Redox-Active Heterotrimetallic Ate Complexes

• Applicant: Professor Dr. Sjoerd Harder
• Subject Area: Inorganic Molecular Chemistry - Synthesis and Characterisation
• Term: since 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 537002245

“Ate” complexes (e.g. LiAlH4) are generally defined as heterobimetallic complexes of an electropositive metal cation (Li+) and complex anion of a less electropositive metal (AlH4ˉ). However, the transition of “ate” complex to mixed aggregate (e.g. nBuLi/KOtBu) is smooth. Whatever the definition, it is clear that a combination of metals leads to cooperativity often resulting in reactivities that can never be reached by the components alone. Our herein proposed contribution to “ate” chemistry entails two novelties: (1) We introduce metal-centered anions to “ate” chemistry. We recently reported a highly unusual sodium magnesyl complex consisting of Na+ cations and (BDI*)Mgˉ anions (BDI* = bulky beta-diketiminate ligand). (2) We synthesize mixed aggregates with unusual, partially unknown, Mg-metal bonds and explore their reactivity. Earlier attempts to create a first Mg-Ca bond by salt metathesis between (BDI*)MgˉNa+ and (BDI)CaI failed. However, mixing the dimer [(BDI*)MgˉNa+]2 with dimeric [Ca(NR2)2]2 gave exclusively a mixed aggregate (BDI*)MgˉNa+ / Ca(NR2)2 in which (BDI*)Mgˉ and R2Nˉ anions are bridged by Na+ and Ca2+ cations giving for a unique [Na-Mg-Ca]3+ fragment with formal oxidation states NaI-Mg0-CaII. The charge distribution shows considerable electron-transfer from Mg0 to CaII, suggesting some MgI-CaI character. Also first Mg-Sr and Mg-Ba bonding could be achieved. The project consists of three parts: I. Creating new unconventional Mg-metal bonds by ion exchange reactions II. Explore the bond characteristics by DFT methods III. Investigation of novel heterotrimetallic complexes in bond activation I. We investigate possibilities to create: Mg-alkali metal bonds (Li-Cs), heterotrimetallic Na-Mg-Al(or Ga) bonds, magnesyl borate complexes like (BDI*)Mg-BR3ˉ Na+, mixed aggregates with Mg-transition metal bonding (Mg-TM, TM = Mn, Fe, Co, Ni) and first Mg-lanthanide complexes. II. We analyze highly unusual Na-Mg-metal bonds with DFT methods, Natural-Bond-Orbitals analysis and Atoms-In-Molecules. This is an essential part of the project that allows us to understand their nature. III. We focus on reactivity of novel heterotrimetallic complexes. Considering the often spectacular reactivity of heterobimetallic complexes, it is clear that the complexes synthesized in this project will display high and unusual reactivity emerging from cooperativity between three different metals. As reactivity is hard to predict, standard reactivity tests will be performed: thermal stability, stability in aromatic solvents, stability under N2, reactivity with various substrates like H2, N2O, O2, H2C=CH2, CO2, CO, Ph3P=CH2 and Me3SiN3. This experience will broaden our understanding of metal-metal bonding and generate new possibilities in low-valent main group metal chemistry.

• DFG Programme: Research Grants