An indication of spin-transition accompanied by an order-disorder structural transformation in [Ni(phpyNO)2(NCS)2] (phpyNO = tert-butyl 5-phenyl-2-pyridyl nitroxide)

Abstract

Reaction of nickel(II) thiocyanate and tert-butyl 5-phenyl-2-pyridyl nitroxide (phpyNO) afforded a 2p–3d–2p heterospin triad [Ni(phpyNO)₂(NCS)₂]. The compound crystallizes in the orthorhombic Pbcn space group. The whole molecule is crystallographically independent. The torsion angles around Ni–O–N–C_2py are 26.8(4) and 27.3(4)° at 400 K, indicating appreciable orbital overlaps of the radical π* and nickel(II) 3d_x²−y²/3d_z² orbitals. In a low-temperature region, the torsion was enhanced, and the space group changed to monoclinic P2₁/c with a doubled asymmetric unit volume. The χ_mT value was practically null below ca. 140 K and, on heating to 400 K, gradually increased and reached 1.30 cm³ K mol⁻¹. A van't Hoff analysis suggests a spin transition at T_1/2 = 530(20) K. Density functional theory calculation reproduced ground S_total = 0 with singlet-triplet gaps of 910 and 1263 K for the 140 K structure, and the gap was reduced to 297 K at 400 K. Consequently, the present compound can be considered as an incomplete spin-crossover material, as a result of T_1/2 located above the experimental temperature window.