Phase and redox shifted four iron/four sulfur clusters: fluorous analogs of metalloenzyme cofactors

Abstract

Reactions of (1) [Q]₂[Fe₄S₄(SC(CH₃)₃)₄] and the fluorous thiols HS(CH₂)_nR_f8 (n = 2, 3; R_f8 = (CF₂)₇CF₃)), or (2) [Na]₂[Fe₄S₄(S(CH₂)_nR_f8)₄] (n = 2, 3) and [PhCH₂P((CH₂)₃R_f6)₃][Br] or [PPN][Cl] (PPN = Ph₃PNPPh₃), give the title compounds [Q]₂[Fe₄S₄(S(CH₂)_nR_f8)₄], comprised of a fluorous dianion and in some cases fluorous cations, with (1) Q/n = Ph₄P/2 (4, 67%), Ph₄P/3 (5, 67%), Me₄N/3 (69%), and Ph₃P(CH₂)₂R_f6/2 (73%) or (2) PhCH₂P((CH₂)₃R_f6)₃/2 (14, 39%), PhCH₂P((CH₂)₃R_f6)₃/3 (15, 63%), and PPN/2 (36%). The educt [Ph₃P(CH₂)₂R_f6]₂[Fe₄S₄(SC(CH₃)₃)₄] is in turn prepared from FeCl₃, HSC(CH₃)₃/CH₃ONa, and [Ph₃P(CH₂)₂R_f6][I], and the educts [Na]₂[Fe₄S₄(S(CH₂)_nR_f8)₄] from [Na]₂[Fe₄S₄(SC(CH₃)₃)₄] and HS(CH₂)_nR_f8. The SCH₂¹H and ¹³C NMR signals of these paramagnetic salts appear 8.7–10.3 and 32.3–34.9 ppm downfield from those of the corresponding thiols, but the chemical shifts of other signals are nearly normal. The UV-visible spectra show bands similar to those of non-fluorous analogs (290–298 nm and 406–415 nm; ε = 25 700 and 19 200 M⁻¹ cm⁻¹ for 5). The singly fluorous salts are soluble in organic solvents of moderate polarity, but not in fluorous solvents. The doubly fluorous salts 14, 15 are soluble in all fluorous solvents assayed, with partition coefficients of >99.65 : <0.35 (CF₃C₆F₁₁/toluene) and 93.2–93.1 : 6.9–6.8 (FC-72/THF). Cyclic voltammograms carried out using a platinum working microelectrode show that 4 is 0.08 V thermodynamically easier to reduce than 5.