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Reports: DNI451842-DNI4: Hydrazone-Based Rotary Switches as Proton-Relay Systems

Ivan Aprahamian, Dartmouth College

Introduction and Objectives The aim of this project is to gain a fundamental understanding of how to control and exploit the vehicle-type proton transfer in imidazole-containing hydrazone-based rotary switches. It is expected that this knowledge will enable us to develop in the future highly conductive and efficient polymer electrolyte membrane fuel cells that can operate at high temperatures and under anhydrous conditions. We have previously shown how coordination-coupled deprotonation (CCD) can be used in initiating a cascade of proton relays that culminate in the activation of two different switches (1 and 2) using a single input (Scheme 1). This multistep switching cascade is an early example of a dynamically switched compound acting as the input to another, and a first step towards the use of the imidazole-containing hydrazone switches in vehicle-type proton transfers.

Results and Discussion We have recently discovered that replacing the methyl-imidazole group in 1 with imidazole (3) leads to a completely different outcome: The coordination of 3 with zinc(II) does not lead to switching (Scheme 2). We rationalized this result with the existence of a very strong intramolecular H-bond between the imidazole N-H proton and the ester group in the rotor part of the switch. This H-bond “locks” the imidazole ring in place and prevents it from rotating upon coordination.

Nonetheless, coordination leads to deprotonation, which changes the pH of the solution. In order to take advantage of this process, we coupled the CCD with a newly developed pH-responsive fluorophore (morpholinyl-containing BODIPY (MBD)), which resulted in the turn “on” of its fluorescence emission (Fig. 1).

 

In order to expand the scope of our system (i.e., use a single input in generating multiple outputs) we coupled the CCD initiated signaling event with an amplification mechanism (Fig. 2). We did this by using the released acid in catalyzing the hydrolysis of an imine bond, leading to the detachment of a quencher group (p-nitrophenyl) from a fluorophore (anthraldehyde). This process was monitored using 1H NMR spectroscopy and fluorometery. We established that the maximum turnover number under the conditions we used is 10, which means that at best each proton leads to 10 outputs! Next we took advantage of the reversible nature of CCD to regulate the catalysis and toggle it between the “on” and “off” states, and thus demonstrated how CCD can be used in signaling and catalysis driven signal amplification.