Fluoride-ferrocenium

Recognition of fluoride in aqueous media is particularly difficult due to the strongly hydrated nature of the anion. Shinkai and co-workers have demonstrated that ferrocene-boronic acid 27 acts as a selective redox sensor for fluoride which operates in H20 [23]. The favourable interaction between boron and fluoride (a hard acid and hard base, respectively) generates a stability constant of 700 M"1 for the fluoride-ferrocenium complex. Stability constants for both the bromide and chloride complexes are <2 M"1. 

A very unusual mixed-metal receptor has been prepared by Jurkschat and co-workers [68]. Macrocycle 102 comprises two ferrocene reporter units linked together by two Lewis acidic organotin spacers. Electrochemical measurements in dichloromethane solution show anion-induced cathodic shifts of the ferrocene/ferrocenium redox couple of 130 mV for chloride, 210 mV for fluoride and 480 mV for dihydrogenphosphate. 

It has been involved in many industrial explosions. Explodes on contact with aluminum + barium nitrate + potassium nitrate + water. Forms explosive mixtures with aluminum powder + titanium dioxide, ethylene glycol (240°C), cotton lint (245°C), furfural (270°C), lactose, metal powders (e.g., aluminum, iron, magnesium, molybdenum, nickel, tantalum, titanium), sulfur, titanium hydride. Reaction with ethanol + heat forms the explosive ethyl perchlorate. Violent reaction or ignition under the proper conditions with aluminum + aluminum fluoride, barium chromate + mngsten or titanium, boron + magnesium + silicone rubber, ferrocenium diammine-tetrakis(thiocyanato-N) chromate(l —), potassium hexacyanocobaltate(3—), A1 +... 

Molina and co-workers have investigated the urea-crown ether functionalised ferrocene 27 [25]. This receptor-produced anion induced cathodic shifts in the ferrocene/ferrocenium redox couple of 52 mV with fluoride and 190 mV with dihydrogenphosphate. In acetonitrile solution on addition of 2 equivalents of K ions a dramatic attenuation in the anion-induced cathodic shift was observed, with dihydrogenphosphate giving rise to a shift of only 50 mV. 

The receptor 73, based on an azaferrocenophane structure bearing two urea groups as linkers between the redox active (ferrocene) and fluorescent (naphthalene) signalling subimits, also shows both fluorescent and electrochemical sensing of fluoride [51]. On addition of excess fluoride it displays an enhancement factor of 13 in the naphthalene emission bands at 362 and 380 nm in DMF solution and a cathodic shift of the ferrocene/ferrocenium couple of 190 mV in DMSO electrolyte solution. 

A related system 70 containing two boronic esters has been developed by Aldridge et al., who observed that the bis-fluoride adduct undergoes a color change from orange to green as the ferrocene is aerobically oxidized to ferrocenium. The same group has also developed a boronic acid with dimethyl amine 71 to act as a ditopic receptor for HF. ... 

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