Improved complexation of paraquats with crown ether‐based pyridyl cryptands

Abstract

The commonly used paraquat guest N,N′‐dimethyl‐4,4′‐bipyridinium bis(hexafluorophosphate) (7) was targeted for structural alterations to improve the binding constants with crown ethers and cryptands. The association constant was improved by one order of magnitude (to Ka = 1.00 × 106 mol L−1) with the dibenzo‐30‐crown‐10 pyridyl cryptand host 5 by changing the N‐methyl groups to benzyl and the counterions from PF6 to TFSI, that is, 11. Moreover, through addition of a p‐bromobenzyloxy moiety on the 4‐position of the pyridyl ring of cryptand 18, an association constant of Ka = 5.35 × 106 mol L−1 was achieved with paraquat 11.

“Reverse” pyridyl cryptands as hosts for viologens

“…Surprisingly, however, for all of the complexes, the Δ H values correlate linearly with their Δ S values (Figure ). This has been observed for other pseudorotaxane systems as well [7],[9]…”

“…The fact that this binding constant is 6‐fold higher than that of the BMP32C10‐based analog 2 with PQ(PF 6 ) 2 is partially due to the (i) use of a less polar solvent DCM relative to acetone, (ii) the introduction of the N‐benzyl moieties, and (iii) the TFSI counterions, as demonstrated elsewhere [7]. In particular, the benzyl moieties provide more acidic hydrogen atoms for interaction with the electron pairs of the host, as reflected in the increased Δ H value, as observed previously [7]. However, even with these mitigating factors, reverse cryptand 4 is not as good a host as its isomer 3 , whose K a with PQ(PF 6 ) 2 is 2.0 × 10 5 mol −1 in acetone at 25°C,[3d] a factor of six higher than that of 2 · 8 in DCM.…”

“…Isothermal titration calorimetry (ITC) determined the strength of complexation: Δ H = −9.68 (±0.34) kcal/mol and K a = 6.13 (±0.50) × 10 3 mol −1 in acetone at 25°C [Δ S = −15.2 (±0.6) eu]. This very surprising result is a 270‐fold reduction in binding compared to that of 1 with the same guest under the same conditions [Δ H = −18.8 kcal/mol and K a = 1.12 × 10 6 mol −1 ] (Δ S = −35.4 eu) [7]. The experiment was repeated in triplicate after base washing, to make sure the pyridine ring of 2 was not protonated, only to give the same results.…”

“…ITC analysis of the complexation of dibenzyl viologen bis(trifluoromethylsulfonyl)imide ( 8 )[7] by 4 (Figure ) afforded the binding constant in DCM at 25°C: K a = 3.50 (±0.2) × 10 4 mol −1 with Δ H = −10.4 (±0.4) kcal/mol [Δ S = −14.0 (±1.4) eu]. The fact that this binding constant is 6‐fold higher than that of the BMP32C10‐based analog 2 with PQ(PF 6 ) 2 is partially due to the (i) use of a less polar solvent DCM relative to acetone, (ii) the introduction of the N‐benzyl moieties, and (iii) the TFSI counterions, as demonstrated elsewhere [7]. In particular, the benzyl moieties provide more acidic hydrogen atoms for interaction with the electron pairs of the host, as reflected in the increased Δ H value, as observed previously [7].…”

Supramolecular Pseudorotaxane Polymers from Biscryptands and Bisparaquats

“…For most crown ether based complexes, K a ≤ 10 3 M –1 , leading to maximum DP = 32 at 1 M concentration assuming that no cyclics form; i.e., only oligomers will form. The pyridyl dibenzo-30-crown-10 cryptand ( 1a ) with dimethyl paraquat PF 6 ( 2a ) in acetone has an association constant of 1 × 10 5 M –1 , while the same cryptand with dibenzyl paraquat TFSI ( 2b ) in DCM has an association constant of 1 × 10 6 M –1 . Appropriate monomers with these association constants would afford DP = 316 and DP = 1000 at 1 M concentration, respectively, i.e., truly high molecular weight polymers.…”