Cite this: CrystEngComm, 2013, 15, 2795 Received 24th October 2012, Accepted 5th February 2013 Self-assembly, concomitant photochemical processes, and improvement of the yield of [2 + 2] photoreactions from supramolecular arrays via mechanochemical assistance3 DOI: 10.1039/c3ce26752k www.rsc.org/crystengcomm Alexander Bricen ˜o,* a Dayana Leal, ab Gabriela Ortega, a Graciela Dı ´az de Delgado, b Edgar Ocando a and Liz Cubillan a Novel examples of self-assembly, structural transformations and concomitant photochemical processes from supramolecular tern- ary assemblies directed by charge-assisted hydrogen bonds are shown. In addition, mechanochemical assistance is also shown as a potential tool in order to improve the yield of [2 + 2] photoreactions in the solid state. Chemical synthesis performed in the solid state is gaining interest as an important green alternative for the preparation of new and conventional compounds. 1,2 A particular case of solid state reactions is represented by topochemical [2 + 2] cycloaddition of olefins. In recent years, distinctive supramolecular approaches have been exploited in order to direct stereo- and regiocontrolled photoproducts. 2,3 On the other hand, mechanochemical activation methods have received significant attention in the last years due to attractive advantages such as: shorter reaction times, reduction of costs, energy and waste combined with high yields in comparison with traditional synthesis in liquid-phase. 1 This alternative provides the possibility of affording a greater level of complexity and sophistication in the study of solid state transformations, which could involve sequential synthesis steps from molecular activation in order to obtain complex molecular compounds to the preparation of functional supramolecular arrangements self- assembled via mechanochemistry. Recently, such approach has been exploited to carry-out stereocontrolled synthesis of cyclobu- tane derivatives from topochemical [2 + 2] cycloaddition reac- tions. 3,4 In this context, as part of ongoing efforts on the preparation and design of photoreactive solids 5 we have reported unprecedented examples of concomitant [2 + 2] cycloaddition solid state reactions from co-crystals self-assembled via mechan- ochemistry (Scheme 1). 5a Herein, we have prepared two novel ionic ternary assemblies based on carboxylic acids and stilbazole or stilbene derivatives: (2Cl-HStb + )(Hmal 2 )(H 2 Fu) 0.5 (1) and (2,29-H 2 bpe 2+ )(Fu 22 )(H 2 Fu) (2) (where HStb + : stilbazolium; H 2 bpe 2+ : stilbenium; Hmal 2 : hydrogen maleate; Fu 22 : fumarate and H 2 Fu: fumaric acid). Both compounds are photoreactive upon UV-irradiation. In addition, the reinvestigation of two other photoreactive assemblies, (2,29-bpe)(H 2 Fu) 5a (3) and trans-3-(3- pyridyl)acrylic acid 6 (4), was also undertaken. As UV-irradiation of 1 quantitatively yielded the expected photodimer, the photoreac- tion of 2, 3 and 4 were improved until obtaining nearly quantitative yields of all the products via sequential grinding- irradiation steps.{ Crystals of 1 and 2 were obtained by mixing trans-2-chloro- stilbazole (2-Cl-Stb) with maleic acid (H 2 Mal) and 1,2-bis(2- pyridyl)ethylene (2,29-bpe) with fumaric acid (H 2 Fu) in molar ratio 1 : 1 and 1 : 2 in methanol, respectively. These compounds can also be obtained as crystalline single phases by direct grinding of the starting compounds in short periods of time (30–60 min) via liquid-assisted grinding 7 in the presence of a few drops of methanol (see ESI, 3 Fig. S1). The combination of H 2 Mal with 2-Cl-Stb in solution produces an unexpected ternary crystal: (Hmal 2 )(2Cl-HStb + )(H 2 Fu) 0.5 (1). Formation of H 2 Fu is due to the partial cis–trans isomerisation of H 2 Mal to H 2 Fu at RT (Scheme 2). Such isomerisation has been observed previously by Rao and co-workers in the presence of other nitrogen-containing heterocycles and polar solvents. 8 This process was monitored by 1 H-NMR in order to evaluate the degree of cis–trans conversion in solution as a function of time in the presence of DMSO and CH 3 OH at RT. This analysis revealed an a Instituto Venezolano de Investigaciones Cientı ´ficas (IVIC), Apartado 21827, Caracas 1020-A, Venezuela. E-mail: [email protected]; Fax: +58-212-5041350; Tel: +58-212-5041320 b Universidad de Los Andes (ULA), Apartado 40, La Hechicera, Me ´rida 5251, Venezuela 3 Electronic supplementary information (ESI) available: Experimental details. 1 H- NMR spectra and PXRD patterns. CCDC 900648 and 900649. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c3ce26752k Scheme 1 CrystEngComm COMMUNICATION This journal is ß The Royal Society of Chemistry 2013 CrystEngComm, 2013, 15, 2795–2799 | 2795 Downloaded by Texas A & M University on 20 March 2013 Published on 07 February 2013 on http://pubs.rsc.org | doi:10.1039/C3CE26752K View Article Online View Journal | View Issue
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Cite this: CrystEngComm, 2013, 15, 2795
Received 24th October 2012,Accepted 5th February 2013
Self-assembly, concomitant photochemical processes,and improvement of the yield of [2 + 2]photoreactions from supramolecular arrays viamechanochemical assistance3
DOI: 10.1039/c3ce26752k
www.rsc.org/crystengcomm
Alexander Briceno,*a Dayana Leal,ab Gabriela Ortega,a Graciela Dıaz de Delgado,b
Edgar Ocandoa and Liz Cubillana
Novel examples of self-assembly, structural transformations and
concomitant photochemical processes from supramolecular tern-
ary assemblies directed by charge-assisted hydrogen bonds are
shown. In addition, mechanochemical assistance is also shown as
a potential tool in order to improve the yield of [2 + 2]
photoreactions in the solid state.
Chemical synthesis performed in the solid state is gaining interestas an important green alternative for the preparation of new andconventional compounds.1,2 A particular case of solid statereactions is represented by topochemical [2 + 2] cycloaddition ofolefins. In recent years, distinctive supramolecular approacheshave been exploited in order to direct stereo- and regiocontrolledphotoproducts.2,3 On the other hand, mechanochemical activationmethods have received significant attention in the last years due toattractive advantages such as: shorter reaction times, reduction ofcosts, energy and waste combined with high yields in comparisonwith traditional synthesis in liquid-phase.1 This alternativeprovides the possibility of affording a greater level of complexityand sophistication in the study of solid state transformations,which could involve sequential synthesis steps from molecularactivation in order to obtain complex molecular compounds to thepreparation of functional supramolecular arrangements self-assembled via mechanochemistry. Recently, such approach hasbeen exploited to carry-out stereocontrolled synthesis of cyclobu-tane derivatives from topochemical [2 + 2] cycloaddition reac-tions.3,4 In this context, as part of ongoing efforts on thepreparation and design of photoreactive solids5 we have reportedunprecedented examples of concomitant [2 + 2] cycloadditionsolid state reactions from co-crystals self-assembled via mechan-ochemistry (Scheme 1).5a Herein, we have prepared two novel
ionic ternary assemblies based on carboxylic acids and stilbazoleor stilbene derivatives: (2Cl-HStb+)(Hmal2)(H2Fu)0.5 (1) and(2,29-H2bpe2+)(Fu22)(H2Fu) (2) (where HStb+: stilbazolium;H2bpe2+: stilbenium; Hmal2: hydrogen maleate; Fu22: fumarateand H2Fu: fumaric acid). Both compounds are photoreactive uponUV-irradiation. In addition, the reinvestigation of two otherphotoreactive assemblies, (2,29-bpe)(H2Fu)5a (3) and trans-3-(3-pyridyl)acrylic acid6 (4), was also undertaken. As UV-irradiation of1 quantitatively yielded the expected photodimer, the photoreac-tion of 2, 3 and 4 were improved until obtaining nearlyquantitative yields of all the products via sequential grinding-irradiation steps.{
Crystals of 1 and 2 were obtained by mixing trans-2-chloro-stilbazole (2-Cl-Stb) with maleic acid (H2Mal) and 1,2-bis(2-pyridyl)ethylene (2,29-bpe) with fumaric acid (H2Fu) in molar ratio1 : 1 and 1 : 2 in methanol, respectively. These compounds canalso be obtained as crystalline single phases by direct grinding ofthe starting compounds in short periods of time (30–60 min) vialiquid-assisted grinding7 in the presence of a few drops ofmethanol (see ESI,3 Fig. S1).
The combination of H2Mal with 2-Cl-Stb in solution producesan unexpected ternary crystal: (Hmal2)(2Cl-HStb+)(H2Fu)0.5 (1).Formation of H2Fu is due to the partial cis–trans isomerisation ofH2Mal to H2Fu at RT (Scheme 2). Such isomerisation has beenobserved previously by Rao and co-workers in the presence ofother nitrogen-containing heterocycles and polar solvents.8 Thisprocess was monitored by 1H-NMR in order to evaluate the degreeof cis–trans conversion in solution as a function of time in thepresence of DMSO and CH3OH at RT. This analysis revealed an
aInstituto Venezolano de Investigaciones Cientıficas (IVIC), Apartado 21827, Caracas
Tel: +58-212-5041320bUniversidad de Los Andes (ULA), Apartado 40, La Hechicera, Merida 5251,
Venezuela
3 Electronic supplementary information (ESI) available: Experimental details. 1H-NMR spectra and PXRD patterns. CCDC 900648 and 900649. For ESI andcrystallographic data in CIF or other electronic format see DOI: 10.1039/c3ce26752k Scheme 1
CrystEngComm
COMMUNICATION
This journal is � The Royal Society of Chemistry 2013 CrystEngComm, 2013, 15, 2795–2799 | 2795
important effect of the solvent used on the rate of isomerisation.The reaction is slower in DMSO (93 days) than in CH3OH (30 days)to achieve quantitative conversion to trans-H2Fu (Fig. S2, ESI3).Given this dynamic behaviour of maleic acid in solution in thepresence of 2-Cl-Stb, the solid formed was never obtained as acrystalline single phase with a reasonable chemical purity. Theself-assembly of 1 was only successfully achieved via directgrinding from the three components in the appropriate molarratio (Scheme 2).
The asymmetric unit of 1 contains one hydrogen maleate anion(Hmal2), one stilbazolium cation (2Cl-HStb+) and half molecule ofH2Fu, which occupies a special position. The crystal structure of 1consists of a self-assembly of anions and cations linked by charge-assisted carboxylate–pyridinium supramolecular synthons [N1–H1N…O4: 2.681(6) Å] (Fig. 1(a)). This ionic moiety forms dimericunits via self-complementary hydrogen bonding interactionsbetween HMal2 anions [C15–H15…O2: 3.483(7) Å]. Adjacentsupramolecular units are connected through fumaric acidmolecules via H-bonding interactions between Hmal2 anionsand H2Fu molecules [O5–H5OH…O4: 2.601(5) Å], leading to a 2Dnetwork. The 3D structure is achieved by p–p interactions directedby organic cations. The relative orientation of the molecules allowsthe existence of close contacts between the double bonds either inpairs of 2Cl-HStb+ cations or between an Hmal2 anion and a H2Fu
molecule, suitable for [2 + 2] cycloaddition in the solid state9
(Fig. 1(b, c)). In particular, this last one is organized in a crisscrossmanner. Thus, such an arrangement may lead to a novel exampleof a concomitant solid state reactivity pattern upon UV-irradia-tion.5a
Compound 2 consists of a novel self-assembly between 2,29-bpeand H2Fu. The asymmetric unit of 2 contains one half stilbeniumdication (2,29-H2bpe)2+, one half fumarate anion (Fu22) and onehalf H2Fu. The crystal structure of 2 consists of a novel self-assembly between cations and anions via charge-assisted carbox-ylate–pyridinium synthons [N1–H1N…O3: 2.607(3) Å]. This supra-molecular motif is extended along the b-axis to generate H-bondedzig-zag ribbons (Fig. 2(a)). These ribbons are linked through H2Fuvia self-complementary lateral C–H…O bonding interactionsbetween carboxylic and C–H bond from adjacent organic cations.Additionally, the proton of the acidic groups in H2Fu moleculeadopts an antiplanar configuration forming H-bonded chains withFu22 anions via O–H…O charge-assisted hydrogen bonds [O1–H1OH…O4: 2.594(3) Å], which are intercalated between 2,29-bpemolecules. These interactions lead to 2D H-bonded sheets parallelto the bc-plane. The 3D structure is achieved by p–p interactionsbetween the sheets related by simple translation. The relativeorientation of the molecules allows the presence of close contactsbetween the double bonds of the same kind of molecules suitablefor [2 + 2] cycloaddition. This structure displays structural featuressimilar to the (2,29-bpe)(H2Fu) co-crystal with stoichiometry ratio(1 : 1) previously reported by us,5a which produces unprecedentedconcomitant topochemical [2 + 2] cycloaddition reactions in asingle crystal. Thus, such arrangement also anticipates thepossibility of a similar behaviour in the solid state uponirradiation.
Encouraged by the formation of different stoichiometricvariations of H2Fu and 2,29-bpe via mechanochemistry,5a weevaluated the possibility of interconversion between both phases.10
Thus, we studied the transformation of 2 to 3 as a function ofgrinding time. Analysis of the PXRD patterns of the equimolarmixture of 2 with 2,29-bpe after 15 min revealed the incipientformation of 3 and the complete disappearance of 2 (molar ratio1 : 1). Likewise, compound 3 can be interconverted to 2, byaddition of stoichiometric amounts of H2Fu via mechanochem-
Fig. 1 (a) View of the 2D H-bonded network in the structure of 1, showinghydrogen bond interactions. (b) Interactions between double bonds ofstilbazolium cations and carboxylic acid molecules (c).
Fig. 2 (a) View of the 2D H-bonded network in the structure of 2, showinghydrogen bond interactions. (b) Interactions between double bonds of2,29-H2bpe2+ (c) Fu22 anions and carboxylic acid molecules.
Scheme 2
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istry after 15 min (Fig. 3). Complete transformations of bothphases are achieved after 30 min of liquid-assisted grinding inmethanol, along with an increase in crystallinity, which is reflectedby the decrease of background radiation of the patterns and theaugmentation of intensity. It is noteworthy that these transforma-tions involve an interesting proton transfer process between thecomponents, transforming an ionic assembly 2 into a neutral co-crystal 3 and viceversa. Structural differences between bothH-bonded supramolecular arrays are shown in Fig. 4.
The 1H-NMR characterisation of the solids obtained after UVirradiation of 1 and 2 for 3–4 days confirms the cycloadditionreaction in both cases. The spectrum of the crude solid obtainedfrom 1 at 350 nm reveals the quantitative formation of rctt-1,3-bis(4-pyridyl)-2,4-bis(2-chlorophenyl)cyclobutane head to tail iso-mer (2-Cl-dpcb; yield: 100%, see Scheme 3) together with thepartial isomerisation of H2Mal to H2Fu (55 : 45) (Fig. S3, ESI3). Theyield of the photodimer remains consistent when 1 is irradiated ata shorter wavelength (302 nm). Furthermore, cis–trans isomerisa-tion was not observed (Fig. S3, ESI3).
As expected, the 1H-NMR spectrum of the irradiated solid 2 at302 nm showed the formation of two concomitant photoproductscharacteristic of rctt-tetrakis(2-pyridyl)cyclobutane (2,29-tpcb) and
rctt-1,2,3,4-cyclobutanetretracarboxylic acid (H4Cbtc) with ca. 78%and 27% of yield (see Scheme 3), respectively (Fig. S4(a), ESI3). Thereactivity observed for 2 is similar to that found previously in the(2,29-bpe)(H2Fu) co-crystal (3) with (1 : 1) stoichiometry, whichproduces a mixture of the photodimers 2,29-tpcb and H4Cbtc.5a
Apparently, the existence of a greater number of H-bondinginteractions in the structure of 2 reduces the mobility of the acidmolecules enabling a higher yield of the product.
Toda and co-workers in 1987 were, perhaps, the first to report acatalytic effect of mixing upon the stereospecificity of aphotoreaction. In this case, a diol was used as host compoundin the photoreaction of a chalcone derivative.11a Different host–guest ratios were used and the photoreactions were carried out incycles of irradiation and shaking. Thus, the combination of solid-state host–guest assembly, irradiation, and mixing in the solidstate, drove the stereoselective photoreaction of the chalcone.More recently, MacGillivray and co-workers have re-explored asimilar approach called Mechano-assisted supramolecular cataly-sis11b in order to drive solid state [2 + 2] photoreactions directed bya linear template. Although these results provide interestingapplications for the study of solid state photoreactions, its scope islimited due to the difficulty in the prediction of solid state crystalpacking, together with extremely limited molecular mobility in thesolid state to promote the self-assembly or disassembly of the pairsubstrate/catalyst (or template). Nevertheless, we have recognizedthat such approach could be exploited in order to improve theyield of any photoreaction until near quantitative yield, simply bythe combination of multisteps number of grinding-irradiationcycles of the resulting molecular solid state solution (photopro-duct, remaining photoactive target and possible template). Takingadvantage of this approach, we re-evaluated the photoreactivity of2 and of two other known organic assemblies reported by us:(2,29-bpe)(H2Fu) (3) and trans-3-(3-pyridyl)acrylic acid (4). A secondgrinding cycle was applied to compound 2 irradiated at 302 nm for
Fig. 3 XRPD patterns showing the conversion from compound 2 to 3 (a) andviceversa 3 to 2 (b), as a function of grinding time via mechanochemistry.
Fig. 4 Comparative view of H-bonded layers found for the structures of 2 and 3.
Scheme 3 Molecular diagram of photoproducts obtained.
This journal is � The Royal Society of Chemistry 2013 CrystEngComm, 2013, 15, 2795–2799 | 2797
30 min and after exposed to UV-irradiation during 2 days again.The new 1H-NMR spectrum of the solid obtained after the secondirradiation showed an increase in yields for both photoproducts:96% for 2,29-tpcb whereas the yield of H4Cbtc only rose to 35%(Fig. S4(a), ESI3). Thus, a third grinding-irradiation cycle was tried.In this attempt, ca. 40% fresh 2,29-bpe was added to the solid andco-ground for 30 min. A subsequent third exposure to UV-irradiation for 2 days also was carried-out. The spectrum of thesolid obtained showed the presence of the same signals togetherwith an additional complex set of signals (Fig. S4(b), ESI3), whichsuggests the formation of an additional unidentified photopro-duct. Apparently, the presence of both cyclobutanes compromisesthe topochemical control of the initial reaction. Only a smallincrease on the yield of H4Cbtc was observed (45%).
In order to illustrate the origin of the photoreactivity of 3 and 4,short contacts between the double bonds of pairs of molecules areshown in Fig. 5.
Compound 3 was obtained via mechanochemistry. Upon UV-irradiation for 3 days it produces a mixture of photodimers2,29-tpcb and H4Cbtc with yields of 90% and 60%, respectively. Asecond grinding-irradiation cycle was then carried-out. Thereactivity in both cycles was monitored by 1H-NMR spectroscopy.These spectra revealed an improvement in the yield of bothphotoproducts, reaching almost quantitative conversion withyields of 100% and 92%, respectively (Fig. S5, ESI3). Likewise,trans-3-(3-pyridyl)acrylic acid (4) crystallises in a b-structure, whichis photoactive in the solid state leading to rctt-3,4-bis(3-pyridyl)-cyclobutane-1,2-dicarboxylic acid (3-bpcd). However its yield ismodest, ca. 60%.6 The yield of the photoreaction was alsoimproved very close to the quantitative conversion (98%), usingthe procedure described above for 2 and 3. Fig. S6, ESI,3 shows theNMR spectra of the different cycles evaluated.
In summary, we report unprecedented examples, to the best ofour knowledge, of ternary supramolecular arrays self-assembledvia mechanochemistry that lead to different solid state reactivitypatterns. These systems represent examples of novel sequentialmechanochemical processes, including self-assembly and struc-tural transformations. Compound 1 upon UV-irradiation leads tothe quantitative [2 + 2] cycloaddition of 2Cl-Stb and theconcomitant cis–trans isomerisation of H2Mal into H2Fu. Inaddition, such isomerisation is observed either in solution or inthe solid state. Likewise, the interconversion between compound 2and 3 can be controlled by modifying the composition of thereaction mixture via mechanochemistry. Compounds 2 and 3represent unique examples of concomitant [2 + 2] photoreactionsin single crystals built-up from the same components. We alsoshowed that mechanochemical assistance via multiple grinding
and irradiation steps provides an interesting green route to highyields of photoproducts. This approach opens a window ofopportunities to develop efficient routes for the preparation and/orimprovement of the yield of new and conventional cyclobutane-like stereoisomers from stoichiometric solids.5d Such results maybe improved using the method described recently by MacGillivrayand co-workers based on the use of a vortex mixer that permitsautomated grinding and simultaneous UV- irradiation of the solidsample.12 This alternative can be very helpful in order to overcomethe limitations imposed by the topochemical postulates in solidstate photoreactions. In these reactions, the yield of the product isseriously affected by important changes on distances and relativeorientations between the pairs of photoactive species due to largemolecular movements or structural rearrangements during thecourse of the reaction. Further studies on improving [2 + 2]photoreactions from supramolecular arrays via mechanochemicalassistance are in progress.
Acknowledgements
We thank FONACIT (grant LAB-97000821) for financialsupport and M. A. Ramos and R. Atencio (INZIT) for technicalassistance.
References{ Crystal data for 1: C19H16ClNO6, Mt = 389.78, triclinic, space group P1, T =293(2) K, a = 8.687(2), b = 8.900(2), c = 13.039(3) Å, a = 104.06(3), b= 97.67(3),c = 105.15(3)u, U = 922.8(3) Å3, Z = 2, m(Mo Ka) = 0.243 mm21, rcalcd = 1.403g cm23. Rint = 0.058, R1(F2) = 0.096, wR(F2) = 0.21, S = 1.12 for 2296independent reflections (I > 2s(I)). CCDC 900648.
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