Nickel(II) Complexes as Paramagnetic Shift and paraCEST Agents

Study of paraCEST response on six-coordinated Co(II) and Ni(II) complexes of a pyridine-tetraamide-based ligand

This study highlights the successful synthesis of a potential ligand, 2,2',2'',2'''-((pyridine-2,6-diylbis(methylene))bis(azanetriyl))tetraacetamide (PATA), along with its corresponding Co(II) and Ni(II) complexes for paraCEST-based agents. X-ray diffraction data confirmed that both the complexes are six coordinated with distorted octahedral geometries, but only the [Co(PATA)]2+ complex has a good structural feature to show paraCEST activity. After a thorough characterization of the ligand and both of its complexes, various studies, including solution-state magnetic properties, redox properties, temperature, and pH variation studies, were carried out. [Co(PATA)]2+ remained inert in the presence of competing ions, under acidic conditions, at high temperatures, and in the physiological pH range. The paraCEST response of [Co(PATA)]2+ has been measured in the presence of HEPES buffer medium, and a high paraCEST feature was discovered at both 37 and 25 °C. The pH variation paraCEST studies were carried out and the exchange rate constant of the probe at 37 and 25 °C was also determined. However, due to the fast exchange of water protons, the [Ni(PATA)(OH2)]2+ complex remained inactive in the CEST process.

Comparative paraCEST effect of amide and hydroxy groups in divalent cobalt and nickel complexes of tripyridine-based ligands

Co(II) and Ni(II) complexes of two tri-pyridine-based ligands with two hydroxy and two amide exchangeable protons (TDTA) and with six amide exchangeable protons (TMTP) were investigated for application as paraCEST-based magnetic resonance imaging (MRI) contrast agents. The two hydroxy groups present in the TDTA ligand were found to be passive while the amide group was active towards the CEST process. In the case of the Co(II) and Ni(II) complexes of the TMTP ligand, all three coordinated amide groups participated in the exchange process, and excellent CEST signals were observed. The X-ray structure of the four complexes revealed the seven-coordinate geometry of Co(II) complexes and the six-coordinate geometry of Ni(II) complexes. The presence of amide protons and hydroxy protons in the complexes was detected by the NMR method. The stability of the complexes in solution at high temperatures, in different pH ranges and acidic conditions, in the presence of competing cations, and biologically relevant anions was investigated. Potentiometric titrations were carried out to determine the ligand's protonation constants and the complexes' thermodynamic stability constant at 25.0 °C and I = 0.15 mol L-1 NaClO4. ParaCEST studies of [Co(TMTP)]2+ and [Ni(TMTP)]2+ at variable pH and variable pulse power are highlighted.

Di-Pyridine-Containing Macrocyclic Triamide Fe(II) and Ni(II) Complexes as ParaCEST Agents

Fe(II) and Ni(II) paraCEST contrast agents containing the di-pyridine macrocyclic ligand 2,2',2″-(3,7,10-triaza-1,5(2,6)-dipyridinacycloundecaphane-3,7,10-triyl)triacetamide (DETA) are reported here. Both [Fe(DETA)]²⁺ and [Ni(DETA)]²⁺ complexes were structurally characterized. Crystallographic data revealed the seven-coordinated distorted pentagonal bipyramidal geometry of the [Fe(DETA)]·(BF₄)₂·MeCN complex with five coordinated nitrogen atoms from the macrocyclic ring and two coordinated oxygen atoms from two amide pendant arms. The [Ni(DETA)]·Cl₂·2H₂O complex was six-coordinated in nature with a distorted octahedral geometry. Four coordinated nitrogen atoms were from the macrocyclic ring, and two coordinated oxygen atoms were from two amide pendant arms. [Fe(DETA)]²⁺ exhibited well-resolved sharp proton resonances, whereas very broad proton resonances were observed in the case of [Ni(DETA)]²⁺ due to the long electronic relaxation times. The CEST peaks for the [Fe(DETA)]²⁺ complex showed one highly downfield-shifted and intense peak at 84 ppm with another shifted but less intense peak at 28 ppm with good CEST contrast efficiency at body temperature, whereas [Ni(DETA)]²⁺ showed only one highly shifted intense peak at 78 ppm from the bulk water protons. Potentiometric titrations were performed to determine the protonation constants of the ligand and the thermodynamic stability constant of the [M(DETA)]²⁺ (M = Fe, Co, Ni, Cu, Zn) species at 25.0 °C and I = 0.15 mol·L^-1 NaClO₄. Metal exchange studies confirmed the stability of the complexes in acidic medium in the presence of physiologically relevant anions and an equimolar concentration of Zn(II) ions.

Distinct Coordination Chemistry of Fe(III)-Based MRI Probes

ConspectusContrast agents are used in approximately 40% of all magnetic resonance imaging (MRI) procedures to improve the quality of the images based on the distribution and dynamic clearance of the agent. To date, all clinically approved contrast agents are Gd(III) coordination complexes that serve to shorten the longitudinal (T₁) and transverse (T₂) proton relaxation times of water. Recent interest in replacing Gd with biologically relevant metal ions such as Mn or Fe has led to increased interest in the aqueous coordination chemistry of their complexes. In this Account, we focus on high-spin Fe(III) complexes that have been recently reported as MRI contrast agents or probes in our laboratory.The highly Lewis acidic Fe(III) center has distinct coordination chemistry in aqueous solutions, facilitating alternative strategies in the design of MRI probes. To illustrate this, we describe different classes of Fe(III) MRI probes with a focus on macrocyclic complexes and multinuclear complexes such as self-assembled metal organic polyhedra (MOP). Our initial efforts focused on macrocyclic complexes of Fe(III) in order to tune spin and oxidation states with the goal of stabilizing high-spin Fe(III) in reducing biological environments. Our probes feature six-coordinate Fe(III) complexes of 1,4,7-triazacyclononane with hydroxypropyl, phosphonate, or carboxylate pendant groups to produce Fe(III) complexes that shorten proton T₁ times predominantly from second-sphere or outer-sphere interactions at neutral pH. Analogues with pentadentate macrocyclic ligands have an inner-sphere water that does not exchange rapidly on the NMR time scale, yet these complexes are effective relaxation agents. Fe(III) macrocyclic complexes in this class can be modified to modulate their biodistribution and pharmacokinetic clearance in mice. The goal of these studies is for the Fe(III) agents to clear as extracellular fluid agents and produce profiles similar to those of Gd agents. Finally, studies of multimeric Fe(III) complexes are of interest to produce probes that give large proton relaxivity. In this approach the two Fe(III) centers are connected through aryl linkers as demonstrated for several macrocyclic complexes. Even more tightly connected Fe(III) centers are produced in a Fe(III) self-assembled cage with relaxivity of 21 mM^-1 s^-1 at 4.7 T, 37 °C in the presence of serum albumin to which it is tightly bound. This cage enhances contrast of the vasculature as a blood pool agent and accumulates in tumors. Finally, we present our perspectives on the further development of Fe(III) complexes for various applications in MRI.

CoII Complexes as Liposomal CEST Agents

Three paramagnetic CoII macrocyclic complexes containing 2-hydroxypropyl pendant groups, 1,1',1'',1'''-(1,4,8,11-tetraazacyclotetradecane-1,4,8,11-tetrayl)tetrakis- (propan-2-ol) ([Co(L1)]2+ , 1,1'-(4,11-dibenzyl-1,4,8,11-tetraazacyclotetradecane-1,8-diyl)bis(propan-2-ol) ([Co(L2)]2+ ), and 1,1'-(4,11-dibenzyl-1,4,8,11-tetraazacyclotetradecane-1,8-diyl)bis(octadecan-2-ol) ([Co(L3)]2+ ) were synthesized to prepare transition metal liposomal chemical exchange saturation transfer (lipoCEST) agents. In solution, ([Co(L1)]2+ ) forms two isomers as shown by 1 H NMR spectroscopy. X-ray crystallographic studies show one isomer with 1,8-pendants in cis-configuration and a second isomer with 1,4-pendants in trans-configuration. The [Co(L2)]2+ complex has 1,8-pendants in a cis-configuration. Remarkably, the paramagnetic-induced shift of water 1 H NMR resonances in the presence of the [Co(L1)]2+ complex is as large as that observed for one of the most effective LnIII water proton shift agents. Incorporation of [Co(L1)]2+ into the liposome aqueous core, followed by dialysis against a solution of 300 mOsm L-1 produces a CEST peak at 3.5 ppm. Incorporation of the amphiphilic [Co(L3)]2+ complex into the liposome bilayer produces a more highly shifted CEST peak at -13 ppm. Taken together, these data demonstrate the feasibility of preparing CoII lipoCEST agents.

Applications for Transition-Metal Chemistry in Contrast-Enhanced Magnetic Resonance Imaging

Contrast-enhanced magnetic resonance imaging (MRI) is an indispensable tool for diagnostic medicine. However, safety concerns related to gadolinium in commercial MRI contrast agents have emerged in recent years. For patients suffering from severe renal impairment, there is an important unmet medical need to perform contrast-enhanced MRI without gadolinium. There are also concerns over the long-term effects of retained gadolinium within the general patient population. Demand for gadolinium-free MRI contrast agents is driving a new wave of inorganic chemistry innovation as researchers explore paramagnetic transition-metal complexes as potential alternatives. Furthermore, advances in personalized care making use of molecular-level information have motivated inorganic chemists to develop MRI contrast agents that can detect pathologic changes at the molecular level. Recent studies have highlighted how reaction-based modulation of transition-metal paramagnetism offers a highly effective mechanism to achieve MRI contrast enhancement that is specific to biochemical processes. This Viewpoint highlights how recent advances in transition-metal chemistry are leading the way for a new generation of MRI contrast agents.

Dual use of colorimetric sensor and selective copper removal from aqueous media with novel p(HEMA-co-TACYC) hydrogels: Cyclen derivative as both monomer and crosslinker

Within the scope of this study, p(2-hydroxyethyl methacrylate-co-tetraacrylic cyclen) (p(HEMA-co-TACYC)) hydrogels were synthesized for the first time in the literature using a tetraacrylic cyclen (TACYC) as both functional monomer and crosslinker. The hydrogels designed especially for Cu²⁺ ions showed colorimetric sensor behavior selective for Cu²⁺ ions in all aqueous media (deionized, tap, river and sea water) and in metal ion mixtures. The p(HEMA-co-TACYC) hydrogels forming a stable complex with Cu²⁺ ions simultaneously showed properties of being a good adsorbent material. The hydrogels have reuse capacity as both sensor and adsorbent material. Changing the amount of TACYC in the hydrogel structure changes the maximum adsorption capacity for Cu²⁺ ions. The Langmuir and Freundlich adsorption constants for Cu²⁺ ion adsorption of the hydrogels, acting as selective adsorbent in all aqueous media and metal ion mixtures, were determined.

Exploring Inner-Sphere Water Interactions of Fe(II) and Co(II) Complexes of 12-Membered Macrocycles To Develop CEST MRI Probes

Several paramagnetic Co(II) and Fe(II) macrocyclic complexes were prepared with the goal of introducing a bound water ligand to produce paramagnetically shifted water ¹H resonances and for paramagnetic chemical exchange saturation transfer (paraCEST) applications. Three 12-membered macrocycles with amide pendent groups including 1,7-bis(carbamoylmethyl)-1,4,7,10-tetraazacyclodocane (DCMC), 4,7,10-tris(carbamoylmethyl)-,4,7,10-triaza-12-crown-ether (N3OA), and 4,10-bis(carbamoylmethyl)-4,10-diaza-12-crown-ether (NODA) were prepared and their Co(II) complexes were characterized in the solid state and in solution. The crystal structure of [Co(DCMC)]Br₂ featured a six-coordinated Co(II) center with distorted octahedral geometry, while [Co(NODA)(OH₂)]Cl₂ and [Co(N3OA)](NO₃)₂ were seven-coordinated. The analogous Fe(II) complexes of NODA and NO3A were successfully prepared, but the complex of DCMC oxidized rapidly to the Fe(III) form. Similarly, [Fe(NODA)]²⁺ oxidized over several days, forming crystals of the Fe(III) complex isolated as the μ-O bridged dimer. Magnetic susceptibility values and paramagnetic NMR spectra of the Fe(II) complexes of NODA and N3OA, as well as Co(II) complexes of DCMC, NODA, and N3OA, were consistent with high spin complexes. CEST peaks ranging from 60 ppm to 70 ppm, attributed to NH groups of the amide pendents, were identified. Variable-temperature ¹⁷O NMR spectra of Co(II) and Fe(II) NODA complexes were consistent with rapid exchange of the water ligand with bulk water. Notably, the Co(II) and Fe(II) complexes presented here produced substantial paramagnetic shifts of bulk water ¹H resonances, independent of having an inner-sphere water.