Bimetallic Reactivity. Synthesis of Bimetallic Complexes of Macrocyclic Binucleating Ligands Containing 6- and 4-Coordinate Sites and Their Reactivity with Dioxygen and Other Oxidants

Abstract

General methods are described for the synthesis of amine macrocyclic ligand systems which can contiguously accommodate two metals, one in a 6-coordinate site and the other in a 4-coordinate location. The amine complexes of the type [M(amine ligand)(H+)2]n+, where M is in the 6-coordinate site and the other site contains two protons, are obtained from the previously described imine complexes, [M(imine ligand)(H+)2]n+, by BH4- reduction. The macrocyclic framework consists of two diamine links, one at each site. Ligands incorporating the trimethylenediamine (tn) and ethylenediamine (en) links were prepared, namely, the four ligands containing the tntn, tnen, entn, and enen combinations. Bimetallic complexes of the amine ligands are prepared under mild conditions from the monometallic complexes [M(amine ligand)(H+)2]n+ by addition of the second metal. The structures of these complexes were studied by 1H NMR spectrometry and by X-ray diffraction. The monometallic imine complexes, [CoII(imine ligand)(H+)2]2+, are oxidized to cobalt(III) complexes by ferrocinium ions (fc+) when en links are present in the 6-coordinate site but this is not the case when tn is present in this site. All of the amine complexes, [CoII(amine ligand)(H+)2]2+, are oxidized by fc+ to give stable cobalt(III) complexes and it is concluded that the rigidity of the imine ligands prevents the ligand from adjusting to the stereochemical demands of cobalt(III) when the 6-coordinate site contains the larger tn link in the imine systems. This stereochemical impediment is relaxed in the more flexible amine systems. This effect is called mechanical coupling. Ferrocinium ion oxidation of the bimetallic amine complexes, [CoII(amine ligand)CoIICl]+, leads only to the formation of the mixed oxidation state complexes, [CoIII(amine ligand)CoIICl]2+, where the cobalt(III) is in the 6-coordinate site. Unlike the imine dicobalt(II) complexes which are unreactive to dioxygen, the amine ligand dicobalt(II) complexes readily react with dioxygen to give the mixed oxidation state complexes, [CoII(amine ligand)CoIII(Cl)(X)]+, where now the cobalt(III) ion occupies the 4-coordinate site. The inability to form stable dicobalt(III) complexes with these amine ligands, despite the fact that each site is capable of supporting the cobalt(III) states, is ascribed to various forms of coupling between the metals in the two sites. Three forms of coupling are considered and discussed; these are, mechanical coupling which refers to ligand constraints, through-space electrostatic interactions between metals, and through-bond coupling which refers to covalent interaction between the metals via the bridging ligands.