Synthesis, morphology, structure, and magnetic characterization of layered cobalt hydroxyisocyanates

Article abstract of DOI:10.1021/ic702260s

New layered pink cobalt hydroxyisocyanates have been prepared by controlled hydrolysis of aqueous solutions of CoCl₂·6H₂O. Co(OH)_1.4(NCO)_0.6·0.6H₂O (1) is formed when urea is used as the hydrolysis agent and mannitol as the stabilizer, while Co(OH)_1.25(NCO)_0.75·0.2H₂O (2) is formed when the hydrolysis agent is changed to hexamethylenetetramine and NaOCN is added to the solution. IR spectroscopy of 1 and 2 indicates that the OCN ^- is N-bonded to Co²⁺. The X-ray powder data for 1 could be indexed using an intergrowth model consisting of both rhombohedral (R3m (166); a = 3.2031(1) A, c = 23.6876(11) A, hydrotalcite-like, 3R₁) and hexagonal (P 63/mmc (194); a = 3.2005(2) A, c = 15.8303(5) A, Manasseite-like, 2H₁) polytypes. Rietveld refinement of the X-ray powder data was performed using a two polytype model and 30% random substitution of the OH^- ions by OCN^-. Although it is less crystalline, the XRD data for 2 can be indexed using a rhombohedral-symmetry cell with unit cell parameters, a = 3.158 A and c = 21.57 A. Both 1 and 2 exhibit magnetically ordered ground states with a saturation magnetization of ca. 2.0 μ_B. The magnetization data is consistent with 3D ferromagnetic ordering of edge-share octahedral-Co ²⁺ layers with effective spin S′ = 1/2.

Full text of DOI:10.1021/ic702260s

Inorg. Chem. 2008, 47, 3234-3242
Synthesis, Morphology, Structure, and Magnetic Characterization of
Layered Cobalt Hydroxyisocyanates
Yi Du and Dermot O’Hare*
Chemistry Research Laboratory, Department of Chemistry, UniVersity of Oxford, Mansfield Road,
Oxford, U.K. OX1 3TA
Received November 16, 2007
New layered pink cobalt hydroxyisocyanates have been prepared by controlled hydrolysis of aqueous solutions of
CoCl2 ·6H2O. Co(OH)1.4(NCO)0.6 · 0.6H2O (1) is formed when urea is used as the hydrolysis agent and mannitol as
the stabilizer, while Co(OH)1.25(NCO)0.75 · 0.2H2O (2) is formed when the hydrolysis agent is changed to
hexamethylenetetramine and NaOCN is added to the solution. IR spectroscopy of 1 and 2 indicates that the OCN-
is N-bonded to Co2+. The X-ray powder data for 1 could be indexed using an intergrowth model consisting of both
¯
rhombohedral (R3m (166); a ) 3.2031(1) Å, c ) 23.6876(11) Å, hydrotalcite-like, 3R1) and hexagonal (P 63/mmc
(194); a ) 3.2005(2) Å, c ) 15.8303(5) Å, Manasseite-like, 2H1) polytypes. Rietveld refinement of the X-ray
powder data was performed using a two polytype model and 30% random substitution of the OH- ions by OCN-.
Although it is less crystalline, the XRD data for 2 can be indexed using a rhombohedral-symmetry cell with unit cell
parameters, a ) 3.158 Å and c ) 21.57 Å. Both 1 and 2 exhibit magnetically ordered ground states with a
saturation magnetization of ca. 2.0 µB. The magnetization data is consistent with 3D ferromagnetic ordering of
1
edge-share octahedral-Co2+ layers with effective spin S′ ) /2.
Introduction
tions of these materials as heterogeneous catalysts;4,5 how-
ever, significant interest from materials chemists and physi-
cists has been prompted by the observation of long-range
interlayer magnetic interactions in a range of organically
pillared cobalt hydroxides and hydroxysulfates.2,6–12
To date, the synthesis of crystalline single-phase samples
of R-cobalt hydroxides has proven to be difficult because
the R-phase is metastable and typically transforms rapidly
to the ꢀ-phase during synthesis or upon storage in strong
alkaline media.13–15 Many attempts have been made to
prepare the R-cobalt hydroxide by electrochemical and
Cobalt (II) hydroxides are well-known to crystallize in two
polymorphic forms, R and ꢀ. The ꢀ-form has a brucite-like
structure consisting of a closed-packed array in which the
divalent metal ion is located in an octahedral site generated
by six hydroxyl oxygen atoms, while the R-cobalt hydroxides
are reported to be isostructural with hydrotalcite-like com-
pounds consisting of positively charged Co(OH)2-x layers
and charge balancing anions (e.g., NO3 , CO32-, Cl-, etc.)
-
in the interlayer gallery.1 It is of major interest to both
chemists and physicists due primarily to the rigidity of the
metal–oxygen network brought about by the µ3-OH and the
edge sharing octahedral.2 The chemical interest mainly stems
from the intercalation chemistry3,4 and the potential applica-
(5) Didier, T.; Bernard, C. CATTECH 2003, 7, 206.
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Plouet, M.; Mhiri, T.; Kurmoo, M. Chem. Mater. 2005, 17, 2612.
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2004, 65, 11.
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1993, 32, 2463.
* E-mail: dermot.ohare@chem.ox.ac.uk. Fax: +44 1865 272690. Tel:
+44 1865 285130.
(9) Demessence, A.; Rogez, G.; Rabu, P. Chem. Mater. 2006, 18, 3005.
(10) Takada, T.; Bando, Y.; Kiyama, M.; Miyamoto, H.; Sato, T. J. Phys.
Soc. Jpn. 1966, 21, 2726.
(1) Kamath, P. V.; Annal Therese, G. H.; Gopalakrishnan, J. J. Solid State
Chem. 1997, 128, 38.
(11) Drillon, M.; Panissod, P.; Rabu, P.; Souletie, J.; Ksenofontov, V.;
Gulich, P. Phys. ReV. B 2002, 65, 104404.
(2) Mohsen, B. S.; Vilminot, S.; Richard-Plouet, M.; Andre, G.; Mhiri,
T.; Kurmoo, M. Chem. Comm.(Cambridge, United Kingdom) 2004,
2548.
(12) Rujiwatra, A.; Kepert, C. J.; Claridge, J. B.; Rosseinsky, M. J.;
Kumagai, H.; Kurmoo, M. J. Am. Chem. Soc. 2001, 123, 10584.
(13) Liu, Z.; Ma, R.; Osada, M.; Takada, K.; Sasaki, T. J. Am. Chem. Soc.
2005, 127, 13869.
(3) Yamanaka, S.; Kawaji, H.; Hotehama, K.; Ohashi, M. AdV. Mater.
1996, 8, 771.
(4) Cavani, F.; Trifiro, F.; Vaccari, A. Catal. Today 1991, 11, 173.
(14) Gaunand, A.; Lim, W. L. Powder Technol. 2002, 128, 332.
3234 Inorganic Chemistry, Vol. 47, No. 8, 2008
10.1021/ic702260s CCC: $40.75  2008 American Chemical Society
Published on Web 03/13/2008

Characterization of Layered Co Hydroxyisocyanates
water and ethanol to give the final concentrations of 10, 60 and 60
mM, respectively. The reaction solution was then heated to 90 °C
in the absence of magnetic stirring under a N2 atmosphere. After
2 h at 90 °C, a suspension containing bright pink crystals formed.
The precipitate was filtered and washed thoroughly with deionized
water, anhydrous ethanol, and acetone several times and, finally,
air-dried at room temperature to give an isolated yield of 70%.
The resulting crystals were pale pink in color with a uniform flower-
like crystal morphology.
Figure 1. Reaction scheme for the hydrolysis of urea and the production
of HNCO.
Material Characterization. IR spectra of the samples as KBr
discs were recorded over the range of 600-4000 cm-1 on a Mattson
Galaxy Series 6020 FT-IR spectrometer. Sixteen scans were
recorded with a scan resolution of 8 cm-1. Elemental analysis (EA)
was performed by the analytical services department of the
Inorganic Chemistry Laboratory, Oxford University. C, H, and N
contents were calculated by quantitatively digesting the sample
through oxidative combustion. Other elements were analyzed using
inductively coupled plasma (ICP) atomic emission spectroscopy.
Thermogravimetric analysis (TGA) were carried out on a Rheo-
metric STA-1500H machine. The sample (ca. 10 mg) was mounted
in a corundum crucible and heated at a rate of 1 °C min-1 between
30 and 700 °C under a flow of argon. Powder X-ray diffraction
(XRD) patterns were recorded on a PANalytical X’Pert Pro
instrument in reflection mode with Cu KR radiation (λ ) 1.54178
Å). The accelerating voltage was set at 40 kV with 40 mA current.
The diffractometer was equipped with X’Celerator detector. Scan
rates were typically 0.002° s-1 (continuous scan, step size 0.008°,
time per step 500s). Scanning electron microscopy was performed
on a JSM 840F at 5 kV, samples coated with Pt before analysis.
High-resolution transmission electron microscopy (HRTEM) and
selected area electron diffraction (SAED) were performed on a
JEOL 4000EX at 400 kV. A JEOL 2000FX instrument operating
at 200 kV was used for energry dispersive X-ray (EDX) analysis.
Samples were dispersed in ethanol and loaded onto copper grids
supporting Formvar film. Magnetic measurements were made using
a Quantum Design MPMS-5 SQUID magnetometer. The suscep-
tibility was determined in an applied field of 1 kG/5G after cooling
of the sample in both zero applied field (ZFC) and the measuring
field (FC). The saturated moment and hysteresis loop was measured
at 2 K for ZFC in fields up to 50 kG.
chemical synthesis including urea and ammonia precipitation
as well as sonication-assisted methods.2,15–18 Unfortunately,
most of the R-cobalt hydroxides synthesized are either poorly
crystalline and/or adopt a turbostratically disordered struc-
ture,15–18 in which the layers are randomly oriented about
the c-axis. In addition, the crystallites rarely adopt a uniform
or well defined morphology but typically occur as ag-
gregates.13 There is one structural report of green-colored
single-crystal R-cobalt hydroxides which was prepared using
hexamethylenetetramine (HMT) as the hydrolysis agent.13
Rietveld refinement of the powder XRD suggests a structural
model involving tetrahedral cobalt coordination.19 Although
a pink-colored form of an R-cobalt hydroxide phase has been
reported17 no structural data on this material has been
published. Therefore, it is of interest to search for novel
routes for the preparation of crystalline materials with well-
defined, uniform particle size and morphology. In this paper,
we have studied the use of both urea/mannitol and hexam-
ethylenetetramine/NaOCN mixtures to prepare layered cobalt
hydroxyisocyanates. In these reactions, the slow hydrolysis
of urea or hexamethylenetetramine controls the pH, while
mannitol acts as a stabilizer to prevent transformation to a
ꢀ-type cobalt hydroxide.
Experimental Section
Synthesis of Co(OH)1.4(NCO)0.6 ·0.6H2O (1). Pink color Co-
(OH)1.4(NCO)0.6 ·0.6H2O (1) was prepared using a homogeneous
precipitation method. CoCl2 ·6H2O (0.47 g), mannitol (0.03 g), and
urea (6 g) were dissolved in 200 cm3 of a 9:1 mixture of deionized
water and ethanol to give the final concentrations of 10, 1, and
500 mM, respectively. The deionized water was purged with N2
overnight and the addition was carried out with stirring under an
ambient pressure N2 atmosphere. The reaction solution was then
heated to 90 °C in the absence of magnetic stirring. After 4 h at 90
°C, a suspension containing bright pink microcrystals formed. The
precipitate was filtered and washed thoroughly with deionized water,
anhydrous ethanol, and acetone several times and, finally, air-dried
at room temperature to give an isolated yield of 60%. The material
has uniform hexagonal platelet-like crystal morphology.
Results
Synthesis. The homogeneous precipitation method has
been used previously to prepare crystalline layered cobalt
hydroxides.13 In this paper, we have exploited the slow
hydrolysis reaction of urea in basic solution (Figure 1) as a
method to enable us to control the nucleation and growth of
highly ordered, phase-pure cobalt hydroxyisocyanates. Ad-
dition of a mixture of CoCl2 · 6H2O, mannitol, and urea to a
nonstirred, deionized water/ethanol mixture at 90 °C results
in the phase pure synthesis of pink, highly crystalline,
hexagonal platelets of cobalt hydroxyisocyanate in 60% yield.
Our detailed characterization including elemental analysis
(Table 1), IR, thermogravimetric analysis (TGA), EDX
(Supporting Information Figure S1), X-ray diffraction, and
TEM are all consistent with a formulation of Co(OH)1.4-
(NCO)0.6 · 0.6H2O (1). We postulate that isocyanic acid
(OCNH) formed as a result of the hydrolysis of urea (Figure
1) reacts to displace some of the OH- ions from the
coordination sphere of the Co2+ ions forming cobalt isocy-
anate (Co-NCO) bonds. At present it is unknown whether
Synthesis of Co(OH)1.25(NCO)0.75 · 0.2H2O (2). Co(OH)1.25
-
(NCO)0.75 ·0.2H2O (2) was prepared using a similar homogeneous
precipitation method as outlined for (1). CoCl2 ·6H2O (0.47 g),
NaOCN (0.78 g) and hexamethylenetetramine, HMT (2 g) were
dissolved in 200 cm3 of a 9:1 mixture of N2 saturated deionized
(15) Xu, Z.; Zeng, H. Chem. Mater. 1999, 11, 67.
(16) Xu, R.; Zeng, H. Chem. Mater. 2003, 15, 2040.
(17) Rajamathi, M.; Kamath, P. V. Int. J. Inorg. Mater. 2001, 3, 901.
(18) Rajamathi, M.; Kamath, P. V.; Seshadri, R. Mater. Res. Bull. 2000,
35, 271.
(19) Ma, R.; Liu, Z.; Takada, K.; Fukuda, K.; Ebina, Y.; Bando, Y.; Sasaki,
T. Inorg. Chem. 2006, 45, 3964.
Inorganic Chemistry, Vol. 47, No. 8, 2008 3235

Du and O’Hare
Table 1. Summary of Elemental Microanalysis Data
elemental composition obs (calc) %
compound
morphology
N
C
H
Co
formula
1
2
hexagonal platelets
flower
6.56(7.06)
8.78(9.10)
5.75(6.06)
7.99(7.80)
2.22(2.21)
1.55(1.45)
49.9(49.6)
50.2(51.0)
Co(OH)1.4(NCO)0.6 ·0.6H2O
Co(OH)1.25(NCO)0.75 ·0.2H2O
NCO for OH substitution happens in solution before
condensation of the Co(OH)2 layers or as a result of NCO-
intercalation in solid Co(OH)2. Nevertheless, the color of
the material and the XPS data (Supporting Information Figure
S2) strongly suggests that the sample contains exclusively
octrahedrally coordinated Co2+ ions.
Surprisingly, if the controlled precipitation reaction is
performed in the absence of urea and mannitol but instead
we use hexamethylenetetramine (HMT) as the hydrolysis
agent in the presence of sodium cyanate (NaOCN), then the
flower-like morphology crystals of a different layered cobalt
hydroxide phase are isolated in 70% yield. On the basis of
analytical and spectroscopic data, this material may be
formulated as Co(OH)1.25(NCO)0.75 · 0.2H2O (2).
IR Spectroscopy. Vibrational spectroscopy has proved to
be the principal method for studying the existence and
bonding mode of the OCN anion in metal complexes. Three
normal IR active vibrational modes for the free OCN ion
are the antisymmetric stretching (VCN), the pseudosymmetric
stretching (VCO) and the bending (δNCO). They occur at
2160(s), (1296 and 1204)(m), and (635 and 628)(m) cm-1,
respectively.20–22 The positions of these bands are shifted
upon coordination to metal centers. The FT-IR of 1 and 2
are shown in Figure 2 together with the IR of sodium cyanate
(NaOCN) for reference. The presence of the OCN- ion in
both 1 and 2 is clearly shown by the absorptions at ca. 2219,
634 and 2263, 641 cm-1 for 1 and 2, respectively, which
may be assigned to (VCN).20–26 However, the OCN- anion,
is potentially ambidentate and one may distinguish if this
anion is nitrogen bonded (i.e., isocyanate: -NCO) or oxygen
bonded (i.e., cyanate: -OCN) to a metal center by reference
to the frequency of the remaining weaker pseudosymmetric
stretching VCO mode.27,28 The published literature indicates
for metal isocyanate complexes (N-bonded) only a single
band assigned to VCO is observed with a positive shift
compared to free OCN to ca. 1350–1300 cm-1.22,25,29,30 For
metal cyanates (O-bonded) a negative shift in frequency is
predicted for the VCO. For 1, we clearly resolve a band at
1340 cm-1 which we assign to VCO of a Co-NCO moiety.
Unfortunately, we do not observe a similar absorbance in
the IR spectrum in 2; however, this mode is commonly
reported to be a weak feature. On balance, we favor a similar
Co-NCO coordination for 2. This is supported by a review
of the literature on OCN- containing metal complexes which
indicates that almost all first row transition metals contain
Figure 2. FT-IR spectra of (a) 1, (b) 2, and (c) NaNCO. .
Figure 4. Results of Rietveld refinement for 1: experimental (black),
calculated (red), and the difference spectrum (blue) XRD data. The inset
shows a magnification of the high 2θ region. Miller indices for relatively
strong Bragg reflections are indicated for rhombohedral and hexagonal (in
bracket) polytypes, respectively.
Figure 3. TGA traces for (a) 1 and (b) 2.
3236 Inorganic Chemistry, Vol. 47, No. 8, 2008

Characterization of Layered Co Hydroxyisocyanates
Figure 5. Various possible unit cell choices for 1. (a) Intralayer (a ≈ a0, c ≈ 2a0 sin 60°). (b) Interlayer (b ≈ 6c0) related to the rhombohedral/ hexagonal
j
model (a, b, and c are cell parameters from the Pmm2 space group, a0 and c0 are cell parameters from the R3m space group).
Figure 6. TEM images of 1: (a) perpendicular to c-axis, (b) parallel to c-axis.
Table 2. Summary of the Crystallographic Data for 1 Based on a
Mixed Phase Refinementa
Table 3. Atomic Positions and Thermal Parameters for 1 Based on an
Intergrowth of Two Polytypes of Rhombohedral and Hexagonal
Symmetry
rhombohedral model
hexagonal model
P63/mmc (194)
j
space group R3m (166)
a (Å)
c (Å)
V (Å3)
Z
3.2031(1)
23.6876(11)
210.47(1)
3
3.2005(2)
15.8303(5)
140.43(1)
2
Rhombohedral Model
atom
site
x
y
z
SOF
U [Å2]
Co1
O1/N1
C1
O2
O3
3a
6c
6c
6c
6c
0
0
0
0
0
0
0
0
0
0
0
1
0.0195(11)
0.023(2)b
0.023(2)b
0.023(2)b
0.089(6)
calc. formula Co(OH)1.4(NCO)0.6 ·0.6H2O Co(OH)1.4(NCO)0.6 ·0.6H2O
sum.
0.62337(13)
0.5722(5)
0.5214(5)
0.5013(6)
0.7/ 0.3a
0.3a
phase
fractions
73.3(10)%
26.7(10)%
0.3a
0.3a
a Fitted Rwp ) 1.10%, Rp ) 0.76%, -BnkRwp ) 2.11%, Rp ) 1.60%,
and ꢁ2 ) 3.29.
Hexagonal model
N-bonded OCN ligands.22,31 In 2006, Faus and co-workers
reported they were the first to structurally characterize a
atom
site
x
y
z
SOF
U [Å2]
Co1
O1/N1
C1
2a
4f
4f
4f
4f
0
0
0
1
0.0195(11)
0.021(2)b
0.021(2)b
0.021(2)b
0.074(6)
1/3
1/3
1/3
1/3
2/3
2/3
2/3
2/3
0.06509(17)
0.1420(7)
0.2186(8)
0.2510(8)
0.7/0.3a
0.3a
0.3a
0.3
(20) Mavis, B.; Akinc, M. Chem. Mater. 2006, 18, 5317.
(21) Talismanova, M. O.; Sidorov, A. A.; Novotortsev, V. M.; Aleksandrov,
G. G.; Nefedov, S. E.; Eremenko, I. L.; and Moiseev, I. I. Russian
Chem. Bull., Int. Ed. 2001, 50, 2251.
O2
O3
(22) Martinez-Lillo, J.; Armentano, D.; De Munno, G.; Lloret, F.; Julve,
M.; Faus, J. Inorg. Chim. Acta 2006, 359, 4343.
a Statistically disordered with 70% OH- and 30% OCN-, SOF(O3) )
SOF(H2O) ) 0.3. b Thermal parameters constrained to be equal.
Inorganic Chemistry, Vol. 47, No. 8, 2008 3237

Du and O’Hare
Figure 7. Structural model of 1 generated from Rietveld refinement based on an intergrowth model of two polytypes of rhombohedral (a) and hexagonal
symmetry (b). Co, O, N, and C are presented by light blue, red, dark blue, and white spheres, respectively. The average occupancy for the hydroxyl and NCO
groups is 70% and 30%, respectively.
Table 4. Selected Bond Lengths and Angles for 1 Based on an
1–5% begins at temperatures below 100 °C, which corre-
sponds to the loss of interlayer water, to give the dehydrated
Intergrowth of Two Polytypes of Rhombohedral and Hexagonal
Symmetry
materials Co(OH)1-x(NCO)x (x ) 0.6 and 0.75). Further
rhombohedral model
hexagonal model
decomposition events we ascribe to the dehydration of the
layers and decomposition of the OCN- ligands, to finally
give a calcined product which has a powder XRD pattern
corresponding to CoO. We observe a slight inconsistency
between the percentage water in these materials as measured
by elemental analysis and TGA which we think this is due
to a small variation from sample to sample in the amount of
absorbed water or cointercalated water. The figures given
in Table 1 are the best fit to the elemental analysis data.
Structural Characterization. The X-ray powder diffrac-
tion pattern of 1 is shown in Figure 4. At first inspection,
the pattern has the features expected for a rhombohedral
symmetry R-type cobalt hydroxide similar to that reported
in the literature.13,19 For example, two prominent low-angle
reflections at d ) 7.83 and 3.92 Å could be assigned to 003
and 006 Bragg reflections of the hydrotalcite-like phase with
Co1-O1/N1
2.11463(3)
1.21136(2)
1.2045(13)
119.010(1)
Co1-O1/N1
2.11574(7)
1.21768(2)
1.21225(2)
119.143(1)
N1-C1
N1-C1
C1-O2
Co1-N1-C1
C1-O2
Co1-N1-C1
M-OCN complex.22 The bending vibration of interlayer
water molecules gives rise to absorptions at 1650 cm-1 in
layered double hydroxide (LDH) spectra.32 Therefore, we
assign the 1624 cm-1 absorbance for 1 to intercalated water
which is in agreement with the XRD, elemental analysis,
and TGA data. Both the TGA and elemental analysis data
for 2 suggests that 2 contains almost no water and this
supported the absence of an equivalent band in the IR
spectrum. Finally, the absence of any absorptions ca. 1500
cm-1 in the IR data of in either 1 or 2 suggest that no CO3
ions are present either as impurity phases or by cointerca-
lation.
Thermogravimetric Analysis. TGA was performed on
1 and 2 in order to investigate the number of water molecules
cointercalated and to gain insight into the decomposition
processes that occur. TGA traces for both samples are given
in Figure 3. Both TGA traces share a similar shape. Mass
loss occurs in two/three stages. The initial mass loss of ca.
2-
(23) Sato, K.; Yoshinari, T.; Kintaichi, Y.; Haneda, M.; Hamada, H. Appl.
Catal. B: EnViron. 2003, 44, 67.
(24) Nelson, J. N. J. Chem. Soc. A 1969, 1597. a. S. M.
(25) Sabatini, A.; Bertini, I. Inorg. Chem. 1965, 4, 959.
(26) Maki, A.; Decius, J. C. J. Chem. Phys. 1958, 28, 1003.
(27) Mavis, B.; Akinc, M. J. Am. Ceram. Soc. 2006, 89, 471.
(28) Carbacho, H.; Ungerer, B.; Contreras, G. J. Inorg. Nuc. Chem. 1970,
32, 579.
(29) Anderson, S. J.; Brown, D. S.; Finney, K. J. J. Chem. Soc., Dalton
Trans. 1979, 152.
(30) Fawcett, J.; Peacock, R. D.; Russell, D. R. J. Chem. Soc., Dalton Trans.
1987, 567.
(31) D’Agnano, A.; Gargano, M.; Malitesta, C.; Ravasio, N.; Sabbatini,
L. J. Electron Spectrosc. Relat. Phenom. 1991, 53, 213.
(32) Williams, G. R.; O’Hare, D. Solid State Sci. 2006, 8, 971.
Figure 8. XRD pattern for 2.
3238 Inorganic Chemistry, Vol. 47, No. 8, 2008

Characterization of Layered Co Hydroxyisocyanates
indexed using an orthorhombic unit cell (Pmm2 (25)) with
lattice constants a ) 3.1823(5), b ) 47.130(8), and c )
5.5125(10) Å, This orthorhombic cell is related to the
rhombohedral cell (a ≈ a0, c ≈ 2a0 sin 60°, b ≈ 6c0, where
a0 and c0 are the approximate lattice parameters of the
rhombohedral/ hexagonal unit cell) as shown in Figure 5.
We have carefully considered the validity of both models
and have performed a parallel refinement using the orthor-
hombic cell model. Since the orthorhombic cell does not
index any additional observed reflections and the final
agreement factors for the refinement using the orthorhombic
cell model are no better even though we have greater number
of least-squares variables and the electron microscopy (see
below), we have chosen to report the results obtained using
the intergrowth model below.
Figure 9. Plots of ꢁmolT vs T and ꢁmol-1 vs T following zero-field cooling
between 2 and 300 K for (1 0), (2 O).
Furthermore compound 1 was also been studied by high-
resolution transmission electron microscopy (HRTEM) and
selected area electron diffraction (SAED) with a view to
probing the symmetry of the crystalline domains. Figure 6a
and b shows TEM images of 1. The corresponding selected
area electron diffraction (SAED) pattern (inset in Figure 6b)
shows a diffraction pattern which can be indexed as a two-
dimensional in-plane hexagonal lattice. This implies that the
platelets are lying on their {001} zone axis direction. The
HREM image (Figure 6a) shows lattice fringes separated by
0.76 nm. On the basis of all our microscopy data, we believe
it is best to consider the structure of 1 as an intergrowth of
rhombohedral and hexagonal symmetry polymorphs.
In order to build trial models for structural refinement,
we then considered the nature of individual cobalt hydroxide
layers and the isocyanate location. It is possible that the
structure may be analogous to hydrotalcite-like layered
double hydroxides,15 in which the positive charge originates
from partial substitution of divalent metal cations by trivalent
cations in a brucite-like octahedral layer. However, this is
not the case here because of the lack of Co3+ in the R-form.
From the X-ray photoemission spectroscopy (XPS) spectrum,
we observe no photoemission from a Co3+ ion (see Sup-
porting Information Figure S2). Furthermore, the existence
of Co3+ would result in a gray/black colored sample rather
than the pure pink color of our product.
Figure 10. (a) Plot of molar magnetization (M) vs field (T) for 1 at 70
(0), 10 (O), and 2 K (∆). (b) Plot of molar magnetization (M) vs field (T)
for 2 70 (0), 10 (O), and 2 K (∆).
unit cell parameter, c ) 23.52 Å (hydrotalcite-like, 3R1).33
The Bragg reflection at d ) 1.60 Å could be indexed as 110
which allows us to determine the unit cell parameter a )
3.20 Å. However, on closer inspection, we find that not all
the observed Bragg reflections could be indexed on this unit
cell. For example, two prominent low-angle reflections at d
) 2.60, 2.25, 1.90 Å could only be indexed to the 102, 104,
and 106 Bragg reflections of a hexagonal symmetry cell
with c ) 15.68 Å (manasseite-like, 2H2),33 the 110 Bragg
reflection at d ) 1.60 Å allows us to deduce hexagonal unit
cell parameter a ) 3.20 Å (see bracketed reflections inset
in Figure 4). Using these two unit cells, it is possible to index
the entire X-ray powder diffraction pattern. However, the
X-ray powder diffraction (XRD) data may also be fully
Another possibility is the existence of additional Co2+
cations at interstitial (tetrahedral) sites. Recently, the Sasaki
group19 have reported the results of a Rietveld refinement
on a green color Cl- ion intercalated R-cobalt hydroxide.
Their model was based on a rhombohedral symmetry phase
with the Co2+ cations in both octahedral and tetrahedral
coordination sites. The refinement clearly demonstrated that
one-fifth to one-sixth of the Co2+ in octahedral sites were
replaced by pairs of tetrahedrally coordinated Co2+ on either
side of the hydroxide plane; the material can be represen-
ted by a structure formula [Coocta0.828Cotetra0.348(OH)2]0.348+
-
Cl0.348 · 0.45H2O. In their example, the material has a
characteristic green color. Cobalt complexes typically fall
into two general groups; pink-red octahedral complexes and
intensely blue tetrahedral complexes. Green can be identified
as a mixture of tetrahedral and octahedral complexes.
(33) Ramesh, T. N.; Rajamathi, M.; Kamath, P. V. J. Solid State Chem.
2006, 179, 2386.
Inorganic Chemistry, Vol. 47, No. 8, 2008 3239

Du and O’Hare
Table 5. Summary of the Magnetic Data for 1 and 2
samples
µeff (µB)
C (emu K mol-1
)
θ (K)
MSAT (µB)
coercive field (Oe)
remanent magnetization (µB)
Co(OH)1.4(NCO)0.6 ·0.6H2O (1)
Co(OH)1.25(NCO)0.75 ·0.2H2O (2)
5.26
5.38
3.46
3.62
-5.11
-2.72
∼2.00
∼2.00
80
270
0.02
0.09
Therefore, we believe that our pink cobalt hydroxyisocyan-
ates materials are entirely composed of octahedral cobalt
centers.
The phase fractions of the rhombohedral and hexagonal
symmetry polymorphs were also allowed to refine, and this
resulted in a composition of ca. 70% rhombohedral and 30%
hexagonal forms. We managed to refine the atomic position
with separate successive cycles. Selected bond lengths and
angles for 1 are given in Table 4. The agreement indices
and the Estimated Standard Deviations (esds) on the refined
parameters reflect that this refinement is less than ideal. Even
after all our efforts, we still observe some poor agreements
between the observed and calculated data between 50 and
60° in 2θ. However, we would like to highlight that this
does represent a rare example of a quantitative refinement
of the X-ray powder diffraction data from a hydrotalcite-
like phase prepared at low temperature.
Finally, a third model involving hydroxyl ion substitution
was considered.1 In this model, the isocyanate anions are
incorporated into the octahedral layers by replacing some
of the OH- ions and directly coordinated to the Co2+ cations.
In this model, the cobalt centers all remain as octahedrally
coordinated 2+ cations. The longer rod-like OCN- would
expand the interlayer distance to ∼8 Å. Thus, a novel
structure model based on 30% hydroxyl substitution by
OCN- is proposed. A Rietveld refinement was performed
on a rigid body fragment model with soft constraints on the
bond lengths.
The summary of the refinement parameters is given in
Table 2, final atomic positions and thermal parameters are
presented in Table 3, and the experimental, calculated, and
the difference XRD patterns for 1 are shown in Figure 4.
The structural model generated from the results of the Ri-
etveld refinement for 1 are shown in Figure 7. The structure
consists of sheets of cobalt octahedra and these layers are
stacked along the c-direction. We see no evidence on any
OCN- ordering in either the electron diffraction or X-ray
diffraction data. The OCN- ions are arranged perpendicular
to the layer planes. Each cobalt octahedra is linked through
either edge-sharing hydroxyl or isocyanate ions. A µ3-NCO
ligand has been observed previously for Co2+ ions21 and
other metal compounds.22,33,34 In the refinement, a rigid body
model was first employed and a composite O/N atom was
used corresponding to the chemical composition with a fixed
Cooct-O/N ) 2.11 Å. The NdC and CdO bond lengths in
the NCO- ligand were also fixed at 1.20 Å, which are close
to the values found of 1.179(5) and 1.190(5) Å for a µ2-
NCO ligand bridging two Co2+ centers.19,21 Moreover, an
oxygen atom was introduced in each phase model at (0, 0,
0.498) and (1/3, 2/3, 0.251), respectively, with a fractional
occupancy set to model the amount of cointercalated water.
The XRD data for 2 (Figure 8) indicates that this phase is
less crystalline than 1. Nevertheless, the XRD data may be
indexed using a single rhombohedral hydrotalcite-like cell
with unit cell parameters, a ) 3.158 and c ) 21.57 Å. The
full-width at half-maximum (fwhm) of the 003 Bragg
reflection for 2 is 0.267° which is significantly larger than
0.094° found for the equivalent 003 reflection in the
rhombohedral phase in 1. These data are clearly not of
sufficient quality or angular range to enable us to perform a
structural refinement.
Direct Current Magnetic Measurements. The molar
magnetic susceptibility of 1 and 2 was studied as a function
of temperature (2-300 K) and applied magnetic field (0–5
T). A plot of the temperature dependence of ꢁmolT for both
1 and 2 after cooling in zero field is shown in Figure 9. Both
1 and 2 behave as paramagnets above 50 K. Above 50 K,
the magnetic susceptibilities can be fitted to the Curie–Weiss
law (Figure 9) with effective molar magnetic moments µeff
) 5.26 and 5.38 µB and negative Weiss constants of -5.1
and -3.6 K for 1 and 2, respectively. The observed effective
magnetic moments are higher than the spin-only value
predicted for an S ) 3/2 ion (µSO ) 3.87 µB), but they do fall
within the experimental range (4.7–5.4 µB) found for
4
octahedral Co2+which would be expected to adopt a T1g
ground state.9,36,37
On cooling below 50 K, ꢁmolT for both 1 and 2 exhibit a
shallow minimum before rapidly increasing at low temper-
ature. This slight decrease in ꢁmolT has been observed by
Rabu and Drillon et al., for ꢀ-Co(OH)2 and Co2(NO3)(OH)3,
and was ascribed to spin–orbit coupling effects for Co2+ ion
stabilizing a Kramers doublet ground-state and/or competing
(34) Nicola, C. D.; Effendy; Fazaroh, F.; Pettinari, C.; Skelton, B. W.;
Somers, N.; White, A. H. Inorg. Chim. Acta 2005, 358, 720.
(35) Luo, J.; Zhou, X. G.; Weng, L. H.; Hou, X. F. Acta Crystallogr. Sect.
C 2003, 59, 519.
Figure 11. Tempereature dependence of the dc magnetization in ∼5 Oe
and the in- an out-of-phase ac magnetization for 1 between 2 and 40 K
under an oscillating magnetic field of 3.5 Oe. (inset) Frequency dependence
of ac susceptibility (ꢁ′′) for 1 between 2 and 10 K under an oscillating
magnetic field of 3.5 Oe.
(36) Kurmoo, M.; Kumagai, H.; Hughes, S. M.; Kepert, C. J. Inorg. Chem.
2003, 42, 6709–6722.
(37) Drillon, M.; Panissod, P. J. Magn. Magn. Mater. 1998, 188, 93.
3240 Inorganic Chemistry, Vol. 47, No. 8, 2008

Characterization of Layered Co Hydroxyisocyanates
Figure 12. (a) SEM image of 1. (b) SEM image of 2. A trace amount (<1%) of cobalt impurity phase was observed in 1.
in-plane magnetic interactions between Co2+ moments.8 The
large increase in ꢁmolT at low temperature suggests the onset
of a 3D ordered state.11,38,39
change the in the gradient of the M(H) curves; the data is
consistent with both ferromagnetic inplane and interplane
interactions.
The field dependence of the molar magnetization, M(H),
for 1 at three different temperatures is shown in Figure 10a.
At 70 K, the plot of M vs H is linear as expected for a
paramagnet. Least squares fitting of M(H) at 70 K for an S
) 3/2 ion allows us to determine an average isotropic g-value
for the Co2+ ions of 2.64. Below 10 K, the M vs H data
deviate from the behavior predicted by the Brillouin function.
At 2 K, the M(H) curve rises rapidly with increasing field
and tends toward a saturation magnetization (MSAT) of approx
2.0 µB. At 2 K, 1 exhibits a narrow hysteresis loop with a
coercive field of 80 Oe. Similar M(H) curves can be observed
for 2 as shown in Figure 10b. For 2, an isotropic <g> value
of 2.72 may be deduced from the least-squares fitting of
M(H) at 300 K. At 2 K, the M(H) curve tends toward a
similar saturation magnetization of ∼2.00 µB. The summary
of dc magnetic data for both 1 and 2 are in Table 5. The
saturation magnetization (MSAT) is predicted to be gS µB for
an ensemble of ferromagnetically aligned spins (S). Thus for
1 and 2, we would expect MSAT of 3.96 and 4.08 µB,
Alternating Current Magnetic Measurements. The
temperature dependence of ac magnetic susceptibility for 1
was measured in the temperature range 2–40 K under an
oscillating magnetic field of 3.5 Oe (under a near-zero dc
field of 5 Oe) (Figure 11). Spontaneous magnetic ordering
for 3D magnets is characterized by a discontinuous jump in
dc magnetization which coincides with the peak in the in-
phase ac magnetization and to the nonzero out of phase ac
magnetization. The sharp peaks in both real (in phase) and
imaginary (out of phase) components of the ac susceptibilities
are taken to be indicative of a ferromagnetic 3D ordering
transition. The Curie temperature, TC, of the sample, was
determined to be 4 and 3 K for 1 and 2 (ac susceptibility
measurement for 2 is not shown) by inspection of the real
and imaginary components of the ac susceptibilities (Figure
11).
The frequency dependence of imaginary (ꢁ′′) components
of the magnetic susceptibility for 1 between 2 and 10 K is
shown inset in Figure 11. The maximum in ꢁ′′ occur ca. 3
K but becomes smaller and shifts to higher temperatures with
increasing frequency. The real component, ꢁ′ varies with
temperature in the same way as ꢁ′′ but with much larger
susceptibility. Below 5 K, ꢁ′′ rises from zero to a maximum
at 3 K. The observed nonzero ꢁ′′ below 4 K confirms the
transition to a magnetically ordered ground-state with a net
spontaneous magnetization. The spontaneous magnetization
could be further proved by a sharp increase of dc magnetiza-
tion and by a peak in both the in- and out-of-phase
components of the ac magnetization as shown in Figure
10.35,39
3
respectively, if the Co2+ ions adopt a well defined S ) /2
ground-state at low temperature. However, lower than
expected saturation magnetization values has been observed
previously for several magnetically ordered layered Co2+
hydroxides. This has been ascribed to 3D ferromagnetic
ordering of the Co2+ sites in which the octahedral Co2+ ions
adopt a ground-state Kramers doublet giving an effective S′
35,39
1
) /2.
Layers of cobalt hydroxides such as ꢀ-Co(OH)2 and
Co2(NO3)(OH)3 exhibit ferromagnetic inplane interactions.
At low temperature in the ordered state, these compounds
exhibit metamagnetic behavior.8 The magnetic properties of
several organic intercalated layered cobalt hydroxides with
large interlayer separations have been studied in detail by
Kurmoo, Drillon, Kepert, and Rosseinsky.11,35–38 These
materials show ferromagnetic coupling between these widely
spaced layers, and this has been ascribed to long-range
dipolar interactions.9,11,36,40 For 1 or 2, we see no evidence
of any metamagnetism which would be indicated by a sudden
Crystal Morphology. Figure 12a shows the SEM images
of 1 following crystallization at 90 °C. The sample is
composed of uniform hexagonal crystalline platelets with
widths in the range 6–7 µm and thickness ca. 20 nm. The
SEM image of 1 shows the presence of a small amount of
an cobalt impurity phase. In contrast, the SEM image of 2
(Figure 12b) which also forms at 90 °C in the presence of
NaNCO yields flower-like morphology crystallites with a
radius around 0.4-1 µm. The difference in morphology may
be ascribed to the fact that a larger number of smaller nuclei
(38) Williams, D. L.; Smith, D. W.; Stoufer, R. C. Inorg. Chem. 1967, 6,
590.
(39) Figgis, B. N.; Nyholm, R. S. J. Chem. Soc. 1954, 12.
(40) Kurmoo, M. Chem. Mater. 1999, 11, 3370.
Inorganic Chemistry, Vol. 47, No. 8, 2008 3241

Du and O’Hare
nucleate during the condensation of 2, and these aggregate
to form spherical structures.
bohedral and 30% hexagonal polytypes. Both 1 and 2 have
3D ferromagnetically ordered ground states with a net
saturation magnetization corresponding to ordered Co2+ ions
Conclusions
1
with an effective S′ ) /2.
A series of pink layered cobalt hydroxyisocyanates
(hexagonal sheet platelets/flower-like structures) have been
synthesized and characterized. IR spectroscopy shows that
1 and 2 both contain the N-bonded Co-NCO ligands, A
Rietveld refinement has been performed for 1. The structure
is composed of layers of divalent cobalt(II) ions in octahedral
coordination with 30% of hydroxyl replaced by OCN-
anions. The extended structure is best considered as an
intergrowth of containing both approximately 70% rhom-
Acknowledgment. The authors thank Clarendon Scholar-
ship Fund for financial support to Y.D. We thank Dr. H.
Wang for XPS measurements.
Supporting Information Available: EDX for 1; XPS spectrum
of 1. This material is available free of charge via the Internet at
http://pubs.acs.org.
IC702260S
3242 Inorganic Chemistry, Vol. 47, No. 8, 2008