Iron spin-equilibrium complex

This report has been written in order to demonstrate the nature of spin-state transitions and to review the studies of dynamical properties of spin transition compounds, both in solution and in the solid state. Spin-state transitions are usually rapid and thus relaxation methods for the microsecond and nanosecond range have been applied. The first application of relaxation techniques to the spin equilibrium of an iron(II) complex involved Raman laser temperature-jump measurements in 1973 [28]. The more accurate ultrasonic relaxation method was first applied in 1978 [29]. These studies dealt exclusively with the spin-state dynamics in solution and were recently reviewed by Beattie [30]. A recent addition to the study of spin-state transitions both in solution and the... 

Drabent and Wajda (1980) Spin equilibrium in six-coordinate iron(II) complexes [213]. 

The phenomenon of spin equilibrium in octahedral complexes was first reported by Cambi and co-workers in a series of papers between 1931 and 1933 describing magnetic properties of tris(iV,iV-dialkyldithio-carbamato)iron(III) complexes. By 1968 the concept of a thermal equilibrium between different spin states was sufficiently well established that the definitive review by Martin and White described the phenomenon in terms which have not been substantially altered subsequently (112). During the 1960s the planar-tetrahedral equilibria of nickel(II) complexes were thoroughly explored and the results were summarized in comprehensive reviews published by Holm and coworkers in 1966 and 1973 ( 79, 80). Also, in 1968, Busch and co-workers... 

There have been very few reports of the Raman spectra of spin-equilibrium complexes. In one experiment the presence of both high-spin and low-spin isomers of an iron(II) Schiff base complex was observed by the resonance Raman spectra of the imine region (11). The temperature dependence of the spectra was recorded for both solid and solution samples. Recently differences were described in the resonance Raman spectra of four- and six-coordinate nickel(II) porphyrin complexes which undergo coordination-spin equilibria. These studies are extensions of a considerable literature on spin state effects on the Raman spectra of iron porphyrins and hemes. There are apparently no reports of attempts to use time-resolved Raman spectra for dynamics experiments. 

The Raman laser temperature-jump technique has been used in studies of a variety of spin-equilibrium processes. It was used in the first experiment to measure the relaxation time of an octahedral spin-equilibrium complex in solution (14). Its applications include investigations of cobalt(II), iron(II), iron(III), and nickel(II) equilibria. 

Mossbauer spectroscopy of the 57Fe nucleus has been extensively used to investigate aspects of spin equilibria in the solid state and in frozen solutions. A rigid medium is of course required in order to achieve the Mossbauer effect. The dynamics of spin equilibria can be investigated by the Mossbauer experiment because the lifetime of the excited state of the 57Fe nucleus which is involved in the emission and absorption of the y radiation is 1 x 10 7 second. This is just of the order of the lifetimes of the spin states of iron complexes involved in spin equilibria. Furthermore, the Mossbauer spectra of high-spin and low-spin complexes are characterized by different isomer shifts and quad-rupole coupling constants. Consequently, the Mossbauer spectrum can be used to classify the dynamic properties of a spin-equilibrium iron complex. 

The dynamics of an octahedral spin equilibrium in solution was first reported in 1973 for an iron(II) complex with the Raman laser temperature-jump technique (14). A relaxation time of 32 10 nsec was observed. Subsequently, further studies have been reported with the use of this technique, with ultrasonic relaxation, and with photoperturbation. Selected results are presented in Table III. 

The spin-equilibrium dynamics of iron(III) complexes in solution have been examined with the techniques of Raman laser temperature-jump, ultrasonic relaxation, and photoperturbation. The complexes investigated, the relaxation times observed, and one of the derived rate constants are presented in Table IV. Many of the relaxation times are quite short, and some of the original temperature-jump results (45) were found to be inconsistent with more accurate ultrasonic experiments (20) and later photoperturbation experiments (102). It has not been possible to repeat some of these laser temperature-jump observations. Instead, the expected absorbance changes and isosbestic points were found to occur within the heating rise time of the laser pulse, consistent with the ultrasonic and photoperturbation experiments (20). Consequently, none of the original Raman laser temperature-jump results is included in Table IV. 

The EPR spectrum of a spin-equilibrium complex can be used to establish a lower limit to the spin state lifetimes of the order of 10 10 second. In an important paper in 1976, Hall and Hendrickson reported observation of EPR signals for both the high-spin and the low-spin isomers of iron(III) dithiocarbamate complexes at 4 12 K as powders, glasses, and doped solids (71). This resolved the question whether these complexes possess distinct high-spin and low-spin states. It also sets a lower limit on their interconversion lifetimes. Similarly, the observation of signals for both the high-spin and low-spin states of [Co(terpy)22+] (97) leads to the same conclusions about this complex. In both cases the interconversion rates in solution have proved too fast to measure, with lifetimes of less than 10-9 second indicated. The solution measurements were undertaken, of course, at room temperature and the EPR measurements at close to 4 K. Significant differences in the rates of solid and solutions at room temperature are still possible. 

Mossbauer spectroscopy can only be used to obtain rates of interconversion if the lifetimes are close to 10 7 second. As described in Section III,E a few examples satisfying this condition have been found. Some questions remain over the quantitative interpretation of the data. Nevertheless, spin-equilibrium relaxation lifetimes have been estimated from Mossbauer temperature-dependent linewidths for two salts of an iron(III) complex, [Fe(acpa)2]+. The lifetimes are of the order 10 5—10 7 second over temperature ranges from 100 to 300 K (109, 111). 

The spin state lifetimes in solution of the complexes II and III have been measured directly with the laser Raman temperature-jump technique189). Changes in the absorbance at 560 nm (CT band maximum) following the T-jump perturbation indicate that the relaxation back to equilibrium occurs by a first-order process. The spin-state lifetimes are r(LS) = 2.5 10 6 s and r(HS) =1.3 10 7 s. The enthalpy change is AH < 5 kcal mol-1, in good agreement with that derived from x(T) data in Ref. 188. The dynamics of intersystem crossing processes in solution for these hexadentate complexes and other six-coordinate ds, d6, and d7 spin-equilibrium complexes of iron(III), iron(II), and cobalt(II) has been discussed by Sutin and Wilson et al.u°). 

T2(Oh) 1A1(Oh) spin equilibrium in iron(II) complexes based on the hy-drotris(l-pyrazolyl)borate ligand, was established for [Fe(HB(me-pz)3)2] (for abbreviations see Sect. 8.1) in the solid state 172>173 and for [Fe(HB(pz)3)2] in solution174 . Sutin et al.17S studied the dynamics of the spin interconversion in CH2C12/CH30H solutions with the laser Raman temperature-jump technique between 0 and 25 °C. The relaxation was observed to be first order with a lifetime of 32 10 ns, independent of temperature and concentration over the range studied. The ki and k j values for the process... 

Since it can be predicted that depopulation of the (high-spin) sextet level in favour of the (low-spin) doublet level should be accompanied by a contraction of the [FeS6] core in spin transition systems, it is to be expected that the position of the spin equilibrium K will depend on pressure. This has been experimentally verified for several tris(dithiocarbamato)iron(III) complexes ita chloroform solution at pressures up to 5000 atmospheres at room temperature. On the assumption that the molecular volume contraction AV arises entirely as a result of the spin change, equation (15) can be applied to the calculation of the volume change. Values of A V of the order of 5 cm3 mol-1 were obtained for systems with a wide variety of N substituents.289,290 The values correspond to a contraction of the Fe—S bond length of about 0.1 A, during the 5 A - 2T transition. 

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