The molecular and electronic structure of symmetrically and asymmetrically coordinated, non-heme iron complexes containing [Fe^III(μ-N)Fe^IV]⁴⁺ (S = 3/2) and [Fe^IV(μ-N)Fe^IV]⁵⁺ (S = 0) cores

Photolysis of [LFe^III(nadiol)(N₃)] (2) in dry CH₃CN at 20 °C produces red-brown crystals of the dinuclear symmetrically coordinated complex [{L(nadiol)Fe}₂(μ-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(μ-N)]PF₆ (7). Photolysis of an equimolar CH₃CN solution of [L(Ph₂acac)Fe^III(N₃)]ClO₄ (3) (Ph₂acac^- is the monoanion 1,3-diphenylpropane-1,3-dionate) and [L(Cl₄-cat)Fe^III(N₃)] (Cl₄-cat^2- = tetrachlorocatecholate dianion) produces the asymmetrically coordinated species [L(Ph₂acac)-Fe^III=N=Fe^IV(Cl ₄-cat)L]ClO₄ (8) in 50% yield. The (μ-oxo)diferric complexes [{L(acac)Fe^III}₂(μ-O) ](ClO₄)₂ (4) (acac^- = pentane-2,4-dionate) and [{L(nadiol)Fe^III}₂(μ-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- (μ-N)Fe^III]⁴⁺ cores, whereas 7 contains the linear, symmetric [Fe^IV(μ-N)Fe^IV]⁵⁺ core with an Fe^IV-N_b bond length of 1.694(1) Å. The Fe^III-N_b bond length of 1.785(7) Å in 8 is longer than the Fe^IV-N_b bond length of 1.695(7) Å at 100 K (Δ = 0.09 Å). The electronic structure of these complexes has been characterized by Mössbauer, 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 highspin Fe^III ions which exhibit the usual antiferromagnetic coupling of the [Fe^III(μ-O)Fe^III]⁴⁺ core (J = - 90 and - 95cm^-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(μ-N)Fe]⁴⁺ core as containing a highspin ferric ion (S = 5/2) and a low-spin ferryl ion (S=1) has formal character only.