Structural, spectroscopic and magnetic properties of M[R2P(E)NP(E)R′2]2 complexes, M = Co, Mn, E = S, Se and R, R′ = Ph or iPr. Covalency of M–S bonds from experimental data and theoretical calculations

Abstract

The S/Se-containing bidentate ligands LH of the type R₂P(E)NHP(E)R′₂, E = S, Se and R, R′ = Ph or ⁱPr have been employed to synthesize ML₂ (M = Mn, Co) complexes which contain the biologically important MS₄ core. Theoretical calculations on the LH and L⁻ forms of the ligands probe the geometric and electronic changes induced by the deprotonation of the LH form, which are correlated with structural data from X-ray crystallography. These results reflect the flexibility of the ligands, which enables them to be rather versatile with respect to the formation of ML₂ complexes with varied geometries and MEPNPE metallacycle conformations. A series of old and new ML₂ complexes have been synthesized and their structural, spectroscopic and magnetic properties characterized in detail. The nephelauxetic ratio β of the CoL₂ complexes provides evidence of covalent interactions, whereas the EPR properties of the MnL₂ complexes are interpreted on the basis of predominant ionic interactions, between the metal center and the ligands, respectively. Additional evidence for the existence of covalent interactions in the CoL₂ complexes (R = Ph, ⁱPr, or mixed Ph/ⁱPr), is offered by comparisons between their ³¹P NMR. The aforementioned notations are supported by extensive theoretical calculations on the ML₂ (E = S, R = Me) modelled structures, which probe the covalent and ionic character of the M–S bonds when M = Co or Mn. Wider implications of the findings of the present study on the M–S covalency and its importance in the active sites of various metalloenzymes are also discussed.