Chromium Metal Nanoparticles, Their Reactivity and Reactions

Zerovalent chromium nanoparticles (2.2±0.2 nm in size) are prepared in the liquid phase (THF) by reduction of CrCl${}_2$ with lithium naphthalenide ([LiNaph]). The deep black Cr(0) nanoparticle suspensions in THF are colloidally and chemically highly stable. On the other hand, the Cr(0) nanoparticles are highly reactive, e. g., when in contact to O${}_2$, H${}_2$O, or other oxidizing agents. To probe the reactivity of the Cr(0) nanoparticles in the liquid phase near room temperature (50–80 °C), they are reacted with different coordinatively demanding, N−H, S−H or O−H acidic reactants. This includes the amines 2,2′-dipyridylamine (HDPA), 2-(1H-imidazol-2-yl)pyridine (HImPy) and carbazole (HCbz), the thiol 2-mercaptopyridine (HMPy), and benzoic acid (HBz) as a carboxylate. As a result, the five novel compounds [Cr(DPA)${}_3$], [Cr(ImPy)${}_3$] ⋅ HImPy, [Cr(MPy)${}_3$] ⋅ 0.5 Tol (Tol: toluene), [Na${}_2$Cr(Cbz)${}_4$(THF)${}_3$], and [Cr${}_2$(Bz)${}_4$(THF)${}_2$] are obtained. [Cr(DPA)${}_3$] and [Cr(ImPy)${}_3$] ⋅ HImPy show Cr(III) coordinated by nitrogen only; [Cr(MPy)${}_3$] ⋅ 0.5 Tol shows a coordination of Cr(III) by both N and S atoms. [Na${}_2$Cr(Cbz)${}_4$(THF)${}_3$] and [Cr${}_2$(Bz)${}_4$(THF)${}_2$] contain Cr(II) and exhibit an infinite chain-like structure and pairs of chromium atoms with fourfold binding.