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The molecular and electronic structure of symmetrically and asymmetrically coordinated, non-heme iron complexes containing [Fe-III(mu-N)Fe-IV](4+) (S=3/2) and [Fe-IV(mu-N)Fe-IV](5+) (S=0) cores
Citation key ISI:000078555300038
Author Justel, T and Muller, M and Weyhermuller, T and Kressl, C and Bill, E and Hildebrandt, P and Lengen, M and Grodzicki, M and Trautwein, A X and Nuber, B and Wieghardt, K
Pages 793-810
Year 1999
ISSN 0947-6539
Journal Chem. Eur. J
Volume 5
Number 2
Month FEB
Abstract Photolysis of [LFeIII(nadiol)(N-3)] (2) in dry CH3CN at 20 degrees C produces red-brown crystals of the dinuclear symmetrically coordinated complex [\L(nadiol)Fe\(2)(mu-N)] (6) in 40 \% yield (L = 1,4,7-trimethyl-1,4,7-triazacyclononane and nadiol(2-)=naphthaline-2,3-diolate). One-electron oxidation of 6 in dry CH2Cl2 with one equivalent of ferrocenium hexafluorophosphate generates blue crystals of [\L(nadiol)Fe-IV\(2)(mu-N)]PF6 (7). Photolysis of an equimolar CH3CN solution of [L(Ph(2)acac)Fe-III(N-3)]ClO4 (3) (Ph(2)acac(-) is the monoanion 1,3-diphenylpropane-1,3-dionate) and [L(Cl-4-cat)Fe-III(N-3)] (Cl-4-cat(2-)=tetrachlorocatecholate dianion) produces the asymmetrically coordinated species [L(Ph(2)acac)Fe-III=N=Fe-IV(Cl-4-cat) L]ClO4 (8) in 50\% yield. The (mu-oxo)diferric complexes [\L(acac)Fe-III\(2)(mu-O)](ClO4)(2) (4) (acac(-) = pentane-2,4-dionate) and [\L(nadiol)Fe-III\(2)(mu-O)] (5) have also been prepared for comparison with complexes 6-8. Complexes 4-8 have been characterized by single-crystal X-ray crystallography. Complexes 6 and 8 contain mixed-valent [Fe-IV(mu-N)Fe-III](4+) cores, whereas 7 contains the linear, symmetric [Fe-IV(mu-N)Fe-IV](5+) core with an Fe-IV-N-b bond length of 1.694(1) Angstrom. The Fe-III-N-b bond length of 1.785(7) Angstrom in 8 is longer than the Fe-IV-N-b bond length of 1.695(7) Angstrom at 100 K (Delta=0.09 Angstrom). The electronic structure of these complexes has been characterized by Mossbauer, electron paramagnetic resonance (EPR), resonance Raman (RR) and UV/Vis spectroscopy, electrochemistry and magnetic susceptibility measurements, and by molecular orbital calculations by local density approximation. These studies reveal that 4 and 5 contain two equivalent high-spin Fe-III ions which exhibit the usual antiferromagnetic coupling of the [Fe-III(mu-O)Fe-III](4+) core (J=-90 and -95 cm(-1)) to a diamagnetic ground state. Similarly, 7 is diamagnetic even at room temperature and contains two equivalent Fe-IV ions, which are strongly antiferromagnetically coupled to also yield an S = 0 ground state. On the other hand, complexes 6 and 8 display an S = 3/2 ground state and two nonequivalent, strongly antiferromagnetically coupled iron sites with partially delocalized valencies. Therefore, a description of the [Fe(mu-N)Fe](4+) core as containing a high-spin ferric ion (S=5/2) and a low-spin ferryl ion (S=1) has formal character only.
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