Direct electrochemical synthesis and crystal structure of copper(I) complex with 2′-hydroxy-5′-methyl-acetophenone and triphenylphosphine ligands

Article abstract of DOI:10.1081/SIM-200059211

A new complex of Cu with 2′-hydroxy-5′-methyl-acetophenone (HMAP) and triphenylphosphine (PPh₃) was prepared by the electrochemical oxidation of Cu in acetonitrile solutions. The final product, (MAP)Cu(PPh₃)₂, was crystallized from bulk solution and characterized by microanalysis, IR, Raman, ¹H NMR together with x-ray crystallographic determinations. The results showed that the complex was a chelate structure in which Cu(I) ion was coordinated with two oxygen atoms of the deprotonated ligand of MAP anion to form a six member ring, and two PPh ₃ molecules participated in the coordination. The parameters of the complex were monoclinic, space group of P2₁/n, a = 9.048(2) A, b = 26.405(6) A, c = 15.606(4) A, β = 95.990(5)°, Z = 4, R = 0.057 for 8428 unique reflections. The details of synthesis, reaction mechanism on the electrode and the effect of PPh3 on the coordinating processes are discussed. Copyright

Full text of DOI:10.1081/SIM-200059211

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Direct Electrochemical Synthesis and
Crystal Structure of Copper(I) Complex with
2′‐hydroxy‐5′‐methyl‐acetophenone and
Triphenylphosphine Ligands
Yaxian Yuan a , Jianlin Yao a , Cheng Lin a , Yong Zhang a & Renao Gu a
a Department of Chemistry , Suzhou University , Suzhou, P.R. China
Published online: 15 Feb 2007.
To cite this article: Yaxian Yuan , Jianlin Yao , Cheng Lin , Yong Zhang & Renao Gu (2005) Direct Electrochemical Synthesis
and Crystal Structure of Copper(I) Complex with 2′‐hydroxy‐5′‐methyl‐acetophenone and Triphenylphosphine Ligands,
Synthesis and Reactivity in Inorganic, Metal-Organic, and Nano-Metal Chemistry, 35:5, 385-390, DOI: 10.1081/SIM-200059211
To link to this article: http://dx.doi.org/10.1081/SIM-200059211
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Synthesis and Reactivity in Inorganic, Metal-Organic and Nano-Metal Chemistry, 35:385–390, 2005
Copyright # 2005 Taylor & Francis, Inc.
ISSN: 0094-5714 print/1532-2440 online
DOI: 10.1081/SIM-200059211
Direct Electrochemical Synthesis and Crystal Structure of
Copper(I) Complex with 20-hydroxy-50-methyl-acetophenone
and Triphenylphosphine Ligands
Yaxian Yuan, Jianlin Yao, Cheng Lin, Yong Zhang, and Renao Gu
Department of Chemistry, Suzhou University, Suzhou, P.R. China
compounds are the appropriate and common materials for the
A new complex of Cu with 20-hydroxy-50-methyl-acetophenone chemical vapor deposition (CVD) of copper in the microelec-
(HMAP) and triphenylphosphine (PPh3) was prepared by the elec-
trochemical oxidation of Cu in acetonitrile solutions. The final
product, (MAP)Cu(PPh3)2, was crystallized from bulk solution
tronics (Jain et al., 1991). So far, different methods were
employed to prepare complexes. Among them the direct elec-
trochemical synthesis provided a well-controlled way to form
the Cu(I) by applying an appropriate voltage between the
showed that the complex was a chelate structure in which Cu(I) anode and cathode (Hart et al., 1994; Tuck, 1979; Annan
and characterized by microanalysis, IR, Raman, 1H NMR
together with x-ray crystallographic determinations. The results
ion was coordinated with two oxygen atoms of the deprotonated
ligand of MAP anion to form a six member ring, and two
PPh3 molecules participated in the coordination. The parameters
of the complex were monoclinic, space group of P21/n, a 5
et al., 1991; Tuck, 1993; Gu et al., 1997; Mabrouk et al.,
1993). Furthermore, no oxidant and reductant was used in the
electrochemical method, which avoided the possible compli-
cated redox processes resulted by the redox regents. The oxi-
˚
˚
˚
9.048(2) A, b 5 26.405(6) A, c 5 15.606(4) A, b 5 95.990(5)8,
Z 5 4, R 5 0.057 for 8428 unique reflections. The details of syn- dation of the metal electrode and the reduction of the ligands
thesis, reaction mechanism on the electrode and the effect of
PPh3 on the coordinating processes are discussed.
were carried out by the so-called ‘clean’ electron which was
removed or added on the anode and cathode by the applied
potential. This method decreased the contamination due to
Keywords Copper, crystal structure, direct electrochemical synthesis
the extraneous chemicals in the reaction processes, and also
in the final products. Most importantly, the variable applied
potential provided high selectivity and enabled the production
of the expected final products (Hart et al., 1994). In this article,
we have reported the direct electrochemical synthesis and
crystal structure of a new Cu(I) complex of 20-hydroxy-50-
methyl-acetophenone (HMAP) and triphenylphosphine
(PPh3), (MAP)Cu(PPh3)2.
INTRODUCTION
The transition metal complexes of 20-hydroxy-50-methyl
acetophenone (HMAP) are of interest because of their antimi-
crobial activities (Jha and Verma, 2003). There are a few
reports about the bivalence metal complexes (Jha and Verma,
2003; Naikwade et al., 2000; Reddy, 1971). However, the
low valence metal complexes have attracted more attention
because of their potential applications. For example, the
complexes of Cu(I) are especially interesting not only for
their variability of structure, but also for their application in
synthetic organic chemistry as important reagents and in the
biological system as an essential element of a number of
enzymes (Ramaprabhu, 1999). Moreover, the Cu(I)-organic
EXPERIMENTAL
Physical Measurements
HMAP(Aldrich) was used as received. Solvents were
purified by standard methods. Elemental analysis for C and
H were performed on an Italy Carlo Erba MOD-1106 microa-
nalysis instrument. Small amount of complex was dissolved in
concentrated HNO3 and then dilute solution to ppm level. The
metal ion concentration was determined by Hitachi 180-80
atomic absorption spectrometer. IR spectra were recorded on
a Nicolet 550 FTIR spectrometer using KBr discs. Raman
spectra were measured on a micro-Raman system of LabRam
Received 30 May 2004; accepted 22 February 2005.
This work was partially supported by the key organic synthesis
laboratory of Jiangsu Province of Suzhou University.
Address correspondence to Renao Gu, Yaxian Yuan, Departmentof
Chemistry, Suzhou University, Suzhou 215006, China. E-mail:
yuanyaxian@suda.edu.cn
1
I with a low power He-Ne laser source. H NMR spectrum
was obtained on an Inova 400 MHz instrument in CDCl3.
385

386
Y. YUAN ET AL.
Electrochemical Synthesis of (MAP)Cu(PPh3)2
ligands in a ‘one pot’ procedurce. The stoichiometry of the
electrochemical reaction is indicated by the electrochemical
efficiency (Ef, mol/F), defined as moles of metal dissolved
from the anode per Faraday of charge (Gu et al., 1997). In
the present study, the value of Ef of about 1.02 mol/F
implied that the monovalent metal (I) complex was formed at
first. The electrochemical system can be represented as
Pt(2)jCH3CN, PPh3, HMAP, electrolytejCu(þ), and the
electrode reaction can be described as follows:
The electrosynthesis of metal complex was carried out in
acetonitrile using Pt as the cathode and Cu as a sacrificial
anode according to the technique described by Tuck and in
our previous work (Tuck, 1979; Annan et al., 1991; Tuck,
1993; Gu et al., 1997). The ligand of HMAP (0.1553 g) and
PPh3 (0.2620 g) were dissolved in CH3CN (40 ml) containing
a small amount of Et4NClO4 as the supporting electrolyte.
The electrodes were treated with HNO3, and washed and
dried before use. All operations were carried out under the pro-
tection of the dry Ar with rigorous exclusion of air using the
standard vacuum line techniques. The voltage was regulated
to obtain an initial current of 20 mA. After a half-hour electro-
lysis, the Cu piece was rinsed, dried and weighed for determin-
ing the electrochemical efficiency (Ef). The final solution
became yellow-green and it was kept in the fridge (2108C)
overnight to form crystal. Yellow crystal was separated out
from the bulk solution and rinsed by the cooling CH3CN and
ether twice, respectively, then dried in vacuum for further
measurements.
cathode: HMAP þ e ! MAP2 þ 1/2H2
anode: Cu 2 e ! Cuþ
overall reaction: Cu þHMAP þ 2PPh3
! (MAP)Cu(PPh3)2 þ 1/2H2
The TG/DTG curves showed that the complex was stable
up to 250.618C followed by a decomposition stage. Finally,
the residual weight of 11.68% suggests that the residue may
be CuO [(calc./found)%: 10.79/11.68].
The Role of PPh3 in the Coordination
It is well known that PPh3 serves as a common ligand in the
metal coordination chemistry. Based on the theory of hard and
soft acids and bases, low valence metal ion (Cuþ) is a class of
soft Lewis acids, while the PPh3 is a typical soft base (Parson,
1963). Therefore, the stabilization of the low valence metal ion
was increased by the interaction of PPh3 with the metal ion,
which resulted in an increase of the softness by increasing
the electron density on the metal ions. Generally, more than
one PPh3 molecule coordinated with one Cuþ ion to form
Cu(PPh3)þn cation. Therefore, this cation is a soft acceptor
ion that can accommodate a wide range of coordination
anions to form air-stable complexes (Hart et al., 1994). In the
direct electrochemical synthesis, the electrochemical efficiency
Ef value of 1.02 mol/F indicated that the metal Cu was
oxidized to Cuþ ion. As a consequence, the central metal ion
was Cuþ, which was stabilized by the soft neutral ligand
of PPh3 quickly. Following that, the anion of MAP2, which
was generated at the cathode, participated to form a stable
complex.
Data Collection and Refinement of the Crystal Structure
A yellow prism crystal of C45H39CuO2P2 having approxi-
mate dimensions of 0.80 ꢀ 0.20 ꢀ 0.10 mm was mounted on
a glass fiber. All measurements were made on a Rigaku
Mercury CCD area detector with graphite monochromated
˚
Mo-Ka radiation (l ¼ 0.71070 A). The data were collected
at a temperature of 280 + 18C to a maximum 2u value of
55.08. A total of 720 oscillation images were collected. A
sweep of data was done using v scans from 280.0 to 100.08
in 0.58 step, at x ¼ 45.08 and f ¼ 0.08. Lorentz-polarization
and adsorption corrections were applied. Of the 41420 reflec-
tions that were collected, 8428 were unique (Rint ¼ 0.087);
equivalent reflections were merged. Data were collected and
processed using CrystalClear (Rigaku). The structure was
solved by direct methods (SIR97) and expanded using Fourier
techniques (Altomare et al., 1999). The non-hydrogen atoms
were refined anisotropically. Hydrogen atoms were refined
using the riding model. The final cycle of full-matrix least-
squares refinement on F2 was based on 4893 observed
reflections and 491 variable parameters and converged
(largest parameter shift was 0.00 times its esd) with unweighted
and weighted agreement factors of R1 ¼ 0.057, wR2 ¼ 0.120.
All calculations were performed using the CrystalStructure
crystallographic software package. The detail crystallographic
data was listed in the next section.
As a comparison, potential was applied at the Cu electrode
in the non-aqueous solutions that contained HMAP, but
without the PPh3. In this experiment, a black solid was
produced. However, red-brown gas was observed on the
surface when nitric acid was dropped on the black solid. This
observation indicated the existence of Cu metal in the final
products. In this case, the Ef value was also about 1.00 mol/F.
Therefore, it can be inferred that the low valence Cuþ ion
was produced in the first electrochemical step and coordinated
with the MAP2 anion. The results also showed that the
complex Cu(MAP) underwent disproportionation quickly to
RESULTS AND DISCUSSION
Electrochemical Synthesis of Complex
The electrochemical oxidation of Cu provided a convenient form Cu metal and a stable Cu(II) complex (MAP)2Cu(II).
route to its coordination compound in high yield with the title Therefore, the complex of HMAP with low valence of Cuþ

ELECTROCHEMICAL SYNTHESIS AND CRYSTAL STRUCTURE OF COPPER(I)
387
ion was not obtained in the present synthesis. Possible mechan-
ism may involve:
TABLE 2
Selected IR and Raman bands of HMAP and (MAP)Cu(PPh3)2
HMAP
IR
(MAP)Cu(pph3)2
Raman
IR
Raman
634(s)
178(w)
227(w)
339(w)
437(m)
471(s)
513(s)
522(s)
694(s)
188(w)
253(m)
420(w)
605(w)
782(s)
828(s)
748(s)
488(w)
527(w)
548(w)
1H NMR Spectra
963(s)
1096(s)
1204(s)
1434(s)
1492(s)
1602(s)
1620(s)
3053(m)
1
The H NMR data of ligand HMAP and the complex are
1025(m)
1143(m)
1217(s)
1247(s)
1295(s)
1323(s)
1369(s)
1491(s)
1619(s)
1644(s)
2924(s)
2955(s)
3017(s)
501(w)
636(w)
704(s)
615(m)
693(m)
892(w)
listed in Table 1 together with their assignment. The absence
of the peak due to –OH in the spectrum of the complex indi-
cated that the ligand of HMAP was deprotonated during the
electrolysis and coordinated with the Cuþ through the O atom.
781(m)
897(m)
964(m)
1246(s)
1326(s)
1351(w)
1378(m)
1596(w)
1634(s)
2926(s)
2980(w)
3018(m)
3055(s)
958(w)
1000(s)
1028(m)
1098(m)
1157(m)
1185(w)
1350(m)
1374(m)
1412(m)
1572(sh)
1584(s)
1604(sh)
2928(w)
3005(w)
3055(s)
3140(w)
3168(w)
IR and Raman Spectra
The bands of the IR and Raman spectra of ligands HMAP
and PPh3 are listed in Table 2 together with the complex.
The structure of the free ligand and the Raman spectra of
free ligand and complex are presented in Figures 1 and 2,
respectively. It can be seen that the carbonyl group (C55O)
was bonded directly to one of the ring atoms, and the OH
group was attached to the carbon atom at a- position with
respect to the C55O group.
It is well known that adjacent C55O and OH groups can
interact each other and form a six-member quasi-ring through
the intra-molecular hydrogen bond with increased molecular
stability (Norma et al., 1990; Rostkowska et al., 2002). For
such structure, the vibrational modes of C55O and –OH have
a large shift in frequencies in the vibrational spectra. In the
high frequency region ranging from 2900 cm21 to
3600 cm21, in which a broad band at 3017 cm21 in IR
spectrum and a weak band at 3018 cm21 in Raman spectrum
were observed, respectively, together with several C–H
stretching vibrational modes both in the IR and Raman
spectra. The former bands can be assigned to the stretching
vibrational mode of –OH. Their frequency red-shifted with
ꢁs ¼ strong, m ¼ medium, w ¼ weak, sh ¼ shoulder.
several tens of wavenumbers as compared with normal –OH
group bonded to the ring. In the C55O vibrational band
region (normally higher than 1700 cm21), relevant bands
were not detected in their IR and Raman spectra. However,
the bands of 1644 cm21 in IR spectrum and 1634 cm21 in the
Raman spectrum have been observed, which can be assigned
to the stretching vibration mode of C55O of each ligands,
respectively (Annan et al., 1991; Mabrouk et al., 1993). The
large red-shift in frequencies of the –OH and C55O vibrational
TABLE 1
1H NMR spectral data (d. ppm) of HMAP and its complex
Compound
HMAP
(MAP)Cu(pph3)2
OH
12.09 (s)
6.87–7.51
(m)
Aromatic
protons
CH3–CO–
ph–CH3
7.26–7.36 (m)
2.62 (s)
2.32 (s)
2.63 (s)
2.32 (s)
ꢁs ¼ singlet, m ¼ multiplet.
FIG. 1. The molecular structure of HMAP.

388
Y. YUAN ET AL.
features are noted. Firstly, the –OH stretching vibrational
mode of the free ligand was not observed both in IR and
Raman spectra of the complex due to the deprotonation of
the –OH group and formation of an anion of MAP2 at the
cathode during the electrochemical synthesis (Annan et al.,
1991; Gu et al., 1997; Tuck, 1993). Therefore, it is reasonable
to assume that the ligand coordinated with the metal atom
through its deprotonated ion, i.e., the oxygen atom of the OH
group was bonded directly with the central metal ion.
Secondly, the stretching mode of C55O of the free HMAP at
1644 cm21 (IR) and 1634 cm21 (Raman) were downshifted
to 1602 cm21 and 1604 cm21, respectively. The large shift
indicated the interaction of the C55O with the central metal
ions. For such coordination modes, a six-member ring of –
O–C–C–C–O– and Cu formed between metal ion and
ligand of HMAP is proposed. As a consequence, the electron
was delocalized over the six-member ring, which caused a
remarkable red shift of the C55O stretching vibrational mode
in its complex. However, the slight shift in frequency of
PPh3 in the complex showed that it played the role of a
neutral ligand and stabilized the complex. In the present
study, new broad bands at 420 cm21 and 527 cm21 were
observed in the Raman spectrum of the complex, which
FIG. 2. Normal Raman spectra of HMAP, PPh3 and (MAP)Cu(PPh3)2.
modes was mainly due to the formation of intra-molecular
hydrogen bond and the conjugate effect of C55O with the
double bond (C55C) of the benzene ring (Norma et al., 1990;
Rostkowska et al., 2002; Vrielynck et al., 1994). In the IR
spectra of the complex, the main characteristic vibrational
modes of PPh3 located at 3053 cm21, 748 cm21, 694 cm21
,
522 cm21, 513 cm21 were observed unambiguously (Jiang
et al., 1993). By comparing the IR and Raman spectra of the
complex to that of free ligands, some important spectral
TABLE 4
˚
Selected bond lengths (A) and angles (8)
Cu(1)–P(1)
Cu(1)–O(1)
2.2360(10)
1.994(3)
1.830(4)
1.812(5)
1.821(4)
1.329(6)
TABLE 3
Summary of crystal data of (MAP)Cu(PPh3)2
P(1)–C(10)
P(1)–C(22)
Empirical formula
Formula weight
Crystal system
Space group
C45H39CuO2P2
737.30
P(2)–C(34)
O(1)–C(1)
Monoclinic
P21/n
P(1)–Cu(1)–P(2)
P(2)–Cu(1)–O(1)
P(2)–Cu(1)–O(2)
Cu(1)–P(1)–C(10)
Cu(1)–P(1)–C(22)
Cu(1)–P(2)–C(34)
Cu(1)–O(1)–C(1)
Cu(1)–P(2)
130.32(5)
˚
119.22(11)
99.81(10)
123.3(2)
108.5(2)
112.9(2)
126.0(3)
2.2160(10)
2.089(3)
1.832(4)
1.818(5)
1.831(4)
1.231(6)
105.57(11)
101.68(10)
88.7(1)
a(A)
9.048(2)
26.405(6)
15.606(4)
95.990(5)
3708.2(15)
4
˚
b(A)
˚
c(A)
b(8)
V (A )
3
˚
Z value
Crystal dimensions (mm)
cal(g/cm3)
0.80 ꢀ 0.20 ꢀ 0.10
1.321
55.0
Cu(1)–O(2)
P(1)–C(16)
D
2umax(8)
F(000)
P(2)–C(28)
P(2)–C(40)
O(2)–C(8)
1536.00
41420
8428
Total reflections measured
Unique
P(1)–Cu(1)–O(1)
P(1)–Cu(1)–O(2)
O(1)–Cu(1)–O(2)
Cu(1)–P(1)–C(16)
Cu(1)–P(2)–C(28)
Cu(1)–P(2)–C(40)
Cu(1)–O(2)–C(8)
Residuals: R1 (I . 2.00s (I))
Residuals: wR2 (I . 2.00s(I))
Max shift/error in final cycle
0.057
0.120
0.00
3
˚
112.3(2)
111.6(2)
119.4(2)
127.9(3)
Maximum peak in final diff. map (e/A ) 0.60
3
Minimum peak in final diff. map (e/A ) 20.48
˚
R1 ¼ SjjFoj 2 jFcjj/S jFoj and wR2 ¼ [S (w (Fo2 – Fc2)2)/S
w(Fo2)2]1/2

ELECTROCHEMICAL SYNTHESIS AND CRYSTAL STRUCTURE OF COPPER(I)
389
also consistent with the reported values for a [Cu(PPh3)2]þ
complex. The angle of P(1)–Cu–P(2) is dependent on the
donor capacity of the anion either directly or via the changes
in the steric profile of the anion. The stronger the donor
capacity, the smaller this angle.
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coordinated with Cu via its oxygen atom to form a chelate
structure, and the two neutral PPh3 ligands participated in
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Crystal Structural of (MAP)Cu(PPh3)2
Norma, B. C.; Lawrence, H. D.; Stephen, E. W. Introduction to
Infrared and Raman Spectroscopy (3rd ed.); Academic Press:
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The crystallographic data are summarized in Table 3.
The selected bond lengths and angles are listed in Table 4.
The molecular structure of the complex is illustrated in
Figure 3.
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The (MAP)Cu(PPh3) complex crystallized as a mono-
nuclear structure, in which the central Cu(I) ion was coordi-
nated with two O atoms of the HMAP, i.e., one is from the
C55O, the other through the deprotonated OH group.
The chelate structure comprised of the six membered ring of
–C(6)–C(1)–O(1)–Cu(1)–O(2)–C(8)–C(6)–. The bond of
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˚
˚
Cu(1)–O(1) ¼ 1.994(3)A, Cu(1)–O(2) ¼ 2.089(3)A are very
close to the previously reported values for the complexes
with similar structure, while the angle of O(1)–Cu(1)–O(2)
of 88.7(1)8 in the six member ring was larger than the pre-
viously reported values of that with four membered rings
formed between two O atoms of the ligand and the central
metal ion (Yang et al., 2001). The Cu-P bond lengths are
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