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Synthesis, structure and physical properties of an iron(II) complex with four ferrocenyl groups
Polyhedron 24 (2005) 2409–2412 www.elsevier.com/locate/poly
Synthesis, structure and physical properties of an iron(II) complex with four ferrocenyl groups Lingqin Han, Masayuki Nihei, Hiroki Oshio
*
Department of Chemistry, University of Tsukuba, Tennodai 1-1-1, Tsukuba 305-8571, Japan Received 5 October 2004; accepted 14 December 2004 Available online 29 June 2005
Abstract A novel tridentate ligand with two ferrocenyl groups and its iron(II) complex were synthesized and these structures and physical properties are presented. The pentanuclear iron(II) complex is composed of one central ion(II) ion and four ferrocenyl groups, in which the central iron(II) ion was coordinated by two tridentate ligands. Electrochemical measurement for the complex in MeCN showed irreversible redox waves, and the redox process can be understood by EC mechanism. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Tridentate nitrogen ligand; Ferrocenyl group; Polynuclear complex; Iron; Multi-nuclear complex; Iron(II)
1. Introduction Tridentate N-heterocyclic ligands have been used over the last century as effective and stable complexing agents for transition metal ions. These ligands have been used to build supramolecular coordination polymers [1– 4] and denderimers [5] leading to new materials with interesting catalytic, photochemical and redox properties [6]. Synergy of metal ions in multi-nuclear complexes have been attracting much attention from view points of studying physical and catalytic properties [7]. Ferrocenyl units are frequently employed as redox-active sites in supramolecular assemblies [8], owing to their well-behaved redox activity as well as their synthetic versatility. A number of ferrocenyl-functionalized bipyridines [9–12] and terpyridines [11,13–15] have been synthesized for these reasons. Tridentate nitrogen ligand 2,6-di(pyrazol-1-yl)pyridine and its derivatives, which are considered as analogues of terpyridines, ligate an iron(II) ion and complex molecules have been known to *
Corresponding author. Tel.: +81298534238; fax: +81298534238. E-mail address: [email protected] (H. Oshio).
0277-5387/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.poly.2005.03.120
show spin-crossover phenomena [16,17]. Thus, combination of such tridentate ligands with multi-ferrocenyl groups might be expected to be a good candidate showing multi-functionalities. We report here syntheses, structures and physical properties of novel tridentate nitrogen ligand with two ferrocenyl groups (2,6-di(3-ferrocenylpyrazol-1-yl)pyridine = dppFc2), as well as its pentanuclear iron(II) complexes, [Fe(dppFc2)2](BF4)2.
2. Experimental 2.1. Syntheses All reagents were purchased and used without further purification. 3(5)-(ferrocen-1-yl)pyrazole was synthesized according to the literature [18]. 2.1.1. 2,6-Bis(3-ferrocenylpyrazol-1-yl)pyridine(dppFc2) Potassium hydride (0.80 g, 2.0 mmol) was slowly added to a solution of 3(5)-(ferrocen-1-yl)pyrazole (0.51 g, 2.0 mmol) in dry diglyme (10 ml) and stirred for 10 min at r.t. under N2. 2,6-Dibromopyridine
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(0.24 g, 1.0 mmol) was added to the resulting slurry and the mixture was stirred for 63 h at 90 °C. The reaction mixture was quenched with 100 ml water and yellow suspension was extracted with dichloromethane. Organic phase was evaporated and obtained crude products was purified with silica-gel column chromatography with dichloromethane/hexane (1/1, v/v) as an eluent to yield 0.24 g of dppFc2 as orange powder (0.41 mmol, 41%). Anal. Calc. for C31H25N5Fe2: C, 64.28; H, 4.35; N, 12.09. Found: C, 64.18; H, 4.62; N, 12.11%. 1H NMR (270 MHz, CDCl3, 25 °C): d 8.51(d, J = 2.7, 2H, pz), 7.89 (m, 3H, py), 6.54 (d, J = 2.7, 2H, pz), 4.79 (s, 4H, Cp), 4.34 (s, 4H, Cp), 4.11 (s, 10H, Cp).
2.3. Physical measurements
2.1.2. [Fe(dppFc2)2](BF4)2 A solution of Fe(BF4)2 Æ 6H2O (44 mg, 0.13 mmol) in degassed acetone (5 ml) was added to the orange suspension of dppFc2 (35 mg, 0.06 mmol) in acetone (5 ml) and stirred for 30 min to obtain deep orange solution. Addition of diethylether to the deep orange solution gave brown powder. Recrystallization from acetonitrile and diethylether gave deep red crystals of [Fe(dppFc2)2](BF4)2. Anal. Calc. for C62H50B2F8Fe5N10 Æ 4H2O: C, 51.00; H, 4.00; N, 9.59. Found: C, 51.18; H, 3.91; N, 9.50%.
3.1. Descriptions of crystal structures: dppFc2 and [Fe(dppFc2)2](BF4)2 Æ 3CH3CN
2.2. X-ray crystallography Each crystal of dppFc2 and [Fe(dppFc2)2](BF4)2 Æ 3CH3CN was mounted on a glass capillary, and data were collected at 30 °C (Bruker SMART APEX diffractometer coupled with a CCD area detector with ˚ ) radiagraphite monochromated Mo Ka(k = 0.71073 A tion). The structure was solved by direct methods and expanded by using Fourier techniques using SHELXTL program. Crystal data: [dppFc2]: C31H25Fe2N5, M = 579.26, monoclinic space group P21/n, a = 14.246(2), ˚ , b = 90.055(3)°, V = b = 9.793(1), c = 17.910(3) A ˚ 3, Z = 4. A total of 10 770 were collected 2498.6(7) A (1.83° < h < 23.26°) of which 3580 unique reflections (Rint = 0.0546) were measured. Residual R and wR were 0.0424 and 0.1297, respectively, from the refinement on F2 with I > 2r(I). [[Fe(dppFc2)2](BF4)2 Æ 3CH3CN]: C62H54F8Fe5N13B2, M = 1506.11, monoclinic space group C2/c, a = 21.864(3), b = 21.536(2), c = 29.389(3) ˚ , b = 102.721(2)°, V = 13498(3) A ˚ 3, Z = 8. A total of A 30 072 were collected (1.34° < h < 23.30°) of which 9712 unique reflections (Rint = 0.0499) were measured. Residual R and wR were 0.0539 and 0.1459, respectively, from the refinement on F2 with I > 2r(I). Empirical absorption corrections by SADABS (G.M. Sheldrick, 1994) were carried out. In the structure analyses, nonhydrogen atoms were refined with anisotropic thermal parameters. Hydrogen atoms were included in calculated positions and refined with isotropic thermal parameters riding on those of the parent atoms.
DC magnetic susceptibility data were collected by using a Quantum Design MPMS-XL SQUID magnetometer at temperatures ranging from 1.8 to 300 K. Cyclic voltammetry measurements were carried out in a standard one-compartment cell under N2 equipped with a platinum-wire counter electrode, a grassy carbon electrode and a saturated calomel electrode (SCE) with an ALS 620A electrochemical analyzer.
3. Results and discussion
Single crystals of dppFc2 for X-ray crystallography were obtained by slow evaporation of dichloromethane/hexane solution. An ORTEP diagram of dppFc2 is shown in Fig. 1. dppFc2 crystallizes in monoclinic space group P21/n. Two ferrocenyl groups are attached to 3-position of pyrazole rings and show an anti-parallel conformation with each other. Cyclopentadienyl rings are coplanar with three heterocyclic rings of tridentate ligand moiety, which indicates weak p-conjugation between them. [Fe(dppFc2)2](BF4)2 Æ 3CH3CN crystallizes in monoclinic space group C2/c, and the unit cell contains two crystallographic independent molecules. Each central iron(II) ions show both significantly distorted octahedral surroundings composed of six nitrogen atoms from two tridentate dppFc2 ligands (Fig. 2). Coordination bond lengths are in the range of ˚ , characteristic of bond lengths for 2.134(5)–2.190(4) A a high spin iron(II) ion. Bite angles of tridentate ligands to iron(II) ion (N1–Fe1–N2 and N4–Fe1–N5) are 73.1(1)° and 72.67(9)°, respectively. Distortion can be quantified using R parameters defined by Guionneau et al. [19] R parameter is the sum of the deviations from 90° of the 12 bite angles in the coordination sphere, and the large R value represents large distortion of octahedron from ideal one. The R values are estimated to be 168.62° and 169.28° for two complex cations, which
Fig. 1. ORTEP drawing of dppFc2.
L. Han et al. / Polyhedron 24 (2005) 2409–2412
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Fig. 3. Cyclic voltammogram of [Fe(dppFc2)2](BF4)2 at a glassy carbon electrode in 0.1 mol dm3 [Bu4N][PF6]/MeCN at a scan rate of 0.1 V s1.
˚) Fig. 2. ORTEP drawing of [Fe(dppFc2)2]2+. Selected bond lengths (A and angles (°): Fe(1)–N(1) 2.134(5), Fe(1)–N(4) 2.135(5), Fe(1)–N(2) 2.181(4), Fe(1)–N(5) 2.185(3), Fe(2)–C(10) 2.018(4), N(1)–Fe(1)–N(2) 73.1(1), N(4)–Fe(1)–N(2) 106.9(1), N(1)–Fe(1)–N(5) 107.3(1), N(4)– Fe(1)–N(5) 72.67(9), N(2)#1–Fe(1)–N(5) 102.9(1), N(2)–Fe(1)–N(5) 87.1(1), N(1)–Fe(1)–N(5)#1 107.33(9), N(4)–Fe(1)–N(5)#1 72.7(1). Symmetry operation #1: x, y, z + 1/2.
show good agreement with the values for high spin iron(II) complexes with tridentate nitrogen ligands. In [Fe(dppFc2)2](BF4)2 Æ 3CH3CN, conformations of two ferrocenyl groups in the ligand are anti-parallel and two cyclopentadienyl rings are distorted from three heterocyclic rings plane with torsion angles of C8–C9– C10–C14 being 140.0(5)°, which may be due to the steric hindrance between ferrocenyl groups. 3.2. Redox properties of dppFc2 and [Fe(dppFc2)2](BF4)2 In the electrochemical measurements, the tridentate ligand dppFc2 undergoes reversible one-step 2e oxi0 dation on two ferronenyl groups at E0 ¼ 0.47 V versus SCE. In the cyclic voltammogram of [Fe(dppFc2)2](BF4)2 with potential scans between 0.00 and 0.90 V versus SCE showed two irreversible redox couples (Fig. 3). The first couple of oxidation 0 and re-reduction waves was observed at E01 ¼ 0.47 V versus SCE, which is same as the value of dppFc2. In the first couple, re-reduction current is much larger than that in oxidation wave. When the positive limit of the potential scan is 0.55 V, the large re-reduction wave does not appear (Fig. 3). While the 0 second redox couple at E02 ¼ 0.63 V showed smaller re-reduction current than that in oxidation. By comparison of the redox potentials, the first redox couple
was assigned to dppFc2 =dppFc2 2þ and irreversibility of two-step redox process in [Fe(dppFc2)2]2+ can be interpreted by the EC mechanism and dissociation equilibrium in MeCN solution as shown in Scheme 1. Before oxidation of the two ferrocenyl moiety, [Fe(dppFc2)2]2+ species is dominant in MeCN solution, while after the oxidation electronic repulsion between central iron(II) ion and ferrocenyl cations may cause the ligand dissociation. 3.3. Magnetic susceptibility measurements of [Fe(dppFc2)2](BF4)2 Æ 3CH3CN Temperature dependence of magnetic susceptibilities for [Fe(dppFc2)2](BF4)2 Æ 3CH3CN was measured in the temperature range of 2–300 K (Fig. 4). At room temperature the vmT value is 3.35 emu mol1 K, which corresponds to the spin-only value for a non-interacting high spin FeII ion. vmT values are almost constant down to 80 K, followed by sudden decrease below 50 K which is due to the zero-field splitting of the high spin iron(II) ion. The temperature independent vmT values indicates [Fe(dppFc2)2](BF4)2 Æ 3CH3CN is in the high spin state in all temperature range measured. An iron(II) complex with similar ferrocenyl-substituted tridentate ligand shows spin conversion between high- and low-spin state at Tc = ca. 370 K, in which ferrocenyl group attaches to 4-position on the pyridine ring with ester bond [20]. The absence of spin conversion in [Fe(dppFc2)2](BF4)2 Æ 3CH3CN can be attributable to the steric hindrance
[Fe(dppFc2)2]
2 dppFc2
2+
6+
+ Fe
2+
E0'2 = 0.63 V
E0'1 = 0.47 V
[Fe(dppFc2)2]2+
2 dppFc2 + Fe2+c
Scheme 1.
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Acknowledgements This work was partially supported by a Grant-in-aid for Scientific Researches from the Ministry of Education, Culture, Sports, Science and Technology, Japan, and by the COE and TARA projects of University of Tsukuaba.
References
Fig. 4. Plot of vmT vs. T for [Fe(dppFc2)2](BF4)2 Æ 3CH3CN.
between ferrocenyl groups which causes significant distortion of coordination geometry and stabilizes the high spin state in the central iron(II) ion.
4. Conclusion Tridentate ligand with two ferrocenyl groups give an opportunity to assemble four redox-active centre by coordination to iron(II) ion. The complex with four ferrocenyl groups undergoes two irreversible redox process in EC mechanism.
5. Supplementary material Crystallographic data reported in this paper have been deposited with Cambridge Crystallographic Data Centre as supplementary publication no. CCDC 251833 and 251834. These data can be obtained free of charge via www.ccdc.cam.ac.uk/conts/retrieving.html (or from the Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336 033; or [email protected]).
[1] M. Kimura, T. Horai, T. Muto, K. Hanabusa, H. Shirai, Chem. Lett. (1999) 1129. [2] S. Kelch, M. Rehahn, Macromolecules 32 (1999) 5818. [3] U. Velten, B. Lahn, M. Rehahn, Macromol. Chem. Phys. 198 (1997) 2789. [4] U.S. Schubert, C. Eschbaumer, Angew. Chem. Int. Ed. 41 (2002) 2892. [5] U.S. Schubert, C.H. Weidl, C.N. Moorefield, G.R. Baker, G.R. Newkome, Polym. Preprints 40 (1999) 940. [6] B.J. Holliday, C.A. Mirkin, Angew. Chem. Int. Ed. 40 (2001) 2022. [7] A. Harriman, R. Ziessel, Chem. Commun. (1996) 1707. [8] A.M. DeBlass, C. DeSantis, L. Fabbrizzi, M. Liccheli, P. Pallavicini, A. Poggi, in: V. Balzani (Ed.), Supramolecular Chemistry, Kluwer Academic Publishers, London, 1995, pp. 87– 103. [9] S. Achar, C.E. Immoos, M.G. Hill, V.J. Catalano, Inorg. Chem. 36 (1997) 2314. [10] N. Sachsinger, C.D. Hall, J. Organomet. Chem. 531 (1997) 55. [11] I.R. Butler, N. Burke, L.J. Hobson, H. Findenegg, Polyhedron 19 (1992) 2435. [12] B. Ko¨nig, M. Nimtz, H. Zierg, Tetarahedron 22 (1995) 6267. [13] U. Siemeling, U. Vorfeld, B. Neumann, H.G. Stammler, P. Zanello, F. Fabrizi de Biani, Eur. J. Inorg. Chem. (1999) 5. [14] M.E. Padilla-Tosta, R. Martı´nezMa´nez, J. Soto, J.M. Loris, Tetrahedron 51 (1998) 12039. [15] I.R. Butler, S.J. McDonald, M.B. Hursthouse, K.M. AdulMalik, Polyhedron 14 (1995) 529. [16] J. Elhaı¨k, V.A. Money, S.A. Barrett, C.A. Kilner, I.R. Evans, M.A. Halcrow, J. Chem. Soc., Dalton Trans. (2003) 253. [17] J.M. Holland, J.A. McAllister, C.A. Kilner, M. Thornton-Pett, A.J. Bridgeman, M.A. Halcrow, J. Chem. Soc., Dalton Trans. (2002) 548. [18] N. Almirante, A. Cerri, G. Fedrizzi, G. Marazzi, M. Santagostinio, Tetrahedron Lett. 39 (1998) 3287. [19] P. Guionneau, M. Marchivie, G. Bravic, J.-F. Le´tard, D. Chasseau, J. Mater. Chem. (2002) 2546. [20] Unpublished data.
Synthesis, structure and physical properties of an iron(II) complex with four ferrocenyl groups Lingqin Han, Masayuki Nihei, Hiroki Oshio
*
Department of Chemistry, University of Tsukuba, Tennodai 1-1-1, Tsukuba 305-8571, Japan Received 5 October 2004; accepted 14 December 2004 Available online 29 June 2005
Abstract A novel tridentate ligand with two ferrocenyl groups and its iron(II) complex were synthesized and these structures and physical properties are presented. The pentanuclear iron(II) complex is composed of one central ion(II) ion and four ferrocenyl groups, in which the central iron(II) ion was coordinated by two tridentate ligands. Electrochemical measurement for the complex in MeCN showed irreversible redox waves, and the redox process can be understood by EC mechanism. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Tridentate nitrogen ligand; Ferrocenyl group; Polynuclear complex; Iron; Multi-nuclear complex; Iron(II)
1. Introduction Tridentate N-heterocyclic ligands have been used over the last century as effective and stable complexing agents for transition metal ions. These ligands have been used to build supramolecular coordination polymers [1– 4] and denderimers [5] leading to new materials with interesting catalytic, photochemical and redox properties [6]. Synergy of metal ions in multi-nuclear complexes have been attracting much attention from view points of studying physical and catalytic properties [7]. Ferrocenyl units are frequently employed as redox-active sites in supramolecular assemblies [8], owing to their well-behaved redox activity as well as their synthetic versatility. A number of ferrocenyl-functionalized bipyridines [9–12] and terpyridines [11,13–15] have been synthesized for these reasons. Tridentate nitrogen ligand 2,6-di(pyrazol-1-yl)pyridine and its derivatives, which are considered as analogues of terpyridines, ligate an iron(II) ion and complex molecules have been known to *
Corresponding author. Tel.: +81298534238; fax: +81298534238. E-mail address: [email protected] (H. Oshio).
0277-5387/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.poly.2005.03.120
show spin-crossover phenomena [16,17]. Thus, combination of such tridentate ligands with multi-ferrocenyl groups might be expected to be a good candidate showing multi-functionalities. We report here syntheses, structures and physical properties of novel tridentate nitrogen ligand with two ferrocenyl groups (2,6-di(3-ferrocenylpyrazol-1-yl)pyridine = dppFc2), as well as its pentanuclear iron(II) complexes, [Fe(dppFc2)2](BF4)2.
2. Experimental 2.1. Syntheses All reagents were purchased and used without further purification. 3(5)-(ferrocen-1-yl)pyrazole was synthesized according to the literature [18]. 2.1.1. 2,6-Bis(3-ferrocenylpyrazol-1-yl)pyridine(dppFc2) Potassium hydride (0.80 g, 2.0 mmol) was slowly added to a solution of 3(5)-(ferrocen-1-yl)pyrazole (0.51 g, 2.0 mmol) in dry diglyme (10 ml) and stirred for 10 min at r.t. under N2. 2,6-Dibromopyridine
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(0.24 g, 1.0 mmol) was added to the resulting slurry and the mixture was stirred for 63 h at 90 °C. The reaction mixture was quenched with 100 ml water and yellow suspension was extracted with dichloromethane. Organic phase was evaporated and obtained crude products was purified with silica-gel column chromatography with dichloromethane/hexane (1/1, v/v) as an eluent to yield 0.24 g of dppFc2 as orange powder (0.41 mmol, 41%). Anal. Calc. for C31H25N5Fe2: C, 64.28; H, 4.35; N, 12.09. Found: C, 64.18; H, 4.62; N, 12.11%. 1H NMR (270 MHz, CDCl3, 25 °C): d 8.51(d, J = 2.7, 2H, pz), 7.89 (m, 3H, py), 6.54 (d, J = 2.7, 2H, pz), 4.79 (s, 4H, Cp), 4.34 (s, 4H, Cp), 4.11 (s, 10H, Cp).
2.3. Physical measurements
2.1.2. [Fe(dppFc2)2](BF4)2 A solution of Fe(BF4)2 Æ 6H2O (44 mg, 0.13 mmol) in degassed acetone (5 ml) was added to the orange suspension of dppFc2 (35 mg, 0.06 mmol) in acetone (5 ml) and stirred for 30 min to obtain deep orange solution. Addition of diethylether to the deep orange solution gave brown powder. Recrystallization from acetonitrile and diethylether gave deep red crystals of [Fe(dppFc2)2](BF4)2. Anal. Calc. for C62H50B2F8Fe5N10 Æ 4H2O: C, 51.00; H, 4.00; N, 9.59. Found: C, 51.18; H, 3.91; N, 9.50%.
3.1. Descriptions of crystal structures: dppFc2 and [Fe(dppFc2)2](BF4)2 Æ 3CH3CN
2.2. X-ray crystallography Each crystal of dppFc2 and [Fe(dppFc2)2](BF4)2 Æ 3CH3CN was mounted on a glass capillary, and data were collected at 30 °C (Bruker SMART APEX diffractometer coupled with a CCD area detector with ˚ ) radiagraphite monochromated Mo Ka(k = 0.71073 A tion). The structure was solved by direct methods and expanded by using Fourier techniques using SHELXTL program. Crystal data: [dppFc2]: C31H25Fe2N5, M = 579.26, monoclinic space group P21/n, a = 14.246(2), ˚ , b = 90.055(3)°, V = b = 9.793(1), c = 17.910(3) A ˚ 3, Z = 4. A total of 10 770 were collected 2498.6(7) A (1.83° < h < 23.26°) of which 3580 unique reflections (Rint = 0.0546) were measured. Residual R and wR were 0.0424 and 0.1297, respectively, from the refinement on F2 with I > 2r(I). [[Fe(dppFc2)2](BF4)2 Æ 3CH3CN]: C62H54F8Fe5N13B2, M = 1506.11, monoclinic space group C2/c, a = 21.864(3), b = 21.536(2), c = 29.389(3) ˚ , b = 102.721(2)°, V = 13498(3) A ˚ 3, Z = 8. A total of A 30 072 were collected (1.34° < h < 23.30°) of which 9712 unique reflections (Rint = 0.0499) were measured. Residual R and wR were 0.0539 and 0.1459, respectively, from the refinement on F2 with I > 2r(I). Empirical absorption corrections by SADABS (G.M. Sheldrick, 1994) were carried out. In the structure analyses, nonhydrogen atoms were refined with anisotropic thermal parameters. Hydrogen atoms were included in calculated positions and refined with isotropic thermal parameters riding on those of the parent atoms.
DC magnetic susceptibility data were collected by using a Quantum Design MPMS-XL SQUID magnetometer at temperatures ranging from 1.8 to 300 K. Cyclic voltammetry measurements were carried out in a standard one-compartment cell under N2 equipped with a platinum-wire counter electrode, a grassy carbon electrode and a saturated calomel electrode (SCE) with an ALS 620A electrochemical analyzer.
3. Results and discussion
Single crystals of dppFc2 for X-ray crystallography were obtained by slow evaporation of dichloromethane/hexane solution. An ORTEP diagram of dppFc2 is shown in Fig. 1. dppFc2 crystallizes in monoclinic space group P21/n. Two ferrocenyl groups are attached to 3-position of pyrazole rings and show an anti-parallel conformation with each other. Cyclopentadienyl rings are coplanar with three heterocyclic rings of tridentate ligand moiety, which indicates weak p-conjugation between them. [Fe(dppFc2)2](BF4)2 Æ 3CH3CN crystallizes in monoclinic space group C2/c, and the unit cell contains two crystallographic independent molecules. Each central iron(II) ions show both significantly distorted octahedral surroundings composed of six nitrogen atoms from two tridentate dppFc2 ligands (Fig. 2). Coordination bond lengths are in the range of ˚ , characteristic of bond lengths for 2.134(5)–2.190(4) A a high spin iron(II) ion. Bite angles of tridentate ligands to iron(II) ion (N1–Fe1–N2 and N4–Fe1–N5) are 73.1(1)° and 72.67(9)°, respectively. Distortion can be quantified using R parameters defined by Guionneau et al. [19] R parameter is the sum of the deviations from 90° of the 12 bite angles in the coordination sphere, and the large R value represents large distortion of octahedron from ideal one. The R values are estimated to be 168.62° and 169.28° for two complex cations, which
Fig. 1. ORTEP drawing of dppFc2.
L. Han et al. / Polyhedron 24 (2005) 2409–2412
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Fig. 3. Cyclic voltammogram of [Fe(dppFc2)2](BF4)2 at a glassy carbon electrode in 0.1 mol dm3 [Bu4N][PF6]/MeCN at a scan rate of 0.1 V s1.
˚) Fig. 2. ORTEP drawing of [Fe(dppFc2)2]2+. Selected bond lengths (A and angles (°): Fe(1)–N(1) 2.134(5), Fe(1)–N(4) 2.135(5), Fe(1)–N(2) 2.181(4), Fe(1)–N(5) 2.185(3), Fe(2)–C(10) 2.018(4), N(1)–Fe(1)–N(2) 73.1(1), N(4)–Fe(1)–N(2) 106.9(1), N(1)–Fe(1)–N(5) 107.3(1), N(4)– Fe(1)–N(5) 72.67(9), N(2)#1–Fe(1)–N(5) 102.9(1), N(2)–Fe(1)–N(5) 87.1(1), N(1)–Fe(1)–N(5)#1 107.33(9), N(4)–Fe(1)–N(5)#1 72.7(1). Symmetry operation #1: x, y, z + 1/2.
show good agreement with the values for high spin iron(II) complexes with tridentate nitrogen ligands. In [Fe(dppFc2)2](BF4)2 Æ 3CH3CN, conformations of two ferrocenyl groups in the ligand are anti-parallel and two cyclopentadienyl rings are distorted from three heterocyclic rings plane with torsion angles of C8–C9– C10–C14 being 140.0(5)°, which may be due to the steric hindrance between ferrocenyl groups. 3.2. Redox properties of dppFc2 and [Fe(dppFc2)2](BF4)2 In the electrochemical measurements, the tridentate ligand dppFc2 undergoes reversible one-step 2e oxi0 dation on two ferronenyl groups at E0 ¼ 0.47 V versus SCE. In the cyclic voltammogram of [Fe(dppFc2)2](BF4)2 with potential scans between 0.00 and 0.90 V versus SCE showed two irreversible redox couples (Fig. 3). The first couple of oxidation 0 and re-reduction waves was observed at E01 ¼ 0.47 V versus SCE, which is same as the value of dppFc2. In the first couple, re-reduction current is much larger than that in oxidation wave. When the positive limit of the potential scan is 0.55 V, the large re-reduction wave does not appear (Fig. 3). While the 0 second redox couple at E02 ¼ 0.63 V showed smaller re-reduction current than that in oxidation. By comparison of the redox potentials, the first redox couple
was assigned to dppFc2 =dppFc2 2þ and irreversibility of two-step redox process in [Fe(dppFc2)2]2+ can be interpreted by the EC mechanism and dissociation equilibrium in MeCN solution as shown in Scheme 1. Before oxidation of the two ferrocenyl moiety, [Fe(dppFc2)2]2+ species is dominant in MeCN solution, while after the oxidation electronic repulsion between central iron(II) ion and ferrocenyl cations may cause the ligand dissociation. 3.3. Magnetic susceptibility measurements of [Fe(dppFc2)2](BF4)2 Æ 3CH3CN Temperature dependence of magnetic susceptibilities for [Fe(dppFc2)2](BF4)2 Æ 3CH3CN was measured in the temperature range of 2–300 K (Fig. 4). At room temperature the vmT value is 3.35 emu mol1 K, which corresponds to the spin-only value for a non-interacting high spin FeII ion. vmT values are almost constant down to 80 K, followed by sudden decrease below 50 K which is due to the zero-field splitting of the high spin iron(II) ion. The temperature independent vmT values indicates [Fe(dppFc2)2](BF4)2 Æ 3CH3CN is in the high spin state in all temperature range measured. An iron(II) complex with similar ferrocenyl-substituted tridentate ligand shows spin conversion between high- and low-spin state at Tc = ca. 370 K, in which ferrocenyl group attaches to 4-position on the pyridine ring with ester bond [20]. The absence of spin conversion in [Fe(dppFc2)2](BF4)2 Æ 3CH3CN can be attributable to the steric hindrance
[Fe(dppFc2)2]
2 dppFc2
2+
6+
+ Fe
2+
E0'2 = 0.63 V
E0'1 = 0.47 V
[Fe(dppFc2)2]2+
2 dppFc2 + Fe2+c
Scheme 1.
2412
L. Han et al. / Polyhedron 24 (2005) 2409–2412
Acknowledgements This work was partially supported by a Grant-in-aid for Scientific Researches from the Ministry of Education, Culture, Sports, Science and Technology, Japan, and by the COE and TARA projects of University of Tsukuaba.
References
Fig. 4. Plot of vmT vs. T for [Fe(dppFc2)2](BF4)2 Æ 3CH3CN.
between ferrocenyl groups which causes significant distortion of coordination geometry and stabilizes the high spin state in the central iron(II) ion.
4. Conclusion Tridentate ligand with two ferrocenyl groups give an opportunity to assemble four redox-active centre by coordination to iron(II) ion. The complex with four ferrocenyl groups undergoes two irreversible redox process in EC mechanism.
5. Supplementary material Crystallographic data reported in this paper have been deposited with Cambridge Crystallographic Data Centre as supplementary publication no. CCDC 251833 and 251834. These data can be obtained free of charge via www.ccdc.cam.ac.uk/conts/retrieving.html (or from the Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336 033; or [email protected]).
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