High-Pressure Synthesis and Characterization of Li2Ca3[N2]3-An Uncommon Metallic Diazenide with [N2]2- Ions

Dinitrogen (N-2) 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 [N-2](2-) and [N-2](4-) ions, respectively. Here, we report on the HP/HT synthesis of the first ternary diazenide. Li2Ca3[N-2](3) (space group Pmma, no. 51, a = 4.7747(1), b = 13.9792(4), c = 8.0718(4) angstrom, Z = 4, wR(p) = 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 N-N bond lengths of d(NN) = 1.34(2)-1.35(3) angstrom exceeding those of previously known, binary diazenides. In fact, the refined N-N distances in Li2Ca3[N-2](3) would rather suggest the presence of [N-2](3-) radical ions. Also, characteristic features of the N-N stretching vibration occur at lower wavenumbers (1260-1020 cm(-1)) than in the binary phases, and these assignments are supported by first-principles phonon calculations. Ultimately, the true character of the N-2 entity in Li2Ca3[N-2](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 [N-2](2-) units according to the formula (Li+)(2)(Ca2+)(3)([N-2](2-))(3)center dot(e(-))(2).