Chemistry Reference
In-Depth Information
6.7 Structure Determination of a Material Produced
by Precipitation from Solution
A process that has been studied widely in relation to phenomena such as chiral
symmetry breaking, spontaneous resolution and chiral amplification is the reaction
(Fig.
18
) of [Co(H
2
O)
2
{(OH)
2
Co(en)
2
}
2
](SO
4
)
2
(denoted 1) with NH
4
Br to give the
chiral complex
cis
-[CoBr(NH
3
)(en)
2
]Br
2
(denoted 2). This reaction is historically
important, as 2 was one of the first octahedral metal complexes to be resolved into
D
and
stereoisomers, some years after Werner predicted that octahedral ions
M(en)
2
XY should exist as enantiomeric pairs.
Two crystal structures containing 2, determined by single-crystal XRD, were
reported previously for single crystals prepared by recrystallization of 2. In one case
[
128
], the single crystal was grown from a homochiral sample [(+)
589
-enantiomer],
and the crystal structure has the chiral space group P2
1
2
1
2
1
with one molecule in the
asymmetric unit. In the other case [
129
], the single crystal was a dihydrate of 2,
prepared by crystallization using a racemic mixture of 2, and the crystal structure is
racemic with nonchiral space group C2/c.
Clearly, these structures do not necessarily correspond to the solid product
formed directly in the reaction - as rapid precipitation of 2 occurs from the solution
in which the reaction is carried out, the material obtained directly from the reaction
is a microcrystalline powder. Indeed, powder XRD indicates [
130
] that the product
obtained directly from the reaction is a new solid phase of 2, which differs from the
structures of 2 reported previously. Structure determination of this new phase of
2 was carried out [
130
] directly from powder XRD data using the direct-space
genetic algorithm technique for structure solution followed by Rietveld refinement.
In the crystal structure (Fig.
18b
), the
cis
-[CoBr(NH
3
)(en)
2
]
2+
complexes are
arranged in two different types of chain, propagated along the
a
-axis and
b
-axis
respectively, with neighbouring complexes in each type of chain linked by
N-H
L
Br
-
H-N interactions. Along the
a
-axis, neighbouring repeat units are
related by translation and the chain is relatively straight, whereas along the
b
-axis, neighbouring repeat units are related by the 2
1
symmetry operation and
the chain is helical. With regard to chirality, the most important feature of this result
is the fact that the structure is racemic (nonchiral space group P2
1
/n, with one
formula unit in the asymmetric unit). Studies of this reaction under a wide range
of experimental conditions led consistently to this new racemic phase of 2, and
conglomerate phases of 2 were never obtained. The implications of this result in
relation to previous reports of spontaneous induction of chirality in this system are
discussed in detail elsewhere [
130
].
6.8 Exploiting Anisotropic Thermal Expansion in Structure
Solution
As discussed in Sect.
2.6
, structure solution from powder XRD data by the tradi-
tional approach relies upon the availability of accurate values of the extracted
