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Structure and properties of iron–cyclam complex of 2-aminophenol
Polyhedron 20 (2001) 493– 499 www.elsevier.nl/locate/poly
Structure and properties of iron–cyclam complex of 2-aminophenol Luiz C.G. Vasconcellos a,*, Cristiane P. Oliveira a, Eduardo E. Castellano b, Javier Ellena b, I´caro S. Moreira a b
a Departamento de Quı´mica Orgaˆnica e Inorgaˆnica, Uni6ersidade Federal do Ceara´, Cx Postal 12200, 60455 -760 Fortaleza, Ceara´, Brazil Departamento de Fı´sica e Informa´tica, Instituto de Fı´sica de Sa˜o Carlos, Uni6ersidade de Sa˜o Paulo, C.P. 369, 13560 Sa˜o Carlos (SP), Brazil
Received 14 March 2000; accepted 25 October 2000
Abstract The cis-[Fe(cyclam)Cl2]Cl and trans-[Fe(cyclam)Cl2]BF4 complexes were prepared by a new synthetic procedure. These isomers were characterized by elemental analysis, electronic spectroscopy (umax = 238, 334 and umax = 238, 302 and 355 nm for cis- and trans-[Fe(cyclam)Cl2]+, respectively) and electrochemistry (E1/2 = 212 and 83 mV; vs. Ag AgCl, KCl 3.5 M, 25°C v =0.1 M NaTFA, pH 3.0 for cis- and trans-[Fe(cyclam)Cl2]+, respectively). The reaction of cis-[Fe(cyclam)Cl2]+ isomer with 2-aminophenol (catH3) ligand yields the cis-[Fe(cyclam)(qH)]2 + complex, where (qH) is the quinonoid oxidized form of the (catH3) ligand that is originated by an intramolecular electron transfer reaction. The cis-[Fe(cyclam)(qH)](PF6)2 complex crystallizes in the space group P21/n, with two different values for Fe–N bond length, due to the s and p distinct contributions. The electrochemical analysis of this species shows three one-electron sequential processes: E1/2 (I) = −316 mV, E1/2 (II) = +361 mV and E1/2 (III)= + 1.013 mV assigned to (sqH/catH3), (qH/sqH) and (Fe(III)/Fe(II)) redox process, respectively. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Iron– cyclam; Quinone-related species; 2-Aminophenol; 1,4,8,11-Tetraazacyclotetradecane; Macrocycle; X-ray structures
1. Introduction The chemistry of transition-metal complexes involving catechol-related ligands such as 2-aminophenol has been of great interest in the development of multi-electron transfer catalyst for O2 reduction, fuel cells and photosensitizers [1], as well as on the studies of oxidative cleavage of biomolecules [2,3] and cyclohexene epoxidation [4]. The electron delocalization between the metal and these o-benzoquinone-related ligands, which is the main characteristic of this class of compounds, depends on the extension of mixing of the metal d orbitals and the molecular orbitals of the ligand [5]. Changes in the oxidation state of the ligand can alter the extend of orbital mixture [6]. Studies on the iron – cyclam systems reported in the literature are rare when compared with those on the ruthenium – cyclam, ruthenium –tetra* Corresponding author. Fax: +55-85-288-9978. E-mail address: [email protected] (L.C.G. Vasconcellos).
amines and ruthenium –bis(2,2%-bipyridine) complexes with ortho-phenylene ligands [1,7,8]. The major problem with the study of iron –amine derivative complexes is their preparation due to the basic medium required for the reactions causing iron hydrolysis. As a result, the research on these systems did not expand comparatively to the ruthenium –amine chemistry. The use of the macrocyclic ligand, the four-dentate all saturated 1,4,8,11-tetraazacyclotetradecane (cyclam, Scheme 1), would mimic some of the amines properties and proportionates a very stable environment around the metal center due to its polydentate attributes [9]. Therefore, the study of [Fe(cyclam)L]n + complexes is of interest to establish a suitable comparative system with ruthenium –amine derivatives, regarding their properties and reactivity. The 2-aminophenol (catH3) can be sequentially oxidized to the semiquinone (sqH) and quinone (qH) related forms (Scheme 2). The (catH3) form behaves as a p-innocent ligand once it has no p* orbital available to accept electron
0277-5387/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 2 7 7 - 5 3 8 7 ( 0 0 ) 0 0 6 2 1 - 5
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density from the metal when coordinated to both, nitrogen and oxygen atoms. However, in the semiquinonoid (sqH) and quinonoid (qH) oxidation states, a large p system is spread all over the molecule, including the metal binding sites of the ligand. Once established the metal to ligand p back-bonding, a remarkable chemical stabilization is provided due to the aromatic behavior of the pentagon ring having the iron atom as participant [6,10].
2. Experimental
2.1. Chemical and reagents All chemicals were analytical reagent grade or better and were used without further purification. Iron dichloride (FeCl2·4H2O) from Aldrich was the start material for the synthesis of the complexes. The cyclam (1,4,8,11-tetraazacyclotetradecane) and 2-aminophenol ligands were also purchased from Aldrich. Double distilled water was used throughout the experiments.
2.2. Synthesis Although the syntheses of the cis and trans-[Fe(cyclam)Cl2]+ isomer complexes are reported in the literature [11], we have developed a new and suitable preparation method.
2.2.1. cis-[Fe(cyclam)Cl2]Cl A total of 280 mg (1.4 mmol) of FeCl2·4H2O were dissolved in 20 ml of methanol and added to a 20 ml of degassed methanol solution containing 282 mg (1.4 mmol) of the cyclam ligand. Although a dark green
color has immediately developed, the reaction was allowed to proceed during 3 h under argon flow at room temperature (r.t). The resulting solution was then exposed to the atmospheric oxygen. The yellow powder formed, the cis-[Fe(cyclam)Cl2]Cl complex was collected by filtration, washed with cold methanol, dried and stored under vacuum. Yield better than 45%. The dark brown filtrate resulted from the cis-[Fe(cyclam)Cl2]Cl separation was further used in the synthesis of the trans-[Fe(cyclam)Cl2](BF4). Elemental analysis: Anal. Calc. for C10H24Cl3FeN4: C, 33.1; H, 6.6; N, 15.4. Found: C, 32.9; H, 6.7; N, 15.2%.
2.2.2. trans-[Fe(cyclam)Cl2](BF4) The dark brown filtrate from the cis-[Fe(cyclam)Cl2]Cl preparation was refluxed for 1 h and then exposed to the atmospheric oxygen at r.t. The addition of 2 ml of a 45% aqueous solution of HBF4 originated a yellow –greenish solid that was collected by filtration, washed with cold methanol, dried and stored under vacuum. Yield better than 40%. Elemental analysis: Anal. Calc. for C10H24BCl2F4FeN4: C, 30.0; H, 5.8; N, 13.5. Found: C, 29.7; H, 5.6; N, 13.7%. 2.2.3. cis-[Fe(cyclam)(qH)](PF6)2 The cis-[Fe(cyclam)(qH)](PF6)2 complex was synthesized by mixing equimolar amounts of cis-[Fe(cyclam)Cl2]Cl and 2-aminophenol in 1:1 methanol –water, under argon atmosphere. In a typical preparation, 10 ml of the solvent mixture where used to dissolve 100.0 mg (0.24 mmol) of cis-[Fe(cyclam)Cl2]Cl followed by addition of 26.3 mg (0.24 mmol) of 2-aminophenol under vigorous stirring. The color change from pale yellow to dark blue was immediately developed. After 5 h of reaction the product was precipitated by adding 100 mg of NH4PF6 followed by the addition of 20 ml of cold 1:1 methanol –diethyl ether solvent mixture. The dark blue microcrystals formed were separated by filtration, dried and stored under vacuum. Yield better than 60%. The purity of the product was verified by liquid chromatography (HPLC). Elemental analysis: Anal. Calc. for C16H29F12FeP2N5O: C, 29.4; H, 4.4; N, 10.7. Found: C, 29.7; H, 4.3; N, 10.5%. 2.3. Apparatus and techniques
Scheme 1. 1,4,8,11-Tetraazacyclotetradecane (cyclam).
Scheme 2. The 2-aminophenol and its redox forms.
The electronic spectra were acquired in a Hitachi U-2000 spectrophotometer. The analysis of the electronic spectra was also performed using Gaussian deconvolution procedures in order to obtain more detailed results of energy and extinction coefficients values. The electrochemical experiments were performed on a BAS-100BW electrochemical analyzer from Bioanalytical Systems, using a regular 5 ml electrochemical cell, with glassy carbon, platinum and Ag AgCl (3.5 M
L.C.G. Vasconcellos et al. / Polyhedron 20 (2001) 493–499 Table 1 Crystal data, data collection details and structure refinement results Color/shape Empirical formula Temperature Crystal system Space group Unit cell dimensions (25 reflections in the 16B2qB35 range) a (A, ) b (A, ) c (A, ) i (°) V (A, 3) Z Dcalc (Mg m−3) Absorption coefficient (mm−1) Diffactometer/scan Radiation/wavelength (A, ) Crystal size (mm3) q Range for data collection (°) Index ranges Reflections collected Independent/observed reflections [IB2|(I)] Absorption correction [20] Refinement method Computing a Goodness-of-fit on F 2 weight parameters Final R indices [I\2|(I)] R indices (all data)
SHELXL-97
red/prism C16H29N5OFe[PF6]2 293(2) K monoclinic P21/n
9.248(1) 16.133(3) 17.521(2) 103.94(1) 2537.1(6) 4 1.710 6.994 Enraf–Nonius CAD-4/ –2q Cu Ka (graphite monochrom.)/1.54184 0.15×0.065×0.025 3.7852q565.0 −15h510, −15k518, −205l520 5571 4301 [Rint = 0.0533]/2420 analytical full-matrix least-squares on F 2 CAD-4 [21], XCAD4 [22], SHELXS-97 [23], SHELXL-97 [24] 1.288 0.120, 2.550 R1 = 0.0834, wR2 = 0.2337 R1 = 0.1599, wR2 = 0.2696
a Data collection, data reduction, structure solution and refinement, respectively.
KCl) as working, auxiliary and reference electrodes, respectively. Aqueous solutions of sodium trifluoroacetate, v = 0.1 M, pH 3.0, containing 1.0×10 − 3 M of the complex were commonly used on the experiments. Any other electrolyte solution used is specified in the text. Hydrogen and carbon NMR spectra were recorded in a Bruker AC500 spectrometer. D2O was used as solvent with small amounts of DSS as internal standard. The infrared spectra were taken on a Shimadzu spectrometer, FTIR-8300, using solid sample dissolved in KBr pellets. Liquid chromatograms (HPLC) were acquired on a Shimadzu chromatograph equipped with a model LC10AD pump and SPD-M10A photo diode-array UV – Vis spectrophotometer detector. A SH wrisorb ODS column (250×4.6 mm i.d., 5 mm particles; Alltech) was used in all analytical experiments.
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3. Structure determination and refinement Crystal data, collection, reduction and refinement results for the cis-[Fe(cyclam)(qH)](PF6)2 complex are summarized in Table 1. Each hydrogen atom was stereochemicaly positioned and refined riding on the carbon atom to which it is bonded. In the refinement procedure all non-hydrogen atoms were treated anisotropically, while the hydrogen atoms were set isotropically with an isotropic thermal parameter 1.2 times the equivalent isotropic displacement parameter of the carbon atom to which each one is bonded. The two hexafluorophosphate anions presented some degree of disorder. This disorder is higher in the PF6 anion centered on P2, whose fluorine atoms were modeled with partial occupation factors.
4. Results and discussion The alternative synthetic route for cis-[Fe(cyclam)Cl2]Cl and trans-[Fe(cyclam)Cl2](BF4) was developed and the results compared with those reported in the literature [12,13]. This new procedure is quite simple and selective on the preparation of both isomers. The pure cis-[Fe(cyclam)Cl2]Cl complex was easily obtained by direct reaction of FeCl2·4H2O with cyclam in dried methanol. Anaerobic condition was used in the beginning of the reaction, followed by later exposition to the atmospheric oxygen. The residue of this reaction was refluxed for 1 h leading to the formation of trans[Fe(cyclam)Cl2](BF4) upon tetrafluoroborate acid solution addition. The electronic spectrum of cis-[Fe(cyclam)Cl2]Cl complex (in aqueous, HCl 0.1 M solution) has shown absorption bands at umax = 238 nm (m= 5.5× 103 M − 1 cm − 1) and umax = 334 nm (m= 1.8×103 M − 1 cm − 1), both assigned to ligand to metal, pp(Cl) 3dp, charge transfer transitions [14,15]. These transitions occur in the spectrum of trans-[Fe(cyclam)Cl2]BF4 complex at umax = 238 nm (m= 7.5× 103 M − 1 cm − 1) and umax = 302 nm (m= 2.3× 103 M − 1 cm − 1). An additional band for the trans complex at umax = 355 nm (m= 1.0×103 M − 1 cm − 1) was assigned to d–d transition. The presence of more than two bands, that were observed for analogue ruthenium complexes [16], is probably due to the interchange of high-low spin configuration of these iron complexes [13]. Studies on four-coordinate macrocycle ligand containing weak field axial ligands like Cl−, Br−, I−, CH3CO2− and BF4− have shown that the average ligand field is not large enough to cause electron pairing [17]. The cyclic voltammograms of the isomer complexes have shown clear difference in the electrochemical behavior. The one-electron processes Fe(III)/Fe(II) with E1/2 = 212 and 83 mV were observed for the cis and
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trans-[Fe(cyclam)Cl2]+ ion complexes, respectively. Both cyclic voltammograms show a single reversible wave, suggesting that no aquation processes were present in the time scale of the experiment at room temperature. The distinct values observed for the redox potentials of the cis-[Fe(cyclam)Cl2]+ and trans-[Fe(cyclam)Cl2]+ isomers are probably due to their different symmetries (C26 and C2h, respectively), that would cause energy variation of the HOMO orbitals. The conversion of cis to trans-[Fe(cyclam)Cl2]+ was observed by cyclic voltammetry. The trans-[Fe(cyclam)Cl2]+ redox wave starts to appear after approximately 30 min of the cis complex dissolution. The conversion was observed to be favored by heating. The infrared spectra of trans-[Fe(cyclam)Cl2](BF4) complex shown absorption bands at 810 and 896 cm − 1 that are characteristic of this conformation, while the spectra of the cis-[Fe(cyclam)Cl2]Cl complex presented bands at 793, 805, 850 and 860 cm − 1 as reported in the literature [11,16]. The imine band (intense band at ca. 1600 cm − 1) that would be originated from the cyclam dehydrogenation side reactions was not observed [12]. The electronic spectrum of the cis-[Fe(cyclam)(qH)](PF6)2 complex presented six bands that are assigned in Table 2. Two of these bands occurred in the visible range at 19 231 and 16 447 cm − 1, both assigned to MLCT, one charge transfer from the metal to the p* of the HNC terminal and the other charge transfer from the metal to the p* on OC terminal of the (qH) ligand, respectively. By comparing the data above with that Table 2 The electronic spectrum data for cis-[Fe(cyclam)(qH)](PF6)2 complex umax (nm)/log m
Assignment
202/4.26 271/3.70 342/3.23 358/3.17 520/3.44 608/3.51
ILCT p p * (qH) ILCT p p* (qH) MLCT dp (Fe) p* (qH)(HNC) (2) MLCT dp (Fe) p*(qH)(OC) (2) MLCT dp (Fe) p* (qH)(HNC) (1) MLCT dp (Fe) p* (qH)(OC) (1)
Fig. 1. The electronic spectra of the cis-[Fe(cyclam)(qH)](PF6)2 complex in aqueous medium, showing the spectra changes under argon and Zn – Hg amalgam.
reported for the [Ru(bpy)2(qH)](PF6)2 complex (20 500 and 17 400 cm − 1) [1], the two bands for cis-[Fe(cyclam)(qH)](PF6)2 complex are lower in energy. The possible explanation for these energy changes (DE = 1269 and 953 cm − 1, respectively for each band) could be the better overlap of the 4d ruthenium and the pp*(qH) orbitals compared with the similar effect of the 3d iron orbitals. A qualitative discussion of the cis-[Fe(cyclam)(qH)]2 + spectrum suggests that the overlap between filled 3d (t2g) orbitals and (HNC) pp* orbital is greater than that for the (OC) pp* orbital, which is directly related with the electronegativity of the nitrogen, compared to that of oxygen. As a consequence, the MLCT band assigned to the Fe(3d) (pp*)HNC transition occurs at more energetic position than that for Fe(3d) (pp*)OC transition (DE = 2784 cm − 1). This energy difference can be directly related to the radial size of these pp* orbitals. This later conclusion was supported by the X-ray diffraction Fe –N(1) bond 1.867(7) A, , comparatively to Fe –O(1) 1.908(6) A, bond. The normal distance of Fe –N(cyclam) single bond is averaged at 2.02(2) A, in this complex, as will be discussed in the crystal structure section. The complex was reduced to its cis-[Fe(cyclam)(catH3)]2 + form by dissolving the cis-[Fe(cyclam)(qH)]2 + species in aqueous solution containing Zn –Hg amalgam as reducing agent. As a consequence, the two bands originally at 520 and 608 nm in the spectrum of quinone-related iron complex (Fig. 1) lower their intensities. The fact that no MLCT bands were observed in the spectrum of this reduced species strongly suggests that the quinonoid iron(II) complex originates the catecholoid iron(II). In this former complex the MLCT transitions are restricted due to the long distance of the iron 3d6 orbitals from the p* system of the aromatic ring. The differential pulse voltammetry (DPV) curves for the free 2-aminophenol ligand (catH3) show two peaks, E1/2 = − 224 and −20 mV for the one-electron sequential processes (sqH)/(catH3) and (qH)/(sqH), respectively. The DPV for the complex cis-[Fe(cyclam)(qH)](PF6)2, (Fig. 2) presents three waves at −316, + 361 and + 1013 mV assigned to the (sqH)/(catH3), (qH)/(sqH) and Fe(III)/Fe(II) redox processes, respectively. Upon coordination the (Fe(II) –sqH)/(Fe(II) –catH3) potential is 92 mV more negative than that observed in the free ligand, indicating the electrochemical stabilization of the (Fe(II) –sqH) species due to the five-membered ring formation, with iron(II) back-bonding contribution. The (Fe(II) –qH)/(Fe(II) –sqH) potential is 380 mV more positive than that observed in the free ligand, suggesting the accumulation of charge on the five-membered ring, contributing to the stabilization of the semiquinonoid form by binding Fe(II).
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Fig. 2. The differential pulse voltammetry of the complex cis-[Fe(cyclam)(qH)](PF6)2. Potential vs. Ag AgCl. Aqueous medium, v= 0.1 mol l − 1, NaTFA pH 3.0.
Fig. 3. The 1H NMR spectra cyclam)(qH)](PF6)2 in D2O.
for
the
complex
cis-[Fe(-
497
[Ru(bpy)2(qH)]2 + complex, DE = 704 mV, reflects the poorer back-bonding effect from the 3d orbitals of iron metal center to the (qH) ligand, even when the ruthenium metal center is coordinated to a p-acceptor ligand such as 2,2%-bipyridine. This observation is reinforced by the redox potential value for the Ru(III/II) process, E1/2 = 1520 mV, that is 1.5 times bigger in magnitude order than that for Fe(III/II) similar process. These data well illustrate the remarkable ability of the (qH) ligand on the M(II) stabilization toward its oxidized M(III) state. The Fe(III/II) redox process at + 1013 mV accounts for the great stability achieved by the Fe(II) due to the (qH) ligand coordination that could be well explained by an aromaticity like behavior of the pentametalocycle ring [18]. The complex was shown to be stable in aqueous solution at least for 1 week. Also, no hydrolysis product has been detected when the complex was dissolved in basic aqueous solution. Sharp peaks were registered in the hydrogen 1H NMR spectrum of the cis-[Fe(cyclam)(qH)]2 + complex (Fig. 3) which is a good indicative of the absence of paramagnetic species. The hydrogen signals (Scheme 3) of the coordinated cyclam ligand appear in the range of 1.4 –3.8 ppm. The coordinated (qH) ligand presented four signals at 7.11, 7.23, 8.14 and 8.94 ppm, assigned to the H4%, H5%, H3% and H6% hydrogen atoms, respectively. The 13C NMR spectrum of the cis-[Fe(cyclam)(qH)]2 + complex (Fig. 4(b)) presented 16 carbon signals. Ten of these signals are grouped in the l range of 20–60 ppm assigned to the cyclam carbons: (l=22.7 and 23.2 ppm; C13 and C6, respectively); (l= 47.7, 48.5 and 50.0 ppm; C7, C14 and C9, respectively); (l=50.9 ppm; C2 and C12 and l= 51.4 ppm; C5) and (l= 55.9 and 56.6 ppm; C10 and C3, respectively).
Scheme 3. Carbons and protons labelling.
For the analogue [Ru(bpy)2(qH)]2 + complex the redox potentials for the processes (sqH)/(catH3) and (qH)/(sqH) are E1/2 = − 660 and +44 mV, respectively [1]. The difference of the potentials between (sqH)/ (catH2) and (qH)/(sqH) redox process for the cis-[Fe(cyclam)(qH)]2 + complex, DE = 677 mV, compared with the difference between similar processes for the
Fig. 4. (a) The 13C NMR Dept 135 and (b) the cis-[Fe(cyclam)(qH)](PF6)2 in D2O.
13
C NMR for
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the (qH) ligand, respectively. The two signals with the bigger chemical shifts, l= 180.8 and 188.3 ppm, were assigned to the (C2%) and (C1%) carbons of the coordinated quinone ligand, respectively. The 13C NMR Dept 135 spectrum shown ten inverted signals of the secondary carbons of the cyclam ligand in the range of 22.7 –56.6 ppm. The four carbon signals in the upright position were assigned to the (qH) ligand. As expected, no signal from the imine or carboxylic carbon was observed (Fig. 4(a)).
5. Structural results
Fig. 5. ORTEP view of the cis-[Fe(cyclam)(qH)]2 + ion complex, showing the 30% probability ellipsoids. Table 3 Selected bond lengths (A, ) and bond angles (°) Bond lengths Fe–O(1) Fe–N(1) Fe–N(2) Fe–N(3) Fe–N(4) Fe–N(5) O(1)–C(6) N(1)–C(1) N(2)–C(9) N(2)–C(10) N(3)–C(11) N(3)–C(12) N(4)–C(14) N(4)–C(15)
1.908(6) 1.867(7) 2.008(7) 2.005(7) 2.049(6) 2.016(7) 1.335(10) 1.277(10) 1.485(12) 1.482(11) 1.514(11) 1.475(11) 1.459(12) 1.497(12)
N(5)–C(7) N(5)–C(16) C(1)–C(2) C(1)–C(6) C(2)–C(3) C(3)–C(4) C(4)–C(5) C(5)–C(6) C(7)–C(8) C(8)–C(9) C(10)–C(11) C(12)–C(13) C(13)–C(14) C(15)–C(16)
1.507(12) 1.438(12) 1.443(11) 1.438(12) 1.286(13) 1.459(17) 1.387(16) 1.371(13) 1.523(14) 1.511(15) 1.461(14) 1.484(13) 1.548(15) 1.535(16)
Bond angles O(1)–Fe–N(2) O(1)–Fe–N(3) O(1)–Fe–N(4) O(1)–Fe–N(5) N(1)–Fe–O(1) N(1)–Fe–N(2) N(1)–Fe–N(3) N(1)–Fe–N(4) N(1)–Fe–N(5) N(2)–Fe–N(4) N(2)–Fe–N(5) N(3)–Fe–N(2) N(3)–Fe–N(4) N(3)–Fe–N(5) N(5)–Fe–N(4) C(6)–O(1)–Fe C(1)–N(1)–Fe C(7)–N(5)–Fe C(9)–N(2)–Fe C(10)–N(2)–C(9)
174.7(3) 92.7(3) 87.1(3) 92.1(3) 81.6(3) 94.1(3) 94.3(3) 168.4(3) 93.6(3) 97.3(3) 91.4(3) 84.4(3) 89.1(3) 171.2(3) 83.8(3) 113.9(5) 118.0(6) 118.3(6) 119.4(6) 109.9(8)
C(10)–N(2)–Fe C(11)–N(3)–Fe C(12)–N(3)–Fe C(12)–N(3)–C(11) C(14)–N(4)–C(15) C(14)–N(4)–Fe C(15)–N(4)–Fe C(16)–N(5)–C(7) C(16)–N(5)–Fe C(9)–C(8)–C(7) N(2)–C(9)–C(8) N(3)–C(12)–C(13) N(5)–C(7)–C(8) C(11)–C(10)–N(2) C(10)–C(11)–N(3) C(12)–C(13)–C(14) N(4)–C(14)–C(13) N(4)–C(15)–C(16) N(5)–C(16)–C(15)
107.8(5) 109.6(5) 120.6(6) 110.7(7) 108.3(8) 118.7(6) 106.4(6) 110.3(8) 111.4(6) 114.7(9) 112.1(9) 113.7(8) 112.2(8) 109.0(7) 107.8(7) 111.6(9) 111.8(8) 107.1(8) 107.3(8)
The four peaks at l =117.9, 122.3, 123.1 and 128.0 ppm were assigned to C4%, C5%, C3% and C6% carbons of
Fig. 5 is an ORTEP [19] view of the molecule cis-[Fe(cyclam)(qH)](PF6)2 showing the 30% probability ellipsoids. Table 3 gives a list of selected bond lengths and angles [20 –24]. The Fe atom is six-coordinated by the four nitrogen atoms of the cyclam, by one oxygen and one nitrogen atom of the (qH) ligand. The coordination polyhedron is a distorted octahedron. The four cyclam to the iron (Fe –N) bond lengths are equal within three standard deviations. The (qH) group is planar and the oxygen atom O1 deviate from the least square plane through the (qH) group 0.019(6) A, . This plane is almost coplanar with the plane through Fe, N2 and N4 [3.6(3)°] and perpendicular with respect to the one through Fe, N3 and N5 [89.2(1.3)°]. In the macrocycle the C–N [average 1.48(2) A, ] and C–C distances [average 1.51(3) A, ] and the C–C–N [average 110(3)°], C–N–C [average 110(1)°] and C–C– C angles [113(2)°] are normal for this type of ring. Elongated ellipsoids of some macrocycle C atoms may indicate the typical conformational flexibility of a large ring system. These results along with those discussed on the spectroscopic and electrochemical studies are strong evidence that the reaction of cis-[Fe(cyclam)Cl2]Cl complex with 2-aminophenol occurred by the oxidation of the ligand, together with two protons losses, one from the OH terminal and the other from the NH2 terminal, producing the unstable semiquinonoid radical coordinated to Fe(III). This later species experiment an intramolecular electron transfer redox reaction where the metal center is reduced to Fe(II) and the (sqH) form is oxidized to the quinonoid (qH) form. The semiquinonoid radical coordinated to the metal could not be isolated or even observed on the experiments. The result of the reaction is the product cis-[Fe(cyclam)(qH)](PF6)2 (Scheme 4). This mechanism is in agreement with that observed for the similar catecholate tetraammine ruthenium complex [25] in which greater metal center stability is achieved after intramolecular redox process leading to the quinonoid form of the ligand, and an extensive electron delocalization [6,18].
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Scheme 4. The mechanism of intramolecular redox reaction.
6. Supplementary material Crystallographic data for the structural analysis have been deposited with the Cambridge Crystallographic Data Centre, CCDC no. 139960. Copies of this information may be obtained free of charge from The Director, CCDC, 12 Union Road, Cambridge, CB2 1EZ, UK (fax: + 44-1223-336033; e-mail: deposit@ ccdc.cam.ac.uk or www: http://www.ccdc.cam.ac.uk).
Acknowledgements This work was partially supported by Brazilian agencies CAPES, CNPq, FAPESP and FUNCAP.
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[7] R.S. Silva, E. Tfouni, Inorg. Chem. 31 (1992) 3313. [8] P. Belser, A. von Zelewsky, M. Zehnder, Inorg. Chem. 20 (1981) 3098. [9] C.K. Poon, A.W.M. To, Inorg. Chem. 18 (1979) 1277. [10] A. Vlcek, Jr., Comments Inorg. Chem. 16-4 (1994) 207. [11] P.K. Chan, C.K. Poon, J. Chem. Soc., Dalton Trans. (1976) 858. [12] S.S. Isied, Inorg. Chem. 19 (1980) 911. [13] W. Nam, R. Ho, J.S. Valentin, J. Am. Chem. Soc. 113 (1991) 7052. [14] D.D. Walker, H. Taube, Inorg. Chem. 20 (1981) 2828. [15] C.M. Che, S.S. Kwong, C.K. Poon, T.F. Lai, T.C.W. Mak, Inorg. Chem. 24 (1985) 1359. [16] C.K. Poon, C.M. Che, J. Chem. Soc., Dalton Trans. (1981) 1336. [17] J.C. Dabrowiak, P.H. Merrell, D.H. Busch, Inorg. Chem. 11 (1972) 1979. [18] H. Massui, Ph.D. Thesis, York University, North York, Ont., Canada, 1993. [19] L. Zsolnai, H. Pritskow, ZORTEP. Graphic Program for PC for the Analysis of Crystalline Structure, University of Heidelberg, Heidelberg, Germany, 1996. [20] C. Katayama, Acta. Crystallogr., Sect. A 42 (1986) 19. [21] Enraf-Nonius, CAD-4-PC. Version 1.2. Enraf– Nonius, Delft, The Netherlands, 1993. [22] K. Harms, S. Wocadlo, XCAD-4. Program for Processing CAD-4 Diffractometer Data, University of Marburg, Marburg, Germany, 1995. [23] G.M. Sheldrick, SHELXS86. Program for Crystal Structure Resolution, University of Go¨ttingen, Go¨ttingen, Germany, 1987. [24] G.M. Sheldrick, SHELX97. Programs for Crystal Structure Analysis (Release 97-2), University of Go¨ttingen, Go¨ttingen, Germany, 1997. [25] M.J. Clarke, Inorg. Chem. 23 (1984) 385.
Structure and properties of iron–cyclam complex of 2-aminophenol Luiz C.G. Vasconcellos a,*, Cristiane P. Oliveira a, Eduardo E. Castellano b, Javier Ellena b, I´caro S. Moreira a b
a Departamento de Quı´mica Orgaˆnica e Inorgaˆnica, Uni6ersidade Federal do Ceara´, Cx Postal 12200, 60455 -760 Fortaleza, Ceara´, Brazil Departamento de Fı´sica e Informa´tica, Instituto de Fı´sica de Sa˜o Carlos, Uni6ersidade de Sa˜o Paulo, C.P. 369, 13560 Sa˜o Carlos (SP), Brazil
Received 14 March 2000; accepted 25 October 2000
Abstract The cis-[Fe(cyclam)Cl2]Cl and trans-[Fe(cyclam)Cl2]BF4 complexes were prepared by a new synthetic procedure. These isomers were characterized by elemental analysis, electronic spectroscopy (umax = 238, 334 and umax = 238, 302 and 355 nm for cis- and trans-[Fe(cyclam)Cl2]+, respectively) and electrochemistry (E1/2 = 212 and 83 mV; vs. Ag AgCl, KCl 3.5 M, 25°C v =0.1 M NaTFA, pH 3.0 for cis- and trans-[Fe(cyclam)Cl2]+, respectively). The reaction of cis-[Fe(cyclam)Cl2]+ isomer with 2-aminophenol (catH3) ligand yields the cis-[Fe(cyclam)(qH)]2 + complex, where (qH) is the quinonoid oxidized form of the (catH3) ligand that is originated by an intramolecular electron transfer reaction. The cis-[Fe(cyclam)(qH)](PF6)2 complex crystallizes in the space group P21/n, with two different values for Fe–N bond length, due to the s and p distinct contributions. The electrochemical analysis of this species shows three one-electron sequential processes: E1/2 (I) = −316 mV, E1/2 (II) = +361 mV and E1/2 (III)= + 1.013 mV assigned to (sqH/catH3), (qH/sqH) and (Fe(III)/Fe(II)) redox process, respectively. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Iron– cyclam; Quinone-related species; 2-Aminophenol; 1,4,8,11-Tetraazacyclotetradecane; Macrocycle; X-ray structures
1. Introduction The chemistry of transition-metal complexes involving catechol-related ligands such as 2-aminophenol has been of great interest in the development of multi-electron transfer catalyst for O2 reduction, fuel cells and photosensitizers [1], as well as on the studies of oxidative cleavage of biomolecules [2,3] and cyclohexene epoxidation [4]. The electron delocalization between the metal and these o-benzoquinone-related ligands, which is the main characteristic of this class of compounds, depends on the extension of mixing of the metal d orbitals and the molecular orbitals of the ligand [5]. Changes in the oxidation state of the ligand can alter the extend of orbital mixture [6]. Studies on the iron – cyclam systems reported in the literature are rare when compared with those on the ruthenium – cyclam, ruthenium –tetra* Corresponding author. Fax: +55-85-288-9978. E-mail address: [email protected] (L.C.G. Vasconcellos).
amines and ruthenium –bis(2,2%-bipyridine) complexes with ortho-phenylene ligands [1,7,8]. The major problem with the study of iron –amine derivative complexes is their preparation due to the basic medium required for the reactions causing iron hydrolysis. As a result, the research on these systems did not expand comparatively to the ruthenium –amine chemistry. The use of the macrocyclic ligand, the four-dentate all saturated 1,4,8,11-tetraazacyclotetradecane (cyclam, Scheme 1), would mimic some of the amines properties and proportionates a very stable environment around the metal center due to its polydentate attributes [9]. Therefore, the study of [Fe(cyclam)L]n + complexes is of interest to establish a suitable comparative system with ruthenium –amine derivatives, regarding their properties and reactivity. The 2-aminophenol (catH3) can be sequentially oxidized to the semiquinone (sqH) and quinone (qH) related forms (Scheme 2). The (catH3) form behaves as a p-innocent ligand once it has no p* orbital available to accept electron
0277-5387/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 2 7 7 - 5 3 8 7 ( 0 0 ) 0 0 6 2 1 - 5
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density from the metal when coordinated to both, nitrogen and oxygen atoms. However, in the semiquinonoid (sqH) and quinonoid (qH) oxidation states, a large p system is spread all over the molecule, including the metal binding sites of the ligand. Once established the metal to ligand p back-bonding, a remarkable chemical stabilization is provided due to the aromatic behavior of the pentagon ring having the iron atom as participant [6,10].
2. Experimental
2.1. Chemical and reagents All chemicals were analytical reagent grade or better and were used without further purification. Iron dichloride (FeCl2·4H2O) from Aldrich was the start material for the synthesis of the complexes. The cyclam (1,4,8,11-tetraazacyclotetradecane) and 2-aminophenol ligands were also purchased from Aldrich. Double distilled water was used throughout the experiments.
2.2. Synthesis Although the syntheses of the cis and trans-[Fe(cyclam)Cl2]+ isomer complexes are reported in the literature [11], we have developed a new and suitable preparation method.
2.2.1. cis-[Fe(cyclam)Cl2]Cl A total of 280 mg (1.4 mmol) of FeCl2·4H2O were dissolved in 20 ml of methanol and added to a 20 ml of degassed methanol solution containing 282 mg (1.4 mmol) of the cyclam ligand. Although a dark green
color has immediately developed, the reaction was allowed to proceed during 3 h under argon flow at room temperature (r.t). The resulting solution was then exposed to the atmospheric oxygen. The yellow powder formed, the cis-[Fe(cyclam)Cl2]Cl complex was collected by filtration, washed with cold methanol, dried and stored under vacuum. Yield better than 45%. The dark brown filtrate resulted from the cis-[Fe(cyclam)Cl2]Cl separation was further used in the synthesis of the trans-[Fe(cyclam)Cl2](BF4). Elemental analysis: Anal. Calc. for C10H24Cl3FeN4: C, 33.1; H, 6.6; N, 15.4. Found: C, 32.9; H, 6.7; N, 15.2%.
2.2.2. trans-[Fe(cyclam)Cl2](BF4) The dark brown filtrate from the cis-[Fe(cyclam)Cl2]Cl preparation was refluxed for 1 h and then exposed to the atmospheric oxygen at r.t. The addition of 2 ml of a 45% aqueous solution of HBF4 originated a yellow –greenish solid that was collected by filtration, washed with cold methanol, dried and stored under vacuum. Yield better than 40%. Elemental analysis: Anal. Calc. for C10H24BCl2F4FeN4: C, 30.0; H, 5.8; N, 13.5. Found: C, 29.7; H, 5.6; N, 13.7%. 2.2.3. cis-[Fe(cyclam)(qH)](PF6)2 The cis-[Fe(cyclam)(qH)](PF6)2 complex was synthesized by mixing equimolar amounts of cis-[Fe(cyclam)Cl2]Cl and 2-aminophenol in 1:1 methanol –water, under argon atmosphere. In a typical preparation, 10 ml of the solvent mixture where used to dissolve 100.0 mg (0.24 mmol) of cis-[Fe(cyclam)Cl2]Cl followed by addition of 26.3 mg (0.24 mmol) of 2-aminophenol under vigorous stirring. The color change from pale yellow to dark blue was immediately developed. After 5 h of reaction the product was precipitated by adding 100 mg of NH4PF6 followed by the addition of 20 ml of cold 1:1 methanol –diethyl ether solvent mixture. The dark blue microcrystals formed were separated by filtration, dried and stored under vacuum. Yield better than 60%. The purity of the product was verified by liquid chromatography (HPLC). Elemental analysis: Anal. Calc. for C16H29F12FeP2N5O: C, 29.4; H, 4.4; N, 10.7. Found: C, 29.7; H, 4.3; N, 10.5%. 2.3. Apparatus and techniques
Scheme 1. 1,4,8,11-Tetraazacyclotetradecane (cyclam).
Scheme 2. The 2-aminophenol and its redox forms.
The electronic spectra were acquired in a Hitachi U-2000 spectrophotometer. The analysis of the electronic spectra was also performed using Gaussian deconvolution procedures in order to obtain more detailed results of energy and extinction coefficients values. The electrochemical experiments were performed on a BAS-100BW electrochemical analyzer from Bioanalytical Systems, using a regular 5 ml electrochemical cell, with glassy carbon, platinum and Ag AgCl (3.5 M
L.C.G. Vasconcellos et al. / Polyhedron 20 (2001) 493–499 Table 1 Crystal data, data collection details and structure refinement results Color/shape Empirical formula Temperature Crystal system Space group Unit cell dimensions (25 reflections in the 16B2qB35 range) a (A, ) b (A, ) c (A, ) i (°) V (A, 3) Z Dcalc (Mg m−3) Absorption coefficient (mm−1) Diffactometer/scan Radiation/wavelength (A, ) Crystal size (mm3) q Range for data collection (°) Index ranges Reflections collected Independent/observed reflections [IB2|(I)] Absorption correction [20] Refinement method Computing a Goodness-of-fit on F 2 weight parameters Final R indices [I\2|(I)] R indices (all data)
SHELXL-97
red/prism C16H29N5OFe[PF6]2 293(2) K monoclinic P21/n
9.248(1) 16.133(3) 17.521(2) 103.94(1) 2537.1(6) 4 1.710 6.994 Enraf–Nonius CAD-4/ –2q Cu Ka (graphite monochrom.)/1.54184 0.15×0.065×0.025 3.7852q565.0 −15h510, −15k518, −205l520 5571 4301 [Rint = 0.0533]/2420 analytical full-matrix least-squares on F 2 CAD-4 [21], XCAD4 [22], SHELXS-97 [23], SHELXL-97 [24] 1.288 0.120, 2.550 R1 = 0.0834, wR2 = 0.2337 R1 = 0.1599, wR2 = 0.2696
a Data collection, data reduction, structure solution and refinement, respectively.
KCl) as working, auxiliary and reference electrodes, respectively. Aqueous solutions of sodium trifluoroacetate, v = 0.1 M, pH 3.0, containing 1.0×10 − 3 M of the complex were commonly used on the experiments. Any other electrolyte solution used is specified in the text. Hydrogen and carbon NMR spectra were recorded in a Bruker AC500 spectrometer. D2O was used as solvent with small amounts of DSS as internal standard. The infrared spectra were taken on a Shimadzu spectrometer, FTIR-8300, using solid sample dissolved in KBr pellets. Liquid chromatograms (HPLC) were acquired on a Shimadzu chromatograph equipped with a model LC10AD pump and SPD-M10A photo diode-array UV – Vis spectrophotometer detector. A SH wrisorb ODS column (250×4.6 mm i.d., 5 mm particles; Alltech) was used in all analytical experiments.
495
3. Structure determination and refinement Crystal data, collection, reduction and refinement results for the cis-[Fe(cyclam)(qH)](PF6)2 complex are summarized in Table 1. Each hydrogen atom was stereochemicaly positioned and refined riding on the carbon atom to which it is bonded. In the refinement procedure all non-hydrogen atoms were treated anisotropically, while the hydrogen atoms were set isotropically with an isotropic thermal parameter 1.2 times the equivalent isotropic displacement parameter of the carbon atom to which each one is bonded. The two hexafluorophosphate anions presented some degree of disorder. This disorder is higher in the PF6 anion centered on P2, whose fluorine atoms were modeled with partial occupation factors.
4. Results and discussion The alternative synthetic route for cis-[Fe(cyclam)Cl2]Cl and trans-[Fe(cyclam)Cl2](BF4) was developed and the results compared with those reported in the literature [12,13]. This new procedure is quite simple and selective on the preparation of both isomers. The pure cis-[Fe(cyclam)Cl2]Cl complex was easily obtained by direct reaction of FeCl2·4H2O with cyclam in dried methanol. Anaerobic condition was used in the beginning of the reaction, followed by later exposition to the atmospheric oxygen. The residue of this reaction was refluxed for 1 h leading to the formation of trans[Fe(cyclam)Cl2](BF4) upon tetrafluoroborate acid solution addition. The electronic spectrum of cis-[Fe(cyclam)Cl2]Cl complex (in aqueous, HCl 0.1 M solution) has shown absorption bands at umax = 238 nm (m= 5.5× 103 M − 1 cm − 1) and umax = 334 nm (m= 1.8×103 M − 1 cm − 1), both assigned to ligand to metal, pp(Cl) 3dp, charge transfer transitions [14,15]. These transitions occur in the spectrum of trans-[Fe(cyclam)Cl2]BF4 complex at umax = 238 nm (m= 7.5× 103 M − 1 cm − 1) and umax = 302 nm (m= 2.3× 103 M − 1 cm − 1). An additional band for the trans complex at umax = 355 nm (m= 1.0×103 M − 1 cm − 1) was assigned to d–d transition. The presence of more than two bands, that were observed for analogue ruthenium complexes [16], is probably due to the interchange of high-low spin configuration of these iron complexes [13]. Studies on four-coordinate macrocycle ligand containing weak field axial ligands like Cl−, Br−, I−, CH3CO2− and BF4− have shown that the average ligand field is not large enough to cause electron pairing [17]. The cyclic voltammograms of the isomer complexes have shown clear difference in the electrochemical behavior. The one-electron processes Fe(III)/Fe(II) with E1/2 = 212 and 83 mV were observed for the cis and
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trans-[Fe(cyclam)Cl2]+ ion complexes, respectively. Both cyclic voltammograms show a single reversible wave, suggesting that no aquation processes were present in the time scale of the experiment at room temperature. The distinct values observed for the redox potentials of the cis-[Fe(cyclam)Cl2]+ and trans-[Fe(cyclam)Cl2]+ isomers are probably due to their different symmetries (C26 and C2h, respectively), that would cause energy variation of the HOMO orbitals. The conversion of cis to trans-[Fe(cyclam)Cl2]+ was observed by cyclic voltammetry. The trans-[Fe(cyclam)Cl2]+ redox wave starts to appear after approximately 30 min of the cis complex dissolution. The conversion was observed to be favored by heating. The infrared spectra of trans-[Fe(cyclam)Cl2](BF4) complex shown absorption bands at 810 and 896 cm − 1 that are characteristic of this conformation, while the spectra of the cis-[Fe(cyclam)Cl2]Cl complex presented bands at 793, 805, 850 and 860 cm − 1 as reported in the literature [11,16]. The imine band (intense band at ca. 1600 cm − 1) that would be originated from the cyclam dehydrogenation side reactions was not observed [12]. The electronic spectrum of the cis-[Fe(cyclam)(qH)](PF6)2 complex presented six bands that are assigned in Table 2. Two of these bands occurred in the visible range at 19 231 and 16 447 cm − 1, both assigned to MLCT, one charge transfer from the metal to the p* of the HNC terminal and the other charge transfer from the metal to the p* on OC terminal of the (qH) ligand, respectively. By comparing the data above with that Table 2 The electronic spectrum data for cis-[Fe(cyclam)(qH)](PF6)2 complex umax (nm)/log m
Assignment
202/4.26 271/3.70 342/3.23 358/3.17 520/3.44 608/3.51
ILCT p p * (qH) ILCT p p* (qH) MLCT dp (Fe) p* (qH)(HNC) (2) MLCT dp (Fe) p*(qH)(OC) (2) MLCT dp (Fe) p* (qH)(HNC) (1) MLCT dp (Fe) p* (qH)(OC) (1)
Fig. 1. The electronic spectra of the cis-[Fe(cyclam)(qH)](PF6)2 complex in aqueous medium, showing the spectra changes under argon and Zn – Hg amalgam.
reported for the [Ru(bpy)2(qH)](PF6)2 complex (20 500 and 17 400 cm − 1) [1], the two bands for cis-[Fe(cyclam)(qH)](PF6)2 complex are lower in energy. The possible explanation for these energy changes (DE = 1269 and 953 cm − 1, respectively for each band) could be the better overlap of the 4d ruthenium and the pp*(qH) orbitals compared with the similar effect of the 3d iron orbitals. A qualitative discussion of the cis-[Fe(cyclam)(qH)]2 + spectrum suggests that the overlap between filled 3d (t2g) orbitals and (HNC) pp* orbital is greater than that for the (OC) pp* orbital, which is directly related with the electronegativity of the nitrogen, compared to that of oxygen. As a consequence, the MLCT band assigned to the Fe(3d) (pp*)HNC transition occurs at more energetic position than that for Fe(3d) (pp*)OC transition (DE = 2784 cm − 1). This energy difference can be directly related to the radial size of these pp* orbitals. This later conclusion was supported by the X-ray diffraction Fe –N(1) bond 1.867(7) A, , comparatively to Fe –O(1) 1.908(6) A, bond. The normal distance of Fe –N(cyclam) single bond is averaged at 2.02(2) A, in this complex, as will be discussed in the crystal structure section. The complex was reduced to its cis-[Fe(cyclam)(catH3)]2 + form by dissolving the cis-[Fe(cyclam)(qH)]2 + species in aqueous solution containing Zn –Hg amalgam as reducing agent. As a consequence, the two bands originally at 520 and 608 nm in the spectrum of quinone-related iron complex (Fig. 1) lower their intensities. The fact that no MLCT bands were observed in the spectrum of this reduced species strongly suggests that the quinonoid iron(II) complex originates the catecholoid iron(II). In this former complex the MLCT transitions are restricted due to the long distance of the iron 3d6 orbitals from the p* system of the aromatic ring. The differential pulse voltammetry (DPV) curves for the free 2-aminophenol ligand (catH3) show two peaks, E1/2 = − 224 and −20 mV for the one-electron sequential processes (sqH)/(catH3) and (qH)/(sqH), respectively. The DPV for the complex cis-[Fe(cyclam)(qH)](PF6)2, (Fig. 2) presents three waves at −316, + 361 and + 1013 mV assigned to the (sqH)/(catH3), (qH)/(sqH) and Fe(III)/Fe(II) redox processes, respectively. Upon coordination the (Fe(II) –sqH)/(Fe(II) –catH3) potential is 92 mV more negative than that observed in the free ligand, indicating the electrochemical stabilization of the (Fe(II) –sqH) species due to the five-membered ring formation, with iron(II) back-bonding contribution. The (Fe(II) –qH)/(Fe(II) –sqH) potential is 380 mV more positive than that observed in the free ligand, suggesting the accumulation of charge on the five-membered ring, contributing to the stabilization of the semiquinonoid form by binding Fe(II).
L.C.G. Vasconcellos et al. / Polyhedron 20 (2001) 493–499
Fig. 2. The differential pulse voltammetry of the complex cis-[Fe(cyclam)(qH)](PF6)2. Potential vs. Ag AgCl. Aqueous medium, v= 0.1 mol l − 1, NaTFA pH 3.0.
Fig. 3. The 1H NMR spectra cyclam)(qH)](PF6)2 in D2O.
for
the
complex
cis-[Fe(-
497
[Ru(bpy)2(qH)]2 + complex, DE = 704 mV, reflects the poorer back-bonding effect from the 3d orbitals of iron metal center to the (qH) ligand, even when the ruthenium metal center is coordinated to a p-acceptor ligand such as 2,2%-bipyridine. This observation is reinforced by the redox potential value for the Ru(III/II) process, E1/2 = 1520 mV, that is 1.5 times bigger in magnitude order than that for Fe(III/II) similar process. These data well illustrate the remarkable ability of the (qH) ligand on the M(II) stabilization toward its oxidized M(III) state. The Fe(III/II) redox process at + 1013 mV accounts for the great stability achieved by the Fe(II) due to the (qH) ligand coordination that could be well explained by an aromaticity like behavior of the pentametalocycle ring [18]. The complex was shown to be stable in aqueous solution at least for 1 week. Also, no hydrolysis product has been detected when the complex was dissolved in basic aqueous solution. Sharp peaks were registered in the hydrogen 1H NMR spectrum of the cis-[Fe(cyclam)(qH)]2 + complex (Fig. 3) which is a good indicative of the absence of paramagnetic species. The hydrogen signals (Scheme 3) of the coordinated cyclam ligand appear in the range of 1.4 –3.8 ppm. The coordinated (qH) ligand presented four signals at 7.11, 7.23, 8.14 and 8.94 ppm, assigned to the H4%, H5%, H3% and H6% hydrogen atoms, respectively. The 13C NMR spectrum of the cis-[Fe(cyclam)(qH)]2 + complex (Fig. 4(b)) presented 16 carbon signals. Ten of these signals are grouped in the l range of 20–60 ppm assigned to the cyclam carbons: (l=22.7 and 23.2 ppm; C13 and C6, respectively); (l= 47.7, 48.5 and 50.0 ppm; C7, C14 and C9, respectively); (l=50.9 ppm; C2 and C12 and l= 51.4 ppm; C5) and (l= 55.9 and 56.6 ppm; C10 and C3, respectively).
Scheme 3. Carbons and protons labelling.
For the analogue [Ru(bpy)2(qH)]2 + complex the redox potentials for the processes (sqH)/(catH3) and (qH)/(sqH) are E1/2 = − 660 and +44 mV, respectively [1]. The difference of the potentials between (sqH)/ (catH2) and (qH)/(sqH) redox process for the cis-[Fe(cyclam)(qH)]2 + complex, DE = 677 mV, compared with the difference between similar processes for the
Fig. 4. (a) The 13C NMR Dept 135 and (b) the cis-[Fe(cyclam)(qH)](PF6)2 in D2O.
13
C NMR for
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the (qH) ligand, respectively. The two signals with the bigger chemical shifts, l= 180.8 and 188.3 ppm, were assigned to the (C2%) and (C1%) carbons of the coordinated quinone ligand, respectively. The 13C NMR Dept 135 spectrum shown ten inverted signals of the secondary carbons of the cyclam ligand in the range of 22.7 –56.6 ppm. The four carbon signals in the upright position were assigned to the (qH) ligand. As expected, no signal from the imine or carboxylic carbon was observed (Fig. 4(a)).
5. Structural results
Fig. 5. ORTEP view of the cis-[Fe(cyclam)(qH)]2 + ion complex, showing the 30% probability ellipsoids. Table 3 Selected bond lengths (A, ) and bond angles (°) Bond lengths Fe–O(1) Fe–N(1) Fe–N(2) Fe–N(3) Fe–N(4) Fe–N(5) O(1)–C(6) N(1)–C(1) N(2)–C(9) N(2)–C(10) N(3)–C(11) N(3)–C(12) N(4)–C(14) N(4)–C(15)
1.908(6) 1.867(7) 2.008(7) 2.005(7) 2.049(6) 2.016(7) 1.335(10) 1.277(10) 1.485(12) 1.482(11) 1.514(11) 1.475(11) 1.459(12) 1.497(12)
N(5)–C(7) N(5)–C(16) C(1)–C(2) C(1)–C(6) C(2)–C(3) C(3)–C(4) C(4)–C(5) C(5)–C(6) C(7)–C(8) C(8)–C(9) C(10)–C(11) C(12)–C(13) C(13)–C(14) C(15)–C(16)
1.507(12) 1.438(12) 1.443(11) 1.438(12) 1.286(13) 1.459(17) 1.387(16) 1.371(13) 1.523(14) 1.511(15) 1.461(14) 1.484(13) 1.548(15) 1.535(16)
Bond angles O(1)–Fe–N(2) O(1)–Fe–N(3) O(1)–Fe–N(4) O(1)–Fe–N(5) N(1)–Fe–O(1) N(1)–Fe–N(2) N(1)–Fe–N(3) N(1)–Fe–N(4) N(1)–Fe–N(5) N(2)–Fe–N(4) N(2)–Fe–N(5) N(3)–Fe–N(2) N(3)–Fe–N(4) N(3)–Fe–N(5) N(5)–Fe–N(4) C(6)–O(1)–Fe C(1)–N(1)–Fe C(7)–N(5)–Fe C(9)–N(2)–Fe C(10)–N(2)–C(9)
174.7(3) 92.7(3) 87.1(3) 92.1(3) 81.6(3) 94.1(3) 94.3(3) 168.4(3) 93.6(3) 97.3(3) 91.4(3) 84.4(3) 89.1(3) 171.2(3) 83.8(3) 113.9(5) 118.0(6) 118.3(6) 119.4(6) 109.9(8)
C(10)–N(2)–Fe C(11)–N(3)–Fe C(12)–N(3)–Fe C(12)–N(3)–C(11) C(14)–N(4)–C(15) C(14)–N(4)–Fe C(15)–N(4)–Fe C(16)–N(5)–C(7) C(16)–N(5)–Fe C(9)–C(8)–C(7) N(2)–C(9)–C(8) N(3)–C(12)–C(13) N(5)–C(7)–C(8) C(11)–C(10)–N(2) C(10)–C(11)–N(3) C(12)–C(13)–C(14) N(4)–C(14)–C(13) N(4)–C(15)–C(16) N(5)–C(16)–C(15)
107.8(5) 109.6(5) 120.6(6) 110.7(7) 108.3(8) 118.7(6) 106.4(6) 110.3(8) 111.4(6) 114.7(9) 112.1(9) 113.7(8) 112.2(8) 109.0(7) 107.8(7) 111.6(9) 111.8(8) 107.1(8) 107.3(8)
The four peaks at l =117.9, 122.3, 123.1 and 128.0 ppm were assigned to C4%, C5%, C3% and C6% carbons of
Fig. 5 is an ORTEP [19] view of the molecule cis-[Fe(cyclam)(qH)](PF6)2 showing the 30% probability ellipsoids. Table 3 gives a list of selected bond lengths and angles [20 –24]. The Fe atom is six-coordinated by the four nitrogen atoms of the cyclam, by one oxygen and one nitrogen atom of the (qH) ligand. The coordination polyhedron is a distorted octahedron. The four cyclam to the iron (Fe –N) bond lengths are equal within three standard deviations. The (qH) group is planar and the oxygen atom O1 deviate from the least square plane through the (qH) group 0.019(6) A, . This plane is almost coplanar with the plane through Fe, N2 and N4 [3.6(3)°] and perpendicular with respect to the one through Fe, N3 and N5 [89.2(1.3)°]. In the macrocycle the C–N [average 1.48(2) A, ] and C–C distances [average 1.51(3) A, ] and the C–C–N [average 110(3)°], C–N–C [average 110(1)°] and C–C– C angles [113(2)°] are normal for this type of ring. Elongated ellipsoids of some macrocycle C atoms may indicate the typical conformational flexibility of a large ring system. These results along with those discussed on the spectroscopic and electrochemical studies are strong evidence that the reaction of cis-[Fe(cyclam)Cl2]Cl complex with 2-aminophenol occurred by the oxidation of the ligand, together with two protons losses, one from the OH terminal and the other from the NH2 terminal, producing the unstable semiquinonoid radical coordinated to Fe(III). This later species experiment an intramolecular electron transfer redox reaction where the metal center is reduced to Fe(II) and the (sqH) form is oxidized to the quinonoid (qH) form. The semiquinonoid radical coordinated to the metal could not be isolated or even observed on the experiments. The result of the reaction is the product cis-[Fe(cyclam)(qH)](PF6)2 (Scheme 4). This mechanism is in agreement with that observed for the similar catecholate tetraammine ruthenium complex [25] in which greater metal center stability is achieved after intramolecular redox process leading to the quinonoid form of the ligand, and an extensive electron delocalization [6,18].
L.C.G. Vasconcellos et al. / Polyhedron 20 (2001) 493–499
499
Scheme 4. The mechanism of intramolecular redox reaction.
6. Supplementary material Crystallographic data for the structural analysis have been deposited with the Cambridge Crystallographic Data Centre, CCDC no. 139960. Copies of this information may be obtained free of charge from The Director, CCDC, 12 Union Road, Cambridge, CB2 1EZ, UK (fax: + 44-1223-336033; e-mail: deposit@ ccdc.cam.ac.uk or www: http://www.ccdc.cam.ac.uk).
Acknowledgements This work was partially supported by Brazilian agencies CAPES, CNPq, FAPESP and FUNCAP.
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