An invited Meeting Report on the first European Conference on Molecular Spintronics, held in Bologna (Italy)

Controlling the dynamics of spins on surfaces is pivotal to the design of spintronic1 and quantum computing2 devices. Proposed schemes involve the interaction of spins with graphene to enable surface-state spintronics3,4, and electrical spin-manipulation4-11. However, the influence of the graphene environment on the spin systems has yet to be unraveled12. Here we explore the spin-graphene interaction by studying the classical and quantum dynamics of molecular magnets13 on graphene. While the static spin response remains unaltered, the quantum spin dynamics and associated selection rules are profoundly modulated. The couplings to graphene phonons, to other spins, and to Dirac fermions are quantified using a newly-developed model. Coupling to Dirac electrons introduces a dominant quantum-relaxation channel that, by driving the spins over Villain’s threshold, gives rise to fully-coherent, resonant spin tunneling. Our findings provide fundamental insight into the interaction between spins and graphene, establishing the basis for electrical spin-manipulation in graphene nanodevices.

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The homoleptic mononuclear compound [Co(bpp-COOMe) ](ClO ) (1) (bpp-COOMe=methyl 2,6-di(pyrazol-1-yl)pyridine-4-carboxylate) crystallizes in the monoclinic C2/c space group, and the cobalt(II) ion possesses a pseudo-octahedral environment given by the two mer-coordinated tridentate ligands. Direct-current magnetic data, single-crystal torque magnetometry, and EPR measurements disclosed the easy-axis nature of this cobalt(II) complex, which shows single-molecule magnet behavior when a static field is applied in alternating-current susceptibility measurements. Diamagnetic dilution in the zinc(II) analogue [Zn(bpp-COOMe) ](ClO ) (2) afforded the derivative [Zn Co (bpp-COOMe) ](ClO ) (3), which exhibits slow relaxation of magnetization even in zero field thanks to the reduction of dipolar interactions. Theoretical calculations confirmed the overall electronic structure and the magnetic scenario of the compound as drawn by experimental data, thus confirming the spin-phonon Raman relaxation mechanism, and a direct quantum tunneling in the ground state as the most plausible relaxation pathway in zero field.

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The molecular structures and magnetic properties of homoleptic iron(ii) compounds [Fe(bpp-COOMe)](ClO) (1) and [Fe(bpp-triolH)](ClO) (2) have been investigated to ascertain their spin crossover (SCO) behaviour. In these hexacoordinated complexes, the bpp (2,6-bis(pyrazol-1-yl)pyridine) ligands adopt a mer-mer coordination mode and carry COOMe or C(O)NHC(CHOH)para substituents, respectively, on the central pyridyl ring. In spite of the almost equal donor power of the ligands to the iron(ii) centre, the two compounds feature different spin state configurations at room temperature. Compound 1 displays a highly-distorted octahedral environment around the iron(ii) centre, which adopts a high spin (HS) state at all temperatures, even under an external applied pressure up to 1.0 GPa. By contrast, 2 is characterized by a more regular octahedral coordination around the metal ion and exhibits a low spin (LS) configuration at or below room temperature. However, it shows a thermally-induced SCO behaviour at T > 400 K, along with Light-Induced Excited Spin State Trapping (LIESST) at low temperature, with T = 38 K. Since DFT (U)M06/6-311+G(d) geometry optimizations in vacuo indicate that both complexes should adopt a HS state and a highly-distorted coordination geometry, the stabilization of a LS configuration in 2 is ultimately ascribed to the effect of intermolecular hydrogen bonds, which align the [Fe(bpp-triolH)] cations in 1D chains and impart profound differences in the geometric arrangement of the ligands.

S2 1. Synthesis General. Unless otherwise noted, reagents and solvents were of commercial origin and used without further purification. Naphthalene was resublimed and NaSCN was recrystallized from acetone before use. Dichloromethane and diethyl ether for the synthesis of 1a•Et2O were purified using an Inert Technologies solvent purification system. For the synthesis of 2a4CHCl32Et2O and 2b anhydrous dichloromethane (Sigma-Aldrich) was directly used, while chloroform was washed with water, stirred over CaCl2 overnight and distilled under N2; diethyl ether was pre-dried over CaCl2 overnight and distilled from its sodium benzophenone ketyl solution under N2; n-hexane was dried over 4A molecular sieves; all these solvents were degassed by three freeze-pump-thaw cycles. 1 The H2tpda ligand was prepared as described elsewhere, 2,3 recrystallized from boiling methanol or 2-propanol and checked by 1 H NMR spectroscopy and melting point measurement. All reactions involving chromium(II) complexes were carried out under Ar or N2 atmosphere using Schlenk techniques or glovebox methods. Elemental analysis was carried out by Microlab Kolbe (Oberhausen, Germany, 1a•Et2O) or on a Carlo Erba EA1110 CHNS-O automatic analyzer (2a4CHCl32Et2O and 2b). The IR spectra were measured on a Nicolet 6700 FT-IR spectrometer using a Smart iTR accessory between 600 and 4000 cm-1 with 4 cm-1 resolution (1a•Et2O) or on a JASCO 4700 FT-IR spectrometer between 400 and 4000 cm-1 with 2 cm −1 resolution (2a4CHCl32Et2O and 2b). Electrospray Ionization Mass Spectrometry (ESI-MS) was performed on an Agilent Technologies 6310A Ion Trap LC-MS(n) spectrometer. Synthesis of [Cr3(dpa)4Cl2]•Et2O (1a•Et2O). Complex 1a was prepared following a literature procedure 4,5 and isolated as the monodiethylether solvate 6,7 by layering a dichloromethane solution with diethyl ether, yielding a crop of dark green crystals in 60% yield after a week of diffusion. Anal. Calcd for

The structure of pentachromium(ii) extended metal atom chain [Cr(tpda)Cl] (2), which behaves as a single molecule magnet at low temperature, was investigated by Density Functional Theory (DFT) calculations and spectroscopic studies without the constraints of a crystal lattice (Htpda = N,N-bis(pyridin-2-yl)pyridine-2,6-diamine). DFT studies both in the gas phase and including CHCl solvent effects indicate that an unsymmetric structure (C point group), with pairs of formally quadruply-bonded metal ions and one terminal metal center, is slightly more stable (2.9 and 3.9 kcal mol) than a symmetric structure (D point group). Isotopically-labelled samples (2-d and 2-d) have then been prepared to aid in molecular symmetry determination by combined H andH NMR studies in dichloromethane solution. The spectra are strongly suggestive of a symmetric (D) framework, indicating fast shuttling between the two unsymmetric forms over the timescale of NMR experiments. Procedures for a high-yield Pd-free synthesis of Htpda and for site-selective post-synthetic H/D exchange of aromatic Htpda hydrogens are also reported.