Synthesis and characterization of chromium-isothiocyanate-4-methylpyridine complexes

Article abstract of DOI:10.1016/j.ica.2011.07.023

The synthesis and single crystal X-ray structure of trans-[HNC ₆H₇][Cr(NCS)₄(NC₆H₇) ₂] (1) and mer-[Cr(NCS)₃(NC₆H₇) ₃] (2) are reported. Compound 1 was synthesized by refluxing chromium powder and thiourea in 4-methylpyridine. The isothiocyanate ligand is believed to be generated from the isomerization of thiourea to ammonium thiocyanate during synthesis. Compound 2 was prepared from CrCl₃ and KSCN in 4-methylpyridine. The reaction conditions for both compounds yielded a mixture of [Cr(NCS)_x(NC₆H₇)_{6- x}]^3-x isomers (x = 0-6), which were analyzed by HPLC. The thermal properties of trans-[HNC₆H₇][Cr(NCS) ₄(NC₆H₇)₂] were analyzed by thermogravimetric analysis/differential scanning calorimetry-mass spectroscopy (TGA/DSC-MS), and the compound was found to oxidize to CrO₂ in three major weight loss steps when heated to 1000 °C in dry air.

Full text of DOI:10.1016/j.ica.2011.07.023

Inorganica Chimica Acta 377 (2011) 14–19
Contents lists available at SciVerse ScienceDirect
Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Synthesis and characterization of
chromium-isothiocyanate-4-methylpyridine complexes
a
a
a
Jennifer L. Young a, Jerry D. Harris a, , Joshua A. Benjamin , Jannah E. Fitch , Daniel F. Nogales ,
⇑
Joanna R. Walker a, Brian J. Frost b, Aaron Thurber c, Alex Punnoose c
a Department of Chemistry, Northwest Nazarene University, Nampa, ID 83686, United States
b Department of Chemistry, University of Nevada, Reno, Reno, NV 89557, United States
c Department of Physics, Boise State University, Boise, ID 83725, United States
a r t i c l e i n f o
a b s t r a c t
Article history:
The synthesis and single crystal X-ray structure of trans-[HNC6H7][Cr(NCS)4(NC6H7)2] (1) and mer-
[Cr(NCS)3(NC6H7)3] (2) are reported. Compound 1 was synthesized by refluxing chromium powder and
thiourea in 4-methylpyridine. The isothiocyanate ligand is believed to be generated from the isomeriza-
tion of thiourea to ammonium thiocyanate during synthesis. Compound 2 was prepared from CrCl3 and
Received 29 January 2011
Received in revised form 14 July 2011
Accepted 15 July 2011
Available online 28 July 2011
KSCN in 4-methylpyridine. The reaction conditions for both compounds yielded
a mixture of
3ꢀx
[Cr(NCS)x(NC6H7)6ꢀx
]
isomers (x = 0–6), which were analyzed by HPLC. The thermal properties of
Keywords:
Chromium
Isothiocyanate
Methylpyridine
Crystal structure
Thermal analysis
trans-[HNC6H7][Cr(NCS)4(NC6H7)2] were analyzed by thermogravimetric analysis/differential scanning
calorimetry–mass spectroscopy (TGA/DSC–MS), and the compound was found to oxidize to CrO2 in three
major weight loss steps when heated to 1000 °C in dry air.
Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction
compounds, (where M = Mn, Fe, Co, Ni, and Cu and NC6H7 = 4-
methylpyridine), from metal powders and thiourea in refluxing 4-
The synthesis of chromium-isothiocyanate-amine compounds
dates back more than one hundred years with the report of
Reinecke’s salt, NH4[Cr(NCS)4(NH3)2] [1]. More recently, the syn-
thesis and structural characterization of several chromium-isothi-
ocyanate-N-heterocyclic containing compounds have been
reported. The majority of the interest in these compounds has been
in their use as anions in charge transfer salts with tetrathiafulva-
lene and its derivatives [2–12]. Chromium-isothiocyanate-N-het-
erocyclic containing compounds have also been prepared to
study oxo-bridged chromium(III) dimmers [13], and extended me-
tal atom chains of chromium [14–17]. Others have prepared these
compounds to investigate the nitrido-chemistry of chromium (i.e.,
[Cr(N)(NCS)(phen)]ꢀ) [18], to use them as building blocks in poly-
metallic coordination polymers [19], to explored the coordination
chemistry of polypyridyl ligands toward chromium(III) [20], and
to use as a redox stable dye substitute for studying electron trans-
port in dye-sensitized solar cells [21].
methylpyridine [22–24]. This synthetic route provides an alterna-
tive to the traditional method of preparing these compounds by
reacting metal salts with thiocyanate salts and donor ligands or
from M(SCN)n and donor ligands. However, in the case of chro-
mium, many of the reactions yielded a mixture of products. Here
we report the synthesis and structural characterization of two chro-
mium-isothiocyanate-4-methylpyridine
compounds,
trans-
[HNC6H7][Cr(SCN)4(NC6H7)2] (1) and mer-[Cr(SCN)3(NC6H7)3] (2)
and the thermal characterization of trans-[HNC6H7][Cr(SCN)4
(NC6H7)2].
2. Experimental
2.1. Techniques and materials
Reactions with metal powders and thiourea were preformed
both under nitrogen and open to the air. Reactions with metal salts
were performed on the bench, open to air. Reaction solvents were
freshly distilled under nitrogen from the appropriate drying agent
[25]. Wash solvents, chromium powder, 99% (Strem Chemicals),
CrCl3ꢁ6H2O (J.T. Baker), thiourea (Fisher Scientific), and potassium
thiocyanate (Mallinckrodt) were used as received. The chromium
powder was stored in a nitrogen filled glovebox.
Our interest in this chemistry was to build upon our initial work,
where we reported the synthesis of several [M(NCS)x(NC6 H7)y]
⇑
Corresponding author. Address: Department of Chemistry, Northwest Nazarene
University, 623 S. University Blvd., Nampa, ID 83686, United States. Tel.: +1 208 467
8883; fax: +1 208 467 8676.
E-mail address: jdharris@nnu.edu (J.D. Harris).
0020-1693/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2011.07.023

J.L. Young et al. / Inorganica Chimica Acta 377 (2011) 14–19
15
2.2. Synthesis of [HNC6H7][Cr(NCS)4(NC6H7)2]ꢁNC6H7 (1)
2.6. X-ray crystallography
Chromium powder (0.27 g, 5 mmol), thiourea (1.52 g,
20 mmol), and 4-methylpyridine (20 mL) were added to a Schlenk
flask. The solution was refluxed under nitrogen overnight, and the
wine-red solution was cooled in an ice bath to induce precipitation.
The solution was filtered, and the deep red precipitate washed
with hexanes. The product was dissolved in acetone, filtered to re-
move unreacted metal (0.063 g), and evacuated to dryness. Yield:
1.573 g (53%). X-ray quality crystals were grown by cooling a sat-
urated solution of the product dissolved in 4-methylpyridine from
140 °C to room temperature over 100 h using an Omega Engineer-
ing CN3251 furnace controller and power supply to ramp the tem-
perature of an oil bath. The product is soluble in acetonitrile,
acetone, dichloromethane, and sparingly soluble in methanol.
Crystals of 1 and 2 suitable for X-ray diffraction were obtained
as described above. Data for both 1 and 2 were collected at
100( 2) K on a Bruker APEX CCD diffractometer with Mo K
ation (k = 0.71073 Å) and detector-to-crystal distance of
4.94 cm. A full sphere of data was collected utilizing four sets of
frames, 600 frames per set, with 0.5° rotation about between
a radi-
a
x
frames, and an exposure time of 10 s per frame. Data integration,
correction for Lorentz and polarization effects, and final cell refine-
ment were performed using SAINTPLUS and corrected for absorption
using SADABS. Both structures were solved using direct methods fol-
lowed by successive least-squares refinement on F2 using the SHEL-
XTL 5.12 software package [26]. All non-hydrogen atoms were
refined anisotropically. Hydrogens of the protonated solvents were
located in the density maps and the remaining hydrogen atoms
were placed on the appropriate carbon atoms using the standard
riding model. Relevant crystallographic data and data collection
parameters are summarized in Table 1.
IR/cmꢀ1 /Mꢀ1 cmꢀ1): 239
: v(CN): 2063 (s). UV–Vis: kmax/nm (e
(2.29 ꢂ 104), 283 (9.76 ꢂ 103), 325 (1.58 ꢂ 104), 402 (119), 539
(121).
2.7. Thermal analysis
2.3. Synthesis of 3[Cr(NCS)3(NC6H7)3]ꢁ4(NC6H7) (2)
The thermal degradation of compound 1 was characterized
using a Mettler Toledo TGA/DSC1 thermal analyzer, connected to
a Pfieffer Vacuum ThermoStar mass spectrometer. Thermal charac-
terization was preformed in both air and nitrogen environments,
with ramp rates of 10–50 °C/min from 50 to 1000 °C.
CrCl3ꢁ6H2O (1.0660 g, 4 mmol), KSCN (1.1666 g, 12 mmol), and
4-methylpyridine (20 mL) were placed in a round-bottom flask
and refluxed for a week. The crude product was precipitated with
hexanes (100 ml) and filtered. The solid was dissolved in a minimal
amount of dichloromethane, filtered and then precipitated with
the addition of hexanes (100 ml) and cooling in an ice bath. The
product was filtered, washed with pentane and dried under vac-
uum. Yield: 1.4395 g (57%). X-ray quality purple crystals were
grown by slow evaporation of a solution of the product dissolved
in dichloromethane. The product is soluble in acetonitrile, acetone,
2.8. X-ray photoelectron spectroscopy
X-ray photoelectron spectra were recorded using a Physical
Electronics Versaprobe equipped with a 100 W monochromated
dichloromethane, and sparingly soluble in methanol. IR/cmꢀ1
:
v
(CN): 2056 (s). UV–Vis: kmax/nm (
e
/Mꢀ1 cmꢀ1): 284 (2.92 ꢂ 104),
Table 1
Crystal data and structure refinement for 1 and 2.
332 (5.11 ꢂ 104), 537 (475).
[HNC6H7][Cr(SCN)4
(NC6H7)2]ꢁNC6H7
3[Cr(NCS)3(NC6H7)3]ꢁ
4(NC6H7)
Empirical formula
Formula weight
T (K)
C
28H29CrN8S4
C87H91Cr3N22S9
1889.36
100(2)
2.4. Spectroscopy
657.83
100(2)
Infrared spectra of the compounds were measured on a Thermo
Fisher Scientific Nicolet IR200 using a Performer single-reflection
ATR cell with a diamond crystal. Ultraviolet-visible spectra were
measured for the compounds dissolved in acetonitrile using a Var-
ian Cary 100 Bio spectrophotometer.
k (Å)
0.71073
monoclinic
C2/c
30.8545(17)
12.0236(6)
19.4041(13)
90
118.787(2)
90
6308.9(6)
8
0.71073
Crystal system
Space group
a (Å)
b (Å)
c (Å)
triclinic
ꢀ
P1
15.837(8)
16.630(9)
19.174(10)
91.634(11)
106.623(10)
94.323(11)
4818(4)
2
a
(°)
b (°)
c
(°)
V (Å3)
2.5. Chromatography
Z
Dcalc (Mg/m3)
1.385
1.302
All synthetic products were analyzed by thin layer chromatog-
raphy (TLC) and high-performance liquid chromatography (HPLC).
TLC analysis was done by spotting acetone solutions of each com-
pound on both silica and alumina plates. The compounds were
eluted with dichloromethane, ethanol and 50/50, 70/30 and 90/
10 (dichloromethane/ethanol) mixtures. For HPLC characteriza-
tion, freshly prepared acetonitrile solutions of the compounds
were injected into a Hewelett Packard Series 1050 HPLC with a
diode array detector. The samples were isocratic eluted with an
acetonitrile/water (80%/20%) mixed solvent through a reverse-
phase, C-18 column. The water contained 0.1% trifluoroacetic acid.
The UV-Vis detectors on the HPLC monitored wavelengths 254 and
328 nm. Products from several chromium/thiourea reactions were
also separated by flash column chromatography using silica gel.
Compound 1 was eluted with dichloromethane, and the remaining
material was eluted with a 50/50 mixture of dichloromethane/
ethanol.
Absorption coefficient
0.660
0.582
(mmꢀ1
)
Crystal size (mm3)
h range for data collection
(°)
1.14 ꢂ 0.34 ꢂ 0.24
0.33 ꢂ 0.09 ꢂ 0.03
1.85–32.44
1.61–25.00
Index ranges
ꢀ46 6 h 6 46
ꢀ18 6 k 6 18
ꢀ29 6 l 6 29
54 999
11 280
[Rint = 0.0295]
SADABS
ꢀ18 6 h 6 18
ꢀ19 6 k 6 19
ꢀ22 6 l 6 22
52 005
16 978
[Rint = 0.0792]
SADABS
Reflections collected
Independent reflections
Absorption correction
Data/restraints/parameters
11280/0/486
16978/0/1090
0.965
Goodness-of-fit (GOF) on F2 1.054
Final R indices
[I > 2 (I)]
R1 = 0.0316,
R1 = 0.0527
Rw = 0.1137
R1 = 0.0975,
Rw = 0.1261
793880
r
Rw = 0.0855
R1 = 0.0366
Rw = 0.0902
793879
R indices (all data)
CCDC No.

16
J.L. Young et al. / Inorganica Chimica Acta 377 (2011) 14–19
Table 2
Al Ka X-ray source. Samples were pressed into indium foil and se-
Selected bond lengths (Å) and bond angles (°) for 1.
cured under a molybdenum mask. Sample charging was minimized
using low energy electrons and Ar ions, and spectra were shifted
and referenced to the C 1s peak at 284.8 eV. Samples were sputter
Bond
Bond length (Å)
Bond
Bond angle (°)
Cr(1)–N(1)
Cr(1)–N(2)
Cr(1)–N(3)
Cr(1)–N(4)
Cr(1)–N(5)
Cr(1)–N(6)
1.9885(8)
1.9961(8)
1.9963(8)
1.9889(8)
2.0741(8)
2.0730(8)
Cr(1)–N(1)–C(1)
Cr(1)–N(2)–C(2)
Cr(1)–N(3)–C(3)
Cr(1)–N(4)–C(4)
N(1)–Cr(1)–N(3)
N(2)–Cr(1)–N(4)
N(5)–Cr(1)–N(6)
N(1)–Cr(1)–N(2)
N(1)–Cr(1)–N(6)
N(2)–Cr(1)–N(6)
166.00(8)
166.66(8)
170.64(8)
169.05(8)
179.74(3)
179.37(3)
179.61(3)
88.42(3)
cleaned using a 4 kV, 5
l
A Ar ion beam rastered over a 2 ꢂ 2 mm
area for 1 min at an Ar pressure of ꢃ5 ꢂ 10ꢀ6 Pa.
3. Results and discussion
91.24(3)
The unit cell of compound
1 consist of eight trans-
89.32(3)
[Cr(SCN)4(NC6H7)2]ꢀ anions and 16 solvent molecules, eight of
which are protonated and serve as counter ions. Fig. 1 shows the
product. In the solid, bound and unbound solvent molecule inter-
leave in columns that run normal to the (1 0 0) plane. All of the sol-
vent molecules are paired in the structure, with the nitrogen atoms
pointing toward each other. A hydrogen atom was located between
the two nitrogen atoms. The distance between N7–N8 is 2.686 Å,
and is consistent with what was found for the hydrogen-bonded
solvents in compounds [HNC6H7]2[Mn(SCN)4(NC6H7)2]ꢁ2HNC6H7,
[HNC6H7]2[Fe(SCN)4(NC6H7)2]ꢁ2HNC6H7, and [HNC6H7][Cu(SCN)3
(NC6H7)2]ꢁHNC6H7, where the N–N solvent separation ranges from
2.629 to 2.687 Å [23]. The nitrogen bound isothiocyanate ligand is
consistent with hard-soft acid-base chemistry. The hard Cr3+ Lewis
acid bonds to the harder Lewis base end of the ambidentate thio-
cyanate ligand. The trans-[Cr(SCN)4(NC6H7)2]ꢀ anion has a near-
octahedral geometry, with longer bonds to the coordinated solvent
than to the isothiocyanate ligands. The Cr–NCS bond lengths range
from 1.9885(8) to 1.9963(8) Å and the Cr–NC6H7 bonds lengths are
2.0730(8) and 2.0741(8) Å (Table 2). The isothiocyanate ligands are
nearly linear, with N@C@S bond angles ranging from 178.40(9)° to
179.33(9)°. Chromium–N@C bond angles range from 166.00(8)° to
170.64(8)°. All bond lengths and angles are consistent with what
have been observed in other trans-[Cr(NCS)4L2]ꢀ molecules
[4,5,7,9,11].
2062 cmꢀ1. This very intense band is attributed to the antisymmet-
ric (CN) of the isothiocyanate ligand.
The unit cell of compound contains six mer-[Cr(NCS)3
v
2
(NC6H7)3] molecules (three which are crystallographically unique)
and eight solvent molecules (four which are crystallographically
unique). The mer-[Cr(NCS)3(NC6H7)3] molecules stack in columns
normal to the (1 1 0) plane, and the ligands are bound to the
chromium in the meridional coordination geometry. One of the
mer-[Cr(NCS)3(NC6H7)3] molecules is shown in Fig. 2. The mer-
[Cr(NCS)3(NC6H7)3] molecules have near octahedral geometry,
where again the Cr–NCS bonds are slightly shorter than the
Cr–NC6H7 bonds. The Cr–NCS bond distances average 1.997 Å and
The infrared spectrum of [HNC6H7][Cr(SCN)4(NC6H7)2]ꢁNC6H7
was also measured.
A prominent band was observed at
Fig. 2. Thermal ellipsoid diagram of mer-[Cr(SCN)3(NC6H7)3] (2). Atoms are
represented by ellipsoids at 50% probability. All hydrogen atoms have been omitted
for clarity.
Table 3
Selected average bond lengths (Å) and average bond angles (°) for 2.
Bond
Average
Range
Cr–NCS
Cr–NC6H7
1.997
2.100
1.982(3)–2.007(3)
2.082(3)–2.113(3)
Cr–N@C
N@C@S
SCN–Cr–NCS
SCN–Cr–NC6H7
H7C6N–Cr–NC6H7
167.9
179.3
177.90
178.86
177.84
153.1(3)–175.4(3)
178.8(3)–179.8(4)
176.79(12)–178.61(12)
178.35(12)–179.44(12)
176.67(12)–179.61(11)
Fig. 1. Thermal ellipsoid diagram of [HNC6H7][Cr(SCN)4(NC6H7)2]ꢁNC6H7 (1). Atoms
are represented by ellipsoids at 50% probability. With the exception of the
protonated solvent, all hydrogen atoms have been omitted for clarity.

J.L. Young et al. / Inorganica Chimica Acta 377 (2011) 14–19
17
range from 1.982(3) to 2.007(3) Å, where as the Cr–NC6H7 bond
distances average 2.100 Å and range from 2.082(3) to 2.113(3) Å
(Table 3). The isothiocyanate ligands are nearly linear, with N@C@S
bond angles ranging from 178.8(3)° to 179.8(4)°. Chromium–N@C
bond angles range from 153.1(3)° to 175.4(3)°. Chromium–
nitrogen bond lengths for the neutral mer-[Cr(NCS)3(NC6H7)3] are
consistent with those reported above for trans-[Cr(SCN)4
(NC6H7)2]ꢀ, and other six coordinate [Cr(NCS)x(L)y] anions and
cations and the only other mer-[Cr(NCS)3(L)y] compound reported
[3–7,9,11,20,21].
Compound 1 was prepared from a reaction of chromium pow-
der and thiourea in refluxing 4-methylpyridine, but the mecha-
nism for the reaction is not fully understood. It is well
documented that solutions of NH4SCN can be used to prepare thio-
urea in good yield [27,28], and at temperatures near the boiling
point of 4-methylpyridine (145 °C) the equilibrium concentration
ratio of NH4SCN to thiourea (NH4SCN:S@C(NH2)2) is approximately
0.75:0.25 in propanol and butanol [29]. The most probable path-
way for the reaction is that in refluxing 4-methylpyridine some
of the thiourea is converted to NH4SCN, and the metal reacts with
the thiocyanate ions in solution and pulls the isomerization reac-
tion (Eq. (1)) to the right.
thiourea
ratios
of
1:2
yielded
neutral
compounds
[M(NCS)2(NC6H7)4] (M = Mn, Fe, Co, Ni, and Cu), whereas ratios of
1:5 yielded [M(NCS)x (NC6H7)6ꢀx
(2ꢀx)
]
anions, with x = 4 for Mn
and Fe and x = 3 for Cu. To explore the effect of reaction stoichiom-
etry with chromium, products were prepared and characterized
from reactions with chromium to thiourea ratios systematically
varied from 1:1 to 1:6. Crystals of 1 were crystallographically char-
acterized from reactions with Cr:thiourea ratios of 1:1, 1:3, 1:4,
and 1:6. Crystals obtained from reactions with Cr:thiourea ratios
of 1:2 and 1:5 yielded the cationic species [Cr(NCS)2(NC6H7)4]+.
However, due to excessive anion disorder a publishable structure
could never be obtained, despite numerous attempts. These at-
tempts either yielded poor quality crystals containing
[Cr(NCS)2(NC6H7)4]+ or crystals of 1. Attempts were also made to
prepare [Cr(NCS)2(NC6H7)4]+ from Cr2(SO4)3 and KSCN and from
CrCl3 and KSCN in refluxing 4-methylpyridine, but these attempts
also yielded either crystals of 1 or poor quality crystals. However,
the neutral compound, [Cr(NCS)3(NC6H7)3] (2), was prepared from
stoichiometric amounts of CrCl3 and KSCN in refluxing 4-methyl-
pyridine, and was obtained in modest yield (57%).
To better understand the influence of the Cr:thiourea ratio on
the synthesis and to better understand the crystallographic results,
the product from each reaction was analyzed by chromatography.
High-performance liquid chromatography revealed that all of the
reactions produced multiple products (Table 4), or mixtures of
S@CðNH2Þ ꢀ SCNꢀ þ NH4
ð1Þ
þ
2
This equilibrium provides an anhydrous thiocyanate source and
was previously utilized to prepare both neutral compounds with
the 10 possible structural and geometric isomers of
3ꢀx
[Cr(NCS)x(NC6H7)6ꢀx
]
(x = 0–6). An example of a chromatogram
the composition [M(NCS)2(NC6H7)4] (M = Mn, Fe, Co, Ni, and Cu)
obtained for Cr:thiourea reaction ratio of 1:5 is provided in
Fig. 3. In the figure, the presence of at least four species is clearly
observed. In all of the reactions with chromium powder and
(2ꢀx)
and [M(NCS)x(NC6H7)6ꢀx
]
anions, with x = 4 for Mn and Fe
and x = 3 for Cu [22–24]. While the above equilibrium accounts
for the presence of the isothiocyanate ion, it fails to account for
the observed oxidation in the metal powders. An obvious oxidant
is oxygen from the air, but reactions with chromium were run un-
der nitrogen as well as open to the air, and all reactions yielded the
same oxidation. When products prepared anaerobically and aero-
bically were analyzed by HPLC (see below), reactions with the
same stoichiometry had the same product composition. Addition-
ally, all of the above mentioned Mn-, Fe-, Co-, Ni- and Cu-contain-
ing compounds were prepared from metal powders under strictly
anaerobic conditions. Another potential oxidation source is the sol-
vent, but it was dried and distilled prior to use. The chromium
powder may have contained an oxide impurity, and the product
was simply the result of oxidized metal reacting, leaving behind
the unoxidized metal. These reactions were not quantitative and
chromium powder always remained at the end of a reaction. How-
ever, the amount of unreacted metal powder changed based on the
stoichiometry of the reaction, with 77.5% of the powder remaining
when the Cr:thiourea ratio was 1:1 and only 21.2% of the metal
remaining for reactions with Cr:thiourea ratios of 1:6. One last con-
Table 4
3ꢀx
Percent composition of each [Cr(NCS)x(NC6H7)6ꢀx
material, as determined by HPLC for each reaction stoichiometry.
]
isomer (x = 0–6) in the isolated
Reaction
stiochoimetry
[Cr(NCS)4
[Cr(NCS)3
(NC6H7)3]
All other isomers
(NC6H7)2]ꢀ
of [Cr(NCS)x
3ꢀx
(NC6H7)6ꢀx
]
Cr + 1 SC(NH2)2
Cr + 2 SC(NH2)2
Cr + 3 SC(NH2)2
Cr + 4 SC(NH2)2
Cr + 5 SC(NH2)2
Cr + 6 SC(NH2)2
CrCl3 + 3 KSCN
93
83
96
96
94
98
34
1
14
2
1
2
6
3
2
3
4
1
6
1
60
þ
sideration is the reduction of NH4 to NH3 and H2 and/or the
reduction of protonated solvent to 4-methylpyridine and H2. This
reduction would require two electrons per H2 formed. In refluxing
4-methylpyridine, any NH3 or H2 generated would be out-gassed.
However no attempts were made to isolate or characterize any
gasses that may have been produced during the reactions. Addi-
tionally, with the exception of copper, all of the metals that have
yielded crystalline products using this synthetic protocol have
standard reduction potentials that are negative compared to the
reduction potential of hydrogen (0.0 V) [30]. In short, the mecha-
nism for the reaction is not fully understood. It is still under inves-
tigation and results will be reported elsewhere when available.
In the initial reports of the synthesis of the [M(NCS)x
(NC6H7)6ꢀx]
(2ꢀx) (x = 2–4 and M = Mn, Fe, Co, Ni, and Cu) complexes
prepared by this route [22–24], it was believed that the synthesis
was quantitative, with the metal to thiourea ratio dictating which
thermodynamically stable product would be formed. Metal to
Fig. 3. HPLC chromatogram of the product from the reaction of chromium powder
and thiourea at a 1:5 stoichiometric ratio. The peaks A and B correspond to
[Cr(NCS)3(NC6H7)3] and [Cr(NCS)4(NC6H7)2]ꢀ, respectively.

18
J.L. Young et al. / Inorganica Chimica Acta 377 (2011) 14–19
Fig. 4. Thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), and mass spectrometry (MS) curves for the heating of [HNC6H7][Cr(SCN)4(NC6H7)2]ꢁNC6H7
to 1000 °C in dry air at a rate of 20 °C/min. ⁄The mass spectrograms for m/z = 76 and m/z = 44 are shown at 50% and 5%, respectively, to keep all mass signals on the same scale.
thiourea, [Cr(NCS)4(NC6H7)2]ꢀ anion is the predominate species
present and appears to be the thermodynamically most stable spe-
cies for the reaction conditions, provided adequate thiourea is
present. Crystals characterized containing [Cr(NCS)2(NC6H7)4]+ cat-
ions were likely minor components in the reactions, but may have
preferentially crystallized and/or been randomly selected for X-ray
characterization.
Attempts were made to separate individual isomers by column
chromatography, and it was possible to isolate pure quantities of
[Cr(NCS)4(NC6H7)2]ꢀ, which aided the assignment of bands in the
chromatogram. However, a solvent system was never found for
successfully purifying [Cr(NCS)3(NC6H7)3] and [Cr(NCS)2
(NC6H7)4]+.
approximately 135 °C, which is close to the boiling point of 4-
methylpyridine (145 °C), and has a maximum weight loss at
190 °C. The principle species observed by the mass spectrometer
during this step are water and 4-methylpyridine. The observed
weight loss is consistent with the loss of a non-coordinated 4-
methyopyridine molecule in the structure. The endotherm in the
DSC curve at 245 °C corresponds to the melting of the material,
as little mass is lost at this temperature. The second major weight
loss starts at about 225 °C, has a maximum weight loss at 340 °C
and corresponds to the loss of both 4-methylpyridine and thiocya-
nate. Taken together, the first two steps constitute a 56% weight
loss, consistent with the loss of all of the 4-methylpyridine. How-
ever, a prominent species observed during the second weight loss
þ
To better understand the synthetic process, the reaction with a
1:6 stoichiometry was also studied during reflux. Periodically
throughout the reaction, beginning at 30 min into reflux, the reac-
tion was stopped and the composition was analyzed by HPLC. At
each interval, at least five different isomers were observed, includ-
ing those in Fig. 3, but in differing quantities. It is well documented
step is CS2 (m/z = 76), indicating that the thiocyanate ligand also
begins to decompose during this step. During the second decompo-
sition step N2S+ (m/z = 60) is also observed in the mass spectrom-
eter, along with species with mass to charge ratios of 52, 26 and
38, consistent with the presence of CN2 (m/z = 52) and C2N+ (m/
þ
z = 38). However m/z = 52, 26 and 38 species are also produced as
fragments during ionization of 4-methylpyridine, and are always
present whenever 4-methylpyridine is detected in the mass spec-
trometer, making it difficult to determine if CN2 or C2N are pro-
duced during thermal decomposition. This was confirmed by
recording the mass spectrogram when 4-methylpyridine was
evaporated separately in the TGA.
0
that chromium(III) with an octahedral ligand field has a t2g3eg
electronic configuration and is considered substitutionally inert,
whereas octahedral Cr2+ has an electronic configuration of t2g3eg
1
and is substitutionally labile [20,31]. The authors hypothesize that
even in refluxing 4-methylpyrdine (145 °C), ligand exchange is
slow and the observed mixture is the result of slow ligand ex-
change of Cr3+. However, given enough time (18–24 h), the reac-
tions eventually proceeds to a thermodynamic equilibrium, with
[Cr(NCS)4(NC6H7)2]ꢀ being the most stable product. To minimize
ligand exchange during HPLC characterization, solutions were
freshly prepared prior to injection. When HPLC solutions were ana-
lyzed 18–24 h after preparation, they contained many more com-
pounds (peaks).
The authors have an interest in metal oxides [32–37], so the
thermal properties of compound 1 were probed, since it could be
obtained in high purity. The thermal properties of 1 were studied
by thermogravimetric analysis/differential scanning calorimetry–
mass spectroscopy (TGA/DSC–MS). It has been previously shown
for several metal thiocyanate compounds (Bi(SCN)3, Cd(SCN)2,
Pb(SCN)2, and Cu(SCN)2) that the thiocyanate ligand decomposes
upon heating in air to yield CO2, SO2 and N2 [38]. Compound 1
was found to thermally decompose in air with three major weight
loss steps (Fig. 4). The first step has an onset temperature of
Several groups have reported thermal decomposition data for
an extensive list of [Cr(NCS)6]3ꢀ and [Cr(NCS)4(base)2]ꢀ salts,
including [HNC6H7][Cr(NCS)4(NC6H7)2]ꢁNC6H7 [39–42]. At 350 °C
under high vacuum, Cherkasova and Semyannikov observed the
+
presence of CN+, S+, C2N+, CS+, ðCNÞ2þ, N2S , S2þ, CS2þ, SðCNÞ2
þ
þ
and S6 during the thermolysis of K3[Cr(NCS)6] [43]. Under a flow
of dry air and at ambient pressure, many of the masses for these
species were also observed during our experiments. However,
CS+ and S2 have the same masses as CO2 and SO2þ, respectively,
making it impossible to distinguish which chemical species were
released. Calu et al. reported that [HNC6H7][Cr(NCS)4
(NC6H7)2]ꢁNC6H7 decomposed in an ambient atmosphere by loss
of (CN)2 from 335 to 430 °C and by loss of S from 430 to 515 °C
to yield a final residue of CrS [41]. The results reported here and
below do not support Calu’s findings.
þ
þ
When measured under a nitrogen atmosphere, only the first
two weight loss steps were observed, and above 400 °C the weigh

J.L. Young et al. / Inorganica Chimica Acta 377 (2011) 14–19
19
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slowly decreases to a total weight loss of 76%. The onset tempera-
ture for the third weight loss step in air starts at 430 °C and has a
maximum weight loss at 480 °C. The onset temperature also corre-
sponds closely to the initiation of oxidation of the sample, as
apparent from the exotherm in the DSC curve. This weight loss cor-
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CO2, SO2 and NO2 (m/z = 44, 64 and 46, respectively) and a small
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The synthesis of [M(NCS)x(NC6H7)y] compounds from metal
powders and thiourea in refluxing 4-methylpyridine for many of
the first row transition metals (M = Cr, Mn, Fe, Co, Ni, and Cu) pro-
vides an interesting and alternative to the traditional routes of
their synthesis. In the case of chromium, because of the slow ligand
exchange kinetics of Cr3+, reactions yielded multiple products of
3ꢀx
composition [Cr(NCS)x(NC6H7)6ꢀx
]
(x = 1–6). The compound
[HNC6H7][Cr(NCS)4(NC6H7)2]ꢁNC6H7 was found to thermally
decompose to yield CrO2 when heated to 1000 °C in dry air.
Acknowledgments
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We gratefully acknowledge the financial support from the Na-
tional Science Foundation through Grants DMR-0840265, MRI-
0722699 and CHE-0226402, the U.S. Department of Education
through Grant FIPSE-P116Z090244, NASA Idaho Space Grant Con-
sortium, the M.J. Murdock Charitable Trust, and Northwest Naza-
rene University’s Science, Math Associates.
Appendix A. Supplementary material
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A complete listing of anisotropic displacement parameters,
interatomic distances, angles, hydrogen coordinates and displace-
ment parameters and structure factor amplitudes for the reported
compounds are available upon request from the authors. CCDC
793879 and 793880 contain the supplementary crystallographic
data for compounds 1 and 2. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif. Supplementary data associ-
ated with this article can be found, in the online version, at
doi:10.1016/j.ica.2011.07.023.
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