Intramolecular C–H‥O hydrogen bonding reduces cation complexation strength in a fluorescent crown ether

Abstract

Complexes of the fluorescent crown ether, N,N-bis(9-anthrylmethyl)-4,13-diaza-18-crown-6, with NaI 3 and KSCN reveal the presence of intramolecular C-H…O hydrogen bonds that appear to control conformation and lead to reduced cation binding affinity.For more than a decade, the fluorescent properties of macrocycles have proved fascinating. 1 A hope has been that fluorescent residues appended to macrocycles would afford lariat ethers 2 that could function as cation sensors. 3 Indeed, de Silva and de Silva demonstrated 'fluorescent signaling' in N-(9-anthrylmethyl)aza-15-crown-5 and -18-crown-6 nearly 15 years ago. 4 Anthracene has proved particularly popular as the fluorescent element. It has, for example, been incorporated into crowns and cryptands used as probes of phospholipid membrane fluidity. 5 It has been attached to one crown, to two crowns, and via the 9-and 10-positions of a single molecule to a macrocycle to afford proton sensors. 6 Two anthracenes incorporated into a macrocycle 7 and two anthracenes attached to a crown afforded a crown-cryptand photoswitch. 8 Anthracene has also been incorporated as the fluorescent element for sensing Cu 2+ ions 9 and d-glucosamine. 10 Very recently, Kubo, Ishige and Sakurai have prepared and studied N,NA-bis(9-anthrylmethyl)-4,13-diaza-18-crown-6 1 for use as a fluorescent cation sensor. In the latter case, some cation binding selectivity and cation-induced fluorescence change were observed, but the results were generally modest. This is surprising for such a widely used and strongly fluorescent residue. We now report the first solid state structures of 1 bound by either Na + or K + . Intimate C-H…O hydrogen bonding between the sidearm and macrorings clearly decrease the ability of 1 to form stable cation complexes and, in turn, diminish their utility as sensors.Compound 1 was prepared by heating commercial 9-chloromethylanthracene, 4,13-diaza-18-crown-6, 11 and Na 2 CO 3 in MeCN for 72 h. Crystallization from toluene afforded 1 as yellow needles (68%, mp 195-196 °C). 12 Kubo and co-workers report 1 in 90% yield as yellow needles (from CHCl 3 ) having mp 189-190 °C. The NaI 3 complex of 1 was isolated from a mixture of 1 and NaI (1+1) in THF that was not protected from the air. The rhombohedral crystals obtained (dark yellow to brown) had the stoichiometry 1·NaI 3 ·(THF) 2 and decomposed on melting, presumably with loss of THF, at ca. 215 °C. A projection of the complex is shown in Fig. 1.There are several notable features about this complex. First, the macroring is in the D 3d conformation, as expected. The average Na + -O distance is 2.46 Å and the Na + -N distances are 3.03 and 2.92 Å. These values are typical for diaza-18-crown-6 Na + complexes. The sidearms are in the anti conformation, one being above, and the other below, the mean plane of the macroring heteroatoms. The anti orientation of the anthryl sidearms effectively excludes the anion from the cation's solvation sphere. It is especially interesting that the two (disordered) THF molecules (normally excellent...

Photoinduced Formation of a Cryptand from a Coronand: An Unexpected Switch in Cation Binding Affinity

“…Bond lengths and bond angles are reported in the Supporting Information. They are similar to values reported for related macrocylic compounds 9b…”

Palladium-Catalyzed Amination in the Synthesis of Polyazamacrocycles Containing a 1,3-Disubstituted Benzene Moiety

“…An increase in the catalyst loading from 4 mol% to 8 mol% provided a better yield of the target macrocycle 3i, as well as of other products of benzene diamination (10,11 in the benzene ring on the macrocycle ring size. In the compounds with smaller cycles the signal of this hydrogen atom is shifted relatively downfield (d 2-H 7.31 in 3b and 6.96 in 3h), while an increase in the cycle size leads to a substantial upfield shift (d 2-H 6.48 in 3c, 6.30 in 3d, and 6.09 in 3f). It means that this hydrogen atom is sensitive to the close proximity of the nitrogen or oxygen atoms of the aliphatic part of the molecule in smaller cycles.…”

“…In the majority of cases, aromatic groups are separated from the nitrogen atom by at least one methylene or methine group. [6][7][8][9][10][11] According to literature data, a better response to the complexation is achieved when the nitrogen atom is directly linked to an aromatic substituent, but such compounds are still rare. [12][13][14] Recently, using the method of palladium-catalyzed amination of haloarenes proposed by Buchwald and Hartwig, 15 we have elaborated an efficient approach to the synthesis of polyazamacrocycles using palladium-catalyzed diamination of dihaloarenes with linear polyamines.…”

Synthesis and Photophysical Properties of Polyazacrown Ethers with Appended Naphthyl or Anthracenyl Units

“…The interaction leading to this CT excited state can be favoured by the intramolecular hydrogen bond between the α-hydrogen of anthracene and the proximate oxygen atom of the macrocycle recently observed in similar compounds. [15] This kind of interaction could also be responsible for the slight spectral differences between MAA and 1A. The difference in behaviour between 1A and the azacrown compounds containing two or four anthracene moieties can be assigned to the presence, in the latter compounds, of conformers in which an anthracene-anthracene interaction is present (even in the ground state), which accounts for the presence of the broad, excimer-type emission with maximum around 480 nm.…”