Low-valent Alkaline Earth Metal Complexes: Synthesis, Structure and Reactivity

• Applicant: Professor Dr. Sjoerd Harder
• Subject Area: Inorganic Molecular Chemistry - Synthesis and Characterisation
• Term: since 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 491060547

The serendipitous discovery of the first Zn(I) complex, Cp*ZnZnCp*, has stimulated research activities in low-valent s-block metals chemistry. This led in 2007 to discovery of the first Mg(I) complexes in the form of (BDI)MgMg(BDI), a compound class that received enormous attention and was found to be a versatile reducing agent in chemistry (BDI = -diketiminate ligand). Apart from rare Be(0) or Be(I) species and a Ca(I) complex, there are hitherto no examples for (BDI)AeAe(BDI) complexes of the heavier alkaline earth (Ae) metals Ca, Sr and Ba. Our attempts to isolate a (BDI)CaCa(BDI) complex resulted in 2e-reduction of either the aromatic solvent or N2 and formation of (BDI)Ca(arene)Ca(BDI) or (BDI)Ca(N2)Ca(BDI) complexes. Theoretical analyses in cooperation with Frenking (Marburg) showed that d-orbitals on Ca play an essential role in N2 activation. The key to isolation of the Ca-N2 complex is a superbulky -diketiminate ligand that, due to 3-pentyl substituents, enables use of alkane solvents. An even bulkier BDI ligand was used in an effort to cleave the Mg-Mg bond in (BDI)MgMg(BDI) which would result in a highly reactive, hitherto unknown, (BDI)Mg● radical. This attempt led to isolation of a unique (BDI)MgˉNa+ complex in which the zerovalent Mg center carries a formal charge of -1. This new class of magnesyl sodium complexes features an electron-rich Mg center and displays reactivity that is a complete opposite of that of common Mg complexes with positively charged Mg2+ centers.Project aims:1) Deepening our knowledge on bonding in an extended series of (BDI)Ca(arene)Ca(BDI) and (BDI)Ca(N2)Ca(BDI) complexes using spectroscopic methods (NMR, EPR and Calcium Valence-to-Core X‑ray Emission Spectroscopy studies) and theory (partially in collaboration). Our main interest is the importance of d-orbitals in bonding.2) Extending this chemistry to the heavier Ae metals Sr and Ba by design of bulkier ligands (L) and attempts to isolate LAeAeL, the original target of our studies. 3) Studying arene or N2 functionalization and the extreme redox-reactivity of LAe(arene)AeL, LAe(N2)AeL and LAeAeL complexes.4) Isolation of LMg● radicals using superbulky ligands. 5) Investigating the reactivity of the new compound class (BDI)MgˉNa+ with nucleophilic Mg centers that is currently a no-man’s land. 6) Developing a general route to LAeˉM+ complexes for the heavier Ae metals Ca, Sr and Ba. 7) Use the (BDI)MgˉNa+ reagent to create Mg-metal bonds following a salt metathesis route with MXn (X = halide, M = s-, p- or d-block metal). 8) Theoretical analysis of new complexes with DFT, NBO and AIM methods.We aim to run the project with two PhD students and forecast an output of 8-10 full papers which will likely be published in high-impact journals.

• DFG Programme: Research Grants