Spin state crossover in iron II

Interest toward spin-state crossover in iron(II) complexes is increasing because of potential technological applications,198199 such as intelligent contrast agents for biomedical imaging, as temperature/pressure threshold indicators, and as optical elements... 

The electronic spin-state crossover in [Fe(HB(pz)3)2] has also been observed in the fine structure of its fC-edge x-ray absorption spectrum [38]. The changes in the x-ray absorption spectra of [Fe(HB(pz)3)2] are especially apparent between 293 and 450 K at ca. 25 eV, as is shown in Fig. 5. The 293 K x-ray absorption spectral profile observed in Fig. 5 for [Fe(HB(pz)3)2] has been reproduced [39] by a multiple photoelectron scattering calculation, a calculation that indicated that up to 33 atoms at distances of up to 4.19 A are involved in the scattering. As expected, the extended x-ray absorption fine structure reveals [38] no change in the average low-spin iron(II)-nitro-gen bond distance of 1.97 A in [Fe(HB(pz)3)2] upon cooling from 295 to 77 K. 

The temperature dependence of the isomer shift and quadrupole splitting for the high-spin and low-spin iron(II) states in [Fe(HC(3,5-(CH3)2pz)3)2](BF4)2 and details of the fits and their temperature dependence may be found elsewhere [46]. The extent of the spin-state crossover is shown in Fig. 18, a figure which clearly indicates that the spin-state crossover in [Fe(HC(3,5-(CH3)2pz)3)2](BF4)2 stops at 50 percent. In contrast it should be noted that, in the structurally very similar [Fe(HC(3,5-(CH3)2pz)3)2]l2 complex, [49] the spin-state crossover is 100 percent complete at 4.2 K. 

Reedijk and co-workers have studied the iron(II) spin-state crossover in [Fe(teec)6]X2, where X is BF4, CIO4, or PFg, and teec is the monodentate l-(2-chloroethyl)tetrazole ligand. Depending upon the anion and the rate of precipitation, these complexes exhibit differing, partial or complete, spin-state crossover behavior with gradual or sharp, one- or two-step, transitions both with and without hysteresis. 

The relaxation fits of the Mossbauer spectra of [Fe(HB(pz)3)2] yield [30] the temperature dependence of both the population of the iron(II) high-spin and low-spin states and the relaxation rate between these two states. The resulting population of the high-spin state has a striking resemblance to that of the magnetic moment shown in Fig. 1 and these populations provide clear support both for the spin-state crossover and for the difference in populations upon heating and cooling. 

As was mentioned above, the [Fe(HB(3,5-(CH3)2pz)3)2] complex represents a classic example [27, 28] of an iron(II) spin-state crossover that may be induced in a high-spin complex upon cooling. The room temperature crystal structure of this complex [26] reveals a structure rather similar to that of [Fe(HB(pz)3)2], but with a substantially longer average iron-nitrogen bond... 

The pressure effects on spin relaxation dynamics for these iron(II) complexes have been examined using laser flash photolysis techniques. For Fe(pyim) the two spin states are in equilibrium with a K = 0.56 in 298 K acetone with a partial molar volume difference AV = +8.1 cm mol [34]. Photoexcitation (2ex = 532 nm) leads to transient bleaching of the low spin isomer s MLCT bands followed by first order relaxation to the original spectrum with a 45-ns lifetime. Transient bleaching and subsequent return of the MLCT absorption was attributed to formation of the HS isomer and subsequent spin relaxation. The pressure dependence of the relaxation lifetimes was used to determine the activation volumes of the spin relaxation rates for a variety of FeL in different solvents. It was found that AV j fell into a remarkably narrow range of values (-5.5 + 1 cm mol ) and it was concluded that the spin crossover for these species follows a common mechanism via a transition state located midway between the high and low spin states [33]. 

In Chapters 6 and 7 the data for both high-spin and low-spin iron(II) and iron(III) complexes were discussed. Although these are the common electronic configurations for these oxidation states, there is also a minority group of compounds which show either ligand field crossover from high-spin to low-spin configuration or intermediate spin states, and in addition there are some uncommon oxidation states. 

In contrast to iron(II) spin-state crossover complexes which usually show a cooperative behavior, iron(III) complexes exhibit a gradual, noncooperative, spin-state transition. There has been extensive Mossbauer spectral studies of these transitions, including ambient and high-pressure studies of the iron(III) trisdithiocarbamate complexes, the first iron complexes which were reported " in 1931 to undergo a spin-state transition. 

S. Decurtins, P. Gutlich, C.P. Kohler, H. Spiering, A. Hauser, Light-induced excited spin state trapping in a transition-metal complex the hexa-l-propyltetrazole-iron(II) tetrafluoroborate spin crossover system. Chem. Phys. Lett. 105, 1 (1984)... 

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