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In nature, oxidizing metalloenzymes usually bind O 2 in the form of peroxo intermediates that frequently requires for activation a further protonation, for which the formation of hydroperoxo species is proposed. In dinuclear metalloenzymes, which typically feature Fe, Mn, and Cu, structures and electrophilic reactivity of peroxo, superoxo, and hydroperoxo intermediates are generally difficult to study due to their reactive and transient nature, which is why more stable peroxo Co III Co III complexes might be helpful models. Here, we present the molecular structures of a series of μ-1,2-peroxo, μ-1,2-superoxo, and μ-1,1-hydroperoxo Co III Co III complexes and the variation of their electrophilic hydrogen-atom-transfer (HAT) and oxygen-atom-transfer (OAT) reactivity. The μ-1,2-peroxo complex exhibits no electrophilic reactivity, whereas oxidation to the μ-1,2-superoxo complex activates electrophilic HAT reactivity, while protonation to the μ-1,1-hydroperoxo complex activates electrophilic OAT reactivity. The latter occurs by an associated mechanism with the μ-1,1-hydroperoxo ligand being the electrophilic OAT agent. Protonation to the μ-1,1-hydroperoxo elongates the O-O bond, while oxidation to the μ-1,2-superoxo shortens it relative to the μ-1,2-peroxo. The protonation of the μ-1,2-peroxo complex could be made accessible by the introduction of remote electron-donating substituents into a recently reported μ-1,2-peroxo Co III Co III complex that increases the μ-1,2-peroxo basicity by more than five p K a units. Hence, the initially counterintuitive increase of electron donation by the ligand environment increases the μ-1,2-peroxo basicity allowing its protonation to a more electrophilic μ-1,1-hydroperoxo ligand. More generally, HAT and OAT reactivity is correlated with accessibility of the following intermediate.

The singlet states of cyclopentadienyl (Cp) cations are considered as true prototypes of an antiaromatic system. Unfortunately, their high intrinsic reactivity inhibited their isolation in the solid state as a salt, and controlled reactions are also scarce. Here we present the synthesis and solid state structure of the room-temperature-stable Cp cation salt [Cp(C 6 F 5 ) 5 ] + [Sb 3 F 16 ] - . Although the aromatic triplet state of the [Cp(C 6 F 5 ) 5 ] + cation is energetically favoured in the gas phase according to quantum chemical calculations, coordination of the cation by either [Sb 3 F 16 ] - or C 6 F 6 in the crystal lattice stabilizes the antiaromatic singlet state, which is present in the solid state. The calculated hydride and fluoride ion affinities of the [Cp(C 6 F 5 ) 5 ] + cation are higher than those of the perfluorinated tritylium cation [C(C 6 F 5 ) 3 ] + . Reactions of [Cp(C 6 F 5 ) 5 ] + [Sb 3 F 16 ] - with CO, which probably yields the corresponding carbonyl complex, and of radical Cp(C 6 F 5 ) 5 ∙ with selected model substrates (Cp 2 Fe, (Ph 3 C∙) 2 and Cp*Al) are also presented., (© 2024. The Author(s), under exclusive licence to Springer Nature Limited.)

In nature, nonheme iron enzymes use dioxygen to generate high-spin iron(IV)=O species for a variety of oxygenation reactions. Although synthetic chemists have long sought to mimic this reactivity, the enzyme-like activation of O 2 to form high-spin iron(IV) = O species remains an unrealized goal. Here, we report a metal-organic framework featuring iron(II) sites with a local structure similar to that in α-ketoglutarate-dependent dioxygenases. The framework reacts with O 2 at low temperatures to form high-spin iron(IV) = O species that are characterized using in situ diffuse reflectance infrared Fourier transform, in situ and variable-field Mössbauer, Fe Kβ x-ray emission, and nuclear resonance vibrational spectroscopies. In the presence of O 2 , the framework is competent for catalytic oxygenation of cyclohexane and the stoichiometric conversion of ethane to ethanol.

Peroxo complexes are key intermediates in water oxidation catalysis (WOC). Cobalt plays an important role in WOC, either as oxides CoO x or as {Co III (μ - 1,2-peroxo)Co III } complexes, which are the oldest peroxo complexes known. The oxidation of {Co III (μ - 1,2-peroxo)Co III } complexes had usually been described to form {Co III (μ - 1,2-superoxo)Co III } complexes; however, recently the formation of {Co IV (μ - 1,2-peroxo)Co III } species were suggested. Using a bis(tetradentate) dinucleating ligand, we present here the synthesis and characterization of {Co III (μ - 1,2-peroxo)(μ-OH)Co III } and {Co III (μ-OH) 2 Co III } complexes. Oxidation of {Co III (μ - 1,2-peroxo)(μ-OH)Co III } at -40 °C in CH 3 CN provides the stable {Co III (μ - 1,2-superoxo)(μ-OH)Co III } species and activates electrophilic reactivity. Moreover, {Co III (μ - 1,2-peroxo)(μ-OH)Co III } catalyzes water oxidation, not molecularly but rather via CoO x films. While {Co III (μ - 1,2-peroxo)(μ-OH)Co III } can be reversibly deprotonated with DBU at -40 °C in CH 3 CN, {Co III (μ - 1,2-superoxo)(μ-OH)Co III } undergoes irreversible conversions upon reaction with bases to a new intermediate that is also the decay product of {Co III (μ - 1,2-superoxo)(μ-OH)Co III } in aqueous solution at pH > 2. Based on a combination of experimental methods, the new intermediate is proposed to have a {Co II (μ-OH)Co III } core formed by the release of O 2 from {Co III (μ - 1,2-superoxo)(μ-OH)Co III } confirmed by a 100% yield of O 2 upon photocatalytic oxidation of {Co III (μ - 1,2-peroxo)(μ-OH)Co III }. This release of O 2 by oxidation of a peroxo intermediate corresponds to the last step in molecular WOC.

The observation of single-molecule magnetism in transition-metal complexes relies on the phenomenon of zero-field splitting (ZFS), which arises from the interplay of spin-orbit coupling (SOC) with ligand-field-induced symmetry lowering. Previous studies have demonstrated that the magnitude of ZFS in complexes with 3d metal ions is sometimes enhanced through coordination with heavy halide ligands (Br and I) that possess large free-atom SOC constants. In this study, we systematically probe this "heavy-atom effect" in high-spin cobalt(II)-halide complexes supported by substituted hydrotris(pyrazol-1-yl)borate ligands (Tp tBu,Me and Tp Ph,Me ). Two series of complexes were prepared: [Co II X(Tp tBu,Me )] ( 1-X ; X = F, Cl, Br, and I) and [Co II X(Tp Ph,Me )(Hpz Ph,Me )] ( 2-X ; X = Cl, Br, and I), where Hpz Ph,Me is a monodentate pyrazole ligand. Examination with dc magnetometry, high-frequency and -field electron paramagnetic resonance, and far-infrared magnetic spectroscopy yielded axial ( D ) and rhombic ( E ) ZFS parameters for each complex. With the exception of 1-F , complexes in the four-coordinate 1-X series exhibit positive D -values between 10 and 13 cm -1 , with no dependence on halide size. The five-coordinate 2-X series exhibit large and negative D -values between -60 and -90 cm -1 . Interpretation of the magnetic parameters with the aid of ligand-field theory and ab initio calculations elucidated the roles of molecular geometry, ligand-field effects, and metal-ligand covalency in controlling the magnitude of ZFS in cobalt-halide complexes.

Synthetic routes to the crystallization of two new box-like complexes, [Au 6 (Triphos) 4 (CuBr 2 )](OTf) 5 ·(CH 2 Cl 2 ) 3 ·(CH 3 OH) 3 ·(H 2 O) 4 ( 1 ) and [Au 6 (Triphos) 4 (CuCl 2 )](PF 6 ) 5 ·(CH 2 Cl 2 ) 4 ( 2 ) (triphos = bis(2-diphenylphosphinoethyl)phenylphosphine), have been developed. The two centrosymmetric cationic complexes have been structurally characterized through single-crystal X-ray diffraction and shown to contain a CuX 2 - (X = Br or Cl) unit suspended between two Au(I) centers without the involvement of bridging ligands. These colorless crystals display green luminescence (λ em = 527 nm) for ( 1 ) and teal luminescence (λ em = 464 nm) for ( 2 ). Computational results document the metallophilic interactions that are involved in positioning the Cu(I) center between the two Au(I) ions and in the luminescence.

Herein, we report the synthesis, isolation, and characterization of two cationic organobismuth(II) compounds bearing N,C,N pincer frameworks, which model crucial intermediates in bismuth radical processes. X-ray crystallography uncovered a monomeric Bi(II) structure, while SQUID magnetometry in combination with NMR and EPR spectroscopy provides evidence for a paramagnetic S = 1/2 state. High-resolution multifrequency EPR at the X-, Q-, and W-band enable the precise assignment of the full g - and 209 Bi A -tensors. Experimental data and DFT calculations reveal both complexes are metal-centered radicals with little delocalization onto the ligands.

Reaction of Co(II) nitrate with 2-methylimidazole (2mIm) yields ZIF-67, the structure of which features Co(II) ions in pseudo-tetrahedral coordination geometry. Strong antiferromagnetic interactions between Co(II) ions mediated by the 2mIm ligands lead to antiferromagnetic ordering at 22 K. Postsynthetic treatment of Co(II) ZIF-67 with 5-methyltetrazole (5mT) results in the loss of crystallinity and magnetic order. The local structure of the Co(II) ions was probed by a combination of diffuse-reflectance electronic absorption spectroscopy and Co K-edge X-ray absorption spectroscopy (in the XANES and EXAFS regions). Upon reaction with 5mT, the 4 A 2 (F)- 4 T 1 (F) and 4 A 2 (F)- 4 T 1 (P) transitions at 1140 and 585 nm, respectively, of the pseudo-tetrahedral Co(II) center in ZIF-67 become less prominent and are replaced by transitions at 990 and 475 nm attributable to the 4 T 1g (F)- 4 T 2g (F) and 4 T 1g (F)- 4 T 1g (P) transitions of a pseudo-octahedral Co(II) center, respectively. Furthermore, the 1s-3d pre-edge absorption feature in the Co K-edge XANES spectrum loses intensity during this reaction, and the edge feature becomes more sharp, consistent with a change from pseudo-T d to pseudo-O h geometry. EXAFS analysis further supports the proposed change in geometry: EXAFS data for ZIF-67 are well fitted to four Co-N scatterers at 1.99 Å, whereas the data for the 5mT-substituted compound are best fitted with 6 Co-N scatterers at 2.14 Å. Our results support the conclusion that a six-coordinate, pseudo-O h geometry is adopted upon ligand substitution. The increase in coordination number directly increases the Co-N bond distances, which in turn weakens magnetic exchange interactions. No magnetic ordering is found in the 5mT-substituted materials.

Atom-efficient syntheses of the tetraethylammonium Roussin black sulfur and selenium salts ((Et 4 N)[Fe 4 E 3 (NO) 7 ], E = S, Se) as well as their 15 N-labeled counterparts are described herein. Broken-symmetry DFT calculations were conducted on both complexes to model an antiferromagnetic interaction between the apical {FeNO} 7 unit, S ap = 3/2, and the three basal {Fe(NO) 2 } 9 units, S bas = 1/2. The calculated J values are -1813 and -1467 cm -1 for the sulfur and selenium compounds, respectively. The mechanism for antiferromagnetic exchange in both compounds was deduced to be direct exchange on the basis of the partially overlapping magnetic orbitals with orbital density only residing on the Fe-centers. The obtained Mössbauer parameters are most consistent with the calculated M S = 0 broken-symmetry state for both complexes. The values for J have been determined with variable-temperature 15 N NMR experiments. Values of -1660 and -1430 cm -1 for the sulfur and selenium compounds, respectively, were obtained by fits to the variable-temperature NMR data, further validating the broken-symmetry M S = 0 model of the electronic structure.

Iron-based extended metal atom chains (EMACs) are potentially high-spin molecules with axial magnetic anisotropy and thus candidate single-molecule magnets (SMMs). We herein compare the tetrairon(ii), halide-capped complexes [Fe4(tpda)3Cl2] (1Cl) and [Fe4(tpda)3Br2] (1Br), obtained by reacting iron(ii) dihalides with [Fe2(Mes)4] and N2,N6-di(pyridin-2-yl)pyridine-2,6-diamine (H2tpda) in toluene, under strictly anhydrous and anaerobic conditions (HMes = mesitylene). Detailed structural, electrochemical and Mössbauer data are presented along with direct-current (DC) and alternating-current (AC) magnetic characterizations. DC measurements revealed similar static magnetic properties for the two derivatives, with χMT at room temperature above that for independent spin carriers, but much lower at low temperature. The electronic structure of the iron(ii) ions in each derivative was explored by ab initio (CASSCF-NEVPT2-SO) calculations, which showed that the main magnetic axis of all metals is directed close to the axis of the chain. The outer metals, Fe1 and Fe4, have an easy-axis magnetic anisotropy (D = -11 to -19 cm-1, |E/D| = 0.05-0.18), while the internal metals, Fe2 and Fe3, possess weaker hard-axis anisotropy (D = 8-10 cm-1, |E/D| = 0.06-0.21). These single-ion parameters were held constant in the fitting of DC magnetic data, which revealed ferromagnetic Fe1-Fe2 and Fe3-Fe4 interactions and antiferromagnetic Fe2-Fe3 coupling. The competition between super-exchange interactions and the large, noncollinear anisotropies at metal sites results in a weakly magnetic non-Kramers doublet ground state. This explains the SMM behavior displayed by both derivatives in the AC susceptibility data, with slow magnetic relaxation in 1Br being observable even in zero static field.

Coordination complexes that possess large magnetic anisotropy (otherwise known as zero-field splitting, ZFS) have possible applications in the field of magnetic materials, including single molecule magnets (SMMs). Previous studies have explored the role of coordination number and geometry in controlling the magnetic anisotropy and SMM behavior of high-spin ( S = 3/2) Co(II) complexes. Building upon these efforts, the present work examines the impact of ligand oxidation state and structural distortions on the spin states and ZFS parameters of pentacoordinate Co(II) complexes. The five complexes included in this study ( 1 - 5 ) have the general formula, [Co(Tp Ph2 )( L X,Y )] n + (X = O, S; Y = N, O; n = 0 or 1), where Tp Ph2 is the scorpionate ligand hydrotris(3,5-diphenyl-pyrazolyl)borate(1-) and L X,Y are bidentate dioxolene-type ligands that can access multiple oxidation states. The specific L X,Y ligands used herein are 4,6-di- tert -butyl substituted o -aminophenolate and o -aminothiophenolate ( 1 and 2 , respectively), o -iminosemiquinonate and o -semiquinonate radicals ( 3 and 4 , respectively), and o -iminobenzoquinone ( 5 ). Each complex exhibits a distorted trigonal bipyramidal geometry, as revealed by single-crystal X-ray diffraction. Direct current (dc) magnetic susceptibility experiments confirmed that the complexes with closed-shell ligands ( 1 , 2 , and 5 ) possess S = 3/2 ground states with negative D -values (easy-axis anisotropy) of -41, -78, and -30 cm -1 , respectively. For 3 and 4 , antiferromagnetic coupling between the Co(II) center and o -(imino)semiquinonate radical ligand results in S = 1 ground states that likewise exhibit very large and negative anisotropy (-100 > D > -140 cm -1 ). Notably, ZFS was measured directly for each complex using far-infrared magnetic spectroscopy (FIRMS). In combination with high-frequency and -field electron paramagnetic resonance (HFEPR) studies, these techniques provided precise spin-Hamiltonian parameters for complexes 1 , 2 , and 5 . Multireference ab initio calculations, using the CASSCF/NEVPT2 approach, indicate that the strongly negative anisotropies of these Co(II) complexes arise primarily from distortions in the equatorial plane due to constrictions imposed by the Tp Ph2 ligand. This effect is further amplified by cobalt(II)-radical exchange interactions in 3 and 4 .

This work represents an important step in the quest to make hetero multi metallic molecules featuring specific metal types and complicated metal ratios. The rational design, synthesis, and characterization of a complex hetero tri metallic single-source molecular precursor for the next generation sodium-ion battery cathode material, Na 2 Mn 2 FeO 6 , is described. A unique pentametallic platform [Mn II (ptac) 3 -Na-Mn III (acac) 3 -Na-Mn II (ptac) 3 ] ( 1 ) was derived from the known polymeric structure of [NaMn II (acac) 3 ] ∞ , through a series of elaborate design procedures, such as mixed-ligand, unsymmetric ligand, and mixed-valent approaches. Importantly, the application of those techniques results in a molecule with distinctively different transition metal positions in terms of ligand environment and oxidation states. An isovalent substitution of Fe III for the central Mn III ion forms the target hetero tri metallic precursor [Mn II (ptac) 3 -Na-Fe III (acac) 3 -Na-Mn II (ptac) 3 ] ( 3 ) with an appropriate metal ratio of Na:Mn:Fe = 2:2:1. The arrangement of metal ions and ligands in this pentametallic assembly was confirmed by single crystal X-ray investigation. The unambiguous assignment of the positions and oxidation states of the Periodic Table neighbors Fe and Mn in 3 has been achieved by a combination of investigative techniques that include synchrotron resonant diffraction, X-ray multiwavelength anomalous diffraction, X-ray fluorescence spectroscopy, Mössbauer spectroscopy, and gas-phase DART mass spectrometry. The hetero tri metallic single-source precursor 3 was shown to exhibit a clean decomposition pattern yielding the phase-pure P2-Na 2 Mn 2 FeO 6 quaternary oxide with high uniformity of metal ion distribution as confirmed by electron microscopy.

The bis(2-pyridylthio)methanidopalladium(II) pincer complex (1), containing a Pd-C bond, was obtained from the reaction of bis(2-pyridylthio)methane (H 2 L) with palladium(II) acetate in toluene under reflux. When palladium(II) trifluoroacetate was used, H 2 L reacted to generate the tetrakis(pyridine-2-thiol)palladium(II) complex (2). Complex 2 was converted to a heterobimetallic palladium(II)-iron(II) paddlewheel complex (3) upon treatment with iron(II) triflate in the presence of a base in acetonitrile at room temperature.

Zeolitic imidazolate frameworks (ZIFs) with open-shell transition metal nodes represent a promising class of highly ordered light harvesting antennas for photoenergy applications. However, their charge transport properties within the framework, the key criterion to achieve efficient photoenergy conversion, are not yet explored. Herein, we report the first direct evidence of a charge transport pathway through node-to-node communication in both ground state and excited state ZIFs using the combination of paramagnetic susceptibility measurements and time-resolved optical and X-ray absorption spectroscopy. These findings provide unprecedented new insights into the photoactivity and charge transport nature of ZIF frameworks, paving the way for their novel application as light harvesting arrays in diverse photoenergy conversion devices.