Novel dichloro(bis{2-[1-(4-methylphenyl)-1H-1,2,3-triazol-4-yl-κN3]pyridine-κN})metal(II) coordination compounds of seven transition metals (Mn, Fe, Co, Ni, Cu, Zn and Cd)

Article abstract of DOI:10.1016/j.poly.2018.03.026

The synthesis, characterization, DFT and, in two cases, the structure of seven novel dichloro(bis{2-[1-(4-methylphenyl)-1H-1,2,3-triazol-4-yl-κN³]pyridine-κN})metal(II) coordination compounds ([M(L²)₂Cl₂]), containing transition metals of groups 7–12, are described. Both experimentally measured magnetic moment and DFT calculations showed that d⁵ Mn(II) (with μ_eff = 5.62 B.M., S = 5/2), d⁶ Fe(II) (with μ_eff = 5.26 B.M., S = 2), d⁷ Co(II) (with μ_eff = 3.98 B.M., S = 3/2), d⁸ Ni(II) (with μ_eff = 3.00 B.M., S = 1) and d⁹ Cu(II) (with μ_eff = 1.70 B.M., S = ?) are all paramagnetic, while d¹⁰ Zn(II) and Cd(II) are diamagnetic with S = 0. DFT calculations on the possible isomers of these coordination compounds, showed that the cis–cis–trans and the trans–trans–trans isomers, with the pyridyl groups trans to each other, are the lowest in energy. The trans–trans–trans isomers were experimentally characterized by X-ray crystallography for [Ni(L²)₂Cl₂] and [Zn(L²)₂Cl₂]·L² in this study. In the solid state the coordination compounds are connected by intermolecular hydrogen bonds, mainly involving the chloride atoms, to form 3D supramolecular structures. Computational chemistry calculations, using Natural Bonding Orbital calculations, identified these inter-molecular hydrogen bonds, C–H?Cl, by a donor–acceptor interaction from a filled lone pair NBO on Cl to an empty antibonding NBO on (C–H). The inter-molecular hydrogen bonds were also identified by QTAIM determined bonding paths between Cl and the respective hydrogen. The theoretically calculated computational chemistry results thus give an understanding on a molecular level why in the solid state where inter-molecular forces and packing play a role, the trans–trans–trans isomers are mostly obtained.

Full text of DOI:10.1016/j.poly.2018.03.026

Accepted Manuscript
Novel dichloro(bis{2-[1-(4-methylphenyl)-1H-1,2,3-triazol-4-yl-κNc]pyri-
dine-κN})metal(II) coordination compounds of seven transition metals (Mn, Fe,
Co, Ni, Cu, Zn and Cd)
J. Conradie, M.M. Conradie, K.M. Tawfiq, M.J. Al-Jeboori, S.J. Coles, C.
Wilson, J.H. Potgieter
PII:
S0277-5387(18)30151-7
https://doi.org/10.1016/j.poly.2018.03.026
POLY 13092
DOI:
Reference:
Polyhedron
To appear in:
Received Date:
Accepted Date:
23 December 2017
25 March 2018
Please cite this article as: J. Conradie, M.M. Conradie, K.M. Tawfiq, M.J. Al-Jeboori, S.J. Coles, C. Wilson, J.H.
Potgieter,
Novel
dichloro(bis{2-[1-(4-methylphenyl)-1H-1,2,3-triazol-4-yl-κNc]pyridine-κN})metal(II)
coordination compounds of seven transition metals (Mn, Fe, Co, Ni, Cu, Zn and Cd), Polyhedron (2018), doi: https://
doi.org/10.1016/j.poly.2018.03.026
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Novel dichloro(bis{2-[1-(4-methylphenyl)-1H-1,2,3-triazol-4-yl-N3]pyridine-
N})metal(II) coordination compounds of seven transition metals (Mn, Fe, Co, Ni, Cu, Zn
and Cd)
J. Conradie1*, M.M. Conradie1, K.M. Tawfiq2,3, M.J. Al-Jeboori3, S.J. Coles4 C. Wilson5 and J.H.
Potgieter2,6*
1. Department of Chemistry, University of the Free State, P.O. Box 339, Bloemfontein, 9300, South
Africa
2. Division of Chemistry and Environmental Science, Manchester Metropolitan University,
Manchester, M1 5GD, UK
3. Department of Chemistry, College of Education for Pure Science (Ibn Al-Haitham), University of
Baghdad, Baghdad, Iraq
4. EPSRC National Crystallography Service, Chemistry, University of Southampton, Southampton,
SO17 1BJ, England
5. School of Chemistry, University of Glasgow, Joseph Black Building. University Avenue,
Glasgow, G12 8QQ, Scotland
6. School of Chemical and Metallurgical Engineering, University of the Witwatersrand, Private Bag
X3, Wits, 2050, South Africa
*Contact author details:
Name: Jeanet Conradie
Tel: +27-51-4012194
Fax: +27-51-4017295
e-mail: conradj@ufs.ac.za
Abstract
The synthesis, characterization, DFT and, in two cases, the structure of seven novel dichloro(bis{2-
[1-(4-methylphenyl)-1H-1,2,3-triazol-4-yl-N3]pyridine-N})metal(II) coordination compounds
([M(L2)2Cl2]), containing transition metals of groups 7 – 12, are described. Both experimentally
measured magnetic moment and DFT calculations showed that d5 Mn(II) (with µeff = 5.62 B.M., S =
5/2), d6 Fe(II) (with µeff = 5.26 B.M., S = 2), d7 Co(II) (with µeff = 3.00 B.M., S = 3/2), d8 Ni(II)

(with µeff = 3.00 B.M., S = 1), d9 Cu(II) (with µeff = 1.70 B.M., S = ½), are all paramagnetic, while
d10 Zn(II) and Cd(II) are diamagnetic with S = 0. DFT calculations on the possible isomers of these
coordination compounds, showed that the cis-cis-trans and the trans-trans-trans isomers, with the
pyridyl groups trans to each other, are the lowest in energy. The trans-trans-trans isomers were
experimentally characterized by x-ray crystallography for [Ni(L2)2Cl2] and [Zn(L2)2Cl2].L2 in this
study. The coordination compounds are connected by intermolecular hydrogen bonds, mainly
involving the chloride atoms, to form 3D supramolecular structures. Computational chemistry
calculations, using Natural Bonding Orbital calculations, identified these inter-molecular hydrogen
bonds, C−H···Cl, by a donor-acceptor interaction from a filled lone pair NBO on Cl to an empty
antibonding NBO on (C-H). The inter-molecular hydrogen bonds were also identified by QTAIM
determined bonding paths between Cl and the respective hydrogen. The theoretically calculated
computational chemistry results thus give an understanding on a molecular level, while in the solid
state where inter-molecular forces and packing play a role, the trans-trans-trans isomers are mostly
obtained.
Keywords
(1,2,3-triazol-4-yl)pyridine; coordination compound; DFT; bonding-path; donor-acceptor
TOC graphics and text
DFT calculations on the optimized structure of two dichloro(bis{2-[1-(4-methylphenyl)-1H-1,2,3-
triazol-4-yl-N3]pyridine-N})zinc(II) molecules (6) show C−H···Cl donor-acceptor interactions,
similar to those observed in the X-ray structures of (6) and related nickel(II) structures.

Highlights
 Group 7 – 12 metal(II)-(1,2,3-triazol-4-yl)pyridine coordination compounds
 H-bonded 3D supramolecular metal-(1,2,3-triazol-4-yl)pyridine solid state structures
 Experimental spin state in agreement with DFT calculated values
 QTAIM inter-molecular C−H···Cl hydrogen bonds
 NBO inter-molecular LP(Cl)  BD*(C-H) donor-acceptor interactions
1
Introduction
The 1,3-dipolar cycloaddition “click” reaction between an azide and an alkyne to give a 1,2,3-
triazole was reported by Huisgen in 1961 [1]. In 2002 the Copper(I)-catalyzed Azide-Alkyne
Cycloaddition (CuAAC) to prepare a 1,2,3-triazole was reported [2,3]. The existence of relatively
basic nitrogen atoms in the 1,2,3-triazole rings, and the possibility of introducing additional donor
groups in the substituents (Figure 1), made the CuAAC “click” reaction an attractive method to
prepare differently substituted 1,2,3-triazoles. These compounds have been used as ligands to
coordinate to various metal ions that display a range of applications such as in electrochemical and
photochemical studies, in supramolecular chemistry, magnetism, metal-ion sensing and catalysis
[4]. The reasons for the success of the “click” reaction, is that it is easy to carry out and is widely
applicable. It is not affected by a variety of functional groups, and can be carried out with a variety
of Cu(I) catalysts and solvents, including aqueous conditions. The Cu(I) catalysts overcome the
high activation energy barrier of the non-catalyzed Huisgen reaction by changing the mechanism of
the reaction. A large variety of copper catalysts can be used for the CuAAC reaction, on condition
that the maximum concentration of Cu(I) species is generated during the reaction. The pre-catalyst
can be a Cu(II) salt (usually CuSO4) together with a reducing agent (often sodium ascorbate) or a
Cu(I) compound in the presence of a base or amine ligand and a reducing agent to prevent oxidation
to Cu(II). Some strong oxidising cupric salts or complexes such as Cu(OAc)2 also work. The
solvent is very flexible from organic to aqueous, with the most commonly used combination water
+ an alcohol (t-BuOH, MeOH or EtOH). The key role of the solvent or solvent mixture is to
solubilize the substrates and Cu(I) catalyst in order to ensure rapid reactions. Such aqueous
conditions are very useful for biochemical conjugations, as well as for organic syntheses.
We recently reported on the synthesis of a series of differently substituted 1,2,3-triazole
chromophores, the substituted 2-(1-phenyl-1H-1,2,3-triazol-4-yl)pyridine ligands [5], see Figure 2
left. These versatile ligands were found to coordinate to various first row transition metals, such as
manganese, cobalt and nickel [6]. Here we extend the series to include more first row transition
metal(II) coordination compounds, copper and zinc, as well as a second row transition metal(II)
coordination compound, cadmium, containing the 2-(1-(4-methyl-phenyl)-1H-1,2,3-triazol-1-

yl)pyridine chromophore (Figure 2 with R = CH3). This series of seven novel coordination
compounds is the first series of pyridyl-triazole based transition metal coordination compounds
where seven different transition metals are coordinated to the same 1,2,3-triazole chromophore,
namely 2-(1-(4-methyl-phenyl)-1H-1,2,3-triazol-1-yl)pyridine.
Figure 1: The Cu(I) catalyzed “click” reaction between an azide and alkyne to produce a
1,2,3-triazole that can be functionalized with different donor atoms or groups.
Figure 2: Synthesis of 2-pyridyl-(1,2,3)-triazole ligands from an azide and alkyne by the Cu(I)
catalyzed “click” reaction (reaction left). Synthesis of the various dichloro(bis{2-[1-(4-R-
phenyl)-1H-1,2,3-triazol-4-yl--N3]pyridine-N})metal(II) coordination compounds, M = Mn
(1), Fe (2), Co (3), Ni (4), Cu (5), Zn (6) and Cd (7) (reaction right), with the structure of 1,2,3-
triazole chromophores, 2-(1-(4-R-phenyl)-1H-1,2,3-triazol-1-yl)pyridine shown in the middle.
R = H for the ligand L1, and R = CH3 for the ligand L2.

2
Methods and Materials
2.1 Synthesis of 2-(1-(4-methyl-phenyl)-1H-1,2,3-triazol-1-yl)pyridine (L2)
The ligand, 2-(1-(4-methyl-phenyl)-1H-1,2,3-triazol-1-yl)pyridine (L2), was synthesized and
characterized as described previously [5,7]. A mixture of 1-azido-4-methylbenzene (0.75 g; 5.63
mmol) and 2-ethynylpyridine (0.69 g; 6.75 mmol, 1.2 eq) was dissolved in a 1:1 mixture of
water/tert–butyl alcohol (100 ml). After stirring for 20 min, a solution of CuSO4.5H2O (0.41 g;
1.64 mmol) in water (10 ml) was added dropwise followed by a freshly prepared solution of Na-
ascorbate (0.37 g; 1.85 mmol) in water (5 ml). The mixture was allowed to stir for 24h at RT, and
then an aqueous ammonia solution (15%; 50 ml) was added. The mixture was stirred for a further
20 min, and then extracted with dichloromethane (2x100 ml). The organic phase was washed twice
with water (2x100ml) and filtered through celite to remove trapped Cu(I)-salts ([Cu(NH3)6]+). The
combined organic layer was washed with brine (2x100ml), and then dried over MgSO4. The organic
solvent was removed under vacuum to give the crude product as a bright yellow solid with yield of
(0.98g; 74%). Recrystallisation from a mixture of CH2Cl2: CH3OH (1:1) gave the product as
colourless crystals (0.93g; 70%), mp. 128-129 °C. ATR / IR: (cm-1): 3128, 3099, 2947, 2919,
1597, 1592, 1566, 1549, 1471, 1271, 1238, 1212, 1176, 1148, 1031, 998, 813 and 784, 745. NMR
data (ppm), δH (400 MHZ, CD2Cl2- d2): 8.60-8.58 (1H, ddd, 1JHH = 0.92 Hz , 2JHH = 1.83Hz, 3JHH
=
2
3
5.04Hz, H14), 8.57 (1H, s, H8), 8.21-8.18 (1H, td, 1JHH = 0.92 Hz , JHH = 1.37Hz, JHH = 7.79Hz ,
1
2
H11), 7.82-7.77 (1H, dt, JHH = 1.83Hz, JHH = 7.79Hz, H12), 7.70-7.67 (2H, d, JHH = 8.70Hz, Ar-
1
2
H2,6), 7.35-7.33 (2H, d, JHH = 8.24Hz, Ar-H3,5), 7.26-7.23 (1H, ddd, JHH = 1.37Hz, JHH = 4.58Hz,
3J = 7.33, H13), 2.41 (3H, s, CH3, H7); δc (100.63MHZ, CD2Cl2-d2): 21.14 (C7), 120.34 (C11),120.37
(C8), 120.56 (C3,C5), 123.26 (C13), 130.56 (C2,C6), 135.05 (C4), 137.13 (C12), 139.40 (C1), 149.14
(C9), 149.89 (C14), 150.48 (C10). These assignments were confirmed using DEPT 13C (135°), 1H-1H
1
COSY and H-13C HMQC two-dimensional correlations. NMR spectra are provided in the
Electronic Supplementary Information. HRMS (P+NSI): [M+H]+ (100%): m/z = 237.1135
calculated for (C14H13N4); found 237.1133. The fragment of the molecular ion plus H+ [(M-
N2)+H]+ (15%): calculated for (C14H13N2); found m/z = 209.1133.

2.2 Synthesis of the 2-(1-(4-methylphenyl)-1H-1,2,3-triazol-1-yl)pyridine-metal coordination
compounds [M(L2)2Cl2]
2.2.1 Dichloro(bis{2-[1-(4-methylphenyl)-1H-1,2,3-triazol-4-yl-N3]pyridine-N})manganese(II)
(1)
[Mn(L2)2Cl2] was prepared by stirring a solution of anhydrous MnCl2 (0.041 g, 0.32 mmol) in
CH3OH (10 ml). A solution of the ligand L2 (0.15 g, 0.65 mmol, 2eq) in CH2Cl2 (10 ml), was added
dropwise to it. A resulting pale yellow precipitate was obtained after stirring for 8-10h at RT. The
solvent was then reduced in volume by a half under vacuum distillation before it was filtered and
washed twice with cold methanol and then diethyl ether. A pale yellow solid was obtained and
isolated to yield a precipitate that give (0.195 g, 0.32 mmol, yielded 80%), mp. 324-326°C. ATR /
IR: (cm-1): 3068, 3055, 3022, 1606, 1595, 1575, 1521, 1473, 1446, 1253, 1253, 1062, 1044, 1011,
1000, 979, 861, 812, 784, 719. UV-Vis (DMSO) λmax: [Mn(L2)2Cl2] showed absorption bands at
257 nm, εmax = 88450 dm3mol-1 cm-1, 287 nm, εmax = 40200 dm3mol-1cm-1, 682 nm, εmax = 13
dm3mol-1cm-1. [Mn(L2)2Cl2] showed a value of µeff = 5.62 B.M. HRMS TOF (ESI+) (water:
acetonitrile = 1:3) with the highest molecular weight ion peak matching, was observed at m/z =
562.1202 (80%) and is related to [[Mn(L2)2Cl2] - Cl]+. The calculated value for [C28H24ClMnN8]+ is
562.1193. ɅM (DMSO) = 50 Ω-1cm2mol-1. Elemental Anal. Calc. for C28H24N8Cl2Mn: C, 56.2; H,
4.0; N 18.7. Found: C, 56.0; H, 4.1; N 18.4%.
2.2.2 Dichloro(bis{2-[1-(4-methylphenyl)-1H-1,2,3-triazol-4-yl-N3]pyridine-N})iron(II) (2)
For the preparation of [Fe(L2)2Cl2], the method used was analogous to that for [Mn(L2)2Cl2]. An
amount of 0.041 g, 0.32 mmol of anhydrous FeCl2 and 0.15 g, 0.63 mmol of L2 were used, and an
identical work-up procedure gave the required compound as a bright yellow solid. The isolated
precipitate gave (0.20 g, 0.33 mmol, yield 83%), mp. 310-312 °C. ATR / IR: (cm-1); 3063, 3047,
3025, 1605, 1595, 1571, 1522, 1473, 1448, 1267, 1258, 1063, 1054, 1015, 1004, 886, 815, 786,
553. UV-Vis (DMSO) λmax: [Fe(L2)2Cl2] showed absorption bands at 259 nm, εmax = 65500 dm3mol-
1cm -1, 287 nm, εmax = 52000 dm3mol-1cm-1, 326nm, εmax = 4783 dm3mol-1 cm -1, 908 nm, εmax = 85
dm3mol-1cm-1. [Fe(L2)2Cl2] showed a value of µeff = 5.26 B.M. HRMS TOF (ESI+) (water:
acetonitrile = 1:3) with the highest molecular weight ion peak matching,, was observed at m/z =
563.1135 (80%) and is attributed to [[Fe(L2)2Cl2] - Cl]+. The calculated value for
-1
[(C28H24N8MnCl)]+ is 563.1162. ɅM (DMSO) = 43 Ω-1cm2 mol . Elemental Anal. Calc. for
C28H24N8Cl2Fe: C, 56.1; H, 4.0; N 18.7. Found: C, 56.0; H, 4.1; N 18.8%.

2.2.3 Dichloro(bis{2-[1-(4-methylphenyl)-1H-1,2,3-triazol-4-yl-N3]pyridine-N})cobalt(II) (3)
For the preparation of [Co(L2)2Cl2], the method used was analogous to that for [Mn(L2)2Cl2]. An
amount of 0.060 g, 0.21 mmol of CoCl2.6H2O and 0.11g, 0.50 mmol of L2 were used, and an
identical work-up procedure gave the required compound as a bright pink solid. The isolated
precipitate gave (0.11 g, 0.18 mmol, yield 73%), mp. 346-348°C. ATR / IR: (cm-1); 3045, 3024,
1609, 1595, 1574, 1521, 1475, 1450, 1262, 1245, 1065, 1056, 1018, 1005, 871, 814, 786, 755. UV-
Vis (DMSO): The [Co(L2)2Cl2] showed absorption bands at 252 nm, εmax = 45200 dm3mol-1cm-1,
257 nm, εmax = 77100 dm3mol-1cm-1, 287nm, εmax = 26967 dm3mol-1cm-1, 615 nm, εmax = 56
dm3mol-1cm-1, 678 nm, εmax = 89 dm3mol-1cm-1. The [Co(L2)2Cl2] showed a value of µeff = 3.00
B.M. HRMS TOF (MALDI) with the highest molecular weight ion peak matching, was observed
at m/z = 566.1 (100%) and is related to [[Co(L2)2Cl2] - Cl]+. The calculated value for
[(C28H24N8FeCl)]+ is 566.1. ɅM (DMSO) λmax = 48 Ω-1cm2mol-1. Elemental Anal. Calc. for
C28H24N8Cl2Co: C, 55.8; H, 4.0; N 18.6. Found: C, 55.9; H, 3.9; N 18.8%.
2.2.4 Dichloro(bis{2-[1-(4-methylphenyl)-1H-1,2,3-triazol-4-yl-N3]pyridine-N})nickel(II) (4)
For the preparation of [Ni(L2)2Cl2], the method used was as described for [Mn(L2)2Cl2]. An amount
of 0.050 g, 0.21 mmol of NiCl2.6H2O and 0.10 g, 0.42 mmol of L2 were used, and an identical
work-up procedure gave the required compound as a pale blue solid. The isolated precipitate gave
(0.11 g, 0.18 mmol, yield 73%), mp. 340°C (decomp.). ATR / IR: (cm-1); 3038, 3021, 3010,
1612, 1596, 1577, 1521, 1476, 1451, 1264, 1247, 1067, 1058, 1007, 874, 813, 786, 756, 720. UV-
Vis (DMSO) λmax: [Ni(L2)2Cl2] showed absorption bands at 257nm, εmax = 102150 dm3mol-1cm-1,
285 nm, εmax = 42300 dm3mol-1cm-1, 408 nm, εmax = 18 dm3mol-1cm-1, 668 nm, εmax = 9 dm3mol-
1cm-1. [Ni(L2)2Cl2] showed a value of µeff = 3.00 B.M. HRMS TOF (ESI+) (water: acetonitrile =
1:3) with the highest molecular weight ion peak matching, was observed at m/z = 565.1167 (40%)
and is related to [[Ni(L2)2Cl2] - Cl]+. The calculated value for [(C28H24N8NiCl)]+ is 565.1166. ɅM
(DMSO) = 45 Ω-1cm2mol-1. Elemental Anal. Calc. for C28H24N8Cl2Ni: C, 55.9; H, 4.0; N 18.6.
Found: C, 55.8; H, 4.0; N 18.5%. A good single crystal for X-ray structural analysis was obtained
by slow evaporation of a hot DMSO:CH3CN = 1:9 solution of the [Ni(L2)2Cl2].
2.2.5 Dichloro(bis{2-[1-(4-methylphenyl)-1H-1,2,3-triazol-4-yl-N3]pyridine-N})copper(II) (5)
For the preparation of [Cu(L2)2Cl2], the method used was similar to that for [Mn(L2)2Cl2]. An
amount of 0.062 g, 0.46 mmol of anhydrous CuCl2 and 0.21 g, 0.92 mmol of L2 were used, and an
identical work-up procedure gave the required compound as a pale green solid. The isolated
precipitate gave (0.24 g, 0.39 mmol, yield 91%), mp. 274-276°C. ATR / IR: (cm-1); 3068, 3058,
3025, 1606, 1594, 1575, 1516, 1477, 1449, 1267, 1250, 1063, 1042, 1029, 862, 817, 779, 754, 716.

UV-Vis (DMSO) λmax: [Cu(L2)2Cl2] showed absorption bands at 257 nm, εmax = 52222 dm3mol-1cm-
1, 286 nm, εmax = 35556 dm3mol-1cm-1, 908 nm, εmax = 85 dm3mol-1cm-1. [Cu(L2)2Cl2] showed a
value of µeff = 1.70 B.M. HRMS (P+NSI); (CH3OH)/(NH4OAC) with the highest molecular weight
ion peak matching, was observed at m/z = 594.1547 (45%) and is attributed to [(Cu(L2)2)+ +
(CH3COO-)]+. The calculated value for [C28H24CuN8]+ + (CH3COO-)]+ is 594.1540. ɅM (DMSO) =
31 Ω-1cm2mol-1. Elemental Anal. Calc. for C28H24N8Cl2Cu: C, 55.4; H, 4.0; N 18.5. Found: C, 55.2;
H, 4.1; N 18.6%.
2.2.6 Dichloro(bis{2-[1-(4-methylphenyl)-1H-1,2,3-triazol-4-yl-N3]pyridine-N})zinc(II) (6)
For the preparation of [Zn(L2)2Cl2], the method used was as described for that of the [Mn(L2)2Cl2].
An amount of anhydrous ZnCl2 of 0.19 g, 1.40 mmol and 0.66 g, 2.8 mmol of L2 were used, and an
identical work-up procedure gave the required compound as a white solid. The isolated precipitate
gave (0.18 g, 0.39 mmol, yield 78%), mp. 318-320°C. ATR / IR: (cm-1); 3064, 3041, 3021, 1607,
1570, 1517, 1475, 1448, 1270, 1239, 1073, 1057, 1075, 1006, 864, 812, 774, 754, 718. UV-Vis
-1
(DMSO) λmax: [Zn(L2)2Cl2] showed absorption bands at 258 nm, εmax = 47931 dm3mol-1cm , 288
nm, εmax = 31379 dm3mol-1cm-1. NMR data (ppm), δH (400MHZ, DMSO-d6): 9.23 (1H, s, H8), 8.66-
1
2
8.65 (1H,d, JHH = 4.12Hz, H14), 8.11-8.09 (1H, dd, JHH = 7.79Hz, JHH = 0.92Hz, H11), 7.97-7.92
1
2
(1H, dt, JHH = 7.79 Hz, JHH = 1.83Hz, H12), 7.89-7.87 (2H,d, JHH = 8.24Hz, Ar-H2,6), 7.43-7.38
13
(3H, m, H13 and Ar-H3,5), 3.22 (3H,s,CH3,H7); CNMR (100.63MHZ, DMSO-d6) δC/ppm: 20.49
(C7), 119.83 (C11), 120.09 (C2, C6), 121.13 (C8), 123.25 (C13), 130.14 (C3, C5), 134.26 (C4), 137.31
(C12), 138.49 (C1), 148.80 (C9), 149.33 (C14), 149.54 (C10). These assignments were confirmed
using DEPT 13C (135°), 1H-1H COSY and 1H-13CHMQC two-dimensional correlation
spectroscopy. NMR spectra are provided in the Electronic Supplementary Information. HRMS TOF
(ESI+) (water: acetonitrile = 1:3) with the highest molecular weight ion peak matching, was at m/z
= 571.1129 (80%) and is related to [[Zn(L2)2Cl2] - Cl]+. The calculated value for [(C28H24N8ZnCl)]+
is 571.1104. ɅM(DMSO) = 8 Ω-1cm2mol -1. Elemental Anal. Calc. for C28H24N8Cl2Zn: C, 55.2; H,
4.0; N 18.4. Found: C, 55.4; H, 3.8; N 18.2%. A good single crystal of [Zn(L2)2Cl2].L2 for X-ray
structural analysis was obtained by slow evaporation of a hot CH3OH solution of the [Zn(L2)2Cl2].
2.2.7 Dichloro(bis{2-[1-(4-methylphenyl)-1H-1,2,3-triazol-4-yl-N3]pyridine-N})cadmium(II)
(7)
For the preparation of [Cd(L2)2Cl2], the method used was as described for that of the [Mn(L2)2Cl2].
An amount of anhydrous CdCl2 of 0.15 g, 0.82 mmol and 0.39 g, 1.65 mmol of L2 were used, and
an identical work-up procedure gave the required compound as a white solid, and the isolated
precipitate gave (0.18 g, 0.27 mmol, yield 78%), mp. 306-308°C. ATR / IR: ῡ(cm-1); 3102, 1627,
1607, 1572, 1521, 1469, 1449, 1264, 1239, 1062, 1015, 1004, 815, 781, 749, 720. UV-Vis (DMSO)

λmax: [Cd(L2)2Cl2] showed absorption bands at 256 nm, ε max = 33448 dm3mol-1cm-1, 260 nm, ε max
=
31724 dm3mol-1cm-1, 287 nm, εmax = 29655 dm3mol-1cm-1. NMR data (ppm), δH (400 MHZ,
1
DMSO-d6): 9.26 (1H, s, H8), 8.66-8.67 (1H,d, JHH = 4.12Hz, H14), 8.11-8.10 (1H, dd, JHH
=
7.73Hz, H11), 7.98-7.95 (1H, dt, 1JHH = 7.79Hz, 2JHH = 1.83Hz, H12), 7.89-7.87 (2H,d, JHH = 8.24Hz,
Ar-H2,6), 7.43-7.41 (3H, m, H13 and Ar-H3,5), 3.23 (3H, s, CH3); 13CNMR (100.63MHZ, DMSO-d6)
δC/ppm: 20.50 (C7), 119.98 (C11), 120.12 (C2, C6), 121.27 (C8), 123.39 (C13), 130.17 (C3, C5),
134.23 (C4), 137.52 (C12), 138.58 (C1), 148.47 (C9), 148.97 (C14), 149.50 (C10). These assignments
1
1
were confirmed using DEPT 13C (135°), H-1H COSY and H-13C HMQC two-dimensional
correlation spectroscopy. NMR spectra are provided in the Electronic Supplementary Information.
HRMS TOF (ESI+) (water: acetonitrile = 1:3) with the highest molecular weight ion peak
matching, was observed at m/z = 621.0856 (100%) and is assigned to [[Cd(L2)2Cl2] - Cl]+ The
calculated value for [(C28H24N8CdCl)]+ is 621.0846. ɅM (DMSO) = 16 Ω-1cm2mol-1. Elemental
Anal. Calc. for C28H24N8Cl2Cd: C, 51.3; H, 3.7; N 17.1. Found: C, 51.3; H, 3.8; N 17.3%.
2.3 Instruments and measurement parameters
Infrared (ATR-FTIR IR) spectra were recorded using a smart diamond ATR attachment on a
Thermo-Nicolet FT-IR Spectrometer (AVATAR 320) over the range 4000 to 400 cm-1. Mass
spectra were performed at the EPSRC Mass Spectrometry Service Centre, University of Wales,
Swansea and University of Sheffield. The instrument used was the ‘WATERS LCT premier’, the
ionization was electrospray (ESI+), the solvent was water/acetonitrile (1:3), while the ionization
was electrospray (ESI+ and ES-). Thermofisher LTQ Orbitrap XL was used to analyse volatile
molecules in the mass range m/z 50–2000 or m/z 200–4000 Daltons. NMR spectra (1H, 13C, COSY,
13C–1H correlated NMR) were recorded on an ECS-400 MHz, JEOL multi nuclear FT spectrometer,
1
using Optiplex 380 Delta 5.02 software, with tetramethylsilane (TMS) as an internal standard for H
NMR analysis. Chemical shifts were reported in ppm downfield from tetramethylsilane (TMS), at
298 K, with coupling constants (J) reported in Hertz (Hz). Standard abbreviations indicating
multiplicity were used as follows: m = multiplet, t = triplet, d = doublet and s = singlet. UV-Vis
spectra were obtained on a PerkinElmer Lambda 40 UV/Vis spectrometer. Magnetic susceptibility
is measured with a Gouy magnetic susceptibility balance. The gram magnetic susceptibility for a
substance is calculated from:
χg = (Cbal) (l) (R – Ro) / (109) (m)
Where l = height of sample in the tube in units of centimeters, m = mass of the sample in units of
grams, R = reading for tube plus sample, Ro = reading for the empty tube and Cbal = balance
calibration constant = 1.0. The molar magnetic susceptibility is then calculated from the gram
magnetic susceptibility using the following equation.

χm = (χg) (molar mass)
The effective magnetic moment for a particular substance is calculated from the molar magnetic
susceptibility [8] using the following equation (T represents the Kelvin temperature (294 K)):
µeff = 2.83 [(χm ) (T)] 1/2
The calculated µeff values for the [M(L2)2Cl2] coordination compounds are given in the
experimental characterization data.
2.4 X-ray diffraction
Single crystal X-ray diffraction measurements for [Ni(L2)2Cl2] and [Zn(L2)2Cl2].L2 were performed
using a Rigaku SPIDER RAXIS image plate detector and Rigaku AFC12 goniometer equipped with
an enhanced sensitivity (HG) Saturn724+ detector mounted at the window of an FR-E+ SuperBright
molybdenum rotating anode generator with HF Varimax optics (100µm focus) respectively. Data
were processed and empirical absorption corrections were carried out using Crystal Clear SM-
Expert [9]. The structures were solved by direct methods using SHELXS-97 within OLEX2 [10].
2
All refinements on Fo by full-matrix least squares refinement were performed using the SHELXL-
97 program package within OLEX2 [11]. All non-hydrogen atoms for the Zn coordination
compound were refined with anisotropic displacement parameters however the free ligand is
disordered over an inversion centre and was modelled as 0.5 occupied with isotropic displacement
parameters. No distance restraints were applied or required. Hydrogen atoms were added at
calculated positions and included as part of a riding model with C-H (aromatic) 0.93 Å UISO
=
1.2Ueq (C); C-H (methyl) 0.96 Å UISO = 1.5Ueq (C). Perspective drawings [11] of the molecular
structure of [Ni(L2)2Cl2] and [Zn(L2)2Cl2].L2, also showing the atom numbering scheme used, are
shown in Figure 3 and Figure 4. Crystallographic data are presented in Table 2 with selected bond
lengths, bond angles and torsion angles in Table 3. Additional crystallographic data is provided in
the Electronic Supplementary Information.
2.5 Theoretical approach
Density functional theory (DFT) calculations were performed with the B3LYP functional as
implemented in the Gaussian 09 package [12] using the triple-ζ basis set 6-311G(d,p), except for Cd
where the Stuttgart/Dresden (SDD) pseudopotential was used to describe the metal electronic core,
while the metal valence electrons were described using the def2-TZVPP basis set [13]. Since the
coordination compounds of this study are paramagnetic, all the different spin states were considered
when performing the DFT calculations. Calculations were done unrestricted in the gas phase. All
structures were confirmed as true minimum structures by a frequency analysis, i.e. no imaginary

frequencies. The input coordinates for the molecules were constructed using Chemcraft [14].
Natural bond orbital (NBO) analysis (using the NBO 3.1 module [15] in Gaussian 09), as well as an
electronic density analysis (using Bader’s quantum theory of atoms in molecules (QTAIM)
[16,17,18], as implemented in ADF2017 [19,20,21]) were performed on an optimized structure of
two [Zn(L2)2Cl2] molecules. The input coordinates for the latter were obtained from the crystal
data, also presented in this study. The optimized coordinates of the DFT calculations are provided
in the Supporting Information.
3
Results and discussion
3.1 Characterization
The dichloro(bis{2-[1-(4-methylphenyl)-1H-1,2,3-triazol-1-yl)pyridine}-metal
coordination
compounds [M(L2)2Cl2], synthesized from a 2:1 mole ratio of the 2-(1-(4-methyl-phenyl)-1H-1,2,3-
triazol-1-yl)pyridine and the metal chloride. These compounds were characterized by FT-IR, MS,
elemental analysis, NMR (Zn and Cd), UV-Vis, melting points, magnetic moments, conductivity
measurements, single crystal X-ray diffraction (Ni and Zn) and computational chemistry
calculations, see Supplementary Information for experimental spectra.
Comparison of the IR spectra of the [M(L2)2Cl2] coordination compounds with that of the 2-(1-(4-
methyl-phenyl)-1H-1,2,3-triazol-1-yl)pyridine ligand (L2), shows that characteristic bands were
shifted due to complex formation (see Table 1 and Supplementary Information). For example,
ν(C=N) stretching band of the pyridine moiety is observed at a value around 1624–1606 cm-1 for
the various [M(L2)2Cl2] coordination compounds, which is shifted to higher wavenumbers than in
the free ligand (1597 cm-1). This indicate coordination of the nitrogen of the C=N pyridine moiety
to the different metal atoms. The region for ν(C=C)Ar bands of phenyl ring in complexes, are around
ν = 1598-1580 cm-1 and 1500-1470 cm-1 [22,23]. The ν(C=N)triazole absorption band of the triazole
moiety at 1566 cm-1 in the free ligand is detected around 1572–1595 cm-1 in the [M(L2)2Cl2]
coordination compounds, while the ν(C=C)triazole absorption band of the triazole moieties which
appear at 1549 cm-1 in the free ligand is detected around 1549–1566 cm-1 in the [M(L2)2Cl2]
coordination compounds, as indicated in Table 1.
Table 1. IR frequencies in wavenumber (cm-1) units of the ligand (L2) and the [M(L2)2Cl2]
coordination compounds.
ν (C=N)pyridine
ν(C=C)Ar.
,
ν(C=C)triazole
ν (N-
N)triazole
ν(N=N)triazole ν(C-N)
Compound

ν(C=N)triazole
L2
L2
1597, 1592, 1566
1606, 1595, 1575
1605, 1595, 1571
1609, 1595, 1575
1612, 1596, 1577
1606, 1594, 1575
1607, ------, 1570
1624, 1603, 1572
1543
1556
1555
1554
1566
1556
1549
1564
1144,1036
1164,1062
1152,1062
1152,1065
1152,1067
1156,1063
1153,1062
1156,1062
1517
1521
1522
1522
1521
1516
1517
1520
[Mn(L2)2Cl2] (1)
[Fe(L2)2Cl2] (2)
[Co(L2)2Cl2] (3)
[Ni(L2)2Cl2] (4)
[Cu(L2)2Cl2] (5)
[Zn(L2)2Cl2] (6)
[Cd(L2)2Cl2] (7)
[Mn(L2)2Cl2] (1)
[Fe(L2)2Cl2] (2)
[Co(L2)2Cl2] (3)
[Ni(L2)2Cl2] (4)
[Cu(L2)2Cl2] (5)
[Zn(L2)2Cl2] (6)
[Cd(L2)2Cl2] (7)
The experimentally measured room temperature magnetic moment for the paramagnetic
[M(L2)2Cl2] coordination compounds (1) – (4) of this study are consistent with high spin
complexes, namely Mn (µeff = 5.62 B.M., S = 5/2), Fe (µeff = 5.26 B.M., S = 2), Co (µeff = 3.00
B.M., S = 3/2), Ni (µeff = 3.00 B.M., S = 1), while [Cu(L2)2Cl2] (5) (µeff = 1.70 B.M., S = ½) can
only be low spin. The molar conductivity measurements of the [M(L2)2Cl2] coordination
compounds were conducted using 10-3 M solutions of [M(L2)2Cl2] in DMSO. The molar
conductivities ranged from 6 - 50 Ω-1 cm2 mol-1 at 294 K. The low values indicate that the chloride
anions bind to the metal ions as coligands and do not ionize. Low conductivity values are indicative
of coordination compounds having 1:2 metals:ligand stoichiometry of the type ML2Cl2, where L
acts as a bidentate ligand [24]. Higher than expected conductivity values are usually due to the
possible displacement of one chlorine atom by one molecule of the solvent DMSO in the
[M(L2)2Cl2] coordination compounds, producing intermediate behaviour [ML2(Cl)(DMSO)].Cl
between those of non-electrolytes and 1:1 electrolytes. Similar behaviours were observed for
several coordination compounds, mainly measured in DMSO solvent, because this solvent is a
strong donor with profitable steric properties [25,26]. The 13C and 1H NMR spectra of [Zn(L2)2Cl2]
(6) and [Cd(L2)2Cl2] (7) show no impurities, including no traces of the ligand L2, see
Supplementary Information.
3.2 X-ray structures
The molecular structure of the coordination compounds [Ni(L2)2Cl2] and [Zn(L2)2Cl2].L2 are
presented here. Perspective drawings of the molecular structures, also showing the atom numbering
schemes, are given in Figure 3 for [Ni(L2)2Cl2] and Figure 4 for [Zn(L2)2Cl2].L2. Crystallographic
data are given in Table 2 with selected bond lengths, bond angles and torsion angles in Table 3. The
structures can be described as trans-trans-trans, since the Cl, NPy and Ntriazole are orientated trans to
each other respectively in [Ni(L2)2Cl2] and [Zn(L2)2Cl2].L2. A trans-trans-trans isomer was also
previously found for [Ni(L)2Br2] with L = 2-[1-(4-cyclohexyl)-1H-1,2,3-triazol-4-yl]pyridine [27].
Both [Ni(L2)2Cl2] and [Zn(L2)2Cl2].L2 crystallized in the P21/c space group with the metal centre
lying on an inversion symmetry. However, a free ligand L2, crystallized in a 1:1 ratio with

[Zn(L2)2Cl2]. The structures of the molecules [Ni(L2)2Cl2] and [Zn(L2)2Cl2].L2 are very similar as
can be seen from the overlay of the two structures in Figure 5. The bond lengths and angles of the
two structures differ very slightly, see data in Table 3.
Figure 3. View of [Ni(L2)2Cl2] showing the atom labelling scheme. The asymmetric unit
contains one ligand, one Cl and the Ni atom which lies on an inversion centre, the second
ligand and Cl atom are generated by symmetry (-x, 1-y, 1-z). Displacement ellipsoids for non-
hydrogen atoms are drawn at 50% probability level.

Figure 4. View showing atom labelling scheme of [Zn(L2)2Cl2].L2. Displacement ellipsoids for
the atoms refined with anistropic adps and spheres for those with isotropic adps are drawn at
50% probability level. The asymmetric unit contains one coordinated ligand, one Cl and the
Zn which lies on an inversion centre and 0.5 free ligand molecule. The free ligand molecule is
disordered over an inversion centre and the second orientation is omitted for clarity.
Table 2. Crystallographic data for the [Ni(L2)2Cl2] and [Zn(L2)2Cl2].L2.
Compound
[Ni(L2)2Cl2]
C28H24Cl2N8Ni
602.16
[Zn(L2)2Cl2].L2
C42H36Cl2N12Zn
845.1
Empirical formula
Formula weight
Temperature
Wavelength
Crystal system
Space group
293(2) K
0.71075 Å
Monoclinic
P21/c
100(2) K
0.71075 Å
Monoclinic
P21/c
Unit cell dimensions
a = 10.7323(7) Å
b = 12.9118(7) Å
c = 9.7218(5) Å
 
a = 15.279(8) Å
b = 12.919(6) Å
c = 9.866(5) Å
 
 
 

 
 
Volume
1303.17(13) Å3
1901.1(16) Å3
Z
2
2
Density (calculated)
1.535 Mg / m3
1.476 Mg / m3
0.985 mm1
0.837 mm1
Absorption coefficient
F(000)
Crystal
620
872
plate; Colourless
plate; Colourless
0.09  0.04  0.01 mm3
0.07  0.04  0.01 mm3
Crystal size
 range for data collection 3.16  27.47°
2.64  27.51°
Index ranges
  h     k     h     k  
  l  
8726
  l  
16822
Reflections collected
Independent reflections
2969 [Rint = 0.039]
4328 [Rint = 0.075]
99.20%
99.20%
Completeness to  = 27.47°
Absorption correction
Semiempirical from
equivalents
Semiempirical from
equivalents
Max. and min. transmission 1.000 and 0.675
1.000 and 0.733
Refinement method
Full-matrix least-squares on Full-matrix least-squares on
F2
F2
Data / restraints / parameters 2969 / 0 / 179
4328 / 0 / 252
1.20
Goodness-of-fit on F2
1.02
Final R indices [F2 >
R1 = 0.040, wR2 = 0.087
R1 = 0.087, wR2 = 0.158
2(F2)]
R indices (all data)
Largest diff. peak and hole
R1 = 0.057, wR2 = 0.094
0.56 and 0.54 e Å3
R1 = 0.113, wR2 = 0.171
0.46 and 0.72 e Å3
Table 3. Selected bond lengths (Å) and bond (°) for the [Ni(L2)2Cl2] and [Zn(L2)2Cl2].L2.
[Ni(L2)2Cl2] [Zn(L2)2Cl2].L2
Bond distance (Å)
M1-N1
M1-N8
2.1015(19) 2.144(3)
2.0739(19) 2.191(4)
M1-Cl1
N1C2
N1C6
N8N9
N8C7
N9N10
N10C11
N10C12
Bond angle (°)
N8i-M1-N8
N8-M1-N1i
N8-M1-N1
N1i-M1-N1
Cl1-M1-Cl1i
C2N1C6
2.4123(6)
1.341(3)
1.352(3)
1.316(3)
1.357(3)
1.352(3)
1.353(3)
1.428(3)
2.4615(14)
1.341(5)
1.346(5)
1.316(5)
1.363(5)
1.364(5)
1.352(5)
1.434(5)
180
180
100.41(8)
79.59(8)
180
180
117.9(2)
77.78(13)
102.22(13)
180
180
119.0(4)

C2-N1-M1
C6-N1-M1
127.46(16) 125.5(3)
114.55(15) 115.4(3)
Symmetry transformations used to generate equivalent atoms:
(i) x+1,y+1,z+1 for [Ni(L2)2Cl2] and (i) x,y+1,z+1 for [Zn(L2)2Cl2].L2
Figure 5. Overlay [Ni(L2)2Cl2] (red) and [Zn(L2)2Cl2] (green). The root means square (RMS)
overlay values, when using the metal and the six atoms of the octahedral coordination
polyhedron, is 0.078.
Intermolecular H-bonding interaction involving the chloride atoms in [Ni(L2)2Cl2] form a 3D
supramolecular structure [28][29], see Figure 6. The zinc structure is different as it contains
molecules of the free ligand L2 and [Zn(L2)2Cl2] molecules. The molecules in the layers of the
[Zn(L2)2Cl2].L2 structure are also held together by hydrogen bonds between [Zn(L2)2Cl2] and L2
molecules, see Figure 7. The intermolecular separations suggest there are hydrogen bonds between
both the coordinated and free ligand and between free ligand molecules but are not analysed further
given the disordered nature of the free ligand.

Figure 6. Partial packing for [Ni(L2)2Cl2] showing an intermolecular C-H---Cl hydrogen bonding
interaction between Cl1…H11_$1-C11_$1; Cl1…H11_$1 2.44Å, Cl1_C11_$1 3.366 Å and the
angle C11_$1-H11_41…Cl1 is 174.2 °, $1 signifies symmetry code 1-x, ½-y,3/2-z.
Figure 7. Partial packing for [Zn(L2)2Cl2].L2 showing (Intermolecular C-H…Cl hydrogen bonding
interaction between Cl1…H11_$1-C11_$1; Cl1…H11_$1 2.55Å, Cl1_C11_$1 3.473 Å and the
angle C11_$1-H11_41…Cl1 is 170.1°, $1 signifies symmetry code x, 3/2-y,-1/2+z).

3.3 DFT study
The relative orientation of the Cl, NPy and Ntriazole molecules around the metal respectively, in each
of 1 - 9, can lead to five geometrical isomers, namely cis-trans-cis, cis-cis-cis, cis-cis-trans, trans-
trans-trans, and trans-cis-cis, as shown in Figure 8 below. DFT calculations on the different
isomers and possible spin states of the series of [M(L1)2Cl2] coordination compounds, L1 with R =
H (Figure 2) and M = Mn (1), Fe (2), Co (3), Ni (4), Cu (5), Zn (6) and Cd (7), confirmed the
experimentally measured spin states for 1 – 7. Thus, d5 Mn(II) (with µeff = 5.62 B.M., S = 5/2), d6
Fe(II) (with µeff = 5.26 B.M., S = 2), d7 Co(II) (with µeff = 3.00 B.M., S = 3/2), d8 Ni(II) (with µeff =
3.00 B.M., S = 1), d9 Cu(II) (with µeff = 1.70 B.M., S = ½), are all paramagnetic, while d10 Zn(II)
and Cd(II) are diamagnetic with S = 0.
Since the cis-cis-trans and the trans-trans-trans isomers for the [M(L1)2Cl2] coordination
compounds, both with NPy trans to each other, are generally equi-energetic within 0.15 eV, both
isomers could experimentally be possible. In Table 5 DFT calculations for the lowest energy spin
state of each isomer of the dichloro{bis[2-(1-(4-methylphenyl)-1H-1,2,3-triazol-4-yl-N3]pyridine-
N]}metal(II), [M(L2)2Cl2], with a methyl substituent on the phenyl ring (L2 = 2-(1-(4-
methylphenyl)-1H-[1,2,3-triazol]-4-yl)pyridine), are given.
In this study, the trans-trans-trans isomer was obtained both for [Ni(L2)2Cl2], 4, and [Zn(L2)2Cl2],
6. Only in one case a cis-cis-trans isomer for this kind of coordination compounds was obtained till
date [30], namely for cis-cis-trans [Mn(L3)2Cl2], L3 with R = OCH3 (Figure 2) [6], while the isomer
trans-trans-trans isomers were reported for [Ni(L)2Br2] [27], [Co(L3)2Cl2] [6] and [Ni(L3)2Cl2] [6].

Figure 8. Geometrical isomers for [ML2Cl2] coordination compounds. R = CH3 for
coordination compounds 1 – 9 of this study. The isomers are defined by the relative
orientation of the Cl, NPy and Ntriazole molecules around the metal respectively. R = H for the
unsubstituted ligand, L1, R = CH3 for the CH3 substituted ligand, L2, and R = OCH3 for the
OCH3 substituted ligand, L3.
Table 4. Relative Electronic energies E (eV) for the indicated spin states and geometrical
isomers of [M(L1)2Cl2]. L1 with R = H (Figure 2). The energy of the lowest energy isomer is
indicated in bolt font.
Isomera
S
E (eV) E (eV) E (eV)
S
0
E (eV) E (eV) E (eV) E (eV)
Mnb
Cob
1.07
0.80
0.71
0.56
0.74
0.45
0.24
0.04
0.00
0.22
-
Cu
0.33
0.35
0.15
0.00
0.20
-
-
-
-
-
-
-
-
-
-
Fe
Nib
1.20
1.21
0.97
0.93
1.11
0.50
0.29
0.06
0.00
0.25
-
Zn
0.40
0.21
0.00
0.11
0.28
-
-
-
-
-
-
-
-
-
-
Cd
ctc
ccc
cct
ttt
tcc
ctc
ccc
cct
ttt
tcc
ctc
ccc
cct
ttt
1/2 1.90
1.69
1.11
0.88
0.63
0.61
0.89
1.18
0.97
0.78
1.95
0.91
0.48
0.27
0.10
0.00
0.19
0.29
0.12e
1.49
1.35
1.64
0.00
0.15
d
-
-
-
-
-
-
-
-
-
-
3/2 1.78
c
1
2
c
2.19
1.64
5/2 0.33
0.17
-
-
-
-
-
-
0.00
-
0.14
tcc
d
-
a See Figure 8 for the geometry of the different isomers.
b from reference [6].
c geometry did not converge
d optimized to the cct isomer
e optimized to a 5-coordinated coordination compound, not 6-coordinated
Table 5. Relative Electronic energies E (eV) for the indicated spin states and geometrical
isomers of [M(L2)2Cl2]. L2 with R = CH3 (Figure 2). The energy of the lowest energy isomers
are indicated in bolt font.
Isomera E (eV) E (eV) E (eV) E (eV) E (eV) E (eV) E (eV)
Mn
Fe
Co
Ni
Cu
Zn
Cd
S = 5/2 S = 4/2 S = 3/2 S = 1
S = 1/2 S = 0
S = 0
0.28
0.12 b
0.00
0.15
ctc
ccc
cct
ttt
0.33
0.17
0.00
0.14
0.48
0.27
0.10
0.00
0.48
0.24
0.04
0.00
0.51
0.25
0.06
0.00
0.33
0.35
0.15
0.00
0.39
0.20
0.00
0.11

tcc
a
0.20
0.23
0.26
0.20
0.28
a
a optimized to the cct isomer
b optimized to a 5-coordinated coordination compound, not 6-coordinated
To shed more light on the experimental observation that the trans-trans-trans isomers are generally
experimentally favoured for metal(II)-(1,2,3-triazol-4-yl)pyridine coordination compounds,
although DFT calculations predict that both the cis-cis-trans and the trans-trans-trans isomers
could experimentally be possible, we present here results for a di-molecular model of [Zn(L2)2Cl2],
where two [Zn(L2)2Cl2] molecules were optimized together. The resulting optimized di-molecular
structure was evaluated by Bader’s quantum theory of atoms in molecules (QTAIM) method, as
well as by the Weinhold natural bonding orbital (NBO) method to evaluate the nature of the
intermolecular hydrogen bonds between chlorine and H-atoms by theoretical computational
chemistry methods. Bader’s definition of an atom in a molecular system, is based purely on the
electronic charge density, while zero flux surfaces divide atoms. The position the nuclei of atoms,
an atom critical point, ACP (3, -3), is determined by a local maximum of electron density, with the
electron density decreasing in all three perpendicular directions of space. The atoms are connected
by bond paths with a bond critical point, BCP (3, -1), along the bond where the shared electron
density reaches a minimum [31]. QTAIM calculated electron density at H-bond critical points
correlates well with experimental hydrogen bond strengths [32,33,34]. Typical calculated
topological parameters for hydrogen-bonds, i.e. X−H···Y through-space interactions, are
0.002−0.04 au for the electron density and 0.02−0.15 au for the Laplacian of the electron density
[35,36] at the H···Y BCP. The inter-molecular bond paths identified for the optimized di-molecular
model of [Zn(L2)2Cl2], are shown in Figure 9 with the related topological parameters summarized in
Table 6. All QTAIM calculated inter-molecular bonds are C−H···Cl bonds with electron density
and Laplacian of the electron density values that fall well within the typical values for hydrogen
bonds. The shortest and strongest QTAIM identified inter-molecular C−H···Cl bonds numbered
148 and 163 in Figure 9, are the same as experimental observed for [Zn(L2)2Cl2] as shown in Figure
7.

Figure 9. Visualization of the optimized di-molecular model of [Zn(L2)2Cl2], showing bond-
paths (BP) coloured according to the value of the electron density; blue (high density) to green
to red (low density). Bond critical point (BCP) numbers, related to inter-molecular bonds, are
indicated. Colour code of atoms (online version): Zn (off-white), C (grey), N (blue), Cl
(green), H (white).
Table 6. Selected QTAIM calculated topological parameters related to the intermolecular
hydrogen bonds in the optimized di-molecular model of [Zn(L2)2Cl2] shown in Figure 9.
BCP
Atoms
involved
inter-
atomic
distance
/ Å
BP
Electron Laplacian of
length density electron
density
-3

 / e a0  e a0
/ Å
CP # 190
CP # 148
CP # 147
CP # 170
CP # 163
CP # 164
Cl33-H108 3.056
Cl33-H112 2.425
Cl33-H115 2.867
3.077 0.0051
2.443 0.0171
2.875 0.0072
2.917 0.0067
2.446 0.0170
3.018 0.0057
0.0156
0.0510
0.0220
0.0205
0.0508
0.0174
Cl96-H21
Cl96-H18
Cl96-H14
2.907
2.427
2.999
The Weinhold NBO method can be used to describe intermolecular interactions from a natural bond
orbital, donor-acceptor viewpoint. In the optimized di-molecular model of [Zn(L2)2Cl2], a lone pair
on Cl acts as donor to donate electron density into an empty antibonding orbital of nearby C-H as
acceptors. Nine LP(Cl33)  BD*(C-H) from Cl33 to the three nearest hydrogens (H108, H112 and
H115) as shown in Figure 10 and tabulated in Table 7, are identified by the NBO calculation. The
LP(3) (Cl33)  BD*(1) (C111-H112) donor-acceptor interaction of 14.853 kJ·mol-1 is the
strongest, and is the same as the experimental C-H…Cl hydrogen bonding interaction observed in
the solid state for [Zn(L2)2Cl2] as shown in Figure 7. Due to symmetry, nine similar LP(Cl96) 

BD*(C-H) from Cl 96 to the three nearest hydrogens (H14, H18 and H21) exist (not shown in
Figure 10, values tabulated in Table 7). The theoretically obtained donor-acceptor interaction from
a filled lone pair NBO on Cl to an empty antibonding NBO on (C-H), and the QTAIM determined
bonding paths between Cl and nearby hydrogens on a neighboring molecule, thus give an
understanding on a molecular level, why in the solid state where inter-molecular forces and packing
play a role, the trans-trans-trans isomers are mostly obtained.

Figure 10. Selected intermolecular donor–acceptor interactions (between LP on Cl33 and
BD* of the indicated CH bonds) involved in intermolecular interactions in the optimized di-
molecular model of [Zn(L2)2Cl2]. Top left show atom numbers of the atoms involved in the
selected interactions. The natural bond orbital (NBO) plots utilise a contour of 0.03 e/Å3.
Colour code of atoms (online version): Zn (turquois), C (grey), N (blue), Cl (green), H (white).

Table 7. Second order perturbation theory interaction energies, E(2), and calculated NBO
occupations, for the LP (1-centre nonbonded lone pair) and BD (2-centre bond) NBOs
involved in intermolecular interactions in the optimized di-molecular model of [Zn(L2)2Cl2].
Donor
Acceptor
E(2) / kJ·mol-1
LP(1) Cl33
LP(1) Cl33
LP(2) Cl33
LP(2) Cl33
LP(3) Cl33
LP(3) Cl33
LP(3) Cl33
LP(4) Cl33
LP(4) Cl33
LP(1) Cl96
LP(1) Cl96
LP(2) Cl96
LP(2) Cl96
LP(3) Cl96
LP(3) Cl96
LP(3) Cl96
LP(4) Cl96
LP(4) Cl96
Donor
BD*(1) C107-H108 0.335
BD*(1) C111-H112 2.678
BD*(1) C107-H108 1.799
BD*(1) C114-H115 3.640
BD*(1) C107-H108 0.377
BD*(1) C111-H112 14.853
BD*(1) C114-H115 2.218
BD*(1) C107-H108 0.293
BD*(1) C111-H112 5.188


















BD*(1) C13-H14
BD*(1) C17-H18
BD*(1) C13-H14
BD*(1) C20-H21
BD*(1) C13-H14
BD*(1) C17-H18
BD*(1) C20-H21
BD*(1) C13-H14
BD*(1) C17-H18
0.377
2.594
2.092
3.347
0.502
14.602
1.841
0.335
5.104
Occupancy / e– Acceptor
Occupancy / e–
LP(1) Cl33 1.995
LP(2) Cl33 1.991
LP(3) Cl33 1.976
LP(4) Cl33 1.922
LP(1) Cl96 1.995
LP(2) Cl96 1.991
LP(3) Cl96 1.976
LP(4) Cl96 1.921
BD*(1) C107-H108 0.015
BD*(1) C111-H112 0.026
BD*(1) C114-H115 0.017
BD*(1) C13-H14
BD*(1) C17-H18
BD*(1) C20-H21
0.015
0.026
0.016
4
Summary
A first comprehensive series of seven pyridyl-triazole based transition metal coordination
compounds, where seven different transition metals, M = Mn (1), Fe (2), Co (3), Ni (4), Cu (5), Zn
(6) and Cd (7), coordinated to the same 1,2,3-triazole chromophore, namely 2-(1-(4-methyl-
phenyl)-1H-1,2,3-triazol-1-yl)pyridine, has been successfully synthesized and characterized. DFT
calculations on the possible spin states of the coordination compounds, are in agreement with the
results of the experimentally measured magnetic moment for the paramagnetic coordination
compounds (Mn, Fe, Co, Ni and Cu). DFT calculations on the possible isomers of the coordination
compounds, showed that the cis-cis-trans and the trans-trans-trans isomers, with the pyridyl groups

trans to each other, are the lowest in energy. Single crystal diffraction studies of [Ni(L2)2Cl2] and
[Zn(L2)2Cl2].L2, reported in this study, showed them to crystalise as trans-trans-trans isomers. The
experimentally observed inter-molecular hydrogen bonds, X−H···Cl, in the solid state X-ray
structure of [Zn(L2)2Cl2] can from a computational chemistry point of view be described by a
donor-acceptor interaction from a filled lone pair NBO on Cl to an empty antibonding NBO on (C-
H). The inter-molecular hydrogen bonds can also be described by the QTAIM determined bonding
path between Cl and the respective hydrogen.
5
Supplementary material
CCDC 1813109 and 1813110 contains the supplementary crystallographic data for the crystals of
this study. 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, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail:
deposit@ccdc.cam.ac.uk. Experimental IR, UV/vis, NMR, MS, additional crystallographic data
and optimized coordinates of the DFT calculations associated with this article can be found at
http://dx.doi.org/10.1016/j.dib.2018.xx.xxx.
6
Acknowledgements
The National Mass Spectroscopy Centre at the University of Wales, Swansea is thanked for
supplying the mass spectrometry data. XRD data and structures were supplied by the National
Crystallograhy Service at the University of Southampton. KT expresses his gratitude to the Iraqi
Government for financial support to conduct the research reported in the UK. This work has
received support from the South African National Research Foundation and the Central Research
Fund of the University of the Free State, Bloemfontein, South Africa. The High Performance
Computing facility of the University of the Free State and the Centre for High Performance
Computing CHPC of South Africa are gratefully acknowledged for computer time.
7
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Graphical Abstract
DFT calculations on the optimized structure of two dichloro(bis{2-[1-(4-methylphenyl)-1H-1,2,3-
triazol-4-yl-N3]pyridine-N})zinc(II) molecules (6) show C−H···Cl donor-acceptor interactions,
similar to those observed in the X-ray structures of (6) and related nickel(II) structures.