A Porous Array of Clock Qubits.

Realizing atomic-level spatial control over qubits, the fundamental units of both quantum information processing systems and quantum sensors, constitutes a crucial cross-field challenge. Toward this end, embedding electronic-spin-based qubits within the framework of a crystalline porous material is a promising approach to create precise arrays of qubits. Realizing porous hosts for qubits would also impact the emerging field of quantum sensing, whereby porosity would enable analytes to infuse into a sensor matrix. However, building viable qubits into a porous material is an appreciable challenge because of the extreme sensitivity of qubits to local magnetic noise. To insulate these frameworks from ambient magnetic signals, we borrowed from atomic physics the idea to exploit clock transitions at avoided level crossings. Here, sensitivity to magnetic noise is inherently limited by the flat slope of the so-called clock transition. More specifically, we created an array of clocklike qubits within a metal-organic framework by combining coordination chemistry considerations with the fundamental concept of atomic clock transitions. Electron paramagnetic resonance studies verify a clocklike transition for the hosted cobalt(II) spins in the framework [(TCPP)Co0.07Zn0.93]3[Zr6O4(OH)4(H2O)6]2, the first demonstration in any porous material. The clocklike qubits display lifetimes of up to 14 µs despite abundant local nuclear spins, illuminating a new path toward proof-of-concept quantum sensors and processors with high inherent structural precision.

CO Binding at a Four-Coordinate Cobaltous Porphyrin Site in a Metal-Organic Framework: Structural, EPR, and Gas Adsorption Analysis.

The variable-temperature CO binding at a four-coordinate cobaltous porphyrin complex within a metal-organic framework is investigated using a combined array of infrared, X-ray diffraction, EPR, and CO adsorption analysis. Single-crystal X-ray diffraction experiments provide the first crystallographically characterized example of a noniron first-row transition metal porphyrin carbonyl adduct. Variable-temperature electron paramagnetic resonance spectroscopy supports the structural observation and reveals conversion of the dicarbonyl complex to a monocarbonyl species as temperature is increased. Finally, CO adsorption analysis data enable quantification of the Co-CO interaction, providing a binding enthalpy of hads = -29(2) kJ/mol. This value is nearly twice that observed for O2 binding in the same compound and is attributed to a stronger σ interaction for Co and CO vs O2. These results demonstrate the ability of MOFs to enable a thorough investigation of CO binding in metalloporphyrins.

Incorporation of Pyrazine and Bipyridine Linkers with High-Spin Fe(II) and Co(II) in a Metal-Organic Framework.

A series of isoreticular metal-organic frameworks (MOFs) of the formula M(BDC)(L) (M = Fe(II) or Co(II), BDC = 1,4-benzenedicarboxylate, L = pyrazine (pyz) or 4,4'-bipyridine (bipy)) has been synthesized and characterized by N2 gas uptake measurements, single crystal and powder X-ray diffraction, magnetometry, X-ray absorption spectroscopy, and Mössbauer spectroscopy. These studies indicate the formation of a permanently porous solid with high-spin Fe(II) and Co(II) centers that are weakly coupled, consistent with first-principles density functional theory calculations. This family of materials represents unusual examples of paramagnetic metal centers coordinated by linkers capable of mediating magnetic or electronic coupling in a porous framework. While only weak interactions are observed, the rigid 3D framework of the MOF dramatically impacts the properties of these materials when compared with close structural analogues.

A five-coordinate heme dioxygen adduct isolated within a metal-organic framework.

The porphyrinic metal-organic framework (MOF) PCN-224 is metalated with Fe(II) to yield a 4-coordinate ferrous heme-containing compound. The heme center binds O2 at -78 °C to give a 5-coordinate heme-O2 complex. For the first time, this elusive species is structurally characterized, revealing an Fe(III) center coordinated to superoxide via an end-on, η(1) linkage. Mössbauer spectroscopy supports the structural observations and indicates the presence of a low-spin electronic configuration for Fe(III). Finally, variable-temperature O2 adsorption data enable quantification of the Fe-O2 interaction, providing a binding enthalpy of -34(4) kJ/mol. This value is nearly half of that observed for comparable 6-coordinate, imidazole-bound heme-O2 complexes, a difference that further illustrates the importance of axial ligands in biological heme-mediated O2 transport and storage. These results demonstrate the ability of a MOF, by virtue of its rigid solid-state structure, to enable isolation and thorough characterization of a species that can only be observed transiently in molecular form.

A structurally-characterized peroxomanganese(iv) porphyrin from reversible O2 binding within a metal-organic framework.

The role of peroxometal species as reactive intermediates in myriad biological processes has motivated the synthesis and study of analogous molecular model complexes. Peroxomanganese(iv) porphyrin complexes are of particular interest, owing to their potential ability to form from reversible O2 binding, yet have been exceedingly difficult to isolate and characterize in molecular form. Alternatively, immobilization of metalloporphyrin sites within a metal-organic framework (MOF) can enable the study of interactions between low-coordinate metal centers and gaseous substrates, without interference from bimolecular reactions and axial ligation by solvent molecules. Here, we employ this approach to isolate the first rigorously four-coordinate manganese(ii) porphyrin complex and examine its reactivity with O2 using infrared spectroscopy, single-crystal X-ray diffraction, EPR spectroscopy, and O2 adsorption analysis. X-ray diffraction experiments reveal for the first time a peroxomanganese(iv) porphyrin species, which exhibits a side-on, η2 binding mode. Infrared and EPR spectroscopic data confirm the formulation of a peroxomanganese(iv) electronic structure, and show that O2 binding is reversible at ambient temperature, in contrast to what has been observed in molecular form. Finally, O2 gas adsorption measurements are employed to quantify the enthalpy of O2 binding as hads = -49.6(8) kJ mol-1. This enthalpy is considerably higher than in the corresponding Fe- and Co-based MOFs, and is found to increase with increasing reductive capacity of the MII/III redox couple.

An Efficient Synthesis of 4(5)-Benzyl-L-Histidines Employing Catalytic Transfer Hydrogenolysis at Elevated Temperatures.

An efficient two-step synthesis of 4(5)-benzyl-L-histidine from L-histidine was developed. A Pictet-Spengler reaction between L-histidine and benzaldehyde in the presence of excess strong base yielded 4-phenylspinacine within one hour. Catalytic transfer hydrogenolysis in methanol at reflux using ammonium formate rapidly converted 4-L-phenylspinacine to 4(5)-benzyl-L-histidine within five minutes. No racemization of the final product 4(5)-benzyl-L-histidine was observed using the Marfey reagent. To show the utility of this methodology, a series of fluorinated benzylhistidines is presented.