Redox Potential and Electronic Structure Effects of Proximal Nonredox Active Cations in Cobalt Schiff Base Complexes Alexander H. Reath, Joseph W. Ziller, Charlene Tsay, Austin J. Ryan, and Jenny Y. Yang* Department of Chemistry, University of California, Irvine, California 92697, United States * S Supporting Information ABSTRACT: Redox inactive Lewis acidic cations are thought to facilitate the reactivity of metalloenzymes and their synthetic analogues by tuning the redox potential and electronic structure of the redox active site. To explore and quantify this effect, we report the synthesis and characterization of a series of tetradentate Schiff base ligands appended with a crown-like cavity incorporating a series of alkali and alkaline earth Lewis acidic cations (1M, where M = Na + ,K + , Ca 2+ , Sr 2+ , and Ba 2+ ) and their corresponding Co(II) complexes (2M). Cyclic voltammetry of the 2M complexes revealed that the Co(II/I) redox potentials are 130 mV more positive for M = Na + and K + and 230-270 mV more positive for M = Ca 2+ , Sr 2+ , and Ba 2+ compared to Co(salen- OMe) (salen-OMe = N,N′-bis(3-methoxysalicylidene)-1,2-diaminoethane), which lacks a proximal cation. The Co(II/I) redox potentials for the dicationic compounds also correlate with the ionic size and Lewis acidity of the alkaline metal. Electronic absorption and infrared spectra indicate that the Lewis acid cations have a minor effect on the electronic structure of the Co(II) ion, which suggests the shifts in redox potential are primarily a result of electrostatic effects due to the cationic charge. ■ INTRODUCTION Nonredox active Lewis acidic metal cations play a key role in a diverse set of biological and synthetic transition metal complexes that mediate redox activity. In biological systems, the Ca 2+ ion found in the oxygen evolution complex (OEC) in Photosystem II is critical for water oxidation activity. 1 In synthetic transition metal complexes, the presence of Lewis acidic metals are known to promote C-H oxidation, 2 oxygen atom transfer, 3 olefin hydrogenation, 4 and oxygen reduction 5 reactions, as well as facilitate electron transfer reactions. 6 One of the proposed roles that proximal redox inactive metal cations play in promoting reactivity is tuning the redox potential of the reaction site. Redox tuning by incorporation of redox inactive cations has been reported in several synthetic systems including mono- 4,7 and multimetallic manganese 8 and triiron 9 clusters incorporating Lewis acid cations through oxo- bridges. Additionally, pendant crown ethers encapsulating alkali or alkaline earth metals have been appended onto molybde- num, 10 ferrocene, 11 and iron pyridinediimine 12 complexes. The shifts in the reversible redox potential denote a change in the absolute energy of the molecular orbital participating in electron transfer. We were interested in investigating how the Lewis acid cations engender this change. An inductive effect due to a modification of the ligand field would result in changes to the electronic structure (or molecular orbitals) of the redox active cation. In contrast, an electrostatic effect would uniformly shift the molecular orbitals on the redox active metal due to the electric field potential of the proximal cation. To elucidate the source of the change in redox potential due to adjacent Lewis acidic cations, we synthesized a series of cobalt(II) Schiff base complexes with an appended crown functionality containing a series of alkali and alkaline earth metal cations (2M in Chart 1, M = Na + ,K + , Ca 2+ , Sr 2+ , and Ba 2+ ). These compounds are well-suited to investigate the difference between inductive and electrostatic effects. The ligand provides a cavity similar in size to 18-crown-6, and can enclose a variety of ions with minimal effect on the coordination geometry of the Co(II) ion. Within this framework, the Co(II/I) couple is reversible, allowing a direct handle on changes in redox potential. The similar ligand environments permit facile comparisons in electronic structure Received: February 2, 2017 Published: February 27, 2017 Article pubs.acs.org/IC © 2017 American Chemical Society 3713 DOI: 10.1021/acs.inorgchem.6b03098 Inorg. Chem. 2017, 56, 3713-3718 Downloaded via UNIV OF CALIFORNIA IRVINE on September 27, 2018 at 17:57:12 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.