Selective recognition of fluoride and acetate by a newly designed ruthenium framework: experimental and theoretical investigations

Abstract

An effective anion sensor, [Ru(II)(bpy)(2)(H(2)L(-))](+) (1(+)), based on a redox and photoactive {Ru(II)(bpy)(2)} moiety and a new ligand (H(3)L = 5-(1H-benzo[d]imidazol-2-yl)-1H-imidazole-4-carboxylic acid), has been developed for selective recognition of fluoride (F(-)) and acetate (OAc(-)) ions. Crystal structures of the free ligand, H(3)L and [1](ClO(4)) reveal the existence of strong intramolecular and intermolecular hydrogen bonding interactions. The structure of [1](ClO(4)) shows that the benzimidazole N-H of H(2)L(-) is hydrogen bonded with the pendant carboxylate oxygen while the imidazole N-H remains free for possible hydrogen bonding interaction with the anions. The potential anion sensing features of 1(+) have been studied by different experimental and theoretical (DFT) investigations using a wide variety of anions, such as F(-), Cl(-), Br(-), I(-), HSO(4)(-), H(2)PO(4)(-), OAc(-) and SCN(-). Cyclic voltammetry and differential pulse voltammetry established that 1(+) is an excellent electrochemical sensor for the selective recognition of F(-) and OAc(-) anions. 1(+) is also found to be a selective colorimetric sensor for F(-) or OAc(-) anions where the MLCT band of the receptor at 498 nm is red shifted to 538 nm in the presence of one equivalent of F(-) or OAc(-) with a distinct change in colour from reddish-orange to pink. The binding constant between 1(+) and F(-) or OAc(-) has been determined to be logK = 7.61 or 7.88, respectively, based on spectrophotometric titration in CH(3)CN. The quenching of the emission band of 1(+) at 716 nm (λ(ex) = 440 nm, Φ = 0.01 at 298 K in CH(3)CN) in the presence of one equivalent of F(-) or OAc(-), as well as two distinct lifetimes of the quenched and unquenched forms of the receptor 1(+), makes it also a suitable fluorescence-based sensor. All the above experiments, in combination with (1)H NMR, suggest the formation of a 1:1 adduct between the receptor (1(+)) and the anion (F(-) or OAc(-)). The formation of 1:1 adduct {[1(+)·F(-)] or [1(+)·OAc(-)]} has been further evidenced by in situ ESI-MS(+) in CH(3)CN. Though the receptor, 1(+), is comprised of two N-H protons associated with the coordinated H(2)L(-) ligand, only the free imidazole N-H proton participates in the hydrogen bonding interactions with the incoming anions, while the intramolecularly hydrogen bonded benzimidazole N-H proton remains intact as evidenced by the crystal structure of the final product (1). The hydrogen bond mediated anion sensing mechanism, over the direct deprotonation pathway, in 1(+) has been further justified by a DFT study and subsequent NBO analysis.

Electronic structures and selective fluoride sensing features of Os(bpy)2(HL2−) and [{Os(bpy)2}2(μ-HL2−)]2+(H3L: 5-(1H-benzo[d]imidazol-2-yl)-1H-imidazole-4-carboxylic acid)

“…The Os1-N1(imidazole) and Os1-O1-(carboxylate) bond lengths involving HL 2− in 1 of 2.059(2) Å and 2.116(18) Å, respectively, are close to the earlier reported Ru-N(imidazole): 2.049(5) Å/2.054(4) Å and Ru-O(carboxylate): 2.102(4) Å/2.103(3) Å bond lengths for the corresponding [Ru II (bpy) 2 (H 2 L − )]ClO 4 /[Ru II (bpy) 2 (HL 2− )], respectively. 13 The Os1-N1(imidazole) and Os1-O1(carboxylate) bond lengths in 1 also match well with the reported {Os II (bpy) 2 } complexes incorporating a partially deprotonated 4,5-bis(benzimidazol-2-yl) imidazole ligand. 17 The average Os-N(bpy) bond length of 2.038(8) Å in 1 matches reasonably well with the reported distances in the analogous complexes.…”

“…14, 1266.89 and 1330. at 1625 cm −1 , 1621 cm −1 and 1619 cm −1 , respectively, are close to that of the free ligand (H 3 L) value of 1627 cm −1 . 13 The ν(ClO 4 ) frequencies of [2](ClO 4 ) 2 appear at 1100 cm −1 and 600 cm −1 . The expected N-H vibration of the coordinated HL 2− in the complexes could not be clearly identified, due to the presence of a broad water peak (KBr disk) around the region of 3500-3300 cm −1 .…”

“…2d,6 The mixed valent osmium complexes generally exhibit more complex electronic spectra and faster relaxation process, leading to the EPR-silent situation, primarily due to the stronger spin-orbit coupling effects (spin-orbit coupling constant λ/cm −1 : Os III , 3000; Ru III , 1000). 12 The present article originates from our interest in exploring the osmium chemistry of the recently introduced structurally characterised 13 potential non-innocent ligand framework 5-(1H-benzo [d]imidazol-2-yl)-1H-imidazole-4-carboxylic acid (H 3 L) incorporating multiple dissociable N-H protons, and which also has the potential to bridge the metal fragments in an unsymmetrical fashion. 14 Herein, we report the synthesis, characterisation and electronic structural aspects of newly designed mononuclear and dinuclear osmium-bipyridine complexes incorporating partially deprotonated dianionic HL 2− , [Os II (bpy) 2 (HL 2− )] n (1 n ) and [(bpy) 2 Os II (μ-HL 2− )Os II (bpy) 2 ] n (2 n ).…”

Comprehensive Review in Current Developments of Imidazole-Based Medicinal Chemistry

“…Further study revealed that the anion sensing mechanism was mediated by hydrogen bonds. The free N‐H group in benzimidazole could form hydrogen bonds with the pendant carboxylate oxygen while the imidazole N‐H was responsible for possible hydrogen‐bond interaction with the anions . It has been found that transition metals can be used as structural elements to build up anion receptors, enhancing hydrogen‐bond donor tendencies and favoring the assembling of the molecular framework.…”

Synthesis and characterization of ruthenium(II) complexes incorporating 4′-phenyl-terpyridine and triphenylphosphine

“…They are used as light harvesters in dye-sensitized solar cells [2][3][4], luminescent emitters in light-emitting electrochemical cells [5][6][7], potential anticancer and imaging agents in phototherapy [8][9][10], sensors for ions [11][12][13][14][15][16] and small molecules [17,18], photocatalysts for water splitting [19,20], hydrogen production [21][22][23][24], CO 2 reduction [21,23,25], and many other chemical reactions [23,[26][27][28][29], components in mixed valence systems [30][31][32][33][34][35], light upconversion systems [36][37][38][39] and molecular memory devices [40][41][42].…”