Magnetic Coordination Compounds and More… a Long and Successful Story: A Tribute to M. Julve and F. Lloret

This Special Issue of Magnetochemistry, entitled “Magnetic Coordination Compounds and More... a Long and Successful Story: A Tribute to M. Julve and F. Lloret”, was organized to commemorate the partial retirement in 2024, institutionally speaking, rather than scientifically, of two extraordinary chemists and friends, Francisco Lloret Pastor and Miguel Julve Olcina, both faculty members of the Department of Inorganic Chemistry and the Institute of Molecular Science at the University of Valencia, after an active and successful research and teaching career.

Sadly, on 9 July 2024, while preparing this Special Issue, the chemical community lost one of its distinguished scholars, Prof. Miguel Julve. Miguel was a great friend, collaborator, and mentor for his graduates and postdoctoral associates. He always had his desk door open to anyone who wanted to ask his advice, eat some fruit, or simply listen to his last jokes. He was proud of his work and enjoyed talking about it almost as much as about his grandson Sebastian. He will be sorely missed by colleagues, friends, his wife María José, and daughters Andrea, Ingrid, and Michaela. Fortunately, we have Paco to help us fill the huge gap left by Miguel.

The two protagonists of this Special Issue had established an extraordinary scientific and personal relationship between each other for almost 48 years, since their undergraduate studies, then doctoral theses, later postdoctoral stays, and finally when they joined the Department of Inorganic Chemistry at the University of Valencia as faculty members. Francisco Lloret Pastor and the late Miguel Julve Olcina were two good friends who shared, in a hybrid mode, the passion for making new complex inorganic magnetic systems and creating an experimental and theoretical understanding of them.

We believe that the personal Laudatio entitled “Miguel Julve and Francisco Lloret, a Friendly Pair of Two Exceptional Coordination Chemists in Molecular Magnetism”, written by their good friend and close colleague Michel Verdaguer, from the Sorbonne University in Paris (France), says everything about their scientific and academic careers [1]. This laudatio summarizes, in three parts, the scientific trajectory of the Coordination Chemistry team lead by Miguel and Paco at the University of Valencia, their most important results, and, finally, the strong friendly link between them (Figure 1).

Following a chronological order, the first contribution in this Special Issue by B. Casanovas, R. Vicente, M. Font-Bardía, and M. Salah El Fallah, from the University of Barcelona (Spain), ref. [2], nicely describes six new polynuclear compounds prepared with Mn^II, R-salicylaldehyde oximes, and 9-anthracenecarboxylato. Their crystal structures contain trinuclear Mn^III₃ units forming chains, dimers, or monomers, and their magnetic properties show antiferromagnetic interactions in three cases and a combination of antiferromagnetic and ferromagnetic interactions in the other three. The contribution includes a magneto-structural correlation between the magnetic exchange and the intramolecular Mn^III–N–O–Mn^III torsion angle. Furthermore, two of these compounds showed slow relaxation of magnetization.

The contribution by A. Palii, S. Zilberg, and B. Tsukerblat, from the Federal Research Center of Problems of Chemical Physics and Medicinal Chemistry RAS (Russia), the Ariel University (Israel), and the Ben-Gurion University of the Negev (Israel), ref. [3], reports on the theoretical foundations of the rational design of molecular cells for quantum cellular automata (QCA) devices with optimized properties. The vibronic approach was applied to address the analysis of the cell–cell response and energy dissipation, which are the two key properties of such molecular cells. This study shows that in order to reach a good compromise between low energy dissipation and a strong cell–cell response, the use of weakly interacting mixed valence molecules with weak electron delocalization as cells is recommended. Some relevant results from a recent ab initio study on electron transfer and vibronic coupling are briefly discussed with the idea of controlling the key parameters of molecular QCA cells.

J. Löhr, M. Font-Bardia, J. Mayans, and A. Escuer, from the University of Barcelona (Spain), ref. [4], studied the magnetic properties of a series of manganese clusters with {Mn^III₆Mn^IINa₂}, {Mn^III₃Mn^IINa}, and {Mn^III₃} metallic cores, prepared by the reaction of manganese halides with polydentate NO₃ Schiff bases. Their magnetic properties show spin ground states of 19/2, 7/2, and 12/2 for the {Mn^III₆Mn^IINa₂}, {Mn^III₃Mn^IINa}, and {Mn^III₃} clusters, respectively, including the presence of field-induced slow relaxation of the magnetization for the trinuclear complexes.

The contribution by C. Andrei Spinu, D. O. T. A. Martins, T. Mocanu, M. Hillebrand, J.-P. Sutter, F. Tuna, and M. Andruh, from the University of Bucharest (Romania), the Romanian Academy, the University of Manchester (UK), and the University of Toulouse (France), ref. [5], presents two new 2p–3d complexes of the type (Et₃NH)[ML(hfac)₂] [M = Mn and Co], obtained with a nitronylnitroxide radical. The magnetic measurements show strong antiferromagnetic M(II)-Rad interactions, confirmed by EPR spectra.

A. L Świtlicka, from the University of Silesia (Poland), ref. [6], provides an updated overview of the different topologies and magneto-structural correlations in Co^II compounds with dicyanamide as a ligand.

The contribution by B. Gil-Hernández, S. Millan, I. Gruber, C. Janiak, C. J. Gómez-García, and J. Sanchiz, from the University of La Laguna (Spain), Heinrich Heine University (Germany), and the University of Valencia (Spain), ref. [7], reports the X-ray single crystal structure and magnetic properties of two new mesoxalate-bridged heptanuclear copper(II) compounds, formulated as (NH₄)₄[Cu₇(Hmesox)₆(H₂O)₈]·10H₂O and [Ru(bpy)₃]₄[Cu₇(Hmesox)₆Cl₂]Cl₂·2CH₃CN·12H₂O. Magnetic studies show the presence of antiferromagnetic interactions in the former compound and the coexistence of ferro- and antiferromagnetic interactions in the latter that, additionally, shows luminescent properties arising from the [Ru(bpy)₃]²⁺ cation, supporting its multifunctionality.

W. X. C. Oliveira, V. G. Araújo, C. B. Pinheiro, M. Julve, and C. L. M. Pereira, from the Federal University of Minas Gerais (Brazil) and the University of Valencia (Spain), ref. [8], investigated the synthesis, structure, and magnetic characterization of the hexanuclear copper(II) complex [Cu₆(en)₄(eg)₂(pyox)₄]·3eg·en·12H₂O, with en = ethylenediamine, eg = ethylene glycol, and H₂pyox = 4-(1H-pyrazole-4-yl)phenylene-N-oxamic acid. This hexacopper(II) complex is built from two linear tricopper(II) complexes containing [Cu(en)]²⁺ moieties connected to a [Cu(eg)] unit by two pyox²⁻ ligands and a [eg]²⁻ ligand. DC magnetic studies reveal a strong antiferromagnetic Cu···Cu interaction within the trinuclear subunits, mediated by the alkoxide and pyrazolate bridges.

J. D. Tempesta, F. Faria Paiva, L. A. Ferreira, R. M. R. da Silva, L. D. G. Botelho, I. M. L. Rosa, C. Cesar Candido, A. Marcio Gomes, W. C. Nunes, G. P. Guedes, and M. Vanda Marinho, from the Universities of Alfenas, Goiás, Rio de Janeiro, and Fluminense (Brazil), ref. [9], report the synthesis, structure, and magnetic properties of the heterobimetallic 15-MC-5 metallacrown-based Cu–Ce complex [CeCu₅(5mpzHA)₅(NO₃)(H₂O)₇]·2NO₃·7H₂O. Magnetic measurements show the presence of antiferromagnetic interactions inside the heterometallic complex.

The article by P. Escamilla, N. Moliner, D. Armentano, E. Pardo, J. Ferrando-Soria, and T. Grancha, from the University of Valencia (Spain) and the University of Calabria (Italy), ref. [10], reports an interesting example of the partial post-synthetic metal exchange of Zn^II metal ions by Co^II ones in the water-stable three-dimensional CaZn₆-MOF 1, containing the amino acid S-methyl-L-cysteine. This ion exchange yields two novel MOFs with increasing contents of Co^II ions of 4 % and 8%. This post-synthetic metal exchange methodology led to MOFs that cannot be obtained by direct synthesis, featuring a modulation of the magnetic properties and field-induced slow relaxation of the magnetization.

A. Colin, Y. Wang, F. Lambert, N. Bridonneau, N. Suaud, R. Guillot, E. Rivière, Z. Halime, N. Guihéry, S.-I. Ohkoshi, and T. Mallah, from the University of Paris-Saclay, the University of Toulouse (France), and The University of Tokyo (Japan), ref. [11], undertook a systematic study on a trinuclear Co^II-containing complex assembled using the non-innocent hexahydroxytriphenylene bridging ligand. The characterization of this complex shows that the central ligand undergoes up to four reversible redox processes, leading to species with different optical properties. The exchange coupling among the electrons of the bridge resulted in a spin doublet (S = 1/2) localized close to one of the three Co²⁺ ions, as demonstrated by the experimental magnetic data.

The article by T. T. da Cunha, J. H. de Araujo-Neto, M. E. Alvarenga, F. Terra Martins, E. F. Pedroso, D. L. Mariano, W. C. Nunes, N. Moliner, F. Lloret, M. Julve, and C. L. M. Pereira, form the Universities of Minas Gerais, São Paulo, Goiás, Rio de Janeiro, and Fluminense (Brazil) and the University of Valencia (Spain), ref. [12], reports on the synthesis, crystal structures, and magnetic properties of four air-stable mononuclear lanthanide(III) complexes with the ligand N-(2,4,6-trimethylphenyl)oxamate. The magnetic properties show the typical behavior for the ground state terms of the Ln^III ions. AC magnetic measurements reveal the presence of slow magnetic relaxation without the presence of a DC field only for the last compound, whereas a DC field is needed to observe slow magnetic relaxation behavior in the other three compounds.

I. F. Diaz-Ortega, Y. Ye, J. Jover, E. Ruiz, E. Colacio, and J. M. Herrera, from the Universities of Granada, Almería, and Barcelona (Spain), ref. [13] report the synthesis of a series of nine mononuclear Ln^III complexes with LnN₄O₂Cl₂ and LnN₄O₄ coordination spheres and rare hexagonal bipyramidal geometries exhibiting field-induced slow magnetization relaxation (SMR).

The review by R. Rabelo, L. M. Toma, A. Bentama, S.-E. Stiriba, R. Ruiz-García, and J. Cano, from the University of Valencia (Spain), the Federal University of Goiás (Brazil), and the University of Fès (Morocco), ref. [14], presents the current trends and future directions of the use of mononuclear six-coordinate Co^II spin crossover and single-ion magnet complexes with opto-, electro-, or chemo-active ligands for the preparation of multifunctional and multiresponsive magnetic devices for applications in molecular spintronics and quantum computing technologies. The review gives an illuminating insight in how these spin-crossover cobalt(II) molecular nanomagnets are suited for the preparation of devices on different supports, like metal molecular junctions, carbon nanomaterials, metal–organic frameworks, and metal–covalent organic frameworks in order to measure the single-molecule electron transport and quantum coherence properties, which are two major challenges in single-molecule spintronics (SMSs) and quantum information processing.

F. S. Delgado, L. Cañadillas-Delgado, J. Rodríguez-Carvajal, Ó. Fabelo, and J. Pasán, from the University of La Laguna (Spain) and Institut Laue-Langevin (France), ref. [15], investigate the magnetic structure of the molecular compound [Mn(mal)(H₂O)]_n (mal = dianion of malonic acid) with unpolarized neutron diffraction. The combined neutron diffraction and magnetometry results leads to a comprehensive understanding of how structural and symmetry factors influence the magnetic properties of malonate-based manganese compounds.

The contribution by S. Benmansour, C. Cerezo-Navarrete, and C. J. Gómez-García, from the University of Valencia (Spain), ref. [16], reports on the synthesis and characterization of two isostructural layered MOFs with the asymmetric ligand chlorocyananilato and Eu^III or Dy^III as metal ions. Both compounds show a (4,4)-layered square structure with square cavities. The magnetic properties show the presence of a field-induced slow relaxation of the magnetization in the Dy^III derivative at low temperatures that follows direct and Orbach relaxation mechanisms.

N. Marino, F. Lloret, M. Julve, and G. De Munno, from the University of Calabria (Italy) and the University of Valencia (Spain), ref. [17], show the simultaneous synthesis of up to three copper compounds: the Cu^II-containing compound Cu^II(bpm)₃](I₃)(I) (1), the mixed valence Cu^II/Cu^I compound {[Cu^I(I₃)Cu^II(I)(bpm)₂](I₃)}_n (2), and a fully reduced Cu^I compound: {[Cu^I₂(μ-I)₂(bpm)]}_n (3). Compound 1 consists of a rare tris(2,2′-bipyrimidine)copper(II) monomeric dication, charge balanced by both iodide and triiodide anions. Compound 2 consists of a regular alternating μ-bpm/di-μ-iodide copper(I) chain. Interestingly, the mixed valence Cu^II/Cu^I compound 3 consists of a rare, regular alternating mixed-valent Cu^II-Cu^I μ bpm copper chain, with a very weak antiferromagnetic coupling between well-separated paramagnetic Cu^II ions.

The contribution by J. W. Maciel, L. H. G. Kalinke, R. Rabelo, M. E. Alvarenga, F. Terra Martins, N. Moliner, and D. Cangussu, from the Federal University of Goiás (Brazil) and University of Valencia (Spain), ref. [18], presents the synthesis and characterization of three isostructural lanthanide(III) compounds with the ligand N-(4-carboxyphenyl)oxamic acid (H₃pcpa) formulated as {[Ln₂(Hpcpa)₃(H₂O)₅]}_n, with Ln = Dy^III, Ho^III, and Er^III. Their structures consist of neutral zig–zag chains of Ln^III ions, with Hpcpa^2– ligands acting as bridges. The Dy^III derivative shows a field-induced slow relaxation of the magnetization with a reciprocating thermal behavior below 5 K for H = 0.25 T, whereas the Ho^III compound shows maxima of the magnetic entropy from 3 to 6 K for ΔH > 2 T.

V. Jornet-Mollá, C. J. Gómez-García, M. J. Dolz-Lozano, and F. M. Romero, from the University of Valencia (Spain), ref. [19], present the synthesis and characterization of a series of isostructural coordination polymers with a homoditopic picolinato ligand (L₁), formulated as [(CH₃)₂NH₂][Ln(L₁)₂]·H₂O·CH₃COOH with Ln = Eu, Gd, Tb, Dy, and Ho. Their crystal structures show 3D-anionic lattices with triangular cavities. AC magnetic susceptibility measurements indicate that the Gd, Tb, and Dy derivatives show field-induced slow relaxation of the magnetization with the Gd and Dy derivatives following direct and Orbach relaxation mechanisms. The Dy derivative also shows slow relaxation of the magnetization in the absence of an external field. The Gd derivative is one of the very few reported Gd^III compounds showing slow relaxation of its magnetization.

The contribution by C. Pejo, S. Valiero, C. Rojas-Dotti, G. P. Guedes, J. Cano, M. A. Novak, R. Chiozzone, M. G. F. Vaz, and R. González, from the University of the Republic (Uruguay), of the Fluminense Federal University (Brazil), and University of Valencia (Spain), ref. [20], reports on two tetranuclear complexes formulated as [M₄{(py)₂C(OH)O}₄(O₂CPh)₄] with M = Co and Ni, whose structures show [M₄O₄] cubane-like cores. The magnetic properties, analyzed with a two-J model including magnetic anisotropy, indicate the presence of dominant ferromagnetic interactions within the tetranuclear cores. DFT calculations support the experimental magnetic properties.

The list of the contributions of this Special Issue is completed by G. Abellán-Dumont, J. M. Clemente-Juan, and C. Giménez-Saiz, from the University of Valencia (Spain), ref. [21], who report the new polyoxometalate [Co₇(OH)₆(H₂O)₂(CH₃COO)₄(PW₉O₃₄)₂]¹³⁻, whose crystal structure consists of two trilacunary heptadentate B-α-[PW₉O₃₄]⁹⁻ fragments encapsulating a hepta-cobalt di-cubane-like {Co^II₆Co^IIIO₈} core. The magnetic properties of this compound have been fitted with an anisotropic exchange model in the low-temperature regime and show the presence of ferromagnetic interactions between Co^II ions with Co–O–Co angles close to orthogonality, whereas the Co^II ions connected through the central Co^III ion present a weak antiferromagnetic coupling.

The list of contributions, a total of twenty-one, in this commemorative Special Issue that pays tribute to the key contributions of Paco and Miguel in the field of coordination chemistry and magnetochemistry clearly shows that Miguel and Paco have not only left a long list of key contributions in coordination chemistry and magnetic materials but also an endless list of collaborators and friends in many different countries. The positive answers to participating in this Special Issue were immediate and enthusiastic by all contributors, as can be read in their commemorative words for each contribution. They were all impressed by the scientific exchange and friendship of Paco and Miguel. We would like to thank all of them for their high-level contributions in this Special Issue devoted to these two distinguished scholars. We are sure that Paco will enjoy and Miguel would also have enjoyed reading all of them as much as we will miss Miguel.