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Description
This thesis demonstrates the validity of a new strategy to create highly redox- responsive H-bond dimers based on electron transfer induced proton transfer. The underlying principle is straightforward: a reduction increases the negative charge on a H-acceptor or oxidation increases the positive charge on a H-donor will increase the strength of a H-bond. However, it is possible oxidation or reduction could also lead to full proton transfer. If this occurs across the H-bond, the primary H-bonds will remain, but the secondary H-bonds will change. This can lead to an increase in unfavorable secondary interactions, which would counteract the effect of the initial proton transfer, but, with proper design, proton transfer could lead to an increase in favorable secondary interactions, which would enhance the effect of initial transfer. For this work, a 3 H-bond DAD array (D = H donor; A = H-acceptor) that contains a N-methyl-4,4'-bipyridinium or “monoquat” redox couple, H(MQ+)H, was synthesized. 1HNMR studies show H(MQ+)H forms a three H bond dimer with the non-electroactive ADA array, O(NH)O, in CH2Cl2 with a Kassoc = 507 M−1. This modest Kassoc is typical for DAD-ADA dimers due to the three, favorable primary H-bonds being counterbalanced by four unfavorable secondary H-bonds. Cyclic voltammetry studies of H(MQ+)H in CH2Cl2 show no significant shift in the E1/2 of the first reduction with addition of O(NH)O but a significant positive shift in the second reduction upon addition of only 1 equivalent. The maximum ∆E1/2= 0.311V corresponds to a 1.8×105 increase in binding strength, giving a very large binding constant of 9.1×107 M−1 in the fully reduced state. This value is consistent with that expected for a DDD-AAA complex in which all the primary and secondary H-bonds are favorable. Such a complex would be formed if proton transfer across the central H-bond occurs upon addition of the second electron. This conclusion is supported by CV’s with monoquat itself, indicating that the fully reduced monoquat anion is sufficiently basic to deprotonate O(NH)O. Overall, this work supports the hypothesis that, with proper design, proton transfer can be used to amplify the effect of electron transfer in redox-responsive H-bonding systems.
