Nature and Topology of Metal-Oxygen Binding Sites in Zeolite Materials: 17 O High-Resolution EPR Spectroscopy of Metal-Loaded ZSM-5.

Determining structural models is pivotal to the rational understanding and development of heterogeneous catalytic systems. A paradigmatic case is represented by open-shell metals supported on oxides, where the catalytic properties crucially depend on the nature of the metal-oxygen bonds and the extent of charge and spin transfer. Through a combination of selective 17 O isotopic enrichment and the unique properties of open-shell s-state monovalent Group 12 cations, we derive a site-specific topological description of active sites in an MFI zeolite. We show that just a few selected sites out of all possible are populated and that the relative occupancies depend on the specific properties of the metal, and we provide maps of charge and spin transfer at the metal-oxygen interface. This approach is not restricted to zeotype materials, rather it is applicable to any catalysts supported on oxygen-containing materials.

A Rare Example of Four-Coordinate Nonoxido Vanadium(IV) Alkoxide in the Solid State: Structure, Spectroscopy, and Magnetization Dynamics.

The distorted tetrahedral [V(OAd)4] alkoxide (OAd = 1-adamantoxide, complex 1) is the first homoleptic, mononuclear vanadium(IV) alkoxide to be characterized in the solid state by X-ray diffraction analysis. The compound crystallizes in the cubic P4̅3 n space group with two highly disordered, crystallographically independent molecules in the asymmetric unit. Spin Hamiltonian parameters extracted from low temperature X- and Q-band electron paramagnetic resonance (EPR) experiments performed for polycrystalline samples of 1, both in the concentrated (bulk) form and diluted in the diamagnetic [Ti(OAd)4] analogue, reveal a fully axial system with g z < g x, g y and A z ≫ A x, A y. Complex 1 has also been characterized by alternate current susceptometry with varying temperature (3-30 K) and static magnetic field (up to 8.5 T), showing field-induced slow relaxation of the magnetization with relaxation times ranging from ca. 3 ms at 3 K to 0.02-0.03 ms at 30 K, in line with relevant results described recently for other potential molecular quantum bits. Pulsed EPR measurements, in turn, disclosed long coherence times of ca. 4 µs at temperatures lower than 40 K, despite the presence of the H-rich ligands. The slow spin relaxation in 1 is the first observed for a tetracoordinate nonoxido vanadium(IV) complex, and results are compared here to those generated by square-pyramidal VIV(O)2+ and trigonal prismatic V4+ with oxygen donor atom sets. Considering that the number of promising d1 complexes investigated in detail for slow magnetization dynamics is still small, the present work contributes to the establishment of possible structural/electronic correlations of interest to the field of quantum information processing.

Scaling Up Electronic Spin Qubits into a Three-Dimensional Metal-Organic Framework.

Practical implementation of highly coherent molecular spin qubits for challenging technological applications, such as quantum information processing or quantum sensing, requires precise organization of electronic qubit molecular components into extended frameworks. Realization of spatial control over qubit-qubit distances can be achieved by coordination chemistry approaches through an appropriate choice of the molecular building blocks. However, translating single qubit molecular building units into extended arrays does not guarantee a priori retention of long quantum coherence and spin-lattice relaxation times due to the introduced modifications over qubit-qubit reciprocal distances and molecular crystal lattice phonon structure. In this work, we report the preparation of a three-dimensional (3D) metal-organic framework (MOF) based on vanadyl qubits, [VO(TCPP-Zn2-bpy)] (TCPP = tetracarboxylphenylporphyrinate; bpy = 4,4'-bipyridyl) (1), and the investigation of how such structural modifications influence qubits' performances. This has been done through a multitechnique approach where the structure and properties of a representative molecular building block of formula [VO(TPP)] (TPP = tetraphenylporphyrinate) (2) have been compared with those of the 3D MOF 1. Pulsed electron paramagnetic resonance measurements on magnetically diluted samples in titanyl isostructural analogues revealed that coherence times are retained almost unchanged for 1 with respect to 2 up to room temperature, while the temperature dependence of the spin-lattice relaxation time revealed insights into the role of low-energy vibrations, detected through terahertz spectroscopy, on the spin dynamics.

A two-qubit molecular architecture for electron-mediated nuclear quantum simulation.

A switchable interaction between pairs of highly coherent qubits is a crucial ingredient for the physical realization of quantum information processing. One promising route to enable quantum logic operations involves the use of nuclear spins as protected elementary units of information, qubits. Here we propose a simple way to use fast electronic spin excitations to switch the effective interaction between nuclear spin qubits and the realization of a two-qubit molecular architecture based on highly coherent vanadyl moieties to implement quantum logic operations. Controlled generation of entanglement between qubits is possible here through chemically tuned magnetic coupling between electronic spins, which is clearly evidenced by the splitting of the vanadium(iv) hyperfine lines in the continuous-wave electron paramagnetic resonance spectrum. The system has been further characterized by pulsed electron paramagnetic resonance spectroscopy, evidencing remarkably long coherence times. The experimentally derived spin Hamiltonian parameters have been used to simulate the system dynamics under the sequence of pulses required to implement quantum gates in a realistic description that includes also the harmful effect of decoherence. This demonstrates the possibility of using this molecular complex to implement a control-Z (CZ) gate and simple quantum simulations. Indeed, we also propose a proof-of-principle experiment based on the simulation of the quantum tunneling of the magnetization in a S = 1 spin system.

Structural Effects on the Spin Dynamics of Potential Molecular Qubits.

Control of spin-lattice magnetic relaxation is crucial to observe long quantum coherence in spin systems at reasonable temperatures. Such a control is most often extremely difficult to achieve, because of the coexistence of several relaxation mechanisms, that is direct, Raman, and Orbach. These are not always easy to relate to the energy states of the investigated system, because of the contribution to the relaxation of additional spin-phonon coupling phenomena mediated by intramolecular vibrations. In this work, we have investigated the effect of slight changes on the molecular structure of four vanadium(IV)-based potential spin qubits on their spin dynamics, studied by alternate current (AC) susceptometry. The analysis of the magnetic field dependence of the relaxation time correlates well with the low-energy vibrational modes experimentally detected by time-domain THz spectroscopy. This confirms and extends our preliminary observations on the role played by spin-vibration coupling in determining the fine structure of the spin-lattice relaxation time as a function of the magnetic field, for S = 1/2 potential spin qubits. This study represents a step forward in the use of low-energy vibrational spectroscopy as a prediction tool for the design of molecular spin qubits with long-lived quantum coherence. Indeed, quantum coherence times of ca. 4.0-6.0 µs in the 4-100 K range are observed for the best performing vanadyl derivatives identified through this multitechnique approach.

EPR approaches to heterogeneous catalysis. The chemistry of titanium in heterogeneous catalysts and photocatalysts.

Paramagnetic species are often involved in catalytic or photocatalytic reactions occurring at the solid-gas interface of heterogeneous catalysts. In this contribution we will provide an overview of the wealth and breadth of information that can be obtained from EPR in the characterization of paramagnetic species in such systems, illustrating the advantages that modern pulsed EPR methodologies can offer in monitoring the elementary processes occurring within the coordination sphere of surface transition-metal ions. To do so we selected three representative systems, where titanium ions in low oxidation states act as active catalytic sites, trying to outline the methodological approaches which characterize the application of EPR techniques and the questions that can be answered and addressed relative to the characterization of heterogeneous catalytic materials.

Quantum Coherence Times Enhancement in Vanadium(IV)-based Potential Molecular Qubits: the Key Role of the Vanadyl Moiety.

In the search for long-lived quantum coherence in spin systems, vanadium(IV) complexes have shown record phase memory times among molecular systems. When nuclear spin-free ligands are employed, vanadium(IV) complexes can show at low temperature sufficiently long quantum coherence times, Tm, to perform quantum operations, but their use in real devices operating at room temperature is still hampered by the rapid decrease of T1 caused by the efficient spin-phonon coupling. In this work we have investigated the effect of different coordination environments on the magnetization dynamics and the quantum coherence of two vanadium(IV)-based potential molecular spin qubits in the solid state by introducing a unique structural difference, i.e., an oxovanadium(IV) in a square pyramidal versus a vanadium(IV) in an octahedral environment featuring the same coordinating ligand, namely, the 1,3-dithiole-2-thione-4,5-dithiolate. This investigation, performed by a combined approach of alternate current (ac) susceptibility measurements and continuous wave (CW) and pulsed electron paramagnetic resonance (EPR) spectroscopies revealed that the effectiveness of the vanadyl moiety in enhancing quantum coherence up to room temperature is related to a less effective mechanism of spin-lattice relaxation that can be quantitatively evaluated by the exponent n (ca. 3) of the temperature dependence of the relaxation rate. A more rapid collapse is observed for the non-oxo counterpart (n = 4) hampering the observation of quantum coherence at room temperature. Record coherence time at room temperature (1.04 µs) and Rabi oscillations are also observed for the vanadyl derivative in a very high concentrated material (5 ± 1%) as a result of the additional benefit provided by the use of a nuclear spin-free ligand.

Unraveling the Catalytic Synergy between Ti(3+) and Al(3+) Sites on a Chlorinated Al2 O3 : A Tandem Approach to Branched Polyethylene.

An original step-by-step approach to synthesize and characterize a bifunctional heterogeneous catalyst consisting of isolated Ti(3+) centers and strong Lewis acid Al(3+) sites on the surface of a chlorinated alumina has been devised. A wide range of physicochemical and spectroscopic techniques were employed to demonstrate that the two sites, in close proximity, act in a concerted fashion to synergistically boost the conversion of ethylene into branched polyethylene, using ethylene as the only feed and without any activator. The coordinatively unsaturated Al(3+) ions promote ethylene oligomerization through a carbocationic mechanism and activate the Ti(3+) sites for the traditional ethylene coordination polymerization.

Room-Temperature Quantum Coherence and Rabi Oscillations in Vanadyl Phthalocyanine: Toward Multifunctional Molecular Spin Qubits.

Here we report the investigation of the magnetic relaxation and the quantum coherence of vanadyl phthalocyanine, VOPc, a multifunctional and easy-processable potential molecular spin qubit. VOPc in its pure form (1) and its crystalline dispersions in the isostructural diamagnetic host TiOPc in different stoichiometric ratios, namely VOPc:TiOPc 1:10 (2) and 1:1000 (3), were investigated via a multitechnique approach based on the combination of alternate current (AC) susceptometry, continuous wave, and pulsed electron paramagnetic resonance (EPR) spectroscopy. AC susceptibility measurements revealed a linear increase of the relaxation rate with temperature up to 20 K, as expected for a direct mechanism, but τ remains slow over a very wide range of applied static field values (up to ∼5 T). Pulsed EPR spectroscopy experiments on 3 revealed quantum coherence up to room temperature with T(m) ∼1 µs at 300 K, representing the highest value obtained to date for molecular electronic spin qubits. Rabi oscillations are observed in this nuclear spin-active environment ((1)H and (14)N nuclei) at room temperature also for 2, indicating an outstanding robustness of the quantum coherence in this molecular semiconductor exploitable in spintronic devices.

Quantum coherence in a processable vanadyl complex: new tools for the search of molecular spin qubits.

Electronic spins in different environments are currently investigated as potential qubits, i.e. the logic units of quantum computers. These have to retain memory of their quantum state for a sufficiently long time (phase memory time, Tm) allowing quantum operations to be performed. For molecular based spin qubits, strategies to increase phase coherence by removing nuclear spins are rather well developed, but it is now crucial to address the problem of the rapid increase of the spin-lattice relaxation rate, T1-1, with increasing temperature that hampers their use at room-temperature. Herein, thanks to the combination of pulsed EPR spectroscopy and AC susceptometry we evidence that an evaporable vanadyl complex of formula VO(dpm)2, where dpm- is the anion of dipivaloylmethane, presents a combination of very promising features for potential application as molecular spin-qubit. The spin-lattice relaxation time, T1, studied in detail through AC susceptometry, decreases slowly with increasing temperature and, more surprisingly, it is not accelerated by the application of an external field up to several Teslas. State-of-the art phase memory times for molecular spin systems in protiated environment are detected by pulsed EPR also in moderate dilution, with values of 2.7 µs at 5 K and 2.1 µs at 80 K. Low temperature scanning tunnel microscopy and X-ray photoelectron spectroscopy in situ investigations reveal that intact molecules sublimated in ultra-high vacuum spontaneously form an ordered monolayer on Au(111), opening the perspective of electric access to the quantum memory of ensembles of spin qubits that can be scaled down to the single molecule.

Probing the coordinative unsaturation and local environment of Ti³âº sites in an activated high-yield Ziegler-Natta catalyst.

The typical activation of a fourth generation Ziegler-Natta catalyst TiCl4/MgCl2/phthalate with triethyl aluminum generates Ti(3+) centers that are investigated by multi-frequency continuous wave and pulse EPR methods. Two families of isolated, molecule-like Ti(3+) species have been identified. A comparison of the experimentally derived g tensors and (35,37)Cl hyperfine and nuclear-quadrupole tensors with DFT-computed values suggests that the dominant EPR-active Ti(3+)  species is located on MgCl2(110) surfaces (or equivalent MgCl2 terminations with tetra-coordinated Mg). O2 reactivity tests show that a fraction of these Ti sites is chemically accessible, an important result in view of the search for the true catalyst active site in olefin polymerization.

EPR investigation of TiCl3 dissolved in polar solvents--implications for the understanding of active Ti(III) species in heterogeneous Ziegler-Natta catalysts.

Multi-frequency continuous-wave and pulsed EPR techniques are employed to investigate Ti(III)-chloro complexes obtained by dissolving TiCl3 in anhydrous and hydrated methanol. Two distinctly different species, characterized by different g matrices are observed in the two cases. Hyperfine sublevel correlation (HYSCORE) spectroscopy is found to be a powerful method to identify the type of nuclei surrounding the Ti(3+) ion. For the first time, the hyperfine and nuclear quadrupole data of Ti(III)-bound (35/37)Cl nuclei are reported together with (1)H and (13)C hyperfine data of the coordinated methanol molecules. DFT modelling allows interpreting the measured spin Hamiltonian parameters in terms of microscopic models of the solvated species. The theoretical observable properties (g matrix, (35/37)Cl, (1)H and (13)C hyperfine tensors) are in quantitative agreement with the experiments for two families of complexes: [TiCln(CH3OH)6-n]((3-n)+) (with n ranging from 1 to 3) and [Ti(CH3OH)5(OH)](2+) or [Ti(CH3OH)5(OCH3)](2+). The first complex is observed in anhydrous methanol, while the second type of complex is observed when water is added to the solution, the presence of OH(-) and/or CH3O(-) species being promoted by water hydrolysis. The results obtained for the frozen solutions are critically compared to EPR spectra recorded for a MgCl2-supported Ti-based Ziegler-Natta model catalyst.

Probing the redox chemistry of titanium silicalite-1: formation of tetrahedral Ti3+ centers by reaction with triethylaluminum.

Transition-metal ions with open-shell configurations hold promise in the development of novel coordination chemistry and potentially unprecedented redox catalysis. Framework-substituted Ti(3+) ions with tetrahedral coordination are generated by reductive activation of titanium silicalite-1 with triethylaluminum, an indispensable co-catalyst for heterogeneous Ziegler-Natta polymerization catalysts. Continuous-wave and pulse electron paramagnetic resonance methods are applied to unravel details on the local environment of the reduced transition metal-ions, which are shown to be part of the silica framework by detection of (29)Si hyperfine interactions. The chemical accessibility of the reduced sites is probed using ammonia as probe molecule. Evidence is found for the coordination of a single ammonia molecule. Comparison to similar systems, such as TiAlPO-5, reveals clear differences in the coordination chemistry of the reduced Ti sites in the two solids, which may be understood considering the different electronic properties of the solid frameworks.