Spin crossover in iron(II) tris(2-(2′-pyridyl)benzimidazole) complex monitored by variable temperature methods: synchrotron powder diffraction, DSC, IR spectra, Mössbauer spectra, and magnetic susceptibility

Theoretical Investigation of the Electronic Structure of Fe(II) Complexes at Spin-State Transitions

The electronic structure relevant to low spin (LS)↔high spin (HS) transitions in Fe(II) coordination compounds with a FeN₆ core are studied. The selected [Fe(tz)₆]²⁺ (1) (tz = 1H-tetrazole), [Fe(bipy)₃]²⁺ (2) (bipy = 2,2′-bipyridine), and [Fe(terpy)₂]²⁺ (3) (terpy = 2,2′:6′,2″-terpyridine) complexes have been actively studied experimentally, and with their respective mono-, bi-, and tridentate ligands, they constitute a comprehensive set for theoretical case studies. The methods in this work include density functional theory (DFT), time-dependent DFT (TD-DFT), and multiconfigurational second order perturbation theory (CASPT2). We determine the structural parameters as well as the energy splitting of the LS–HS states (ΔE_HL) applying the above methods and comparing their performance. We also determine the potential energy curves representing the ground and low-energy excited singlet, triplet, and quintet d⁶ states along the mode(s) that connect the LS and HS states. The results indicate that while DFT is well suited for the prediction of structural parameters, an accurate multiconfigurational approach is essential for the quantitative determination of ΔE_HL. In addition, a good qualitative agreement is found between the TD-DFT and CASPT2 potential energy curves. Although the TD-DFT results might differ in some respect (in our case, we found a discrepancy at the triplet states), our results suggest that this approach, with due care, is very promising as an alternative for the very expensive CASPT2 method. Finally, the two-dimensional (2D) potential energy surfaces above the plane spanned by the two relevant configuration coordinates in [Fe(terpy)₂]²⁺ were computed at both the DFT and CASPT2 levels. These 2D surfaces indicate that the singlet–triplet and triplet–quintet states are separated along different coordinates, i.e., different vibration modes. Our results confirm that in contrast to the case of complexes with mono- and bidentate ligands, the singlet–quintet transitions in [Fe(terpy)₂]²⁺ cannot be described using a single configuration coordinate.

Synthesis, infrared spectra, thermal analyses and structural studies of half-sandwich Fe(III)/Fe(II) complex containing pyridine-2,6-dicarboxylate and 1,10-phenanthroline

A new pyridine-2,6-dicarboxylate iron(III)/iron(II) complex [Fe(phen)(3)][Fe(2)(PDC)(4)].3CH(3)OH was synthesized and characterized (where PDC=pyridine-2,6-dicarboxylate, phen=1,10-phenanthroline) by using elemental analysis, IR spectroscopy and thermal analyses (TGA and DTA). The molecular structure of the complex has been determined by single-crystal X-ray diffraction. The complex is mixed-ligands and the IR spectra display bands characteristic of coordinated mixed-ligand bases. All the IR results are in agreement with the X-ray crystal result. The bond lengths indicate that this complex has [Fe(phen)(3)](2+) cation where Fe(II) ion is in typical low-spin state, and in counter ions, [Fe(PDC)(2)](-) are both in high-spin state.

Synthesis, X-ray crystal structures and biomimetic and anticancer activities of novel copper(II)benzoate complexes incorporating 2-(4′-thiazolyl)benzimidazole (thiabendazole), 2-(2-pyridyl)benzimidazole and 1,10-phenanthroline as chelating nitrogen donor ligands

Cu(BZA)(2)(EtOH)(0.5) (1) was generated by the reaction of copper(II) hydroxide with benzoic acid (BZAH). [Cu(TBZH)(2)(BZA)](BZA).0.5TBZH.H(2)O (2) and [Cu(2-PyBZIMH)(2-PyBZIM)(BZA)].1.66EtOH (3) were obtained when 1 reacted with Thiabendazole (TBZH) and 2-(2-pyridyl)benzimidazole (2-PyBZIMH), respectively. [Cu(BZA)(2)(phen)(H(2)O)] (4) was isolated from the reaction of benzoic acid and 1,10-phenanthroline (phen) with copper(II)acetate dihydrate. Molecular structures of 2, 3 and 4 were determined crystallographically. 2 and 3 are hydrogen bonded dimers and trimers, respectively. The copper centres in complexes 2 and 3 are bis-chelate derivatives that have N(4)O ligation and their geometry is very similar being approximately square-pyramidal. However whereas in complex 2 both TBZH ligands are neutral in 3 one of the 2-PyBZIMH chelators is deprotonated on each copper. The structural results for 4 represent a re-examination of this crystallographically known compound for which no hydrogen atom coordinates have been previously reported. It crystallises as a hydrogen bonded dimmer and is a mono-chelate of phen with each copper centre possessing N(2)O(3) ligation and square pyramidal geometry. The catalase and superoxide dismutase (SOD) activities of the four complexes along with those of the known phenanthroline complexes [Cu(mal)(phen)(2)] and [Cu(phendione)(3)](ClO(4))(2) (malH(2)=malonic acid and phendione=1,10-phenanthroline-5,6-dione) were investigated. Complexes 1-4, the metal free ligands and a simple copper(II) salt were assessed for their cancer chemotherapeutic potential against the hepatocellular carcinoma (Hep-G(2)) and kidney adenocarcinoma (A-498) cell lines. TBZH, 2-PyBZIMH and benzoic acid when uncoordinated to a metal centre offer poor chemotherapeutic potential. copper(II) benzoate is significantly more active than the free acid. The bis-chelate derivatives [Cu(TBZH)(2)(BZA)](BZA).0.5TBZH.H(2)O (2) and [Cu(2-PyBZIMH)(2-PyBZIM)(BZA)].1.66EtOH (3) elicit a significant cytotoxic response to the cancer cell lines tested. Replacing TBZH and 2-PyBZIMH with phen to give [Cu(BZA)(2)(phen)(H(2)O)] (4) does not significantly increase the anti-cancer activity.

Interplay of Spin Conversion and Structural Phase Transformations: Re-Entrant Phase Transitions in the 2-Propanol Solvate of Tris(2-picolylamine)iron(II) Dichloride

Crystal structures, magnetic and thermodynamic properties of the spin-crossover compound tris(2-picolylamine)iron(II) dichloride (with 2-propanol solvent molecules) have been measured in the temperature range from 15 to 293 K. X-ray diffraction, SQUID, and calorimetric experiments all showed two first-order phase transitions with hysteresis loops, a narrow one at T(1) approximately 196 K and a broad, triangular one covering the range 153<T(2)<166 K. Crystal structures were analysed at fourteen temperatures in a cooling cycle and at seven temperatures in a heating cycle. They reveal a complex, temperature-dependent ordering behaviour of both the complex cations and the alcohol molecules. A phenomenological model accounting for spin conversion, solvent ordering and coupling between the two processes describes the observed phase transitions and ordering phenomena reasonably well. Similarities and differences in the behaviour of the corresponding ethanol solvate with the same crystal architecture are discussed. It is concluded that spin-crossover behaviour depends as much on molecular properties as it does on intermolecular interactions, both of the spin active and the spin inactive components.