Copper phosphates and phosphinates with pyridine/pyrazole alcohol co-ligands: Synthesis and structure

Article abstract of DOI:10.1016/j.ica.2011.03.063

Copper phosphates, [Cu(dtbp)₂(pzet)₂]·H ₂O (1) and [Cu(dtbp)₂(pyme)₂] (2), as well as copper phosphinate, [Cu(dppi)₂(pyet)₂] (3) have been synthesized by the reaction of copper acetate with di-tert-butyl phosphate (dtbp) or diphenyl phosphinate (dppi) in the presence of pyridine base having hydroxyl group, namely, 3,5-dimethylpyrazole-2-ethanol (pzet) or 2-(hydroxymethyl)pyridine (pyme) or 2-(2-hydroxyethyl)pyridine (pyet). Single crystal X-ray diffraction studies reveal that copper ion in all the three complexes is bonded to two phosphoryl ions (P(O)O^-) and two pyridine co-ligands. The crystal structure of 1 reveals that the hydroxyl group of the CH₂CH₂OH moiety of pzet ligand exhibits a positional disorder between the non-bonding position and the bonding position with respect to the central copper ion along the Jahn-Teller axis. Hence, the structure of 1 can be considered to exhibit both 'square-planar' and 'octahedral' coordination geometries simultaneously for the copper ion in the same complex. A similar situation for the -OH groups has not been observed in the complexes 2 and 3 and hence the coordination geometry around the copper ion is axially elongated octahedron.

Full text of DOI:10.1016/j.ica.2011.03.063

Inorganica Chimica Acta 372 (2011) 347–352
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Inorganica Chimica Acta
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Copper phosphates and phosphinates with pyridine/pyrazole alcohol
co-ligands: Synthesis and structure
Ramasamy Pothiraja a, Malaichamy Sathiyendiran a, Alexander Steiner b, Ramaswamy Murugavel a,
⇑
a Department of Chemistry, Indian Institute of Technology – Bombay, Powai, Mumbai 400 076, India
b Department of Chemistry, University of Liverpool, Crown Street, Liverpool L69 7ZD, UK
a r t i c l e i n f o
a b s t r a c t
Article history:
Available online 3 April 2011
Copper phosphates, [Cu(dtbp)2(pzet)2]ꢀH2O (1) and [Cu(dtbp)2(pyme)2] (2), as well as copper phosphi-
nate, [Cu(dppi)2(pyet)2] (3) have been synthesized by the reaction of copper acetate with di-tert-butyl
phosphate (dtbp) or diphenyl phosphinate (dppi) in the presence of pyridine base having hydroxyl group,
namely, 3,5-dimethylpyrazole-2-ethanol (pzet) or 2-(hydroxymethyl)pyridine (pyme) or 2-(2-hydroxy-
ethyl)pyridine (pyet). Single crystal X-ray diffraction studies reveal that copper ion in all the three com-
plexes is bonded to two phosphoryl ions (P(O)Oꢁ) and two pyridine co-ligands. The crystal structure of 1
reveals that the hydroxyl group of the CH2CH2OH moiety of pzet ligand exhibits a positional disorder
between the non-bonding position and the bonding position with respect to the central copper ion along
the Jahn–Teller axis. Hence, the structure of 1 can be considered to exhibit both ‘square-planar’ and ‘octa-
hedral’ coordination geometries simultaneously for the copper ion in the same complex. A similar situ-
ation for the –OH groups has not been observed in the complexes 2 and 3 and hence the coordination
geometry around the copper ion is axially elongated octahedron.
Dedicated to S.S. Krishnamurthy.
Keywords:
Copper phosphate
Copper phosphinate
Synthesis
Spectroscopy
X-ray structures
Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction
reactions, etc. [8–13,15–22]. As a part of our ongoing research, we
have used di-tert-butyl phosphate and diphenyl phosphinic acid as
Complexation to copper enhances the biological activity of a
wide variety of organic ligands [1–3]. For example, the copper
complex of 3,5-dimethylpyrazole has higher antimicrobial activity
(testing on Staphylococcus aureus, Escherichia coli and Candida
albicans) than 3,5-dimethylpyrazole [4,5] and the copper complex
of 2-acetyloxybenzoic acid (copper(II) aspirinate) is a more effec-
tive anti-inflammatory agent than aspirine itself [3,6,7]. Hence it
is very important to study the interaction of copper(II) ion with
various ligands, which are actively involved in the biological
metabolism. Since phosphorus based ligands are playing an impor-
tant role in many biological reactions, the study of interaction of
phosphorus containing ligands with copper(II) is very important.
Copper phosphate or phophonate or phosphinates based research
is mostly focused on clusters or complexes having more than one
copper ions in the view of interesting magnetic interactions and
structural nature [8–15]. The structure and interaction of phospho-
rus based ligands with copper ions in monomeric complexes are
relatively less investigated [16].
phosphorus containing ligands along with the co-ligands, 3,5-
dimethylpyrazole-2-ethanol (pzet), 2-(hydroxymethyl)pyridine
(pyme) and 2-(2-hydroxyethyl)pyridine (pyet), and synthesized
copper phosphate and phosphinate complexes, [Cu(dtbp)2(p-
zet)2]ꢀH2O (1), [Cu(dtbp)2(pyme)2] (2) and [Cu(dppi)2(pyet)2] (3).
We have investigated herein the interaction of these phosphorus
containing ligands with copper ion, in the presence of hydroxyl
group containing Lewis bases with the aid of single crystal X-ray dif-
fraction studies. One of the interesting aspects of these co-ligands is
that these Lewis bases possess additional hydroxyl group for coordi-
nation to metal either through OH or O- depending on the pH of the
solution [23,24].
2. Experimental
2.1. General
All the starting materials and the products were found to be sta-
ble toward moisture and air, and hence no specific precaution was
taken to rigorously exclude air during the preparation or handling
of the compounds. Elemental analyses were performed on a Carlo
Erba (Italy) Model 1106 Elemental Analyzer. Infrared spectra were
recorded on a Perkin-Elmer Spectrum One spectrometer as KBr di-
luted discs. UV–Vis spectra were obtained on a Shimadzu UV-260
We have been studying various metal phosphates, phosphinates
and phosphonates for purposes such as realizing framework models,
precursors for ceramic materials, and models for hydrolase
⇑
Corresponding author. Tel.: +91 22 25767163; fax: +91 22 25767152.
E-mail address: rmv@chem.iitb.ac.in (R. Murugavel).
0020-1693/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2011.03.063

348
R. Pothiraja et al. / Inorganica Chimica Acta 372 (2011) 347–352
spectrophotometer. Magnetic susceptibility was measured on a
PAR vibrating sample magnetometer. Commercial grade solvents
were purified by employing conventional procedures and were dis-
tilled prior to their use [25]. Commercially available starting mate-
rials such as Cu(OAc)2ꢀH2O (Fluka), pyridine (S.d.Fine-Chem.), PCl3
(S.d.Fine-Chem.), ButOH (S.d.Fine-Chem.), conc. HCl (Merck), AlCl3
(S.d.Fine-Chem.), diphenylphosphinic acid (Lancaster), 2-(hydroxy-
methyl) pyridine (Aldrich) and 2-(2-hydroxyethyl) pyridine
(Aldrich) were used as received. 3,5-Dimethylpyrazole-2-ethanol
was prepared by condensation of acetylacetone with H2NNHCH2-
CH2OH. Di-tert-butylphosphate (dtbp-H) has been synthesized
and purified as described previously [26].
1180 (s), 1056 (vs), 978 (s), 825 (m), 716 (m). UV–Vis (CH3OH,
nm): 288, 742. TGA: temperature range, °C (% weight loss): 103–
185 (47.6); 186–325 (15.5); 326–792 (5.3). DSC (°C): 144 (endo);
160 (endo); 211 (endo).
lfound = 1.73 BM, g = 2.09 (300 K), 2.08
(77 K), 2.03 (77 K, CH2Cl2).
2.4. Synthesis of 3
To a solution of Cu(OAc)2ꢀH2O (100 mg, 0.5 mmol in 20 mL
methanol), dppi-H (218 mg, 1 mmol) and 2-(2-hydroxyethyl) pyr-
idine (0.25 mL, ꢂ2 mmol) were added. The resulting solution was
stirred for 2 min and the solvent was removed under vacuum.
The resulting waxy solid was dissolved in Et2O/MeOH (1:1) and
kept at room temperature for crystallization. X-ray diffraction
quality crystals were obtained after 2 weeks. Yield: 0.6 g (81%).
Mp 115 °C. Anal. Calc. for C38H38CuN2O6P2 (Mr = 744.2): C, 61.33;
H, 5.15; N, 3.76. Found: C, 59.2; H, 5.5; N, 4.3%. IR (KBr, cmꢁ1):
3120 (br), 3070 (m), 2946 (w), 2913 (m), 2874 (m), 2827 (w),
1562 (m), 1491 (s), 1449 (m), 1235 (w), 1208 (vs), 1092 (vs),
1065 (s), 1028 (m), 918 (vs), 770 (s), 691 (m). UV–Vis (CH3OH,
nm): 288. TGA: temperature range, °C (% weight loss): 76–175
(33.1); 176–716 (41.5). DSC (°C): 109 (endo); 64 (endo).
2.2. Synthesis of 1
A methanol solution of dtbp-H (210 mg, 1 mmol in 25 mL) was
added to a solution of copper acetate (100 mg, 0.5 mmol in 5 mL
methanol). To the resulting solution, solid pzet (140 mg, 1 mmol)
was added under constant stirring. The stirring was continued for
15 min, solvent was removed under vacuum, and resulting blue so-
lid was dissolved in THF. Blue crystals of 1 were obtained from this
solution after 72 h. The crystals were re-dissolved in diethyl ether
and kept at ꢁ15 °C to obtain dark blue single crystals of X-ray dif-
fraction quality. Yield: 279 mg (72%). Anal. Calc. for C30H62Cu-
N4O11P2 (Mr = 780.3): C, 46.2; H, 8.0; N, 7.2. Found: C, 45.9; H,
7.9; N, 6.9%. IR (KBr, cmꢁ1): 3435 (br), 2975 (vs), 2928 (s), 1636
(w), 1557 (s), 1479 (s), 1444 (s), 1394 (s), 1368 (s), 1318 (s),
1249 (s), 1180 (s), 1063 (vs), 985 (vs), 913 (s), 875 (s), 829 (s),
818 (s), 780 (s), 706 (s), 604 (s), 516 (s). UV–Vis (CH3OH, nm):
304, 770.
lfound = 1.81 BM, g = 2.16 (300 and 77 K).
2.5. Structure determination
A suitable crystal of each compound, crystallized as described
vide supra, was used for the diffraction studies. The diffraction
intensity data have been obtained on a Nonius CAD4 diffractome-
ter for 1, on a STOE AED2 diffractometer for 2, and on a Bruker
Smart Apex diffractometer for 3. The structure solution was
achieved by direct methods as implemented in SHELXS-97 and the
final refinement of the structures was carried using full least-
squares methods on F2 using SHELXL-97 [27]. The positions of hydro-
gen atoms attached to oxygen or nitrogen atoms were identified
from the successive difference Fourier maps and were included
in further calculations and refinement. The C–H hydrogen atoms
were placed on calculated positions and refined using a riding
model. The structure of 3 contains a void in the lattice, which
has been filled with both a water dimer (50%) and a disordered
MeOH (50%). Other details pertaining to data collection, structure
solution and refinement are given in Table 1.
2.3. Synthesis of 2
Cu(OAc)2ꢀH2O (199 mg, 1 mmol) was dissolved in methanol
(40 mL) and dtbp-H (420 mg, 2 mmol) was added. To this,
2-(hydroxymethyl) pyridine (0.2 mL, ꢂ2 mmol, in acetonitrile
30 mL) was added. The resulting solution was filtered and the fil-
trate was kept at room temperature for crystallization. X-ray dif-
fraction quality single crystals were obtained after 1 week. Yield:
0.51 g (73%). Mp 139 °C. Anal. Calc. for
C28H50CuN2O10P2
(Mr = 700.2): C, 48.0; H, 7.2; N, 4.0. Found: C, 47.8; H, 7.4; N,
4.2%. IR (KBr, cmꢁ1): 2979 (m), 2934 (m), 1365 (m), 1250 (s),
Table 1
Crystal data for compounds 1–3.
1
2
3
Identification code
Empirical formula
Formula weight
T (K)
rm61
30H62Cu1N4O11P2
780.32
mur57
C28H50Cu1N2O10P2
700.2
rp275
C39H46CuN2O9P2
812.26
C
293(2)
133(2)
100(2)
k (Å)
0.71073
monoclinic
P21/n
12.389(3)
10.263(2)
16.314(2)
90.87(2)
2074.1(7)
2
1.249
0.658
834
0.6 ꢃ 0.5 ꢃ 0.5
2.05–24.98
3641/20/246
1.118
0.71073
monoclinic
P21/n
13.126(3)
9.390(2)
14.880(3)
112.15(3)
1698.6(6)
2
1.369
0.791
742
0.3 ꢃ 0.2 ꢃ 0.2
2.62–24.89
2938/0/296
1.027
0.71073
monoclinic
C2/c
13.8457(7)
15.5038(8)
17.8197(9)
108.693(1)
3623.4(3)
4
1.489
0.752
1700
0.3 ꢃ 0.1 ꢃ 0.1
4.19–25.02
3199/5/243
1.078
Crystal system
Space group
a (Å)
b (Å)
c (Å)
b (°)
V (Å3)
Z
Dc (Mg/m3)
Absolute coefficient (mmꢁ1
F(0 0 0)
)
Crystal size (mm3)
h range (°)
Data/restraints/parameters
Goodness-of-fit on F2
R1 [I > 2
R2 [I > 2
r
r
(I)]
(I)]
0.0483
0.1345
0.0311
0.0757
0.0404
0.1152

R. Pothiraja et al. / Inorganica Chimica Acta 372 (2011) 347–352
349
The high value of magnetic moment for 3 compared to 2 may indi-
cate a partial contribution of the orbital angular momentum via
spin–orbit coupling [5].
3. Results and discussion
3.1. Synthesis and spectral characterization
Although the composition of compounds 1–3 could be estab-
lished from their analytical and spectroscopic data, the solid-state
structure of all three compounds have been established by single
crystal X-ray diffraction studies in order to obtain information on
the molecular structure, including the role played by the –OH
groups of the pyridinic co-ligands side arms. The details of the
molecular structures are described below.
The reaction between one equivalent of copper acetate and two
equivalents of dtbp-H in the presence of two equivalents of pzet in
methanol yields crystalline [Cu(dtbp)2(pzet)2]ꢀH2O (1) in good
yield (Scheme 1). Similarly, the compounds, [Cu(dtbp)2(pyme)2]
(2) and [Cu(dppi)2(pyet)2] (3) have been synthesized by the reac-
tion of copper acetate with their corresponding phosphoryl ligand
and co-ligand (1). Compounds 1–3 have been characterized by
using elemental analysis, IR and UV–Vis spectroscopic studies,
magnetic moment measurements and single crystal X-ray diffrac-
tion studies. The IR spectra of compounds 1–3 show broad peaks
around 3400 cmꢁ1 indicating the presence of free OH group from
the alcohol side-arms of the co-ligands. The absence of any infrared
absorption in the region of 2700–2500 cmꢁ1 indicates the com-
plete neutralization of dtbp-H/dppi-H in 1–3. Strong bands ob-
served at 1180, 1063 and 985 cmꢁ1 for 1, 1180, 1056 and
978 cmꢁ1 for 2 and 1208, 1092 and 918 cmꢁ1 for 3, are due the
POO and M–O–P vibrations [21].
3.2. Molecular structure of 1
Dark blue single crystals suitable for the diffraction studies
were obtained by dissolving the compound [Cu(dtbp)2(pzet)2]ꢀH2O
(1) in diethyl ether and storing the solution at ꢁ5 °C for 24 h. The
compound crystallizes in the monoclinic P21/n space group with
half the molecule shown in Fig. 1 in the asymmetric part of the unit
cell. The molecular structure reveals a monomeric structure for
this compound. Two dtbp ligands coordinate to the central copper
ion in a monodentate fashion through the de-protonated phos-
phate oxygen atom as in the case of the five coordinated copper–
dtbp–phen complex [Cu(phen)(dtbp)2(OH2)] [16]. However, the
dtbp ligands adopt a trans configuration around the metal in the
present case. The two pzet ligands are also coordinated to the cen-
tral metal ion through the pyrazolyl nitrogen atom in a trans con-
figuration, thus providing a square planar geometry around the
metal. Quite interestingly, the hydroxyl group of the CH2CH2OH
moiety in pzet ligands shows a positional disorder (which can best
The UV–Vis spectra of these complexes exhibit bands around
750 and 300 nm. These absorptions can be assigned to the d–d
(2Eg ? 2T2g) transitions and combination of LMCT and p⁄ transi-
p–
tions, respectively. The ESR spectra of compounds 2 and 3 recorded
as polycrystalline powder at 77 K as well as at room temperature
show broad peaks with less resolved hyperfine structure [5]. The
room temperature magnetic moments (
leff = 1.73 BM for 2 and
2.16 BM for 3) are consistent with the monomeric Cu2+ complex.
Scheme 1. Synthesis of [Cu(dtbp)2(pzet)2]ꢀH2O (1), [Cu(dtbp)2(pyme)2] (2) and [Cu(dppi)2(pyet)2] (3).

350
R. Pothiraja et al. / Inorganica Chimica Acta 372 (2011) 347–352
Fig. 1. Molecular structure of [Cu(dtbp)2(przet)2]ꢀH2O (1) showing (a) octahedral and (b) square planar geometry around copper atoms. Selected bond lengths (Å) and angles
(°) Cu(1)–O(1) 1.939(2), Cu(1)–N(2) 2.025(2), Cu(1)–O(5) 2.485(6), P(1)–O(2) 1.460(3), P(1)–O(1) 1.504(2), P(1)–O(4) 1.572(2), P(1)–O(3) 1.593(3), O(1)–Cu(1)–O(1a)
180.0(2), N(2)–Cu(1)–N(2a) 180.0(2), O(1)–Cu(1)–N(2) 91.5(1), O(1a)–Cu(1)–N(2) 88.5(1), O(1)–Cu(1)–O(5) 94.0(2), O(1a)–Cu(1)–O(5) 86.0(2), N(2)–Cu(1)–O(5) 83.3(2),
N(2a)–Cu(1)–O(5) 96.8(2), O(2)–P(1)–O(1) 116.5(2), O(2)–P(1)–O(4) 113.3(2), O(1)–P(1)–O(4) 105.6(1), O(2)–P(1)–O(3) 113.5(2), O(1)–P(1)–O(3) 106.9(2), O(4)–P(1)–O(3)
99.4(2), P(1)–O(1)–Cu(1) 133.8(2).
be termed as rotational disorder about C–C axis) with two positions
for the hydroxyl group with equal occupancy (0.5:0.5) as schemat-
ically shown in Fig. 2. Since the CH2CH2OH chain is also conform-
ationally flexible, one of the disordered –OH groups folds in such a
manner that it is coordinated to the metal ion from the axial posi-
tion, thus providing an octahedral geometry. The other half of the
hydroxyl group points away from the metal center and is engaged
in hydrogen bonding with the lattice water molecule present in the
crystal (Fig. 1). For simplicity sake, these two positions are referred
as ‘‘–OH in’’ and ‘‘–OH out’’.
The molecular structure of this complex is drawn twice in Fig. 1
(once with the atomic coordinates of ‘‘–OH in’’ and once with the
atomic coordinates of ‘‘–OH out’’) to clearly show both ‘octahedral’
and ‘square-planar’ coordination geometries for the copper ion in
the same complex.
The Cu–O(P) distance in 1 (1.939(2) Å) is slightly shorter than
that observed for [Cu(phen)(dtbp)2(OH2)] (1.974(3) and
1.975(4) Å) [16], but is longer than that observed in the case of cop-
per polymer, [Cu(dtbp)2]n, 1.89(5) Å [15]. It should be noted that
when dtbp ligand occupies the axial position in an octahedral com-
plex, e.g., [Cu(dtbp)2(imz)4] [28], this distance gets elongated to
2.475(1) Å. The observed Cu–N distance (2.025(2) Å) in 1 is compa-
rable to the Cu–N(imidazole) distance in [Cu(dtbp)2(imz)4]
(2.006(2) and 2.045(2) Å). The Cu–O(axial) bond in the ‘‘–OH in’’
is elongated, as expected, and is in consistent with the Jahn–Teller
elongation in d9 copper systems (2.485(6) Å). The cis and trans L–
Cu–L angles around the metal ion do not vary much from their
ideal values 90 and 180°, respectively. The cis angles in 1 are in
the range 83.3(2)–96.8(2)° while the observed trans angles are ex-
actly 180° due to the center of symmetry lying at the metal center.
The crystal structure of 1 shows interesting H-bonding interac-
tions assisted by four unique hydrogen bonds. For example, the –
CH2CH2OH group (O(5)) of the pzet ligand when present in ‘‘–OH
in’’ form, forms an intramolecular hydrogen bond to the P@O group
(O(2)) of the coordinated dtbp ligand (2.663 Å). When the same
group is present in the ‘‘–OH out form’’, it is engaged in hydrogen
bonding with the lattice water (O(50)ꢀ ꢀ ꢀO(6) 2.827 Å). In addition to
this there are two more hydrogen bonds involving the lattice water
and the phosphoryl oxygen atoms (O(2)ꢀ ꢀ ꢀO(6) 2.894 and 2.778 Å).
Thus the presence of a lattice water molecule (O(6)) in the crystal
of 1 along with the conformationally flexible –CH2CH2OH group is
responsible for the 1D supramolecular assembly formation shown
in Fig. 3.
3.3. Molecular structure of 2
Compound 2 has been crystallized from acetonitrile/methanol
mixture. The centrosymmetric compound 2 crystallizes in the
monoclinic P21/n space group with half of the molecule in the
asymmetric part of the unit cell. The molecular structure of 2
(Fig. 4) can best be described as the ‘‘–OH in’’ form of 1. Thus,
the pyme ligands in 2 form five-membered chelate rings. The ab-
sence of ‘‘–OH out’’ form in 2 could be ascribed to the shorter –
CH2OH side chain on the pyridine. The coordination geometry
around the central copper(II) ion 2 hence is axially elongated octa-
hedron. The metal is surrounded by two phosphate oxygen atoms
(Cu1–O1, 2.010(2) Å), two pyridine nitrogen atoms (Cu1–N1,
1.988(2) Å), and two –CH2OH groups (Cu1–O5, 2.357(2) Å). Each
of the ligand types binds the metal ion in a trans configuration as
shown in Fig. 4. The bond distances are in agreement with the val-
ues described above for compound 1. The cis angles around the me-
tal ion do not deviate appreciably from its ideal value 90°
(78.16(6)–101.84(6)°), while the observed trans angles are 180°
due to molecule’s center of symmetry at the metal center.
Fig. 2. Schematic representation showing the role of disordered –OH group in the
molecular structure of 1.

R. Pothiraja et al. / Inorganica Chimica Acta 372 (2011) 347–352
351
Fig. 3. Formation of 1D supramolecular assembly in 1 aided by intra- and intermolecular H-bonds. Hydrogen atoms and the methyl groups on pyrazole and tert-butyl
substituents are omitted for clarity. O5 and O50 represent the two positions for the ethanol oxygen.
3.4. Molecular structure of 3
The singe crystals of 3 obtained from acetonitrile/methanol
solution, crystallize in monoclinic C2/c space group with half of
the molecule occupying the asymmetric part of the unit cell. The
molecular structure of 3 is very similar to 2 (Fig. 6) although the
co-ligand used in 3 is pyet and not pyme. Similarly, the dtbp ligand
in 2 has been replaced by dppi in 3. Hence the coordination geom-
etry around the central copper(II) ion 3 is Jahn–Teller elongated
octahedron. The metal is surrounded by two phosphate oxygen
atoms (Cu1–O2, 2.025(2) Å), two pyridine nitrogen atoms (Cu1–
N1 2.017(2) Å) and two oxygen atoms from two pyet side chains
(Cu1–O1, 2.385(2) Å). As in the case of 1 and 2 described above,
each of the ligand types in 3 bind the metal ion in a trans configu-
ration; the bond distances mentioned are also in agreement with
the corresponding values found in 1 and 2, and similar complexes.
The cis angles around the metal ion do not deviate appreciably
from its ideal value 90° (87.0(1)–93.0(1)°) while the observed trans
angles are 180° owing to molecule’s center of symmetry lying on
the metal center.
Fig. 4. Molecular structure of [Cu(dtbp)2(pyme)2] (2). Selected bond lengths (Å) and
angles (°): Cu(1)–O(1) 2.010(2), Cu(1)–O(5) 2.357(2), Cu(1)–N(1) 1.988(2),
P(1)–O(1) 1.511(2), P(1)–O(2) 1.495(2), P(1)–O(3) 1.588(2), P(1)–O(4) 1.586(2);
3.5. Comparison of structures of 1–3
O(1)#1–Cu(1)–O(1) 180.0, O(5)#1–Cu(1)–O(5) 180.0, N(1)#1–Cu(1)–N(1) 180.0,
O(1)–Cu(1)–O(5) 93.84(6), O(1)–Cu(1)–O(5)#1 86.16(6), N(1)–Cu(1)–O(1) 91.30(7),
N(1)–Cu(1)–O(1)#1 88.70(7), N(1)–Cu(1)–O(5) 78.16(6), N(1)–Cu(1)–O(5)#1
101.84(6), O(2)–P(1)–O(1) 115.16(9), O(2)–P(1)–O(4) 105.92(8), O(1)–P(1)–O(4)
111.93(8), O(2)–P(1)–O(3) 112.03(9), O(1)–P(1)–O(3) 109.85(8), O(4)–P(1)–O(3)
The molecular structures of 1–3 are essentially similar in the
sense that they all contain Cu(II) ion which is surrounded by either
two dialkyl phosphate or diaryl phosphinate ligands existing in a
monodentate terminal fashion and two chelating pyrazole or pyr-
idine co-ligands bearing a alcohol side chain (either –CH2OH or –
CH2CH2OH). In the case of 1, however, the high flexibility of the
–CH2CH2OH chain leads to both monodentate and chelating forms
of the pyrazole ethanol. This is probably aided by both crystal
packing effects as well as the length of the chain itself. The confor-
mation of the six- or five-membered chelate rings in the structure
of 1–3 is pictorially depicted in Fig. 7, which reveals the highly
non-planar nature of the rings. Because of the presence of –OH
groups, uncoordinated phosphoryl P@O groups, and also lattice
water (in case of 1), a number of intra- and intermolecular hydro-
gen bonds are formed in the crystal lattice.
101.00(8). Symmetry transformations used to generate equivalent atoms: (#1)
2 ꢁ x, ꢁy, 2 ꢁ z.
The intramolecular hydrogen bond (O5–H1ꢀ ꢀ ꢀO2, 2.600(3) Å)
creates a six-membered metallocycle Cu–O–H–O–P–O, which fur-
ther stabilizes the molecular structure. The crystals of 2 do not pos-
sess lattice water molecule as in 1. However, the pyridine C–H
bonds act as the source of intermolecular hydrogen bonding with
the adjacent molecule’s phosphate oxygen atoms to form a 1D
chain like structure as shown in Fig.
3.368(3); C(13)–H(13)ꢀ ꢀ ꢀO(2) 3.301(3) Å).
5
(C(12)–H(12)ꢀ ꢀ ꢀO(4)
Fig. 5. Hydrogen bonding pattern in [Cu(dtbp)2(pyme)2] (2) (hydrogen atoms on the methyl groups are removed for the clarity).

352
R. Pothiraja et al. / Inorganica Chimica Acta 372 (2011) 347–352
phosphorus based acids). The conformationally very flexible etha-
nol side chain of the co-ligand in 1 led to the realization of both
square planar and octahedral coordination environments for cop-
per in this complex. Thus the results presented here complement
our earlier studies on copper dtbp complexes with strictly mono-
dentate [12,28], strictly bidentate chelating [13,16] and bidentate
bridging ligands [21,22], as well as copper monoaryl phosphate
complexes [11].
Acknowledgements
This work was supported by DST, New Delhi. R.P. thanks CSIR,
Delhi for a senior research fellowship. The authors thank the SAIF,
IIT-Bombay for spectral data and National Single Crystal Diffraction
Facility for the X-ray data for 1.
Appendix A. Supplementary material
CCDC 791906–791908 for compounds 1–3 contain the supple-
mentary crystallographic data for this paper. These data can be ob-
tained free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif. Supplementary
data associated with this article can be found, in the online version,
at doi:10.1016/j.ica.2011.03.063.
Fig. 6. Molecular structure of [Cu(dppi)2(pyet)2] (3). Selected bond lengths (Å) and
angles (°): Cu(1)–O(1) 2.385(2), Cu(1)–O(2) 2.025(2), Cu(1)– N(1) 2.017(2), P(1)–
O(3) 1.508(2), P(1)–O(2) 1.517(2); O(3)–P(1)–O(2) 115.9(1), O(1)–Cu(1)–O(1)#1
180.0(1), O(2)–Cu(1)–O(2)#1 180.0(1), N(1)–Cu(1)–N(1)#1 180.0, O(2)–Cu(1)–O(1)
90.9(1), O(2)–Cu(1)–O(1)#1 89.1(1), N(1)–Cu(1)–O(1) 87.0(1), N(1)#1–Cu(1)–O(1)
93.0(1), N(1)–Cu(1)–O(2) 88.7(1), N(1)–Cu(1)–O(2)#1 91.4(1). Symmetry transfor-
mations used to generate equivalent atoms: (#1) 2 ꢁ x, ꢁy, 1 ꢁ z, (#2) 1 ꢁ x, y, 1/
2 ꢁ z.
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We have presented in this contribution the synthesis and struc-
tural characterization of a series of Cu(II) complexes bearing
monoprotic phosphorus acids (dialkyl phosphate and diaryl phos-
phinate). The use of a co-ligand in the form of 2-pyrazole ethanol
and 2-pyridine methanol/ethanol in these reactions serves to block
the oligomerization through chelation, and hence only monomeric
compounds have been isolated (in contrast to isolation of
polymeric and oligomeric complexes isolated earlier with