Mikio Nakamura a,b,c and Masashi Takahashi d

1. Spin States of Highly Deformed Iron(III) Porphyrin Complexes Studied by 57Fe Mössbauer Spectroscopy Mikio Nakamuraa,b,c and Masashi Takahashid a Department of Chemistry, School of Medicine, Toho University Ota-ku, Tokyo 143–8510, Japan b Research Center for Materials with Integrated Properties, Toho University Funabashi, Chiba 274-8510, Japan c Division of Chemistry, Graduate School of Science, Toho University Funabashi, Chiba 274-8510, Japan d Department of Chemistry, Faculty of Science, Toho University Funabashi, Chiba 274-8510, Japan “Gütlich, Bill, Trautwein: Mössbauer Spectroscopy and Transition Metal Chemistry@Springer-Verlag 2009”

2. Introduction: spin state and deformation of porphyrin ring dz2 dx2-y2 ruffled deformation saddled deformation low spin state S = 1/2 high spin state S = 5/2 intermediate spin state S = 3/2 Fig. 2 Two modes of deformation of porphyrin ring: atoms shown by open circles are above the least square plane, and those by the filled circles are below the plane. Fig. 1. Spin states adopted by Fe(III)-porphyrin complexes Elucidation of the electronic structures and spin states of iron porphyrin complexes is of importance to understand the function of naturally occurring heme proteins. A number of investigations have been carried out to this end using many physicochemical methods. Among them iron-57 Mössbauer spectroscopy is a powerful and appropriate tool. The spin states of iron(III) in porphyrin complexes are controlled by the nature and number of the axial ligands through the strength of the ligand field splitting (Fig. 1). The deformation of the porphyrin ring, which induces certain specific interactions between the iron d orbitals and the porphyrin ring, also has considerable influence on the spin state [ref. 1]. A number of naturally occurring heme proteins exhibit porphyrin ring deformation. Hence the elucidation of the deformation effect is an important subject. Two modes of deformation are well-known: ruffle and saddle (Fig. 2), which result in different effects on the electronic configuration. In this paper we will focus mainly on FeIII-OETPP complexes, which adopt the saddled deformation. The abbreviations of the porphyrins and ligands appearing in this paper are listed in the last slide. Ref. 1. M. Nakamura, Coord. Chem. Rev. 250 (2006) 2271–2294.

3. Spin states of Fe(III) porphyrin complexes Fig. 3 shows the spectra of saddle-type Fe(III) OETPP complexes: [Fe(OETPP)Cl], [Fe(OETPP)(thf)2]ClO4 and [Fe(OETPP)(Him)2]ClO4. [Fe(OETPP)Cl] has a small quadrupole splitting (DEQ) value and is assigned to the S = 5/2 state, while [Fe(OETPP)(Him)2]ClO4 has a larger DEQ value and a rather small isomer shift (d) value corresponding to anS = 1/2 state. [Fe(OETPP)(thf)2]ClO4 shows a very large DEQ and rather large d values, indicating a large imbalance in d electron population, and is assigned to the S = 3/2 state. Five-coordinate porphyrin complexes with an axial halide ligand usually show the S = 5/2 state. Interestingly the saddled OETPP complex with an iodide ion in the axial position has a large DEQ value (3.05 mm s–1 at 77 K), indicating the S = 3/2 state [ref. 2]. This demonstrates that control of the spin state using the axial ligand is easier in non-planar porphyrin complexes. Indeed the porphycene complex, [Fe(EtioPc)I] also adopts the S = 3/2 state [ref. 3]. [Fe(OETPP)Cl] S = 5/2 [Fe(OETPP)(thf)2]ClO4 S = 3/2 [Fe(OETPP)(Him)2]ClO4 S =1/2 Fig. 3. 57Fe Mössbauer spectra of OETPP complexes at room temperature in different spin states. Ref. 2. M. Nakamura et al., Chem. Commun., (2002) 1198–1199. Ref. 3. Y. Ohgo et al., Inorg. Chem., 41 (2002) 4527–4529.

4. L = Him, d = 0.18, DEQ = 1.82 mm s–1 S = 1/2 L = dmap, d = 0.19, DEQ = 2.21 mm s–1 S = 1/2 L = py, d = 0.32, DEQ = 2.76 mm s–1 S = 3/2–1/2 L = 4-CNpy, d = 0.37, DEQ = 3.26 mm s–1 S = 3/2 L = thf, d = 0.41, DEQ = 3.65 mm s–1 S = 3/2 Spin states of [Fe(OETPP)L2]ClO4 Fig. 4 demonstrates that the ligand field strength of the axial ligand L determines the spin state. Strongly splitting ligands [imidazole (Him) and 4-dimethylaminopyridine (dmap)] lead to the pure S = 1/2 state, while the weakly splitting ligand tetrahydrofuran (thf) gives a pure S = 3/2 spin state. Intermediate strength ligands such as pyridine (py) and 4-cyanopyridine (4-CNpy) result in new spin-crossover complexes as discussed later [ref. 4]. Interestingly the importance of the ring deformation is also confirmed in the case of the ruffled deformation: [Fe(TEtPrP)(thf)2]ClO4 also has the S = 3/2 state (d = 0.24 and DEQ = 3.80 mm s–1) [ref. 5]. Ref. 4. T. Ikeue et al, Angew. Chem. Int. Ed. 40 (2001) 2617 – 2620. Ref. 5. T. Sakai et al., J. Am. Chem. Soc., 125 (2003) 13028 – 13029. Fig. 4 Mössbauer spectra at room temperature of six-coordinate complexes [Fe(OETPP)L2]ClO4.

5. Spin-crossover in [Fe(OETPP)L2]ClO4 (L = py , 4-CNpy) The Mössbauer spectra of 4-CNpy and py show very interesting temperature dependence [ref. 4]. In both complexes the spectra change as the temperature is lowered. The 4-CNpy complex exhibits a new doublet below 230 K and the relative intensities for the site increase on decreasing the temperature. The d and DEQ values of the new site (blue) are 0.20 and 2.70 mm s–1 at 77 K, while those of the other site (green) are 0.57 and 3.03 mm s–1. This clearly indicates that two spin states co-exist at 77 K and implies a spin-crossover between S=3/2 and S=1/2. Although no new peak is observed in the py complex, both d and DEQ values decrease on lowering the temperature (Fig. 8 in a later slide), reaching values of 0.25 and 2.29 mm s–1, respectively at 77 K. This is obviously the low spin state. Thus the py complex is also a spin-crossover complex. The difference in the Mössbauer behaviour of the complexes is due to the time scale of the spin-crossover transition. Fig. 5 Mössbauer spectra of py and 4-CNpy complexes at room temperature and 77 K.

6. Magnetic moments and 13C nmr chemical shifts of [Fe(OETPP)L2]ClO4 L = thf L = py L = 4-CNpy L = dmap L = Him L = dmap L = py L = Him L = 4-CNpy Fig. 6. Temperature dependence of magnetic moments of [Fe(OETPP)L2]ClO4 in the crystalline state (a) and in CH2Cl2 solution (b). Fig. 7. Temperature dependence of 13C chemical shifts of the meso13C atoms in CD2Cl2 solution. Spin-crossover in py and 4-CNpy complexes is confirmed by other physicochemical methods [ref.4]: the magnetic moments of py and 4-CNpy complexes are definitely those of spin-crossover complexes (Fig. 6). Interestingly the magnetic moments determined by the Evans method in CH2Cl2 indicate that spin-crossover occurs in the py complexes (Fig. 6 inset). Furthermore, the 13C chemical shifts of the meso carbon atoms in CD2Cl2 also show spin-crossover for the py complex (Fig. 7). DtrsH and DtrsS of the py complex are estimated to be 16.9 kJ mol–1 and 66.6 J K–1 mol–1 from the chemical shift values. Thus spin-crossover between intermediate and low spin states in the py and 4-CNpy complexes is definite.

7. Spin states of [Fe(OMTPP)L2]ClO4 OETPP-Py (S=3/2– 1/2) OMTPP-Py (S =1/2) OMTPP-DMAP (S = 1/2) OETPP-DMAP (S = 1/2) Fig. 8 Temperature dependence of DEQ of OMTPP and OETPP complexes. Fig. 9 Temperature dependence of magnetic moments of OMTPP and OETPP complexes. We extend the discussion to the methyl analogues [Fe(OMTPP)L2]+ with py and dmap [ref. 6]. Mössbauer spectra indicate that both py and dmap complexes maintain the S = 1/2 state over the temperature range 77 – 300 K (Fig. 8), and this is confirmed by SQUID magnetometry (Fig. 9). These results unexpectedly suggest that the spin states in the OMTPP complexes are quite different from those of the structurally related OETPP complexes. Furthermore, the magnetic moments measured for the solution samples confusingly show that the OMTPP-py complex behaves as a spin-crossover complex just like the OETPP-py complex [ref. 7]. Ref. 6. Y. Ohgo et al., Eur. J. Inorg. Chem. (2004) 798–809. Ref. 7. T. Ikeue et al., Inorg. Chem. 42 (2003) 5560–5571.

8. Origin of difference in spin-crossover behaviour [Fe(OMTPP)(py)2]ClO4 V = 19.81 Å3 Fe–Nax = 2.058 Å Fe–Np = 1.963 Å d = 1.406 g cm–3 V = 18.77 Å3 Fe–Nax = 2.024 Å Fe–Np = 1.973 Å d = 1.460 g cm–3 [Fe(OETPP)(py)2]ClO4 V = 23.19 Å3 Fe–Nax = 1.993 Å Fe–Np = 1.957 Å d = 1.388 g cm–3 V = 32.08 Å3 Fe–Nax = 2.201 Å Fe–Np = 1.985 Å d = 1.296 g cm–3 Fig. 10 Temperature-dependent orientation change of the pyridine ligands in the cavities of [Fe(OMTPP)(py)2]ClO4 and [Fe(OETPP)(py)2]ClO4 from 298 K (left) to 80 K (right). The key is the diference in the molecular structures, especially in the cavity around the axial ligand [ref. 6]. While the Fe–Nax lengths of OMTPP-py hardly change with temperature, those of OETPP-py contract on lowering the temperature. This difference is induced by a difference in the molecular packing: OETPP-py molecules are more loosely packed than OMTPP-py molecules. This leads to a large contraction of the cavity size (V) of OETPP-py on cooling. Thus we can conclude that the large cavity around the axial ligands is essential and important for the spin-crossover process.

9. Abbreviations porphyrins (OMTPP)H2 2,3,7,8,12,13,17,18-octamethyl-5,10, 15,20-tetraphyenylporphyrin (OETPP)H2 2,3,7,8,12,13,17,18-octaethyl-5,10, 15,20-tetraphyenylporphyrin (TEtPrP)H2 5,10,15,20-tetrakis(1-ethylpropyl)-porphyrin axial ligands (EtioPc)H2 2,7,12,17-tetraethyl-3,6,11,18-tetramethylporphycene