Solution Structure of Ln(III) Complexes with Macrocyclic Ligands Through Theoretical Evaluation of ¹H NMR Contact Shifts

Abstract

Herein, we present a new approach that combines DFT calculations and the analysis of Tb(III)-induced (1)H NMR shifts to quantitatively and accurately account for the contact contribution to the paramagnetic shift in Ln(III) complexes. Geometry optimizations of different Gd(III) complexes with macrocyclic ligands were carried out using the hybrid meta-GGA TPSSh functional and a 46 + 4f(7) effective core potential (ECP) for Gd. The complexes investigated include [Ln(Me-DODPA)](+) (H(2)Me-DODPA = 6,6'-((4,10-dimethyl-1,4,7,10-tetraazacyclododecane-1,7-diyl)bis(methylene))dipicolinic acid, [Ln(DOTA)(H(2)O)](-) (H(4)DOTA = 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetate), [Ln(DOTAM)(H(2)O)](3+) (DOTAM = 1,4,7,10- tetrakis[(carbamoyl)methyl]-1,4,7,10-tetraazacyclododecane), and related systems containing pyridyl units (Ln = Gd, Tb). Subsequent all-electron relativistic calculations based on the DKH2 approximation, or small-core ECP calculations, were used to compute the (1)H hyperfine coupling constants (HFCCs) at the ligand nuclei (A(iso) values). The calculated A(iso) values provided direct access to contact contributions to the (1)H NMR shifts of the corresponding Tb(III) complexes under the assumption that Gd and Tb complexes with a given ligand present similar HFCCs. These contact shifts were used to obtain the pseudocontact shifts, which encode structural information as they depend on the position of the nucleus with respect to the lanthanide ion. An excellent agreement was observed between the experimental and calculated pseudocontact shifts using the DFT-optimized geometries as structural models of the complexes in solution, which demonstrates that the computational approach used provides (i) good structural models for the complexes, (ii) accurate HFCCs at the ligand nuclei. The methodology presented in this work can be classified in the context of model-dependent methods, as it relies on the use of a specific molecular structure obtained from DFT calculations. Our results show that spin polarization effects dominate the (1)H A(iso) values. The X-ray crystal structures of [Ln(Me-DODPA)](PF(6))·2H(2)O (Ln = Eu or Lu) are also reported.

Spectroscopic Properties of a Family of Mono‐ to Trinuclear Lanthanide Complexes

“…Figure represents the molecular geometries of the [Eu 2 L 2 ] 2+ and [Eu 3 L 3 ] 3+ complexes calculated at the HF/LCECP/3‐21G level. The solid‐state and solution structures of the [Ln L 1 ] + complexes have been reported in previous works , . Herein, theoretical calculations were carried out to estimate the intramolecular Ln ··· Ln distances in the binuclear and trinuclear [Ln 2 L 2 ] 2+ and [Ln 3 L 3 ] 3+ entities.…”

“…In previous works,, it has been shown that the macrocyclic ligand H 2 L 1 (Scheme ) forms eight‐coordinate complexes with Ln ions both in the solid state and in solution. The presence of the methyl substituents in positions 1 and 7 of the cyclen moiety introduces a certain degree of steric hindrance on the picolinate pendants that prevents the coordination of water molecules.…”

Investigating the Complexation of the Pb2+/Bi3+ Pair with Dipicolinate Cyclen Ligands

“…The hard nature of the metal ion is more evident in the case of the Eu 3+ and Lu 3+ complexes reported previously, for which the bond distances involving the oxygen atoms of the ligand are even shorter when compared to the distances to nitrogen atoms. 30 The distances between Bi and the donor atoms of the cyclen unit are ca. 0.1 Å longer than the average Bi−N distance observed for [Bi(dota)] − (2.53 Å).…”

Beyond Bleaney's Theory: Experimental and Theoretical Analysis of Periodic Trends in Lanthanide‐Induced Chemical Shift

“…[11] Susceptibility tensor fitting results (Figure 1) ] complexes reconstructed by Spinach [9] using the best-fit magnetic susceptibility tensor. Positive PCS is shown in red, negative -in blue, transparency indicates the absolute value normalized for all complexes (see the ESI for further information).…”