Characterization, molecular structures and fluorescent properties of Pd(II) and Ni(II) complexes with 1-benzyl-2-methylimidazole

Article abstract of DOI:10.1016/j.poly.2012.11.047

The paper presents a combined experimental and computational study of Pd(II) and Ni(II) complexes containing chloride, thiocyanate and 1-benzyl-2-methylimidazole ligands. The complexes were studied by IR, ¹H NMR, UV-Vis spectroscopy and X-ray crystallography. Electronic structures of the complexes have been calculated using the density functional theory (DFT) method. Spin-allowed electronic transitions have been calculated using the time-dependent DFT method and the UV-Vis spectrum has been discussed on this basis. Luminescence properties of the complexes have been examined.

Full text of DOI:10.1016/j.poly.2012.11.047

Polyhedron 50 (2013) 452–460
Contents lists available at SciVerse ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Characterization, molecular structures and fluorescent properties of Pd(II) and
Ni(II) complexes with 1-benzyl-2-methylimidazole
⇑
´
J.G. Małecki , A. Maron
Department of Crystallography, Institute of Chemistry, University of Silesia, 9th Szkolna St., 40-006 Katowice, Poland
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 27 July 2012
Accepted 27 November 2012
Available online 6 December 2012
The paper presents a combined experimental and computational study of Pd(II) and Ni(II) complexes con-
taining chloride, thiocyanate and 1-benzyl-2-methylimidazole ligands. The complexes were studied by
IR, 1H NMR, UV–Vis spectroscopy and X-ray crystallography. Electronic structures of the complexes have
been calculated using the density functional theory (DFT) method. Spin-allowed electronic transitions
have been calculated using the time-dependent DFT method and the UV–Vis spectrum has been dis-
cussed on this basis. Luminescence properties of the complexes have been examined.
Ó 2012 Elsevier Ltd. All rights reserved.
Keywords:
Palladium(II) and nickel(II)
1-Benzyl-2-methylimidazole
X-ray structure
Electronic structure
DFT calculations
Fluorescence
1. Introduction
complexes is valuable as a means to the prediction of their proper-
ties [13–16].
Palladium(II) complexes currently attract a considerable inter-
est because of their potentially beneficial pharmacological proper-
ties. Palladium complexes with aromatic N-containing ligands, e.g.
derivatives of pyridine, quinoline, pyrazole and 1,10-phenanthro-
line, have shown very promising antitumor characteristics [1–4].
Some of these complexes, especially the trans analogs with nonpla-
nar heterocyclic amine ligands, have been found to overcome mul-
tifactorial cisplatin resistance in human ovarian cell lines [5,6]. On
the other hand, Pd(II) complexes have been widely explored due to
their catalytic efficiency, e.g. for various carbon–carbon and car-
bon–nitrogen bond-forming reactions [7–11]. Whilst most studies
of palladium have concentrated on the reactivity of its complexes,
little information about their electronic structures has been ob-
tained. On the other hand four-coordinate palladium(II) complexes
with square-planar geometry and various N-heteroaromatic li-
gands are one large class of useful building blocks for producing
interesting molecular structures by combination of coordination
chemistry and hydrogen bonding [12].
Here is reported an experimental and quantum chemical study
of two cationic palladium(II) complexes with pyrazole derivative
ligands. The quantum chemical study included a characterization
of the molecular and electronic structures of the complex by anal-
ysis of optimized molecular geometry, electronic populations by
using the natural bond orbitals scheme. The latter was used to
identify the nature of the interactions between the ligands and
the central ion. The calculated density of states showed the inter-
actions and influences the orbital composition in the frontier elec-
tronic structure. The time dependent density functional theory
(TD-DFT) was finally used to calculate the electronic absorption
spectrum. Based on a molecular orbital scheme, these results al-
lowed the interpretation of the UV–Vis spectrum obtained at an
experimental level.
2. Experimental
All reagents used for the synthesis of the complexes are com-
mercially available and have been used without further purifica-
tion. All operations were carried out in air.
Complexes with N-heteroaromatic ligands and such metals as
ruthenium(II), osmium(II) and rhenium(I) are by far the most stud-
ied due to their luminescent properties. In contrast d8 metal com-
plexes have been much less studied. The luminescent properties of
palladium(II) complexes strongly depend on their electronic struc-
ture. For this reason, study of the electronic structures of such
2.1. Synthesis of [Pd(X)2(PhCH2ImCH3)2] where X = Cl; SCN complexes
1-Benzyl-2-methylimidazole (0.32 cm3, ꢀ2 mmol) was dis-
solved in acetonitrile (10 cm3) and then slowly added to warm
solution of PdCl2 (0.18 g, 1 mmol) (in the synthesis of thiocyanate
complex the NH4SCN (0.15 g, 2 mmol) was added) in acetonitrile
⇑
Corresponding author.
E-mail address: gmalecki@us.edu.pl (J.G. Małecki).
0277-5387/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.poly.2012.11.047

´
453
J.G. Małecki, A. Maron / Polyhedron 50 (2013) 452–460
(30 cm3). The mixture was refluxed for about 2 h and after the time
the reaction solution was filtered and yellow single crystals were
obtained by slow evaporation of the solvent. Yield: 88% for chloride
complex (1) and 92% for thiocyanate analog (2).
of ‘‘d’’ and ‘‘p’’ polarization functions. The TD-DFT (time dependent
density functional theory) method [22] was employed to calculate
the electronic absorption spectrum of the complex in the solvent
PCM (Polarizable Continuum Model) model. In this work, 80 singlet
excited states were calculated as vertical transitions for the com-
plex. A natural bond orbital (NBO) analysis was also made for the
complex using the NBO 5.0 package [23] included in GAUSSIAN09.
Natural bond orbitals are orbitals localized on one or two atomic
centers, that describe molecular bonding in a manner similar to a
Lewis electron pair structure, and they correspond to an orthonor-
mal set of localized orbitals of maximum occupancy. NBO analysis
(1): IR (KBr): 3155
1351 d(C–CH in the plane); 1274, 1155 d(N–CH in the plane); 1067 d(CH3)
750, 695 d(Im ring)
1H NMR (400 MHz, CDCl3)
mArH; 2927 mCH; 1543 mCN, 1497 mC@C; 1454,
;
.
d
7.43–7.26 (m, Ph), 7.17 (d,
J = 1.8 Hz, Im), 7.13–7.06 (m, Ph), 6.74 (d, J = 1.8 Hz, Im), 5.03 (s,
CH2), 2.92 (s, CH3).
UV–Vis (acetonitrile, k (nm): 401.5, 276.1, 228.4, 211.5.
(2): IR (KBr): 3152
mArH; 2927
mCH; 2117
m
(CN from SCN); 1550 mCN
,
provides the contribution of atomic orbitals (s, p, d) to the NBO
r
1503 C@C; 1448, 1354 d(C–CH in the plane); 1281, 1166 d(N–CH in the plane)
m
;
and hybrid orbitals for bonded atom pairs. In this scheme, three
p
1077 d ; 723 m(SC from SCN)
.
NBO hybrid orbitals are defined, bonding orbital (BD), lone pair
(LP), and core (CR), which were analyzed on the atoms directly
bonded to or presenting some kind of interaction with the palla-
dium atom. The contribution of a group (ligands, central ion) into
a molecular orbital was calculated using Mulliken population anal-
ysis. GAUSSIAN2.2 [24] was used to calculate group contributions to
the molecular orbitals and to prepare the partial density of states
(DOS) spectra. The DOS spectra were created by convoluting the
molecular orbital information with Gaussian curves of unit height
and FWHM (full width at half maximum) of 0.3 eV.
(CH3)
1H NMR (400 MHz, CDCl3) d 7.47–6.98 (m, Ph; Im), 6.76 (d,
J = 1.8 Hz, Im), 5.09 (d, J = 13.2 Hz, CH2), 2.92 (s, CH3).
UV–Vis (acetonitrile, k (nm) (log
212.7.
e): 400.1, 325.6, 264.6, 232.5.
2.2. Synthesis of [Ni(NCS)2(PhCH2ImCH3)4]ꢁCH3OH
The 1-benzyl-2-methylimidazole (0.64 cm3, ꢀ4 mmol) was
added to the solution of NiCl2ꢁ6H2O (0.24 g, ꢀ1 mmol) and NH4SCN
(0.15 g, 2 mmol) in methanol (50 cm3). The mixture was refluxed
for about 2 h and after the time the reaction solution was filtered
and blue single crystals were obtained by slow evaporation of
the solvent.
2.5. Crystal structures determination and refinement
X-ray intensity data were collected with graphite monochro-
mated Mo Ka radiation at temperature of 295.0(2) K, with x scan
(3): IR (KBr): 3119, 3030
1537 CN, 1495 C@C; 1451, 1415 1351 d(C–CH
1140 d(N–CH in the plane); 988 d(CH3); 726 m(SC from SCN)
UV–Vis (methanol, k (nm): 1069.7, 647.8, 414.5, 267.6, 264.3,
258.2, 252.2, 247.2, 211.4.
m
ArH; 2954
m
CH; 2094 m(CN
;
from SCN)
m
m
plane); 1280,
in the
mode. Lorentz, polarization and empirical absorption correction
using spherical harmonics implemented in SCALE3 ABSPACK scal-
ing algorithm [CrysAlis RED, Oxford Diffraction Ltd., Version
1.171.29.2] were applied. The structure was solved by Patterson
method and subsequently completed by difference Fourier recy-
cling. All the non-hydrogen atoms were refined anisotropically
using full-matrix, least-squares technique. All the hydrogen atoms
were found from difference Fourier synthesis after four cycles of
anisotropic refinement, and refined as riding on the parent carbon
atom with individual isotropic temperature factor equal 1.2 times
the value of equivalent temperature factor of the parent atom. The
OLEX2 [25] and SHELXS97, SHELXL97 [26] programs were used for all the
calculations.
.
2.3. Physical measurements
Infrared spectra were recorded on a Nicolet Magna 560 spectro-
photometer in the spectral range of 4000–400 cmꢂ1 using KBr pel-
lets. Electronic spectra were measured on a Lab Alliance UV–Vis
8500 spectrophotometer in the range of 600–180 nm in methano-
lic solution. 1H NMR spectrum was obtained at room temperature
in DMSO-d6 using a Bruker 400 MHz spectrometer. Luminescence
measurements were made in ethanolic solutions on an F-2500 FL
spectrophotometer at room temperature.
Quantum yields of fluorescence were calculated using follow
Gradsg2s
equation: Us
¼
Ustd
, where Us – quantum yield of unknown
3. Results and discussion
Gradstdg2
std
sample; Ustd – quantum yield of naphthalene as reference at
313 nm equal to 0.21 [17]; Grads and Gradstd are the gradients from
the plot of integrated fluorescence intensity versus the solutions
absorbance at the excitation wavelength and gs and gstd are the
refractive indices of the solvents. Samples were prepared with
absorbance less than 0.1 at excitation wavelengths in order to min-
imize re-absorption effects and avoid inner-filter effects, which
may perturb the quantum yields values. Solvent (ethanol) using
in the measurements was spectroscopic grade and checked for
background fluorescence.
3.1. Spectroscopic characterization and molecular structure of the
complexes
The complexes were synthesized in a simple reaction between
palladium(II) or nickel(II) dichloride, ammonium thiocyanate and
1-benzyl-2-methylimidazole in acetonitrile in the case of palla-
dium complexes (1) and (2) or methanol in (3) solutions. The reac-
tions have been carried out using molar ratio 1:2:2 – palladium
complexes and 1:4:2 nickel(II) complex. The IR spectra of the com-
plexes present imidazole C@N and C@C stretches bands with max-
ima at 1550–1537 and 1503–1495 cmꢂ1 ranges. The mCN and mCS
frequencies of thiocyanato ligands present maxima at 2117 and
723 cmꢂ1 in the case of palladium complex (2) and 2094,
726 cmꢂ1 in nickel(II) complex (3). The strong band above
2100 cmꢂ1 is characteristic for S-bonded end-on NCSꢂ coordina-
tion and for N-bonded complexes, generally the C–N stretching
band is in a lower region around 2050 cmꢂ1. However, the frequen-
cies of the bands are sensitive to other factors like coexisting li-
gands and the structure of the compounds were determined
using X-ray analysis. While the M–S–C angles of S-bonded
2.4. Computational methods
Calculations were carried out using GAUSSIAN09 [18] program.
Molecular geometry of the singlet ground state of complex was
fully optimized in the gas phase at the B3LYP/DZVP level of theory
[19,20]. The frequency calculation was carried out, verifying
whether the optimized molecular structure corresponds to energy
minimum, thus only positive frequencies were expected. The DZVP
basis set [21] was used to describe the palladium atom and the ba-
sis set used for the lighter atoms (C, N, O, H, S) was 6-31G with a set

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J.G. Małecki, A. Maron / Polyhedron 50 (2013) 452–460
454
Table 1
Crystal data and structures refinement details of [PdCl2(PhCHImCH)2] (1), [Pd(SCN)2(PhCHImCH)2] (2) and [Ni(NCS)2(PhCHImCH)4]ꢁCH3OH (3) complexes.
Empirical formula
C22H24Cl2N4Pd (1)
C24H24N6PdS2 (2)
C46H48N10NiS2, CH4O (3)
Formula weight
T (K)
Crystal system
Space group
Unit cell dimensions
a (Å)
521.75
567.01
895.84
295.0(2)
triclinic
295.0(2)
monoclinic
C2/c
295.0(2)
triclinic
ꢀ
ꢀ
P1
P1
7.7343(6)
8.1076(5)
9.7896(6)
100.719(5)
94.589(6)
113.906(6)
543.10(6)
1
31.611(2)
6.1127(4)
12.7977(9)
90
92.346(7)
90
2470.9(3)
4
1.524
9.8141(4)
12.5425(6)
19.7021(9)
82.035(4)
85.048(3)
83.202(4)
2378.86(19)
2
b (Å)
c (Å)
a
(°)
b (°)
c
(°)
V (Å3)
Z
Dcalc (Mg/m3)
1.595
1.116
264
1.251
0.541
944
Absorption coefficient (mmꢂ1
F(000)
)
0.944
1152
Crystal dimensions (mm)
h (°)
Index ranges
0.34 ꢃ 0.20 ꢃ 0.04
3.65–25.05
ꢂ9 6 h 6 9,
ꢂ9 6 k 6 9,
ꢂ11 6 l 6 11
4094
0.17 ꢃ 0.09 ꢃ 0.04
3.39–25.05
ꢂ37 6 h 6 36,
ꢂ7 6 k 6 7,
ꢂ15 6 l 6 15
8794
0.24 ꢃ 0.05 ꢃ 0.04
3.35–25.05
ꢂ11 6 h 6 11,
ꢂ14 6 k 6 14,
ꢂ23 6 l 6 23
17804
Reflections collected
Independent reflections (Rint
Data/restraints/parameters
Goodness-of-fit (GOF) on F2
)
1926 (0.0263)
1926/0/134
1.036
2181 (0.0336)
2181/0/152
1.063
8397 (0.0360)
8397/0/567
0.980
Final R indices [I > 2
r
(I)]
R1 = 0.0238,
wR2 = 0.0572
R1 = 0.0251,
wR2 = 0.0578
0.362 and ꢂ0.376
R1 = 0.0269,
wR2 = 0.0624
R1 = 0.0390,
wR2 = 0.0670
0.430 and ꢂ0.306
R1 = 0.0476,
wR2 = 0.1007
R1 = 0.0875,
wR2 = 0.1122
0.289 and ꢂ0.187
R indices (all data)
Largest difference peak and hole
thiocyanato ligand in complexes are bent around 110°, the M–N–C
angles of N-bonded isothiocyanato ligands are close to linear.
The 1H NMR spectra of (1) and (2) complexes show characteris-
tic signals of imidazole derivative hydrogens. The signals at 6.74,
5.03 and 2.92 ppm are attributed to C(2)H, CH2 and CH3 protons
of 1-benzyl-2-methylimidazole, respectively. The proton NMR
spectrum for nickel(II) complex was not measured due to para-
magnetic d8 configuration of nickel central ion in octahedral ligand
field.
In the molecular structure of nickel(II) complex (3) the thiocyanate
ligands are bonded to central ion by nitrogen donor atoms occur
the isothiocyanate form. The differences in the bonding of thiocy-
anate ligands in the palladium and nickel complexes are visible in
the angles between central ions and pseudohalide ligands equal to
106.35(10)° in (2) and 159.0(2)°, 163.3(3)° in complex (3) adequate
to S- and N-bonding mode of NCSꢂ. The Pd–N distances in both
complexes are very close and by about 0.1 Å shorter than Ni–
N(Im) distances. The Pd(1)–Cl(1) bond length fall within a usual
range (2.298–2.354 Å) reported for four-coordinate palladium(II)
complexes [27]. The Pd(1)–S(CN) and Ni(1)–N(CS) bonds lengths
are also normal. Square planar environments of palladium as well
as octahedral geometry in complex (3) are distorted and the devi-
ation from the expected 90° bond angles comes from the angles be-
tween thiocyanato and imidazole derivative ligands. The
deviations of N(Im)–M–S(N) angles from the right angle do not ex-
ceed 3°.
In the crystal structures of the (1) and (3) complexes several
weak inter- and intramolecular hydrogen bonds exist [28] given
in Table 3. The packing of molecules of (1) is presented on the
Fig. 2 and as one can see the complex molecules form layers along
a axis linking by C(4)–H(4A)ꢁ ꢁ ꢁCl(1) hydrogen bond. In the struc-
ture of thiocyanate complex (2) an infinite one-dimensional chain
along b axis is created by interaction between sulfur atoms gener-
ated by 1 ꢂ x, y, ꢂ1/2 ꢂ z symmetry. In the case of nickel(II) com-
plex (3) except the hydrogen bonds the stacking contacts as
ꢀ
The complexes crystallize in triclinic P1 (1) and (3) and mono-
clinic C2/c complex (2) space groups. Moreover nickel(II) complex
(3) crystallizes in a form solvated by one methanol molecule. De-
tails concerning crystal data and refinement are gathered in Ta-
ble 1. Fig. 1 presents molecular structures, and selected bond
distances and angles are presented in Table 2. The slightly in-
creases values of R1 and wR2 indices equal to 0.0476 and 0.1122
in the case of complex (3) are from the quality of measured crystal.
In the structure of this complex the sulfur S(2) atom is disordered
what on one hand is resulted from the quality of the crystal and
on the other hand is connected with the short contact interactions
between S(2)/(2A) and C(27)–H(27A), C(43)–H(43) atoms of 1-ben-
zyl-2-methylimidazole ligands generated by symmetry operations.
The S(2A)ꢁ ꢁ ꢁH(43)-C(43)[x, ꢂ1 + y, z] and S(2A)ꢁ ꢁ ꢁH(27A)–C(27)
[2 ꢂ x; 1 ꢂ y; 2 ꢂ z], S(2)ꢁ ꢁ ꢁH(27A)–C(27)[2 ꢂ x; 1 ꢂ y; 2 ꢂ z] dis-
tances are 2.946 and 2.876, 2.832 Å respectively. From the second
reason the dynamical disorder could be considered and determina-
tion of the structure in the low temperature should eliminate this
factor. Palladium(II) atoms in the complexes (1) and (2) have
square planar environments with the 1-benzyl-2-methylimidazole
ligands bonded to the metal centre through the imidazole nitrogen
atoms and thiocyanate by sulfur (complex 2). The halide and imid-
azole derivative ligands in complexes (1) and (2) are mutually trans
disposed. The structure of octahedral complex (3) with 1-benzyl-2-
methylimidazole ligands in the equatorial plane and mutually
trans NCSꢂ ligand may be described by D4h Schoenflies symbol
and the palladium(II) complexes (1) and (2) have D2h symmetry.
presented on Fig. 2 are visible. The C–Hꢁ ꢁ ꢁ
imidazole C(35)[ꢂ1 + x; y; z]ꢁ ꢁ ꢁH(16B)C(16), C(24)–H(24)[1 + x; y;
-stacking between phenyl rings
p distances between
z]ꢁ ꢁ ꢁC(40) and parallel-dispaced
p
C(7)–C(11)ꢁ ꢁ ꢁC(7)–C(11)[2 ꢂ x, ꢂy, 1 ꢂ z] are 3.224, 2.818 and
3.380 Å respectively.
3.2. Electronic structure
To obtain an insight in electronic structures and bonding
properties of the complex, calculations using the density functional

´
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J.G. Małecki, A. Maron / Polyhedron 50 (2013) 452–460
Fig. 1. Molecular structures of [PdCl2(PhCHImCH)2] (1), [Pd(SCN)2(PhCHImCH)2] (2) and [Ni(NCS)2(PhCHImCH)4]ꢁCH3OH (3) complexes with anisotropic displacement
parameters shown at the 50% probability level. On the structure of complex (3) the hydrogen atoms and solvent molecule were omitted for clarity.
theory (DFT) method with the B3LYP functional of GAUSSIAN09 were
carried out. Before the calculations, their geometry was optimized
in singlet states using the DFT method with the B3LYP functional.
In general, the predicted bond lengths and angles are in good
agreement with the values based on the X-ray crystal structure
data, and the general trends observed in the experimental data
are well reproduced in the calculations as one can see from the
data given in Table 2. Vibrational harmonic frequency analyses of
the optimized geometries were utilized to ensure that it is true lo-
cal minimum having no imaginary frequencies. From the data col-
lected in this table, it may be seen that the bond lengths are
0
maximally elongated by ꢀ0.1 ÅA in the calculated gas phase struc-
tures, while the bond angles changed maximally by 2° except the
Ni–NCS bonds which the negligence of intermolecular interactions

´
J.G. Małecki, A. Maron / Polyhedron 50 (2013) 452–460
456
Table 2
population analyses, are 0.44, 0.29 form (1), (2) and 1.55 form com-
plex (3) respectively. This is a result of charge donation from the
ligands to the metal center and as one can see the charge flows
are much greater in the case of square planar palladium complexes
than octahedral nickel(II) complex (3). These differences relate
both to the number of imidazole derivative ligands in the coordina-
tion spheres and the difference between chloride and thiocyanate
ligands. M–N(Im) interactions have Coulomb-type character, which
are visible in the calculated Wiberg bond indices with values con-
siderably lower than one, 0.4, 0.2 in complexes (1), (2) and 0.1 in
complex (3). The chloride and thiocyanate ligands show also
non-covalent bonding with palladium and nickel with Wiberg indi-
ces 0.6 and 0.2 respectively.
Selected bond lengths (Å) and angles (°) with the optimized geometry values for
[PdCl2(PhCHImCH)2] (1), [Pd(SCN)2(PhCHImCH)2] (2) and [Ni(NCS)2(PhCHImCH)4]-
ꢁCH3OH (3) complexes.
(1)
(2)
Exp.
Exp.
Calc.
Calc.
Exp.
Calc.
Bond lengths (Å)
Pd(1)–Cl(1)
Pd(1)–N(1)
Pd(1)–S(1)
S(1)–C(12)
N(3)–C(12)
2.3011(6)
2.0202(17)
2.38
2.08
2.023(2)
2.3202(8)
1.656(3)
1.144(4)
2.08
2.42
1.69
1.17
Angles (°)
Analysis of the frontier molecular orbitals is useful for under-
standing the spectroscopic properties, such as electronic absorp-
tion and emission spectra of organometallic complexes. The
overlap partial densities of states (DOS) in terms of Mulliken pop-
ulation analysis were calculated using the GAUSSIAN program. As one
can see from the Fig. 3 the interaction of isothiocyanate ligands
with nickel(II) is weaker compared with palladium(II) complex
(2). Moreover interaction of chloride ligands with Pd(II) in complex
(1) is also stronger than between NCSꢂ and Ni(II).
N(1)–Pd(1)–Cl(1)
N(1)–Pd(1)–S(1)
Pd(1)–S(1)–C(12)
N(3)–C(12)–S(1)
89.40(5)
89.26
86.05(7)
106.35(10) 105.80
176.2(3) 178.17
85.50
(3)
Exp.
Calc.
Bond lengths (Å)
Ni(1)–N(1)
Ni(1)–N(3)
Ni(1)–N(5)
Ni(1)–N(7)
Ni(1)–N(9)
Ni(1)–N(10)
S(1)–C(45)
S(2)–C(46)
N(9)–C(45)
N(10)–C(46)
2.133(2)
2.135(2)
2.1339(19)
2.126(2)
2.077(3)
2.073(3)
1.633(3)
1.663(8)
1.147(3)
1.137(3)
2.16
2.17
2.17
2.16
2.06
2.05
1.69
1.69
1.19
1.19
Fig. 4 presents the simplified molecular orbital diagrams for
palladium complexes (1) and (2). The splitting of d palladium orbi-
tals is specific to square planar D4h symmetry of the complexes.
The eg, a1g, b2g and b1g terms consist mainly of the palladium d
orbitals with some admixture of halide ligand orbitals. The imidaz-
ole derivative ligands play role in HOMO ꢂ 4 (dxz) and LUMO orbi-
tals. In the thiocyanate complex (2) an increase in energy of the
Angles (°)
( )
pꢄxy
N(10)–Ni(1)–N(9)
N(1)–Ni(1)–N(7)
N(9)–Ni(1)–N(7)
N(10)–Ni(1)–N(1)
N(9)–Ni(1)–N(1)
N(7)–Ni(1)–N(1)
N(10)–Ni(1)–N(5)
N(9)–Ni(1)–N(5)
N(7)–Ni(1)–N(5)
N(1)–Ni(1)–N(5)
N(10)–Ni(1)–N(3)
N(9)–Ni(1)–N(3)
N(7)–Ni(1)–N(3)
N(1)–Ni(1)–N(3)
N(5)–Ni(1)–N(3)
Ni(1)–N(9)–C(45)
Ni(1)–N(10)–C(46)
N(9)–C(45)–S(1)
N(10)–C(46)–
177.71(8)
89.88(9)
91.45(9)
88.94(8)
89.24(8)
88.34(8)
92.58(8)
89.28(8)
89.98(8)
177.73(10)
89.74(9)
89.00(9)
177.67(8)
93.95(8)
87.74(8)
159.0(2)
163.3(3)
178.7(3)
162.6(13)
[169.6(4)]
178.8
90.8
92.7
88.6
90.2
88.5
92.0
89.2
88.5
177.7
89.9
89.1
177.7
92.7
88.1
170.2
178.1
179.9
179.2
HOMO is visible and the differences in energies between b2g
and b1g
HOMO–LUMO gap in nickel(II) complex (3) is higher with value
(
rꢄx2ꢂy2 ) levels equals to 3.73 and 3.11 eV respectively;
equal to 4.04 eV.
3.3. Absorption and emission electronic spectra
UV–Vis spectra of the palladium(II) complexes exhibit bands
with maxima at 401.5, 276.1, 228.4, 211.5 nm and 400.1, 325.6,
264.6, 232.5. 212.7 nm. The nature of the electronic transitions
has been analyzed using time-dependent DFT (TDDFT) based on
the optimized singlet state geometry. The electronic transitions
were calculated by use of the long range corrected version of the
B3LYP functional – CAM-B3LYP which employs the Coulomb-atten-
uating method. This functional provides a better estimation of exci-
tation energies and oscillators strengths, especially for transitions
with charge-transfer character. In Table 4, several calculated elec-
tronic transitions and their assignments to the experimental
absorption bands are gathered. The assignment of the calculated
orbital excitations to the experimental bands was based on an over-
view of the composition and relative energy to the orbitals HOMO
and LUMO involved in the electronic transitions.
S(2)[S(2A)]
Table 3
Hydrogen bonds for [PdCl2(PhCHImCH)2] (1) and [Ni(NCS)2(PhCHImCH)4]ꢁCH3OH (3)
complexes (Å and °).
D–Hꢁ ꢁ ꢁA
(1)
C(4)–H(4A)ꢁ ꢁ ꢁCl(1) #1
(3)
C(4)–H(4A)ꢁ ꢁ ꢁN(10)
C(26)–H(26C)ꢁ ꢁ ꢁN(10)
C(37)–H(37C)ꢁ ꢁ ꢁN(9)
C(40)–H(40)ꢁ ꢁ ꢁN(8)
d(D–H)
0.96
d(Hꢁ ꢁ ꢁA)
d(Dꢁ ꢁ ꢁA)
<(DHA)
135
As it may be seen from the data, ligand-to-metal charge transfer
transitions are dominant in the electronic spectrum of the chloride
complex (1). However in the spectrum of the complex charge
transfer transitions from imidazole derivative ligands to b1g term
2.80
3.545(3)
0.96
0.96
0.96
0.93
2.61
2.60
2.54
2.57
3.311(3)
3.229(3)
3.264(4)
2.882(4)
130
123
132
100
(pIm ? d
) play significant role in the energy range up to
x2ꢂy2
236 nm. The transition between a1g and b1g terms is calculated
only at lowest energy band (390.1 nm). The metal-to-ligand charge
transfer transitions were calculated at the energy close to 220 nm
and the highest experimental band is attributed to ligand-to-ligand
charge transfer processes. In case of the spectrum of thiocyanate
complex (2) the transitions between terms result from splitting,
in D2h ligand field, palladium 4d orbitals were calculated in the
Symmetry transformations used to generate equivalent atoms: #1 1 ꢂ x, 2 ꢂ y, ꢂz.
for the gas phase caused significant differences between experi-
mental and calculated values.
whole spectrum with admixture of LMCT transitions (pL/SCN ? b1g).
The formal charges of palladium and nickel central ions are +2
in the complexes but the calculated charges, obtained from natural
The difference in the character of the spectra refers to lumines-
cence property of the complexes.

´
457
J.G. Małecki, A. Maron / Polyhedron 50 (2013) 452–460
Fig. 2. Crystal packing and contacts in the crystal structures of the [PdCl2(PhCHImCH)2] (1), [Pd(SCN)2(PhCHImCH)2] (2) and [Ni(NCS)2(PhCHImCH)4] (3) complexes.
The lowest state of the nickel(II) ion is 3F term, which splits into
3B2g/3Eg and 3A2g/3Eg in the D4h symmetry. The levels are then fur-
ther split by spin–orbit coupling but this coupling is small that can
3
3
3A2g 3T2g and T1g terms in the field with Oh symmetry and B1g
, ,

´
J.G. Małecki, A. Maron / Polyhedron 50 (2013) 452–460
458
for complexes (1), (2) and (3) respectively, were observed. Fig. 5
presents the excitation and fluorescence spectra. The red shifts of
the emission maxima are typical to 3d and 4d metal(II) complexes
and the emissions originating from the charge transfer states, de-
rived from the excitations involving a d
? pligand transitions.
p
The quantum efficiencies are equal to 0.003 for complex (1), and
0.011 for (2) and 0.131 for (3) and as can be seen thiocyanate li-
gands considerably increase the luminescence. Moreover the quan-
tum efficiency of palladium complex (3) is ten times bigger than
[Ni(NCS)2(PhCHImCH)4]ꢁCH3OH (3).
In the electronic spectrum of [Pd(SCN)2(PhCHImCH)2] (2) the
eg/pL/SCN ? dx2ꢂy2
/
p⁄
(b1g) transitions (MLCT character) were
ImCH
calculated in the energy range close to excitation wavelength. Sim-
ilarly, MLCT transitions about 320 nm were calculated for complex
(3) but with large admixture of LLCT (pIm
fluorescence has less intensity and efficiency. In the case of chlo-
ride complex (1) the LMCT ( Im ? d ) transitions predominate
that explain its weak fluorescence.
?
p⁄Im) and thus the
p
x2ꢂy2
Summarizing, the new palladium(II) and nickel(II) complex
with 1-benzyl-2-methylimidazole ligand have been synthesized.
The molecular structures of the complexes are determined by X-
ray, and spectroscopic properties as infrared, 1H NMR, UV–Vis
spectra were studied. Based on the crystal structures, computa-
tional studies were carried out in order to determine their
electronic structures. Electronic spectra were calculated using of
TD-DFT method and character of transitions was commented in
connection with structure of molecular orbitals. Emission proper-
ties of the complexes have been examined. Based on the electronic
structures of the complexes showed the contributions of ligands in
terms resulted from splitting of palladium or nickel d orbitals in
the complexes the emission originating from the lowest energy
metal to ligand charge transfer (MLCT) state, derived from the exci-
Fig. 3. The overlap partial density-of-states diagrams.
be neglected. As a consequence, the first and second spin allowed
transition bands observed on the UV–Vis spectrum (1069.7,
3
647.8 nm) of complex (3) are assigned to B2g/3Eg(3F) ? 3B1g(3F)
and 3A2g/3Eg(3F) ? 3B1g(3F), respectively. The third absorption band
3
(414.5 nm) results from
a
spin allowed transition A2g/3Eg
(3P) ? 3B1g (3F). The value of the ligand field parameter 10Dq of
the complex calculated on the basis of electronic bands positions
is equal to 9348 cmꢂ1 and the Racah parameter B is equal to
1048 cmꢂ1. In consequence the nepheloauxetic parameter close
to one confirms Coulomb-type character of nickel(II)–ligands inter-
actions in the complex.
Emission characteristic of the complexes have been prepared in
optically diluted ethanolic solutions at room temperature. The
measurement of quantum yields have been examined by compar-
tation involving d
?
p
ligand transitions. In the UV–Vis spectrum of
chloride complex (1) these kind of transitions play role at the high-
er energies where are covered by LLCT transitions (pIm
p⁄Im).
p
?
ative method using naphthalene in ethanol as
a standard
The ligand-to-ligand charge transfer transitions occurred at the exci-
tation energy of nickel(II) complex (3) are also responsible for low-
er, compared with [Pd(SCN)2(PhCHImCH)2], fluorescence efficiency
(
UST = 0.21). The solutions of the complexes were excited in the
range between 250 and 500 nm and the fluorescence with maxima
at 491, 385 and 388 nm by the excitations at 340, 303 and 311 nm
Fig. 4. Splitting of 4d palladium(II) orbital diagram for the (1) and (2) complexes.

´
459
J.G. Małecki, A. Maron / Polyhedron 50 (2013) 452–460
Table 4
The calculated electronic transitions and its assignments to the experimental absorption bands for palladium complexes (1) and (2).
The most important orbital excitations
Character
k (nm)
f
Exp. k (nm)
(1)
H-1 ? LUMO (97%)
H-10 ? LUMO (94%)
H-5 ? LUMO (87%)
H-11 ? LUMO (91%)
a
1g ? b1g
L ? b1g
L/Cl ? b1g
L ? b1g
390.1
328.4
321.2
316.7
308.2
270.9
236.7
220.9
217.2
211.9
0.0038
0.0204
0.0013
0.0028
0.0002
0.1422
0.3705
0.0559
0.0149
0.0201
401.5
p
p
p
p
p
p
a1g
a1g
pL/Cl
H-7 ? LUMO (97%)
L/Cl ? b1g
L/dPd ? b1g
L/dPd ? b1g
H-14 ? LUMO (17%), H-12 ? LUMO (79%)
H-14 ? LUMO (80%), H-12 ? LUMO (15%)
H-1 ? L + 2 (32%), HOMO ? L + 1 (41%)
H-1 ? L + 4 (41%), HOMO ? L + 3 (49%)
H-2 ? L + 3 (30%), H-2 ? L + 5 (66%)
276.1
228.4
211.5
?
?
?
p⁄L; b2g
p⁄L; b2g
?
?
p⁄
p⁄L
L
p⁄
L
(2)
H-9 ? LUMO (10%), H-3 ? LUMO (48%), H-1 ? LUMO (37%)
H-9 ? LUMO (51%), H-8 ? LUMO (41%)
H-9 ? LUMO (26%), H-8 ? LUMO (56%), H-4 ? LUMO (12%)
H-4 ? LUMO (20%), H-3 ? LUMO (24%), H-1 ? LUMO (49%)
H-8 ? LUMO (53%), H-6 ? LUMO (18%), H-4 ? LUMO (22%)
H-12 ? LUMO (99%)
p
p
p
L ? b1g; eg ? b1g
L ? b1g
L ? b1g; eg ? b1g
;
p
SCN ? b1g
393.2
328.9
323.2
294.3
280.4
265.7
229.3
214.7
0.0063
0.0236
0.5346
0.3616
0.3226
0.0056
0.0117
0.0118
400.1
325.6
e
g ? b1g
;
pSCN ? b1g
pL ? b1g; eg ? b1g
a
a
a
1g ? b1g
1g ? b1g
1g ? b1g
264.6
232.5
H-12 ? LUMO (11%), H-8 ? LUMO (17%), H-4 ? LUMO (47%), H-1 ? LUMO (13%)
H-12 ? LUMO (82%)
; pL ? b1g; eg ? b1g; pSCN ? b1g
L denotes 1-benzyl-2-methylimidazole.
Appendix A. Supplementary material
CCDC 882436, CCDC 883502 and CCDC 885047 contain the sup-
plementary crystallographic data for [PdCl2(PhCHImCH)2] (1),
[Pd(SCN)2(PhCHImCH)2] (2) and [Ni(NCS)2(PhCHImCH)4]ꢁCH3OH
(3) complexes. These data can be obtained free of charge via
http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the
Cambridge Crystallographic Data Centre, 12 Union Road, Cam-
bridge CB2 1EZ, UK; fax: +44 1223 336 033; or e-mail:
deposit@ccdc.cam.ac.uk.
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ꢂy2
orbital (SCNꢂ (39%), PhCHImCH (18%)). In the case of nickel com-
plex (3) excited states are localized on 1-benzyl-2-methylimidaz-
ole ligands (100%).
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