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Synthesis and crystal structure of a mononuclear iron(III) (η2-acetato) complex of a β-cis folded salen type ligand
Polyhedron 23 (2004) 963–967 www.elsevier.com/locate/poly
Synthesis and crystal structure of a mononuclear iron(III) (g2-acetato) complex of a b-cis folded salen type ligand q Jes us Sanmartın *, Ana M. Garcıa-Deibe, Matilde Fondo, Debora Navarro, Manuel R. Bermejo Departemento de Quımica Inorganica, Facultade de Quımica, Universidade de Santiago de Compostela, Campus Sur, Santiago de Compostela 15782, Spain Received 25 November 2003; accepted 3 December 2003
Abstract Fe(H2 SB)(g2 -O2 CMe) MeCN [H4 SB: N,N 0 -bis(5-hydroxysalicylidene)-1,4-diaminobutane] was synthesised via an electrochemical procedure and its crystal structure was determined. The metal ion assumes a distorted octahedral coordination geometry that involves the two O-atoms of a chelating acetate and the N2 O2 compartment of the bisdeprotonated diimine, which is asymmetrically folded in a b-cis arrangement. Moderate hydrogen bonds between adjacent complex molecules, which also interact with lattice acetonitrile molecules, lead to a linear interaction, predominant in the crystal packing. Ó 2004 Elsevier Ltd. All rights reserved. Keywords: Iron complexes; Acetate ligands; g2 -Coordination modes; Schiff bases; b-Cis arrangements
1. Introduction Iron(III) complexes of salen type ligands have received considerable attention as potential models for biologically important enzymes [1] and oxidation catalysts [2]. As a result, some crystal structures of Fe(salen)X complexes, where X is a potentially chelating or bridging ligand such as catecol, hydroquinone, phenanthrenesemiquinone or acetylacetone, have been described [3]. Carboxylato complexes in general and acetato ones, in particular, have also been subject to much interest [4], since they are present in some important enzymes. Among the coordinating modes of the acetate ligand, g1 and l modes are very common, whereas the chelating behaviour is not very frequent both for iron proteins and complexes [5]. In the course of our efforts to prepare building-blocks [6] capable of linking two remote moieties yielding triq Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.poly.2003.12.011. * Corresponding author. Tel.: +34-981-591076; fax: +34-981-597525. E-mail address: [email protected] (J. Sanmartın).
0277-5387/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.poly.2003.12.011
nuclear complexes [7], we have obtained Fe(H2 SB)(g2 O2 CMe) MeCN, where H4 SB is N ,N 0 -bis(5-hydroxysalicylidene)-1,4-diaminobutane (Fig. 1).
2. Experimental 2.1. Materials and methods Chemicals of the highest commercial grade available (Aldrich) were used as received. An iron metal plate (Aldrich) was washed with a dilute hydrochloric acid solution prior to electrolysis. Elemental analyses were performed on a Carlo Erba EA 1108 analyser. Mass spectra (ES) were recorded on a LC/MSD Hewlett–Packard 1100 spectrometer with methanol and formic acid (3%) as solvent. The samples had been previously dissolved in dimethylsulfoxide. NMR spectra were recorded on a Bruker 300 AC spectrometer, using DMSO as solvent. Infrared spectra were recorded as KBr pellets on a Bio-Rad FTS 135 spectrophotometer in the range of 4000–600 cm1 . Magnetic susceptibility measurements were performed
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J. Sanmartın et al. / Polyhedron 23 (2004) 963–967 (7)
(8)
HO
OH
(2)
HO
OH
(1) (6)
N
(3)
N
H4SB
(4) (5)
Fig. 1. Schematic representation of H4 SB.
at room temperature on pulverised samples using a Sherwood Scientific magnetic susceptibility balance. 2.2. Ligand synthesis N ,N 0 -Bis(2,5-dihydroxybencylidene-1,4-diaminobutane), H4 SB, has been prepared by condensation in chloroform (80 ml) of 2,5-dihydroxybenzaldehyde (1.0 g, 7.2 mmol) with 1,4-diaminobutane (0.32 g, 3.6 mmol). The solution was heated at reflux over a 3 h period and subsequently concentrated in vacuum. The orangey precipitate formed was collected by filtration, washed with diethyl ether (15 ml), and dried in vacuum. Yield: 1.11 g (93%). m.p.: 195 °C. Anal. Calc. for C18 H20 N2 O4 : C, 65.85; H, 6.10; N, 8.54. Found: C, 65.56; H, 5.89; N, 8.38%. ES-MS (0 V): 329.0 m=z (82%, Mþ ). FT-IR (KBr): m(O–H)OH 3434 (b) and 3258 (b), m(C@N) 1643 (s), m(C–H) 1461 (vs), m(C–O) 1258 (m). 1 H NMR (DMSO-d6 , 300 MHz) 12.71 (2H, s, H1 ), 8.99 (2H, s, H2 ), 8.46 (2H, s, H3 ), 6.72–6.40 (6H, m, H6–8 ), 3.60 (4H, t, H4 ), 1.67 (4H, t, H5 ). 2.3. Complex synthesis An acetonitrile solution (80 cm3 ) of H4 SB (0.100 g, 0.305 mmol), dipyridyl ketone (0.112 g, 0.609 mmol) and acetic acid (0.036 g, 0.609 mmol), containing about 40 mg of tetramethylammonium perchlorate, was electrolysed [8] for 1 h 40 min using a 10 mA current intensity with an initial voltage of 11.1 V. Caution: Although no problem has been encountered in this work, all perchlorate compounds are potentially explosive, and should be handled in small quantities with great care! After filtration of the resulting solution, its slow evaporation allowed the obtaining of blackish plate-shaped crystals of Fe(H2 SB)(g2 -O2 CMe) MeCN, suitable for X-ray diffraction studies. Yield: 0.037 g (25%). Anal. Calc. for C22 H24 FeN3 O6 : C, 54.79; H, 5.02; N, 8.71. Found: C, 53.97; H, 4.99; N, 8.53%. ES-MS: 383.1 m=z (100%, Mþ –O2 CMe–MeCN). FT-IR (KBr):m(O–H)OH 3323 (s), m(C@N) 1613 (vs), masym (CO2 ) 1558 (s), m(C–H) 1472 (vs), msym (CO2 ) 1455 (sh,s) m(C–O) 1296 (vs) cm1 . l (295 K): 6.2 BM.
Table 1 Crystal and structure O2 CMe) MeCN
refinement
Empirical formula Molecular weight Crystal system Space group a (A) b (A) c (A) b (°) 3 ) V (A T (K) Z l (Mo Ka) (mm1 ) Number of reflections collected Number of independent reflections Rint Data/restraints/parameters R1 , wR2 ½I > 2rðIÞ R1 , wR2 (all data) 3 ) Residuals (e A
data
for
Fe(H2 SB)(g2 -
C22 H24 FeN3 O6 482.29 monoclinic P 21 =n (No. 14) 10.417(2) 19.244(3) 12.179(2) 113.481(3) 2239.1(6) 293(2) 4 0.717 9331 3783 0.0589 3783/0/298 0.0495, 0.0963 0.1147, 0.1242 0.605, )0.507
2.4. Single-crystal X-ray diffraction studies X-ray data were collected at room temperature on a Bruker Smart-CCD-1000 diffractometer, using graphite monochromated Mo Ka radiation (k ¼ 0:71073 A). Data collection and processing were carried out using the Bruker S M A R T programs [9] and empirical absorption correction was applied using S A D A B S [10]. The structure was solved by direct methods, and refined by full-matrix least-squares based on F 2 using S H E L X -97 software [11]. All non-hydrogen atoms were anisotropically refined. Hydrogen atoms were included at geometrically calculated positions with thermal parameters derived from the parent atoms. Table 1 provides a summary of crystal data, data collection and refinement parameters.
3. Results and discussion The obtaining of H4 SB had been previously reported [7], however, its detailed synthesis and characterisation had not been previously described. With respect to Fe(H2 SB)(g2 -O2 CMe) MeCN, this was obtained by an electrochemical method [8] in an attempt to prepare an iron complex of di-2-pyridylmethanediol and N ,N 0 bis(5-hydroxysalicylidene)-1,4-diaminobutane ligands. The analytical, ES-MS and IR data led us to postulate the obtaining of Fe(H2 SB)(g2 -O2 CMe) MeCN, its magnetic moment at room temperature being in agreement with a high-spin d5 octahedral configuration [12]. The substantial decrease of the m(C@N) band (30 cm1 ) joint to the increase of the m(C–O) frequency (38 cm1 ), compared to the free ligand spectrum, are in agreement with the participation of the phenol and imine donor
J. Sanmartın et al. / Polyhedron 23 (2004) 963–967
atoms in the coordination to the metal centre [8]. The observation of only one of the O–H vibration bands, at about 3323 cm1 , reveals the bideprotonation of H2 SB2 , where the 5-hydroxy positions on each benzylidene ring remain protonated. The two strong bands arising from the C–O stretching vibrations show a frequency difference of 103 cm1 , which according to the literature [5e] corresponds to a chelating coordination mode of the acetate group. 3.1. Single-crystal X-ray diffraction Fe(H2 SB)(g2 -O2 CMe) MeCN
studies
of
The molecular structure of the complex is represented in Fig. 2, with the atom labelling scheme used in Table 2, which lists the most relevant bond distances and angles. The asymmetric unit consists of a discrete molecule of Fe(H2 SB)(g2 -O2 CMe) interacting with a lattice acetonitrile molecule. In Fe(H2 SB)(g2 -O2 CMe) MeCN, the iron(III) centre is coordinated in a distorted octahedral fashion by H2 SB2- acting as a typical N2 O2 -donor and a chelating acetate ligand. The latter occupies two of the equatorial coordination sites [O(5) and O(6)], whilst the Schiff base fills a meridian [O(4)N(2)N(1)] of the octahedron and the remaining equatorial position [O(1)]. This asymmetric folding of the diimine conforms to the b-cis arrangement shown in Fig. 3. Despite it being easier to achieve with a flexible spacer such as (CH2 )4 , it is a not very frequent arrangement in mononuclear octahedral complexes of salen-type ligands. The majority of these are trans arranged (Fig. 3), but the b-cis conformation is adopted by Fe(salen)X complexes when X behaves as cis chelating [3a]. Electrochemical studies on M(R2 salen)(N0 Bu)2 (where M ¼ Mo or W) allowed the con-
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Table 2 and bond angles (°) for Fe(H2 SB)(g2 Selected bond distances (A) O2 CMe) MeCN Bond lengths Fe(1)–O(1) Fe(1)–O(4) Fe(1)–N(1) Fe(1)–N(2) Fe(1)–O(5) Fe(1)–O(6) Fe(1)–C(20)
1.863(3) 1.903(3) 2.136(4) 2.081(4) 2.131(3) 2.186(3) 2.505(5)
Bond angles O(1)–Fe(1)–O(4) O(1)–Fe(1)–N(1) O(1)–Fe(1)–N(2) O(1)–Fe(1)–O(5) O(1)–Fe(1)–O(6) O(4)–Fe(1)–N(1) O(4)–Fe(1)–N(2) O(4)–Fe(1)–O(5) O(4)–Fe(1)–O(6) N(1)–Fe(1)–O(6) N(2)–Fe(1)–O(5) N(2)–Fe(1)–O(6) N(2)–Fe(1)–N(1) O(5)–Fe(1)–N(1) O(5)–Fe(1)–O(6)
96.24(14) 87.82(14) 107.49(15) 97.82(15) 157.62(14) 175.86(14) 87.85(14) 91.53(14) 90.73(13) 85.79(14) 154.60(14) 93.97(14) 90.16(15) 88.71(14) 60.64(13)
Torsion angles N(1)–C(8)–C(9)–C(10) C(8)–C(9)–C(10)–C(11) C(9)–C(10)–C(11)–N(2)
)69.9(7) 63.1(8) )88.9(6)
H bonds D–H A O(2)–H(2A) O(6)a O(3)–H(3A) N(1S)
d(D–H) 0.68(5) 0.67(5)
d(H A) d(D A) \(DHA) 2.09(6) 2.741(6) 162(7) 2.15(6) 2.782(9) 158(8)
a Symmetry transformations used to generate equivalent atoms: x 1; y; z.
Fig. 3. Schematic representation and nomenclature of two possible geometries for complexes of the type M(N2 O2 )X2 .
Fig. 2. ORTEP view of Fe(H2 SB)(g2 -O2 CMe). Ellipsoids are drawn at 50% probability. The acetonitrile molecule and parentheses are omitted for clarity.
clusion that the adoption of the b-cis conformation is apparently dictated by electronic rather than steric factors [13]. Although the ribonucleotide reductase iron protein (RR B2) [5c] and other model polynuclear iron complexes, such as [Fe2 (Py)2 (O2 CMe)5 (OH2 )]Et4 N or [Fe3 (O)(O2 CMe)7 (OH2 )]Et4 N [5d], contain terminal g2 acetato groups, this coordination mode is rather infrequent both for Fe(III) and Fe(II) ions [5]. Thus, to the best of our knowledge Fe(H2 SB)(g2 -O2 CMe) MeCN is the first example of a mononuclear iron (g2 -acetato)
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Fig. 4. Stick representation of three molecules of Fe(H2 SB)(g2 -O2 CMe) connected by H bonds between the acetate and one of the outer phenol groups. Each complex molecule is also interacting with an acetonitrile molecule through the remaining protonated phenol group but they have been omitted for clarity.
complex, at least of salen type ligands. This cis chelating behaviour of the acetate ligand could be allowed by the flexibility of the Schiff base ligand. Thus, [FeIII (salen)(lO2 CMe)]n shows linear polymerisation through bridging bidentate acetate groups [14] with a typical trans arrangement of the more rigid ligand (Fig. 3). By contrast, salbn2 , which contains the same spacer as H2 SB2 , assumes the b-cis folding even with monodentate ligands such as MeO in [Fe(salbn)(l-OMe)]2 [12]. The flexibility, as well as the strain, of the gauche conformed (CH2 )4 chain of H2 SB2 is revealed by the torsion angles, that also show that one of the bonds is almost eclipsed (Table 2). In general, although the Fe–N and Fe–O distances related to the Schiff base ligand could be considered within the usual ranges reported for Fe(III) complexes of N,O-donor ligands [14–16], the equatorial bonds (in trans positions with respect to the acetate group) are shorter than the apical ones, Fe(1)–(O1) being especially short. With regard to the acetate group, both angles and distances are within the common ranges found for Fe(III) complexes [5d,15] and even for Fe(II) ones [5b,5d], since they are not very different in that respect. The presence of the outer phenol groups is significant, since they give rise to an H bonding scheme, apart from some C–H p interactions, that is essential for the crystal packing. One of the external phenolic O atoms, O(3), interacts with an acetonitrile molecule, whereas the other one is connected to an acetate group of a neighbouring complex molecule. This interaction leads to an almost linear ‘‘polymerisation’’ based on moderate H bonds [17] along the a direction (Fig. 4). Since the external phenolic groups, O(2) and O(3), remain protonated and uncoordinated, they allow the additional, and previously confirmed, possibility to behave as a building-block [7].
4. Conclusions We have synthesised by an electrochemical procedure Fe(H2 SB)(g2 -O2 CMe) MeCN. The Fe(III) ion assumes a distorted octahedral coordination geometry, afforded by the N2 O2 -donor set of the dianionic diimine and two oxygen atoms of an acetate group. Despite its simplicity, two features of Fe(H2 SB)(g2 -O2 CMe) MeCN can provide some interest from the point of view of the coordination chemistry: (i) the acetate is coordinated in a hitherto unfamiliar g2 mode for mononuclear iron complexes; (ii) the Schiff base ligand is b-cis folded. These seem to be related, since the atypical chelating coordination of the acetate group could be favoured by the diimine flexibility, and the cis chelating behaviour of the acetate involves the b-cis folding of the Schiff base.
5. Supplementary material Crystallographic data for the structural analysis are deposited with the Cambridge Crystallographic Data Centre, CCDC with deposition number CCDC 220996. Copies of this information can be obtained from Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK (fax: +44-1223-336033; e-mail: [email protected] or www: http://www. ccdc.cam.ac.uk).
Acknowledgements The authors are grateful to Xunta de Galicia (PGIDIT03PXIB20901PR) for financial support.
J. Sanmartın et al. / Polyhedron 23 (2004) 963–967
References [1] (a) L. Que Jr., A.E. True, Prog. Inorg. Chem. 38 (1990) 97; (b) L. Que Jr., Coord. Chem. Rev. 50 (1983) 73. [2] A. Boettcher, M.W. Grinstaff, J.A. Labinger, H.B. Gray, J. Mol. Catal. A 113 (1996) 191. [3] (a) R.B. Lauffer, R.H. II Heistand, L. Que Jr., Inorg. Chem. 22 (1983) 50; (b) R.H. II Heistand, A.L. Roe, L. Que Jr., Inorg. Chem. 21 (1982) 676. [4] (a) W.H. Armstrong, A. Spool, G.C. Papaevthymiou, R.B. Frankel, S.J. Lippard, J. Am. Chem. Soc. 106 (1984) 3653; (b) I.M. Arafa, H.M. Goff, S.S. David, B.P. Murch, L. Que Jr., Inorg. Chem. 26 (1987) 2779. [5] (a) S. Ogo, R. Yamahara, M. Roach, T. Suenobu, M. Aki, T. Ogura, T. Kitagawa, H. Masuda, S. Fukuzumi, Y. Watanabe, Inorg. Chem. 41 (2002) 5513; (b) D. Coucouvanis, R.A. III Reynolds, W.R. Dunham, J. Am. Chem. Soc. 117 (1995) 7570; (c) P. Nordlund, H. Eklund, J. Mol. Biol. 232 (1993) 123; (d) R.A. Reynolds III, W.R. Dunham, D. Coucouvanis, Inorg. Chem. 37 (1998) 1232; (e) P. Poganiuch, S. Liu, G.C. Papaefthymiou, S.J. Lippard, J. Am. Chem. Soc. 113 (1991) 4645. [6] J. Sanmartın, A.M. Garcıa-Deibe, M. Fondo, D. Navarro, M.R. Bermejo, Inorg. Chem. Commun. 7 (2004) 311. [7] (a) S.M. Kagwanja, D.J. Jones, J.P. Maher, J.A. McCleverty, Polyhedron 13 (1994) 2615;
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(b) S.M. Kagwanja, D.J. Jones, J.A. McCleverty, Polyhedron 16 (1997) 1439. J. Sanmartın, M.R. Bermejo, A.M. Garcıa-Deibe, M. Maneiro, C. Lage, A.J. Costa-Filho, Polyhedron 19 (2000) 185. S M A R T : Bruker Molecular Analysis Research Tool V 5.5054, Bruker AXS Inc., Madison, WI, USA, 1998. S A D A B S : Area-Detector Absorption Correction, Siemens Industrial Automation Inc., Madison, WI, USA, 1996. G.M. Sheldrick, S H E L X -97 (S H E L X S -97 and S H E L X L -97): Programs for Crystal Structure Analyses, University of G€ ottingen, G€ ottingen, Germany, 1998. S. Negoro, H. Asada, M. Fujiwara, T. Matsushita, Inorg. Chem. Commun. 6 (2003) 357. M. Motevalli, A.D. Oduwole, B.C. Parkin, R. Ramnauth, A.C. Sullivan, N. Kaltsoyannis, J. Chem. Soc., Dalton Trans. (2003) 3591. G. Liu, A.M. Arif, F.W. Bruenger, S.C. Miller, Inorg. Chim. Acta 287 (1999) 109. M. Mikuriya, Y. Hashimoto, S. Nakashima, Chem. Commun. (1996) 295. (a) L. He, Q.-F. Shi, S.-H. Gou, M. Qian, C.-Y. Duan, Acta Crystallogr., Sect. C 46 (1999) 46; (b) T. Nakamura, E. Kuranuki, K. Niwa, M. Fujiwara, T. Matsushita, Chem. Lett. (2000) 248; (c) J. Reglinski, M. Patykiewicz, Acta Crystallogr., Sect. E 58 (2002) m543; (d) Y. Miyasato, Y. Nogami, M. Ohba, H. Sakiyama, H. Okawa, Bull. Chem. Soc. Jpn. 76 (2003) 1009. T. Steiner, Angew. Chem. Int. Ed. 41 (2002) 48.
Synthesis and crystal structure of a mononuclear iron(III) (g2-acetato) complex of a b-cis folded salen type ligand q Jes us Sanmartın *, Ana M. Garcıa-Deibe, Matilde Fondo, Debora Navarro, Manuel R. Bermejo Departemento de Quımica Inorganica, Facultade de Quımica, Universidade de Santiago de Compostela, Campus Sur, Santiago de Compostela 15782, Spain Received 25 November 2003; accepted 3 December 2003
Abstract Fe(H2 SB)(g2 -O2 CMe) MeCN [H4 SB: N,N 0 -bis(5-hydroxysalicylidene)-1,4-diaminobutane] was synthesised via an electrochemical procedure and its crystal structure was determined. The metal ion assumes a distorted octahedral coordination geometry that involves the two O-atoms of a chelating acetate and the N2 O2 compartment of the bisdeprotonated diimine, which is asymmetrically folded in a b-cis arrangement. Moderate hydrogen bonds between adjacent complex molecules, which also interact with lattice acetonitrile molecules, lead to a linear interaction, predominant in the crystal packing. Ó 2004 Elsevier Ltd. All rights reserved. Keywords: Iron complexes; Acetate ligands; g2 -Coordination modes; Schiff bases; b-Cis arrangements
1. Introduction Iron(III) complexes of salen type ligands have received considerable attention as potential models for biologically important enzymes [1] and oxidation catalysts [2]. As a result, some crystal structures of Fe(salen)X complexes, where X is a potentially chelating or bridging ligand such as catecol, hydroquinone, phenanthrenesemiquinone or acetylacetone, have been described [3]. Carboxylato complexes in general and acetato ones, in particular, have also been subject to much interest [4], since they are present in some important enzymes. Among the coordinating modes of the acetate ligand, g1 and l modes are very common, whereas the chelating behaviour is not very frequent both for iron proteins and complexes [5]. In the course of our efforts to prepare building-blocks [6] capable of linking two remote moieties yielding triq Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.poly.2003.12.011. * Corresponding author. Tel.: +34-981-591076; fax: +34-981-597525. E-mail address: [email protected] (J. Sanmartın).
0277-5387/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.poly.2003.12.011
nuclear complexes [7], we have obtained Fe(H2 SB)(g2 O2 CMe) MeCN, where H4 SB is N ,N 0 -bis(5-hydroxysalicylidene)-1,4-diaminobutane (Fig. 1).
2. Experimental 2.1. Materials and methods Chemicals of the highest commercial grade available (Aldrich) were used as received. An iron metal plate (Aldrich) was washed with a dilute hydrochloric acid solution prior to electrolysis. Elemental analyses were performed on a Carlo Erba EA 1108 analyser. Mass spectra (ES) were recorded on a LC/MSD Hewlett–Packard 1100 spectrometer with methanol and formic acid (3%) as solvent. The samples had been previously dissolved in dimethylsulfoxide. NMR spectra were recorded on a Bruker 300 AC spectrometer, using DMSO as solvent. Infrared spectra were recorded as KBr pellets on a Bio-Rad FTS 135 spectrophotometer in the range of 4000–600 cm1 . Magnetic susceptibility measurements were performed
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J. Sanmartın et al. / Polyhedron 23 (2004) 963–967 (7)
(8)
HO
OH
(2)
HO
OH
(1) (6)
N
(3)
N
H4SB
(4) (5)
Fig. 1. Schematic representation of H4 SB.
at room temperature on pulverised samples using a Sherwood Scientific magnetic susceptibility balance. 2.2. Ligand synthesis N ,N 0 -Bis(2,5-dihydroxybencylidene-1,4-diaminobutane), H4 SB, has been prepared by condensation in chloroform (80 ml) of 2,5-dihydroxybenzaldehyde (1.0 g, 7.2 mmol) with 1,4-diaminobutane (0.32 g, 3.6 mmol). The solution was heated at reflux over a 3 h period and subsequently concentrated in vacuum. The orangey precipitate formed was collected by filtration, washed with diethyl ether (15 ml), and dried in vacuum. Yield: 1.11 g (93%). m.p.: 195 °C. Anal. Calc. for C18 H20 N2 O4 : C, 65.85; H, 6.10; N, 8.54. Found: C, 65.56; H, 5.89; N, 8.38%. ES-MS (0 V): 329.0 m=z (82%, Mþ ). FT-IR (KBr): m(O–H)OH 3434 (b) and 3258 (b), m(C@N) 1643 (s), m(C–H) 1461 (vs), m(C–O) 1258 (m). 1 H NMR (DMSO-d6 , 300 MHz) 12.71 (2H, s, H1 ), 8.99 (2H, s, H2 ), 8.46 (2H, s, H3 ), 6.72–6.40 (6H, m, H6–8 ), 3.60 (4H, t, H4 ), 1.67 (4H, t, H5 ). 2.3. Complex synthesis An acetonitrile solution (80 cm3 ) of H4 SB (0.100 g, 0.305 mmol), dipyridyl ketone (0.112 g, 0.609 mmol) and acetic acid (0.036 g, 0.609 mmol), containing about 40 mg of tetramethylammonium perchlorate, was electrolysed [8] for 1 h 40 min using a 10 mA current intensity with an initial voltage of 11.1 V. Caution: Although no problem has been encountered in this work, all perchlorate compounds are potentially explosive, and should be handled in small quantities with great care! After filtration of the resulting solution, its slow evaporation allowed the obtaining of blackish plate-shaped crystals of Fe(H2 SB)(g2 -O2 CMe) MeCN, suitable for X-ray diffraction studies. Yield: 0.037 g (25%). Anal. Calc. for C22 H24 FeN3 O6 : C, 54.79; H, 5.02; N, 8.71. Found: C, 53.97; H, 4.99; N, 8.53%. ES-MS: 383.1 m=z (100%, Mþ –O2 CMe–MeCN). FT-IR (KBr):m(O–H)OH 3323 (s), m(C@N) 1613 (vs), masym (CO2 ) 1558 (s), m(C–H) 1472 (vs), msym (CO2 ) 1455 (sh,s) m(C–O) 1296 (vs) cm1 . l (295 K): 6.2 BM.
Table 1 Crystal and structure O2 CMe) MeCN
refinement
Empirical formula Molecular weight Crystal system Space group a (A) b (A) c (A) b (°) 3 ) V (A T (K) Z l (Mo Ka) (mm1 ) Number of reflections collected Number of independent reflections Rint Data/restraints/parameters R1 , wR2 ½I > 2rðIÞ R1 , wR2 (all data) 3 ) Residuals (e A
data
for
Fe(H2 SB)(g2 -
C22 H24 FeN3 O6 482.29 monoclinic P 21 =n (No. 14) 10.417(2) 19.244(3) 12.179(2) 113.481(3) 2239.1(6) 293(2) 4 0.717 9331 3783 0.0589 3783/0/298 0.0495, 0.0963 0.1147, 0.1242 0.605, )0.507
2.4. Single-crystal X-ray diffraction studies X-ray data were collected at room temperature on a Bruker Smart-CCD-1000 diffractometer, using graphite monochromated Mo Ka radiation (k ¼ 0:71073 A). Data collection and processing were carried out using the Bruker S M A R T programs [9] and empirical absorption correction was applied using S A D A B S [10]. The structure was solved by direct methods, and refined by full-matrix least-squares based on F 2 using S H E L X -97 software [11]. All non-hydrogen atoms were anisotropically refined. Hydrogen atoms were included at geometrically calculated positions with thermal parameters derived from the parent atoms. Table 1 provides a summary of crystal data, data collection and refinement parameters.
3. Results and discussion The obtaining of H4 SB had been previously reported [7], however, its detailed synthesis and characterisation had not been previously described. With respect to Fe(H2 SB)(g2 -O2 CMe) MeCN, this was obtained by an electrochemical method [8] in an attempt to prepare an iron complex of di-2-pyridylmethanediol and N ,N 0 bis(5-hydroxysalicylidene)-1,4-diaminobutane ligands. The analytical, ES-MS and IR data led us to postulate the obtaining of Fe(H2 SB)(g2 -O2 CMe) MeCN, its magnetic moment at room temperature being in agreement with a high-spin d5 octahedral configuration [12]. The substantial decrease of the m(C@N) band (30 cm1 ) joint to the increase of the m(C–O) frequency (38 cm1 ), compared to the free ligand spectrum, are in agreement with the participation of the phenol and imine donor
J. Sanmartın et al. / Polyhedron 23 (2004) 963–967
atoms in the coordination to the metal centre [8]. The observation of only one of the O–H vibration bands, at about 3323 cm1 , reveals the bideprotonation of H2 SB2 , where the 5-hydroxy positions on each benzylidene ring remain protonated. The two strong bands arising from the C–O stretching vibrations show a frequency difference of 103 cm1 , which according to the literature [5e] corresponds to a chelating coordination mode of the acetate group. 3.1. Single-crystal X-ray diffraction Fe(H2 SB)(g2 -O2 CMe) MeCN
studies
of
The molecular structure of the complex is represented in Fig. 2, with the atom labelling scheme used in Table 2, which lists the most relevant bond distances and angles. The asymmetric unit consists of a discrete molecule of Fe(H2 SB)(g2 -O2 CMe) interacting with a lattice acetonitrile molecule. In Fe(H2 SB)(g2 -O2 CMe) MeCN, the iron(III) centre is coordinated in a distorted octahedral fashion by H2 SB2- acting as a typical N2 O2 -donor and a chelating acetate ligand. The latter occupies two of the equatorial coordination sites [O(5) and O(6)], whilst the Schiff base fills a meridian [O(4)N(2)N(1)] of the octahedron and the remaining equatorial position [O(1)]. This asymmetric folding of the diimine conforms to the b-cis arrangement shown in Fig. 3. Despite it being easier to achieve with a flexible spacer such as (CH2 )4 , it is a not very frequent arrangement in mononuclear octahedral complexes of salen-type ligands. The majority of these are trans arranged (Fig. 3), but the b-cis conformation is adopted by Fe(salen)X complexes when X behaves as cis chelating [3a]. Electrochemical studies on M(R2 salen)(N0 Bu)2 (where M ¼ Mo or W) allowed the con-
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Table 2 and bond angles (°) for Fe(H2 SB)(g2 Selected bond distances (A) O2 CMe) MeCN Bond lengths Fe(1)–O(1) Fe(1)–O(4) Fe(1)–N(1) Fe(1)–N(2) Fe(1)–O(5) Fe(1)–O(6) Fe(1)–C(20)
1.863(3) 1.903(3) 2.136(4) 2.081(4) 2.131(3) 2.186(3) 2.505(5)
Bond angles O(1)–Fe(1)–O(4) O(1)–Fe(1)–N(1) O(1)–Fe(1)–N(2) O(1)–Fe(1)–O(5) O(1)–Fe(1)–O(6) O(4)–Fe(1)–N(1) O(4)–Fe(1)–N(2) O(4)–Fe(1)–O(5) O(4)–Fe(1)–O(6) N(1)–Fe(1)–O(6) N(2)–Fe(1)–O(5) N(2)–Fe(1)–O(6) N(2)–Fe(1)–N(1) O(5)–Fe(1)–N(1) O(5)–Fe(1)–O(6)
96.24(14) 87.82(14) 107.49(15) 97.82(15) 157.62(14) 175.86(14) 87.85(14) 91.53(14) 90.73(13) 85.79(14) 154.60(14) 93.97(14) 90.16(15) 88.71(14) 60.64(13)
Torsion angles N(1)–C(8)–C(9)–C(10) C(8)–C(9)–C(10)–C(11) C(9)–C(10)–C(11)–N(2)
)69.9(7) 63.1(8) )88.9(6)
H bonds D–H A O(2)–H(2A) O(6)a O(3)–H(3A) N(1S)
d(D–H) 0.68(5) 0.67(5)
d(H A) d(D A) \(DHA) 2.09(6) 2.741(6) 162(7) 2.15(6) 2.782(9) 158(8)
a Symmetry transformations used to generate equivalent atoms: x 1; y; z.
Fig. 3. Schematic representation and nomenclature of two possible geometries for complexes of the type M(N2 O2 )X2 .
Fig. 2. ORTEP view of Fe(H2 SB)(g2 -O2 CMe). Ellipsoids are drawn at 50% probability. The acetonitrile molecule and parentheses are omitted for clarity.
clusion that the adoption of the b-cis conformation is apparently dictated by electronic rather than steric factors [13]. Although the ribonucleotide reductase iron protein (RR B2) [5c] and other model polynuclear iron complexes, such as [Fe2 (Py)2 (O2 CMe)5 (OH2 )]Et4 N or [Fe3 (O)(O2 CMe)7 (OH2 )]Et4 N [5d], contain terminal g2 acetato groups, this coordination mode is rather infrequent both for Fe(III) and Fe(II) ions [5]. Thus, to the best of our knowledge Fe(H2 SB)(g2 -O2 CMe) MeCN is the first example of a mononuclear iron (g2 -acetato)
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Fig. 4. Stick representation of three molecules of Fe(H2 SB)(g2 -O2 CMe) connected by H bonds between the acetate and one of the outer phenol groups. Each complex molecule is also interacting with an acetonitrile molecule through the remaining protonated phenol group but they have been omitted for clarity.
complex, at least of salen type ligands. This cis chelating behaviour of the acetate ligand could be allowed by the flexibility of the Schiff base ligand. Thus, [FeIII (salen)(lO2 CMe)]n shows linear polymerisation through bridging bidentate acetate groups [14] with a typical trans arrangement of the more rigid ligand (Fig. 3). By contrast, salbn2 , which contains the same spacer as H2 SB2 , assumes the b-cis folding even with monodentate ligands such as MeO in [Fe(salbn)(l-OMe)]2 [12]. The flexibility, as well as the strain, of the gauche conformed (CH2 )4 chain of H2 SB2 is revealed by the torsion angles, that also show that one of the bonds is almost eclipsed (Table 2). In general, although the Fe–N and Fe–O distances related to the Schiff base ligand could be considered within the usual ranges reported for Fe(III) complexes of N,O-donor ligands [14–16], the equatorial bonds (in trans positions with respect to the acetate group) are shorter than the apical ones, Fe(1)–(O1) being especially short. With regard to the acetate group, both angles and distances are within the common ranges found for Fe(III) complexes [5d,15] and even for Fe(II) ones [5b,5d], since they are not very different in that respect. The presence of the outer phenol groups is significant, since they give rise to an H bonding scheme, apart from some C–H p interactions, that is essential for the crystal packing. One of the external phenolic O atoms, O(3), interacts with an acetonitrile molecule, whereas the other one is connected to an acetate group of a neighbouring complex molecule. This interaction leads to an almost linear ‘‘polymerisation’’ based on moderate H bonds [17] along the a direction (Fig. 4). Since the external phenolic groups, O(2) and O(3), remain protonated and uncoordinated, they allow the additional, and previously confirmed, possibility to behave as a building-block [7].
4. Conclusions We have synthesised by an electrochemical procedure Fe(H2 SB)(g2 -O2 CMe) MeCN. The Fe(III) ion assumes a distorted octahedral coordination geometry, afforded by the N2 O2 -donor set of the dianionic diimine and two oxygen atoms of an acetate group. Despite its simplicity, two features of Fe(H2 SB)(g2 -O2 CMe) MeCN can provide some interest from the point of view of the coordination chemistry: (i) the acetate is coordinated in a hitherto unfamiliar g2 mode for mononuclear iron complexes; (ii) the Schiff base ligand is b-cis folded. These seem to be related, since the atypical chelating coordination of the acetate group could be favoured by the diimine flexibility, and the cis chelating behaviour of the acetate involves the b-cis folding of the Schiff base.
5. Supplementary material Crystallographic data for the structural analysis are deposited with the Cambridge Crystallographic Data Centre, CCDC with deposition number CCDC 220996. Copies of this information can be obtained from Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK (fax: +44-1223-336033; e-mail: [email protected] or www: http://www. ccdc.cam.ac.uk).
Acknowledgements The authors are grateful to Xunta de Galicia (PGIDIT03PXIB20901PR) for financial support.
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