High-pressure synthesis and characterization of Li2Ca3[N2]3--an uncommon metallic diazenide with [N2]2- ions.

ABSTRACT

Dinitrogen (N2) ligation is a common and well-characterized structural motif in bioinorganic synthesis. In solid-state chemistry, on the other hand, homonuclear dinitrogen entities as structural building units proved existence only very recently. High-pressure/high-temperature (HP/HT) syntheses have afforded a number of binary diazenides and pernitrides with [N2](2­) and [N2](4­) ions, respectively. Here, we report on the HP/HT synthesis of the first ternary diazenide. Li2Ca3[N2]3 (space group Pmma, no. 51, a = 4.7747(1), b = 13.9792(4), c = 8.0718(4) Å, Z = 4, wRp = 0.08109) was synthesized by controlled thermal decomposition of a stoichiometric mixture of lithium azide and calcium azide in a multianvil device under a pressure of 9 GPa at 1023 K. Powder X-ray diffraction analysis reveals strongly elongated N­N bond lengths of dNN = 1.34(2)­1.35(3) Å exceeding those of previously known, binary diazenides. In fact, the refined N­N distances in Li2Ca3[N2]3 would rather suggest the presence of [N2](3·­) radical ions. Also, characteristic features of the N­N stretching vibration occur at lower wavenumbers (1260­1020 cm(­1)) than in the binary phases, and these assignments are supported by first-principles phonon calculations. Ultimately, the true character of the N2 entity in Li2Ca3[N2]3 is probed by a variety of complementary techniques, including electron diffraction, electron spin resonance spectroscopy (ESR), magnetic and electric conductivity measurements, as well as density-functional theory calculations (DFT). Unequivocally, the title compound is shown to be metallic containing diazenide [N2](2­) units according to the formula (Li(+))2(Ca(2+))3([N2](2­))3·(e(­))2.