Iron(II) complexes containing thiophene-substituted "bispicen" ligands Spin-crossover, ligand rearrangements, and ferromagnetic interactions

Article abstract of DOI:10.1139/V10-086

The synthesis and characterization of three new tetradentate "bispicen-type" ligands containing a substituted thiophene heterocycle are described [2,5-thienyl substituents = H (7), Ph (8), or 2-thienyl (9)]. Iron(II) bis(thiocyanate) coordination complexe

Full text of DOI:10.1139/V10-086

954
Iron(II) complexes containing thiophene-
substituted ‘‘bispicen’’ ligands — Spin-crossover,
ligand rearrangements, and ferromagnetic
interactions
Haojin Cheng, Brandon Djukic, Hilary A. Jenkins, Serge I. Gorelsky, and
Martin T. Lemaire
Abstract: The synthesis and characterization of three new tetradentate ‘‘bispicen-type’’ ligands containing a substituted thi-
ophene heterocycle are described [2,5-thienyl substituents = H (7), Ph (8), or 2-thienyl (9)]. Iron(II) bis(thiocyanate) coor-
dination complexes containing 7–9 were prepared, and the electronic and variable-temperature magnetic properties of
complexes containing 7 (10) and 9 (12) are described. Complex 10 features a gradual and incomplete spin crossover in the
solid state, and 12 remains high-spin over the entire temperature range. Complex 11 is extremely unstable and rearranges
to another iron(II) complex (13), which was structurally characterized. The temperature-dependent magnetic properties of
13 are described as a one-dimensional ferromagnetic chain, with interchain antiferromagnetic interactions and (or) zero-
field splitting dominant at low temperatures. The magnetic analysis is corroborated by the molecular packing and density
functional theory calculations, which suggest intermolecular interactions between coordinated thiocyanate ligands bearing
a significant spin density.
Key words: spin crossover, iron(II), thiophene, ferromagnetic interactions.
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Resume : On decrit la synthese et la caracterisation de trois nouveaux ligands tetradentates de type bispicene contenant un
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heterocycle thiophene substitue [substituants 2,5-thienyl = H (7), Ph (8) ou 2-thienyl (9)]. On a prepare les complexes de
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coordination bis(thiocyanate) de fer(II) contenant les ligands 7–9 et on decrit les proprietes magnetiques en fonction de la
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temperature des complexes contenant le ligand 7 (10) et le ligand 9 (12). Le complexe 10 par une inversion graduelle et
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incomplete de spin a l’etat solide alors que le complexe 12 garde un spin eleve sur toute la plage de temperature. Le com-
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plexe 11 est extremement instable et il se rearrange en un autre complexe du fer(II) (13) dont on a caracterise la structure.
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On decrit les proprietes magnetiques du complexe 13 en fonction de la temperature comme une chaıne ferromagnetique
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unidimensionnelle avec des interactions antiferromagnetiques interchaınes et (ou) un dedoublement de champ dominant
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nul a basses temperatures. L’analyse magnetique est corroboree par des calculs d’empilement moleculaire et par la theorie
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de la fonctionnelle de la densite, ce qui suggere qu’il existe des interactions intermoleculaires entre les ligands thiocyana-
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tes porteurs d’une densite de spin significative.
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Mots-cles : inversion de spin, fer(II), thiophene, interactions ferromagnetiques.
chromism have all now been reported. A number of
research groups have also reported single-component ionic-
coordination complexes containing SCO cations and electri-
cally conducting anions as fascinating SCO conductors.4 We
are also interested in SCO conduting materials and our ap-
proach is focused on combining SCO properties with electri-
cal conductivity in novel metallopolymer materials.5 To
date, all reported SCO conductors have contained iron(III),
and we thought it would be interesting to produce SCO con-
ductors instead containing iron(II).
Introduction
Spin-crossover (SCO) in iron(II) coordination complexes
provides the best example of molecular bistability.1 There
are many examples of iron(II) complexes exhibiting abrupt
spin state transitions and concomitant thermal hysteresis,
confering true bistability on these materials.2 Exploration of
new avenues for research with SCO complexes is underway,
including coupling SCO with other properties in single-
component materials (so-called multifunctional materials).3
SCO complexes also exhibiting magnetic exchange cou-
pling, liquid crystalline behaviour, porosity, and photo-
As an initial foray into this research, we have designed
Received 3 March 2010. Accepted 14 June 2010. Published on the NRC Research Press Web site at canjchem.nrc.ca on 18 August 2010.
H. Cheng, B. Djukic, and M.T. Lemaire.1 Department of Chemistry, Brock University, 500 Glenridge Avenue, St. Catharines, ON L2S
3A1, Canada.
H.A. Jenkins. Department of Chemistry, McMaster University, 1280 Main Street West, Hamilton, ON L8S 4M1, Canada.
S.I. Gorelsky. Centre for Catalysis Research and Innovation, Department of Chemistry, University of Ottawa, 10 Marie Curie, Ottawa,
ON K1N 6N5, Canada.
1Corresponding author (e-mail: mlemaire@brocku.ca).
Can. J. Chem. 88: 954–963 (2010)
doi:10.1139/V10-086
Published by NRC Research Press

Cheng et al.
955
Fig. 1. Structural relationship between known bispicen ligands and
a thienyl-substituted ligand produced herein.
and the preparation, electronic, and variable-temperature
magnetic properties of mononuclear iron(II) coordination
complexes containing these ligands, including the observa-
tion of SCO. We also report an unusual ligand-centered
structural rearrangment in solutions containing these coordi-
nation complexes, which produces new iron complexes. One
of these complexes features significant intermolecular ferro-
magnetic interactions.
Experimental
Table 1. Crystallographic data and structure refinement for 13.
General procedures
Empirical formula
Formula mass
Habit
C58H40FeN10S4
1061.09
Rod
All reagents were commercially available and used as re-
ceived unless otherwise stated. Deaerated and anhydrous
solvents were obtained from a Puresolve PS MD-4 solvent-
purification system, and all air- and (or) moisture-sensitive
reactions were carried out using standard Schlenk techni-
Color
Orange
Crystal size (mm)
Crystal system
Space group
Z
0.37ꢀ0.13ꢀ0.10
Monoclinic
C2/c
1
ques, unless otherwise stated. H/13C NMR spectra were re-
corded on
a Bruker Advance 300 (or 600) MHz
4
spectrometer (as indicated) with a 7.05 (or 14.1) T Ultra-
shield magnet using deuterated solvents. FTIR spectra were
recorded on a Shimadzu IR/Affinity spectrometer as KBr
discs or thin films on KBr plates. EI and FAB mass spectra
were obtained using a Kratos Concept 1S High Resolution
E/B mass spectrometer, and ESI mass spectra were obtained
using a Bruker HCT Plus Proteineer LC–MS. Room temper-
ature vis–NIR spectra were recorded on a Shimadzu 3600
UV–vis–NIR spectrophotometer as solutions in appropriate
solvents. Spectra at 77 K were obtained as frozen ethanol
glasses in 5 mm NMR tubes immersed in a liquid N2 Dewar.
Elemental analyses were carried out by Guelph Chemical
Laboratories LTD, Guelph, ON, Canada.
˚
a (A)
17.343(4)
17.136(4)
17.227(4)
90
108.315(5)
90
–16£h£21
–16£k£21
–21£I£20
4860(2)
˚
b (A)
˚
c (A)
a (8)
b (8)
g (8)
Collection ranges
3
˚
V (A )
Dcalcd. (Mg m–3)
m (mm–1)
1.450
0.535
2192
F(000)
q range for data collection (8)
Observed reflections
Independent reflections
Data/restraints/parameters
GOF on F2
Final R1 indices [I>2s(I)]
wR2 indices (all data)
Largest diff. peak and hole (e A )
1.88–26.00
26762
4768 (Rint = 0.0713)
4768/3/335
1.326
0.0670, 0.1791
0.1162, 0.2037
0.816 and –0.524
X-ray crystallography
An orange rod was mounted on a MiTeGen mount, and
placed in the cold stream at 100 K. Data were collected on
a SMART APEX II diffractometer with Mo Ka radiation (l
˚
= 0.71073 A) located at the McMaster Analytical X-ray Dif-
fraction Facility (MAX). Data were collected using omega
and phi scans, integrated, and a numerical face-indexed ab-
sorption correction was applied with a secondary absorption
correction using redundant data (SADABS). The space
group chosen was C2/c, based on systematic absences. Data
were solved using direct methods (SHELXS-97), and refined
using least-squares techniques. All non-hydrogen atoms
were refined anisotropically; hydrogen atoms were located
and then treated as riding on their constituent atoms and up-
dated after each cycle of refinement. The NCS ligand
showed a 73:27 disorder over two positions, which were
symmetry-related to each other due to the twofold axis at
Fe1. The non-bonded pyridine also showed disorder between
N5 and C13/C17, which was similar in magnitude to the
NCS disorder, and due to some intermolecular interactions
between these moieties, we chose to couple the C/N disorder
in the pyridine ring to the NCS disorder.
–3
˚
new tetradentate ligands with structural features that could
enable electropolymerization of the precursor iron(II) coor-
dination complexes. We also wanted the ligand to coordi-
nate the metal ion close to the polymer backbone to
facilitate stronger interactions among the coordinated SCO
unit and the conducting polymer. Inspection of the SCO lit-
erature led us to the bis(2-pyridylmethyl)-diamine type (bis-
picen) reported by Toftlund and co-workers in the 1980s
(Fig. 1).6 These tetradentate ligands feature an ethyl or
propyl spacer between the 2-pyridylmethylamine substitu-
ents, which could easily be replaced with a polymerizable
thiophene heterocycle substituted at the 3,4-ring positions
(bispicth 7). Also, cis-iron(II) complexes containing bispicen
(or other derivatives) and two equivalents of thiocyanate
have been observed to exhibit SCO, including abrupt transi-
tions with small thermal hysteresis cycles.
The possibility of choosing a lower-symmetry space
group to solve the structure was investigated, with the ex-
pectation that the disorder would be resolved; however,
solving the data in Cc (the c-glide is obvious in the recipro-
cal lattice) gave a highly correlated structure, with the same
Herein, we describe the multistep syntheses of three new
bis-(2-pyridylmethyl) ligands containing thienyl substituents
Published by NRC Research Press

956
Can. J. Chem. Vol. 88, 2010
Scheme 1. Preparation of ligands. Reagents and conditions: (a) concentrated H2SO4/HNO3. (b) Phenylboronic acid, 6 mol% Pd(PPh3)4,
K2CO3(aq), dimethoxyethane, 13 h reflux. (c) 2-Thienylboronic acid, 6 mol% Pd(PPh3)4, K2CO3(aq), dimethoxyethane, 8 h reflux. (d) Sn,
HCl/EtOH. (e) KOH(aq), 2-pyridinecarboxaldehyde, NaBH4, MeOH, 1 h reflux.
Scheme 2. Preparation of cis-iron(II) complexes 10 and 12.
Scheme 3. Ligand structural rearrangement in solutions of complex 11.
Fig. 2. ORTEP view of 13. Thermal ellipsoids are drawn at the
50% probability level.
disorder of the NCS ligand. Twin refinement indicated the
presence of the twofold rotation axis in the molecule, and
in the end, it was decided that C2/c was the correct space
group. In the final cycles of refinement, R1 = 6.70%, wR2 =
20.37% (Table 1).
Electrochemical measurements
Cyclic voltammetry (CV) experiments were performed
with a Bioanalytical Systems Inc. Epsilon electrochemical
workstation. Compounds were dissolved in anhydrous sol-
vent (CH3CN) and deaerated by sparging with N2 gas for
20 min. Solution concentrations were approximately
10–3 mol/L in analyte containing 0.1 mol/L supporting elec-
trolyte (Bu4NPF6). A typical three-electrode setup was used
including a platinum working electrode, Ag-wire pseudo-
reference electrode, and a platinum-wire auxiliary electrode.
Ferrocene was used in all cases as an internal standard and
was oxidized at a potential of +0.51 V in our setup; all po-
tentials quoted are versus the ferrocene redox potential. Scan
rates for CV experiments were, generally, 100 mV/s.
Variable-temperature magnetic susceptibility
measurements
Variable-temperature magnetic susceptibility measure-
ments were recorded on a superconducting quantum interfer-
ence device (SQUID) magnetometer (Quantum Design
MPMS) with a 5.5 T magnet (temperature range: 1.8 to
400 K) in an external field of 5000 Oe. Samples were care-
fully weighed into gelatin capsules, with empty gelatin
capsules above and below to eliminate background contribu-
Published by NRC Research Press

Cheng et al.
957
˚
Table 2. Selected bond lengths (A) and
angles (8) for 13.
7.00 mL) was then added dropwise, maintaining a tempera-
ture under 30 8C. Once the addition was complete, the reac-
tion mixture was allowed to react for an additional 3 h and
then poured over *160 g of ice. Upon melting of the ice,
the solid residue was recovered by vacuum filtration and
washed with water to produce a light yellow powder. Re-
crystallization from methanol afforded 7.64 g (52%) of pure
material. Mp 135–136 8C. The FTIR spectrum of 1 is iden-
tical to that previously reported.1013C NMR (75.5 MHz,
CDCl3): d 140.3, 113.4 ppm. MS (EI+): m/z 332 (M+,
100%).
˚
Bond lengths (A)
Fe—N(1)
Fe—N(2)
Fe—N(4)
Fe—N(4’)
2.203(3)
2.293(3)
2.127(5)
1.983(10)
1.502(6)
1.469(6)
1.474(5)
1.642(7)
1.642(8)
1.338(5)
1.404(5)
1.375(5)
1.388(5)
1.163(7)
1.163(8)
C(9)—C(18)
C(10)—C(24)
C(5)—C(6)
C(30)—S(2)
C(30’)—S(2’)
N(2)—C(6)
N(2)—C(8)
N(3)—C(6)
N(3)—C(7)
N(4)—C(30)
N(4’)—C(30’)
3,4-Dinitro-2,5-diphenylthiophene (2)
Phenylboronic acid (0.20 g, 1.66 mmol) was added to a
Schlenk flask and flushed with N2 gas. Then, 1.8 mL of
H2O and 5 mL of 1,2-dimethoxyethane were added to the
reaction flask, which was sparged with N2 for 30 min. After
that, K2CO3 (0.50 g, 3.62 mmol), 1 (0.10 g, 0.60 mmol),
6 mol% Pd(PPh3)4 (0.04 g, 0.04 mmol) were added to the
flask, respectively, and the mixture was refluxed at 65 8C
under nitrogen for 13 h. The reaction mixture was washed
with water and extracted into CH2Cl2. The combined or-
ganic layers were dried over MgSO4 and concentrated to
dryness. The crude product was chromatographed over silica
gel using 1:3 CH2Cl2:hexane as eluent to yield 0.16 g (80%)
of bright yellow crystal. Mp 145–147 8C. FTIR (KBr): 3448
(m, br), 3059 (w), 1963 (w), 1542 (s), 1524 (s), 1448 (m),
1395 (s), 1327 (s), 1261 (m), 1079 (m), 902 (m), 748 (s),
Bond angles (8)
N(1)–Fe–N(2)
N(1)–Fe–N(4)
N(1)–Fe–N(4’)
N(2)–Fe–N(4)
N(2)–Fe–N(4’)
Fe–N(1)–C(5)
Fe–N(2)–C(6)
Fe–N(2)–C(8)
Fe–N(4)–C(30)
Fe–N(4’)–C(30’)
N(4)–C(30)–S(2)
N(4’)–C(30’)–S(2’)
73.46(11)
92.9(4)
87.2(12)
91.0(3)
100.2(10)
118.1(2)
108.7(2)
136.5(3)
166.3(6)
163(3)
1
691 (s) cm–1. H NMR (300 MHz, CDCl3): d 7.54 (m, 10H)
ppm. 13C NMR (75.5 MHz, CDCl3): d 140.8, 136.8, 130.9,
129.3, 129.1, 128.1 ppm. MS (EI+): m/z 326 (M+, 100%).
HR-MS (EI+) calculated for [C16H10N2O4S]+: 326.03613;
found: 326.03557.
177.4(10)
178(2)
tions from the gelatin, which were loaded into plastic straws,
and attached to the sample transport rod. Diamagnetic cor-
rections were made using Pascal’s constants.
3’,4’-Dinitro-2,2’:5’,2@-terthiophene (3)
Thiophene-2-boronic acid (3.29 g, 25.68 mmol) was
added to a Schlenk flask and flushed with N2. Then,
1.8 mL of H2O and 5 mL of 1,2-dimethoxyethane were
added to the reaction flask, which was sparged with N2 for
0.5 h. After that, K2CO3 (0.25 g, 1.81 mmol), 1 (3.1 g,
9.34 mmol), 6 mol% Pd(PPh3)4 (0.65 g, 0.56 mmol) were
added to the flask, respectively, and the mixture was re-
fluxed at 100 8C for 8 h. The reaction mixture was washed
with water and extracted into CH2Cl2. The combined or-
ganic layers were dried over MgSO4 and concentrated to
dryness. The crude product was chromatographed over silica
gel using 1:3 CH2Cl2:hexane as eluent to yield 0.05 g (48%)
Computational details
All density functional theory (DFT) calculations were per-
formed using the Gaussian 03 package using the B3LYP hy-
brid functional and the DZVP basis set for all atoms.7 Tight
SCF convergence criteria were used for all calculations. The
converged wave functions were tested to confirm that they
correspond to the ground-state surface. The evaluation of
atomic charges and spin densities was performed using the
natural population analysis (NPA).8 The analysis of molecu-
lar orbitals in terms of fragment orbital contributions were
carried out using the AOMix program.9 Time-dependent
DFT (TD-DFT) calculations at the B3LYP/DZVP level
were performed to calculate the absorption spectra as previ-
ously described.9b
1
of yellow solid. H NMR (300 MHz, CDCl3): d 7.20 (m,
2H), 7.57 (m, 2H), 7.62 (m, 2H) ppm. 13C NMR
(75.5 MHz, CDCl3): d 133.8, 131.3, 131.2, 128.4, 128.1,
96.13 ppm. MS (FAB+): m/z 338 [M+, 100%]. HR-MS
(EI+) calculated for [C12H6N2O4S3]+: 337.94897; found:
337.94872.
Synthesis
3,4-Diaminothiopheneꢁ2HCl (4)
2,5-Dibromo-3,4-dinitrothiophene (1)
Concentrated HCl (25 mL) was added to a three-neck
round-bottom flask and sparged with N2 for 0.5 h. Then, 1
(1.28 g, 3.80 mmol) was carefully combined with the HCl,
cooled in an ice-water bath. Tin (mossy) metal (3.19 g,
26.89 mmol) was added slowly to maintain a temperature
H2SO4 (18 mol/L, 40 mL) was added to a three-neck
round-bottom flask then purged with N2 for 30 min and
cooled in an ice-water bath. Under N2, 2,5-dibromothio-
phene (10.74 g, 5.00 mL, 44.37 mmol) was added slowly to
maintain a temperature below 20 8C. HNO3 (16 mol/L,
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958
Can. J. Chem. Vol. 88, 2010
Fig. 3. Molecular packing of 13 with a view down the ac diagonal. Intermolecular S2ꢁꢁꢁS2’ and S2’ꢁꢁꢁS2’ contacts are indicated by dashed
lines.
Fig. 4. Left: Visible–NIR spectrum of 10 in ethanol solution at 298 K (grey curve) and as a frozen ethanol glass at 77 K (black curve).
Right: Cyclic voltammogram of a 10–3 mol/L solution of 12 in CH3CN, containing 0.1 mol/L Bu4NPF6.
Fig. 5. Variable-temperature magnetic properties of 10 (&), 12 (~) (left) and 13 (^) (right). External magnetic field of 5000 Oe was
applied in all experiments. Magnetic data was acquired between 2–350 K for 10 and between 5–325 K for 12 and 13. The best fit to a 1-D
Bonner–Fisher chain model is indicated as a solid line.
3,4-Diamino-2,5-diphenylthiopheneꢁ2HCl (5)
between 25–30 8C. After stabilizing at 25 8C, the reaction
continued until all the tin metal was consumed and then
placed in a refrigerator overnight. The solid precipitate was
recovered by vacuum filtration and washed with diethyl
ether and acetonitrile until the wash was colorless to afford
To a mixture of 2 (0.33 g, 1.00 mmol) in absolute ethanol
(30 mL) and concentrated HCl (60 mL) was added tin
(mossy) metal (3.63 g, 30.58 mmol) in small portions. The
resulting mixture was stirred at room temperature for 15 h
in air. A pale yellow solid was obtained, which was col-
lected by vacuum filtration and washed with H2O to afford
0.24 g (71%) of product. Mp 205–207 8C. FTIR (KBr):
1
0.64 g (83%) of white solid. H and 13C NMR spectra and
the FTIR spectrum of 4 are identical to those previously re-
ported.11 MS (EI+): m/z 114 [(M – 2HCl)+, 100%].
Published by NRC Research Press

Cheng et al.
959
Fig. 6. Calculated spin density distribution in 13. NPA-derived atomic spin density is shown for most important contributors. H atoms are
not shown for clarity.
3330 (m, d), 3047 (w), 2920 (w), 2848 (w), 1616 (m), 1595
(m), 1523 (m), 1489 (m), 1429 (s), 1313 (w), 970 (m), 754
MgSO4. The solution was filtered by passing through a Cel-
ite pad. The filtrate was concentrated by rotary evaporation,
and pentane was used to precipitate impurities, which were
removed by gravity filtration. The filtrate was then concen-
trated to dryness and chromatographed over neutral alumina
using 2:1 hexane:EtOAc, followed by 1:2 hexane:EtOAc as
eluent to yield 0.053 g (17%) of a bright yellow viscous oil.
FTIR (KBr): 3447 (s, br), 3105 (m), 2960 (w), 2923 (m),
2851 (w), 1594 (s), 15.7 (m), 1436 (m), 1261 (w), 1097 (w,
1
(s), 703 (m), 621 (w), 573 (w) cm–1. H NMR (300 MHz,
CDCl3): d 3.67 (br, s, 4H), 7.30 (m, 2H), 7.45 (m, 4H), 7.56
(m, 4H) ppm. 13C NMR (75.5 MHz, CDCl3): d 134.4, 133.2,
129.1, 127.6, 126.7, 116.5 ppm. MS (FAB+): m/z 266 [(M –
2HCl)+, 100%]. HR-MS (EI+) calculated for [C16H14N2S]+:
266.08777; found: 266.08880.
1
br), 801 (w), 757 (m) cm–1. H NMR (300 MHz, CDCl3): d
3’,4’-Diamino-2,2’:5’,2@-terthiopheneꢁ2HCl (6)
To a mixture of 3 (0.27 g, 0.80 mmol) in absolute ethanol
(30 mL) and concentrated HCl (60 mL) was added tin
(mossy) metal (2.84 g, 23.96 mmol) in small portions. The
resulting mixture was stirred at room temperature for 15 h
in air. A deep yellow solid was collected by vacuum filtra-
tion and washed with H2O to afford 0.28 g (88%) of the
product. Melting point and FTIR spectrum (KBr) are identi-
cal to those previously report.11 1H NMR (300 MHz,
CDCl3): d 3.76 (br, s, 4H), 7.11 (m, 4H), 7.30 (m, 2H)
ppm. 13C NMR (150.9 MHz, CDCl3): d 135.9, 133.6, 127.8,
124.0, 123.9, 110.13 ppm. MS (FAB+): m/z 278 [(M –
2HCl)+, 100%].
4.45 (m, 6H), 6.01 (s, 2H), 7.21 (dd, 2H, J = 5.2, 1.8 Hz),
7.39 (d, 2H, J = 7.8 Hz), 7.67 (td, 2H, J = 7.5, 1.8 Hz),
8.60 (d, 2H, J = 4.5 Hz) ppm. 13C NMR (75.5 MHz,
CDCl3): d 158.5, 149.3, 139.5, 136.7, 122.2, 121.8, 97.6,
51.4 ppm. MS (FAB+): m/z 297 [(M+, 100%)], 93 [(M –
C10H10N3S)+, 62%]. HR-MS (EI+) calculated for
[C16H16N4S]+: 296.10951; found: 296.10957.
N,N’-Bis(2-pyridylmethyl)-3,4-diamino-2,5-
diphenylthiophene (8)
Compound 5 (0.12 g, 0.35 mmol) was added to an oven-
dried three-neck round-bottom flask fitted with a reflux con-
denser. The apparatus was then flushed with N2. Then, KOH
(0.04 g, 0.69 mmol) was dissolved in methanol (20 mL) in a
round-bottom flask, the solution was sparged with N2 for
0.5 h, and transferred by syringe into the three-neck round-
bottom flask. The mixture was stirred for 0.5 h followed by
the addition of 2-pyridinecarboxaldehyde (0.22 g, 0.20 mL,
2.07 mmol). The reaction mixture was refluxed for 1 h
while protected from light by covering with foil. The reac-
tion was cooled to RT followed by adding NaBH4 (0.26 g,
6.90 mmol), and the mixture was again refluxed at 75 8C
for 1 h. The reaction mixture was diluted into Na2CO3 solu-
tion (pH 9.5) and extracted into diethyl ether. The organic
extracts were combined and dried over MgSO4. Concentra-
tion by rotary evaporation and chromatography over neutral
alumina using CH2Cl2 as eluent provided 0.12 g of a vibrant
N,N’-Bis(2-pyridylmethyl)-3,4-diaminothiophene (7)
Compound 4 (0.20 g, 1.07 mmol) was added to an oven-
dried three-neck round-bottom flask fitted with a reflux con-
denser. The apparatus was then flushed with N2 for 5 min.
NaOH (0.086 g, 2.15 mmol) was dissolved in methanol
(20 mL), the solution was sparged with N2 for 0.5 h and
transferred by syringe into the three-neck flask. The mixture
was stirred for 0.5 h, and then 2-pyridinecarboxaldehyde
(0.24 g, 0.20 mL, 2.25 mmol) was added. The reaction mix-
ture was refluxed for 1.5 h and protected from light by cov-
ering with foil. NaBH4 (0.21 g, 5.67 mmol) was added in
small portions followed by a reflux for 14 h under N2. The
solvent was removed by rotary evaporation. The residue was
extracted into CH2Cl2, washed with water and dried over
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960
Can. J. Chem. Vol. 88, 2010
orange oil, which is very unstable. 1H NMR (300 MHz,
CDCl3): d 4.35 (br s, 6H), 7.59–7.13 (m, 16H), 8.49 (dd,
2H) ppm. HR-MS (FAB+) calculated for [C28H25N4S]+:
449.17999; found: 449.17897.
solid. UV–vis (MeOH): lmax = 650 nm. FTIR (KBr): 3424
(m, br), 3194 (w, br), 2921 (m), 2851 (w), 2071 (s), 1602
(w), 1508 (w), 1261 (w), 1102 (m), 1018 (m), 802 (m), 757
(m), 698 (m) cm–1. MS (FAB+): m/z 620 [(M+, 3%)], 562
[(M – SCN)+, 18%], 503 [(M – 2SCN)+, 17%]. Anal. calcd.
for C30H24N6S3Feꢁ2C2H6O (found %): C 59.21 (59.21), H
5.54 (4.97), N 11.21 (11.00)
N,N’-Bis(2-pyridylmethyl)-3’,4’-diamino-2,2’:5’,2@-
terthiophene (9)
Compound 6 (0.10 g, 0.28 mmol) was added to an oven-
dried three-neck round-bottom flask fitted with a reflux con-
denser. The apparatus was then flushed with N2 for 5 min.
Then, KOH (0.032 g, 0.57 mmol) was dissolved in methanol
(20 mL) and the solution was sparged with N2 for 0.5 h. The
solution was transferred by syringe into the three-neck
round-bottom flask. The mixture was stirred for 0.5 h fol-
lowed by the addition of 2-pyridinecarboxaldehyde (0.18 g,
0.16 mL, 1.71 mmol). The reaction mixture was refluxed for
1 h while protected from light by covering with foil. The re-
action was cooled to RT, and NaBH4 (0.22 g, 5.70 mmol)
was carefuly added and the mixture was again refluxed for
1 h under N2. The reaction mixture was diluted into
NaHCO3 solution (pH 9.5) and extracted into diethyl ether.
The organic extracts were combined and dried over MgSO4.
Concentration by rotary evaporation provided 0.10 g (76%)
of a brown-yellow oil, which required no further purifica-
tion. FTIR (KBr): 3428 (s), 2922 (s), 2853 (m), 1593 (m),
1466 (m, br), 1431 (m), 1402 (m), 1219 (w, br), 1148 (m),
[FeII(9)(NCS)2] (12)
Compound 9 (0.12 g, 0.25 mmol) was added to a Schlenk
flask and flushed with N2. Then, methanol (8 mL) was
added to the flask, which was sparged with N2 for 0.5 h.
Fe(BF4)2ꢁ6H2O (0.085 g, 0.25 mmol) was added to the solu-
tion, and the mixture was stirred for 0.5 h under N2. KSCN
(0.10 g, 1.00 mmol) was dissolved in 10 mL of H2O, and
this solution was added to the reaction. A dark green precip-
itate was observed, which was isolated by vacuum filtration
to afford 0.09 g (57%) of dark green solid. UV–vis (MeOH):
lmax (3) = 656 nm (400 mol/L–1cm–1). FTIR (KBr): 3425
(m, br), 3222 (w), 3107 (w), 3075 (w), 2921 (w), 2851 (w),
2081 (s), 2063 (s), 1602 (m), 1572 (w), 1484 (w), 1440 (w),
1411 (m), 1232 (w), 1101 (w), 899 (w), 760 (w), 697 (m)
cm–1. MS (FAB+): m/z 632 [(M+, 20%)], 574 [(M – SCN)+,
95%].Anal calcd. for C26H20N6S5Fe (found %): C 49.38
(49.30), H 3.19 (2.96), N 13.30 (12.86).
1
1045 (m, br), 691 (s) cm–1. H NMR (300 MHz, CDCl3): d
C58H40N10S4Fe (13)
Complex 11 (20 mg) was recrystallized in 1:1
MeOH:CH2Cl2 (4 mL) to produce orange rod-shaped crys-
tals over a period of two weeks. FTIR (KBr): 2961 (m),
2922 (m), 2855 (m), 2060 (s), 1628 (m), 1597 (w), 1470
(w), 1437 (w), 1261 (m), 1096 (s), 1024 (s), 802 (s), 754
(m), 694 (m) cm–1. MS (FAB+): m/z 1002 [(M – NCS)+,
8%)], 445 [(C28H21N4S)+, 100%)], 353 [(C22H15N3S)+,
52%)]. Anal. calcd. for C58H40N10S4Feꢁ0.5CH2Cl2 (found
%): C 63.86 (63.90), H 3.73 (2.84), N 12.63 (12.13).
4.38 (s, 4H), 4.92 (br, 2H), 7.04 (m, 2H), 7.16 (m, 4H), 7.25
(m, 4H), 7.61 (m, 2H), 8.50 (d, 2H, J = 4.8 Hz) ppm. 13C
NMR (150.9 MHz, CDCl3): d 159.0, 149.1, 138.7, 136.5,
135.7, 127.2, 125.2, 124.9, 122.1, 116.1, 52.6 ppm. MS
(FAB+): m/z 461 [(M+, 52%)], 369 [(M – C6H6N)+, 100%].
HR-MS (FAB+) calculated for [C24H21N4S3]+: 461.09284;
found: 461.09655.
[FeII(7)(NCS)2] (10)
A 25 mL Schlenk flask was charged with 7 (0.025 g,
0.08 mmol) and flushed with N2. Methanol (8 mL) was
then added to the flask, which was sparged with N2 for
0.5 h. Fe(BF4)2ꢁ6H2O (0.028 g, 0.08 mmol) was added to
the solution and the mixture was stirred for 0.5 h under N2.
KSCN (0.033 g, 0.338 mmol) was dissolved in 10 mL of
H2O and was added to the mixture noted above. Gradually,
a green precipitate was observed, which was isolated by
vacuum filtration to afford 0.032 g (80%) of green powder.
UV–vis (MeOH): lmax (3) = 640 nm (400 mol/L–1 cm–1).
FTIR (KBr): 3448 (s, br), 3159 (m, br), 2920 (m), 2060 (s,
br), 1603 (m), 1425 (m), 787 (m), 762 (m) cm–1. MS
(FAB+): m/z 468 [(M+, 17%)], 410 [(M – SCN)+, 100%],
351 [(M – 2SCN)+, 99%]. HR-MS (FAB+) calculated for
[C18H16N6S3Fe]+: 467.98843; found: 467.99476.
Results and discussion
Ligand synthesis and coordination chemistry
Ligand preparation is outlined in Scheme 1. 2,5-Dibromo-
3,4-dinitrothiophene (1) was prepared by nitration of com-
mecrially available 2,5-dibromothiophene in a modified lit-
erature procedure.10 We chose to use non-fuming
concentrated acids and generated 1 in reasonably good yield.
To generate the 2,5-disubstituted ligands (where the sub-
stituents are phenyl or 2-thienyl), we performed Suzuki–
Miyuara cross-coupling reactions between 1 and phenylbor-
onic acid or 2-thienylboronic acid. 3,4-Dinitro-substituted
thiophenes (1–3) were reduced with a 30-fold excess of
mossy tin and the 3,4-diamino-substituted thiophenes, as di-
hydrochloride salts, precipitated out in each case.11 Tetra-
dentate ligands were prepared in one pot by neutralization
of the dihydrochloride salts with KOH, followed by conden-
sation reactions in methanol with 2-pyridinecarboxaldehyde
to generate the imine intermediates and subsequent imine re-
duction with NaBH4. Ligands 7–9 are very unstable oils
(particularly ligand 8, which is so unstable we could not
generate a quality 13C NMR spectrum prior to decomposi-
tion) and had to be purified quickly and used directly in co-
ordination reactions. To our knowledge, these instability
[FeII(8)(NCS)2] (11)
A solution of 8 (0.10 g, 0.22 mmol) in 10 mL of MeOH
was degassed by sparging with N2 gas for 0.5 h.
Fe(BF4)2ꢁ6H2O (0.08 g, 0.22 mmol) was added to generate
a yellow-green solution. A solution of KSCN (0.09 g,
0.89 mmol) in 10 mL of water was added under N2, fol-
lowed by the addition of 10 mL of diethyl ether to instantly
produce a green precipitate. The solid residue was isolated
by vacuum filtration to afford 0.069 g (50%) of bright green
Published by NRC Research Press

Cheng et al.
961
issues are not observed with similar reported tetradentate li-
gands that do not feature thiophene substitution, which could
in all cases be purified by short-path vacuum distillation.6
Our attempts to purify 7–9 in this manner in all cases re-
sulted in decomposition. We speculate that the observed in-
stability likely arises from oligomerization or polymerization
of the thiophene heterocycle, and (or) from nucleophilic re-
action of the amino N atoms at the 3,4 thiophene ring posi-
tions. Unfortunately, our spectral data does not allow us to
unambiguously identify the products of the decomposition.
We have some mass spectral evidence that would seem to
suggest that ligands 8–9, in solution and over time, undergo
a similar rearrangement that is observed in complexes 11
and 12 (vida infra).
X-ray quality crystals and, to our surprise, a structurally re-
arranged material was indicated. Complex 13 contains two
coordinated molecules of rearranged 8. For clarity, the li-
gand rearrangement is indicated in Scheme 3.
An Oakridge thermal ellipsoid plot (ORTEP) of the mo-
lecular structure of 13 is shown in Figure 2. The metal cen-
ter falls on a twofold axis, rendering each coordinated ligand
equivalent. Coordinate bond lengths (Table 2) are consistent
with an oxidation state assignment of +2 for the metal ion.
Each molecule of rearranged 8 is coordinated through N1
and N2 of a bidentate pyridine–imidazole-type fragment of
the molecule. The other imidazole-type ring atom N3 fea-
tures a covalently bound and uncoordinated 2-pyridylmethy-
lene substituent. Crystallographically equivalent thiocyanate
ligands are coordinated cis to the iron center, and are disor-
dered over two positions.
Iron(II) complexes were prepared by coordination of 7–9
with iron(II) tetrafluoroborate in deaerated methanol solu-
tion, followed by addition of excess aqueous KSCN to gen-
erate green precipitates of complexes 10 or 12 (Scheme 2).
Complexes 10 and 12 are analytically pure powders, stable
to air in the solid state, but very unstable in solution, includ-
ing deaerated solutions. It is likely that oligomerization or
polymerization of the thienyl substituents is at least one de-
stabilizing factor in these materials, but we are not exactly
sure what the pathway(s) for decomposition in these com-
plexes is (are). Mass spectral data suggests that decomposi-
tion leads to a complex mixture of products. Although X-ray
diffraction data could not be obtained from samples of 10 or
12 because of solution instability issues, we could use FTIR
spectroscopy (nC:N) to identify the stereochemistry about
the metal centre. Complexes 10 or 12 feature a strong
‘‘doublet’’ between 2060–2080 cm–1, indicating a cis stereo-
chemistry, which has been observed in other similar reported
complexes. In fact, the energy of this band has been previ-
ously correlated to the electronic ground state of the com-
plex and the energies observed for complexes 10 or 12
Along the ac diagonal, molecules of 13 pack in a one-di-
mensional chain structure, with intermolecular SꢁꢁꢁS contacts
between coordinated and disordered thiocyanate ligands
among adjacent molecules along the chain (Fig. 3). As noted
in the Experimental section, the NCS ligand exhibits a 73:27
disorder over two positions. The SꢁꢁꢁS contacts vary from
˚
shorter [3.54(1) A] than the sum of the van der Waals radii
for the interaction between S2’–S2’ to longer (3.85 and 4.40
˚
A) than the sum of the van der Waals radii for the interac-
tion between S2–S2’ and S2–S2, respectively. This packing
structure is possibly implicated in the unusual variable-tem-
perature magnetic properties of 13 because no other close
intermolecular contacts were found in the X-ray structure
between sites carrying a significant amount of spin density
(vida infra).
Red solutions of 12 deposit a dark red powder from which
FTIR and mass spectrometric data indicate a similar struc-
tural rearrangement occurs. Unforunately, we could not ob-
tain X-ray quality crystals from solutions containing 12.
Solutions of 10 decompose to a mixture of products over
time. To our knowledge, similar structural rearrangements
have not been reported for other iron(II) bis(thiocyanate)
complexes containing tetradentate bis(2-pyridylmethyl)-type
ligands, suggesting that the thienyl substituent or the elec-
tronic effect of this substituent is possibly involved in the
mechanism for this rearrangement.
5
suggest a significant population of the high spin (HS) T2
state at room temperature.
Electrochemistry
The electrochemical properties of complexes 10 and 12
were investigated with cyclic voltammetry, and as an exam-
ple, the CV of 12 is presented in Fig. 4 (right). Each voltam-
mogram features a number of broad and irreversible
processes versus ferrocene. Cathodic scans are unremarkable
and each complex exhibits irreversible waves at potentials
greater than –1.5 V, which are attributed to pyridine ring re-
ductions. Over anodic potentials, an irreversible oxidation
process centered at +0.4 V is observed in the voltammo-
grams of 10 and 12. A similar wave at approximately +0.4
V is observed in the voltammogram of uncoordinated 7 and
is likely a ligand-centered oxidation (dehydrogenation of
NH bond). Beyond +0.4 V, 10 exhibits a very broad irrever-
sible oxidation at +1.3 V and 12 features irreversible proc-
esses at +0.8, +1.0, and +1.3 V, which we attribute to a
combination of terthienyl and iron(II) oxidation processes.
We attempted to electropolymerize complex 12 by repeated
Complex 11, which was prepared under identical condi-
tions to 10 or 12, is unstable in the solid state and solution,
making characterization difficult; however, based on our
spectroscopic data, we are confident in the purported struc-
ture. As opposed to 10 or 12, complex 11 features a single
sharp strong band at 2070 cm–1, which strongly suggests an
unusual trans disposition of the thiocyanate ligands (and a
HS ground state).
Attempts to recrystallize deaerated solutions of 10–12
consistently resulted in solution color changes, from green
to red over a period of days, which we anticipated was the
result of iron(II) oxidation. Red solutions of 11 deposited
Published by NRC Research Press

962
Can. J. Chem. Vol. 88, 2010
scans over the terthienyl oxidation potential; however, we
observed no indication for any successful electropolymeriza-
tion reactions.
points toward an incomplete spin-crossover in these materi-
als.6
The variable-temperature magnetic properties of 13 are
completely different. At room temperature, the magnetic
moment is significantly higher than anticipated for a mag-
netically isolated mononuclear iron(II) complex (Fig. 5,
right). With decreasing temperature, we see a gradual in-
crease in magnetic moment, indicating intermolecular ferro-
magnetic interactions are operative. The magnetic moment
reaches a plateau of 8.1 BM at approximately 8 K, and then
slightly decreases. Low-temperature (5 K) magnetization
versus field experiments provided no indication of ferromag-
netic ordering, with a saturation magnetization of 4.6 NB at
3.5 T, which is a typical value for HS iron(II) (see Supple-
mentary data). In the molecular packing of 13, close inter-
molecular SꢁꢁꢁS contacts were observed between coordinated
thiocyanate ligands from adjacent molecules, suggesting a
possible pathway for magnetic exchange coupling. We used
DFT at the B3LYP/DZVP level to calculate the structure
and spin density of complex 13, and found significant spin
delocalization onto the coordinated thiocyanate ligands
(0.16 on each NCS, Fig. 6), suggesting that the magnetic ex-
change pathway in 13 is possibly that of a 1-D S = 2 ferro-
magnetic chain. The fit to a Bonner–Fisher 1-D chain
model12 (eq. [1], g = 2.13 and J/k = +7.5 K) containing a
correction for interchain magnetic interactions (eq. [2] zJ’/k
= –2.2 K) is good between 320 and 30 K, and then deviates
at low temperature.
Visible–NIR spectroscopy
Ethanol solutions of complexes 11 and 12 exhibit absorp-
tion maxima at 650 nm (3 = 400 mol/L–1 cm–1), with absorp-
tion intensities that are not temperature-dependent. To help
provide the assignments of the absorption bands in 11 and
12, the absorption spectrum of 13 was calculated using TD-
DFT at the B3LYP/DZVP level. The calculated spectrum of
13 in the visible region contains a main band at 638 nm
with the oscillator strength (f) of 0.0040 and a weaker band
at 629 nm (f = 0.0011). The 638 nm band originates from
the b-spin HOMO ? LUMO excitation with the b-spin
HOMO being the Fe dp – NCS p orbital (35% Fe and 65%
NCS contributions) and the b-spin LUMO being the Fe ds –
pyridine–imidazole p* orbital (12% Fe and 85% L). As a
result, this absorption band has a mixed metal-to-ligand and
ligand-to-ligand charge transfer (MLCT/LLCT) character.
The 629 nm band originates from a mixture of several elec-
tron excitations (the two principal components are the a-spin
HOMO-4 ? LUMO and b-spin HOMO-4 ? LUMO excita-
tions). The room-temperature spectrum of 10 in ethanol also
features a weak absorption at 640 nm; however, upon cool-
ing to 77 K, a large increase in absorption intensity is ob-
served concomitant with the growth of a shoulder on the
low-energy side of the 640 nm absorption. The color of the
solution becomes intensely dark green from the pale green/
yellow color observed at room temperature (Fig. 4, left).
These observations suggest an electronic change, likely
spin-crossover is occurring in solutions containing 10. The
new absorption feature centered at 700 nm is likely the
band of the low-spin state of 10.
ꢀ
ꢁ
Ng2b2eSðS þ 1Þ 1 þ u
½1ꢂ
cchainT ¼
3kT
1 ꢃ u
ꢀ
ꢁ
1
zJ0
kT
½2ꢂ
cmT ¼
ꢃ
cchain
T
where
Variable-temperature magnetic properties
2JSðS þ 1Þ
kT
The variable-temperature magnetic properties of 10, 12,
and 13 were probed via SQUID magnetometry and the data
are presented as plots of the effective magnetic moment
(meff) versus temperature (Fig. 5). The magnetic properties
of 12 feature very little temperature dependence and the ob-
served magnetic moment values suggest a high-spin ground
state for this complex. The magnetic moment of 12 de-
creases rapidly below 40 K, which likely results from a
combination of zero-field splitting (ZFS) and intermolecular
antiferromagnetic interactions.
Data from variable-temperature visible–NIR spectroscopy
indicated spin-crossover was operative in solutions of 10. In
the solid state, variable-temperature magnetic susceptibility
data indicates a gradual and incomplete spin-crossover with-
out thermal hysteresis in powder samples of 10. The mag-
netic moment of 10 at the highest measured temperature
(350 K) is 4.95 mB and a gradual decrease is observed with
decreasing temperature to 2 K, where the observed magnetic
moment is 2.4 mB, which is higher than anticipated for a
complete crossover to the low-spin (LS) state (theoretical
value is 0 BM). However, the profile of the data, including
the higher than expected moments at very low temperature,
is very similar to that observed by Toftlund for structurally
similar iron(II) complexes (without thienyl substituents) and
u ¼ coth
ꢃ
kT
2JSðS þ 1Þ
Since there are no other significant intermolecular mag-
netic exchange pathways between atoms/fragments with
high spin density in the molecular packing of 13, which
would suggest that another competing antiferromagnetic in-
terchain coupling is dominant at lower temperatures, it is
also reasonable to assume that the deviation from 1-D chain
behaviour at lower temperatures is a result of ZFS of the
iron(II) S = 2 ground state. The relatively large value of J/k
is suprising, especially considering that this exchange path-
way is intermolecular. Typically, weak magnetic interac-
tions are mediated in an intramolecular fashion through
ambidentate N and S coordinated and bridging thiocyanate
between metal ions within bimetallic complexes or coordi-
nation polymers.13 We could find no other reported exam-
ples of intermolecular ferromagnetic coupling mediated by
coordinated thiocyanate ligands.
Conclusions
We have described the synthesis of three new thiophene-
containing ‘‘Toftlund-like’’ bispicen ligands (7–9). In appa-
rent contrast to the bispicen family of ligands, 7–9 are unsta-
Published by NRC Research Press

Cheng et al.
963
A.; Britten, J. F.; Lemaire, M. T. Inorg. Chem. 2009, 48 (2),
699. doi:10.1021/ic801233x. PMID:19053331.; (b) Djukic,
B.; Lemaire, M. T. Inorg. Chem. 2009, 48 (22), 10489.
doi:10.1021/ic9015542. PMID:19831361.
ble oils, which we could isolate and characterize, but must
be coordinated quickly, or else suffer decomposition. Coor-
dination of 7 and 9 with iron(II) bis(thiocyanate) resulted in
the anticipated cis-pseudo-octahedral complexes (10 and
12), which were characterized and featured incomplete spin-
crossover (10), or HS iron(II) (12). Coordination of 8 with
iron(II) bis(thiocyanate) produced a very unstable complex
(11), which in solution undergoes a ligand-centered struc-
tural rearrangment to produce a stable complex 13, featuring
intriguing variable-temperature magnetic properties. Our
analysis indicates that the temperature-dependent magnetic
behaviour of 13 is best-described as a one-dimensional fer-
romagnetic chain with interchain antiferromagnetic interac-
tions and (or) ZFS at low temperatures, which reduces the
magnetization. We are currently exploring the synthesis of
derivatives of 13, including other iron(II) complexes with
different ligand structures as precursors to novel iron(II)
containing spin-crossover conductors.
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Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.;
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Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J.
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Supplementary data
Supplementary data for this article are available on the
journal Web site (canjchem.nrc.ca). CCDC 758104 contains
the X-ray data in CIF format for this manuscript. These data
can be obtained, free of charge, via www.ccdc.cam.ac.uk/
conts/retrieving.html (or from the Cambridge Crystallo-
graphic Data Centre, 12 Union Road, Cambridge CB2 1EZ,
UK; fax +44 1223 336033; or deposit@ccdc.cam.ac.uk).
Acknowledgement
MTL acknowledges the Natural Sciences and Engineering
Research Council of Canada (NSERC) and the Canadian
Foundation for Innovation (CFI) for support of this research.
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