Transition metal complexes with sulfur ligands Part CII. Changes in configuration and reactivity caused by alkyl substituents R in [Fe(L)('N_RS₄')] complexes (LCO, NO⁺, NO, PMe₃, N₂H₄; RCH₃, CH₂CH₂CO₂CH₃, CH₂CH₂CO₂^-)

In search for complexes revealing functional and structural characteristics of the active sites of nitrogenases the iron(II) complexes [Fe(L)('N_ES₄')] (E = CH₂CH₂COOCH₃; L = CO (2), PMe₃ (11), N₂H₄ (9)) and [Fe('N_ES₄')]_x (7), [Fe(L)('N_MeS₄')] (L = CO (1), PMe₃ (10), N₂H₄ (8), NO (13), NO⁺ (12)) and [Fe('N_MeS₄')]_x (6), Na[Fe(CO)('N_AS₄')] (16) (A = CH₂CH₂COO^-) and {Na[Fe('N_AS₄')}_x (15) as well as the free ligands 'N_ES₄'-H₂·HCl (5), 'N_MeS₄'-H₂·HCl (4) and Na₃-'N_AS₄' (14) were synthesized. 1 was characterized by X-ray structure analysis. It crystallizes in the space group P2₁/c with a = 753.9(4), b = 1283.7(8), c = 1931.9(10) pm, β = 95.86(4)°, Z = 4. Unexpectedly, the [Fe(L)('N_RS₄')] complexes (R = CH₃, CH₂CH₂CO₂CH₃, CH₂CH₂CO₂Na) show different coordination geometries and considerably higher reactivities in comparison to the corresponding [Fe(L)('N_HS₄')] complexes with the parent ligand 'N_HS₄'^2- (2,2′-bis(2-mercapto-phenylthio)diethylamine(2-)). These differences can be traced back to the fact that the N atom is a tertiary amine in 'N_RS₄'^2- and a secondary amine in the 'N_HS₄'^2- ligand. Repulsive interactions among the three N substituents in the 'N_RS₄'^2- ligands lead to a weakening of the FeN bond, to a meso configuration of the [Fe('NS₄')] core and ultimately to higher reactivities of the [Fe(L)('N_RS₄')] complexes in comparison to the [Fe(L)('N_HS₄')] patent compounds. The low-spin complexes [Fe(CO)('N_MeS₄')] (1) and [Fe(CO)('N_HS₄')] (3) may serve as examples: [Fe(CO)('N_HS₄')] (3) has C₁ symmetry and a short FeN distance (207.2(8) pm), whereas [Fe(CO)('N_MeS₄')] (₁) reveals C₅ symmetry and a long FeN bond (218.7(4) pm). Such long FeN bonds can only be observed in the patent [Fe(L)('N_HS₄')] complexes with σ ligands when electrons occupy antibonding σ* molecular orbitals.