Ligand-based modification of the structures and optical properties of new silver(I)-rhenate(VII) oxide/organic hybrid solids

Article abstract of DOI:10.1021/ic901749r

A new series of silver(I)-rhenate(VII) hybrids was systematically prepared under hydrothermal conditions from eight different N-donor organic ligands (isonicotinate=inca, pyrazine-2-carboxylate=pzc, 1,2,4-triazole=tro, pyridazine=pda, 4,4′-bipyridine=bpy,

Full text of DOI:10.1021/ic901749r

Inorg. Chem. 2009, 48, 11265–11276 11265
DOI: 10.1021/ic901749r
Ligand-Based Modification of the Structures and Optical Properties of New
Silver(I)-Rhenate(VII) Oxide/Organic Hybrid Solids
Haisheng Lin, Xiaomeng Wu, and Paul A. Maggard*
Department of Chemistry, North Carolina State University, Raleigh North Carolina 27695-8204
Received September 3, 2009
A new series of silver(I)-rhenate(VII) hybrids was systematically prepared under hydrothermal conditions from eight
different N-donor organic ligands (isonicotinate=inca, pyrazine-2-carboxylate=pzc, 1,2,4-triazole=tro, pyridazine=pda,
4,40-bipyridine=bpy, 1,2-bis(4-pyridyl)-ethane=dpa, 2,3-bis(2-pyridyl)pyrazine=bpp, and tetra-2-pyridinylpyrazine =
tpp), and their resulting structures and optical properties were investigated. The reactions targeted a 1:1 molar ratio of
Ag/Re, and new hybrid solids were prepared with the compositions Ag(bpp)ReO4 (1), Ag(tpp)ReO4 H2O (2),
Ag(Hinca)2ReO4 H2O (3), Ag(tro)ReO4 (4), Ag(pda)ReO4 1/2H2O (5), Ag(Hpzc)ReO4 (6), Ag2(Hpzc)(pzc)-
3
3
3
(H2O)ReO4 (7), Ag(bpy)ReO4 (8), and Ag(dpa)2ReO4 (9). Hybrid solids 1, 2, and 3 each exhibit low-dimensional
structures, consisting of [Ag2(bpp)4]2þ and [Ag2(Hinca)4]2þ dimers in 1 and 3, respectively, and [Ag(tpp)]nnþ chains
in 2. Hybrid solids 4 and 5 contain a [Ag(tro)]þ chain and a [Ag3(pda)3]3þ cyclic trimer, respectively, that are both
ReO4-bridged into layered structures. Both 6 and 8 consist of ligand-pillared “AgReO4” layers, while 7 is a Re-deficient
analogue of 6 that contains ligand-pillared [Ag2(H2O)ReO4]þ layers where H2O replaces the missing ReO4- anion.
The hybrid networks of 8 and 9 are interpenetrating, owing to the length of the bpy and dpa ligands, and consist of bpy-
pillared “AgReO4” layers and ReO4-filled [Ag(dpa)2]þ diamond-type networks that are 2-fold and 6-fold interpenetrat-
ing, respectively. Their optical properties and thermal stabilities were investigated using UV-vis transmittance, X-ray
photoelectron spectroscopy, and thermogravimetric analysis. The measured properties were analyzed with respect to
the varying structural modifications. The Ag-ReO4 network dimensionalities, Ag coordination environments, and the
ligand lengths and geometries are found to play important roles in the absorption coefficients, bandgap sizes, and
whether the structure collapses softly to give condensed AgReO4, respectively.
Introduction
Charge Transfer (MMCT) excitation between the electron-
donating d10 and electron-accepting d0 configurations,2 the
bandgap sizes of these solids occur ∼0.5-1.5 eV lower than
that of the alkali-metal analogues which have only a d0 metal
cation (e.g., AgNbO3 versus NaNbO3). For example, AgN-
The optical properties of heterometallic oxides that include
both an early and a late transition metal have attracted
growing interest owing to their relatively small bandgap sizes
and potential uses in efficiently harvesting solar energy.1 In
particular, metal-oxides containing a d10 electron configura-
tion (e.g., Cuþ, Agþ) in combination with a d0 transition-
metal cation (e.g., Re7þ, W6þ, Nb5þ) have recently been
investigated for their optical properties and for use as
photocatalytic materials.2-6 Owing to a Metal-to-Metal
bO3,2a Ag2MoO4,2b AgVO3, and Ag3VO4 exhibit band-
2c
edge absorption in the visible part of the spectrum and have
been investigated for use as visible-light photocatalysts for
the production of H2/O2 from water under visible light.
However, their synthetic preparation has proceeded primar-
ily by conventional solid-state methods. There is therefore a
severely restricted ability to investigate new structural or
particle modifications to more deeply understand and tune
their optical properties.
Solution-based hydrothermal synthetic techniques have
enabled the incorporation of varying N-donor ligands as
flexible structural components within metal-oxide frame-
works, that is, as metal-oxide/organic hybrid solids.7 This
synthetic approach has yielded an immense structural diversity
and a growing ability to tune physical properties expressed in
*To whom correspondence should be addressed. E-mail: paul_maggard@
ncsu.edu. Phone: (þ1) 919-515-3616. Fax: (þ1) 919-515-5079.
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2009, 38, 253.
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2009 American Chemical Society
Published on Web 11/03/2009
pubs.acs.org/IC

11266 Inorganic Chemistry, Vol. 48, No. 23, 2009
Lin et al.
metal oxides.8-14 The ligands function in a variety of roles to
direct the structural arrangement, including as (1) charge-
compensating cations, (2) bridging or terminating ligands
coordinated to the metal sites, and (3) altering the local
coordination environments. Thus, these methods can be used
to alter a metal-oxide network and can serve as a gateway to
more deeply probing the properties of heterometallic oxides.
However, even with the identification of the “building
blocks” in solution, there still exists substantial challenges
in predicting their structures and resultant properties owing
to the numerous possible arrangements of three-dimensional
structures. While many relatively small advances have been
reported, it is rare when a large and diverse set of ligands is
utilized to unravel the effects of synthetic conditions, coordi-
nation preferences, and ligand geometries.15
Scheme 1. A Series of Selected N-Donor Organic Ligands Used in
the Synthesis of New Silver(I)-Rhenate(VII) Hybrid Solids and Their
Different Coordination Modes to Ag
Our current research efforts have significantly expanded
the known structural diversity of hybrid solids containing a
combination of d0 with d10 transition metals,16-22 and which
we have used to modify their bandgap sizes and photocata-
lytic reactivities. Recent research shows that, for example,
“MReO4” (M = Agþ or Cuþ) layers derived from parent
MReO4 phases can be pillared by bridging or metal-coordi-
nated ligands, leading to new hybrid metal-oxide/organic
structures including M(pyz)ReO4 (M = Cu, Ag),16,17 Cu-
(pzc)2(H2O)2ReO4,18 M(pzc)2(H2O)2AgReO4 (M=Co, Ni),19
and Cu(pzc)2AgReO4.20 The latter two examples can rever-
sibly absorb interlayer water and consist of a chiral network,
respectively. Hybrid solids in these systems can also undergo
subsequent ligand-mediated structural transformations, such
have been primarily based on only two or three different
ligands.
as for Cu(bpy)ReO4 and Cu(bpy)2ReO4 1/2H2O,21 and that
3
are accompanied by a significant modulation of their band-
gap sizes. Further, some of the first reported to exhibit
photocatalytic activity were recently found in the related
Herein, a diverse and systematic investigation of Ag-
(I)-Re(VII) hybrid structures (at a Ag/Re molar ratio of
1:1) was carried out using a series of eight different ligands,
shown in Scheme 1, with varying lengths and geometric
arrangement of N-donor atoms. The underlying emphasis
is to more deeply probe the structural origins of their charge-
transfer absorptions, modulation of bandgap sizes, as well
as optical absorption coefficients. The hybrids were charac-
terized structurally via X-ray diffraction, and their properties
investigated using UV-vis transmittance, X-ray photoelec-
tron spectroscopy, and thermogravimetric analysis. The
nine new hybrids were analyzed for the effect of the ligand
on both their structures and their properties, in order to
reveal the roles of the Ag coordination environments,
Ag-ReO4 network dimensionalities, and ligand lengths
and geometries.
hybrid vanadates [Ag(L)]4V4O12 xH2O (L=bpy, x=2; L=
3
dpa, x=4) and Ag4(pzc)2V2O6.22 However, these investigations
(8) Cheetham, A. M. Science 1994, 264, 794.
€
(9) Pope, M. T.; Muller, A. Angew. Chem., Int. Ed. Engl. 1991, 30, 34.
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Chem. 2003, 42, 7342. (b) M€uller, A.; Reuter, H.; Dillinger, S. Angew. Chem.,
Int. Ed. Engl. 1995, 34, 2328.
(11) (a) Yaghi, O. M. Nature 1999, 402, 276. (b) Yaghi, O. M. J. Am. Chem.
Soc. 1998, 120, 8571.
(12) Yan, B.; Maggard P. A. Inorganic Chemistry in Focus III; Meyer, G.,
Naumann, D., Weseman, L., Eds.; Wiley-VCH: Weinheim, 2006; Vol. 17,
pp 251-266.
(13) (a) Zheng, L.; Wang, X.; Wang, Y.; Jacobson, A. J. J. Mater. Chem.
2001, 11, 1100. (b) Zhang, C.; Liu, S.; Xie, L.; Gao, B; Sun, C.; Li, D. J. Mol.
Struct. 2005, 753, 40.
(14) (a) Hagrman, D. E.; Zubieta, J. J. Solid State Chem. 2000, 152, 141.
(b) LaDuca, R. L.; Finn, R.; Zubieta, J. Chem. Commun. 1999, 1669.
(c) Hagrman, P. J.; Finn, R. C.; Zubieta, J. Solid State Sci. 2001, 3, 745.
(15) (a) Eddaoudi, M.; Kim, J.; Wachter, J. B.; Chae, H. K.; O’Keeffe, M.;
Yaghi, O. M. J. Am. Chem. Soc. 2001, 123, 4368. (b) Sun, D.; Cao, R.; Liang,
Y.; Shi, Q.; Hong, M. J. Chem. Soc., Dalton Trans. 2002, 1847. (c) Kitagawa, S.;
Kitaura, R.; Noro, S. Angew. Chem., Int. Ed. 2004, 43, 2334. (d) Zeng, M. H.;
Zhang, W. X.; Sun, X. Z.; Chen, X. M. Angew. Chem., Int. Ed. 2005, 44, 3079.
(e) Liu, G.-X.; Huang, Y.-Q.; Chu, Q.; Okamura, T.-A.; Sun, W.-Y.; Liang, H.;
Ueyama, N. Cryst. Growth Des. 2008, 8(9), 3233.
(16) Lin, H.; Maggard, P. A. Inorg. Chem. 2007, 46, 1283.
(17) Lin, H.; Yan, B.; Boyle, P. D.; Maggard, P. A. J. Solid State Chem.
2006, 179, 37.
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2004, 43, 5537.
Experimental Section
General Procedures. All starting materials were purchased
commercially and used without further purification. Hydro-
thermal conditions were used to synthesize each hybrid solid,
which involved heat sealing the starting materials and solvent
inside an FEP Teflon pouch (300 ꢀ 400). A reagent amount of
deionized water was used as the solvent in each of the reactions.
The pouch was then placed inside a 125 mL Teflon-lined
stainless steel reaction vessel that was backfilled with ∼40 mL
(∼33%) of deionized water before closing. After holding the
reaction at a fixed temperature (120 °C for 3 days for hybrids
1-7 and 9; 140 °C for 3 days for 8), it was then slowly cooled to
room temperature at a rate of 6 °C h-1. The resulting products
were filtered, washed with deionized water, and dried and
weighed in air. The phase purity of each hybrid was >95%
according to powder X-ray diffraction data (see Supporting
Information, Figure 1S).
(19) Maggard, P. A.; Yan, B.; Luo, J. Angew. Chem., Int. Ed. 2004, 44,
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Article
Inorganic Chemistry, Vol. 48, No. 23, 2009 11267
Table 1. Single Crystal Data and Structure Refinement Details for Hybrid Solids 1-9
1
2
3
4
formula
C28H20AgN8O4Re
triclinic
P1, 1
296(2)
9.7798(5)
10.9114(5)
14.0695(10)
102.678(3)
105.191(3)
104.828(2)
1332.96(13)
2.059
C24H18AgN6O5Re
triclinic
P1, 2
C12H12AgN2O9Re
monoclinic
P21/c, 4
223(2)
7.3679(7)
26.197(2)
9.0160(9)
90.0
105.664(5)
90.0
1675.6(3)
2.467
8.436
C4H6Ag2N6O8Re2
triclinic
P1, 2
crystal system
space group, Z
temperature, K
296(2)
296(2)
˚
a, A
8.3697(3)
11.6210(4)
13.5757(5)
101.646(2)
103.515(2)
106.524(2)
1178.74(7)
2.154
6.011
53808
6721
0.027, 0.059
6.3515(3)
7.7186(3)
14.2016(7)
89.449(2)
81.686(2)
89.229(2)
688.83(5)
4.119
20.367
26117
4568
0.025,0.050
˚
b, A
˚
c, A
R, deg
β, deg
γ, deg
3
˚
V, A
F, g/cm
μ, mm-1
5.324
20900
5195
0.040,0.077
total reflections
unique reflections
R1, wR2 [I > 2σ(I)]a
92852
5506
0.037,0.044
5
6
7
8
9
formula
C
24H30Ag6N12O27Re6
C5H4AgN2O6Re
monoclinic
P21/n, 4
223(2)
5.7889(3)
12.2433(6)
13.6640(6)
90.0
92.2874(18)
90.0
967.67(8)
3.310
14.53
C10H8Ag2N4O9Re
orthorhombic
Pca21, 4
173(2)
22.4400(7)
5.2048(2)
13.8475(4)
90.0
90.0
90.0
1617.33(9)
2.999
9.920
C10H8AgN2O4Re
C24H24AgN4O4Re
orthorhombic
Pnnn, 2
crystal system
space group, Z
temperature, K
monoclinic
C2/c, 4
296(2)
25.2811(9)
15.6042(5)
15.2652(5)
90.0
117.421(2)
90.0
5345.4(3)
3.334
15.757
orthorhombic
Pbca, 8
173(2)
11.3441(23)
13.2000(26)
15.3016(31)
90.0
90.0
90.0
2291.3(8)
2.982
12.272
296(2)
˚
a, A
6.3716(3)
13.1646(5)
14.8622(6)
90.0
90.0
90.0
1246.64(9)
1.936
5.673
˚
b, A
˚
c, A
R, deg
β, deg
γ, deg
3
˚
V, A
F, g/cm
μ, mm-1
total reflections
unique reflections
R1, wR2 [I > 2σ(I)]a
34966
4699
0.036, 0.078
57306
4781
0.025,0.033
68948
9247
0.025,0.054
75390
4139
0.046, 0.097
19042
1148
0.052, 0.184
P
P
P
P
a R1= ||Fo| - |Fc||/ |Fo|; wR2=[ w(Fo2 - Fc2)2/ w(Fo2)2]1/2; w=σF
.
-2
Synthesis of Ag(bpp)2ReO4 (1), Ag(tpp)ReO4 H2O (2),
3
colorless prism crystals in ∼68% yield based on Ag. 9 was
prepared from a mixture of dpa (= 1,2-bis(4-pyridyl)-ethane)
(73.6 mg, 0.20 mmol), Ag2O (23.2 mg, 0.10 mmol), Re2O7 (48.4 mg,
0.10 mmol), and H2O (0.3 g, 16.7 mmol) as colorless bar-shaped
crystals in ∼70% based on Ag.
Ag(Hinca)2ReO4 H2O (3), Ag(tro)ReO4 (4), Ag(pda)ReO4
3
3
1/2H2O (5), Ag(Hpzc)ReO4 (6), Ag2(Hpzc)(pzc)(H2O)ReO4 (7),
Ag(bpy)ReO4 (8), and Ag(dpa)2ReO4 (9). 1 was prepared from a
reaction of bpp (= 2,3-bis(2-pyridyl)pyrazine) (93.7 mg, 0.40 mmol),
Ag2O (23.2 mg, 0.10 mmol), Re2O7 (48.4 mg, 0.10 mmol), and
H2O (0.4 g, 22.2 mmol) as colorless block crystals in ∼30% yield
based on Ag. 2 was prepared from a mixture of tpp (= tetra-
2-pyridinylpyrazine) (77.6 mg, 0.20 mmol), Ag2O (23.2 mg,
0.10 mmol), Re2O7 (48.4 mg, 0.10 mmol), and H2O (0.4 g,
22.2 mmol) as yellow bar-shaped crystals in ∼98% yield based
on Ag. 3 was prepared from a reaction of Hinca (= isonicotinic
acid) (24.6 mg, 0.20 mmol), Ag2O (11.6 mg, 0.05 mmol), Re2O7
(24.2 mg, 0.10 mmol), and H2O (0.2 g, 11.1 mmol) as colorless
bar-shaped crystals in ∼64% yield based on Ag. 4 was synthe-
sized from a reaction of tro (=1,2,4-triazole) (13.4 mg, 0.20 mmol),
Ag2O (23.2 mg, 0.10 mmol), Re2O7 (96.8 mg, 0.20 mmol), and
H2O (0.4 g, 22.2 mmol) as colorless block crystals in ∼76%
yield based on Ag. 5 was prepared from a mixture of pda
(= pyridazine) (16.0 mg, 0.20 mmol), Ag2O (23.2 mg, 0.10 mmol),
Re2O7 (48.4 mg, 0.10 mmol), and H2O (0.4 g, 22.2 mmol) as
colorless plate-shaped crystals in ∼78% yield based on Ag. 6
was synthesized from a reaction of Hpzc (24.8 mg, 0.20 mmol),
Ag2O (23.2 mg, 0.10 mmol), Re2O7 (48.4 mg, 0.10 mmol), and
H2O (0.05 g, 2.80 mmol) as colorless bar-shaped crystals
in ∼84% yield based on Ag. 7 was prepared from a mixture of
Hpzc (= pyrazine-2-carboxylic acid) (59.5 mg, 0.48 mmol),
Ag2O (55.7 mg, 0.24 mmol), Re2O7 (58.1 mg, 0.12 mmol), and
H2O (0.2 g, 11.1 mmol) as colorless bar-shaped crystals in ∼80%
yield based on Ag. 8 was prepared from a mixture of bpy (= 4,40-
bipyridine) (31.2 mg, 0.20 mmol), Ag2O (23.2 mg, 0.10 mmol),
Re2O7 (96.8 mg, 0.20 mmol), and H2O (0.72 g, 40.0 mmol) as
Crystallographic Structure Determination of 1-9. Suitable
single crystals of 1-9 were selected and mounted on a nylon
loop with a small amount of NVH immersion oil. All X-ray
measurements were made on a Bruker-Nonius X8 Apex2 CCD
diffractometer using graphite-monochromatized Mo KR radia-
˚
tion (λ=0.71073 A). The frame integration was performed using
the SAINT program.23 The resulting raw data were scaled and
corrected for absorption using a multiscan averaging of sym-
metry-equivalent data via the SADABS program.24 Each struc-
ture was solved by direct methods and refined by full-matrix
least-squares fitting on F2 using SHELX-97.25 All non-hydro-
gen atoms were refined with anisotropic thermal parameters.
Hydrogen atoms on the organic ligands were introduced at
idealized positions and were allowed to ride on the parent
carbon atoms. Details of the crystal parameters, data collec-
tions, and structure refinements for all hybrid solids are sum-
marized in Table 1. Selected interatomic distances and angles are
listed in Table 2. Further refinement details are provided in the
Supporting Information.
Optical Properties. Transmittance spectra for pressed pellets
of each powdered sample were measured in a wavelength range
(23) SAINTþ, version 7.07B; Bruker-Nonius: Madison, WI, 2004.
(24) SADABS, version 2.1; Bruker-Nonius: Madison, WI, 2004.
(25) Sheldrick, G.M. SHELXTL NT, Software Package for Refinement of
Crystal Structures, ver. 5.10; Bruker Analytical X-ray Instruments, Inc.:
Madison,WI, 1998.

11268 Inorganic Chemistry, Vol. 48, No. 23, 2009
Lin et al.
˚
Table 2. Selected Interatomic Distances (A) in Hybrid Solids 1-9
Table 2. Continued
˚
atom1 atom2 distance (A) atom1 atom2 distance (A)
˚
˚
atom1 atom2 distance (A) atom1 atom2 distance (A)
˚
O4
N1
N2
2.954(2)
2.141(2)
2.132(2)
O3
O4
1.726(2)
1.733(2)
Ag(bpp)2ReO4 (1)
Ag1
Ag1
Ag1
N1
N4
N5
N7
N8
2.431(5)
2.415(5)
2.438(5)
2.345(5)
2.412(5)
Re1
O1
O2
O3
O4
1.706(5)
1.724(5)
1.690(5)
1.690(5)
Ag(dpa)2ReO4 (9)
2.316(9) Re1
Ag1
N1
O1
1.707(8)
of 200-800 nm at room temperature on a UV-vis spectro-
photometer (UV-3600, Shimadzu). To account for the regular
reflection losses at the boundary surfaces in the transmittance
measurements, each compound was measured twice with two
different pellet thicknesses, as detailed previously.26 Resultant
absorption spectra were calculated using the Lambert Law (eq 1)
and evaluated as R (cm-1) versus wavelength (nm):
Ag(tpp)ReO4 H2O(2)
3
N1
N2
N4
N5
N6
2.469(2)
Re1
O1
1.705(2)
1.709(3)
1.712(3)
1.712(3)
2.408(2)
2.468(2)
2.393(2)
2.409(3)
O2
O3
O4
Ag(Hinca)2ReO4 H2O (3)
3
R ¼ ð1=ΔxÞðlogð1=R2Þ -logð1=R1ÞÞ
ð1Þ
O6
O6
N1
N2
2.826(3)
2.894(3)
2.111(3)
2.117(3)
Re1
O5
O6
O7
O8
1.710(4)
1.723(4)
1.710(4)
1.721(3)
where R is the absorption coefficient in cm-1; Δx= x2 - x1,
where x1 and x2 are the thicknesses of the two sample pellets.
X-ray Photoelectron Spectroscopy (XPS). The valence-band
XPS spectra were recorded on a RIBER LAS-3000 spectrometer
using Mg KR monochromatized radiation (hν=1253.6 eV). The
base pressure of the analysis chamber was ∼1 ꢀ 10-7 Torr.
Sample sizes were ∼1 cm square, and the diameter of the X-ray
spot was set at 2-3 mm. The binding energies of the valence-
band electronic structures were calibrated by fixing the C 1s
core-level peak at 284.8 eV for each sample.
Ag(tro)ReO4 (4)
Ag1
Ag2
O1
O3
N1
N4
O5
O8
N2
N5
2.625(4)
2.690(4)
2.146(4)
2.155(4)
2.773(4)
2.707(4)
2.146(4)
2.140(4)
Re1
O1
O2
O3
O4
O5
O6
O7
O8
1.720(3)
1.712(3)
1.723(3)
1.737(3)
1.730(3)
1.720(3)
1.719(3)
1.738(3)
Re2
Thermogravimetric Analyses (TGA). The thermal stability
and decomposition of the hybrids were measured on a TA
Instruments TGA Q50 under flowing nitrogen gas and heating
to 500 °C at a programmed rate of 5 °C/min. Weighed amounts
(∼20 mg) of each hybrid solid were loaded onto Pt pans,
equilibrated and tared at room temperature, and the data
plotted as the starting weight (%) versus temperature (°C).
Post-TGA residuals were characterized by powder X-ray
diffraction in transmission mode on an Inel XRG 3000
diffractometer fitted with a CPS 120 position sensitive detec-
tor and using Cu KR1 radiation from a sealed tube X-ray
source.
Ag(pda)ReO4 1/2H2O (5)
3
Ag1
Ag2
O6
O11
N2
N3
O2
O5
O9
N4
N5
O1
O6
O8
O9
N1
N6
2.690(9)
2.802(9)
2.179(9)
2.177(9)
2.747(9)
2.616(9)
2.492(11)
2.224(9)
2.222(8)
2.579(8)
2.681(9)
2.598(8)
2.527(12)
2.273(9)
2.283(9)
Re1
Re2
Re3
O1
O2
O3
O4
O5
O6
O7
O8
1.715(7)
1.721(8)
1.718(9)
1.718(9)
1.713(8)
1.724(8)
1.708(9)
1.712(8)
1.678(11)
1.663(18)
1.733(14)
1.718(8)
3.575(1)
3.410(2)
3.557(1)
O9
Ag3
O10
O11
O12
Ag2
Ag3
Ag3
Results and Discussion
Ag1
Ag2
The structures and properties of Ag(I)-Re(VII) hybrid
solids have been systematically investigated by varying the
geometry and local coordination sites of the organic ligands,
shown in Scheme 1. The resultant hybrid structures are
described within separate sections below, and range from
0D (molecules), to 1D (chains), to 2D (layered), and also to
interpenetrating 3D (pillar-layered or diamondoid) net-
works. A listing of all near-neighbor interatomic distances
can be found in Table 2 for each of the hybrid structures of
1-9. All symmetry-unique atoms are labeled on at least one
representative figure of each structure, a consistent coloring
scheme is used, and all hydrogen bonds are drawn as dashed
lines. Topological analyses were carried out using TOPOS27
for structures consisting of extended two- or three-dimen-
Ag(Hpzc)ReO4 (6)
Ag1
O2
O3
O4
O5
N1
N2
2.573(3) Re1
O3
O4
O5
O6
1.730(2)
1.728(2)
1.714(2)
1.700(2)
2.648(3)
2.842(3)
2.552(3)
2.284(2)
2.249(2)
Ag2(Hpzc)(pzc)(H2O)ReO4 (7)
Ag1
Ag2
O1
O3
O7
N1
N3
O5
O6
O8
N2
N4
2.537(2)
2.492(2)
2.785(2)
2.253(3)
2.277(2)
2.608(2)
2.397(2)
2.878(2)
2.229(3)
2.240(3)
Re1
O6
O7
O8
O9
1.728(2)
1.721(2)
1.727(2)
1.722(3)
2.140(3)
2.115(3)
˚
sional coordination networks (cutoff distance of ∼3.0 A) and
O5-H O1
O5-H O3
3 3 3
described herein using recently proposed conventional nota-
tions.28
3 3 3
€
€
(26) (a) Kortum, G.; Haug, P. Z. Naturforsch. 1953, 8a, 372. (b) Kortum G.
Reflectance Spectroscopy; Springer-Verlag: New York, 1969.
(27) Blatov, V. A. TOPOS, A Multipurpose Crystallochemical Analysis
with the Program Package; Samara State University: Russia, 2004.
(28) Friedrichs, O. D.; O’Keeffe, M.; Yaghi, O. M. Acta Crystallogr.,
Sect. A 2003, A59, 22.
Ag(bpy)ReO4 (8)
Re1
Ag1
O2
O3
3.002(2)
2.869(2)
O1
O2
1.717(2)
1.731(2)

Article
Inorganic Chemistry, Vol. 48, No. 23, 2009 11269
Figure 2. Structural view of 2, Ag(tpp)ReO4 H2O, showing the extended
3
nþ
[Ag(tpp)]n and [(ReO4-)(H2O)]n chains aligned down the a-axis direc-
tion. Hydrogen bonds are drawn as dashed lines with labeled bond
distances.
Figure 1. Structural view of 1, Ag(bpp)2ReO4, with hydrogen bonds
drawn as dashed lines and symmetry-unique atom types labeled. Hydro-
gen atoms are omitted for clarity.
˚
2.505(3) A, respectively) to form a collinear chain.
The two chains, [Ag(tpp)]n and [(ReO4-)(H2O)]n, are
nþ
(i). Monodentate and Multidentate Bridging Ligands
Yielding the 0D and 1D Structures of 1, 2, and 3. Ag-
(bpp)2ReO4 (1; bpp = 2,3-bis(2-pyridyl)pyrazine). The
single crystal X-ray characterization of 1 reveals a mo-
lecular (0D) structure that is composed of [Ag2(bpp)4]2þ
dimers that are interconnected via hydrogen bonding to
hydrogen bonded to each other via the oxygen atoms of
ReO4/H2O and the hydrogen atoms of the tpp ligands
˚
at ∼2.32-2.48 A. Thus, while the Ag coordination geo-
metry is similar to that in 1 with the bpp ligand, the
addition of two pyridine groups on the tpp ligand affords
the extended bridging coordination in the chains of 2.
-
the ReO4 anions, that is, C-H O at ∼2.463-
3 3 3
˚
(5)-2.501(5) A (i.e., the - H O distance throughout
the text) down the a-axis, as shown in Figure 1. There is
Ag(Hinca)2ReO4 H2O (3; inca=isonicotinate). Shown
3 3 3
3
in Figure 3, the structure of 3 contains dimeric “Ag2-
(inca)4(ReO4)2” molecular species that are linked to each
other via hydrogen bonding interactions. Each Ag atom is
coordinated in a slight-distorted seesaw geometry, which
only one symmetry-unique Ag site that is coordinated by
˚
five nitrogen atoms (Ag-N at 2.345(5)-2.438(5) A) in a
distorted square-pyramidal geometry, twice by two dif-
ferent bpp ligands, and once by a single bpp ligand. Two
of the bpp ligands bridge between the two Ag atoms,
while the other two bpp ligands are terminating on each
Ag, with one of the nitrogen atoms remaining uncoordi-
nated. Each [Ag2(bpp)4]2þ dimer is hydrogen bonded to
˚
is formed by two oxygen atoms (Ag1-O6 at 2.826(3) A
and 2.894(3) A) from ReO4 and two nitrogen atoms
-
˚
˚
˚
(Ag1-N1, -N2 at 2.111(3) A, 2.117(3) A, respectively)
from two Hinca ligands. The axial N1-Ag1-N2 bond
angle is nearly linear, at ∼173.0°, as expected for a seesaw
geometry. The carboxylate group of the inca ligand
remains uncoordinated to any Ag atoms and must instead
be protonated for charge balancing, that is, as Hinca.
However, the ReO4- anion functions in a bridging role,
leading to a relatively close Ag1-Ag1 dimer at 3.4811(7)
-
four neighboring tetrahedral ReO4- anions. The ReO4
tetrahedra are nearly regular with Re-O distances of
˚
1.690(5)-1.724(5) A, with a narrow range of O-Re-O
angles from ∼106.9°-111.3°. Two oxygen atoms of each
-
ReO4 are hydrogen-bonded to two neighboring
[Ag2(bpp)4]2þ dimers. Thus while the structure of 1 is
considered to be molecular based upon the bpp ligand
coordination, the weaker hydrogen-bonding interactions
lead to some layer-like structural connectivity.
˚
A. Within the bc-plane, shown in Figure 3B, neighboring
˚
Hinca ligands are oriented face-to-face (∼3.75 A, π-π
interaction) with hydrogen bonding between the carbox-
˚
˚
ylate groups (O-H O at 2.012(3) A and 2.091(3) A).
The ReO4 anions and uncoordinated water molecules
3 3 3
-
Ag(tpp)ReO4 H2O (2, tpp=tetra-2-pyridinylpyrazine).
The yellow needle-like crystals of 2 consist of [Ag(tpp)]n
3
nþ
form hydrogen-bonded [(ReO4-)(H2O)]n zigzag chains
˚ ˚
O8 at 1.992(2) A, 2.038(3) A,
chains that are oriented down the a-axis, as illustrated in
Figure 2. The [Ag(tpp)]nnþ chains are constructed from a
single symmetry-unique Ag atom that is coordinated in a
distorted square-pyramidal geometry by five nitrogen
atoms from three different tpp ligands, at Ag-N dis-
(O1w-H O7,
respectively) down the a-axis. The distances and angles
3 3 3
3 3 3
-
for ReO4 are closely similar to 1 and 2, with Re-O
˚
distances of ∼1.710(4)-1.723(4) A and O-Re-O angles
of ∼107.3-111.3°. However, the terminating rather than
bridging role of the Hinca ligand results in a molecular
structure of “Ag2(inca)4(ReO4)2” dimeric species that are
hydrogen bonded to each other and to water.
˚
tances ranging from 2.393(3)-2.469(3) A. Each tpp
ligand chelates via three nitrogen atoms to Ag, with two
other nitrogen atoms bridging to the neighboring Ag sites
to form the extended chain. By contrast, previously
reported M(I)/tpp (M=Ag, Cu) cationic chains contain
(ii). Ligands with Neighboring N-Donor Groups Yield-
ing the 2D Structures of 4 and 5. Ag(tro)ReO4 (4; tro=
1,2,4-triazole). The structure of 4 consists of corrugated
three distinct
M
coordination sites,29 while the
[Ag(tpp)]nnþ chain in 2 consists of just one. The structure
of 2 also contains tetrahedral ReO4 anions and H2O
nþ
[Ag(tro)ReO4]n layers composed of [Ag(tro)]n zigzag
chains that are cross-linked by ReO4 anions that
-
-
molecules that are linked via hydrogen bonds
˚
coordinate to the Ag sites, as shown in Figure 4. The
[Ag(tro)]n zigzag chain consists of two symmetry-un-
ique Ag atoms that alternate down the b-axis and are
coordinated in a distorted seesaw geometry via the nitro-
gen atoms of two bridging tro ligands each at Ag-N
nþ
(O1w-H1w O3, O1w-H2w O4 at 2.053(3) A and
3 3 3
3 3 3
(29) (a) Burkholder, E.; Zubieta, J. Solid State Sci. 2004, 6, 1421. (b) Allis,
D. G.; Rarig, R. S.; Burkholder, E.; Zubieta, J. J. Mol. Struct. 2004, 688, 11.

11270 Inorganic Chemistry, Vol. 48, No. 23, 2009
Lin et al.
Figure 3. Local (A) and extended (B) structural views of 3, [Ag(Hinca)2ReO4 H2O], with all symmetry-unique atoms and hydrogen bonding interactions
3
labeled. The polyhedral view in (B) is aligned down the a-axis, with red polyhedra=Ag-centered and blue polyhedra=ReO4.
Figure 4. (A) Structural view of a single layer in Ag(tro)ReO4 (4) with the symmetry-unique atom types labeled, and (B) an ∼[010] polyhedral view of the
layers stacked down the c axis; Red polyhedra=Ag-centered and blue polyhedra=ReO4.
-
Figure 5. (A) Structural view of a single layer in Ag(pda)ReO4 1/2H2O (5), showing seven trigonal-planar [Ag3(pda)3]3þ clusters bridged by ReO4
3
anions;(B)An∼[010] polyhedralviewoftwo oftheselayersstackedin5 with interlayerhydrogenbondsdrawnasdashed lines;Red polyhedra=Ag-centered
and blue polyhedra=ReO4.
-
˚
distances of 2.140(4)-2.155(4) A and to two ReO4
N-Ag-N angle of ∼170° and form the backbone of
the [Ag(tro)]nnþ zigzag chain. Alternatively, a corrugated
“AgReO4” chain can also be described that is oriented
˚
anions each at Ag-O distances of 2.625(4)-2.773(4) A.
The tro ligands coordinate to the axial sites with a

Article
Inorganic Chemistry, Vol. 48, No. 23, 2009 11271
Figure 6. Structural views of Ag(Hpzc)ReO4 (6), showing the (A) “AgReO4” layer with atom types labeled, and (B) an ∼[100] polyhedral view. Red
polyhedra=Ag-centered and blue polyhedra=ReO4.
˚
2.747(9) A, 2.616(9) A, and 2.492(11) A, respectively).
˚
˚
down the a-axis and which is cross-linked by the tro
ligands. The distorted ReO4 tetrahedra exhibit bond
-
-
The Ag3 site is bonded to four oxygen atoms from ReO4
˚
angles and distances consistent with the structures pre-
viously described above. This layer structure has resulted
from the greater number of bridging ReO4- anions that
serve to cross-link the Ag atoms within a single layer, as
anions (Ag3-O1, -O6, -O8, -O9, at 2.579(8) A,
˚
˚
˚
2.681(9) A, 2.598(8) A, and 2.527(12) A, respectively)
and that occupy the equatorial sites of the distorted-
octahedral coordination geometry. There are three sym-
metry-unique ReO4- tetrahedra, two of which (Re1 and
Re3) are bridging between the Ag atoms on two different
[Ag3(pda)3]3þ clusters, while the other (Re2) is bridging
between three neighboring [Ag3(pda)3]3þ clusters, shown
in Figure 5A. Analogous to that described above for 4, the
layered structure of 5 has resulted from the bridging
-
compared to the non-bridging ReO4 anions in 1 and 2
that result from the chelating and more sterically crowded
tpp and bpp ligands.
Ag(pda)ReO4 1/2H2O (5; pda = pyridazine). The hy-
3
brid solid 5 exhibits a layered structure consisting of
[Ag(pda)ReO4]n layers stacked down the a-axis, as shown
in Figure 5. This layer is constructed from a remarkable
trigonal-planar cluster of three Ag atoms, that is, as a
[Ag3(pda)3]3þ cyclic trimer in Figure 5a, that features a
slightly distorted [Ag3N6] core with N-Ag-N bond
angles between ∼170.0°-173.9°. This [Ag3(pda)3]3þ tri-
mer is unusual but structurally similar to that reported for
pyrazolate-bridged CuI/AgI molecular species,30 and here
represents the first such structural motif synthesized in a
hybrid solid. Each [Ag3(pda)3]3þ cluster is also coordi-
ReO4 anions that serve to cross-link the [Ag3(pda)3]3þ
-
cyclic trimers, as compared to the lower-dimensional
structures of 1-3 with more ligand-crowded Ag sites
and non-bridging ReO4- anions.
(iii). Small Asymmetric Bridging Ligands Leading to
the Pillar-Layered Structures of 6 and 7. Ag(Hpzc)ReO4
(6; pzc = pyrazine-2-carboxylate). The structure of 6 is
composed of neutral “AgReO4” layers that are cross-
linked by Hpzc ligands, shown in Figure 6, with a shortest
-
nated to seven bridging ReO4 anions that extend the
structure in the bc-plane to form the [Ag(pda)ReO4]n
layer.
˚
interlayer distance of ∼2.79 A. A single symmetry-unique
“AgReO4” layer stacks down the c-axis to generate the full
three-dimensional structure in the monoclinic space
group P21/n. Within the layer, there is a single symme-
try-unique Ag site that has a distorted-octahedral coor-
dination geometry, with the equatorial positions
occupied by one carboxylate group (Ag1-O2 at
Including both the N-donor and anionic ReO4- ligands,
the [Ag3(pda)3]3þ cluster contains three different Ag
coordination geometries, a seesaw (Ag1), a distorted
square-pyramidal (Ag2), and a distorted octahedral
(Ag3) coordination environment. Each Ag is coordinated
to two N atoms on the pda ligands at distances ranging
2.573(3) A) and three bridging ReO4- anions (Ag1-O3,
˚
˚
˚
˚
-O4, -O5, at 2.648(3) A, 2.842(3) A, and 2.552(3) A,
respectively). The axial positions are bonded by two
nitrogen groups from pzc ligands above and below the
˚
from 2.177(9)-2.283(9) A, with the differences in coordi-
nation geometries owing to the asymmetric arrangement
of the ReO4 ligands. The Ag1 site is bonded to two
-
˚
˚
layer (Ag1-N1, -N2, at 2.284(2) A and 2.249(2) A,
respectively), completing the octahedral environment.
Each tetrahedral ReO4 bridges between three different
oxygen atoms on two ReO4- tetrahedra (Ag1-O6, -O11
˚
˚
at 2.690(9) A and 2.802(9) A, respectively), to give the
seesaw coordination geometry. The distorted square-
pyramidal geometry of Ag2 is completed by three oxygen
atoms from three ReO4- anions (Ag2-O2, -O5, -O9, at
-
Ag atoms, with the fourth oxygen vertex located alter-
nately above or below the layer, shown in Figure 6A. A
topological analysis of 6 shows that it is classified as a 3,5-
€
c net with 2 nodes (Ag and Re atoms) and the Schlafli
(30) (a) Raptis, R. G.; Fackler, J. P. Inorg. Chem. 1988, 27, 4179. (b) Dias,
H. V. R.; Polach, S. A.; Wang, Z. J. Fluorine Chem. 2000, 103, 169. (c) Fujisawa,
K.; Ishikawa, Y.; Miyashita, Y.; Okamoto, K. I. Chem. Lett. 2004, 33, 66.
(d) Dias, H. V. R.; Diyabalanage, H. V. K.; Eldabaja, M. G.; Elbjeirami, O.;
Rawashdeh-Omary, M. A.; Omary, M. A. J. Am. Chem. Soc. 2005, 127, 7489.
(e) Dias, H. V. R.; Gamage, C. S. P.; Keltner, J.; Diyabalanage, H. V. K.; Omari, I.;
Eyobo, Y.; Dias, N. R.; Roehr, N.; McKinney, L.; Poth, T. Inorg. Chem. 2007, 46,
2979. (f) Yang, G.; Raptis, R. G. Inorg. Chim. Acta 2007, 360, 2503. (g) Rasika
Dias, H. V.; Singh, S.; Campana, C. F. Inorg. Chem. 2008, 47, 3943.
symbol {63}{69;8}.
Each Hpzc ligand chelates to a single Ag atom via the
carboxylate and nitrogen groups, and also bridges to a
neighboring Ag atom through its opposing nitrogen atom
to form the [Ag(Hpzc)]þ chains oriented down the c-axis.
The Hpzc ligands in a single chain are all oriented in the
same direction, while the ligand orientations of neighboring

11272 Inorganic Chemistry, Vol. 48, No. 23, 2009
Lin et al.
Figure 7. Stuctural views of Ag2(Hpzc)(pzc)(H2O)ReO4 (7), showing the (A) [Ag2(H2O)ReO4]n layer with atom types labeled, and (B) an ∼[010]
polyhedral view with hydrogen bonds as dashed lines; red polyhedra=Ag-centered and blue polyhedra=ReO4.
Figure 8. (A) A single “AgReO4” layerofAg(bpy)ReO4 (8) with63 hexagonal netsand allsymmetry-unique atomtypes labeled, and (B) ∼[010] polyhedral
view of its interpenetrating structure; Red polyhedra=Ag-centered and blue polyhedra=ReO4.
chains are reversed to cancel any net dipole moment, as
required for space group P21/n. While the carbonyl end
layers consist of two symmetry-unique Ag sites, distorted
square-pyramidal (Ag1) and distorted trigonal-bipyrami-
dal (Ag2) coordination environments that alternate down
the [Ag2(pzc)(Hpzc)]þ chains. Each Ag1 atom is chelated
by two nitrogen atoms and two oxygen atoms from two
pzc ligands above and below (Ag1-N1, -N3 at 2.253(3)
˚
(C=O at ∼1.21 A) of the carboxylate group preferen-
tially coordinates to the Ag atom, the alcoholic end
˚
(C-OH at ∼1.32 A) remains protonated and uncoordi-
nated. These carboxylate distances are consistent with
˚
˚
˚
˚
those of the Hinca ligand of 3 (C=O, C-OH at ∼1.22 A
A and 2.277(2) A; Ag1-O1, -O3 at 2.537(2) A and
˚
˚
2.492(2) A, respectively), and is also coordinated to
and ∼1.34 A, respectively), that is uncoordinated and
-
protonated as well. Thus, while the terminating Hinca
ligand in 3 yields a molecular-type structure, the intro-
duction of a para nitrogen group in the Hpzc ligand of 6
results in ligand-bridged “AgReO4” layers. Similar pil-
lared “AgReO4” layers are also found in Ag(pyz)ReO4,17
showing that the opposing nitrogen atoms on the ligand
(sans the carboxylate functionality) are key to forming
this type of hybrid structure.
one oxygen atom from the ReO4 anion (Ag1-O7 at
2.785(2) A). Similarly, Ag2 is coordinated to two pzc
˚
ligands through the opposing nitrogen atoms (Ag2-N2, -N4
at 2.229(3) A and 2.240(3) A, respectively) and to two
˚
˚
-
ReO4 anions via its oxygen atoms (Ag2-O6, -O8 at
2.397(2) A and 2.878(2) A, respectively). Additionally,
Ag2 is equatorially bonded to one H2O molecule
˚
˚
˚
(Ag2-O5 at 2.608(2) A) that hydrogen bonds to neighboring
˚
Ag2(Hpzc)(pzc)(H2O)ReO4 (7; pzc = pyrazine-2-car-
boxylate). The structure of 7 is the only one of the nine
hybrids described herein to crystallize in a non-centro-
symmetric space group, Pca21. Further, it is the only one
with a Ag/Re molar ratio of 2:1 rather than 1:1, as for the
other hybrids. Shown in Figure 7, the hybrid structure of
7 is composed of asymmetric [Ag2(H2O)ReO4]þ layers
that are stacked down the c-axis with pzc/Hpzc ligands
bridging between the Ag atoms in each layer, in analogy
with the structural role of the Hpzc ligands in 6. These
carboxylate groups (O5-H O at ∼2.1 A) shown as
3 3 3
dashed lines in Figure 7.
A
topological analysis shows that the [Ag2-
(H2O)ReO4]þ layer in 7 is a new topology with a 3,4-c
net with 3 nodes (two Ag and one Re atom) and the
2
2
4
2
€
Schlafli symbol {6;8 }{6 ;8 }{6 ;8}. As before for 6, each
tetrahedral ReO4- anion bridges between three different
Ag sites in 7, with the fourth oxygen vertice located
between the layers in a polar arrangement, shown in
Figure 7B. Thus, an undulating [Ag2ReO4]þ layer is

Article
Inorganic Chemistry, Vol. 48, No. 23, 2009 11273
þ
Figure 9. (A) Simplified view of a single diamond-type Ag(dpa)2 network, upper, and the 6-fold interpenetration of the same diamond-type net in
Ag(dpa)2ReO4 (9), lower. Each node represents a Ag atom. (B) An ∼[100] polyhedral view of 9, with red polyhedra=Ag-centered tetrahedra and blue
polyhedra=ReO4.
formed in 7 that is structurally similar to 6, but that is
Re-deficient owing to the 2:1 (versus 1:1) Ag/Re molar ratio.
Further, the layer structure has been able to compensate for
this deficiency by incorporating a coordinated water mole-
cule on the same position as the missing ReO4- anion.
(iv). Longer Bridging Ligands Yielding the Interpene-
trating Structures of 8 and 9. Ag(bpy)ReO4 (8; bpy =
4,40-bipyridine). The structure of 8 is a three-dimensional
pillar-layered framework that is 2-fold self-interpenetrat-
ing, as illustrated in Figure 8 for both a single layer (A)
and multiple layers (B). The network is composed of a
single symmetry-unique “AgReO4” layer that consists of
Ag(dpa)2ReO4 (9; dpa = 1,2-bis(4-pyridyl)-ethane).
Shown in Figure 9, use of the slightly longer and more
flexible dpa ligand in 9 results in a three-dimensional
[Ag(dpa2)]þ diamond-type network that is 6-fold inter-
penetrating. The structure represents a three-dimen-
sional
interpenetration) with a 4c-net that is uninodal with
dia
topology
(class
Ia
for
6-fold
6
the Schlafli symbol {6 }. The overall structure is highly
€
symmetric and contains only one symmetry-unique Ag
and Re atom. The [Ag(dpa2)]þ network is constructed
from tetrahedrally coordinated Ag via the N donors of
˚
four dpa ligands (Ag-N at 2.316(9) A), Figure 9A. The
-
˚
˚
alternating tetrahedral ReO4 anions and Ag-centered
open channels (dimensions of ∼6.1 A ꢀ ∼6.1 A from N1
trigonal bipyramids that are arranged into 63 hexagonal
nets and are pillared via the bridging bpy ligands. A
topological analysis indicates that the interpenetrating
framework of 8 represents a three-dimensional hms to-
pology (class IIa) with 3,5-connectivity and a 2-nodal
to N10) are filled by five other symmetry-related nets as
well as the ReO4 anionic guests. This network type is
-
relatively common, and has been found for Cu-
(bpy)2ReO4 1/2H2O and for several other metal-organ-
3
ic frameworks.21,31,32 However, the tetrahedral ReO4
-
3
9
€
(Re, Ag) net, with the Schlafli symbol {6 }{6 ;8}. The
hybrid network of 8 is iso-structural to that recently
reported for Cu(bpy)ReO4,21 with the replacement of
Cuþ for Agþ, and thus is described here only briefly.
Within each layer, each ReO4- anion bridges via its O
vertices to three different Agþ in the layer (Ag-O of
anions in 9 are not involved in bridging between the Ag
sites, which is their typical role in all other silver(I)-
rhenate(VII) hybrids reported herein. The charge-bal-
-
ancing ReO4 anions have a much narrower range of
˚
tetrahedral Re-O distances (1.701(1) A) and O-Re-O
angles (109.2-109.7°). Thus, use of the longer dpa
ligand (versus bpy or pzc in 6 and 8) results in the only
hybrid herein with tetrahedrally coordinated Ag atoms,
as compared to the more typical trigonal bipyramidal or
˚
2.869(2)-3.002(2) A), with the fourth vertex oriented
either above or below the layer, as found similarly in
the silver-rhenate layers of 6 and 7. The coordination
environment of Ag consists of three O atoms from three
-
seesaw coordination geometries. The absence of ReO4
-
different ReO4 as well as two N atoms from two
pyrazine ligands located above and below each layer
from the Ag coordination environment prevents the
formation of any extended Ag-ReO4 connectivity, as
found for hybrids 4-8.
Optical Properties. Measurements of the optical ab-
sorption coefficients and bandgap sizes were carried out
to investigate the changes arising from the new structural
˚
˚
(2.141(2) A and 2.132(2) A for Ag1-N1 and -N2,
respectively). The layer-to-layer distance is determined
˚
by the length of the bpy ligand at ∼7.08 A, and which
allows for intervening layers that interpenetrate through
the hexagonal faces of the “AgReO4” net via the bpy
ligands. Thus, the bridging role of the bpy ligand in 8
results in a pillared structure similar to that of 6 and 7
(sans the carboxylate group), but the longer ligand length
significantly expands the interlayer spacing and results in
the 2-fold interpenetration of the pillared network.
(31) (a) Blake, A. J.; Champness, N. R.; Chung, S. S. M.; Li, W.-S.;
Schroder, M. Chem. Commun. 1997, 1005. (b) Blake, A. J.; Champness, N. R.;
Khlobystov, A. N.; Lemenovskii, D. A.; Li, W.-S.; Schroder, M. Chem. Commun.
1997, 1339.
(32) Carlucci, L.; Ciani, G.; Proserpio, D. M.; Sironi, A. J .Chem. Soc.,
Chem. Commun. 1994, 1325.

11274 Inorganic Chemistry, Vol. 48, No. 23, 2009
Lin et al.
Measurements of R of hybrid solids 1-9 were taken on
a UV-vis spectrophotometer using transmittance meth-
ods in the range of 200-800 nm, and the resultant spectra
are plotted as R (cm-1) versus wavelength (nm) in
Figure 10A. These spectra include data for the previously
reported pillar-layered structure of AgReO4(pyz) (10,
pyz = pyrazine) and the ligand-free AgReO4 (11) for
comparison. The measured values of R for these hybrids
all fall within one of two different groups, which either
exhibit larger bandgap sizes and higher absorption coef-
ficients (3, 4, 5, 8, 9, and 11) or smaller bandgap sizes in
the visible range with lower absorption coefficients (1, 2,
6, 7, and 10). A steep onset of absorption occurs at ∼350 nm
for the first group and at ∼450 nm for the second group.
Previous theoretical and experimental studies on Ag(I)-
Re(VII) solids have shown that the band-edge absorption
occurring at around 340 nm in AgReO4 can be assigned
to a MMCT transition (AgI f ReVII).34-36 Further, the
shift of the absorption edge by ∼0.8 eV to lower energies
is consistent with a Ligand-to-Metal Charge Transfer
(LMCT) excitation, as was investigated previously in
AgReO4(pyz) and AgReO4.17 Thus, the steep rise in absorp-
tion in the first group owes to a MMCT transition, while
that of the second group to a LMCT transition.
The optical bandgap size for each of the hybrid solids
can be calculated from the respective wavelength-depen-
dent absorption coefficient spectrum. Electronic transi-
tion probabilities are given by the following eq 2 when the
energy of the photon exceeds the bandgap energy:37
n
ðRhυÞ µ ðhυ -EgÞ
ð2Þ
where R is the absorption coefficient (cm-1), hυ is the
photon energy (eV), Eg is the band gap energy (eV), and
n=2 when the transition is direct, and n=1/2 when the
transition is indirect. Bandgap sizes (Eg) can be estimated
from the linear portion of the plots of (Rhυ)n versus hυ
when extrapolating to zero. The plots of (Rhυ)2 versus hυ,
shown in Figure 10B, yield bandgap sizes that are con-
sistent with the observed colors and also suggest that each
of the lowest energy excitations are direct. The calculated
bandgap sizes for each compound are 2.89 eV (1), 2.60 eV
(2), 3.70 eV (3), 3.75 eV (4), 3.30 eV (5), 2.90 eV (6), 2.96
eV (7), 3.71 eV (8), 3.46 eV (9), 2.91 eV (10; AgReO4-
(pyz)), and 3.68 eV (11; AgReO4). Hybrid solids in the
group with the higher values of R show a range of
bandgap sizes from ∼3.30-3.75 eV, while those in the
second group with lower values of R show a range of
bandgap sizes from ∼2.60-2.96 eV. For hybrids in the
former group, that is, 3, 4, 5, 8, 9, 11 assigned to a MMCT
excitation, there is a close correlation of R with the
Ag-ReO4 network dimensionality but not with the
bandgap size. For example, as R decreases in the order
of 11 > 8 > 4 > 5 > 3 > 9, the network dimensionality
of Ag-ReO4 (minus the ligand) decreases from 3D
condensed (11) to 2D layers (8, 4, 5) to 0D molecular
(3) and to no connectivity (9). The ligands have a
predominant role in altering the Ag-ReO4 network
Figure 10. (A) A plot of the optical absorption coefficients, R (cm-1
)
versus wavelength (nm), and (B) a plot of (Rhυ)2 vs hυ for hybrids 1-9, 10
(AgReO4(pyz)), and 11 (AgReO4).
modifications (e.g., coordination environment, network
dimensionality, etc.) of the Ag(I)-Re(VII) hybrids. The
absorption coefficient, R (cm-1), determines the rate of
light absorption in a solid as a function of depth (at R-1
the intensity drops to ∼36% of its starting intensity). It is
one of the most critical but underexplored parameters of
semiconductor materials for efficiently capturing solar
energy for conversion into fuels and/or electricity; for
example, a low R leads to light being poorly absorbed in a
material.33 Generally, semiconductors exhibit a sharp
edge in their absorption coefficient spectra, since only
light of sufficient energy will excite an electron across the
band gap. Thus, R depends on both the solid and on the
wavelength of light. As studied previously in AgReO4 and
AgReO4(pyz),17,34,35 the Ag(I)-Re(VII) hybrids are pro-
mising candidates for light absorption at relatively low
visible-light energies owing to a MMCT excitation be-
tween the electron-donating d10 (Ag) and electron-accept-
ing d0 (Re) configurations. Thus, an investigation of their
optical properties was conducted herein to probe how the
atomic-level structure alters their light-absorption char-
acteristics.
(36) Spitaler, J.; Ambrosch-Draxal, C.; Nachbauer, E.; Belaj, F.; Gomm,
H.; Netzer, F. Phys. Rev. B 2003, 67, 115127/1.
(37) (a) Bube, R. H. Electrons in Solids; Academic Press, Inc.: San Diego,
1988; (b) Van Leeuwen, R. A.; Hung, C. J.; Kammler, D. R.; Switzer, J. A.
J. Phys. Chem. 1995, 99, 15247.
(33) Honsberg C.; Bowden, S.; Photovoltaics CDROM, II ed., 2008.
(34) Kunkely, H.; Vogler, A. Inorg. Chim. Acta 2004, 357, 1317.
(35) Otto, J. W.; Vassiliou, J. K.; Porter, R. F.; Ruoff, A. L. J. Phys.
Chem. Solids 1992, 53, 631.

Article
Inorganic Chemistry, Vol. 48, No. 23, 2009 11275
Figure 12. Thermogravimetric analyses (TGA) of hybrids 1-9, plotted
as weight (%) versus temperature (°C).
and 7 with the next highest Eg sizes that range from
∼2.89-2.96 eV. The next lowest set of valence band
energies for 5, 9, 11, 8, 3 and 4 also closely follows the same
trend toward increasing Eg from ∼3.30-3.75 eV. Owing to
the nature of the bandgap transition, that is, LMCT for the
first five hybrids and MMCT for the next six, the valence
band is composed primarily of the Ag d-orbitals in the latter
and the ligand-based π-orbitals in the former. Obviously,
there is significant orbital mixing between the Ag 4d and
ligand π-orbitals and a gradual progression of their mixture
in the valence band, and both are significant factors in
determining the resultant Eg of the hybrids.
The hybrid network dimensionality (e.g., clusters,
chains) and local coordination environments should effect
the bandgap sizes, such as via the strength of the d-orbital-
to-ligand interactions or in the band broadening for
extended versus molecular networks. Described above,
the local Ag coordination environments vary quite sig-
nificantly among the hybrids 1-9, and are dependent on
the N-donor positions, the presence of carboxylate
groups, and the geometry and size of the ligand. These
include a distorted seesaw {AgN2O2} coordination geo-
metry in 3 and 4, a distorted {AgN4} tetrahedral geometry
in 9, a distorted {AgN2O4} octahedral geometry in 6, and
distorted {AgN5} or {AgN2O3} square pyramidal or
trigonal bipyramidal coordination geometries in 1, 2, 7,
8, and 10. The structure of 5 contains Ag-centered seesaw,
octahedral, and square-pyramidal coordination geome-
tries. The greater d-orbital interactions of the distorted
square-pyramidal/trigonal-bipyramidal and octahedral
coordination geometries in 1, 2, 6, 7, and 10 raise the
filled Ag 4d10 orbitals to higher energies and results in
their relatively smaller bandgap sizes. By comparison, the
tetrahedral and seesaw coordination geometries of 3, 4, 9,
and 11 result in weaker d-orbital interactions, a lower-
energy valence band, and increased bandgap sizes. By
contrast, no trends were found between the Ag-ReO4
network dimensionality and the bandgap sizes, owing to
the relatively much weaker interaction between the Re 5d-
orbitals and the Ag- or ligand-based orbitals.
Figure 11. XPS valence-band spectra of hybrids 1-9, 10 (AgReO4-
(pyz)) and 11 (AgReO4), ordered according to decreasing binding
energies.
dimensionality, and thus, the absorption coefficients that
are dependent on the electronic transition probabilities.
X-ray Photoelectron Spectroscopy (XPS). Electronic
structure calculations on Ag(I)/Cu(I)-Rhenate(VII) hy-
brids have shown previously that the conduction band is
composed primarily of unfilled Re 5d orbitals, while the
valence band originates from mixture of Ag 4d, O 2p, and
-
N 2p orbitals. As described above, the ReO4 anions in
hybrid structures 1-9 exhibit nearly identical tetrahedral
coordination geometries, while the coordination environ-
ments and connectivities of the Ag sites vary substantially
as a result of the different ligands. Thus, while the valence
bands are expected to exhibit a larger variation in en-
ergies, the conduction bands are more fixed at similar
energy levels.
The XPS results show that the valence band energies of
the hybrids, shown in Figure 11, closely correlate with the
changes in their bandgap energies (Eg) (central peak
positions arranged from smallest to largest Eg: 2, 3.85 eV;
1, 4.40 eV; 10, 4.45 eV; 6, 4.50 eV; 7, 4.52 eV; 5, 4.80 eV; 9,
4.95 eV; 11, 5.10 eV; 3, 5.18 eV; 8, 5.20 eV; 4, 5.45 eV). For
example, the highest valence band energy was measured for
2 which has the lowest Eg of 2.60 eV, followed by 1, 10, 6,
Thermogravimetric Analysis (TGA). Many hybrid metal-
oxides/organics exhibit high thermal stabilities, can be
thermally converted to new or known condensed phases
after removal of the ligand, and reversibly absorb water or
other small molecules. The TGA results for hybrid solids
1-9 are shown in Figure 12. The post-TGA products were
characterized using Powder X-ray Diffraction (PXRD)

11276 Inorganic Chemistry, Vol. 48, No. 23, 2009
Lin et al.
and these results provided in the Supporting Information
(Figure 2S). The TGA results can be separated into two
groups, one group (1, 2, 8, and 9) that decomposes into
Ag(s) and other amorphous solids, and another group that
decomposes to yield the condensed AgReO4 (4, 5, 6, and 7)
or the ReO2 (3) phase.
resulted in a soft collapse of the hybrid structures to give
AgReO4 in the products.
Conclusions
A series of nine new silver(I)-rhenate(VII) oxide/organic
hybrids was synthesized. The use of large multidentate
chelating ligands resulted in the molecular and chain-like
In the first group, the strongly chelating bpp and tpp
ligands in 1 and 2 produce two of the most thermally
stable hybrid structures at temperatures of up to ∼300-350 °C.
Further, heating to ∼500 °C results in a weight change
corresponding to only a partial loss of the ligands
(∼20.2%, calcd. 56.7% for 1; ∼21.8%, calcd. 50.8%
for 2), although the lattice H2O in 2 is easily removed
at ∼100 °C (∼2.3%, calcd. 2.4%). Both post-TGA
residuals contain a similar mixture of Ag(s) and an
amorphous material. The same residual products were
found for 8 and 9 as well, with the bridging bpy and dpa
ligands. In 8, removal of the bpy ligand begins at ∼300 °C
and extends to ∼480°C, whiletheremovalofthe dpaligand
in 9 extends from ∼150 to ∼480 °C. The thermal stability of
9, which is lower than any of the other hybrids, is because
of the absence of any close Ag-ReO4 interactions. Thus, all
of the larger and chelating ligands (bpp, tpp, bpy, and dpa)
inhibited the soft collapse of the metal-oxide framework,
and instead yielded Ag(s) and amorphous products rather
than crystalline AgReO4.
structures of Ag(bpp)ReO4 and Ag(tpp)ReO4 H2O with
3
limited Ag-ReO4 connectivity. Conversely, shorter bridging
ligands yielded the layered and pillared-types of hybrid
structures of Ag(tro)ReO4, Ag(pda)ReO4 1/2H2O, Ag-
3
(Hpzc)ReO4, and Ag2(Hpzc)(pzc)(H2O)ReO4, respectively,
each with a two-dimensional Ag-ReO4 connectivity. Lastly,
the interpenetrating networks of Ag(bpy)ReO4 and Ag-
(dpa)2ReO4 are found with the longest bridging ligands.
Measured bandgap sizes show that the hybrid solids fall into
two groups, and exhibit either larger bandgap sizes of
∼3.30-3.75 eV owing to MMCT excitations, or smaller
bandgap sizes of ∼2.60-2.96 eV from LMCT excitations.
For the hybrid group with the larger bandgap sizes and
MMCT, the absorption coefficients are relatively larger and
closely correlate with the Ag-ReO4 network dimensionality.
The XPS spectra reveal that the bandgap sizes are primarily
determined by changes in the valence band energies that are a
function of the Ag coordination geometries and d-orbital
energies. TGA results reveal the highest possible thermal
In the second group, the thermal decomposition of
hybrids 5, 6, and 7 leads to the low temperature loss of
their respective pda and pzc ligands at ∼200-250 °C and
the formation of condensed AgReO4. Interestingly, while
this transformation occurs in a single weight-loss step for
5 and 6, the weight loss of 7 exhibits four distinct steps. On
heating 7 to ∼500 °C, an ∼2.4% weight loss of coordi-
nated water from 120-160 °C (calcd. 2.5%) occurs,
followed by an ∼16.7% weight loss at ∼160-240 °C
that corresponds to the removal of one pzc ligand per
formula (calcd. 16.9%), and followed by another ∼16.8%
weight loss of the remaining pzc ligand at ∼280-300 °C,
resulting in the formation of AgReO4 and Ag. The
intermediate products are unknown and have not yet
been structurally characterized. The fourth and last
weight loss for 7 at >400 °C is due to the decomposition
of AgReO4. The post-TGA products of 4 were also
determined to be a mixture of AgReO4 and Ag, with the
loss of tro ligands from ∼220-360 °C (∼16.9%, calcd.
16.2%). Lastly, the decomposition of 3 exhibits two
weight-loss steps of ∼41% extending from 180 to 500 °C,
and that corresponds to the loss of both lattice water and
inca ligands (calcd. 42.5%), yielding a mixture of Ag and
ReO2. Thus, all of the shorter ligands (tro, pda, and pzc)
stability of ∼300 °C for Ag(bpp)ReO4, Ag(tpp)ReO4 H2O,
3
and Ag(bpy)ReO4, while for the smaller ligands in Ag-
(tro)ReO4, Ag(pda)ReO4 1/2H2O, and Ag(Hpzc)ReO4 there
3
is a lower-temperature structural collapse to yield condensed
AgReO4. In summary, the nature of the coordinating ligand
plays a critical role in directing the structure of each
hybrid, and therefore can be of great utility in tuning their
optical properties in the future design of new materials for
solar-energy conversion.
Acknowledgment. The authors acknowledge support of
this research from the donors of the American Chemical
Society Petroleum Research Fund (#46803-AC10), and
from the Chemical Sciences, Geosciences and Biosciences
Division, Office of Basic Energy Sciences, Office of
Science, U.S. Department of Energy (DE-FG02-
07ER15914), and also assistance with the collection of
single crystal X-ray data (P. Boyle).
Supporting Information Available: Crystallographic data for
1-9 in CIF format, tables of atomic coordinates and equivalent
isotropic displacement for each compound, powder X-ray dif-
fraction results for all prepared compounds including for the as-
synthesized hybrids and post-TGA residuals. This material is
available free of charge via the Internet at http://pubs.acs.org.