17- and 19-electron complexes [Fe^III(η⁵-C₅R₅)(S ₂CNMe₅)L]ⁿ⁺ (n = 1, 0): Electronic structure and substitution and redox chemistry. Formation of [Fe^IV(η⁵-C₅R₅)(dtc)2] and characterization of both 17e and 19e states of a transition-metal complex

Oxidation of [FeCp*(η¹-dtc)(CO)₂], 1 (Cp* = η⁵-C₅Me₅, dtc = S₂CNMe₂), or [FeCp*(η²-dtc)(CO)], 2, using [Fe^IIICp₂]⁺X^- (X^- = PF₆^- or BF₄^-, Cp = η⁵-C₅H₅) in THF cleanly gives [Fe^IIICp*(η²-dtc)(CO)]⁺X^-, 2⁺X^-, as macrocrystalline green, thermally stable, but substitution labile, salts. The substitution of CO in 2⁺PF₆^- by various solvents (CH₂Cl₂, THF, CH₃COCH₂, CH₂CN) (visible spectroscopy) follows pseudo-first-order kinetics but shows clearly the influence of the incoming solvent ligand on the substitution rate and, hence, is in good agreement with an associative mechanism. Displacement of the labile solvent ligand in these complexes by a phosphine results in the 17-electron (17e) cations [Fe^IIICp*(η²-dtc)(L)]⁺PF ₆^-, L = PPh₃ (7⁺PF₆^-) or η¹-dppe (8⁺PF₆^-). The same reaction in the presence of the anionic ligands CN^-, SCN^-, and Cl^- affords the corresponding neutral 17e Fe^III complexes (respectively compounds 11, 13, and 14). All these 17e complexes were characterized by IR, ESR, and Mössbauer spectroscopies and elemental analysis. The cations were reduced to isostructural neutral Fe^II complexes using 1 equiv of [Fe¹Cp(C₆Me₆)] in THF or oxidized to the robust green 18e Fe^II complex [Fe⁶(η⁵-C₅Me₅)(η ⁵-S₂CNMe₂)₂]⁺-PF ₆^-, 9⁺PF₆^-, using Na⁺dtc^-·2H₂O. [Fe^IV(η⁵-C₅(CH₂C ₆H₅)₅)(η²-S₂CNMe ₂)₂]⁺PF₆^-, 16⁺PF₆^-, was structurally characterized, and the dihapto mode of coordination of both dtc ligands was established. The 19e Fe^III species 9 was shown to be an intermediate which further reduced H₂O. It could be alternatively synthesized by reduction of the 18e precursor 9⁺PF₆^- using 1 equiv of [Fe¹Cp(C₆Me₆)] or by addition of anhydrous Na⁺dtc^- to 3⁺PF₆^- in MeCN at -40 °C. The 19e complex 9 showed an ESR spectrum indicating an axial symmetry (two g values) in contrast with the ESR spectra of all the 17e species (2⁺-14) which show three g values characteristic of a rhombic distortion (for instance, the very close model 13). The Mössbauer doublet of 9 very slowly evolved to the new doublet of the thermally stable 17e complex 9′. In MeCN solution, the transformation of the blue complex 9 to the purple 17e complex 9′ was much more rapid (above -40 °C) as indicated by the rhombic spectrum of 9′ in frozen solution and by low-temperature ¹³C NMR. In toluene, however, the 19e complex 9 showed a remarkable stability up to room temperature, which allowed recording of the ¹³C spectrum in d⁸-toluene. MO calculations have been performed on models for the 17e and 19e bis-dtc Fe^III complexes. They suggest that the 17e species should have some significant sulfur spin density. The 19e species is found to have its odd electron occupying an antibonding metal-centered orbital. The cyclic voltammogram of 9⁺PF₆^- under continuous scanning for the monoelectronic reduction and the two monoelectronic reductions showed the decrease of the waves of 9^-PF₆^- and the concomitant increase of those due to the partially decoordinated dtc complexes formed upon reduction. This permits an interpretation of the CV in terms of a triple-square scheme involving 9^+/0/-, ^+/0/-, and solvent (DMF) adducts in 18- and 19e states.