Manganese(iii)-mediated cyclodimerization of a hydrazinyl derivative generating an unprecedented 1,2,3,5,6-substituted leuco-verdazyl ring

Article abstract of DOI:10.1039/b917410a

The one-pot reaction of Mn(OAc)₃ with two equivalents of 6-(pyridine-2-ylhydrazonomethyl)phenol (Hphp) in methanol leads to a unique leuco-verdazyl-type ring. The remarkable cyclodimerization reaction is proposed to involve a number of one-electron oxidative steps mediated by Mn³⁺ ions, as well as a very uncommon 1,2-radical aryl migration and a nucleophilic attack of the solvent, i.e. methanol. The Royal Society of Chemistry 2010.

Full text of DOI:10.1039/b917410a

View Article Online / Journal Homepage / Table of Contents for this issue
PAPER
www.rsc.org/dalton | Dalton Transactions
Manganese(III)-mediated cyclodimerization of a hydrazinyl derivative
generating an unprecedented 1,2,3,5,6-substituted leuco-verdazyl ring†
Jinkui Tang,b Sanjit Nayak,a Jose´ Sa´nchez Costa,c Arturo Robertazzi,d Roberta Pievo,a Ilpo Mutikainen,e
Olivier Roubeau, f Simon J. Teat,g Patrick Gamez*a and Jan Reedijk*a
Received 24th August 2009, Accepted 5th November 2009
First published as an Advance Article on the web 4th December 2009
DOI: 10.1039/b917410a
The one-pot reaction of Mn(OAc)3 with two equivalents of 6-(pyridine-2-ylhydrazonomethyl)phenol
(Hphp) in methanol leads to a unique leuco-verdazyl-type ring. The remarkable cyclodimerization
reaction is proposed to involve a number of one-electron oxidative steps mediated by Mn3+ ions, as well
as a very uncommon 1,2-radical aryl migration and a nucleophilic attack of the solvent, i.e. methanol.
pathway is to use an organic halide in place of formaldehyde and
Introduction
Ba(OH)2 instead of KHSO4 to produce 1,3,5-substituted verdazyls
with C2-symmetry.11,12
Verdazyl rings are a very important class of compounds. Indeed,
once they are monoelectronically oxidized, the resulting molecules
In the course of investigations aimed at preparing polynuclear
represent one of the very few families of neutral, air-stable organic
radicals that can be synthesized using conventional techniques
and isolated as solvent-free pure crystalline materials.1–4 Such
stable radicals, characterized by their unpaired electron, are of
fundamental interest and have been extensively investigated for
their potentially interesting magnetic properties.5–8
metal complexes with attractive magnetic properties, the potential
tridentate ligand 6-(pyridine-2-ylhydrazonomethyl)phenol (Hphp,
Scheme 2) has been used with copper(II) chloride to generate a
1D coordination polymer.13 Amazingly, the reaction of Hphp with
manganese(III) acetate in methanol gives rise to an unexpected,
and quite remarkable cyclodimerization reaction of the organic
ligand, forming a leuco-verdazyl-type compound.
Verdazyl radicals (Scheme 1; 3) are commonly obtained by
in situ one-electron oxidation of leuco-verdazyl molecules (2),
which are synthesized by reaction of a formazan derivative (1)
with formaldehyde in the presence of potassium hydrogensulfate
(Scheme 1).9,10
Scheme 2 6-(pyridine-2-ylhydrazonomethyl)phenol (Hphp).
In the present paper, this new 1,2,3,5,6-substituted leuco-
verdazyl-type ring is described. A mechanism for its MnIII-
mediated formation is proposed, based on earlier literature data
and on DFT results, as well as on the reaction of the original ligand
Scheme 1 General synthetic route to 1,3,5-substituted verdazyl radicals.10
II
Hphp with Mn , which does not lead to any ligand transformation,
in contrast to MnIII.
This typical synthetic procedure allows the preparation of sym-
metrical 1,3,5-trisubstituted verdazyl compounds.1 An alternative
Results and discussion
aLeiden Institute of Chemistry, Gorlaeus Laboratories, Leiden Univer-
The reaction of manganese(III) acetate with Hphp in methanol pro-
duces yellow block-shaped single crystals, whose X-ray diffraction
analysis unambigously illustrates the formation of an 1,2,3,5,6-
sity, P.O. Box 9502, 2300 RA, Leiden, The Netherlands. E-mail:
p.gamez@chem.leidenuniv.nl; Fax: 31 715274671
bState Key Laboratory of Rare Earth Resource Utilization, Changchun
substituted leuco-verdazyl molecule (4, Fig. 1), resulting from a
cyclodimerization of Hphp, associated with a 1,2-migration of a
phenol group and nucleophilic attack of the solvent, i.e. methanol.
Compound 4 crystallizes in the monoclinic space group P21/n.
A representation of the solid-state structure of 4 is shown in Fig. 1.
Details for the structure solution and refinement are summarized
in Table 1. The ring bond lengths ranging from 1.295(2) to
Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun
130022, P. R. China
cDepartament de Qu´ımica Inorga`nica, Universitat de Barcelona, Diagonal,
647, 08028 Barcelona, Spain
dCNR-INFM SLACS and Dipartimento di Fisica, Universita` di Cagliari,
S.P. Monserrato-Sestu Km 0.700, I-09042 Monserrato, Italy
eDepartment of Chemistry, Laboratory of Inorganic Chemistry, University of
Helsinki, P.O. Box 55 (A.I. Virtasenaukio 1), FIN-00014, Helsinki, Finland
f
˚
Instituto de Ciencia de Materiales de Arago´n, CSIC and Universidad de
1.451(2) A are listed in Table 2. The molecular structure of 4
Zaragoza, Plaza San Francisco, s/n, 50009 Zaragoza, Spain
also exhibits strong intramolecular hydrogen bonds involving the
phenolic protons H37A and H47A (Fig. 1 and Table 2).
The formation of this unique heteroatomic six-membered ring
is obviously initiated by MnIII ions. Manganese(III) salts are indeed
gAdvanced Light Source, Lawrence Berkeley National Laboratory, Berkeley,
California 94720, USA
† CCDC reference numbers 711637 and 713054. For crystallographic data
in CIF or other electronic format see DOI: 10.1039/b917410a
This journal is
The Royal Society of Chemistry 2010
Dalton Trans., 2010, 39, 1361–1365 | 1361
©

View Article Online
Table 1 Crystal data and structure refinement details for the compounds
3-(diphenol-2-yl)-2,5-(dipyridin-2-yl)-6-methoxy-leuco-verdazyl (4) and
[Mn(php)2] (5)
Compound
4
5
Empirical formula
Formula weight/g mol-1
Crystal system
Space group
Crystal colour
C25H22N6O3
454.49
monoclinic
P21/n
yellow
C24H20MnN6O2
479.40
orthorhombic
Pbcn
yellow
Scheme 3 Two possible radicals that may be produced by MnIII-based
oxidative deprotonation of Hphp.
Temperature/K
173(2)
150(2)
˚
a/A
8.248(2)
15.584(3)
17.206(3)
95.15(3)
2202.7(8)
1.370
0.094
952
2.71–27.50
5039
3967
20.661(2)
8.9103(10)
11.1550(12)
90
2053.6(4)
1.551
0.949
988
3.37–33.56
3113
2596
˚
b/A
˚
c/A
employing the Gaussian03 suite of programs.16 It should be
stressed that both ab initio and DFT methods face serious
difficulties when employed on radical systems.17 In particular, DFT
undergoes the so-called self-interaction error, which may lead to an
unphysical delocalization of the unpaired electron over the entire
molecule. On the other hand, unrestricted Hartree-Fock (uHF)
and subsequent MP2 calculations may wrongly describe the total
spin of the system, leading to spin contamination. To prevent
these inconveniences, restricted open-shell calculations, such as
roHF and roMP2, may be carried out. In the present study, the
spin of both radicals has been checked and fortunately no spin
contamination has been observed. Thus, full MP2/6-31+G(d,p)
optimizations (based on uHF) have been carried out on radical-C
and radical-N. In addition, PCM18 single-point calculations have
been performed on the optimized structures (at the same level
of theory) to simulate the effect of methanol solvation, and to
evaluate their relative stabilities. The data obtained suggest that
radical-N is more likely to be initially formed during the reaction
[Scheme 4(a)]. Indeed, radical-N is found to be more stable than
radical-C by about 63 kJ mol-1.
b/◦
V/A
3
˚
r
calcd/Mg m-3
m/mm-1
F(000)
q for data collection/◦
Collected reflections
Independent reflections
Rint
0.0429
0.0617
0.1031
1.044
0.0445
0.0541
0.1274
1.043
R [I > 2s(I)]
wR (all data)
Goodness of fit on F2
a
3
˚
Largest diff. peak and hole/e A
0.24 and -0.23
0.92 and -0.51
ꢀ
a Goodness-of-fit S = [ w(Fo2 - F2c )2/(n - p)1/2, where n is the number of
reflections and p the number of parameters.
˚
Table 2 Bond lengths (A) characterizing the leuco-verdazyl ring and the
intramolecular hydrogen-bonding interactions in 4
˚
Bond length/A
N1–N2
N1–C6
N4–C3
O37–H37A ◊ ◊ ◊ N2
1.372(2)
1.445(2)
1.399(2)
2.755(2)
N4–N5
N2–C3
N5–C6
O47–H47A ◊ ◊ ◊ N22
1.422(2)
1.295(2)
1.451(2)
2.698(2)
Scheme 4(b) depicts the proposed mechanism of formation of 4
from radical-N. Once formed, radical-N reacts (step a, Scheme 4)
=
with the C N bond of a second Hphp molecule, leading to the
radical A. Actually, such attack of a double bond by a hydrazinyl-
type radical has been reported earlier.19 A subsequently loses
a H∑ to produce B (step b), which is in turn attacked by the
H∑ to generate the (radical-N)-type moiety C (step c). Similarly
to step a, the radical-N C undergoes an intermolecular attack
=
of the C N bond, producing the cyclic radical D (step d).
Then, D experiences a 1,2-radical phenol migration to form the
carboradical intermediate E (step e). A number of MnIII-initiated
1,4- and 1,5-radical aryl migrations have been described,20,21 but
examples of 1,2-radical aryl migrations are scarce.22 The final
step towards 4, i.e. step f , is the oxidation of the carboradical
E by Mn(OAc)3, followed by nucleophilic attack of the resulting
carbocation by methanol.
To corroborate both the formation of organic radical species
and of Mn2+ ions, the reaction generating compound 4 has been
followed by X-band EPR spectroscopy. Thus, the EPR spectrum
depicted in Fig. 2 was obtained. The spectrum is characterized by
the presence of two signals, hence suggesting the presence of two
different types of paramagnetic species. The EPR resonance with
a characteristic six-line splitting pattern (Fig. 2A; g = 2.0045, A
= 100 G) is typical for a high-spin Mn(II) center. The six lines
are due to hyperfine interactions between the unpaired electrons
and the I = 5/2 55Mn nucleus. The very sharp signal at g =
2.0023 (corresponding to the g value for a free electron; Fig. 2B)
Fig. 1 Representation of the molecular structure of 1,3-(diphenol-
2-yl)-2,5-(dipyridin-2-yl)-6-methoxy-leuco-verdazyl (4). Only the H atoms
involved in hydrogen bonding interactions (dashed lines) are shown for
clarity.
known to mediate (one-electron) oxidative coupling reactions
involving radical intermediates.14,15 Thus, the creation of 4 is most
likely initiated by the MnIII-based formation of a radical species
from the ligand Hphp. Two different radicals may be produced via
one-electron oxidation of Hphp, which are depicted in Scheme 3.
The stability of both potential radicals, namely radical-C
and radical-N (Scheme 3), has been theoretically investigated,
1362 | Dalton Trans., 2010, 39, 1361–1365
This journal is
The Royal Society of Chemistry 2010
©

View Article Online
Scheme 4 Proposed mechanism for the MnIII-based formation of 4. (a) Formation of radical-N; (b) cyclodimerization of two molecules of Hphp with
1,2-migration of a phenol group and nucleophilic attack of methanol.
characterizes an organic radical. These EPR data confirm the
formation of MnII ions from MnIII and the presence of organic
radicals during the production of compound 4.
To verify the crucial involvement of MnIII species in the
II
formation of 4, the ligand Hphp was used with a Mn salt. Thus, as
anticipated, the reaction of Hphp with manganese(II) perchlorate,
using the same experimental conditions as for the preparation of 4,
does not result in any modification of the original ligand. Instead,
II
a typical octahedral Mn coordination compound is obtained,
namely [Mn(php)2] (5) that crystallizes in the orthorhombic space
group Pbcn. Crystal data and structure refinement for 5 are also
given in Table 1 and selected bond distances and angles are listed
in Table 3. A representation of the molecular structure of 5,
determined by single-crystal X-ray analysis, is shown in Fig. 3.
The octahedral environment around the Mn2+ ion Mn1 is
formed by two deprotonated php ligands, coordinated to the
metal centre through their pyridine N, imine N and phenol O
donor atoms (Fig. 3). The Mn–N and Mn–O bond distances
are at normal ranges for this type of MnN4O2 chromophore.23
Fig. 2 (a) EPR spectrum recorded during the formation of compound 4;
(b) zoomed-in portion of the spectrum showing the organic radical signal
with g = 2.0023.
The octahedron is slightly distorted with an axial angle N8_a–
Mn1–N8 of 162◦, 18◦ away from the ideal value of 180◦. This
distortion is most likely due to the two small bite angles (N8–
Mn1–O16 = 81.09(5)◦ and N8–Mn1–N1 = 71.27(5)◦) of the
planar, rigid php ligand. The molecular structure of 5 exhibits
strong intermolecular hydrogen bonding interactions involving the
This journal is
The Royal Society of Chemistry 2010
Dalton Trans., 2010, 39, 1361–1365 | 1363
©

View Article Online
◦
a
˚
The resulting light-brown solution was left unperturbed for the
slow evaporation of the solvent. Yellow block crystals, suitable
for X-ray diffraction analysis, were formed after three days. Yield:
24.4 mg, 43% (based on the ligand Hphp). Mw = 454.49 g mol-1.
Elemental analysis (%) calcd for C25H22N6O3: C 66.07, H 4.88, N
18.49; found: C 66.22, H 5.47, N 18.59. IR data for 4: n = 1589
(vs), 1572 (w), 1491 (s), 1466 (s),1437 (vs), 1334 (w), 1293 (s), 1250
(s), 1189 (s), 1145 (w), 1080 (s), 1009 (s), 960 (s), 887 (w), 805 (s),
757 (vs), 668 (s), 637 (s), 506 (w), 465 (s), 336 (s) cm-1.
Table 3 Selected bond lengths (A) and angles ( ) for 5
˚
Bond length/A
Bond angle/◦
Mn1–O16
Mn1–N1
Mn1–N8
2.0954(13)
2.2506(14)
2.2556(13)
O16–Mn1–O16_a
O16_a–Mn1–N1
N1_a–Mn1–N1
N1_a–Mn1–O16
N8_a–Mn1–N8
91.31(5)
94.47(5)
93.24(5)
94.47(5)
161.61(5)
N7–
2.7504(18)
H7A ◊ ◊ ◊ O16
a Symmetry operation: a = 1 - x, y, 1/2 - z.
[Mn(php)2] (5)
A solution of 181 mg (0.50 mmol) of Mn(ClO4)2·6H2O in 10 mL
of methanol was added to a stirred methanolic solution (15 mL)
of Hphp (53.3 mg, 0.25 mmol). The resulting light yellow solution
was left unperturbed to allow the slow evaporation of the solvent.
Block yellow-orange single crystals of [Mn(php)2], suitable for
X-ray diffraction analysis, were obtained after one day. Yield =
46.1 mg (77%, based on the ligand Hphp). Mw = 479.40 g mol-1.
Elemental analysis (%) calcd for C24H20MnN6O2: C 60.13, H 4.21,
N 17.53; found: C 60.22, H 4.03, N 17.45. IR data for 5: n = 2748
(w), 1616 (vs), 1595 (s), 1538 (s), 1465 (s), 1442 (m), 1424 (s), 1378
(m), 1331 (s), 1311 (s), 1285 (s), 1262 (w), 1196 (s), 1150 (s), 1115
(s), 1091 (s), 1037 (s), 1003 (s), 941 (s), 883 (s), 788 (m), 747 (s),
733 (s), 653 (s), 584 (s), 500 (m), 476 (s), 404 (vs) cm-1.
Fig. 3 Schematic representation of the molecular structure of [Mn(php)2]
(5).
phenolic oxygen atom O16 and the hydrazino donor nitrogen atom
N7 (Fig. 4 and Table 3). This hydrogen bonding network gives rise
to a supramolecular 1D chain (Fig. 4).
X-Ray crystal structure determinations
Crystallographic data and refinement details are given in Table 1.
The intensity data for 4 were recorded on a Nonius Kappa CCD
˚
diffractometer with Mo Ka radiation (l = 0.71073 A) at 173(2) K.
The intensity data were corrected for Lorentz and polarization
effects, and for absorption. The programs COLLECT,24 SHELXS–
97,25 and SHELXL–9726 were used for data reduction, structure
solution, and structure refinement, respectively. The non-hydrogen
atoms were refined anisotropically. The H atoms were introduced
incalculated positions andrefinedwithfixedgeometry with respect
to their carrier atoms.
Fig. 4 Illustration showing the hydrogen-bonding interactions (green
˚
lines; N7–H7A ◊ ◊ ◊ O16 = 2.7504(18) A) in 5, producing a 1D supramolec-
The intensity data for 5 were recorded at 150(2) K on a
yellow needle on beamline 11.3.1 of the Advanced Light Source
synchrotron at Lawrence Berkeley National Laboratory (l =
ular chain along the c-axis.
˚
0.7749 A from silicon 111 monochromator), using a Bruker Apex
II CCD diffractometer. The structure was solved with SIR97,27 and
refined over F2 with SHELXL–97.26 All non-hydrogens were refined
anisotropically, while hydrogens, found in difference Fourier maps,
were refined with a riding model at geometrical position on their
carrier atom.
Experimental section
General
All chemicals were used as commercially obtained without further
purification. The ligand Hphp was prepared following a procedure
reported earlier.13 Elemental analyses (C, H, N) were carried out
on a Perkin-Elmer 2400 series II analyzer. FTIR spectra were
recorded with a Perkin-Elmer Paragon 1000 FTIR spectropho-
tometer, equipped with a Golden Gate ATR device, using the
reflectance technique (n = 4000–300 cm-1).
CCDC–711637 (4) and CCDC–713054 (5) contain the supple-
mentary crystallographic data for this paper.†
Conclusions
In summary, an unprecedented 1,2,3,5,6-pentasubstituted leuco-
verdazyl-type ring has been generated from a simple hydrazinyl
compound via different one-electron oxidative steps mediated by
The X-band EPR spectrum was obtained at room temperature
with a Bruker EMXplus spectrometer.
III
Mn3+ ions. The involvement of Mn species in the oxidation
1,3-(Diphenol-2-yl)-2,5-(dipyridin-2-yl)-6-methoxy-leuco-verdazyl
(4)
II
is corroborated by the use of Mn , which yields an expected
coordination compound, showing a non-altered php ligand. The
III
Compound 4 was synthesized from Mn(OAc)3·2H2O (134.0 mg,
0.50 mmol) and Hphp (53.3 mg, 0.25 mmol) in methanol (25 mL).
versatility of this unique Mn -based reaction is currently being
investigated with other related hydrazinyl compounds. In addition,
1364 | Dalton Trans., 2010, 39, 1361–1365
This journal is
The Royal Society of Chemistry 2010
©

View Article Online
the leuco-verdazyl molecule 4 presents N and O donor atoms,
which can certainly be used to bind metal ions and therefore
in the preparation of polynuclear coordination compounds with
potentially interesting properties.
12 A. R. Katritzky, S. A. Belyakov, H. D. Durst, R. X. Xu and N. S. Dalal,
Can. J. Chem., 1994, 72, 1849–1856.
13 J. Tang, J. S. Costa, A. Pevec, B. Kozlevcar, C. Massera, O. Roubeau,
I. Mutikainen, U. Turpeinen, P. Gamez and J. Reedijk, Cryst. Growth
Des., 2008, 8, 1005–1012.
14 K. Asahi and H. Nishino, Tetrahedron, 2005, 61, 11107–11124.
15 B. B. Snider, Chem. Rev., 1996, 96, 339–363.
Acknowledgements
16 M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb,
J. R. Cheeseman, J. A. Montgomery, Jr., T. Vreven, K. N. Kudin, J. C.
Burant, J. M. Millam, S. S. Iyengar, J. Tomasi, V. Barone, B. Mennucci,
M. Cossi, G. Scalmani, N. Rega, G. A. Petersson, H. Nakatsuji, M.
Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T.
Nakajima, Y. Honda, O. Kitao, H. Nakai, M. Klene, X. Li, J. E.
Knox, H. P. Hratchian, J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo,
R. Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi,
C. Pomelli, J. Ochterski, P. Y. Ayala, K. Morokuma, G. A. Voth, P.
Salvador, J. J. Dannenberg, V. G. Zakrzewski, S. Dapprich, A. D.
Daniels, M. C. Strain, O. Farkas, D. K. Malick, A. D. Rabuck, K.
Raghavachari, J. B. Foresman, J. V. Ortiz, Q. Cui, A. G. Baboul, S.
Clifford, J. Cioslowski, B. B. Stefanov, G. Liu, A. Liashenko, P. Piskorz,
I. Komaromi, R. L. Martin, D. J. Fox, T. Keith, M. A. Al-Laham, C. Y.
Peng, A. Nanayakkara, M. Challacombe, P. M. W. Gill, B. G. Johnson,
W. Chen, M. W. Wong, C. Gonzalez and J. A. Pople, GAUSSIAN,
Gaussian, Inc., Pittsburgh, PA, 2003.
Support from the Graduate Research School Combination “Catal-
ysis” and the COST program Action D35/0011 is acknowledged.
Coordination of some of our research by the FP6 Network of
Excellence “Magmanet” (contract number 515767) is also kindly
acknowledged. The Advanced Light Source is supported by the
Director, Office of Science, Office of Basic Energy Sciences, of
the US Department of Energy under Contract No. DE-AC02-
05CH11231. J. Tang thanks Changchun Institute of Applied
Chemistry (Start-up) and National Natural Science Foundation
of China (Grants 20871113 and 20921002) for their financial
support.
17 F. Jensen, Introduction to Computational Chemistry, Wiley & Sons,
References
Chichester, 1999.
1 B. D. Koivisto and R. G. Hicks, Coord. Chem. Rev., 2005, 249, 2612–
2630.
2 R. Kuhn and H. Trischmann, Angew. Chem., 1963, 75, 294–295.
3 F. A. Neugebauer, Angew. Chem., Int. Ed. Engl., 1973, 12, 455–464.
4 F. A. Neugebauer, H. Fischer and R. Siegel, Chem. Ber., 1988, 121,
815–822.
5 K. Mukai, D. Shiba, K. Mukai, K. Yoshida, H. Hisatou, K. Ohara, Y.
Hosokoshi and N. Azuma, Polyhedron, 2005, 24, 2513–2521.
6 J. B. Gilroy, S. D. J. McKinnon, B. D. Koivisto and R. G. Hicks, Org.
Lett., 2007, 9, 4837–4840.
7 D. J. R. Brook and G. T. Yee, J. Org. Chem., 2006, 71, 4889–4895.
8 M. J. Plater, S. Kemp, E. Coronado, C. J. Gomez-Garcia, R. W.
Harrington and W. Clegg, Polyhedron, 2006, 25, 2433–2438.
9 L. Keeney and M. J. Hynes, Dalton Trans., 2005, 133–138.
10 R. Kuhn and H. Trischmann, Monatsh. Chem., 1964, 95, 457–479.
11 P. R. Serwinski, B. Esat, P. M. Lahti, Y. Liao, R. Walton and J. Lan,
J. Org. Chem., 2004, 69, 5247–5260.
18 V. Barone and M. Cossi, J. Phys. Chem. A, 1998, 102, 1995–2001.
19 B. S. Sherigara, I. Pinto and K. I. Bhat, Mikrochim. Acta, 1994, 113,
61–69.
20 L. El Kaim, L. Grimaud and E. Vieu, Org. Lett., 2007, 9, 4171–4173.
21 A. Studer and M. Bossart, Tetrahedron, 2001, 57, 9649–9667.
22 A. J. Clark, and S. M. Rooke, Organic Reaction Mechanisms 1995, John
Wiley & Sons, Ltd, Chichester, 1997.
23 D. A. Bardwell, J. C. Jeffery and M. D. Ward, J. Chem. Soc., Dalton
Trans., 1995, 3071–3080.
24 Nonius, COLLECT, Nonius BV, Delft, The Netherlands, 2002.
25 G. M. Sheldrick, SHELXS-97 Program for Crystal Structure Determi-
nation, University of Gottingen, Germany, 1997.
¨
26 G. M. Sheldrick, SHELXL-97-2 Program for Crystal Structure Refine-
ment, University of Gottingen, Germany, 1997.
¨
27 A. Altomare, M. C. Burla, M. Camalli, G. L. Cascarano, C. Giacov-
azzo, A. Guagliardi, A. G. G. Moliterni, G. Polidori and R. Spagna,
J. Appl. Crystallogr., 1999, 32, 115–119.
This journal is
The Royal Society of Chemistry 2010
Dalton Trans., 2010, 39, 1361–1365 | 1365
©