Probing the Relationships Between Structure, Reactivity, and Redox Distribution in Trinuclear Transition Metal Complexes

Abstract

Polydentate templating ligands enable the synthesis of transition metal clusters that can be used to systematically probe open questions in the chemistry of cooperative small molecule activation. Changes in ligand design, metal identity, or oxidation state, for instance, can have significant impacts on metal-metal interactions, reactivity with small molecule substrates, and even delocalization of redox within the metal cluster core. Our lab has previously demonstrated that the hexadentate ligand tbsLH6 (1,3,5-(tBuMe2SiNH-o-C6H4NH)3C6H9) allows for the synthesis of open-shell, trinuclear first-row transition metal clusters capable of multi-electron reactivity. We sought to expand our understanding of the interplay between cluster structure, redox distribution, and multi-electron reactivity by further exploration of the chemistry and spectroscopy of trinuclear complexes traversing the periodic table from (tbsL)Cr3(thf) to (tbsL)Ni3. We found that one-electron reduction of clusters of the type (tbsL)M3(solv) (M: Mn, THF; Fe, THF, py; Co, py; Ni) resulted in dramatic reorganization to form a pseudo C3-symmetric series of anionic clusters: [(tbsL)M3]– (M: Mn, Fe, Co, Ni). The members of the anionic series were largely found to adopt well-isolated, high-spin grounds states, while the homovalent starting materials displayed a range of low, intermediate, and high-spin states depending on the metal identity. The convergence of the electronic structure of the clusters upon the introduction of mixed-valency was found to arise from extensive delocalization. Moving to an earlier transition metal complex, the trichromium cluster (tbsL)Cr3(thf) was found to exhibits steric- and solvation-controlled reactivity with organic azides to form three distinct products: a symmetrized bridging imido complex (tbsL)Cr3(µ3–NBn), a terminally-bound imido complex (tbsL)Cr3(µ1–NMes), and a nitride complex (tbsL)Cr3(µ3–N). The nitride complex was formed via terminal N-atom excision from the mesityl azide – the same substrate which gives rise to the terminal imido complex when the reaction with (tbsL)Cr3(thf) in benzene solution. The reactivity of this complex demonstrates the ability of the templating ligand to produce a cluster that can access distinct single-site and cooperative reactivity controlled by either substrate steric demands or reaction media. In order to probe the redox distribution of symmetric and asymmetric polynuclear complexes such as those described above, we turned to an advanced X-ray technique known as multiwavelength anomalous diffraction (MAD), which combines the structural resolution of single-crystal X-ray diffraction with the oxidation state resolution of X-ray absorption spectroscopy (XAS). We first benchmarked a method for the application of this technique to small molecule transition metal clusters by the systematic comparison of a series of trinuclear iron complexes, for which the resulting MAD data could be compared to 57Fe Mössbauer spectra. We then applied MAD to the study of local chromium oxidation states in (tbsL)Cr3(µ1–NDipp), (tbsL)Cr3(µ3–NBn), and (tbsL)Cr3(µ3–NPh)(µ1–NPh). Evidence of asymmetric redox load distribution was found for both terminal and bridging imido architectures.