Shaping distinct magnetic interactions in molecular compounds
Research highlights
► Nine new oxalates, with 3d elements, showing interesting characteristics were synthesized. ►The oxalate units and the positive or negative groups have induced various magnetic properties. ► The Mössbauer data revealed two different positions for each valence state of iron in molecule. ► There is a competition of anti-ferromagnetic couplings inside the magnetic units and between them. ► An overall ferri-magnetic long range ordering was demonstrated unambiguously.
Introduction
Iron oxalates have been part of our daily life for decades, starting years ago with photograph processing, or more recently as food preservation materials. Nevertheless, this class of compounds remains a permanent alternative source of tailoring many new materials with improved properties, especially for magnetic, optical or sensing devices.
Molecular oxalates of 3d-transition metals (M) provide a large variety of physical properties, including those magnetic ([1] and references therein). The oxalate entities could build layers, polymer-like structures and even large single molecules, via an adequate selection of the 3d metal ionic state, ligands, and bridging groups. Thus, were produced various architectures with complementary electrical, catalytic or optical properties which give rise to poly-functional 2D or 3D magnets [2]. In the past decades many series of oxalato-bridged hetero-polynuclear complexes were reported [3 and references therein]. Their magnetic properties were tuned using molecular chemistry.
Among them, the hetero-combinations show ferri, ferro (F) or anti-ferromagnetic (AF) behavior [4], [5] or metal-like conductivity concomitantly with ferromagnetism [6]. When containing multiple valence 3d ions, the complexes show noticeable different properties as a function of packing, valence and spin state of each individual ion.
A new series of homo and hetero nuclear oxalates containing either a positive (NH4)+ or a negative (OH)− group are reported in the following sections. The peculiarity of the two series is derived from the presence of either two or eight oxalate groups, at variance to the classical reported oxalate compounds containing one or, more often, three oxalate units. The actual compounds have revealed very interesting magnetic properties and local couplings, as observed both from magnetic and Mössbauer temperature and field dependent measurements.
Section snippets
Experimental
A new series of oxalate compounds, with general formula (NH4)7+x[FeIII3−xMx(C2O4)8]·6H2O, [FeIII3−xMx(C2O4)2(OH)5−x]·2H2O and, respectively, (M,M′=FeII, or bivalent 3d element while x and y took values =1) have been prepared. It was started using reagent grade quality chemicals Fe(NO3)3·9H2O, Co(NO3)2·6H2O,Cu(NO3)2·3H2O, Mn(NO3)2·6H2O, FeCl2·4H2O and oxalic acid (H2C2O4·2H2O). The main differences in the processing routes are related to pH, molar oxalate
Infrared spectroscopy data
The IR spectra of the listed compounds are usually dominated by two strong bands in the ranges 1680–1550 cm−1 (asymmetric) and 1400–1300 cm−1 (symmetric) arising from C–O stretching vibrations. In the case of (C2O4)2− anion the assignation is more complicated because (C2O4)2− can act as mono, bi, tri and tetra-dentate ligand. In Fig. 1 the IR spectra of two selected compounds are shown.
The IR spectra of the –a series (first three compounds from Table 1) are similar, but definitely different in
Discussion
The most clear result is that all these compounds (except Fe2Co–a, see detailed discussion in [8]) have long range ordering to an essentially anti- ferromagnetic state with TN ranging between 10 K (FeFeCu-h) and 36.8 K (Fe2Co-h). Therefore, the inter-cluster interaction is quite effective in coupling their magnetic moments. Besides, from above MS results it is evident an anti-ferromagnetic intra-molecular coupling. In all compounds, below TN, relaxation processes attributed to domain motion were
Conclusions
A series of nine new oxalates with interesting characteristics, containing 3d transitional elements and alternatively positive NH4 or negative OH groups, were successfully synthesized.
Four dedicated programs for cell configuration and electron structure calculations were used to solve the powder diffractograms.
The oxalate groups have demonstrated their influence in inducing a large diversity of magnetic properties of the investigated compounds, but it appears that they affect less Mössbauer
Acknowledgements
The financial support of CNCSIS-UEFISCSU via contract 235/2007 of “IDEI”—Romanian National Program and, respectively, MINCYT Spain project MAT08/1077 are acknowledged, firstly and deeply. Authors like to thank especially to Dr. Jean-Marc Greneche for the logistic support of a few measurements.
References (12)
- et al.
The influence of the packing ligand on spin state and magnetic interactions in new oxalates with 3d-transition metals
J. Magn. Magn. Mater.
(2007) Molecular Magnetism
(1993)- et al.
Polyfunctional two (2D) and three (3D) dimensional oxalate bridged bimetallic magnets
Monatshefte für Chem.
(2003) - et al.
New extended magnetic systems based on oxalate and iron(III) ions
Inorg. Chem.
(2008) - et al.
Ferrimagnetic mixed-valency and mixed-metal tris(oxalato)iron(iii) compounds: synthesis, structure, and magnetism
Inorg. Chem.
(1996) - et al.
Design of metal-complex magnets. syntheses and magnetic properties of mixed-metal assemblies {NBu4[ MCr( ox)3]}x (NBu4+=Tetra(n-buty1)ammonium ion; ox2-=oxalate ion; M=Mn2+, Fe2+, Co2+, Ni2+, Cu2+, Zn2+)
J. Am. Chem. Soc.
(1992)
Cited by (1)
Cobalt chromite obtained by thermal decomposition of oxalate coordination compounds
2014, Ceramics InternationalCitation Excerpt :Magnetic measurements have been investigated using a superconducting quantum interference device (SQUID) at three different temperatures (5, 70 and 120 K) into a magnetic field of 6 T. Frequencies of 10 and 100 Hz were used. The selection of oxalic acid as complexing agent is justified by its coordination versatility, multiple coordination capacity, as a monodentate ligand (bonded to the oxygen atoms of the same carboxylic group), as well as a bidentate ligand (bonded to the oxygen atoms from each carboxylic group) or as a tetradentate ligand (double bridging between two or four metal ions) [27]. Several heteropolynuclear oxalate complexes have been reported as precursors for spinel-type oxides in the past decade.