Figure - uploaded by Mikio Nakamura
Content may be subject to copyright.
Source publication
Der axiale Ligand bestimmt den Spinzustand von Eisen(III)-Porphyrinkomplexen in Sattelkonformation (siehe Bild). Liganden L wie Imidazol und 4-Dimethylaminopyridin, die gewöhnlich ein starkes Ligandenfeld erzeugen, bewirken einen reinen S=1/2-Zustand, wohingegen THF einen reinen S=3/2-Zustand zur Folge hat. Intermediäre Liganden wie Pyridin und 4-C...
Contexts in source publication
Context 1
... in the case of the solid sample, the m eff value of 3 in solution decreased from 3.5 m B (298 K) to 2.6 m B (193 K), which suggests the spin-crossover process also occurs in solution. Table 2 shows the 1 H NMR chemical shifts at 223 K. The large downfield shifts of the CH 2 and ligand (L) signals on going from 2 to 4 suggest that the spin densities at the b-pyrrole and the ligand carbon atoms increase from 2 34. ...Similar publications
A key step in cytochrome P450 catalysis includes the spin-state crossing from low spin to high spin upon substrate binding and subsequent reduction of the heme. Clearly, a weak perturbation in P450 enzymes triggers a spin-state crossing. However, the origin of the process whereby enzymes reorganize their active site through external perturbations,...
Citations
With the development of molecular science, more and more bistability systems are used as the carriers of molecular-based devices, such as temperature sensor, light switch and information storage devices. Spin crossover compounds are widely studied in bistability systems at present. With the appropriate external perturbations (such as temperature, pressure, magnetic field, light, and guest/chemical environment), transition between high spin state and low spin state may occur, which confers a memory effect or switching responses on the system. In this paper, we briefly introduce the research advances of spin crossover in recent years, including the concept, significance and species of spin crossover system, especially the Fe(H) and Fe(M) species. The perspective of future development in this field and the research frontiers nowadays are also briefly mentioned.
In this mini-review, we have highlighted our works on metal complexes having saddle-distorted dodecaphenylporphyrin (DPP) and its derivative as ligands in the light of enhancement of the Lewis acidity of a metal center coordinated by the porphyrin. The important point through this mini-review is ill-overlap of the out-of-plane lone pairs of pyrrole nitrogen atoms with σ-orbitals of the metal center bound to the saddle-distorted porphyrin core. The enhanced Lewis acidity of the central metal ions enabled us to construct stable molecular complexes through axial coordination using metal–DPP (M(DPP)) moieties (M = MoV or SnIV) and molecular or ionic entities with Lewis-basic coordination sites, including Keggin-type polyoxometallates (POM), which are known to have weak Lewis basicity and thus hard to coordinate to metal ions. A discrete 1:2 complex with a Ru-substituted POM performs catalytic substrate oxidation reactions in organic solvents. A 1:1 complex between SnIV(DPP) and a Keggin-type POM exhibited photoinduced electron transfer, in which the SnIV(DPP) moiety acts as an electron donor and the POM as an electron acceptor. Besides POM, other electron acceptors, including μ3-oxo trinuclear RuIII clusters and anthraquinone, having carboxyl groups as a linker unit also formed stable complexes with DPP-metal complexes as axial ligands to perform photoinduced electron transfer. Successful photoreactions of the M(DPP)-acceptor complexes are mainly enabled by the enhanced Lewis acidity of the DPP-metal complexes for the stabilization of the assemblies and also by lowering the oxidation potential of the porphyrin ligand to gain larger driving force of electron transfer to form an electron-transfer state with avoiding intersystem crossing. The stability and photochemical behavior are in sharp contrast to those for metal complexes with planar porphyrins as ligands.
The free unpaired electron in Fe3+ ions cannot be directly removed, and needs a transfer pathway with at least four steps to overcome the high energy barriers to form Fe4+ ions. Finer changes in the electronic structure of Fe3+ ions than spin conversion were identified through deeper analysis of the diffraction, spectral and electrochemical data for six nonplanar iron porphyrins. Fe3+ ions can form four d electron tautomers as the compression of the central ion is increased. This indicates that the Fe3+ ion undergoes a multistep electron transfer where the total energy gap of electron transfer is split into several smaller gaps to form high-valent Fe4+ ions. We find that the interchange of these four electron tautomers is clearly related to the core size of the macrocycle in the current series. The large energy barrier to produce iron(IV) complexes is overcome through a gradient effect of multiple energy levels. In addition, a possible porphyrin Fe3+• radical may be formed from its stable isoelectronic form, porphyrin Fe3+, under strong core contraction. These results indicate the important role of heme distortion in its catalytic oxidation functions.
A first systematic study upon the preparation and exploration of a series of iron 10-thiacorroles with simple halogenido (F, Cl, Br, I), pseudo-halogenido (N3 , I3 ) and solvent-derived axial ligands (DMSO, pyridine) is reported. The compounds were prepared from the free-base octaethyl-10-thiacorrole by iron insertion and subsequent ligand-exchange reactions. The small N4 cavity of the ring-contracted porphyrinoid results in an intermediate spin (i.s., S=3/2) state as the ground state for the iron(III) ion. In most of the investigated cases, the i.s. state is found unperturbed and independent of temperature, as determined by a combination of X-ray crystallography and magnetometry with (1) H NMR-, EPR-, and Mössbauer spectroscopy. Two exceptions were found. The fluorido iron(III) complex is inhomogenous in the solid and contains a thermal i.s. (S=3/2)→high spin (h.s., S=5/2) crossover fraction. On the other side, the cationic bis(pyridine) complex resides in the expected low spin (l.s., S=1/2) state. Chemically, the iron 10-thiacorroles differ from the iron porphyrins mainly by weaker axial ligand binding and by a cathodic shift of the redox potentials. These features make the 10-thiacorroles interesting ligands for future research on biomimetic catalysts and model systems for unusual heme protein active sites.
The physicochemical properties of a series of the six-coordinated iron(III) diazaporphyrinate complexes, [Fe(DAzP)L-2](+/-) where the axial ligands (L) are DMAP, CN-, HIm, Py, 3-ClPy, 4-CNPy, 3-CNPy, 3,5-Cl2Py, THF, have been examined by combined analyses of NMR, EPR, Mossbauer spectrometry, and SQUID magnetometry. The solution and solid-state studies clarified that the complexes with 3-ClPy, 4-CNPy, 3-CNPy, 3,5-Cl2Py are in the less common spin-crossover process between S = 3/2 and S = 1/2 during the temperature change. The results clearly indicate that the complexes showed the switching behaviors of the electronic structure or the magnetic properties responsive to the external stimuli.
Spin transitions generally occur in compounds of octahedrally coordinated 3d transition metal ions. These transitions can be induced by external perturbations such as light, heat, pressure, magnetic field, and chemical substitution. Transition metal cyanides are one such material, which exhibit reversible spin transition while perturbed with light at T < 10 K. Here we report the first-principles (DFT+U) study of anhydrated KCoFe(CN)6. We find that the complete spin transition from the low spin ground sate (S=0) to a high spin (S=2) state takes place due to intra-atomic and inter-atomic charge transfers in two steps. In the first step a d-electron is transferred from Fe to Co through cyanide ligand, which is followed by the d-electron rearrangement in the Co. This spin transition is strongly correlated with the internal lattice, and we find as large as 10% extension of the Co-N bond via a Jahn-Teller active (tetragonally distorted) lattice in the intermediate spin (S=1) state. The calculated energy required for this transition is in agreement with experiments. We further predict that this spin transition in such materials can be induced, and further tuned, by external pressure to enable realization of such reversible transitions at ambient temperatures.
Ironman oder Schwächling? Ligandenfeldstärken werden üblicherweise mithilfe der empirischen spektrochemischen Reihe beschrieben. Obwohl Cyanid als Starkfeldligand fest etabliert ist, lassen Beispiele aus jüngster Zeit, z. B. die High‐Spin(S=2)‐Zustände von [CrII(CN)5]3− und [FeII(tpp)(CN)]− , Zweifel an der Einordnung dieses Liganden aufkommen. tpp=meso‐Tetraphenylporphinat.
Bromido-(3,3′,4,4′,8,8′,9,9′-octaethyl-2,2′-bidipyrrinato)iron(III) crystallizes in two different polymorphs, space groups and C2/c, with Z = 2 and 8, respectively, and slightly different densities. The Fe-N and Fe-Br bond lengths vary significantly with the polymorph indicating the presence of intermediate spin (S = 3/2) species admixed with different amounts of high spin (S = 5/2) iron(III) compounds in the triclinic resp. monoclinic form.
1H and 13C NMR chemical shifts of iron porphyrin complexes are determined mainly by the spin densities at the peripheral carbon and nitrogen atoms caused by the interaction between paramagnetic iron 3d and porphyrin molecular orbitals. This review describes how the half-occupied iron 3d orbitals such as dπ(dxz, dyz), dxy, d, and d- interact with a specific porphyrin molecular orbital and affect the 1H and 13C NMR chemical shifts in planar, ruffled, saddled, and domed complexes. Revealing the relationship between the orbital interactions and NMR chemical shifts is quite important to determine the fine electronic structures of synthetic iron porphyrin complexes as well as naturally occurring heme proteins.
