High-Spin and Incomplete Spin-Crossover Polymorphs in Doubly Chelated [Ni(L)₂Br₂] (L = tert-Butyl 5-Phenyl-2-pyridyl Nitroxide)

Abstract

Complexation of nickel­(II) bromide with tert-butyl 5-phenyl-2-pyridyl nitroxide (phpyNO) gave two morphs of doubly chelated [Ni­(phpyNO)2Br2] as a 2p–3d–2p heterospin triad. The α phase crystallizes in the orthorhombic space group Pbcn. An asymmetric unit involves a half-molecule. The torsion angle around Ni–O–N–C2py is as small as 6.5(3)° at 100 K and 7.0(6)° at 400 K, guaranteeing an orthogonal arrangement between the magnetic radical π* and metal 3d x 2–y 2 and 3d z 2 orbitals. Magnetic study revealed the high-spin ground state with the exchange coupling constant 2J/k B = +288(5) K, on the basis of a symmetrical spin Hamiltonian. The β phase crystallizes in the monoclinic space group P21/n. The whole molecule is an independent unit. The Ni–O–N–C2py torsion angles are 24.2(6) and 37.2(5)° at 100 K and 10.4(7) and 25.9(6)° at 400 K. A magnetic study revealed a very gradual and nonhysteretic spin transition. An analysis based on the van’t Hoff equation gave a successful fit with the spin-crossover temperature of 134(1) K, although the susceptibility did not reach the theoretical high-spin value at 400 K. Density functional theory calculation on the β phase showed ground S total = 0 in the low-temperature structure while S total = 2 in the high-temperature structure, supporting the synchronized exchange coupling switch on both sides. Consequently, the β phase can be recognized as an “incomplete spin crossover” material, as a result of conflicting thermal depopulation effects in a high-temperature region.

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)

“…In addition, vibrational contribution DS vib is present as well in total DS. The previous works on the [Ni(phpyNO) 2 X 2 ] reported reasonable DS ¼ 26.3(1) and 24.0(7) J K À1 mol À1 for X ¼ Cl 11 and Br,12 respectively. It should be noted that comparably small DS ¼ 12(2) J K À1 mol À1 was obtained here.…”

“…Now let us overview the results on the derivatives with X ¼ Cl, 11 Br, 12 and NCS (this study). The T 1/2 order is as follows: T 1/2 (Br) < T 1/2 (Cl) < T 1/2 (NCS) ( Table 3).…”

“…Complex [Ni(phpyNO) 2 (NCS) 2 ] was facilely prepared by mixing solutions of the paramagnetic ligand and commercially available Ni(NCS) 2 in an established manner. 11,12 The complex was characterized by means of IR spectroscopic, elemental, and X-ray crystallographic analyses, clarifying the absence of solvent molecules.…”

“…A number of ligands having a tert-butyl nitroxide (aminoxyl) group have been developed for building blocks in 2p-3d heterospin 5,9 as well as 2p-4f heterospin 10 compounds. There have been only two examples reported at present on the spin transition between S total ¼ 0 % 2, 11,12 where two S ¼ 1/2 paramagnetic ligands and an S ¼ 1 nickel(II) centre are involved. What "spin crossover" covers may be extended to multi-centred systems showing an essentially similar spinmultiplicity switch at their ground state.…”

“…What "spin crossover" covers may be extended to multi-centred systems showing an essentially similar spinmultiplicity switch at their ground state. [11][12][13] The spin state is regulated by ferro-/antiferromagnetic balance continuum in multi-centred systems, 11 in place of the Hund/auau principles, or potential/kinetic exchange interactions, in one-centred systems.…”