Zoltán Garda

Viktória Nagy

Aurora Rodríguez-Rodríguez

Rosa Pujales-Paradela

Véronique Patinec

Goran Angelovski

Éva Tóth

Ferenc K Kálmán

David Esteban-Gómez

Raphaël Tripier

Viktoria Nagy

Raphaël Tri- Pier

Carlos Platas-Iglesias

Gyula Tircsó

Unexpected Trends in the Stability and Dissociation Kinetics of Lanthanide(III) Complexes with Cyclen-Based Ligands across the Lanthanide Series

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INTRODUCTION

Lanthanide complexes based on the macrocyclic 1,4,7,10tetraazacyclododecane (cyclen) containing different types and numbers of pendant arms were intensively investigated in the last decades due to the important medical and bioanalytical applications of their metal complexes. [START_REF] Stasiuk | The Ubiquitous DOTA and Its Derivatives: The Impact of 1,4,7,10-Tetraazacyclododecane-1,4,7,10-Tetraacetic Acid on Biomedical Imaging[END_REF][START_REF] Wahsner | Chemistry of MRI Contrast Agents: Current Challenges and New Frontiers[END_REF] For example, gadolinium complexes of cyclen derivatives are currently used as contrast agents in magnetic resonance imaging (MRI), 3 while analogous complexes with other lanthanide ions are currently being investigated as paramagnetic [START_REF] Stasiuk | The Ubiquitous DOTA and Its Derivatives: The Impact of 1,4,7,10-Tetraazacyclododecane-1,4,7,10-Tetraacetic Acid on Biomedical Imaging[END_REF] H chemical exchange saturation transfer (paraCEST), 4-6 19 F [START_REF] Chalmers | F-Lanthanide Complexes with Increased Sensitivity for 19 F-MRI: Optimization of the MR Acquisition[END_REF][START_REF] Placidi | Aryl-Phosphonate Lanthanide Complexes and Their Fluorinated Derivatives: Investigation of Their Unusual Relaxometric Behavior and Potential Application as Dual Frequency 1 H/ 19 F MRI Probes[END_REF][START_REF] Kadjane | Dual-Frequency Calcium-Responsive MRI Agents[END_REF] and paraSHIFT [START_REF] Harvey | Moving the Goal Posts: Enhancing the Sensitivity of PARASHIFT Proton Magnetic Resonance Imaging and Spectroscopy[END_REF][START_REF] Mason | PARASHIFT Probes: Solution NMR and X-Ray Structural Studies of Macrocyclic Ytterbium and Yttrium Complexes[END_REF] MRI contrast agents. Furthermore, many lanthanide cyclen-based luminescent complexes have been developed for application as optical probes in optical imaging or in cellulo studies. [START_REF] Eliseeva | Lanthanide Luminescence for Functional Materials and Bio-Sciences[END_REF][START_REF] Bünzli | Lanthanide Luminescence for Biomedical Analyses and Imaging[END_REF][START_REF] Delbianco | Highly Water-Soluble Triazacyclononane Europium Complexes to Detect Ligand Binding with Time-Resolved FRET Microscopy[END_REF][START_REF] Starck | Structural Control of Cell Permeability with Highly Emissive Europium(III) Complexes Permits Different Microscopy Applications[END_REF] Whatever the targeted medical or biomedical application of a lanthanide complex, the chelate must present a high stability to avoid the toxic effects associated to the dissociation of the complex in vivo. [START_REF] Grobner | Gadolinium -A Specific Trigger for the Development of Nephrogenic Fibrosing Dermopathy and Nephrogenic Systemic Fibrosis?[END_REF][START_REF] Layne | Gadolinium Deposition and the Potential for Toxicological Sequelae -A Literature Review of Issues Surrounding Gadolinium-Based Contrast Agents[END_REF] Nowadays it is widely recognised that a high kinetic inertness with respect to their dissociation is more important than a high thermodynamic stability of the complex. [START_REF] Baranyai | The Role of Equilibrium and Kinetic Properties in the Dissociation of Gd[DTPA-Bis(Methylamide)] (Omniscan) at near to Physiological Conditions[END_REF][START_REF] Baranyai | Dissociation Kinetics of Open-Chain and Macrocyclic Gadolinium(III)-Aminopolycarboxylate Complexes Related to Magnetic Resonance Imaging: Catalytic Effect of Endogenous Ligands[END_REF] For instance, it has been shown that lanthanide complexes with cyclen derivatives containing four acetamide pendant arms are extremely inert, [START_REF] Pasha | Synthesis and Characterization of DOTA-(Amide)4 Derivatives: Equilibrium and Kinetic Behavior of Their Lanthanide(III) Complexes[END_REF] which allowed their safe in vivo application, [START_REF] Funk | Oxidative Conversion of a Europium(II)-Based T1 Agent into a Europium(III)-Based ParaCEST Agent That Can Be Detected in Vivo by Magnetic Resonance Imaging[END_REF][START_REF] Ratnakar | Modulation of CEST Images in Vivo by T1 Relaxation: A New Approach in the Design of Responsive PARACEST Agents[END_REF][START_REF] Fernando | Breaking the Barrier to Slow Water Exchange Rates for Optimal Magnetic Resonance Detection of ParaCEST Agents[END_REF] in spite of their low thermodynamic stabilities compared with those of the carboxylate analogue DOTA. [START_REF] Pasha | Synthesis and Characterization of DOTA-(Amide)4 Derivatives: Equilibrium and Kinetic Behavior of Their Lanthanide(III) Complexes[END_REF] We recently reported a series of ligands based on the cyclen platform functionalized with both acetate and picolinate pendant arms (Chart 1). The first member of this series to be investigated was H4DO3APA, which forms lanthanide complexes with thermodynamic stabilities and kinetic inertness clearly inferior to those of DOTA. [START_REF] Regueiro-Figueroa | Lanthanide Dota-like Complexes Containing a Picolinate Pendant: Structural Entry for the Design of Ln III -Based Luminescent Probes[END_REF] More recently, we also reported the 1,4-H3DO2APA and 1,7-H3DO2APA ligands, whose lanthanide complexes were shown to possess some dramatically different properties in spite of having identical donor sets. [START_REF] Rodríguez-Rodríguez | Definition of the Labile Capping Bond Effect in Lanthanide Complexes[END_REF] For instance, the exchange rate of the coordinated water molecule in [Gd (1,4-DO2APA)] was found to be three orders of magnitude faster than that of [Gd(1,7-DO2APA)], which was attributed to the weak coordination of the innersphere water molecule in the former due to the capping bond effect. [START_REF] Rodríguez-Rodríguez | Definition of the Labile Capping Bond Effect in Lanthanide Complexes[END_REF][START_REF] Zhang | Understanding Lanthanoid(III) Hydration Structure and Kinetics by Insights from Energies and Wave Functions[END_REF][START_REF] Zhang | Labile Capping Bonds in Lanthanide(III) Complexes: Shorter and Weaker[END_REF] In subsequent works we also showed that Y 3+ and Gd 3+ complexes of regioisomeric pyclen-based ligands showed rather different thermodynamic stabilities and dissociation kinetics. [START_REF] Le Fur | The Role of the Capping Bond Effect on Pyclen Nat Y 3+ / 90 Y 3+ Chelates: Full Control of the Regiospecific N-Functionalization Makes the Difference[END_REF][START_REF] Le Fur | A Coordination Chemistry Approach to Fine-Tune the Physicochemical Parameters of Lanthanide Complexes Relevant to Medical Applications[END_REF] Chart 1. Structures of the ligands discussed in this work.

Given the very different exchange kinetics of the coordinated water molecules observed for the Gd 3+ complexes of 1,4-DO2APA and 1,7-DO2APA, we sought to analyze whether the different arrangement of the ligand donor atoms around the Ln 3+ ions also had an effect on the thermodynamic stability and dissociation kinetics of the complexes. Thus, herein we report protonation constants of the ligands and the stability constants of the metal complexes formed with several Ln 3+ and divalent ions available in vivo (Mg 2+ , Ca 2+ , Zn 2+ and Cu 2+ ). For this purpose, we employed a combination of potentiometric, relaxometric and spectrophotometric measurements. The dissociation kinetics of the Ln 3+ complexes was investigated by following the exchange reactions of the correspond-ing complexes with Cu 2+ at different pH values. A computational study was performed to gain insight at the molecular level into the trends observed across the lanthanide series. We will show for the first time that the rates of dissociation can vary dramatically across the lanthanide series, and that the different arrangement of the donor atoms around the metal ion may result in dissociation rates differing up to six orders of magnitude. Finally, we report paraCEST studies that provided the exchange rate of the coordinated water molecule in [Yb (1,7-DO2APA)].

RESULTS AND DISCUSSION

Ligand protonation constants. The protonation constants of 1,4-DO2APA 3-and 1,7-DO2APA 3-, defined as in eqn (1), were determined using pH-potentiometric titrations in 0.15 M NaCl. The logKi H values are compared with those of DO3APA 4-and DOTA 4-in Table 1. [START_REF] Regueiro-Figueroa | Lanthanide Dota-like Complexes Containing a Picolinate Pendant: Structural Entry for the Design of Ln III -Based Luminescent Probes[END_REF][START_REF] Chaves | The Stability of the Metal Complexes of Cyclic Tetra-Aza Tetra-Acetic Acids[END_REF]

𝐾 𝑖 𝐻 = [H 𝑖 L] [H 𝑖-1 L][H + ] with i = 1, 2,….., 5 (1)
The protonation constants of 1,4-DO2APA 3-and 1,7-DO2APA 3-are very similar, as expected for two ligands having identical nature of their protonation sites. The first and second protonation processes are associated to two nitrogen atoms of the macrocycle in trans position, which allows a larger charge separation of the protonated sites. 31 The remaining three protonation constants are attributed to the stepwise protonation of the three carboxylate groups of the ligand or additional N atoms of the macrocyclic unit. The first protonation constant is two logK units lower than that reported for DOTA [START_REF] Evbuomwan | CEST and PARACEST Agents for Molecular Imaging[END_REF]-, which might be related, at least in part, to the higher negative charge of the latter. The total basicity (logKi H , i = 1-4) of 1,4-DO2APA 3-and 1,7-DO2APA 3-is somewhat lower than that of DOTA 4-, but slightly higher than for DO3APA 3-(Table 1). The values of the first protonation constants logK1 H are significantly reduced on increasing the concentration of the background electrolyte from 0.15 M to 1 M (NaCl), which can be attributed to the formation of relatively stable Na + complexes with these ligands. [START_REF] Regueiro-Figueroa | Lanthanide Dota-like Complexes Containing a Picolinate Pendant: Structural Entry for the Design of Ln III -Based Luminescent Probes[END_REF] Stability constants of the metal complexes. The stability constants of the Mg 2+ , Ca 2+ and Zn 2+ complexes with 1,4-DO2APA 3-and 1,7-DO2APA 3-were determined using direct pH-potentiometric titrations. The stability constants are defined as in eqn (2), while the protonation constants of the complexes are expressed in eqn (3):

𝐾 ML = [ML] [M][L]
(2)

𝐾 MH 𝑖 L = [MH 𝑖 L] [MH 𝑖-1 L][H + ] (3)
The two ligands form rather weak complexes with Mg 2+ and Ca 2+ , the corresponding stability constants being several orders of magnitude lower than those of DOTA 4-and DO3APA 4-(Table 1). This comparison should be taken with caution due to the different ionic strengths used for stability constant determination. However, the Zn 2+ complexes present thermodynamic stabilities comparable to those of the DOTA 4-and DO3APA 4-analogues. It is noteworthy that the Ca 2+ complex of 1,4-DO2APA 3-is almost four orders of magnitude more stable than that of 1,7-DO2APA 3-.

The stability constants of the Cu 2+ complexes could not be determined by direct potentiometric titrations, as the formation of a protonated complex was quantitative at the beginning of the titrations at pH1.7. Thus, the formation of the complexes was followed by monitoring the changes of the d-d absorption bands of the metal/ligand systems as a function of proton concentration. At pH1 the two systems present an absorption band in the visible spectrum at ca. 705 nm ( = 125 M -1 cm -1 ) whose intensity decreases upon increasing proton concentration (Fig. 1, see also Fig. S1, Supporting Information). Under these conditions, the absorption spectra are very similar to that measured for the Cu 2+ :DO3A 3-system, [START_REF] Takács | Solution Structures, Stabilities, Kinetics, and Dynamics of DO3A and DO3A-Sulphonamide Complexes[END_REF] suggesting that the diprotonated form of the complex dominates the solution speciation at this pH possesses a similar structure. In the case of [Cu(H2DOTA)], [START_REF] Riesen | Metal Complexes of Macrocyclic Ligands. Part XXIII. Synthesis, Properties, and Structures of Mononuclear Complexes with 12-and 14-Membered Tetraazamacrocycle-N,N′,N″,N‴-Tetraacetic Acids[END_REF] an X-ray structure evidenced coordination of the four nitrogen atoms of the macrocycle and two oxygen atoms of carboxylate groups, while the two uncoordinated arms are protonated. Table 1. Ligand protonation constants as well as stability and protonation constants of metal complexes of 1,4-DO2APA 3-and 1,7-DO2APA 2.

The simultaneous analysis of the spectroscopic and potentiometric data afforded the stability and protonation constants reported in Table 1. The logKCuL values obtained for the complexes of 1,4-DO2APA 3-and 1,7-DO2APA 3-are slightly higher than those reported for DO3APA and DOTA 4-. [START_REF] Chaves | The Stability of the Metal Complexes of Cyclic Tetra-Aza Tetra-Acetic Acids[END_REF][START_REF] Delgado | Metal Complexes of Cyclic Tetra-Azatetra-Acetic Acids[END_REF] The corresponding pM values of the complexes containing picolinate groups are clearly higher than that calculated for the complex with DOTA 4-. The speciation diagrams (Fig. S2-S8, Supporting Information) evidence the formation of protonated complex species below pH  6. Complex dissociation occurs only at rather acidic pH values (< pH 3).

The stability constants of representative Ln 3+ complexes were obtained by using both pH-potentiometry and the relaxometric technique (Gd 3+ ), 31 or by potentiometric titrations only (La 3+ and Lu 3+ ). The relaxivity of the [Gd(1,4-DO2APA)] complex measured at 20 MHz and 25 ºC (pH = 4.5) amounts to r1p = 4.2 mM -1 s -1 , in good agreement with that reported previously. [START_REF] Rodríguez-Rodríguez | Definition of the Labile Capping Bond Effect in Lanthanide Complexes[END_REF] In the case of [Gd(1,7-DO2APA)] the relaxivity measured in the same conditions (r1p = 2.7 mM -1 s -1 ) is somewhat higher than that measured previously without controlling the ionic strength (r1p = 2.0 mM -1 s -1 ). This is likely due to a faster water exchange of the coordinated water molecule in 0.15 M NaCl, which is expected to increase the inner-sphere contribution to relaxivity. [START_REF] Thompson | On the Role of the Counter-Ion in Defining Water Structure and Dynamics: Order, Structure and Dynamics in Hydrophilic and Hydrophobic Gadolinium Salt Complexes[END_REF] The relaxivities of the two complexes increase below pH 4.5 reaching a value of 13.0 mM -1 s -1 (Fig. 2) at pH 3, which corresponds to the relaxivity of [Gd(H2O)8] 3+ . [START_REF] Powell | Structural and Dynamic Parameters Obtained from 17 O NMR, EPR, and NMRD Studies of Monomeric and Dimeric Gd 3+ Complexes of Interest in Magnetic Resonance Imaging: An Integrated and Theoretically Self-Consistent Approach[END_REF] The analysis of the relaxivity data provided the stability constants of the two Gd 3+ complexes, which turned out to be rather similar (Table 1). The stability constants determined using relaxometry and pH-potentiometry are in good agreement. The stability constants determined for the Gd 3+ complexes resulted to be six-seven orders of magnitude lower than that reported for [Gd(DO3APA)] -, [START_REF] Regueiro-Figueroa | Lanthanide Dota-like Complexes Containing a Picolinate Pendant: Structural Entry for the Design of Ln III -Based Luminescent Probes[END_REF] and seven-eight orders of magnitude lower than the stability constants reported for [Gd(DOTA)] -. [START_REF] Cacheris | Thermodynamic Study of Lanthanide Complexes of 1,4,7-Triazacyclononane-N,Nʹ,Nʺ-Triacetic Acid and 1,4,7[END_REF][START_REF] Kumar | Stability, and Structure of Gadolinium(III) and Yttrium(III) Macrocyclic Poly(Amino Carboxylates)[END_REF] This highlights the dramatic effect that removing one of the carboxylate groups of the ligand causes on the thermodynamic stability of the complexes. The stability constants of the complexes with 1,4-DO2APA 3-increase across the lanthanide series from La 3+ to Lu 3+ , with logK = logKLuL-logKLaL = 3.2. This trend is observed for most lanthanide complexes as a consequence of the increasing charge density of the metal ion due to the lanthanide contraction. [START_REF] Regueiro-Figueroa | Understanding Stability Trends along the Lanthanide Series[END_REF] The stability constants of 1,4-DO2APA 3-complexes increases along the series, as usually observed for Ln 3+ complexes. In contrast, the stability of the complexes with 1,7-DO2APA 3-increases one order of magnitude from La 3+ to Gd 3+ and then decreases from Gd 3+ to Lu 3+ . This stability trend is rather uncommon, though has been observed for the complexes of DTPA 5-. [START_REF] Sarka | Synthesis, Equilibrium and NMR Studies of Lanthanide(III) Complexes of the N-Mono(methylamide) and N'-Mono(methylamide) Derivatives of Diethylenetriamine-N,N,N',N'',N''pentaacetic Acid[END_REF] A fully reversed stability trend with the stability decreasing across the series was also observed for a few systems, and attributed to the weakening of some Ln 3+ -donor bonds due to steric constraints. [START_REF] Roca-Sabio | Macrocyclic Receptor Exhibiting Unprecedented Selectivity for Light Lanthanides[END_REF] As a result, a smaller increase of the overall electrostatic interaction between the ligand and the Ln 3+ ion across the 4f period does not compensate the more negative hydration energies of the lanthanide ions. [START_REF] Regueiro-Figueroa | Understanding Stability Trends along the Lanthanide Series[END_REF] Dissociation kinetics. Both the free metal ion and the ligand released as a result of complex dissociation are potentially toxic (the former being more toxic), and thus metal-complexes for medical applications must have high kinetic inertness. The inertness of metal complexes with respect to dissociation is often assessed by measuring dissociation rates under acidic conditions, sometimes in the presence of Zn 2+ or Cu 2+ metal ions as ligand scavengers. Generally, the dissociation of Gd 3+ complexes with cyclen-based ligands such as DOTA 4-or DO3A 3-occurs slowly following the proton-assisted dissociation mechanism, while the presence of metal ions such as Zn 2+ and Cu 2+ in most of the cases does not affect the dissociation rates. [START_REF] Gündüz | Coordination Properties of GdDO3A-Based Model Compounds of Bioresponsive MRI Contrast Agents[END_REF][START_REF] Baranyai | Comprehensive Evaluation of the Physicochemical Properties of Ln III Complexes of Aminoethyl-DO3A as pH-Responsive T1-MRI Contrast Agents[END_REF] The inertness of the [Ln(1,4-DO2APA)] and [Ln(1,7-DO2APA)] complexes (Ln = La, Gd or Yb) was assessed by studying the rates of the metal exchange reactions occurring with Cu 2+ ions (Eq 4) at 25 ºC by following the formation of the corresponding Cu 2+ complex at 300 nm (Eq (4)).

LnL + Cu 2+  CuL -+ Ln 3+ (4)
Where L denotes either the 1,4-DO2APA 3-or 1,7-DO2APA 3-chelator. In the presence of 10-40 fold excess of the Cu 2+ -ion, the transmetallation reaction of the complexes can be treated as a pseudo-first-order process whose rate can be expressed by eqn (5):

- 𝑑[LnL] 𝑡 𝑑𝑡 = 𝑘 𝑑 [LnL] 𝑡 ( 5
)
where kd is a pseudo-first-order rate constant and [LnL]t is the concentration of the LnL species at time t. The rates of the transmetallation reactions of [Ln(1,4-DO2APA)] complexes were found to be either directly proportional to the H + concentration (Ln=La 3+ and Yb 3+ ) or showed saturation-like behavior (Gd 3+ ). For all systems studied an increase in Cu 2+ concentration was found to slow the dissociation (Figure 3, see also Figure S9, Supporting Information). For the [Ln(1,7-DO2APA)] complexes (Ln=La 3+ and Gd 3+ ) saturation-like behavior was observed with hydrogen ion concentration, while the Yb 3+ system shows a second order dependence on hydrogen ion concentration (Fig. 4). The pseudo-first-order rate constants characterizing the dissociation of the Ln 3+ complexes formed with the 1,7-DO2APA ligand are independent of the Cu 2+ ion concentration and thus kinetic experiments were performed while keeping the Cu 2+ ion concentration constant (20 fold excess was utilized in all kinetic experiments). The total concentration of the complexes can be expressed by considering the species present in solution under the conditions applied in the kinetic studies. Obviously the majority of the complex is present in its LnL form, but lowering the pH may result in the formation of mono-and diprotonated intermediate species (LnHL and LnH2L), while in the presence of a scavenging metal ion (such as Cu 2+ used in the studies), the formation of a dinuclear intermediate (LnLCu) may occur by involving weakly bound (to Ln 3+ ions) or uncoordinated donor atom(s) in Ln 3+ complexes (the latter complex may even protonate to form a Ln(HL)Cu intermediate). These reaction pathways are summarized in Chart 2. (6) Taking this into account and using eqn (5) the following rate equation is obtained:

- 𝑑[Ln(L)] tot 𝑑𝑡 = 𝑘 d [Ln(L)] 𝑡𝑜𝑡 = 𝑘 LnL [Ln(L)] + 𝑘 H [Ln(HL)] + +𝑘 H H [Ln(H 2 L)] + 𝑘 Cu [Ln(L)Cu] + 𝑘 Cu H [Ln(HL)Cu] (7)
The rate constants in eqn (7) characterize the rate of the spontaneous (kLnL), proton-assisted (kH, kH H ), copper-assisted (kCu) and proton-copper-assisted (kCuH) dissociation pathways. Taking into account the different reaction pathways shown in Chart 2 and the equations determining the KH, KH H , KCu and KCu H equilibrium constants, the pseudo-first-order rate constant (kd) can be expressed by eqn (8):

𝑘 d = 𝑘 LnL + 𝑘 1 [H + ] + 𝑘 2 [H + ] 2 + 𝑘 3 [Cu n+ ]+𝑘 4 [Cu 2+ ][H + ] 1 + 𝐾 H [H + ] + 𝐾 H H [H + ] 2 + 𝐾 Cu [Cu 2+ ] + 𝐾 Cu H [Cu 2+ ][H + ] (8)
where

KH=[Ln(HL)]/[Ln(L)][H + ], KH H =[Ln(H2L)]/[Ln(HL)][H + ], KCu=[Ln(L)Cu]/[Ln(L)][Cu], KCu H =[Ln(HL)Cu]/[Ln(L)Cu][H + ], k1=kH•KH, k2= kH H •KH•KH H , k3=kCu
•KCu, and k4= kCu H KCu H . Eqn (8) takes into account all the rational/possible dissociation pathways expected to occur under the applied conditions. As for other systems, this expression can be simplified to fit the rate constants determined as a function of pH (for both systems) and Cu 2+ ion concentration (for the complexes of dissociation of 1,4-DO2APA). For instance, the rate of the spontaneous (kLnL) dissociation of complexes with macrocyclic ligands derived from DOTA 4-is generally extremely low, and therefore its accurate determination is very challenging (for the [Gd(DOTA)] -the values lie in the range of 510 -8 to 6.710 -11 s -1 ). [START_REF] Baranyai | Dissociation Kinetics of Open-Chain and Macrocyclic Gadolinium(III)-Aminopolycarboxylate Complexes Related to Magnetic Resonance Imaging: Catalytic Effect of Endogenous Ligands[END_REF][START_REF] Wang | A Kinetic Investigation of the Lanthanide DOTA Chelates. Stability and Rates of Formation and of Dissociation of a Macrocyclic Gadolinium(III) Polyaza Polycarboxylic MRI Contrast Agent[END_REF] In the present work, we could obtain reliable data for the rate of spontaneous dissociation only in a few cases (Table 2). For all other systems the given rate constant was fixed to 0 during the data refinement. On the other hand, involvement of the metal ion in the dissociation of 1,7-DO2APA complexes could not be confirmed and thus the terms characterizing this pathway (rate constants kCu and kCu H and equilibrium constants KCu and KCu H ) were neglected during the data refinement. For the complexes of 1,4-DO2APA 3-the value of the pseudo-first-order rate constant decreases with increasing Cu 2+ concentration, indicating that the dinuclear intermediate formed in the reaction acts as a "dead-end" complex. In agreement with this phenomenon the data fitting returned the stability constants of the dinuclear intermediates (KCu), but the rate constants of the metalassisted dissociation had to be neglected. Altogether, these observations agree with literature data, where the acid assisted dissociation (characterized by k1 and k2 rate constants) was recognized to be the most important pathway for the dissociation of Ln 3+ complexes formed with DOTA 4-and its derivatives.

The rate constants obtained by fitting the kd first-order dissociation rate constants are presented and compared in Table 2. The results show that the complexes of the two regioisomeric ligands behave very differently as far as their dissociation kinetics concerns. The comparison of the k1 rate constants shows that removal of an acetate pendant arm from the DO3APA 4-ligand results in a considerable drop in the inertness of the complexes formed with Ln 3+ ions. [START_REF] Regueiro-Figueroa | Lanthanide Dota-like Complexes Containing a Picolinate Pendant: Structural Entry for the Design of Ln III -Based Luminescent Probes[END_REF] Indeed, the Gd 3+ complexes of DO2APA 3 exhibit rate constants 3-4 orders of magnitude higher than that of the Eu(DO3APA) chelate. This is not unexpected, since a similar increase in the rate constants characterizing the acid-assisted dissociation process can be observed when the data of Gd(DOTA) and Gd(DO3A) are compared. [START_REF] Takács | Solution Structures, Stabilities, Kinetics, and Dynamics of DO3A and DO3A-Sulphonamide Complexes[END_REF] More striking are the differences observed when the data of the two isomeric complexes are compared. The complexes of the 1,4-regioisomer behave very much like the majority of the complexes formed with DOTA 4-and DOTA derivative ligands: the rate constants of acid-assisted dissociation decrease with decreasing size of the Ln 3+ ions. However, the complexes of the 1,7-regioisomer become considerably less inert as the size of the Ln 3+ ions gets smaller due to the lanthanide contraction. As an interplay between the two opposite trends, the ratios of the k1 values of the complexes with the two ligands are 2, 16 and 2.510 5 for the La 3+ , Gd 3+ and Yb 3+ complexes, respectively. To the best of our knowledge, the complexes of 1,7-DO2APA 3-represent the first example of Ln 3+ complexes with DOTA derivatives that show an increase in the rate constants of acid-assisted dissociation across the lanthanide series. The different trends observed for the complexes of 1,4-DO2APA and 1,7-DO2APA are very likely related to the labile capping bond effect, which was introduced to explain the differences in terms of water exchange rates of Gd 3+ complexes formed with the 1,4-and 1,7-regioisomers investigated here. Complexes with regioisomeric ligands derived from a pyclen platform were also found to present significant differences in terms of kinetic inertness. [START_REF] Le Fur | The Role of the Capping Bond Effect on Pyclen Nat Y 3+ / 90 Y 3+ Chelates: Full Control of the Regiospecific N-Functionalization Makes the Difference[END_REF][START_REF] Le Fur | A Coordination Chemistry Approach to Fine-Tune the Physicochemical Parameters of Lanthanide Complexes Relevant to Medical Applications[END_REF] However, the amplitude of the differences in the dissociation rates are considerably less pronounced for the pyclen derivatives. Furthermore, pyclen derivatives present k1 values four orders of magnitude lower. This can be rationalized in terms of different rigidity of the parent macrocycles (cyclen vs. pyclen). The pyclen macrocycle is considerably more rigid than cyclen, and thus the energy cost required to rearrange the complex structure during the dissociation process is expected to be higher for the more rigid pyclen derivatives. The Z-spectra (25 ºC, saturation time 10 s) recorded from a 15 mM solution of [Yb(1,7-DO2APA)] present a prominent CEST feature at 176 ppm that is already visible at low saturation powers (3% at B1 = 5 T) and reaches 33% at B1 = 30 T (Fig. 5). The CEST peak becomes broader and shifts to 162 ppm at 37 ºC (29% at B1 = 30 T, Fig. S10, Supporting Information). The analysis of the CEST spectra using Bloch equations [START_REF] Zaiss | QUESP and QUEST Revisited -Fast and Accurate Quantitative CEST Experiments[END_REF] provides a kex 298 value of (15.9 + 1.6)  10 3 s -1 . This value is similar to that estimated for the Gd 3+ analogue using 1 H NMRD and [START_REF] Layne | Gadolinium Deposition and the Potential for Toxicological Sequelae -A Literature Review of Issues Surrounding Gadolinium-Based Contrast Agents[END_REF] O NMR measurements (kex 298 = 8.8  10 3 s -1 ). [START_REF] Eliseeva | Lanthanide Luminescence for Functional Materials and Bio-Sciences[END_REF] Water exchange is faster at 37 ºC with a kex 310 value of (51.6 + 0.9)  10 3 s -1 . Thus, CEST experiments confirm the very slow water exchange of the coordinated water molecule in [Ln(1,7-DO2APA)] complexes, as a result of a strong Ln-Owater interaction. Slow water exchange rates were also reported recently for [Ln(AAZTA)(H2O)] -(Ln = Tm, Yb) derivatives in spite of their negative charge. [START_REF] Delli Castelli | An Unconventional ParaCEST MRI Probe[END_REF][START_REF] Baranyai | Combined NMR, DFT and X-Ray Studies Highlight Structural and Hydration Changes of [Ln(AAZTA)] -Complexes across the Series[END_REF] These results represent another evidence that positively charged Ln 3+ complexes (or their derivatives) are not a prerequisite to achieve slow water exchange rates, useful for the design of paraCEST agents.

; f k2, M -2 s -1  2.1(3)10 5 4.810 -4 (Eu 3+ )  KH 7.9(9)10 2 2.0(3)10 4   KCu 6(2)    Yb 3+ k0, s -1 1.0(5) 10 -7    k1, M -1 s -1 5.8(3)10 -3 1.48(7)10 3 2.7910 -3  k2, M -2 s -1  6(1)10 6   KH     KCu 13 (
CEST studies. Chemical exchange saturation transfer experiments allow estimating the exchange rates of protons in slowto-intermediate exchange with bulk water. [START_REF] Ratnakar | Modulation of Water Exchange in Europium(III) DOTA-Tetraamide Complexes via Electronic Substituent Effects[END_REF][START_REF] Viswanathan | Multi-Frequency PARACEST Agents Based on Europium(III) DOTA-Tetraamide Ligands[END_REF][START_REF] Delli Castelli | An Unconventional ParaCEST MRI Probe[END_REF] Among the different Ln 3+ ions, Yb 3+ induces relatively large pseudocontact shifts without extensive line broadening due to paramagnetic relaxation effects. [START_REF] Funk | Critical Analysis of the Limitations of Bleaney's Theory of Magnetic Anisotropy in Paramagnetic Lanthanide Coordination Complexes[END_REF][START_REF] Blackburn | Spectroscopic and Crystal Field Consequences of Fluoride Binding by [YbDTMA] 3+ in Aqueous Solution[END_REF][START_REF] Do | Understanding the Quenching Effects of Aromatic C-H-and C-D-Oscillators in Near-IR Lanthanoid Luminescence[END_REF] In a previous work, we showed that the coordinated water molecule present in [Gd(1,7-DO2APA)] shows a very low water exchange rate. [START_REF] Rodríguez-Rodríguez | Definition of the Labile Capping Bond Effect in Lanthanide Complexes[END_REF] Thus, we performed CEST experiments using the Yb 3+ analogue to assess the water exchange rate of the coordinated water molecule and provide further experimental evidence of the labile capping bond phenomenon. 31G(d,p) level following the methodology described before [START_REF] Regueiro-Figueroa | Toward the Prediction of Water Exchange Rates in Magnetic Resonance Imaging Contrast Agents: A Density Functional Theory Study[END_REF] (see also computational details below). The model systems investigated include two second-sphere water molecules, which were found to be crucial to provide a good description of the environment around the coordinated water molecule. [START_REF] De Blas | Hyperfine Coupling Constants on Inner-Sphere Water Molecules of Gd(III)-Based MRI Contrast Agents[END_REF] These water molecules are involved in hydrogen-bonding interactions with the inner-sphere water molecule and oxygen atoms of carboxylate groups, but they do not coordinate to the metal ion. Our previous relaxometric and luminescence study pointed to the presence of a hydration equilibrium in [Gd(1,4-DO2APA)] involving a nine-coordinated species with an inner-sphere water molecule and an eight-coordinated species lacking the coordinated water molecule. [START_REF] Rodríguez-Rodríguez | Definition of the Labile Capping Bond Effect in Lanthanide Complexes[END_REF] We therefore initiated the computational study by exploring the potential energy surface of the [Ln(1,4-DO2APA)(H2O)]•2H2O systems (Ln = La, Gd or Yb) by increasing the distance between the metal ion and the oxygen atom of the coordinated water molecule. The corresponding relaxed potential energy surface scans are shown in Fig. 6. Our results indicate that the hydrated species is clearly more stable for the La 3+ complex, while the situation is reversed for Yb 3+ . In the case of the Gd 3+ complex the q = 1 and q = 0 species (q is the number of coordinated water molecules) present very similar energies, which is in nice agreement with the hydration number of q = 0.55 estimated from the analysis of 1 H NMRD and 17 O NMR data. •2H2O complexes and the numbering scheme adopted for the donor atoms of the ligands. The [Yb(1,7-DO2APA)(H2O)]•2H2O complex presents a capped square antiprismatic coordination geometry in which an oxygen atom of the picolinate group (O3) occupies the capping position. The steric compression around the capping position results in a rather long Yb-O3 distance (2.385 Å) compared to the distances involving the oxygen atoms of acetate groups (Yb-O1 = 2.278 Å; Yb-O2 = 2.320 Å). This is in line with the capping bond effect introduced recently by our group. [START_REF] Rodríguez-Rodríguez | Definition of the Labile Capping Bond Effect in Lanthanide Complexes[END_REF][START_REF] Le Fur | The Role of the Capping Bond Effect on Pyclen Nat Y 3+ / 90 Y 3+ Chelates: Full Control of the Regiospecific N-Functionalization Makes the Difference[END_REF][START_REF] Le Fur | A Coordination Chemistry Approach to Fine-Tune the Physicochemical Parameters of Lanthanide Complexes Relevant to Medical Applications[END_REF] In [Yb(1,4-DO2APA)(H2O)]•2H2O, the capping position is occupied by the coordinated water molecule, which presents a very long Yb-O1w distance (2.483 Å) compared to that obtained for the 1,7-DO2APA derivative (2.390 Å). As a result, the most stable for of the Yb 3+ complex with 1,4-DO2APA does not contain a coordinated water molecule. Thus, the change in hydration state on going from the La 3+ complex (q=1) to Yb 3+ (q=0) is a direct consequence of the steric compression around the capping water binding site. In [Ln(1,7-DO2APA)(H2O)]•2H2O complexes the coordinated water molecule occupies a coordination position in the upper square plane of the SAP coordination polyhedron, and thus remains tightly bound throughout the lanthanide series from La 3+ to Yb 3+ .

The bond distances of the metal coordination environment in lanthanide complexes generally decrease across the lanthanide series due to the lanthanide contraction. [START_REF] Seitz | The Lanthanide Contraction Revisited[END_REF] As a consequence, an analysis of the strength of the Ln-donor bonds in terms of bond distances is not straightforward. Thus, we calculated the electron densities at the critical points of the Ln-donor bonds (BCP), which have been shown to provide a measure of the strength of the concerned bonds. [START_REF] Zhang | Understanding Lanthanoid(III) Hydration Structure and Kinetics by Insights from Energies and Wave Functions[END_REF][START_REF] Zhang | Labile Capping Bonds in Lanthanide(III) Complexes: Shorter and Weaker[END_REF] The results of this analysis are presented in Fig. 8 (see also Table S1, Supporting Information). The Ln-O bonds involving carboxylate groups present higher BCP values than the Ln-N bonds, as would be expected considering the hard nature of the Ln 3+ ions in Pearson's classification. All Ln-donor bonds are characterised by positive values of the Laplacian of the electron density at the bond critical points ( 2 BCP, Table S1, Supporting Information) and BCP values well below 0.2 au. This indicates that the electron density is locally depleted at the bond critical points, pointing to ionic interactions. [START_REF] Kerridge | Quantification of F-Element Covalency through Analysis of the Electron Density: Insights from Simulation[END_REF][START_REF] Wu | Insight into the Nature of M-C Bonding in the Lanthanide/Actinide-Biscarbene Complexes: A Theoretical Perspective[END_REF] The electron densities of the critical points at the Ln-Owater bonds are higher for the complexes of 1,7-DO2APA 3-, which is in agreement with the slow water exchange rate measured for the Gd 3+ complex. [START_REF] Zhang | Understanding Lanthanoid(III) Hydration Structure and Kinetics by Insights from Energies and Wave Functions[END_REF][START_REF] Zhang | Labile Capping Bonds in Lanthanide(III) Complexes: Shorter and Weaker[END_REF] For the [Ln(1,7-DO2APA)(H2O)]•2H2O complexes, the BCP characterizing the donor atoms delineating the upper plane of the square antiprism (O1, O2 and O1w) increase across the lanthanide series, which indicates that the concerned bonds become stronger as a result of the increased positive charge density of the metal ion. However, the Ln-O3 bond, which involves the oxygen atom of the picolinate group, weakens across the lanthanide series due to an increasing compression around the capping bond. Thus, the weaker interaction of the metal ion with the picolinate oxygen atom appears to be responsible for the decrease in complex stability for the smaller Ln 3+ ions. The same effect is likely responsible for the increasing acid-catalyzed dissociation rates observed across the 4f period. The acid-catalyzed dissociation of the complex proceeds through the formation of a protonated species involving negatively charged carboxylate groups of the ligands. Once protonated, a cascade of proton transfer events causes the protonation of the more basic amine nitrogen atoms, provoking complex dissociation. The relatively weak coordination of the picolinate group likely facilitates its decoordination upon protonation, so that the proton can be more easily transferred to the amine nitrogen atoms.

The trends obtained for the BCP values in the complexes of 1,4-DO2APA 3-indicate that all Ln-O bonds involving the carboxylate groups of the ligand become stronger across the lanthanide series, explaining the increasing stability of the complexes as well as the increasing kinetic inertness with respect to complex dissociation. The astonishing increase of the kinetic inertness on going from Gd 3+ to Yb 3+ is the result of the more compact structure of the complex associated to the reduced coordination number in the q = 0 Yb 3+ complex. The coordinated water molecule in the complexes with 1,7-DO2APA becomes more tightly bound across the series, while the reverse situation is observed for the analogues of 1,4-DO2APA.

CONCLUSIONS

We have presented a detailed analysis of the thermodynamic stability and dissociation kinetics of lanthanide complexes with two cyclen-based regioisomeric ligands. The two series of complexes present different stability trends across the lanthanide series: The complexes of 1,4-DO2APA show the typical increase in stability across the series, while the stability of the 1,7-DO2APA analogues decreases on going from Gd 3+ to Lu 3+ . The rates of dissociation of 1,4-DO2APA complexes decrease slightly from La 3+ to Gd 3+ and then drop dramatically for the Yb 3+ complex, an effect that is related to a more tight coordination of the ligand, associated to the higher positive charge density of the metal ion and a reduced coordination number.

The rates of dissociation of the complexes with 1,7-DO2APA show an unprecedented increase on moving to the right along the lanthanide series. DFT studies show that this is likely related to the coordination of the picolinate oxygen atom at the sterically demanding apical position. The coordination of the picolinate group becomes weaker on decreasing the ionic radius of the Ln 3+ ion, facilitating the decoordination of this group upon protonation. The coordination of the picolinate oxygen atom at the apical position leaves the water molecule tightly coordinated at the upper square plane of the square antiprismatic coordination polyhedron. As a result, the coordinated water molecule exhibits a low exchange rate with bulk water, as demonstrated by CEST experiments for the Yb 3+ complex. While the low kinetic inertness of this complex precludes any potential application as MRI contrast agent, this result shows that the slow water exchange rates required for CEST agents may be attained placing water molecules at noncapping positions.

In summary, the arrangement of the donor atoms of the ligand around the Ln 3+ ion plays a crucial role in the thermodynamic and kinetic properties of the complexes. We have shown that dissociation rates may vary up to three orders of magnitude across the lanthanide series. This unprecedented observation shows that the kinetic inertness of a complex may not be assured when one lanthanide ion is changed by another. Furthermore, the rates of dissociation of lanthanide complexes may vary by up to six orders of magnitude by changing the arrangement of the donor atoms around the metal ion. These issues must be carefully considered in ligand design in addition to the number and type of donor atoms. The concentration of the ligands was determined by pHpotentiometric titration in the presence and absence of a large excess of CaCl2. To obtain the protonation constants of 1,4-DO2APA 3-and 1,7-DO2APA 3-acidified (HCl) ligand solutions (2 mM) were titrated with 0.15 M NaOH in the presence of 0.15 M NaCl as ionic strength. The stability and protonation constants of the Mg 2+ , Ca 2+ , Cu 2+ (by using 1.0 M ionic background for this particular system) and Zn 2+ complexes were determined by pH potentiometric titration where the metal to ligand ratios were 1:1 using 1.7 mM ligand concentration (100-200 V(mL) NaOH -pH data pairs were recorded). The pH-potentiometric titrations were carried out with a Metrohm 888 Titrando workstation with the use of a Metrohm 6.0234.100 combined electrode in the pH range of 1.70-11.80. For the calibration of the pH meter, KH-phtalate (pH = 4.005) and borax (pH = 9.177) buffers were used. The titrated samples (6 mL) were thermostated at 25 ºC and mechanically stirred under inert atmosphere (N2). The method proposed by Irving et al. [START_REF] Irving | A Study of Some Problems in Determining the Stoicheiometric Proton Dissociation Constants of Complexes by Potentiometric Titrations Using a Glass Electrode[END_REF] was used to calculate the H + concentrations from the measured pH values. The Irving factor and the ion product of water (pKw=13.75) was determined in the same HCl -NaOH titration experiment. In the case of the Cu 2+ complexes pH-potentiometry was supported by spectrophotometric measurements performed on batch samples, since the formation of protonated Cu 2+ complexes was found to be quantitative even near pH=1.5 (thus only the deprotonation events of the complexes could be characterized in the pH range of 1.70-11.80). The absorbance spectra were recorded with a Cary 100 Bio spectrophotometer at 25 ºC, using 1.0 cm cells. For determining the stability constants of the Cu 2+ complexes 8 separated samples were prepared with different acid concentrations (in the acid concentration range of 0.01 -0.70 M) and the absorbance data measured at 7 wavelengths in the 650 -800 nm range were used for the calculations. In these experiments, the ionic strength of the samples was set to 1.0 M (cHCl+cNaCl=1.0 M), and thus the protonation constants of the ligands were also determined under the same conditions (Table 1). Beside the pH-potentiometric data, the measured absorbance and molar absorptivity data determined independently were taken into account to determine the equilibrium constants.

Due to the relatively slow formation of the Ln 3+ complexes, the so called "out-of-cell" technique was utilized to determine their stability constants. Samples with total volume of 2.50 mL containing the Ln 3+ ion (2 mM) and the ligand (2 mM) were prepared in the pH range of about 3.0 -6.5. The ionic strength in the samples was set to 0.15 M by using NaCl. The closed samples were kept at 25 ºC for 4 weeks to reach the equilibrium and then their pH and relaxivity (for the Gd 3+ complexes) were measured. These data allowed determining the stability constants by using the pH-potentiometric (by knowing the acid concentration of the samples and their pH at equilibrium) and 1 H relaxometric method (by using the pH and the relaxivities of the samples). These two sets of data were fitted simultaneously.

The equilibrium constants have been calculated from the ti-tration data with the program PSEQUAD. [START_REF] Zekany | Computational Methods for the Determination of Formation Constants SE -8[END_REF] Kinetic studies. The rates of the dissociation of the Ln 3+ complexes in metal exchange reactions taking place with the Cu 2+ ion were studied by spectrophotometry at 300 nm in thermostated cells (1.0 cm, 25 °C) with Cary 100 Bio (1,4-DO2APA 3-) and Jasco V770 (1,7-DO2APA 3-) spectrophotometers in the presence of 0.15 M NaCl. The concentration of the [Ln (1,4-DO2APA)] and [Ln(1,7-DO2APA)] complexes was 0.3 mM and 0.2 mM, respectively. The Cu 2+ ion was used in 10-to 40-fold excess for the [Ln(1,4-DO2APA)] complexes, while a 10-fold excess was applied in the case of [Ln(1,7-DO2APA)] complexes. A non-coordinating buffer at 0.05 M concentration was used to keep the pH constant (DMP: 1,4-dimethylpiperazine, pKa=4.17). The exchange reactions were studied in the pH range 3.4-5.0. For the calculation of the pseudo-first-order rate constants (kd), the absorbance values measured at different t times were fitted to the following equation:

𝐴 𝑡 = (𝐴 0 -𝐴 𝑒 )𝑒 -𝑘 𝑑 𝑡 + 𝐴 𝑒 (7) where A0, At and Ae are the absorbance values measured at the start, at time t and at equilibrium, respectively. Relaxometric measurements. Measurements of longitudinal relaxation times (T1) were performed by using a Bruker Minispec MQ-20 NMR Analyzer operating at 0.49 T (corresponding to 20 MHz proton Larmor frequency) at 25.0+0.2 °C (set by a circulating water bath). The T1 values were determined with the inversion recovery method (180° -τ -90°) by averaging 4-6 data points obtained at 10 different τ delay values. NMR studies. CEST spectra have been recorded at 25 and 37 ⁰C, at diverse radiofrequency fields (B1 = 2.5, 5, 10, 15, 20, 25 and 30 µT) on a Bruker Advance 300 MHz spectrometer. Z-spectra of 15 mM complex aqueous solutions were performed using a saturation time of 10 s and a 2 ppm frequency resolution. The pH of the solution was adjusted to pH 7.0 by adding 0.01 to 0.1 M NaOH or HCl solutions. The exact concentration of the solutions was determined by elemental analysis of the complexes.

DFT calculations. Full geometry optimizations of the [Ln(1,4-DO2APA)(H2O)]•2H2O, [Ln(1,7-DO2APA)(H2O)]•2H2O (Ln = La, Gd or Yb) and [Yb(1,4-DO2APA)] systems were performed employing DFT calculations at the TPSSh/LCRECP/6-31G(d,p) [START_REF] Tao | Climbing the Density Functional Ladder: Nonempirical Meta-Generalized Gradient Approximation Designed for Molecules and Solids[END_REF] level with the Gaussian 09 package (Revision E.01). 63 LCRECP refers to the large-core quasi-relativistic pseudopotential of Dolg el al. and its associated (7s6p5d)/[5s4p3d]-GTO valence basis set. [START_REF] Dolg | Energy-Adjusted Pseudopotentials for the Rare Earth Elements[END_REF] Solvent effects were included by using the polarizable continuum model (PCM), in particular the integral equation formalism (IEFPCM) variant. [START_REF] Tomasi | Quantum Mechanical Continuum Solvation Models[END_REF] No symmetry constraints have been imposed during the optimizations. An ultrafine integration grid and the default SCF energy convergence threshold (10 -8 a. u.) were used. The stationary points found on the potential energy surfaces as a result of geometry optimizations were characterized via frequency analysis. The electron density () and its Laplacian ( 2 ) at the bond critical points (BCPs) were computed with the Multiwnf program (version 3.2). [START_REF] Lu | A Multifunctional Wavefunction Analyzer[END_REF]

c

  Data in 0.1 M KCl from references 24 and 34. d Data in 0.1 M Me4NNO3 from references 30 and 35. e Data determined using a 1 M (cHCl+cNaCl) ionic strength. f Data in 0.1 M NaCl from reference 38. g Data from reference 39. h pM are defined as -log[M]free for [M]tot = 1M and [L]tot = 10 M.

Figure 1 .

 1 Figure 1. VIS spectra of the solutions containing nearly equimolar amounts of Cu 2+ and 1,4-DO2APA (cCu2+=2.99 mM, cLig=3.03 mM) as a function of acid concentration (see inset). The dashed spectrum corresponds to aqueous Cu 2+ in 1.0 M NaCl.

Figure 2 .

 2 Figure 2. Relaxivity pH profiles of the equilibrated [Gd(1,4-DO2APA)] (squares) and [Gd(1,7-DO2APA)] (circles) complexes ([Gd(L)] = 1.5 × 10 -3 M normalized to 1.0 × 10 -3 M, T = 25 °C, I = 0.15 M NaCl, B = 0.47 T). The solid lines correspond to the fits of the data to obtain the stability constants given in Table2.

Figure 3 .

 3 Figure 3. Pseudo-first-order rate constants of dissociation of [Ln(1,4-DO2APA)] complexes (Ln 3+ = La 3+ (triangles, shades of green), Gd 3+ (squares, shades of green) or Yb 3+ (circles, shades of blue)) as a function of H + ion concentration measured by using a large excess of exchanging Cu 2+ ion (for Gd cCu2+ = 3.33, 5.95, 9.28, and 11.9 mM corresponding to 11.5, 20.5, 30 and 41 fold metal excess, respectively; in other Ln 3+ systems 11 (3.37 mM, closed symbols) and 31 (10.1 mM, open symbols) fold Cu 2+ excess was applied).

Figure 4 .

 4 Figure 4. Pseudo-first-order rate constants of dissociation of [Ln(1,7-DO2APA)] complexes (Ln 3+ = La 3+ (green triangles), Gd 3+ (green squares) or Yb 3+ (blue circles)) as a function of H + ion concentration measured by using large excess of exchanging Cu 2+ ion (cCu2+ = 2.02 mM;10.1 fold metal excess). The lines represent the fits of the data as explained in the text.

Chart 2 .

 2 Dissociation mechanisms of LnL complexes. The total concentration of the LnL complexes in kinetic experiments ([LnL]tot) can be expressed as the sum of the concentrations of all the different Ln-containing species present in solution: [Ln(L)] tot = [LnL] + [Ln(HL)] + [Ln(H 2 L)] + [Ln(L)Cu] + +[Ln(HL)Cu]

25

 25

Table 2 .

 2 Dissociation rate and equilibrium constants characterizing the dissociation of the [Ln(1,4 DO2APA)] and [Ln(1,7 DO2APA)] complexes (I =0.15 M NaCl, 25 ºC, Ln = La, Gd or Yb) and related systems provided for comparison.

  This work. Values within parenthesis represent the statistical errors of the fits in the last significant digits. b Data from Ref 24. c Data from Ref 46. d Data from Ref 32. e Data from Ref 45. f Data from Ref 43.

Figure 5 .

 5 Figure 5. CEST spectra of [Yb(1,7-DO2APA] in H2O (15 mM, pH 7, saturation time 10 s) recorded using different saturation powers at 25 (top) and 37 ºC (bottom).

  Computational study. The unusual variation of the stability constants and dissociation rate constants across the lanthanide series prompted us to perform a computational study to gain insight into the structural origin of these effects at the molecular level. The structures of the [Ln(1,4-DO2APA)(H2O)]•2H2O and [Ln(1,7-DO2APA)(H2O)]•2H2O systems (Ln = La, Gd or Yb) were optimized at the TPSSh/LCRECP/6-

25

 25

Figure 6 .

 6 Figure 6. Relaxed potential energy surface scans computed for the [Ln(1,4-DO2APA)(H2O)]•2H2O systems (Ln = La, Gd or Yb) by increasing the distance between the lanthanide ion and the oxygen atom of the coordinated water molecule in steps of 0.1 Å.

  Figure 7. Optimized geometries of the [Yb(1,4-DO2APA)(H2O)]•2H2O (a) and [Yb(1,7-DO2APA)(H2O)]•2H2O (b) systems obtained with DFT calculations.

Figure 8 .

 8 Figure 8. Electron densities at the bond critical points (BCP) involving Ln-O bonds in [Ln(1,4-DO2APA)(H2O)]•2H2O (Ln = La, Gd), [Yb(1,4-DO2APA)] and [Ln(1,7-DO2APA)(H2O)]•2H2O (Ln = La, Gd, Yb) complexes.

  The metal salts and other materials used in equilibrium experiments were of the highest analytical grade. Standardized Na2H2EDTA was used to determine the concentration of the MgCl2, CaCl2, CuCl2, ZnCl2 and LnCl3 solutions by complexometric titration in the presence of eriochrome black T (MgCl2), Patton & Reeder (CaCl2), murexid (CuCl2,) and xylenol orange (ZnCl2 and LnCl3) as the indicators.

  3-determined by pH-potentiometry (0.15 M NaCl, 25 ºC) and literature data for related systems. Determined by simultaneous fitting of pH-potentiometric and UV-vis spectrophotometric data. b Determined by 1 H relaxometry.

	Metals / Ligand		1,4-DO2APA 3-	1,7-DO2APA 3-	DO3APA 4-, c	DOTA 4-, d
		logK1 H	10.21(1); 9.44(5) e	10.15(2); 9.25(6) e 9.21	12.09
		logK2 H	9.46(1); 9.55(2) e	9.26(2); 9.32(2) e	8.94	9.76
		logK3 H	4.17(2); 4.19(5) e	4.22(2); 4.29(5) e	4.82	4.56
		logK4 H	3.65(3); 3.82(4) e	3.25(3); 3.49(4) e	3.52	4.09
		logK5 H	1.95(3); 1.95(3) e	2.38(3); 2.41(3) e	1.39	-
		logKi H (i = 1-4) 27.49; 27.00 e	26.88; 26.35 e	26.49	30.50
	Mg	logKMgL	6.59(1)	7.25(8)	10.44	11.91
		logKMgHL	-	-	6.89	3.92
		logKMgH2L	-	-	6.37	-
		pM h	6.0	6.0	8.2	6.2
	Ca	logKCaL	12.72(1)	8.96(7)	14.82	17.23
		logKCaHL	4.28(5)	-	4.59	3.54
		logKCaH2L	-	-	4.32	4.19
		pM h	8.8	6.1	12.4	11.1
	Cu	logKCuL	23.84(3) a, e	23.52 a, e	23.20	22.25
		logKCuHL	4.10(2)	3.93(3)	4.17	3.78
		logKCuH2L	2.97(1)	2.80(3)	3.31	3.77
		logKCuH3L	-	-	1.97	-
		pM h	19.9	19.9	20.8	16.2
	Zn	logKZnL	19.36(4)	20.23(4)	20.25	21.09
		logKZnHL	3.92(3)	3.55(3)	4.42	4.18
		logKZnH2L	3.28(1)	2.84(1)	3.06	3.52
		logKZnH3L	-	-	1.98	-
		pM h	15.4	16.6	17.8	15.0
	La	logKLaL	15.96(1)	15.26(1)	21.17	21.7 f
		logKLaHL			2.55
		pM h	12.0	11.6	18.8	15.6
	Gd	logKGdL	16.98(2), 17.12(2) b 16.33(2) a ,	23.31	24.7 f , 25.3 g
				16.69(2) b
		logKGdHL	-	-	2.65	-
		pM h	13.1	12.7	20.9	18.6
	Lu	logKLuL	19.6(2), 19.21(2) b	15.24(2) a ,	22.82	25.4 f
				14.55(2) b
		logKLuHL	-	-	2.48	-
		pM h	15.7	11.6	20.4	19.3
	a

ACKNOWLEDGMENT

The research was funded by the GINOP-2.3.2-15-2016-00008 project supported by the EU and co-supported by the European Regional Development Fund, by the Hungarian National Research, Development and Innovation Office (NKFIH K-120224 and 128201 projects), the bilateral Hungarian-Spanish science and technology cooperation program (2019-2.1.11-TÉT-2019-00084) and the COST Action CA15209 European Network on NMR Relaxometry. Authors C. P.-I., D. E.-G. and A. R.-R. also thank Ministerio de Economía y Competitivad (CTQ2016-76756-P) for generous financial support and Centro de Supercomputación de Galicia (CESGA) for providing the computer facilities.

ASSOCIATED CONTENT Supporting Information

The Supporting Information is available free of charge on the ACS Publications website at DOI: XXXXX. Additional kinetic data, speciation diagrams, CEST spectra and DFT data (PDF).

AUTHOR INFORMATION Corresponding Authors

*E-mail: gyula.tircso@science.unideb.hu (G. T.) *E-mail: aurora.rodriguez@udc.es (A. R.-R.)

Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Notes

The authors declare no competing financial interest.

TOC Graphic:

The arrangement of the ligand donor atoms around the lanthanide ion provokes dramatic differences in the thermodynamic stabilities and dissociation kinetics of lanthanide complexes, as demonstrated by investigating the complexes with two isomeric cyclen-based ligands, containing a picolinate arm and two acetate arms either in positions 1,7 or 1,4 of the macrocyclic structure.


Antoine Roquilly

Gerald Chanques

Sigismond Lasocki

Arnaud Foucrier

Brice Fermier

Hugues De Courson

Cedric Carrie

Marc Danguy

Des Deserts

Clément Gakuba

Jean-Michel Constantin

Implementation of French recommendations for the prevention and the treatment of hospital-acquired pneumonia: a cluster-randomized trial

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Bram Rochwerg

Sharon Einav

Dipayan Chaudhuri

Jordi Mancebo

Tommaso Mauri

Yigal Helviz

Ewan Goligher

Samir Jaber

The role for high flow nasal cannula as a respiratory support strategy in adults: a clinical practice guideline

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Silvana A D'ippolito

Adriana D Ballarini

Santiago M Rosas

Laurence Pirault-Roy

Catherine Especel

Florence Epron

Carlos L Pieck
email: pieck@fiq.unl.edu.ar

Laurence Pirault

Influence of the Ir content and the support on the thiotolerance of the Ir/SiO2-Al2O3 catalysts used in selective ring opening of decalin

Keywords: selective ring opening, decalin, Ir/SiO2-Al2O3, thiotolerance

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Introduction

Selective ring opening (SRO) is a particular case of hydrocracking of cyclic hydrocarbons where the endocyclic C-C bonds should be broken without modifying the molecular weight of the molecules. SRO can be used to enhance the light cycle oil (LCO) fraction and thus contribute to meet the growing demand for diesel fuel. LCO products consists mainly of aromatic compounds of two rings, which can be hydrogenated and then opened by SRO to obtain linear or mono-branched paraffins leading to an improved cetane index for Diesel fuel application. [1,2] Moreover, the sulphur content of the LCO fractions is about 0.2 to 2 wt% and the industrial fractions must be hydrotreated in a first stage to decrease the S content between 6 to 16 ppm. [3][4][5] Ring opening reactions involving C6 cycles are favored using bifunctional catalysts. [6][7][8] Among metals, such as Rh, Ir, Ru, and Ni, with moderate to high hydrogenolytic activity, Ir is the most selective to ring opening products. [9] Kubička et al. proved that Brønsted acid sites of the support play an important role due to the need of skeletal isomerization before ring opening. [10] The incorporation of Pt or Ir on acidic materials decreases the strength of Brønsted acid sites of the support, enhances isomerization, and ring opening of decalin. [7,11,12] On the other hand, the acid function can lead to excessive cracking lowering the yield to ring opening products. For this reason, it was proposed the use of noble metal catalysts supported on mesoporous silica-alumina whose acid function is lower compared to zeolites, but still enough to allow the ring contraction reaction which facilitates the hydrogenolysis of the C-C bond on the metal sites. [13][14][15][16][17][18] As reported above, the oil fractions to be processed in SRO contain sulphur compounds that are known to cause deactivation of noble metal catalysts. [START_REF] Paál | Catalytic Naphtha Reforming[END_REF][START_REF] Fujikawa | [END_REF][21][22] An improvement of the tolerance to sulphur is observed when noble metals are deposited on acid supports, related to the modifications of the electronic properties of the metals due to their interaction with the Brønsted acid sites of the support (electron acceptor), with an electron transfer from the metal to the support. [21,22] This weakens the metal-sulphur bond and disfavors strong chemisorption of sulphur compounds. [23,24] Non-zeolitic acid supports such as SiO2-Al2O3 could generate moderate or strong resistance to sulphur. [2,[25][26][27][28] For example, Nassreddine et al. stated that iridium catalysts supported on SiO2-Al2O3 present high stability in the presence of 200 ppm of H2S during SRO of tetralin. [24] The objective of the present work is to study the influence of the content of iridium and of the nature of the support (SiO2-Al2O3 of various proportions) in selective ring opening of decalin, in the absence and presence of sulphur compounds, in order to determine the formulation with the best balance between the acid and metal functions.

Experimental

Supports

SIRAL 70 and 80 (amorphous silica alumina, SiO2-Al2O3) provided by SASOL were used as supports. SIRAL 70 and SIRAL 80 contain 71.5 and 79.0 wt% of SiO2, respectively. 12 In the following, they will be named S70 and S80, respectively. The specific surface areas were 377 m 2 g -1 and 337 m 2 g -1 for S70 and S80, respectively. In order to eliminate any organic impurities that it could have due to the adsorption of impurities from the environment and manipulation the supports were calcined during 4 h at 450 °C (10 °C min -1 , air, 60 cm 3 min -1 ).

Moreover, this treatment standardizes the moisture content of the support. The procedure involves calcination of the support and its subsequent storage in closed bottles inside a desiccator with silicagel.

Catalysts preparation

The support was impregnated with HCl (0.2 mol L -1 , 1.5 cm 3 g -1 ) and left 1 h at rest. H2IrCl6 was then incorporated to obtain 1, 1.5, and 2 wt% of iridium. The impregnated samples were shaken gently for 1 h, then dried at 70 °C in a thermostatic bath until dry powder was obtained and left 12 h in an oven at 120 °C. Lastly, the catalysts were calcined (air, 60 cm 3 min -1 , 300 °C, 4 h) and reduced (H2, 60 cm 3 min -1 , 500 °C, 4 h). The catalysts were called xIr/Sy, where "x" stand for the nominal Ir content (1, 1.5, or 2 wt%) and "Sy" for S70 and S80 supports.

Determination of metallic and chlorine contents

An X-ray Model EDX-720 (Shimadzu) fluorescence spectrometer was used to determine the composition of the catalysts.

Temperature-programmed desorption (TPD) of pyridine

Samples were impregnated with pyridine, and the excess removed in a hood. Before, the experiments, they were treated in nitrogen flow (40 cm 3 min -1 ) for 1 h at 110 °C to eliminate physisorbed pyridine. The temperature was increased at 10 °C min -1 to a final value of 750 °C. The reactor exhaust was connected to a flame ionization detector (FID) to measure the amount of desorbed pyridine.

Isomerization of 3,3-dimethyl-1-butene (33DMB1)

The reaction was performed in a microreactor using 100 mg of catalyst. At first, the catalyst was reduced in situ (H2, 60 cm 3 min -1 , 450 °C, 1 h). After the sample was cooled in N2 (30 cm 3 min -1 ) to the reaction temperature (100 or 125 °C), the reaction was started by addition of 33DMB1, with a partial pressure of 20.9 kPa in N2 and a flow rate of 15.27 mmol h -1 .

Products were analyzed on-line by gas chromatography.

Temperature-programmed reduction (TPR)

TPR experiments were carried out at atmospheric pressure using a reductive mixture of H2-N2 (5% H2 v/v, 10 cm 3 min -1 ). Samples (200 mg) were heated at 10 °C min -1 from 25 °C up to 700 °C. The reactor outlet was connected to a TCD to obtain the TPR profile. Before the TPR measurements the samples were calcined in situ at 400 °C with flowing air 60 cm 3 min -1 g -1 for 1 h.

X-ray Photoelectron Spectroscopy

XPS analysis were carried out on samples reduced ex situ at 500 °C during 2 h under pure H2 (60 cm 3 min -1 ) with a Kratos Axis Ultra DLD spectrometer using a monochromatic Al Kα source (1486.6 eV) operating at 180W (12mA, 15kV). Operating pressure of the spectrometer was 9 x 10 -8 Pascal. The charge neutraliser system was operated for all analyses.

High-resolution spectra were recorded using an analysis area of 300µm x 700 µm and 40 eV pass energy. This pass energy corresponds to Ag 3d5/2 FWHM of 0.55 eV. Data were acquired with 0.1 eV steps. The binding energy was calibrated using Si2p binding energy fixed at 103.9 eV as an internal reference. The following binding energies regions were recorded: C1s, O1s, Al2p, Si2p, Ir4f. Atomic concentration ratios were calculated using sensitivity factors provided by the manufacturer. Peak fitting was achieved with Casa XPS software (version 2.3.17) and Gaussian-Lorentzian profiles (Lorentzian 30%) where used with adding asymmetry peak-shape for Ir4f metallic.

Determination of the metal dispersion by dynamic chemisorption of CO

Metal dispersion of the Ir particles was determined by CO chemisorption where calibrated pulses of CO were injected in a stream of nitrogen that flowed over the sample until saturation. For that purpose, the sample was reduced (H2, 500 °C, 10 °C min -1 ) for 1 h. subsequently nitrogen was flowed over the sample for 1 h at 500 °C to remove adsorbed hydrogen. Finally, the sample was cooled to room temperature in nitrogen and pulses of 0.6 moles of CO were sent to the reactor. The CO that was not chemisorbed was quantitatively converted into CH4 over a kieselguhr-supported Ni catalyst and detected in a flame ionization detector. It was assumed stoichiometry of one CO per surface Ir. [29,30]

Cyclopentane (CP) hydrogenolysis

The reaction was performed at atmospheric pressure in a conventional fixed bed glass microreactor. The reaction temperature was controlled by using a Novus N1100 controller.

Samples were reduced with H2 at 500 °C for 1 h before reaction. The conditions used were: reaction temperature = 225 °C, CP flow rate = 0.36 cm 3 h -1 , catalyst mass= 80 mg, H2 flow rate = 36 cm 3 min -1 , reaction time = 2 h. The products were analyzed on a Thermo Scientific Trace 1300 gas chromatograph equipped with a capillary column Phenomenex ZB-1 connected online. The activity in CP hydrogenolysis was determined from the conversion extrapolate at zero reaction time.

Ring opening of decalin

The stainless-steel autoclave reactor and the reaction conditions were described previously. [31] Pure decalin (37.5% cis isomer and a cis/trans ratio of 0.60) and mixtures of decalin doped with thiophene (S= 10, 15, and 20 ppm) were used. The reaction conditions were: volume of decalin = 25 cm 3 , temperature = 300 and 350 °C, hydrogen pressure = 3 MPa, catalyst loading = 1 g, and stirring rate = 1360 rpm. It was checked that diffusional limitations due to mass transfer were negligible in these reaction conditions. The samples were analyzed at the end of the reaction (6 h) by a Thermo Scientific Trace 1300 gas chromatography apparatus using with a Phenomenex ZB-5 capillary column. The identification of the reaction products was carried out on GC-MS Saturno 2000 spectrometer coupled to a GC Varian 3800 using the same chromatographic column. The experimental reproducibility (catalytic test and analysis) was checked by (i) undertaken the same experiment twice and (ii) for each experiment by injecting three times the same collected sample. The yields and conversion were calculated by averaging the three values. Thus, it was estimated that the conversion and yield values were obtained with a precision of ± 1.5%. [32] 3 Results and discussion

Characterization of the catalysts

Table 1 shows the percentages of chlorine and iridium determined by fluorescence together with the values of total acidity and distribution of acid sites (weak, moderate, and strong) obtained from the TPD traces of pyridine.

It can be seen in Table 1 that the iridium content deviates by 2.5 to 15 % from the nominal value, the 1.5Ir catalyst presenting the highest difference. One can also note that the chlorine content increases with the metal content whatever the support. Moreover, for the same metal content, the chlorine contents are higher on the S70 support than on the S80 one. a Values reported previously [12] , Weak: T<300 °C; Moderate: 300<T<500 °C; Strong: T>500 °C.

The TPD profiles of pyridine are shown in Figure 1, while the TPD of pyridine of the supports were previously reported. [12] Contrary to the support alone, both series of Ir catalysts present mostly strong acid sites (desorption temperature above 500 °C). It can be observed in Table 1 that for the same metallic content, both series have similar total acidity (difference < 7%). In both series, the increase in the iridium content correlated to an increase in chlorine content produces a displacement of the peaks at lower desorption temperatures but increases the total acidity. Ozimek et al. proved that the incorporation of chlorine ions on alumina produce an increase of the acidity. [33,34] It was also demonstrated that the increase in the total acidity is due to the deposition of both chlorine and iridium species. [12] It is noteworthy that, whatever the Ir content and the support (S70 or S80), catalysts present a huge majority of strong acid sites (Table 1) contrary to the supports alone. The support acidity was previously studied by FTIR spectroscopy of pyridine (Py-FTIR) in order to discriminate Lewis from Brønsted acid sites. [32] At 150 °C, the amounts of Brønsted acid sites were of 81 and 72 µmol of Py gCat -1 on S70 and S80, respectively, much lower than the number of Lewis acid sites (148 and 144 µmol of Py gCat -1 , respectively), with a Brønsted/Lewis ratio of ca. 0.5. The total numbers of acid sites (Lewis + Brønsted) determined at 150 °C were of 229 and 216 µmol of Py gCat -1 for S70 and S80, respectively, which gives a ratio between S70 and S80 of 1.06, similar to the ratio determined in the present study from the TPD of pyridine (1.01).

The isomerization reaction of 33DMB1 occurring by protonic mechanism is also useful to evaluate the Brønsted acid sites. [35] At reaction temperatures lower than 300 °C only two isomers are formed (2,3-dimethyl-2-butene and 2,3-dimethyl-1-butene) resulting from the reaction on Brønsted acid sites, Lewis acid sites being not involved in their formation. [36][37][38] In the present study, as the reaction was carried out at 100 and 125 °C, the selectivity to 2,3dimethyl-2-butene and 2,3-dimethyl-1-butene was 100 %, confirming that the reaction only is catalyzed by Brønsted acid sites. at both reaction temperatures. It was demonstrated that the conversion of 33DMB1 depends mainly on the support, and is less affected by the metal deposition, the metal being inactive in the reaction. [39] That means that probably the same trends would be observed for 1.5Ir and 2Ir.

The much higher activity of the Ir/S70 catalyst compared with Ir/S80 in the 33DMB1 isomerization would suggest a higher ability of this catalyst to protonate olefins, i.e. more Brønsted acid sites, which seems contradictory with the results arising from pyridine adsorption/desorption followed by TPD or FTIR. However, one has to keep in mind that pyridine needs stronger Brønsted acid sites than olefins to be protonated. [40] For example, pyridine adsorption onto alumina support only gives rise to bands characteristic of pyridine species coordinated on Al 3+ Lewis acid sites, whereas the same alumina support, pretreated in the same conditions, present a non-negligible activity in 33DMB1 isomerization. [41] Consequently, it can be inferred from the results of 33DMB1 isomerization, that Ir/S70 is more active than Ir/S80 for olefin isomerization, with probably more Brønsted acid sites of moderate or even weak strength. This can be due to the highest chlorine contents of the It/S70 catalysts compared to the Ir/S80 ones.

Figure 3 reports TPR profiles of the both series of catalysts. Considering the hydrogen consumption, in all the catalysts the iridium is reduced to the zerovalent state. It should be recalled that after the pretreatment at 400 °C Ir is present in the form of IrO2 species. 1Ir/Sy catalysts present two reduction peaks overlapped. The peak at lower reduction temperature is attenuated by increasing the metal content to 1.5 wt% and appears as a shoulder; while, on 2wt% Ir catalysts, only one well defined peak is observed. The presence of these two reduction peaks can be attributed to a heterogeneous deposition of the iridium species. [42,43] Furthermore, in Figure 3 the maximum reduction peak of the 1.5/Sy and 2Ir/Sy catalysts appears at higher temperatures than the corresponding 1Ir/Sy. This trend, already observed for Ir/Al2O3 catalysts can be associated with a stronger metal-support interaction. [44] Figure 3. TPR traces of both catalyst series.

Additional XPS analysis was performed in order to corroborate the strong metal interaction observed by TPR experiments. Figure 4 shows that the peak due to metallic Ir (4f7/2) appears at 61.85 and 61.06 eV for the 1 and 1.5 catalysts, respectively supported on S80. According to the literature, the Ir 4f7/2 (metallic) peak should appear at 61 +/-0.2 eV. The observed displacement was attributed to a strong metal support interaction. [45,46] a Determined from metal dispersion supposing spherical particles. [START_REF] Bergeret | Handbook of Heterogeneous Catalysis[END_REF] Cyclopentane hydrogenolysis is used to evaluate the metallic function of the catalysts for hydrogenolysis activity. It is known that this reaction needs large ensembles of metal atoms to be produced. [47] It can be seen in Table 2 that the activity increases with the Ir content in both series, while the turn over frequency (TOF), molecules of CP converted per Ir surface atom per second, does practically not change because of the small differences in the mean size of the metallic particles as calculated from the metal dispersion. [START_REF] Bergeret | Handbook of Heterogeneous Catalysis[END_REF]

Decalin reaction without sulphur

Decalin reaction was carried out at 325 and 350 °C. The reaction products were lumps as cracking (C1-C9), ring opening (RO, C10), ring contraction (RC), and dehydrogenated products (DH) naphthalene and other heavy dehydrogenated products. [START_REF] D'ippolito | [END_REF] Figure 5 shows the conversion values and yields to different reaction products obtained during decalin reaction at 325 and 350 °C. It can be seen that the decalin conversion logically increases with the reaction temperature. The increase of the Ir content from 1 to 2 wt% leads to a small increase of the conversion whatever the support. For a given Ir content, the conversion values are practically the same for S70 and S80. This similarity between both series has been observed

for CP conversion, pyridine thermodesorption, and amount of Brønsted acid sites as it was determined by Py-FTIR, and then attributed to strong Brønsted acid sites, the only difference being the amount of moderate and weak Brønsted acid sites determined by isomerization of 33DMB1. Then, it can be inferred that these moderate and weak Brønsted acid sites do not play a determining role in the reaction. The small increase of the conversion with the Ir content is an unexpected result. Catalysts with higher Ir contents have higher hydrogenolytic activity (Table 2) and higher acidity (Table 1). There is no thermodynamic limitation because decalin reaction is irreversible.

Therefore, if the reaction time or the amount of catalyst is very high, 100 % of decalin conversion could be achieved. We did not obtain 100 % of conversion. Nonetheless, in order to eliminate the possibility that all the catalysts have the same conversion due to an excess of catalyst into the reactor, additional experiments using 0.5 g of catalysts were performed.

Table 3 shows the conversion of decalin and the yield to different reaction products obtained using 0.5 g of catalyst at 350 °C. It must be noted that the three catalysts have practically the same conversion and distribution of products, as observed for 1 g of catalyst. Consequently, it could be concluded that the increase of metal content from 1 to 2 wt% has little influence on decalin conversion as it was previously reported for RuS2/HY zeolite and Pt/Beta zeolite. [50,51] Table 3. Conversion of decalin and yield to different reaction products obtained using 0. It can be observed in Figure 5 that, for all the catalysts, the increase in the reaction temperature leads to an increase in the yield to ring opening, cracking, and dehydrogenated products; and to a decrease in the formation of ring contraction products; this behaviour is more noticeable on the catalysts supported on SIRAL 70. These results are consistent with many publications that postulate that the SRO mechanism of molecules with C6 rings proceeds via consecutive reactions: initially formation of ring contraction products, followed by ring opening and finally cracking. [7,9,10,18,24,[52][53][54][55][56] The yield to cracking products increases a little with the iridium content, but as the increase in Ir content induces an increase in both hydrogenolytic activity on the metallic sites and acidity, it is not possible to infer which type of site, metallic or acidic, is responsible of the formation of cracking products. The increase of the Ir content has practically no influence on the yield to ring opening products. Monteiro et al. found that for metal contents greater than 1 wt%, the acid function of the catalyst becomes the limiting step of the reaction rate. [51]

Decalin reaction with sulphur

The oil fraction to be treated for the selective ring opening process could contain sulphur compounds, mainly thiophenes which are strong deactivating compound of the catalysts. [57][58][59][60] Sulphur is considered as a poison of the metallic function because it adsorbs on the metal sites but, it could be also adsorbed on the acid sites of the support. [START_REF] Paál | Catalytic Naphtha Reforming[END_REF][START_REF] Fujikawa | [END_REF][21][22]61,62] Figure 6 shows the conversion values obtained using decalin contaminated with different amounts of thiophene at 350 °C. At first, the effect of sulphur on the decalin conversion was studied on the support alone, i.e. without iridium. Values of conversion and yield using decalin without sulphur of the supports were previously reported. [12] The addition of 10 ppm of S decreases the conversion of about 20 % on both supports. This could be associated to thiophene adsorption on acid sites. [62] Figure 6. Conversion as a function of the sulphur content (ppm) obtained on decalin reaction at 350 °C.

Before analyzing the S influence on activity and selectivity of the Ir catalysts, it is important to determinate the S/Irsurf atomic ratio used in each decalin reaction. Table 4 gives the S/Irsurf ratios calculated taking into account that the total amount of S incorporated with the decalin is adsorbed on superficial Ir atoms of each catalyst. These values vary between 0.19 to 0.46.

The activity in decalin conversion for both Ir catalysts series decreases more drastically than that of the supports alone, as the sulphur content increases due to the loss of metal activity and some loss of acid function. However, it is observed that the catalysts with the highest Ir content are less deactivated. It can be seen that with 2 wt% Ir, the one on S80 is a little more active than the one on S70 for S of 15 and 20 ppm. This behaviour could be due to the higher metal dispersion of the 2Ir/S80 catalyst compared to 2Ir/S70 catalyst (Table 2). It is interesting to analyze the decline of conversion relative to conversion obtained using pure decalin defined as Δconversion = (Xpure decalin -Xdecalin with S)/Xpure decalin (where X is conversion) as a function of the S/Irsurf atomic ratio. It can be seen in Figure 7 a direct relation between the fall of the conversion and the increase of the S/Irsurf ratio. Sulphur strongly decreases the metallic activity because it can block neighboring metal atoms simply due to its size of the same order of the one of the metals. [63] Similar conclusions were reported by Fischer et al. who suggested that sulphur adatom has an effective blocking radius extending over a number of nearest and next-nearest neighboring metal atoms. [64] The toxicity of sulphur on metal depends on the amount of sulphur, for example when S/Pt = 0.5 the metal is chemically inert. [65] This value corresponds to the total coverage of Pt by sulphur. [66] It was reported a higher adsorption of sulphur on iridium than on Pt which was attributed to its electronic properties, that is, lower electronic affinity (2.12 eV for Pt and 1.6 eV for Ir). [65] According to the values of S/Irsurf ratio reported in Table 4 and considering the aforementioned works related to sulphur influence on the metallic activity, it could be expected that metallic function is partially and totally deactivated by S. It was found that the catalysts evaluated in the decalin conversion in the presence of different amounts of sulphur practically do not present coke deposit, as evaluated by temperature programmed oxidation (results not shown), contrary to the catalysts evaluated in the absence of sulphur.

Consequently, it can be inferred that the deactivation observed is due to sulphur poisoning.

Finally, for a similar S/Irsurf ratio = 0.30  0.04, 1.5Ir/Sy presents the highest decalin conversion compared to 1Ir/Sy and 2Ir/Sy. In other words, it seems more resistant to moderate sulphur loading (10 ppm of S). For highest S/Irsurf ratio (S/Irsurf ratio  0.4), 1.5Ir/Sy is more strongly deactivated than 2Ir/Sy. Table 5 shows the conversion values of cis and trans decalin using feed doped with 0, 10, 15, and 20 ppm of sulphur. As expected, in the case of the metal catalysts, the conversion of cis and trans decalin decreases as the increases of the amount of sulphur on the decalin due to the sulphur adsorption on Ir particles. Curiously, for the supports alone, the conversion of trans-decalin increases by using decalin with 10 ppm of sulphur, while the conversion of cis-decalin is strongly decreased. Considering the different molecular configurations of these isomers, it can be inferred that the presence of thiophene prevents (or affects) cis to trans-decalin stereoisomerization. [67] As a result, the formation of trans-decalin is decreased.

The evolution of the various types of products is presented in Figure 8. The yield to cracking products increases with the addition of 10 ppm and 15 ppm of S on both catalysts series while the yield to RC decreases. Sulphur addition strongly decreases the formation of dehydrogenated products (Figure 8). Considering that dehydrogenation reactions are catalyzed by the metal function, the results are consistent with the poisoning of the metallic function. Recently, Catherin et al. found similar results in SRO of decalin, the addition of S to Ru catalysts supported on zeolite HY, decreased coke formation and dehydrogenated products. [50] Therefore, some increase of the cracking reaction in the presence of sulphur could be due to the lower deposition of coke, as compared with the experiments in the absence of sulphur. Monteiro et al. postulated that the main effect of the S is to modify the balance between the acid and metal activity, which alters the reactivity of the intermediate products and favors the cracking reactions. 50 However, the strong decrease of metal function at the highest S concentration (20 ppm) leads to a lower yield to dehydrogenated products and consequently less cracking products since dehydrogenated products are more easily cracked than saturated compounds. [START_REF] Parera | Catalytic Naphtha Reforming. Science and Technology[END_REF] Additionally, Figure 8 shows that in the presence of 10 ppm of S, the cracking products reach a maximum with 1.5Ir/Sy catalysts and this maximum corresponds to a minimum in the ring contraction products. When increasing the concentration of S to 15 ppm, this maximum was found with 2Ir/Sy catalysts which have the minimum yield to RC products. This shift of the maximum and the minimum to higher metal contents as the sulphur concentration increases shows that isomerization is preponderant when the metal function is deactivated by sulphur.

Blanco et al. emphasize that the addition of S on supported noble metals causes changes in the mechanism of reaction because of the poisoning of the metal function. [START_REF] Blanco | [END_REF] Figure 9 shows the yield to RO products as a function of the sulphur content on decalin. The yield to ring opening products decreases with the amount of sulphur on decalin reaction medium. As expected, higher iridium content, lower decrease in the yield to RO products. On S70, for the same S/IrSurf ratio (0.31) the highest yield in RO products (38%) is obtained with 1.5Ir. However, for higher S/IrSurf value, (0.4), and whatever the support, 2Ir/Sy is the less deactivated with a yield  30%. This behaviour is consistent with a bifunctional mechanism proposed to ring opening reaction of decalin. Sulphur blocks the active metallic sites decreasing the high hydrogenolytic activity of iridium. [61,70] Probably, sulphur causes a decrease in the size of metallic ensembles that inhibits formation of the αβ-adsorption complexes which are esencial for hydrogenolysis reaction catalyzed by metals. [71,72] However, some influence of the sulphur on the acid function cannot be discarded because the decalin conversion on the supports alone was also influenced by the presence of sulphur. It can be inferred that higher metallic content favors tolerance to sulphur. Also, the higher thiotolerance exhibited by the catalysts 2Ir/S70 and 2Ir/S80, can be attributed to the strong metal-support interaction, evidenced by TPR, leading to the formation of electrodeficient metallic particles, which in interaction with the Brønsted acid sites of the support decreases the strength of the S-Metal bond. [27] This could be also due to more accessible metal atoms (because of the higher metal content and a similar metal dispersion) since the amount of catalyst and the duration of the catalytic test are the same. Finally, for these two catalysts, the test was performed using 0.5 g of catalyst instead of 1 g but maintaining contant the S/IrSurf ratio. Obviously, the amount of decalin was also reduced to 12.5 cm 3 . Results are reported in Table 6. Surprisingly, the conversions obtained with two times less catalyst are superior to those attained with 1g of catalyst and the decline of conversion (Δ Conversion) is around 0.16, much lower than that observed with 1 g of catalyst for the same S/IrSurf ratio (Δ Conversion > 0.5). In the experimental conditions used, the metal charges of the catalysts have little influence on the activity and yield using pure decalin because the acid function limits the reaction rate.

Whatever the Ir content and the support, the same trends are observed with a decrease in conversion proportional to the amount of S per Ir surface atom. However, for a moderate S loading (S/IrSurf  0.30) 1.5 Ir/Sy appears less poisoned than 1Ir/Sy and 2Ir/Sy, while for higher amounts of S (S/IrSurf  0.40), the 2Ir is more resistant to S poisoning. This could be attributed to the strong metal-support interaction that leads to the formation of electrodeficient particles, which in interaction with the Brønsted acid sites of the support decreases the strength of the S-Metal bond, as well as the largest available amount of iridium surface particles.

In the presence of sulphur, the yield to dehydrogenated products was almost zero due to the deactivation of the Ir activity by sulphur. Unfortunately, the sulphur also leads to an increase of the cracking products because a change of the reaction pathway.

Although the catalyst supported on SIRAL 80 presented a slight advantage over SIRAL 70 in thiotolerance for S = 15 and 20 ppm, the supports have a lower incidence on sulphur tolerance than the metallic content.

Acknowledgment

Figure 1 .

 1 Figure 1. TPD of pyridine traces of both catalyst series.

Figure 2 .

 2 Figure 2. Initial and final activity on the isomerization of 33DMB1 reaction at reaction temperature 100 and 125 °C.

Figure 2

 2 Figure 2 displays the activity of 1Ir/S70 and 1Ir/S80 for the 33DMB1 conversion at initial and final reaction time for both reaction temperatures. 1Ir/S70 catalyst is more active than 1Ir/S80

Figure 4 .

 4 Figure 4. X-ray photoelectron spectra (Ir 4f core levels) for 1Ir/S80 and 1.5Ir/S80 samples reduced ex situ at 500 °C under H2.

Figure 5 .

 5 Figure 5. Decalin conversion and yield to cracking (CR), ring contraction (RC), ring opening (RO), and dehydrogenated products (DH) obtained on the decalin reaction (6 h).

Figure 7 .

 7 Figure 7. Relative conversion loss due to sulphur as a function of S/Irsurf for both catalysts series. (Δconversion = (Xpure decalin -Xdecalin with S)/Xpure decalin ).

Figure 8 .

 8 Figure 8. Yield to cracking, ring contraction (RC), and dehydrogenated (PD) products as a function of the ppm of sulphur on decalin obtained at 6 h reaction (T = 350 ° C).

Figure 9 .

 9 Figure 9. Yield to ring opening products as a function of the S content on decalin reaction obtained at 6 h reaction (T = 350 ° C). Values between brackets correspond to the S/Irsurf ratio.

Table 1 .

 1 Chorine and iridium content (wt%), total acidity and distribution of acid sites.

	Catalyst	Cl	Ir	Total Acidity	Acid Amount (mol Py g -1 )
		(wt%)	(wt%)	(mol Py g -1 )	Weak	Moderate	Strong
	S70 a	-	-	1119	128	493	498
	1Ir/S70	0.76	0.93	1763	95	565	1103
	1.5Ir/S70	1.07	1.30	1899	25	493	1381
	2Ir/S70	1.10	1.95	2189	84	696	1409
	S80 a	-	-	1104	303	384	417
	1Ir/S80	0.61	0.96	1820	85	626	1109
	1.5Ir/S80	0.71	1.28	1780	88	608	1084
	2Ir/S80	0.96	1.80	2084	110	761	1213

Table 2 .

 2 Values

	Catalyst	Metal dispersion
		(%)

of the metal dispersion determined from CO chemisorption, Ir particle diameter, the activity in cyclopentane conversion, and the corresponding TOF (reaction temperature = 225 °C, CP flow rate = 0.36 cm 3 h -1 , catalyst mass= 80 mg, H2 flow rate = 36 cm 3 min -1 , reaction time = 2 h).

Ir particle diameter (nm) a TOF CP (s -1 ) Activity (mol CP g -1 s -1 )

	1Ir/S70	45	2.5	0.38	8.4
	1.5Ir/S70	34	3.2	0.37	8.6
	2Ir/S70	32	3.4	0.35	11.4
	1Ir/S80	41	2.7	0.39	8.0
	1.5Ir/S80	39	2.8	0.34	8.7
	2Ir/S80	40	2.8	0.32	11.8

Table 4 .

 4 S/Irsurf atomic ratio used on decalin reaction doped with 10, 15 and 20 ppm of thiophene.

	Catalyst		S/Ir surf atomic ratio
		10 ppm	15 ppm	20 ppm
	1Ir/S70	0.32	-	-
	1.5Ir/S70	0.30	0.46	-
	2Ir/S70	0.22	0.32	0.43
	1Ir/S80	0.34	-	-
	1.5Ir/S80	0.27	0.40	-
	2Ir/S80	0.19	0.28	0.37

Table 5 .

 5 Conversion of trans and cis decalin (considering the initial amount of cis and trans decalin) obtained at 6 h reaction (T = 350 °C) with 0, 10, 15, and 20 ppm of sulphur on the feed.

		Trans-decalin conversion (%)	Cis-decalin conversion (%)
	S content (ppm)
		0	10	15	20	0	10	15	20
	S70	22.7	31.5	-	-	83.2	36.1	-	-
	1Ir/S70	77.8	42.2	-	-	97.1	68.6	-	-
	1.5Ir/S70	78.1	59.3	27.5	-	97.3	65.9	45.8	-
	2Ir/S70	80.5	66.2	50.3	38.9	97.7	69.6	60.5	47.9
	S80	26.8	36.2	-	-	96.3	57.3	-	-
	1Ir/S80	74.1	42.1	-	-	96.3	72	-	-
	1.5Ir/S80	73.7	63.9	32.6	-	96.3	68.7	34.1	-
	2Ir/S80	79.8	72.3	62.1	45.3	97.1	60.5	51.1	48.4

Table 6 .

 6 Influence of the sulphur on decalin reaction using equal S/IrSurf ratio.

	Catalyst	Weigh	Sulphur	S/Ir	Conversion		Yield (%)
		(g)	(ppm)		(%)	CR	RC	RO	DH
	2Ir/S70	1.0	20	0.43	42.27	5.05	8.27	28.80	0.15
	2Ir/S70	0.5	10	0.43	52.60	9.77	8.99	32.09	1.74
	2Ir/S80	1.0	20	0.37	46.45	8.05	6.58	31.32	0.50
	2Ir/S80	0.5	10	0.37	53.91	6.52	9.86	35.50	2.07
	4 Conclusions

The authors warmly thank Christine Canaff from IC2MP for the XPS experiments and the results analysis, performed in particular conditions due to the COVID-19 pandemic.