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R.W. Saalfrank, A. Scheurer, Universität Erlangen-Nürnberg, Germany V. Schünemann, Technische Universität Kaiserslautern, Germany A.X. Trautwein, Universität.

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Presentation on theme: "R.W. Saalfrank, A. Scheurer, Universität Erlangen-Nürnberg, Germany V. Schünemann, Technische Universität Kaiserslautern, Germany A.X. Trautwein, Universität."— Presentation transcript:

1 R.W. Saalfrank, A. Scheurer, Universität Erlangen-Nürnberg, Germany V. Schünemann, Technische Universität Kaiserslautern, Germany A.X. Trautwein, Universität zu Lübeck, Germany Trinuclear Mixed-Valent Oxo-Centered Iron Complexes: Fully Localised, Partially and Fully Delocalised Valencies

2 Motivation Our interest in polynuclear supramolecular {Fe 3 O} complexes stems from the importance of oxo-centered polyiron aggregates as model compounds for iron-oxo proteins, oxidation catalysts, and corrosion inhibitors. Such oxo-centered complexes, related to basic iron acetate [Fe III 3 O(O 2 C-R) 6 (H 2 O) 3 ] +, are easily accessible (Fig. 1, left): Deprotonation of pentadentate ligand H 2 L 1 with triethylamine and treatment of the resulting dianion L 2- with a solution of iron(III) chloride, yielded after workup the triple- helical, μ 3 -oxo-centered, trinuclear and mixed-valent complex [Fe II Fe III 2 O(L 1 ) 3 ] (2) as dark-green crystals (Fig. 1, right: stereoview) [1]. Fig. 1 Stereoview of 2: Fe II dark blue, Fe III gold, C white, N blue, O red

3 The lack of counterion in the solid state implies intramolecular charge compensation and therefore mixed-valence character for [Fe II Fe III 2 O(L 1 ) 3 ] (2). This is confirmed by a Mössbauer spectrum of a powder sample recorded at 4.2K. The spectrum exhibits two quadrupolar doublets with a peak area ratio of about 1:2 (Fig. 2). The doublet with quadrupole splitting E Q =2.64 mms -1 and an isomer shift =0.95 mms -1 suggests a high-spin iron(II) species and the doublet with E Q =1.83 mms -1 and =0.53 mms -1 a high-spin iron(III) species. From this view 2 appears as a localised mixed-valent trinuclear complex with ground state of the type Fe 2+ -Fe 3+ -Fe 3+ as observed also in other cases [2]. A more rigorous characterisation of the valencies in 2, however, requires a more detailed investigation (vide infra), including the variation of temperature. Fig. 2

4 Fully localised vs. fully delocalised valency The complex [Fe 3 O(OAc) 6 (3-Et-py) 3 ]. CH 3 CCl 3 (3), with 3-Et-py representing 3-ethylpyridine (Fig. 3, left), exhibits I =0.54 mms -1, E QI =1.04 mms -1 for the ferric and II =1.26 mms -1, E QII =1.54 mms -1 for the ferrous site at T=43 K [3]. At room temperature only one doublet with =0.65 mms -1 and E Q =0.93 mms -1 is seen (Fig. 3, right); this corresponds to full delocalisation of the excess electron over the three iron sites. If the solvate molecule CH 3 CCl 3 is replaced by toluene or benzene, however, the complex remains valence trapped up to room temperature. Fig. 3 (Mössbauer spectra taken from [3]) Stereoview of 3, Fe gold, C white, N blue, O red, S yellow

5 Partially valence delocalised states The Mössbauer spectra of [Fe II Fe III 2 O(L 1 ) 3 ] (2) in the temperature range between 4.2 and 332 K are shown in Fig. 4 [4]. A consistent analysis of all spectra requires four doublets. They are pairwise related by the area ratio I:II and III:IV of 1:2. From and 2 appears as mixed-valent iron complex which exhibits a partially delocalised ground state: Fe 2.2+ -Fe 2.9+ - Fe 2.9+ (vide infra). With increasing temperature the subspectra III and IV are gaining weight. They represent Fe 2.5+ - Fe 2.75+ -Fe 2.75+, a structurally slightly different molecular state of [Fe II Fe III 2 O(L 1 ) 3 ] (2) in the solid. The two complexes Fe 2.2+ - Fe 2.9+ -Fe 2.9+ and Fe 2.5+ -Fe 2.75+ -Fe 2.75+ do not significantly change their respective delocalisation pattern of the excess electron, however, their relative amounts do change with increasing temperature: at 4.2 K the relative amounts are 90 % and 10 %, and at 332 K they are 45 % and 55 %. Fig. 4 II I IV III

6 Fig. 4: The spectrum taken at 4.2 K is dominated by two doublets, I and II, with intensity ratio 1:2 and with the isomer shifts and quadrupole splittings of I =0.99 mms -1, E QI =2.65 mms -1 and II =0.52 mms -1, E QII =1.85 mms -1. It has been found in an accompanying study that the isomer shift of the ferric sites of [Fe II Fe III 2 O(L 1 ) 3 ] (2) ( =0.52 mms -1 ) is enhanced compared to the value of the ferric sites of the corresponding hetero-nuclear complexes [M II Fe III 2 O(L 1 ) 3 ] with M II =Ni II, Co II and Cu II, i.e. =0.46 mms -1. In turn, the isomer shift of the ferrous site ( =0.99 mms -1 ) is significantly lower than the corresponding -values expected for 5N/1O ligation; e.g. isomer shifts of various octahedrally coordinated iron(II)-pyridine complexes [Fe II (pyridine) 4 X 2 ] with X=Cl, Br, I, NCO are ~1.1 mms -1 at 4.2 K. Both the enhancement of the isomer shift of the ferric sites as well as the reduction of the isomer shift of the ferrous site can be explained on the basis of partial electron (valence) delocalisation in the ground state of [Fe II Fe III 2 O(L 1 ) 3 ] (2) according to the relation,Fe (2+x)+ = x Fe 2+ + (1-x),Fe 3+,Fe (3-y)+ = y,Fe 2+ + (1-y),Fe 3+ and x = 2y. From this it has been concluded that 2 exhibits a partially delocalised ground state at 4.2 K, i.e. Fe 2.2+ -Fe 2.9+ -Fe 2.9+. A corresponding estimate with subspectra III and IV ( III =0.80 mms -1 and IV =0.60 mms -1 at 20K) yields Fe 2.5+ -Fe 2.75+ -Fe 2.75+ [4]. The main contribution to the observed temperature dependence of isomer shifts in Fig. 4 is due to the second-order Doppler shift (SOD). For comparison, SOD accounts for ~ 0.13 mms -1 in the temperature interval between 4.2 and 300 K for [Fe II (pyridine) 4 X 2 ] complexes [5]. If the temperature dependence of isomer shifts in 2 were mainly due to the temperature-induced changes of electron delocalisation, then the isomer shifts I and II as well as III and IV would gradually merge pairwise with increasing temperature according to the above relation. Such behavior was not observed in the present case, therefore it was concluded that the clusters Fe 2.2+ -Fe 2.9+ -Fe 2.9+ and Fe 2.5+ -Fe 2.75+ - Fe 2.75+ do not change their respective delocalisation pattern of valencies.

7 Fig. 5 Solvent vs. packing effect upon valence delocalisation Electron (valence) delocalisation is very much affected by solvent and packing. Fig. 4 has shown the temperature- dependent Mössbauer spectra of [Fe II Fe III 2 O(L 1 ) 3 ] (2) in the solid state with partially delocalised valencies over the whole temperature range. Fig. 5 dis- plays the Mössbauer spectra of 2 at 4.2K (a) and at 160 K (b) in frozen THF [6]. Only three doublets are required to fit the spectra at 160 K. Two of them (I and II) are consistent with partially localised valencies, i.e. Fe 2.3+ -Fe 2.85+ - Fe 2.85+, while the third (III; =0.62 mms -1 ) is consistent with full valence delocalisation, i.e. Fe 2.67+ -Fe 2.67+ -Fe 2.67+. This apparently delocalised component accounts for as much as 70% of the molecules at 160 K.

8 Electron exchange in asymmetrically substituted hetero-nuclear {M II Fe III 2 O} complexes The oxo-centered iron(II,III) complex [Fe II Fe III 2 O(L 2 ) 3 (OAc) 3 ] (5) was synthesised starting from HL 2 (4) under aerobic conditions with iron(II) acetate. In addition, 5 can be converted to complexes 6-8 by co-ligand exchange or nickel(II) acetate (Fig. 6) [7,8]. The asymmetrically substituted µ 3 -oxo centered complexes [M II Fe III 2 O(L 2 ) 3 (O 2 C-R) 3 ] (M II =Fe II, Ni II ; R=Me, Ph) (5-8) possess a metal-to- ligand-to-co-ligand ratio (M:L:co-L) of 1:1:1, and the absence of a counterion requires intramolecular charge compensation, i.e. mixed-valence character for 5-8. Fig. 6

9 The X-ray structure of [Fe II Fe III 2 O(L 2 ) 3 (OBz) 3 ] (6) reveals: The iron centers Fe1/Fe2 are linked by two ligands (L 2 ) -, Fe2/Fe3 by one (L 2 ) - ligand and a benzoate ion, and Fe1/Fe3 by two benzoate bridges. As a consequence, all three iron ions in the mixed-valent complex 6 are differently octahedrally coordinated (Fig. 7). The effect of the (OAc) - co-ligands on the {Fe II Fe III 2 O} core in 5 is different from the effect of the (OBz) - co-ligands on the iron core in 6, leading to the situation that in 6 the difference in coordination among iron sites is larger than in 5. This situation is reflected in the Mössbauer spectra (Fig. 8). Fig. 7 Stereoview of 6 with numbering of the iron ions: Fe II blue, Fe III gold, C white, N blue (small), O red, S yellow

10 The Mössbauer spectra of [Fe II Fe III 2 O(L 2 ) 3 (O 2 C-R) 3 ] (5,6) (R=Me, Ph) are shown in Fig. 8 [7,8]. The spectra of 5 and 6 at 300 K are almost identical; both exhibit two quadrupolar doublets with an area ratio of 1:2 for the Fe II and the Fe III ions. However, at 4.2 K the Mössbauer spectra of 5 and 6 differ considerably. Unlike for 5 the spectrum of 6 shows at 4.2 K a rather broad signature of the Fe II doublet as compared with the Fe III doublet, i.e. the experimental data (inset in Fig. 8d) are best fitted by three doublets ( E Q =2.77, 2.66, 2.66 mms -1, =1.19, 1.12, 0.96 mms -1, =0.25, 0.25, 0.25mms -1 ), which represent Fe II with a relative area ratio of 11,11,11 %, respectively (Table 1) [7]. The presence of partial electron (valence) delocalisation at 4.2 K in the present case is excluded, since the hetero-nuclear all-ferric complexes [Ni II Fe III 2 O(L 2 ) 3 (O 2 C-R) 3 ] (7,8) (R=Me, Ph) exhibit comparable isomer shifts at 77 K, i.e. 0.50 mms -1 and 0.49 mms -1, as the ferric sites in 5 and 6 at 4.2 K (Table 1). Fig. 8

11 As a result of the different coordination of the iron sites (Fe1)3N/3O, (Fe2)4N/2O, (Fe3)2N/4O in [Fe II Fe III 2 O(L 2 ) 3 (OBz) 3 ] (6), there exist evidently three configurations with unlike energies (Fe1) II (Fe2) III (Fe3) III, (Fe1) III (Fe2) II (Fe3) III, (Fe1) III (Fe2) III (Fe3) II for 5 and 6. Even if the difference among these energies is small and can be overcome at elevated temperature by thermal activation, at low temperature one expects that only the configuration with the lowest energy is populated, corresponding to only one Fe II doublet (instead of three doublets) in the Mössbauer spectrum at 4.2 K. However, assuming a slow electron-tunneling exchange mechanism in 6, one might arrive at a situation, at which all three configurations remain nearly equally populated, even at very low temperature. Thermal fluctuations at 300 K then equalise the ligand-field strength around each iron site in 5 and in 6, such that it is not possible to resolve the different (Fe1) II, (Fe2) II, (Fe3) II components in the Mössbauer spectra.

12 Summary Mixed-valent oxo-centered trinuclear metal complexes provide model systems for systematic studies of the phenomenon of electron (valence) delocalisation. Such investigations have shown that the rate of electron transfer is dramatically affected by the environment of the complex, especially by solvent and/or packing, and by the nature of the ligands directly coordinated to the metal sites. References [1] R.W. Saalfrank, S. Trummer, H. Krautscheid, V. Schünemann, A.X. Trautwein, S. Hien, C. Stadler and J. Daub, Angew. Chemie. Int. Ed. Engl. 35, 2206 (1996) [2] D.N. Hendrickson, in Mixed Valency Systems: Applications in Chemistry, Physics and Biology, K. Prassides (Ed.), Kluwer, Dordrecht, 1991, p. 67 [3] C.-C. Wu, H.G. Jang, A.L. Rheingold, P. Gütlich and D.N. Hendrickson, Inorg. Chem. 35, 4137 (1996) [4] V. Coropceanu, V. Schünemann, C. Ober, M. Gerdan, A.X.Trautwein, J. Köhler and R.W. Saalfrank, Inorg. Chim. Acta 300-302, 875 (2000) [5] D.P.E. Dickson and F.J. Berry, Mössbauer Spectroscopy, Cambridge University Press, Cambridge, UK, 1986, p. 88 [6] C. Stadler, J. Daub, J. Köhler, R.W. Saalfrank, V. Coropceanu, V. Schünemann, C. Ober, A.X. Trautwein, S.F. Parker, M. Poyraz, T. Inomata and R.D. Cannon, J. Chem. Soc., Dalton Trans. 3373 (2001) [7] R.W.Saalfrank, A. Scheurer, U. Reimann, F. Hampel, C. Trieflinger, M. Büschel, J. Daub, A.X. Trautwein, V. Schünemann and V. Coropceanu, Chem. Eur. J. 11, 5843 (2005) [8] R.W. Saalfrank, A. Scheurer, K. Pokorny, H. Maid, U. Reimann, F. Hampel, F. W. Heinemann, V Schünemann and A.X. Trautwein, Eur. J. Inorg. Chem. 1383 (2005)


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