36) AE,120X-Red may be equated to E,'(o") -Ek(R") with good approximation, since the redox processes are reversible here. (37) Planar quadridentate systems with an attached tail containing a donor functionality capable of axial coordination have been the subject of considerable recent interest.] Such combinations of planar quadridentate and axial coordination occur frequently in biological systems; examples include the axial imidazole and planar heme in hemoglobin and myoglobin* and the axial benzimidazole and quasi-planar corrin in vitamin B,2.3 Cytochrome c is a six-coordinate system containing a planar heme with the axial sites occupied by histidine imidazole and methionine t h i~e t h e r .~ Prompted by the last, we have synthesized the ligandsalenH,, containing a thioether tail ( Figure 1). The purpose was to determine the coordinating ability of the thioether tail, since, although thioether coordination occurs in the biological system, under normal circumstances thioether ligands coordinate only weakly with the first transition elements in their usual oxidation states6 We wished to know whether the chelate effect achieved by tail incorporation would enhance the stability or whether other factors were necessary.
Synthesis and Stereochemical ConsiderationsThe synthesis, originating from (S)-methionine, is shown in Figure 1, as is the structure of the sought-after metal complexes. The structural requirement for the tail coordination, namely, that the tail carbon atom emanating from the ethylenediamine backbone be axially disposed, is consistent with observations on substituted salen complexes for which it has been found that the ethylenediamine substituents prefer an axial dispo~ition.~ Results W e have isolated crystalline salmet complexes of copper(II), Figure 1. Method of preparation of the salmetH, ligand and its complexes. nickel(II), and iron(III), the latter containing two Nmethylimidazole (N-meim) ligands without which a crystallizable compound could not be obtained. The solid crystalline complexes [Cu(salmet)] and [Ni(salmet)] are dark blue and deep red, respectively, and are quite unlike the colors obtained for the corresponding [Cu(salen)] and [Ni(salen)] complexes. The color of the copper complex is consistent with thioether coordination6, but new absorption bands could arise in square-planar Cu(I1) and Ni(I1) salen and salmet complexes because of oxygen-metal interaction between neighboring molecules in the solids. So it is not possible to confidently transfer information from the solid to solution where we show there is no evidence for thioether coordination. Electronic spectra of [Cu(salmet)] were recorded in chloroform, benzene, methanol. dioxane, dichloromethane, and nitrobenzene. In all cases the visible spectrum (300-800 nm) wa5 identical with that of [Cu(salen)] in the same solvent, indicating lack of coordination of the sulfur in these media. Similarly, electronic spectra of [Ni(salmet)] (diamagnetic)
