Jeffrey R. Long

The synthesis of molecules exhibiting strong magnetic exchange coupling between contracted lanthanide 4f electrons and a radical spin presents an important challenge relevant to the design of single-molecule magnets and solid-state magnetic materials. Concurrently, the isolation of a planar benzene trianion radical remains an unrealized target. Here, we report the synthesis and characterization of the dilanthanide compounds [(CpiPr5Ln)2(μ-η6:η6-C6H6)][Fe(N(SiMe3)2)3] (Ln = Y, Gd, Tb, Dy, Ho, Er, Tm; CpiPr5 = pentaisopropylcyclopentadienyl), featuring cationic inverse sandwich complexes in which two Ln centers are bridged by a planar benzene trianion radical. These compounds are synthesized via one-electron oxidation of (CpiPr5Ln)2(μ-η6:η6-C6H6) using Fe(N(SiMe3)2)3 as a mild outer-sphere oxidant. Single-crystal X-ray diffraction reveals inverse sandwich structures featuring a planar benzene bridge and structural signatures of extended δ-bonding along the Cp–Ln–Bz–Ln–Cp axis. Continuous-wave electron paramagnetic resonance spectroscopy of the congener containing diamagnetic Y3+ supports assignment of an S = 1/2 benzene trianion radical. Magnetometry measurements on the Gd analogue reveal the presence of remarkably strong ferromagnetic coupling (J = +55.6(6) cm–1) between the S = 7/2 Gd3+ centers and the benzene radical, giving rise to a well-isolated S = 15/2 ground state. These observations are explained through a molecular orbital analysis, which indicates that the δ-symmetric (CpiPr5)− ligands lower the energy of the Ln3+ dδ (dxy and dx2–y2) orbitals. Importantly, this electronic structure facilitates enhanced covalency with the benzene trianion radical, leading to greater than 30% metal character in the radical-based singly occupied molecular orbital (SOMO). The results establish a new regime of radical-mediated lanthanide exchange coupling in which substantial ground-state metal character within the radical SOMO drives strong intraatomic-like ferromagnetic exchange akin to Hund’s first rule.

We elucidate new structure–property relationships for phase-change gas adsorption in metal–organic frameworks, resulting in a balanced material capable of separating tricomponent mixtures of the methylamines.

The decarbonization of fossil fuel combustion streams and air is imperative to achieve negative carbon emissions and requires discovery of new materials that exhibit high CO2 capacities, long-term stability, and minimal energy input for the release of pure CO2. Numerous candidate sorbents have been reported, but none meet all of these requirements simultaneously. Our strategy for creating a material that does is centered on designing a crystalline molecular polyamine that retains porosity throughout CO2 absorption and desorption while achieving a high CO2 uptake capacity, thereby enabling meaningful CO2 capture and release under mild conditions. Here, we show that porous crystals of 2,3,6,7,14,15-hexakis(aminomethyl)triptycene (C20H8(CH2NH2)6, TriptH) capture CO2 from flue gas or air to form a porous ammonium carbamate network solid. The reversibility of the transformation is monitored in situ using powder X-ray diffraction, diffuse reflectance infrared Fourier transform spectroscopy, and solid-state nuclear magnetic resonance spectroscopy. Breakthrough analyses reveal that TriptH achieves a high CO2 capacity of 5.3 mmol/g under humid conditions, long-term oxidative and thermal stability through the course of 660 absorption–desorption cycles, and an unprecedented ability to capture CO2 from humid, low-concentration streams and release it with little or no temperature change and vacuum pressures as high as 100 mbar.

Magnetic exchange coupling between high-anisotropy spin centers is a key requirement for single-molecule magnets with high operating temperatures. Here, we report the synthesis and characterization of the dinuclear iron(II) and cobalt(II) complexes [(iPr3Si)(Dipp)NFe]2(m-(NSiMe3)2C6Me3H) (m-Fe2), [(iPr3Si)(Dipp)NFe]2(p-(NSiMe3)2C6Me4) (p-Fe2), and [(iPr3Si)(Dipp)NCo]2(p-(NSiMe3)2C6Me4) (p-Co2), with each metal center residing in a linear, two-coordinate geometry. In these complexes, the two high-magnetic-anisotropy metal centers are linked through a meta- or para-bis(amido)arene, which mediates ferro- or antiferromagnetic superexchange, respectively. Variable-temperature magnetic susceptibility data reveal antiferromagnetic coupling in the para-substituted complexes, giving S = 0 ground states with exchange constants of −3.56(5) cm–1 for p-Fe2 and −9.82(15) cm–1 for p-Co2. In stark contrast, the meta-linker in m-Fe2 mediates weak ferromagnetic exchange (+0.44(1) cm–1), leading to an S = 4 ground state. Here, the coexistence of ferromagnetic coupling and strong single-ion anisotropy gives rise to single-molecule-magnet behavior with a relaxation barrier of 139 cm–1, the highest yet reported for a multinuclear transition metal complex, and the coupling markedly suppresses the fast through-barrier quantum relaxation that plagues related mononuclear species. To gain a microscopic understanding of the magnetic exchange in this family of complexes, we employed a combination of far-infrared magnetospectroscopy and computation. While a spin-only model reproduces the spectroscopic energy levels well, the substantial unquenched orbital angular momentum at the FeII and CoIIcenters inspired a fully ab initio treatment of the electronic structure, crystal-field splitting, and exchange constants. These calculations indicate that both the sign and the magnitude of the exchange constant depend strongly on the linker geometry.

Metal-organic frameworks with coordinatively unsaturated metal sites (open metal sites) capable of engaging in orbital interactions with π-acidic gases are of interest for enabling ambient-temperature gas separations, such as hydrogen isotope separations. In view of the weakly π-acidic nature of H₂, we sought to strengthen π-backbonding-mediated H₂ adsorption through pore confinement effects. Toward that end, we synthesized and characterized the ultramicroporous metal-organic framework Cu_xZn_5-xCl_4-yH_z(bbta)₃ (Cu^IZn-MFU-4; H₂bbta = 1H,5H-benzo(1,2-d:4,5-d')bistriazole), featuring π-basic trigonal pyramidal Cu^I sites that reside within 7 Å of one another at their closest. Gas adsorption measurements reveal an H₂ adsorption enthalpy of -38 kJ/mol, exceeding that of the larger-pore analog (Cu^IZn-MFU-4l; -33 kJ/mol) and representing the strongest H₂ adsorption yet achieved in a metal-organic framework. The stronger H₂ adsorption in Cu^IZn-MFU-4 is attributed to a combination of pore confinement effects and the increased σ-accepting nature of the Cu^I sites caused by a more electron-withdrawing bbta^2- linker, as supported by structural, spectroscopic, and computational evidence. With the strongest H₂ adsorption, equilibrium isotope effects in Cu^IZn-MFU-4 lead to a D₂/H₂ selectivity (as estimated by ideal adsorbed solution theory) of 1.35 even at 298 K, approaching the values reported below 200 K for conventional porous materials.

Diamine functionalization of the metal–organic framework Mg₂(dobpdc) (dobpdc^4– = 4,4′-dioxidobiphenyl-3,3′-dicarboxylate) significantly enhances its selectivity for CO₂ capture from flue gases and air. The structure and CO₂ capacity of such materials are typically assessed using bulk techniques that rely on averaging signal over large ensembles of unit cells, obscuring local heterogeneities, such as variations in CO₂ occupancy across individual nanocrystals. To resolve this limitation, we demonstrate a multimodal, nanoscale characterization of Mg₂(dobpdc) appended with 1,3-diaminopropane. By employing recently developed characterization techniques at progressively smaller length scales, we uncover insights from correspondingly smaller populations of unit cells. First, we use parallel-beam 3D electron diffraction (3D ED) to identify a prominent expansion in lattice parameters upon desorption of CO₂, as observed at the level of single nanocrystals. Second, we use convergent-probe 4D scanning transmission electron microscopy (4D-STEM) to quantify associated differences in lattice strain as a function of gas loading and diamine appending. These measurements sample small subvolumes within individual nanocrystals. Finally, we apply infrared scattering scanning near-field optical microscopy (IR s-SNOM) to confirm variable CO₂ chemisorption across adsorption sites at the surface of single nanocrystals. This multimodal, multiscale approach allows us to map heterogeneity within individual nanocrystals. Collectively, these findings emphasize the importance of local, nanoscale characterization of metal–organic frameworks in revealing previously unresolvable features that impact their performance.

Mixed-valence complexes featuring lanthanide-lanthanide bonding have recently been shown to act as single-molecule magnets with unprecedented operating temperatures and magnetic coercivities. Here, we present the synthesis and detailed examination of the electronic structure, bonding, and magnetic properties of mixed-valence trinuclear clusters (C₅ⁱPr₅)₃Ln₃H₃I₂ (Ln = Tb, Dy, Ho, Er, and Tm). Near-infrared and X-ray absorption spectra, together with computational results, confirm these clusters possess a three-center, one-electron σ bond. This metal-metal bonding leads to strong intermetal exchange coupling, resulting in magnetic behaviors that starkly contrast with typical multinuclear lanthanide complexes. Notably, structural, spectroscopic, and computational studies of the thulium cluster reveal valence delocalization through a bonding orbital of 5d-parentage between the three Tm centers. This observation represents the first example of a nontraditional electronic structure for thulium involving 5d rather than 4f orbitals. Magnetic analysis reveals a complex interplay between single-ion magnetic anisotropy and ferromagnetic exchange, governing the overall magnetic anisotropy of these clusters. Magnetic susceptibility measurements for Ln = Tb-Er indicate thermally well-isolated high-moment ground states arising from strong magnetic coupling, although the maximum values are lower than those expected for complete parallel alignment of the σ and 4f electrons. Computational analyses suggest that collinear alignment of the local anisotropy axes results in out-of-plane anisotropy for Ln = Er and Tm, whereas noncollinear alignment induces in-plane anisotropy for Ln = Tb, Dy, leading to distinct magnetic relaxation properties. Together, the results highlight the diverse magnetic behaviors that can be realized through lanthanide-lanthanide bonding and outline a synthetic path forward toward maximizing the magnetic anisotropy in f-element clusters.

Transition metal nitrides are reactive intermediates in biological and industrial processes. Chemists have synthesized molecular model complexes of such reactive species to understand their function and electronic requirements for new applications. However, molecular chemistry can suffer from intra- and intermolecular decomposition pathways, which preclude further discovery of unknown reactive species. Metal-organic frameworks offer an opportunity for creating long-lived forms of such species with the vacuum of the pore suppressing degradation while simultaneously enabling substrate access for controlled reactivity studies. Here, we report the characterization of an elusive terminal cobalt nitride species generated through photolysis or thermolysis of a site-isolated cobalt azide within the evacuated metal-organic framework CoN₃-MFU-4l. The first crystal structure of such a species is presented, with vibrational, X-ray absorption, and electron paramagnetic resonance spectroscopies providing further direct evidence for its formation while elucidating its electronic structure. The system additionally enables subsequent reactivity studies with selected substrates, revealing a unique ambiphilic behavior for a metal nitride species.

Cooperative gas adsorption in metal–organic frameworks (MOFs) is a rare phenomenon that generally involves long-range communication between multiple binding sites. We demonstrate a MOF containing cobalt(II)–methyl sites that selectively and reversibly capture two carbon monoxide (CO) molecules per site, leading to record-high adsorption capacities at ambient temperatures and pressures. Gas adsorption and structural, spectroscopic, and computational analyses support a mechanism in which binding of one CO molecule triggers a spin transition, followed by binding of a second CO molecule and migratory insertion of the first CO molecule into the cobalt–methyl bond to form an acetyl. The greater binding affinity associated with the second CO results in sigmoidal adsorption isotherms, a hallmark of cooperativity and phase-change materials, despite the absence of long-range interactions within the framework.

Dysprosium sandwich complexes incorporating sterically encumbered cyclic ligands represent a superior class of mononuclear single-molecule magnets. Here, we report the synthesis of the heteroleptic complex Dy(BC₄Ph₅)(C₅ⁱPr₅), incorporating one borolide and one cyclopentadienide ligand. Strong axial coordination by both ligands engenders high-temperature single-molecule magnet behavior, with a relaxation barrier of U_eff = 1536(15) cm^-1 and a blocking temperature of T_b = 65 K. These values are similar to those of related bis(borolide) complexes yet considerably larger than in a related bis(cyclopentadienide) species, [Dy(C₅ⁱPr₅)₂]⁺. To understand this trend, we carried out single-crystal X-ray diffraction and computational analyses, which revealed reduced ligand-ligand repulsion in Dy(BC₄Ph₅)(C₅ⁱPr₅) versus a related bis(borolide) complex, but with a weaker axial field owing to the cyclopentadienide ligand. This was further understood through calculations of the effective ligand charges across the series. In all, Dy(BC₄Ph₅)(C₅ⁱPr₅) operates at one of the highest temperatures among single-molecule magnets, significantly higher than [Dy(C₅ⁱPr₅)₂]⁺. Although offsetting axial field strength and ligand-ligand repulsions limit Dy(BC₄Ph₅)(C₅ⁱPr₅) from surpassing the best single-molecule magnets, these results demonstrate that high-temperature magnetism can be modulated by incorporating ligands with different charges and axiality and suggest that heteroleptic complexes with carefully chosen ligands could operate at still higher temperatures.