Design of end-on cyanato bridged trinuclear Cu(II) Schiff base complex with salen type Schiff base ligand: synthesis, structural investigation and DFT study

Article abstract of DOI:10.1080/00958972.2017.1287354

One ONNO-donor tetradentate Schiff base ligand LH₂ was derived from the condensation of salicylaldehyde and 1,3-diaminopropane and reacted with Cu(NO₃)₂·6H₂O and NaNCO to yield one trinuclear complex with molecular formula [Cu₃L₂(μ_1,1-NCO)₂]. The synthesized complex was characterized by IR, UV–vis spectroscopy, and electrochemical analysis. Single-crystal X-ray diffraction study explores that the two terminal copper atoms adopt square pyramidal geometry, whereas the central copper atom situated at the inversion center is surrounded by four phenoxo oxygens and two end-on cyanato anions to adopt an octahedral geometry. The ONNO-tetradentate Schiff base ligand coordinates with the copper(II) ion via two oxygen atoms of the phenoxo-group and two nitrogen atoms from the imine moiety. A theoretical density functional theory (DFT) calculation was also carried out to supplement the experimental results. All the DFT calculations were done in gas phase.

Full text of DOI:10.1080/00958972.2017.1287354

Journal of Coordination Chemistry
ISSN: 0095-8972 (Print) 1029-0389 (Online) Journal homepage: http://www.tandfonline.com/loi/gcoo20
Design of end-on cyanato bridged trinuclear Cu(II)
Schiff base complex with salen type Schiff base
ligand: Synthesis, structural investigation and DFT
study
Sandeepta Saha, Chirantan Roy Choudhury, Guillaume Pilet, Antonio
Frontera & Samiran Mitra
To cite this article: Sandeepta Saha, Chirantan Roy Choudhury, Guillaume Pilet, Antonio Frontera
&
Samiran Mitra (2017): Design of end-on cyanato bridged trinuclear Cu(II) Schiff base complex
with salen type Schiff base ligand: Synthesis, structural investigation and DFT study, Journal of
Coordination Chemistry, DOI: 10.1080/00958972.2017.1287354
To link to this article: http://dx.doi.org/10.1080/00958972.2017.1287354
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Date: 08 February 2017, At: 21:40

Publisher: Taylor & Francis
Journal: Journal of Coordination Chemistry
DOI: http://dx.doi.org/10.1080/00958972.2017.1287354
Design of end-on cyanato bridged trinuclear Cu(II) Schiff base complex with
salen type Schiff base ligand: Synthesis, structural investigation
and DFT study
†
‡
§
¶
SANDEEPTA SAHA , CHIRANTAN ROY CHOUDHURY* , GUILLAUME PILET , ANTONIO FRONTERA
†
,
and SAMIRAN MITRA*
†
Department of Chemistry, Jadavpur University, Raja S. C. Mullick Road, Kolkata-700032, India
Department of Chemistry, West Bengal State University, Barasat, Kolkata-700126, India
Groupe de Crystallographieet Ingénierie Moléculaire, Laboratoire des Multimatériaux et Interfaces, UMR 5615
‡
§
CNRS-Université Claude Bernard Lyon 1, Bât. Chevreul, 43 bd du 11 Novembre 1918, 69622 Villeurbanne Cedex,
France
¶
Departamen de Química, Universitat de les Illes Balears, Crta. de Valldemossa kmm 7.5, 07122
Palma de Mallorca (Baleares), Spain
Department of Chemistry, Heritage Institute of Technology, Chowbaga Road, Anandapur, Kolkata – 700107, India
2
One ONNO-donor tetradentate Schiff base ligand LH was derived from the condensation of
salicylaldehyde and 1,3-diaminopropane and reacted with Cu(NO ) ·6H O and NaNCO to yield
3
2
2
one trinuclear complex with molecular formula [Cu
3
L
2
2
(μ1,1-NCO) ]. The synthesized complex
was characterized by IR, Uv-vis spectroscopy and electrochemical analysis. Single-crystal X-ray
diffraction study explores that the two terminal copper atoms adopt square pyramidal geometry,
whereas the central copper atom situated at the inversion center is surrounded by four phenoxo
oxygens and two end-on cyanato anions to adopt an octahedral geometry. The ONNO-
tetradentate Schiff base ligand coordinates with the copper(II) ion via two oxygen atoms of the
phenoxo-group and two nitrogen atoms from the imine moiety. A theoretical DFT calculation
was also carried out to supplement the experimental results. All the DFT calculations were done
in gas phase.
Keywords: Tetradentate Schiff base; Trinuclear copper complex; X-ray study; DFT Calculation
*Corresponding authors. Email: samiranju92@gmail.com (S. Mitra); crchoudhury2000@yahoo.com (C.
Choudhury)
1

1
. Introduction
The construction of transition metal Schiff base complexes having supramolecular arrays based
on covalent or hydrogen-bonding interactions along with pseudohalide-bridging is an area of
great research activity in inorganic chemistry due to their interesting properties [1, 2]. Their
physical, optical, and electronic properties have been explored in different coordination
environments with organic chelators, blockers, and suitable bridging units [3-5]. In a few
multicopper enzymes, it has been established by X-ray crystallography [6], MCD spectroscopy
[7] and magnetic studies [8] that a trinuclear array of copper centers may be the essential
-
+
functional unit of the cofactors which catalyze the 4e /4H reduction of dioxygen to water. The
key factor that leads the researchers to synthesize such type of complexes is the exchange
coupling between paramagnetic centers and possible magneto-structural correlation in molecular
systems, which develops a few new functional molecular-based materials [9-12].
-
-
-
Pseudohalide anions (N , SCN , NCO , etc.) are versatile ligands for obtaining
3
multinuclear 1D, 2D or 3D coordination networks. These bridging ligands also play an important
role in the magnetic exchange pathways between paramagnetic centers and, in general, show
various magnetic interactions which are attributed to the diversity of coordination modes and to
their ability to transmit magnetic interaction between metal centers [13, 14]. The cyanate anion,
like the most widely studied azide, can also act as a bridging ligand between two metal ions. The
most common bridging modes are: end-to-end (a) and end-on fashion (b, c) (scheme 1). The
most comprehensive literature refers to azide bridging ligand: the end-to-end bridging mode
creates antiferromagnetic coupling [15] and the end-on mode results in ferromagnetic coupling
[16]. But, these behaviors are not so clear for the cyanato bridging ligands as the number of
reported complexes that have been fully characterized is too low to permit magneto-structural
correlations [14]. The study of the ability of cyanato ligand as a super-exchange pathway
between copper(II) ions is difficult as most of the published compounds for which structural and
magnetic data are available also have another type of bridge in their structures. It is difficult to
separate the contribution of the cyanato ligand to the total coupling constant. Only four such
compounds are bridged exclusively by cyanato ligands, [Cu (μ -NCO) )(terp) (H O) ](PF ) ,
2
1,1
2
2
2
2
6 2
1
[Cu
2
(μ1,3-NCO)(L )](ClO
4
3
) ·MeCN·1.5EtOH, [Cu
2
(μ1,3-NCO)
2
(EtMe
4
dien)
2
](ClO4)
2
and
[{CuL(μ1,3-NCO)}
n
] [17a-d]. Magnetic data for these complexes prove the high versatility of the
2

cyanato ligand in its super-exchange pathway. Also a few trinuclear copper complexes of LH2
are reported in the literature [18] and in all the cases the ligand acts as tetradentate N O -donor.
2
2
The common factor between the structures is that they mainly consist of Cu
the factor of counter anion.
3 2 4
N O core regardless
Scheme 1.
Herein, we describe the synthesis of a newly designed trinuclear copper(II) complex having
molecular formula [Cu ] (1), where LH is the 2:1 condensation product of
3
L
2
(μ1,1-NCO)
2
2
salicylaldehyde and 1,3-diaminopropane. The complex has been characterized by elemental
analysis, IR, UV-vis spectroscopy and cyclic voltammetry. The crystal structure of the
synthesized complex has been determined by single-crystal X-ray diffraction study. Theoretical
density functional theory (DFT) study has also been carried out to supplement the experimental
data.
2
2
. Experimental
.1. Materials
Salicylaldehyde, 1,3-diaminopropane and copper nitrate were purchased from Sigma-Aldrich
(USA) and used without purification. All solvents were AR grade and used as received.
2
.2. Physical measurements
The infrared spectrum of the investigated complex was recorded on a Perkin–Elmer Spectrum 2
-1
FT-IR spectrophotometer with KBr discs (4000–400 cm ). Elemental analyses (C, H and N)
were carried out using a Perkin–Elmer 2400 II elemental analyzer. Ground-state absorptions for
the complex were recorded at 300 K with a Perkin–Elmer Lambda-35 (UV–vis)
spectrophotometer using HPLC grade acetonitrile. Cyclic voltammetric study was carried out on
3

CH 660E electrochemistry system using tetrabutylammonium perchlorate as supporting
-1
electrolyte at scan rate of 50 mV sec in acetonitrile. A platinum working electrode, one
platinum wire auxiliary electrode and saturated calomel electrode (SCE) were used as standard
three electrode configurations.
2
2
.3. Synthesis of the ligand and complex
.3.1. Synthesis of LH . LH was synthesized by refluxing 2 mmol of salicylaldehyde (0.21 mL)
2
2
and 1 mmol of 1,3-diaminopropane (0.08 mL) for 1 h in methanol and was used without
purification [18a].
2
.3.2. Synthesis of [Cu L (µ -NCO) ] (1). Cu(NO ) ·6H O (0.2813 g, 1.5 mmol) and LH
3 2 1,1 2
3 2
2
2
(0.268 g, 1 mmol) were dissolved in 25 mL acetonitrile and then an aqueous solution (5 mL) of
sodium cyanate (0.651 g, 1 mmol) was added dropwise to the solution. The green mixture was
then stirred for 30 min and the resulting solution was filtered and kept for slow evaporation.
After a couple of weeks green needle-shaped crystals appeared and were found suitable for X-ray
diffraction study. Yield: 42% (0.351 g). Anal. Calc. for [C H Cu N O ]: C, 51.71; H, 3.86; N,
3
6
32
3
6
6
1
0.05. Found: C, 51.63; H, 3.93; N, 10.18%.
2
.4. X-ray crystallography
To collect the X-ray diffraction data single-crystal of the complex was mounted on an Oxford
Diffraction Gemini diffractometer equipped with graphite monochromated Mo Kα (λ =
0
.71069 Å) fine-focus sealed tubes. Intensity data were collected at 100(1) using φ and ω scans.
Crystal data were collected using a Gemini (Oxford Diffraction, 2006). Data refinement and
reduction were performed using CrysAlis RED [19], (Oxford Diffraction, 2002) software.
Multiscan absorption corrections were applied empirically to the intensity values (Tmin = 0.46
and Tmax = 0.90) using SADABS [20]. The structures were solved by direct methods using the
2
program SIR-97 [21] for the complex and refined with full-matrix least-squares based on F
using program CRYSTALS [22]. All non-hydrogen atoms were refined anisotropically. C-bound
hydrogen atoms were placed geometrically and refined using a riding model approximation. The
molecular graphics and crystallographic illustrations for the complex were prepared using
4

CAMERON [23] and ORTEP [24]. All relevant crystallographic data and structure refinement
parameters for the complexes are summarized in table S1.
2
.5. Computational details
The gas-phase geometry of the investigated complex was optimized without any symmetry
restrictions with spin-unrestricted DFT calculations using the hybrid DFT (B3LYP [25] and
PBEPBE [26]) methods and several basis sets (6-31G* and 6-311+G* [27]) with the Gaussian 09
software package [28]. The initial structure of the complex was generated from its X-ray
crystallographic data. No constraint was applied in the calculations and all atoms were free to
optimize. The charge distribution in the complex has been computed using the Mulliken method
[
29] for deriving atomic charges at the B3LYP/6-311+G* level of theory. We have also
computed the chemical hardness using the equation proposed in the literature at the B3LYP/6-
11+G* level of theory [30]. That is, η = ½ × (I–A), where I and A are the vertical ionization
potential and vertical electron affinity, respectively. On the basis of the optimized ground and
3
excited state geometries, the absorption and emission spectral properties in acetonitrile (CH
were calculated by the time-dependent density functional theory (TD-DFT) [31] approach
3
CN)
associated with the conductor-like polarizable continuum model (CPCM) [32]. We computed the
lowest forty singlet–singlet transitions and forty singlet–triplet transitions in absorption and
emission processes, respectively, and the results of the TD calculations were qualitatively similar
to the observed spectra. GaussSum program version 2.2 was used to calculate the molecular
orbital contributions from groups or atoms [33].
3
3
. Results and discussion
.1. Synthesis
2
LH was synthesized using commercially available reagents following the process described
earlier [18]. In recent years, our group has reported a number of trinuclear complexes using
ONNO donor Schiff base ligands and in most of the cases the central metal atom is copper
[
18a, 34]. Interestingly, most of the trinuclear complexes reported so far contain an additional
anion bridge. For example, in such studies, we synthesized two trinuclear copper(II) complexes
using the same type of ligands as that of LH with copper(II) acetate [34c] and Cu(II) bromide
18a]. When we used acetate, it behaves as a bridging chelating ligand [34c] but when we used
2
[
5

bromide [18a] it acted as a monodentate ligand as bromide is devoid of any bridging ability.
Keeping all this in mind, we used sodium cyanate to understand the role of cyanate in the final
nuclearity of the synthesized copper(II) complex with one compartmental ligand LH
2
to furnish
the trinuclear complex [Cu L (µ -NCO) ] (1), where the desired square-pyramidal – octahedral
3
2
1,1
2
–
square-pyramidal topology of the triad is achieved with the support from cyanato bridging
co-ligand which acted in a monodentate bridging mode.
3
.2. Crystal structure descriptions
ORTEP diagram of the complex with atom numbering scheme of the trimeric asymmetric unit is
presented in figure 1. Selected bond lengths and angles are given in table 1. The centrosymmetric
complex shows a linear trinuclear array of three Cu(II) ions, where two μ-phenolato oxygen
-
atoms of the deprotonated Schiff base ligand and one monoatomic bridging μ -NCO anion
1
,1
bridge each terminal copper atom with the central copper atom. Both the cyanato bridges for the
terminal Cu1 and central Cu2 centers are in a syn-syn fashion but are mutually trans to each
other, due to inversion symmetry.
The equatorial plane of the central Cu2 is formed by two bridging phenoxo-oxygen atoms
(O21 and its symmetry-related counterpart O21′) from two Schiff base ligands and two nitrogen
atoms (N31 and N31′) from two bridging cyanates, while the axial positions of the octahedron
are occupied by two bridging phenoxo-oxygen atoms O1 and O1′ (symmetry related
counterpart). As the central Cu2 atom is located on an inversion center, all three trans angles are
very close to 180° (actual values: 179.994°, 179.994° and 179.995° for N31′-Cu2-N31, O21-
Cu2-O21′ and O1-Cu2-O1′, respectively). In the coordination sphere of Cu2 in the complex, the
equatorial Cu2-O21(phenoxo) and Cu2-N31(cyanato) bond lengths are 2.003(4) Å and
1
2
.957(5) Å, respectively, which are distinctly shorter than the axial Cu2-O1(phenoxo) distance of
.409(4) Å (table 1). The coordination geometry of the central Cu(II) ion has typical Jahn-Teller
9
distortion, as it is expected for a Cu(II) d system, from ideal octahedral geometry [35].
In the complex, two terminal copper atoms are five-coordinate, bearing identical
environments, due to the presence of crystallographic inversion symmetry. The coordination
number five for Cu(II) is very common, where it may possess either square-pyramidal (SP) or
trigonal-bipyramidal (TBP) geometry [36]. For a five-coordinate metal center, the distortion of
structure from SP to TBP can be evaluated by the Addison distortion index, τ defined as the ratio
6

of the mean in-plane Cu-L bond distance to the out-of-plane Cu-L bond distance [37], = [|θ -
Φ|/60], where θ and Φ are the two largest coordination angles and τ is 0 for perfect SP and 1 for
ideal TBP geometry [38]. The calculated τ value for the complex is 0.326 which clearly suggests
a distorted SP geometry for the terminal copper atoms in the complex. The basal plane of each of
the two equivalent terminal copper centers is formed by the two imine nitrogen atoms (N9 and
N13) and two phenolic oxygen atoms (O1 and O21). Cu1 is displaced from the mean equatorial
plane constructed by two imine nitrogen atoms and two phenolic oxygen atoms of the Schiff
base ligand by 0.244(3) Å, toward the apical nitrogen (N31). The two six-membered CuONC
3
chelate rings possess half-chair conformation, while the CuN C chelate ring has a pronounced
2
3
boat conformation. Cu-O equatorial bond lengths around the terminal Cu1 ions are 1.927(4) Å to
.937(4) Å, while the Cu1-N equatorial distances are 1.935(5) Å and 1.978(5) Å. All the four
1
bond distances are much shorter than the axial Cu1-N31 bond distance of 2.682(5) Å. The
Cu1-imine nitrogen and Cu1-phenolic oxygen bond distances are in the ranges observed for
similar systems [39]. Sorrel and Jameson have pointed out that the linear N-Cu-N bond enjoys a
strong σ-overlapping between the ligand and the copper orbitals [40]. The extent of such overlap
is determined by the steric demands of the participating ligands. In the present study, the
Cu2-N31 distance is fairly high. This could be attributed to the presence of steric interference of
the coordinating linear cyanato ligand, which is expected to participate in stronger σ-overlapping
in the OCN-Cu(II)-NCO links. Interestingly, bridging –NCO group coordinates to the central
Cu2 along equatorial plane via N-atom, whereas it occupies the axial position of the terminal
Cu1 atom. The distortion from ideal square pyramidal geometry is also reflected from the
difference in cis-angles lie in the basal plane which vary from 82.44(17)° to 92.99(18)° and
angles in trans position vary from 158.83(19)° to 163.23(19)°.
The Cu1····Cu2 distance is 2.954 Å, which is comparable to similar type systems [41].
Packing diagrams of the complex are shown in figures 2 and S1. None of the intramolecular
H-bonds were detected in the trimer. But the weak intermolecular non-classical interactions
C14-H141···O33 = 2.44 Å along with C16-H161···O1 = 2.56 Å and C19-H191···N31 = 2.62 Å
and stacking interaction propagate the molecule along the c-axis.
7

3
.3. FT-IR spectrum
The infrared spectrum (figure S2) of the complex is very much consistent with the structural
-1
data. In the title complex, a strong and sharp absorption band at 2100 cm in the spectrum of the
-1
complex is due to ν(NCO) and a band around 1615 cm may be assigned to the imine stretching
frequency i.e. coordination of the nitrogen atom of the imine group to the metal ions [41, 42].
-1
The phenolic ν(Ar-O) band is observed in the frequency region 1178-1184 cm , providing
evidence for coordination to the metal ions through the deprotonated phenolic oxygen atoms
-1
-1
[
43]. Sharp bands at ~498 cm and ~423 cm correspond to the Cu-N and Cu-O stretching
-1
frequencies, respectively. A characteristic peak for ν(C=C) is located at 1545 cm [43].
3
.4. Electronic spectrum
The electronic spectrum of 1 was measured in acetonitrile and is depicted in figure 3. Multiple
transitions are the characteristics of the spectrum for 1 and the bands are attributed primarily to
the charge-transfer transitions within the tri-coordinated Schiff base ligand with varying amounts
of anionic coligand-p orbital and metal-d orbital participation. To gain deeper insight into the
electronic properties of the transitions involved in the optical absorption processes, we
investigated 1 by means of TD-DFT in acetonitrile. The most relevant transitions involved, along
with their energy, character, oscillator strengths of 1 are listed in table 2. The spectrum shows
two strong absorption bands in the region 233 nm and 269 nm. These bands are clearly charge-
transfer in origin [44]. The band around 382 nm is assigned to the n–π* transition of the
azomethine group. The visible spectrum of the complex displays a single broad band at 673 nm.
These spectral features are consistent with the five-coordinate geometry of the Cu(II) complex.
In general, typical five-coordinate Cu(II) complexes of SP or distorted SP geometries exhibit a
band in the range 550–660 nm, whereas the corresponding TBP complexes usually show a band
at λ > 800 nm with a higher-energy shoulder [45]. This indicates that the solution spectrum of
the complex is adequately explained by a d–d transition for the Cu(II) ion in a distorted SP
2
2
geometry where the absorption band observed in the visible region results from a E → B
1
transition. Based on TD-DFT analysis for 1, lower-energy transition around 673 nm is computed
2
at 714 nm (1.7344 eV, f = 0.0177) as π(L1) + π(L2) + 3d (Cu3) + π(NCO2) → 3d (Cu3) +
yz
z
2
2
π*(L2) ILCT/ LMCT transition. The transition near 382 nm (3.2024 eV, f = 0.0228, λ
=
theo
8

2
3
87 nm) is computed as π(L1) + π(L2) + π(NCO1) + π(NCO2) → 3d
z
(Cu3) + π*(L2)
ILCT/LLCT transition.
3
.5. Electrochemical study
Cyclic voltammogram of 1 (figure 4) shows two quasi-reversible reductive responses on the
negative side of SCE at -0.219 and -1.04V, respectively, and one irreversible oxidative response
on the positive side of SCE at 0.49 V. When the complex is cathodically scanned (0.0 to -1.5 V),
1
2
the reduction peak at E = -0.219 V and the peak at E = -1.04 V are observed. When the
pc
pc
2
sweep is reversed at -1.5 V in the anodic direction we can observe an oxidation wave (E pa
)
1
corresponding to a one-electron redox process. Moreover, the first reaction (E = -0.219 V) is a
1
/2
-1
-1
single peak with a peak current intermediate between those of single-step 1e and 2e reactions;
it therefore demonstrates the process of two one-electron steps that correspond to the redox
process of the two terminal copper(II) ions.
The electrochemical behavior of the trinuclear copper(II) complexes can be interpreted
according to the following scheme:
II
II
II
II
III
II
Positive potential, Cu Cu Cu
Cu Cu Cu ;
II
II
II
II
I
II
I
I
I
negative potential, Cu Cu Cu
This is similar to the other trinuclear copper(II) complexes [46].
e-1
Cu Cu Cu
Cu Cu Cu .
-
II
III
II
II
II
II
Cu Cu Cu
At Positive Potential: Cu Cu Cu
II II +e-1
II +2e-1
II
II
I
I
I
I
At Negative Potential: Cu Cu Cu
Cu Cu Cu
Cu Cu Cu
3
.6. Geometry optimization, charge distribution and electronic structure
The geometry of 1, as derived from X-ray crystallography, was optimized without any symmetry
restrictions by spin-unrestricted DFT calculations using the hybrid DFT (B3LYP and PBEPBE)
methods with several basis sets (figure 5) and the GAUSSIAN 09 software package [28]
(figure 5). The calculated bond lengths and angles are in agreement with the values based upon
the X-ray crystal structure data (table 1). A difference between the experimental and theoretical
studies is that computations were done in the gas phase, whereas the X-ray data were obtained in
the solid phase for the complex under consideration. The main differences are found in the
i
Cu1-N31 (i = 1-x, 1-y, 1-z) and Cu2-O1 distances that are overestimated using the DFT
9

methods. This is likely due to crystal packing effects that are not considered in the theoretical
calculations. Regarding the coordination angles, the level of theory that provides better
agreement is the B3LYP/6-311+G*, especially in the O21-Cu1-N9 angle (table 1).
All the nitrogen and oxygen atoms in the complex bear negative charges, while the
carbon atoms and the copper center bear positive charges. The calculated charges on the copper
atoms in the complex are +1.058 e for Cu1 and +1.029 e for Cu2 which are considerably lower
than the formal charge +2, that confirms a significant charge donation from the ligands. In the
compound, the calculated Mulliken atomic charges for the atoms N31, O1 and O31 are less than
-1: -0.522, -0.433 and -0.420 e, respectively. So, all the electron density in valence 3s and 3p
orbitals is lower than the predicted values. Similarly, the electron density of these atoms is lower
than expected values. All these results indicate electron transfer from the donor atoms to the
central metal. Calculated highest-occupied molecular orbital (HOMO) and lowest-unoccupied
molecular orbital (LUMO) frontier orbitals of the synthesized complex are shown in figures 6
and 7, respectively, and they show relevant differences. In fact, in the complex the HOMO is
mainly localized on one of the Schiff base phenyl rings and terminal Cu, while in the LUMO,
major contributions come from the metal and, to a lesser extent from the terminal Schiff base.
Since the fimaj electron transfer occurs from the HOMO to LUMO, it can be inferred that the
first electron-transfer in the complex is related to be LMCT. The HOMO–LUMO energy
separation is useful for estimating the kinetic stability of molecules. A large HOMO–LUMO gap
can be associated with high kinetic stability and low chemical reactivity [47]. The orbital energy
level analysis for the complex at the B3LYP level shows that the HOMO–LUMO gap is 2.7 eV
for α-spin and 2.4 eV for β-spin, which suggests that this complex is stable.
3
.7. Frontier molecular orbital compositions
The partial frontier molecular orbital (FMO) compositions of 1 along with the HOMO–LUMO
energy gap are listed in tables 3 and 4. The complex was grouped according to the following
scheme: i) O1, C2-C8, N9, C10-C12, N13, C14-C20, O21 as L1; ii) the remaining Schiff base
i
i
i
moiety as L2; iii) N31, C32, O33 as NCO1 and iv) N31 , C32 , O33 as NCO2. In the ground
state the HOMO, HOMO-1, HOMO-2 and HOMO-3 of 1 mainly consist of π orbitals from the
2
Schiff base ligand moieties. Whereas, LUMO consists of 3d
z
(Cu3) with a considerable
*
participation of π (L2) and LUMO+1 to LUMO+3 consist of alternating contributions from the
10

ligands (L1 and L2) in the α-spin state. But, the scenario for the-β state is quite different. The
ground state of HOMO and HOMO-1 consist of π(L1) and d (Cu3). The HOMO-2 consist of
yz
π(L2) and d (Cu3) and HOMO-3 mainly of π(L1). It is worth noting that LUMO of 1 are alike
yz
2
2
*
to 3d of Cu1. LUMO+1 consist of π*(NCO1) and 3d (Cu3). LUMO+2 are composed of π (L1)
z
z
2
and LUMO+3 consist of 3d
z
(Cu3).
4
. Conclusion
Using the cyanate anion as one of the bridging ligands, we have synthesized and structurally
characterized one rare end-on cyanato bridged trinuclear copper(II) Schiff base complex.
Elemental analyses, IR, electronic spectra and cyclic voltammogram are in agreement with the
structure of the complex. The ONNO-tetradentate Schiff base ligand coordinates with the
copper(II) ion via two oxygens of the phenoxo-group and two nitrogens from the imine moiety.
The large value of Cu2-N31 distance shows the presence of steric interference of the
coordinating linear cyanato ligand. DFT calculations were used for calculating structural,
electronic and orbital properties of the complex and all values correlate with the structural data.
All the DFT calculations were done in gas phase.
Appendix A. Supporting Information
Supplementary information contains crystallographic data for 1 having CCDC No. 882002 along
with other necessary tables and figures. These data can be obtained free of charge via
www.ccdc.cam.ac.uk.
Acknowledgement
S. Saha acknowledges UGC (University Grants Commission, New Delhi, Govt. of India) for
awarding senior research fellowship.
Funding
Our thanks are also extended to University Grants Commission, New Delhi, India for financial
assistance to S. Mitra as emeritus fellow [F.6-6/EMERITUS-2013-14-GEN-2214/(SA-II)] to
carry out this work. C.R. Choudhury acknowledges DST- FIST (Project No. SR/FST/CSI-
2
46/2012) New Delhi, Govt. of India for instrumental support under capital heads.
11

Disclosure statement
No potential conflict of interest was reported by the authors.
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16

Figure 1. ORTEP diagram of 1 with atom numbering scheme (Thermal ellipsoids are plotted at
0% probability level).
4
17

Figure 2. Packing diagrams of 1.
18

Figure 3. UV-vis spectrum of 1 in acetonitrile.
19

Figure 4. Cyclic voltammogram of 1.
20

Figure 5. The theoretical optimized geometric structure of 1.
21

Figure 6. Molecular orbital surfaces and energy level for the HOMO of 1 (Computed at B3LYP
level; EHOMO = 8.55 eV).
22

Figure 7. Molecular orbital surfaces and energy level for the LUMO of 1 (Computed at B3LYP
level; ELUMO = 3.30 eV).
23

Table 1. Important bond lengths (Å) and angles (°) for 1. The calculated distances and angles correspond to the
B3LYP/6-311+G* level of theory.
X-ray
B3LYP/6-31G*
B3LYP/6-311+G*
PBEPBE/6-311+G*
Cu1-O1
Cu1-O21
Cu1-N9
Cu1-N13
Cu1-N31
Cu2-O1
Cu2-O21
Cu2-N31
1.928(4)
1.937(5)
1.936(5)
1.978(5)
2.682(5)
2.409(3)
2.003(4)
1.957(4)
82.45(16)
93.00(17)
158.83(19)
79.49(16)
163.24(19)
91.86(18)
97.81(19)
94.92(17)
117.39(17)
69.83(17)
180.00
1.911
1.896
1.926
1.899
2.281
2.717
1.944
1.923
83.30
90.55
165.39
81.49
152.50
93.45
98.60
92.37
113.98
70.34
179.94
94.13
179.89
85.77
1.976
2.054
2.018
2.033
2.475
2.718
2.096
1.971
80.54
91.40
165.57
87.92
165.57
87.92
97.89
100.65
108.52
64.01
177.45
85.29
179.45
81.40
1.963
2.033
1.989
2.011
2.382
2.718
2.097
1.949
79.98
91.63
163.25
89.84
171.24
89.49
98.08
100.32
106.56
62.96
178.83
85.06
179.49
80.75
i
O1-Cu1-O21
O1-Cu1-N9
O1-Cu1-N13
O1-Cu1-N31
O21-Cu1-N9
i
O21-Cu1-N13
N9-Cu1-N13
i
N9-Cu1-N31
i
i
N13-Cu1-N31
O1-Cu2-O21
i
O1-Cu2-O1
O21-Cu2-N31
95.3(2)
O21-Cu2-O21
180.00
i
O21 -Cu2-N31
84.8(2)
Symmetry code: i= [3666.00]= 1-x, 1-y, 1-z.
24

Table 2. Main calculated vertical transitions with compositions, vertical excitation energies and oscillator strength of
in acetonitrile based on the lowest lying triplet excited state.
1
Composition
E (eV)
3.2024
Oscillator strength (f)
λtheo (nm)
Assignment
λexp (nm)
H − 4 → L (14%)
H − 13 → L (19%)
H − 11 → L (15%)
H − 6 → L (10%)
H − 4 → L (53%)
H − 13 → L (30%)
H − 11 → L (23%)
H − 12 → L (46%)
H − 10 → L (19%)
H − 6 → L (20%)
H − 2 → L (58%)
H − 3 → L (16%)
0.0228
387.15
ILCT/LLCT
LMCT
382
ILCT/LMCT
3.1624
3.0316
2.9677
0.0145
0.0127
0.0116
392.05
408.96
417.77
LLCT/LMCT
ILCT/LLCT
LMCT
ILCT/LMCT
ILCT/LMCT
ILCT/LMCT
LLCT/LMCT
1.7344
0.0177
714.84
ILCT/LMCT
ILCT/LMCT
673
25

Table 3. Frontier molecular orbital composition (%) in the excited state of 1 for α-spin.
Contribution (%)
Energy
Orbital
α- MO
Main Bond Type
(
eV)
Cu1
Cu2
Cu3
L1
L2 NCO1
NCO2
2
2
2
2
2
2
2
2
1
1
1
1
1
1
1
04
03
02
01
00
99
98
97
96
95
94
93
L+5
0.25
24
0
0
0
2
0
0
1
0
1
3
4
44
0
0
0
0
0
1
1
1
0
4
4
22
1
5
4
0
0
z z z
3d (Cu1) + 3d (Cu2) + 3d (Cu3)
L+4
-1.37
-1.42
-1.6
3
96
3
0
0
π*(L2)
π*(L1)
π*(L2)
L+3
0
96
0
0
0
L+2
2
97
0
0
0
L+1
-1.63
-2.55
-5.25
-5.35
-5.6
0
98
0
0
0
π*(L1)
2
LUMO
HOMO
H-1
61
2
38
83
13
75
13
12
7
0
0
3d (Cu3) + π*(L2)
z
13
83
12
77
13
24
1
0
π(L1) + π(L2)
0
0
1
π(L1) + π(L2)
π(L1) + π(L2)
π(L1) + π(L2)
H-2
1
8
2
H-3
-5.65
-6.06
-6.21
0
1
8
H-4
4
37
40
28
21
π(L1) + π(L2) + π(NCO1) + π(NCO2)
π(L1) + π(NCO1) + π(NCO2)
H-5
1
HOMO - LUMO gap = -2.7 eV
26

Table 4. Frontier molecular orbital composition (%) in the excited state of 1 for β-spin.
Contribution (%)
Energy
Orbital
β- MO
Main Bond Type
(
eV)
Cu1 Cu2 Cu3
L1
L2 NCO1
NCO2
2
203
202
201
200
199
198
197
196
195
194
193
192
L+5
-1.38
-1.41
-1.59
-1.62
-2.52
-2.85
-5.25
-5.33
-5.58
-5.62
-6.1
0
0
0
0
5
59
1
1
0
1
3
2
0
0
0
2
0
0
0
1
0
1
3
9
85
14
96
2
14
85
1
0
0
0
2
0
55
7
2
0
2
0
2
0
0
π*(L1) + 3d
z
(Cu3)
(Cu3)
2
z
L+4
0
0
π*(L1) + 3d
2
L+3
0
0
3d (Cu3)
z
L+2
97
1
0
0
π*(L1)
2
z
L+1
37
9
1
1
π*(NCO1) + d
(Cu3)
2
LUMO
HOMO
H-1
7
9
9
3d (Cu1)
z
74
22
84
2
24
73
2
0
0
π(L1) + 3dyz(Cu3)
π(L1) + 3dyz(Cu3)
π(L2) + 3dyz(Cu3)
π(L1) + π(NCO2)
π(L1) + π(L2) + π(NCO2)
π(L1) + π(NCO2)
1
1
H-2
11
1
1
H-3
84
19
53
11
22
32
H-4
8
42
1
H-5
-6.21
4
HOMO - LUMO gap = -2.4 eV
27

Graphical abstract
28