Grid Expansion: a Rhombiclike [L₄Fe₂(Ag₂)₂] Complex Containing Ag₂ Dumbbells at Two Vertices

The pyrazolate-based ditopic ligand HL forms a strongly hydrogen-bonded corner complex dimer [Fe^II(HL)₂]₂(BF₄)₄ (<b>1</b>) with a [2 × 2] gridlike arrangement of four ligand strands. The two empty vertices can then be filled with {Ag₂}²⁺ dumbbells, yielding the unprecedented diferric complex [L₄Fe^III₂(Ag^I₂)₂]­(BF₄)₆ (<b>2</b>) that features a rhombiclike structure with an almost planar hexagon of metal ions

Mixed-Spin [2 × 2] Fe₄ Grid Complex Optimized for Quantum Cellular Automata

The new pyrazolate-bridged proligand 4-methyl-3,5-bis­{6-(2,2′-bipyridyl)}­pyrazole (^MeLH) has been synthesized. Similar to its congener that lacks the backbone methyl substituent (^HLH) it forms a robust Fe^II₄ grid complex, [^MeL₄Fe^II₄]­(BF₄)₄. The molecular structure of [^MeL₄Fe^II₄](BF₄)₄·2MeCN has been elucidated by X-ray diffraction, revealing two high-spin (HS) and two low-spin (LS) ferrous ions at opposite corners of the rhombic metal ion arrangement. SQUID and ⁵⁷Fe Mössbauer data for solid material showed that this [HS–LS–HS–LS] configuration persists over a wide temperature range, between 7 and 250 K, while spin-crossover sets in only above 250 K. According to Mössbauer spectroscopy a [1HS–3LS] configuration is present in solution at 80 K. Thus, the methyl substituent in [^MeL]⁻ leads to a stronger ligand field compared to parent [^HL]⁻ and hence to a higher LS fraction both in the solid state and in solution. Cyclic voltammetry of [^MeL₄Fe^II₄]­(BF₄)₄ reveals four sequential oxidations coming in two pairs with pronounced stability of the di-mixed-valence species [^MeL₄Fe^II₂Fe^III₂]⁶⁺ (<i>K</i>_C = 3.35 × 10⁸). The particular [HS–LS–HS–LS] configuration as well as the di-mixed-valence configuration, both with identical spin or redox states at diagonally opposed vertices of the grid, make this system attractive as a molecular component for quantum cellular automata

Spin-State Versatility in a Series of Fe₄ [2 × 2] Grid Complexes: Effects of Counteranions, Lattice Solvent, and Intramolecular Cooperativity

The new compartmental proligand 4-bromo-3,5-bis­{6-(2,2′-bipyridyl)}­pyrazole (HL^Br) was synthesized and shown to form robust [2 × 2] grid complexes [Fe^II₄L^Br₄]­X₄ with various counteranions (X^– = PF₆^–, ClO₄^–, BF₄^–, Br^–). The grid [Fe^II₄L^Br₄]⁴⁺ is stable in solution and features two high-spin (HS) and two low-spin (LS) ferrous ions in frozen MeCN, and its redox properties have been studied. Six all-ferrous compounds [Fe₄L^Br₄]­X₄ with different counteranions and different lattice solvent (<b>1a–f</b>) were structurally characterized by X-ray diffraction, and their magnetic properties were investigated by Mössbauer spectroscopy and SQUID magnetometry. Variations in spin-state for the crystalline material range from the [4HS] via the [3HS-1LS] to the [2HS-2LS] forms, with some grids showing thermal spin crossover (SCO). The series of [Fe^II₄L^Br₄]⁴⁺ compounds allowed us to establish experimentally well-grounded correlations between structural distortion of the {FeN₆} coordination polyhedra, quantified by using continuous shape measures, and the grid’s spin-state pattern. These correlations evidenced pronounced cooperativity for the multistep SCO transitions within the grid, imparted by the strain effects of the rigid bridging ligands, and a high stability of the dimixed-spin configuration <i>trans</i>-[2HS-2LS] that has identical sites at opposite corners of the grid. The results are in good agreement with recent quantum chemical calculations for such molecular [2 × 2] grids featuring strongly elastically coupled vertices

Mixed-Spin [2 × 2] Fe₄ Grid Complex Optimized for Quantum Cellular Automata

The new pyrazolate-bridged proligand 4-methyl-3,5-bis­{6-(2,2′-bipyridyl)}­pyrazole (^MeLH) has been synthesized. Similar to its congener that lacks the backbone methyl substituent (^HLH) it forms a robust Fe^II₄ grid complex, [^MeL₄Fe^II₄]­(BF₄)₄. The molecular structure of [^MeL₄Fe^II₄](BF₄)₄·2MeCN has been elucidated by X-ray diffraction, revealing two high-spin (HS) and two low-spin (LS) ferrous ions at opposite corners of the rhombic metal ion arrangement. SQUID and ⁵⁷Fe Mössbauer data for solid material showed that this [HS–LS–HS–LS] configuration persists over a wide temperature range, between 7 and 250 K, while spin-crossover sets in only above 250 K. According to Mössbauer spectroscopy a [1HS–3LS] configuration is present in solution at 80 K. Thus, the methyl substituent in [^MeL]⁻ leads to a stronger ligand field compared to parent [^HL]⁻ and hence to a higher LS fraction both in the solid state and in solution. Cyclic voltammetry of [^MeL₄Fe^II₄]­(BF₄)₄ reveals four sequential oxidations coming in two pairs with pronounced stability of the di-mixed-valence species [^MeL₄Fe^II₂Fe^III₂]⁶⁺ (<i>K</i>_C = 3.35 × 10⁸). The particular [HS–LS–HS–LS] configuration as well as the di-mixed-valence configuration, both with identical spin or redox states at diagonally opposed vertices of the grid, make this system attractive as a molecular component for quantum cellular automata

Heavy element production in a compact object merger observed by JWST

The mergers of binary compact objects such as neutron stars and black holes are of central interest to several areas of astrophysics, including as the progenitors of gamma-ray bursts (GRBs)1, sources of high-frequency gravitational waves (GW)2 and likely production sites for heavy element nucleosynthesis via rapid neutron capture (the r-process)3. Here we present observations of the exceptionally bright gamma-ray burst GRB 230307A. We show that GRB 230307A belongs to the class of long-duration gamma-ray bursts associated with compact object mergers4–6, and contains a kilonova similar to AT2017gfo, associated with the gravitational-wave merger GW1708177–12. We obtained James Webb Space Telescope mid-infrared (mid-IR) imaging and spectroscopy 29 and 61 days after the burst. The spectroscopy shows an emission line at 2.15 microns which we interpret as tellurium (atomic mass A=130), and a very red source, emitting most of its light in the mid-IR due to the production of lanthanides. These observations demonstrate that nucleosynthesis in GRBs can create r-process elements across a broad atomic mass range and play a central role in heavy element nucleosynthesis across the Universe.</p