Investigations into bis(triphenylphosphine)copper(I) complexes with cyclic derivatives of methylpyruvate thiosemicarbazones

Article abstract of DOI:10.1016/j.poly.2010.04.009

The syntheses of four compounds, obtained by the reaction of methylpyruvate thiosemicarbazone (Hmpt) and its methyl (Me-Hmpt) and allyl (Allyl-Hmpt) derivatives with bis(triphenylphosphine)copper(I) acetate, are reported. The compounds [Cu(PPh₃)₂(pt_c)(Hpt _c)]·H₂O (1), [Cu(PPh₃) ₂(Me-pt_c)] (2), [Cu₂(PPh₃) ₂μ-S(Me-pt)μ-S(Me-pt_c)]·H₂O (3) and [Cu(PPh₃)₂(Allyl-pt_c)] (4) [H₂pt = pyruvic acid thiosemicarbazone and Hpt_c = cyclized pyruvic acid thiosemicarbazone, Me = methyl and Allyl are radical substituents on the amino nitrogen] were characterized by elemental analysis, IR, ¹H NMR, and by X-ray crystallography. Compound 3 was also studied by EPR because of the presence in the compound of two copper atoms in two different oxidation states. During the complexation reaction, the thiosemicarbazone ligands tend to undergo a cyclization reaction that leads to the formation of a six-member heterocyclic ring. All four compounds present the [Cu(PPh₃)₂] ⁺ fragment and constant but different coordination situations. Compound 1 contains two cyclic ligand molecules, one protonated and the other deprotonated, bound as monodentate through the sulfur. Compounds 2 and 4 present a single deprotonated cyclic SN bidentate ligand molecule, while compound 3 contains copper(I) and copper(II), and two ligand molecules, one of which is linear and behaves as SNO tridentate and the other is cyclic and behaves as bridging μSN.

Full text of DOI:10.1016/j.poly.2010.04.009

Polyhedron 29 (2010) 2134–2141
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Polyhedron
journal homepage: www.elsevier.com/locate/poly
Investigations into bis(triphenylphosphine)copper(I) complexes with cyclic
derivatives of methylpyruvate thiosemicarbazones
a
Marisa Belicchi-Ferrari a, Franco Bisceglie a, Enrico Buluggiu b, Giorgio Pelosi a, , Pieralberto Tarasconi
*
a Dipartimento di Chimica Generale ed Inorganica, Chimica Analitica, Chimica Fisica, Viale G.P. Usberti 17A, Università degli Studi di Parma, 43124 Parma, Italy
b Dipartimento di Sanità Pubblica – Sezione di Fisica, Via Volturno 39, Università degli Studi di Parma, 43124 Parma, Italy
a r t i c l e i n f o
a b s t r a c t
Article history:
The syntheses of four compounds, obtained by the reaction of methylpyruvate thiosemicarbazone (Hmpt)
and its methyl (Me-Hmpt) and allyl (Allyl-Hmpt) derivatives with bis(triphenylphosphine)copper(I) ace-
Received 3 February 2010
Accepted 4 April 2010
Available online 27 April 2010
tate, are reported. The compounds [Cu(PPh3)2(ptc)(Hptc)]ꢀH2O (1), [Cu(PPh3)2(Me-ptc)] (2), [Cu2(PPh3)2
l-
S(Me-pt)
l-S(Me-ptc)]ꢀH2O (3) and [Cu(PPh3)2(Allyl-ptc)] (4) [H2pt = pyruvic acid thiosemicarbazone and
Hptc = cyclized pyruvic acid thiosemicarbazone, Me = methyl and Allyl are radical substituents on the
amino nitrogen] were characterized by elemental analysis, IR, 1H NMR, and by X-ray crystallography.
Compound 3 was also studied by EPR because of the presence in the compound of two copper atoms
in two different oxidation states. During the complexation reaction, the thiosemicarbazone ligands tend
to undergo a cyclization reaction that leads to the formation of a six-member heterocyclic ring. All four
compounds present the [Cu(PPh3)2]+ fragment and constant but different coordination situations. Com-
pound 1 contains two cyclic ligand molecules, one protonated and the other deprotonated, bound as
monodentate through the sulfur. Compounds 2 and 4 present a single deprotonated cyclic SN bidentate
ligand molecule, while compound 3 contains copper(I) and copper(II), and two ligand molecules, one of
Keywords:
Copper(I)
Thiosemicarbazone
Mixed-valence complex
which is linear and behaves as SNO tridentate and the other is cyclic and behaves as bridging lSN.
Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction
on thiosemicarbazone complexes of copper(I) stabilized in its low-
er oxidation state by the presence of triphenylphosphine [12,13].
In bis(triphenylphosphine) copper(I) complexes present in the
Cambridge Structural Database [14], aside from the two donor spe-
cies P and S, the remaining space around the copper ion is usually
occupied by a fourth ligand, mainly a halide [4,15,16]. Complexes
of Cu(I) without halides are quite novel. We have verified that in
our Cu(I) bis(triphenylphosphine) thiosemicarbazones, the absence
of halides or other soft species influences the coordination, and the
imino nitrogen becomes a suitable donor for coordination and al-
lows the ligand to show chelating behaviour. In the course of these
synthetic studies we also observed that under certain conditions
carbonyl containing thiosemicarbazones can undergo cyclization
reactions, with consequent dramatic changes in their coordination
properties.
Notwithstanding the large amount of literature about thiosemi-
carbazones, the interest in their chemistry is still growing and at-
tracts more and more research groups. These compounds present
interesting chemical properties, both as organic molecules and also
as ligands in metal complexes. The main reason for which they are
being studied is connected with their biological and, more specifi-
cally, pharmacological properties since they present antiviral, anti-
bacterial or antitumoral activities [1–9]. Their chemical properties
are also of remarkable interest thanks to the co-presence of nitro-
gen and sulfur in the thiosemicarbazide fragment, elements that
are the main chemical species that in biological systems such as
metalloproteins and metalloenzymes coordinate and modulate
the redox properties of transition metal ions. The predictability
of their chemistry is moreover very often elusive because of the
plurality of properties that these compounds show in dependence
of physical factors and the chemical nature of the substituents.
Recently our attention has been devoted to complexes of thio-
semicarbazones with Cu(I), since there are few studies on copper(I)
thiosemicarbazones [4–7,10,11], probably in connection with their
poor solubility in many solvents. We have lately published papers
In this work we report the syntheses of four compounds ob-
tained by the reaction of methylpyruvate thiosemicarbazone
(Hmpt) and its methyl (Me-Hmpt) and allyl (Allyl-Hmpt) deriva-
tives with bis(triphenylphosphine)copper(I) acetate. The obtained
metal complexes are: [Cu(PPh3)2(ptc)(Hptc)]ꢀH2O (1), [Cu(PPh3)2
(Me-ptc)] (2), [Cu2(PPh3)2
l
-S(Me-pt)
l
-S(Me-ptc)]ꢀH2O (3) and
[Cu(PPh3)2(Allyl-ptc)] (4) [Me = methyl and Allyl are substituents
on the amino nitrogen]. Under the preparative conditions, the
starting ligand Hmpt is converted into its hydrolyzed linear
H2pt = pyruvic acid thiosemicarbazone or cyclic Hptc = cyclized
* Corresponding author. Tel.: +39 0521 905 420; fax: +39 0521 905 557.
E-mail address: giorgio.pelosi@unipr.it (G. Pelosi).
0277-5387/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2010.04.009

M. Belicchi-Ferrari et al. / Polyhedron 29 (2010) 2134–2141
2135
Scheme 2 reports the structures of the ligands used in the
experiments.
O
O
H3C
O
CH3
N
H3C
NH
2.3. Synthesis of the complexes
N
H
S
N
S
N
R
To 20 mL of a hot stirred methanolic solution of the proper li-
gand was added an amount of solid [Cu(PPh3)2(CH3COO)] (for 1:
0.500 g of the ligand, 2.71 mmol, and 1.757 g of metal salt,
2.71 mmol; for 2: 0.610 g of the ligand, 2.94 mmol, and 1.905 g
of metal salt, 2.94 mmol; for 3: 0.484 g of the ligand, 2.34 mmol,
and 1.511 g of metal salt, 2.34 mmol; for 4: 0.515 g of the ligand,
2.56 mmol, and 1.657 g of metal salt, 2.56 mmol), together with
an additional amount of 20 mL of methanol. The mixture was al-
lowed to reach the reflux temperature. When the inorganic salt
was completely dissolved, the resulting solution changed colour
from pale yellow to dark orange, and was left under magnetic stir-
ring for 2 h. The solution was then cooled down to room tempera-
ture, and by slow evaporation of the solvent, crystals suitable for X-
ray diffraction were obtained.
RHN
Scheme 1. Chemical drawing of the linear ligand Hmpt (left) and cyclic ligand Hptc
(R = H, methyl and allyl). In the cyclic molecule, as consequence of the
nucleophilic attack of the terminal amino group on the ester group, a methanol
molecule is eliminated and the ligand actually becomes a cyclic derivative of
pyruvic acid thiosemicarbazone (Hptc).
a
pyruvic acid thiosemicarbazone form (see Scheme 1). All of the
complexes have been characterized by elemental analysis, IR, 1H
NMR spectroscopy and by X-ray crystallography. The co-presence
in compound 3 of two copper ions, one of which is possibly oxi-
dised, has also required EPR spectroscopy analysis.
2.3.1. [Cu(PPh3)2(ptc)(Hptc)]ꢀH2O (1)
2. Experimental
Yield: 1.61 g, 73% (based on metal). M.p.: 165 °C. FT-IR (KBr,
cmꢁ1): 3443 m, br, 3221 ms,
m
(NH); 3042 mw,
m(CHaromatic);
2.1. Materials and techniques
1684 ms,
m
(C@O); 1594 ms,
m(CN); 745 s, (CS). Anal. Calc. for
m
C
44H41N6O3P2S2Cu: C, 59.28; H, 4.64; N, 9.43; S, 7.19. Found: C,
Cu(PPh3)2(CH3COO) was prepared by reduction of Cu(CH3COO)2
using a fourfold excess of PPh3 in CH3OH at reflux temperature for
3 h. The white powder formed in the reaction flask was filtered to
separate the product from Ph3PO, washed first with EtOH and then
with diethyl ether. Cu(CH3COO)2ꢀH2O was procured from Carlo-
Erba, while PPh3 was purchased from Aldrich. C, H, N and S analy-
ses were obtained with a Carlo-Erba 1108 instrument. IR spectra
were recorded using KBr pellets on a Nicolet 5PC FT-IR spectropho-
tometer in the 4000–400 cmꢁ1 range. 1H NMR spectra were re-
corded on a Bruker Avance spectrometer at 300 MHz in d6-DMSO
with TMS as the internal reference. The magnetic properties were
examined by EPR spectroscopy in the range from room tempera-
ture to liquid nitrogen temperature. X-band resonance spectra
were recorded in the derivative form by a 9-GHz Varian V-4502
58.90; H, 4.65; N, 9.31; S, 7.42%. 1H NMR data (d, ppm; DMSO-
d6): 2.15 (s, 6H, CH3–), 7.23 (t, b, 18H, PPh3 Hp + Ho), 7.47 (t, 12H,
PPh3 Hm), 9.88 (s, 1H, NH), 11.78 (bs, 2H, NH).
2.3.2. [Cu(PPh3)2(Me-ptc)] (2)
Yield: 1.33 g, 61% (based on metal). M.p.: 163 °C. FT-IR (KBr,
cmꢁ1): 3049 m,
1430 m, 695 m,
Cu: C, 66.16; H, 4.87; N, 5.64; S, 4.31. Found: C, 66.01; H, 4.87; N,
5.48; S, 4.20%. 1H NMR data (d, ppm; DMSO-d6): 2.15 (s, 3H, CH3–),
3.35 (s, 3H, CH3-N), 7.25 (t, b, 18H, PPh3 Hp + Ho), 7.45 (t, 12H, PPh3
Hm).
m
(CHaromatic); 1660 s,
m(C@O); 1606 m, m(CN);
m
(NCS); 745 s, (CS). Anal. Calc. for C41H36N3OP2S-
m
spectrometer in the phase-sensitive detection mode with
a
2.3.3. [Cu2(PPh3)2
Yield: 1.40 g, 60% (based on metal). M.p.: 180 °C. FT-IR (KBr,
cmꢁ1): 3439 m, br,
(NH); 3042 mw, (CHaromatic); 1695 ms,
1657 s, (C@O); 1606 m, (CN); 1454 m, 695 ms, (NCS); 742
ms, (CS). Anal. Calc. for C46H45N6O4P2S2Cu2: C, 55.30; H, 4.54; N,
l
-S(Me-pt)
l
-S(Me-ptc)]ꢀH2O (3)
100 kHz field modulation. The spectrometer was equipped with a
cylindrical cavity, rotating magnet and a continuous flow variable
temperature device.
m
m
m
m
m
m
8.42; S, 6.42. Found: C, 54.90; H, 4.53; N, 8.24; S, 6.65%.
2.2. Preparation of the ligands
The methylpyruvate thiosemicarbazones were prepared by con-
densation of methylpyruvate with different thiosemicarbazides in
a 1:1 molar ratio in methanol using the procedure reported in
the literature [17]. In this way methylpyruvatethiosemicarbazone
(Hmptꢀ0.5H2O, used to prepare complex 1) methylpyruvate-N1-
methylthiosemicarbazone (Me-HmptꢀH2O, used to prepare com-
plexes 2 and 3) and methylpyruvate-N1-allylthiosemicarbazone
(Allyl-Hmpt, used to prepare complex 4) were synthesized.
2.3.4. [Cu(PPh3)2(Allyl-ptc)] (4)
Yield: 1.44 g, 73% (based on metal). M.p.: 170 °C. FT-IR (KBr,
cmꢁ1): 3050 mw,
m(CHaromatic); 1664 s,
m(C@O); 1605 m, m(CN);
1427 m, 684 s,
m
(NCS); 746 s, (CS). Anal. Calc. for C43H38N3OP2SCu:
m
C, 67.04; H, 4.97; N, 5.46; S, 4.16. Found: C, 66.87; H, 4.97; N, 5.37;
S, 4.01%. 1H NMR data (d, ppm; DMSO-d6): 2.18 (s, 3H, CH3–), 4.22
(m, 2H, N-CH2), 5.12 (m, 1H, allylic H cis + trans), 5.88 (m, 1H,
allylic H gem), 7.25 (t, b, 18H, PPh3 Hp + Ho), 7.37 (t, 12H, PPh3 Hm).
O
CH3
O
O
CH3
CH3
H
N
H
N
H
N
O
N
O
O
N
N
H3C
S
H3C
S
H3C
S
HN
HN
H2N
CH3
Hmpt
Me-Hmpt
Allyl-Hmpt
Scheme 2. Chemical drawings of the ligands used in the reactions.

2136
M. Belicchi-Ferrari et al. / Polyhedron 29 (2010) 2134–2141
Table 1
Selected crystal data.
Compound
(1)
(2)
(3)
(4)
Formula
Molecular mass
Space group
a (Å)
b (Å)
c (Å)
C
44H41Cu1N6O3P2S2
C41H36Cu1N3O1P2S1
744.308
P21/c
14.897 (4)
9.606(2)
26.153(5)
90
C46H45Cu2N6O4P2S2
999.059
P21/n
10.288(3)
12.911(4)
35.205(10)
90
C43H38Cu1N3O1P2S1
770.346
P21/c
15.108(4)
9.567(3)
26.854(5)
90
891.46
P21/n
11.309(2)
12.953(2)
30.006(4)
90
a
(°)
b (°)
92.21(1)
90
4392(1)
4
93.65(1)
90
3734(1)
4
95.97(1)
90
4651(2)
4
91.04(1)
90
3880(2)
4
c
(°)
V (Å3)
Z
F(0 0 0)
1848
1.348
0.71
1544
1.323
2.43
2060
1.427
1.12
1600
1.318
0.73
Dcalc (Mg/m3)
l
(mmꢁ1
)
k (Å)
0.71069
1.54184
0.71069
0.71069
Radiation
Mo Ka
Cu Ka
Mo K
a
Mo Ka
h Range (°)
3–23.29
3–59.99
3–27.58
3–25.27
hkl range
Crystal size (mm)
ꢁ12, 12; ꢁ14, 14; ꢁ33, 33
ꢁ16, 16; 0, 10; 0, 29
0.2 ꢂ 0.1 ꢂ 0.1
6058
5541
442
ꢁ13, 13; ꢁ16, 16; ꢁ45, 45
0.5 ꢂ 0.4 ꢂ 0.4
57 243
10 663
559
ꢁ18, 18; ꢁ11, 11; ꢁ32, 32
0.2 ꢂ 0.1 ꢂ 0.1
36 223
0.4 ꢂ 0.3 ꢂ 0.2
37 720
No. of measured reflections
No. of unique reflections
No. of refined parameters
Maximum and minimum residues,
6319
531
0.37, ꢁ0.25
7043
469
0.74, ꢁ0.43
1.08, ꢁ0.89
1.01, ꢁ0.66
D
F map (e Åꢁ3
)
P
P
R = ||Fo| ꢁ |Fc||/ |Fo|
0.0554
0.0572
0.1665
0.0716
0.0849
Rw2
0.1332
0.2049
0.1811
2
2
2
2
1=½r2ðF2oÞ þ ð0:0377PÞ þ 9:66Pꢃ 1=½r2ðFo2Þ þ ð0:1256PÞ ꢃ 1=½r2ðFo2Þ þ ð0:0941PÞ þ 5:55Pꢃ 1=½r2ðF2oÞ þ ð0:0225PÞ þ 12:15Pꢃ
Weighting scheme
P ¼ ðmaxðF2o; 0Þ þ 2F2c Þ=3.
2.4. X-ray crystallography
ence of metal ions can catalyze the reaction by coordinating the
carbonyl oxygen and, in this way, increasing the electrophilicity
Relevant data concerning collection and details of the structure
refinements are summarized in Table 1. Intensity data for 1, 3 and
4 were collected on a SMART 1000 Bruker AXS diffractometer with
of the carbonyl carbon, contributing also to the stabilization of
the tetrahedral intermediate.
On setting up the reactions, in all four cases a 1:1 ratio between
the linear ligand and the inorganic salt was used, but the products
present different coordination stoichiometries. Compound 1 con-
tains two cyclic ligands, one protonated and the other deproto-
nated, bound as monodentate through the sulfur, compounds 2
and 4 present a single deprotonated bidentate cyclic ligand mole-
cule, while compound 3 contains two copper atoms in different
oxidation states, (I) and (II), and two ligand molecules one of which
is linear and behaves as SNO tridentate and the other is cyclic and
Mo Ka radiation and those of 2 on a CAD4 diffractometer with Cu
Ka
radiation. The crystals of compound 4 were small and affected
by twinning. The data of 2, collected on the CAD4 diffractometer,
were not corrected for absorption, while for compounds 1, 3 and
4, absorption corrections were carried out using SADABS [18]. The
structures were solved using direct methods (SIR-97 [19]). Refine-
ments were carried out by full matrix least-squares cycles, SHEL-
XL97 [20], for all compounds. Most hydrogen atoms were
calculated with standard geometries and were refined in riding
positions. The program PARST [21] was employed for the geometrical
description and PLUTON [22] and MERCURY [23] for the drawings.
behaves as bridging lSN.
The soft Cu(I) metal binds the soft sulfur atom in 1 and also the
nitrogen of intermediate hardness in 2 and 4 of the thiosemicarba-
zone chain, leaving free the harder donor atom oxygen. In this way
hydrolysis of the ester group of the ligand can occur, resulting in
ring formation (Scheme 3).
3. Results and discussion
Despite the syntheses being carried out in methanol, the water
dissolved and the pH variation due to the presence of the acetate
anion leads to the modification of the ligand. Complex 3 is isolated
as a dinuclear valence mixed compound with a ligand in the linear
form and another one in its cyclic form. Probably because of slow
evaporation of the solvent in the open atmosphere (with a conse-
quently more oxygenated solution), the first step hypothesized is
a partial oxidation of copper(I) to copper(II). Consequently the
harder Cu(II) coordinates via SNO a hydrolized ligand, preventing
its cyclization. The Cu(I) metal centre binds via the soft P,P donor
atoms and the other soft S,S donor atoms, stabilizing its lower oxi-
dation state. It was verified that crystallizing the complex under a
N2 atmosphere, only the copper(I) complex 2 was isolated.
In the 1H NMR spectrum of diamagnetic complex 1, the pres-
ence of a signal at 9.88 ppm integrated for 1 proton, typical of
the aminic NH terminal group, confirms the deprotonation of only
one ligand. Moreover, the lack of a resonance due to the second NH
3.1. Synthesis and spectroscopy
The ligands were synthesized as previously described [17]. The
complexes were obtained by reacting the proper ligand with
bis(triphenylphosphine) copper(I) acetate. Both ligands and com-
plexes were produced in good yields. The ligands were all isolated
in their linear form [17]. In complexes 1, 2 and 4, the ligands un-
dergo cyclization.
A requirement for the cyclization is the presence of a carbonyl
group sufficiently distant from the terminal nitrogen to form five
or six-membered rings. This usually takes place at relatively high
temperatures and is promoted by the presence of a metal centre
[24]. The phenomenon can be interpreted in terms of a larger ther-
modynamic stability of the cyclized compound with respect to the
linear one and to the presence of an energetic barrier between the
two forms that can be overcome at high temperatures. The pres-

M. Belicchi-Ferrari et al. / Polyhedron 29 (2010) 2134–2141
2137
CH3
R
PPh3
PPh3
Cu
R
NH
S
N
CH3
NH
S
PPh3
Cu
N
O
PPh3
O-
O
H
O
N
O
O
O
H
N
CH3
CH3
O
CH3
CH3
-CH3COOH
PPh3
PPh3
S
R
PPh3
Cu
N
S
N-
N
PPh3
R
H
Cu
+ H2O
CH3
OH
N
O
H
- CH3OH
N-
O
O
H3C
N
CH3
- H2O
PPh3
Cu
PPh3
S
N-
N-R
N
H3C
O
Scheme 3. Example of complex formation and cyclization process.
terminal proton and also the absence of the –OCH3 ester moiety
could explain the ligand cyclization. In complexes 2 and 4, because
of the cyclization and deprotonation of the ligands bearing a sub-
stituent on the amino terminal group, no NH and –OCH3 ester sig-
nals were revealed.
As one can see, apart from the expected higher intensity at low
temperature, the shapes of the two powder spectra are almost
identical. Both are typical of an S = ½ isolated unpaired spin. Their
behaviours are close to that shown by polycrystals of the analo-
gous copper complex [13]. However, in contrast with that case,
here we do not observe any evidence of splitting due to a hyperfine
or super-hyperfine interaction.
As regards to the IR spectra, the absorptions in the 3443–
3221 cmꢁ1 region are attributed to
m(NH) stretching frequencies
for complexes 1 and 3. The endocyclic C@O vibrations fall in the
range 1684–1657 cmꢁ1, in good agreement with the carbonyl
stretchings of analogous azathymine derivatives [25]. In complex
3 the vibration at 1695 cmꢁ1 is attributed to the coordinated
C@O group and is shifted downfield by ca. 20 cmꢁ1 with respect
to the same vibration of the free ligand [17a]. For all the complexes
The spectroscopic Landé factor is rhombic with two principal
values close to
gmax ¼ 2:173 ꢄ 0:002;
ð1Þ
gmed ¼ 2:067 ꢄ 0:005:
m
(C@S) is significantly shifted to lower frequencies, showing that
The third value is more uncertain because of the large overlap of
the related peak with the principal absorption. We may only esti-
mate it as extremely low, close to the free electron value of 2.0023.
From a quantitative point of view, the splitting factor g mea-
sured for a crystal does not necessarily coincide with the molecular
g, as observed in magnetically diluted structures. In fact, it is well
known that exchange coupling among the ions may average the
true single-ions g values [26]. Fortunately, in the case of the pres-
the ligands bind the metal via the sulfur atoms.
An electron paramagnetic resonance study was performed for
compound 3. The sample was in the polycrystalline form owing
to the difficulty to find single crystals of dimensions large en-
ough to give an acceptable signal to noise ratio. Resonance
absorptions recorded at temperatures of 295 K and 77 K are
shown in Fig. 1.

2138
M. Belicchi-Ferrari et al. / Polyhedron 29 (2010) 2134–2141
The measured g value appears to be somewhat low when com-
pared with the usual values shown by a Cu(II) ion in octahedral
environments of oxygen or nitrogen ligands, generally found in
the ranges g = 2.35–2.25, g\ ꢆ 2.10–2.05. As anticipated, this is
k
very likely due to the presence of sulfur in the in-plane coordina-
tion. It is well known, in fact, that Cu(II)–S or Cu(II)–Se
r-bonds
are strongly covalent and may be responsible for a large reduction
of the g values and of the hyperfine coupling constant at the copper
nucleus [31,32].
Recently, we were able to isolate some single crystals with a
measurable signal. Preliminary results at the X-band frequency
evidenced a main S = ½ absorption with intensity strongly depen-
dent on the direction of the static magnetic field. A spurious weak
signal was also observed, appearing only for H along some special
directions. The nature of this signal at present is not yet
understood.
The g tensor of the main peak was determined by the use of the
three-plane method, first introduced by Geusic and Brown [33],
and the principal values resulting at room temperature were:
gmax = 2.178 0.002, gmed = 2.093 0.002, gmin = 2.016 0.002.
These values are in substantial agreement with those of Eq. (1),
confirming the rhombic nature of the polycrystal spectrum (Fig. 1).
From a quantitative point of view, they compare reasonably well
with typical g values reported for planar cupric complexes with
sulfur and nitrogen ligands. For example, Anufrienko and Zeif
[34] obtained g = 2.14, g\ ꢆ 2.042 by an X-band EPR study of pow-
k
ders of Cu2+
a
-thiopicolinanilide and Buluggiu et al. [31] measured
values in the ranges gz = 2.127–2.152, gx = 2.023–2.027, gy = 2.041–
2.047 from X- and Q-band spectra of powders of trans-bis(thiosem-
icarbazide) Cu(II) nitrate and single crystals of Cu(II) doped trans-
bis(thiosemicarbazide) Ni(II) sulfate trihydrate complexes. In all
these cases two sulfur and two nitrogen atoms are bound to the
metal in trans positions.
Fig. 1. X-band EPR powder spectra of complex 3 at room and liquid nitrogen
temperatures.
On the basis of these examples we may assume gmax of Eq. (1) as
the g value orthogonal to the coordination plane. Its somewhat lar-
ger value, as compared to those of Refs. [31,34], may be related to
the lower extent of the unpaired spin delocalization in our complex
due to the presence of one only in-plane Cu–S bond.
ent complex there is no doubt about this because the point sym-
metry of the crystal makes all the Cu(II) ions magnetically undis-
tinguishable. Thus, the crystal g and single-ion g values must
necessarily coincide.
We conclude that the magnetic study of compound 3 testifies
the presence of a single magnetic copper species with features in
line with the proposed crystallographic mixed valence view.
The value of g, as known, is directly influenced by the delocal-
ization degree of the unpaired spin in the complex and can, to
some extent, be related to the molecular-orbital arrangement of
the metal–ligand bonds [27,28]. In a simple crystal-field vision,
for the case of an elongated octahedral Cu2+ complex (3d9, 2D con-
3.2. Crystal structures of the complexes
figuration) with rhombic in-plane distortion, both the dx2
and
ꢁy2
Fig.
2
represents
a
PLUTON drawing of the complex
d3z2
orbitals belong to the same irreducible representation and
ꢁr2
[Cu(PPh3)2(ptc)(Hptc)]ꢀH2O (1). The copper(I) ion tetrahedrally
coordinates two triphenylphosphines and two cyclic ligands
through the sulfur atoms. One of the cyclic ligands is protonated
and is connected to the corresponding deprotonated nitrogen of
a second molecule by a hydrogen bond [N1ꢀ ꢀ ꢀN4 2.688(7) Å, N1–
Hꢀ ꢀ ꢀN4 163.3(3)°] (Fig. 2). The average planes of the two rings form
an angle of 72.8(2)°. The copper coordination bond distances are
2.365(2) and 2.333(2) Å for the Cu–S bonds, while the Cu–P bond
distances are 2.271(1) and 2.271(2) Å. The copper coordination is
distorted from an ideal tetrahedron and the angles S–Cu–S and
P–Cu–P are 117.48(6)° and 118.25(6)°, respectively.
can mix to give for the Cu(II) unpaired electron an orbital ground
state of the form
wAg
¼
adx2
þ nd3z2
ð2Þ
ꢁy2
ꢁr2
where n ꢅ
Some rhombicity may be hypothesized due to the strongly
covalent character of the in-plane Cu–S -bond, which should also
a and a
2 + n2 = 1.
r
be responsible for the low g values observed.
Taking into account the time spent by the unpaired electron on
the ligands, by assuming a LCAO orbital description of the mag-
netic complex, in a second-order approximation we can write
[29,30]
The packing is formed by a linear network of hydrogen bonds
between the rings along the
x
direction [N5ꢀ ꢀ ꢀO1
gz ꢆ 2:0023 ꢁ 8
gx ꢆ 2:0023 ꢁ 2ð
gy ꢆ 2:0023 ꢁ 2ð
a
2b2z k0=
D
Exy
2
(ꢁx + 1, ꢁy + 2, ꢁz) 2.819(7) Å, N5–H6ꢀ ꢀ ꢀO1 166(6)° and N2ꢀ ꢀ ꢀO2
(ꢁx + 2, ꢁy + 2, ꢁz) 2.825(7) Å, N2–H2ꢀ ꢀ ꢀO2 160(6)°]. Alternating
on both sides of this ribbon, protrude the tpp groups. The phenyls,
in their turn, contribute to the packing through edge-to-face phe-
nyl interactions [with Pꢀ ꢀ ꢀP distances of 8.346 and 8.041 Å] and
these interactions develop along the y direction. The crystallization
water molecules occupy interstitial sites and seem to play no rele-
vant role in the packing. In fact the distances Owꢀ ꢀ ꢀO1 3.102(10)
pffiffiffi
a
a
þ
ꢁ
3nÞ b2k0=
D
Eyz
Exz
ð3Þ
x
pffiffiffi
2
3nÞ b2k0=
D
y
where k0 is the spin–orbit coupling constant for the free Cu(II) ion (a
value close to ꢁ828 cmꢁ1) and
D
Exy and
D
Exz
,
are the optical
splittings EðdxyÞ ꢁ Eðdx2ꢁy2 Þ and Eðdxy;yzÞ ꢁ Eðdx2ꢁy2 Þy,zrespectively.

M. Belicchi-Ferrari et al. / Polyhedron 29 (2010) 2134–2141
2139
2.241(1) Å. The crystal structure is stabilized by a tight sixfold phe-
nyl embrace (6PE) [35–38] with a Pꢀ ꢀ ꢀP distance of 6.942 Å and
other two phenyl–phenyl interactions.
The crystal structure of compound 3, [Cu2(PPh3)2l-S(Me-pt)l-
S(Me-ptc)], is reported in Fig. 4. In this compound we have found
two copper atoms in two different coordination geometries: one
is tetrahedral and the other square planar. Noteworthy is the pres-
ence of both linear and cyclic ligands in the same molecule. In the
course of the reaction the linear methylpyruvate has hydrolyzed
losing the ester methyl group and has become a carboxylic acid.
According to the observed geometry around the copper ions,
square planar in one case and tetrahedral in the other, taking into
account the bond distances and based on the electroneutrality
principle, a mixed valence Cu(I)/Cu(II) system can be proposed.
Consistent with this model, the linear ligand behaves as tridentate,
binding Cu(II) (Cu2 in Fig. 4) through S, N and O, while the cyclic
ligand bridges the two copper centres using its sulfur atom to bind
the softer Cu(I) (Cu1 in Fig. 4) and a nitrogen atom for the harder
Cu(II). Moreover, the sulfur of the linear ligand is shared between
Cu(II) and Cu(I). We had already observed a similar case [13] in
which the bridging was made by an acetate ion and by a sulfur
atom of the SNO chromophore system of the linear Et–pt ligand.
The geometries are therefore slightly different.
The coordination distances for square planar Cu2 are: Cu2–
S1(linear) 2.289(2), Cu2–N3(linear) 1.937(4), Cu2–O1(linear) 1.976(4),
Cu2–N4(cyclic) 1.948(4) Å. Those of the tetrahedral copper(I) frag-
ment are: Cu1–S2(cyclic) 2.381(2), Cu1–S1(linear) 2.389(1) Å, and
the two with the phosphorus atoms of the tpp ligands are Cu1–
P1 2.312(2) and Cu1–P2 2.276(2) Å. The angle S1–Cu1–S2 is
106.04(6)° and that of P1–Cu1–P2 is 118.56(5)°. The tetrahedral
coordination around copper(I) is therefore fairly regular.
Dimer-like molecules are formed by hydrogen bonds involving
the water oxygen, the aminic nitrogen and the uncoordinated O2
atom (N1–Hꢀ ꢀ ꢀO1W 2.897(8) Å 149.3(3)° and O1W–Hꢀ ꢀ ꢀO2i
2.997(9) Å 176(5)° i = 1 ꢁ x, ꢁy, 2 ꢁ z). The water molecule acts a
bridge between the two complexes related by a centre of symme-
try at ½ 0 1. Also a weak interaction between the copper(II) atom
with the hydrazine nitrogen contribute to form these dimers
Fig. 2. PLUTON rendering of the complex [Cu(PPh3)2(Hptc)(ptc)] (1).
and Owꢀ ꢀ ꢀO2 3.042(9) Å are in the range of normal van der Waals
contacts. Packing details are reported as Supplementary material.
The structure of compound 2, [Cu(PPh3)2(Me-ptc)], is depicted
in Fig. 3. In this case there is a single ptc deprotonated ring bound
as a bidentate ligand to the bis(triphenylphosphine)copper(I) cat-
ion through the sulfur and a nitrogen. The coordination is still tet-
rahedral, but since the ligand forms a four-member chelate ring,
the bite angle is obviously very small [S1–Cu1–N2 67.64(8)°] and
leaves more room for the angle between the two tpp molecules
[P1–Cu1–P2 125.22(4)°]. Also the coordination distances are influ-
enced by the binding mode of ptc: the N–Cu bond is the shortest
one [2.080(3) Å] while that with the sulfur is much longer, Cu–S
2.513(1) Å, and those with phosphorus are 2.246(1) and
Fig. 4. PLUTON rendering of the complex fragment [Cu2(PPh3)2l-S(Me-pt)l-S(Me-
Fig. 3. PLUTON rendering of the complex [Cu(PPh3)2(Me-ptc)] (2).
ptc)] of compound 3.

2140
M. Belicchi-Ferrari et al. / Polyhedron 29 (2010) 2134–2141
that interact with the metal centre. In particular in these com-
pounds the absence of softer competitors made the imino nitrogen
a suitable donor for coordination to the soft copper(I) ion, besides P
and S species. In the present work all the ligands with methyl and
allyl substituents on the terminal nitrogen have been isolated in
the linear form. Differently, during the complexation reaction with
bistriphenylphosphine copper(I) acetate, the starting ligand Me-
Hmpt is converted into its hydrolyzed linear form Me-H2pt, meth-
ylpyruvic acid thiosemicarbazone, or cyclic Me-Hptc form. The
cyclization is probably favoured by a carbonyl group sufficiently
distant from the terminal nitrogen to form, in this case, a six-mem-
bered ring. The cyclic ligands behave as a monodentate ligand
through the sulfur atom in complex 1 or as SN bidentate ligands
in complexes 2 and 4. Particularly interesting is compound 3
where, probably because of a slow evaporation of the solvent in
the open atmosphere, a partial oxidation of copper(I) to copper(II)
occurred. One linear hydrolyzed ligand coordinates Cu(II) as a SNO
tridentate ligand and a second cyclic deprotonated ligand behaves
as bridging lSN ligand.
Acknowledgments
The magnetic measurements were performed at the EPR labora-
tory of the Dipartimento di Sanità Pubblica (Sezione di Fisica). We
are indebted to Dr. Mario Ghillani for valuable technical assistance
in all phases of the magnetic experimental work.
This work was supported by Italian MURST (PRIN 2007) in the
frame of the project ‘Design and synthesis of copper complexes
for developing bioinorganic target-specific drugs’ (Coordinator
Prof. C. Santini) and by the Consorzio Interuniversitario di Ricerca
in Chimica dei Metalli nei Sistemi Biologici (CIRCMSB).
Fig. 5. PLUTON rendering of the complex [Cu(PPh3)2(Allyl-ptc)] (4).
(Cu2ꢀ ꢀ ꢀN2i 3.292(5) Å). In addition these dimers are linked by an-
other hydrogen bond between the water oxygen and the O1 coor-
dinated to copper(II) (Ow–Hꢀ ꢀ ꢀO1ii 2.939(7) Å 164(5)° ii = x ꢁ 1, y,
z) to form ribbons that develops along the x axis. These ribbons
are then joined together by long Pꢀ ꢀ ꢀP interactions of 8.477 and
8.677 Å that involve the triphenylphosphines to form sheets paral-
Appendix A. Supplementary data
CCDC 764401, 764402, 764403 and 764404 contain the supple-
mentary crystallographic data for compounds 1, 2, 3 and 4. These
data can be obtained free of charge via http://www.ccdc.cam.a-
c.uk/conts/retrieving.html, or from the Cambridge Crystallographic
Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44)
1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk. Supplementary
data associated with this article can be found, in the online version,
at doi:10.1016/j.poly.2010.04.009.
ꢀ
lel to the (1 0 1) plane.
The crystal structure of compound [Cu(PPh3)2(Allyl-ptc)] (4) is
reported in Fig. 5. As in complex 2, in this case there is a single
ptc deprotonated ring, bound as a bidentate ligand to the bis(tri-
phenylphosphine)copper(I) cation through the sulfur and a nitro-
gen atoms. The coordination is still tetrahedral, the ligand forms
a four-membered chelate ring with a small bite angle [S1–Cu1–
N2 67.34(15)°] and the angle between the two tpp molecules
[P1–Cu1–P2 126.98(6)°] is much larger. The coordination distances
are N2–Cu1 bond [2.091(5) Å], that with the sulfur is Cu1–S1
2.518(2) Å and those with phosphorus are 2.239(2) and
2.250(2) Å. All the coordination distances and angles are compara-
ble with those of complex 2. The allyl substituent on the aminic
nitrogen presents a certain degree of disorder with the terminal
C7 carbon atom, statistically distributed in two positions.
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