complex with two different coordination

Inorganic Chemistry Communications 84 (2017) 109–112

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Synthesis and structural characterization of a hemiacetal and aldehyde bound diiron(III) complex with two different coordination numbers: A product by oxidative cleavage of carbon\\nitrogen single bond Dhrubajyoti Mondal ⁎, Kisholoy Bhattacharya Department of Inorganic Chemistry, Indian Association for the Cultivation of Science, 2A & 2B Raja S.C. Mullick Road, Jadavpur, Kolkata 700032, India

a r t i c l e

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Article history: Received 12 June 2017 Received in revised form 19 July 2017 Accepted 5 August 2017 Available online 07 August 2017 Keywords: Dinuclear iron(III) complex Hemiacetal iron(III) complex Metal-oxo C-H bond activation

a b s t r a c t The work in this report presents the synthesis, structural and spectroscopic characterization of a hemiacetal and aldehyde bound diiron(III) complex 1 with two different coordination numbers supported by a tetradentate N2O2 donor H2L ligand, despite the fact that no aldehyde molecules have been added to the solution. The complex 1 is a phenoxo and alkoxo bridged complex and it was generated when oxo-transfer reagent iodosylbenzene was treated to a methanolic solution containing Fe(BF4)2·6H2O and deprotonated H2L ligand. The Fe⋯Fe separation of this complex is 3.158 Å. © 2017 Elsevier B.V. All rights reserved.

The oxidation of organic compounds mediated by transition metal complexes has been an area of intensive research because of their industrial implications and relevance to metalloenzymatic systems [1]. Among these types of metalloenzymes, heme and nonheme iron enzymes participate in many metabolically important oxidative transformations by activating dioxygen [2–11]. The key intermediate in the catalytic cycle of dioxygen activation of these types of iron enzymes is high-valent oxoiron(IV) species that effect the oxygenation of organic substrates. For example, oxoiron(IV) porphyrin π-cation radicals in Cytochrome P450 are believed to responsible for the oxidative metabolism and detoxification of xenobiotics and a number of important biochemical transformations [2–4]. Nonheme iron enzymes are highly versatile in nature and nonheme oxoiron(IV) complexes bearing different types of tetra and pentadentate ligands have been synthesized and characterized by different spectroscopy [12–15]. The reactivities of these nonheme iron(IV)-oxo complexes are studied in various oxidation reactions, such as aliphatic hydroxylation, olefin epoxidation, alcohol oxidation, C\\H bond activation of alkylaromatics, N-dealkylation, aromatic hydroxylation, and the oxidation of sulfides and phosphines [16–17]. Both heme and nonheme iron enzymes participate in oxidative N-dealkylation reactions in nature. The oxidative N-dealkylation of N,Ndialkylamines by CYP 450 and their model compounds has been intensively investigated and it is proposed that this type of oxidation reaction occurs via two different mechanisms, such as an electron transferproton transfer (ET-PT) or a hydrogen atom transfer (HAT) mechanism ⁎ Corresponding author. E-mail addresses: [email protected], [email protected] (D. Mondal).

http://dx.doi.org/10.1016/j.inoche.2017.08.007 1387-7003/© 2017 Elsevier B.V. All rights reserved.

[2,18–19]. The Escherichia coli AlkB protein has been known to play an important role in alkylated DNA damage repair and it has been proposed that a nonheme oxoiron(IV) species takes part as an active oxidant for oxidative N-dealkylation reaction [20]. Wonwoo Nam and co-workers have reported the oxidative N-dealkylation of N,Ndimethylaniline by [(N4Py)FeIV(O)]2+ and [(TMC)FeIV(O)]2 + species where N-methylaniline was formed as a major product with the concurrent formation of CH2O and they have proposed that the N-dealkylation reaction is occurred via an electron transfer-proton transfer (ET-PT) mechanism [21]. Nonheme diiron enzymes have also attracted great interest as they carry out a wide variety of reactions despite having very similar active sites [22–29]. For example Methane monooxygenase (MMO) is an enzyme that catalyzes the methane oxidation reaction and the main active species is a diamond core, (FeIV)2(μ-O)2 species with Fe-Fe separation of 2.46 Å [30]. Generally such types of high valent iron-oxo complexes have been synthesized from their low valent precursor by using different types of terminal oxidant such as iodosyl benzene, hydroperoxides, peracids, KHSO5, NaOX (X = Cl or Br), O3 or R3NO. Aldehydes and ketones reacts reversibly with ROH or RO− in alcoholic solution by various catalysts (protic or Lewis acids) and transforms to acetals or ketals via the hemiacetal or hemiketal intermediate, respectively. However, isolation of the intermediate hemiacetal or hemiketal is relatively far difficult compared to that of the corresponding stable acetal or ketal. There are only a few structurally characterized examples containing stable hemiacetal or hemiketals bound metal complexes [31–34]. So the synthesis of acetal bound metal complexes is a great challenge from the view point of metal coordination chemistry.

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D. Mondal, K. Bhattacharya / Inorganic Chemistry Communications 84 (2017) 109–112

Here in this work we report the synthesis and characterization (Scheme S1) of a new type of sterically constrained tetradentate phenol based flexible N2O2 donor H2L ligand (H2L = N,N-bis-(3,5-di-isopropyl-2hydroxybenzyl)-N′,N′-diisopropylethylenediamine). The H2L ligand is then used to synthesis of a hemiacetal and aldehyde bound diiron(III) complex 1 by treating Fe(BF4)2·6H2O as an metal precursor and iodosyl benzene as an terminal oxidizing agent (Scheme 1). Synthesis spectroscopic characterization and crystal structure of complex 1 have been reported in this paper and details are given in the Supporting Information. Metal-induced oxidative C\\N bond cleavage appears to be the key step in this drastic ligand transformation reaction. The diphenol N2O2 ligand (H2L) has been prepared by Mannich type reaction of N,N-diisopropylethylenediamine, 2,4-diisopropylphenol and paraformaldehyde in methanol solvent by using 1:2:2 mixture. Reaction of equimolar amount H2L ligand, Fe(BF4)2·6H2O and 2 equivalents of triethylamine in methanol yielded a light brown colored solution under N2 atmosphere and then one equivalent of oxo-transfer reagent iodosyl benzene was added to get a dark brown solution. Slow evaporation of the solvent at 4 °C yielded needle shaped browned colored complex [(FeIII2(L)(LHemiacetal)(LAldehyde)], (1) suitable for X-ray analysis. [LHemiacetal]3 − and [LAldehyde]− are deprotonated form of 2-(bis(3,5diisopropyl-2-hydroxybenzyl)amino)-1-methoxyethanol and 2-hydroxy-3,5-diisopropylbenzaldehyde ligand, respectively and both these oxidized ligands are originated from H2L ligand by oxidative cleavage of carbon nitrogen single bond (see later). Here it is important to mention that our main objective of this reaction was to activate the C\\H bond of isopropyl substituent [35] on the phenolic group but surprisingly we get the crystals of complex 1. Selected IR band for H2L ligand and compound 1 have been listed in the Experimental Section (SI). IR spectra of the complex 1 show all the characteristic bands of the coordinated ligands and it is shown in Fig. S1. The spectra display a strong signature band at 1618 cm− 1 owing to ν(C_O) vibration together with a sharp aldehydic C\\H stretch at ca. 2958 cm−1, both providing the signatures for an aldehyde group in this molecule. The absorption band of bridging hemiacetal in 1 can be deduced by peaks at 1074 and 605 cm−1 which are characteristic for aliphatic ethers. UV–Vis spectrum of this complex is shown in Fig. S2 and it exhibits bands at 321 (4200), 403 (2200), 451 (2100) and 578 (1100) nm. The higher energy band at 321 nm appears due to intraligand π-π* transition of H2L ligand. The bands observed in the range 403–451 nm can be assigned as ligand to metal charge transfer (phenolate pπ → FeIII dσ*) transitions, while the band around 578 nm can be assigned to pπ → dπ* transition [36]. The ESI-MS spectrum of complex 1 is shown in Fig. S3. The molecular ion peak appeared at m/z 525.49 is due to [(FeIII Hemiacetal (L ) + H+] ionic species, also confirmed by the perfect matching of the observed and simulated isotopic distribution patterns (Fig. S3, inset). Crystal structure of compound 1 has been determined and the molecular structures and atom-labeling scheme for this compound is displayed in Fig. 1. The selected metrical parameters are summarized in Table S2. Compound 1 crystallizes in monoclinic space group P21/c

with four molecular mass units accommodated per unit cell. The asymmetric unit consists of a neutral unsymmetrical dinuclear diferric [(FeIII Hemiacetal )(LAldehyde)] entity and two ferric ions are connected by 2(L)(L a phenoxo oxygen O(4) atom and by the deprotonated alkoxo oxygen atom O(3) of the hemiacetal moiety. The crystal structure of complex 1 (Fig. 1) reveals that the two iron ions are in distinct coordination sites with two different coordination numbers. The Fe(1) center is pentacoordinated, showing an distorted trigonal bipyramidal geometry, τ = 0.59; [τ = (β − α) / 60] [37]. The coordination geometry of the iron center Fe(1) are occupied by the N(1), O(1), O(2) and O(3) donor atoms from the [LHemiacetal]3− ligand and O(4) donor atom from bridging μphenoxo, resulting in the FeO4N core. Here the equatorial plane around Fe(1) are occupied by O(1), O(2) and bridging O(3) donor atoms, whereas the apical positions are occupied by the N(1) and bridging O(4) atoms. The iron(III) ion is 0.112 Å above the equatorial plane. The three bond angles 119.06(17)° [O(2)\\Fe(1)\\O(3)], 119.20(18)° [O(2)\\Fe(1)\\O(1)] and 120.70(18)° [O(1)\\Fe(1)\\O(3)] in the equatorial plane around Fe(1) are almost same and close to 120°. The trans angle 155.86(17)° [O(4)\\Fe(1)\\N(1)] is slightly less than 180°. The equatorial bonds are shorter than the axial bonds as expected for TBP geometry (Fig. S4). On the other hand Fe(2) is in distorted octahedral. The coordination geometry around this ion are occupied by N(2), O(4) and O(5) donor atoms from the [L]2 − ligand, O(6) and O(7) from [LAldehyde]− ligand and O(4) donor atom from bridging μ-alkoxo group of [LHemiacetal]3− ligand, resulting in the FeO5N core. N(3) atom contains the bulky isopropyl substituents and for this reason it remains uncoordinated to the Fe(2) ion. The basal positions are occupied by O(3), O(6), O(7) and N(2) donor atoms whereas the axial positions are occupied by O(4) and O(5) donor atoms. The trans angles O(6)—Fe(2)—O(3) 159.08(18)°, O(5)—Fe(2)—O(4) 170.84(17)° and O(7)—Fe(2)—N(2) 176.78(19)° are close to linearity as expected for an octahedral geometry. The Fe(2)\\O(5) bond [1.870(4) Å] trans to the bridging phenoxo oxygen O(4) is shorter than the Fe(2)\\O(6) [1.923(5) Å] due to the trans influence of the phenoxo group. The average trigonal bipyramidal Fe(1)—Ophenolate length (1.85 Å) is shorter than the average octahedral Fe(2)\\Ophenolate bond length (1.90 Å) suggesting relatively strong iron oxygen overlap in TBP, which is consistent with the lower coordination number. The Fe(1)\\O(3)\\Fe(2) [103.62(19)°] and Fe(1)\\O(4)\\Fe(2) [102.21(19)°] bond angles are almost similar and these two bridges are asymmetric where Fe(1)\\O(3), Fe(2)\\O(3); Fe(1)\\O(4) and Fe(2)\\O(4) bond distances are 1.983(4), 2.034(4) 1.973(4) and 2.083(4) Å, respectively. The Fe—Fe separation of this molecule is 3.158 Å. To the best of our knowledge, there is so far only one reported work [38] which contains diferric compound with octahedral and trigonal bipyramidal coordination geometries. Bond valence sum (BVS) calculations [39] have been also applied to confirm the oxidation states of the iron centers in 1 and it shows that both the iron centers are in +3 oxidation state (Table S3). The most interesting part this complex is the presence of tetradentate (LHemiacetal)3 − and bidentate (LAldehyde)− ligand

Scheme 1. Synthetic scheme for the formation of hemiacetal and aldehyde bound diiron(III) complex 1.


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Fig. 1. Molecular structure and atom-labeling scheme of complex 1 with thermal ellipsoids drawn at the 30% probability label. Hydrogen atoms have been omitted for clarity.

moieties in the crystal structure. The (LHemiacetal)3 − ligand formed by the solvent methanol and the 2-(bis(2-hydroxy-3,5diisopropylbenzyl)amino)acetaldehyde, bridges Fe(1) and Fe(2) through the oxygen atom O(3) with a μ-η1,η1 binding mode. The C(63)\\O(3) and C(63)\\O(8) bond lengths around tetrahedral hemiacetal carbon are 1.391(8) and 1.435(8) Å, respectively. The observed C(41)\\O(7) distance of 1.237(8) Å of (LAldehyde)− ligand appears to correspond for carbonyl functionality. We have also proposed a possible mechanistic pathway (see supporting information) to explain the generation of these two oxidized ligands from the sterically constrained tetradentate H2L ligand via C\\N single bond cleavage pathway. Here we believe that possibly a highly reactive species (I) [(L)2FeIV2O2] is first generated after the addition of PhIO. One iron(IV)-oxo centre abstracts a H atom from the methylene group [(2,4 diisopropylphenol)CH2)2N—CH2—CH2—N(i—Pr)2] of the supporting ligand and other iron(IV)-oxo centre abstracts an aliphatic H atom from the one of the benzyl group [((2,4 diisopropylphenol)CH2)2N—CH2—CH2—N(i—Pr)2] of an external sacrificing HL ligand. Then through rebound mechanism [40] species (IV) is formed. In the next step C\\N single bond cleavage is occurred and 2-(bis(2-hydroxy-3,5-diisopropylbenzyl)amino)acetaldehyde and 2-hydroxy-3,5-diisopropylbenzaldehyde are generated. As a result two amines get eliminated from the ligand backbone. Identities of these byproducts P1 (M+ = 335.6) and P2 (M+ = 102.2) have been established by ESI-MS spectroscopy. The aldehyde oxygen atom of 2(bis(2-hydroxy-3,5-diisopropylbenzyl)amino)acetaldehyde coordinates between two iron(III) ions and as a result the carbonyl carbon becomes more electrophilic in nature and leads to the formation of hemiacetal bound complex 1 via reaction with solvent methanol molecule. So the steric crowding of ligand plays an important role to generate diiron(III) complex with two different coordination numbers. In summary, we report the synthesis and structural characterization of a hemiacetal and aldehyde bound diiron(III) complex [(FeIII2(L)(LHemiacetal)(LAldehyde)] with two different coordination numbers by using oxo-transfer reagent PhIO. The mother aldehyde of (LHemiacetal)3− and (LAldehyde)− are not added to the solution as external

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