Acidity and photolability of ruthenium salen nitrosyl and aquo complexes in aqueous solutions

The photochemical behavior of nitrosyl complexes Ru(salen)(NO)(OH2)+ and Ru(salen)(NO)Cl (salen = N,N′-ethylenebis-(salicylideneiminato) dianion) in aqueous solution is described. Irradiation with light in the 350–450 nm range resulted in nitric oxide (NO) release from both. For Ru(salen)(NO)Cl secondary photoreactions also resulted in chloride aquation. Thus, in both cases the final photoproduct is the diaquo cation RuIII(salen)(OH2)2+, for which pKa’s of 5.9 and 9.1 were determined for the coordinated waters. The pKa of the Ru(salen)(NO)(OH2)+ cation was also determined as 4.5 ± 0.1, and the relative acidities of these ruthenium aquo units are discussed in the context of the bonding interactions between Ru(III) and NO.

Graphical abstractPhotolyses of Ru(salen)(NO)(OH2)+ and Ru(salen)(NO)Cl in aqueous solution at different pH’s result in the same photoproduct, RuIII(salen)RuIII(salen)(OH2)2+, with NO release. Photolysis of the chloro complex results also in chloride release via sequential photoreactions. The acidity of the coordinated water of Ru(salen)(NO)(OH2)+ (pKa = 4.5) indicates that the ruthenium should have a Ru(III) character in this complex.Figure optionsDownload full-size imageDownload as PowerPoint slide