Regiospecificity in the Exchange Coupling of the Spins of Copper(II) Ion Coordinated with the Ring Nitrogen Atoms and N-tert-Butylaminoxyl Radical Attached as a Substituent on the Pyridine and N-Phenylimidazole Rings

• Published in: Inorganic chemistry Vol. 37; no. 9; pp. 2273 - 2280
• Main Authors: Ishimaru, Yoichiro, Kitano, Makoto, Kumada, Harumi, Koga, Noboru, Iwamura, Hiizu
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 04.05.1998
• ISSN: 0020-1669; 1520-510X
• DOI: 10.1021/ic970602s

Complexes of bis(hexafluoroacetylacetonato)copper(II) [Cu(hfac)2] ligated with 3- and 4-(N-oxyl-tert-butylamino)pyridines (3NOPy and 4NOPy) and N-{3- and 4-(N-oxyl-tert-butylamino)phenyl}imidazoles (3NOIm and 4NOIm) were prepared. The 1:2 complexes [Cu(hfac)2(3NOPy and 4NOPy)2] have hexacoordinated octahedral structures in which the two pyridyl nitrogen atoms are coordinated to the copper(II) ions in the trans configuration. The 1:1 complex [Cu(hfac)2(4NOPy)] has a distorted pentacoordinated pseudo-head-to-tail cyclic dimer structure in which the oxygen atom of the aminoxyl group of one complex is situated at a distance of 2.79 Å from the copper ion of the other complex. The magnetic properties of these and two other complexes, [Cu(hfac)2(3NOIm and 4NOIm)2], were investigated using a SQUID susceptometer. Temperature dependence studies of the χmol T values of [Cu(hfac)2(4NOPy)2] revealed that the two aminoxyl radicals interact ferromagnetically with the copper(II) ion (S = 1/2) with an exchange parameter J/k B = 60.4 ± 3.3 K to produce a quartet ground state (S = 3/2). On the other hand, the χmol T value of the isomeric [Cu(hfac)2(3NOPy)2] was nearly constant at 0.5 emu K mol-1 in the temperature range 5−300 K. A rectangular four-spin model was fitted to the χmol T vs T plot for complex [Cu(hfac)2(4NOPy)] to give J/k B = 58.6 and 58.5 K. Similarly, the J values for [Cu(hfac)2(3NOIm and 4NOIm)2] were negative and positive, respectively, but their absolute J values were too small to determine in both cases.