Novel Square Planar Copper(II) Complexes with Imino or Nitronyl Nitroxide Radicals Exhibiting Large Ferro- and Antiferromagnetic Interactions

Abstract

This paper reports the synthesis, crystal structures, and magnetic properties of two copper(II) complexes (1, 2) of general formula Cu(tfac)2(radical)2 (tfac = trifluoroacetate; radical = (1) 2-phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (NITPh) or (2) 2-phenyl-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazoline-1- oxyl (IMPh)). They crystallize in the monoclinic P2(1)/n space group with the following parameters: (1) a = 13.212(2) A, b = 9.136(1) A, c = 15.587(2) A, beta = 114.61(1) degrees, Z = 2; (2) a = 11.059(2) A, b = 15.289(1) A, c = 10.694(2) A, beta = 114.20(1) degrees, Z = 2. In both complexes the copper(II) ion is coordinated to two radicals in a slightly distorted square planar surrounding. The copper(II)-radical exchange couplings are antiferromagnetic for the nitronyl nitroxide (NITPh) complex (1) and ferromagnetic in the case of the imino nitroxide (IMPh) analogue (2). The ground state has been found to be a spin-doublet for 1 and the spin-quartet for 2. No thermal population of the highest states has been observed, indicating copper(II)-radical couplings of magnitude of J > 500 cm-1.

Radical ligand-containing single-molecule magnets

“…The bent metal-ligand bonding configuration with Cu O N angle of 123.9(4) • offers appropriate symmetry for overlap of nitroxyl * and copper d x 2 −y 2 orbitals, enabling strong antiferromagnetic coupling (J ≤ −225 cm −1 [97]; Table 1). While long axial bonds of the NO groups coordinated to Cu II results in moderate ferromagnetic coupling between nitroxide radical and the metal ion [91,98], short equatorial coordination induces strong antiferromagnetic exchange interactions [95,96], as rationalized by extended Hückel calculations [98]. Axial coordination of the nitroxide leads to an orthogonality between the ligand * orbital and the metal d x 2 −y 2 orbital resulting in ferromagnetic coupling.…”

“…A limitation of these ligands, however, is the poor ability of the weakly Lewis basic N O group to coordinate to a metal center. Nevertheless, enhanced binding of monodentate nitroxides can often be achieved through employment of electron withdrawing groups, such as (1,1,1,5,5,5)-hexafluoroacetylacetonate (hfac) [88][89][90][91][92], halides [93], and halocarboxylate [94,95], as ancillary ligands in order to increase the Lewis acidity of metal ions and generate compounds of the form L n M(L ) 2 (L = nitroxide radical; L = ancillary ligand; n = 1 or 2). The first structurally [88] and magnetically [96] characterized complex of this form, (tempo)Cu(hfac) 2 (tempo = 2,2,6,6-tetramethylpiperidinyl-1-oxyl), comprises a distorted square pyramidal Cu II center with the nitroxyl oxygen atom occupying a basal coordination site.…”

Ni(II), Co(II) and Mn(II) tris(pyrazolyl)borate complexes with 2,6-di-tert-butyl-4-carboxy-phenol: Formation of coordinated phenoxyl radical

“…Metal-radical systems are being studied as potential magnetic materials [1][2][3][4][5], as well as models of intermediates in various biochemical processes [6][7][8]. The stability of such radicals (both free and in metal complexes) is a key problem in synthesis of such systems, and only a few classes of radicals have been used, mostly represented by nitronyl nitroxides, imino nitroxides, verdazyls, semiquinones, TCNE and TCNQ [9][10][11][12][13][14][15].…”

Synthesis, crystal structure and magnetism of two cobalt(II) complexes with imino and nitronyl nitroxides

“…For complex 2, two nitronyl nitroxide radicals are coordinated to the cobalt(II) atoms in equatorial positions. The Co-O-N angle is about 117°, which favors the overlap of the radical magnetic orbital (p*) and the cobalt(II) magnetic orbitals (d x 2 Ày 2 and d z 2 ), resulting in strong antiferromagnetic interaction [35].…”