Studies on functionalised macrocyclic ligands

The work presented in this thesis hinges on three main topics: a) the coordination chemistry of symmetric and asymmetric derivatives of [9]aneN3 towards lanthanide ions; b) the transition metal co-ordination chemistry of nitrile and amino derivatives of [9]aneN3 and [15]aneN3O2; c) the use of macrocyclic ligands for the synthesis of polymeric Ag' complexes. Chapter 3 describes the Ln"' complexes of the ligand obtained by Schiffbase condensation of 1,4,7-tris(2-aminoethyl)-1,4,7-triazacyclononane (L) with three molar equivalents of sodium pyruvate using the Ln0' ion as templating agent. The mononuclear complexes [Ln(La)] (Ln"' = Y10, Sm"', Gd"', Dy"', Eu", Yb"', La"') have been prepared and characterised, and in most cases the crystal structure has also been determined. NMR spectroscopic studies on the diamagnetic [Y(La)] and [La(La)] complexes and on paramagnetic [Yb(La)] and [Sm(La)] complexes have been carried out. Variable temperature 1H NMR behaviour of the [Y(La)] and [Yb(La)] complexes has also been investigated. Hydrolysis experiments on the [La(La)] complex in D20 at neutral and acidic pH have been performed in order to determine the stability of such a complex in in vivo conditions. Moreover, lanthanide properties such as relaxivity of the Gd"' complex and Dysprosium Induced Shift (DIS) have been determined in order to obtain information either about the efficiency as contrast agent of the Gd"' complex and the number of water molecules bound to the metal centre. After the study on the nine co-ordinate complexes [Ln(La)] discussed in Chapter 3, Chapter 4 reports the Ln°1 complexes obtained by changing the ketone employed for the Schiff-base condensation with the triamine L. Two different acetylphosphonate monoesters have been used in order to form novel nine co-ordinate Ln"' complexes: the synthesis of [Ln(Lb)] (Ln"' = Y"', Gds", Yb"', La'y') and [Ln(Lc)] (Ln"' = Y"", Gd"', Eu") complexes has been achieved by Schiffbase condensation of the triamine (L) with methyl sodium acetyl phosphonate and methoxybenzyl sodium acetyl phosphonate, respectively, using the Ln01 ion as templating agent. These Ln"' complexes have been studied again by X-ray crystallography and NMR spectroscopy. Since the nine co-ordinate lanthanide complexes such as [Ln(Lb)] and [Ln(L`)] contain three chiral phosphorus centres, four possible diastereomers, each of them with two enantiomers, could be distinguished and NMR spectroscopic studies have been carried out in order to determine the four different diastereomers present in solution. As in Chapter 3, hydrolysis experiments on the Y" complexes with Lb and Lc and relaxivity ofthe Gd1° complexes have been determined. The ligands discussed in Chapter 5 have been synthesised in order to provide a set of seven or eight donor atoms for the co-ordination of lanthanide ions, leaving one or two co-ordination sites available for the binding of water molecules. Therefore, 4,7-bis(2-aminoethyl)-1,4,7-triazacyclononane (L2), 1-(carboxymethyl)-4,7-bis(2-aminoethyl)-1,4,7-triazacyclononane (HL3) and 1-(2-hydroxyethyl)-4,7-bis(2-aminoethyl)-1,4,7-triazacyclononane (HL4) have been synthesised and then reacted with two equivalents of sodium pyruvate, methyl sodium acetyl phosphonate or methoxybenzyl sodium acetyl phosphonate using the Ln" ion as templating agent. A large number of lanthanide complexes with formulations [Ln(L2a)(CH3CO2)], [Ln(L2b)(CH3CO2)], [Ln(L2°)(CH3CO2)], [Ln(L3a)] [Ln(L3b)], [Ln(L4a)] and [Ln(L4b)] have been synthesised and characterised. The single crystal X-ray diffraction analysis of [Gd(L2a)(CH3CO2)]"CH30H, the 1H and 13C NMR spectra and the hydrolysis experiments on the V" complexes with L 2a, L2b, L2c, L3a, L3b, L 4a and L 4b are reported. Relaxivity of the Gd" complexes and Dysprosium Induced Shift on [Dy(L2a)(CH3CO2)], [Dy(L3a)] and [Dy(L4a)] have been determined. Chapter 6 describes the co-ordination chemistry of symmetric and asymmetric derivatives of [9]aneN3 towards transition metal ions. The two ligands tris(cyanomethyl)- and tris(2-cyanoethyl)-1,4,7-triazacyciononane (L' and L5, respectively) form peculiar complexes with Cull: using Cu(BF4)2.4H20 in MeOH at 65°C, the methanolysis of two nitriles with formation of imino-ether groups have produced square-based pyramidal Cull complexes. However, from the reaction of L5 with CuC12.2H2O in CH3CN at room temperature, the distorted square-based pyramidal Cull complex [Cu(L5)C12] with the nitrite pendant arms left uncoordinated has been formed. Cull and Zn" complexes with 1-(2-aminoethyl)-1,4,7-triazacyclononane (L7), Mn", Nil', Cull and Zn" complexes with L2, Mn", Cull and Zn" complexes with 1,4,7-tris(3-aminopropyl)-1,4,7 triazacyclononane (L) and Mn" and Zn" complexes with HL3 and HL4 have been prepared and characterised, and in most cases the crystal structure has also been determined. Furthermore, the EPR spectra of the Cull complexes and the 13C NMR spectroscopic data for the Zn" complexes are reported. Synthesis, solution studies and structural characterisation of complexes with [15]aneN302 derivatives are the topics of Chapter 7. The two ligands 1,4,7-tris(cyanomethyl)-1,4,7-triaza-10,13-dioxacyclopentadecane (L8) and 1,4,7-tris(2-aminoethyl)-1,4,7-triaza-10,13-dioxacyclopentadecane (L9) have been synthesised and their co-ordination chemistry towards transition and posttransition metal ions (Cull, Zn°, Cd" and Pbll) has been studied. Most of the complexes have been structurally characterised and they all show interesting structures: the pendant nitrile arms of L8 are not involved in co-ordination of the metal except for the Pbl' crystal structure and with L9 only larger metal ions such as Cd" and Pbll are bound to all the donor atoms of the ligand. The protonation equilibria of the two ligands and the formation of the Cu", Zn", Cd" and Pb" complexes with L8 and L9 have been studied by means of potentiometric measurements. The protonation constants of the ligands and the stability constants of the complexes are reported and compared to other ligands with similar macrocyclic framework and donor atoms. In order to further investigate the structural features of the complexes in solution, 'H NMR spectra of diamagnetic complexes have also been recorded at various temperatures. In Chapter 8, the nitrite pendant arm derivatives L', L5, L8 and L1° have been used as building blocks for the synthesis of extended inorganic architectures by reaction with Ag'. The complexes {[Ag(L')]PF6},,,, {[Ag(L')]BF4}oo, {[Ag(L8)]BF4}. and [Ag(L10)]PF202 have been prepared and structurally characterised. Analogously to other complexes of the same type prepared in the Schröder group, these compounds show nuclearity and dimensionality strictly dependent upon the number and length of the nitrite functionalised pendant arms present in the ligand: these act as linkers between different metal centres. Ag' complexes of [9]aneN3 and [15]aneN302 derivatives bearing three 7-methylquinoline pendant arms have also been prepared and characterised. The crystal structure of the Ag' complex with 1,4,7-tris(7-methylquinolyl)-1,4,7-triaza-10,13-dioxacyclopentadecane (L12) shows theformation of a dimer {[Ag2.5(L12)(PF202)2.5"CH3CN]2w}i th one Ag' co-ordinated linearly by two heterocyclic N-donors and bridging the two units

Studies on functionalised macrocyclic ligands

The work presented in this thesis hinges on three main topics: a) the coordination chemistry of symmetric and asymmetric derivatives of [9]aneN3 towards lanthanide ions; b) the transition metal co-ordination chemistry of nitrile and amino derivatives of [9]aneN3 and [15]aneN3O2; c) the use of macrocyclic ligands for the synthesis of polymeric Ag' complexes. Chapter 3 describes the Ln"' complexes of the ligand obtained by Schiffbase condensation of 1,4,7-tris(2-aminoethyl)-1,4,7-triazacyclononane (L) with three molar equivalents of sodium pyruvate using the Ln0' ion as templating agent. The mononuclear complexes [Ln(La)] (Ln"' = Y10, Sm"', Gd"', Dy"', Eu", Yb"', La"') have been prepared and characterised, and in most cases the crystal structure has also been determined. NMR spectroscopic studies on the diamagnetic [Y(La)] and [La(La)] complexes and on paramagnetic [Yb(La)] and [Sm(La)] complexes have been carried out. Variable temperature 1H NMR behaviour of the [Y(La)] and [Yb(La)] complexes has also been investigated. Hydrolysis experiments on the [La(La)] complex in D20 at neutral and acidic pH have been performed in order to determine the stability of such a complex in in vivo conditions. Moreover, lanthanide properties such as relaxivity of the Gd"' complex and Dysprosium Induced Shift (DIS) have been determined in order to obtain information either about the efficiency as contrast agent of the Gd"' complex and the number of water molecules bound to the metal centre. After the study on the nine co-ordinate complexes [Ln(La)] discussed in Chapter 3, Chapter 4 reports the Ln°1 complexes obtained by changing the ketone employed for the Schiff-base condensation with the triamine L. Two different acetylphosphonate monoesters have been used in order to form novel nine co-ordinate Ln"' complexes: the synthesis of [Ln(Lb)] (Ln"' = Y"', Gds", Yb"', La'y') and [Ln(Lc)] (Ln"' = Y"", Gd"', Eu") complexes has been achieved by Schiffbase condensation of the triamine (L) with methyl sodium acetyl phosphonate and methoxybenzyl sodium acetyl phosphonate, respectively, using the Ln01 ion as templating agent. These Ln"' complexes have been studied again by X-ray crystallography and NMR spectroscopy. Since the nine co-ordinate lanthanide complexes such as [Ln(Lb)] and [Ln(L`)] contain three chiral phosphorus centres, four possible diastereomers, each of them with two enantiomers, could be distinguished and NMR spectroscopic studies have been carried out in order to determine the four different diastereomers present in solution. As in Chapter 3, hydrolysis experiments on the Y" complexes with Lb and Lc and relaxivity ofthe Gd1° complexes have been determined. The ligands discussed in Chapter 5 have been synthesised in order to provide a set of seven or eight donor atoms for the co-ordination of lanthanide ions, leaving one or two co-ordination sites available for the binding of water molecules. Therefore, 4,7-bis(2-aminoethyl)-1,4,7-triazacyclononane (L2), 1-(carboxymethyl)-4,7-bis(2-aminoethyl)-1,4,7-triazacyclononane (HL3) and 1-(2-hydroxyethyl)-4,7-bis(2-aminoethyl)-1,4,7-triazacyclononane (HL4) have been synthesised and then reacted with two equivalents of sodium pyruvate, methyl sodium acetyl phosphonate or methoxybenzyl sodium acetyl phosphonate using the Ln" ion as templating agent. A large number of lanthanide complexes with formulations [Ln(L2a)(CH3CO2)], [Ln(L2b)(CH3CO2)], [Ln(L2°)(CH3CO2)], [Ln(L3a)] [Ln(L3b)], [Ln(L4a)] and [Ln(L4b)] have been synthesised and characterised. The single crystal X-ray diffraction analysis of [Gd(L2a)(CH3CO2)]"CH30H, the 1H and 13C NMR spectra and the hydrolysis experiments on the V" complexes with L 2a, L2b, L2c, L3a, L3b, L 4a and L 4b are reported. Relaxivity of the Gd" complexes and Dysprosium Induced Shift on [Dy(L2a)(CH3CO2)], [Dy(L3a)] and [Dy(L4a)] have been determined. Chapter 6 describes the co-ordination chemistry of symmetric and asymmetric derivatives of [9]aneN3 towards transition metal ions. The two ligands tris(cyanomethyl)- and tris(2-cyanoethyl)-1,4,7-triazacyciononane (L' and L5, respectively) form peculiar complexes with Cull: using Cu(BF4)2.4H20 in MeOH at 65°C, the methanolysis of two nitriles with formation of imino-ether groups have produced square-based pyramidal Cull complexes. However, from the reaction of L5 with CuC12.2H2O in CH3CN at room temperature, the distorted square-based pyramidal Cull complex [Cu(L5)C12] with the nitrite pendant arms left uncoordinated has been formed. Cull and Zn" complexes with 1-(2-aminoethyl)-1,4,7-triazacyclononane (L7), Mn", Nil', Cull and Zn" complexes with L2, Mn", Cull and Zn" complexes with 1,4,7-tris(3-aminopropyl)-1,4,7 triazacyclononane (L) and Mn" and Zn" complexes with HL3 and HL4 have been prepared and characterised, and in most cases the crystal structure has also been determined. Furthermore, the EPR spectra of the Cull complexes and the 13C NMR spectroscopic data for the Zn" complexes are reported. Synthesis, solution studies and structural characterisation of complexes with [15]aneN302 derivatives are the topics of Chapter 7. The two ligands 1,4,7-tris(cyanomethyl)-1,4,7-triaza-10,13-dioxacyclopentadecane (L8) and 1,4,7-tris(2-aminoethyl)-1,4,7-triaza-10,13-dioxacyclopentadecane (L9) have been synthesised and their co-ordination chemistry towards transition and posttransition metal ions (Cull, Zn°, Cd" and Pbll) has been studied. Most of the complexes have been structurally characterised and they all show interesting structures: the pendant nitrile arms of L8 are not involved in co-ordination of the metal except for the Pbl' crystal structure and with L9 only larger metal ions such as Cd" and Pbll are bound to all the donor atoms of the ligand. The protonation equilibria of the two ligands and the formation of the Cu", Zn", Cd" and Pb" complexes with L8 and L9 have been studied by means of potentiometric measurements. The protonation constants of the ligands and the stability constants of the complexes are reported and compared to other ligands with similar macrocyclic framework and donor atoms. In order to further investigate the structural features of the complexes in solution, 'H NMR spectra of diamagnetic complexes have also been recorded at various temperatures. In Chapter 8, the nitrite pendant arm derivatives L', L5, L8 and L1° have been used as building blocks for the synthesis of extended inorganic architectures by reaction with Ag'. The complexes {[Ag(L')]PF6},,,, {[Ag(L')]BF4}oo, {[Ag(L8)]BF4}. and [Ag(L10)]PF202 have been prepared and structurally characterised. Analogously to other complexes of the same type prepared in the Schröder group, these compounds show nuclearity and dimensionality strictly dependent upon the number and length of the nitrite functionalised pendant arms present in the ligand: these act as linkers between different metal centres. Ag' complexes of [9]aneN3 and [15]aneN302 derivatives bearing three 7-methylquinoline pendant arms have also been prepared and characterised. The crystal structure of the Ag' complex with 1,4,7-tris(7-methylquinolyl)-1,4,7-triaza-10,13-dioxacyclopentadecane (L12) shows theformation of a dimer {[Ag2.5(L12)(PF202)2.5"CH3CN]2w}i th one Ag' co-ordinated linearly by two heterocyclic N-donors and bridging the two units

High spin Fe(III)-doped nanostructures as T1 MR imaging probes

Magnetic Resonance Imaging (MRI) T1 contrast agents based on Fe(III) as an alternative to Gd-based compounds have been under intense scrutiny in the last 6-8 years and a number of nanostructures have been designed and proposed for in vivo diagnostic and theranostic applications. Excluding the large family of superparamagnetic iron oxides widely used as T2 -MR imaging agents that will not be covered by this review, a considerable number and type of nanoparticles (NPs) have been employed, ranging from amphiphilic polymer-based NPs, NPs containing polyphenolic binding units such as melanin-like or polycatechols, mixed metals such as Fe/Gd or Fe/Au NPs and perfluorocarbon nanoemulsions. Iron(III) exhibits several favorable magnetic properties, high biocompatibility and improved toxicity profile that place it as the paramagnetic ion of choice for the next generation of nanosized MRI and theranostic contrast agents. An analysis of the examples reported in the last decade will show the opportunities for relaxivity and MR-contrast enhancement optimization that could bring Fe(III)-doped NPs to really compete with Gd(III)-based nanosystems. This article is categorized under: Diagnostic Tools > In Vivo Nanodiagnostics and Imaging Diagnostic Tools > Diagnostic Nanodevices Therapeutic Approaches and Drug Discovery > Nanomedicine for Oncologic Disease

GdDOTAGA(C18)2: an efficient amphiphilic Gd(iii) chelate for the preparation of self-assembled high relaxivity MRI nanoprobes

A new amphiphilic GdDOTA-like complex functionalized with two octadecyl chains was synthesised and incorporated into the bilayer of liposomes and dendrimersomes. (1)H NMR relaxometric studies and in vivo MRI experiments on mice bearing a syngeneic melanoma tumour have shown a great improvement in performance

Optimizing the relaxivity of MRI probes at high magnetic field strengths with binuclear GdIIIComplexes

The key criteria to optimize the relaxivity of a Gd(III) contrast agent at high fields (defined as the region 65 1.5 T) can be summarized as follows: (i) the occurrence of a rotational correlation time \u3c4R in the range of ca. 0.2\u20130.5 ns; (ii) the rate of water exchange is not critical, but a \u3c4M < 100 ns is preferred; (iii) a relevant contribution from water molecules in the second sphere of hydration. In addition, the use of macrocycle-based systems ensures the formation of thermodynamically and kinetically stable Gd(III) complexes. Binuclear Gd(III) complexes could potentially meet these requirements. Their efficiency depends primarily on the degree of flexibility of the linker connecting the two monomeric units, the absence of local motions and the presence of contribution from the second sphere water molecules. With the aim to maximize relaxivity (per Gd) over a wide range of magnetic field strengths, two binuclear Gd(III) chelates derived from the well-known macrocyclic systems DOTA-monopropionamide and HPDO3A (Gd2L1 and Gd2L2, respectively) were synthesized through a multistep synthesis. Chemical Exchange Saturation Transfer (CEST) experiments carried out on Eu2L2 at different pH showed the occurrence of a CEST effect at acidic pH that disappears at neutral pH, associated with the deprotonation of the hydroxyl groups. Then, a complete 1H and 17O NMR relaxometric study was carried out in order to evaluate the parameters that govern the relaxivity associated with these complexes. The relaxivities of Gd2L1 and Gd2L2 (20 MHz, 298 K) are 8.7 and 9.5 mM 121 s 121, respectively, +77% and +106% higher than the relaxivity values of the corresponding mononuclear GdDOTAMAP-En and GdHPDO3A complexes. A significant contribution of second sphere water molecules was accounted for the strong relaxivity enhancement of Gd2L2. MR phantom images of the dinuclear complexes compared to GdHPDO3A, recorded at 7 T, confirmed the superiority of Gd2L2. Finally, ab initio (DFT) calculations were performed to obtain information about the solution structure of the dinuclear complexes

Surprising Complexity of the [Gd(AAZTA)(H2O)2]− Chelate Revealed by NMR in the Frequency and Time Domains

[Abstract] Typically, Ln(III) complexes are isostructural along the series, which enables studying one particular metal chelate to derive the structural features of the others. This is not the case for [Ln(AAZTA)(H2O)x]− (x = 1, 2) systems, where structural variations along the series cause changes in the hydration number of the different metal complexes, and in particular the loss of one of the two metal-coordinated water molecules between Ho and Er. Herein, we present a 1H field-cycling relaxometry and 17O NMR study that enables accessing the different exchange dynamics processes involving the two water molecules bound to the metal center in the [Gd(AAZTA)(H2O)2]− complex. The resulting picture shows one Gd-bound water molecule with an exchange rate ∼6 times faster than that of the other, due to a longer metal–water distance, in accordance with density functional theory (DFT) calculations. The substitution of the more labile water molecule with a fluoride anion in a diamagnetic-isostructural analogue of the Gd-complex, [Y(AAZTA)(H2O)2]−, allows us to follow the chemical exchange process by high-resolution NMR and to describe its thermodynamic behavior. Taken together, the variety of tools offered by NMR (including high-resolution 1H, 19F NMR as a function of temperature, 1H longitudinal relaxation rates vs B0, and 17O transverse relaxation rates vs T) provides a complete description of the structure and exchange dynamics of these Ln-complexes along the series.This research was supported by the Università del Piemonte Orientale (Ricerca locale FAR2019). F.C., L.T., and M.B. acknowledge the financial support from the Ministero dell’Università e della Ricerca (PRIN 2017A2KEPL project “Rationally designed nanogels embedding paramagnetic ions as MRI probes”). This work was carried out within the framework of the COST CA15209 Action “European Network on NMR Relaxometry”Italia. Ministero dell'Università e della Ricerca; PRIN-2017A2KEP

Thermodynamic stability, kinetic inertness and relaxometric properties of monoamide derivatives of lanthanide(III) DOTA complexes

A complete thermodynamic and kinetic solution study on lanthanide(III) complexes with monoacetamide (DOTAMA, L1) and monopropionamide (DOTAMAP, L2) derivatives of DOTA (DOTA = 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid) was undertaken with the aim to elucidate their stability and inertness in aqueous media. The stability constants of GdL1 and GdL2 are comparable, whereas a more marked difference is found in the kinetic inertness of the two complexes. The formation of the Eu(III) and Ce(III) complexes takes place via the formation of the protonated intermediates which can deprotonate and transform into the final complex through a OH-assisted pathway. GdL2 shows faster rates of acid catalysed decomplexation with respect to GdL1, which has a kinetic inertness comparable to GdDOTA. Nevertheless, GdL2 is one order of magnitude more inert than GdDO3A. A novel DOTAMAP-based bifunctional chelating ligand and its deoxycholic acid derivative (L5) were also synthesized. Since the coordinated water molecule in GdL2 is characterized by an exchange rate ca. two orders of magnitude greater than in GdL1, the relaxivity of the macromolecular derivatives of GdL5 should not be limited by the slow water exchange process. The relaxometric properties of the supramolecular adduct of GdL5 with human serum albumin (HSA) were investigated in aqueous solution by measuring the magnetic field dependence of the H-1 relaxivity which, at 20 MHz and 298 K, shows a 430% increase over that of the unbound GdL5 chelate. Thus, Gd(III) complexes with DOTAMAP macrocyclic ligands can represent good candidates for the development of stable and highly effective bioconjugate systems for molecular imaging applications

First in vivo MRI study on theranostic dendrimersomes

Amphiphilic Janus-dendrimers are able to self-assemble into nanosized vesicles named dendrimersomes.We recently synthesized the 3,5-C12-EG-(OH)4 dendrimer that generates dendrimersomes with very promising safety and stability profiles, that can be loaded with different contrast agents for in vivo imaging. In this contribution, nanovesicles were loaded with both the Magnetic Resonance Imaging (MRI) reporter GdDOTAGA(C18)2 and the glucocorticoid drug Prednisolone Phosphate (PLP), in order to test their effective potential as theranostic nanocarriers on murine melanoma tumour models. The incorporation of GdDOTAGA(C18)2 into the membrane resulted in dendrimersomes with a high longitudinal relaxivity (r1 = 39.1 mM−1 s−1, at 310 K and 40 MHz) so that, after intravenous administration, T1-weighted MRI showed a consistent contrast enhancement in the tumour area. Furthermore, the nanovesicles encapsulated PLP with good efficiency and displayed anti-tumour activity both in vitro and in vivo, thus enabling their practical use for biomedical theranostic applications

Semi-Rigid (Aminomethyl) Piperidine-Based Pentadentate Ligands for Mn(II) Complexation

Two pentadentate ligands built on the 2-aminomethylpiperidine structure and bearing two tertiary amino and three oxygen donors (three carboxylates in the case of AMPTA and two carboxylates and one phenolate for AMPDA-HB) were developed for Mn(II) complexation. Equilibrium studies on the ligands and the Mn(II) complexes were carried out using pH potentiometry, 1H-NMR spectroscopy and UV-vis spectrophotometry. The Mn complexes that were formed by the two ligands were more stable than the Mn complexes of other pentadentate ligands but with a lower pMn than Mn(EDTA) and Mn(CDTA) (pMn for Mn(AMPTA) = 7.89 and for Mn(AMPDA-HB) = 7.07). 1H and 17O-NMR relaxometric studies showed that the two Mn-complexes were q = 1 with a relaxivity value of 3.3 mM-1 s-1 for Mn(AMPTA) and 3.4 mM-1 s-1 for Mn(AMPDA-HB) at 20 MHz and 298 K. Finally, the geometries of the two complexes were optimized at the DFT level, finding an octahedral coordination environment around the Mn2+ ion, and MD simulations were performed to monitor the distance between the Mn2+ ion and the oxygen of the coordinated water molecule to estimate its residence time, which was in good agreement with that determined using the 17O NMR data