Characterization of the Fe(III)-Tiron System in Solution through an Integrated Approach Combining NMR Relaxometric, Thermodynamic, Kinetic, and Computational Data

The Fe(III)-Tiron system (Tiron = 4,5-dihydroxy-1,3-benzenedisulfonate) was investigated using a combination of 1H and 17O NMR relaxometric studies at variable field and temperature and theoretical calculations at the DFT and NEVPT2 levels. These studies require a detailed knowledge of the speciation in aqueous solution at different pH values. This was achieved using potentiometric and spectrophotometric titrations, which afforded the thermodynamic equilibrium constants characterizing the Fe(III)-Tiron system. A careful control of the pH of the solution and the metal-to-ligand stoichiometric ratio allowed the relaxometric characterization of [Fe(Tiron)3]9–, [Fe(Tiron)2(H2O)2]5–, and [Fe(Tiron)(H2O)4]− complexes. The 1H nuclear magnetic relaxation dispersion (NMRD) profiles of [Fe(Tiron)3]9– and [Fe(Tiron)2(H2O)2]5– complexes evidence a significant second-sphere contribution to relaxivity. A complementary 17O NMR study provided access to the exchange rates of the coordinated water molecules in [Fe(Tiron)2(H2O)2]5– and [Fe(Tiron)(H2O)4]− complexes. Analyses of the NMRD profiles and NEVPT2 calculations indicate that electronic relaxation is significantly affected by the geometry of the Fe3+ coordination environment. Dissociation kinetic studies indicated that the [Fe(Tiron)3]9– complex is relatively inert due to the slow release of one of the Tiron ligands, while the [Fe(Tiron)2(H2O)2]5– complex is considerably more labile.


■ INTRODUCTION
It has long been known that chelators containing catechol functional groups play an important biological role. For example, the presence of this chemical moiety characterizes the siderophores, compounds involved in the bacterial Fe 3+ sequestration. 1 In fact, the ability of catechol ligands to coordinate stably various metal ions has been used in a number of therapies. In general, catechol chelators show a remarkable affinity toward metal ions in high oxidation states. In particular, disodium 4,5-dihydroxy-1,3-benzenedisulfonate (Tiron) is a water-soluble and nontoxic ligand capable of strongly coordinating different metal ions and therefore of potential interest for use in chelation therapy. 2 In the chemical field, Tiron is known for analytical applications, primarily as a chelating agent used in the determination of trace metals. For example, Tiron is used as a colorimetric reagent of various metal ions, among which are Fe 3+ , Al 3+ , and Ti 4+ , for the sequestration of Pb 2+ , spectrofluorimetric determination of Cu 2+ , and spectrophotometric detection of Th 4+ and Bi 3+ . 3−5 More recently, it has been proposed for uses in electrochemistry concerning the preparation of redox flow batteries or modified glass electrodes. 6,7 However, Tiron is best known for its ability to form very stable Fe 3+ complexes. This characteristic explains its wide-spread use as a complexometric indicator for the spectrophotometric detection of Fe 3+ ions. 8−10 The solution chemistry of the Fe(III)-Tiron system is quite complex and it is strongly affected by pH and ligand-to-metal molar ratios. As shown by UV−vis spectrophotometric data, three distinct coordination compounds can be identified in aqueous solution in different pH zones. 11,12 Each species is characterized by a well-defined stoichiometry, which determines its state of hydration and therefore its reactivity, stability, and color. At pH values below 2, the turquoise-green solution is due to the presence of a complex in which only one unit of Tiron is coordinated to the metal center, which completes its coordination sphere with four water molecules (q = 4): [Fe(Tiron)(H 2 O) 4 ] − . In the pH range of 4−5, the solution turns purple, indicating the coordination of a second Tiron with displacement of two Fe 3+ -bound water molecules and the formation of the bishydrated complex (q = 2): [Fe(Tiron) 2 (H 2 O) 2 ] 5− . Finally, above pH 7, the coordination sphere of the ferric ion is occupied by three bidentate Tiron ligands and therefore a q = 0 complex is present in the bright red solution: [Fe(Tiron) 3 ] 9− (Scheme 1).
In the past, various studies have considered this system, and a number of data have been reported. However, detailed and complete characterization of each of the species present in solution is still missing. For instance, Ozutsumi et al. used EXAFS measurements to show that the three complexes share the same octahedral geometry characterized by Fe−O bond lengths of 200 pm. 13 UV−vis spectrophotometry has been the main source of information on other physical−chemical properties of this system, such as thermodynamic stability, dissociation kinetics, and pH speciation. 11,12,14 These early studies proved that all of the complexes of the Fe-Tiron system are thermodynamically extremely stable. The cumulative stability constants (log β values) of [Fe(Tiron)(H 2 O) 4 ] − , [Fe(Tiron) 2 (H 2 O) 2 ] 5− , and [Fe(Tiron) 3 ] 9− species are 18.7, 33.4, and 44.8, respectively. These three complexes are paramagnetic with high-spin iron and therefore their solutions are particularly suitable for NMR relaxometry studies. Fastfield-cycling relaxometry (FFC-NMR) consists in the investigation of the dependence of the longitudinal nuclear magnetic relaxation rate (1/T 1 = R 1 ) of the solvent protons on the applied magnetic field in a dilute solution of the solute. In the case of paramagnetic complexes, the analysis of these data, the so-called NMRD dispersion profiles, allows to accurately evaluate a series of important molecular parameters related to the structural and dynamic properties of the system. Among the most relevant, it is worth indicating the hydration number q, the distance r MH between the metallic center and the protons of the coordinated water molecule, its average lifetime τ M in the inner coordination sphere, the molecular reorientation rate of the complex 1/τ R , and the electronic relaxation times of the metal ion T 1,2e . 15 Fe 3+ , with five unpaired electrons in the 3d orbitals, a 6 S electronic ground state, and a high magnetic moment (μ eff = 5.9 B.M.) is very well suited to be investigated through this technique.
In this work, we present a detailed 1 H NMRD relaxation study of the Fe-Tiron system. Additional information on the exchange kinetics of the coordinated water molecule(s) is provided by the measurement and analysis of the 17 O reduced transverse relaxation rates (R 2r = 1/T 2r ) and chemical shift (Δω r ) of the bulk water as a function of temperature (280− 350 K) at high field (11.74 T). To select the pH range in which the different complexes have a largely dominant population (>95%), an accurate species distribution diagram is required, which was obtained from potentiometric and spectrophotometric titration data. These data were accurately remeasured under conditions of ferric ions and Tiron concentrations suitable for the relaxometric study. Moreover, DFT computational calculations and spectrophotometric measurements were performed to obtain a complete and accurate description of the structural and dynamic characteristics of the species present in aqueous solutions of the Fe-Tiron system. Finally, to complete the study, we also evaluated their thermodynamic stability and kinetic inertness. We believe that the integrated use of these complementary techniques is able to provide a rather complete and accurate picture of the structure, dynamics, and properties of paramagnetic species in solution not easily accessible otherwise. ulse width of 3.5 μs, and the reproducibility of the data was within ±0.5%. The Fe 3+ concentration was estimated by 1 H NMR (Bruker Advance III Spectrometer equipped with a wide bore 11.7 T magnet) measurements using Evans's method. 16 17 O NMR Measurements. The spectra were acquired on a Bruker Avance III spectrometer (11.7 T) using a 5   pH Measurements and Titrations. A Metrohm 785 DMP Titrino titration workstation and a Metrohm-6.0233.100 combined electrode were used. Equilibrium measurements were carried out at a constant ionic strength (0.15 M NaNO 3 or NaCl) in 6 mL samples at 25°C. Solutions were stirred and continuously purged with N 2 . Titrations were performed in a pH range of 1.7−11.7. KH-phthalate (pH = 4.005) and borax (pH = 9.177) buffers were used to calibrate the pH meter. For calculation of [H + ] from measured pH values, the method proposed by Irving et al. was used. 17 A 0.01 M HNO 3 solution was titrated with the standardized NaOH solution in the presence of 0.15 M NaNO 3 . Differences between the measured (pH read ) and calculated pH (−log[H + ]) values (pA) were used to obtain the equilibrium H + concentration from the pH values, measured in the titration experiments (pA = 0.02). For equilibrium calculations, the stoichiometric water ionic product (pK w ) is also needed to calculate [H + ] values in basic conditions. The V NaOH −pH read data pairs of HNO 3 −NaOH or HCl−NaOH titration obtained in the pH range of 10.5−12 have been used to calculate the pK w value (pK w = 13.77). For calculation of the equilibrium constants, the program PSEQUAD was used. 18 Kinetic The total concentration of the [Fe(Tiron) x ] (4x−3)− complexes was 0.1 mM, while the concentration of CDTA was 20−80 times higher, to guarantee pseudo-first-order conditions. The temperature was maintained at 25°C and the ionic strength of the solutions was kept constant using 0.15 M NaNO 3 . For keeping the pH values constant at pH = 5.0 and 5.5, 0.01 M piperazine buffers were used. At pH > 5.5, the buffer was not used since the CDTA excess was able to maintain a constant pH value due to the protonation equilibria between HCDTA and H 2 CDTA species (log K 2 H = 6.08, 0.15 M NaNO 3 , 25°C 19 ). The pseudo-first-order rate constants (k d ) were calculated by fitting the absorbance data to eq 1 where A t , A 0 , and A p are the absorbance values at time t, the start of the reaction, and at equilibrium, respectively. The calculation of the kinetic parameters was performed by the fitting of the absorbance− time pairs with the Micromath Scientist computer program (version 2.0, Salt Lake City, Utah  24 The integration grid was set with the integral = superfinegrid option. Frequency calculations were subsequently used to corroborate that the optimized geometries corresponded to stationary points on the potential energy surface. Hyperfine coupling constants (A/ℏ = 2π × a iso ) were calculated using the ORCA software package (5.0.3) 25,26 at the uTPSSh/Def2-TZVPP level. 20,21 These calculations incorporated the resolution of identity and chain of spheres approximation (RIJCOSX) 27,28 with the aid of the Def2/J 29 auxiliary basis set. Spin−orbit coupling effects were considered with the spin−orbit mean-field method [SOMF-(1X)]. 30 The integration grids were increased from the defaults using the IntAcc 5.0 and AngularGrid 7 keywords. Zero-field splitting (ZFS) parameters were computed with N-electron valence perturbation theory to second order (SC-NEVPT2) 31,32 on the top of complete active space self-consisted field (CASSCF) 33−35 calculations with the Def2-TZVPP basis set. The active space consisted of five electrons distributed in the five metal-based 3dorbitals [CAS (5,5)], with the state-average CASSCF calculation incorporating one sextet, 24 quartet, and 75 doublet roots. All calculations were performed with the aid of the resolution of identity (RI) approximation for both Coulomb and exchange (RI-JK) using the Def2/JK auxiliary basis set. 29,36 Bulk water solvent effects in all ORCA calculations were introduced with the SMD model developed by Truhlar, 37   The protonation scheme of Tiron was well characterized by both spectroscopic and potentiometric methods. 11,12 These studies reveal that the first and second protonations of Tiron occur at two phenolate O-donor atoms.
Comparison of the protonation constants obtained in 0.15 M NaNO 3 , 0.2 M KCl, or 1.0 M KNO 3 indicates that log K i H values of Tiron are independent of the ionic strength (Table  1). Tiron forms very stable complexes with the Fe 3+ ion. 12 Consequently, the determination of the equilibrium constants characterizing the species formed in the Fe(III)-Tiron system based only on pH-potentiometric studies is impossible. However, the interaction between the Fe 3+ ion and   The pH-potentiometric data of the Fe(III)-Tiron system at 1:2 metal-to-ligand concentration ratio indicate base consuming processes in the pH range of 6.0−9.0. These processes can be accounted for by the formation of [Fe(Tiron) 2  Relaxometric Characterization. 1/T 1 NMR relaxation measurements were performed as a function of pH, at 298 K and 32 MHz, to evaluate relaxivity and identify the pH range in which each species prevails ( Figure 4). The ability to increase the relaxation rate is called relaxivity (r 1 ), and it measures the relaxation rate enhancement of water proton nuclei normalized to a 1 mM concentration of the paramagnetic ion. The sample was prepared following a well-established procedure reported   It is well-established that the relaxivity can be considered as the sum of three contributions describing the different ways in which the modulation of the dipolar coupling between the electron magnetic moment of the metal ion and the nuclear magnetic moment of the water protons can occur. One is associated with the water molecule(s) directly bound to the metal ion (inner sphere; IS), the other arises from water molecules interacting with polar groups of the ligand through long-lived hydrogen bonds (second sphere; SS), and the third due to bulk water molecules diffusing in the proximity of the paramagnetic complex (outer sphere, OS) The most important contribution is that associated with r 1 IS , which is directly proportional to the number of coordinated water molecules q. In fact, the inner-sphere relaxivity is given by the following expression 39 In eq 9, g is the electron g-factor, r M−H the distance between the electron and nuclear spins, μ B the Bohr magneton, γ the proton gyromagnetic ratio, S the total spin (5/2 for a high-spin Fe 3+ complex), ω I the proton resonance frequency, and ω S the Larmor frequency of the Fe 3+ electron spin. The correlation time τ di is given by eq 10, where τ R is the rotational correlation time and T ie are the longitudinal (i = 1) and transverse (i = 2) relaxation times of the electron spin r 1 IS scales with q and therefore making the reasonable assumption of a comparable contribution of r 1 OS in the three complexes, we should expect that  47,48 The SS contribution to the relaxivity of [Fe(Tiron) 3 ] 9− can be assessed by measuring the 1/T 1 1 H NMRD profiles over a wide range of frequency values and analyzing the data using the Freed's equation for the OS mechanism and eqs 8−10, suitable also for the SS mechanism by making some reasonable assumptions. 49,50 The experimental profiles, shown in Figure 5, were measured over the proton Larmor frequency range of 0.01−500 MHz at four different temperatures (283, 288, 298, and 310 K) and at pH = 7.4. Typical values of the Fe 3+ complexes were assigned to some parameters that describe the OS contribution: the diffusion coefficient (D), its activation energy (E D ), and the distance of the closest approach (a) of the proton nuclei of outer-sphere water molecules to the paramagnetic ion (Table 2). 19 For the fit of the data, also some of the parameters of the SS contribution were fixed to reasonable values. The number of SS water molecules was set to five (q SS = 5), while their distance from the metal ion, r SS , was set to 3.5 Å. This assumption was based on DFT calculations performed on the [Fe(Tiron) 3 ] 9− ·5H 2 O system ( Figure S2), which contains three second-sphere water molecules hydrogen-bonded to the three negatively charged sulfonate groups that lie close to the C 3 symmetry axis of the complex. Two additional secondsphere water molecules are involved in hydrogen bonds with The remaining three sulfonate groups are rather far away from the metal center, and thus second-sphere water molecules bonded to these groups are not expected to provide a significant contribution to relaxivity, which depends on 1/r 6 . The second-sphere water molecules display r SS distances in the range of 3.2−5.3 Å, with an average 1/r 6 value that corresponds to r = 3.5 Å. The average life of the water molecules of the second-sphere τ M SS is rather short and therefore such as not to influence relaxivity at any frequency value. Typically, τ M SS is fixed at a value of 1 ns. The structure obtained with DFT is very similar to that determined by X-ray diffraction, which shows two sets of Fe−O bond distances of 1.950 and 2.109 Å with an average value of 2.03 Å. 51 In the calculated structure, the Fe−O bond lengths vary in the range of 2.01−2.09 Å, with an averaged value of 2.05 Å. The latter value is in excellent agreement with that determined in solution with EXAFS measurements (2.04 Å). 13 The best-fit parameters are reported in Table 2. We obtained an excellent fit of the r 1 profiles of [Fe(Tiron) 3 ] 9− on the basis of a τ R SS of 52.7 ps and an associated activation energy, E R SS , of 15.3 kJ mol −1 . The parameters characterizing the relaxation of the electron spin, the mean square transient ZFS energy (Δ 2 ), and its correlation time (τ v ) assume values in the normal range reported for Fe 3+ chelates and fully comparable to those calculated for [Fe(CDTA)(H 2 O)] − . 19 As previously observed, the variation of r 1 with temperature is well reproduced if the zero-field splitting energy Δ is allowed to vary with temperature, according to an Arrhenius behavior with activation energy E Δ ( Table 2). 19 From the calculated SS and OS contributions to the NMRD profile at 298 K ( Figure  5), we can conclude that the r 1 SS component represents a contribution of about 65−70% to total relaxivity.
[Fe(Tiron) 2 19 This simple consideration highlights the lack of a clear relationship between the relaxivity and hydration state, suggesting also in this case the presence of a marked contribution of the SS mechanism. The NMRD profiles are reported in Figure 6 and need to be analyzed taking into account all three contributions to r 1 . DFT calculations suggest the presence of two second-sphere water molecules at a close distance with respect to the paramagnetic center, which led us to assume the presence of two water molecules (q SS = 2) and a r SS value of 3.25 Å ( Figure S3). Furthermore, with regard to the analysis of SS and OS contributions, we used the same procedure described for [Fe(Tiron) 3 ] 9− , setting different parameters at reasonable values.
The presence of two metal-bound water molecules requires consideration of a strong IS contribution to relaxivity (eqs 3−5). Based on the literature data, we fixed the metal−proton  Information on the exchange dynamics of the two bound water molecules was obtained from recording and analyzing reduced 17 O transverse relaxation rates (1/T 2r ) and chemical shifts (Δω r ) data of an aqueous solution of the complex at 11.7 T (Figure 7). The transverse relaxation rates decrease as the temperature decreases, a behavior that indicates an intermediate/slow exchange regime, in which τ M is not negligible compared to the transverse relaxation time of the two coordinated water molecules (τ M ∼ T 2M ). Such a process is dominated by the scalar mechanism, which depends on the square of the hyperfine coupling constant A O /ℏ. The chemical shifts, Δω r , are directly proportional to A O /ℏ and therefore provide a complementary set of data. The 17 O NMR data were analyzed using the Swift−Connick equations. 52 We carried out a global fit of the 1 H NMRD profiles and 17 O NMR data, which is well known to be able to provide accurate and reliable values of the molecular parameters that influence relaxation. An excellent fit of the 1 H NMRD and 17 O NMR data was obtained with the parameters reported in Table 2. The analysis provided a value for τ R (70 ps), which is entirely in line with those found for the corresponding complexes of Mn(II) or Gd(III) of similar molecular mass. The values of the electron relaxation parameters, Δ 2 and τ V , fall within the range of values typical of the previously investigated Fe 3+ complexes. 19 The average residence lifetime of the coordinated water molecules, τ M = 272 ns, is rather long compared to those found for [Fe(EDTA)(H 2 O)] − (0.9 ns) and [Fe(CDTA)-(H 2 O)] − (36 ns). 1 9 It is possible that in [Fe-(Tiron) 2 (H 2 O) 2 ] 5− , where the metal ion is hexacoordinated, there is a lower degree of steric hindrance near the water coordination sites and therefore a stronger interaction with the Fe 3+ ion. Finally, the parameters relating to the SS contribution are fully comparable with those calculated for [Fe(Tiron) 3 ] 9− , the only substantial difference being the number of secondsphere water molecules, i.e., two vs five. In summary, the relaxivity of [Fe(Tiron) 2 (H 2 O) 2 ] 5− is dominated by the IS mechanism that provides a contribution of about 77% to r 1 , while the SS mechanism is only about 8% of the observed relaxivity (r 1 = 6.5 mM −1 s −1 ; 3 T and 298 K). The latter, although rather small, is not a negligible contribution and without taking it into consideration, the best-fit procedure provides unsatisfactory results.
[      6 ] 3+ . 19 The molecular correlation time τ R is quite small, in excellent agreement with the reduced molecular mass of this complex. The results of the best-fit procedure are insensitive to the consideration of the presence of a contribution from SS. The presence of four water molecules in the inner coordination sphere of the metal ion makes the IS contribution largely dominant.
For all three complexes, we used the same value for the distance of closest approach of the OS water molecules. The value used is in line with previous studies on Fe 3+ and other small complexes and it was fixed during the fit. Furthermore, it is worth noting that the fitting results are insensitive to variations of this parameter in the range of 3.4−3.6 Å (see Figures S5−S7).
The values of the bond distances obtained with DFT provide some hints on the trend observed for water exchange. For [Fe(H 2 O) 6 ] 3+ , variable pressure 17 O NMR measurements afforded an activation volume ΔV ‡ = −5.4 cm 3 mol −1 , 53 which points to an associative interchange mechanism. The introduction of Tiron ligands introduces some steric hindrance around the metal ion and makes the charge of the complex increasingly negative, an effect that is reflected in increased Fe−O water and Fe−O Tiron distances (Table 3). Thus, it is likely that the water exchange mechanism takes a more dissociative character as the negative charge of the complex increases. The faster water exchange in [Fe(Tiron) 2 (H 2 O) 2 ] 5− is therefore probably the result of a favorable dissociative pathway facilitated by relatively weak Fe−O water bonds. The absolute values of the hyperfine coupling constants A O /ℏ, calculated by DFT, decrease as the Fe−O water distances increase, as would be expected. The trend predicted by DFT is in good agreement with the results obtained with 17 O NMR experiments, which provides confidence on the results of the fits.
We have shown previously that electron relaxation affects the inner-sphere contribution to relaxivity at the imaging fields. Electron relaxation arises from the modulation of the zero-field splitting (ZFS) energy due to fluctuations of the metal coordination environment caused by vibrations and collisions with solvent molecules. Furthermore, a static ZFS mechanism was also shown to provide a significant contribution to electron relaxation. The ZFS lifts the degeneration of the magnetic sublevels of the S = 5/2 electronic ground state even in the absence of a magnetic field, generating three Kramers doublets, as discussed previously for both Mn 2+ and Fe 3+ complexes. Axial (D) and rhombic (E) ZFS parameters of the Tiron complexes investigated here were estimated using ab initio NEVPT2 calculations in an attempt to gain information of the factors that influence electron relaxation in Fe 3+ complexes.    Table 4. Definitions and equations used for the evaluation of the kinetic data are reported in the Supporting Information As it is shown in Figures S10 and S11, the k d values increase with the increase of [H + ] and [CDTA] t especially at pH < 6.0. The transchelation reaction of Fe(III) complexes takes place by the relatively slow dissociation of [Fe(Tiron) 2 ] 5− and [Fe(Tiron) 3 ] 9− species, which is followed by a fast reaction between the free Fe 3+ ion and the exchanging CDTA ligand. The transchelation reaction can occur via the spontaneous dissociation of [Fe(Tiron) 3 ] 9− (k 0 , eq S3), proton-(k 1 , eq S5), and CDTA-assisted dissociation (k 4 , eqs S8 and S9) of [Fe(Tiron) 2 ] 5− through the formation of protonated *-[FeHL 2 ] (K Fe(HLd 2 ) , eq S4) or ternary *[FeL 2 (CDTA)] intermediates (K FeLd 2 CD , eq S6), respectively. Interestingly, the stability constant of the [Fe(Tiron) 3 ] 9− complex obtained by the kinetic studies (log K FeLd 3 = 9.20, Table 4) agrees well with the log K FeLd 3 value determined by pH-potentiometric and spectrophotometric studies (log K FeLd 3 = 9.83(2), Table 3

■ CONCLUSIONS
The bidentate ligand Tiron finds common use in chemical research and various industrial applications. In analogy with numerous other mono-and bidentate ligands, Tiron can coordinate with Fe 3+ in aqueous solution in a variety of ways, giving rise to metal complexes that differ in stoichiometry, hydration state, and overall electric charge. These species have a range of existence strictly controlled by the pH of the solution and dependent on the ligand-to-metal-ion molar ratio.
The detailed characterization of these species is a difficult challenge and an open problem. This work has shown that the combined use of potentiometric data, 1 H and 17 O NMR relaxometric studies, and theoretical calculations represents a very effective approach to obtain relevant structural and dynamic information on each of the species of the Fe-Tiron system. Equilibrium data obtained by the combination of pH potentiometry and vis spectrophotometry allows the accurate determination of the stability and protonation constants of with CDTA reveal that the dissociation rate of [Fe(Tiron) 3 ] 9− is relatively slow (t 1/2 = 4.1 h, 25°C) due to the slow decoordination of the first Tiron ligand. The multinuclear and multifrequency NMR relaxometric data provide a set of consistent and sufficiently accurate molecular parameters that well-describe the structure, hydration state, molecular tumbling motion, and dynamics of the solvent exchange process of the species present in different pH ranges of the Fe 3+ /Tiron system. This information is very useful and extremely hard to obtain through other experimental procedures. Theoretical calculations are of considerable help in guiding the analysis of the relaxometric data and the correct interpretation of the obtained molecular parameters. In particular, DFT calculations provide information on the number of second-sphere water molecules and their distance to the paramagnetic center, as well as the 17 O hyperfine coupling constants responsible for the scalar contribution to the NMR transverse relaxation rates and chemical shifts. Furthermore, CASSCF/NEVPT2 calculations provide information on electronic relaxation, which is still poorly understood. The results reported here suggest that the symmetry of the metal coordination environment plays a significant role in electron relaxation.
In conclusion, the results presented here represent a step forward toward the development of an effective methodology to understand the behavior in aqueous media of paramagnetic species characterized by a complex pH-dependent speciation.