Route to Useful Metallomonomers: Step-Wise Construction of Bimetallic Triangles by Site-Specific Metalation

  • Rajarshi Sarkar
  • Zaihong Guo
  • Tarak Nath Burai
  • Charles N. Moorefield
  • Chrys Wesdemiotis
  • George R. NewkomeEmail author


Three bimetallic triangles were constructed via a step-wise assembly of tailored terpyridine building blocks. Oligomeric trimer 2 was obtained by reacting ligand 1 with Ru(II). Subsequent cyclization of trimer 2 with Zn(II), Cd(II), and Fe(II) resulted in the formation of bimetallic triangles 3, 4, and 5 in high yield, respectively. All triangles were characterized by NMR spectroscopy, ESI and travelling-wave ion mobility-mass spectrometry (MS), as well as gradient MS, which provided insight into their stabilities.


Terpyridine Metallomonomers Terpyridine–metal complex Bimetal Triangle TWIM Zinc Cadmium Iron 

1 Introduction

Assembly of supramolecular constructs via the spontaneous association of tailored organic modules has witnessed significant progress in the last two decades. Pioneering work by Lehn [1, 2, 3] followed by contributions from Stang et al. [4, 5, 6, 7, 8], Fujita and colleagues [9], Newkome et al. [10, 11, 12, 13], Schmittel and colleagues [14, 15], and many others [16, 17, 18, 19, 20] have revealed the significance of metal–ligand coordinative interactions to build complex architectures. As proposed by Lehn, self-assembled supramolecules are generally an equilibrium distribution of all possible constructs possessing comparable stabilities [20, 21]. Thus, most supramolecular assemblies utilize symmetric, homoleptic building blocks to produce a single, discrete molecule. Based on homoleptic ligands, numerous metallomacrocycles such as triangles [22], squares and rectangles [23], pentagons [24], hexagons [25], and other polygons [26] have been synthesized. In contrast, albeit to a lesser extent, mixtures of binary and multiple structures have resulted in heteroleptic assemblies [27]. Consequently, construction of intricate, supramolecular architectures through the combination of two or more monomers still remains challenging. Various reaction conditions have been developed to isolate the thermodynamically most stable species in a quantitative or near quantitative yield from a heteroleptic assembly [15, 28]. These protocols have paved the way for the synthesis of higher-ordered, macromolecular architectures via directed assembly [25, 26, 29, 30].

With respect to both homo- and hetero-leptic utility, terpyridine-based monomers have gained tremendous popularity due, in part, to the synthetic ease necessary to instill pre-designed structural features and their ability to form complexes with different transition metals with predictable stability [31, 32, 33, 34]. For example, we have assembled a molecular “Sierpiński hexagonal gasket” [6, 13, 35, 36, 37], molecular isomeric bow-tie and butterfly structures [28] via combination of step-wise and dynamic assembly protocols, the first, three-metal polygon [38] by modifying ligand structures and taking advantage of kinetic and thermodynamic properties of different \(\langle {\text{tpy}} {-} {\text{M}}^{\text{II}} {-} {\text{tpy}}\rangle\) bonds, as well as 3D architectures that uniquely equilibrate between large and small Archimedean structures [39, 40, 41, 42, 43]. Recently, Wang et al. [44] have reported step-wise synthesis of a terpyridine-based, chair-like, hexameric metallo-macrocycle as well as other interesting complex structures [36, 45, 46]. Herein, we report a general, high-yield, step-wise construction of three basic terpyridine-based bimetallic triangles opening the door to the ready availability of commercial building blocks to precise materials’ nanoarchitectonics [47].

2 Results and Discussion

Initially, the 60°-based bisterpyridine 1 (Scheme 1) was readily prepared [35] from 4,5-dibromo-veratrole and 4′-(4-boronatophenyl)[2,2′:6′,2″]-terpyridine using a Suzuki cross-coupling reaction. Ligand 1 was reacted with Ru(DMSO)4Cl2 [48] to obtain the oligomeric trimer 2. A 3:2 mixture of 1 and Ru(DMSO)4Cl2 in CHCl3:MeOH (1:1 v/v) was refluxed for 24 h. The reaction mixture was cooled to 25 °C then dried in vacuo to give a red residue, which was purified by column chromatography (Al2O3) using satd. KNO3:H2O:MeCN (1:1:35 v/v/v) as eluent to afford (48%) pure trimer 2. Its 1H NMR spectrum (Fig. 1) displayed two very close yet distinct 3′,5′-terpyridine singlets at 9.00 (arm A) and 8.99 (arm B) ppm, respectively, corresponding to two complexed terpyridines as well as another singlet at 8.76 ppm (arm C) corresponding to uncomplexed terpyridine in an exact 1:1:1 ratio. The 1H NMR spectrum also contains three singlets at 4.01, 3.99 and 3.97 ppm corresponding to three different –OCH3 protons, which support the proposed structure. All other aromatic protons were assigned with the help of 2D-COSY NMR data. Additional support for structure 2 was provided by ESI–mass spectrometry (MS; Fig. S16, Supporting Information) via a series of peaks at m/z 1291.9, 840.9, and 615.7 corresponding to 2+, 3+ and 4+ charge states, respectively. Upon obtaining trimer 2, it was subjected to subsequent cyclization using Zn(II), Cd(II), and Fe(II) to obtain the desired bimetallic triangular macrocycles.
Scheme 1

Synthesis of oligomeric trimer 2 and subsequent cyclization to obtain bimetallic triangles 35 from trimer 2

Fig. 1

Stacked 1H NMR spectra (500 MHz) of trimer 2, triangle 4, 3, and 5 (from bottom to the top) in CD3CN

To a 1:1 MeOH/CHCl3 (v/v) solution of trimer 2, a methanolic solution of Zn(NO3)2·6H2O was added in a precise 1:1 ratio. The reaction mixture was stirred at 25 °C for 30 min, then a methanolic solution of NH4PF6 was added to affect a counter-ion exchange to \({\text{PF}}_{6}^{ - } .\) The light orange colored precipitate was filtered and washed repeatedly with MeOH to remove excess NH4PF6 affording (~ 99%) bimetallic macrocycle 3 without any further purification. Triangle 3 was subsequently characterized using NMR spectroscopy and ESI- and travelling-wave ion mobility (TWIM)-MS. Terpyridyl-based spectral asymmetry is evident in 1H NMR data in which (Fig. 1) three distinct 3′,5′-proton signals at 9.05, 9.04, and 9.00 ppm resulting from arms A, B, and C, respectively, with a precise integration ratio of 1:1:1, were observed. The 6,6″-protons (at 7.87 ppm), corresponding to arm C of 3, experience an upfield shift (Δδ = − 0.8 ppm) and 3′,5′-protons (at 9.00 ppm) from arm C experience a downfield shift (Δδ = 0.24 ppm) upon complexation. The NMR data also include signals from three different –OCH3 protons at 4.07, 4.06, and 4.05 ppm with an integration ratio of 1:1:1, supporting the proposed structure. Another notable feature of the 1H NMR data is the three sharp aromatic singlets at 7.30, 7.29, and 7.27 ppm with an exact integration ratio of 1:1:1, which ascribes to aromatic protons g, h, and i, respectively. All other aromatic protons were assigned by using 2D-COSY and 2D-NOESY NMR data.

Further evidence for triangle 3 was obtained from the ESI–MS data (Fig. 2a), which showed a series of dominant peaks at m/z 420.6, 533.4, 703.7, 987.3, and 1553.4 corresponding to charge states from 6+ to 2+ via the loss of a varying number of \({\text{PF}}_{6}^{ - }\) anions. The isotope pattern for each peak fits with the simulated isotope pattern. TWIM-MS data (Fig. 2b) provided additional support for triangle 3 by showing a set of single and narrow bands for charge states 6+ to 3+. This also supports the presence of a single discrete structure.
Fig. 2

a ESI–MS spectrum with simulated and experimental isotope pattern for the 3+ species and b 2D ESI–TWIM-MS plot (mass-to-charge ratio vs. drift time) of bimetallic triangle 3. The charges of intact assemblies are marked

Trimer 2 was cyclized with Cd(NO3)2·4H2O in a similar procedure to obtain (~ 99%) heterometallic triangle 4, also as a light orange precipitate that was filtered and washed repeatedly with MeOH. Triangle 4 was also characterized by 1H NMR data (Fig. 1) showing three distinct singlets for 3′,5′-protons (at 9.00, 8.99, and 8.93 ppm), –OCH3 protons (at 4.03, 4.02, and 4.01 ppm) and aromatic protons g, h, and i (at 7.27, 7.24, and 7.22 ppm) with the expected 1:1:1 integration ratio. The 3′,5′-protons (at 8.93 ppm) and 6,6″-protons (at 8.10 ppm) from arm C showed expected downfield (Δδ = 0.17 ppm) and upfield shift (Δδ = − 0.57 ppm), respectively, upon complexation. All other aromatic protons were assigned with the aid of 2D-COSY and 2D-NOESY NMR data.

The marked differences between the 1H NMR data of triangles 3 and 4 are the signals from 3′,5′- and 6,6″-protons from arm C. The 3′,5′-proton signal from arm C of triangle 3 appears downfield (Δδ = 0.07 ppm) and the 6,6″-protons appear upfield (Δδ = − 0.23 ppm) when compared to triangle 4. In the ESI–MS analysis of complex 4 (Fig. S19, Supporting Information), a series of dominant peaks were generated at m/z 428.6, 543.7, 716.2, 1002.9, and 1577.7 corresponding to charge states from 6+ to 2+ resulting from loss of \({\text{PF}}_{6}^{ - }\) anions. The experimental isotope distribution pattern for each peak fits well with the theoretical isotope distribution pattern. TWIM-MS data (Fig. S20, Supporting Information) of 4 further show a set of narrow bands for charge states 6+ to 3+ supporting the desired assignment.

Using a similar procedure, triangle 5 was obtained by reacting oligomeric trimer 2 with FeCl2·4H2O In this case, after stirring for 4 h at 25 °C, the reaction mixture was dried in vacuo and subjected to column chromatography (SiO2) using a mixed solvent system of satd. KNO3:H2O:MeCN 1:1:30 (v/v/v), as eluent. The pure fraction was dried and washed repeatedly with water to remove excess KNO3 to give the product, which was dissolved in MeOH and excess NH4PF6 was added to change the counterion from \({\text{NO}}_{3}^{ - }\) to \({\text{PF}}_{6}^{ - } .\) The PF6 salt was filtered and washed with a copious amount of MeOH to remove excess NH4PF6. Bimetallic triangle 5 was thereby obtained (72%) as a magenta color solid, whose 1H NMR spectrum (Fig. 1) exhibited a similar characteristic pattern to 3 and 4. Thus, the spectrum for 5 contains signals from three 3′,5′-protons (at 9.24, 9.07, and 9.06 ppm for arms B, A, and C, respectively), three –OCH3 protons (at 4.03, 4.02, and 4.01 ppm) and three phenyl protons g, h, and i (at 7.28, 7.27, and 7.25 ppm) as expected with an anticipated 1:1:1 integration ratio. Both the 3′,5′-protons (at 9.24 ppm) and 6,6″ protons (at 7.18 ppm) from arm C experience a larger upfield shift (Δδ = 0.48) and downfield shift (Δδ = − 1.49 ppm), respectively, upon complexation. All other aromatic protons were assigned by using 2D-COSY and 2D-NOESY NMR data. The ESI–MS spectrum (Fig. 3a) shows a series of peaks at m/z 419.4, 532.3, 701.6, and 983.7 corresponding to the charge states from 6+ to 2+ generated by the loss of \({\text{PF}}_{6}^{ - }\) ions. The experimental isotope pattern for each peak is in complete agreement with the calculated isotope pattern. TWIM-MS data (Fig. 3b) showed a series of single and discrete bands for charge states 6+ to 3+ providing further supportive structural evidence. Gradient MS (gMS2) experiments were performed on triangles 3 - 5 to assess their relative stabilities. The 3 + ion for each triangle (Fig. 4) was exposed to collisionally activated dissociation at increasing collision energy. gMS2 data revealed that complexes 35 have comparable stabilities. All of the triangles are stable up to a nominal collision energy of 30 eV. Macrocycles 3 and 4 start dissociating at ca. 35 eV and completely dissociate at 42 and 45 eV. While compound 5 remains intact up to 35 eV and completely disintegrates at 46 eV. The center-of-mass collision energies calculated from the complete dissociation of 3, 4, and 5 are 1.68 eV, 1.77 eV and 1.85 eV, respectively.
Fig. 3

a ESI–MS spectrum with simulated and experimental isotope pattern for the 3+ species and b 2D ESI–TWIM-MS plot (mass-to-charge ratio vs. drift time) of bimetallic triangle 5. The charges of intact assemblies are marked

Fig. 4

ESI–TWIM-gMS2 plot of the 3+ charge state of bimetallic triangle a 3, b 4, and c 5

Photophysical properties of 25 were studied using steady state absorption spectroscopy and fluorescence spectroscopy. Their absorption spectra are presented in Fig. 5a. Ligand 1 exhibits typical ligand to ligand π–π* charge transfer (CT) bands at 285 and 330 nm localized on terpyridine-phenyl subunits. Complexation with Ru(II) results in a new metal–ligand CT (MLCT) band at 483 nm due to the CT active Ru(II) center; whereas, 5 possesses a second MLCT band at 575 nm arising from the characteristic Fe(II)–tpy absorption band. The emission intensities of bimetallic triangles 35 are lower than the intensities of trimer 2 at the excitation wavelength of 480 nm (Fig. 5b) probably due to the quenching effect of Zn(II), Cd(II), and Fe(II) [49].
Fig. 5

a Normalized UV–visible spectra of 25. b Corrected emission spectra of 25 by at the excitation wavelength λex = 480 nm. All photoluminescence spectra are corrected for fluctuation in absorbance at excitation wavelengths λex = 480 nm

3 Conclusions

We have constructed step-wise three bimetallic triangles utilizing site-specific metalation. Oligomeric trimer 2 was synthesized from ligand 1. Subsequent cyclization of trimer 2 with Zn(II) and Cd(II) gave triangles 3 and 4 in quantitative yields. On the other hand, cyclization of 2 with Fe(II) followed by purification by column chromatography afforded triangle 5 in high yield. All of the triangles were characterized by 1H, COSY, NOESY, and 13C NMR spectroscopy as well as ESI and TWIM-MS. gMS2 experiments provided insight into their stability.



The authors gratefully acknowledge funding from the National Science Foundation (CHE-1151991 GRN and CHE-1308307 C.W.), the Ohio Board of Regents, and the James and Vanita Oelschlager funding via The University of Akron.

Supplementary material

10904_2019_1223_MOESM1_ESM.docx (4 mb)
Experimental procedures and characterization data including COSY, NOESY, and 13C NMR, ESI–MS, tandem mass spectrum, and UV/vis absorption spectrum. Supplementary material 1 (DOCX 4099 kb)


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Copyright information

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Authors and Affiliations

  • Rajarshi Sarkar
    • 1
    • 2
  • Zaihong Guo
    • 2
  • Tarak Nath Burai
    • 2
  • Charles N. Moorefield
    • 2
    • 3
  • Chrys Wesdemiotis
    • 2
    • 4
  • George R. Newkome
    • 2
    • 4
    • 5
    Email author
  1. 1.School of Technology Management & EngineeringNMIMSIndoreIndia
  2. 2.Department of Polymer ScienceThe University of AkronAkronUSA
  3. 3.Reese Technology CenterDendronix LLCLubbockUSA
  4. 4.Department of ChemistryThe University of AkronAkronUSA
  5. 5.Center for Molecular Biology and BiotechnologyFlorida Atlantic UniversityJupiterUSA

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