The symmetry breaking effects for quantum tunneling of the magnetization in Mn12-acetate, a molecular nanomagnet, represent an open problem. We present structural evidence that the disorder of the acetic acid of crystallization induces sizable distortion of the Mn(III) sites, giving rise to six different isomers. Four isomers have symmetry lower than tetragonal and a nonzero second-order transverse magnetic anisotropy, which has been evaluated using a ligand field approach. The result of the calculation leads to an improved simulation of electron paramagnetic resonance spectra and justifies the tunnel splitting distribution derived from the field sweep rate dependence of the hysteresis loops.

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-Zn-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.

We use the strategy of diamagnetic substitution for obtaining information on the crystal field effects in paramagnetic rare earth ions using the homologous series of compounds with the diamagnetic tropolonato ligand, Ln(Trp)(HBPz(3))(2), and the paramagnetic semiquinone ligand, Ln(DTBSQ)(HBPz(3))(2), (DTBSQ = 3,5-di-tert-butylsemiquinonato, Trp = tropolonate, HBPz(3)= hydrotrispyrazolylborate) for Ln = Sm(iii), Eu(iii), Gd(iii), Tb(iii), Dy(iii), Ho(iii), Er(iii) or Yb(iii). The X-ray crystal structure of a new form of tropolonate derivative is presented, which shows, as expected, a marked similarity with the structure of the semiquinonate derivative. The Ln(Trp)(HBPz(3))(2) derivatives were then used as a reference for the qualitative determination of crystal field effects in the exchange coupled semiquinone derivatives. Through magnetisation and susceptibility measurements this empirical diamagnetic substitution method evidenced for Er(iii), Tb(iii), Dy(iii) and Yb(iii) derivatives a dominating antiferromagnetic coupling. The increased antiferromagnetic contribution compared to other radical-rare earth metal complexes formed by nitronyl nitroxide ligands may be related to the increased donor strength of the semiquinone ligand.

A tetrairon(III) single-molecule magnet is deposited using a thermal evaporation technique in high vacuum. The chemical integrity is demonstrated by time-of-flight secondary ion mass spectrometry on a film deposited on Al foil, while superconducting quantum interference device magnetometry and alternating current susceptometry of a film deposited on a kapton substrate show magnetic properties identical to the pristine powder. High-frequency electron paramagnetic resonance spectra confirm the characteristic behavior for a system with S = 5 and a large Ising-type magnetic anisotropy. All these results indicate that the molecules are not damaged during the deposition procedure keeping intact the single-molecule magnet behavior.

The strongest antiferromagnetic coupling to Gd(III) so far reported was found for the complex [Gd(Hbpz(3))(2)(dtbsq)] small middle dot2 CHCl(3) (1; Hbpz(3)=hydrotris(pyrazolyl)borate; dtbsq=3,5-di-tert-butylsemiquinonato; see structure). At 245 K the magnetic susceptibility of 1 is lower than expected for two uncorrelated spins of 7/2 and 1/2, and the lowering of chiT with increasing temperature suggests that this is due to antiferromagnetic interaction between Gd(III) and the radical.

The coherence time of the 17‐electron, mixed sandwich complex [CpTi(cot)], (η8‐cyclooctatetraene)(η5‐cyclopentadienyl)titanium, reaches 34 μs at 4.5 K in a frozen deuterated toluene solution. This is a remarkable coherence time for a highly protonated molecule. The intramolecular distances between the Ti and H atoms provide a good compromise between instantaneous and spin diffusion sources of decoherence. Ab initio calculations at the molecular and crystal packing levels reveal that the characteristic low‐energy ring rotations of the sandwich framework do not yield a too detrimental spin‐lattice relaxation because of their small spin–phonon coupling. The volatility of [CpTi(cot)] and the accessibility of the semi‐occupied, non‐bonding dnormalz2 orbital make this neutral compound an ideal candidate for single‐qubit addressing on surface and quantum sensing in combination with scanning probe microscopy.

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

Valence tautomerism (VT) defines reversible interconversions between two or more redox isomers. It is established that these interconversions can be stimulated by temperature and light irradiation. [1] For example, the diamagnetic [Co III -(Me 2 tpa)(DBCat)]PF 6 ·C 6 H 5 CH 3 complex (1) (Me 2 tpa = bis(6-methyl-(2-pyridylmethyl)) (2-pyridylmethyl)amine, DBCat = 3,5-di-tert-butylcatecholato) was found to undergo a thermally induced interconversion in the solid state yielding the redox isomer characterized by the high-spin Co II -semiquinonato (hs-Co II -SQ) charge distribution (see Scheme 1). [2,3] The observed transition can be formally described as the result of an entropy-driven intramolecular electron transfer involving the donor catecholato and the cobalt(III) acceptor. At cryogenic temperatures, laser irradiation of the solid compound at 904 nm, where a ligand-tometal charge transfer (LMCT) occurs, was found by bulk magnetic measurements [2] to induce the same process, affording the hs-Co II -semiquinonato species as a metastable phase in 90 % yield with a rather long lifetime (two weeks at 9 K).Soft X-ray absorption spectroscopy (XAS) is an elementsensitive synchrotron-based technique and provides a powerful tool to study the electronic and chemical structure of a specific atom and its coordination environment. It is particularly powerful in the magnetic study of 3d metal complexes. [4] With the additional asset of very high detection sensitivity, XAS has been effectively used in the characterization of systems with multiple quasi degenerated electronic states, [5] including very diluted and nanostructured systems. [6] We have found that for 1 this technique not only yields this important information, but also intrinsically provides the perturbation for inducing interconversion between the two redox isomers. This is an unprecedented result and we believe it to be particularly important for the study of all the complexes exhibiting photochromism. Figure 1 shows the temperature dependence of the cobalt L 3 -edge X-ray absorption spectra of 1 (the L 2,3 spectra are shown in Figure S1 of the Supporting Information). The spectra were obtained with a X-ray flux of 10 8 photons s À1 on Scheme 1. The two different electronic configurations involved in VT process. Figure 1. Temperature dependence of the normalized L 3 -edge X-ray absorption spectra of 1, acquired with a low X-ray photon flux on the sample. The underlying light gray and dark gray curves show the theoretical spectra expected for pure Co II and Co III states, respectively (see text for details).[**] We acknowledge ESRF for provision of synchrotron radiation facilities. We thank ID08 beamline staff for assistance in using the beamline and Dr. N. Brookes for fruitful discussions. This work was funded by EU through project MOLSPINQIP (STREP 211284) and NoE MAGMANet (NMP3- CT-2005-515767) and by the Italian CNR through the PROMO project and Commessa CNR PM.P05.012: "Nanosistemi organizzati di magneti molecolari".Supporting information for this article ...