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 {Fe(III)Ln(III)}(2) neutral molecular squares of general formula [Fe{HB(pz)(3)}(CN)(mu-CN)(2)Ln(NO3)(2)(pyim)(Ph3PO)](2)center dot 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-(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 [Ln(III)(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 P (1) over bar 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)(NO3)(2)(pyim)(Ph3PO)](+) 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 center dot center dot center dot Ln(III) distances across the two cyanido-bridges range from ca. 5.48/5.46 up to ca. 5.58/5.61 angstrom. The molecular squares in 1-7 are interlinked through hydrogen bonds, weak pi...pi stacking and very weak C-H center dot center dot center dot pi 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 chi(M)'' maxima moved us to estimate their energy barriers through ln(chi(M)''/chi(M)') vs. 1/T plots, obtaining values from 25 to 40 cm(-1).