The molecular and electronic structure of symmetrically and asymmetricallycoordinated, 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

Photolysis of [LFeIII(nadiol)(N-3)] (2) in dry CH3CN at 20 degrees C produc es 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-triazac yclononane and nadiol(2-)=naphthaline-2,3-diolate). One-electron oxidation of 6 in dry CH2Cl2 with one equivalent of ferrocenium hexafluorophosphate g enerates 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)aca c(-) is the monoanion 1,3-diphenylpropane-1,3-dionate) and [L(Cl-4-cat)Fe-I II(N-3)] (Cl-4-cat(2-)=tetrachlorocatecholate dianion) produces the asymmet rically 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)](C lO4)(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. Comple xes 6 and 8 contain mixed-valent [Fe-IV(mu-N)Fe-III](4+) cores, whereas 7 c ontains 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) A ngstrom 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 magnet ic susceptibility measurements, and by molecular orbital calculations by lo cal density approximation. These studies reveal that 4 and 5 contain two eq uivalent high-spin Fe-III ions which exhibit the usual antiferromagnetic co upling of the [Fe-III(mu-O)Fe-III](4+) core (J=-90 and -95 cm(-1)) to a dia magnetic ground state. Similarly, 7 is diamagnetic even at room temperature and contains two equivalent Fe-IV ions, which are strongly antiferromagnet ically coupled to also yield an S = 0 ground state. On the other hand, comp lexes 6 and 8 display an S = 3/2 ground state and two nonequivalent, strong ly antiferromagnetically coupled iron sites with partially delocalized vale ncies. 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 c haracter only.