Hydrothermal synthesis, structural, Raman, and luminescence studies of Am[M(CN)2]3·3H2O and Nd[M(CN)2]3·3H2O (M=Ag, Au): Bimetallic coordination polymers containing both trans-plutonium and transition metal elements

Article abstract of DOI:10.1016/j.jssc.2007.08.032

The polymeric compounds consisting of the man-made element, americium, and gold and silver dicyanides were prepared under mild hydrothermal conditions at 120 °C. It was found that the americium ion and the transition metal ions are interconnected through cyanide bridging in the compounds. Given the similarities in the radii of americium and neodymium, crystals of the latter were also characterized for comparison purposes. The four compounds are isostructural and crystallize in the hexagonal space group, P6₃/mcm, with only slight differences in their unit cell parameters. Crystallographic data (MoKα, λ=0.71073 A): Am[Ag(CN)₂]₃·3H₂O (1), a=6.7205(10) A, c=18.577(3) A, V=726.64(19), Z=2; Am[Au(CN)₂]₃·3H₂O (2), a=6.666(2) A, c=18.342(3) A, V=705.9(4), Z=2; Nd[Ag(CN)₂]₃·3H₂O (3), a=6.7042(4) A, c=18.6199(14) A, V=724.77(8), Z=2; and Nd[Au(CN)₂]₃·3H₂O (4), a=6.6573(13) A, c=18.431(4) A, V=707.2(2), Z=2. The coordination around the Am and/or Nd consists of six N-bound CN^- groups resulting in a trigonal prismatic arrangement. Three capping oxygen atoms of coordinated water molecules complete the tricapped trigonal prismatic coordination environment, providing a total coordination number of nine for the f-elements. Raman spectroscopy, which compliments the structural analyses, reveals that the four compounds display strong signals in the ν_CN stretching region. When compared with KAg(CN)₂ or KAu(CN)₂, the ν_CN stretching frequencies for these compounds blue-shift due to bridging of the dicyanometallate ions with the f-element ions. There is subsequent reduction in electron density at the cyanide center. Compared with the silver systems, the ν_CN frequency appears at higher energy in the gold dicyanide complexes. This shift is consistent with the structural data where the carbon-nitrogen bond distance is found to be shorter in the gold dicyanides.

Full text of DOI:10.1016/j.jssc.2007.08.032

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Journal of Solid State Chemistry 180 (2007) 3121–3129
www.elsevier.com/locate/jssc
Hydrothermal synthesis, structural, Raman, and luminescence studies of
Am[M(CN)2]3 ꢀ 3H2O and Nd[M(CN)2]3 ꢀ 3H2O (M ¼ Ag, Au):
Bimetallic coordination polymers containing both trans-plutonium
and transition metal elements
Ã
Zerihun Assefaa, , Katrina Kalachnikovab, Richard G. Hairec, Richard E. Sykorab
aDepartment of Chemistry, North Carolina A&T State University, Greensboro, NC 27411, USA
bDepartment of Chemistry, University of South Alabama, Mobile, AL 36688, USA
cChemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA
Received 21 May 2007; received in revised form 16 August 2007; accepted 31 August 2007
Available online 14 September 2007
Abstract
The polymeric compounds consisting of the man-made element, americium, and gold and silver dicyanides were prepared under mild
hydrothermal conditions at 120 1C. It was found that the americium ion and the transition metal ions are interconnected through cyanide
bridging in the compounds. Given the similarities in the radii of americium and neodymium, crystals of the latter were also characterized
for comparison purposes. The four compounds are isostructural and crystallize in the hexagonal space group, P63/mcm, with only slight
˚
˚
differences in their unit cell parameters. Crystallographic data (MoKa, l ¼ 0.71073 A): Am[Ag(CN)2]3 ꢀ 3H2O (1), a ¼ 6.7205(10)A,
˚
˚
˚
c ¼ 18.577(3) A, V ¼ 726.64(19), Z ¼ 2; Am[Au(CN)2]3 ꢀ 3H2O (2), a ¼ 6.666(2)A, c ¼ 18.342(3) A, V ¼ 705.9(4), Z ¼ 2; Nd[Ag(CN)2]3 ꢀ
˚
˚
˚
˚
3H2O (3), a ¼ 6.7042(4) A, c ¼ 18.6199(14) A, V ¼ 724.77(8), Z ¼ 2; and Nd[Au(CN)2]3 ꢀ 3H2O (4), a ¼ 6.6573(13) A, c ¼ 18.431(4) A,
V ¼ 707.2(2), Z ¼ 2. The coordination around the Am and/or Nd consists of six N-bound CNꢁ groups resulting in a trigonal prismatic
arrangement. Three capping oxygen atoms of coordinated water molecules complete the tricapped trigonal prismatic coordination
environment, providing a total coordination number of nine for the f-elements. Raman spectroscopy, which compliments the structural
analyses, reveals that the four compounds display strong signals in the nCN stretching region. When compared with KAg(CN)2 or
KAu(CN)2, the nCN stretching frequencies for these compounds blue-shift due to bridging of the dicyanometallate ions with the f-element
ions. There is subsequent reduction in electron density at the cyanide center. Compared with the silver systems, the nCN frequency appears
at higher energy in the gold dicyanide complexes. This shift is consistent with the structural data where the carbon–nitrogen bond distance
is found to be shorter in the gold dicyanides.
r 2007 Elsevier Inc. All rights reserved.
Keyword: Trans-plutonium complexes; Coordination polymer; Emission; Hydrothermal syntheses
1. Introduction
cyanide-bridged, bimetallic systems prepared by assem-
bling cyanometallates and transition metal and/or lantha-
nide building blocks exhibit fascinating structural,
magnetic, electrochemical, and magneto-optical properties
[6–11]. The importance of metal containing polymers for
new materials is quite extensive and Au(I)-, Ag(I)-, and
Pt(II)-based polymers are of particular importance because
of their unique photophysical properties [12–14].
The design and synthesis of self-assembling supramole-
cular systems is of intense interest as a rational route for
generating important functional materials. One facet in this
crystal engineering approach is the use of relatively weak-
bonding interactions, such as hydrogen bonding [1–5].
Gold(I) centers are known to form weakly bonding
aurophilic interactions which have strengths that are on the
same order-of-magnitude as that for hydrogen bonds [15].
Ã
Corresponding author. Tel.: +1 336 4336741; fax: +1 336 334 7124.
E-mail addresses: zassefa@ncat.edu (Z. Assefa),
rsykora@jaguar1.usouthal.edu (R.E. Sykora).
0022-4596/$ - see front matter r 2007 Elsevier Inc. All rights reserved.
doi:10.1016/j.jssc.2007.08.032

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The association usually extends to yield larger oligomers
and one- or two-dimensional polymers. In general terms,
‘‘aurophilic’’ bonding of this type is now widely accepted as
the most prominent example of a general phenomenon of
metallophilicity, and is recognized as a major factor in
determining supramolecular structures and properties
[16–18]. Hence, the chemistry of Au(I) is satiated with
systems that are polymeric by virtue of these Au–Au
interactions. In this regard, the linear building block
dicyanoaurate, [Au(CN)2]ꢁ, is a convenient unit with
which to explore the use of aurophilicity as a supramole-
cular design element [16–20]. This simple anion has been
extensively studied over the years due it its ability to form
coordination polymers through cyanide bridging. It can
form polymers in a fashion similar to other metal–cyanide
anions, where the central Au(I) atom is prone to forming
gold–gold bonds both in solution [20–21] and in the
solid state [22–26]. Similar structural features are also
exhibited in Ag(I) and Pt(II) systems displaying what are
known as ‘‘argentophilic’’ and ‘‘platinophilic’’ interactions,
respectively.
While a majority of the synthetic procedures of these
systems follow conventional solution routes, interest has
also been shown in the less-conventional solvothermal
techniques. Many homometallic cyanide-bridged copper(I)
systems constructed through hydrothermal techniques by
using CuCN and aromatic imine ligands as starting
materials have been reported recently [27,28]. The first
cyanide-bridged bimetallic complex prepared by solvother-
mal techniques, an Fe(II)–Cu(I) complex, was reported in
2003 by Colacio et al. [29]. Of the vast array of gold
complexes reported, an ISI literature search provides
Pb[Au(CN)2]2(H2O) as the only compound synthesized
recently using a hydrothermal technique [30].
The synthesis of trans-plutonium compounds, especially
as single crystals, is complicated by the limited availability
of the materials and restriction on the experimental
procedures that can be adapted due to their high radio-
toxicity. Single-crystal X-ray studies in most cases are not
possible given the radioactive nature that restricts handling
and reduces the stability of the crystals. As a result, most of
the potential compounds of these are unexplored due to the
special equipment and facilities needed to deal with such
materials, as well as crystal deterioration that arises due to
self-irradiation.
In this paper, we report an example of a trans-plutonium
element linked to a transition metal through cyanide
bridging to form a three-dimensional coordination poly-
meric structure. Only a handful of coordination polymeric
compounds have been described involving the trans-
uranium element, neptunium [31,32]. To the best of our
knowledge, prior examples of coordination polymers
involving the trans-plutonium elements do not exist.
The polymeric compounds between the man-made
element, americium, and gold and silver dicyanides were
prepared using the less-conventional hydrothermal syn-
thetic procedure. When compared to the more traditional,
slow evaporation technique, the hydrothermal synthetic
procedure is especially useful for more-rapid single-crystal
growth, which helps counter the internal, radioactive-
induced crystal deterioration. The neodymium system is
reported for structural comparison, since Nd3+ has a
similar ionic size to that of Am3+
.
2. Experimental Section
2.1. Materials and methods
The americium used in these studies was made available
through the US Department of Energy. Nd2O3 (Research
Chemicals, 99.9%), NdCl3 (Strem Chemicals, 99.9%),
KAg(CN)2 (Alfa-Aesar, 99.9%), and KAu(CN)2 (Alfa-
Aesar, 99.99%) were used as received. Triply distilled water
was used in all reactions. An aqueous AmCl3 solution was
prepared by dissolving Am metal in 1 M HCl, evaporating
the resulting solution to dryness, and then dissolving the
solid product in H2O. An aqueous solution of Nd(NO3)3
was prepared by dissolving Nd2O3 in concentrated HNO3,
evaporating the solution to dryness, and then dissolving the
resultant solid in H2O. The isotopic purity of the
americium was determined to be 499.9% 243Am by
mass spectrometric analysis. It should be noted that special
training and experimental facilities are required in order
to perform work with highly radioactive transuranium
materials, such as americium. Preparations of the
Am[M(CN)2]3 ꢀ 3H2O (M=Ag, Au) were conducted in
glove boxes in accordance with the established policies for
handling radioactive materials at the Oak Ridge National
Laboratory. To decrease the likelihood of radioactive
contamination, the established policies required that
americium samples selected for X-ray and spectroscopic
analyses be doubly contained.
2.2. Syntheses of Am[M(CN)2]3 ꢀ 3H2O (M=Ag, 1; Au, 2)
The synthesis of Am[Ag(CN)2]3 ꢀ 3H2O, (1), proceeded
by adding AmCl3 (40 mL, 0.095 M) to a quartz reaction
vessel that had been loaded with solid KAg(CN)2
(2.269 mg, 11.4 mmol). The reaction vessel was flame sealed
and heated to 120 1C, where the reaction/crystallization
occurred under autogenously generated pressure. After
67 h, the sample vessel was allowed to self-cool to room
temperature. The product consisted of yellow crystals of 1
in the form of hexagonal plates. The synthesis of
Am[Au(CN)2]3 ꢀ 3H2O (2) was similar to the synthesis of
compound 1, except that KAu(CN)2 (3.212 mg, 11.1 mmol)
was used. Crystals of 2 were pink and also consisted of
hexagonal plates.
2.3. Syntheses of Nd[M(CN)2]3 ꢀ 3H2O (M=Ag, 3; Au, 4)
The syntheses of compounds 3 and 4 were performed in
a manner similar to that described for the americium
compounds above. For compound 3, Nd(NO3)3 (85.3 mL,

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3123
0.233 M) and KAg(CN)2 (11.865 mg, 59.6 mmol) were
reacted, while NdCl3 (91.4 mL, 0.225 M) and KAu(CN)2
(17.769 mg, 61.7 mmol) were used in the synthesis of 4.
Crystals in the form of purple hexagonal plates for 3 and 4
were recovered from the reactions as the major solid
products. Powder X-ray diffraction experiments revealed
the presence of several minor unindexed lines that were not
consistent with either the desired products or starting
materials. The purity of 3 and 4 was estimated to be ꢂ95%
based on these studies. However, X-ray and spectro-
scopical studies were conducted on single crystals which
provided reproducible results. Average yields for 3 and 4
based on a number of reactions are 55%.
The determination of integrated intensities and a global-
cell refinement were performed with the Bruker SAINT
(v 6.02) software package using a narrow-frame, integra-
tion algorithm. A semi-empirical absorption correction was
applied using SADABS [33]. The program suite SHELXTL
(v 5.1) was used for space group determination (XPREP),
structure solution (XS), and least-squares refinement (XL)
[34]. A Patterson function was used to locate the heavy
atom positions. The C, N, and O atomic positions were
located in difference maps. The hydrogen atoms of the
water molecules were not located or calculated. The
scattering factor, real and imaginary dispersion terms,
and the linear absorption coefficient for americium [35]
were manually inputted into the SHELXTL instruction
files. The final refinements for 1, 3, and 4 included
anisotropic displacement parameters for all atoms and a
secondary extinction parameter; for 2, only the americium
and gold atoms were refined anisotropically. Experimental
challenges included very small crystals coupled with the
double containment required for the X-ray experiments,
which resulted in a lower quality X-ray structure for 2.
Crystallographic details for compounds 1–4 are listed in
Table 1. Further details of the crystal structure investiga-
tions may be obtained from the Fachinformationzentrum
Karlsruhe, D-76344 Eggenstein-Leopoldshafen, Germany
(Fax: (+49)7247-808-666; Email: crysdata@fiz-karlsruhe.
de) on quoting the depository numbers CSD-417896 for 1,
CSD-417897 for 2, CSD-417898 for 3, and CSD-417899 for 4.
2.4. Single-crystal X-ray diffraction
Single crystals of compounds 1–4 were selected for the
X-ray diffraction studies. The crystals of Am[Ag(CN)2]3 ꢀ
3H2O (1) and Am[Au(CN)2]3 ꢀ 3H2O (2) were sealed in
quartz capillaries and then placed inside polyethylene tubes
used for secondary containment. The crystals of
Nd[Ag(CN)2]3 ꢀ 3H2O (3) and Nd[Au(CN)2]3 ꢀ 3H2O (4)
were mounted on the tips of quartz fibers. The crystals
were aligned on a Bruker SMART APEX CCD X-ray
diffractometer with a digital camera. Intensity measure-
ments were performed using graphite monochromated,
MoKa radiation from a sealed X-ray tube equipped with a
monocapillary collimator. The intensities of reflections of a
sphere were collected by a combination of three sets of
exposure frames. Each set had a different f angle for
the crystal, and each exposure covered a range of 0.31 in o.
A total of 1800 frames were collected with exposure
times per frame of 15 s for 1, 20 s for 2, and 30 s for
compounds 3 and 4.
2.5. Raman spectroscopy
Raman spectroscopy was performed using an argon-ion
laser (Coherent, model 306) and a double-meter spectro-
meter (Jobin-Yvon Ramanor model HG.2S). The resolution
Table 1
Crystallographic data for Am[Ag(CN)2]3 ꢀ 3H2O (1), Am[Au(CN)2]3 ꢀ 3H2O (2), Nd[Ag(CN)2]3 ꢀ 3H2O (3), and Nd[Au(CN)2]3 ꢀ 3H2O (4)
Formula
Am[Ag(CN)2]3 ꢀ 3H2O
Am[Au(CN)2]3 ꢀ 3H2O
Nd[Ag(CN)2]3 ꢀ 3H2O
Nd[Au(CN)2]3 ꢀ 3H2O
Formula mass
Color and habit
Crystal system
Space group
776.84
Yellow, hexagonal plate
1044.13
678.02
Purple, hexagonal plate
945.31
Purple, hexagonal plate
Pink, hexagonal plate
Hexagonal
P63/mcm (No. 193)
6.666(2)
Hexagonal
P63/mcm (No. 193)
6.7205(10)
18.577(3)
726.64(19)
2
Hexagonal
P63/mcm (No. 193)
6.7042(4)
18.620(1)
724.77(8)
2
Hexagonal
P63/mcm (No. 193)
6.6573(13)
18.431(4)
707.2(2)
2
˚
a (A)
˚
c (A)
18.342(6)
705.9(4)
3
V (A )
˚
Z
2
21
T (1C)
21
0.71073
21
21
˚
l (A)
0.71073
50.36
0.71073
56.54
3.107
0.71073
56.54
4.440
2ymax
56.58
3.551
rcalcd (g cm–3
)
4.913
36.463
4793
m(MoKa) (mm–1
)
9.212
6567
7.525
6464
34.643
6247
Reflections collected
Independent reflections
Data/restraints/parameters
R(F)a for F2o42s(Fo2)a
Rw(F2o)b
360 [R(int) ¼ 0.0406]
360/0/24
0.0149
254 [R(int) ¼ 0.3069]
254/0/15
0.0404
360 [R(int) ¼ 0.0266]
360/0/24
0.0141
350 [R(int) ¼ 0.0520]
350/0/24
0.0304
0.0371
0.0881
0.0505
0.0677
P
P
aRðFÞ ¼ jjFoj ꢁ jFcjj= jFoj .
h
h
i.
i
1=2
ꢀ
ꢁ
P
P
2
bRwðF2oÞ ¼
w F2o ꢁ F2c
wF4o
.

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3124
of the monochromator at 514.5 nm is 0.5 cmꢁ1. The
monochromator is interfaced with a personal computer;
scanning and data collections are controlled by LabSpec
(version 3.04) software. Signal detection is acquired with a
water-cooled photo-multiplier tube (Hamamatsu R636).
The 528 nm wavelength was the principal argon laser line
used for excitation in these experiments. The reported
spectra are uncorrected for instrumental responses, but the
monochromator positions and wavenumber accuracies were
calibrated daily using the Hg line at 18312.7 cmꢁ1 (546.1 nm)
from an external source. Additional calibration was
conducted using the tri-azaphospha-adamantane ligand
(TPA), which exhibits several sharp and intense Raman
bands in the region of interest.
hydrothermal technique, a method that has also been used
recently in the preparation of other transition metal
cyanides [39–40]. This method is particularly attractive
for the americium systems, and single crystals of
Am[M(CN)2]3 ꢀ 3H2O (M=Ag(1), Au(2)) can be grown
with small quantities of reactants (e.g., 0.92 mg Am) in
sealed reaction tubes.
3.2. Crystal structures
Compounds 1–4 all exhibit the same structure, crystal-
lizing in the hexagonal space group, P63/mcm, and are
isostructural to the previously reported Eu[Ag(CN)2]3 ꢀ 3H2O
[26]. This structural type has also been reported for several
additional tris(dicyanoargentate)lanthanide trihydrates or
tris(dicyanoaurate)lanthanide trihydrates [38,41–42]. Due
to the structural similarities of the 1-4 compounds reported
here, for brevity, only the structure of Am[Ag(CN)2]3 ꢀ
3H2O (1) will be discussed in detail; some important
comparisons with the other structures will be addressed.
Table 1 contains the crystallographic data for these four
compounds, and selected bond lengths for compounds 1–4
are given in Tables 2 and 3.
Fig. 1 shows the coordination geometry around the
americium atom with the atomic labeling schemes. The
environment for the Am3+ ion in 1 consists of six N-bound
CNꢁ groups coordinated approximately end-on in a
trigonal prismatic arrangement. In addition, three water
molecules cap the rectangular faces of the prism. The
resultant tricapped trigonal prismatic coordination envir-
onment of the Am site is shown in Fig. 1. The overall
cyanide coordination is also shown at one site, but the
three-dimensional interaction is clearly evident in the
packing diagram shown in Fig. 2. The tricapped trigonal
prismatic coordination geometry around the Am3+ is a D3h
2.6. Emission spectroscopy
The 514.5 nm line was used for the excitation of the
americium complexes. The luminescence spectra were
collected using an Instrument SA optical system consisting
of a monochromator (model 1000 M) attached to CCD,
PMT, and IR detectors. Additional spectra of higher
resolution were collected using a HG2.S monochromator.
Data analyses were performed with Grams/32 software
(Galactic, version 5.1).
3. Results and discussion
3.1. Material syntheses
Given the limited quantities of americium available, the
reaction conditions were first optimized by formation of
Nd[M(CN)2]3 ꢀ 3H2O (M ¼ Ag(3), Au(4)) before the synth-
eses of Am[M(CN)2]3 ꢀ 3H2O (M ¼ Ag (1), Au (2)) were
attempted. These conditions are given in the experimental
section. Initial hydrothermal reactions between Nd(NO3)3
and KAg(CN)2 were attempted at 170 1C, but under these
conditions the dicyanoargentate anions decompose, result-
ing in the reduction of Ag+ and formation of silver metal.
As determined by single-crystal X-ray characterization,
Nd(CO3)(OH) [36] was also isolated under the above
conditions. However, high-quality crystals of compounds 3
or 4 are isolated as the sole crystalline products when
NdCl3 is reacted with KAg(CN)2 or KAu(CN)2 in a 1:3
ratio at 120 1C. When Nd(NO3)3 is reacted with KAu(CN)2
at 120 1C, Nd[Au(CN)2]3 ꢀ 3H2O is formed as well as an
unidentified yellow microcrystalline powder. Reaction
times from 16 h to 3 days have been tried and this period
was found adequate for the syntheses. Once the synthetic
procedures were optimized on the neodymium systems,
compounds 1 and 2 were prepared under similar reaction
conditions.
Table 2
˚
Selected bond distances (A) for Am[Ag(CN)2]3 ꢀ 3H2O (1), and
Nd[Ag(CN)2]3 ꢀ 3H2O (3)
Am[Ag(CN)2]3 ꢀ 3H2O
Nd[Ag(CN)2]3 ꢀ 3H2O
Am(1)–N(1) ꢃ 6
Am(1)–O(1) ꢃ 3
C(1)–N(1)
2.570(5)
2.496(7)
1.132(7)
2.072(6)
3.3603(5)
Nd(1)–N(1) x 6
Nd(1)–O(1) x 3
C(1)–N(1)
2.585(2)
2.478(3)
1.134(3)
2.059(3)
3.3521(2)
Ag(1)–C(1) ꢃ 2
Ag(1)–C(1) x 2
Ag(1)?Ag(1)0
Ag(1)?Ag(1)0
Table 3
˚
Selected bond distances (A) for Am[Au(CN)2]3 ꢀ 3H2O (2) and
Nd[Au(CN)2]3 ꢀ 3H2O (4)
Am[Au(CN)2]3 ꢀ 3H2O
Nd[Au(CN)2]3 ꢀ 3H2O
Previous reports have outlined the syntheses of similar
tris-(dicyanoargentate) lanthanide trihydrates or tris-
(dicyanoaurate) lanthanide trihydrates either by slow
evaporation [18,26,37] or by gel growth [38] methods.
Our synthetic approach for compounds 1–4 was to use the
Am(1)–N(1) ꢃ 6
Am(1)–O(1) ꢃ 3
Au(1)–C(1) ꢃ 2
C(1)–N(1)
2.60(3)
2.51(4)
Nd(1)–N(1) ꢃ 6
Nd(1)–O(1) ꢃ 3
Au(1)–C(1) ꢃ 2
C(1)–N(1)
2.598(7)
2.479(12)
1.984(10)
1.14(1)
2.06(4)
1.07(5)
3.3330(11)
Au(1)?Au(1)0
Au(1)?Au(1)0
3.3287(6)

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Fig. 1. An illustration of the tricapped trigonal prismatic coordination environment around the Am3+ centers in Am[Ag(CN)2]3 ꢀ H2O (1). 50%
displacement ellipsoids are shown.
Fig. 2. The packing diagram for Am[Ag(CN)2]3 ꢀ 3H2O (1) as viewed down the a-axis. Green Am, red ¼ O and blue ¼ nitrogen atoms of the cyanide
ligand.
site symmetry. The three O atoms of the water molecules
are coplanar with the Am atom, by symmetry. The Am–O
found in other dicyanoargentates [26,41]. This arrangement
is shown clearly in the packing diagram of Fig. 2.
˚
distance of 2.496(7) A is shorter than the Am–N distance of
˚
2.570 A. The average bond length between the Am atom
In the structure of 1, the [AmN6O3] polyhedra are
arranged in layers found in the crystallographic (ab) plane.
As shown in Fig. 2, these alternating layers of Ag atoms
and Am polyhedra are bridged with cyanide linkages
resulting in an overall three-dimensional framework for
Am[Ag(CN)2]3 ꢀ 3H2O (1). The silver atoms in 1 form a
and the coordinated water molecules is comparable to the
Am–O distance of 2.434(16) found for the coordinated
water molecule in (C7H5O3)3Am ꢀ H2O [43]. Our structure
˚
consists of one unique Am–N distance of 2.570(5) A, which
is within the range of Am–N distances found in other
˚
compounds. For comparison, an Am–N distance of 2.50 A
Kagome lattice, also found in the ab plane that separates
´
the layers of americium polyhedra. As shown in Fig. 3,
every Ag atom has four nearest Ag neighbors, with
is reported for AmN (americium nitride) [44], while longer
Am–N bond distances of 2.617(2) and 2.626(2) are found in
(C10N2H8)(C5NS2H10)3Am [45]. Each silver atom in
compound 1 is coordinated to the carbon atoms of two
cyanide anions, resulting in nearly linear Ag(CN)ꢁ2 units, as
˚
uniform Ag?Ag separations of 3.3603(5) A.
Upon replacing silver by gold, and/or Am3+ by Nd3+
the overall structural features remain unchanged. However,
subtle differences are evident that could affect some of the

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Z. Assefa et al. / Journal of Solid State Chemistry 180 (2007) 3121–3129
3126
Fig. 4. Raman spectrum of: (a) KAg(CN)2; (b) Nd[Ag(CN)2]3 ꢀ 3H2O; and
(c) Am[Ag(CN)2]3 ꢀ 3H2O.
Table 4
Assignment of Raman bands (cmꢁ1
)
Am[Au(CN)2]3
2176
Nd[Au(CN)2]3
Nd[Ag(CN)2]3
2160
Assignment
2166
2158
491
n(CN)
nCN)
n(AuC)
n(AuC)
n(AgC)
n(AgC)
Fig. 3. Drawing showing the Kagome lattice contained in the structure of
´
474
compounds 1–4. Au?Au or Ag?Ag interactions are indicated by the
dashed lines. Every Ag or Au atom is surrounded by four other silver or
gold atoms placed in a rectangular arrangement.
412
384
347
330
d(AuCN)
d(AgCN)
308 (sh)
289
274 (sh)
spectroscopic properties of the complexes. In 2, the
˚
Au?Au separation is slightly smaller (3.3330(11) A) than
226
183
152
120
79
Lattice modes
Lattice modes
Lattice modes
Lattice modes
n(Au–Au)
the Ag?Ag separation in 1. When compared with the Am
compounds, the M?M (M ¼ Ag, Au) distances are
smaller for the corresponding neodymium compounds.
For example, the Ag?Ag and Au?Au distances are
˚
3.3521(2) and 3.3287(6) A in the Nd compounds 3 and 4,
The small size of the americium samples precluded the collection of a
reliable data in the low-frequency region.
respectively. This contributes to the smaller unit cell
volumes observed in the gold compounds (ꢂ2.6%) as
compared with the silver compounds with the same f-
element species (i.e. Am or Nd). Although gold has a
higher Z than silver, the reduction in molecular volume can
be traced to the smaller covalent radii arising from the
larger ‘‘relativistic contraction’’ for gold [46,47]. Thus, the
bonding interactions reflect that, gold is more prone for
covalent interaction than silver [48].
similar ionic radii. However, the small size of the
Am[Ag(CN)2]3 ꢀ 3H2O crystals results in a weak Raman
signal and required a large number of data collection to
counter the background noise. The shift of the nCN to
higher energy for compound 3 is consistent with data
reported for most cyanometalates that show a blue-shifting
upon nitrile binding. This shift can be rationalized as due
to a substantial electron density withdrawal from the
antibonding N lone pair upon bridging with f-element ions.
Only metals such as Tl and Pb, that participate in back-
bonding to the CN group, are known to exhibit red-shifted
nCN symmetric stretching bands [50].
The Raman spectra of the gold dicyanide complexes are
shown in Fig. 5 and cover the nCN stretching region. The
spectrum of KAu(CN)2 shown in Fig. 5a has a single band
at 2162 cmꢁ1. In the neodymium complex the band splits
into two: a strong band at 2166 cmꢁ1 and a much weaker
one at ꢂ2158 cmꢁ1 in a ratio of 1:20, respectively. The
3.3. Raman studies
The Raman spectra for the silver dicyanide complexes
are shown in Fig. 4 in the nCN stretching region; the band
assignments are given in Table 4. As shown in Fig. 4a, the
CN symmetric stretch for KAg(CN)2 is observed at
2145 cmꢁ1, consistent with the previously reported value
for this compound [49]. Upon coordination with Nd3+, the
n
CN band blue-shifts by ꢂ15 cmꢁ1 and a single strong band
appears at 2160 cmꢁ1. No significant change is observed on
the nCN symmetric stretching band when neodymium is
replaced by the actinide, americium, due to their very

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Z. Assefa et al. / Journal of Solid State Chemistry 180 (2007) 3121–3129
3127
Fig. 5. Raman spectrum of: (a) KAu(CN)2; (b) Nd[Au(CN)2]3 ꢀ 3H2O;
and (c) Am[Au(CN)2]3 ꢀ 3H2O.
Fig. 6. Raman spectra of the compounds covering the low-energy
(50–550 cmꢁ1) vibrational region: (a) KAg(CN)2; (b) Nd[Ag(CN)2]3 ꢀ
3H2O; (c) KAu(CN)2; and (d) Nd[Au(CN)2]3 ꢀ 3H2O.
spectrum shown in Fig. 5c corresponds to the Am[Au(CN)2]3 ꢀ
3H2O system, where the nCN symmetrical stretching band is
significantly blue-shifted as compared to either KAu(CN)2
or Nd[Au(CN)2]3 ꢀ 3H2O. This band appears at 2176 cmꢁ1
.
The blue shift in the nCN stretch indicates bridging of gold
and americium, which results in a substantial electron
density removal from the cyanide group. When compared
with the silver systems, the nCN frequency appears at a
higher energy for the gold dicyanide complexes. The shift
to higher frequency is consistent with the structural data
where the carbon–nitrogen bond distance is shorter in the
gold dicyanide systems. In the Nd–Ag compound the CN
˚
bond distance is 1.134(3) A, with error of the C–N distance
of 1.14(1) found in the Nd–Au system. However, the CN
˚
bond distance is reduced from 1.132 to 1.07 A on going
from the Am–Ag to the Am–Au system, indicating bond
strengthening in the latter system.
Fig. 7. Emission spectrum of Am[Ag(CN)2]3 ꢀ 3H2O (1) obtained, using
the 514.5 nm argon-ion laser line as an excitation source.
In the low-energy region, the neodymium compounds
provide detailed and well-defined spectral profiles.
Although not clearly defined in the KAg(CN)2 spectrum
(Fig. 6a), the Nd[Ag(CN)2]3 ꢀ 3H2O system has a well-
resolved doublet (Fig. 6b) at 412 and 384 cmꢁ1, which is
assignable to the nAg–C stretch. A broad band centering at
289 cmꢁ1 is also observed with shoulders at 308 and
274 cmꢁ1, which is blue-shifted compared with that for
KAg(CN)2 system, (observed at 249 cmꢁ1). These bands
are assigned to the Ag–CN bending modes. In the
Nd[Au(CN)2]3 ꢀ 3H2O system, the Au–C stretching appears
(Fig. 6d) as a doublet at blue-shifted positions of 474 and
491 cmꢁ1, when compared with the silver compounds. The
blue shift is consistent with a stronger Au–C bond inferred
complexes, the sample sizes available for analysis combined
with the reduced signal intensity in the low-frequency
vibrational region limited our ability to collect as suitable
Raman data on the americium compounds. However, the
structural comparisons indicate that the M–C (M ¼ Ag,
Au) distances in the americium complexes are longer than
those of the neodymium systems and a red shift would be
anticipated in the nMC vibrational modes for the Am
compounds.
3.4. Photoluminescence studies
The photoluminescence spectrum of crystalline
Am[Ag(CN)2]3 ꢀ 3H2O is shown in Fig. 7. The 514.5 nm
laser line, which is essentially in resonance with the
˚
from the M–C distances (2.001 vs. 2.063 A in Au–C vs.
5L6 level, was used to excite the sample. As is evident
0
Ag–C, respectively). The Au–CN bending mode in
Nd[Au(CN)2]3 ꢀ 3H2O is also blue-shifted to 347 and
330 cmꢁ1 compared with the values for the Ag–CN modes
(Fig. 6b), and the Au–CN bend in KAu(CN)2 (Fig. 6c).
Additional Raman bands are observed in the lower-energy
region at 226, 183, 152, and 120 cmꢁ1; all bands are
assigned to lattice modes. The much stronger band at
79 cmꢁ1 is assignable to the Au–Au stretching [51].
5
from the absorption spectrum shown in Fig. 8, the L6 has
0
the largest absorbance of the f–f transitions within the
americium excited levels. Excitation to this level with the
514.5 nm laser line produces emission bands originating
from two different excited states. The broad weak band
5
7
centering at ꢂ16700 cmꢁ1 corresponds to the L6 - F1
0
0
transition [52,53].
The most intense band appears at 14,350 with a
broad shoulder 14,250 cmꢁ1. In this region, americium
Although, it would be desirable to compare the
lanthanide Raman data with those for the actinide

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Z. Assefa et al. / Journal of Solid State Chemistry 180 (2007) 3121–3129
3128
transition metals are interconnected through cyanide
bridging.
The isostructural neodymium compounds were prepared
for comparison with the americium compounds. All four of
these compounds studied crystallize in the same hexagonal
space group with only slight differences occurring in their
unit cell parameters. The coordination around the Am or
Nd consists of six CNꢁ groups coordinated through the N
atoms resulting in a trigonal prismatic environment. Three
oxygen atoms of coordinated water molecules complete the
tricapped trigonal prismatic coordination environment for
Am3+ providing a total coordination number of nine.
Raman spectroscopy, which compliments the structural
work, reveals that all of the compounds display strong
signals in the nCN stretching region. When compared with
KAg(CN)2 or KAu(CN)2, the nCN stretching frequency in
all the compounds blue-shift due to bridging and sub-
stantial electron density removal from the cyanide center.
When compared with the silver systems, the nCN stretching
frequency appears at higher energy in the gold dicyanide
complexes. The shift to higher frequency is consistent with
the structural data where the carbon–nitrogen bond
distance decreases in the gold dicyanide systems when
compared with the silver system.
Fig. 8. Solid-state absorption spectrum of a Am[Ag(CN)2]3 ꢀ 3H2O single
crystal. [Absorbance is in arbitrary units.]
luminescence spectar are known of overlapping emissions
originating from two different excited levels [52–59]. Based
on the established assignments, the higher energy of the
emission (with a maximum at 14350 cmꢁ1) corresponds to
5
7
0
the D1 - F1’ transitions [54–59], while the lower-energy
side mainly corresponds to contributions from the
5
7
0
0
L6 - F2 transitions. When compared with the americium
iodates and borosilicate matrices reported earlier [60–61],
the most intense band for the americium emission in the
Am[Ag(CN)2]3 ꢀ 3H2O system is red-shifted by 250 cmꢁ1
.
The relative intensities of the emissions from the two excited
states are temperature dependent. In the spectrum collected
at liquid nitrogen temperature, the lower-energy emission
Acknowledgments
Support for this work was provided by the Division of
Chemical Sciences, Geosciences and Biosciences, OBES,
USDOE, under Contract DE-AC05-00OR22725 with Oak
Ridge National Laboratory, managed by UT-Battelle,
LLC. The 243Am isotope used in these studies was supplied
by the USDOE through its production program at ORNL.
Dr. Radu Custelcean and Dr. Bruce Moyer at ORNL are
thanked for their generous allocation of X-ray diffract-
ometer time. The authors would also like to thank Travis
H. Bray at Auburn University for single-crystal X-ray data
sets collected on compounds 3 and 4 and Peter Khalifah at
the University of Massachusetts for measuring powder
X-ray diffraction patterns on compounds 3 and 4. ZA also
acknowledges the support from the National Oceanic and
Atmospheric Administration Educational Partnership Pro-
gram (NOAA-EPP) Award no. NA06OAR4810187 to
North Carolina A&T State University.
5
0
originating from the L6 state has comparable intensity to
that of the higher-energy band corresponding to the
5
7
0
0
D1 - F1 transition. The lower-energy band also shows a
well-resolved shoulder at ꢂ14,000 cmꢁ1. The relative
decrease of the emission intensity originating from the
0
5
lower excited state ( D1 ) is indicative of the reduced
5
thermal deactivation from the upper L6 level. The presence
0
of water molecules in the coordination sphere of the Am ion
is expected to bridge the gap and facilitate cascading of the
excitation to the lower excited level. Radiative transitions
from this emitting level to the various ground levels
enhances the emission originating from this state.
Several attempts to observe emission from the
Am[Au(CN)2]3 ꢀ 3H2O were not successful, although
Am3+ is one of the strong, spectral-emitting actinide ions.
The americium emission in the gold complex is quenched,
even at liquid nitrogen temperature, indicating the presence
of an intense, non-radiative mechanism responsible for the
dissipation of the excitation energy.
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