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Design and synthesis of new models for diiron biosites
www.elsevier.nl/locate/jinorgbio Journal of Inorganic Biochemistry 79 (2000) 41–46
Design and synthesis of new models for diiron biosites V.M. Trukhan a, O.N. Gritsenko a, E. Nordlander b,1, A.A. Shteinman a,* a
Institute of Problems of Chemical Physics, Russian Academy of Sciences, 142432 Chernogolovka, Russia b Inorganic Chemistry 1, Chemical Center, Lund University, Box 124, SE-221 00 Lund, Sweden Received 3 June 1999; received in revised form 17 November 1999; accepted 23 November 1999
Abstract In order to mimic dinuclear active sites of some non-heme diiron proteins, ten new polydentate and potentially dinucleating ligands have been synthesized. Each ligand contains a carboxylate moiety designed to bridge two metal atoms. These central carboxylate moieties are derived from substituted benzoic acids that in turn are linked to terminal nitrogen or oxygen donors by spacers so that framework-type polydentate ligands similar to the polypeptide frames in diiron metallobiosites are formed. Reaction of these ligands with Fe(ClO4)3P9H2O leads to ferric m-oxo-m-carboxylato iron complexes of the general formulas [Fe2O(L)2(H2O)2](ClO4)2 and [Fe2O(L)(BzO)](ClO4)2 (Lsligand), containing one or two immobilized bridging carboxylates, respectively. While X-ray crystallography shows that some of these complexes are dimers or network polymers in the solid state, electrospray ionization mass spectrometry (ESMS) and spectroscopic data ¨ (UV–Vis, NMR, Mossbauer) indicate that they dissociate to monomeric Fe2O units in dilute CH3CN solutions. q2000 Elsevier Science Inc. All rights reserved. Keywords: Diiron biosites; Modeling; Framework ligands; Fe(III) complexes
1. Introduction Dinuclear oxo- and hydroxo-bridged iron complexes have recently received much attention in bioinorganic chemistry [1–3] owing to the wide occurrence of such units in organisms and their involvement in a range of biological functions such as the storage and transport of dioxygen (hemerythrin, Hr), DNA synthesis (ribonucleotide reductase R2 protein, RNR), alkane oxidation (methane monooxygenase, MMO), etc. The active sites of non-heme iron proteins such as Hr, RNR and MMO contain two iron atoms bridged by at least one endogenous carboxylate and usually one or two oxo-, hydroxo- or aqua-bridges derived from exogenous water and capped by endogenous nitrogen and oxygen donor groups [1–5]. Despite the common structural characteristics of their active sites, the functional diversity is one of the most striking aspects of this class of proteins [6]. These diiron biosites have been characterized by spectroscopic and magnetic methods [4,5], and X-ray crystal structures of diiron cores have been determined for Hr [7], the R2 subunit of RNR [8], MMO [9] and some other related proteins. In the case of RNR and MMO, a great number of hypotheses regarding the mechanisms of oxygen activation * Corresponding author. Fax: q7-096-515-3588; e-mail: [email protected] 1 Also corresponding author. E-mail: [email protected]
by the diiron clusters in these proteins have been put forth, but elucidation of these mechanisms remains one of the most challenging unsolved problems in bioinorganic chemistry [1–6]. Insights into the mechanisms of RNR and MMO— which are structurally closely related with respect to the coordination environment of their diiron clusters—may be achieved by synthesis of structural models of their active sites and investigation of these model complexes in functional mimicry of the enzymes. A number of Fe2O complexes have been synthesized [10,11] to model the spectroscopic and structural properties of m-oxo-m-carboxylato diiron sites in non-heme iron proteins. In many cases, such biomimetic complexes have been found to be relatively unstable [12]. Efforts have therefore been made to prepare framework polydentate ligands that may stabilize this type of complex [13]: examples include ligands in which multidentate nitrogen bases that bind to metals in a capping manner are linked to alkoxo or phenoxo groups capable of bridging two metals [14]. The use of framework dinucleating ligands is promising for modeling of this type of biosite because such ligands permit some preorganization of the iron coordination sphere. Considering the fact that the carboxylate bridges in the above-mentioned enzymes are parts of the protein [15] while the oxygen bridges in the diiron sites are derived from exogenic water or
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dioxygen and probably take part in catalytic transformations [16], it is of interest to use polydentate ligands in which the nitrogen bases are linked to a carboxylate moiety for structural and functional modeling of the active sites. Until recently, only diiron complexes of framework ligands in which the bridging ligand moieties are phenoxo or alkoxo groups were known. Here, we describe the preparation of a number of new framework ligands and the reactions of some of these ligands to form new iron-oxo complexes [17–19]. These ligands contain central carboxylates, designed to bridge Fe–O–Fe cores, that are connected to nitrogenous base(s) by spacer(s) (e.g. –CH2CO–, –CH2CH2CH2–). A preliminary investigation of the catalytic activities of some of the reported diiron complexes in alkane oxidation is also reported. 2. Experimental The ligands described here were designed on the basis of relative ease of synthesis and potential fit to a dinuclear oxobridged metal (iron) core as determined by molecular modeling using the HyperChem 5.0 software package. Commercial chemicals were used without further purification. All reactions were carried out under an atmosphere of dry, oxygen-free nitrogen, using solvents which were freshly distilled from appropriate drying agents. Microanalyses were performed in the Analytical Laboratory of the Institute of Problems of Chemical Physics (Chernogolovka). NMR spectra were recorded on Varian Unity 300 MHz (Lund University) or Tesla 200 MHz (Chernogolovka) spectrometers. Mass spectrometric measurements were carried out on a unique home-built high resolution time-of-flight mass spectrometer [22] in the laboratory of Professor A.F. Dodonov of the Institute of Energy Problems in Chemical Physics (Chernogolovka). X-ray data collection was carried out with a Siemens SMART CCD area detector at Lund University and a KUMA automatic diffractometer at the Institute of Problems of Chemical Physics (Chernogolovka). Further experimental details may be found in Refs. [17–22]. 3. Results and discussion 3.1. Syntheses of framework dinucleating ligands Two unsuccessful attempts to prepare dinuclear iron(III) complexes containing polydentate ligands with connected nitrogen bases and bridging carboxylate moieties have been reported [23,24] (vide infra). Furthermore, a bis-m-carboxylato diiron(II) complex of a dipyridylbenzoyl amine which acts as a pentadentate dinucleating ligand has recently been reported [25]. This complex has an unusual syn–anti configuration of ligand-based carboxylate bridges and a relatively ˚ In order to create more large Fe–Fe distance ()4.6 A). adequate models for non-heme diiron biosites we have undertaken the design and synthesis of new dinucleating framework ligands with bridging carboxylates.
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Our efforts have resulted in a first generation of framework ligands that is shown in Fig. 1 [20]. Ligands 1–10 (Fig. 1) are based on substituted benzoic acids which contain the central (and potentially bridging) carboxylates. Spacers of different length and type (–CH2O–, –CH2CH2CH2–, –CH2C6H4CH2–) were chosen to connect the terminal donor groups (pyridine, dipicolylamine, dipyridyl, carboxylate) to the central benzoate moieties. The methyl esters of salicylic and resorcylic acids were taken as starting reagents to produce ligands with one and two terminal arms, designed to form complexes with two and one ligand-based bridging carboxylates, respectively. The fundamental reaction of these syntheses is the condensation of phenols or amines with bromoalkanes in the presence of a base that can consume the hydrobromic acid formed during the coupling reaction [20]. Typical synthetic routes are depicted in Fig. 2. The structures of the final products were in all cases confirmed by NMR and mass spectrometry [20], and were in some cases also substantiated by subsequent formation of coordination complexes whose structures were determined by X-ray crystallography (vide infra). 3.2. Diiron model complexes It is evident that the ligands in Fig. 1 should permit the synthesis of diiron complexes in which the bridging carboxylate(s) is (are) a part of the ligand. Such diiron complexes should also contain the labile coordination sites that are necessary for catalysis. Some of these ligands open a possibility to enrich the iron coordination spheres by oxygen donors, which predominate in the coordination spheres of the diiron clusters in RNR and MMO. Using the above-mentioned ligands, we have initiated the preparation and characterization of diiron complexes containing immobilized ligand-based bridging carboxylates and having labile coordination sites occupied by water molecules. Ligand 1 (2-(pyrid-2-ylmethoxy)benzoic acid) is potentially tridentate. Coordination of two molecules of 1 to an Fe–O–Fe core requires coordination of two more monodentate ligands (e.g. solvent) per metal in order for the irons to be hexa-coordinate. However, the formation of a trinuclear, rather than dinuclear, complex with a triply bridging central oxo ligand is observed in the reaction of 1 with Fe(ClO4)3P9H2O in methanol [24]. The resultant complex, [(MeOH)(H2O)2Fe3(m3-O)(1)6](ClO4)7 (11), has been characterized by magnetometry and cyclic voltammetry, IR ¨ and Mossbauer spectroscopy, elemental analysis and X-ray crystallography 2. The crystal structure of 11 reveals that the 2 Selected crystallographic data for complexes: 12: [Fe2O(2)2(H2O)2](ClO4)2P13H2O, C36H56Cl2Fe2N4O30, space group ˚ Vs46812 A ˚ 3, Zs24, Cu Ka radiation, ms4.039 Pn-3, as36.040(1) A, mmy1, 10 812 observed reflections, 3604 reflections used in the structure refinement (I)2s(I)), R1(I)s0.0996, wR2s0.2543. For further crystallographic details, see Ref. [21]. 14: [Fe2O(5)(OBz)]2(ClO4)4PC2H5OHP3H2O, C88H100Cl4Fe4N12O29, ˚ as72.648(2), bs as13.685(3), bs14.468(4), cs15.666(2) A,
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Fig. 1. Framework ligands containing potentially bridging/dinucleating carboxylate moieties.
pyridine-carboxylate ligands coordinate through their carboxylate moieties only, as depicted in Fig. 3. Presumably, the formation of 11 is kinetically/thermodynamically favored over the formation of the desired dinuclear complex. ˚ 3, F(000)s1114.0, Mo Ka radi14.468(3), gs15.666(2)8, Vs2567.32 A ation, ms0.74 mmy1, Dcs1.398 Mg my3, 18 506 unique reflections, R1s0.1164 for 2209 reflections (Fo)4sFo), wR2s0.5288. 16: C84H80Cl8Fe4N12O36.75, space group P1¯ , as16.4575(3), bs ˚ as77.521(1), bs89.245(1), gs 17.8346(2), cs19.3790(3) A, ˚ 3, Zs2, Dcs1.410 Mg my3, ms0.788 86.432(1)8, Us5542.9(2) A mmy1, F(000)s2404, 31 848 reflections [Fo)2sFo] used in the structure refinement. R1(F)s0.0973, wR2(F2)s0.2047. For further crystallographic details, see Ref. [19].
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In order to saturate the coordination sphere of the irons in a hypothetical dinuclear iron-oxo complex to a greater extent, the potentially tetradentate ligand 2-[(2,2-bipyrid-6-yl)methoxy]benzoic acid (2) (cf. Fig. 1), which is closely related to 1, was prepared [20]. Reaction of the sodium salt of 2 with Fe(ClO4)3P9H2O in an acetonitrile/methanol mixture led to the formation of a diiron(III) complex of the formulation [Fe2O(2)2(H2O)2](ClO4)2 (12, Fig. 4) which contains a di-m-carboxylato-diiron core that resembles the diiron cluster present in hemerythrin. Complex 12 was characterized by electrospray ionization mass spectrometry ¨ (ESMS), UV–Vis, NMR and Mossbauer spectroscopy as well as elemental analysis and X-ray crystallography 2
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Fig. 2. Synthetic routes to (a) 2-(2,29-dipyrid-6-yl)methoxybenzoic acid (2) [17,20], and (b) 2,6-di{3-[di(pyridylmethyl)amino]propoxy}benzoic acid (5) [19,20].
Fig. 3. Schematic drawing of the molecular structure of the cation of [(MeOH)(H2O)2Fe3(m3-O)(1)6](ClO4)7 (11).
[17,21]. In the solid state, the structure of the compound is polymeric; the crystal structure shows that it consists of dinuclear Fe2O units that are interconnected by the dipyridylcarboxylate ligands which coordinate so that the carboxylate moiety of each ligand bridges one Fe2O unit, while the dipyridyl group simultaneously chelates an iron atom of an adjacent Fe2O unit, thus forming an extended polymer (Fig. 5). However, ESMS (Mqs738; [Fe2O(2)2]2q) and spectroscopic [17] data indicate that in acetonitrile solution, complex 12 exists as a monomeric diiron complex in accordance with its depiction in Fig. 4. It is likely that there is a solution
equilibrium between the monomer and polymer, which involves dissociation of particular ligand groups. In efforts to mimic the diiron active site of MMO and RNR, the ligand 2,6-bis{3-[N,N-di(2-pyridylmethyl)amino]propoxy}benzoic acid (5) (Fig. 1) has been used to prepare a ferric iron-oxo complex which is proposed to be [Fe2O(5)(H2O)2](ClO4)2 (13) (Fig. 4) [18,19]. This formulation is supported by microanalysis and spectroscopic data but our efforts to grow crystals of 13 suitable for X-ray diffraction experiments have thus far been unsuccessful. Complex 13 contains labile coordination sites (the coordinated water molecules) at which a second carboxylate, which is not a part of the framework ligand, can coordinate to form, for example, [Fe2O(5)(OBz)](ClO4)2 (14) and [Fe2O(5)(ClCH2CO2)](ClO4)2 (15) (Fig. 6) [18]. The MMO diiron core has been shown to exhibit a similar behavior, in that an exogenous acetate was found to coordinate to the diiron cluster in the original structure determination of the oxidized form of the MMO hydroxylase protein [9]. However, the X-ray crystal structure 2 of 14 shows that, at least in the case of benzoic acid, the coordination of the additional carboxylate is accompanied by a dimerization to form a complex in which two diiron units are interconnected via ligand 5. Electrospray mass spectrometry indicates that
Fig. 4. Proposed molecular structures of [Fe2O(2)2(H2O)2](ClO4)2 (12) and [Fe2O(5)(ClCH2CO2)](ClO4)2 (13).
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Fig. 5. Schematic drawing of the polymeric solid-state structure containing Fe–O–Fe units linked by molecules of 2 (cf. text).
both the dimeric and monomeric forms of 14 are present in acetonitrile solution; it is possible that there is an equilibrium between these two forms (Fig. 6). Under strongly acidic conditions (excess of chloroacetic acid), further coordination of chloroacetate to 15 leads to the formation of [{Fe2OL(ClCH2CO2)2}2](ClO4)4 (16), the X-ray crystal structure 2 of which shows that it has a helical form (cf. Fig. 6) [19]. This ‘dimer of dimers’ consists of two hemerythrin-like Fe–O–Fe units that are bridged by two chloroacetates and linked to each other via two ligands 5. The carboxylate moieties of the two framework ligands form a hydrogen-bonded pair at the center of the helix. 3.3. Some features of diiron complex-mediated alkane oxidation by hydrogen peroxide A preliminary investigation of the catalytic activities of the complexes [Fe2O(5)L](ClO4)2 (Ls2H2O (13), C6H5CO2y (14), ClCH2CO2y (15), CH3CO2y) in alkane (methane, isopentane, cyclohexane) oxidation in acetonitrile solution, using hydrogen peroxide as the oxidant, has been undertaken. The formation of methanol (1–2 turnovers) during methane oxidation and cyclohexanol/cyclohexanone (30 turnovers) during cyclohexane oxidation was detected. These results are comparable to those observed for hydroperoxideassisted oxidation of a number of linear and cyclic alkanes by the diiron complexes [Fe2O(L)2(H2O)2](ClO4)4 (Lsbipy, 5,59-Me2-bipy, 5,59-(CH2Cl)2-bipy, phen, 5NO2-phen) [26]. Opposite sensitivities of W0 and alcohol/ ketone ratio to changes in L are observed during cyclohexane oxidation (Table 1), indicating that metal-based oxidation
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Fig. 6. Proposed/observed transformations in acetonitrile solution of iron-oxo complexes based on 2,6-di{3-[di(pyridylmethyl)amino]propoxy}benzoic acid (5) (RsCH3, C6H5, ClCH2). Table 1 W0 and alcohol/ketone ratios for cyclohexane oxidation by the diiron complexes [Fe2O(5)L](ClO4)2 (Ls2H2O (13), C6H5CO2y (14), CH3CO2y) in acetonitrile solution L
(H2O)2
C6H5CO2y
CH3CO2y
W0 (mM hy1) Alcohol/ketone
1.0 2.5
1.2 1.4
1.8 1.3
may be taking place. However, self-oxidation and destruction of the catalytic complex as well as observation of alkylhydroperoxide intermediates, which is evidence for a radical chain mechanism, have demonstrated that these complexes are ill-adapted for oxygen-transfer catalysis. 4. Summary and conclusions (i) The framework polydentate ligands 1–10 containing potentially bridging carboxylate moieties as well as terminal
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nitrogen and oxygen donor groups have been synthesized. Several iron-oxo complexes based on these ligands have been prepared and the solid-state and solution structures of some of these complexes have been established. (ii) Preliminary information shows that at least some of these ligands seem to be too flexible to provide the required kinetic stability that will enable their diiron complexes to effect biomimetic oxidation of substrates by hydrogen peroxide via a catalytic oxygen-transfer mechanism. We are presently attempting the preparation of oxo- or hydroxo-bridged diiron complexes that may serve as model complexes for the active sites of RNR or MMO. Ultimately, we wish to use such biomimetic complexes to mimic and investigate the catalytic reactions of these oxidative diiron enzymes. Thus far, our efforts have been centered on the synthesis of iron complexes, but the preparation of manganese, copper and nickel complexes of 1–10 as well as related ligands is also being undertaken.
Acknowledgements This work was supported by grants from the Russian Foundation for Basic Research (No. 97-03-32253), INTAS (No. 97-1289), the Swedish Natural Science Research Council (NFR) and the Royal Swedish Academy of Sciences. We ¨ thank Dr N.S. Ovanesyan for assistance with Mossbauer measurements and Dr K.B. Jensen (University of Southern Denmark, Odense) for some of the mass spectrometric measurements on complexes 13, 15 and 16.
References [1] I. Bertini, H.B. Gray, S.J. Lippard, J.S. Valentine (Eds.), Bioinorganic Chemistry, University Science Books, Mill Valley, CA, 1994. [2] J. Reedijk (Ed.), Bioinorganic Catalysis, Marcel Dekker, New York, 1993.
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[3] R.H. Holm, P. Kennepohl, E.I. Solomon, Chem. Rev. 96 (1996) 2239–2314. [4] B.J. Wallar, J.D. Lipscomb, Chem. Rev. 96 (1996) 2625–2657. [5] A.M. Valentine, S.J. Lippard, J. Chem. Soc., Dalton Trans. (1997) 3925–3932. [6] D.M. Kurtz Jr., J. Biol. Inorg. Chem. 2 (1997) 159–167. [7] R.E. Stenkamp, Chem. Rev. 94 (1994) 715–726. [8] P. Nordlund, H. Eklund, J. Mol. Biol. 232 (1993) 123–164. [9] A.C. Rosenzweig, C.A. Frederic, S.J. Lippard, P. Nordlund, Nature 366 (1993) 537–543. [10] D.M. Kurtz Jr., Chem. Rev. 90 (1990) 585–606. [11] L. Que Jr., J. Chem. Soc., Dalton Trans. (1997) 3933–3940. ´ E.C. Wilkinson, L. Que Jr., M. Fontecave, Angew. Chem., [12] S. Menage, Int. Ed. Engl. 34 (1995) 203–205. [13] L. Berchet, M.-N. Collomb-Dunand-Sauthier, P. Dubordeaux, W. Moneta, A. Deronzier, J.-M. Latour, Inorg. Chim. Acta 282 (1998) 243–246. [14] D.E. Fenton, H. Okawa, Chem. Ber./Recl. Trav. Chim. Pays-Bas 130 (1997) 433–442. [15] N. Elango, R. Radhakrishnan, W.A. Froland, B.J. Wallar, C.A. Earhart, J.D. Lipscomb, D.H. Ohlendorf, Protein Sci. 6 (1997) 556– 568. [16] A.A. Shteinman, FEBS Lett. 362 (1995) 5–9. [17] V.M. Trukhan, A.A. Shteinman, Russ. Chem. Bull. 46 (1997) 202– 203. [18] V.M. Trukhan, V.V. Polukhov, I.V. Sulimenkov, N.S. Ovanesyan, N.A. Kovalchuk, A.F. Dodonov, A.A. Shteinman, Kinet. Catal. 39 (1998) 788–791. [19] V.M. Trukhan, C.G. Pierpont, K.B. Jensen, E. Nordlander, A.A. Shteinman, Chem. Commun. (1999) 1193. [20] V.M. Trukhan, E. Nordlander, A.A. Shteinman, unpublished results. ¨ B. [21] V.M. Trukhan, S.G. Mkoyan, L.O. Atovmyan, H. Schwoppe, Krebs, V.I. Kozlovski, A.F. Dodonov, N.S. Ovanesyan, V.V. Strelets, A.A. Shteinman, unpublished results. [22] A. Dodonov, V. Kozlovski, A. Loboda, V. Raznikov, I. Soulimenkov, A. Tolmachev, A. Kraft, H. Wollnik, Rapid Commun. Mass Spectrom. (1997) 1649–1656. [23] A. Hazell, K.B. Jensen, K.J. McKenzie, H. Toftlund, J. Chem. Soc., Dalton Trans. (1993) 3249–3255. [24] V.M. Trukhan, I.L. Eremenko, N.S. Ovanesyan, A.A. Pasynskii, I.A. Petrunenko, V.V. Strelets, A.A. Shteinman, Russ. Chem. Bull. 45 (1996) 1981–1987. [25] C. Hemmert, M. Verelst, J.-P. Tuchagues, Chem. Commun. (1996) 617–618. [26] O.N. Gritsenko, G.N. Nesterenko, V.S. Kulikova, A.A. Shteinman, Kinet. Catal. 38 (1997) 639–645.
StyleTag -- Journal: JIB (Journal of Inorganic Biochemistry)
Article: 6338
Design and synthesis of new models for diiron biosites V.M. Trukhan a, O.N. Gritsenko a, E. Nordlander b,1, A.A. Shteinman a,* a
Institute of Problems of Chemical Physics, Russian Academy of Sciences, 142432 Chernogolovka, Russia b Inorganic Chemistry 1, Chemical Center, Lund University, Box 124, SE-221 00 Lund, Sweden Received 3 June 1999; received in revised form 17 November 1999; accepted 23 November 1999
Abstract In order to mimic dinuclear active sites of some non-heme diiron proteins, ten new polydentate and potentially dinucleating ligands have been synthesized. Each ligand contains a carboxylate moiety designed to bridge two metal atoms. These central carboxylate moieties are derived from substituted benzoic acids that in turn are linked to terminal nitrogen or oxygen donors by spacers so that framework-type polydentate ligands similar to the polypeptide frames in diiron metallobiosites are formed. Reaction of these ligands with Fe(ClO4)3P9H2O leads to ferric m-oxo-m-carboxylato iron complexes of the general formulas [Fe2O(L)2(H2O)2](ClO4)2 and [Fe2O(L)(BzO)](ClO4)2 (Lsligand), containing one or two immobilized bridging carboxylates, respectively. While X-ray crystallography shows that some of these complexes are dimers or network polymers in the solid state, electrospray ionization mass spectrometry (ESMS) and spectroscopic data ¨ (UV–Vis, NMR, Mossbauer) indicate that they dissociate to monomeric Fe2O units in dilute CH3CN solutions. q2000 Elsevier Science Inc. All rights reserved. Keywords: Diiron biosites; Modeling; Framework ligands; Fe(III) complexes
1. Introduction Dinuclear oxo- and hydroxo-bridged iron complexes have recently received much attention in bioinorganic chemistry [1–3] owing to the wide occurrence of such units in organisms and their involvement in a range of biological functions such as the storage and transport of dioxygen (hemerythrin, Hr), DNA synthesis (ribonucleotide reductase R2 protein, RNR), alkane oxidation (methane monooxygenase, MMO), etc. The active sites of non-heme iron proteins such as Hr, RNR and MMO contain two iron atoms bridged by at least one endogenous carboxylate and usually one or two oxo-, hydroxo- or aqua-bridges derived from exogenous water and capped by endogenous nitrogen and oxygen donor groups [1–5]. Despite the common structural characteristics of their active sites, the functional diversity is one of the most striking aspects of this class of proteins [6]. These diiron biosites have been characterized by spectroscopic and magnetic methods [4,5], and X-ray crystal structures of diiron cores have been determined for Hr [7], the R2 subunit of RNR [8], MMO [9] and some other related proteins. In the case of RNR and MMO, a great number of hypotheses regarding the mechanisms of oxygen activation * Corresponding author. Fax: q7-096-515-3588; e-mail: [email protected] 1 Also corresponding author. E-mail: [email protected]
by the diiron clusters in these proteins have been put forth, but elucidation of these mechanisms remains one of the most challenging unsolved problems in bioinorganic chemistry [1–6]. Insights into the mechanisms of RNR and MMO— which are structurally closely related with respect to the coordination environment of their diiron clusters—may be achieved by synthesis of structural models of their active sites and investigation of these model complexes in functional mimicry of the enzymes. A number of Fe2O complexes have been synthesized [10,11] to model the spectroscopic and structural properties of m-oxo-m-carboxylato diiron sites in non-heme iron proteins. In many cases, such biomimetic complexes have been found to be relatively unstable [12]. Efforts have therefore been made to prepare framework polydentate ligands that may stabilize this type of complex [13]: examples include ligands in which multidentate nitrogen bases that bind to metals in a capping manner are linked to alkoxo or phenoxo groups capable of bridging two metals [14]. The use of framework dinucleating ligands is promising for modeling of this type of biosite because such ligands permit some preorganization of the iron coordination sphere. Considering the fact that the carboxylate bridges in the above-mentioned enzymes are parts of the protein [15] while the oxygen bridges in the diiron sites are derived from exogenic water or
0162-0134/00/$ - see front matter q2000 Elsevier Science Inc. All rights reserved. PII S 0 1 6 2 - 0 1 3 4 ( 9 9 ) 0 0 2 4 4 - 5
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dioxygen and probably take part in catalytic transformations [16], it is of interest to use polydentate ligands in which the nitrogen bases are linked to a carboxylate moiety for structural and functional modeling of the active sites. Until recently, only diiron complexes of framework ligands in which the bridging ligand moieties are phenoxo or alkoxo groups were known. Here, we describe the preparation of a number of new framework ligands and the reactions of some of these ligands to form new iron-oxo complexes [17–19]. These ligands contain central carboxylates, designed to bridge Fe–O–Fe cores, that are connected to nitrogenous base(s) by spacer(s) (e.g. –CH2CO–, –CH2CH2CH2–). A preliminary investigation of the catalytic activities of some of the reported diiron complexes in alkane oxidation is also reported. 2. Experimental The ligands described here were designed on the basis of relative ease of synthesis and potential fit to a dinuclear oxobridged metal (iron) core as determined by molecular modeling using the HyperChem 5.0 software package. Commercial chemicals were used without further purification. All reactions were carried out under an atmosphere of dry, oxygen-free nitrogen, using solvents which were freshly distilled from appropriate drying agents. Microanalyses were performed in the Analytical Laboratory of the Institute of Problems of Chemical Physics (Chernogolovka). NMR spectra were recorded on Varian Unity 300 MHz (Lund University) or Tesla 200 MHz (Chernogolovka) spectrometers. Mass spectrometric measurements were carried out on a unique home-built high resolution time-of-flight mass spectrometer [22] in the laboratory of Professor A.F. Dodonov of the Institute of Energy Problems in Chemical Physics (Chernogolovka). X-ray data collection was carried out with a Siemens SMART CCD area detector at Lund University and a KUMA automatic diffractometer at the Institute of Problems of Chemical Physics (Chernogolovka). Further experimental details may be found in Refs. [17–22]. 3. Results and discussion 3.1. Syntheses of framework dinucleating ligands Two unsuccessful attempts to prepare dinuclear iron(III) complexes containing polydentate ligands with connected nitrogen bases and bridging carboxylate moieties have been reported [23,24] (vide infra). Furthermore, a bis-m-carboxylato diiron(II) complex of a dipyridylbenzoyl amine which acts as a pentadentate dinucleating ligand has recently been reported [25]. This complex has an unusual syn–anti configuration of ligand-based carboxylate bridges and a relatively ˚ In order to create more large Fe–Fe distance ()4.6 A). adequate models for non-heme diiron biosites we have undertaken the design and synthesis of new dinucleating framework ligands with bridging carboxylates.
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Our efforts have resulted in a first generation of framework ligands that is shown in Fig. 1 [20]. Ligands 1–10 (Fig. 1) are based on substituted benzoic acids which contain the central (and potentially bridging) carboxylates. Spacers of different length and type (–CH2O–, –CH2CH2CH2–, –CH2C6H4CH2–) were chosen to connect the terminal donor groups (pyridine, dipicolylamine, dipyridyl, carboxylate) to the central benzoate moieties. The methyl esters of salicylic and resorcylic acids were taken as starting reagents to produce ligands with one and two terminal arms, designed to form complexes with two and one ligand-based bridging carboxylates, respectively. The fundamental reaction of these syntheses is the condensation of phenols or amines with bromoalkanes in the presence of a base that can consume the hydrobromic acid formed during the coupling reaction [20]. Typical synthetic routes are depicted in Fig. 2. The structures of the final products were in all cases confirmed by NMR and mass spectrometry [20], and were in some cases also substantiated by subsequent formation of coordination complexes whose structures were determined by X-ray crystallography (vide infra). 3.2. Diiron model complexes It is evident that the ligands in Fig. 1 should permit the synthesis of diiron complexes in which the bridging carboxylate(s) is (are) a part of the ligand. Such diiron complexes should also contain the labile coordination sites that are necessary for catalysis. Some of these ligands open a possibility to enrich the iron coordination spheres by oxygen donors, which predominate in the coordination spheres of the diiron clusters in RNR and MMO. Using the above-mentioned ligands, we have initiated the preparation and characterization of diiron complexes containing immobilized ligand-based bridging carboxylates and having labile coordination sites occupied by water molecules. Ligand 1 (2-(pyrid-2-ylmethoxy)benzoic acid) is potentially tridentate. Coordination of two molecules of 1 to an Fe–O–Fe core requires coordination of two more monodentate ligands (e.g. solvent) per metal in order for the irons to be hexa-coordinate. However, the formation of a trinuclear, rather than dinuclear, complex with a triply bridging central oxo ligand is observed in the reaction of 1 with Fe(ClO4)3P9H2O in methanol [24]. The resultant complex, [(MeOH)(H2O)2Fe3(m3-O)(1)6](ClO4)7 (11), has been characterized by magnetometry and cyclic voltammetry, IR ¨ and Mossbauer spectroscopy, elemental analysis and X-ray crystallography 2. The crystal structure of 11 reveals that the 2 Selected crystallographic data for complexes: 12: [Fe2O(2)2(H2O)2](ClO4)2P13H2O, C36H56Cl2Fe2N4O30, space group ˚ Vs46812 A ˚ 3, Zs24, Cu Ka radiation, ms4.039 Pn-3, as36.040(1) A, mmy1, 10 812 observed reflections, 3604 reflections used in the structure refinement (I)2s(I)), R1(I)s0.0996, wR2s0.2543. For further crystallographic details, see Ref. [21]. 14: [Fe2O(5)(OBz)]2(ClO4)4PC2H5OHP3H2O, C88H100Cl4Fe4N12O29, ˚ as72.648(2), bs as13.685(3), bs14.468(4), cs15.666(2) A,
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Fig. 1. Framework ligands containing potentially bridging/dinucleating carboxylate moieties.
pyridine-carboxylate ligands coordinate through their carboxylate moieties only, as depicted in Fig. 3. Presumably, the formation of 11 is kinetically/thermodynamically favored over the formation of the desired dinuclear complex. ˚ 3, F(000)s1114.0, Mo Ka radi14.468(3), gs15.666(2)8, Vs2567.32 A ation, ms0.74 mmy1, Dcs1.398 Mg my3, 18 506 unique reflections, R1s0.1164 for 2209 reflections (Fo)4sFo), wR2s0.5288. 16: C84H80Cl8Fe4N12O36.75, space group P1¯ , as16.4575(3), bs ˚ as77.521(1), bs89.245(1), gs 17.8346(2), cs19.3790(3) A, ˚ 3, Zs2, Dcs1.410 Mg my3, ms0.788 86.432(1)8, Us5542.9(2) A mmy1, F(000)s2404, 31 848 reflections [Fo)2sFo] used in the structure refinement. R1(F)s0.0973, wR2(F2)s0.2047. For further crystallographic details, see Ref. [19].
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In order to saturate the coordination sphere of the irons in a hypothetical dinuclear iron-oxo complex to a greater extent, the potentially tetradentate ligand 2-[(2,2-bipyrid-6-yl)methoxy]benzoic acid (2) (cf. Fig. 1), which is closely related to 1, was prepared [20]. Reaction of the sodium salt of 2 with Fe(ClO4)3P9H2O in an acetonitrile/methanol mixture led to the formation of a diiron(III) complex of the formulation [Fe2O(2)2(H2O)2](ClO4)2 (12, Fig. 4) which contains a di-m-carboxylato-diiron core that resembles the diiron cluster present in hemerythrin. Complex 12 was characterized by electrospray ionization mass spectrometry ¨ (ESMS), UV–Vis, NMR and Mossbauer spectroscopy as well as elemental analysis and X-ray crystallography 2
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Fig. 2. Synthetic routes to (a) 2-(2,29-dipyrid-6-yl)methoxybenzoic acid (2) [17,20], and (b) 2,6-di{3-[di(pyridylmethyl)amino]propoxy}benzoic acid (5) [19,20].
Fig. 3. Schematic drawing of the molecular structure of the cation of [(MeOH)(H2O)2Fe3(m3-O)(1)6](ClO4)7 (11).
[17,21]. In the solid state, the structure of the compound is polymeric; the crystal structure shows that it consists of dinuclear Fe2O units that are interconnected by the dipyridylcarboxylate ligands which coordinate so that the carboxylate moiety of each ligand bridges one Fe2O unit, while the dipyridyl group simultaneously chelates an iron atom of an adjacent Fe2O unit, thus forming an extended polymer (Fig. 5). However, ESMS (Mqs738; [Fe2O(2)2]2q) and spectroscopic [17] data indicate that in acetonitrile solution, complex 12 exists as a monomeric diiron complex in accordance with its depiction in Fig. 4. It is likely that there is a solution
equilibrium between the monomer and polymer, which involves dissociation of particular ligand groups. In efforts to mimic the diiron active site of MMO and RNR, the ligand 2,6-bis{3-[N,N-di(2-pyridylmethyl)amino]propoxy}benzoic acid (5) (Fig. 1) has been used to prepare a ferric iron-oxo complex which is proposed to be [Fe2O(5)(H2O)2](ClO4)2 (13) (Fig. 4) [18,19]. This formulation is supported by microanalysis and spectroscopic data but our efforts to grow crystals of 13 suitable for X-ray diffraction experiments have thus far been unsuccessful. Complex 13 contains labile coordination sites (the coordinated water molecules) at which a second carboxylate, which is not a part of the framework ligand, can coordinate to form, for example, [Fe2O(5)(OBz)](ClO4)2 (14) and [Fe2O(5)(ClCH2CO2)](ClO4)2 (15) (Fig. 6) [18]. The MMO diiron core has been shown to exhibit a similar behavior, in that an exogenous acetate was found to coordinate to the diiron cluster in the original structure determination of the oxidized form of the MMO hydroxylase protein [9]. However, the X-ray crystal structure 2 of 14 shows that, at least in the case of benzoic acid, the coordination of the additional carboxylate is accompanied by a dimerization to form a complex in which two diiron units are interconnected via ligand 5. Electrospray mass spectrometry indicates that
Fig. 4. Proposed molecular structures of [Fe2O(2)2(H2O)2](ClO4)2 (12) and [Fe2O(5)(ClCH2CO2)](ClO4)2 (13).
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Fig. 5. Schematic drawing of the polymeric solid-state structure containing Fe–O–Fe units linked by molecules of 2 (cf. text).
both the dimeric and monomeric forms of 14 are present in acetonitrile solution; it is possible that there is an equilibrium between these two forms (Fig. 6). Under strongly acidic conditions (excess of chloroacetic acid), further coordination of chloroacetate to 15 leads to the formation of [{Fe2OL(ClCH2CO2)2}2](ClO4)4 (16), the X-ray crystal structure 2 of which shows that it has a helical form (cf. Fig. 6) [19]. This ‘dimer of dimers’ consists of two hemerythrin-like Fe–O–Fe units that are bridged by two chloroacetates and linked to each other via two ligands 5. The carboxylate moieties of the two framework ligands form a hydrogen-bonded pair at the center of the helix. 3.3. Some features of diiron complex-mediated alkane oxidation by hydrogen peroxide A preliminary investigation of the catalytic activities of the complexes [Fe2O(5)L](ClO4)2 (Ls2H2O (13), C6H5CO2y (14), ClCH2CO2y (15), CH3CO2y) in alkane (methane, isopentane, cyclohexane) oxidation in acetonitrile solution, using hydrogen peroxide as the oxidant, has been undertaken. The formation of methanol (1–2 turnovers) during methane oxidation and cyclohexanol/cyclohexanone (30 turnovers) during cyclohexane oxidation was detected. These results are comparable to those observed for hydroperoxideassisted oxidation of a number of linear and cyclic alkanes by the diiron complexes [Fe2O(L)2(H2O)2](ClO4)4 (Lsbipy, 5,59-Me2-bipy, 5,59-(CH2Cl)2-bipy, phen, 5NO2-phen) [26]. Opposite sensitivities of W0 and alcohol/ ketone ratio to changes in L are observed during cyclohexane oxidation (Table 1), indicating that metal-based oxidation
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Fig. 6. Proposed/observed transformations in acetonitrile solution of iron-oxo complexes based on 2,6-di{3-[di(pyridylmethyl)amino]propoxy}benzoic acid (5) (RsCH3, C6H5, ClCH2). Table 1 W0 and alcohol/ketone ratios for cyclohexane oxidation by the diiron complexes [Fe2O(5)L](ClO4)2 (Ls2H2O (13), C6H5CO2y (14), CH3CO2y) in acetonitrile solution L
(H2O)2
C6H5CO2y
CH3CO2y
W0 (mM hy1) Alcohol/ketone
1.0 2.5
1.2 1.4
1.8 1.3
may be taking place. However, self-oxidation and destruction of the catalytic complex as well as observation of alkylhydroperoxide intermediates, which is evidence for a radical chain mechanism, have demonstrated that these complexes are ill-adapted for oxygen-transfer catalysis. 4. Summary and conclusions (i) The framework polydentate ligands 1–10 containing potentially bridging carboxylate moieties as well as terminal
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nitrogen and oxygen donor groups have been synthesized. Several iron-oxo complexes based on these ligands have been prepared and the solid-state and solution structures of some of these complexes have been established. (ii) Preliminary information shows that at least some of these ligands seem to be too flexible to provide the required kinetic stability that will enable their diiron complexes to effect biomimetic oxidation of substrates by hydrogen peroxide via a catalytic oxygen-transfer mechanism. We are presently attempting the preparation of oxo- or hydroxo-bridged diiron complexes that may serve as model complexes for the active sites of RNR or MMO. Ultimately, we wish to use such biomimetic complexes to mimic and investigate the catalytic reactions of these oxidative diiron enzymes. Thus far, our efforts have been centered on the synthesis of iron complexes, but the preparation of manganese, copper and nickel complexes of 1–10 as well as related ligands is also being undertaken.
Acknowledgements This work was supported by grants from the Russian Foundation for Basic Research (No. 97-03-32253), INTAS (No. 97-1289), the Swedish Natural Science Research Council (NFR) and the Royal Swedish Academy of Sciences. We ¨ thank Dr N.S. Ovanesyan for assistance with Mossbauer measurements and Dr K.B. Jensen (University of Southern Denmark, Odense) for some of the mass spectrometric measurements on complexes 13, 15 and 16.
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