More Like this

1. Magnetic anisotropy and slow magnetic relaxation processes of cobalt( ii )-pseudohalide complexes

   https://doi.org/10.1039/C9DT00644C  

   Cui, Hui-Hui ; Zhang, Yi-Quan ; Chen, Xue-Tai ; Wang, Zhenxing ; Xue, Zi-Ling ( January 2019 , Dalton Transactions)

   Three mononuclear six-coordinate Co( ii )-pseudohalide complexes [Co(L)X 2 ] with two N-donor pseudohalido coligands occupying the cis -positions (X = NCS − ( 1 ), NCSe − ( 2 ) or N(CN) 2 − ( 3 )), and a five-coordinate complex [Co(L)(NCO)][B(C 6 H 5 ) 4 ] ( 4 ) [L = 1,4,7,10-tetramethyl-1,4,7,10-tetraazacyclododecane (12-TMC)] have been prepared and structurally characterized. Easy-plane magnetic anisotropy for 1–3 and easy-axis anisotropy for 4 were revealed via the analyses of the direct-current magnetic data, high-frequency and -field EPR (HFEPR) spectra and ab initio theoretical calculations. They display slow magnetic relaxations under an external applied dc field. Typically, two slow relaxation processes were found in 1 and 2 while only one relaxation process occurs in 3 and 4 . The Raman-like mechanism is found to be dominant in the studied temperature range in 1 . For 2–4 , the Raman process is dominant in the low temperature region, while the Orbach mechanism dominates in the high temperature range. 

   more » « less
2. Magnetic anisotropy of two tetrahedral Co( ii )-halide complexes with triphenylphosphine ligands

   https://doi.org/10.1039/D2DT00121G  

   Lv, Wei ; Cui, Hui-Hui ; Chen, Lei ; Zhang, Yi-Quan ; Chen, Xue-Tai ; Wang, Zhenxing ; Ouyang, Zhong-Wen ; Xue, Zi-Ling ( May 2022 , Dalton Transactions)

   Recently, the choice of ligand and geometric control of mononuclear complexes, which can affect the relaxation pathways and blocking temperature, have received wide attention in the field of single-ion magnets (SIMs). To find out the influence of the coordination environment on SIMs, two four-coordinate mononuclear Co( ii ) complexes [NEt 4 ][Co(PPh 3 )X 3 ] (X = Cl − , 1; Br − , 2) have been synthesized and studied by X-ray single crystallography, magnetic measurements, high-frequency and -field EPR (HF-EPR) spectroscopy and theoretical calculations. Both complexes are in a cubic space group Pa 3̄ (No. 205), containing a slightly distorted tetrahedral moiety with crystallographically imposed C 3 v symmetry through the [Co(PPh 3 )X 3 ] − anion. The direct-current (dc) magnetic data and HF-EPR spectroscopy indicated the anisotropic S = 3/2 spin ground states of the Co( ii ) ions with the easy-plane anisotropy for 1 and 2. Ab initio calculations were performed to confirm the positive magnetic anisotropies of 1 and 2. Frequency- and temperature-dependent alternating-current (ac) magnetic susceptibility measurements revealed slow magnetic relaxation for 1 and 2 at an applied dc field. Finally, the magnetic properties of 1 and 2 were compared to those of other Co( ii ) complexes with a [CoAB 3 ] moiety. 

   more » « less
3. Magnetic anisotropies and slow magnetic relaxation of three tetrahedral tetrakis(pseudohalido)–cobalt( ii ) complexes

   https://doi.org/10.1039/D1NJ01916C  

   Chen, Shu-Yang ; Lv, Wei ; Cui, Hui-Hui ; Chen, Lei ; Zhang, Yi-Quan ; Chen, Xue-Tai ; Wang, Zhenxing ; Ouyang, Zhong-Wen ; Yan, Hong ; Xue, Zi-Ling ( September 2021 , New Journal of Chemistry)

   Three mononuclear tetrakis(pseudohalido)-cobalt( ii ) complexes (Ph 4 P) 2 [Co(E) 4 ] (E = N 3 − , 1; NCO − , 2; NCS − , 3) have been synthesized and structurally characterized. Each compound contains a distorted tetrahedral Co 2+ ion coordinated by four pseudohalide ligands. The magnetic properties of 1–3 have been studied using direct-current magnetic measurements and high-frequency and -field EPR spectroscopy (HFEPR), suggesting easy-axis magnetic anisotropy for 1 and 2 and easy-plane anisotropy for 3. Analysis of the HFEPR spectra yielded D values of −5.23 and +3.63 cm −1 for 2 and 3, respectively. The absence of the EPR signal in 1 is consistent with a large, negative value of the zero-field splitting (ZFS) parameter D in 1. The nature of magnetic anisotropies of 1–3 has also been confirmed by ab initio calculations. The calculated D values are consistent with those determined using magnetometry and HFEPR studies. Alternating current (AC) magnetic susceptibilities reveal slow magnetic relaxation under an applied magnetic field, thus indicating that 1–3 are field-induced single-ion magnets (SIMs). 

   more » « less
4. Magnetic anisotropy and relaxation behavior of six-coordinate tris(pivalato)-Co( ii ) and -Ni( ii ) complexes

   https://doi.org/10.1039/C8DT01554F  

   Chen, Shu-Yang ; Cui, Hui-Hui ; Zhang, Yi-Quan ; Wang, Zhenxing ; Ouyang, Zhong-Wen ; Chen, Lei ; Chen, Xue-Tai ; Yan, Hong ; Xue, Zi-Ling ( January 2018 , Dalton Transactions)

   Experimental and theoretical studies of magnetic anisotropy and relaxation behavior of six-coordinate tris(pivalato)-Co( ii ) and -Ni( ii ) complexes (NBu 4 )[M(piv) 3 ] (piv = pivalate, M = Co, 1 ; M = Ni, 2 ), with a coordination configuration at the intermediate between an octahedron and a trigonal prism, are reported. Direct current magnetic data and high-frequency and -field EPR spectra (HFEPR) of 1 have been modeled by a general Hamiltonian considering the first-order orbital angular momentum, while the spin Hamiltonian was used to interpret the data of 2 . Both 1 and 2 show easy-axis magnetic anisotropies, which are further supported by ab initio calculations. Alternating current (ac) magnetic susceptibilities reveal slow magnetic relaxation at an applied dc field of 0.1 T in 1 , which is characteristic of a field-induced single-ion magnet (SIM), but 2 does not exhibit single-ion magnetic properties at 1.8 K. Detailed analyses of relaxation times show a dominant contribution of a Raman process for spin relaxation in 1 . 

   more » « less
5. Co( ii ) complexes of tetraazamacrocycles appended with amide or hydroxypropyl groups as paraCEST agents

   https://doi.org/10.1039/d3dt01572f  

   Raymond, Jaclyn J. ; Abozeid, Samira M. ; Sokolow, Gregory E. ; Bond, Christopher J. ; Yap, Constance E. ; Nazarenko, Alexander Y. ; Morrow, Janet R. ( July 2023 , Dalton Transactions)

   Co( ii ) complexes of 1,4,7,10-tetraazacyclododecane (CYCLEN) or 1,4,8,11-tetraazacyclotetradecane (CYCLAM) with 2-hydroxypropyl or carbamoylmethyl (amide) pendants are studied with the goal of developing paramagnetic chemical exchange saturation transfer (paraCEST) agents. Single-crystal X-ray diffraction studies show that two of the coordination cations with hexadentate ligands, [Co(DHP)] 2+ and [Co(BABC)] 2+ , form six-coordinate complexes; whereas two CYCLEN-based complexes with potentially octadentate ligands, [Co(THP)] 2+ and [Co(HPAC)] 2+ , are seven-coordinate with only three of the four pendant groups bound to the metal center. 1 H NMR spectra of these complexes suggest that the six-coordinate complexes are present as a single isomer in aqueous solution. For the complexes which are seven-coordinate in the solid state, one is highly fluxional in aqueous solution on the NMR time scale ([Co(HPAC)] 2+ ), whereas the NMR spectrum of [Co(THP)] 2+ is consistent with an eight-coordinate complex with all pendants bound. Co( ii ) complexes of CYCLEN derivatives show CEST effects of low intensity that are assigned to NH or OH groups of the pendants. One complex, [Co(DHP)] 2+ , shows a highly-shifted CEST peak at 113 ppm versus bulk water, attributed to OH protons. However, the CEST effect is largest for two Co( ii ) CYCLAM-based complexes with coordinated amide groups that undergo NH proton exchange. All five complexes are inert towards dissociation in buffered solutions containing carbonate and phosphate and towards trans-metalation by excess Zn( ii ). These data give insight into the production of an intense CEST effect for tetraazamacrocyclic complexes with pendant groups containing NH or OH exchangeable protons. The intense and highly shifted CEST peak(s) of the CYCLAM-based complexes suggest that they are promising for further development as paraCEST agents. 

   more » « less