Synthesis of 2,6-di(pyrazol-1-yl)-4-bromomethylpyridine, and its conversion to other 2,6-di(pyrazol-1-yl)pyridines substituted at the pyridine ring

Article abstract of DOI:10.1016/j.tet.2006.10.051

Two routes to 2,6-di(pyrazol-1-yl)-4-hydroxymethylpyridine (1) from 2,6-dihydroxy-isonicotinic acid, in four and six steps, are reported. Reaction of 1 with 48% HBr yields 2,6-di(pyrazol-1-yl)-4-bromomethylpyridine (2), which is a powerful precursor to a range of new tridentate ligands for transition metals functionalised at the pyridine ring. As a proof of principle, we describe the further elaboration of 2 to give two 2,6-di(pyrazol-1-yl)pyridines bearing nucleobase substituents, and the back-to-back ligand 1,2-bis[2,6-di(pyrazol-1-yl)pyrid-4-yl]ethane. Crystal structures of two of these new derivatives are presented.

Full text of DOI:10.1016/j.tet.2006.10.051

Tetrahedron 63 (2007) 291–298
Synthesis of 2,6-di(pyrazol-1-yl)-4-bromomethylpyridine,
and its conversion to other 2,6-di(pyrazol-1-yl)pyridines
substituted at the pyridine ring
*
ˆ
ꢀ
Jerome Elhaık, Christopher M. Pask, Colin A. Kilner and Malcolm A. Halcrow
¨
School of Chemistry, University of Leeds, Woodhouse Lane, Leeds LS2 9JT, UK
Received 11 August 2006; revised 4 October 2006; accepted 19 October 2006
Available online 3 November 2006
Abstract—Two routes to 2,6-di(pyrazol-1-yl)-4-hydroxymethylpyridine (1) from 2,6-dihydroxy-isonicotinic acid, in four and six steps, are
reported. Reaction of 1 with 48% HBr yields 2,6-di(pyrazol-1-yl)-4-bromomethylpyridine (2), which is a powerful precursor to a range of new
tridentate ligands for transition metals functionalised at the pyridine ring. As a proof of principle, we describe the further elaboration of 2 to
give two 2,6-di(pyrazol-1-yl)pyridines bearing nucleobase substituents, and the back-to-back ligand 1,2-bis[2,6-di(pyrazol-1-yl)pyrid-4-yl]-
ethane. Crystal structures of two of these new derivatives are presented.
Ó 2006 Elsevier Ltd. All rights reserved.
1. Introduction
The synthesis of 2,6-di(pyrazol-1-yl)pyridines substituted at
the pyridine ring is more difficult,1 because of the poorer
availability of many suitable 4-substituted-2,6-dibromopyr-
idine reagents and because yields of Eq. 1 can be drastically
reduced when carried out using 2,6-dibromopyridines bear-
ing electron-withdrawing substituents.8 Palladium catalysis
of Eq. 1 increases the rate of reaction, but does not signifi-
cantly improve the yields obtained.11
We have been studying the metal complex chemistry of 2,6-
di(pyrazol-1-yl)pyridine and 2,6-di(pyrazol-1-yl)pyrazine
ligands for some time.1 The iron(II) complexes of these
ligands have proved to be particularly fruitful, in that they
undergo thermal spin-crossover transitions at accessible
temperatures (typically 200–300 K), which are amenable
to detailed study.2–6 Materials that undergo spin-crossover
near room temperature are of great current interest, because
their thermochromism lends itself to a variety of applications
in display and information storage devices.7 Others have
also found 2,6-dipyrazolylpyridine derivatives to be effec-
tive sensitisers for lanthanide ions, which have been incorpo-
rated into sensors for biological molecules.8,9 Synthesis of
2,6-di(pyrazol-1-yl)pyridine derivatives bearing alkyl or
substituents at the 3- or 4-positions of the pyrazole ring is
generally straightforward, and achieved in no more than
three steps from commercially available starting materials.
The key step here is the nucleophilic coupling of pre-formed
pyrazoles with 2,6-dibromopyridine (Eq. 1), yielding the
final tridentate ligand in yields of 50–70%.10
We5 and others12 have recently addressed this problem by
using commercially available 2,6-dihydroxy-isonicotinic
acid as a precursor to new 4-substituted-2,6-dibromo-
pyridine derivatives. We report here in full two synthetic
pathways to 2,6-di(pyrazol-1-yl)-4-hydroxymethylpyridine
(1) from this starting material, and its conversion to 2,6-
di(pyrazol-1-yl)-4-bromomethylpyridine (2). Compound 2
is a potentially useful precursor to other 4-substituted-2,6-
di(pyrazol-1-yl)pyridine derivatives bearing a range of
pendant substituents. These might be designed to incorpo-
rate redox-active or fluorescent substituents onto their
complex compounds to dope them onto solid supports, or
to link coordinated metal centres covalently or through
supramolecular interactions. As a proof of principle, we
also describe the synthesis of three new ligands derived
from 2 intended for the latter application.
ð1Þ
2. Results and discussion
The synthesis of 1 was achieved by two different routes,
shown as A and B in Scheme 1. Steps (i), (ii) and (v) of
this scheme followed literature procedures.13,14 While
method A is shorter, step (iii) proceeds in a relatively low
Keywords: Dipyrazolylpyridine; Ligand; Supramolecular; Crystal structure.
* Corresponding author. Tel.: +44 113 343 6506; fax: +44 113 343 6565;
e-mail: m.a.halcrow@leeds.ac.uk
0040–4020/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2006.10.051

292
J. Elha¨ık et al. / Tetrahedron 63 (2007) 291–298
yield of 37%, and was also somewhat unreliable in our
hands. Although it involves extra protection and deprotec-
tion steps, we have found route B in Scheme 1 to be higher
yielding and more reliable overall. It also has the advantage
of avoiding the expensive reagent POBr3 used to prepare the
2,6-dibromoisonicotinic acid ethyl ester starting material for
route A (attempts to perform step (iii) using 2,6-dichloroiso-
nicotinic acid ethyl ester were unsuccessful). Using route B,
1 can be reliably prepared on a multi-gram scale. Bromina-
tion of 1 proceeds readily in 48% HBr, yielding 2 in 73%
yield after the usual work-up (Scheme 2).
As a proof of principle, we have pursued the synthesis of new
ligands bearing nucleobase groups, which could have use in
metal-containing host–guest complexes. The only related
nucleobase/polyimine conjugates, we are aware of in the
literature, are the nucleobase-containing 2,2-bipyridine
derivatives prepared by Ward et al. to promote photo-energy
transfer between supramolecularly linked donors and accep-
tors in solution,15 and a thymine-substituted 2,2:60,200-
terpyridine communicated by Constable.16 Deprotonation
of cytosine with NaH, and reaction of the resultant anion
with 2 in the presence of catalytic KI, cleanly affords 3 in
69% yield. Howeversimilar reactions using guanine, adenine
or thymine all yielded mixtures of products containing both
1
mono- and di-alkylated nucleobases by H NMR, which
were not amenable to separation because of their poor solu-
bilities in solvents other than DMSO. This problem was
overcome in the guanine case by reaction of 2 with 2-
amino-6-chloropurine, cleanly yielding the monoalkylation
product 4, which was in turn dechlorinated to 5by acid hydro-
lysis.17 Although pure by NMR, as-precipitated 5 appears to
retain some water upon drying by microanalysis. This sug-
gestion was supported by its ES mass spectrum, which con-
tained a strong peak at m/z¼393 assignable to [M+H2O+H]+.
Scheme 1. The two syntheses of 1 used in this study. Reagents and condi-
tions: (i) POBr3, 130 ꢀC, 3 h, 82%; (ii) EtOH, concd H2SO4 (cat.), reflux,
15 h, 93%; (iii) 2 equiv Kpz, 2-methoxyethylether, 130 ꢀC, 5 days, 40%;
(iv) 5 equiv NaBH4, EtOH, reflux, 3 h, 78%; (v) POCl3, 150 ꢀC,
24 h, 72%; (vi) MeOH, concd H2SO4 (cat.), reflux, 15 h, 84%; (vii) 5 equiv
NaBH4, EtOH, reflux, 3 h, 85%; (viii) 3,4-dihydropyran, pyridinium
para-toluenesulfonate, CH2Cl2, rt, 16 h, 80%; (ix) 2 equiv Napz, 2-methoxy-
ethylether, 130 ꢀC, 5 days, 81%; (x) pyridinium para-toluenesulfonate,
EtOH, 55 ꢀC, 3 h, 80%.
Pale yellow plates of formula [32H]ClO4 crystallised from
a MeNO2/Et2O solution of 3 and Fe[ClO4]2$6H2O.
Scheme 2. Conversion of 1 to 2, and the further derivatisation of 2. Reagents and conditions: (i) 48% HBr, reflux, 4 h, 73%; (ii) Na[cytosinate], KI (cat.), DMF,
50 ꢀC, 5 h, 69%; (iii) Na[9-chloroguaninate], KI (cat.), DMF, 50 ꢀC, 5 h, 95%; (iv) 0.1 M HCl, reflux, 4 h, 67%; (v) BuLi, Et2O, ꢁ78 ꢀC then 1,2-dibromo-
ethane, rt, 24 h, 50%.

J. Elha¨ık et al. / Tetrahedron 63 (2007) 291–298
293
Figure 1. Partial packing diagram of [32H]ClO4, showing the protonated dimeric supramolecules in the lattice and their association with the perchlorate anions.
Thermal ellipsoids are at the 50% probability level, except for H atoms, which have arbitrary radii. For clarity, only one orientation of the (disordered) ClO4ꢁ
anion is shown. Symmetry codes: (i) 1/2ꢁx, 3/2ꢁy, 1ꢁz; (ii) ꢁx, y, 1/2ꢁz.
Presumably, the proton in this compound originated from the
weakly acidic MeNO2 solvent. The structure contains a cen-
trosymmetric dimeric supramolecule of 3, with their two
cytosine moieties linked by two N–H/O hydrogen bonds
(Fig. 1). The two N3 atoms of these groups are spanned by
a shared proton [H(20) in Fig. 1], which lies on the crystal-
lographic inversion centre in the structural refinement. This
supramolecular motif has been seen before in other
[(Cyt)2H]+ (cyt¼cytosine or a 1-alkylcytosine derivative)
co-crystals.18–20 In contrast to at least one previous exam-
ple,19 the [(Cyt)2H]+ moiety in [32H]+ is perfectly coplanar.
The dimeric supramolecules are linked into 1-D chains run-
ning parallel to the crystallographic [101] vector, through
hydrogen bonding to the perchlorate anions (Fig. 1). The
cytosinyl and 2,6-di(pyrazol-1-yl)pyrid-4-yl groups in each
molecule of 3 are mutually close to perpendicular, with a
dihedral angle of 76.67(5)ꢀ between their least squares
planes. The 1-D chains in the lattice therefore form a set
of approximately square cavities, with two adjacent sides
formed from [(Cyt)2H]+ protonated dimers, and the other
two by 2,6-di(pyrazol-1-yl)pyrid-4-yl groups. These cavities
˚
have approximate dimensions 3.7ꢂ3.9ꢂ8.3 A, and are filled
by two more 2,6-di(pyrazol-1-yl)pyrid-4-yl substitutents
held in place by p–p interactions with the [(Cyt)2H]+ groups
(Fig. 2). Since the space-filling heterocyclic groups are
attached to different neighbouring hydrogen-bonded chains,
this motif does not result in a self-penetrating network. The
pyrazole and pyridine rings in 3 adopt the near-coplanar,
transoid conformation that is usually observed in crystals
of 2,6-di(pyrazol-1-yl)pyridine derivatives (Fig. 1).1,21
Lithiation of 2 at ꢁ78 ꢀC in diethyl ether, followed by
stirring at room temperature for 24 h, yielded the back-
to-back ligand 6 in variable yields of up to 50% (Scheme
2). Addition of the oxidising agent 1,2-dibromoethane to
the reaction appeared to improve its reliability, without sig-
nificantly increasing the yield obtained. Related syntheses
of 1,2-bis(pyrid-4-yl)ethane by the oxidative lithiation of
Figure 2. Partial packing diagram of [32H]ClO4, showing the approximately square cavities formed by the hydrogen-bonded chains of molecules. All atoms
have arbitrary radii, while the two 2,6-di(pyrazol-1-yl)pyrid-4-yl substitutents filling one of the cavities are also shown, and de-emphasised. The orientation of
the unit cell in the view is shown in inset (not to scale).

294
J. Elha¨ık et al. / Tetrahedron 63 (2007) 291–298
conformation.1,21 The two unique molecules of 6 in the crys-
tal associate through p–p interactions into centrosymmetric
BAAB tetrads zigzagging along the unit cell b direction
(via the symmetry transformation 1ꢁx, ꢁy, 1ꢁz), while
the remaining 2,6-di(pyrazol-1-yl)pyrid-4-yl fragment in
molecule A also forms a centrosymmetric p–p interaction
with its own symmetry equivalent related by ꢁx, ꢁy, 2ꢁz.
3. Conclusions
This work has demonstrated that 2 can be a powerful precur-
sor to a range of 2,6-di(pyrazol-1-yl)pyridines substituted at
the pyridyl group, which could allow spin-transition centres
to be linked together, to be combined with other functional-
ity in the same molecule, or to be tethered onto surfaces.
While we have studied the iron(II) complexes of 15,6 and
223 in some detail, unfortunately 3, 5 and 6 are of limited
value as ligands in themselves since their homoleptic iron(II)
complexes are intractably insoluble and non-crystalline. The
amorphous nature of the complexes is also reflected in the
spin transitions they undergo upon cooling,1 which are
extremely gradual and proceed to <50% completion.23
None-the-less, there is very wide scope for conversion of 2
into a wide range of other tridentate ligands bearing pendant
functionality, whose iron complexes may behave more
promisingly. The solubility and crystallinity problems
referred to above might also be alleviated by preparing
analogues of 3, 5 and 6 bearing alkyl substituents at the
4-positions of the pyrazole rings in steps (iii) or (ix) of
Scheme 1 (cf. Eq. 1, R2 s H). We are actively investigating
all these possibilities.
Figure 3. Structure of the whole unique molecule in the crystal of 6$2/
3CH2Cl2. Thermal ellipsoids are at the 50% probability level, except for
H atoms, which have arbitrary radii. The second half molecule in the struc-
ture is similar, but has coplanar 2,6-di(pyrazol-1-yl)pyrid-4-yl substitutents
rather than near-perpendicular ones as here.
4-picoline have been reported previously.22 While the iden-
tity of 6 was confirmed by NMR and mass spectrometry,
microanalysis consistently implied that the recrystallised
product also contained lattice solvent. That was confirmed
by a structure determination from a crystal of stoichiometry
6$2/3CH2Cl2 (Fig. 3). These crystals contain one whole mol-
ecule (molecule ‘A’) and one centrosymmetric half molecule
(molecule ‘B’) of 6, which differ in the relative dispositions
of their two 2,6-di(pyrazol-1-yl)pyrid-4-yl fragments
(Scheme 3). In the whole molecule these moieties are almost
perpendicular to each other, with a dihedral angle between
their least squares planes of 83.12(2)ꢀ (Fig. 3), while in the
half molecule (not shown in Fig. 3) they are strictly coplanar
by symmetry. As for [32H]ClO4, the three heterocycles in
each of the unique 2,6-di(pyrazol-1-yl)pyrid-4-yl moieties
in 6$2/3CH2Cl2 all exhibit the usual near-coplanar, transoid
4. Experimental
4.1. General
2,6-Dibromoisonicotinic acid ethyl ester13 and 2,6-dichloro-
isonicotinic acid14 were prepared by the literature proce-
dures. All other reactions were carried out in air using
non-pre-dried AR-grade solvents unless otherwise stated,
and all reagents were purchased commercially and used as
supplied. Elemental microanalyses were performed by the
University of Leeds School of Chemistry microanalytical
service. NMR spectra were obtained on a Bruker ARX250
spectrometer, operating at 250.1 MHz (1H) and 62.9 MHz
(13C). Electron impact mass spectra were obtained on
a VG Autospec instrument, while electrospray mass spectra
were run from MeCN solution using a Micromass LCT TOF
spectrometer. Melting points are uncorrected. [CAUTION:
While we have experienced no difficulty in handling the
iron complexes described below, metal-organic perchlorates
are potentially explosive and should be handled with due
care in small quantities.]
4.2. Syntheses
4.2.1. 2,6-Di(pyrazol-1-yl)isonicotinic acid ethyl ester. A
solution of pyrazole (5.80 g, 86 mmol) in 2-methoxyethyl-
ether (300 mL) was added to solid KH (3.55 g, 89 mmol)
under N2, and the resultant suspension stirred at room tem-
perature for 1 h. 2,6-Dibromoisonicotinic acid ethyl ester
Scheme 3. Different conformations of molecule A and half molecule B in
the crystal structure of 6$2/3CH2Cl2. Molecule A is shown in Figure 3.

J. Elha¨ık et al. / Tetrahedron 63 (2007) 291–298
295
(8.50 g, 28 mmol) was then added in one portion, and the
mixture stirred at 130 ꢀC for 5 days. After cooling, a large
excess of cold water was added to the mixture, yielding
a white solid, which was collected and washed with water.
Yield 3.1 g, 40%. Found C, 59.4; H, 4.7; N, 24.6%. Calcd
for C14H13N5O2: C, 59.4; H, 4.6; N, 24.7%. Mp 147–
149 ꢀC (lit.13 143–144 ꢀC). EI mass spectrum: m/z 284
pyran CH2 6), 1.42–1.93 (m, 6H, pyran CH2 3–5). 13C NMR
(CDCl3): d 154.4 (2C, Py C2/6), 151.0 (1C, Py C4), 121.1
(2C, Py C3/5), 98.9 (1C, pyran CH), 66.5, 62.9 (both 1C,
pyCH2O and pyran CH2 6), 20.1, 25.6 and 30.7 (all 1C, pyran
CH2 3–5).
4.2.5. 2,6-Di(pyrazol-1-yl)-4-(tetrahydropyran-2-yl-
oxymethyl)pyridine. A solution of pyrazole (3.19 g,
47 mmol) in 2-methoxyethylether (170 mL) was added to
NaH (1.12 g, 47 mmol) under N2, and the resultant suspen-
sion stirred at room temperature for 1 h. 2,6-Dichloro-4-
tetrahydropyranyloxypyridine (4.20 g, 16 mmol) was then
added to the reaction in one portion, and the mixture stirred
at 130 ꢀC for 5 days. After cooling, a large excess of cold wa-
ter was added to the mixture, yielding a brown oil that was
collected by decantation, washed with water and dried in va-
cuo. Any 2-methoxyethylether remaining in the product by
NMR was removed with additional aqueous washes. Yield
4.2 g, 81%. Found C, 63.1; H, 5.8; N, 21.8%. Calcd for
C17H19N5O2: C, 62.8; H, 5.9; N, 21.5%. EI mass spectrum:
m/z 326 [M+H]+. 1H NMR (CDCl3): d 8.48 (d, 3.3 Hz, 2H,
Pz H5), 7.79 (s, 2H, Py H3/5), 7.68 (d, 1.7 Hz, 2H, Pz H3),
6.40 (dd, 1.7 and 3.3 Hz, 2H, Pz H4), 4.55 and 4.83 (both
d, 14.0 Hz, both 1H, pyCH2O), 4.70 (t, 3.4 Hz, 1H, pyran
CH), 3.48 and 3.82 (both m, both 1H, pyran CH2 6), 1.45–
1.96 (m, 6H, pyran CH2 3–5). 13C NMR (CDCl3): d 154.6
(2C, Py C2/6), 150.6 (1C, Py C4), 142.6 (2C, Pz C5), 127.5
(2C, Pz C3), 107.9 and 108.3 (both 2C, Py C3/5 and Pz
C4), 98.8 (1C, pyran CH), 62.4 and 67.6 (both 1C, pyCH2O
and pyran CH2 6), 19.5, 25.8 and 30.8 (all 1C, pyran CH2 3–5).
1
[M+H]+. H NMR (CDCl3): d 8.50 (d, 2.2 Hz, 2H, Pz H5),
8.32 (s, 2H, Py H3/5), 7.74 (d, 1.7 Hz, 2H, Pz H3), 6.46
(dd, 1.7 and 2.2 Hz, 2H, Pz H4), 4.40 (q, 7.1 Hz, 2H, CH2),
1.39 (t, 7.1 Hz, 3H, CH3). 13C NMR (CDCl3): d 164.0 (1C,
CO2), 150.7 (2C, Py C2/6), 143.6 (1C, Py C4), 142.7 (2C, Pz
C3), 127.1 (2C, Pz C5), 109.1 and 108.3 (both 2C, Py C3/5
and Pz C4), 62.1 (1C, CH2), 14.2 (1C, CH3).
4.2.2. 2,6-Dichloroisonicotinic acid methyl ester. A solu-
tion of 2,6-dichloroisonicotinic acid (16.5 g, 86 mmol) in
MeOH (135 mL) was treated with concd H2SO4 (1.2 mL)
and refluxed for 15 h. After the mixture was cooled to
room temperature, a brown solid precipitated and was col-
lected. Yield 14.9 g, 84%. Mp 71–73 ꢀC (lit.24 80–81 ꢀC).
Found C, 40.8; H, 2.6; N, 6.7%. Calcd for C7H5NO2Cl2:
C, 40.8; H, 2.5; N, 6.8%. EI mass spectrum: m/z 206 [M]+.
1H NMR (CDCl3): d 7.77 (s, 2H, Py H3/5), 3.95 (s, 3H,
CH3). 13C NMR (CDCl3): d 163.2 (1C, CO2Me), 151.4
(2C, Py C2/6), 142.4 (1C, Py C4), 122.6 (2C, Py C3/5), 53.3
(1C, CH3).
4.2.3. 2,6-Dichloro-4-hydroxymethylpyridine. NaBH4
(5.70 g, 0.15 mol) was added in portions to a solution of
2,6-dichloro-4-ethoxycarbonylpyridine (6.25 g, 30 mmol)
in dry EtOH (570 mL) at 0 ꢀC. The mixture was then heated
to reflux for 3 h under N2 atmosphere. The resultant solution
was then cooled, and evaporated to dryness. The residue was
taken up in H2O (430 mL), and excess NaBH4 was destroyed
by the addition of 1 M HCl (110 mL). The mixture was neu-
tralised with Na2CO3 and extracted with CH2Cl2. The
organic layer was dried over MgSO4, and solvent was
removed, affording a pale brown solid. Yield 4.5 g, 85%.
Mp 129–131 ꢀC (lit.25 133 ꢀC). Found C, 40.5; H, 3.1; N,
7.9%. Calcd for C6H5NOCl2: C, 40.5; H, 2.8; N, 7.9%. EI
mass spectrum: m/z 178 [M+H]+. 1H NMR (CDCl3):
d 7.21 (s, 2H, Py H3/5), 4.68 (s, 2H, CH2). 13C NMR
(CDCl3): d 160.0 (2C, Py H2/6), 149.4 (1C, Py C4), 120.9
(2C, Py C3/5), 61.1 (1C, CH2).
4.2.6. 2,6-Di(pyrazol-1-yl)-4-(hydroxymethyl)pyridine
(1). Method A: Solid NaBH4 (1.75 g, 46 mmol) was added
in portions to a solution of 2,6-di(pyrazol-1-yl)isonicotinic
acid ethyl ester (2.63 g, 9.3 mmol) in anhydrous ethanol
(190 mL) at 0 ꢀC. After the addition was complete, the mix-
ture was heated to reflux under N2 for 3 h. The solution was
cooled to room temperature, and evaporated to dryness. Wa-
ter (140 mL) was added to the residue, and excess NaBH4
was destroyed by the addition of 1 M HCl (35 mL). The
mixture was then neutralised with Na2CO3, and extracted
with CH2Cl2. The organic layer was dried over MgSO4
and evaporated to dryness, affording a bright brown solid
product. Yield 1.74 g, 78%. Method B: A solution of 2,6-
bis(pyrazol-1-yl)-4-tetrahydropyranyloxypyridine (3.85 g,
12 mmol) and PPTS (0.29 g, 1.2 mmol) in EtOH (100 mL)
was stirred at 55 ꢀC for 3 h. The solvent was then removed
in vacuo, and the residue redissolved in Et2O (80 mL). The
solution was extracted with water, separated, dried over
MgSO4 and concentrated in vacuo to yield a white solid
that was used without further purification. Yield 2.3 g,
80%. Found C, 60.0; H, 4.6; N, 29.0%. Calcd for
C12H11N5O: C, 59.7; H, 4.6; N, 29.0%. Mp 128–130 ꢀC.
4.2.4. 2,6-Dichloro-4-(tetrahydropyran-2-yloxymethyl)-
pyridine. A solution of pyridinium para-toluenesulfonate
(1.05 g, 4.4 mmol) in CH2Cl2 (30 mL) was added dropwise
with stirring, at room temperature, to a solution of 2,6-
dichloro-4-hydroxymethylpyridine (3.75 g, 21 mmol) and
3,4-dihydropyran (3.69 g, 44 mmol) in CH2Cl2 (600 mL).
The mixture was stirred at room temperature overnight.
The solvent was then removed in vacuo, and Et2O was added
(300 mL). The solution was extracted with water, separated,
dried over MgSO4 and concentrated in vacuo. Silica flash
column chromatography in 85:15 pentane/Et2O afforded
the pure product as a white solid. Yield 4.4 g, 80%. Mp
59–61 ꢀC. Found C, 50.6; H, 5.1; N, 5.2%. Calcd for
C11H13NO2Cl2: C, 50.4; H, 5.0; N, 5.3%. EI mass spectrum:
m/z 262 [M+H]+. 1H NMR (CDCl3): d 7.18 (s, 2H, Py H3/5),
4.41 and 4.70 (both d, 15.1 Hz, both 1H, pyCH2O), 4.64 (t,
3.2 Hz, 1H, pyran CH), 3.49 and 3.76 (both m, both 1H,
1
EI mass spectrum: m/z 242 [M+H]+. H NMR (CDCl3):
d 8.50 (d, 2.6 Hz, 2H, Pz H5), 7.82 (s, 2H, Py H3/5), 7.72
(d, 1.7 Hz, 2H, Pz H3), 6.45 (dd, 1.7 and 2.6 Hz, 2H, Pz
H4), 4.81 (s, 2H, CH2). 13C NMR (CDCl3): d 157.0 (2C,
Py C2/6), 150.0 (1C, Py C4), 142.3 (2C, Pz C3), 127.2 (2C,
Pz C5), 107.9 and 106.7 (both 2C, Py C3/5 and Pz C4),
63.3 (1C, CH2).
4.2.7. 2,6-Di(pyrazol-1-yl)-4-(bromomethyl)pyridine (2).
Compound 1 (2.75 g, 11 mmol) was refluxed in 48% HBr

296
J. Elha¨ık et al. / Tetrahedron 63 (2007) 291–298
(70 mL) for 4 h. The mixture was then cooled to room tem-
perature, poured onto 40 g of ice, made basic with a saturated
solution of Na2CO3, and extracted with CH2Cl2 (4ꢂ50 mL).
The combined organic extracts were dried over MgSO4 and
the solvent was removed under vacuum, yielding a light
brown solid. Pure product was isolated by flash silica column
chromatography in 4:1 pentane/ether. Yield 2.54 g, 73%.
Found C, 47.4; H, 3.5; N, 22.6%. Calcd for C12H10N5Br:
C, 47.2; H, 3.6; N, 22.9%. Mp 130–132 ꢀC. EI mass
4.0; N, 35.2%. Calcd for C17H14N10O$1.5H2O: C, 50.9; H,
4.3; N, 34.9%. Mp 217–219 ꢀC. EI mass spectrum: m/z
749 [2M+H]+, 393 [M+H2O+H]+, 374 [M]+. 1H NMR
({CD3}2SO): d 9.03 (d, 2.5 Hz, 2H, Pz H5), 8.47 (s, 1H,
Gu H8), 7.96 (d, 1.7 Hz, 2H, Pz H3), 7.71 (s, 2H, Py H3/5),
7.09 (br s, 2H, NH2), 6.74 (dd, 1.7 and 2.5 Hz, 2H, Pz H4),
5.66 (s, 2H, CH2). 13C NMR ({CD3}2SO): d 160.4 (1C,
Gu C6), 154.5 (1C, Gu C4), 152.8 (2C, Py C2/6), 150.3
(1C, Gu C2), 150.1 (1C, Py C4), 143.7 (1C, Gu C8), 143.2
(2C, Pz C3), 128.6 (2C, Pz C5), 123.5 (1C, Gu C5), 109.0
and 107.0 (both 2C, Py C3/5 and Pz C4), 45.8 (1C, CH2).
1
spectrum: m/z 306 [M(81Br)+H]+, 304 [M(79Br)+H]+. H
NMR (CDCl3): d 8.51 (d, 2.4 Hz, 2H, Pz H5), 7.86 (s, 2H,
Py H3/5), 7.74 (d, 1.6 Hz, 2H, Pz H3), 6.46 (dd, 1.6 and
2.5 Hz, 2H, Pz H4), 4.45 (s, 2H, CH2). 13C NMR (CDCl3):
d 152.1 (2C, Py C2/6), 150.5 (1C, Py C4), 142.6 (2C, Pz
C3), 127.1 (2C, Pz C5), 109.3, 108.1 (both 2C, Py C3/5 and
Pz C4), 30.0 (1C, CH2).
4.2.11. 1,2-Bis[2,6-di(pyrazol-1-yl)pyrid-4-yl]ethane
dichloromethane solvate (6). Compound 2 (0.61 g,
2.0 mmol) was suspended in dry diethyl ether (10 mL) under
N2 at ꢁ78 ꢀC. Butyllithium (1.25 mL of a 1.6 M solution in
hexanes, 2.0 mmol) was then added, and the reaction stirred
at ꢁ78 ꢀC for 1 h. 1,2-Dibromoethane (0.42 g, 2.0 mmol)
was added, and the reaction warmed to room temperature
and stirred for 2 h. The mixture was then quenched with wa-
ter (50 mL), the organic layer was separated and the aqueous
layer extracted with dichloromethane (3ꢂ30 mL). The com-
bined organic extracts were dried over MgSO4 and the sol-
vent removed under reduced pressure to yield a pale
brown solid. Recrystallisation from CH2Cl2/pentane yielded
a white polycrystalline solid, which was isolated by filtration
and dried in vacuo. The presence of CH2Cl2 in the product,
suggested by microanalysis, was confirmed crystallographi-
cally (see below). Yield 0.25 g, 50%. Found C, 59.6; H, 4.2;
N, 28.8%. Calcd for C24H20N10$½CH2Cl2: C, 60.0; H, 4.3;
N, 28.5%. Mp 183–185 ꢀC. EI mass spectrum: m/z 449
4.2.8. 1-([2,6-Di(pyrazol-1-yl)pyridin-4-yl]methyl)cyto-
sine (3). A solution of cytosine (0.11 g, 1.0 mmol) and a cat-
alytic amount of KI (0.030 g) in dry DMF (10 mL) was
added to NaH (0.024 g, 1.0 mmol) at 0 ꢀC under N2. The
mixture was stirred for 30 min at room temperature, and a so-
lution of 2 (0.31 g, 1.0 mmol) in dry DMF (10 mL) was then
added and the mixture stirred for 5 h at 50 ꢀC under N2. After
cooling to room temperature, addition of water (100 mL) af-
forded a white precipitate. This was collected, dried in vacuo
over P2O5 and analysed without further purification. Yield
0.23 g, 69%. Found C, 57.3; H, 4.1; N, 33.1%. Calcd for
C16H14N8O: C, 57.5; H, 4.2; N, 33.5%. Mp 308 ꢀC. EI
1
mass spectrum: m/z 669 [2M+H]+, 335 [M+H]+. H NMR
({CD3}2SO): d 8.92 (d, 2.6 Hz, 2H, Pz H5), 7.85 (d,
1.7 Hz, 2H, Pz H3), 7.81 (d, 7.2 Hz, Ct H6), 7.65 (s, 2H,
Py H3/5), 7.18 (br s, 2H, NH2), 6.62 (dd, 1.7 and 2.6 Hz,
1
[M+H]+. H NMR (CDCl3): d 8.55 (d, 2.7 Hz, 4H, Pz H5),
7.78 (s, 4H, Py H3/5), 7.75 (d, 1.7 Hz, 4H, Pz H3), 6.48
(dd, 1.8 and 2.7 Hz, 4H, Pz H4), 3.17 (s, 4H, CH2). 13C
NMR (CDCl3): d 154.8 (4C, Py C2/6), 149.3 (2C, Py C4),
141.3 (4C, Pz C3), 126.1 (4C, Pz C5), 108.2, 106.9 (both
4C, Py C3/5 and Pz C4), 35.2 (2C, CH2).
2H, Pz H4), 5.79 (d, 7.2 Hz, Ct H5), 5.06 (s, 2H, CH2). 13
C
NMR ({CD3}2SO): d 166.6 (1C, Ct C4), 162.7 (1C, Ct
C2), 154.5 (2C, Py C2/6), 150.1 (1C, Py C4), 146.6 (1C, Ct
C6), 143.0 (2C, Pz C3), 128.5 (2C, Pz C5), 108.9, 107.3
(both 2C, Py C3/5 and Pz C4), 94.5 (1C, Ct C5), 51.4 (1C,
CH2).
4.2.12. [Fe(3)2][ClO4]2. A mixture of Fe[ClO4]2$6H2O
(0.11 g, 0.30 mmol) and 3 (0.20 g, 0.60 mmol) in CH3NO2
(30 mL) was refluxed for 30 min. The resultant orange solu-
tion was cooled to room temperature, then concentrated in
vacuo to ca. 10 mL. Storage at room temperature resulted
in a mustard yellow powder, that was very sparingly soluble
and was analysed without further purification. Yield 0.14 g,
52%. Found C, 41.3; H, 3.3; N, 24.2%. Calcd for
C32H28N16FeCl2O10: C, 41.6; H, 3.1; N, 24.3%. ES mass
spectrum: m/z 823 [56Fe(3)235ClO4]+, 669 [2(3)+H]+, 362
[56Fe(3)2]2+, 335 [3+H]+. IR (Nujol) 3407br, 3325br,
3198br, 1730s, 1688m, 1652s, 1621s, 1545s, 1524s,
1337m, 1356m, 1277m, 1261m, 1208s, 1174m, 1095vs,
1044m, 972m, 955s, 917m, 849m, 801m, 784s, 773s,
4.2.9. 6-Chloro-9-[2,6-di(pyrazol-1-yl)pyridin-4-yl]-
methyl-9H-purin-2-ylamine (4). Compound 4 was pre-
pared by the same method as for 3, using 2-amino-6-
chloropurine (0.17 g, 1.0 mmol). The product precipitated
as a white solid after the aqueous quench. Yield 0.37 g,
95%. Found C, 51.6; H, 3.7; N, 35.2%. Calcd for
C17H13ClN10: C, 52.0; H, 3.3; N, 35.6%. Mp 231–233 ꢀC.
EI mass spectrum: m/z 393 [M(35Cl)+H]+. 1H NMR
({CD3}2SO): d 9.07 (d, 2.5 Hz, 2H, Pz H5), 8.50 (s, 1H,
Pu H8), 7.99 (d, 1.7 Hz, 2H, Pz H3), 7.75 (s, 2H, Py H3/5),
7.13 (br s, 2H, NH2), 6.62 (dd, 1.7 and 2.5 Hz, 2H, Pz H4),
5.70 (s, 2H, CH2). 13C NMR ({CD3}2SO): d 160.4 (1C, Pu
C6), 154.5 (1C, Pu C4), 152.9 (2C, Py C2/6), 150.3 (1C, Py
C4), 150.1 (1C, Pu C2), 143.7 (1C, Pu C8), 143.2 (2C, Pz
C3), 128.6 (2C, Pz C5), 123.5 (1C, Pu C5), 108.9 and 107.0
(both 2C, Py C3/5 and Pz C4), 45.8 (1C, CH2).
754m, 624s cmꢁ1
.
4.2.13. [Fe(5)2][ClO4]2$3H2O. This compound was pre-
pared by the method described in 4.2.12, using 5 (0.24 g,
0.60 mmol). The product was a sparingly soluble yellow-
brown powder. Yield 0.14 g, 61%. Found C, 38.6; H, 2.9;
N, 26.2%. Calcd for C34H28N20FeCl2O10$3H2O: C, 38.6;
H, 3.2; N, 26.5%. ES mass spectrum: m/z 749 [2(5)+H]+,
547 [56Fe(5)(H2O)35ClO4]+, 529 [56Fe(5)35ClO4]+, 393
[5+H2O+H]+, 374 [5+H]+. IR (Nujol) 3470br, 3330br,
3116br, 1693m, 1613s, 1563s, 1521s, 1507m, 1404m,
4.2.10. 9-[2,6-Di(pyrazol-1-yl)pyridin-4-yl]methylgua-
nine sesquihydrate (5). Compound 4 (0.70 g, 1.8 mmol)
was refluxed in 0.1 M HCl (150 mL) for 4 h. After cooling
and neutralisation with aq KOH, the product precipitated
as a yellow solid, which was filtered and analysed without
further purification. Yield 0.47 g, 67%. Found C, 51.2; H,

J. Elha¨ık et al. / Tetrahedron 63 (2007) 291–298
297
1310m, 1282m, 1209m, 1173m, 1095vs, 1048m, 1000m,
971s, 911m, 858w, 785m, 760s, 730m, 643m, 623s,
4.3.1. Single crystal X-ray structure determination of
[32H]ClO4. The asymmetric unit contains half of a hydro-
gen-bonded dimer of molecules lying across the inversion
centre [1/4, 3/4, 1/2]. The two halves of this dimer share
a proton (H20), which lies on this inversion centre, yielding
a protonated dimer cation. The asymmetric unit also con-
tains half of a ClOꢁ4 anion, which is disordered across the
crystallographic C2-axis [0, y, 1/4]; the Cl atom Cl(26) was
modelled as lying on this axis. Two disorder orientations
of the anion O atoms were modelled: O(27A) and O(28A)
(occupancy 0.5) form a C2-symmetric partial anion with
their symmetry equivalents and O(27B)–O(30B) (occupancy
0.25) form a complete partial anion environment. Combin-
ing partial anions ‘A’, ‘B’ and the symmetry equivalent of
‘B’ yields a complete anion occupancy. All Cl–O bonds
600m cmꢁ1
.
4.2.14. [Fe(6)][ClO4]2$H2O. A solution of 6 (0.17 g,
0.39 mmol) in acetone (60 mL) was added dropwise to a so-
lution of Fe[ClO4]2$6H2O (0.14 g, 0.39 mmol) in acetone
(20 mL), yielding a yellow precipitate. The reaction was
stirred for a further 90 min after addition was complete.
The pale yellow solid was isolated by filtration, washed at
the filter with acetone, dichloromethane and methanol and
dried in vacuo. Yield 0.17 g, 59%. Found C, 39.7; H, 3.0;
N, 19.2%. Calcd for C24H20Cl2FeN10O8$H2O: C, 40.0; H,
3.1; N, 19.4%. IR (Nujol) 3403br, 3116m, 1632s, 1581m,
1524m, 1406m, 1339m, 1287m, 1216w, 1178w, 1095vs,
1058m, 976s, 892w, 870w, 781m, 623m cmꢁ1. The com-
pound was insufficiently soluble to give a detectable ES
mass spectrum.
˚
were restrained to 1.44(2) A, and O/O distances within a
˚
given disorder orientation to 2.35(1) A. All non-H atoms
except for anion disorder orientation ‘B’ were refined aniso-
tropically. All H atoms were first located in the difference
map, thus confirming the assignments of the N2 and C5
atoms of each pyrazole ring in the structure. For the final re-
finements, however, the C-bound H atoms were all placed in
calculated positions and refined using a riding model. N-
4.3. Single crystal X-ray structure determinations
Crystals of [32H]ClO4 were obtained serendipitously, from
an attempted recrystallisation of [Fe(3)2][ClO4]2 from
CH3NO2 over a period of several days. Diffraction data for
this compound were collected using a Bruker X8 Apex dif-
fractometer, using graphite-monochromated Mo Ka radia-
tion generated by a rotating anode. Single crystals of 6$2/
3CH2Cl2 were obtained by slow diffusion of pentane vapour
into a CH2Cl2 solution of the compound, and were analysed
using a Nonius KappaCCD area detector diffractometer,
with graphite-monochromated Mo Ka radiation from
a sealed tube source. Experimental details from the structure
determinations are given in Table 1. The structures were
solved by direct methods (SHELXS 9726) and refined by
full matrix least squares on F2 (SHELXL 9727). All crystal-
lographic figures were prepared using XSEED,28 which
incorporates POVRAY.29 The CCDC reference numbers
for the structures are 617656 ([32H]ClO4) and 617655
(6$2/3CH2Cl2).
bound H(20), H(25A) and H(25B) were refined freely with
2
˚
a common Uiso thermal parameter of 0.092(9) A . Since
H(20) lies on a special position, however, this may not in
fact correspond to the true location of this H atom.17
4.3.2. Single crystal X-ray structure determination of
6$2/3CH2Cl2. The asymmetric unit contains one molecule
of the compound lying on a general position; a second half
molecule with the CH2–CH2 bond spanning the inversion
centre at [1/2, 1/2, 1/2]; and a wholly occupied molecule
of CH2Cl2 occupying a general position. No disorder was de-
tected during refinement, and no restraints were applied. All
non-H atoms were refined anisotropically, while all H atoms
were located in the difference map, but placed in calculated
positions and refined using a riding model in the final least
squares cycles.
Acknowledgements
Table 1. Experimental details for the single crystal structure determinations
in this study
The authors thank the EPSRC (J.E.) and the Leverhulme
Trust (C.M.P.) for funding.
[32H]ClO4
6$2/3CH2Cl2
Formula
Mr
C32H29ClN16O6
769.16
Monoclinic
C2/c
22.066(3)
12.4986(13)
12.4529(14)
—
92.455(5)
—
3431.2(7)
4
1.489
C24.67H21.33Cl1.33N10
505.12
Triclinic
References and notes
Crystal system
Space group
ꢁ
P1
1. Halcrow, M. A. Coord. Chem. Rev. 2005, 249, 2880.
2. (a) Holland, J. M.; McAllister, J. A.; Lu, Z.; Kilner, C. A.;
Thornton-Pett, M.; Halcrow, M. A. Chem. Commun. 2001,
577; (b) Holland, J. M.; McAllister, J. A.; Kilner, C. A.;
Thornton-Pett, M.; Bridgeman, A. J.; Halcrow, M. A.
J. Chem. Soc., Dalton Trans. 2002, 548; (c) Money, V. A.;
Evans, I. R.; Halcrow, M. A.; Goeta, A. E.; Howard, J. A. K.
Chem. Commun. 2003, 158.
˚
a (A)
10.1953(2)
12.5985(3)
14.8638(4)
97.1977(10)
100.9265(11)
102.4931(9)
1802.44(7)
3
1.396
0.233
150(2)
37,949
˚
b (A)
˚
c (A)
a (deg)
b (deg)
g (deg)
3
˚
V (A )
Z
Dc (g cmꢁ3
)
m (mmꢁ1
T (K)
Measured reflections
)
0.183
3. (a) Holland, J. M.; Barrett, S. A.; Kilner, C. A.; Halcrow, M. A.
Inorg. Chem. Commun. 2002, 5, 328; (b) Elhaık, J.; Kilner,
150(2)
34,227
3388
0.051
0.058
0.182
¨
C. A.; Halcrow, M. A. CrystEngComm 2005, 7, 151.
Independent reflections
Rint
R(F)
8237
0.095
0.048
0.138
4. (a) Elhaık, J.; Money, V. A.; Barrett, S. A.; Kilner, C. A.;
¨
Radosavljevic Evans, I.; Halcrow, M. A. Dalton Trans. 2003,
wR(F2)
2053; (b) Money, V. A.; Elhaık, J.; Evans, I. R.; Halcrow,
¨

298
J. Elha¨ık et al. / Tetrahedron 63 (2007) 291–298
M. A.; Howard, J. A. K. Dalton Trans. 2004, 65; (c) Money, 16. Constable, E. C.; Fallahpour, R.-A. J. Chem. Soc., Dalton
V. A.; Evans, I. R.; Elhaık, J.; Halcrow, M. A.; Howard,
¨
J. A. K. Acta Crystallogr. 2004, B60, 41.
Trans. 1996, 2389.
17. Mullah, K. B.; Bentrude, W. G. J. Org. Chem. 1991, 56, 7218.
18. (a) Fujinami, F.; Ogawa, K.; Arakawa, K.; Shirotake, S.; Fujii,
S.; Tomita, K. Acta Crystallogr. 1979, B35, 968; (b) Vijay-
Kumar, S.; Sakore, T. D.; Sobell, H. M. Nucleic Acids Res.
1984, 12, 3649; (c) Gdaniec, M.; Brycki, B.; Szafran, M.
J. Chem. Soc., Perkin Trans. 2 1988, 1775; (d) Schimanski,
A.; Freisinger, E.; Erxleben, A.; Lippert, B. Inorg. Chim.
Acta 1998, 283, 223; (e) Salam, M. A.; Aoki, K. Inorg.
Chim. Acta 2000, 311, 15.
5. Money, V. A.; Elhaık, J.; Halcrow, M. A.; Howard, J. A. K.
¨
Dalton Trans. 2004, 1516.
6. Carbonera, C.; Costa, J. S.; Money, V. A.; Elhaık, J.; Howard,
ꢀ
¨
J. A. K.; Halcrow, M. A.; Letard, J.-F. Dalton Trans. 2006,
3058.
7. Spin Crossover in Transition Metal Compounds I–III; Gutlich,
€
P., Goodwin, H. A., Eds.; Topics in Current Chemistry; 2004;
Vols. 233–235.
´
€
8. (a) Rodruıgez-Ubis, J. C.; Sedano, R.; Barrosso, G.; Juanes, O.;
19. Kruger, T.; Bruhn, C.; Steinborn, D. Org. Biomol. Chem. 2004,
2, 2513.
Brunet, E. Helv. Chim. Acta 1997, 80, 86 and 621 (correction);
(b) Yuan, J.; Wang, G.; Majima, K.; Matsumoto, K. Anal.
Chem. 2001, 73, 1869; (c) Brunet, E.; Juanes, O.; Sedano, R.;
20. Armentano, D.; De Munno, G.; Di Donna, L.; Sindona, G.;
Giorgi, G.; Salvini, L. J. Am. Soc. Mass Spectrosc. 2004, 15, 268.
21. (a) Bessel, C. A.; See, R. F.; Jameson, D. L.; Churchill, M. R.;
Takeuchi, K. J. J. Chem. Soc., Dalton Trans. 1992, 3223; (b)
Halcrow, M. A.; Kilner, C. A.; Thornton-Pett, M. Acta
Crystallogr. 2000, C56, 213; (c) Calderazzo, F.; Englert, U.;
Hu, C.; Marchetti, F.; Pampaloni, G.; Passarelli, V.; Romano,
A.; Santi, R. Inorg. Chim. Acta 2003, 344, 197; (d) Brien,
K. A.; Garner, C. M.; Klausmeyer, K. K.; Feazell, R. P.
J. Chem. Crystallogr. 2005, 35, 875.
´
Rodruıgez-Ubis, J. C. Photochem. Photobiol. Sci. 2002, 1,
613; (d) Yuan, J.; Tan, M.; Wang, G. J. Lumin. 2004, 106, 91.
9. (a) Sueda, S.; Yuan, J.; Matsumoto, K. Bioconjugate Chem.
2002, 13, 200; (b) Ye, Z.; Tan, M.; Wang, G.; Yuan, J. Anal.
Chem. 2004, 76, 513; (c) Ye, Z.; Tan, M.; Wang, G.; Yuan, J.
ꢀ
´
Talanta 2005, 65, 206; (d) Gonzalez-Martınez, M. A.;
Penalva, J.; Rodrıguez-Urbis, J. C.; Brunet, E.; Maquieira,
´
A.; Puchades, R. Anal. Bioanal. Chem. 2006, 384, 1540.
10. Jameson, D. L.; Goldsby, K. A. J. Org. Chem. 1990, 55, 4992.
11. Sun, X.; Yu, Z.; Wu, S.; Xiao, W.-J. Organometallics 2005,
24, 2959.
22. (a) Ito, Y.; Kobayashi, K.; Saegusa, T. J. Organomet. Chem.
1986, 303, 301; (b) Jahangir; Wang, J.; Smith, R. W.;
MacLean, D. B.; Holland, H. L. Can. J. Chem. 1990, 68, 587.
€
12. Vermonden, T.; Branowska, D.; Marcelis, A. T. M.; Sudholter,
23. Elhaık, J.; Pask, C. M.; Halcrow, M. A. Unpublished data.
¨
E. J. R. Tetrahedron 2003, 59, 5039.
13. Fallahpour, R. A. Synthesis 2000, 1138.
14. Adamczyk, M.; Akireddy, S. R.; Reddy, R. E. Tetrahedron
2002, 58, 6951.
24. Mello, J. V.; Finney, N. S. J. Am. Chem. Soc. 2005, 127, 10124.
25. (a) Wibaut, J. P.; Overhoff, J. Recl. Trav. Chim. Pays-Bas 1933,
52, 55; (b) Rabe, P.; Spreckelsen, O.; Wilhelm, L.; Hoter, H.
J. Prakt. Chem. 1938, 151, 65.
15. (a) White, C. M.; Gonzalez, M. F.; Bardwell, D. A.; Rees, L. H.;
Jeffery, J. C.; Ward, M. D.; Armaroli, N.; Calogero, G.;
Barigelletti, F. J. Chem. Soc., Dalton Trans. 1997, 727; (b)
Armaroli, N.; Barigelletti, F.; Calogero, G.; Flamigni, L.;
White, C. M.; Ward, M. D. Chem. Commun. 1997, 2181; (c)
Ward, M. D.; White, C. M.; Barigelletti, F.; Armaroli, N.;
Calogero, G.; Flamigni, L. Coord. Chem. Rev. 1998, 171, 481.
26. Sheldrick, G. M. Acta Crystallogr. 1990, A46, 467.
27. Sheldrick, G. M. SHELXL 97, Program for the Refinement
€
of Crystal Structures; University of Gottingen: Gottingen,
€
Germany, 1997.
28. Barbour, L. J. J. Supramol. Chem. 2003, 1, 189.
29. POVRAY v. 3.5; Persistence of Vision Raytracer Pty. Ltd: Wil-
liamstown, Victoria, Australia, 2002; http://www.povray.org.