Low spin Fe(II) complexes formed of monosubstitued 2,6-bis(2-benzimidazolyl)pyridine ligands

Article abstract of DOI:10.1016/j.poly.2016.11.009

Five ligands as monosubstituted derivatives of the basic skeleton of 2,6-bis(2-benzimidazyl)pyridine were synthesized, and characterized by NMR and IR spectra along with the X-ray structure analysis. Their complexation gave a set of hexacoordinate Fe(II) complexes showing predominantly the diamagnetism until ambient temperature. Some temperature-independent paramagnetism along with an onset of the spin transition is also detected.

Full text of DOI:10.1016/j.poly.2016.11.009

Accepted Manuscript
Low spin Fe(II) complexes formed of monosubstitued 2,6-bis(2-benzimidazol-
yl)pyridine ligands
Cyril Rajnák, Ján Titiš, Olaf Fuhr, Mario Ruben, Roman Boča
PII:
S0277-5387(16)30589-7
DOI:
http://dx.doi.org/10.1016/j.poly.2016.11.009
POLY 12316
Reference:
Polyhedron
To appear in:
Received Date:
Revised Date:
Accepted Date:
28 July 2016
4 November 2016
5 November 2016
Please cite this article as: C. Rajnák, J. Titiš, O. Fuhr, M. Ruben, R. Boča, Low spin Fe(II) complexes formed of
monosubstitued 2,6-bis(2-benzimidazolyl)pyridine ligands, Polyhedron (2016), doi: http://dx.doi.org/10.1016/
j.poly.2016.11.009
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Low spin Fe(II) complexes formed of monosubstitued 2,6-bis(2-benzimidazyl)pyridine ligands
Cyril Rajnák, Ján Titiš, Olaf Fuhr, Mario Ruben and Roman Boča
1
Low spin Fe(II) complexes formed of monosubstitued
2,6-bis(2-benzimidazolyl)pyridine ligands
Cyril Rajnáka,b,c, Ján Titiša, Olaf Fuhrb,d, Mario Rubenb,c* and Roman Bočaa*
a Department of Chemistry, FPV, University of SS Cyril and Methodius, 91701 Trnava,
Slovakia, E-mail: cyril.rajnak@ucm.sk
b Institute of Nanotechnology, Karlsruhe Institute of Technology (KIT), 76021 Karlsruhe,
Germany
c IPCMS-CNRS, Université de Strasbourg, 23, rue du Loess, f-67034 Strasbourg, France
d Karlsruhe Nano-Micro Facility (KNMF), Karlsruhe Institute of Technology (KIT), 76021
Karlsruhe, Germany
Abstract
Five ligands as monosubstituted derivatives of the basic skeleton of 2,6-bis(2-
benzimidazyl)pyridine were synthesized, and characterized by NMR and IR spectra along
with the X-ray structure analysis. Their complexation gave a set of hexacoordinate Fe(II)
complexes showing predominantly the diamagnetism until ambient temperature. Some
temperature-independent paramagnetism along with an onset of the spin transition is also
detected.
Key words
Crystal structure, derivates of 2,6-bis(2-benzimidazolyl)pyridine, Fe(II) complex, magnetic
data.
1. Introduction
The condensation product of the o-phenylenediamine with pyridine-2,6-dicarboxylic acid
results in the formation of 2,6-bis(2-benzimidazolyl)pyridine, abbr. bzimpy. The bzimpy
ligand became one of the most useful starting materials for more complicated organic
compounds e.g. anchoring and ancillary ligands used to coordination chemistry [1-3]. The
benzimidazole group is also known as a good provider of greater coordination bonding due to
strong σ donor effect [4]. This ligand has been widely combined with metal ions yielding a
number of transition metal complexes that were studied from various aspects in detail [5]. The
high and low spin Fe(II) complexes with six-memberd ring ligands were studied largely [6-
13]. One target of investigation was oriented to magnetic studies as Fe(II) complexes of the
formula [Fe(bzimpy)2]X2·sol are susceptible for spin crossover from the low-spin state S = 0
to the high-spin state S = 2 [14-16]; X – monoanion, sol – crystal solvent. Also deprotonation
of the basic ligand bzimpy to bzimpy-1H gave neutral Fe(II) complex with interesting magnetic
behaviour [17].
Herein we are reporting about synthesis and characterization of new ligands L2 through L6
as derivatives of the basic ligand L1 = bzimpy (Scheme 1). Their complexation with Fe(II)
salts produced a set of hexacoordinate complexes [Fe(Ln)2](ClO4)2 and [Fe(Ln-1H)2] whose
magnetic properties, after structure determination, were studied in detail (Scheme 2).

Low spin Fe(II) complexes formed of monosubstitued 2,6-bis(2-benzimidazyl)pyridine ligands
Cyril Rajnák, Ján Titiš, Olaf Fuhr, Mario Ruben and Roman Boča
2
H3C
H
H
H
N
N
N
N
N
N
N
N
N
N
L2, Me-bzimpy,
2-[6-(1H-benzimidazol-2-yl)-pyridin-2-yl]-1-
methyl-1H-benzimidazole
L1, bzimpy,
2,6-bis(2-benzimidazolyl)pyridine
CH3
CH3
H3C
CH
H2C
H
H
N
N
N
N
N
N
N
N
N
N
L3, Et-bzimpy,
L4, iPr-bzimpy,
2-[6-(1H-benzimidazol-2-yl)-pyridin-2-yl]-1- 2-[6-(1H-benzimidazol-2-yl)-pyridin-2-yl]-1-
ethyl-1H-benzoimidazole
isopropyl-1H-benzimidazole
Br
H
N
N
N
H
N
N
N
N
N
N
N
L5, tBu2Bz-bzimpy,
2-[6-(1H-benzimidazol-2-yl)-pyridin-2-yl]-1-
(3,5-di-tert-butyl-benzyl)-1H-
benzoimidazole
L6, BrBz-bzimpy,
2-[6-(1H-benzimidazol-2-yl)-pyridin-2-yl]-1-
(4-bromo-benzyl)-1H-benzimidazole
Scheme 1. Sketch of the ligands L1 through L6.

Low spin Fe(II) complexes formed of monosubstitued 2,6-bis(2-benzimidazyl)pyridine ligands
Cyril Rajnák, Ján Titiš, Olaf Fuhr, Mario Ruben and Roman Boča
3
H
H
R
N
H
O
O
N
N
N
a
b
N
N
N
OH
OH
N
N
N
N
L1
L2-L6
-
R
H
(ClO4
)
R
N
0
2
N
N
N
N
N
N
N
N
N
N-
N-
c
d
Fe2+
Fe2+
N-
N-
N
N
N
N
N
N
H
R
R
2-5
2d-5d
Scheme 2. Synthetic path: a) phosphoric acid (200°C), o-phenylenediamine, b) NaH, SA c)
CH3OH, Fe(H2O)6(ClO4)2, Ar, 60°C, d) Na2CO3, CH2Cl2, Ar, 39°C
2. Experimental
2.1. Chemicals and handling
All chemicals of reagent grade quality were purchased and used as received (Sigma-
Aldrich, Merck). The solvents dichloromethane, methanol for column chromatography were
used without further purification. Other solvents, acetonitrile, N,N-dimethylformamide and
toluene were distilled with dehydrating agent CaH2 under inert argon atmosphere.
Dichloromethane for synthesis was purified analogously. 1H and 13C NMR measurements of
ligands were conducted in DMSO-d6.
CAUTION. Handling with metal–organic perchlorates is potentially dangerous due to
their explosive properties. It should be handled with care in small quantities. Especially high-
temperature magnetic measurements are risky.
2.2. Physical Measurements
Elemental analysis was carried out by the vario MICRO cube analyzer (Elementar
Analysensysteme) and Flash 2000 CHNS/O apparatus (Thermo Scientific). The NMR spectra
of ligands were measured using FT-NMR Spectrometer (Avance III 500, Bruker) and
analyzed by TOPSPIN 1.2. Prior to using, the solid KBr for FT-IR measurements was kept
against absorption of moisture in an oven at 60°C. FT-IR spectra were measured in KBr
pellets (Spectrum GX, Perkin Elmer). Absorption UV-Vis spectra were recorded by two-beam
UV-Vis-NIR spectrophotometer (Cary 500 Scan). Electrospray ionization time of flight (ESI-
TOF) mass spectrometry was acquired on a micrOTOF-Q II (Bruker) spectrometer in the
positive mode of ion polarity. Melting points were studied with thermo-microscopy via a
Kofler hot-stage microscope at 4°C·min-1 and reported without corrections.
2.3. Synthesis of 2,6-bis(2-benzimidazolyl)pyridine (L1)
Pyridine-2,6-dicarboxylic acid (6.68 g, 40 mmol) was stirred with o-phenylenediamine
(9.52 g, 88 mmol) in syrupy 85% phosphoric acid (60-70 cm3) at 220°C for overnight [18,

Low spin Fe(II) complexes formed of monosubstitued 2,6-bis(2-benzimidazyl)pyridine ligands
Cyril Rajnák, Ján Titiš, Olaf Fuhr, Mario Ruben and Roman Boča
4
19]. The blue coloured melt was poured into 1000 cm3 of vigorously stirred cold water for 1
hour. The blue precipitate was collected by filtration through a Büchner funnel, then slurried
in 1000 cm3 of 10 % aqueous sodium carbonate for 1 hour to obtain ochroid-pink colour. The
resulting solid was filtered off and recrystallized from hot methanol solution (250 cm3) and
activated charcoal to give long, white needles. Yield: 5.70 g, 46 %. Melting point 353 -
355°C. ESI-TOF MS (CH2Cl2): m/z = 334.11 M+Na+. 1H NMR (500 MHz, DMSO-d6, 25°C)
δ(ppm) 12.99 (s, 2H), 8.33 (d, J = 7.5, 2H), 8.17 (t, J = 7.5Hz, 1H), 7.75 (t, J = 4.5, 4H), 7.31
(q, J = 2.5, 4H). 13C NMR (125 MHz, DMSO-d6, 25°C) δ(ppm) 150.47, 147.73, 139.19,
123.04, 121.35, 79.14. FT-IR (KBr): ν/cm-1 = 3178, 3061, 1775, 1623, 1601(m), 1591,
1573(m), 1491, 1459(s), 1436(s), 1383, 1320(s), 1277(s), 1230, 1146, 1115, 1102, 1023,
1013, 995, 963, 929, 902, 880, 845, 822, 803, 767, 742(s), 693, 655, 622, 570, 557. Elemental
analysis calc. C19H13N5: C, 73.3; H, 4.21; N, 22.5. Found: C, 73.0; H, 4.45; N, 22.2 %. X-ray
structure analysis: C19H13N5·3CH3OH, M = 407.47 g mol-1.
2.4. Synthesis of ligands L2 through L6
Monosubstituted ligands based upon the bzimpy skeleton have been synthesized by a
general recipe as follows. The substituting agent (SA) was MeI, EtI, iPrI, 3,5-di-(tert-butyl)-
benzyl bromide, and 4-bromobenzyl bromide for L2 through L6, respectively [20].
N,N-dimethylformamide was distilled with dehydrating agent CaH2 under inert argon
atmosphere. A 250 cm3 three necked flask was charged with bzimpy (2.30 g, 7.38 mmol) and
100 cm3 of freshly distilled dimethylformamide and then degassed. NaH (dispersion 60 %,
0.29 g, 7.38 mmol) was suspended in 10 cm3 of dry dimethylformamide under an inert
atmosphere and then added drop-by-drop to the basic solution cooled down at ca 0°C under
argon. The mixture was allowed to stand at room temperature for 1 h. The mixture was cooled
again to 0°C, SA (5.54 mmol) was added dropwise and the mixture was heated to 40°C for
overnight. The purification by column chromatography was carried out on Al2O3
dichlomethane and methanol (150:1). The crude product was recrystallized from appropriate
solvent (30 cm3). A single crystal of good quality was used for the X-ray diffraction studies.
Data for L2. Yield: 0.53 g, 22 %, white powder. Melting point: 241 - 243°C. Thin layer
chromatography (CH2Cl2/CH3OH; 150:1): Rf ~ 0.48. ESI-TOF MS (CH2Cl2): m/z = 348.13,
(theoretical m/z = 348.12 M+Na+. 1H NMR (500 MHz, DMSO-d6, 25°C) δ(ppm) 12.79 (s,
1H), 8.44 (dd, J = 7.75, 1H), 8.32 (dd, J = 7.75Hz, 1H), 8.18 (t, J = 7.75, 1H), 7.76 (dd, J = 8,
2H), 7.69 (s, 2H), 7.37 (t, J = 7.75, 1H), 7.32-7.26 (m, 3H), 4.33 (s, 3H). 13C NMR (125 MHz,
DMSO-d6, 25°C) δ(ppm) 149.03, 148.69, 148.63, 146.76, 140.93, 137.54, 135.91, 124.12,
122.16, 121.27, 120.72, 118.39, 116.94, 109.69, 31.59. FT-IR (KBr): ν/cm-1 = 3392, 3057,
2946, 1932, 1891, 1769, 1594(m), 1572(m), 1468, 1435(s), 1396, 1380, 1315, 1276, 1262,
1230, 1155, 1072, 1006, 994, 972, 928, 903, 856, 824(m), 742(s), 658, 618, 588, 565, 455,
426. UV/VIS (CH2Cl2): λmax (ε, M-1cm-1) = 327 (34088). Elemental analysis calc. C20H15N5:
C, 73.8; H, 4.65; N, 21.5. Found: C, 73.3; H, 4.75; N, 21.6 %. X-ray structure analysis:
C20H15N5·CH3OH, M = 357.41 g mol-1, CCDC 1038552.
Data for L3. Yield: 0.39 g, 16 %, white powder. Melting point: 244 - 246°C. Thin layer
chromatography (CH2Cl2/CH3OH; 150:1): Rf ~ 0.36. ESI-TOF MS (CH2Cl2): m/z = 340.16
M+H+. 1H NMR (500 MHz, DMSO-d6, 25°C) δ(ppm) 12.75 (s, 1H), 8.44 (dd, J = 8, 1H),
8.32 (dd, J = 7.5Hz, 1H), 8.19 (t, J = 7.75, 1H), 7.77 (t, J = 8.25, 2H), 7.69 (s, 2H), 7.36 (t, J
= 7.5, 1H), 7.31-7.26 (m, 3H), 5.01 (q, J = 7.25Hz, 2H), 1.27 (s, J = 7.25 3H). 13C NMR (125
MHz, DMSO-d6, 25°C) δ(ppm) 150.20, 150.01, 149.04, 148.17, 142.25, 138.72, 135.96,
125.25, 123.33, 122.41, 122.15, 119.66, 110.86, 15.32. FT-IR (KBr): ν/cm-1 = 3457, 3051,
2977, 2933, 1593(m), 1571(m), 1467, 1443(s), 1424, 1380, 1354, 1331, 1314, 1274, 1258,
1229, 1196, 1153, 1133, 1111, 1074, 1005, 976, 861, 824(m), 742(s), 714, 658, 618, 583, 564,

Low spin Fe(II) complexes formed of monosubstitued 2,6-bis(2-benzimidazyl)pyridine ligands
Cyril Rajnák, Ján Titiš, Olaf Fuhr, Mario Ruben and Roman Boča
5
522, 461, 427. UV/VIS (CH2Cl2): λmax (ε, M-1cm-1) = 328 (33522). Elemental analysis calc.
C21H17N5: C, 74.3; H, 5.05; N, 20.6. Found: C, 73.1; H, 4.93; N 20.4 %. X-ray structure
analysis: C21H17N5·(CH3)2SO·H2O, M = 435.54 g mol-1, CCDC 1038554.
Data for L4. Yield: 0.29 g, 11 %, white powder. Melting point: 208 - 210°C. Thin layer
chromatography (CH2Cl2/CH3OH; 150:1): Rf ~ 0.33. ESI-TOF MS (CH2Cl2): m/z = 354.17
M+H+. 1H NMR (500 MHz, DMSO-d6, 25°C) δ(ppm) 12.71 (s, 1H), 8.44 (dd, J = 8Hz, 1H),
8.20 (t, J = 7.75Hz, 1H), 8.08 (dd, J = 7.5, 1H), 7.88 (d, J = 7.5, 1H), 7.75 (d, J = 8Hz, 1H),
7.66 (s, 2H), 7.33-7.24 (m, 4H), 5.32 (sx, J = 6.75Hz, 1H), 1.66 (d, J = 7Hz, 6H).13C NMR
(125 MHz, DMSO-d6, 25°C) δ(ppm) 150.35, 150.31, 150.24, 148.23, 142.96, 138.69, 133.76,
126.08, 122.82, 122.15, 121.98, 119.96, 113.08, 48.87, 21.11. FT-IR (KBr): ν/cm-1 = 3453,
3065, 2300, 2969, 2937, 1923, 1888, 1852, 1770, 1612, 1590(m), 1571(m), 1484, 1444(s),
1419, 1391, 1380, 1336, 1315, 1285, 1253, 1225, 1192, 1164, 1133, 1105, 1090, 1051, 1009,
993, 979, 926, 901, 856, 828(m), 766, 749, 739(s), 661, 639, 565, 535, 516, 478, 446, 423.
UV/VIS (CH2Cl2): λmax (ε, M-1cm-1) = 358 (21694), 308(27786). Elemental analysis calc.
C22H19N5: C, 74.8; H, 5.42; N, 19.8. Found: C, 74.2; H 5.22; N, 19.8 %. X-ray structure
analysis: C22H19N5, M = 353.42 g mol-1, CCDC 1038556.
Data for L5. Yield: 0.45 g, 12 %, white powder. Melting point: 214 - 216°C. Thin layer
chromatography (CH2Cl2/CH3OH; 150:1): Rf ~ 0.596. ESI-TOF MS (CH3CN): m/z = 514.30
M+H+. 1H NMR (500 MHz, DMSO-d6, 25°C) δ(ppm) 12.83 (s, 1H), 8.40 (d, J = 7.5, 1H),
8.24 (d, J = 8Hz, 1H), 8.14 (t, J = 7.75, 1H), 8.02 (d, J = 8, 1H), 7.68 (s, 2H), 7.40 (t, J =
7.75Hz, 1H), 7.33-7.27 (m, 4H), 7.07 (s, 1H), 6.84 (s, 2H), 6.32 (s, 2H), 0.94 (s, 18H). 13C
NMR (125 MHz, DMSO-d6, 25°C) δ(ppm) 150.25, 150.13, 150.02, 149.22, 148.03, 142.15,
138.66, 136.91, 136.72, 125.12, 123.52, 122.59, 121.92, 121.21, 120.64, 119.72, 111.28,
48.19, 34.08, 30.80. FT-IR (KBr): ν/cm-1 = 3676, 3629, 3376, 3053, 2963(s), 2904, 2867,
2520, 1981, 1931, 1901, 1770, 1670, 1597(s), 1573(s), 1515, 1461, 1447(s), 1405, 1395,
1385, 1363, 1346, 1330, 1316, 1290, 1278, 1249(m), 1231, 1201, 1164, 1153, 1117, 1096,
1075, 1037, 1009, 994, 971, 928, 890, 879, 863, 823(s), 803, 763, 745(s), 714, 658, 618, 593,
580, 566, 545, 523, 457, 427, 397. UV/VIS (CH2Cl2): λmax (ε, M-1cm-1) = 326 (43750).
Elemental analysis calc. C34H35N5: C, 79.5; H, 6.87; N, 13.6. Found: C, 79.6; H, 6.73; N, 13.1
%. X-ray structure analysis: C34H35N5·(CH3)2SO·H2O, M = 609.81 g mol-1, CCDC 1038557.
Data for L6. The crude product was recrystallized from toluene (2 x 10 cm3). Yield: 0.49
g, 14 %, white powder. Melting point: 276-278°C. Thin layer chromatography
(CH2Cl2/CH3OH; 150:1): Rf ~ 0.451. ESI-TOF MS (CH2Cl2/CH3OH): m/z = 482.08 M+H+.
1H NMR (500 MHz, DMSO-d6, 25°C) δ(ppm) 12.61 (s, 1H), 8.40 (dd, J = 10Hz, 1H), 8.30
(dd, J = 7.5Hz, 1H), 8.15 (t, J = 7.75, 1H), 7.83 (q, J = 7.75, 2H), 7.69 (s, 2H), 7.38-7.24 (m,
6H), 6.93 (d, J = 8.5, 2H), 6.34 (s, 2H). 13C NMR (125 MHz, DMSO-d6, 25°C) δ(ppm)
149.99, 149.71, 149.17, 147.93, 142.18, 138.83, 137.27, 136.58, 131.58, 131.45, 128.63,
125.19, 123.77, 122.79, 122.18, 120.32, 119.82, 111.20, 47.38. FT-IR (KBr): ν/cm-1 =
3397(s), 3060, 2917, 2849, 1598, 1574, 1488, 1458(s), 1436(m), 1418(m), 1354, 1329, 1314,
1277, 1253, 1234, 1218, 1169, 1156, 1068, 1010, 994, 969, 895, 859, 822, 807, 797, 765, 752,
745(s), 659, 586, 576, 568, 556, 476, 457, 429. Elemental analysis calc. C26H18N5Br: C, 65.0;
H, 3.78; N, 14.7. Found: C, 65.0; H, 3.83; N, 14.1 %.
2.5. Synthesis of Fe(II) complexes [Fe(Ln)2](ClO4)2
Mononuclear Fe(II) complexes containing ligands L2 through L5 have been synthesized by
the following general recipe. A 100 cm3 two necked round-bottomed flask was charged with
ligand Ln (0.31 mmol) dissolved in methanol (50 cm3) and/or acetonitrile for L5. Argon was
passed through the solution for 30 min. to remove dissolved air and then Fe(ClO4)2
hexahydrate (0.039 g, 0.15 mmol) was added once. The mixture was stirred at 65 - 70°C

Low spin Fe(II) complexes formed of monosubstitued 2,6-bis(2-benzimidazyl)pyridine ligands
Cyril Rajnák, Ján Titiš, Olaf Fuhr, Mario Ruben and Roman Boča
6
overnight with colour changing to violet-pink. Single crystals were grown by using
ACN/Et2O.
Data for [Fe(L2)2](ClO4)2 (2). Yield: 0.12 g, 86 %. Melting point 284 - 286°C. ESI-TOF
MS (CH2Cl2): m/z = 353.09 M2+. UV/VIS (Nujol): νmax/103 cm-1 (relat. absorb.) = 27.100
(0.926), 19.763 (0.615), 18.149 (0.639), 14.085 (0.319). UV/VIS (CH2Cl2): λmax (ε, M-1cm-1)
= 321 (57962), 369 (47770), 574 (6688). FT-IR (KBr): ν/cm-1 = 3408, 3060, 1608, 1573,
1519, 1489, 1469(s), 1423, 1349, 1324(m), 1261, 1231, 1109(s), 1089(s), 1019, 930, 911,
858, 805, 744(s), 674, 626(m), 430. Elemental analysis calc. C40H30Cl2N10FeO8: C, 53.1; H,
3.34; N, 15.5. Found: C, 53.0; H, 3.71; N, 15.9 %.
Data for [Fe(L3)2](ClO4)2 (3). Yield: 0.11 g, 80 %. Melting point 268-270°C. ESI-TOF
MS (CH2Cl2): m/z = 367.12 M2+. UV/VIS (Nujol): νmax/103 cm-1 (relat.absorb.) = 27.027
(0.913), 19.960 (0.574), 18.149 (0.601), 14.184 (0.300). UV/VIS (CH2Cl2): λmax (ε, M-1cm-1)
= 321 (56481), 369 (48765), 574 (6481). FT-IR (KBr): ν/cm-1 = 3446, 3065, 2975, 1607,
1573, 1512, 1461(s), 1385, 1323, 1232, 1190, 1101(s), 928, 866, 814, 744(s), 622(m), 430,
380. Elemental analysis calc. C42H34Cl2FeN10O8: C, 54.0; H, 3.67; N, 15.0. Found: C, 54.6; H,
4.00; N, 15.3 %.
Data for [Fe(L4)2](ClO4)2 (4). Yield: 0.14 g, 79 %. Melting point 204-206°C. ESI-TOF
MS (CH2Cl2): m/z = 381.13 M2+. UV/VIS (Nujol): νmax/103 cm-1 (relat absorb.) = 27.027
(0.762), 19.802 (0.460), 17.637 (0.492), 14.184 (0.219). UV/VIS (CH2Cl2): λmax (ε, M-1cm-1)
= 321 (60943), 370 (43098), 573 (6061). FT-IR (KBr): ν/cm-1 = 3448, 3066, 2979, 1606,
1573, 1499, 1479, 1455(s), 1375, 1335, 1323, 1290, 1258, 1232, 1218, 1105(s), 1018, 929,
865, 822, 801, 764, 743(s), 675, 656, 623(m), 561, 431. Elemental analysis calc.
C44H38Cl2FeN10O8: C, 54.96; H, 3.98; N, 15.7. Found: C, 55.1; H, 4.01; N, 15.7 %.
Data for [Fe(L5)2](ClO4)2 (5). Used reagents: L5 (0.251 g, 0.49 mmol) dissolved in
acetonitrile (80 cm3); iron(II) perchlorate hexahydrate (0.062 g, 0.25 mmol. Yield: 0.25 g, 80
%. Melting point 252-254°C. ESI-TOF MS (CH2Cl2): m/z = 541.26 M2+. UV/VIS (Nujol):
νmax/103 cm-1 (relat. absorb.) = 26.954 (0.586), 17.452 (0.347), 14.184 (0.102). UV/VIS
(CH2Cl2): λmax (ε, M-1cm-1) = 321 (60738), 370 (49329), 573 (7047). FT-IR (KBr): ν/cm-1 =
3370, 3187, 3068, 2962(s), 2905, 2867, 1777, 1662(w), 1602(m), 1573, 1510, 1465(s), 1444,
1395, 1364, 1325, 1276, 1233, 1200, 1102(s), 1020, 1011, 927, 899, 868, 823, 808, 762,
745(s), 712, 675,660, 623(s), 565, 431. Elemental analysis calc. C68H70Cl2N10FeO8: C, 63.7;
H, 5.50; N, 10.9. Found: C, 63.4; H, 5.77; N, 11.2 %.
2.6. Synthesis of Fe(II) complexes with deprotonated ligands
Iron(II) complexes with monodeprotonated ligands L2-1H through L5-1H were prepared as
follows. In a 100 cm3 two necked round-bottomed flask a crude solid complex
[Fe(Ln)2](ClO4)2 was dissolved in dichloromethane (50 cm3) under argon atmosphere. After
30 min. of stirring, a solid Na2CO3 was added and the mixture stirred at 39°C for overnight
resulting in a blue colour.
Data for [Fe(L2-1H)2] (2d). Used reagents: [Fe(L2)2](ClO4)2 (0.051 g, 0.06 mmol); Na2CO3
(0.024 g, 0.23 mmol, 4eq.). Yield: 0.04 g, 95 %. Melting point 225 - 227°C. ESI-TOF MS
(CH2Cl2): m/z = 705.19 M+H+. FT-IR (KBr): ν/cm-1 = 3360, 3055, 2964, 1604, 1571, 1501,
1487, 1452, 1433, 1415, 1326, 1262(s), 1228, 1095(s), 1023(s), 863, 803(s), 741(s), 622, 428.
UV/VIS (Nujol): νmax/103 cm-1 (relat. absorb.) = 30.488 (0.809), 27.397 (0.560), 16.835
(0.393), 12.391 (0.184). UV/VIS (CH2Cl2/CH3OH; 5:1): λmax (ε, M-1cm-1) = 322 (39942), 368
(25073), 578 (3499). Elemental analysis calc. C40H28FeN10: C, 68.2; H, 4.01; N, 19.9. Found:
C, 68.4; H, 4.44; N 19.7 %. X-ray structure analysis: C40H28FeN10·(CH3CH2)2O·CH3OH, M =
810.74 g mol-1, CCDC 1038553.

Low spin Fe(II) complexes formed of monosubstitued 2,6-bis(2-benzimidazyl)pyridine ligands
Cyril Rajnák, Ján Titiš, Olaf Fuhr, Mario Ruben and Roman Boča
7
Data for [Fe(L3-1H)2] (3d). Used reagents: [Fe(L3)2](ClO4)2 (0.090 g, 0.10 mmol); Na2CO3
(0.041 g, 0.39 mmol, 4 eq.). Yield: 0.07 g, 92 %. Melting point 210 – 212 °C. There is a
continuous mass loss of 7 % until 210 oC accompanied by an exothermic peak. ESI-TOF MS
(CH2Cl2): m/z = 733.22 M+H+. FT-IR (KBr): ν/cm-1 = 3392, 3055, 2964, 1631, 1603, 1573,
1434(s), 1381, 1349, 1327, 1092(s), 1019, 868, 803(s), 742, 627, 429. UV/VIS (Nujol):
νmax/103 cm-1 (relat. absorb.) = 26.882 (0.722), 16.695 (0.523), 12.3916 (0.271). UV/VIS
(CH2Cl2/CH3OH; 5:1): λmax (ε, M-1cm-1) = 307 (39934), 347 (38944), 598 (5281). Elemental
analysis calc. C42H32FeN10: C, 69.8; H, 4.26; N, 18.5. Found: C, 69.4; H, 4.35; N, 18.9 %. X-
ray structure analysis: C42H32FeN10·3CH3CN, M = 855.79 g mol-1, CCDC 1038555.
Data for [Fe(L4-1H)2] (4d). Used reagents: [Fe(L4)2](ClO4)2 (0.080 g, 0.08 mmol); Na2CO3
(0.035 g, 0.33 mmol, 4 eq.). Yield: 0.06 g, 87 %. Melting point 160-162 °C. There is a
continuous mass loss of 5 % until 210 oC accompanied by a broad exothermic peak at 160 oC,
then a sharp exothermic peak at 180 oC, and a sharp endothermic peak at 200 oC. ESI-TOF
MS (CH2Cl2): m/z = 760.25 M A79. UV/VIS (Nujol): νmax/103 cm-1 (relat. absorb.) = 26.954
(0.841), 16.863 (0.504), 14.025 (0.254). UV/VIS (CH2Cl2/CH3OH; 5:1): λmax (ε, M-1cm-1) =
323 (52339), 369 (16959), 579 (1754) A80. FT-IR (KBr): ν/cm-1 = 3057, 2929, 2857, 1739,
1664, 1595, 1572, 1448(s), 1388, 1326, 1315, 1289, 1256, 1229, 1191, 1159, 1133, 1100(s),
1009, 993, 930, 912, 854, 824, 808, 767, 745(s), 656, 623, 565, 533, 428, 386. Elemental
analysis calc. C44H36FeN10: C, 69.5; H, 4.77; N, 18.4. Found: C, 69.0; H, 4.34; N, 18.2 %.
Data for [Fe(L5-1H)2] (5d). Used reagents: [Fe(L5)2](ClO4)2 (0.312 g, 0.24 mmol); Na2CO3
(0.103 g, 0.97 mmol, 4eq.). Yield: 0.26 g, 97 %. Melting point 220 – 222 °C. There is a
continuous mass loss of 4 % until 220 oC accompanied by an exothermic peak; another
exothermic peak appears at 280 oC. ESI-TOF MS (CH2Cl2): m/z = 1081.51 M+H+
A81.UV/VIS (Nujol): νmax/103 cm-1 (relat. absorb.) = 27.548 (1.242), 16.260 (0.721), 13.298
(0.315), 12.107 (0.279). UV/VIS (CH2Cl2/CH3OH; 5:1): λmax (ε, M-1cm-1) = 308 (48860), 356
(40717), 580 (3909) A82. FT-IR (KBr): ν/cm-1 = 3375, 3058, 2962, 2903, 2867, 1602, 1572,
1543, 1476, 1435(s), 1394, 1394, 1363, 1325, 1278, 1249, 1198, 1155, 1092(s), 1016, 979,
927, 899, 867, 850, 823, 803, 741(s), 711, 673, 659, 622, 594, 433. Elemental analysis calc.
C68H68FeN10: C, 75.5; H, 6.34; N, 13.0. Found: C, 75.3; H, 6.28; N, 13.5 %.
2.7. X-ray structure determination
Single-crystal X-ray diffraction experiments and data collection for ligands L1 through L6
were conducted using Xcalibur S CCD (Oxford Diffraction) at 293 K. Intensities were
recorded with a ω scans. Corrections for Lorentz, polarization and multi-scan absorption
effects were applied. Diffraction data of 2d and 3d were collected on STOE IPDS II or STOE
IPDS 2T diffractometers at low temperature. Graphite-monochromated Mo Kα radiation, λ =
0.71093 Å, was used. The structures were either solved by charge-flipping or direct methods
and subsequent Fourier differences techniques. Non-hydrogen atoms were refined
anisotropically against F2 by common least-squares methods. The programs SUPERFLIP,
SHELX (ver. 2014/7), OLEX2 and MERCURY have been used for structure determination,
refinement and drawing [21-26]. The hydrogen atoms were refined with fixed distances from
the parent carbon atoms.
2.8. Magnetic data collection
The magnetic measurements were conducted using a SQUID apparatus (MPMS-XL7,
Quantum Design) in the RSO mode of detection. The polycrystalline sample was
encapsulated in a diamagnetic gelatine-made sample holder. The susceptibility taken at B =
0.1 or 0.5 T has been corrected for the underlying diamagnetism and transformed to the

Low spin Fe(II) complexes formed of monosubstitued 2,6-bis(2-benzimidazyl)pyridine ligands
Cyril Rajnák, Ján Titiš, Olaf Fuhr, Mario Ruben and Roman Boča
8
effective magnetic moment calculated as µeff/µB = 798(χ′T)1/2 (SI units). For high-temperature
range above 300 K an oven has been installed. Due to a different sample holder the analyzed
data does not necessarily follow the 300 K tail and some discontinuity is registered.
3. Results and discussion
3.1. Organic synthesis
The 2,6-bis(2-benzimidazolyl)pyridine, as the bearer of N-donors atoms, was synthesized
many times by the Phillips condensation reaction [27, 28]. One of the easiest ways to attach a
new substituent to the skeleton of this ligand leads by N-alkylation of benzimidazole rings.
The Williams synthetic route has looked as the promising for the preparation of
unsymmetrical bzimpy ligands extended on the benzimidazole rings [29, 30]. The ratio of
reactants, ligand: base: alkyl halides, was selected to minimize the effect of dialkylation. Five
novel ligands L2 - L6 were synthesized. Four of them (L1-L5) were used to coordination with
iron salts. Due to the worse solubility of L6 (toluene), it has not been to try yet.
3.2. Molecular and crystal structure
Crystal data, information about data collection and structure analysis for ligands L1
through L6 were forwarded to Electronic Supplementary Information (ESI). The data referring
to the Fe(II) complexes with deprotonated ligands, 2d and 3d, are listed in Table 1.
Table 1 Crystal data for Fe(II) complexes with deprotonated ligands
Abbr.
2d
C45H42FeN10O2
810.74
3d
C48H41FeN13
855.79
Empirical formula
Formula weight /g mol-1
Crystal system
Space group
monoclinic
C2/c
monoclinic
C2/c
Temperature /K
150(2)
180(2)
Crystal size /mm
Z
0.23 × 0.03 × 0.02
4
0.32 × 0.30 × 0.26
4
a / Å
b / Å
c / Å
α /°
14.4272(12)
15.3787(8)
17.9306(14)
90
15.4016(6)
16.2551(8)
16.7971(7)
90
β /°
106.845(6)
90
3807.6(5)
1.414
0.451
10073/4021
4021 / 0 / 275
1.008
101.431(3)
90
4121.8(3)
1.379
0.419
11353/3871
3871 / 0 / 264
1.043
γ /°
V /Å3
Calculated density Dc /g cm-3
Absorption coefficient /mm-1
Reflections collected /unique
Data / restraints / parameters
Goodness-of-fit
Final R indices (I > 2σ(I))
R1 = 0.0944
wR2 = 0.2056
R1 = 0.1349
wR2 = 0.2235
2.415 / -0.732
1038553
R1 = 0.0667
wR2 = 0.1771
R1 = 0.0815
wR2 = 0.1865
1.377 / -0.761
1038555
R indices (all data)
Largest diff. peak and hole / e Å -3
CCDC No

Low spin Fe(II) complexes formed of monosubstitued 2,6-bis(2-benzimidazyl)pyridine ligands
Cyril Rajnák, Ján Titiš, Olaf Fuhr, Mario Ruben and Roman Boča
9
Table 2 Bond lengths (Å) within the chromophore of 2d and 3d.
T = 180 K
1.906(3)
1.906(3)
1.965(3)
1.965(3)
1.976(3)
1.976(3)
2d
T = 150 K
1.972(4)
1.972(4)
1.900(3)
1.900(3)
1.939(4)
1.939(4)
3d
Fe(1)-N(1)
Fe(1)-N(1)#1
Fe(1)-N(3)
Fe(1)-N(3)#1
Fe(1)-N(4)
Fe(1)-N(4)#1
Fe(1)-N(3)
Fe(1)-N(3)#1
Fe(1)-N(4)
Fe(1)-N(4)#1
Fe(1)-N(2)
Fe(1)-N(2)#1
The structure of the compound 2d·is formed by molecular units (Z = 4) and solvent
molecules of diethyl ether and methanol which are present in the crystal lattice. The Fe(II)
centre is hexacoordinate with two deprotonated ligands in the coordination environment (Fig.
1). The bond lengths of Fe-N(pyridine) equals to 1.900 Å and those of Fe-N(imidazole) are
1.972 and 1.939 Å, respectively (Table 2). This indicates the low-spin state of the complex at
150 K. Two benzene rings belonging to the neighbouring molecular units, show a perfect π-π
stacking at the C…C contact of 3.67 Å.
Fig.1 The structure of Fe(II) complexes 2d with deprotonated ligands at 150 K. Hydrogen
atoms and crystal solvent molecules are omitted for clarity. Left – molecular structure, right –
the π-π stacking between benzene rings.
Fig. 2 The structure of Fe(II) complexes 3d with deprotonated ligands at 180 K. Hydrogen
atoms and crystal solvent molecules are omitted for clarity. Left – molecular structure, right –
the π-π stacking between benzene rings.

Low spin Fe(II) complexes formed of monosubstitued 2,6-bis(2-benzimidazyl)pyridine ligands
Cyril Rajnák, Ján Titiš, Olaf Fuhr, Mario Ruben and Roman Boča
10
Fig. 3 The structure of Fe(II) complexes 1d·H2O with deprotonated ligands at 293 K [9].
Hydrogen atoms and crystal solvent molecules are omitted for clarity. Left – molecular
structure, right – the π-π stacking between benzene rings.
The molecular and crystal structure of 3d is essentially analogous to 2d (Fig. 2). The
shortest bond lengths Fe-N(pyridine) within the chromophore equal to 1.906 Å and those of
Fe-N(imidazole) are 1.965 and 1.976 Å, respectively. Centroid-centroid distances between the
stacked benziimidazole rings are 3.675 Å for 2d and 3.724 for 3d.
The analogous Fe(II) complex with the deprotonated ligand bzimpy-1H (abbr. 1d)
possesses the distances of Fe-N(pyridine) = 1.918 Å and Fe-N(imidazole) = 1.950 Å,
respectively [17]; the molecular and crystal structure of 1d·H2O is viewed in Fig. 3.
3.3. Spectral data and electronic structure
Electronic spectra for complexes containing protonated bzimpy ligands (2 − 5), as well as,
for complexes containing deprotonated ligands (2d – 5d) have been measured in solid state
(nujoll mull) and are shown in Fig. S43 in ESI. One can easily recognize the almost identical
spectral profile for all compounds, that is characterized by a small peak at ~17 180 cm-1
followed by a more intense peaks located at ~27 400 and 31 900 cm-1.
To elucidate the electronic structure of complexes under study and also the origin of the
spectral transitions, we have calculated electronic spectra for three complexes 1d, 2d and 3d
containing deprotonated bzimpy derivatives [31]. To carry out this objective, the time-
dependent restricted Kohn-Sham formalism has been chosen, due to the proven low-spin (S =
0) ground state of the 1d. According to the structural similarities of 1d with 2d and 3d, we
suspect to such magnetic behaviour also in the later two complexes. Calculations have been
performed on X-ray structure coordinates employing hybrid B3LYP functional and def2-
TZVP basis set [32-34].
The main results are compiled in Figs. 4 and 5. From these, we can see that the calculated
electronic spectra are in good agreement with the experimental ones. Furthermore, we can
recognize similarities in electronic structure, especially for complexes 2d and 3d. Complex 1d
shows slight differences. Due to the complex nature of the excited states, we have analyzed
the principal electronic transitions in terms of difference densities. This approach has helped
us to recover the simple single-electron picture of the transitions that is depicted in Fig. S44
(for 1d) and Fig. S45 (for 3d) in ESI. For complex 2d the excitations are identical with those
of 3d. As can be seen (Fig. 4), in 1d the transitions under 20 000 cm-1 can be characterized as
ligand-to-metal charge transfer (LMCT) due to mainly ligand character of the highest
occupied orbitals. At higher energies (>20 000 cm-1) the electron relocations to π-character
metal-ligand orbitals can be identified. In case of 2d and 3d the metal-to-ligand charge
transfers (MLCT) are dominated with relocations to pyridyl moiety of the bzimpy-1H ligands.

Low spin Fe(II) complexes formed of monosubstitued 2,6-bis(2-benzimidazyl)pyridine ligands
Cyril Rajnák, Ján Titiš, Olaf Fuhr, Mario Ruben and Roman Boča
11
0.5
0.4
0.3
0.2
0.1
0.0
0.6
0.5
0.4
0.3
0.2
0.1
0.0
0.4
0.3
0.2
0.1
0.0
3d
π(Fe[d]+bzim)->π(py)
2d
π(Fe[d]+bzim)->π(py)
1d
Exp
TD-DFT
TD-DFT
Exp
TD-DFT
π(bzimpy)->π(Fe[d]+py)
π(bzim)->π(Fe[d]+py)
π(bzim)->Fe[d]
Fe[d]->π(py)
Fe[d]->π(py)
10
15
20
25
30
35
40
10
15
20
25
30
35
40
10
15
20
25
30
35
40
wavenumber /103 cm-1
wavenumber /103 cm-1
wavenumber /103 cm-1
Fig. 4 Comparison of measured and calculated electronic spectra of complexes 1d, 2d and 3d
(bzim = benzimidazolyl moiety of the bzimpy-1H ligands, py = pyridyl moiety of the bzimpy-1H
ligands). Calculated intensities were divided by a factor 3×105.

Low spin Fe(II) complexes formed of monosubstitued 2,6-bis(2-benzimidazyl)pyridine ligands
Cyril Rajnák, Ján Titiš, Olaf Fuhr, Mario Ruben and Roman Boča
12
Fig. 5 Frontier orbitals of 1d, 2d and 3d.
-0.40
-0.45
protonated
-0.50
upon deprotonation
-0.55
-0.60
L1
L2
L3
L4
L5
Fig. 6 Calculated natural atomic charges on nitrogen donor atoms (▼- on pyridyl moiety; ●,■-
on benzimidazolyl moieties) of free bzimpy ligands.
Effect of the ligand substituents on electronic structure has been studied on free ligands
(L1 – L5 and L1-1H – L5-1H X-ray structures). Natural population analysis (B3LYP/6-311G*)
has been performed to obtain natural charges on nitrogen donor atom [35]. Results are shown
in Fig. 6 (see also Figs. S46 – S50 in ESI). According to these findings, deprotonation of the
benzimidazolyl subunits increases electron density on nitrogen donors of the benzimidazolyls
and decreases electron density on the pyridyl nitrogen. Introduction of methyl substituent on
the benzimidazolyl and further extension of the alkyl chain affect the electronic structure
slightly (moderate variation of the negative charge from L1 to L5). Significantly greater
impact brings the deprotonation of the ligands. Recently studies have shown that the low-spin
state of such complexes is stabilized by electron-withdrawing pyridyl subtituents, but also by
electron-donating substituents located on the peripheral ligand subunits (such as a
benzimidazolyl) [36, 37]. Alkyls and phenyls are weakly electron-donating groups, thus the
low-spin is stabilized in complexes under study.
3.4. Magnetic data
Magnetic data for [Fe(L2)2](ClO4)2 (2) is shown in Fig. 5. At T = 1.9 K the effective
magnetic moment adopts a value of µeff = 0.58 µB. On heating it shows only a small increase
to µeff = 1.57 µB at the room temperature. Such a behaviour is characteristic for the S = 0 state
with some temperature-independent paramagnetism (TIP; it appears as an effect of the
presence of close-lain excited states). It is confirmed that 2 is a low-spin Fe(II) complex until
T = 300 K.

Low spin Fe(II) complexes formed of monosubstitued 2,6-bis(2-benzimidazyl)pyridine ligands
Cyril Rajnák, Ján Titiš, Olaf Fuhr, Mario Ruben and Roman Boča
13
6
5
4
3
2
1
0
6
5
4
3
2
1
0
6
5
4
3
2
1
0
heating
cooling
2d
2d-HT
2
µ
µ
µ
µ
µ
µ
0
50
100
150
200
250
300
0
50
100
150
200
250
300
300
350
400
450
500
550
T/K
T/K
T/K
Fig. 7 Magnetic data for [Fe(L2)2](ClO4)2 and [Fe(L2-1H)2] complexes.
Magnetic data for the complex [Fe(L2-1H)2] (2d) is also displayed in Fig. 5. At T = 1.9 K
the value of µeff = 0.91 µB reflects the S = 0 state with some paramagnetic impurity (as
confirmed by the magnetic susceptibility). On heating the effective magnetic moment slowly
increases until T ≈ 47 K and the increase continues up to the ambient temperature where µeff =
1.45 µB. High-temperature measurements (with the installed “owen”) were conducted in the
cooling mode T = 551 - 300 K as well as in the heating direction T = 301 - 501 K. These data
superimpose and at the end of the heating (T = 501 K) µeff = 2.07 µB. Above T > 500 K an on-
set of the spin transition is indicated based upon a more rapid increase of the effective
magnetic moment.
6
5
4
3
2
1
0
6
5
4
3
2
1
0
6
5
4
3
2
1
0
heating
cooling
3
3d
3d-HT
µ
µ
µ
µ
µ
µ
0
50
100
150
200
250
300
0
50
100
150
200
250
300
300
350
400
450
500
550
T/K
T/K
T/K
Fig. 8 Magnetic data for [Fe(L3)2](ClO4)2 and [Fe(L3-1H)2] complexes.
Fig. 6 brings the magnetic data for the complex [Fe(L3)2](ClO4)2 (3). At T = 1.9 K the
effective magnetic moment is only µeff = 0.52 µB and on heating it increases gradually to µeff =
1.25 µB at T = 300 K. Again the diamagnetic (S = 0) state is confirmed until room
temperature.
Magnetic data for [Fe(L3-1H)2] (3d) is also shown in Fig. 6. At T = 1.9 K the value of µeff =
0.92 µB and this system behaves analogously to the previous case so that 3 is low-spin (S = 0)
until T = 300 K. High-temperature measurements (with the installed “owen”) in the heating
mode T = 354 - 552 K and in the cooling mode T = 547 - 469 K, however, are different. On
heating the effective magnetic moment slowly increases from µeff = 0.62 µB at T = 354 K to µeff
= 1.38 µB at T = 552 K. On cooling the effective magnetic moment stays almost constant
between µeff = 1.39 µB at T = 546 K and µeff = 1.36 µB at T = 469 K.
Magnetic data for [Fe(L4)2](ClO4)2 (4) is shown in Fig. 7 and it is essentially similar to
that for 3 and 2. To this end, 4 is a low-spin Fe(II) complex until T = 300 K. For [Fe(L4-1H)2]
(4d) at T = 1.9 K the value of µeff = 0.37 µB and on heating it slightly increases to µeff = 1.50 µB
at T = 300 K also confirming the diamagnetic state, S = 0.

Low spin Fe(II) complexes formed of monosubstitued 2,6-bis(2-benzimidazyl)pyridine ligands
Cyril Rajnák, Ján Titiš, Olaf Fuhr, Mario Ruben and Roman Boča
14
6
5
4
3
2
1
0
6
4
4d
5
4
µ
µ
3
µ
µ
2
1
0
0
50
100
150
200
250
300
0
50
100
150
200
250
300
T/K
T/K
Fig. 9 Magnetic data for [Fe(L4)2](ClO4)2 and [Fe(L4-1H)2] complexes.
Fig. 8 brings magnetic data for [Fe(L4)2](ClO4)2 (5). At T = 1.9 K the effective magnetic
moment is only µeff = 0.55 µB ; on heating the it increases to µeff = 1.83 µB at the room
temperature. These data match the diamagnetic state (S = 0) until ambient temperature.
Magnetic data for [Fe(L5-1H)2] (5) is also shown in Fig. 8. The sample was cycled between 5 K
– 360 K – 5 K – 390 K – 15 K. On the first heating the effective magnetic moment starts from
the value of 0.72 µB at T = 5 K and increases to µeff = 2.32 µB at T = 360 K. On the second
heating the effective magnetic moment starts from a higher value 0.80 µB at T = 5 K and
increases to µeff = 2.48 µB at T = 390 K. Most likely the solvent was slowly evaporated by
heating which slightly influences the measured magnetic moment of the specimen.
6
5
4
3
2
1
0
6
5
4
3
2
1
0
5
5d
first heating
first cooling
second heating
µ
µ
µ
µ
0
50
100
150
200
250
300
0
50 100 150 200 250 300 350 400
T/K
T/K
Fig. 10 Magnetic data for [Fe(L5)2](ClO4)2 and [Fe(L5-1H)2] complexes.
Some more data about the related complexes have been retrieved from literature [15-17]
and the corresponding magnetic functions are redrawn in Fig. 9. and the transition
temperature is listed in Table 3. A comparison of the data presented in Figs. 4 through 8 leads
to the following findings.

Low spin Fe(II) complexes formed of monosubstitued 2,6-bis(2-benzimidazyl)pyridine ligands
Cyril Rajnák, Ján Titiš, Olaf Fuhr, Mario Ruben and Roman Boča
15
6
5
4
3
2
1
0
6
heating
cooling
heating
cooling
1
1d
5
4
3
2
1
0
6
5
4
3
2
1
0
µ
µ
µ
µ
350
400
450
0
50 100 150 200 250 300 350 400 450 500
0
50 100 150 200 250 300 350 400 450 500
T/K
T/K
6
5
4
3
2
1
0
6
5
4
3
2
1
0
heating
1'
1''
cooling
µ
µ
µ
µ
heating, n = 4
cooling, n = 0
0
50 100 150 200 250 300 350 400 450 500
0
50 100 150 200 250 300 350 400 450 500
T/K
T/K
Fig. 11 Comparison of magnetic data for [Fe(L1)2](ClO4)2·0.25H2O, [Fe(L1-1H)2]·H2O,
Fe(bzimpy)2](BPh4)2⋅nH2O (1’) and [Fe(bzimpy)2](SO4)⋅2H2O (1’’) complexes [15-17].
Table 3. Comparison of magnetic behaviour with literature data.
Compound
Behaviour
S = 0 to S = 2, Tc = 300 K
High-spin, S = 2
Ref.
[15]
[15]
[16]
[Fe(L1)2](BPh4)2⋅4H2O
[Fe(L1)2](BPh4)2
[Fe(L1)2](ClO4)2⋅0.25H2O
Low-spin until r.t.
S = 0 to S = 2, T1/2↑ = 409 K, T1/2↓ = 397 K, ∆T = 12 K
Low-spin until r.t.
[Fe(L1-1H)2]⋅H2O
[17]
S = 0 to S = 2, T1/2↑ = 424 K
Low-spin until r.t.
Low-spin until r.t.
[Fe(L1)2]SO4⋅2H2O
[Fe(Ltz)2](ClO4)2·2H2O
unpublished
[17]
S = 0 to S = 2, T1/2 = 323 K, ∆T = 35 K
1. The low-temperature magnetic data for [Fe(Ln)2](ClO4)2 type complexes are influenced
by some paramagnetic impurity (PI), probably traces of Fe(III). In such a case the effective
magnetic moment on heating stays almost constant (the PI follows the Curie-Weiss law).
2. A constant slope of the µeff vs T function reflects a temperature-independent
paramagnetism originating in the presence of the close-lying excited states (arising from the
5T2g crystal-field term).
3. There is no indication of the spin transition until the ambient temperature. However,
above 300 K an on-set of the spin crossover is visible in 2d and 3d.
4. In the previously reported complex 1, the transition temperature is T1/2↑ = 409 K and
T1/2↓ = 397 K giving rise to the hysteresis width of ∆T = 12 K.
5. In 1d, however, T1/2↑ = 424 K is rather high and probably the spin transition is
incomplete until highest temperature of the data taking (500 K); also the release of the crystal
water can modify the cooling path.
6. In 1’ four molecules of the crystal water are lost on the heating and the complex turns to
the high-spin state that remains high-spin on the cooling.

Low spin Fe(II) complexes formed of monosubstitued 2,6-bis(2-benzimidazyl)pyridine ligands
Cyril Rajnák, Ján Titiš, Olaf Fuhr, Mario Ruben and Roman Boča
16
7. An analogous liberation of the solvent molecules in 3d causes that the cooling path is
different from the heating one.
8. In the sulfato complex [Fe(bzimpy)2](SO4)⋅2H2O (1’’) the on-set of the spin crossover
is registered.
4. Conclusions
Five derivatives of the basic bzimpy ligand have been prepared and fully characterized (X-
bzimpy). These were used in synthesizing [Fe(X-bzimpy)2](ClO4)2 complexes as well as the
deprotonated analogues [Fe(X-bzimpy-1H)2]. Magnetic studies reveal that all these systems are
low-spin until room temperature with some on-set of the spin crossover above the room
temperature.
Acknowledgments
Slovak grant agencies (APVV-14-0078, VEGA 1/0534/16 and VEGA 1/0534/16) and Marie
Curie fellowship (contract No. PITN-GA-2009-238804) are acknowledged for the financial
support. We thank the KIT-INT staff for their support during measurements and Dr. Šalitroš
to help with high temperature SQUID measurements.
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in the online version, at
http://dx.doi.org/. CCDC 1038552, 1038554, 1038556 and 1038557 contains the
supplementary crystallographic data for L2, L3, L4, and L5. CCDC 1038555 and 1038553
contains the supplementary crystallographic data for 2d and 3d. 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.
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Low spin Fe(II) complexes formed of monosubstitued 2,6-bis(2-benzimidazyl)pyridine ligands
Cyril Rajnák, Ján Titiš, Olaf Fuhr, Mario Ruben and Roman Boča
19
Synopsis
We have prepared five new ligands, monosubstituted derivatives of the basic 2,6-bis(2-
benzimidazyl)pyridine (bzimpy) ligand. Reactions of these ligands with Fe(II) ion gave a set
of hexacoordinate complexes showing predominantly the diamagnetism until ambient
temperature. Furthermore, some temperature-independent paramagnetism along with an onset
of the spin transition has also been detected.