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Chem. , 2016, 55 (9), pp 4537–4544. DOI: 10.1021/acs.inorgchem.6b00371. Publication Date (Web): April 21, 2016. Copyright © 2016 American Chemical ...
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Spheroid Metallacycles and Metallocavitands with Calixarene- and/or Cleft-Shaped Receptors on the Surface Bhaskaran Shankar, Rajendiran Marimuthu, Shankar Deval Sathiyashivan, and Malaichamy Sathiyendiran* School of Chemistry, University of Hyderabad, Hyderabad 500 046, India S Supporting Information *

ABSTRACT: Flexible hexatopic ligands, 1,2,3,4,5,6-hexakis(1H-naphtho[2,3-d]imidazol-1-ylmethyl)benzene (L2) and 1,2,3,4,5,6-hexakis(4,5-diphenylimidazol-1-ylmethyl)benzene (L3), containing six neutral naphthanoimidazolyl and 4,5diphenylimidazolyl N donors were synthesized and used to assemble M6L6L′-type [M = Re(CO)3, L = anionic angular rigid NN donors, and L′ = flexible hexatopic N donors] spheroid metallacycles. These molecules with a diameter of ∼17 Å were obtained from Re2(CO)10, H−L (imidazole, benzimidazole, and naphthanoimidazole), and L′ [1,2,3,4,5,6hexakis(benzimidazol-1-ylmethyl)benzene (L1), L2, and L3] in a one-step process. Ligands L2 and L3 were characterized by elemental analysis, electrospray ionization time-of-flight mass spectrometry (ESI-TOF-MS), and 1H NMR spectroscopy. Metallacycles 1−5 were characterized by elemental analysis, ESI-TOF-MS, Fourier transform infrared spectroscopy, and singlecrystal X-ray diffraction analysis. Molecules 1, 2, and 4 can be considered as metallocavitands and contain multiple solventaccessible receptors, i.e., two metallocalix[3]arene units and six/four calix[4]arene-/cleft-shaped receptors, on the surface. Guests such as acetone molecules could be accommodated in the calix[4]arene/cleft-shaped receptor of the metallocavitands.

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INTRODUCTION

units at the top and bottom of the spheroid and six calix[4]arene-shaped cavities on the circular space (Figure 1)].12 The synthetic approach was unique in creating eight exocyclic calixarene-shaped receptors on the surface, which, in principle, can accommodate eight water/acetone/dimethyl sulfoxide (DMSO) molecules through multiple H-bondacceptor functional units present on the walls of the calixarene-shaped framework.12 Notably, a M3L3L″-type (L″ = neutral tritopic N donors) hemispheroid provides only one exocyclic H-bond-donor functional unit, i.e., three exo-C−H bonds from its one metallocalix[3]arene framework,12,4b whereas a M6L6L′-type spheroid, which is formed by the fusion of two M3L3L″ units, contains two metallocalix[3]arene units and six calix[4]arene-shaped receptors on the surface because of the additional arrangement of L and L′ in the spheroid (Figure S1 in the Supporting Information, SI). The above-mentioned design is useful for constructing larger spheroid metallocavitands with a well-defined size and uniformly positioned multiple exocyclic receptors using predesigned organic donors. To explore further, we envisioned that modulation of both an angular NN-donor motif and hexatopic N donors by varying a steric unit would provide spheroid metallocavitands with improved properties, i.e., larger exocyclic cavities that can accommodate more than one guest molecule or larger size guest molecules. In this study, three neutral flexible hexatopic donors, 1,2,3,4,5,6-hexakis(benzimidazol-1-ylmethyl)benzene (L 1),

The design of metallocavitands, i.e., multimetallic complexes where metal coordination is required for cavity formation,1 using various metal cores and organic donors has attracted much interest because of their supramolecular phenomena such as host−guest encapsulation, catalysis, and unique regioselectivity.1,2 Currently, the focus in this field is to functionalize the metallocavitand frameworks with various functional groups, suitably modulate the inner cavities of metallocavitands to accommodate a specific guest molecule, and design metallocavitands with multiple exocyclic receptors.1−11 We have earlier reported the one-pot fac-Re(CO)3-core-directed synthetic approach, i.e., a combination of Re2(CO)10, rigid NN donors (H−L), and flexible hexatopic N donors (L′) (Scheme 1), for spheroid metallocavitands with two distinct solventaccessible recognition sites [two exocyclic metallocalix[3]arene Scheme 1. Synthetic Approach to a Spheroid Metallocavitanda

○ = fac-Re(CO)3, H−L = rigid NN donor, and L′ = flexible hexatopic N donor.12

a

© XXXX American Chemical Society

Received: February 13, 2016

A

DOI: 10.1021/acs.inorgchem.6b00371 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 1. Chemdraw structure of spheroid [{(Re(CO)3benzimidazolate)3}2L1] (I).12 Metallocalix[3]arene unit with a set of three exo-C−H bonds as H-bond donors (left, bottom). Open form of I (for clarity) with six calix[4]arene-shaped receptor units with guest molecules (yellow) (right, bottom).

Figure 2. Neutral hexatopic N-donor ligands.

achieved using Re2(CO)10, rigid angular NN donors (H−L), and flexible hexatopic N donors (L′) in a one-step approach.12 The ligands were characterized by elemental analysis, electrospray ionization time-of-flight mass spectrometry (ESI-TOFMS), and 1H NMR spectroscopy. Complexes 1−5 were characterized by elemental analysis, ESI-TOF-MS, Fourier transform infrared (FTIR) spectroscopy, and single-crystal Xray diffraction analysis.

1,2,3,4,5,6-hexakis(1H-naphtho[2,3-d]imidazol-1-ylmethyl)benzene (L2), and 1,2,3,4,5,6-hexakis(4,5-diphenylimidazol-1ylmethyl)benzene (L3) (Figure 2), were used to assemble spheroid-shaped metallacycles and metallocavitands (1−5). Ligands L2 and L3 were designed and synthesized to increase the width of the arms, providing larger size spheroids and exocyclic calix[4]arene-shaped receptors with a large internal cavity. NN donors such as imidazole (H-imz), benzimidazole (H-benz), and naphthanoimidazole (H-nimz) are good building blocks for constructing metallocalix[3]arenes as metallocavitands.4,12,13 Moreover, benzimidazolyl- and imidazolyl-based flexible ligands are used for constructing metallosupramolecules.6,12−15 The self-assembly of spheroids was

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RESULTS AND DISCUSSION Synthesis and Characterization of Ligands. Ligands L2 and L3 were synthesized from H-nimz/4,5-diphenylimidazole, 1,2,3,4,5,6-hexakis(bromomethyl)benzene, and KOH in tetraB

DOI: 10.1021/acs.inorgchem.6b00371 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry hydrofuran (THF; Scheme 2). Both ligands were air- and moisture-stable. Scheme 2. Synthesis of the L2 and L3 Ligands

L2 is sparingly soluble in DMSO, whereas L3 is soluble in polar organic solvents. The 1H NMR spectrum of L2 showed two triplets, two doublets, and three singlet chemical resonances all well-separated in the aromatic region, which were assigned to the H7, H6, H5, H8, H9, H4, and H2 protons of the nimz unit of L2 (Figures 3 and S2 in the SI). All six nimz

Figure 4. Experimental (blue) and calculated (red) ESI-TOF mass spectra of [L2 + H]+ and [L3 + H]+.

Synthesis and Characterization of Complexes 1−5. The solvothermal treatment of Re2(CO)10, NN donors (H−L= Hnimz, H-benz, or H-imz), and L′ (L1, L2, and L3) in a mixture of toluene/acetone afforded metallacycles 1−5 [1·(C3H6O)2, where 1 = [{(Re(CO)3nimz)3}2L1]; 2·(C3H6O)4(H2O)2, where 2 = [{(Re(CO)3imz)3}2L2]; 3·C7H8, where 3 = [{(Re(CO)3nimz)3}2L2]; 4·(C3H6O)2, where 4 = [{(Re(CO)3imz)3}2L3]; [{(Re(CO)3benz)3}2L3] (5); Scheme 3]. The complexes were air- and moisture-stable and poorly soluble in organic solvents. FTIR spectra of the complexes showed strong bands in the 2030−1870 cm−1 region, which are characteristic of a fac-Re(CO)3 unit in an asymmetric environment.6 ESI-TOF mass spectra of 1−5 showed molecular ion peaks (m/z 3485.2730 for [1 + H] + , 3185.1431 for [2 + H]+, 3784.4000 for [3 + H]+, 3497.2953 for [4 + H]+, and 3797.5331 for [5 + H]+), consistent with theoretical values (Figures 5 and S7−S11 in the SI). Crystal Structures of Complexes 1−5. Single crystals of complexes 1−5 were obtained by the solvothermal reactions in a mixture of toluene/acetone. The solid-state structures of the complexes were confirmed by single-crystal X-ray diffraction analyses (Tables 1 and S1 in the SI). The molecules were characterized as M6L6L′-type metallacycles, consisting of six facRe(CO)3 cores, six anionic L units, and one neutral L′ (Figures 6−9 and S12−S16 in the SI). Complexes 1 and 3−5 have a spheroid or pseudospheroid structure, whereas complex 2 has a star-shaped structure. Metallacycles 1−5 can also be viewed as a [2 + 1] assembly of metallocalix[3]arene units and a propellershaped or sandwiched L′ unit. In complexes 1−4, the three alternatively arranged anionic L units and three fac-Re(CO)3 cores, [Re−L−Re−L−Re−L], constituted the metallocalix[3]arene unit with a cone conformation because of a syn,syn,syn arrangement of the three L units. The metallocalix[3]arene units in complex 5 are in a partial cone conformation with a syn,anti,syn arrangement of the three benzimidazolate units. The Re···Re distances and dihedral angles between the best plane passing through the Re···Re···Re atoms and L in the metallocalix[3]arene unit are very similar in complexes 1−4 (Table 2). The two metallocalix[3]arene motifs exhibited a staggered conformation in complexes 1, 2, 4, and 5, whereas an eclipsed arrangement was observed in complex 3. L′ in the complexes acts as a hexamonodentate N donor and has a syn,anti,syn,anti,syn,anti conformation (type I) in complexes 1, 2, and 4. A similar type of symmetrical conformation was observed in an acyclic ruthenium complex and I bearing an L1 ligand. However, an unsymmetrical arrangement of the six arms

Figure 3. Partial 1H NMR spectra of L2 (top) and H-nimz (bottom) in DMSO-d6.

units in L2 were equivalent in solution. The aromatic protons of L2 shifted upfield compared to the free H-nimz protons. The methylene protons (−CH2−) of L2 appeared as a singlet resonance at δ 5.72. The proton ratio of the nimz and methylene protons was 7:2, confirming the stoichiometry of L2. The disappearance of the NH proton peak, the well-separated signals for nimz, and a singlet for methylene protons indicate that L2 was formed. The 1H NMR spectrum of L3 showed that the signals in the δ 7.38−7.00 region, corresponding to H2 of imidazolyl and the protons of the two phenylene units of L3, merged. However, the methylene protons (−CH2−) appeared as a singlet chemical resonance at δ 4.79. In general, the methylene protons in R−CH2−R1 (R = benzene and R1 = imidazolyl/ benzimidazolyl) appear either as a singlet with/without broadening or a doublet with a geminal coupling constant in the δ 5.50−6.00 region.6,15−17 The unusual upfield shift of the −CH2− proton signal of L3 compared to those of the known di/tri/hexatopic imidazolyl/benzimidazolyl ligands6,15−17 and L2 indicates that the −CH2− protons experienced ring-current effects; i.e., they were located in the shielding zone of the arene units, from the adjacent phenylene units. The results indirectly support the formation of L3 (Figures S3 and S4 in the SI). The formation of L2 and L3 was further confirmed by ESI-TOF-MS, exhibiting their molecular ion peaks (m/z 1159.4583 for [L2 + H]+ and 1472.6329 for [L3 + H]+; Figures 4 and S5 and S6 in the SI). C

DOI: 10.1021/acs.inorgchem.6b00371 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Scheme 3. Synthesis of Spheroid Metallacycles 1−5

Figure 5. Experimental (blue) and calculated (red) ESI-TOF mass spectra of [1 + H]+, [2 + H]+, [3 + H]+, [4 + H]+, and [5 + H]+.

arene-shaped exocyclic cavities on the circumference. Among these cavities, the two oppositely arranged cavities have similar size, larger than that of the remaining two exocyclic cavities. Two solvent molecules, one guest in each exocyclic cavity, were observed in the cavity of the larger calix[4]arene unit. The C O group of the accommodated acetone in the cavity was directed inward, contacting the −CH2− protons of methylene walls through C−H···O bonds (acetone 1, C70−H/C74−H··· O19(acetone), d = 2.38/2.30 Å, θ = 141/146°; acetone 2, C72−H/C75−H···O20(acetone), d = 2.31/2.43 Å, θ = 140/ 139°; Figure S18 in the SI). The two exocyclic metallocalix[3]arene frameworks at the top and bottom of complex 1 provided a convergent arrangement of a set of three exo-C−H bonds, acting as Hbond donors.13 A similar type of arrangement was observed in

of L′ as a syn,syn,anti,anti,syn,anti conformation (type II) was observed in complexes 3 and 5 (Figure S17 in the SI). Although a hexatopic N donor can have different conformations, the type II conformation of L2/L3 in complexes 3 and 5 was observed for the first time.16 Three heterocyclic units of L′ were above the central benzene plane with a vertical head-tohead syn conformation, and three heterocyclic units were below the plane in all of the complexes. Spheroid 1 can be considered as a metallocavitand containing six calixarene-shaped receptors on the surface (Figure 6). The arrangement of neutral benzimidazolyl units and anionic naphthanoimidazolate motifs provided four pseudocalix[4]D

DOI: 10.1021/acs.inorgchem.6b00371 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Table 1. Crystal Data for the Structure Determinations of 1−5 chemical formula fw cryst syst space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Z T (K) λ (Å) Dcalc (g cm−3) μ (mm−1) R1 [I > 2σ(I)]a wR2 (all data)b GOF a

1·(C3H6O)2

2·(C3H6O)4(H2O)2

3·C7H8

4·(C3H6O)2

5

C144H96N24O20Re6 3599.64 monoclinic P21/c 16.3674(12) 27.1222(19) 34.577(3) 90 120.171(11) 90 13270(2) 4 296(2) 1.54184 1.802 11.046 0.1119 0.3481 0.888

C126H96N24O24Re6 3447.46 triclinic P1̅ 14.609(4) 15.011(4) 16.039(5) 107.270(5) 91.454(5) 105.883(5) 3208.8(16) 1 100(2) 0.71073 1.784 5.714 0.0447 0.1187 0.985

C169H104N24O18 Re6 3785.96 monoclinic P21/c 18.1372(15) 25.086(2) 33.454(3) 90 102.1180(10) 90 14882(2) 4 100(2) 0.71073 1.730 4.937 0.0714 0.1835 1.040

C144H108N24O20Re6 3611.74 triclinic P1̅ 15.6048(13) 15.9121(13) 16.8834(14) 114.3050(10) 94.3710(10) 98.5480(10) 3734.1(5) 1 100(2) 0.71073 1.606 4.913 0.0455 0.1006 1.010

C162H108N24O18Re6 3795.92 monoclinic C2/c 28.127(4) 19.365(3) 28.965(4) 90 108.018(2) 90 15003(4) 4 100(2) 0.71073 1.681 4.895 0.0584 0.1396 1.057

R1 = ∑||Fo| − |Fc||/∑|Fo|. bwR2 = {∑[w(Fo2 − Fc2)2]/∑[w(Fo2)2]}1/2.

Figure 7. Molecular structures of 2. For clarity, guest solvent molecules are omitted in part A; the top calix[3]arene [(Re(CO)3imz)3] unit and six carbonyl groups from the bottom of the calix[3]arene unit are omitted in part B. Color code: lavender, imz and Re(CO)3; blue, L2; C (turquoise), H (rose), O (red), acetone; green, Re in part B. Figure 6. Molecular structures of 1. For clarity, guest solvent molecules are omitted in parts A (top view) and B (side view); two calixarene units with guest acetone molecules (space-filled and stick views) in parts C and D (CO units are omitted; H atoms are omitted in part D; other parts are shown as thin sticks). Color code: lavender, nimz and Re(CO)3; blue, L1; C (turquoise), H (rose), O (red), acetone. Figure 8. Molecular structures of 4 [space-filled views: A and B]. For clarity, the top calix[3]arene [(Re(CO)3imz)3] unit and six carbonyl groups from the bottom calix[3]arene unit are omitted in part C. Color code: lavender, imz and Re(CO)3; blue, L3; C (turquoise), H (rose), O (red), acetone; green, Re in part C.

M6L6L′-type spheroid I, M3L3L″-type hemispheroid, and ionic M3L3 and M2L‴2 [L‴ = di(1H-naphtho[2,3-d]imidazolyl-1yl)methane] metallocavitands.4,12,13 Star-shaped complex 2 can be considered as a metallocavitand, containing four exocyclic cleft-shaped receptors and two metallocalix[3]arene exocyclic receptors on the surface. These solvent-accessible cleft-shaped receptors consisted of six neutral nimz units of L2, arranged as cleft−cleft−(π···π wall)− cleft−cleft−(π···π wall), as shown in Figure 7B. The size of each cleft {width = ∼5.56 Å, bottom; ∼11.15 Å, top [centroid(terminal arene)···centroid(terminal arene)]; height = 7.67 Å; dihedral angle of two nimz units = ∼88°} was sufficient to accommodate diverse guest molecules.1 In the crystal packing, complex 2 showed four acetone molecules and

two water molecules in the cleft framework. The accommodated acetone molecules were located deep inside the cleft by directing the CO groups inward and contacted the walls through C−H···O bonds [acetone 1: C29−H(methylene)··· O10(acetone), d = 2.30 Å, θ = 142°; C63−H···O10(acetone), d = 2.41 Å, θ = 130°; acetone 2: C29B−H(methylene)··· O11(acetone), d = 2.38 Å, θ = 109°; C49−H(imidazolate)··· O11(acetone), d = 2.65 Å, θ = 148°; Figure S18 in the SI]. E

DOI: 10.1021/acs.inorgchem.6b00371 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

units [dihedral angle = 11°; distance range (plane to atoms) = 3.02−3.95 Å] in complex 3. The two metallocalix[3]arene units in complex 3 were arranged face-to-face and contacted each other through edgeto-face C−H···π interactions between the terminal benzene moieties of the naphthanoimidazolate units [dihedral angle = 73°; distance = 3.7 Å (C148/48···C36/37)]. Two adjacent molecules of complex 3 interacted with each other via nonclassical H-bonding interactions using the carbonyl O atom and three exo-C−H atoms from the metallocalix[3]arene unit (Figure S19 in the SI), i.e., forming trifurcated H bonds [(C−H)3···OC−Re; C12/22/34···O5; distance = 3.51/3.58/ 3.22 Å; C−H···O = 162/169/152°]. This further supports that the metallocalix[3]arene units of spheroids can act as H-bond donors by providing three exo-C−H donors to H-bondacceptor molecules. Similar interactions were observed in the M3L3L″-type metallocalix[3]arene units in which one DMSO molecule was located in an exocyclic cavity, interacting with the benzimidazolate C−H groups through trifurcated H bonds.13a

Figure 9. Molecular structures of 3 (A−C) and 5 (D−F). For clarity, CO units and H atoms are omitted in parts C, E, and F. Color code: lavender, nimz and benz and Re(CO)3; blue, L2 and L3.

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CONCLUSION Hexatopic ligands L2 and L3 containing six neutral N-donor units were synthesized and used to assemble M6L6L′-type neutral heteroleptic spherical and star-shaped metallocavitands using an angular NN donor (H−L) and Re2(CO)10 in a onestep approach. Metallocavitands 1, 2, and 4 contained metallocalix[3]arene and calix[4]arene units as the exocyclic receptors. The guest molecule, acetone, occupied the calix[4]arene/cleft-shaped exocyclic cavity on the circumference of complexes 1, 2, and 4. Except complex 5, all of the complexes showed two convergent arrangements of a set of three exo-C− H H-bond donors from the two metallocalix[3]arene units of metallocavitands. Anionic linkers imidazolate and benzimidazolate were found to be good building blocks to construct spheroids with multiple exocyclic cavities by combining with imidazolyl/benzimidazolyl/naphthanoimidazolyl-based hexatopic donors. However, naphthanoimidazolate as an anionic linker is suitable to construct highly compact spheroids with naphthanoimidazolyl-based hexatopic donors but not spheroids with multiple exocyclic cavities on the surface. The results showed that the arrangement of the anionic motif and the sizes of both anionic/neutral donors affected both the overall size of the molecules and the size of the exocyclic cavity. Moreover, the results provide design controls on the number and size of the cavities on the surface of metallocavitands by modulating the framework of building blocks. The construction of large spheroid/star-shaped molecules with multiple receptors on the surface and decoration of the framework with alkyl/alkoxyl/ hydroxyl/thiol groups to increase and tune the solubility are underway.

Table 2. Some Important Parameters Including the Sizes of the Complexes (Å), Re···Re Distances (Å) in the Metallocalix[3]arene Motif, and Dihedral Angles between Two L Units (deg) size of the spheroid (radius) major axisa minor axisb Re···Re

anionic linker

1

2

3

4

5

8.49 7.87 6.396 6.329 6.407 6.316 6.399 6.428 47 47 48 43 51 46

8.50 9.92 6.345 6.377 6.363

8.52 10.34 6.362 6.359 6.385 6.293 6.377 6.405 65 59 66 64 56 61

8.26 9.12 6.368 6.326 6.387

8.15 8.62 6.346 6.288 6.344

77 73 69

77 65 72

77 74 80

a

Center of the benzene ring of the hexatopic donor to the center of three carbonyl O atoms. bCenter of the benzene ring of the hexatopic donor to the H atom pointing outward of the arene.

Complex 4 contained two exocyclic receptors and two metallocalix[3]arene units on the surface. In the crystal structure, complex 4 accommodated two acetone molecules in its two exocyclic cavities. Similar to complex 1, each exocyclic cavity provided the space for one acetone molecule, as shown in Figure 8. Complex 3 had only two exocyclic metallocalix[3]arene units, similar to complexes 1, 2, and I.12 The surface of complexes 3 and 5 was completely packed because of the intramolecular edge-to-face C−H···π and/or intramolecular π···π-stacking interactions between the anionic linker and neutral heterocyclic arene motifs and/or between the arene units of neutral hexatopic donors. In particular, complex 3 had a highly compact structure containing six anionic nimz and six neutral nimz units, contacting each other through noncovalent interactions. A strong intramolecular offset π−π-stacking interaction was observed between the two naphthanoimidazolyl

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EXPERIMENTAL SECTION

General Data. Re2(CO)10, toluene, HBr in acetic acid, 2,3diaminonaphthalene, formic acid, 4,5-diphenyl-1H-imidazole, KOH, and mesitylene were procured from commercial sources and used as received. 1,3,5-Tris(bromomethyl)-2,4,6-trimethylbenzene, 18a 1,2,3,4,5,6-hexakis(bromomethyl)benzene,18b H-nimz,18c and L116a were prepared. FTIR spectra were recorded on a JASCO-5300 FTIR spectrometer. Elemental analyses were performed on a Flash EA series 1112 CHNS analyzer. 1H NMR spectra were recorded on a Bruker AVANCEIII 400 instrument. ESI-TOF mass spectra were recorded on a Bruker maXis mass spectrometer. Synthesis of 1,2,3,4,5,6-Hexakis(1H-naphtho[2,3-d]imidazol-1ylmethyl)benzene (L2). A mixture of H-nimz (402.82 mg, 2.39 F

DOI: 10.1021/acs.inorgchem.6b00371 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Anal. Calcd for C162H108N24O18Re6: C, 51.26; H, 2.87; N, 8.86. Found: C, 51.36; H, 2.81; N, 8.95. IR (KBr, cm−1): 2032 (s), 1890 (s). X-ray Crystallography. Intensity data of crystal of 1 was carried out at 296(2) K on an Oxford CCD X-ray diffractometer (Xcalibur, Eos, Gemini) equipped with a Cu Kα radiation (λ = 1.54184 Å) source. Data reduction was performed using CrysAlisPro 1.171.37.35h (release 09-02-2015 CrysAlis171.NET). Intensity data of suitably sized crystals of 2−5 were carried out at 100(2) K on a Bruker Smart Apex CCD area detector system [λ(Mo Kα) = 0.71073 Å], with a graphite monochromotor. The data were reduced using SAINT PLUS. The structures were solved by direct methods using SHELXS-97,19 which revealed the atomic positions, and refined using the SHELXL-2014/7 program (within the WinGX program package).19b,c Non-H atoms were refined anisotropically. Some of the lattice solvent molecules that could not be modeled and hence their contribution to the intensities were excluded using the SQUEEZE option in PLATON. The detailed crystallographic data of 1−5 are given in Table 1.

mmol), KOH (177 mg, 3.15 mmol), and THF (23 mL) was stirred at room temperature for 5 h. Hexakis(bromomethyl)benzene (252.3 mg, 0.4 mmol) was added to the solution, and the reaction mixture was then stirred continuously for 48 h. The solvent was removed under reduced pressure; the residue was poured into water (20 mL). The resulting precipitate was filtered off, washed several times with water, and dried. Yield: 87% (402 mg). 1H NMR (400 MHz, DMSO-d6): δ 8.31 (s, 6 H, H2), 7.99 (s, 6 H, H4), 7.81 (d, JHH = 6.4 Hz, 6 H, H8), 7.65 (s, 6 H, H9), 7.48 (d, JHH = 6.4 Hz, 6 H, H5), 7.22 (t, JHH = 6.4 Hz, 6 H, H7), 7.15 (t, JHH = 6.4 Hz, 6 H, H6), 5.72 (s, 12 H,  CH2). ESI-TOF-MS. Calcd for C78H54N12 ([M + H]+): m/z 1159.4667. Found: m/z 1159.4583. Anal. Calcd for C78H54N12: C, 80.81; H, 4.69; N, 14.50. Found: C, 80.92; H, 4.61; N, 14.36. Synthesis of 1,2,3,4,5,6-Hexakis(4,5-diphenylimidazol-1ylmethyl)benzene (L3). A mixture of 4,5-diphenyl-1H-imidazole (1057 mg, 4.79 mmol), KOH (362 mg, 6.45 mmol), and THF (23 mL) was stirred at room temperature for 5 h. Hexakis(bromomethyl)benzene (503.2 mg, 0.79 mmol) was added to the solution, and the reaction mixture was stirred continuously for another 48 h. The solvent was removed under reduced pressure; the residue was poured into water (20 mL). The white precipitate was filtered off, washed several times with water, and dried. Yield: 96% (1120 mg). 1H NMR (400 MHz, DMSO-d6): δ 7.38−7.00 region where six merged peaks were obtained (H2, H7−H11), 4.79 (s, 12 H, −CH2−). 13C NMR (100.5 MHz, DMSO-d6): δ 44.45, 126.66, 126.79, 128.06, 129.24, 129.46, 130.30, 131.01, 135.09, 136.89. ESI-TOF-MS. Calcd for C102H78N12 ([M + H]+): m/z 1472.6577. Found: m/z 1472.6329. Anal. Calcd for C102H78N12: C, 83.24; H, 5.34; N, 11.42. Found: C, 83.15; H, 5.29; N, 11.36. Synthesis of [{((Re(CO)3)(μ-nimz))3}2(L1)] (1). A mixture of Re2(CO)10 (101.1 mg, 0.155 mmol), H-nimz (51.7 mg, 0.3073 mmol), L1 (44.1 mg, 0.051 mmol), toluene (10 mL), and acetone (5 mL) in a Teflon flask was placed in a steel bomb. The bomb was kept in an oven maintained at 160 °C for 48 h and then cooled to 25 °C. Yield: 54% (100.5 mg, weight of crystals). ESI-TOF-MS. Calcd for C138H84N24O18Re6 ([M + H]+): m/z 3485.38. Found: m/z 3485.27. Anal. Calcd for C138H84N24O18Re6: C, 47.58; H, 2.43; N, 9.65. Found: C, 47.65; H, 2.51; N, 9.58. IR (KBr, cm−1): 2027 (s), 1896 (s). Synthesis of [{((Re(CO)3)(μ-imz))3}2(L2)] (2). Pale-yellow crystals of 2·(C3H6O)4(H2O)2 were obtained by following a procedure similar to that for 1, using a mixture of Re2(CO)10 (100.5 mg, 0.154 mmol), Himz (21.3 mg, 0.313 mmol), L2 (59.4 mg, 0.051 mmol), toluene (10 mL), and acetone (5 mL). Yield: 50% (88.3 mg, weight of crystals). ESI-TOF-MS. Calcd for C114H72N24O18Re6 ([M + H]+): m/z 3185.28. Found: m/z 3185.14. Anal. Calcd for C114H72N24O18Re6: C, 43.01; H, 2.28; N, 10.56. Found: C, 43.12; H, 2.32; N, 10.48. IR (KBr, cm−1): 2027 (s), 1906 (s), 1873 (s). Synthesis of [{((Re(CO)3)(μ-nimz))3}2(L2)] (3). Orange crystals of 3· C7H8 were obtained by following a procedure similar to that for 1 using Re2(CO)10 (101.6 mg, 0.156 mmol), H-nimz (52.1 mg, 0.31 mmol), L2 (59.8 mg, 0.051 mmol), toluene (10 mL), and acetone (5 mL). Yield: 45% (90.3 mg, weight of crystals). ESI-TOF-MS. Calcd for C162H96N24O18Re6 ([M + H]+): m/z 3784.47. Found: m/z 3784.40. Anal. Calcd for C162H96N24O18Re6: C, 51.42; H, 2.56; N, 8.88. Found: C, 51.34; H, 2.51; N, 8.76. IR (KBr, cm−1): 2021 (s), 1901 (s). Synthesis of [{((Re(CO)3)(μ-imz))3}2(L3)] (4). Crystals of 4·(C3H6O)2 were obtained by following a procedure similar to that for 1 using Re2(CO)10 (100 mg, 0.153 mmol), H-imz (20.87 mg, 0.306 mmol), L3 (75.2 mg, 0.051 mmol), toluene (5 mL), and acetone (4 mL). Yield: 74% (137 mg, weight of crystals). ESI-TOF-MS. Calcd for C138H96N24O18Re6 ([M + H]+): m/z 3497.47; Found: m/z 3497.29. Anal. Calcd for C138H96N24O18Re6: C, 47.42; H, 2.77; N, 9.62. Found: C, 47.52; H, 2.71; N, 9.56. IR (KBr, cm−1): 2016 (s), 1890 (s). Synthesis of [{((Re(CO)3)(μ-benz))3}2(L3)] (5). Crystals of 5 were obtained by following a procedure similar to that for 1 using Re2(CO)10 (200.3 mg, 0.307 mmol), H-benz (73.2 mg, 0.62 mmol), L3 (151 mg, 0.102 mmol), toluene (10 mL), and acetone (5 mL). Yield: 33% (130 mg, weight of crystals). ESI-TOF-MS. Calcd for C162H108N24O18Re6 ([M + H]+): m/z 3797.57. Found: m/z 3797.53.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b00371. Spectral data of ligands, ESI-TOF mass spectra of compounds, and additional figures of metallacycles (PDF) X-ray crystallographic data for 1−5 in CIF format (CIF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] or [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the University of Hyderabad, UGC-UPE-II, DSTSERB (EMR/2015/000627), for financial support. B.S. thanks the UGC for the award of a Dr. D. S. Kothari Postdoctoral Fellowship.

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DEDICATION This paper is dedicated to Prof. M. Ravikanth, Department of Chemistry, IIT-Bombay, India. REFERENCES

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DOI: 10.1021/acs.inorgchem.6b00371 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.6b00371 Inorg. Chem. XXXX, XXX, XXX−XXX