A novel class of tetrairon(III) single-molecule magnets with graphene-binding groups

Tripods of general formula R’–O–CH₂C(CH₂OH)₃ are excellent site-specific ligands for the preparation of functionalized Fe₄ single-molecule magnets. Herein, we describe the synthesis and characterization of two novel complexes designed to bind graphene surfaces, in which the R group consists of an alkyl spacer –(CH₂)_n– (n = 6 and 10) and a terminal pyrenyl moiety. The site-specific ligand substitution on [Fe₄(OMe)₆(dpm)₆] (Hdpm = dipivaloylmethane) with the new tripods has been studied with ²H NMR on isotopically-enriched samples, revealing that, once formed, these clusters are stable in solution over long timescales. It was not possible to isolate the new compounds as crystalline solids, nevertheless they were chemically characterized by elemental analysis and ¹H NMR. The presence of the pyrenyl ending groups prompted us to investigate the effect of metal complexation on fluorescence, and a full pyrene-to-iron cluster excitation energy transfer was observed. The analysis of the magnetic behaviour revealed an S = 5 ground spin state with a negative zero-field splitting parameter D = ?0.42 cm^?1.     

A ligand-field study of the ground spin-state magnetic anisotropy in a family of hexanuclear Mn(III) single-molecule magnets.

A ligand field analysis of two structurally related hexanuclear Mn(iii) coordination complexes reveals that the observed difference in their ground spin-state anisotropy originates from the difference in projection coefficients of the single-ion anisotropy to spin states of different total spin quantum-number, S, rather than the geometrical distortions of the metal ions. Furthermore we show that the single-ion second order anisotropy induces fourth and higher order anisotropy terms to the ground spin states of the studied systems, as a consequence of spin-state mixing effects due to the comparable magnitude of the single-ion second order anisotropy and the isotropic exchange parameters.     

Heterometallic [Mn5-Ln4] single-molecule magnets with high anisotropy barriers

The reaction of [Mn6O2(Piv)(10)(4-Me-py)(2.5)(PivH)(1.5)] (1) (py: pyridine, Piv: pivilate) with N-methyldiethanolamine (mdeaH2) and Ln(NO3)3 x 6 H2O in MeCN leads to a series of nonanuclear compounds [Mn5Ln4(O)6(mdea)2(mdeaH)2(Piv)6(NO3)4(H2O)2]2 MeCN (Ln=Tb(III) (2), Dy(III) (3), Ho(III) (4), Y(III) (5)). Single-crystal X-ray diffraction shows that compounds 2-5 are isostructural, with the central core composed of two distorted {Mn(IV)Mn(III)Ln2O4} cubanes sharing a Mn(IV) vertex, representing a new heterometallic 3d-4f motif for this class of ligand. The four new compounds display single-molecule magnet (SMM) behaviour, which is modulated by the lanthanide ion used. Moreover, the values found for Delta(eff) and tau(o) for 3 of 38.6 K and 3.0 x 10(-9) s respectively reveal that the complex 3 exhibits the highest energy barrier recorded so far for 3d-4f SMMs. The slow relaxation of the magnetisation for 3 was confirmed by mu-SQUID measurements on an oriented single crystal and the observation of M versus H hysteresis loops below 1.9 K.     

A family of hexanuclear Mn(III) single-molecule magnets

In an attempt to employ salicylic acid (HOsalH), 2,6-dihydroxy benzoic acid {(HO)₂PhCO₂H}, and naphthalene-1,8-dicarboxylic acid {1,8-naph(CO₂H)₂} in Mn(III) salicylaldoximate chemistry as a means to alter the structural identity of the hexanucluear clusters usually obtained from this reaction system, we have isolated a family of hexanuclear Mn(III) complexes based on salicyladloxime (saoH₂) and 2-hydroxy-1-naphthaldehyde oxime (naphthsaoH₂). Five hexanuclear clusters, [Mn₆O₂(sao)₆(HOsal)₂(EtOH)₄]·EtOH (1·EtOH), [Mn₆O₂(sao)₆{1,8-naph(CO₂Me)(CO₂)}₂(MeOH)₆]·3MeOH (2·3MeOH), [Mn₆O₂(naphthsao)₆{1,8-naph(CO₂Et)(CO₂)}₂(EtOH)₆] (3·2MeOH), [Mn₆O₂(naphthsao)₆(MeCO₂)₂(EtOH)₄]·2H₂O (4·2H₂O), and [Mn₆O₂(naphthsao)₆{(HO)₂PhCO₂}₂(EtOH)₄]·4EtOH (5·4EtOH), have been synthesized and characterized by single-crystal X-ray crystallography. The magnetic properties of 3, 4, and 5 are discussed.     

A polyoxometalate-based single-molecule magnet with an S = 21/2 ground state

Ligand modification transforms a polyoxometalate-anchored cubane-type [Mn(III)(3)Mn(IV)O(4)] core into a centrosymmetric [Mn(III)(6)Mn(IV)O(8)] di-cubane cluster, and restores the slow magnetization relaxation characteristics typical for [Mn(4)O(4)] cubane-based single-molecule magnets.     

A novel tripodal ligand with organosulfur alligator clips for deposition of tetrairon(III) single-molecule magnets on gold

A propeller-like tetrairon(III) complex functionalized with two 1,2-dithiolan-3-yl groups was synthesized and magnetically characterized. The compound has formula [Fe₄(thioctic)₂(dpm)₆] and was specifically designed to be grafted on gold surfaces. It was prepared by reacting [Fe₄(OMe)₆(dpm)₆] (Hdpm = dipivaloylmethane) with a new tripodal ligand, H₃thioctic, obtained by esterification of 2,2-bis(hydroxymethyl)-propane-1,3-diol with (±)-α-lipoic acid (also known as thioctic acid). Direct current and alternating current magnetic measurements revealed single-molecule magnet behaviour with an effective anisotropy barrier of 14.0(1) K resulting from a high spin (S = 5) ground state and an easy-axis anisotropy.     

Slow magnetic relaxation of a ferromagnetic Ni(II)5 cluster with an S = 5 ground state

Complex [Ni 5{pyCOpyC(O)(OMe)py} 2(O 2CMe) 4(N 3) 4(MeOH) 2].2MeOH.2.6H 2O ( 1.2MeOH.2.6H 2O) was synthesized by the reaction of Ni(O 2CMe) 2.4H 2O with pyCOpyCOpy and NaN 3 in refluxing MeOH. It crystallizes in the monoclinic C2/ c space group and consists of five Ni (II) atoms in a helical arrangement. Direct current magnetic susceptibility studies reveal ferromagnetic interactions between the Ni (II) ( S = 1) ions, stabilizing an S = 5 ground state. Alternating current susceptibility experiments revealed the existence of out-of-phase signals indicative of slow magnetic relaxation. Analysis of the signals showed that they are composite, suggesting more than one relaxation process, while analysis of their magnitudes suggests not all molecules undergo slow magnetic relaxation. Magnetization field-sweep experiments revealed hysteresis at 1.8 K, and magnetization decay experiments clearly verified the appearance of slow magnetic relaxation at that temperature.     

A novel undecametallic iron(III) cluster with an S = (11)/(2) spin ground state

The reaction of [NEt(4)](2)[Fe(2)OCl(6)] with sodium benzoate, 4,6-dimethyl-2-hydroxypyrimidine (dmhp), and 1,1,1-tris(hydroxymethyl)ethane (H(3)thme) gives the undecametallic compound [NEt(4)][Fe(11)O(4)(O(2)CPh)(10)(thme)(4)(dmhp)(2)Cl(4)]. X-ray crystallography, EPR spectroscopy, bulk magnetic susceptibility studies, and low-temperature single-crystal magnetic measurements were used to characterize the compound. Magnetic measurements indicate an S = (11)/(2) ground state with the parameters g = 2.03 and D = -0.46 cm(-)(1). Single-crystal magnetic studies show hysteresis of molecular origin at T < 1.2 K with fast quantum mechanical tunneling at zero field.     

One pot grafting of tetrairon(III) single molecule magnets on silicon

Herein we report on the Si grafting of two Fe₄ derivatives, [Fe₄(Lⁱ)₂(tmhd)₆], in which tmhd is 2,2,6,6-tetramethylheptane-3,5-dionate and H₃Lⁱ = R–C(CH₂OH)₃ is a tripodal ligand with R = CH₂CH–CH₂–O–CH₂ (H₃L¹) and CH₂CH–(CH₂)₉–O–CH₂ (H₃L²). These complexes were specifically designed to be directly anchored on the H-terminated silicon surface via the hydrosilylation reaction. The complexes were grafted by a one pot route based on the photoinduced hydrosilylation followed by a ligand exchange step in the same reaction solution. The resulting decorated surfaces were characterized using X-ray photoelectron spectroscopy (XPS), attenuated total reflection infrared spectroscopy (ATR-IR) and atomic force microscopy (AFM).     

Single-chain magnet (NEt4)[Mn2(5-MeOsalen)2Fe(CN)6] Made of Mn(III)-Fe(III)-Mn(III) trinuclear single-molecule magnet with an S(T) = 9/2 spin ground state

The cyano-bridged trinuclear compound, (NEt(4))[Mn(2)(salmen)(2)(MeOH)(2)Fe(CN)(6)] (1) (salmen(2)(-) = rac-N,N'-(1-methylethylene)bis(salicylideneiminate)), reported previously by Miyasaka et al. (ref 19d) has been reinvestigated using combined ac and dc susceptibility measurements. The strong frequency dependence of the ac susceptibility and the slow relaxation of the magnetization show that 1 behaves as a single-molecule magnet with an S(T) = (9)/(2) spin ground state. Its relaxation time (tau) follows an Arrhenius law with tau(0) = 2.5 x 10(-)(7) s and Delta(eff)/k(B) = 14 K. Moreover, below 0.3 K, tau saturates around 470 s, indicating that quantum tunneling of the magnetization becomes the dominant process of relaxation. (NEt(4))[Mn(2) (5-MeOsalen)(2)Fe(CN)(6)] (2) (5-MeOsalen(2)(-) = N,N'-ethylenebis(5-methoxysalicylideneiminate)) is a heterometallic one-dimensional assembly made of the trinuclear [Mn(III)(SB)-NC-Fe(III)-CN-Mn(III)(SB)] (SB is a salen-type Schiff-base ligand) motif similar to 1. Compound 2 has two types of bridges, a cyano bridge (-NC-) and a biphenolate bridge (-(O)(2)-), connecting Mn(III) and Fe(III) ions and the two Mn(III) ions, respectively. Both bridges mediate ferromagnetic interactions, as shown by modeling the magnetic susceptibility above 10 K with g(av) = 2.03, J(Mn)(-)(Fe)/k(B) = +6.5 K, and J'/k(B) = +0.07 K, where J' is the exchange coupling between the trimer units. The dc magnetic measurements of a single crystal using micro-SQUID and Hall-probe magnetometers revealed a uniaxial anisotropy (D(T)/k(B) = -0.94 K) with an easy axis lying along the chain direction. Frequency dependence of the ac susceptibility and time dependence of the dc magnetization have been performed to study the slow relaxation of the magnetization. A mean relaxation time has been found, and its temperature dependence has been studied. Above 1.4 K, both magnetic susceptibility and relaxation time are in agreement with the dynamics described in the 1960s by R. J. Glauber for one-dimensional systems with ferromagnetically coupled Ising spins (tau(0) = 3.7 x 10(-)(10) s and Delta(1)/k(B) = 31 K). As expected, at lower temperatures below 1.4 K, the relaxation process is dominated by the finite-size chain effects (tau'(0) = 3 x 10(-)(8) s and Delta(2)/k(B) = 25 K). The detailed analysis of this single-chain magnet behavior and its two regimes is consistent with magnetic parameters independently estimated (J'and D(T)) and allows the determination of the average chain length of 60 nm (or 44 trimer units). This work illustrates nicely a new strategy to design single-chain magnets by coupling ferromagnetically single-molecule magnets in one dimension.     

Mn4 single-molecule magnets with a planar diamond core and S=9

The syntheses and magnetic properties are reported for three Mn₄ single-molecule magnets (SMMs): [Mn₄(hmp)₆(NO₃)₂(MeCN)₂](ClO₄)₂·2MeCN (3), [Mn₄(hmp)₆(NO₃)₄]·(MeCN) (4), and [Mn₄(hmp)₄(acac)₂(MeO)₂](ClO₄)₂·2MeOH (5). In each complex there is a planar diamond core of Mn^III ₂Mn^II ₂ ions. An analysis of the variable-temperature and variable-field magnetization data indicate that all three molecules have intramolecular ferromagnetic coupling and a S=9 ground state. The presence of a frequency-dependent alternating current susceptibility signal indicates a significant energy barrier between the spin-up and spin-down states for each of these three Mn^III ₂Mn^II ₂ complexes. The fact that these complexes are SMMs has been confirmed by the observation of hysteresis in the plot of magnetization versus magnetic field measured for single crystals of complexes 3 and 4. The hysteresis loops for both of these complexes exhibit steps characteristic of quantum tunneling of magnetization. Complex 4 shows its first step at zero field, whereas the first step for complex 3 is shifted to −0.10 T. This shift is attributable to weak intermolecular antiferromagnetic exchange interactions present for complex 3.     

Magnetic and optical bistability in tetrairon(III) single molecule magnets functionalized with azobenzene groups

Tetrairon(III) complexes known as "ferric stars" have been functionalized with azobenzene groups to investigate the effect of light-induced trans-cis isomerization on single-molecule magnet (SMM) behaviour. According to DC magnetic data and EPR spectroscopy, clusters dispersed in polystyrene (4% w/w) exhibit the same spin (S = 5) and magnetic anisotropy as bulk samples. Ligand photoisomerization, achieved by irradiation at 365 nm, has no detectable influence on static magnetic properties. However, it induces a small but significant acceleration of magnetic relaxation as probed by AC susceptometry. The pristine behaviour can be almost quantitatively recovered by irradiation with white light. Our studies demonstrate that magnetic and optical bistability can be made to coexist in SMM materials, which are of current interest in molecular spintronics.     

Magneto-structural correlations: synthesis of a family of end-on azido-bridged manganese(II) dinuclear compounds with S = 5 spin ground state

The preparation of a series of multidentate pyridyl-imine ligands, L1-L3, and their reactivity with the Mn(II)/N3- system is described (L1 = [N,N-bis(pyridine-2-yl)benzylidene]ethane-1,2-diamine; L2 = [N,N-bis(pyridine-2-yl)benzylidene]propane-1,3-diamine, and L3 = [N,N-bis(pyridine-2-yl)benzylidene]butane-1,4-diamine). Complexes comprising dinuclear end-on bis(mu-azido)-bridged manganese(II) units of formulas [Mn2(L1)2(N3)4][Mn2(L1)2(N3)2(CH3OH)2](ClO4)2 (two cocrystallized dinuclear units, 1.2), [Mn2(L2)2(N3)2](ClO4)2 (3), and [Mn2(L3)2(N3)2](ClO4)2 (4) have been synthesized. The crystal structures of complexes 1-4 as well as their magnetic properties are presented. Each manganese atom of cocrystallized complexes in compound 1.2 is heptacoordinated, displaying Mn-N-Mn angles, theta, of 102.53(12) and 101.70(12) degrees and Mn...Mn distances of 3.5091(7) and 3.4680(7) A. On the other hand, each manganese center in compounds 3 and 4 is located within an octahedral coordination environment, the complexes displaying theta angles of 104.29(11) and 103.60(18) degrees , respectively, and Mn...Mn vectors of 3.5371(7) and 3.5338(10) A, respectively. Magnetic susceptibility studies revealed the presence of intramolecular ferromagnetic superexchange, yielding an S = 5 spin ground state in all complexes. Fitting of the experimental data led to coupling constants, intermolecular exchange values, and g factors (in the J/zJ'/g format) of 0.77 cm(-1)/0.01 cm(-1)/2.20 (1.2), 2.04 cm(-1)/0.01 cm(-1)/1.99 (3), and 1.75 cm(-1)/-0.05 cm(-1)/2.04 (4), respectively (using H = -2JS1S2 as the convention for the Heisenberg spin-Hamiltonian). These results are consistent with predictions from recent DFT calculations performed on end-on bis(mu-N3-)-bridged Mn(II) dinuclear complexes. A plot of experimental J vs theta, including data from the only preexisting compound of this kind, reveals a linear relationship, which could be the first evidence of a possible magneto-structural correlation between these two parameters.     

A family of enneanuclear iron(II) single-molecule magnets

Complexes [Fe₉(X)₂(O₂CMe)₈{(2‐py)₂CO₂}₄] (X^?=OH^? ( 1 ), N₃^? ( 2 ), and NCO^? ( 3 )) have been prepared by a route previously employed for the synthesis of analogous Co₉ and Ni₉ complexes, involving hydroxide substitution by pseudohalides (N₃^?, NCO^?). As indicated by DC magnetic susceptibility measurements, this substitution induced higher ferromagnetic couplings in complexes 2 and 3 , leading to higher ground spin states compared to that of 1 . Variable‐field experiments have shown that the ground state is not well isolated from excited states, as a result of which it cannot be unambiguously determined. AC susceptometry has revealed out‐of‐phase signals, which suggests that these complexes exhibit a slow relaxation of magnetization that follows Arrhenius behavior, as observed in single‐molecule magnets, with energy barriers of 41 K for 2 (τ₀=3.4×10^?12 s) and 44 K for 3 (τ₀=2.0×10^?11 s). Slow magnetic relaxation has also been observed by zero‐field ⁵⁷Fe Mössbauer spectroscopy. Characteristic integer‐spin electron paramagnetic resonance (EPR) signals have been observed at X‐band for 1 , whereas 2 and 3 were found to be EPR‐silent at this frequency. ¹H NMR spectrometry in CD₃CN has shown that complexes 1 – 3 are stable in solution.     

Using Ln(III)[15-MCCuII((N)(S)-pheHA(-5)]3+ complexes to construct chiral single-molecule magnets and chains of single-molecule magnets

The {DyIII[15-MC-CuII(N)(S)-pheHA(-5)]}3+ complex displays slow magnetic relaxation behavior in a frozen solution at low temperature, whereas the analogous HoIII structure does not exhibit similar behavior.     

Magnetic anisotropy of two trinuclear and tetranuclear Cr(III)Ni(II) cyanide-bridged complexes with spin ground states S = 4 and 5

The trinuclear and the tetranuclear complexes [[iPrtacnCr(CN)3]2[Ni(cyclam)]](NO3)2.5H2O 1 (cyclam = 1,4,8,11-tetraazacyclotetradecane, iPrtacn = 1,4,7-tris-isopropyl-1,4,7-triazacyclononane) and [[iPrtacnCr(CN)3Ni(Me2bpy)2]2](ClO4)4.2CH3CN 2 (Me2bpy = 4,4-dimethyl-2,2-bipyridine) were synthesized by reacting (iPrtacn)Cr(CN)3 with [Ni(cyclam)](NO3)2 and [Ni(Me2bpy)2(H2O)2](ClO4)2, respectively. The crystallographic structure of the two compounds was solved. The molecular structure of complex 1 consists of a linear Cr-Ni-Cr arrangement with a central Ni(cyclam) unit surrounded by two Cr(iPrtacn)(CN)3 molecules through bridging cyanides. Each peripheral chromium complex has two pending CN ligands. Complex 2 has a square planar arrangement with the metal ions occupying the vertices of the square. Each Cr(iPrtacn)(CN)3 molecule has two bridging and one non-bridging cyanide ligands. The magnetic properties of the two complexes were investigated by susceptibility vs. temperature and magnetization vs. field studies. As expected from the orthogonality of the magnetic orbitals between Cr(III) (t2g3) and Ni(II) (e(g)2) metal ions, a ferromagnetic exchange interaction occurs leading to a spin ground states S = 4 and 5 for 1 and 2, respectively. The magnetization vs. field studies at T = 2, 3 and 4 K showed the presence of a magnetic anisotropy within the ground spin states leading to zero-field splitting parameters obtained by fitting the data D4 = 0.36 cm(-1) and D5 = 0.19 cm(-1) (the indices 4 and 5 refer to the ground states of complexes 1 and 2, respectively). In order to quantify precisely the magnitude of the axial (D) and the rhombic (E) anisotropy parameters, High-field high frequency electron paramagnetic resonance (HF-HFEPR) experiments were carried out. The best simulation of the experimental spectra (at 190 and 285 GHz) gave the following parameters for 1: D4 = 0.312 cm(-1), E4/D4 = 0.01, g4x = 2.003, g4y = 2.017 and g4z = 2.015. For complex 2 two sets of parameters could be extracted from the EPR spectra because a doubling of the resonances were observed and assigned to the presence of complexes with slightly different structures at low temperature: D5 = 0.154 (0.13) cm(-1), E5/D5 = 0.31 (0.31) cm(-1), g4x = 2.04 (2.05), g4y = 2.05 (2.05) and g4z = 2.03 (2.02). The knowledge of the magnetic anisotropy parameters of the mononuclear Cr(iPrtacn)(CN)3, Ni(cyclam)(NCS)2 and Ni(bpy)2(NCS)2 complexes by combining HF-HFEPR studies and calculation using a software based on the angular overlap model (AOM) allowed to determine the orientation of the local D tensors of the metal ions forming the polynuclear complexes. We, subsequently, show that the anisotropy parameters of the polynuclear complexes computed from the projection of the local tensors are in excellent agreement with the experimental ones extracted from the EPR experiments.