The area of molecular magnetic coolants has developed rapidly in recent years. A large number of competitive candidates have been reported, with the cooling performances chasing each other. In this account, four explicit strategies, namely, increasing ground-state spin, reducing magnetic anisotropy, weakening magnetic interactions, and lowering the molecular weight, are proposed from the theoretical viewpoint towards improving the magnetocaloric effect (MCE). According to this guidance, these successful strategies are discussed to pursue excellent magnetic coolants. This is accompanied by a discussion of the representative examples reported by our group. The magnetic entropy change increases from one compound to another, which in the most pronounced cases is suggestive of being the largest MCE in magnetic coolants.

A series of heterometallic 3d–Gd³⁺ complexes based on a lanthanide metalloligand, [M(H₂O)₆][Gd(oda)₃]⋅3 H₂O [M=Cr³⁺ (1-Cr)] (H₂oda=2,2′-oxydiacetic acid), [M(H₂O)₆][MGd(oda)₃]₂⋅3 H₂O [M=Mn²⁺ (2-Mn), Fe²⁺ (2-Fe) and Co²⁺ (2-Co)], and [M₃Gd₂(oda)₆(H₂O)₆]⋅12 H₂O [M=Ni²⁺ (3-Ni), Cu²⁺ (3-Cu), and Zn²⁺ (3-Zn)], are reported. Magnetic and heat-capacity studies revealed a significant impact on the magnetocaloric effect depending on the anisotropy of the 3d transition metal ions, as confirmed by comparison of the observed maximum values of −ΔS_m between complexes 2-Co and 1-Cr. In these two complexes, the 3d metal ions have the same spin (S=3/2 for Co²⁺ and Cr³⁺ ions), and the theoretical calculation suggested a larger −ΔS_m value for 2-Co (47.8 J K⁻¹ kg⁻¹) than 1-Cr (37.5 J K⁻¹ kg⁻¹); however, the significant anisotropy of Co²⁺ ions in 2-Co, which can result in smaller effective spins, gives a smaller value of −ΔS_m for 2-Co (32.2 J K⁻¹ kg⁻¹) than for 1-Cr (35.4 J K⁻¹ kg⁻¹) at ΔH=9 T.

A Hoffman-like coordination polymer with appreciable porosity and uncoordinated pyridyl groups, namely, [Fe(2,5-bpp){Au(CN)₂}₂]⋅x Solv (2,5-bpp=2,5-bis(pyrid-4-yl)pyridine; Solv=solvent), was synthesised and characterised. A series of fascinating spin-crossover behaviours with abrupt, stepwise and hysteretic features were obtained by exchange with a range of protic solvents (ethanol, n-propanol, isopropyl alcohol, sec-butanol and isobutanol). Guest–host hydrogen-bonding interactions involving the H-accepting site of the framework are primarily responsible for the pronounced cooperativity of these spin-crossover behaviours. Meanwhile, the tunable critical temperatures over a range of about 130 K are presumably attributable to a certain degree of competition between internal pressure and local electronic influences of solvents.

Based on the analogous kagomé [Co₃(imda)₂] layers (imda=imidazole-4,5-dicarboxylate), a family of pillar-layered frameworks with the formula of [Co₃(imda)₂(L)₃]⋅(L)_n⋅xH₂O (1: L=pyrazine, n=0, x=8; 2: L=4,4′-bipyridine, n=1, x=8; 3: L=1,4-di(pyridin-4-yl)benzene, n=1, x=13; 4: L=4,4′-di(pyridin-4-yl)-1,1′-biphenyl, n=1, x=14) have been successfully synthesized by a hydrothermal/solvothermal method. Single-crystal structural analysis shows a significant increase in the interlayer distances synchronized with the extension of the pillar ligands, namely, 7.092(3) (1), 10.921(6) (2), 14.780(5) (3), and 19.165(4) Å (4). Despite the wrinkled kagomé layers in complexes 2–4, comprehensive magnetic characterizations revealed weakening of interlayer magnetic interactions and an increase in the degree of frustration as the pillar ligand becomes longer from 1 to 4; this leads to characteristic magnetic ground states. For compound 4, which has the longest interlayer distance, the interlayer interaction is so weak that the magnetic properties observed within the range of temperature measured would correspond to the frustrated layer.

The transformation of Mn^II glycolates (glc) between the three-dimensional coordination polymer [Mn(glc)₂]_n (1) and discrete mononuclear phase [Mn(glc)₂(H₂O)₂] (2) can be reversibly switched by water molecules, which dramatically change the magnetocaloric effect (MCE) of Mn^II glycolates from the maximum of 6.9 J kg⁻¹ K⁻¹ in 1 to 60.3 J kg⁻¹ K⁻¹ in 2. This case example reveals that the effect of magnetic coupling on MCE plays a dominant role over that of other factors such as magnetic density for 3d-type magnetic refrigerants.

A record anisotropy barrier (319 cm⁻¹) for all d-f complexes was observed for a unique Fe^II-Dy^III-Fe^II single-molecule magnet (SMM), which possesses two asymmetric and distorted Fe^II ions and one quasi-D_5h Dy^III ion. The frozen magnetization of the Dy^III ion leads to the decreased Fe^II relaxation rates evident in the Mössbauer spectrum. Ab initio calculations suggest that tunneling is interrupted effectively thanks to the exchange doublets.

Two kinds of inorganic gadolinium(III)-hydroxy “ladders”, [2×n] and [3×n], were successfully trapped in succinate (suc) coordination polymers, [Gd₂(OH)₂(suc)₂(H₂O)]_n⋅ 2n H₂O (1) and [Gd₆(OH)₈(suc)₅(H₂O)₂]_n⋅4n H₂O (2), respectively. Such coordination polymers could be regarded as alternating inorganic–organic hybrid materials with relatively high density. Magnetic and heat capacity studies reveal a large cryogenic magnetocaloric effect (MCE) in both compounds, namely (ΔH=70 kG) 42.8 J kg⁻¹ K⁻¹ for complex 1 and 48.0 J kg⁻¹ K⁻¹ for complex 2. The effect of the high density is evident, which gives very large volumetric MCEs up to 120 and 144 mJ cm⁻³ K⁻¹ for complexes 1 and 2, respectively.

A series of heterometallic [Ln^III_xCu^II_y] complexes, [Gd₂Cu₂]_n (1), [Gd₄Cu₈] (2), [Ln₉Cu₈] (Ln=Gd, 3⋅Gd; Ln=Dy, 3⋅Dy), were successfully synthesized by a one-pot route at room temperature with three kinds of in situ carbonyl-related reactions: Cannizzaro reaction, aldol reaction, and oxidation. This strategy led to dysprosium analogues that behaved as single-molecule magnets (SMMs) and gadolinium analogues that showed significant magnetocaloric effect (MCE). In this study a numerical DFT approach is proposed by using pseudopotentials to calculate the exchange coupling constants in three polynuclear [Gd_xCu_y] complexes; with these values exact diagonalization or quantum Monte Carlo simulations have been performed to calculate the variation of the magnetic entropy involved in the MCE. For the [Dy₉Cu₈] complexes, local magnetic properties of the Dy^III centers have been determined by using the CASSCF+RASSI method.

The comprehensive study reported herein provides compelling evidence that anion templates are the main driving force in the formation of two novel nanoscale lanthanide hydroxide clusters, {Gd₃₈(ClO₄)₆} (1) and {Gd₄₈Cl₂(NO₃)} (2), characterized by single-crystal X-ray crystallography, infrared spectroscopy, and magnetic measurements. {Gd₃₈(ClO₄)₆}, encapsulating six ClO₄⁻ ions, features a cage core composed of twelve vertex-sharing {Gd₄} tetrahedrons and one Gd⋅⋅⋅Gd pillar. When Cl⁻ and NO₃⁻ were incorporated in the reaction instead of ClO₄⁻, {Gd₄₈Cl₂(NO₃)} is obtained with a barrel shape constituted by twelve vertex-sharing {Gd₄} tetrahedrons and six {Gd₅} pyramids. What is more, the cage-like {Gd₃₈} can be dynamically converted into the barrel-shaped {Gd₄₈} upon Cl⁻ and NO₃⁻ stimulus. To our knowledge, it is the first time that the linear M-O-M′ fashion and the unique μ₈-ClO₄⁻ mode have been crystallized in pure lanthanide complex, and complex 2 represents the largest gadolinium cluster. Both of the complexes display large magnetocaloric effect in units of J kg⁻¹ K⁻¹ and mJ cm⁻³ K⁻¹ on account of the weak antiferromagnetic exchange, the high N_Gd/M_W ratio (magnetic density), and the relatively compact crystal lattice (mass density).

The solvothermal reaction of manganese(II) acetylacetonate [Mn(acac)₂], 1,1,1-tris(hydroxymethyl)ethane (H₃thme), 1,1,1-tris(hydroxymethyl)propane (H₃thmp), and (CH₃)₃CCO₂H leads to a series of mixed-valence Mn^III₄Mn^II₈clusters, [Mn^III₄Mn^II₈(μ₅-O)₂(μ₃-OMe)₂(thme)₄(Me₃CCO₂)₁₀(H₂O)₂] (1), [Mn^III₄Mn^II₈(μ₅-O)₂(μ₃-OMe)₂(thme)₄(Me₃CCO₂)₁₀(MeOH)₂] (2), [Mn^III₄Mn^II₈(μ₅-O)₂(μ₃-OMe)₂(thmp)₄(Me₃CCO₂)₁₀(H₂O)₂] (3), and [Mn^III₄Mn^II₈(μ₅-O)₂(μ₃-OMe)₂(thmp)₄(Me₃CCO₂)₁₀(MeOH)₂] (4). The Mn^III₄Mn^II₈ cores of the complexes can be described as a central rhomboid [Mn₄O₆] layer sandwiched by two [Mn₄O₇] layers. Alternating current (AC) magnetization studies reveal that complexes 1–4 possess the S = 4 spin ground state and behave as single-molecule magnets (SMMs). All four of the compounds clearly show slow magnetic relaxation behavior, but out-of-phase AC peaks above 1.8 K were only obtained for 3 and 4, thereby revealing that the ground-state anisotropy of four Mn₁₂ clusters are effectively modified by intra- and intermolecular interactions by substituting the tripodal ligands and terminal ligands.

A family of linear Dy₃ and Tb₃ clusters have been facilely synthesized from the reactions of DyCl₃, the polydentate 3-methyloxysalicylaldoxime (MeOsaloxH₂) ligand with auxiliary monoanionic ligands, such as trichloroacetate, NO₃⁻, OH⁻, and Cl⁻. Complexes 1–5 contain a nearly linear Ln₃ core, with similar Ln⋅⋅⋅Ln distances (3.6901(4)–3.7304(3) Å for the Dy₃ species, and 3.7273(3)–3.7485(5) Å for the Tb₃ species) and Ln⋅⋅⋅Ln⋅⋅⋅Ln angles of 157.036(8)–159.026(15)° for the Dy₃ species and 157.156(8)–160.926(15)° for the Tb₃ species. All three Ln centers are bridged by the two doubly-deprotonated [MeOsalox]²⁻ ligands and two of the four [MeOsaloxH]⁻ ligands through the N,O-η²-oximato groups and the phenoxo oxygen atoms (Dy-O-Dy angles=102.28(16)–106.85(13)°; Tb-O-Tb angles=102.00(11)–106.62(11)°). The remaining two [MeOsaloxH]⁻ ligands each chelate an outer Ln^III center through their phenoxo oxygen and oxime nitrogen atoms. Magnetic studies reveal that both Dy₃ and Tb₃ clusters exhibit significant ferromagnetic interactions and that the Dy₃ species behave as single-molecule magnets, expanding upon the recent reports of the pure 4f type SMMs.

An unprecedented dodecanuclear Co^III₃Co^II₉ and a dinuclear Co^II₂ cluster were synthesized facilely from reactions of different coblt(II) salts with the ligand (1H-benzimidazol-2-yl)methanol. The cobalt ions in the Co₁₂ supercluster are linked into a disclike structures through μ₃-O_L, μ-O_L, μ₃-O^2–, μ_1,1-N₃^– and μ_1,1,1-N₃^– bridges. Magnetic studies reveal that strong ferromagnetic coupling through double end-on (EO) azido bridges exists in 1, while both ferromagnetic coupling through the μ₃-O_L and μ-O_L pathways and antiferromagnetc coupling through the μ_1,1,1-N₃^– pathway exist in 2, which results in a ferrimagnetic behaviour of 2.

Hydrothermal reactions of CdCl₂, oxalic acid, 1,3,5-benzenetricarboxylic acid and o-phenylene diamine (o-PD) yielded three novel coordination polymers [NaCd₄(btc)₃(H₂bbim)₄(H₂O)₂]·2H₂O (1), [KCd₄(btc)₃(H₂bbim)₄(H₂O)₂]·2H₂O (2) and [Cd(H₂bbim)(H₂bimbdc)(HbimbdcH)]·H₂O (3) [btc = 1,3,5-benzene tricarboxylate, H₂bbim = 2,2′-bisbenzimidazole, HbimbdcH₂ = 1-(2-Benzimidazolyl)-3,5-benzene dicarboxylic acid]. The benzimidazole derivatives were in situ generated via hydrothermal metal/ligand condensation of oxalic acid or 1,3,5-benzenetricarboxylic acid with o-phenylenediamine. Both 1 and 2 have novel single-walled nanotubular coordination structures and show strong red photoluminescence at room temperature.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

The reaction of freshly synthesized [Ag(NH₃)₂](OH), 2,1,3-benzoselenadiazole (bsd) and benzene- or cyclohexane-1,3,5-tricarboxylic acid (H₃btc/H₃ctc) results in two new coordination polymers containing 2D Ag₃(btc) and Ag₃(ctc) hexagonal motifs, namely [Ag₃(btc)(bsd)₆]·0.5H₂O (1) and [Ag₃(ctc)(bsd)₃]·1.5bsd·3.5H₂O (2). Complex 1 is a 3D supramolecular framework extended by deformed 2D Ag₃(btc) honeycomb nets via Se···N synthons formed between the dangling monodentate bsd groups, whereas 2 is a 3D metal–organic framework of nanoscale cages made up from ideal Ag₃(ctc) honeycomb layers and μ₂-bridging bsd ligands.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)

One-step in situ oxidations of 2,9-dimethyl-1,10-phenanthroline to 9-methyl-1,10-phenanthroline-2-carboxylic acid and 1,10-phenanthroline-2,9-dicarboxylic acid were carried out by utilizing HNO₃ as the oxidant in the presence of Cu^II, Ni^II, and Cu^II–Ln^III salts under hydrothermal conditions, showing that different transition-metal ions can tune the oxidation products through different coordination modes in metal complexes of the in situ formed ligands. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)

Hydrothermal reactions of CdCl₂·2.5H₂O or ZnCl₂, sodium dicyanamide, and sodium azide yield two interesting photoluminescent coordination polymers, namely, [Cd{Cd₃(μ₃-OH)(dcadtzH)₃(H₂O)₆}₂] (1) and [Zn₅(μ₃-OH)₂(μ-OH)₂(dcadtz)₂]·2H₂O (2) [dcadtzH₃ = bis(5-tetrazolyl)amine], in which triangular [Cd₃(μ₃-OH)]⁵⁺ cluster subunits in 1 and 1D [Zn₅(μ₃-OH)₂(μ-OH)₂]_n⁶ⁿ⁺ chain subunits in 2 are linked by in situ generated bis(tetrazole)amine ligands to generate an interesting 2D coordination net and a 3D microporous metal–organic framework, respectively. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)

Five new copper(II) complexes have been isolated from the reaction system of Cu^II/N(CN)₂^–/HNu (HNu = pyrazole or MeOH) by Cu²⁺-mediated in situ nucleophilic additions of methanol or pyrazole to dicyanamide; they are [Cu(HdcadMeOH)₂](ClO₄)₂·2H₂O (3), [Cu(dca)₂(3-nic)₂]_n·(HdcaMeOH)_2n (4), [Cu(dcapz)(dcadMeOH)] (5), [Cu₃(dcadpz)₂(Hpz)₂(MeCN)₂](ClO₄)₂(NO₃)₂ (6), and [Cu₆(dcadpz-2H)₂(μ-pz)₆] (7) [HdcadMeOH = bis(methoxycarbimido)amine, HdcaMeOH = (methoxycarbimido)cyanoamine, dcadMeOH^– = bis(methoxycarbimido)aminato, dcapz^– = (pyrazolecarbimido)cyanoaminato, dcadpz^– = bis(pyrazolecarbimido)aminato, Hpz = pyrazole, 3-nic = nicotinamide]. X-ray crystallography revealed that 3 and 5 are mononuclear complexes, 6 is a trinuclear complex, 7 is a cyclic hexanuclear cluster, whereas 4 is composed of neutral one-dimensional [Cu(μ_1,5-dca)₂(3-nic)₂]_n coordination chains with HdcaMeOH guest molecules located between these chains. In the five complexes, the 1:1 and 1:2 nucleophilic addition reactions occurred between dca^– and methanol or pyrazole molecules under controlled conditions. In complexes 3, 5, 6, and 7, the addition products generated in situ act as bridging or chelating ligands, whereas in compound 4 the addition product acts as the guest molecule. The magnetic properties of complexes 6 and 7 were studied by variable-temperature magnetic susceptibility and magnetization measurements. In 6, significant ferromagnetic coupling exists between the three S = 1/2 spins within the Cu₃ unit through the dcadpz^– bridges, whereas in 7, strong antiferromagnetic couplings exist between the neighboring copper ions through the [dcadpz-2H]^3– and pz^– bridges with an intratrimer exchange interaction of –250.6 K and an intertrimer coupling interaction of –69.1 K. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)

To study the conformations of 1,2,3,4,5,6-cyclohexanehexacarboxylic acid (H₆L), eleven new coordination polymers have been isolated from hydrothermal reactions of different metal salts with 1e,2a,3e,4a,5e,6a-cyclohexanehexacarboxylic acid (3e+3a, H₆L^I) and characterized. They are [Cd₁₂(μ₆-L^II)(μ₁₀-L^II)₃(μ-H₂O)₆(H₂O)₆]⋅16.5 H₂O (1), Na₁₂[Cd₆(μ₆-L^II)(μ₆-L^III)₃]⋅27 H₂O (2), [Cd₃(μ₁₃-L^II)(μ-H₂O)] (3), [Cd₃(μ₆-L^III)(2,2′-bpy)₃(H₂O)₃]⋅2 H₂O (4), [Cd₄(μ₄-L^VI)₂(4,4′-Hbpy)₄(4,4′-bpy)₂(H₂O)₄]⋅9.5 H₂O (5), [Cd₂(μ₆-L^II)(4,4′-Hbpy)₂(H₂O)₁₀]⋅5 H₂O (6), [Cd₃(μ₁₁-L^VI)(H₂O)₃] (7), [M₃(μ₉-L^II)(H₂O)₆] (M=Mn (8), Fe (9), and Ni (10)), and [Ni₄(OH)₂(μ₁₀-L^II)(4,4′-bpy)(H₂O)₄]⋅6 H₂O (11). Three new conformations of 1,2,3,4,5,6-cyclohexanehexacarboxylate, 6e (L^II), 4e+2a (L^III) and 5e+1a (L^VI), have been derived from the conformational conversions of L^I and trapped in these complexes by controlling the conditions of the hydrothermal systems. Complexes 1 and 2 have three-dimensional (3D) coordination frameworks with nanoscale cages and are obtained at relatively low temperatures. A quarter of the L^I ligands undergo a conformational transformation into L^II while the others are transformed into L^III in the presence of NaOH in 2, while all of the L^I are transformed into L^II in the absence of NaOH in 1. Complex 3 has a 3D condensed coordination framework, which was obtained under similar reaction conditions as 1, but at a higher temperature. The addition of 2,2′-bipyridine (2,2′-bpy) or 4,4′-bipyridine (4,4′-bpy) to the hydrothermal system as an auxiliary ligand also induces the conformational transformation of H₆L^I. A new L^VI conformation has been trapped in complexes 4–7 under different conditions. Complex 4 has a 3D microporous supramolecular network constructed from a 2D L^III-bridged coordination layer structure by π-π interactions between the chelating 2,2′-bpy ligands. Complexes 5–7 have different frameworks with L^II/L^VI conformations, which were prepared by using different amounts of 4,4′-bpy under similar synthetic conditions. Both 5 and 7 are 3D coordination frameworks involving the L^VI ligands, while 6 has a 3D microporous supramolecular network constructed from a 2D L^II-bridged coordination layer structure by interlayer N_4,4′-HbpyH⋅⋅⋅O(L^II) hydrogen bonds. 3D coordination frameworks 8–11 have been obtained from the H₆L^I ligand and the paramagnetic metal ions Mn^II, Fe^II, and Ni^II, and their magnetic properties have been studied. Of particular interest to us is that two copper coordination polymers of the formulae [{Cu^II₂(μ₄-L^II)(H₂O)₄}{Cu^I₂(4,4′-bpy)₂}] (12 α) and [Cu^II(Hbtc)(4,4′-bpy)(H₂O)]⋅3 H₂O (H₃btc=1,3,5-benzenetricarboxylic acid) (12 β) resulted from the same one-pot hydrothermal reaction of Cu(NO₃)₂, H₆L^I, 4,4′-bpy, and NaOH. The Hbtc²⁻ ligand in 12 β was formed by the in situ decarboxylation of H₆L^I. The observed decarboxylation of the H₆L^I ligand to H₃btc may serve as a helpful indicator in studying the conformational transformation mechanism between H₆L^I and L^II–VI. Trapping various conformations in metal-organic structures may be helpful for the stabilization and separation of various conformations of the H₆L ligand.