Redox Reactions with Calcium‐Metal Nanoparticles

Calcium is generally a highly reactive alkaline-earth metal, but as bulk metal, it exhibits only low reactivity at ambient conditions due to small surface area, low solubility, and/or passivation. The reactivity can be significantly enhanced when using small-sized calcium nanoparticles. In this regard, we describe the first synthesis of Ca(0) nanoparticles, 5.4 +/- 1.2 nm in size, by TMEDA-supported reduction of CaI2 with lithium naphthalenide in toluene (TMEDA: N,N,N ',N '-tetramethylethylenediamine). We also show Ca(0) nanoparticles to be substantially different from so-called "Rieke calcium". The high reactivity can be used for redox reactions, for instance, with [Cp2MoCl2] and the sterically demanding beta-diketiminate ligand HDippNacNac (Dipp: 2,6-iPr2C6H3) or with [(Ph3P)AuCl] as a derivative of a ligand-stabilized noble metal. The Ca(0)-nanoparticle-driven reactions result in the novel compounds [{(DippNacNac)(thf)Ca}2(naph)] (1) with a rare naphthalenide dianion, [{(DippNacNac)(thf)CaMo(Cp)H}2(fulvalene)] (2) with unusual MoH -> Ca dative bonding, and [Au9(PPh3)8](naph)(tmeda)0.5 (3) with a non-charged body-centered cubic Au9 cluster core of zerovalent gold. ... mehrCa(0) nanoparticles and the compounds 1-3 are characterized by electron microscopy, X-ray diffraction (single crystals, powders), spectroscopy (IR, UV-Vis, NMR, EPR), and computation. Exemplarily, the formation of 1-3 points to the potential of nanosized alkaline-earth metals for chemical syntheses and the different reactivity of nanosized and bulk metals.