Cobalt(III) Metallacryptates and Their Guest Cation-Exchange in Solution Monitored by ⁵⁹Co NMR

Abstract

One-pot reactions of the catechol-scaffolding aroylbis­(N,N-diethylthiourea) H2Lcat with mixtures of CoCl2 and MCl (M+ = Cs+, Rb+, K+, Tl+, or NH4 +) or with a Co­(NO3)2/TlNO3 mixture lead to the self-assembly of a series of cationic Co­(III) metallacryptates of the general formula [M ⊂ {Co2(Lcat)3}]+ (M+ = Cs+, Rb+, K+, Tl+, or NH4 +). Crystalline PF6 – salts were obtained after workup with (n-Bu4N)­(PF6), and the single-crystal structures of all five metallacryptates have been determined. Depending on the nature of the guest cations, the directional interactions between guest cations and the metallacryptand {Co2(Lcat)3} are either weak coordination contacts or hydrogen bonds. The bonding mode and the size of the guest ions slightly influence the molecular skeleton of the host molecule. These small structural variations also exist in solution and could be detected by means of 59Co NMR spectroscopy, which is shown to be a unique tool for an easy characterization of such compounds. 59Co NMR chemical shifts are extraordinarily sensitive to the guest cation in the metallacryptates, and time-arrayed 59Co NMR experiments show that cation-exchange processes in biphasic organic/aqueous systems can be studied in detail. This leads to insights into the relative rates of cation exchange, as well as the relative conditional distribution coefficients of such Co­(III) metallacryptates between the aqueous and organic phases. Thus, the extent and the relative rate of the NH4 + ion exchange in [NH4 ⊂ {Co2(Lcat)3}]­(PF6) by Cs+ and K+ ions across the organic/aqueous phase boundary at room temperature have been studied by in situ 59Co NMR experiments. Preliminary 59Co NMR experiments show that the K+ ion in [K ⊂ {Co2(Lcat)3}]­(PF6) can be removed by its competitive complexation with the highly potassium-selective [2.2.2]­cryptand, to give a transient 59Co NMR signal of the relatively unstable “empty” {Co2(Lcat)3} complex, which slowly decomposes in solution.

Ammonium‐Iron(III) metallacryptate inclusion complexes based on Aroylbis(N,N‐diethylthioureas): Synthesis and structure

“…The parameters of the hydrogen bonds closely resemble those found in the analogous NH4 +inclusion complex with the Co(III) metallacryptates [Co2(L 2 )3]. [20] 4. CONCLUSION Two ammonium-iron(III) metallacryptate inclusion compounds derived from pyridine-and catecholscaffolding aroylbis(N,N-diethylthiourea) ligands, H2L 1 and H2L 2 , have been synthesized and structurally characterized by elemental analyses, spectroscopic methods such as IR and HRMS, and single crystal X-ray diffraction.…”

“…Their bifunctional coordinative sites, namely the pyridine-or catechol-scaffolding block and the aroylthiourea moieties, enable heterotrinuclear complexes. [16][17][18][19][20][21][22] Some representatives are presented in figure 2. These trinuclear systems are topologically analogous with organic-based host-guest cryptates.…”

“…Indeed, the complexes could be perceived as inclusion compounds arising from the capture of guest ions in central cavities of {2}-metallacryptands. [17,[20][21] It is interesting that not only metal ions, but also nonmetal ions like NH4 + are encapsulated in the metallamacrocycles. Spectroscopic studies on the exchange of the NH4 + ion in the Co(III) {2}metallacryptate, {NH4 ⊂ [Co2(L 2 )3]}(PF6) revealed the high lability of the guest cation.…”

“…Spectroscopic studies on the exchange of the NH4 + ion in the Co(III) {2}metallacryptate, {NH4 ⊂ [Co2(L 2 )3]}(PF6) revealed the high lability of the guest cation. [20] In consideration of the topological similarity between Fe(III) and In(III) or Co(III) {2}-metallacryptates (figure 2), the present work reports the synthesis and structure determination of two new NH4 + -inclusion complexes with Fe(III) {2}-metallacryptands developed from the aroyl-bis(N,N-diethylthiourea) ligands H2L 1 and H2L 2 .…”

Isotopomeric Elucidation of the Mechanism of Temperature Sensitivity in 59Co NMR Molecular Thermometers

“…More “flexible” ligands could be envisioned to produce species with greater temperature-dependent inner-coordination spheres, thereby producing higher Δδ/Δ T . However, recent studies by us − and others , suggest that bond-distance expansion alone is not the dominant feature. In a prior work, we posited a model wherein structural rigidity imparted by the ligand shell engenders long-lived vibrations caused by collisions of a given 59 Co NMR thermometer with the surrounding solvent (Figure ) and thus temperature-driven structures and Δδ/Δ T .…”