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 counter-anions are bound to the cations in all cases. Electrochemical data, in concert with electron paramagnetic resonance (EPR) studies, show that nickel(II)/nickel(I) redox is modulated by the secondary metals; the heterogeneous electron transfer (ET) 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 that were not noted in prior work, we conclude that the placement and identity of both the secondary metal cations and their associated counter-anions can afford unique changes in the (electro)chemical behavior of heterobimetallic species.