Multistep synthesis, reactivity and X-ray structure of the anisole-terminated iron(II) polyhalogenoclathrochelates and their monoribbed-functionalized macrobicyclic derivatives

Multistep synthetic pathway towards a series of the anisoleboron-capped ribbed-functionalized iron(II) cage complexes was developed. Their hexachloroclathrochelate precursor was obtained by the template condensation of three dichloroglyoximate chelating ligand synthons with two molecules of 4-methoxyphenylboronic acid as a Lewis-acidic cross-linking agent on the iron(II) ion as a matrix. It easily underwent a stepwise nucleophilic substitution with S2- and O2-dinucleophilic aliphatic (ethanedithiolate) or aromatic (pyrocatecholate) agents, forming the stable X2 (X = S or O)-six-membered ribbed substituent(s) at a quasiaromatic cage framework. Performing these reactions under the different reaction conditions (i.e., at various hexachloroclathrochelate-to-nucleophile molar ratios, a wide range of temperatures and a series of the solvents) allowed to control a predominant formation of its mono-, di- or triribbed-substituted macrobicyclic derivatives. Thus obtained iron(II) di- and tetrachloroclathrochelates can undergo their post-synthetic transformations with active nucleophilic agents. The latter complexes underwent a further nucleophilic substitution with the anionic derivative of n-butanthiol, thus giving the hexasulfide macrobicyclic compound with two functionalizing n-alkyl substituents in one of its three chelate α-dioximate fragments and two apical biorelevant anisole substituents. The obtained iron(II) clathrochelates, possessing a low-spin electronic d6 configuration, were characterized using elemental analysis, MALDI-TOF mass spectrometry, UV–Vis, 1H and 13C{1H} NMR spectroscopies, and by the single-crystal X-ray diffraction experiments for the hexachloroclathrochelate precursor, its dichlorotetrasulfide macrobicyclic derivative and the monoribbed-functionalized hexasulfide cage complex. In all their molecules, the encapsulated iron(II) ion is situated in the centre of its FeN6-coordination polyhedron, the geometry of which is intermediate between a trigonal prism and a trigonal antiprism with the distortion angles φ from 21.4 to 23.4°. Halogen bonding between the polyhalogenoclathrochelate molecules in their crystals is observed.


Introduction
Monoribbed-functionalized iron(II) clathrochelates and bisclathrochelates (Scheme 1, on top), the cage framework(s) of which contain two chelate α-dioximate fragments with hydrophobic substituents in them, while the third moiety of this type is decorated with functionalizing (first of all, biorelevant) groups, are known [1,2] to be the most efficient allosteric (so-called "topological") inhibitors in the transcription systems of nucleic acids [3][4][5][6][7], as well as the prospective antifibrillogenic agents tested [8] in a model insulin fibrillization reaction. In the latter case, a concentration-dependent inhibition of the insulin fibril formation by these mono-and bis-clathrochelate bioeffectors has been observed [8]. They caused a change in the kinetics of this reaction and a decrease in the amount of formed fibrils (up to 70%), as well as a substantial decrease in their diameters and in formation of the corresponding superfibrillar clusters. Therefore, the aforementioned iron(II) complexes are proposed [8] as prospective drug candidates for the treatment of various neurodegenerative disorders, such as Alzheimer's, Parkinson's and Creutzfeldt-Jakob's diseases, type II diabetes and amyloidosis. On the other hand, the aryl-heterocyclic organic compounds, the molecules of which contain the terminal 4-methoxyphenyl (anisole) functionalizing group (their examples are shown in Scheme 1, on bottom), are reported [9][10][11] to possess the hydrogen-bond acceptor properties, thus being the prospective drug candidates for the treatment of Alzheimer's disease. In particular, the use of heterocyclic derivatives of anisole suppresses the formation of pathogenic β-amyloids (Aβ 40 and Aβ 42 ) via the inhibition of γ-secretase ferment [9]. On the other hand, the absence of a given biorelevant group in the molecules of these potent heterocyclic Alzheimer's disease drug candidates caused a decrease in their given bioactivity [10]. Therefore, its presence seems to be important from the point of view of the molecular design of prospective pharmaceutical candidates for drug therapy of various neurodegenerative diseases.
However, all of the aforementioned cage and bis-cage iron(II) complexes are formed by a cross-linking of their chelating α-dioximate ligand synthons with strongly Lewis-acidic fluoroboron cross-linking agents, such as BF 3 ·O(C 2 H 5 ) 2 . Thus formed tetrahedral apical fragments O 3 BF of the corresponding macropolycyclic molecules usually possess a very low reactivity and, therefore, their further post-synthetic functionalization (modification) seems to be a hardly possible or even impossible. This hampered an ability to introduce various vector and biorelevant (including anisole) groups into their caging or bis-caging ligands for a target delivery of their metal complexes, as the probable bioeffectors, to a given biosystem, thus improving their bioactivity. Therefore, we recently developed [12] a general synthetic approach that allowed to obtain the metal clathrochelates with functionalizing substituents or groups both in their ribbed chelate α-dioximate moieties and in their apical cross-linking fragments as well. However, these substituents (groups) are typically reactive and, therefore, they can undergo their unwanted chemical transformations or side reactions in a course of the multistep preparation of the target clathrochelate complex of a given symmetry and an improved functionality; the presence of these terminal polar groups can also hamper its chromatographic isolation. So, they should be protected from these unwanted transformations (i.e., their chemical reactions and physical adsorption) using the suitable protecting groups, known from classical organic chemistry [13,14], including, in particular, a formation of their ethers. In the present paper, we describe the multistep synthetic pathway towards a series of the first anisole-terminated ribbed-functionalized iron(II) cage complexes starting from their initially prepared hexachloroclathrochelate precursor with two apical biorelevant substituents of this type at its cage framework. Earlier, another synthetic strategy allowing to obtain the target cage compounds with a terminal vector fragment has been evaluated [12]. It included an isolation of the suitable clathrochelate precursors with an apical reactive group, followed by their postsynthetic transformation through the imine or hydrazonate condensation Scheme 1 Chemical drawings of the earlier-elaborated most efficient iron(II) clathrochelate bioeffectors (on top) and those of prospective anisole-containing drug candidates (on bottom) with a suitable pharmacophoric amine or hydrazine component, giving the corresponding imine (hydrazonate) derivatives. However, thus formed Schiff bases can easily undergo a cleavage of their C = N bond in the diluted biological aqueous solutions under physiological conditions. Contrary, the biorelevant anisole group is chemically stable under these conditions and the use of an initially functionalized hexachloroclathrochelate precursor allowed to escape the aforementioned final stage of an introduction of the suitable pharmacophoric vector group into the target macropolycyclic molecule and to avoid its unwanted side chemical reactions as well.

Results and discussion
On the first stage, we obtained the reactive anisoleboron-capped hexachloroclathrochelate Fe(Cl 2 Gm) 3 (B4-C 6 H 4 OCH 3 ) 2 , using the template condensation by Scheme 2 of three dichloroglyoximate chelating ligand synthons with two molecules of 4-methoxyphenylboronic acid as a Lewisacidic cross-linking agent on the iron(II) ion as a matrix. Thus obtained apically functionalized macrobicyclic precursor easily underwent a stepwise nucleophilic substitution with S 2 -and O 2 -dinucleophilic aliphatic (ethanedithiolate) or aromatic (pyrocatecholate) agents, forming the stable X 2 (X = S or O)-six-membered ribbed substituent(s) in the α-dioximate chelate fragment(s) of its quasiaromatic cage framework [15]. Performing these reactions under the different reaction conditions (i.e., at various hexachloroclathrochelate-to-nucleophile molar ratios, a wide range of temperatures and a series of the solvents) allowed to control a predominant formation of the target mono-, di-or triribbedsubstituted macrobicyclic derivatives of the aforementioned hexachloroclathrochelate precursor.
The obtained iron(II) di-and tetrachloroclathrochelates, the molecules of which contain two and one X 2 -sixmembered ribbed fragment(s), respectively, can undergo Scheme 2 Template synthesis of the anisoleboron-capped iron(II) hexachloroclathrochelate precursor Fe(Cl 2 Gm) 3 (B4-C 6 H 4 OCH 3 ) 2 and its further chemical transformations their further post-synthetic transformations with the same or different active nucleophilic agents, thus giving their macropolycyclic derivatives with two or three non-equivalent chelate α-dioximate fragments. Dichloromethane and chloroform were found to be the most suitable solvents for a successful proceeding of these reactions, allowed to obtain the target clathrochelate products in the high yields and to avoid the side reactions of a complete destruction of their cage framework. First of all, we performed a stepwise nucleophilic substitution of the macrobicyclic precursor Fe(Cl 2 Gm) 3 (B4-C 6 H 4 OCH 3 ) 2 using the anionic derivatives of 1,2-ethanedithiol, which were generated in situ in the presence of triethylamine as organic base. This gave its tetraand dichloroclathrochelate derivatives Fe(Cl 2 Gm) 2 The latter complex underwent a further nucleophilic substitution with an anionic derivative of n-butanthiol as a S-nucleophile, generated in situ in the presence of triethylamine. This gave its hexasulfide derivative Fe((S 2 -Nx) 2 (S-n-C 4 H 9 ) 2 Gm)(B4-C 6 H 4 OCH 3 ) 2 with two functionalizing n-alkyl substituents in one of its three chelate α-dioximate fragments of a quasiaromatic cage framework and two apical anisole substituents as well. We also obtained the monoribbed-substituted iron(II) tetrachloroclatrochelate Fe(Cl 2 Gm) 2 PrchGm)(B4-C 6 H 4 OCH 3 ) 2 , the molecule of which contains one pyrocatecholate ribbed α-dioximate fragment. However, its further functionalization was found to be hindered: the corresponding clathrochelate products were obtained in the low yields and their chromatographic isolation was complicated as well. Therefore, we focused on a post-synthetic functionalization of the aforementioned diand tetrasulfideclathrochelate derivatives of ethanedithiol, as described above in more details.
The obtained iron(II) clathrochelates, possessing a lowspin electronic d 6 configuration, were characterized using elemental analysis, MALDI-TOF mass spectrometry, UV-Vis, 1 H and 13 C{ 1 H} NMR spectroscopies, and by the single-crystal X-ray diffraction experiments as well.
Positive range of the MALDI-TOF mass spectra of these new macrobicyclic complexes (see SI, Figs. S7 -S11) contain the peaks of their molecular ions [ M] +• and those of the corresponding ionic associates [ M + Cat + ] + with Na + and K + ions as well. These peaks have the characteristic isotopic distributions, which are in good agreement with those theoretically calculated.
Numbers and positions of the signals in the solution 1 H and 13 C{ 1 H} NMR spectra of the obtained dimethoxyl-terminated diamagnetic iron(II) clathrochelates (their NMR spectra are presented in SI, Figs. S12 -S20), as well as the ratios of the integral intensities of the 1 H NMR signals of protons of the apical aromatic fragments and the terminal methoxyl groups in them, those of the X 2 -aliphatic and X 2 -aromatic moietie(s) and of the functionalizing ribbed substituents at a cage framework, confirmed a given composition and symmetry of their macrobicyclic molecules.
Solution UV-Vis spectra of the hexachloroclathrochelate precursor Fe(Cl 2 Gm) 3 (B4-C 6 H 4 OCH 3 ) 2 and their obtained derivatives contain one asymmetric intensive (ε ~ 2·10 4 mol L cm -1 ) band in the visible range with maxima in the range 440 -480 nm. These bands were assigned to the metal-toligand Fed → Lπ* charge transfers (MLCTs) characteristic of a given type of the tris-α-dioximate iron(II) clathrochelates [16]. If the introduction of one pyrocatecholate ribbed fragment into an encapsulating macrobicyclic ligand almost did not affect a position of the aforementioned complex MLCT band in the visible range, passing to the di-, tetraand hexasulfide iron(II) cage complexes under study caused a substantial (up to 30 nm) and a gradual (with an increment of approximately 10 nm per one S 2 -containing ribbed fragment) longwave shift of its maximum. The performed deconvolution of these spectra into their Gaussian components (see SI, Figs. S1 -S6, Table S1) gave two or three individual bands in the near UV-visible range, while their more far UV ranges contain a series of the bands assigned to the π-π* transitions in their quasiaromatic macrobicyclic tris-α-dioximate framework and in the apical aromatic substituents at it as well. Their maxima are substantially shifted as compared with those in UV spectra of the corresponding chelating α-dioximate and cross-linking boron-containing ligand synthons.
General views of the macrobicyclic molecules of the hexachloroclathrochelate precursor Fe(Cl 2 Gm) 3 (B4-C 6 H 4 OCH 3 ) 2 , and those of its dichlorotetrasulfide derivative Fe(Cl 2 Gm)(S 2 -Nx) 2 (B4-C 6 H 4 OCH 3 ) 2 and of the monoribbed-functionalized hexasulfide complex Fe(S 2 -Nx) 2 ((S-n-C 4 H 9 ) 2 Gm)(B4-C 6 H 4 OCH 3 ) 2 , which were obtained using the single-crystal X-ray diffraction experiments, are shown in Figs. 1, 2, 3; main geometrical parameters of their cage frameworks are compiled in Table 1. Asymmetric units of their crystals contain a half of the hexachloroclathrochelate molecule Fe(Cl 2 Gm) 3 (B4-C 6 H 4 OCH 3 ) 2 , two independent dichlorotetrasulfideclathrochelate entities Fe(Cl 2 Gm) 3 (B4-C 6 H 4 OCH 3 ) 2 and four solvent benzene molecules as well, and one hexasulfide macrobicyclic molecule Fe(S 2 -Nx) 2 ((S-n-C 4 H 9 ) 2 Gm) (B4-C 6 H 4 OCH 3 ) 2 , respectively. In all these molecules, the encapsulated iron(II) ion is situated in the centre of its  (Table 1) are characteristic for the boron-capped iron(II) clathrochelates [2,16]. Absence of the intramolecular interactions between their apical anisole substituents at a macrobicyclic framework and its ribbed fragments allowed a free rotation around the corresponding ordinary B -C bonds between them. As a result, the mean planes of their aromatic fragments fall in the range 34.4(1) -64.4(2)° with the corresponding torsion angles O -B -C -C varying from 2.5(2) to 23.7(2)°; these fragments are almost coplanar in all their clathrochelate molecules. The hydrophobic interactions were found to dominate in the corresponding crystals. However, in those of the hexachloroclathrochelate Fe(Cl 2 Gm) 3 (B4-C 6 H 4 OCH 3 ) 2 and its dichloroclathrochelate derivative Fe(Cl 2 Gm)(S 2 -Nx) 2 (B4-C 6 H 4 OCH 3 ) 2 , the halogen bonding between their macrobicyclic entities is also observed. Despite a difference in the number of their ribbed chlorine atoms, these clathrochelate molecules form the similar crystal motifs through the Cl…π interactions between them. The intermolecular C -Cl bond is directed towards the π-conjugated system of a chelating α-dioximate fragment of the neighboring clathrochelate molecule (Fig. 4). The values of r i (Cl…N) in the crystal Fe(Cl 2 Gm) 3 (B4-C 6 H 4 OCH 3 ) 2 are substantially lower than those in the crystal Fe(Cl 2 Gm) (S 2 -Nx) 2 (B4-C 6 H 4 OCH 3 ) 2 {3.080(3) -3.093(3)Å versus 3.242(7) -4.001(8) Å, respectively}. Two independent molecules A and B of the complex Fe(Cl 2 Gm)(S 2 -Nx) 2 (B4-C 6 H 4 OCH 3 ) 2 form the parallel chains in its crystal. In that of the hexachloroclathrochelate Fe(Cl 2 Gm) 3 (B4-C 6 H 4 OCH 3 ) 2 , four remaining chlorine atoms of the same macrobicyclic molecule (i.e. those not included in a halogen bonding) form the intermolecular C-H…Cl bonds. Moreover, in all these crystals, π-systems of the ribbed α-dioximate fragments of their quasiaromatic macrobicyclic frameworks, which are not involved in the aforementioned halogen bonding, are included in the intermolecular C-H…π interactions.

Conclusions
Thus, the multistep general synthetic pathway towards the first anisoleboron-capped ribbed-functionalized iron(II) cage complexes was developed. Their initially prepared hexachloroclathrochelate precursor, containing two apical biorelevant substituents at its capping boron atoms, easily underwent a stepwise nucleophilic substitution with S 2 -and O 2 -dinucleophilic aliphatic (1,2-ethanedithiolate) or aromatic (pyrocatecholate) agents, forming the stable X 2 (X = S or O)-six-membered ribbed substituent(s) in the α-dioximate chelate fragment(s) of a quasiaromatic cage framework. Thus obtained apically functionalized iron(II) di-and tetrachloroclathrochelates, the molecules of which contain two and one fragment(s) of this type, respectively, can undergo their further post-synthetic transformations with the same or different active nucleophiles, to give the macropolycyclic derivatives with two or three non-equivalent chelate α-dioximate fragments and two apical biorelevant anisole groups. These apically and ribbed-functionalized iron(II) macrobicyclic complexes seem to be the prospective drug candidates (prodrugs) allowing a target delivery of cage molecules of these intracomplexes to a given biosystem for their further bioscreening.
Analytical data (C, H, N contents) were obtained with a Carlo Erba 1106 microanalyzer. MALDI-TOF mass spectra were recorded with and without a matrix using a MALDI-TOF-MS Bruker Autoflex II (Bruker Daltonics) mass spectrometer in reflectomol mode. The ionization was induced by a UV-laser with a wavelength of 337 nm. The samples were applied to a nickel plate, and 2,5-dihydroxybenzoic acid was used as the matrix. The accuracy of measurements was 0.1%.
Thin layer chromatography (TLC) experiments were performed using a Silica Gel 60 F254 foil (Merk). 1 H and 13 C NMR spectra were recorded from the solutions in CD 2 Cl 2 with Varian Inova 400 and Bruker Avance 600 spectrometers. The measurements were performed using the residual signals of these deuterated solvents.
UV-Vis spectra of the solutions in dichloromethane were recorded in the range 220 -800 nm with a Varian Cary 60 spectrophotometer. The individual Gaussian components of these spectra were calculated using the Fityk program [19].
A solution of triethylamine (0.107 ml, 0.78 mmol) and 1,2-ethanedithiol (0.035 ml, 0.39 mmol) in chloroform (30 ml) was added dropwise to the stirring solution of the complex Fe(Cl 2 Gm) 3 (B4-C 6 H 4 OCH 3 ) 2 (0.15 g, 0.20 mmol) in chloroform (50 ml). The reaction mixture was refluxed for 3 h, the obtained red solution with a precipitate was filtered off, the filtrate was evaporated, then the resulting solid residue was washed with methanol (10 ml), diethyl ether (5 ml) and hexane (10 ml). Then it was extracted with dichloromethane (5 ml) and the extract was separated using column chromatography on silica gel (eluent: chloroform). The first and second elutes were discarded, and the third elute was collected and evaporated to dryness. The obtained solid residue was dried in vacuo. Yield: 0.05 g (35%  (19). This complex was also characterized using the single-crystal XRD experiment.
All these structures were solved using the SHELXT method [23] and refined by a full-matrix least squares method against F 2 of all data using the SHELXL-2014 [24] and OLEX2 [25] programs. Non-hydrogen atoms were refined in an anisotropic approximation. The positions of hydrogen atoms were calculated and included in the refinement in an isotropic approximation by the riding model with the U iso (H) = 1.5U eq (C) for methyl groups and 1.2U eq (C) for other atoms, where U eq (X) are equivalent thermal parameters of the parent atoms. Experimental details and the results of these refinements are listed in Table S2 (see SI).
Crystallographic information files are available from the Cambridge Crystallographic Data Center upon a request (https:// ccdc. cam. ac. uk/ struc ture, deposition numbers are 2,190,194 -2,190,196).