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1. Metal binding and interdomain thermodynamics of mammalian metallothionein-3: enthalpically favoured Cu ⁺ supplants entropically favoured Zn ²⁺ to form Cu ₄ ⁺ clusters under physiological conditions

   https://doi.org/10.1039/d2sc00676f  

   Mehlenbacher, Matthew R. ; Elsiesy, Rahma ; Lakha, Rabina ; Villones, Rhiza Lyne ; Orman, Marina ; Vizcarra, Christina L. ; Meloni, Gabriele ; Wilcox, Dean E. ; Austin, Rachel N. ( January 2022 , Chemical Science)

   Metallothioneins (MTs) are a ubiquitous class of small metal-binding proteins involved in metal homeostasis and detoxification. While known for their high affinity for d 10 metal ions, there is a surprising dearth of thermodynamic data on metals binding to MTs. In this study, Zn 2+ and Cu + binding to mammalian metallothionein-3 (MT-3) were quantified at pH 7.4 by isothermal titration calorimetry (ITC). Zn 2+ binding was measured by chelation titrations of Zn 7 MT-3, while Cu + binding was measured by Zn 2+ displacement from Zn 7 MT-3 with competition from glutathione (GSH). Titrations in multiple buffers enabled amore »detailed analysis that yielded condition-independent values for the association constant ( K ) and the change in enthalpy (Δ H ) and entropy (Δ S ) for these metal ions binding to MT-3. Zn 2+ was also chelated from the individual α and β domains of MT-3 to quantify the thermodynamics of inter-domain interactions in metal binding. Comparative titrations of Zn 7 MT-2 with Cu + revealed that both MT isoforms have similar Cu + affinities and binding thermodynamics, indicating that Δ H and Δ S are determined primarily by the conserved Cys residues. Inductively coupled plasma mass spectrometry (ICP-MS) analysis and low temperature luminescence measurements of Cu-replete samples showed that both proteins form two Cu 4 + –thiolate clusters when Cu + displaces Zn 2+ under physiological conditions. Comparison of the Zn 2+ and Cu + binding thermodynamics reveal that enthalpically-favoured Cu + , which forms Cu 4 + –thiolate clusters, displaces the entropically-favoured Zn 2+ . These results provide a detailed thermodynamic analysis of d 10 metal binding to these thiolate-rich proteins and quantitative support for, as well as molecular insight into, the role that MT-3 plays in the neuronal chemistry of copper.« less
2. Heterometallic [Cu–O–M] ²⁺ active sites for methane C–H activation in zeolites: stability, reactivity, formation mechanism and relationship to other active sites

   https://doi.org/10.1039/d1cy00687h  

   Adeyiga, Olajumoke ; Panthi, Dipak ; Odoh, Samuel O. ( August 2021 , Catalysis Science & Technology)

   The formation and reactivities of [Cu–O–M] 2+ species (M = Ti–Cu, Zr–Mo and Ru–Ag) in metal-exchanged zeolites, as well as stabilities of these species towards autoreduction by O 2 elimination are investigated with density functional theory. These species were investigated in zeolite mordenite in search of insights into active site formation mechanisms, the relationship between stability and reactivity as well as discovery of heterometallic species useful for isothermal methane-to-methanol conversion (MMC). Several [Cu–O–M] 2+ species (M = Ti–Cr and Zr–Mo) are substantially more stable than [Cu 2 O] 2+ . Other [Cu–O–M] 2+ species, (M = Mn–Ni and Ru–Ag) havemore »similar formation energies to [Cu 2 O] 2+ , to within ±10 kcal mol −1 . Interestingly, only [Cu–O–Ag] 2+ is more active for methane activation than [Cu 2 O] 2+ . [Cu–O–Ag] 2+ is however more susceptible to O 2 elimination. By considering the formation energies, autoreduction, cost and activity towards the methane C–H bond, we can only conclude that [Cu 2 O] 2+ is best suited for MMC. Formation of [Cu 2 O] 2+ is initiated by proton transfer from aquo ligands to the framework and proceeds mostly via dehydration steps. Its μ-oxo bridge is formed via water-assisted condensation of two hydroxo groups. To evaluate the relationship between [Cu 2 O] 2+ and other active sites, we also examined the formation energies of other species. The formation energies follow the trend: isolated [Cu–OH] + < paired [Cu–OH] + < [Cu 2 O] 2+ < [Cu 3 O 3 ] 2+ . Inclusion of Gibbs free-energy corrections indicates activation temperatures of 257, 307 and 327 and 331 °C for isolated [Cu–OH] + , paired [Cu–OH] + , [Cu 2 O] 2+ and [Cu 3 O 3 ] 2+ , respectively. The provocative nature of the lower-than-expected activation temperature for isolated [Cu–OH] + species is discussed.« less
3. Evaluating electrochemical accessibility of 4f ⁿ 5d ¹ and 4f ⁿ⁺¹ Ln( ii ) ions in (C ₅ H ₄ SiMe ₃ ) ₃ Ln and (C ₅ Me ₄ H) ₃ Ln complexes

   https://doi.org/10.1039/D1DT02427B  

   Trinh, Michael T. ; Wedal, Justin C. ; Evans, William J. ( October 2021 , Dalton Transactions)

   The reduction potentials (reported vs. Fc + /Fc) for a series of Cp′ 3 Ln complexes (Cp′ = C 5 H 4 SiMe 3 , Ln = lanthanide) were determined via electrochemistry in THF with [ n Bu 4 N][BPh 4 ] as the supporting electrolyte. The Ln( iii )/Ln( ii ) reduction potentials for Ln = Eu, Yb, Sm, and Tm (−1.07 to −2.83 V) follow the expected trend for stability of 4f 7 , 4f 14 , 4f 6 , and 4f 13 Ln( ii ) ions, respectively. The reduction potentials for Ln = Pr, Nd, Gd, Tb,more »Dy, Ho, Er, and Lu, that form 4f n 5d 1 Ln( ii ) ions ( n = 2–14), fall in a narrow range of −2.95 V to −3.14 V. Only cathodic events were observed for La and Ce at −3.36 V and −3.43 V, respectively. The reduction potentials of the Ln( ii ) compounds [K(2.2.2-cryptand)][Cp′ 3 Ln] (Ln = Pr, Sm, Eu) match those of the Cp′ 3 Ln complexes. The reduction potentials of nine (C 5 Me 4 H) 3 Ln complexes were also studied and found to be 0.05–0.24 V more negative than those of the Cp′ 3 Ln compounds.« less
4. Synthesis and crystallographic characterization of diphenylamide rare-earth metal complexes Ln (NPh ₂ ) ₃ (THF) ₂ and [(Ph ₂ N) ₂ Ln (μ-NPh ₂ )] ₂

   https://doi.org/10.1107/S2056989020009998  

   Palumbo, Chad T. ; Kotyk, Christopher M. ; Ziller, Joseph W. ; Evans, William J. ( September 2020 , Acta Crystallographica Section E Crystallographic Communications)

   Studies of the coordination chemistry between the diphenylamide ligand, NPh 2 , and the smaller rare-earth Ln III ions, Ln = Y, Dy, and Er, led to the structural characterization by single-crystal X-ray diffraction crystallography of both solvated and unsolvated complexes, namely, tris(diphenylamido-κ N )bis(tetrahydrofuran-κ O )yttrium(III), Y(NPh 2 ) 3 (THF) 2 or [Y(C 12 H 10 N) 3 (C 4 H 8 O) 2 ], 1-Y , and the erbium(III) (Er), 1-Er , analogue, and bis[μ-1κ N :2(η 6 )-diphenylamido]bis[bis(diphenylamido-κ N )yttrium(III)], [(Ph 2 N) 2 Y(μ-NPh 2 )] 2 or [Y 2 (C 12 H 10 N)more »6 ], 2-Y , and the dysprosium(III) (Dy), 2-Dy , analogue. The THF ligands of 1-Er are modeled with disorder across two positions with occupancies of 0.627 (12):0.323 (12) and 0.633 (7):0.367 (7). Also structurally characterized was the tetrametallic Er III bridging oxide hydrolysis product, bis(μ-diphenylamido-κ 2 N : N )bis[μ-1κ N :2(η 6 )-diphenylamido]tetrakis(diphenylamido-κ N )di-μ 3 -oxido-tetraerbium(III) benzene disolvate, {[(Ph 2 N)Er(μ-NPh 2 )] 4 (μ-O) 2 }·(C 6 H 6 ) 2 or [Er 4 (C 12 H 10 N) 8 O 2 ]·2C 6 H 6 , 3-Er . The 3-Er structure was refined as a three-component twin with occupancies 0.7375:0.2010:0.0615.« less
5. Cooperative C–H activation of pyridine by PBP complexes of Rh and Ir can lead to bridging 2-pyridyls with different connectivity to the B–M unit

   https://doi.org/10.1039/D1SC01850G  

   Cao, Yihan ; Shih, Wei-Chun ; Bhuvanesh, Nattamai ; Zhou, Jia ; Ozerov, Oleg V. ( November 2021 , Chemical Science)

   Pyridine and quinoline undergo selective C–H activation in the 2-position with Rh and Ir complexes of a boryl/bis(phosphine) PBP pincer ligand, resulting in a 2-pyridyl bridging the transition metal and the boron center. Examination of this reactivity with Rh and Ir complexes carrying different non-pincer ligands on the transition metal led to the realization of the possible isomerism derived from the 2-pyridyl fragment connecting either via B–N/C–M bonds or via B–C/N–M bonds. This M–C/M–N isomerism was systematically examined for four structural types. Each of these types has a defined set of ligands on Rh/Ir besides 2-pyridyl and PBP. A pairmore »of M–C/M–N isomers for each type was computationally examined for Rh and for Ir, totaling 16 compounds. Several of these compounds were isolated or observed in solution by experimental methods, in addition to a few 2-quinolyl variants. The DFT predictions concerning the thermodynamic preference within each M–C/M–N isomeric match the experimental findings very well. In two cases where DFT predicts <2 kcal mol −1 difference in free energy, both isomers were experimentally observed in solution. Analysis of the structural data, of the relevant Wiberg bond indices, and of the ETS-NOCV partitioning of the interaction of the 2-pyridyl fragment with the rest of the molecule points to the strength of the M–C(pyridyl) bond as the dominant parameter determining the relative M–C/M–N isomer favorability. This M–C bond is always stronger for the analogous Ir vs. Rh compounds, but the nature of the ligand trans to it has a significant influence, as well. DFT calculations were used to evaluate the mechanism of isomerization for one of the molecule types.« less