BIMETALLIC REACTIVITY - SYNTHESIS OF BIMETALLIC COMPLEXES OF MACROCYCLIC BINUCLEATING LIGANDS CONTAINING 6-COORDINATE AND 4-COORDINATE SITES AND THEIR REACTIVITY WITH DIOXYGEN AND OTHER OXIDANTS
Dg. Mccollum et al., BIMETALLIC REACTIVITY - SYNTHESIS OF BIMETALLIC COMPLEXES OF MACROCYCLIC BINUCLEATING LIGANDS CONTAINING 6-COORDINATE AND 4-COORDINATE SITES AND THEIR REACTIVITY WITH DIOXYGEN AND OTHER OXIDANTS, Journal of the American Chemical Society, 118(6), 1996, pp. 1365-1379
General methods are described for the synthesis of amine macrocyclic l
igand 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 obtaine
d 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 trimethylen
ediamine (tn) and ethylenediamine (en) links were prepared, namely, th
e four ligands containing the tntn, tnen, entn, and enen combinations.
Bimetallic complexes of the amine ligands are prepared under mild con
ditions from the monometallic complexes [M(amine ligand)(H+)(2)](n+) b
y addition of the second metal. The structures of these complexes were
studied by H-1 NMR spectrometry and by X-ray diffraction. The monomet
allic imine complexes, [Co-II(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, [Co-II(amine ligand)
(H+)(2)](2+), are oxidized by fc(+) to give stable cobalt(III) complex
es 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 flexibl
e amine systems. This effect is called mechanical coupling, Ferrociniu
m ion oxidation of the bimetallic amine complexes, [Co-II(amine ligand
)(CoCl)-Cl-II](+), leads only to the formation of the mixed oxidation
state complexes, [Co-II(amine ligand)(CoCl)-Cl-II](2+), where the coba
lt(III) is in the 6-coordinate site. Unlike the imine dicobalt(II) com
plexes which are unreactive to dioxygen, the amine ligand dicobalt(II)
complexes readily react with dioxygen to give the mixed oxidation sta
te complexes, [Co-II(amine ligand)Co-III(Cl)(X)](+), where now the cob
alt(III) ion occupies the 4-coordinate site. The inability to form sta
ble dicobalt(III) complexes with these amine ligands, despite the fact
that each site is capable of supporting the cobalt(III) states, is as
cribed to various forms of coupling between the metals in the two site
s. Three forms of coupling are considered and discussed; these are, me
chanical coupling which refers to ligand constraints, through-space el
ectrostatic interactions between metals, and through-bond coupling whi
ch refers to covalent interaction between the metals via the bridging
ligands.