Hetero-tri-spin systems: an alternative stairway to the single molecule magnet heaven?

The search for molecule-based magnetic materials has stimulated over the years the development of extremely rich coordination chemistry. Various combinations of spin carriers have been investigated and illustrated by a plethora of hetero-spin complexes: 3d-nd, 3d-4f, 2p-3d, and 2p-4f. More recently, two other classes of hetero-spin complexes have grown rapidly: compounds containing three different paramagnetic metal ions, or one radical and two different paramagnetic metal ions (all within the same molecular entity). Such new classes of systems represent a challenge both from a synthetic and theoretical point of view. Indeed, the synthetic control and the understanding of the spin topology effect on the overall magnetic behavior from first-principles is a difficult problem to be solved. The presence of different spin carriers in a single molecule makes such compounds particularly interesting because they offer the possibility of developing new magnetic properties, different from those of hetero-bi-spin or homo-spin systems. A critical overview taking the case of 2p-3d-4f complexes is the focus of this perspective paper. An original organic picture of the state-of-art in this field and new hints about the main directions that should be pursued to achieve hetero-tri-spin systems with large anisotropy barriers, low quantum tunneling of magnetization and, possibly, large blocking temperatures are provided in this article through an analysis based on numerically revisiting already published data and a critical survey of the literature reported so far. The reasons for the limited success obtained for the largely used 3d-2p-4f topology are given along with the ones explaining the failure for the 2p-4f-3d case. The still never synthesized linear 2p-3d-4f spin topology seemed to be the most promising one based on the results obtained for the unique closed hetero-tri-spin closed triangular system synthesized so far.

Luminescence thermometry based on one-dimensional benzoato-bridged coordination polymers containing lanthanide ions.

Three 1D coordination polymers with benzoate bridges have been assembled in the presence of 18-crown-6-ether (18C6): 1∞[Tb(PhCOO)3(H2O)(EtOH)]·0.5(18C6) 1, 1∞[Eu(PhCOO)3(H2O)2]·0.5(18C6) 2, 1∞[Nd(PhCOO)3(H2O)2]·0.5(18C6) 3. Compounds 2 and 3 are isomorphous. The crown ether molecules co-crystallize with the resulting 1D coordination polymers and play an important role in the supramolecular architecture of the crystals. A molecular alloy was prepared in a similar way to compound 1 using TbCl3·6H2O and EuCl3·6H2O in a molar ratio of 95 : 5. The EuIII ions have statistically substituted the TbIII ions in the host lattice The luminescence thermometry performance of the Tb0.95Eu0.05 system was investigated using pulsed excitation into TbIII absorption at 352 nm. The maximum Sr value is 1.88% K-1 at 80 K which is slightly reduced at 1.60% K-1 at 313 K. Time-gated emission spectroscopy, employed here for the first time, allows us to reduce the spectral overlap of Tb and Eu emissions in the 610 to 625 nm range by 100% at 80 K, from 18 to 9%. Compound 1 as well as the molecular alloy, Tb0.95Eu0.05, show X-ray induced luminescence.

SMM Behavior Tuned by an Exchange Coupling LEGO Approach for Chimeric Compounds: First 2p-3d-4f Heterotrispin Complexes with Different Metal Ions Bridged by One Aminoxyl Group.

Coordination compounds containing three different spin carriers (2p, 3d, and 4f), with the general formula [MIIDyIII(LH)(hfac)5] (M = Co, Ni, Mn, Zn), have been obtained using Mannich ligands decorated with a nitronyl-nitroxide fragment. The synthetic approach is general and leads to binuclear 3d-4f complexes, the two metal ions being bridged by one aminoxyl group and by one oxygen atom arising from a hfac- ligand. Triangular spin topology affords significant 2p-3d, 3d-4f, and 2p-4f exchange interactions. For the [CoIIDyIIIRad] derivative obtained using a nitronyl-nitroxide chiral ligand, a high energy barrier (∼200 cm-1) and a slow relaxation behavior below 30 K were found and rationalized by ab initio calculations. The improvement of magnetic properties comes from the synergy of optimal single ions properties and exchange couplings contributions where the CoII-Rad interaction becomes the leading one. The role played by this interaction is clearly proved by the investigation of the magnetic properties of the [ZnIIDyIIIRad] derivative, with a much lower energy barrier (12.7 cm-1) and by the lack of SMM behavior of the previously reported [CoIIDyIIIRad] compound ( Chem. Commun . 2017 , 53 , 6504 ), with a linear topology of spin carriers and a negligible CoII-Rad interaction.

On the role played by the chirality of ligands on the aggregation of heterometallic CuII -HgII complexes.

Copper(II) complexes constructed from enantiopure Schiff-base ligands derived from o-vanillin and methionine or from their racemic mixture have been employed as metalloligands towards mercury(II) ions. The nature of the heterometallic CuII -HgII complexes (coordination polymers or oligonucler species), resulted from the reactions of the copper(II) complexes with HgCl2 or HgI2 , is different. The enantiopure metalloligands lead to chiral coordination polymers, 1∞ [{Cu(R-/S-valmet)(H2 O)}2 HgCl2 ]·5H2 O·CH3 OH and 2∞ [{Cu2 (R-/S-valmet)2 (µ-H2 O)(H2 O)2 }HgI2 ]·4H2 O. The reaction of the racemic mixture of the metalloligands with HgX2 affords discrete complexes: [{Cu(R,S-valmet)(H2 O)2 }2 HgCl2 ]·2H2 O and [{Cu(R,S-valmet)(H2 O)(CH3 OH)}2 Hg2 I4 ].

Heterometallic 3d-4d coordination polymers assembled from trans-[RuIII(L)(CN)2]- tectons and 3d cations.

Five new cyanido-bridged heterometallic coordination polymers have been obtained by reacting PPh4[RuIII(salpn)(CN)2]·H2O (1) and AsPh4[RuIII(valen)(CN)2]·8.5H2O (2) (H2salpn and H2valen being Schiff-base proligands resulting from the condensation reaction of salicylaldehyde with 1,3-propanediamine and, respectively, o-vanillin with 1,2-ethanediamine) with divalent transition metal perchlorate salts: ∞1[{RuIII(salpn)(CN)2}3{MII(DMF)3}2](ClO4)·4DMF (MII = Mn, 3; Co, 4) and ∞2[{RuIII(valen)(CN)2}4{MII(DMF)3}2{MII(DMF)4}](ClO4)2·4DMF (MII = Mn, 5; Co, 6; Ni, 7), respectively. The dicyanido species, trans-[Ru(salpn)(CN)2]- and trans-[Ru(valen)(CN)2]-, act as metalloligands with the 3d metal ions. Compounds 3 and 4 are isostructural one-dimensional (1D) coordination polymers with a ladder topology. Each MII ion is hexacoordinated by three cyanido groups arising from three {Ru(salpn)(CN)2} units and by the oxygen atoms from three DMF molecules, which are coordinated at meridional positions. Compounds 5-7 are also isostructural, their structures consisting of 2D networks with a herringbone topology. The magnetic susceptibility measurements of these 1D and 2D systems reveal the presence of dominating RuIII-MII antiferromagnetic (AF) interactions in compounds 3, 4, 5 and 6, while ferromagnetic RuIII-NiII interactions are observed in 7. All these compounds stay in their paramagnetic state down to 1.8 K except compound 4 which possesses a 3D ordered AF ground state.

Copper complexes for biomedical applications: Structural insights, antioxidant activity and neuron compatibility.

Copper coordinated with amino acid residues is essential for the function of many proteins. In addition, copper complexed to free l-Histidine, as [Cu(His)2], is used in the treatment of the neurodegenerative Menkes disease and of cardioencephalomyopathy. This study was aimed to coordinate copper(II) with four small ligands (l-Serine, l-Histidine, Urea and Biuret) and to evaluate structural features, stability, antioxidant activity and neuronal compatibility of the resulting complexes. All complexes were synthesized with CuCl2 and purified by precipitation in alcohol. Elemental composition, X-rays diffraction and FTIR indicated that the complexes were in form of [Cu(ligand)2] and exhibited tridentate (l-Histidine), bidentate (l-Serine and Biuret) or monodentate (Urea) coordination with copper. UV-Vis absorbance profiles in physiologically relevant solutions and cyclic voltammetry revealed that, contrarily to [Cu(Urea)2Cl2] and [Cu(Biuret)2Cl2], the [Cu(Ser)2] and [Cu(His)2Cl2] complexes were stable in different media including water, physiological saline and intestinal-like solutions. All complexes and their ligands had antioxidant capacity as evaluated by DPPH (1,1-diphenyl-2,2-picrylhydrazyl) and DPD (N,N-diethyl-p-phenylenediamine) methods, and the [Cu(His)2Cl2] complex was the most potent. Neuronal compatibility was assessed through cell viability measurements using cultured neurons derived from mouse P19 stem cells. Although only [Cu(His)2Cl2] showed a good neurocompatibility (about 90% at concentrations up to 200 µM), the cytotoxicity of the other copper complexes was lower compared to equivalent concentrations of CuCl2. These findings open new perspectives for the use of these copper complexes as antioxidants and possibly as therapeutic agents for neurodegenerative diseases. Furthermore, study of these complexes may help to improve chelation therapy for copper dysfunctions.

Aggregation of [Ln] clusters by the dianion of 3-formylsalicylic acid. Synthesis, crystal structures, magnetic and luminescence properties.

Three isostrucutral dodecanuclear clusters with the general formula [Ln12(fsa)12(µf3-OH)12(DMF)12]·nDMF (fsa2- is the dianion of 3-formylsalicylic acid; Ln = Eu 1, Gd 2, Dy 3) have been obtained from the reaction of fromylsalicyclic acid (H2fsa), tetrabutylammonium hydroxide and Ln(NO3)3·6H2O in methanol/DMF. Their structure consists of four vertex-sharing heterocubanes. Each heterocubane unit is assembled from four LnIII ions, three µ3-OH groups and one µ3-oxygen atom arising from the fsa2- carboxylato group. The photophysical properties of the europium derivative investigated at both 300 and 80 K sustain a relative intense emission obtained under low power LED excitation at 420 nm. The dysprosium derivative shows a SMM behavior with an effective energy barrier Ueff of 22.9 cm-1, while the thermodynamical properties of Gd12 confirm a large magnetocaloric effect: S(7 T) - S(0 T) = 20R = 166 J mol-1 K), typical for isotropic GdIII derivatives, with ΔS = S(7 T) - S(0 T) = 1.7R for each GdIII ion.

Enantiopure versus Racemic Mixture in Reversible, Two-Step, Single-Crystal-to-Single-Crystal Transformations of Copper(II) Complexes.

The reaction of chiral sodium complexes, 1∞ [Na(S-valmetH)]⋅H2 O (1-S) and 1∞ [Na(R-valmetH)]⋅H2 O (1-R), with copper(II) acetate affords chiral one-dimensional coordination polymers with the formulas 1∞ [Cu(S-valmet)(H2 O)]⋅H2 O (2-S) and 1∞ [Cu(R-valmet)(H2 O)]⋅H2 O (2-R) (R/S-valmetH2 are Schiff base proligands resulting from the condensation reactions between o-vanillin and R/S-methionine). The copper ions are connected by the carboxylato groups belonging to the amino-acid moieties, resulting in infinite chains showing syn-anti out-of-plane bridging mode. The circular dichroism spectra of 1-S, 1-R, 2-S, and 2-R confirm their enantiomeric nature. Compounds 2-S and 2-R undergo a two-step single-crystal-to-single-crystal transformation, with the elimination of the lattice and coordinated water molecules: 1∞ [Cu(S-valmet)(H2 O)]⋅H2 O (2-S)→1∞ [Cu(S-valmet)]⋅H2 O (3-S⋅H2 O)→1∞ [Cu(S-valmet)] (3-S) and 1∞ [Cu(R-valmet)(H2 O)]⋅H2 O (2-R)→1∞ [Cu(R-valmet)]⋅H2 O (3-R⋅H2 O)→1∞ [Cu(R-valmet)] (3-R), respectively. During these transformations, every pair of face-to-face chains present in 2-S (or 2-R) has been "zipped up" into a chiral double chain through the removal of the aqua ligands and their replacement by the carboxylato oxygen atoms from the neighboring chain. Consequently, each carboxylato group now bridges three copper ions. The conversion of the single chains, 2-S and 2-R, into the double chains, 3-S and 3-R, is accompanied by a change of the strength of the exchange interactions between the copper ions: weak antiferromagnetic couplings are observed in compound 2-S (J/kB =-1.23(5) K, H=-2J ΣSi Si+1 ) and relatively strong in compound 3-S (J/kB =-76.0(8) K). When the racemic mixture of the ligands, R,S-valmetH2 , is employed, in the same experimental conditions, a racemic mixture of mononuclear compounds, [Cu(R,S-valmet)(H2 O)2 ]⋅H2 O (4-RS), is obtained. Compound 4-RS also undergoes a SCSC transformation with the elimination of the lattice and one of the coordinated water molecules, resulting in a racemic mixture of chiral chains, 1∞ [Cu(R-valmet)(H2 O)]⋅1∞ [Cu(S-valmet)(H2 O)] (5-RS). In this compound, the coupling of the copper(II) ions within the chains is weak and ferromagnetic (J/kB =+0.10(2) K). These results prove that the chirality of the valmetH2 ligands (optically pure or racemic mixture) plays a key role in the self-assembly process of the copper(II) complexes.

Heterotrimetallic complexes in molecular magnetism.

This paper reviews the most representative examples of heterometallic complexes containing three different paramagnetic metal ions, focusing on their magnetic properties. These compounds show a rich structural variety, ranging from discrete species to coordination polymers of various dimensionalities. The general synthetic strategies leading to heterotrimetallic complexes are discussed and illustrated.

Triphenylbismuth(v) di[(iso)nicotinates] - transmetallation agents or divergent organometalloligands? First organobismuth(v)-based silver(i) coordination polymers.

The reaction of Ph3BiCl2 with alkali salts of isonicotinic and nicotinic acids afforded Ph3Bi[O(O)CC5H4N-4]2 (1) and Ph3Bi[O(O)CC5H4N-3]2 (2), respectively, which were characterized by multinuclear NMR spectroscopy in solution, mass spectrometry and IR spectroscopy in the solid state. Their molecular structures were established by single-crystal X-ray diffraction. For both 1 and 2 the molecules contain a trigonal bipyramidal C3BiO2 core, with the phenyl groups in equatorial positions. The potential use of 1 and 2 as ditopic organometalloligands was investigated. The reaction of 1 or 2 with Me3SnCl (1 : 2 molar ratio) resulted in carboxylato ligand exchange and the formation of Me3Sn[O(O)CC5H4N-4] (3) and Me3Sn[O(O)CC5H4N-3] (4) besides Ph3BiCl2. The crystals of both 3 and 4 contain 1-D coordination polymers built through intermolecular N → Sn interactions. The treatment of Ni[S2P(OiPr)2]2 with 1 and 2, respectively, resulted, in addition to di(carboxylato)nickel(ii) derivatives, in isolation of Ph3Bi and the disulphane [(iPrO)2P(S)S]2. New coordination polymers were obtained by reacting 1 and 2 with various silver(i) salts: [Ag{Ph3Bi[O(O)CC5H4N-4]2}(OTf)] (5), [Ag{Ph3Bi[O(O)CC5H4N-3]2}(OTf)]·CH2Cl2 (6·CH2Cl2), [Ag{Ph3Bi[O(O)CC5H4N-4]2}](SbF6)·2THF (7·2THF), [Ag{Ph3Bi[O(O)CC5H4N-3]2}](SbF6)·CH2Cl2 (8·CH2Cl2) and [Ag{Ph3Bi[O(O)CC5H4N-3]2}(NO3)]·CH2Cl2 (9·CH2Cl2). The crystal structures of 5 and 6 can be described as 1-D chains linked by triflate bridges in pairs of chains and 2-D networks, respectively. Compound 7 features a 2-D grid-like topology of the network, with tecton 1 acting as a tridentate ligand through both nitrogen and one oxygen atoms. The linker 2 molecules adopt either cis (compound 6) or trans (compounds 8 and 9) conformation. Complexes 8 and 9 are 1-D chain polymers exhibiting zig-zag and wavy motifs, respectively. The dimensionality of the structures is extended by the presence of supramolecular interactions (ππ, AgAg, AgO).

Three different types of bridging ligands in a 3d-3d'-3d'' heterotrimetallic chain.

A one-pot synthesis of a 3d-3d'-3d'' heterotrimetallic coordination polymer with double diphenoxido, single cyanido and bis-bidentate oxalate as alternating bridges which exhibits an overall antiferromagnetic behaviour has been developed.

Synthesis, Crystal Structures, and EPR Studies of First MnIIILnIII Hetero-binuclear Complexes.

A new family of binuclear complexes [MnIIILnIII(dpm)4(MeO)2(MeOH)2] is reported (where Ln = LaIII (1), PrIII (2), and EuIII(3)). These compounds were obtained from a one-pot reaction between 2,2,6,6-tetramethyl-3,5-heptanodione (Hdpm), MnII, and the respective LnIII salt in the presence of sodium methoxide. The derivative containing the diamagnetic ion LaIII has been synthesized in order to characterize the local anisotropy of the MnIII ion. High-field electron paramagnetic resonance (HFEPR) spectroscopy shows that the MnIII ion, with an elongated octahedral geometry in all compounds, has a significant axial zero-field splitting and a small rhombic anisotropy. Additionally, the HFEPR measurements indicate that there is almost no exchange between the spin carriers in these compounds, all of which exhibit field-induced slow relaxation of the magnetization.

Cobalt(II) Ions Connecting [CoII4] Helicates into a 2-D Coordination Polymer Showing Slow Relaxation of the Magnetization.

The reactions of cobalt(II) perchlorate with a diazine tetratopic helicand, H4L, in the presence of sodium carbonate afford two coordination polymers constructed from tetranuclear anionic helicates as building blocks: ∞3[Co4L3Na4(H2O)4]·4H2O (1) and ∞2[Co5L3Na2(H2O)9]·2.7H2O·DMF (2). The tetranuclear triple-stranded helicates, {CoII4L3}4-, are connected in 1 by sodium(I) ions and in 2 by sodium(I) and cobalt(II) ions (H4L results from the condensation reaction between 3-formylsalicylic acid and hydrazine). The crystal structures of the two compounds have been solved. In both compounds the anionic helicates interact with the assembling cations through the carboxylato oxygen atoms. Compound 2 features chains resulting from connecting the tetranuclear helicates through cobalt(II) ions. The analysis of the magnetic properties of compounds 1 and 2 evidenced a dominant antiferromagnetic coupling for 1, resulting in a diamagnetic ground state. In contrast, the magnetic behavior of 2 is dominated at low temperature by the CoII ion which connects the antiferromagnetically coupled {CoII4} helical moieties. The ac magnetic measurements for 2 reveal the occurrence of slow relaxation of the magnetization that is due to the single, uncorrelated cobalt(II) ions, which are diluted in an essentially diamagnetic matrix of {CoII4} moieties (ΔEeff = 26.7 ± 0.3 cm-1 with τ0 = (2.3 ± 0.2) × 10-6 s).

A chimeric design of heterospin 2p-3d, 2p-4f, and 2p-3d-4f complexes using a novel family of paramagnetic dissymmetric compartmental ligands.

End-off bicompartmental ligands bearing a nitronyl-nitroxide arm have been designed for synthesizing various heterospin molecular systems. These ligands can selectively interact with 3d and 4f metal ions, leading to 2p-4f, 2p-3d, and 2p-3d-4f complexes. The magnetic properties of the 2p-4f and 2p-3d-4f complexes have been investigated and rationalized by theoretical calculations.

Synthesis, Crystal Structures, and Magnetic Properties of Two Novel Cyanido-Bridged Heterotrimetallic {CuIIMnIICrIII} Complexes.

The self-assembly process between the heteroleptic [CrIII(phen)(CN)4]- and [CrIII(ampy)(CN)4]- metalloligands and the heterobimetallic {CuII(valpn)MnII}2+ tecton afforded two heterotrimetallic complexes of formula [{CuII(valpn)MnII(µ-NC)2CrIII(phen)(CN)2}2{(µ-NC)CrIII(phen)(CN)3}2]·2CH3CN (1) and {[CuII(valpn)MnII(µ-NC)2CrIII(ampy)(CN)2]2·2CH3CN}n (2) [phen = 1,10-phenanthroline, ampy = 2-aminomethylpyridine, and H2valpn = 1,3-propanedyilbis(2-iminomethylene-6-methoxyphenol)]. The crystal structure of 1 consists of neutral CuII2MnII2CrIII4 octanuclear units, where two [Cr(phen)(CN)4]- anions act as bis-monodentate ligands through cyanide groups toward two manganese(II) ions from two [CuII(valpn)MnII]2+ units to form a [{Cu(valpn)Mn}2Cr2(CN)4]6+ square motif. Two [Cr(phen)(CN)4]- pendant anions in 1 are bound to the copper(II) ions with cis-trans geometry with respect to the bridging [Cr(phen)(CN)4]- anion. Compound 2 is a sheet-like coordination polymer, where chains constituted by {CrIII(ampy)(CN)4} spacers act as bis-monodentate ligands toward the manganese(II) ions belonging to the {CuII(valpn)MnII} nodes, which are interlinked by another {CrIII(ampy)(CN)4} unit that acts as a bridge between the copper(II) and manganese(II) ions of adjacent chains. Magnetic susceptibility measurements in the temperature range of 1.9-300 K were performed for 1 and 2. An overall antiferromagnetic behavior is observed for 1, the ground spin state being described by a spin triplet from the square motif plus two magnetically isolated spin triplets from the two peripheral chromium(III) ions. Ferrimagnetic chains with interacting spins 1/2 (resulting spin of the trimetallic {CuII(valpn)MnII(µ-NC)CrIII} fragment) and 3/2 (spin from the bis-monodentate [CrIII(ampy)(CN)4]- with weak interchain ferromagnetic interactions across the cyanide bridge between the chromium(III) and the copper(II) ion from adjacent chains [θ = +3.83(2) cm-1]) occur in 2, resulting into a ferromagnetic ordering below 3.5 K. The values of the magnetic coupling between the Cu(II) and Mn(II) ions through the double phenoxide bridge [J = -63.1(2) (1) and -62(3) cm-1 (2)] and those between the Cr(III) and the Mn(II) across the single cyanide bridge [J = -7.08(5) and -4.86(6) cm-1 (1) and -8.59(3) cm-1 (2)] agree with the values reported for these exchange pathways in other magnetostructural studies.

Synthesis, Crystal Structures, Magnetic Properties, and Theoretical Investigation of a New Series of NiII-LnIII-WV Heterotrimetallics: Understanding the SMM Behavior of Mixed Polynuclear Complexes.

The polynuclear compounds containing anisotropic metal ions often exhibit efficient barriers for blocking of magnetization at fairly arbitrary geometries. However, at variance with mononuclear complexes, which usually become single-molecule magnets (SMM) under the sole requirement of a highly axial crystal field at the metal ion, the factors influencing the SMM behavior in polynuclear complexes, especially, with weakly axial magnetic ions, still remain largely unrevealed. As an attempt to clarify these conditions, we present here the synthesis, crystal structures, magnetic behavior, and ab initio calculations for a new series of NiII-LnIII-WV trimetallics, [(CN)7W(CN)Ni(H2O)(valpn)Ln(H2O)4]·H2O (Ln = Y 1, Eu 2, Gd 3, Tb 4, Dy 5, Lu 6). The surprising finding is the absence of the magnetic blockage even for compounds involving strongly anisotropic DyIII and TbIII metal ions. This is well explained by ab initio calculations showing relatively large transversal components of the g-tensor in the ground exchange Kramers doublets of 1 and 4 and large intrinsic tunneling gaps in the ground exchange doublets of 3 and 5. In order to get more insight into this behavior, another series of earlier reported compounds with the same trinuclear [WVNiIILnIII] core structure, [(CN)7W(CN)Ni(dmf)(valdmpn)Ln(dmf)4]·H2O (Ln = GdIII 7, TbIII 8a, DyIII 9, HoIII 10), [(CN)7W(CN)Ni(H2O)(valdmpn)Tb(dmf)2.5(H2O)1.5]·H2O·0.5dmf 8b, and [(CN)7W(CN)Ni(H2O)(valdmpn)Er(dmf)3(H2O)1]·H2O·0.5dmf 11, has been also investigated theoretically. In this series, only 8b exhibits SMM behavior which is confirmed by the present ab initio calculations. An important feature for the entire series is the strong ferromagnetic coupling between Ni(II) and W(V), which is due to an almost perfect trigonal dodecahedron geometry of the octacyano wolframate fragment. The reason why only 8b is an SMM is explained by positive zero-field splitting on the nickel site, precluding magnetization blocking in complexes with fewer axial Ln ions. Further analysis has shown that, in the absence of ZFS on Ni ion, all compounds in the two series (except those containing Y and Gd) would be SMMs. The same situation arises for perfectly axial ZFS on Ni(II) with the main anisotropy axis parallel to the main magnetic axis of Ln(III) ions. In all other cases the ZFS on Ni(II) will worsen the SMM properties. The general conclusion is that the design of efficient SMMs on the basis of such complexes should involve isotropic or weekly anisotropic metal ions, such as Mn(II), Fe(III), etc., along with strongly axial lanthanides.

Binuclear Lanthanide-Radical Complexes Featuring Two Centers with Different Magnetic and Luminescence Properties.

Binuclear complexes with general formula [Ln2(hfac)6(H2O)2(dppnTEMPO)] (LnIII = Gd, Tb, and Dy) have been obtained using the paramagnetic ligand 1-piperidinyl-4-[(diphenylphosphinyl)amino]-2,2,6,6-tetramethyl (dppnTEMPO) as a bridge. One of the lanthanide ions is ferromagnetically coupled with the TEMPO moiety. Two of the complexes (Dy and Tb) show slow relaxation of the magnetization, and the non-magneto-equivalence of the two LnIII ions was clearly observed. The ab initio CASSCF calculations were employed to confirm this behavior, as well as to rationalize the Ln-Rad interaction. The simulations of the magnetic properties were allowed by the insights given by the calculations. The inequivalence of the TbIII ions was also proved by emission spectroscopy.

Slow Relaxation of Magnetization in an Isostructural Series of Zinc-Lanthanide Complexes: An Integrated EPR and AC Susceptibility Study.

We report the synthesis, structure, and spectroscopic and dynamic magnetic properties of a series of heterodinuclear complexes, [ZnLn(LH4 )2 ](NO3 )3 ⋅6 H2 O (Ln=Nd, Tb, Dy, Ho, Er, and Yb), with the singly deprotonated form of a new compartmentalized Schiff-base ligand, LH5 . The Ln(III) ions in these systems show a distorted square-antiprism geometry with an LnO8 coordination sphere. EPR spectroscopy and DC magnetic studies have shown that the anisotropic nature of the complexes is far more complicated than predicted on the basis of a simple electrostatic model. Among the investigated systems, only the Dy(III) derivative showed single-ion magnet behavior, in zero and an applied magnetic field, both in pure polycrystalline samples and in a series of polycrystalline samples with different degrees of dilution at the single-crystal level in the isostructural Y(III) derivative. The rich dynamics observed as functions of frequency, field, and temperature reveals that multiple relaxation mechanisms are at play, resulting in a barrier of 189 cm(-1) , which is among the highest reported for a dinuclear Zn-Dy system. Analysis of the dynamic behavior as a function of dilution degree further evidenced the persistence of non-negligible intermolecular interactions, even at the lowest concentration of 1 %.

A new family of [Cu(II)Ln(III)M(V)] heterotrimetallic complexes (Ln = La, Gd, Tb; M = Mo, W): model systems to probe exchange interactions and single-molecule magnet properties.

Four isostructural trinuclear 3d-4f-4(5)d heterotrimetallic complexes, with the general formula [L(2)CuLn(H2O)5(µ-NC)M(CN)7], were obtained from the association of binuclear 3d-4f complexes and {M(V)(CN)8}(3-) metalloligands (M = Mo, Ln = La ; M = W, Ln = La ; M = Mo, Ln = Gd ; M = Mo; Ln = Tb , where H2L(2) = 1,2-ethanediylbis(2-iminomethylene-6-methoxy-phenol)). The metalloligand coordinates through a single-cyanido group at the apical position of the copper(ii) ion belonging to the {Cu(II)Ln(III)} binuclear complex. The analysis of the magnetic data for the La(iii) derivatives (compounds and ), in the 1.85-300 K temperature range, shows a weak ferromagnetic exchange interaction between Cu(II) and Mo(V)/W(V) ions across the cyanido bridge (JCuM/kB = 3.6(6) K; g = 2.23(5) for and JCuM/kB = 3.8(6) K, g = 2.21(5) for , with H = -2JCuMSCu·SM). These results were used to simulate the magnetic properties of compound , using the isotropic spin Hamiltonian H = -2JCuMoSCu·SMo - 2JCuGdSCu·SGd. The resulting magnetic interaction between Cu(II) and Gd(III) ions via the phenoxo-bridge was found to be weakly ferromagnetic (JCuGd/kB = +4.5(2) K with JCuMo/kB = +3.6(2) K, gGd = gCu = 2.00 and gMo = 1.98). The dc magnetic properties for compound also show a predominant ferromagnetic interaction, while the ac magnetic measurements indicate the presence of the slow relaxation of the magnetization below 3.5 K.

First coordination compounds based on a bis(imino nitroxide) biradical and 4f metal ions: synthesis, crystal structures and magnetic properties.

The synthesis, crystal structures and magnetic properties of two families of heterospin complexes containing lanthanide ions and a bis(imino nitroxide) biradical (IPhIN = 1-iodo-3,5-bis(4',4',5',5'-tetramethyl-4',5'-dihydro-1H-imidazole-1'-oxyl)benzene) are reported: in [Ln2(hfac)6(IPhIN)(H2O)2] compounds, two lanthanide ions [Ln = Gd(III) (1) and Dy(III) (2)] are coordinated to the biradical, and in [Ln(hfac)3(IPhIN)(H2O)] compounds, one lanthanide ion (Ln = Tb(III) (3), Gd(III) (4) or Dy(III) (5)) is coordinated to the biradical. Ferromagnetic intramolecular magnetic interactions between Gd(III) and the biradical were found for 1 and 4, while intramolecular magnetic interactions between the radicals were ferro- and antiferromagnetic, respectively. Compound 2 shows a field induced slow relaxation of magnetization, which (under an external applied field of 2 kOe) exhibits an activation energy barrier of ΔE/kB = 27 K and a pre-exponential factor of 1.4 × 10(-8) s. To support the magnetic characterization of compound 3ab initio calculations were also performed.