Trivalent Cations Slow Electron Transfer to Macrocyclic Heterobimetallic Complexes

Incorporation of secondary redox-inactive cations into
heterobimetallic
complexes is an attractive strategy for modulation of metal-centered
redox chemistry, but quantification of the consequences of incorporating
strongly Lewis acidic trivalent cations has received little attention.
Here, a family of seven heterobimetallic complexes that pair a redox-active
nickel center with La3+, Y3+, Lu3+, Sr2+, Ca2+, K+, and Na+ (in the form of their triflate salts) have been prepared on a heteroditopic
ligand platform to understand how chemical behavior varies across
the comprehensive series. Structural data from X-ray diffraction analysis
demonstrate that the positions adopted by the secondary cations in
the crown-ether-like site of the ligand relative to nickel are dependent
primarily on the secondary cations’ ionic radii and that the
triflate counteranions are bound to the cations in all cases. Electrochemical
data, in concert with electron paramagnetic resonance studies, show
that nickel(II)/nickel(I) redox is modulated by the secondary metals;
the heterogeneous electron-transfer rate is diminished for the derivatives
incorporating trivalent metals, an effect that is dependent on steric
crowding about the nickel metal center and that was quantified here
with a topographical free-volume analysis. As related analyses carried
out here on previously reported systems bear out similar relationships,
we conclude that the placement and identity of both the secondary
metal cations and their associated counteranions can afford unique
changes in the (electro)chemical behavior of heterobimetallic species.