Transition metal complexes with sulfur ligands Part CII. Changes in configuration and reactivity caused by alkyl substituents R in [Fe(L)(‘N RS 4’)] complexes (LCO, NO +, NO, PMe 3, N 2H 4; RCH 3, CH 2CH 2CO 2CH 3, CH 2CH 2CO 2−)

Abstract

In search for complexes revealing functional and structural characteristics of the active sites of nitrogenases the iron(II) complexes [Fe(L)(‘N ES 4’)] (E = CH 2CH 2COOCH 3; L = CO ( 2), PMe 3 ( 11), N 2H 4 ( 9)) and [Fe(‘N ES 4’)] x ( 7), [Fe(L)(‘N MeS 4’)] (L = CO ( 1), PMe 3 ( 10), N 2H 4 ( 8), NO ( 13), NO + ( 12)) and [Fe(‘N MeS 4’)] x ( 6), Na[Fe(CO)(‘N AS 4’)] ( 16) (A = CH 2CH 2COO −) and {Na[Fe(‘N AS 4’)} x ( 15) as well as the free ligands ‘N ES 4’-H 2·HCl ( 5), ‘N MeS 4’-H 2·HCl ( 4) and Na 3-‘N AS 4’ ( 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, β = 95.86(4)°, Z = 4. Unexpectedly, the [Fe(L)(‘N RS 4’)] complexes (R = CH 3, CH 2CH 2CO 2CH 3, CH 2CH 2CO 2Na) show different coordination geometries and considerably higher reactivities in comparison to the corresponding [Fe(L)(‘N HS 4’)] complexes with the parent ligand ‘N HS 4’ 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 RS 4’ 2− and a secondary amine in the ‘N HS 4’ 2− ligand. Repulsive interactions among the three N substituents in the ‘N RS 4’ 2− ligands lead to a weakening of the FeN bond, to a meso configuration of the [Fe(‘NS 4’)] core and ultimately to higher reactivities of the [Fe(L)(‘N RS 4’)] complexes in comparison to the [Fe(L)(‘N HS 4’)] patent compounds. The low-spin complexes [Fe(CO)(‘N MeS 4’)] ( 1) and [Fe(CO)(‘N HS 4’)] ( 3) may serve as examples: [Fe(CO)(‘N HS 4’)] ( 3) has C 1 symmetry and a short FeN distance (207.2(8) pm), whereas [Fe(CO)(‘N MeS 4’)] ( 1) reveals C 5 symmetry and a long FeN bond (218.7(4) pm). Such long FeN bonds can only be observed in the patent [Fe(L)(‘N HS 4’)] complexes with σ ligands when electrons occupy antibonding σ* molecular orbitals.