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DOI: 10.1039/C8DT05065A (Paper) Dalton Trans., 2019, 48, 7425-7431

fac-Re(CO)3-based neutral heteroleptic tetrahedrons†

Ramar Arumugam , Bhaskaran Shankar , K. R. Soumya and Malaichamy Sathiyendiran *
School of Chemistry, University of Hyderabad, Hyderabad -500 046, India. E-mail: msathi@uohyd.ac.in

Received 22nd December 2018 , Accepted 11th April 2019

First published on 11th April 2019

Abstract

Four new flexible ditopic nitrogen donors possessing a xylene spacer and 2-phenylbenzimidazolyl or its derivatives as a coordinating unit and one rigid bis-chelating ligand consisting of two 2-hydroxyphenylbenzimidazolyl motifs and a central phenylene spacer were synthesized and further used with Re2(CO)10 for making a new family of neutral, heteroleptic tetrahedral-shaped supramolecular coordination complexes via a one-pot approach. The new ligands and the complexes were characterized using various analytical and spectroscopic methods. The molecular structures of the complexes were determined using single crystal X-ray diffraction analysis, which reveal that four rhenium cores are arranged in the vertices, and four ligands are at the edges of the tetrahedron.

Introduction

The design of supramolecular coordination complexes (SCCs) with various shapes and sizes has been going on during the past three decades due to their beautiful architectures, which can be assembled in a one-step approach, and their importance both in the materials and medicinal fields.1–9 Various metal sources including naked metal ions and partially protected metal precursors are used as connectors for ligand structural frameworks in the SCCs. Among the metal sources, a fac-Re(CO)3-directed approach is one of the most versatile methods for making neutral heteroleptic SCCs via a one-pot approach.6–9 Examples of various types of fac-Re(CO)3-based SCCs are helicate, mesocate, bowl, square, rectangle, trigonal-/tetragonal prisms, and spheroid.6–9 Surprisingly, the fac-Re(CO)3 core-based homoleptic or heteroleptic tetrahedron i.e., having four metal ions arranged in a tetrahedral topology, is scarce.4,5 The lack of a fac-Re(CO)3-based approach for the tetrahedron may be due to the difficulty in predesigning ligands for the stereoelectronic requirement of the fac-Re(CO)3 core, which provides three orthogonal acceptor sites and requires two two-electron donors and one anionic one-electron donor (Fig. 1). Predesigned ligands (rigid–rigid or rigid–flexible or flexible–flexible ligand motifs) for the fac-Re(CO)3 core-based tetranuclear SCC have so far yielded tetranuclear square- or heteroleptic rectangle- or zigzag-shaped 2D-SCCs.8 Since 2008, we have been designing various types of ligands for making new types of heteroleptic fac-Re(CO)3-based SCCs via new bonding combinations in a one-pot approach.9 Our research has been yielding synthetic approaches for neutral heteroleptic SCCs with aesthetically pleasing architectures and which have potential applications in various fields. Herein, we report a new fac-Re(CO)3-based synthetic approach for neutral, heteroleptic tetrahedral-shaped SCCs. The combination of rigid bis-chelating donors with a rigid phenyl spacer and neutral flexible ditopic nitrogen donors with Re2(CO)10 in a one-pot approach provides a neutral, heteroleptic M4L2L′2-type tetrahedron with two missing edges (Scheme 1).
image file: c8dt05065a-f1.tif
Fig. 1 Chemicals used for the work. (a) Stereoelectronic requirement of the fac-Re(CO)3 core, (b) rigid bis-chelator (H2-RBC = H2-L) and (c) N-donor.

image file: c8dt05065a-s1.tif
Scheme 1 Synthetic approach to heteroleptic M4L2L′2-type SCC with a tetrahedral shape (gray) with two missing edges (gray dotted line).

Results and discussion

Synthesis and characterization of ligands

Neutral phenyl or substituted phenyl at the 2-position of the benzimidazolyl-based ditopic N-donor ligands (L2–L5) were obtained using 1,3-di(bromomethyl)benzene, the corresponding benzimidazole and KOH in DMF.10 The synthetic approach used to prepare L2–L5 is similar to that for the benzimidazolyl-based ditopic ligands.10 The ligands are air- and moisture-stable and are soluble in polar organic solvents. The 1H NMR spectra of L2–L5 showed a single peak around ∼5.5 ppm corresponding to methylene protons (see Fig. S2–S5 in the ESI†). The benzimidazolyl protons of all the ligands displayed as well-separated two doublets and two triplets, indicating the unsymmetrical nature of the benzimidazolyl protons due to the formation of ligands. The rigid ligand (H2-RBC = H2-L, Fig. 1) was synthesized using 2-hydroxyphenylbenzimidazole (HO-PBz-H) and 1,4-dibromobenzene in the presence of CuI/Cs2CO3 in DMF. The sharp singlet at δ 7.71 ppm for the protons of the central phenylene motif, and a 4[thin space (1/6-em)]:[thin space (1/6-em)]18 proton ratio for –C6H4– protons and two (HO-PBz) units of protons, confirm the rigid ligand (Fig. S1 in ESI†). The ESI-MS spectra of the ligands (Fig. S6–S10 in ESI†) show molecular ion peaks, further supporting the formation of the product.

The molecular structure of L4 was confirmed using single crystal X-ray diffraction (SCXRD) analysis (Fig. 2).11,12 The two 3,4,5-trimethoxyphenylbenzimidazolyl motifs are trans to each other and are perpendicular to the central arene motif. Both nitrogen atoms (N1 and N4) are directed on the same side. The dihedral angle between the two imidazolyl units is 42°. The distance between two nitrogen donor atoms is 11.5 Å (N1⋯N4) in L4.


image file: c8dt05065a-f2.tif
Fig. 2 Molecular structure of L4 (left: H atoms are removed).

Synthesis and characterization of SCCs

The solvothermal heating of Re2(CO)10, H2-RBC and Ln and toluene yielded SCCs (1–5) with/without lattice toluene molecule(s) (Scheme 2). The SCCs are air and moisture stable, and moderately soluble in polar organic solvents. The FT-IR spectra of the complexes displayed three strong bands in the region of 2020–1800 cm−1, characteristic of the fac-Re(CO)3 motifs in the asymmetric environment.9 The 1H NMR spectrum of 4 is discussed here due to the clear pattern i.e., without decomposition or free ligand Ln impurity. The 1H NMR spectra of 1 and 2 indicated that the chemical resonance pattern is similar to that of 4 (Fig. S11–S13 in ESI†). However, there were additional peaks which are similar to the free ligand Ln (L1 for 1 and L2 for 2), which may be due to either decomposition of the complex while heating or the presence of impure ligands. No clear 1H NMR spectra was obtained for the complexes 3 and 5. The 1H NMR spectrum of 4 in DMSO-d6 displayed well-separated peaks compared to both the free ligands. Both upfield and downfield shifts were observed for the protons of the ligands of 4. In particular, eight peaks are present in the region of 6.8–4 ppm in 4. However, only one single peak for methylene protons was observed for the uncoordinated L1 ligand in the region. Further, the methylene (–CH2–) protons appeared as two doublets with coupling constant consistent with geminal coupling (J = 18 Hz). Among the two doublets of the methylene protons, one is near the chemical resonance of the methylene protons of the free ligand and the other proton is upfield shifted. The results suggest that complex 4 remains as an SCC structure in the solution and the upfield shift for the protons of the complex compared to the free ligand is due to neighbouring aromatic motifs. All the remaining complexes also displayed a similar pattern like that of 4. The results are further supported by ESI-MS studies. The mass spectra of the complexes show a molecular ion peak with isotopic distribution peaks which match with theoretical values. Further, the mass spectra of the complexes displayed peaks corresponding to the consecutive loss of ligand(s) and Re(CO)3 core(s). In addition, the mass spectra showed the mass of the [M2LLn] motif, where M = Re(CO)3 (Fig. S14–S18 in ESI†).
image file: c8dt05065a-s2.tif
Scheme 2 Synthesis of 1.

Molecular structures of SCCs

The molecular structures of 1–5 were confirmed by SCXRD studies, which showed that the complexes adopt an M4L2L′2-type SCC architecture (Fig. 3–6 and Fig. S24–S32 in ESI†). The complexes 1–5 can be viewed as a [4 + 2 + 2] assembly of four fac-Re(CO)3 cores, two dianionic ligand (RBC2− = L2−) motifs, and two neutral nitrogen ligands (Ln). The arrangements of four rhenium atoms and four ligand motifs provide an overall distorted tetrahedron shape with two missing edge units to the complexes. The four fac-Re(CO)3 cores are considered as the four vertices and the four ligand motifs are represented as the four edges of the tetrahedron in the complexes. Three types of edges are present in the tetrahedra (the rigid RBC ligand provides shorter edge {Re-RBC-Re, d(Re⋯Re) = 13.5 Å), the flexible Ln offers longer edge (Re-Ln-Re, d(Re⋯Re) = ∼15.3 Å) and the missing edge (Re⋯Re, d(Re⋯Re) = ∼11.2 Å}) (Table 1). Due to the different edges and two missing edges, the distorted tetrahedron has scalene triangular faces. Each scalene triangular face (Δ) has one anionic ligand edge, one neutral ligand edge and one missing edge (Fig. 3c and Fig. S28–S32 in ESI,†Table 1).
image file: c8dt05065a-f3.tif
Fig. 3 Various representations of molecular structure of 1 (A-C). Ball and stick model without hydrogen atoms (A) and space-filling view (B). Face view of the highlighted tetrahedron with a thin stick model of 1; two dotted lines represent two missing edges (C). Helical arrangement of ligands in 1; the blue line is L1 and chelating atoms (O–C–C–C–N) are shown to indicate a Δ or Λ configuration at each of the four vertices; Re1Re1Re2Re2 adopts a ΛΔΛΛ twisted conformation (D). Color code: red = ligand RBC2−, blue = L1, Re(CO)3 = green or gray (C) and green or red (O).

image file: c8dt05065a-f4.tif
Fig. 4 Helical arrangement of a dinuclear anionic ligand (A)- and a neutral ligand (B) in 1. Two chelating (O–C–C–C–N) atoms are colored in blue and in purple (A and C) to show the difference clearly in the top-view (C). Top view with two chelating motifs of A (top, C) and the neutral motif of B (bottom, C) highlighted. Intramolecular π⋯π stacking interactions between the phenyl of the N donor and the arene motif of the phenoxybenzimidazolyl motif (D). Hydrogen atoms are omitted. C = gray, N = blue, O = red, Re = green.

image file: c8dt05065a-f5.tif
Fig. 5 Two views of the molecular structures of 2 (top) and 5 (bottom). Carbon atoms of the rigid ligand are shown in red, C atoms of the flexible ligand are shown in aqua. H atoms are omitted for clarity. Color code: C = gray for CO, N = blue, O = red and Re = green.

image file: c8dt05065a-f6.tif
Fig. 6 Different edge arrangement of the Ln motif (blue) in the tetrahedrons 1–5. Red indicates a rigid ligand (RBC), and the dotted line indicates a missing edge.
Table 1 Twist angles (φ) between two chelating units of RBC and three Re⋯Re distances in a scalene triangle (Δ) in 1–5. The parameters were calculated without the solvent
Comp. φ (°) d of Δ (Å)
Re-RBC-Re Re-Ln-Re Re⋯Re
1 78, 87 13.6, 13.54 15.27 11.28
2 74, 81 13.48 15.10 11.33
3 70, 74 13.50 14.91 11.63, 10.46
4 72 13.51 14.90 10.63, 11.63
5 87, 78 13.55, 13.47 15.22 11.16

The arrangement of the four ligands in the complexes provides a helical SCC architecture (Fig. 4, 5 and Table 1). For example, the two chelating motifs of the RBC strand (Re-RBC-Re) in 1 are twisted with respect to each other with a twist angle of RBC i.e., the angle between the planes of the two Re–O(chel)–N(chel) units, of 78/87°. The flexible L1 takes a syn-conformation with an anti-cofacial arrangement of two phenylbenzimidazolyl units and is arranged in a helical fashion in 1 (Fig. 3–6 and Fig. S28–S32 in ESI†).

The four Re⋯Re distances in 1 are mentioned in Table 1. The rhenium core adopts a distorted octahedral geometry and is surrounded by three carbon atoms from three carbonyl ligands, N∩O from chelating units and N from neutral benzimidazolyl motif. The bond distances between Re–C, Re–N(bbenz), Re–N(chel), and Re–O(chel) are normal in 1 and consistent with values found for dinuclear fac-Re(CO)3-based helicates/mesocates possessing similar coordinating cores i.e. a hydroxyphenylbenzimidazolyl chelating unit, a benzimidazolyl motif, and three CO units.9d

The complexes 2 and 4 are almost similar to that of 1 (Fig. 5). The four methoxy (OCH3) groups of the L2 motif are directed away from the tetrahedron 2. However, the complexes 3 and 5 differ from those of 1, 2, and 4 with respect to the arrangement of two neutral ligand motifs in the tetrahedron edges (Fig. 5 and 6). Both anionic ligand motifs take similar edges in all the complexes. The two missing edges in 1 and 4 are now occupied by Ln in 2, 3 and 5 as shown in Fig. 6. The four exo-cavities present in the four faces of the tetrahedra of 3 and 4 were occupied by the methoxy unit of Ln i.e., one methoxy motif sits one face of the tetrahedron. Multiple weak C–H⋯π contacts were found between the OCH3 and the framework (phenylene units) of the cavity.

It is worth mentioning that the number of methoxy units, (OCH3), in the complexes is increased from complex 2 to complex 4i.e., four (OCH3) units in 2, eight (OCH3) units in 3, and twelve (OCH3) units in 4. Moderate-to-strong intramolecular π⋯π stacking interactions were found between the benzimidazolyl unit, in particular the imidazolyl motif, of RBC and the phenyl motif of Ln (dihedral angle = 13.6°; and dC63–C47/C59–C49/N6–C44 = 3.44/3.5/3.8 Å for 1). Four such π⋯π stacking interactions are found in all the complexes. In the case of complex 5, in addition to the intramolecular π⋯π stacking interactions, the C–H⋯π contacts are found between the planar 1,3-benzodioxole ring and the benzimidazolyl unit of the chelating motif.

It is worth mentioning that the bis-chelating ligand (H2-RBC = HO∩N-C6H4-N∩OH, where N∩OH is 2-(2-hydroxyphenyl)-benzimidazolyl) used in this work is rigid, whereas the ditopic N donor Ln is flexible (Ln = N-CH2-C6H4-CH2-N, where N = 2-(phenyl) benzimidazolyl). A similar bis-chelating flexible ligand (H2-FBC = HO∩N-CH2-C6HR3-CH2-HO∩N, where R = H or Me) i.e., a methylene unit incorporated as a connector for the chelating motif (HO∩N) and spacer (C6HR3) and a flexible ditopic nitrogen donor (Lm = N-CH2-C6H4-CH2-N, where N = benzimidazolyl) provided neutral dinuclear unsaturated heteroleptic helicate/mesocate with Re2(CO)10.9d The formation of the tetranuclear tetrahedron instead of dinuclear helicate/mesocate while using the rigid bis-chelating ligand may be due to the steric hindrance between these two ligands since the distance between two bis-chelating donors either in the rigid H2-RBC ligand (d(Re-RBC-Re) = 11.3 Å) or in the flexible H2-FBC ligand (d(Re-RBC-Re) = 13.1 Å) is comparable.9d The attempt to obtain a single crystal of SCC with Re2(CO)10, H2-RBC, and Lm is fruitless. Therefore, we suggest that the phenyl/substituted phenyl at the 2-position of the benzimidazolyl of Ln may play a major role for directing from the dinuclear helicate/mesocate to the tetrahedron assembly. The role of the fused benzene ring of benzimidazole in the ligand (Ln) as a steric motif may not be omitted.13

Conclusions

A family of the fac-Re(CO)3 core-based heteroleptic tetrahedra with two missing edges was obtained using new bonding combinations i.e. Re2(CO)10, a rigid bis-chelating donor possessing a phenyl spacer, a flexible ditopic nitrogen donor possessing a xylene spacer and 2-phenylbenzimidazolyl or its derivatives coordinating unit via a one-pot approach. The tetrahedra are neutral, heteroleptic, and possess scalene triangle faces. To the best of our knowledge, the reported synthetic approach is the first example of a design approach for making fac-Re(CO)3-based tetrahedra. The result provides a way to prepare fac-Re(CO)3-based heteroleptic tetrahedra with a tunable exterior via a simple one-pot method. The construction of fac-Re(CO)3-based tetrahedra with similar building units by tuning the spacer is in progress.

Experimental details

General data

The starting materials, Re2(CO)10, o-phenylenediamine, benzaldehyde, piperonal, p-anisaldehyde, 3,5-dimethoxybenzaldehyde, 3,4,5-trimethoxybenzaldehyde, NaHSO3, 1,3-di(bromomethyl)benzene, 1,4-dibromobenzene, copper(I) iodide, 1,10-phenanthroline, cesium carbonate, toluene, acetone and dimethylformamide (DMF) were procured from commercial sources and used as received. 2-Phenyl-1H-benzimidazole, 2-(4-methoxyphenyl)-1H-benzimidazole, 2-(3,5-dimethoxyphenyl)-1H-benzimidazole, 2-(3,4,5-trimethoxyphenyl)-1H-benzimidazole, 2-(1,3-benzodioxole)-1H-benzimidazole and L1 were synthesized using procedures reported in the literature.101H NMR spectra were recorded on Bruker Avance III 400 and 500 MHz spectrometers. FT-IR spectra were recorded on a JASCO-5300 FT-IR spectrometer. Elemental analyses were performed on a Flash EA series 1112 CHNS analyser. The ES mass spectra were recorded on a Bruker maXis mass spectrometer.
Synthesis of H2-RBC. The mixture of (2-hydroxyphenyl)-1H-benzimidazole (6.0 g, 28.5 mmol), 1,4-dibromobenzene (2.8 g, 11.9 mmol), 1,10-phenanthroline (0.85 g, 4.7 mmol), CuI (0.45 g, 2.4 mmol) and Cs2CO3 (16.3 g, 50 mmol) was taken in a Schlenk flask under a N2 atmosphere. Dry DMF (30 mL) was added to the mixture, which was heated under reflux for 48 h. The mixture was extracted using CHCl3/H2O three times. The organic layer was separated, washed with brine solution and dried using anhydrous Na2SO4. The solvent was removed using vacuum. The crude H2-RBC was eluted as a white powder using column chromatography using EA/hexane (20/80). The eluted white solid was again separated using column chromatography using the same solvent mixture to obtain pure H2-RBC. Yield: 20% (1.18 g). ESI (HR-MS). Calcd for C32H23N4O2 [M + H]+: m/z 495.1821. Found: m/z 495.1775. 1H NMR (400 MHz, DMSO-d6): δ 11.91 (s, 2H, -OH), 7.86–7.84 (m, 2H, H4), 7.71 (s, 4H, phenylene), 7.41–7.31 (m, 8H, H5,6,4′,5′), 7.18 (d, J = 7.9 Hz, 2H, H2′), 7.0 (d, J = 8.2 Hz, 2H, H7), and 6.82 (t, J = 8.0 Hz, 2H, H3′).

General synthetic approach for L2–L5

A mixture of KOH and phenylbenzimidazole in DMF was stirred at room temperature for 2 h. 1,3-Di(bromomethyl)benzene was added to the solution. The reaction mixture was stirred for 24 h. The reaction was quenched by adding ice water (∼100 mL). The powder was collected by filtration, washed with hexane and dried.
Synthesis of L2. A white powder was obtained by the treatment of 2-(4-methoxyphenyl)-1H-benzimidazole (800 mg, 3.6 mmol), 1,3-di(bromomethyl)benzene (471 mg, 1.8 mmol), KOH (400 mg, 7.1 mmol) and DMF (10 mL). Yield: 61% (600 mg). ESI (HR-MS). Calcd for C36H31N4O2 [M + H]+: m/z 551.2447. Found: m/z 551.2456. 1H NMR (400 MHz, DMSO-d6): δ 7.69 (d, J = 7.92 Hz, 2H, H4), 7.39 (d, J = 8.8 Hz, 4H, H2′,6′), 7.29 (d, J = 7.92 Hz, 2H, H5), 7.24 (t, J = 7.32 Hz, 3H, H7,11), 7.17 (t, J = 7.08 Hz, 2H, H10,12), 6.96 (d, J = 7.72 Hz, 2H, H6), 6.88 (d, J = 8.84 Hz, 4H, H3′,5′), 6.54 (s, 1H, H9), 5.42 (s, 4H, H8) and 3.7 (s, 6H, -OCH3). 13C NMR (500 MHz, DMSO-d6) 160.65, 153.68, 143.17, 138.14, 136.18, 130.81, 129.68, 125.57, 123.92, 122.81, 122.63, 122.50, 119.53, 114.51, 111.22, 55.72, 47.73.
Synthesis of L3. A white powder was obtained by the treatment of 2-(3,5-dimethoxyphenyl)-1H-benzimidazole (800 mg, 3.1 mmol), 1,3-di(bromomethyl)benzene (415 mg, 1.5 mmol), KOH (353 mg, 6.3 mmol) and DMF (10 mL). Yield: 98% (941 mg). ESI (HR-MS). Calcd for C38H35N4O4 [M + H]+: m/z 611.2658. Found: m/z 611.2651. 1H NMR (400 MHz, DMSO-d6): δ 7.71 (d, J = 7.84 Hz, 2H, H4), 7.32 (d, J = 7.92 Hz, 2H, H7), 7.27–7.16 (m, 5H, H5,10–12, 6.87 (d, J = 7.4 Hz, 3H, H6,9), 6.75 (d, J = 2.16 Hz, 4H, H2′,6′), 6.61 (s, 2H, H4′), 5.50 (s, 4H, H8), 3.64 (s, 12H, –OCH3). 13C NMR (400 MHz, DMSO-d6) 160.90, 153.36, 142.93, 138.23, 136.33, 132.18, 129.86, 125.57, 124.42, 123.22, 122.70, 119.77, 111.26, 107.30, 102.39, 55.68, 47.94.
Synthesis of L4. A white powder was obtained by the treatment of 2-(3,4,5-trimethoxyphenyl)-1H-benzimidazole (800 mg, 2.8 mmol), 1,3-di(bromomethyl)benzene (371 mg, 1.4 mmol), KOH (320 mg, 5.7 mmol) and DMF (10 mL). Yield: 91% (854 mg). ESI (HR-MS). Calcd for C40H39N4O6 [M + H]+: m/z 671.2869. Found: m/z 671.2869. 1H NMR (400 MHz, DMSO-d6): δ 7.70 (d, J = 7.84 Hz, 2H, H4), 7.32 (d, J = 7.96 Hz, 2H, H10,12), 7.24 (t, J = 7.86 Hz, 3H, H5,11), 7.18 (t, J = 7.12 Hz, 2H, H7), 6.91–6.88 (m, 6H, H2′,6′,6), 6.83 (s, 1H, H9), 5.54 (s, 4H, H8), 3.70 (s, 6H, –OCH3) and 3.60 (s, 12H, –OCH3). 13C NMR (500 MHz, DMSO-d6) 153.47, 153.36, 142.93, 139.19, 138.41, 136.45, 129.83, 125.63, 125.51, 124.07, 123.10, 122.68, 119.64, 111.09, 106.86, 60.54, 56.16, 47.96.
Synthesis of L5. A white powder was obtained by the treatment of 2-(1,3-benzodioxole)-1H-benzimidazole (802 mg, 3.4 mmol), 1,3-di(bromomethyl)benzene (443 mg, 1.7 mmol), KOH (379 mg, 6.8 mmol) and DMF (10 mL). Yield: 85% (831 mg). ESI (HR-MS). Calcd for C36H27N4O4 [M + H]+: m/z 579.2031. Found: m/z 579.2027. 1H NMR (400 MHz, DMSO-d6): δ 7.67 (d, J = 7.92 Hz, 2H, H4), 7.27 (d, J = 7.88 Hz, 2H, H5), 7.22 (t, J = 7.08 Hz, 3H, H7,11), 7.15 (t, J = 7.88 Hz, 2H, H10,12), 7.09 (d, J = 1.48 Hz, 2H, H2′), 6.94–6.90 (m, 4H, H6,5′), 6.86 (d, J = 8.04 Hz, 2H, H6′), 6.56 (s, 1H, H9), 6.09 (s, 4H, Ha) and 5.43 (s, 4H, H8).

General synthetic approach for 1–5

A mixture of Re2(CO)10, Ln and H2-RBC in toluene in a Teflon flask was placed in a steel bomb. The bomb was placed in an oven maintained at 160 °C for 48 h and then cooled to 25 °C. The resulting crystalline products were separated by filtration, washed with distilled hexane and air-dried.
Synthesis of 1. Greenish yellow crystals of 1 were obtained from Re2(CO)10 (100.2 mg, 0.1536 mmol), H2-RBC (76.3 mg, 0.1542 mmol), L1 (73.2 mg, 0.1492 mmol), toluene (10 mL), and acetone (2 mL). Yield: 26% (120 mg; crystals). Anal. calcd for C144H92N16O16Re4: C, 56.76; H, 3.04; N, 7.35. Found: C, 56.69; H, 3.12; N, 7.41. ESI (HR-MS). Calcd for C144H93N16O16Re4 [M + H]+: m/z 3048.5194. Found: m/z 3048.7418. FT-IR (KBr, cm−1): ν = 2017(s), 1898 and 1862(s).
Synthesis of 2. Greenish yellow crystals of 2 were obtained from Re2(CO)10 (100.5 mg, 0.154 mmol), H2-RBC (76.4 mg, 0.1545 mmol), L2 (82 mg, 0.1498 mmol), toluene (10 mL) and acetone (2 mL). Yield: 12% (56 mg; crystals). Anal. calcd for C148H100N16O20Re4: C, 56.12; H, 3.18; N, 7.08. Found: C, 56.27; H, 3.12; N, 7.23. ESI (HR-MS). Calcd for C148H101N16O20Re4 [M + H]+: m/z 3168.5618. Found: m/z 3168.8069. FT-IR (KBr, cm−1): ν = 2013(s), 1903 and 1862(s). 1H NMR (400 MHz, DMSO-d6): δ 8.56 (d, J = 8.6 Hz, 4H), 8.58 (d, J = 8.56 Hz, 4H), 7.83 (d, J = 8.6 Hz, 2H), 7.62–7.14 (t,m, 32 H, compound + toluene), 6.9 (m, 9H), 6.19 (d, J = 7.2 Hz, 4H), 5.98 (d, J = 8.7 Hz, 4H), 5.76 (t, J = 7.4 Hz, 4H), 5.4 (m, 8H), 5.3 (m, 8H), 5.58 (d, J = 18.2 Hz, 4H) and 3.76 (s, 6H).
Synthesis of 3. Greenish yellow crystals of 3 were obtained from Re2(CO)10 (100.8 mg, 0.1548 mmol), H2-RBC (76.2 mg, 0.1541 mmol), L3 (91 mg, 0.1488 mmol), toluene (10 mL) and acetone (2 mL). Yield: 22% (109 mg; crystals). Anal. calcd for C152H108N16O24Re4: C, 55.53; H, 3.31; N, 6.82. Found: C, 55.43; H, 3.38; N, 6.75. ESI (HR-MS). Calcd for C152H109N16O24Re4 [M + H]+: m/z 3288.6041. Found: m/z 3288.7196. FT-IR (KBr, cm−1): ν = 2012(s), 1900 and 1867(s).
Synthesis of 4. Greenish yellow crystals of 4 were obtained from Re2(CO)10 (100.2 mg, 0.1536 mmol), H2-RBC (76 mg, 0.1537 mmol), L4 (100 mg, 0.1485 mmol), toluene (10 mL), and acetone (2 mL). Yield: 31% (155 mg; crystals). Anal. calcd for C156H116N16O28Re4: C, 54.99; H, 3.43; N, 6.58. Found: C, 55.43; H, 3.38; N, 6.75. ESI (HR-MS). Calcd for C156H117N16O28Re4 [M + H]+: m/z 3408.6464. Found: m/z 3408.6287. FT-IR (KBr, cm−1): ν = 2013(s), 1899 and 1863(s). 1H NMR (400 MHz, DMSO-d6): δ 8.65 (d, J = 8.4 Hz, 4H), 7.99 (d, J = 4H), 7.89 (t, J = 8 Hz, 4H), 7.44–6.96 (t, t, and m, 52 H, compound + toluene), 6.81 (s, 2H), 6.79 (t, J = 7.8 Hz, 2H), 6.31 (d, J = 8.4 Hz, 4H), 6.05 (d, J = 7.2 Hz, 4H), 5.65 (t, J = 7.4 Hz, 4H), 5.51 (d, J = 18.8 Hz, 4H), 5.37 (d, J = 8 Hz, 4H), 5.24–5.19 (t and s, 8H), 4.37 (d, J = 18.8 Hz, 4H), 3.38 (s, 8H, –OCH3) and 3.29 (s, 10H, –OCH3).
Synthesis of 5. Green crystals of 5 were obtained from Re2(CO)10 (100.3 mg, 0.1537 mmol), H2-RBC (76 mg, 0.1537 mmol), L5 (86.3 mg, 0.1488 mmol) in toluene (10 mL) and acetone (2 mL). Yield: 17% (84 mg; crystals). Anal. calcd for C148H92N16O24Re4: C, 55.15; H, 2.88; N, 6.95. Found: C, 55.21; H, 2.83; N, 7.06. ESI (HR-MS). Calcd for C148H93N16O24Re4 [M + H]+: m/z 3224.4788. Found: m/z 3224.4948. FT-IR (KBr, cm−1): ν = 2017(s), 1899 and 1863(s).

X-ray crystallography

Intensity data of crystals of 1–5 were collected on a Bruker D8 Quest diffractometer [λ(Mo Kα) = 0.71073 Å]. The structures were solved by direct methods using SHELXS-9711 and refined using the SHELXL-2018/3 program (within the WinGX program package).11b,c Non-H atoms were refined anisotropically. Two methoxy units in 3 and 4 are disordered. Two 1,3-benzodioxole units are disordered in 5. The majority of the solvent molecules in the complexes could not be modelled correctly, and hence their contribution to the intensities was excluded using the SQUEEZE option in PLATON.11d The intensity data of crystal of L4 were collected on an Oxford CCD X-ray diffractometer (Xcalibur, Eos, Gemini) [λ(Cu Kα) = 1.54184 Å] and data reduction was performed using CrysAlisPro 1.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We thank the University of Hyderabad and the DST-SERB (EMR/2015/000627) for financial support. Dedicated to Prof. V. Chandrasekhar on the occasion of his 60th birthday.

Notes and references

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  12. The SCXRD-data of L4 is poor. However, the data supports the structure L4.
  13. The preparation of both rigid and flexible ligands with an imidazolyl core, instead of a benzimidazolyl core, is underway in our laboratory for assembling rhenium-core-based SCCs.

Footnote

† Electronic supplementary information (ESI) available: Experimental section and crystallographic data of L4, and 1–5. CCDC 1886926, 1885873, 1885870, 1547013, 1885869 and 1885872. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c8dt05065a
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