{"status":"ok","message-type":"work","message-version":"1.0.0","message":{"indexed":{"date-parts":[[2025,10,12]],"date-time":"2025-10-12T03:10:28Z","timestamp":1760238628283,"version":"build-2065373602"},"reference-count":59,"publisher":"MDPI AG","issue":"17","license":[{"start":{"date-parts":[[2020,8,20]],"date-time":"2020-08-20T00:00:00Z","timestamp":1597881600000},"content-version":"vor","delay-in-days":0,"URL":"https:\/\/creativecommons.org\/licenses\/by\/4.0\/"}],"content-domain":{"domain":[],"crossmark-restriction":false},"short-container-title":["Molecules"],"abstract":"<jats:p>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)\u00b0 and 36.2(4)\u00b0 at 100 K vs. 25.7(10)\u00b0 and 32.3(11)\u00b0 at 400 K. The magnetic behavior was almost diamagnetic below ca. 100 K, and the \u03c7mT value reached 1.04 cm3 K mol\u22121 at 400 K. An analysis using the van\u2019t Hoff equation indicates a possible spin transition at T1\/2 &gt;&gt; 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\u00b0 while the Stotal = 2 state has those of 11.9\u00b0. 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.<\/jats:p>","DOI":"10.3390\/molecules25173790","type":"journal-article","created":{"date-parts":[[2020,8,20]],"date-time":"2020-08-20T21:13:42Z","timestamp":1597958022000},"page":"3790","update-policy":"https:\/\/doi.org\/10.3390\/mdpi_crossmark_policy","source":"Crossref","is-referenced-by-count":5,"title":["A Hidden Coordination-Bond Torsional Deformation as a Sign of Possible Spin Transition in Nickel(II)-Bis(nitroxide) Compounds"],"prefix":"10.3390","volume":"25","author":[{"given":"Yukiya","family":"Kyoden","sequence":"first","affiliation":[{"name":"Department of Engineering Science, The University of Electro-Communications, Chofu, Tokyo 182-8585, Japan"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"ORCID":"https:\/\/orcid.org\/0000-0001-9088-2526","authenticated-orcid":false,"given":"Takayuki","family":"Ishida","sequence":"additional","affiliation":[{"name":"Department of Engineering Science, The University of Electro-Communications, Chofu, Tokyo 182-8585, Japan"}],"role":[{"role":"author","vocabulary":"crossref"}]}],"member":"1968","published-online":{"date-parts":[[2020,8,20]]},"reference":[{"key":"ref_1","doi-asserted-by":"crossref","unstructured":"G\u00fctlich, P., and Goodwin, H.A. (2004). Spin Crossover in Transition Metal Compounds I, II, and III, Springer.","DOI":"10.1007\/b96439"},{"key":"ref_2","doi-asserted-by":"crossref","unstructured":"Halcrow, M.A. (2013). Spin-Crossover Materials: Properties and Applications, John Wiley & Sons, Inc.","DOI":"10.1002\/9781118519301"},{"key":"ref_3","doi-asserted-by":"crossref","first-page":"1703862","DOI":"10.1002\/adma.201703862","article-title":"Spin Crossover Nanomaterials: From Fundamental Concepts to Devices","volume":"30","author":"Rat","year":"2018","journal-title":"Adv. Mater."},{"key":"ref_4","doi-asserted-by":"crossref","first-page":"87","DOI":"10.1038\/s41578-019-0146-8","article-title":"Molecular magnetism: From chemical design to spin control in molecules, materials and devices","volume":"5","author":"Coronado","year":"2020","journal-title":"Nat. Rev. Mater."},{"key":"ref_5","doi-asserted-by":"crossref","first-page":"176","DOI":"10.1016\/j.ccr.2017.03.024","article-title":"Emerging trends in spin crossover (SCO) based functional materials and devices","volume":"346","author":"Kumar","year":"2017","journal-title":"Coord. Chem. Rev."},{"key":"ref_6","doi-asserted-by":"crossref","first-page":"2152","DOI":"10.1002\/anie.200602205","article-title":"Control of magnetic properties through external stimuli","volume":"46","author":"Sato","year":"2007","journal-title":"Angew. Chem. Int. Ed."},{"key":"ref_7","doi-asserted-by":"crossref","first-page":"303","DOI":"10.1039\/C1CS15165G","article-title":"Playing with organic radicals as building blocks for functional molecular materials","volume":"41","author":"Ratera","year":"2012","journal-title":"Chem. Soc. Rev."},{"key":"ref_8","doi-asserted-by":"crossref","first-page":"3","DOI":"10.1016\/j.poly.2013.03.010","article-title":"What role has organic chemistry played in the development of molecule-based magnets?","volume":"66","author":"Iwamura","year":"2013","journal-title":"Polyhedron"},{"key":"ref_9","doi-asserted-by":"crossref","unstructured":"Arom\u00ed, G., and Real, J.A. (2016). Special Issue \u201cSpin Crossover (SCO) Research\u201d. Magnetochemistry, 2.","DOI":"10.3390\/magnetochemistry2030028"},{"key":"ref_10","doi-asserted-by":"crossref","unstructured":"Takahashi, K. (2018). Special Issue \u201cSpin-Crossover Complexes\u201d. Inorganics, 6.","DOI":"10.3390\/inorganics6010032"},{"key":"ref_11","doi-asserted-by":"crossref","unstructured":"Kitazawa, T. (2019). Special Issue \u201cSynthesis and Applications of New Spin Crossover Compounds\u201d. Crystals, 9.","DOI":"10.3390\/cryst9080382"},{"key":"ref_12","doi-asserted-by":"crossref","first-page":"7767","DOI":"10.1039\/C5TC90128F","article-title":"Themed Issue \u201cSpin-State Switches in Molecular Materials Chemistry\u201d","volume":"3","author":"Hayami","year":"2015","journal-title":"J. Mater. Chem. C"},{"key":"ref_13","doi-asserted-by":"crossref","first-page":"6737","DOI":"10.1021\/acsomega.8b01095","article-title":"Spin-crossover temperature predictable from DFT calculation for iron(II) complexes with 4-substituted pybox and related heteroaromatic ligands","volume":"3","author":"Kimura","year":"2018","journal-title":"ACS Omega"},{"key":"ref_14","doi-asserted-by":"crossref","first-page":"10163","DOI":"10.1021\/acs.inorgchem.0c01296","article-title":"Odd\u2013Even Effect on the Spin-Crossover Temperature in Iron (II) Complex Series Involving an Alkylated or Acyloxylated Tripodal Ligand","volume":"59","author":"Kashiro","year":"2020","journal-title":"Inorg. Chem."},{"key":"ref_15","doi-asserted-by":"crossref","first-page":"392","DOI":"10.1021\/ar00167a004","article-title":"Toward molecular magnets: The metal-radical approach","volume":"22","author":"Caneschi","year":"1989","journal-title":"Acc. Chem. Res."},{"key":"ref_16","doi-asserted-by":"crossref","first-page":"134","DOI":"10.1016\/j.ccr.2018.02.002","article-title":"Magnetism in one-dimensional metal\u2013nitronyl nitroxide radical system","volume":"378","author":"Meng","year":"2019","journal-title":"Coord. Chem. Rev."},{"key":"ref_17","doi-asserted-by":"crossref","first-page":"14008","DOI":"10.1021\/ja065623c","article-title":"Strong Ferromagnetic Exchange Couplings in Copper(II) and Nickel(II) Complexes with a Paramagnetic Tridentate Chelate Ligand, 2,2\u2032-Bipyridin-6-yl tert-Butyl Nitroxide","volume":"128","author":"Osanai","year":"2006","journal-title":"J. Am. Chem. Soc."},{"key":"ref_18","doi-asserted-by":"crossref","first-page":"2733","DOI":"10.1021\/cm070698g","article-title":"tert-Butyl 2-Pyridyl Nitroxide Available as a Paramagnetic Chelate Ligand for Strongly Exchange-Coupled Metal\u2212Radical Compounds","volume":"19","author":"Okazawa","year":"2007","journal-title":"Chem. Mater."},{"key":"ref_19","doi-asserted-by":"crossref","first-page":"8859","DOI":"10.1021\/ic800937n","article-title":"Magneto-structure relationship in copper(II) and nickel(II) complexes chelated with stable tert-butyl 5-phenyl-2-pyridyl nitroxide and related radicals","volume":"47","author":"Okazawa","year":"2008","journal-title":"Inorg. Chem."},{"key":"ref_20","doi-asserted-by":"crossref","first-page":"1917","DOI":"10.1016\/j.poly.2008.10.049","article-title":"Strong intramolecular ferromagnetic couplings in nickel(II) and copper(II) complexes chelated with tert-butyl 5-methoxy-2-pyridyl nitroxide","volume":"28","author":"Okazawa","year":"2009","journal-title":"Polyhedron"},{"key":"ref_21","doi-asserted-by":"crossref","first-page":"1306","DOI":"10.1039\/C5DT04218F","article-title":"A designed room-temperature triplet ligand from pyridine-2,6-diyl bis(tert-butyl nitroxide)","volume":"45","author":"Kawakami","year":"2016","journal-title":"Dalton Trans."},{"key":"ref_22","doi-asserted-by":"crossref","first-page":"3120","DOI":"10.1016\/j.tetlet.2013.04.008","article-title":"Ground triplet pyrimidine-4,6-diyl bis(tert-butyl nitroxide) as a paramagnetic building block for metal-organic frameworks","volume":"54","author":"Homma","year":"2013","journal-title":"Tetrahedron Lett."},{"key":"ref_23","doi-asserted-by":"crossref","first-page":"17874","DOI":"10.1039\/C8NJ04180F","article-title":"A triplet biradical with double bidentate sites based on tert-butyl pyridyl nitroxide as a candidate for strong ferromagnetic couplers","volume":"42","author":"Okazawa","year":"2018","journal-title":"New J. Chem."},{"key":"ref_24","doi-asserted-by":"crossref","first-page":"183","DOI":"10.1016\/j.poly.2013.04.004","article-title":"Magnetic study on radical-gadolinium(III) complexes. Relationship between the exchange coupling and coordination structure","volume":"66","author":"Ishida","year":"2013","journal-title":"Polyhedron"},{"key":"ref_25","doi-asserted-by":"crossref","first-page":"10794","DOI":"10.1021\/ic501496a","article-title":"Strongest exchange coupling in gadolinium(III) and nitroxide coordination compounds","volume":"53","author":"Kanetomo","year":"2014","journal-title":"Inorg. Chem."},{"key":"ref_26","doi-asserted-by":"crossref","first-page":"8140","DOI":"10.1021\/acs.inorgchem.6b01072","article-title":"Strongest Ferromagnetic Coupling in Designed Gadolinium(III)\u2013Nitroxide Coordination Compounds","volume":"55","author":"Kanetomo","year":"2016","journal-title":"Inorg. Chem."},{"key":"ref_27","doi-asserted-by":"crossref","first-page":"3310","DOI":"10.1021\/acs.inorgchem.6b02685","article-title":"Giant Exchange Coupling Evidenced with a Magnetization Jump at 52 T for a Gadolinium-Nitroxide Chelate","volume":"56","author":"Kanetomo","year":"2017","journal-title":"Inorg. Chem."},{"key":"ref_28","doi-asserted-by":"crossref","first-page":"5732","DOI":"10.1021\/acs.inorgchem.5b00300","article-title":"Relationship between torsion and anisotropic exchange coupling in a TbIII-radical-based single-molecule magnet","volume":"54","author":"Baker","year":"2015","journal-title":"Inorg. Chem."},{"key":"ref_29","doi-asserted-by":"crossref","first-page":"3557","DOI":"10.1021\/ic00268a028","article-title":"Orbital symmetries and magnetic interaction between copper (II) ions and the o-semiquinone radical. Magnetic studies of (di-2-pyridylamine)(3,5-di-tert-butyl-o-semiquinonato)copper(II) perchlorate and bis(bis(3,5-di-tert-butyl-o-semiquinonato)copper(II))","volume":"26","author":"Kahn","year":"1987","journal-title":"Inorg. Chem."},{"key":"ref_30","doi-asserted-by":"crossref","first-page":"213148","DOI":"10.1016\/j.ccr.2019.213148","article-title":"Unusual metal centres\/coordination spheres in spin crossover compounds","volume":"407","year":"2020","journal-title":"Coord. Chem. Rev."},{"key":"ref_31","doi-asserted-by":"crossref","first-page":"10743","DOI":"10.1021\/acs.inorgchem.9b00885","article-title":"High-Spin and Incomplete Spin-Crossover Polymorphs in Doubly Chelated [Ni(L)2Br2] (L = tert-Butyl 5-Phenyl-2-pyridyl Nitroxide)","volume":"58","author":"Kyoden","year":"2019","journal-title":"Inorg. Chem."},{"key":"ref_32","doi-asserted-by":"crossref","first-page":"1835","DOI":"10.1021\/acs.chemmater.7b05357","article-title":"A new S = 0 \u21c4 S = 2 \u201cSpin-crossover\u201d scenario found in a Nickel(II) Bis(nitroxide) system","volume":"30","author":"Homma","year":"2018","journal-title":"Chem. Mater."},{"key":"ref_33","doi-asserted-by":"crossref","first-page":"16009","DOI":"10.1039\/D0RA02041A","article-title":"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)","volume":"10","author":"Kyoden","year":"2020","journal-title":"RSC Adv."},{"key":"ref_34","doi-asserted-by":"crossref","first-page":"97","DOI":"10.1007\/b95414","article-title":"Structural Aspects of Spin Crossover. Example of the [FeIILn(NCS)2] Complexes","volume":"234","author":"Guionneau","year":"2004","journal-title":"Top. Curr. Chem."},{"key":"ref_35","unstructured":"Lluncll, M., Casanova, D., Circra, J., Bofill, J.M., Alcmany, P., Alvarez, S., Pinsky, M., and Avnir, D. (2005). SHAPE, v2.1, University of Barcelona."},{"key":"ref_36","doi-asserted-by":"crossref","first-page":"012002","DOI":"10.1088\/1757-899X\/202\/1\/012002","article-title":"Magneto-Structural Relationship on Strong Exchange Interactions between Chelating Nitroxide Radical and Transition-Metal Spins","volume":"202","author":"Okazawa","year":"2017","journal-title":"IOP Conf. Ser. Mater. Sci. Eng."},{"key":"ref_37","doi-asserted-by":"crossref","first-page":"3238","DOI":"10.1039\/B204087E","article-title":"Layered nickel hydroxide salts: Synthesis, characterization and magnetic behaviour in relation to the basal spacing","volume":"12","author":"Taibi","year":"2002","journal-title":"J. Mater. Chem."},{"key":"ref_38","doi-asserted-by":"crossref","first-page":"452","DOI":"10.1006\/jssc.1999.8147","article-title":"3D Long-Range Magnetic Ordering in Layered Metal\u2013Hydroxide Triangular Lattices 25 \u00c5 Apart","volume":"145","author":"Kurmoo","year":"1999","journal-title":"J. Solid State Chem."},{"key":"ref_39","first-page":"20140792","article-title":"Nickel hydroxides and related materials: A review of their structures, synthesis and properties","volume":"471","author":"Hall","year":"2015","journal-title":"Proc. R. Soc. A Math. Phys. Eng. Sci."},{"key":"ref_40","unstructured":"Kahn, O. (1993). Molecular Magnetism, VCH."},{"key":"ref_41","unstructured":"Boca, R. (1999). Theoretical Foundations of Molecular Magnetism: Current Methods in Inorganic Chemistry, Elsevier."},{"key":"ref_42","doi-asserted-by":"crossref","first-page":"5737","DOI":"10.1063\/1.440939","article-title":"Valence bond description of antiferromagnetic coupling in transition metal dimers","volume":"74","author":"Noodleman","year":"1981","journal-title":"J. Chem. Phys."},{"key":"ref_43","doi-asserted-by":"crossref","first-page":"10757","DOI":"10.1039\/b907148b","article-title":"Density functional theory for transition metals and transition metal chemistry","volume":"11","author":"Cramer","year":"2009","journal-title":"Phys. Chem. Chem. Phys."},{"key":"ref_44","doi-asserted-by":"crossref","unstructured":"Pantazis, D.A. (2019). Assessment of Double-Hybrid Density Functional Theory for Magnetic Exchange Coupling in Manganese Complexes. Inorganics, 7.","DOI":"10.3390\/inorganics7050057"},{"key":"ref_45","doi-asserted-by":"crossref","first-page":"1062","DOI":"10.1246\/cl.200413","article-title":"Practically Diamagnetic Macrocycle Consisting of Nickel-Biradical Heterospins with the Largest Out-of-Plane Torsion at Coordination Bonds","volume":"49","author":"Ito","year":"2020","journal-title":"Chem. Lett."},{"key":"ref_46","doi-asserted-by":"crossref","first-page":"441","DOI":"10.1021\/j100785a001","article-title":"Van der Waals volumes and radii","volume":"68","author":"Bondi","year":"1964","journal-title":"J. Phys. Chem."},{"key":"ref_47","doi-asserted-by":"crossref","first-page":"12001","DOI":"10.1088\/1757-899X\/515\/1\/012001","article-title":"Moving Molecules in Crystalline Solids: Gradual Structure Transition and Spin Transition\/Crossover","volume":"515","author":"Ishida","year":"2019","journal-title":"IOP Conf. Ser. Mater. Sci. Eng."},{"key":"ref_48","doi-asserted-by":"crossref","first-page":"8221","DOI":"10.1021\/ja00071a035","article-title":"Bistability in the [CoII(semiquinonate)2] to [CoIII(catecholate)(semiquinonate)] valence-tautomeric conversion","volume":"115","author":"Adams","year":"1993","journal-title":"J. Am. Chem. Soc."},{"key":"ref_49","doi-asserted-by":"crossref","first-page":"10630","DOI":"10.1021\/ja991476p","article-title":"Light Induced Excited Pair Spin State in an Iron(II) Binuclear Spin-Crossover Compound","volume":"121","author":"Letard","year":"1999","journal-title":"J. Am. Chem. Soc."},{"key":"ref_50","doi-asserted-by":"crossref","first-page":"10144","DOI":"10.1021\/ic101536q","article-title":"Spin-Transition-Like Behavior on One Side in a Nitroxide-Copper(II)-Nitroxide Triad System","volume":"49","author":"Okazawa","year":"2010","journal-title":"Inorg. Chem."},{"key":"ref_51","doi-asserted-by":"crossref","first-page":"11516","DOI":"10.1021\/ja102163d","article-title":"Ferro- and Antiferromagnetic Coupling Switch Accompanied by Twist Deformation around the Copper(II) and Nitroxide Coordination Bond","volume":"132","author":"Okazawa","year":"2010","journal-title":"J. Am. Chem. Soc."},{"key":"ref_52","doi-asserted-by":"crossref","first-page":"5853","DOI":"10.1021\/acs.inorgchem.6b00140","article-title":"A Copper-Nitroxide Adduct Exhibiting Separate Single Crystal-to-Single Crystal Polymerization\u2212Depolymerization and Spin Crossover Transitions","volume":"55","author":"Ovcharenko","year":"2016","journal-title":"Inorg. Chem."},{"key":"ref_53","doi-asserted-by":"crossref","first-page":"341","DOI":"10.1016\/j.ccr.2014.11.015","article-title":"Electron Paramagnetic Resonance of Switchable Copper-Nitroxide-Based Molecular Magnets: An Indispensable Tool for Intriguing Systems","volume":"289","author":"Fedin","year":"2015","journal-title":"Coord. Chem. Rev."},{"key":"ref_54","doi-asserted-by":"crossref","first-page":"11247","DOI":"10.1021\/ja00150a023","article-title":"A New Type of Thermally Induced Spin Transition Associated with an Equatorial \u2194 Axial Conversion in a Copper(II)-Nitroxide Cluster","volume":"117","author":"Belorizky","year":"1995","journal-title":"J. Am. Chem. Soc."},{"key":"ref_55","doi-asserted-by":"crossref","first-page":"3484","DOI":"10.1021\/ic951295y","article-title":"Spin-Transition and Ferromagnetic Interactions in Copper(II) Complexes of a 3-Pyridyl-Substituted Imino Nitroxide. Dependence of the Magnetic Properties upon Crystal Packing","volume":"35","author":"Luneau","year":"1996","journal-title":"Inorg. Chem."},{"key":"ref_56","unstructured":"(2010\u20132015). Crystal Structure, Rigaku\/MSC. Version 4.2."},{"key":"ref_57","doi-asserted-by":"crossref","first-page":"3","DOI":"10.1107\/S2053229614024218","article-title":"Crystal structure refinement with SHELXL","volume":"71","author":"Sheldrick","year":"2015","journal-title":"Acta Crystallogr. Sect. C Struct. Chem."},{"key":"ref_58","doi-asserted-by":"crossref","first-page":"532","DOI":"10.1021\/ed085p532","article-title":"Diamagnetic Corrections and Pascal\u2019s Constants","volume":"85","author":"Bain","year":"2008","journal-title":"J. Chem. Educ."},{"key":"ref_59","unstructured":"Frisch, M.J., Trucks, G.W., Schlegel, H.B., Scuseria, G.E., Robb, M.A., Cheeseman, J.R., Scalmani, G., Barone, V., Petersson, G.A., and Nakatsuji, H. (2019). Gaussian 16, Revision C.01, Gaussian Inc."}],"container-title":["Molecules"],"original-title":[],"language":"en","link":[{"URL":"https:\/\/www.mdpi.com\/1420-3049\/25\/17\/3790\/pdf","content-type":"unspecified","content-version":"vor","intended-application":"similarity-checking"}],"deposited":{"date-parts":[[2025,10,11]],"date-time":"2025-10-11T10:04:04Z","timestamp":1760177044000},"score":1,"resource":{"primary":{"URL":"https:\/\/www.mdpi.com\/1420-3049\/25\/17\/3790"}},"subtitle":[],"short-title":[],"issued":{"date-parts":[[2020,8,20]]},"references-count":59,"journal-issue":{"issue":"17","published-online":{"date-parts":[[2020,9]]}},"alternative-id":["molecules25173790"],"URL":"https:\/\/doi.org\/10.3390\/molecules25173790","relation":{},"ISSN":["1420-3049"],"issn-type":[{"type":"electronic","value":"1420-3049"}],"subject":[],"published":{"date-parts":[[2020,8,20]]}}}