17- and 19-Electron Complexes [FeIII(η5-C5R5)(S2CNMe2)L]n+ (n = 1, 0): Electronic Structure and Substitution and Redox Chemistry. Formation of [FeIV(η5-C5R5)(dtc)2] and Characterization of both 17e and 19e States of a Transition-Metal Complex

Abstract

Oxidation of [FeCp*(η1-dtc)(CO)2], 1 (Cp* = η5-C5Me5, dtc = S2CNMe2), or [FeCp*(η2-dtc)(CO)], 2, using [FeIIICp2]+X- (X- = PF6- or BF4-, Cp = η5-C5H5) in THF cleanly gives [FeIIICp*(η2-dtc)(CO)]+X-, 2+X-, as microcrystalline green, thermally stable, but substitution labile, salts. The substitution of CO in 2+PF6- by various solvents (CH2Cl2, THF, CH3COCH3, CH3CN) (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 [FeIIICp*(η2-dtc)(L)]+PF6-, 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 FeIII 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 FeII complexes using 1 equiv of [FeICp(C6Me6)] in THF or oxidized to the robust green 18e FeIV complex [FeIV(η5-C5Me5)(η2-S2CNMe2)2]+PF6-, 9+PF6-, using Na+dtc-·2H2O. [FeIV(η5-C5(CH2C6H5)5)(η2-S2CNMe2)2]+PF6-, 16+PF6-, was structurally characterized, and the dihapto mode of coordination of both dtc ligands was established. The 19e FeIII 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 [FeICp(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 FeIII 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/-, 9‘+/0/-, and solvent (DMF) adducts in 18- and 19e states.