• Edward R T Tiekink

      Articles written in Journal of Chemical Sciences

    • Structural and electronic properties of six-coordinate Lewis-base adducts of iron(III) Schiff-base complexes. Crystal structure and magnetism of the high-spin complex [(imidazole)Fe(salen)(NCS)]-0·5 MeOH and of a related complex containing two dissimilar axial ligands

      Brendan J Kennedy Keith S Murray Peter R Zwack Ernst Horn Michael R Snow Edward R T Tiekink

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      The structural and electronic properties of two high-spin iron (III) Schiff-base complexes [(imd)Fe(salen)(NCS)] (I) and [(imd)Fe(salen)(N3)] (II) are described. Crystals ofI as a hemimethanol solvate, (C3H4N2)Fe(C16H14N2O2)(NCS). 0·5 CH3OH, crystallize in the triclinic space group$$P\bar 1$$ with unit cell dimensionsa=14·278(4),b=16·932(4),c=9·057(4) Å, α=97·40(3), β=92·07(3), γ=85·32(2)° andZ=4. The structure was refined by a block-matrix least-squares procedure to finalR 0·050 andRw 0·057 using 3127 unique reflections with I>2·5 σ(I). Complex I is one of the few structurally characterized examples of six-coordinate Lewis-base adducts in which there are two different axial ligands. The in-plane Fe-N distances, 2·12 Å (av) are characteristic ofS=5/2 FeIII. The Fe-N (imidazole) distance of 2·149(5) Å is similar to that in related high-spin imidazole-FeIII complexes. Non-linear Fe-NCS bonding is observed (Fe-N-C=159·0(5)°) as in related six-coordinate FeIII porphyrin and methemoglobin isothiocyanate species. Mössbauer spectra ofI andII between 295 and 4·2 K are symptomatic ofS=5/2 FeIII. The nature of the asymmetry of the quadrupole doublet as a function of temperature points to zero field splitting of the6A1 ground state having a negative value of the axial ZFS parameter,D. These are new additions to the few known examples of negativeD in FeIII-tetradentate chelating systems, and contrast for instance with aquo-imidazole FeIII salen adducts which we have recently shown to have positiveD values. Spin-hamiltonian analyses of the 300-4·2 KχFe/T data, assuming rhombic ligand field symmetry, yielded the following parameter sets;I:g=1·98±0·02,D=−2·4±0·2 cm−1,E=0·6±0·2 cm−1.II:g=2·00±0·01,D=−1·4±0·2 cm−1,E=0·4±0·2 cm−1. In the 77 KX-band ESR spectra of neat powdered samples ofII: a line atg=4·4 confirms the rhombic splitting, while the related spectrum forI shows a broad line atg∼2·1. This somewhat unusual line position probably arises through solid state exchange effects since ag 4·3 signal is observed for a frozen glass spectrum.

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