There has been a rapid expansion in the use of molecular magnets for both low- and ultra low-temperature cooling applications in recent years, and here we review the chemical variation and magnetothermal properties of reported molecular coolers, structuring the review by structural dimensions, metal-ions involved and ligands employed. This review provides an overview of the developments in designing better low-temperature magnetic refrigerants, and includes description of new 3D-materials that, in some ways, out-perform traditional magnetic coolants. Thus, this review should serve as both a tutorial for many newcomers and a summary of progress for researchers who are active in the field.

High-Nuclearity 3d-4f Clusters as Enhanced Magnetic Coolers and MolecularMagnets. -The Co II /Co III (9:1) mixed compounds (III) and the Ni II compounds (V) are isostructural and crystallize in the monoclinic space group P21/m with Z = 2 (single crystal XRD). (IIIa) and (Va) exhibit the largest magnetocaloric effects among any known 3d-4f complexes, which is significant for their potential applications in magnetic cooling technology in the ultralow temperature range. Compounds (IIIb) and (Vb) display slow relaxation of the magnetization.

A previously developed solid-state route (dimensional reduction) has led to the first molecular clusters containing the face-capped octahedral [Re6(μ3-Q)8]2+ core (Q = S, Se). Among these is the protonated cluster [Re6Se7(SeH)I6]3-, whose substitution reactions have been examined in an effort to create a set of site-differentiated precursors for forming multicluster assemblies. Reaction with Et3P under reflux with different conditions of reactant mole ratio and time affords the clusters fac-[Re6Se8(PEt3)3I3]1- (2), trans- (3) and cis-[Re6Se8(PEt3)4I2] (4), [Re6Se8(PEt3)5I]1+ (5), and [Re6Se8(PEt3)6]2+ (6). The synthesis of fully substituted 6 in quantitative yield required the forcing conditions of 20 equiv of phosphine in refluxing DMF for 3 days. Reaction of 4 with AgBF4 in dichloromethane/acetonitrile gave cis-[Re6Se8(PEt3)4(MeCN)2]2+ (7); a similar reaction of 5 yielded [Re6Se8(PEt3)5(MeCN)]2+ (8) and that of [Re6Se7(SeH)I6]3- led to fully substituted [Re6Se8(MeCN)6]2+ (9). The structures of clusters 2−9 as Bu4N+ (2), iodide (5), SbF6 - (7), or BF4 - (6, 8, 9) salts were proven by X-ray crystallography. No significant variance was observed in the face-capped geometry of the core when bound by iodine, phosphine, and nitrile ligands alone or in combination. Terminal Re−P/I/N bond lengths were similarly independent of co-ligands. The combination of ligands of different lability in clusters 2−9 should make them of considerable utility in producing linked cluster assemblies/materials wherein labile ligands are displaced by bridging ligands. The simplest of these reactions, the direct coupling of two [Re6Se8]2+ cluster cores, is demonstrated here. When thermolyzed at 180 °C for 24 h under dynamic vacuum, the compounds [8]X2 (X = BF4 -, SbF6 -) lose acetonitrile and condense to form two new compounds containing the dicluster [Re12Se16(PEt3)10]4+ (10), whose structure has been established by X-ray methods. This cluster is composed of two [Re6Se8]2+ portions which are centrosymmetrically connected by two Re−(μ4-Se) bonds to form a rhomboidal Re2Se2 unit. The bridge bonds are ca. 0.1 Å longer than the Re−(μ3-Se) bonds of the remainder of the core; they have the same value as in the bridging rhombs present in the two-dimensional phase Re6Se8Cl2, which is a precursor solid to molecular [Re6Se8]2+ clusters by dimensional reduction. The synthesis of 10 is directed by the structure of 8, which can give only one logical product upon deligation of acetonitrile. The synthesis concept is potentially extendable to other types of oligomerized clusters.

Lanthanide ions and complexes occupy a special position in developing synthetic nucleases capable of catalyzing the hydrolytic cleavage of RNA or DNA. Stimulated by a number of serendipitous lanthanide complexes that feature the common active-site structure of dinuclear metallo-phosphodiesterases, rational design and synthesis of polynuclear lanthanidehydroxo species via ligand-controlled hydrolysis of the lanthanide ions were attempted. The efforts yielded a series of highly sophisticated yet structurally well-defined lanthanide-hydroxo complexes. These materials are potentially applicable to the study of biomimetic catalysis of phosphate diester cleavage. Research highlights are described in this Feature Article.