Intramolecular Energy Transfer Involving Heisenberg Spin-Coupled Dinuclear Iron−Oxo Complexes

The synthesis, structure, and physical properties of a series of oxo-bridged dinuclear Fe(III) complexes containing
pendant naphthalene groups are described. The compounds [Fe2O(O2CCH2−C10H7)(tren)2](BPh4)(NO3)2 (8), [Fe2O(O2CCH2−C10H7)(TPA)2](ClO4)3 (9), Fe2O(O2CCH2−C10H7)2(Tp)2 (10), and Fe2O((O2CCH2CH2)2−C10H6)(Tp)2 (11) (where
tren is tris(2-aminoethyl)amine, TPA is tris(2-pyridyl)amine, and Tp is hydrotrispyrazolylborate) have been characterized
in terms of their structural, spectroscopic, magnetic, and photophysical properties. All four complexes exhibit moderately
strong intramolecular antiferromagnetic exchange between the high-spin ferric ions (ca. −130 cm-1 for H = −2JS1·S2). Room-temperature steady-state emission spectra for compounds 8−11 in deoxygenated CH3CN solution reveal
spectral profiles similar to methyl-2-naphthyl acetate and [Zn2(OH)(O2CCH2−C10H7)2(TACN-Me3)2](ClO4) (13, where
TACN-Me3 is N,N,N-1,4,7-trimethyltriazacyclononane) but are significantly weaker in intensity relative to these latter
two compounds. Time-resolved emission data for the iron complexes following excitation at 280 nm can be fit to
simple exponential decay models with τobsS1 = 36 ± 2, 32 ± 4, 30 ± 5, and 39 ± 3 ns for compounds 8−11,
respectively. The decays are assigned to the S1 → S0 fluorescence of naphthalene; all of the lifetimes are less
than that of the zinc model complex (τobsS1 = 45 ± 2 ns), indicating quenching of the S1 state by the iron−oxo core.
Nanosecond time-resolved absorption data on [Zn2(OH)(O2CCH2−C10H7)2(TACN-Me3)2](ClO4) reveal a feature at
λmax = 420 nm that can be assigned as the T1 → Tn absorption of the naphthalene triplet; the rise time of 50 ±
10 ns corresponds to an intersystem crossing rate of 2 × 107 s-1. A similar feature (though much weaker in
intensity) is also observed for compound 8. The order-of-magnitude reduction in the T1 lifetime of the pendant
naphthalene for all of the iron−oxo complexes (τobsT1 = 5 ± 2 μs vs 90 ± 10 μs for [Zn2(OH)(O2CCH2−C10H7)2(TACN-Me3)2](ClO4)) indicates quenching of the naphthalene triplet with an efficiency of >90%. Neither the naphthalene
radical cation nor the reduced FeIIFeIII species were observed by transient absorption spectroscopy, implying that
energy transfer is the most likely origin for the quenching of both the S1 and T1 states. Spectral overlap considerations
strongly support a Förster (i.e., dipolar) mechanism for energy transfer from the S1 state, whereas the lack of
phosphorescence from either the free naphthyl ester or the Zn model complex suggests Dexter transfer to the
diiron(III) core as the principal mechanism of triplet quenching. The notion of whether spin exchange within the
diiron(III) core is in part responsible for the unusual ability of the iron−oxo core to engage in energy transfer from
both the singlet and triplet manifolds of naphthalene is discussed.