Crystal structures and dielectric properties of two salts of nickel-bis-1,2-dithiolene with noncentrosymmetric organic cations

Two nickel-bis-1,2-dithiolene salts, [1,4-dimethyl-DABCO][Ni(mnt)₂] (1) and (TMSF)₂[Ni(mnt)₂] (2) (mnt²⁻ = maleonitriledithiolate, 1,4-dimethyl-DABCO²⁺ = 1,4-dimethyl-1,4-diazoniabicyclo[2.2.2]octane and TMSF⁺ = trimethylsulfonium), were synthesized and characterized by elemental analysis, IR spectra, thermal analysis and single crystal structures. Both 1 and 2 crystallize in monoclinic space group C2/c, with a formula and cell parameters of C₁₆H₁₈N₆NiS₄, a = 16.9688(8) Å b = 9.3003(5) Å 13.4090(6) Å and β = 103.244(2)° for 1 at 293 K versus C₁₄H₁₈N₄NiS₆, a = 17.108(3) Å b = 10.2932(16) Å 14.071(2) Å and β = 118.845(4)° for 2 at 293 K. Moreover, two salts show similar packing structure, with the arrangements of alternating layer of anions and cations, and the layers are parallel to the crystallographic ab-plane. Both 1 and 2 display analogous dielectric behavior, which dielectric permittivity is almost a constant with ε′ ≈ 12 at the temperature below 270 K for 1 versus 215 K for 2, and two steps of dielectric relaxations appear in the higher temperature regime, which originate from the molecular dipole orientation and the ion migration.