Spin-crossover complexes

A series of NFS spectra of the spin-crossover complex [Fe(tpa)(NCS)2] were recorded over a wide temperature range [45]. A selection of spectra around the spin-crossover transition temperature is shown in Fig. 9.13. At 133 K, the regular quantum-beat structure reflects the quadrupole splitting from the pure high-spin (HS) phase, and the envelope of the spectrum represents the dynamical beating with a minimum around 200 ns. Below the transition, at 83 K, the QBs appear with lower frequency because of smaller AEq of the low-spin (LS) phase. Here the minima of... 

Iron-Ligand Vibrations in Spin-Crossover Complexes... 

Paulsen H, Trautwein AX (2004) Density Functional Theory Calculations for Spin Crossover Complexes. 235 197-219 Penades S,see Rojo J (2002) 218 45-92 Perrio C, see Lasne M-C (2002) 222 201-258... 

Abstract Metal complexes including the dmit2- (1,3-dithio-2-thione-4,5-dithiolato) ligand are the only class of metal bis-dithiolenes to give rise to superconductive molecular materials. This chapter first focuses on the description of these superconductive phases. Further sections describe the association of M(dmit)2 moieties with three types of magnetic molecules, i.e., metalloceniums, radical cations, and spin crossover complexes. 

Keywords Iron(II) spin crossover complexes Multidentate ligands Ligand design Synthesis Magnetic properties... 

To the best of our knowledge no Fe(II) spin crossover complexes have been created with these type of ligands. 

These complexes are either high-spin or low-spin depending on the nature of the ligand X. It has been possible to create spin-crossover complexes [20] in a few cases, such as X=NCS. ... 

Nakano et al. have demonstrated that Fe(III) spin crossover complexes adsorbed on the surface of silicon dioxide retain their spin crossover behaviour [220]. EPR and 57Fe Mossbauer spectral data indicated that the spin transitions observed are similar to those of the neat solid materials used, i.e. [Fe(acpa)2]PF6, [Fe(acpa)2]BPh4 (Hacpa=N-(l-acetyl-2-propylidene)(2-pyri-dylmethyl)amine) and [Fe(bzpa)2]PF6 (Hbzpa=(l-benzoylpropen-2-yl)(2-pyridylmethyl)amine). 

H. Paulsen, A. X. Trautwein. Density functional theory calculations for spin crossover complexes. In P. Giitlich H.A. Goodwin, editors. Spin Crossover in Transition Metal Compounds III, volume 235 of Top Curr Chem, pages 197-219. Springer-Verlag, 2004. 

Dynamic Jahn-Teller Character of Manganese(IH) Spin-Crossover Complex [Mn(taa)] (H3taa = tris(l-(2-azolyl)-2-azabuten-4-yl)amine)... 

Dynamic Jahn-Teller Character of Manganese(lll) Spin-Crossover Complex... 

Jahn-Teller effects are rarely adopted in theoretical treatments of spin-crossover phenomena, except Kambara s model for Fe(II) complexes [24] and Bersuker s for Fe(III) complexes [19,25]. In the Kambara theory, abrupt spin-crossover transitions are given an FO => LS character, while gradual transitions are of the HS LS type. However, experimental evidences of cooperative Jahn-Teller transitions (FO HS) have not yet been reported for Fe(II) spin-crossover complexes. Kambara s theory ignores the spin-orbit interaction and appears to overestimate the Jahn-Teller coupling, producing an unphysical (for Fe(II) complexes) FO => HS transition. 

The most interesting possibility indicated by this phase diagram is that the FO phase may be present as a metastable phase below Tjt = 26 K, instead of the HS phase above rJT. It suggests that physical properties of the metastable phase may change at rJT. Recently, a photo-induced metastable phase of an Fe(II) spin-crossover complex [Fe(2-pic)3]Cl2 EtOH (2-pic = 2-aminomethylpyridine) was reported [26-28]. Its properties are remarkably different from those of the metastable HS phase quenched by rapid cooling. Lowered symmetry accompanying the Jahn-Teller deformation has... 

Reiher M (2002) Theoretical study of the Fe(phen)2(NCS)2 spin-crossover complex with reparametrized density functionals. Inorg. Chem. 41 6928-6935... 

Comparing the bond lengths for the two temperature measurements, it is apparent that the M1 O and Cr-O bond lengths are similar for the two temperatures. The Mn-N bond lengths, however, decrease from an average of 2.095 to 2.014 A (that is, by 0.081 A) between 293 and 10 K. This value is far too large to be due to thermal contraction alone and serves as a first indication that the [Co(bpy)3]2+ cation has become a spin crossover complex in this lattice. 

Figure 17-E-3 Schematic representation of the potential wells for the LS ( A,) and the HS (ST2) states of an iron(II) spin crossover complex. The nuclear coordinate is the bond length. Thermal spin crossover is observed when AEHL — kT.

---