Magnetocaloric efficiency tuning through solvent-triggered 3D to 2D interconversion in holmium(III)-based dynamic MOFs.

The neutral holmium(III) oxalate octadecahydrate {[Ho2(ox)3(H2O)6]·12H2O}n of mixed hexagonal/decagonal (63·103) 3D net topology shows important changes in the magnetocaloric efficiency upon dehydration/rehydration by heating and water vapor exposition to give the holmium(III) oxalate decahydrate {[Ho2(ox)3(H2O)6]·4H2O}n of hexagonal (63) 2D net topology through the intermediacy of the elusive amorphous anhydrous compound {Ho2(ox)3}n.

Enantiomeric Complexes Based on Ruthenium(III) and 2,2'-Biimidazole: X-ray Structure and Magnetic Properties.

We have prepared and characterized two Ru(III) compounds based on the 2,2'-biimidazole (H2biim) ligand, namely, a single complex of formula cis-[RuCl2(H2biim)2]Cl·4H2O (1) and a racemic mixture of formula {cis-[RuCl2(H2biim)2]Cl}2·4H2O (2), which contains 50% of Ru(III) complex 1. Both compounds crystallize in the monoclinic system with space groups C2 and P21 for 1 and 2, respectively. These complexes exhibit the metal ion bonded to four nitrogen atoms from two H2biim molecules and two chloride ions, which balance part of the positive charges in a distorted octahedral geometry. Significant differences are observed in their crystal packing, which leads to the observation of differences in their respective magnetic behaviors. Despite having imidazole rings in both compounds, π-π stacking interactions occur only in the crystal structure of 2, and the shortest intermolecular Ru···Ru separation in 2 is consequently shorter than that in 1. Variable-temperature dc magnetic susceptibility measurements performed on polycrystalline samples of 1 and 2 reveal different magnetic behaviors at low temperatures: while 1 behaves pretty much as a magnetically isolated mononuclear Ru(III) complex with S = 1/2, 2 exhibits the behavior of an antiferromagnetically coupled system with S = 0 and a maximum in the magnetic susceptibility curve at approximately 3.0 K.

pH-Switching of the luminescent, redox, and magnetic properties in a spin crossover cobalt(ii) molecular nanomagnet.

The ability of mononuclear first-row transition metal complexes as dynamic molecular systems to perform selective functions under the control of an external stimulus that appropriately tunes their properties may greatly impact several domains of molecular nanoscience and nanotechnology. This study focuses on two mononuclear octahedral cobalt(ii) complexes of formula {[CoII(HL)2][CoII(HL)L]}(ClO4)3·9H2O (1) and [CoIIL2]·5H2O (2) [HL = 4'-(4-carboxyphenyl)-2,2':6',2''-terpyridine], isolated as a mixed protonated/hemiprotonated cationic salt or a deprotonated neutral species. This pair of pH isomers constitutes a remarkable example of a dynamic molecular system exhibiting reversible changes in luminescence, redox, and magnetic (spin crossover and spin dynamics) properties as a result of ligand deprotonation, either in solution or solid state. In this last case, the thermal-assisted spin transition coexists with the field-induced magnetisation blockage of "faster" or "slower" relaxing low-spin CoII ions in 1 or 2, respectively. In addition, pH-reversible control of the acid-base equilibrium among dicationic protonated, cationic hemiprotonated, and neutral deprotonated forms in solution enhances luminescence in the UV region. Besides, the reversibility of the one-electron oxidation of the paramagnetic low-spin CoII into the diamagnetic low-spin CoIII ion is partially lost and completely restored by pH decreasing and increasing. The fine-tuning of the optical, redox, and magnetic properties in this novel class of pH-responsive, spin crossover molecular nanomagnets offers fascinating possibilities for advanced multifunctional and multiresponsive magnetic devices for molecular spintronics and quantum computing such as pH-effect spin quantum transformers.

Lipoic Acid-Functionalized Hexanuclear Manganese(III) Nanomagnets Suitable for Surface Grafting.

Highly anisotropic single-molecule magnets (SMMs) have attracted much interest in the field of molecular magnetism because of their spin features and potential technological applications. Additionally, a great effort has been devoted to the functionalization of such molecule-based systems which are made with ligands containing functional groups suitable to connect SMMs to junction devices or to perform their grafting on surfaces of different substrates. We have synthesized and characterized two lipoic acid-functionalized and oxime-based Mn(III) compounds, of formula [Mn6(µ3-O)2(H2N-sao)6(lip)2(MeOH)6][Mn6(µ3-O)2(H2N-sao)6(cnph)2(MeOH)6]}·10MeOH (1) and [Mn6(µ3-O)2(H2N-sao)6(lip)2(EtOH)6]·EtOH·2H2O (2) [H2N-saoH2 = salicylamidoxime, lip = lipoate anion, cnph = 2-cyanophenolate anion]. Compound 1 crystallizes in the space group Pi of the triclinic system and 2 crystallizes in the space group C2/c of the monoclinic system. In the crystal, neighboring Mn6 entities are linked using non-coordinating solvent molecules, which are H-bonded to N atoms of -NH2 groups of amidoxime ligand. In addition, Hirshfeld surfaces of 1 and 2 were calculated to study the variety of intermolecular interactions and the different levels of importance that take place in their crystal lattice; this type of computed study is the first time performed on Mn6 complexes. The study of the magnetic properties of 1 and 2 through dc magnetic susceptibility measurements reveals the coexistence of ferromagnetic and antiferromagnetic exchange couplings between the Mn(III) metal ions in both compounds, the latter being the predominant magnetic interaction. A spin S = 4 value of the ground state was obtained using isotropic simulations of the experimental magnetic susceptibility data for both 1 and 2. Ac magnetic susceptibility measurements show features typical of slow relaxation of the magnetization in 1 and 2, which indicate that SMM behavior takes place in both compounds.

Trinuclear Cobalt(II) Triple Helicate with a Multidentate Bithiazolebis(oxamate) Ligand as a Supramolecular Nanomagnet.

The cobalt(II)-mediated self-assembly of the potentially tris(chelating) N,N'-2,2'-(4,4'-bithiazole)bis(oxamate) (dabtzox) ligand gives a new metal-organic supramolecular nanomagnet of formula K6Co3(dabtzox)3·8H2O·MeOH (1) featuring a unique linear triple-stranded trinuclear structure of the helicate type.

A rare isostructural series of 3d-4f cyanido-bridged heterometallic squares obtained by assembling [FeIII{HB(pz)3}(CN)3]- and LnIII ions: synthesis, X-ray structure and cryomagnetic study.

A new series of cyanido-bridged {FeIIILnIII}2 neutral molecular squares of general formula [Fe{HB(pz)3}(CN)(µ-CN)2Ln(NO3)2(pyim)(Ph3PO)]2·2CH3CN [Ln = Ce (1), Pr (2), Nd (3), Gd (4), Tb (5), Dy (6) and Er (7); {HB(pz)3}- = hydrotris(pyrazolyl)borate, pyim = 2-(1H-imidazol-2-yl)pyridine and Ph3PO = triphenylphosphine oxide] were obtained by reacting the low-spin [Fe{HB(pz)3}(CN)3]- species with the preformed [LnIII(pyim)(NO3)2(pyim)(Ph3PO)]+ complex anions (generated in situ by mixing the nitrate salt of each Ln(III) ion with pyim and Ph3PO molecules). Single-crystal X-ray diffraction studies show that 1-7 are isostructural compounds that crystallize in the triclinic P1̄ space group. Their crystal structures consist of centrosymmetric cyanido-bridged {FeIIILnIII}2 molecular squares where two [Fe{HB(pz)3}(CN)3]- units adopt bis-monodentate coordination modes towards two [LnIII(pyim)(NO3)2(pyim)(Ph3PO)]+ moieties. The cis-oriented convergent sites from both low-spin FeIII and LnIII fragments form a quasi square-shaped molecule in which the 3d and 4f ions alternatively occupy the corners of the square. Both FeIII ions show a distorted octahedral surrounding (C3v symmetry), whereas the LnIII ions exhibit a distorted muffin-like geometry (Cs symmetry) in 1-7. The intramolecular FeIII⋯LnIII distances across the two cyanido-bridges range from ca. 5.48/5.46 up to ca. 5.58/5.61 Å. The molecular squares in 1-7 are interlinked through hydrogen bonds, weak π⋯π stacking and very weak C-H⋯π type interactions into three-dimensional supramolecular networks. The analysis of the solid-state direct-current (dc) magnetic susceptibility data of 1-7 in the temperature range 1.9-300 K reveals the occurrence of weak intra- and intermolecular antiferromagnetic interactions. The small intramolecular antiferromagnetic couplings in 4 compare well with those previously reported for parent systems. Although the coexistence of the spin-orbit coupling (SOC) of the low-spin iron(III) and lanthanide(III) ions in the remaining compounds together with the ligand field effects mask the visualization and make difficult the evaluation of the possible magnetic interactions in them, we were able to do it through a SOC model applied on exact or effective Hamiltonians. Frequency-dependent alternating current magnetic susceptibility signals in the temperature range 2.0-9.0 K under zero and non-zero static fields were observed for 5-7 which indicate slow magnetic relaxation (SMM) behavior. The usual absence of χ''M maxima moved us to estimate their energy barriers through ln(χ''M/ χ'M) vs. 1/T plots, obtaining values from 25 to 40 cm-1.

Holmium(III) Single-Ion Magnet for Cryomagnetic Refrigeration Based on an MRI Contrast Agent Derivative.

The coexistence of field-induced blockage of the magnetization and significant magnetocaloric effects in the low-temperature region occurs in a mononuclear holmium(III) diethylenetriamine-N,N,N',N″,N″-pentaacetate complex, whose gadolinium(III) analogue is a commercial MRI contrast agent. Both properties make it a suitable candidate for cryogenic magnetic refrigeration, thus enlarging the variety of applications of this simple class of multifunctional molecular nanomagnets.

Ferro- and Antiferromagnetic Interactions in Oxalato-Centered Inverse Hexanuclear and Chain Copper(II) Complexes with Pyrazole Derivatives.

Two novel copper(II) complexes of formulas {[Cu(4-Hmpz)4][Cu(4-Hmpz)2(µ3-ox-κ2O1,O2:κO2':κO1')(ClO4)2]}n (1) and {[Cu(3,4,5-Htmpz)4]2[Cu(3,4,5-Htmpz)2(µ3-ox-κ2O1,O2:κO2':κO1')(H2O)(ClO4)]2[Cu2(3,4,5-Htmpz)4(µ-ox-κ2O1,O2:κ2O2',O1')]}(ClO4)4·6H2O (2) have been obtained by using 4-methyl-1H-pyrazole (4-Hmpz) and 3,4,5-trimethyl-1H-pyrazole (3,4,5-Htmpz) as terminal ligands and oxalate (ox) as the polyatomic inverse coordination center. The crystal structure of 1 consists of perchlorate counteranions and cationic copper(II) chains with alternating bis(pyrazole)(µ3-κ2O1,O2:κO2':κO1'-oxalato)copper(II) and tetrakis(pyrazole)copper(II) fragments. The crystal structure of 2 is made up of perchlorate counteranions and cationic centrosymmetric hexanuclear complexes where an inner tetrakis(pyrazole)(µ-κ2O1,O2:κ2O2',O1'-oxalato)dicopper(II) entity and two outer mononuclear tetrakis(pyrazole)copper(II) units are linked through two mononuclear aquabis(pyrazole)(µ3-κ2O1,O2:κO2':κO1'-oxalato)copper(II) units. The magnetic properties of 1 and 2 were investigated in the temperature range 2.0-300 K. Very weak intrachain antiferromagnetic interactions between the copper(II) ions through the µ3-ox-κ2O1,O2:κO2':κO1' center occur in 1 [J = -0.42(1) cm-1, the spin Hamiltonian being defined as H = -J∑S1,i · S2,i+1], whereas very weak intramolecular ferromagnetic [J = +0.28(2) cm-1] and strong antiferromagnetic [J' = -348(2) cm-1] couplings coexist in 2 which are mediated by the µ3-ox-κ2O1,O2:κO2':κO1' and µ-ox-κ2O1,O2:κ2O2',O1' centers, respectively. The variation in the nature and magnitude of the magnetic coupling for this pair of oxalato-centered inverse copper(II) complexes is discussed in the light of their different structural features, and a comparison with related oxalato-centered inverse copper(II)-pyrazole systems from the literature is carried out.

Field-induced slow magnetic relaxation and magnetocaloric effects in an oxalato-bridged gadolinium(iii)-based 2D MOF.

The coexistence of field-induced slow magnetic relaxation and moderately large magnetocaloric efficiency in the supra-Kelvin temperature region occurs in the 2D compound [Gd(ox)3(H2O)6]n·4nH2O (1), a feature that can be exploited in the proof-of-concept design of a new class of slow-relaxing magnetic materials for cryogenic magnetic refrigeration.

Electroswitching of the single-molecule magnet behaviour in an octahedral spin crossover cobalt(ii) complex with a redox-active pyridinediimine ligand.

Thermal-assisted spin crossover and field-induced slow magnetic relaxation coexist in the solid state for the mononuclear cobalt(ii) complex with the non-innocent 2,6-bis(N-4-methoxyphenylformimidoyl)pyridine ligand. One-electron oxidation of the paramagnetic low-spin CoII ion (SCo = 1/2) to the diamagnetic low-spin CoIII ion (SCo = 0) leads to the electroswitching of the slow magnetic relaxation in acetonitrile solution.

On the magneto-structural role of the coordinating anion in oxamato-bridged copper(ii) derivatives.

We herein present the synthesis, spectroscopic analysis, description of the crystal structures and magnetic properties of four new complexes of the formula [{Cu(opba)(H2O)1.2}{Cu(dmphen)(SCN)}2]·dmf (1), [{Cu(opba)}2{Cu(dmphen)Cl}4]·1.5dmf·2.5dmso (2), [{Cu(opba)}2{Cu(dmphen)Br}4]·dmf·2.3dmso (3) and [{Cu(opba)}{Cu(dmphen)(dca)}2]n (4) [H4opba = N,N'-1,2-phenylenebis(oxamic acid), dmphen = 2,9-dimethyl-1,10-phenanthroline and dca = dicyanamide anion]. 1 is a neutral tricopper(ii) complex where an inner [Cu(opba)]2- fragment adopts a bis-bidentate coordination mode towards two outer [Cu(dmphen)(NCS)]+ units. 2 and 3 are bis-trinuclear species where two oxamato-bridged [Cu(opba){Cu(dmphen)X}2] [X = Cl- (2) and Br- (3)] tricopper(ii) entities are connected by two single X ions involving the central and one of the peripheral copper(ii) ions. 4 is a neutral chain made up of oxamato-bridged [Cu(opba){Cu(dmphen)(dca)}2] fragments linked through a single end-to-end dicyanamide ligand that connects the central copper(ii) ion with one of the peripheral copper(ii) ions. Magnetic susceptibility measurements on polycrystalline samples of 1-4 in the temperature range 1.9-300 K show the occurrence of strong antiferromagnetic interactions between the copper(ii) ions through the oxamate bridge [J = -328(2) (1), -288(2) (2), -431(2) (3) and -370(1) cm-1 (4), the Hamiltonian being defined as H = -J(S1·S2 + S1·S3)] and a weak ferromagnetic coupling across the equatorial-axial exchange pathway provided by the single halide bridge [j = +3.08(3) (2) and +2.34(1) cm-1 (3)]. These values are analyzed by simple orbital symmetry considerations and compared with those reported in the literature for polynuclear copper(ii) complexes with these bridging ligands.

Field-induced slow relaxation of magnetisation in an anionic heterotetranuclear [ZnIIRe] system.

The compound (NBu4)4[ZnII{ReIVCl4(µ-ox)}3] (1) [NBu4+ = tetra-n-butylammonium cation and ox2- = oxalate dianion] is the first example of an oxalato-bridged ZnII system coordinated to a 5d metal ion that exhibits slow relaxation of magnetisation.

Spin Crossover in Double Salts Containing Six- and Four-Coordinate Cobalt(II) Ions.

The preparation and spectroscopic and structural characterization of three cobalt(II) complexes of formulas [Co(tppz)2](dca)2 (1), [Co(tppz)2][Co(NCS)4]·MeOH (2), and [Co(tppz)2][Co(NCO)4]·2H2O (3) [tppz = 2,3,5,6-tetrakis(2-pyridyl)pyrazine and dca = dicyanamide] are reported here. Compounds 1-3 have in common the presence of the cationic [Co(tppz)2]2+ entity where each mer-tridentate tppz ligand coordinates to the cobalt(II) ion equatorially through two pyridyl donors and axially via the pyrazine, completing the six-coordination. The electroneutrality is achieved by the organic dca group (1) and the anionic tetrakis(thiocyanato-κN)cobaltate(II) (2) and tetrakis(cyanato-κN)cobaltate(II) (3) complexes. Direct current (dc) magnetic susceptibility measurements of 1 in the temperature range 1.9-400 K show the occurrence of a thermally induced spin crossover behavior of the [Co(tppz)2]2+ unit from a high spin (S = 3/2) at higher temperatures to a low-spin (S = 1/2) at lower temperatures, with the low spin phase being reached at T ≤ 200 K. X-band electron paramagnetic resonance (EPR) measurements in solution at low temperatures were used to characterize the low spin state. An analytical expression based on the combination of the spin-orbit coupling and both first- and second-order Zeeman effects for a d7 electronic configuration was used to fit the magnetic data of 1, the values of the best-fit parameters being Cvib = 0.1367(9), λ = -168(2) cm-1, α = 1.12(1), Δ = 1626(15) cm-1, and gLS = 2.12(1). The magnetic behavior of the four-coordinate cobalt(II) ions [Co(NCS)4]2- (2) and [Co(NCO)4]2- (3) with a 4A2 ground state overlaps with the spin crossover of the [Co(tppz)2]2+ entity, the abrupt decrease of the χMT product below 15.0 K being due to zero-field splitting effects between the spin components |±1/2> and |±3/2>. The combined analysis of the dc magnetic data and the Q-band EPR spectra in the solid state of 2 and 3 led to the following sets of best-fit parameters: Cvib = 0.105(5), λ = -170(4) cm-1, α = 1.10(2), Δ = 1700(25) cm-1, gLS = 2.10(1), gHS = 2.27(1), and |D| = 3.80(2) cm-1 (2) and Cvib = 0.100(1), λ = -169(5) cm-1, α = 1.10(3), Δ = 1500(30) cm-1, gLS = 2.10(1), gHS = 2.28(1), and |D| = 4.30(2) cm-1 (3). Some evidence of slowing of the relaxation of the magnetization has been found in the out-of-phase ac signal at very low temperatures under applied dc fields of 0.1-0.4 T for 3, suggesting the occurrence of single-ion magnet behavior of its [Co(NCO)4]2- anionic entity.

Mass effect on the equienergetic high-spin/low-spin states of spin-crossover in 4,4'-bipyridine-bridged iron(II) polymeric compounds: synthesis, structure, and magnetic, Mössbauer, and theoretical studies.

The suitability of the system [Fe(4,4'-bipy)(H(2)O)(2)(NCX)(2)].(4,4'-bipy), where 4,4'-bipy stands for 4,4'-bipyridine and X = S (1) and Se (2), as a precursor for the synthesis of new polymeric spin-crossover compounds has been studied. The reaction of 1 or 2 with bt (2,2'-bithiazoline) afforded the polymeric compounds of formula [Fe(4,4'-bipy)(bt)(NCX)(2)] (X = S (3), Se (4)). Compounds 3 and 4 are isostructural, but only the crystal structure of 3 has been fully determined. It crystallizes in the orthorhombic system, Fdd2 space group, Z = 24, with a = 38.962(8) A, b = 11.545(2) A, c = 30.889(6) A, V = 13895(5) A(3). The structure consists of linear chains constituted by trans-4,4'-bipy linked iron(II) ions; two cis equatorial positions are occupied by two pseudohalide ligands, and the remaining positions are filled by the bidentate bt ligand. Investigation of their magnetic properties and Mössbauer spectra has revealed the occurrence of a low-spin (LS) <--> high-spin (HS) conversion involving 12% (3, S) and 20% (4, Se) of the Fe(II) ions. The thermal variation of the HS fraction is gradual with onset temperatures as low as 60 K. A theoretical approach based on the Ising-like model, completed with molecular vibrations, through harmonic oscillators, fits the data successfully, leading to an energy gap of 65 cm(-1) (3) and 86 cm(-1) (4) between the lowest LS and HS levels, and an average vibration frequency ohgrmacr;(LS) of 382 cm(-1) (3) and 365 cm(-1) (4) in the LS state. The ca. 1.05 omega(LS(3))/omega(LS(4)) ratio is close to the ca. 1.09 Se/S molar mass ratio. The simple electrovibrational Ising-like model permits us to explain, for the first time, a mass effect through the molecular vibrations in a spin-crossover complex that is in the unusual situation of equienergy among the HS and LS states.