Exploring the coordinative adaptation and molecular shapes of trinuclear Cu II 2 MII (M = Zn/Cd) complexes derived from salen type Schiff bases: Structural and theoretical studies

Article abstract of DOI:10.1039/c5dt04941e

Three new trinuclear hetero-metallic complexes [(CuL)₂Zn(NCS)₂] (1), [(CuL^R)₂Zn(NCS)(μ_1,1-NCS)] (2) and [(CuL^R)₂Cd(μ_1,3-NCS)₂] (4) have been synthesized using [CuL] and [CuL^R] as "metalloligands" (where H₂L = N,N′-bis(salicylidene)-1,3-propanediamine and H₂L^R = N,N′-bis(2-hydroxybenzyl)-1,3-propanediamine). All three complexes are characterized by elemental analysis, spectroscopic methods and single crystal XRD. Complex 1 is an angular trinuclear species, in which two terminal four-coordinate square planar "metalloligands" [CuL] are coordinated to a central Zn(ii) through double phenoxido bridges along with two mutually cis nitrogen atoms of terminal isothiocyanate ions as is usually found in such complexes. In contrast, in complex 2, the two terminal "metalloligands" [CuL^R] are square pyramidal, as one of the SCN^- ions makes an unusual μ_1,1-NCS bridge between copper centers while the other one coordinates to Zn(ii) through a N atom in a usual fashion making its geometry also square pyramidal. For 4 which possesses an angular trinuclear structure, in addition to double phenoxido bridges from two terminal [CuL^R], both the SCN^- ions are S-bonded to Cd(ii) and form a bridge (cis-μ_1,3-SCN) between Cd(ii) and each of the terminal Cu(ii) ions. This structure is different from its unreduced analogue in which NCS^- was N-terminal coordinated to Cd(ii) (3/3′). All the structures have been optimized using density functional theory (DFT) calculations. It has been found that for H₂L, optimized structures like 1 and 2 differ only by 0.4 kcal mol^-1 but the H₂L^R structure 2 is more stable by 5.5 kcal mol^-1 than the structure resembling 1. For Cd(ii) complexes also, H₂L optimized structures such as 3 and 4 do not differ significantly in energy (1.0 kcal mol^-1) but the H₂L^R structure 4 is more stable than that of 3 by 4.6 kcal mol^-1. In fact, structure 4 has been found to be the most stable one among the other possible isomers of H₂L^R.

Full text of DOI:10.1039/c5dt04941e

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Exploring the coordinative adaptation and molecular shapes of the trinuclear
CuII MII (M = Zn/Cd) complexes derived from salen type Schiff bases:
2
Structural and theoretical studies
Alokesh Hazari,a,b Lakshmi Kanta Das,c Antonio Bauzá,d Antonio Frontera,d* and
Ashutosh Ghosha*
aDepartment of Chemistry, University College of Science, University of Calcutta, 92, A.P.C.
Road, Kolkataꢀ700 009, India. Eꢀmail: ghosh_59@yahoo.com
bDepartment of Chemistry, Government General Degree College Singur, Jalaghata, Hooghlyꢀ
712409
cDepartment of Chemistry, Government General Degree College KharagpurꢀII, Ambigeria,
Madpur, Paschim Medinipurꢀ721149
dDepartament de Química, Universitat de les IllesBalears, Crta. deValldemossa km 7.5, 07122
Palma de Mallorca (Baleares), Spain; E–mail: toni.frontera@uib.es
Three new trinuclear heteroꢀmetallic complexes [(CuL)2Zn(NCS)2] (1), [(CuLR)2Zn(NCS)(
and [(CuLR)2Cd( 1,3ꢀNCS)2] (4) have been synthesized using [CuL] and [CuLR] as “metalloligands”
ꢁ1,1ꢀNCS)] (2)
ꢁ
(where H2L = N,N′ꢀbis(salicylidene)ꢀ1,3ꢀpropanediamine and H2LR = N,N′ꢀbis(2ꢀhydroxybenzyl)ꢀ1,3ꢀ
propanediamine). All three complexes are characterized by elemental analysis, spectroscopic methods and
single crystal XRD. Complex 1 is an angular trinuclear species, in which two terminal fourꢀcoordinate
square planar “metalloligands” [CuL] are coordinated to a central Zn(II) through double phenoxido
bridges along with two mutually cis nitrogen atoms of terminal isothiocyanate ions as is usually found in
such complexes. In contrast, in complex 2, the two terminal “metalloligands” [CuLR] are square
pyramidal, as one of the SCN− ions makes an unusual ꢁ1,1−NCS bridge between copper centers while the
other one coordinates to Zn(II) through N atomin a usual fashion making its geometry also square
pyramidal. For 4 which possesses an angular trinuclear structure, in addition to double phenoxido
bridges from two terminal [CuLR], both the SCN− ions are Sꢀbonded to Cd(II) and form a bridge (cisꢀ
ꢁ1,3−SCN) between Cd(II) and each of the terminal Cu(II) ions. This structure is different from its
unreduced analogue in which NCS− was Nꢀterminal coordinated to Cd(II) (3/3′). All the structures have
been optimized using density functional theory (DFT) calculations. It has been found that for H2L,
optimized structures like 1 and 2 differ only by 0.4 kcal/mol but for H2LR structure 2 is more stable by 5.5
kcal/mol than the structure resembling 1. For Cd(II) complexes also, with H2L optimized structures as 3
and 4 do not differ significantly in energy (1.0 kcal/mol) but for H2LR structure 4 is more stable than that
of like 3 by 4.6 kcal/mol. In fact, structure 4 has been found to be the most stable one among the other
possible isomers with H2LR.
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Introduction
Interest in the coordination complexes containing thiocyanate ion, a linear triatomic
pseudohalide1, is growing because of its various coordination modes which can broadly be
classified as terminal or bridging (Scheme 1)2ꢀ5. Two types of terminal coordination of SCN−
ligand are observed: Sꢀcoordinated SCN− termed as thiocyanate6 (I, Scheme 1) and Nꢀ
coordinated SCN−, known as isothiocyanate7 (II, Scheme 1). The former is usually bent with M–
S–C angle ranging between 80 and 110° whereas in the latter M–N–C angles vary within a range
of 110–180°.6ꢀ7 On the other hand, for bridging SCN− ligand, thirteen multidentate bridging
modes are possible theoretically but not all of them have been observed.6ꢀ7 The most commonly
observed bridging modes are ꢁ1,1ꢀSCN (III)8, ꢁ1,1ꢀNCS (IV)9 and cisꢀꢁ1,3ꢀNCS (V)10. In few
cases, rare ꢁ1,1,3ꢀSCN (VI)11 and ꢁ1,1,1,3ꢀSCN (VII)12 modes have also been found. The observed
coordination mode of thiocyanate depends on several factors e.g. hardness13 and coordination
number of the metal centers14, nature of coligands and their stochiometric ratios15ꢀ16, steric,
electronic and symbiotic effects17ꢀ19, etc. Recently, we have shown that Hꢀbonding can also be
the determining factor for a particular coordination mode in Cd(II).15a
In our recent studies, on the heteroꢀmetallic complexes, synthesized with the help of
“metalloligand” [ML] [where H2L = N,N′ꢀbis(salicylidene)ꢀ1,3ꢀpropanediamine or N,N′ꢀ
bis(salicylidene)ꢀ1,4ꢀbutanediamine or N,N′ꢀbis(αꢀmethylsalicylidene)ꢀ1,3ꢀpropanediamine etc.
M = Cu(II)/Ni(II)], it is shown that the trinuclear species are conformationally very flexible and
can exist in various shapes that allow different coordination modes of anionic coligands.20ꢀ24 It is
rather fascinating to observe that “metalloligands” with the reduced Schiff bases produce
complexes having very different shapes and bonding compared to their unreduced
analogues.15a,25 These phenomena encourage us to synthesize some more trinuclear complexes
using the unreduced and reduced Schiff bases exploiting the competitive coordination modes of
thiocyanate ligand. We would also like to investigate the effect of central metal ion on the shapes
of the molecule and coordination modes of thiocyanate.
Here, we present the syntheses and Xꢀray structures of three new coordination complexes
[Zn(CuL)2(NCS)2] (1), [Zn(CuLR)2(NCS)(ꢁ1,1ꢀNCS)] (2), and [Cd(CuLR)2(ꢁ1,3ꢀNCS)2] (4).
Complex 1 is derived from unreduced Schiff base and complexes 2 and 4 are from reduced
Schiffꢀbase ligands. 1 is a bent trinuclear with Nꢀcoordinated SCN– as is usually found in many
such species while in complex 2, one of the SCN− is coordinated to Zn(II) with N and the other
2

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one forms a bridge (ꢁ1,1ꢀNCS) between the two Cu(II). On the other hand, in complex 4, two
SCN− ions form bridges (ꢁ1,3ꢀSCN) between Cd(II) and each of the terminal Cu(II) ions, being
cisꢀSꢀbonded to Cd(II). Complex 4 is a very different structure from its unreduced analogue
reported previously (3/3′)20b where the two thiocyanates are cisꢀNꢀbonded to the central Cd(II).
Theoretical study has been performed using DFT calculations at the BP86/def2ꢀTZVP level of
theory to rationalize the differences in the structures and bonding in these and similar complexes
of Zn(II) and Cd(II) with unreduced and reduced Schiff base ligands.
Scheme 1 Observed coordination modes of SCN− ion.
Experimental
Starting Materials.
The salicylaldehyde, 1,3ꢀpropanediamine and sodium borohydride were purchased from
Lancaster and were of reagent grade. They were used without further purification.
Caution! Perchlorate 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.
Synthesis of the Schiff Base Ligand, N,N′ꢀbis(salicylidene)ꢀ1,3ꢀpropanediamine (H2L)
andthe “metalloligand” [CuL]
The Schiff base ligand was synthesized by a standard method.26 5 mmol of 1,3ꢀpropanediamine
(0.42 mL) was mixed with 10 mmol of the salicylaldehyde (1.04 mL) in methanol (20mL). The
resulting solution was refluxed for ca. 2 h and allowed to cool. This yellow colored methanolic
solution was added to a methanolic solution (20 mL) of Cu(ClO4)2·6H2O (1.852 g, 5 mmol) to
prepare the “metalloligand” [CuL].27
Synthesis of the reduced Schiff base ligand, N,N'ꢀbis(2ꢀhydroxybenzyl)ꢀ1,3ꢀpropanediamine
(H2LR) and its “metalloligand” [CuLR]
The diꢀSchiff base ligand (H2L) of 1,3ꢀpropanediammine (0.42 mL) and salicyldehyde (1.04 mL)
was synthesized as stated above. Then 20 mL (5 mmol) of this prepared methanolic ligand
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solution (H2L) was cooled to 0°C, and solid sodium borohydride (380 mg, 10 mmol) was added
slowly to this methanolic solution with stirring. After completion of addition, the resulting
solution was acidified with concentrated HCl (10 mL) and then evaporated to drynesson a water
bath.28 The reduced Schiff base ligand H2LR was extracted from the solid mass with methanol,
and this methanol solution (ca. 20 mL) was used for the preparation of “metalloligand” (CuLR)
following a procedure similar to that for [CuL].
Syntheses of the complexes [(CuL)2Zn(NCS)2] (1) and [(CuLR)2Zn(NCS)(µ1,1ꢀNCS)] (2)
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 solution
(1 mL) of ammonium thiocyanate (0.159 g, 2 mmol), was added to this solution. The mixture
was stirred for 1 h and then filtered. The filtrate was allowed to stand overnight when dark green
prismatic shaped Xꢀray quality single crystals of 1 appeared at the bottom of the vessel. The
bright green rhombic shaped single crystal of complex 2 was synthesized by the same procedure
as described for complex 1 but using the “metalloligand” [CuLR] instead of [CuL]. The crystals
were washed with a methanolꢀwater mixture and dried in desiccators containing anhydrous
CaCl2 and then characterized by elemental analysis, spectroscopic methods and single crystal Xꢀ
ray diffraction.
Complex 1: Yield: 0.626 g (72%). Anal.calc. for: C36H32Cu2N6O4S2Zn, C 49.74; H 3.71; N 9.67
%; found: C 49.52; H 3.68; N 9.85%. IR (KBr): ν(C=N) 1614 cm−1, ν(SCN) 2091 and 2063
cm−1, ν(CS) 856 cm−1. UV/vis: [λmax in nm (εmaxin M−1 cm−1)] (MeCN) = 601(167), 356(19672)
and λmax (solid, reflectance) = 595 and 372 nm.
Complex 2: Yield: 0.570 g (65%). Anal.calc. for: C36H40Cu2N6O4S2Zn, C 49.28; H 4.60; N
9.58%; found: C 49.32; H 4.56; N 9.82%. IR (KBr): ν(N–H) 3208 cm−1, ν(SCN) 2080 and 2010
cm−1, ν(CS) 885 and 786 cm−1. UV/vis: [λmax in nm (εmaxin M−1 cm−1)] (MeCN) = 591(135),
393(16500) and λmax (solid, reflectance) = 596 and 400 nm.
Syntheses of the complex [(CuLR)2Cd(µ1,3ꢀNCS)2] (4)
Complex 4 was synthesised of following the similar procedure as described for complex 2 but
using anhydrous Cd(ClO4)2 (0.311 g, 1 mmol) instead of Zn(ClO4)2·6H2O(0.372 g, 1 mmol).The
square shaped green colored crystals of 4 were washed with a methanol-water mixture and dried
in a desiccators containing anhydrous CaCl2 and then characterized by elemental analysis,
spectroscopic methods, and Xꢀray diffraction.
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Complex 4: Yield: 0.572 g (62%). Anal.calc. for: C36H40CdCu2N6O4S2, C 46.78; H 4.36; N
9.09%; found: C 46.52; H 4.56; N 9.22%. IR (KBr): ν(N–H) 3226 cm−1, ν(SCN) 2078 and 2063
cm−1, ν(CS) 765 cm−1. UV/vis: [λmax in nm (εmax in M−1 cm−1)] (MeCN) = 587(346),
400(14207); λmax (solid, reflectance) = 616 and 399 nm.
Physical Measurements
Elemental analyses (C, H, and N) were performed using a PerkinꢀElmer 2400 series II CHN
analyzer. IR spectra in KBr pellets (4000−400 cm−1) were recorded using a Perkinꢀ Elmer RXI
FTꢀIR spectrophotometer. Electronic spectra in methanol (800−300 nm) and solid state
(800−300 nm) were recorded in a Hitachi U3501 spectrophotometer. Powder Xꢀray diffraction
patterns are recorded on a powder Xꢀray diffractometer (Model: X’Pert Powder, PANalytical) at
45 kV voltage and 40 mA current and calibrated with a standard silicon sample, using Niꢀfiltered
CuꢀKα (λ = 0.15418 nm) radiation.
Crystallographic data collection and refinement
Wellformed single crystals of each complex was mounted on a BrukerꢀAXS SMART APEX II
diffractometer equipped with a graphite monochromator and Mo Kα (λ = 0.71073 Å) radiation.
The crystals were positioned 60 mm from the CCD, and frames (360) were measured with a
counting time of 5 s. The structures were solved using the Patterson method through the
SHELXS 97 program. Non hydrogen atoms were refined with independent anisotropic
displacement parameters, while difference Fourier synthesis and leastꢀsquares refinement
showed the positions of any remaining nonꢀhydrogen atoms. The nonꢀhydrogen atoms were
refined with anisotropic thermal parameters. The hydrogen atoms bound to carbon atoms were
included in geometric positions and given thermal parameters equivalent to 1.2 (or 1.5 for methyl
groups) times those of the atom to which they were attached. Hydrogen atoms that bonded to N
or O were located in a difference Fourier map and refined with distance constraints. Successful
convergence was indicated by the maximum shift/error of 0.001 for the last cycle of the leastꢀ
squares refinement. The structures of 1 has a disorder in the position of C20 and the disorder has
been refined isotropically. In this structure a total of six hydrogen atoms [two each at C19, C20
and C21] can not be located from Fourier map. The data quality for 4 are not good because the
crystal gave a weak diffraction pattern. Absorption corrections were carried out using the
SADABS program,29 while all calculations were made via SHELXS 97,30 SHELXL 97,31
PLATON 99,32 ORTEPꢀ32,33 WINGX system verꢀ1.6434a and refined using SHELXLꢀ201434b,c
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Data collection, structure refinement parameters, and crystallographic data for the three
complexes are given in Table 1.
Theoretical methods
The energies of all complexes included in this study were fully optimized at the BP86/def2ꢀ
TZVP level of theory. We have used the crystallographic coordinates as starting points for the
optimizations. The calculations have been performed by using the program TURBOMOLE
version 7.0.35 Solvent effects have been taken into account using the COSMO continuum
model.36
Results and discussion
Syntheses the complexes
The Schiffꢀbase ligand (H2L), the reduced Schiffꢀbase ligand (H2LR) and their Cu(II) complexes,
[CuL] and [CuLR] were synthesized using the reported procedures.26ꢀ28Both the “metalloligands”,
[CuL] and [CuLR] on reaction with zinc perchlorate hexahydrate and ammonium thiocyanate in
2:1:2 molar ratio in MeOHꢀH2O medium (10:1, v/v) resulted in two new trinuclear heteroꢀ
metallic complexes, [(CuL)2Zn(NCS)2] (1) and [(CuLR)2Zn(NCS)(ꢁ1,1ꢀNCS)] (2), respectively.
Complex [(CuLR)2Cd(ꢁ1,3−NCS)2] (4) was prepared similarly by using [CuLR] and anhydrous
cadmium perchlorate (Scheme 2). [CuL] was used to prepare complexes 3/3′ as reported
earlier.20b We have measured the powder Xꢀray diffraction patterns for all three new complexes.
The diffraction patterns confirm their phase purity (Figs. S1ꢀS3 in SI)
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Scheme 2. Formation of complexes 1, 2, 4 (new) and 3/3′(reported earlier).
IR and solid state UVꢀVis spectra
Besides elemental analysis, all the complexes were initially characterized by IR spectra. The
“metalloligands” [CuL] and [CuLR] are neutral and do not have any anionic coligand, whereas
complexes, 1, 2 and 4 contain thiocyanate ion (Figs. S4−S6, in SI). In the spectra of complex 1, a
strong and sharp band at 1614 cm−1 due to the azomethane ν(C=N) group of the Schiff base is
found.37 On the other hand, there is no band in the region 1620−1650 cm−1 in the spectra of 2
and 4 due to the reduction of the imines group.38 Moreover, a moderately strong and sharp peak
at 3208 and 3226 cm−1 due to a N−H stretching vibration, for complexes 2 and 4 respectively,
also shows that the imines group of the Schiff base is reduced.38 In addition, all the complexes
show the characteristic CN bands for thiocyanato ligand. In the spectra of all three complexes
this CN band is split with peaks at 2091, 2080 and 2078 cm−1 along with shoulders at 2063, 2010
and 2063 cm−1 for 1, 2 and 4, respectively (Figs. S4, S5 and S6 in SI).The splitting of the band in
case of complex 2 is indicative of the presence of two different types of coordinated thiocynato
ions in agreement with its crystal structure. Although complexes 1 and 4 contain only one type of
coordinated thiocyanate (Nꢀterminal for 1 and µ1,3ꢀSCN for 4) but bond distances and angles of
the two SCN− ions in each of the complexes are slightly diferent which presuamably explain the
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splitting of CN bands. On the other hand, the CS stretching bands for thiocyanato ligands are of
low intensity and are overlapped with other bands in the same region of the respective
complexes. Nevertheless, by comparing the spectra of similar compounds we assigned the CS
stretching bands at 856 and 765 cm−1 for 1 and 4 respectively. For complex 2, two CS stretching
bands at 885 and 786 cmꢀ1 were observed attributable to the presence of two different types of
thiocyanato ligands. We have also compared the theoretical (DFT BP86/def2ꢀTZVP) and
experimental stretching CN and CS values of the thiocyanate ligands. The results are
summarized in Table S6 and a very good agreement has been found between the theoretical and
experimental values, thus validating the experimental assignment and giving reliability to the
level of theory used in this manuscript.
The electronic spectra of these complexes were recorded in acetonitrile solutions and in
solid state.The synthesized complexes show a broad absorption band in the visible region at 601,
591, and 587 nm in acetonitrile (Fig. S8) and 595, 596, and 616 nm in the solid state (Fig. S9)
for 1, 2 and 4 respectively, while the band maxima for “metalloligands” [CuL] and [CuLR]
appear at 602 and 605 nm in solution andat 600 and 635 nm in solid, respectively attributed to
d−d transitions of Cu(II) ions. Besides these bands, an absorption band in the range of 356−410
nm was observed in acetonitrile solution (Fig. S7) as well as in solid state (Fig. S9), assignable to
ligandꢀtoꢀmetal charge transfer transitions for all three complexes and two “metalloligands”,
[CuL] and [CuLR].
Structures description of the complexes
Complex 1
The angular trinuclear structure of 1 having molecular formula [(CuL)2Zn(NCS)2] and consisting
of two terminal “metalloligand” [CuL] (where H2L
=
N,N'ꢀbis(salicylidene)ꢀ1,3ꢀ
propanediamine), one central Zn(II) ion and two isothiocyanato ligands is shown in Fig. 1
together with the atomic numbering scheme. Dimensions of the metal coordination sphere of 1
are given in Table S1 in Supporting Information. Each copper atom is bonded to two oxygen
atoms and two nitrogen atoms of the dianionic tetradentate Schiff base ligand [L2ꢀ]. Both the Cu–
O bond distances are shorter than Cu–N distances [Table S1]. The trans angles around the Cu
centers [168.23(1)–169.30(1)° for Cu(1) and 170.31(1)–171.16(1)° for Cu(2)] indicate that they
suffer a tetrahedral distortion from the ideal square planar geometry. The tetrahedral distortion
around Cu centers is also apparent from the dihedral angles [Table S3] between the two N–Cu–O
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planes, which are 9.13(1)° and 1.24(1)° for Cu(1) and Cu(2), respectively. This angle is 0° for a
perfectly squareꢀplanar arrangement, and 90° for a perfect tetrahedral arrangement. This
geometry is also confirmed by the soꢀcalled τ4 index39 that measures the distortion between a
perfect tetrahedron (
[360°– (α + β)]/141°, with α and β (in °) being the two largest angles around the metal center.
The 4 values for Cu(1) and Cu(2) are 0.159 and 0.131, respectively.The four donor atoms in the
τ4 = 1) and a perfect square planar geometry (τ4 = 0) with the formula: τ4 =
τ
equatorial plane around both the Cu atoms also show a considerable r.m.s. deviations (Table S4).
The Zn–Cu and Cu–Cu distances are given in Table S5.
Fig. 1 The structure of 1 with ellipsoids at 50% probability. Hydrogen atoms have been omitted
for clarity.
The sixꢀcoordinated zinc has a distorted octahedral environment, constructed by the
coordination of four oxygen atoms from the two “metalloligands” [CuL] and two mutually cis
nitrogen atoms of isothiocyanate ligands. The Zn–O bond distances are greater than the Zn–N
distances (Table S1). The range of the cis [66.42(1)–105.74(2)°] and the trans [154.31(1)–
159.55(1)°] angles around zinc centre indicate significant distortion from its ideal octahedral
geometry.
Complex 2
The structure of 2, having molecular formula [(CuLR)2Zn(
ꢀ1,1ꢀNCS)(NCS)] is shown in Fig. 2
together with the atomic numbering scheme. Dimensions in the metal coordination sphere are
compared with 1 in Table S1. The bent trinuclear structure of 2 contains two terminal reduced
“metalloligand” [CuLR] (where H2LR =N,N'ꢀbis(2ꢀhydroxybenzyl)ꢀ1,3ꢀpropanediamine), two
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thiocyanato ligands (one is Nꢀterminal and the other is
ꢀ1,1ꢀNCS bridging) and a central Zn(II)
metal ion.Thus, the geometry is very different from that observed in complex 1. The terminal
Cu(II) atoms have a fiveꢀcoordinated distorted square pyramidal geometry unlike 1, described
above. The basal plane of the two terminal Cu(II) centres is formed by two phenoxido oxygen
atoms and two imines nitrogen atoms of the dianionic tetradentated reduced Schiff base ligand
[LR]2− and the axial positionis occupied by the nitrogen atom of the ꢁ1,1ꢀbridging thiocyanato
ligand. The basal Cu–O bond distances in each of the terminal copper atoms are shorter than the
basal Cu–N distances and the axial Cu–N bond distances are longer than the basal bonds (Table
S1).The root mean squared (r.m.s.) deviation of the four basal atoms from their mean plane
(Table S4) and the range of cis angles [78.64(2)−108.20(2)° for Cu(1) and 78.65(2)–107.98(2)°
for Cu(2)] and the trans angles [155.55(2)–167.20(2)° for Cu(1) and 156.03(2)–167.5(2)° for
Cu(2)] indicate the distortion from the ideal square piramidal geometry.The distortions of this
geometry from square pyramid to trigonal bipyramid have been calculated by the Addison
parameter (τ).40 The value of τ isdefined as the difference between the two largest donor metal–
donor angles divided by 60, τ is 0 for the ideal square pyramidand 1 for the trigonal bipyramid.
The τ values of Cu(1) and Cu(2)are0.195 and 0.191 respectively,indicating that the geometry is
only slightly distorted from its ideal square pyramidal geometry. The Zn–Cu and Cu–Cu
distances in 2 are smaller than those in 1 (Table S5).The Cu(1)−Zn(1)−Cu(2) angle is 89.0(1)°
which is slightly greater than that in complex 1 (85.7(2)°).
Fig. 2 The structure of 2 with ellipsoids at 50% probability. Hydrogen atoms have been omitted
for clarity.
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Unlike structure 1, here the central zinc atom is pentaꢀcoordinated by four phenoxido
oxygen atoms of two reduced Schiff base ligands, and an Nꢀterminal thiocyanato ligand. The
Zn−O bond distances are larger than Zn−N like complex 1 but all the Zn−O and Zn−N distances
are smaller than those in 1 (Table S1). Here also, the zinc atom shows some distortions of
coordination geometry from the square pyramid to the trigonal bipyramid which is clear from the
ranges of cis [73.19(2)−118.98(2)°] and trans angles [122.8(2)−151.51(2)°]. This distortion is
also determined from Addison parameter (τ = 0.053) which shows only very slight distortion
from ideal geometry.
Complex 4
The structure of 4, having molecular formula [(CuLR)2Cd(
ꢀ1,3ꢀNCS)2] is shown in Fig. 3 together
with the atomic numbering scheme.
Fig. 3 The structure of 4 with ellipsoids at 50% probability. Hydrogen atoms have been omitted
for clarity.
Dimensions in the metal coordination sphere of 4 are compared with 3/3′ (analogous trinuclear
structures derived from unreduced “metalloligand” [CuL], reported earlier shown in Fig. 3A) in
Table S2. The bent trinuclear structure 4 consists of two reduced “metalloligand” [CuLR], two
bridging (ꢀ1,3ꢀNCS) thiocyanato ligands and a Cd(II) metal ion. The geometry of 4 is very
dissimilar from that observed in our reported complex 3/3′ as both the metal centers, Cu and Cd
have unusual coordination behaviour towards thiocyanato ligands. Like 3/3′, the central
cadmium atom, Cd(1) has a six coordinate octahedral environment but the coordinating atoms
and their relative arrangement around it are somewhat different with respect to 3/3′. It is bonded
with the four oxygen atoms from the two “metalloligands”[CuLR] and two mutually cis sulphur
(cis nitrogen in case of 3/3′) atoms from two bridging ꢀ
1,3ꢀSCN− ions (Nꢀterminal thiocyanates
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for 3/3′). The distortion from the octahedral geometry of Cd(1) is clear from the range of cis
[68.3(4)–105.8(3)°] and trans [158.8(4)−172.5(3)°] angles. Each Cu(II) is bonded with the two
nitrogen atoms and two oxygen atoms from the dianionic tetradentate reduced Schiff base ligand
[LR]2−and one nitrogen atom from bridging
ꢀ
1,3ꢀSCN− ion, forming a square pyramidal
coordination environment. The Cu–O bond distances in 4 are smaller than Cu–N distances just
like other two structures (1 and 2) described above and also in 3/3′. But it is noticeable that the
Cu−N distances in 4 are considerably larger than that in 3/3′. The r.m.s. deviation of the four
basally coordinating atoms from the mean plane passing through them is considerably greater
compared to 3/3′(Table S4).
Fig. 3A The structures of 3/3′(Reported) with ellipsoids at 50% probability. Hydrogen atoms
have been omitted for clarity.
The distortion of ideal square pyramidal geometry has been confirmed from the Addison
parameters (τ) [0.347 and 0.252 for Cu(1) and Cu(2), respectively] and the range of cis
[82.9(5)−109.3(5)° for Cu(1) and 83.7(4)–110.8(5)° for Cu(2)] and the trans angles [148.8(5)–
169.6(5)° for Cu(1) and 153.8(4)–168.9(5)° for Cu(2)]. The Cd–Cu distances (Table S5) is
almost similar to those found in complex 3/3′ but the Cu–Cu separation (5.856Å) is much longer
than that of 3/3′(4.264 Å) as a consequence of a slight bent structure in 4 compared to 3/3′. The
Cu(1)−Cd(1)−Cu(2) angle [124.51(2)° for 4] is considerably greater than those for 3 and 3′
[79.84(2)° and 98.84(1)° respectively] although all of them possess bent structure.
DFT studies of the complexes
Three new complexes have been synthesized and Xꢀray characterized, two of them (2 and 4)
using reduced Schiff base and one (1) with unreduced Schiff base ligand of 1,3ꢀ
propanediammine and salicyldehyde (see Fig. 4). The structure of 1 is bent trinuclear with Nꢀ
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coordinated SCN– as usual in many published complexes. However, the complexes with reduced
ligand are rather unusual. On one hand, complex 2 (Cu2Zn), one SCN− is coordinated to Zn(II)
with N (usual) but other one forms a bridge (ꢁ1,1−NCS) between the two Cu(II) in sharp contrast
to the unreduced complex, 1. On the other hand, in complex 4, two SCN− ions are Sꢀbonded to
Cd(II) as we found earlier, however in 4 the thiocyanate ions are cis to Cd(II), whereas they were
trans in the reported structure (complex 5 in Fig. 5), and form a (ꢁ1,3ꢀSCN) bridge to the terminal
Cu(II) ions. In case the unreduced Schiff base is used with the Cu2Cd core, the usual bent
structure having cis coordinated Nꢀbonded isothiocyanate is formed as published before (see
complexes 3/3′ in Fig. 5). In this DFT study we wish to rationalize the differences in structures
by means of theoretical calculations.
Fig. 4 Newly synthesized complexes 1, 2 and 4. H atoms are omitted for clarity (apart from those
belonging to the N–H groups).
Fig. 5 Xꢀray structures of known complexes 3/3′ and 5. H atoms are omitted for clarity (apart
from those belonging to the N–H groups).
Rationalization of the Zn complexes.
As aforementioned, the reduction of the Schiff base provokes a significant change in the
coordination sphere of the metal centers. In order to evaluate energetically the stability of the
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different complexes, we have fully optimized the geometries of complexes 1 and 2 starting from
the crystallographic coordinates (see A and D in Fig. 6). Moreover, we have also optimized two
hypothetical complexes, denoted as 1a and 2a (see B and C in Fig. 6) where the reduced ligand is
used in the optimization of 1a and the unreduced ligand in the optimization of 2a. We have
compared the isomerisation energies and in case of the unreduced ligand, both isomers (1 and
2a) are basically isoenergetic (ꢁE1 = 0.4 kcal/mol), therefore both complexes are energetically
equivalent. Therefore, the observation of complex 1 in the solid state is likely due to packing
forces. In contrast, for the reduced ligand isomeric complexes 1a and 2, the isomer observed
experimentally is more stable (ꢁE2 = –5.5 kcal/mol) thus explaining the formation of complex 2
with the bridged ꢁ1,1ꢀNCS ligand.
Fig. 6 BP86/def2ꢀTZVP optimized complexes and isomerisation energies of complexes 1 and 2.
H atoms are omitted for clarity (apart from those belonging to the N–H groups).
Rationalization of the Cd complexes.
Using the same strategy described above for the Zn complexes to evaluate energetically the
stability of the different complexes, we have fully optimized the geometries of Cd complexes, 3
and 4 starting from the crystallographic coordinates (see A and D in Fig. 7). Moreover, we have
also optimized two hypothetical complexes, denoted as 3a and 4a (see B and C in Fig. 7) where
the reduced ligand is used in the optimization of 3a and the unreduced in ligand in the
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optimization of 4a. We have compared the isomerisation energies and in case of the unreduced
ligand, both isomers (3 and 4a) are similar in energy (ꢁE3 = –1.0 kcal/mol) being 3 slightly more
stable. Therefore the observation of complex 3 instead of 4a in the solid state is likely due to
packing forces. In contrast, for the reduced ligand isomeric complexes 3a and 4, the isomer
observed experimentally is considerably more stable (ꢁE4 = –4.6 kcal/mol) thus explaining the
formation of complex 4 with the bridged (µ1,3ꢀSCN) ligand. Interestingly, the optimization of
complex 3a, using the reduced ligand in a bent trinuclear with Nꢀcoordinated SCN– conformation
as a starting point converges to a trans (µ1,3ꢀSCN, Nꢀcoordinated) bridged conformation (see Fig.
7). This further demonstrates that the bent trinuclear Nꢀcoordinated cis conformation is not stable
if the reduced ligand is used. Finally, we have also compared the cis/trans isomers of complex 4
(µ1,3ꢀSCN bridge). The optimized trans isomer (4t) is shown in Fig. 7E and it is lesser stable than
the cis (ꢁE5 = –5.9 kcal/mol), in agreement with the experimental observation.
At this point, we have also analyzed an unexpected experimental finding. That is,
we have previously shown that the N coordination to Cd is energetically favored over the S
coordination in Cd(NCS)2 complexes and verified by searches in the CSD.25 However, the Xꢀray
analysis of complex 4 reveals that the µ1,3ꢀSCN bridge is coordinated to Cd via the S end and to
the Cu via the N end. We have previously observed25 the same coordination mode in a similar
complex (see complex 5 in Fig. 5). DFT calculations demonstrated25 that the N coordination to
Cd is more favorable in 5 than the S coordination (µ1,3ꢀSCN bridge) and rationalized the
experimental structure (opposite coordination mode) showing that additional interactions
observed in the solid state Xꢀray structure compensate the energetic difference between
bothisomers. As a matter of fact, complex 3a (Cd–N coordination, µ1,3ꢀSCN bridge) is more
favorable (ꢁE6 = 1.3 kcal/mol) than 4t (Cd–S coordination, µ1,3ꢀSCN bridge), as shown in Fig. 7.
However, in these complexes and also complex 5 the relative disposition of the µ1,3−SCN
bridges is trans. Interestingly, when complex 4 is optimized with the µ1,3ꢀSCN bridges is cis and
Cd–N coordination (denoted as 4i, see Fig. 7F), the bridging µ1,3ꢀSCN ligand breaks the Cu–S
coordination bond and the optimization converges to a minimum structure where the ligands
becomes monodentate (ꢀ1ꢀNCS) and the Sꢀend is establishing two intramolecular N–H···S bonds
(see Fig. 7F). However, this conformation is higher in energy than 4 (the one observed
experimentally). Therefore, in case of having the µ1,3ꢀSCN bridges is cis conformation, the most
favorable binding mode to Cd is via the S end. This is likely due to the different coordination
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angle preference that is the Cd–NCS coordination requires a larger Cd–X–C angle value than the
Cd–SCN coordination (X = N, S). Therefore, the Cd–SCN coordination mode is favored in µ1,3ꢀ
SCN bridges when the organic ligands are in cis (higher steric hindrance) thus requiring lower
coordination angles.
Fig. 7 BP86/def2ꢀTZVP optimized complexes and isomerization energies of complexes 3 and 4.
H atoms are omitted for clarity (apart from those belonging to the N–H groups).
Comparison of the Zn and Cd complexes with reduced ligand.
Another issue that we want to rationalize using DFT calculations is the sharp difference between
the complexes of Zn and Cd with the reduced ligand. That is, on one hand in complex 2, one
SCN− is coordinated to Zn(II) and the other one forms a bridge (µ1,1ꢀNCS) between both Cu(II)
(see Fig. 4). On the other hand, in complex 4 both pseudohalides are µ1,3−SCN bridged ligands.
We have evaluated energetically the difference between complex 2 and the hypothetical complex
4 using Zn instead of Cd, denoted as 4Zn. Moreover, we have also evaluated the energetic
difference between complex 4 and the hypothetical complex 2 using Cd instead of Zn, denoted as
2Cd. The optimized geometries of the hypothetical complexes and the energetic results are
gathered in Fig. 8. For Zn, it can be observed that the conformation observed experimentally
(complex 2, Fig. 8A) is significantly more stable (ꢁE8 = +27.1 kcal/mol) than the hypothetical
complex of Zn (4Zn, Fig. 8B) in the conformation adopted by the experimental Cd complex (µ1,3ꢀ
SCN). Similarly, for Cd, it can be observed that the conformation observed experimentally
(complex 4, Fig. 8D) is significantly more stable (ꢁE9 = –8.6 kcal/mol) than the hypothetical
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complex of Cd (2Cd, Fig. 8B) in the conformation adopted by the experimental Zn complex (µ1,1ꢀ
NCS). These energetic results are useful to rationalize the different geometry observed for Cd
and Zn complexes with the reduced organic ligand and the SCN−coligands. Finally, in compound
4 we have evaluated energetically the formation of a selfꢀassembled dimer in the solid state
governed by the formation of two symmetrically related N–H···N interactions (see ESI, Fig.
S10).
Fig. 8 BP86/def2ꢀTZVP optimized complexes and isomerization energies of complexes 2 and 4.
H atoms are omitted for clarity (apart from those belonging to the N–H groups).
Conclusion
In this present work, we synthesize two Cu2Zn (1 and 2) and one Cu2Cd (4) trinuclear complexes
using unreduced and reduced forms of N,N′ꢀbis(salicylidene)ꢀ1,3ꢀpropanediamine and
thiocyanate ion. The unreduced ligand produces usual bent trinuclear complexes both with Zn(II)
(1) and Cd(II) (3/3′, reported earlier) in which two thiocyanate ions show usual terminal Nꢀ
coordination. However, the complexes with the corresponding reduced Schiff base ligand have
very different structures and coordination modes of thiocyanate: in the Zn(II) complex (2), one
of the thiocyanate ions is terminally Nꢀcoordinated to Zn(II) but the other one rather unusually
bridges (
ꢀ
1,1ꢀNCS) the two terminal [CuLR] units while in the Cd(II) complex (4), the two
thiocyanate ions link the central Cd(II) to each of the terminal [CuLR] units in cisꢀ
ꢀ1,3ꢀNCS
mode. These structural differences have been rationalized by DFT study. The unusual structure
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of Zn(II) complexes as of 2 is found to be stabilized only with the reduced form of the ligand.
For Cd(II) complexes with unreduced ligand, the terminal Nꢀcoordinated and bridged ( 1,3ꢀNCS)
ꢀ
structures are similar in energies but for reduced ligand the latter is clearly more stable
explaining the formation of 4. Although Nꢀcoordination of thiocyanate to Cd(II) is energetically
favoured over the S coordination, the µ1,3ꢀSCN bridge between central Cd(II) and terminal Cu(II)
is the most favourable in cis conformation and coordinated to Cd via the S end, most likely due
to smaller Cd–S–C angle. The structures of the molecules with the reduced ligand are also
central metal ion specific as replacements of Zn by Cd in 2 and Cd by Zn in 4 make the
corresponding structures unfavourable. The study therefore demonstrate that for this kind of
hingeꢀlike flexible trinuclear clusters, a slight difference in ligand system or change of metal ion
can stabilize different coordination mode of thiocyanate yielding structures various geometries.
Acknowledgement
A.H. is thankful to CSIR for Senior Research Fellowship [Sanction No. 09/028 (0889)/2012ꢀ
EMRꢀI]. Crystallography was performed at the DSTꢀFIST, India, funded Single Crystal
Diffractometer Facility at the Department of Chemistry, University of Calcutta. The authors also
thank the Department of Science and Technology (DST), New Delhi, India, for financial support
(SR/S1/IC/0034/2012). A.B. and A.F. thank DGICYT of Spain (projects CTQ2014ꢀ57393ꢀC2ꢀ1ꢀ
P and CONSOLIDER INGENIO CSD2010–00065, FEDER funds) for funding. We thank the
CTI (UIB) for free allocation of computer time.
Notes
Electronic supplementary information (ESI) available: The electronic supplementary
information file contains TablesS1−S6 and Figs. S1–S10.CCDC1442236, 1442237 and 1442238
for 1, 2 and 4 respectively.
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Table 1 Crystal data and structure refinement of complexes 1, 2 and 4
1
2
4
Formula
C36H32Cu2N6O4S2Zn C36H40Cu2N6O4S2Zn C36H40CdCu2N6O4S2
M
869.29
877.35
924.37
Crystal System
Monoclinic
P21/c
Triclinic
Triclinic
Space Group
P
1
P
1
a/Å
b/Å
c/Å
12.3347(5)
10.0906(4)
28.5707(1)
90.000(5)
101.824(1)
90.000(5)
3480.6(3)
4
10.631(3)
11.708(3)
15.231(4)
84.236(3)
86.873(3)
70.058(3)
1772.7(8)
2
9.918(5)
10.653(5)
19.107(5)
77.290(5)
87.044(5)
67.076(5)
1812.5(13)
2
α/°
β
/°
γ
/°
V/Å3
Z
Dc/g cmꢀ3
µ/mmꢀ1
F (000)
R(int)
1.659
1.644
1.694
2.065
2.027
1.908
1768
900
936
0.055
0.040
0.046
Total Reflections
39791
11616
20344
Unique reflections 6384
5803
6320
4751
4070
5231
I>2σ(I)
R1a, wR2b
GOFcon F2
R (all)
0.0392, 0.1138
1.04
0.0534, 0.1619
1.03
0.0910, 0.2890
1.12
0.0597
0.0826
0.1025
Temp (K)
293
296
293
aR1 = ∑||Fo|−|Fc||/∑|Fo|,bwR2 (Fo ) = [∑[w(Fo − Fc2)2/∑w Fo ] and cGOF = [∑[w(Fo − Fc2)2/(Nobs– Nparams)]
2
2
4 ½
2
½
22

Page 23 of 23
Dalton Transactions
View Article Online
DOI: 10.1039/C5DT04941E
Graphical Abstract
Exploring the coordinative adaptation and molecular shapes of the trinuclear
CuII MII (M = Zn/Cd) complexes derived from salen type Schiff bases:
2
Structural and theoretical studies
Alokesh Hazari,a,b Lakshmi Kanta Das,c Antonio Bauzá,d Antonio Frontera,d* and
Ashutosh Ghosh a*
Structural analyses and DFT studies of three new hetero-metallic compounds and other related
species established that slight modification in ligand system or change in central metal ion can
stabilize various interesting novel structures due to different coordination modes of thiocyanate
ion and flexibility of the trinuclear core.