Sodium Aminotroponiminates: Ligand‐Induced Disproportionation, Mixed‐Metal Compounds, and Exceptional Activity in Polymerization Catalysis

Abstract

Sodium complexes of aminotroponiminate (ATI) ligands have been reacted with a range of neutral donor ligands. Upon addition of crown ethers, they undergo an unusual ligandinduced disproportionation reaction with formation of [Na(A-TI) 2 ] À sodiate complex anions. The same structural motif has also been found in the first well-defined mixed-metal ATI complexes, which have been accessed starting from monometallic sodium and potassium species. The mixed-metal compounds confirm the possibility of ATIs to act as ditopic ligands. Using the polymerization of e-caprolactone as a model system, structure-reactivity-relationships in sodium and mixed-metal ATI compounds have been studied by comparing the reactivity of [Na(ATI)] and [Na(ATI) 2 ] À structural motifs. These studies revealed trends in the catalyst activity depending on the nuclearity of the complex and the substitution pattern at the ATI ligand. Exceptionally high activities were obtained for dinuclear sodium sodiates of type [Na(crown) 2 ][Na(ATI) 2 ], making them the most active alkali metal initiators for this reaction. The organometallic and polymeric compounds presented in this work have been characterized by techniques including (VT-)NMR spectroscopy, single-crystal X-ray diffraction, gel permeation chromatography, mass spectrometry, and DFT calculations.

New Perspectives for Aminotroponiminates: Coordination Chemistry, Redox Behavior, Cooperativity, and Catalysis

“…The substitution pattern at the nitrogen atoms of the ATI ligands was shown to have a significant influence on the HOMO/LUMO energies (Figure , bottom). This makes ATI ligands with electron donating substituents at nitrogen more suitable for ligand‐to‐metal (L→M) bonding of type E , which was confirmed experimentally . DFT calculations and NMR spectroscopic studies suggest that the coordination mode E is not only relevant in the solid state, but also in solution {e.g.…”

“…The coordination mode E was first analyzed in detail for the sodium complex [Na(ATI Ph/ i Pr )(thf)], for which it leads to a one‐dimensional coordination polymer in the solid state (Figure ) , . Related ATI complexes of Na and K have been shown to adopt the same coordination mode in the solid state …”

“…These compounds form coordination polymers in the solid state (Figure ). According to the hard/soft acid/base principle, potassium ions interact with the softer ligand backbone and sodium atoms are found in the harder N , N ‐binding pocket …”

“…The ionic sodium ATI complexes 33‐R and 34‐R , each of which contain the complex anion [Na(ATI) 2 ] – (Scheme a), have been reported to be highly active as initiators for the polymerization of ε‐caprolactone (full conversion after 1 min at –30 °C with 0.04–0.1 mol‐% catalyst loading) . They exhibit synergistic effects, i.e.…”

Alkali‐Metal Aminotroponiminates: Selectivities and Equilibria in Reversible Radical Coupling of Delocalized π‐Electron Systems

“…In these calculations, the coordination spheres of the metal centers were saturated by THF ligands. Although bonding between the π‐electron cloud of the C 7 ligand backbone and the metal center plays an important role in diamagnetic alkali‐metal ATIs, DFT calculations suggest that this is not the case for the radical intermediates discussed in this work (for details see the Supporting Information). In agreement with the results from EPR spectroscopy, the spin density in the calculated radical intermediates 1‐Li‐int and 3‐Li‐int is delocalized through the π‐electron system of the ATI ligands (Figure , Table ).…”