[Cr(dpa)(ox)2]-: A new bis-oxalato building block for the design of heteropolymetallic systems

Article abstract of DOI:10.1039/b104469a

The new complexes of formulae PPh₄[Cr(dpa)(ox)₂] (1), AsPh₄[Cr(dpa)(ox)₂] (2), Hdpa[Cr(dpa)(ox)₂]·4H₂O (3), Rad[Cr(dpa)(ox)₂]·H₂O (4) and Sr[Cr(dpa)(ox)₂]₂·8H₂O (5) [PPh₄=tetraphenylphosphonium cation; AsPh₄=tetraphenylarsonium cation; dpa=2,2′-dipyridylamine; ox=oxalate dianion; Rad=2-(4-N-methylpyridinium)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazol-1- oxyl-3-N-oxide] have been prepared and characterised by single-crystal X-ray diffraction. The structures of 1-4 consist of discrete [Cr(dpa)(ox)₂]^- anions, tetraphenyl- phosphonium (1), tetraphenylarsonium (2), monoprotonated Hdpa (3) and univalent radical (4) cations and uncoordinated water molecules (2-4). The chromium environment in 1-4 is distorted octahedral with Cr-O bond distances between 1.982(2)-1.946(2) A and Cr-N bonds of 2.0716(17)-2.048(3) A. The angles subtended at the chromium atom by the two oxalates are 83.6(2)-81.71(8)°, whereas the N-Cr-N angles are 87.76(7)-86.24(9)°. The [Cr(dpa)(ox)₂]^- unit of 1-4 is also present in 5 but it acts as a chelating ligand through its two oxalato groups towards divalent strontium cations, yielding heterobimetallic zig-zag chains that run parallel to the a axis. Each chain is formed of diamond-shaped units sharing the strontium atoms, while the two other corners are occupied by two crystallographically independent chromium atoms. The [Cr(dpa)(ox)₂]^- unit in 5 retains the environment observed in 1-4 and the strontium atom is coordinated to eight oxalate oxygens from four oxalate ligands. The two crystallographycally independent chromium centres within each double chain have opposite chirality. However, the adjacent double chains are related by an inversion centre resulting in achiral layers parallel to the ac plane. The magnetic properties of 1-5 have been investigated in the temperature range 1.9-290 K. A quasi Curie law behaviour is observed for 1-3 and 5 in agreement with their crystal structures, whereas a significant antiferromagnetic interaction between the chromium(III) and the radical centre occurs in the case of 4. The synthetic possibilities offered by the use of the heteroleptic species [CrL(ox)₂]^- (L=α-diimine-type ligand) as a ligand towards metal ions is analysed and discussed in the light of the available structural results.

Full text of DOI:10.1039/b104469a

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[Cr(dpa)(ox) ]—: a new bis-oxalato building block for the design of
2
heteropolymetallic systems. Crystal structures and magnetic
properties of PPh [Cr(dpa)(ox) ], AsPh [Cr(dpa)(ox) ],
4
2
4
2
Hdpa[Cr(dpa)(ox) ] Æ 4H O, Rad[Cr(dpa)(ox) ] Æ H O and
2
2
2
2
Sr[Cr(dpa)(ox) ] Æ 8H O (dpa = 2,2º-dipyridylamine)
2 2
2
Rodrigue Lescouezec,a Gabriela Marinescu,b M. Carmen Munoz,c Dominique Luneau,d
Marius Andruh,b Francesc Lloret,a Juan Faus,a Miguel Julve,*a Jose Antonio Mata,e
Rosa Llusarf and Joan Canog
a Departament de Qu•mica Inorganica/Instituto de Ciencia Molecular, Facultat de Qu•mica de la
Universitat de V alencia, Dr. Moliner 50, 46100 Burjassot (V alencia), Spain.
E-mail: miguel.julve=uv.es
b Inorganic Chemistry L aboratory, Faculty of Chemistry, University of Bucharest, Str. Dumbrava
Rosie nr. 23, 70254 Bucharest, Romania
c Departamento de F•sica Aplicada, Universidad Politecnica de V alencia, Camino de V era s/n,
46071 V alencia, Spain
d Service de Chimie Inorganique et Biologique (CNRS UMR 5046), DMFMC, CEA-Grenoble,
17 rue des Martyrs, 38054 Grenoble cedex 09, France
e Departament de Qu•mica Inorganica i Organica, Campus Riu Sec de la Universitat Jaume I,
POB 224, 12080 Castello, Spain
f Departament de Ciencies Experimentals, Campus Riu Sec de la Universitat Jaume I, POB 224,
12080 Castello, Spain
g L aboratoire de Chimie Inorganique (CNRS UMR 8613), Universite de Paris-Sud, 91405 Orsay,
France
Received (in Strasbourg, France) 21st May 2001, Accepted 10th July 2001
First published as an Advance Article on the web 19th September 2001
The new complexes of formulae PPh [Cr(dpa)(ox) ] (1), AsPh [Cr(dpa)(ox) ] (2), Hdpa[Cr(dpa)(ox) ] É 4H O (3),
4
2
4
2
2
2
Rad[Cr(dpa)(ox) ] É H O (4) and Sr[Cr(dpa)(ox) ] É 8H O (5) [PPh \ tetraphenylphosphonium cation; AsPh \
2
2
2 2
2
4
4
tetraphenylarsonium cation; dpa \ 2,2@-dipyridylamine; ox \ oxalate dianion; Rad \ 2-(4-N-methylpyridinium)-
4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazol-1-oxyl-3-N-oxide] have been prepared and characterised by
single-crystal X-ray di†raction. The structures of 1–4 consist of discrete [Cr(dpa)(ox) ]~ anions, tetraphenyl-
2
phosphonium (1), tetraphenylarsonium (2), monoprotonated Hdpa (3) and univalent radical (4) cations and
uncoordinated water molecules (2È4). The chromium environment in 1È4 is distorted octahedral with CrÈO bond
distances between 1.982(2)È1.946(2) A and CrÈN bonds of 2.0716(17)È2.048(3) A. The angles subtended at the
chromium atom by the two oxalates are 83.6(2)È81.71(8)¡, whereas the NÈCrÈN angles are 87.76(7)È86.24(9)¡. The
[Cr(dpa)(ox) ]~ unit of 1È4 is also present in 5 but it acts as a chelating ligand through its two oxalato groups
2
towards divalent strontium cations, yielding heterobimetallic zig-zag chains that run parallel to the a axis. Each
chain is formed of diamond-shaped units sharing the strontium atoms, while the two other corners are occupied by
two crystallographically independent chromium atoms. The [Cr(dpa)(ox) ]~ unit in 5 retains the environment
2
observed in 1È4 and the strontium atom is coordinated to eight oxalate oxygens from four oxalate ligands. The two
crystallographycally independent chromium centres within each double chain have opposite chirality. However, the
adjacent double chains are related by an inversion centre resulting in achiral layers parallel to the ac plane. The
magnetic properties of 1È5 have been investigated in the temperature range 1.9È290 K. A quasi Curie law
behaviour is observed for 1È3 and 5 in agreement with their crystal structures, whereas a signiÐcant
antiferromagnetic interaction between the chromium(III) and the radical centre occurs in the case of 4. The synthetic
possibilities o†ered by the use of the heteroleptic species [CrL(ox) ]~ (L \ a-diimine-type ligand) as a ligand
2
towards metal ions is analysed and discussed in the light of the available structural results.
The Ðrst studies of the coordinating ability of 2,2@-dipyridyl-
amine (dpa) showed the great versatility of this ligand, which
can act not only as a bidentate ligand through its two pyridyl
nitrogen atoms1h5 but also as a bridging ligand after removal
of the amine hydrogen atom.5,6 Interestingly, the free dpa
ligand has a dimeric structure7 in which two dpa molecules
are linked by NÈHÉ É ÉN hydrogen bonds, each dpa group
exhibiting the anti-syn conÐguration [see Scheme 1(a)]. Most
of the reported crystal structures of dpa-containing metal
complexes concern the copper(II) ion with metal-to-dpa molar
1224
New J. Chem., 2001, 25, 1224È1235
DOI: 10.1039/b104469a
This journal is ( The Royal Society of Chemistry and the Centre National de la Recherche ScientiÐque 2001

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was reÑuxed under continuous stirring for 1.5 h. Tetra-
phenylphosphonium (1) or tetraphenylarsonium (2) chloride (2
mmol), dissolved in the minimum amount of warm water, was
added to the previously Ðltered deep violet solution. A Ðrst
crop of a pink (1) or violet (2) crystalline solid was obtained
on standing. Single crystals as red prisms (1) or violet parallel-
epipeds (2) were grown from the mother liquor by slow evapo-
ration at room temperature. Yield ca. 40% (1 and 2). Anal.
calc. for C
H
CrN O P (1): C, 61.81; H, 3.93; N, 5.69;
38 29
3 8
found: C, 61.33; H, 3.86; N, 5.59%. Anal. calc. for
C
H
AsCrN O (2): C, 58.34; H, 3.71; N, 5.37; found: C,
38 29
3 8
58.01; H, 3.67; N, 5.29%.
Scheme 1
Preparation of Hdpa[Cr(dpa)(ox) ] Æ 4H O (3). The prep-
ratios of 1 : 1 and 1 : 2,8,9 the dpa ligand acting as a bidentate
ligand through its two pyridyl nitrogen atoms [anti-anti con-
formation, Scheme 1(b)]. Although the resulting species are
usually mononuclear, polynuclear complexes with a 1 : 1
Cu(II) : dpa molar ratio were obtained in the presence of suit-
able bridging ligands such as hydroxo,8h chloro,8f oxam-
idato8c and carbonato.8d Some years ago, copper(I) complexes
with dpa were investigated as models for copper-containing
plant hormone binding sites10 and electrochemical studies
showed that dpa favours the one-electron reduction from
[Cu(dpa) ]2` to [Cu(dpa) ]` to a higher extent that the more
2
2
aration of 3 is analogous to that of 1 and 2 except for the
addition of a concentrated aqueous solution of HdpaCl
(generated by reaction of stoichiometric amounts of dpa and
dilute hydrochloric acid) to the Ðltered aqueous violet solution
containing the [Cr(dpa)(ox) ]~ species. A Ðrst crop of a crys-
2
talline pink solid of 3 was formed on standing after half a day.
Plate-like crystals of 3 suitable for X-ray di†raction separated
from the mother liquor on slow evaporation at room tem-
perature. Yield ca. 50%. The crystals of 3 lost water molecules
after a few days. Anal. calc. for C
H, 4.20; N, 13.06; found: C, 44.56; H, 4.13; N, 12.95%.
H
CrN O (3): C, 44.81;
24 27
6 12
2
2
rigid 2,2@-bipyridine (bpy) and 1,10-phenanthroline (phen).11
Very recently, the unprecedented monodentate coordination
mode of dpa through one of its two pyridyl atoms has been
Preparation of Rad[Cr(dpa)(ox) ] Æ H O (4). This complex
2
2
was prepared as the previous one but the the radical cation as
an iodide salt dissolved in the minimum amount of water (1
mmol, 10 cm3) was added to the Ðltered violet aqueous solu-
observed in the monuclear complex [W(dpa)(CO) ].12 Finally,
5
the uncommon monodeprotonated form of the dpa molecule
can act also as a ligand towards Ðrst- and second-row tran-
sition metal ions, adopting either chelating [Scheme 1(c)]13 or
bridging [Scheme 1(d,e)]14h19 coordination modes. Very
interesting linear trinuclear complexes were reported in this
latter case with the 2,2@-dipyridylamido exhibiting the syn-syn
conÐguration.15h19 Variable-temperature magnetic suscepti-
bility studies on the trinuclear complexes revealed a remark-
able diversity of magnetic behaviours ranging from strong
antiferromagnetic coupling (tricopper and trinickel)15,16 to
spin crossover (tricobalt)17a,c and passing through Curie law
behaviour (trichromium).18
In the context of our current research work devoted to the
design of heterometallic complexes using as precursors hetero-
leptic chromium(III) mononuclear complexes that can be used
as ligands,20h22 we have synthesised a new family of com-
pounds containing the bis(oxalato)(2,2@-dipyridylamine)chro-
mium(III) unit. Their preparation, structural characterisation
and magnetic properties are presented here.
tion containing the [Cr(dpa)(ox) ]~ complex (1 mmol, 25
2
cm3). 4 separates as brown prismatic crystals from the mother
liquor on slow evaporation at room temperature. Yield ca.
35%. Anal. calc. for C
N, 12.61; found: C, 48.54; H, 4.43; N, 12.52%.
H
CrN O (4): C, 48.67; H, 4.50;
27 30
6 11
Preparation of Sr[Cr(dpa)(ox) ] Æ 8H O (5). The addition
2 2
2
of complex 1 (0.25 mmol) to an aqueous solution of lithium
perchlorate (0.25 mmol, 20 cm3) under continuous stirring
a†ords a violet solution containing the lithium salt of
[Cr(dpa)(ox) ]~ and a white precipitate of PPh ClO . The
2
4
4
white solid was removed by Ðltration and strontium chloride
(0.125 mmol) dissolved in the minium amount of water added
to the violet solution. Red prisms of 5 separated from this
solution on slow evaporation at room temperature. The yield
is practically quantitative. Anal. calc. for C
(5): C, 32.64; H, 3.30; N, 8.15; found: C, 32.51; H, 3.23; N,
8.05%.
H
Cr N O Sr
28 34
2 6 24
Experimental
Physical techniques
Materials
IR spectra (4000È400 cm~1) were recorded on a Bruker IF
S55 spectrophotometer with samples prepared as KBr pellets.
Variable temperature (1.9È290 K) magnetic susceptibilty mea-
surements on polycrystalline samples were carried out with a
Quantum Design SQUID operating at 0.5 T in the high
temperature range (20È290 K) and at 250 G at T \ 20 K in
order to avoid saturation phenomena. Diamagnetic correc-
tions for the constituent atoms were estimated from PascalÏs
constants24 as [404 ] 10~6 (1), [399 ] 10~6 (2),
[343 ] 10~6 (3), [332 ] 10~6 (4) and [248 ] 10~6 (5) cm3
mol~1 [per mol of Cr(III)].
2,2@-Dipyridylamine, chromium(III) chloride hexahydrate, stron-
tium(II) chloride hexahydrate, sodium oxalate, tetraphenyl-
phosphonium chloride and tetraphenylarsonium chloride
monohydrate were purchased from commercial sources and
used as received. The radical salt 2-(4-N-methylpyridinium)-
4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazol-1-oxyl-3-N-oxide
iodide was prepared as reported in the literature.23 Elemental
analyses (C, H, N) were carried out by the Microanalytical
Service of the Universidad Autonoma de Madrid. The P : Cr
(1), As : Cr (2) and Sr : Cr (5) molar ratios (1 : 1 for 1 and 2
and 0.5 : 1 for 5) were determined by electron microscopy at
the Servicio Interdepartamental de Investigacion of the Uni-
versity of Valencia.
Computational methodology
The computational strategy used in the present work here has
been fully described elsewhere25 and we will recall here only
the relevant aspects. Density functional theory calculations26
were done to evaluate the coupling constants concerning the
the chromium(III)Èradical and radicalÈradical pairs [see
Preparation of XPh [Cr(dpa)(ox) ], X = P (1) and As (2).
4
2
An aqueous suspension (40 cm3) containing chromium(III)
chloride (2 mmol), dpa (2 mmol) and sodium oxalate (4 mmol)
New J. Chem., 2001, 25, 1224È1235
1225

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Table 1 Crystallographic data for PPh [Cr(dpa)(ox) ] (1), AsPh [Cr(dpa)(ox) ] (2), Hdpa[Cr(dpa)(ox) ] É 4H O (3), Rad[Cr(dpa)(ox) ] É H O (4)
4
2
4
2
2
2
2
2
and Sr[Cr(dpa)(ox) ] É 8H O (5)
2 2
2
1
2
3
4
5
Formula
FW
Crystal system
Space group
a/A
b/A
c/A
a/¡
C
738.61
Monoclinic
P2
9.2002(11)
13.4712(16)
13.7964(16)
90
97.574(3)
90
1695.0(3)
2
293(2)
4.44
H
CrN O P
C
782.56
Monoclinic
P2
9.2221(5)
13.471(3)
13.8725(17)
90.0
97.418(7)
90.0
1709.0(4)
2
293(2)
13.54
H
CrN O As
C
643.52
Triclinic
P1
9.294(2)
11.380(3)
14.292(3)
93.78(2)
95.838(14)
111.39(2)
1391.6(5)
2
H
CrN O
C
666.57
H
CrN O
C
H
Cr N O Sr
24
38 29
3
8
38 29
3
8
24 27
6
12
27 30
6
11
28 34
2 6
1030.23
Triclinic
P1
Monoclinic
C2/c
21.999(8)
16.248(4)
17.111(4)
90
110.18(3)
90
5741(3)
8
1
1
9.457(2)
12.095(2)
17.850(4)
99.95(3)
99.57(3)
104.87(3)
1895.5(7)
2
b/¡
c/¡
U/A
Ž
3
Z
T /K
293(2)
4.86
3611
298(7)
4.72
6964
293(2)
20.68
7110
k(Mo-Ka)/cm~1
No. indep. reÑ.
No. obs. reÑ.
[I P 2p(I)]
13 778
2452
9433
2050
3108
6964
4338
R
0.0203
0.0350
0.0531
0.0275
0.0340
0.0751
0.03680
0.0473
0.1249
0.0740
0.0654
0.1616
0.1006
0.0551
0.1405
int
Ra
R b
w
a R \ &(pF o [ o F p)/& o F o. b R \ [& ( o F o2 [ o F o2)2]/[& o F o2]1@2.
o
c
o
w
w
o
c
w
o
Scheme 2 and Fig. 9(a)] in 4. Separate calculations were
carried out for the high-spin (quartet and triplet spin states for
the Cr(III)Èrad and radÈrad pairs, respectively) and broken-
symmetry low-spin states (triplet and singlet spin states for the
Cr(III)Èrad and radÈrad pairs, respectively). The hybrid
B3LYP method27 has been used in the calculations as imple-
mented in GAUSSIAN 98,28 mixing the exact HartreeÈFock
exchange with BeckeÏs expression for the exchange29 and
using the LeeÈYangÈParr correlation functional.30 Basis sets
of triple-f31 with two extra p polarization functions (chrom-
ium atom) and of double-f quality32 (atoms other than
chromium) were used in our calculations.
symmetry low spin (E ) states through eqn. (1):
ls
2(E [ E )
ls
hs
J \
(1)
S (S ] 1) [ S (S ] 1)
hs hs ls ls
where S and S are the values of the total spin for the high-
hs
ls
and low-spin states, respectively, dealing with the Cr(III)Èrad
and radÈrad pairs. The calculations were done on the real
structures of the CrÈrad and radÈrad pairs as well as for the
mononuclear paramagnetic units. Finally, spin density calcu-
lations were performed through Mulliken33 and natural bond
order (NBO)34 analyses giving the same qualitative results.
The high spinÈlow spin energy gap (J) was evaluated from
X-Ray data collection and structure reÐnement
the calculated energies of the high spin (E ) and broken-
hs
Crystals were mounted on Bruker Smart CCD (1 and 4) and
EnrafÈNonius CAD-4 (2, 3 and 5) di†ractometers and used for
data collection. Di†raction data were collected at room tem-
perature by using graphite-monochromated Mo-Ka radiation
(j \ 0.710 73 A) with the u [ 2h method. Nominal crystal-to-
detector distances of 4.0 and 5.9 cm were used for 1 and 4,
respectively. The unit cell parameters were determined from
least-squares reÐnement on the setting angles from 25 centred
reÑections in the range 12 \ h \ 20¡ (2, 3 and 5). In the case
of 1 and 4, the unit cell parameters were based upon least-
squares reÐnement of three-dimensional centroids of 512
reÑections. No signiÐcant Ñuctuations were observed in the
intensities of three (2, 3 and 5) standard reÑections monitored
periodically thoughout data collection. An hemisphere of data
for 1 was collected based on one u-scan run (starting u \ 28¡)
at / \ 0 with the detector at 2h \ 28¡. For this run, a total of
606 frames were collected at 0.3¡ intervals and 30 s per frame.
A total of 1271 (1) and 1321 (4) frames of data were collected
using a narrow-frame method with scan widths of 0.3¡ in u
and exposure times of 30 (1) and 10 s (4) per frame. Intensity
data for 1È5 were corrected for Lorentz and polarisation
e†ects and absorption corrections made. The di†raction
frames of 1 and 4 were integrated using the SAINT package35
and corrected for absorption with SADABS.36 A summary of
the crystallographic data and structure reÐnement is given in
Table 1.
Table 2 Selected bond lengths (A) and angles (¡)a for compounds 1
and 2
1
2
CrÈN(1)
CrÈN(2)
CrÈO(1)
CrÈO(2)
CrÈO(3)
CrÈO(4)
2.0716(17)
2.0581(17)
1.9558(12)
1.9642(15)
1.9716(15)
1.9582(12)
2.092(7)
2.054(6)
1.944(4)
1.948(5)
1.977(5)
1.966(5)
O(1)ÈCrÈO(2)
O(1)ÈCrÈO(4)
O(1)ÈCrÈN(1)
O(1)ÈCrÈN(2)
O(1)ÈCrÈO(3)
O(2)ÈCrÈO(4)
O(2)ÈCrÈN(1)
O(2)ÈCrÈN(2)
O(2)ÈCrÈO(3)
N(1)ÈCrÈO(4)
N(1)ÈCrÈN(2)
N(1)ÈCrÈO(3)
O(4)ÈCrÈN(2)
O(4)ÈCrÈO(3)
N(2)ÈCrÈO(3)
83.25(6)
174.02(6)
89.68(6)
93.49(6)
92.77(6)
92.64(6)
172.67(6)
90.74(7)
87.76(7)
94.56(6)
87.76(7)
94.53(7)
90.90(6)
82.70(6)
173.34(6)
83.6(2)
174.4(2)
89.0(2)
93.9(2)
92.5(2)
93.1(2)
172.3(2)
91.2(2)
88.1(2)
94.5(2)
87.6(3)
93.9(3)
90.7(2)
82.9(2)
173.5(2)
The structures of 1È5 were solved by direct methods and
reÐned by the full-matrix least-squares method on F2. The
computations were performed with the SHELXTL 5.10
a Estimated standard deviations in the last signiÐcant digits are given
in parentheses.
1226
New J. Chem., 2001, 25, 1224È1235

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package37 (1 and 4) and SHELX 86 and SHELXL 97
programs38 (2, 3 and 5). All non-hydrogen atoms were reÐned
anisotropically. The amine hydrogen atom of dpa in 1È4 and
the proton of Hdpa` in 3 were found in di†erence maps and
reÐned isotropically. The remaining hydrogen atoms of 1È5
were set in calculated positions and reÐned isotropically with
a common Ðxed isotropic thermal parameter. Concerning
complex 1, the crystal was modeled as a racemic twin with a
Flack parameter of 0.13 improving the residual parameters.
The Ðnal full-matrix least-squares reÐnement on F2, mini-
mizing the function &w[( o F o )2 [ ( o F o )2]2 with w \
o
c
1/[p2(F 2) ] (mP)2 ] nP] and P \ (F 2 ] 2 F 2)/3 [m
o
o
c
\ 0.0331 (1), 0.0457 (2), 0.1105 (3), 0.1098 (4) and 0.0869 (5);
n \ 0 (1), 1.0914 (2), 2.0131 (3), 0.0 (4) and 1.7274 (5), con-
verged at the values of R and R listed in Table 1. The values
w
Fig. 1 Perspective view of compound 1 along with the atom num-
bering. The thermal ellipsoids are drawn at the 50% probability level
and the hydrogen atoms have been omitted for clarity.
of f, f @ and f A were taken from ref. 39. The molecular plots
were drawn with the ORTEP program.40 Selected bond
lengths and angles are gathered in Tables 2 (1 and 2), 3 (3), 4
(4) and 5 (5).
CCDC reference numbers 161709È161713. See http://
www.rsc.org/suppdata/nj/b1/b104469a/ for crystallographic
data in CIF or other electronic format.
linked through hydrogen bonds between the amine hydrogen
[H(3)]
and
one
oxalate
oxygen
[O(8a)]
atom
[N(3)É É ÉO(8a) \ 2.97(10) (1) and 2.98(10)
(2) and N(3)È
A
H(3)É É ÉO(8a) \ 152.0(1) (1) and 138.6(1)¡ (2); a \ 1 ] x, y, z],
leading to a chain of chromium atoms that runs parallel to the
x axis (Fig. 3).
Results and discussion
Each chromium atom in 1 and 2 is coordinated by two dpa
nitrogen and four oxalate oxygen atoms, taking on a distorted
octahedral geometry. The narrow bites of the bidentate dpa
[87.76(7) (1) and 87.6(3)¡ (2) for N(1)ÈCrÈN(2)] and ox ligands
[83.25(6) (1) and 83.6(2)¡ (2) for O(1)ÈCrÈO(2), 82.70(6) (1) and
82.9(2)¡ (2) for O(4)ÈCrÈO(3)] are the main factors accounting
for the distortion of the metal environment from the ideal
octahedron. The four CrÈO(ox) bond distances are very close
[values varying in the ranges 1.9558(17)È1.9716(15) (1) and
1.944(4)È1.977(5) A (2)] and they compare well with the
average value reported in the parent heteroleptic
[Cr(phen)(ox) ]~ [1.948(2)È1.962(4) A],22,42 [Cr(bpm)(ox) ]~
IR characterisation
The most relevant features of the IR spectrum of free dpa are
the sharp peaks at 3250m (NÈH stretching), 1605s and 1566m
(NÈH deformation and C2N and C2C stretchings) and 768s
and 735m cm~1 (CÈH out-of-plane deformation). The shift
towards higher wavenumbers of the NÈH stretching of dpa in
the IR spectra of 1È5 [medium and broad absorption at 3435
(1), 3445 (2), 3440 (3), 3455 (4) and 3420 (5) cm~1] indicates
that it is coordinated.3 The broadening observed in all these
high frequency features suggests the presence of hydrogen
bonds in 1È5,41 as conÐrmed by X-ray di†raction. The out-of-
plane bending vibrations of the phenyl rings of the tetra-
phenylphosphonium (1) and tetraphenylarsonium (2) cations
occur at 690, 722 and 762 cm~1 for the former and at 693, 740
and 753 cm~1 for the latter. The occurrence of a medium
absorption at 1656 cm~1 in the IR spectrum of 3, which is
assigned to the NÈH deformation of the protonated amine of
dpa, is in agreement with the presence of Hdpa` in this
complex. In this respect, it deserves to be pointed out that this
peak is located at 1654 cm~1 in the IR spectrum of
2
2
[1.945(6)
],21 [Cr(hm)(ox) ]~ [1.974(2)
]43 and
A
A
2
[Cr(bpy)(ox) ]~ [1.953(2) A]20c mononuclear species (phen \
2
1,10-phenanthroline, bpm \ 2,2@-bipyrimidine, hm \ hista-
mine and bpy \ 2,2@-bipyridine). The CrÈN(dpa) bond
distances [2.0716(17) and 2.0581(17) A (1); 2.092(7) and
2.054(6) A (2)] are somewhat longer than the CrÈO(ox) ones.
The two ox ligands do not exhibit signiÐcant deviations from
planarity and they form dihedral angles of 68.85(6) (1) and
68.86(18)¡ (2). The carbonÈcarbon bond distance in the ox
HdpaClO . The peaks of the the nitronyl nitroxide radical in
ligands is as expected for
a single CÈC bond [C(1)È
4
the IR spectra of 4 (main peaks at 1638, 1563, 1421, 1380,
C(2) \ 1.566(3) (1) and 1.533(11) A (2); C(3)ÈC(4) \ 1.550(3) (1)
1322, 1172 and 833 cm~1) are in the same positions as those
observed in the spectra of its iodide salt, suggesting that the
radical is uncoordinated in 4. Finally, as far as the oxalato
peaks are concerned, slight but signiÐcant di†erences are
observed between the IR spectra of 5 and those of 1È4: three
peaks at 1709s, 1680s and 1654s cm~1 (5) and at 1716s, 1700s
and 1680s cm~1 (1È4) [l (CO)]; two peaks at 1409s and 1264
as
cm~1 (5) and at 1360s and 1240m (1 and 2), 1380s and 1245m
(3) and 1370s and 1235m cm~1 (4) [l (CO)]; a sharp medium
s
intensity peak at 810 (5), and at 805 (1 and 2) and 800 cm~1 (3
and 4) [d(OCO)]. The di†erent coordination mode of oxalato
in 5 vs. that in 1È4 (see structural discussion) would account
for these spectral di†erences.
Description of the X-ray structures
PPh [Cr(dpa)(ox) ] (1) and AsPh [Cr(dpa)(ox) ] (2). The
4
2
4
2
crystal structures of the isostructural complexes 1 and 2 are
made up of discrete tris-chelated [Cr(phen)(ox) ]~ anions and
2
either PPh ` (1) (Fig. 1) or AsPh ` (2) cations (Fig. 2) that are
Fig. 2 Perspective view of compound 2 along with the atom num-
bering. The thermal ellipsoids are drawn at the 30% probability level
and the hydrogen atoms have been omitted for clarity.
4
4
held together by electrostatic forces and Van der Waals
interactions. Interestingly, the tris-chelated anions are
New J. Chem., 2001, 25, 1224È1235
1227

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Fig. 3 A view along the x axis of the chain arrangement of the [Cr(dpa)(ox) ]~ units in 1 connected by hydrogen bonds.
2
and 1.546(11) A (2)]. The average values for the peripheral and
inner CÈO bonds are 1.216(3) (1) [1.216(10) (2)] and 1.289(2)
(1) [1.298(9) A (2)], respectively, the greater double bond char-
acter of the free CÈO bonds accounting for the shorter values.
The dpa molecule acts as a bidentate ligand towards the chro-
mium atom through its two pyridine nitrogen atoms, N(1) and
N(2). The six-membered CrÈN(1)ÈC(9)ÈN(3)ÈC(10)ÈN(2)
chelate ring adopts the boat conformation with the Cr and
N(3) atoms 0.56 (1) [0.227(3) (2)] and 0.23 (1) [0.237(5) A (2)],
respectively, below the plane deÐned by the remaining four
atoms. The individual pyridine rings in the dpa ligand are
essentially planar [maximum deviation 0.021 A at C(9) in 1
and 2], but the ligand itself is not, with a 26.1(1) (1) [25.3(3)¡
(2)] dihedral angle between the planes deÐned by the pyridine
rings. This value is very close to that of the free dpa (23¡)7 and
this twist serves to relieve the steric hindrance within the
ligand. The bond distances and angles about chemically
equivalent atoms in the two pyridine rings are practically
identical and there is no distortion about the bridging nitro-
gen [N(3)ÈC(9) \ 1.374(3) (1) and 1.372(10) A (2); N(3)È
6.807(1) (1) and 6.8(1) A (2). No signiÐcant graphite-like
interactions are observed between the pyridine rings of adja-
cent [Cr(dpa)(ox) ]~ units. The chromiumÈchromium separa-
2
tion within the hydrogen bonded chain [CrÉ É ÉCr(a) \ 9.20(1)
(1) and 9.11(8) A (2)] is much longer than the shortest
chromiumÈchromium separation [CrÉ É ÉCr(b) \ 7.43(1) (1) and
7.43(9) A (2)]. The shortest CrÉ É ÉX distances are 6.467(1) (1)
and 6.506(1) A (2).
Hdpa[Cr(dpa)(ox) ] Æ 4H O (3). The structure of complex 3
2
2
consists of discrete [Cr(dpa)(ox) ]~ anions, monoprotonated
2
Hdpa` cations (see Fig. 5) and crystallisation water molecules,
which are held together by electrostatic forces, Van der Waals
interactions and an extensive network of hydrogen bonds
involving the water molecules, four oxalate oxygen atoms
[O(3), O(5), O(6) and O(8)] and the Hdpa` cation (see
end of Table 3). The hydrogen bonds within the
[Cr(dpa)(ox) ]~/Hdpa` ion pair are shown in Fig. 6.
2
As in 1 and 2, the environment of the chromium atom is
distorted octahedral, the largest deviations from the ideal
octahedron being caused by the narrow bites of the bidentate
dpa [86.53(11)¡] and ox ligands [81.95(10) and 82.26(9)¡ for
O(1)ÈCrÈO(2) and O(3)ÈCrÈO(4), respectively]. The four CrÈ
O(ox) bond distances vary in the range 1.962(2)È1.982(2) A
and they are somewhat shorter than the CrÈN(dpa) ones
[2.048(3) and 2.067(3) A]. The two ox ligands are planar and
they form a dihedral angle of 74.72(9)¡. The carbonÈcarbon
bond distances [1.553(5) and 1.527(5) A for C(11)ÈC(12) and
C(13)ÈC(14), respectively] and the average values for the per-
C(10) \ 1.375(3) (1) and 1.380(10) (2)]. The average values of
A
the intra-ring CÈC and CÈN bond distances are 1.376(4) (1)
[1.372(12) (2)] and 1.347(3) (1) [1.343(10) A (2)], respectively.
The bulky XPh ` cations [X \ P (1) and As (2)] exhibit the
4
expected tetrahedral shape with average XÈC bond distances
of 1.797(3) (1) and 1.919(8) A (2), and average CÈXÈC angles of
109.5(1) (1) and 109.5(4)¡ (2).
Interestingly, edge-to-face phenylÈphenyl interactions44
between the XPh ` cations occur along the y axis (see Fig. 4),
4
the XÉ É ÉX(b) (b \ [x, y ] 0.5, 1 [ z) distances involved being
Fig. 5 Perspective view of the Hpda` and [Cr(dpa)(ox) ]~ ions of
2
compound 3 with the atom numbering. The thermal ellipsoids are
drawn at the 30% probability level. The hydrogen atoms and the
water molecules have been omitted for clarity.
Fig. 4 A projection of the crystal packing of 2 through the x axis.
1228
New J. Chem., 2001, 25, 1224È1235

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Table 3 Selected bond lengths (A) and angles (¡)a for compound 3
Chromium environment
CrÈN(1)
CrÈO(1)
CrÈO(3)
2.048(3)
1.968(2)
1.982(2)
CrÈN(2)
CrÈO(2)
CrÈO(4)
2.067(3)
1.971(2)
1.962(2)
N(1)ÈCrÈN(2)
N(1)ÈCrÈO(3)
N(1)ÈCrÈO(4)
N(2)ÈCrÈO(2)
N(2)ÈCrÈO(4)
O(2)ÈCrÈO(1)
O(3)ÈCrÈO(1)
O(1)ÈCrÈO(4)
86.53(11)
173.29(9)
91.13(10)
170.02(10)
96.08(10)
81.95(10)
91.97(9)
N(1)ÈCrÈO(2)
N(1)ÈCrÈO(1)
N(2)ÈCrÈO(3)
N(2)ÈCrÈO(1)
O(2)ÈCrÈO(3)
O(2)ÈCrÈO(4)
O(3)ÈCrÈO(4)
90.42(10)
94.53(10)
95.25(10)
88.81(10)
88.86(10)
93.48(10)
82.26(9)
172.74(9)
dpa ligand
N(1)ÈC(1)
C(1)ÈC(2)
C(3)ÈC(4)
N(3)ÈC(5)
N(2)ÈC(6)
C(6)ÈC(7)
C(8)ÈC(9)
1.365(4)
1.358(5)
1.354(5)
1.385(4)
1.356(4)
1.392(5)
1.387(5)
N(1)ÈC(5)
C(2)ÈC(3)
C(4)ÈC(5)
N(3)ÈC(6)
N(2)ÈC(10)
C(7)ÈC(8)
C(9)ÈC(10)
1.341(4)
1.395(5)
1.400(5)
1.390(4)
1.360(4)
1.372(5)
1.369(5)
N(1)ÈC(1)ÈC(2)
C(2)ÈC(3)ÈC(4)
C(4)ÈC(5)ÈN(1)
C(4)ÈC(5)ÈN(3)
C(5)ÈN(3)ÈC(6)
N(3)ÈC(6)ÈC(7)
C(7)ÈC(8)ÈC(9)
C(9)ÈC(10)ÈN(2)
N(2)ÈC(6)ÈC(7)
122.8(3)
119.4(3)
120.9(3)
119.6(3)
127.4(3)
117.9(3)
118.9(3)
123.6(3)
121.4(3)
C(1)ÈC(2)ÈC(3)
C(3)ÈC(4)ÈC(5)
C(5)ÈN(1)ÈC(1)
N(1)ÈC(5)ÈN(3)
N(3)ÈC(6)ÈN(2)
C(6)ÈC(7)ÈC(8)
C(8)ÈC(9)ÈC(10)
C(10)ÈN(2)ÈC(6)
118.6(3)
119.9(3)
118.3(3)
119.5(3)
120.6(3)
120.1(3)
118.6(3)
117.2(3)
Hpda` cation
N(4)ÈC(15)
C(15)ÈC(16)
C(17)ÈC(18)
N(6)ÈC(19)
N(5)ÈC(20)
C(20)ÈC(21)
C(22)ÈC(23)
1.359(59)
1.346(6)
1.367(6)
1.357(5)
1.320(5)
1.392(5)
1.380(6)
N(4)ÈC(19)
C(16)ÈC(17)
C(18)ÈC(19)
N(6)ÈC(20)
N(5)ÈC(24)
C(21)ÈC(22)
C(23)ÈC(24)
1.340(5)
1.400(6)
1.397(5)
1.407(5)
1.336(5)
1.382(6)
1.364(6)
N(4)ÈC(15)ÈC(16)
C(16)ÈC(17)ÈC(18)
C(18)ÈC(19)ÈN(4)
C(18)ÈC(19)ÈN(6)
C(19)ÈN(6)ÈC(20)
N(6)ÈC(20)ÈC(21)
C(21)ÈC(22)ÈC(23)
C(23)ÈC(24)ÈN(5)
N(5)ÈC(20)ÈC(21)
120.5(4)
120.4(4)
118.7(3)
121.9(3)
129.5(3)
119.2(3)
118.7(4)
123.8(4)
123.6(3)
C(15)ÈC(16)ÈC(17)
C(17)ÈC(18)ÈC(19)
C(19)ÈN(4)ÈC(15)
N(4)ÈC(19)ÈN(6)
N(6)ÈC(20)ÈN(5)
C(20)ÈC(21)ÈC(22)
C(22)ÈC(23)ÈC(24)
C(24)ÈN(5)ÈC(20)
118.7(4)
119.3(4)
122.3(3)
119.3(3)
117.2(3)
118.0(4)
118.8(4)
117.1(4)
Hydrogen bondsb
D
A
DÉ É ÉA
2.642(4)
2.900(4)
2.85(2)
2.82(2)
2.89(2)
2.84(2)
2.76(2)
2.78(2)
2.80(2)
2.73(2)
nDÈHÉ É ÉA
150(5)
170(5)
N(4)
N(6)
O(10)
O(11)
O(12)
O(12)
O(9)
O(9)
O(9)
O(10)
N(5)
O(5)
O(5)
O(3)
O(6)
O(8)
O(10)
O(11)
O(12)
O(11)
a Estimated standard deviations in the last signiÐcant digits are given in parentheses. b D \ donor; A \ acceptor.
ipheral [1.221(4) A] and inner CÈO [1.293(4) A] bonds of the
oxalate ligands are in agreement with those observed in 1 and
2. The dpa molecule acts as a bidentate ligand towards the
chromium atom through its two pyridine nitrogen atoms, N(1)
and N(2) thus exhibiting the anti-anti conformation. The
resulting six-membered CrÈN(1)ÈC(5)ÈN(3)ÈC(6)ÈN(2) chelate
ring adopts the boat conformation with the Cr and N(3)
atoms 0.298(1) and 0.304(2) A, respectively, below the plane
deÐned by the remaining four atoms. The individual pyridine
rings in the dpa ligand are planar [maximum deviation
0.012(3) A at C(3)]. However, they form a dihedral angle of
32.57(15)¡, a value that is somewhat greater than those
observed for the dpa ligand in 1 and 2. The occurrence of
the monoprotonated Hdpa` species as a counterion in 3
allows the comparison of its structure and that of coord-
inated dpa in the same system for the Ðrst time (see Table
3). Previous structural reports on the complexes [Cu(dpa) -
2
(NCO)](SO ) É dpa É H O9f
and
[Mo (CF CO ) -
4 1@2
2
2
3
2 4
(CF CO H) ] É 2dpa14 revealed the coexistence of both coor-
dinated and free dpa molecules in the same compound, the
3
2
2
New J. Chem., 2001, 25, 1224È1235
1229

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Table 4 Selected bond lengths (A) and angles (¡)a for compound 4
Chromium environment
CrÈN(1)
CrÈO(1)
CrÈO(5)
2.049(2)
1.946(2)
1.977(2)
CrÈN(2)
CrÈO(3)
CrÈO(7)
2.070(2)
1.958(2)
1.9677(19)
N(1)ÈCrÈN(2)
N(1)ÈCrÈO(5)
N(1)ÈCrÈO(7)
N(2)ÈCrÈO(3)
N(2)ÈCrÈO(7)
O(3)ÈCrÈO(1)
O(5)ÈCrÈO(1)
O(1)ÈCrÈO(7)
86.24(9)
175.94(9)
94.23(9)
173.28(9)
94.23(9)
83.47(9)
91.58(9)
171.90(8)
N(1)ÈCrÈO(3)
N(1)ÈCrÈO(1)
N(2)ÈCrÈO(5)
N(2)ÈCrÈO(1)
O(3)ÈCrÈO(5)
O(3)ÈCrÈO(7)
O(5)ÈCrÈO(7)
90.62(9)
92.48(9)
93.97(9)
90.73(9)
89.59(9)
91.93(9)
81.71(8)
Radical cation
O(11)ÈN(11)
N(11)ÈC(21)
N(11)ÈC(22)
C(22)ÈC(23)
C(22)ÈC(24)
C(22)ÈC(25)
C(28)ÈC(29)
C(29)ÈC(30)
N(13)ÈC(30)
N(13)ÈC(31)
1.272(2)
1.353(4)
1.498(4)
1.516(5)
1.526(5)
1.557(4)
1.395(4)
1.366(5)
1.344(4)
1.476(4)
O(12)ÈN(12)
N(12)ÈC(21)
N(12)ÈC(25)
C(25)ÈC(26)
C(25)ÈC(27)
C(21)ÈC(28)
C(28)ÈC(33)
C(32)ÈC(33)
N(13)ÈC(32)
1.264(4)
1.355(4)
1.496(4)
1.513(5)
1.517(5)
1.454(4)
1.401(4)
1.358(5)
1.346(4)
O(11)ÈN(11)ÈC(21)
O(11)ÈN(11)ÈC(22)
C(21)ÈN(11)ÈC(22)
N(11)ÈC(22)ÈC(25)
N(11)ÈC(22)ÈC(23)
N(11)ÈC(22)ÈC(24)
C(23)ÈC(22)ÈC(25)
C(23)ÈC(22)ÈC(24)
N(11)ÈC(21)ÈC(28)
N(11)ÈC(21)ÈN(12)
C(21)ÈC(28)ÈC(33)
C(28)ÈC(33)ÈC(32)
C(33)ÈC(32)ÈN(13)
C(32)ÈN(13)ÈC(31)
C(30)ÈN(13)ÈC(32)
126.8(3)
120.0(2)
112.7(2)
101.1(2)
109.5(3)
106.4(3)
115.7(3)
109.0(3)
126.0(3)
108.1(3)
121.7(3)
120.4(3)
121.6(3)
119.8(3)
119.5(3)
O(12)ÈN(12)ÈC(21)
O(12)ÈN(12)ÈC(25)
C(21)ÈN(12)ÈC(25)
N(12)ÈC(25)ÈC(22)
N(12)ÈC(25)ÈC(26)
N(12)ÈC(25)ÈC(27)
C(26)ÈC(25)ÈC(22)
C(26)ÈC(25)ÈC(27)
N(12)ÈC(21)ÈC(28)
C(21)ÈC(28)ÈC(29)
C(29)ÈC(28)ÈC(33)
C(28)ÈC(29)ÈC(30)
C(29)ÈC(30)ÈN(13)
C(30)ÈN(13)ÈC(31)
126.2(3)
121.0(3)
112.7(3)
101.1(2)
106.4(3)
109.3(3)
114.1(3)
110.4(3)
126.0(3)
121.6(3)
116.7(3)
120.5(3)
121.3(3)
120.7(3)
a Estimated standard deviations in the last signiÐcant digits are given in parentheses.
free dpa adopting the anti-syn and anti-anti conformation
modes, respectively. The presence of the proton H(4) bound to
N(4) [1.17(6) A for H(4)ÈN(4)] and its hydrogen bond with
N(5) accounts for the anti-anti conformation of Hdpa` and
the quasi coplanarity of its two pyridine rings [the dihedral
angle between the N(4)ÈC(15)ÈC(16)ÈC(17)ÈC(18)ÈC(19) and
N(5)ÈC(20)ÈC(21)ÈC(22)ÈC(23)ÈC(24) mean planes is only
3.21(12)¡]. Bond distances and angles about chemically equiv-
alent atoms in dpa are practically identical whereas signiÐcant
di†erences are observed within Hpda`, which are attributed
to the asymmetry of the N(4)ÈH(4)É É ÉN(5) intramolecular
hydrogen bond: 1.320(5) [N(5)ÈC(20)] vs. 1.340(5) A [N(4)È
C(19)] and 1.336(5) [N(5)ÈC(24)] vs. 1.359(5) A [N(4)ÈC(15)],
for instance. Partial pÈp overlap between pyridine rings of
pairs of Hdpa` cations [3.82(4) A] and [Cr(dpa)(ox) ]~
2
anions [4.00(2) ] along the x axis are observed (see Fig. 7).
A
The shortest chromium-chromium separation is 6.32(4) A
[CrÉ É ÉCr(a) with a \ [x, [y, 1 [ z].
Rad[Cr(dpa)(ox) ] Æ H O (4). The structure of 4 consists of
2
2
[Cr(dpa)(ox) ]~ anions, p-N-methylpyridinium nitronyl nitr-
2
oxide radicals (Rad`) and a water molecule of crystallisation
(see Fig. 8). Hydrogen bonds involving the uncoordinated
water molecule (O), the amine nitrogen atom of dpa [N(3)]
and three oxalate oxygens [O(4), O(6) and O(7)] [2.855(4),
2.893(4) and 2.899(5) A for OÉ É ÉO(4a), N(3)É É ÉO(6b) and
OÉ É ÉO(7), respectively with 167(4), 175(4) and 172(3)¡ for OÈ
H(20)É É ÉO(7), OÈH(10)É É ÉO(4a) and N(3)ÈH(3N)É É ÉO(6b);
symmetry codes: a \ 3/2 [ x, 1/2 [ y, 2 [ z and b \ x,
1 [ y, 1/2 ] z] provide additional stability to the lattice.
The chromium environment is distorted octahedral as in
1È3, the main distortion being caused by the short bite of the
bidentate dpa [86.24(9)¡ for N(1)ÈCrÈN(2)] and ox ligands
[83.47(9) and 81.71(8)¡ for O(3)ÈCrÈO(1) and O(5)ÈCrÈO(7),
respectively]. The pyridyl rings of dpa are planar [largest
deviations are 0.17 A at C(3) for one ring and 0.28 A at C(9)
for the second ring] but the ligand as a whole is not [the
Fig. 6
A view of the hydrogen bonding between Hdpa` and
[Cr(dpa)(ox) ]~ ions in 3.
2
1230
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O(11)ÈN(11)ÈC(21)ÈN(12)ÈO(12)
p
system containing the
unpaired electron. This is a rather small value compared with
those generally reported in substituted nitronyl nitr-
oxides.23,45h49 The shortest distance between the two radical
cations is 3.750(7) A for O(11)É É ÉN(11c) (symmetry code
c \ 1 [ x, y, 1/2 [ z). The dihedral angle between the two
NO groups is 34.7¡. From previous theoretical work, it is
known that short contacts between two NO groups may lead
to noticeable magnetic interactions.47 Their magnitude and
sign rely both on the separation and the relative orientation of
the p* orbitals that contain the unpaired electrons. In com-
pound 4, the distance between the two NO groups and their
relative orientation are far from the optimum 3 in a head-
A
to-tail arrangement. This is assumed not to favour a strong
overlap, and hence only a weak antiferromagnetic interaction
is expected to occur between the radicals. In addition, the
shortest distance between the radical cation and the chro-
mium complex involves one of the NO groups [N(12)ÈO(12)]
and the O(2) oxalato oxygen atom [3.410(4)
A
for
N(12)É É ÉO(2)]. These two contacts lead to a discrete Cr Rad
2
2
unit, which is depicted in Scheme 2. The overall distance
separating the NO group from the Cr(III) is 4.65(1) A and the
shortest metalÈmetal separation is 6.793(1) A. A very weak
pÈp interaction between the pyridyl rings of dpa ligands of
adjacent [Cr(dpa)(ox) ]~ units is also observed, the signiÐcant
Fig. 7 A projection of the crystal packing of 3 through the x axis.
2
slipping of the rings leading to a very slight overlap. Finally,
there is a very weak overlap between the pyridine rings of two
adjacent radicals, the separation between the two centres of
the pyridyl rings being 4.29 A. Given that the spin density on
the pyridine rings is extremely weak, the magnetic interactions
through this pathway is neglected.
dihedral angle between the two pyridyl rings is 26.98¡]. As in
1È3, the six-membered CrÈN(1)ÈC(5)ÈN(3)ÈC(6)ÈN(2) chelate
ring adopts the boat conformation with the Cr and N(3)
atoms being 0.764 and 0.218 A, respectively, below the plane
deÐned by the remaing four atoms. The two ox ligands are not
strictly planar and the oxygen atoms are slightly twisted
around the CÈC bond so that the carbon atoms for both ox
ligands are in the mean plane of the six atoms but the oxygen
atoms are ^0.60 A away in one ox ligand and ^0.50 A away
in the second one. This results in dihedral angles between the
two OÈCÈO planes of 5.22¡ at one ox ligand and 6.60¡ at the
other one. The dihedral angle between the mean planes of the
two ox ligands is 76.07¡. The carbonÈcarbon bond distances
[1.533(4) and 1.540(4) A for C(11)ÈC(12) and C(13)ÈC(14),
respectively] and the main values for the peripheral [1.215(4)
A] and inner CÈO [1.297(4) A] bonds of the ox ligands are
comparable with those observed in 1È3.
Sr[Cr(dpa)(ox) ] Æ 8H O (5). The structure of compound 5
2 2
2
is made up of crystallographically independent hetero-
trinuclear species of formula MSr[Cr(dpa)(ox) ] N (Fig. 9) that
2 2
grow along the x axis through oxalato bridges, yielding
neutral double zigzag chains (Fig. 10). Each chain is formed of
diamond shaped units sharing the strontium atoms, while the
other two corners are occupied by chromium atoms having
opposite chirality. Adjacent double chains are related by an
inversion centre leading to an achiral arrangement in the xz
plane (Fig. 10). An extensive network of hydrogen bonds (see
end of Table 5) involving all the water molecules, the dpa
amine nitrogen atoms and nine of the sixteen oxalate oxygens
contributes to the stabilisation of the structure. The interchain
pÈp interactions are very weak, the shortest separation
between pyridyl rings of dpa ligands being ca. 4 A.
The two crystallographically independent chromium atoms
[Cr(1) and Cr(2)] are six-fold coordinated with two nitrogen
atoms from a bidentate dpa ligand and four oxygen atoms
from two bis-bidentate oxalate groups building a distorted
octahedron around each metal atom. The CrÈN bond lengths
[mean value 2.05 A] are somewhat longer than the CrÈO ones
The bond distances and angles found for the radical cation
agree with previous Ðndings for N-alkylpyridinium nitronyl
nitroxide radicals.23,45,46 The pyridyl ring is planar and it
makes a dihedral angle of 8.96¡ with the mean plane of the
Fig. 8 Perspective view of compound 4 along with the atom num-
bering. The thermal ellipsoids are drawn at the 30% probability level.
Scheme 2 Exchange coupling patterns within the Cr Rad unit.
2
2
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Fig. 9 Perspective drawings of the crystallographically independent unit of compound 5 with the atom numbering. The thermal ellipsoids are
drawn at the 30% probability level. The hydrogen atoms and water molecules have been omitted for clarity.
[average value 1.97 A]. The angles subtended at the chro-
mium atom by the dpa [values of 86.4(13) to 87.5(4)¡] and ox
[values varying in the range 82.3(3)È82.8(3)¡] ligands are in
agreement with those observed in the previous structures. The
dpa ligands exhibit the anti-anti conformation and they are
bound to the chromium atoms through the pyridine nitrogen
atoms. The pyridyl rings of the dpa ligands are planar as
expected but the entire dpa ligands deviate signiÐcantly from
planarity [37.2(6) and 21.2(6)¡ for the dpa ligands at Cr(1) and
Cr(2), respectively]. The six-membered chelate rings made by
the bidentate dpa groups at each chromium atom adopt the
boat conformation with signiÐcant deviation of the chromium
with divalent strontium.50 The OÈSrÈO bite angles of the bis-
bidentate oxalato ligands are very acute [values varying in the
range 61.8(3)È63.1(3)¡]. The carbonÈcarbon bond lengths of
the oxalate ligands agree with those observed in 1È4 [CÈC
bond distance varying between 1.543(16) and 1.559(18) A].
The CÈO bonds within each oxalato are signiÐcantly di†erent
Mthe shortest and longest values being 1.206(14)
[C(12)È
A
O(4)] and 1.279(14) A [C(25)ÈO(9)]N as expected due to the
di†erent nature and charge of the cations they bridge.
The intrachain metalÈmetal separations are 5.925(3)
[Cr(1)É É ÉSr(1)], 5.962(3) [Cr(2)É É ÉSr(1)], 7.245(4) [Cr(1)
É É ÉCr(2)] and 9.457(2) [Sr(1)É É ÉSr(1a); a \ 1 ] x, y, z]. The
A
atoms [0.295(5) and 0.270(5)
A for Cr(1) and Cr(2),
shortest homo- and heterometallic interchain metalÈmetal dis-
respectively] and the corresponding amine nitrogens M0.261(8)
[N(3)] and 0.242(8) A [N(6)]N from the plane deÐned by the
remaining four dpa atoms. The oxalato ligands are planar and
they form dihedral angles of 78.9(3) [Cr(1)] and 80.6(3)¡
[Cr(2)].
The strontium atom is eight-fold coordinated by eight
oxygens of four bis-bidentate oxalate ligands. The SrÈO bond
distances vary in the range 2.607(10)È2.729(10) A, values which
are in agreement with those observed in reported structures
tances are 6.731(5) [Cr(1)É É ÉCr(1b); b \ [x, [y, 1 [ z],
6.445(6) [Cr(2)É É ÉCr(2c); c \ 1 [ x, [y, [z] and 10.291(5)
[Sr(1)É É ÉSr(1d); d \ 1 [ x, 1 [ y, 1 [ z].
A
Magnetic properties of 1–5
The temperature dependence of s T for 1È5 [s being the
M
M
magnetic susceptibility per mol of chromium(III)] in the tem-
perature range 1.9È100 K is shown in Fig. 11. At 290 K, the
values of s T are 1.87 (1È3 and 5) and 2.24 (4) cm3 K mol~1.
M
These values are as expected for a magnetically isolated spin
quartet (S \ 3/2; 1È3 and 5) and for a pair of non-
Cr
interacting quartet (S \ 3/2) and doublet (S \ 1/2) spin
Cr
rad
states (4). The value of s T for 1È3 and 5 remains practically
constant down to 30 K and it exhibits a small decrease at
M
lower temperatures to reach 1.63 (1), 1.74 (2), 1.61 (3) and 1.80
Fig. 11 s T vs. T plots for complexes 1È5. The solid line for 4 corre-
M
sponds to the best Ðt using eqn. (2) (see text). The inset shows the the
low temperature region in detail.
Fig. 10 View of the double chain arrangement of 5 in the xz plane.
1232
New J. Chem., 2001, 25, 1224È1235

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Table 5 Selected bond lengths (A) and angles (¡)a for compound 5
Chromium environment
Cr(1)ÈN(1)
Cr(1)ÈN(2)
Cr(1)ÈO(1)
Cr(1)ÈO(2)
Cr(1)ÈO(5)
Cr(1)ÈO(6)
2.063(9)
2.044(9)
1.978(8)
1.987(8)
1.968(8)
1.954(7)
Cr(2)ÈN(4)
Cr(2)ÈN(5)
Cr(2)ÈO(9)
Cr(2)ÈO(10)
Cr(2)ÈO(13)
Cr(2)ÈO(14)
2.035(9)
2.058(10)
1.974(8)
1.969(7)
1.971(8)
1.976(8)
N(1)ÈCr(1)ÈN(2)
N(1)ÈCr(1)ÈO(2)
N(1)ÈCr(1)ÈO(5)
N(1)ÈCr(1)ÈO(1)
N(1)ÈCr(1)ÈO(6)
N(2)ÈCr(1)ÈO(5)
N(2)ÈCr(1)ÈO(2)
N(2)ÈCr(1)ÈO(1)
N(2)ÈCr(1)ÈO(6)
O(2)ÈCr(1)ÈO(5)
O(2)ÈCr(1)ÈO(1)
O(2)ÈCr(1)ÈO(6)
O(5)ÈCr(1)ÈO(1)
O(5)ÈCr(1)ÈO(6)
O(1)ÈCr(1)ÈO(6)
86.4(4)
92.9(4)
175.1(3)
92.7(4)
92.5(3)
92.4(4)
177.7(4)
95.6(4)
91.7(3)
88.5(4)
82.3(3)
90.6(3)
92.1(4)
82.8(3)
171.3(3)
N(4)ÈCr(2)ÈN(5)
N(2)ÈCr(2)ÈO(14)
N(4)ÈCr(2)ÈO(10)
N(4)ÈCr(2)ÈO(9)
N(4)ÈCr(2)ÈO(13)
N(5)ÈCr(2)ÈO(10)
N(5)ÈCr(2)ÈO(14)
N(5)ÈCr(2)ÈO(9)
N(5)ÈCr(2)ÈO(13)
O(10)ÈCr(2)ÈO(14)
O(10)ÈCr(2)ÈO(9)
O(10)ÈCr(2)ÈO(13)
O(14)ÈCr(2)ÈO(9)
O(14)ÈCr(2)ÈO(13)
O(9)ÈCr(2)ÈO(13)
87.5(4)
93.5(4)
92.4(4)
90.6(4)
175.9(4)
91.9(3)
94.4(4)
174.1(4)
94.0(4)
171.5(3)
82.6(3)
91.4(3)
91.3(4)
82.6(3)
88.2(4)
Strontium environment
Sr(1)ÈO(3)
2.607(10)
2.729(10)
2.631(10)
2.638(8)
Sr(1)ÈO(11)
Sr(1)ÈO(12)
Sr(1)ÈO(15b)
Sr(1)ÈO(16b)
2.658(10)
2.671(9)
2.705(9)
2.659(9)
Sr(1)ÈO(4)
Sr(1)ÈO(7a)
Sr(1)ÈO(8a)
O(3)ÈSr(1)ÈO(7a)
O(3)ÈSr(1)ÈO(8a)
O(3)ÈSr(1)ÈO(16b)
O(3)ÈSr(1)ÈO(11)
O(3)ÈSr(1)ÈO(12)
O(3)ÈSr(1)Èo(15b)
O(3)ÈSr(1)ÈO(4)
O(7a)ÈSr(1)ÈO(8a)
O(7a)ÈSr(1)ÈO(16b)
O(7a)ÈSr(1)ÈO(11)
O(7a)ÈSr(1)ÈO(12)
O(7a)ÈSr(1)ÈO(15b)
O(7a)ÈSr(1)ÈO(4)
O(8a)ÈSr(1)ÈO(16b)
87.4(3)
70.0(3)
125.7(3)
142.4(3)
141.9(3)
76.6(3)
61.8(3)
63.1(3)
144.7(3)
70.9(3)
75.3(3)
125.1(3)
144.2(3)
134.4(3)
O(8a)ÈSr(1)ÈO(11)
O(8a)ÈSr(1)ÈO(12)
O(8a)ÈSr(1)ÈO(15b)
O(8a)ÈSr(1)ÈO(4)
O(16b)ÈSr(1)ÈO(11)
O(16b)ÈSr(1)ÈO(12)
O(16b)ÈSr(1)ÈO(15b)
O(16b)ÈSr(1)ÈO(4)
O(11)ÈSr(1)ÈO(12)
O(11)ÈSr(1)ÈO(15b)
O(11)ÈSr(1)ÈO(4)
O(12)ÈSr(1)ÈO(15b)
O(12)ÈSr(1)ÈO(4)
O(15b)ÈSr(1)ÈO(4)
72.9(3)
126.5(3)
145.3(3)
87.9(3)
85.2(3)
70.7(3)
61.8(3)
70.8(3)
62.4(3)
141.0(3)
122.6(3)
86.0(3)
140.4(3)
67.9(3)
Possible hydrogen bonds
N(3)É É ÉO(17c)
N(6)É É ÉO(18)
2.86(2)
2.82(2)
2.786(19)
2.860(18)
2.94(2)
2.94(2)
2.900(13)
2.91(2)
O(9)É É ÉO(20)
2.853(14)
3.04(2)
2.86(2)
2.92(2)
2.99(2)
2.735(13)
2.98(2)
2.94(2)
O(11)É É ÉO(23)
O(11)É É ÉO(24)
O(15)É É ÉO(19b)
O(15)É É ÉO(22d)
O(16)É É ÉO(18)
O(16)É É ÉO(24d)
O(18)É É ÉO(23b)
O(3)É É ÉO(17)
O(3)É É ÉO(22)
O(4)É É ÉO(24)
O(4)É É ÉO(20b)
O(5)É É ÉO(19)
O(7)É É ÉO(22b)
O(8)É É ÉO(21)
2.853(16)
a Estimated standard deviations in the last signiÐcant digits are given in parentheses. b Symmetry codes: a \ x ] 1, y, z; b \ x [ 1, y, z; c \ x,
2 [ y, 1 [ z; d \ x [ 2, y, z.
(5) cm3 K mol~1 at 1.9 K. Zero-Ðeld splitting e†ects (D)
data through the isotropic Hamiltonian of eqn. (2):
and/or very weak magnetic interactions between the local spin
H \ [J[S É S
Cr1 rad1
] S É S ] [ J@[S
É S
]
Cr2 rad2
rad1 rad2
quartets would account for the small decrease in s T of 1È3
M
and 5 in the low temperature region. This is not surprising in
] g HbS ] g HbS ] g HbS
Cr1
Cr1
HbS
Cr2
Cr2
rad1
rad1
the light of their structures [mononuclear Cr(III) units in 1È3
and oxalato-bridged bimetallic Cr(III)ÈSr(II) chains in 5]. It
can also be concluded that the efficiency of the NÈHÉ É ÉO(ox)
hydrogen bonding pattern of 1 and 2 in mediating magnetic
interactions between the Cr(III) ions is extremely low, if any.
] g
] D [S 2 [ 1/3S(S ] 1)]
(2)
rad2
rad2
Cr
z
where J and J@ refer to the exchange coupling parameters
within the CrÈrad and radÈrad pairs, g \ g
and g
\
Cr1
Cr2
rad1
g
are the Lande factors for the Cr(III) and radical centres,
rad2
Interestingly, s T for 4 exhibits a signiÐcant decrease at
while the last term accounts for an axial zero-Ðeld splitting for
the Cr(III) ion with S \ 3/2. No theoretical analytical expres-
sion is known for this Hamiltonian and the theoretical
analysis of the magnetic data of 4 has been performed by
means of the VPMAG program.51 The energy matrix is built
for this tetrameric entity considering all the microstates for
two close and very small values of the external magnetic Ðeld
M
T \ 100 K and it reaches a value of 1.25 cm3 K mol~1 at 1.9
K. Such a value is clearly below those of complexes 1È3 and 5,
indicating that signiÐcant antiferromagnetic interactions occur
between the spins of the chromium and radical cations.
Taking into account the Cr(1)Èrad(1)Èrad(2)ÈCr(2) tetrameric
entity present in 4 (Scheme 2), we have analysed its magnetic
New J. Chem., 2001, 25, 1224È1235
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(H). This allows us to calculate the magnetic susceptibility at
pair (Scheme 2), one can see how the polarisation of the spin
zero Ðeld as s \ dM/dH where M is the magnetisation. The
density of the neighbouring NO group by that of the 2p
M
z
best agreement between the experimental and calculated sus-
orbital of the oxalate oxygen through the p pathway is
ceptibility data is obtained for J \ [3.1 cm~1, J@ \ [12.5
symmetry-adapted and it accounts for the weak but signiÐcant
antiferromagnetic coupling observed in the CrÈrad pair.
In summary, the new building block of formula
cm~1, g \ 1.97 and g \ 2.01, o D o \ 0.91 cm~1 with
Cr
rad
R \ 3.1 ] 10~5 MR is the agreement factor deÐned as
& [(s T ) (i) [ (s T ) (i)]2/& [(s T ) (i)]2). The computed
[Cr(dpa)(ox) ]~ has been prepared and structurally character-
i
M
obs
M
calc
i
M
obs
2
values of the g factors and D are physically reasonable and the
calculated curve (solid line in Fig. 11) matches the magnetic
data very well.
ised with the diamagnetic XPh ` [X \ P (1) and As (2)] and
4
the unprecedented monoprotonated Hdpa` (3) species as
counterions. The use of a radical cation as counterion
(complex 4) allowed us to observe signiÐcant antiferromag-
netic interactions through space in the Cr(III)Èrad and radÈrad
pairs. Finally, the structure of the bimetallic double zig-zag
chain occurring in 5 shows the potential versatility of the
The values of the antiferromagnetic interactions for both
the Cr(III)Èrad and radÈrad pairs in 4 are small but signiÐcant.
In order to substantiate both the nature and magnitude of
these magnetic interactions, we have carried out density func-
tional theory (DFT) calculations on the real structures of the
radÈrad and Cr(III)Èrad pairs present in 4 (Scheme 2) and also
on the isolated radical and mononuclear chromium(III)
complex. The calculated values of J ([5 cm~1) and J@ ([18.2
cm~1) agree both in sign and magnitude with those obtained
[Cr(dpa)(ox) ]~ unit to generate interesting self-assembling
2
frameworks with divalent cations, which could exhibit inter-
esting magneto-optical properties. Further work using this
building block with paramagnetic metal ions is currently
under way.
from the Ðt. Spin density calculations on the [Cr(dpa)(ox) ]~
2
[Scheme 3(a)] and Rad` [Scheme 3(b)] units allowed us to
Acknowledgements
understand the nature of the magnetic interactions through
the intermolecular contacts in 4. In the case of the radical
[Scheme 3(b)], a positive spin density of ca. one-half electron
is distributed on each NO unit and a negative spin density of
ca. 1/5 electron is located on the carbon atom that is bound to
both NO fragments. The negative spin density on this carbon
atom, as well as the others that appear in the system, are due
to the spin polarization mechanism. In such a mechanism, one
atom having a spin density tends to induce a spin density of
the opposite sign on the neighbouring atoms. The same
mechanism is operative through space (intermolecular
interactions) and thus, the NO group of one radical unit in 4
polarizes the spin density of the NO group of the neighbour-
ing unit, which is in a quasi-eclipsed conformation (see
Scheme 2) and the antiferromagnetic coupling results in the
radÈrad pair. An analogous situation occurs in the CrÈrad
pair. The three unpaired electrons of the Cr(III) ion in octa-
Financial support from the Spanish Direccion General de
Investigacion Cient•Ðca y Tecnica (DGICYT) through Pro-
jects PB97-1397 and PB98-1044, the Romanian Ministry of
Education (Grants 30C/CNCSIS and 1D/CNCSIS) and the
TMR Program from the European Union (Contract
ERBFMRXCT98-0181) is acknowledged. Thanks are also
extended to the Centre dÏInformatica de la Universitat de
Valencia for computer resource support.
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9
Scheme 3 Spin density maps of the Cr (a) and Rad (b) units.
Opposite signs of spin density values are denoted by the white and
shaded areas.
Scheme 4
1234
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