Mössbauer and electron paramagnetic resonance study of the double-exchange and Heisenberg-exchange interactions in a novel binuclear Fe(II/III) delocalized-valence compound

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Abstract
In this paper we present the characterization by UV-VIS, Mössbauer, and EPR spectroscopy of [L2Fe2(μ-OH)3](ClO4)2⋅2CH3OH⋅2H2O, with L=N,N′,N″-trimethyl-1,4,7-triazacyclononane, a novel dimeric iron compound, which is shown to possess a central exchange-coupled delocalized-valence Fe(II/III) pair. Complete delocalization of the excess electron in the dimeric iron center is concluded from the indistinguishability of the two iron sites in Mössbauer spectroscopy. Mössbauer, EPR, and magnetic susceptibility data imply a system spin St =9/2 for the ground state. This finding is explained as being a consequence of the double-exchange interaction which is generated by the delocalized electron. Experimental values obtained from UV-VIS, Mössbauer, and EPR spectroscopy are for the double-exchange parameter B=1300 cm−1, the g factors gx,y =2.04 and gz =2.3, the parameters for zero-field splitting D=4 cm−1 and E≊0 cm−1, and for the hyperfine parameters ΔEQ =−2.14 mm s−1, Ax,y =−21.2 T, Az =−27 T, and δ=0.74 mm s−1. From our temperature-dependent studies we assign to the first excited state a spin-octet with an excitation energy Δ>175 cm−1. From this value a lower bound of −235 cm−1 has been deduced for the exchange-coupling constant J. In the framework of a simplified description of the iron atoms by unperturbed 3d orbitals, the values of the A tensor components as well as the quadrupole splitting ΔEQ can be interpreted in a consistent manner by assuming the excess electron being delocalized over two dσ orbitals centered at the two iron sites of the dimer and directed along the iron–iron axis as the z direction.