Transition Ion Strikes Back: Large Magnetic Susceptibility Anisotropy in Cobalt(II) Clathrochelates

Abstract

Transition-metal complexes are rarely considered as paramagnetic tags for NMR spectroscopy due to them generally having relatively low magnetic anisotropy. Here we report cobalt(II) cage complexes with the largest (among the transition-metal complexes) axial anisotropy of magnetic susceptibility, reaching as high as 12.6 × 10(-32) m(3) at room temperature. This remarkable anisotropy, which results from an unusual trigonal prismatic geometry of the complexes and translates into large negative value of the zero-field splitting energy, is high enough to promote reliable paramagnetic pseudocontact shifts at the distance beyond 2 nm. Our finding paves the way toward the applications of cobalt(II) clathrochelates as future paramagnetic tags. Given the incredible stability and functionalization versatility of clathrochelates, the fine-tuning of the caging ligand may lead to new chemically stable mononuclear single-molecule magnets, for which magnetic anisotropy is of importance.

Intramolecular Spin State Locking in Iron(II) 2,6-Di(pyrazol-3-yl)pyridine Complexes by Phenyl Groups: An Experimental Study

“…In addition, these chemical shifts closely follow the linear dependence on temperature as δ ~ T −1 ( Figure 2) expected at high T (>200 K) for systems with a g value close to 2 and a moderate zero-field splitting [35]. As there are no deviations stemming from a spin transition [29] (their other source is large magnetic anisotropy [36][37][38]), it does not occur between 240 and 330 K. This is quantified by the Evans method [25,26], which measures magnetic susceptibility based on a difference in chemical shifts of an inert substance, e.g., TMS, in a solution of a paramagnetic compound and in a pure solvent recorded simultaneously. Within the experimental error, which is fairly large due to the limitations of the method [29], the resulting χT value is constant with temperature at 3.5-4.0 cm 3 mol −1 K ( Figure 1), thereby corroborating the HS locking of the complexes Fe(L1)2OTf2, Fe(L2)2OTf2 and Fe(L2)2(BF4)2.…”

The Peculiarities of Ionization, Fragmentation and Association (Macrobicyclization) of Pseudoclathrochelate Zinc, Cobalt, Iron and Manganese(II) tris-Pyrazoloximates in the LDI Mass Spectra

“…«топологи-ческих лекарств» [3][4][5][6] и перспективных парамагнитных проб для использования в структурной биологии. [7,8] В связи с этим представляет несомненный интерес изуче-ние устойчивости и путей фрагментации этих клеточных комплексов как в растворах, так и в газовой фазе. Ранее [9] методом ББА масс-спектрометрии были изучены пути фрагментации гидроксиборсодержащих клатрохелатов железа(II) -производных алициклических диоксимов (Схема 1).…”

Mechanistic Investigation of Well-Defined Cobalt Catalyzed Formal E-Selective Hydrophosphination of Alkynes

“…The rationale for EPR silent signature of Co(II) ion in a high-spin state ( S = 3/2) is well established already in the literature. 58,[69][70][71][72][73][74] When an alkyne is added to the in-situ generated species, the EPR signal is re-generated, The spin Hamiltonian parameters extracted for A', reproduce only part of the experimental spectrum, which leave out some of the spectral features in 320-350 mT range (see Figure 3 marked with # symbol). Origin of these EPR signals is due to an unknown radical impurity.…”