TRANSITION-METAL COMPLEXES WITH SULFUR LIGANDS .104. PROTONATION AND ALKYLATION OF THIOLATE DONORS OF [FE(CO)2(S4)] AND [FE(CO)(S5)] - SYNTHESES AND PROPERTIES OF FE(II) COMPLEXES WITH OPEN-CHAIN THIOETHER LIGANDS (S-4(2-) = 1,2-BIS(2-MERCAPTOPHENYLTHIO)ETHANE(2-) - S-5(2-) = 2,2'-BIS(2-MERCAPTO-PHENYLTHIO)DIETHYLSULFIDE(2-))

[Fe(CO)2('S4')] and [Fe(CO)('S5')] could be reversibly protonated in t wo steps by strong acids such as CF3SO3H yielding species whose nu(CO) bands are shifted to higher wavenumbers by approximately 40 cm-1 per equivalent of acid. The nu(CO) shifts are explained by protonation of the thiolate donors leading to a decrease of electron density at the m etal centers and consecutive weakening of Fe-CO pi backbonds. While th e protonated species could be detected in solution only, analogous iso electronic complexes which are alkylated at the thiolate donors were i solated and fully characterized spectroscopically and by X-ray structu re analyses. Upon reaction with one or two equivalents of oxonium salt s R3OBF4 (R = Me, Et), [Fe(CO)2('S4')] yielded [Fe(CO)2(R-'S4')]BF4 (R = Me: [1a]BF4, R = Et: [1b]BF4) and [Fe(CO)2(R2-'S4')](BF4)2 (R = Me: [3a](BF4)2, R = Et: [3b]BF4)2). In an analogous way, [Fe(CO)('S5')] y ielded [Fe(CO)(R-'S5')]BF4 (R = Me: [2a]BF4, R = Et: [2b]BF4) and [Fe( CO)(R2-'S5')](BF4)2 (R = Et: [5](BF4)2). The 'asymmetrically' alkylate d [Fe(CO)2(Me-'S4'-Et)](BF4)2, [4](BF4)2, was obtained by reacting [Fe (CO)2('S4')] first with Me3OBF4 and subsequently with Et3OBF4. Further complexes obtained by alkylation were [Fe(CO)(Bz-'S5')]PF6 ([2c]PF6), containing the benzylated 'S5' ligand, [Fe(CO)(I)(Me2-'S4')]FeI4 ([6] FeI4) and [Fe(I)2(Me2-'S4')] (7). Except 7, all Fe(II) complexes are d iamagnetic containing low-spin Fe(II) centers. Per step of alkylation, the nu(CO) frequencies are raised by approximately 40 cm-1 in the cas e of [Fe(CO)2('S4')] and by approximately 32 cm-1 in case of [Fe(CO)(' S5')]. These nu(CO) shifts are explained in the same way as for the pr otonated species and corroborate the assumption that protonation takes place at the thiolate donors. X-ray structure determinations were car ried out for [1b]BF4, [3a](BF4)2, [3b](BF4)2, [5](BF4)2, [6]FeI4 and 7 . Although the nu(CO) shifts indicate large differences of the electro n densities at the iron centers, in a remarkable way the average Fe-S distances of approximately 228 pm remain practically invariant in all low-spin Fe(II) complexes regardless of the charge or the degree of al kylation. This is traced back to the transformation of Fe-S(thiolate) sigma-donor bonds into Fe-S(thioether) sigma-donor-pi-acceptor bonds u pon alkylation. This lowers the electron densities at the iron centers but leaves Fe-S distances unchanged because weakening of the sigma bo nds is compensated by the formation of additional pi-acceptor bonds. T he mono- and dialkylated complexes hydrolyze much more rapidly than th e neutral parent complexes and allow the facile synthesis of the corre sponding sulfur ligands in the free state. The complexes further prove that 3d metals can form stable complexes not only with crown thioethe rs but also with open chain thioethers. Correlation of states of proto nation of iron sulfur ligand complexes with electron densities at the metal center and expected redox potentials allows the hypothesis to be made that reducing the N2 molecule at either Fe-S or Mo-S sites of th e cofactor in nitrogenases requires previous protonation of the S dono rs.