Trinuclear, tetranuclear, and polymeric CuII complexes from the first use of 2-pyridylcyanoxime in transition metal chemistry: Synthetic, structural, and magnetic studies

Article abstract of DOI:10.1021/ic102286r

The first use of 2-pyridylcyanoxime, (py)C(CN)-NOH, in transition metal chemistry is described. Depending on the nature of the metal starting material and the reaction conditions employed, the Cu^II/(py)C(CN)NOH system has provided access to complexes [Cu₃O{(py)C(CN)NO} ₃(NO₃)-(H₂O)₂(MeOH)] (1), [Cu ₄O{(py)C(CN)NO}₄(O₂CMe)₂] (2), [Cu₄(OH)₂{(py)C(CN)NO}₂(O₂CPh) ₄]_2n·n[Cu₄(OH)₂-{(py)C(CN)NO} ₂(O₂CPh)₄](3), and [Cu{(py)C(CN)NO} ₂]_n (4). The molecule of 1 consists of three Cu ^II atoms in a strictly equilateral arrangement bridged by a central μ₃-oxide group. The molecule of 2 consists of a tetrahedron of Cu^II atoms held together by a central μ₄-oxide ion, four η¹:η¹:η¹;;μ-(py)C(CN) NO^- ligands and two η¹:η¹;;μ- MeCO₂^- groups. The crystal structure of 3 consists of [Cu₄(OH)₂{(py)C(CN)NO}₂(O₂CPh) ₄]_2n double chains and discrete cluster [Cu ₄(OH)₂{(py)C(CN)NO}₂-(O₂CPh) ₄] molecules. The crystal structure of 4 consists of neutral polymeric chains based on centrosymmetric mononuclear [Cu{(py)C(CN)NO} ₂] units. The Cu^II atoms are doubly bridged by the oximate groups of two η¹:η¹:η¹:μ-(py) C(CN)NO^- ligands. Variable-temperature, solid-state direct current (dc) magnetic susceptibility studies were carried out for 1-4. The data indicate very strong antiferromagnetic exchange interactions for 1-3. The obtained J values are discussed in depth on the basis of the structural parameters of the complexes, literature reports, and existing magnetostructural correlations.

Full text of DOI:10.1021/ic102286r

ARTICLE
pubs.acs.org/IC
Trinuclear, Tetranuclear, and Polymeric CuII Complexes
from the First Use of 2-Pyridylcyanoxime in Transition Metal
Chemistry: Synthetic, Structural, and Magnetic Studies
Albert Escuer,*,† Gina Vlahopoulou,†,‡ Spyros P. Perlepes,‡ and Franz A. Mautner§
†Departament de Química Inorgꢀanica and Institut de Nanociꢀencia i Nanotecnologia, Universitat de Barcelona (IN2UB),
Martí I Franquꢁes 1-11, 08028-Barcelona, Spain
‡Department of Chemistry, University of Patras, 265 04 Patras, Greece
§Institut f€ur Physikalische und Theoretische Chemie, Technische Universit€at Graz, A-8010 Graz, Austria
S Supporting Information
b
ABSTRACT: The first use of 2-pyridylcyanoxime, (py)C(CN)-
NOH, in transition metal chemistry is described. Depending on
the nature of the metal starting material and the reaction
conditions employed, the CuII/(py)C(CN)NOH system has
provided access to complexes [Cu3O{(py)C(CN)NO}3(NO3)-
(H2O)2(MeOH)] (1), [Cu4O{(py)C(CN)NO}4(O2CMe)2]
(2), [Cu4(OH)2{(py)C(CN)NO}2(O2CPh)4]2n n[Cu4(OH)2-
3
{(py)C(CN)NO}2(O2CPh)4] (3), and [Cu{(py)C(CN)NO}2]n (4). The molecule of 1 consists of three CuII atoms in a strictly
equilateral arrangement bridged by a central μ3-oxide group. The molecule of 2 consists of a tetrahedron of CuII atoms held together
by a central μ4-oxide ion, four η1:η1:η1:μ-(py)C(CN)NO- ligands and two η1:η1:μ-MeCO2- groups. The crystal structure of 3
consists of [Cu4(OH)2{(py)C(CN)NO}2(O2CPh)4]2n double chains and discrete cluster [Cu4(OH)2{(py)C(CN)NO}2-
(O2CPh)4] molecules. The crystal structure of 4 consists of neutral polymeric chains based on centrosymmetric mononuclear
[Cu{(py)C(CN)NO}2] units. The CuII atoms are doubly bridged by the oximate groups of two η1:η1:η1:μ-(py)C(CN)NO-
ligands. Variable-temperature, solid-state direct current (dc) magnetic susceptibility studies were carried out for 1-4. The data
indicate very strong antiferromagnetic exchange interactions for 1-3. The obtained J values are discussed in depth on the basis of
the structural parameters of the complexes, literature reports, and existing magnetostructural correlations.
’ INTRODUCTION
polymeric products.12 There is currently a renewed interest in
the chemistry of metal oxime systems.13-22 2-pyridyl oximes23
and 2,6-pyridyl dioximes24 (Chart 1) are currently popular ligands
for a variety of research objectives, including μ2-μ4 behaviors; the
activation of 2-pyridyl oximes by 3d-metal centers toward
further reaction is also becoming a fruitful area of research.25
Such anionic ligands have been central “players” in several areas
of single-molecule26,27 and single-chain28 magnetism.
Two classes of compounds which currently attract the intense
interest of inorganic chemists are molecular clusters1 and co-
ordination polymers2 (also known as organic-inorganic hybrid
polymers in the case where the metal-organic connectivity is
interrupted by “inorganic” bridges). There are many reasons for
this,3-7 not least of which is the aesthetic beauty of their struc-
tures. For example, polynuclear three-dimensional (3d)-metal
complexes often display interesting and occasionally novel magnetic
properties, including high ground-state spin values,6 and single-
molecule magnetism behavior.7 The 3d-metal ions have attrac-
tive features for a new generation of coordination polymers. By
carefully selecting the ligand (mainly the organic one) and the
metal ion, researchers aim to tune the physical properties and
thus realize various applications of this class of compounds in
catalysis, electrical conductivity, luminescence, magnetism, mo-
lecular electronics, nonlinear optics, sensing, zeolitic behavior,
gas storage, and medicine.8 Coordination polymers are also
significant from a structural chemistry perspective.9
Efforts to date by us25a,b,26,27,29 and other groups24,28,30 have
mainly concentrated on 2-pyridyl oximes bearing H, Me, Ph,
NH2, and 2-pyridyl groups as the R moiety and on 2,6-pyridyl
dioxime ligands with R = H, Me, and NH2. We have very recently
extended our work to 2-pyridylcyanoxime (2-(oximino)-2-pyr-
idylacetonitrile; (py)C(CN)NOH in Chart 1). The presence of
the cyano functionality is expected to alter, because of its
potential coordination capability and different electronic proper-
ties and hydrogen bonding effects, the coordination chemistry of
this ligand (and hence the identity of the resultant products) in
comparison with that of the (py)C(R)NOH (R = H, Me, Ph,
2-pyridyl, etc.) ligands. There are no literature reports of any
There are now several empirically established approaches to a
variety of metal clusters10 and polymers.2,11 One fertile route is
the use of oxime ligands, since the oximate function is a good
bridging group and thus fosters formation of polynuclear or
Received: November 15, 2010
Published: February 24, 2011
r
2011 American Chemical Society
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|

Inorganic Chemistry
ARTICLE
Chart 1. Structural Formulae and Abbreviation of the
Ligands Discussed in the Text
1491s, 1466vs, 1418sh, 1394s, 1345 m, 1305s, 1270s, 1232vs, 1154 m,
1105 m, 1061 m, 1037vs, 783vs, 734w, 709vs, 668 m.
[Cu4(OH)2{(py)C(CN)NO}2(O2CPh)4]2n n[Cu4(OH)2{(py)C(CN)-
3
NO}2(O2CPh)4] nMeCN (3 MeCN). Solid (py)C(CN)NOH (0.029 g,
3
3
0.20 mmol) was added to a pale blue solution of Cu(O2CPh)2 2H2O
3
(0.068 g, 0.20 mmol) in MeCN (30 mL). The solid immediately
dissolved followed by an abrupt color change to dark green. The solution
was stirred for 15 min, filtered, and layered with Et2O/n-hexane (20 mL,
50:50 v/v). Slow mixing gave small green crystals of the product after
2 days. The crystals were collected by filtration, washed with Et2O (3 ꢀ
3 mL), and dried in air. The yield was ∼60%. Anal. Calcd for C42H30N6-
O12Cu4 (C2H3N)1/3 (MW, 1078.6): C 47.51; H, 2.90; N, 8.22%.
3
Found: C, 47.65; H, 3.01; N, 8.40%. IR (KBr, cm-1): 3425mb, 2215 m,
1593s, 1550vs, 1500w, 1472 m, 1439w, 1401s, 1362 m, 1307vw, 1224 m,
1110vw, 1039w, 778vw, 715 m, 678w.
[Cu{(py)C(CN)NO}2]n (4). A pale pink solution of (py)C(CN)NOH
(0.40 mmol) in MeCN (10 mL) was added dropwise to a stirred pale
green solution of Cu(BF4)2 6H2O (0.069 g, 0.20 mmol) in the same
3
solvent (10 mL). The resulting dark green solution was stirred for a
further 1 h and then allowed to slowly evaporate at room temperature.
X-ray quality green crystals of the product grew over a period of
6-7 days. The crystals were collected by filtration, washed with Et2O (4 ꢀ
2 mL), and dried in air. The yield was ∼65%. Anal. Calcd for C14H8N6-
O2Cu (MW, 355.8): C, 47.26; H, 2.27; N, 23.62%. Found: C, 47.50; H,
2.29; N, 23.80%. IR (KBr, cm-1): 3335wb, 3087w, 2213 m, 1601s,
1486w, 1453vs, 1407vs, 1371s, 1353w, 1271w, 1235vs, 1165s, 1108w,
1064s, 1036vs, 1019w, 976w, 785vs, 744w, 708s, 652w, 499w, 415 m.
Physical Measurements. Magnetic susceptibility measurements
were carried out on polycrystalline samples with a DSM5 Quantum
Design susceptometer working in the range 30-300 K under magnetic
fields of 0.3 T and under a field of 0.03T in the 30-2 K range to avoid
saturation effects. Diamagnetic corrections were estimated from Pascal
Tables. Infrared spectra (4000-400 cm-1) were recorded from KBr pellets
on a Bruker IFS-125 FT-IR spectrophotometer. Microanalyses (C, H, N)
were performed by the in house facilities of the University of Barcelona.
X-ray Crystallography. The X-ray single-crystal data of com-
pounds 1-4 were collected on a Bruker-AXS SMART APEX CCD
diffractometer with graphite-monochromator utilizing Mo KR radiation
(λ = 0.71073 Å), with ω-scans at 100(2) K. The crystallographic data,
conditions retained for the intensity data collection, and some features
of the structure refinements are listed in Table 1. Data processing,
including Lorentz-polarization and absorption corrections using the
SADABS33 computer program were performed. The structures were
solved by direct methods using the SHELXS-8634 computer program,
and refined by full-matrix least-squares methods, using the SHELXL-93
program incorporated in the SHELXTL/PC V 5.03 suite.34 All non-
hydrogen atoms were refined anisotropically. The H atoms attached to
C were added theoretically and treated as riding on concerned parent
atoms. H atoms attached to O atoms were located from difference
Fourier maps, applied with isotropic U values, and included on fixed
positions in the final refinement cycles. In case of 1, occupancy of
0.6667 for H(42) and occupancy of 0.3333 for C(11), H(11A), H(11B),
and H(11C), respectively, were applied. Occupancy of 0.50 was applied
for atoms of disordered solvent water molecule in 2, and for disordered
atoms of MeCN solvent molecule in 3, respectively.
transition metal clusters or polymers of neutral or anionic (py)C-
(CN)NOH (only a TlI/(py)C(CN)NO- compound has been
reported31), but its 2-pyridyl oxime and cyano functionalities
suggested a rich potential for the isolation of such complexes.
The crystal structure of the free ligand was reported32 in 1999.
In the present work, we report the first use of the (py)C-
(CN)OH ligand in transition metal chemistry by describing CuII
clusters and polymers of the anionic form of the ligand which
exhibit unusual structural features and interesting magnetic
characteristics.
’ EXPERIMENTAL SECTION
Syntheses. All manipulations were performed under aerobic con-
ditions using reagents (mainly from Sigma-Aldrich Inc. and Fluka AG)
and solvents as received. The ligand 2-pyridylcyanoxime, (py)C(CN)NOH,
was synthesized from 2-pyridylacetonitrile, KNO2, and glacial MeCO2H
at -50 °C following the reported method.32 Cu(O2CPh)2 2H2O was
3
prepared in high yield by the direct reaction of aqueous solutions
of CuSO4 5H2O and NaO2CPh in an 1:2 ratio; the compound was
3
characterized by microanalyses and IR spectroscopy.
[Cu3O{(py)C(CN)NO}3(NO3)(H2O)2(MeOH)] (1). Solid (py)C(CN)-
NOH (0.029 g, 0.20 mmol) was added to a blue solution of Cu(NO3)2
3
2.5H2O (0.047 g, 0.20 mmol) in MeOH (30 mL). The solid immedi-
ately dissolved, and the resulting dark green solution was stirred for 2 h,
filtered, and allowed to slowly evaporate at room temperature. X-ray quality,
cubic dark green crystals of the product slowly grew over a period
of 10 days. The crystals were collected by filtration, washed with Et2O
(3 ꢀ 3 mL), and dried in air. Yield 65%. Anal. Calcd for C22H20N10O10Cu3
(MW, 775.1) C, 34.09; H, 2.60; N, 18.07%. Found: C, 34.37; H, 2.47;
N, 18.31%. IR (KBr, cm-1): 3440mb, 1601 m, 1506 m, 1471s, 1419 m,
1383vs, 1339 m, 1306 m, 1213s, 1159w, 1110w, 1061w, 1037s, 784w,
711 m, 668vw, 553w, 422w.
[Cu4O{(py)C(CN)NO}4(O2CMe)2] 0.25H2O (2 0.25H2O). Solid
3
3
Plots for publication were generated with ORTEP3 for Windows and
(py)C(CN)NOH (0.029 g, 0.20 mmol) was added to a green solution
of [Cu2(O2CMe)4(H2O)2] (0.040 g, 0.10 mmol) in CH2Cl2 (25 mL).
The solid soon dissolved, and the resulting dark green solution was
stirred for 1 h, filtered, and layered with Et2O (20 mL). Slow mixing gave
polyhedral green crystals of the product after 2 days. The crystals were
collected by filtration, washed with Et2O (3 ꢀ 3 mL), and dried in air.
plotted with Pov-Ray programs.35
CCDC-799893-799896 (for 1-4) contain the supplementary
crystallographic data for this paper. See http://www.rsc.org/ for crystal-
lographic data in .cif or other electronic format.
’ RESULTS AND DISCUSSION
The yield was ∼80%. Anal. Calcd for C32H22N12O9Cu4 0.25H2O
3
Syntheses and IR Spectra. Our general approach for the
(MW, 977.3): C, 39.33; H, 2.32; N, 17.20%. Found: C, 38.98; H,
2.42; N, 17.02%. IR (KBr, cm-1): 3447mb, 2224 m, 1602vs, 1559vs,
isolation of CuII/(py)C(CN)NO- compounds has been to treat
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Inorganic Chemistry
ARTICLE
Table 1. Crystal Data and Structure Refinement for Compounds 1-4
(1)
(2)
(3)
(4)
formula
mol wt
cryst system
space group
a/Å
C
22H20Cu3N10O10
C32H22.5Cu4N12O9.25
977.32
C42.67H31Cu4N6.33O12
1078.61
triclinic
C14H8CuN6O2
355.81
775.12
trigonal
R3
monoclinic
I2/a
triclinic
P1
P1
15.801(2)
15.801(2)
19.599(24)
90
15.875(3)
25.921(5)
17.456(4)
90
13.546(2)
15.635(2)
16.036(3)
80.00(3)
78.65(3)
77.25(2)
3218 (1)
3
4.1724(8)
6.963(1)
11.361(2)
91.37(1)
91.03(1)
100.72(1)
324.1(1)
1
b/Å
c/Å
R/deg
β/deg
γ /deg
V/Å3
90
94.71(3)
90
120
4237(1)
6
7159(3)
8
Z
T/K
100(2)
1.822
100(2)
100(2)
100(2)
F
calcd/g m-3
1.814
1.670
1.823
μ/mm-1
2.312
2.418
2.027
1.706
cryst size/mm3
θmax-θmin/deg
no. reflns collected
R(int)
0.11 ꢀ 0.20 ꢀ 0.28
26.3-1.8
5224
0.16 ꢀ 0.25 ꢀ 0.32
26.0-1.4
27505
0.12 ꢀ 0.20 ꢀ 0.30
26.4-1.3
25410
0.22 ꢀ 0.28 ꢀ 0.35
26.4-1.8
2513
0.0500
1906
0.0433
0.0814
0.029
data
7031
12919
1291
params
145
521
893
106
Ra [I > 2σ(I)]
0.0618
0.1544
1.242/-0.566
0.0512
0.0748
0.0543
R
2b (all data)
residual/e Å-3
0.1185
0.1781
0.1309
0.896/-0.501
1.115/-0.809
1.726/-0.522
a R(Fo) = ∑||Fo| - |Fc||/∑|Fo|. b Rω(Fo)2 = {∑[ω((Fo)2 - (Fc)2)2]/∑[ω((Fo)4]}1/2
.
a variety of CuII sources with the ligand. Addition of external base
for the deprotonation of the oxime group is not necessary
because of the increased acidity of cyanoximes in comparison
with other monoximes. The presence of the cyano group close
to the oxime fragment makes the acidity of cyanoximes about
103-105 times greater than that of common oximes.32 A variety
of reactions were explored with different solvents and under
different reagent ratios, and other conditions before the follow-
ing successful procedures were identified.
we were not able to further characterize.
2½Cu2ðO2CMeÞ4ðH2OÞ2ꢁ þ 4ðpyÞCðCNÞNOH
CH2Cl2
s
½Cu OfðpyÞCðCNÞNOg ðO CMeÞ ꢁ
f
4
2
4
2
þ 6MeCO2H þ 3H2O
ð2Þ
The 1:1 reaction between Cu(O2CPh)2 2H2O and (py)C-
3
(CN)NOH in MeCN afforded a dark green solution from which
was subsequently isolated [Cu4(OH)2{(py)C(CN)NO}2(O2C-
The 1:1 reaction between Cu(NO3)2 2.5H2O and (py)C-
3
Ph)4]2n n[Cu4(OH)2{(py)C(CN)NO}2(O2CPh)4] (3) in
3
60% yield, eq 3. The “wrong” CuII to (py)C(CN)NOH (4:4)
reaction ratio employed for the preparation of this complex did
not prove detrimental to its formation. With the identity of 3
established by single-crystal X-ray crystallography, the stoichio-
metric reaction ratio (4:2) was employed and led to the pure
compound, albeit in lower yield.
(CN)NOH in MeOH afforded a dark green solution from which
was subsequently isolated [Cu3O{(py)C(CN)NO}3(NO3)-
(H2O)2(MeOH)] (1) in 65% yield. Its formation is summar-
ized in eq 1. It is remarkable that the μ3-O2- bridge is stable in an
acidic reaction mixture. Small variations in the CuII/(py)C-
(CN)NOH ratio also gave complex 1.
3CuðNO3Þ2 3 2:5H2O þ 3ðpyÞCðCNÞNOH þ MeOH
12nCu2ðO2CPhÞ2 2H2O þ 6nðpyÞCðCNÞNOH
3
MeOH
MeCN
s
½Cu OfðpyÞCðCNÞNOg ðNO ÞðH OÞ ðMeOHÞꢁ
f
3
3
2
3
2
s
½Cu ðOHÞ fðpyÞCðCNÞNOg ðO CPhÞ ꢁ
f
4
2
2
2
4 2n
þ 5HNO3 þ 4:5H2O
ð1Þ
n½Cu4ðOHÞ fðpyÞCðCNÞNOg2ðO2CPhÞ4ꢁ
The nature of the anion present in the CuII starting material
affects the product identity. The 1:2 reaction between [Cu2-
(O2CMe)4(H2O)2] and (py)C(CN)NOH (CuII/ligand =1:1)
in CH2Cl2 afforded a dark green solution which gave [Cu4O-
{(py)C(CN)NO}4(O2CMe)2] (2) in high yield (∼80%) upon
addition of Et2O. The preparation of the complex is summarized
in eq 2. The nature of the solvent is crucial for the identity and
crystallinity of the product; reaction schemes involving alcohols
(MeOH, EtOH) gave non-crystalline, acetate-free materials that
þ 212nPhCO2H þ 18nH2O
ð3Þ
3
In the above-mentioned complexes, the CuII to (py)C(CN)-
NOH ratio is high (1:1 in 1 and 2, and 2:1 in 3). We wondered
if a compound with a lower ratio could be capable of existence.
To achieve this goal we used a CuII source with a poorly
coordinating anion and a low CuII to ligand reaction ratio. Thus,
the 1:2 reaction between Cu(BF4)2 6H2O and (py)C(CN)NOH
3
in MeCN gave the coordination polymer [Cu{(py)C(CN)NO}2]n
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Inorganic Chemistry
ARTICLE
(4) in 65% yield upon conventional workup; its formation is
summarized in eq 4.
nCuðBF4Þ2 6H2O þ 2nðpyÞCðCNÞNOH
3
MeCN
s
½CufðpyÞCðCNÞNOg ꢁ þ 2nHBF þ 6nH O ð4Þ
f
4
2
2 n
The presence of coordinated MeOH/H2O molecules in 1 and
hydroxo ligands in 3 is manifested by one broad band of medium
intensity at 3440 and 3425 cm-1, respectively, assigned to
ν(OH); their broadness and relatively low frequency are both
indicative of hydrogen bonding. The in-plane deformation
mode of the 2-pyridyl ring of free (py)C(CN)NOH at 633 cm-1
shifts upward in the spectra of 1 (668 cm-1), 2 (668 cm-1),
3 (678 cm-1), and 4 (652 cm-1), confirming the involvement
of the ring-N atom in coordination.36 Several bands appear in the
1605-1365 cm-1 region for the four complexes; contributions
from the ν(CdN)oximate, δ(OH) [in 1 and 3 at wavenumbers
higher than 1580 cm-1], νas(CO2) and νs(CO2) [in 2 and 3],
and ν(NO)nitrate (in 1) are expected in this region, but overlap
with the stretching vibrations of the aromatic (py)C(CN)NO-
ring renders assignments and discussion of the coordination
shifts difficult. The bands at 1559 and 1418 cm-1 in the spectrum
of 2 are tentatively assigned to the νas(CO2) and νs(CO2)
carboxylate modes, respectively.37 The parameter Δ, where
Δ = νas(CO2) - νs(CO2), is 141 cm-1, smaller than that for
Figure 1. Partially labeled Pov-Ray plot of the trinuclear complex 1 (top)
and 2D H-bonded arrangement of its triangular molecules (bottom).
NaO2CMe 3H2O (164 cm-1), as expected for the bidentate
3
bridging mode of the acetate ligation.37a The very strong band at
1383 cm-1 in the spectrum of 1 is characteristic of the presence
of ionic nitrates and is assigned to the ν3(E0) [νd(NO)] mode of
the D3h NO3- ion.38 Its appearance is rather surprising because
the molecule of 1 contains a coordinated nitrato group, and
provides strong evidence for the replacement of the coordinated
Table 2. Selected Interatomic Distances (Å) and Angles
(deg) for Compound 1
Cu(1)-O(1)
Cu(1)-O(2)
Cu(1)-O(3a)
1.957(3)
1.954(1)
2.540(3)
Cu(1)-N(1)
Cu(1)-N(3)
Cu(1)-O(4)
1.981(3)
2.002(3)
2.365(4)
NO3 by Br- ions during the preparation of the KBr pellet.39
-
Color changes were not observed during the pellet preparation.
The medium intensity band at 1055 for free (py)C(CN)NOH
can be assigned to the ν(NO)oxime mode;13,29e the wavenumber
of this vibration increases to ∼1100 cm-1 in the spectra of the com-
plexes. This shift to higher frequencies has been discussed29e,40
and is in accord with the fact that upon deprotonation and
oximate-O coordination, there is a higher contribution of NdO
to the electronic structure of the oximate group; consequently
the ν(NO) vibration shifts to a higher wavenumber relative to
that for the free oxime ligand.
N(3)-O(1)
C(7)-N(4)
1.301(5)
1.141(4)
Cu(1)-O(1)-N(3c)
Cu(1)-N(3)-O(1e)
Cu(1)-O(2)-Cu(1c,e)
113.4(3)
126.3(2)
114.42(9)
Chart 2. Crystallographically Established Coordination
Modes of the (py)C(CN)NO-, NO3-, MeCO2-, and
PhCO2- Ligands in Complexes 1-4
Description of the Structures. [Cu3O{(py)C(CN)NO}3(NO3)-
(H2O)2(MeOH)] (1). A partially labeled plot of the neutral tri-
nuclear unit present in compound 1 is shown in Figure 1 (top).
Selected bond parameters are listed in Table 2. The molecule
consists of three CuII atoms in a triangular arrangement bridged
by central μ3-oxide atom O(2). Each edge of the Cu3 triangle is
bridged by an η1:η1:η1:μ-(py)C(CN)NO- ligand (I, Chart 2),
with the pyridyl and oximate nitrogen atoms chelating a CuII
atom and forming a five-membered chelate ring, and the oximate
oxygen atom binding a neighboring metal ion. The nitrogen
atom of the η1:η1:η1:μ3-NO3- group (II, Chart 2) and the oxide
atom are located on a 3-fold symmetry axis; the triangle is thus
equilateral with a Cu Cu distance of 3.285(1) Å. The nitrato
3 3 3
nitrogen and the oxide atoms are 2.451(4) Å above and 0.469(4) Å
below the Cu3 plane, respectively. The O(4) atom, from a dis-
ordered H2O/MeOH ligand, completes six-coordination at each
metal ion.
in this geometry. As is most often the case for CuII atoms, the JT
distortion is a pronounced axial elongation. The JT axis in 1 is the
The CuII atoms are near-octahedral (4 þ 1 þ 1 coordination)
exhibiting a Jahn-Teller (JT) distortion, as expected for a d9 ion
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O(nitrate)-Cu-O(H2O/MeOH) axis, with Cu-O distances of
2.540(3) and 2.365(4) Å, respectively. The Cu-N-O-Cu
torsion angles are 11.1(3)°. The angle between the mean plane
of the almost planar (py)C(CN)NO- ligand and the Cu3 plane
is 11.08(6)°. The non-coordinated cyano groups interact by
means of H bonds with the O(4) H2O/MeOH oxygen atoms of
neighboring molecules (O N = 2.977(5) Å) creating a two-
3 3 3
dimensional (2D) supramolecular arrangement of Cu3 units
placed at 120° (Figure 1, bottom). A last structural feature of 1
deserves a comment: The oximate C-N [1.309(5) Å] and N-O
[1.301(5) Å] bond lengths are slightly larger and smaller than the
corresponding ones [C-N = 1.287(2) Å, N-O = 1.367(2) Å] in
the free ligand32 indicating that this anionic group is delocalized.
Using the metallacrown terminology,30c,d complex 1 is an inverse-
9-MCCuII{(py)C(CN)N0}-3 metallacrown.
Complex 1 is a rare example of a discrete cluster containing the
{Cu3(μ3-O)}4þ core;41 CuII6 clusters42 and coordination poly-
mers based on CuII,II,II3(μ3-O) units are also known.43 However,
it is the first such complex with an oximate ligation. Compound 1
is structurally similar to the members of a small family30c,d,41e,44
of triangular CuII oximato complexes containing the {Cu3(μ3-
X)}5þ core, where the X- ligand is a hydroxo or an alkoxo group.
These complexes follow some general trends: (i) The CuII atoms
and the oximate ligands define a plane; (ii) the central μ3-donor
is well out of this plane; (iii) axial/apical coordination sites (often
4 þ 1) are occupied by anions or solvent molecules; (iv) tetra-
hedral anions (e.g., SO42-, ClO4-) act as tripodal bridges to one
side of the molecule, whereas carboxylate or related groups are
bound to opposite sides of the molecule, and (v) the central μ3-
OH- group (if present) participates in a strong intramolecular H
bond. Except being the first oximato complex with the {Cu3(μ3-
O)}4þ core, complex 1 is also unique because it consists of two
parallel planes, one containing the three CuII atoms and the other
the extremely rare η1:η1:η1:μ3 NO3- group.
Figure 2. Structure of the tetranuclear molecule present in complex 2
(top) and labeled Pov-Ray plot of its {Cu4(μ4-O)(μ-ONR)4(μ-O2C-
Me)2} core (bottom); the apical coordination bonds are shown as thin
black lines.
Table 3. Selected Interatomic Distances (Å) and Angles
(deg) for Compound 2
Cu(1)-O(5)
Cu(1)-O(6)
Cu(1)-O(8)
Cu(1)-N(1)
Cu(1)-N(2)
Cu(2)-O(1)
Cu(2)-O(3)
Cu(2)-O(5)
Cu(2)-N(4)
Cu(2)-N(5)
1.947(3)
2.264(3)
1.954(3)
2.030(4)
2.015(4)
1.949(3)
2.204(4)
1.957(3)
2.004(4)
1.975(4)
Cu(3)-O(4)
Cu(3)-O(5)
Cu(3)-O(9)
Cu(3)-N(7)
Cu(3)-N(8)
Cu(4)-O(2)
Cu(4)-O(5)
Cu(4)-O(7)
Cu(4)-N(10)
Cu(4)-N(11)
2.020(3)
1.933(3)
2.280(3)
1.994(4)
1.989(4)
2.260(3)
1.939(3)
1.941(3)
2.020(4)
1.984(4)
[Cu4O{(py)C(CN)NO}4(O2CMe)2] 0.25H2O (2 0.25H2O). A
3
3
plot of the structure of complex 2 0.25H2O is shown in Figure 2.
3
Selected interatomic distances and angles are listed in Table 3.
The crystal structure of the complex is composed of [Cu4O-
{(py)C(CN)NO}4(O2CMe)2] molecules and H2O interstitial
solvent molecules; the latter do not exhibit strong H-bond
interactions and will not be further discussed. The tetranuclear
molecule consists of a tetrahedron of CuII atoms held together by
a central O(5) μ4-oxide ion, four η1:η1:η1:μ-(py)C(CN)NO-
ligands (I, Chart 2) and two η1:η1:μ-MeCO2 groups (III,
-
O(1)-N(2)
O(2)-N(5)
1.292(5)
1.292(5)
O(3)-N(8)
O(4)-N(11)
1.274(5)
1.293(5)
Chart 2). Each of the four diatomic oximate groups (O(1)/N(2),
O(2)/N(5), O(3)/N(8), O(4)/N(11)) spans one edge of the
tetrahedron, while each of the remaining two edges is bridged
-
Cu(1)-O(5)-Cu(2)
Cu(1)-O(5)-Cu(3)
Cu(1)-O(5)-Cu(4)
113.1(2)
109.5(2)
108.2(2)
Cu(2)-O(5)-Cu(3)
Cu(2)-O(5)-Cu(4)
Cu(3)-O(5)-Cu(4)
111.8(2)
102.9(2)
111.2(2)
by an MeCO2 group (O(6)/O(7), O(8)/O(9)). The core is
{Cu4(μ4-O)}6þ; if we consider the diatomic oximate and the
triatomic carboxylate groups as parts of the core, then this is
becoming {Cu4(μ4-O)(μ-ONR)4(μ-O2CMe)2}, where RNO-
is (py)C(CN)NO- (Figure 2, bottom). The Cu4(μ4-O) tetra-
atoms lie out of their respective basal planes toward the apical
donor atoms. As expected the Cu(1)-O(6) [2.264(3) Å],
Cu(2)-O(3) [2.204(4) Å], Cu(3)-O(9) [2.280(3) Å], and
Cu(4)-O(2) [2.260(3) Å] bond distances are the longest.
Analysis of the shape-determining bond angles using the ap-
proach of Reedijk, Addison, and co-workers45 yields values for
the trigonality index, τ, of 0.07, 0.06, 0.35, and 0.08 for Cu(1),
Cu(2), Cu(3), and Cu(4), respectively (τ = 0 and 1 for perfect square
pyramidal and trigonal bipyramidal geometries, respectively).
No significant intra- or intermolecular H bonds are present
hedron is moderately distorted, the Cu Cu distances being
3 3 3
between 3.045(1) and 3.258(1) Å, the Cu-O-Cu angles
varying from 102.9(2) to 113.1(2)°, and the Cu-O(5) bond
lengths being in the range 1.933(3)-1.957(3) Å.
Each of the CuII atoms is in a slightly (Cu(1), Cu(2), Cu(4))
or moderately (Cu(3)) distorted square pyramidal coordination
environment (N2O2þO donor set), with atoms O(6), O(3),
O(9), and O(2) occupying the apical positions of Cu(1), Cu(2),
Cu(3), and Cu(4), respectively. Thus each MeCO2- group links
basal-apical sites belonging to different CuII atoms. The CuII
in the crystal structure of 2 0.25H2O. The 2-pyridyl rings of the
3
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Table 4. Selected Interatomic Distances (Å) and Angles
(deg) for the Three Tetranuclear Subunits of Compound 3
Isolated Subunit Cu(4)/Cu(5)
Cu(4)-O(10)
Cu(4)-O(19)
Cu(4)-O(17)
Cu(4)-N(5)
Cu(4)-N(9)
1.911(3) Cu(5)-O(11)
1.890(3) Cu(5)-O(17)
2.495(3) Cu(5)-O(19)
1.980(5) Cu(5)-O(19f)
1.999(4) Cu(5)-O(21)
1.943(4)
1.979(4)
2.466(3)
1.886(3)
1.985(4)
Cu(5f)-O(19)-Cu(4) 122.7(2) Cu(5)-O(19)-Cu(5f) 95.4(1)
Cu(4)-O(19)-Cu(5) 87.1(1)
Linked Subunits Cu(2)/Cu(1) and Cu(6)/Cu(3)
Cu(2)-O(8)
1.963(3) Cu(1)-O(7)
1.888(3) Cu(1)-O(13)
2.621(3) Cu(1)-O(15)
1.988(4) Cu(1)-O(15c)
2.014(4) Cu(1)-O(9)
2.546(3)
1.930(3)
1.961(3)
2.333(3)
1.912(3)
1.987(3)
Cu(2)-O(15)
Cu(2)-O(13)
Cu(2)-N(2)
Cu(2)-N(7)
Cu(2)-O(2a)
Figure 3. Partially labeled Pov-Ray plot of the {Cu4(μ3-OH)2(μ-OR)2-
(μ-ONR0)2(μ-O2CR00)2} core that is present in the discrete Cu4 cluster
subunit of complex 3 [RO- and R00CO2- = PhCO2 , R0NO- = (py)C-
-
(CN)NO-]. The symbol f is used for symmetry generated atoms. The
dashed lines indicate H bonds.
Cu(2)-O(15)-Cu(1c) 122.7(2) Cu(1)-O(15)-Cu(1c) 95.2(1)
Cu(2)-O(15)-Cu(1)
93.1(1)
atoms from the two μ3-OH- groups (O(19), O(19f)) and two
η2:μ benzoate ligands (O(17), O(17f); IV, Chart 2). The shared
face of the two cubanes is the Cu(5)O(19)Cu(5f)O(19f) one. Peri-
pheral ligation is provided by two η1:η1:η1:μ-(py)C(CN)NO-
Cu(6)-O(6)
1.925(3) Cu(3)-O(3)
1.890(3) Cu(3)-O(1)
2.608(4) Cu(3)-O(4e)
1.971(4) Cu(3)-O(4)
2.005(4) Cu(3)-O(5)
2.851(4)
1.952(3)
1.959(3)
2.293(3)
1.899(3)
1.990(3)
Cu(6)-O(4)
Cu(6)-O(1e)
-
ligands (I, Chart 2) and two syn-syn η1:η1:μ-PhCO2 groups
Cu(6)-N(1)
(O(10)/O(11) and symmetry equivalents; V, Chart 2). The
core is {Cu4(μ3-OH)2(μ-OR)2}4þ, where RO- is PhCO2-. If
we consider the diatomic oximate and the syn-syn carboxylate
groups as parts of the core, then this is becoming {Cu4(μ3-OH)2-
(μ-OR)2(μ-ONR0)2(μ-O2CR00)2}, where R0NO- is (py)C(CN)-
NO- and R00CO2- is PhCO2-; this view of the core is illustrated
in Figure 3. The triply bridging hydroxo oxygen atom O(19)
forms two strong bonds [1.890(3) and 1.886(3) Å] to Cu(4) and
Cu(5f), and one weaker [2.466(3) Å] to Cu(5). The bridging
Cu(4,5)-O(17) distances [2.495(3) and 1.979(4) Å] are fully
asymmetric; this is a consequence of the basal/apical coordina-
tion of this atom. The Cu(4)-N(9)-O(21f)-Cu(5f) torsion
angle is 10.3(5)°.
Atoms Cu(4) and Cu(5) adopt slightly distorted square
pyramidal coordination geometries with τ values of 0.17 and
0.18, respectively. The chromophores are Cu(4)N2O3 and Cu-
(5)O5. The apical positions for Cu(4) and Cu(5) are occupied by
the doubly bridging benzoate atom O(17) and the triply bridging
hydroxo atom O(19), respectively. The hydroxo atom O(19) is
at basal coordination sites of Cu(4) and Cu(5f), and at the apical
site of Cu(5).
Cu(6)-N(3)
Cu(6)-O(14)
Cu(6)-O(4)-Cu(3)
Cu(6)-O(4)-Cu(3e)
120.7(2) Cu(3)-O(4)-Cu(3e)
94.7(1)
95.7(2)
four (py)C(CN)NO- ligands form π-π stacking interactions
with symmetry-related 2-pyridyl rings of adjacent Cu4 cluster
molecules creating a 3D network.
Complex 2 is the first structurally characterized oximato com-
plex containing the {Cu4(μ4-O)}6þ core. This core is present
in more than 100 tetranuclear CuII clusters, most of which have
the general formula [Cu4OX10-xLx]x-4, where X- is most often
Cl- or Br- and L is a monodentate ligand, and contain the
{Cu4(μ4-O)(μ-X)6} core.46
[Cu4(OH)2{(py)C(CN)NO}2(O2CPh)4]2n n[Cu4(OH)2{(py)C-
(CN)NO}2(O2CPh)4] nMeCN (3 MeCN). The crystal structure
3
3
3
of 3 MeCN consists of [Cu4(OH)2{(py)C(CN)NO}2(O2C-
3
Ph)4]2n chains, discrete [Cu4(OH)2{(py)C(CN)NO}2(O2CPh)4]
clusters, and interstitial solvent MeCN molecules. The latter do
not give H-bonds and will not be further discussed. There are
three independent Cu4 subunits in the triclinic unit cell of the
complex, each located on an inversion center; thus there are
six crystallographically independent CuII atoms. The three Cu4
subunits have very similar interatomic distances and angles, and
thus only the subunit containing Cu(4), Cu(5) and symmetry
equivalents will be discussed in some detail. The Cu(4)/Cu(5)
Cu4 subunit appears well isolated (cluster subunit) in the crystal,
while the Cu(1)/Cu(2) and Cu(3)/Cu(6) tetranuclear subunits
give double chains of tetramers. Selected interatomic distances
are listed in Table 4.
There is one strong intramolecular H bond with the hydroxo
oxygen (O(19), O(19f)) as the donor and the uncoordinated
benzoate oxygen (O(18f), O(18)) as the acceptor; the O(19f)
3 3 3
O(18) distance is 2.604(6) Å. There is a weak π-π stacking
-
interaction between the aromatic ring of the one of the PhCO2
groups and the pyridyl ring of the (py)C(CN)NO- ligand; the
rings are quasi parallel with a distance between centroids of
3.609(8) Å.
As mentioned above, the packing of complex 3 MeCN
3
(Figure 4, left) reveals the formation of polymeric chains through
alternation of the Cu(1)/Cu(2) and Cu(3)/Cu(6) tetranuclear
subunits. The intertetramer linkage is provided by two η1:η2:μ-
The four CuII atoms (Cu(4), Cu(5) and symmetry equiv-
alents) of the cluster subunit are located at four vertices of a
defective dicubane47 (two cubanes sharing one face and each
missing one vertex, Figure 3) and bridged by means of oxygen
-
PhCO2 groups (VI, Chart 2) per tetramer (Figure 4, right);
these groups are η2:μ in the Cu(4)/Cu(5) discrete subunit. In
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Figure 4. Packing of complex 3 MeCN showing the chains of the
3
alternating Cu(1)/Cu(2) and Cu(3)/Cu(6) Cu4 subunits and the
discrete Cu(4)/Cu(5) tetranuclear cluster units (left), and the 1D
arrangement of the Cu(1)/Cu(2) and Cu(3)/Cu(4) subunits by means
of carboxylato bridges (right); in the right view the η1:η1:μ PhCO2
-
groups are not shown for clarity.
Figure 5. Partially labeled Pov-Ray plot of the asymmetric unit of
complex 4 (top) and the 1D arrangement of the Cu{(py)C(CN)NO}2
units (bottom).
the chain the η1:η2:μ-PhCO2- and 2-pyridyl rings of neighboring
Cu4 subunits are parallel at 3.688(8) and 3.555(8) Å, providing
significant interactions. These interactions bring the Cu4 sub-
units closer allowing the free, that is, uncoordinated benzoate
oxygen atoms of one subunit to axially coordinate CuII atoms of
two neighboring Cu4 subunits. In the discrete cluster subunit,
Cu(4) exhibits a square pyramidal coordination, whereas the
corresponding Cu(2) and Cu(6) atoms are becoming octahedral
(4 þ 2 coordination) in the chain. The PhCO2- groups have also
been modified; in the discrete Cu(4)/Cu(5) Cu4 subunit, the
O(18)-O(17)-Cu(5)-O(19f) torsion angle is 3.15(3)°, and
the dihedral angle between the O(17)-C(61)-O(18) carbox-
Table 5. Selected Interatomic Distances (Å) and Angles
(deg) for the Asymmetric Unit of Compound 4
Cu(1)-N(1)
Cu(1)-(1a)
2.009(3) Cu(1)-N(3)
2.633(3) O(1)-N(1)
2.007(3)
1.292(4)
N(1)-Cu(1)-N(3) 80.9(1)
N(1)-Cu(1)-N(3c)
99.1(1)
Cu(1) Cu(1b)
3 3 3
4.1724(8) Cu(1)-N(1)-O(1)-Cu(1b) 79.1(3)
-
ylate plane and the PhCO2 aromatic ring is 17.7(6)°; the
unit is shown in Figure 5 (top). Selected interatomic distances
and angles are listed in Table 5. Its crystal structure consists of
neutral polymeric chains based on centrosymmetric mononuc-
lear [Cu{(py)C(CN)NO}2] units; a portion of one such chain is
shown in Figure 5 (bottom). The CuII atoms are doubly bridged
by the oximate groups of two η1:η1:η1:μ-(py)C(CN)NO-
ligands (I, Chart 2).
corresponding parameters for the Cu(1)/Cu(2) and Cu(3)/
Cu(6) tetranuclear subunits in the chain are 28.1(6)°/26.1(6)°
and 30.7(6)°/40.2(6)°, respectively. The intertetramer inter-
actions from the axial benzoate coordination to Cu(2) and Cu(6)
also induce intracluster bonding parameter changes; the most
significant change is in the Cu-N-O-Cu torsion angle, which
is 10.3(5)° for the discrete Cu(4)/Cu(5) cluster subunit, and
6.9(5)° and 0.7(4)° for the Cu(1)/Cu(2) and Cu(3)/Cu(6)
tetranuclear subunits, respectively, of the chain.
Each ligand chelates one CuII atom forming two 5-membered
CuNCCN chelating rings, while its deprotonated oximate oxy-
gen atom is terminally bound to a neighboring metal ion; the
cyano group of the ligand remains uncoordinated.
Complex 3 is the first defective dicubane CuII cluster with
oximate ligation. Several Cu4 clusters containing the {Cu4(μ3-
OH)2}6þ unit, most of them with the defective dicubane motif,
have been structurally characterized.44b,48 This unit is also a
recognizable fragment of higher nuclearity CuII clusters.49 Of
some relevance to the structure of 3 is complex [Cu4(μ3-OH)2-
(O2CMe)6(4pds)2]n,50 where 4pds is the twisted ligand 4,40-
dithiobis(pyridine). The compound has an 1D structure com-
posed of an alternate linking of one rhomboid (and not defective
dicubane because of the lack of doubly bridging oxygen atoms)
{Cu4(μ3-OH)2(O2CMe)6} unit and two 4pds ligands. The
structural motif of 3 with the simultaneous presence of polymeric
chains and isolated clusters is unprecedented in transition metal
oximate chemistry.
The coordination geometry about the CuII center is described
as elongated octahedral (4 þ 2); the two symmetry-related
oximate oxygen atoms define the JT axis of the metal [Cu(1)-
O(1a) = 2.633(3) Å]. The equatorial bond angles deviate from
the ideal 90° value because of the restriction imposed from the
chelating ring, with the N(1)-Cu(1)-N(3) angle being
80.9(1)°. The metal metal distance is 4.1724(8) Å. Significant
3 3 3
H bonds or/and π-π stacking interactions were not located in
the crystal structure. Compound 4 is the first chain polymer of
any transition metal ion containing double oximato bridges.
Magnetic Measurements and Correlations. Variable-
temperature direct current (dc) magnetic susceptibility studies
were performed on powdered samples of compounds 1-4 in
applied fields of 0.3 T (300-30 K) and 0.03 T (30-2.0 K). The
[Cu{(py)C(CN)NO}2]n (4). Complex 4 crystallizes in the tri-
clinic space group P1. A partially labeled plot of its asymmetric
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ARTICLE
theoretical study on complexes containing the {CuII (μ3-O/
3
OH)(μ-ONR0)3}þ/2þ unit, Tangoulis, Tsipis, Kessissoglou,
and co-workers have derived30c the linear correlation J (cm-1) =
1068.4R - 937.7, where R is the distance of the μ3-O2-/OH-
group from the centroid of the Cu3 triangle. The R value for 1
is 0.469 Å and the predicted J value is -436.6 cm-1, in good
agreement with the value derived from the fitting procedure
(-485 cm-1).
The χMT values at room temperature for 2 and 3 are 0.37 and
0.08 cm3 K mol-1, respectively. These values are much lower
than the expected one (1.50 cm3 K mol-1 with g = 2.0) for a
system comprising four noninteracting CuII centers. The χMT
products rapidly decrease with decreasing temperature, reaching
a constant value very close to zero below ∼100 K. The χM values
show a continuous decrease upon cooling, and the curves show
broad minima at ∼85 K (2) and ∼130 K (3), but below these
temperature they slightly increase because of the presence of
small amounts of paramagnetic, possibly monomeric impurities.
The maxima in the χM versus T plots should be located above
room temperature in both cases. The just described behavior is
indicative of very strong antiferromagnetic interactions between
the CuII centers.
Figure 6. Plot of the χMT product versus T for compounds 1 (diamonds),
2 (dot centered circles), 3 (squares), and 4 (triangles) . The solid lines are
the fits of the data; see the text for the fit parameters.
Chart 3. Main Superexchange Pathways for Compounds 1-3a
Close inspection of the molecular structure of 2 reveals that
the main exchange interactions should be through the central
μ4-O2- group and the oximate groups (N(2)/O(1) and N(11)/
O(4) in Figure 2) that connect basal coordination sites of the
CuII atoms. The weak basal-apical interactions can be neglected
in the fitting procedure. On this basis, the fitting of the experi-
mental data was performed by using the analytical equation
derived from the spin Hamiltonian of eq 6. J1 corresponds to the
four exchange interactions mediated by the μ4-O2- group, and J2
to the two interactions mediated by the central oxide group
and one oximate bridge (Chart 3, middle). Best fit parameters are
J1 = -81 cm-1, J2 = -484 cm-1, and g = 2.154.
aWeak interactions as those derived from axial-equatorial interactions
have been suppressed in the simplified cores.
data are plotted as χMT versus T in Figure 6. The room
temperature χMT value for complex 1 is 0.46 cm3 K mol-1. This
value is much lower than the value of 1.125 cm3 K mol-1 with g =
2.0 expected for a complex of three noninteracting S = 1/2 spins,
indicating a very strong antiferromagnetic coupling even at 300 K.
The χMT product decreases slightly with decreasing temperature
in the range 300-150 K range to a value of 0.41 cm3 K mol-1
that agrees very well with the expected one for an S = 1/2 ground
state. The curve shows a plateau around 150 K before a more
rapid decrease of χMT below this temperature to a minimum
value of 0.25 cm3 K mol-1 at 2.0 K. The decrease below ∼150 K
is attributed to the antisymmetric exchange interaction51 which
plays an important role in the magnetic behavior of triangular
CuII complexes.44c,51 Because of the 3-fold symmetry of the
complex (Chart 3, left) and to the antisymmetric exchange, the
fitting of the experimental data was performed in the 300-150 K
range by means of the analytical expression derived from the
isotropic spin Hamiltonian given by eq 5.
^ ^
^ ^
^ ^
^ ^
H ¼ - J1ðS1 S3 þ S1 S4 þ S2 S3 þ S2 S4Þ
3
3
3
3
^ ^
^ ^
- J2ðS1 S2 þ S3 S4Þ
ð6Þ
3
3
Comparison with similar systems is not possible because there
have been no reports for oximato complexes containing the
{Cu4(μ4-O)}6þ core. The J1 value can be compared with the
value of -120 cm-1 obtained for complex [Cu4(μ4-O)-
(bahped)2](ClO4)2,53 where bahped2- is the dianion of 1,3-
bis-[30-aza-40-(3-methyl-20-hydroxyphenyl)prop-4-en-10-yl]-ethane-
1,2-diamine; this complex is the first of this family not having
any additional oligoatomic bridge between CuII atoms other than
the μ4-O2- ion. The J2 value is not unexpected in view of the
remarkable ability of the oximato bridges to mediate strong
antiferromagnetic exchange between CuII centers.12,23,30c,d,54
The three Cu4 subunits that are present in the crystal structure
of 3 have very similar interatomic distances and angles, and
we thus assume that they are magnetically similar. According to
the interaction picture shown in Chart 3 (right), the analysis of
the experimental data was based on the spin Hamiltonian of eq 7.
J1 is the coupling constant that corresponds to the double hydroxo/
oximato bridge, and J2 describes the exchange interaction through
the single syn-syn carboxylato bridge.
^ ^
^ ^
^ ^
H ¼ - JðS1 S2 þ S1 S3 þ S2 S3Þ
ð5Þ
3
3
3
The best-fit parameters are J = -485 cm-1 and g = 2.091. As it
is very common in oximato-bridged CuII complexes,12,23,41d the
superexchange interaction is strongly antiferromagnetic. Com-
parison with similar systems is not easy because the number of
the magnetically studied CuII triangles is not large and additional
axial bridging often modifies the value of the coupling constant. It
should be mentioned at this point that the relationship of J values
^ ^
^ ^
^ ^
^ ^
H ¼ - J1ðS1 S2 þ S3 S4Þ - J2ðS1 S3 þ S2 S4Þ ð7Þ
3
3
3
3
Best fit parameters are J1 = -854 cm-1 and g = 2.101, with J2
undetermined. With a so large value for J1, it is mathematically
not reliable to try to calculate J2. We performed fits fixing J2 in the
to the Cu-O-Cu angle in complexes possessing the {CuII (μ3-
3
O}4þ core has been discussed and analyzed.52 In an excellent
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-100 to 0 cm-1 range (the J2 = 0 value corresponds to the
dinuclear model) checking that J2 did not modify the excellent
quality (Figure 6) of the fits. On the basis of the linear correlation
between -J/n and a (n is the number of carboxylato bridges
and a the Cu-O-Cu angle), established by Chaudhuri’s/
provided access to two new clusters (1, 2), one 1D coordination
polymer (4) and to one compound containing both chains and
isolated cluster molecules (3). The obtained products are
structurally interesting in multiple ways, as described in detail,
and their structural motifs have not been seen before in the
coordination chemistry of 2-pyridyl oximes. Compounds 1-3
also provide additional examples of CuII complexes with extremely
strong (but also variable) antiferromagnetic coupling between
the metal ions.
Wieghardt’s group for CuII,II complexes bridged by syn, syn
2
carboxylates,55 we would expect a value close to ∼50 cm-1 for J2.
The room temperature χMT value for complex 4 is 0.41 cm3 K
mol-1. This value is close to the expected value of 0.375 cm3 K
mol-1 with g = 2.0 for one S = 1/2 spin. The χMT product shows
a constant value down to 50 K, and for lower temperatures it
decreases to a value of 0.13 cm3 K mol-1 at 2 K, indicating a very
weak antiferromagnetic coupling. Experimental data were fitted
to the analytical expression derived from the Hamiltonian:56
Our initial results described in this work suggest that reactions
of (py)C(CN)NOH with CuII in the presence of coligands with a
strong coordination ability other than nitrates and carboxylates
(e.g., Cl-, Br-, N3-, OCN-, SCN-, SO42-, β-diketonates, etc.)
and with other paramagnetic 3D-metal ions (MnII, MnIII, FeII,
FeIII, CoII, and NiII) promise to deliver many new clusters and
coordination polymers with interesting properties. We are also
pursuing the employment of (py)C(CN)NO- for the synthesis
of mixed 3d/4f-metal clusters to study their magnetic and optical
properties. A final goal of our future work will be to give a bio-
inorganic chemistry flavor in our efforts. Since (i) (py)C(CN)-
^ ^
H ¼ - ∑JðSi Si þ 1
Þ
ð8Þ
3
Best fit parameters were J = -1.33(1) cm-1 and g = 2.086(2).
This weak interaction is in good agreement with the structural
data which only show axial-equatorial interactions between
neighbor CuII atoms along the chain.
NOH has the propensity toward formation of CuII (μ3-O)
3
The great difference between the J2 value for 2 (-484 cm-1
)
species, and (ii) redox-active triangular CuII complexes are found
at the active sites of important metallobiomolecules,5,41a,42 the
road is open for the synthesis and study of bioinorganic models
based on the 2-pyridylcyanoximate ligand, as successfully carried
out by Raptis’ group with the use of pyrazolates.41a,d
and the J1 value for 3 (-854 cm-1) seems, at a first glance,
surprising. Both refer to exchange interactions through the O2-
or OH-/oximate bridging system. However, in the light of
recent density functional theory (DFT) calculations on triangu-
lar {CuII (μ-O/OH)(μ-ONR0)3}þ/2þ systems,30c which have
3
correlated J with the degree of the pyramidal distortion of the
Cu3O(H) fragment (this distortion is expressed by the R param-
eter mentioned above or the equivalent Cu-O(H)-Cu bond
angles), we may extrapolate the conclusion to isolated Cu-
O(H)/oximate-Cu fragments that are present in higher nucle-
arity CuII clusters; thus, the strong antiferromagnetic coupling
may be tuned by the Cu-O(H)-Cu bond angle. The only
known compound containing dinuclear Cu-OH-/oximate-Cu
units is the centrosymmetric trinuclear complex [Cu3(OH)2-
(L)2(H2O)2](NO3)2,57 where L- is the monoanion of 2-[2-(R-
pyridyl)ethyl]imino-3-butanone oxime. The Cu-O-Cu bond
angle is 118.6(1)° and the J value is ∼ -900 cm-1. This value
compares well with the -854 cm-1 value (J1) obtained for 3
exhibiting an average Cu-O-Cu bond angle of ∼122°. On the
contrary, the average Cu-O-Cu bond angle within the Cu-
’ ASSOCIATED CONTENT
S
Supporting Information. Crystallographic data for com-
b
plexes 1, 2 0.25H2O, 3 MeCN, and 4 in CIF format. This
material is available free of charge via the Internet at http://pubs.
3
3
acs.org.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: albert.escuer@ub.edu.
’ ACKNOWLEDGMENT
O
2-/oximate-Cu fragments of complex 2 (the Cu1-O5-Cu2
This work was financially supported by the CICYT projects
CTQ2009-07264 and excellence in research ICREA-Academia
Award.
and Cu3-O5-Cu4 angles in the real structure, see Figure 2) is
112.2°, significantly smaller than the corresponding bond angles
in 3 and [Cu3(OH)2(L)2(H2O)2](NO3)2.57 It seems that , the
tetrahedral O2- coordination, which gives smaller Cu-O-Cu
angles, is responsible for the much weaker antiferromagnetic
J2 interaction in the oxo-bridged cluster 2 compared with the
J1 interaction in the hydroxo-bridged compound 3 (-484 cm-1
vs -854 cm-1). A great pyramidal distortion at the oxygen
bridge (approaching sp3) leads to a situation where less efficient
magnetic orbital overlap with the oxygen bridge occurs, thus
significantly reducing antiferromagnetic coupling.48h
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