Structural diversity in heteroleptic dipyrrinato copper(II) complexes

Article abstract of DOI:10.1016/j.ica.2013.09.044

Synthesis and characterization of the heteroleptic dipyrrinato complexes [Cu₂(2-pcdpm)₂(acac)₂] (1), [Cu ₂(2-pcdpm)₂(hfacac)₂] (2), [Cu ₃(4-OHdpm)₂(acac)₄]

Full text of DOI:10.1016/j.ica.2013.09.044

Inorganica Chimica Acta 409 (2014) 518–527
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Structural diversity in heteroleptic dipyrrinato copper(II) complexes
Rakesh Kumar Gupta a, Rampal Pandey a, Amit Kumar a, K.V. Ramanujachary b, Samuel E. Lofland c,
Daya Shankar Pandey a,
⇑
a Department of Chemistry, Faculty of Science, Banaras Hindu University, Varanasi 221 005, India
b Department of Chemistry and Biochemistry, Rowan University, Glassboro, NJ 08028, USA
c Department of Physics and Astronomy, Rowan University, Glassboro, NJ 08028, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 17 June 2013
Received in revised form 17 September 2013
Accepted 24 September 2013
Available online 10 October 2013
Synthesis and characterization of the heteroleptic dipyrrinato complexes [Cu2(2-pcdpm)2(acac)2] (1),
[Cu2(2-pcdpm)2(hfacac)2]
(2),
[Cu3(4-OHdpm)2(acac)4]
(3),
[Cu3(4-OHdpm)2(hfacac)4]
(4),
[Cu2(3-OHdpm)(acac)3] (5), [Cu2(3-OHdpm)(hfacac)3] (6), [Cu(2-OHdpm)(acac)] (7) and [Cu(2-
OHdpm)(hfacac)] (8) (acac = acetylacetone; hfacac = hexafluoroacetylacetone; 2-pcdpm = 4-(2-methoxy-
pyridyl)phenyldipyrromethene; 3-pcdpm = 4-(3-methoxypyridyl)phenyldipyrromethene; 4-pcdpm =
4-(4-methoxypyridyl)phenyldipyrromethene; 4-/3-/2-OHdpm = 4-/3-/(2-hydroxyphenyl)dipyrrometh-
ene) have been described. The complexes 1–8 have been characterized by elemental analyses, ESI-MS,
IR, UV–Vis, EPR and electrochemical (CV and DPV) studies. Electronic absorption spectra of 1–8 clearly
suggested that position of the absorption bands depend on the nuclearity and coordination geometry
about the copper(II) centres. Structures of 1, 2, 3 and 7 have been determined by X-ray single crystal anal-
yses. The spectral and structural studies revealed that copper(II) in 1 and 2 adopted square pyramidal
geometry, whereas in 7 a square planar geometry. On the other hand in trinuclear complex 3 one cop-
per(II) adopted square planar, while other two square pyramidal geometry. In their cyclic voltammo-
grams, 1–6 exhibited an irreversible oxidation and two reduction waves whereas 7 and 8 displayed
one oxidation and reduction wave. Variable temperature magnetic susceptibility measurements sug-
gested paramagnetic nature of the complexes 1–3. The X-ray photoelectron spectroscopic (XPS) studies
indicated that copper(II) centres in complex 3 are present in two different coordination environments.
Room temperature EPR studies on 1–8 (g|| > g\ > 2.003) suggested a typical d9 copper(II) with unpaired
Keywords:
Copper
Controlled nuclearity
Dipyrrin
Crystal structure
EPR
Electrochemical
electron lying in a dx2
orbital.
Ày2
Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction
interest, particularly in the field of magneto-chemistry and
material science [6]. Polynuclear copper complexes are also inter-
esting because these enable us to understand the molecular fea-
tures determining the nature of exchange coupling between
neighbouring metal centres through bridging ligands [7]. In addi-
tion, polynuclear copper complexes have also been attractive due
to their DNA cleavage activity and relevance to the active site of
numerous metalloproteins [8].
Similarly, meso-substituted dipyrrins have attracted attention
of many research groups because of their ease of synthesis, rich
optical properties and tendency to form homo-/heteroleptic com-
plexes with various metal ions [9]. By manipulation of these prop-
erties numerous charge neutral complexes and supramolecular
entities have been synthesized and extensively studied [10]. The
optical and electrochemical properties of dipyrrins and their com-
plexes depend on the meso-substituents and can be modulated by
incorporating electron withdrawing/releasing substituents [11]. In
addition, dipyrrin derivatives appended with pyridyl, carboxylic,
nitrile or imidazolyl groups have been used in construction of
the polymeric networks exhibiting attractive topologies [12].
The supramolecular architectures have attracted substantial
current interest owing to their unusual and exciting properties
[1]. In this context designing and synthesis of the multidentate li-
gands and creation of extended structures based on it have drawn
special attention [2]. Usually, extended systems are obtained by
interaction of the metal ions with organic ligands possessing two
or more donor sites oriented in a particular position [3]. A variety
of supramolecular structures viz. triangles, squares, molecular rect-
angles etc. have been synthesized using organic ligands with di-
verse backbone and functionalities [4]. Further, it has been
shown that the creation of these ensembles can be controlled by
changing geometrical characteristics of the metal centre, the coor-
dination modes, the shape and size of ligands [5]. At the same time,
polynuclear copper complexes have received enormous current
⇑
Corresponding author. Tel.: +91 542 6702480; fax: +91 542 2368174.
E-mail address: dspbhu@bhu.ac.in (D.S. Pandey).
0020-1693/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.ica.2013.09.044

R.K. Gupta et al. / Inorganica Chimica Acta 409 (2014) 518–527
519
Depending on the nature of coordination sites along with the shape
and size of ligands various self-assembly modes have been ob-
served [13]. Although, a large number of supramolecular systems
based on Cu(acac)2, Cu(hfacac)2 and dipyrrin ligands have been re-
ported the development of systems with new geometries and
structures are still demanding [14]. With an objective of develop-
ing systems based on meso-substituted dipyrrins we have chosen
two sets of ligands appended with pyridyl and phenolic groups
and synthesized heteroleptic dipyrrinato copper complexes exhib-
iting structural diversity. Through this contribution we describe
the synthesis, characterization and magnetic properties of some
new copper complexes based on dipyrrins containing pyridyl and
phenolic functionalities and acac/hfacac ancillary ligands.
1579 cmÀ1. The diagnostic bands associated with mC@O vibrations
of the coordinated acac (1593, 1; 1579, 3; 1580, 5; 1583, 7;
Cu(acac)2, 1578 cmÀ1) and hfacac (1649, 2; 1647, 4; 1648, 6;
1646, 8; Cu(hfacac)2, 1646 cmÀ1) were displayed at their usual
positions [9,16]. Vibrations due to both dipyrrin moiety and mC@O
associated with coordinated acac/hfacac in the spectra of respec-
tive complexes strongly suggested formation of the heteroleptic
dipyrrinato complexes [9,16].
The electronic absorption spectra of 1–8 were acquired in
CH3CN and resulting spectra are shown in Fig. 1. All the complexes
exhibited analogous spectral features and displayed strong absorp-
tions in the low energy region at ꢀ400–500 nm (488, 1; 486, 2;
484, 3; 483, 4; 486, 5; 488, 6; 490, 7; 489 nm, 8) due to p–
p⁄ tran-
sitions of the dipyrrinato chromophores. These also exhibited
bands in the high energy region at ꢀ350 and 290 nm (346, 294,
1; 345, 293, 2, 333, 257, 3; 333, 257, 4; 315, 259, 5; 317, 263, 6;
295, 7; 354, 298 nm, 8) due to ligand-centred intraligand
transitions [16]. It is noteworthy to mention that the trinuclear
2. Results and discussion
2.1. Synthesis and characterization
complexes
3 and 4 displayed an additional broad band at
The dipyrromethanes, 4-(2-methoxypyridyl)phenyldipyrrome-
ꢀ333 nm, while 5 and 6 at ꢀ315 nm, which may arise due to
different coordination geometry about the copper(II) centres in
these complexes (Table 1) [17].
thane,
4-(3-methoxypyridyl)phenyldipyrromethane
and
4-(4-methoxypyridyl)phenyldipyrromethane were synthesized by
the reaction of respective aldehydes [4-(2-methoxypyridyl)benzal-
dehyde/4-(3-methoxypyridyl)benzaldehyde/4-(4-methoxypyr-
idyl)benzaldehyde] with an excess of pyrrole in presence of
catalytic amounts of trifluoroacetic acid (TFA) following the litera-
ture procedures [15c]. An anlogous method has been employed for
the synthesis of 4-/3-/2-hydroxyphenyldipyrromethanes [15]. The
metal complexes were synthesized by reaction of the metal precur-
sors Cu(acac)2 and Cu(hfacac)2ÁH2O with in situ generated dipyrro-
methenes obtained by oxidation of the corresponding dipyr
romethanes using an equimolar amount of 2,3-dichloro-5,6-dicy-
ano-1,4-benzoquinone (DDQ) in benzene. A simple synthetic strat-
egy adopted for preparation of the complexes 1–8 is depicted in
Scheme 1.
The precursor complexes reacted with 4-(2-methoxypyr-
idyl)phenyldipyrromethene and afforded cyclic dimeric complexes
1 and 2, whereas 4-(3-methoxypyridyl)phenyldipyrromethene or
4-(4-methoxypyridyl)phenyldipyrromethene reacted with afore-
said metal precursors to form trinuclear complexes 3 and 4
(Scheme 1) via cleavage of the etheratic (C–O) bond [15c]. There-
fore, to investigate the possibility of formation of 3 and 4 directly
from the reactions of 4-hydroxyphenyldipyrromethene and appro-
priate metal precursor, parallel reactions were run under analo-
gous conditions. Expectedly, these afforded trinuclear complexes
3 and 4. It prompted us to examine the reactivity of precursor com-
plexes with 3-/2-hydroxyphenyldipyrromethenes under analogous
conditions (Scheme 1). Interestingly, 3-hydroxyphenyldipyrro-
methene gave binuclear complexes 5 and 6, on the other hand
2-hydroxyphenyldipyrromethene gave mononuclear complexes 7
and 8.
The complexes 1–8 are crystalline, non hygroscopic solids, sta-
ble at room temperature and soluble in common organic solvents
such as methanol, ethanol, benzene, chloroform, dichloromethane,
acetone, dimethylsulfoxide, dimethylformamide and insoluble in
hexane, petroleum ether and diethyl ether. The complexes under
investigation have been characterized by satisfactory elemental
analyses, spectral (ESI-MS, IR, UV–Vis, EPR), electrochemical (CV
and DPV) and pH titration studies. Stoichiometry and composition
of the complexes 1–8 have also been supported by ESI-MS studies.
Resulting data along with assignments are summarized in Table S5
and spectra depicted through Figs. S1–S8. Presence of the various
peaks and overall spectral patterns are consistent with formulation
of the respective complexes.
2.2. Crystal structures
Structuresof1, 2, 3and7havebeendeterminedbysinglecrystalX-
ray diffraction analysis (Fig. 2 and 3). Details about the crystallo-
graphic data and selected geometrical parameters are gathered
through Tables 2–4. The complexes 1 and 2 crystallizes in triclinic
system with Pbar1 space group. The complex 1 consists of two
2-pcdpm ligands and two Cu(acac) units creating a rectangular cen-
trosymmetric dimeric structure. The metal centre in this molecule
(Fig. 2a) adopted a distorted square pyramidal geometry wherein
equatorial positions are occupied by the pyrrolic nitrogen (N1, N2)
from one dipyrromethene and O1, O2 from acac. The Cu–N(dpm)
and Cu–O(acac) bond distances at the base of square pyramid are
[Cu–N1, 1.963(10); Cu–N2, 1.938(11); Cu–O1, 1.987(9); Cu–O2,
1.927(9) Å], which are comparable to other related systems
[12,16,18]. The bond angles around copper centre are [O2–Cu1–N2,
170.9(3); O2–Cu1–N, 88.5(6); N2–Cu1–N1, 90.9(7); O2–Cu1–O1,
89.6(4) and N2–Cu1–O1, 89.5(6)°], and are close to square pyramidal
environment. The copper atom is displaced slightly above the square
plane(N1, N2, O1, O2)towardtheapicalsiteby0.357 Å,whichisoccu-
pied by pyridyl nitrogen from an adjacent molecule to form cyclic di-
mer (Cu–N3, 2.558 Å, Table 3). The Cu–N3(pyridyl) bond distance is
significantly longer relative to Cu–N(dpm) and Cu–O(acac) distances
indicating occurrence of the Jahn–Teller distortion [18]. The Cu–Cu
bond distance is 12.34 Å, which is longer than the closest intermolec-
ular Cu–Cu distance of 9.170 Å. Each acac moiety is twisted relative to
dipyrrin unit by a dihedral angle of 13.0°.
Unlike complex 1, the asymmetric unit of 2 contains two mole-
cules. In the distorted square pyramidal arrangement about the
metal centre, the base of square pyramid is formed by oxygen
(O1, O2) from the hfacac and N donor atoms (N1, N2) from the
dipyrrin ligands. The Cu–N(dpm) and Cu–O(hfacac) bond distances
fall in the range of 1.933–1.955 and 1.993–2.037 Å, respectively.
Bond angles about the metal centre are in the range of 86.5–
169.4°, which is comparable to the bond angles in other square
pyramidal complexes [12,16,18]. The metal centre copper is dis-
placed above the plane represented by (N1, N2, O1, O2) toward
the apical site by 0.506 Å, which is occupied by pyridyl nitrogen
from an adjacent ligand to form cyclic dimer [Cu1–N3, 2.328(3);
Cu2–N6, 2.291(3) Å, Table 3]. The Cu–N(pyridyl) distance is signif-
icantly longer than Cu–N(dpm) and Cu–O(acac) indicating occur-
rence of the Jahn–Teller distortion [18]. The Cu–Cu distance is
ꢀ11.72 Å, which is longer than the closest intermolecular Cu–Cu
IR spectra of 1–8 displayed characteristic bands due to dipyrrin
moiety at ꢀ990, 1022, 1244, 1335, 1394, 1545, 1552 and

520
R.K. Gupta et al. / Inorganica Chimica Acta 409 (2014) 518–527
R
N
O
O
N
O
Cu
R
O
N
N
N
Cu(acac)
2
N
N
Cu(hfacac)2
O
R
R
Cu
O
O
R = CH3 = 1
R = CF3 = 2
N
HN
4-(2-methoxypyridyl)phenyl-
dipyrromethene
R
R
N
O
N
O
O
O
H
N
Cu
N
OH
O
Cu(acac)2
Cu(hfacac)2
or
or
N
R
O
H
O
R
R
N
O
O
Cu
O
C
u
N
HN
R
O
O
N
HN
N
HN
R
4-(3-methoxypyridyl)phenyldipyrromethene
4-(4-methoxypyridyl)phenyldipyrromethene
or 4-hydroxyphenyldipyrromethene
R = CH3, 3;
R = CF3, 4;
R
OH
R
H
O
OH
N
O
Cu(acac)2
Cu(acac)2
R
O
O
R
C
u
N
N
O
Cu(hfacac)2
Cu(hfacac)2
N
C
u
C
u
O
O
O
R
O
N
NH
R
R
R
R = CH3, 5;
R = CF3, 6;
R
3-hydroxyphenyldipyrromethene (5 and 6)
2-hydroxyphenyldipyrromethene (7 and 8)
R = CH3, 7;
R = CF3, 8;
Scheme 1. Synthesis of complexes 1–8.
Table 1
Electronic spectral data.
Complexes
kmax, nm (e )
, MÀ1 cmÀ1
1
2
3
4
5
6
7
8
488 (1.21 Â 105), 346 (3.98 Â 104), 294 (1.54 Â 105)
486 (1.05 Â 105), 345 (3.42 Â 104), 293 (2.12 Â 105)
484 (2.79 Â 105), 333 (8.26 Â 104), 257 (8.26 Â 104)
483 (2.56 Â 105), 333 (6.65 Â 104), 257 (5.50 Â 104)
486 (2.04 Â 105), 315 (6.07 Â 104), 259 (4.17 Â 104)
488 (1.80 Â 105), 317 (4.46 Â 104), 263 (3.51 Â 104)
490 (1.64 Â 105), 295 (1.06 Â 105)
489 (1.64 Â 105), 354 (2.84 Â 104), 298 (6.36 Â 104)
Complex 3 crystallizes in monoclinic system with P21/n space
group. Structural studies revealed that it is a trinuclear complex
containing three copper centres, two 4-OH-dpm, and four acac
moieties (Fig. 3a). The Cu1 in this complex adopted almost square
planar geometry completed by pyrrolic nitrogen (N1, N2) from the
dipyrrin and O1, O2 from chelating ligand acac. The Cu–N and
Cu–O bond distances [Cu1–N1, 1.980(5); Cu1–N2, 1.970(6); Cu1–O1,
1.952(5); Cu1–O2, 1.947(5); Å] and bond angles about the metal
centre [O1–Cu1–O2, 92.6(2); O1–Cu1–N2, 177.7(2); O2–Cu1–N2,
86.7(2); O1–Cu1–N1, 89.3(2); O2–Cu1–N1, 177.2(2); N2–Cu1–N1,
Fig. 1. UV–Vis spectra of complexes 1–8 in CH3CN (c, 10 lM) at room temperature.
distance of ꢀ7.103 Å. The hfacac moiety is twisted relative to
dipyrrin unit by ꢀ37.38°. The fluorine atoms from hfacac exhibited
disorder over two positions of CF3 units.

R.K. Gupta et al. / Inorganica Chimica Acta 409 (2014) 518–527
521
Fig. 2. Molecular structure of metallacycles 1 (a) and 2 (b) with partial atom numbering scheme (hydrogen atoms omitted for clarity).
Fig. 3. Molecular structure of trinuclear 3 (a) and mononuclear 7 (b) with the partial numbering scheme (hydrogen atoms have been omitted for clarity).

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R.K. Gupta et al. / Inorganica Chimica Acta 409 (2014) 518–527
Table 2
Crystal data and structure refinement parameters for 1, 2, 3 and 7.
1
2
3
7
Empirical formula
Crystal system
Space group
a (Å)
b (Å)
c (Å)
C
52H46Cu2N6O6
C52H34Cu2F12N6O6
triclinic
P1
C50H50Cu3N4O10
monoclinic
P21/n
8.801(18)
24.965(5)
22.396(5)
90.00
91.65(3)
90.00
4918.9(17)
4
C20H18CuN2O3
monoclinic
P21/c
7.112(15)
13.516(3)
19.323(5)
90.00
100.67(3)
90.00
1825.3(7)
2
triclinic
P1
ꢀ
ꢀ
9.170(5)
9.537(5)
13.656(5)
79.761(5)
88.390(5)
80.500(5)
1159.1(10)
1
9.431(4)
12.422(5)
21.389(9)
92.411(3)
96.553(4)
98.142(3)
2459.9(18)
2
a
(deg)
b (deg)
c
(deg)
V (Å3)
Z
k (Å)
Color and habit
T (K)
0.71073
red, block
293(2)
0.71073
red, block
293(2)
0.71073
red green, block
293(2)
0.71073
red, needle
293(2)
Reflections collected
Reflections/restraint/parameters
3119
1279/0/ 100
1.401
0.975
1.157
R1 = 0.053
wR2 = 0.143
R1 = 0.059
wR2 = 0.138
17616
8634/0/731
1.612
0.968
0.938
R1 = 0.048
wR2 = 0.120
R1 = 0.069
wR2 = 0.126
11089
10937/0/615
1.428
1.346
1.076
R1 = 0.084
wR2 = 0.238
R1 = 0.130
wR2 = 0.231
4163
4121/0/ 239
1.448
1.218
1.091
R1 = 0.073
wR2 = 0.204
R1 = 0.081
wR2 = 0.208
Dcalc (Mg mÀ3
)
l
(mmÀ1
GOF on F2
Final R indices I > 2
)
r
(I)a
R indices (all data)a
P
P
P
P
a
2
R1
=
||Fo| À Fc||/ |Fo|;R2 = { [w(Fo À Fc2)2]/ [wFo4]}1/2
.
Table 3
Selected bond lengths (Å) and angles (°) for 1 and 2.
Table 4
Selected bond lengths (Å) and angles (°) for 3 and 7.
Bond length (Å)
Bond angle (°)
Bond length (Å)
Bond angle (°)
1
3
Cu–N1
Cu–N2
Cu–O1
Cu–O2
Cu–N3
C12–O3
C16–O3
1.96(10)
1.94(11)
1.99(9)
1.93(9)
2.56
O2–Cu1–N2
O2–Cu1–N1
N2–Cu1–N1
O2–Cu1–O1
N2–Cu1–O1
N1–Cu1–O1
C12–O3–C16
170.9(3)
88.5(6)
90.9(7)
89.6(4)
89.5(6)
170.5(3)
118.8(11)
Cu1–N1
Cu1–N2
Cu1–O1
Cu1–O2
Cu2–N3
Cu2–N4
Cu2–O3
Cu2–O4
Cu2–O5
Cu3–O6
Cu3–O7
Cu3–O8
Cu3–O9
Cu3–O10
C18–O3
C38–O6
1.98(5)
1.97(6)
1.95(5)
1.95(5)
1.98(5)
1.98(6)
2.48
1.97(5)
1.97(5)
2.62
1.94(3)
1.93(5)
1.92(5)
1.92(5)
1.36(7)
1.37(8)
O1–Cu1–O2
O1–Cu1–N2
O2–Cu1–N2
O1–Cu1–N1
O2–Cu1–N1
N2–Cu1–N1
O5–Cu2–O4
O5–Cu2–N4
O4–Cu2–N4
O5–Cu2–N3
O4–Cu2–N3
N4–Cu2–N3
O5–Cu2–O3
O4–Cu2–O3
N4–Cu2–O3
N3–Cu2–O3
O10–Cu3–O9
O10–Cu3–O7
O9–Cu3–O7
O10–Cu3–O8
O9–Cu3–O8
O7–Cu3–O8
92.6(2)
177.7(2)
86.7(2)
89.3(2)
177.2(2)
91.5(2)
90.1(2)
88.7(2)
176.8(2)
173.5(2)
90.5(2)
90.4(2)
91.2
1.38(13)
1.42(12)
2
Cu1–N1
Cu1–N2
Cu1–O1
Cu1–O2
Cu1–N3
Cu2–N4
Cu2–N5
Cu2–O5
Cu2–O4
Cu2–N6
C18–O3
C21–O3
C44–O6
C47–O3
1.95(3)
1.93(3)
2.02(2)
1.99(2)
2.33(3)
1.95(3)
1.95(3)
1.99(2)
2.03(2)
2.29(3)
1.37(4)
1.43(4)
1.37(4)
1.43(4)
N2–Cu1–N1
N2–Cu1–O2
N1–Cu1–O2
N2–Cu1–O1
N1–Cu1–O1
O2–Cu1–O1
N4–Cu2–N5
N4–Cu2–O5
N5–Cu2–O5
N4–Cu2–O4
N5–Cu2–O4
O5–Cu2–O4
N4–Cu2–N6
N5–Cu2–N6
O5–Cu2–N6
O4–Cu2–N6
C18–O3–C21
C44–O6–C47
91.9(12)
169.4(13)
89.3(11)
88.6(11)
160.6(12)
86.9(10)
91.1(12)
168.1(11)
88.8(11)
88.8(11)
156.3(11)
86.6(10)
100.5(12)
111.0(11)
90.7(11)
92.3(11)
117.4(3)
114.6(3)
89.2
95.4(13)
93.7(14)
94.2(2)
176.3(2)
85.7(2)
86.3(3)
172.8(3)
93.3(2)
7
Cu–N1
Cu–N2
Cu–O1
Cu–O2
C11–O3
O3–H1
1.97(5)
1.95(5)
1.93(4)
1.94(4)
1.34(11)
0.820
O2–Cu1–N2
O2–Cu1–N1
N2–Cu1–N1
O2–Cu1–O1
N2–Cu1–O1
N1–Cu1–O1
C11–O3–H1
178.7(19)
89.0(18)
91.8(19)
90.3(17)
88.9(18)
176.9(19)
109.5
91.5(2)°] are normal. On the other hand geometry around Cu2 is dis-
torted square pyramidal defined by two pyrrolic nitrogen (N3, N4)
from the dipyrrin unit O4 and O5 from acac and phenolic oxygen
(O3). Various Cu–N and Cu–O bond lengths and angles around the
copper(II) are in good agreement with the square pyramidal geome-
try [18,19]. Further, it is observed that Cu2–O3 (phenolic oxygen;
2.477 Å) is significantly longer relative to Cu2–N (dipyrrin) and
Cu2–O (acac) suggesting Jahn–Teller distortion [18]. The Cu3 also
adopted square pyramidal geometry wherein four positions are
occupied by acac oxygen and the fifth one by phenolic oxygen
(O6). The Cu–O bond distances [Cu3–O7, 1.915(3); Cu3–O8,
1.925(5); Cu3–O9, 1.922(5); Cu3–O10, 1.920(5) Å] are normal.
Elongated Cu3–O6bond distance(phenolicoxygen, 2.615 Å)relative
to Cu3–O(acac) is indicative of the Jahn–Teller distortion (Table 4).
The bond angles about Cu3 are comparable to those observed for
Cu2. The distances between Cu1Á Á ÁCu2, Cu2Á Á ÁCu3 and Cu1Á Á ÁCu3
are 10.803, 10.719, 14.572 Å, respectively. It is worth mentioning
that the ligating sites about copper centres in this complex are en-
tirely different, Cu1 is coordinated with one acac and one 4-OHdpm
unit, Cu2 with one acac, one 4-OHdpm and phenolic oxygen while
Cu3 with two acac and phenolic oxygen.

R.K. Gupta et al. / Inorganica Chimica Acta 409 (2014) 518–527
523
Complex 7 crystallizes in monoclinic system with P21/c space
group and presents a simple mononuclear complex. The metal cen-
tre in this complex adopted square planar coordination geometry
(Fig. 3b). The Cu–N and Cu–O bond lengths [Cu1–N1, 1.973(5);
Cu1–N2, 1.952(5); Cu1–O1, 1.931(4); Cu1–O2, 1.939(4) Å] in this
complex are comparable with other related systems [12,16,18].
The twist angle between two chelating acac and 2-OHdpm is
ꢀ22.16°. As expected meso-phenyl substituents are twisted out
of the plane with respect to dipyrrin moiety by ꢀ77°.
of the metal ligand bond [17,20]. As expected the X-band EPR
displayed well resolved characteristic four line axial spectra with
a nuclear hyperfine spin (I = 3/2). The g|| > g\ > 2.003 observed for
complexes is typical for d9 copper(II) and a ground state doublet
indicated that unpaired electron resides in
a
dx2
orbital
Ày2
[17,20]. Notably, 2, 4 and 6 having hfacac unit exhibit an additional
hyperfine F-coupling. The complexes 1–8 exhibited paramagne-
tism corresponding to a single electron at room temperature.
Weak interaction studies on 1 revealed creation of a cavity
2.4. X-ray photoelectron spectral studies
through C–HÁ Á Á
p
(3.723 Å) interactions (Fig. S9) whereas C–HÁ Á ÁF
(3.347 Å) type of interactions (Fig. S10) in 2 resulted in 1D parallel
ribbons. Conversely, 3 exhibited a zig–zag 1D polymer through
weak hydrogen bonding interactions (2.836, 3.034, 2.982 Å;
To ascertain two different coordination environments about the
copper(II) centres in 3 XPS studies have been performed. The XPS
data of 3 revealed that characteristic binding energy peaks arise
from 2p core electrons. The 2p3/2 and 2p1/2 peak of 3 showed a
shoulder at 933.90, 953.80 eV as well as another intense peak from
2p3/2 and 2p1/2 for copper(II) appeared at higher energy 937.05
Fig. S11), while C–HÁ Á Á
p
(3.693 Å) and C–HÁ Á ÁN (3.563 Å) interac-
tions in 7 leads to dimeric head-to-head arrangements (Fig. 4).
(
D
eV, 2p3/2; 3.15 eV), 956.95 eV (
[17,21]. The difference in binding energy (
and ( eV, 2p1/2; 3.15 eV) may be attributed to different coordina-
D
eV, 2p1/2; 3.15 eV) (Fig. 6)
2.3. EPR spectral studies
D
eV, 2p3/2; 3.15 eV)
D
EPR spectral studies on 1–8 have been performed at room tem-
perature in CH2Cl2 solution. Ensuing parameters are given in
Table S6 and the spectra depicted in Fig. 5 and S11–S16. The com-
plexes exhibited g|| and g\ values in the range 2.0449–2.2163 and
2.0193–2.0434, respectively which are comparable to other cop-
per(II) complexes exhibiting square planar or square pyramidal
geometry [20,23]. Observed g values for the complexes under
study are less than 2.3 and in agreement with covalent character
tion environment about the copper(II) centres in this complex.
2.5. Electrochemical studies
The electrochemical behavior of 1–8 have been investigated by
cyclic (CV) and differential pulse voltammetry (DPV) in CH3CN
(c, 100 lM) using 0.1 M tetrabutylammonium perchlorate
Fig. 4. Dimeric structure resulting from head-to-head C–HÁ Á Áp and C–HÁ Á ÁN interactions in 7.
Fig. 5. EPR spectra of 7 (a) and 8 (b) in CH2Cl2 at room temperature.

524
R.K. Gupta et al. / Inorganica Chimica Acta 409 (2014) 518–527
CuII/I redox couple, associated with the different environment of
copper(II) complexes [17,22]. In their cyclic voltammogram the
complexes 7 and 8 displayed one electron irreversible oxidation
wave (Epa) at 0.948 and 0.969 V assignable to CuII/III. On the other
hand, the reduction wave (Epc À0.734, 7; À0.753 V, 8) has been as-
signed to CuII/I redox couple [17,22].
In their DPV 1–6 exhibited an oxidation peak (Epa, 0.721, 1;
0.732, 2; 0.740, 3; 0.745, 4; 0.645, 5; 0.648 V, 6) due to CuII/III
and two reduction peaks (Epc1, À0.531, 1; À0.541, 2; À0.251, 3;
À0.261, 4; À0.268, 5; À0.298 V, 6) and (Epc2, À1.231, 1; À1.230,
2; À1.048, 3; À0.968, 4; À1.139, 5; À1.235 V, 6) due to CuII/I redox
couple in different environment. However, 7 and 8 exhibited oxi-
dation peaks at 0.835 and 0.998 V, 8) assignable to CuII/III and only
one reduction peak (Epc, À0.752, 7; À0.809 V, 8) due to CuII/I. The
difference in oxidation and reduction potentials for these com-
plexes may be attributed to the difference in ligand framework
and is consistent with reported values [17,22].
Fig. 6. XPS spectra of Cu(2p) core in 3.
2.6. Variable temperature magnetic studies
[(n-Bu)4N]ClO4 as a supporting electrolyte at a scan rate of 100 mV
sÀ1 in the range +2.0 to À2.0 V and the redox potentials were ref-
erenced to Fc/Fc+ couple (0.1 V). Ensuing data is summarized in
Table S7 and spectra depicted in Fig. 7 and S18–S25. The cyclic vol-
tammograms of 1–6 exhibited one electron oxidation wave (Epa
0.648, 1; 0.673, 2; 0.501, 3; 0.732, 4; 0.783, 5; 0.795 V 6) assignable
to CuII/III and two reduction waves (Epc1 À0.172, 1; À0.193, 2;
À0.221, 3; À0.253, 4; À0.129, 5; À0.205 V 6) and (Epc2 À1.281, 1;
À1.341, 2; À1.021, 3; À1.053, 4; À0.789, 5; À0.798 V, 6) due to
The magnetic susceptibilities of 1–3 were measured as a func-
tion of temperature. Curie plots (Fig. 8) indicate that these are
paramagnetic to lowest temperature, with the Weiss temperature
h < 1 K. For both 1 and 2 the effective moment is 1.91 BM/Cu, coin-
ciding with that of copper(II), whereas in 3 the effective moment of
2.29 BM/Cu may be associated with the metal–metal cooperativity
between the three copper(II) centres in both square planar and
square pyramidal geometries.
Fig. 7. The cyclic voltammograms 3 (a) and 4 (b) of MeCN (c, 100
l
M) at room temperature.
40
60
50
40
30
20
10
0
80
(a)
(b)
(c)
35
70
60
50
40
30
20
10
0
30
25
20
15
10
5
0
0
10
20
30
40
50
0
10
20
30
40
50
0
10
20
30
40
50
Temperature (K)
Temperature (K)
Temperature (K)
Fig. 8. Curie plot for complexes 1 (a), 2 (b) and 3 (c).

R.K. Gupta et al. / Inorganica Chimica Acta 409 (2014) 518–527
4. Experimental
525
4.1. Materials and methods
The synthetic manipulations were performed in deaerated sol-
vents under nitrogen atmosphere and solvents were purified by
standard literature procedure prior to their use [24]. 4-/3-/2-
hydroxybenzaldehyde, 4-/3-/2-picolylchloride, 2,3-dicloro-5,6-
dicyano-1,4-benzo-quinone (DDQ), pyrrole, tetrabutylammonium
perchlorate [(n-Bu)4N]ClO4, Cu(acac)2 and Cu(hfacac)2Á Â H2O were
procured from Sigma Aldrich Chemical Co. Pvt. Ltd., and used as
received without further purifications. The ligands 4-(2-methoxy-
pyridyl)phenyldipyrromethane,
4-(3-methoxypyridyl)phenyldi-
pyrromethane, 4-(4-methoxypyridyl)phenyldipyrromethane were
prepared and purified following our earlier procedure, [15c] while
4-/3-/2-hydroxyphenyldipyrromethane (OHdpm) by literature
procedures [15c,16].
Elemental analyses for C, H, and N were performed on an Exeter
CE-440 CHN analyzer in the microanalytical laboratory of the
Department of Chemistry, Faculty of Science, Banaras Hindu Uni-
versity, Varanasi, India. Infrared and electronic absorption spectra
were acquired on a Varian 3300 FT-IR and Shimadzu UV-1601
spectrophotometers, respectively. The X-band EPR spectra were
acquired on a Bruker EMX-1444 EPR spectrometer, calibrated with
diphenylpicrylhydrazyl, DPPH (g = 2.0037). The g value has been
directly calculated with the help of Bruker WINEPR software. XPS
was recorded on Thermoscientific Multilab 2000 equipment,
Fig. 9. Titration plots (pH) for 3–8 (methanol/water 3:7, c, 1 mM) upon addition of
LiOHÁH2O (water, c, 100 mM) at room temperature.
2.7. pH titration studies
Along with dipyrrin unit the ligands 4-/3-/2-OHdpm may also
coordinate with the metal centre through phenolic oxygen either
in protonated or deprotonated form. To confirm the coordination
behavior of these ligands in 3–8 pH titration studies (methanol/
water 3:7; c, 1 mM) have been performed. The complexes were ti-
trated against a base LiOHÁH2O (water; c, 100 mM) using pH elec-
trode to monitor the changes and pKa values calculated from the
change in slope of the titration curve (Fig. 9). From the titration
curves it is clear that two equiv of LiOHÁH2O are required for the
deprotonation of complexes 3 and 4 suggesting two protons are
simultaneously withdrawn at almost the same pKa (5.65, 3; 5.48,
4) [23].
On the other hand, 5–8 consumes only one equiv of LiOHÁH2O
indicating deprotonation of only one proton (pKa, 6.33, 5; 6.11, 6;
6.92, 7; 6.78, 8) [23]. The lower pKa value for 3 and 4 may be
attributed to rather strong coordination of the phenolic oxygen
with metal centre. The complexes 5 and 6 exhibited relatively
high pKa values relative to 3 and 4, which may be attributed to
weaker coordination through the phenolic oxygen. On the other
hand, high pKa values for 7 and 8 may be due to uncoordinated
phenolic oxygen which has been verified by single crystal X-ray
analysis. Conclusively, from pH titration studies we can say that
3 and 4 behave like dibasic, whereas 5–8 act as monobasic weak
acids.
employing Mg K
a X-rays as the source (hm = 1253.4 eV). Electro-
spray ionization mass spectrometric (ESI-MS) data were obtained
on a THERMO Finningan LCQ Advantage Max ion trap mass spec-
trometer. The samples (10 lL) were dissolved in dichlorometh-
ane/acetonitrile (3:7, v/v) and introduced into the ESI source
through a Finningan surveyor auto sampler. The mobile phase
(MeOH/MeCN: H2O, 90: 10) flowed at a rate of 250 lL/min. The
ion spray voltage was set at 5.3 kV and capillary voltage at 34 V.
The MS scan runs up to 2.5 min and the spectra print-outs were
averaged of over 10 scans. Electrochemical measurements were
performed on a CHI-620c electrochemical analyzer. All the mea-
surements were carried out in a one compartment cell equipped
with a glassy carbon working, platinum wire counter, and Ag/Ag+
reference electrode under nitrogen atmosphere. The magnetic sus-
ceptibility measurements were made on a Quantum Physical Prop-
erties Measurement System between 3 and 200 K and fields up to
10 kOe. The pH titration studies were carried out using a Eutech
PHTEST10BNC pH meter with a BNC electrode, directly giving pH
of the solution. The electrode was calibrated with standard buffer
solution pH 4, 7 and 10, respectively before the experiment. Equi-
librium measurements were performed on 10.0 mL of solution
(methanol/water 3:7, c, 1 mM) of 3–8 in a cell.
3. Conclusion
4.2. Synthesis
In summary, through this work we have reported a series of
heteroleptic dipyrrinato copper complexes bearing peripheral
coordination sites appended with pyridyl and phenolic groups.
The ligand 4-(2-methoxypyridyl)phenyldipyrromethane having
2-pyridyl substituent forms binuclear metallacycles with
Cu(acac)2 and Cu(hfacac)2, while 3-/4-pyridyl derivative under-
go etheretic C–O bond cleavage and forms a trinuclear complex.
The reactions of 4-/3-/2-hydroxyphenyldipyrromethenes with
precursor complexes gave trinuclear, binuclear and mononu-
clear complexes, respectively. Thus, the present work clearly
demonstrated that position and nature of the ligating site other
than dipyrrin core influences the complexation behavior and
supramolecular assembly of the heteroleptic dipyrrinato
complexes.
4.2.1. [Cu2(2-pcdpm)2(acac)2] (1)
2,3-Dichloro-5,6-dicyano-1,4-benzoquinone (0.114 g, 0.50 mmol)
dissolved in benzene (60 mL) was added dropwise over ꢀ30 min to
an ice cooled solution of 4-(2-methoxypyridyl)phenyldipyrrome-
thane (0.164 g, 0.50 mmol) in 40 mL of CH2Cl2 and stirred for 1 h.
Triethylamine (1 mL) and Cu(acac)2 (0.131 g, 0.50 mmol) were suc-
cessively added to the reaction mixture and stirred for 10 min. It
was concentrated under reduced pressure to dryness and the crude
product purified by column chromatography [SiO2; CH2Cl2: MeOH
(1%)] to afford a red/green film. Yield: 32% (0.319 g). Anal. Calc. for
C
52H46Cu2N6O6: C, 63.86; H, 4.74; N, 8.59. Found: C, 63.92; H, 4.68;
N, 8.78%. ESI-MS. Calcd, found: m/z 979.0502, 979.2038 [M]+. IR
(KBr pellets, cmÀ1): 592, 730, 811, 989, 1022, 1205, 1244, 1335,

526
R.K. Gupta et al. / Inorganica Chimica Acta 409 (2014) 518–527
1393, 1514, 1552, 1593, 3385. UV–Vis (MeCN, kmax nm,
cmÀ1); 488 (1.21 Â 105), 346 (3.98 Â 104), 294 (1.54 Â 105).
e
MÀ1
3360. UV–Vis (MeCN, kmax nm,
295 (1.06 Â 105).
e
MÀ1 cmÀ1); 490 (1.64 Â 105),
4.2.2. [Cu2(2-pcdpm)2(hfacac)2] (2)
4.2.8. [Cu(2-OHdpm)(hfacac)] (8)
This complex was prepared following the above procedure for 1
using [Cu(hfacac)2ÁH2O] (0.248 g, 0.50 mmol) in place of Cu(acac)2.
Yield: 30% (0.357 g). Anal. Calc. for C52H34Cu2F12N6O6: C, 52.31; H,
2.87; N, 7.04. Found: C, 53.02; H, 2.57; N, 7.52%. ESI-MS. Calcd,
found: m/z 1193.9358, 1195.1094 [M+2]+. IR (KBr pellets, cmÀ1):
586, 670, 733, 820, 997, 1032, 1143, 1210, 1340, 1379, 1543,
It was prepared following the above procedure for 3 using
2-hydroxyphenyldipyrromethane (0.113 g, 0.50 mmol) and
[Cu(hfacac)2ÁH2O] (0.248 g, 0.50 mmol). Yield: 65% (0.328 g). Anal.
Calc. for C20H12CuF6N2O3: C, 47.49; H, 2.39; N, 5.54. Found: C,
47.91; H, 2.30; N, 5.59%. ESI-MS. Calcd, found: m/z 506.0148,
506.0121 [M]+. IR (KBr pellets, cmÀ1): 548, 725, 775, 891, 1001,
1031, 1146, 1216, 1244, 1342, 1383, 1456, 1599, 1648, 3353. UV–
1603, 1649. UV–Vis (MeCN, kmax nm,
e
MÀ1 cmÀ1); 486
(1.05 Â 105), 345 (3.42 Â 104), 293 (2.12 Â 105).
Vis (MeCN, kmax nm,
e
MÀ1 cmÀ1); 489 (1.64 Â 105), 354 (2.84 Â
104), 298 (6.36 Â 104).
4.2.3. [Cu3(4-OHdpm)2(acac)4] (3)
The complex 3 was prepared following above procedure for 1
starting from 4-(3-methoxypyridyl)phenyldipyrromethane/4-(4-
methoxypyridyl)phenyldipyrromethane/4-hydroxyphenyldipyr-
romethane (0.164 or 0.113 g, 0.50 mmol) and [Cu(acac)2] (0.131 g,
0.50 mmol). Yield: 28% (0.295 g). Anal. Calc. for C50H50Cu3N4O10: C,
56.89; H, 4.58; N, 5.31. Found: C, 57.02; H, 4.73; N, 5.52%. ESI-MS.
Calcd, found: m/z 1055.5749, 1055.0565 [M]+. IR (KBr pellets,
cmÀ1): 612, 730, 775, 812, 990, 1022, 1205, 1244, 1335, 1394,
4.3. X-ray structure determinations
Crystals suitable for single crystal X-ray diffraction analyses
were obtained by slow diffusion of hexane over dichloromethane
solution of the respective complexes. Single crystal X-ray data on
1 were collected on Enarf-Nonius CAD4 diffractometer at Jadavpur
University, Kolkata, 2 and 3 on a OXFORD DIFFRACTION X CALI-
BUR-S at Indian Institute of Technology, Powai, Mumbai, India
and for 7 on R-AXIS RAPID II diffractometer at National Institute
of Advanced Industrial Science and Technology (AIST), Osaka, Ja-
pan, at room temperature with Mo K
Structures were solved by direct methods (SHELXS 97) and refined
by full-matrix least square calculations on F2
SHELX 97) [25]. All
the non-H atoms were treated anisotropically. H-atoms attached
to the carbon were included as fixed contribution and were geo-
metrically calculated and refined with the SHELX riding model. Com-
puter program PLATON was used for analyzing the interaction and
stacking distances [26].
1515, 1552, 1579, 3323. UV–Vis (MeCN, kmax nm,
e
MÀ1 cmÀ1);
484 (2.79 Â 105), 333 (8.26 Â 104), 257 (8.26 Â 104).
a radiation (k = 0.71073 Å).
4.2.4. [Cu3(4-OHdpm)2(hfacac)4] (4)
It was prepared following the above procedure for 3 except that
[Cu(hfacac)2ÁH2O] (248 mg, 0.50 mmol) was used in place of
[Cu(acac)2] (0.131 g, 0.50 mmol). Yield: 22% (0.327 g). Anal. Calc.
for C50H24Cu3F24N4O10: C, 40.38; H, 1.63; N, 3.77. Found: C,
40.75; H, 1.69; N, 3.98%. ESI-MS. Calcd, found: m/z 1487.3460,
1488.0368 [M+1]+. IR (KBr pellets, cmÀ1): 592, 673, 773, 797,
825, 890, 1005, 1038, 1144, 1209, 1263, 1341, 1383, 1406, 1461,
(
1552, 1605, 1647, 3282. UV–Vis (MeCN, kmax nm,
e
MÀ1 cmÀ1);
Acknowledgements
483 (2.56 Â 105), 333 (6.65 Â 104), 257 (5.50 Â 104).
The authors thank to the Department of Science and Technology
(DST), New Delhi, India for financial assistance through the
Scheme (SR/S1/IC/-25/2011), and Council of Scientific and Indus-
trial Research (CSIR) New Delhi, India for the award of Junior Re-
search Fellowship to RKG (No. 09/013(0210)/2009-EMR-I). We
are also thankful to the Head, Department of Chemistry, Faculty
of Science, Banaras Hindu University, Varanasi (U.P.) India for
extending laboratory facilities, Prof. S Natarajan, Indian Institute
of Science, Bangalore, India for providing XPS data, Prof. S. Bhat-
tacharya, Jadavpur University Kolkata, Indian Institute of Technol-
ogy, Powai, Mumbai and Prof. Q. Xu National Institute of Advanced
Industrial Science and Technology (AIST), Osaka, Japan for extend-
ing single crystal X-Ray facility.
4.2.5. [Cu2(3-OHdpm)(acac)3] (5)
This complex was also prepared using the method described for
starting from 3-hydroxyphenyldipyrromethane (0.113 g,
3
0.50 mmol) and [Cu(acac)2] (0.131 g, 0.50 mmol). Yield: 45%
(0.296 g). Anal. Calc. for C30H31Cu2N2O7: C, 54.70; H, 4.74; N,
4.25. Found: C, 54.96; H, 4.58; N, 4.68%. ESI-MS. Calcd, found: m/
z 658.6683, 658.0975 [M]+. IR (KBr pellets, cmÀ1): 454, 612, 730,
781, 937, 1025, 1216, 1241, 1275, 1334, 1355, 1377, 1551, 1580,
3317. UV–Vis (MeCN, kmax nm,
e
MÀ1 cmÀ1); 486 (2.04 Â 105),
315 (6.07 Â 104), 259 (4.17 Â 104).
4.2.6. [Cu2(3-OHdpm)(hfacac)3] (6)
It was prepared following the above procedure for
3
using 3-hydroxyphenyldipyrromethane (0.113 g, 0.50 mmol) and
[Cu(hfacac)2ÁH2O] (0.248 g, 0.50 mmol). Yield: 42% (0.224 g). Anal.
Calc. for C30H31Cu2F18N2O7: C, 36.73; H, 1.33; N, 2.85. Found: C,
36.91; H, 1.48; N, 2.79%. ESI-MS. Calcd, found: m/z 983.4967,
985.8985 [M+2]+. IR (KBr pellets, cmÀ1): 590, 730, 777, 836, 898,
999, 1035, 1143, 1215, 1251, 1340, 1397, 1454, 1551, 1595, 1648,
Appendix A. Supplementary material
CCDC 879368, 879364, 879365 and 879363 contains the sup-
plementary crystallographic data for complexes (1), (2), (3) and
(7), respectively. These data can be obtained free of charge from
The Cambridge Crystallographic Data Centre via www.ccdc.cam.a-
c.uk/data_request/cif. Supplementary data associated with this
article can be found, in the online version, at http://dx.doi.org/
10.1016/j.ica.2013.09.044.
3285. UV–Vis (MeCN, kmax nm,
e
MÀ1 cmÀ1); 488 (1.80 Â 105), 317
(4.46 Â 104), 263 (3.51 Â 104).
4.2.7. [Cu(2-OHdpm)(acac)] (7)
This complex was synthesized following the method described
for 3 starting using 2-hydroxyphenyldipyrromethane (0.113 g,
0.50 mmol) and [Cu(acac)2] (0.131 g, 0.50 mmol). Yield: 67%
(0.265 g). Anal. Calc. for C20H18CuN2O3: C, 60.37; H, 4.56; N, 7.04.
Found: C, 60.58; H, 4.74; N, 7.25%. ESI-MS. Calcd, found: m/z
397.9145, 398.9597 [M+1]+. IR (KBr pellets, cmÀ1): 544, 750, 888,
993, 1027, 1180, 1210, 1248, 1338, 1398, 1515, 1560, 1583,
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