Dinitrogen complexation and reduction at low-valent calcium

Calcium catches dinitrogen

Although lithium reduces dinitrogen, the other alkali and alkaline Earth metals have proven largely inert to the gas under ambient conditions. Rösch et al. report that with just the right β-diketiminate ligand and an assist from potassium as terminal reductant, calcium can mediate dinitrogen reduction. Crystallography and spectroscopic characterization revealed a product in which doubly reduced dinitrogen adopted a side-on bridging motif between two calcium centers. A subsequent reaction with coordinated tetrahydrofuran appeared to release diazene.

Science, this issue p. 1125

Abstract

Here we report that attempted preparation of low-valent Ca^I complexes in the form of LCa-CaL (where L is a bulky β-diketiminate ligand) under dinitrogen (N₂) atmosphere led to isolation of LCa(N₂)CaL, which was characterized crystallographically. The N₂²ˉ anion in this complex reacted in most cases as a very potent two-electron donor. Therefore, LCa(N₂)CaL acts as a synthon for the low-valent Ca^I complex LCa-CaL, which was the target of our studies. The N₂²ˉ anion could also be protonated to diazene (N₂H₂) that disproportionated to hydrazine and N₂. The role of Ca d orbitals for N₂ activation is discussed.