Role of temperature and time in the formation of infinite -M-O-M-linkages and isolated clusters in MOFs: A few illustrative examples

Article abstract of DOI:10.1021/ic800621q

The role of temperature and time of reaction in the formation of metal-organic frameworks (MOFs) has been studied in two systems of compounds, manganese oxybis(benzoate) and manganese trimellitates, and the results compared and contrasted with other similar studies in the literature. The investigation reveals the formation of six different phases in oxybis(benzoate) and three phases in trimellitate systems. The low-temperature phases in both systems of compounds possess Mn₄ cluster units linked by the carboxylate ligands, while the high-temperature phase, irrespective of the duration of the reaction, has a three-dimensional structure with -Mn-O-Mn- linkages with brucite-related layers pillared by oxybis(benzoate) and Kagome-related layers pillared by trimellitate ligands. In all of the preparation, the reactions appear to have thermodynamic control as a function of the temperature. The isolation of low-dimensional structures in manganese oxybis(benzoate) at moderate time and temperature indicates possible kinetic control. The formation of reactive low-dimensional phases has been rationalized by considering the local charge distribution around the Mn site and also invoking a possible dissolution-recrystalization mechanism.

Full text of DOI:10.1021/ic800621q

Inorg. Chem. 2008, 47, 8451-8463
Forum
Role of Temperature and Time in the Formation of Infinite -M-O-M-
Linkages and Isolated Clusters in MOFs: A Few Illustrative Examples
Partha Mahata, Manikanda Prabu, and Srinivasan Natarajan*
Framework Solids Laboratory, Solid State and Structural Chemistry Unit, Indian Institute of
Science, Bangalore 560 012, India
Received April 9, 2008
The role of temperature and time of reaction in the formation of metal-organic frameworks (MOFs) has been
studied in two systems of compounds, manganese oxybis(benzoate) and manganese trimellitates, and the results
compared and contrasted with other similar studies in the literature. The investigation reveals the formation of six
different phases in oxybis(benzoate) and three phases in trimellitate systems. The low-temperature phases in both
4
systems of compounds possess Mn cluster units linked by the carboxylate ligands, while the high-temperature
phase, irrespective of the duration of the reaction, has a three-dimensional structure with -Mn-O-Mn- linkages
with brucite-related layers pillared by oxybis(benzoate) and Kagome-related layers pillared by trimellitate ligands.
In all of the preparation, the reactions appear to have thermodynamic control as a function of the temperature. The
isolation of low-dimensional structures in manganese oxybis(benzoate) at moderate time and temperature indicates
possible kinetic control. The formation of reactive low-dimensional phases has been rationalized by considering the
local charge distribution around the Mn site and also invoking a possible dissolution-recrystalization mechanism.
Introduction
usefulness of the strong directional coordination bonds and
the control achieved in the design of predetermined frame-
works. Some of the MOF compounds, thus prepared, exhibit
Metal-organic framework (MOF) compounds have at-
tracted considerable attention because of their unique struc-
tures that combine the coordination versatility of the metal
ion and the multibinding modes of the organic ligands. The
relatively easier possibility of tuning the pore sizes, the
introduction of functionality, and their possible use in
catalysis, sorption, separation, and related processes have
provided the much required impetus to study this class of
materials. Intense research activity during the past decade
or so has resulted in the isolation of many MOF compounds
2
good host-guest properties indicating remarkable structural
robustness and flexibility, very large pore volume and surface
3
area, etc.
(
2) (a) Wong-Foy, A. G.; Lebel, O.; Matzger, A. J. J. Am. Chem. Soc.
2007, 129, 15740. (b) Cussen, E. J.; Claridge, J. B.; Rosseinsky, M. J.;
Kepert, C. J. Am. Chem. Soc. 2002, 124, 9574. (c) Ma, S.; Sun, D.;
Simmons, J. M.; Collier, C. D.; Yuan, D.; Zhou, H. C. J. Am. Chem.
Soc. 2008, 130, 1012. (d) Maji, T. K.; Lemur, K.; Chang, H. C.;
Matsuda, R.; Kitagawa, S. Angew. Chem., Int. Ed. 2004, 43, 3269.
(
e) Lee, E. Y.; Jang, S. Y.; Suh, P. M. J. Am. Chem. Soc. 2005, 127,
374. (f) Rowsell, J. L. C.; Yaghi, O. M. J. Am. Chem. Soc. 2006,
1
of varying composition and structure. The diversity exhib-
6
ited by these compounds stands as a testimony to the
128, 1304. (g) Chen, B. L.; Ockwig, N. W.; Millward, A. R.; Contreras,
D. S.; Yaghi, O. M. Angew. Chem., Int. Ed. 2005, 44, 4745. (h)
Kesanli, B.; Cui, Y., Jr.; Bittner, E. W.; Bockrath, B. C.; Lin, W.
Angew. Chem., Int. Ed. 2005, 44, 72.
*
To whom correspondence should be addressed. E-mail: snatarajan@
sscu.iisc.ernet.in.
(3) (a) Li, H.; Eddaoudi, M.; O’Keeffe, M.; Yaghi, O. M. Nature 1999,
402, 276. (b) Chui, S. S. Y.; Lo, S. M. F.; Charmant, J. P. H.; Orpen,
A. G.; Williams, I. D. Science 1999, 283, 1148. (c) Chae, H. K.;
Siberio-Perez, D. Y.; Kim, J.; Go, Y. B.; Eddaoudi, M.; Matzger, A. J.;
O’Keeffe, M.; Yaghi, O. M. Nature 2004, 427, 523. (d) Wong-Foy,
A. G.; Matzger, A. J.; Yaghi, O. M. J. Am. Chem. Soc. 2006, 128,
3439. (e) Lin, X.; Jia, J.; Zhao, X.; Thomas, K. M.; Blake, A. J.;
Walker, G. S.; Champness, N. R.; Hubberstey, P.; Schroder, M. Angew.
Chem., Int. Ed. 2006, 45, 7358. (f) Zhao, X.; Xiao, B.; Fletcher, A. J.;
Thomas, K. M.; Bradshaw, D.; Rosseinsky, M. J. Science 2004, 306,
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1) (a) Banerjee, R.; Phan, A.; Wang, B.; Knobler, C.; Furukawa, H.;
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D.; Ruiz-Molina, D.; Veciana, J. Chem. Soc. ReV. 2007, 36, 770. (c)
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4
3, 2334. (d) Eddaoudi, M.; Moler, D. B.; Li, H.; Chen, B.; Reineke,
T. M.; O’Keeffe, M.; Yaghi, O. M. Acc. Chem. Res. 2001, 34, 319.
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(
1
999, 38, 2639. (f) Maji, T. K.; Matsuda, R.; Kitawaga, S. Nat. Mater.
007, 36, 770. (g) Ferey, G.; Draznieks, C. M.; Serre, C.; Millange,
2
F.; Dutour, J.; Surble, S.; Margiolaki, I. Science 2005, 309, 2040.
1
0.1021/ic800621q CCC: $40.75  2008 American Chemical Society
Inorganic Chemistry, Vol. 47, No. 19, 2008 8451
Published on Web 09/29/2008

Mahata et al.
Recently, there has been some effort to prepare MOFs
based on a reticular synthesis approach. Thus, designed
increase in the the reaction temperature, through an entropy-
driven dehydration pathway. It has been shown that
4
10
organic linkers have been employed as “struts” or bridges
to connect the metal centers, forming new compounds with
interesting adsorption properties. In spite of these develop-
compounds with extended -M-O-M- connectivity also
1
1
have higher thermal stability as well. A similar trend was
also observed by Kitagawa and co-workers during their study
5
1
2
ments, our understanding of the formation of such com-
pounds continues to be poor. One of the persistent issues
with respect to the synthesis of framework solids is that their
formation appears to be more kinetically driven rather than
based on thermodynamic control. Thus, it may be possible
to isolate many kinetically favored interesting new structures
enroute to the thermodynamically favored stable ones. Thus,
the trial and error approach appears to be more appealing
to study the formation of extended framework structures. In
light of this, it may be worth noting that a reactive
intermediate, based on a simple four-membered ring (zero-
dimensional), has been isolated in the family of zinc
phosphates and shown to be reactive, giving rise to structures
of the formation of cobalt pyridinedicarboxylates. Here a
two-dimensional hydrated structure at low temperature gives
way to a three-dimensional structure with infinite one-
dimensional -M-O-M- linkages at higher temperatures.
In a related study, the use of temperature and composition
as the variables gave rise to three different homochiral
1
3
cadmium camphorates. Postsynthesis modification studies
have been carried out, which also establishes the importance
of temperature in the control of dimensionality in nickel
14
diphosphonate structures. During the course of our studies
toward the understanding of the formation of structures with
extended networks, we have recently reported the role of
temperature in the formation of manganese oxybis(benzoate)
6
15
of higher dimensionality. Studies of this nature are not many
structures. In a continuation of this theme, we have taken
on the MOF compounds because the focus appears to be
more on understanding the role of organic ligands such as
carboxylates, amines, etc. Intense research activity over the
up the study of manganese-based MOF compounds prepared
using two different aromatic poly(carboxylic acids), 4,4′-
oxybis(benzoic acid) (OBA), and trimellitic acid (1,2,4-
benzenetricarboxylic acid, BTC). In this paper, we present
a brief overview of the role of time and temperature of
reaction in the formation of MOFs along with our findings
on the manganese benzenecarboxylate phases.
7
years, however, has provided examples of MOFs with more
than two structures formed with the same combination of
8
metal and organic ligands.
Mechanistic studies, especially on the formation of solids
with extended structures, are difficult to perform because of
the large number of variables involved in the synthesis.
Systematic studies on the role of simple variables such as
the temperature and time of the reaction can be investigated,
Experimental Section
Materials. The reagents needed for the synthesis of the
compounds are Mn(OAC)
9%], 4,4′-oxybis(benzoic acid), 1,2,4-benzenetricarboxylic acid
Lancaster (Lancaster, U.K.), 99%], imidazole [Ranbaxy (Gurgaon,
2
·4H
2
O [Ranbaxy (Gurgaon, India),
8b,9
9
although such studies are rare.
One of the emerging trends
[
in the area of MOFs appears to be the study of the possible
competition between the thermodynamic and kinetic factors
in the formation of such structures. There have been some
studies toward this direction, which indicates that in some
cases the thermodynamic considerations appear to be more
important than the kinetic ones. Cheetham and co-workers
provided the first insight on the influence of the reaction
temperature during the formation of cobalt succinate phases,
which indicated the formation of denser higher dimensional
structures with extended -M-O-M- connectivity upon an
India), 99%], and KOH [CDH (New Delhi, India), 99%]. The water
used was double distilled through a Millipore membrane.
(
a) Synthesis of Mn-OBA Compounds. A typical reaction
mixture containing Mn(OAc) ·4H O (0.245 g, 1 mM), 4,4′-
oxybis(benzoic) acid (OBA) (0.26 g, 1 mM), imidazole (0.068 g,
mM), and KOH (0.11 g, 2 mM) and 8 mL of water was heated
2
2
1
in a poly(tetrafluoroethylene) (PTFE)-lined stainless steel autoclave
at different times (1-5 days) and temperatures (75-220 °C) under
autogenous pressure. The synthesis conditions and other related
parameters are summarized in Table 1. The synthesis efforts resulted
in a total of six different phases identified by single-crystal X-ray
diffraction (XRD). The products of the synthesis were initially
characterized by powder XRD studies, which indicated that in some
of the preparations mixed-phase products are obtained. In most of
the cases, we observed a reasonable yield for the products, which
are typically in the range of 70-75% based on Mn.
(
4) (a) Ferey, G. Chem. Soc. ReV. 2008, 37, 191. (b) Rosi, N. L.; Eddaoudi,
M.; Kim, J.; O’Keeffe, M.; Yaghi, O. M. CrystEngComm 2002, 4,
4
01.
(
5) (a) Yaghi, O. M.; O’Keeffe, M.; Ockwig, N. W.; Chae, H. K.;
Eddaoudi, M.; Kim, J. Nature 2003, 423, 705. (b) Eddaoudi, M.; Kim,
J.; Rosi, N.; Vodak, D.; Wachter, J.; O’Keeffe, M.; Yaghi, O. M.
Science 2002, 295, 469. (c) Devic, T.; David, O.; Valls, M.; Marrot,
J.; Couty, F.; Ferey, G. J. Am. Chem. Soc. 2007, 129, 12614.
(b) Synthesis of Mn-1,2,4-BTC Compounds. A typical reaction
mixture containing Mn(OAc)
2
2
·4H O (0.245 g, 1 mM), 1,2,4-
(
6) (a) Rao, C. N. R.; Natarajan, S.; Choudhury, A.; Neeraj, S.; Ayi, A. A.
Acc. Chem. Res. 2001, 34, 80. (b) Ayi, A. A.; Choudhury, A.;
Natarajan, S.; Neeraj, S.; Rao, C. N. R. J. Mater. Chem. 2001, 11,
benzenetricarboxlic acid (0.21 g, 1 mM), imidazole (0.068 g, 1
1
181. (c) Natarajan, S.; Wullen, L. V.; Klein, W.; Jansen, M. Inorg.
(10) (a) Forster, P. M.; Burbank, A. R.; Livage, C.; Ferey, G.; Cheetham,
A. K. Chem. Commun. 2004, 368. (b) Forster, P. M.; Stock, N.;
Cheetham, A. K. Angew. Chem., Int. Ed. 2005, 44, 7608.
(11) Forster, P. M.; Cheetham, A. K. Angew. Chem., Int. Ed. 2002, 41,
457.
(12) Tong, M. L.; Kitagawa, S.; Chang, H. C.; Ohba, M. Chem. Commun.
2004, 418.
Chem. 2003, 42, 6265.
(
(
7) Chesnut, D. J.; Hagrman, D.; Zapf, P. J.; Hammond, R. P.; Laduca,
R., Jr.; Haushalter, R. C.; Zubieta, J. Coord. Chem. ReV. 1999, 190,
7
37.
8) (a) Rao, C. N. R.; Natarajan, S.; Vaidhyanathan, R. Angew. Chem.,
Int. Ed. 2004, 43, 1466. (b) Wang, X. L.; Qin, C.; Wang, E. B.; Li,
Y. G.; Su, Z. M.; Xu, L.; Carlucci, L. Angew. Chem., Int. Ed. 2005,
(13) Zhang, J.; Bu, X. Chem. Commun. 2008, 444.
(14) Gao, Q. M.; Guillou, N.; Nogues, M.; Ferey, G.; Cheetham, A. K.
Chem. Mater. 1999, 11, 2937.
4
4, 5824.
9) Cheetham, A. K.; Rao, C. N. R.; Feller, R. K. Chem. Commun. 2006,
780.
(
4
(15) Mahata, P.; Sundaresan, A.; Natarajan, S. Chem. Commun. 2007, 4471.
8452 Inorganic Chemistry, Vol. 47, No. 19, 2008

Infinite -M-O-M- Linkages and Isolated Clusters in MOFs
Table 1. Data for the Manganese Oxybis(benzoate) Structures and Manganese Trimellitates
a
H2O/M2+b density (g cm-3
M2+/COO- M2+/10 Å
3
3
c
compound
synthesis temperature (°C)/time (h)
)
dimensionality
Manganese Oxybis(benzoate) Structures
KMn2(OH)(H2O)6(OBA)2 (1)
Mn(H2O)3(OBA)·H2O (2)
Mn(H2O)(OBA) (3)
Mn(H2O)2(OBAH)2 (4)
Mn(H2O)(Im)2(OBA) (5)
Mn2(OH)2(OBA) (6)
75/24
3 (2)
4 (3)
1 (1)
2 (2)
1 (1)
0 (0)
1.681
1.578
1.664
1.572
1.465
2.001
1.25
1.5
2
2
1
2.59
2.49
3.04
1.56
1.9
3 (2)
2 (0)
2 (0)
2 (0)
1 (0)
3 (2)
110/24
150/24
150/48
180/72
220/24
3
6.02
Manganese Trimellitates
Mn2(OH)(H2O)2(BTC)·2H2O (7) 110/24
2 (1)
2 (1)
0 (0)
2.096
2.017
2.424
2.33
1.33
2.67
6.28
4.06
7.56
3 (0)
3 (0)
3 (2)
Mn(H2O)(BTCH)·H2O (8)
180/48
220/24
Mn3(BTC)2 (9)
a
Based on the first observation of the phase (lowest temperature and smallest reaction time). The first number in the H2O/Co2+ column refers to the total
b
c
water content, and the second, given in parentheses, refers to coordinated water molecules. The first number in the dimensionality column refers to the total
dimensionality and the second to the inorganic dimensionality.
Table 2. Crystal Data and Structure Refinement Parameters for Compounds 1, 4, 5, 7, and 8 Isolated during the Present Study
1
4
5
7
8
empirical formula
fw
C28H29O17KMn2
786.46
C28H22O12Mn
605.40
C20H18N4O6Mn
465.32
C9H12O11Mn2
406.03
C9H8O8Mn
299.09
cryst syst
space group
a (Å)
b (Å)
c (Å)
orthorhombic
Pbca (No. 61)
11.4256(9)
17.3545(13)
31.182(2)
90.0
90.0
90.0
6183.0(8)
8
triclinic
triclinic
triclinic
triclinic
P1 (no.1)
5.0218(11)
5.0344(11)
25.438(5)
88.593(3)
88.101(3)
84.484(4)
639.6(2)
P 1j (No. 2)
P 1j (No. 2)
P 1j (No. 2)
8.496(2)
9.581(2)
6.6108(8)
9.2901(11)
11.2290(13)
111.538(2)
92.512(2)
95.010(2)
3178.0(13)
2
6.3999(14)
7.3424(15)
10.868(2)
92.197(4)
100.979(3)
100.017(4)
492.35(18)
2
13.225(3)
98.988(4)
92.752(4)
96.294(4)
1054.5(4)
2
R (deg)
ꢁ
(deg)
γ (deg)
3
volume (Å )
Z
1
temperature (K)
273(2)
1.681
1.033
293(2)
1.572
0.586
0.710 73
2.40-28.02
R1 ) 0.0635, wR2 )
0.1635
R1 ) 0.0666, wR2 )
0.1683
293(2)
1.465
0.670
0.710 73
2.17-28.02
R1 ) 0.0495, wR2 )
0.1322
R1 ) 0.0598, wR2 )
0.1392
293(2)
2.096
2.043
0.710 73
1.96-27.99
R1 ) 0.0495, wR2 )
0.1117
R1 ) 0.0644, wR2 )
0.1187
293(2)
2.017
1.376
0.710 73
3.29-27.99
R1 ) 0.0481, wR2 )
0.0992
R1 ) 0.0589, wR2 )
0.1033
-
3
Fcalcd (g cm
)
-
1
µ (mm
)
wavelength (Å)
θ range (deg)
R index [I > 2σ(I)]
0.710 73
2.21-28.02
R1 ) 0.0790, wR2 )
a
0
.1246
R1 ) 0.1493, wR2 )
.1457
a
R (all data)
0
a
R1 ) ||Fo| - |Fc||/∑|Fo|; wR2 ) (∑[w(Fo2 - Fc )]/∑[w(Fo ) ]) . w ) 1/[F (Fo) + (aP) + bP]. P ) [max(Fo, O) + 2(Fc) ]/3, where a ) 0.0447 and
2
2 2 1/2
2
2
2
2
b ) 1.3185 for 1, a ) 0.1183 and b ) 0.6211 for 4, a ) 0.0757 and b ) 0.4166 for 5, a ) 0.0483 and b ) 1.6860 for 7, and a ) 0.0236 and b ) 1.6728
for 8.
1
6
mM), and KOH (0.11 g, 1 mM) and 8 mL of water was heated in
a PTFE-lined stainless steel autoclave at different times (1-5 days)
and temperatures (75-220 °C) under autogenous pressure. The
synthesis conditions and other related parameters are summarized
in Table 1. We have not been able to isolate any insoluble solid-
phase products from the reaction mixture when the reactions were
carried out at 75 °C, which always resulted in a clear solution.
The synthesis efforts at other temperatures resulted in a total of
three different phases identified by single-crystal XRD. The products
were characterized by powder XRD studies, which indicated a
mixture of products in some of the preparations. In most of the
cases, we observed a reasonable yield for the products, which are
typically in the range of 65-70% based on Mn.
SAINTPLUS, and an empirical absorption correction was applied
1
7
using the SADABS program. The structure was solved by
SHELXS97 and refined using SHELXL97 present in the WinGx
suit of programs (version 1.63.04a). Final refinement included
atomic positions for all of the atoms, anisotropic thermal parameters
for all of the non-hydrogen atoms, and isotropic thermal parameters
for all of the hydrogen atoms. Full-matrix least-squares refinement
1
8
18
1
9
2
19
against F was carried out using the WinGx package of programs.
Details of the structure solution and final refinements for the new
compounds identified in the present investigation, KMn (OH)-
(OBA)
O)(BTCH)·H
2
(
(
(
(
H
5), Mn
2
O)
6
(OBA)
2
(1), Mn(H
2
O)
2
(OBAH)
2 2 2
(4), Mn(H O)(Im)
2
(OH)(H
2
O) (BTC)·2H
2
2
O (7), and Mn(H
2
2
O
8), are given in Table 2. CCDC 683492 (1), 683493 (4), 683494
5), 678766 (7), and 683495 (8) contain the crystallographic data
for this paper. The numbers for the other compounds are CCDC
48374 [Mn(H O) (OBA) ·H O (2)], 648375 [Mn(H O)(OBA) (3)],
Single-Crystal Structure Determination. A suitable single
crystal of each compound was carefully selected under a polarizing
microscope and glued carefully to a thin glass fiber. The single-
crystal data were collected on a Bruker AXS SMART Apex CCD
diffractometer at 293(2) K. The X-ray generator was operated at
6
2
3
2
2
(
(
(
(
16) SMART (V 5.628), SAINT (V 6.45a), XPREP, and SHELXTL; Bruker
AXS Inc.: Madison, WI, 2004.
5
0 kV and 35 mA using Mo KR (λ ) 0.710 73 Å) radiation. Data
17) Sheldrick, G. M. Siemens area correction absorption correction
program; University of G o¨ ttingen: G o¨ ttingen, Germany, 1994.
18) Sheldrick, G. M. SHELXL-97 program for crystal structure solution
and refinement; University of G o¨ ttingen: G o¨ ttingen, Germany, 1997.
19) WinGx suite for small-molecule single crystal crystallography: Far-
rugia, J. L. J. Appl. Crystallogr. 1999, 32, 837.
were collected with a ω scan width of 0.3°. A total of 606 frames
were collected in three different settings of ꢀ (0, 90, and 180°),
keeping the sample-to-detector distance fixed at 6.03 cm and the
detector position (2θ) fixed at -25°. The data were reduced using
Inorganic Chemistry, Vol. 47, No. 19, 2008 8453

Mahata et al.
6
37649 [Mn
2
(OH)
2
(OBA) (6)], and 637648 [Mn
3
(BTC)
2
(9)]. These
dicarboxylic acid, NaOH, and water in the ratio 1:1:2:555
data can be obtained free of charge from the Cambridge Crystal-
lographic Data Centre (CCDC) via www.ccdc.cam.ac.uk/data_
request/cif.
were heated at 150 °C/24 h followed by at 180 °C/24 h
(Table 3). Similar to the behavior observed with the cobalt
10
succinate phases, a two-dimensional compound possessing
water-coordinated octahedral cobalt centers was isolated at
Results
1
50 °C and a three-dimensional compound with one-
dimensional -M-O-M- linkages was obtained at 180 °C
Figure 2). This study suggests that it is likely that the two-
Here we briefly describe the results of the earlier studies
on three different systems of compounds as a function of
the variation in the reaction temperature.
(
dimensional phase could have transformed into the three-
dimensional compound because the reaction mixture yielded
only the two-dimensional compound at 150 °C. Though the
result of this study appears to be similar to that observed by
(a) Cobalt Succinate Phases. Cobalt succinates is the first
system of compounds investigated to study the effect of
temperature because they represent a well-established family
10
1
0,20
Cheetham and co-workers, it does not indicate the possible
role of time in the formation of the different phases. This
study also shows the formation of -Co-O-Co- linkages
at higher temperatures accompanied by a decrease in the
coordinated water molecules bound to the central cobalt
atom.
of compounds.
Cheetham and co-workers carried out a
systematic study of the role of temperature in the formation
of the various cobalt succinate phases by keeping the same
1
0
reaction mixture. During the study, five cobalt succinate
phases were identified when cobalt(II) hydroxide was reacted
with succinic acid in a 1:1 molar ratio at five different
temperature in the range of 60-250 °C (Figure 1). This
study, the first of its kind, clearly showed the formation of
one-dimensional structures with hydrated metal centers up
to 100 °C, two-dimensional structures with hydrated metal
centers at 150 °C, and three-dimensional structures possess-
ing -M-O-M- connectivities at higher temperatures
(c) Cadmium Camphorate Phases. Recently, Bu and co-
workers reported the role of temperature and the composition
of the reaction mixture in the formation of a family of
13
homochiral cadmium camphorates (Table 3 and Figure 3).
In this study, 6 mL of ethanol was layered onto a mixture
of Cd(NO · 4H O (0.0815 g), Na CO (0.0253 g), D-
3
)
2
2
2
3
camphoric acid (0.0512 g), and 2 mL of water at room
temperature. Slow diffusion over several days yielded a
hydrated three-dimensional compound with one-dimensional
(Table 3). The notable outcome of the study is that the water
coordinating with the cobalt atoms in the structures continu-
ously decreases with increasing temperature, suggesting an
increased entropy contribution. In addition, the loss of water
molecules from the coordination sphere of the cobalt atoms
forces the nearby cobalt atoms to come closer and form the
-
(
2
M-O-M- linkages. When a mixture of Cd(NO
0.1546 g), Na CO (0.0528 g), D-camphoric acid (0.1075),
g of water, and 6.1 g of ethanol were heated at 140 °C/3
3
)
2
2
·4H O
2
3
days, a three-dimensional structure with fewer water mol-
ecules was isolated. An anhydrous three-dimensional struc-
-
M-O-M- linkages, thereby increasing the total density
of the system. This study paved the way for the investigation
of other variables such as the reaction time, which would
probe the role of thermodynamic and kinetic factors in the
formation of extended MOF structures.
ture was isolated when a mixture of Cd(NO
g), Na CO (0.0530 g), D-camphoric acid (0.1004 g), 2.4 g
of water, and 4.44 g of ethylene glycol was heated at 180
C/5 days. In this study, the composition was varied along
)
3 2
2
·4H O (0.1507
2
3
°
In a subsequent paper, Cheetham and co-workers using a
high-throughput investigation studied the role of the pH,
temperature, concentration, and time of synthesis of the
with the temperature in obtaining the products. The total
water content of the different products obtained decreases
with increasing reaction temperature, similar to those ob-
1
0b
cobalt succinate MOFs. This study revealed the presence
of two new phases in addition to the five compounds isolated
10
served by Cheetham and co-workers and Kitagawa and co-
workers.
d) Manganese Oxybis(benzoate) Phases. As mentioned
1
2
1
0a
during their study on the role of temperature. From this,
it appears that there is competition between the many closely
related phases and the reaction mixture is under a constant
change of equilibrium at any given period of time and
temperature. Additionally, the study also reveals the impor-
tance and subtle relationship needed between the temperature,
acid/base ratio (concentration) and time in determining the
favorable structure.
(
earlier, we have been interested in the study of the effect of
temperature on a particular composition during the synthesis
1
5
of MOF compounds. During this study, we have investi-
gated the formation of manganese oxybis(benzoate) phases
by reacting a mixture of Mn(OAc)
2
2
· 4H O (0.245 g, 1 mM),
4
,4′-oxybis(benzoic acid) (0.26 g, 1 mM), imidazole (0.068
g, 1 mM), and NaOH (0.08 g, 2 mM) and 8 mL of water in
a 23 mL autoclave in the temperature range of 80-220 °C
for 24 h. The reaction yielded three distinct phases with two-
and three-dimensional structures. Similar to the earlier
(b) Cobalt Pyridine-3,4-dicarboxylate Phases. Kitagawa
and co-workers investigated the effect of both the time and
temperature on the formation of cobalt pyridine-3,4-dicar-
1
2
boxylate phases. In this study, CoCl
2
2
·6H O, pyridine-3,4-
1
0
observations, the water molecules were found to be
coordinated with the metal center at low temperatures and
at high temperatures, the three-dimensional structures pos-
sessed two-dimensional -M-O-M- linkages. The three-
dimensional structure has a brucite-related two-dimensional
lattice, which was observed for the first time.
(
20) (a) Livage, C.; Egger, C.; Ferey, G. Chem. Mater. 2001, 13, 410. (b)
Livage, C.; Egger, C.; Ferey, G. Chem. Mater. 1999, 11, 1546. (c)
Long, L. S.; Chen, X. M.; Tong, M. L.; Sun, Z. G.; Ren, Y. P.; Huang,
R. B.; Zheng, L. S. J. Chem. Soc., Dalton Trans. 2001, 2888. (d)
Livage, C.; Egger, C.; Nogues, M.; Ferey, G. J. Mater. Chem. 1998,
8
, 2743.
8454 Inorganic Chemistry, Vol. 47, No. 19, 2008

Infinite -M-O-M- Linkages and Isolated Clusters in MOFs
Figure 1. Structures of the five cobalt succinate phases: Co(H2O)4(C4H4O4), Co(H2O)2(C4H4O4), Co4(H2O)2(OH)2(C4H4O4)3 ·2H2O, Co6(OH)2(C4H4O4)5 ·
H2O, and Co5(OH)2(C4H4O4)4 (from bottom to top). The left-side vertical bar indicates the approximate temperature scale. and the time indicates the first
observation of the phase.
Though three distinct structures have been isolated at three
different temperatures, it is likely that there may be other
related phases because an unidentified phase was observed
at low temperatures (80 °C; Figure S4 in the Supporting
Information). This suggests that other parameters such as
the duration of the reaction (time of reaction) may be of some
importance. To test this hypothesis and to validate the
argument that the time of reaction can also play a significant
role in determining the kinetic versus thermodynamic control
carried out. Thus, we employed a reaction composition
identical with that employed to study the effect of temper-
1
5
ature but performed the reaction using KOH as the base
instead of NaOH at varying time intervals (typically 1-5
days) at each temperature. It was observed from our related
studies that the use of KOH in the reaction mixture gives
2
1
rise to good quality single crystals. Because we do not
expect to form single-phase solid products in all of our
1
0
in the formation of MOFs, a new set of experiments were
(21) Mahata, P.; Sen, D.; Natarajan, S. Chem. Commun. 2008, 1278.
Inorganic Chemistry, Vol. 47, No. 19, 2008 8455

Mahata et al.
Table 3. Data for the Cobalt Succinates, Cobalt Pyridine-3,4-dicarboxylates, and Cadmium Camphorates
compound
synthesis temperature (°C)/time (h) H2O/M2
+a
density (g cm-3
)
M2+/COO- M2+/10 Å
3
3
dimensionality
b
Cobalt Succinates
Co(H2O)4(C4H4O4)
Co(H2O)2(C4H4O4)
60/20
100/20
4 (4)
2 (2)
1.945
1.926
2.085
2.197
2.337
1
2
2.67
2.8
3
4.74
5.50
7.28
8.05
8.88
1 (0)
1 (0)
2 (2)
3 (2)
3 (2)
1
Co4(H2O)2(OH)2(C4H4O4)3 ·2H2O 150/20
1 ( /2)
1
Co6(OH)2(C4H4O4)5 ·H2O
190/20
250/20
/6 (0)
Co5(OH)2(C4H4O4)4
0 (0)
Cobalt Pyridine-3,4-dicarboxylates
3 (2)
Co(H2O)2(C3H3NO4)·H2O
Co3(OH)2(C3H3NO4)2(H2O)2
150/24
150/24 + 180/24
2.038
2.361
1.5
2
4.44
7.39
2 (0)
3 (1)
2
2
/3 ( /3)
Cadmium Camphorates
3
1
3
Cd2(H2O)3(C10H14O4)2
Cd4(H2O)(C10H14O4)4 ·H2O
Cd2(C10H14O4)2
a
RT/several days
140/72
180/120
/2 ( /2)
1.557
1.901
1.948
2
2.25
2
2.78
3.58
3.78
3 (1)
3 (1)
3 (1)
1
/2 ( /4)
0 (0)
The first number in the H2O/Co2+ column refers to the total water content, and the second, given in parentheses, refers to the coordinated water molecules.
The first number in the dimensionality column refers to the total dimensionality and the second to the inorganic dimensionality.
b
reactions, it would be desirable to have a mixture of single-
heating the reaction mixture at 150 °C for 48 h. The single-
crystal structure reveals that it also has a two-dimensional
structure but with free carboxylic acid groups. The asym-
crystalline products rather than a mixture of powder samples
because the structural characterization can be made by using
the single-crystal XRD. This prompted us to study the effect
of time and temperature by using KOH as the secondary
base in all of our reactions. The resulting product of each
experiment was analyzed using powder XRD studies, and
the results are presented as a two-dimensional temperature
vs time plot in Figure 4 (for the experimental powder XRD
patterns, see Figure S6-S10 in the Supporting Information).
The time dependence study revealed the presence of three
new additional phases, making a grand total of six phases
in the manganese oxybis(benzoate) family of compounds
2
+
metric unit has one crystallography independent Mn ion,
two OBAH anions [monoanionic 4,4′-oxybis(benzoic acid)],
2
+
and two coordinated water molecules. The octahedral Mn
ions are coordinated by four carboxylate oxygen atoms and
2
+
two water molecules. The connectivity between Mn ions
and the carboxylate groups forms a two-dimensional layer.
The short O-O contact distances (2.62 and 2.66 Å) between
the free carboxylic acid groups of the two adjacent layers
suggest that the layers are strongly hydrogen bonded with
each other.
(
Figure 5). Here we describe briefly the structures of the new
Compound 5 was also formed as a mixture of phases by
heating the reaction mixture at 180 °C for 72 h. The single-
crystal X-ray structure studies reveal a one-dimensional
structure. The asymmetric unit of 5 has one crystallographi-
compounds isolated in the present study.
Compound I was obtained as a pure single-phase material
during the reaction at 75 °C/24 h. The simulated powder
XRD pattern of this phase matched with the previously
unknown phase obtained using NaOH as a base at 80 °C.
2+
cally independent Mn ion, one OBA anion, two imidazole
molecules, and one coordinated water molecule. The octa-
+
2+
Compound 1 has a three-dimensional structure with K ions
hedral Mn ions are coordinated by three carboxylate
as part of the framework. The asymmetric unit of I consists
oxygen atoms, two nitrogen atoms of two different imidazole
molecules, and one coordinated water molecule. The con-
nectivity between the Mn ion and OBA forms a simple
2
+
of two crystallographically independent octahedral Mn
+
2+
ions, one K ion, two 4,4′-oxybis(benzoate) (OBA) anions,
one µ
Mn(1) is coordinated by two carboxylate oxygen atoms, three
O molecules (two of them have µ -connectivity), and one
-OH group, and Mn(2) is coordinated by three carboxylate
3
-OH, and six coordinated water molecules. Thus,
one-dimensional structure with the bound imidazole mol-
ecules hanging as a pendent in the interchain region. The
structure is stabilized by many different weak interactions,
which include the hydrogen-bonding (O-H· · ·O, C-H· · ·O,
and N-H· · ·O) and N-H· · ·π interactions. Compound 6 has
athree-dimensionalstructurewithtwo-dimensional-M-O-M-
connected layers that closely resemble those observed in
H
2
2
µ
3
oxygen atoms, one water molecule, and two µ
The K ion is five-coordinated with one carboxylate oxygen
3
-OH groups.
+
atom and four water molecules (two of them have µ
connectivity). The structure contains a rhombus-shaped
tetranuclear cluster of the formula [Mn (µ -OH) (H O)
2
-
1
5
brucite and related structures.
4
3
2
2
8
O10],
The powder XRD patterns of each preparation were
compared with the simulated XRD patterns of each com-
pound (1-6), generated from the single-crystal X-ray
structures, which indicated few interesting observations.
Thus, the previously unidentified phase formed at low
temperature (T < 100 °C) during our studies on the effect
formed by the two Mn atoms, Mn(1) and Mn(2), which are
related by the center of symmetry. A -OH group [O(1)]
has a µ
3
-connectivity linking Mn(1), Mn(2), and Mn(2a). The
O) O polyhedra, forming
Mn clusters are connected by K(H
4
2
4
two-dimensional mixed-metal layers, which are connected
by the OBA units, completing the three-dimensional struc-
ture.
1
5
of temperature was found to correlate with the simulated
+
XRD pattern of the K phase, compound 1. It may be noted
Compound 2 has a pseudo-two-dimensional structure, and
has a two-dimensional structure. Compound 4 was
isolated from a reaction that produced multiple phases by
that we were unable to obtain a suitable single crystal when
NaOH was used in the reaction mixture, which precluded
this identification. From Figure 4, it becomes apparent that
1
5
3
15
8456 Inorganic Chemistry, Vol. 47, No. 19, 2008

Infinite -M-O-M- Linkages and Isolated Clusters in MOFs
Figure 2. Structures of two the cobalt pyridine-3,4-dicarboxylate phases: Co(H2O)2(C3H3NO4).H2O (bottom) and Co3(OH)2(C3H3NO4)2(H2O)2 (top). The
left-side bar indicates the approximate temperature scale.
this phase persists up to 4 days at low temperatures and
vanishes at higher temperatures irrespective of the duration
of the cases, irrespective of the time of the reaction, the
highest temperature phase always corresponds to the brucite-
related structure, compound 6, having two-dimensional
-M-O-M- linkages. The time of the reaction definitely
has a bearing on the formation of 6 because we observe this
phase at 180 °C after heating for 5 days (Figure 4e). From
the present study, it becomes clear that the total water in the
structures, whether coordinated or free water, continuously
decreases with increasing temperature and time. Less water
at higher temperatures (T > 180 °C) is likely driven by the
increased entropic contribution associated with the release
2
+
of the reaction. The formation of Mn clusters at low
2
2
temperatures has been known. Thus, it is not totally
unexpected to observe the presence of Mn cluster units in
4
the present study. Interestingly, when the reaction mixture
was heated for a longer duration, two more new compounds,
4
and 5 with two- and one-dimensional structures, have been
isolated. Of these, 4 appears to be more stable compared to
the imidazole-bound structure (5), which forms only when
the reaction mixture was heated at 180 °C for 3 days. In all
Inorganic Chemistry, Vol. 47, No. 19, 2008 8457

Mahata et al.
Figure 3. Structures of the three cadmium camphorate phases: Cd2(H2O)3(C10H14O4)2, Cd4(H2O)(C10H14O4)4 ·H2O, and Cd2(C10H14O4)2 (from bottom to
top). The left-side bar indicates the approximate temperature scale.
of water. The present observations show that there are subtle
relationships that exist between the temperature and time of
reaction, which indirectly influences the formation of more
stable compounds of higher dimensionality with -M-O-M-
linkages. In addition, the decreasing dielectric constant of
water at high temperature under hydrothermal conditions may
also favor higher coordination numbers for the carboxylate
oxygen atoms as well.
(e) Manganese Trimellitate (Benzene-1,2,4-tricarboxy-
late) Phases. Similar to the manganese oxybis(benzoate)
structures, we have carried out investigations on the role of
time and temperature in the formation of manganese trim-
ellitate phases. Thus, a reaction mixture of Mn(OAc)
2
2
·4H O
(0.245 g, 1 mM), trimellitic acid (0.21 g, 1 mM), imidazole
(0.068 g, 1 mM), and KOH (0.11 g, 2 mM) and 8 mL of
water was heated in a 23 mL autoclave in the temperature
8458 Inorganic Chemistry, Vol. 47, No. 19, 2008

Infinite -M-O-M- Linkages and Isolated Clusters in MOFs
of symmetry. A -OH group [O(2)] has a µ
linking Mn(1), Mn(2), and Mn(2a) and a µ
oxygen [O(6)] connecting Mn(1) and Mn(2). The Mn
3
-connectivity
3
-carboxylate
4
clusters are connected by trimelliate anions, forming a three-
dimensional structure with one-dimensional channels oc-
cupied by two extraframework water molecules [O(100) and
O(200)]. This structure is closely related to the structure of
2+
1
obtained by the reaction of Mn and OBA in the presence
of KOH.
Compound 8 was formed as a mixture of phases (7 and
) by heating the reaction mixture at 180 °C for 48 h. The
8
single-crystal XRD studies reveal a three-dimensional struc-
ture with one-dimensional water-filled channels. The asym-
metric unit of this compound consists of one crystallograph-
2
+
ically independent Mn ion, one dianionic trimelliate anion,
and one coordinated and one extraframework water mol-
Figure 4. Temperature vs time plot of the various manganese oxybis(ben-
zoate) phases isolated and identified using powder XRD patterns. The
various phases were identified by comparing the powder XRD pattern with
the simulated pattern generated from the single-crystal structure. Color code:
2+
ecules. The Mn ion is bound with four carboxylate oxygen
atoms and two µ -H O molecules. The µ -H O molecules
connect the Mn centers to form a dimer of the formula
Mn (µ -H O) ], which are linked through the bidentate
carboxylate groups, giving rise to a one-dimensional struc-
ture, which are further connected by trimelliate anions,
forming the three-dimensional structure with one-dimensional
channels occupied by the extraframework water molecules.
A careful study of the powder XRD patterns of the
products of the reaction between Mn and trimellitic acid
and a comparison with the simulated XRD patterns, gener-
ated from the single-crystal XRD structures, reveal few
generalities (Figure 6). The notable observation is that there
is no solid-phase product for the reaction carried out at low
temperatures (T < 100 °C) irrespective of the time of the
reaction (1-5 days). The Mn
forms initially at 110 °C, appears to be stable up to 200 °C.
Interestingly, the related structure, the Mn dimer compound
1
, red; 2, green; 3, blue; 4, orange; 5, black; 6, pink.
2
2
2
2
range of 75-220 °C for varying durations (1-5 days). The
resulting product of each experiment was subjected to powder
XRD studies, and the results are presented as a two-
dimensional temperature vs time plot in Figure 6 (for the
experimental powder XRD patterns, see Figures S11-S15
in the Supporting Information). During the study of this
system, we did not observe any water-insoluble crystalline
products at lower temperatures (T < 100 °C). The higher
temperature reactions yielded three distinct crystalline com-
pounds with three-dimensional structures, 7-9 (Figure 7).
Similar to the earlier observations, water molecules were
found to be coordinated with the metal center at lower
temperatures, and at higher temperature, the three-dimen-
sional structure has two-dimensional -M-O-M- linkages.
The compound formed at 220 °C, 9, has a distorted Kagome-
related two-dimensional lattice, which is the only example
of a single-atom-bridged -M-O-M- Kagome layer in
[
2
2
2
2 8
O
2
+
4
cluster compound (7), which
2
8, starts to form around 180 °C when the reaction mixture
was heated for 2 days and appears to be stable up to 200
2
1
MOFs. Of the three different compounds, identified and
isolated during the present study, compound 9 has been
°
C. This indicates a very narrow range of stability for this
phase (∼20 °C). Similar to the observation in the oxybis-
benzoate) structures, the formation of Kagome-related
M-O-M- linked layers has some dependence on the time
of reaction. The formation of compound 9 was observed at
80 °C after 3 days (Figure 6), while it forms only at 220
C when heated for 24 h (Figure 6). This indicates that
compound 9 may be the kinetically stable phase at higher
temperatures (220 °C/24 h), while it could be the thermo-
dynamically stable phase because the formation was observed
at lower temperature (180 °C) when the reactions were
carried for a longer duration (>3 days). This observation is
2
1
described recently by us and the other two structures (7
and 8) would be presented briefly here.
(
-
Compound 7 has been obtained as a pure single-phase
material when the reaction mixture was heated at 110 °C
for 24 h. The structure determined by single-crystal XRD
studies indicates a three-dimensional network with one-
dimensional water-filled channels. The asymmetric unit of
1
°
2+
7
consists of two crystallographically independent Mn ions,
one trimellitate anion, one µ -OH, and two coordinated and
two extraframework water molecules. The structure contains
a rhombus-shaped tetranuclear cluster of the formula [Mn (µ
OH) (H O) 12], formed by the two octahedrally coordinated
Mn atoms, Mn(1) and Mn(2), which are related by the center
3
4
3
-
2
2
4
O
10,13
consistent with other similar observations in the literature,
indicating a trend in the formation of the MOFs where a
subtle balance between the kinetic and thermodynamic
factors would be required to isolate the preferred phase. In
addition, the total water content of the system also decreases
continuously when the reaction temperature is increased,
indicating a reasonable entropy-driven control.
(
22) (a) Piligkos, S.; Rajaraman, G.; Soler, M.; Kirchner, N.; Slageren,
J. V.; Birchen, R.; Parsons, S.; Gudel, H. U.; Kortus, J.; Wernsdorfer,
W.; Christou, G.; Brechin, E. K. J. Am. Chem. Soc. 2005, 127, 5572.
(
b) Tasiopoulos, A. J.; Vinslava, A.; Wernsdorfer, W.; Abboud, K. A.;
Christou, G. Angew. Chem., Int. Ed. 2004, 43, 2117. (c) Ruiz, D.;
Sun, Z.; Albela, B.; Folting, K.; Ribas, J.; Christou, G.; Hendrickson,
D. N. Angew. Chem., Int. Ed. 1998, 37, 300.
Inorganic Chemistry, Vol. 47, No. 19, 2008 8459

Mahata et al.
Figure 5. Structures of the six manganese oxybis(benzoate) phases: KMn2(OH)(H2O)6(OBA)2 (1), Mn(H2O)3(OBA)·H2O (2), Mn(H2O)(OBA) (3),
Mn(H2O)2(OBAH)2 (4), Mn(H2O)(Im)2(OBA) (5), and Mn2(OH)2(OBA) (6) (from bottom to top). The left-side bar indicates the approximate temperature
scale, and the time indicates the first observation of the phase.
1
0
Discussion
that of the cobalt succinate phases reported earlier (Table 3).
We have also estimated the above parameters for the cobalt
The above results on manganese oxybis(benzoate) and
trimellitate compounds can be compared with other similar
studies available in the literature.
oxybis(benzoate) system of compounds, a systematic increase
with respect to density, M /COO , and the number of M
12
13
pyridinedicarboxylate and the cadmium camphorate phases,
1
0,12,13,15
which also show similar behavior. In addition, there is an overall
dimensionality increase in all of the cases. It is likely that these
changes are a consequence of the changing coordination around
the central metal ions and represent trends that are common to
the five systems of compounds described here.
In the manganese
2+
-
2+
ions per 1000 Å as a function of the temperature can be
observed (Table 1 and Figure 4). This observation is similar to
8460 Inorganic Chemistry, Vol. 47, No. 19, 2008

Infinite -M-O-M- Linkages and Isolated Clusters in MOFs
observed during the synthesis of cadmium camphorate phases
1
3
at low temperatures. In addition, we also observe the
formation of another three-dimensional phase, compound 8,
which is formed as a mixture and appears to be in equilibrium
with compounds 7 and 9. One way to rationalize the
formation of these intermediate phases would be to consider
the distribution of charges within the structure, especially at
the metal site. This approach has been employed gainfully
1
3
earlier.
To estimate the charge distribution, we have followed the
1
3
model employed earlier by considering the coordination
environment around each Mn site. In compound I, Mn(1) is
surrounded by one triply coordinated (with two different
2
+
+
Mn ions and one K ion) and one bidentate (with two
Figure 6. Temperature vs time plot of the various manganese trimellitate
phases isolated and identified using powder XRD patterns. The various
phases were identified by comparing the powder XRD pattern with the
simulated pattern generated from the single-crystal structure. Color code:
2
+
3
different Mn ions) carboxylate groups, one µ -OH group,
two µ -H O molecules, and one terminal H O molecule, so
2 2
2
1
1
-
the charge at this site is thus [(-1 × /
3
) + (-1 × /
2
)]COO
7
, red; 8, green; 9, blue.
1
-
5
+
[-1 ×
3
/ ]
OH + [2]Cd
)
/
6
. The Mn(2) atom is
During the present study, we have also made efforts to
surrounded by one monodentate, one bidentate (with two
2
+
identify the role of the time of reaction in the formation of
hybrid compounds. The earlier study by Cheetham and co-
workers using a high-throughput investigation on the cobalt
different Mn ions), and one triply coordinated (with two
2
+
+
different Mn ions and one K ion) carboxylate groups,
two µ -OH groups, and one terminal water molecule.
Therefore, the charge at this site is /
surrounded by one triply coordinated carboxylate group (with
two Mn ions and one K ion), two µ
molecules, and two terminal water molecules. So, the charge
of the K site is / . The total charge on the two Mn atoms
3
and the one K atom is 1+, which is compensated for by the
presence of the free COO of the OBA unit. Similar
3
8
b
1
succinates revealed the formation of two new phases in
2
-. The K atom is
1
0
addition to the five phases identified earlier. From the
results of the present study (Table 1), it becomes clear that
the role of the time of reaction does not follow any simple
systematic behavior. From Table 1, the number of metal ions
per carboxylate appears to show a continuous increase as a
function of the time and temperature; with the only anomaly
being the one-dimensional compound with bound imidazole,
compound 5. It has been postulated that at higher temperature
there is a competition between the water molecules that may
be bound to the metal and the carboxylate present in solution,
leading to the formation of a higher dimensional structure
2
+
+
2 2
-H O water
2
-
calculations can be performed on the compounds 2-6, which
reveals that the charge on the Mn site is zero (all of the five
compounds have only one crystallographically independent
2+
Mn ion). Out of the five compounds, compounds 2-5 have
-
a ratio of Mn and COO of 1:2, which suggests that the
10,15
with -M-O-M- connectivity.
In addition, it is known
transformation from one structure to the other may not alter
the charge on the Mn site. Thus, the structural changes can
be brought about by the replacement of a few neutral ligand
that most of these compounds are formed by a mechanism
2
3
involving dissolution and recrystallization, which implies
that there is a continuous change in the equilibrium in
solution. During the present study, the loss of the carboxylate
unit paved the way for the imidazole molecules to bind with
the metal center, giving rise to this rather unusual, but not
totally unexpected, one-dimensional coordination polymer
that may be reactive. To this end, it may be noted that, during
the study on zinc phosphate based open-framework struc-
tures, it has been postulated that the one-dimensional
structures could be precursors for the structure of higher
2
molecules such as H O and imidazole or by a change in the
coordination number/mode of the carboxylate group. The
change in the coordination number/mode of the carboxylate
groups has been observed earlier in obtaining related cobalt
succinate structures. This type of rationalizing permits the
presence of the intermediate phases at moderate tempera-
ture as well as the presence of a mixture of phases, when
the total charges at the Mn site are similar, as has been the
case for the four structures (2-5) isolated during the study.
In addition, this also indicates that the phases may have
comparable energy. Compound 6 also has a charge of zero
at the Mn site with a ratio of Mn:COO ) 1:1, which
requires the presence of an OH group attached to the Mn
site to maintain charge equilibrium. This situation gives rise
to a -Mn-(OH)-Mn-(OH)-Mn- type of linkage that
resembles the layers commonly encountered in layered
double hydroxides.
1
0
2
4
dimensionality.
The formation of a Mn
4
cluster compound, compounds 1
-
and 7, at T ∼ 100 °C with three-dimensional connectivity is
also intriguing. One way of rationalizing this is by consider-
ing other known manganese cluster compounds in the
-
22
literature, which are typically prepared at T < 100 °C, and
it may be expected that such clusters could form in our
preparations. Similar clustering of metal ions has also been
The charge distribution calculations on the three manga-
nese trimellitate phases, 7-9, reveal that the compounds 7
and 8 have zero charge at the Mn site whereas 9 has two
(
23) Walton, R. I.; Norquist, A. J.; Neeraj, S.; Natarajan, S.; Rao, C. N. R.;
O’Hare, D. Chem. Commun. 2001, 1990.
24) Choudhury, A.; Neeraj, S.; Natarajan, S.; Rao, C. N. R. J. Mater.
Chem. 2001, 11, 1537.
(
1
1
different charges, Mn(1) ) /12 and Mn(2) ) /
6
-. Because
Inorganic Chemistry, Vol. 47, No. 19, 2008 8461

Mahata et al.
Figure 7. Structures of the three manganese trimelliate phases: Mn2(OH)(H2O)2(BTC).2H2O (1), Mn(H2O)(BTCH).H2O (2) and Mn3(BTC)2 (3) (from
bottom to top). The left-side bar indicates the approximate temperature scale, and the time indicates the first observation of the phase.
the trimellitate anion is triply charged, one would expect
different ratios between the Mn and the carboxylate ligand.
In 7, the distribution of the charges at the two Mn sites is
being compensated for by the presence of one trimellitate
unit and a -OH group. In 8, the charge at the Mn site is
compensated for by the presence of a dianionic trimelliate
unit. In 9, the two differently charged Mn sites are fully
compensated for by the presence of one trianionic trimelliate
unit. The similar charge distribution along with a comparable
ratio of Mn and carboxylate suggests that compound 8 could
be an intermediate phase, which may be structurally closer
to compound 7. This is the observed behavior as compound
8 was isolated during the time-dependent study and also
observed within a narrow temperature range (∼20 °C).
Conclusions
The dependence of simple variables such as the reaction
time and temperature under hydrothermal conditions has been
investigated during the preparation of manganese oxybis-
(benzoate) and manganese trimelliate phases. The studies
show that denser three-dimensional structures with infinite
-M-O-M- layers are formed at high temperatures ir-
8462 Inorganic Chemistry, Vol. 47, No. 19, 2008

Infinite -M-O-M- Linkages and Isolated Clusters in MOFs
respective of the duration of the reaction. The appearance
of low-dimensional structures at reasonably high tempera-
tures (∼180 °C) and a longer duration (∼3 days) indicates
a subtle balance between the local coordination environment
of manganese, the available carboxylate units, the water
molecules, and the secondary base. This shows a possible
kinetic role in the formation of such structures, which is also
suggested by the charge distribution calculations at the Mn
site. The results of the present study appear to favor
overwhelmingly thermodynamic control, and many more
such investigations would be required to arrive at a more
meaningful correlation and also to establish the close and
subtle relationships between thermodynamic and kinetic
control in the formation of MOF compounds.
Acknowledgment. S.N. thanks the Department of Science
and Technology (DST), Government of India, for the award
of a research grant, and the authors thank the Council of
Scientific and Industrial Research, Government of India, for
the award of a fellowship (to P.M.) and a research grant.
S.N. also thanks the DST, Government of India, for the award
of the “RAMANNA” fellowship. The Board of Research on
Nuclear Sciences, Department of Atomic Energy, Govern-
ment of India, is thanked for their support.
Supporting Information Available: X-ray crystallographic data
in CIF format for compounds 1, 4, 5, 7, and 8 and Figures S1-S15.
This material is available free of charge via the Internet at
http://pubs.acs.org.
IC800621Q
Inorganic Chemistry, Vol. 47, No. 19, 2008 8463