A R T I C L E S
Appelhans et al.
blocks is relatively straightforward, understanding and control-
ling the factors that affect the structures and properties of both
chiral and achiral metal-organic frameworks remains a fun-
damental challenge in the field. Structural control is best
exercised in systems that combine a geometrically rigid ligand
and a metal with a well-defined coordination geometry. Flexible,
polydentate ligands, like many of the natural carboxylic acids,
have less predictable structural behavior. Composition and
structure, as would be expected, also depend on an array of
synthetic variables, and some general trends in hydration and
connectivity have been identified.13 The prediction and control
of structure is further complicated because reactions can occur
under either thermodynamic or kinetic control.14,15 In chiral
systems, the additional complications of ligand racemization16
and conglomerate formation are also important. Finally, the
existence (or nonexistence) of a predictable structural relation-
ship between chiral and racemic frameworks is also of consider-
able interest.9,12,14,17 Series of readily accessible frameworks
based on available chiral ligands are ideal systems to advance
the understanding of these areas.
Figure 1. The three forms of tartaric acid.
new phases, with unique phase behavior for each of the metals.
By the combination of synthetic, calorimetric, and computational
studies, we have been able to identify and begin to distinguish
the kinetic and thermodynamic factors controlling phase be-
havior in this complex system. We focus in particular on the
anhydrous phases of Ca, Sr, and Ba, allowing the determination
of relative stabilities by computational methods without the
complications arising from different levels of hydration.
Results and Discussion
1. Hydrothermal Synthesis of Alkaline Earth Tartrates. The
reactions of tartaric acid with calcium, strontium, and barium
were studied from room temperature to 220 °C. Tartaric acid
exists in three crystalline forms (Figure 1), excluding consid-
eration of polymorphs and hydrates: the chiral D- or L-tartaric
acid (S,S- and R,R- respectively), the racemic D,L-tartaric acid
(racemic mixture of the enantiomers), and the achiral diastere-
omer, meso-tartaric acid (R,S-tartaric acid). Reactions were
carried out with each of the three forms. The reactions yield 18
phases between room temperature and 220 °C, including the
five previously known phases. Eleven new crystal structures
are reported (Table 1). Structural descriptions of the new
anhydrous phases 2, 7, 11, 13, and 17, which were used in the
computational and calorimetry studies, are included in the text.
Complete structural descriptions of all new hydrate phases are
included in the Supporting Information.
1.1. Calcium Tartrates. The reactions of calcium acetate with
tartaric acid yield seven different phases between room tem-
perature and 220 °C (Table 2). Throughout this paper, con-
nectivity is described using the InOm notation.13 In is the
dimensionality of inorganic (metal-oxygen-metal) connectiv-
ity. Om is the dimensionality of organic connectivity, excluding
organic connectivity that is redundant with inorganic connectiv-
ity. The sum, n + m, is the overall connectivity and must be
less than or equal to 3. For example, a compound with both
3-D inorganic and 3-D organic connectivity would be described
as I3O0 because the organic connectivity does not increase the
overall connectivity.
Subsequent to our previous work18 on magnesium tartrates,
in which we found that the magnesium tartrate system exhibited
unexpected complexity, we became interested in studying the
other alkaline earth tartrates in order to better understand the
importance of ligand chirality, cation identity, and thermody-
namic and kinetic factors in controlling phase diversity and
behavior. The study of tartaric acid and its salts has a long
history, and as early as 1935 the space group of the enantiopure
room-temperature phase, calcium tartrate tetrahydrate, was
determined.19 Complete structures of calcium tartrate tetrahy-
drate and strontium tartrate trihydrate were subsequently
reported in 1968.20 Despite this early start, the investigation of
new alkaline earth tartrates has been limited and continues to
focus on room-temperature phases. The room-temperature
barium tartrate phase has been synthesized and its structure
determined21 and the structure of the room-temperature phase,
strontium tartrate tetrahydrate, solved.22 However, even with
this recent work, only five unique structures of calcium,
strontium, and barium tartrates were collected in the CCDC prior
to this work. In this work we describe a comprehensive study
of the synthesis of a series of calcium, strontium, and barium
tartrate frameworks from room temperature to 220 °C. The
results reveal a very rich and complex system, generating 13
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Phase Behavior. In the calcium tartrate system, no isomer-
ization of the tartaric acid ligand is observed, even at high
temperatures. Calcium forms a unique phase from each of the
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