Unusual Polynuclear Complexes
FULL PAPER
1
2
ˆ
^
parameters E1,2/D1,2 =0.04, E3/D3 =0.09 (with HZFS =D[Sz ꢀ3
ing between the coordinating amino group of the glucose
backbone and the benzyl protected oxygen atom O4 of
neighboring building blocks. In the case of complex 2 a
strong hydrogen bond supported by the Ni-(R)O···H-O(R)-
Ni fragment is observed which is the result of an increased
basicity based on the preset steric orientation of the two un-
derlying terminal alkoxido groups.
For complex 1 antiferromagnetic interactions with a re-
sulting S=0 ground state are observed, whereas for complex
2 the observed interactions are all ferromagnetic leading to
an S=3 ground state. The nickel(II) complex 2 is the first
example of a ferromagnetically coupled trinuclear nickel(II)
complex. The magnetic interactions for both complexes can
be interpreted in terms of superexchange via the involved
oxygen bridges. Consequently, the steric presettings given by
the glucose backbone of the ligand have a strong influence
on the observed magnetic interactions.
2
2
^
^
S(S+1)+E/D (Sx ꢀSy )]). Nevertheless, this only leads to a
marginal improvement of the fit quality, which is a finger-
print of the herewith related problem of overparametriza-
tion when a full set of ZFS parameters is used for fitting
magnetic susceptibility data.
To the best of our knowledge complex 2 is the first trian-
gular nickel(II) complex with an overall ferromagnetic inter-
action within the Ni3 core. Nevertheless, for linear trinuclear
complexes ferromagnetic interactions between the adjacent
nickel(II) ions have been reported,[39] and date back to an
early paper of Ginsberg et al.,[40] which in most cases exhibit
an additional weak antiferromagnetic coupling between the
two terminal nickel(II) ions. Moreover, for triangular sys-
tems generally an antiferromagnetic interaction is observed,
independent of the molecular symmetry for isosceles[41] and
equilateral Ni3 triangles.[42]
However, for di-[43] and tetranuclear[44] nickel(II) com-
plexes magnetostructural correlations have been put for-
ward. These correlations indicate a Ni-O-Ni border line
angles of about 93.5 and 99.08 for the corresponding m2-O
and m3-O bridged di- and tetranuclear systems, respectively,
below which a ferromagnetic interaction is observed. Inter-
estingly, for complex 2 the Ni-(m2-O)-Ni angles are observed
at about 898, whereas the Ni-(m3-O)-Ni angles are found
within range from 82 to 998, which is consistent with the ob-
served overall ferromagnetic interactions within the isosce-
les Ni3 triangle.
Experimental Section
Physical measurements: Melting points are given uncorrected and were
determined via a VEB Analytik Dresden HMK 72/41555. Thermogravi-
metric analyses (TGA) for powdered samples were performed on a
NETZSCH STA409PC Luxx apparatus under a constant flow of nitrogen
ranging from room temperature up to 10008C with a heating rate of
18Cminꢀ1. Infrared and Raman spectra were recorded on a BRUKER
IFS 55 EQUINOX spectrometer. UV/Vis spectra were recorded on a
VARIAN CARY 5000 UV/Vis-NIR-spectrometer. Mass spectra were
measured on a Bruker MAT SSQ 710 spectrometer. Elemental analyses
were determined on a LECO CHNS/932 Analyser and a VARIO EL III.
Magnetic susceptibilities were obtained from a powdered sample of 1
and from a paraffin-triturated sample of 2 using a Quantum-Design
MPMSR-5S SQUID magnetometer equipped with a 5 Tesla magnet in
the range from 300 to 2 K with an applied magnetic field of 5000 Oe (for
details see ref. [45]). The experimental magnetic susceptibility data were
fitted using the program package julX version 1.3 which allows spin-
Hamiltonian simulations of the data by a full-matrix diagonalization ap-
proach and includes the treatment of paramagnetic impurities in the fit-
ting procedure.[46]
Conclusion
The 2-aminoglucose ligand benzyl 2-amino-4,6-O-benzyli-
dene-2-deoxy-a-d-glucopyranoside (HL) has been used to
generate appropriate {Cu(L)} and {Ni(L)} building blocks
which via self-assembly afford the tetranuclear m4-hydroxido
bridged copper(II) complex 1 and the trinuclear alcoholate
bridged nickel(II) complex 2, respectively. Complex 1 repre-
sents the first example of a m4-hydroxido bridged copper
complex not supported by a macrocyclic framework, where-
as complex 2 is a rare example of a trinuclear nickel com-
plex with a bis-m3-alkoxido bridged Ni3 core structure repre-
senting an isosceles triangle. The hydrogen-bonding proper-
ties of the employed ligand, providing both hydrogen-bond
donors and acceptors in a rigid preset orientation given by
the glucose backbone, allows for intramolecular hydrogen
bonding between the building blocks in a self-complementa-
ry fashion. In the case of copper(II) this leads to a rationale
for how specific tetranuclear core structures can be generat-
ed by the choice of the appropriate functionalization of the
2-aminoglucose backbone.
Syntheses: The ligand benzyl 2-amino-4,6-O-benzylidene-2-deoxy-a-d-
glucopyranoside (HL) was synthesized starting from N-acetyl-a-d-gluco-
pyranosamine according to the published procedures.[23] All chemicals
were purchased from commercial suppliers and used without further pu-
rification.
CAUTION: In general, perchlorate salts of metal complexes with organic
ligands are potentially explosive. While the present complex has not
proved to be shock sensitive, only small quantities should be prepared
and great care is recommended.
[(m4-OH)Cu4(L)4(MeOH)3(H2O)](ClO4)3 (1):
A
solution of Cu-
(ClO4)2·6H2O (519 mg, 1.40 mmol) in methanol (6.6 mL) was added to a
solution of the ligand HL (500 mg, 1.40 mmol) in chloroform (20 mL) at
room temperature. After 7 d of slow evaporation complex 1 was obtained
as blue prismatic crystals of 1·2.7MeOH which were suitable for X-ray
diffraction. The crystals were isolated, washed with a small amount of
methanol, and dried in air. Yield: 198 mg (28%). Decomposition inter-
ꢀ
val=191–2228C; IR (KBr): n˜ =3448 (br, n O H), 3326 and 3260 (m, n
Both polynuclear complexes 1 and 2 exhibit the rare coor-
dination of the trans-ee-configurated donor atoms N2 and
O3 of the glucose backbone which is assumed for chitosan-
based metal complexes. The carbohydrate backbone adopts
ꢀ
ꢀ
N H), 3169, 3067, and 3033 (w, n C H arom.), 2934 and 2911 (w, nas
CH2), 2874 (w, ns CH2), 1617 (w, n C=C), 1456 (m, d CH2), 1385 (m),
ꢀ
ꢀ
ꢀ
1200 (w, n C O), 1132, 1122, and 1093 (vs, n Cl O and C O), 1069, 1055,
ꢀ
and 1026 (vs, n C O), 990 (s), 927 (m), 764 (m), 737 (m), 701 (s), 668
(m), 624 (m) cmꢀ1; Raman (solid): n˜ =3066 (s, n C H arom.), 2943 (m,
ꢀ
4
for both cases the stable C1 chair conformation. The core
ꢀ
nas CH2), 2875 (m, ns CH2), 1606 (m, n C=C), 1028 (w, n C O), 1003 (s, n
C O), 930 (m) cmꢀ1; UV/Vis (CHCl3): lmax (e)=278 (11.100), 696 nm
ꢀ
structure of both complexes is stabilized by hydrogen bond-
Chem. Eur. J. 2009, 15, 1261 – 1271
ꢂ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1269