Synthesis and structures of crystalline solvates formed of pyridinium N-phenoxide (Reichardt's-type) betaine dyes and alcohols

Abstract

Four new betaine dyes of the Reichardt's type featuring two tolyl substituents in different attachment (2)(3)(4) or being an analogous 1-naphthyl derivative (5) have been synthesized and described with reference to their negative solvatochromism. The dinaphthyl derivative 5 and also a corresponding intermediate nitrophenol 9d were shown with variable-temperature NMR spectroscopy to form conformational atropisomers in solution. Crystal structures of seven alcoholic solvent complexes including the unsubstituted parent compound (1) and different alcohols [1ÁEtOH (1 : 1), 1Ái-PrOH (1 : 1), 2ÁMeOH (1 : 2), 3Ái-PrOH (1 : 1), 3Án-BuOH (1 : 1), 4ÁEtOH (1 : 2) and 5ÁMeOH (1 : 3)] have been studied using X-ray diffraction methods. The betaine dyes exist as genuine zwitterion ground state structures in the solid state with twisted conformation between the phenoxide and pyridinium rings. With the exception of 3a [3Ái-PrOH (1 : 1)], the alcohol guests in the complexes are hydrogen bonded to the oxygen atom of the phenoxide units, whereas in the i-PrOH complex of 3, the alcohol forms trimeric clusters without specific interaction with the betaine framework. From the packing modes in the crystals, the solvated betaines may be classified either as co-ordination type inclusion compounds with pocket-like host arrangements [1ÁEtOH (1 : 1), 1Ái-PrOH (1 : 1), 2ÁMeOH (1 : 2), 3Án-BuOH (1 : 1), 4ÁEtOH (1 : 2) and 5ÁMeOH (1 : 3)], or a true clathrate with channel-type topology [3Ái-PrOH (1 : 1)].

Micro- vs Macrosolvation in Reichardt’s Dyes

“…The twisting between these rings prevents full π-conjugation, rendering 1 as zwitterionic neutral molecules with a large dipole moment. The interplanar angle of 1a was determined to 55° by X-ray diffraction analysis. , However, the charge-transfer process is generally not affected by coplanarity because both the unhindered betaine 1b and the nearly orthogonal ortho isomer of 1a show remarkable solvatochromic properties. , The phenolate unit of 1 is highly basic, capable of interacting as hydrogen bond acceptor with hydrogen bond donor (HBD) solvents and Lewis acids. In contrast, interactions of 1 with Lewis bases are weak due to the delocalization of the positive charge in the pyridinium ring.…”

“…The interplanar angle of 1a was determined to 55°by X-ray diffraction analysis. 14,15 However, the charge-transfer process is generally not affected by coplanarity because both the unhindered betaine 1b and the nearly orthogonal ortho isomer of 1a show remarkable solvatochromic properties. 16,17 The phenolate unit of 1 is highly basic, capable of interacting as hydrogen bond acceptor with hydrogen bond donor (HBD) solvents and Lewis acids.…”

Solvatomorphism of Reichardt's dye

“…7,11 However, reports on the solid-state behaviour of 1 are rare. [12][13][14] The high solubility of 1 in a wide range of solvents makes it ideal for investigating the influence of crystallisation solvent on packing in the solid state. The only crystal structures of 1 that have been reported to date are the solvates of ethanol and iso-propanol, which are isostructural.…”

“…The only crystal structures of 1 that have been reported to date are the solvates of ethanol and iso-propanol, which are isostructural. 12 Crystal structures of the protonated form of Reichardt's dye have also been obtained by crystallisation in the presence of nitric and sulphuric acid. 14 Here, we report a systematic study of solvatomorphism of 1 based on crystallisation experiments in a wide range of different solvent systems.…”

Reichardt’s dye: the NMR story of the solvatochromic betaine dye

“…Surveying the literature reveals many papers that describe the properties of this dye, [3][4][5][6][7][8][9] but only a few describe the 1 H data [10][11][12][13] and 13 C NMR assignments 10 in any detail. Much to our surprise, some of the critical chemical shift values we observed were significantly different from those previously reported.…”