Caterpillar Track Complexes in Template-Directed Synthesis and Correlated Molecular Motion

Small alterations to the structure of a star-shaped template totally change its mode of operation. The hexapyridyl template directs the conversion of a porphyrin dimer to the cyclic hexamer, but deleting one pyridine site changes the product to the cyclic decamer, while deleting two binding sites changes the product to the cyclic octamer. This surprising switch in selectivity is explained by the formation of 2:1 caterpillar track complexes, in which two template wheels bind inside the nanoring. Caterpillar track complexes can also be prepared by binding the hexapyridyl template inside the 8- and 10-porphyrin nanorings. NMR exchange spectroscopy (EXSY) experiments show that these complexes exhibit correlated motion, in which the conrotatory rotation of the two template wheels is coupled to rotation of the nanoring track. In the case of the 10-porphyrin system, the correlated motion can be locked by binding palladium(II) dichloride between the two templates.

ShiqiLiu, Dmitry V. Kondratuk, Sophie A. L. Rousseaux, Guzmµn Gil-Ramírez, Melanie C. OSullivan, Jonathan Cremers,T im D. W. Claridge,and Harry L. Anderson* Abstract: Small alterations to the structure of as tar-shaped template totally change its mode of operation. The hexapyridyl template directs the conversion of ap orphyrin dimer to the cyclic hexamer,b ut deleting one pyridine site changes the product to the cyclic decamer,while deleting two binding sites changes the product to the cyclic octamer.T his surprising switch in selectivity is explained by the formation of 2:1 caterpillar trackcomplexes,inwhichtwo template wheels bind inside the nanoring.C aterpillar trackc omplexes can also be prepared by binding the hexapyridyl template inside the 8-and 10-porphyrin nanorings.N MR exchange spectroscopy (EXSY) experiments show that these complexes exhibit correlated motion, in which the conrotatory rotation of the two template wheels is coupled to rotation of the nanoring track. In the case of the 10-porphyrin system, the correlated motion can be locked by binding palladium(II) dichloride between the two templates.
Template-directed synthesis is ap owerful strategy for using reversible non-covalent interactions to control the formation of covalent bonds, [1] and many spectacular examples have been reported recently. [2] We have used oligo-pyridines to direct the synthesis of zinc-porphyrin nanorings, [3] and shown that several small template molecules can cooperate to direct the formation of larger macrocycles,a ccording to aV ernier principle. [4] Here we demonstrate that as ingle template hub can be modified to direct the formation of several different products,simply by deleting some of the binding sites on the template ( Figure 1). Coupling of linear porphyrin dimer P2 in the presence of the regular D 6h T6 template gives the cyclic hexamer c-P6 (Figure 1a). [3b,d] If twoadjacent pyridine binding sites of the T6 template are deleted to give T4,t hen the main product becomes the cyclic octamer c-P8 (Figure 1b), or if only one binding site is deleted, the main product is the cyclic decamer c-P10 (Figure 1c). Thef ormation of c-P10 from P2 directed by T5 is ac ase of Vernier templating, because 10 is the lowest common multiple of 2and 5, whereas formation of c-P8 directed by T4,i llustrates an ew type of non-Vernier cooperative templating. [5] We call the 1:2n anoring template assemblies c-P8·(T4) 2 and c-P10·(T5) 2 "caterpillar track" complexes because of their resemblance to the wheeled treads commonly used on bulldozers,t anks and tractors.Inc-P8·(T4) 2 and c-P10·(T5) 2 the template wheels do not rotate,b ut similar complexes c-P8·(T6) 2 and c-P10·(T6) 2 can be prepared which undergo correlated motion, like at urning caterpillar track.  Figure 2; the product distributions shown here were obtained using the compounds with R = OC 8 H 17 .T he percentages are isolatedy ields, for the main product of each reaction, and analytical yields for byproducts.
[*] S. Liu, [+] Dr.D .V .K ondratuk, [+] Dr.S.A.L.Rousseaux, [+] Dr.G. Them olecular structures of the compounds used in this study are shown in Figure 2. Despite their structural similarity,t emplates T4, T5 and T6 have dramatically different effects on the palladium-catalyzed oxidative coupling of porphyrin dimer P2,a ss een from the gel permeation chromatography (GPC) traces of crude reaction mixtures in Figure 1. No detectable trace of c-P8 or c-P10 is formed when the reaction is carried out in the presence of T6,b ut these larger nanorings are isolated in yields of 14 %a nd 20 %, respectively,byusing templates T4 and T5 (with R = OC 8 H 17 , n-octyloxy). Similar reactions were also carried out using the version of P2 with R = t-Bu, giving c-P8 and c-P10 in isolated yields of 29 %a nd 18 %, respectively (see Figure S33 in the Supporting Information for GPC traces). Previously we isolated c-P10 as an unexpected byproduct during the synthesis of c-P30 by the Vernier coupling of alinear porphyrin decamer in the presence of T6; [4a] the formation of ac aterpillar track complex now explains how c-P10 was formed in this reaction. [6] Thed iscovery of "caterpillar track" template effects prompted us to investigate the structures and stabilities of the complexes c-P8·(T4) 2 and c-P10·(T5) 2 .U V/Vis/NIR and 1 HNMR titrations showed that 1:2c omplexes are formed in solution with high allosteric cooperativity;i nb oth cases,t he corresponding 1:1c omplexes do not form in detectable concentrations ( Figure S35-S37). Then anorings c-P8 and c-P10 also form similar 1:2c aterpillar track complexes with the regular T6 template.T he optimized geometries of c-P8·(T6) 2 and c-P10·(T6) 2 ,from molecular mechanics calculations ( Figure 3) are similar to the geometries of c-P8·(T4) 2 and c-P10·(T5) 2 deduced from solution-phase small-angle Xray scattering (SAXS) studies ( Figures S48 and S49). In both structures,all the zinc atoms lie in essentially the same plane, and in c-P8·(T6) 2 ,t he two template units overlap at the center. 1 HNMR exchange spectroscopy (EXSY) experiments were carried out to elucidate the dynamics of the complexes c-P8·(T4) 2 , c-P8·(T6) 2 , c-P10·(T5) 2 ,a nd c-P10·(T6) 2 .T hese experiments demonstrated that the two coordinatively saturated complexes, c-P8·(T4) 2 and c-P10·(T5) 2 ,a re static,w ith no exchange between template environments,w hereas the two complexes with free pyridine sites, c-P8·(T6) 2 and c-P10·(T6) 2 ,u ndergo caterpillar track motion of the type shown schematically for c-P8·(T6) 2 in Figure 4. The1DEXSY spectra in Figure 5d emonstrate this concerted motion by revealing exchange signals only between environments that are related by a6 0 8 8rotation of the template (m,a, a,b, l,c,a nd c,d); signals corresponding to ar otation of 1208 8 (m,b and l,d)a re absent on the time-scale of the experiment, confirming that the observed exchange peaks are due to intramolecular rotation of bound T6,and not due to random exchange of the template units.Under the same conditions,an identical 1D EXSY experiment on c-P8·(T4) 2 showed no exchange signals between template protons ( Figure S43), confirming that the exchange peaks observed in c-P8·(T6) 2 result from intramolecular motion.
As econd set of 1D EXSY NMR experiments,t argeting the porphyrin nanoring protons,w as performed to confirm that the nanoring and template rotation in c-P8·(T6) 2 are concerted (i.e.that the motions indicated by the blue and red arrows in Figure 4p roceed at the same rate). The ortho proton 7 on the porphyrin aryl side-groups and template proton d were chosen for irradiation in these kinetic experiments (Figure 5a nd S43). Spectra were recorded under various mixing times (100-600 ms) to determine the relative rates of exchange.A ryl proton 7 generates two types of exchange signals corresponding to 1) the rotation of the aryl  . .
Similar 1D EXSY NMR experiments on c-P10·(T6) 2 showed that caterpillar track motion occurs faster in this system than in c-P8·(T6) 2 ,with arate constant of 10 AE 2s À 1 for a608 8rotation, resulting in the observation of exchange signals corresponding to more than one 608 8 rotation. When the noncoordinated b-pyridyl proton (a' ' in Figure 3) is selectively irradiated, agradual build-up of exchange signals corresponding to a608 8rotation of the template is observed to b' ',followed by the appearance of exchange signals corresponding to a1 208 8 rotation to c' ',a nd finally by signals corresponding to a1808 8 rotation to d' ' ( Figure S45). Thedelayed appearance of signals c' ' and d' ' is ac lear signature of as equential step-wise exchange process. [7] This kinetic behavior demonstrates that the template undergoes as tep-wise rotation within the nanoring,r ather than exchanging through dissociation/association.
Several systems have been reported in which intramolecular rotary motion can be locked by binding am etal cation, [8][9][10][11] which suggest that we might be able to control the motion in am olecular caterpillar track by coordinating am etal between the two templates ( Figure 6). Thed istance

Angewandte
Chemie between the two central nitrogen atoms in the energyminimized geometry of c-P10·(T6) 2 (Figure 3b)i s6 .8 , which is too far to chelate am etal atom, but molecular modeling shows that these pyridine sites can easily move closer together if the complex adopts ah elical twist (Figure S50). We tested this idea by carrying out a 1 HNMR titration ( Figure S41). When one equivalent of [PdCl 2 -(PhCN) 2 ]i sa dded to as olution of c-P10·(T6) 2 in CDCl 3 , new sharp signals appear corresponding to the formation of ac omplex of stoichiometry c-P10·(T6) 2 ·PdCl 2 .T he signal from the central a-pyridyl protons shifts from 8.74 ppm in c-P10·(T6) 2 to 8.17 ppm in c-P10·(T6) 2 ·PdCl 2 ,w hile other protons are essentially unaffected ( Figure S31), confirming that palladium binds between the two central nitrogens.T he resonances corresponding to the T6 template protons in this palladium complex are sharper than those in c-P10·(T6) 2 , suggesting that the motion has become locked. 1D EXSY NMR experiments on c-P10·(T6) 2 ·PdCl 2 showed no exchange signals between template protons,c onfirming that palladium effectively prevents the caterpillar track motion in this complex ( Figure S46d).