Transition metal complexes with sulfur ligands Part CII. 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)

Dedicated to Professor Helmut Werner on the occasion of his 60th birthday.
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Abstract

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’-H2·HCl (5), ‘NMeS4’-H2·HCl (4) and Na3-‘NAS4’ (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)(‘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 ‘NHS42− (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 ‘NRS42− and a secondary amine in the ‘NHS42− ligand. Repulsive interactions among the three N substituents in the ‘NRS42− 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’)] patent 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 C5 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)(‘NHS4’)] complexes with σ ligands when electrons occupy antibonding σ* molecular orbitals.

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