Reversible Guest Removal and Selective Guest Exchange with a

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Reversible guest removal and selective guest exchange with a covalent dinuclear wheel-and-axle metallorganic host constituted by halfsandwich Ru(II) wheels connected by a linear diphosphine axle Alessia Bacchi, Susan Bourne, Giulia Cantoni, Silvia A.M. Cavallone, Simona Mazza, Gift Mehlana, Paolo Pelagatti, and Lara Righi Cryst. Growth Des., Just Accepted Manuscript • Publication Date (Web): 04 Mar 2015 Downloaded from http://pubs.acs.org on March 5, 2015

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Crystal Growth & Design

Reversible guest removal and selective guest exchange with a covalent dinuclear wheel-and-axle metallorganic host constituted by half-sandwich Ru(II) wheels connected by a linear diphosphine axle Alessia Bacchi,§ Susan Bourne,⊥ Giulia Cantoni,§ Silvia A. M. Cavallone,§ Simona Mazza,§¶ Gift Mehlana,⊥ Paolo Pelagatti,*§ Lara Righi§ §

Dipartimento di Chimica, Università degli Studi di Parma, Parco Area Scienze 17/A, 43124 Parma, Italy. ⊥ Chemistry Department, University of Cape Town, Room 6.41 P D Hahn Building, 7700 Rondebosch, South Africa

The organometallic unit {[(p-cymene)RuCl2]2[4,4′-bis-(diphenylphosphino)biphenylene]} revealed to be a good building block for the construction of wheel-and-axle (waa) crystalline scaffolds able to incorporate different organic solvents. In fact, the synthesis carried out in THF, dichloromethane, toluene and p-xylene led to the corresponding solvate complexes. A not solvate form (apohost) could instead be isolated from diethyl ether. However, this showed a much lower crystallinity than the solvate forms, to indicate the necessity of including solvent molecules for the construction of a crystalline lattice, as expected for waa compounds. Thermal treatment of the THF solvate led to the extrusion of the solvent with isolation of a new apohost framework. The desolvation process occurred with partial loss of crystallinity which, however, was completely restored after sorption of THF vapors with rebuilding of the starting THF solvate. THF could also be exchanged with p-xylene by vapor uptake process, while exposure to other aromatics, such as benzene, toluene, o- and m-xylene led to partial guest exchanges. The use of a more branched guest, such as p-cymene, completely blocked the exchange. THF could be exchanged also with terminal alkynes, such as phenylacetylene and 4-ethynyltoluene, although a final stable host/guest compound was isolated only with 4-ethynyltoluene. With phenylacetylene the host-guest species is extremely unstable at room temperature and it quickly releases the acetylenic with formation of the apohost system isolated from diethyl ether. The monitoring by XRPD analysis of the p-xylene and phenylacetylene uptakes gave evidences that the exchange processes occur with complete retention of crystallinity, thus pointing out the flexibility of the crystalline networks involved in the aforementioned dynamic processes.

Prof. Paolo Pelagatti Dept. of Chemistry Università degli Studi di Parma Area delle Scienze 17/A, 43124 Parma, Italy Phone: +39 0521 905426, Fax: +39 0521 905557, Email: [email protected]

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Dipartimento di Chimica, Università degli Studi di Parma, Parco Area Scienze 17/A, 43124

Parma, Italy; [email protected]. ⊥ Chemistry Department, University of Cape Town, Room 6.41 P D Hahn Building, 7700 Rondebosch, South Africa ABSTRACT

The

organometallic

unit

{[(p-cymene)RuCl2]2[4,4′-bis-(diphenylphosphino)biphenylene]}

revealed to be a good building block for the construction of wheel-and-axle crystalline scaffolds able to incorporate different organic solvents. In fact, the synthesis carried out in THF, dichloromethane, toluene and p-xylene led to the corresponding solvates. A apohost could instead be isolated from diethyl ether. However, this showed a lower crystallinity than the solvates, as usually expected for waa compounds. Thermal extrusion of THF led to a new


2

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apohost framework. The desolvation process occurred with partial loss of crystallinity which, however, was completely restored after sorption of THF vapors with rebuilding of the starting THF solvate. THF could also be exchanged with p-xylene by vapor uptake process, while exposure to other aromatics, such as benzene, toluene, o- and m-xylene led to partial guest exchanges. The use of a more branched guest, such as p-cymene, completely blocked the exchange. THF could be exchanged also with phenylacetylene and 4-ethynyltoluene, although a final stable host/guest compound was isolated only with the last. The monitoring by XRPD analysis of the p-xylene and phenylacetylene uptakes gave evidences that the exchange processes occur with complete retention of crystallinity, thus pointing out the flexibility of the crystalline networks involved in the aforementioned dynamic processes.

Introduction There is a continuous interest on crystalline materials able to reversibly incorporate small molecules into the crystal lattice due to the potential impact they can have on several applications, such as gas storage/separation, heterogeneous catalysis and development of sensors. Metal containing materials characterized by rigid crystalline networks, high porosity and absorbing ability towards important chemicals, such as hydrogen, carbon dioxide, methane and others, receive a lot of attention, and nowadays they are popular among the scientific community as Metal-Organic-Frameworks (MOF).1 However, it has been demonstrated that a permanent porosity is not a necessary prerequisite in order to have crystalline materials with the propensity to uptake volatile guests,2-3 since crystals based on soft intermolecular interactions can reversibly rearrange their crystal lattices in order to allow the entering and removal of small guests. These materials led to the definition of “porosity without pores”.4 In the last years we focused our research on the preparation of metal-containing crystalline solids based on wheel-and-axle (waa)


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shaped molecules, to say molecules made by two relatively bulky end groups connected by a rigid linear spacer. It is well known that dumbbell shaped molecules have the tendency to crystallize as solvates.5 The inclusion of solvent molecules (guest) is necessary in order to fill the voids present in the apohost frameworks, voids which are due to the irregular morphology of the waa building blocks. In the presence of weak intermolecular interactions the crystal lattices of such materials can show attractive dynamic behaviors which bring to the reversible removal of the guest species, allowing for the continuous transformation between the solvate species and the apohost one through heterogeneous solid/gas processes. For the construction of such compounds we have so far followed three different synthetic designs. The first considers the positioning of the metal M into the central axle (Scheme 1a, waad, wheel-and-axle diols). The axle is the structural synthon D…M…D, where D is the donor belonging to a bulky OH-functionalized pybased ligand (wheel) in a square-planar coordination, typically governed by Pd(II) or Pt(II).6-7 The other two synthetic approaches consider the positioning of the metal into the wheels as halfsandwich units of the type [(η6-arene)RuX2] (Scheme 1b, waamo, wheel-and-axlemetallorganic). In these cases the axle can be covalent or supramolecular in character. In the covalent waamo the axle is a divergent linear ligand (Scheme 1c),8 while in the supramolecular waamo the axle is based on the dimerization of COOH9-10 or CONH211 functions present on suitable divergent ligands (Scheme 1). As regards the waad complexes, we have demonstrated that systems of the type trans-{[α-4-(pyridyl)benzhydrol]2PdCl2} can absorb vapors of acetone through an heterogeneous solid/gas reaction, giving rise to an acetone-solvate where the solvent molecule is hydrogen-bonded to the OH function present on the benzhydrol wheels. The extrusion of the guest molecule can be accomplished by thermal treatment of the solid solvate with restoration of the starting unsolvated complex. These processes occur with structural


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continuity, that’s crystallinity of the sample is maintained throughout the steps as evidenced by XRPD monitoring. Recently, we showed that supramolecular waamo compounds based on functionalized benzoic acid ligands (Scheme 1b) can trigger on the absorption and release of acetone through heterogeneous solid/gas processes, provided the fulfillment of some structural requirements. These concern the length of the supramolecular axle9, the hydrogen-bond acceptor character of the X ligands12,13 and the presence of hydrogen-bond acceptor functionalities on the aminobenzoic acid ligand. 10 Again, these transformations occurred with retention of crystallinity. Covalent waamos containing linear ligands such as 4,4ʹ-bipy, 4-cyanopyridyne, 1,2-trans-(4pyridyl)ethylene and 1,2-bis-(4-pyridyl)ethane (Scheme 1c) resulted instead inert towards gasuptake processes.8 The observed inertness was attributed to an excessive rigidity of the host skeleton which impeded the entrance/exiting of the guest. With the aim of verifying the possibility of obtaining covalent waamo complexes with higher host capacity, the nature of the central linear axle was significantly modified, replacing the N-containing ligands listed above with the linear bis-phosphine ligand 4,4′-bis-(diphenylphosphino)biphenylene.14 With respect to most of the previous ligands the bis-phosphine ligand in this work represents a longer and, because of the PPh2 moieties, more irregular spacer, characteristics which could have a profound impact on the clathrating properties of the corresponding waamos. The ligand was then used to connect two [(η6-p-cymene)RuCl2] units thus giving rise to the dinuclear system {[(pcymene)RuCl2]2[4,4′-bis-(diphenylphosphino)biphenylene]} (1 in Scheme 2). The aromatic character of the spacer was considered to be a good prerequisite in order to favor the inclusion of aromatic guests through π-π or C-H/π interactions. In this work we present a study on the hostguest properties of the molecular building block 1 towards different aromatic volatile organic compounds (VOCs), such as benzene, toluene, p-cymene, xylene isomers and phenylacetylenes,


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showing the preparation of different solvates from solution processes, or through heterogeneous solid-gas (uptake) or solid-liquid (slurry) reactions. The possibility of interconverting the different solvates into each other will also be shown. Particular attention will be given to the definition of a structural continuity governing the uptake processes. These studies assume particular importance in light of the significance that half-sandwich Ru(II) complexes have as homogeneous catalysts15 and potential anticancer drugs,16-17 where the knowledge of possible pseudopolymorphs is of fundamental importance for aspects concerning solubility and storage.

Scheme 1 General scheme of the wheel-and-axle systems prepared in our laboratory Experimental Section All the reactions were performed under an atmosphere of dry nitrogen, using standard Schlenk techniques and anhydrous solvents. The ligand 4,4′-bis(diphenylphosphino)biphenylene was synthesized by following a literature procedure (see Supporting Information).14 [(pcymene)RuCl2]2 was synthesized by following the standard procedure.18 All the volatile organic compounds used in the uptake experiments were utilized without prior purification, except THF


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which was distilled over Na/benzophenone. 1H NMR spectra were recorded on a Bruker AV-300 or AV-400 spectrophotometer, the chemical shift values are refereed to TMS. The 31P{1H}NMR spectra were recorded on a Bruker AV-400 (161.9MHz), the chemical shift values are referred to the external standard H3PO4 (85%). The FT-IR ATR spectra were collected by means of a Nicolet-Nexus (Thermo-Fisher) spectrophotometer, in the range 4000-400 cm-1 with a diamond ATR plate. Elemental analyses were performed by using a FlashEA 1112 Series CHNS-O analyzer (ThermoFisher) with gas-chromatographic separation. The mass-spectra were collected by using a ThermoFisher DSQII single quadrupole spectrometer, equipped with a DIP (direct insertion probe) for the direct analysis of solid samples. The source temperature was 200 °C. The DIP-analyses conducted for determining the presence of the adsorbed guest species were carried out under isothermal conditions. For the temperature values see the experimental details. In order to maintain the spectra free from signals arising from decomposition processes of the organometallic entities, the applied temperatures were much lower than the decomposition temperatures of the organometallics. Powder XRD patterns were collected using Cu Kα radiation with a Thermo ARL X’TRA powder diffractometer equipped with a Thermo Electron solid state detector. The data collection of the p-xylene and phenylacetylene uptakes were performed in the 6-14° and 5-17° 2θ intervals, respectively. In both cases the step size was 0.05° and the scan rate was 1°2θmin-1 at a temperature of 25°C. TGA analyses were performed with a Perkin Elmer TGA7 apparatus, with a constant purge of dry nitrogen. The desorption kinetics of THF from complex 1⋅⋅1.5THF were carried out using a TA-Q500 instruments with Universal Analysis 2000 software (see Supporting Information for experimental details). Synthesis


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{[(p-cymene)RuCl2]2[4,4′′-bis(diphenylphosphino)biphenylene]}

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(1)

[(p-cymene)RuCl2]2

(97 mg, 0.158 mmol) and 4,4′-bis(diphenylphosphino)biphenylene (86 mg, 0.165 mmol) were introduced in a Schlenk tube equipped with a magnetic bar and treated with diethyl ether (30 ml). The mixture was stirred at room temperature overnight obtaining a light red solid. After filtration the solid was washed with diethyl ether and vacuum dried for several hours. Yield: 156 mg (0.137 mmol, 87%). M.p.: 190 °C (dec.). Anal. Calcd. (found) for C56H56Cl4P2Ru2: C, 59.28 (58.95); H, 4.94 (4.75). 1H NMR (CDCl3, 295K, δ, ppm): 1.12 (d, 12H, CH(CH3)2), 1.89 (s, 6H, CH3), 2.88 (sept, 2H, CH(CH3)2), 5.03 (d, 4H, p-cymene), 5.23 (d, 4H, p-cymene), 7.37 (m, 12H, Ph), 7.55 (m, 4H, Ph), 7.89 (m, 12H, Ph).

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P{1H} NMR (CDCl3, 295K, δ, ppm): 24 (s). TGA

(from 25 °C to 190 °C, T-ramp: 5 °C min-1): observed loss < 1%. No signal referable to diethyl ether were detected by MS-EI(+)-DIP analysis (probe temperature from 90 °C to 120 °C). {[(p-cymene)RuCl2]2[4,4′′-bis(diphenylphosphino)biphenylene]}⋅⋅1.5C4H8O (1⋅⋅1.5THF) [(pcymene)RuCl2]2 (257mg, 0.42mmol) was placed in a Schlenk tube equipped with a magnetic bar. The compound was dissolved in dry THF (30 ml) under stirring, then a THF solution (20 ml) of the bis-phosphine ligand (220 mg, 0.42 mmol) was added dropwise observing the immediate precipitation of a red microcrystalline solid. After completion of the ligand addition the mixture was stirred at room temperature for 5 hours, the red precipitate was filtered off, washed with THF and diethyl ether and finally vacuum dried for several hours. Yield: 460 mg (0.470 mmol, 90%). M.p.: 270°C (browning). Anal. Calcd. (found) for C62H68Cl4O1.5P2Ru2: C, 61.62 (61.56), H, 5.68 (5.75). 1H NMR (CD2Cl2, 295K, δ, ppm): 1.14 (d, 12H, CH(CH3)2), 1.86 (m, 6H, THF), 1.90 (s, 6H, CH3), 2.80 (sept., 2H, CH(CH3)2), 3.73 (m, 6H, THF), 5.05 (d, 4H, p-cymene), 5.24 (d, 4H, p-cymene), 7.46 (m, 12H, Ph), 7.63 (m, 4H, Ph), 7.84 (m, 12H, Ph).

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P{1H}NMR

(CD2Cl2, 295K, δ, ppm): 24 (s). IR (ATR, cm-1): 3039 ν(C-H)ar; 2967 νas(C-H)al; 2866 νs(C-H)al;


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1481 ν(C=C)ar; 1436 ν(P-Ph). TGA (isotherm at 120°C for 20 hours): observed loss: 9.7%; calculated loss for 1.5 molecules of THF: 8.7%. MS-EI(+)-DIP (probe temperature: 100°C): m/z = 71 [THF]+. A saturated dichloromethane solution afforded single crystals suitable for X-ray analysis corresponding to the complex 1⋅⋅2DCM (see Crystallographic Section). {[(p-cymene)RuCl2]2[4,4′′-bis(diphenylphosphino)biphenylene]}⋅⋅0.5CH2Cl2

(1⋅⋅0.5DCM)

As for 1⋅⋅1.5THF but using dichloromethane (30 ml + 20 ml) instead of THF. The brick-red product was precipitated by adding diethyl ether and washed with THF and diethyl ether and finally vacuum dried for several hours. Yield: 165 mg (0.141 mmol, 80%). M.p.: 222-224 °C (dec.). Anal. Calcd. (found) for C56.5H57Cl5P2Ru2: C, 57.65 (58.05); H, 4.84 (5.10). 1H NMR (CDCl3, 295K, δ, ppm): 1.12 (d, 12H, CH(CH3)2), 1.89 (s, 6H, CH3), 2.88 (sept, 2H, CH(CH3)2), 5.03 (d, 4H, p-cymene), 5.23 (d, 4H, p-cymene), 5.32 (s, 1H, DCM), 7.39 (m, 12H, Ph), 7.55 (m, 4H, Ph), 7.88 (m, 12H, Ph). 31P{1H} NMR (CDCl3, 295K, δ, ppm): 24 (s). TGA (from 25°C to 190°C with a T-ramp of 5 °Cmin-1): observed loss: 2.84%; calculated loss for 0.5 molecules of DCM: 2.72%. MS-EI(+)-DIP (probe temperature: 90°C): m/z = 83.9 [DCM]+. {[(p-cymene)RuCl2]2[4,4′′-bis(diphenylphosphino)biphenylene)⋅⋅1.5C7H8 (1⋅⋅1.5tol) As for 1⋅⋅1.5THF but using toluene as solvent. The final product precipitated as red microcrystalline powder which was filtered off and washed with toluene and diethyl ether and finally vacuum dried. Yield: 383 mg (0.301 mmol, 70%). M.p.: 200 °C (browning). Anal. Calcd. (found) for C67H68Cl4P2Ru2: C, 62.91 (62.60); H, 6.38 (6.29). 1H NMR (CD2Cl2, 295K, δ, ppm): 1.15 (d, 12H, CH(CH3)2), 1.90 (s, 6H, p-cymene), 2.34 (s, 4.5H, CH3 toluene), 5.05 (d, 4H, p-cymene), 5.25 (d, 4H, p-cymene), 7.26 (m, 7.5H, toluene), 7.44 (m, 12H, Ph), 7.63 (m, 4H, Ph), 7.84 (m, 12 H, Ph). 31P{1H} NMR (CD2Cl2, 295K, δ, ppm): 24 (s). TGA (from 25 °C to 200 °C, 5 °C min-


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): observed loss: 8.4%; calculated loss for 1.5 molecules of toluene: 8.9%. MS-EI(+)-DIP (probe

temperature: 90 °C): m/z = 92.6 {[(p-cymene)RuCl2]2[4,4′′-bis(diphenylphosphino)biphenylene]}⋅⋅0.5C8H10

(1⋅⋅0.5px)

[(p-

cymene)RuCl2]2 (100 mg, 0.163 mmol) was introduced in a Schlenk tube equipped with a magnetic bar, together with p-xylene (30 ml). Under stirring the suspension was added of 4,4′bis(diphenylphosphino)biphenylene (85 mg, 0.163 mmol) and the mixture was stirred at room temperature overnight. After filtration, the red-orange microcrystalline solid was washed with pxylene and diethyl ether and then vacuum dried for several hours. Yield: 157 mg (134 mmol, 82%). M.p.: 190 °C (browning). Anal. Calcd. (found) for C60H61Cl4P2Ru2: C, 60.66 (60.45); H, 5.18 (5.21). 1H NMR (CDCl3, 395K, δ, ppm): 1.12 (d, 12H, CH(CH3)2), 1.89 (s, 6H, CH3 cymene), 2.33 (s, 3H, CH3 p-xylene), 2.88 (sept, 2H, CH(CH3)2), 5.03 (d, 4H, p-cymene), 5.23 (d, 4H, p-cymene), 7.08 (m, 2H, p-xylene), 7.42 (m, 12H, Ph), 7.56 (m, 4H, Ph), 7.87 (m, 12H, Ph). 31P{1H} NMR (CDCl3, 295K, δ, ppm): 24 (s). TGA (from 25°C to 190 °C with a T-ramp of 5 °C min-1): observed loss: 3.8%; loss calculated for 0.5 molecules of p-xylene: 4.5%. MS-EI(+)DIP (probe temperature: 90 °C): m/z = 106 [C8H10]+; 91 [C7H7]+. Uptake experiments The compound (30 mg) was introduced in an open vial which was placed in a tube of an H-cell (see Supporting Information). In the other tube 10 ml of the liquid guest were introduced. The Hcell was closed and a gentle vacuum was applied maintaining the apparatus at room temperature during the entire experiments. The amount of liquid guest assures a fast saturation and a constant partial pressure during the uptake process. The progress of the reaction was monitored by 1H NMR analysis on small portions of the starting sample. In no cases color changes were observed


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during the uptake processes. At the end of the reaction the compound was analyzed by TGA, 1H NMR and XRPD analysis. Host-Guest systems obtained subjecting 1⋅1.5THF to the uptake of organic volatile compounds Uptake of p-xylene: {[(p-cymene)RuCl2]2[4,4′′-bis(diphenylphosphino)biphenylene}⋅⋅C8H10 (1⋅⋅px) 1H NMR (CDCl3, 295K, δ, ppm): 1.12 (d, 12H, CH(CH3)2), 1.90 (s, 6H, CH3 p-cymene), 2.33 (s, 6H, CH3 p-xylene), 2.88 (sept, 2H, CH(CH3)2), 5.03 (d, 4H, p-cymene), 5.23 (d, 4H, pcymene), 7.01 (s, 4H, p-xylene), 7.41 (m, 12H, Ph), 7.55 (m, 4H, Ph), 7.89 (m, 12H, Ph). 31

P{1H} NMR (CD2Cl2, 295K, δ, ppm): 24.2 (s). TGA (from 25 °C to 200 °C, 5 °C min-1):

observed loss: 9.18%; calculated loss for one molecule of p-xylene: 9.35%. MS-EI(+)-DIP (probe temperature: 90 °C): m/z = 106 [C8H10]+; m/z = 91 [C7H7]+; m/z = 77 [C6H5]+. Uptake

of

phenylacetylene:

{[(p-cymene)RuCl2]2[4,4′′-

bis(diphenylphosphino)biphenylene}·0.3C8H6 (1·0.3PhC2H) 1H NMR (CDCl3, 295K, δ, ppm) before vacuum: 1.13 (d, 12H, CH(CH3)2), 1.90 (s, 6H, CH3), 2.80 (m, 2H, CH(CH3)2), 3.16 (s, 0.3H, PhC≡C-H), 5.05 (d, 4H, p-cymene), 5.24 (d, 4H, p-cymene), 7.44 (m, 12H, Ph), 7.62 (m, 4H, Ph), 7.88 (m, 12H, Ph). TGA (form 25 °C to 190 °C, 5 °C min-1): observed loss: 2.7%; calculated loss for 0.3 molecules of phenylacetylene: 3%. MS-EI(+)-DIP (probe temperature: 90 °C): m/z = 102 [C8H6]+, 77 [C6H5]+. After vacuum the 1H NMR spectrum and XRPD trace correspond to those of 1. MS-EI(+)-DIP and FTIR-ATR analysis did not show any trace of phenylacetylene. Uptake

of

4-ethynyltoluene:

{[(p-cymene)RuCl2]2[4,4′′-

bis(diphenylphosphino)biphenylene}⋅⋅1.5C9H8 (1⋅⋅1.5MeTol) 1H NMR (CDCl3, 195K, δ, ppm): 1.12 (d, 12H, CH(CH3)2), 1.90 (s, 6H, CH3 p-cymene), 2.38 (s, 4.5H, CH3 4-ethynyltoluene), 2.88 (m, 2H, CH(CH3)2), 3.05 (s, 1.5H, H-C≡C), 5.03 (d, 4H, p-cymene), 5.23 (d, 4H, p-


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cymene), 7.15 (d, 3H, 4-ethynyltoluene), 7.41-7.56 (m, 18H, Ph+4-ethynyltoluene), 7.89 (m, 12H, Ph). TGA (from 25 °C to 190 °C, 5°C min-1): observed loss: 15.6%; calculated loss for 1.5 molecules of 4-ethynyltoluene: 13.30%. MS-EI(+)-DIP (probe temperature: 90 °C): m/z = 115 [C9H7]+, 89 [C7H4]+. The characterization data of the mixed-guest products obtained after uptake of benzene, toluene, o-xylene and meta-xylene are reported in the Supporting Information. The characterization of the product obtained after uptake of p-cymene gave data equivalent to those of 1·1.5THF. X-ray crystallography Single crystal X-ray diffraction data were collected using the Mo Kα radiation (λ = 0.71073 Å) at T = 293 K on a SMART APEX2 diffractometer. Data quality was very poor due to twinning and to weak diffraction caused by the small crystal dimensions. Nevertheless it was possible to determine the overall structural arrangement, without discussing the finest geometric details. The collected intensities were corrected for Lorentz and polarization factors and empirically for absorption by using the SADABS program.19 The structure was solved by direct methods using SIR201120 and refined by full-matrix least-squares on all F2 using SHELXL201421 implemented in the Olex2 package.22 Hydrogen atoms were introduced in calculated positions. Anisotropic displacement parameters were refined for all non-hydrogen atoms. Geometric restraints and thermal parameter restraints were necessary to model the highly mobile p-cymene moiety. Hydrogen bonds have been analyzed with SHELXL201421 and PARST9723 and extensive use was made of the Cambridge Crystallographic Data Centre packages24,25 for the analysis of crystal packing. Table 1 summarizes crystal data and structure determination results for 1·2DCM. Crystallographic data (excluding structure factors) of 1.2DCM have been deposited with the


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Table 2 Crystal data and structure refinement for 1.2DCM 1·2DCM Empirical formula C58H60P2Cl8Ru2 Formula weight 1304.83 Temperature/K 293(2) Crystal system monoclinic Space group P21/c a/Å 9.709(16) b/Å 18.44(3) c/Å 16.53(3) α/° 90 β/° 99.02(2) γ/° 90 3 2922(9) Volume/Å Z 2 3 1.395 ρcalcg/cm -1 µ/mm 0.792 F(000) 1256.0 Radiation MoKα (λ = 0.71073) 2Θ range for data collection/° 3.332 to 35.074 Reflections collected 12620 Independent reflections 1838 [Rint = 0.2658, Rsigma = 0.1546] Data/restraints/parameters 1838/15/268 2 Goodness-of-fit on F 1.016 Final R indexes [I>=2σ (I)] R1 = 0.0827, wR2 = 0.1879 Final R indexes [all data] R1 = 0.1838, wR2 = 0.2478 -3 0.41/-0.36 Largest ∆F max/min / e Å

Cambridge Crystallographic Data Centre as supplementary publications nos. CCDC 1043550. 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; e-mail: [email protected]). The structural analysis of 1-1.5THF was attempted by powder diffraction with conventional CuKα source. The red-orange crystalline powder was deposited on a zero background sample


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holder and the collection conditions was : 3-55° 2θ range, with 3sec/0.02° scan rate. The analysis of the diffraction data were carried out by EXPO2014 suite.26 The diffraction pattern indicates the presence of a crystalline phase with a defined sequence of peaks. The de-convolution procedure was obtained by a profile fitting performed on the basis of Pearson VII model it was possible to retrieve a set of reflections. The unit cell dimensions was hunted by applying the N-TREOR indexing procedure with a set of 22 lines found in 5-22 degrees 2θ range. Among the different solutions, the monoclinic unit cell with dimensional parameters closely related to the homologous 1·2DCM and associated to the best figures of merit (M(20) = 13, F 20 = 42), was selected. Afterwards, a check of systematic absences was carried out with a conventional routine implemented in EXPO2014 giving the P21/c space group as the most probable symmetry with a profile fitting having R = 3.88. The resulting unit cell and related symmetry reflect a close analogy with 1·2DCM. The presence of a sufficiently resolved diffraction pattern is a fundamental prerequisite for the structure determination by powder methods. Actually a series of attempts of structure solving was realized either with direct and with reciprocal space based approaches. The preliminary results, essentially obtained by simulated annealing runs, are promising in locating the molecular di-nuclear dumbbell in the monoclinic unit cell and further work is necessary in order to fix the THF molecules arrangement. Nevertheless, the current model, as indicated by the agreement factors determined from the profile fitting of PXRD pattern, can be considered as sufficiently reliable. The crystal data obtained from the profile fitting (Figure S15) are listed in the Table 2.


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Table 2 Crystal data from the profile fitting of the powder XRD pattern based on 1·1.5 THF structural model

Crystal system Space group a/Å b/Å c/Å α/° β/° γ/° Volume/Å3 Radiation 2Θ range for data collection/° Profile function Backgrournd Agreement factors

1·1.5 THF monoclinic P21/c 9.571(4) 19.251(8) 16.148(9) 90 94.44(5) 90 2965(3) CuKα (λ = 1.5405Å) 3 to 55 Pearson VII Chebyshev polynomial (19 parameters) Rp= 0.118 Rwp=0.156, RB=0.18

Results and Discussion The ligand 4,4′-bis(diphenylphosphino)biphenylene was chosen because is well suited for behaving as rigid linear covalent axle in the target waamo scaffold. The biphenylene skeleton was thought useful in order to promote inclusion of aromatic guests through π/π or C-H…π interactions. The ligand was synthesized following a literature reported method.14 In order to investigate the clathrating properties of the waamo scaffold, the free ligand and [(pcymene)RuCl2]2 were reacted in several organic solvents. For all the isolated products the inclusion of the solvent was investigated by means of several techniques, such as 1H NMR, TGA, MS-EI(+) DIP and elemental analysis. Initially, in order to test the possibility of isolating a apohost system, the reaction was carried out in diethyl ether at room temperature. Although neither the free ligand nor the ruthenium


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precursor are soluble in diethyl ether, after a prolonged stirring at room temperature a light-red solid was isolated (1 in Scheme 2).

Scheme 2. Synthesis of the half-sandwich Ru(II) complexes The characterization of 1 revealed an unsolvated not-solvate form. In fact, no signals belonging to diethyl ether were found in the MS-EI(+)-DIP analysis of a solid sample of 1 and the 1H NMR spectrum of the complex recorded in CDCl3 showed only the signals corresponding to the organometallic scaffold with no traces of diethyl ether. The aromatic p-cymene protons gave rise to two doublets centered at 5.03 and 5.23 ppm, respectively, the others signals being in the expected regions. The 31P{1H} NMR spectrum recorded in CDCl3 showed only one singlet at 24 ppm, a much higher value than that found for the free ligand which indicates that both P donors are bound to ruthenium in the same coordination environment. Then, the bridging bisphosphine ligand coordinates two pseudo-octahedral Ru(II) half-sandwich units, where the coordination sphere of each metal is defined by one phosphine, two chloride ligands and a η6coordinated p-cymene ring. On the basis of crystallization evidences (see Crystallographic


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Sections) a mutually transoid disposition of the half-sandwich units is assumed for all the isolated complexes described below. Later on, the reaction between the free ligand and the ruthenium precursor was conducted in dichloromethane at room temperature (Scheme 2, compound 1·0.5DCM). From the final red solution a microcrystalline brick-red solid was isolated. The characterization of the purified complex

led

to

the

formulation

{[(p-cymene)RuCl2]2[4,4′-

bis(diphenylphosphino)biphenylene}⋅0.5CH2Cl2 (1⋅⋅0.5DCM). The MS-EI(+)-DIP analysis revealed the typical isotopic signal of CH2Cl2. The 1H NMR spectroscopic pattern was as that found for 1, with the exception of the signal belonging to dichloromethane (5.32 ppm). The 31

P{1H} NMR spectrum was identical to that of 1 with a singlet at 24 ppm. With the aim of

testing the effect of a coordinating solvent, the synthesis was later on conducted in THF. This synthetic protocol led to the fast precipitation of a microcrystalline red product during stirring, corresponding

to

{[(p-cymene)RuCl2]2[4,4′-bis(diphenylphosphino)biphenylene}⋅1.5C4H8O

(1⋅⋅1.5THF in Scheme 2). The presence of THF was confirmed by MS-EI(+)-DIP analysis as well as by 1H NMR spectroscopy, where the ethereal protons gave rise to two multiplets at 1.86 and 3.73 ppm, respectively. The

31

P{1H} NMR spectrum showed again the singlet at 24 ppm.

Complex 1·1.5THF was again isolated after washing of 1 with THF, as inferred by 1H NMR analysis and XRPD analysis. This THF triggered conversion was confirmed by slurry, that’s suspending complex 1 in THF and stirring the suspension at room temperature overnight (Figure S10). Complexes 1 and 1·0.5DCM are stable as solids at room temperature, while for 1·1.5THF a prolonged (several weeks) storing at room temperature on the bench of the laboratory provokes a partial loss of the included solvent, as demonstrated by 1H NMR spectroscopy. This loss can be prevented on storing the sample in a refrigerator. All the complexes are well soluble in


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chlorinated solvents, such as chloroform and dichloromethane, while result practically insoluble in all the other common organic solvents. The CHCl3 and CH2Cl2 solutions resulted stable if left to evaporate at room temperature, without signs of decomposition or color changes. The formation of X-ray quality single crystals was attempted by refrigeration of saturated CHCl3 or CH2Cl2 solutions, slow diffusion of a number of different (anti)solvents into CHCl3 or CH2Cl2 solutions of the complexes as well as by slow evaporations of saturated solutions of chloroform, dichloromethane and dichloroethane. Single crystals suitable for X-ray analysis were isolated by slow evaporation of a chloroform dichloromethane solution of 1⋅⋅1.5THF. The X-ray analysis revealed the formation of a new solvate species, corresponding to {[(p-cymene)RuCl2]2[4,4′bis(diphenylphosphino)biphenylene]}⋅2CH2Cl2 (1⋅⋅2DCM) (see Crystallographic Section). A prolonged vacuum pumping of complex 1·2DCM led to 1·0.5DCM, as evidenced by 1H NMR spectroscopy and XRPD analysis. With the aim of verifying the possibility of synthesizing solvate species containing larger aromatic guests, the ligand 4,4′-bis(diphenylphosphino)biphenylene was later on reacted with [(p-cymene)RuCl2]2 in toluene and p-xylene. In both cases the reactions occurred smoothly leading

to

solvates

of

bis(diphenylphosphino)biphenylene]}⋅1.5C7H8

formula (1·1.5tol)

{[(p-cymene)RuCl2]2[4,4′and

{[(p-cymene)RuCl2]2[4,4′-

bis(diphenylphosphino)biphenylene]}⋅0.5C8H10 (1·0.5px) (Scheme 2), respectively. The presence of the solvents was confirmed by MS-EI(+)-DIP analysis. In the 1H NMR spectrum of 1·1.5tol the methyl protons of the guest are well visible as a singlet at 2.34 ppm, while in the case of 1·0.5px the methyl protons of the guest fall at 2.33 ppm. As expected, the 31P{1H} NMR spectra returned a singlet at 24 ppm in both cases. Also these two complexes are well soluble


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only in chlorinated solvents and all the attempts to grow X-ray quality single crystals were unsuccessful. From the synthetic results it appears evident the tendency of the waamo scaffold to form solvate species, as expected for waa compounds.5 Study of the host-guest properties of the Ru-complexes Initially, we were interested to verify the possibility of transforming complex 1⋅⋅1.5THF into complex 1 by THF extrusion. Vacuum alone was not sufficient even if prolonged overnight. However, complete desolvation could be accomplished by heating the complex at 120 °C for 13 hours, as evidenced by isothermal TGA analysis (Figure S16). The 1H NMR spectrum of the residual TGA sample did not contain any residual THF peak, the other signals being practically equivalent to those found in the spectrum of the starting solvate species (Figure S4). Although not evident by a microscope inspection, the thermally induced desolvation provoked a partial loss of crystallinity, as indicated by the poorly resolved XRPD analysis (Figure S7). The XRPD trace of this new material did not correspond to that of complex 1, thus indicating the formation of a new apohost polymorph (1', Scheme 3 and Figure S11).

Scheme 3 Scheme of the dynamic transformations observed with complexes 1⋅⋅1.5THF and 1′ by heterogeneous solid-gas reactions.


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Very nicely, the exposure of 1′′ to vapors of THF at room temperature for 24 hours gave 1⋅⋅1.5THF back as confirmed by 1H NMR spectroscopy. Importantly, the absorption of THF led to complete restoration of crystallinity as inferred by XRPD analysis (Scheme 3). These findings, together with the aforementioned conversion of 1 into the THF solvate by slurry, clearly indicate that complexes 1, 1′′ and 1⋅⋅1.5THF have flexible and dynamic crystalline networks able to rearrange reversibly into each other in response to external stimuli, such as uptake of volatile organic solvents, thermal induced desolvation, and contact with the liquid guest. Later on, we directed our attention on the possibility of exchanging THF contained in 1⋅⋅1.5THF with other organic solvents by heterogeneous solid-gas uptake experiments. As new entering guests we chose aromatic VOCs, such as benzene, toluene, xylene isomers, and p-cymene. These guests were chosen since their significance in health and environmental concerns and because their separation is hampered by the similarity of their physical-chemical properties, this being particularly true for the xylene isomers.27,28,29 In addition, we tested also two acetylenic substrates, phenylacetylene and 4-ethynyltoluene. These were chosen because they are typical impurities of olefins obtained by naphtha cracking, which are difficult to remove because of their close boiling points with olefins.30 Moreover, acetylenic compounds are known as catalyst poisons in many olefin polymerization processes.31 Solid 1⋅⋅1.5THF was then exposed to vapors of the different aromatic VOCs at room temperature, monitoring the progress of the exchange processes by 1H NMR spectroscopy for 24 hours. The final host-guest ratios were determined by 1H NMR spectroscopy and TGA analysis (when conversion of the starting material was complete), and all the final compounds were analyzed by X-ray powder diffraction. The results are collected in Table 3.


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Table 3. Guest-exchange processes by complex 1⋅⋅1.5THF with aromatic VOCs based on the equation 1·1.5THF(s) + xVOC(g) (Guest) ==> 1·xGuest(s) + 1.5THF(g). Host refers to the organometallic scaffold corresponding to 1.

Entry

Guest

Time of vapor exposure

Final

Residual THF/Host

(hours)a

Guest/Host ratiob

ratiob

1

Benzene

24

1.4/1

1/1

2

toluene

24

1.5/1

0.5/1

3

p-xylene

24c

1/1

0/1

4

m-xylene

24

0.8/1

0.2/1

5

o-xylene

24

1.8/1

0.2/1

6

p-cymene

24

0/1

1.5/1

7

phenylacetylene

24

0.35/1d

0/1

8

4-ethynyltoluene

24

1/1

1/1

72

1/1.5

0/1

a

All the exchange processes have been carried out at r.t.. bDetermined by 1H NMR spectroscopy.

c

The reaction is complete after 5 hours, as established by XRPD monitoring. dThe solvate is

extremely unstable in the absence of phenylacetylene vapors


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Because of the intrinsic limitation of NMR spectroscopy to exactly quantify the amount of a given species in solution, the data collected in Table 3 must not be considered as an exact quantification of the adsorbed guest, but rather a measure of the ability of the newly incoming guest to extrude the initially present THF. From Table 3 it can then be inferred that only the linear p-xylene was able to completely remove THF forming the new solvate species 1⋅⋅px (entry 3), which considers a 1:1 Host/px ratio. The XRPD trace of 1·px is equivalent to that recorded for 1·0.5px synthesized in solution, but the peaks are systematically shifted to lower 2θ values, in agreement with a higher content in guest species (Figure S13). With the other guests different amounts of THF were still visible in the 1H NMR spectra of the final products. Substituted benzenes appear more able than the simple benzene to exchange THF (compare entry 1 with entries 2, 3, 4 and 5), while a branched guests, such as p-cymene (entry 6), completely blocked the reaction. For the xylene isomers, who are characterized by very similar vapor pressure values (6.6, 8.29 and 8.75 mmHg for ox, mx and px, respectively) the results are quite intriguing. In fact, the residual amounts of THF are quite comparable (0.2, 0.2 and 0 with ox, mx and px, respectively), but the amount of absorbed guest is significantly different (compare entries 3, 4 and 5). In the absence of precise structural information is at present hard to rationalize such a behavior, but the higher efficiency of p-xylene in removing THF might indicate the presence of channels in the solid state framework of complex 1⋅⋅1.5THF, through which the linear p-xylene can move more easily. Moreover, the higher exchange ability found for the xylene isomers with respect to benzene and toluene seems to indicate that C-H…π interactions play an important role during the guest exchange reactions. Finally, the XRPD traces of the final products show different patterns, pointing out that the matrics of the host network is adjusted according to the guest, thus pointing out a breathing behavior of the host framework (Figure S14).


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In order to verify if the uptake processes occurred with retention of crystallinity, the p-xylene uptake was monitored by XRPD analysis in the 6-14° 2θ interval, where signals of the two involved phases are well distinguishable. During the five hours of reaction no traces of amorphous intermediate phases were detected, thus pointing out that the exchange process occurred with complete retention of crystallinity (Figure 1).

Figure 1. Monitoring by XRPD analysis of the p-xylene uptake by complex 1⋅⋅1.5THF On the basis of the different guest exchange velocities found with the xylene-isomers, some competition experiments were carried out in order to highlight possible guest uptake selectivity. Then, solid 1⋅⋅1.5THF was exposed to vapors of equimolar mixtures of o/p-xylene as well as of m/p-xylene at room temperature. In both cases, regrettably, no selectivity was observed but rather a reduced rate in the THF exchange. In fact, in the case of the o/p-xylene mixture, the 1H NMR signals of THF were still visible after 18 hours of reaction, with a complex/o-xylene/p-


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xylene/THF ratio of 1/0.9/0.4/0.4. After 30 and 72 hours the ratios remained practically unchanged. In the case of m/p-xylene mixture the signals of the methyl groups of the aromatic guests were almost coincident thus preventing a correct estimation of the ratios. However, after 24 hours of exposure the THF signals were disappeared with an approximate complex/xylenes ratio of 1/1. Quite unexpectedly, also when complex 1·1.5THF was exposed to an equimolar mixture of p-xylene and p-cymene, no selectivity was observed, and the 1H NMR spectrum recorded after 24 hours showed an approximate complex/THF/p-xylene/p-cymene ratio of 1/0.7/1.4/0.4. The missed selectivity can be understood on considering the sorption properties of complex 1·0.5px. In fact, when subjected to an equimolar mixture of p-xylene and p-cymene at room temperature 1·0.5px absorbed a respectable amount of p-cymene, the complex/p-xylene/pcymene ratio being 1/0.5/0.7 after 24 hours of exposure. Thus, the presence of p-xylene makes the crystal network more weary and able to absorb p-cymene, contrarily to what is seen with 1·1.5THF. As regards acetylenic guests, the uptake of phenylacetylene led, after 24 hours of exposure, to a 1H NMR spectrum free from THF signals and containing a weak signal corresponding to the acetylenic proton whose integration brought to a rough complex/phenylacetylene ratio of 1/0.3 (1·0.3PhC2H).


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Figure 2 XRPD monitoring of the phenylacetylene uptake by complex 1·1.5THF. Inset: superimposition of the XRPD traces of complex 1 (red line) with those of 1·xPhC2H.

The phenylacetylene uptake was then monitored by XRPD analysis for 17 hours (Figure 2). During the uptake the signals belonging to the starting complex reduced significantly while new signals corresponding to another crystalline phase appeared (1·xPhC2H). The solid stored at room temperature in the absence of guest vapors decomposed quickly, as evidenced by XRPD analysis which gave back a trace very similar to that of 1 (inset in Figure 2, blue trace). Thus, under phenylacetylene vapors the guest exchange process led to the formation of a new host/guest system which, upon guest removal, quickly decomposed. The high quality of the


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diffractogram observed under phenylacetylene atmosphere points out a high crystallinity, higher than that of the guest free species, as expected for waa compounds. The instability of 1·xPhC2H impeded the quantification of the absorbed phenylacetylene. A different behavior was observed with 4-ethynyltoluene. In fact, the vapors uptake by complex 1·1.5THF led, after 24 hours, to a complex/THF/4-ethynyltoluene ratio of 1/1/1, the complete exchange of THF being reached after 72 hours, with a complex/4-ethynyltoluene ratio of 1/1.5 (1·1.5EtTol). This new host/guest system resulted stable at room temperature without any loss of guest within 24 hours, as evidenced by 1H NMR spectroscopy. The complete extrusion of THF from the starting material observed with the acetylenic substrates confirms the importance of the guest linearity for an efficient exchange process as in the case of p-xylene. Again, the higher guest exchange efficiency observed with 4-ethynyltoluene together with the higher stability of the corresponding host-guest system, confirm the importance of C-H…π interactions during the uptake processes. The faster kinetic of exchange observed with p-xylene can be understood on considering the lower boiling point, and then higher volatility, of p-xylene (138.2°C), with respect to phenylacetylene (142-144°C) and 4-ethynyltoluene (168-170 °C). The ability of the organometallic building-block 1 to generate responsive crystalline frameworks is certainly ascribable to its waa geometry combined with the weak intermolecular interactions which connect the different molecular building-blocks, as evidenced in the structural characterization of 1·2DCM. Desorption kinetic of THF by 1⋅1.5THF and sorption kinetic of p-xylene by complex 1′ A series of mass loss vs. time curves were obtained for the isothermal desorption of 1⋅⋅1.5THF. The desorption curves were converted to the extent of reaction (α) vs. time curves32 and then fitted to standard kinetic models33 based on geometrical and diffusion rate law to determine the


26

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rate constant for desorption of THF (see Figures S17-18). Based on the correlation coefficients for each temperature, the chosen model was the Ginstling-Brounshtein mechanism (D4, diffusion model) which showed the best linearity at each temperature (Table S1). D4 model is for the formation of a non-porous material where the gaseous product diffuses through the solid phase product and diffusion is the rate-limiting stage. The drawing of an Arrhenius plot (ln k vs. 1/T) led to the determination of an activation energy E of 77.8 kJ mol-1 (Figure S19). This value is certainly lower than those found in other systems where THF is hydrogen bonded to the host scaffold,34,35,36 but in line with other desolvation processes.37,38 A kinetic study was also performed in order to analyze the uptake of p-xylene vapor by complex 1′′. Four different kinetic runs were performed at 23, 27, 30 and 33 °C, continuously recording the weight increase as a function of time. The amount of p-xylene uptake in all four experiments was found to be 9.74±0.65% (expected value for a Host-Guest = 1:1 ratio was 9.35%). The XRPD analysis of the final product was equivalent to that of 1·px. The α vs. time curves (Figure S20) indicated that the p-xylene absorption by the not-solvate complex 1′′ follows a deceleratory behavior. This suggests that the mechanism could be based on geometrical model (R2 and R3) or diffusion based model (D2, D3 and D4). The best data fitting at each temperature was found for the contracting volume model R3 or the diffusion model D4 model (Table S2). The two models lead to activation energies of 109.9 kJ mol-1 and 119.4 kJ mol-1, respectively (Figures S21-22). Unfortunately, the lack of structural data for the p-xylene solvate impedes to discriminate between the two mechanistic models. There are many instances in the literature where more than one solid-state model can be used to describe a particular process.39,40 This behavior is a further confirmation of the dynamic nature of the involved crystal lattices corresponding to a dynamic porosity of the title compounds (Scheme 3).


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Page 28 of 41

Crystallographic Section The crystal structure of the solvate form 1·2DCM was determined by single crystal X-ray diffraction and reveals the arrangement of the complex molecules and how the host framework accommodates the DCM guest. In 1·2DCM the ruthenium displays a pseudo-octahedral geometry, where the η6-coordinated p-cymene, two chloride ligands and the phosphorous atom of the ligand complete the coordination sphere. The molecular structure of 1·2DCM is shown in Figure 3. The {[(p-cymene)RuCl2]2[4,4’-bis(diphenylphosphino)biphenylene]} complex is centrosymmetric and the central biphenyl linker is perfectly planar, while the ruthenium units are oriented in opposite directions with respect to the central linker. The examination of thermal displacement parameters suggests that the biphenyl linker is relatively rigid (Figure 3), while the phenyl rings attached to the phosphorous atoms are highly mobile, as well as the p-cymene units coordinated to ruthenium atoms.

Figure 3. Molecular structure of complex 1, with labelling and displacement ellipsoids drawn at the 30% probability level for sake of clarity.


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In absence of functional groups that could promote strong directional interactions such as conventional hydrogen bonds, the supramolecular assembly is based on close packing and shape complementarity, assisted by CH…Cl short contacts. As already observed in the literature for many wheel-and-axle systems the host is organized in arrays of parallel, slightly offset, molecules with short interactions between phenyl hydrogens and metal bound chlorides. These arrays are assembled in layers with short contacts between the CH of the p-cymene and the chlorides reminiscent of the inverted piano stool motif, a recurrent pattern for half-sandwich ruthenium units (Figure 4).41 The guest DCM molecules are accommodated in the hollows defined between consecutive layers of complex host guest molecules (Figure 5).


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Figure 4. Crystal packing of the host molecules in layers for 1·2DCM. The edge-on view of the layer is shown in the inset.


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Figure 5. Arrangement of the DCM guest molecules in the space between the layers of 1·2DCM. The structure of the THF-solvate complex tentatively was solved from powder diffraction data. The quality of the refined structure is sufficient to evaluate the main features of the crystal packing, and compare it to the one observed for the DCM-solvate, that has a similar unit cell, with only a significant distortion in the beta angle (Table 1 and 2 and Experimental Section). The complex molecules are in fact arranged in the same layers as in 1.2DCM (Figure 6), while the guest THF molecules are slightly displaced in the host framework, being closer each other than the DCM guests. This is evident in Figure 7, where the arrangement of pairs of THF molecules in the packing is shown. The packing is apparently looser than in the DCM solvate, as the host molecules alone leave a void space for the guest of 305Å3 in the DCM-solvate and of 663Å3 in the THF-solvate, Figure 8.


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Page 32 of 41

Figure 6. Crystal packing of 1.2THF shown in the same orientation as 1.2DCM in Figure 2. THF is represented to show that it fits close to the host molecules.

Figure 7. Arrangement of the THF guest molecules in the space between the layers.


32

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Figure 8. Comparison of the voids occupied by the guest molecules in the two different solvates, DCM (left) and THF (right).

Conclusions The

wheel-and-axle

shaped

organometallic

system

{[(p-cymene)RuCl2]2[4,4′-bis-

(diphenylphosphino)biphenylene} revealed to be a good building-block for the fabrication of different crystalline solvates with a number of organic solvents, such as DCM, THF, toluene and p-xylene. A not-solvate complex could be obtained carrying the synthesis out in a volatile and poorly coordinating solvent such as diethyl ether (1), this however at the expense of crystallinity. The thermally induced THF extrusion from the THF-solvate (1·1.5THF) led to the formation of a poorly crystalline not-solvate species (1′′) which, on the basis of XRPD data, resulted a polymorphic form of 1. However, sorption of vapors of THF quickly restored the lost crystallinity, giving 1·1.5THF back smoothly. The possibility of exchanging THF with other volatile organic compounds resulted to be strongly dependent on the nature of the incoming guest. In fact, the exposure of 1·1.5THF to vapors of different benzene derivatives, such as benzene, toluene, o-, m- and p-xylene as well as p-cymene, led to the fast and complete


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displacement of THF only in the case of p-xylene, with formation of the new solvate 1·px which considers a 1:1 host/guest ratio. With the others monosubstitued benzenes the conversion was not complete even after 24 hours of exposure, while with p-cymene, a disubstituted and more branched guest, the exchange reaction was completely inhibited. The p-xylene solvate complex 1·px was also formed by uptake of p-xylene vapors by the apohost compound 1′′. The monitoring of the p-xylene uptake by XRPD analysis revealed a structural continuity during the heterogeneous reactions, with no formation of intermediate amorphous phases. The THF exchange occurs also with terminal alkynes, such as phenylacetylene and 4-ethynyltoluene. In the case of phenylacetylene the complete removal of THF occurs after 24 hours, but the newly formed host/guest compound was very unstable and quickly lost phenylacetylene at room temperature with concomitant loss of crystallinity. The new apohost framework has an XRPD diffractogram very similar to that of 1. Monitoring of the uptake by XRPD analysis revealed that the disappearance of the signals belonging to the THF solvate occurred concomitantly with the formation of a new crystalline phase that we assume to correspond to a phenylacetylenecontaining host/guest system, again with structural continuity. The exchange of THF with 4ethynyltoluene was much slower, and it reached completion only after 72 hours. However, the so formed host/guest compound was completely stable at room temperature, indicating that C-H/π interactions, possibly between the methyl group of the guest and the phenyl ring of the host scaffold, can play an important role in the stabilization of the corresponding host/guest compound. These could be essential also during the uptake of the benzene derivatives, for which, however, linearity and branchiness of the guest are fundamental issues. The different degrees of solvation observed for the compounds isolated by solution syntheses and the different host/guest ratios found in the products isolated by the uptake experiments could be justified by the absence


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of strong directional interactions in the host framework, which would be expected to exert a higher degree of control over the final supramolecular assembly. The definition of a plausible mechanism describing the THF-exchange processes needs the knowledge of the structural details of 1·1.5THF. Attempts aimed at defining the solid structure of 1·1.5THF from XRPD data are currently under way in our laboratory.

AUTHOR INFORMATION Corresponding Author *Paolo Pelagatti, Dipartimento di Chimica, Università degli Studi di Parma, Area delle Scienze 17/A, 43124, Parma, Italy. Tel. +39 0521 905426- Fax: +39 0521 905557. Email: [email protected]. Present Addresses ¶

Ecole Polytechnique Fédérale de Lausanne

EPFL SB ISIC LSCI BCH 3305 (Bât. BCH) CH-1015 Lausanne, Switzerland Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENT


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The Laboratorio di Strutturistica M. Nardelli of the Dipartimento di Chimica of the University of Parma is thanked for X-ray facilities. Dr. Guglielmina Gnappi is thanked for TGA measurements. Dr. Ferdinando Vescovi is thanked for XRPD analyses. SUPPORTING INFORMATION AVAILABLE 1

H and

31

P{1H}NMR spectra, XRPD traces, TGA trace of the desolvation of 1·1.5THF,

sorption/desorption kinetics, XRPD data,

and crystallographic data in cif format. This

information is available free of charge via Internet at http://pubs.acs.org/.

ABBREVIATIONS THF, tetrahydrofuran, DCM, dichloromethane, px, p-xylene; tol, toluene; PhC2H, phenylacetylene; EtTol, 4-ethyltoluene


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For Table of Contents Use Only


Dipartimento di Chimica, Università degli Studi di Parma, Parco Area Scienze 17/A, 43124 Parma, Italy. ⊥ Chemistry Department, University of Cape Town, Room 6.41 P D Hahn Building, 7700 Rondebosch, South Africa

The crystalline THF solvate {[(p-cymene)RuCl2]2[4,4′-bis-(diphenylphosphino)biphenylene]} gives rise to an interesting solid-state reactivity based on the reversible THF extrusion and THF exchange with a number of aromatic volatile organic compounds (VOCs). The solid-state processes occur with retention of crystallinity. The guest exchange efficiency strongly depend on the guest linearity.


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Reversible Guest Removal and Selective Guest Exchange with a

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