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FULL PAPER
DOI: 10.1039/a908648j J. Chem. Soc., Dalton Trans., 2000, 915–918 915
This journal is © The Royal Society of Chemistry 2000
Structure, magnetic properties and epoxidation activity of iron(
III
)
salicylaldimine complexes
Huey-Lih Shyu,
aHo-Hsiang Wei,*
bGene-Hsaing Lee
cand Yu Wang
ca
Chung-Tai Institute of Health Science and Technology, Taiwan
bDepartment of Chemistry, Tamkang University, Tamsui, Taiwan.
E-mail: [email protected]
c
Instrumentation Center of College of Science, National Taiwan University, Taipei, Taiwan
Received 1st November 1999, Accepted 25th January 2000
Two new iron() complexes with salicylaldimine ligands, [Fe(L1)Cl]ⴢCH
3CN (1) and [Fe(L2)Cl(H2O)]ⴢ1/3CH3Cl (2) [H2L1= N,N⬘-(1,1-dimethylethylene)bis(salicylaldimine), H2L2= N,N⬘-(1,1-dimethylethylene)bis(3-methoxysali-cylaldimine)], have been prepared and characterized. The X-ray crystal structure analysis shows that the iron() ion in complex 1 is in a distorted square pyramidal environment, while in 2, the iron() center is six-coordinate with distorted octahedral geometry. The Schiff base ligands L1 and L2 occupied the basal sites, with chloride and water occupying the apical sites. Magnetic susceptibility and Mössbauer effect measurements showed a high-spin configuration (S = 5/2) for Fe() in 1 and 2. The temperature dependence of the magnetic data for complexes 1 and 2 is analyzed in terms of the spin-Hamiltonian formalism, which gives an accurate estimate of the zero-field splitting of the ground state of the iron() ions as D = 10.2 cm⫺1 (E= 3.4 cm⫺1) and D= 7.2 cm⫺1 (E= 2.4 cm⫺1, zJ⬘ = ⫺0.5 cm⫺1), respectively, in the above two complexes. It has been found for the first time that complexes 1 and 2 activate the epoxidation of cis-stilbene by NaOCl. With 1, the epoxides were produced in 40% yield with cis : trans= 20:80, and with 2, the epoxides were produced in 90% yield with cis : trans= 40:60.
Introduction
Oxygen activation and transfer by cytochrome P-450 has attracted the attention of organic chemists in particular, since it catalyses the mono-oxygenation of various compounds, both biotic and exobiotic, with high stereo- and regioselectivity under mild conditions. The P-450-catalyzed oxygen transfer reaction was proposed by Groves and Nemo to proceed through an oxohaem catalytic intermediate.1,2 Furthermore, Groves and co-workers have reported that simple iron() porphyrins are good models for the reaction site of cytochrome P-450.2 From these basic results, many optically active iron() and manganese() porphyrins were synthesized and have been found to be efficient catalysts for the epoxidation of olefins.3–6
Parallel to these metal–porphyrin complexes, the use of manganese() complexes of substituted salen ligands [salen = N,N-(ethylene)bis(salicylideneaminate)] as a catalyst for the epoxidation of olefins has been widely investigated.7–11 An increase in the enantioselectivities for these substrates has been achieved by modulating the steric and electronic properties of the catalyst through varying the substitution pattern at the salicylidene moiety and the diamine fragment.9 Surprisingly, very little attention has been paid to studies on the epoxidation of olefins using salen–iron() complexes. Although several substituted iron()–salen complexes have been investigated as models for catechol dioxygenase,12 to our knowledge, no example of olefin epoxidation using iron()–salen catalysts has been reported.
Here, we report the preparation, crystal structures, magnetic properties, and the first catalytic activity in the epoxidation of cis-stilbene with NaOCl of mononuclear iron() complexes with substituted salen ligands (Scheme 1).
Experimental
Materials
The salicylaldehyde, 3-methoxysalicylaldehyde, 1,1
⬘-dimethyl-ethylenediamine, ferric chloride, cis-stilbene and cis- and trans-stilbene oxide used were all commercial samples from Aldrich. The sodium hypochlorite solution was from Riedel-de Haën.
Syntheses of [Fe(L1)Cl]ⴢCH
3CN (1) and [Fe(L 2)Cl(H
2O)]ⴢ 1/3CH3Cl (2)
The Schiff bases H2L1 and H2L2 were obtained from the reac-tion of salicylaldehyde or 3-methoxysalicylaldehyde and 1,1 ⬘-dimethylethylenediamine in methanol, respectively. The iron() complexes with H2L1 and H2L2 were obtained according to standard methods. A methanol solution containing anhydrous FeCl3 and the ligand was refluxed for 7 h in air and the desired powder iron() complexes 1 and 2 were filtered off. Single crystals of 1 and 2 were grown from acetonitrile and chloro-form, respectively. Anal. found: C, 55.93; H, 4.98; N, 9.95. Calc. for C18H18N3O2FeClⴢCH3CN (1): C, 56.30; H, 4.96; N, 9.85%. IR (KBr) ν(CN)/cm⫺1: 1611. Found: C, 48.03; H, 4.70; N, 5.32. Calc. for C20H24N2O5FeClⴢ1/3CHCl3 (2): C, 48.53; H, 4.81; N, 5.57%. IR (KBr) ν(CN)/cm⫺1: 1607.
Crystallography
Crystal data. Complex 1. C20H21N3O2FeCl, M= 426.70, orthorhombic, space group Pbca, a= 14.701(4), b = 13.552(4), c= 19.941(3) Å, crystal size = 0.20 × 0.20 × 0.20 mm, V = 3972.7(16) Å3, D
c (Z= 8) = 1.427 g cm⫺3, µ = 9.008 cm⫺1, 2θmax= 50.0⬚, N = 3490, No= 1370, R = 0.049, R⬘ = 0.049.
916 J. Chem. Soc., Dalton Trans., 2000, 915–918
Complex 2. C20.33H24.33N2O5FeCl2, M= 503.17, trigonal, space group R3¯, a= 25.189(7), c = 18.061(6) Å, crystal size = 0.25 × 0.30 × 0.40 mm, V= 9924(4) Å3, D
c (Z= 18) = 1.515 g cm⫺3, µ = 8.290 cm⫺1, 2θmax= 50.0⬚, N = 4284, No= 1868,
R= 0.069, R⬘ = 0.070.
The X-ray crystal data were collected at 298 K using an Enraf-Nonius CAD4 diffractometer equipped with graphite-monochromated Mo-Kα radiation (λ = 0.70930 Å), 2θ scan mode. The N independent reflections and No with I > 2.0σ(I) were observed. The structures were solved by location of heavy atoms using a Patterson map and refined (based on F) by a full-matrix least-squares method using the NRCVAX software package.13
CCDC reference number 186/1822.
See http://www.rsc.org/suppdata/dt/a9/a908648j/ for crystal-lographic files in .cif format.
Physical measurements
The IR spectra were recorded on a Bio-Rad FTS40 FTIR spec-trophotometer as KBr pellets in the 4000–400 cm⫺1 region and the X-band EPR spectra were recorded at 77 K for the complexes as powders on a Bruker ESC-106 spectrometer. The temperature dependence of the magnetic susceptibility of polycrystalline samples was measured between 4 and 300 K in a field of 1 T using a SQUID magnetometer. Diamag-netic corrections were made using Pascal’s constants.14 The Mössbauer spectra of the powdered samples at 80 K were observed with an ASA 600 Mössbauer spectrometer, with a 57Co(Rh) radiation source. Iron foil was used as a standard for isomer shifts.
Reaction of NaOCl with cis-stilbene catalyzed by [Fe(L1)Cl]ⴢ CH3CN (1) and [Fe(L2)Cl(H2O)]ⴢ1/3CHCl3 (2)
An unbuffered, undiluted aqueous solution of commercial sodium hypochlorite (13% w/w, 0.40 ml, 0.56 mmol) was slowly added to a solution of cis-stilbene (0.101 g, 0.56 mmol) in CH2Cl2 (10 ml) and complex 1 (0.01 g, 0.03 mmol) or complex 2 dissolved in methylene chloride (10 ml). After complete addition, the mixture was stirred for 10 h and the methylene chloride removed under reduced pressure. The organic products were extracted with diethyl ether, dried with Na2SO4 and filtered. Analysis by NMR spectroscopy, using 1,4-dibromo-benzene in CDCl3 as an internal standard, showed the presence of cis-stilbene oxide (singlet, δ 4.25) and a trans-stilbene oxide (singlet, δ 3.85).
Results and discussion
Crystal structures of complexes 1 and 2
The X-ray crystal structures (without the solvent molecules) of complexes 1 and 2 are illustrated in Figs. 1 and 2, respectively. Selected bond distances and angles relevant to the iron coordination sphere are listed in Tables 1 and 2.
The coordination geometry around each iron atom can be described as a distorted square pyramid for 1 and an elongated distorted octahedron for 2. In compound 1 the equatorial sites are occupied by the N2O2 donor atoms of the ligand L1, with average bond distances of Fe–N= 1.971(5) and Fe–O= 1.868(5) Å, and the apical chloride atom, with Fe–Cl = 2.4468(24) Å. In compound 2, two apical sites are occupied by the chloride and the water oxygen atom, with Fe–Cl= 2.322(3) and Fe–O(5)= 2.209(6) Å, and the basal coordination plane consists of the N2O2 donor atoms of the HL
2 ligand, with average bond distances of Fe–N= 2.074(6) and Fe–O = 1.890(2) Å.
Since the space group of 1 has an inversion center, the 8 molecules have to be present as pairs with opposite chirality. The central C–N–N–C chelate ring of the 1,1 ⬘-dimethylethyl-enediimine ligand in complexes 1 (Fig. 1) and 2 (Fig. 2) has to
be present in a λ form and a δ form, respectively, and in Fig. 3 both chiralities of 2 are shown. The displacements of the Fe() ions from O(1), O(2), N(2), and N(1) equatorial planes of com-plexes 1 and 2 are 0.266 and 0.141 Å, respectively, these are smaller than that of 0.46 Å observed for Fe(salen)Cl.15,16 The displacement of iron from the N2O2 plane reflects the strain exerted by the ligand structure. The smaller displacement of
Fig. 1 Perspective view of complex 1 with the atom numbering
scheme. The solvent CH3CN is omitted for clarity. Thermal ellipsoids
are drawn at the 30% probability level.
Fig. 2 Perspective view of complex 2 with the atom numbering scheme. The solvent CHCl3 is omitted for clarity. Thermal ellipsoids are drawn at the 30% probability level.
Table 1 Selected bond distances (Å) and angles (⬚) for [Fe(L1)Cl]ⴢ
CH3CN 1 Fe–Cl Fe–O(2) Fe–N(2) C(11)–O(4) C(18)–O(2) Cl–Fe–O(1) Cl–Fe–N(1) O(1)–Fe–O(2) N(1)–Fe–N(2) O(1)–Fe–N(2) 2.4468(24) 1.866(5) 1.953(5) 1.318(8) 1.317(9) 100.02(18) 98.62(18) 92.32(22) 80.65(24) 167.06(23) Fe–O(1) Fe–N(1) C(5)–N(1) C(12)–N(2) Cl–Fe–O(2) Cl–Fe–N(2) O(1)–Fe–N(1) N(2)–Fe–O(2) O(2)–Fe–N(1) 1.869(5) 1.990(6) 1.270(10) 1.270(10) 100.49(17) 91.79(18) 92.29(22) 90.68(23) 159.23(24)
Table 2 Selected bond distances (Å) and angles (⬚) for [Fe(L2)Cl(H 2O)]ⴢ 1/3CHCl3 2 Fe–Cl Fe–O(1) Fe–N(1) C(1)–O(1) C(7)–N(1) Cl–Fe–O(5) Cl–Fe–O(2) Cl–Fe–N(2) O(5)–Fe–O(2) O(1)–Fe–O(2) N(1)–Fe–N(2) O(2)–Fe–N(1) 2.322(3) 1.888(5) 2.082(6) 1.311(9) 1.251(12) 174.23(16) 93.49(19) 90.42(21) 87.49(21) 99.54(21) 80.3(3) 167.1(3) Fe–O(5) Fe–O(2) Fe–N(2) C(16)–O(2) C(10)–N(2) Cl–Fe–O(1) Cl–Fe–N(1) O(5)–Fe–O(1) O(5)–Fe–N(1) O(2)–Fe–N(2) O(1)–Fe–N(2) 2.209(6) 1.892(5) 2.066(6) 1.331(9) 1.283(11) 98.18(19) 93.63(21) 87.25(21) 90.09(25) 88.93(24) 167.48(24)
J. Chem. Soc., Dalton Trans., 2000, 915–918 917 Fe() ion from the N2O2 plane in 2 may be due to the second
apical ligand (water), which makes hydrogen bonds between the hydrogens of H2O and the oxygen atoms of the basal ligand. As shown in Fig. 3, the closest intermolecular interactions occur between the [Fe(L2)Cl(H
2O)] molecules which are held together in pairs by hydrogen bonding in which O(5) makes hydrogen-bonding contacts to atoms O(1A) and O(2A) at distances of 2.863(7) and 2.808(7) Å, respectively. However, the two water molecules do not hydrogen-bond with each other [O(5)ⴢ ⴢ ⴢ O(5a) = 3.560(9) Å]. Studies of iron()–salen systems have shown that the metal-to-imine nitrogen bond distance is sensitive to the electronic spin state of the metal ion.17,18 In iron() complexes, the metal-to-imine nitrogen distances are in the range 2.00–2.10 Å for the high-spin state and in the range 1.93–1.96 Å for the low-spin case. In the structures of complexes 1 and 2 reported here, the mean Fe–N bond distances of 1.971(5) Å for 1 and 2.074(6) Å for 2, respectively, suggest that the metal ion in 1 and 2 is in the high-spin state, which is consistent with the results of room temperature magnetic susceptibility and Mössbauer effect measurements (see below).
EPR and Mössbauer spectra
The powder X-band EPR spectra at 77 K of both complexes show a weak absorption signal near g= 4.50 and strong broad lines at g≈ 2.0 with line widths of 0.08–0.20 T. The width of the individual absorption lines making up the pattern for complex 2 is estimated at 0.20 T and is attributed primarily to inter-molecular interactions. The line near g= 4.50, observed in the spectra of 1 and 2, is generally associated with rhombically distorted high-spin Fe() complexes and is predicted to occur as the ratio of the zero-field splitting (ZFS) parameters E:D approaches 1 : 3 the rhombic limit.19
Mössbauer spectra at 80 K for powdered samples of 1 and 2 consist of single asymmetric quadrupole-split doublets. A representative spectrum of 2 is shown in Fig. 4. The values of the isomer shift (IS) and quadrupole splitting (QS), obtained by computer fitting two unconstrained Lorentzian lines to the spectra, are: IS (QS)= 0.52 mm s⫺1 (0.96 mm s⫺1) for 1 and 0.56 mm s⫺1 (0.90 mm s⫺1) for 2. These parameters lie in the range expected for typical high-spin (S= 5/2) square-pyramidal FeN4O2 Schiff-base systems.19,20 It is interesting to note that the QS value of 1 is larger than that of 2. The rather larger displacement of the iron atom from the basal plane in complex 1 would be expected to increase the ligand field distortion and lead to a larger QS value.
Fig. 3 View of the packing of the molecules in complex 2. Weak intermolecular hydrogen bonds are shown by dotted lines.
Magnetic properties
The temperature dependence of χmT for complexes 1 and 2 shows a similar pattern and the entire characteristic expected for a monomeric Fe() complex with a rather large zero-field splitting of the 6A
1 state.20,21 A representative plot of χmT vs. T for 2 is illustrated in Fig. 5. Above ca. 65 K the χmT value is 4.52 cm3 K mol⫺1 and is independent of temperature. Below 65 K it begins to decrease quite rapidly, with a marked curvature in the plot, reaching a value of 1.92 cm3 K mol⫺1 at 4 K. This behavior is generally reminiscent of a 6A
1 state, having con-sidered zero-field splitting and weak intermolecular magnetic interactions.19–21
The observed susceptibilities were analyzed with the rhombic spin Hamiltonian19–21 and it was assumed that E : D approached the rhombic limit of 1 : 3.
H= gµBHSiz⫹ D[Siz2⫺ (1/3)S(S ⫹ 1)] ⫹ E(Six2⫺ Siy2) (1)
The data cannot be well-described solely by eqn. 1; a molecular field correction must be applied to eqn. 1 to account for the presence of additional intermolecular exchange:
χm= χ[1 ⫺ (2zJ⬘/Ng2µB2)χ] (2) where J⬘ is the intermolecular interaction parameter, and z is the number of nearest neighbors around a given magnetic molecule in the crystal lattice.
Careful and systematic variation of D, E, and zJ⬘ showed that acceptable fits could be obtained for the following parameter values: D= 10.2 (E = 3.4, zJ⬘ = 0 cm⫺1, g= 2.0) for 1 and D= 7.2 (E = 2.4, J⬘ = 0.5 cm⫺1, z= 1, g = 2.0) for 2. The
Fig. 4 Mössbauer spectra at 80 K for complex 2. The solid line was plotted using the fitting parameters reported in the text.
Fig. 5 Temperature dependence of χmT for complex 2. The solid line was calculated with the fitting parameters reported in the text.
918 J. Chem. Soc., Dalton Trans., 2000, 915–918
weak intermolecular antiferromagnetic exchange (J⬘) observed in 2 presumably originates through the pairwise interactions described above and is shown in Fig. 4. If a superexchange pathway via H-bonded fragments of the type Fe–OH2– O(salen)–Fe is involved, then J⬘ would be expected to be small, as is observed.
Epoxidation
Sodium hypochlorite was found to react readily with substituted derivatives of the salen–Fe() complexes 1 and 2 in the pres-ence of cis-stilbene to produce trans- and cis-epoxides in good yield. With 1, epoxides were produced in 40% yield with cis : trans= 20:80, and with 2, epoxides were produced in 90% yield with cis : trans= 40:60. These data clearly show that the cis-stilbene produced higher yields of trans-epoxide, the prod-uct mixtures containing 20 and 40% of the corresponding cis isomer, and the relative reactivity of epoxidation was sensitive to the substitution on the Schiff-base ligands. It has been observed that cis-stilbene reacts with iron–porphyrin complexes to give high yields of cis-stilbene epoxide containing a minor amount of the corresponding trans isomer.1–5 Therefore, it is somewhat surprising that the epoxidation of cis-stilbene with NaOCl catalyzed by complexes 1 and 2 gave high yields of trans-stilbene epoxide, similarly to the epoxidation of cis-olefins catalyzed by Mn(salen) derivatives, which essentially favors the production of trans-epoxide products.7–9 At this stage, no clear mechanism accounts for the selective formation of trans-epoxides from cis-olefins by using complexes 1 and 2. We pro-pose at this moment that the mechanism for the Mn(salen)-catalyzed cis-stilbene epoxidation8c,22–24 including a radical intermediate with rotational collapse could explain the present results. More detailed studies on the mechanism and enantiose-lectivity for the present Fe(salen)-catalyzed epoxidation are cur-rently being made in this laboratory.
In summary, the results indicate that under the same con-ditions, epoxidation of cis-stilbene with NaOCl catalyzed by complex 1 provided 80% selectivity for trans-stilbene oxide but only 40% conversion, while when catalyzed by complex 2, the conversion of epoxidation of cis-stilbene rose to 90%. The higher conversion and trans-stilbene oxide formation with complex 2 could be due either to steric or electronic effects of the OCH3 group in positions C(2) and C(15) of the phenoxide moiety of the L2 ligand. It has been observed by Kochi7 and Jacobsen and co-workers8a,23,24 that (salen)manganese() com-plexes with bulky groups at C(3) and C(3⬘) of the salen moiety make excellent catalysts for the epoxidation of cis-olefins. These results indicated that the yield and the stereo-selectivity of the metal–salen-complex-catalyzed olefin-epoxidation is strongly dependent on the steric9b,24 and electronic25 effects of the sub-stituents on the salen moiety.
Acknowledgements
The authors thank the National Science Council of Taiwan for financial support via a grant (NSC-89-2113-M-032-009).
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![Table 2 Selected bond distances (Å) and angles ( ⬚) for [Fe(L 2 )Cl(H 2 O)] ⴢ 1/3CHCl 3 2 Fe–Cl Fe–O(1) Fe–N(1) C(1)–O(1) C(7)–N(1) Cl–Fe–O(5) Cl–Fe–O(2) Cl–Fe–N(2) O(5)–Fe–O(2) O(1)–Fe–O(2) N(1)–Fe–N(2) O(2)–Fe–N(1) 2.322(3)1.888(5)2.082(6)1.311(9) 1.251](https://thumb-ap.123doks.com/thumbv2/9libinfo/8873936.249435/2.919.471.832.625.787/table-selected-bond-distances-angles-chcl-fe-cl.webp)
