Solvate-Dependent Spin Crossover and Exchange in Cobalt(II) Oxazolidine Nitroxide Chelates

Abstract

Two oxazolidine nitroxide complexes of cobalt(II), [Co(II)(L(•))2](B(C6F5)4)2·CH2Cl2 (1) and [Co(II)(L(•))2](B(C6F5)4)2·2Et2O (2), where, L(•) is the tridentate chelator 4,4-dimethyl-2,2-bis(2-pyridyl)oxazolidine N-oxide, have been investigated by crystallographic, magnetic, reflectivity, and theoretical (DFT) methods. This work follows on from a related study on [Co(II)(L(•))2](NO3)2 (3), a multifunctional complex that simultaneously displays magnetic exchange, spin crossover, and single molecule magnetic features. Changing the anion and the nature of solvation in the present crystalline species leads to significant differences, not only between 1 and 2 but also in comparison to 3. Structural data at 123 and 273 K, in combination with magnetic data, show that at lower temperatures 1 displays low-spin Co(II)-to-radical exchange with differences in fitted J values in comparison to DFT (broken symmetry) calculated J values ascribed to the sensitive influence of a tilt angle (θ) formed between the Co(dz(2)) and the trans-oriented O atoms of the NO radical moieties in L(•). Spin crossover in 1 is evident at higher temperatures, probably influenced by the solvate molecules and crystal packing arrangement. Complex 2 remains in the high-spin Co(II) state between 2 and 350 K and undergoes antiferromagnetic exchange between Co-radical and radical-radical centers, but it is difficult to quantify. Calculations of the magnetic orbitals, eigenvalue plots, and the spin densities at the Co and radical sites in 1 and 2 have yielded satisfying details on the mechanism of metal-radical and radical-radical exchange, the radical spins being in π*NO orbitals.

Use of the TCNQF42− Dianion in the Spontaneous Redox Formation of [FeIII(L−)2][TCNQF4⋅−]

“…The crystallographic bond lengths and angles (Tables and ) are used to assign the redox level of the tridentate ligand and TCNQF 4 . For the cationic Fe species in 1 , the nitroxide N−O (N3−O1) bond length is 1.4177(19) Å, which is clearly appropriate for the hydroxylamino anionic form of the ligand (L − ) . The Fe−O (1.8551(12) Å) and Fe−N (1.9869(16) and 1.9553(15) Å) bond lengths are consistent with earlier studies on [Fe III (L − ) 2 ](BPh 4 ), so we can confidently assign the cationic species as [Fe III (L − )] + where the Fe III ion is low‐spin.…”

“…and [Fe II (L . ) 2 ][BF 4 ] 2 were synthesised as described previously . Li 2 TCNQF 4 was prepared according to literature methods .…”

“…The cobalt analogues are represented by the dication, [Co II (L . ) 2 ] 2+ and monocation [Co III (L − ) 2 ] + , which exhibit similar redox processes to the iron complexes above as well as a host of other interesting properties such as solvate dependent exchange/spin‐crossover and single‐molecule magnet (SMM) like behaviour . In the cobalt case, the S =3/2 [Co II (L . )…”

Spectroscopic, Structural, and Kinetic Investigation of the Ultrafast Spin Crossover in an Unusual Cobalt(II) Semiquinonate Radical Complex

“…Examples of SCO in metal complexes containing radical ligands that are directly bound to the metal ions are rather rare, presumably due to difficulties in identifying suitable target molecules, and in preparing these complexes . One of us has initiated a research endeavor to synthesize such complexes and to study the influence of the coordinated ligand radical on the SCO properties of the metal ion.…”

“…[3,19,20] Examples of SCO in metal complexes containing radical ligandst hat are directly bound to the metal ions are rather rare, presumably due to difficulties in identifying suitable targetm olecules, and in preparing these complexes. [21][22][23][24][25][26][27][28] One of us has initiated ar esearch endeavor to synthesize such complexes and to study the influence of the coordinated ligand radicalo nt he SCO properties of the metal ion. Thus we succeeded in demonstratingt he occurrence of SCO in an octahedral Fe II bis(imino)acenaphthalene radicalc omplex [29] as well as in an octahedral Co II semiquinonate radicalcomplex.…”