Absorption and luminescence as a function of pH for carboxylic acid-functionalized ReI tricarbonyls

Six new ester- and carboxylic acid-derivatives of the fac-ReI(CO)3(2,2′-bipyridine)Cl and [fac-ReI(CO)3(2,2′-bipyridine)(pyridine)]CF3SO3 type have been prepared, in which the functions reside on the bipy or py fragments, in their 5- and 3-positions, respectively: the compounds are ReI(CO)3(bipyCOOCH3)Cl (2), ReI(CO)3(bipyCOOH)Cl (3), [ReI(CO)3(bipyCOOH)(pyCOOCH3)]CF3SO3 (4), [ReI(CO)3(bipyCOOCH3)(pyCOOH)]CF3SO3 (5), [ReI(CO)3(bipyCOOCH3)(pyCOOCH3)]CF3SO3 (6) and [ReI(CO)3(bipyCOOH)(pyCOOH)]CF3SO3 (7). These ReI complexes are air- and moisture-stable, and it has been possible to study their properties in aqueous solution (water–dioxane 1:4), as a function of pH. The pKas of the carboxylic acid(s)-containing species have been determined by means of potentiometric titration experiments and the absorbing and emitting properties of the six complexes related to the deprotonation processes. In particular, it has been possible to ascertain that the MLCT band of the rhenium chromophore is related, as expected, to an electron transfer process taking place always towards the bipy fragments, even in the py containing species 4–7, independently on the substituent (ester, carboxylic acid or carboxilate) residing on it. Interestingly, the sharp and intense luminescence versus pH variation found for 3 makes it a suitable candidate for a luminescence-based pH sensor.