Solvent-Induced Encapsulation of Cobalt(II) Ion by a Boron-Capped tris-Pyrazoloximate

Abstract

Boron-cross-linked cobalt(II) pseudoclathrochelate was obtained by the template reaction of 2-acetylpyrazoloxime, phenylboronic acid, and a new DMF cobalt(II) solvato complex with a decachloro-closo-decaborate dianion. As confirmed by singlecrystal X-ray diffraction, this complex crystallizes with two symmetry-independent cobalt(II) pseudoclathrochelate cations, one decachloro-closo-decaborate dianion, one benzene, one dichloromethane solvent molecule, and two molecules of DMF. The latter act as pseudocapping fragments to the monocapped trispyrazoloximate ligands by forming N−H•••O hydrogen bonds with their pyrazole groups. The Co II N 6 -coordination polyhedra adopt a nearly ideal TP geometry with distortion angles φ equal to 1.22(16) and 2.58(17)°for two symmetry-independent pseudoclathrochelate cations, both containing the encapsulated cobalt(II) ion in its high-spin state (Co−N 2.115(4)−2.198(3) Å). Magnetic properties of this complex were studied both by dc-magnetometry and by solution-state NMR spectroscopy to reveal a high magnetic anisotropy, thus suggesting a large magnetic susceptibility tensor anisotropy (25.8 × 10 −32 m 3 at 298 K) and a large negative zero-field splitting energy (−85 cm −1 ). The results of magnetometry studies in the ac magnetic field suggest a single molecule magnet behavior of this TP complex with an effective magnetization reversal barrier of approximately 130 cm −1 . Its pseudocapping DMF molecules that form H-bonds with tris-pyrazoloximate fragments are easy to substitute by strong H-bond acceptors, such as chloride ions and di-and tetramethylureas, thus affecting the magnetic properties of a whole pseudomacrobicyclic paramagnetic system.

Unexpected Side Products of Chemical Transformations in Cobalt(II) Pseudoclathrochelates: An X‐Ray Diffraction Study

“…An attempted crystallization of the complex 2 with an H‐bonded DMF molecule (Scheme ) from its solution in a chloroform‐heptane mixture, which was left for some time on a sunlight in air, unexpectedly produced yellow crystals of a chloroform solvate of the complex 1 earlier synthesized by our group.…”

“…Although it is generally accepted that (pseudo)clathrochelates of transition metals can be easily obtained by a simple template condensation of appropriate oximes, boronic acids and metal salts, our study clearly shows that other compounds can be sometimes identified among the products of their synthesis or chemical transformations occurring in different media. Therefore, a further insight (such as from powder X‐ray diffraction or in situ paramagnetic NMR spectroscopy) is needed for optimizing of synthetic pathways towards this unique class of transition metal complexes for their future SMM and MRI applications.…”

“…According to the collected X‐ray diffraction data, its two symmetry‐equivalent pseudoclathrochelate species are hold together by N−H⋅⋅⋅O hydrogen bonds (N⋅⋅⋅O 2.713(4)–2.769(3) Å, ∠NHO 148.51(18)–169.5(2)°) with a sulphate anion acting both as a pseudocapping and as a μ 2 ‐bridging fragment. As a result, their CoN 6 ‐coordination polyhedra (Co−N 2.117(2)–2.167(2) Å) adopt a nearly ideal TP geometry with the distortion angle ϕ equal to 2.16(13)° often found in pseudoclathrochelates with an encapsulated cobalt(II) ion in the high spin state . Note that only two oxygen atoms of the sulfate anion are involved in the above H‐bonds, so the resulting binuclear complex 3 (Figure ) has a slightly angular geometry with an angle Co(1)S(1)Co(1) of 150.24(2)°.…”

“…A popular synthetic strategy towards cage complexes of 3d ‐metals (clathrochelates) recently emerged as prospective single molecule magnets (SMM)‐ and paramagnetic probes for magnetic resonance imaging (MRI) is the use of monocapped tripodal ligands pseudo‐cross‐linked via hydrogen bonding by a counterion or by a neutral H‐bond acceptor (Scheme ). The resulting decrease in the ligand field strength compared to macropolycyclic covalent analogues with a quasiaromatic cage system (regular clathrochelates) produced high‐spin cobalt(II) pseudoclathrochelates that featured a very large axial anisotropy of magnetic susceptibility tensor owing to their unusual trigonal‐prismatic (TP, the distortion angle ϕ =0°) coordination geometry …”

“…Chemical design of these cobalt(II) pseudoclathrochelates is, however, a huge synthetic challenge, as only a boron capping atom ensures a TP coordination geometry of such complexes; otherwise, they have a trigonal‐antiprismatic (TAP, ϕ =60°) geometry unfavourable for their use as SMMs or paramagnetic probes for MRI . In addition, H‐bonded boron‐capped 3 d ‐metal pseudoclathrochelates (Scheme ) are only relatively stable, unlike their macropolycyclic analogues formed by covalent and strong coordination bonds, and can readily exchange pseudocapping H‐bonded species (Scheme ) or undergo intramolecular cyclization reactions (Scheme ) .…”