An unprecedented "linear-bent" isomerism in tri-nuclear Cu 2IIZnII complexes with a salen type di-Schiff base ligand

Article abstract of DOI:10.1039/c2dt31011b

Two new trinuclear hetero-metallic copper(ii)-zinc(ii) complexes [(CuL)₂Zn(N₃)₂] (1A and 1B) have been synthesized using [CuL] as a so-called "metalloligand" (where H ₂L = N,N′-bis(salicylidene)-1,3-propanediamine) and structurally characterized. Complexes 1A and 1B have the same molecular formula but crystallize in different crystal systems (triclinic for 1A and monoclinic for 1B) with space group P1 for 1A and P2₁/c for 1B. 1A is an angular trinuclear species, in which two terminal four-coordinate square planar "metalloligand" [CuL] are coordinated to a central Zn(ii) through double phenoxido bridges. The Zn(ii) is in a six-coordinate distorted octahedral environment being bonded additionally to two mutually cis nitrogen atoms of terminal azide ions. In complex 1B, in addition to the double phenoxido bridge, the two terminal Cu(ii) ions are linked to the central Zn(ii) via a μ_-l,l azido bridge giving rise to a square pyramidal environment around the Cu(ii) ions and consequently the structure becomes linear. These two species can be considered as "linear-bent" isomers. EPR spectra and ESI mass spectra show that the two isomers are identical in solution. The DFT calculation reveals that the energy of 1A is 7.06 kcal mol^-1 higher than that of 1B. The existence of both isomers in the solid state suggests that crystal packing interactions in 1A are more efficient and probably compensate for the difference in energy.

Full text of DOI:10.1039/c2dt31011b

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PAPER
II
An unprecedented “linear-bent” isomerism in tri-nuclear Cu2 ZnII
complexes with a salen type di-Schiff base ligand†
Lakshmi Kanta Das,a Sung-Woo Park,b Seung Joo Choc and Ashutosh Ghosh*a
Received 9th May 2012, Accepted 22nd June 2012
DOI: 10.1039/c2dt31011b
Two new trinuclear hetero-metallic copper(II)–zinc(II) complexes [(CuL)2Zn(N3)2] (1A and 1B) have been
synthesized using [CuL] as a so-called “metalloligand” (where H2L = N,N′-bis(salicylidene)-1,3-
propanediamine) and structurally characterized. Complexes 1A and 1B have the same molecular formula
but crystallize in different crystal systems (triclinic for 1A and monoclinic for 1B) with space group
ˉ
P1 for 1A and P21/c for 1B. 1A is an angular trinuclear species, in which two terminal four-coordinate
square planar “metalloligand” [CuL] are coordinated to a central Zn(II) through double phenoxido
bridges. The Zn(II) is in a six-coordinate distorted octahedral environment being bonded additionally to
two mutually cis nitrogen atoms of terminal azide ions. In complex 1B, in addition to the double
phenoxido bridge, the two terminal Cu(II) ions are linked to the central Zn(II) via a μ-l,l azido bridge
giving rise to a square pyramidal environment around the Cu(II) ions and consequently the structure
becomes linear. These two species can be considered as “linear-bent” isomers. EPR spectra and ESI mass
spectra show that the two isomers are identical in solution. The DFT calculation reveals that the energy
of 1A is 7.06 kcal mol−1 higher than that of 1B. The existence of both isomers in the solid state suggests
that crystal packing interactions in 1A are more efficient and probably compensate for the difference in
energy.
of coordination modes, large number of isomers are expected to
Introduction
form with an azide ion7 but only a few are reported in the
literature.8
Isomerism is an important aspect of the chemistry of coordi-
nation compounds.1 Isomers are substances that have the same
number and kinds of atoms with different arrangements. Various
types of isomerism, e.g. ionization, hydrate, linkage, polymeriz-
ation, geometric (cis–trans, fac–mer), optical etc. have been well
Recently, we have focused our attention on the synthesis of
some hetero-metallic polynuclear complexes based on neutral
copper(II) complexes of salen type di-Schiff base ligands.9 It has
been found that the nature of the anionic coligands or counter
anions play an important role to determine nuclearity and topo-
logy of the resulting hetero-metallic polynuclear complexes.9b,c,e
In most of them, the oxygen atoms of two “metalloligands”
coordinate to a central-metal ion along with bridging anionic
coligands such as carboxylate, nitrate and perchlorate which act as
an additional bridge between two adjacent metal centers to result
in linear trinuclear complexes.10 On the other hand, when these
anionic coligands are monodentate, (e.g. chloride, perchlorate,
thiocyanate) the resulted trinuclear complexes can either be bent11
or linear.12 Hence, using azide ion, both bent and linear trinuclear
complexes can potentially be formed as both bridging and
monodentate coordination modes of it are well documented.
However, to our knowledge there is only one example9d of bent
trinuclear adducts (Cu2Mn) but no linear trinuclear species
containing this type of “metalloligand” is reported.
recognized since the time of Werner.2 The N3 ion is a versatile
−
ligand that can coordinate to metal ions in a variety of modes.3 It
can act as a monodentate ligand as well as a bridging linker in
complexes.4 As a bridging bidentate ligand, azide binds the
metal either in the EO (end on) or EE (end-to-end) mode.5 The
possible tridentate bridging coordination modes of azido ligand
are μ-1,1,1-N3 and μ-1,1,3-N3.4c The rare variety of coordination
modes e.g. μ-1,1,1,1-N3, μ-1,1,3,3-N3 and μ-1,1,1,3,3,3-N3 have also
been reported in some compounds.4b,d,6 Considering the variety
aDepartment of Chemistry, University College of Science, University of
Calcutta, 92, A.P.C. Road, Kolkata-700 009, India.
E-mail: ghosh_59@yahoo.com
bCenter for Superfunctional Materials, Department of Chemistry,
Pohang University of Science and Technology, San 31, Hyojadong,
Namgu, Pohang 790-784, Korea
cDepartment of Cellular Molecular Medicine, College of Medicine,
Chosun University, Gwangju 501-759, Republic of Korea
†Electronic supplementary information (ESI) available: The electronic
supplementary information file contains Fig. S1–S5. CCDC 879879 and
879880 for 1A and 1B respectively. For ESI and crystallographic data in
CIF or other electronic format see DOI: 10.1039/c2dt31011b
In the present paper, we report the formation of two new iso-
meric hetero-metallic coordination compounds [(CuL)2Zn(N3)2]
(1A and 1B) of Zn(II) using [CuL] as a “metalloligand” where
H2L = N,N′-bis(salicylidene)-1,3-propanediamine (Scheme 1).
Isomer 1A is an angular trinuclear species having the terminal
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Scheme 1 Formation of the complexes (1A and 1B).
coordination mode of azide while 1B, a linear triangular
complex is formed through the μ-l,l-azido bridges. Therefore,
they may be considered as a pair of “linear-bent” isomers. To
our knowledge, both linear and bent structures of such trinuclear
species are known but with different sets of ligands or coligands
and therefore are not isomeric. Complexes 1A and 1B are thus
the first report of these types of “linear-bent” isomers.
for complex formation. To a methanolic solution (20 mL) of
Cu(ClO4)2·6H2O (1.852 g, 5 mmol) was added a methanolic
solution of H2L (5 mmol, 10 mL) to prepare the “metalloligand”
[CuL] as reported earlier.13
Synthesis of the complex [(CuL)2Zn(N3)2] (1A)
The precursor complex [CuL] (0.718 g, 2 mmol) was dissolved
in methanol (20 mL) and then a water solution (1 mL) of
Zn(ClO4)2·6H2O (0.372 g, 1 mmol) followed by an aqueous sol-
ution (1 mL) of sodium azide (0.130 g, 2 mmol) was added to
this solution. The mixture was stirred for 1 h at room temp-
erature. The filtrate was allowed to stand overnight at room temp-
erature when reddish brown X-ray quality single crystals
appeared at the bottom of the vessel. The crystals were washed
with a methanol–water mixture and dried in a desiccator contain-
ing anhydrous CaCl2 and then characterized by elemental analy-
sis, spectroscopic methods and X-ray diffraction.
Experimental
Starting materials
The salicylaldehyde and 1,3-propanediamine were purchased
from Lancaster and were of reagent grade. They were used
without further purification.
Caution! Perchlorate and azide salts of metal complexes with
organic ligands are potentially explosive. Only a small amount
of material should be prepared and it should be handled with
care.
Complex 1A: (Yield: 0.600 g, 72%), Anal. calc. for
C34H32ZnCu2N10O4: C, 48.78; H, 3.85; N, 16.73; % found: C,
48.58; H, 3.62; N, 16.61; %. λmax (MeOH): 601 and 358 nm,
λmax (solid, reflectance): 580 and 365 nm. IR (KBr): ν(CvN)
1614 cm−1; ν(N3) 2056 cm−1. HRMS (m/z, ESI+): found for
[(CuL)H]+ = 343.96 (calc. 344.07), [(CuL)Na]+ = 365.94 (calc.
Synthesis of the Schiff base ligand (H2L) N,N′-bis(salicylidene)-
1,3-propanediamine and the “metalloligand” [CuL]
The Schiff base ligand was synthesized by a standard method.
Briefly, 5 mmol of 1,3-propanediamine (0.42 mL) was mixed
with 10 mmol of the salicylaldehyde (1.04 mL) in methanol.
The resulting solution was refluxed for ca. 2 h, and allowed to
cool. The yellow coloured methanolic solution was used directly
366.05), [(CuL)2H]+ = 686.92 (calc. 687.13), [(CuL)2Na]+
=
708.85 (calc. 709.11), [Zn(CH3OH)4(N3)2Na]+ = 299.05 (calc.
299.04).
11010 | Dalton Trans., 2012, 41, 11009–11017
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Table 1 Crystal data and structure refinement of complexes 1A and 1B
Synthesis of the complex [(CuL)2Zn(N3)2] (1B)
Complexes
1A
1B
Complex 1B was prepared by mixing the same components as
for 1A. However, after mixing the components, the mixture was
heated in a water bath (ca. 15 min) until the volume of the
mixture reduced to almost half of the initial volume. A green
crystalline compound was separated during reduction of the
volume. The crystals were isolated by filtration. Some of these
green crystals were of diffraction quality. The filtrate on slow
evaporation at room temperature yielded isomer 1A which is
brown in colour.
Complex 1B: (Yield: 0.505 g, 60%), Anal. calc. for
C34H32ZnCu2N10O4: C, 48.78; H, 3.85; N, 16.73; % found: C,
48.99; H, 3.89; N, 16.69; % λmax (MeOH): 599 and 357 nm,
λmax (solid, reflectance): 627 and 360 nm. IR (KBr): ν(CvN)
1617 cm−1; ν(N3) 2052 cm−1. HRMS (m/z, ESI+): found for
[(CuL)H]+ = 343.97 (calc. 344.07), [(CuL)Na]+ = 365.95 (calc.
366.05), [(CuL)2H]+ = 686.91 (calc. 687.13), [(CuL)2Na]+ = 708.89
(calc. 709.11), [Zn(CH3OH)4(N3)2Na]+ = 299.08 (calc. 299.04).
Formula
M
Crystal system
Space group
a (Å)
b (Å)
c (Å)
C34H32Cu2N10O4Zn
837.19
C34H32Cu2N10O4Zn
837.19
Monoclinic
P21/c
9.453(5)
11.123(5)
15.869(5)
90
102.039(5)
90
1631.9(12)
2
1.704
2.078
852
0.028
23 309
5181
3851
0.0309, 0.0813
293
Triclinic
ˉ
P1
9.8880(3)
11.9657(3)
14.0680(4)
86.702(1)
77.540(1)
89.604(1)
1622.56(8)
2
1.714
2.090
852
0.036
19 256
6766
5338
0.0430, 0.1285
293
α (°)
β (°)
γ (°)
V (Å3)
Z
Dc (g cm−3
)
μ (mm−1
F (000)
R(int)
)
Total reflections
Unique reflections
I > 2σ(I)
R1, wR2
Temp (K)
G-o-F value
1.05
1.04
Physical measurements
Elemental analyses (C, H and N) were performed using a
Perkin-Elmer 2400 series II CHN analyzer. IR spectra in KBr
pellets (4000–500 cm−1) were recorded using a Perkin-Elmer
RXI FT-IR spectrophotometer. Electronic spectra in methanol
and in solid state (750–300 nm) were recorded in a Hitachi
U-3501 spectrophotometer. Powder X-ray diffraction patterns are
recorded on a Bruker D-8 advance diffractometer operated at
40 kV voltage and 40 mA current and calibrated with a standard
silicon sample, using Ni-filtered Cu-Kα (α = 0.15406 nm) radi-
ation. The X-band (9.1 GHz) EPR spectra were recorded using a
JEOL JES-FA 200 instrument. The electrospray ionization mass
spectrometry (ESI-MS positive) spectra were recorded with a
Micromass Qtof YA 263 mass spectrometer.
Results and discussion
Syntheses
The Schiff-base ligand (H2L) and its Cu(II) complex, [CuL] were
synthesized using the reported procedure.13 The Cu(II) complex
on reaction with zinc perchlorate hexahydrate and the sodium
azide in MeOH–H2O medium (10 : 1, v/v) resulted in a pair of
isomers, [(CuL)2Zn(N3)2] (1A) and [(CuL)2Zn(N3)2] (1B)
depending upon the temperature. Complex 1A separated as a
brown solid when the solution was stirred after mixing the com-
ponents at room temperature. The diffraction quality rhombic
shaped reddish-brown single crystals of 1A were obtained on
keeping the filtrate overnight at open atmosphere. A comparison
of the powder XRD patterns of the brown product (Fig. 1) which
separated on stirring with that of the simulated powder XRD
pattern of the single crystal clearly show that it is pure isomer
1A. However, when the mixture is warmed in order to reduce the
volume, then the green crystalline compound along with X-ray
quality single crystals are separated from the hot solution. The
green product is pure 1B as is evident from its powder XRD
pattern (Fig. 1). When this green compound (1B) is dissolved in
methanol and the solution is kept at room temperature for slow
evaporation of solvent, the brown crystals of 1A separate. Simi-
larly, a methanol solution of 1A yields the green crystalline com-
pound of 1B if the solvent is evaporated in a water bath.
However, isomers 1A and 1B show hardly any change in their
respective powder XRD patterns and colour on heating the solid
compounds up to 200 °C, after which they start to decompose.
Therefore it can be concluded that isomers 1A and 1B are inter-
convertible in solutions but not in the solid state.
Crystallographic data collection and refinement
Suitable single crystals of each complexes were mounted on a
Bruker-AXS SMART APEX II diffractometer equipped with a
graphite monochromator and Mo-Kα (λ = 0.71073 Å) radiation.
The crystals were positioned at 60 mm from the CCD. Frames
(360) were measured with a counting time of 5 s. The structures
were solved using Patterson method by using the SHELXS 97.
Subsequent difference Fourier synthesis and least-square refine-
ment revealed the positions of the remaining non-hydrogen
atoms. Non-hydrogen atoms were refined with independent ani-
sotropic displacement parameters. Hydrogen atoms were placed
in idealized positions and their displacement parameters were
fixed to be 1.2 times larger than those of the attached non-hydro-
gen atom. Successful convergence was indicated by the
maximum shift/error of 0.001 for the last cycle of the least
squares refinement. Absorption corrections were carried out
using the SADABS program.14 All calculations were carried out
using SHELXS 97,15 SHELXL 97,16 PLATON 99,17
ORTEP-3218 and WINGX system ver-1.64.19 Data collection
and structure refinement parameters and crystallographic data for
the two complexes are given in Table 1. The selected bond
lengths and bond angles are summarized in Table 2.
IR and UV-Vis spectra
Besides elemental analysis, both the complexes were initially
characterized by IR spectra. There is no significant difference
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between the IR spectra of the two complexes (Fig. S1 and S2,
ESI†). The precursor “metalloligand” [CuL] is neutral and
obviously does not have any counter anion whereas the
appearance of the characteristic bands for azide at 2056 and
2052 cm−1 for isomers 1A and 1B, respectively in their IR
spectra indicate the formation of the trinuclear adducts. A strong
and sharp band due to the azomethine ν(CvN) group of the
Schiff base was found at 1614 and 1617 cm−1 for complexes 1A
and 1B, respectively. The rest of the spectral patterns and band
positions of both the complexes are very similar to the precursor,
[CuL].
The electronic spectra were measured in CH3OH as well as in
the solid state. The electronic spectra of “metalloligand”, 1A and
1B in methanolic solution and their solid state diffuse reflectance
spectra are shown in Fig. 2 and the spectral parameters are given
in Table 3. Both the complexes show a sharp single absorption
band near 358 and 357 nm in methanol and 365 and 360 nm in
the solid state for 1A and 1B, respectively, attributed to ligand-
to-metal charge transfer transitions. Besides this band, a broad
absorption band is observed in the visible region at 601 and
599 nm in methanol and 580 and 627 nm in solid state for 1A
and 1B , respectively. This lower energy band is typical of d–d
transitions of Cu(II) ions with a square based environment. In
methanol, the two isomers and the mononuclear [CuL] precursor
complex show very similar electronic spectra indicating a square
planar environment around Cu(II) in all of them. However, in
solid state the band positions of lower energy d–d transition in
complexes 1A and 1B are distinctly different. This band at
Table 2 Bond distances (Å) and angles (°) for complexes 1A and 1B
Complex 1A
Complex 1B
Cu(1)–O(1)
Cu(1)–O(2)
Cu(1)–N(1)
Cu(1)–N(2)
1.925(2)
1.905(2)
1.963(3)
1.954(3)
1.941(3)
Cu(1)–O(1)
Cu(1)–O(2)
Cu(1)–N(1)
Cu(1)–N(2)
Cu(1)–N(3)a
1.944(2)
1.928(2)
1.942(2)
1.972(2)
2.584(2)
Cu(2)–O(3)
Cu(2)–O(4)
Cu(2)–N(3)
1.915(2)
1.965(3)
Cu(2)–N(4)
Zn(1)–O(1)
Zn(1)–O(2)
Zn(1)–O(3)
1.976(4)
2.175(2)
2.312(2)
2.278(2)
Zn(1)–O(1)
Zn(1)–O(2)
Zn(1)–N(3)
2.183 (2)
2.134(2)
2.068(2)
Zn(1)–O(4)
Zn(1)–N(5)
2.155(2)
2.008(4)
Zn(1)–N(8)
2.027(3)
O(1)–Cu(1)–O(2)
O(1)–Cu(1)–N(1)
O(1)–Cu(1)–N(2)
O(2)–Cu(1)–N(1)
O(2)–Cu(1)–N(2)
N(1)–Cu(1)–N(2)
O(3)–Cu(2)–O(4)
O(3)–Cu(2)–N(3)
O(3)–Cu(2)–N(4)
O(4)–Cu(2)–N(3)
O(4)–Cu(2)–N(4)
N(3)–Cu(2)–N(4)
O(1)–Zn(1)–O(2)
O(1)–Zn(1)–O(3)
O(1)–Zn(1)–O(4)
O(1)–Zn(1)–N(5)
O(1)–Zn(1)–N(8)
O(2)–Zn(1)–O(3)
O(2)–Zn(1)–O(4)
O(2)–Zn(1)–N(5)
O(2)–Zn(1)–N(8)
O(3)–Zn(1)–O(4)
O(3)–Zn(1)–N(5)
O(3)–Zn(1)–N(8)
O(4)–Zn(1)–N(5)
O(4)–Zn(1)–N(8)
N(5)–Zn(1)–N(8)
80.85(10)
91.32(12)
167.21(12)
165.26(13)
93.26(11)
96.75(12)
78.30(10)
92.28(13)
169.95(12)
170.01(14)
92.10(12)
97.47(15)
67.15(8)
O(1)–Cu(1)–O(2)
O(1)–Cu(1)–N(1)
O(1)–Cu(1)–N(2)
O(1)–Cu(1)–N(3)a
O(2)–Cu(1)–N(1)
O(2)–Cu(1)–N(2)
O(2)–Cu(1)–N(3)a
N(1)–Cu(1)–N(2)
N(1)–Cu(1)–N(3)a
N(2)–Cu(1)–N(3)a
81.71(6)
92.57(6)
159.39(6)
76.07(5)
165.63(6)
91.77(6)
73.58(6)
97.88(6)
92.28(6)
120.94(6)
O(1)–Zn(1)–O(2)
O(1)–Zn(1)–N(3)
O(1)–Zn(1)–O(1)a
O(1)–Zn(1)–O(2)a
O(1)–Zn(1)–N(3)a
O(2)–Zn(1)–N(3)
O(2)–Zn(1)–O(2)a
O(2)–Zn(1)–N(3)a
N(3)–Zn(1)–N(3)a
71.86(5)
96.39(6)
180.00
108.14(5)
83.61(6)
98.15(6)
180.00
96.33(9)
158.58(8)
101.30(13)
96.05(13)
75.21(8)
95.02(8)
94.43(11)
160.46(13)
66.55(9)
154.22(13)
97.85(11)
91.48(13)
98.90(13)
98.83(14)
81.85(6)
180.00
Symmetry elementa = (−x, 1 − y, −z).
Fig. 1 Powder XRD patterns of complexes 1A and 1B.
Fig. 2 Left: UV-Vis spectra in methanol. Right: UV-Vis spectra in solid state.
11012 | Dalton Trans., 2012, 41, 11009–11017 This journal is © The Royal Society of Chemistry 2012

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Table 3 UV-Vis spectral parameters of the “metalloligand”, 1A and
1B
The copper atoms are each bonded to two oxygen atoms and
two nitrogen atoms of the Schiff-base ligand. Both the Cu–O
[Cu(1)–O(1) 1.925(2) Å, Cu(1)–O(2) 1.905(2) Å Cu(2)–O(3)
1.941(3) Å and Cu(2)–O(4) 1.915(2) Å] and Cu–N [Cu(1)–N(1)
1.963(3) Å, Cu(1)–N(2) 1.954(3) Å, Cu(2)–N(3) 1.965(3) Å and
Cu(2)–N(4) 1.976(4) Å] bond distances around the Cu(1) are
slightly shorter than those in Cu(2). The trans angles around
Cu(1) and Cu(2) are O(1)–Cu(1)–N(2) 167.21(12)°, O(2)–
Cu(1)–N(1) 165.26(13)°, O(3)–Cu(2)–N(4) 169.95(12)° and
O(4)–Cu(2)–N(3) 170.01(14)° indicate that Cu(1) suffers more
tetrahedral distortion than Cu(2) from the ideal square planar
geometry. The greater tetrahedral distortion around Cu(1) is also
clear in the dihedral angles between the two N–Cu–O planes,
which are 15.51 and 4.00° for Cu(1) and Cu(2) respectively.
This angle is 0° for a perfectly square-planar arrangement, and
90° for a perfect tetrahedral arrangement. The four donor atoms
in the equatorial plane show r.m.s. deviations of 0.188 and
0.049 Å for Cu(1) and Cu(2) respectively with the copper atoms
0.008(1) Å, 0.003(1) Å from the plane. The Zn(1)⋯Cu(1) dis-
tance [3.207(1) Å] is slightly greater than Zn(1)⋯Cu(2) distance
[3.188(1) Å]. The Cu⋯Cu separation is 4.475 Å.
λmax(nm) (ξ, M−1 cm−1
in CH3OH
)
λmax(nm) in solid
state
Complex
Metalloligand
1A
1B
598 (161) and 360 (18 940)
601(169) and 358 (31 088)
599 (193) and 357 (29 180)
594 and 370
580 and 365
627 and 360
The structure of 1B is shown in Fig. 4 together with the
atomic numbering scheme. Dimensions in the metal coordination
sphere are given in Table 2. The complex consists of a centro-
symmetric trinuclear structure showing all three metal atoms
(two terminal Cu atoms and central Zn atom) in a linear disposi-
tion. Thus the geometry is very different from that observed in
1A. The central zinc atom, Zn(1) has a six coordinate octahedral
environment being bonded to the four oxygen atoms of the two
different “metalloligands” at distances 2.183(2) and 2.134(2) Å.
The two trans axial positions are occupied by the bridging nitro-
gen atom N(3) of the azide at distance of 2.068(2) Å. It is to be
noted that the average Zn–O bond distance in 1B (2.159 Å) is
slightly shorter but the average Zn–N distance (2.068 Å) is
longer than those in 1A [av. Zn–O = 2.230 Å and av. Zn–N =
2.018 Å in 1A]. Due to the presence of centre of inversion, all
Fig. 3 The structure of complex 1Awith ellipsoids at 30% probability.
580 nm for 1A (no Cu–N axial bond) is 47 nm blue shifted than
that of the complex 1B (627 nm). The absence of the axial inter-
action around Cu(II) in 1A is expected to shift this absorption
band towards higher energy.20
Description of the structures
Both structures consist of trinuclear neutral molecules having
same general formula [(CuL)2Zn(N3)2]. Both the structures
contain two [CuL] moieties (where L represents the deprotonated,
tetradentate, di-Schiff-base ligand) held together by zinc azide.
The structure of 1A is shown in Fig. 3 together with the
atomic numbering scheme. Dimensions in the metal coordination
sphere are given in Table 2. The structure contains six-coordinate
zinc in a distorted octahedral environment together with two
four-coordinate square planar copper atoms. The zinc atom is
bonded to four oxygens from the two different “metalloligands”,
at distances 2.175(2), 2.312(2), 2.278(2) and 2.155(2) Å all of
which bridge to a copper atom. In addition, the zinc atom is
bonded to two mutually cis nitrogen atoms of terminal azide
ligands at distances of 2.008(4) and 2.027(3) Å which are con-
siderably shorter than Zn–O distances. Both the cis [66.55(9)–
101.30(13)°] and trans [154.22(13)–160.46(13)°] angles indicate
significant distortions from ideal octahedral geometry around
zinc atom.
Fig. 4 The structure of complex 1B with ellipsoids at 30% probability.
Dalton Trans., 2012, 41, 11009–11017 | 11013
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Fig. 5 Left: A scalene triangle formed by three metal atoms is indicated by the green lines for complex 1A. Right: Three metal atoms are collinear in
complex 1B as shown by the green line.
the trans angles are ideal (180°), but the cis angles [71.86(5)–
108.14(5)°] deviate considerably from the ideal value (90°).
The two terminal Cu(II) ions possess a penta-coordinate,
square pyramidal coordination sphere. The basal plane in the
copper ion is constituted by the two nitrogen atoms and two
oxygen atoms from the “metalloligand”. The azide ion bridges
the copper atom to the axial position at a distance Cu(1)–N(3)
2.584(2) Å to complete the square pyramidal geometry. The so
called Addison parameter (τ) is 0.104 for the copper atom. This
value is 0 for the ideal square pyramid and 1 for the trigonal
bipyramid.21 The average Cu–O bond distance in complex 1B
[1.936 Å] is longer than that [1.922 Å] in complex 1A but the
average Cu–N distance in complex 1B [1.957 Å] is shorter than
that [1.965 Å] in complex 1A. The r.m.s. deviation of the four
basally coordinating atoms from the mean plane passing through
them is 0.260 Å. The metal atom is 0.058(1) Å from this plane
towards the axially coordinating nitrogen atom. The two Zn⋯Cu
distance [2.935(2) Å] is considerably shorter than those found in
complex 1A but the Cu⋯Cu separation [5.870 Å] is much
longer than that of 1A as a consequence of the linear structure.
The two isomers crystallize in the different crystal systems
polymorphism is unsuitable in this case because two new bonds
(Cu–N) are created in the linear complex (1B) with respect to
angular species (1A). Therefore, we prefer to consider them as a
pair of “linear-bent” isomers where the binding mode of the
azide ions is different.
A CSD search (Nov 2011) reveals that a large number of com-
plexes in which [CuL] (H2L is a symmetrical tetradentate salen
type Schiff base ligand) acts as “metalloligand” have been
reported. There are 66 complexes in all, of which 24 are dinuc-
lear, 31 are trinuclear, 10 are tetranuclear and one is hexanuclear.
Among the trinuclear complexes 21 are linear and 10 are bent.
The trinuclear complexes in which Zn(II) is the central metal ion
are rare. There are only two such complexes and both of them
are linear: one containing bridging nitrite23 and the other con-
taining acetate.24 In both compounds, in addition to the phenox-
ido bridges, the anionic coligand also acts as a bridge between
the Cu(II) and Zn(II) which is a common feature of this type of
linear trinuclear structure. To our knowledge, complex 1A of the
present work is the first report of a bent trinuclear CuII2ZnII
system. Moreover, complexes 1A and 1B present the first
example of “linear-bent” isomerism for this type of trinuclear
complexes containing any metal ions with any anionic
coligands.
ˉ
(triclinic for 1A and monoclinic for 1B) with space group P1 for
1A and P21/c for 1B and their molecular geometries are signifi-
cantly different (Fig. 5). In 1A the two Cu atoms and the zinc
atom are located in the vertices of a scalene triangle in which
two terminally coordinated cis-azide ions hang from a common
vertex (containing Zn). However, in case of the other isomer
(1B) the three metal atoms are colinear where both trans-azide
ions link (μ-l,l-azido bridges) the central Zn(II) to a terminal
Cu(II). It should be noted that the term “linkage isomer” is
defined as “two or more coordination compounds in which the
donor atom of at least one of the ligands is different”.8 Therefore,
the two structures should not be considered as linkage isomers
as both of them are a result of the same donor atoms of the
ligands (Schiff base and azide). Again, the structures of 1A and
1B are fundamentally different from another interesting solid
state phenomenon, “polymorphism” which is described as “at
least two different arrangements of a molecules with identical
chemical composition and the same molecular connectivity in
the solid state but lead to identical liquid and vapor states with
the condition that no bonds are broken or created”.22 Although,
both the complexes have identical chemical compositions with
two different arrangements in the solid state, the term
Theoretical study
We also performed geometry optimization of 1A and 1B using
density functional theory (DFT) Becke 3 parameters Lee-Yang-
Parr functionals (B3LYP)25 with the 6-31G* basis set for all
atoms except for the effective core potential LANL2DZ basis set
for Cu and Zn.26 In the optimized structure of 1A, the central
Zn(II) coordinates six atoms to form a distorted octahedral struc-
ture similar to the crystal structure. It is bonded to four oxygens,
at distances 2.184, 2.259, 2.318 and 2.151 Å. While Zn–N
distances with azide ligands are 2.062 and 2.106 Å, which are
also considerably shorter than Zn–O distances. The Zn⋯Cu
distances are 3.211 and 3.188 Å. The Cu⋯Cu separation is
4.359 Å and the Cu–Zn–Cu angle is 85.9°. There are only small
differences between the calculated and solid state structures. For
1B, Zn is also coordinated by six atoms to form an octahedral
structure. It is bonded to the four oxygen atoms of the two differ-
ent salen complexes at distances 2.165 and 2.210 Å. The two
11014 | Dalton Trans., 2012, 41, 11009–11017
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Fig. 6 Stacking diagram of complexes 1A (Left) and 1B (Right). Distances in Å.
Fig. 7 Solid state EPR spectra of complexes 1A and 1B at room
temperature.
axial positions are occupied by the bridging nitrogen atom N of
the azide at a distance of 2.164 Å. The Cu–Zn–Cu and N–Zn–N
angles are 180°.
Fig. 8 EPR spectra of complexes 1A and 1B in methanol at 77 K.
Although the energy of 1A is 7.06 kcal mol−1 higher than 1B,
1A is also obtained experimentally. Inspection of the two iso-
meric structures from crystallography suggests that 1A is more
tightly packed with a wider contact surface area. As a result, the
cell volume of 1A (1622.56 Å3) is smaller than that of 1B
(1631.90 Å3). It is noted that van der Waals and π⋯π stacking
interactions27 could play major roles. Based on the X-ray struc-
ture, the π⋯π interaction in 1A is 3.468 Å, while that in 1B is
3.756 Å which is 0.288 Å longer than in 1A (Fig. 6). This also
supported by the DFT calculations in which the π⋯π interaction
in 1A is 3.243 Å, while that in 1B is 3.492 Å which is 0.249 Å
longer than in 1A (Fig. S3, ESI†). Then, the structure 1A is
better stabilized than 1B. Thus both 1A and 1B are likely to be
compatible in crystallization.
an intense signal at g⊥ ca. 2.055 for 1A and 2.066 for 1B having
a typical derivative line shape. These are well established fea-
tures in the EPR spectra of mainly mononuclear Cu(II) com-
plexes (S = 1/2) in polycrystalline samples. The trend in g value
(g|| > g⊥ > 2.00) in both the cases suggested that the unpaired
electron in the copper(II) ion is in the dx −y orbital. Therefore, a
square based geometry around the copper(II) is expected in both
the hetero-metallic complexes. The different splitting pattern in
the EPR signal in the solid state for both the complexes indicate
the presence of different square based environments around
copper(II). The hyperfine structure due to 63Cu having a nuclear
spin (I = 3/2) was not observed, perhaps due to a strong spin–
spin interaction, characteristic of concentrated paramagnetic
salts.
2
2
To reduce the broadening of the signal in the complexes and
to resolve the hyperfine interactions and super-hyperfine inter-
actions if any, the EPR spectra of these hetero-metallic com-
plexes in methanol were recorded at 77 K (Fig. 8) and the
parameters are given in Table 4. Both the complexes show very
similar splitting patterns in the EPR signal in solution. It consists
of the superposition of two spectra which can be divided into
two axially symmetric spectra. One is due to the mononuclear
complex (S = 1/2) and the other is assigned to the binuclear
complex (S = 1).28 The EPR spectra of the mononuclear
EPR spectra
Electron Paramagnetic Resonance (EPR) spectroscopy in present
paper is used to obtain the structural information about the two
CuII2ZnII hetero-metallic complexes. EPR spectra of 1A and 1B
were obtained in the solid state at room temperature and in
methanol (10−3 M) at 77 K. Room temperature EPR spectra
(Fig. 7) of both of the complexes in polycrystalline form consist
of a weak shoulder at g|| ca. 2.127 for 1A and 2.181 for 1B and
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Table 4 EPR parameters of complexes 1A and 1B in methanolic solution
Complex
gzz
gyy
gxx
giso
Azz(G)
Ayy
Axx
D(G)
a
(1A)1
2.2487
2.1336
2.2503
2.1279
2.0518
2.0638
2.0492
2.0589
2.0095
2.0638
2.0083
2.0589
2.1033
2.0871
2.1026
2.0819
164
70
160
66
—
—
—
—
—
—
—
—
(1A)2
(1B)1
(1B)2
287
291
a 1 for mononuclear and 2 for binuclear species.
complex consists of an axially symmetric EPR spectrum with
parallel components (g|| = 2.25) and an intense perpendicular
component (g⊥ = 2.05). The quartet hyperfine structure has been
observed only on the parallel component due to the interaction
of unpaired electrons of copper(II) with 63,65Cu having nuclear
spin I = 3/2. The EPR spectra of the binuclear complex consists
of an intense doublet (g ca. 2.05) due to fine-structure transitions
(ΔMs = 1) having a zero-field splitting (ZFS) of about 290 G
confirming the formation of a binuclear copper complex (S = 1).
The presence of a seven line hyperfine structure (2nI + 1
hyperfine component, for binuclear copper complexes n = 2 and
I = 3/2 for 63Cu) also confirmed the formation of the binuclear
complex. The similar splitting patterns in the EPR spectra of
both complexes in solution indicate that they lose their solid
state identity and become identical in solution.
From the X-ray crystal structure, it is clear that complexes 1A
and 1B exhibit distorted square planar geometry around the
Cu(II) ions consisting of two nitrogen and two oxygen atoms
with unequal bond lengths. In addition to the regular four-fold
coordination, 1B exhibits axial coordination of nitrogen [Cu–
N(apical) > Cu–N(equatorial)] from the azide group. The EPR
signals in solution seem to be due to the formation of mono-
nuclear species with [CuN2O2] moieties and a binuclear
complex with the [Cu2N4O4] moiety. The existence of mono-
nuclear and binuclear species is also confirmed by ESI mass
spectra. If only these species show EPR signals in solution, one
can expect the superhyperfine structure in EPR spectra for these
complexes. The absence of a superhyperfine structure in the
present EPR studies may be because of the presence of
in-equivalent nitrogens in the coordination polyhedra around the
Cu(II) ions.
no peak corresponding to the molecular ion of the trinuclear
species is observed for any of the compounds and both of them
show the presence of almost identical species. Moreover Zn(II) is
found only as an isolated species, [Zn(CH3OH)4(N3)2Na]+.
Therefore, from the mass spectra it can be concluded that in
methanolic solution the identity of both the trinuclear complexes
is lost. The presence of a seven line hyperfine structure in the
EPR spectra due to binuclear Cu(II) complexes in methanolic
solution is also supported by mass spectroscopy.
Conclusions
Two hetero-metallic CuII2ZnII complexes have been synthesised
and characterized by X-ray crystallography and other spectro-
scopic analyses. From X-ray crystal structures, it is evident that
1A is an angular trinuclear species where azide ions act as
monodentate ligands whereas 1B is linear with μ-l,l bridging
azide ion. These two complexes (1A and 1B) have the same
chemical formulation but different structures. However, they can
not be termed as “linkage isomers” or “polymorphs”; instead
they present a new class of isomerism. We termed this unprece-
dented class as “linear-bent” isomerism. The terminal and EO
coordination modes of the azide ion are responsible for the
creation of these two isomers. Electronic spectra, EPR spectra
and electrospray ionization mass spectrometry reveal that these
two complexes exist in the solid state only but they lose their
identity in solution and become identical. A DFT calculation
shows that isomer 1B is more (7.06 kcal mol−1) stable than
isomer 1A. However, the energy of dimerization is more favor-
able in 1A through the stronger crystal packing that compensates
for the difference in the relative energy of the isomers. As a
result, both complexes become isolable in the solid state.
Electrospray ionization mass spectrometry
The electrospray ionization mass spectra (ESI-MS positive) of
1A and 1B, were recorded in methanolic solution (Fig. S4 and
S5, ESI†). The spectra of both complexes show a similar pattern.
The spectra of complex 1A shows the base peak at m/z = 708.85
(100%) which can be assigned to the cationic species
[(CuL)2Na]+ and a peak at m/z = 686.92 due to the [(CuL)2H]+.
The other peaks at m/z = 343.96, 365.94 and 299.05 are due the
presence of cationic species [(CuL)H]+, [(CuL)Na]+ and [Zn
(CH3OH)4(N3)2Na]+. On the other hand, complex 1B displays
the base peak at m/z = 343.97 (100%) can be assigned to the cat-
ionic “metalloligand” [(CuL)H]+. The appearance of other peaks
at 365.95, 686.91, 708.89 and 299.08 may be attributed to the
presence of [(CuL)Na]+, [(CuL)2H]+, [(CuL)2Na]+ and
[Zn(CH3OH)4(N3)2Na]+ respectively in 1B. It is to be noted that
Acknowledgements
L. K. D is thankful to CSIR, India, the award of a Junior
Research Fellowship [Sanction No. 09/028 (0805)/2010-
EMR-I]. Crystallography was performed at the DST-FIST, India,
funded Single Crystal Diffractometer Facility at the Department
of Chemistry, University of Calcutta. We also thank
Dr R. M. Kadam, Radiochemistry Division, BARC, for his valu-
able suggestions regarding the investigation of EPR spectra.
References
1 S. F. A. Kettle, Physical Inorganic Chemistry, Oxford University Press,
2002, 43.
11016 | Dalton Trans., 2012, 41, 11009–11017
This journal is © The Royal Society of Chemistry 2012

View Article Online
2 (a) G. B. Kauffman, J. Chem. Educ., 1959, 36, 521; (b) A. Werner and
V. L. King, Ber. Dtsch. Chem. Ges., 1911, 44, 1887; (c) D. Das,
I. R. Laskar, A. Ghosh, A. Mondal, K.-i. Okamoto and N. R. Chaudhuri,
J. Chem. Soc., Dalton Trans., 1998, 3987; (d) S. S. Massoud, Polyhe-
dron, 1994, 13, 3127.
3 C. Adhikary and S. Koner, Coord. Chem. Rev., 2010, 254, 2933.
4 (a) S. Naiya, C. Biswas, M. G. B. Drew, C. J. Gómez-García,
J. M. Clemente-Juan and A. Ghosh, Inorg. Chem., 2010, 49, 6616;
(b) A. Escuer and G. Aromí, Eur. J. Inorg. Chem., 2006, 4721 and refer-
ences therein; (c) M. A. S. Goher and T. C. W. Mak, Inorg. Chim. Acta,
1985, 99, 223; (d) G. S. Papaefstathiou, S. P. Perlepes, A. Escuer,
R. Vicente, M. Font-Bardia and X. Solans, Angew. Chem., Int. Ed., 2001,
40, 884.
11 (a) J. M. Epstein, B. N. Figgis, A. H. White and A. C. Willis, J. Chem.
Soc., Dalton Trans., 1974, 1954; (b) F. Ercan and O. Atakol, Z. Kristal-
logr., 2006, 221, 735.
12 (a) Y.-F. Song, G. A. V. Albada, J. Tang, I. Mutikainen, U. Turpeinen,
C. Massera, O. Roubeau, J. S. Costa, P. Gamez and J. Reedijk, Inorg.
Chem., 2007, 46, 4944; (b) Y. Song, P. Gamez, O. Roubeau,
I. Mutikainen, U. Turpeinen and J. Reedijk, Inorg. Chim. Acta, 2005,
358, 109; (c) A. Datta, C. R. Choudhury, P. Talukder, S. Mitra,
L. Dahlenburg and T. Matsushita, J. Chem. Res., 2003, 642;
(d) B. M. Ates, F. Ercan, I. Svoboda, H. Fuess and O. Atakol, Acta Crys-
tallogr., Sect. E: Struct. Rep. Online, 2008, 64, m481; (e) Y. F. Song,
G. A. V. Albada, M. Quesada, I. Mutikainen, U. Turpeinen and
J. Reedijk, Inorg. Chem. Commun., 2005, 8, 975.
5 (a) J. Ribas, A. Escuer, M. Monfort, R. Vicente, R. Cortěs, L. Lezama
and T. Rojo, Coord. Chem. Rev., 1999, 193–195, 1027; (b) S. Dalai,
P. S. Mukherjee, T. Mallah, M. G. B. Drew and N. R. Chaudhuri, Inorg.
Chem. Commun., 2002, 5, 472; (c) S. Mukherjee, B. Gole, Y. Song and
P. S. Mukherjee, Inorg. Chem., 2011, 50, 3621; (d) P. S. Mukherjee,
T. K. Maji, A. Escuer, R. Vicente, J. Ribas, G. Rosair, F. A. Mautner and
N. R. Chaudhuri, Eur. J. Inorg. Chem., 2002, 943; (e) S. Mukherjee,
Y. P. Patil and P. S. Mukherjee, Dalton Trans., 2012, 41, 54.
13 M. G. B. Drew, R. N. Prasad and R. P. Sharma, Acta Crystallogr., Sect.
C: Cryst. Struct. Commun., 1985, 41, 1755.
14 SAINT, version 6.02; SADABS, version 2.03, Bruker AXS, Inc., Madison,
WI, 2002.
15 G. M. Sheldrick, SHELXS 97, Program for Structure Solution, University
of Göttingen, Germany, 1997.
16 G. M. Sheldrick, SHELXL 97, Program for Crystal Structure Refinement,
University of Göttingen, Germany, 1997.
6 G. S. Papaefstathiou, A. Escuer, R. Vicente, M. Font-Bardia, X. Solans
and S. P. Perlepes, Chem. Commun., 2001, 2414.
17 A. L. Spek, PLATON. Molecular Geometry Program, J. Appl. Crystal-
logr., 2003, 36, 7.
7 S. Naiya, S. Biswas, M. G. B. Drew, C. J. Gómez-García and A. Ghosh,
Inorg. Chim. Acta, 2011, 377, 26.
8 P. Chaudhuri, T. Weyhermtiller, E. Bill and K. Wieghardt, Inorg. Chim.
Acta, 1996, 252, 195.
9 (a) P. Mukherjee, M. G. B. Drew, A. Figuerola and A. Ghosh, Polyhe-
dron, 2008, 27, 3343; (b) S. Biswas, S. Naiya, M. G. B. Drew,
C. Estarellas, A. Frontera and A. Ghosh, Inorg. Chim. Acta, 2011, 366,
219; (c) S. Biswas and A. Ghosh, Polyhedron, 2011, 30, 676;
(d) S. Biswas, S. Naiya, C. J. Gómez-García and A. Ghosh, Dalton
Trans., 2012, 41, 462; (e) S. Biswas and A. Ghosh, Indian J. Chem.,
Sect. A: Inorg., Bio-inorg., Phys., Theor. Anal. Chem., 2011, 50A, 1356;
(f) L. K. Das, A. Biswas, A. Frontera and A. Ghosh, Polyhedron, 2012,
DOI: 10.1016/j.poly.2012.04.029.
18 L. J. J. Farrugia, J. Appl. Crystallogr., 1997, 30, 565.
19 L. J. J. Farrugia, J. Appl. Crystallogr., 1999, 32, 837.
20 S. Roy, P. Mitra and A. K. Patra, Inorg. Chim. Acta, 2011, 370, 247.
21 A. W. Addison, T. N. Rao, J. Reedijk, J. Van Rijn and G. C. Verschoor,
J. Chem. Soc., Dalton Trans., 1984, 1349.
22 J. Elguero, Cryst. Growth Des., 2011, 11, 4731.
23 D. Ulku, L. Tatar, O. Atakol and S. Durmus, Acta Crystallogr., Sect. C:
Cryst. Struct. Commun., 1999, 55, 1652.
24 C. Fukuhara, K. Tsuneyoshi, N. Matsumoto, S. Kida, M. Mikuriya and
M. Mori, J. Chem. Soc., Dalton Trans., 1990, 3473.
25 A. D. Becke, J. Chem. Phys., 1993, 98, 5648.
26 C. E. Check, T. O. Faust, J. M. Bailey, B. J. Wright, T. M. Gilbert and
L. S. Sunderlin, J. Phys. Chem. A, 2001, 105, 8111.
10 (a) Z.-L. You and H.-L. Zhu, Z. Anorg. Allg. Chem., 2004, 630, 2754;
(b) Y.-F. Feng, H.-Y. Wu, B.-L. Zhu, S.-R. Wang and W.-P. Huang, Acta
Crystallogr., Sect. E: Struct. Rep. Online, 2007, 63, m1107;
(c) S. Thakurta, J. Chakraborty, G. Rosair, J. Tercero, M. S. E. Fallah,
E. Garribba and S. Mitra, Inorg. Chem., 2008, 47, 6227; (d) R. Shakya,
A. Jozwiuk, D. R. Powell and R. P. Houser, Inorg. Chem., 2009, 48,
4083.
27 (a) K. S. Kim, P. Tarakeshwar and J. Y. Lee, Chem. Rev., 2000, 100,
4145; (b) E. C. Lee, D. Kim, P. Jurečka, P. Tarakeshwar, P. Hobza and
K. S. Kim, J. Phys. Chem. A, 2007, 111, 3446; (c) I. Geronimo,
E. C. Lee, N. J. Singh and K. S. Kim, J. Chem. Theory Comput., 2010, 6,
1931.
28 S. D. Pramodini, R. K. S. Hemakumar and R. M. Kadam, Inorg. Chem.,
2006, 45, 2193.
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