TRANSITION-METAL COMPLEXES WITH SULFUR LIGANDS .120. PROTONATION AND ALKYLATION OF THE THIOLATO DONORS IN [FE(CO)(NHS4)] - EFFECTS ON THE STRUCTURAL, ELECTRONIC, AND REDOX PROPERTIES OF METAL-SULFUR COMPLEXES

The effects of protonation and alkylation at the sulfur donors in meta l-sulfur complexes have been investigated the specific example of [Fe- (CO)(''NHS4'')] (1). The 18 valence electron (VE) complex 1 consists o f a low-spin Fe-II center and the dithioether thiolato amine ligand '' NHS4''(2-) (= 2,2'-bis(2-mercaptophenylthio)diethylamine(2 -)). comple x 1 can be reversibly protonated at the two thiolato donors;this resul ts in an increase in v(CO) of 35 cm(-1) after the first protonation an d 45 cm(-1) after the second. Alkylation of I with one or two equivale nts of the oxonium salts R(3)OBF(4) (R = Me, Et) yields [Fe(CO)(''NHS4 ''-R)]BF4 (R = Me: 4, Et: 5), [Fe-(Co)(''NHS4''-R(2))](BF4)(2) (R = Me : 6, Et: 7), and the methyl ethyl derivative [Fe-(Co)(''NHS4''-Me-Et)] (BF4)(2) (8). An increase in nu(CO) of 31 - 32 cm(-1) is observed for each successive alkylation. Due to the C-1 symmetry of 1, complexes 4, 5, and 8 are formed as 1:1 mixtures of two diastereomers, whereas 6 a nd 7 are present as only one stereoisomer. Acidic hydrolysis of 4, 6, and 7 liberates the corresponding ligands [''NH2S4''-R(n)](BF4)(n) (9- 11, n = 1,2), which were isolated as the ammonium tetrafluoroborate sa lts. The molecular structure of 8a has been elucidated by X-ray struct ure analysis. This shows that the Fe-N and Fe-S bonds in the [FeNS4] c ore do not change after alkylation of the thiolato donors despite the Delta nu(CO) of ca. 60 cm(-1) between 1 and 8, which indicates a disti nct decrease in electron density at the Fe center. This decrease can b e rationalized by increased pi-acceptor character of the sulfur donors upon protonation or alkylation. The change in electron density at the [Fe-(''NS4'')] core is further corroborated by cyclic voltammetry. Fo r each successive protonation or alkylation of 1, the redox couple pot entials shift by 600-800 mV. As a consequence, the diethyl derivative, for example, gives rise to reduced species (19 or 20 VE) which are no t observed for 1. The implications of these results for the reduction of N-2 within the coordination sphere of the FeMo cofactor of nitrogen ases are discussed.