Elucidating the Structural Chemistry of a Hysteretic Iron(II) Spin-Crossover Compound From its Copper(II) and Zinc(II) Congeners

Annealing [FeL₂][BF₄]₂ ⋅ 2 H₂O (L=2,6-bis-[5-methyl-1H-pyrazol-3-yl]pyridine) affords an anhydrous material, which undergoes a spin transition at T_1/2=205 K with a 65 K thermal hysteresis loop. This occurs through a sequence of phase changes, which were monitored by powder diffraction in an earlier study. [CuL₂][BF₄]₂ ⋅ 2 H₂O and [ZnL₂][BF₄]₂ ⋅ 2 H₂O are not perfectly isostructural but, unlike the iron compound, they undergo single-crystal-to-single-crystal dehydration upon annealing. All the annealed compounds initially adopt the same tetragonal phase but undergo a phase change near room temperature upon re-cooling. The low-temperature phase of [CuL₂][BF₄]₂ involves ordering of its Jahn–Teller distortion, to a monoclinic lattice with three unique cation sites. The zinc compound adopts a different, triclinic low-temperature phase with significant twisting of its coordination sphere, which unexpectedly becomes more pronounced as the crystal is cooled. Synchrotron powder diffraction data confirm that the structural changes in the anhydrous zinc complex are reproduced in the high-spin iron compound, before the onset of spin-crossover. This will contribute to the wide hysteresis in the spin transition of the iron complex. EPR spectra of copper-doped [Fe_0.97Cu_0.03L₂][BF₄]₂ imply its low-spin phase contains two distinct cation environments in a 2:1 ratio.     

Variable temperature far and mid FT-IR as a valuable tool to determine the spin transition temperature of iron(II) spin-crossover coordination compounds

The thermally induced spin transition behaviour of selected iron(II) coordination compounds with substituted tetrazole ligands has been monitored using variable temperature FT-IR spectroscopy. The reliability of these results is discussed and compared with independent analytical techniques such as SQUID measurements and ⁵⁷Fe Mössbauer spectroscopy proofing variable temperature IR spectroscopy to be a valuable tool in the determination of the spin transition temperature T_1/2.     

Temperature dependence of the 1J(11B19F) spin–spin coupling in BF3 molecule

The ¹J(¹¹B¹⁹F) spin–spin coupling of gaseous BF₃ was observed in ¹¹B NMR spectra as a function of density in a wide range of temperatures. Following the extrapolation of the measured values to the zero‐density limit, the coupling constant free from intermolecular effects ¹J₀(¹¹B¹⁹F) was obtained for each temperature. In contrast to previous investigations, the final results indicate a nonlinear dependence of ¹J₀(¹¹B¹⁹F) on temperature. In the corresponding ab initio calculations of spin–spin coupling constants performed at the coupled cluster singles and doubles (CCSD) level to obtain a reliable result for this coupling constant we had to take into account large vibrational corrections. Copyright © 2009 John Wiley & Sons, Ltd.     

Uncompensated magnetization in the layered molecular antiferromagnet {N(n-C5H11)4[MnFe(ox)3]}∞

Studies on the magnetic properties of the molecular antiferromagnetic material {N(n-C₅H₁₁)₄[Mn^IIFe^III(ox)₃]}_∞, carried out by various physical techniques (AC/DC magnetic susceptibility, magnetization, heat capacity measurements and Mössbauer spectroscopy) at low temperatures, have been presented. Different experimental observations complement each other and provide a clue for the observation of an uncompensated magnetization below the Néel temperature and short-range correlations persisting high above T_N. It is understood that the honeycomb layered structure of the compound contains non-equivalent magnetic sub-lattices, (Mn^II–ox–Fe^III_A–...) and (Mn^II–ox–Fe^III_B–...), where different responses of the Fe^III_A and Fe^III_B spin sites towards an external magnetic field might be responsible for the observation of the uncompensated magnetization in this compound at T < T_N. The present magnetic system is an S = 5/2 2-D Heisenberg antiferromagnet system with the intralayer exchange parameter J/k_B = −3.29 K. A very weak interlayer exchange interaction was anticipated from the spin wave modeling of the magnetic heat capacity for T < 0.5T_N. The positive sign of the coupling between the layers has been concluded from the Mössbauer spectrum in the applied magnetic field. Frustration in the magnetic interactions gives rise to the uncompensated magnetic moment in this compound at low temperatures.     

Solid‐state 1H → 19F/19F → 1H CP/MAS NMR study of poly(vinylidene fluoride)

Solid‐state ¹H → ¹⁹F and ¹⁹F → ¹H cross‐polarization magic angle spinning (CP/MAS) NMR spectra have been investigated for a semicrystalline fluoropolymer, namely poly(vinylidene fluoride) (PVDF). The ¹H → ¹⁹F CP/MAS spectra can be fitted by five Lorentzian functions, and the amorphous peaks were selectively observed by the DIVAM CP pulse sequences. Solid‐state spin‐lock experiments showed significant differences in T_1ρ^F and T_1ρ^H between the crystalline and amorphous domains, and the effective time constants, T_HF* and T_1ρ*, which were estimated from the ¹H → ¹⁹F CP curves, also clarify the difference in the strengths of dipolar interactions. Heteronuclear dipolar oscillation behaviour is observed in both standard CP and ¹H → ¹⁹F inversion recovery CP (IRCP) experiments. The inverse ¹⁹F → ¹H CP‐MAS and ¹H → ¹⁹F CP‐drain MAS experiments gave complementary information to the standard ¹H → ¹⁹F CP/MAS spectra in a manner reported in our previous papers for other fluoropolymers. The value of N_F/N_H (where N is a spin density) estimated from the CP‐drain curve is within experimental error equal to unity, which is consistent with the chemical structure. Copyright © 2001 John Wiley & Sons, Ltd.     

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).     

Phase transitions and molecular motions in [Cd(H2O)6](BF4)2 studied by DSC, H and F NMR and FT-MIR

Two solid phase transitions of [Cd(H₂O)₆](BF₄)₂ occurring on heating at T_C2=183.3 K and T_C1=325.3 K, with 2 K and 5 K hysteresis, respectively, were detected by differential scanning calorimetry (DSC). High value of entropy changes indicated large orientational disorder of the high temperature and intermediate phase. Nuclear magnetic resonance (¹H NMR and ¹⁹F NMR) relaxation measurements revealed that the phase transitions at T_C1 and T_C2 were associated with a drastic and small change, respectively, of the both spin-lattice relaxation times: T₁(¹H) and T₁(¹⁹F). These relaxation processes were connected with the “tumbling” motions of the [Cd(H₂O)₆]²⁺, reorientational motions of the H₂O ligands, and with the iso- and anisotropic reorientation of the BF₄⁻ anions. The cross-relaxation effect was observed in phase III. The line width and the second moment of the ¹H and ¹⁹F NMR line measurements revealed that the H₂O reorientate in all three phases of the title compound. On heating the onset of the reorientation of 3 H₂O in the [Cd(H₂O)₆]⁺², around the three-fold symmetry axis of these octahedron, causes the isotropic reorientation of the whole cation. The BF₄⁻ reorientate isotropically in the phases I and II, but in the phase III they perform slow reorientation only about three- or two-fold axes. A small distortion in the structure of BF₄⁻ as well as of [Cd(H₂O)₆]²⁺ is postulated. The temperature dependence of the bandwidth of the O-H stretching mode measured by Fourier transform middle infrared spectroscopy (FT-MIR) indicated that the activation energy for the reorientation of the H₂O did not change much at the T_C2 phase transition.     

Two‐Step Spin Transition in a 1D FeII 1,2,4‐Triazole Chain Compound

A thermochromic 1D spin crossover coordination (SCO) polymer [Fe(βAlatrz)₃](BF₄)₂ ? 2 H₂O ( 1? 2 H₂O), whose precursor βAlatrz, (1,2,4‐triazol‐4‐yl‐propionate) has been tailored from a β‐amino acid ester is investigated in detail by a set of superconducting quantum interference device (SQUID), ⁵⁷Fe Mössbauer, differential scanning calorimetry, infrared, and Raman measurements. An hysteretic abrupt two‐step spin crossover (T_1/2^↓=230 K and T_1/2^↑=235 K, and T_1/2^↓=172 K and T_1/2^↑=188 K, respectively) is registered for the first time for a 1,2,4‐triazole‐based Fe^II 1D coordination polymer. The two‐step SCO configuration is observed in a 1:2 ratio of low‐spin/high‐spin in the intermediate phase for a 1D chain. The origin of the stepwise transition was attributed to a distribution of chains of different lengths in 1? 2 H₂O after First Order Reversal Curves (FORC) analyses. A detailed DFT analysis allowed us to propose the normal mode assignment of the Raman peaks in the low‐spin and high‐spin states of 1? 2 H₂O. Vibrational spectra of 1? 2 H₂O reveal that the BF₄^? anions and water molecules play no significant role on the vibrational properties of the [Fe(βAlatrz)₃]²⁺ polymeric chains, although non‐coordinated water molecules have a dramatic influence on the emergence of a step in the spin transition curve. The dehydrated material [Fe(βAlatrz)₃](BF₄)₂ ( 1 ) reveals indeed a significantly different magnetic behavior with a one‐step SCO which was also investigated.     

Synthesis,stability and reactivity of cationic carbonyl complexes of rhodium(I)

Summary Trans-[RhCl(CO)L₂] (L = PPh₃, AsPh₃ or PCy₃) react with AgBF₄ in CH₂Cl₂ to give the novel species [Rh-(CO)L₂]⁺ [BF₄]^–.nCH₂Cl₂ (n = 1/2 or 1 1/2) (1–3), which we believe to be stabilised by weak solvent interaction. The corresponding stibine compound cannot be isolated by the same process, instead [Rh(CO)₂(SbPh₃)₃]⁺ [BF₄]^– (7) is formed when the reaction is carried out in the presence of CO. When reactions designed to prepare [Rh(CO)L₂]⁺ [BF₄]^– are performed in the presence of CO, or [Rh(CO)L₂]⁺ [BF₄]^– complexes are reacted with CO, [Rh(CO)₂L₂]⁺ [BF₄]^– (L = PPh₃, AsPh₃ or PCy₃) (4–6) are formed. If Me₂CO is used as solvent in the preparation of [Rh(CO)L₂]⁺ [BF₄]^– (L = PPh₃ or AsPh₃), then the products are the four-coordinate [Rh(CO)L₂-(Me₂CO)]⁺ [BF₄]^– (8,9) species. The complexes have been characterised by i.r., ³¹P and ¹H n.m.r. spectroscopy and elemental analyses.     

Transfer of spin density in molecular complexes of paramagnetic metalloporphyrins

Using the¹H and¹³C NMR method, we have observed delocalization of spin density from the paramagnetic metalloporphyrins to aromatic nitro compounds in chloroform solutions. We have shown that spin transfer occurs directly from the metal atoms. The effectiveness of spin transfer varies in the order Co²⁺ > Mn³⁺ > Fe³⁺ > Cu²⁺, correlating with the accessibility of the d_z2 orbital of the metal containing the unpaired electrons.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 5, pp. 1009–1015, May, 1990.We thank A. B. Solov'eva and G. V. Ponomarev for providing the metalloporphyrins.     

Semiconducting poly(dicyanoacetylenes)

Poly(dicyanoacetylene) (PDCA) has been synthesized and characterized. The pristine polymer has EPR g-value, linewidth, unpaired spin concentration, spin—spin relaxation time (T₂), and room temperature dc conductivity (σ_RT) very similar to those of pristine cis-polyacetylene (PA), but shorter spin—lattice relaxation time (T₁). Saturation doping with iodine has little effect on most EPR characteristics of the polymer except for a slight increase in T₁. The doped PDCA has σ_RT value of only 5 X 10^-9 (Ω cm)^-1, indicating either low carrier concentration and/or carrier mobility. Partial cyclization of the nitrile groups by heating at 400°C of PDCA produces l-PDCA with significant increases in unpaired spin concentration and σ_RT but marginal effects on other properties. Saturation doping of l-PDCA with iodine increases σ_RT to 7 × 10^-3 (Ω cm)^-1 without appreciable changes in EPR characteristics. The dopants in both polymers can be removed by evacuation indicating only weak charge transfer interactions. The possible stereoelectronic contribution toward the property differences between the PDCA polymers and PA are discussed.     

Prediction of Extremely Strong Antiferromagnetic Superexchange in Silver(II) Fluorides: Challenging the Oxocuprates(II)

Strong magnetic coupling between the spins of unpaired electrons is an essential ingredient of many fascinating physical phenomena. Here we report calculations using the hybrid HSE06 functional of magnetic superexchange constants, J , for a series of low‐dimensional Cu^II and Ag^II binary and ternary systems with fluoride and oxide ligands. The calculations correctly reproduce the sign and size of the magnetic superexchange constants for prototypical antiferromagnetic (AFM) 1D (J_1D ) and 2D (J_2D ) systems, while overestimating the absolute values of J by about 11 %. We find that [AgF][BF₄], a quasi‐1D system with linear infinite [Ag^IIF⁺] chains, is predicted to exhibit an unprecedented strong AFM superexchange via one atom (F), with J_1D about −300 meV. Compression of [AgF][BF₄] to 10 GPa should lead to a further increase in AFM interactions with J_1D reaching −360 meV at 10 GPa.     

NMR Spectra of Guest–Host Complexes of Boron and Silicon Fluorides with Crown Ethers in Nonaqueous Solutions

¹⁹F, ¹¹B, and ²⁹Si NMR spectroscopy was used to examine the behavior of the guest–host complexes (BF₃· H₂O)₂· 18-crown-6 · 2H₂O, (BF₃· H₂O)₂· DCH-6B, and (DCH-6A · H₃O)SiF₅in acetone (DCH-6B and DCH-6A are the cis-anti-cis- and cis-syn-cis-isomers of dicyclohexano-18-crown-6, respectively). It was shown that molecular boron fluoride complexes undergo partial solvolysis in acetone to yield BF₃· acetone as the main product; the ionic pentafluorosilicate complex does not experience significant solvolysis transformations.     

Zum Spingleichgewicht5T2−1A1 in Fe(II)py 2 phen(NCS) 2

Zusammenfassung Die magnetischen, spektralen und strukturellen Eigenschaften des gemischten Amminkomplexes [Fe(II)py ₂ phen(NCS)₂] werden diskutiert. Der Orbitalparamter liegt mit 11 700 cm^–1 nahe der Spinpaarungsenergie für den mittleren kovalenten Fall 11 000 bis 12 000 cm^–1 und erlaubt so ein Spingleichgewicht zwischen den Spektralzuständen⁵T₂–¹A₁. Dieses Gleichgewicht wird durch magnetische Messungen bei verschiedenen Temperaturen nachgewiesen.

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On the spin equilibrium⁵T₂–¹A₁ in [Fe(II)py₂phen(NCS)₂]

The magnetic, spectral and structural properties of the mixed ammine [Fe(II)py ₂ phen(NCS)₂] are discussed. The orbital parameter of about 11 700 cm^–1 near to the spin-pairing energy for the mean covalent case 11 000 to 12 000 cm^–1 allows a spin equilibrium between the spectral states⁵T₂–¹A₁, put in evidence by the magnetic measurements at different temperatures.

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Mit 3 Abbildungen

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Herrn Prof. Dr.H. Nowotny gewidmet.     

Nickel complexes of 1-methylimidazoline-2(3H)-thione: crystal structures of trans-dichlorotetrakis(1-methylimidazoline-2(3H)-thione)nickel(II) and tetrakis[1-methylimidazoline-2(3H)-thione]nickel(II) tetrafluoroborate

Summary Crystal structures have been determined for two nickel complexes of the monodentate S-donating ligand 1-methyl imidazoline-2(3H)-thione (mimtH). The parainagnetic trans-octahedral complex, [Ni(mimtH)₄Cl₂], crystallises in an orthorhombic unit cell (a=12.459(1),b=13.078(1),c=15.406(1)Å, V=2510.24ų,Z=4, space group Pbca). Final conventional R from 1848 observed data [F>4(F)] is 0.0273. The Ni–Cl distance is. 2.537(1) Å and the mean Ni–S distance is 2.479 Å.The diamagnetic complex, [Ni(mimtH)₄](BF₄)₂, contains a distorted square-planar cation which is H-bonded. to [BF₄]^– anions. The complex crystallises in an orthorhombic unit cell [a=9.810(1),b=14.585(1),c=20.120(2)Å, V=2878.8ų,Z=4, space group Pbcn]. Final conventional R from 1756 observed data [F>4(F)] is 0.0629. The average Ni–S distance is 2.216Å.     

Sliding Polymeric Layers and Anion Displacement Coupled with Spin Crossover in Two-Dimensional Networks of [Fe(hbtz)2(CH3CN)2](BF4)2

The abrupt high spin (HS)→low spin (LS) transition (T^↓_1/2=136 K) in [Fe(hbtz)₂(CH₃CN)₂](BF₄)₂ (hbtz=1,6-di(tetrazol-2-yl)hexane) is finished at 100 K and further thermal treatment influences the spin crossover. Subsequent heating involves a change of the spin state in the same way (T^↑_1/2=136 K) on cooling. In contrast, cooling below 100 K triggers different behavior and T^↑_1/2 is shifted to 170 K. The extraordinary structural changes that occurred below 100 K are responsible for the observed diversity of properties. A unique feature of the low-temperature phase is the rebuilding of the anion network expressed by a shift of anions inside the polymeric layer at a distance of 1.2 Å as well as the relative shift of neighboring layers at over 4 Å. These structural alterations, connected with a phase transition, become the origin of the strain, which in most cases causes crystal cleaving. In a sample composed from crystals crushed as a result of the phase transition or as a result of mechanical crumbling, the hysteresis loop vanishes; however, annealing the sample allows to its partial restoration. A replacement of acetonitrile by other nitriles leads to preservation of the polymeric structure and spin crossover, but no phase transition follows.     

Spectroscopic Study of the Interaction of the Pd(acac)(C3-acac)PPh3Complex with BF3OEt2 in the Presence of PPh3

The interaction between the components of a catalytic system Pd(acac)(C³-acac)PPh₃+nPPh₃+ mBF₃OEt₂(where n= 1–4, m= 0.25–4, and acac is the acetylacetonate ligand) in benzene is examined by UV and IR spectroscopy. With a relative excess of PPh₃(n> m), acacH and [Pd(acac)(PPh₃)₂]⁺BF^– ₄were the main products, whereas BF₂acac and a polynuclear complex of PdF₂with PPh₃also containing Pd²⁺(BF^– ₄)₂units were formed with a relative excess of BF₃OEt₂(n< m).     

Adduct of nonamethylferrocenecarbaldehyde with HBF4 and its transformations

Some reactions of the adduct of nonaniethylferrocenecarbaldehyde with HBF₄, [Me₅C₅FeC₅Me₄CH⁺(OH)]BF₄ ^–, and its transformation into the paramagnetic complex [Me₅C₅Fe^+-C₅Me₄CH=O]BF₄ ^– were studied. The structure of the complex was established from the chemical properties, the data from IR spectroscopy, mass spectrometry, and¹H,¹³C, and¹⁹F NMR spectroscopy and confirmed by independent synthesis.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 12, pp, 2979–2983, December, 1996.     

Synthesis,Magnetic Properties and X‐ray Structure Analysis of a 1‐D Chain Iron(II) Spin Crossover Complex with wide Hysteresis

The reaction of [FeL(MeOH)₂] (L being a tetradentate [N₂O₂]^2? coordinating Schiff base like ligand [([3,3′]‐[1,2‐phenylenebis(iminomethylidyne)]bis(2,4‐pentane‐dionato)(2‐)N,N′,O²,O²′], MeOH = methanol) with 4,4′‐bipyridine (bipy) results in the formation of a new iron(II ) spin crossover coordination polymer of the formula [FeL(bipy)] ( 1 ). T‐dependent susceptibility measurements revealed an abrupt HS ? LS spin transition with an approximately 18 K‐wide thermal hysteresis loop (T_1/2↑ = 237 K and T_1/2↓ = 219 K). The isolation of crystals suitable for X‐ray structure analysis allowed the determination of the motive of the molecule structure of the first 1‐D chain compound with hysteresis in the HS form at 250 K. Despite the low qualtity of the data, we were able to obtain some insight into the interplay of covalent and elastic interactions that are both responsible for the high cooperative interactions during the spin transition in this compound.     

Co(II) and Co(III) macrocyclic boron-bearing dioximates

Syntheses are reported for CoD₃(BF)₂ and [CoD₃(BF)₂]BF₄,where H ₂ D is dimethylglyoxime, -benzyldioxime, or cyclohexanedione dioxime. The IR spectra at 400–4000 cm ^–1 have been measured, as have the electronic absorption spectra and the¹H,¹³C,and ¹¹BNMR spectra; a comparison is made with the spectra of the analogous iron(II) complexes.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 26, No. 3, pp. 375–377, May–June, 1990.