Conformational flexibility of xanthene‐based covalently linked dimers

Abstract

The conformational flexibility of three covalently linked dimers consisting of two xanthene‐based moieties connected by a diphenyl ether linker was studied using NMR spectroscopy, X‐ray crystallography, and density functional theory (DFT) calculations. The three dimers interconvert as a function of pH: the doubly cationic dimer (Xan+)2 exists in acidic solutions (pH < 0.5), the mono‐alcohol monocation Xan+–Xan‐OH at intermediate pH values (pH = 1–3), and the neutral diol at the highest pH‐values (pH > 3). Each dimer exhibits conformational degrees of freedom associated with rotations of either the xanthene moiety or of the diphenyl ether (DPE) linker. The barriers for rotation of the xanthylium moiety were evaluated using DFT calculations, yielding values of 23 kcal/mol for (Xan+)2 and 11 kcal/mol for (Xan‐OH)2, respectively. The rotational barrier for the diphenyl ether linker in Xan+–Xan‐OH (15 kcal/mol) was experimentally determined using variable temperature NMR measurements. The relative orientation of the two –OH groups in (Xan‐OH)2 diol was investigated in solution and the solid state using NMR spectroscopy and X‐ray crystallography. The conformer observed in the solid state was found to be the In–Out conformer, while free rotation of the xanthenol units is thought to occur on the NMR timescale at room temperature. These studies are relevant for the design of linkers for efficient water oxidation catalysts. Copyright © 2016 John Wiley & Sons, Ltd.

Conformational analysis of diols: Role of the linker on the relative orientation of hydroxyl groups

“…The instability of the C form of BP(OH) 2 was not investigated further. It might involve an intramolecular Friedel‐Crafts reactivity as was observed previously for the DPE(OH) 2 dication …”

“…As pH is decreased, the PB form of XAN(OH) 2 is formed and can be identified by its absorption bands at 363 and 460 nm. The PB pK a values were determined from photometric titration data and were found to correlate with the electronic factors that affect the stability of the cationic forms: pK a = 1.3 for MONO(OH), pK a1 = 0.2 and pK a2 = 2.6 for DPE(OH) 2 , pK a1 = 1.3 and pK a2 = 3.5 for XAN(OH) 2 (Figure B). It was found that the cationic forms of MONO(OH) and DPE(OH) 2 react readily with water to form PB derivatives, while the equivalent conversion of XAN(OH) 2 required several hours and elevated temperatures.…”

“…Comparison of relative energies of the In‐In , In‐Out , and Out‐Out conformers indicates that the most stable conformation of DPE(OH) 2 is the In‐Out structure, while XAN(OH) 2 and BP(OH) 2 adopt the desirable In‐In structure as their most stable form. The stabilization of the In‐In structure relative to the other conformers is most pronounced for BP(OH) 2 .…”

“…Three types of linkers were investigated, namely, diphenyl ether, xanthene, and biphenyl. The conformational analysis of DPE(OH) 2 has been reported previously, and it was found that the diphenyl linker is flexible, enabling the relatively fast rotation of two alcohol units . In this work, we extend the conformational analysis to additional model dimers, XAN(OH) 2 and BP(OH) 2 , and investigate the relative orientation of two alcohol moieties using X‐ray crystallography, NMR spectroscopy, and density functional theory (DFT) calculations.…”

“…The compound was synthesized according to a previously published procedure . 1 H NMR (500 MHz, DMSO‐ d6 ) δ H 5.66 (d, J = 8.8 Hz, 2 H, H12), 6.02 (s, 2 H, OH), 6.87‐7.06 (m, 14 H, H2, H2′, H11, H10, H5, H4, and H4′), 7.13‐7.18 (m, 4 H, H3′, and H5′), 7.26 (m, 2 H, H3), 7.53 (d, J = 7.3 Hz, 2 H, H9) 13 C NMR (125 MHz, DMSO‐ d6 ) δ C 68.23 (C7), 116.07 (C2′), 116.64 (C2), 118.54 (C6′), 120.93 (C12), 122.90 (C11), 123.08 (C10), 127.52 (C9), 128.02 (C3′), 128.60 (C5′, C5), 128.64 (C4′), 128.77 (C3), 128.80 (C6), 128.87 (C4), 138.40 (C8), 149.52 (C1), 149.74 (C1′), 155.10 (C13).…”