Stabilities of cooperatively formed cyclic pseudorotaxane dimers Nori Yamaguchi and Harry W. Gibson* Department of Chemistry, Virginia Polytechnic Institute and State University, Blacksburg, Virginia, USA. E-mail: hwgibson@vt.edu Received (in Cambridge, UK) 8th February 1999, Accepted 24th March 1999 Systematic analysis of the stabilities of cyclic pseudorotax- ane dimers formed between complimentary homoditopic molecules demonstrates that cooperative cyclic dimerization can be minimized by increasing the length of one component via an aliphatic spacer. Recently, we reported preparation of supramolecular linear polymer 5b (Scheme 1) with up to 9.1 repeat units in which monomeric homoditopic molecules 1 and 2b containing dibenzo-24-crown-8 (DB24C8) and dibenzylammonium hexa- fluorophosphate moieties, respectively, are linked non-cova- lently via pseudorotaxane complexes in equimolar concentrated solutions ( > 1.0 M in CD 3 COCD 3 –CDCl 3 , 1 : 1 v/v at 295 K). 1 Unsurprisingly, cyclic dimer 4b (Scheme 1) was preferen- tially formed in equimolar dilute solutions ( < 1.0 3 10 23 M in CD 3 COCD 3 –CDCl 3 , 1 : 1 v/v at 295 K) as observed in other cases. 2–6 In pursuit of more efficient construction of supramo- lecular polymers 5, we speculated that by mismatching the lengths of the aliphatic spacer units in the homoditopic molecules (e.g. 1 and 2),† the equilibrium process may be reversed to favor linear extension (e.g. 3 and then to 5) even in dilute conditions largely due to a greater steric penalty associated with the corresponding cyclic dimer complex (e.g. 4).‡ Here, we investigate the stabilities of cyclic and linear dimer complexes based on complementary homoditopic mole- cules whose spacer segments were varied systematically. The 1 H NMR spectra of dilute equimolar solutions of 1 and 2 [Fig. 1(a)–(c)] revealed four sets of N-CH 2 signals corre- sponding to (i) uncomplexed moieties of the ditopic guest molecule [H(2) u ], (ii) complexed moieties in cyclic dimer [H(4)], and (iii) complexed and (iv) uncomplexed moieties in the linear dimer [H(3) c and H(3) u , respectively] on the basis of slow exchange on the NMR time scale.§ Integration of H(3) c and H(3) u gave a ratio of 1:1 for each solution,¶ indicating that the signals assigned to H(3) c and H(3) u arise from the same species. The signal assignments were properly made based on our previous investigation 1 and the 1 H NMR spectrum [Fig. 1(d)] of a dilute solution of DB24C8 and 2b which exhibited three sets of N-CH 2 signals corresponding to uncomplexed ammonium salt moieties of 2b [H(2b) u ]∑ and complexed and uncomplexed ammonium salt moieties of 6b [H(6b) c and H(6b) u , re- spectively].** The signals for H(6b) c and H(6b) u were integrated to be 1 : 1; thus complex 6b was exclusively formed, confirming that the two signals assigned to 6b (and to 3) arise from the same species. The considerable downfield chemical shift observed for H(6b) u with respect to H(2b) u in Fig.1(d) (Dd = 0.17 ppm) is presumably a consequence of interaction(s) between the pseudorotaxane and free ammonium salt moieties in 6b (and by analogy in 3); e.g. ‘intramolecular’ p-stacking between a benzo ring of complexed DB24C8 and the terminal phenyl ring of the free ammonium salt moiety achieved by folding of the flexible aliphatic spacer, as illustrated in Scheme 2.†† These spectroscopic observations allowed us to conclude that the signals in the region of d 4.35 to 4.45 in Fig. 1(a)–(c) correspond to H(3) u , and that only 3 and 4 exist and cyclic or linear oligomers 5 are not present in detectable amounts in these dilute solutions. Since the concentrations of each species (1, 2, 3 and 4) at equilibrium are readily known, one can estimate the association Scheme 1 Cartoon illustrations of formation of the linear dimer complex 3 and cyclic dimer complex 4 from homoditopic molecules 1 and 2 in substantially dilute conditions. Scheme 2 Cartoon illustrations of formation of the 1 : 1 dimer complex 6b from DB24C8 and homoditopic molecule 2b in substantially dilute conditions. Chem. Commun., 1999, 789–790 789