Synthesis and Electrochemical and Spectroscopic Characterization of 4,7-diamino-1,10-phenanthrolines and Their Precursors

New approaches to the synthesis of 4,7-dichloro-1,10-phenanthrolines and their corresponding 9H-carbazol-9-yl-, 10H-phenothiazin-10-yl- and pyrrolidin-1-yl derivatives were developed. Their properties have been characterized by a combination of several techniques: MS, HRMS, GC-MS, electronic absorption spectroscopy and multinuclear NMR in both solution and solid state including 15N CP/MAS NMR. The structures of 5-fluoro-2,9-dimethyl-4,7-di(pyrrolidin-1-yl)-1,10-phenanthroline (5d), 4,7-di(9H-carbazol-9-yl)-9-oxo-9,10-dihydro-1,10-phenanthroline-5-carbonitrile (6a) and 4,7-di(10H-phenothiazin-10-yl)-1,10-phenanthroline-5-carbonitrile (6b) were determined by single-crystal X-ray diffraction measurements. The nucleophilic substitutions of hydrogen followed by oxidation produced compounds 6a and 6b. The electrochemical properties of selected 1,10-phenanthrolines were investigated using cyclic voltammetry and compared with commercially available reference 1,10-phenanthrolin-5-amine (5l). The spatial distribution of frontier molecular orbitals of the selected compounds has been calculated by density functional theory (DFT). It was shown that potentials of reduction and oxidation were in consistence with the level of HOMO and LUMO energies.


Introduction
1,10-Phenanthrolines are well known bidentate nitrogen ligands used in coordination chemistry for the analysis of transition metal ions and homogenous catalysis [1][2][3][4]. Their coordination abilities are the result of the contribution of lone pairs of electrons on both nitrogen atoms in the characteristic shape of heterotricyclic systems. They have been found applications in supramolecular chemistry, luminescent sensors, and photosensitizers for solar cells [5][6][7][8][9][10][11][12][13][14][15]. 1,10-Phenanthrolines have also received much attention as potential targets for anticancer drug development [16][17][18][19][20][21]. Mainly there are two methods for the synthesis of compounds containing 1,10-phenanthroline core. The first one is the classical Skraup and Friedlander transformations. The second method is used in the synthesis of 4,7-dichloro-1,10-phenanthroline derivatives through the condensation of Meldrum's acid, orthoesters, and ortho-phenylenediamine derivatives, followed by sequential thermal cyclization, decarboxylation and treatment with refluxing phosphoryl chloride [22,23]. The synthesis of 4,7-dichloro-1,10-phenanthrolines offered a possibility of further transformation through replacing halogen atoms in C4 and C7 positions. In the literature there are few methods describing their modification into amino derivatives [24,25]. In this paper, we report new approaches to the synthesis of the aforementioned compounds with in-depth spectroscopic and electrochemical characterization. In order to improve the understanding of the chemical behavior of novel 4,7-dichloro-1,10-phenanthrolines and their amino derivatives, a 15 N CP/MAS NMR technique was employed to differentiate and characterize the nitrogen atoms presented in their structures, and to show the influence of substituents on their chemical shifts. Additionally, computation and X-ray studies have been carried out of on the novel 4,7-disubstituted-1,10-phenanthrolines. The investigation of the electrochemical properties of these newly designed compounds may provide a promising tool to demonstrate their stability towards oxidative and reductive conditions for applications, such as advanced synthesis and catalysis.

Synthesis of 4,7-dichloro-1,10-phenanthrolines
In the current study, the functionalization of selected 1,10-phenanthroline derivatives at R, R 1 and R 2 positions was the focus, which have not been fully exploited and may serve as interesting building blocks. The classical Skraup-Doebner-Miller reaction offers the simplest and fastest transformation. To prepare 1,10-phenanthrolines we first attempted to adopt a one-step Skraup-Doebner-Miller cyclocondensation of ortho-phenylenediamines 1 with unsaturated carbonyl compounds. However, like in previous work by other authors, this procedure failed, possibly due to an intermolecular condensation [26]. Instead, the synthesis of 4,7-dichloro-1,10-phenanthroline derivatives 4 has been carried out using a three-step condensation of 2,2-dimethyl-1,3-dioxane-4,6-dione (Meldrum's acid), orthoesters, and ortho-phenylenediamines 1 (Scheme 1) [22,23]. Compounds 4 were prepared with high yield, even on the scale of several grams. The first yield-determining step depends on the double nucleophilic addition of ortho-phenylenediamines (1) to the vinyl group of an intermediate obtained from the condensation of Meldrum's acid and an orthoester (Scheme 2).
Anilines are poor N-nucleophiles. Their reactivity strongly depends on the solvents used, and substituents in their structures. In a series of experiments it was noticed that a higher isolation yield (up to 94%) was obtained for molecule 2d which possesses a methyl group as R 1 substituent and hydrogen atoms as R (Scheme 2). Replacing the methyl group by a halogen atom (F, Cl, Br) or an electron-withdrawing group like CN or COOH in aniline decreases the isolated yield. Additionally, it is observed that the yield of products was dependent on the orthoester used. The use of trimethyl orthoformate produced the highest yields of compounds 2a, 2b, 2c, 2d and 2g, followed by a lower yield using triethyl orthoacetate, and the lowest was triethyl orthoformate of molecules 2f, 2h, 2i, 2j and 2l. This reactivity could be explained by increased steric interactions between Meldrum's acid and the orthoesters, which hampers the transformation (Scheme 2). In the syntheses of heterocycles 2, appropriate esters were produced depending on the ortho esters used as reactants. The hydrogen atom in carboxyl group was replaced by methyl or ethyl fragment giving the compounds 2m and 2n, respectively. It is of importance that the synthesis of products 2 was accompanied by side reactions which led to benzo [d]imidazole type molecules, for example during the synthesis of compound 2f 1H-benzo[d]imidazole-6-carboxylic acid was isolated. During the next step, diketene was generated in situ from molecule 2. This process relies on a thermally induced decarboxylation and acetone elimination. Highly reactive ketene underwent a cyclocondensation with the aryl substituent leading to compound 3 (Scheme 2). It is noticed that the rate of this reaction was not very sensitive to the presence of substituents at the C5 position (R 1 ). This method is suitable for a variety of useful functional groups, including ester, CN, alkyl substituents and halogen atoms, with the exception however of COOH, which reacted with electrophilic ketene through disturbing cyclocondensation to give very complicated reaction mixture. Carbonyl groups at C4 and C7 positions in heterocycle 3 can be replaced by chlorine atoms to prepare 4,7-dichloro-1,10-phenanthrolines 4. In a series of experiments, molecule 3 was treated with phosphoryl chloride giving quantitatively products 4 (Scheme 1) [22,23]. 4,7-Dichloro-1,10-phenanthrolines were obtained in high yields (ca. 68-93%) with the exception of compounds 4e (48%), 4l (32%) and 4m (38%). The lower yields of these molecules could be explained by the hydrolysis of their R 1 substituents, i.e., the CN, COOMe and COOEt groups and formation of carboxylic acid groups [27]. To avoid the hydrolysis of CN and COOAlk groups, and a reverse reaction of the 4,7-dichloro-1,10-phenanthrolines 4 to form the 1,10-dihydro-1,10-phenanthroline-4,7-diones 3, the excess of phosphoryl chloride should be evaporated under reduced pressure. This strategy should facilitate hydrolysis of the excess of phosphoryl chloride. It is noticed that the use of basic solutions during the hydrolysis procedure causes the competitive reversion of product 4 to molecule 3. Hydroxide ion can substitute for chloride atom at the C4/C7 positions. Surprisingly during the isolation of compound 4e an unexpected product 4f was obtained, which is explained as a hydrolysis of the heterocycle 4e. It is worth noting that during this chemical transformation 4,7-dichloro-1,10-phenanthroline-5-carboxylic acid was not isolated or identified (Scheme 3).

Synthesis of 4,7-diamino-1,10-phenanthrolines
The introduction of substituents R 2 , such as a chlorine atom in the 1,10-phenanthroline skeleton, opened opportunities for the further functionalization. Five 4,7-dipyrrolidinyl-1,10-phenanthrolines 5 with four novel structures were synthesized from heterocycles 4 by microwave-assisted nucleophilic aromatic substitution with a 10-fold excess of pyrrolidine. The nitrogen R 2 substituents should increase the electron density on nitrogen atoms in 1,10-phenanthrolines (Scheme 1) [24]. Reactions were carried out in a sealed vial in a microwave reactor at 130 • C (Scheme 1). MW irradiation improves the reactivity of the substitutions, shortening the reaction time to give the substitution products in better yields. In the absence of MW irradiation, the substitution reaction failed to initiate. The yields of products depend on the substituent R 1 , which may influence the outcome of the reaction through steric and/or electronic effect. Unexpectedly R 1 fluorine or chlorine atoms failed to undergo substitution. This phenomenon can be explained by a steric hindrance attributed to the already introduced pyrrolidinyl rings and too high an electron density on the carbon atom at C5 position. Substituents R 2 in the pyridine rings in 1,10-phenanthrolines 4 tended to undergo more S N Ar type substitutions due to the presence of nitrogen atoms which are able to accommodate a negative charge in the intermediate state.
Four 4,7-di(9H-carbazol-9-yl)-1,10-phenanthrolines (5f, 5g, 5h and 5m) and four 4,7-di(10H-phen othiazin-10-yl)-1,10-phenanthroline derivatives (5i, 5j, 5k and 5n) with seven novel structures were synthesized from molecules 4 by nucleophilic aromatic substitution with 9H-carbazole and 10H-phenothiazine, respectively (Scheme 1). Reagents were stirred for sixteen hours under reflux. The chemistry was based on inexpensive, commercially available reagents and easily synthesized heterocycles 4 which possess methyl, CN, fluorine or hydrogen atom as R 1 . The yields of 9H-carbazole derivatives 5 were lower than for 10H-phenothiazine derivatives. In all cases the purification of 9H-carbazole derivatives 5 required chromatographic methods to receive pure products. However, the purification of 10H-phenothiazine could be carried out by crystallization from the mixture of CH 2 Cl 2 (or THF) and hexane to afford products even on a multigram scale. The highest yield and less complex reaction mixture of 10H-phenothiazine derivatives can be attributed to the better nucleophilic properties of 10H-phenothiazine anion than 9H-carbazole. Wu et al. obtained the X-ray structure of 4,7-di(10H-phenoxazin-10-yl)-1,10-phenanthroline [28], which was cocrystallized with THF as a guest molecule located in the crystalline frameworks. The THF molecule is surrounded by two 10H-phenoxazine rings and 1,10-phenanthroline core. 1 H-NMR and 13 C-NMR studies showed that the presence of signals from THF for 10H-phenothiazine derivatives 5i, 5j and 5n (Supporting Information). It is worth noting that in the case of molecule 5k, with a methyl group as R 1 , no residual THF signals have been detected. 9H-Carbazole derivatives have shown similar supramolecular inclusion processes to compounds 5i, 5j and 5n, in which the solvent is located in a cage made by 1,10-phenanthroline derivatives, however, with smaller intensity.

X-Ray Studies
Crystals of compounds 5d, 6a and 6b were mounted in turn on a Gemini A Ultra Oxford Diffraction automatic diffractometer equipped with a CCD detector used for data collection. X-ray intensity data were collected with graphite monochromated MoK α radiation at room temperature, with ω scan mode. Details concerning crystal data and refinement are gathered in Table 1 in ESI. Lorentz, polarization and empirical absorption correction using spherical harmonics implemented in SCALE3 ABSPACK scaling algorithm were applied [31]. The structures were solved by a direct method and subsequently completed by a difference Fourier recycling. All the non-hydrogen atoms were refined anisotropically using full-matrix, least-squares techniques. The Olex2 [32] and SHELXS, SHELXL [33] programs were used for all the calculations. Atomic scattering factors were incorporated in the computer programs. The performed investigations on the X-ray studies were inspired by the work of G. Zucchi et al. and K.J. Shaffer et al. [25,28]. Presented here compounds crystalize in the triclinic P-1 space group as a solvate with two CHCl 3 molecules 5d and two THF and one methanol molecules in the case of compounds 6a and 6b crystalizes in monoclinic P2 1 /c space group with two THF molecules. The molecular structures are displayed as ORTEP representation in Figure 1a-c. The intra-and intermolecular hydrogen bonds in the structures of compounds 5d and 6a, 6b are listed in Table S2 in the Supporting Information. The 1,10-pPhenanthroline moiety is in a planar orientation with a ring puckering amplitude of 0.105 in molecule 5d and 0.05 and 0.07 in the cases of compounds 6a and 6b, respectively. In the structure of heterocycle 5d C-Cl . . . π interactions are visible, while in molecules 6a and 6b π . . . π stacking occurs between phenanthroline rings with centroid-centroid distances of 3.89 Å and 3.78 Å and shift distances of 1.91 Å and 1.07 Å, respectively (Table S3; Supporting Information). The interaction between the nitrile C(13)-N(3) group causes flattening of 10H-phenothiazine ring in the position C7. The 10H-phenothiazine ring in position C4 shows a characteristic, for this type of heterocyclic compounds, deviation of the sulfur atom position from the plane delimited by the benzene rings. The structures of compounds 5d, 6a and 6b are constitutionally related to 4,7-di(10H-phenoxazin-10-yl)-1,10-phenanthroline and 4,7-di(9H-carbazol-9-yl)-1,10-phenanthroline [25,28]. Similarly to two other compounds which are composed of a 1,10-phenanthroline core and two peripheral amino substituents, the 9H-carbazol-9-yl and 10H-phenoxazin-10-yl substituents are strongly twisted in relation to the 1,10-phenanthroline rings with a larger angle (~87 • ) between 1,10-phenanthroline and substituent planes in the position C7 than C4 (~83 • ). It is noticed that bond lengths between carbon atoms at C4/3 and C7/8 positions and pyrrolidinyl, 9H carbazol-9-yl and 10H-phenothiazin-10-yl groups 1.360 Å, 1.420 Å and 1.434(3) Å, respectively, are comparable to the values of 1.42 Å in 4,7-di(9H-carbazol-9-yl)-1,10-phenanthroline or 1.43 Å for 4,7-di(10H-phenoxazin-10-yl)-1,10-phenanthroline. The π-conjugated system derived from 1,10-phenanthroline core and two peripheral 9H-carbazole or 10H-phenoxazine rings showed less extended delocalization than that of pyrrolidinyl group over the entire π-conjugated systems, because the lone pair of nitrogen atom and the 1,10-phenanthroline core are not in the same plane. 9H-carbazole and 10H-phenothiazine rings are perpendicular to the 1,10-phenanthroline moiety [25,28].

NMR Studies
13 C CP/MAS NMR spectra of compounds 4 and 5 displayed readily discernible aliphatic carbon signals such as methyl, and distinguishable resonance lines attributed to aromatic carbons. In general, the signals in the solution and in the solid state are in good agreement (Tables S5 and S6; Supporting Information). To investigate the character of nitrogen atoms which are responsible for their bidentate coordination abilities, studies using the 15 N CP/MAS NMR technique were carried out, which is a highly sensitive probe and allows being monitored even in subtle changes in molecular constitution and electronic structures. Due to the symmetry of 4e, 5c and 5f molecules only one or two lines are presence in 15 N CP/MAS NMR spectrum, respectively (Table S7; Supporting Information). Comparing the experimental CP/MAS 15 N chemical shifts of selected heterocyles 4 and 5, the influence of substituents originated from chlorine atoms, methyl group and N-donor fragments i.e., NH 2 , pyrrolidine, 9H-carbazole and 10H-phenothiazine could be observed. It is worth noting that the 15 N chemical shift of a free 1,10-phenanthroline (or 2,9-dimethyl-1,10-phenanthroline) molecule is equal to −74.4 (or −80.2) ppm in the absolute scale [34,35]. A significant impact on the 15 N chemical shifts of pyridine nitrogen has been seen with pyrrolidine and 10H-phenothiazine substituents (Table  S7; Supporting Information). For pyridine rings, the dominant contribution to nitrogen shielding is associated with a n → π* excitation, and low transition energies correlate with large deshieldings. Interestingly, 9H-carbazole and 10H-phenothiazine fragments have higher impact on one of the two pyridine nitrogen atoms. One is significantly deshielded (downfield effect, larger δ) with 11.14 ppm of difference for molecule 5k or 23.78 ppm of difference for compound 5h (Table S7). In contrast, N-donor substituents i.e., NH 2 , pyrrolidine, 9H-carbazole and 10H-phenothiazine are characterized by the more negative value of 15 N chemical shift (−249.48 to −317.08 ppm).

Cyclic Voltammetry
The electrochemical behaviour of selected substituted 1,10-phenanthroline derivatives in acetonitrile was studied by cyclic voltammetry on a glassy carbon electrode. Compounds 4 substituted by chlorine atoms (R 2 substituent) yield rather complex cyclic voltammograms mainly in the negative potential regions ( Figure 2). In the case of all compounds 4 the first reduction wave corresponds to the formation of anion radical and according to literature the subsequent cleavage of chlorine from position R 2 in the overall ECE process follows [36,37]. The compounds 4b and 4i were found to be the most easily reduced because of the presence of chlorine and fluorine atoms as R 1 . In the case of compound 4i the first reduction step may be attributed to the cleavage of chlorine (R 1 ) from primarily formed radical anion. Calculated HOMO and LUMO spatial distributions of sterically symmetrical compounds are shown in Table 1 and results for other electrochemically investigated compounds are summarized in Tables S8 and S9 in the Supporting Information. A linear relationship of LUMO energy on reduction potential was observed for compounds from series 4 and 5 (except compounds 5a and 5c) showing that the first reduction step proceeds on 1,10-phenanthroline rings and an anion radical is formed ( Figure 3). Compounds 5a and 5c are substituted with pyrrolidine and their reduction may follow another mechanism. These two compounds differ from the others also by high values of their dipole moments as indicated in Table 2. The substituent effect on the reduction and oxidation potentials of selected compounds was summarized in Table 2. Methyl group as R substituent significantly influenced reduction potential of compounds towards more negative values (compounds 4g, 4k, 5c compare to 4a, 4d, 5a, respectively). The oxidation of all compounds 4 may lead to the formation of cation radicals [38]. The dependences of the highest occupied molecular orbital energy on the first oxidation potential were linear for both series of compounds, substituents R 2 with chlorine atoms 4 and N-heterocycles 5. Their independence is related to their different oxidation mechanism, resulting from the different electroactive site visible from the spatial distribution of HOMO orbitals ( Table 1). Oxidation of phenothiazine is known in the literature [39,40]. Detailed investigation of reduction and oxidation mechanism of both series will be further plan of our research. Figure 3C shows a linear relationship between the energy difference of HOMO and LUMO orbitals and the potential gap ∆E obtained experimentally.

3.3.3.
Step C Modifications to existing procedure described in the literature [22,23] rely on the evaporation of excess phosphoryl chloride under reduced pressure, and the addition of CH 2 Cl 2 or CHCl 3 into reaction mixture after alkalified by NaOH solution, because of the very exothermic hydrolysis of residual phosphoryl chloride. Next the crude products were purified by chromatography and crystallization from CH 2 Cl 2 .

Conclusions
This research has focused on the synthesis of 27 4,7-disubstituted-1,10-phenanthrolines, including their 4,7-dichloro-, 4,7-di(9H-carbazol-9-yl)-, 4,7-di(10H-phenothiazin-10-yl)-and 4,7-di(pyrrolidin-1-yl) derivatives, giving 23 novel compounds. The presented protocols allowed us to synthesize the targeted compounds more efficiently, with yields up to 96%. The structures of the obtained molecules were proved by a combination of varies techniques, such as NMR, GC-MS, MS, HRMS, UV-Vis and X-ray crystallography. A variety of substituents (methyl, halogen (F, Cl and Br), CN, 9H-carbazole, pyrrolidine, 10H-phenothiazine, COOEt and COOH groups) were chosen in order to represent different electronic features. For the first time 15 N CP/MAS-NMR spectra of selected 4,7-disubstituted-1,10-phenanthroline derivatives were elucidated to differentiate the nitrogen nucleus and to give an insight into their characteristics. The electrochemical studies showed the influence of substituents on the redox properties of synthesized compounds. Compounds with methyl as R substituent were the most difficult ones to reduce. On the contrary, compounds substituted with 9H-carbazole as R 2 had the highest oxidation potentials and were the most stable ones against oxidative processes. Compounds substituted with phenothiazine and pyrrolidine as R 2 were the most easily oxidized due to the oxidation of the substituent R 2 . Phenothiazine derivatives were also stronger electron acceptors and were more facile to reduction than other compounds. Regarding the largest potential gap, methylated compounds 4g, 4k and compound 5f containing 9H-carbazole are the most stable structures against oxidative and reductive processes.