Ag(I) and Cu(I) [2 × 2] Chiral Grids Containing Pyrimidine Ligands with

ARTICLE pubs.acs.org/crystal

Ag(I) and Cu(I) [2
2] Chiral Grids Containing Pyrimidine Ligands with Camphor Moieties. Arene Encapsulation M. Carmen Carri
on, Isabel M. Ortiz, F
elix A. Jal
on, and Blanca R. Manzano* Departamento de Química Inorg
anica, Org
anica y Bioquímica, Universidad de Castilla-La Mancha, Facultad de Químicas-IRICA, Avda. C. J. Cela, 10, 13071 Ciudad Real, Spain

Ana M. Rodríguez Departamento de Química Inorg
anica, Org
anica y Bioquímica, Universidad de Castilla-La Mancha, Escuela T
ecnica Superior de Ingenieros Industriales, Avda. C. J. Cela, 3, 13071 Ciudad Real, Spain

Jos
e Elguero Instituto de Química M
edica, Centro de Química Org
anica “Manuel Lora-Tamayo”, CSIC, Juan de la Cierva, 3, E-28006 Madrid, Spain

bS Supporting Information ABSTRACT: The C2 symmetric chiral ligand 4,6-bis[(4S,7R)-7,8,80 -trimethyl4,5,6,7-tetrahydro-4,7-methaneindazol-2-yl]pyrimidine (bpzCampm) was synthesized and reacted with Cu(I) and Ag(I) salts to give four chiral [2
2] grid-type structures: [M4(bpzCampm)4]X4, M = Cu(I), Ag(I); X = BF4, PF6. The structures of the two BF4 derivatives were determined by X-ray diffraction. The grids contain facing ligands that are not parallel and, in contrast to the situation found in previous examples, the anions are not hosted within the cavities but toluene molecules are. In the case of the copper derivative, π-stacking interactions are established between the pyrimidine rings and these toluene molecules and others situated in the intercationic regions, giving rise to the formation of π-stacked columns with alternating pyrimidine and toluene molecules. Due to the chirality of bpzCampm, chiral cavities are generated in the crystalline structure. From NMR studies, it was concluded that the copper grids are maintained in solution but this is not the case for the silver derivatives. The UVvis spectra of the copper complexes show an MLCT band, higher for the PF6 derivative. Some derivatives exhibit fluorescence with a high influence of the solvent and counteranion in the intensity of the emission.

’ INTRODUCTION Chirality is an essential element of life. The self-assembly of chiral supramolecular species is common in vivo. To the chemist, however, the design and preparation of chiral artificial selfassembled systems is an interesting and challenging objective.1 Metalorganic species including MOF2 (metal organic frameworks) that contain chiral cavities have received a great deal of attention in recent years3 due to their potential applications in enantioselective synthesis and separation, sensing, catalysis, and optical devices.4 There are two strategies to generate chiral metalorganic systems and these are outlined below. The first approach concerns the use of achiral components to make chiral species.58 In this case, although individual chiral crystals may be obtained, the bulk material is often a racemic mixture of crystals, thus limiting their potential uses. There have also been cases of spontaneous resolution under supramolecular control9 and of absolute asymmetric synthesis or chiral amplification.10 The second strategy involves the use of chiral components, usually chiral ligands.1114 Both approaches have been r 2011 American Chemical Society

extensively used for the synthesis of polynuclear helical complexes13,15 and a wide variety of self-assembled chiral species with different dimensionalities have also been described, including discrete molecular species14 that have found applications in asymmetric catalysis and in enantioselective sensing.16 However, examples of chiral [2
2] grid-type inorganic arrays are scarce.17 On the one hand, chiral species are obtained when two types of metal center are consecutively introduced in an anti disposition. In this case, the introduction of the second metal involves a combination of two precursors of the same chirality (Figure 1, A) through a diastereoselective self-assembly process.18 On the other hand, molecular grids of type B (Figure 1B) are also chiral (D4 symmetry) because of the special way in which the ligand strands enfold the cations.19,20 In these two types of grid both enantiomers are present, a fact that limits their applications. However, if enantiomerically pure ligands Received: December 23, 2010 Revised: March 1, 2011 Published: March 22, 2011 1766

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Chart 1

Figure 1. Two types of chiral grids that do not contain chiral ligands.

are used, enantiopure species can be obtained with all the crystals possessing the same handedness. These systems have potential applications mainly in the field of asymmetric catalysis or enantioselective separation. Molecular [2
2] grids of type B, containing a chiral ligand derived from pinene, have been reported.19 We recently reported the formation of a new type of Cu(I) [2
2] grid containing cavities with a shape different to that of other previously described examples.21 This new material was obtained through the use of bis-chelating ligands derived from pyrimidine and pyrazole with nonparallel coordination vectors (Chart 1). In all cases, one anion was hosted in each cavity and the effect that the presence of amino groups oriented toward the inside of the cavities had on this hosting was evaluated.21b In the work described here, we decided to prepare chiral [2
2] grids by using a new homochiral C2 symmetric ligand that contains pyrazole heterocycles derived from camphor, an important constituent of the chiral pool and the source of many useful chiral derivatives.22 The new ligand is 4,6-bis[(4S,7R)-7,8,80 trimethyl-4,5,6,7-tetrahydro-4,7-methaneindazol-2-yl]pyrimidine (bpzCampm) (1, Chart 2), a derivative that could also find other applications as a chiral chelate N-donor ligand.23 The C2 symmetry is likely to be advantageous for applications in asymmetric catalysis.24 This ligand may behave as tetradentate, more specifically, as bis-bidentate and may give rise to [2
2] grids through the reaction with tetrahedral centers. These grids will have chiral cavities and intermolecular chiral regions could also be generated in the crystalline structure. We decided to explore the formation of the chiral grids with two different metals, Cu(I) and Ag(I), in order to study the influence of the metal center. The silver center is less stereochemically rigid and gives rise to more labile derivatives with N-donor ligands and it seemed likely that the grid structure would not be maintained in solution. This possibility could be easily determined by 1H NMR spectroscopy because the formation of the grid would remove the C2 symmetry of the ligand. We were also interested in evaluating the effect on the final structure of noncovalent forces such as anion-π and ππ interactions. Analysis of the influence of the bulky and apolar camphor groups in the ligands was another goal of the work. It was envisaged that these groups could change the steric hindrance in the cavities formed with respect the previously described grids and may create hydrophobic forces.

’ EXPERIMENTAL SECTION General Comments. All reactions were carried out under a nitrogen atmosphere using standard Schlenk techniques. Solvents were freshly distilled from the appropriate drying agents and degassed before use. Elemental analyses were performed with a Thermo Quest Flash EA1112 microanalyzer. IR spectra were recorded on microcrystalline solids with an ATR system on IRPRESTIGE-21 Shimadzu (4000700 cm1) spectrophotometers. Mass spectrometry measurements were carried out on a matrix assisted laser desorption ionization-time-of-flight (MALDI-TOF) Applied Biosystems Voyager DE STR system or with a Q-q-TOF hybrid analyzer with an electrospray ionization source (ESI-TOF) QStar Elite Applied Biosystems

Chart 2. Ligand bpzCampm and Numbering Scheme

spectrometer. 1H, 13C{1H}, and 19F NMR spectra were recorded on Varian Unity 300, Varian Gemini 400 and Inova 500 spectrometers. Chemical shifts (ppm) are relative to tetramethylsilane (1H, 13C NMR) and CFCl3 (19F NMR). Coupling constants (J) are in Hertz. The NOE difference spectra were recorded with a 5000 Hz spectrum width, an acquisition time of 3.27 s, a pulse width of 90°, a relaxation delay of 4 s, an irradiation power of 510 dB, and a number of scans of 240. For 1H13C g-HMQC and g-HMBC spectra, the standard VARIAN pulse sequences were used (VNMR 6.1 C software). The spectra were acquired using 7996 (1H) and 25133.5 Hz (13C) spectrum widths; 16 transients of 2048 data points were collected for each of the 256 increments. For variable temperature spectra, the probe temperature ((1 K) was controlled by a standard unit calibrated with a methanol reference. In the NMR data, s, d, and b refer to singlet, doublet, and broad, respectively. The carbon resonances are singlets. See Chart 2 for numbering. The optical rotations were measured with a JASCO P-2000 Polarimeter in acetone. UVvisible and Fluorescence spectra were recorded on an Uvikon-XS spectrofluorimeter. The starting materials [Cu(CH3CN)4]X25 (X = PF6, BF4) and (4S,7R)-7,8,80 -trimethyl-4,5,6,7-tetrahydro-4,7-methane-2-indazole26 were prepared according to literature procedures. AgX (X = BF4and PF6) were purchased from Aldrich and used without further purification. X-ray Crystallography. A summary of crystal data collection and refinement parameters for 3 and 5 is given in Table 1. See Supporting Information for the data of 1. Single colorless crystals of the ligand bpzCampm (1) and [Ag(bpzCampm)]4(BF4)4 3 2C7H8 (5 3 2C7H8), and an orange crystal of [Cu(bpzCampm)]4(BF4)4 3 4C7H8 (3 3 4C7H8) were placed in a Bruker-Nonius X8 APEXII CCD area-detector diffractometer equipped with a graphite-monochromated MoKR radiation source (λ = 0.71073 Å). Data were integrated using SAINT,27 which also applied corrections for Lorentz and polarization effects. An absorption correction was performed with the program SADABS.28 The software package SHELXTL29 was used for space group determination, structure solution, and refinement. The structures were solved by direct methods, completed with difference Fourier syntheses, and refined with full-matrix least-squares minimization w(Fo2  Fc2)2. All nonhydrogen atoms were refined with anisotropic displacement parameters. The positions of hydrogen atoms were calculated geometrically and were allowed to ride on their parent carbon atoms with fixed isotropic U. The absolute structure of compound 1 could not be determined as no significant anomalous scatters were present. Friedel pairs in the data have been merged and not used as independent data. For complex 3, the toluene 1767

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Table 1. Crystal Data and Structure Refinement for 3 3 4C7H8 and 5 3 2C7H8 3 3 4C7H8

5 3 2C7H8

mol formula

C130H156B4Cu4F16N24

C118H142Ag4B4F16N24

fw

2656.23

2675.28

T (K)

180(2)

180(2)

wavelength (Å)

0.71073

0.71073

cryst syst

tetragonal

tetragonal

space group

I4122

I4122

a (Å)

29.141(2)

29.131(1)

b (Å) c (Å)

29.141(2) 17.527(3)

29.131(1) 18.230(1)

R (deg)

90

90

β (deg)

90

90

γ (deg)

90

90

V (Å3)

14884(3)

15470(1)

Z

4

4

D (calcd) (g/cm3)

1.185

1.149

absorp coeff (mm1) F(000)

0.633 5536

0.564 5480

cryst size (mm3)

0.32
0.22
0.19

0.55
0.42
0.33

index ranges

36 e h e 36

36 e h e 33

36 e k e 36

35 e k e 36

21 e l e 21

22 e l e 22

reflns collected

39554

61912

independent reflns

7239 [R(int) = 0.0928]

7837 [R(int) = 0.1223]

data/restraints/params GOF on F2

7239/60/390 0.811

7837/73/380 0.896

final R indices [I > 2σ(I)]

R1 = 0.0712

R1 = 0.0641

wR2 = 0.1769

wR2 = 0.1508

R indices (all data)

R1 = 0.1732

R1 = 0.1130

wR2 = 0.1956

wR2 = 0.1704

absolute structure param

0.04(4)

0.05(5)

largest diff. peak and hole (e Å3)

0.660 and 0.335

0.478 and 0.631

molecule inside the cavities show a undetectable methyl group and has been refined such a benzene molecule. Refinement for 3 and 5 included application of the squeeze procedure30 to model diffuse electron density and chemical formula and derivates quantities are given without solvent. For 3, the electron density appears in four voids per unit cell, presumably occupied by four acetone molecules of solvent highly disordered (43 electron per void approximately). For 5, the electron density appears in four substantial voids with 348 electrons per void which can correspond to 28 molecules of toluene solvent. The BF4 anions and toluene molecules in 3 and 5 appears disordered and some restraints were used. Syntheses of the New Derivatives. The synthesis of the ligand 4,6-bis[(4S,7R)-7,8,80 -trimethyl-4,5,6,7-tetrahydro-4,7-methane-indazol-2-yl]-pyrimidine (bpzCampm), 1, is described in the Supporting Information [Cu(bpzCampm)]4(PF6)4, 2. To a solution of 43.3 mg (0.116 mmol) of [Cu(CH3CN)4]PF6 in 10 mL of CH2Cl2 was added a solution of 50.0 mg (0.116 mmol) of bpzCampm in 10 mL of CH2Cl2, giving instantaneously an orange solution, which was stirred at room temperature for 2 h. The solution is concentrated under vacuum to 1 mL, and 3 mL of Et2O were added to precipitate the product, that was then filtered and washed with 2
5 mL of Et2O. The obtained yellow-orange solid was dried under vacuum. Yield: 34.8 mg (43%). Anal. Calcd for 2 3 3CH2Cl2: C, 45.66; H, 4.80; N, 11.92. Found: C, 45.44; H, 4.97; N, 11.47. 1H NMR (acetone-d6, 500 MHz, 25 °C): δ = 8.80 (s, 1H, H2), 8.41 (s, 1H, H30 ),

8.40 (s, 1H, H30 ), 8.16 (s, 1H, H5), 3.05 (bs, 2H, H40 ), 2.23 (m, 2H, H-pzCam), 1.95 (m, 2H, H-pzCam), 1.38 (m, 2H, H-pzCam), 1.26 (m, 2H, H-pzCam), 1.13 (s, 3H, Me70 ), 1.06 (s, 3H, Me70 ), 1.02 (s, 3H, MepzCam), 1.00 (s, 3H, Me-pzCam), 0.76 (s, 3H, Me-pzCam), 0.64 (s, 3H, Me-pzCam) ppm. 13C{1H} NMR (acetone-d6, 125 MHz, 25 °C): δ = 169.6 (C7a0 ), 158.2 (C4,6 þ C2), 134.5 (C3a0 ), 134.4 (C3a0 ), 120.9 (C30 ), 120.7 (C30 ), 92.0 (C5), 60.4 (C70 ), 60.3 (C70 ), 50.9 (C80 ), 50.8 (C80 ), 47.1 (C40 ), 47.0 (C40 ), 33.0 (C60 ), 26.9 (C50 ), 26.8 (C50 ), 20.1 (Me8a0 ), 19.1 (Me8a0 ), 18.0 (Me8b0 ), 17.9 (Me8b0 ), 9.4 (Me70 ), 9.3 (Me70 ) ppm. 19F NMR (acetone-d6, 282 MHz, 25 °C): δ = 73.6 (d, 6F, J = 703.4 Hz) ppm. IR (ATR) ν/cm1: 2960, 2927, 2872, ν(CH); 1598, ν(CdN) or ν(CdC); 1469, 744, δ(CH); 837, ν(PF). MS (ESITOFþ, NBA): m/z (rel. int. %): 1128 [Cu2(bpzCampm)2PF6þ, 1.0], 919 [Cu(bpzCampm)2þ, 100.0], 529 [Cu(bpzCampm)þ þ 2 H2O, 6.0], 491 [Cu(bpzCampm)þ, 3.0]. [Cu(bpzCampm)]4(BF4)4, 3. The synthesis of 3 is similar to that of 2. The amounts of products were as follows: 36.5 mg (0.116 mmol) of [Cu(CH3CN)4]BF4 and 50.0 mg (0.116 mmol) of bpzCampm. Compound 3 is also yellow to orange in color. Yield 36.4 mg (45%). Anal. Calcd for 3 3 6CH2Cl2: C, 46.76; H, 4.99; N, 11.90. Found: C, 46.79; H, 5.18; N, 12.25. 1H NMR (acetone-d6, 500 MHz, 25 °C): δ = 8.87 (s, 1H, H2), 8.48 (s, 2H, H30 ), 8.18 (s, 1H, H5), 3.04 (bs, 2H, H40 ), 2.24 (m, 2H, H-pzCam), 1.94 (m, 2H, H-pzCam), 1.37 (m, 2H, H-pzCam), 1.27 (m, 2H, H-pzCam), 1.13 (s, 3H, Me70 ), 1.06 (s, 3H, Me70 ), 1.02 1768

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Crystal Growth & Design (s, 3H, Me-pzCam), 0.99 (s, 3H, Me-pzCam), 0.89 (s, 3H, Me-pzCam), 0.76 (s, 3H, Me-pzCam) ppm. 19F NMR (acetone-d6, 282 MHz, 25 °C): δ = 152.7 (s, 4F) ppm. IR (ATR) ν/cm1: 2960, 2927, 2870, ν(CH); 1598, ν(CdN) or ν(CdC); 1471, 743, δ(CH); 1055, 1037, ν(BF). MS (ESI-TOFþ, NBA): m/z (rel. int. %): 1069 [Cu2(bpzCampm)2BF4þ, 12.3], 919 [Cu(bpzCampm)2þ, 100.0], 491 [Cu(bpzCampm) þ, 17.1], 429 [(bpzCampm), 20.0]. MS (MALDITOFþ, NBA): m/z (rel. int. %): 2228 [Cu4(bpzCampm)4(BF4)3þ, 0.3], 984 [Cu2(bpzCampm)2þ, 4.0], 919 [Cu(bpzCampm)2þ, 100], 491 [Cu(bpzCampm)þ, 10.0], 429 [(bpzCampm), 15.0]. Crystals of 3 3 4C7H8 suitable for X-ray diffraction were obtained by diffusion of the starting reagents using three phases. The lower phase was [Cu(CH3CN)4](BF4) (11.0 mg, 0.035 mmol) solved in 4 mL of dichloromethane, a phase of 4 mL of acetone was put in the middle, and the upper phase contained bpzCampm (15.0 mg, 0.035 mmol) solved in 10 mL of diethyl ether. After 10 days of diffusion, the solution was concentrated and crystals were obtained by diffusion of toluene in gas phase in the mixture of solvents. [Ag(bpzCampm)]4(PF6)4, 4. To a solution of 23.5 mg (0.093 mmol) of AgPF6 in 5 mL of acetone and protected from the light was added a solution of 40.0 mg (0.093 mmol) of bpzCampm in 10 mL of acetone, giving a colorless solution, which was stirred at room temperature for 7 h. The obtained suspension is filtered and the precipitate washed with 2
5 mL of Et2O. The resulting white solid was dried under vacuum. A microcrystalline product was obtained by crystallization of the starting reagents in CH2Cl2/ acetone/dibutylether. Yield: 47.1 mg (69%). Anal. Calcd for 4 3 3.5C3H6O: C, 46.98; H, 5.14; N, 11.45. Found: C, 46.45; H, 5.46; N, 10.97. 1H NMR (acetone-d6, 500 MHz, 25 °C): δ = 8.91 (s, 1H, H2), 8.41 (s, 2H, H30 ), 8.07 (s, 1H, H5), 3.01 (bs, 2H, H40 ), 1.98 (bs, 2H, H6ax0 ), 1.41 (bs, 2H, H6ec0 ), 1.29 (bs, 2H, H5ec0 ), 1.20 (s, 6H, Me-pzCam), 1.04 (s, 6H, Me-pzCam), 0.73 (s, 6H, Me-pzCam) ppm. 13C{1H} NMR (acetone-d6, 125 MHz, 25 °C): δ = 172.2 (C7a0 ), 159.0 (C4,6/C2), 157.4 (C4,6/C2), 132.0 (C3a0 ), 120.0 (C30 ), 93.0 (C5), 60.3 (C70 ), 50.6 (C80 ), 47.0 (C40 ), 33.4 (C60 ), 27.1 (C50 ), 20.2 (Me8a0 ), 18.1 (Me8b0 ), 9.8 (Me70 ) ppm. 19F NMR (acetone-d6, 282 MHz, 25 °C): δ = 73.6 (d, 6F, J = 703.5 Hz) ppm. IR (ATR) ν/cm1: 2960, 2914, 2870, ν(CH); 1585, ν(CdN) or ν(CdC); 1454, 749, δ(CH); 835, ν(PF). MS (ESI-TOFþ, NBA): m/z (rel. int. %): 1217 [Ag2(bpzCampm)2PF6þ, 1.2], 963 [Ag(bpzCampm)2þ, 45.0], 713 [Ag(bpzCampm)þ þ 3 C3H6O, 100.0], 535 [Ag(bpzCampm)þ, 22.0]. [Ag(bpzCampm)]4(BF4)4, 5. The synthesis of 5 is similar to that of 4. The amounts of reagents were as follows: 40.0 mg (0.093 mmol) of bpzCampm and 18.1 mg (0.093 mmol) of AgBF4. 5 is also a white solid. Microcrystalline product could be obtained by crystallization of the starting reagents in CH2Cl2/acetone/dibutylether. Yield: 48.3 mg (71%). Anal. Calcd for 5 3 2.5CH2Cl2 3 4C3H6O: C, 42.34; H, 4.85; N, 9.40. Found: C, 42.42; H, 5.00; N, 9.12. 1H NMR (acetone-d6, 400 MHz, 25 °C): δ = 9.04 (s, 1H, H2), 8.54 (s, 2H, H30 ), 8.16 (s, 1H, H5), 3.04 (bs, 2H, H40 ), 2.12 (bs, 2H, H5ax0 ), 1.99 (bs, 2H, H6ax0 ), 1.41 (bs, 2H, H6ec0 ), 1.27 (bs, 2H, H5ec0 ), 1.20 (s, 6H, Me-pzCam), 1.04 (s, 6H, Me-pzCam), 0.73 (s, 6H, Me-pzCam) ppm. 19 F NMR (acetone-d6, 282 MHz, 25 °C): δ = 151.2 (s, 4F) ppm. IR (ATR) ν/cm1: 2962, 2926, 2868, ν(CH); 1593, ν(CdN) or ν(CdC); 1469, 743, δ(CH); 1070, 1028, ν(BF). MS (ESI-TOFþ, NBA): m/z (rel. int. %): 1159 [Ag2(bpzCampm)2BF4þ, 1.1], 963 [Ag(bpzCampm)2þ, 45.0], 713 [Ag(bpzCampm)þ þ 3 C3H6O, 100.0], 537 [Ag(bpzCampm)þ, 34.0]. Crystals of 5 3 2C7H8 suitable for X-ray diffraction were obtained by diffusion of toluene in gas phase into a solution of the complex in acetone.

’ RESULTS AND DISCUSSION Synthesis of the Ligand bpzCampm, 1. The new chiral ligand

bpzCampm, 1, the structure of which is shown in Chart 2, is a derivative of camphor and contains four chiral centers. This compound can be synthesized from (4S,7R)-7,8,80 -trimethyl-

ARTICLE

4,5,6,7-tetrahydro-4,7-methane-2-indazole, HpzCam. This pyrazole has been described previously,31 its X-ray structure determined,32 and a series of bidentate or tridentate ligands were obtained upon reaction with other heterocycles.31,33 Tris(pyrazolyl)phosphane oxide, tris(pyrazolyl)hydroborate34 and other tripod ligands35 containing this pyrazole or related systems have also been described.36 The most similar ligand is that derived from 2,6-dibromopyridine, i.e. 2,6-bis[(4S,7R)-7,8,80 trimethyl-4,5,6,7-tetrahydro-4,7-methaneindazol-2-yl]pyridine, bpzCampy, although this is a tridentate ligand.37 The coordination chemistry of this kind of ligand has barely been explored. Some reactions with metal centers, such as Pd(II), Cu(II) or Rh(I), have been reported to give mono- or dinuclear derivatives.33,36,37 The synthesis of the ligand was carried out by deprotonation of the campho[2,3-c]pyrazole, HpzCam, with KH followed by reaction with 4,6-dichloropyrimidine. Only the isomer reflected in Chart 2 is obtained as deduced from the NOE observed between H5 of the pyrimidine ring and H30 of the pyrazole fragment. See the Supporting Information for more details concerning the synthesis, NMR and X-ray diffraction characterization of 1. Synthesis and General Characterization of the New Metallic Derivatives. The ligand bpzCampm was reacted with Cu(I) and Ag(I) salts containing BF4 or PF6 counteranions in a 1:1 ratio, yielding in all cases the expected [2
2] grids (Scheme 1). The complexes, once in the solid state, are stable in air. The derivatives crystallized with solvent molecules. Ag(I) compounds should be stored with the exclusion of light in order to avoid reduction to Ag(0). The IR spectra of 25 showed bands corresponding to the stretching frequencies of the counteranions. The ν(PF) (complexes 2 and 4) or ν(BF) (3 and 5) bands were split, indicating a possible decrease in the symmetry of the anions. In the mass spectra, peaks corresponding to the fragments [M2L2X]þ, [ML2]þ, and [ML]þ (M = Cu, Ag) were observed. In the case of 3, a tetranuclear fragment [Cu4L4X3]þ was also detected in the MALDI spectrum, an observation that points to the formation of the proposed [2
2] grid. Solid-State Characterization: Crystal Structures of Complexes 3 and 5. The structures of complexes 3 and 5 were determined by X-ray diffraction. The crystallographic data are gathered in Table 1 and a selection of bond lengths and angles is given in Table 2. The molecular structures of 3 3 4C7H8 and 5 3 2C7H8 are shown in Figure 2. Both complexes crystallize with toluene molecules, the positions of which will be explained below. It can clearly be seen that both derivatives have a [2
2] grid structure that is formed despite the large volume of the camphor moiety. It is noteworthy that these chiral grids contain 16 chiral centers (four for each ligand). These products crystallize in the chiral space group I4122 of the tetragonal system, and only one-fourth of the grid is crystallographically independent, with the whole grid generated by means of two C2 binary rotation axes containing Cu1/Ag1 and Cu2/Ag2 atoms. The MN distances (Table 2) are in the range expected and are longer for the silver derivative. The distances to the pyrazolyl rings are shorter, in accordance with their higher basicity. The pyrimidine and pyrazole heterocyclic rings are nearly coplanar and the metal centers are practically situated in the plane defined by these rings (torsion angles between 3 and 6°), exhibiting a distorted tetrahedral environment by coordination of two ligands in a bis-bidentate chelate fashion. The bite angles are smaller than 1769

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Scheme 1. Formation of the [2
2] Grids 25

Table 2. Distances (Å) and Angles (deg) Found for Complexes 3 3 4C7H8 and 5 3 2C7H8 (bite angles are represented in bold)a complex 3

complex 5 Distances (Å)

Cu(1)N(1) = Cu(1)N(1)1 1.974(6)

Ag(1)N(1) = Ag(1)N(1)1 2.236(5)

1

Ag(1)N(3)2 = Ag(1)N(3) 2.393(6)

2

Ag(2)N(5) = Ag(2)N(5)2 2.221(5)

2

Ag(2)N(4)2 = Ag(2)N(4) 2.351(5)

Cu(1)N(3) = Cu(1)N(3) 2.105(7) Cu(2)N(5) = Cu(2)N(5) 1.967(7) Cu(2)N(4) = Cu(2)N(4) 2.103(7) Angles (deg) N(1)Cu(1)N(1)1 143.1(4)

N(1)Ag(1)N(1)3 153.5(3)

N(3) Cu(1)N(3) 113.3(4)

N(3)3Ag(1)N(3) 122.1(3)

N(5) Cu(2)N(5) 145.5(4)

N(5)Ag(2)N(5)4 152.5(3)

N(4) Cu(2)N(4) 116.2(4) N(1)Cu(1)N(3)1 = N(1)1Cu(1)N(3) 122.8(3)

N(4)4Ag(2)N(4) 117.7(3) N(1)Ag(1)N(3)3 = N(1)3Ag(1)N(3) 122.1(2)

N(1)1Cu(1)N(3)1 = N(1)Cu(1)N(3) 78.8(3)

N(1)3Ag(1)N(3)3 = N(1)Ag(1)N(3) 72.0(2)

N(5) Cu(2)N(4) = N(5)Cu(2)N(4) 80.4(3)

N(5)4Ag(2)N(4)4 = N(5)Ag(2)N(4) 70.8(2)

N(5)Cu(2)N(4) = N(5) Cu(2)N(4) 118.6(3)

N(5)Ag(2)N(4)4= N(5)4Ag(2)N(4) 125.1(2)

1 2 2

2

2

2

2

Symmetry transformations used to generate equivalent atoms: (1) y þ 1/2, x  1/2, z þ 1/2; (2) y þ 3/2, x þ 3/2, z þ 1/2; (3) y, x, z þ 1; (4) y þ 1, x þ 1, z þ 1.

a

those corresponding to a regular tetrahedral environment and are in the range 7880° and 7072° for the copper and silver derivatives, respectively. The dihedral angle formed between each pair of facing ligands is 19.5° for 3 and 13.1° for 5. These values are clearly lower than those found in copper grids with bis(pyrazol-1-yl) ligands without the camphor groups,21 a fact that demonstrates the flexibility of this type of grid and makes them suitable for accommodating different substrates. If the size of the cavities generated is calculated from the distances between opposite C2 and C5 atoms of pyrimidine rings of facing ligands (Chart 3), the values obtained for 3 are 6.78 and 7.66 Å. The cavity is bigger in the silver structure, with values for W and L of 7.34 and 7.94 Å, respectively. The four metal centers define an approximate square (MMM angles of 87° and 92°) with longer MM distances for the silver derivative (6.57 Å in 5 and 6.30 Å in 3). A simplified drawing of the obtained grids is indicated in Chart 4, where the black and gray rectangles represent the ligands that are above and below the metal centers, respectively, and the point of the triangles reflects the orientation of the CH2CH2 groups of the indazole 6-membered ring. Considering the enantiomerically pure character of the ligands this is the only grid that can be formed. The aliphatic fragments of the ligands are gathered in the corners of the grids. It is noticeable that two types of corners are formed, a fact that has consequences for the crystalline structure (see below).

In contrast to the situation found in the grids described previously by us,21 it is remarkable that in both derivatives reported here the two cavities are not occupied by the anions. This is not favorable if the Coulombic attractions are considered but it is feasible that the lower dihedral angle between the facing ligands and the presence of the camphor groups create steric hindrance that prevents the hosting of anions in the cavities. In any case, complex 3 acts as a cavitand since one toluene molecule is hosted in each cavity. These toluene molecules interact through weak double π-stacking interactions38 with the two pyrimidine rings of the ligands that define the cavity (see Figure 3). In addition, there are toluene molecules in the intercationic regions in the positions represented in Figure 3 and these also exhibit π-stacking interactions with the pyrimidine rings of the ligands (see Table 3 for the distances and angles for these interactions with both types of toluene molecule). In this way, both sides of the pyrimidine ring of each ligand exhibit π-stacking interactions with toluene molecules. The anions are also situated outside the grids but in a different level than the toluene molecules (see Figure 2a) and these interact through weak hydrogen bonds with two adjoining grids and with the nearest exterior toluene. In the case of 5, on refining the structure some electronic density that could correspond to toluene was detected inside the cavities but it was not possible to localize the solvent molecules clearly. It is possible that the larger size of the cavities makes it much more difficult to establish the double π-stacking 1770

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Figure 2. Molecular structures of (a) 3 3 4C7H8 and (b) 5 3 2C7H8.

Chart 3

Chart 4

Figure 3. π-Stacking interaction of the internal and external toluene molecules with the pyrimidine rings in complex 3 3 4C7H8.

interactions detected in the case of 3 and more positions are therefore accessible to the molecules, which in turn leads to higher disorder. In any case, and in a similar way to that found in 3, the anions, bonded through hydrogen bonds, and toluene molecules, exhibiting π-stacking interactions (see Table 3), are located in the intercationic regions. Comparison of the parameters for the π-stacking interaction in the two molecules reveals that the interaction is stronger in the silver derivative (exterior toluene). The fact that the ligands are less divergent in this molecule possibly makes the orientation of the pyrimidine rings more favorable. In the crystalline structure, and removing the solvents, a packing index39 (percent filled space) of 46.5% for 3 and 47.8% for 5 has been calculated. A perspective of the packing mode of

both complexes along the c axis is presented in Figure 4. The channels formed due to the packing of the grids that extend along c axis have the form of a square prism with a side of 3.2 or 3.8 Å (for 3 and 5, respectively). The value has been determined considering the van der Waals radii of the nearest carbon atoms of the two pyrimidine rings of two facing ligands (C15C15 or C13C13 for 3 and 5, respectively). In the case of 3, if we consider the π-stacking interactions between the pyrimidine rings with the interior and exterior toluene molecules, columns of π-stacking interactions are formed along both the a and b axes. The asymmetry of the ligands is transferred to the grids, to their cavities and to the whole structure. In this way, the anions are located in holes lacking symmetry, formed by the sides of two grids. These chiral cavities should be interesting for the recognition of chiral anions.40 The grid corners that are of two types (Chart 4) define two different unions in the net, as can be seen in Figure 4. 1771

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Behavior of 25 in Solution. The 1H and 19F NMR spectra

of all derivatives and the 13C{1H} NMR spectra of the more soluble complexes 2 and 4 were recorded. In the case of complex 2, which is slightly more soluble than the others, different monoand bidimensional experiments were carried out in order to assign all the resonances in the NMR spectrum. For the rest of the derivatives, assignments were made by comparison with 2 or with the ligand itself. As expected, coordination of the ligand generally leads to a shift in the aromatic proton resonances to lower field. This effect is more marked for the copper derivatives than for the silver complexes, possibly due to the stronger character of the CuN bonds. For the pyrimidine ring, the shift is more marked for H2 than for H5, in agreement with the fact that H2 points toward the inside of the cavity of the grid. In the case of the copper derivatives 2 and 3, six signals were observed in the 1H NMR spectrum for the methyl groups instead of the three observed for the free ligand; this observation is due to the disappearance of the ligand C2 axis. The same number of Table 3. Distances (Å) and Angles (deg) for the π-Stacking Interactions in Complexes 3 and 5a,b π-stacking complex

ring

3 3 4C7H8 interior toluene

d(centcent) 4.16

d(centplane)

5 3 2C7H8 exterior toluene

3.76 3.71

β

3.58 cent pm-pl tol 12.8 30.6 3.89 cent tol-pl pm

exterior toluene

R

signals was observed at low temperature (80 °C). All of these findings are consistent with the existence of the grid structure in solution. However, for the silver derivatives 4 and 5, only three signals were detected for the methyl groups at room temperature, which indicates that the tetranuclear structure present in the solid state is not retained in solution. At 80 °C, six signals were observed for the methyl groups of complex 5 along with two signals for each H40 and H30 protons, indicating an asymmetric structure. This situation may arise from the presence of the grid structure at low temperature, although a slow interchange of the silver centers between the two positions of the ligand can also explain the observed spectrum. It can be concluded that, as stated in the introduction, ligand 1 is very useful to provide information about the behavior of these grids in solution. The 13C{1H} NMR spectra of 2 and 4 give the same information concerning the symmetry of the ligand. The chirality of these complexes led us to measure the specific optical rotation. The data are gathered in Table 4. The sign of the optical rotation of the derivatives indicates that all the complexes rotate light in the opposite sense to the ligand. Differences between derivatives 25 indicate that in solution there is a different behavior depending on the metal center (as previously observed by NMR) and the anion. The values for the copper derivatives are quite similar but are very different to those of the silver complexes, where the influence of the anion is

20.6

Table 4. Specific Optical Rotation Values for the Ligand bpzCampm and Complexes 25 in Acetone

3.50 cent tol-pl pm 19.4 21.4 3.76 cent pm-pl tol

3.73

3.38 cent tol-pl pm 3.46 cent pm-pl tol

5.5 24.3 21.1

[R]D20

compd

10.13

bpzCampm (1) [Cu(bpzCampm)]4(PF6)4 (2)

R represents the dihedral angle between the two planes and β the angle formed between the centroid-centroid and centroid-to-plane lines. b Cent = centroid, pm = pyrimidine, tol = toluene. a

6.42

[Cu(bpzCampm)]4(BF4)4 (3)

2.01

[Ag(bpzCampm)]4(PF6)4 (4)

46.36

[Ag(bpzCampm)]4(BF4)4 (5)

68.52

Figure 4. Unit cell of complexes (a) 3 3 4C7H8 and (b) 5 3 2C7H8 along the c axis. The position of exterior toluene molecules (in pink), interior toluene molecules (in red), and anions (in green) is indicated. The red arrow indicates one direction of π-stacking where the pyrimidine and toluene rings alternate. 1772

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Figure 5. UVvis spectra for ligand 1, derivatives 2, 3, and the indicated mixtures. (a) Dichloromethane: blue, 1; green, 2; red, 3; pink, 2 þ 4 NaBF4. (b) Methanol: blue, 1; purple, 2, orange, 3; gray, 3 þ 4 NH4PF6.

important. It is noticeable that for the silver complexes the value of the specific optical rotation is not similar to that of the free ligand, as one would expect for total decoordination, which indicates that another structure is formed in solution where the anion plays an important role. All these data indicate that the chirality in solution arises from the supramolecular structure. The UVvis spectra of the ligand and the copper derivatives 2 and 3 were registered in methanol and dichloromethane. The results are shown in Figure 5. Three bands were observed for the ligand (the data for the dichloromethane solution are 280, 305, and 313 nm). In the complexes these bands were also observed but they were shifted and in some cases the band at 305 nm, which in the ligand was observed as a shoulder, is no longer seen. The metallic derivatives exhibit a band at around 375 nm of variable intensity and this is ascribed to an MLCT band. It is possible to see that the behavior of the different metallic solutions studied is much more similar in methanol than in dichloromethane. The MLCT band has a higher intensity for derivative 2 especially in dichloromethane solution (ε = 12 360 M1 cm1). The intensity of this band in dichloromethane did not diminish on addition of four equivalents of NaBF4 to a solution of complex 2, which indicates that the PF6 anion exerts a positive effect on the intensity of the MLCT band. The effect of concentration (1
105 to 1
106 M range) on the ε value was also evaluated for complex 2 and the variation found was rather small, indicating that a process of association does not occur in solution (see the Supporting Information). The fluorescence of the ligand and derivatives 2 and 3 was also measured in the same two solvents at room temperature (Figure 6). No fluorescence was observed upon excitation of the MLCT band. However, luminescence was detected upon excitation in the regions about 280 or 315 nm (excitation at the maximum of the bands in each case) and the same bands were observed in both cases. A band with a shoulder was observed for the ligand in dichloromethane solution at 319 nm. However, for the methanol solution the fluorescence was very weak. Weak emission was also observed for all the methanol solutions of the complexes and that of complex 2 in dichloromethane. In other words, only the dichloromethane solution of complex 3, containing the BF4 anion, exhibited a considerable emission. In this case, two bands were observed due to the vibronic structure.41 Dichloromethane solutions of complex 2 þ 4 NaBF4 (solution I) or complex 3 þ 4 NH4PF6 (solution II) were also studied. Weak

Figure 6. Emission spectra of 1, 2, 3, and the indicated mixtures after excitation at the maximum of the band that appears in the range 310316 nm. The bands due to 3 (methanol), 2 (methanol), and that of 3 þ 4 NH4PF6 (methanol) are overlapped. dark blue, 1/CH2Cl2; green, 2/CH2Cl2; red, 3/CH2Cl2; light blue, 1/MeOH; purple, 2/MeOH; orange, 3/MeOH; dark gray, 3 þ 4 NH4PF6/MeOH; light gray, 3 þ 4 NH4PF6/CH2Cl2; pink, 2 þ 4 NaBF4/CH2Cl2.

emission was also observed in both cases. This indicates that the PF6 anion has a weakening effect over the luminescence of the complexes. The position of the bands for the complexes, very similar to that of the ligand, and the vibronic structure observed in some cases, make us to assign them as emissions involving the coordinated heterocyclic ligands.4143 Concerning the fact that no CT (charge transfer) emission is observed, it has been stated that these signals are typically weak and short-lived in Cu(I) complexes,44 and it is also proposed that distortions around the metal center leads to a reduction of the excited-state lifetime and eventually to cancel the luminescence.45 As a conclusion from both studies, it is possible to say that the PF6 anion has a marked effect on the photochemical behavior of the grid derivatives, but this effect of the anion can only be observed in dichloromethane solutions. Methanol cancels the effect of the counteranion. It is possible that in methanol the ions are more solvated and the “ionic pairs or aggregates” are more dissociated.

’ CONCLUSIONS A new chiral enantiopure ligand that can act as a bis-bidentate chelate has been synthesized. Reaction of this ligand with Cu(I) 1773

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Crystal Growth & Design and Ag(I) salts led to the synthesis of the first chiral complexes with a [2
2] grid structure derived from pyrimidine. In the solid state structures the opposite ligands are slightly divergent and generate cavities in which toluene molecules rather than anions are hosted. In the case of the copper derivative with the BF4 anion (3) columns of π-stacking with alternate toluene and pyrimidine rings are formed in two different directions. Chiral cavities are generated in the crystalline structure. The asymmetry of the ligands is transferred to the grids, to their cavities and to the whole structure. In this way, the anions are located in asymmetric holes, formed by the sides of two grids. These chiral cavities should be interesting for the recognition of chiral anions. The structure of the copper grids is maintained in acetone solution at room temperature. This is not the case for the silver derivatives. The optical rotation of the complexes in solution has a different sign to that of the ligand and depends on the metal and counteranion, a fact that indicates that the chirality arises from the supramolecular structure. In the photochemical studies of the copper derivatives, the presence of the PF6 anion leads to a higher intensity MLCT band in the UVvis spectra. Fluorescence was observed for some derivatives. A strong influence of the solvent and the counteranion on the intensity of the emission was detected.

’ ASSOCIATED CONTENT

bS

Supporting Information. Data for the synthesis and characterization of 1, X-ray crystallographic files in CIF format, tables and ORTEP representations. Data of photophysical properties. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

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