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 β-diketiminate ligand H^DippNacNac (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. Ca(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.