„Tuning“ the Ligand Field by Ligand Replacement

1. Thermal and light-induced spin transition in iron compoundsP. Gütlich et al.Institut für Anorganische Chemie und Analytische ChemieJohannes Gutenberg-Universität MainzStaudingerweg 9, D-55099 Mainz, GermanyE-mail: guetlich@uni-mainz.dehttp://ak-guetlich.chemie.uni-mainz.de

2. Thermal spin transition (spin crossover) is a widely observed phenomenon in coordination chemistry of 3d transition metal ions with d4 up to d7 electron configuration. It was first observed in iron(III) compounds around 1930 by the italian researchers Cambi and coworkers, but is now well established and extensively studied in many iron(II) compounds, last but not least by application of the powerful tool of Mössbauer spectroscopy. Spin crossover in iron(II) compounds may be observed if the ligand field strength is such that the difference between the lowest “vibronic” energy levels of the high spin state 5T2 and the low spin state 1A1 state is comparable with thermal energy kBT (kB = Boltzmann constant). The spin transition behaviour can be influenced by chemical alteration, e.g. ligand replacement, change of non-coordinating anion and solvent molecule, substitution of spin state changing metal by another metal, e.g. substitution of iron by zink. It can also be influenced by physical perturbation such as irradiation with light, application of pressure or a magnetic field. For a comprehensive coverage of spin crossover research see: - Gütlich, P.; Hauser, A.; Spiering, H.Angew. Chem. Int. Ed. Engl. 1994, 33, 2024. - Gütlich, P.; Goodwin, H.A. (Eds.), Spin Crossover in Transition Metal Compounds, Top. Curr. Chem. Vol 233, 234, 235, Springer, Berlin Heidelberg New York The influence of the ligand molecules on the spin state of the central iron(II) ions is demonstrated with the following two examples and their temperature dependent Mössbauer spectra shown in the next picture.

3. [FeII(phen)3]X2 [FeII(phen)2(NCS)2] HS LS 300 K 300K 188 K 240 K 186 K 184 K 80 K 180 K LS LS 5 K 77 K „Tuning“ the Ligand Field by Ligand Replacement

4. [FeII(phen)3]X2 (phen = 1.10-phenanthroline) is a typical low spin compound with the characteristic Mössbauer spectra as shown on the left; the isomer shift is ca. 0.2 mm s-1 (relative to -iron) and the quadrupole splitting ca. 0.5 mm s-1, nearly independent of temperature. If one of the relatively strong phen ligands, which occupies two coordination positions of the octahedron, is replaced by two monofunctional NCS- groups, the average ligand field strength becomes weaker than the mean spin pairing energy and the compound [FeII(phen)2(NCS)2] adopts high spin (HS) character at room temperature. The Mössbauer spectrum at 300 K shows the typical features of an iron(II) HS compound with isomer shift of ca. 1 mm s-1 and large quadrupole splitting of ca. 3 mm s-1. However, the compound [FeII(phen)2(NCS)2] fulfils the condition for thermal spin crossover to occur, viz. ΔEHL kBT. On lowering the temperature, the compound changes spin state from high spin to low spin near 180 K as is well documented by the Mössbauer spectra as a function of temperature, which was first reported by I. Dezsi et al. in 1967. Since then more than 200 spin crossover compounds of iron(II) and iron(III) have been studied by Mössbauer spectroscopy (see e.g. P. Gütlich, H.A. Goodwin (eds.), Spin Crossover in Transition Metal Compounds, Springer Series “Topics in Current Chemistry, Vol. 233, 234, 235, Berlin Heidelberg 2004). [Fe(ptz)6](BF4)2, where ptz stands for the ligand molecule 1-propyl-tetrazole as shown in the picture, is another iron(II) coordination compound exhibiting thermal spin crossover with a spin transition temperature T1/2 of ca. 135 K. The HS phase is nearly colourless, the LS phase is red. Thus the thermally induced spin transition can be easily followed by optical spectroscopy in the uv/visbile range. The 57Fe Mössbauer spectra clearly indicate the transition near 135 K between the HS phase (quadrupole doublet shown in red) and the LS phase (singlet shown in blue). Whereas the quadrupole doublet is typical for iron(II) in the HS state, the singlet of the LS state is rarely observed and points at a quasi regular octahedral surroundings at the iron centre.

5. C H 3 7 N N N N Thermal spin transition in [Fe(ptz)6](BF4)2 followed by optical spectroscopy 300 K Six ptz molecules coordinate via N atoms to the iron center and form the twofold positive cationic complex ion. Two BF4¯ serve as counter ions. 1A1→1T2 80 K 1A1→1T1 The complex compound [Fe(ptz)6](BF4)2 is in the high spin (HS) state 5T2 (t2g4eg2) at room temperature down to ca. 135 K. The optical spectrum of the colorless crystal shows a weak absorption band for the 5T2→5E transition. At 135 K, thermal spin transition to the low spin (LS) state 1A1 (t2g6) takes place on further cooling with dramatic color change from white (HS) to red (LS). The optical spectrum shows two spin-allowed (but parity-forbidden) singlet-singlet absorption bands. E. W. Müller, J. Ensling, H. Spiering, P. Gütlich, Inorg. Chem., 22(14), 2074-8 1983

6. Thermal spin transition in [Fe(ptz)6](BF4)2 followed by Mössbauer spectroscopyand magnetic susceptilibility measurements The 57Fe Mössbauer spectra of [Fe(ptz)6](BF4)2 recorded as a function of temperature clearly indicate the spin transition between HS state (quadrupole doublet shown in red) and LS state (singlet shown in blue) at ca. 135 K. The HS quadrupole doublet is mainly caused by noncubic electron distribution in the (Jahn-Teller-active) t2g4eg2 configuration. The singlet of the LS state arises from the t2g6 points at a quasi regular octahedral surroundings at the iron centre. The spin transition occurs with hysteresis as shown by the magnetic susceptibility measurements, from which the effective magnetic moment can be derived. 155 K HS 136 K Rel. Transmission (%) 98 K LS v (mm/s) E. W. Müller, J. Ensling, H. Spiering, P. Gütlich, Inorg. Chem., 22(14), 2074-8 1983

7. Mössbauer spectroscopy is ideally suited to follow the light-induced spin state conversion in this system as exemplified in the next viewgraph A polycrystalline sample of [Fe(ptz)6](BF4)2 was cooled to 15 K. Before irradiation the sample is in the LS state and shows the typical Mössbauer spectrum of the LS state (upper left). After irradiating with green light (xenon lamp with filters or 514 nm band of an Ar ion laser) at 15 K the sample is quantitatively converted to the metastable HS state (middle left). The asymmetry in the intensity of the two components of the quadrupole doublet is due to the plate-like shape of the crystals (texture effect). Thermal relaxation on a 15 minute-timescale sets in at 50 K (lower left and upper right: the sample was heated for 15 minutes at 50 K and then cooled to the measuring temperature of 15 K in two runs). Thermal relaxation to the stable LS state is complete at 10 K. On further heating to 150 K the sample undergoes again thermal spin transition at 135 K to the (now stable) HS state. This photophysical phenomenon was first observed by S. Decurtins, P. Gütlich, C. P. Köhler, H. Spiering, A. Hauser, Chem. Phys. Lett., 105(1), 1-4 1984 and became known as “Light-InducedExcited Spin State Trapping” (LIESST). The processes involved in the LIESST effect are well understood on the basis of ligand field theory (Gütlich, P.; Hauser, A.; Spiering, H. Angew. Chem. Int. Ed. Engl. 1994, 33, 2024. The next viewgraph explains the mechanisms of LIESST and reverse-LIESST.

8. 15 K no light 50 K (30 ') 15 K green light 100 K Relative Transmission (%) 50 K (15 ') 150 K 15 K green light 5T2  5E Light-Induced Spin Crossover in [Fe(ptz)6](BF4)2 LS HS

9. eg t2g Light-Induced Excited Spin State Trapping (LIESST) The mechanism of LIESST and Reverse-LIESST is explained on the basis of ligand field theory. Green ligth (e.g. 514 nm from Ar ion laser) affords the spin-allowed (but parity-forbidden) transition 1A1 → 1T1,2. Preferred relaxation occurs via two subsequent intersystem crossing processes 1T1,2 → 3T1,2 → 5T2, which are favoured by spin-orbit coupling. The metastable HS state 5T2 has an extremely long lifetime (typically minutes near 50 K). Backconversion from the metastable HS state to the LS state can be achieved by irradiation with red light (e.g. 820 from Kr laser), whereby the first transition step 5T2 → 5E is followed by two intersystem crossing processes 5E → 3T1,2 → 1A1 back to the stable LS state.