Stefan Drueke

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

In this paper we present the characterization by UV‐VIS, Mossbauer, 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 Mossbauer spectroscopy. Mossbauer, 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, Mossbauer, 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. Fr...

Synthese von [LRe(CO)3]+-Komplexen mit makrocyclischen Liganden Die Kristallstrukturen von 1,4,7-Triazacyclononan-tricarbonylrhenium( I)-thiocyanat und 1,4,7-Trithiacyclononan-tricarbonylrhenium( I) -bromid-hemihydrat / Synthesis of [LRe(CO)3]+ Complexes with Macrocyclic Ligands The Crystal Structures of 1,4,7-Triazacyclononane-tricarbonyl-rhenium(I) Thiocyanate and of 1,4,7-Trithiacyclononane-tricarbonyl-rhenium(I) Bromide Hemihydrate

The tridentate macrocyclic ligands 1,4,7-triazacyclononane, its thia analogue 1,4,7-trithiacyclononane, 1-oxa-4,7-diazacyclononane, and N,N′,N″-trimethyl-1 ,4,7-triazacyclononane react with Re(CO)5Br in dimethylformamide to yield colorless, air-stable [LRe(CO)3]Br complexes (L = tridentate macrocycle). With Mn(CO)5Br only the cyclic amines react to afford yellow-orange [LMn(CO)3]Br complexes whereas 1,4,7-trithiacyclononane does not give stable complexes of manganese. The crystal structures of [([9]aneN3)Re(CO)3](NCS) and [([9]aneS3)Re(CO)3]Br · 1/2 H2O have been determined. The former crystallizes in the orthorhombic space group P bca (a = 12/763(4), b = 12.662(5), c = 17.472(9) Å; Z = 8); whereas the latter crystallizes in the monoclinic space group P21/c (a = 9.444(1). b = 11.797(2), c = 28.095(3) Å. β = 90,86(1); Z = 8).

Spectroscopic Studies of Spin State and Electron Delocalization in a Binuclear Mixed-Valence Fe(II/III) Complex

A brief historical survey is given of the “doubleexchange” interaction, by which the spins of the paramagnetic centers in mixed-valence systems are ferromagnetically coupled through quantum-mechanical electron delocalization. The spectroscopic evidences, obtained by UV-visible spectrometry, EPR, and Mossbauer spectroscopy, for electron delocalization and ferromagnetic spin coupling in the binuclear triply ligand-bridged Fe(II/III) molecular complex [L2Fe2(μ-OH)3] (C104)2 • 2CH3OH-2H2O, with L = N, N’,N”-trimethyl 1, 4, 7-triazacyclononane are summarized. Values for the double-exchange parameter B (±1300 cm-1) and the Heisenberg exchange-coupling constant J (-180 cm-1 in JS1 S2) in the dimeric iron unit are inferred from UV-visible and magnetic-susceptibility data. The electric and magnetic hyperfine parameters, deduced from the Mossbauer spectra, correlate with an “odd” electron that is delocalized over two do orbitals, oriented along the Fe-Fe axis. A condition for electron delocalization, obtained with the PKS model, is briefly discussed.