Fabrizio Moro

A delocalized arene-bridged diuranium single-molecule magnet

Single-molecule magnets (SMMs) are compounds that, below a blocking temperature, exhibit stable magnetization purely of molecular origin, and not caused by long-range ordering of magnetic moments in the bulk. They thus show promise for applications such as data storage of ultra-high density. The stability of the magnetization increases with increasing ground-state spin and magnetic anisotropy. Transition-metal SMMs typically possess high-spin ground states, but insufficient magnetic anisotropies. Lanthanide SMMs exhibit large magnetic anisotropies, but building high-spin ground states is difficult because they tend to form ionic bonds that limit magnetic exchange coupling. In contrast, the significant covalent bonding and large spin-orbit contributions associated with uranium are particularly attractive for the development of improved SMMs. Here we report a delocalized arene-bridged diuranium SMM. This study demonstrates that arene-bridged polyuranium clusters can exhibit SMM behaviour without relying on the superexchange coupling of spins. This approach may lead to increased blocking temperatures.

Surface Supramolecular Organization of a Terbium(III) Double-Decker Complex on Graphite and its Single Molecule Magnet Behavior

The two-dimensional self-assembly of a terbium(III) double-decker phthalocyanine on highly oriented pyrolitic graphite (HOPG) was studied by atomic force microscopy (AFM), and it was shown that it forms highly regular rectangular two-dimensional nanocrystals on the surface, that are aligned with the graphite symmetry axes, in which the molecules are organized in a rectangular lattice as shown by scanning tunneling microscopy. Molecular dynamics simulations were run in order to model the behavior of a collection of the double-decker complexes on HOPG. The results were in excellent agreement with the experiment, showing that-after diffusion on the graphite surface-the molecules self-assemble into nanoscopic islands which align preferentially along the three main graphite axes. These low dimension assemblies of independent magnetic centers are only one molecule thick (as shown by AFM) and are therefore very interesting nanoscopic magnetic objects, in which all of the molecules are in interaction with the graphite substrate and might therefore be affected by it. The magnetic properties of these self-assembled bar-shaped islands on HOPG were studied by X-ray magnetic circular dichroism, confirming that the compounds maintain their properties as single-molecule magnets when they are in close interaction with the graphite surface.

Synthesis of a Uranium(VI)-Carbene: Reductive Formation of Uranyl(V)-Methanides, Oxidative Preparation of a [R2C═U═O]2+ Analogue of the [O═U═O]2+ Uranyl Ion (R = Ph2PNSiMe3), and Comparison of the Nature of UIV═C, UV═C, and UVI═C Double Bonds

We report attempts to prepare uranyl(VI)- and uranium(VI) carbenes utilizing deprotonation and oxidation strategies. Treatment of the uranyl(VI)-methanide complex [(BIPMH)UO(2)Cl(THF)] [1, BIPMH = HC(PPh(2)NSiMe(3))(2)] with benzyl-sodium did not afford a uranyl(VI)-carbene via deprotonation. Instead, one-electron reduction and isolation of di- and trinuclear [UO(2)(BIPMH)(μ-Cl)UO(μ-O){BIPMH}] (2) and [UO(μ-O)(BIPMH)(μ(3)-Cl){UO(μ-O)(BIPMH)}(2)] (3), respectively, with concomitant elimination of dibenzyl, was observed. Complexes 2 and 3 represent the first examples of organometallic uranyl(V), and 3 is notable for exhibiting rare cation-cation interactions between uranyl(VI) and uranyl(V) groups. In contrast, two-electron oxidation of the uranium(IV)-carbene [(BIPM)UCl(3)Li(THF)(2)] (4) by 4-morpholine N-oxide afforded the first uranium(VI)-carbene [(BIPM)UOCl(2)] (6). Complex 6 exhibits a trans-CUO linkage that represents a [R(2)C═U═O](2+) analogue of the uranyl ion. Notably, treatment of 4 with other oxidants such as Me(3)NO, C(5)H(5)NO, and TEMPO afforded 1 as the only isolable product. Computational studies of 4, the uranium(V)-carbene [(BIPM)UCl(2)I] (5), and 6 reveal polarized covalent U═C double bonds in each case whose nature is significantly affected by the oxidation state of uranium. Natural Bond Order analyses indicate that upon oxidation from uranium(IV) to (V) to (VI) the uranium contribution to the U═C σ-bond can increase from ca. 18 to 32% and within this component the orbital composition is dominated by 5f character. For the corresponding U═C π-components, the uranium contribution increases from ca. 18 to 26% but then decreases to ca. 24% and is again dominated by 5f contributions. The calculations suggest that as a function of increasing oxidation state of uranium the radial contraction of the valence 5f and 6d orbitals of uranium may outweigh the increased polarizing power of uranium in 6 compared to 5.

Encapsulation of single-molecule magnets in carbon nanotubes

Next-generation electronic, photonic or spintronic devices will be based on nanoscale functional units, such as quantum dots, isolated spin centres or single-molecule magnets. The key challenge is the coupling of the nanoscale units to the macroscopic world, which is essential for read and write purposes. Carbon nanotubes with one macroscopic and two nanoscopic dimensions provide an excellent means to achieve this coupling. Although the dimensions of nanotube internal cavities are suitable for hosting a wide range of different molecules, to our knowledge, no examples of molecular magnets inserted in nanotubes have been reported to date. Here we report the successful encapsulation of single-molecule magnets in carbon nanotubes, yielding a new type of hybrid nanostructure that combines all the key single-molecule magnet properties of the guest molecules with the functional properties of the host nanotube. The findings may pave the way to the construction of spintronic or ultrahigh-density magnetic data storage devices.

The Inherent Single-Molecule Magnet Character of Trivalent Uranium†

SMMs are often based on transition-metal clusters, but significant attention has recently focused on complexes of single and multiple lanthanoid ions, because the crystal-field splitting of the lowest Russell–Saunders multiplet engenders large magnetic anisotropies. [5–11] These anisotropies are responsible for high relaxation barriers and therefore slow magnetic relaxation. However, well isolated high-spin ground states are difficult to achieve within polynuclear lanthanoid SMMs because the valence 4f orbitals have limited radial extension and are usually energetically incompatible with ligand orbitals. These inherent 4f orbital properties give predominantly ionic interactions and results in weak magnetic exchange coupling with neighboring spin centers, with very few exceptions. [7, 12] In principle, actinoids, and in particular uranium, possess properties that render these ions ideal candidates from which to construct SMMs. This is because, compared to the lanthanoids, uranium exhibits enhanced crystal field splitting, [13, 14] as well as increased covalency, the latter enabling significant spin couplings in polynuclear systems, [15] and therefore both stronger magnetic exchange and anisotropies can be envisaged. This premise was realized recently with reports of several closely related single-ion pyrazolylborate uranium(III) complexes which were shown to display SMM behavior. [14, 16–20] In addition, two neptunium complexes were shown to display slow relaxation of the magnetization. [15, 21] The fact that all uranium(III) SMMs reported to date are closely related to each other raises the question as to how sensitive SMM behavior in trivalent uranium is to the composition of the coordination sphere and its symmetry. SMM behavior in all published examples is much more pronounced in an external field than in zero field, which suggests that quantum tunneling of the magnetization plays a significant role in shortening the relaxation times. However, in principle, low-symmetry crystal field components cannot induce tunneling of the magnetization, because uranium(III) is a Kramers half-integer angular momentum ion. Also the nuclear spin I = 0o f 238 U cannot induce tunneling of the

Spectroscopic determination of crystal field splittings in lanthanide double deckers

We have investigated the crystal field splitting in the archetypal lanthanide-based single-ion magnets and related complexes (NBu4)+[LnPc2]−·2dmf (Ln = Dy, Ho, Er; dmf = N,N-dimethylformamide) by means of far infrared and inelastic neutron scattering spectroscopies. In each case, we have found several features corresponding to direct crystal field transitions within the ground multiplet. The observation of three independent peaks in the holmium derivative enabled us to derive crystal field splitting parameters. In addition, we have carried out CASSCF calculations. We show that exploiting the interplay of CASSCF calculation (for the composition of the states) and advanced spectroscopic measurements (for accurate determination of the energies) is a very powerful approach to gain insight into the electronic structure of lanthanide-based single-molecule magnets.

A ring of rings and other multicomponent assemblies of cages

Ring a ring of roses: Spectacular nanoscale molecular assemblies have been created by design of individual polymetallic components that are then linked together through simple reactions. These include an assembly where six octametallic rings surround a dodecametallic central ring.

An unsupported uranium-rhenium complex prepared by alkane elimination

Understanding the nature of metal–metal bonds is fundamentally important to furthering our understanding of chemical bonding. This is particularly relevant to the fblock elements because there is continued debate over the degree of covalency in interactions involving f-block metal centres. Although compounds containing metal–metal bonds involving dand p-block elements are well known, and now even examples from the s-block have been reported, examples of uranium–metal complexes remain scarce. Prior to our initiation of a programme of research investigating uranium–metal bonds, structurally characterised examples of species with U M bonds were limited to the p-block derivatives [{(Ar) ACHTUNGTRENNUNG(tBu)N}3USiACHTUNGTRENNUNG(SiMe3)3] (Ar=3,5-Me2C6H3), [(hC5H5)3USnPh3], [7] and [(h-C5H4SiMe3)3UE ACHTUNGTRENNUNG(h5-C5Me5)] (E= Al, Ga). We have been investigating the chemistry of uranium– metal complexes supported by tripodal triamido ligands, which has resulted in the characterisation of the U Ga complex [(Tren) ACHTUNGTRENNUNG(THF)UGa ACHTUNGTRENNUNG(NAr’CH)2] [1, Ar’=2,6iPr2C6H3; Tren TMS =N(CH2CH2NSiMe3)3], [10] the U Re complex [(Tren)URe ACHTUNGTRENNUNG(h5-C5H5)2] (2), and two U Re complexes [(Ts) ACHTUNGTRENNUNG(THF)URe ACHTUNGTRENNUNG(h5-C5H5)2] [3, Ts =HCACHTUNGTRENNUNG(SiMe2NAr)3; Ar=3,5-Me2C6H3) and [(Ts)URe ACHTUNGTRENNUNG(h5-C5H5)2] (4). Complexes 1–4 are noteworthy because they exhibit s and p contributions within the uranium–metal interactions, although the latter component is clearly very weak. Compounds 1 and 2 were prepared by salt elimination, whereas complexes 3 and 4 were prepared by amine elimination. Although alkane elimination has proven to be a very successful strategy for preparing rare earth–metal bonds, it has not yet been applied to the synthesis of uranium–metal bonds; uranium alkyls tend to suffer from thermal instability, and since alkyls are reducing, the redox chemistry of uranium might be anticipated to interfere with uranium– metal bond formation. In search of a suitable uranium–alkyl complex with which to test whether or not alkane elimination can be a useful strategy for constructing uranium–metal bonds, our attention was drawn to the cyclometallated alkyl triamidoamine complex [U{N(CH2CH2NSiMe2tBu)2(CH2CH2NSiMetBuCH2)}] (5) developed by Scott and co-workers. [15] Complex 5 is stable at room temperature, in an inert atmosphere, and straightforward to prepare. This presents an excellent opportunity to examine whether alkane elimination is compatible with the construction of uranium–metal bonds. Herein, we show for the first time that a uranium–metal bond can be prepared by formal alkane elimination. Additionally, we report that the U Re interaction is predominantly ionic with a weak p contribution involving the U and Re centres, as revealed by analyses of the calculated electron density and frontier Kohn–Sham orbitals. The addition of toluene to a mixture of purple 5 and yellow rhenocene hydride resulted in a dark red solution. Following work-up, red crystals of [(Tren)URe ACHTUNGTRENNUNG(h5C5H5)2] [6, Tren DMSB =N(CH2CH2NSiMe2tBu)3]) were isolated in 46 % yield (Scheme 1). The H NMR spectrum exhibits paramagnetically shifted resonances over the range d= 19.85 to +5.64 ppm. The meff of 6 ranged from 0.31 to 2.86 mB over the temperature range 1.8–300 K (Figure 1), and clearly tends to zero, which is characteristic of a H4 uranium(IV) complex. The crystal structure of complex 6 is illustrated in Figure 2 with selected bond lengths. The uranium centre is five-coordinate, but, setting the U(1) N(4) bond to one [a] B. M. Gardner, Dr. J. McMaster, F. Moro, Dr. W. Lewis, Prof. A. J. Blake, Dr. S. T. Liddle School of Chemistry, University of Nottingham University Park, Nottingham, NG7 2RD (UK) Fax: (+44) 115-951-3563 E-mail : stephen.liddle@nottingham.ac.uk Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201100682.

Grafting Derivatives of Mn6 Single-Molecule Magnets with High Anisotropy Energy Barrier on Au(111) Surface

We study the magnetic properties of two new functionalized single-molecule magnets belonging to the Mn 6 family (general formula [Mn (III)6O2(R-sao)6(O2C-th)2L(4-6)], where R=H (1) or Et (2), HO2C-th=3-thiophene carboxylic acid, L=EtOH, H2O and saoH2 is salicylaldoxime) and their grafting on the Au(111) surface. Complex 1 exhibits spin ground-state S=4, as the result of ferromagnetic coupling between the two antiferromagnetic Mn (III) 3 triangles, while slight structural changes in complex 2, switch the dominant magnetic exchange interactions from anti- to ferromagnetic, enhancing the spin ground-state to S=12 and, consequently, the effective energy barrier for the relaxation of magnetization. Direct-current and alternating-current magnetic susceptibility measurements show that the functionalized complexes preserve the main magnetic properties of the corresponding not-functionalized Mn 6 clusters (i.e., total spin value and magnetic behavior as a function of temperature), though a reduction of the anisotropy barrier is observed in complex 2. For both complexes, the -O2C-th functionalization allows the direct grafting on Au(111) surface by liquid-phase deposition. X-ray photoemission spectroscopy demonstrates that the stoichiometry of the molecular cores is preserved after grafting. Scanning tunneling microscopy (STM) reveals a sub-monolayer distribution of isolated clusters with a slightly higher coverage for complex 1. The cluster stability in the STM images and the S-2p energy positions demonstrate, for both derivatives, the strength of the grafting with the gold surface.

Addressing the magnetic properties of sub-monolayers of single-molecule magnets by X-ray magnetic circular dichroism

We report on a comparative study of electronic and magnetic properties of Mn6 single-molecule magnets (SMMs) grafted on gold surface. Two derivatives with spin-ground states S=4 and S=12 have been functionalized with 3-tp-CO2- (3-thiophene carboxylate, tpc) ligands and characterized as thick films (TFs) as well as sub-monolayers (sMLs) by synchrotron based techniques. X-ray absorption spectroscopy at the Mn L2,3 edges shows the modification of the spectral lineshape in the sMLs with respect to the TFs suggesting that the local symmetry at the Mn sites changes once the molecules are deposited on gold surface. In spite of this, the expected MnIII oxidation state is preserved. X-ray magnetic circular dichroism (XMCD) spectra show that the total magnetic moment is only given by spin part because of the quenched orbital moment. Moreover, variable temperature and variable field XMCD spectra reveal an effective decrease of the Mn spin moment for both derivatives.