A Hidden Coordination-Bond Torsional Deformation as a Sign of Possible Spin Transition in Nickel(II)-Bis(nitroxide) Compounds

• Published in: Molecules (Basel, Switzerland) Switzerland), 2020-08, Vol.25 (17), p.3790
• Main Authors: Kyoden, Yukiya, Ishida, Takayuki
• Format: Article
• Language: English
• Subjects: aminoxyl; Antiferromagnetism; Complex formation; Cyanates; Density functional theory; Diamagnetism; Energy levels; exchange interaction; Level crossings; Ligands; Magnetic properties; magneto-structural relation; Molecular structure; Nickel; Nickel - chemistry; Nitrogen Oxides - chemistry; Optimization; Phase diagrams; Spin transition; Temperature
• ISSN: 1420-3049; 1420-3049
• DOI: 10.3390/molecules25173790

Complex formation of nickel(II) tetrafluoroborate and -butyl 5-phenyl-2-pyridyl nitroxide (phpyNO) in the presence of sodium cyanate gave a discrete molecule [Ni(phpyNO) (X) ] (X = NCO). The Ni-O-N-C 2 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 value reached 1.04 cm K mol at 400 K. An analysis using the van't Hoff equation indicates a possible spin transition at >> 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-C 2 torsion angles of 32.7° while the = 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.