Ruthenium cryptates with an unusual selectivity for nitrate

Abstract

The synthesis of two new tripodal complexes [Ru(L3)](PF(6))(2) and [Ru(L4)](PF(6))(2), encapsulating a ruthenium(II) cation, has been successfully achieved and the products fully characterized, including by X-ray structural determination. The smaller cavity, built around a tris(2-aminoethyl)amido scaffold demonstrated only moderate and predictable interactions with a range of anions and no significant spectroscopic change with nitrate, chloride and bromide, although dihydrogen phosphate did result in an almost stoichiometric precipitation. The expansion of the cavity to include the more rigid 1,3,5-benzenetricarbonylamide group creates a larger cavity, which shows a decrease in the emission on the introduction of chloride, bromide, hydrogen sulfate and nitrate salts, with the (1)H NMR titrations giving a surprisingly high binding affinity for nitrate over the smaller and simpler halides.

A Tripodal Ruthenium(II) Polypyridyl Complex with pH Controlled Emissive Quenching

“…The methylene protons in the complex are diastereotopic and appear as an AB set presenting as doublets at δ = 5.24 and 5.43 ppm with the typical geminal coupling of 14.5 Hz, which is good evidence of the proposed structure in solution. These are significantly downfield of the corresponding protons in the free ligand, and also downfield of the methylene protons in the previously reported[21b] fac ‐alcohol complex [Ru(bipy‐CH 2 OH) 3 ](PF 6 ) 2 ( δ =4.60 ppm), probably due to an increased deshielding effect from the adjacent aromatic ring. There are small but significant differences between these two compounds in the bipyridine proton signals, most notably with the H 3 proton.…”

“…For example, a tris(bipyridine‐imidazolium) ligand reported by Sato, gave a 91 % yield with Fe 2+ , but only 46 % with Ru 2+ , whilst a more recent example Nabeshima and co‐workers show similar differences,[1l] and the systems reported by Oyler et al, and Weizman et al only achieved yields in the region of 25 % despite using high reaction temperatures. In our experience, yields are often observed to be disappointingly low, especially with the more rigid of systems, but high dilution conditions can overcome this to a certain extent, with a yield of 85 % being achieved in one exceptional case . The tether itself can also play a crucial role in the determination of the metal centered stereochemistry.…”

“…The tether itself can also play a crucial role in the determination of the metal centered stereochemistry. In several examples it has been shown to determine the metal centered helicity,, , [21c], and can even be removed to form a true fac orientated inert chelate ,. [21b], [21c],…”

“…Yet the rewards of isolating a species with a long lived fluorescent unit such as [Ru(bipy) 3 ] 2+ adjacent to an enclosed three dimensional cavity offers significant opportunities for the selective recognition of small molecules . It has been shown to be of interest in the selective detection of anions,[4b], , [21a] but this has not been replicated with the successful recognition of cations. Nabeshima et al have reported a cavity with a suitable extended cavity space with a degree of selectivity for divalent cations such as Ca 2+ and Mg 2+ , but the protonation of a central nitrogen resulted in some interesting and unexplained effects.…”

Artificial receptors for nitrate: a comprehensive overview

“…15). 39 The tripodal ligand consists of an anion binding amide functionality and metal chelating bipyridyl groups, and its Ru(II) complex is investigated for anion binding studies. Structural analysis of the [Ru28] 2+ complex shows the presence of a cavity in the amide cleft suitable for an anionic species.…”