PtII)-Based Supramolecular Coordination

Article pubs.acs.org/IC

Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

Heterobimetallic (FeII/PtII)‑Based Supramolecular Coordination Complexes Using 1,1′-Ferrocene Dicarboxylate: Self-Assembly and Interaction with Carbon Dots Achintya Jana, Saptarshi Mandal, Khushwant Singh, Prolay Das,* and Neeladri Das* Department of Chemistry, Indian Institute of Technology Patna, Patna 801106, Bihar, India Inorg. Chem. Downloaded from pubs.acs.org by UNITED ARAB EMIRATES UNIV on 01/12/19. For personal use only.

S Supporting Information *

ABSTRACT: The synthesis and characterization of a new pyrazine-based ditopic organoplatinum(II) complex having a bite angle of 180° is reported. The facile and efficient syntheses are described of three discrete neutral Fe(II)/Pt(II) heterobimetallic SCCs with Pt(II) acceptor clips of different binding angles, 0, 120, and 180°. These new SCCs were characterized by multinuclear NMR and mass spectrometry. Electrochemical response of these ferrocene containing self-assembled ensembles was studied using cyclic voltammetry. The diplatinum acceptor organometallic clips significantly quench the fluorescence of highly emitting carbon quantum dots (CD), while the self-assembled macrocycles tend to nullify the quenching effect of the organometallic clips. Interestingly, the inefficient quenching of CD fluorescence by these SCCs was found to be directly related to the angular disposition of the binding sites in the Pt(II) based organometallic clips.

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INTRODUCTION Design and synthesis of self-assembled metallasupramolecular architectures has been investigated extensively in recent past.1 Study of coordination-driven self-assembled ensembles has thus emerged as a popular research area.1,2 The versatility of this approach is due to the numerous metal ions (with various coordination geometry) that can be employed in conjugation with different kinds of ligands (with variation in denticity and ligating nuclei/center).3 In recent years, the phrase “supramolecular coordination complex (SCC)” has been coined as a general term by Stang to describe discrete, cyclic coordination complexes assembled using predesigned starting materials.2c In the realm of SCCs, the use of flexible ligands is quite interesting as well as challenging.3,4 This is because unlike rigid tectons that yield architectures of predefined dimensions flexible tectons are less predictable as far as the shape and size of the self-assembled product is concerned.5 1,1′-Difunctionalized ferrocenes are examples of flexible donor ligands that have been used in the design of SCCs. More specifically, 1,1′ferrocenedicarboxylate is an example of commercially available flexible ligand that has been used as a donor tecton in the selfassembly reactions.6 Ferrocene (FcH) is an 18-electron organometallic sandwich complex. Use of ferrocene units as donor ligands along with © XXXX American Chemical Society

suitable acceptor tectons (containing Lewis acidic metal centers) would yield heterometallic as well as functionalized metallasupramolecular systems that would be redox-active.7 However, there are relatively fewer reports that incorporate 1,1′-disubstituted ferrocenes in discrete ensembles because of the inherent rotational flexibility of the two cyclopentadienyl (cp) rings. In the present case, 1,1′-ferrocenedicarboxylate is considered a flexible ligand because there may exist various conformations of the ferrocenyl rings (shown in Chart S1).6 As a result, the directionality between the ligating centers (coming from the ferrocene) is variable and in turn renders the outcome of a self-assembly reaction. Literature survey indicated that ferrocene motifs have been incorporated in discrete as well as polymeric SCCs.7 There is considerable research interest in such ferrocene-bearing metallasupramolecular systems because of their potential as stimuli-responsive materials (such as molecular switches) and redox sensors.8 For example, anion recognition and sensing of Pd(II)/Pt(II) based macrocycles were investigated by using cyclic voltammetry, and this was possible due to the presence of ferrocene in the backbone.7 SCCs containing multiple ferrocenyl groups have also shown Received: October 30, 2018

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DOI: 10.1021/acs.inorgchem.8b03058 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry Chart 1. Chemical Structure of Three Ditopic Pt(II) Molecular Acceptor Tectons (1−3)

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biomedical applications such as interactions with DNA.9 A thorough literature survey of metallacycles and metallacages having multiferrocenyl units reveals synthesis of heterometallic ensembles derived from complementary acceptor tectons having Sn(IV), Te(IV),10 Mo,11 Re,12 Ni(II), Pd(II), Pt(II), Cu(I), Ag(I), Zn (II), and Ga13 ions. In continuation with our research interest to design SCCs, here we describe synthesis and characterization of three new heterobimetallic Fe(II)/Pt(II) macrocycles, wherein depending on the bite angle of the Pt(II) acceptor unit, the flexible ferrocene unit adapts a suitable conformer to yield the most entropically favored ensemble. Electrochemical response of the macrocycles is also elucidated. Previously, it has been reported that organoplatinum compounds and their SCCs have the potential to act as sensors for different guest molecules that are relatively electron-poor (such as nitroaromatics, C60, solvents, etc.).14 However, the interactions of organometallic compounds (derived from PtII) or metallacycles (derived from organometallic precursors) with carbon quantum dots (CD) have not yet been explored. CD is the newest member in the carbon nanofamily with excellent fluorescence and negligible photobleaching property. These have been used in the research related to bioimaging, drug delivery, biosensors, light-emitting displays (LEDs), and so on.15 Photoexcited CDs act as excellent electron acceptors or donors as shown in previous reports.16 The tuning and modulation of the fluorescence of CD is critical to its use in various design as probing agents. As such, the modulation of photoluminescence of CD with neutral supramolecular frameworks is a scarcely researched area. An interesting attribute of such studies would be to explore the effect of angular disposition of entities/tectons that are used in the construction of discrete supramolecular frameworks. Moreover, we are curious to explore the effect on the tuning of photophysical properties of CDs, using macrocycles of various shape and size. Herein, the three newly synthesized discrete neutral Fe(II)/Pt(II) heterobimetallic SCCs with Pt(II) acceptor clips of different binding angle 0, 120, and 180° are scrutinized for their ability to tune the fluorescence of blue-emitting CDs. We found definite correlation of quenching behavior between the organometallic clips and the corresponding macrocycles that are function of the binding angle of the acceptor clip.

RESULT AND DISCUSSION Synthesis and Characterization of Pyrazine-Based Organometallic Clip 3. Our research group previously reported two aforementioned 2,6-disubstituted rigid pyrazinebased Pt(II) containing acceptor clips (1 and 2 in Chart 1) and their application in the synthesis of SCCs.17,18 Compound 3 was however unknown in literature. As shown in Chart 1, the three compounds differ from each other with respect to the angle between the two Lewis acidic Pt(II) reactive centers. We were curious to study their (1−3) mode of binding with the flexible 1,1′-ferrocenedicarboxylate when subjected to selfassembly in 1:1 stoichiometric ratio. Therefore, synthesis of 3 was planned and this molecule would be a valuable addition to the library of Pt(II)-based ditopic acceptor tectons reported in the literature to date. Complex 3 was synthesized in two steps starting from 2,5dibromopyrazine. In the first step, 2,5-diethynylpyrazine was obtained (Scheme 1) using Sonogashira cross-coupling Scheme 1. Synthesis of Organoplatinum Complex 3

conditions between 2,5-dibromopyrazine and trimethylsilane acetylene followed by removal of trimethylsilane protecting groups. In the second step, 2,5-diethynylpyrazine in toluene was reacted with trans-PtI2(PEt3)2 (2 equiv) in the presence of triethylamine and CuI (catalyst) at room temperature for 16 h (Scheme 1). The desired organometallic complex (3) was obtained as an air-stable yellow colored solid. 3 was soluble in common polar organic solvents. It was characterized by FT-IR and NMR spectroscopies and mass spectrometry. The presence of an intense peak at 2108 cm−1 in the FTIR spectrum of 3 suggested the presence of an ethynyl functional group in this B


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Inorganic Chemistry

Figure 1. ORTEP presentation of 3 (30% thermal ellipsoid) (green, Pt; violet, I; orange, P; blue, N; black, C). Hydrogen atoms are omitted for the sake of clarity.

Scheme 2. Design and Synthesis of Macrocycles 4−6

C


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Inorganic Chemistry

Figure 2. (a) 31P{1H} NMR and (b) 1H NMR spectra of macrocycle 6 recorded in CDCl3.

molecule. The 1H NMR spectrum of 3 is very simple with only three sets of signals. The peak at 8.30 ppm is due to the two chemically equivalent hydrogens in the pyrazine ring. The methyl and methylene hydrogens in the triethylphosphine units show up as multiplets between 1.12 and 2.26 ppm. The 31 1 P{ H} NMR spectrum of 3 exhibited one sharp singlet at 8.66 ppm along with two 195Pt satellites peaks (1JP−Pt = 1147 Hz). The presence of a single sharp peak in 31P NMR implies that all four phosphorus nuclei ligated to the two Pt(II) centers are chemically equivalent. The molecular structure of 3 was also unambiguously supported by single-crystal X-ray crystallographic analysis. X-ray Crystallographic Analysis of 3. X-ray quality single crystals of 3 were obtained by slow evaporation of its dichloromethane solution at ambient temperature. The ORTEP presentation of 3 is shown in Figure 1. The molecule crystallizes in triclinic space group P1̅. In the structural analysis, we observed slightly distorted square-planar geometry at the two Pt(II) centers, and the cis angles (at the Pt centers) were between 89 and 91°. The Pt−pyrazine−Pt angle was measured to be 180°; therefore, 3 is a molecular tecton with bite angle of 180°. The two Pt nuclei are separated by a distance of 11.835 Å. Unusual bond angle/lengths were not observed in the crystallographic analysis of 3. Selected bond and crystallographic parameters are shown in Tables S1 and S2. Self-Assembly of SCCs. Inspired by the versatility of coordination-driven self-assembly protocol for the construction of nanoscopic supramolecular ensembles having various shapes and sizes, we were interested to incorporate the flexible ferrocenedicarboxylate in the skeletal framework of a metallamacrocycle. SCCs bearing two different metals would render the supramolecular framework heterobimetallic in nature. These are interesting materials since such materials are anticipated to show exciting properties like electronic coupling via the two different metallic centers.6 Commercially available ferrocene derivatives such as 1,1′-ferrocenedicarboxylic acid have been often utilized to yield such bimetallic macrocycles that possess unique electrochemical and optical properties. Both Pt(II)- and ferrocene-based materials have excellent electrochemical and fluorescence modulating properties. Therefore, we wanted to incorporate both the metal ions to include their properties synergistically or additively in our designed SCCs. It was envisioned that incorporation of ferrocene would render the corresponding macrocycle redoxactive. Moreover, it was anticipated that use of acceptor

tectons (1, 2, or 3) of different directionality (0, 120, and 180°) would yield macrocycles not only of different sizes but also with different numbers of ferrocenyl units. Additionally, these self-assembled reactions would also confirm the flexible nature of the ferrocene-based ligands in general. Simultaneously, these reactions would also attempt to enrich the library of discrete multiferrocenyl supramolecules as well as that of flexible self-assembled ensembles. In a typical self-assembly reaction, a diplatinum acceptor organometallic clip (1, 2, or 3) was initially treated with silver nitrate (2 equiv) that yielded the corresponding dinitrate derivative. The precipitated silver halide residue was removed by filtration. Subsequently, an aqueous solution of 1,1′ferrocenedicarboxylate was added in a 1:1 stoichiometric ratio to the dinitrate solution (filtrate). The reaction was stirred for 15 h. This resulted in gradual precipitation of a product that was washed with water and n-pentane and finally recrystallized from chloroform to get the respective product as microcrystalline yellow solid in reasonably good yields (>90%). Products of these reactions, as depicted in Scheme 2, were characterized using NMR spectroscopy and high-resolution mass spectrometry (ESI-TOF-MS). The 31P{1H} NMR spectrum of 4−6 were recorded and showed a sharp singlet (4: 18.42 ppm, 5: 18.69 ppm, and 6: 18.70 ppm) along with a pair of 195Pt satellite peaks (1JPPt = 1265 Hz for 4,1JPPt = 1251 Hz for 5, and 1JPPt = 1247 Hz for 6) (Figure 2 and the Supporting Information). Relative to precursor 3, the 31P signal of 6 showed a significant downfield shift which confirms the presence of new ligand−metal (PtII− O) coordinate bonds substituting Pt−I bonds present in 3. 1H NMR spectra of the products (4−6) indicated presence of both reactants, i.e., donor (ferrocenedicarboxylate) and acceptor clips (1, 2, or 3). Additionally, the 1H NMR spectra of products (4−6) also confirmed the formation of highly symmetrical structures in pristine form. As an illustrative example, the 1H NMR spectrum of 6 is shown in Figure 2. Herein, the signal at 8.27 ppm corresponds to the two hydrogens in pyrazine ring of the acceptor unit, while the signals at 4.72 and 4.26 ppm are due to the hydrogens present in the ferrocene units. Presence of the PEt3 groups ligated to Pt(II) centers was confirmed from the signals in the range of 2.0−1.24 which are due to hydrogens in the ethyl groups bound to phosphorus nuclei. Thus, NMR experiments corroborated the coordination of the donor and acceptor tectons in a self-assembly reaction to yield 6. Similarly, all signals in 1H NMR were assigned precisely (Supporting D


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Inorganic Chemistry

Figure 3. Experimental (bottom) and theoretical (top) ESI-TOF-MS spectra of macrocycles [M + H]+ (a) M = 4, (b) M = 5, and (c) M = 6.

Figure 4. Simulated molecular models (ball-and-stick representation) of 4−6 optimized by PM6 method (light gray, C; orange, P; dark cyan, Pt; and blue, N). Hydrogen atoms are shown for clarity.

units of acceptor and two units of donor tectons. Formation of [2 + 2] unit is evident from the ESI-TOF-MS spectra of products 5 and 6 in which peaks at m/z 2522.20 (5) and 2522.18 (6) are observed corresponding to the respective [M + H]+ species. Herein, [M] represents the molar mass of the intact dimeric [2 + 2] macrocycle. The mass spectrum signals for each [M + H]+ species were examined for isotopic resolution. An excellent match with the respective theoretically predicted isotopic distribution was observed (Figure 3). Thus, ESI-TOF-MS spectra of macrocycles 4−6 confirmed formation of discrete [1 + 1] or [2 + 2] self-assembled macrocyclic architecture as shown in Scheme 2. PM6 Molecular Modeling of Self-Assembled Ensembles 4−6. Attempts to obtain X-ray quality single crystals of the macrocycles were unsuccessful. Therefore, structural information (size and shape) of macrocycles (4−6) was obtained from their optimized geometry obtained by using the PM6 semiempirical molecular orbital method.19 The energyminimized structures suggest that the different conformers of

Information) for products 4 and 5. Signals corresponding to the ferrocenyl group were observed in the range 4.8−4.2 ppm. The formation of 4−6 was confirmed from their respective 1H DOSY NMR experiment wherein a single trace was observed in the corresponding spectrum (Supporting Information). In characterization of SCCs, mass spectrometry has been routinely employed as a powerful analytical technique to confirm the formation of discrete and finite species over oligomeric/polymeric materials.1a In the present study, ESIMS data were recorded to rationalize the stoichiometry of the reactant tectons in the products of the self-assembly reactions depicted in Scheme 2. In the ESI-TOF-MS spectrum of 4, the peak at m/z 1461.36 is due to the [4 + H]+ charged species, wherein 4 represents a [1 + 1] neutral macrocycle formed due to the reaction of one unit each of PtII2 acceptor (1) and ferrocenedicarboxylate donor tecton. However, for the other two reactions, ESI-TOF-MS analysis suggests formation of [2 + 2] neutral self-assembled macrocyclic species. In this context, [2 + 2] implies a self-assembled discrete unit composed of two E


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Inorganic Chemistry

Figure 5. Cyclic voltammograms of (a) 4, (b) 5, and (c) 6 in dichloromethane containing n-Bu4NPF6 as supporting electrolyte on a Pt electrode at a scan rate of 100 mV/s.

(concentration = 0.1 mmol), and Bu4NPF6 was used as the supporting electrolyte. A single peak was observed for 4 and the corresponding E1/2 value was 0.82 V vs Ag/AgCl. This wave, in the cyclic voltammogram of 4, is assigned to [Fe(II)/ Fe(III)] redox couple due to the ferrocene motif. The separation between the anodic oxidation peak and the cathodic counter peak is about 150 mV. In the case of 5 and 6, two ferrocenyl oxidations were observed (ca. 0.82 and 0.65 V vs Ag/AgCl for 5 and 0.84 and 0.65 V vs Ag/AgCl for 6). Comparison of the E1/2 data suggested that redox potentials are very close for the SCCs. In other words, exchanging one Pt(II)-based acceptor linker by another has marginal effect on the ferrocenyl/ferrocenium potential. The potential difference (ΔE) between individual [Fe(II)/Fe(III)] redox couples of 5 and 6 is 0.17 and 0.19 V, respectively. The appearance of two closely separated reduction−oxidation waves in the cyclic voltammograms of 5 and 6 suggest that the two ferrocene units in these macrocycles are nonequivalent. This also hints at stepwise and sequential oxidation of the two ferrocenyl moieties in 5 and 6. The observation of two waves corresponding to ferrocenyl/ferrocenium potential also suggests that there is electronic communication between these two active centers of the respective macrocyclic cage. SCCs with more than one ferrocene units, in which a potential difference is observed between the individual

ferrocenedicarboxylate participate in the three self-assembly reactions described herein. In other words, the flexibility of the donor ligand is evident from its conformational changes during coordination-driven self-assembly reaction with acceptor tectons of different bond directionality. Moreover, a consequence of the flexible nature of the donor tecton is the absence of any particular geometric shape. Self-assembly of rigid tectons is usually associated with the yield of macrocyclic framework with a definite shaped polygon (such as square, rhomboid, hexagon, etc.). The sizes of these SCCs were determined as well. The farthest distances between two opposite Pt centers in the case of metallacycles 4−6 were found to be 0.74, 1.41, and 1.39 nm, respectively. This suggests that SCCs have nanoscopic dimensions having structural features of nanometer size (1− 100 nm). In all cases, the platinum centers have slightly distorted square planar geometry. The optimized structures (Figure 4) suggest the ferrocenyl spacer groups adopt synclinal staggered conformation in 4, anticlinal staggered in 5, and synclinal staggered in 6. The Fe−Fe distances in 5 and 6 were measured to be 16.5 and 21.0 Å, respectively. Electrochemical Response of Macrocycles 4−6. Cyclic voltammetry (CV) spectra were recorded for 4−6 to obtain insight into their electrochemical properties and these are depicted in Figure 5. SCCs were dissolved in dichloromethane F


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Inorganic Chemistry

incubated at room temperature with 4−6 (4 μM) for 15 min. An increase in absorbance of macrocycles with slight shift in wavelength was observed, which indicates interaction of CDs with the SCCs. The increase in absorbance was quite significant for the mixture of CD and 5, which displayed an absorbance maximum at 358 nm. A decrease in UV−vis absorbance of the exciplex (CD−macrocycle union) was observed by decreasing the concentration of 4−6 (Figure S14). Steady-state fluorescence measurements of CD show excitation-dependent emission spectra (Figure 6B). When excited with 365 nm, the CD solution displays emission maxima at 430 nm. Coincidentally, all the macrocycles (4−6) as well as organometallic complexes (1−3) also show fluorescence emission maxima at around 430 nm when they are excited at 365 nm. However, CD has very large emission intensity compared to the macrocycles and organometallic complexes (Figure 7). The effect of various concentrations of the organometallic complexes and macrocycles on the fluorescence intensity of CD was studied. Quenching of fluorescence of CD was observed with increasing concentration of the organometallic complexes and macrocycles. However, fluorescence intensity of CD increased with first few additions for 1, 3, 5, and 6 that ultimately lead to quenching with further addition of these compounds. Interestingly, about 2-fold increase in fluorescence intensity of CDs was observed with the addition of 5 at low concentration (2 μM) with CD solution. This is consistent with UV absorbance spectra that also display a significant increase in the absorption of CD upon addition of 5 and point toward formation of exciplexes. While a similar phenomenon was observed for the ferrocene derivative 1,1′-ferrocenedicarboxylate (L1), the quenching of CD fluorescence by L1 was insignificant (Figure S15). For 2 and 4, no such enhancement of fluorescence intensity was found to occur. F0/F versus [Q] was plotted (Figure S16) to determine the Stern−Volmer constant, where F0 and F are fluorescence intensities in absence and presence of quencher, respectively, and [Q] is the quencher concentration. The Stern−Volmer plots yield upward curvature, concave toward the y-axis, which indicates both dynamic and static quenching.23 To further investigate if the ferrocene moieties (present in 4−6) have any role in the observed fluorescence modulation of CDs by 4−6, steady-state fluorescence spectra of CD were

[Fe(II)/Fe(III)] redox couples, has been previously reported in literature. For example, similar response to the ferrocenyl/ ferrocenium potential was observed by Crowley and coworkers in their Pd(II)-based macrocycles having more than one ferrocene motifs.20 In another example, Chandrasekhar and Thirumoorthi reported 1,1′-ferrocenedicarboxylate bridged and organotin/tellurium based macrocycles that demonstrate two quasi-reversible oxidation processes assigned to the ferrocene based redox couples.10 In such supramolecular frameworks, the magnitude of electronic communication between the [Fe(II)/ Fe(III)] redox couples has been estimated by calculating the comproportionation constants (Kc = 10ΔE/59.15mV).10,21 In the case of 5 and 6, Kc was calculated to be 748 (5) and 1630 (6). On the basis of previous literature reports, the magnitude of Kc obtained for 5 and 6 implies that these two macrocycles belong to an intermediate category between class I (noncoupling) and class II (weakly coupled) systems as per Robin−Day classifications.10,21 Additionally, we observed that the macrocycles were stable even after five electrochemical cycles (Figure S13). Macrocycle 4 has one ferrocenyl group, while 5 and 6 have two such groups. Accordingly, 4 showed a smaller peak current relative to that for 5 and 6 (Figure S12). Thus, there is some consistency in the response of the Fc+/0 couple and the number of ferrocene units present in the respective molecule. Results of the electrochemical results are also summarized in Table 1. Table 1. Electrochemical Data for Macrocycles 4−6a compound

E1/2, V (ΔEp, mV)b

Kcc

4 5 6

0.82 (150) 0.82 (143), 0.65 (70) 0.84 (156), 0.65 (90)

7.48 × 102 1.63 × 103

a

Measured in CH2Cl2 containing [n-Bu4N][PF6] at a scan rate of 100 mV s−1. The peak potentials are with respect to Ag/AgCl. bE1/2 = (Epc + Epa)/2. cKc = 10ΔE/59.15mV at 298 K.

Interaction with CD. CD was synthesized with citric acid and cysteamine by hydrothermal method following previously reported literature.22 The CD displays an absorption maximum at 350 nm (Figure 6A). The CD solution (0.04 μg/mL) were

Figure 6. (A) UV−vis absorption spectra of CD and macrocycles. (B) Steady-state fluorescence spectra of CD. G


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Inorganic Chemistry

Figure 7. Steady-state fluorescence spectra of CD solution in the presence of organometallic clips and macrocycles: (A) CD and 1, (B) CD and 4, (C) CD and 2, (D) CD and 5, (E) CD and 3, (F) CD and 6.

Table 2. Fluorescence Emission Decays of CD in Absence and Presence of 1−3 (tectons) and 4−6 (SCCs) compound

CD

CD + 1

CD + 2

CD + 3

CD + 4

CD + 5

CD + 6

average fluorescence lifetime (ns)

6.11

5.42

5.91

4.49

5.95

5.98

4.68

recorded in the presence of a previously reported24 neutral homometallic Pt-macrocycle (7 in Figure S17). In 7, 1,4benzenedicarboxylates bridge Pt(II) centers of different acceptors units instead of 1,1′-ferrocenedicarboxylates in 4. It was observed that 7 shows higher quenching efficiency (Figure S17) than that of its heterobimetallic analogue (4). Hence, it

can be concluded that ferrocene moieties do not have any role in the observed interaction between CD and heterobimetallic Pt-SCCs (4−6). The contribution of dynamic quenching of CD by the acceptor tectons (1−3) and the corresponding SCCs was investigated by fluorescence lifetime measurements (Figure H


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Inorganic Chemistry S18). The average fluorescence lifetimes of CD, organometallic tectons 1−3, and SCCs 4−6 are illustrated in Table 2. The lifetime of fluorescence of CD decreased after coincubation with the organometallic clips and macrocycles. The decrease was more prominent for the clips as compared to that of the macrocycles. The decrease in fluorescence lifetime of the CD in the presence of clips (1−3) and macrocycles indicates diffusion-driven collision of these molecules with CD, which leads to dynamic quenching of the CD fluorescence.23 However, energy transfer becomes more efficient with linearity in a molecule and subsequently fluorescence lifetime becomes shorter. The most significant decrease in fluorescence lifetime of CD was observed for 3, which projects a bite angle of 180°. The decrease in lifetime was also noteworthy for 6, which was derived from organometallic compound 3. Fluorescence intensity of CD is highly related to the strength and extent of H-bonding of the surface functional groups with solvent molecules. As such, decrease in the extent of Hbonding is responsible for the increase of fluorescence intensity of CDs with polar surface functionality.25 Citric acid and cysteamine derived aqueous-soluble CD used here are decorated with polar functional groups like −OH, −COOH, NH2, and SH (Table S3). In presence of the tectons and macrocycles, few hydrogen bonds between functional groups of CD and solvent are replaced with noncovalent interactions between CD and the acceptor clips/macrocycles. At higher concentrations, clips and macrocycles overcrowd the functional groups on CD surface which ultimately lead to fluorescence quenching of CD through intersystem crossing (IC) and photoinduced electron transfer (PET). The extent of interaction of macrocycles with CD is also a function of the bite angles of the macrocycles. However, in some cases, where there is significant interaction between pyrazine moiety and the CD, at lower concentration fluorescence enhancement was observed due to the inherent back electron transfer tendency of pyrazine-nitrogen to CD. Halogens and heavy atoms are known to facilitate the conversion of excited singlet state to excited triplet. These long-lived triplet states may return to the ground state by nonradiative decay or be quenched to the ground state by the quencher.26 We believe exciplexes are being formed by addition of acceptor molecules and macrocycles, which are responsible for the increase or decrease in the fluorescence intensity of CD. Among 1−3, quenching capacity is highest for 2 (120°), followed by 3 (180°) and 1 (0°). An increase in bite angle from 120 to 180° reduces interaction of CD with Pt(II) that subsequently decreases the potential of the organometallic clips to quench the fluorescence of CD. The insignificant quenching capacity of 1 (0°) is probably due to incorporation of two additional phenylacetylene moiety that suppress the electron accepting capacity of Pt(II) from CD. However, after macrocycle formation, the efficiency of fluorescence quenching of CD is 5 > 4 > 6 (with corresponding bite angles 120° > 0° > 180°). Since ferrocene acts as an electron donor, the incorporation of L1 into the macrocycles holds back the electron transfer from CD to the Pt(II). Moreover, ferrocene, being an efficient electron donor, can also transfer electrons to the CD. Remarkably, macrocycle 4 affects fluorescence quenching of CD even at low concentration, while two of its constitutive tectons, i.e., 1 and L1, individually first enhance the fluorescence intensity of CD before quenching the same at higher concentrations. In macrocycle 4, two Pt(II) moieties are almost fixed at 0.74 nm distance, whereas in 1, Pt(II) moieties

can rotate, leading to reduced interaction or charge transfer to CD. The interaction of the organometallic tectons and macrocycles with surface functional groups of CD are predominantly controlled by the orientation of the donors and acceptors present. This is evident from changes in fluorescence with change in bite angles. The change in fluorescence is an outcome of the balance between fluorescence quenching tendency of iodine, platinum, and carboxylate and enhancement due to interaction with pyrazine present in the acceptor tectons (1−3) and SCCs. In summary, the organometallic acceptors and their corresponding neutral macrocycles have definite contribution in tuning the fluorescence of CD owing to their distinct angular disposition.

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CONCLUSIONS In conclusion, we report herein the synthesis of a new Pt(II)based ditopic acceptor linker (3), which is an addition to the library of Pt(II)-based linear ditopic acceptor tectons reported in the literature. Subsequently, using 3 and two other Pt(II)based ditopic tectons (1 and 2), three neutral and discrete SCCs (4, 5, and 6) were synthesized. 4−6 are heterobimetallic since 1,1′-ferrocenedicarboxylate was used as the complementary donor tecton in conjugation with Pt(II)-based acceptor tectons (having different bond directionality). 4−6 were self-assembled in very high yields and were subsequently characterized satisfactorily using NMR and mass-spectrometry. Energy-minimized structures obtained from molecular modeling (PM6 semiempirical molecular orbital method) indicated that these have nanoscalar supramolecular cyclic frameworks. The fact that 1,1′-ferrocenedicarboxylate is a flexible ligand is evident from the different conformations adapted by the ferrocenyl moieties in these macrocycles. As expected, 4−6 are electrochemically active due to the presence of the ferrocene motif. Interestingly, unlike a previously reported heterobimetallic Pt(II)-based neutral SCCs with 1,1′-ferrocenedicarboxylate groups bridging metal centers, electronic communication is observed between the ferrocene units present in the macrocycles reported in this work. This observation hints that the strength and nature of such interaction between nonequivalent redox centers may be controlled by structural changes in the nature of the macrocyclic framework. We also studied the photophysical properties of the macrocycles and their organometallic synthons in the light of their ability to quench fluorescence emission of CD. Noncovalent interaction studies of CDs with two different transition metals in a single organometallic neutral macrocycle are reported first time to the best of our knowledge. This may pave the way toward potential use of platinum based organometallics as regulator for CD fluorescence with appropriate design. Organoplatinum compound 2 was found to be an excellent quencher for CD employed in the study. In future, host−guest chemistry of these redox-active supramolecular metallacycles can be explored by observing changes in their electrochemical signatures upon hosting a suitable guest in the macrocyclic cavity. Additionally, studies related to biological interaction of these heterobimetallic and charge-neutral SCCs may be undertaken in future.

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EXPERIMENTAL SECTION

General Details. All chemicals including anhydrous solvents used in this work were purchased from commercial sources and used without further purification. Triethylamine was freshly distilled prior use. All air-sensitive reactions were carried out under nitrogen I


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Inorganic Chemistry

δ18.70 (1JPPt = 1247 Hz). IR (ATR): 2968, 2936, 2879, 2106, 1709, 1616, 1456, 1377, 1315, 1252, 1178, 1135, 1032, 911, 831, 763, 738, 627 cm−1. ESI-TOF-MS. Calcd for ([M + H]+): m/z 2522.29. Found: 2522.18. Interaction with Carbon Dot (CD). CD used in this study were reported in our earlier literature.22 In short, citric acid and cysteamine are mixed in aqueous solution and heated in a stainless-steel autoclave at 120 °C for 6 h. Product was then purified by centrifugation and filtering with a 0.22 μm syringe filter. UV−visible absorption spectra of CD and macrocycles were taken on UV-2550 spectrophotometer (Shimadzu, Japan). Steady-state fluorescence emission measurements were carried out in Fluoromax-4P spectrofluorometer (Horiba Jobin Yvon). Both absorption and fluorescence measurement were carried out in quartz cuvette with 1 cm path length, using ethanol as solvent. Fluorescence quenching studies was done at excitation wavelength of 365 nm. All the fluorescence data in the presence of macrocycles and organometallic molecules have been normalized with respect to that of CDs. Time-resolved fluorescence emission decays were measured at room temperature by a time-resolved fluorescence spectrophotometer (LifeSpec-II, Edinburgh Instruments, UK) using picosecond time-correlated single-photon-counting (TCSPC) technique.

atmosphere. 2,5-Diethynylpyrazine27 was prepared by following a previously reported literature protocol. SCC 7 was synthesized using our earlier literature report.24 Melting points were recorded using SRS EZ-Melt automated melting point apparatus by capillary methods and uncorrected. FTIR spectra were obtained using a PerkinElmer Spectrum 400 FTIR spectrophotometer. 1H and 31P NMR spectra were measured on Bruker 400 MHz spectrometer. MS data were recorded using a Bruker Impact ESI-Q-TOF system. All the samples were diluted in methanol and infused directly via a standard ESI source using a syringe pump (flow rate = 180 μL/h). The system was previously calibrated in the mass range of 50−3000 m/z using a tune mix solution. Further data were processed using Data Analysis 4.2 software. Theoretical isotope patterns were obtained from Bruker Isotope Pattern software. Molecular modeling (PM6 semiempirical molecular orbital method) was carried out with Gaussian 09. Synthesis of 3. In a glovebox, trans-diiodobis(triethylphosphine)platinum(II) (1.60 g, 2.33 mmol) and 2,5-diethynylpyrazine (0.10 g, 0.78 mmol) were loaded in a Schlenk flask (100 mL). This flask was linked to a Schlenk line, and the contents were dissolved in anhydrous toluene (40 mL). Freshly distilled triethylamine (16 mL) was added under nitrogen, and reaction was stirred for 15 min at room temperature prior addition of CuI (22 mg, 0.11 mmol). The reaction mixture was allowed to stir at room temperature for several hours. Overnight reaction resulted in precipitation of triethylammonium iodide, which was removed by filtration. Solvent was evaporated from the filtrate on a rotary evaporator to yield a yellow product that was purified by column chromatography on silica gel. Initially, 1% ethyl acetate in hexane was used as elutant, and then gradually the polarity was gradually increased to 3% ethyl acetate in hexane to isolate 3 in pristine form as a yellow solid. Organometallic Complex 3. Yellowish solid. Yield: 0.37 g, 38%. Mp 216−219 °C. 1H NMR (400 MHz, CDCl3): δ 8.30 (s, 2H, Ar− H), 2.26−2.17 (m, 24H, −CH2−), 1.20−1.12 (m, 36H, −CH3). 31P NMR (162 MHz, CDCl3): δ 8.66 (1JPPt = 1147 Hz). IR (ATR): 2962, 2929, 2874, 2108, 1456, 1407, 1296, 1249, 1183, 1031, 911, 760, 728 cm−1. HRMS (ESI, m/z): Calculated for C32H62I2N2P4Pt2 ([M + H]+): 1242.12. Found: 1242.14. General Protocol for Synthesis of Pt(II)-Based Macrocycles (4−6). Using organometallic platinum compounds 1, 2, and 3, we obtained macrocycles 4, 5, and 6, respectively. The organoplatinum compound (1, 2, and 3, 0.01 mmol) was dissolved in acetone and AgNO3 (3.4 mg, 0.02 mmol) was added to it in one portion. The reaction mixture was stirred for 12 h at room temperature in the absence of light. This resulted in precipitation of AgI that was removed by filtration over a Celite pad. The filtrate was yellow in color to which an aqueous solution (0.5 mL) of disodium ferrocenedicarboxylate (3.2 mg, 0.01 mmol) was added dropwise with constant stirring. This reaction mixture was allowed to stir at room temperature for 15 h. This resulted in gradual precipitation of the product that was washed with water and n-pentane and finally recrystallized from chloroform to get the respective product as microcrystalline yellow solid in reasonably good yields (>90%). Macrocycle 4. Yield: 21 mg, 93%. 1H NMR (400 MHz, CDCl3): δ 8.67 (s, 2H, Ar−H), 7.50 (s, 2H, Ar−H), 7.48−7.41 (m, 2H, Ar−H), 7.32−7.24 (m, 4H, Ar−H), 4.73 (s, 4H), 4.26 (s, 4H), 2.11−2.00 (m, 24H, −CH2−), 1.30−1.20 (m, 36H, −CH3). 31P NMR (162 MHz, CDCl3): δ 18.42 (1JPPt = 1265 Hz). IR (ATR): 2966, 2934, 2877, 2114, 1622,1584, 1508, 1457, 1376, 1317, 1154, 1033, 887, 761, 684 cm−1. ESI-TOF-MS. Calcd for ([M + H]+): m/z1461.36. Found: 1461.35. Macrocycle 5. Yield: 13 mg, 91%. 1H NMR (400 MHz, CDCl3): δ 8.09 (s, 4H, Ar−H), 4.72 (s, 8H), 4.26 (s, 8H), 2.02 (m, 48H, −CH2−), 1.27−1.23 (m, 72H, −CH3). 31P NMR (162 MHz, CDCl3): δ 18.69 (1JPPt = 1251 Hz). IR (ATR): 2966, 2933, 2879, 2107, 1706, 1619, 1489, 1457, 1378, 1315, 1231, 1153, 1033, 920, 871, 762, 737, 618 cm−1. ESI-TOF-MS. Calcd for ([M + H]+): m/z 2522.29; Found: 2522.20. Macrocycle 6. Yield: 13.2 mg, 92%. 1H NMR (400 MHz, CDCl3): δ 8.27 (s, 4H, Ar−H), 4.71 (s, 8H), 4.26 (s, 8H), 2.00 (m, 48H, −CH2−), 1.24 (m, 72H, −CH3). 31P NMR (162 MHz, CDCl3):

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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.8b03058. 1

H and 31P{1H} NMR spectra of organometallic complex 3 and SCCs 4−6, DOSY NMR spectra of 4− 6, X-ray crystallographic analysis of 3, details of cyclic voltammetry experiment, steady-state fluorescence spectra of CD solution in the presence of L1, Stern−Volmer plots and time-resolved fluorescence decay of CD (PDF) Accession Codes

CCDC 1849562 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Tel.: +91-612-3028057 (P.D.). *E-mail: [email protected] or [email protected]. Tel.: +919631624708, + 91-612-3028023 (N.D.). ORCID

Achintya Jana: 0000-0003-3087-8212 Prolay Das: 0000-0002-2774-8479 Neeladri Das: 0000-0003-3476-1097 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS N.D. thanks the Indian Institute of Technology (IIT) Patna for infrastructural and financial assistance. A.J. and K.S. thank UGC, New Delhi, India, for Research Fellowship. S.M. thanks IIT Patna for an Institute Research Fellowship. We also acknowledge SAIF-IIT Patna for access to instrumental facilities (ESI-MS and SC-XRD). We acknowledge SAIFPanjab University for NMR facilities. J


Article

Inorganic Chemistry

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(6) Das, N.; Arif, A. M.; Stang, P. J.; Sieger, M.; Sarkar, B.; Kaim, W.; Fiedler, J. Self-Assembly of Heterobimetallic Neutral Macrocycles Incorporating Ferrocene Spacer Groups: Spectroelectrochemical Analysis of the Double Two-Electron Oxidation of a Molecular Rectangle. Inorg. Chem. 2005, 44, 5798−5804. (7) Xu, L.; Wang, Y.-X.; Chen, L.-J.; Yang, H.-B. Construction of multiferrocenyl metallacycles and metallacages via coordinationdriven self-assembly: from structure to functions. Chem. Soc. Rev. 2015, 44, 2148−2167. (8) (a) Scottwell, S. Ø.; Barnsley, J. E.; McAdam, C. J.; Gordon, K. C.; Crowley, J. D. A ferrocene based switchable molecular folding ruler. Chem. Commun. 2017, 53, 7628−7631. (b) Scottwell, S. Ø.; Elliott, A. B. S.; Shaffer, K. J.; Nafady, A.; McAdam, C. J.; Gordon, K. C.; Crowley, J. D. Chemically and electrochemically induced expansion and contraction of a ferrocene rotor. Chem. Commun. 2015, 51, 8161−8164. (c) Crowley, J. D.; Steele, I. M.; Bosnich, B. Protonmotive Force: Development of Electrostatic Drivers for Synthetic Molecular Motors. Chem. - Eur. J. 2006, 12, 8935−8951. (9) Cook, T. R.; Vajpayee, V.; Lee, M. H.; Stang, P. J.; Chi, K.-W. Biomedical and Biochemical Applications of Self-Assembled Metallacycles and Metallacages. Acc. Chem. Res. 2013, 46, 2464−2474. (10) Chandrasekhar, V.; Thirumoorthi, R. 1,1’-Ferrocenedicarboxylate-Bridged Redox-Active Organotin and -tellurium-Containing 16Membered Macrocycles: Synthesis, Structure, and Electrochemistry. Organometallics 2007, 26, 5415−5422. (11) Cotton, F. A.; Daniels, L. M.; Lin, C.; Murillo, C. A. Square and Triangular Arrays Based on Mo24+ and Rh24+ Units. J. Am. Chem. Soc. 1999, 121, 4538−4539. (12) Bera, J. K.; Clérac, R.; Fanwick, P. E.; Walton, R. A. Assemblies of multiply bonded [Re2]n+ cores possessing bond orders of 3 or 3.5 that are linked by dicarboxylate bridges. J. Chem. Soc., Dalton Trans. 2002, 2168−2172. (13) Uhl, W.; Spies, T.; Haase, D.; Winter, R.; Kaim, W. Synthesis of a Large Organometallic Macrocycle Comprising Four Ga−Ga Bonds and Four Bridging Ferrocene Dicarboxylato Ligands. Organometallics 2000, 19, 1128−1131. (14) (a) Saha, M. L.; Yan, X.; Stang, P. J. Photophysical Properties of Organoplatinum(II) Compounds and Derived Self-Assembled Metallacycles and Metallacages: Fluorescence and its Applications. Acc. Chem. Res. 2016, 49, 2527−2539. (b) Shanmugaraju, S.; Mukherjee, P. S. Self-Assembled Discrete Molecules for Sensing Nitroaromatics. Chem. - Eur. J. 2015, 21, 6656−6666. (c) Chen, L.-J.; Ren, Y.-Y.; Wu, N.-W.; Sun, B.; Ma, J.-Q.; Zhang, L.; Tan, H.; Liu, M.; Li, X.; Yang, H.-B. Hierarchical Self-Assembly of Discrete Organoplatinum(II) Metallacycles with Polysaccharide via Electrostatic Interactions and Their Application for Heparin Detection. J. Am. Chem. Soc. 2015, 137, 11725−11735. (d) Zhang, M.; Saha, M. L.; Wang, M.; Zhou, Z.; Song, B.; Lu, C.; Yan, X.; Li, X.; Huang, F.; Yin, S.; Stang, P. J. Multicomponent Platinum(II) Cages with Tunable Emission and Amino Acid Sensing. J. Am. Chem. Soc. 2017, 139, 5067−5074. (15) Jelinek, R. In Carbon Quantum Dots: Synthesis, Properties and Applications; Springer International Publishing: Cham, 2017. (16) Zhu, J.; Sun, S.; Jiang, K.; Wang, Y.; Liu, W.; Lin, H. A highly sensitive and selective fluorimetric probe for intracellular peroxynitrite based on photoinduced electron transfer from ferrocene to carbon dots. Biosens. Bioelectron. 2017, 97, 150−156. (17) Bhowmick, S.; Jana, A.; Singh, K.; Gupta, P.; Gangrade, A.; Mandal, B. B.; Das, N. Coordination-Driven Self-Assembly of Ionic Irregular Hexagonal Metallamacrocycles via an Organometallic Clip and Their Cytotoxicity Potency. Inorg. Chem. 2018, 57, 3615−3625. (18) Bhowmick, S.; Chakraborty, S.; Das, A.; Rajamohanan, P. R.; Das, N. Pyrazine-Based Organometallic Complex: Synthesis, Characterization, and Supramolecular Chemistry. Inorg. Chem. 2015, 54, 2543−2550. (19) Stewart, J. J. P. Optimization of parameters for semiempirical methods V: Modification of NDDO approximations and application to 70 elements. J. Mol. Model. 2007, 13, 1173−1213. (20) Findlay, J. A.; McAdam, C. J.; Sutton, J. J.; Preston, D.; Gordon, K. C.; Crowley, J. D. Metallosupramolecular Architectures Formed

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PtII)-Based Supramolecular Coordination

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