Abstract
We report magnetic and EPR (electron paramagnetic resonance) spectroscopy studies of [Cu2(flu)4(dmf)2] (flu = flufenamate and dmf = dimethylformamide), which has CuII ions in tetracarboxylate "paddle wheel" dinuclear units. Susceptibility measurements at 10 < T < 275 K allowed the evaluation of an antiferromagnetic intradinuclear exchange coupling J0 = -294 ± 5 cm-1 between CuII ions (Hex = "J0 S1·S2). EPR experiments at 300 K in powder and single-crystals at 9.5 and 34.4 GHz indicated g// = 2.373, g⊥ = 2.073 and zero field splitting parameters D = (-0.334 ± 0.001) cm"1 and E ca. 0. EPR signal intensity measurements at X-band in the range 4 < T < 295 K indicated that J0 = "283 ± 5 cm"1. A higher limit |J´| < 5×10-3 cm-1 for the interdinuclear exchange coupling between neighbor units at ca.14.24 Å was estimated from the angular variation of the single crystal spectra around the magic angles. The results are discussed in terms of the structure of the dinuclear unit and the bridges connecting CuII ions and compared with values reported for similar compounds.
Highlights
Studies of copper dinuclear compounds,[1] the simplest coupled magnetic systems, allow evaluating the exchangeEPR and Magnetic Studies of a Carboxylate-Bridged Dinuclear Copper(II) CompoundJ
Changes of the geometry of the Cu–O–C–O–Cu bridges vary the overlap between Cu dx2-y2 and O 2px orbitals, which in turn produce variations of J0.2 So, for “paddle wheel” units described by [Cu2(m-OOCR)4L2], the R groups are most important in changing the electronic structure of the bridges, but the L groups may vary structural constraints, changing local symmetry and bond geometry at the dinuclear unit, producing geometrical differences between the structures of the four O–C–O bridges
Even if a value |J0| ca. 280-300 cm-1 is a good guess for all tetracarboxylate “paddle wheel” copper(II) dinuclear units,[2] explaining the small differences that may occur for different cases requires a detailed analysis of the symmetry and dimensions of the units, that may change from case to case
Summary
Studies of copper dinuclear compounds,[1] the simplest coupled magnetic systems, allow evaluating the exchangeEPR and Magnetic Studies of a Carboxylate-Bridged Dinuclear Copper(II) CompoundJ. Studies of copper dinuclear compounds,[1] the simplest coupled magnetic systems, allow evaluating the exchange. EPR and Magnetic Studies of a Carboxylate-Bridged Dinuclear Copper(II) Compound. Paramagnetic resonance spectroscopy (EPR) are the techniques commonly employed to evaluate the exchange coupling and the zero field splitting. A large number of dinuclear CuII compounds with four O-C-O bridges and the classic “paddle wheel” structure, reported since the classical works of Bleaney and Bowers,[1] and Guha[3] on copper acetate, are found in the Cambridge Crystallographic Database.[4] They may be described by the general formula [Cu2(m-OOCR)4L2] with many possible R and L. The two axial ligands (L) are important for the coupling between neighbor units; when they are absent, the units tend to combine in chains or rings.[5,6] The great structural diversity of the “paddle wheel” units reported over ca. Interesting quantum phase transitions produced by randomly distributed interactions between one unit and the “bath” of other units in the lattice have been recently observed by EPR.[6,8]
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