Supramolecular Macrocyclic Pd(II) and Pt(II) Squares and Rectangles

Oct 7, 2015 - Homi Bhabha National Institute, Training School Complex, Mumbai 400094, India. § Department of Chemistry, Texas A&M University, P.O. Bo...
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Supramolecular Macrocyclic Pd(II) and Pt(II) Squares and Rectangles with Aryldithiolate Ligands and their Excellent Catalytic Activity in Suzuki C−C Coupling Reaction K. V. Vivekananda,† S. Dey,*,† D. K. Maity,‡ N. Bhuvanesh,§ and V. K. Jain† †

Chemistry Division, Bhabha Atomic Research Centre, Mumbai 400 085, India Homi Bhabha National Institute, Training School Complex, Mumbai 400094, India § Department of Chemistry, Texas A&M University, P.O. Box 30012, College Station, Texas 77842-3012, United States ‡

S Supporting Information *

ABSTRACT: Addition of 1,4-benezenedithiol and 4,4′-biphenyldithiol to M(OTf)2 (M = cis-[Pt(PEt3)2]2+ or cis-[Pd(dppe)]2+) (dppe = 1,2-bis(diphenylphosphino)ethane) gave self-assembled tetranuclear complexes [M2{S(C6H4)nS}]2(OTf)4 (n = 1, 2). The same reaction with 1,4-benezenedimethanethiol yielded octanuclear supramolecular coordination complexes (SCC) [M2{SCH2C6H4CH2S}]4(OTf)8. These complexes were characterized by NMR, mass, and UV−vis spectroscopies, cyclic voltammetry, as well as density functional theory studies and represent the first examples of SCCs constructed by thiolate groups and square-planar metal ions. The rectangular shape of tetranuclear complexes and square shape of octanuclear complex are confirmed by single-crystal structures and computational studies. The palladium complexes showed excellent catalytic activity in Suzuki C−C cross-coupling reactions with high turnover numbers (2 × 107), even with low catalyst loading.

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INTRODUCTION The metal−ligand coordination bonds are stronger and more directional than hydrogen bonds or noncovalent interactions such as π−π stacking, van der Waals contacts, etc., which are commonly used in the development of metallo-supramolecular assemblies or supramolecular coordination complexes (SCCs).1−4 The SCCs are formed as discrete and single thermodynamically controlled product favored by a metaldirected self-assembly process involving metal ions and organic ligands. In the past decade several synthetic strategies have been evolved for high-yield SCC as metallacycles or metallacages with well-defined shapes and sizes; the best known of these are the directional bonding,1,3 symmetry interactions,5 and weak link approaches.2 The solubility of SCCs over metal− organic frameworks (MOF) make them better candidates as functional materials for solution-based applications, such as gas storage,1 molecular sensing,6 drug delivery,7 enzyme mimics,8 and cavity-controlled homogeneous catalysis.9−11 The use and applications primarily depend on the metal ions, functional groups incorporated in the ligands used as building blocks, and the cavity size of the metallacycles. Among the transition series, d8 metal ions such as Rh(I), Pd(II), and Pt(II) have been extensively used to construct the supramolecular structures due to their well-defined square planar geometry. By capping two cis-positions with phosphines/amines of the metal square plane, a library of closed and © XXXX American Chemical Society

discrete two- (square, triangle, rectangle, etc.) and threedimensional (tetrahedra, cubes, octahedra, pyramids, etc.) architectures have been constructed by employing several pyridyl-based ligands.1,3,4,12−14 The ligands containing S atom, in particular the thiolates, have hardly been employed to design SCC. This could be possibly due to high propensity of thiolate to act as a bridging ligand thus leading to oligomerization of resulting metal complexes, which are either insoluble or sparingly soluble in organic solvents, limiting their utility as functional material. Nevertheless, the metal−sulfur linkage, being a stronger bond, can be exploited to construct rigid structures. Besides metal ion stereochemistry, the ligand structure is equally critical to determine the overall architecture. A bridging ligand capable to coordinate multiple metal ions together in a well-defined configuration must be conformationally rigid or at least restricted.15 With respect to the above considerations, the S atom linked to the rigid aromatic backbone (e.g., phenyl, biphenyl) in combination with another donor atom at para- or meta-position is a good choice in the formation of supramolecular structures. Using carboxylic acid as another group the S-based mercaptobenzoic acid ligands have been employed to construct supramolecular structures of tin16 and gold17 Received: April 9, 2015

A

DOI: 10.1021/acs.inorgchem.5b00806 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

160H, PPh), 31P{1H} NMR (acetone-d6): δ 63.5 (s), 19F{1H} NMR (acetone-d6): δ −77.1 (s). [Pt2(PEt3)4(S(C6H4)S)]2(OTf)4 (3b). To a methanolic solution (10 mL) of 2b (70.5 mg, 0.097 mmol) was added methanolic solution (7 mL) of 1a (7.0 mg, 0.049 mmol); the yellow solution was stirred for 8 h. The solvent was evaporated in vacuuo, and the residue was washed with hexane and ether and extracted with dichloromethane (5 mL). Few drops of hexane were added to yield pale yellow crystals of 3b (52.0 mg, 0.020 mmol, 83%), mp 210 °C (dec). Anal. Calcd for C64H128F12O12P8Pt4S8: C, 29.54; H, 4.96; S, 9.86; Found: C, 30.08; H, 4.78; S, 9.72%. 1H NMR (400 MHz, CD3OD): δ 1.30 (dt, 3JP,H (d) = 17.2 Hz, 3JH,H (t) = 7.6 Hz, 72 H, PCH2CH3), 2.29 (m, 48H, PCH2), 7.49 (s, 8H, C6H4), 31P{1H} NMR (162 MHz, CD3OD): δ 13.3 (s, 1 JPt−P = 2911 Hz), 195Pt{1H} NMR (64 MHz, CD3OD): δ −4359 (t,1JPt−P = 2951 Hz), 19F{1H} NMR (376 MHz, CD3OD): δ −80.0 (s). [Pt2(PEt3)4(S(C12H8)S)]2(OTf)4 (4b). Prepared similar to 3b, using 2b (72.3 mg, 0.099 mmol) and 1b (11 mg, 0.05 mmol) and recrystallized from dichloromethane−hexane mixture to yield the title complex as a pale yellow solid (45 mg, 0.016 mmol, 66%), mp > 230 °C (dec). Anal. Calcd for C76H136F12O12P8Pt4S8: C, 33.14; H, 4.98; S, 9.31; Found: C, 33.60; H, 4.90; S, 9.40%. 1H NMR (400 MHz, CD3OD): δ 1.35 (dt, 3JP,H (d) = 17.2 Hz, 3JH,H (t) = 7.6 Hz, 72H, PCH2CH3), 2.32 (m, 48H, PCH2), 7.20 (d, 3JHH = 8.4 Hz, 8H, m-H, C12H8), 7.49 (d, 3JHH = 8.4 Hz, 8H, o-H, C12H8), 31P{1H} NMR (162 MHz, CD3OD): δ 11.9 (s, 1JPt−P = 2907 Hz), 195Pt{1H} NMR (86 MHz, CD3OD): δ −4369 (t, 1JPt−P = 2917 Hz), 19F{1H} NMR (376 MHz, CD 3 OD): δ −79.9 (s); electrospray ionization mass spectrometry (ESI-MS; ion, relative intensity): m/z 1638 ([M − Pt(PEt3)2 − 2PEt3 − 3OTf]+1, 11%), 1227 ([M − 2OTf]2+, 100%), 1109 ([M − 2OTf − 2PEt3]2+, 23%), 769 ([M − 3OTf]3+, 60%), 651 ([M − OTf]4+, 29%), 539 ([M − 4OTf]4+, 16%), 430 ([M − 4OTf − 4C6H5S]4+, 16%). [Pt2(PEt3)4(S(CH2C6H4CH2)S)]4(OTf)8 (5b). Prepared similar to 3b, using 2b (66.0 mg, 0.09 mmol) and 1c (7.8 mg, 0.046 mmol) and recrystallized from acetone−hexane mixture to yield the title complex as a colorless solid (38.5 mg, 0.007 mmol, 64%), mp 141 °C. Anal. Calcd for C136H272F24O24P16Pt8S16: C, 30.72; H, 5.16; S, 9.65; Found: C, 31.23; H, 5.08; S, 10.25%. 1H NMR (300 MHz, acetone-d6): δ 0.90−1.40 (m, 144 H, PCH2CH3, peak for PCH2 group merged with the solvent peak at δ 2.05), 4.97 (s, 16 H, SCH2), 7.90 (s, 16 H, C6H4), 31P{1H} NMR (121 MHz, acetone-d6): δ 12.9 (s, 1JPt−P = 2833 Hz); ESI-MS (ion, relative intensity): m/z 1216 ([M − 3OTf]4+, 17%), 1062 ([M]5+, 69%), 1047 ([M − OTf − 8SCH2C6H4]4+, 100%), 927 ([M − 3OTf − 5CH2S]5+, 28%), 811 ([M − 3OTf ]6+, 27%), 778 ([M − 8OTf − 5CH2S]5+, 29%), 746 ([M − 5OTf − 2CH2S]6+, 17%), 687 ([M − 8OTf ]6+, 59%), 463 ([M − 2OTf − 8PEt3 − 8CH2S]8+, 20%). Suzuki-Miyaura Cross-Coupling of Aryl Bromide with Phenylboronic Acid. An oven-dried flask was charged with aryl halide (1 mmol), aryl boronic acid (1.3 mmol), palladium complex 4a (0.1 mmol, equivalent to 0.4 mol % of Pd), 1,4-dioxane (2.0 mL), and aqueous (aq) K2CO3 (2 mmol, 1 mL) and was placed on an oil bath at 100 °C under a nitrogen atmosphere, and the reaction mixture was stirred until maximum conversion of aryl halide to product occurred. The reaction mixture was cooled to room temperature, quenched with water (10 mL), and neutralized by dropwise addition of dilute HCl (aq). The mixture was extracted with hexane (6 × 10 mL), washed with water (2 × 10 mL), and dried over anhydrous Na2SO4. The solvent of the extract was removed with rotary evaporator, and the resulting residue was analyzed by 1H NMR spectroscopy. Density Functional Theory Calculations. Full geometry optimization of macrocyclic complexes of Pd(II) and Pt(II) was performed by applying a DFT functional, namely, BP86. The BP86 is a generalized gradient approximation (GGA) functional that combines Becke’s 1988 exchange functional with Perdew’s 1986 correlation functional. SARCZORA basis sets for platinum, Gaussian type atomic basis functions, 631G(d,p) for H, C, P, and S atoms and 3-21G for Pd atom are applied for all the calculations. Basis sets for Pt and Pd atoms were obtained from Extensible Computational Chemistry Environment Basis Set

complexes via aurophilic and hydrogen-bonding interactions. The supramolecular assemblies of Pd(II) and Pt(II) complexes derived from mercaptobenzoic acid via intermolecular Hbonding have been recently synthesized by us.18 The flexible and hemilabile heteroligands containing phosphine and chalcogen (S, Se) groups are also used for the synthesis of palladium and platinum macrocyclic structures via weak-link approach,2 whereas the ligands containing two S donor atoms symmetrically separated by a rigid aromatic core (Scheme 1) Scheme 1. Aryldithiol Ligands

have not been explored as building blocks to prepare functional SCCs of Pd and Pt. These ligands are versatile to act in bridging mode as well as the same S donor atom that can bridge two different metal centers. In fact the gold complexes19 of 1a and 1b aggregate in one-dimensional arrays and a Pd(II) complex20 has been reported as dimeric open structure with the analogous heavier chalcogenolates, that is, biphenylene-4,4′-diselenolate. With the above perspective and our current interest in the applications of platinum group metal complexes we chose 1,4aryl dithiolates of varying flexibility (Scheme 1) to construct self-assembled supramolecular structures. Here we report the first examples of rigid Pd(II) and Pt(II)-based macrocyclic tetranuclear rectangles and octanuclear squares of dithiolates and have also assessed the catalytic activity of Pd complexes in Suzuki C−C coupling reactions.

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EXPERIMENTAL SECTION

[Pd2(dppe)2(S(C6H4)S)]2(OTf)4 (3a). To a methanolic solution (10 mL) of 1a (6.7 mg, 0.047 mmol), 2a (75.6 mg, 0.094 mmol) was added in methanol (7 mL). The wine-red solution was stirred for 6 h. The solvent was evaporated in vacuuo, and the red residue was washed with ether and hexane and extracted with dichloromethane (5 mL). Few drops of hexane were added to yield orange crystals of 3a (49.6 mg, 0.017 mmol, 72%; dppe = 1,2-bis(diphenylphosphino)ethane), mp > 230 °C. Anal. Calcd for C120H104F12O12P8Pd4S8: C, 49.77; H, 3.62; S, 8.86; Found: C, 49.72; H, 3.53; S, 9.46%. 1H NMR (CDCl3): δ 2.60−2.85 (m, 8H, PCH2), 3.25−3.45 (m, 8H, PCH2), 5.98 (s, 8H, C6H4), 7.26−7.33 (m, 8H, p-H, C6H5), 7.45−7.69 (m, 32H, o/m-H, C6H5), 31P{1H} NMR (CDCl3): δ 60.9 (s), 19F{1H} NMR (CDCl3): δ −77.9 (s). [Pd2(dppe)2(S(C12H8)S)]2(OTf)4 (4a). Prepared similar to 3a, using 2a (80.1 mg, 0.10 mmol) and 1b (11 mg, 0.05 mmol) and recrystallized from dichloromethane−hexane mixture to yield orange crystals of 4a (51 mg, 0.017 mmol, 67%), mp 188 °C (dec). Anal. Calcd for C132H112F12O12P8Pd4S8: C, 52.01; H, 3.70; S, 8.42; Found: C, 51.60; H, 3.52; S, 8.94%. 1H NMR (acetone-d6): δ 2.58−3.15 (m, 8H, PCH2; another multiplet for PCH2 merged with the base of the peak for H2O present in the solvent), 6.45 (d, 3JHH = 8.4 Hz, 8H, o-H, C12H8), 6.56 (d, 3JHH = 8.4 Hz, 8H, m-H, C12H8), 7.53 (t, 3JHH = 7.2 Hz, 16H, m-H, Ph), 7.65 (br s, 24H, m/p-H, Ph), 7.71 (t, 3JHH = 7.6 Hz, 8H, p-H, Ph), 7.75−7.85 (m, 16H, o-H, Ph), 7.85−7.96 (m, 16H, o-H, Ph), 31P{1H} NMR (acetone-d6): δ 62.7 (s), 19F{1H} NMR (acetone-d6): δ −78.7 (s). [Pd2(dppe)2(SCH2(C6H4)CH2S)]4(OTf)8 (5a). Prepared similar to 3a, using 2a (76.1 mg, 0.095 mmol) and 1c (8.1 mg, 0.048 mmol) and recrystallized from acetone−hexane mixture to yield yellow crystals of 5a (36.6 mg, 0.007 mmol, 52%), mp > 230 °C. Anal. Calcd for C248H224F24O24P16Pd8S16: C, 50.45; H, 3.82; S, 8.69; Found: C, 50.06; H, 3.63; S, 8.77%. 1H NMR (acetone-d6): δ 2.65−3.10 (m, 32H, PCH2), 3.28 (s, 16H, SCH2), 6.33 (s, 16H, C6H4), 7.30−7.90 (m, B

DOI: 10.1021/acs.inorgchem.5b00806 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Scheme 2. Syntheses of Palladium and Platinum SCCs

Database, Pacific Northwest National Laboratory.21 The quasiNewton−Raphson-based algorithm was applied to perform geometry optimization to locate the minimum energy structure in each case. All these calculations were performed by applying GAMESS suite of ab initio programs on a Linux cluster platform.22

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and integration of the 1H NMR signals suggest the metal-toligand ratio is 2:1 with the general formula [M2{SX(C6H4)nXS}]m(OTf)2m (m = 2 or 4) of the complexes. The chemical shift for the phenylene protons of the coordinated dithiolate ligands are shielded in the range of Δδ = 0.9−1.2 ppm for Pd complexes, whereas it is deshielded to Δδ ≈ 0.6 ppm for analogous Pt complexes (see Table S1 in the Supporting Information). The sharp singlet for benzylic and benzene ring protons of aryldithiolate ligand for 3a, 5a, 3b, and 5b indicates that the rotation about the H2C−C (phenylene) bond is rapid on the NMR time scale at room temperature. In the case of 4a and 4b, appearance of two doublets suggests that α and β biphenylene protons are equivalent and also the rotation of biphenylene rings is rapid. This rules out the possibility of existence of inner and outer environments, unusually observed in other macrocyclic rigid structures.14,24 The 1JPt−P values of platinum complexes (2833−2912 Hz) are significantly reduced as compared to subunit 2b (∼3704 Hz) due to two strong trans influencing thiolate groups.18 All the Pt nuclei are magnetically equivalent, as a triplet in 195Pt NMR spectra for 3b and 4b (see Figure S15 in the Supporting

RESULTS AND DISCUSSION

Syntheses and Spectroscopy. Treatment of metal cistriflate complexes (2a and 2b) with aryldithiols in 2:1 stoichiometry at room temperature afforded corresponding palladium (3a, 4a, 5a) and platinum (3b, 4b, 5b) complexes in nearly quantitative yields (Scheme 2). The resulting complexes were characterized by analytical and spectroscopic data and also by X-ray structural analyses. Although the complexes are of high molecular weights (5a: 5904 Da; 5b: 5317 Da) and possesses four or eight charges, they are highly soluble in acetone, methanol, and dichloromethane. A sharp singlet in the 31 P NMR spectra with associated Pt satellites in the case of the Pt complexes are consistent with the highly symmetric macrocyclic structures.4,23 The singlet at ca. −78 ppm in 19F NMR spectra is indicative of triflate anion. The microanalysis C

DOI: 10.1021/acs.inorgchem.5b00806 Inorg. Chem. XXXX, XXX, XXX−XXX

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(Table 1). The UV−vis spectra of 3a and 4a are characterized by a long wavelength, strong and relatively broad band at ∼449

Information) is observed due to coupling with two equivalent 31 P nuclei. The ESI-MS measurement gave convincing evidence for the formation of 4b and 5b. The prominent peaks were observed for 4b corresponding to the intact structure minus triflate anions at m/z: 1227 ([M-2OTf]+), 769 ([M-3OTf]3+), and 651 ([M-OTf]4+) strongly support the formation of proposed structure. In the case of 5b, peaks at m/z: 1216 ([M3OTf]4+), 1063 ([M]5+), 811 ([M-3OTf]6+), 687 ([M8OTf]6+), and ([M-4OTf]8+) indicate the formation of octanuclear structure (see Figures S20 and S21 in the Supporting Information). The above complexes were also crystallized from the reactions of 1 and 2 when performed in 1:1 ratio showing the formation of identical products. This is confirmed by the experiments involving stepwise addition of ligand to metal (Figure 1) and metal to ligand (see Figure S16 in the

Table 1. UV−vis Absorptiona and Electrochemical Datab,c of Complexes complex 3a 4a 5a 3b 4b

λmax (ε) 327 (48 800), 369 (sh, 18 400), 447 (27 500) 327 (60 400), 388 (28 000), 449 (31 800) 327 (110 500), 346 (sh, 78 100) 328 (19 600), 364 (18 700) 339 (44 500), 382 (sh, 20 400)

red 1

red 2

red 3

d

ox

−1.07

−1.68

−1.95

0.98

−1.18

−1.59

−1.93

d

−1.25

−2.15

−2.42

d

−1.45

−1.69

−2.19

d

−1.29

−1.59

−2.26

Wavelengths λmax at the absorption maxima in nm in acetone, molar extinction coefficients in M−1cm−1. bFrom cyclic voltammetry at 50 mV s−1 scan rate, peak potentials in V vs FeCp2/FeCp2+. cMeasurement in CH3CN/Me4NPF6. dNot observed. a

nm (2.76 ev) in the visible region responsible for the distinct orange color of the complexes (Figure 2). While the absorption

Figure 1. 31P NMR spectra (162 MHz, CD3OD) obtained from the mixture of 2a upon stepwise addition of 1c in an NMR tube (ligand variation). (A) 2a:1c = 2:0.25; (B) 2a:1c = 2:0.75; (C) 2a:1c = 2:1.25.

Supporting Information) employing 2a and 1c. The formation of 5a was monitored by 31P NMR spectroscopy. The Figure 1 reveals the existence of at least two products or intermediates at δ 63.4 and 63.8 ppm along with the expected complex 5a at δ 62.9 ppm when ligand concentration was less than 1 equiv (Figure 1A,B). When it was above 1 equiv (Figure 1C) the yield of 5a became quantitative (>90%). Similar experiments keeping fixed ratio of metal (2a) to ligand (1c), 31P NMR spectra were monitored versus time (see Figures S17−S19 in the Supporting Information). In case of low ratio 2:0.5, an equilibrium existed between 2a and 5a with the intermediates, whereas the higher ratios of 2:1 and 2:1.5 yielded 5a as major product with the same spectral features as observed in the case of metal-to-ligand variation experiments. These studies support the formation of thermodynamically favored rigid and cyclic products, which are sufficiently stable in solid as well as in solution for several months without any dissociation as observed by NMR spectra in methanol and acetone. Surprisingly the complex trans-Pd(PEt3)2(OTf)2 upon reaction with 1a and 1c resulted only in insoluble oligomeric products of the general formula [Pd2(PEt3)4{SXC6H4XS}]m(OTf)2m as indicated by microanalysis. The absorption spectra of the complexes, orange 3a and 4a, yellow 5a, and pale yellow 3b and 4b, were recorded in acetone

Figure 2. UV−vis spectrum of [Pd2(dppe)2(S(C6H4)S)]2(OTf)4 (3a) in acetone solution.

of the 5a and platinum complexes are blue-shifted in the nearUV region (see Figures S22 and S23 in the Supporting Information). Such long wavelength absorption was confirmed as transitions from the E orbital to the mixed orbital contributed from phosphine and metal in our mixed-ligand complexes [MCl(ECH2CH2NMe2)(PR3)] (E = S, Se, Te).25 The hypsochromic shift of the absorption band on replacement of Pd by Pt keeping common combination of donor atoms of the ligands also supports the above assignment. The high molar extinction coefficient of ∼31 800 M−1 cm−1 indicates a symmetry-allowed transition with a good orbital overlap. Electrochemistry. Cyclic voltammetry (CV) of the complexes were measured by using three electrode cell in CH3CN/0.05 M Me4NPF6, cycling between −2.5 and 1.5 V (Table 1). The CV measurements on one representative Pd and Pt complexes in acetone and dichloromethane did not show detectable peaks. The electrochemical peak potentials of all the palladium complexes in general exhibited three irreversible reduction steps, while complex 4a showed one D

DOI: 10.1021/acs.inorgchem.5b00806 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry irreversible oxidation step. The overall charge of the complexes, that is, +4 for tetranuclear 3 and 4 and +8 for octanuclear 5, is considered to be spread over the whole molecules; as a result, the localized charge on all the metal centers should be similar in all the complexes. So the oxidation at M(II) centers or the reduction at S center, which is in its most reduced form, are chemically less acceptable. The peak potential assigned to 0.98 V is due to the oxidation taking place primarily at the thiolate site, whereas the most negative peak potential at −1.93 V is due to the irreversible reduction at the phosphine ligand in 4a.25 Although the processes involved corresponding to the other two negative peak potentials (−1.18 and −1.59 V) are not clearly understood, they could possibly be originating from two reduction processes involving metal centers where Pd(II) is reduced to Pd(0), as one of the peak potentials is quasireversible in nature. It is reported that the dithiolate complexes [M(Ph2C2S2)2] (M = Ni, Pd, Pt) exhibited two quasi-reversible reduction processes at the metal center and one oxidation at the thiolate center, whereas the selenide clusters [Pd5(μ3Se)4(dppe)4]+2 showed three reduction steps.26 Although the oxidation peak potential was not observed in 3a, the little variation in three negative peak potentials indicate similar positioning of the frontier orbitals involved both in 3a and 4a, as observed in absorption spectra. However, by changing the ligand backbone from “phenylene” to “benzenedimethylene” moiety in 5a a significant negative shift of the peak potentials, particularly of the most negative peak potential (∼0.49 V), was observed. This result supports the observed blue shift of the long wavelength band in absorption spectra while comparing 3a/4a versus 5a. The platinum complexes 3b and 4b also showed three irreversible reduction steps at the more negative side in comparison to Pd analogues, and no oxidation step was observed. The most negative potential shift of ∼0.24 V in platinum complex 3b was observed relative to the Pd analogue 3a. The other two negative potentials (−1.45, 1.69 V) in 3b may be attributed to the reduction at PtII center as above. Crystal Structures. The structures of macrocycles 3a, 4a, and 5a were unambiguously determined by X-ray crystallography. The crystal lattices of 3a, 4a, and 5a consist of rectangular or square cations with well-separated, but electrostatically interacting, anions. The asymmetric unit cell of 3a and 4a comprised of half rectangle and two triflate anions, in the case of 3a partially occupied dichloromethane (3a·0· 36CH2Cl2), is also associated with the crystal lattice. Similarly the asymmetric unit cell of 5a contains only a half square, two triflate anions, and one acetone molecule as solvent. In case of 5a, the Z is 1 for the unit cell, and Z′ is 0.5, as the asymmetric unit doubles there will be eight triflate anions and two acetone molecules in the unit cell. Some of the solvent was also squeezed out. The entire rectangle or square is generated via an inversion center that is located at the midpoint between the two diagonal S atoms. The molecules 3a and 4a consist of two dithiolate ligands and four Pd atoms (m = 2) forming rectangular-shaped macrocycles; the four dppe chelating ligands attached to four Pd atoms form two boats of “Pd2(dppe)2S2” unit at head and tail position. Interestingly the molecule 5a is a square-shaped octanuclear (m = 4) and comprises four similar boats at four corners. All the three structures 3a, 4a, and 5a have common feature, that is, hinged four-membered “Pd2S2”ring, capped with dppe ligand, which are bridged via phenylene, biphenylene, and benzenedimethylene groups, respectively (Figures 3−5). Although half of the phenyl groups of the dppe ligand lie

Figure 3. Molecular structure of [Pd2(dppe)2(S(C6H4)S)]2(OTf)4 (3a) ellipsoids drawn at 50% probability. The dppe phenyl groups, hydrogen atoms, and triflate ions are omitted for clarity.

Figure 4. Molecular structure of [Pd2(dppe)2(S(C12H8)S)]2(OTf)4 (4a) ellipsoids drawn at 50% probability. The dppe phenyl groups, hydrogen atoms, and triflate ions are omitted for clarity.

inside the macrocycle and close to the phenylene rings, the metal−S−C angle and S−C bond help in bending and lengthening of the bridge, thus successfully self-assemble into macrocycles. Note that the self-assembly of Pd(dppp)(OTf)2 and pyrazine is prevented due to the steric crowding of phenyl groups of the phosphine ligand, whereas molecular squares are formed with 1,4-dicyanobenzene.23 The hinging of Pd2S2 rings is reflected in the dihedral angles (∼127−144°) deviated from the flat conformation (θ = 180°), which has been established due to the attractive donor−acceptor interactions between the dz2 and pz orbitals of the metal atoms.27,28 The Pd···Pd distances in 3a (3.190 Å), 4a (3.254 Å), and 5a (3.258, 3.306 Å) within the same “Pd2S2” core fall in the expected range of binuclear thiolato-bridged palladium complexes (Tables 2 and 3).29−31 Although numerous sulfido- and thiolato-bridged palladium and platinum complexes of diverse nuclearity are reported,29−31 the present macrocyclic structures are the first examples of supramolecular coordination complexes with thiolate ligands. The rectangular shape of 3a and 4a is given by four sulfur atoms of the bridging ligands at the four corners. The length of the smaller rectangle 3a as defined by edge-to-edge S···S E

DOI: 10.1021/acs.inorgchem.5b00806 Inorg. Chem. XXXX, XXX, XXX−XXX

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Table 3. Selected Interatomic Distances [Å] and Angles [deg] of Complex 5a·(CH3COCH3)2 Pd1−S1 Pd1−S2 Pd2−S1 Pd2−S2 Pd1−P1 Pd1−P2 S1−C27 Pd1···Pd2a S1···S2 θb θc S1−Pd1−S2 P1−Pd1−P2 Pd1−S1−Pd2 Pd1−S1−C27

2.3584(10) 2.3890(11) 2.3679(10) 2.3458(11) 2.2603(12) 2.2855(11) 1.847(4) 3.258 3.024 127.12 132.94 79.11(4) 84.40(4) 87.15(3) 99.78(13)

Pd3−S3 Pd3−S4 Pd4−S3 Pd4−S4 Pd4−P7 Pd4−P8

2.3541(11) 2.3493(11) 2.3545(12) 2.3642(10) 2.2605(12) 2.3048(13)

Pd3···Pd4a S3···S4

3.306 3.032

S1−C27−C28 S4−C34−C31 S2−C94−C91

110.1(3) 110.1(3) 109.3(3)

Distance between two Pd atoms within the same “Pd2S2” core. Dihedral angle between S1−Pd1−S2 and S1−Pd2−S2 planes. c Dihedral angle between S3−Pd3−S4 and S3−Pd4−S4 planes. a b

Figure 5. Molecular structure of [Pd 2 (dppe) 2 (SCH2 (C 6 H4 )CH2S)]4(OTf)8 (5a) ellipsoids drawn at 50% probability. The dppe phenyl groups, hydrogen atoms, and triflate ions are omitted for clarity.

π···π stacking is 3.8 Å. The comparison between the two rectangular-shaped rigid structures of 3a and 4a reflects the increased π−π stacking of the phenyl group of the thiolate ligand and metal···metal interaction in 3a. The two phenyl groups in 3a are tightly packed and aligned parallel to each other within the cavity, whereas in 4a, the C−C single bond between two phenyl groups helps in bending of the biphenyl group thus creating slight space to allow the twisting of the adjacent phenyl rings by 26.47°. The square-shaped highly symmetric structure of octanuclear complex 5a is different from that of rectangular-shaped tetranuclear complexes 3a and 4a. The structure of 5a consists of four “Pd2S2” units capped by dppe ligands, which occupy the corners of a square, held together by bridging dimethylene phenyl dithiolate groups. Since both the Pd atoms of the Pd2S2 unit lie outside the macrocycle, two S2 and two S4 sulfur atoms residing inside define the corners of a square. Though the molecular shape looks like a square, its lengths are not perfectly equal possibly due to two linkers, benzylic C atom and S atom of tetrahedral geometry. The dimensions of the cavity as defined by the edge-to-edge S···S distances are 8.8 and 9.0 Å, whereas the diagonal S···S distance is 13.7 Å. Macrocyclic structure of analogous platinum complexes may show interesting features. Thus, attempts were made to elucidate structures of such complexes, after several attempts crystals of 3b could be obtained. Partial data collection (∼40%) on a crystal sealed in a capillary with mother liquor at room temperature showed approximate structure similar to its palladium analogue 3a, and the structural features are undeniable (see Figure S28 and Table S3 in the Supporting Information). Thus, by introducing one flexible methylene group in the ligand backbone, the same reaction conditions easily yield lower-order (m = 2) tetranuclear to higher-order (m = 4) octanuclear supramolecular coordination complexes. The effect of ligand rigidity on structural changes has been observed in the case of catecholamide-forming supramolecular structures of Fe(III) and Ga(III) by addition of methylene group in the ligand.5 Computational Results. Minimum-energy structures of S(C6H4)S2−, S(C6H4)2S2−, SCH2(C6H4)SCH22−, Pd(dppe)2+, and Pt(PEt3)22+ units as well as complexes 3a, 3b, 4a, 4b, and

Table 2. Selected Interatomic Distances [Å] and Angles [deg] of Complexes 3a·0.36CH2Cl2 and 4a Pd1−S1 Pd1−S2 Pd2−S2 Pd2−S1 Pd1−P1 Pd1−P2 Pd2−P3 Pd2−P4 Pd1···Pd2a Pd1···Pd2b S1···S2c π···π stacking θd S1−C53/C27 S2−C56/C36 S1−Pd1−S2 P1−Pd1−P2 P1−Pd1−S P2−Pd1−S2 S1−Pd2−S2 P4−Pd2−S12 S2−Pd2−P4 P3−Pd2−P4

3a·0.36CH2Cl2

4a

2.3691(6) 2.4209(6) 2.4049(6) 2.3723(6) 2.2796(6) 2.2737(6) 2.2881(6) 2.2675(6) 3.190 7.647 3.311 3.386 135.1 1.783(2) 1.783(2) 87.454(18) 83.62(2) 90.522(19) 98.39(2) 87.75(2) 91.26(2) 172.89(2) 83.27(2)

2.4020(9) 2.3992(9) 2.3761(9) 2.4239(9) 2.2907(9) 2.2641(10) 2.2616(9) 2.2840(10) 3.254 11.717 3.369 3.830 144.1 1.793(3) 1.783(3) 89.12(3) 83.98(3) 94.34(3) 92.54(3) 89.12(3) 95.82(3) 172.85(3) 84.67(4)

Distance between Pd1 and Pd2 atoms within the same “Pd2S2” core. Distance between Pd1 and Pd2 atoms separated by dithiolate ligand. c Distance between S1 and S2 atoms within the same “Pd2S2” core. d Dihedral angle between S1−Pd1−S2 and S1−Pd2−S2 planes. a b

distance is 6.3 Å; the height as defined by S1···S2 distance in a same Pd2S2 unit is 3.3 Å, while the distance between the two centroids of benzene rings of dithiolate ligand, that is, π···π stacking is 3.4 Å (Figure 3). Similarly, the length and height of the larger rectangle, 4a, are 10.7 and 3.3 Å, respectively, and the F

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Inorganic Chemistry

Table 4. Selected Interatomic Distances (Å) and Energy Parameters of Complexes Calculated Applying BP86 DFT Functional Pd Complex

Pt Complex

M = Pd/Pt

3a

4a

5a

3b

4b

M−S M−P M···M stability (kcal/mol) HOMO−LUMO gap (eV)

2.47−2.50 2.36−2.37 3.19 −869.7 1.53

2.47−2.51 2.35−2.38 3.33 −827.7 1.43

2.46−2.50 2.35−2.38 3.04−3.06 −1476.7 2.21

2.53−2.54 2.45 3.46 −925.6 1.84

2.52−2.53 2.44−2.46 3.51−3.53 −887.4 1.70

5a are obtained with various possible input structures based on different spatial orientations of units. Optimization of structures are performed by applying a DFT functional, namely, BP86 that is known to work well for Pd/Pt-based complexes considering mixed atomic basis functions. The optimized structures look similar to the available X-ray structures, and selected bond distance parameters are displayed in Table 4. Calculated Pd−S and Pd−P bond distances of isolated complexes 3a, 4a, and 5a are ∼0.1 Å longer than that of X-ray data. However, though Pd···Pd distance is predicted accurately in case of 3a, an error of +0.2 Å is observed for 4a and 5a with respect to experimental data. This result is quite expected based on the present DFT functional considered. Corresponding Pt−S, Pt−P, and Pt···Pt distance parameters are calculated to be very close for the two Pt complexes. On the basis of calculated energy of the optimized structures of small units of the complexes, stabilization energies of the Pd and Pt complexes are calculated and are listed in Table 4. Stabilization of 5a needs an extra −CH2− group in the ligand to form the complex. Several attempts to obtain the structure similar to complex 5a with the ligands S(C6H4)S2−, S(C6H4)2S2−, and metal unit Pd(dppe)2+ failed due to spatial constraint. Similar attempts to generate trinuclear structures of the general formula [M2{SX(C6H4)nXS}]3(OTf)6 were also unsuccessful. Frontier orbitals corresponding to highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of all the five complexes are plotted in Figures 6 and 7. Clearly, mainly S atoms and phenyl rings of the dithiolate ligands participate in the corresponding HOMO of 3a, 3b, 4a, and 4b. In case of LUMO of these complexes, orbitals of Pd/Pt metals and P atoms actively participate. For 5a frontier orbitals HOMO and LUMO are rather localized; however, participation of orbitals is similar to complexes 3 and 4. Calculated energy gaps between HOMO and LUMO orbitals are shown in Table 4. On the basis of the MO situations resulting from the DFT calculations, the low-energy band is assigned to HOMO (S and phenyl ring orbitals)→ LUMO (phosphine and metal oribtals) electronic transition.25b The calculated values of the HOMO−LUMO gap increase from tetranuclear (1.53 eV for 3a) to octanuclear (2.21 eV for 5a) complexes. This trend qualitatively supports the observed blue shift (∼101 nm) in the absorption spectra and more negative shift (0.47 V) of the reduction peak potential in cyclic voltammetry of the Pd complexes. The HOMO−LUMO gap also increases when it is compared with the analogous Pt complexes (1.84 eV for 3b). The similar effect observed in the absorption spectra, and a negative shift of ∼0.24 V of the reduction peak potential, may be attributed to more destabilization of LUMO composed by phosphine ligand. The stronger back-bonding interaction of Pt/P versus Pd/P dominates over the chelating dppe (3a) versus monodentate PEt3 (3b); in the case of aryl phosphine the interaction is expected to be more than alkyl phosphine.

Figure 6. Plots of HOMO (i) and LUMO (ii) of Pd complexes 3a, 4a, and 5a based on corresponding optimized minimum-energy structures calculated applying DFT method.

Figure 7. Plots of HOMO (i) and LUMO (ii) of Pt complexes 3b and 4b based on corresponding optimized minimum-energy structures calculated applying DFT method. G

DOI: 10.1021/acs.inorgchem.5b00806 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Table 5. Suzuki−Miyaura Cross-Coupling of Aryl Halide with Phenylboronic Acida

entry

R

X

complex

mol % of “Pd”

time (hrs)

yield (%)b

TON

1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21.

4−CH3 4−CH3 4−CH3 4−CH3 4−CH3 4−CH3 4−CH3 4−CH3 2,6−CH3 2,6−CH3 2,4,6−CH3 2,4,6−CH3 2,4,6−CH3 2−CHO 4−CHO 4−COCH3 4−COCH3 4−COCH3 4−COCH3 3−COCH3 4−CHO

Br Br Br Br Br Br Br Br Br Br Br Br Br Br Br Br Br Br Br Cl Cl

3a 4a 4a 4a 4a 5a 5a 5a 5a 3a 3a 3a 5a 5a 3a 3a 3a 3a 3a 3ac 3ac

0.04 0.4 0.04 0.004 0.0004 0.8 0.08 0.008 0.8 0.8 0.4 0.8 0.8 0.08 0.04 0.04 0.0004 0.000 04 0.000 004 4 2

10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 8 16 16

90 96 90 67 39 97 82 62 58 70 48 82 83 95 94 99 95 93 85 43 57

2250 240 2250 16 750 97 500 121 1025 7750 73 88 120 103 104 1188 2350 2475 237 500 2325 000 21 250 000 11 29

a c

Reaction conditions: aryl halide (1.0 mmol), aryl boronic acid (1.3 mmol), K2CO3 as base (2 mmol), dioxane (2 mL), H2O (1 mL). bIsolated yield. TBAB (5 mmol) used.

Catalysis. The potential of sulfur-based metal complexes as homogeneous catalysts has been neglected for a long time due to the belief that sulfur acts as catalyst poison.32 Recently Vanjari et.al. have demonstrated the role of sulfur as a promoter rather than catalyst poison in C−C bond-formation reactions.33 Ananikov et al. have employed one-dimensional nanoparticles of in situ prepared [Pd(SCy)2]n as catalyst for the addition of cyclohexanethiol to alkynes leading to the formation of Markovnikov-type product.34 Although supramolecular dendritic palladium complexes35 and self-assembled palladium complexes with amphiphilic phosphine ligands36 have been used as catalysts in Suzuki and Heck reactions, the catalytic activity of multinuclear chalcogenolate complexes is hardly explored. However, catalytic activity of mono- and binuclear palladium chalcogenolate complexes in C−C bond formation has been reported recently.37,38,39a,b To assess the suitability of macrocyclic palladium complexes as homogeneous catalyst in C−C coupling reaction, the activity of these complexes is examined in Suzuki coupling reactions. On the basis of our recent experiments on optimization study of reaction conditions in Suzuki coupling reactions using palladium chalcogenolate/chalcogeno ether complex as catalysts, the solvent dioxane, base K2CO3 and temperature 100 °C were used as reaction conditions.39 It is evident from Table 5 that the complexes 3a, 4a, and 5a efficiently catalyzed the coupling reaction of both electron-rich and -poor aryl bromides with phenylboronic acids, with more than 90% isolated yield. To start the investigation the deactivated 4-bromotoluene was used as a substrate, since its coupling reaction can be conveniently monitored. The reactions proceeded smoothly with 0.04 mol % of “Pd” (entries 1, 3). The palladium complexes were active at loading as low as 1 × 10−4 mol % leading to slightly lower yield but higher TON up to 97 500

(entries 4, 5, and 8) under the same reaction conditions and time. The palladium supramolecular complexes also promote the coupling of bulky or sterically demanding substrates. The reaction of 2,6-dimethylbromobenzene and 2-bromomesitylene with phenylboronic acid gave moderate to good yields of biaryls (entries 9−13). Under the same conditions cross-coupling of electron-poor aryl bromides gave quantitative yields of the corresponding biaryls (entries 14−16). After lowering the catalyst concentration gradually to 1 × 10−6 mol % of Pd, the complex was highly active to yield 85% product with a very high TON of 20 million in case of bromoacetophenone as substrate (entries 17−19). These results are better than the carbene−thiolato Pd(II) complexes37 but comparable with respect to the catalytic activity of phosphino−thiolato Pd(II) complexes.38 The present Pd complexes are superior precatalysts than the supramolecular cationic Pd complexes with flexible coordination cage.9a Encouraged by these results, the more challenging coupling reactions of aryl chloride were attempted. The coupling of less reactive chlorobenzene exhibited very low yield. Nevertheless moderate yields were achieved for the electron-deficient aryl chloride in the presence of TBAB as a promoter (entries 20−21). A general comparative study on the relative activities of the three macrocyclic complexes showed comparable results (entries 1 vs 3; 12 vs 13; 14 vs 15) under the same reaction conditions. However, the critical comparison revealed that the activities of 3a, 4a are slightly higher than that of 5a (entries 9 vs 10). The similar trend was observed after careful consideration of the amount of catalyst used, for example, 0.04 mol % of Pd of 3a and 4a, resulted in 90% yield (entries 1 and 3), whereas twice the concentration, that is, 0.08 mol % of Pd of 5a, reached a corresponding yield of 82% (entry 7); this H

DOI: 10.1021/acs.inorgchem.5b00806 Inorg. Chem. XXXX, XXX, XXX−XXX

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ACKNOWLEDGMENTS One of the authors (K.V.V.) is thankful to the department of Atomic Energy for the award of Senior Research Fellowship.

is reflected in the difference of their TONs. These results indicate that the complexes 3a and 4a produce more catalytically active species after relieving the strain associated with the rigid macrocyclic structures in comparison to the more flexible and easily accessible larger macrocyclic complex 5a. The tetranuclear complexes 3a and 4a are also estimated by DFT calculations as less stable by ∼607 and 649 kcal/mol, respectively, relative to the octanuclear complex 5a (Table 5). Although the actual nature of the active species is not clearly known, the high activity upon lowering of Pd loading40 could possibly be due to the formation of Pd-nanoparticles (PdNPs). The excellent activity of the previously reported palladacycles in Suzuki and Heck C−C coupling reaction is now attributed to the formation of PdNPs.41 In the present case the thiolate ligands may stabilize the PdNPs, which can act as catalysts or precatalyst.

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CONCLUSIONS Self-assembled cationic macrocyclic Pd(II) and Pt(II) complexes have been synthesized as a single product by addition reaction from the simple and commercially available three aryldithiol ligands and metal triflates. These ligands, while holding the metals through bridging mode, facilitate stabilizing rigid unique supramolecular coordination complexes. The rigid donor S atom, coupled with flexible CH2 group, control the sizes, shapes, and cavities of the Pd and Pt macrocycles. The octanuclear complexes are of higher stability and absorb in lower wavelength as compared to the tetranuclear analogues. The excellent catalytic activity of palladium complexes has been demonstrated in Suzuki C−C cross coupling reactions. ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.5b00806. CCDC-1049141 (for 3a·0.36CH 2 Cl 2 ), 1049142 (for 4a), and 1049143 (for 5a·(CH3COCH3)2) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac. uk/data_request/cif. Details of general procedures of the experiments, crystallographic and structure refinement data tables of all complexes, 1H, 19F{1H}, and 31P{1H} NMR spectra of all the complexes, 195Pt{1H} NMR spectra of 4b, 31 1 P{ H} NMR spectra obtained from the addition of 2a to 1c (stepwise and fixed ratio vs time), ESI-mass spectra of 4b and 5b, UV−vis spectra of 3b and 5a, cyclic voltammograms of 4a, 5a, 3b, and 4b, molecular structure of 3b, 1H NMR spectra of 4-methylbiphenyl, 2,6-dimethylbiphenyl, 2,4,6-trimethylbiphenyl, 4-acetylbiphenyl, biphenyl-2-carboxaldehyde, biphenyl-4-carboxaldehyde. (PDF) Single-crystal X-ray diffraction studies of 3a, 4a, and 5a. (CIF)

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AUTHOR INFORMATION

Corresponding Author

*Phone: 91 22 25592589. E-mail: [email protected]. Notes

The authors declare no competing financial interest. I

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J

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