Synthesis, Structural Characterization, and Electronic Properties of the Ph₄P⁺ Salts of the Mixed Terminal Ligand Cubanes Fe₄S₄(Et₂Dtc)_n(X)_4-n²-(X = Cl^-, PhS^-) (n = 1, 2). Two Different Modes of Ligation on the [Fe₄S₄]²⁺ Core

Mixed terminal ligand iron—sulfur clusters of the type [Ph₄Pl'>Fe₄S₄(X)_4-n,(Et₂NCSS)_n (X = SPh, Cl, n = 2 and X = Cl, n = 1) have been synthesized in good yields by the reaction of [Ph₄P]₂Fe₄S₄(L)₂(X)₂(L = SPh, Cl and X = Cl) with 2 equiv of Et₂NCSSNa•3H₂O and [Ph₄P]₂Fe₄S₄CI₄ with 1 equiv of Et₂NCSSNa.3H₂O, respectively. The synthesis and crystal structures of [Ph₄P]₂Fe₄S₄(Cl)₂(Et₂NCSS)₂ [X = SPh (I), Cl (II), and [Ph₄P]₂Fe₄S₄Cl₃(Et₂NCSS) (III)] are described in detail. Complexes I and II crystallize in the monoclinic space group C2/c with cell constants a = 17.486 (3) Å, b = 18.240 (3) Å, c = 24.198 (3) Å, and β = 109.73 (1)° and a = 20.409 (6) Å, b = 13.873 (3) Å, c = 25.739 (9) Å, and β = 118.94 (2)°, respectively. Complex III crystallizes in the monoclinic space group P2 /c with cell constants a = 15.836 (2) Å, b = 17.803 (2) Å, c = 22.496 (3) Å, and β = 101.44 (1)°. In the structures of I and II the non-hydrogen atoms were refined anisotropically and hydrogen atoms were included in the structure factor calculation but not refined. In the structure of III the carbon atoms were refined isotropically while all other non-hydrogen atoms were assigned anisotropic temperature factors. The hydrogen atoms were treated as in I and II. Refinement by full-matrix least squares of 661 parameters on 3226 data for I, 343 parameters on 1919 data for II, and 402 parameters on 5038 data for III gave final R values 0.079, 0.056, and 0.051, respectively. The anions in I and II are located on crystallographic 2-fold axes. The mean Fe—S* bond lengths in I, II, and III are 2.291, 2.307, and 2.317 A, respectively. The Fe₄S₄ units in I—III contain two structurally distinct iron sites. One of the sites has tetrahedrally coordinated iron atoms with PhS^- and Cl^- terminal ligands. The second site is rather uncommon and shows the iron atoms coordinated by the Et₂NCSS ligands in a bidentate fashion. The Fe—Fe distances are longer for the iron atoms associated with the chelating Et₂NCSS^- ligands (3.053 (3) A for I, 3.045 (4) A for II, and 2.936 (2) A for III). The Fe—SPh bond in I is 2.281 (4) A. The Fe-Cl bonds in II and III are found at 2.249 (4) and 2.231 (2) A, respectively. The Et₂NCSS” ligand coordination in I and II is asymmetric with unequal Fe-S(Dt) distances of 2.552 (4) A and 2.436 (4) and 2.578 (4) and 2.421 (4) A, respectively. In contrast the corresponding distances in III are 2.478 (2) and 2.469 (2) A. The electronic ¹H NMR and solution magnetic studies are reported. Zero-field Mossbauer spectra of I—III show two well-resolved quadrupole doublets. At 77 K, 1S₁ = 0.47, ΔE_Q1= 1.06, IS₂ = 0.64, and ΔE_Q2 = 1.84 mm/s for I, IS₁ = 0.53, ΔE_Q1 -1.06, IS₂ = 0.62, ΔE_Q2 = 1.85 mm/s for II, and IS₁ =0.51, ΔE_Q1 = 1.07, IS₂ = 0.64, and ΔE_Q2 = 2.13 mm/s for III. Cyclic voltammetric studies for I—III and double-potential step chronoamperometry for I are described.