Leoní A. Barrios

Molecular prototypes for spin-based CNOT and SWAP quantum gates

We show that a chemically engineered structural asymmetry in [Tb2] molecular clusters renders the two weakly coupled Tb3+ spin qubits magnetically inequivalent. The magnetic energy level spectrum of these molecules meets then all conditions needed to realize a universal CNOT quantum gate. A proposal to realize a SWAP gate within the same molecule is also discussed. Electronic paramagnetic resonance experiments confirm that CNOT and SWAP transitions are not forbidden.

Heterodimetallic [LnLn′] Lanthanide Complexes: Toward a Chemical Design of Two-Qubit Molecular Spin Quantum Gates

A major challenge for realizing quantum computation is finding suitable systems to embody quantum bits (qubits) and quantum gates (qugates) in a robust and scalable architecture. An emerging bottom-up approach uses the electronic spins of lanthanides. Universal qugates may then be engineered by arranging in a molecule two interacting and different lanthanide ions. Preparing heterometallic lanthanide species is, however, extremely challenging. We have discovered a method to obtain [LnLn′] complexes with the appropriate requirements. Compound [CeEr] is deemed to represent an ideal situation. Both ions have a doubly degenerate magnetic ground state and can be addressed individually. Their isotopes have mainly zero nuclear spin, which enhances the electronic spin coherence. The analogues [Ce2], [Er2], [CeY], and [LaEr] have also been prepared to assist in showing that [CeEr] meets the qugate requirements, as revealed through magnetic susceptibility, specific heat, and EPR. Molecules could now be used for quantum information processing.

A Molecular Pair of [GdNi3] Tetrahedra Bridged by Water Molecules

The authors thank the Generalitat de Catalunya for the prize ICREA Academia 2008 (G.A.) and Grant 2009SGR-1459 (E.C. and E.R.) and Spanish MCI through CTQ2009–06959 (G.A., L.B.), MAT2009-13977- C03 (M.E.), and CTQ2008–06670-C02–01 (E.C., ER). The advanced light source (S.J.T.) is supported by the U.S. Department of Energy (DE-AC02–05CH11231).

Lanthanide Contraction within a Series of Asymmetric Dinuclear [Ln2] Complexes

A complete isostructural series of dinuclear asymmetric lanthanide complexes has been synthesized by using the ligand 6-[3-oxo-3-(2-hydroxyphenyl)propionyl]pyridine-2-carboxylic acid (H3L). All complexes have the formula [Ln2(HL)2(H2L)(NO3)(py)(H2O)] (Ln = La (1), Ce (2), Pr (3), Nd (4), Sm (5), Eu (6), Gd (7), Tb (8), Dy (9), Ho (10), Er (11), Tm (12), Yb (13), Lu (14), Y (15); py = pyridine). Complexes of La to Yb and Y have been crystallographically characterized to reveal that the two metal ions are encapsulated within two distinct coordination environments of differing size. Whereas one site maintains the coordination number (nine) through the whole series, the other one increases from nine to ten owing to a change in the coordination mode of an NO3(-) ligand. This series offers a unique opportunity to study in detail the lanthanide contraction within complexes of more than one metal. This analysis shows that various representative parameters proportional to this contraction follow a quadratic decay as a function of the number n of f electrons. Slaters model for the atomic radii has been used to extract, from these decays, the shielding constant of 4f electrons. The average of O⋅⋅⋅O distances within the coordination polyhedra shared by both metals and of the Ln⋅⋅⋅Ln separations follow also a quadratic decay, therefore showing that such dependence holds also for parameters that receive the contribution of two lanthanide ions simultaneously. The magnetic behavior has been studied for all nondiamagnetic complexes. It reveals the effect of the spin-orbit coupling and a weak antiferromagnetic interaction between both metals. Photoluminescent studies of all the complexes in the series reveal a single broad emission band in the visible region, which is related to the coordinated ligand. On the other hand, the Nd, Er, and Yb complexes show features in the near-IR region due to metal-based transitions.

Coordination Complexes Exhibiting Anion···π Interactions: Synthesis, Structure, and Theoretical Studies

The polydentate ligand 2,4,6-tris(dipyridin-2-ylamino)-1,3,5-triazine (dpyatriz) in combination with the Cu(ClO 4) 2/CuX 2 salt mixtures (X (-) = Cl (-), Br (-), or N 3 (-)) leads to the formation of molecular coordination aggregates with formulas [Cu 3Cl 3(dpyatriz) 2](ClO 4) 3 ( 2), [Cu 3Br 3(dpyatriz) 2](ClO 4) 3 ( 3), and [Cu 4(N 3) 4(dpyatriz) 2(DMF) 4(ClO 4) 2](ClO 4) 2 ( 4). These complexes consist of two dpyatriz ligands bridged via coordination to Cu (II) and disposed either face-to-face in an eclipsed manner ( 2 and 3) or parallel and mutually shifted in one direction. The copper ions complete their coordination positions with Cl (-) ( 2), Br (-) ( 3), or N 3 (-), ClO 4 (-), and N, N-dimethylformamide (DMF) ( 4) ligands. All complexes crystallize together with noncoordinate ClO 4 (-) groups that display anion...pi interactions with the triazine rings. These interactions have been studied by means of high level ab initio calculations and the MIPp partition scheme. These calculations have proven the ClO 4 (-)...[C 3N 3] interactions to be favorable and have revealed a synergistic effect from the combined occurrence of pi-pi stacking of triazine rings and the interaction of these moieties with perchlorate ions, as observed in the experimental systems.

Synthesis and Properties of a Family of Unsymmetric Dinuclear Complexes of LnIII (Ln = Eu, Gd, Tb)

A new ligand has been synthesized with the aim of favoring distinct coordination environments within lanthanide polynuclear complexes. It has led to the formation of three unsymmetrical [Ln(III)(2)] (Ln = Gd, Tb, Eu) complexes, exhibiting weak antiferromagnetic coupling and, for Eu and Tb, high single-ion magnetic anisotropy. All of these attributes are necessary for these clusters to behave as possible 2qubit quantum gates.

Designed Topology and Site-Selective Metal Composition in Tetranuclear [MM′⋅⋅⋅M′M] Linear Complexes

The ligand 1,3-bis[3-oxo-3-(2-hydroxyphenyl)propionyl]benzene (H(4)L), designed to align transition metals into tetranuclear linear molecules, reacts with M(II) salts (M=Ni, Co, Cu) to yield complexes with the expected [MMMM] topology. The novel complexes [Co(4)L(2)(py)(6)] (2; py=pyridine) and [Na(py)(2)][Cu(4)L(2)(py)(4)](ClO(4)) (3) have been crystallographically characterised. The metal sites in complexes 2 and 3, together with previously characterised [Ni(4)L(2)(py)(6)] (1), favour different coordination geometries. These have been exploited for the deliberate synthesis of the heterometallic complex [Cu(2)Ni(2)L(2)(py)(6)] (4). Complexes 1, 2, 3 and 4 exhibit antiferromagnetic interactions between pairs of metals within each cluster, leading to S=0 spin ground states, except for the latter cluster, which features two quasi-independent S=1/2 moieties within the molecule. Complex 4 gathers the structural and physical conditions, thus allowing it to be considered as prototype of a two-qbit quantum gate.

Synthesis of a novel heptacoordinated Fe(III) dinuclear complex: experimental and theoretical study of the magnetic properties.

A new functionalized bis-pyrazol-pyridine ligand has been prepared by reaction with hydrazine of the corresponding bis-β-diketone precursor, also unprecedented. The aerobic reaction of this ligand with ferrous thiocyanate in the presence of ascorbic or oxalic acid affords the dinuclear complex of seven-coordinate Fe(III), [Fe₂(H₄L2)₂(ox)(NCS)₄] (1), as revealed by single crystal X-ray diffraction. This may represent an entry into a new family of [Fe₂] compounds with heptacoordinate metal centres. The capacity of this unusual chromophore to undergo magnetic super-exchange was investigated by means of bulk magnetization and DFT calculations. Both approaches confirmed the presence of antiferromagnetic interactions within the molecule. The theoretical investigation has served to describe the magnetic orbitals of Fe(III) in this unusual coordination geometry, as well as the exchange mechanism. A brief review of the scarce number of iron heptacoordinate complexes reported in the literature is also included and discussed.

The Use of a Bis(phenylpyrazolyl)pyridyl Ligand to Prepare [Mn4] and [Mn10] Cage Complexes

Many branches in molecular magnetism live on the production of novel transition-metal (TM) coordination clusters. This has been the source of high-spin molecules, most single-molecule magnets (SMMs), or the possible future qubits for quantum computing. Without doubt, the great advances made in these areas have benefited, for the most part, from the preparation of molecules through so-called “serendipitous self-assembly”. Even though the designed preparation of clusters with predetermined structures and properties is highly desirable, it continues to be a huge synthetic challenge. Therefore, there is no reason to think that the “nonpredictable” approach will cease to be instrumental in the near future. For this, synthetic coordination chemists will keep providing original methods of preparation of polynuclear TM complexes. Typically, these are produced from reactions involving one or more small ligands with donor atoms capable of bridging metals, normally exhibiting several potential coordination modes. By contrast, the synthesis of large multidentate ligands with numerous donor atoms is far less common in this context. We have recently prepared a new ligand with two aligned hydroxyphenylpyrazolyl units separated by a pyridine group (2,6-bis[5-(2-hydroxyphenyl)pyrazol-3-yl]pyridine, H4L, Scheme 1). [10] Hydroxyphenylpyrazolyl derivatives or related pyrazolinoles have proven to be excellent ligands for the assembly of TM clusters with novel structures. The build up of more than one such moiety on the same molecule promises to be an open door to a variety of unprecedented architectures. Ligand H4L is well suited, for example, to stimulate the aggregation of a sequence of five closely spaced metals within a cluster (Scheme 2B). We report herein very promis-

Guest-, Light- and Thermally-Modulated Spin Crossover in [FeII2] Supramolecular Helicates

A new bis(pyrazolylpyridine) ligand (H2 L) has been prepared to form functional [Fe2 (H2 L)3 ](4+) metallohelicates. Changes to the synthesis yield six derivatives, X@[Fe2 (H2 L)3 ]X(PF6 )2 ⋅xCH3 OH (1, x=5.7 and X=Cl; 2, x=4 and X=Br), X@[Fe2 (H2 L)3 ]X(PF6 )2 ⋅yCH3 OH⋅H2 O (1 a, y=3 and X=Cl; 2 a, y=1 and X=Br) and X@[Fe2 (H2 L)3 ](I3 )2 ⋅3 Et2 O (1 b, X=Cl; 2 b, X=Br). Their structure and functional properties are described in detail by single-crystal X-ray diffraction experiments at several temperatures. Helicates 1 a and 2 a are obtained from 1 and 2, respectively, by a single-crystal-to-single-crystal mechanism. The three possible magnetic states, [LS-LS], [LS-HS], and [HS-HS] can be accessed over large temperature ranges as a result of the structural nonequivalence of the Fe(II) centers. The nature of the guest (Cl(-) vs. Br(-) ) shifts the spin crossover (SCO) temperature by roughly 40 K. Also, metastable [LS-HS] or [HS-HS] states are generated through irradiation. All helicates (X@[Fe2 (H2 L)3 ])(3+) persist in solution.