TRANSITION-METAL COMPLEXES WITH SULFUR LIGANDS .102. CHANGES IN CONFIGURATION AND REACTIVITY CAUSED BY ALKYL SUBSTITUENTS-R IN [FE(L)(NRS4)] COMPLEXES (L=CO, NO+, NO, PME3, N2H4 R=CH3, CH2CH2CO2CH3, CH2CH2CO2-)

In search for complexes revealing functional and structural characteristics of the active sites of nitrogenases the iron(II) complexes [Fe(L)('NES4')] (E=CH2CH2COOCH3; L=CO (2), PMe3 (11), N2H4 (9)) and [Fe('NES4')]x (7), [Fe(L)('NmeS4')] (L=CO (1), PMe3 (10), N2H4 (8), NO (13), NO' (12)) and [Fe('NMeS4')]x (6), Na[Fe(CO)('NAS4')] (16) (A=CH2CH2COO-) and {Na[Fe('NAS4')}x (15) as well as the free ligands 'NES4'-H-2-HCl (5), 'NMeS4'-H-2-HCl (4) and Na3-'NAS4' (14) were synthesized. 1 was characterized by X-ray structure analysis. It crystallizes in the space group P2(1)/c with a=753.9(4), b=1283.7(8), c=1931.9(10) pm, beta=95.86(4)-degrees, Z=4. Unexpectedly, the [Fe(L)('NRS4')] complexes (R=CH3, CH2CH2CO2CH3, CH2CH2CO2Na) show different coordination geometries and considerably higher reactivities in comparison to the corresponding [Fe(L)('NHS4')] complexes with the parent ligand 'NHS4'2- (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 'NRS4'2- and a secondary amine in the 'NHS4'2- ligand. Repulsive interactions among the three N substituents in the 'NRS4'2- ligands lead to a weakening of the Fe-N bond, to a meso configuration of the [Fe('NS4')] core and ultimately to higher reactivities of the [Fe(L)('NRS4')] complexes in comparison to the [Fe(L)('NHS4')] parent compounds. The low-spin complexes [Fe(CO)('NMeS4')] (1) and [Fe(CO)('NHS4')] (3) may serve as examples: [Fe(CO)('NHS4')] (3) has C1 symmetry and a short Fe-N distance (207.2(8) pm), whereas [Fe(CO)(['NMeS4')] (1) reveals C(s) symmetry and a long Fe-N bond (218.7(4) pm). Such long Fe-N bonds can only be observed in the parent [Fe(L)('NHS4')] complexes with sigma ligands when electrons occupy antibonding sigma* molecular orbitals.