Does the Sign of the Cu-Gd Magnetic Interaction Depend on the Number of Atoms in the Bridge?

Article abstract of DOI:10.1002/chem.201504238

Several theoretical investigations with CASSCF methods confirm that the magnetic behavior of Cu-Gd complexes can only be reproduced if the 5d Gd orbitals are included in the active space. These orbitals, expected to be unoccupied, do present a low spin density, which is mainly due to a spin polarization effect. This theory is strengthened by the experimental results reported herein. We demonstrate that Cu-Gd complexes characterized by Cu-Gd interactions through single-oxygen and three-atom bridges consisting of oxygen, carbon, and nitrogen atoms, present weak ferromagnetic exchange interactions, whereas complexes with bridges made of two atoms, such as the nitrogen-oxygen oximato bridge, are subject to weak antiferromagnetic exchange interactions. Therefore, a bridge with an odd number of atoms induces a weak ferromagnetic exchange interaction, whereas a bridge with an even number of atoms supports a weak antiferromagnetic exchange interaction, as observed in pure organic compounds and also, as in this case, in metal-organic compounds with an active spin polarization effect.

Full text of DOI:10.1002/chem.201504238

DOI: 10.1002/chem.201504238
Full Paper
&
Ferromagnetic Interactions
Does the Sign of the Cu–Gd Magnetic Interaction Depend on the
Number of Atoms in the Bridge?
Jean-Pierre Costes,*[a, b] Carine Duhayon,[a, b] Sonia Mallet-Ladeira,[a, b] Sergiu Shova,[a, b, c] and
Laure Vendier[a, b]
Dedicated to Professor Dante Gatteschi
Abstract: Several theoretical investigations with CASSCF
methods confirm that the magnetic behavior of Cu–Gd com-
plexes can only be reproduced if the 5d Gd orbitals are in-
cluded in the active space. These orbitals, expected to be
unoccupied, do present a low spin density, which is mainly
due to a spin polarization effect. This theory is strengthened
by the experimental results reported herein. We demon-
strate that Cu–Gd complexes characterized by Cu–Gd inter-
actions through single-oxygen and three-atom bridges con-
sisting of oxygen, carbon, and nitrogen atoms, present weak
ferromagnetic exchange interactions, whereas complexes
with bridges made of two atoms, such as the nitrogen–
oxygen oximato bridge, are subject to weak antiferromag-
netic exchange interactions. Therefore, a bridge with an odd
number of atoms induces a weak ferromagnetic exchange
interaction, whereas a bridge with an even number of atoms
supports a weak antiferromagnetic exchange interaction, as
observed in pure organic compounds and also, as in this
case, in metal–organic compounds with an active spin polar-
ization effect.
Introduction
having unoccupied 3dx2
orbitals. Such an observation con-
Ày2
firms the preponderant role played by the 3dx2
orbitals,
Ày2
Since the discovery of ferromagnetic Cu–Gd interactions in a tri-
nuclear Cu-Gd-Cu complex[1] a lot of studies aimed at evaluat-
ing the nature and magnitude of the magnetic interaction be-
tween a lanthanide ion and a second spin carrier, have been
performed in the last two decades, as demonstrated by the
number of reviews devoted to the magnetic properties of
these 3d–4f complexes.[2–6] Meanwhile, these experimental re-
sults have allowed the development of theoretical calculations
directed toward a better understanding of the ferromagnetic
Cu–Gd interactions.[7–14] Studies of CoII–Gd complexes have
provided an additional and very interesting piece of informa-
tion.[15,16] Indeed, ferromagnetic interactions are observed in
the complexes presenting high-spin CoII ions and singly occu-
which is in agreement with theoretical results. We have previ-
ously published,[17] along with the group of Matsumoto,[18]
a synthetic pathway yielding tetranuclear [Cu–Gd]2 complexes.
This method is based on a stepwise process involving original
mononuclear copper complexes, therefore, we decided to find
a route allowing an easy preparation, isolation, and characteri-
zation of these key copper complexes. The structural determi-
nations of three mononuclear copper complexes used as li-
gands in the syntheses of polynuclear Cu–Gd entities confirm
the interest of this experimental process. The anionic mononu-
clear Cu precursors, obtained with ligands possessing three
functions that can be deprotonated, yield structurally charac-
terized tetranuclear [Cu–Gd]2 complexes after reaction with Gd
ions and ancillary ligands. The alternate arrangement of the Cu
and Gd ions favors Cu–Gd interactions through bridges involv-
ing single atom (phenoxo) or three atoms (amidato) bridges
and avoids direct Cu–Cu and Gd–Gd interactions. According to
the same process, a neutral Cu precursor, in which the Cu ion
is surrounded by one phenoxo oxygen atom and three differ-
ent nitrogen atoms (amide, imine, and pyridine) gives a trinu-
clear triangular Cu-Gd-Cu complex possessing three atoms
amidato Cu–Gd bridges only. Furthermore, we have shown in
a recent paper that the Cu–Gd interaction through a single
oxime bridge is antiferromagnetic.[19] A supplementary exam-
ple is also described in the present paper, thanks to the struc-
tural determination of a tetranuclear [Cu–Gd]2 complex involv-
ing a neutral Cu precursor having a unique deprotonated
oxime function introducing Cu–Gd interactions through the
pied 3dx2
orbitals (S=3/2), whereas these interactions
Ày2
become antiferromagnetic with low-spin CoII ions (S=1= )
2
[a] Dr. J.-P. Costes, Dr. C. Duhayon, Dr. S. Mallet-Ladeira, Prof. S. Shova,
Dr. L. Vendier
Laboratoire de Chimie de Coordination du CNRS
205, route de Narbonne, BP 44099, 31077 Toulouse Cedex 4 (France)
E-mail: jean-pierre.costes@lcc-toulouse.fr
[b] Dr. J.-P. Costes, Dr. C. Duhayon, Dr. S. Mallet-Ladeira, Prof. S. Shova,
Dr. L. Vendier
UniversitØ de Toulouse; UPS, INPT, 31077 Toulouse Cedex 4 (France)
[c] Prof. S. Shova
Department of Chemistry, Moldova State University
A. Mateevici Str. 60, 2009 Chisinau (Moldova)
Supporting information for this article (containing the crystallographic data
in CIF format and the magnetic data) is available on the WWW under
http://dx.doi.org/10.1002/chem.201504238.
Chem. Eur. J. 2016, 22, 2171 – 2180
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ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
aldehyde (0.62 g, 5.1”10À3 mol) in ethanol (20 mL) was heated to
reflux for ten minutes under stirring. The solution was left to stand,
overnight, yielding yellow crystals that were filtered off and dried.
Yield: 1.1 g (64%). 13C{1H} NMR (100.62 MHz, 208C, [D6]DMSO): d=
24.37 (s, CH2), 24.93 (s, CH2), 31.56 (s, CH2), 33.81 (s, CH2), 53.01 (s,
NHCH), 70.26 (s, NCH), 115.56 (s, ArC1), 116.85 (s, ArCH), 117.78 (s,
ArCH), 118.79 (s, ArCH), 118.89 (s, ArCH), 118.98 (s, ArC1), 127.83 (s,
ArCH), 132.62 (s, ArCH), 133.97 (s, ArCH), 134.03 (s, ArCH), 160.57 (s,
ArC2OH), 160.86 (s, ArC2OH), 165.48 (s, C=N), 168.96 ppm (s,
OCNH); elemental analysis calcd for C20H22N2O3 (334.4): C 71.0, H
6.6, N 8.3; found: C 70.7, H 6.4, N 8.2.
L2Cu(C5H12N) (1): A mixture of L2H3 (0.34 g, 1”10À3 mol), Cu(OA-
c)2·H2O (0.20 g, 1”10À3 mol), and piperidine (0.30 g, 3.5”10À3 mol)
was heated and stirred for one hour. The solution was cooled to
room temperature, filtered off and left overnight, giving crystals
that were isolated by filtration. Yield: 0.30 g (80%). IR (ATR): n˜ =
3301 (w), 2939 (w), 1630 (s), 1611 (s), 1543 (s), 1516 (m), 1446 (m),
1433 (m), 1364 (m), 1314 (s), 1263 (s), 1163 (w), 1031 (w), 907 (m),
756 (w), 756 (w), 707 cmÀ1 (m); elemental analysis calcd for
C25H30CuN3O3 (484.1): C 62.0, H 6.3, N 8.7; found: C 61.8, H 6.3, N
8.6.
two atoms oximato bridges. These experimental results based
on the number of atoms involved in the Cu–Gd bridges, com-
bined to recently published theoretical data, give interesting
information on the general M–Gd magnetic interaction.
Experimental Section
Materials: Salicylaldehyde, hexafluoroacetylacetone (Hhfa), 3-(per-
fluorobutyryl)-(À)camphor (HScamph), 3-(perfluorobutyryl)-(+)cam-
phor (HRcamph), Cu(OAc)2·2H2O, and GdCl3·6H2O, (Aldrich) were
used as purchased. [Gd(hfa)3]·2H2O[20] (hfa=hexafluoroacetylaceto-
nato), [Gd(hfa)2(Hhfa)(CH3CO2)],[21] and [L11Cu][19] were prepared as
previously described. High-grade solvents (diethyl ether, dimethyl-
formamide, acetone, and methanol) were used for the syntheses of
ligands and complexes.
Physical measurements: C, H, and N elemental analyses were car-
ried out at the Laboratoire de Chimie de Coordination Microanalyt-
ical Laboratory in Toulouse, France. IR spectra were recorded with
a Perkin–Elmer Spectrum 100FTIR by using the ATR mode. 1D and
1
2D H and 13C NMR spectra were acquired at 400.16 (500.33) MHz
L3Cu(C5H12N) (2): A mixture of 2-hydroxy-N-{2-[(1-methylethylidene)
amino]ethyl}benzamide (0.44 g, 2”10À3 mol) and salicylaldehyde
(0.25 g, 2”10À3 mol) in methanol (20 mL) was heated to reflux for
thirty minutes and then left to cool to room temperature under
stirring. Cu(OAc)2·H2O (0.40 g, 1”10À3 mol) and piperidine (0.50 g,
5.9”10À3 mol) were added. The solution was heated to reflux and
stirred for two hours. From the cooled solution, a violet precipitate
appeared. It was filtered off and washed with 2-propanol and di-
ethyl ether. Yield: 0.39 g (90%). IR (ATR): n˜ =2923 (w), 2862 (w),
2501 (w), 1632 (s), 1593 (s), 1564 (s), 1519 (s), 1464 (m), 1438 (s),
1391 (s), 1380 (s), 1345 (m), 1311 (m), 1310 (m), 1257 (m), 1194 (w),
1156 (w), 1034 (m), 903 (m), 838 (w), 752 (s), 704 (m), 652 cmÀ1 (w);
elemental analysis calcd for C21H25CuN3O3 (431.0): C 58.5, H 5.8, N
9.7; found: C 58.1, H 5.6, N 9.5.
L4Cu(C5H12N) (3): A mixture of N-(2-amino-2-methylpropyl)-2-hydrox-
ybenzamide (0.42 g, 2”10À3 mol) and salicylaldehyde (0.25 g, 2”
10À3 mol) in methanol (20 mL) was heated to reflux for thirty mi-
nutes and then left to cool under stirring. Cu(OAc)2·H2O (0.40 g, 1”
10À3 mol) and piperidine (0.50 g, 5.9”10À3 mol) were added. The
solution was heated to reflux and stirred for two hours. From the
cooled solution, a violet precipitate appeared. It was filtered off
and washed with 2-propanol and diethyl ether. Yield: 0.74 g (80%).
IR (ATR): n˜ =2958 (w), 2857 (w), 2580 (w), 2482 (w), 1622 (s), 1594
(s), 1563 (s), 1530 (s), 1466 (m), 1440 (s), 1397 (m), 1386 (s), 1341
(m), 1317 (m), 1310 (m),1263 (m), 1186 (w), 1147 (w), 1029 (m), 888
(m), 849 (w), 759 (s), 704 (m), 658 cmÀ1 (w); elemental analysis
calcd for C23H29CuN3O3 (459.0): C 60.2, H 6.4, N 9.2; found: C 59.8,
H 6.3, N 9.0.
[L10Cu]·MeOH (4): A mixture of N-(2-amino-2-methylpropyl)-2-hy-
droxy-3-methoxybenzamide (0.24 g, 1”10À3 mol), pyridinecarboxal-
dehyde (0.11 g, 1”10À3 mol), Cu(OAc)2·H2O (0.20 g, 1”10À3 mol)
and piperidine (0.25 g, 3”10À3 mol) in methanol (10 mL) was
heated to reflux 20 min under stirring and set aside for three days.
Crystals were isolated by filtration and washed by acetone and di-
ethyl ether. Yield: 0.20 g (48%). IR (KBr): n˜ =3422 (l), 2950 (w), 2833
(w), 1643 (w), 1598 (s), 1573 (s), 1546 (s), 1464 (m), 1436 (s), 1381
(m), 1369 (s), 1338 (m), 1322 (w), 1261 (m), 1232 (s), 1198 (m), 1149
(w), 1089 (w), 1062 (m), 1023 (w), 956 (w), 856 (m), 820 (w), 774
(m), 739 (s), 679 (w), 630 cmÀ1 (w); elemental analysis calcd for
C19H23CuN3O4: C 54.2, H 5.5, N 10.0; found: C 53.9, H 5.3, N 9.8.
(1H) or 100.63 MHz (125.82 MHz) (13C) on Bruker Avance 400 or 500
spectrometers by using [D6]DMSO as solvent. Chemical shifts are
given in [ppm] versus trimethylsilane (TMS) (1H and 13C). Magnetic
data were obtained with a Quantum Design MPMS SQUID suscep-
tometer. Magnetic susceptibility measurements were performed in
the temperature range 2–300 K under a 0.1 T applied magnetic
field, and diamagnetic corrections were applied by using Pascal’s
constants.[22] Isothermal magnetization measurements were per-
formed up to 5 T at 2 K. The magnetic susceptibilities have been
computed by exact calculations of the energy levels associated to
the spin Hamiltonian through diagonalization of the full matrix
with a general program for axial symmetry,[23] and with the MAG-
PACK program package4[24] in the case of magnetization. Least-
squares fittings were accomplished with an adapted version of the
function-minimization program MINUIT.[25]
Synthesis
Ligands: The syntheses of N-(2-amino-2-methylpropyl)-2-hydroxy-
benzamide,[17c] 2-hydroxy-N-{2-[(1-methylethylidene)amino]ethyl}-
benzamide,[17c] and N-(2-amino-2-methylpropyl)-2-hydroxy-3-me-
thoxybenzamide[17b] “half-units” ligands have been previously de-
scribed.
N-(2-Aminocyclohexyl)-2-hydroxybenzamide (L1H2):
A mixture of
phenyl salicylate (2.14 g, 1”10À2 mol) and 1,2-diaminocyclohexane
(1.14 g, 1”10À2 mol) in propane-2-ol (40 mL) was heated to reflux
for thirty minutes and then cooled down to room temperature
under stirring. The white precipitate, which appeared upon cool-
ing, was filtered off and washed with diethyl ether. Yield: 1.2 g
(51%). 1H NMR (400 MHz, 208C, [D6]DMSO): d=1.20–1.30 (m, 4H;
CH2), 1.64–1.68 (m, 2H; CH2), 1.87–1.90 (m, 2H; CH2), 2.64–2.69 (m,
1H; CH), 3.63–3.65 (m, 1H; CH), 6.72 (t, J=8 Hz, 1H; C5H), 6.82 (d,
J=8 Hz, 1H; C3H), 7.27 (td, J=1.8, 8 Hz, 1H; C4H), 7.86 (dd, J=1.8,
8 Hz, 1H; C6H), 9.39 ppm (l, 1H; NH); 13C{1H} NMR (100.62 MHz,
208C, [D6]DMSO): d=24.77 (s, CH2), 25.09 (s, CH2), 32.02 (s, CH2),
33.90 (s, CH2), 54.83 (s, NHCH), 55.35 (s, NH2CH), 116.90 (s, ArC1),
117.26 (s, ArC3H), 118.51 (s, ArC5H), 129.08 (s, ArC6H), 133.34 (s,
ArC4H), 162.52 (s, ArC2OH), 168.98 ppm (s, OCNH); elemental anal-
ysis calcd for C13H18N2O2 (234.3): C 66.6, H 7.7, N 12.0; found: C
66.2, H 7.5, N 11.8.
2-Hydroxy-N-(2-{[(2-hydroxyphenyl)methylene]amino}cyclohexyl)benz-
amide (L2H3): A mixture of L1H2 (1.20 g, 5.1”10À3 mol) and salicyl-
[L2Cu(dmf)Gd(hfa)2(dmf)]2 (5): This complex was obtained by slow
crystallization of a solution of [L2Cu](C5H12N) (0.05 g, 1.0”10À4 mol)
Chem. Eur. J. 2016, 22, 2171 – 2180
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Full Paper
and [Gd(hfa)3]·2H2O (0.08 g, 1.0”10À4 mol) in DMF (2 mL). Yield:
(0.08 g, 71%). IR (ATR): n˜ =2936 (w), 2864 (w), 1648 (s), 1601 (m),
1573 (s), 1530 (s), 1471 (m), 1439 (m), 1396 (m), 1345 (w), 1249 (s),
1207 (s), 1142 (s), 1102 (m), 1074 (w), 955 (w), 919 (w), 844 (w), 797
(m), 764 (m), 740 (m), 658 (m), 638 cmÀ1 (w); elemental analysis
calcd for C72H70Cu2F24Gd2N8O18 (2232.9): C 38.7, H 3.2, N 5.0; found:
C 38.7, H 3.0, N 4.8.
[L3CuGd(hfa)2(dmf)]2 (6): This complex was obtained by slow crystal-
lization of a solution of [L3Cu](C5H12N) (0.04 g, 1.0”10À4 mol) and
[Gd(hfa)3]·2H2O (0.08 g, 1.0”10À4 mol). In DMF (2 mL) Yield:
(0.10 g, 49%); IR (ATR): n˜ =2939 (w), 1668 (m), 1648 (s), 1600 (m),
1576 (s), 1555 (m), 1530 (s), 1508 (m), 1475 (m), 1437 (w), 1418 (w),
1405 (m), 1384 (w), 1335 (w), 1258 (s), 1189 (s), 1127 (s), 1107 (m),
1045 (w), 906 (w), 847 (w), 793 (m), 754 (m), 659 (m), 615 cmÀ1 (w).
elemental analysis calcd for C58H44Cu2F24Gd2N6O16 (1978.6): C 35.2,
H 2.2, N 4.2; found: C 35.0, H 2.1, N 4.0.
[L4CuGd(Rcamph)2(CH3OH)]2 (7): To a solution of [L4Cu](C5H12N)
(0.15 g, 3.3”10À4 mol) in methanol (10 mL) was added GdCl3·6H2O
(0.12 g, 3.3”10À4 mol), 3-(perfluorobutyryl)-(+)camphor (0.23 g,
6.5”10À4 mol) (RcamphH), and piperidine (0.10 g, 1.2”10À3 mol).
The mixture was stirred and heated to reflux for 15 min and then
cooled to room temperature. The resulting lilac precipitate was fil-
tered off and dried. It was dissolved in diethyl ether and slow diffu-
sion of methanol yielded crystals suitable for XRD analysis. Yield:
0.26 g (60%). IR (ATR): n˜ =3402 (l), 2964 (m), 1645 (s), 1600 (s),
1577 (s), 1541 (s), 1520 (m), 1476 (w), 1449 (w), 1414 (m), 1343 (m),
1291 (w), 1225 (s), 1211 (s), 1195 (m), 1177 (s), 1106 (m), 1078 (m),
1039 (w), 931 (w), 891 (m), 812 (w), 760 (m), 748 (m), 705 (w),
628 cmÀ1 (w); elemental analysis calcd for C104H122Cu2F28Gd2N6O14
(2653.7): C 47.1, H 4.6 N 3.2; found: C 46.8, H 4.5, N 3.0.
[L4CuGd(Scamph)2(CH3OH)]2 (8): The same experimental process as
described for the synthesis of compound 7 but by using 3-(per-
fluorobutyryl)-(À)camphor (ScamphH) yielded the corresponding
complex 8. Yield: 0.27 g (62%). IR (ATR): n˜ =3400 (l), 2962 (m),
1644 (s), 1600 (s), 1577 (s), 1541 (s), 1521 (m), 1476 (w), 1449 (w),
1414 (m), 1342 (m), 1290 (w), 1224 (s), 1211 (s), 1195 (m), 1177 (s),
1106 (m), 1078 (m), 1038 (w), 931 (w), 890 (m), 811 (w), 759 (m),
748 (m), 705 (w), 627 cmÀ1 (w); elemental analysis calcd for
C104H122Cu2F28Gd2N6O14 (2653.7): C 47.1, H 4.6, N 3.2; found: C 46.8,
H 4.6, N 3.1.
[L5CuGd(hfa)2(dmf)]2 (9): A solution of 2-hydroxy-N-{2-[(1-methyl-
ethylidene)amino]ethyl}benzamide (0.22 g, 1”10À3 mol) and o-van-
illin (0.15 g, 1”10À3 mol) in methanol (15 mL) was heated to reflux
and stirred for 20 min. Copper acetate (0.2 g, 1”10À3 mol) and pi-
peridine (0.3 g, 3.5”10À3 mol) were then added and heating was
pursued for 30 min. Then the solution was evaporated and the res-
idue was dissolved in dichloromethane. Evaporation of this solu-
tion yielded a pasty product. The copper complex (0.046 g, 1”
10À4 mol) and [Gd(hfa)3]·2H2O (0.082 g, 1”10À4 mol) were mixed in
DMF (2 mL). Red crystals appeared a few days later. They were col-
lected by filtration and dried. Yield: 0.045 g (47%). IR (ATR): n˜ =
2944 (w), 1667 (m), 1649 (s), 1603 (m), 1576 (m), 1541 (m), 1521
(m), 1505 (s), 1472 (m), 1457 (m), 1439 (m), 1402 (m), 1380 (w),
1334 (w), 1295 (m), 1246 (s), 1223 (m), 1193 (s), 1131 (s), 1092 (m),
1059 (m), 984 (w), 949 (w), 900 (w), 851 (w), 791 (m), 766 (w), 747
(m), 739 (m), 709 (w), 683 (w), 658 cmÀ1 (m); elemental analysis
calcd for C66H62Cu2F24Gd2N8O20 (2184.8): C 36.3, H 2.9, N 5.1; found:
C 36.0, H 2.8, N 4.9.
1208 (s), 1143 (s), 1099 (w), 1061 (w), 950 (w), 860 (w), 817 (w), 796
(m), 741 (m), 660 (s), 642 cmÀ1 (w); elemental analysis calcd for
C62H67Cu2F18GdN6O17 (1794.6): C 41.5, H 3.8, N 4.7; found: C 41.2, H
3.7, N 4.5.
[(L11Cu)2Gd2(CH3CO2)2(hfa)4(H2O)] (11): A mixture of L11Cu (0.07 g,
2.07”10À4 mol) and [Gd(hfa)2(Hhfa)(CH3CO2)] (0.17 g, 2.07”
10À4 mol) in CH2Cl2 (10 mL) was heated to reflux for ten minutes
and concentrated to half volume. Slow diffusion of pentane yield-
ed crystals suitable for XRD analysis. Yield: 0.04 g (20%). IR (ATR):
n˜ =2975 (w), 1655 (m), 1633 (m), 1608 (w), 1542 (m), 1525 (m),
1471 (w), 1446 (m), 1396 (w), 1305 (w), 1249 (s), 1188 (s), 1130 (s),
1011 (w), 949 (w), 891 (w), 791 (m), 756 (w), 659 cmÀ1 (m); elemen-
tal analysis calcd for C54H50Cu2F24Gd2N6O17: (1952.6) C 33.2, H 2.6 N
4.3; found: C 32.9, H 2.6 N 4.2.
Crystallographic data collection and structure determination for
the complexes 1, 2, 3, 4, 5, 10, and 11: Crystals of compounds 1,
2, 3, 4, 5, 10, and 11 were kept in the mother liquor until they
were dipped into oil. The chosen crystals were mounted on a Mite-
gen micromount and quickly cooled down to 180 (compounds 2,
3, and 5), 100 (compound 11), or 293(2) K (compounds 1, 4, and
10). The selected crystals of compounds 1 (red, 0.625”0.15”
0.075 mm3),
2
(violet, 0.15”0.125”0.05 mm3),
(red, 0.25”0.20”0.15 mm3),
3
(dark purple,
(blue,
0.18”0.15”0.04 mm3),
4
5
0.375”0.25”0.075 mm3), 10 (red, 0.325”0.15”0.125 mm3), and 11
(blue, 0.375”0.25”0.075 mm3) were mounted on a Bruker Kappa
Apex II (compounds 2 and 3), an Oxford Diffraction Gemini (com-
pound 11), or a Stoe Imaging Plate Diffractometer System (IPDS)
(1, 4, 5, 10) using molybdenum (l=0.71073 Š) and equipped with
an Oxford Cryosystems cooler device. The unit cell determination
and data integration were carried out by using XRED,[26] CrysAlis
RED, or SAINT packages.[27–29] The structures have been solved by
using SIR92,[30] SUPERFLIP,[31] or SHELXS-97[32] and they have been
refined by least-squares procedures by using the software pack-
ages CRYSTALS[33] or WinGX version 1.63.[34] Atomic scattering fac-
tors were taken from the international tables for X-ray crystallogra-
phy.[35] All hydrogen atoms were refined by using a riding model.
When it was possible, all non-hydrogen atoms were anisotropically
refined. Drawings of the molecules have been performed with the
program CAMERON.[36] CCDC 1055186 (1), 1055187 (2), 1055188
(3), 1055189 (4), 1055190 (5), 1055191 (10), and 1055192 (11) con-
tain the supplementary crystallographic data for this paper. These
data can be obtained free of charge from The Cambridge Crystallo-
graphic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
Crystal data for compound 1: C25H31CuN3O3; M=485.07; monoclinic;
P21/c; Z=4; a=11.252(2), b=19.285(4), c=11.357(2) Š; a=g=90,
b=112.66(3)8; V=2274.2(8) Š3; 16356 collected reflections; 4458
unique reflections (Rint =0.0612); R=0.0393; Rw =0.0538 for 2759
contributing reflections [I>2s(I)].
Crystal data for compound 2: C21H25CuN3O3; M=430.98; ortho-
rhombic; Pbca; Z=8; a=10.065(5), b=16.999(5), c=22.395(5) Š;
a=b=g=908; V=3832(2) Š3; 14911 collected reflections; 3365
unique reflections (Rint =0.0314); R=0.0280, Rw =0.0673 for 2705
contributing reflections [I>2s(I)].
Crystal data for compound 3: C23H29CuN3O3; M=459.03; monoclin-
ic; P21/c; Z=4; a=10.8652(7), b=15.9059(9), c=13.0538(8 Š; a=
g=90, b=109.225(3)8; V=2130.2(2) Š3; 34713 collected reflec-
tions; 4356 unique reflections (Rint =0.0304); R=0.0249; Rw =
0.0645 for 3716 contributing reflections [I>2s(I)].
[(L10Cu)2Gd(hfa)3](C3H6O)3 (10): Addition of [Gd(hfa)3]·2H2O (0.05 g,
6.1”10À5 mol) to L4Cu (0.05 g, 1.2”10À4 mol) in acetone (5 mL)
gave a solution that was filtered and set aside. Crystals appeared
three days later. Yield: 0.05 g (45%). IR (KBr): n˜ =2974 (w), 1656 (s),
1580 (s), 1539 (s), 1495 (m), 1449 (m), 1398 (s), 1343 (w), 1254 (s),
Crystal data for compound 4: C19H23CuN3O4; M=420.94; monoclin-
ic; P21/n; Z=4; a=11.271(2), b=11.792(2), c=14.480(3) Š; a=g=
90, b=110.21(3)8; V=1806.0(6) Š3; 12982 collected reflections;
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3549 unique reflections (Rint =0.0617); R=0.0378; Rw =0.0603 for
2553 contributing reflections [I>2s(I)].
Crystal data for compound 5: C72H70Cu2F24Gd2N8O18; M=2232.94;
¯
triclinic; P1; Z=2; a=11.558(5), b=12.101(5), c=18.338(5) Š; a=
75.141(5), b=87.455(5), g=65.439(5)8; V=2249.1(15) Š3; 21836
collected reflections; 8003 unique reflections (Rint =0.0568); R=
0.0479; Rw =0.1210 for 6803 contributing reflections [I>2s(I)].
Crystal data for compound 10: C60H59Cu2F18GdN6O15; M=1730.5; or-
thorhombic; Pca21; Z=4; a=25.650(5), b=21.370(4), c=
12.614(3) Š; a=b=g = 90.008; V=6914(2) Š3; 43520 collected re-
flections; 10948 unique reflections (Rint =0.1107); R=0.0545; Rw =
0.1070 for 7304 contributing reflections [I>2s(I)].
Crystal data for compound 11: C54H50Cu2F24Gd2N6O17; M=1952.58;
monoclinic; P21/n (No.14); Z=4; a=18.9557(2), b=18.7083(1), c=
20.0267(2) Š; b=95.4704(8)8; V=7069.7(1) Š3; 120093 collected re-
flections; 17445 unique reflections (Rint =0.037); R=0.046; Rw =
0.051 for 13743 contributing reflections [I>3s(I)].
Relevant parameters for complex [L4CuGd(Scamph)2(CH3OH)]2 (7):
Space group: P21; lattice parameters: a=17.874(5), b=15.377(5),
c=20.789(5)) Š; a=g=90, b=112.829(5)8; V=5266(3) Š3. These
data do confirm that complex 8 is tetranuclear with the expected
C94H88Cu2F28Gd2N4O16 formula and DMF molecules coordinated to
the Cu and Gd ions. The non-centrosymmetry comes from the use
of a pure enantiomeric (S)-camphorate ligand. The cif data are not
given due to problems coming from refinement of the fluorinated
camphorate chain.
Scheme 1. Schematic representation of the various ligands used within this
study. L2H3 =2-hydroxy-N-(2-{[(2-hydroxyphenyl)methylene]amino}cyclohex-
yl)benzamide, L3H3 =2-hydroxy-N-(2-{[(2-hydroxyphenyl)methylene]amino}
ethyl)benzamide, L4H3 =2-hydroxy-N-(2-{[(2-hydroxyphenyl)methylene]ami-
no}-2-methylpropyl)benzamide, L5H3 =2-hydroxy-N-(2-{[(2-hydroxy-3-meth-
oxyphenyl)methylene]amino}ethyl)benzamide, L6H3 =2-hydroxy-N-(2-{[(2-hy-
droxy-3-methoxyphenyl)methylene]amino}-2-methylpropyl)benzamide,
L7H3 =2-hydroxy-N-(2-{[(2-hydroxyphenyl)methylene] amino}ethyl)-3-meth-
oxybenzamide, L7H3 =2-hydroxy-N-(2-{[(2-hydroxyphenyl)methylene]ami-
no}ethyl)-3-methoxybenzamide, L8H3 =2-hydroxy-N-(2-{[(2-
Results and Discussion
Syntheses
The non-symmetric compartmental ligands LnH3 with n going
from 2 to 9, and L10H2, used in this study have been prepared
according to an experimental stepwise procedure,[17b,c] by reac-
tion of “half-unit” ligands, possessing the amide function, with
hydroxyphenyl)methylene]amino}-2-methylpropyl)-3-methoxybenzamide,
L9H3 =2-hydroxy-N-(2-{[(2-hydroxy-3-methoxyphenyl)methylene]amino}-2-
methylpropyl)-3-methoxybenzamide, L10H2 =2-hydroxy-3-methoxy-N-{2-
methyl-2-[(pyridine-2-ylmethylidene]amino]propyl)benzamide.
salicylaldehyde,
o-vanillin,
or
pyridinecarboxaldehyde
(Scheme 1). Note that we previously reported several “half-
unit” ligands, namely, 2-hydroxy-N-{2-[(1-methylethylidene)
amino]ethyl}benzamide,
N-(2-amino-2-methylpropyl)-2-hy-
droxybenzamide,[17c] and N-(2-amino-2-methylpropyl)-2-hy-
droxy-3-methoxybenzamide[17b], whereas the preparation of N-
(2-aminocyclohexyl)-2-hydroxybenzamide has been recently re-
ported.[37] The LnH3 ligands (n=2 to 9) possess identical inner
N2O2 coordination sites with one amide, one imine, and two
phenol functions. They slightly differ by their diamino chain
but mainly by their outer coordination sites that involve two
(O2), three (O2O), and four (O2O2) oxygen atoms. The ligand
L10H2 is made of one amide, one imine, one phenol, and one
pyridine functions, the pyridine function replacing a phenol
function. The monometallic copper complexes are readily ob-
tained by reaction of the ligands dissolved in methanol with
copper(II) acetate in the presence of piperidine as deprotonat-
ing agent. The desired [LnCu]pipH (pipH=piperidine) (n=2 to
9) and L10Cu complexes are obtained as crystals from the re-
sulting reaction media with satisfying yields. The complexes 1–
3 are ionic whereas complex 4 is neutral, due to the presence
of three or only two ligand functions that can be deprotonat-
ed. The heterometallic Cu–Gd complexes are isolated by reac-
tion of these precursors with [Gd(hfa)3]·2H2O (hfa=hexafluoro-
acetylacetonato ligand) or GdCl3·6H2O and the 3-(perfluorobu-
tyryl)-camphor ligands (i.e., ScamphH or RcamphH) in metha-
nol. According to chemical analysis, they are well represented
by the formulae [LnCuGd(X)2] (Ln =L2 to L9 trianionic ligands)
with or without DMF or methanol molecules (X=nitrato, hfa,
or camph anion). Due to the dianionic nature of the L10H2
ligand, the analytical results confirm a 2:1 Cu/Gd ratio and the
formulation [(L10Cu)2Gd(hfa)3], with replacement of the two
water molecules present in [Gd(hfa)3]·2H2O by two L10Cu frag-
ments. On the contrary the L11Cu copper complex with
a
unique oxime function is known to react with
[Gd(hfa)3]·2H2O to give a dinuclear [L11CuGd(hfa)3(H2O)] entity
that was characterized by analytical results and magnetic prop-
erties.[19b] The structural determination of the complex resulting
from reaction of [L11Cu] with [Gd(hfa)2(Hhfa)(CH3CO2)] indicates
that we are dealing with a tetranuclear complex giving a new
example of oximato Cu-N-O-Gd bridges.
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Description of the structures
function in the two complexes 1 and 2, thus giving 1D chains.
The alternate arrangement of (R,R)-L2Cu and (S,S)-L2Cu units in
complex 1 yields a racemate chain. The head-to-tail arrange-
ment of two L4Cu units yields a hydrogen-bonded dinuclear
entity in complex 3.
The asymmetric units of complexes 1, 2, and 3 comprise the
monoanionic entities L2Cu, L3Cu, or L4Cu with their cationic pi-
peridinium counterparts (Figure 1 for complex 1 and Fig-
ures S1 and S2 in the Supporting Information for complexes 2
and 3, respectively). They crystallize in the monoclinic space
Replacement of a phenol function by a pyridine nitrogen
atom in complex 4 gives a neutral [L10Cu] species (Figure S3 in
the Supporting Information) with the CuII ion in the dianionic
N3O coordination site. The copper ion remains in a square-
planar environment but the presence of cycles involving 6-5-5
atoms around the Cu ion instead of a 6-5-6 surroundings in
complexes 1–3 shortens the CuÀO and CuÀN(amide) bonds.
Crystals were obtained for complexes 5 and 7. Due to their
similarity, we only describe in detail complex 5. In these com-
plexes two heteronuclear Cu–Gd entities are assembled in
a head-to-tail arrangement through the oxygen atoms of the
amidato groups to form a double (Cu-N-C-O-Gd) bridge lead-
ing to a tetranuclear entity (Figure 2). In the dinuclear Cu–Gd
Figure 1. View of the mononuclear complex [L2Cu(C5H12N)] (1).
groups P21/c (complexes 1 and 3) or Pbca (complex 2). The CuII
ions are in the trianionic N2O2 coordination sites, linked in
a square-planar environment to the amidato and imine nitro-
gen atoms and the two phenoxo oxygen atoms, with similar
CuÀO and CuÀN bond lengths for complexes 1, 2 and 3 (see
Table 1), the amide oxygen atom being not involved in the co-
ordination. There are hydrogen bonds involving the free amide
oxygen atom, the piperidinium cation, and the phenoxo
oxygen atom located on the phenyl cycle bearing the amide
Figure 2. View of the tetranuclear complex 5. Selected bond lengths [Š] and
angles [8]: Cu2ÀN2 1.934(5), Cu2ÀN1 1.951(4), Cu2ÀO1 1.914(4), Cu2ÀO2
1.977(4), Cu2ÀO4 2.388(4), O3ÀC23 1.290(7), Gd1ÀO2 2.373(4), Gd1ÀO1
2.411(4), Gd1ÀO3 2.252(4), Gd1ÀO5 2.374(4), Gd1ÀO6 2.390(4), Gd1ÀO7
2.469(4), Gd1ÀO8 2.480(4), Gd1ÀO9 2.450(4) Š; Gd1-O1-Cu2 102.7(1), Gd1-
O2-Cu2 102.1(1)8.
units, the two ions are doubly bridged by two phen-
Table 1. Selected bond lengths and angles for the mononuclear copper complexes
1–4.
oxo oxygen atoms belonging to the L2 ligand with
Cu···Gd separations of 3.393(1) Š. The bridging net-
work is reported in Scheme 2a. The copper ion is
penta-coordinate, with a long axial CuÀO(dmf) bond
length (i.e., 2.388(2) Š). The gadolinium ions are
eight-coordinate, surrounded by five supplementary
oxygen atoms coming from two ancillary hfa mole-
cules and one DMF molecule. The bridging networks
Cu-(O2)-Gd are not planar, with hinge angles between
the (O-Cu-O) and (O-Gd-O) planes equal to 20.8(1)8.
The intermolecular copper–copper separations are
shorter (i.e., 8.516(2) Š) than the Gd···Gd distances
(i.e., 10.891(2) Š). They preclude any significant inter-
action of magnetic nature between these tetranu-
clear units.
[L2Cu(C5H12N)] (1) [L3Cu(C5H12N)] (2) [L4Cu(C5H12N)] (3) [L10Cu] (4)
bond lengths [Š]
Cu1ÀN1
1.932(2)
1.923(2)
1.877(2)
1.913(2)
1.912(2)
1.936(2)
1.902(2)
1.911(2)
1.914(1)
1.950(1)
1.907(1)
1.908(1)
1.883(2)
1.952(2)
1.862(2)
Cu1ÀN2
Cu1ÀO1
Cu1ÀO2
Cu1ÀN3
1.995(2)
1.254(3)
OÀC amide
angles [8]
O1-Cu1-O2
O1-Cu1-N1
N2-Cu1-N1
O2-Cu1-N2
O1-Cu1-N3
N2-Cu1-N3
1.260(3)
1.266(3)
1.263(2)
86.06(7)
95.56(8)
86.31(9)
92.11(8)
86.50(6)
94.86(7)
86.05(8)
92.79(7)
87.47(5)
94.35(5)
85.41(6)
93.53(5)
98.16(9)
84.88(9)
97.52(9)
81.09(9)
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Scheme 2. Bridging motive in a) complexes 5 and 7 and b) complex 10.
Figure 4. View of the tetranuclear [Cu–Gd]2 complex 11. Selected bond
lengths [Š] and angles [8]: Cu1ÀN1 1.963(4), Cu1ÀN2 1.946(4), Cu1ÀN3
1.993(4), Cu1ÀO1 1.926(3), Cu1ÀO8 2.289(3), Cu2ÀN4 1.953(4), Cu2ÀN5
1.959(4), Cu2ÀN6 1.997(4), Cu2ÀO3 1.917(3), Cu2ÀO9 2.327(3), Gd1ÀO2
2.318(3), Gd1ÀO4 2.382(3), Gd1ÀO5 2.584(3), Gd1ÀO6 2.453(4), Gd1ÀO7
2.499(3), Gd1ÀO8 2.489(3), Gd1ÀO9 2.486(3),Gd1ÀO10 2.406(3), Gd1ÀO11
2.439(3), Gd2ÀO4 2.479(3), Gd2ÀO5 2.569(3), Gd2ÀO7 2.390(3), Gd2ÀO12
2.435(3), Gd2ÀO13 2.469(4), Gd2ÀO14 2.357(3), Gd2ÀO15 2.400(3), Gd2ÀO16
2.424(3), Gd2ÀO17 2.409(4), Gd1···Gd2 3.8678(3) Š; Cu1-N3-O2 123.8(3), Cu2-
N6-O4 126.6(3), Cu1-O8-C33 136.3(3), Cu2-O9-C33 128.1(3), Gd1-O2-N3
117.4(2), Gd2-O4-N6 128.5(2), Gd1-O5-Gd2 97.3(1), Gd1-O7-Gd2 104.6(1),
Gd1-O4-Gd2 105.4(1)8.
Figure 3. View of the trinuclear complex 10. Selected bond lengths [Š] and
angles [8]: Cu1ÀN4 2.018(8), Cu1ÀN5 1.932(8), Cu1ÀN6 1.890(8), Cu1ÀO4
1.875(7), Cu2ÀO1 1.880(6), Cu2ÀN1 1.971(9), Cu2ÀN2 1.957(8), Cu2ÀN3
1.889(7), O2ÀC11 1.289(11), O5ÀC29 1.324(12), Gd1ÀO2 2.258(8), Gd1ÀO5
2.280(7), Gd1ÀO7 2.425(9), Gd1ÀO8 2.400(5), Gd1ÀO9 2.409(7), Gd1ÀO10
2.467(7), Gd1ÀO11 2.437(8), Gd1ÀO12 2.373(7) Š; Cu1-O1-Cu2 94.71(3), Cu1-
O4-Cu2 98.02(4)8.
The asymmetric unit of complex 10 consists of the trinuclear
neutral [(L10Cu)2Gd(hfa)3] unit, shown in Figure 3, and three
acetone molecules crystallizing in the orthorhombic space
group Pca21. Two L10Cu molecules are linked to a gadolinium
ion through their oxygen amide atom (2.258(8) and 2.280(7) Š
for GdÀO2 and GdÀO5, respectively) and the three ions are lo-
cated at the apexes of a practically perfect isosceles triangle
(Gd···Cu1=5.837(2), Gd···Cu2=5.897(2) Š, with Cu1···Cu2=
3.457(2) Š). The copper ions are five-coordinate, linked to the
N3O inner coordination site of the dideprotonated L10 ligand,
the fifth axial coordination coming from the phenoxo function
Scheme 3. Bridging motive in complex 11.
given in Scheme 3. The copper ions are bridged together by
an acetate group occupying the axial position of each copper
ion and linking the two copper ions in an anti–anti coordina-
tion mode with a Cu···Cu distance of 6.278(4) Š whereas it che-
lates at the same time a Gd ion in a syn–syn mode. The two
Gd ions are linked by three different bridges, a supplementary
m-acetato-1,2-k-O,-2-k-O’, a m-oximato-1,2-k-O, and a water
molecule bridged in a m-1,2-k-O mode, according to the kappa
of the neighboring L10Cu unit (Cu1ÀO1=2.751(1) and Cu2À convention. The two Gd ions are nona-coordinate. The Gd2 ion
O4=2.655(1) Š and Cu-O-Cu angles of 94.7(1) and 98.0(1)8, re-
spectively), thus inducing a triangular arrangement of the pres-
ent trinuclear complex (see Scheme 2b). The gadolinium ions
are again eight-coordinate to the six oxygen atoms belonging
to three hfa ligands and to two oxygen amide atoms coming
from the two L10Cu units.
Complex 11 is a tetranuclear complex made of two L11Cu
units linked to two different Gd ions by the oxygen atoms of
their oxime functions, as shown in Figure 4. A schematic repre-
sentation of the metal ions interaction pathways with the
numbering of the oxygen atoms retained in the structure is
is linked to the three oxygen atoms of the bridges and to six
oxygen atoms coming from three chelating hexafluoroacetyl-
acetonato ligands whereas the Gd1 ion is coordinated to an
acetate ligand and to a chelating hexafluoroacetylacetonato
ligand, linked to an oximato oxygen atom and to four oxygen
atoms from the three bridges. The single oximato Gd1ÀO2
bond (2.318(3) Š) is shorter than the oximato-bridged one
(2.382(3) Š for Gd1ÀO4 and 2.479(3) Š for Gd2ÀO4). The Gd1
ion is chelated by the two acetate ligands with bonds varying
from 2.453(4) to 2.499(3) Š but the h2 oxygen atom of the
bridging acetate group presents two different bond lengths
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(2.390(3) Š for Gd2ÀO7 vs. 2.499(3) Š for Gd1ÀO7). The water
molecule is located in a practically symmetrical position in be-
tween the two Gd ions (2.584(3) vs. 2.569(3) Š). This results in
a Gd···Gd distance of 3.868(3) Š. Although the Cu1···Gd1 and
Cu2···Gd1 distances are equivalent, that is, 4.030(3) and
4.054(3) Š, the Cu2···Gd2 distance is larger (4.636(4) Š).
double phenoxo Cu-(O2)-Gd bridged form and the lower value
to the single amidato bridge (Cu-N-C-O-Gd’), which is in agree-
ment with previous results.[17,18] The magnetic studies of the
other [Cu–Gd]2 complexes (i.e., complexes 6–9) give similar
data, which are reported in Table 2, the thermal variations of
the cMT versus T plots appearing in the Supporting Information
(Figures S4–S7). Table 2 also reports the magnetic data coming
from previously published tetranuclear complexes.[17,18] Confir-
Magnetic properties
The magnetic susceptibility of the tetranuclear Cu2–
Table 2. Interaction parameters found in tetranuclear [Cu–Gd]2 complexes possessing
alternate double phenoxo and amidato bridges.
Gd2 complexes 5, 6, 7, 8, and 9 have been measured
in the temperature range 2–300 K in a 0.1 T applied
magnetic field. As an example the thermal variation
of the cMT versus T plot of complex 5 is shown in
Figure 5, with cM being the molar magnetic suscepti-
bility of the tetranuclear complex, corrected for the
diamagnetism of the ligands.[22] At 300 K, the cMT
value is equal to 16.53 cm3 molÀ1 K, which is in agree-
ment with the expected value for two CuII ions and
two GdIII ions, which are uncoupled (i.e.,
16.50 cm3 molÀ1 K with g=2). Lowering the tempera-
ture causes the cMT value to increase and reach
a value of 29.34 cm3 molÀ1 K at 2 K, which is larger
than expected (2”10 cm3 molÀ1 K) for two uncorrelat-
Complex
J [cmÀ1
]
J’ [cmÀ1
]
g
R
Reference
[L2CudmfGd(hfa)2dmf]2
[L3CuGd(hfa)2dmf]2
4.17
1.95
3.20
0.77
0.46
0.54
0.61
0.51
0.53
2.4
0.82
0.64
0.82
0.58
1.98
7”10À5 compound 5
1.99 4.6”10À5 compound 6
[L4CuGd(hfa)2 MeOH]2
2.00
2.00
3”10À5 [17a]
6”10À5 compound 7
[L4CudmfGd(camph+)2dmf]2 2.44
[L4CudmfGd(camph-)2dmf]2
[L5 CuGd(hfa)2dmf]2
2.87
3.85
6.2
5.89
2.80
7.96
6.94
2.04 1.5”10À5 compound 8
2.04 1.1”10À5 compound 9
[L5CuGd(hfa)2 MeOH]2
[L6CuGd(NO3)2OH2]2
2.02
1.99
1.99
1.99
1.99
2”10À5 [18a]
4”10À5 [17b]
1”10À4 [17b]
1”10À5 [17b]
1”10À5 [17d]
[L7CuGd(NO3)2OH2]2
[L8CuGd(NO3)2MeOH]2
[L9CuGd(NO3)2MeOH]2
mation of the nature of the ground state and, consequently of
the nuclearity of complex 8 is afforded by the field depend-
ence of the magnetization M at 2 K (Figure S8 in the Support-
ing Information). Increasing the field up to 5 T causes the mag-
netization to approach the saturation value of 16 Nb units,
which is consistent with a S=8 and g=2 system.
The thermal variation of the cMT versus T plot for the trinu-
clear complex 10 goes from 8.5 cm3 molÀ1 K at 300 K to
9.11 cm3 molÀ1 K at 2 K. As this variation is weak, Figure 6 only
reports the data from 100 to 2 K. In that case the experimental
data can be properly fitted by using an Hamiltonian, taking
into account Cu–Gd and Cu–Cu interactions, that is, H=
ÀJ(SCuSGd+SCu’SGd)ÀJ’(SCuSCu’)+ÆijgibHjSij, which yields J=
Figure 5. Temperature dependence of the product cMT for complex 5 at
a 0.1 T applied magnetic field. The solid line corresponds to the best data fit
(see text).
0.40(2) cmÀ1, J’=À2.60(5) cmÀ1, g=1.99, and R=7.5”10À4
.
ed pairs of ferromagnetically coupled CuII and GdIII ions but
smaller than the maximum value (36 cm3 molÀ1 K), which is at-
tributable to a S=8 spin state resulting from the ferromagnet-
ic coupling of two Cu–Gd pairs. Such a behavior is consistent
with the simultaneous occurrence of two ferromagnetic Cu-Gd
interactions, which operate within each Cu–Gd pair and
These values allow to fit the field dependence of the magneti-
zation M at 2 K (Figure S9 in the Supporting Information), with
a saturation value at 5 T equal to 8.7 Nb corresponding to an
S=9/2 and g=2 ground state. The magnetization curve does
between two pairs, respectively.
A quantitative analysis
based on a “dimer of dimer” model directly derived from the
structural data and by using the Hamiltonian H=
ÀJ(SCuSGd+SCu’SGd’)ÀJ’(SCuSGd’+SCu’SGd)+ÆijgibHjSij with the terms
gauged by the J and J’ parameters accounting for the spin ex-
change and the last term accounting for the Zeeman contribu-
tions where i=Cu and Gd, and j=x, y, and z, leads to J=
4.17(5) cmÀ1, J’=0.77(5) cmÀ1, and g=1.99 with an agreement
factor R (R=S[(cT)obsÀ(cT)calcc]2/S[(cT)obs]2) equal to 7”10À5. The
two coupling constants characterizing complex 5 differ by an
order of magnitude so that their respective pathways are
easily recognized, the larger value corresponding to the
Figure 6. Temperature dependence of the product cMT for complex 10 at
a 0.1 T applied magnetic field. The solid line corresponds to the best data fit
(see text).
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not present a sigmoidal form because the J(Cu,Gd) interaction
is ferromagnetic, with a value which is in complete agreement
with those obtained in the tetranuclear species.
camph) able to chelate the Gd ion is added to the reaction
medium. With nitrato anions, ligands having O4 and O3 outer
coordination sites are needed to obtain tetranuclear com-
plexes. The solvent (MeOH or DMF) completes the Gd coordi-
nation sphere, yielding eight-coordinate Gd ions with the li-
gands L2–L4 and nine-coordinate Gd ions with the ligands L5–
L7. Note that the Gd ions remain nine-coordinate in the case of
ligands L8 and L9 with their O4 outer coordination sites, due to
the absence of coordinated solvent. Methanol never enters the
Cu coordination sphere whereas DMF can coordinate in the
axial position with ligands presenting methyl groups on their
diamino chain (i.e., ligands L2, L4, and L6). Two Cu–Gd interac-
tion pathways are present in these complexes, one through
a double phenoxo bridge and the other one through the ami-
dato bridge. The magnetic studies demonstrate that ferromag-
netic Cu–Gd interactions are active in the two cases, with J
values varying from 2 to 10 cmÀ1 for the double phenoxo
bridge and from 0.4 to 1.1 cmÀ1 for the amidato bridge (J’ pa-
rameter).[17,18] A view at Table 2 indicates that the larger J
values involve complexes possessing nitrato ligands in the Gd
coordination sphere. The organic hfa and camph ligands,
which imply larger steric constraints, yield lower J values. On
the contrary, it is difficult to find a correlation for the J’ param-
eter, as for example the change from nine-coordinate to eight-
coordinate of the gadolinium coordination spheres.
In the tetranuclear complex 11, the product cMT remains
practically constant and equal to 16.15 cm3 molÀ1 K from 300 to
50 K, where it begins to decrease till
a
value of
13.49 cm3 molÀ1 K at 2 K (Figure 7). A value of 16.5 cm3 molÀ1 K
is expected at room temperature for two Cu and two Gd ions,
which do not interact. In view of the structural determination,
we have to take into account at least two different Cu–Gd in-
teractions along with a Gd–Gd interaction. Nevertheless, we
Figure 7. Temperature dependence of the product cMT for complex 11 at
a 0.1 T applied magnetic field. The solid line corresponds to the best data fit
(see text).
can eliminate the Cu–Cu interaction, because the anti–anti ace-
tate bridge links the two Cu ions in their axial position, where
the spin density is expected to be almost zero. Concerning the
Gd–Gd interaction, the m-acetato-1,2-k-O,-2-k-O’ bridge is
known to induce ferromagnetic interactions,[38–40] whereas the
bridging oxygen atoms of water molecules and oxime func-
tions[41] are expected to yield antiferromagnetic interactions. As
these interactions are always weak, these antagonist effects
should give a very weak parameter. We have previously shown
that the Cu–Gd interaction through an oxime bridge is antifer-
romagnetic.[19] The best fit obtained with the Hamiltonian H=
We tried to simplify the magnetic analysis with the design of
the new L10H2 ligand in which pyridine replaces one phenol
function. Two possible gadolinium coordination modes to the
neutral copper complex were expected, that is, through the
single phenoxo oxygen atom or through the oxygen atom of
the amide function. The structural determination of complex
10 confirms a coordination through the amide bridge, but the
triangular arrangement of this complex introduces a slight an-
tiferromagnetic Cu–Cu interaction favored by the weak axial
coordination of each Cu ion to the phenoxo oxygen atom of
the neighboring L10Cu unit, thus yielding a double phenoxo
bridge (Scheme 2b) with long Cu1ÀO1 (2.751(1) Š) and Cu2À4
(2.655(1) Š) bond lengths. The magnetization curve at 2 K
tends to the expected value for an S=9/2 ground state with-
out showing a sigmoidal form (Figure S9 in the Supporting In-
formation). With help of that complex it becomes clear that
the Cu–Gd interactions through the three atoms amidato
ÀJ1(SCu1SGd1)ÀJ2(SCu2 Gd1+SCu2SGd2)ÀJ3(SGd1SGd2)+ÆijgibHjSij, which
S
yields J1(Cu,Gd)=À0.36(5) cmÀ1, J2(Cu,Gd)=À0.19(5) cmÀ1,
J3(Gd,Gd)=À0.023(5) cmÀ1, g=1.99, and R=1”10À5, which is
in complete agreement with our expectation. Due to the pres-
ence of several interaction parameters, this result is subject to
caution, concerning the type of the very weak Gd–Gd interac-
tion but this supplementary example confirms that the Cu–Gd
interaction through the oxime bridge is antiferromagnetic,
which is in agreement with our previous results.[19]
bridge are comprised in the range 0.4–1 cmÀ1
.
We have recently shown that the Cu–Gd interaction through
the oximato bridge made of an even number of atoms, that is,
nitrogen and oxygen atoms, is surprisingly antiferromagnetic
in complexes formulated as [(L11Cu)2Gd(NO3)3(H2O)] and
[L11CuGd(hfa)3].[19] The use of [Gd(hfa)2(Hhfa)(CH3CO2)] as start-
ing material instead of [Gd(hfa)3]·2H2O allowed the isolation
and structural determination of a tetranuclear complex. Un-
fortunately, the presence of different Cu–Gd interactions along
with Gd–Gd interaction pathways does not simplify the mag-
netic study. Nevertheless, this complex does confirm the anti-
ferromagnetic character of the Cu–Gd interaction through the
oximato bridge.
Discussion
These results clearly demonstrate that the use of ligands pos-
sessing amide, imine, and phenoxo functions yield tetranuclear
[Cu–Gd]2 complexes with an alternate arrangement of the Cu
and Gd ions linked by bridges involving one oxygen atom
(phenoxo bridge) or three atoms (O-C-N) for the interaction
through the amide bridge. Whatever the nature of the outer
coordination site is, O2 for the ligands L2, L3, and L4, O3 for the
ligands L5, L6, and L7, O4 for the ligands L8 and L9, tetranuclear
complexes are isolated if an ancillary ligand (namely, hfa or
Previous and present results indicate that the 3dx2 orbitals
Ày2
play an important role in the Cu–Gd magnetic interaction.
Chem. Eur. J. 2016, 22, 2171 – 2180
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Indeed the Cu–Gd complexes with double phenoxo bridges
(edge-sharing complexes) give the largest ferromagnetic inter-
actions. In these complexes, the main structural parameter is
the M-(O2)-Gd hinge angle, defined as the angle between the
O-M-O and O-Gd-O planes of the bridging unit. A hinge angle
Whether the appearance of a spin density in the 5d orbitals re-
sults from a spin delocalization of monoelectronic origin, or
from a spin polarization phenomenon of bi-electronic nature,
is not clear. This question would require some restricted versus
unrestricted calculations.[54] For convenience, and as frequently
done, we use hereafter the term spin polarization for the intro-
duction of spin densities from remote unpaired electrons.
The results reported in this paper furnish an experimental
support playing again in favor of an active spin polarization in
3d Gd complexes. Indeed we show that oxygen phenoxo
bridges (single atom bridges) and deprotonated O-C-N amide
bridges (three-atom bridges) transmit ferromagnetic interac-
tion when the oximato bridge (two-atom bridge) transmits an-
tiferromagnetic interaction. When spin polarization is active an
odd number of bridging atoms in between the ions bearing
the spins favors a parallel alignment of the spins, whereas an
even number of bridging atoms favors an antiparallel align-
ment.[55–57] So this could explain why the oxime function, with
its nitrogen and oxygen atoms in between the Cu and Gd
ions, is responsible for the presence of an antiferromagnetic
Cu–Gd interaction when the phenoxo function with its unique
oxygen atom and the amide bridge with three oxygen, carbon
and nitrogen atoms give a ferromagnetic Cu–Gd interaction.
As spin polarization is transmitted through bonds,[55] we can
understand that the complexes with planar M-O2-Gd cores
of 1.7(2)8 is associated with
a J coupling constant of
10.1 cmÀ1 [42]
,
whereas a hinge angle of 41.6(2)8 gives a J value
of 0.2 cmÀ1.[43] Similar examples are found in Ni–Gd com-
plexes.[44,45] Furthermore, a powerful example is also given by
the Co–Gd complexes in which CoII ions are in a high-spin
state and penta-coordinate or in a low-spin state and square-
planar coordinate.[15] So the complex [L3CoGd(thd)2(MeOH)]2
(thd=tetramethylheptanedionato), in which the Co ion is in
a low-spin state, presents an antiferromagnetic interaction
(J(Co,Gd)=À0.9 cmÀ1) through the double phenoxo bridge.[15b]
And the complex [L5Co(MeOH)Gd(thd)(MeCOO)]2, with its
penta-coordinate and high-spin state Co ion, gives ferromag-
netic interactions through the double phenoxo (J(Co,Gd)=
0.40 cmÀ1) and amidato (J’(Co,Gd)=0.24 cmÀ1) bridges.[15a] In
the low-spin case the 3dx2
cobalt orbitals are unoccupied
Ày2
and the Co–Gd interaction is antiferromagnetic,[15b] whereas
the Co–Gd interaction becomes ferromagnetic in the high-spin
case with a single occupation of the 3dx2 orbital.[15a,16] More-
Ày2
over, the entire set of dinuclear M–Gd complexes in which the
M ions do possess singly occupied dx2
orbitals (M=CuII,[42]
Ày2
NiII,[44,45] CoII(high-spin state),[15,16,46] FeII(high-spin state),[46,47] or
MnII (high-spin state)[46,48,49] do present ferromagnetic interac-
tions through the double phenoxo bridge. On the contrary,
weak antiferromagnetic interactions through the double phe-
noxo bridge are observed in CoII (low-spin state)–Gd,[15a] CrIII–
favor a better interaction between the s 3dx2
the weakly populated 5d Gd orbitals.[42,45]
orbitals and
Ày2
Conclusions
Gd,[50,51] and MnIII–Gd compounds,[52,53] with unoccupied 3dx2
This paper reports on a straightforward process for easy prepa-
ration and isolation of genuine mononuclear copper com-
plexes, accomplished by the use of piperidinium counterions.
In these complexes the copper ions are chelated in a N2O2 co-
ordination site by ligands with three functions that can be de-
protonated and a supplementary oxygen atom that belongs to
an amide function, which, at a later stage, is able to coordinate
to Gd ions to yield heterotetranuclear [Cu–Gd]2 complexes.
The resulting [Cu–Gd]2 complexes, in which the Cu and Gd
ions are alternately arranged, clearly shows that Cu–Gd interac-
tions through bridges made of single (oxygen) or three
(oxygen, carbon, and nitrogen) atoms present ferromagnetic
interactions. We also report a new example of an antiferromag-
netic Cu–Gd interaction through the oximato bridge made of
two (nitrogen and oxygen) atoms. These experimental obser-
vations imply that spin polarization, first expected thirty years
ago by Dante et al.,[1] then retained in recent theoretical calcu-
lations, is the mechanism that allows the reproduction of
strength and sign of the magnetic Cu–Gd interactions in com-
plexes with Cu and Gd ions. The angular dependence of the
ferromagnetic interactions present in simple 3d-Gd complexes,
Ày2
orbitals. These results have a double interest. As they highlight
the role of the dx2
orbital of the 3d ions, they give a direct
Ày2
confirmation for the participation of the 5d Gd orbitals to the
interaction mechanism. Some 5d orbitals of Gd being of the
same symmetry as the 3dx2
molecular orbital of the transi-
Ày2
tion-metal atom, delocalization may take place between them,
which will result in the appearance of some spin density in
these Gd orbitals. This small delocalization tail of the magnetic
orbital centered on the transition-metal atom will induce
a magnetic coupling with the unpaired electrons of the Gd
atom. Furthermore the angular dependence of the J parameter
eliminates the participation of the spherical 6s orbital.
Till now, there appeared several theoretical papers devoted
to the understanding of the Cu–Gd interaction.[7–14] We must
keep in mind that in the case of Ln ions, the 4f orbitals are
shielded and that the 6s and 5d orbitals are the outer orbitals.
Although the 5d Gd orbitals are expected to be empty, it has
been shown that these 5d orbitals must be included in the cal-
culations to reproduce the experimental J values.[7,9,10,14] So
Ruiz et al. demonstrated that the 5d Gd orbitals have a positive
spin population, which has to be taken into account to calcu-
late the J parameters, and that the 5d spin population does
neither come from a 5d–4f mixing nor from a spin transfer of
the 3d orbitals.[14] Their conclusion indicates that the spin po-
larization of the metal–ligand bonding electron pairs involving
the formally empty 5d orbitals is the predominant mechanism.
in which the 3d ions have half-occupied 3dx2
orbitals, plays
Ày2
in favor of spin-populated 5d Gd orbitals and eliminates the 6s
orbital participation. Furthermore the presence of singly occu-
pied 3dx2 orbitals is needed for the observation of ferromag-
Ày2
netic 3d Gd interactions, the 3d ions with unoccupied 3dx2
Ày2
orbitals yield antiferromagnetic 3d-Gd interactions. Hence, by
Chem. Eur. J. 2016, 22, 2171 – 2180
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Published online on January 7, 2016
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