Vanadyl as a Spectroscopic Probe of Tunable Ligand Donor Strength in Bimetallic Complexes

Abstract

Incorporation of secondary metal ions into heterobimetallic complexes has emerged as an attractive strategy for rational tuning of compounds' properties and reactivity, but direct solution-phase spectroscopic interrogation of tuning effects has received less attention than it deserves. Here, we report the assembly and study of a series of heterobimetallic complexes containing the vanadyl ion, [VO] 2+ , paired with monovalent cations (Cs + , Rb + , K + , Na + , and Li + ) and a divalent cation (Ca 2+ ). These complexes, which can be isolated in pure form or generated in situ from a common monometallic vanadyl-containing precursor, enable experimental spectroscopic and electrochemical quantification of the influence of the incorporated cations on the properties of the vanadyl moiety. The data reveal systematic shifts in the V−O stretching frequency, isotropic hyperfine coupling constant for the vanadium center, and V(V)/V(IV) reduction potential in the complexes. These shifts can be interpreted as charge density effects parametrized through the Lewis acidities of the cations, suggesting broad potential for the vanadyl ion to serve as a spectroscopic probe in multimetallic species.

Trivalent Cations Slow Electron Transfer to Macrocyclic Heterobimetallic Complexes

“…Similar features were measured for all the heterobimetallic species, albeit shifted in each case to higher energies, similar to observations made in prior work (see Figures S40−S66). [8][9][10]34 Upon cation incorporation, both the d−d and CT transitions undergo blue shifts relative to the precursor complex [Ni]. As shown in Figure 4, the CT transitions undergo a shift from 418 nm for [Ni] to 370 nm for [Ni,La], whereas the d−d transitions shift more modestly from 550 to 535 nm for the same range of complexes as can be seen in [ 33,35 Electrochemical Studies.…”

“…This relationship could be indicative of charge density effects being operative in this system, wherein the cationic charge of the secondary cations exerts an electrostatic influence over, of primary importance, the phenoxide moieties that bridge to the nickel center. 10,38 In such a case, the phenoxide O atoms could be subject to an influence from the secondary metal cations that is not unlike the influence cast over the coordinated water molecules in the metal−aqua complexes for which the pK a values were tabulated. 16 EPR Studies.…”

“…Recently, we have gathered evidence for the hypothesis that, in cases where bridging ligands are present between redox-active metals and the incorporated secondary electropositive metal cations, there is an induced diminishment of the donor power of the bridging ligands that depends on the identity of the secondary cation. 10 Other frameworks for understanding the origin of tuning effects have also been developed, including the concepts of cation-induced electric fields, 13,14 cation-driven structural deformations, 15 dependent behaviors. 2 Regardless of the interpretation around their origins, the use of cations to modulate redox chemistry is currently attracting significant attention as this strategy appears broadly useful.…”

Synthesis, Isolation, and Study of Heterobimetallic Uranyl Crown Ether Complexes

“…In order to generate isolable uranyl-crown complexes, we developed a Pt­(II)-templated macrocycle (denoted 1 ) with a Reinhoudt-type ligand scaffold that presents an 18-crown-6-like site for binding of the uranyl ion, UO 2 2+ . In prior work, we have used crown-containing ligands to access lanthanide-binding Ni­(II), Pd­(II), Zn­(II), and V­(IV) , complexes, but here, a new method was developed for installation of a platinum center into the macrocyclic framework with the goal of imparting stability and enabling future spectroscopic studies with 195 Pt NMR (see Experimental Section in Supporting Information for details). We found that 1 could be prepared from the previously reported [H 2 ,Ba] using Ba­(OH) 2 ·8H 2 O as a base and Pt­(DMSO) 2 Cl 2 as a metalating agent (Scheme ).…”

Redox Processes Involving Oxygen: The Surprising Influence of Redox-Inactive Lewis Acids

“…One of the more broadly studied effects of Lewis acids on electron transfer involves the conjugation of redox-active metal complexes to crown-ethers (Figure ). − Extensive studies on mono- ( 1 ) and bimetallic ( 2 ) manganese Schiff-base complexes first appeared in the 1990s that aimed to elucidate the properties of manganese-dependent enzymes (including the OEC). − While salen-type related ligands have been shown to be useful for binding or sensing of a wide variety of redox-inactive metals via coordination to O atom donors (e.g., phenoxide), inclusion of crown-ether motifs enables stronger binding of Lewis acidic ions with stoichiometric control. Exposing complex 1 to Li + , K + , Ca 2+ , or Ba 2+ leads to incorporation of a single redox-inactive metal into the crown-ether moiety, resulting in a positive shift in reduction potential for the Mn III /Mn II redox couple .…”