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First Example of Cationic Cyclopentadienyliron Based Chromene Complexes and Polymers: Synthesis, Characterization, and Biological Applications

  • A. S. Abd-El-AzizEmail author
  • E. G. El-Ghezlani
  • M. M. Elaasser
  • T. H. Afifi
  • R. M. Okasha
Article
  • 41 Downloads

Abstract

The present work encompasses development of a new class of active antimicrobial agents of the ɳ6-arene-ɳ5-cyclopentadienyliron (II) complexes and polymers to expand the existing family of antimicrobials. The design of the first example of these cationic organoiron complexes, incorporating chromene moieties, was successfully accomplished in good yield via the Steglich esterification, followed by the nucleophilic aromatic substitution reactions to isolate their polymeric analogues. Meanwhile, the structural identities of the new materials were confirmed using 1H and 13C NMR, IR, and elemental analysis. The molecular weight of the new polymeric materials was estimated using viscosity measurements, and was found to be in the range of 10,000 and 31,000. The chromene-containing organoiron complexes displayed reversible reduction behaviors as their functionalized polymers. Meanwhile, the TGA thermogram revealed initial weight losses due to the cleavage of the cationic iron moieties, while the second weight losses were dependent upon the etheric linkages in the polymer backbones. This study, therefore, focuses on the exploration of the antimicrobial and antitumor performance of the new materials along with their structure activity relationship.

Keywords

Cationic organoiron Steglich esterification Antimicrobial agent Antitumor behavior 

1 Introduction

Chromene scaffolds are established as one of the most powerful medicinal molecules that are involved in numerous biomedical applications due to their exceptional pharmacological and biological features. Their derivatives demonstrate antimicrobial and antifungal [1, 2, 3, 4, 5], antioxidant [6, 7], antileishmanial [8], antitumor [9, 10], hypotensive [11], antiproliferative [12], local anaesthetic [13], antiallergenic [14, 15], and central nervous system (CNS) activities and effects [16] while administrating treatment of schizophrenia and Alzheimer’s disease [17]. Our current objective from utilizing these chromene compounds is to purge the resistance in microbial organisms and tumor cells.

Innovations in drug design have dawned a new era through the introduction of metallic moieties into these materials. The assembly of biologically active molecules with metallic moieties manipulates the properties of these compounds and enhances their performance as drugs. The majority of this work encompasses inorganic chelating agents with metals, such as Cu, Zn, Pt, Ru, As, and Au [18, 19, 20]. Recently, great efforts have been devoted to design organometallic architectures with new medicinal properties [21, 22, 23, 24]. Moreover, a few of these compounds have even entered clinical trials as promising anticancer, antimalarial, antimicrobial, or diagnostic agents [21].

In particular, organoiron compounds have penetrated medicinal fields with some prominent examples of ferrocene. For instance, ferrocene derivatives of tamoxifen, known as ‘ferrocifens’, displayed remarkable effects on breast cancer patients, especially those who have resisted cancer drugs [25, 26]. It is also believed that the redox behavior of the iron center performs an imperative role in the treatment process [27]. In fact, researchers’ interest has been directed to the organometallic antimicrobial agents, [28, 29, 30, 31, 32, 33, 34] with the assumption that the functionalized materials in the presence of a metal in the antimicrobial drug will provide a new mechanism of action that bypasses resistance mechanisms in the drug-resistant microbes. This attention is motivated by the recognized biological activity of cisplatin [35], an anticancer platinum-containing drug, and ferroquine, an antimalarial drug containing iron. Ferroquine is a well-known drug against the chloroquine-resistant strains of Plasmodium falciparum [36]. A subcellular probe of P. falciparum after treatment with ferroquine displays an increase in the reactive oxygen species (ROS), which were implicated in causing oxidative damage to the cells, eventually killing the parasite [36]. This mechanism of action is due to the redox activity of ferrocene, which under physiological conditions, oxidizes to the 17-electron ferrocenium cation that catalyzes the in vivo generation of the ROS [32, 33, 34, 36].

Our interests have been pledged to the design of novel neutral and cationic sandwich organoiron complexes and polymers with tailored properties [37, 38, 39, 40, 41, 42, 43]. Unlike ferrocene, which is at the forefront of organometallic medicinal chemistry, curiosity about the biological activity of the cationic iron complexes is low, despite its rich redox nature, since [η6-arene-η5-CpFe]+ oxidizes to the 17-electron di-cation compound, which is a stronger oxidizing agent than ferrocene [44]. Under a physiological environment, this redox behavior could catalyze the generation of the ROS and induce oxidative stress similar to the effect of the ferrocene described above, which consequently damages cells and acts as a defense strategy employed against a broad spectrum of microbes [45, 46]. Thus, the investigation of [η6-arene-η5-CpFe]+ as an antimicrobial agent is substantiated. The present work tested this assumption by evaluating the antimicrobial activity of these complexes as molecules and functional moieties in the organometallic complexes. Antimicrobial organometallic complexes containing chromene moieties are yet to be explored. As it is acknowledged that chromenes have immense potential in the biomedical field, developing antimicrobial and anticancer organometallics containing chromene is attractive and, therefore, a key objective of this study.

Herein, we are introducing the first example of cationic cyclopentadienyliron-containing chromene molecules with the exploration of their antiproliferative abilities.

2 Experimental

2.1 Instrumentation

A Bruker Avance NMR spectrometer (1H, 300 MHz and 13C, 75 MHz) was used to characterize all the synthesized complexes in DMSO-d6 or acetone-d6 with the chemical signals referenced to the solvent residual signal in ppm. Attenuated total reflection Fourier transform IR (ATR-FTIR) absorption spectroscopic measurements were acquired on a Bruker FTIR spectrometer Alpha-P. The cyclic voltammetric experiments were carried out on a Princeton Applied Research/EG&G Model 263 potentiostat/galvanostat, using a glassy carbon working electrode, a Pt counter electrode, and an Ag reference electrode. The experiments, which were carried out at a scan rate between 0.1 and 1.5 Vs−1 and at a temperature between 25 and − 25 °C under a nitrogen atmosphere in a degassed propylene carbonate solvent with tetrabutylammonium hexafluorophosphate as supporting electrolyte, were externally referenced to a DMF solution of ferrocene. The elemental analyses were performed on a CE-440 Elemental Analyser, Exeter Analytical, Inc. The thermogravimetric analysis (TGA) was conducted in platinum pans under nitrogen at a heating rate of 10 °C on the TA Instruments TGA Q500. The scanning electron micrographs (SEM) of the complexes were obtained from a TM3000 Hitachi Instrument with a broad magnification range of 15 × to 30,000 ×. The instrument contains three beam conditions “5 kV, 15 kV and Analysis” with high sensitivity backscattered electron detectors that encompass four independent segments. The sample is usually vacuum coated with a thin layer of gold before observation.

2.2 Materials and Solvents

All chemicals, reagents, and solvents were purchased from Sigma-Aldrich and were used without any further purification. These materials were 4-hydroxybenzoic acid, resorcinol, malononitrile, 2- and 3-thiophenecarboxaldehyde, 3- and 4-chlorobenzaldehyde, 3-bromobenzaldehyde, 3-fluorobenzaldehyde, 3-nitrobenzaldehyde, 2-nitrobenzaldehyde, 4-tert-butylbenzaldehyde, piperidine, 1,4-dichlorobenzene, aluminum powder, aluminum chloride, ammonium hexafluoro-phosphate (NH4PF6), potassium carbonate, 4,4-bis(4-hydroxyphenyl) valeric acid, dicyclohexylcarbodiimide (DCC), and 4-dimethylaminopyridine (DMAP). All solvents were dried and stored over 3 Å molecular sieves before use. These solvents were dimethyl sulfoxide-d6 (DMSO-d6), acetone-d6, N, N′-dimethylformamide (DMF), dichloromethane (DCM), and absolute ethanol. Diethyl ether and hydrochloric acid were used as received.

2.3 Biological Measurements

2.3.1 Antimicrobial Assay

All the microbroth antibiotic susceptibility testing was carried out according to Overy et al. [47], using the following pathogens: methicillin-resistant Staphylococcus aureus ATCC 33591 (MRSA); S. warneri ATCC 17917; vancomycin-resistant Enterococcus faecium EF379 (VRE); Pseudomonas aeruginosa ATCC 14210; Proteus vulgaris ATCC 12454; and Candida albicans ATCC 14035. The complexes were serially diluted to generate a range of eight concentrations (128 µg/mL to 1 µg/mL) in a final well volume concentration of 2% DMSO (aq). Each plate was comprised of three uninoculated positive controls, three untreated negative controls, and an appropriate concentration range of a control antibiotic (vancomycin for MRSA and S. warneri, rifampicin for VRE, gentamycin for P. aeruginosa, ciprofloxacin for P. vulgaris, and nystatin for C. albicans). The optical density of the plate was recorded, using a Thermo Scientific Varioskan Flash plate reader at 600 nm at time zero and then again after the incubation of the plates for 22 h at 37 °C. After subtracting the time zero OD600 from the final reading, the percentages of the microorganism survival relative to vehicle control wells were calculated.

2.3.2 Antitumor Activity Assay

The tested human carcinoma cell lines were obtained from the American Type Culture Collection (ATCC, Rockville, MD). The cells were grown on a RPMI-1640 medium, supplemented with a 10% inactivated fetal calf serum and 50 µg/mL gentamycin. The cells were maintained at 37 °C in a humidified atmosphere with 5% CO2 and were subcultured two to three times a week. For the antitumor assays, the tumor cell lines were suspended in a medium at a cell density of 5x104 cells/well in Corning® 96-well tissue culture plates and then incubated for 24 h. The tested compounds were then added to 96-well plates (six replicates) to achieve eight concentrations for each compound. Six vehicle controls with media or 0.5% DMSO were run for each of the 96 well plates. After incubating for 24 h, the number of the viable cells was determined by the MTT assay [48, 49]. Briefly, the media was removed from the 96-well plates and was replaced with 100 µL of a fresh culture RPMI 1640 medium without phenol red. Then, 10 µL of the 12 mM MTT stock solution (5 mg of MTT in 1 mL of PBS) was added to each well, including the untreated controls. The 96 well plates were then incubated at 37 °C and 5% CO2 for 4 h. An 85 µL aliquot of the medium was removed from the wells, and 50 µL of DMSO was added to each well, mixed thoroughly with the pipette, and incubated at 37 °C for 10 min. Additionally, the optical density was measured at 590 nm with the microplate reader (SunRise, TECAN, Inc, USA) to determine the number of viable cells. The percentage of viability was calculated as [1-(ODt/ODc)] × 100%, where the ODt is the mean optical density of the wells treated with the tested sample, and the ODc is the mean optical density of the untreated cells. The relation between the surviving cells and the drug concentration is plotted to get the survival curve of each tumor cell line after treatment with the specified compound. The 50% inhibitory concentration (IC50), the concentration required to cause toxic effects in 50% of the intact cells, was estimated from the graphic plots of the dose–response curve for each concentration, using the GraphPad Prism software (San Diego, CA. USA) [48, 49].

2.4 Synthesis and Characterization

2.4.1 Synthesis of Complexes 4, 5 and 6

0.97 mmol, of 2, 3a or 3b was combined with 0.97 mmol, 0.26 g of ɳ6-arene-ɳ5-cyclopentadienyliron carboxylic acid 1 in a 10 mL of DMF in the presence of DMAP. Then, a DCC solution, 0.97 mmol, was added to the reactants over a 15-min period. The reaction was carried out for 72 h. under a nitrogen environment and was precipitated in a 10% HCl solution. The product was collected, washed, and dried under a vacuum.

2.4.1.1 Complex 4

Yield 38%, brown powder. The calculated molecular weight 831.95 g/mol. ATR-FTIR; γmax/cm: 3094 (NH2), 1695 (C=O), 1232 (C–O). 1H NMR data δH (300 MHz, acetone-d6): 8.09 (3H, d, J = 8.7 Hz, uncomplexed-H), 7.95 (2H, s, NH2), 7.61 (1H, m, Ar–H), 7.42 (3H, d, J = 8.4 Hz, Ar–H), 7.17(1H, s, Ar–H), 6.84 (2H, d, J = 6.6 Hz, complexed-H), 6.54 (3H, d, J = 6.6 Hz, complexed-H), 6.45 (1H, s, Ar–H), 5.30 (5H, s, Cp), 4.97 (1H, s, pyran-H). 13C NMR δC (75 MHz, acetone-d6): 171.54 (CO), 166.47, 160.15, 156.79, 148.59, 145.91, 140.99, 134.69, 103.92 (quat-C), 120.26, 132.17 (uncomplexed-C), 133.18, 130.76, 128.33, 125.63, 121.67, 119.46 (Ar–C), 118.73 (CN), 86.97, 77.48 (complexed-C), 79.70 (Cp–C), 64.77 (C–CN), 57.38 (C–N), and 35.71 (pyran-C). The Elemental Analysis of C32H22ClF6FeN2O4PS: Calc %C 50.12, %H 2.89, %N 3.65, and found: %C 50.09, %H 2.93, and %N 3.66.

2.4.1.2 Complex 5

Yield 43%, yellowish powder. The calculated molecular weight 904.82 g/mol. ATR-FTIR; γmax/cm: 3201 (NH2), 1651 (C=O), 1250 (C–O). 1H NMR data δH (300 MHz, DMSO-d6): 8.10 (2H, s, NH2), 7.98 (2H, d, J = 8.7 Hz, Ar–H), 7.36 (4H, m, Ar–H), 6.95 (1H, s, Ar–H), 6.85 (4H, dd, J = 6.6 Hz, Ar–H), 6.59 (2H, d, J = 6.0 Hz, Ar–H), 6.40 (2H, d, J = 6.3 Hz, Ar–H), 5.28 (5H, s, Cp), 5.01 (1H, s, pyran-H). 13C NMR δC (75 MHz, DMSO-d6): 171.18 (CO), 166.01, 159.12, 155.78, 149.34, 140.23, 133.09, 130.11, 126.34, and 104.21 (quat-C), 130.86 and 121.24 (uncomplexed-C), 132.83, 131.17, 129.40, 125.46, 121.44, 120.35, and 114.16, (Ar–C), 119.02 (CN), 86.14 (Cp–C), 79.08, and 76.96 (complexed-C), 65.22 (C–N), 55.98 (C–CN), 34.87 (pyran-C). Elemental Analysis for C34H23BrClF6FeN2O4P: Calc: %C 48.63, %H 2.76, %N 3.34, and found %C 48.60, %H 2.79, and %N 3.31.

2.4.1.3 Complex 6

Yield 46%, yellowish powder. The calculated molecular weight 860.37 g/mol. ATR-FTIR; γmax/cm−1: 3198 (NH2), 1660 (C=O), 1200 (C–O). 1H NMR data δH (300 MHz, acetone-d6): 8.12 (2H, d, J = 8.7 Hz, Ar–H), 7.66 (2H, s, NH2), 7.40 (4H, m, Ar–H), 6.83 (4H, dd, J = 6.9 Hz, Ar–H), 6.53(2H, d, J = 6.0 Hz, Ar–H), 6.37 (2H, d, J = 6.3 Hz, Ar–H), 5.29 (5H, s, Cp), 5.09 (1H, s, pyran-H). 13C NMR δC (75 MHz, DMSO-d6): 171.20 (CO), 166.19, 159.73, 156.63, 150.87, 140.63, 134.19, 130.69, 128.16, and 103.77 (quat-C), 131.86 and 120.15 (uncomplexed-C), 133.03, 131.31, 129.84, 126.46, 121.52, 120.60, and 113.36, (Ar–C), 116.29 (CN), 86.60 (Cp–C), 79.39 and 77.07 (complexed-C), 64.37 ((C–N), 56.17 (C–CN), 35.30 (pyran-C). Elemental Analysis for C34H23Cl2F6FeN2O4P: Calc: %C 51.35, %H 2.92, %N 3.52, and found %C 51.34, %H 2.96, and %N 3.54.

2.4.2 Synthesis of Complexes 8 and 9

Similarly, 0.50 g, 0.48 mmol of the bimetallic organoiron 7, was added to 0.48 mmol of compound 2a or 2b in a 50 mL round bottom flask to synthesize complexes 8 and 9, respectively. Subsequently, 0.09 g, 0.48 mmol of DCC and 0.10 g, 0.96 mmol of DMAP were added to the mixture. The reaction was conducted in an ice bath for 30 min under a N2 environment for 48 h. 0.08 g, 0.48 mmol of NH4PF6 was added to the reaction solution in a 10% HCl solution to precipitate the product.

2.4.2.1 Complex 8

Yield 58%, yellowish powder. The calculated molecular weight 1291.51 g/mol. ATR-FTIR; γmax/cm−1: 3103 (NH2), 2938 (Ar–C), 1650 (C=O), 1244 (C–O–C), 556 (C–Cl). 1H NMR data δH (300 MHz, DMSO-d6): 7.41 (5H, m, uncomplexed Ar–H), 7.29 (6H, t, J = 4.2 Hz, uncomplexed Ar–H), 7.16 (1H, d, J = 9.0 Hz, Ar–H), 7.05 (2H, m, Ar–H), 6.88 (2H, s, NH2), 6.80 (4H, t, J = 3.6 Hz, complexed Ar–H), 6.42 (4H, d, J = 6.6 Hz, complexed Ar–H), 5.28 (10H, s, Cp–H), 4.88 (1H, s, pyran-H), 2.59 (2H, br s, CH2), 2.43 (2H, br s, CH2), 1.73 (3H, s, CH3). 13C NMR δc (75 MHz, acetone-d6): 172.51 (CO), 161.24, 155.77, 152.58, 148.21, 139.09, 134.26, 107.28, and 105.26 (quat-C), 131.12 and 121.62 (uncomplexed Ar–C), 130.20, 127.90, 122.46, 120.41, 119.45, and 111.16 (Ar–C), 119.18 (CN), 80.88 and 77.64 (complexed Ar–C), 88.27 (Cp–C),, 66.79 (C–CN), 59.29 (C–N), 46.77 and 37.33 (C–CH2), 37.18 (pyran-C), and 28.35 (C–CH3). The Elemental Analysis of C53H42 Cl2F12Fe2N2O5P2S: Calc: %C 49.29, %H 3.28, %N 2.17, and found %C 49.34, %H 3.35, and %N 2.18.

2.4.2.2 Complex 9

Yield: 49%, brown powder. The calculated molecular weight 1291.51 g/mol. ATR-FTIR; γmax/cm−1: 3099 (NH2), 1762 (C=O), 1245.02 (C–O–C), 826.24 (C–Cl). 1H NMR data δH (300 MHz; DMSO-d6): 7.37 (6H, m, uncomplexed Ar–H), 7.24 (6H, m, uncomplexed Ar–H), 7.08 (1H, s), 6.89 (3H, m), 6.77 (4H, dd, J = 3.0 Hz, complexed Ar–H), 6.39 (5H, d, J = 7.2 Hz, complexed Ar–H), 5.25 (10H, s, Cp–H), 5.13 (1H, s, pyran-H), 2.52 (2H, br s, CH2), 2.40 (2H, br s, CH2), 1.70 (3H, s, CH3). 13C NMR δc (75 MHz; DMSO-d6): 171.33 (CO), 160.07, 151.03, 150.38, 149.66, 147.95, 146.01, 131.77, and 103.50 (quat-C), 129.20 and 120.01 (uncomplexed Ar–C), 129.75, 126.82, 125.28, 124.30, 118.16, and 109.75 (Ar–C), 120.88 (CN), 79.27 and 76.29 (complexed Ar–C), 86.62 (Cp–C), 64.79 (C–CN), 55.98 (C–N), 35.72 and 29.79 (C–CH2), 35.18 (pyran-C), and 26.78 (C–CH3). The Elemental Analysis of C53H42 Cl2F12Fe2N2O5P2S: Calc: %C 49.28, %H 3.28, %N 2.17, and found %C 49.29, %H 3.30, and % N 2.18.

2.4.3 Synthesis of Complexes 1015

Following the same protocol, 0.96 g, 0.92 mmol of the bimetallic organoiron 7 and 0.92 mmol of 3af were dissolved in DMF in a round bottom flask. A total of 0.11 g of DMAP and 0.92 mmol of the DCC solution were added to the reaction solution. The reaction was conducted in an ice bath for 30 min and under N2 for 48 h. The product was precipitated in 10% HCl, and 1.84 mmol of NH4PF6 was added.

2.4.3.1 Complex 10

Yield: 51%, yellow powder. The calculated molecular weight 1364.37 g/mol. ATR-FTIR; γmax/cm−1: 3084 (NH2), 2936 (Ar–C), 1760 (C=O), 1655 (C=C), 1244 (C–O–C), 829 (C–Br). 1H NMR data δH (300 MHz; acetone-d6): 7.48 (6H, m, uncomplexed Ar–H), 7.33 (6H, dd, J = 8.1 Hz, uncomplexed Ar–H), 7.15 (1H, d, J = 7.8 Hz, Ar–H), 6.88 (2H, s, NH2), 6.79 (4H, d, J = 6.0 Hz, complexed Ar–H), 6.50 (4H, d, J = 6.9 Hz, complexed Ar–H), 6.30 (2H, s, Ar–H), 5.37 (10H, s, Cp–H), 4.84 (1H, s, pyran-H), 2.65 (2H, d, J = 14.4 Hz, CH2), 2.48 (2H, d, J = 18.9 Hz, CH2), 1.80 (3H, s, CH3). 13C NMR δc (75 MHz; acetone-d6): 172.29 (CO), 161.10, 152.39, 151.45, 149.23, 147.86, 133.94, 123.24, and 104.98 (quat-C), 131.74, 131.48, 131.07, 129.87, 127.74, 119.42, 111.72, and 110.94 (Ar–C), 130.76 and 121.41 (uncomplexed Ar–C), 119.80 (CN), 80.56 and 77.28 (complexed Ar–C), 87.94 (Cp–C), 66.14 (C–CN), 58.75 (C–N), 46.53 and 37.04 (C–CH2), 41.37 (pyran-C), and 28.01 (C–CH3). Elemental Analysis for C55H43BrCl2F12Fe2N2O5P2: Calc: %C 48.42, %H 3.18, %N 2.05, and found: %C 48.49, %H 3.21, and %N 2.04.

2.4.3.2 Complex 11

Yield: 52%, brown yellowish powder. The calculated molecular weight 1319.92 g/mol. ATR-FTIR; γmax/cm−1: 3101 (NH2), 2192 (CN), 1755 (C=O), 556 (C–Cl).1H NMR δH (300 MHz; acetone-d6): 7.51 (5H, d, J = 9.9 Hz, uncomplexed Ar–H), 7.33 (9H, m, uncomplexed Ar–H), 7.08 (1H, m, Ar–H), 6.87 (2H, s, NH2), 6.78 (4H, d, J = 5.7 Hz, complexed Ar–H), 6.49 (4H, d, J = 6.6 Hz, complexed Ar–H), 5.35 (10H, s, Cp–H), 4.82 (1H, s, pyran-H), 2.68 (2H, t, J = 8.6 Hz, CH2), 2.48 (2H, t, J = 7.8 Hz, CH2), 1.79 (3H, s, CH3).13C NMR δc (75 MHz; acetone-d6): 171.83 (CO), 160.45, 155.88, 151.68, 150.28, 148.88, 146.62, 132.37, and 104.04 (quat-C),129.75 and 120.62 (uncomplexed Ar–C), 130.33, 129.17, 128.55, 122.15, 118.70, 112.90, and 110.27 (Ar–C), 119.87 (CN), 87.26 and 76.88 (complexed Ar–C), 79.79 (Cp–C), 65.62 (C–CN), 55.82 (C–N), 45.56, and 30.27 (C–CH2), 36.21 (pyran-C), and 27.33 (C–CH3). Elemental Analysis for C55H43Cl3F12Fe2N2O5P2: Calc: %C 50.05, %H 3.28, %N 2.12, and found: %C 50.09, %H 3.31, and %N 2.13.

2.4.3.3 Complex 12

Yield 63%, light yellow powder. The calculated molecular weight 1341.58 g/mol. ATR-FTIR; γmax/cm−1: 3158 (NH2), 3216 (C–H), 2191 (CN), 809 (C–Cl). 1H NMR data δH (300 MHz; acetone-d6): 7.49 (5H, t, J = 8.4 Hz, uncomplexed Ar–H), 7.35 (7H, m, uncomplexed Ar–H), 7.16 (3H, dd, J = 8.7 Hz, Ar–H), 6.88 (2H, s, NH2), 6.81 (4H, t, J = 6.0 Hz, complexed Ar–H), 6.50 (4H, d, J = 6.3 Hz, complexed Ar–H), 6.20 (1H, s, 1H, Ar–H), 5.38 (10H, s, Cp–H), 4.76 (1H, s, pyran-H), 2.68 (2H, t, J = 8.4 Hz, CH2), 2.50 (2H, t, J = 7.8 Hz, CH2), 1.81 (3H, s, CH3), 1.29 (9H, s, CH3). 13C NMR δc (75 MHz; acetone-d6): 172.20 (CO), 160.87, 152.24, 151.14, 149.82, 147.85, 143.48, 133.94, 122.19, 120.09, and 104.95 (quat-C), 130.67 and 121.38 (uncomplexed Ar–C), 129.86, 129.17, 128.12, 126.25, 119.16, 115.94, and 110.81 (Ar–C), 80.53 and 77.16 (complexed Ar–H), 87.90 (Cp–C), 66.03 (C–CN), 59.57 (C–N), 46.37 and 37.06 (C–CH2), 41.31 (pyran-C), 31.63 and 27.98 (C–CH3). The Elemental Analysis of C59H52Cl2F12Fe2N2O5P2: Calc: %C 52.82, %H 3.91, %N 2.09, and found: %C 52.92, %H 3.98, and %N 2.13.

2.4.3.4 Complex 13

Yield 56%, yellowish powder. The calculated molecular weight 1303.46 g/mol. ATR-FTIR; γmax/cm−1: 3094 (NH2), 2929 (Ar–C), 1753 (C=O), 1649 (C–N), 1456 (C–F), 1244 (C–O–C), 556 (C–Cl). 1H NMR data δH (300 MHz, DMSO-d6): 7.34 (13H, m, uncomplexed Ar–H), 7.00 (4H, br s, Ar–H), 6.79 (4H, s, complexed Ar–H), 6.41 (4H, s, complexed Ar–H), 5.26(10H, s, Cp–H), 4.85 (1H, s, pyran-H), 2.39 (2H, br s, CH2), 2.07 (2H, br s, CH2), 1.71 (3H, br s, CH3). 13C NMR δC (75 MHz, DMSO-d6): 170.55 (CO), 163.87, 159.76, 150.50, 147.86, 145.56, 141.33, 131.37, and 103.11 (quat-C), 129.30, 128.13, 123.05, 117.75, 113.72, 113.35, and 109.30 (Ar–C), 128.82 and 119.61 (uncomplexed Ar–C), 117.48 (CN), 78.85 and 75.75 (complexed Ar–C), 86.34 (Cp–C), 66.62 (C–CN), 54.92 (C–N), 35.22 and 31.00 (C–CH2), 34.28 (pyran-C), and 26.53 (C–CH3). The Elemental Analysis of C55H43Cl2F13Fe2N2O5P2: Calc: %C 50.68, %H 3.33, %N 2.15, and found: %C 50.69, %H 3.34, and %N 2.15.

2.4.3.5 Complex 14

Yield 52%, brown yellowish powder. The calculated molecular weight 1319.92 g/mol. ATR-FTIR; γmax/cm−1: 3101 (NH2), 2192 (CN), 1755 (C=O), 556 (C–Cl). 1H NMR δH (300 MHz; acetone-d6): 7.49 (5H, d, J = 9.9 Hz, uncomplexed Ar–H), 7.30 (9H, m, uncomplexed Ar–H), 7.07 (1H, d, J = 9.0 Hz, Ar–H), 6.86 (2H, s, NH2), 6.75 (4H, d, J = 7.5 Hz, complexed Ar–H), 6.46 (4H, d, J = 7.2 Hz, complexed Ar–H), 5.33 (10H, s, Cp–H), 4.83 (1H, s, pyran-H), 2.64 (2H, t, J = 7.9 Hz, CH2), 2.46 (2H, t, J = 7.5 Hz, CH2), 1.77 (3H, s, CH3). 13C NMR δc (75 MHz; acetone-d6) 172.11 (CO), 161.02, 158.59, 152.22, 151.34, 147.93, 145.38, 133.93, 119.80, 112.69, and 104.96 (quat-C), 130.73 and 121.41 (uncomplexed Ar–C), 130.94, 130.39, 129.66, 119.48, 119.29, 111.67, and 110.89 (Ar–C), 80.84 and 77.25 (complexed Ar–C), 87.35 (Cp–C), 64.97 (C–CN), 58.90 (C–N), 46.28 and 36.83 (C–CH2), 41.05 (pyran-C), and 28.03 (C–CH3). Elemental Analysis for C55H43Cl3F12Fe2N2O5P2: Calc: %C 50.05, %H 3.28, %N 2.12, and found: %C 50.08, %H 3.35, and %N 2.13.

2.4.3.6 Complex 15

Yield 61%, brown yellowish powder. The calculated molecular weight 1330.47 g/mol. ATR-FTIR; γmax/cm−1: 3092 (NH2), 2929 (Ar–C), 2171 (CN), 1526 (NO2), 1243 (C–O–C), 557 (C–Cl). 1H NMR data δH (300 MHz, acetone-d6): 8.12 (2H, s, NH2), 7.75 (1H, d, J = 7.8 Hz, Ar–H), 7.66 (1H, t, J = 7.5 Hz, Ar–H), 7.49 (5H, m, uncomplexed Ar–H), 7.33 (5H, d, J = 9.6 Hz, uncomplexed Ar–H), 7.16 (1H, d, J = 7.2 Hz, Ar–H), 6.88 (2H, m, Ar–H), 6.77 (4H, d, J = 7.2 Hz, complexed Ar–H), 6.48 (4H, d, J = 7.2 Hz, complexed Ar–H), 5.34 (10H, s, Cp–H), 5.06 (1H, s, pyran-H), 2.65 (2H, d, J = 8.4 Hz, CH2), 2.49 (2H, d, J = 8.7 Hz, CH2), 1.79 (3H, s, CH3). 13C NMR data δc (75 MHz, acetone-d6): 172.22 (CO), 161.14, 157.84, 152.26, 149.82, 148.64, 147.99, 134.00, 112.87, and 105.00 (quat-C), 135.30, 131.19, 130.99, 123.19, 123.01, 119.56, and 111.15 (Ar–C), 130.72 and 121.38 (uncomplexed Ar–C), 120.64 (CN), 87.91 and 77.24 (complexed Ar–C), 80.50 (Cp–C), 66.28 (C–CN), 58.31 (C–N), 46.46 and 37.00 (C–CH2), 41.26 (pyran-C), and 28.00 (C–CH3). The Elemental Analysis of C55H43Cl2F12Fe2N3O7P2: Calc: %C 49.65, %H 3.26, %N 3.16, and found: %C 49.69, %H 3.31, and %N 3.18.

2.4.4 Synthesis of Polymers 17, 18, 19, 20 and 21

0.08 mmol of complexes 1015 were added to 0.08 mmol of bisphenol A (16) in a 25 mL round bottom flask. 0.38 mmol, 0.05 g of potassium carbonate was added to the mixture in 0.3 mL of DMF. The reaction was achieved under N2 and darkness for 72 h. 0.16 mmol of NH4PF6 was added to the reaction solution in a 10% HCl solution to precipitate polymers 17, 18, 19, 20, and 21.

2.4.4.1 Polymer 17

Yield 67%, yellow powder. ATR-FTIR; γmax/cm−1: 3134 (N–H), 1796 (C=O), 1640 (C=C), 1215 (C–O–C), 668 (C–Br). 1H NMR data δH (300 MHz, DMSO-d6): 7.29 (19H, m, uncomplexed Ar–H), 7.04 (3H, d, J = 9.0 Hz, Ar–H), 6.69 (2H, d, J = 9.0 Hz, Ar–H), 6.27 (8H, s, complexed Ar–H), 5.22 (10H, s, Cp–H), 5.05 (1H, s, pyran-H), 2.39 (2H, s, CH2), 2.08 (2H, s, CH2) 1.71 (3H, s, CH3), 1.62 (6H, s, (CH3)2, bisphenol A). 13C NMR data δC (75 MHz, DMSO-d6): 171.77 (CO), 161.06, 156.10, 154.63, 152.36, 148.62, 147.03, 141.16, 137.67, 132.94, 131.00, and 114.23 (quat-C), 134.21, 133.09, 130.75, 130.11, 128.81, 128.24, and 115.66 (Ar–C), 129.62 and 120.84 (uncomplexed Ar–C), 118.48 (CN), 78.80 (Cp–C), 75.86 (complexed Ar–C), 62.49 (C–CN), 58.19 (C–N), 45.31 and 33.37 (C–CH2), 37.67 (pyran-C), 31.61 and 27.85 (C–CH3).

2.4.4.2 Polymer 18

Yield 61%, yellow powder. ATR-FTIR; γmax/cm−1: 3376 (N–H), 3205 (C–H), 2170 (CN), 529 (C–Cl). 1H NMR data δH (300 MHz, DMSO-d6): 7.36 (11H, d, J = 15.0 Hz, Ar–H), 7.23 (9H, d, J = 9.0 Hz, Ar–H), 7.04 (5H, s, Ar–H and NH2), 6.26 (8H, s, complexed Ar–H), 5.22 (10H, s, Cp–H), 4.70 (1H, s, pyran-H), 2.37 (2H, s, CH2), 2.07 (2H, s, CH2), 1.71 (3H, s, CH3), 1.36 (6H, s, CH3), 1.24 (9H, s, CH3). 13C NMR data δC (75 MHz, DMSO-d6): 172.18 (CO), 157.07, 152.17, 148.57, 147.28, 133.64, and 131.02 (quat-C), 129.61, 128.23, 126.54, 123.77, 122.14, 120.60, 118.46, and 115.67, (Ar–C), 130.09 and 120.83 ((uncomplexed Ar–C) 120.01 (CN), 78.76 (Cp–C), 75.95 (complexed Ar–C), 64.23 (C–CN), 57.27 (C–N), 46.66 and 37.73 (C–CH2), 36.66 (pyran-C), 31.66 and 27.87 (C–CH3).

2.4.4.3 Polymer 19

Yield 78%, yellow powder. ATR-FTIR; γmax/cm−1: 3345 (N–H) 2781 (Ar–C), 2196 (CN), 1261 (C–O–C), 1070 (C–F). 1H NMR data δH (300 MHz, DMSO-d6): 7.32 (15H, m, Ar–H), 6.99 (6H, dd, J = 9.5 Hz, Ar–H), 6.69 (4H, d, J = 9.9 Hz Ar–H), 6.26 (8H, s, complexed Ar–H), 5.22 (10H, s, Cp–H), 4.73 (1H, s, pyran-H), 2.39 (4H, s, (CH2)2, 1.70 (3H, s, CH3), 1.61 (6H, s, (CH3)2, bisphenol A).13C NMR data δC (75 MHz, DMSO-d6): 172.28 (CO), 167.33, 162.66, 156.64, 152.62, 145.60, 142.08, and 132.40 (quat-C), 119.07 (CN), 130.51, 120.74 (uncomplexed-C), 130.07, 127.86, 126.43, 124.72, 123.22, 122.96, 117.67, and 116.38 (Ar–CH), 78.54 (Cp–C), 75.97 (complexed-C), 65.30 (C–CN), 46.01 and 32.52 (C–CH2), 36.73 (pyran-C), 27.97 and 25.84 (C–CH3).

2.4.4.4 Polymer 20

Yield 74%, yellow powder. ATR-FTIR; γmax/cm−1: 3204 (N–H), 2200 (CN), 1741 (C=O), 438 (C–Cl). 1H NMR data δH (300 MHz, DMSO-d6): 7.20 (19H, m, Ar–H), 6.68 (2H, d, J = 9.0 Hz, Ar–H), 6.26 (8H, s, complexed Ar–H), 5.21 (10H, s, Cp–H), 4.98 (1H, s, pyran-H), 2.40 (s, 2H), 2.05 (s, 2H), 1.70 (s, 3H), 1.61 (s, 6H). 13C NMR data δC (75 MHz, DMSO-d6):172.42 (CO), 156.81, 152.42, 150.90, 149.21, 144.78, 140.74, and 131.03 (quat-C) 118.99 (CN), 64.41 (C–CN), 58.17 (C–N), 130.77, 130.37, 129.62, 129.50, 128.99, 128.64, 128.25, 120.62, 120.01, 119.23, 118.42, and 115.70 (Ar–C), 130.07 and 120,85 (uncomplexed Ar–C), 78.78 (Cp–C), 75.93 (complexed Ar–C), 45.79 and 36.59 (C–CH2), 37.32 (pyran-C), 32.46 and 31.53 (C–CH3).

2.4.4.5 Polymer 21

Yield 48%, yellow powder. ATR-FTIR; γmax/cm−1: 3234 (N–H) 2821 (Ar–C), 2191 (CN), 1496 (NO2), 1270 (C–O–C), 504 (C–Cl). 1H NMR data δH (300 MHz, DMSO-d6): 7.23 (16H, m, Ar–H), 7.02 (4H, d, J = 7.5 Hz, Ar–H), 6.94 (2H, d, J = 8.4 Hz Ar–H), 6.65 (2H, d, J = 8.4 Hz, Ar–H), 6.60 (1H, d, J = 8.7 Hz, Ar–H), 6.22 (8H, s, complexed Ar–H), 5.18 (10H, s, Cp–H), 4.96 (1H, s, pyran-H), 2.33 (2H, s, CH2), 2.01,(2H, s, CH2), 1.69 (3H, s, CH3), 1.59 (6H, s, (CH3)2, bisphenol A).13C NMR data δC (75 MHz, DMSO-d6): 171.68 (CO), 156.07, 151.86, 149.74, 148.58, 140.80, and 134.81 (quat-C), 135.81, 131.31, 130.78, 130.07, 128.24, 127.56, 126.09, 124.37, 122.97, 120.80, 119.28, 118.36, 115.73, and 115.38 (Ar–C), 129.49 and 120.63 (uncomplexed Ar–C), 119.63 (CN), 78.77 (Cp–C), 75.96 (complexed Ar–C), 64.57 (C–CN), 56.46 (C–N), 45.88 and 33.30 (C–CH2), 48.08 (pyran-C), 31.52 and 27.91 (C–CH3).

3 Results and Discussion

3.1 Synthesis and Characterization of Complexes 415

The cationic complexes were prepared via the Steglich esterification under mild reaction conditions. For instance, the chloroarene complexes 4, 5, and, 6 were investigated by generating reactions with different chromene species. The cationic iron complexes, containing chromene moieties, were prepared by the reaction of a 1:1 molar ratio of the ɳ6-chloroarene-ɳ5-cyclopentadienyliron carboxylic acid complex 1 with the desired chromenes 2 and 3, using DMAP and DCC as coupling reagents, Scheme 1. All reactions proceeded smoothly and allowed for the isolation of the desired ɳ6-arene-ɳ5-cyclopentadienyliron (II) complexes with the ether linkage as yellow solids in 46–50% yields.
Scheme 1

Schematic representation for the synthesis of mono-organoiron complexes (4, 5, and 6)

The successful synthesis of these complexes is clearly exemplified by the 1H NMR, in acetone-d6, of this example in which the Cp, pyran, and amino protons were all found as singlets at 5.30, 4.87, and 7.95 ppm, respectively, for compound 4. The complexed protons were shifted up from the uncomplexed aromatic ones in the form of duplets at 6.84 and 6.54 ppm as anticipated. Moreover, the 13C NMR spectrum of complex 4 illustrates that the pyran carbon appeared as an upward peak at 35.71 ppm while the quaternary carbon of the C–CN was at 64.77 ppm. Furthermore, the cyclopentadienyl peak surfaced at 79.70 ppm as an intense peak. The complexed aromatic CH carbons emerged at 77.48 ppm and 86.97 ppm, whereas the quaternary complexed aromatic carbons resonated at 132.17 and 103.92 ppm. Additionally, the uncomplexed aromatic peaks reverberated further downfield at 120.26 and 132.17 ppm, and the quaternary carbon belongs to the ester carbon atom, located at 171.54 ppm.

As an extension of this synthetic strategy, complexes 815 were prepared via the reaction of the bimetallic organoiron complex 7 with the chromene derivatives 2a,b and 3af in the presence of the catalytic amount of DMAP. The DCC solution was added to the mixture dropwise for a 15-min period. Subsequently, the reaction was conducted in an ice bath under an N2 atmosphere. The obtained products were purified from the DHU and precipitated in the NH4PF6 acidic solution. The desired complexes 815 were isolated as yellow powders in a higher than 50% yield, Scheme 2.
Scheme 2

Schematic representation of the synthesis of complexes 815

The ATR-FTIR absorption spectra of the new complexes exhibited bands of the amino, Cp–C, and ester groups around 3100, 2900, and 1650 cm−1, respectively. The characterization of these complexes was also performed, using the 1H and 13C NMR in DMSO-d6. For example, in the 1H NMR spectrum of complex 8, a doublet at δ = 6.50 ppm, corresponding to the protons in the p-dichloroarene and attached to the chloride atom, was shifted downfield to δ = 6.42 ppm. Furthermore, the uncomplexed protons of 8 were shifted as a multiplet from δ = 7.48 ppm to δ = 7.41 ppm. The Cp protons of this complex showed a downfield shift at δ = 5.28 ppm in comparison to the δ = 5.36 ppm of the p-dichloroarene moiety in complex 7. The integration of the proton signals in the 1H NMR disclosed that the singlet peak at δ = 4.88 ppm corresponded to the pyran proton of the chromene moieties while the broad singlet peak at δ = 6.88 ppm was associated to the two protons, which were bonded to the nitrogen atom of the chromene motif.

3.2 Synthesis and Characterization of Polymers Containing Chromene Moieties

It has been well established that the type of the pendent group incorporated into the polymeric chain has a strong influence on the obtained properties of these materials, which is one of the strategic techniques of developing new classes of macromolecules with desired features. This work demonstrates the first use of chromene moieties as pendant groups of the cationic organoiron macromolecules. The organoiron polymers, shown in Scheme 3, were prepared via the nucleophilic aromatic substitution polymerization of the p-dichlorobenzene complexes 1013 and 15 with bisphenol A 16 in the presence of a strong base such as potassium carbonate. These reactions allowed for the formation of polyaromatic ethers containing pendent chromene moieties 1721 in yields, ranging from 54 to 67%. Traditionally, this class of polymers was synthesized at high temperatures in the presence of strong electron-withdrawing groups. However, the presence of the electron-withdrawing cyclopentadienyliron moiety pendant to the dichloroarene allowed these polymerization reactions to occur at room temperature over a period of 72 h. All of the organoiron polymers were soluble in polar aprotic solvents such as DMF and DMSO and had varying degrees of solubility in solvents such as acetone, dichloromethane, and acetonitrile.
Scheme 3

Schematic representation of the synthesis of polymers 1721

The 1H and 13C NMR analysis of the cyclopentadienyliron-coordinated chromene moieties is a beneficial technique to verify the success of the polymerization reactions. The 1H NMR spectrum of the p-dichlorobenzene complex 7 is very simple with the cyclopentadienyl resonance appearing as a singlet at 5.26 ppm, and the complexed aromatic protons resonating as two singlets at 6.79 and 6.41 ppm. Upon polymerization, the resonances, corresponding to both peaks, underwent noticeable upfield shifts to become a one singlet peak at 6.26 ppm for polymer 19. The appearance of one singlet, representing the cyclopentadienyl protons in the polymers’ spectra at 5.22 ppm, indicated that there was no starting complex remaining. Furthermore, the complexed aromatic protons were shifted upfield in polymers 1721 to 6.22–6.27 ppm while the quaternary complexed aromatic carbons appeared at 132.40 ppm. The pyran proton resonated as a singlet between 4.73 ppm and 5.05 ppm for polymers 1721. The 13C NMR spectra of the organoiron polymers were also essential in evaluating the polymerization reactions. In particular, the presence of one Cp resonance from 78.50 to 78.80 ppm for polymers 1721 with etheric bridges revealed a successful polymer formation. The presence of one complexed aromatic resonance (C–H) from 75.86 to 75.97 ppm for all the synthesized polymers demonstrated an equivalence of the complexed aromatic CH carbons. Additionally, the signals for pyran-C appeared at the positive peaks around 36–37 ppm.

3.3 Molecular Weight Determination of the Functionalized Polymers

The orthodox method to establish the molecular weight of the polymers is Gel Permeation Chromatography (GPC). However, using the GPC in this situation was impracticable due to the presence of the cationic metal moieties in our synthesized polymers. Applying the Mark-Houwink equation, the molecular weight of the new polymers can be determined by its intrinsic viscosity [50].
$$ \left[ \eta \right] = KM^{a} . $$
(1)
Although not being an ideal model for the organometallic polymers, polystyrene with MW 20800 was selected as reference and utilized for quantitative measurement. The intrinsic viscosity is relative to the molecular weight of the polymers, as demonstrated in Eq. (1), which can be rearranged to:
$$ M = \left( {\frac{\left[ \eta \right]}{K}} \right)^{{\frac{1}{a}}} $$
(2)
where [η] is the intrinsic viscosity; M is the molecular weight of the polymer; (K) is the Mark–Houwink parameter value, linked to the polymer–solvent system; and (a) is the Mark–Houwink parameter value, linked to the flexibility and shape of the polymer chain [51].
Based on the calculations from Eq. (2), the approximate molecular weight for polymers 1721 and polystyrene are disclosed in Table 1, along with their intrinsic viscosity [52, 53].
Table 1

Calculated intrinsic viscosities and molecular weights of polymers 1721

Polymer no.

Intrinsic viscosity [η]

Molecular weight

17

82.71 ± 0.30

31,000

18

71.31 ± 0.22

25,000

19

87.69 ± 1.32

34,000

20

50.29 ± 0.43

15,000

21

36.63 ± 0.51

10,000

Polystyrene

55.54 ± 0.71

18,000

3.4 Redox Activity Properties of Complexes 415

The electrochemical properties of the new cationic organoiron complexes and macromolecules were examined, using cyclic voltammetry (CV). The CV’s measurements were obtained in a 0.1 M Bu4NPF6 solution as a supporting electrolyte with an Ag/AgCl reference electrode, a glassy carbon working electrode, and a Pt auxiliary electrode at different temperatures, ranging from − 13 to 25 °C. It is renowned that the cationic arene complexes of the cyclopentadienyliron undergo reversible reduction processes to produce neutral nineteen-electron iron complexes [54].

The mono-organoiron complexes investigated here were observed to exhibit electrochemical behavior, which was consistent with the analogous complexes studied previously [55]. The chemical reversibility of the first reduction step of the mono-organoiron complexes, [4]+, [5]+, and [6]+ stemmed the generation of their neutral 19e counterparts, which was discovered to be highly dependent on the temperature and on the substituent of each complex. In the case of the glassy carbon working electrode, the 18e/19e couple was determined to be chemically reversible at 0 °C for all the mono-metallic complexes, [4]+, [5]+ and [6]+. However, the effect of the temperature on the electrochemical behavior of the complexed arene species was evident upon the transfer of the second electron. It was witnessed that the thiophenyl substituted chromene [4]0 complex was considerably more stable than [5]0 and [6]0, which decomposed rapidly.

The electrochemical behavior of the ɳ6-chloroarene-ɳ5-cyclopentadienyliron complexes was further extended to the bimetallic CpFe+ complexes, [8]+2 to [15]+2. Interest in this series of complexes arises from earlier work of Bard and co-workers [56, 57], who divulged that in the case of similar complexes, the reduction of one metal moiety is strictly tailed by the reduction of the second metal moiety at a more negative potential. In other words, the electrochemical behavior of this class of material suggests some measurable degree of interaction between the iron centers.

To gain a better understanding of the electrochemical behavior of the cyclopentadienyliron complexes, several bimetallic complexes containing chromene moieties were explored at various temperatures. For example, Fig. 1a shows the cyclic voltammogram of complexes 8, 9, and 10 at 0 °C with a sweep rate of 0.2 V/s. The CVs displayed that the 18e cationic iron centers underwent reduction to give the neutral 19e complexes at the range of about E1/2 = − 1.24 V.
Fig. 1

Representative of cyclic voltammogram of complexes 8, 9, 10 (a) and polymer 19 (b) in 0.1 M Bu4NPF6 in propylene carbonate, scan rate = 0.2 V/s, at 0 °C

The electrochemical properties of the functionalized polymers exhibited similar behavior as their complexed precursors. It is apparent from the CV measurements that there was only one redox process due to the reversible reduction of the cationic eighteen-electron iron centers to the nineteen-electron neutral iron centers. For instance, the CV of polymer 19, functionalized with the fluoro group, is shown in Fig. 1b and presented a one-electron reduction step of the iron center pendant to the polymer backbone at a scan rate of 0.2 V/s.

3.5 Morphological Study of the Polymeric Materials

The surface texture of the new macromolecules 1721 in the powder form was explored using the scanning electron microscopy (SEM). The SEM images of these materials revealed an amorphous character of the polymeric residues, Fig. 4. The residue of polymer 17 implied an agglomeration of the different particle sizes and shapes. The large substrate particles were of different sizes with sharp edges that were surrounded with irregular, amorphous particles, Fig. 2a. The SEM images of polymer 19 and 20 had no specific shape and emerged as globular, irregular, and amorphous, where the particles were less rough and more solid-like in appearance, Fig. 2b and 2e, respectively. The micrographs of polymer 18 appeared fluffy and very granular with some particles having smooth surfaces and sharp edges, Fig. 2c. The well-defined fluffy particles were observed in the polymer 21’s residues with particles having oval shapes and almost uniform in size, Fig. 2d.
Fig. 2

Representative of morphology images SEM of polymers 17 (a), 20 (b), 18 (c), 21 (d), and 19 (e)

3.6 Thermal Stability of the Polymeric Materials

The thermal behavior of the chromene containing organoiron polymers were assessed via the thermogravimetric analysis (TGA). As anticipated, all the metallated polymers, 1721, experienced a 15 to 26% weight loss between 158 and 213 °C, which corresponds to the cleavage of the cyclopentadienyliron hexafluorophosphate moieties. Following this initial weight loss, the polymers encountered second weight losses that were dependent upon the etheric linkages in their backbones. The weight percentages lost were comparable in all polymers.

The TGA thermograms of the organoiron polymer 19 and its corresponding monomer (13) are illustrated in Fig. 3. Polymer 19 displayed a three-step weight loss of 21%, beginning at 175 °C and ending at 291 °C, followed by a loss of 54% of its weight at 580 °C and, eventually, an overall loss of 83% at Tendset 950 °C. However, its monomer analogue, experiencing a two-step weight loss, starting at 220 °C and ending at 250 °C, recorded 16% of its total weight loss.
Fig. 3

TGA thermogram of polymer 19 and its monomer analogue (13)

It is worth noting that the incorporation of the chromene moieties into the polymeric materials did not remarkably alter their thermal stability, which is characterized by the onset of the rapid degradation of the polymers, Table 2. All the polymers were thermally stable up to 180 °C and were degraded in two steps. On the other hand, the first onset of the rapid degradation (Tonset) corresponds to the cleavage of the CpFe2+ moiety and the subsequent thermal degradation of the cyclopentadienyl ligands as formerly reported [58]. The resulting organic compound-iron composite was stable up to 362 °C, after which the rapid weight loss and volatilization occurred at the second Tonset. The TGA data for the functionalized polymers is provided in Table 2.
Table 2

Thermal analysis of functionalized polymers 1721

Polymer

Weight loss (%)

Tonset (°C)

Tendset (°C)

17

22

181

291

43

362

580

18

15

165

259

70

327

749

19

21

175

291

54

375

580

20

26

213

327

31

387

555

21

15

158

273

60

708

946

3.7 Biological Study of Complexes 1 and 415

3.7.1 Microbial Activity of Complexes 1 and 415

The new complexes were evaluated for their activity against the infection-causing microorganisms, including methicillin-resistant Staphylococcus aureus (MRSA), vancomycin-resistant Enterococcus faecium (VRE), and Staphylococcus warnerii (S warnerii). Previously, the ferrocene molecule was reported to be inactive against microorganisms directly, but has shown enhancements in the antimicrobial activity via the oxidative damage caused by the reactive oxygen species (ROS) [32]. In contrast to the ferrocene, the [ɳ6-arene-ɳ5-CpFe]+ complexes were active against several species of Gram-positive bacteria [43]. As can be noticed from Table 3, the ɳ6-chloroarene-ɳ5-cyclopentadienyliron carboxylic acid, complex 1, showed moderate activity with an IC50 of 70.24 μM against MRSA, 59.03 μM against S. warnerii, and did not exhibit activity against VRE. Also, the bimetallic organoiron complex 7 showed slight microbial effects against MRSA and S. warnerii, while there was no activity against VRE.
Table 3

Microbial activity of complexes 1, 2b and 415

Complex

Epc (V)

MRSA IC50 (µM)

S. warnerii IC50 (µM)

VRE IC50 (µM)

1

 

70.24

59.03

>75

2b

 

66.23

>75

>75

4

− 1.26

17.60

35.82

12.91

5

− 1.33

23.58

22.71

16.48

6

− 1.30

41.93

15.03

30.16

7

 

68.56

30.47

>75

8

− 1.29

2.40

2.05

8.39

9

− 1.32

4.79

4.58

15.81

10

− 1.34

3.65

3.05

8.74

11

− 1.35

5.01

4.87

10.32

12

− 1.43

12.34

6.41

9.74

13

− 1.39

8.56

7.39

2.12

14

− 1.37

4.25

7.92

7.56

15

− 1.46

21.46

15.89

47.31

Vancomycin

 

0.81

0.50

Rifampicin

 

0.87

The presence of the chromene moieties, functionalized with different groups, affect the microbial activity of these complexes. A structure–activity relationship (SAR) investigation was carried out to understand the critical parameters that influence the activity of these complexes as molecules and moieties in the complexes. To this end, an electron-donating alkyl group was introduced into the arene ligand to alter their properties such as the redox activity of the iron centers. The CV study indicated that the tert-methyl groups decreased the Epc, as demonstrated in Table 3. Furthering the SAR investigation, the methyl group of compound 12 was replaced by a fluoro group, yielding compound 13, as seen in Scheme 2. This substitution increased the Epc from − 1.43 V in (12) to − 1.39 V in (13) and led to an evident improvement in the antimicrobial activity against MRSA (IC50 = 8.58 μM). Similarly, the chromene-containing organoiron complexes 11 and 14 exhibited increased activity against the MRSA in comparison with complexes 12 and 13 when substituted by a chloro group. Additionally, replacing different withdrawing groups (bromo and thio) resulted in increased activity against the MRSA when evaluated against the methyl-substituted analogue, as shown in Fig. 4 and Table 3. The activity of the chromene molecules containing cationic iron complexes has been manipulated by the type of the substituent on the phenyl ring. As demonstrated, the lower activity has been associated with the presence of the electron donating group (t-butyl group). Meanwhile, exchanging this substituent with electron withdrawing groups (halogens groups) greatly impacted these complexes at the microbial level, particularly against MRSA. In fact, the degree of the electronegativity, the bond length and the position of the halogen substituents play a crucial role in the performance of these complexes against the tested organisms; the activity of the tested complex has the following order: 10 > 14 > 11 > 13. Furthermore, complexes 8 exhibited the strongest activity against MRSA and S. warnerii, which could be attributed to the lower aromaticity of the thiophene ring compared to the phenyl ring.
Fig. 4

Microbial activity of complexes 1, 2b and 415

3.7.2 Cytotoxicity of Complexes 1 and 415

The human hepatocellular carcinoma (HepG-2), the adenocarcinoma human alveolar basal epithelial cell (A-549), and the human breast adenocarcinoma (MCF-7) cell lines were obtained from the American Type Culture Collection (ATCC, Rockville, MD).

The in vitro growth inhibitory rates (%) and the inhibitory growth activity (as measured by IC50) of the synthesized compounds were investigated in comparison with the well-known anticancer standard drugs Cisplatin and Imatinib (2-substituted aminopyrimidine derivative; Gleevec®), using the MTT viability assay. However, the results revealed that the tested compounds showed high variation in the inhibitory growth rates and the activity against the tested tumor cell lines in a concentration-dependent manner. In addition, the activity denoted a considerable variation, according to the type of the tumor cell line, and the difference between the inhibitory activities of all compounds with the different concentrations was statistically significant (P ˂ 0.001). The highest activity against the human hepatocellular carcinoma (HepG-2) cell line was measured for complex 13 with an IC50 value of 25.6 μM, compared with the reference drug Imatinib (32.9 μM), followed by the complexes 11, 10, 9, 12, 8, 14, and 15, respectively, Table 4. Additionally, the highest detected inhibitory activities against the human breast carcinoma (MCF-7) cell line was measured for complex 9, followed by 13, 10, 12, 11, 8, 14, and 15, respectively, presented in Fig. 5 and Table 4. Interestingly, the measured IC50 values for complexes 9 and 13 against the human breast carcinoma (MCF-7) and liver carcinoma (HepG-2) cell lines revealed that these compounds exhibited antitumor activities better than those detected for the reference drug Imatinib. In addition, the order of activity against the lung carcinoma cell line (A 549) was 11, 14, 9, 8, 10, and 13, respectively. Moreover, the complexes 12 and 15 were less active among their analogues against the lung tumor cell line. Complexes 4, 5, and 6 did not show appreciable activity against all the cell lines. Membrane-active antimicrobial agents can also interact with mammalian cell membranes, causing harm, a development that could limit their clinical applications [33, 47, 59]. The toxicity of these cationic complexes against the human foreskin BJ fibroblast cells was evaluated. Complexes 12, 4, 5, and 6 appear to be non-toxic towards this cell line under assay conditions. The bimetallic complexes 815 exhibited activity against all the tested cell lines in comparison to the complexes that have one iron moiety in their structure, Fig. 5 and Table 4. Furthermore, complexes 1 and 4 did not show any activity against these cell lines.
Table 4

Cytotoxicity activity of complexes (1 and 415)

Complex

IC50 (µM)

IC50 values (µM) against tumor cell lines

Fibroblast (BJ)

Hep-G2

MCF-7

A-549

1

> 128

> 128

> 128

> 128

4

127.0 ± 4.3

> 128

> 128

121.8

5

> 128

125.4

> 128

> 128

6

> 128

> 128

125.6 ± 2.7

> 128

7

> 128

> 128

> 128

> 128

8

96.0 ± 5.8

53.7

56.4 ± 3.8

57.1

9

30.4 ± 2.6

48.8

15.6 ± 0.6

54.2

10

46.3 ± 5.2

44.7

45.4 ± 3.0

57.5

11

70.6 ± 3.4

35.1

51.8 ± 3.2

44.6

12

> 128

52.9

46.9 ± 2.4

108.8

13

14.2 ± 0.8

25.6

29.3 ± 3.1

90.2

14

55.01 ± 4.7

82.3

114.7 ± 5.1

49.3

15

30.5 ± 5.8

124.9

126.8 ± 6.3

113.7

Zinc pyrithione

5.06 ± 0.8

   

Cisplatin

 

12.3

14.9

35.8

Imatinib

 

32.9

30.6

21.7

Fig. 5

Cytotoxicity activity of complexes (1 and 415)

The biological activity of the cationic organoiron agents (antimicrobial and anticancer) is influenced by an interaction of some parameters that highlight the relationship between the structure and the biological selectivity of the drugs. It seems that at least part of the growth or the proliferation-inhibiting effects of the compounds could be mainly due to the functional group and the substitution at the 2, 3, or 4 positions of the phenyl ring on the chromene moieties.

3.7.3 Biological Study of the Polymeric Materials

To improve the antimicrobial activity, the polymers were functionalized with the known antimicrobial agents to afford the active antimicrobial polymers. The CLSI microbroth dilution antimicrobial assay protocol [60, 61] was used to evaluate the in vitro antimicrobial activity of these functionalized polymers to gain insight into their IC50s and MIC90s. All the polymers were active against the Gram-positive MRSA, VRE, and S. warnerii, but were inactive against the Gram-negative bacteria P. aeruginosa and P. vulgaris, Fig. 6 and Table 5. Comparatively, the bromo-functionalized polymer 17 was more active against the three gram-positive bacteria in comparison to its meta-fluoro and tert-butyl-functionalized polymer. Furthermore, the chloro-functionalized polymer was more potent than the o-nitro substituted chromene, as evidenced by their lower MIC50s and IC50s as presented in Table 5. As an example, the IC50 of the MRSA of polymer 17, which was functionalized with the bromo group in the position C3 was two times higher than that of the o-nitro substituent in the position C2, implying a more potent activity. Moreover, the chloro functionalized polymer 20 was more active against VRE than the other four tested polymers. The functionalization of the chloro groups drastically enhanced the activity. For instance, the IC50 for polymer 20 was 5.4 µg/mL against VRE while it was 8.8 µg/mL for polymer 19, which was functionalized with the fluoro group. The most active compound against S. warnerii was the bromo substituted chromene, 17, (IC50 = 5.1 µg/mL) followed by polymer 19 (fluoro group) (IC50 = 6.0 µg/mL), with the lowest active polymer, supported by the lower IC50 value of 8.2 µg/mL, being polymer 18.
Fig. 6

Microbial activity of the functionalized polymers 1721

Table 5

Microbial activity of the functionalized polymers 1721

Polymer

MRSA

VRE

S. Warnerii

MIC90 (µg/mL)

IC50 (µg/mL)

MIC90 (µg/mL)

IC50 (µg/mL)

MIC90 (µg/mL)

IC50 (µg/mL)

17

8

4.7 ± 0.9a

16

7.5 ± 0.1

8

5.1 ± 0.0

18

8

9.1 ± 0.3

16

6.4 ± 0.3

16

8.2 ± 1.7

19

8

5.8 ± 0.3

16

8.8 ± 0.2

8

6.0 ± 1.1

20

16

3.3 ± 1.9

32

5.4 ± 0.1

32

3.2 ± 1.2

21

16

8.9 ± 1.1

32

8.0 ± 0.4

32

10.1 ± 0.7

Vancomycin

 

0.81

 

0.50

 

Rifampicin

 

 

 

0.87

aData represented in terms of mean ± SD

The toxicity of the new functionalized polymers against the following human cell lines was evaluated: foreskin BJ fibroblast, HTB-26, HCT-116, and MCF-7. The polymers were non-toxic to the BJ fibroblast cells in the concentration of 64 µg/mL under assay conditions. Polymer 17 displayed activity against all the tested tumor cell line agents and was non-toxic to the foreskin BJ fibroblast cells. For instance, the IC50 of polymer 17 was 41.07 and 47.23 µg/mL against the HTB-26 and the HCT-116 cell lines, respectively. Polymer 18 exhibited activity against the HTB-26, which had an IC50 of 54 µg/mL.

4 Conclusion

A new class of cationic organoiron complexes and polymers containing chromene motifs has been developed to exploit their potential as drug-like candidates. The microbial assay of the redox-active [η6-arene-η5-CpFe]+ complexes revealed an oxidative damage to the examined microorganisms. Moreover, the cationic charge on the synthesized complexes assisted in disrupting the microbial cell membrane, which, subsequently, improved the antimicrobial effect. A structure–activity relationship (SAR) investigation was conducted to gain insight into critical parameters that control the activity of these complexes. For instance, complexes with the halogenated substituents displayed a strong activity against MRSA in comparison to the methyl derivative. The cytotoxic activity of the new cationic complexes, particularly with the fluoro and chloro moieties, exhibited a strong performance against the HepG-2 and MCF-7 cell lines. Furthermore, the toxicity against the human foreskin BJ fibroblast cells was enhanced with the fluoro or thiophene substituents at the 3-position compared to the position of 2 or 4. Additionally, the polymeric materials showed significant activities against the three Gram-positive bacteria. The SAR confirmed that the biological activity of these polymers depends on the position and the functionalized group on the chromene moiety.

Notes

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Copyright information

© Springer Science+Business Media, LLC, part of Springer Nature 2019

Authors and Affiliations

  1. 1.Department of ChemistryUniversity of Prince Edward IslandCharlottetownCanada
  2. 2.The Regional Center for Mycology and BiotechnologyAl-Azhar UniversityCairoEgypt
  3. 3.Department of ChemistryTaibah UniversityMadinahSaudi Arabia

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