Switching through Coordination‐Coupled Proton Transfer

Abstract

The binding of transition metals, such as Zn 2+ , Ni 2+ , and Cd 2+ , with protein residues often leads to proton displacement. [1] This process occurs, for example, in the reaction centers (RCs) of photosynthetic bacteria, [2] and leads to conformational changes and modification of the pK a values of the surrounding amino acid residues. These changes eventually slow down the photosynthetic process. A similar deprotonation event also takes place in cation-diffusion facilitators (CDFs), in which the coordination-coupled deprotonation (CCD) provides the energetic basis for metal efflux. [3] The CCD process has considerable potential for molecular switches, [4] as it opens the way to a new and far-reaching switching mechanism in which acid modulations [5] can be brought about without the addition of protons. [6] This possibility could be of interest, for example, in cases in which molecular switches need to be activated under mild or neutral conditions. The CCD process in RCs and CDFs is associated with metal binding to histidine residues. Therefore, NÀH-containing molecular switches, such as the pH-activated hydrazone-based rotary switches [7] that we have been developing, might be suitable for mimicking this bioinorganic process. As it happens, the coordination of transition metals to hydrazones can lead, in certain cases, to N À H deprotonation. [8] With this possibility in mind, we set out to develop a new switching mechanism that takes advantage of the CCD process.We recently reported the four-step pH-activated switching cycle of a tristable hydrazone-based molecular switch (QPH) with a quinolinyl group as the stator and an ethyl 2pyridylacetate derivative as the rotor. [7b] This molecular switch can be viewed as a tridentate ligand that can accommodate a transition metal by coordination with either the pyridine or carbonyl moiety in the rotor, the quinolinyl group in the stator, and the imine or N À H nitrogen atoms in the axle. We explored the binding of this molecular switch to Zn II , which has been shown to bring about CCD in RCs, CDFs, and the deprotonation of hydrazones, [8] to discern whether the deprotonation process could be useful in activating the molecular rotor.The UV/Vis spectrum of QPH in CH 3 CN shows an absorption maximum at l max = 393 nm (Figure 1). Upon titration with zinc(II) perchlorate (Zn(ClO 4 ) 2 ), the absorption maximum shifted bathochromically with an accompanying hyperchromic change that reached saturation at 8 equivalents of Zn 2+ (l max = 452 nm). This shift in UV/Vis absorption is an indication that Zn 2+ coordinates strongly with QPH. A Job plot (see Figure S4 in the Supporting Information) derived from the UV/Vis spectra shows a vertex around 0.65, which suggests a 2:1 binding stoichiometry between QPH and Zn 2+ . [9] The fact that no isosbestic points were observed during the UV/Vis titrations indicates that more than two chromophoric species are involved in the process, and thus the binding stoichiometry cannot be 1:1. Binding constants were determined as K 1 = 5.7...

Manipulating Liquid‐Crystal Properties Using a pH Activated Hydrazone Switch

“…[8] The use of chemical input in controlling the supramolecular assembly of LCs is very scarce in general. Our interest in developing a complementary approach that relies on controllable molecularly defined transitions [10] (as opposed to bulk effects) [11] in manipulating the long-range order of supramolecular LC assemblies, has led us to the use of our chemically activated hydrazone-based switches [12] for such an application. Our interest in developing a complementary approach that relies on controllable molecularly defined transitions [10] (as opposed to bulk effects) [11] in manipulating the long-range order of supramolecular LC assemblies, has led us to the use of our chemically activated hydrazone-based switches [12] for such an application.…”

“…The original isomer ratio (Figure 1 d) is reached within a few minutes, thereby completing an entire switching cycle. [12] To better understand the structural changes that 1 undergoes upon switching, DFT calculations (B3LYP-D3/6-31G*) [14] were carried out using a dichloromethane solvent model. [12] To better understand the structural changes that 1 undergoes upon switching, DFT calculations (B3LYP-D3/6-31G*) [14] were carried out using a dichloromethane solvent model.…”

Resonance‐Assisted Hydrogen Bonding as a Driving Force in Synthesis and a Synthon in the Design of Materials

“…RAHB-to-radical/ion transformation in aBi-PhOH-PF10-TiO 2 system. [298,299] The 1 HNMR spectrumo fH Li nC D 3 CN consists of two sets of signals at d = 12.8 and 15.8 ppm, indicating its presence as am ixture of the E-a nd Z-hydrazone isomeric forms (Scheme 64). Coordinating ability of aR AHB toward different metal ions.…”

“…[296] Chem. [299] It was concluded that ac oordination-coupled intramolecular protont ransfer is responsible for the E$Z isomerization of HL in complexation with Zn 2 + . 2016, 22,1 6356-16398 www.chemeurj.org 2016 Wiley-VCH Verlag GmbH &Co. KGaA, Weinheim lution of cobalt(II) nitrate and H 3 Lf ailed, whereas when ethylenediamine was used as as tarting material, the target compound was obtained (Scheme 63).…”

“…Eur.J. [299] The crystal structure of this complex shed furtherl ight on the complexation process, the Zn 2 + cation being pentacoordinate, with one HL unit, one triflate anion, and one water molecule (Scheme 64). [297] The nature of the donora nd acceptorc enters in the RAHB system also strongly affects the coordination ability of the corresponding molecule.…”