Abstract
Complex formation of nickel(II) tetrafluoroborate and tert-butyl 5-phenyl-2-pyridyl nitroxide (phpyNO) in the presence of sodium cyanate gave a discrete molecule [Ni(phpyNO)2(X)2] (X = NCO). The Ni-O-N-Csp2 torsion angles were reduced on heating; 33.5(5)° and 36.2(4)° at 100 K vs. 25.7(10)° and 32.3(11)° at 400 K. The magnetic behavior was almost diamagnetic below ca. 100 K, and the χmT value reached 1.04 cm3 K mol−1 at 400 K. An analysis using the van’t Hoff equation indicates a possible spin transition at T1/2 >> 400 K. Density functional theory calculation shows that the singlet-quintet energy gap decreases as the structural change from 100 to 400 K. The geometry optimization results suggest that the diamagnetic state has the Ni-O-N-Csp2 torsion angles of 32.7° while the Stotal = 2 state has those of 11.9°. The latter could not be experimentally observed even at 400 K. After overviewing the results on the known X = Br, Cl, and NCS derivatives, the magnetic behavior is described in a common phase diagram. The Br and Cl compounds undergo the energy level crossing of the high-/low-spin states, but the NCS and NCO compounds do not in a conventional experimental temperature range. The spin transition mechanism in this series involves the exchange coupling switch between ferro- and antiferromagnetic interactions, corresponding to the high- and low-spin phases, respectively.
Highlights
Coordination compounds provide us a great opportunity for the development of spin-transition materials [1,2,3,4,5,6,7,8]
We have proposed the M-O-N-Csp2 torsion angle |φ| as a bond gives rise to an overlap between magnetic orbitals and antiferromagneticorbitals interaction
The novelty of spin transition mechanism resides in the ferro- and antiferromangetic exchange coupling switch, the present spin transition mechanism resides in the ferro- and antiferromangetic exchange coupling corresponding to the HS and LS phases, respectively
Summary
Coordination compounds provide us a great opportunity for the development of spin-transition materials [1,2,3,4,5,6,7,8]. Iron(II) (3d6 )-based spin-crossover (SCO) materials are the most intensively studied [9,10,11,12,13,14], because magnetic and chromic changes would be drastic between the low-spin (LS) S = 0 diamagnetic and high-spin (HS) S = 2 paramagnetic states. According to the “metal–radical approach” proposed by Gatteschi et al [15,16], direct metal-radical coordination bonds afford considerable magnetic exchange interaction. A number of ligands based on 2-azaaryl tert-butyl nitroxides (aminoxyls) have been explored for 2p-3d [17,18,19,20,21,22,23] as well as 2p-4f heterospin systems [24,25,26,27,28]. The exchange coupling in radical-coordinated copper(II) (3d9 ) and nickel(II) (3d8 ) complexes is the best understood in the
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