Spin State In Solution Research Articles

Electronic Control of Spin-Crossover Properties in Four-Coordinate Bis(formazanate) Iron(II) Complexes.

The transition between spin states in d-block metal complexes has important ramifications for their structure and reactivity, with applications ranging from information storage materials to understanding catalytic activity of metalloenzymes. Tuning the ligand field (ΔO) by steric and/or electronic effects has provided spin-crossover compounds for several transition metals in the periodic table, but this has mostly been limited to coordinatively saturated metal centers in octahedral ligand environments. Spin-crossover complexes with low coordination numbers are much rarer. Here we report a series of four-coordinate, (pseudo)tetrahedral Fe(II) complexes with formazanate ligands and demonstrate how electronic substituent effects can be used to modulate the thermally induced transition between S = 0 and S = 2 spin states in solution. All six compounds undergo spin-crossover in solution with T1/2 above room temperature (300–368 K). While structural analysis by X-ray crystallography shows that the majority of these compounds are low-spin in the solid state (and remain unchanged upon heating), we find that packing effects can override this preference and give rise to either rigorously high-spin (6) or gradual spin-crossover behavior (5) also in the solid state. Density functional theory calculations are used to delineate the empirical trends in solution spin-crossover thermodynamics. In all cases, the stabilization of the low-spin state is due to the π-acceptor properties of the formazanate ligand, resulting in an “inverted” ligand field, with an approximate “two-over-three” splitting of the d-orbitals and a high degree of metal–ligand covalency due to metal → ligand π-backdonation. The computational data indicate that the electronic nature of the para-substituent has a different influence depending on whether it is present at the C–Ar or N–Ar rings, which is ascribed to the opposing effect on metal–ligand σ- and π-bonding.

Small Structural Variations Have Large Effects on the Assembly Properties and Spin State of Room Temperature High Spin Fe(II) Iminopyridine Cages.

Small changes in steric bulk at the terminus of bis-iminopyridine ligands can effect large changes in the spin state of self-assembled Fe(II)-iminopyridine cage complexes. If the added bulk is properly matched with ligands that are either sufficiently flexible to allow twisted octahedral geometries at the Fe centers or can assemble with unusual mer configurations at the metals, room temperature high spin Fe(II) cages can be synthesized. These complexes maintain their high spin state in solution at low temperatures and have been characterized by X-ray crystallographic and computational methods. The high spin M2L3 meso-helicate and M4L6 cage complexes display longer N-Fe bond distances and larger interligand N-Fe-N bond angles than their diamagnetic counterparts, and these structural changes invert the ligand selectivity in narcissistic self-sorting and accelerate subcomponent exchange rates. The paramagnetic cages can be easily converted to diamagnetic cages by subcomponent exchange under mild conditions, and the intermediates of the exchange process can be visualized in situ by NMR analysis.

Manipulating and quantifying spin states in solution as a function of pressure and temperature.

Monitoring the spin states of species in solution is a crucial aspect of understanding magnetic properties as well as spin-labile sensing, supramolecular, catalytic and biochemical processes. Herein, we describe the first quantitative variable-pressure and variable-temperature method of determining spin states in solution, demonstrate that it is accurate, and identify a simultaneous T and P sensor system.

A Simple Method of Predicting Spin State in Solution.

A simple method, using density functional theory (DFT), of predicting spin-state in advance of synthesis is reported. Specifically, an excellent correlation is observed between the switching temperatures (T1/2) in CDCl3 solution of five spin-crossover (SCO)-active [FeII(Lazine)2(NCBH3)2] complexes and the DFT-calculated (and observed) 15N NMR chemical shift (δNA) of the five different azine-substituted 1,2,4-triazole ligands employed, Lazine = 4-(4-methylphenyl)-3-phenyl-5-(azine)-1,2,4-triazole, where azine = pyridine, pyridazine, 4-pyrimidine, pyrazine, and 2-pyrimidine. To test the generality of this finding, DFT was also employed to readily predict the δNA values for a family of 16 literature ligands, known as bppX,Y [X,Y-substituted 2,6-(pyrazol-1-yl)pyridines], which have produced 16 SCO-active [FeII(bppX,Y)2](Z)2 complexes (Z = BF4 or in one case PF6) in (CD3)2CO solution: again an excellent correlation was found between the computed δNA and the observed T1/2. These correlations represent a key advance in the field, as they allow a simple DFT calculation on a modified ligand to be used to reliably predict, before synthesis of the ligand or complex, the T1/2 that would result from that modification. Achieving such easily predictable tuning of T1/2, and hence of spin-state, is a significant step forward in the field of SCO and also has big implications in many other fields in which spin-state is key, including catalysis, metallo-enzyme modeling studies, and host-guest chemistry.

A high-spin square-planar Fe(ii) complex stabilized by a trianionic pincer-type ligand and conclusive evidence for retention of geometry and spin state in solution

Square-planar high-spin Fe(ii) molecular compounds are rare and the only three non-macrocyclic or sterically-driven examples reported share a common FeO4 core. Using an easily modifiable pincer-type ligand, the successful synthesis of the first compound of this type that breaks the FeO4 motif was achieved. In addition, we present the first evidence that geometry and spin state persist in solution. Extensive characterization includes the first high-field EPR and variable field/temperature Mössbauer spectra for this class of compounds. Analysis of the spectroscopic data indicates this complex exhibits a large and positive zero-field splitting tensor. Furthermore, the unusually small ΔEQ value determined for this compound is rationalized on the basis of DFT calculations.

ESR and magnetic studies of octahedral [Fe(III)(Cl)(pcd)(H2O)(DMSO)] (pcd = pyridine-2,6-dicarboxylato) compound showing Fe(III) species with different spin states in solution

By direct synthesis has been obtained the octahedral complex [Fe(III)(Cl)(pdc)(H2O)(DMSO)] 1 (pdc=pyridyn-2,6-dicarboxylato), which shows a supramolecular structure and a clear and large molecular distortion. UV–Vis spectrum of 1 shows at 741nm, 836nm (ε=100M−1cm−1) and 970nm (ε=1600M−1cm−1) typical iron d–d transitions. FIR spectrum of 1 shows vibrations υFeO, υFeN and υFeCl. NMR 1H spectroscopy is typical of paramagnetic compounds. The susceptibility data fit to a Hamiltonian, Hex=−2JS1·S2, using two models, give close exchange interaction constant values of: J=−1.803cm−1, g=1.810 and J=−1.102cm−1, g=1.832. X-band ESR spectra of powder sample of 1 at 300K/77K show broad signals at H≈56.70mT/64.53mT, characteristic of ferromagnetic absorptions. X-band spectra of 1 at 77K in methanol solution at different concentrations show typical signals of Fe(III) species with high spin, S=5/2, quantum mixed spin state (qms), 5/2, 3/2, which are present in ESR spectra of peroxidase enzymes, and a low spin S=1/2. 1 has been used as catalyst in the guaiacol test, characteristic of peroxidase enzymes, giving positive by visual observation and by UV–Vis spectroscopy. By ERS spectroscopy has been observed the characteristic π-cationic-radical ferryl, enzyme covalently modified, when the peroxidase enzyme is active. The presence of Fe(III) species with qms observed by ESR spectra of 1, and its catalytic activity, both characteristic of peroxidase enzymes, are a significant conclusion of the work.

Thermal Spin Crossover in Binuclear Iron(II) Helicates: Negative Cooperativity and a Mixed Spin State in Solution

Comparison of the thermal spin crossover transition in solution of a Fe(II) dinuclear triple helicate with the analogous mononuclear complex shows that the helicate transition takes place over a wider temperature range in a two step process, implying negative cooperativity.

Spin-state equilibria in non-aqueous solution and quantum-mechanical investigations of iron(II) and nickel(II) complexes with 4-substituted 2,6-bis(benzimidazol-2-yl)pyridines

Cationic complexes with a series of tridentate ligands, L = 4X-substituted 2,6-bis(benzimidazol-2-yl)pyridines, [ML2][ClO4]2(M = Fe or Ni; X = H, OH or Cl), were isolated and characterized, together with the free pyridines, by elemental analysis, Fourier-transform IR, 1H NMR and UV/VIS spectroscopy. The syntheses were performed via condensation of o-phenylenediamine with 4-substituted pyridine-2,6-dicarboxylic acids. Ligand-field parameters were estimated for the nickel complexes. The [FeL2]2+ species show thermally induced spin-crossover behaviour (1A1→5T2g) which has been investigated in methanol, nitromethane and 20%(v/v) dimethylformamide in MeOH. The behaviour is complicated by two complex dissociation equilibria, for which equilibrium constants have been evaluated. Ligand substitution is reflected in a change of the spin state in solution (µexptl= 2.50, X = H; 4.19, OH; and 4.49 µB, Cl at 295 K, in MeOH) and in the metal-to-ligand charge-transfer band (500–557 nm); when M = Fe and X = H there is a pronounced spin-crossover equilibrium in methanolic solution (µexptl= 1.31–3.45 µB for 213–328 K). A small variation of the magnetic moments when M = Fe and X = OH (µexptl= 3.77–4.73 µB at 220–332 K) might indicate a temperature-variable population of the 5Eg sublevel or variation in hydrogen bonding. The results are compared with quasi-relativistic quantum-mechanical calculations, and the spin-crossover behaviour of the new ligands, L, with substituents X = CHO, NH2, CN, Me, NO2, OH, CONH2, COCl, SH, F, Cl, Br or I has been estimated. The differences in the calculated heats of formation between the high-and low-spin forms of [FeL2]2+ when plotted against Δδ(=1H NMR para increment for substituents X in benzene) show a turning point in the region around X = H and in this region spin-crossover behaviour is observed. Outside this region there is very little or no such behaviour and it is therefore possible to predict the spin-crossover behaviour for other substituents X from the Δδ value.