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1. Cobalt(II)-Substituted Cysteamine Dioxygenase Oxygenation Proceeds through a Cobalt(III)-Superoxo Complex

   https://doi.org/10.1021/jacs.4c01871  

   Li, Jiasong; Duan, Ran; Liu, Aimin (July 2024, Journal of the American Chemical Society)

   DeBeer, Serena (Ed.)

   We investigated the metal-substituted catalytic activity of human cysteamine dioxygenase (ADO), an enzyme pivotal in regulating thiol metabolism and contributing to oxygen homeostasis. Our findings demonstrate the catalytic competence of cobalt(II)- and nickel(II)-substituted ADO in cysteamine oxygenation. Notably, Co(II)-ADO exhibited superiority over Ni(II)-ADO despite remaining significantly less active than the natural enzyme. Structural analyses through X-ray crystallography and cobalt K-edge excitation confirmed successful metal substitution with minimal structural perturbations. This provided a robust structural basis, supporting a conserved catalytic mechanism tailored to distinct metal centers. This finding challenges the proposed high-valent ferryl-based mechanism for thiol dioxygenases, suggesting a non-high-valent catalytic pathway in the native enzyme. Further investigation of the cysteamine-bound or a peptide mimic of N-terminus RGS5 bound Co(II)-ADO binary complex revealed the metal center’s high-spin (S = 3/2) state. Upon reaction with O2, a kinetically and spectroscopically detectable intermediate emerged with a ground spin state of S = 1/2. This intermediate exhibits a characteristic 59Co hyperfine splitting (A = 67 MHz) structure in the EPR spectrum alongside UV–vis features, consistent with known low-spin Co(III)-superoxo complexes. This observation, unique for protein-bound thiolate-ligated cobalt centers in a protein, unveils the capacities for O2 activation in such metal environments. These findings provide valuable insights into the non-heme iron-dependent thiol dioxygenase mechanistic landscape, furthering our understanding of thiol metabolism regulation. The exploration of metal-substituted ADO sheds light on the intricate interplay between metal and catalytic activity in this essential enzyme. 

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2. A five-coordinate cobalt bis(dithiolene)–phosphine complex [Co(pdt) ₂ (PTA)] (pdt = phenyldithiolene; PTA = 1,3,5-triaza-7-phosphaadamantane)

   https://doi.org/10.1107/S2056989020005447  

   Williams, DaShawn; Brannon, Jacob P.; Chandrasekaran, Perumalreddy; Stieber, S. Chantal (May 2020, Acta Crystallographica Section E Crystallographic Communications)

   The title compound, bis(1,2-diphenyl-2-sulfanylideneethanethiolato-κ 2 S , S ′)(1,3,5-triaza-7-phosphaadamantane-κ P )cobalt(II) dichloromethane hemisolvate, [Co(pdt) 2 (PTA)]·0.5C 2 H 4 Cl 2 or [Co(C 14 H 10 S 2 ) 2 (C 6 H 12 N 3 P)]·0.5C 2 H 4 Cl 2 , contains two phenyldithiolene (pdt) ligands and a 1,3,5-triaza-7-phosphaadamantane (PTA) ligand bound to cobalt with the solvent 1,2-dichloroethane molecule located on an inversion center. The cobalt core exhibits an approximately square-pyramidal geometry with partially reduced thienyl radical monoanionic ligands. The supramolecular network is consolidated by hydrogen-bonding interactions primarily with nitrogen, sulfur and chlorine atoms, as well as parallel displaced π-stacking of the aryl rings. The UV–vis, IR, and CV data are also consistent with monoanionic dithiolene ligands and an overall Co II oxidation state. 

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3. Semiquinone radical-bridged M ₂ (M = Fe, Co, Ni) complexes with strong magnetic exchange giving rise to slow magnetic relaxation

   https://doi.org/10.1039/d0sc03078c  

   Chakarawet, Khetpakorn; Harris, T. David; Long, Jeffrey R. (August 2020, Chemical Science)

   null (Ed.)

   The use of radical bridging ligands to facilitate strong magnetic exchange between paramagnetic metal centers represents a key step toward the realization of single-molecule magnets with high operating temperatures. Moreover, bridging ligands that allow the incorporation of high-anisotropy metal ions are particularly advantageous. Toward these ends, we report the synthesis and detailed characterization of the dinuclear hydroquinone-bridged complexes [(Me 6 tren) 2 MII2(C 6 H 4 O 2 2− )] 2+ (Me 6 tren = tris(2-dimethylaminoethyl)amine; M = Fe, Co, Ni) and their one-electron-oxidized, semiquinone-bridged analogues [(Me 6 tren) 2 MII2(C 6 H 4 O 2 − ˙)] 3+ . Single-crystal X-ray diffraction shows that the Me 6 tren ligand restrains the metal centers in a trigonal bipyramidal geometry, and coordination of the bridging hydro- or semiquinone ligand results in a parallel alignment of the three-fold axes. We quantify the p -benzosemiquinone–transition metal magnetic exchange coupling for the first time and find that the nickel( ii ) complex exhibits a substantial J < −600 cm −1 , resulting in a well-isolated S = 3/2 ground state even as high as 300 K. The iron and cobalt complexes feature metal–semiquinone exchange constants of J = −144(1) and −252(2) cm −1 , respectively, which are substantially larger in magnitude than those reported for related bis(bidentate) semiquinoid complexes. Finally, the semiquinone-bridged cobalt and nickel complexes exhibit field-induced slow magnetic relaxation, with relaxation barriers of U eff = 22 and 46 cm −1 , respectively. Remarkably, the Orbach relaxation observed for the Ni complex is in stark contrast to the fast processes that dominate relaxation in related mononuclear Ni II complexes, thus demonstrating that strong magnetic coupling can engender slow magnetic relaxation. 

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4. Slow magnetic relaxation in cobalt N-heterocyclic carbene complexes

   https://doi.org/10.1039/D0DT02286A  

   Saber, Mohamed R.; Przyojski, Jacob A.; Tonzetich, Zachary J.; Dunbar, Kim R. (August 2020, Dalton Transactions)

   The combined experimental and theoretical investigation of the magnetic properties of the cobalt( ii ) NHC complexes (NHC = N-heterocyclic carbene); [Co(CH 2 SiMe 3 ) 2 (IPr)] ( 1 ), [CoCl 2 (IMes) 2 ] ( 2 ) and [Co(CH 3 ) 2 (IMes) 2 ] ( 3 ) revealed a large easy plane anisotropy for 1 ( D = +73.7 cm −1 ) and a moderate easy axis anisotropy for 2 ( D = −7.7 cm −1 ) due to significant out-of-state spin–orbit coupling. Dynamic magnetic measurements revealed slow relaxation of the magnetization for 1 ( U eff = 22.5 K, τ 0 = 3 × 10 −7 s, 1000 Oe) and for 2 ( U eff = 20.2 K, τ 0 = 1.73 × 10 −8 s, 1500 Oe). The molecular origin of the slow relaxation phenomena was further supported by the retention of AC signal in 10% solutions in 2-MeTHF which reveals a second zero field AC signal in 1 at higher frequencies. Compound 3 was found to be an S = 1/2 system. 

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5. A Molecular Tetrahedral Cobalt–Seleno-Based Complex as an Efficient Electrocatalyst for Water Splitting

   https://doi.org/10.3390/molecules26040945  

   Abdullahi, Ibrahim Munkaila; Masud, Jahangir; Ioannou, Polydoros-Chrisovalantis; Ferentinos, Eleftherios; Kyritsis, Panayotis; Nath, Manashi (February 2021, Molecules)

   The cobalt–seleno-based coordination complex, [Co{(SePiPr2)2N}2], is reported with respect to its catalytic activity in oxygen evolution and hydrogen evolution reactions (OER and HER, respectively) in alkaline solutions. An overpotential of 320 and 630 mV was required to achieve 10 mA cm−2 for OER and HER, respectively. The overpotential for OER of this CoSe4-containing complex is one of the lowest that has been observed until now for molecular cobalt(II) systems, under the reported conditions. In addition, this cobalt–seleno-based complex exhibits a high mass activity (14.15 A g−1) and a much higher turn-over frequency (TOF) value (0.032 s−1) at an overpotential of 300 mV. These observations confirm analogous ones already reported in the literature pertaining to the potential of molecular cobalt–seleno systems as efficient OER electrocatalysts. 

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