Second-Coordination-Sphere Assisted Selective Colorimetric Turn-on

Jun 30, 2017 - The metal(II) fluoride complexes [F⊂Co(mBDCA-5t-H3)]2– (3) and [F⊂Zn(mBDCA-5t-H3)]2– (4) were both fully characterized, includi...
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Second-Coordination-Sphere Assisted Selective Colorimetric Turn-on Fluoride Sensing by a Mono-Metallic Co(II) Hexacarboxamide Cryptand Complex Julia M. Stauber,† Glen E. Alliger,† Daniel G. Nocera,*,‡ and Christopher C. Cummins*,† †

Department of Chemistry, Massachusetts Institute of Technology, 77 Massachusetts Ave, Cambridge, Massachusetts 02139, United States ‡ Department of Chemistry and Chemical Biology, Harvard University, 12 Oxford Street, Cambridge, Massachusetts 02138, United States S Supporting Information *

while the second-coordination sphere offers preorganized hydrogen-bond-donating N−H groups. The monometalated cryptand motif was inspired by the work of Borovik et al.19−21 and others22,23 who have achieved modulation of the secondarycoordination sphere through the use of ligands containing Hbond donating moieties. Herein, we present the design, synthesis, and anion binding properties of the colorimetric fluoride sensor [Co(mBDCA-5tH3)]− (1). We also present the preparation, characterization, and fluoride-binding properites of the diamagnetic Zn(II) analogue ([Zn(mBDCA-5t-H3)]−, 2). The corresponding fluoride-bound cobalt and zinc complexes have also been isolated and characterized. Controlled monometalation was achieved through treatment of a thawing DMF slurry of mBDCA-5t-H6 with thawing DMF solutions of KHMDS and Co(OAc)2 or Zn(OAc)2 (Scheme 1). This protocol afforded the potassium salts of the monometalated Co(II) ([K(DMF)3][1]) and Zn(II) ([K(DMF)3][2]) salts as turquoise and colorless powders in 74 and 84% yields, respectively. Although the described procedure resulted in selective monometalation of the cryptand, 1H NMR analysis typically indicated contamination with free mBDCA-5t-H6 impurity (ca. 10%). Purification was achieved by exploiting the anion receptor properties of the free cryptand. Preliminary studies indicated that potassium salts of 1 and 2 were unreactive with respect to Cl− and Br−; however, the free cryptand was shown to bind these anions. The sequestration of Cl− and Br− by the cryptand effectively breaks up the intra- and intermolecular hydrogenbonding networks,11,15 rendering the resulting cryptand encapsulated anions fully soluble in solvents such as THF. Treatment of the potassium salts of 1 and 2 with [TBA]X (X = Cl or Br) in THF enabled separation of the sparingly soluble TBA+ complexes of 1 and 2 from the THF-soluble cryptandencapsulated Br− or Cl− salts.24 Crystals of [K(DMF)3][1] suitable for an X-ray diffraction study were grown, and the solid-state structure is shown in Figure 1. There are only a few reports in the literature of monometallic complexes of binucleating macrocycles,25,26 most of which include lanthanide complexes of phenolate-spaced ligands,27 and

ABSTRACT: The preparation of a selective turn-on colorimetric fluoride sensor was achieved through single cobalt(II) ion insertion into a macrobicyclic cryptand. Monometallic [Co(mBDCA-5t-H3)]− (1) and [Zn(mBDCA-5t-H3)]− (2) complexes were prepared in 74 and 84% yields, respectively. Structural characterization of 1 confirmed the presence of a proximal hydrogen-bonding network consisting of carboxamide N−H donors. The reaction of 1 with F− was accompanied by a distinct colorimetric turn-on response in mixed aqueous/organic media, and 1 was capable of selective fluoride sensing in the presence of large quantities of potentially competitive anions. Complex 1 represents a unique example of a fluoride sensor wherein selective F− binding takes place directly at a transition-metal center and induces a color change based upon metal-centered transitions. The metal(II) fluoride complexes [F⊂Co(mBDCA-5t-H3)]2− (3) and [F⊂Zn(mBDCA-5t-H3)]2− (4) were both fully characterized, including single crystal X-ray analyses. Fluoride binding is synergistic involving hydrogen-bond donors from the second-coordination sphere together with metal(II) ion complexation. The fluoride ion is an attractive target for sensor design due to its critical role in dental care,1 osteoperosis treatment,2 skeletal and dental fluorosis,3 and the safety of drinking water.4 Currently studied fluoride sensors typically depend on molecular interactions between the receptor and fluoride anion including the complexation of fluoride to a main-group Lewis acidic center5,6 or the utilization of compounds containing preorganized N−H or O−H groups designed to provide hydrogen bond stabilization to the anion.7,8 Recently, Bowman-James expanded the field of anion recognition to include tren-based polyamide cryptands evincing high affinity for anions including fluoride.9−13 We targeted selective and colorimetric detection of fluoride through complexation directly to a transition-metal center. This was achieved through installation of a single cobalt(II) ion within one of the tren binding pockets of the hexacarboxamide cryptand mBDCA-5t-H6.14−18 Insertion of a single transition-metal ion into the cryptand provides a Lewis acidic binding site for fluoride, © XXXX American Chemical Society

Received: May 24, 2017

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DOI: 10.1021/acs.inorgchem.7b01335 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry

Scheme 1. General Procedures to Synthesize Mono-metalated Complexes, [M(mBDCA-5t-H3)]− (M = Co(II), 1; Zn(II), 2), and Mono-metalated Fluoride Complexes, [F⊂M(mBDCA-5t-H3)]2− (M = Co(II), 3; Zn(II), 4)

symmetry is preserved on the NMR time scale for the compound in solution at 25 °C. Due to the paramagnetic d7 Co(II) center, a super-WEFT (water eliminated Fourier transform)31 1H NMR spectrum (Figure S3) of [TBA][1] was collected that allowed for observation of all 13 broadened and shifted resonances. The resonance observed at δ 5.10 ppm (Δν1/2 = 50 Hz) is assigned to the protons of the three equivalent tert-butyl groups, and the remaining 12 resonances span a range of −20 to 133 ppm in DMSO-d6. The solid-state FTIR spectra of 1 (Figure S8) and 2 (Figure S13) contain diagnostic amide N−H vibrations that are absent in the spectra of related bimetallic cryptand complexes,17 and are consistent with single-metal-ion coordination and an intact protonated second-coordination sphere. The electronic absorbance spectrum of 1 measured in DMF (Figure S6) is characterized by bands located at λmax 428 and 613 nm.23,32−35 Complex 1 displays an S = 3/2 electronic configuration at 25 °C in DMSO-d6, as determined by the Evans method (μeff = 3.67 μB).36 The X-band EPR spectrum (Figure S7) of a frozen solution of 1 in DMF at 10 K has axial symmetry with effective g values of g⊥ = 4.21 and g∥ = 2.03. A characteristic, eight-line 59Co hyperfine pattern is observed at gz according to an Az component of the hyperfine coupling tensor of 80 × 10−4 cm−1. These data confirm an S = 3/2 ground state and are in line with features observed for similar trigonal monopyramidal cobalt(II) complexes.32,33,37 When measured between 1.3 and −1.6 V vs Fc/Fc+, the cyclic voltammogram of complex 1 (Figure S37) displayed an irreversible process located at Epa = 329 mV attributed to the CoII/CoIII couple.37 Initial fluoride binding studies were carried out through treatment of a pyridine solution of [TBA][1] with 1 equiv [TBA][F]·3H2O at 25 °C (Scheme 1). This reaction resulted in a rapid color change from blue to purple with clean formation of a new paramagnetic species after workup as determined by 1H NMR spectroscopy. The product of this reaction, [TBA]2[F⊂Co(mBDCA-5t-H3)] ([TBA]2[3]), is characterized by 13 paramagnetically broadened and shifted resonances in its super-WEFT 1H NMR spectrum (Figure S24), a m/z value of 462.68 (462.71, calcd, Figure S21), and a λmax of 572 nm (Figure S22). Purple single crystals of sufficient quality for an X-ray diffraction study were grown, and the solid-state structure is shown in Figure 2. The cobalt(II) ion in 3 resides in a tetrahedral coordination geometry with stabilization of the axial F− ligand by six hydrogen bonds provided by the three carboxamide N−H moieties of the secondary-coordination sphere (avg. d(F−N) 2.888(6) Å) and the three internally directed aryl C−H groups (avg. d(F−C) 2.961(3) Å) of the phenylene spacers. Preparation of the analogous zinc(II) fluoride complex, [F⊂Zn(mBDCA-5t-H3)]2− (4), proceeded in a similar fashion through treatment of a pyridine solution of [TBA][2] with [TBA][F]·3H2O (Scheme 1). Isolation of this diamagnetic

Figure 1. Solid-state molecular structure of [Co(mBDCA-5t-H3)]− (1) with thermal ellipsoids rendered at the 50% probability level with PLATON.30 DMF solvent molecules, K+ counterion, and selected H atoms are omitted for clarity. τ4 = 0.86.

octahedral copper(II) complexes of a hexa-imino ligand.28 In contrast, the solid-state structure of 1 features the Co(II) center coordinated in a trigonal monopyramidal geometry29 within one of the tren binding pockets. Additionally, Figure 1 establishes the presence of the vacant second-coordination sphere provided by the metal-free half of the cryptand. Within the unmetalated half, one N−H group is engaged in an intramolecular H-bonding interaction (N302−O102 2.926(5) Å) that is similar to the two observed in the structure of the free cryptand.17 A second N−H group is externally directed to form an intermolecular H-bond with a carbonyl oxygen of a neighboring 1 molecule (N102− O701 2.834(4) Å). The intermolecular H-bonding interactions give rise to an extended network of cryptands similar to the infinite two-dimensional sheets observed for the free anion receptor.15,17 These interactions serve as the basis for the solubility differences between the monometalated complexes and the cryptand-encapsulated anions15 that enable the described purification procedure (vide supra). The identities of 1 and 2 have been confirmed by ESI-MS(−), each with m/z values corresponding to the monoanionic species (1: 906.42; calcd, 906.42, Figure S5. 2: 911.40; calc’d, 911.42, Figure S12). The 1H NMR spectrum of complex 2 (Figure S10) features a singlet located at δ 1.31 ppm (27H) assigned to the tert-butyl protons, and three resonances each with integrations of 3H are observed for the three distinct protons located on the phenylene spacers. These data indicate that an effective C3 axis of B

DOI: 10.1021/acs.inorgchem.7b01335 Inorg. Chem. XXXX, XXX, XXX−XXX

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even observed in the presence of >1000 equiv Cl−, Br−, and I−. The anions investigated for interference in this system were chosen to reflect species that are commonly examined when assessing the performance of fluoride sensors.38 Complex 1 was also capable of selective F− detection upon the addition of an aqueous solution of F−, Cl−, Br−, and I− (82:18 DMF/H2O, v/v). The detection of F− by 1 in the presence of water is noteworthy considering that fluoride has the highest hydration enthalpy of all water stable anions (ΔH° = −504 kJ/ mol),39 consequently rendering its detection in the presence of water extremely difficult.5,40 The instability and low solubility of 1 in neat water prevented further investigation of fluoride sensing in neat H2O. Due to the basic character of F−, efforts were made to rule out the possible involvement of OH− ions that could form during sensing experiments carried out in the presence of H2O. The putative cobalt(II) hydroxide species, [OH⊂Co(mBDCA-5tH3)]2− (5), was generated in situ through treatment of DMF and DMSO solutions of 1 with excess CsOH (section S12). The UV−vis properties of 3 and 5 resemble each other, as would be expected considering the similar nature of F− and OH− as axial anionic ligands. The 1H NMR spectrum of 5, however, displays a tert-butyl resonance at δ −0.49 ppm, differing from that of 3 by 1.96 ppm. Furthermore, the remaining 12 resonances observed in the super-WEFT 1H NMR spectrum of 5 are at markedly different chemical shifts from those of 3. Distinct m/z values were also observed by ESI-MS(−) for 5 (461.71) when compared with a pure sample of 3 (462.68). Upon close inspection, no traces of the spectroscopic features characteristic of 5 were present in ESI-MS(−) and 1H NMR data recorded for pure 3, further substantiating the difference between these two species and the lack of involvement of OH− during F− sensing. The binding constants (Ka) of fluoride with 1 were calculated in pyridine, MeCN, DMF, and DMSO by following UV−vis titration experiments and monitoring the growth of the absorption located at 572 nm. The Ka values span a range of 1.3 × 103 to 2.8 × 102 M−1 and demonstrate a significant solvent dependence according to the following trend: pyridine > MeCN > DMF > DMSO. These values are within the range reported for other fluoride sensors in the literature measured in organic solvents or mixed aqueous/organic media and indicate a modest affinity of 1 for the fluoride ion.6,8,24,39,41 The reversibility of F− binding to 1 was assessed through treatment of 3 with the free mBDCA-5t-H6 cryptand (1 equiv). Generation of the cryptand encapsulated fluoride anion, [F⊂(mBDCA-5t-H6)]−,15 was indeed observed by 19F NMR spectroscopy (section S9), therefore indicating F− binding to 1 is reversible. We note that B(C6F5)3 also abstracts F− from 3 with conversion back to 1 and generation of [FB(C6F5)3]− (section S11).42 The reversibility of F− binding to 1 may be due in part to an elongated and weakened Co−F bond resulting from the extensive H-bonding contributions of the second-coordination sphere. However, scant structural data are available for tetrahedral cobalt(II) fluoride complexes with which to compare the Co−F distance in 3 of 2.0554(18) Å.43 In order to assess the sensitivity of sensor 1 to F−, UV−vis experiments were conducted to determine the fluoride limit of detection (LOD). Although it is not suitable for fluoride detection in neat water, complex 1 displays high sensitivity to F−, with a LOD of 2−5 ppm in pyridine solution at 25 °C (section S6), and would provide adequate fluoride detection in mixed aqueous/organic media.8,44

Figure 2. Solid-state molecular structure of [F⊂Co(mBDCA-5t-H3)]2− (3) with thermal ellipsoids rendered with PLATON30 at the 50% probability level. TBA+ counter cations, disorder, and non-hydrogenbonding H atoms are removed for clarity. τ4 = 0.91.

analogue allowed for observation of the 19F NMR resonance (Figure S28). The 19F NMR spectrum displays a higher order multiplet pattern resulting from F− coupling with the carboxamide N−H and aryl C−H protons and is consistent with its 1H NMR spectrum that features the corresponding doublets centered at δ 10.06 (1JH−F = 26 Hz), and 9.94 (1JH−F = 15 Hz) ppm, respectively. Similar features have been observed by 1 H and 19F NMR spectroscopy for related examples of cryptandencapsulated fluoride anions.12,15 There is a distinct and immediate color change associated with the coordination of fluoride to 1. Accordingly, the fluoride binding properties of 1 were studied using absorption spectroscopy. The sensing ability of 1 was tested in DMF solution as shown in Figure 3. Addition of the TBA+ salts of Cl−, Br−, I−,

Figure 3. Top: images of DMF solutions of 1 treated with 3 equiv of various anions. Bottom: UV−vis spectra of [TBA][1] solutions (2.5 mM, DMF, 25 °C) treated with the listed anions.

HPO42−, OAc−, NO2−, NO3−, and BH4− to a solution of 1 did not result in any visually detectable changes in the color of the solution, nor were any substantial differences in the absorption spectrum observed. In stark contrast, when [TBA][F]·3H2O was added to this mixture, pronounced absorption changes were observed that matched signatures of the pure complex 3 (λmax 572 nm), therefore indicating that 1 has excellent selectivity for fluoride in the presence of excess (ca. 3 equiv) amounts of potentially competitive anions. Detection of fluoride by 1 was C

DOI: 10.1021/acs.inorgchem.7b01335 Inorg. Chem. XXXX, XXX, XXX−XXX

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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.7b01335. Experimental details and characterization data (PDF) Accession Codes

CCDC 1524850−1524854 contain 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]. *E-mail: [email protected] ORCID

Julia M. Stauber: 0000-0001-9783-907X Daniel G. Nocera: 0000-0001-5055-320X Christopher C. Cummins: 0000-0003-2568-3269 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by NSF under the NSF Center CHE1305124. Dr. Nazario Lopez is thanked for assistance with synthetic procedures, and Professor François Gabbai ̈ and Dr. Shiyu Zhang are thanked for helpful discussions. Dr. Jonathan Becker is thanked for assistance with X-ray crystallography.

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REFERENCES

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DOI: 10.1021/acs.inorgchem.7b01335 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry

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DOI: 10.1021/acs.inorgchem.7b01335 Inorg. Chem. XXXX, XXX, XXX−XXX