The preparation, characterization, and X-ray structure are reported for the single-molecule magnet (PPh4)[Mn12O12(O2CPh)16(H2O)4]·8(CH2Cl2) (2). Complex 2 crystallizes in the triclinic space group P1̄, which at 213 K has a = 17.2329(2), b = 17.8347(2), c = 26.8052(2) Å, α = 90.515(2), β = 94.242(2), γ = 101.437(2)°, and Z = 2. The salt consists of PPh4 + cations and [Mn12O12(O2CPh)16(H2O)4]- anions. The (Mn12O12)15+ core of the anion is formed by an external ring of eight Mn atoms bridged by μ3−O2- ions to an internal tetrahedron of four Mn atoms. Because of disorder in both phenyl rings and solvate molecules, it was difficult to use bond valence sum values to determine definitively the oxidation state of each Mn atom. There is a Mn4O4 cubane unit in the internal part of the molecule and these Mn atoms are all MnIV ions. For the eight “external” Mn atoms the bond valence sum values did not define well their oxidation states. For these eight Mn atoms, it was not possible to determine whether a trapped-valence MnIIMnIII 7 or an electronically delocalized description is appropriate. High-frequency EPR (HFEPR) data were collected for the previously structurally characterized MnIV 4MnIII 7MnII valence-trapped salt (PPh4)[Mn12O12(O2CEt)16(H2O)4] (1) at 328.2 and 437.69 GHz. In the high magnetic field the crystallites orient and the HFEPR spectra are pseudo−single-crystal like, not powder patterns. The spectral features are attributed to the fine structure expected for a S = 19/2 complex experiencing axial zero-field splitting D Ŝ z 2, where D = −0.62 cm-1. The sign of D was definitively determined by the temperature dependence of the spectrum. Complex 2 exhibits one out-of-phase ac magnetic susceptibility (χ‘ ‘M) signal in the 3−6 K range. The temperature of the χ‘ ‘M peak is frequency dependent, as expected for a single-molecule magnet. The rate at which the direction of magnetization reverses from “up” to “down” was evaluated from χ‘ ‘M data collected at various frequencies (1−1512 Hz) of oscillation of the ac magnetic field. This gives magnetization relaxation rates in the 2.86−4.51 K range for complex 2 and in the 3.2−7.2 K range for complex 1. Rates were also determined in the 1.80−2.50 K range for complex 1 via magnetization decay experiments. In this latter case, the polycrystalline sample is magnetically saturated in a large dc field. After the magnetic field is rapidly decreased to zero, the decay of the magnetization to zero is monitored. The rates evaluated by both the frequency dependence of the out-of-phase ac signal and dc relaxation decay experiments for complex 1 fit on an Arrhenius plot to give an activation energy of U eff = 57 K and a preexponential rate of 1/τ0 = 7.2 × 107 s-1. From the HFEPR data, complex 1 has a S = 19/2 ground state with D = −0.62 cm-1. This gives a potential-energy barrier of U = 79 K for the double-well potential-energy diagram. The value of U eff is less than the barrier height U, because when individual [Mn12 -] anions convert from spin “up” to spin “down”, they can not only...

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NiCl2-4SC(NH2)2 (DTN) is a quantum S = 1 chain system with strong easy-pane anisotropy and a new candidate for the Bose-Einstein condensation of the spin degrees of freedom. ESR studies of magnetic excitations in DTN in fields up to 25 T are presented. Based on analysis of the single-magnon excitation mode in the high-field spin-polarized phase and previous experimental results [ Phys. Rev. Lett. 96, 077204 (2006)], a revised set of spin-Hamiltonian parameters is obtained. Our results yield D = 8.9 K, Jc = 2.2 K, and J a,b = 0.18 K for the anisotropy, intrachain, and interchain exchange interactions, respectively. These values are used to calculate the antiferromagnetic phase boundary, magnetization and the frequency-field dependence of two-magnon bound-state excitations predicted by theory and observed in DTN for the first time. Excellent quantitative agreement with experimental data is obtained. PACS numbers: 75.40.Gb, 75.10.Jm Antiferromagnetic (AFM) quantum spin-1 chains have been the subject of intensive theoretical and experimental studies, fostered especially by the Haldane conjecture [1]. Due to quantum fluctuations, an isotropic spin-1 chain has a spin-singlet ground state separated from the first excited state by a gap ∆ ∼ 0.41J [2], where J is the exchange interaction. As shown by Golinelli et al. [3], the presence of a strong easy-plane anisotropy D can significantly modify the excitation spectrum, so that the gap size is not determined by the strength of the AFM quantum fluctuations exclusively, but depends on the dimensionless parameter ρ = D/J. The Haldane phase is predicted to survive up to ρ c = 0.93 [4], where the system undergoes a quantum phase transition. For ρ > ρ c the gap reopens, but its origin is dominated by the anisotropy D, and the system is in the so-called large-D regime. While the underlying physics of Haldane chains is fairly well understood, relatively little is known about the magnetic properties (and particularly the elementary excitation spectrum) of nonHaldane S = 1 AFM chains in the large-D phase. Intense theoretical work and numerous predictions [3,4,5,6,7,8,9,10] make the experimental investigation of large-D spin-1 chains a topical problem in low-dimensional magnetism.Recently, weakly-coupled spin-1 chains have attracted renewed interest due to their possible relevance to the fieldinduced Bose-Einstein condensation (BEC) of magnons. When the field H, applied perpendicular to the easy plane, exceeds a critical value H c1 (defined at T = 0), the gap closes and the system undergoes a transition into an XY -like AFM phase with a finite magnetization and AFM magnon excitations. If the spin Hamiltonian has axial symmetry with respect to the applied field, the AFM ordering can be described as BEC of magnons by mapping the spin-1 system into a gas of semi-hard-core bosons [11]. The applied field plays the role of a chemical potential, changing the boson population. In accordance with mean-field BEC theory [12,13,14], the phasediagram boundary for a three-dimensional system sh...

High-field and -frequency electron paramagnetic resonance (HFEPR) spectroscopy has been used to study two complexes of high-spin manganese(III), d(4), S = 2. The complexes studied were (tetraphenylporphyrinato)manganese(III) chloride and (phthalocyanato)manganese(III) chloride. Our previous HFEPR study (Goldberg, D. P.; Telser, J.; Krzystek, J.; Montalban, A. G.; Brunel, L.-C.; Barrett, A. G. M.; Hoffman, B. M. J. Am. Chem. Soc. 1997, 119, 8722-8723) included results on the porphyrin complex; however, we were unable to obtain true powder pattern HFEPR spectra, as the crystallites oriented in the intense external magnetic field. In this work we are now able to immobilize the powder, either in an n-eicosane mull or KBr pellet and obtain true powder pattern spectra. These spectra have been fully analyzed using spectral simulation software, and a complete set of spin Hamiltonian parameters has been determined for each complex. Both complexes are rigorously axial systems, with relatively low magnitude zero-field splitting: D approximately -2.3 cm(-)(1) and g values quite close to 2.00. Prior to this work, no experimental nor theoretical data exist for the metal-based electronic energy levels in Mn(III) complexes of porphyrinic ligands. This lack of information is in contrast to other transition metal complexes and is likely due to the dominance of ligand-based transitions in the absorption spectra of Mn(III) complexes of this type. We have therefore made use of theoretical values for the electronic energy levels of (phthalocyanato)copper(II), which electronically resembles these Mn(III) complexes. This analogy works surprisingly well in terms of the agreement between the calculated and experimentally determined EPR parameters. These results show a significant mixing of the triplet (S = 1) excited state with the quintet (S = 2) ground state in Mn(III) complexes with porphyrinic ligands. This is in agreement with the experimental observation of lower spin ground states in other metalloporphyrinic complexes, such as those of Fe(II) with S = 1.

The trigonal pyramidal complex [Mn4O3Cl(O2CCH3)3(dbm)3], where dbm- is the monoanion of dibenzoylmethane, functions as a single-molecule magnet. High-field EPR data are presented for an oriented microcrystalline sample to characterize the electronic structure of the MnIVMnIII 3 complex. These data show that the complex has a S = 9/2 ground state, experiencing axial zero-field splitting (DŜ z 2) with D = −0.53 cm-1 and a quartic zero-field splitting (B 4 0Ô4 0)with B 4 0 = −7.3 × 10-5 cm-1. Magnetization versus external magnetic field data were collected for an oriented single crystal in the 0.426−2.21 K range. At temperatures below 0.90 K hysteresis is seen. Steps are seen on each hysteresis loop. This is clear evidence that each MnIVMnIII 3 complex functions as a single-molecule magnet that is magnetizable. Furthermore, the steps on the hysteresis loops are due to resonant magnetization quantum mechanical tunneling. In response to an external field each molecule reverses its direction of magnetization not only by being thermally activated over a potential-energy barrier, but by the magnetization tunneling through the barrier. Additional evidence for resonant magnetization tunneling was found in the change in the temperature at which the out-of-phase ac magnetic susceptibility is observed as a function of an external dc field. The results of magnetization relaxation experiments carried out in the 0.394−0.700 K range are presented. These data are combined with the ac susceptibility data taken at higher temperatures to give an Arrhenius plot of the logarithm of the magnetization relaxation rate versus inverse absolute temperature. The temperature-dependent part of this plot gives an activation barrier of 11.8 K. Below 0.6 K the relaxation rate is independent of temperature with a rate of 3.2 × 10-2 s-1. This S = 9/2 single-molecule magnet exhibits a tunneling of its direction of magnetization at a rate of 3.2 × 10-2 s-1 in the 0.394−0.600 K range. Thus, resonant magnetization tunneling is seen for a half-integer-spin (S = 9/2) ground-state magnet in the absence of an external magnetic field. The transverse component of the small magnetic field from the nuclear spins is probably the origin of this tunneling.

With the discovery of molecular complexes exhibiting slow relaxation of the magnetization and magnetic hysteresis at low temperature, research activity in the field of molecular magnetism based on coordination compounds has experienced spectacular growth. [1] These nanomagnets, called single-molecule magnets (SMMs), [1][2][3] straddle the quantum/ classical interface showing quantum effects, such as quantum tunneling of the magnetization and quantum phase interference, and have potential applications in molecular spintronics, ultra-high density magnetic information storage, and quantum computing at the molecular level. [3] The motivation of much of this research activity has been provided by the prospect of integrating SMMs into nanosized devices. The origin of the SMM behavior is the existence of an energy barrier that prevents reversal of the molecular magnetization, [1] although the currently observed energy barriers are (relatively) low and therefore SMMs act as magnets only at very low temperature. To increase the height of the energy barrier and therefore to improve the SMM properties, systems with large spin-ground states and/or with large magnetic anisotropy are required. The early examples of SMMs were clusters of transition metal ions, [2] but recently mixed 3d/4f metal aggregates, [4] low-nuclearity 4f metal complexes, [5] and even mononuclear complexes (called single-ion magnets, SIMs) of lanthanide, [6] actinide, [7] and transition-metal ions [8] have been reported to exhibit slow relaxation of the magnetization.It should be noted that for integer-spin systems with D < 0 fast quantum tunneling of the magnetization (QTM) through the mixing of AE Ms levels may suppress the observation of slow magnetic relaxation through a thermally activated mechanism. QTM is promoted by transverse zero-field splitting (E), hyperfine interactions, and/or dipolar interactions. [1] The application of a small direct current (dc) field, stabilizing the negative Ms levels with regard to the positive ones, may remove the degeneracy of the AE Ms levels on either side of the energy barrier, tilting the system out of resonance and, on occasion, enabling the thermally activated mechanism. For non-integer spin systems with D < 0, the mixing of the degenerate ground state AE Ms levels through transverse anisotropy (E) is forbidden, thus favoring observation of the thermally activated relaxation process. [9] This situation, together with the fact that mononuclear species can exhibit larger anisotropies than their multinuclear counterparts (the

High-frequency and high-field electron paramagnetic resonance (HFEPR) spectroscopy (using frequencies of approximately 90-550 GHz and fields up to approximately 15 T) has been used to probe the non-Kramers, S = 1, Ni(2+) ion in a series of pseudotetrahedral complexes of general formula NiL(2)X(2), where L = PPh(3) (Ph = phenyl) and X = Cl, Br, and I. Analysis based on full-matrix solutions to the spin Hamiltonian for an S = 1 system gave zero-field splitting parameters: D = +13.20(5) cm(-1), /E/ = 1.85(5) cm(-1), g(x) = g(y) = g(z) = 2.20(5) for Ni(PPh(3))(2)Cl(2). These values are in good agreement with those obtained by powder magnetic susceptibility and field-dependent magnetization measurements and with earlier, single-crystal magnetic susceptibility measurements. For Ni(PPh(3))(2)Br(2), HFEPR suggested /D/ = 4.5(5) cm(-1), /E/ = 1.5(5) cm(-1), g(x) = g(y) = 2.2(1), and g(z) = 2.0(1), which are in agreement with concurrent magnetic measurements, but do not agree with previous single-crystal work. The previous studies were performed on a minor crystal form, while the present study was performed on the major form, and apparently the electronic parameters differ greatly between the two. HFEPR of Ni(PPh(3))(2)I(2) was unsuccessful; however, magnetic susceptibility measurements indicated /D/ = 27.9(1) cm(-1), /E/ = 4.7(1), g(x) = 1.95(5), g(y) = 2.00(5), and g(z) = 2.11(5). This magnitude of the zero-field splitting ( approximately 840 GHz) is too large for successful detection of resonances, even for current HFEPR spectrometers. The electronic structure of these complexes is discussed in terms of their molecular structure and previous electronic absorption spectroscopic studies. This analysis, which involved fitting of experimental data to ligand-field parameters, shows that the halo ligands act as strong pi-donors, while the triphenylphosphane ligands are pi-acceptors.