Supramolecular Architectures of Pt (II) Complexes Controlled by

Article pubs.acs.org/crystal

Supramolecular Architectures of Pt(II) Complexes Controlled by Hydrogen Bonds and by Guest Molecules Alessia Bacchi,* Mauro Carcelli, Paolo Pelagatti, Gabriele Rispoli, and Dominga Rogolino Dipartimento di Chimica Generale ed Inorganica, Chimica Analitica, Chimica Fisica, Parco Area delle Scienze 17/A, 43124 Parma, Italy S Supporting Information *

ABSTRACT: The synthesis, the characterization, and the X-ray structures of cis- and trans-Pt(II) complexes with α-(4-pyridyl)benzhydrol are reported. The crystal organization of several solvates is presented and compared with particular regard to the role of the hydrogen bond in the formation of the supramolecular architecture and how this is regulated by the presence of guest solvent molecules. It is shown that these networks are flexible as a response to the insertion of small molecules. trans-Complexes reproduce the “venetian blinds” pattern observed for the palladium analogs [Bacchi, A.; Bosetti, E.; Carcelli, M.; Pelagatti, P.; Rogolino, D.; Pelizzi, G. Inorg. Chem. 2005, 44, 431 and Bacchi, A.; Bosetti, E.; Carcelli, M. CrystEngComm 2005, 7, 527], while cis-complexes persistently give supramolecular rectangles assisted by the concomitant presence of OH···OH hydrogen bonds and edge-to-face interactions between aromatic rings inside the rectangles. These rectangles are packed by taking advantage of OH···Cl interactions unless the guests provide other means of satisfying the hydrogen bond donor/acceptor balance.

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INTRODUCTION Supramolecular systems based on coordination compounds have received much attention because of their potential use as sensors, probes, photonic devices, and catalysts and in host− guest chemistry.1−7 In particular, great effort has been devoted to the design of crystalline species capable of incorporating small molecules into the lattice through weak interactions, which might be broken at will.8 The interest in potential applications of host−guest materials has rapidly extended to the fields of separation, storage, heterogeneous catalysis, sensor devices, and solid-state “green chemistry”.9−12 One very successful approach to these applications is the design of materials with permanent porosity such as MOFs,4,9,12 which can incorporate guest molecules by shape and size selectivity. The different approach that we are pursuing considers a crystalline material as a supramolecular device that can rearrange collectively to the presence or the removal of a guest by a reversible modification of the crystal packing assisted by the formation and breaking of soft interactions. In this case, porosity is not a permanent property of the starting material (apohost), but it is created dynamically when triggered by an external stimulus. Wheel-and-axle diols (WAADs)13 have proven to be good candidates to promote dynamic porosity. They are constituted by two bulky and relatively rigid end groups (wheels) bonded to a linear rigid link (axle) (Figure 1): their molecular shape frustrates the achievement of a unique compact packing that rather may be realized by inclusion of suitable small molecules filling the voids. Recently, we studied the supramolecular architecture and the solid-state reactivity of transition metal complexes containing pyridylbenzhydrol ligands.14−17 These organic/inorganic systems present a N(pyridyl)−M−N© 2011 American Chemical Society

(pyridyl) axle and two sterically encumbered ends with a hydroxyl function that can be involved in the crystal organization (Figure 1).

Figure 1. WAADs.

The propensity to clathration observed for the palladium complexes with pyridylbenzhydrol ligands can be related to their irregular shape, which promotes the reversible uptake of volatile guests by solid−gas processes.15 With the aim to extend the supramolecular architectures accessible with pyridylbenzhydrol ligands, we focused our attention on the use of the Pt(II) ion. Platinum can form cis and trans dipyridyl complexes. The trans geometry leads to complexes similar to the Pd-WAAD (Figure 1), while the cis configuration could give rise to different supramolecular structures. Received: September 9, 2011 Revised: November 15, 2011 Published: November 22, 2011 387

dx.doi.org/10.1021/cg201185c | Cryst. Growth Des. 2012, 12, 387−396

Crystal Growth & Design

Article

Figure 2. Examples of supramolecular rectangles with cis-Pt(II)(LOH)2 synthons. trans-Pt(LOH)2Cl2 (1t). The ligand (100 mg, 0.14 mmol) was dissolved in 5 mL of dimethylsulfoxide and slowly added at room temperature to 2 mL of a water solution of K2PtCl4 (29 mg, 0.07 mmol). The solution turned yellow within few minutes. It was stirred at room temperature for 24 h: on concentrating the solution, a tiny yellow precipitate was formed, which was filtered off and washed with water. Recrystallization from chloroform gives 1t. Yield, 78%. IR (cm−1): νOH = 3156 (br). 1H NMR (CDCl3, ppm): δ 8.81 (d, 4H, Hortho py), 7.35−7.23 (m, 24H, Harom + Hmeta py), 3.64 (s, br, 2H, OH, D2O exchangeable). 195Pt{1H}-NMR (CDCl3, ppm): δ −1936. Anal. calcd for C36H30Cl2N2O2Pt: C, 54.87; H, 3.84; N, 3.56. Found: C, 54.95; H, 3.72; N, 3.29. Crystal Structure Determination. Mo Kα radiation (λ = 0.71073 Å) on a SMART CCD diffractometer is used for all compounds. All data are collected at room temperature (293 K). Lorentz, polarization, and absorption corrections are applied.37 Structures are solved by direct methods using SIR9738 and refined by full-matrix least-squares on all F2 using SHELXL9739 implemented in the WingX package.40 Hydrogen atoms were mostly introduced in idealized positions riding on their carrier atoms; where data were of sufficient quality, the hydrogens of OH groups were located from Fourier maps. Anisotropic displacement parameters were refined for all nonhydrogen atoms, except for compounds 1t·2(CH3)2CO and 1c, see below. Tables 2 and 3 summarize crystal data and structure determination results. The crystals of 1t·2(CH3)2CO were seriously twinned, and the refinement was carried out on a model comprising two images of the complex and of the guest, whose occupancy was refined to 0.88:0.12. Crystals of 1c were also twinned, and the data reduction was carried out on two individuals rotated by 180° around [010] in real space. The poor quality of the resulting structural refinement reflects the impossibility of a complete description of the twinning in both cases. Hydrogen bonds have been analyzed with SHELXL9739 and PARST97;41 extensive use was made of the Cambridge Crystallographic Data Centre packages42,43 for the analysis of crystal packing. Crystallographic data (excluding structure factors) for 1c, 1c·(CH3)2CO, 1c·1/2CHCl3, 1c·2CH3OH, 1c·2.5THF, 1c·CH3OH, 1c·3CH3COOH, 1t, 1t·2(CH3)2CO, 1t·2DMSO, and 1t·4DMSO have been deposited with the Cambridge Crystallographic Data Centre as supplementary publications nos. CCDC 833886−833896. Copies of the data can be obtained free of charge on application to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK [Fax: (+44)1223-336-033; email: [email protected]].

There are several examples of Pt-containing supramolecular metallacycles or metallacages that encourage the use of this metal as a useful building block to construct a wide range of supramolecular architectures.18−22 In general, square or rectangle metallacycles are obtained by using four metal ions with cis coordination geometry as corners and four ditopic ligands as sides23 or by using two metal ions and two “bent” ligands.24−27 Recently, cis-platinum complexes are used with ditopic ligands to obtain metallacalix[4]arenes.28 Anyway, in the synthesis of metallamacrocycles, the pyridyl ligands were used as covalent spacers.29−36 On the contrary, we try to build metallacycles based on Pt(II) complexes with pyridylbenzhydrol ligands, whose self-assembling is assisted by noncovalent interaction, such as the hydrogen bonds involving the hydroxyl functions of the organic ligands and the anions (Figure 2). Herein, we report the synthesis, the characterization, and the X-ray structures of cis- and trans-Pt(II) complexes with α-(4pyridyl)benzhydrol (LOH). The crystal organization of several solvates is presented and compared, with particular regard to the role of the hydrogen bond in the formation of the supramolecular architecture and how this is regulated by the presence of guest solvent molecules.

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EXPERIMENTAL SECTION

General. The ligand LOH (Aldrich, 75%) was recrystallized from methanol. NMR spectra (1H and 195Pt{1H}) were recorded at 27 °C on a Bruker Avance 400 FT spectrophotometer. K2PtCl4 in D2O (−1628 ppm) was used as a reference for 195Pt{1H}-NMR spectra. IR spectra were obtained with a Nicolet 5PCFT-IR spectrophotometer in the 4000−400 cm−1 range, in ATR mode (diamond plate). Elemental analyses were performed by using a Carlo Erba model EA 1108 apparatus. Synthesis and Characterization. cis-Pt(LOH)2Cl2 (1c). The ligand (100 mg, 0.14 mmol) was dissolved in 30 mL of boiling ethanol. The solution was allowed to cool at room temperature, and then, 5 mL of a water solution of K2PtCl4 (29 mg, 0.07 mmol) was added. A precipitate immediately appeared, and the solution was light pink: within 24 h, the color turns to light yellow. The reaction mixture was stirred at room temperature for 3 days and then concentrated in vacuum, and the precipitate was filtered off and washed with water. Yield: 61%. IR (cm−1): νOH = 3311 (br). 1H NMR (CDCl3): δ 8.64 (d, 4H, Hortho py), 7.37−7.19 (m, 24H, Harom + Hmeta py), 2.91 (s, br, 2H, OH, D2O exchangeable). 195Pt{1H}-NMR (CDCl3, ppm): δ −1985. Anal. calcd for C36H30Cl2N2O2Pt: C, 54.87; H, 3.84; N, 3.56. Found: C, 54.52; H, 3.77; N, 3.45.

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RESULTS AND DISCUSSION Synthesis of the Complexes and Solution Behavior. LOH is reacted with K2PtCl4 following different synthetic 388



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Table 1. Crystallization Conditions (T = 25 °C, Slow Evaporation of Saturated Solutions) starting compd 1c 1c 1c 1c 1c 1c and 1t 1t 1t 1t

crystallization solvent toluene, acetonitrile, ethyl acetate (1:1:1) acetic acid acetone CHCl3 THF CH3OH/CHCl3 CHCl3 acetone DMSO

room temperature. In these conditions, the ligand reacts completely; the cis-product precipitates and can be isolated in good yields. From the filtrate, a mixture of cis- and transisomers (about 1:1) can be recovered, as evidenced by NMR measurements. Therefore, both isomers 1c and 1t are formed in ethanol solution, but the former can be separated by crystallization. cis−trans Isomerization has been excluded by proton NMR measurements: Crystals of 1c have been dissolved in CDCl3 at −40 °C, and a unique set of signals is present. No isomerization has been detected in the sample by heating to 60 °C or after 3 days at room temperature. Analogous behavior has been found for cis- and trans-complexes of Pt(II) with diethyl(pyridin-4-ylmethyl)phosphate.44 The use of methanol instead of ethanol leads to the coprecipitation of the two isomers (1:1.5 cis to trans molar ratio), since in this solvent their solubility is evidently similar. The concurrent formation of both isomers can be explained on considering the steric hindrance of the phenyl rings, which probably favors the formation of the trans-isomer, that can be detected by NMR already after 6 h of reaction, when most of the ligand is still unreacted. Effectively, it has been previously reported that bulky ligands need longer reaction times and hamper the formation of the cis-product.45−47 With the aim to isolate the pure trans-isomer (1t), synthesis has been attempted by Kauffman's method,48 but again, both isomers have been obtained. Moreover, crystallization of the crude product from methanol/CHCl3 afforded crystals of a 1:2 host:methanol solvate of 1c [cis-Pt(LOH)2Cl2·2CH3OH]. The reaction between K2PtCl4 and LOH has been conducted also at 50 °C in ethanol for 2 h. However, also in this case, a mixture of the cis- and trans-isomers has been isolated (at about 4:1 molar ratio). Longer reaction times resulted in extensive decomposition. From the recrystallization of the crude mixture from methanol/CHCl3, a 1:1 methanol solvate of 1c was obtained [cis-Pt(LOH)2Cl2·CH3OH].

product 1c 1c·3CH3COOH 1c·(CH3)2CO 1c·1/2CHCl3 1c·2.5THF 1c·CH3OH or 1c·2CH3OH 1t 1t·2(CH3)2CO 1t·2DMSO or 1t·4DMSO

Figure 3. Molecular structures of trans-Pt(LOH)2Cl2 (1t) (top) and cis-Pt(LOH)2Cl2 (1c) (bottom) in the apohost compounds.

approaches, to obtain the cis and trans isomers of the complex separately. cis-Pt(LOH)2Cl2 (1c) has been finally prepared by stirring K2PtCl4 with 2 equiv of LOH in ethanol over 3 days at

Table 2. Crystal Data and Structure Refinement for Compounds 1t, 1t·2(CH3)2CO, 1t·2DMSO, and 1t·4DMSO compd formula molecular weight crystal system space group Z a (Å) b (Å) c (Å) α (°) β (°) γ (°) V (Å3) ρ (Mg m−3) θ range for data collection (°) independent reflections reflections collected data/restraints/parameters goodness-of-fit on F2 R1 [I > 2σ(I)] wR2 [I > 2σ(I)] R1 (all data) wR2 (all data)

1t

1t·2(CH3)2CO

1t·2DMSO

1t·4DMSO

C36H30Cl2N2O2 Pt 788.61 monoclinic P21/n 4 10.718(1) 17.391(2) 16.807(2)

C42H42Cl2N2O4Pt 904.77 monoclinic P21/n 2 7.288(1) 18.572(3) 14.618(2)

C40H42Cl2N2O4PtS2 944.87 monoclinic P21/n 2 7.672(2) 18.482(4) 14.479(3)

C44H54Cl2N2O6PtS4 1101.12 monoclinic P21/n 2 9.3556(8) 10.5786(9) 24.03(2)

95.796(2)

104.342(2)

106.31(1)

97.300(1)

3116.8(6) 1.681 1.69−23.28 4477 [R(int) = 0.0423] 19838 4477/0/396 0.995 0.0312 0.0763 0.0431 0.0795

1916.9(5) 1.568 1.81−26.41 3926 [R(int) = 0.0423] 19042 3926/0/79 1.244 0.2595 0.5683 0.2724 0.5726

1970.5(8) 1.592 1.83−26.75 4191 [R(int) = 0.0379] 21093 4191/0/238 1.004 0.0237 0.0646 0.0363 0.0682

2359(2) 1.591 1.71−26.73 4995 [R(int) = 0.0440] 24563 4995/0/272 0.967 0.0364 0.0927 0.0596 0.1004

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1c

C36H30Cl2N2O2Pt 788.61 triclinic P1̅ 4 13.83(4) 14.24(4) 17.57(6) 81.27(6) 80.12(5) 89.1(1) 3369(18) 1.555 1.45−22.37 12862 [R(int) = 0.0000] 12862 12862/2/236 2.151 0.2560 0.5170 0.2560 0.5170

compd

formula molecular weight crystal system space group Z a (Å) b (Å) c (Å) α (°) β (°) γ (°) V (Å3) ρ (Mg m−3) θ range for data collection (°) independent reflections reflections collected data/restraints/parameters goodness-of-fit on F2 R1 [I > 2σ(I)] wR2 [I > 2σ(I)] R1 (all data) wR2 (all data) 3500.7(8) 1.607 2.24−27.92 5899 [R(int) = 0.0441] 10284 5899/0/441 1.005 0.0579 0.1355 0.0864 0.1501

98.76(1)

C39H36Cl2N2O3Pt 846.69 monoclinic P21/a 4 13.593(2) 14.738 (1) 17.680(3)

1c·(CH3)2CO C36.5H30.5Cl3.5N2O2Pt 848.3 triclinic P1̅ 4 13.526(1) 15.172(1) 17.955(1) 79.538(1) 71.919(1) 86.574(1) 3444.5(4) 1.636 1.21−23.27 9867 [R(int) = 0.0696] 29738 9867/0/815 0.846 0.0370 0.0711 0.0744 0.0773

1c·1/2CHCl3 C37H34Cl2N2O3Pt 820.65 triclinic P1̅ 4 13.822(1) 14.991(1) 17.094(1) 78.330(1) 71.749(1) 85.893(1) 3294.1(4) 1.655 1.28−28.28 15632 [R(int) = 0.0381] 40458 15632/1/837 0.964 0.0240 0.0537 0.0344 0.0560

1c·CH3OH C42H42Cl2N2O8Pt 968.77 triclinic P1̅ 2 9.577(1) 13.027(2) 16.796(2) 89.144(2) 85.349(2) 81.510(2) 2065.7(5) 1.558 1.58−25.06 7250 [R(int) = 0.0531] 15901 7250/0/496 1.153 0.0497 0.1094 0.0855 0.1335

1c·3CH3COOH

3548.3(6) 1.591 1.35−26.40 7279 [R(int) = 0.0515] 38443 7279/1/442 0.865 0.0242 0.0427 0.0397 0.0449

93.515(2)

C38H38Cl2N2O4Pt 849.71 monoclinic P21/n 4 20.256(2) 8.335(1) 21.056(2)

1c·2CH3OH 1c·2.5THF

C46H50Cl2N2O4.50Pt 968.87 triclinic P1̅ 2 9.576(2) 13.448(4) 17.558(5) 96.543(3) 80.661(3) 78.431(3) 2160(1) 1.489 1.19−23.30 6150 [R(int) = 0.0804] 16798 6150/24/500 1.141 0.0392 0.0719 0.0932 0.0939

Table 3. Crystal Data and Structure Refinement for Compounds 1c, 1c·(CH3)2CO, 1c·1/2CHCl3, 1c·CH3OH, 1c·3CH3COOH, 1c·2CH3OH, and 1c·2.5THF

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Finally, 1t has been isolated in very good yields by reacting K2PtCl4 and LOH (2 equiv) in DMSO for 24 h with recrystallization of the crude product from chloroform. Compounds 1c and 1t have been fully characterized by 1H NMR, 195Pt{1H}-NMR, IR, elemental analysis, and X-ray diffraction analysis. They can be differentiated for the stretching of the OH group, which is at 3311 and 3156 cm−1 for the cisand trans-isomer, respectively. In the 1H NMR spectrum, coordination to the metal leads to a deshielding (0.1−0.3 ppm) of the resonance of the ortho protons of the pyridine ring vs the free ligand, indicating a slight decrease in electron density upon coordination. The shift is greater for the trans-complex, as already observed for other chloro compounds with pyridine derivatives.12 The 3J(195Pt−1H) coupling constants are not observed, probably because satellites are broad and less intense on high field instruments, because of chemical shift anisotropic relaxation.45 The 195Pt resonances for 1c and 1t (−1985 and −1936 ppm, respectively) are in accord with literature data.45 A large number of recrystallizations have been carried out, with the aim to investigate the possible crystalline architectures that can be realized. In particular, solvents with different hydrogen bond donor/acceptor features have been chosen. Crystals of 1c have been obtained by slow evaporation of a mixture of toluene, acetonitrile, and ethyl acetate (1:1:1), while the crystallization in acetone, tetrahydrofuran, chloroform, and acetic acid afforded the corresponding solvates (Table 1): Their structures and the role of noncovalent interactions in the definition of the supramolecular motives are described in the solid-state section. Crystals of 1t suitable for X-ray analysis have been obtained by slow evaporation of a chloroform solution, while crystallization in DMSO afforded two different solvates [trans-Pt(LOH)2Cl2·2DMSO and trans-Pt(LOH)2Cl2·4DMSO, Table 1]. No traces of 1c or of cationic DMSO-coordinated species have been detected, as possible for chloro complexes with pyridine ligands.49 Solid-State Structures. In analogy with our previous approach,14,15,17,50,51 herein, we analyze the influence of the guest on the packing of the apohost, to get insight into the flexibility of the networks as a response to the insertion of small molecules and as a hint to the comprehension of the mechanisms for desolvation. We base our present analysis mainly on the screening of the hydrogen bonds interactions, since these are usually the most stabilizing and the most directional features that can drive the structural arrangement, even if it is recognized that also less evident or popular interactions are important in the definition of the supramolecular aggregation in the solid.52−54 trans-Pt(LOH)2Cl2 (1t). The trans-Pt(LOH)2Cl2 molecule (Figure 3) presents the wheel-and-axle shape whose packing and host−guest features have been extensively discussed in the last years, especially for the analogous palladium complexes.14,15 This new series of trans-platinum(II) complexes, in fact, replicates the same features previously observed, confirming their robustness. The apohost (1t), the 1:2 host:acetone solvate [1t·2(CH 3 ) 2 CO], and the 1:2 host:DMSO solvate (1t·2DMSO) are isostructural to the respective palladium complexes14 (Tables 2 and 3): they present a layer arrangement of the complex molecule containing arrays of guests hydrogenbonded to the terminal carbinol OH groups (Figure 4). The alteration of the stoichiometry with the insertion of a higher amount of guest G in 1t·4DMSO (1:4 host:guest) destroys the layered pattern, since one pair of DMSO molecules forms the canonical trans-Pt(LOH)2Cl2·2G system, while the other pair is intercalated between the complexes (Figure 4).

Figure 4. Crystal packing of 1t·2G compounds (in the figure, G = acetone) (top) and of 1t·4DMSO (bottom). Guest molecules are evidenced in the spacefill mode.

cis-Pt(LOH)2Cl2 (1c). In all of the structures where the cisPt(LOH)2Cl2 unit is observed, it presents the same conformation (Figure 3), with only small discrepancies regarding the orientation of the carbinol phenyls. The pyridine rings form a dihedral angle of 72° due to steric crowding on the coordination plane, and the carbinol phenyls are arranged as a propeller around the −COH center. We analyze the architectures generated by these V-shaped cis-Pt(LOH)2Cl2 building units in presence of a series of guest molecules in the compounds characterized in this work. In the apohost complex 1c, two nearly identical nonequivalent molecules are present in the asymmetric unit, and they are linked in homomeric centrosymmetric supramolecular rectangles by OH···OH hydrogen bonds, where for each complex one OH acts as donor and the other as acceptor (Figure 5).

Figure 5. Supramolecular rectangle in the structure of 1c. Rectangle dimensions are defined as (e) 13.4, (f) 11.5, and (g) 7.8 Å. Table 5 reports the dimensions observed in the solvate compounds.

The carbinol aryls and the pyridines inside the rectangle are oriented to form multiple edge-to-face contacts, reminiscent of phenyl embraces.55 Details on hydrogen bond geometry are 391



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Table 4. Hydrogen Bond Geometry (Å, °) for Host···Host (HH) and Host···Guest (HG) Interactionsa compd 1c

1c·(CH3)2CO 1c·1/2CHCl3

1c·CH3OH

1c·3CH3COH

1c·2CH3OH

1c·2.5THF

1t 1t·2(CH3)2CO 1t·2DMSO 1t·4DMSO a

interaction cis-Pt(LOH)2Cl2·nG forming supramolecular rectangles HH rectangle 1 to 1 OH···O (−x, 2 − y , −z) HH rectangle 2 to 2 OH···O (1 − x, 1 − y, −z) HH rectangle 1 to 2 OH···Cl (3 − x, 1 − y, −z) HH rectangle 2 to 1 OH···Cl (2 − x, 1 − y, −z) HH OH···O (1 − x, 1 − y, 1 − z) HH OH···Cl (x −1/2, 1/2 − y, z) HH rectangle 1 to 1 OH···O (1 − x, 1 − y, −z) HH rectangle 2 to 2 OH···O (2 − x, −y, 2 − z) HH rectangle 1 to 2 OH···Cl (x, y, z − 1) GH rectangle 2 Cl3CH···Cl (x, y, z − 1) HH rectangle 1 to 1 OH···O (2 − x, −y, −z) HH rectangle 2 to 2 OH···O (1 − x, 1 − y, −z) HH rectangle 2 to 1 OH···Cl HG1 rectangle 1 OH···OHCH3 (x, y, z − 1) G2H rectangle 2 CH3OH···Cl (1 − x, 1 − y, 1 − z) HH OH···O (2 − x, 1 − y, −z) HG1 OH···OHAc (x + 1, y, z) G1G2 AcOH···OHAc G2G1 AcOH···OHAc G3G3 AcOH···OHAc (−x, 2 − y, 1 − z) 1c·nG not forming supramolecular rectangles HG1 OH···OHMe HG2 OH···OHMe G2H MeOH···Cl (1 − x, 1 − y, −z) HG1 OH···THF (x + 1, y − 1, z) HG2 OH···THF (x, y, z + 1) 1t·nG HH OH···Cl (x + 1, y, z) HH OH···Cl (x − 1, y, z) HG OH···OC(CH3)2 HG OH···OS(CH3)2 HG OH···OS(CH3)2

D···A (Å)

D−H···A (°)

2.88 2.84 3.38 3.32 2.87 3.30 2.96 3.31 3.19 3.50 2.98 2.83 3.23 2.78 3.26 2.99 2.90 2.61 2.69 2.63

155 149 167 102 157 169 161 135 166 154 174 159 170 171 172 172 171 148 170 149

2.72 2.73 3.14 2.73 2.84

170 175 178 166 134

3.16 3.16 2.78 2.69 2.71

158 170 145 157 163

Independent guest molecules are labelled as Gn. The s.u.'s values are in the order of the last decimal digits.

rectangles are equivalent. The layers are organized in the ab plane [a = 13.593(2), b = 14.738(1) Å, and γ = 90°], which indeed has rather similar metrics as in the triclinic 1c [a = 13.83(4), b = 14.24(4) Å, and γ = 89.1(1)°] (Tables 2 and 3). The surfaces of the layer are made by the carbinol phenyls; the acetone molecules are nested between these, thus slightly changing the direction of stacking of the layers, as shown by the similar length of the c-axis in 1c and 1c·(CH3)2CO [17.680(3) and 17.57(6) Å], although with different α and β angles (Tables 2 and 3). cis-Pt(LOH)2Cl2 2:1 Chloroform Solvate (1c·1/2CHCl3). Two nonequivalent but very similar complex molecules are present in the asymmetric unit, along with one chloroform molecule, in a 2:1 host:guest stoichiometry. The cis-Pt(LOH)2Cl2 units are arranged exactly as in the apohost compound, but the chloroform molecules destroy the cross-linking between the herringbone arrays in the [110] direction by substituting the OH···Cl with the Cl3CH···Cl−Pt hydrogen bonds to rectangle(2); rectangle(1) does not interact with the guest, and its pendant OH donors remain unsatisfied (Figure 7a). cis-Pt(LOH)2Cl2 1:1 Methanol Solvate (1c·CH3OH). The introduction of a methanol guest with significant hydrogenbonding attitudes confirms that the layers can be cut by breaking the OH···Cl hydrogen bonds along the [110] direction and replacing them by a host−guest interaction.

found in Table 4. The two nonequivalent rectangles thus assembled have identical conformation. Rectangle(1) (Figure 6a) employs the two OH groups involved as acceptors in the rectangle as hydrogen bond donors toward two chloride ligands belonging to two adiacent rectangles(2) to form a chain along the [110] direction. Concomitantly, the pendant OH donors of rectangle(2) link to two chlorides of two rectangles(1) and cross-link the previous arrays in the [110] direction forming a layer in the ab plane. Figure 6b shows the resulting packing pattern, consisting of a herringbone arrangement of rectangles that will constitute the basis of all of the other packing observed in this work. The layers are stacked along the [001] vector through phenyl−phenyl contacts. In summary, each cisPt(LOH)2Cl2 unit employs both OH as hydrogen bond donors and employs one OH and one chloride as acceptors as described above. The remaining OH and Cl potential acceptors interact with CH groups and consolidate the layer networks. We now analyze the influence of the insertion of one or more guest molecules in the framework. cis-Pt(LOH)2Cl2 1:1 Acetone Solvate [1c·(CH3)2CO]. The inclusion of one acetone guest in the packing preserves the layer structure made by cross-linked herringbone chains of supramoleculars rectangles observed in the apohost compound. In fact, the layer organization is here symmetrized as only one type of rectangle is present and all of the interactions between 392



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Figure 7. Organization of supramolecular rectangles and their interaction with the guests in (a) 1c·1/2CHCl3, (b) 1c·CH3OH, and (c) 1c·3CH3COOH. Guest molecules are evidenced in spacefilling mode; hydrogen bonds are dashed.

Figure 6. Packing arrangement of 1c (carbinol aryls and hydrogens omitted for clarity): (a) chains of supramolecular rectangles linked by OH···Cl hydrogen bonds and (b) layers of supramolecular rectangles made by the chains displayed in (a) linked by OH···Cl hydrogen bonds. Two chains are represented, one colored in green for clarity. Shaded inset: scheme showing the hydrogen bond connectivity of the layers.

Here, two independent methanol molecules concur to segregate the herringbone chains inside the layers: one CH3OH donates a OH···Cl hydrogen bond to break the cross-linking along [110], while the second CH3OH accepts the hydrogen bond by the pendant OH groups. CH3OH molecules are inserted between chains as in a zipper. The structure is isostructural to 1c·1/2CHCl3 showing the robustness of the herringbone chain motif (Figure 7b). cis-Pt(LOH)2Cl2 1:3 Acetic Acid Solvate (1c·3CH3COOH). The cis-Pt(LOH)2Cl2 building unit crystallized accompanied by three molecules of acetic acid. The supramolecular rectangle pattern is conserved similarly to all of the structures discussed above. The guest is a strong hydrogen bond donor and acceptor, and it destroys the connectivity between the supramolecular rectangles by acting on the hierarchy of donor/acceptors according to Etter's rules.56 Because acetic acid is in the structure both the best donor and the best acceptor, the three guest molecules dimerize through the pervasive R22(8) synthon.57 Each supramolecular rectangle corresponds stoichiometrically to three acetic acid dimers. The next best donors are the −OH groups left pendant after the formation of the supramolecular rectangle. These interact with the carbonyl acceptors of a pair of acetic acid dimers (Figure 7c). Because there are no more donors in the structure, the

Figure 8. Crystal packing of 1c·3CH3COOH: (a) column of supramolecular rectangles decorated by acetic acid dimers hydrogen bonded to the rectangles and intercalated between the columns (green); (b) interdigitation of columns by stacking of coordination rectangles. Hydrogen bonds are dashed.

chlorides of the supramolecular rectangle and the carbonyls of the remaining dimer are not involved in further hydrogen bonds. Therefore, the structure is made by isolated rectangles perfectly stacked in columns along a, surrounded by acetic acid dimers, two connected to the rectangles and one intercalated. Columns are interdigitated by self-intercalation of the rectangle planar coordination systems (Figure 8). Although frequent, the supramolecular rectangle observed so far is not the only possible supramolecular arrangement for cisPt(LOH)2Cl2. A change in stoichiometry and nature of the guests revealed a different organization of the complex. 393



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the best way to employ all of them in the packing is by assembling the layers sketched in Figure 6. The insertion of small guests produces effects depending on their hydrogenbonding nature. Acetone is a moderate acceptor, worse than OH and Cl; therefore, its presence does not alter the assembly of the layers, but it intercalates on the layer surface. CHCl3 is a moderate donor in competition with OH; in fact, it cuts the layer structure by replacing one OH···Cl hydrogen bond with a CH···Cl interaction; methanol works in the same way and replaces one OH···Cl interaction with one MeOH···Cl and one OH···(OH)Me hydrogen bonds. The best hydrogen bond acceptors, that is, the carbonyl atoms of the acetic acid dimers, compete with the metal-bound chlorides59 and destroy the cross-linking of the supramolecular rectangles. One aspect is worth noting in this analysis: The packing of cis-Pt(LOH)2Cl2 involves a vast variety of intermolecular interactions much richer than the limited set of hydrogen bonds described here and surely cannot be predicted by the common considerations based on the stability of single interactions. Nevertheless, the careful comparison of the same molecule packed in slightly different environments perturbed by the presence of diverse guests allows us to highlight the most conserved features, that necessarily must be related to the most stable or the easiest formed interactions. We can therefore rank the importance of the factors involved in the supramolecular assemblies on the basis of their robustness. Effectively, supramolecular rectangles are persistent in this series of compounds. cis-Pt(LOH)2Cl2 is a reliable metallatecton, and this can be ascribed to the concomitant presence of OH···OH hydrogen bonds and of edge-to-face interactions between aromatic rings inside the rectangles. It is worth noting that the dimensions of the rectangles are well conserved (Table 5). These rectangles are

Figure 9. Organization of supramolecular rectangles and methanol molecules in 1c·2CH3OH. Left: hydrogen-bonding systems linking two equivalent rectangles and four MeOH molecules, equivalent in pairs (different colors). Right: stacking of complexes by CH···Cl interactions. Hydrogen bonds are dashed.

Figure 10. Crystal packing organization of 1c·2.5THF. One THF molecule is disordered around a center of symmetry, the disordered image is shaded in gray, and hydrogen bonds are dashed.

Table 5. Dimensions (Å) of the Supramolecular Rectangles in the cis-Pt(LOH)2Cl2·nG Seriesa

cis-Pt(LOH)2Cl2 1:2 Methanol Solvate (1c·2CH3OH). The complex donates two OH···O hydrogen bonds to two independent CH3OH molecules. One of the guests donates a OH···Cl hydrogen bond to a chloride group of a second complex, forming a dimeric system where two complexes are bridged by one pair of equivalent MeOH molecules, while the other guests decorate the periphery of the system (Figure 9). These units are stacked by superimposition of the rectangle planar coordination moiety. cis-Pt(LOH)2Cl2 1:2.5 THF Solvate (1c·2.5THF). Analogously, cis-Pt(LOH)2Cl2 is hydrogen bonded to two THF molecules, one disordered around a center of symmetry. The cis-Pt(LOH)2Cl2·2THF systems are stacked by superimposition of the rectangle planar coordination groups, while one THF molecule is intercalated in the structure (Figure 10). A significant feature that emerges from compounds 1c·CH3OH and 1c·2.5THF is the stacking of the cisPt(LOH)2Cl2 moieties with a Pt···Pt distance of 4.5 and 4.7 Å, respectively, that is the result of CH···Cl interactions. Close stacking of PtX2Cl2 systems is not uncommon in the crystallographic literature,58 but the average contact distance is shorter (3.47 Å). The elongation of the contacts in the present compounds is presumably due to the hindrance of the pyridine rings tilted out of the platinum coordination plane.

compd 1c 1c·(CH3)2CO 1c·1/2CHCl3 1c·CH3OH 1c·3CH3COOH a

mol A mol B mol mol mol mol

A B A B

(e) Pt···Pt (Å)

(f) N1···C2′ (Å)

(g) C19···C19′ (Å)

13.4 13.2 13.1 13.2 13.2 13.2 13.7 12.9

11.5 11.3 11.2 11.2 11.2 11.2 11.2 11.0

7.8 7.7 7.3 7.2 7.8 7.3 7.2 7.9

For labels and references, see Figures 3 and 5.

then packed by taking advantage of OH···Cl interactions unless the guests provide other means of satisfying the hydrogen bond donor/acceptor balance. The comparison between the supramolecular properties of the cis-Pt(LOH)2Cl2 and the transPt(LOH)2Cl2 complexes evidentiate the importance of the molecular shape in the optimization of close packing. The wheel-and-axle shape of the trans-Pt(LOH)2Cl2 unit in fact is best suited to assemble in arrays of offset molecules that can switch between OH···Cl and OH···guest hydrogen bonds according to the so-called “venetian blinds mechanism”,14−17 while the use of OH acceptors for direct host···host hydrogen bonds would not be sterically accessible. The use of metal complexes opportunely designed with peripheral donor/acceptor groups is a promising way to obtain new supramolecular units, new metallamacrocycles, and,

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CONCLUSIONS Some general considerations can be driven by the comparison of the above results. The apohost species 1c contains two hydrogen bond donors (OH) and four acceptors (OH, Cl), and 394



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ultimately, new crystalline materials that join the robustness of the coordination bonds with the flexibility of hydrogen bonds.60−62

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ASSOCIATED CONTENT

S Supporting Information *

X-ray crystallographic information files (CIF) for compounds 1c, 1c·3CH3COOH, 1c·(CH3)2CO, 1c·1/2CHCl3, 1c·2.5THF, 1c·CH3OH, 1c·2CH3OH, 1t, 1t·2(CH3)2CO, 1t·2DMSO, and 1t·4DMSO. 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].

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ACKNOWLEDGMENTS The Laboratorio di Strutturistica 'M. Nardelli' and the CIM (Centro Interdipartimentale di Misure) “G. Casnati” of the University of Parma are thanked for technical assistance and instrument facilities. Thanks are due to Dr. Maria Mita for the synthesis of some complexes.

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Supramolecular Architectures of Pt (II) Complexes Controlled by

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