Anion-Induced Structural Diversity and Optical Chromism in a Series of Cyano-Bridged Heterometallic 3d-4f Coordination Polymers

The self-assembly reaction of the neutral dicyano-bis(1,10-phenanthroline) iron(II) complex with lanthanide ions (Ln = Eu(III), Gd(III), Er(III)) provided two different classes of heterometallic cyano-bridged 3d-4f architectures depending on the nature of the counteranion, irrespective of the size of the 4f metal ion. Tetranuclear oligomers with a squared Fe₂Ln₂ core were isolated when using nitrate salts, whereas unusual 1D polymeric chains were obtained when resorting to triflate salts under the same synthetic conditions. It is shown that the different structural motifs have a remarkable impact on the thermal stability and the optical properties of the compounds, which display a notable optical ipsochromism of the parent Fe(II) complex upon coordination with the Ln ion. This effect is significantly more pronounced in the polymeric chain than in the Fe₂Ln₂ oligomer both in solution and in the solid state. Structural evidence suggests that this behavior is likely related to the geometry of the CN-Ln bridge. On the other hand, more extended π-stacking interactions in the oligomer give rise to a broad charge-transfer absorption (600-1500 nm), making this compound promising as NIR absorber. Density Functional Theory calculations and electrochemical studies demonstrate that the observed negative chromism originates from the stabilization of a mixed metal/cyanide character HOMO with respect to a phenanthroline-centered LUMO.

Heterobimetallic cyanide-bridged FeIII(μ-CN)MII complexes (M = Mn and Cu): synthesis, structure and magnetism

Systematic magneto-structural studies reveal ‘[FeIII(Tp)(CN)2(μ-CN)MnIICl(L1/L2)]′ (both antiferromagnetic) and [FeIII(Tp)(CN)2(μ-CN)CuII(L1/L2)]+′(L1 ferro- and L2 antiferromagnetic).

The rational construction of diamond-like dysprosium–hexacyanometallate frameworks featuring dynamic magnetic behaviour

Four novel diamond-like dysprosium–hexacyanometallate frameworks featuring slow magnetic relaxation have been rationally constructed by a feasible building block strategy using hexacyanometallate to link superparamagnetic dimeric units.

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^IIILn^III}₂ neutral molecular squares of general formula [Fe{HB(pz)₃}(CN)(μ-CN)₂Ln(NO₃)₂(pyim)(Ph₃PO)]₂·2CH₃CN [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 P1̄ space group. Their crystal structures consist of centrosymmetric cyanido-bridged {Fe^IIILn^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⋯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.

Cyanido-bridged {FeIIILnIII} heterobimetallic chains assembled through the [FeIII{HB(pz)3}(CN)3]− complex as metalloligand: synthesis, crystal structure and magnetic properties

The first isostructural series of cyanido-bridged 3d–4f chains showing a unique zig-zag motif have been obtained by using the heteroleptic [Fe^III{HB(pz)₃}(CN)₃]⁻ complex as a metalloligand towards the preformed [Ln^III(bpdo)(NO₃)₂(H₂O)]⁺ species.

X-ray Structure and Magnetic Properties of Heterobimetallic Chains Based on the Use of an Octacyanidodicobalt(III) Complex as Metalloligand

The assembly of [Co2III(μ-2,5-dpp)(CN)8]2− anions and [MII(CH3OH)2(DMSO)2]2+ cations resulted into the formation of two heterobimetallic 1D coordination polymers of formula [MII(CH3OH)2(DMSO)2(μ-NC)2Co2III(μ-2,5-dpp)(CN)6]n·4nCH3OH [M = CoII (1)/FeII (2) and 2,5-dpp = 2,5-bis(2-pyridyl)pyrazine. The [Co2III(μ-2,5-dpp)(CN)8]2− metalloligand coordinates the paramagnetic [MII(CH3OH)2(DMSO)2]2+ complex cations, in a bis-monodentate fashion, to give rise to neutral heterobimetallic chains. Cryomagnetic dc (1.9–300 K) and ac (2.0–13 K) magnetic measurements for 1 and 2 show the presence of Co(II)HS (1) and Fe(II)HS (2) ions (HS – high-spin), respectively, with D values of +53.7(5) (1) and −5.1(3) cm−1 (2) and slow magnetic relaxation for 1, this compound being a new example of SIM with transversal magnetic anisotropy. Low-temperature Q-band EPR study of 1 confirms that D value is positive, which reveals the occurrence of a strong asymmetry in the g-tensors and allows a rough estimation of the E/D ratio, whereas 2 is EPR silent. Theoretical calculations by CASSCF/NEVPT2 on 1 and 2 support the results from magnetometry and EPR. The analysis of the ac magnetic measurements of 1 shows that the relaxation of M takes place in the ground state under external magnetic dc fields through dominant Raman and direct spin-phonon processes.

The Influence of Halide Substituents on the Structural and Magnetic Properties of Fe6Dy3 Rings

We report the synthesis and magnetic properties of three new nine-membered Fe(III)-Dy(III) cyclic coordination clusters (CCCs), with a core motif of [Fe₆Dy₃(μ-OMe)₉(vanox)₆(X-benz)₆] where the benzoate ligands are substituted in the para-position with X = F (1), Cl (2), Br (3). Single crystal X-ray diffraction structure analyses show that for the smaller fluorine or chlorine substituents the resulting structures exhibit an isostructural Fe₆Dy₃ core, whilst the 4-bromobenzoate ligand leads to structural distortions which affect the dynamic magnetic behavior. The magnetic susceptibility and magnetization of 1-3 were investigated and show similar behavior in the dc (direct current) magnetic data. Additional ac (alternating current) magnetic measurements show that all compounds exhibit frequency-dependent and temperature-dependent signals in the in-phase and out-of-phase component of the susceptibility and can therefore be described as field-induced SMMs. The fluoro-substituted benzoate cluster 1 shows a magnetic behavior closely similar to that of the corresponding unsubstituted Fe₆Dy₃ cluster, with U_eff = 21.3 K within the Orbach process. By increasing the size of the substituent toward 4-chlorobenzoate within 2, an increase of the energy barrier to U_eff = 36.1 K was observed. While the energy barrier becomes higher from 1 to 2, highlighting that the introduction of different substituents on the benzoate ligand in the para-position has an impact on the magnetic properties, cluster 3 shows a significantly different SMM behavior where U_eff is reduced in the Orbach regime to only 4.9 K.

In situ generation of Ph3PO in cyanido-bridged heterometallic {FeIIILnIII}2 molecular squares (Ln = Eu, Sm)

Two new examples of cyanido-bridged {Fe^IIILn^III}₂ molecular squares, with pyim and PPh₃O as capping ligands at the Ln^III sites, exhibit weak antiferromagnetic interactions [Ln = Eu (1), Sm (2), pyim = 2-(1H-imidazol-2-yl)pyridine, PPh₃PO = triphenylphosphine oxide].

A novel octacyanido dicobalt(iii) building block for the construction of heterometallic compounds

The first bimetallic 3d cyanido-bearing complex with 2,5-dpp as a bridge, (Ph₄P)₂[Co₂^III(μ-2,5-dpp)(CN)₈]·2H₂O, is obtained and used as a metalloligand with the [Mn(MAC)(H₂O)₂]²⁺ cation to build a {Co^IIIMn^II} heterobimetallic chain.

A turn-on luminescent europium probe for cyanide detection in water

A luminescent europium probe that responds to cyanide directly in water with a large nine-fold turn-on of the EuIII centered time-gated luminescence is presented. Unlike other CN- probes reported, the mechanism of action of EuIII-Lys-HOPO does not rely on reaction of CN- with the probe, but on direct coordination of CN- to the EuIII ion concomitant with displacement of three inner-sphere water molecules. This unusual coordination of CN- with a lanthanide ion in aqueous solution was confirmed by luminescence lifetime measurements.