Tetraanionic N2O2-coordinating ligands as potential building blocks for supramolecular magnetic networks

Article abstract of DOI:10.1039/c3dt50177a

A bisoxamate ligand containing three different types of coordination sites was designed and synthesized. The developed synthetic strategy was adopted to prepare a related 1,2-bis(2-hydroxybenzamido)benzene-derived ligand. Nickel(ii) complexes of both the

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Tetraanionic N2O2-coordinating ligands as potential
building blocks for supramolecular magnetic
networks†
Cite this: DOI: 10.1039/c3dt50177a
Received 17th January 2013,
Accepted 19th February 2013
DOI: 10.1039/c3dt50177a
Magdalena Milek, Alexander Witt, Carsten Streb, Frank W. Heinemann and
Marat M. Khusniyarov*
www.rsc.org/dalton
A bisoxamate ligand containing three different types of coordi- prepared a tetranuclear species [Fe(LPhenCu)3]4− (Scheme 1) by
nation sites was designed and synthesized. The developed syn- reacting a preformed [Fe(LPhen)3]2+ with a copper(II) source.6
thetic strategy was adopted to prepare
a related 1,2-bis- Since both phenanthrolines and bisoxamates bind strongly to
(2-hydroxybenzamido)benzene-derived ligand. Nickel(II) complexes most transition metal ions, competitive metal binding to the
of both the novel ligands were obtained and characterized by different coordination sites of LPhen is expected, rendering the
X-ray crystallography, NMR, electronic absorption spectroscopy, controlled synthesis of such heterometallic architectures a
and theoretical calculations.
challenging task.
Targeting the synthesis of multidimensional magnetic
materials, our approach to the design of o-phenylene-bisoxa-
mate ligands employs the creation of auxiliary weakly binding
“soft” coordination sites at the phenylene unit that are not
competing with the central “hard” N2O2 and the two side
“intermediate” OvC–CvO oxamate coordination sites. Appro-
priate metal ions added to a preformed M2M1M2-bisoxamate
and capable of coordinating to the auxiliary sites can conse-
quently act as bridges offering the possibility to control the
formation of metallosupramolecular structures. Thus, we
decided to introduce two “soft” thiophene groups to o-pheny-
lene-bisoxamate (Scheme 2), since thiophenes are known to be
weakly coordinating ligands with diverse coordination modes
including η1(S) and η2–η5 that form supramolecular architec-
tures when coordinated to Ag(I) or Rh(II) ions.9–11 Here, we
report the synthesis of the o-phenylene-bisoxamate ligand12
H4L1 bearing two thiophene groups at the phenylene unit and
our first results on the coordination chemistry of this novel
o-Phenylene-bisoxamates coordinated by a metal ion M1 at the
central position represent versatile building blocks for con-
structing multidimensional magnetic materials (Scheme 1).
This is due to the ability of metal bisoxamates to coordinate
additional metal ions M2 through the four carbonyl groups,
thus forming multimetallic complexes, chain structures, as
well as two- and three-dimensional networks of interacting
paramagnetic metal ions that find applications as single-
molecule magnets, single-chain magnets, and high-dimensional
molecule-based magnets.1–3 Metal ions (M2) coordinated to
the periphery of the ligand act as bridges being actively
involved in the formation of metallosupramolecular chains
and multidimensional networks, whereby the intrinsic mag-
netic properties of
perturbed.
a
monomer M2M1M2-bisoxamate are
Another approach to creating molecular magnetic materials
with novel and enhanced physical properties consists of the
controlled synthesis of metallosupramolecular assemblies via
a rational ligand design.4 In this context, modifications at the
o-phenylene-bisoxamate backbone that could assist the for-
mation of supramolecular structures are very scarce.5–8 The
only example of a successful ligand modification leading to a
multimetallic structure was reported by Aukauloo et al., who
ligand, where
a
monometallic complex (TBA)2[NiL1]
Friedrich-Alexander-University Erlangen-Nürnberg, Department of Chemistry and
Pharmacy, Egerlandstr. 1, 91058 Erlangen, Germany.
E-mail: marat.khusniyarov@chemie.uni-erlangen.de; Fax: +49 9131 8527367;
Tel: +49 9131 8527464
†Electronic supplementary information (ESI) available: Syntheses, electronic
absorption spectra, crystallographic and computational details are included.
CCDC 850435, 848195, 848406, 850436. For ESI and crystallographic data in CIF
or other electronic format see DOI: 10.1039/c3dt50177a
Scheme 1 o-Phenylene-bisoxamates: general structure (left) and ligand
modification by Aukauloo et al.6 (right).
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(TBA = tetra-n-butyl-ammonium) has been obtained and bromide 2, is easily prepared from o-phenylenediamine in a
characterized (Scheme 2). Moreover, the established synthetic three-step procedure18 including protection by tosylation, sub-
route towards H4L1 can be easily transferred to other related sequent bromination, followed by deprotection of the product.
ligand systems as we demonstrated by preparing the 1,2-bis- The boronic acid 1 and the aryl bromide 2 are subsequently
(2-hydroxybenzamido)benzene derived ligand H4L2 and its nickel fused via a double Suzuki cross-coupling reaction using
complex (TBA)2[NiL2] (Scheme 2). Note that similarly to metal [PdCl2(dppf)] (dppf = 1,1′-bis(diphenylphosphino)ferrocene) as
bisoxamates, metal complexes of 1,2-bis(2-hydroxybenzamido)- a catalyst. After purification via column chromatography the
benzenes are known to form multimetallic spin coupled product 3 can be isolated with yields up to 84%. In the final
species with high potential for molecular magnetism.13,14
H4L1 in its protected doubly ethylated form H2L1 can be tected ligand H2L1 is obtained with yields up to 77%. The
step ethyl oxalyl chloride is allowed to react with 3 and the pro-
Et
Et
prepared via a multistep synthetic route that includes one C–C total yield of the 8-step synthesis of H2L1Et is 40%.
cross-coupling reaction (Scheme 3). After commercially avail-
A related ligand H4L2 in its protected doubly acylated form
able 2,5-dimethylthiophene is doubly brominated,15 one H2L2 can be prepared by adopting the synthetic protocol
Ac
bromine atom is replaced with a hydrogen via lithiation and established for H2L1Et, except the last step (Scheme 3): acetyl-
subsequent hydrolysis.16 A corresponding boronic acid 1 is salicylic acid chloride, generated in situ from acetylsalicylic
then obtained via a boronate ester in a one-pot reaction.17 The acid and oxalyl chloride, reacts with 3 to give the target com-
second component for the cross-coupling reaction, aryl pound H2L2 with a yield of 45%. The total yield of the 8-step
Ac
synthesis of H2L2 is 23%. The obtained H2L2 can be depro-
Ac
Ac
tected, if necessary, by the action of 6 M NaOH to give H4L2 in
a quantitative yield on a small scale (50 mg). However, scaling
up of this procedure is tedious due to difficulties in controlling
precisely the pH value during the work-up on larger scales.
Crystals of the protected ligand H2L1 suitable for X-ray
Et
analysis were obtained by slow diffusion of diethyl ether into
an acetonitrile solution. The ligand crystallizes in the P21/n
space group (Fig. 1). Five-membered thiophene rings make
dihedral angles of 61° and 62° with the six-membered pheny-
lene ring, thus the three π-systems are essentially electronically
uncoupled. The two thiophenes are rotated in the same direc-
tion, thus forming an anti-parallel conformation in the
Scheme 2 Protonated ligands of this work.
Scheme 3 Multistep synthesis of protected ligands H2L1Et and H2L2
.
Ac
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Fig. 3 Molecular structure of (TBA)2[NiL1], thermal ellipsoids are drawn at the
50% probability level, TBA+ cations and water molecules are omitted.
Fig. 1 Molecular structure of H2L1Et, thermal ellipsoids are drawn at the 50%
probability level.
All three ligands H2L1Et, H2L2Ac, and H4L2 are colorless
compounds as confirmed by their electronic absorption
spectra (see ESI†) featuring high intensity absorption bands
exclusively in the UV region (200–340 nm). These bands are
assigned to π→π* and n→π* intraligand transitions.
After H2L1 and H2L2 are deprotected and deprotonated
Et
Ac
by the action of aqueous NaOH, addition of a Ni(II) salt and
(TBA)Br results in the formation and precipitation of target
complexes (TBA)2[NiL1] and (TBA)2[NiL2], respectively. Sharp
signals in 1H NMR spectra and the X-ray structures of the com-
plexes confirm the presence of a diamagnetic NiII ion in both
compounds.
X-ray quality crystals of (TBA)2[NiL1] were obtained from an
EtOH–H2O mixture; the complex crystallizes in the P21/c space
group (Fig. 3). The fourfold deprotonated ligand coordinates
to the Ni ion through the two oxygen and two nitrogen donors.
The Ni–N (1.819(2) and 1.821(2) Å) and Ni–O (1.8819(19) and
Fig. 2 Molecular structure of H4L2, thermal ellipsoids are drawn at the 50%
probability level.
crystal.19 The tetradentate metal coordination site in H2L1 is 1.890(2) Å) bond distances in (TBA)2[NiL1] are similar to those
Et
non-planar: the two planes of the OOC–CON oxamic acid back- found in other o-phenylene-bis(oxamato) low-spin Ni(II) com-
bones have dihedral angles of 44° and 47° with the six-mem- plexes.8,20,21 Five-membered thiophene rings form dihedral
bered phenylene ring. Rotation of oxamic acid backbones in angles of 53° and 54° with a six-membered phenylene ring
opposite directions results in the appearance of an O6⋯H2– that are slightly reduced compared to the respective angles in
N2 intramolecular hydrogen bond in the crystal structure. the metal-free H2L1Et. The two thiophene substituents are in
Additionally an intermolecular O2*⋯H1–N1 hydrogen bond parallel conformation in crystalline (TBA)2[NiL1].19 The
was found in the crystal lattice (see ESI†).
o-phenylene-bis(oxamato)nickel(II) fragment is nearly planar
The crystals of H4L2 suitable for X-ray structure determi- that results in strong electronic coupling between the
nation were obtained by recrystallization from CHCl3. The π-systems of two oxamate moieties and the phenylene ring.
Crystals of (TBA)2[NiL2] suitable for X-ray analysis were
ˉ
compound crystallizes in the P1 space group (Fig. 2). Two five-
membered thiophene rings form dihedral angles of 43° and obtained from a CHCl3 solution on cooling; the complex crys-
86° with the central phenylene plane that indicates a weak tallizes in the P21/n space group (Fig. 4). The fourfold deproto-
electronic coupling between the π-systems of one thiophene nated ligand is coordinated to the Ni ion through two oxygen
and the phenylene moiety. The tetradentate metal coordi- and two nitrogen atoms building a distorted square-planar
nation site of the metal-free ligand is non-planar: two nearly environment around the central metal ion. Short Ni–N (1.885(3)
planar salicylamide groups show dihedral angles of 45° and and 1.870(2) Å) and Ni–O (1.853(2) and 1.852(2) Å) bond dis-
50° with the plane of the phenylene, thereby an intramolecular tances in (TBA)2[NiL2] compare well with those observed for a
hydrogen bond O3⋯H1A–N1 is built. Two additional intra- similar low-spin Ni(II) complex without thiophene substitu-
molecular hydrogen bonds and one intermolecular hydrogen ents.22 Two thiophene rings form dihedral angles of 49° and
bond are formed via –NH and –OH groups (see ESI†).
54° with the central phenylene plane, thus the π-systems of
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In summary, a synthetic strategy has been developed and
consequently utilized to prepare
a phenylene-bisoxamate
derived ligand featuring three different types of coordination
sites (H2L1Et). Our synthetic approach can be adopted to
modify other related ligand systems as demonstrated by pre-
paring 1,2-bis(2-hydroxybenzamido)benzene derivatives H2L2
Ac
and H4L2. The novel ligands form stable mononuclear Ni(II)
complexes that were characterized by X-ray crystallography,
NMR and electronic absorption spectroscopy as well as theo-
retical calculations. The obtained ligands represent attractive
building blocks to create multidimensional magnetic materials
in a controllable way that is currently under investigation in
our laboratory.
We thank the Fonds der Chemischen Industrie (Liebig
Fellowships for CS and MMK) and the Deutsche Forschungs-
gemeinschaft (DFG Research Grant KH 279/2-1) for financial
support. Friedrich-Alexander-University Erlangen-Nürnberg
and the Lehrstuhl für Anorganische und Allgemeine Chemie
(Prof. Karsten Meyer) are acknowledged for general support.
Fig. 4 Molecular structure of (TBA)2[NiL2], thermal ellipsoids are drawn at the
50% probability level, two TBA+ cations and a water molecule are omitted.
these three fragments are essentially uncoupled. The metal
coordination site N2O2 is slightly twisted: the Ni1N1N2 plane
shows a dihedral angle of 10° with the Ni1O1O4 plane that is
likely due to a too small ligand cavity to accommodate a low-
spin Ni(II) ion without being distorted. Consequently, the two
salicylamide groups are slightly twisted and not coplanar with
the central phenylene fragment: the C6-rings of the salicyl-
amide groups have dihedral angles of 21° and 22° with the six-
membered central phenylene ring. Thus, significant electronic
coupling is expected between the π-systems of the metal
coordination site and the central phenylene unit in
(TBA)2[NiL2], albeit it is weaker than that in the nearly planar
(TBA)2[NiL1].
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visible regions that are absent in the spectra of the correspond-
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shoulders for (TBA)2[NiL1] and (TBA)2[NiL2], respectively, both
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