Mössbauer studies of Fe x Zn1−x (4-amino-1,2,4-triazole)3(NO3)2 complexes possessing the1 A 1⇄5 T 2 spin transition

Mössbauer studies were carried out for the mixed complexes Fe_xZn_1?x(ATr)₃(NO₃)₂ (0.2≤x≤1) having a polynuclear chain structure, for which we had earlier found a significant decrease in the¹A₁ ⁵T₂ spin transition temperature when iron was replaced with zinc. For these complexes, we have found for the first time a tendency toward an increase in the chemical shifts and quadrupole splitting of iron atoms in the low-spin state with the degree of their replacement by other metal atoms. A correlation between these Mössbauer spectral data and the spin transition temperature was found. The results of these studies are explained in terms of the model of steric strains in molecular fragments of the chain structure of the complexes appearing when iron atoms are replaced by zinc atoms.     

Anomalous spin state of iron(II) in 57Fe Mössbauer emission spectra of 57Co-Labeled tris(2-methylphenanthroline) cobalt(II) perchlorate

⁵⁷Fe Mössbauer emission spectra of the ⁵⁷Co labeled complex compound [⁵⁷Co(2-CH₃-phen)₃] (ClO₄)₂·2H₂O have been measured as a function of temperature between 293 and 4.6 K. The spectra exclusively show high-spin iron(II) resonances beside a small fraction of an high-spin iron(II) species, whereas the corresponding iron(II) compound is known to exhibit thermally induced high-spin ⁵T_2g(O_h) ? low spin ¹A_1g(O_h) transition. The electronic nature of the anomalous spin state has been found to be ⁵A₁(D₃) by a theoretical treatment of the temperature dependence of the quadrupole splitting. The results are in good agreement with those obtained from Mössbauer absorption measurements on [⁵⁷Fe_0.01Co_0.99(2-CH₃-(phen)₃] (ClO₄)₂·2H₂O.     

Mössbauer Studies on Spin Crossover in Schiff Bases Complexes of Iron (III)

The Mössbauer spectra of the imidazole adducts of tetradentate Schiff bases complexes of Iron (III), (FeLIm₂)⁺ have been measured. Based on the Mössbauer spectra and magnetic moments, the spin-equilibrium of Low-spin ? high-spin for [Fe(bzacen)(lm)₂)B(ph)₄ was observed.     

Effect of powder dispersion on the1 A 1 ⇄5 T 2 spin transition in Fe(II) complexes with 4-amino-l,2,4-triazole

A series of Fe(II) complexes with 4-amino-1,2,4-triazole ground in an agate mortar for 10 min is studied by Mössbauer spectroscopy. Strong effects of powder dispersion both on the¹ A ₁ ?⁵ T ₂ spin transition and on the structure dynamic characteristics of the complexes are found. Thus at 295 K the high-spin form of Fe(II) appears in the samples or its fraction increases; the ionicity of Fe-N bonds and the extent of distortion of the octahedral environment of iron atoms for the low-spin phases of the complexes also increase. It is established that powder dispersion markedly affects the probability of the Mössbauer effect and the vibrational spectrum of the lattice of coordination compounds. For both the low- and high-spin phases of the complexes, it is reported that the vibrational spectrum is “softened.” The main reason for these effects is supposed to be defectiveness rather than the size of the particles due to mechanical activation of the powder.     

Simplified formula for the1 A 1⇄5 T 2 spin transition temperature in Fe(II) complexes

A formula relating the¹A₁?⁵T₂ spin transition temperature (T_c) in Fe(II) complexes to characteristics of the compounds is derived. With certain assumptions, T_c is determined by the splitting parameter Δ_LS of e_g- and t_2g-orbitals for the low-spin complexes and by the frequency ratio of normal vibrations of the low- and high-spin phases. For the group of compounds possessing spin transitions, the values of Δ_LS are found and analyzed. Correlations between T_c and Δ_LS are established; the values of the change in the probability of the Mössbauer effect are correlated with those of entropy of spin transition. The correlations are substantiated. It is concluded that for mononuclear Fe(II) complexes possessing sharp spin transitions, T_c may not be significantly higher than for Fe(Phy)₂(BF₄)₂ (T_c=282 K).     

Synthesis and Mössbauer Spectroscopy Studies on the Spin States of tris(2-Pyridylimine) Iron(ii) Complexes

Some new tris(2-pyridylimine) complexes of iron(II) such as FeL₃X₂ (L = substituted imine ligand, 2-py-CR₁=NR₂, where R₁ or R₂=H, CH₃(Me) and C₄H₅(Ph), X=ClO₄^? or NCS^?, have been synthesized and their electronic structures in the solid state examined by magnetic measurement and ⁵⁷Fe Mössbauer spectroscopic studies. All the perchlorate complexes FeL₃(ClO₄)₂ are low-spin electronic ground state of Fe(II). The dithiocyanato-complexes, FeL₃(NCS)₂; the spin states of Fe(II) are dependent on substituent R₁ and R₂. The two complexes Fe(2-py-CH=NH)₃(NCS)₂ and Fe(2-py-CPh=NH)₃(NCS)₂ are spin-intermediate (S=1) and Fe(2-py-CH=NPh)₃(NCS)₂ shows a thermally inducing spin transition of ⁵T?¹A₁, whereas the other derivatives remain a low-spin (S=0, ¹A₁) ground state of Fe(II) at three temperatures of 298, 202 and 78 K.     

Mössbauer study of the influence of ligands and anions of the second coordination sphere in Fe(II) complexes with 1,2,4-triazole and 4-amino-1,2,4-triazole on the temperature of the 1A 1⇆5 T 2 spin transitions

For Fe(II) complexes with 1,2,4-triazole (Tr) and 4-amino-1,2,4-triazole (ATr) in low-spin phases, it has been revealed that the ligand and anion of the second coordination sphere affect the electronic state of the central atom and the symmetry of its local environment. For low-spin phases, a correlation between the Mössbauer spectrum parameters—the chemical shift and the bandwidth of the weakly resolved quadrupole doublet—and the spin transition temperature T_c has been established. The mechanism of the influence of complex composition on the spin transition temperature is accounted for by assuming that T_c is determined by the lattice energy of a complex compound.     

Spin transition in heptanuclear star-shaped iron(III)–antimony(V) NCS- and CN-bridged compounds

The precursor [Fe^III(L)Cl] (LH₂ = N,N′-bis(2′-hydroxy-benzyliden)-1,6-diamino-3-azahexane) has been prepared and Mössbauer spectroscopy assigned a high-spin (S = 5/2) state at room temperature. The precursor is combined with the bridging units [Sb^V(X)₆]^? (X = CN^?, NCS^?) to yield star-shaped heptanuclear clusters [(LFe^III–X)₆Sb^V]Cl₅. The star-shaped compounds are in general high-spin systems at room temperature. On cooling to 20 K some of the iron(III) centers switch to the low-spin state as indicated by Mössbauer spectroscopy, i.e. multiple electronic transitions. While the cyano-bridged complex performs a multiple spin transition the thiocyanate-compound shows no significant population at both temperatures.     

Mesophase and magnetic behavior in cobalt(II) and iron(II) compounds

The metal complexes with long alkyl chains [Co(C16-terpy)₃](BF₄)₂ (1) and [Fe(C16-terpy)₂](BF₄)₂ (2) were synthesized and the physical properties of the complex were characterized by magnetic susceptibility, Mössbauer spectroscopy, polarizing optical microscopy, differential scanning calorimetry, and X-ray scattering, where C16-terpy is 4′-hexadecyloxy-2,2′:6′,2′′-terpyridine. Variable-temperature magnetic susceptibility measurements and/or Mössbauer studies revealed that the complex 1 exhibited unique spin transition (T_1/2↓ = 217 K and T_1/2↑ = 260 K) induced by structural phase transition, and the complex 2 was in the low-spin state in the temperature region of 5–400 K before the first mesophase transition. The cobalt(II) and iron(II) complexes exhibited liquid-crystal properties in the temperature range of 371–528 K and 466–556 K, respectively. After mesophase transition, the complex 1 exhibited only slight spin transition (T_1/2↓ = 266 K and T_1/2↑ = 279 K), and the complex 2 was in the low-spin state. The compounds with multifunction, i.e., magnetic property and liquid-crystal properties, are important in the development of molecular materials.     

Synthesis and Mossbauer Spectra Studies of Mono-Binuclear Iron(III) Complexes with a Quinquedentate Ligand Derived from Saucylaldehyeds and Diethylenetriamine

Mono- and bi-nuclear iron(III) complexes of general formula [FeXL] and [LFe-Y-FeL](Bph₄)₂ have been prepared, and their spin state of iron atom in the complexes has been studied by means of the temperature dependence of the Mössbauer spectra, electronic spectra and magnetic measurement, where X is a mono- dentate ligand such as Cl-, NCS-, NCO-, N3-, pyridine and L denotes a quinquedentate Schiff base derived from salicylaldehyde and diethylenetriamine, and Y denotes bridged ligand such as pyrazine(pyr), 4,4′-bipyridine(bpy) and 4,4′-vinylenebipyridine(vibpy). On the basis of the Mössbauer and magnetic data, it was concluded that these complexes were all the high-spin (S = 5/2) slate. The effect of gamma ray irradiation for these complexes has been discussed.     

121Sb-Mössbauer-Spektren von Antimon(V)-Verbindungen. II

¹²¹Sb Mössbauer Spectra of Antimony (V) Compounds. II The Mössbauer resonance effect of ¹²¹Sb has been studied at 4.2 K in a series of 11 antimony(V) compounds which are in most cases bridged. Isomer shift and quadrupole splitting variations are considered in the light of results of X-ray and vibrational spectroscopy. Preparation and the vibrational spectra of [NMe₄][SbCl₄(n₃)₂] are described. Within the limitations of available structural data related tin compounds are compared with those of corresponding ¹¹⁹Sn Mössbauer spectra.     

Mössbauer spectra of the tetrakis-(1,8-naphthyridine) iron(II)perchlorate in external magnetic fields. Evidence of slow relaxation ln paramagnetic iron(II)

⁵⁷Fe Mössbauer spectra of [FeL₄] (ClO₄)₂ where L = 1,8-naphthyridine have been measured at 4.2°K in external magnetic fields up to 55 kG parallel to the direction of the γ-rays. The spectra have been fitted in the spin hamiltonian approximation assuming an orbital singlet ground state of the ⁵D multiplet of Fe²⁺. The fit of the spectra is not unique, yet the possible spin hamiltonian parameter sets found lead to a spin doublet ground state split by less than 1 cm^?1. The transition probabilities for spin-lattice relaxation have been calculated for those ground states. Orbach processes via excited spin hamiltonian states cannot be neglected. The results explain the fluctuations observed in the spectra in low external magnetic fields (10 kG).The spin hamiltonian parameters provide information on the orbital energy levels. Therefrom the reduction of the quadrupole splitting by spin—orbit coupling results to be small thus explaining the extremely large quadrupole splitting of 4.54 mm/sec.     

Study on the valence fluctuation and the magnetism of an iron mixed-valence complex, (n-C4H9)4N[FeIIFeIII(mto)3](mto = C2O3S)

In the case of iron mixed-valence complexes whose spin states are situated in the spin-crossover region, conjugated phenomena coupled with spin and charge are expected. In general, the Fe site coordinated by six S atoms is in the low-spin state, while the Fe site coordinated by six O atoms is in the high-spin state. From this viewpoint, we have synthesized and investigated physical properties for an monothiooxalato-bridged (mto = C₂O₃S) iron mixed-valence complex, (n-C₄H₉)₄N[Fe^IIFe^III(mto)₃], consisting of Fe^IIIO₃S₃ and Fe^IIO₆ octahedra, which behaves as a ferrimagnet with its magnetic transition temperature of T_N = 38 K and Weiss temperature of θ = ?93 K. From the analysis of ⁵⁷Fe Mössbauer spectra of ⁵⁷Fe enriched complexes, (n-C₄H₉)₄N[⁵⁷Fe^IIFe^III(mto)₃] and (n-C₄H₉)₄N[Fe^II57Fe^III(mto)₃], the charge transfer between Fe^II and Fe^III exists in the paramagnetic phase. Considering the time window of ⁵⁷Fe Mössbauer spectroscopy, the time scale of the valence fluctuation is at least slower than 10^?7 s. In order to confirm the valence fluctuation between Fe^II and Fe^III, we investigated the dielectric constant and found an anomalous enhancement attributed to the Fe valence fluctuation between 170 and 250 K.     

New polynuclear 4,4ˈ-bis-1,2,4-triazole Fe(II) spin crossover compounds

Two novel polynuclear Fe(II) spin crossover materials of formula 〚Fe(btr)₃〛 〚Fe(btr)₂(H₂O)₂〛(anion)₄, where btr = 4,4′-bis–1,2,4-triazole and anion = BF₄^–, PF₆^–, have been prepared and their spin transition characteristics studied over the temperature range 5–300 K. They both reveal incomplete spin crossover behaviour. Two different Fe(II) lattice sites of the FeN₆ and FeN₄O₂ type are distinguished by ⁵⁷Fe Mössbauer spectroscopy. The first site is responsible for the SC behaviour whereas the second one remains high-spin throughout the whole temperature range. This explains why it is not possible to switch all the Fe(II) ions to the low-spin state by application of hydrostatic pressure for the BF₄^– derivative. The temperature dependence of the population of these sites has been carefully analysed by Mössbauer spectroscopy.     

Tris [2-(2′-pyridyl)benzimidazole]iron(II) complexes. Some new examples of 5T2 — 1A1 spin equilibria

Mössbauer spectra and magnetic susceptibilities for three iron (II) complexes of 2-(2′-pyridyl)benzimidazole have been measured in the temperature range 80–300°K. The results are consistent with the existence of ⁵T₂ — ¹A₁ spin equilibria in all three complexes.     

Synthesis and immobilization of molecular switches onto titaniumdioxide nanowires

The precursor [Fe^III(L)Cl (L = N,N′-bis(2′-hydroxy-3′-methyl-benzyliden)-1,7-diamino-4-azaheptane) is combined with [Mo(CN)₈]^4? yields a star shaped nona-nuclear cluster, [Mo^IV{(CN)Fe^III(L)}₈]Cl₄. This Fe₈Mo molecule is a high-spin system at room temperature. On cooling to 20 K some of the iron(III) centres in the molybdenum(IV)-star switch to the low-spin state as proven by Mössbauer spectroscopy. This molecule was deposited on TiO₂ nanowires by electrostatic interactions between the cluster cations and the surface functionalized titanium oxide nanowire. The synthesis and surface binding of the multistable molecular switch was demonstrated using IR and UV–Vis spectroscopy (high-resolution) transmission electron microscopy ((HR)TEM) and Mössbauer spectroscopy. High- and low-temperature Mössbauer spectra indicate that the spin state transition of the free cluster molecules is preserved after surface binding. The above results emphasize the possibility of fabricating molecule-based low-dimensional structures by using traditional bottom-up approaches based on the electrostatic interaction between the cluster cations and polymer functionalized nanowires. These results can be generalized for the application to both charged and non-charged molecules.     

Rapid cooling experiments and use of an anionic nuclear probe to sense the spin transition of the 1D coordination polymers [Fe(NH2trz)3]SnF6n x H2O (NH2trz=4-amino-1,2,4-triazole)

[Fe(NH₂trz)₃]SnF₆ ? n H₂O (NH₂trz=4‐amino‐1,2,4‐triazole; n=1 ( 1 ), n=0.5 ( 2 )) are new 1D spin‐crossover coordination polymers. Compound 2 exhibits an incomplete spin transition centred at around 210 K with a thermal hysteresis loop approximately 16 K wide. The spin transition of 2 was detected by the Mössbauer resonance of the ¹¹⁹Sn atom in the SnF₆^2? anion primarily on the basis of the evolution of its local distortion. Rapid‐cooling ⁵⁷Fe Mössbauer and superconducting quantum interference device experiments allow dramatic widening of the hysteresis width of 2 from 16 K up to 82 K and also shift the spin‐transition curve into the room temperature region. This unusual behaviour of quenched samples on warming is attributed to activation of the molecular motion of the anions from a frozen distorted form towards a regular form at temperatures well above approximately 210 K. Potential applications of this new family of materials are discussed.     

Studies on the reaction products of bis(2-furanthiocarboxyhydrazidato)m(II) with pyridine-2- and -4-carboxaldehydes

Condensation of bis(2-furanthiocarboxyhydradatometal(II), M(fth)₂; [M (II) = Mn, Fe, Co, Ni, Cu and Zn] with pyridine-2- and -4-carboxaldehydes gave complexes of the formula M(pfth)₂ [pfth? = pyridine-2-carboxaldehyde-2-furanthiocarboxyhydrazonato], Ni(Ifth)₂, Zn(Ifth)₂, Cu(Ifth) and Co(Ifth)₃, (Ifth? = pyridine-4-carboxaldehyde-2-furanthiocarboxyhydrazonato). The magnetic and electronic spectral studies coupled with photoacoustic or Mössbauer spectra suggested octahedral geometry for the M(II) complexes with low-spin states for Co(Ifth)₃ and Fe(pfth)₂. IR and ¹H NMR spectral studies of diamagnetic complexes suggested bonding through “azomethine” nitrogen and “thiolo” sulphur. IR spectra also showed the involvement of pyridine ring nitrogen in coordination in all the complexes except Cu(Itfh), Co(Ifth)₃, and Zn(Ifth)₂. Some of the compounds possessed antimicrobial activity.     

The preparation and tin-119 Mössbauer spectra of new adducts of the type organotetrachlorostannate(IV) base (11)

New adducts Et₄N(RSnCl₄ · L) (R butyl and phenyl; L pyridine, trimethylamine, tri-n-butylphosphine, triphenylphosphine oxide, triphenylarsine oxide, dimethylsulphoxide, dimethylformamide) have been isolated and their Mössbauer spectra studied. Definite structural assignments are made for the phosphine and phosphine oxide adducts from Mössbauer quadrupole splitting data. Comments are made on the choice of partial splitting values for phenyl and butyl groups.     

Spin Crossover and Valence Tautomerism in Neutral Homoleptic Iron Complexes of Bis(pyridylimino)isoindolines

Homoleptic iron complexes of six bis(pyridylimino)isoindoline (bpi) ligands with different substituents (H, Me, Et, tBu, OMe, NMe₂) at the 4‐positions of the pyridine moieties have been prepared and studied with regard to temperature‐dependent spin and redox states by a combination of ⁵⁷Fe Mössbauer spectroscopy, SQUID magnetometry, single‐crystal X‐ray diffraction analysis, X‐band EPR, and ¹H NMR spectroscopy. While the H‐, methyl‐, and ethyl‐substituted complexes remain in a pure high‐spin state irrespective of the temperature, the 4‐tert‐butyl‐substituted derivative shows spin‐crossover behavior. The methoxy‐ and dimethylamino‐substituted compounds were found to easily undergo oxidation. In the crystalline state, valence tautomeric behavior was observed for the methoxy derivative as a thermally activated charge‐transfer transition, accompanied by a spin crossover above 200 K. The valence tautomerism leads to a chelate with one of the bpi ligands as a dianion radical L^2?. and with an effective spin of S=2.