Combining topochemical [2+2] photoreactions and hydrothermal isomerisation for the regioselective and quantitative preparation ofrtct-pyridylcyclobutanes

Article abstract of DOI:10.1039/c2nj20855e

A non-photochemical route for the regioselective and quantitative preparation of rtct-pyridylcyclobutane isomers is achieved from the combination of solid state [2+2] photoreactions and controlled isomerisations promoted either by Lewis or Bronsted acids under hydrothermal conditions. The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2012.

Full text of DOI:10.1039/c2nj20855e

View Article Online / Journal Homepage / Table of Contents for this issue
Dynamic Article Links
NJC
Cite this: New J. Chem., 2012, 36, 554–557
www.rsc.org/njc
LETTER
Combining topochemical [2+2] photoreactions and hydrothermal
isomerisation for the regioselective and quantitative preparation of
rtct-pyridylcyclobutanesw
Yennifer Hill, Maholy Linares and Alexander Briceno*
˜
Received (in Montpellier, France) 6th October 2011, Accepted 25th November 2011
DOI: 10.1039/c2nj20855e
6
a
A non-photochemical route for the regioselective and quantitative
preparation of rtct-pyridylcyclobutane isomers is achieved from the
combination of solid state [2+2] photoreactions and controlled
isomerisations promoted either by Lewis or Brønsted acids under
hydrothermal conditions.
promoted by polymolybdates (POMs). These isomers were iso-
lated as counterions into inorganic–organic hybrid solids and
obtained in high to fair yields. Likewise, we reported the asymme-
trical thermal stereomutation of head to tail rctt-1,3-bis(2-
pyridyl)2,4-bis(4-pyridyl)cyclobutane [rctt-2,4-tpcb-ht; tpcb =
tetrapyridylcyclobutane] to the rttt-isomer via metal assistance
2
+ 6b
(
Mn ). Later, Vittal and co-workers have reported that the
0
Recent efforts have been made in the implementation of supra-
molecular strategies to direct reactivity of olefins either in the
photoproduct from 4,4 -bpe (rctt-4,4-tpcb) isomerizes slowly
in solution to the rcct- and rtct-isomers at room temperature
7a
catalyzed by CF CO H acid. Also, the same dimer at 100 1C
3 2
1
–3
4
or in solution. In both cases, supramolecular
solid state
assistance based on the use of auxiliary molecules, and/or
metal centres is commonly used to drive regiocontrolled synthesis
of single cyclobutane stereoisomers. Particularly, in the solid
state such strategies provide an exquisite regio- and stereo-
undergoes a quantitative conversion to the rtct-4,4-tpcb
7b,8
isomer in the presence of either trifluoroacetic acid or HCl.
Based on these antecedents, we have evaluated different
factors such as: electronic effect of the relative position of
the N atom on the pyridyl ring, temperature, heating time,
metal centres and acid assistance, among others in order to
tune quantitative transformations. Herein the potential of such
a strategy is demonstrated by the synthesis, structural characteri-
sation, solid state reactivity and isomerisation studies of different
1–3
selective control combined with quantitative yields. However,
in spite of the well known advantages of solid state reactions over
the same reactions in solution, such strategies have been successful
almost exclusively in obtaining rctt-cyclobutane-like compounds,
but are unsuitable for preparation of other stereochemical
configurations that are difficult to access (i.e., rcct, rtct and/or
head-to-tail isomers). The preference for this configuration is a
direct consequence of the syn orientation of the available func-
tional groups commonly used on the homo-template (–OH,
1
rctt-stilbene dimers (Scheme 1). The H-NMR characterisation of
the organic products obtained under different experimental condi-
tions reveals the selective isomerisation of the different rctt-dimers
into rcct-, or rtct-isomers, depending on temperature, metal or acid
assistance. Under hydrothermal conditions, quantitative isomeri-
sation of rctt-dimers into rtct-dimers was observed in all the cases.
The cyclobutane derivatives were prepared from controlled
1c
COOH and pyridines) exploited for the alignment of the olefins.
Interestingly, although this stereochemistry is not the most
thermodynamically stable, it is the most favourable in the
products obtained from direct irradiation of olefins also in
[
2+2] cycloaddition reactions in the solid state of trans-bis
0 0
5
solution. Thus, the development of strategies focused on the
(
4-pyridyl)ethylene (4,4 -bpe), trans-bis(2-pyridyl)ethylene (2,2 -bpe)
0
improvement of regioselective synthesis of not easily accessible
cyclobutanes continues to be an exciting challenge in organic
photochemistry. In this regard, we reported the combination of
topochemical cycloaddition with hydrothermal methods for the
regioselective preparation of different rtct-pyridylcyclobutanes
and trans-1-(2-pyridyl)-2-(4-pyridyl)ethylene (2,4 -bpe), according
6,9,10
to previously published procedures.
The different photo
1
products obtained were characterised by H-NMR spectroscopy
before the isomerisation in order to verify the chemical purity of
starting cyclobutanes (see ESIw).
6
Based on our earlier works, we promptly evaluated the
Laboratorio de Sı ´n tesis y Caracterizacio´n de Nuevos Materiales,
Centro de Quı´mica, Instituto Venezolano de Investigaciones Cientı´ficas
effect of the metal centre on the isomerisation of the photo-
2+ 3+
dimers, which were heated in the presence of Mn and Al
(IVIC), Apartado 21827, Caracas 1020-A, Venezuela.
E-mail: abriceno@ivic.gob.ve; Fax: +58-212-5041350;
Tel: +58-212-5041320
metal centres upon reflux in methanol/water for 24 h. The
1
H-NMR characterisation of the products isolated after the
w Electronic supplementary information (ESI) available: Experimental
details, H-NMR spectra. CCDC 831805–831807. For ESI and crystallo-
thermal treatment confirmed the isomerisation of some cyclo-
butane derivatives. The spectra showed the presence of the
same signals observed for starting compounds, together with
1
graphic data in CIF or other electronic format see DOI: 10.1039/
c2nj20855e
5
54 New J. Chem., 2012, 36, 554–557
This journal is c The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2012

View Article Online
contrast to those cases described above. The signals observed
display different chemical shift values compared to the corres-
ponding protons in the starting rctt-isomers. These values are
5
characteristic of different symmetrical isomers (rtct-configuration).
All the spectra revealed a quantitative isomerisation (100%). In
order to gain better insight about this process, a new set of
experiments in the presence of a Brønsted-acid were also evaluated.
Thus, taking advantage of the use of 1,2,3,4 benzenetetracarboxylic
acid (bta) as an efficient template to direct reactivity of olefins in the
solid state and its ability to give interesting supramolecular
Scheme 1
11
assembly directed by charge-assisted hydrogen bonds, here,
we extended the use of bta either to assist the isomerisation
process or as a potential crystallisation agent of the resulting
isomers. The dimers were heated at 140 1C in the presence of bta
under hydrothermal conditions, using bta : dimer in a molar
an additional complex set of signals characteristic of an asymme-
trical product. These changes can be easily monitored in the
region of the aliphatic methine protons (3–5.4 ppm, see ESIw). All
the compounds undergo partial isomerisation to give rcct and rctt
mixture in different ratios, depending on the starting compound,
1
0
0
ratio 1 : 1. The H-NMR spectra showed similar results to those
except for the dimers from 2,2 -bpe and 2,4 -bpe (h–t), where the
3+
observed for the isomerisation in the presence of Al (quantitative
partial formation of the rtct-isomers was also observed in the
1
H-NMR spectra. The compositions of the resulting mixtures
1
were determined by H-NMR analysis and are summarised in
isomerisation into the rtct-isomer). As anticipated, the
0
0
resulting stereochemistry for rtct-4,4 -tpcb and rtct-2,4 -tpcb-ht
isomers was confirmed by single crystal X-ray diffraction
2+
Table 1. In all the cases the isomerisation degree was always slightly
2ꢀ
0
3+
2+
analysis. Crystals of [(bta )ꢁ(rtct-4,4 -H tpcb) ]ꢁ5H O (5) and
2
2
higher in the presence of Al than Mn solution. It is important
to note that the formation of rcct isomers is not commonly
observed, to our knowledge, from direct photoreaction of stilbenes
2ꢀ 4+
0
[
(bta )ꢁ(rtct-2,4 -H
4
tpcb-ht)0.5 ]ꢁ1.28H
2
O (6) were obtained by
crystallisation of corresponding bta–rtct-isomer mixture after
4f,7b,8
in solution,
heating.
the rctt-isomer always being the favoured one.
0
The asymmetric unit of 5 contains half stilbenium (rtct-4,4 -
Nevertheless, under such conditions it is possible to achieve
moderate yields (36–52%), such a possibility can be viewed as a
useful route to drive the synthesis of these stereoisomers. In
addition, the rcct-isomer obtained from the isomerisation of 1b
yields a chiral isomer (Fig. S6, ESIw). This result suggests that this
route can also be exploited in order to induce chirality from achiral
2
+
2ꢀ
tpcb) cation lies on a twofold-axis, one bta anion and five
disordered lattice water molecules.z The crystal structure of 5
forms a 3D-hydrogen bonded network (Fig. 1(a)), which can
be described as 2D-hydrogen bonded layers built-up from self-
2ꢀ
assembly of bta anions linked by charge-assisted hydrogen
12
bonding between carboxylic and carboxylate groups
˚
cyclobutane derivatives.
Encouraged by these results, we evaluated the effect of Al
3
+
[O1ꢁ ꢁ ꢁO5: 2.492(4) and O7ꢁ ꢁ ꢁO3: 2.526(4) A] (Fig. 1(b)). Such
H-bonded networks are extended in the bc-plane and display
˚
salt on the same transformations under hydrothermal conditions
rhomboidal windows with distances ca. 7.2 ꢂ 8.7 A. In each
at 140 1C for 24–48 h, in order to achieve quantitative conver-
2+
layer, every window is penetrated by one 4-pyridyl unit from
2
sions. On the other hand, the Mn ion was discarded in the
study under hydrothermal conditions due to the effect of a
paramagnetic ion on NMR line broadening, which is difficult
+
(
rtct-4,4-H tpcb)
2
cations, whereas the remaining pyridyl
moieties are oriented parallel to 2D-hydrogen networks. These
cations are distributed above and below these layers in an
alternate fashion. These layers are sustained through multiple
hydrogen bonding interactions among pyridinium, pyridyl,
carboxylate and carboxylic moieties and disordered lattice
water molecules located into intricate channels that run along
1
to be removed completely from the solution. The H-NMR
spectrum of the products obtained after the hydrothermal treat-
ment either of the mixture obtained from the study upon reflux in
3+
the presence of Al or using fresh rctt-dimers did not show the
presence or formation of asymmetrical stereoisomers (rcct), in
Table 1 Product distribution of 1a–1e isomers determined by
H-NMR at different thermal conditions; upon reflux or hydrothermal
1
2+
3+
conditions (HC) in the presence of Mn , Al or bta
2+
Reflux with Mn
3+
Upon HC with Al or bta
Dimer
2
3
4
2
3
4
1a
1b
1c
1d
1e
44
62
52
54
—
—
38
48
46
—
56
ꢀ
—
—
—
—
14
—
—
—
—
86
100
100
100
100
—
ꢀ
ꢀ
—
3+
Reflux with Al
1
1
1
1
a
b
c
44
59
13
48
—
41
49
52
56
—
38
—
Fig. 1 (a) View of the stacking of the crystal structure of 5. (b) 2D
H-bonded network formed by charge-assisted hydrogen bonding
between anions in the bc-plane.
d
This journal is c The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2012
New J. Chem., 2012, 36, 554–557 555

View Article Online
Fig. 2 View of the H-bonded layers linked by the charge-assisted
carboxylate–pyridinium hydrogen bonds in the structure of 6.
the b-direction. The percentage of solvent-accessible space in
Fig. 3 View of H-bonded chains in the crystal structure of 7.
these channels in the crystal structure corresponds to 21.8%
3
2132 A per unit cell) of the total volume (9780 A ).
3
˚
˚
(
reactions were carried out in the absence of metal centres or
bta under identical experimental conditions. In addition, in
these assays the formation of side products due to the cyclo-
reversion reaction and/or other asymmetrical stereomutations
was not observed, in contrast to the thermal isomerisation of
The asymmetric unit of 6 contains one half stilbenium (rtct-
4+
0
2
,4 -tpcb)
cation lies about a twofold axis, two halves of
2ꢀ
crystallographically independent bta anions, one is located on
a twofold axis, while the other lies about an inversion centre and
one water of crystallisation with partial occupation. The crystal
structure of 6 forms a 3D-hydrogen bonded network (Fig. 2),
which can be seen as a sinusoidal 2D-hydrogen bonded network
1
2
cyclobutanes at elevated temperatures. These results reveal
the regio- and stereospecific nature of all the isomerisations
upon hydrothermal control.
2ꢀ
self-assembled from bta
anions linked by charge-assisted
At the present stage, based on these observations and further
examination of our previously reported works on isomerisation of
pyridyl cyclobutanes some insight can be drawn: (a) the isomeri-
sation of this kind of compounds is closely related to the intrinsic
strain associated with a four membered ring combined with
carboxylate–pyridinium supramolecular synthons. This array is
constructed from zigzag ribbons connected via charge assisted
2ꢀ
hydrogen bonding between one bta anion and 4-pyridinium
0
4+
tpcb-ht) unit along the c-direction
units from the (rtct-2,4 -H
˚
4
[
N1ꢁꢁꢁO1: 2.657(3) A]. These ribbons are linked by the second
repulsive electrostatic charges generated for the presence of
6,13
2ꢀ
bta
anion through remaining 2-pyridyl units from (rtct-
4+
ionisable groups (pyridyl and –COOH) on the rings.
In
0
2
,4 -H tpcb-ht) cations also via carboxylate–pyridinium inter-
particular, a remarkable electronic effect either of the relative
position of the N atom on the pyridyl ring or the presence of
neutral substituent groups is observed. (b) Either acidity or
temperature plays a fundamental role in the stereoselective
control of the resulting isomers, where the rcct-isomer is a
metastable intermediate, which is formed in a first step upon
mild heating, then is transformed to the most stable isomer (rtct)
at highest temperatures. Similar results have been observed by
4
˚
actions [N2ꢁꢁꢁO8: 2.405(3) A]. These interactions generate
H-bonded layers parallel to the ac-plane. Adjacent layers are
stacked via self-complementary hydrogen bonding between water
molecules of crystallisation and oxygen atoms of carboxylic
groups. In this structure the preferred formation of intra-
molecular hydrogen bonds between carboxylate and carboxylic
groups is observed, in contrast to that observed for 5.
7
0
Encouraged by the above results, we have expanded the
tolerance of this approach in relation to stilbazole derivatives.
In particular, the isomerisation from the rctt-head to tail dimer of
Vittal et al., in the isomerisation of rctt-4,4 -tpcb to rtct-isomer
promoted by HCl. (c) The intelligent choice of the substituent
groups on the cyclobutane ring can be exploited to fine-tune
quantitative and regio- and stereoselective synthesis either of rcct
or rtct isomers, depending on temperature.
4
-Cl-Stb (rctt-4-Cl-dpcb-ht) under hydrothermal conditions at
40 1C leads to the formation exclusively of the asymmetrical
1
stereoisomer (rcct) in high yield ca. 86%, in contrast to those
cases found using stilbene derivatives. A remarkable feature in
these processes is the apparent isomerisation of the pyridyl ring.
Suitable crystals bearing bta-rcct-4-Cl-dpcb-ht isomer mixture (7)
were also obtained. The asymmetric unit of 7 contains one
molecule of the resulting dimer located in a general position
with no imposed symmetry; one half of bta lies about an
inversion centre, and one water of crystallisation with partial
occupation. The crystal structure of 7 forms 1D-hydrogen
bonded chains self-assembled via heterodimeric hydrogen-
bonded synthons based on the interaction between carboxylic
and pyridyl groups in an alternate fashion, producing hydrogen
bonded rings along the (101)-direction [N1ꢁꢁꢁO3: 2.594(3) and
In summary, we have demonstrated the ability either of a
Lewis or Brønsted acid combined with an increase in temperature
to promote the partial or total isomerisation in solution of rctt-
isomers either to rcct or rtct. We have established a non-photo-
chemical route to prepare quantitatively and regio- and stereo-
selectively rtct-tpcb compounds via hydrothermal-assisted
isomerisation. This novel approach opens a window to develop
efficient routes for the preparation and/or improvement of the
yield of new and conventional cyclobutane-like stereoisomers that
are difficult or impossible to access either in solution or by known
solid state routes, including the possibility of obtaining chiral
cyclobutanes. This alternative can be very helpful in order to
14
overcome the limitations imposed by the topochemical postulate
˚
N2ꢁꢁꢁO2: 2.725(3) A] (Fig. 3). Such interactions are neutral in
for obtaining regioselective photoproducts with such stereochemical
requirements from crystalline assemblies. In addition, these rtct-
cyclobutane derivatives represent novel attractive multitopic
contrast to those observed for 5 and 6.
A striking feature of the isomerisation processes observed
for the different tpcb is that these did not occur when the
15
building blocks for the self-assembly of metal–organic polyhedra,
5
56 New J. Chem., 2012, 36, 554–557
This journal is c The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2012

View Article Online
1
b
MOFs, supramolecular hydrogen bonded assemblies and parti-
9a,16
cularly the study of supramolecular isomerism.
(g) T. Caronna, R. Liantonio, T. A. Logothetis, P. Metrangolo,
T. Pilati and G. Resnati, J. Am. Chem. Soc., 2004, 126, 4500;
Further works
(
(
h) G. K. Kole, G. K. Tan and J. J. Vittal, Org. Lett., 2010, 12, 128;
i) G. K. Kole, G. K. Tan and J. J. Vittal, CrystEngComm, 2011,
on the use of such molecules in the self-assembly of multi-
component supramolecular arrays, as well as the induction of
chirality on achiral cyclobutane derivatives via hydrothermal
isomerisation are in progress.
1
3, 3138; (j) X. Mei, S. Liu and C. Wolf, Org. Lett., 2007, 9, 2729;
(k) W.-Z. Zhang, Y.-F. Han, Y.-J. Lin and G.-X. Jin, Organometallics,
2010, 29, 2842; (l) K. A. Wheeler, J. D. Wiseman and R. C. Grove,
CrystEngComm, 2011, 13, 3134.
(a) M. B. Hickey, R. F. Schlam, C. Guo, T. H. Cho, B. B. Snider
and B. M. Foxman, CrystEngComm, 2011, 13, 3146; (b) M. J. Vela,
B. B. Snider and B. M. Foxman, Chem. Mater., 1998, 10, 3167;
This work was supported in part by FONACIT, grant
G-2005000433. We thank FONACIT (grant LAB-97000821)
for partial financial support. TSU Alberto Fuentes (NMR
laboratory, IVIC).
3
4
(c) G. Dıaz de Delgado, K. A. Wheeler, B. B. Snider and
´
B. M. Foxman, Angew. Chem., Int. Ed. Engl., 1991, 30, 420.
(a) M. Pattabiraman, A. Natarajan, R. Kaliappan, J. T. Mague and
V. Ramamurthy, Chem. Commun., 2005, 4542; (b) S. Yamada,
N. Uematsu and K. Yamashita, J. Am. Chem. Soc., 2007,
129, 12100; (c) D. G. Amirsakis, M. A. Garcia-Garibay, S. J. Rowan,
J. F. Stoddart, A. J. P. White and D. J. Williams, Angew. Chem., Int.
Ed., 2001, 40, 4256; (d) R. Kaliappan, M. V. S. N. Maddipatla,
L. S. Kaanumalle and V. Ramamurthy, Photochem. Photobiol. Sci.,
Notes and references
z Crystal data for 5: C22
26 2 t
H N O13, M = 526.45, orthorhombic, space
˚
group Fdd2, T = 293(2) K, a = 27.455(7), b = 36.891(9), c = 9.656(2) A,
.
3
ꢀ1
ꢀ3
˚
U = 9781(4) A , Z = 16, m(Mo-Ka) = 0.12 mm , rcalcd = 1.430 g cm
2
2
R
int = 0.035, R
independent reflections (I > 2s(I)). CCDC 831805. Crystal data for 6:
= 895.80, monoclinic, space group P2/n, T =
1
(F ) = 0.058, wR(F ) = 0.16, S = 1.050 for 3579
2007, 6, 737; (e) Y. Nishioka, T. Yamaguchi, M. Yoshizawa and
44 34.56 4 t
C H N O17.28, M
˚
93(2) K, a = 11.2070(18), b = 10.5278(15), c = 17.198(3) A,
3
M. Fujita, J. Am. Chem. Soc., 2007, 129, 7000; (f) S. Yamada,
Y. Nojiri and M. Sugawara, Tetrahedron Lett., 2010, 51, 2533.
5 J. Vansant, S. Toppet, G. Smets, J. P. Declercq, G. Germain and
2
ꢀ1,
˚
b = 96.718(4), U = 2015.1(5) A , Z = 2, m(Mo-Ka) = 0.12 mm
ꢀ3
2
(F ) = 0.058, wR(F ) = 0.16, S =
2
r
calcd = 1.476 g cm . Rint = 0.037, R
.04 for 2878 independent reflections (I > 2s(I)). CCDC 831806. Crystal
1
1
M. V. Meerssche, J. Org. Chem., 1980, 45, 1565.
data for 7: C62
H
47.24Cl
4
N
4
O
8.63, M
t
= 1128, triclinic, space group P 1% ,
6 (a) A. Bricen
2008, 3275; (b) Y. Hill and A. Bricen
3930.
˜
o, A. Fulgence, Y. Hill and R. Atencio, Dalton Trans.,
˚
T = 293(2) K, a = 10.254(3), b = 10.381(3), c = 13.085(5) A,
˜
o, Chem. Commun., 2007,
3
˚
a = 86.63(3), b = 87.56(3), g = 75.39(2)1, U = 1344.9(7) A , Z = 2,
ꢀ
1
ꢀ3
2
m(Mo-Ka) = 0.284 mm
,
r
calcd = 1.393 g cm . Rint = 0.024, R
.052, wR(F ) = 0.153, S = 1.05 for 4015 independent reflections
I > 2s(I)). CCDC 831807.
1
(F ) =
7 (a) A. M. P. Peedikakkal, C. S. Y. Peh, L. L. Koh and J. J. Vittal,
Inorg. Chem., 2010, 49, 6775; (b) A. M. P. Peedikakkal, L. L. Koh
and J. J. Vittal, Chem. Commun., 2008, 441; (c) G. K. Kole,
G. K. Tan and J. J. Vittal, J. Org. Chem., 2011, 76, 7860.
2
0
(
1 For recent reviews in solid state reactivity: (a) Y. Sonoda, Molecules,
2011, 16, 119; (b) I. Weissbuch and M. Lahav, Chem. Rev., 2011,
8
9
D. Liu, N.-Y. Lia and J.-P. Lang, Dalton Trans., 2011, 2170.
(a) A. Briceno, D. Leal, R. Atencio and G. Dıaz de Delgado,
Chem. Commun., 2006, 3534; (b) A. Briceno, Y. Hill, T. Gonzalez
and G. Dıaz de Delgado, Dalton Trans., 2009, 1602;
c) C. Avendano and A. Briceno, CrystEngComm, 2009, 11, 408.
˜
´
1
11, 3236; (c) L. R. MacGillivray, J. Org. Chem., 2008, 73, 3311;
˜
´
(
d) L. R. MacGillivray, G. S. Papaefstathiou, T. Fris
T. D. Hamilton, D.-K. Bucar, Q. Chu, D. B. Varshney and
I. G. Georgiev, Acc. Chem. Res., 2008, 41, 280;
e) M. Nagarathinan, A. M. P. Peedikakkal and J. J. Vittal, Chem.
Commun., 2008, 5277; (f) F. Toda, ed. Organic Solid Sate Chemistry,
special issue. Top. Curr. Chem., 2005, 254, 1–305.
(a) S. Yamada and Y. Tokugawa, J. Am. Chem. Soc., 2009,
31, 2098; (b) R. Santra and K. Biradha, Cryst. Growth Des.,
010, 10, 3315; (c) R. Santra and K. Biradha, CrystEngComm,
008, 10, 1524; (d) A. Papagni, P. Del Buttero, C. Bertarelli,
L. Miozzo, M. Moret, M. T. Pryce and S. Rizzato, New J. Chem.,
010, 34, 2612; (e) I. G. Georgiev, D.-K. Bucar and
L. R. Macgillivray, Chem. Commun., 2010, 46, 4956;
f) B. R. Bhogala, B. Captain, A. Parthasarathy and
V. Ramamurthy, J. Am. Chem. Soc., 2010, 132, 13434;
cic,
ˇ ˇ ´
´
ˇ
(
˜
˜
1
0 T. D. Hamilton, G. S. Papaefstathiou and L. R. MacGillivray,
J. Am. Chem. Soc., 2002, 124, 11606.
1 M. Linares and A. Briceno, New J. Chem., 2010, 34, 587.
˜
(
1
1
1
2 J. E. Baldwin, Chem. Rev., 2003, 103, 1197, and references therein.
3 (a) Y. Kim and D.-Y. Jung, Inorg. Chim. Acta, 2002, 338, 229;
˜
(b) A. Briceno, unpublished results.
14 G. M. J. Schmidt, Pure Appl. Chem., 1971, 27, 647.
15 (a) I. G. Georgiev and L. R. MacGillivray, Chem. Soc. Rev., 2007,
36, 1239; (b) J. J. Vittal, Coord. Chem. Rev., 2007, 251, 1781;
(c) M. Nagarathinam and J. J. Vittal, Angew. Chem., Int. Ed., 2006,
45, 4337.
2
1
2
2
2
ˇ
(
16 B. Moulton and M. J. Zaworotko, Chem. Rev., 2001, 101, 1629.
This journal is c The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2012
New J. Chem., 2012, 36, 554–557 557