Intramolecular Excimer Formation in a Naphthalene-Appended Dinuclear Iron-Oxo Complex

The synthesis, structure, and physical properties of a Heisenberg exchange-coupled cluster containing naphthalene groups are described. [Fe2(0)(02CCH2C1oH7)2(TACN-Me3)2] (3) crystallizes in space group PI with unit cell parameters a = 12.94(2) A, b = 14.84(2) A, c= 15.23(2) A, (alpha) = 101.12(7) (bata) = 90.8(1)°, (gamma) - 114.14(7)°, V = 2605(6) A^3 and Z= 2 with R = 0.0425 and wR2 = 0.1182. Variable-temperature magnetic susceptibility data indicate that the two high-spin Feʹ" centers are antiferromagnetically coupled with J = -105 cm^-1 (H = -2JS1.S2), which is typical for this class of compounds. The room-temperature static emission spectrum of the compound in deoxygenated CHsCN solution is centered near 335 nm and has features reminiscent of both methyl-2-naphthylacetate (1) and [Zn2(OH)(O2CCH2C10H7)2(TACN-Me3)2ʹʹ (2) with the following two caveats: (1) the overall emission intensity is roughly a factor of 10 less than that of the free ester (1, C>r = 0.13) or the Zn" analogue (2, (PHI) = 0.14), and (2) there is significant broadening of the low-energy shoulder of the emission envelope. Time-correlated single photon counting data revealed biphasic emission for 3 with T] = 4.6 ± 1 ns and 12 = 47 ± 1 ns. The latter compares favorably with that found for 2 (tau) = 47 ± 1 ns) and is assigned as the So "~ Si fluorescence of naphthalene. Emission anisotropy, time-gated emission spectra, and nanosecond time-resolved absorption measurements all support the assignment of the 4.6 ns component as being due to a singlet excimer that forms between the two naphthylacetate groups of 3, a process that is likely mediated by the structural constraints of the oxo-bis-carboxylato diiron core. No direct evidence for intramolecular electron and/or energy transfer from the photoexcited naphthyl group to the iron-oxo core was obtained, suggesting that the short-lived excimer may contribute to circumventing such pathways in this type of system.