Cyclic versus Polymeric Supramolecular Architectures in Metal

Article pubs.acs.org/crystal

Cyclic versus Polymeric Supramolecular Architectures in Metal Complexes of Dinucleating Ligands: Silver(I) Trifluoromethanesulfonate Complexes of the Isomers of Bis(di(1Hpyrazolyl)methyl)-1,1′-biphenyl. James R. Gardinier,* Heidi M. Tatlock, Jeewantha S. Hewage, and Sergey V. Lindeman Department of Chemistry, Marquette University, Milwaukee, Wisconsin 53201-1881, United States S Supporting Information *

ABSTRACT: In the search for new examples of systems that selfassemble into cyclic metal−organic architectures, the six isomers of X,Y′-bis(di(1H-pyrazolyl)methane)-1,1′-biphenyl, LXY, and their silver(I) trifluoromethanesulfonate complexes were prepared. Five of the six silver complexes gave crystals suitable for single crystal X-ray diffraction, with only the microcrystalline derivative of 2,3′-bis(di(1H-pyrazolyl)methane)-1,1′-biphenyl, L23, proving to be unsuitable for this analysis. Of the structurally characterized silver(I) complexes, that with L22 showed an unusual trans-spanning chelating coordination mode to silver. At the same time the ligand was also bound to a second silver center giving rise to a cyclic supramolecular isomer with a 22-membered metallacycle. The complex of L34 also gave a cyclic dication but with a remarkable 28-membered metallacycle ring. The remaining three derivatives were polymeric. The results of this study underscore that a 120° angle between dipyrazolylmethyl moieties across aromatic spacers will give rise to a cyclic dication but this is not an exclusive requirement for the formation of cyclic architectures. Also, the supramolecular structures of complexes are assembled via a variety of noncovalent interactions involving the di(pyrazolyl)methyl cation most notably by weak hydrogen bonding interactions involving the methine hydrogen and an oxygen atom of the triflate anion.

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INTRODUCTION

Chart 1. Bis[di(pyrazolyl)methyl]organyls

The discovery of new tectons that can give rise to reproducible self-assembled structures is important for the future development of functional materials.1 An emerging class of tecton for the fabrication and study of supramolecular architectures is based on variants of the di(pyrazolyl)methyl moiety, Figure 1.2−5 This

versatile group can bind metal cations via the nitrogen atoms of the heterocycles while the acidic methine hydrogen or pyrazolyl ring hydrogen atoms provide a reliable platform for directed selfReceived: March 28, 2013 Revised: July 1, 2013

Figure 1. Ditopic di(pyrazolyl)methyl tecton. © XXXX American Chemical Society

A

dx.doi.org/10.1021/cg400466k | Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

Article

Chart 2. X,Y′-Bis[di(pyrazol-1-yl)methyl]-1,1′-biphenyl ligands, LXY, Used in This Study

dications or coordination polymers depending on ligand substitution patterns or on anions. For instance, the silver(I) complexes of a series of alkylidine linked derivatives of the type (pz2CH)2(μ-CH2)n where n = 1−3, gave cyclic [Ag2(μ-L)2]2+ dications when the anion was BF4− or O3SCF3− (= triflate or OTf) but gave oligomeric or polymeric species when the anion was nitrate.3 For phenyl-linked derivatives, pz4m-xyl gave cyclic dications whereas the paraderivative pz4p-xyl gave polymeric 1:1 complexes with silver(I) triflate.4 The cyclic [Ag2(μ-L)2]2+ dication architecture is retained when the m-phenyl linker is replaced by a m-pyridyl, as in pz4lut.5 On the other hand, the 1:1 silver(I) triflate complex of pzDIP4lut has a polymeric structure, presumably as a result of unfavorable steric interactions that would occur between the central pyridyl rings and isopropyl groups in a hypothetical cyclic architecture.5 Since large cyclic structures may prove beneficial for the discovery of new porous materials or might be useful as secondary building units in future hybrid materials, we became interested in determining the geometric limits under which cyclic structures could be formed from silver(I) salts and ligands that have two di(pyrazolyl)methyl groups connected via different poly aryl spacers. Herein we report on the synthesis of six isomers of the new ditopic ligand X,Y′-bis[di(1H-pyrazolyl)methyl]-1,1′biphenyl, LXY, (Chart 2) and their complexes with silver(I) trifluoromethanesulfonate. This particular silver salt was chosen because of the typically good crystallinity exhibited by its complexes, and because it was previously shown to give selectivity for cyclic architectures over polymeric ones in related

assembly via CH···X weak hydrogen bonding interactions. Furthermore, in metal complexes of di(pyrazolyl)methylorganyls the supramolecular structures are also often assembled by a set of cooperative π···π and CH···π interactions, a supramolecular synthon termed the “pyrazolyl embrace”.3b Thus, the ready availability of innumerable variants substituted at the methine carbon (at R of Figure 1) or at the pyrazolyls render this class of tecton especially promising for fecund discoveries in crystal engineering.2 Recently, it was found that silver(I) complexes of ligands with at least two di(pyrazolyl)methyl units connected via various organic spacers (Chart 1) can give rise to either cyclic dimeric

Table 1. Crystallographic Data Collection and Structure Refinement for 1, 3·0.5 toluene, 4·0.5 CH3CN, 5·CH3CN·0.25Et2O, and 6· 1.39 CH3CN formula formula weight crystal system space group temperature [K] a [Å] b [Å] c [Å] α [deg] β [deg] γ [deg] V [Å3] Z Dcalcd [g cm−3] λ [Å] (Cu or Mo Kα) μ.[mm−1] abs. correction F(000) θ range [deg] reflns collected independent reflns T_min/max data/restr./params goodness-of-fit on F2 R1a/wR2b [I > 2σ(I)] R1/wR2 (all data) largest diff. peak/hole/e Å−3 a

1

3·0.5toluene

4·0.5CH3CN

5·CH3CN·0.25Et2O

6·1.39CH3CN

C54H44Ag2F6N16O6S2 1406.91 orthorhombic Pbca 100.0(1) 16.2510(6) 18.0786(7) 18.6411(6) 90.00 90.00 90.00 5476.6(3) 4 1.706 0.7107 0.880 numerical 2832 3.38−29.63 31764 6858 (RInt = 0.0341) 0.853/0.935 6858/0/388 1.044 0.0265/0.0656 0.0304/0.0688 0.660/−0.567

C30.5H26AgF3N8O3S 749.52 monoclinic C2/c 100.0(1) 33.001(2) 11.5470(4) 19.8795(13) 90.00 124.041(9) 90.00 6277.1(6) 8 1.586 1.54178 6.337 multiscan 3032 4.16−73.94 12219 12248 (RInt = 0.0174) 0.51290/1.00000 12248/0/423 1.018 0.0649/0.1758 0.0709/0.1852 2.093/−2.077

C28H23.5AgF3N8.5O3S 723.98 triclinic P1̅ 100.0(1) 8.3100(3) 13.3694(5) 13.6725(6) 76.094(4) 82.749(3) 81.674(3) 1452.32(10) 2 1.656 0.7107 0.833 numerical 730 3.41−29.56 32809 7444 (RInt = 0.0405) 0.828/0.965 7444/0/401 1.079 0.0304/0.0611 0.0392/0.0664 0.442/−0.482

C60H55Ag2F6N18O6.5S2 1526.08 tetragonal P41212 100.0(1) 14.34228(16) 14.34228(16) 32.9099(6) 90.00 90.00 90.00 6769.60(17) 4 1.497 1.54178 5.900 numerical 3092 3.36−73.97 32806 6769 (RInt = 0.0371) 0.156/0.381 6769/57/464 1.065 0.0526/0.1431 0.0560/0.1471 0.733/−0.664

C29.79H26.18AgF3N9.39O3S 760.70 triclinic P1̅ 100.0(1) 14.7117(5) 15.2436(5) 15.7380(6) 87.494(3) 78.842(3) 66.093(3) 3163.23(19) 4 1.597 0.7107 0.770 numerical 1539 3.38−29.64 71399 16183 (RInt = 0.0531) 0.795/0.969 16183/95/911 1.212 0.0705/0.1620 0.0828/0.1671 2.944/−1.139

R1 = Σ∥Fo| − |Fc∥/Σ|Fo|. bwR2 = [Σw(|Fo| − |Fc|)2/Σw|Fo|2]1/2. B



Article

whereupon a colorless precipitate of NaCl forms. Again, the flask originally containing the SOCl2 solution was washed with an additional 10 mL portion of THF to ensure quantitative transfer to the reaction mixture. The suspension of NaCl and S(O)pz2 is allowed to stir 30 min at room temperature, then (10−20 mol %) CoCl2 is added as a powder under a nitrogen blanket. After the blue suspension had been stirred 5 min, the appropriate solid dialdehyde, X,Y′-biphda (1 equiv), was added under a nitrogen blanket. A condenser connected to an oil bubbler (to monitor SO2 evolution) was attached and the reaction mixture was heated at reflux for 15 h (with the exception of L22, which was heated 48 h to maximize yield). Then, after the product mixture had cooled to room temperature, 100 mL of distilled water is added with stirring. The THF soluble portion is separated from the aqueous fraction. The aqueous fraction is extracted with two 50 mL portions ethyl acetate and one 50 mL portion diethyl ether. The combined organic fractions were dried over MgSO4, solvent was removed and the residue was subject to further purification by column chromatography on silica gel. Elution with 2:1 ethyl acetate:hexane afforded the desired compound in the band with an Rf in the range of 0.4−0.7, as indicated below. The quantities of reagents used in a representative synthesis, yields of product obtained, and characterization data along with any additional notes are given for each derivative below. L22. A mixture of 0.377 g (15.7 mmol) of NaH, 1.07 g (15.7 mmol) of pyrazole, 0.57 mL (0.93 g, 1.638 g/mL, 7.9 mmol) of SOCl2, 0.034 g (0.26 mmol) of CoCl2, and 0.550 g (2.62 mmol) of 2,2′-biphda was heated 48 h to afford 0.670 g (57%) of the analytically pure compound as a colorless solid. Rf (2:1 ethyl acetate/hexanes, SiO2): 0.41. mp: 167− 168 °C. Anal. Calcd (obsd) for C26H22N8: C, 69.94 (70.00); H, 4.97 (4.80); N, 25.10 (24.85). 1H NMR (CD3CN): δH 7.60 (d, J = 2 Hz, 2 H, H3pz), 7.48 (d, J = 3 Hz, 2 H, H5pz), 7.46 (d, J = 2 Hz, 2 H, H3pz), 7.44 (s, 2 H, CmethH), 7.43 (td, J = 7.7, 1.3 Hz, 2 H). 7.31 (d, J = 3 Hz, 2 H, H5pz), 7.29−7.25 (br m, 4 H), 6.65 (d, J = 7.2 Hz, 2 H), 6.33 (dd, J = 3, 2 Hz, 2 H, H4pz), 6.26 (dd, J = 3, 2 Hz, 2 H, H4pz). 13C NMR (CD3CN): δC 141.38, 141.36, 138.9, 134.8, 131.3, 130.9, 130.6, 130.2, 129.5, 128.6, 107.2, 107.1, 75.9. Colorless crystalline square plates can be obtained by cooling hot EtOH supersaturated solution (solubility at room temperature ∼0.04 M) to room temperature overnight. L23. A mixture of 0.685 g (28.5 mmol) of NaH, 1.94 g (28.5 mmol) of pyrazole, 1.04 mL (1.70 g, 1.638 g/mL, 14.3 mmol) of SOCl2, 0.0618 g (0.476 mmol) of CoCl2, and 1.00 g (4.76 mmol) of 2,3′-biphda afforded 2.01 g (95%) of the analytically pure compound as a very viscous, colorless gum. Rf (2:1 ethyl acetate/hexanes, SiO2): 0.50. Anal. Calcd (obsd) for C26H22N8: C, 69.94 (70.06); H, 4.97 (5.09); N, 25.10 (25.22). 1H NMR (CD3CN, 400 MHz): δH 7.75 (s, 1 H, CmethH), 7.63 (d, J = 2.4 Hz, 2 H, H5pz), 7.61 (s, 1H, CmethH), 7.57 (d, J = 1.7 Hz, 2 H, H3pz), 7.49 (d, J = 1.7 Hz, 2 H, H3pz), 7.45 (dd, J = 7.5, 1.4 Hz, 1 H), 7.40 (dd, J = 7.8, 1.5 Hz, 1 H), 7.37 (t, J = 6.7 Hz, 1 H), 7.31 (d, J = 2.4 Hz, 2 H, H5pz), 7.23 (dd, J = 7.5, 1.5 Hz, 1 H), 7.10 (d, J = 7.7 Hz, 1 H), 7.06 (d, J = 7.9 Hz, 1 H), 6.95 (d, J = 7.8 Hz, 1 H), 6.71 (s, 1 H), 6.34 (dd, J = 2.4, 1.7 Hz, 2 H, H4pz), 6.25 (dd, J = 2.4, 1.7 Hz, 2 H, H4pz). 13C NMR (CD3CN): δC 141.9, 141.35, 141.26, 140.8, 137.7, 134.9, 131.2, 131.1, 130.7, 130.3, 130.2, 129.8, 129.1, 128.3, 128.2, 127.3, 107.30, 107.26, 77.9, 76.0. L24. A mixture of 0.766 g (31.9 mmol) of NaH, 2.174 g (31.9 mmol) of pyrazole, 1.16 mL (1.90 g, 1.638 g/mL, 16.0 mmol) of SOCl2, 0.207 g (1.59 mmol) of CoCl2, and 1.119 g (5.32 mmol) of 2,4′-biphda afforded 2.18 g (92%) of the analytically pure compound as a colorless solid. Rf (2:1 ethyl acetate: hexanes, SiO2): 0.41. mp: 167−169 °C. Anal. Calcd (obsd) for C26H22N8: C, 69.94 (70.23); H, 4.97 (4.74); N, 25.10 (24.79). 1H NMR (CD3CN, 300 MHz) δH 7.84 (s, 1H, CmethH), 7.65 (d, J = 2 Hz, 2 H, H5pz), 7.64 (s, 1 H), 7.60 (d, J = 1 Hz, 2 H, H3pz), 7.49 (d, J = 1 Hz, 2 H, H3pz), 7.48 (t, J = 7.3 Hz, 1 H), 7.41 (t, J = 7.6 Hz, 1 H), 7.36 (d, J = 2 Hz, 2 H, H5pz), 7.30 (d, J = 7.3 Hz, 1 H), 7.05 (AB m, 4 H), 6.99 (d, J = 7.6 Hz, 1 H), 6.37 (dd, J = 2, 1 Hz, 2 H, H4pz), 6.27 (dd, J = 2, 1 Hz, 2 H, H4pz). 13C NMR (CDCl3, 75.5 MHz) δC 141.0, 140.9, 140.5, 135.7, 133.6, 130.5, 129.9, 129.6, 129.5, 129.2, 129.1, 128.4, 127.2, 127.1, 106.8, 106.6, 77.6, 75.5. L33. A mixture of 0.715 g (29.8 mmol) of NaH, 2.03 g (29.8 mmol) of pyrazole, 1.08 mL (1.77 g, 1.638 g/mL, 14.9 mmol) of SOCl2, 0.0644 g (0.496 mmol) of CoCl2, and 1.04 g (1.00 mmol) of 3,3′-biphda afforded

ligands; the effect of changing anions will be reported elsewhere. The remarkable structures obtained by single crystal X-ray diffraction studies will be discussed including the common noncovalent interactions responsible for organizing the supramolecular structures.

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EXPERIMENTAL SECTION

General Considerations. The known symmetric compounds [1,1′biphenyl]-2,2′-dicarboxaldehyde,6a,b 2,2′-biphda, [1,1′-biphenyl]-3,3′dicarboxaldehyde,6c 3,3′-biphda, and [1,1′-biphenyl]-4,4′-dicarboxaldehyde,6d 4,4′-biphda, were prepared by the Pd(acetate)2-catalyzed homocoupling6a of the appropriate potassium formylphenyltrifluoroborate,7 according to literature procedures. The known compounds [1,1′-biphenyl]-2,4′-dicarboxaldehyde,8a,b 2,4′-biphda, and [1,1′-biphenyl]-3,4′-dicarboxaldehyde,6b 3,4′-biphda, were prepared by known Suzuki coupling reactions. The compound [1,1′-biphenyl]2,3′-dicarboxaldehyde, 2,3′-biphda, is new and its preparation is described below. Pd(PPh3)4 was prepared according to a literature procedure.9 All other chemicals were commercially available and were used as received. Solvents were dried by conventional methods and distilled prior to use. The syntheses of the silver complexes were carried out under a nitrogen atmosphere using standard Schlenk techniques and in foil-covered apparatus to protect AgOTf from light. After complex formation, no special precautions to avoid light or air were taken. Midwest MicroLab, LLC, Indianapolis, Indiana 45250, performed all elemental analyses. IR spectra were recorded for samples as KBr pellets in the 4000−500 cm−1 region on a Nicolet Magna-IR 560 spectrometer. 1 H and 13C NMR spectra were recorded on a Varian 400 MHz spectrometer. Chemical shifts were referenced to solvent resonances at δH 7.26 and δC 77.16 for CDCl3, δH 1.94 and δC 118.26 for CD3CN. Melting point determinations were made on samples contained in glass capillaries using an Electrothermal 9100 apparatus and are uncorrected. Mass spectrometric measurements recorded in ESI(+) mode were obtained on a Micromass Q-TOF spectrometer whereas those performed by using direct-probe analyses were made on a VG 70S instrument. For the ESI(+) experiments formic acid (approximately 0.1% v/v) was added to the mobile phase (CH3CN). Syntheses. [1,1′-Biphenyl]-2,3′-dicarboxaldehyde, 2,3′-biphda. A mixture of 2.73 g (18.2 mmol) 3-formylphenylboronic acid, 2.81 g (15.2 mmol) 2-bromobenzaldehyde, 0.877 g (0.759 mmol) Pd(PPh3)4, 50 mL degassed (N2 purge) DMF, and 50 mL degassed (N2 purge) 2 M aqueous K2CO3 was heated at 100 °C for 14 h. After cooling to room temperature, 50 mL each water and ethyl acetate was added. The organic and aqueous layers were separated. The aqueous layer was extracted to two more 50 mL portions ethyl acetate and one 50 mL portion diethyl ether. The combined organic layers were washed with three 50 mL portions water (to remove residual DMF). The organic portion was dried over MgSO4, filtered, and solvent was removed by vacuum distillation to leave brown oil. The brown oil was subjected to column chromatography on SiO2, using 4:1 hexanes:ethyl acetate as the eluent. Unreacted 2-bromobenzaldehyde elutes first (Rf = 0.65) followed by the desired product (Rf = 0.4). After removal of the solvent from the second band, the pure product was obtained as a pale yellow oil that very slowly crystallized on standing (or more quickly with the aid of scratching against the glass container). Yield: 1.53 g (48%). mp: 54−56 °C. Anal. Calcd (obsd) for C14H10O2: C, 79.98 (80.17); H, 4.79 (4.93). IR (KBr) ν(CO): 1693 cm−1. 1H NMR (CDCl3, 400 MHz) δH 10.10 (s, 1 H, CHO), 9.97 (s, 1 H, CHO), 8.05 (d, J = 8.2 Hz, 1 H), 7.96 (dm, Japp = 7.2, 1.5 Hz, 1 H), 7.92 (s, 1H), 7.71−7.64 (m, 3H), 7.56 (d, J = 7.5 Hz, 1H), 7.46 (t, J = 7.8 Hz, 1H). 13C NMR (CDCl3) δC 193.4, 192.6, 144.9, 139.8, 137.5, 136.6, 134.8, 134.6, 132.0, 131.8, 130.1, 129.7, 129.4, 128.9. General Procedure for the Preparation of Ligands LXY (X, Y = 2−4). This procedure is performed under N2 to prevent inadvertent hydrolysis. A solution of (6 equiv) pyrazole in 30 mL of THF is added via cannula to a suspension of (6 equiv) NaH in 30 mL of THF at a rate slow enough to control hydrogen evolution. The flask originally containing pyrazole is washed with an additional 10 mL portion of THF to ensure quantitative transfer. Next, a solution of (3 equiv) S(O)Cl2 in 30 mL THF is added via cannula to the colorless solution of Na(pz) C



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1.98 g (89%) of the analytically pure compound as a colorless solid. Rf (2:1 ethyl acetate/hexanes, SiO2), 0.55. mp: 180−182 °C. Anal. Calcd (obsd) for C26H22N8: C, 69.94 (69.92); H, 4.97 (4.91); N, 25.10 (25.22). 1H NMR (CD3CN, 400 MHz): δH 7.84 (s, 2H, CmethH), 7.68 (d, J = 2.5 Hz, 4 H, H5pz), 7.59 (d, J = 1.5 Hz, 4 H, H3pz), 7.58 (d, J = 7.8 Hz, 2 H): 7.46 (t, J = 7.8 Hz, 2 H), 7.14 (s, 2 H), 7.05 (d, J = 7.8 Hz, 2 H), 6.36 (dd, J = 2.5, 1.5 Hz, 4 H, H4pz). 13C NMR (CD3CN): δC 141.4, 138.6, 131.1, 130.3, 128.5, 127.3, 126.5, 118.0, 107.4, 78.0. L34. A mixture of 1.20 g (50.0 mmol) of NaH, 3.38 g (49.6 mmol) of pyrazole, 1.80 mL (2.95 g, 1.638 g/mL, 24.8 mmol) of SOCl2, 0.208 g (1.60 mmol) of CoCl2, and 1.73 g (8.21 mmol) of 3,4′-biphda afforded 3.26 g (88%) of the analytically pure compound as a colorless solid. Rf (2:1 ethyl acetate/hexanes, SiO2): 0.70. mp: 131−132 °C. Anal. Calcd (obsd) for C26H22N8: C, 69.94 (70.17); H, 4.97 (5.11); N, 25.10 (25.00). 1H NMR (CD3CN, 300 MHz): δH 7.87 (s, 1 H, CmethH), 7.85 (s, 1 H, CmethH), 7.69 (d, J = 3 Hz, 2 H, H5pz), 7.68 (d, J = 2.5 Hz, 2 H, H5pz), 7.67 (d, J = 7.4 Hz, 1 H), 7.59 (overlapping d, J = 1 Hz each, 4 H, H3pz), 7.55 (part of AB, Japp = 8.5 Hz, 2 H), 7.48 (t, J = 7.8 Hz, 2 H), 7.29 (s, 1 H) 7.12 (part of AB, Japp = 8.5 Hz, 2 H), 7.08 (d, J = 7.8 Hz, 1 H), 6.36 (dd, J = 2, 1 Hz, 2 H, H4pz), 6.35 (dd, J = 2, 1 Hz, 2 H, H4pz). 13C NMR (CD3CN, 75.5 MHz): δC 141.7, 141.31, 141.28, 138.5, 137.2, 131.01, 130.95, 130.3, 128.62, 128.61, 128.1, 127.4, 126.7, 107.3, 78.0, 77.8 (Note: Number of peaks differs from expected because of coincidence, exemplified by the one resonance for C4pz at 107.3 ppm; two were expected). L44. A mixture of 0.277 g (11.5 mmol) of NaH, 0.785 g (11.5 mmol) of pyrazole, 0.42 mL (0.69 g, 1.638 g/mL, 5.8 mmol) of SOCl2, 0.0749 g (0.577 mmol) of CoCl2, and 0.404 g (1.92 mmol) of 4,4′-biphda afforded 0.655 g (76%) of the analytically pure compound as a colorless solid. Rf (2:1 ethyl acetate: hexanes, SiO2): 0.70. mp: 202−204 °C. Anal. Calcd (obsd) for C26H22N8: C, 69.94 (69.66); H, 4.97 (4.61); N, 25.10 (25.25). 1H NMR (CD3CN, 300 MHz): δH 7.86 (s, 2 H, CmethH), 7.69 (d, J = 2.4 Hz, 4 H, H5pz), 7.66 (part of AB, Japp = 8.4 Hz, 4 H), 7.59 (d, J = 1.5 Hz, 4 H, H3pz), 7.14 (part of AB, Japp = 8.4 Hz, 4 H), 6.36 (dd, J = 2.4, 1.5 Hz, 4 H, H4pz). 13C NMR (CDCl3, 75.5 MHz) δC 141.3, 141.0, 135.7, 129.9, 127.7, 127.6, 106.8, 77.6. Ag(L22)(OTf), 1. A solution of 0.350 g (0.784 mmol) of L22 in 5 mL of THF was added to a solution of 0.201 g (0.784 mmol) of AgOTf in 5 mL of THF and copious precipitate formed immediately. To ensure quantitative transfer of reagents, an additional 5 mL portion of THF was added to the flask originally containing L22 and the washings were added to the product mixture. After the suspension had been stirred 1 h, the solid was collected by filtration, washed with two 5 mL portions of Et2O and dried under vacuum to give 0.480 g (87%) of 1 as a colorless solid. mp: 225 °C (decomp). Anal. Calcd (obsd) for C27H22AgF3N8O3S: C, 46.10 (46.42); H, 3.15 (3.23); N, 15.93 (15.90). 1H NMR (CD3CN, 300 MHz): δH 7.70 (d, J = 2 Hz, 2 H, H5pz), 7.61 (d, J = 1.6 Hz, 2 H, H5pz), 7.53 (s, 2 H, CmethH), 7.49 (t, J = 7.8 Hz, 2 H), 7.39 (overlapping d, J = 1 Hz, 4 H, H3pz), 7.28 (t, J = 7.6 Hz, 2H), 7.12 (d, J = 7.9 Hz, 2 H), 6.49 (dd, J = 1.6, 1 Hz, 2 H, H4pz), 6.44 (d, J = 7.6 Hz, 2H), 6.39 (dd, J = 2, 1 Hz, 2 H, H4pz). 13C NMR (CD3CN): δC 143.2, 142.8, 138.3, 133.3, 132.0, 131.6, 131.3, 130.6, 130.1, 128.7, 108.4, 108.2, 76.9. LRMS [ESI(+), m/z] (Int.) [assign.]: 379 (48) [L-pz]+, 447 (100) [HL]+, 469 (6) [NaL]+, 553 (41) [AgL]+, 893 (1) [HL2]+, 910 (4) [(NH4)L2], 915 (2) [NaL2]+, 1001 (10) [AgL2]+. Crystals suitable for single crystal X-ray diffraction were grown by layering an acetonitrile solution with Et2O and allowing solvents to diffuse over 15 h. Ag(L23)(OTf), 2. A solution of 0.503 g (1.96 mmol) of AgOTf in 10 mL of THF was added to a solution of 0.874 g (1.96 mmol) of L23 in 15 mL of THF and copious precipitate formed immediately. To ensure quantitative transfer of reagents, an additional 5 mL portion of THF was added to the flask originally containing AgOTf, and the washings were added to the product mixture. After the suspension had been stirred 1 h, the solid was collected by filtration, washed with two 5 mL portions Et2O and was dried under vacuum to give 1.146 g (87%) of 2 as a colorless solid. Mp, 222 °C (decomp.). Anal. Calcd. (Obsd.) for C27H22F3N8O3SAg: C, 46.10 (46.05); H, 3.15 (3.07); N, 15.93 (16.07). 1 H NMR (CD3CN, 400 MHz) δH 8.14 (d, J = 2.4 Hz, 2 H, H5pz), 8.06 (s, 1 H), 8.01 (s, 1H), 7.86 (br m, 2 H, H3/5pz), 7.56 − 7.48 (overlapping

m, 5 H, Aryl, H3pz, H5pz), 7.40 (td, J = 7.6, 1.2 Hz, 1 H), 7.29 (dd, J = 7.6, 1.2 Hz, 1 H), 7.19 (t, J = 8.0 Hz, 1 H), 6.77 (s, 1 H), 6.76 (d, J = 7.5 Hz, 1 H), 6.66 (d, J = 7.8 Hz, 1 H), 6.60 (d, J = 7.8 Hz, 1 H), 6.53 (dd, J = 2,4, 1.2 Hz, 2 H, H4pz), 6.36 (dd, J = 1.5, 1 Hz, 2 H, H4pz). 13C NMR (CD3CN): δC 143.9, 142.4, 141.6, 140.2, 136.3, 134.8, 134.6, 131.9, 130.9, 130.8, 130.25, 130.24, 129.3, 107.9, 107.6, 76.8, 75.9. LRMS [ESI(+), m/z] (Int.) [assign.]: 311 (28) [L-pz-Hpz]+, 447 (100) [HL]+, 553 (99) [AgL]+, 697 (0.3) [Ag2LCl]+, 893 (0.1) [HL2]+, 910 (0.5) [(NH4)L2], 1001 (5) [AgL2]+. Crystalline blocks unsuitable for single crystal X-ray diffraction were grown by layering an acetonitrile solution over benzene or over toluene and allowing solvents to diffuse. Ag(L24)(OTf), 3. A solution of 0.472 g (1.84 mmol) of AgOTf in 10 mL of THF was added to a solution of 0.820 g (1.84 mmol) of L24 in 15 mL of THF and copious precipitate formed immediately. To ensure quantitative transfer of reagents, an additional 5 mL of portion of THF was added to the flask originally containing AgOTf, and the washings were added to the product mixture. After the suspension had been stirred 1 h, the solid was collected by filtration, washed with two 5 mL portions of Et2O and was dried under vacuum to give 1.155 g (89%) of 3 as a colorless solid. mp: 215 °C (decomp.). Anal. Calcd. (obsd) for C27H22F3N8O3SAg: C, 46.10 (45.98); H, 3.15 (3.27); N, 15.93 (15.77). 1 H NMR (CD3CN): δH 7.93 (s, 1 H, CmethH), 7.91 (d, J = 2.4 Hz, 2 H, H5pz), 7.65 (d, J = 1.7 Hz, 2 H, H3pz), 7.50 (s, 1 H, CmethH), 7.50 (td, J = 7.5, 1.3 Hz, 1 H), 7.49 (d, J = 1.7 Hz, 2 H, H3pz), 7.45 (d, J = 2.6 Hz, 2 H, H5pz), 7.41 (td, J = 7.6, 1.3 Hz, 1 H), 7.28 (dd, J = 7.4, 1.2 Hz, 1 H), 6.99 (part of AB, Japp = 8.3 Hz, 2 H), 6.88 (d, J = 7.8 Hz, 1 H), 6.80 (part of AB, Japp = 8.3 Hz, 2 H), 6.46 (dd, J = 2.4, 1.7 Hz, 2 H, H4pz), 6.29 (dd, J = 2.4, 1.7 Hz, 2 H, H4pz). 13C NMR (CD3CN): δC 142.6, 142.2, 141.8, 141.2, 136.0, 134.9, 132.8, 131.9, 130.9, 130.5, 129.5, 129.3, 128.1, 127.9, 107.6, 107.5, 76.7, 75.9. LRMS [ESI(+), m/z] (Int.) [assign.]: 311 (14) [L-pz-Hpz]+, 379 (11) [L-pz]+, 447 (100) [HL]+, 553 (60) [AgL]+, 697 (6) [Ag2LCl]+, 893 (0.4) [HL2]+, 910 (1.7) [(NH4)L2], 1001 (4.5) [AgL2]+, 1143 (1.3) [Ag2L2Cl]+, 1287 (0.2) [Ag3L2Cl2]+, 1431 (0.1) [Ag4L2Cl3]+. Crystals of 3·0.5 toluene suitable for single crystal X-ray diffraction were grown by layering an acetonitrile solution over toluene and allowing solvents to diffuse over 15 h. Ag(L33)(OTf), 4. A solution of 0.508 g (1.14 mmol) of L33 in 10 mL of THF was added to a solution of 0.292 g (1.14 mmol) of AgOTf in 10 mL of THF and copious precipitate formed immediately. To ensure quantitative transfer of reagents, an additional 5 mL portion of THF was added to the flask originally containing L33, and the washings were added to the product mixture. After the suspension had been stirred 1 h, the solid was collected by filtration, washed with two 5 mL portions of Et2O and was dried under vacuum to give 0.631 g (79%) of 4 as a colorless solid. mp: 230 °C (decomp.). Anal. Calcd (obsd) for C27H22F3N8O3SAg: C, 46.10 (45.93); H, 3.15 (3.17); N, 15.93 (15.68). 1H NMR (CD3CN, 400 MHz): δH 7.87 (s, 2 H, CmethH), 7.82 (d, J = 2.5 Hz, 4 H, H5pz), 7.59 (d, J = 1.8 Hz, 4 H, H3pz), 7.56 (d, J = 7.8 Hz, 2 H), 7.44 (t, J = 7.8 Hz, 2 H), 6.95 (s, 2 H), 6.89 (d, J = 7.8 Hz, 2 H), 6.41 (dd, J = 2.5, 1.8 Hz, 4 H, H4pz). 13C NMR (CD3CN): δC not soluble enough to give detectable signals in reasonable aquisition times ( 2σ(I) for each. Analysis of the data showed negligible crystal decay during collection in each case. Direct methods structure solutions, difference Fourier calculations and full-matrix least-squares refinements against F2 were performed with SHELXTL.12 An empirical absorption correction using spherical harmonics, implemented in SCALE3 ABSPACK13 scaling algorithm was applied to the data for 3·0.5 toluene, while numerical absorption corrections based on Gaussian integration over a E



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Figure 3. Left: Asymmetric unit of [Ag(L22)](OTf), 1. Right: View of dimeric structure in 1 with 22-member metallacycle highlighted. Selected bond distances, (Å): Ag1−N11, 2.1796(14); Ag1−N21, 2.4358(14); Ag1−N41, 2.1799(14); Ag1−O3 2.5952(13). Selected bond angles (deg): N11−Ag1− N21, 99.76(5); N11−Ag1−N41, 158.45(5); N41−Ag1−N21, 100.64(5); N11−Ag1−O3, 94.50(5); N21−Ag1−O3, 81.44(5); N41−Ag1−O3, 95.31(5). multifaceted crystal model were applied to the data for the other crystals. With the exception of the isotropic refinement for atoms of the disordered acetonitrile molecules in 4·0.5 CH3CN and atoms in Et2O of 5·CH3CN·0.25Et2O, all non-hydrogen atoms were refined with anisotropic displacement parameters. Hydrogen atoms were placed in geometrically idealized positions and included as riding atoms. The crystal of 3·0.5 toluene was a TWIN with the components rotated 3 degrees about the y axis. It is noted that the B-level alert in the CheckCIF file (http://checkcif.iucr.org/) for the structure of 3·0.5 toluene that concerns a greater than 100% number of reflections in the twin refinement is a feature of HKLF 5 settings in SHELXL where reflections from both twin components are used, resulting in the error message. The X-ray crystallographic parameters and further details of data collection and structure refinements are given in Table 1.

■

RESULTS Syntheses. The new X,Y′-bis[di(pyrazol-1-yl)methyl]-1,1′biphenyl ligands, LXY, (X, Y = 2−4) used in this study (Chart 2) Table 2. Geometries of Weak Hydrogen-Bonding (C−H···O and C−H···F) Interactions in [Ag(L22)](OTf), 1 donor (D)(−H)···acceptor (A)

D−H (Å)

H···A (Å)

D···A (Å)

D−H···A (deg)

C5−H5···O2 C42−H42···O3 C33−H33···F2

0.95 0.95 0.95

2.50 2.58 2.40

3.398(2) 3.214(2) 3.262(2)

158 125 150

Figure 4. Supramolecular structure of [Ag(L22)](OTf), 1. Top: View down a-axis into a portion of a sheet in the bc-plane which is assembled by CH···O (orange dashed lines) and CH···F (green lines) interactions. Bottom: Side view (down c-axis) of sheet showing the CH···O interactions that assemble sheets along the a-direction.

were prepared by the CoCl2-catalyzed Peterson rearrangement reaction14 between excess (3 equiv) of S(O)pz2 and one equivalent of the appropriate dialdehyde, X,Y′-biphda, exemplified for L22 and L23 in Scheme 1. Excess S(O)pz2 was previously found useful for maximizing the yield of pz4lut and related derivatives. The desired symmetric dialdehyde precursors (2,2′-, 3,3′-, and 4,4′-biphda) were prepared by Pd(OAc)2-catalyzed homocoupling6a of the appropriate potassium formylphenyltrifluoroborate,7 while the other dialdehydes (2,3′-, 2,4′-, and 3,4′biphda6c), were prepared by Suzuki coupling reactions between the appropriate bromobenzaldehyde and formylphenylboronic acid. It is noted that attempts to prepare 2,2′-biphda by Suzuki coupling between 2-formylphenyl boronic acid and 2-bromobenzaldhyde were unsuccessful under a number of different conditions (base, solvent) tested. A higher yielding but longer

route to 2,2-biphda exists6b but was not pursued given the success and ease of the current homocoupling method. Of the various LXY derivatives, the 1H NMR spectrum of L22 was unusual in that steric constraints give rise to a conformation that differentiates pyrazolyls in the di(pyrazolyl)methyl units. Thus, while the spectrum of L33 and L44 each consists of one set of signals for pyrazolyl hydrogens and one singlet resonance for the methine hydrogen, that for L22 has two sets of resonances for each of the pyrazolyl hydrogens and one singlet for the methine hydrogens. Such a pattern of resonances suggests a C2 symmetric structure for L22. Indeed, molecular models of L22 show a lowenergy C2-symmetric conformer (Figure 2) with distinct pyrazolyls but equivalent methine hydrogen atoms. F



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Figure 5. Left: View down c-axis showing polymeric [Ag(L24)+]n chain with partial atom labeling. Right: View of chain down b-axis. Selected bond distances (Å): Ag1−N11, 2.278(2); Ag1−N21, 2.349(3); Ag1−N41, 2.296(3); Ag1−N51, 2.333(3). Selected bond angles (deg): N11−Ag1−N21, 81.92(9); N11−Ag1−N41, 120.42(10); N11−Ag1−N51, 115.07(9); N41−Ag1−N21, 121.66(9); N41−Ag1−N51, 84.91(9); N51−Ag1−N21, 137.11(9).

Table 3. Geometries of Weak Hydrogen-Bonding (C−H···O and C−H···F) and C−H···π Interactions in [Ag(L24)](OTf)· 0.5toluene, 3·0.5toluenea donor (D)(−H)··· acceptor (A) C7−H7···O1 C23−H23···O1 C13−H13···O2 C5−H5···O3 C37−H37···O3 C12−H12···F3 C32−H32···F2 C22−H22···Ct(N51) C4−H4···Ct(C1s)

D−H (Å)

.

D···A (Å)

C−H···O interactions 1.00 2.28 3.237(4) 0.95 2.60 3.339(5) 0.95 2.38 3.278(5) 0.95 2.50 3.151(4) 1.00 2.39 3.340(4) C−H···F interactions 0.95 2.49 3.283(5) 0.95 2.51 3.058(4) C−H···π interactions 0.95 2.76 3.551(4) 0.95 2.81 3.573(4)

D−H···A (deg)

γ (deg)

159 135 158 126 159 141 117 141 138

8.7 14.4

Ct(i) = centroid of ring containing atom i; γ = Angle CH-Ct(i) and normal to plane containing Ct(i).

a

Figure 6. Top: View of a portion of the structure of 3·0.5 toluene down the b- axis that highlights the noncovalent interactions that assemble chains in both the a- and c-directions. Note that only one-half of two chains are shown (compare to the right of Figure 5 or to the bottom of the current figure). Bottom: View of a unit cell with half of the toluene solvent molecules removed to show void-space, (dotted ovals). Key: C− H···O interactions (orange dashed lines), C−H···F interactions (green lines), CH-π interactions (pink dashed lines), Ct(N51) = centroid of pyrazolyl ring containing atom N51.

X-ray diffraction. Also, Ag(L24)(OTf), 3, formed two types of crystals from a toluene/acetonitrile mixture, mainly very thin needles along with a few prisms. The few twinned prisms were suitable for X-ray diffraction and were found to be a toluene hemisolvate. The majority of the sample was highly anisotropic crystalline needles that were too thin for typical single crystal Xray diffraction experiments. Of the structurally characterized derivatives, only [Ag(L22)](OTf), 1, was found to be free from solvent without the need for drying under vacuum whereas crystals of all other complexes contained variable amounts of crystallization solvents. The solvents of crystallization could be removed by prolonged drying under vacuum. Views of the structure of 1 are displayed in Figure 3 along with a list of selected bond distances and angles. The asymmetric unit of 1 consists of one ligand, one silver, and one triflate anion (Figure 3, left). In the asymmetric unit, the ligand binds silver in a unique chelating trans-spanning fashion using one pyrazolyl nitrogen atom from each of two different di(pyrazolyl)methyl groups. The Ag−N41 and Ag−N11 bond distances of 2.1796(14) and 2.1799(14) Å, respectively, are slightly longer than expected for two-coordinate silver(I)−nitrogen bonds, which usually range between 2.10 and 2.14 Å.15 The bond angle N11−Ag1−N41 is slightly bent at 158.45(5)° rather than being linear. Both of the above metrics suggest that coordination number for silver is greater than two. In fact, two asymmetric

In this model, one pyrazolyl from each of the two di(pyrazolyl)methyyl units are forced into an antiparallel faceto-face π−π stacking interaction near the C−C bond that connects the aryl groups. The other two pyrazolyl rings are more distant from the C2-symmetry axis. The reactions between each ligand and one equivalent of silver(I) trifluoromethanesulfonate in THF results in the immediate precipitation of [Ag(LXY)](OTf) as colorless solids. The complexes are soluble in DMF, moderately soluble in acetonitrile, acetone, and halogenated solvents but are insoluble in ethereal and hydrocarbon solvents. Solid State. Crystals suitable for single crystal X-ray diffraction could be obtained for five of the six silver(I) complexes. The structures of the various derivatives are shown in Figures 3−12 while metric data are found in the figure captions and in Tables 2−6. Despite exhaustive attempts using different solvent combinations and varying crystallization rates, [Ag(L23)](OTf), 2, defied forming crystals suitable for single crystal G



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Figure 7. Left: Asymmetric unit of Ag(L33)(OTf)·0.5CH3CN, 4·0.5CH3CN. Right: View down c-axis of polymer chain. Selected bond distances (Å): Ag1−N11, 2.3042(17); Ag1−N21, 2.3624(16); Ag1−N41, 2.2346(17); Ag1−N51, 2.5009(16). Selected bond angles (deg): N11−Ag1−N21, 82.76(6); N11−Ag1−N41, 152.56(6); N11−Ag1−N51, 89.67(5); N41−Ag1−N21, 122.30(6); N41−Ag1−N51, 83.90(6); N51−Ag1−N21, 126.08(5).

Table 4. Summary of Intermolecular Noncovalent Interactions Organizing Supramolecular Structure of 4· 0.5CH3CNa C−F ···Ag interaction C61−F3···Ag1 π···π

C−F (Å)

1.338(2) 3.065(2) Ct−Ct ⊥av dist. (Å) α (°)

Ct(N21)−Ct(N21) 3.446(1) donor (D)−H ···acceptor D−H (A) (Å) C72−H72A···Ct(C1) C7−H7···O3 C13−H13···O3 C12−H12···O2 C23−H23···O2 C35−H35···O1 C37−H37···O1

F···Ag (Å)

0.98 1.00 0.95 0.95 0.95 0.95 1.00

3.301(1) 0 H···A (Å) D···A (Å) 2.91 2.55 2.35 2.49 2.59 2.59 2.19

3.751(8) 3.367(2) 3.119(2) 3.412(3) 3.524(3) 3.314(3) 3.168(2)

C−F···Ag (°) 138 β (°) γ (°) 16.7 16.7 D−H···A γ (°) (°) 144 138 137 164 167 134 165

12

Ct(i) = centroid of ring containing atom i; α = dihedral angle between mean planes x and y. β = angle Ct(i)−Ct(j); γ = angle Ct(i)− Ct(j) and normal to plane containing Ct(j); ⊥av dist. = average of perpendicular distance of centroid i to ring j and of centroid j to ring i. a

Figure 8. Views of interactions that organize the supramolecular structure of 4·0.5 CH3CN. Top: View just off of the b-axis showing the C−H···O interactions (orange dashed lines) that form sheets in the acplane. Bottom: View down a- axis showing the C−H···O (orange dashed lines), π−π (cyan line) and C−F···Ag contacts (green dashed lines) that hold sheets together along the b- direction. Also shown is the C−H···π interactions (pink dashed lines) that hold solvate acetonitrile in the lattice.

Figure 9. Structure of cyclic dication in Ag(L34)OTf, 5·CH3CN· 0.25Et2O with partial atom labeling without hydrogen atoms and with 28-member metallacycle highlighted. Selected bond distances (Å): Ag1−N11, 2.326(5); Ag1−N21, 2.268(4); Ag1−N41, 2.294(4); Ag1− N51, 2.265(4). Selected bond angles (°): N11−Ag1−N21, 85.53(15); N11−Ag1−N41, 117.41(14); N11−Ag1−N51, 121.81(16); N41− Ag1−N21, 123.69(15); N41−Ag1−N51, 86.54(15); N51−Ag1−N21, 125.96(13).

units are connected to each other via longer Ag1−N21 bonds of 2.4358(14) Å to form a cyclic dimer with inversion symmetry (Figure 3, right). The cyclic dication forms a twenty-two member metallacycle where the primary coordination geometry about silver(I) can best be described as either distorted T-shaped (AgN3) or distorted seesaw (AgN3O) depending on one’s view of the long Ag1−O3 2.5952(13) Å contact that is also present. Since the average distance of 2.27 Å for Ag−N(pyrazolyl) bonds falls within the 2.2−2.3 Å range found for other authentically three-coordinate silver complexes with nitrogen donors,15 we favor the former description, with the long Ag···O contact being considered a secondary interaction that assists in the overall organization of the crystal structure.

Thus, in addition to the long Ag···O contact described above, the three-dimensional supramolecular structure of 1 is constructed from three noncovalent interactions (two C−H··· O16 and one C−H···F17 weak hydrogen bonding interactions) whose metrics are summarized in Table 2. Of these, the C−H···F and the shorter of the two C−H···O weak hydrogen bonding interactions give rise to sheets that reside parallel with the bcplane, as shown in Figure 4. The sheets are assembled in the third dimension (along the a- axis) by the longer of the two C−H···O interactions that occurs between the oxygen (O3, that is also in H



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Figure 10. Supramolecular structure of 5·CH3CN·0.25Et2O. Top: View just off [1̅ 1 0] of the pyrazolyl embrace interactions (π···π part of interaction, cyan lines; C−H···π part of interaction, pink dashed lines) that assemble cyclic dications into a 3D porous network. Bottom: View down b- axis of a 2 × 2 supercell with solvent (black spheres) and triflate anions removed from the top two unit cells to show channels. The C−H···O interactions between oxygen atoms of triflate anions and the hydrogen atoms of the cyclic dications are shown as orange dashed lines in the bottom two cells.

Table 5. Geometry of the Interionic Noncovalent Interactions in the Crystal of 5·CH3CN·0.25Et2Oa π···π Ct(CN21)−Ct(N51) donor (D)−H ···acceptor (A) C−H···π C22−H22···Ct(N41) C52−H52···Ct(N11) C72−H72b···Ct(C1) C−H···N C11−H11···N9 C37−H37···N9 C−H···F C34−H34···F1a C−H···O C7−H7··· O3a C23−H23···O2a C36−H36···O1a C53−H53···O2a

Ct−Ct 3.485(3) D−H (Å)

⊥av dist (Å) 3.428(2), 3.379(2) H···A (Å)

α (deg) 4.9(3) D···A (Å)

β (deg)

γ (deg)

14.2 D−H···A (deg)

10.4 γ (deg) 11 7 8

0.95 0.95 0.98

2.99 2.88 2.76

3.667(7) 3.605(6) 3.390(11)

129 134 122

0.95 1.00

2.59 2.50

3.402(11) 3.318(8)

143 139

0.95

2.31

3.123(16)

143

1.00 0.95 0.95 0.95

2.39 2.60 2.28 2.44

3.373(14) 3.352(11) 3.228(19) 3.098(12)

166 136 172 126

a Ct(i) = centroid of ring containing atom i; α = dihedral angle between mean planes x and y. β = angle Ct(i)−Ct(j); γ = angle Ct(i)−Ct(j) and normal to plane containing Ct(j); ⊥av dist. = average of perpendicular distance of centroid i to ring j and of centroid j to ring i.

contact with silver) and an acidic 4-pyrazolyl hydrogen (H42) of a neighboring sheet (Figure 4, bottom). Various views of the structure of Ag(L24)(OTf)·0.5toluene, 3· 0.5 toluene are shown in Figures 5 and 6. The asymmetric unit consists of one ligand, one silver, one triflate anion, and half of a

toluene. As shown in the left of Figure 5, the ligand binds silver centers in a μ−κ2,κ2-mode. The AgC37−C7Ag torsion angle of 109° and H37C37−C7H7 torsion angle of 104°, are indicators of divergent donor groups. In this case, a polymeric structure is formed that propagates in the b-direction along a 21 screw axis. I



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Figure 6) which are filled with toluene molecules (black ellipsoids in the bottom half of the unit cell in Figure 6) that are loosely held in place by a C−H···π interaction (C4−H4··· Ct(C1s), Table 3). The structure of Ag(L33)(OTf)·0.5CH3CN, 4·0.5CH3CN, is shown in Figures 7 and 8. Selected bond distances and angles are provided in the caption of Figure 7. In a manner similar to 3· 0.5toluene, the ligand binds the metal in a μ−κ2,κ2-mode to give tetracoordinate silver(I) (av Ag−N = 2.35 Å) and a polymeric structure that propagates along the a-axis. In the current case, the AgN4 coordination geometry is best described as a distorted seesaw shape given the relatively large N11−Ag1−N41 bond angle of 152.56(6)° and disparate Ag−N bond distances. It is also of interest to note that the di(pyrazolyl)methine groups are located on the same side relative to the aryl−aryl C−C bond where the C7C3−C33C37 torsion angle of 44.2° is close to the dihedral angle between mean planes of aryl rings of the biphenyl moiety (41.3°). The Ag1C7−Ag1aC37 torsion angle of 158° and the H7C7−C37H37 torsion angle of 160° indicate a divergent disposition of donor groups. Views of the interactions that assemble the three-dimensional supramolecular structure are found in Figure 8. A summary of the noncovalent interactions found in the crystal packing arrangement is given in Table 4. The cationic chains are assembled into sheets in the ac-plane by CH···O interactions (orange dashed lines, Figure 8) between oxygens of the triflate anions and the acidic hydrogens methine and pyrazolyl hydrogens of the di(pyrazolyl)methyl groups. The sheets are stacked in the third dimension (along the b-axis) with the aid of a π···π interaction20 between pyrazolyl rings containing N21, a CH···O interaction involving the triflate ion and a hydrogen of the biphenyl moiety (C35−H35···O1, Table 4), and a short C−F···Ag contact (C61− F3···Ag1, Table 4; green dashed lines in bottom of Figure 8). The Ag···F distance in the short contact is in between the sum of the van der Waal radii (3.86 Å) and the sum of covalent radii (2.23 Å) of the atoms.19 The assembly of L33 and AgOTf leaves channels along the a-axis that are filled with CH3CN molecules that are disordered over the inversion center and that are held in place by a weak C−H···π interaction (C72−H72A···Ct(C1), Table 4). The structure of Ag(L34)OTf, 5·CH3CN·0.25Et2O is shown in Figures 9 and 10, while selected bond distances and angles are listed in Figure 9. In 5·CH3CN·0.25Et2O, the bridging bidentate ligands bind tetracoordinate silver (av Ag−N, 2.29 Å) to form a cyclic dication with C2-symmetry. The cyclic dication exhibits a left-handed helical twist that is exclusively found throughout the crystal as indicated by the P41212 space group. It is likely that the bulk sample also has equal amounts of crystals with the enantiomorphous space group P43212 (with right-handed cyclic dications) but we did not pursue this avenue of study further. Regardless, the silver centers in the cyclic dication are separated by 8.391 Å which is longer than the 4.77−5.39 Å range found for the Ag···Ag separation in related cyclic silver complexes of pz4mxyl4 or pzR4lut5 derivatives, as might be suspected. Views of crystal packing of 5·CH3CN·0.25Et2O and associated noncovalent interactions that promote assembly are shown in Figure 10. The metric parameters of the noncovalent interactions are given in Table 5. Each of the four di(pyrazolyl)methyl groups of a cyclic dication participates in a “pyrazolyl embrace” interaction with a di(pyrazolyl)methyl group of a neighboring dication (cyan and dashed pink lines, top of Figure 10) to give a 3D network with channels along b-axis (see the top two unit cells in the 2 × 2 supercell shown in the bottom of Figure 10). As shown in the bottom two unit cells of the 2 × 2 supercell shown

Figure 11. Top: Structure of the silver-containing portion of the asymmetric unit in 6·1.39 CH3CN with hydrogen atoms removed for clarity. Middle: View down a-axis of [Ag(L44)]n polymeric chain. Bottom: View of chain down b-axis. Selected bond distances (Å): Ag1− N41, 2.383(4); Ag1−N51, 2.240(4); Ag1−N41a, 2.237(4); Ag1−N51a, 2.432(4); Ag2−N11, 2.290(5); Ag2−N21, 2.362(5); Ag2−N11a, 2.332(4); Ag2−N21a, 2.291(4). Selected bond angles (deg): N41− Ag1−N51a, 93.86(15); N41a−Ag1-N41, 120.62(15); N41a−Ag1-N51, 139.77(16); N41a−Ag1-N51a, 85.32(15); N51−Ag1−N41, 85.61(16); N51−Ag1−N51a, 125.70(15); N11−Ag2−N11a, 130.94(15); N11− Ag2−N21, 86.22(16); N11−Ag2−N21a, 127.12(16); N11a−Ag2− N21, 112.67(15); N21a−Ag2−N11a, 82.89(15); N21a−Ag2−N21, 120.64(16).

Each silver is chelated by two ligands to form a distorted AgN4 tetrahedron where two of the Ag−N distances (Ag1−N11, 2.278(2) Å and Ag1−N41, 2.296(3) Å) are shorter than the others (Ag1−N21, 2.349(3) Å and Ag1−N51, 2.333(3) Å). The average Ag−N distance of 2.31 Å falls at the low end of the 2.3− 2.4 Å range found in other four-coordinate silver complexes with nitrogen donors.15 The 3D supramolecular structure of 3·0.5toluene is assembled by various C−H···O and C−H···F weak hydrogen bonding interactions and by C−H···π interactions,18 as listed in Table 3 and depicted in Figure 6. The triflate anion is affixed to the cationic polymer backbone by short C-H···O and C−H···F weak hydrogen bonding interactions (orange dashed lines and green lines, respectively, in Figure 6) that serve to assemble chains into sheets in the bc-plane. The shortest and presumably strongest of the interactions occurs between oxygen atoms of the triflate anion and the acidic methine hydrogen atoms and the adjacent 5pyrazolyl hydrogens of the di(pyrazolyl)methyl- tecton. The sheets are assembled into a three-dimensional structure by a concerted set of C−H···π interactions (pink dashed lines, Figure 6) between pyrazolyl groups where the 4-pyrazolyl hydrogen (H22) is donated to the π-cloud of the pyrazolyl ring containing N51. This three-dimensional organization gives rise to channels in the structure along the b-axis (top half of the unit cell shown in J



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Figure 12. Supramolecular structure of 6·1.39CH3CN. (a) View down a-axis showing pyrazolyl embrace interactions (π···π part of interaction, cyan lines; C−H···π part of interaction, pink dashed lines). (b) View of same fragment as in (a) but down the b-axis showing C−H···O interactions (orangedashed lines) that anchor the triflate anion to the cation. (c) View down the b-axis of a 2 × 2 × 2 supercell where anions and solvent are omitted from the top unit cells to show void-space. The bottom left unit cells contain only anions and the bottom right cells include all anions and solvent (black spheres).

solvent fill the channels (see bottom two unit cells, in Figure 12c) and are held in place by the various C−H···O and C−H···F interactions that are listed in Table 6. Since four out of the five structurally characterized [Ag(LXY)](OTf) derivatives showed short C−H···O interactions between the methine hydrogen and triflate oxygen and the exception occurred only for the silver(I) triflate complex of L22 that has inaccessible acidic methine hydrogen atoms hidden within the cation framework, the generality of such a weak hydrogen bonding motif in compounds containing poly(pyrazolyl)methane derivatives was probed via a search of the Cambridge Structural Database (CSD),21 as fully detailed in the Supporting Information. Of the 927 structurally characterized compounds that were deposited with the CSD that contained a di(pyrazolyl)methyl group and an oxygen atom in the crystal lattice, 356 (or 38.4%) had an intermolecular C−H···O contact with metrics that were within the ranges accepted for weak hydrogen bonding interactions (C···O distance between 2.5 to 3.8 Å, and C−H···O angle between 110° to 180°). The seemingly greater propensity of the new complexes to form such C−H···O interactions versus the subset from the CSD search undoubtedly arises from charge-

in the bottom of Figure 10, the triflate anions and solvent molecules reside in the channels and other void-space of the crystal and are anchored to the cationic walls by C−H···N, C− H···O, and C−H···F weak hydrogen bonding interactions and by a C−H···π interaction (C72−H72b···Ct(C1)), see Table 5. The structure of Ag(L44)(OTf), 6·1.39CH3CN, is found in Figures 11 and 12, interatomic distances and angles are found in the Figure 11 caption. The asymmetric unit consists of two silver centers, two ligands, one well-behaved and one disordered triflate anion, and 2.789 acetonitrile molecules distributed over three positions. The silver centers are tetracoordinate (av Ag−N, 2.32 Å) as a result of the ligand binding in bridging bidentate manner. The di(pyrazolyl)methyl groups are oriented on opposite sides of the biphenyl spacer with a Ag1C37a−C7aAg2 torsion angle of 172°. The result is a coordination polymer that propagates along the [1 1 0] direction. The coordination polymers are assembled by three different pyrazolyl embrace interactions (see cyan and dashed pink lines in Figure, 12, and metrical parameters listed in Table 6) into a three-dimensional cationic framework with channels that run along the b-direction (see top two unit cells of 2 × 2 supercell in Figure 12c). The triflate anions and acetonitrile K



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Table 6. Geometry of the Interionic Noncovalent Interactions in the Crystal of 6·1.39CH3CNa π···π Ct(CN21a)−Ct(N21a) Ct(CN41a)−Ct(N41a) Ct(CN51a)−Ct(N51a) donor (D)−H···acceptor (A) C−H···π C22a−H22a···Ct(N11a) C42a−H42a···Ct(N51a) C52−H52···Ct(N41) C−H···O C2−H2···O5 C2a−H2a···O5 C7−H7···O3 C7a−H7a···O2 C13a−H13a···O1 C36−H36···O4 C37−H37···O6 C37a−H37a···O4 C82−H82c··· O3 C−H···F C5−H5···F1 C41−H41···F3

β (°)

γ (°)

D···A (Å)

15.3 18.3 14.4 D−H···A (°)

15.3 18.3 14.4 γ (°)

2.64 2.93 2.97

3.470(6) 3.696(7) 3.699(7)

146 139 135

4 15 14

0.95 0.95 1.00 1.00 0.95 0.95 1.00 1.00 0.98

2.53 2.44 2.26 2.18 2.45 2.47 2.51 2.23 2.50

3.398(11) 3.295(11) 3.257(8) 3.160(8) 3.391(9) 3.330(8) 3.149(9) 3.197(8) 3.458(11)

151 149 171 167 169 150 121 163 167

0.95 0.95

2.54 2.48

3.206(7) 3.402(8)

128 163

Ct−Ct 3.508(3) 3.554(3) 3.683(4) D−H (Å)

⊥av dist. (Å) 3.453(3) 3.374(3) 3.563(3) H···A (Å)

0.95 0.95 0.95

α (°) 0 0 0

Ct(i) = centroid of ring containing atom i; α = dihedral angle between mean planes x and y. β = angle Ct(i)−Ct(j); γ = angle Ct(i)−Ct(j) and normal to plane containing Ct(j); ⊥av dist. = average of perpendicular distance of centroid i to ring j and of centroid j to ring i. a

complexes (both positively charged and charge-neutral species). Many of the C−H···O interactions of the molecular (and some ionic) species arise due to interaction of oxygen-containing solvent molecules (H2O, MeOH, Et2O, etc.) and the methine hydrogen. A similar search probing C−H···F interactions revealed that of 377 structures that contained a di(pyrazolyl)methyl group and an fluorine atom in the crystal lattice, 214 (or 49.1%) had intermolecular C−H···F interactions within the literature ranges: H···F distances between 2.0 and 2.6 Å and C− H···F angles between 90° and 180° (See Supporting Information). In these 377 cases, the fluorine was typically in the form of an anion and there were relatively fewer instances

Figure 13. Electrostatic surface potential map (isodensity value = 0.002) of [Ag(pz2CHPh)2]+ calculated at the B3LYP/LACVP* level.

assistance. It is noted that the subset of 356 structures retrieved from the CSD includes bis- and tris(pyrazolyl)methane derivatives, both in the form of free ligands and their metal

Figure 14. Left: Calculated isotope patterns for [Ag2L2]2+ (top) and [AgL]+ (bottom). Right: Views of the (normalized) experimental isotope patterns of the peaks near m/z = 553 found in the ESI(+) spectra of 4 and 6. L



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Figure 15. Portion of the structures of [Ag2(pz4m-xyl)2]2+ (CSD code NETTOW) and of [Ag(L33)+]n with the skeletal framework (green bars) and joints (black spheres) highlighted. Important dihedral angles are also labeled.

of any peaks for disilver species, even though the structure contained a cyclic dication. As shown in Figure 14, the ESI(+) spectrum of each 4 and 6 is interesting because each has a peak pattern with half-integer m/z spacing centered at m/z = 554 indicative of a [Ag2L2]2+ dication. Based on relative intensity considerations, 33% of the signal near m/z = 553 in the spectrum of 4 is for the dication while remaining signal is for the [AgL]+ monocation. Considering that the solid state structures of 4 and 6 were of coordination polymers, the peaks for di- (and mono-) silver ions further showcases the labile nature of the chains in solution.

(20/377, 5.3%) of molecular compounds (i.e., charge-neutral metal complexes or ligands) than in the search for C−H···O interactions (432/927, 46.6% were molecular compounds). Finally, an electrostatic surface potential map (Figure 13) of a structurally characterized model complex, [Ag(pz2CHPh)2]+ (CSD code AZOTUE), computed at the B3LYP/LACVP* level revealed that the complex is most electron deficient at the methine and 5-pyrazolyl hydrogen positions (blue and cyan areas of Figure 13) and therefore this portion of the complex likely provides the most probable docking site for an anion. Solution. The data from solution NMR and ESI(+) mass spectral studies indicate that the six silver complexes 1−6 are extensively dissociated in CH3CN. If the solid state structures were maintained in solution then one would expect twice as many pyrazolyl resonances in the 1H NMR spectrum than is observed for solutions of each compound. For example, based on the extended structures of the most symmetric derivatives Ag(L33)OTf, 4, and Ag(L44)OTf, 6, shown in Figures 7 and 11, respectively, one might expect two sets of resonances corresponding to hydrogen nuclei of each type of symmetrically distinct pyrazolyl group but only one set of resonances is observed even at low temperature (−35 °C). Similarly, based on the cyclic structure of Ag(L22)OTf, 1, shown in the Figure 3, one would expect four sets of resonances for each type of pyrazolyl hydrogen but only two sets are observed. The fewer sets of observed versus expected resonances might be due to either dissociated complexes or to rapidly exchanging pyrazolyls. We favor the former based on the temperature independence of the spectra and our experience with other similar systems.5 The electrospray ionization mass spectrometric (ESI-MS) data also suggest that 1−4 and 6 are dissociated in solution since the base peak was for either the protonated ligand (at m/z = 447 for HL in 1−4) or a ligand fragment (at m/z = 379 for L-pz, in 6). The spectrum of 5 is an apparent exception since the base peak found at m/z = 554 was for [Ag2(L34)2]2+. The next highest intensity peak (55%) in the spectrum of 5 found at m/z = 447 for the protonated free ligand (HL)+ is an indication of substantial solution fragmentation. Interestingly, Reger and co-workers reported that silver(I) complexes of polytopic di(pyrazolyl)methane ligands which have cyclic dications in the solid state such as either [Ag2(μ-m-[CH(pz)2]2C6H4)2](X)2 (X = BF4, PF6)4 or [Ag2(μ-[CH(pz)2]2(CH2)n)2]2+ (n = 1−3)3a,c will reliably give a weak peak corresponding to [Ag2L2(X)]+ (X = anion) in their ESI(+) mass spectrum whereas such a peak is absent in cases where the solid state structure is of a coordination polymer. However in the case of 5, where the base peak was for a dication, [Ag2(L34)2]2+, the expected weak intensity peaks for the [Ag2L2(OTf or Cl)]+ ions were not observed. Thus, although a peak with an m/z corresponding to [Ag2L2(X)]+ (X = anion) might be a reliable indicator of a cyclic dication, it is not an absolute rule. In fact, the ESI(+) mass spectrum of 1 was devoid

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CONCLUSIONS Previous work with silver(I) trifluoromethanesulfonate complexes of pz4m-xyl or pzR4lut ligands showed that a metadisposition of (120° angle between) two di(pyrazolyl)methyl groups about an aromatic spacer (phenyl or pyridyl) gives cyclic structures. We sought to determine if similar design principles could be uncovered for analogues that have a longer biphenyl spacer linking two di(pyrazolyl)methyl units. The ultimate goals of such a research pursuit would be to find the largest cycle that could be formed from this tecton and to use such design principles for the future construction of new porous materials. It was anticipated (and found) that the biphenyl spacer would give rich structural variety because of the greater flexibility of the ligand versus the pz4m-xyl or pzR4lut ligands. As shown in the left of Figure 15, the variability in structures of metal complexes of the pz4m-xyl or pzR4lut ligands can arise by rotation of the di(pyrazolyl)methyl groups about two axles of the ligand framework. The axles have three main ‘joints’ or pivot points at the methine carbons and at the centroid of the aryl ring. Rotation relative to the central aryl ring can be measured by HCmeth− CArylHAryl torsion angles, τ1 and τ2. The angle between the two axles is locked by the choice of ortho-, meta- and para-isomers. The new ligands with biphenyl spacers have four pivot points (two fixed at the centroids of the aryl rings and two at the methine carbons) and three degrees of rotational freedom. There are two rotations, τ1 and τ2 similar to those in pz4m-xyl or pzR4lut and an additional rotational degree of freedom, ϕ1, the aryl−aryl dihedral angle. For ligands not substituted at a para-aryl position, the CmethCtAryl−CtArylCmeth torsion angle between two di(pyrazolyl)methyl groups is a more informative descriptor of ϕ1. In the current study, the rotational degrees of freedom in L34 and the unexpected binding motif of L22 permitted the formation of two types of cyclic structures that were verified by X-ray diffraction. The larger cyclic structure based on L34 with a 28membered metallacyclic ring has a 120° angle between metal binding units, similar to that in complexes of pz4m-xyl or pzR4lut. On the other hand, the steric demands of the proximal pyrazolyl and aryl rings in L22 give rise to a unique coordination mode, a low coordination number for silver, and a cyclic structure with a M



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(4) Reger, D. L.; Watson, R. P.; Smith, M. D. Inorg. Chem. 2006, 45, 10077−10087. (5) Morin, T. J.; Merkel, A.; Lindeman, S. V.; Gardinier, J. R. Inorg Chem. 2010, 49, 7992−8002. (6) (a) Santos-Filho, E. F.; Sousa, J. C.; Bezerra, N. M. M.; Menezes, P. H.; Oliveira, R. A. Tetrahedron Lett. 2011, 52, 5288−5291. (b) 2,2′biphda: Ligtenbarg, A. G. J.; van den Beuken, E. K.; Meetsma, A.; Veldman, N.; Smeets, W. J. J.; Spek, A. L.; Feringa, B. L. J. Chem. Soc., Dalton Trans. 1998, 263−270. (c) 3,3′-biphda: Kuhnert, N.; Patel, C.; Jami, F. Tetrahedron Lett. 2005, 46, 7575−7579. (d) 4,4′-biphda: Helms, A.; Heiler, D.; McLendon, G. J. Am. Chem. Soc. 1992, 114, 6227−6238. (7) Vedejs, E.; Chapman, R. W.; Fields, S. C.; Lin, S.; Schrimpf, M. R. J. Org. Chem. 1995, 60, 3020−3027. (8) (a) An early report of 2,4′-biphda exists but with no analytical or characterization data: Tanaka, M.; Souma, Y. J. Chem. Soc., Chem. Commun. 1991, 1551−1553. (b) NMR data for 2,4′-biphda: Saha, D.; Chattopadhyay, K.; Ranu, B. C. Tetrahedron Lett. 2009, 50, 1003−1006. (9) Coulson, D. R.; Satek, L. C.; Grim, S. O. Inorg. Synth. 1990, 28, 107. (10) CrysAlisPro, Agilent Technologies,Version 1.171.34.46 (release 25−11−2010 CrysAlis171 .NET), (compiled Nov 25 2010,17:55:46). (11) SAINT+, version 7.23a; SADABS, version 2004/1; Bruker Analytical Xray Systems, Inc.: Madison, WI, U.S.A., 2005. (12) Sheldrick, G. M. SHELXTL, version 6.12; Bruker Analytical X-ray Systems, Inc.: Madison, WI, U.S.A., 2001. (13) SCALE3 ABSPACKAn Oxford Diffraction Program, 1.0.4,gui:1.0.3; Oxford Diffraction, Ltd.: Abingdon, U.K., 2005. (14) (a) Thé, K. I.; Peterson, L. K. Can. J. Chem. 1973, 51, 422−426. (b) Thé, K. I.; Peterson, L. K.; Kiehlmann, E. Can. J. Chem. 1973, 51, 2448−2451. (c) Peterson, L. K.; Kiehlmann, E.; Sanger, A. R.; Thé, K. I. Can. J. Chem. 1974, 52, 2367−2374. (15) Liddle, B. J.; Hall, D.; Lindeman, S. V.; Smith, M. D.; Gardinier, J. R. Inorg. Chem. 2009, 48, 8404−8414. (16) (a) Bernstein, J. Cryst. Growth Des. 2013, 13, 961−964. (b) Gu, Y.; Kar, T.; Scheiner, S. J. Am. Chem. Soc. 1999, 121, 9411−9422. (c) Desiraju, G. J. Acc. Chem. Res. 1996, 29, 441−449. (d) Steiner, T.; Saenger, F. J. Am. Chem. Soc. 1992, 114, 10146−10154. (e) Desiraju, G. J. Acc. Chem. Res. 1991, 24, 290−296. (f) Taylor, R.; Kennard, O. J. Am. Chem. Soc. 1982, 104, 5063−5070. (17) (a) Choudhury, A. R.; Row, T. N. G. Cryst. Growth Des. 2004, 4, 47−52. (b) van den Berg, J. A.; Seddon, K. R. Cryst. Growth Des. 2003, 3, 643. (c) Brammer, L.; Bruton, E. A.; Sherwood, P. New J. Chem. 1999, 23, 965−968. (d) Grepioni, F.; Cojazzi, G.; Draper, S. M.; Scully, N.; Braga, D. Organometallics 1998, 17, 296−307. (e) Thalladi, V. R.; Weiss, H. -C.; Bläser, D.; Boese, R.; Nangia, A.; Desiraju, G. R. J. Am. Chem. Soc. 1998, 120, 8702−8710. (18) (a) Takahashi, H.; Tsuboyama, S.; Umezawa, Y.; Honda, K.; Nishio, M. Tetrahedron 2000, 56, 6185−6191. (b) Tsuzuki, S.; Honda, K.; Uchimaru, T.; Mikami, M.; Tanabe, K. J. Am. Chem. Soc. 2000, 122, 11450−11458. (c) Nishio, M.; Hirota, M.; Umezawa, Y. The CH−π Interaction: Evidence, Nature, and Consequences; Wiley-VCH, Inc: New York, 1998. (d) Umezawa, Y.; Tsuboyama, S.; Honda, K.; Uzawa, J.; Nishio, M. Bull. Chem. Soc. Jpn. 1998, 71, 1207−1213. (e) Braga, D.; Grepioni, F.; Tedesco, E. Organometallics 1998, 17, 2669−2672. (19) (a) Pyykkö, P.; Atsumi, M. Chem.Eur. J. 2009, 15, 186−197. (b) Cordero, B.; Gomez, V.; Platero-Prats, A. E.; Reves, M.; Echeverria, J.; Cremades, E.; Barragan, F.; Alvarez, S. Dalton Trans. 2008, 1477− 9226. (c) Bondi, A. J. Phys. Chem. 1964, 68, 441−451. (20) Janiak, C. J. Chem. Soc., Dalton Trans. 2000, 3885. (21) (a) Allen, F. H. Acta Crystallogr. 2002, B58, 380−388. (b) CSD Version 5.34, updates (Feb 2013).

22-membered metallacycle. It will be of interest to determine the effect of anions and solvent on the τ1, τ2, and the ϕ1 angles in future studies. Also, since the degrees of rotational freedom found for the biphenyl-linked ligands are expected to remain unchanged for p-terphenyl- and p-quarterphenyl-linked derivatives, it will be of interest to determine whether the structural motifs reported here would persist in Ag(OTf) complexes of those analogues with longer spacers. It can be confidently predicted that the coordination modes observed for AgOTf complexes of L22 and those of the ortho-,ortho- disubstituted ligands with p-terphenyl or longer p-polyphenylene spacers will be quite different since the two di(pyrazolyl)methyl groups will be too far apart to act as trans- spanning ligands as found for [Ag(L22)](OTf), 1. It will finally be of interest to examine whether the noncovalent interactions (CH···O and pyrazolyl embrace) that organize the current supramolecular structures will be preserved in derivatives with longer p-polyphenylene spacers.

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ASSOCIATED CONTENT

S Supporting Information *

Details of the CSD search and crystallographic information files (CIF). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS J.R.G. thanks the NSF (CHE-0848515) for financial support. REFERENCES

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