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.

Use of Pyrazole Hydrogen Bonding in Tripodal Complexes to Form Self Assembled Homochiral Dimers

The 3:1 condensation of 5-methyl-1H-pyrazole-3-carboxaldehyde (MepyrzH) with tris(2-aminoethyl)amine (tren) gives the tripodal ligand tren(MePyrzH)₃. Aerial oxidation of a solution of cobalt(II) with this ligand in the presence of base results in the isolation of the insoluble Co(tren)(MePyrz)_3. This complex reacts with acids, HCl/NaClO₄, NH₄ClO₄, NH₄BF₄, and NH₄I to give the crystalline compounds Co(tren)(MePyrzH)₃(ClO₄)₃, {[Co(tren)(MePyrzH_0.5)₃](ClO₄)_1.5}₂ {[Co(tren)(MePyrzH_0.5)₃](BF₄)_1.5}₂ and [Co(tren)(MePyrzH)₃][Co(tren)(MePyrzH)₃]I_2. The latter three complexes are dimeric, held together by three N_pyrazole –H^…N_pyrazolate hydrogen bonds. The structures and symmetries of these homochiral dimers or pseudodimers are discussed in terms of their space group. Possible applications of these complexes by incorporation into new materials are mentioned.