Solution Study, Crystal Structure and Relaxivity Properties of a Gd3+ Complex with an Uncharged Macrocyclic Ligand Bearing Four Amidic Side Arms

Abstract

Equilibrium data on the interaction of DTMA [(DTMA = DOTA tetrakis(methylammide)] with Gd3+ in aqueous solution, properties of the complexes formed in the pH range 0.6–11.8, water proton relaxation rate enhancement, and the crystal structure analysis of the [Gd(DTMA)H2O]3+ complex are reported. In the crystal structure the metal ion is bound to the nitrogen atoms of the tetraazamacrocyclic moiety, to the amidic oxygen atoms, and to an oxygen atom of a water molecule. The nine donors are located at the vertices of a distorted square antiprism, which is capped by the coordinated water oxygen atom in the axial position. In solution [Gd(DTMA)]3+ is not very stable [logKML = 12.8(1)] and gives rise to the formation of [Gd(DTMA)OH]2+ [pKa = 7.9(1)] and [Gd(HDTMA)]4+ [logK(ML+H) = 3.4(1)]. The proton solvent relaxivity of aqueous complex solutions assumes a constant value in the pH range 3–8, increasing at higher and lower pH. For pH > 3 the data are in good agreement with a previous study on the same compound. For pH < 3 a new interpretation is presented, based on the formation of [Gd(HDTMA)]4+ and the release of Gd3+.

Positively Charged Lanthanide Complexes with Cyclen-Based Ligands: Synthesis, Solid-State and Solution Structure, and Fluoride Interaction

“…The mean twist angle (Φ) between the upper and lower planes amounts to 37.1° ([Eu 2 ( L 1 ) 2 F] 5+ ) and 37.6° ([La­( L 2 )­Cl] 2+ ). These values are very similar to those observed in the solid-state structures of Ln III complexes of DOTA, DOTAM, , and DTMA , (Chart ) with SAP geometries. The coordination geometry observed for [La­( L 2 )­Cl] 2+ differs from that observed for the [La­(DOTA)] − complex, which adopts a TSAP geometry in the solid state.…”

Being Excited by Lanthanide Coordination Complexes: Aqua Species, Chirality, Excited-State Chemistry, and Exchange Dynamics

“…e Note the absence of any significant correlation between the Gd-water bond length and the water exchange rate in this series; here there are too many variables at work determining this distance (ligand/geometry changes). For an example where the bond length may be related to the water exchange rate, see ref 270. associated with deprotonation of the bound water, of 7.3 and 7.9 (0.1 M NMe 4 ClO 4 , 298 K), respectively, 34,42,43,101 which are similar to the value of 7.5 reported for [EuDO2AHOE(H 2 O)] + . 41 For systems where water exchange is slow on the NMR time scale and for which τ m > T 1m in eq 6 (defining paramagnetic proton relaxivity, r 1p , where c is the complex concentration, τ m the water exchange lifetime, and T 1m the water proton longitudinal relaxation time), measurements of the pH dependence of proton re- laxivity allow these prototropic processes to be examined.…”

“…The exchange of protons in a lanthanide complexeither of a bound water molecule or of relatively acidic ligand hydrogensis subject to acid or base catalysis. Potentiometric titrations for the cationic Eu and Gd complexes of DTMA revealed p K a values, associated with deprotonation of the bound water, of 7.3 and 7.9 (0.1 M NMe 4 ClO 4 , 298 K), respectively, ,,, which are similar to the value of 7.5 reported for [EuDO2AHOE(H 2 O)] + . For systems where water exchange is slow on the NMR time scale and for which τ m > T 1m in eq 6 (defining paramagnetic proton relaxivity, r 1p , where c is the complex concentration, τ m the water exchange lifetime, and T 1m the water proton longitudinal relaxation time), measurements of the pH dependence of proton relaxivity allow these prototropic processes to be examined…”

Unusually Sharp Dependence of Water Exchange Rate versus Lanthanide Ionic Radii for a Series of Tetraamide Complexes

“…Interestingly, the twist angle between N4 and O4 planes is slightly smaller in Tb( 3 ) (38.0°) than in Eu( 3 ) (38.5°) consistent with a slightly longer Tb−O water (2.427 Å) bond distance (Eu−O water = 2.414 Å). This trend, an increase in Ln−O water bond distance with smaller ionic radii, is opposite that reported for other DOTA-like complexes: for LnDOTA - crystals, Eu−O water (2.480 Å 2.483 Å) > Gd−O water (2.463 Å) > Y−O water (2.435 Å, 2.424 Å) > Lu−O water (2.416 Å); for LnHP-DO3A crystals, Gd−O water (2.50 Å) > Y−O water (2.49 Å 15 ); and for Ln( 5 ) crystals, Gd−O water (2.461 Å) > Dy−O water (2.427 Å 5 ). Furthermore, an analysis of the 1 H and 17 O lanthanide induce shifts (LIS) for Ln( 3 ) complexes indicated that they are isostructural (again, consistent with the M isomer) both in acetonitrile and in water along the Ln 3+ series (see Supporting Information, Figures S1 and S2).…”