Diverse structures and dimensionalities in hybrid frameworks of strontium and lanthanum with isomeric dihydroxybenzoates

Article abstract of DOI:10.1021/ic060917k

Investigations of Sr and La derivatives of 2,3-, 2,4-, 3,5-, and 2,6-dihydroxybenzoic acids have revealed that it is possible to obtain chain and layered structures of these materials. [Sr₂(2,3-DHB) ₄(H₂O)₄]·3H₂O, I, and [Sr(2,4-DHB)₂(H₂O)₄].H₂O, II, are both one-dimensional coordination polymers based on dimers; both the carboxylate and phenolic oxygens bond to the metal in I and only the carboxylate oxygens bond to the metal in II. [Sr(3,5-DHB)₂-(H₂O) ₂]·4H₂O, III, has a one-dimensional chain structure with large channels and involves extended Sr-O-Sr connectivity. [Sr(CH ₃CO₂)(2,4-DHB)(H₂O)₃]·H ₂O, IV, also has a chain structure with extended Sr-O-Sr connectivity, but the chain structure is formed by bridging acetate units, unlike in III where the Dihydroxybenzoate (DHB) is the bridging unit. [Sr(2,6-DHB)₂(H₂O)₂]·0.5H₂O, V, is a two-dimensional coordination polymer where both carboxylate and phenolic oxygens coordinate to the metal. A lanthanum compound with the composition [La(CH₃CO₂)₂(2,4-DHB)H₂O]·0. 18H₂O, VI, with a one-dimensional chain structure with extended La-O-La connectivity, has been prepared. Besides these chain and layered structures, zero-dimensional dimers of rare-earth DHBs with the general formula [La(CH₃CO₂)₂(DHB)(H₂O)₂], with 3,5- (VII), 2,6- (VIII), and 2,3-dihydroxybenzoic acids (IX) have been synthesized. The diverse structures of the strontium and lanthanum DHBs are described. The study demonstrates that it is possible to obtain interesting structures of metal DHBs with different dimensionalities. The structures appear to be controlled largely by geometrical rather than electronic factors.

Full text of DOI:10.1021/ic060917k

Inorg. Chem. 2006, 45, 8227−8238
Diverse Structures and Dimensionalities in Hybrid Frameworks of
Strontium and Lanthanum with Isomeric Dihydroxybenzoates
Meenakshi Dan,†,‡ A. K. Cheetham,*,† and C. N. R. Rao*,‡
Materials Research Laboratory, UniVersity of California, Santa Barbara, California 93106, and
Chemistry and Physics of Materials Unit and CSIR Centre of Excellence in Chemistry,
Jawaharlal Nehru Centre for AdVanced Scientific Research, Jakkur P. O Bangalore 560064, India
Received May 26, 2006
Investigations of Sr and La derivatives of 2,3-, 2,4-, 3,5-, and 2,6-dihydroxybenzoic acids have revealed that it is
possible to obtain chain and layered structures of these materials. [Sr2(2,3-DHB)4(H2O)4] 3H2O, I, and [Sr(2,4-
‚
DHB)2(H2O)4].H2O, II, are both one-dimensional coordination polymers based on dimers; both the carboxylate and
phenolic oxygens bond to the metal in I and only the carboxylate oxygens bond to the metal in II. [Sr(3,5-DHB)2-
(H2O)2]
connectivity. [Sr(CH3CO2)(2,4-DHB)(H2O)3]
but the chain structure is formed by bridging acetate units, unlike in III where the Dihydroxybenzoate (DHB) is the
bridging unit. [Sr(2,6-DHB)2(H2O)2] 0.5H2O, V, is a two-dimensional coordination polymer where both carboxylate
and phenolic oxygens coordinate to the metal. A lanthanum compound with the composition [La(CH3CO2)2(2,4-
DHB)H2O] 0.18H2O, VI, with a one-dimensional chain structure with extended La La connectivity, has been
‚
4H2O, III, has a one-dimensional chain structure with large channels and involves extended Sr
−O−Sr
‚
H2O, IV, also has a chain structure with extended Sr Sr connectivity,
−O−
‚
‚
−O−
prepared. Besides these chain and layered structures, zero-dimensional dimers of rare-earth DHBs with the general
formula [La(CH3CO2)2(DHB)(H2O)2], with 3,5- (VII), 2,6- (VIII), and 2,3-dihydroxybenzoic acids (IX) have been
synthesized. The diverse structures of the strontium and lanthanum DHBs are described. The study demonstrates
that it is possible to obtain interesting structures of metal DHBs with different dimensionalities. The structures
appear to be controlled largely by geometrical rather than electronic factors.
1. Introduction
constitute an important endeavor in view of potential appli-
cations of these materials in sorption, gas storage, and lumi-
nescence.5-7 Research on hybrid materials has been generally
centered on the assembly of organic linkers with d-block
metal ions, with predictable coordination geometries.8-10 The
Organically templated open-framework metal silicates and
phosphates have been reported widely in the literature.1,2 In
the early 1990s, Robson and Hoskins3 used organic linkers
for the first time, to construct open-framework materials.
Since then, there have been many reports in the literature
on the synthesis and characterization of open-framework
metal carboxylates of varying dimensionalities.4 Design and
synthesis of hybrid materials employing carboxylate linkers
(4) (a) Rao, C. N. R.; Natarajan, S.; Vaidhyanathan, R. Angew. Chem.,
Int. Ed. 2004, 43, 1466. (b) Ocwig, N. C.; D-Friedrichs, O.; O’Keefe,
M.; Yaghi, O. M. Acc. Chem. Res. 2005, 38, 176. (c) Forster, P. M.;
Cheetham, A. K. Top. Catal. 2003, 24, 79.
(5) (a) Kitaura, R.; Fujimoto, K.; Noro, S.; Kono, M.; Kitagawa, S. Angew.
Chem., Int. Ed. 2002, 41, 133. (b) Zhao, X.; Xiao, B.; Fletcher, A. J.;
Thomas, K. M.; Bradshaw, D.; Rosseinsky, M. J. Science 2004, 306,
1012.
(6) Rosi, N. L.; Eukert, J.; Eddaoudi, M.; Vodak, D. T.; Kim, J.; O’Keeffe,
M.; Yaghi, O. M. Science 2003, 300, 1127.
(7) (a) Serre, C.; Pelle, F.; Gardant, N.; Fe´rey, G. Chem. Mater. 2004,
16, 1177. (b) Millange, F.; Serre, C.; Marrot, J.; Gardant, N.; Pelle,
F.; Fe´rey, G. J. Mater. Chem. 2004, 14, 642. (c) Thirumurugan, A.;
Natarajan, S. J. Mater. Chem. 2005, 15, 4588.
(8) (a) Forster, P. M.; Burbank, A. R.; O’Sullivan, M. C.; Guillou, N.;
Livage, C.; Fe´rey, G.; Stock, N.; Cheetham, A. K. Solid State Sciences
2005, 1549. (b) Rosi, N. L.; Kim, J.; Eddaoudi, M.; Chen, B.;
O’Keeffe, M.; Yaghi, O. M. J. Am. Chem. Soc. 2005, 127, 1504. (c)
Livage, C.; Guillou, N.; Chaigneau, J.; Rabu, P.; Drillon, M.; Fe´rey,
G. Angew. Chem., Int. Ed. 2005, 44, 6488.
* To whom correspondence should be addressed. E-mail: cnrrao@
jncasr.ac.in (C.N.R.R.), cheetham@mrl.ucsb.edu (A.K.C.). Fax: (+91)80-
22082766 (C.N.R.R.), +1 805 893 8797 (A.K.C.).
† University of California.
‡ Jawaharlal Nehru Centre for Advanced Scientific Research.
(1) (a) Breck, D. W. Zeolite Molecular SieVes; Wiley: New York, 1974.
(b) Meier, W. M.; Oslen, D. H.; Baerlocher, C. Atlas of Zeolite
Structure Types; Elsevier: London, 1996.
(2) Cheetham, A. K.; Fe´rey, G.; Loiseau, T. Angew. Chem., Int. Ed. 1999,
38, 3268.
(3) (a) Hoskins, B. F.; Robson, R. J. Am. Chem. Soc. 1990, 112, 1546.
(b) Abrahams, B. F.; Hoskins, B. F.; Michail, D. M.; Robson, R.
Nature 1994, 369, 727.
10.1021/ic060917k CCC: $33.50
Published on Web 09/02/2006
© 2006 American Chemical Society
Inorganic Chemistry, Vol. 45, No. 20, 2006 8227

Dan et al.
Table 1. Crystal Data and Structure Refinement Parameters for Compounds I-V
parameter
I
II
III
IV
V
empirical formula
crystal system
space group
crystal size (mm)
a (Å)
Sr2C28H31O23
triclinic
P1h
SrC14H20O13
triclinic
P1h
SrC14H18O14
monoclinic
C2/c
0.2 × 0.25 × 0.4
11.703(4)
21.409(6)
7.841(2)
90.0
90.131(5)
90.0
SrC9H15O10
triclinic
P1h
SrC14H14O10.5
triclinic
P1h
0.28 × 0.26 × 0.25
10.240(2)
10.502(2)
16.441(4)
98.156(4)
96.206(4)
96.373(4)
1725.5(7)
2
0.21 × 0.25 × 0.27
0.22 × 0.20 × 0.22
0.24 × 0.15 × 0.2
7.5989(4)
9.3805(5)
11.3938(6)
91.871(1)
95.303(2)
90.687(2)
808.17(7)
2
6.796(2)
7.047(2)
b (Å)
c (Å)
10.461(2)
13.095(3)
101.380(3)
91.542(4)
97.946(4)
902.5(3)
7.719(2)
13.786(3)
99.96(2)
100.05(1)
105.38(3)
692.7(3)
R (deg)
â (deg)
γ (deg)
volume (Å3)
Z
1964.6(10)
4
2
2
formula mass
910.77
1.753
0.71073
3.186
1.98-26.37
18642
-12 e h e 12,
-13 e k e 13,
-20 e l e 20
7016
483.92
1.781
0.71073
3.057
2.01-28.04
7921
-8 e h e 8,
-13 e k e 13,
-17 e l e 17
4102
497.90
1.683
0.71073
2.815
1.90-28.38
7869
-14 e h e 15,
-28 e k e 28,
-10 e l e 10
2276
370.83
1.778
0.71073
3.937
1.54-23.27
2850
-6 e h e 7,
-8 e k e 8,
-15 e l e 12
1948
437.87
1.799
0.71073
3.393
1.80-23.37
3387
-6 e h e 8,
-9 e k e 10,
-12 e l e 12
2318
F
calcd (g cm-3
)
λ (Mo KR; Å)
µ (mm-1
Θ range (deg)
)
total data collected
limiting indices
unique data
refinement method
full-matrix least
squares on |F2|
0.0423
full-matrix least
squares on |F2|
0.0183
full-matrix least
squares on |F2|
0.0427
full-matrix least
squares on |F2|
0.0479
full-matrix least
squares on |F2|
0.0267
Rint
R indexes [I > 2σ(I)]
R1 ) 0.0399,
wR2 ) 0.0777
R1 ) 0.0684,
wR2 ) 0.0886
1.009
R1 ) 0.0310,
wR2 ) 0.0720
R1 ) 0.0387,
wR2 ) 0.0751
1.049
R1 ) 0.0530,
wR2 ) 0.1202
R1 ) 0.0629,
wR2 ) 0.1238
1.194
R1 ) 0.0394,
wR2 ) 0.0950
R1 ) 0.0444,
wR2 ) 0.0972
0.974
R1 ) 0.0283,
wR2 ) 0.0673
R1 ) 0.0359,
wR2 ) 0.0692
0.983
R (all data)
goodness of fit
no. of variables
554
291
152
221
259
largest difference map peak
0.497 and -0.354
0.637 and -0.458
0.773 and -1.480
0.541 and -0.836
0.433 and -0.503
and hole e Å-3
f-block and main group elements are also gaining attention,
the former because they afford materials with high coordina-
tion numbers and variable geometry as well as properties
arising from the f-f electronic transitions. A careful choice
of the organic ligands coupled with the lanthanide ions can
thus lead to new and interesting luminescent materials.7,11
Of the many carboxylic acids employed in recent years for
building hybrid networks, special mention must be made of
the isomeric dicarboxylic acids of benzene, cyclohexane, and
cyclohexene.6,9,10
Dihydroxybenzoic acids are attractive candidates for
forming hybrid frameworks because they can, in principle,
bind to the metal ions by making use of the carboxylate and
hydroxyl oxygens. There are not many reports of metal
dihydroxybenzoates (DHBs) whose structures have been
determined, and most of them are monomeric or one-
dimensional polymeric metal complexes.12-14 No chain with
continuous M-O-M connectivity or layered structures have
been reported hitherto. In the present study, we have
investigated the compounds formed by different isomeric
dihydroxybenzoic acids with a rare earth ion (La3+) and an
equally large alkaline earth metal ion (Sr2+). We have thus
been able to synthesize and characterize several lanthanum
and strontium DHBs possessing zero-, one-, and two-
dimensional structures.
2. Experimental Section
Synthesis. Strontium DHBs, I-V. Compounds I-V were
prepared under mild conditions. For the synthesis of I-III and V,
0.5 mmol of the dihydroxybenzoic acid was dissolved in 5 mL of
deionized water, and 0.5 mmol of strontium carbonate was added
slowly to the solution under constant stirring. The reaction mixture
was heated to 50 °C and stirred for 30 min. Compounds I-III and
V could also be prepared by adding 0.5 mmol of strontium acetate
to a solution of 0.5 mmol of the dihydroxybenzoic acid in 5 mL of
deionized water. For the synthesis of IV, 0.5 mmol of strontium
carbonate was dispersed in 5 mL of deionized water, to which 1.5
mmol of acetic acid was added dropwise, and the mixture was
stirred for 10 min until it formed a clear solution. To this solution,
(12) (a) Suh, M. P.; Min, K. S.; Ko, J. W.; Choi, H. J. Eur. J. Inorg. Chem
2003, 1373. (b) Wang, H.-Y.; Gao, S.; Ng, S. W. Acta Crystallogr.
2005, E61, m2639. (c) Glowiak, T.; Kozlowski, H.; Erre, L. S.; Micera,
G.; Gulinati, B. Inorg. Chim. Acta 1992, 202, 43. (d) Litos, C.; Terzis,
A.; Raptopoulou, C.; Rontoyianni, A.; Karaliota, A. Polyhedron 2006,
25, 1337. (e) Griffith, W. P.; Nogueira, H. I. S.; Parkin, B. C.;
Sheppard, R. N.; White, A. J. P.; Williams, D. J. J. Chem. Soc., Dalton
Trans. 1995, 1775. (f) Micera, G.; Erre, L. S.; Cariati, F.; Ciani, G.;
Sironi, A. Inorg. Chim. Acta 1985, 108, L1.
(13) Papaefstathiou, G. S.; Darrow, B. G.; MacGillivray, L. R. J. Chem.
Crystallogr. 2002, 32, 191.
(9) (a) Thirumurugan, A.; Rao, C. N. R. J. Mater. Chem. 2005, 15, 3853.
(b) Thirumurugan, A.; Avinash, M. B.; Rao, C. N. R. Dalton Trans.
2006, 221.
(10) Kim, D.-S.; Forster, P. M.; Diaz de Delgado, G.; Cheetham, A. K. J.
Chem. Soc., Dalton Trans. 2004, 3365.
(11) (a) Dan, M.; Cottereau, G.; Rao, C. N. R. Solid State Sciences 2005,
7, 437. (b) Reineke, T. M.; Eddaoudi, M.; Fehr, M.; Kelley, D.; Yaghi,
O. M. J. Am. Chem. Soc. 1999, 121, 1651. (c) Serpaggi, F.; Luxbacher,
T.; Cheetham, A. K.; Fe´rey, G. J. Solid State Chem. 1999, 145, 580.
(14) (a) Cariati, F.; Erre, L.; Micera, G.; Panzanelli, A.; Ciani, G.; Sironi,
A. Inorg. Chim. Acta 1983, 80, 57. (b) Garriba, E.; Micera, G.; Zema,
M. Inorg. Chim. Acta 2004, 357, 2038. (c) Soares-Santos, P. C. R.;
Nogneira, H. I. S.; Paz, F. A. A.; SaFerreira, R. A.; Carlos, L. D.;
Klinowski, J.; Trindade, T. Eur. J. Inorg. Chem. 2003, 3609. (d) Ali,
S. F. M.; Rao, V. R. J. Inorg. Nucl. Chem. 1975, 37, 1041. (e) Soares-
Santos, P. C. R.; Nogueira, H. I. S.; Paz, F. A. A.; Sa Ferreira, R. A.;
Carlos, L. D.; Klinowski, J.; Trindade, T. J. Alloys Compd. 2004, 374,
344.
8228 Inorganic Chemistry, Vol. 45, No. 20, 2006

Structure and Dimensionality in Sr and La Hybrid Frameworks
Table 2. Crystal Data and Structure Refinement Parameters for Compound VI-IX
parameter
VI
VIIa
VIII
IX
empirical formula
crystal system
space group
crystal size (mm)
a (Å)
b (Å)
c (Å)
LaC11H13O9.18
orthorhombic
Pbcn
0.22 × 0.20 × 0.18
15.226(4)
8.322(2)
23.698(5)
90.0
90.0
LaC11H15O10
triclinic
P1h
LaC11H15O10
monoclininc
P2(1)/n
0.3 × 0.27 × 0.3
11.1858(3)
8.3939(1)
16.5029(3)
90.0
104.069(1)
90.0
1503.02(5)
4
LaC11H14O10
monoclininc
P2(1)/n
0.25 × 0.27 × 0.22
11.5638(2)
8.2409(2)
16.6299(2)
90.0
105.601(1)
90.0
1526.38(5)
4
0.28 × 0.25 × 0.20
8.5452(18)
9.1824(19)
10.910(2)
103.651(3)
101.304(3)
110.923(3)
739.0(3)
R (deg)
â (deg)
γ (deg)
90.0
3002.7(12)
8
volume (Å3)
Z
2
formula mass
430.92
1.906
0.71073
2.886
2.18-25.40
20487
-18 e h e 18,
-9 e k e 10,
-25 e l e 28
2722
446.14
2.005
0.71073
2.939
2.02-28.37
6160
-11 e h e 11,
-11 e k e 11,
-14 e l e 14
3262
446.14
1.972
0.71073
2.890
2.00-23.26
5842
-7 e h e 12,
-8 e k e 9,
-18 e l e 18
2158
446.13
1.937
0.71073
2.846
1.93-23.28
5982
-12 e h e 12,
-9 e k e 8,
-18 e l e 13
2196
F
calcd (g cm-3
)
λ (Mo KR; Å)
µ (mm-1
Θ range (deg)
)
total data collected
limiting indices
unique data
refinement method
full-matrix least
squares on |F2|
0.0290
full-matrix least
squares on |F2|
0.0190
full-matrix least
squares on |F2|
0.0344
full-matrix least
squares on |F2|
0.0570
Rint
R indexes [I > 2σ(I)]
R1 ) 0.0246,
wR2 ) 0.0481
R1 ) 0.0366,
wR2 ) 0.0513
1.047
R1 ) 0.0224,
wR2 ) 0.0532
R1 ) 0.0249,
wR2 ) 0.0544
1.038
R1 ) 0.0240,
wR2 ) 0.0587
R1 ) 0.0275,
wR2 ) 0.0597
1.034
R1 ) 0.0296,
wR2 ) 0.0610
R1 ) 0.0366,
wR2 ) 0.0627
0.925
R (all data)
goodness of fit
no. of variables
207
221
215
211
largest difference map peak
0.962 and -0.343
0.761 and -0.924
0.618 and -0.936
0.551 and -1.014
and hole e Å-3
a The unit cell dimensions of the isostructural Y and Gd compound are, for Y, a ) 8.415(6), b ) 8.966(6), c ) 10.713(8) Å, and vol ) 697.1(8) Å3 and,
for Gd, a ) 8.464(2), b ) 9.006(3), c ) 10.778(3) Å, and vol ) 707.4(3) Å3.
0.5 mmol of 2,4-DHB was added slowly, and the mixture stirred
for 30 min. The solutions were filtered and kept undisturbed at 50
°C for 24 h, from which the strontium DHBs, I-V, crystallized
out.
SrC9O10H16: C, 29.06%; H, 4.3%. Found: C, 29.47%; H, 4.41%.
For V, calcd for SrC14O10.5H15: C, 38.3%; H, 3.42%. Found: C,
38.25%; H, 3.81%; %. For VI, calcd for LaC11O9.18H13.36: C, 30.2%;
H, 3.2%. Found: C, 30.43%; H, 3.15%. For VII, VIII, IX, calcd
for LaC11O10H15: C, 29.6%; H, 3.36%. Found for VII, C, 29.4%;
H, 3.48%. Found for VIII, C, 29.33%; H, 3.40%. Found for IX,
C, 29.8%; H, 3.33.
Single-Crystal Structure Determination. A suitable single
crystal of each of the compounds I-IX was carefully selected under
a polarizing microscope and glued at the tip of a thin glass fiber
with cyano-acrylate (superglue) adhesive. Single-crystal structure
determination by XRD was performed on a Siemens Smart-CCD
diffractometer equipped with a normal focus, 2.4 kW sealed tube
X-ray source (Mo KR radiation, λ ) 0.710 73 Å) operating at 40
kV and 40 mA. A hemisphere of intensity data was collected at
room temperature with ω space scans (width of 0.30° and exposure
time of 10 s per frame). Pertinent experimental details of the
structure determination for compounds I-V and VI-IX are
presented in Tables 1 and 2, respectively.
Lanthanum DHBs, VI-IX. Compounds VI-IX were prepared
under mild conditions starting with 0.5 mmol of the dihydroxy-
benzoic acid dissolved in 5 mL of deionized water. To this solu-
tion, 0.5 mmol of lanthanum acetate was added slowly under
constant stirring. The clear solutions were kept undisturbed at 50
°C for 24 h, from which the lanthanum DHBs crystallized out.
In the case of VI and IX good quality crystals suitable for single
crystal structure solution could be obtained only by layer diffusion,
wherein a solution of 0.5 mmol of the corresponding dihydroxy-
benzoic acid in 2 mL of n-butanol was layered on top of a solution
of the 0.5 mmol of the lanthanum acetate in 2 mL of deionized
water.
The products, I-IX, were vacuum filtered and washed with
water. The yields generally exceeded 80%. The compounds were
characterized by powder X-ray diffraction (XRD), which indicated
the products to be new materials and monophasic, the patterns being
consistent with those generated from single-crystal X-ray diffraction.
The simulated and experimental powder XRD patterns for the
compounds are provided in the Supporting Information. The
products form over a wide pH range (4-8) of the starting reaction
mixture and over a range of temperatures varying from room
temperature to 100 °C for both the La and the Sr compounds.
Elemental analyses of I-IX were satisfactory. For I, calcd
for Sr2C28O23H34: C, 36.8%; H, 3.79%. Found: C, 36.87%; H,
3.71%. For II, calcd for SrC14O13H20: C, 34.74%; H, 4.14%.
Found: C, 34.85%; H, 3.97%. For III, calcd for SrC14O14H22: C,
33.5%; H, 4.4%. Found: C, 33.75%; H, 4.55%. For IV, calcd for
The structures were solved by direct methods using SHELXS-
86,15 which readily established the heavy atom position (La and
Sr) and facilitated the identification of the light atoms (O, C, H)
from difference Fourier maps. An empirical absorption correction
based on symmetry equivalent reflections was applied using the
SADABS program.16 All the hydrogen positions were initially
(15) (a) Sheldrick, G. M. SHELXS-86 Program for Crystal Structure
Determination; University of Gottingen: Gottingen, Germany, 1986.
(b) Sheldrick, G. M. Acta Crystallogr., Sect. A 1990, 46, 467.
(16) Sheldrick, G. M. SADABS: Siemens Area Detector Absorption
Correction program; University of Gottingen: Gottingen, Germany,
1994.
Inorganic Chemistry, Vol. 45, No. 20, 2006 8229

Dan et al.
Figure 1. (a) ORTEP plot of [Sr2(2,3-DHB)4(H2O)4]‚3H2O, I. Thermal ellipsoids are shown at 50% probability. Only the asymmetric part is labeled.
Hydrogen atoms are not shown for clarity. (b) One-dimensional chain structure formed in I. Note one of the 2,3-DHB molecules uses its hydroxyl groups
to bond to the Sr atoms with the carboxylate end remaining free. (c) The three-dimensional packing diagram of I, with the interstitial molecules residing in
the channels.
located in the difference Fourier maps, and the hydrogen atoms
were placed geometrically and held in the riding mode for the final
refinement. With the single-crystal X-ray data collected at room
temperature, few of the hydrogen atoms of the water molecules
could not be located from difference Fourier maps. The last cycle
of refinement included atomic positions for all the atoms, anisotropic
thermal parameters for all the non-hydrogen atoms, and isotropic
thermal parameters for all the hydrogen atoms. Full-matrix least-
squares structure refinement against |F2| was carried out using the
SHELXTL-PLUS package of programs.17
Thermogravimetric analysis was carried out with a heating rate
of 2 °C min-1 in an oxygen atmosphere using a Mettler 851eTG/
sDTA coupled to a Blazers ThermoStar 300 AMU mass spectrom-
eter. X-ray thermodiffractometry (Cu KR radiation, λ ) 1.5418
Å) was performed under static air in a Bruker D8 Advance
diffractometer outfitted with a M. Braun Position Sensitive Detector
and an Anton Paar HTK 16 high-temperature stage.
(17) Sheldrick, G. M. SHELXS-93 Program for Crystal Structure Solution
and Refinement; University of Gottingen: Gottingen, Germany, 1993.
8230 Inorganic Chemistry, Vol. 45, No. 20, 2006

Structure and Dimensionality in Sr and La Hybrid Frameworks
Figure 3. (a) ORTEP plot of [Sr(3,5-DHB)2(H2O)2]‚4H2O, III. Thermal
ellipsoids are shown at 50% probability. Only the asymmetric part is labeled.
(b) The chain structure of III with one-dimensional Sr-O-Sr connectivity.
Figure 2. (a) ORTEP plot of [Sr(2,4-DHB)2(H2O)4]‚H2O, II. Thermal
ellipsoids are shown at 50% probability. Hydrogen atoms are not shown
for clarity. (b) The one-dimensional chain structure formed in II by the
bridging of the dimers by the carboxylate end of the DHB molecule,
resulting in the formation of four-membered rings in the chain.
cessful. The DHB units exhibit two different types of
coordination modes that are quite different from those formed
in the DHBs to be discussed later. Both types of DHB unit
bind to the Sr atom in a chelating fashion through the
adjacent hydroxyl groups in the benzene ring with an
O-Sr-O bite angle of 60°. Such a coordination mode of
the 2,3-dihydroxybenzoic acid has been observed earlier
too.12d,e The carboxylate end of one type of DHB unit remains
free, whereas the other type binds to the Sr atom of the
adjacent dimer in a monodentate fashion through its car-
boxylate end. This leads to the formation of 4-rings (defined
by two DHB units and two Sr atoms) which connect the
dimers to form chains parallel to the c axis of the unit cell
(Figure 1b). The Sr-O-Sr connectivity does not run across
the entire length of the chain but is limited to the dimers.
The chains are arranged parallel to the bc plane of the unit
cell. Such a stacking results in the formation of channels
along the c axis of the unit cell, where the interstitial water
molecules reside and form hydrogen bonds with the frame-
work oxygen atoms (Figure 1c).
3. Results and Discussion
3.1. Strontium DHBs, I-V. With the various DHBs
strontium forms one-dimensional coordination polymers and
chain structures as well as two-dimensional networks. Thus,
with 2,3-DHB, Sr forms a one-dimensional coordination
polymer with the formula [Sr2(2,3-DHB)4(H2O)4]‚3H2O, I.
The asymmetric unit of I contains two Sr atoms, 20 oxygen
atoms belonging to the four DHB molecules and four
coordinated water molecules, 28 carbon atoms, and 3
interstitial water molecules (Figure 1a). The Sr atoms are
eight-coordinated and are bridged by coordinating water
molecules to form dimers with Sr-O-Sr connectivity
(Figure 1b). The bridging units were not hydroxo groups
because we could locate the hydrogens of the bridging water
in I and also the subsequent structures described in the article.
Furthermore, hydroxo groups would not compensate the
charge correctly. Attempts to locate or fix the hydrogens at
the carboxylate end of the DHB groups were also unsuc-
Sr with 2,4-DHB forms a dimer-based one-dimensional
coordination polymer with the formula [Sr(2,4-DHB)2-
(H2O)4]‚H2O, II. The asymmetric unit of II contains one Sr
Inorganic Chemistry, Vol. 45, No. 20, 2006 8231

Dan et al.
Figure 4. Three-dimensional packing diagram of III. Note the presence
of the rhombohedral channel (marked by the blue line) down the c axis of
the unit cell.
atom, 12 oxygen atoms belonging to two DHB molecules
and the four coordinated water molecules, 14 carbon atoms,
and one interstitial water molecule (Figure 2a). There are
two types of DHB units in the structure, one binding to a Sr
atom in a monodenate fashion through its carboxylate and
the other binding to two Sr atoms in a monodentate fashion
through the carboxylate (Figure 2b). As in I, the Sr atoms
are eight-coordinated and are bridged by coordinating water
molecules to form dimers with Sr-O-Sr connectivity. The
dimers are joined by carboxylate bridges to form chains
parallel to the a axis of the unit cell and also forming 4-rings
in the chain, as in structure I. Similar to I, the Sr-O-Sr
connection in II is limited to the dimers and does not run
across the entire length of the chain (Figure 2b). There is
extensive hydrogen bonding between the chains and the
interstitial water molecules.
Figure 5. (a) One-dimensional chain structure formed in [Sr(CH3CO2)-
(2,4-DHB)(H2O)3]‚H2O, IV. Note the presence of the acetate molecules in
the structure. (b) Three-dimensional packing diagram of IV.
With 3,5-DHB, Sr forms a chain structure of the composi-
tion, [Sr(3,5-DHB)2(H2O)2]‚4H2O, III. The asymmetric unit
of III contains one Sr atom in a special position, seven
oxygen atoms of which three belong to the interstitial water
molecules, and seven carbon atoms (Figure 3a). Two of the
interstitial water molecules are disordered with 50% oc-
cupancy in each site. The Sr atom is eight-coordinated. The
DHB binds only through carboxylate, and it coordinates to
one Sr atom in a chelating fashion and further bridges two
Sr atoms through a three-coordinating oxygen, thus forming
a one-dimensional chain with Sr-O-Sr connectivity parallel
to the c axis of the unit cell (Figure 3b). The chains are
separated by a unit-cell length along the a axis and are
stacked parallel to each other along the ac plane of the unit
cell. Such layers, in turn, are separated by half a unit-cell
length along the b axis and are shifted by half a unit-cell
length along the a axis, to form large rhombohedral
hydrogen-bonded channels with an aperture of 12.19 × 12.19
Å, measured from Sr to Sr, running down the c axis of the
unit cell (Figure 4). The presence of the channel results in
the large unit cell volume of III, the volume per Sr atom
being the highest of all the Sr DHBs investigated by us.
Interstitial water molecules residing in the interchain region
form multipoint hydrogen bonds with the oxygen atoms of
the chain to stabilize the structure.
Strontium forms a chain structure with 2,4-DHB as well,
the formula being [Sr(CH3CO2)(2,4-DHB)(H2O)3]‚H2O, IV.
Here, the Sr atom is nine-coordinated and binds to one DHB
unit, one acetate, and three water molecules. The asymmetric
unit of IV contains one Sr atom, nine oxygen atoms, nine
carbon atoms, and one interstitial water molecule. Both the
DHB and acetate units bind to the Sr atom through the
carboxylate oxygens. The acetate unit bridges the Sr atoms
8232 Inorganic Chemistry, Vol. 45, No. 20, 2006

Structure and Dimensionality in Sr and La Hybrid Frameworks
Figure 6. (a) Top view of the layer formed in [Sr(2,6-DHB)2(H2O)2]‚0.5H2O, V, by the joining of the dimers; the monodentate DHB molecules of the
dimers are shown in a lighter shade. Note the two different coordination modes of the 2,6-DHB molecule present in the structure. (b) Three-dimensional
packing diagram of the layers; note the presence of 4-ring channels running across the layer.
through the three-coordinating carboxylate oxygens, forming
chains parallel to the a axis of the unit cell, with pendant
DHB groups (Figure 5a). The chains, which have extended
Sr-O-Sr connectivity, are separated by a unit cell length
along the b axis and are arranged in layers within the ab
plane of the unit cell. The layers are separated by a unit cell
length along the c axis with the DHB units protruding from
the layers in an interdigitated manner (Figure 5b). The inter-
stitial water molecules reside in the interlamellar region. Hy-
drogen bonding between the chains and with the interstitial
water molecules renders additional stability to the structure.
With 2,6-DHB, strontium forms a layer structure, [Sr(2,6-
DHB)2(H2O)2]‚0.5H2O, V. The asymmetric unit of V con-
tains one Sr atom, 10 oxygen atoms belonging to two DHB
units and two coordinated water molecules, 14 carbon atoms,
and one interstitial water molecule with 50% occupancy. The
Sr atoms are bridged by coordinating water molecules to
form dimers having Sr-O-Sr connectivity. The DHB unit
exhibits two different coordination modes in this structure.
One 2,6-DHB unit binds to a Sr atom in a monodentate
fashion through the carboxylate end, and the other 2,6-DHB
unit binds to a Sr atom in a chelating fashion through the
carboxylate end and further connects the dimers by bonding
to the Sr atoms of the neighboring dimers through the
hydroxyl groups, thus forming layers parallel to the ab plane
of the unit cell (Figure 6a). Such connectivity results in the
Inorganic Chemistry, Vol. 45, No. 20, 2006 8233

Dan et al.
Table 3. Selected Bond Distances in I-Va
unit contains one La atom, 11 carbon atoms belonging to a
DHB molecule and two acetates, nine O atoms coordinating
to the La atom, of which two are from coordinating water
molecules, and one interstitial water molecule with 36%
occupancy (Figure 7a). The 2,4-DHB bridges the La centers
through its carboxylate end, forming a one-dimensional chain
structure parallel to the b axis of the unit cell. The La centers
in the chain are further bridged by three-coordinating oxygen
atoms of the chelating acetate molecules, thus giving rise to
one-dimensional La-O-La connectivity along the chain
(Figure 7b). Unlike the analogous Sr compound, IV, the
chains in VI are formed by both the DHB and the acetate
units. The chains are arranged parallel to each other in the
bc plane. Two adjacent chains are separated by half a unit-
cell length along the c axis. The layers are separated by half
a unit-cell length along the a axis and are stacked over one
another to complete the three-dimensional packing arrange-
ment in the crystal structure (Figure 7c). The interstitial water
molecules reside in the interlamellar region. The O-H‚‚‚O
type hydrogen bonds formed by the water molecules with
the oxygen atoms of the chain stabilize the structure.
moiety
distance (Å)
moiety
distance (Å)
I
Sr(1)-O(19)
Sr(1)-O(12)
Sr(1)-O(16)
Sr(1)-O(13)
Sr(1)-O(14)
Sr(1)-O(11)
Sr(1)-O(15)
Sr(1)-O(10)
2.524(2)
2.550(4)
2.570(3)
2.589(3)
2.603(3)
2.616(3)
2.652(3)
2.671(3)
Sr(2)-O(5)
Sr(2)-O(9)
Sr(2)-O(3)
Sr(2)-O(1)
Sr(2)-O(10)
Sr(2)-O(2)
Sr(2)-O(4)
Sr(2)-O(11)
2.507(2)
2.565(3)
2.572(3)
2.602(3)
2.631(3)
2.633(3)
2.652(3)
2.677(3)
II
Sr(1)-O(1)
Sr(1)-O(2)
Sr(1)-O(4)
Sr(1)-O(3)
2.523(2)
2.542(2)
2.613(4)
2.590(2)
Sr(1)-O(5)
Sr(1)-O(6)
Sr(1)-O(7)1
Sr(1)-O(7)
2.617(3)
2.665(2)
2.666(2)
2.681(2)
III
IV
Sr(1)-O(2)
Sr(1)-O(2)1
Sr(1)-O(4)1
Sr(1)-O(4)
2.492(3)
2.492(3)
2.603(4)
2.603(4)
Sr(1)-O(2)2
Sr(1)-O(2)3
Sr(1)-O(5)
Sr(1)-O(5)1
2.683(3)
2.683(3)
2.698(3)
2.698(3)
Sr(1)-O(1)
Sr(1)-O(2)1
Sr(1)-O(4)
Sr(1)-O(3)
Sr(1)-O(5)
2.515(4)
2.532(4)
2.656(5)
2.660(4)
2.661(5)
Sr(1)-O(6)
Sr(1)-O(2)
Sr(1)-O(1)2
Sr(1)-O(7)
2.670(5)
2.719(3)
2.755(4)
2.778(4)
V
Sr(1)-O(1)
Sr(1)-O(2)
Sr(1)-O(4)1
Sr(1)-O(3)
2.544(3)
2.565(2)
2.569(2)
2.573(2)
Sr(1)-O(4)
Sr(1)-O(5)
Sr(1)-O(6)2
Sr(1)-O(7)
2.616(2)
2.625(2)
2.657(2)
2.921(2)
We have isolated and characterized three closely related
lanthanum acetate DHBs VII, VIII, and IX of the general
formula [La(CH3CO2)2{CO2C6H3(OH)2}(H2O)2], with 3,5-
DHB, 2,6-DHB, and 2,3-DHB, respectively. These com-
pounds, which contain zero-dimensional (i.e., isolated)
dimers, were obtained even when the reaction temperature
was increased to 100 °C. [La(CH3CO2)2(3,5-DHB)(H2O)2],
VII, has an asymmetric unit containing 22 non-hydrogen
atoms. There is one crystallographically independent La
atom, 11 C atoms belonging to the two acetates and one
DHB molecule, and 10 O atoms, of which two are from water
molecules coordinating to the La center as shown in Figure
8a. The DHB molecule bridges the nine-coordinated La
centers through the carboxylate end, forming the dimer. The
La centers in the dimer are further bridged by the three
coordinating oxygen atoms of the acetate ions, giving rise
to La-O-La connectivity. The dimers are arranged linearly,
parallel to the a axis of the unit cell, and the linear
arrangements are parallel to the ac plane. The DHB rings in
the different chains overlap with each other in an interdigi-
tated fashion, indicating the presence of weak π-π interac-
tion between the benzene rings of the adjacent chains (Figure
8b). The hydroxyl groups of the overlapping benzene rings
are oriented in a staggered fashion, thereby minimizing the
possible electrostatic repulsion between them. Such a stack-
ing of the phenyl rings has been observed in a copper
derivative of 3,5-DHB.13 The O-H‚‚‚O type hydrogen bonds
between the coordinated water molecules and the oxygen
atoms of the carboxylate groups render additional stability
to the structure.
a Symmetry transformations used to generate equivalent atoms: (1) -x
+ 1, -y + 1, -z + 1 and (2) -x + 1, -y + 1, -z in I; (1) -x + 1, -y
+ 1, -z + 1 and (2) -x, -y + 1, -z in II; (1) -x + 2, -y, -z + 0.5; (2)
-x + 2, -y + 2, -z; (3) x, -y + 2, z + 0.5; and (4) -x + 2, -y + 2, -z
+ 1 in III; (1) -x - 1, -y, -z and (2) -x - 2, -y, -z in IV; and (1) -x
+ 2, -y + 1, -z + 2; (2) x + 1, y, z; (3) x - 1, y, z; and (4) -x + 2, -y,
-z + 2 in V.
formation of 4-ring channels running across the layer along
the a axis, and the pendant DHB molecules protrude out from
the layer (Figure 6b). Because the Sr-O-Sr connectivity is
confined to the dimers, the structure can be regarded as two-
dimensional coordination polymer. The interstitial water
molecules reside in the interlamellar region and form
O-H‚‚‚O type hydrogen bonds with the framework oxygen
atoms.
π-π interactions with energies in the 3-10 kcal mol-1
range stabilize molecules in many crystals.18 Compounds I,
IV, and V appear to have significant π-π interactions, with
the minimum centroid-to-centroid distances between the
participating aromatic rings being 3.485 Å and 3.539 Å (I)
and 3.849 Å (IV) and 3.599 Å (V); the dihedral angles are
0.0° in I, IV, and V.
The average Sr-O bond distances in the Sr DHB
frameworks are 2.597 (for Sr1) and 2.605 (for Sr2; I), 2.608
(II), 2.619 (III), 2.661 (IV), and 2.634 Å (V). The average
carboxylate C-O bond distances of the DHB units in I-V
are 1.263, 1.267, 1.260, 1.268, and 1.265 Å, respectively,
consistent with the values observed in other metal DHBs.12-14
The carboxylate C-O bond distance of the acetate moiety
in IV is 1.255 Å. Selected bond distances for compounds
I-V are summarized in Table 3.
In addition to the lanthanum compound, VII, isostructural
yttrium and gadolinium compounds with the formulas
[Y(CH3CO2)2(3,5-DHB)(H2O)2] and [Gd(CH3CO2)2(3,5-
DHB)(H2O)2] were also synthesized, and the structures were
solved by single-crystal XRD. The unit cell parameters of
the compounds are listed in the footnote to Table 1. The
3.2. Lanthanum DHBs, VI-IX. With 2,4-DHB, lantha-
num forms a one-dimensional chain structure of the formula
[La(CH3CO2)2(2,4-DHB)H2O]‚0.18H2O, VI. The asymmetric
(18) Hunter, C. A. Chem. Soc. ReV. 1994, 23, 101.
8234 Inorganic Chemistry, Vol. 45, No. 20, 2006

Structure and Dimensionality in Sr and La Hybrid Frameworks
Figure 7. (a) ORTEP plot of [La(CH3CO2)2(2,4-DHB)H2O]‚0.18H2O, VI. Thermal ellipsoids are shown at 50% probability. Hydrogen atoms are not
shown for clarity. (b) The one-dimensional chain structure of VI, with one-dimensional La-O-La connectivity. (c) Three-dimensional packing diagram of
VI.
cell dimensions and the volume are in the expected order Y
< Gd < La.19
The zero-dimensional dimeric structures obtained with 2,6-
DHB, [La(CH3CO2)2(2,6-DHB)(H2O)2], VIII, and 2,3-DHB,
[La(CH3CO2)2(2,3-DHB)(H2O)2], IX, were similar to the
structure of VII (Figure 8). The dimers, in VIII and IX, are
arranged parallel to each other to form linear arrangements
parallel to the b axis of the unit cell. The hydroxyl groups
of the overlapping benzene rings are oriented in a staggered
fashion just as in VII.
The average La-O bond distances in VI-IX are 2.575,
2.560, 2.559, and 2.55 Å, respectively. The average car-
(19) Shannon, R. D. Acta Crystallogr., Sect. A 1976, 32, 751.
Inorganic Chemistry, Vol. 45, No. 20, 2006 8235

Dan et al.
Figure 8. (a) ORTEP plot of [La(CH3CO2)2(3,5-DHB)(H2O)2], VII, with thermal ellipsoids shown at 50% probability. Only the asymmetric part is labeled.
(b) Three-dimensional packing diagram of VII. Note the staggered alignment of the hydroxyl groups of the overlapping DHB rings.
boxylate C-O bond distances of the DHB group in VI-IX
are 1.263, 1.257, 1.274, and 1.263 Å, respectively, and are
consistent with the values observed in other metal DHBs.12-14
The average carboxylate C-O bond distances of the acetate
group in VI-IX are 1.262, 1.261, 1.263, and 1.258 Å,
respectively. In Table 4, we show the selected bond distances
for compounds VI-IX.
3.3. Thermal Properties. Thermogravimetric analysis of
the strontium DHBs, I-III and V, in an oxygen atmosphere,
showed two distinct mass losses. The Sr compounds lose
water molecules around 100 °C (percentage weight loss for
I, calcd, 13.78%, found, 13.0%; for II, calcd, 18.61%, found,
18.11%; for III, calcd, 21.53%, found, 21.75%; for V, calcd,
10.26%, found, 10.50%). The second mass loss starts at 300
°C and is complete by 500 °C, corresponding to the pyrolysis
of DHB, leaving SrO as the product, as verified by powder
XRD (percentage weight loss for I, calcd, 63.51%, found,
63.80%; for II, calcd, 59.97%, found, 60.11%; for III, calcd,
57.81%, found, 57.55%; for V, calcd, 66.12%, found,
66.0%). In the case of IV, the mass losses occur in three
8236 Inorganic Chemistry, Vol. 45, No. 20, 2006

Structure and Dimensionality in Sr and La Hybrid Frameworks
Figure 9. Thermodiffractograms of VII showing the loss of crystallinity above 200 °C.
Table 4. Selected Bond Distances in VI-IXa
of coordinated water molecules (percentage weight loss for
VII, VIII, and IX, calcd, 8.07%, found, 8.40% for VII,
8.00% for VIII, 8.23% for IX). Variable temperature XRD
data show that sample VII loses its crystallinity at this
stage. In Figure 9 we have given the thermodiffractogram
data of compound VII. The second mass loss for compounds
VI-IX occurs gradually in the 250-350 °C range, corre-
sponding to the loss of acetates (percentage weight loss for
VI, calcd, 27.40%, found, 27.22%; for VII, VIII, and IX,
calcd, 26.46%, found, 27.00% for VII, 26.23% for VIII,
26.66% for IX). The loss of the DHB starts at 375 °C, most
of it being lost at 400 °C (percentage weight loss for VI,
calcd, 29.90%, found, 29.52%; for VII, VIII and IX, calcd,
28.93%, found, 28.10% for VII, 28.29% for VIII, 28.45%
for IX). The final product was found to be La2O3, verified
by powder XRD.
moiety
distance (Å)
moiety
distance (Å)
VI
La(1)-O(3)
La(1)-O(8)
La(1)-O(1W)
La(1)-O(7)
La(1)-O(9)1
2.416(2)
2.494(2)
2.510(2)
2.515(2)
2.587(2)
La(1)-O(6)
La(1)-O(4)
La(1)-O(6)1
La(1)-O(7)1
2.588(2)
2.626(2)
2.673(2)
2.770(2)
VII
VIII
IX
La(1)-O(9)
La(1)-O(4)1
La(1)-O(5)
La(1)-O(1)
La(1)-O(7)
2.464(2)
2.489(2)
2.510(2)
2.546(2)
2.563(2)
La(1)-O(8)1
La(1)-O(6)
La(1)-O(2)
La(1)-O(5)1
2.564(2)
2.577(2)
2.606(2)
2.724(2)
La(1)-O(1)
La(1)-O(2)1
La(1)-O(3)
La(1)-O(4)
La(1)-O(5)
2.510(2)
2.518(3)
2.530(3)
2.534(3)
2.535(3)
La(1)-O(6)
La(1)-O(7)
La(1)-O(8)
La(1)-O(1)1
2.550(3)
2.564(3)
2.591(3)
2.698(3)
La(1)-O(1)
La(1)-O(2)
La(1)-O(3)
La(1)-O(4)
La(1)-O(5)
2.484(3)
2.490(3)
2.500(3)
2.510(5)
2.538(4)
La(1)-O(9)
La(1)-O(6)
La(1)-O(7)
La(1)-O(3)1
2.562(3)
2.562(3)
2.573(3)
2.730(3)
4. Concluding Remarks
We have demonstrated that dihydroxybenzoic acids form
compounds with diverse structures and dimensionalities with
both Sr and La. The chain and layered structures, which are
reported here for the first time, are generally based on metal
DHB dimers, sometimes in combination with acetate. It is
noteworthy that some of the chains have metal-oxygen-
metal connectivity and are therefore extended inorganic
hybrid structures rather than coordination polymers. One of
the strontium compounds with the chain structure, III, has
large zeolite-like channels and is accordingly associated with
the largest unit cell volume among the Sr DHBs reported
here. By and large, the carboxylate unit is responsible for
giving rise to the chain or layered structures, presumably
because it is more acidic than the phenolic groups. We have
examined the various structures in the light of the acid
strengths of the different acids, which are approximately 2,6
> 2,4 > 2,3 > 3,520 but find no strong relation between the
acidity and the mode of coordination to the metal. For
example, the strongest and the second weakest acids, 2,6-
DHB and 2,3-DHB, both exhibit coordination through
carboxylate as well as phenolic groups. We, therefore, believe
a Symmetry transformations used to generate equivalent atoms: (1) -x
+ 0.5, y - 0.5, z and (2) -x + 0.5, y + 0.5, z in VI; (1) -x + 1, -y +
1, -z + 1 in VII; (1) -x + 1, -y + 1, -z - 1 in VIII; and (1) -x + 1,
-y, -z in IX.
steps. It loses the water molecules around 100 °C. The second
mass loss occurs in the range 175-250 °C, corresponding
to the loss of acetate molecules. The third mass loss
corresponding to the loss of DHB starts at 325 °C, leaving
behind SrO as the product (percentage weight loss for IV,
calcd, 19.37%, found, 19.80% for water loss; calcd, 15.88%,
found, 16.10% for the loss of acetate molecules; and calcd,
36.87%, found, 37.0% for DHB).
Thermogravimetric analysis of the lanthanum DHBs,
VI-IX, in oxygen atmosphere, showed similar behavior,
with mass losses occurring in three distinct stages. The first
mass loss for VI occurs between 100 and 200 °C, corre-
sponding to the loss of both interstitial and coordinated water
molecules (percentage weight loss for VI, calcd, 4.92%,
found, 4.80%), whereas for the three compounds, VII-IX,
it occurs between 150 and 200 °C, corresponding to the loss
Inorganic Chemistry, Vol. 45, No. 20, 2006 8237

Dan et al.
that the bonding is largely dictated by geometrical factors.
In view of this it is not surprising that we have not
encountered three-dimensional structures. We would expect
three-dimensional structures if we had another carboxylic
group in the 4-position as suggested recently.9 In the case
of the Sr 2,6-DHB structure, the Sr‚‚‚Sr connectivity is
achieved via the two phenolic groups, presumably on account
of their convenient geometry. It is also interesting that we
obtain only zero-dimensional dimeric lanthanum DHBs and
that these do not transform to chains or layers at higher
temperatures as found in many metal carboxylates.21 The
present study suggests that it would be worthwhile to
examine the structures of the DHBs formed by transition
metals.
Acknowledgment. M.D. is thankful to the International
Center for Materials Research, University of California, Santa
Barbara, for a short-term visitor’s grant. M.D. also thanks
the CSIR (India), for the award of the Senior Research
Fellowship. This work partly made use of facilities by the
MRL program of the National Science Foundation under
DMR05-20415.
Supporting Information Available: Simulated and experi-
mental powder XRD patterns for compounds I-IX. Crystallo-
graphic information files (CIF) for the compounds I-IX. This
material is available free of charge via the Internet at http://
pubs.acs.org.
IC060917K
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