Tuning the O₂ Binding Affinity of Cobalt(II) Centers by Changing the Structural and Electronic Properties of the Distal Substituents on Azole-Based Chelating Ligands

Abstract

The effects of the substituents on the chelating ligands located in the secondary coordination sphere on the O₂ affinity of cobalt(II) centers have been explored. The combination of facially capping tridentate tris(pyrazolyl)borates (= Tp^Me2,4R) and bidentate bis(imidazolyl)borates (= [B(Im ^N-Me)₂MeX]^- ; L^X) yields square-pyramidal cobalt(II) complexes. The structural properties of the substituent groups X attached to the boron center of L^X affect the arrangement of X in the resulting cobalt(II) complexes [Co^II(Tp^Me2,4R)(L^X)]. When the boron-attached moiety of X is a relatively bulky sp³-CH₂Y group (i.e., X:Y = Me:H and nBu: nPr), the alkyl group X faces the cobalt center, whereas for isopropoxy (O iPr) and phenyl (Ph) groups, of which the boron-attached atoms are a less hindered oxygen atom and a planer sp²-carbon, respectively, the X group is arranged away from the cobalt center. This flexible behavior of L^X is reflected in the O₂ affinity of the cobalt(II) center, which depends on the extent to which the complex sphere is shielded by the ligands. The dependence of the cobalt(II) oxidation potential on the X substituent of L^X is inconsistent with the O₂ affinity. On the other hand, the electronic properties of R, which is attached to the fourth position of the pyrazolyl rings in the rigid Tp^Me2,4R ligand, are reflected in the electrochemical properties and O₂ affinity of the cobalt center.