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

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.