{2-[1-(2,6-Diisopropylphenylimino)ethyl]pyridyl}palladium dibromide polymorphs originating from different Br...π and C-H...Br contacts

Article abstract of DOI:10.1002/ejic.200800260

{2-[1-(2,6-Diisopropylphenylimino)ethyl]pyridyl}palladium dibromide (1) was synthesized and crystallized as red crystals from chloroform in the monoclinic space group P2₁/c or as yellow crystals from methanol in the triclinic space group P1. Other solvents, such as THF and acetonitrile, gave mixtures of red and yellow crystals. With very little conformational differences between the molecules in the dimorphs of 1 the crystal packing is a herringbone array in 1-red and a parallel array in 1-yellow, with the supramolecular interactions differing in the Br...π and C-H...Br contacts. Wiley-VCH Verlag GmbH & Co. KGaA, 2008.

Full text of DOI:10.1002/ejic.200800260

FULL PAPER
DOI: 10.1002/ejic.200800260
{2-[1-(2,6-Diisopropylphenylimino)ethyl]pyridyl}palladium Dibromide
Polymorphs Originating from Different Br···π and C–H···Br Contacts
Wenjuan Zhang,[a] Xiubo Tang,[a] Hongwei Ma,[a] Wen-Hua Sun,*[a] and
Christoph Janiak*[b]
Keywords: Palladium complexes / Polymorphism / Supramolecular interactions / C–H···Br / Br···π
{2-[1-(2,6-Diisopropylphenylimino)ethyl]pyridyl}palladium
dibromide (1) was synthesized and crystallized as red crystals
from chloroform in the monoclinic space group P21/c or as
ferences between the molecules in the dimorphs of 1 the
crystal packing is a herringbone array in 1-red and a parallel
array in 1-yellow, with the supramolecular interactions dif-
fering in the Br···π and C–H···Br contacts.
¯
yellow crystals from methanol in the triclinic space group P1.
Other solvents, such as THF and acetonitrile, gave mixtures
of red and yellow crystals. With very little conformational dif-
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2008)
pound
{2-[1-(2,6-diisopropylphenylimino)ethyl]pyridyl}-
Introduction
palladium dichloride, however, showed very low activity
towards ethylene. Based on the improved activity observed
with bromo rather than chloro analogues of the nickel com-
plexes,[17] the transformation of palladium chloride into the
bromide was studied. The catalytic activity did not change,
but for {2-[1-(2,6-diisopropylphenylimino)ethyl]pyridyl}-
palladium dibromide different colors and crystal structures
were observed when grown from different solvents. Here we
report on the structural analysis of the title compound and
discuss the basis of its polymorphism. There is need for the
understanding of intermolecular interactions in the context
of crystal packing in view of the ongoing difficulty involved
in investigating and quantifying structural differences be-
tween polymorphs.[6,7]
Studies on polymorphism are of timely interest.[1–4] Poly-
morphism, the occurence of different crystal structures for
the same chemical entity,[5] is of particular practical, and
hence financial importance, in industrial processes where
different physical properties of polymorphic forms can sub-
stantially alter the viability and quality of a product.[6] Poly-
morphism can be associated with differences in molecular
conformation, molecular orientation, crystal packing, solu-
bility, color etc.[7,8] Polymorphs provide information on
conformational flexibility and are the basis for ab initio
crystal structure predictions.[9] However, it is very complex
to rationalize how and why compounds are efficiently
packed in different forms. The aggregation of flexible mole-
cules depends on the compromise between intra- and inter-
molecular weak bonding interactions in the crystal (exter-
nal or extra-molecular interactions).[10,11] The transforma-
tion of different polymorphs may be triggered by the exter-
nal stresses[7,12] and governed by the intra- and intermo-
lecular forces in the solid state.[13]
Results and Discussion
{2-[1-(2,6-Diisopropylphenylimino)ethyl]pyridyl}palla-
dium dichloride was treated with NaBr to form its bromo
analog 1 in 86% yield [Equation (1)].[18] The resultant palla-
dium dibromide complex 1 is stable in the solid state.
Recrystallization of 1 from chloroform gave red, from meth-
anol yellow crystals. Other solvents, such as THF and ace-
Palladium complexes are widely used as catalysts for car-
bon–carbon coupling reactions.[14,15] In our previous work,
(pyridylimine)metal complexes showed good activity for
ethylene oligomerization and polymerization.[16] The com-
[a] Key Laboratory of Engineering Plastics, Institute of Chemistry, tonitrile, gave mixtures of red and yellow colored crystals
Chinese Academy of Sciences,
[Equation (2)]. Both crystal forms were confirmed to be of
Beijing 100080, China
identical molecular composition by their elemental analyses
and NMR spectra. The two forms must be of similar overall
packing energy since they are concomitant dimorphs. The
IR spectra of the solid yellow and red crystals showed some
slight differences (Figure S1), and IR measurements were
useful to investigate the crystal forms and the color, since
most solvents gave mixtures of the two phases.
Fax: +86-10-62618239
E-mail: whsun@iccas.ac.cn
[b] Institut für Anorganische und Analytische Chemie, Universität
Freiburg,
Albertstr. 21, 79104 Freiburg, Germany
Fax: +49-761-2036147
E-mail: janiak@uni-freiburg.de
Supporting information for this article is available on the
WWW under http://www.eurjic.org or from the author.
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© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2008, 2830–2836

Polymorphic Palladium Complex
Figure 1. Overlay of the molecular structures of 1-red (dark grey)
and 1-yellow (light grey) by specifying the atoms of the PdBr2 moi-
ety as three pairs in the two structures (RMS = 0.0159).
(1)
1-red and 1-yellow is quite similar. Also it is much longer
than Pd···Pd separations which are discussed as differen-
tiating factors in other Pd polymorphs (typically much less
than 4.0 Å, see Supporting Information). The Br2···PdN2C2
heterocycle contact given by the Br···N2 distance is shorter
by 0.3 Å in 1-red than in 1-yellow. It has been suggested
that an active electron delocalization within a metal–N-
heterocyclic chelate ring could exhibit some degree of
“metalloaromaticity”[25,19,20] so that a Br···π contact could
be invoked.[21]
(2)
Both crystals, 1-red and 1-yellow feature the same palla-
dium complex with a highly similar square-planar geometry
around the metal atom and similar coordination features in
terms of bond lengths and angles (Table 1). From an over-
lay of the molecules of 1-red and 1-yellow in Figure 1 the
only obvious conformational difference between them is a
different torsion of the isopropyl group with respect to the
phenyl plane [defined by the angle H–C(iPr)–C–Cipso(Ph),
see Table 1]. The very similar molecular structures provide
little information as to the origin of their packing differ-
ence.
Table 1. Selected bond lengths [Å] and angles [°] in the dimorphs
of 1.
1-red
1-yellow
Pd1–N1
Pd1–N2
Pd1–Br1
Pd1–Br2
2.039(4)
2.040(4)
2.3811(8)
2.3993(7)
79.5(2)
2.039(7)
2.023(6)
2.3955(12)
2.4038(17)
79.6(3)
N1–Pd1–N2
N1–Pd1–Br1
N1–Pd1–Br2
N2–Pd1–Br1
N2–Pd1–Br2
Br1–Pd1–Br2
Torsion angle:
H–C(iPr)–C–Cipso(Ph)
Interplanar angle:
PdN2C2 heterocycle to Ph
95.8(1)
95.4(2)
175.0(1)
174.7(1)
95.6(1)
173.3(2)
174.8(2)
95.2(2)
89.10(3)
89.92(5)
1.7(7), 14.4(7)
86.8(1)
23(1), 25(1)
83.3(3)
Figure 2. Dimeric units in (a) 1-red and (b) 1-yellow. For details,
including symmetry transformations, see Tables 1 and 2.
Each dimorph contains dimeric units (see parts a and b
in Figure 2) which may be seen as “building blocks” of both
The dimorphs are the result of two different ways of crys-
structures. The differences in the intra-dimer, intermo- tal packing of these dimeric building blocks. In 1-red they
lecular distances are provided in Table 2. In the comparative form a herringbone array, in 1-yellow they are parallel (Fig-
packing of Pd and other transition metal polymorphs the ure 3). The different packing of the dimeric building blocks
weak interactions of interest (see Supporting Information) must in turn be due to subtle differences in weak supramo-
are Pd···Pd and Pd···Br contacts. The Pd···Pd separation in lecular interactions.
Eur. J. Inorg. Chem. 2008, 2830–2836
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W. Zhang, X. Tang, H. Ma, W.-H. Sun, C. Janiak
FULL PAPER
Table 2. Intermolecular distances in the “dimeric building
blocks”.[8a,9]
More obvious are differences in the intermolecular C–
H···Br hydrogen bonding interactions between the mole-
cules in 1-red and 1-yellow, respectively. Table 3 provides a
list and Figure 4 a pictorial presentation of the C–H···Br
hydrogen bonding. The molecules of 1-red have 12 C–
H···Br interactions with H···Br Ͻ 3.20 Å (2ϫ6 unique C–
H···Br contacts). The molecules of 1-yellow feature only 6
of such C–H···Br interactions. Often C–H···Br contacts are
noted in the packing discussion of bromine containing
Contact
1-red
1-yellow
Pd···Pd [Å]
Pd···Br2 [Å]
4.446(1)(3)
3.820(1)(3)
4.329(2)(2)
3.846(2)(2)
Plane PdN2C2···PdN2C2 plane:
Centroid···centroid [Å]
Interplanar separation [Å]
Interplanar angle [°]
5.903(3)(3)
3.500(3)
0.0(3)
6.047(2)
3.494(2)
0.0(2)
Angle between the centroid vector
and the normal to the plane [°]
53.6
54.7
Table 3. Intermolecular C–H···Br hydrogen bonds in the dimorphs
of 1.[a]
Br2···N2
3.480(4)(3)
3.780(8)(2)
(2)
(3)
C–H···Br
H···Br
[Å]
C···Br
[Å]
C–H···Br
[°]
Symmetry transformations: 2 – x, 2 – y, –z; –x, –y, –z.
1-red[b]
C–H17A···Br1(1Ј)
C–H18A···Br1(1Ј)
C–H14B···Br2(2)
C–H8B···Br2(4)
C–H9A···Br2(4)
C–H10A···Br2(3)
3.04(1)
3.630(5)
122.9(3)
2.99(1)
3.14(1)
3.18(1)
3.13(1)
2.94(1)
3.612(5)
3.457(6)
4.078(8)
4.047(8)
3.760(6)
125.4(3)
101.5(4)
156.2(5)
160.1(5)
142.0(3)
1-yellow[c]
C–H17A···Br2(1Ј)
C–H14B···Br2(2)
C–H10A···Br1(1Ј)
2.93(1)
3.09(1)
2.96(1)
3.64(1)
3.58(1)
3.784(9)
133.8(7)
113.5(6)
142.4(5)
[a] With H···Br less than 3.20 Å. [b] Plus symmetry-related set of
(1Ј)
C–H···Br bonds to Br1 and Br2. Symmetry relations:
–1 + x, y,
(2)
(3)
(4)
z;
–x, 0.5 + y, 0.5 – z;
–x, –y, –z;
= x, –0.5 – y, 0.5 + z.
[c] Plus symmetry-related set of C–H···Br bonds to Br1 and Br2.
(1Ј)
(2)
Symmetry relations:
x, –1 + y, z;
2 – x, 2 – y, –z.
Figure 3. Packing of the dimeric building blocks in the dimorphs.
(a) “herringbone” array in 1-red and (b) parallel array in 1-yellow.
The aromatic ligand system in 1 would suggest the pres-
ence of differentiating π–π and C–H···π interactions.[22–25]
There are no significant π–π contacts, however, in that they
exhibit rather long centroid-centroid distances (Ͼ4.7 Å) to-
gether with large slip angles (Ͼ35°) and vertical displace-
ments (Ͼ3.0 Å) between the ring centroids. No C–H···π
contacts (with H···centroid Ͻ 3.0 Å) can be found with
PLATON in 1-yellow and only one such interaction is ob-
served in 1-red onto the pyridyl plane with symmetry trans-
formation –x, 1/2 + y, 1/2 –z and d[H···centroid] = 2.97 Å,
d[H···Ќ] = 2.89 Å, and Є[CH···centroid] = 132°.[23]
Figure 4. Molecules in 1 with selected surrounding intermolecular
C–H···Br contacts. (a) 1-red, (b) 1-yellow. For further details, in-
cluding symmetry transformations, see Table 3.
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© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2008, 2830–2836

Polymorphic Palladium Complex
molecules,[26] including the differentiation of polymorphs[27]
as well as related cis-(dibromo)metal[28] and other
(multibromo)metal complexes.[29,30] The range of the C–
H···Br contacts observed in 1 matches the range observed
and discussed for the packing in the compound
[Ph2S=N=SPh2]2[Pd2Br6].[30] C–H···Br contacts may not be
as widely known as C–H···O/N,[31,32] C–H···F[33] or C–
H···Cl hydrogen bonds[34] but the role of C–H···Br–M inter-
actions in crystalline species has been recognized as struc-
ture-directing factors.[35]
A packing analysis with PLATON[36] of structurally re-
lated Pd polymorphs of PdCl2(2,2Ј-bipy),[37] PdCl2(N)2,[38]
[39]
PdCl(C N)(P),
PdCl(P)3,[40]
[Ph4P]2[PdCl3(N)]2,[41]
∧
PdI2(PMe2Ph)2,[42] and PdCl2(cod)[43] revealed the absence
of “strong” N/O–H···O/N hydrogen bonds. Only weaker
supramolecular interactions could be found, predominantly
C–H···Cl hydrogen bonds together with some C–H···π and
occasionally π···π interactions. We see this absence of strong
intermolecular interactions, which would tend to favor only
one packing arrangement, as the basis for the formation of
polymorphs.
Figure 6. Scatterplot for the C–H···Br angle vs. (C–)H···Br distance
distribution from a CSD search.[44]
and yellow for the modifications is often noted, e.g. in
[Co2(ibdpt)2(MeCN)2(H2O)2](ClO4)4,[48] 5-methyl-2-[(2-ni-
trophenyl)amino]-3-thiophenecarbonitrile (ROY),[6,49] TlI
picrate,[5,50] and [RhCl(CO)2(Hpzdp)].[51]
A search in the Cambridge Structure Database (CSD)[44]
retrieved 11899 entries with one or more intermolecular
(C–)H···Br distances within the range 2.6–3.3 Å. The total
of 65000 intermolecular C–H···Br contacts within the 11899
different CSD Refcodes shows that such interactions are
not unusual.[35] The frequency distribution of H···Br dis-
tances (Figure 5) shows an onset at about 2.6 Å with a
steady increase in the number of contacts to ca. 3.1 Å where
the curve levels off. There is no clustering of structures. The
experimental C–H···Br angles fall between 100° and 180°
(Figure 6). There is a preference for linearity with shorter
H···Br distances. In 1 the intermolecular C–H···Br hydrogen
Conclusions
Compound
{2-[1-(2,6-diisopropylphenylimino)ethyl]-
pyridyl}palladium dibromide (1) crystallizes as a red form
from chloroform and as a yellow polymorph from meth-
anol. Its polymorphism is traced to packing differences due
to Br···π contacts between the Br···PdN2C2 heterocycle to-
gether with different C–H···Br bonds.
bonds start at 2.93 Å below the sum of the van der Waals Experimental Section
radii of H (1.20 Å) and Br (1.85 Å). The C–H···Br param-
eter in 1 fit well with the statistical analysis.
General: All manipulations were carried out under an atmosphere
of nitrogen using standard Schlenk techniques. The IR spectra were
obtained on a Perkin–Elmer FT-IR 2000 spectrophotometer by
1
using the KBr disc in the range of 4000–400 cm–1. The H NMR
spectra were recorded on a Bruker DMX-300 instrument with TMS
as the internal standard. Elemental analyses were performed on a
Flash EA 1112 microanalyzer.
THF and toluene were refluxed over sodium/benzophenone until
the purple color appeared and distilled under nitrogen prior to use.
Pd(CH3CN)2Cl2 was prepared in a manner similar to that reported
for the benzonitrile anlogue,[52] and 2-[1-(2,6-diisopropylphen-
ylimino)ethyl]pyridine was synthesized as described.[53] All other
chemicals were obtained commercially and used without further
purification unless stated otherwise.
X-ray Crystallography: Crystals of the red and yellow phases were
selected and mounted on the tip of a glass fiber by means of hydro-
carbon oil. Data collection: RIGAKU R-AXIS RAPID IP CCD
area detector at 293(2) K with graphite monochromated Mo-Kα
radiation (λ = 0.71073 Å). Cell parameters were obtained by global
refinement of the positions of all collected reflections. Intensities
were corrected for Lorentz and polarization effects and an empiri-
cal absorption correction was performed. Structure analysis and
refinement: The structure was solved by direct methods (SHELXS-
97);[54] refinement was done by full-matrix least-squares on F2 using
Figure 5. Histogram (frequency distribution) of the (C–)H···Br dis-
tance distribution from a CSD search.[44]
The packing difference in the dimorphs of 1 also extends
to their color. We note that in better investigated com-
pounds of platinum with aromatic ligands, the emission
properties are often affected by the extent of Pt···Pt and
π–π contacts.[45] There exists a yellow and red polymorph
of [PtCl2(2,2Ј-bipy)],[46] of {Tl[(C4H9N4)Pt(CN)2]}n,[47] in
other polymorphic forms a color difference of red (orange) the SHELXL-97 program suite.[54] All non-hydrogen positions were
Eur. J. Inorg. Chem. 2008, 2830–2836
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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2833

W. Zhang, X. Tang, H. Ma, W.-H. Sun, C. Janiak
FULL PAPER
refined with anisotropic temperature factors. Hydrogen atoms were
placed at calculated positions with an appropriate riding model
(AFIX 43 for aromatic CH, AFIX 13 for aliphatic CH and AFIX
33 for CH3) and an isotropic temperature factor of Ueq(H) = 1.2
Ueq(CH) and 1.5 Ueq(CH3). Details of the X-ray structure determi-
nations and refinements are provided in Table 4. Graphics were
drawn with DIAMOND.[55]
tion by their elemental analyses and NMR spectra. C19H24Br2Pd
(518.61): calcd. C 41.75, H 4.43, N 5.12; found C 41.79, H 4.45, N
5.22. H NMR (300 MHz, CDCl3): δ = 9.86 (d, J = 5.20 Hz, 1 H,
1
pyridyl-H), 8.21 (t, J = 1.09 Hz, 1 H, pyridyl-H), 7.90 (d, J =
7.75 Hz, 1 H, pyridyl-H), 7.81 (t, J = 6.93 Hz, 1 H, pyridyl-H),
7.32 (t, J = 7.35 Hz, 1 H, phenyl-H), 7.25 (d, J = 7.35 Hz, 2 H,
phenyl-H), 3.09 [dd, J = 6.89 Hz, 2 H, -CH(CH3)2], 2.25 (s, 3 H,
CH3-C=N), 1.42 (d, J = 6.67 Hz, 6 H, CH3), 1.12 (d, J = 6.81 Hz,
6 H, CH3) ppm; the isopropyl methyl groups are chemically dif-
ferent due to their orientation to the “front” or “rear” of the PdBr2
reference moiety in connection with the hindered rotation of the
phenyl group around the C–N bond.
Table 4. Crystal data and structure refinement for 1-red and 1-yel-
low.
Compound
1-red
1-yellow
Empirical formula
C19H24Br2N2Pd
546.62
C19H24Br2N2Pd
546.62
Supporting Information (see also the footnote on the first page of
this article): Infrared spectra of 1-red and 1-yellow, a Table with a
description and comparison of the palladium polymorphs.
M [gmol–1
]
Crystal size [mm]
θ range [°]
h; k; l range
Crystal system
Space group
a [Å]
b [Å]
c [Å]
α [°]
β [°]
0.31ϫ0.17ϫ0.16
2.01–27.48
0, 13; –15, 16; –20, 20 0, 11; –9, 11; –18, 18
monoclinic
P21/c
10.191(2)
12.763(3)
15.704(3)
90
95.59(3)
90
2032.9(7)
4
1.786
1072
0.34ϫ0.05ϫ0.04
2.56–27.48
triclinic
¯
Acknowledgments
P1
9.029(2)
9.236(2)
14.029(3)
81.24(3)
89.66(3)
61.10(3)
1009.3(3)
2
We are greatly thankful to the National Natural Science Founda-
tion of China for financial assistance under GrantNo. 20272062
and to the University of Freiburg.
γ [°]
V [Å3]
[1] Recent work on polymorphs: a) A. Kuhn, P. Diaz-Carrasco,
M. E. A. de Dompablo, F. Garcia-Alvarado, Eur. J. Inorg.
Chem. 2007, 3375–3384; b) L. Yu, CrystEngComm 2007, 9,
847–851; c) S. Schnabel, C. Röhr, Z. Naturforsch., Teil B 2007,
62, 1235–1245; d) F. P. A. Fabbiani, L. T. Byrne, J. J. McKin-
non, M. A. Spackman, CrystEngComm 2007, 9, 728–731; e)
M. Gobel, T. M. Klapötke, Z. Anorg. Allg. Chem. 2007, 633,
1006–1017; f) B. M. Casari, V. Langer, Z. Anorg. Allg. Chem.
2007, 633, 1074–1081; g) M. F. Davis, W. Levason, M. E. Light,
R. Ratnani, G. Reid, K. Saraswat, M. Webster, Eur. J. Inorg.
Chem. 2007, 1903–1910; h) E. F. Paulus, F. J. J. Leusen, M. U.
Schmidt, CrystEngComm 2007, 9, 131–143; i) E. M. Zerbe, C.
Wolper, S. R. Pinol, P. G. Jones, A. Blaschette, Z. Anorg. Allg.
Chem. 2007, 633, 593–602; j) S. Busi, R. Fröhlich, M. Lahtinen,
R. Sillanpaa, K. Rissanen, Z. Naturforsch., Teil B 2007, 62,
35–43; k) O. Mundt, G. Becker, J. Baumgarten, H. Riffel, A.
Simon, Z. Anorg. Allg. Chem. 2006, 632, 1687–1709; l) M. Neu-
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Preminger, J. Bernstein, J. Y. Becker, CrystEngComm 2006, 8,
41–50; n) K. Roth, Chem. Unserer Zeit 2006, 40, 398–406.
[2] Recent work on polymorphs involving high-pressure crystalli-
zation: a) F. P. A. Fabbiani, D. R. Allan, S. Parsons, C. R. Pul-
ham, CrystEngComm 2005, 7, 179–186; b) F. P. A. Fabbiani,
D. R. Allan, W. I. F. David, S. A. Moggach, S. Parsons, C. R.
Pulham, CrystEngComm 2004, 6, 504–511.
Z
Dcalcd. [g cm–3
F(000)
]
1.799
536
4.881
µ [mm–1
]
4.847
Max./min. transmission
Reflections collected
Independent reflections
Obsd. refl. [IϾ2σ(I)]
Parameters refined
0.5147/0.3141
18905
4656 (Rint = 0.0471)
3367
0.8287/0.2876
7537
4282 (Rint = 0.0452)
2487
217
217
Max./min. ∆ρ[a] [eÅ–3
R1/wR2 [IϾ2σ(I)][b]
R1/wR2 (all refl.)[b]
]
0.799/–1.241
0.0387/0.1052
0.0585/0.1176
0.993
0.917/–0.899
0.0554/0.1355
0.1108/0.1718
1.026
Goodness-of-fit on F2[c]
Weight. scheme w; a/b[d]
0.0681/0.0000
0.0880/0.0000
[a] Largest difference peak and hole. [b] R1 = [Σ(||Fo| – |Fc||)/Σ|Fo|];
wR2 = {Σ[w(Fo – Fc ) ]/Σ[w(Fo2)2]}1/2. [c] Goodness-of-fit =
2
2 2
{Σ[w(Fo – Fc ) ]/(n – p)}1/2. [d] w = 1/[σ2(Fo2) + (aP)2 + bP] where
P = [max(Fo or 0) + 2Fc ]/3.
2
2 2
2
2
CCDC-271437 (for 1-red) and -271438 (for 1-yellow) contain the
supplementary crystallographic data for this paper. These data can
be obtained free of charge from The Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
[3] Recent experimental and theoretical work on polymorphs: a) P.
McArdle, K. Gilligan, D. Cunningham, R. Dark, M. Mahon,
CrystEngComm 2004, 6, 303–309; b) T. C. Lewis, D. A. Tocher,
G. M. Day, S. L. Price, CrystEngComm 2003, 5, 3–9; c) A. T.
Anghel, G. M. Day, S. L. Price, CrystEngComm 2002, 4, 348–
355; d) M. A. Spackman, J. J. McKinnon, CrystEngComm
2002, 4, 378–392.
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