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Original article
13 (
1
); 3546-3557
doi:
10.1016/j.arabjc.2018.12.006

Aryl ferrocenylmethylesters: Synthesis, solid-state structure and electrochemical investigations

, , , , , , , , , ,
a Department of Chemistry, The University of Jordan, Amman 11942, Jordan
b Technische Universität Chemnitz, Faculty of Natural Sciences, Institute of Chemistry, Inorganic Chemistry, D-09107 Chemnitz, Germany
c Al-Hussein Bin Talal University, Ma’an 71111, Jordan
d Department of Chemistry, Jordan University of Science and Technology, Irbid 22110, Jordan
e Department of Chemistry, Yarmouk University, Irbid 21163, Jordan

⁎Corresponding author. d.taher@ju.edu.jo (Deeb Taher)

Received: , Accepted: ,
© The Author(s) 2018
Disclaimer:
This article was originally published by Elsevier and was migrated to Scientific Scholar after the change of Publisher.

Peer review under responsibility of King Saud University.

Abstract

The synthesis of Fe(η5-C5H4CH2OC(O)R)2, 1,2-/1,3-/1,4-(FcCH2O)2-C6H4 and 1,3,5-(FcCH2O)3-C6H3 is discussed. The solid state structures of five compounds are reported. Electrochemical investigations show reversible Fc/Fc+ redox events. DFT calculations reveal different HOMO-LUMO gaps depending on the aryl substituent pattern.

Abstract

  • Synthesis of ferrocenylmethyl aryl esters; electrochemistry shows reversible Fc/Fc+ events.

  • Molecular solid state structures confirm anti-periplanar orientations of the 1,1′-positioned substituents.

  • DFT calculations revealed different degrees of HOMO-LUMO gaps depending on the positions of the carboxylic ester substituents on the aryl rings.

Abstract

The reaction of [Fe(η5-C5H4CH2OH)2] (1) with 2 equiv of ClC(O)R (2) (a, R = C6H5; b, R = 2-CH3-C6H4; c, R = 3-CH3-C6H4, d, R = 4-CH3-C6H4) produced the corresponding ferrocenyl carboxylates [Fe(η5-C5H4CH2OC(O)R)2] (3a–d). Treatment of [FcCH2OLi] (4-Li) (Fc = Fe(η5-C5H5)(η5-C5H4)) with (ClC(O))2C6H4 (5) (a, 1,2-((ClC(O))2-C6H4; b, 1,3-((ClC(O))2-C6H4; c, 1,4-((ClC(O))2-C6H4) in a 2:1 M ratio gave (FcCH2OC(O))2-C6H4 (6a–c), while with 1,3,5-(ClC(O))3-C6H3 (7) in a 3:1 M ratio produced 1,3,5-(FcCH2OC(O))3-C6H3 (8). All compounds were characterized by NMR (1H, 13C{1H}) and IR spectroscopy, ESI-TOF mass spectrometry and elemental analysis. The molecular structures of 3a–d and 6b in the solid state were determined by single crystal X-ray structure analysis, showing anti-periplanar orientations of the 1,1′-positioned substituents at ferrocene (3a–d). In the cyclic voltammograms of 3a–d, 6a–d and 8 reversible electrochemical redox processes (Fc/Fc+) were observed, ranging between 146 and 164 mV for 3a–d, ca. 100 mV for 6a–d and at 113 mV for 8 using [NnBu4][B(C6F5)4] as the supporting electrolyte. The molecular electronic structure of 3, 6 and 8 was calculated by DFT methods in order to obtain the HOMO and LUMO absolute and relative energies in addition to electron density and distribution within the molecular arrangements. It was found that different degrees of HOMO-LUMO energy gaps within the series, due to a lowering in the LUMO energy depending on the positions of the carboxylic ester substituents on the aryl rings, are in agreement with the electrochemical results obtained.

Keywords

Ferrocenylmethyl
Mono-dicarboxy benzene
Trimesic acid
Solid state structure
Electrochemistry
DFT
1

1 Introduction

Ferrocene has been named as an “exceptional molecule” in a recent review summarizing its chemistry (Astruc, 2017). Ferrocene and its derivatives are widely applied as substrates and building blocks for the synthesis of ferrocenyl-containing organometallic molecules (Stepnicka, 2008; Dai and Hou, 2008; Hildebrandt and Lang, 2013; Vecchi et al., 2015; Hamera, 2015). Due to the promising electronic properties of ferrocene and its easy functionalization, ferrocene derivatives have found many applications in, for example, materials sciences (Van Staveren and Metzler-Nolte, 2004), including electroactive materials (Daniel et al., 2006; Gul et al., 2012; Liang et al., 2009; Ningyu et al., 2000; Megiatto et al., 2010; Chungkyun et al., 2001; Farre et al., 2007; Astruc et al., 2008; Mousty et al., 2009; Baldoli et al., 2006), electrochemical sensors (Wang, 2001; Willner and Willner, 1997; Willner and Katz, 2000; Labande et al., 2002; Armada et al., 2006; Astruc et al., 2008; Djeda et al., 2010), combustion regulators (Chernyi et al., 2012; Sinditskii et al., 2014), efficient (chiral) catalysts in organic synthesis (Wei et al., 2002; Togni and Hayashi, 1995; Diallo et al., 2007; Togni et al., 1998; Bonini et al., 2013; Schaarschmidt et al., 2014; Korb et al., 2014; Korb and Lang, 2014), aerospace materials (Neuse, 1968; Neuse et al., 1988) or medicine (Ornelas, 2011; Kondapi et al., 2006; Zhang, 2008; Biot et al., 2000). Due to the stability of ferrocene in aerobic and aqueous media, ferrocene conjugates with various biomolecules, including peptides, amino acids, carbohydrates, DNA, hormones, dendrimers, steroids, and poly-functional organic compounds have been prepared, some of them exhibiting pronounced biological activity (Van Staveren and Metzler-Nolte, 2004; Allardyce et al., 2005; Hillard et al., 2010; Snegur et al., 2010; Fouda et al., 2007) Recently, molecules with several ferrocenyl units, polymers containing ferrocene and ferrocenyl-functionalized π-conjugated hydrocarbons, i.e. arenes or five- and six-membered heterocycles have attracted great attention with respect to their electrochemical, electronic, electronic communication in their mixed-valent state and magnetic properties (Rulkens et al., 1996; Delgado-Pena et al., 1983; Nagarale et al., 2009; Stone and Smith, 2003; Cha et al., 2014; Abd-El-Aziz and Manners, 2007; Aranzaes et al., 2006; Yu et al., 2006; Hildebrandt et al., 2010; Hildebrandt and Lang, 2011; Speck et al., 2014; Speck et al., 2012; Pfaff et al., 2012; Filipczyk et al., 2014; Pfaff et al., 2014; Filipczyk et al., 2017; Miesel et al., 2015; Hildebrandt et al., 2011; Kaleta et al., 2011; Kaleta et al., 2012; Pfaff et al., 2015; Pfaff et al., 2016; Korb, 2017; Lehrich et al., 2017).

Multi-ferrocenyl compounds, including biferrocenyl complexes in which two ferrocenyl units are connected by different covalent units, can easily form mixed-valent (MV) Fe(II)-Fe(III) species by chemical or electrochemical oxidation (Li et al., 2010; Moriuchi and Hirao, 2007; Guldi et al., 1997). Such compounds have received great attention as electronic wires (Sixt et al., 2010; Nguyen et al., 1999). In particular, such compounds have been used widely for electrochemical studies of the electronic intramolecular communications among covalently linked redox units.

Ferrocenyl esters show many applications (Dong et al., 1997; Huang et al., 2012). These complexes can be synthesized by the reaction of ferrocene acyl chlorides with alcohols (Huang et al., 2012; Ion, 2001; Moore et al., 1993; Kumara and Menon, 2009; Auzias et al., 2009; Murashima et al., 2005; Khan et al., 2007) or in situ functionalization of 1,1′-bisferrocene dicarboxylic acid with N-(3-dimethylaminopropyl)-N′-ethylcarbodiimide (Al-Momani and Lataifeh, 2013) or N,N′-dicyclohexylcarbodiimide (Tranchant et al., 2006; Lee et al., 2009), in presence of catalytic amounts of 4-(dimethylamino)pyridine or 1,1′-carbonylbis-1H-imidazole (Hyodo et al., 2002), coordination of two cyclopentadienyl rings, bearing ester function, with iron (Busetto et al., 2001; Busetto et al., 2002; Ou-Yang et al., 2013; Li et al., 2012; Zhao et al., 2011), or oxidation of 1,1′-ferrocene ketones (Yu et al., 2008). Ferrocenyl esters were also accessible from iodoferrocene and carboxylic acids catalyzed by copper(I) compounds (Epton et al., 1977). Moreover, ferrocenyl esters can be prepared by reacting ferrocenylboric acid with aldehydes in the presence of air using N-heterocyclic carbenes as catalysts (Denisov et al., 2015). The reaction of ferrocenoyl fluoride with substituted phenols and N,N-dimethylaminopyridine in 1-butyl-3-methylimidazolium tetrafluoroborate as efficient trapping solvent for hydrogen fluoride or solvent-free reactions by microwaves (Imrie et al., 2005). Ferrocenoyl imidazolides react rapidly with sodium alkoxides to give ferrocenoyl esters (Imrie et al., 2001).

In continuation of our studies on ferrocenyl-substituted compounds (Taher et al., 2013, 2017a, 2017b, 2018, 2006, 2012, 2018; Taher and Corrigan, 2011; Taher, 2009), this article deals with the synthesis, characterization, molecular solid state structure, electrochemical behavior, and molecular orbital electronic structure calculations of a series of aryl ferrocenylmethylesters of type [Fe(η5-C5H4CH2OC(O)R)2], 1,2-/1,3-/1,4-(FcCH2OC(O))2-C6H4 and 1,3,5-(FcCH2OC(O))3-C6H3 (Fc = Fe(η5-C5H5)(η5-C5H4); R = C6H5, 2-CH3-C6H4, 3-CH3-C6H4, 4-CH3-C6H4).

2

2 Results and discussion

2.1

2.1 Synthesis and characterization

1,1′-Substituted ferrocenes of type [Fe(η5-C5H4CH2OC(O)R)2] (3) (a, R = C6H5; b, R = 2-CH3-C6H4; c, R = 3-CH3-C6H4, d, R = 4-CH3-C6H4) were accessible by the reaction of 1,1′-bis(hydroxymethyl)ferrocene (1) (Korb et al., 2014; Korb and Lang, 2014; Korb, 2017; Korb et al., 2017; Korb et al., 2016) with the corresponding acyl chlorides ClC(O)R (2a–d) in the molar ratio of 1:2 in presence of 2 equiv of Et3N in dichloromethane at ambient temperature (Scheme 1). After appropriate work-up, compounds 3a–d were isolated in a yield of 80–93%. The organometallic carboxylic esters are stable towards hydrolysis (Korb et al., 2017).

Synthesis of ferrocenylmethyl esters 3a–d. (i) Treatment of 1 with 2 equiv of Et3N in dichloromethane at 25 °C; (ii) addition of 2 equiv of ClC(O)R (2a–d), 0 °C, 18 h. Yields are based on 1.)
Scheme 1 Synthesis of ferrocenylmethyl esters 3a–d. (i) Treatment of 1 with 2 equiv of Et3N in dichloromethane at 25 °C; (ii) addition of 2 equiv of ClC(O)R (2a–d), 0 °C, 18 h. Yields are based on 1.)

Di- and tri-ferrocenylmethyl aryl esters 1,2-/1,3-/1,4-(FcCH2OC(O))2-C6H4 (6a–c) and 1,3,5-(FcCH2OC(O))3-C6H3 (8) could be prepared by treatment of [FcCH2OLi] [Fc = Fe(η5-C5H5)(η5-C5H4)] (4-Li) with either benzenedicarboxylic acid chlorides (ClC(O))2C6H4 (5) (a, 1,2-; b, 1,3-; c, 1,4-substituted) or trimesic acid (7) in a 2:1 or 3:1 M ratio in diethyl ether (Scheme 2). After appropriate work-up yellow 6 and 8 could be isolated in good yields.

Synthesis of 6a–c (i) Treatment of 4 with MeLi (1 equiv), diethyl ether, 0 °C; (ii) addition of 0.5 equiv of 5a–c, –78 °C, 18 h. (iii) addition of 0.33 equiv of benzene-1,3,5-tricarbonyl chloride, –78 °C, 18 h. Yields are based on 4.)
Scheme 2 Synthesis of 6a–c (i) Treatment of 4 with MeLi (1 equiv), diethyl ether, 0 °C; (ii) addition of 0.5 equiv of 5a–c, –78 °C, 18 h. (iii) addition of 0.33 equiv of benzene-1,3,5-tricarbonyl chloride, –78 °C, 18 h. Yields are based on 4.)

Compounds 3a-d, 6a–c and 8 can be isolated either by their extraction with water-dichloromethane mixtures of ratio 1:5 (v/v) (3a–d), or filtration of the reaction mixtures through a pad of Celite (6a–c, 8). Crystallization of 3a–d, 6a–c and 8 from chloroform-hexane mixtures of ratio 3:1 (v/v) at −18 °C afforded the title compounds as yellow solids in good yield (Experimental). All compounds are stable towards moisture and air in solution as well as in the solid state. They are soluble in most common organic polar solvents like chloroform, dichloromethane and tetrahydrofuran, but almost insoluble in diethyl ether and hexane.

Compounds 3a–d, 6a–c and 8 were analyzed by spectroscopy (IR, 1H and 13C{1H} NMR), ESI-TOF mass spectrometry and elemental analysis. In addition, the molecular structures of 3a–d and 6b in the solid state were determined by single crystal X-ray diffraction structure analysis. The electrochemical behavior was studied by cyclic voltammetry. DFT calculations were carried out.

The IR spectra of 3a–d, 6a–c and 8 show very characteristic strong carbonyl stretching vibrations between 1700 and 1735 cm−1 (Oparina et al., 2016). They are shifted to lower wave-numbers when compared to 2a–d; 5a–c and 7 (Torun et al., 2005; Kampmeier et al., 1981; Hoffmann et al., 1982). During the course of the reaction of ferrocenemethanol with the acid chloride, the typical υOH at 3200–3500 cm−1 was disappeared, and hence IR spectroscopy is suited to follow-up the reaction progress (Claus et al., 2013).

The 1H and 13C{1H} NMR spectra of 3a–d, 6a–c and 8 are consistent with their anticipated molecular structures and show the predictable resonances and coupling patterns (Deng et al., 2008; Taher et al., 2013, 2017a, 2017b, 2018, 2006, 2012, 2018; Taher and Corrigan, 2011; Taher, 2009). As expected, the CH2 protons give rise to a singlet at ca. 5.1 ppm (Lu et al., 2017), while the C5H4 ferrocenyl protons were observed as pseudo-triplets at 4.2–4.4 ppm (Taher et al., 2013, 2017a, 2017b, 2018, 2006, 2012, 2018; Taher and Corrigan, 2011; Taher, 2009). For 6a–c and 8 additional singlets for the C5H5 protons were found at ca. 4.2 ppm (Taher et al., 2013, 2017a, 2017b, 2018; Taher and Corrigan, 2011). The hydrogen atoms of the C6H5, C6H4 or C6H3 unites resonate between 7.5 and 8.7 ppm, whereby different patterns are observed depending on their substitution (Huang et al., 2012; Yang et al., 2006; Splith et al., 2010).

In the 13C{1H} NMR spectra of 3a–d, 6a–c and 8 the expected signals for the organic/organometallic groups are found (Experimental). Among them, the carbonyl carbons resonating at ca. 167 ppm are most characteristic (Experimental) (Denisov et al., 2015). The appropriate ipso carbon atoms of the aryl rings are observed at ca. 140 ppm.

When one compares the 1H and 13C{1H} NMR chemical shift of 3a–d, 6a–c and 8 (Claus et al., 2013; Gallei et al., 2009) with that ones typical for 1 and 4 a high-field shift is characteristic, which is attributed to the presence of the high electron withdrawing carboxy groups.

The electrospray ionization mass spectrometry (ESI–MS) technique was additionally used to characterize 3a–d, 6a–c and 8. For all compounds the molecular ion [M]+ could be detected (Experimental).

2.2

2.2 Single crystal X-ray structure determination

The molecular structures of 3a–d and 6b in the solid state were determined by single crystal X-ray structure analysis. Pale yellow plates of 3a–d and 6b were obtained by slow evaporation of a chloroform solution containing 3a–d or 6d at ambient temperature. The crystallographic and refinement data of 3a–d and 6b are summarized in Table S1, their molecular structures are depicted in Figs. 1–2 with selected bond distances, angles and torsion angles given in Table 1.

Molecular structures (ORTEP, 50% probability level) of 3a–d with their atom numbering schemes. Hydrogen atoms haven been omitted for clarity. Geometric data are summarized in Table 1. Symmetry operation for generating equivalent atoms: 3a, (A) −x, −y, −z + 2; 3b, (A) −x, −y + 1, −z + 1, 3c, (A) −x, −y, −z; 3d, (A) −x + 1, −y, −z.
Fig. 1 Molecular structures (ORTEP, 50% probability level) of 3a–d with their atom numbering schemes. Hydrogen atoms haven been omitted for clarity. Geometric data are summarized in Table 1. Symmetry operation for generating equivalent atoms: 3a, (A) −x, −y, −z + 2; 3b, (A) −x, −y + 1, −z + 1, 3c, (A) −x, −y, −z; 3d, (A) −x + 1, −y, −z.
Molecular structure (ORTEP, 30% probability level) of 6b with the atom numbering scheme. Hydrogen atoms have been omitted for clarity as well as disordered parts of the structure, which are shown in Figure SI1. Geometric data are summarized in Table 1. The symmetry equivalent atoms of the disordered part were generated by (A) −x + 1, y, −z−1/2 to complete the structure.
Fig. 2 Molecular structure (ORTEP, 30% probability level) of 6b with the atom numbering scheme. Hydrogen atoms have been omitted for clarity as well as disordered parts of the structure, which are shown in Figure SI1. Geometric data are summarized in Table 1. The symmetry equivalent atoms of the disordered part were generated by (A) −x + 1, y, −z−1/2 to complete the structure.
Table 1 Selected bond distances (Å), plane intersections (°) and torsion angles (°)) of 3a–d and 6b.
Compd. 3a 3b 3c 3d 6ba)
Bond distances (Å)
O1—C6 1.462(3) 1.466(4) 1.457(6) 1.468(2) 1.47(3)/1.45(2)
O1—C7 1.337(4) 1.344(4) 1.343(6) 1.341(2) 1.40(3)/1.35(3)
O2 = C7 1.213(4) 1.200(4) 1.208(6) 1.205(2) 1.20(2)/1.18(3)
Fe—Ct 1.6463(2) 1.6465(1) 1.6465(1) 1.6493(1) 1.6347(16)–1.6643(17)
Plane intersections (°)
Ph—CO2 17.9(4) 16.96(15) 3.8(4) 4.14(12) 3(6)
Rms Ph 0.0068 0.0080 0.0077 0.0043 0.0021
Torsion angles (°)
C2—C1—C6—O1 88.6(3) 90.0(4) 87.8(6) 89.6(2) 80(3)/101(2)
C5—C1—C6—O1 92.6(3) 94.0(4) 94.9(5) 91.9(2) 109(3)/84(3)
C1—C6—O1—C7 79.1(3) −170.9(3) 160.7(4) 86.77(19) 86(3)/87(3)

Four of the reported compounds crystallize in the monoclinic crystal system P21/c (3b–d) and C2/c (6b), while 3a crystallizes in triclinic P−1. The structure of 6b was refined disordered over two equally occupied sets of crystallographic sites. The disordering occurs by a rotation of the central 1,3-substituted phenylene core by 17° around C14, resulting in a slight shift of all atoms except of C14 (Fig. S1). The asymmetric unit of 6b also contains half of the molecules, due to a mirror plane through the C14 atom of the phenylene ring which is perpendicular oriented to the aromatic plane. The structures of 3b–d bear methyl groups on different positions of the phenyl ring (Fig. 1). The molecular structures of 3a–d are similar, the topological differences between them are minimal, excluding the isomeric nature of the tolyl group, as indicated by the bond distances and angles. The ferrocenyl groups exhibit ideal staggered conformations with tilt angles of 180°, due to the inversion center, which is present at the position of the Fe atoms. Consequently, the asymmetric unit contains one half of the molecules in the asymmetric unit.

The ester groups of all sandwich compounds are positioned anti towards each other above opposite sites of the ferrocenyl groups. The carboxy planes are rather coplanar with the phenyl(ene) rings showing plane intersections between 3.8(4) and 17.9(4)°, respectively. The most significant difference within the series of 3a–d is the orientation of the ester groups towards the cyclopentadienyl plane, which is represented by the C1–C6–O1–C7 torsion angles (Table 1). In case of 3a and 3d they are rotated perpendicular (3a, 79.1(3); 3d, 86.77(19)°) towards the C1–C6–O1 atoms, whereas a linear arrangement is observed for 3b and 3c (3b, 170.9(3); 3c, 60.7(4)°)

2.3

2.3 Electrochemistry

The electrochemical behavior of 3a–d, 6a–c and 8 was investigated by cyclic voltammetry using anhydrous dichloromethane solutions containing [NnBu4][B(C6F5)4] (0.1 M) as supporting electrolyte (Experimental) (Hildebrandt et al., 2016, 2011; LeSuer et al., 2004; Barrière and Geiger, 2006; Swarts et al., 2009; Nemykin et al., 2010; Miesel et al., 2015). The electrochemical measurements were carried out at 25 °C under an atmosphere of argon and were referenced against the potential of the FcH/FcH+ redox couple (Fc = Fe(ƞ5-C5H4)(ƞ5-C5H5)) (Swarts et al., 2009). The electrochemical data are summarized in Table 2. Exemplary, the voltammograms of 3c and 6c are depicted in Fig. 3, the ones of 6a, 6b, 6c and 8 in Figs. 5 and 3a–c in the ESI (Figs. S3–S5). All compounds show reversible and well-defined redox events for the Fc/Fc+ redox couples (Evans et al., 1983; Kissinger and Heineman, 1983; Mabbott, 1983).

Table 2 Cyclic voltammetry data (potentials vs FcH/FcH+) of 1.0 mM solutions of 3a–d, 6a–c and 8 in anhydrous dichloromethane containing [NnBu4][B(C6F5)4] (0.1 M) as supporting electrolyte at 25 °C (Barrière et al., 2002).
Compd. E0′/ΔEp [mV]a
3a 160/64
3b 164/66
3c 155/67
3d 146/59
6a 96/110
6b 97/65
6c 99/113
8 113/106
a E0′ = Formal potential. ΔEp = Difference between the oxidation and reduction peak potentials (Kissinger and Heineman, 1983; Mabbott, 1983; Taher, 2009; Taher et al., 2006, 2012, 2018; Korb et al., 2017, 2016).
Cyclic voltammograms 3c (solid line) and 6c (dotted line), scan rate 100 mVs−1 in dichloromethane (1.0 mM) at 25 °C, supporting electrolyte [NnBu4][B(C6F5)4] (0.1 M). (SWV Results, n.d).
Fig. 3 Cyclic voltammograms 3c (solid line) and 6c (dotted line), scan rate 100 mVs−1 in dichloromethane (1.0 mM) at 25 °C, supporting electrolyte [NnBu4][B(C6F5)4] (0.1 M). (SWV Results, n.d).
Cyclic voltammograms of 6a (dashed line), 6b (solid line), 6c (dash dot dot line) and 8 (dotted line), scan rate 100 mVs−1 in dichloromethane (1.0 mM) at 25 °C, supporting electrolyte [NnBu4][B(C6F5)4] (0.1 M). (SWV Results, n.d).
Fig. 4 Cyclic voltammograms of 6a (dashed line), 6b (solid line), 6c (dash dot dot line) and 8 (dotted line), scan rate 100 mVs−1 in dichloromethane (1.0 mM) at 25 °C, supporting electrolyte [NnBu4][B(C6F5)4] (0.1 M). (SWV Results, n.d).
Energy profile of the frontier orbitals HOMO and LUMO, and HOMO-LUMO gaps for 3a-d, 6a–c and 8, calculated at the B3LYP/6-31G(d, p) + LANL2DZ level in dichloromethane as solvent.
Fig. 5 Energy profile of the frontier orbitals HOMO and LUMO, and HOMO-LUMO gaps for 3a-d, 6a–c and 8, calculated at the B3LYP/6-31G(d, p) + LANL2DZ level in dichloromethane as solvent.

Within the cyclic voltammetry data, the formal potentials (E0′) of 3a–c, 6a–c and 8 are shifted towards anodic potentials relative to the ferrocene FcH redox event (FcH/FcH+) (Table 2). The E0′ values of the Fc/Fc+ events for 3a–c are more shifted to anodic potentials than for 6a–c and 8, which is in good agreement with the electron withdrawing effect of carboxylic acid species (Taher et al., 2013, 2017a, 2017b, 2018; Taher and Corrigan, 2011).

The large ΔEp value of 6a (110 mV) and 6c (113 mV) of the appropriate single wave suggests that two individual reversible one-electron processes take place in a close potential range, indicating that no electrostatic interaction between both ferrocenyls exists, which is indicated by the solid-state structures, where the two redox active moieties are positioned in opposite directions. A similar behavior was observed for 8 (Fig. 4) and hence a certain thermodynamic stability toward the isovalent species 8 and 83+ indicates significant electrostatic interaction among the three terminal FcCH2 groups as oxidation progresses. The current for 6b is twice as intense as that of 6a and 6c as shown in Fig. 4. Thus, the simultaneous oxidation of the ferrocenyl units in 6b offers a lower thermodynamic stability of mono-oxidized 6b+ towards the isovalent species 6b and 6b2+ and suggests a negligible electronic interaction between the ferrocenyl units (Hildebrandt et al., 2016; LeSuer et al., 2004; Taher et al., 2013; Brunschwig et al., 2002; Brunschwig and Sutin, 1999; D’Alessandro and Keene, 2006; Demadis et al., 2001).

The E0′ value for 1 being exactly 0.00 V, i.e. at the same potential as the free FcH/FcH couple (Claus et al., 2013; Braga et al., 2001). The observed positive shift of the E0′ value, when connecting 1 with a carboxylic group is a consequence of the electron-withdrawing OC(O) moiety from the ferrocenyl group, e.g. 4-bis(2-ferrocenecarboxylate)benzene and 1,4-bis(2-ferrocenecarboxylate)-2′-methylbenzene (E0′ = 296 and 296 mV, respectively) (Huang et al., 2012). In addition, E0′ values of 6a–c shift more negative (formal redox potential are catholically shifted) comparing to direct connection compounds between ferrocenyl and the carboxy group, due to the presence of a CH2 spacer between the ferrocenyl and the carboxy group.

2.4

2.4 DFT calculations

Density Functional Theory (DFT) electronic structure calculations have been carried out in order to obtain frontier orbital energies and electron densities within different frontier molecular orbitals. The calculated geometry and structure parameters obtained by geometry optimization are in good agreement with the experimental data obtained by X-ray structure determination as demonstrated on the example of 3d (Table S2). The calculated absolute energies of the HOMO and LUMO orbitals are summarized in Table 3 together with the HOMO-LUMO energy gap values. It is noted that the energy values of the HOMO orbitals for all compounds are very close to each other, due to a similar substitution pattern of the ferrocenyls. However, it is found that the calculated energy values of the LUMO orbitals of 6a–c and 8 are in general lower than that one of the corresponding compounds 3a–d, due to the lower electron density caused by the additional electron-withdrawing ester functionalities.

Table 3 Absolute HOMO and LUMO energies of 3a–d, 6a–c, and 8, and the HOMO-LUMO gap, calculated at the B3LYP/6-31 g(d, p) + LANL2DZ level in dichloromethane.
Compound HOMO LUMO LUMO-HOMO gap
3a −5.62 −1.34 4.28
3b −5.66 −1.27 4.39
3c −5.66 −1.29 4.37
3d −5.61 −1.26 4.35
6a −5.56 −1.54 4.02
6b −5.56 −1.66 3.90
6c −5.57 −2.01 3.56
8 −5.56 −1.80 3.76

All units are given in eV.

A schematic diagram of the calculated relative frontier orbital energies for 3a–d, 6a–c, and 8 is shown in Fig. 5. A notable decrease of the LUMO orbital energy as a function of the relative positions of the two ferrocenylmethyls on the six-membered aromatic ring in 6a–c is observed, indicating a high electron density delocalization within the phenylene ring in the LUMO by going from the ortho to the para position. However, LUMO orbital energies are found to be almost not affected on the position of the attachment of the two phenyl units to the ferrocenyl fragment in 3a–d.

The frontier MO's for 8 are shown in Fig. 6. The frontier quasi-degenerated HOMO orbitals are localized on one of the three ferrocenyl units. This is also true for 6a–c (Figure S2), where the electron density is mainly localized within the two ferrocenyls groups. As expected, no difference in the electron density on the frontier MOs within the series 3a–d and 6a–c was found, as compared with 5-membered heterocyclic rings featuring heteroatoms (O, S, and Se) (Taher et al., 2018). This can be concluded from the projected density of states (PDOS) (Tables S3 and S4).

The frontier orbitals HOMO−1, HOMO, LUMO, and LUMO+1 of 8, calculated at the B3LYP/6-31G(d, p) + LANL2DZ level in dichloromethane as solvent, with surface isovalue 0.03.
Fig. 6 The frontier orbitals HOMO−1, HOMO, LUMO, and LUMO+1 of 8, calculated at the B3LYP/6-31G(d, p) + LANL2DZ level in dichloromethane as solvent, with surface isovalue 0.03.

3

3 Conclusion

Synthetic methodologies for the preparation of 1,1′-[Fe(η5-C5H4CH2OC(O)R)2] (3) (a, R = C6H5; b, R = 2-CH3C6H4; c, R = 3-CH3C6H4, d, R = 4-CH3C6H4), (FcCH2OC(O))2-C6H4 (6) (a, 1,2-((ClC(O))2C6H4; b, 1,3-((ClC(O))2C6H4; c, 1,4-((ClC(O))2C6H4) and 1,3,5-[(FcCH2C(O))3C6H3] (8) are described. While 3a–c are accessible by the reaction of [Fe(η5-C5H4CH2OH)] with acid chlorides ClC(O)R, compounds 6a–c and 8 were produced upon treatment of FcCH2OLi with (ClC(O))2C6H4 or 1,3,5-(ClC(O))3-C6H3. The molecular solid-state structures of 3a–d and 6b were determined by single X-ray structure analysis showing that these compounds possess ideal anti-periplanar orientations of the substituents. Electrochemical measurements on 3a–d, 6a–c and 8 showed that the substitution pattern of the ester on the aryl ring and the electron-withdrawing effect of the acyl groups influences the electrochemical shift of the formal potential. The respective aryl system possesses a significantly higher Fc/Fc+ redox potential as observed for the more electron-rich ferrocenemethanol (Claus et al., 2013; Braga et al., 2001). The Fc/Fc+ redox potential (E0′) for the disubstituted ferrocenes 3a–c is significantly higher than the one for mono-substituted ferrocene 6a–d and 8, keeping with the electron-withdrawing nature of the carboxylic ester substituents (Taher et al., 2018). on the other hand, the presence of a CH2 spacer between the ferrocenyl and the carboxy group leads to shift the E0′ values of 6a–c to more negative formal redox potential comparing to direct connected compounds for example, (4-bis(2-ferrocenecarboxylate)benzene and 1,4-bis(2-ferrocenecarboxylate)-2′-methylbenzene) (Huang et al., 2012). DFT calculations show different degrees of HOMO-LUMO energy gaps within the series of 6a–c, due to a lowering in the LUMO energy depending on the positions of the carboxylic ester substituents on the aryl rings, in agreement with the electrochemical results obtained.

4

4 Experimental part

4.1

4.1 General procedures

All reactions were carried out under an atmosphere of argon using standard Schlenk techniques. Diethyl ether and n-hexane were purified with a MBRAUN SPS-800 purification system. Dichloromethane was purified by distillation from CaH2.

4.2

4.2 Reagents

1,1′-Di(hydroxymethyl)ferrocene (1) (Claus et al., 2013), 1-(hydroxymethyl)ferrocene (4) (Chernyy et al., 2017; Gallei et al., 2009), were synthesized according to literature procedures.

Tetra-n-butylammoniumtetrakis(pentafluorophenyl)borate was prepared by metathesis of lithium tetrakis(pentafluorophenyl)borate etherate (Boulder Scientific) with tetra-n-butylammonium bromide (LeSuer et al., 2004). All other chemicals were purchased from commercial suppliers and were used as received.

4.3

4.3 Instruments

Infrared spectra were measured with a Thermo Nicolet 200 FT-IR spectrometer. NMR spectra were recorded using an BrukerAvance III 500 FT-NMR spectrometer (1H NMR at 500.303 MHz, 13C NMR at 125.813 MHz) at ambient temperature, unless otherwise noted. Chemical shifts (δ) are reported in parts per million (ppm) relative to tetramethylsilane using the solvent as internal reference (CDCl3: 1H NMR δ = 7.26 ppm; 13C NMR δ = 77.16 ppm) (Fulmer et al., 2010). Coupling constants (J) are reported in Hertz (Hz) and integrations are reported in number of protons. The following abbreviations are used to describe peak patterns: s = singlet, pt = pseudo-triplet, dd = doublet-of-doublets, m = multiplet. The melting points (sealed off in argon flushed capillaries) were determined using a Gallenkamp MFB 595 010 M melting point apparatus. Microanalysis was performed by using a Thermo FLASHEA 1112 Series instrument. High resolution mass spectra were recorded with a BrukermicrOTOF-QII spectrometer with an Apollo II ESI source.

4.4

4.4 Electrochemistry

The electrochemical measurements were carried out under an atmosphere of argon on 1.0 mmol·L−1 anhydrous dichloromethane solutions containing 0.1 mol·L−1 of [NnBu4][B(C6F5)4] as supporting electrolyte utilizing a Voltalab 10 electrochemical laboratory from Radiometer analytical (Barrière et al., 2002; Barrière and Geiger, 2006). For the cyclic voltammetry studies, a three electrode cell with a platinum auxiliary electrode, a glassy carbon working electrode and a Ag/Ag+ reference electrode were used. The working electrode was prepared by polishing it with a Buehler microcloth using Buehler diamond pastes with decreasing sizes (1 to 0.25 μm). The Ag/Ag+ reference electrode was constructed from a silver wire inserted into a Luggin capillary with a Vycor tip containing a solution of 0.01 mol·L−1 [AgNO3] and 0.1 mol·L−1 [NnBu4][B(C6F5)4] in acetonitrile. This Luggin capillary was inserted into a second Luggin capillary with Vycor tip filled with a 0.1 mol·L−1 [NnBu4][B(C6F5)4] solution in dichloromethane. Successive experiments under the same experimental conditions showed that all formal reduction and oxidation potentials were reproducible within ± 5 mV. Experimentally potentials were referenced against a Ag/Ag+ reference electrode but the results are presented referenced against the FcH/FcH+ couple (E0′ = 0.0 V) as required by IUPAC (Gritzner and Kuta, 1984). When decamethylferrocene was used as an internal standard, the experimentally measured potential was converted into E vs FcH/FcH+ by addition of −0.61 V (Nafady and Geiger, 2008). The cyclic voltammograms were taken after typical two scans and are considered to be steady state cyclic voltammograms in which the event pattern differs not from the initial sweep. Finally, the experimental data were processed on Microsoft Excel worksheets.

4.5

4.5 Computational methods

DFT calculations were performed using the GAUSSIAN 09 program package (Frisch et al., 2009). Full geometry optimization for 3a–d; 6a–c, and 8 was performed using the B3LYP density functional method (Parr and Yang, 1989) with the 6-31G(d, p) basis set for C, H, O, and S atoms and the effective core potential LANL2DZ basis set for iron and selenium (method and basis set denoted as: B3LYP/6-31G(d, p) + LANL2DZ). The solvent effects were taken into account by using the conductor-like polarizable continuum model (CPCM) method with dichloromethane as solvent for 3a–d and diethyl ether for 6a–c and 8. Frequency calculations were performed on the optimized geometries at the same level of theory; all computed vibrational transitions have no imaginary frequency implying that each optimized geometry is located at the global minimum point on the potential energy surface. Projected density of states (PDOS) has been analyzed through the calculated orbital populations for all compounds at the same level of theory, using GAUSSSUM 3.0 program (O'Boyle et al., 2008).

4.5.1

4.5.1 General procedure for the preparation of 3a–d

[Fe(η5-C5H4CH2OH)2] (1) (125 mg, 0.51 mmol) was dissolved in 15 mL of dichloromethane. The solution was treated with the respective acid chloride (1.02 mmol) in the presence of triethylamine (0.15 mL, 1.08 mmol) at 0 °C for 2 h and for another 2 h at ambient temperature under continuous stirring. The obtained reaction mixture was filtered through Celite to remove the Et3NHCl precipitate. Water (10 mL) was added to the reaction mixture. Dichloromethane (thrice, 15 mL each) was added to extract the complex from the aqueous solution. The recovered organic fractions were combined and concentrated under reduced pressure to afford a pale yellow solid. The solid was washed with hexane twice (20 mL each) and dried under vacuum. Recrystallization of the solid from chloroform-hexane mixtures of ratio 3:1 (v/v) at −18 °C afforded yellow solids of the title compounds 3a–d.

[Fe(η5-C5H4CH2OC(O)C6H5)2] (3a). Yield: 205 mg (0.45 mmol, 88% based on 1). Anal. Calcd. for C26H22FeO4 (454.3): C, 68.74; H, 4.88 Found C, 68.55; H, 4.85%. Mp.: 145 °C (decomp.). IR (NaCl, cm−1): νCO 1713 s. 1H NMR (CDCl3): δ 8.04 (dd, J = 8.3 Hz, 4H, C6H5), 7.55 (t, J = 7.4 Hz, 4H, C6H5), 7.43 (t, J = 8.0 Hz, 2H, C6H5), 5.14 (s, 4H, CH2), 4.38 (pt, JHH = 1.8 Hz, 4H, C5H4), 4.23 (pt, JHH = 1.8 Hz, 4H, C5H4). 13C{1H} NMR (CDCl3): δ 166.5 (CO), 133.1 (Ci-C6H5), 130.3 (C6H5), 129.8 (C6H5), 128.5 (C6H5), 82.4 (C5H4), 70.2 (C5H4), 69.6 (Ci-C5H4), 63.1 (CH2). HR-ESI-MS (positive ion mode) m/z: Calcd. for C26H22FeO4 [M]+ 454.0862 found 454.0854.

[Fe(η5-C5H4CH2OC(O)-2-CH3-C6H4)2] (3b). Yield: 228 mg (0.47 mmol, 93% based on 1). Anal. Calcd. for C28H26FeO4 (484.3): C, 69.72; H, 5.43. Found C, 69.58; H, 5.43%. Mp.: 149 °C (decomp.). IR (NaCl, cm−1): νCO 1724 s. 1H NMR (CDCl3): δ 7.89 (d, J = 8.2 Hz, 2H, C6H4), 7.38 (t, J = 7.5 Hz, 2H, C6H4), 7.22 (t, J = 7.5 Hz, 4H, C6H4), 5.12 (s, 4H, CH2), 4.37 (pt, J = 1.8 Hz, 4H, C5H4), 4.23 (pt, J = 1.8 Hz, 4H, C5H4), 2.59 (s, 6H, CH3). 13C{1H} NMR (CDCl3): δ 167.5 (CO), 140.3 (Ci-C6H4), 132.1 (Ci-C6H4), 131.8 (C6H4), 130.7 (C6H4), 129.7 (C6H4), 125.81(C6H4), 82.3 (C5H4), 70.2 (C5H4), 69.6 (Ci-C5H4), 62.9 (CH2), 21.9 (CH3). HR-ESI-MS (positive ion mode) m/z: Calcd. for C28H26FeO4 [M]+ 482.1175 found 482.1178.

[Fe(η5-C5H4CH2OC(O)-3-CH3-C6H4)2] (3c). Yield: 212 mg (0.44 mmol, 86% based on 1). Anal. Calcd. for C28H26FeO4 (484.3): C, 69.72; H, 5.43. Found C, 69.63; H, 5.47%. Mp.: 155 °C (decomp.). IR (NaCl, cm−1): νCO 1701 s. 1H NMR (CDCl3): δ 7.84 (d, J = 9.1 Hz, 4H, C6H4), 7.32 (dt, J = 15.0, 7.5 Hz, 4H, C6H5), 5.13 (s, 4H, CH2), 4.38 (pt, J = 1.8 Hz, 4H, C5H4), 4.23 (pt, J = 1.8 Hz, 4H, C5H4), 2.38 (s, 6H, CH3). 13C{1H} NMR (CDCl3): δ 166.7 (CO), 138.3 (Ci-C6H4), , 133.9 (Ci-C6H4), , 130.3 (C6H4), 130.2 (C6H4), 128.4 (C6H4), 126.9 (C6H4), 82.4 (C5H4), 70.3 (C5H4), 69.6 (Ci-C5H4), 63.1 (CH2), 21.4 (CH3). HR-ESI-MS (positive ion mode) m/z: Calcd. for C28H26FeO4 [M]+ 482.1175 found 482.1160.

[Fc(CH2OC(O)-4-CH3-C6H4)2] (3d). Yield: 199 mg (0.41 mmol, 80% based on 1). Anal. Calcd. for C28H26FeO4 (484.3): C, 69.72; H, 5.43. Found C, 69.59; H, 5.50%. Mp.: 158 °C (decomp.). IR (NaCl, cm−1): νCO 1709 s. 1H NMR (CDCl3): δ 7.93 (d, J = 8.2 Hz, 2H, C6H4), 7.22 (d, J = 8.0 Hz, 2H, C6H4), 5.12 (s, 4H, CH2), 4.37 (pt, J = 1.5 Hz, 4H, C5H4), 4.22 (pt, J = 1.5 Hz, 4H, C5H4), 2.39 (s, 6H, CH3). 13C{1H} NMR (CDCl3): δ 166.6 (CO), 143.8 (Ci-C6H4), 129.8 (Ci-C6H4), 129.2 (C6H4), 127.6 (C6H4), 82.5 (C5H4), 70.2 (C5H4), 69.5 (Ci-C5H4), 62.9 (CH2), 21.8 (CH3). HR-ESI-MS (positive ion mode) m/z: Calcd. for C28H26FeO4 [M]+ 482.1175 found 482.1163.

4.5.2

4.5.2 General procedure for the preparation of 6a–d

To [Fe(η5-C5H5)(η5-C5H4CHOH)] (4) (250 mg, 1.16 mmol) dissolved in 15 mL of diethyl ether, 0.73 mL of MeLi (1.60 M, 1.16 mmol) was added in a single portion at 0 °C under continuous stirring for 15 min. The resulting reaction mixture was cooled to −78 °C and the appropriate dibenzoic acid chloride (0.58 mmol) was added dropwise over 10 min. The reaction mixture was slowly warmed to ambient temperature, and stirring was continued overnight. After filtration of the reaction mixture through a pad of Celite, the solvent was removed under reduced pressure to give a yellow solid. The solid was washed twice with hexane (20 mL each) and dried in vacuum. Recrystallization of the solid from chloroform-hexane mixtures of ratio 3:1 (v/v) at −18 °C afforded a yellow solid of the title compounds 6a–c.

1,2-[Fe(η5-C5H5)(η5-C5H4OCH2C(O))2C6H5] (6a). Yield: 195 mg (0.35 mmol, 60% based on 4). Anal. Calcd. for C30H26Fe2O4 (562.2): C, 64.09; H, 4.66. Found C, 63.59; H, 4.61%. Mp.: 202 °C (decomp.). IR (NaCl, cm−1): νCO1734 s. 1H NMR (CDCl3): δ 7.67 (dd, J = 5.7, 3.3 Hz, 2H, C6H4), 7.48 (dd, J = 5.7, 3.3 Hz, 2H, C6H4), 5.03 (s, 4H, CH2), 4.32 (pt, J = 1.5 Hz, 4H, C5H4), 4.21 (pt, J = 1.5 Hz, 4H, C5H4), 4.18 (s, 10H, C5H5). 13C{1H} NMR (CDCl3): δ 167.5 (CO), 132.1 (Ci-C6H4), 131.1 (C6H4), 129.0 (C6H4), 80.9 (C5H4), 70.1 (C5H4), 69.1 (C5H5), 68.3 (Ci-C5H4), 64.3 (CH2). HR-ESI-MS (positive ion mode) m/z: Calcd. for C30H26Fe2O4 [M]+ 562.0525 found 562.0536.

1,3-[Fe(η5-C5H5)(η5-C5H4OCH2C(O))2-cC6H4] (6b). Yield: 240 mg (0.43 mmol, 74% based on 4). Anal. Calcd. for C30H26Fe2O4 (562.2): C, 64.09; H, 4.66. Found C, 63.81; H, 4.56%. Mp.: 180 °C (decomp.). IR (NaCl, cm−1): νCO 1716 s. 1H NMR (CDCl3): δ 8.68 (s, 1H, C6H4), 8.20 (d, JHH = 7.8 Hz, 2H, C6H4), 7.49 (t, JHH = 7.8 Hz, 1H, C6H4), 5.14 (s, 4H, CH2), 4.36 (pt, J = 1.5 Hz, 4H, C5H4), 4.21 (pt, J = 1.5 Hz, 4H, C5H4), 4.20 (s, 10H, C5H5). 13C{1H} NMR (CDCl3): δ 165.7(CO), 134.0 (Ci-C6H4), 131.0 (C6H4), 130.9 (C6H4), 128.7 (C6H4), 81.4 (C5H4), 69.8 (C5H4), 69.0 (C5H5), 68.8 (Ci-C5H4), 63.8 (CH2). HR-ESI-MS (positive ion mode) m/z: Calcd. for C30H26Fe2O4 [M]+ 562.0525 found 562.0548.

1,4-[Fe(η5-C5H5)(η5-C5H4OCH2C(O))2-cC6H4] (6c). Yield: 270 mg (0.48 mmol, 83% based on 4). Anal. Calcd. for C30H26Fe2O4 (562.2): C, 64.09; H, 4.66. Found C, 64.28; H, 4.77%. Mp.: 161 °C (decomp.). IR (NaCl, cm−1): νCO 1710 s. 1H NMR (CDCl3): δ 8.06 (s, 4H, C6H4), 5.14 (s, 4H, CH2), 4.37 (pt, J = 1.5 Hz, 4H, C5H4), 4.22 (pt, J = 1.5 Hz, 4H, C5H4), 4.20 (s, 10H, C5H5). 13C{1H} NMR (CDCl3): δ 165.8 (CO), 134.2 (Ci-C6H4), 129.7 (C6H4), 81.2 (C5H4), 69.9 (C5H4), 69.1 (C5H5), 68.8 (Ci-C5H4), 63.9 (CH2). HR-ESI-MS (positive ion mode) m/z: Calcd. for C30H26Fe2O4 [M]+ 562.0525 found 562.0539.

4.5.3

4.5.3 Preparation of 8

[Fe(η5-C5H5)(η5-C5H4CHOH)] (4) (488 mg, 2.27 mmol) was dissolved in diethyl ether (20 mL) and MeLi (1.60 M, 1.42 mL, 2.27 mmol) was added in a single portion at 0 °C. After 15 min of stirring at this temperature it was cooled to −78 °C and a solution of triacid chloride (0.75 mmol) in 25 mL of diethyl ether was added drop-wise. The thus formed reaction mixture was slowly warmed to ambient temperature and stirring was continued overnight. After filtration through a pad of Celite, all volatiles were removed under reduced pressure to give a yellow residue. The precipitate was washed twice with hexane (20 mL each) and the solid was dried in vacuum. Crystallization of the solid from a mixture of dichloromethane-hexane (ratio 3:1, v/v) at −18 gave a yellow solid of the title compound 8.

1,3,5-[Fe(η5-C5H5)(η5-C5H4OCH2C(O))3-cC6H3] (8). Yield: 495 mg (0.61 mmol, 81% based on 4). Anal. Calcd. for C42H36Fe3O6 (804.3): C, 62.72; H, 4.51. Found C, 62.38; H, 4.59%. Mp.: 191 °C (decomp.). IR (NaCl, cm−1): νCO 1714 s. 1H NMR (CDCl3): δ 8.83 (s, 3H, C6H3), 5.16 (s, 6H, CH2), 4.34 (pt, JHH = 1.8 Hz, 6H, C5H4), 4.20 (pt, JHH = 1.9 Hz, 6H, C5H4), 4.19 (bs, 15H, C5H5). 13C{1H} NMR (CDCl3): δ 164.9 (CO), 134.8 (Ci-C6H3), 131.5 (C6H3), 81.1 (C5H4), 69.8 (C5H4), 69.0 (C5H5), 68.8 (Ci-C5H4), 64.1 (CH2). HR-ESI-MS (positive ion mode) m/z: Calcd. for C30H26Fe2O4 [M]+ 804.0557 found 804.0547.

4.5.4

4.5.4 X-ray crystal structure analysis

Single crystals suitable for X-ray structure determination of 3a–d and 6b were obtained by slow evaporation of a chloroform solution containing 3a–d or 6b at ambient temperature. Crystallographic data for 3a–d and 6b are summarized in Table 1. Data of 3a–d and 6b were collected at <110 K with graphite-monochromated Mo Kα radiation (λ = 0.71073 Å) at an Oxford Gemini S. The molecular structures were solved by direct methods using SHELXS-13 (Sheldrick, 1990) and refined by full-matrix least-squares procedures on F2 using SHELXL-13. (Sheldrick, 2008; Altomare et al., 1999) All non-hydrogen atoms were refined anisotropically and a riding model was employed in the treatment of the hydrogen atom positions, except otherwise noted. Graphics of the molecular structures have been created by using ORTEP. (Farrugia, 2012) In case of 6b, the 5-membered rings have been constrained using AFIX 56.

Acknowledgement

We are grateful to the DAAD for a visiting fellowship (AG) and to the University of Jordan, the Alexander von Humboldt foundation (fellowship S.T.A).

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Appendix A

Supplementary material

Supplementary data to this article can be found online at https://doi.org/10.1016/j.arabjc.2018.12.006.

Appendix A

Supplementary material

The following are the Supplementary data to this article:

Supplementary data 1

Supplementary data 1

Supplementary data 2

Supplementary data 2

Supplementary data 3

Supplementary data 3


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