Ligand Field Strength Mediates Electron Delocalization in Octahedral [((H)L)2Fe6(L')m](n+) Clusters

Abstract

To assess the impact of terminal ligand binding on a variety of cluster properties (redox delocalization, ground-state stabilization, and breadth of redox state accessibility), we prepared three electron-transfer series based on the hexanuclear iron cluster [((H)L)2Fe6(L')m](n+) in which the terminal ligand field strength was modulated from weak to strong (L' = DMF, MeCN, CN). The extent of intracore M-M interactions is gauged by M-M distances, spin ground state persistence, and preference for mixed-valence states as determined by electrochemical comproportionation constants. Coordination of DMF to the [((H)L)2Fe6] core leads to weaker Fe-Fe interactions, as manifested by the observation of ground states populated only at lower temperatures (<100 K) and by the greater evidence of valence trapping within the mixed-valence states. Comproportionation constants determined electrochemically (Kc = 10(4)-10(8)) indicate that the redox series exhibits electronic delocalization (class II-III), yet no intervalence charge transfer (IVCT) bands are observable in the near-IR spectra. Ligation of the stronger σ donor acetonitrile results in stabilization of spin ground states to higher temperatures (∼300 K) and a high degree of valence delocalization (Kc = 10(2)-10(8)) with observable IVCT bands. Finally, the anionic cyanide-bound series reveals the highest degree of valence delocalization with the most intense IVCT bands (Kc = 10(12)-10(20)) and spin ground state population beyond room temperature. Across the series, at a given formal oxidation level, the capping ligand on the hexairon cluster dictates the overall properties of the aggregate, modulating the redox delocalization and the persistence of the intracore coupling of the metal sites.

Figure 1

Illustration displaying the removal of three electrons from an all-ferrous octahedral cluster to give a (left) localized or (right) delocalized state.

Figure 2

Solid-state molecular crystal structures obtained at 100 K for (a, b) the cations [(^HL)₂Fe₆(DMF)₄]²⁺ in 4 and [(^HL)₂Fe₆(DMF)₆]³⁺ in 5, respectively, and (c) the anion [(^HL)₂Fe₆(CN)₆]⁻ in 9. The Fe, C, N, and O atoms are colored orange, gray, blue, and red, respectively. Thermal ellipsoids are set at 50% probability. Hydrogen atoms have been omitted for clarity.

Figure 3

Zero-field ⁵⁷Fe Mössbauer spectra for compounds (a) 4, (b) 5, (c) 6, (d) 7, (e) 8, (f) 9, and (g) 10. Blue (DMF adducts) and red (CN adducts) solid lines represent fits to the data (black dots). The fit parameters are described in the text.

Figure 4

Perpendicular-mode CW X-band EPR spectra of compounds (a) 9, (b) 2, and (c) 5. The simulated spectra are depicted as solid gray lines, and the resulting parameters are described in the text.

Figure 5

Variable-temperature perpendicular-mode CW X-band EPR spectra of [Bu₄N]₃[(^HL)₂Fe₆(CN)₆] (7). The spectra were recorded in MeCN at 0.6325 mW with a modulation amplitude of 0.5 mT.

Figure 6

(a) FTIR spectra of the cyanide species and (b) crystallographic CN distance and cyanide stretching frequency versus the cluster oxidation level for n = 3 (7), 4 (8), 5 (9), and 6 (10). (c) Cyclic voltammograms of [(^HL)₂Fe₆(CN)₆]ⁿ⁻⁶ taken in DMF (red) and PC (maroon), [(^HL)₂Fe₆(MeCN)₆]ⁿ⁺ in MeCN (black), and [(^HL)₂Fe₆(DMF)₆]ⁿ⁺ in DMF (blue). All were taken in 0.1 M [Bu₄N][PF₆] at room temperature.

Figure 7

Electrochemical data to test the reversibility of the observed redox couples for the cyanide and DMF electron-transfer series. (a) Cyclic and differential pulse voltammetry (DPV) data for [(^HL)₂Fe₆(CN)₆]ⁿ⁻⁶ taken at scan rates ranging from 100 to 400 mV/s in DMF. (b) Cyclic voltammetry of [(^HL)₂Fe₆(DMF)₆]ⁿ⁺ taken at scan rates ranging from 10 to 500 mV/s in DMF. (c, d) Plots of current density (j_p) vs the square root of the scan rate (ν^1/2) extracted from the data in (a) and (b), respectively. All of the data were collected in solutions containing 0.1 M [Bu₄N][PF₆] as the supporting electrolyte.

Figure 8

Comproportionation constants versus n (bottom axis) and versus the total number of [Fe₆] valence electrons (top axis). Black circles correspond to the MeCN adduct series, blue squares to the DMF series, and red symbols to the cyanide series in five different solvents.

Figure 9

NIR spectra of [(^HL)₂Fe₆(L′)_m]ⁿ⁺ (L′ = MeCN, DMF) and [(^HL)₂Fe₆(CN)₆]ⁿ⁻⁶: (a) n = 2, m = 4 species 1 (black) and 4 (blue); (b) n = 3, m = 6 species 2 (black), 5 (blue), and 7 (multicolor); (c) n = 4, m = 6 species 3 (black) and 8 (multicolor). The spectra for the acetonitrile and cyanide adducts are offset on the y axis.

Figure 10

Variable-temperature dc magnetic susceptibility data, χ_MT for (a) cyanide species 7 (maroon) and 10 (red), (b) acetonitrile adducts 1 (gray) and 2 (black), and (c) DMF adducts 4 (blue) and 5 (navy blue). Experimental data are shown as open symbols for 0.1 T (squares), 0.5 T (triangles), and 1 T (circles). Solid lines correspond to fits (or simulations) to the models described in the main text.

Figure 11

Average intracore Fe–Fe bond lengths as functions of core oxidation level for [(^HL)₂Fe₆(L′)_m]ⁿ⁺ with L′ = NCMe (black ●), DMF (blue ■), or CN (red ▲).

Figure 12

Reduction potentials (E_1/2) as functions of solvent donor number. Redox couples: [(^HL)₂Fe₆(CN)₆]ⁿ⁻⁶ with n = 3/4 (red △, −13.5 mV/DN), 4/5 (red □, −7.7 mV/DN), and 5/6 (red ○, −4.8 mV/DN); {[(NH₃)₅Ru]₂(pyz)}ⁿ⁺ with n = 4/5 (blue ■, −26.4 mV/DN) and 5/6 (blue ●, −26.3 mV/DN).

Figure 13

Plots of ν_max vs solvent dielectric function (1/D_op – 1/D_s) for 7, 8, and the CT ion.

Scheme 1