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A new series of cyanido-bridged {Fe III Ln III } 2 neutral molecular squares of general formula [Fe{HB(pz) 3 }(CN)(μ-CN) 2 Ln(NO 3 ) 2 (pyim)(Ph 3 PO)] 2 ·2CH 3 CN [Ln = Ce (1), Pr (2), Nd (3), Gd (4), Tb (5), Dy (6) and Er (7); {HB(pz) 3 } - = hydrotris(pyrazolyl)borate, pyim = 2-(1 H -imidazol-2-yl)pyridine and Ph 3 PO = triphenylphosphine oxide] were obtained by reacting the low-spin [Fe{HB(pz) 3 }(CN) 3 ] - species with the preformed [Ln III (pyim)(NO 3 ) 2 (pyim)(Ph 3 PO)] + complex anions (generated in situ by mixing the nitrate salt of each Ln(III) ion with pyim and Ph 3 PO molecules). Single-crystal X-ray diffraction studies show that 1-7 are isostructural compounds that crystallize in the triclinic P 1̄ space group. Their crystal structures consist of centrosymmetric cyanido-bridged {Fe III Ln III } 2 molecular squares where two [Fe{HB(pz) 3 }(CN) 3 ] - units adopt bis-monodentate coordination modes towards two [Ln III (pyim)(NO 3 ) 2 (pyim)(Ph 3 PO)] + moieties. The cis -oriented convergent sites from both low-spin Fe III and Ln III fragments form a quasi square-shaped molecule in which the 3d and 4f ions alternatively occupy the corners of the square. Both Fe III ions show a distorted octahedral surrounding ( C 3v symmetry), whereas the Ln III ions exhibit a distorted muffin-like geometry ( C s symmetry) in 1-7. The intramolecular Fe III ⋯Ln III 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 .

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 (H 2 O) 6 ] n ·4nH 2 O (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.

We herein present the synthesis, spectroscopic analysis, description of the crystal structures and magnetic properties of four new complexes of the formula [{Cu(opba)(H 2 O) 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) [H 4 opba = 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(S 1 ·S 2 + S 1 ·S 3 )] 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.

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.