Reactivity, photolability, and computational studies of the ruthenium nitrosyl complex with a substituted cyclam fac-[Ru(NO)Cl2(κ3N4,N8,N11(1-carboxypropyl)cyclam)]Cl·H2O

Abstract

Chemical reactivity, photolability, and computational studies of the ruthenium nitrosyl complex with a substituted cyclam, fac-[Ru(NO)Cl₂(κ³N⁴,N⁸,N¹¹(1-carboxypropyl)cyclam)]Cl·H₂O ((1-carboxypropyl)cyclam = 3-(1,4,8,11-tetraazacyclotetradecan-1-yl)propionic acid)), (I) are described. Chloride ligands do not undergo aquation reactions (at 25 °C, pH 3). The rate of nitric oxide (NO) dissociation (k_obs-NO) upon reduction of I is 2.8 s⁻¹ at 25 ± 1 °C (in 0.5 mol L⁻¹HCl), which is close to the highest value found for related complexes. The uncoordinated carboxyl of I has a pK_a of ∼3.3, which is close to that of the carboxyl of the non coordinated (1-carboxypropyl)cyclam (pK_a = 3.4). Two additional pK_a values were found for I at ∼8.0 and ∼11.5. Upon electrochemical reduction or under irradiation with light (λ_irr = 350 or 520 nm; pH 7.4), I releases NO in aqueous solution. The cyclam ring N bound to the carboxypropyl group is not coordinated, resulting in a fac configuration that affects the properties and chemical reactivities of I, especially as NO donor, compared with analogous trans complexes. Among the computational models tested, the B3LYP/ECP28MDF, cc-pVDZ resulted in smaller errors for the geometry of I. The computational data helped clarify the experimental acid-base equilibria and indicated the most favourable site for the second deprotonation, which follows that of the carboxyl group. Furthermore, it showed that by changing the pH it is possible to modulate the electron density of I with deprotonation. The calculated NO bond length and the Ru/NO charge ratio indicated that the predominant canonical structure is [Ru^IIINO], but the Ru–NO bond angles and bond index (b.i.) values were less clear; the angles suggested that [Ru^IINO⁺] could contribute to the electronic structure of I and b.i. values indicated a contribution from [Ru^IVNO⁻]. Considering that some experimental data are consistent with a [Ru^IINO⁺] description, while others are in agreement with [Ru^IIINO], the best description for I would be a linear combination of the three canonical forms, with a higher weight for [Ru^IINO⁺] and [Ru^IIINO].