APdCu(Se2)(Se3) (A = K and Rb): New quaternary copper palladium polyselenides with unusual metal-selenium coordination

Article abstract of DOI:10.1021/ic0340130

Two novel metal polyselenides, KPdCu(Se₂)(Se₃) (I) and RbPdCu(Se₂)(Se₃) (II), have been synthesized from solvothermal reactions in superheated ethylenediamine at 160 °C. The isostructural compounds crystallize in the monoclinic space group P2₁/m, Z = 2, with a = 6.145(1) A, b = 7.268(1) A, c = 8.865(2) A, β = 102.41(3)° for I, and a = 6.253(1) A, b = 7.267(1) A, c = 8.993(2) A, β = 102.28(3)° for II. Their crystal structures are two-dimensional networks with _∞²[PdCu(Se₂) (Se₃)]^- anionic layers built from one-dimensional _∞¹[Pd(Se₂) (Se₃)]^2- chains that are stitched together by tetrahedrally coordinated Cu atoms. The DSC data show that I and II are stable up to 400 °C and decompose at ca. 436 and 424 °C, respectively. Both compounds are narrow band-gap semiconductors with estimated band gaps of about 0.7 eV (I) and 0.8 eV (II), respectively. They are the first structurally characterized quaternary copper palladium polychalcogenides with a (Se₂)^2- and a (Se₃)^2- fragment, both exhibiting interesting and unusual metal-selenium coordination.

Full text of DOI:10.1021/ic0340130

Inorg. Chem. 2003, 42, 3723−3727
APdCu(Se2)(Se3) (A ) K and Rb): New Quaternary Copper Palladium
Polyselenides with Unusual Metal−Selenium Coordination
Xuean Chen,† Kieran J. Dilks, Xiaoying Huang, and Jing Li*
Department of Chemistry and Chemical Biology, Rutgers UniVersity,
Piscataway, New Jersey 08854
Received January 8, 2003
Two novel metal polyselenides, KPdCu(Se )(Se3) (I) and RbPdCu(Se2)(Se3) (II), have been synthesized from
2
solvothermal reactions in superheated ethylenediamine at 160 °C. The isostructural compounds crystallize in the
monoclinic space group P21/m, Z ) 2, with a ) 6.145(1) Å, b ) 7.268(1) Å, c ) 8.865(2) Å, â ) 102.41(3)° for
I, and a ) 6.253(1) Å, b ) 7.267(1) Å, c ) 8.993(2) Å, â ) 102.28(3)° for II. Their crystal structures are
2
∞
1
∞
two-dimensional networks with [PdCu(Se2)(Se3)]- anionic layers built from one-dimensional [Pd(Se2)(Se3)]2-
“chains” that are “stitched” together by tetrahedrally coordinated Cu atoms. The DSC data show that I and II are
stable up to 400 °C and decompose at ca. 436 and 424 °C, respectively. Both compounds are narrow band-gap
semiconductors with estimated band gaps of about 0.7 eV (I) and 0.8 eV (II), respectively. They are the first
structurally characterized quaternary copper palladium polychalcogenides with a (Se2)2- and a (Se3)2- fragment,
both exhibiting interesting and unusual metal−selenium coordination.
Introduction
unexplored. In the system of A-M-M′-Q (A ) alkali
metal, M ) group I element, M′ ) group VIII metal, Q )
chalcogen), only several Fe compounds with the crystal
formula of AMFeQ2 (A ) Li, Na, K, and Cs; M ) Cu, Ag;
Q ) S, Se, Te) are known.6-10 These monochalcogenides
prepared by high-temperature solid state reactions or the
molten alkali metal polychalcogenide flux method crystallize
in two different space groups, P3hm1 (for Li and Na salts)
The Pd-containing compounds have attracted much atten-
tion due to the catalytic functions of the metal and its ability
to form polychalcogenide complexes in solution media. In
recent years, a number of ternary Pd polychalcogenides have
been isolated and structurally characterized. These include
(Ph4P)2[Pd(Se4)2] with discrete [Pd(Se4)2]2- anions in which
each Pd2+ is coordinated by two chelating (Se4)2- ligands;1
{(CH3)N(CH2CH2)3N}2[Pd(Se6)2] and (enH)2[Pd(Se5)2] fea-
turing sheetlike, two-dimensional (2D) Pd polyselenide
anionic frameworks;2 Rb2[Pd(Se4)2]‚Se8 containing sheet-
like polyanion [Pd(Se4)2]2- with “intercalated” crownlike
Se8 eight-membered rings;3 and K4[Pd(Se4)2][Pd(Se6)2]
()K2PdSe10) and Cs2[Pd(Se4)2] ()Cs2PdSe8 ) having three-
dimensional (3D) structures constructed from two interpen-
etrating [Pd(Sex)2]2- frameworks (x ) 4 and 6 for the K+
salt, x ) 4 and 4 for the Cs+ salt).4,5 In contrast, the
quaternary Pd transition metal chalcogenides are relatively
and I4/mmm (ThCr2Si2-type, for K and Cs salts), respectively.
2
They are all layered compounds containing [MFeQ2]-
∞
anionic layers of edge-sharing MQ4 and FeQ4 tetrahedra
separated by A+ counterions. In this paper, we reported the
preparation, crystal structures, and optical and thermal
properties of two novel compounds, APdCu(Se2)(Se3) (A )
K and Rb), which are the first quaternary copper palladium
polychalcogenides obtained by solvothermal methods using
ethylenediamine (en) as a reaction medium.
(5) Li, J.; Chen, Z.; Wang, R.-J.; Lu, J. Y. J. Solid State Chem. 1998,
140, 149.
* To whom correspondence should be addressed. E-mail: jingli@
rutchem.rutgers.edu.
(6) Llanos, J.; Contreras-Ortega, C.; Mujica, C.; von Schnering, H. G.;
Peters, K. Mater. Res. Bull. 1993, 28, 39-44.
(7) Mujica, C.; Paez, J.; Llanos, J. Mater. Res. Bull. 1994, 29, 263-
268.
† Current address: Department of Applied Chemistry, Hiroshima Uni-
versity, Higashi-Hiroshima 739-8527, Japan.
(1) Kra¨uter, G.; Dehnicke, K. Chem.-Ztg. 1990, 114, 7-9.
(2) Kim, K.-W.; Kanatzidis, M. G. J. Am. Chem. Soc. 1998, 120, 8124-
8135.
(8) Li, J.; Guo, H.-Y.; Yglesias, R. A.; Emge, T. J. Chem. Mater. 1995,
7, 599-601.
(9) Llanos, J.; Contreras-Ortega, C.; Paez, J.; Guzman, M.; Mujica, C. J.
Alloys Compd. 1993, 201, 103-104.
(3) Wachhold, M.; Kanatzidis, M. G. J. Am. Chem. Soc. 1999, 121, 4189-
4195.
(10) Llanos, J.; Valenzuela, P.; Mujica, C.; Buljan, A.; Ramirez, R. J. Solid
State Chem. 1996, 122, 31-35.
(4) Kim, K.-W.; Kanatzidis, M. G. J. Am. Chem. Soc. 1992, 114, 4878.
10.1021/ic0340130 CCC: $25.00 © 2003 American Chemical Society
Published on Web 05/10/2003
Inorganic Chemistry, Vol. 42, No. 12, 2003 3723

Chen et al.
Table 1. Crystallographic Data for KPdCu(Se2)(Se3) (I) and
RbPdCu(Se2)(Se3) (II)
Experimental Section
Materials. A2Se (A ) K, Rb) compounds were prepared by
reactions of alkali metal and elemental Se in a 2:1 ratio in liquid
ammonia. PdSe2 was prepared by stoichiometric reactions of
elements at 300 °C for 3 days. The other chemicals were used as
purchased without further treatment: CuCl (99%, Aldrich), Se
(99.5%, Strem Chemicals, Inc.), and ethylenediamine (99%,
anhydrous, Aldrich).
I
II
formula
fw
space group
a, Å
b, Å
c, Å
KPdCu(Se2)(Se3)
603.84
P21/m (No. 11)
6.145(1)
7.268(1)
8.865(2)
102.41(3)
386.7(1), 2
5.186
RbPdCu(Se2)(Se3)
650.21
P21/m (No. 11)
6.253(1)
7.267(1)
8.993(2)
102.28(3)
399.3(1), 2
5.408
â, deg
Synthesis. KPdCu(Se2)(Se3) (I) was prepared by weighing and
mixing 0.039 g (0.25 mmol) of K2Se, 0.066 g (0.25 mmol) of PdSe2,
0.025 g (0.25 mmol) of CuCl, and 0.059 g (0.75 mmol) of Se in a
glovebox under an inert atmosphere of argon. The mixture was
then transferred to a thick-walled Pyrex tube, and 0.37 mL of en
was added to it. After the liquid was condensed by liquid nitrogen,
the tube was sealed with a torch under a vacuum of ∼10-3 Torr.
The sample was placed in an oven and heated at 160 °C for one
week, and then cooled to room temperature. The dark-red, column-
like crystals in about 80% yield were isolated by washing the
reaction product with 20% and 80% ethanol followed by drying
with anhydrous diethyl ether. RbPdCu(Se2)(Se3) (II) was synthe-
sized from a solvothermal reaction of 0.062 g (0.25 mmol) of Rb2-
Se, 0.066 g (0.25 mmol) of PdSe2, 0.025 g (0.25 mmol) of CuCl,
and 0.059 g (0.75 mmol) of Se in en (0.37 mL). The same heating
and isolation procedures as previously described for I were used.
Dark-red, columnlike crystals of II were obtained in about 90%
yield. Both I and II appeared to be relatively stable in air and water.
Crystal Structure Determination. Intensity data of the title
compounds were collected at room temperature (293 ( 1 K) on an
Enraf-Nonius CAD4 automatic four-circle diffractometer with
graphite monochromated Mo KR radiation. Cell dimensions were
obtained from least-squares refinements with 20 (I) or 25 (II)
automatically centered reflections in the range 7.89° e θ e 12.36°
(I), 6.58° e θ e 13.59° (II). Three standard reflections were
remeasured after every 2 h. No decay was observed except the
statistic fluctuation in the ranges (2.5% (I) and (4.5% (II). Raw
intensities were corrected for Lorentz and polarization effects, and
for absorption by empirical method based on ψ-scan data. Direct
phase determination and subsequent difference Fourier map syn-
thesis yielded the positions of all atoms, all of which were eventually
subjected to the anisotropic refinements. An extinction correction
was applied to the calculated structure factors and refined to 0.0022-
(5) (I) and 0.0020(3) (II). For compound I, the final full-matrix
least-squares refinements on F2 led to R1 ) 0.0316 and wR2 )
0.0770 for 649 observed reflections (I > 2σ(I)) and 47 variables.
The reliability factors for compound II converged to R1 ) 0.0502
and wR2 ) 0.0841 for 529 observed reflections. The final difference
electron density maps showed no features in both cases. Details of
unit cell parameters, data collection, and structure refinements are
given in Table 1. All computations were performed using the
SHELX97 program package.11 Crystal structure drawings were
produced with SCHAKAL 92.12 Final atomic coordinates and
equivalent isotropic displacement parameters are listed in Table 2.
Thermal Analysis. Differential scanning calorimetry (DSC)
measurements were carried out on a computer-controlled TA
Instrument DSC-2920 analyzer. Powder samples of I (25.830 mg)
and II (19.580 mg) were sealed into two aluminum pans,
respectively. An approximately equal mass of sealed empty
aluminum pan was used as a reference. The samples were heated
V, Å3, Z
d
calcd, g/cm3
λ, Å
0.71073
29.037
0.71073
33.654
µ, mm-1
R indices [I g 2σ(I)],
R1b
0.0316
0.0770
0.0502
0.0841
wR2c
R indices (all data),
R1
wR2
0.0533
0.0850
0.1038
0.0961
∑[w(Fo2 - Fc2)2]
a R1 ) (∑||Fo| - |Fc||)/(∑|Fo|). b wR2 )
.
2
∑w(Fo )2
x
Table 2. Atomic Coordinates and Equivalent Isotropic Temperature
Factorsa (Å2) for KPdCu(Se2)(Se3) (I) and RbPdCu(Se2)(Se3) (II)
atom
x
y
z
Ueq
KPdCu(Se2)(Se3) (I)
Pd
Cu
0.5
0
0.25
0.25
0.25
-0.25
-0.0139(1)
0.25
0.5
0.013(1)
0.021(1)
0.013(1)
0.020(1)
0.015(1)
0.014(1)
0.029(1)
0.1188(3)
0.7285(2)
0.7201(2)
0.0389(2)
0.3155(1)
0.2612(5)
0.6075(2)
0.6367(1)
0.8991(2)
0.6662(1)
0.7198(1)
1.0455(4)
Se(1)
Se(2)
Se(3)
Se(4)
K
RbPdCu(Se2)(Se3) (II)
Pd
Cu
0.5
0
0.25
0.25
0.25
-0.25
-0.0126(3)
0.25
0.5
0.016(1)
0.021(1)
0.014(1)
0.020(1)
0.016(1)
0.015(1)
0.025(1)
0.1108(5)
0.7240(3)
0.7088(4)
0.0432(4)
0.3115(2)
0.2553(4)
0.6099(4)
0.6359(3)
0.8933(3)
0.6597(3)
0.7129(2)
1.0493(3)
Se(1)
Se(2)
Se(3)
Se(4)
Rb
a Ueq is defined as one-third of the trace of the orthogonalized U tensor.
at a rate of 5 °C/min from room temperature to 550 °C under
nitrogen and then cooled under nitrogen gas current. The residues
were examined by powder X-ray diffraction immediately after the
DSC experiments.
Diffuse Reflectance Measurements. Optical diffuse reflectance
spectra were measured at room temperature with a Shimadzu UV-
3101PC double-beam, double-monochromator spectrophotometer.
Data were collected in the wavelength range 250-2000 nm. BaSO4
powder was used as a standard (100% reflectance). A similar
procedure as previously described13 was used to collect and convert
the data using the Kubelka-Munk function.14 The scattering
coefficient (S) was treated as a constant since the average particle
size of the samples used in the measurements was significantly
larger than 5 µm.
Results and Discussion
Structure Description. Since I and II are isostructural,
the discussion will refer mainly to I. KPdCu(Se2)(Se3) (I)
(11) Sheldrick, G. M. SHELX-97: program for structure refinement;
University of Goettingen: Goettingen, Germany, 1997.
(12) Keller, E. SCHAKAL 92: a computer program for graphical
representation of crystallographic models; University of Freiburg:
Freiburg, Germany, 1992.
(13) Li, J.; Chen, Z.; Wang, X.-X.; Proserpio, D. M. J. Alloys Compd.
1997, 262-263, 28-33.
(14) Wendlandt, W. WM.; Hecht, H. G. Reflectance Spectroscopy; Inter-
science: A Division of John Wiley & Sons: New York, 1966.
3724 Inorganic Chemistry, Vol. 42, No. 12, 2003

New Quaternary Cu Pd Polyselenides
Figure 1. View of the KPdCu(Se2)(Se3) (I) structure approximately along
the b-axis. Dashed lines show the coordination environment of the K+
cations.
Table 3. Selected Bond Lengths (Å) and Angles (deg) for
KPdCu(Se2)(Se3) (I) and RbPdCu(Se2)(Se3) (II)
KPdCu(Se2)(Se3) (I)
1
∞
Figure 2. (A) Fragment of the undulating 1D [Pd(Se2)(Se3)]2- “rib-
Pd-Se(1)
Pd-Se(4)
Cu-Se(4)
Cu-Se(3)
Cu-Se(1)
Se(1)-Se(2)
Se(3)-Se(4)
K-Se(2)
2.4515(9) ×2
2.458(1) ×2
2.369(1) ×2
2.411(2)
Se(1)-Pd-Se(4)
Se(1)-Pd-Se(4)
Se(1)-Pd-Se(1)
Se(4)-Pd-Se(4)
Se(3)-Cu-Se(1)
Se(4)-Cu-Se(4)
Se(4)-Cu-Se(1)
Se(4)-Cu-Se(3)
Se(4)-Se(3)-Se(4)
Se(3)-Se(4)-Pd
Se(2)-Se(1)-Pd
Pd-Se(1)-Pd
87.20(4) ×2
92.80(4) ×2
bons” found in KPdCu(Se2)(Se3) (I). The Se‚‚‚Se nonbonding interactions
(less than 3.31 Å) are denoted by dashed lines. (B) Perspective view of the
180
180
85.19(7)
2
∞
2D [PdCu(Se2)(Se3)]- anionic layer along the [001] direction. Both Pd-
Se and Se-Se bonds are drawn with thick solid lines, while the Cu-Se
2.467(2)
2.338(2)
bonds are represent by thin solid lines.
108.08(8)
2.390(1) ×2
112.06(6) ×2
118.77(5) ×2
91.78(6)
Two neighboring Pd2+ centers are doubly bridged by one
(Se2)2- and one (Se3)2- ligands to form the undulating 1D
1∞[Pd(Se2)(Se3)]2- “ribbons”, in which the bridging Se1
atoms of (Se2)2- units and the central Se3 atoms of the
(Se3)2- units are positioned at the peaks of the wave with
the unshared terminal Se2 atoms of (Se2)2- units pointing
away from the curve. The “ribbons” are closely related to
those found in the one-dimensional chain compound BaPd-
S2 and can be viewed as substitutional variants of the
1∞[PdS2]2- type in which (Se2)2- and (Se3)2- occupy the S2-
sites.15 These “ribbons” are propagated by a crystallographic
screw axis resulting in the Se2 atoms appearing on opposite
sides of the Se-Pd-Se linkage, as depicted in Figure 2A.
Interestingly, the central Se3 atoms of the triselenide and
the terminal Se2 atoms of the diselenide units are roughly
coplanar with the central PdSe4 coordination plane, their
deviations from this plane being 0.014(2) and 0.694(2) Å,
respectively. As a consequence, short Se-Se nonbonding
contacts are formed between Se1 and Se3 (3.302(2) Å) as
well as Se2 and Se4 atoms (3.269(2) Å) (see dashed lines in
Figure 2A). The values are definitely less than the sum (3.60
Å) of the Se‚‚‚Se van der Waals radii and may be indicative
of secondary bonding interactions. Similar weak interactions
are also found in Rb2[Pd(Se4)2]‚Se8, as revealed by the Se‚‚‚
Se distances of 3.287-3.465 Å.3 The origin of this weak
interaction is not clear but is probably associated with the
well-known catenation capacity of the chalcogens. The Se-
3.299(3)
3.349(4)
K-Se(2)
K-Se(4)
3.423(3) ×2
107.53(5)
K-Se(3)
3.458(4)
111.05(4) ×2
K-Se(4)
3.541(3) ×2
3.6656(7) ×2
95.67(5)
K-Se(2)
RbPdCu(Se2)(Se3) (II)
Pd-Se(4)
Pd-Se(1)
Cu-Se(4)
Cu-Se(3)
Cu-Se(1)
Se(1)-Se(2)
Se(3)-Se(4)
Rb-Se(2)
Rb-Se(2)
Rb-Se(3)
Rb-Se(4)
Rb-Se(4)
Rb-Se(2)
2.454(2) ×2
2.455(2) ×2
2.362(2) ×2
2.411(4)
Se(4)-Pd-Se(1)
Se(4)-Pd-Se(1)
Se(1)-Pd-Se(1)
Se(4)-Pd-Se(4)
Se(3)-Cu-Se(1)
Se(4)-Cu-Se(4)
Se(4)-Cu-Se(1)
Se(4)-Cu-Se(3)
Se(4)-Se(3)-Se(4)
Se(3)-Se(4)-Pd
Se(2)-Se(1)-Pd
Pd-Se(1)-Pd
87.55(7) ×2
92.45(7) ×2
180
180
84.6(1)
2.479(3)
2.337(4)
107.8(1)
2.383(2) ×2
114.2(1) ×2
117.4(1) ×2
92.8(1)
107.74(9)
110.73(8) ×2
95.46(8)
3.407(3)
3.420(3)
3.524(4)
3.525(3) ×2
3.656(3) ×2
3.6702(7) ×2
2
∞
consists of 2D [PdCu(Se2)(Se3)]- anionic layers separated
by K+ cations (Figure 1). Each Pd2+ center is coordinated
by two Se1 atoms from two (Se2)2- and two Se4 atoms from
two (Se3)2- fragments in a slightly distorted square-planar
geometry, typical for d8-ions. Because the Pd atom resides
on an inversion center, the (Se2)2-and (Se3)2- pairs are
arranged in a trans conformation. The Pd-Se distances of
2.4515(9)-2.458(1) Å (Table 3) compare well with those
found in other polyselenides, e.g., K2PdSe10 (average 2.462-
(5) Å),4 Rb2[Pd(Se4)2]‚Se8 (2.4339(6) Å),3 and Cs2PdSe8
(2.428(1) Å).5 The Se-Pd-Se angles are 87.20(4)-
92.80(4)° for cis and 180° for trans Se atoms, respectively.
(15) Huster, J.; Bronger, W. J. Less-Common Met. 1986, 119, 159-165.
Inorganic Chemistry, Vol. 42, No. 12, 2003 3725

Chen et al.
Se bond distances in the diselenide units (2.338(2) Å) are
slightly shorter than those in the triselenide units (2.390(1)
16
Å), and both are close to those of K2Se3 and K3AuSe13,17
in good agreement with the Se-Se covalent bond distance,
2.3 Å.18 The Se-Se-Se angles of 91.78(6)° are smaller than
the Pd-Se-Se angles (107.53(5)-111.05(4)°), which fol-
lows the general trend observed in other known Pd
polyselenides.3-5
1
The [Pd(Se2)(Se3)]2- “chains” run parallel to the crys-
∞
tallographic b-axis and stack in phase along the [100]
direction. They are “stitched” together by tetrahedrally
coordinated Cu atoms leading to the formation of the 2D
2∞[PdCu(Se2)(Se3)]- anionic layers. Each Cu atom is bonded
at two points to Se4 atoms of two (Se3)2- species from one
1∞[Pd(Se2)(Se3)]2- “chain”, and at the remaining sites to one
bridging Se1 atom of (Se2)2- and one central Se3 atom of
(Se3)2- from the neighboring “chain”. This connection mode
causes the adjacent PdSe4 coordination planes to form an
angle close to the tetrahedral value thereby giving rise to a
Figure 3. Differential scanning calorimetry (DSC) curves of KPdCu(Se2)-
(Se3) (I) and RbPdCu(Se2)(Se3) (II), respectively.
“CsPdCu(Se2)(Se3)” have so far been unsuccessful, possibly
because Cs+ is too large to stabilize this structure type.
It is noted that both (Se2)2- and (Se3)2- polyanions found
in the title compounds behave in a rather unusual manner.
Each (Se2)2- functions as a µ3-bridging ligand bonded to two
Pd atoms and one Cu atom via a single Se1 atom, with
another Se of the diselenide unit (Se2) kept noninteracting,
while each (Se3)2- is 5-fold coordinate to two Cu atoms and
two Pd atoms via its two terminal Se4 atoms and to the other
unusual zigzag structural arrangement of the 1 [Pd(Se2)-
∞
(Se3)]2- “chain”. While the CuSe4 tetrahedra and PdSe4
square planes share common corners, the CuSe4 tetrahedra
themselves are linked together by joining the three Se corners
to form four Se-Se bonds (Se3-Se4 bonds) with neighbor-
ing tetrahedra and the fourth corner occupied by a (Se2)2-
group. The distance of 4.216(2) Å between adjacent Cu
centers is considerably longer than that in elemental Cu (2.56
Å), eliminating any d10‚‚‚d10 Cu(I) interactions. The tetra-
hedral environments around Cu atoms are severely distorted,
with Cu-Se distances divided into two sets (2.369(1) × 2,
2.411(2), 2.467(2) Å). A set of two longest Cu-Se contacts
has the smallest bond angle of 85.19(7)°, while other Se-
Cu-Se angles are between 108.08(8)° and 118.77(5)°. A
similar, highly distorted copper coordination geometry has
also been observed in Cs2Cu2Sb2Se5 (Cu-Se distances 2.410-
(3)-2.637(3) Å; Se-Cu-Se angles 96.29(8)-128.2(1)°).19
As can be seen from Figure 1, the potassium cations are
located between the anionic layers to balance the charge and
stabilize the dangling end of the (Se2)2- ligands. Each K+ is
coordinated to nine Se atoms, five of which are from the
top layer, and the others are from the bottom layer. They
are arranged in a monocapped square-antiprismatic geometry
with the K-Se distances of 3.299(3)-3.6656(7) Å, which
are in the same range as reported for the nine-coordinated
2-
Cu via its central Se3 atom. Chelating and bridging Qn
ligands (Q ) S, Se, Te), that are bound to metal centers
through their terminal Q atoms, are common in polychal-
cogenide chemistry, while the tridentate (Se2)2- dangling
bond and the direct coordination of the central Se atom of
the (Se3)2- unit to a metal are quite rare. Limited examples
of tellurides with bridging µ2-(Te2)2 ligands in a dangling
mode are K4Hf3Te1721 and [K-2,2,2-crypt]2[Mo4(Te2)5(Te3)2-
(en)4].22 There are no corresponding examples mentioned in
the literature for the analogous (Se2)2- ligands, but several
monodentate dangling ligands such as η1-(Se3)2- and η1-
(Se5)2- were previously found in Na3AuSe823 and K3AuSe13,17
respectively. The unusual 5-fold coordinated µ5-(Se3)2- unit
has been encountered in the structure of CuAu(Se3)Se whose
crystals were grown from the molten K2Sex fluxes at 310
°C.24
Physical Properties. The DSC data of the title compounds
(Figure 3) showed a single strong endotherm upon heating,
with the peak position centered at ca. 436 °C for I, and ca.
424 °C for II, respectively. Examination of the DSC residue
by powder XRD revealed that both compounds decomposed
to give similar products: PdSe2, Se, and an unknown
amorphous material.
K+ in K12Mo12Se56 (3.305-3.825 Å).20 The 2 [PdCu(Se2)-
∞
(Se3)]- anionic framework is not subject to a cation effect
as one moves from the smaller K+ to the larger Rb+ cation.
This may arise from the fact that the size difference between
K+ and Rb+ is not large enough to overcome the strong
preference of the Pd2+ cations for square plane coordination.
Attempts to prepare the isostructural cesium analogue
The title compounds are electron precise and are predicted
to be semiconductors. The optical properties were examined
by analyzing their diffuse reflectance data. The Kubelka-
Munk functions14 for I and II were converted from the
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(17) Park, Y.; Kanatzidis, M. G. Angew. Chem. 1990, 102, 945.
(18) Pauling, L. The Nature of the Chemical Bond; Cornell University
Press: Ithaca, New York, 1960.
(19) Chen, Z.; Wang, R.-J.; Dilks, K. J.; Li, J. J. Solid State Chem. 1999,
147, 132-139.
(20) Liao, J. H.; Kanatzidis, M. G. J. Am. Chem. Soc. 1990, 112, 7400-
7402.
(21) Keane, P. M.; Ibers, J. A. Inorg. Chem. 1991, 30, 1327.
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265.
3726 Inorganic Chemistry, Vol. 42, No. 12, 2003

New Quaternary Cu Pd Polyselenides
So far, solvothermal reactions have produced many binary
and ternary polychalcogenides containing long (Qn)2- (n g
2) fragments;25 however, the number of known quaternary
polychalcogenides prepared via this route is very limited.
APdCu(Se2)(Se3) (A ) K, Rb) represents a new structure
type and the first example of a quaternary copper palladium
polychalcogenide with a (Se2)2- and a (Se3)2- fragment, both
exhibiting interesting and unusual bond modes. The suc-
cessful syntheses of the title compounds confirmed that the
mild solvothermal conditions stabilize the polychalcogen
building blocks and allow (Se2)2- and (Se3)2- ions to
incorporate and remain intact in the final structure. It is
believed that the synthetic route is still promising for the
preparation of more metal polychalcogenides with unusual
structures and properties.
Figure 4. Optical absorption spectra of KPdCu(Se2)(Se3) (I) and RbPd-
Cu(Se2)(Se3) (II).
Acknowledgment. The authors would like to thank the
National Science Foundation for its generous support through
Grant DMR-0094872.
diffuse reflectance data (F ) (1 - R∞)2/2R∞, where R∞ is
the relative diffuse reflectance of an infinitely thick layer.
For practical purposes, this can be achieved at a layer depth
of a few millimeters). Plotted in Figure 4 are remission
functions versus the wavelength. The spectra of both
compounds display steep absorption edges, from which an
optical band gap of 0.7 eV is estimated for KPdCu(Se2)-
(Se3) (I), while in the case of RbPdCu(Se2)(Se3) (II) this
band gap is shifted to higher energy by about 0.1 eV. The
band gaps are significantly smaller than those observed for
other 2D and 3D ternary Pd polyselenides (range 1.4-1.6
eV), indicating both compounds to be narrow-gap semicon-
ductors.
Supporting Information Available: ORTEP plots (2) showing
atoms in asymmetric unit as 50% thermal ellipsoids. Tables of
atomic coordinates of all atoms, isotropic and anisotropic thermal
parameters, bond distances and angles, crystallographic data in CIF
format. This material is available free of charge via the Internet at
http://pubs.acs.org.
IC0340130
(25) Sheldrick, W. S.; Wachhold, M. Angew. Chem., Int. Ed. Engl. 1997,
36, 206-224.
Inorganic Chemistry, Vol. 42, No. 12, 2003 3727