Metal-Halide Covalency, Exchange Coupling, and Slow Magnetic Relaxation in Triangular (CpiPr5)3U3X6 (X = Cl, Br, I) Clusters.

The actinide elements are attractive alternatives to transition metals or lanthanides for the design of exchange-coupled multinuclear single-molecule magnets. However, the synthesis of such compounds is challenging, as is unraveling any contributions from exchange coupling to the overall magnetism. To date, only a few actinide compounds have been shown to exhibit exchange coupling and single-molecule magnetism. Here, we report triangular uranium(III) clusters of the type (CpiPr5)3U3X (1-X; X = Cl, Br, I; CpiPr5 = pentaisopropylcyclopentadienyl), which are synthesized via reaction of the aryloxide-bridged precursor (CpiPr5)2U2(OPhtBu)4 with excess Me3SiX. Spectroscopic analysis suggests the presence of covalency in the uranium-halide interactions arising from 5f orbital participation in bonding. The dc magnetic susceptibility data reveal the presence of antiferromagnetic exchange coupling between the uranium(III) centers in these compounds, with the strength of the exchange decreasing down the halide series. Ac magnetic susceptibility data further reveal all compounds to exhibit slow magnetic relaxation under zero dc field. In 1-I, which exhibits particularly weak exchange, magnetic relaxation occurs via a Raman mechanism associated with the individual uranium(III) centers. In contrast, for 1-Br and 1-Cl, magnetic relaxation occurs via an Orbach mechanism, likely involving relaxation between ground and excited exchange-coupled states. Significantly, in the case of 1-Cl, magnetic relaxation is sufficiently slow such that open magnetic hysteresis is observed up to 2.75 K, and the compound exhibits a 100-s blocking temperature of 2.4 K. This compound provides the first example of magnetic blocking in a compound containing only actinide-based ions, as well as the first example involving the uranium(III) oxidation state.

Coercive Fields Exceeding 30 T in the Mixed-Valence Single-Molecule Magnet (CpiPr5)2Ho2I3.

Mixed-valence dilanthanide complexes of the type (CpiPr5)2Ln2I3 (CpiPr5 = pentaisopropylcyclopentadienyl; Ln = Gd, Tb, Dy) featuring a direct Ln-Ln σ-bonding interaction have been shown to exhibit well-isolated high-spin ground states and, in the case of the Tb and Dy variants, a strong axial magnetic anisotropy that gives rise to a large magnetic coercivity. Here, we report the synthesis and characterization of two new mixed-valence dilanthanide compounds in this series, (CpiPr5)2Ln2I3 (1-Ln; Ln = Ho, Er). Both compounds feature a Ln-Ln bonding interaction, the first such interaction in any molecular compounds of Ho or Er. Like the Tb and Dy congeners, both complexes exhibit high-spin ground states arising from strong spin-spin coupling between the lanthanide 4f electrons and a single σ-type lanthanide-lanthanide bonding electron. Beyond these similarities, however, the magnetic properties of the two compounds diverge. In particular, 1-Er does not exhibit observable magnetic blocking or slow magnetic relaxation, while 1-Ho exhibits magnetic blocking below 28 K, which is the highest temperature among Ho-based single-molecule magnets, and a spin reversal barrier of 556(4) cm-1. Additionally, variable-field magnetization data collected for 1-Ho reveal a coercive field of greater than 32 T below 8 K, more than 6-fold higher than observed for the bulk magnets SmCo5 and Nd2Fe14B, and the highest coercive field reported to date for any single-molecule magnet or molecule-based magnetic material. Multiconfigurational calculations, supported by far-infrared magnetospectroscopy data, reveal that the stark differences in magnetic properties of 1-Ho and 1-Er arise from differences in the local magnetic anisotropy of the lanthanide centers.

Selective Adsorption of Oxygen from Humid Air in a Metal-Organic Framework with Trigonal Pyramidal Copper(I) Sites.

High or enriched-purity O2 is used in numerous industries and is predominantly produced from the cryogenic distillation of air, an extremely capital- and energy-intensive process. There is significant interest in the development of new approaches for O2-selective air separations, including the use of metal-organic frameworks featuring coordinatively unsaturated metal sites that can selectively bind O2 over N2 via electron transfer. However, most of these materials exhibit appreciable and/or reversible O2 uptake only at low temperatures, and their open metal sites are also potential strong binding sites for the water present in air. Here, we study the framework CuI-MFU-4l (CuxZn5-xCl4-x(btdd)3; H2btdd = bis(1H-1,2,3-triazolo[4,5-b],[4',5'-i])dibenzo[1,4]dioxin), which binds O2 reversibly at ambient temperature. We develop an optimized synthesis for the material to access a high density of trigonal pyramidal CuI sites, and we show that this material reversibly captures O2 from air at 25 °C, even in the presence of water. When exposed to air up to 100% relative humidity, CuI-MFU-4l retains a constant O2 capacity over the course of repeated cycling under dynamic breakthrough conditions. While this material simultaneously adsorbs N2, differences in O2 and N2 desorption kinetics allow for the isolation of high-purity O2 (>99%) under relatively mild regeneration conditions. Spectroscopic, magnetic, and computational analyses reveal that O2 binds to the copper(I) sites to form copper(II)-superoxide moieties that exhibit temperature-dependent side-on and end-on binding modes. Overall, these results suggest that CuI-MFU-4l is a promising material for the separation of O2 from ambient air, even without dehumidification.

A Trinuclear Gadolinium Cluster with a Three-Center One-Electron Bond and an S = 11 Ground State.

The recent discovery of metal-metal bonding and valence delocalization in the dilanthanide complexes (CpiPr5)2Ln2I3 (CpiPr5 = pentaisopropylcyclopentadienyl; Ln = Y, Gd, Tb, Dy) opened up the prospect of harnessing the 4fn5dz21 electron configurations of non-traditional divalent lanthanide ions to access molecules with novel bonding motifs and magnetism. Here, we report the trinuclear mixed-valence clusters (CpiPr5)3Ln3H3I2 (1-Ln, Ln = Y, Gd), which were synthesized via potassium graphite reduction of the trivalent clusters (CpiPr5)3Ln3H3I3. Structural, computational, and spectroscopic analyses support valence delocalization in 1-Ln resulting from a three-center, one-electron σ bond formed from the 4dz2 and 5dz2 orbitals on Y and Gd, respectively. Dc magnetic susceptibility data obtained for 1-Gd reveal that valence delocalization engenders strong parallel alignment of the σ-bonding electron and the 4f electrons of each gadolinium center to afford a high-spin ground state of S = 11. Notably, this represents the first clear instance of metal-metal bonding in a molecular trilanthanide complex, and the large spin-spin exchange constant of J = 168(1) cm-1 determined for 1-Gd is only the second largest coupling constant characterized to date for a molecular lanthanide compound.

Divalent Lanthanide Metallocene Complexes with a Linear Coordination Geometry and Pronounced 6s-5d Orbital Mixing.

A small but growing number of molecular compounds have been isolated featuring divalent lanthanides with 4fn5dz21 electron configurations. While the majority of these possess trigonal coordination geometries, we previously reported the first examples of linear divalent metallocenes Ln(CpiPr5)2 (Ln = Tb, Dy; CpiPr5 = pentaisopropylcyclopentadienyl). Here, we report the synthesis and characterization of the remainder of the Ln(CpiPr5)2 (1-Ln) series (including Y and excluding Pm). The compounds can be synthesized through salt metathesis of LnI3 and NaCpiPr5 followed by potassium graphite reduction for Ln = Y, La, Ce, Pr, Nd, Gd, Ho, and Er, by in situ reduction during salt metathesis of LnI3 and NaCpiPr5 for Ln = Tm and Lu, or through salt metathesis from LnI2 and NaCpiPr5 for Ln = Sm, Eu, and Yb. Single crystal X-ray diffraction analyses of 1-Ln confirm a linear coordination geometry with pseudo-D5d symmetry for the entire series. Structural and ultraviolet-visible spectroscopy data support a 4fn+1 electron configuration for Ln2+ = Sm, Eu, Tm, and Yb and a 4fn5dz21 configuration for the other lanthanides ([Kr]4dz21 for Y2+). Characterization of 1-Ln (Ln = Y, La) using electron paramagnetic resonance spectroscopy reveals significant s-d orbital mixing in the highest occupied molecular orbital and hyperfine coupling constants that are the largest reported to date for divalent compounds of yttrium and lanthanum. Evaluation of the room temperature magnetic susceptibilities of 1-Ln and comparison with values previously reported for trigonal Ln2+ compounds suggests that the more pronounced 6s-5d mixing may be associated with weaker 4f-5d spin coupling.

Programmable Synthesis of Silver Wheels.

The synthesis of sandwich-shaped multinuclear silver complexes with planar penta- and tetranuclear wheel-shaped silver units and a central anion, [Agn(2-HPB)2(A-)](OTf-)n-1, nAgA, n = 4 or 5 and A- = OH- or F- or Cl-, is reported, along with complete spectroscopic and structural characterization. An NMR mechanistic study reveals that silver complexes were formed in the following order: 2Ag → 3AgH2O → 5AgOH → 4AgOH. The central hydroxides in 4AgOH and 5AgOH exhibit exotic physical properties due to the confined environment inside the complex. The size of these silver wheels can be tuned by changing the central anion or extracting/adding one silver atom. This study provides the facile way to synthesize discrete wheel-shaped multinuclear silver complexes and provides valuable insights into the dynamics of the self-assembly process.

Effect of fermented spent instant coffee grounds on milk productivity and blood profiles of lactating dairy cows.

OBJECTIVE: This study was conducted to evaluate the fermentation characteristics under low mesophilic temperature of spent instant coffee ground (SICG) and to estimate the effect of fermented SICG (FSICG) as alternative feed ingredient on milk productivity of dairy cows. METHODS: In the fermentation trial, fermentation of SICG was performed to investigate changes in characteristics using the microbial mixture (Lactobacillus plantarum, Saccharomyces cerevisiae, and Bacillus subtilis = 1:1:1) for 21 days at 20°C under anaerobic conditions. Molasses was added at 5% of dry mass. In the animal trial, eighteen Holstein Friesian cows were used to evaluate the nutritive value of the FSICG which was fermented for 14 days under the same condition as the fermentation trial. RESULTS: In the fermentation trial, the dry matter (DM) and organic matter content linearly decreased with fermentation time (p<0.001 and p = 0.008, respectively). The acid detergent insoluble nitrogen content linearly decreased with fermentation time (p = 0.037). The microorganism counts linearly increased for Lactobacillus plantarum, Saccharomyces cerevisiae, and Bacillus subtilis across fermentation time (p<0.001). In the animal trial, the DM intake of the control and FSICG treatment were not significantly different, as were milk yield, 4% fat corrected milk, fat-protein corrected milk, and feed to milk conversion content. Fat, protein, lactose, non-fat solids, milk urea nitrogen, and somatic cell counts were also not significantly different in milk composition between treatments. CONCLUSION: FSICG should be considered a sufficient substitute for cottonseed as a feed component, and 5% DM of a dietary FSICG level was appropriate for dairy cow diets.

Synthesis and Structural Characterization of Macrocyclic α-ABpeptoids and Their DNA-Encoded Library.

The first synthesis of macrocyclic α-ABpeptoids with varying lengths is described. X-ray crystal structures reveal that cyclic trimer displays a chair-like conformation with a cct amide sequence and cyclic tetramer has a saddle-like structure with an uncommon cccc amide arrangement. The creation of a DNA-encoded combinatorial library of macrocyclic α-ABpeptoids is described.

Reactive high-spin iron(IV)-oxo sites through dioxygen activation in a metal-organic framework.

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 O2 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 O2 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 Kß x-ray emission, and nuclear resonance vibrational spectroscopies. In the presence of O2, the framework is competent for catalytic oxygenation of cyclohexane and the stoichiometric conversion of ethane to ethanol.

Dinitrogen activation by a penta-pyridyl molybdenum complex.

A new dinitrogen (N2) molybdenum(0) complex supported exclusively by pyridine ligands was synthesized. The X-ray crystal structure of the complex elucidated the activated nature of the N2 ligand, consistent with a low N-N IR stretching frequency. Natural bond orbital (NBO) analyses on this system confirmed a strong π-backdonation arising from the large p orbital character in molybdenum lone pairs. The protonation of the N2 ligand using decamethyl chromocene (CrCp*2) in the presence of lutidinium salt afforded 1.22 equivalents of ammonia (NH3).

Facile Synthesis of α-Boryl-Substituted Allylboronate Esters Using Stable Bis[(pinacolato)boryl]methylzinc Reagents.

Reported herein is the utilization of bis[(pinacolato)boryl]methylzinc halides, whose structures are characterized via single-crystal X-ray analysis, as solid storable reagents for copper-catalyzed coupling with vinyliodonum salts. The reaction proceeds under mild conditions and shows broad scope with respect to vinyliodonium salts, affording various α-boryl-substituted allylboronate esters in good yields. Synthetic applications of the obtained products are also demonstrated.

Fluxional motion in a dinuclear copper(i) complex with a propeller-type ligand: metal hopping on both sides.

The first study of fluxional motion in [Cu2(2-HPB)(MeCN)2Cl2] (1,2-HPB = hexa(2-pyridyl)benzene) is presented. For detailed examination of the fluxional motion mechanism, a monofluorinated derivative of the ligand (MFHPB) and its copper(i) complexes were synthesized and characterized. The solution NMR studies of monofluoro copper(i) complex 1a suggest the existence of five species in equilibrium. NMR spectra and DFT calculations suggest the fluxional motion of 1 resulting in the "metal hopping process" of two copper(i) ions.

Static and dynamic coordination behaviours of copper(i) ions in hexa(2-pyridyl)benzene ligand systems.

Two hexaarylbenzenes having six pyridine substituents (LH and LM) were prepared and six corresponding coordination complexes with copper(i) chloride were synthesized. Monomeric complexes (1, 2a and 2b) and 1D coordination polymers (3, 4 and 5) were synthesized via the judicious concentration control of copper(i) ions and characterized fully by X-ray crystal analysis. The binding modes of the ligands to copper(i) ions in the coordination polymers were dependent on steric effects and solvents to determine the morphology of the coordination polymers. More interestingly, dynamic behaviours of the monomeric complexes (1, 2a and 2b) in the solution phase were studied to show potential applications of molecular machines with fluxional motions.