A homoleptic, self-assembled [2 × 2] square Cu4 complex exhibiting intramolecular ferromagnetic exchange

Article abstract of DOI:10.1039/b201076n

A carboxylic acid-substituted picolinic hydrazone ligand (pcoap) forms a homoleptic [2 × 2] tetranuclear Cu(II)₄ complex with four six-coordinate Cu(II) centers bridged by μ-O alkoxide groups in a square grid structure. The terminal carboxylic groups complete the six-coordinate structures at each copper centre, capping off external coordination positions, with two functioning as carboxylates, and two in neutral carboxylic acid form. Despite very large Cu-O-Cu bridge angles (139.2-140.6°), and short Cu-Cu distances (3.97-4.05 A), which would normally be expected to propagate antiferromagnetic exchange interactions within the grid, strictly orthogonal bridging connections between the metal centers lead to intramolecular ferromagnetic exchange (J = 7.2(3) cm^-1), and an S = 4/2 magnetic ground state.

Full text of DOI:10.1039/b201076n

View Article Online / Journal Homepage / Table of Contents for this issue
A homoleptic, self-assembled [2 × 2] square Cu complex
4
exhibiting intramolecular ferromagnetic exchange
Zhiqiang Xu, Laurence K. Thompson* and David O. Miller
Department of Chemistry, Memorial University of Newfoundland, St. John’s, Newfoundland,
A1B 3X7, Canada. E-mail: lthomp@mun.ca
Received 29th January 2002, Accepted 12th April 2002
First published as an Advance Article on the web 14th May 2002
A carboxylic acid-substituted picolinic hydrazone ligand (pcoap) forms a homoleptic [2 × 2] tetranuclear Cu()4
complex with four six-coordinate Cu() centers bridged by µ-O alkoxide groups in a square grid structure. The
terminal carboxylic groups complete the six-coordinate structures at each copper centre, capping off external
coordination positions, with two functioning as carboxylates, and two in neutral carboxylic acid form. Despite
very large Cu–O–Cu bridge angles (139.2–140.6Њ), and short Cu–Cu distances (3.97–4.05 Å), which would
normally be expected to propagate antiferromagnetic exchange interactions within the grid, strictly orthogonal
Ϫ1
bridging connections between the metal centers lead to intramolecular ferromagnetic exchange (J = 7.2(3) cm ),
and an S = 4/2 magnetic ground state.
Introduction
exhibit intramolecular ferromagnetic coupling. This contrasts
markedly with the report of a comparable phenoxide-bridged
2 × 2] Cu() square grid with orthogonal bridging con-
Ligands based on a picolinic hydrazide framework, e.g. poap
[
(
Scheme 1), have two contiguous coordination pockets juxta-
nections, which was shown to exhibit weak intramolecular
4
antiferromagentic coupling.
Each ligand, e.g. poap, provides four donor atoms and fills
five metal ion coordination positions. As a consequence, for six-
coordinate metal ions, four additional ligands are required at
external coordination positions (Scheme 1). These are usually
Ϫ
solvent (e.g. water molecules) or anions (e.g. NO ). In order
3
to encapsulate the four-metal grid in a homoleptic arrangement
of ligands, one additional coordination site per metal needs to
be satisfied by a suitably positioned ligand donor group. The
ligand pcoap (Scheme 1) has a carboxylic acid group at the
pyridine 6-position, and would form an additional five-
membered chelate ring, thus extending the two coordination
pockets to include five donor atoms (Scheme 1). The synthesis
of this unusual unsymmetric ligand will be described, based on
the differing solubilities of mono- and di-substituted inter-
mediates (Scheme 2), and its [2 × 2] homoleptic square Cu()4
complex discussed. The related fully di-substituted ligand
2
poap (Scheme 1) is tritopic in nature, and self-assembles to
5–9
produce novel square [3 × 3] M grids.
9
Experimental
Physical measurements
Infrared spectra were recorded from Nujol mulls using a
Mattson Polaris FT-IR instrument. Microanalyses were carried
out by Canadian Microanalytical Service, Delta, Canada. UV/
vis spectra were collected from dispersed mulls using a Cary 5E
spectrometer. Mass spectra were obtained using a VG Micro-
mass 7070HS spectrometer. Variable temperature magnetic
data (2–300 K) were obtained using a Quantum Design
MPMS5S SQUID magnetometer with field strengths in the
range 0.1 to 5.0 T. Samples were prepared in aluminium pans,
and mounted inside straws for attachment to the sample trans-
port rod. Background corrections for the sample holder
assembly and diamagnetic components of the complexes were
applied.
Scheme 1
posed conveniently to self assemble transition metal ions M()
M᎐Mn(), Fe(), Co(), Ni(), Cu(), Zn()) into non-
homoleptic homo- and hetero-tetranuclear [2 × 2] square grids,
with alkoxide oxygen atoms bridging the metals in close prox-
imity and metal–metal separations around 4 Å. The large
M–O–M bridge angles lead to antiferromagnetic exchange in
the case of Mn(), Ni() and Co() homo-tetranuclear
systems, but as a result of strict magnetic orbital orthogonality
within the square grids containing Cu(), the copper systems
(
᎐
1–3
2
462
J. Chem. Soc., Dalton Trans., 2002, 2462–2466
This journal is © The Royal Society of Chemistry 2002
DOI: 10.1039/b201076n

View Article Online
Scheme 2
Table
Synthesis of pcoap (Scheme 2)
-Methyloxycarbonyl-2-pyridinecarboxylic acid hydrazide.
1
Crystal data and structure refinement parameters for
compound 1
6
Anhydrous hydrazine (98%, 3.27 g, 0.100 mol in 150 mL
methanol) was added dropwise to a stirred warm solution
of dimethyl-2,6-pyridinedicarboxylate (19.8 g; 0.102 mol in
Empirical formula
Formula weight
Temperature/K
Wavelength/Å
Crystal system
Space group
a/Å
C H N O Cu
52 68 22 33 4
1783.42
190(1)
0.71073
Tetragonal
5
00 mL methanol) over a period of 18 h. A white crystalline
solid formed during the reaction, which was filtered off, washed
with methanol and dried under vacuum. This turned out to
be a mixture of the major product, 6-methyloxycarbonyl-2-
pyridinecarboxylic acid hydrazide (so-called mono-hydrazide),
and a minor product, 2,6-pyridinedicarboxylic acid dihydrazide
P4 (no. 78)
3
13.7944(7)
40.546(4)
7715.3(7)
4
1.535
11.58
c/Å
V/Å
3
Z
Ϫ3
Dcalc/g cm
µ/cm
(
so-called dihydrazide). It is possible to separate out the di-
Ϫ1
hydrazide from the mixture to obtain pure mono-hydrazide by
dissolving the mixture in hot water (the dihydrazide is soluble in
hot water but the monohydrazide is not). However, it is more
convenient to separate the byproduct after the next step in the
overall reaction.
R1
wR2
0.090
0.243
a
2
o
2
c
2
2
o
2
1/2
R = Σ||F | Ϫ |F |/Σ|F |, wR = {Σ[w(F Ϫ F ) ]/Σ[w(F ) ]}
.
1
o
c
o
2
Cu(NO ) ؒ6H O (0.76 g, 2.0 mmol) in 5 mL water and 5 mL
3
2
2
epoap. The methyl ester of iminopicolinic acid was prepared
in situ by reaction of 2-cyanopyridine (12.6 g, 0.120 mol)
with sodium methoxide solution, produced by dissolving
sodium metal (0.23 g, 0.010 mmol) in dry methanol (150 mL).
methanol. The resulting mixture was stirred for a few minutes,
producing a clean deep green solution, which was filtered.
Green crystals formed from the filtrate after several days at
Ϫ1
room temperature. Yield: 81%. IR (Nujol) νmax/cm : 3438(w)
7
.0 g of crude 6-methyloxycarbonyl-2-pyridinecarboxylic acid
(
(
ν H O); 3340(w), 3180(w) (ν NH ); 1696 (ν C᎐N); 1626
2
2
hydrazide (containing small amount of 2,6-pyridine-
a
ν COO); 1015(m) (py); 1760(w) (ν NO ). UV/vis λ/nm: 380,
3
dicarboxylic acid dihydrazide) was added to the above solution,
followed by a few drops of glacial acetic acid, and the mixture
was refluxed for 2 h. A light yellow powder was obtained, which
was filtered off and washed with methanol. Yield: 2.58 g. This
proved to be 2poap, the disubstituted iminoester condensation
product (Scheme 1). 10.5 g of yellow crystals of epoap were
obtained after removal of most of the solvent (methanol) and
4
20(sh), 450(sh), 730, 920(br). Elemental analysis on vacuum
dried sample, found: C, 37.01; H, 2.91; N, 18.92; calc. for (C -
H N O ) (C H N O ) Cu (NO ) (H O) : C, 37.69; H, 3.28;
N, 18.60%.
13
10
5
3
2
13
9
5
3
2
4
3
2
2
8
X-Ray crystallography
Crystallographic data and refinement of the structure. The
diffraction intensities of a green prismatic crystal of 1 were
collected with graphite monochromatized Mo-Kα X-radiation
addition of about 10 mL diethyl ether. Mp: 199–201 ЊC. MS
ϩ
(
m/z): 300 (M ), 299 (M Ϫ 1), 282, 163, 107.
(
rotating anode generator) using a Bruker P4/CCD dif-
pcoap. 3.0 g (0.01 mol) of epoap was refluxed with a solution
fractometer at 193(1) K to a maximum 2θ value of 52.8Њ. The
data were corrected for Lorentz and polarization effects. The
structure was solved by direct methods.
containing 1.0 g NaOH, 20 mL H O and 10 mL EtOH, for
2
4
–5 h. The volume of the clean yellowish solution was reduced
10,11
All atoms except
to about 15 mL, and it was allowed to cool down to room
temperature and neutralized with concentrated HCl. A pale
yellow precipitate was collected by filtration and washed with
cold water, EtOH and diethyl ether. Yield: 1.8 g (63%). Mp:
hydrogens were refined anisotropically. Two hydrogen atoms
attached to carboxylate oxygen atoms O(9) and O(12)] were
[
located in a difference map. Other hydrogen atoms were placed
in calculated positions with isotropic thermal parameters set to
20% greater than their bonded partners, and were not refined.
Neutral atom scattering factors and anomalous-dispersion
terms
3
1
(
20 ЊC (dec.). MS (m/z): 283 (M Ϫ 2), 267 (M Ϫ H O), 223, 194,
2
Ϫ1
69, 105. IR (Nujol) νmax/cm : 3405 (ν OH); 3328, 3264
ν NH ); 2514–2613 (ν NH ); 1702 (ν COOH); 1655 (ν C᎐N);
12
ϩ
13,14
2
were taken from the usual sources. All other
9
97 (py). Elemental analysis on dried sample, found: C, 47.61;
15
calculations were performed with the teXsan crystallographic
software package using a PC.
H, 3.94; N, 21.93; calc. for C H N O ؒ2.3H O: C, 47.79; H,
13
11
5
3
2
4
.81; N, 21.44%.
Selected crystal data for 1 are given in Table 1.
CCDC reference number 178403.
See http://www.rsc.org/suppdata/dt/b2/b201076n/ for crystal-
lographic data in CIF or other electronic format.
Synthesis of [Cu (pcoap ؊ H) (pcaop ؊ 2H) ](NO ) ؒ15H O (1)
4
2
2
3
2
2
pcoap (0.29 g, 1.0 mmol) was added to a warm solution of
J. Chem. Soc., Dalton Trans., 2002, 2462–2466
2463

View Article Online
Table 2 Bond distances (Å) and angles (Њ) for 1
Results and discussion
Structure of [Cu (pcoap ؊ H) (pcaop ؊ 2H) ](NO ) ؒ15H O (1)
N3–Cu1
N20–Cu1
O1–Cu1
N1–Cu1
O10–Cu1
O11–Cu1
1.904(11)
1.983(11)
2.005(8)
2.067(9)
2.274(8)
2.451(11)
O7–Cu4
O8–Cu4
Cu1–Cu2
Cu1–Cu4
Cu2–Cu3
Cu3–Cu4
2.242(8)
2.422(9)
4.047(4)
4.037(4)
4.022(3)
3.973(4)
4
2
2
3
2
2
The structure of 1 is illustrated in Fig. 1, and selected bond
N8–Cu2
N5–Cu2
O4–Cu2
N6–Cu2
O1–Cu2
O2–Cu2
N13–Cu3
O7–Cu3
N10–Cu3
N11–Cu3
O4–Cu3
O5–Cu3
N18–Cu4
N15–Cu4
O10–Cu4
N16–Cu4
N8–Cu2–O1
N5–Cu2–O1
O4–Cu2–O1
N6–Cu2–O1
N8–Cu2–O2
N5–Cu2–O2
O4–Cu2–O2
N6–Cu2–O2
O1–Cu2–O2
N13–Cu3–O7
N13–Cu3–N10
O7–Cu3–N10
N13–Cu3–N11
O7–Cu3–N11
N10–Cu3–N11
N13–Cu3–O4
O7–Cu3–O4
N10–Cu3–O4
N11–Cu3–O4
N13–Cu3–O5
O7–Cu3–O5
N10–Cu3–O5
1.904(11)
1.999(10)
2.004(8)
2.101(11)
2.297(8)
2.328(10)
1.909(11)
1.995(8)
2.026(11)
2.046(11)
2.281(8)
2.428(8)
1.874(10)
2.005(9)
2.025(8)
2.056(10)
N3–Cu1–N20
N3–Cu1–O1
N20–Cu1–O1
N3–Cu1–N1
20–Cu1–N1
170.5(4)
80.0(4)
98.7(3)
79.1(4)
102.1(4)
159.1(3)
113.0(4)
76.5(4)
94.8(3)
92.8(3)
176.9(5)
79.3(4)
99.3(4)
77.9(4)
103.4(4)
157.2(4)
91.6(4)
150.9(3)
169.4(4)
79.3(4)
96.4(4)
79.8(4)
104.5(4)
159.0(3)
113.7(4)
76.1(3)
95.0(3)
92.4(3)
95.8(4)
74.6(4)
91.7(3)
91.4(3)
150.5(3)
139.5(4)
139.6(4)
139.2(4)
140.5(4)
O1–Cu1–N1
N3–Cu1–O10
N20–Cu1–O10
O1–Cu1–O10
N1–Cu1–O10
N8–Cu2–N5
N8–Cu2–O4
N5–Cu2–O4
N8–Cu2–N6
N5–Cu2–N6
O4–Cu2–N6
N11–Cu3–O5
O4–Cu3–O5
N18–Cu4–N15
N18–Cu4–O10
N15–Cu4–O10
N18–Cu4–N16
N15–Cu4–N16
O10–Cu4–N16
N18–Cu4–O7
N15–Cu4–O7
O10–Cu4–O7
N16–Cu4–O7
N18–Cu4–O8
N15–Cu4–O8
O10–Cu4–O8
N16–Cu4–O8
O7–Cu4–O8
Cu1–O1–Cu2
Cu2–O4–Cu3
Cu3–O7–Cu4
Cu4–O10–Cu1
107.7(4)
75.0(4)
93.1(3)
95.1(3)
100.7(4)
76.6(4)
90.9(3)
91.9(4)
151.6(3)
79.9(4)
172.0(5)
96.7(4)
78.7(4)
158.6(4)
104.4(4)
110.6(4)
92.9(3)
76.6(4)
95.5(3)
98.4(4)
90.4(3)
74.4(4)
Fig. 1 Structural representation of 1 (40% thermal ellipsoids).
distances and angles are listed in Table 2. The grid consists of
four six-coordinate copper centers arranged in a square, with
alkoxide bridges connecting the metal atoms. Four ligands are
arranged in parallel pairs in an opposed arrangement above
and below the metal plane, with the carboxylate oxygen atoms
filling terminal coordination positions. Cu–Cu separations fall
in the range 3.97–4.05 Å, typical for systems of this sort, and
Cu–O–Cu angles fall in the range 139.2–140.5Њ. Each copper
ion is tetragonally distorted, undergoing axial elongation
involving the terminal carboxylate and the bridging alkoxide
oxygen atoms (Cu–O distances 2.242–2.451 Å). In-plane copper
ligand contacts fall in the range 1.874–2.101 Å.
The presence of two nitrate groups in the lattice implies that
the combination of four ligands provides six negative charges,
and it is reasonable to assume that each ligand loses a proton
from within the hydrazide framework (not the NH group),
2
leading to a negative charge on the alkoxide oxygen atom. This
is supported by one relatively short Cu–O contact to each
copper ion (1.995–2.025 Å) and C–O bond lengths that have
substantial single bond character. In order to balance overall
charge, two ligands must therefore have two negative charges,
which implies that two carboxylic acid groups are deproton-
ated. Two protons were located in a difference map on O(9) and
O(12), in agreement with this assumption.
The most prominent feature of the structure involves the
encapsulation of the four six-coordinate Cu() centers within
the grid, an unusual feature in related non-homoleptic Cu
4
1–3
[2 × 2] square grid complexes, but similar to the phenoxide-
4
bridged [2 × 2] copper() grid reported by Lehn et al.
Copper() undergoes Jahn–Teller distortion, and, in this case,
each six-coordinate copper ion undergoes a tetragonal elong-
ation. The axis of elongation of each copper center includes
only one bridging oxygen connection, leading to a situation
where the copper equatorial planes are tilted at ∼90Њ to those of
neighboring metal centers in all cases. This orbital orthogonal-
ity situation leads to a significant magnetic consequence, that of
intramolecular ferromagnetic exchange (vide infra).
Fig. 2 Structural representation of lattice contacts between Cu
subunits.
grid
4
The lattice structure involves a large number of water
1–3
molecules, as is typical for grids in this class, and two signifi-
cant intermolecular contacts have been identified, which might
be considered to have magnetic consequences. Fig. 2 shows
close contacts between carboxylate oxygen atoms O(3), O(6),
O(9) and O(12) and intervening water molecules O(20), O(21),
O(23) and O(35) [O(3)–O(35) 2.696; O(35)–O(21) 2.768; (O21)–
O(12) 2.496; O(9)–O(23) 2.518; O(23)–O(20) 2.748; O(20)–O(6)
2.543; O(21)–O(23) 2.641 Å], forming a connected layer of grid
2
464
J. Chem. Soc., Dalton Trans., 2002, 2462–2466

View Article Online
molecules in the xy plane. These contacts are within reason-
able hydrogen bonding distances, and could provide long
intermolecular magnetic exchange pathways between Cu grids
4
(
vide infra).
Magnetic properties
Variable temperature magnetic data (µmol) for 1 measured at
0
.1 T are shown in Fig. 3. The room temperature magnetic
Fig. 4 Magnetization versus field data for 1 at 2 K. The solid line
represents the best fit to the theoretical Brillouin function for S =
/2, g = 2.258.
4
This is remarkable in the sense that the large Cu–O–Cu bridge
angles would have normally been expected to propagate strong
antiferromagentic exchange had the magnetic copper orbitals
been directly involved in oxygen overlap in a bridging arrange-
ment, as is the case for essentially planar tetranuclear Cu (µ-O)4
4
complexes with the metal ions constrained within a macrocyclic
1
8–20
ligand framework.
The extra organizational freedom that
Fig. 3 Variable temperature magnetism of 1 (µmol). The solid line
Ϫ1
represents the best data fit with g = 2.246(7), J = 7.2(3) cm
,
results from the self assembly reaction with the alkoxy-
amidrazone ligands in this class allows the metal ions to adopt a
different, and perhaps preferred, arrangement, with novel
orthogonal bridging connections.
Ϫ6
Ϫ1
2
TIP = 250 × 10 emu mol and θ = Ϫ0.15 K {10 R = 0.58; R =
2
2 1/2
[
Σ (χobs Ϫ χ ) /Σ χ
] ).
calc
obs
moment is 4.01 µB and as temperature is lowered, the value
gradually increases, reaching 5.31 µ at 2 K. This behavior is
B
indicative of the presence of dominant intramolecular ferro-
magnetic exchange. The variable temperature data were fitted to
an isotropic exchange expression derived from the exchange
Hamiltonian (eqn. 1), which assumes a single exchange integral
within the square grid.
Conclusion
Compound 1 represents the first example of a ferromagnetic,
homoleptic square [2 × 2] grid with ligands of this type, and
introduces a useful reactive appendage on the ligand, which can
be further functionalized and used as a potential tether group
for possible surface applications. The ligand synthesis is pos-
sible as a result of the differing solubilities of intermediates in
the reaction sequence, and it is of relevance to note that the
fully functionalized ligand 2poap, which has a second appended
H = ϪJ{S ؒS ϩ S ؒS ϩ S ؒS ϩ S ؒS }
(1)
1
2
2
3
3
4
1
4
The total spin-state combinations, and their energies, were
16
calculated in the normal way using the Kambe approach, and
substituted into the van Vleck equation to generate the
appropriate exchange expression. This was achieved within the
amidrazone group (Scheme 1) forms [3 × 3] M magnetic
9
5–9
grids.
17
software of the MAGMUN4.0 magnetic package. The
experimental data were fitted successfully to the derived mag-
netic model (based on eqn. 1), within MAGMUN4.0, to give g
Acknowledgements
Ϫ1
Ϫ6
Ϫ1
=
2.246(7), J = 7.2(3) cm , ρ = 0, TIP = 250 × 10 emu mol
We thank NSERC (Natural Sciences and Engineering Research
Council of Canada) for financial support, and Dr M. J.
Ferguson, University of Alberta, for X-ray structural data.
and θ = Ϫ0.15 K (ρ is the fraction of paramagnetic impurity,
TIP is temperature independent paramagnetism and θ is a
Weiss-like correction indicative of possible intermolecular
2
2
2 1/2
exchange) {10 R = 0.58; R = [Σ (χ Ϫ χcalc) /Σ χ ] }. The
obs
obs
solid line in Fig. 3 was calculated with these parameters. Many
regression analyses were carried out over an extended range of
variables and, in all cases, a small negative θ value was required
in order to optimize the fitting. This suggests the presence of a
weak intermolecular exchange effect which is antiferromagnetic
in nature. This is tentatively assigned to the close contacts
between grid subunits and water molecules, as described
previously.
References
1
C. J. Matthews, K. Avery, Z. Xu, L. K. Thompson, L. Zhao,
D. O. Miller, K. Biradha, K. Poirier, M. J. Zaworotko, C. Wilson,
A. E. Goeta and J. A. K. Howard, Inorg. Chem., 1999, 38, 5266.
Z. Xu, L. K. Thompson, C. J. Matthews, D. O. Miller, A. E. Goeta
and J. A. K. Howard, Inorg. Chem., 2001, 40, 2446.
2
3 L. K. Thompson, C. J. Matthews, L. Zhao, Z. Xu, D. O. Miller,
C. Wilson, M. A. Leech, J. A. K. Howard, S. L. Heath, A. G.
Whittaker and R. E. P. Winpenny, J. Solid State Chem., 2001, 159,
Confirmation of the expected S = 4/2 ground state was
obtained by measuring magnetization as a function of field at
3
08.
4
T. Rojo, J.-M. Lehn, G. Baum, D. Fenske, O. Waldmann and
2
K (Fig. 4). The magnetization data were fitted successfully to
P. Müller, Eur. J. Inorg. Chem., 1999, 517.
the appropriate Brillouin function with g = 2.258, very close to
the value obtained from the variable temperature analysis (the
solid line in Fig. 4 represents the best fit). This result is entirely
5 L. Zhao, Z. Xu, L. K. Thompson and D. O. Miller, Polyhedron,
2001, 20, 1359.
6
7
8
L. Zhao, Z. Xu, L. K. Thompson, S. L. Heath, D. O. Miller and
M. Ohba, Angew. Chem., Int. Ed., 2000, 39, 3114.
L. Zhao, C. J. Matthews, L. K. Thompson and S. L. Heath, Chem.
Commun., 2000, 265.
O. Waldmann, R. Koch, S. Schromm, P. Müller, L. Zhao and
L. K. Thompson, Chem. Phys. Lett., 2000, 332, 73.
consistent with those reported previously for related Cu square
4
1–3
grids, and is in agreement with the strictly orthogonal bridg-
ing arrangement between adjacent copper magnetic orbitals in
the structure, and the close proximity of the copper() centers.
J. Chem. Soc., Dalton Trans., 2002, 2462–2466
2465

View Article Online
14 D. C. Creagh and W. J. McAuley, International Tables for
Crystallography, ed. A. J. C. Wilson, Kluwer Academic Publishers,
Boston, MA, 1992, vol. C, pp. 219–222, Table 4.2.6.8,.
15 teXsan for Windows, Crystal Structure Analysis Package, Molecular
Structure Corporation, The Woodlands, TX, 1997.
16 K. Kambe, J. Phys. Soc. Jpn., 1950, 5, 38.
17 Z. Xu, K. He, L. K. Thompson and O. Waldmann, MAGMUN4. 0,
Memorial University of Newfoundland, St. John’s, NF, Canada,
2002.
9
O. Waldmann, L. Zhao and L. K. Thompson, Phys. Rev. Lett., 2002,
8, 66401.
0 (a) G. M. Sheldrick, SHELX97, University of Göttingen, Germany,
997; (b) SIR97: A. Altomare, M. Cascarano, C. Giacovazzo and
8
1
1
A. Guagliardi, J. Appl. Crystallogr., 1993, 26, 343.
1
1 P. T. Beurskens, G. Admiraal, G. Beurskens, W. P. Bosman,
R. de Gelder, R. Israel and J. M. M. Smits, The DIRDIF-94 Program
System, Technical Report of the Crystallography Laboratory,
University of Nijmegen, The Netherlands, 1994.
1
2 D. T. Cromer and J. T. Waber, International Tables for X-ray
Crystallography, The Kynoch Press, Birmingham, 1974, vol. IV,
Table 2.2A.
18 V. McKee and S. S. Tandon, J. Chem. Soc., Chem. Commun., 1988,
385.
19 V. McKee and S. S. Tandon, Inorg. Chem., 1989, 28, 2902.
20 V. McKee and S. S. Tandon, J. Chem. Soc., Dalton Trans., 1991, 221.
1
3 J. A. Ibers and W. C. Hamilton, Acta Crystallogr., 1964, 17, 781.
2
466
J. Chem. Soc., Dalton Trans., 2002, 2462–2466