Versatile supramolecular reactivity of zinc-tetra(4-pyridyl)porphyrin in crystalline solids: Polymeric grids with zinc dichloride and hydrogen-bonded networks with mellitic acid

Article abstract of DOI:10.3762/bjoc.5.77

Crystal engineering studies confirm that the zinc-tetra(4-pyridyl)porphyrin building block reveals versatile supramolecular chemistry. In this work, it was found to be reactive in the assembly of both (a) a 2D polymeric array by a unique combination of se

Full text of DOI:10.3762/bjoc.5.77

Versatile supramolecular reactivity of
zinc-tetra(4-pyridyl)porphyrin in crystalline solids:
Polymeric grids with zinc dichloride and
hydrogen-bonded networks with mellitic acid
Sophia Lipstman and Israel Goldberg*
Full Research Paper
Open Access
Address:
Beilstein Journal of Organic Chemistry 2009, 5, No. 77.
School of Chemistry, Sackler Faculty of Exact Sciences, Tel Aviv
University, 69978 Ramat Aviv, Tel Aviv, Israel
doi:10.3762/bjoc.5.77
Received: 19 October 2009
Accepted: 07 December 2009
Published: 11 December 2009
Email:
Israel Goldberg* - goldberg@post.tau.ac.il
* Corresponding author
Guest Editor: C. A. Schalley
Keywords:
© 2009 Lipstman and Goldberg; licensee Beilstein-Institut.
License and terms: see end of document.
coordination polymers; crystal engineering; hydrogen-bonded
networks; porphyrin assemblies; supramolecular chemistry
Abstract
Crystal engineering studies confirm that the zinc-tetra(4-pyridyl)porphyrin building block reveals versatile supramolecular chem-
istry. In this work, it was found to be reactive in the assembly of both (a) a 2D polymeric array by a unique combination of self-
coordination and coordination through external zinc dichloride linkers and (b) an extended heteromolecular hydrogen-bonded
network with mellitic acid sustained by multiple connectivity between the component species.
Introduction
The tetra(4-pyridyl)porphyrin entity in its free-base (TPyP) as The facile formation of 3D aggregates, in which the porphyrin
well as its metallated (MTPyP) forms has an extraordinarily units are inter-coordinated via exocyclic metal ion linkers (e.g.,
rich supramolecular chemistry, playing an important role in the tetrahedral CuI), has also been observed [5]. The latter mode of
construction of diverse polymeric architectures. The TPyPs are coordination polymerization, in which the peripheral pyridyl
readily available [1], and the tendency of the zinc-porphyrin de- sites of different TPyP/MTPyP moieties can be readily bridged
rivative to form polymeric chains by self-coordination between by various metal ion connectors due to the high affinity of the
the peripheral pyridyl sites of one molecule and the zinc center pyridyl N-sites to coordinate to transition metal ions, has
of an adjacent species was demonstrated nearly two decades attracted much attention over the years, leading to the formula-
ago [2,3]. Subsequently, it was discovered that MTPyP can not tion of a large variety of hybrid organic–inorganic 1D ladders
only self-assemble into 1D polymeric arrays, but can also form and ribbons, 2D nets, and 3D supramolecular constructs (repre-
a robust 3D architecture with molecular sieving features [4]. sentative refs [6-11]).
Page 1 of 7
(page number not for citation purposes)

Beilstein Journal of Organic Chemistry 2009, 5, No. 77.
Moreover, a new series of MTPyP-based homomolecular by multiple hydrogen bonding. The same applies to ZnTPyP,
coordination polymers has been reported [12-19]. In particular, when the axial coordination site of the zinc ion is blocked.
supramolecular isomerism characterizes the zinc metallopor-
phyrin compound, which results in a range of coordination In order to expand the library of the available polymeric mater-
aggregates with diverse connectivity patterns [17-19]. The high ials and further explore the different possible modes of self-
propensity of the ZnTPyP moiety to exhibit various modes of assembly, we report here on two new ZnTPyP-based structures
self-coordination can be attributed to the binding flexibility of characterized by either coordination or hydrogen-bonding
the zinc ion, as well as to the multiple potential ligating sites networking. They represent a coordination polymer
(the four pyridyl substituents) of the square-planar porphyrin ZnTPyP·ZnCl2, which crystallizes as a 1,1,2,2-tetrachloro-
framework. Thus, the zinc ion in the porphyrin core can be ethane (TCE) trisolvate (I), and a hydrogen-bonded polymeric
four-coordinate (to the four pyrrole N-sites without any axial network composed of Zn(EtOH)TPyP (where the ethanol
ligation and no option for self-coordination), or, as most solvent occupies and protects the axial coordination site of the
frequently encountered, five-coordinate (binding, in addition, zinc ion) and 1,2,3,4,5,6-benzenehexacarboxylic (mellitic) acid
one axial ligand), or six-coordinate (in an octahedral environ- (1:1), which crystallizes with one molecule of o-dichloro-
ment with two axial ligands on both sides of the porphyrin benzene and three molecules of methanol solvent (II). The
macrocycle). In the five-coordinate case, the ZnTPyP assem- component building blocks are shown in Scheme 1.
blies are either 1D chain-polymeric or 0D square-oligomeric,
whereas in the six-coordinate case, they form either 3D honey- Results and Discussion
comb architectures or 2D square-grid networks [17-19]. Simul- The coordination polymer in structure I was obtained by
taneous appearance of the two coordination modes in a single chance, while attempting to network ZnTPyP with different
homomeric assembly has also been observed, yielding in such a tetracarboxylic acids (see Experimental). It exhibits, however, a
case ladder-type 1D polymeric ribbons [13,15].
uniquely interesting connectivity scheme that combines direct
and through-ZnCl2 porphyrin-to-porphyrin coordination
The hydrogen-bonding capacity of TPyP and MTPyP in (Figure 1), a pattern not previously observed.
network formation has not been explored until recently. The
porphyrin framework is characterized by a square-planar Crystal data for I: C40H24N8·ZnCl2·3C2H2Cl4, M = 1321.82,
symmetry, bearing laterally diverging pyridyl sites. The latter monoclinic, space group P21/c, a = 19.1157(2) Å, b =
are available for hydrogen bonding as proton acceptors with 12.9275(2) Å, c = 21.8495(2) Å, β = 103.396(1)°, V =
complementary components that can act as proton donors. 5252.5(1) Å3, Z = 4, Dc = 1.672 g cm−3, μ(Mo Kα) =
Formulation of extended hydrogen-bonding-sustained networks 1.67 mm−1, 38848 reflections measured, 12452 unique (Rint =
requires ideally a tetradentate proton donor of similar square- 0.045), final R = 0.065 for 8437 reflections with I > 2σ(I) and R
planar symmetry. It has been confirmed that 1,2,4,5-benzene- = 0.099 (wR = 0.200) for all data.
tetracarboxylic acid (B4CA) is perfectly suited for this purpose,
as are hydrogen-bonded dimers of 1,3,5-benzenetricarboxylic The zinc ion in the porphyrin core is five-coordinate with
acid (B3CA) [20,21]. In both cases the TPyP moiety self- square-pyramidal environment. A given porphyrin unit is
assembles with the corresponding acid in appropriate solubil- involved in two direct coordination bonds to its neighbors. One
izing environments into 2D heteromolecular grids held together of the four peripheral pyridyl substituents links to the zinc
Scheme 1: Component building blocks of the supramolecular assembly in I and II.
Page 2 of 7
(page number not for citation purposes)

Beilstein Journal of Organic Chemistry 2009, 5, No. 77.
Figure 1: Fragment of the continuous coordination scheme in I, forming a corrugated layer that is aligned perpendicular to the c-axis of the crystal. (a)
Wireframe presentation, with the exception of the zinc ions which are depicted as small spheres, illustrating the connectivity scheme. (b) Space-filling
view of the coordination polymer. Note that the ZnCl2 bridges and the non-coordinated pyridyl groups point outward from the network. The interpor-
phyrin kinks and voids within the polymeric layer, as well as between neighboring layers, are occupied by the TCE solvent (shown as “ball-and-stick”
molecules).
center of an adjacent species, while the metal ion binds to a the crystal. Such a four-point per porphyrin binding model,
pyridyl group of a third porphyrin unit (at Zn–N = 2.150 Å; all which involves the zinc ion and three of the pyridyl groups,
the Zn–Npyrrole bond lengths are in the range 2.066–2.075 Å). It results in the formation of a 2D grid coordination polymer
is further coordinated to two additional porphyrins with the aid wherein neighboring porphyrins are roughly perpendicular to
of the tetrahedral zinc dichloride connectors (at Zn–N = 2.040 each other. There is a considerable resemblance between the
and 2.046 Å), each bridging between pyridyl groups of two observed connectivity and that found in the recently reported
neighboring moieties. The fourth pyridyl group is not involved “paddle-and-wheel”-like square-grid homomolecular coordina-
in intermolecular coordination, and is rotationally disordered in tion polymer of ZnTPyP with a six-coordinate environment
Page 3 of 7
(page number not for citation purposes)

Beilstein Journal of Organic Chemistry 2009, 5, No. 77.
around the zinc center [17,18]. The inter-coordinated assembly tals [24]. Preferential hydrogen bonding of ZnTPyP to the
represents a markedly corrugated layer, which is aligned normal mellitic acid, over self-coordination (as in I), may occur only if
to the c-axis of the unit cell. The non-coordinated pyridyl the axial coordination ability of the central zinc ion is blocked.
groups and the chloride ions lie above and below the molecular This can be achieved by introducing small zinc-coordinating
surface of this layer. In the crystals the layers are stacked along ligands (e.g., water, MeOH, EtOH, DMF, or DMSO) into the
the c-axis and partly interdigitate into one another. Molecules of crystallization mixture [20], as in the present case. Ideally, the
the TCE crystallization solvent accommodate the interpor- use of tetracarboxylic ligand of square-planar geometry such as
phyrin kinks and voids within the polymeric assembly, as well B4CA is best suited to optimize hydrogen-bonding interactions
as voids in the interface between adjacent layers. The forma- with the complementary tetradentate TPyP moiety [20,21].
tion of coordination polymers bridged by exocyclic zinc ions of When 1,3,5-benzenetricarboxylic acid was used in a similar
tetrahedral geometry has also been observed in supramolecular reaction, it formed hydrogen-bonded dimers first by using one
materials based on the tetra(4-carboxyphenyl)porphyrin COOH function of each monomer to yield an entity with four
building blocks [22,23].
free carboxylic groups to bind to the porphyrin.
Based on previous findings [20,21], the ZnTPyP and mellitic Not surprisingly, therefore, when the mellitic acid was used in
acid components provide excellent building blocks for the this study, two of its carboxylic acid functions (at positions 3
construction of heteromolecular networks sustained by coopera- and 6 of the central benzene ring) did not interact with
tive hydrogen bonding. Both have multiple laterally diverging Zn(EtOH)TPyP, thus mimicking effectively the functionality of
functions, the 4-pyridyl substituents in ZnTPyP acting as proton B4CA. The supramolecular assembly that formed in this case is
acceptors and the carboxylic residues in mellitic acid as comple- depicted in Figure 2.
mentary proton donors. The heteromeric COOH···Npyridyl inter-
action results in a relatively strong hydrogen bond, which Crystal data for II: C40H24N8Zn·C2H5OH·C12H6O12 (crystal-
frequently directs supramolecular organization in organic crys- lization solvent, X = C6H4Cl2·3CH3OH, excluded due to
Figure 2: Space-filling representation of the hydrogen-bonded heteromolecular network in II. Note that every porphyrin unit is in direct hydrogen-
bonding contact through its pyridyl groups with four molecules of mellitic acid and vice versa. Two of the carboxylic groups of the latter point into the
intralayer voids. They are solvated in the crystal by molecules of the methanol solvent (MeOH) that occupy the adjacent voids, the alternating voids
being occupied by the o-dichlorobenzene (DCB) solvent. These flat layers are aligned parallel to the (110) plane of the crystal. In this figure, at every
porphyrin site the EtOH axial ligand is connected to the central zinc ion from below, preventing self-coordination of the ZnTPyP units (as in I).
Page 4 of 7
(page number not for citation purposes)

Beilstein Journal of Organic Chemistry 2009, 5, No. 77.
disorder), M = 1070.28, triclinic, space group P , a = adopt in the above constructs either five-coordinate or six-
11.8660(3) Å, b = 13.8803(4) Å, c = 20.4843(5) Å, α = coordinate geometries, which affects the architecture of the
78.237(2)°, β = 78.185(2)°, γ = 84.288(1)°, V = 3227.1(2) Å3, resulting assembly. Formation of coordination networks by
Z = 2, Dc = 1.101 g cm−3 and μ(Mo Kα) = 0.44 mm−1 (X combining direct porphyrin–porphyrin coordination and
excluded), 35843 reflections measured, 15151 unique (Rint = coordination through an external linker (as in I) has been
0.051), final R = 0.060 for 9766 reflections with I > 2σ(I) and R demonstrated here for the first time. It has further been demon-
= 0.092 (wR = 0.167) for all data. Molecules of the crystalliza- strated that TPyP and ZnTPyP scaffolds may be used also for
tion solvent are included in a severely disordered manner within the formulation of supramolecular assemblies sustained by co-
the interstitial voids of the crystal lattice (the solvent-accessible operative hydrogen bonding with the pyridyl substituents as
voids amount to 35.3% of the crystal volume) and cannot be excellent proton acceptors for compatible proton donating
modeled reliably by discrete atoms. Conventional least-squares species [20,21]. In particular, the strong COOH···Npy interac-
refinement of the complete structural model resulted in tion [24] can be harnessed to this end by reacting the
R1 = 0.10. In the final calculations the contribution of the tetrapyridylporphyrin with a polycarboxylic acid entity, and
disordered solvent was subtracted from the diffraction data by thus inducing cooperative hydrogen-bonding interactions in
the Squeeze procedure [25], a common practice in similar situ- four different directions. The observed structure of compound
ations. The least-squares refinement converged smoothly to a II provides an attractive example of designed formulation of
lower R-value, allowing a precise determination of the such heteromolecular networks. The observed modes of self-
hydrogen-bonded network.
assembly are of further significance to studies of surface-based
crystallizations of monolayer and multilayer hydrogen-bonded
Compound II is characterized by a 1:1 stoichiometry of the networks and metal–organic frameworks on various substrates
Zn(EtOH)TPyP and mellitic acid components. Every porphyrin [26,27], in the context of the design of novel molecular devices.
is efficiently hydrogen bonded to four adjacent acids, and every
acid is hydrogen bonded to four different porphyrins. This Experimental
connectivity scheme results in a fascinating supramolecular grid The ZnTPyP, 1,4,5,8-naphthalenetetracarboxylic acid (NTCA),
sustained by such cooperative COOH···Npy hydrogen bonding 1,2,3,4,5,6-benzenehexacarboxylic (mellitic) acid, 3,4,9,10-
(at N···O within 2.56–2.62 Å). The layered array thus formed perylenetetracarboxylic acid (PTCA), as well as common la-
has an open structure, with voids of two types lined by the two boratory solvents were procured commercially, and used
pairs of the networking components and encircled by four without further purification. The porphyrin was treated with
hydrogen bonds (Figure 2). The two “excess” carboxylic func- different acids under diverse experimental conditions in an
tions of the mellitic acid are solvated by the methanol solvent attempt to synthesize heteromeric hydrogen-bonding networks
that protrudes into 50% of the intralayer voids. The other voids of the interacting components. The coordination polymeric
within the layered array are filled by DCB. Such neighboring compound (I) was first obtained when a methanol solution of
layers are related to one another by inversion in an offset PTCA (in which ZnTPyP is sparingly soluble) was carefully
manner. Within the inversion-related layers the axial EtOH layered at room temperature over a solution of ZnTPyP (0.015
ligands of one layer penetrate into the voids of another layer. mmol) in a 1:1 mixture of TCE and methanol (10 ml). Crystals
The concave surfaces of the five-coordinate porphyrin moieties appeared in the bottom solution after a few days. The same
are located on the outside of the paired layers, inducing further crystalline compound was obtained under reflux conditions
incorporation of crystallization solvent (MeOH) to fill this inter- when 0.05 mmol of ZnTPyP was reacted with 0.05 mmol of
facial space between the paired layers, which stack effectively NTCA in a 1:1:1:1 solvent mixture of TCE, o-chlorophenol/
along the [110] axis of the crystal.
o-dichlorobenzene, ethanol, and N,N-dimethylformamide
(DMF). The resulting solution was refluxed for 16 h, and then
cooled to room temperature and left for crystallization. X-ray
Conclusion
This study confirms the high versatility of the ZnTPyP moiety quality crystals were obtained after four days. It appeared in
as an effective building block in the formulation of supra- both cases that ZnCl2 was formed in situ under the acidic condi-
molecular grids. The square-planar ZnTPyP framework has tions by extracting some of the zinc ions from the metallopor-
diverse and multidentate binding capacities. It can self- phyrin. The porphyrin-mellitic acid hydrogen-bonded
coordinate directly via the Zn-pyridyl bonds to form 2D and 3D compound (II) was obtained when a methanol solution (10 ml)
polymeric arrays [4,18,19]. Then, it can form supramolecular of mellitic acid (0.055 mmol) was carefully layered over a solu-
assemblies of varying dimensionality with the aid of exocyclic tion of 0.010 mmol of ZnTPyP dissolved in a 1:1 mixture
metal ion linkers capable of coordinating simultaneously to (10 ml) of ethanol and o-dichlorobenzene. Sizeable crystals
several neighboring ZnTPyP units [5-11]. The ZnTPyP can appeared after three days. The uniformity of the formed crystal-
Page 5 of 7
(page number not for citation purposes)

Beilstein Journal of Organic Chemistry 2009, 5, No. 77.
line materials was confirmed in each case by repeated measure- Acknowledgement
ments of the unit cell dimensions from several randomly chosen This research was supported by The Israel Science Foundation
single crystals.
(Grant No. 502/08).
References
The diffraction measurements were carried out on a Nonius
KappaCCD diffractometer, using graphite monochromated Mo
Kα radiation (λ = 0.7107 Å). The crystalline samples of the
analyzed compounds were covered with a thin layer of light oil
and freeze-cooled to ca. 110 K in order to minimize solvent
escape, structural disorder, and thermal motion effects, and
increase the precision of the results. The structures were solved
by direct methods (SIR-97) and refined by full-matrix least-
squares on F2 (SHELXL-97). Intensity data were corrected for
absorption effects. All non-hydrogen atoms (except of those of
the disordered pyridyl group and TCE solvent in I and the
disordered solvent in II) were refined anisotropically. The
hydrogens were either found in difference Fourier maps or
located in idealized positions, and were refined using a riding
model with fixed thermal parameters [Uij = 1.2 or 1.5 Uij (eq.)
for the atom to which they are bonded]. No phase transitions of
the two crystalline compounds were detected between room
temperature and 110 K. The two polymeric structure types
contain sizeable voids, which are accommodated by molecules
of crystallization solvent (three molecules of TCE in I, and one
moiety of o-dichlorobenzene and three molecules of methanol
in II). In II, the solvent species could be clearly identified in the
electron-density maps but they were found to be severely
disordered in the lattice and could not be reliably modeled by
discrete atoms. Correspondingly, their contribution to the
diffraction pattern was subtracted by the Squeeze procedure
(commonly used in similar situations) [25], allowing smooth
convergence of the crystallographic refinement and precise
description of the hydrogen-bonded framework.
1. Fleischer, E. B. Inorg. Chem. 1962, 1, 493–495.
doi:10.1021/ic50003a010
2. Collins, D. M.; Hoard, J. L. J. Am. Chem. Soc. 1970, 92, 3761–3771.
doi:10.1021/ja00715a038
3. Fleischer, E. B.; Shachter, A. M. Inorg. Chem. 1991, 30, 3763–3769.
doi:10.1021/ic00019a038
4. Krupitsky, H.; Stein, Z.; Goldberg, I.; Strouse, C. E.
J. Inclusion Phenom. Mol. Recognit. Chem. 1994, 18, 177–192.
doi:10.1007/BF00705820
5. Abrahams, B. F.; Hoskins, B. F.; Michail, D. M.; Robson, R. Nature
1994, 369, 727–729. doi:10.1038/369727a0
6. Hagrman, D.; Hagrman, P. J.; Zubieta, J. Angew. Chem., Int. Ed. 1999,
38, 3165–3168.
doi:10.1002/(SICI)1521-3773(19991102)38:21<3165::AID-ANIE3165>3
.0.CO;2-O
7. Sharma, C. V. K.; Broker, G. A.; Huddleston, J. G.; Baldwin, J. W.;
Metzger, R. M.; Rogers, R. D. J. Am. Chem. Soc. 1999, 121,
1137–1144. doi:10.1021/ja983983x
8. Barkigia, K. M.; Battioni, P.; Riou, V.; Mansuy, D.; Fajer, J.
Chem. Commun. 2002, 956–957. doi:10.1039/b202513m
9. Carlucci, L.; Ciani, G.; Proserpio, D. M.; Porta, F.
Angew. Chem., Int. Ed. 2003, 42, 317–322.
doi:10.1002/anie.200390106
10.Carlucci, L.; Ciani, G.; Proserpio, D. M.; Porta, F. CrystEngComm
2005, 7, 78–86. doi:10.1039/b417709f
11.Ohmura, T.; Usuki, A.; Fukumori, K.; Ohta, T.; Ito, M.; Tatsumi, K.
Inorg. Chem. 2006, 45, 7988–7990. doi:10.1021/ic060358h
12.Lin, K.-J. Angew. Chem., Int. Ed. 1999, 38, 2730–2732.
doi:10.1002/(SICI)1521-3773(19990917)38:18<2730::AID-ANIE2730>3
.0.CO;2-9
13.Diskin-Posner, Y.; Patra, G. K.; Goldberg, I.
J. Chem. Soc., Dalton Trans. 2001, 2775–2782. doi:10.1039/b104961p
14.Pan, L.; Kelly, S.; Huang, X.; Li, J. Chem. Commun. 2002, 2334–2335.
doi:10.1039/b207855d
15.Ring, D. J.; Aragoni, M. C.; Champness, N. R.; Wilson, C.
CrystEngComm 2005, 7, 621–623. doi:10.1039/b515083n
16.George, S.; Goldberg, I. Acta Crystallogr., Sect. E: Struct. Rep. Online
2005, 61, m1441–m1443. doi:10.1107/S1600536805018556
17.Koner, R.; Goldberg, I.
Supporting Information
Supporting information features X-ray data for compounds
I and II.
Acta Crystallogr., Sect. C: Cryst. Struct. Commun. 2009, 65,
m139–m142. doi:10.1107/S0108270109005691
18.Lipstman, S.; Goldberg, I. CrystEngComm 2010, 12, 52–54.
doi:10.1039/b914799c
Supporting Information File 1
X-ray data for compound I.
[http://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-5-77-S1.cif]
19.Seidel, R. W.; Goddard, R.; Föcker, K.; Oppel, I. M. CrystEngComm
2009. doi:10.1039/b913791b
Published on the Web as an advance article.
Supporting Information File 2
X-ray data for compound II.
20.Koner, R.; Goldberg, I. CrystEngComm 2009, 11, 1217–1219.
doi:10.1039/b906538p
21.Koner, R.; Goldberg, I. J. Inclusion Phenom. Macrocyclic Chem. 2009.
doi:10.1007/s10847-009-9611-0
[http://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-5-77-S2.cif]
Published on the Web as an advance article.
22.Diskin-Posner, Y.; Dahal, S.; Goldberg, I. Chem. Commun. 2000,
585–586. doi:10.1039/b001189o
Page 6 of 7
(page number not for citation purposes)

Beilstein Journal of Organic Chemistry 2009, 5, No. 77.
23.Shmilovits, M.; Vinodu, M.; Goldberg, I. Cryst. Growth Des. 2004, 4,
633–638. doi:10.1021/cg0342009
24.Aakeröy, C. B.; Beatty, A. M.; Helfrich, B. A. J. Am. Chem. Soc. 2002,
124, 14425–14432. doi:10.1021/ja027845q
25.Spek, A. L. J. Appl. Crystallogr. 2003, 36, 7–13.
doi:10.1107/S0021889802022112
26.Blunt, M.; Lin, X.; Gimenez-Lopez, M. C.; Schröder, M.;
Champness, N. R.; Beton, P. H. Chem. Commun. 2008, 2304–2306.
doi:10.1039/b801267a
27.Shi, N.; Yin, G.; Han, M.; Jiang, L.; Xu, Z. Chem.–Eur. J. 2008, 14,
6255–6259. doi:10.1002/chem.200702032
License and Terms
This is an Open Access article under the terms of the
Creative Commons Attribution License
(http://creativecommons.org/licenses/by/2.0), which
permits unrestricted use, distribution, and reproduction in
any medium, provided the original work is properly cited.
The license is subject to the Beilstein Journal of Organic
Chemistry terms and conditions:
(http://www.beilstein-journals.org/bjoc)
The definitive version of this article is the electronic one
which can be found at:
doi:10.3762/bjoc.5.77
Page 7 of 7
(page number not for citation purposes)