Controlling the Reactivity of Bifunctional Ligands: Carboxylate-Bridged

Article pubs.acs.org/IC

Controlling the Reactivity of Bifunctional Ligands: CarboxylateBridged Nonheme Diiron(II) Complexes Bearing Free Thiol Groups Nabhendu Pal and Amit Majumdar* Department of Inorganic Chemistry, Indian Association for the Cultivation of Science, 2A & 2B Raja S. C. Mullick Road, Jadavpur, Kolkata-700032, India S Supporting Information *

ABSTRACT: Carboxylate-bridged nonheme diiron(II) complexes, bearing free functional groups in general, and free thiol groups in particular, were sought. While the addition of sodium γ-hydroxybutyrate into a mixture of Fe(BF4)2·6H2O, HN-Et-HPTB, and Et3N afforded the complex [Fe2(N-Et-HPTB)(μ-O2C−(CH2)3−OH)](BF4)2 (2) (where N-EtHPTB is the anion of N,N,N′,N′-tetrakis(2-(1-ethylbenzimidazolyl))-2hydroxy-1,3-diaminopropane), a similar, straightforward process could not be used for the synthesis of diiron(II) complexes with free thiol groups. In order to circumvent this problem, a new class of thiolate bridged diiron(II) complexes, [Fe2(N-Et-HPTB)(μ-SR1)](BF4)2 (R1 = Me (1a), Et (1b), tBu (1c), Ph (1d)) was synthesized. Selective proton exchange reactions of one representative compound, 1b, with reagents of the type HS−R2−COOH yielded the target compounds, [Fe2(N-Et-HPTB)(μ-O2C−R2−SH)](BF4)2 (R2 = C6H4 (3a), CH2CH2 (3b), CH2(CH2)5CH2 (3c)). Redox properties of the complexes 3a−3c were studied in comparison with the complex, [Fe2(N-Et-HPTB)(μ-O2CMe)](BF4)2 (5). Reaction of (Cp2Fe)(BF4) with 1b yielded [FeII2(N-Et-HPTB)(DMF)3](BF4)3·DMF (4) (when crystallized from DMF/diethyl ether), which might indicate the formation of a transient ethanethiolate bridged {FeIIFeIII} species, followed by a rapid internal redox reaction to generate diethyldisulfide and the solvent coordinated diiron(II) complex, 4. This possibility was supported by a comparative cyclic voltammetric study of 1a−1c and 4. Prospects of the complexes of the type 3a−3c as potential building blocks for the synthesis of nonheme diiron(II) complexes covalently attached with other redox active metals has been substantiated by the synthesis of the complexes, [Fe2(N-EtHPTB)(μ-O2C−R2− S)Cu(Me3TACN)](BF4)2 (R = p-C6H4 (7a), CH2CH2 (7b)). All the compounds were characterized by a combination of singlecrystal X-ray structure determinations and/or elemental analysis.

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INTRODUCTION

Successful installations of redox active agents on the models of enzymatic active sites16−18 or as an integral part of the active site model systems in general,19,20 are rare, but not unprecedented in bioinorganic chemistry. Iron−sulfur clusters and iron−porphyrin systems may be considered as the most biologically relevant redox active agents for such purposes. However, active site model systems conjugated with these redox active agents are rarely achieved,17,19−21 probably due to the synthetic complexity, instability/unavailability of suitable building blocks that may be chemically paired up to afford such constructs. Iron−porphyrin systems such as [(TPP)FeOMe]22 and 3:1 site differentiated iron−sulfur cubane type clusters such as [Fe4S4(PPri3)3(X)]1+ (X = SSiPri3, OSiMe2tBu),23 as well as many other mononuclear complexes containing redox active metals such as [Cu(Me3TACN)(MeCN)](BF4)24 are available in the literature, and these may fulfill the criteria of suitable building blocks in the synthesis of the bridged metal clusters.24 However, suitable nonheme diiron(II) complexes that may be chemically paired up with the aforementioned chemical building blocks are hitherto unknown. In this context,

1−5

Carboxylate-bridged nonheme diiron complexes have fascinated chemists, because of their presence in bacterial multicomponent monoxygenases (BMMs), as well as in the flavodiiron proteins (FDPs). BMMs comprise a remarkable class of enzymes and catalyze the oxidation of aliphatic and aromatic hydrocarbons using O2.6−8 The flagship of the BMM family, soluble methane monooxygenase (sMMO), catalyzes the conversion of methane to methanol.7,9−12 On the other hand, FDPs13,14 are responsible for two different biological processes, namely, scavenging O2 in microbes, and the reduction of nitric oxide (NO) to nitrous oxide (N2O).15 The catalytic cycle of sMMO involves multiple redox steps. These steps include generation of the high-valency diiron centers, substrate oxidation, reduction of the diiron(III) resting state to a mixed valent FeIIFeIII species, and finally another reduction to regenerate the active, diiron(II) species.12 Considering the importance of the final redox step to sustain the catalytic cycle, small-molecule model systems that feature a diiron core covalently linked with redox active molecules may be sought and, with suitable systems, possible electron transfer reactions may be studied. © XXXX American Chemical Society

Received: February 6, 2016

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DOI: 10.1021/acs.inorgchem.6b00316 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry Chart 1. Abbreviations and Designations of Compounds

Scheme 1. Schematic Representation for the Synthesis of 1a−1d, 2, 3a−3c, 7a, and 7b

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carboxylate-bridged nonheme diiron(II) complexes bearing free thiol groups may be considered as potential synthons for establishing a covalent linkage with other metal complexes. However, synthesis of such diiron(II) complexes has remained elusive, because of the well-known high affinity of thiols toward iron(II).25−27 Nonheme carboxylate-bridged diiron complexes housed within a multidentate dinucleating N, O donor ligands have been extensively used in the modeling chemistry of carboxylatebridged diiron protein active sites, such as sMMO2,6,28−30 and FNORs.15 This type of systems has also been used for modeling phosphatases (e.g., FeII/FeIII purple acid phosphatases)31−38 and has been studied extensively in molecular magnetism.32,39−41 However, no thiolate-bridged analogue of such complexes has been reported yet. Here, we report a new class of thiolate-bridged nonheme diiron(II) complexes (Chart 1) as precursors to the diiron(II) compounds with free thiol groups. These thiolate-bridged diiron(II) compounds were further utilized for the synthesis of the desired carboxylate bridged diiron(II) complexes with free aromatic/aliphatic, short-chain/long-chain thiol groups (Scheme 1), which were finally utilized for the synthesis of nonheme diiron(II) complexes covalently attached with another redox active metal complex, such as [Cu(Me3TACN)(MeCN)](BF4).24 The present report describes the synthesis, characterization, and redox properties of these three new classes of nonheme diiron(II) complexes, and related ones (Chart 1).

EXPERIMENTAL SECTION

Preparation of Compounds. All reactions and manipulations were performed under a pure argon atmosphere, using either standard Schlenk techniques or an inert atmosphere box. Solvents were dried by following standard procedures.42 In the preparations that follow, all the filtrations were performed through Celite and solvent removal steps were carried out in vacuo inside an inert (argon gas) atmosphere box. Yields reported in each case are for recrystallized compounds and are the average of individual yields obtained from multiple batches of reactions, calculated using corresponding molecular weights of the compounds shown in Tables S1 and S2 in the Supporting Information. Fe(BF4)2·6H2O (97%), NaSMe (90%), NaSEt (90%), NaStBu (90%), NaHMDS, 4-mercaptobenzoic acid (90%), 3-mercaptopropionic acid (99%), 8-mercaptooctanoic acid (95%), ferrocene, ferrocenium tetrafluoroborate, and tetra-n-butylammonium hexafluorophosphate were obtained commercially and used without further purifications. Sodium thiophenolate was prepared from thiophenol and sodium hydride in tetrahydrofuran. H-N-Et-HPTB43 and sodium γ-hydroxybutyrate44 were synthesized according to literature procedures. [Cu(Me3TACN)(MeCN)](BF4) (8) was prepared from Me3TACN and [Cu(MeCN)4)](BF4) in MeCN, following a reported method,24 and was crystallized from MeCN/diethyl ether. The identities of all of the new compounds in Chart 1 (except 7a and 7b) were confirmed by single-crystal X-ray structure determinations. Bulk purity of all the new compounds (except 1c) was further confirmed by elemental analysis. All Fe(II) compounds are extremely air-sensitive in both solid form and in solution. [Fe2(N-Et-HPTB)(μ-SMe)](BF4)2 (1a). To a mixture of H-N-EtHPTB (0.08 mmol, 57.8 mg) and triethyl amine (0.16 mmol, 16.2 mg) B


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Inorganic Chemistry

[Fe2(N-Et-HPTB)(μ-O2CC2H4SH)](BF4)2 (3b). To a colorless solution of 1b (0.1 mmol, 107 mg) in 5 mL of MeCN was added a solution of 3-mercaptopropionic acid (0.1 mmol, 10.7 mg) in 2 mL of MeCN with stirring and the reaction mixture was stirred for 4 h. The resulting colorless solution was evaporated to dryness. The residue was extracted with 2 mL of DMF and filtered, and the filtrate was diffused overnight with Et2O at −35 °C and allowed to stand for 1 day at RT to afford the product as a colorless crystalline solid (87 mg, 78%). Anal. Calcd for C46H54N10O3B2F8S1Fe2 (3b): C, 49.67; H, 4.89; N, 12.59. Found C, 49.93; H, 4.61; N, 12.37. [Fe2(N-Et-HPTB)(μ-O2CC7H14SH)](BF4) (3c). To a colorless solution of 1b (0.02 mmol, 21.4 mg) in 2 mL of MeCN was added a solution of 8-mercaptooctanoic acid (0.02 mmol, 3.7 mg) in 1 mL of MeCN with stirring and the reaction mixture was stirred for 4 h. The resulting colorless solution was evaporated to dryness. The residue was extracted with 1 mL of DMF and filtered, and the filtrate was diffused overnight with Et2O at −35 °C and allowed to stand for 1 day at RT to afford the product as a colorless crystalline solid (15 mg, 63%). Anal. Calcd for C 51 H 64 N 10 O 3 B 2 F 8 S 1 Fe 2 ·0.5(C 3 H 7 NO)·0.5H 2 O (3c· 0.5DMF·0.5H2O): C, 51.35; H, 5.62; N, 11.98. Found C, 51.22; H, 5.71; N, 11.95. [Fe2(N-Et-HPTB)(μ-O2CCH3)](BF4) (5). To a mixture of H-N-EtHPTB (0.08 mmol, 57.8 mg) and triethyl amine (0.16 mmol, 16.2 mg) in 4 mL of MeCN was added a solution of Fe(BF4)2·6H2O (0.16 mmol, 55.7 mg) in 2 mL of MeCN with stirring. After 15 min, a suspension of NaOAc (0.085 mmol, 7 mg) in 2 mL of MeCN was added and the resultant slurry was stirred for 4 h. The reaction mixture was filtered and the colorless filtrate was evaporated to dryness. The residue was extracted with 2 mL of DMF and filtered, and the filtrate was diffused overnight with Et2O at −35 °C and allowed to stand for 1 day at RT to afford the product as a colorless crystalline solid (50 mg, 58%) Anal. Calcd for C45H52N10O3B2F8Fe2·H2O (5·H2O): C, 49.85%; H, 5.02%; N, 12.92%. Found C, 49.56%; H, 5.18%; N, 12.71%. [Fe2(N-EtHPTB)(μ-O2CC6H4S)Cu(Me3TACN)](BF4)2 (7a). To a colorless solution of [(Me3TACN)Cu (MeCN)](BF4) (14.4 mg, 0.04 mmol) in 2 mL of MeCN, was added with stirring a yellow solution of 3a (46.4 mg,0.04 mmol) in 2 mL of MeCN. A solution of NaHMDS (7.32 mg, 0.04 mol) in 1 mL of THF was added into the reaction mixture upon which the yellow color of the solution became more intense. The reaction mixture was stirred for 1 h and the solvent was evaporated to dryness. The yellow residue thus obtained was crystallized from MeCN/diethyl ether to afford the product as yellow crystalline solid (42 mg, 75%). Anal. Calcd for C59H74N13O3B2F8S1Fe2Cu1·H2O (7a·H2O): C, 50.18%; H, 5.42%; N, 12.89%. Found C, 50.08%; H, 5.18%; N, 12.89%. [Fe2(N-EtHPTB)(μ-O2CC6H4S)Cu(Me3TACN)](BF4)2 (7b). To a colorless solution of [(Me3TACN)Cu (MeCN)](BF4) (14.4 mg, 0.04 mmol) in 2 mL of MeCN, was added with stirring a colorless solution of 3b (44.8 mg, 0.04 mmol) in 2 mL of MeCN. A solution of NaHMDS (7.32 mg, 0.04 mol) in 1 mL of THF was added into the reaction mixture and stirring was continued for 1 h. The solvent was evaporated to dryness. The residue thus obtained was crystallized from MeCN/diethyl ether to afford the product as a colorless crystalline solid (40 mg, yield 74%). Anal. Calcd for C55H74N13O3B2F8S1Fe2Cu1· 2MeCN (7b·2MeCN): C, 49.61%; H, 5.65%; N, 14.71%. Found: C, 49.92%; H, 5.61%; N, 14.34%. General Physical Methods. Elemental analysis was recorded using a PerkinElmer 2400 Series II CHNS analyzer. Electrochemical studies of the complexes (10−3 M in MeCN) were performed using a CHI620E electrochemical analyzer (CH Instruments, USA). A threeelectrode setup was employed, consisting of a glassy carbon working electrode, a platinum wire auxiliary electrode, and a silver wire (as the pseudo-reference electrode). Tetra-n-butylammonium hexafluorophosphate (0.1 M) was used as the supporting electrolyte. Electrochemical potentials are referenced internally to the ferrocene/ferrocenium couple at 0.0 V. X-ray Structure Determinations. The structures of compounds 1a−1d, 2, 3a−3c, 4, and 5 (Chart 1) were determined. The unavailability of an X-ray structure of 824 and H-N-Et-HPTB43 prompted us to report one for each here. Diffraction-quality crystals

in 4 mL of MeCN was added a solution of Fe(BF4)2·6H2O (0.16 mmol, 55.7 mg) in 2 mL of MeCN with stirring. After 15 min, a suspension of NaSMe (0.085 mmol, 6.6 mg) in 2 mL of MeCN was added and the resultant slurry was stirred for 4 h. The reaction mixture was filtered and the colorless filtrate was evaporated to dryness. The residue was extracted with 2 mL of DMF and filtered, and the filtrate was diffused overnight with Et2O at −35 °C and allowed to stand for 1 day at room temperature (RT) to afford the product as a colorless crystalline solid (52 mg, 57%). Anal. Calcd for C44H52N10B2F8Fe2S1O1· C3H7NO·H2O (1a·DMF·H2O): C, 49.28%; H, 5.37%; N, 13.45%. Found: C, 49.54%; H, 5.19%; N, 13.35%. [Fe2(N-Et-HPTB)(μ-SEt)](BF4)2 (1b). To a mixture of H-N-Et-HPTB (0.08 mmol, 57.8 mg) and triethyl amine (0.16 mmol, 16.2 mg) in 4 mL of MeCN was added a solution of Fe(BF4)2·6H2O (0.16 mmol, 55.7 mg) in 2 mL of MeCN with stirring. After 15 min, a suspension of NaSEt (0.085 mmol, 7.9 mg) in 2 mL of MeCN was added and the resultant slurry was stirred for 4 h. The reaction mixture was filtered and the colorless filtrate was evaporated to dryness. The residue was extracted with 2 mL of DMF and filtered, and the filtrate was diffused overnight with Et2O at −35 °C and allowed to stand for 1 day at RT to afford the product as a colorless crystalline solid (56 mg, 65%). Anal. Calcd for C45H54N10B2F8Fe2S1O1·C3H7NO (1b·DMF): C, 50.51%; H, 5.39%; N, 13.50%. Found: C, 50.74%; H, 5.17%; N, 13.57%. [Fe2(N-Et-HPTB)(μ-StBu)](BF4)2 (1c). To a mixture of H-N-Et-HPTB (0.08 mmol, 57.8 mg) and triethyl amine (0.16 mmol, 16.2 mg) in 4 mL of MeCN was added a solution of Fe(BF4)2·6H2O (0.16 mmol, 55.7 mg) in 2 mL of MeCN with stirring. After 15 min, a suspension of NaStBu (0.085 mmol, 10.6 mg) in 2 mL of MeCN was added and the resultant slurry was stirred for 4 h. The reaction mixture was filtered and the colorless filtrate was evaporated to dryness. The residue was extracted with 2 mL of DMF and filtered, and the filtrate was diffused overnight with Et2O at −35 °C and allowed to stand for 1 day at RT to afford the product as a colorless crystalline solid (61 mg, 65%). [Fe2(N-Et-HPTB)(μ-SPh) (DMF)](BF4)2 (1d). To a mixture of H-NEt-HPTB (0.08 mmol, 57.8 mg) and triethyl amine (0.16 mmol, 16.2 mg) in 4 mL of MeCN was added a solution of Fe(BF4)2·6H2O (0.16 mmol, 55.7 mg) in 2 mL of MeCN with stirring. After 15 min, a suspension of NaSPh (0.085 mmol, 11.2 mg) in 2 mL of MeCN was added and the resultant slurry was stirred for 4 h. The reaction mixture was filtered and the colorless filtrate was evaporated to dryness. The residue was extracted with 2 mL of DMF and filtered, and the filtrate was diffused overnight with Et2O at −35 °C and allowed to stand for 1 day at RT to afford the product as a colorless crystalline solid (58 mg, 60%). Anal. Calcd for C52H61B2F8Fe2N11O2S1·2H2O (1d·2H2O): C, 50.96; H, 5.35; N, 12.57. Found: C, 50.99; H, 4.69; N, 12.38. Despite multiple attempts, a better elemental analysis could not be obtained for 1d. [Fe2(N-Et-HPTB)(μ-O2CCH2CH2CH2OH)](BF4)2 (2). To a mixture of H-N-Et-HPTB (0.08 mmol, 57.8 mg) and triethyl amine (0.16 mmol, 16.2 mg) in 4 mL of MeCN was added a solution of Fe(BF4)2·6H2O (0.16 mmol, 55.7 mg) in 2 mL of MeCN with stirring. After 15 min, a suspension of sodium γ-hydroxybutyrate (0.085 mmol, 10.7 mg) in 2 mL of MeCN was added and the resultant slurry was stirred for 4 h. The reaction mixture was filtered and the colorless filtrate was evaporated to dryness. The residue was extracted with 2 mL of DMF and filtered, and the filtrate was diffused overnight with Et2O at −35 °C and allowed to stand for 1 day at RT to afford the product as a colorless crystalline solid (55 mg, 60%). Anal. Calcd for C47H56N10O4B2F8Fe2·H2O (2·H2O): C, 50.03; H, 5.18; N, 12.41. Found: C, 50.07; H, 4.82, N, 12.36. [Fe2(N-Et-HPTB)(μ-O2CC6H4SH)](BF4)2 (3a). To a colorless solution of 1b (0.1 mmol, 107 mg) in 5 mL of MeCN was added a suspension of 4-mercaptobenzoic acid (0.1 mmol, 17.1 mg) in 2 mL of MeCN with stirring, and the reaction mixture was stirred for 4 h. The resulting yellow solution was evaporated to dryness. The residue was extracted with 2 mL of DMF and filtered, and the filtrate was diffused overnight with Et2O at −35 °C and allowed to stand for 1 day at RT to afford the product as a light-yellow crystalline solid (98 mg, 84%). Anal. Calcd for C50H54N10O3B2F8S1Fe2·2(C3H7NO) (3a·2DMF): C, 51.48; H, 5.25; N, 12.86. Found: C, 51.35; H, 5.19; N, 12.71. C


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Inorganic Chemistry

Figure 1. Molecular structures of 1a−1d, 4, and H-N-Et-HPTB with 50% probability thermal ellipsoids and partial atom labeling scheme. Hydrogen atoms are omitted for the sake of clarity. Note that, in 4, one DMF is directed toward Fe2 but is not coordinated. The Fe2−O5 distance is 2.408 Å, compared with Fe−ODMF = 2.147 Å for DMF molecules coordinated to Fe2. (determined by XPREP) and were further checked by PLATON46,47 for additional symmetry. Structures were solved by direct methods and refined against all data in the reported 2θ ranges by full-matrix leastsquares on F2 with the SHELXL program suite,48 using the OLEX 249 interface. Hydrogen atoms at idealized positions were included in final refinements. The OLEX 2 interface was used for structure visualization and drawing ORTEP50,51 plots. Crystallographic data and final agreement factors for 12 compounds are provided in Table S1 (1a1d, 2, and HN-Et-HPTB) and Table S2 (3a−3c, 4, 5, and 8).

were obtained as described in the respective syntheses. Single crystals coated with Parabar oil were mounted under a 150 K nitrogen cold stream. Data were collected on a Bruker SMART APEX-II diffractometer with graphite-monochromated Mo Kα radiation (λ = 0.71073 Å) controlled by the APEX 2 (v. 2010.1−2) software package. Raw data were integrated and corrected for Lorentz and polarization effects using the Bruker APEX II program suite.45 Absorption corrections were performed using SADABS. Space groups were assigned by analysis of metric symmetry and systematic absences D


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Inorganic Chemistry

Figure 2. Molecular structures of 2, 3a−3c, 5, and 8 with 50% probability thermal ellipsoids and partial atom labeling scheme. Hydrogen atoms are omitted for the sake of clarity. E


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Inorganic Chemistry Table 1. Selected Bond Distances and Bond Angles for Complexes 1a−1d, 2, 3a−3c, and 5 Bond Distances (Å) parameter

1a

1b

1c

1d

2

3a

3b

3c

5

Fe−OLa

1.983(5) 1.978(6)

1.972(3) 1.956(3)

1.972(2) 1.925(2)

2.004(4) 2.003(4)

2.001(5) 1.993(5)

1.988(5) 1.965(5)

1.991(3) 1.995(3)

1.967(4) 1.985(4)

1.993(3) 1.994(3)

Fe−S

2.438(4) 2.550(4)

2.477(2) 2.483(2)

2.486(2) 2.440(2)

2.428(2) 2.457(2) 2.033(5) 2.039(5)

2.036(5) 2.050(5)

2.037(4) 2.049(4)

2.033(4) 2.037(4)

2.030(3) 2.044(3)

3.587

3.514

3.586

Fe−Ocarboxylate

Fe···Fe parameter Fe−OL−Fe

3.328

3.325

3.114

3.130

3.588 3.534 Bond Angles (deg)

1a 114.3(3)

1b 115.7(2)

1c 106.1(8)

1d 102.7(2)

Fe−S−Fe

83.7(1)

84.2(5)

78.4(5)

79.7(6)

S−Fe−OL

81.8(2) 79.0(2)

80.0(1) 80.2 (1)

84.6(5) 86.9(5)

80.7(1) 85.9 (1)

Ocarboxylate−Fe−OL

τb

2 127.9(2)

96.2 (2) 97.0(2) 0.47 0.51

0.51 0.54

0.29 0.42

0.53c

0.83 0.89

3a 126.7(2)

3b 128.2(2)

3c 125.5(2)

5 128.2(1)

97.5(2) 97.5(2)

96.1(2) 96.5(2)

99.2(2) 97.6(2)

95.6(1) 96.4(1)

0.86 0.94

0.92 0.96

0.82 0.96

0.80 0.89

OL indicated the bridging alkoxo group from the deprotonated N-Et-HPTB. bThe shape parameter, τ,52−55 is defined as (α − β)/60, where α and β are the largest and next largest interligand bond angles, respectively. For an idealized TBP geometry, τ = 1; for an idealized SP geometry, τ = 0. cOne iron center is five-coordinated (τ value shown) while the other one is six-coordinate (sixth coordination is fulfilled by DMF) in 1d. a

ylic acid reagent, the μ2-SEt group may uptake one proton only from the more acidic, carboxylic acid group, and thus may selectively afford the desired carboxylate-bridged diiron(II) complexes with free thiol groups. The addition of NaSR (R = Me, Et, tBu, Ph) into a mixture of Fe(BF4)2·6H2O, HN-EtHPTB and Et3N in acetonitrile yielded the precursor complexes, [Fe2(N-Et-HPTB)(μ-SR1)](BF4)2 (R1 = Me (1a), Et (1b), tBu (1c), Ph (1d)) as crystalline solids in 57%−65% yields (Scheme 1). The use of excess NaSR (4 equiv) did not change the identity or yield of the products. The feasibility of the protonation process of the μ2-SEt group in 1b was confirmed by the isolation of a previously reported compound, [Fe2(N-EtHPTB)(μ-O2CPh)](BF4)2 (6)30 from a reaction mixture of 1b and benzoic acid in MeCN. Selective proton exchange reactions of 1b with reagents of the type HS-R2COOH yielded the desired complexes, [Fe2(N-Et-HPTB)(μO2C−R2−SH)](BF4)2 (R2 = C6H4 (3a), CH2CH2 (3b), CH2(CH2)5CH2 (3c)) featuring free thiol groups (Scheme 1), as analytically pure, yellow (3a)/colorless (3b, 3c) crystalline solids in 67%−84% yields. This simple, selective proton exchange strategy allows access to a completely new set of carboxylate-bridged nonheme diiron(II) complexes containing free thiol groups. The complexes 3a−3c may be considered as potential synthons for the synthesis of thiolate-bridged multicomponent systems, where the second component may be different redox active metal complexes. The prospects of the compounds of the type 3a−3c have been substantiated by the synthesis of two such complexes, 7a and 7b, by reactions of 824 with 3a and 3b in the presence of NaHMDS in MeCN. X-ray Structures. The identities of the 11 complexes (1a− 1d, 2, 3a−3c, 4, 5, 8) and HN-Et-HPTB is confirmed by single-

Refinement details and explanations (wherever applicable) are included in the individual CIFs.

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RESULTS AND DISCUSSION Synthesis. Nonheme carboxylate-bridged diiron(II) complexes, bearing free functional groups in general, and free thiol groups in particular, were sought. The addition of sodium γhydroxybutyrate (NaGHB) into a mixture of Fe(BF4)2·6H2O, H-N-Et-HPTB, and Et3N yielded the compound, [Fe2(N-EtHPTB)(μ-O2C−(CH2)3−OH)](BF4)2 (2) featuring a free hydroxyl group (Scheme 1), as an analytically pure crystalline solid in 60% yield. However, compounds of the type HS−X− COONa (X = alkyl, aryl) are unavailable. Moreover, carboxylate bridged diiron(II) complexes, bearing free thiol groups, may not be obtained directly from a mixture of Fe(II) salt, H-N-Et-HPTB and Et3N, because of (i) the unavoidable deprotonation of the thiol groups, and (ii) the well-known high affinity of thiols toward iron(II).25−27 Attempted synthesis of the target diiron(II) complexes with free thiol groups in the aforementioned straightforward synthetic route using 3mercaptopropionic acid and 4-mercaptobenzoic acid resulted in pale yellow and yellowish orange solids, respectively, which, even after multiple attempts using different solvent and counterion combinations, could not be obtained in crystalline form. At this stage, we planned to use this affinity of thiols toward iron(II) to synthesize thiolate-bridged nonheme diiron(II) complexes. This class of complexes is hitherto unknown and was anticipated to be potential precursor to the desired diiron(II) complexes, bearing free thiol groups. The plan relied on the possibility that, during the reaction of these thiolate-bridged diiron(II) compounds with a mercaptocarboxF


Article

Inorganic Chemistry Scheme 2. Schematic Representation for the Redox Process of 1a−1c and [FeII2(N-Et-HPTB)(MeCN)4](BF4)3a

a

Data taken from ref 56. Reduction potentials are referenced against Cp2Fe+/Cp2Fe.

the potential range of 0.51−0.74 V. However, reaction of 1b (E1/2 = −0.045 V vs Cp2Fe/Cp2Fe+) with (Cp2Fe)(BF4) in MeCN yielded a previously reported complex, [FeII2(N-EtHPTB) (MeCN)4](BF4)356 (confirmed by unit-cell determination of diffraction quality crystals), when crystallized from MeCN/diethyl ether, and [FeII2(N-Et-HPTB)(DMF)3](BF4)3· DMF (4) (Figure 1) when crystallized from DMF/diethyl ether. Compound 4 is very similar to a previously reported compound, [FeII2(N-Et-HPTB)(DMF)4](BF4)356 except that, unlike the reported compound, one of the four DMF molecules in 4 is directed toward one of the two iron(II) centers, but not coordinated to iron (Fe−ODMF distance = 2.408 Å, compared with Fe−ODMF = 2.147 Å for DMF molecules coordinated to Fe(II)). The formations of disulfides and ferrocene in the reactions of 1b and 1d with (Cp2Fe)(BF4) were confirmed by gas chromatography−mass spectroscopy (GC-MS) (see Figure S2 in the Supporting Information). The generation of these solvent-coordinated diiron(II) complexes possibly indicates one-electron oxidation of 1a−1c in the range of Epa = −0.03− 0.08 V to form a transient thiolate-bridged mixed-valence FeIIFeIII species, followed by an internal redox reaction that releases the μ2-SEt group as disulfide with concomitant formation of a solvent (MeCN)-coordinated diiron(II) species, either identical with, or similar to (differing only in the number of coordinated solvent molecules), [FeII2(N-Et-HPTB)(MeCN)4 ](BF 4 ) 3 56 (Scheme 2). The two consecutive oxidations observed in the cyclic voltammograms of 1a−1c in the range of 0.51−0.74 V may then correspond to the redox couples, FeIIFeIII/FeIII2 and ligand oxidation and/or product decomposition respectively for this in situ-generated solvent coordinated diiron(II) compound (Scheme 2). This possibility was confirmed by the cyclic voltammetric study of the complex, [FeII2(N-Et-HPTB) (MeCN)4](BF4)356 (Figure 3) and the almost identical potential values for the two consecutive irreversible oxidation processes observed in the case of 1a−1c in the potential range of 0.51−0.74 V (Scheme 2). The cyclic voltammetric signature of complex 1d was not well-behaved and showed two consecutive irreversible oxidations at 0.11 and 0.60 V. Complex 2 features a quasi-reversible oxidation process at Epa = 0.05 V (E1/2 = 0.0 V, ΔE = 100 mV, ipa/ipc = 6.26), followed by two consecutive irreversible oxidations at 0.62 and 0.78 V (Figure 4). The possible reason for the large deviation of ipa/ipc from the ideal value of 1.00 is the multiple redox processes after

crystal X-ray structure determinations, and the molecular structures are shown in Figures 1 and 2. Despite multiple attempts, single crystals suitable for diffraction could not be obtained for 7a and 7b. Selected bond distances and angles are provided in Table 1. Because of the strain generated by the monodentate thiolate bridge (compared to the bidentate carboxylate bridge), Fe centers in 1a−1d (only one Fe center is five-coordinated in 1d) display a highly distorted geometry (average Ntrans−Fe−S angle of 155.5° ± 4.6°) with τ52−55 (shape parameter) values ranging from 0.29 to 0.54, that fall between an idealized TBP (τ = 1) and an idealized SP (τ = 0) geometry. For the same reason, the Fe−Fe distances in 1a−1d (3.114−3.328 Å) are much shorter than that observed in the case of complexes 2 and 3a−3c (3.514−3.588 Å). The compounds 1a−1d display very similar Fe−S bond lengths and slightly different Fe−S−Fe and S−Fe−OL angles (Table 1). Complexes 2 and 3a−3c display carboxylate-bridged iron(II) centers in a slightly distorted trigonal bipyramidal structure with τ52−55 (shape parameter) values ranging from 0.82 to 0.96. Other metric parameters of 2 and 3a−3c are unexceptional, in comparison with 5 and other previously reported carboxylate bridged diiron(II) complexes in the same ligand platform.30,56 All four phenyl rings of the benzimidazole moieties in 2 and 3a−3c are projected in the same directions as the dangling thiol (3a−3c)/hydroxyl (2) groups, while the Nethyl substituents are projected in the opposite direction (Figure 2). The distances of S/O atoms from the mean plane of the four top-most phenyl ring carbon atoms increase in the order ̊ < 2 (4.070 A) ̊ < 3a (5.804 A) ̊ < 3c (6.385 A) ̊ 3b (3.768 A)

implying more and more separation from the diiron core (see Figure S1 in the Supporting Information). Redox Properties. Cyclic voltammetric traces (multiple scans) for the complexes 1a−1d, 2, 3a−3c, 5, 7a, 7b, 8, and a 1:1 mixture of 5 and 8 were obtained using a glassy carbon working electrode in MeCN and are shown in Figures 3−5. Complex 1b displays a quasi-reversible oxidation process at Epa = 0.01 V (E1/2 = −0.045 V, ΔE = 110 mV, ipa/ipc = 1.59), presumably due to the oxidation of one of the two Fe(II) centers, followed by two consecutive irreversible oxidations at Epa = 0.56 and 0.73 V. The complexes 1a and 1c display an irreversible oxidation at Epa = −0.03 and 0.08 V, respectively, followed by two consecutive irreversible oxidation processes in G


Article

Inorganic Chemistry

Figure 3. Cyclic voltammetric traces (multiple scans, scan rate = 100 mV s−1) for 1a−1d and [FeII2(N-Et-HPTB)(MeCN)4](BF4)356 in MeCN.

oxidation of complex 3c at Epa = 0.01 V (ΔE = 90 mV, ipa/ipc = 2.41) cannot be considered as a quasi-reversible process, because it is closely associated with another oxidation at 0.1 V (Figure 4) and it was also observed that the cyclic votammetric scans of 3c in a smaller potential window and/or a higher scan speed could not improve the ipa/ipc ratio. Compound 3c features two more oxidations at 0.55 and 0.74 V, which may be a result of complicated redox processes involving possible product decomposition and were not further investigated. The geometries of the Fe centers in 2 and 3a−3c are quite similar (τ ranges from 0.82−0.96, see Table 1) with 5 (τ = 0.80, 0.89) and 630 (τ = 0.88, 0.92). The only major structural difference between 5/6 and 2, 3b, 3c is the free hydroxyl (2) and thiol groups (3b and 3c). However, there are multiple methylene

the oxidation at Epa = 0.05 V. Indeed, a cyclic votammetric trace of 2 in the potential range of −0.95 V to 0.4 V improved the ipa/ipc ratio from 6.26 to 1.74 (see Figure 4). Complexes 3a and 3b (Figure 4) display a quasi-reversible oxidation at Epa = 0.03 V (E1/2 = −0.025 V, ΔE = 110 mV, ipa/ipc = 1.5) and 0.04 V (E1/2 = −0.02 V, ΔE = 120 mV, ipa/ipc = 3.06), respectively. Comparing the cyclic voltammograms of 3a and 3b, it becomes obvious that the current ratio of the otherwise quasi-reversible wave in the case of 3b has possibly been highly perturbed by the multiple redox processes that occur after the oxidation at Epa = 0.04 V. Indeed, a cyclic votammetric trace of 3b in the appropriate potential range improved the ipa/ipc ratio from 3.06 to 1.67 (Figure 4). Compounds 3a and 3b showed one more oxidation each, at 0.52 and 0.58 V, respectively. However, the H


Article

Inorganic Chemistry

Figure 4. Cyclic voltammetric traces (multiple scans, scan rate = 100 mV s−1) for 2, 3a−3c, and 5 in MeCN.

3c are almost identical to that of 5 (E1/2 = −0.02 V, ΔE = 120 mV, ipa/ipc = 1.47, and Epa = 0.60 V; see Figure 4). On the other hand, because of the presence of the free thiol group in the para position of the bridging benzoate in 3a, the reduction potential has increased by 120−160 mV, relative to that observed in the

spacers between free hydroxyl/thiol and bridging carboxylate groups in complexes 2, 3b, 3c and, therefore, the possibility of a strong electronic conjugation between the free hydroxyl/thiol groups and carboxylate-bridged diiron(II) core may be ruled out. Consistent with that, the reduction potentials of 2, 3b, and I


Article

Inorganic Chemistry case of 630 (E1/2 = −0.18 V, ΔE = 80 mV, ipa/ipc = 1.1, and Epa = 0.36 V).56 Redox properties of 7a (Figure 5c) and 7b (Figure

were found to be comparatively much less than that observed in the case of 7a. The oxidation events were obtained at Epa = 0.07 and 0.50 V (Figure 5d), compared to Epa = 0.04 and 0.58 V for 3b (see Figure 4). Similarly, the reductions were obtained at Epc = 0.22 and 0.04 V in 7b, compared with Epc = 0.30 V and −0.08 V for 3b. The oxidation at Epa = 0.23 V and the corresponding reduction at Epc = −0.40 V seems to be due to the Cu(I) center bound to the diiron(II) unit in 7b.

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SUMMARY A series of thiolate-bridged nonheme diiron(II) complexes (1a−1d) are synthesized and characterized. These complexes could take part in selective proton exchange reactions with different mercapto carboxylic acids to yield another series of carboxylate-bridged nonheme diiron(II) complexes bearing free thiol groups (3a−3c). Unlike 2, which bears a free hydroxyl group, 3a−3c cannot be synthesized directly from a mixture of iron(II) salt, HN-Et-HPTB ligand, the related carboxylic acids, and a base. While the reduction potentials of 2, 3b, and 3c (containing methylene spacers between free hydroxyl/thiol and bridging carboxylate groups) are almost identical with that of the acetate bridged diiron(II) compound, 5, the presence of a free thiol group in the para position of a bridging benzoate increases the reduction potential by 120−160 mV in 3a than that observed in the case of the benzoate-bridged diiron(II) compound, 6. The hitherto unknown, nonheme carboxylatebridged diiron(II) complexes (3a−3c), featuring free thiol groups, may be considered as potential building blocks for the synthesis of nonheme diiron(II) active site model systems conjugated with other redox active metal complexes. The synthesis, characterization, and cyclic voltammetric study of two such complexes (7a and 7b) are also provided.

Figure 5. Cyclic voltammetric traces (multiple scans, scan rate = 100 mV s−1) for (a) 8, (b) 1:1 mixture of 5 and 8 in MeCN, (c) 7a, and (d) 7b in MeCN.

5d) were also studied in comparison with 8 (Figure 5a) and a 1:1 mixture of 5 and 8 (Figure 5b). Compound 8 shows an irreversible oxidation at Epa = 0.15 V with the corresponding reduction at Epc = −0.32 V (Figure 5a), while 5 shows a reversible oxidation at E1/2 = −0.02 V (Epa = 0.04 V, Epc = −0.08 V) and another irreversible oxidation at Epa = 0.60 V with the corresponding reduction at Epc = 0.28 V (Figure 4). As expected, the 1:1 mixture of 5 and 8 behaved similar to a noninteracting mixture of two individual compounds (Figure 5b) and, as such, the Epa and Epc values corresponding to the reversible oxidation for 5 in the mixture were found to be observed at 0.05 V and −0.1 V, respectively, while the irreversible oxidation was obtained at 0.56 V with the corresponding reduction event at 0.28 V. Similarly, for the copper compound, the reduction (of Cu2+) was obtained at Epc = −0.30 V, while the oxidation event (of Cu1+) is found to be masked (note the broad area just next to 0.05 V in Figure 5b) by the oxidation event of 5 at 0.05 V. In 7a (Figure 5c), the Cu(I) center is in conjugation with the diiron(II) unit through the 4-mercapto benzoate group and, as a consequence, the first oxidation observed in the case of 3a has been shifted from Epa = 0.03 V (Epc = −0.08 V) to Epa = 0.40 V (Epc = −0.17 V) in 7a, while the second oxidation observed in the case of 3a has been shifted from Epa = 0.52 V (Epc = 0.26 V) to Epa = 0.91 V (Epc = 0.53 V) in 7a, thus indicating an ∼400 mV shift for both oxidations related with the diiron unit while the redox events for the Cu(I) fragment were found at Epa = 0.10 V and Epc = −0.57 V, compared with Epa = 0.15 V and Epc = −0.32 V for 8. The reduction potential values for the redox events are also found to be altered in the case of 7b; however, because of the presence of methylene spacers between the diiron(II) center and the Cu(I) center in 7b the shifts in reduction potentials

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b00316. Figures showing the distance of the free thiol form the substituent phenyl groups, GCMS data for diethyl disulfide, diphenyl disulfide and ferrocene, crystallographic data and final agreement factors for compounds 1a−1d, 2, 3a−3c, 4, 5, 8, and H-N-Et-HPTB in tabulated form (PDF) X-ray crystallographic data for compounds 1a−1d, 2, 3a−3c, 4, 5, 8, and H-N-Et-HPTB (CIF)

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AUTHOR INFORMATION

Corresponding Author

* Email: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Grant Nos. SB/S1/IC-43/2013 (DST/SERB, India) and 01(2804)/14/EMR-II (CSIR, India). N.P. acknowledges CSIR, India for a junior research fellowship.

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