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)

摘要

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 P 2 _(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.