A comparative structural study of eleven empty and eight filled hexanuclear metalloprismatic cages reveals helical chirality depending on the twist angle between the upper and lower triangles defined by three metal centres

Süss-Fink, Georg

Therrien, Bruno

English

CHIMIA

CHIRALITY 514CHIMIA 2008, 62, No. 6Chimia 62 (2008) 514–518© Schweizerische Chemische GesellschaftISSN 0009–4293doi:10.2533/chimia.2008.514Inducing Helical Chirality by DeformingHexanuclear Metalloprismatic StructuresBruno Therrien* and Georg Süss-FinkAbstract: A comparative structural study of eleven empty and eight filled hexanuclear metalloprismatic cages re-veals helical chirality depending on the twist angle between the upper and lower triangles defined by three metalcentres.Keywords: Chirality · Hexanuclear complex · Supramolecular assembly · Triangular prismwith the aggregate. Among such functions,chirality is often found in supramolecularchemistry, the overall structure being chi-ral, while the individual building blocks areachiral.Helical chirality is very common innature.[3] DNA for example adopts right-handed or left-handed forms, which canundergo molecular motions such as fold-ing[4] or P M conversion.[5] The assemblyof helicate-type complexes has been exten-sively studied[6] since this term was intro-duced by Lehn in 1987.[7] Among artificialhelical structures, Shionoya and co-workershave developed double-decker sandwichsystems connected by three silver centresand two tris(thiazolyl) ligands that showsP M interconversion in solution (Fig. 1).[8]Similarly, Sherman and co-workers haveobserved helical chirality of carceplex com-pounds upon encapsulation of moleculessuch as (R)-2-butanol, pyrazine, or DMSOin a tetraoxatetrathiahemicarceplex cage.[9]The term twistomers was used to describethe phenomena (Fig. 2). A comparativestudy of structurally characterised capsulesof this type revealed that the twist angle canbe rather significant, up to 21°.Recently, we have observed that cation-ic hexanuclear metalloprisms of the generalformula [(arene)6M6(tpt)2(C2O4)3]6+ (M =Ru; arene = piPrC6H4Me, C6Me6: M = Rh,Ir; arene = C5Me5) containing bridging ox-alato ligands (C2O4) and 2,4,6-tri(pyridine-4-yl)-1,3,5-triazine (tpt) units display heli-cal chirality as well (Fig. 3).[10,11] Moreover,a concerted rotation of the aromatic rings ofthe trigonal tpt subunits creating an addi-tional three-bladed propeller chirality wasobserved. These two stereogenic phenom-ena were observed in the solid-state andwere shown to persist in solution.These observations raised the questionof generality for chiral conformations of*Correspondence: Dr. B. TherrienInstitut de ChimieUniversité de NeuchâtelAvenue de Bellevaux 51, Case postale 158CH-2009 NeuchâtelTel.: + 41 32 718 24 99Fax: + 41 32 718 25 11E-Mail: bruno.therrien@unine.chIntroductionSelf-assembly is a process leading to theformation of discrete nanometer-sized ob-jects or well-defined aggregates in whichthe overall structure is controlled by thesymmetry of the different building blocks.[1]In the case of metallosupramolecular as-semblies, the coordination mode of the met-al centre as well as the symmetry of the li-gandsneeds tobecontrolled inorder toallowthe formation of the desired aggregates.[2]If the assembly possesses special propertiesthat are only observed after aggregation, asupramolecular function must be associatedFig. 1. [Ag3(tris-thiazolyl)] double-decker sandwich system[8a]CHIRALITY 515CHIMIA 2008, 62, No. 6metalloprisms, and whether or not the ste-reodynamic phenomena observed for theseoxalato metalloprisms are unique. In or-der to address this question, we retrievedall structural data of hexanuclear metallo-prisms available and performed a compara-tive study.General RemarksThe present study is divided into twosections. The first section is exclusivelydevoted to empty hexanuclear metallo-prisms in which no encapsulated moleculeplays a role in the chiral conformation ofthe metalloprism. The second section dealswith hexanuclear metalloprisms in which amolecule is encapsulated within the cavityof the prismatic cage. The inclusion of en-capsulated molecule can either enhance orweaken the chirality of the cage, and there-fore these filled hexanuclear metalloprismsare discussed separately.Empty HexanuclearMetalloprismatic CagesThe general structure of the first se-ries of empty M6 cage molecules to bediscussed here involves the short bridg-ing chloro ligands and 2,4,6-tri(pyridine-4-yl)-1,3,5-triazine (tpt) triangular panels.The cationic arene ruthenium compounds[(p-iPrC6H4Me)6Ru6(tpt)2Cl6]6+ and[(C6Me6)6Ru6(tpt)2Cl6]6+[12] as well as thecationic pentamethylcyclopentadienyl rho-dium complex, [(C5Me5)6Rh6(tpt)2Cl6]6+are known.[13] In Fig. 4, the molecular struc-tures of the three chloro-bridged cages arecompared. Due to the proximity of the twotpt units, strong parallel π-stacking interac-tions between the aromatic rings are ob-served. Indeed, the centroid−centroid dis-tances between the corresponding triazinerings is slightly shorter than the averagemetal−metal distances of the chloro bime-tallic connectors (Table 1). Therefore, inthese systems only a slight deviation froma perfectly eclipsed conformation of the twotpt subunits is observed, while the propeller-like rotation of the pyridyl rings is relativelyimportant.Similarly, in the hexanuclear car-bonyl rhenium metalloprisms bridgedby butyloxy or phenyloxy ligands,[(CO)18Re6(tpt)2(OC4H9)6]and[(CO)18Re6-(tpt)2(OCH2Ph)6], the centroid−centroid dis-tances between the triazine rings are slightlyshorter than the average metal−metal dis-tances of the bimetallic connectors (Fig.5).[14] Interestingly, in the analogous car-bonyl rhenium metalloprism bridged by2,2’-bisbenzimidazolato (bbz) ligands,[(CO)18Re6(tpt)2(bbz)3],[15] in which theM−M distances are significantly longer (av-erageRe−Re=5.5Å) than thoseof thealkoxyderivatives [(CO)18Re6(tpt)2(OC4H9)6] and[(CO)18Re6(tpt)2(OCH2Ph)6], the centroid−centroid distance of the two triazine rings iseven shorter (3.4 Å). The two tpt units curveinward to maximise the π-stacking interac-tions of the two triazine rings.Fig. 2. Schematic representation of carceplex twistomers[9]Fig. 3.Fig. 4. Structural comparison (top view and side view) of [(p-iPrC6H4Me)6Ru6(tpt)2Cl6]6+,[12] [(C6Me6)6Ru6(tpt)2Cl6]6+,[12] and [(C5Me5)6Rh6(tpt)2Cl6]6+ [13]CHIRALITY 516CHIMIA 2008, 62, No. 6In the oxalato bridged derivatives,[(C5Me5)6Rh6(tpt)2(C2O4)3]6+, [(C5Me5)6-Ir6(tpt)2(C2O4)3]6+, [(piPrC6H4Me)6Ru6-(tpt)2(C2O4)3]6+ and [(C6Me6)6Ru6(tpt)2-(C2O4)3]6+, the metal−metal distances in thedinuclear clips reach 5.5 Å, thus, in theory,giving more conformational freedom to thesubunits of the cage molecules. In fact, whilethe solid-state structure of the triflate saltsof the rhodium and ruthenium derivativesshow an important deformation of the tri-angular prismatic arrangement in the solidstate, the iridium derivative adopts a per-fectly eclipsed conformation of the prism(Fig. 6). In [(C5Me5)6Ir6(tpt)2(C2O4)3]6+,the two tpt panels are strongly curvedinward to maximise π-stacking interac-tion, while the pyridyl rings remain paral-lel to their triazine core. In contrast, thecations [(C5Me5)6Rh6(tpt)2(C2O4)3]6+,[(piPrC6H4Me)6Ru6(tpt)2(C2O4)3]6+ and[(C6Me6)6Ru6(tpt)2(C2O4)3]6+ show a dis-tinct distortion, the dimetallic oxalato clipsbeing twisted out of the plane of the tpt units,which give rise to a ‘double-rosette’ typehelicity. Furthermore, in these systems, thepyridyl rings are tilted out of the plane of thetriazine subunits thus generating a propeller-like chirality. The tilt of the pyridyl ring canbe as much as 30°, and the twist of the twotpt units up to 15°.The large cationic hexanuclear cagemolecule, [(ant)3Pt6(PEt3)12(tpepe)2]6+ (ant= anthracene), obtained by mixing the mo-lecular clip 1,8-[Pt(PEt3)2(NO3)]2(ant) andthe triangular panels 1,1,1-tris(4-phenyl(4’-ethynylpyridyl)ethane) (tpepe), possessesin the solid state, crystallographically im-posed D3h symmetry (space group R3c).[16]Therefore, this system shows no chirality inthe solid state.Filled Hexanuclear MetalloprismaticCagesThe encapsulation of a flat moleculewithin the cavity of a hexanuclear metallo-prism can either enhance or weaken the chi-Fig. 5. Structural comparison (top view and side view) of[(CO)18Re6(tpt)2(OC4H9)6],[14] [(CO)18Re6(tpt)2(OCH2Ph)6],[14] and[(CO)18Re6(tpt)2(bbz)3][15]Fig. 6. Structural comparison (top view and side view) of [(C5Me5)6Rh6(tpt)2(C2O4)3]6+,[11] [(C5Me5)6Ir6(tpt)2(C2O4)3]6+,[11] [(p-iPrC6H4Me)6Ru6(tpt)2(C2O4)3]6+,[10] and [(C6Me6)6Ru6(tpt)2(C2O4)3]6+[10]Table 1. Chirality and structural parameters for the empty hexanuclear cage moleculesEmpty cage complex Space group M−M a ref.[(p-iPrC6H4Me)6Ru6(tpt)2Cl6]6+ C 2/c 3.7 Å 0.2° [12][(C6Me6)6Ru6(tpt)2Cl6]6+ P 21/n 3.7 Å 1.3° [12][(C5Me5)6Rh6(tpt)2Cl6]6+ P 21/c 3.6 Å 4.4° [13][(CO)18Re6(tpt)2(OC4H9)6] R 3c 3.4 Å 1.4° [14][(CO)18Re6(tpt)2(OCH2Ph)6] P 1 3.5 Å 0.2° [14][(CO)18Re6(tpt)2(bbz)3] C 2/c 5.7 Å 4.9° [15][(C5Me5)6Rh6(tpt)2(C2O4)3]6+ C 2/c 5.5 Å 8.5° [11][(C5Me5)6Ir6(tpt)2(C2O4)3]6+ F d3c 5.5 Å 0.0° [11][(p-iPrC6H4Me)6Ru6(tpt)2(C2O4)3]6+ P 3c1 5.5 Å 2.9° [10][(C6Me6)6Ru6(tpt)2(C2O4)3]6+ C 2/c 5.5 Å 14.8° [10][(ant)3Pt6(PEt3)12(tpepe)2]6+ R 3c 5.5 Å 0.0° [16]atwist angle between the two trigonal panelsCHIRALITY 517CHIMIA 2008, 62, No. 6rality. Moreover, if the encapsulated mol-ecule possesses the proper symmetry andfits perfectly into the cavity, the triangularprismatic conformation can be locked, thussuppressing helical chirality. Table 2 sum-marises chirality and structural parametersfor the filled hexanuclear cage moleculesdiscussed below.Interestingly, the filled hexanuclearpalladoprism (hmtp)⊂[(en)6Pd6(tpt)2-(prz)3]12+ (prz = pyrazine; hmtp = hexa-methoxytriphenylene) shows high symme-try (P3) with a perfectly eclipsed conforma-tion and a perfectly parallel arrangement ofthe π-systems.[17] Thus, this system is notchiral (Fig. 7). Similarly, in the idealisedD3h anionic cage [Mo6(CN)9(CO)18]9− apotassium sits in the small cavity, 5.6 × 5.6Å, thus giving rise to an achiral filled hexa-nuclear metalloprismatic carceplex.[18]However, in the slightly more spa-cious hexanuclear cage molecule[(ant)3Pt6(PEt3)12(tpm)2]6+ (tpm = tris(4-pyridyl)methanol), in which a NO3− anionis found within the cavity, a deformationof the triangular prismatic conformation isobserved, the twist angle between the twotris(4-pyridyl)methanol triangular moietiesbeing 1.3° (Fig. 8A).[16]Water molecules are encapsulated by alarge cationic copper cage, [(qpy)3Cu6(Ph6-hat)2]6+, containing hexaphenylhexaazatri-phenylene (Ph6-hat) triangular panels andquaterpyridine (qpy) connecting ligands(Fig. 8B).[19] The water molecules are disor-deredwithinthecavity(sixpositionswith1/3occupation) and therefore play a minor rolein the overall geometry of [(qpy)3Cu6(Ph6-hat)2]6+. The centroid−centroid distancebetween the two Ph6-hat panels is 7.7Åwith a twist angle of 32.5°.Recently, we have shown that the cat-ionic cage complex [(piPrC6H4Me)6Ru6-(tpt)2(dhbq)3]6+ (dhbq = dihydroxybenzo-quinonato) is capable of encapsulatingPt(acac)2 complex (acac = acetylacetonato)within its cavity.[20] To minimise interac-tions between the methyl groups of theacac moieties and the tpt units, the triangu-lar prism adopts a staggered conformation:The twist angle being 12.5° (Fig. 8C). Theplatinum atom is stacked midway betweenthe two triazine moieties at 3.4 Å from thetriazine planes.In the large palladium cage[(en)6Pd6(tpt)2(Me4bipy)3]12+ (Me4bipy= 2,2’,6,6’-tetramethyl-4,4’-bipyridine),in which two pyrene (pyr)[21] or porphyrin(porph)[22] or Pt(acac)2[23] fit into the cav-ity, the chirality and distortion of the tri-angular prism is amplified, the complexes(pyr)2⊂[(en)6Pd6(tpt)2(Me4bipy)3]12+,(porph)2⊂[(en)6Pd6(tpt)2(Me4bipy)3]12+ and[Pt(acac)2]2⊂[(en)6Pd6(tpt)2(Me4bipy)3]12+ were structurally characterised(Fig. 9). In these systems, the deviationfrom triangular prismatic geometry is quiteTable 2. Chirality and structural parameters for the filled hexanuclear cage moleculesFilled cage complex Space group M−M a ref.(hmtp)⊂[(en)6Pd6(tpt)2(prz)3]12+ P 3 6.6 Å 0.0° [17](K+)⊂[ Mo6(CN)9(CO)18]9- P 21/c 5.6 Å 0.0° [18](NO3-)⊂[(ant)3Pt6(PEt3)12(tpm)2]6+ P 1 5.7 Å 1.3° [16](H2O)2⊂[(qpy)3Cu6(Ph6-hat)2]6+ C 2/c 7.7 Å 32.5° [19][Pt(acac)2]⊂[(p-iPrC6H4Me)6Ru6(tpt)2(dhbq)3]6+ P 21/c 6.8 Å 12.5° [20](pyr)2⊂[(en)6Pd6(tpt)2(Me4bipy)3]12+ C 2221 10.0 Å 33.2° [21](porph)2⊂[(en)6Pd6(tpt)2(Me4bipy)3]12+ P 21/n 9.9 Å 19.6° [22][Pt(acac)2]2⊂[(en)6Pd6(tpt)2(Me4bipy)3]12+ P 21/c 10.4 Å 22.5° [23]Fig. 7. Structural representations (top view and side view) of hexame-thoxytriphenylene in [(en)6Pd6(tpt)2(prz)3]12+[17]Fig. 8. Structural comparison (top view and side view) of the filled systemsNO3− in [(ant)3Pt6(PEt3)12(tpm)2]6+ (A),[16] 2 H2O (disordered) in [(qpy)3Cu6(Ph6-hat)2]6+ (B)[19] and [Pt(acac)2] in [(p-iPrC6H4Me)6Ru6(tpt)2(dhbq)3]6+ (C)[20]CHIRALITY 518CHIMIA 2008, 62, No. 6significant. The twist of the tpt units variesfrom 20−33°, while the average rotation ofthe pyridyl groups with respect to the planeof the triazine core remains low (<8°). Thislarge filled hexanuclear metalloprismaticcage, [(en)6Pd6(tpt)2(Me4bipy)3]12+,as well as the copper derivative,[(qpy)3Cu6(Ph6-hat)2]6+, show the great-est deformation from the triangular pris-matic geometry.ConclusionsThe structural comparison of the nine-teen hexanuclear metalloprismatic cagemolecules described in this article showsthat helical chirality is generated by twist-ing the two trigonal panels of the cages oneagainst the other. Only for four complexesthe X-ray structure analyses reveal a per-fectly eclipsed arrangement of the twotrigonal panels, so that these cages are notchiral in the solid state. The other fifteencages show helical chirality in the solidstate. The encapsulation of flat moleculesin some of the cages may enforce or at-tenuate the helical chirality by influenc-ing the twist between the trigonal panels.The longer the vertical connectors in thesehexanuclear cages are, the greater are theobserved twist angles.Received: February 26, 2008[1] a) J.-M. Lehn, ‘Supramolecular Chemistry– Concepts and Perspectives’Wiley-VCH,Weinheim, 1995; b) J.-M. Lehn, Science2002, 295, 2400.[2] a) L. Pirondini, F. Bertolini, B. Cantadori,F. Ugozzoli, C. Massera, E. Dalcanale,Proc. Natl. Acad. Sci. USA 2002, 99,4911 and references therein; b) M. V.Ovchinnikov, B. J. Holliday, C. A. Mirkin,L. N. Zakharov, A. L. Rheingold, Proc.Natl. Acad. Sci. USA 2002, 99, 4927 andreferences therein; c) Y. K. Kryschenko,S. Russell Seidel, D. C. Muddiman, A. I.Nepomuceno, P. J. Stang, J. Am. Chem.Soc. 2003, 125, 9647; d) J. D. Crowley,A. J. Goshe, B. Bosnich, Chem. Commun.2003, 2824; e) D. C. Caskey, J. Michl, J.Org. Chem. 2005, 70, 5442; f) M. Fujita,M. Tominaga, A. 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Structural comparison (top view and side view) of (pyr)2 (top),[21](porph)2 (middle)[22] and [Pt(acac)2]2 (bottom)[23] encapsulated in the cat-ionic cage [(en)6Pd6(tpt)2(Me4bipy)3]12+

Inducing Helical Chirality by Deforming Hexanuclear Metalloprismatic Structures

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Inducing Helical Chirality by Deforming Hexanuclear Metalloprismatic Structures

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