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)

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(x(2)−y(2))/3d(z(2)) orbitals. In a low-temperature region, the torsion was enhanced, and the space group changed to monoclinic P2(1)/c with a doubled asymmetric unit volume. The χ(m)T value was practically null below ca. 140 K and, on heating to 400 K, gradually increased and reached 1.30 cm(3) 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.