4
,5
2
2
.56–2.74 Å. In one case with a chiral N-donor ligand, a short
II
6
7
b
Cu –I distance (2.337 Å) has also been reported.
Recently from this laboratory, coordination chemistry of thio-
stituents, Ph, H and py, H, mononuclear, [CuI(Htsc)(PPh ) ],
for Ph, Me dinuclear, [(Ph P)Cu(μ-I) (μ-S-Htsc)Cu(PPh )]
3
2
7
l
3
2
3
1
2 2
semicarbazones, an important class of thio-ligands, {R R C v
and Ph, Ph combination, three coordinate mononuclear,
1
3
4
7
N–NH–C (vS)NR R }, has been reported. The substituents at
2
the C atoms of neutral thiosemicarbazones were found to affect
a
Department of Chemistry, Guru Nanak Dev University, Amritsar 143
0
05, India. E-mail: tarlokslobana@yahoo.co.in; Fax: +91-183-2258820
b
Department of Chemistry, Howard University, Washington, DC 20059,
USA
†
CCDC reference numbers 710209–710212 for complexes 1–4. For
crystallographic data in CIF or other electronic format see DOI: 10.1039/
c2dt30068k
Chart 1
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1
7j
(ε L−1 mol−1 cm−1)]: 296 (1.07 × 10 ), 416.5 (0.6776 × 10 ).
4
4
[
CuI(η -S-Htsc) ] complexes were formed. In our curiosity to
2
2
observe the effect of Ph, py substituents at C carbons, we have
come across unusual coordination behaviour of 2-benzoylpyri-
dine thiosemicarbazone (Hbpytsc, R , R = Ph, py) during equi-
molar reactions with copper(I) iodide/bromide in the presence
and absence of triphenyl phosphine. The formation of the
halogen-bridged Cu dinuclear complexes (1, 3) in the presence
of Ph P along with the sulfur-bridged Cu dinuclear complexes
2, 4) and the novelty of 2-benzoylpyridine thiosemicarbazone to
stabilize formation of copper(II)–iodide bonds are described
X-band ESR signals: g = 3.85 and g = 2.05. This complex is
k ⊥
partially soluble in CHCl and soluble in hot CH OH. Data for 2
3
3
1
2
is given below.
II
[Cu
I
2
2
(μ-S-Hbpytsc)
2
(PPh ) ]·CH Cl (2)
3 2 2 2
I
3
Yield: orange, 0.017 g , 42%. Mp: 225–227 °C. Anal. Calcd for
C H Br Cu N S P ·CH Cl : C, 53.69; H, 4.00; N, 7.95%.
Found: C, 53.40; H, 4.21; N, 7.89%. Main IR peaks (KBr,
cm ): ν(N–H) 3485m, 3418s, 3203b (–NH ) 3121b (–NH–);
(
62
54
2
2
8
2
2
2
2
herein.
−1
2
ν(CvN) + δ(N–H) + ν(CvC) 1597s, 1506s; ν(C–N) 1107s,
1
049s, 1026s; ν(P–CPh) 1095s; ν(CvS) 833s (thioamide
Experimental section
1
moiety). H NMR (CDCl , δ, ppm) 7.23–7.51 m (PPh + Ph +
C H + N H ),7.74 (td, C , H), 8.82d (C H), 9.45sb (N H ),
1
3
3
4
1
5
6
7
1
General materials and techniques
2
2
2
4.09s (–N H–). Electronic absorption spectra [CH Cl , λ
2 2 max
Copper(I) halides were prepared by reducing an aqueous solution
of CuSO ·5H O using SO in the presence of NaX (X = Cl, Br,
−1
−1
−1
4
(
ε L mol cm )]: 355 (0.168 × 10 ).
4
2
2
8
I) in water. The ligand 2-benzoylpyridine thiosemicarbazone
was prepared by refluxing thiosemicarbazide with 2-benzoylpyri-
dine in presence of acetic acid in methanol for a period of 24 h.
The elemental general analyses (C, H and N) were obtained
using a Thermoelectron FLASHEA1112 CHNS analyzer. The IR
spectra were recorded using KBr pellets in the range
II
3
1
2
[Cu2 Br (η -N , N , S-bpytsc) ]·2CH CN (3)
2
2
3
PPh (0.045 g, 0.17 mmol) was added to a solution of copper(I)
3
bromide (0.025 g, 0.17 mmol) in CH CN, followed by stirring
for 1 h resulting in formation of a white precipitate. Solid
Hbpytsc ligand (0.031 g, 0.17 mmol) and CH Cl (5 mL) were
added to it, followed by refluxing for 10 min resulting in a clear
dark orange solution, which on slow evaporation of the solution
yielded dark green crystals of [Cu Br(η -N ,N ,S-bpytsc)] (3)
and orange crystals of [Cu Br (μ-S-Hbpytsc) (PPh ) ] (4). Data
for 3, yield, 014 g, 38%; mp. 205–207 °C. Anal. Calcd for
3
000–200 cm−1 on a FTIR-SHIMADZU 8400 Fourier transform
2
2
4
spectrophotometer and on a Pye-Unicam SP-3-300 spectro-
photometer. The UV-visible spectroscopy was done using a
SHIMADZU UV-1601PC spectrophotometer. ESR studies were
carried out using an X-band spectrometer. The melting points
were determined with a Gallenkamp electrically heated appar-
II
3
1
2
2
2
2
3 2
1
C H Cu Br N S ·2CH CN: C, 40.96; H, 3.19; N, 15.93%.
26
22
2
2
8
2
3
atus. H NMR spectra were recorded on a JEOL AL-300 FT
Found: C, 40.78; H, 4.24; N, 16.09%. Main IR peaks (KBr,
spectrometer operating at a frequency of 300 MHz in CHCl -d
3
−
1
cm ): ν(N–H) 3439s, 3271s (–NH ); ν(C–H) 3081s, 3047s;
2
with TMS as the internal reference. Cyclic voltammograms were
recorded on an Autolab Electrochemical System equipped with
PGSTAT20 apparatus driven by GPES software employing a
platinum working electrode and an SCE reference electrode. All
ν(CvN) + δ(N–H) + ν(CvC) 1601s, 1481s, 1433s; ν(C–N)
118s, 1105s, 925s; ν(C–S) 781s, 731s. Complex is partially
soluble in hot CH OH and insoluble in CH CN. Electronic
absorption spectra [CH OH, λ
1
3
3
−1
−1
−1
−
3
−1
(ε L mol cm )]: 296
max
3
solutions were 10 mmol L , and the supporting electrolyte
4
4
was 10−1 mmol L−1 of tetraethyl ammonium perchlorate.
(1.88 × 10 ), 417 (1.16 × 10 ). X-band ESR signals: g = 3.75
k
and g = 2.07. Data for 4 is given below.
⊥
II
3
1
2
2 2 3
[
Cu2 I (η -N ,N ,S-bpytsc) ]·2CH CN (1)
[
2 2 2 3 2
Cu Br (μ-S-Hbpytsc) (PPh ) ] (4)
PPh (0.045 g, 0.17 mmol) was added to a solution of copper(I)
3
Yield: orange, 0.026 g, 47%. Mp: 218–220 °C. Anal. Calcd for
C H Cu I N P S : C, 52.51; H, 3.81; N, 7.90%. Found: C,
iodide (0.025 g, 0.17 mmol) in CH CN, followed by stirring for
3
1
h resulting in formation of a white precipitate. Solid Hbpytsc
62 54
2 2 8 2 2
−1
5
2.84; H, 4.10; N, 7.98%. Main IR peaks (KBr, cm ): ν(N–H)
ligand (0.031 g, 0.17 mmol) and CH Cl (5 mL) were added to
2
2
3
419s, 3225m (−NH ) 3126s (–NH–); ν(CvN) + δNH2
+
these precipitates in CH CN, followed by refluxing for 10 min
2
3
ν(CvC) 1597s, 1585s, 1506s; ν(CvS) 833s (thioamide
resulting in a clear dark orange colored solution, which on slow
1
II
3
1
2
moiety); ν(C–N) 1100s, 1050s; ν(P–C ) 1095s. H NMR
evaporation yielded dark green crystals of [Cu I (η -N ,N ,
Ph
2
2
4
1
(
(
CDCl , δ ppm) 7.29–7.61 m (PPh + Ph + C H + N H ), 7.78
3 3 2
S-bpytsc) ] (1) along with orange crystals of [Cu I (μ-S-
2
2 2
5
6
7
2
td, C , H), 8.95d (C H), 14.03s (–N H). Electronic absorption
spectra [CH Cl , λ (ε L mol cm )]: 343 (0.176 × 10 ).
max
Hbpytsc) (PPh ) ] (2). The crystals were manually separated and
2
3 2
−1
−1
−1
4
characterized. The direct reaction of copper(I) iodide with
2
2
Hbpytsc in CH CN–CH Cl was also carried out which essen-
3
2
2
tially gave complex 1 with an enhanced yield of 75%. Data for
, yield, 0.011 g, 38%; mp. 200–202 °C). Anal. Calcd for
C H Cu I N S ·2CH CN: C, 37.00; H, 2.88; N, 14.39%.
X-ray crystallography
1
The data for compounds 1 and 3 was measured on a Bruker
AXS SMART APEX CCD diffractometer with graphite mono-
chromated Mo-Kα radiation at 100(2) K. Single crystals of 2 and
4 were mounted on an Oxford Diffraction Gemini diffractometer
equipped with graphite monochromated Mo-Kα radiation
2
6
22
2 2
8
2
3
Found: C, 36.88; H, 2.75; N, 13.93%. Main IR peaks (KBr,
−1
cm ): ν(N–H) 3269s (–NH ); ν(C–H) 3096s, 3055s; ν(CvN) +
2
δNH + ν(CvC) 1622s, 1583s, 1480s; ν(C–N) 1056s, 1107s;
2
ν(C–S) 779s, 733s. Electronic absorption spectra [CH OH, λ
3
max
4846 | Dalton Trans., 2012, 41, 4845–4851
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7
II
(
λ = 0.71073 Å) and the unit cell dimensions and intensity data
which are similar to 2 and 4. The formation of Cu complexes
were measured at 200(2) K (2) and 296(2) K (4). The structure
was solved by direct methods using the program SHELXS-97
(1 and 3) using 2-benzoylpyridine thiosemicarbazone in the
I
presence of Ph P along with Cu complexes (2 and 4) under
3
(
1, 2, 4), SHELXTL 6.14 (3) and refined by full-matrix least-
reducing conditions demonstrates the novelty of this thio-ligand
to stabilize copper(II)–halogen bonds, particularly the stabiliz-
ation of copper(II)–iodide bonds, whose occurrence in the litera-
2
14
squares techniques against F using SHELXL-97. Positional
and anisotropic atomic displacement parameters were refined for
all non-hydrogen atoms. Hydrogen atoms bonded to carbon were
placed geometrically and the N–H hydrogen atoms were initially
positioned at sites determined from difference maps, but the
positional parameters of all H atoms were included as fixed con-
tributions riding on attached atoms with isotropic thermal para-
meters 1.2 times those of their carrier atoms. Criteria for a
satisfactory complete analysis were the ratio of rms shift to stan-
dard deviation being less than 0.001, and there being no signifi-
cant features in the final difference maps. Scattering factors from
4
–6
10a
ture is limited and rare among the thio-ligands. The use of
excess PPh merely formed the usual mononuclear tetrahedral
3
I
I
1
7a,10b
Cu complexes, [Cu X(η -S-Hbpytsc)(PPh ) ] (X = Cl, Br).
3
2
With copper(I) chloride and bromide, a related thio-ligand,
namely, benzophenone thiosemicarbazone (Hbztsc) (Ph, Ph sub-
2
stituent at C carbon) formed monomeric tetrahedral complexes,
[CuX(Hbztsc)(PPh ) ] (X = Cl, Br) and with copper(I) iodide, it
3
2
7j
formed a trigonal planar complex, [CuI(Hbztsc)2].
9
the International Tables for X-ray crystallography were used.
Spectroscopy, magnetism and electrochemistry
IR spectroscopy of the complexes has confirmed the presence of
the individual ligands through their ν(N–H), ν(C–H)Me, ν(C–N),
ν(CvC), ν(CvN), ν(O–H) and ν(P–C) vibrational modes. The
Results and discussion
Synthesis
1
H NMR spectra of complexes 1 and 3 showed the absence of a
3
Scheme 1 shows the formation of complexes 1–4. Copper(I)
diagnostic imino proton signal (–N H), and it revealed that
I
iodide with PPh in CH CN formed a white solid which was
Hbpytsc is anionic in these complexes. However, Cu dimers 2
3
3
further reacted with 2-benzoylpyridine thiosemicarbazone
Hbpytsc) in CH Cl . A slow evaporation of the mixture yielded
and 4 showed signals due to various groups including that due to
–N H protons and it confirmed that the ligand Hbpytsc is neutral
3
(
2
2
II
3
two products: a dark green copper(II) compound, [Cu (μ-I) (η -
in these complexes (see the Experimental section). The divalent
state of copper in dinuclear complexes 1 and 3 is confirmed
by measuring their magnetic susceptibility which yielded the
magnetic moments 1.77 and 1.76 B. M. respectively. Complex 1
showed a very broad but symmetrical ESR spectrum with a g
value centered at 2.05. There is partial resolution of the coupling
from the Cu nucleus (I = 3/2) and thus a weak signal at 3.85
2
2
1
2
N , N , S-bpytsc) ] 1, and an orange copper(I) compound,
Cu I (μ-S-Hbpytsc) (PPh ) ] 2. However, the direct reaction of
2
I
2 2 2 3 2
[
copper(I) iodide with Hbpytsc in CH CN–CH Cl mixture in the
3
2
2
absence of Ph P formed only dark green complex 1 (Scheme 1).
3
The behaviour of copper(I) bromide in the presence and absence
of Ph P was similar to that of copper(I) iodide. It formed a dark
3
II
II
3
1
2
green Cu complex, [Cu (μ-Br) (η -N , N , S-bpytsc) ] 3 and
an orange Cu dimer, [Cu Br (μ-S-Hbpytsc) (PPh ) ] 4. It is
could be attributed to g = 3.85 and in that case g = 2.05. This
k ⊥
2
2
2
I
I
ESR datum supports a distorted square pyramidal environment
around the Cu atom (Fig. 1). Complex 3 showed a similar ESR
spectrum. The electronic absorption spectrum of complex 1 in
methanol (Fig. 2) showed two bands, one broad band at 296 nm
2
2
2
3 2
important to point out that in the literature, reactions of copper(I)
halides with the thio-ligands in the presence of tertiary phos-
phines invariably have yielded copper(I) complexes, several of
Scheme 1
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II
stabilizing the Cu –I bond, as this group can accept π-electrons
from the metal.
Crystal and molecular structures
The crystal data are given in Table 1 and the bond parameters are
given in Table 2. Complex 1 is a centrosymmetric dimer,
II
2 2 2
3
1
2
II
1
[
Cu (μ-I) (η -N ,N ,S-bpytsc) ] with each Cu bonded to N ,
2
−
N , S-donor atoms of a tridentate anionic bpytsc ligand (Fig. 4)
Table 2). Each Cu is further bonded to two bridging iodide ions
(
II
II
with one short Cu –I bond, 2.5651(6) Å and one long Cu –I
II
bond, 3.3129(6) Å (Scheme 1). The short Cu –I bond distance
is comparable to the 2.56–2.74 Å observed with N-donor
4,5
ligands, and is somewhat shorter than the 2.601(1) Å observed
10
in a sulfur bridged dimer. The geometry around each metal
center can be described as distorted square pyramid with the
long Cu–I bond occupying the axial position. The atoms, N(1),
N(2), S(1) and I(1) form the square plane with non linear trans
bond angles, N(2)–Cu(1)–I(1), 179.54(10)°; N(1)–Cu(1)–S(1),
Fig. 1 EPR spectrum of complex 1 at 120 K.
1
64.68(10)° suggesting distortion in the geometry. Dimer
II
2 2 2
3
1
2
[Cu Br (η -N , N , S-bpytsc) ] (3) showed similar bonding and
II
structure with one short and one long Cu –Br bond {2.391(1),
.111(1) Å} (Fig. 5). The long Cu(1)–Br(1) bond occupies the
axial position with similar trans bond angles, N(2)–Cu(1)–Br(1),
79.32(5)°; N(1)–Cu(1)–S(1), 164.51(5)°. Complexes 2 and 4
3
1
I
exist as sulfur-bridged Cu
dimers, [Cu X (μ-S-Hb-
2 2
pytsc) (PPh ) ] (X; I 2; Br 4.) Each copper atom is bonded to
2
3 2
one halogen atom, one P atom from the PPh ligand and two S
3
Fig. 2 Electronic absorption spectrum of complex 1.
atoms of the two neutral Hbpytsc ligands (Fig. 6 and 7). The
central Cu S cores are parallelograms in thees two cases due to
2
2
unequal Cu–S bond distances. The bond parameters and struc-
tures are similar to dinuclear copper(I) halide complexes with
7
b,c
thiosemicarbazones reported in the literature.
The Cu⋯Cu
separations are long in all the four complexes {1, 4.126(1); 2,
3
.857(1); 3, 3.227(1); 4, 3.285(1) Å}, more than twice the van
13
der Waals radius of a Cu atom, 2.80 Å.
Packing interactions
Complexes 1 and 3 do not involve intramolecular interactions,
however, in each case the amino hydrogen of one dimer interacts
3
with the N atom and the phenyl group of an adjacent dimeric
4
3
4
unit along the a-axis and vice versa {N H⋯N , N H⋯π(ph): 1,
.130, 2.890 Å, 2.126, 2.892 Å 3} forming 1D chains. These 1D
Fig. 3 Cyclic voltammogram of complexes 1 and 3 at a scan rate of
2
−1
0
.05 V s .
chains interact with one another via I⋯H(ph) 3.01 Å (1) or
Br⋯H(ph), 2.87 Å (3) along the b-axis and form 2D sheets
3
(
Fig. 8). In complexes 2 and 4, hydrazinic hydrogens (N H)
(
due to n → π* and π → π* transitions) and a second at 417 nm,
exhibit short intramolecular interactions with pyridyl nitrogen,
II
attributed to the S → Cu LMCT transition characteristic of
divalent copper(II)–sulfur bonds; complex 3 showed a similar
spectrum. The electronic absorption spectra of Cu complexes 2
3
1
N H⋯N (py) (1.91, 2; 1.92 Å 4). The sulfur-bridged dimers
interact with one another along the a-axis via the pyridyl–phenyl
moieties {C–H(py)⋯π(Ph–P): 2, 2.80, 4, 2.81 Å} forming 1D
chains. These chains interact with one another through a
phenyl–phenyl interaction involving PPh3 and thio-ligands
{C–H(Ph–P)⋯π(Ph at C2): 2, 2.324; 4, 2.321 Å} and form 2D
sheets (Fig. 9).
11
I
and 4 each showed a single weak band at 343 and 355 nm,
respectively. The cyclic voltammogram (CV) of complex 1 dis-
II
III
played a quasi-reversible Cu –Cu redox couple at E = 0.813
1
2
V with a peak-to-peak separation (ΔE ) of 102 mV (Fig. 3). The
p
II
III
CVof complex 3 displays an irreversible Cu –Cu redox couple
with E value of 0.794 V (ΔE , 120 mV). The Cu –Cu redox
p
2
couples of 1 and 3 are higher than in the literature reports and
appear to be responsible for the stabilization of copper(II)–iodide
bonding. The pyridyl group appears to be necessary for
II
III
1
Mechanism
12
The formation of complex 1 appears to be due to a proton
coupled electron transfer (PCET) process wherein copper(I) loses
4848 | Dalton Trans., 2012, 41, 4845–4851
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Table 1 Crystallographic data for complexes 1–4
1
2
3
4
Formula
Mol. weight (amu)
Crystal system
Space group
a (Å)
b (Å)
c (Å)
C
26
H
22Cu
2
I
2
N
8
S
2
C
62
H
54Cu
2
I
2
N
8
P
2
S
2
C
26
H
22Br
2
Cu
2
N
8
S
2
C
62 2 2 8 2 2
H54Br Cu N P S ,
891.56
Monoclinic
C2/c
11.196(1)
22.689(3)
11.979(2)
—
104.885(2)°
—
1418.07
Triclinic
P1ˉ
11.7052(6)
12.3415(4)
13.1671(5)
62.760(4)
64.620(4)
67.984(4)
797.54
Monoclinic
C2/c
10.9775(12)
22.299(2)
11.9300(13)
—
106.245(2)
—
1324.09
Triclinic
P1ˉ
11.8176(18)
12.2666(17)
12.9683(19)
61.805(15)
65.714(14)
67.653(14)
1469.2(4)
1
α (°)
β (°)
γ (°)
V (Å )
3
2940.8(6)
4
1489.66(11)
1
2803.8(5)
4
Z
T (K)
100(2)
2.014
3.721
11154
200(2)
1.581
1.920
100(2)
1.889
4.550
14150
296(2)
1.497
2.256
24020
−3
ρ
calcd (g cm
)
−
1
μ (mm
Absolute reflec.
)
22101
Unique, Rint
Reflec. with I > 2σ(I)
3637, 0.0278
3140
9728, 0.0390
6047
3499, 0.0236
3053
9721, 0.0524
4085
R
wR
0.0343
0. 0871
0.0382
0.0758
0.0237
0.0559
0.0336
0.0546
Table 2 Bond distances (Å) and bond angles (°) for complexes 1–4
Complex 1
Cu(1)–N(2)
Cu(1)–N(1)
Cu–Cu
N(1)–Cu(1)–S(1)
N(2)–Cu(1)–I(1)
N(2)–Cu(1)–N(1)
Complex 3
Cu(1)–N(1)
Cu(1)–N(2)
Cu–Cu
1.990(3)
2.009(3)
4.126(1)
164.68(10)
179.54(10)
80.82(13)
Cu(1)–I(1)
Cu(1)–S(1)
2.5650(6)
2.2292(11)
N(2)–Cu(1)–S(1)
N(1)–Cu(1)–I(1)
S(1)–Cu(1)–I(1)
84.35(9)
99.42(10)
95.44(3)
1.9984(17)
1.9851(16)
3.227(1)
Cu(1)–Br(1)
Cu(1)–S(1)
2.3906(4)
2.2258(6)
N(2)–Cu(1)–Br(1)
N(1)–Cu(1)–S(1)
N(2)–Cu(1)–N(1)
Complex 2
Cu–Cu
Cu–P
Cu–I
P–Cu–S
P–Cu–S*
P–Cu–I
179.32(5)
164.51(5)
80.67(6)
N(2)–Cu(1)–S(1)
N(1)–Cu(1)–Br(1)
S(1)–Cu(1)–Br(1)
84.40(5)
98.97(5)
95.912(16)
II
2
3
1
2
Fig. 4 Molecular structure of [Cu
(μ-I)
2
((η -N , N , S-bpytsc)
2
] (1).
3.857(1)
Cu–S
Cu–S*
S–Cu–I*
S–Cu–I
Cu–S–Cu*
S–Cu–S*
2.4298(7)
2.3644(7)
115.776(19)
106.10(2)
83.20(2)
2.2506(7)
2.5847(4)
117.93(3)
105.27(3)
114.08(2)
96.80(2)
Complex 4
Cu–Cu
Cu–P
Cu–Br
P–Cu–S
P–Cu–Br
S*–Cu–Br
3.285(1)
Cu–S
Cu–S*
P–Cu–S*
Br–Cu–S
Cu–S–Cu*
S–Cu–S*
2.4622(8)
2.3716(7)
106.58(2)
106.79(2)
85.56(2)
94.44(3)
2.2454(8)
2.4152(6)
117.44(3)
116.47(3)
113.79(2)
II
3
1
2
an electron to form copper(II), and this is accompanied by the
2 2
Fig. 5 Molecular structure of [Cu2 (μ-Br) (η -N , N , S-bpytsc) ] (3).
3
loss of a –N H proton from the Hbpytsc ligand resulting in the
−
formation of anionic bpytsc (Scheme 2). It appears that this
Conclusions
proton captures the electron lost by the copper(I) to form a H
atom, which may have recombined with another H atom moiety
In conclusion, the presence of both phenyl and pyridyl substitu-
2
1
2
2
3
1
4
to probably form H , but this could not be proved by experimen-
ents at the C group {Ph(py-N )C vN –N H–C (vS)–N H }
2
2
tal procedures. The pyridyl group appears to be necessary for
stabilizing the Cu –I bond, as this group can accept π-electrons
has imparted an unusual property to 2-benzoylpyridine thiosemi-
carbazone, that of stabilizing Cu –halogen bonds in presence of
II
II
from the metal. A similar mechanism is suggested for the
reducing conditions such as presence of a tertiary phosphine,
formation of complex 3.
Ph P and a thio-ligand. Significantly, this thio-ligand acts as a
3
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Fig. 9 Packing diagram of dimer 2 showing the 2D sheet (dimer 4 has
similar interactions).
2 2 2 3 2
Fig. 6 Molecular structure of [Cu I (μ-S-Hbpytsc) (PPh ) ] (2).
Scheme 2
2
substituents at C only formed mononuclear tetrahedral copper(I)
7
b
halide complexes. The presence of bulky phenyl in place of
hydrogen has altered the bonding behaviour of 2-benzoylpyri-
dine thiosemicarbazone.
Acknowledgements
Financial support from Council of Scientific and Industrial
Research, New Delhi (F.No.9/254(159)/2005-EMR-I), is grate-
fully acknowledged. We are thankful to Prof. M. Zeller, Depart-
ment of Chemistry, Youngstown State University, USA for
partial help in X-ray studies. We thank Prof. R. N. Mukherjee,
Department of Chemistry, IIT Kanpur for recording of ESR
spectra.
2 2 2 3 2
Fig. 7 Molecular structure of [Cu Br (μ-S-Hbpytsc) (PPh ) ] (4).
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