Density-functional computation of (99)Tc NMR chemical shifts

[Tc(NO)Cl2(PPh3)2(CH3CN)] and Its Reactions with 2,2'-Dipyridyl Dichalcogenides

The sparingly soluble technetium(I) complex [TcI(NO)Cl2(PPh3)2(CH3CN)] (1) slowly dissolves during reactions with 2,2'-dipyridyl ditelluride, (2-pyTe)2, 2,2'-dipyridyl diselenide, (2-pySe)2, or 2,2'-dipyridyl disulfide, (2-pyS)2, under formation of deeply colored solutions. Blue (Te compound) or red solids (Se compound) of the composition [{TcI(NO)Cl2(PPh3)2}2{µ2-(2-pyE)2}], E = Te (3), Se (4), precipitate from the reaction solutions upon addition of toluene. They represent the first technetium complexes with dichalcogenides. While [{TcI(NO)Cl2(PPh3)}2{µ2-(2-pyTe)2}] (3) is the sole product, a small amount of a second product, [TcII(NO)Cl2(PPh3)(2-pySe)] (5), was obtained from the respective mother solution of the reaction with the diselenide. From the corresponding reaction between 1 and (2-pyS)2, the technetium(II) compound, [TcII(NO)Cl2(PPh3)(2-pyS)] (6), could be isolated exclusively. The products were studied by single-crystal X-ray diffraction and spectroscopic methods including 99Tc NMR for the technetium(I) products and EPR spectroscopy for the Tc(II) complexes. The experimental results are accompanied by DFT considerations, which help to rationalize the experimental observations.

Pivotal role of 99Tc NMR spectroscopy in solid-state and molecular chemistry

The radioelement Technetium (element 43) pertains to various domains including the nuclear enterprise (i.e., spent nuclear fuel (SNF) reprocessing and nuclear waste remediation) and nuclear medicine (i.e., development of new imaging agents) as well as to the fundamental science of transition metals (i.e., chemical trends in catalytic properties). One method that can provide critical information to improve knowledge in these domains is 99Tc nuclear magnetic resonance (NMR) spectroscopy. The review, presented here, summarizes the pivotal role of 99Tc NMR spectroscopy over the past two decades and presents prospects of the method to tackle challenges in Tc chemistry.

[Tc(NO)(Cp)(PPh3)Cl] and [Tc(NO)(Cp)(PPh3)(NCCH3)](PF6), and Their Reactions with Pyridine and Chalcogen Donors

Reactions of the technetium(I) nitrosyl complex [Tc(NO)(Cp)(PPh3)Cl] with triphenylphosphine chalcogenides EPPh3 (E = O, S, Se), and Ag(PF6) in a CH2Cl2/MeOH mixture (v/v, 2/1) result in an exchange of the chlorido ligand and the formation of [Tc(NO)(Cp)(PPh3)(EPPh3)](PF6) compounds. The cationic acetonitrile complex [Tc(NO)(Cp)(PPh3)(NCCH3)]+ is formed when the reaction is conducted in NCCH3 without additional ligands. During the isolation of the corresponding PF6- salt a gradual decomposition of the anion was detected in the solvent mixture applied. The yields and the purity of the product increase when the BF4- salt is used instead. The acetonitrile ligand is bound remarkably strongly to technetium and exchange reactions readily proceed only with strong donors, such as pyridine or ligands with 'soft' donor atoms, such as the thioether thioxane. Substitutions on the cyclopentadienyl ring do not significantly influence the ligand exchange behavior of the starting material. 99Tc NMR spectroscopy is a valuable tool for the evaluation of reactions of the complexes of the present study. The extremely large chemical shift range of this method allows the ready detection of corresponding ligand exchange reactions. The observed 99Tc chemical shifts depend on the donor properties of the ligands. DFT calculations support the discussions about the experimental results and provide explanations for some of the unusual findings.

Inquiring 199Hg NMR Parameters by Combining Ab Initio Molecular Dynamics and Relativistic NMR Calculations

Ab initio molecular dynamics (AIMD) sampling followed by relativistic density functional theory (DFT) 199Hg NMR calculations were performed for Hg organometallic complexes in water, dimethyl sulfoxide, and chloroform. The spin-orbit coupling, a relativistic effect, is a key factor for predicting δ(Hg) and 1J(Hg-C) accurately, in conjunction with a dynamic treatment of the systems. Good agreement between the theoretical and experimental results is reached by adopting implicit (based on a continuum model) and explicit (solvent molecules treated quantum mechanically) solvation models. Broader trends appearing in the experimental data available in the literature are reproduced by the calculations, and therefore, quantum chemistry is able to assist in the assignment and interpretation of 199Hg NMR data. Less pronounced trends, such as changes in the 199Hg chemical shift in different systems with the same atom types bound to Hg, are too weak to be predicted reliably by the current state-of-the-art theoretical methods based on AIMD sampling and relativistic DFT with hybrid functionals for NMR calculations.

Computational Prediction of Tc-99 NMR Chemical Shifts in Technetium Complexes with Radiopharmaceutical Applications

The Tc-99m nucleus is the most used nuclide in radiopharmaceuticals designed for imaging diagnosis. The metal can exist in nine distinct oxidation states and forms distinct coordination complexes with a variety of chelating agents and geometries. These complexes are usually characterized through Tc-99 NMR that is very sensitive to the Tc coordination sphere. Therefore, predicting Tc-99 NMR might be useful to assist experimentalists in structural characterization. In the present study, we propose three computational protocols for predicting Tc-99 NMR chemical shifts based on density functional theory calculations using relativistic and nonrelativistic Hamiltonians: the relativistic Model 1, the nonrelativistic Model 2, and the empirical nonrelativistic Model 3. In Models 2 and 3, the NMR-DKH basis set was used for all atoms, including the Tc, for which it was developed here. All models were applied for a set of 41 Tc-complexes with metal oxidation states 0, I, and V, for which the Tc-99 chemical shift was available experimentally. The mean absolute deviation and the mean relative deviation were 67 ppm and 4.8% (Model 1), 92 ppm and 6.2% (Model 2), and 65 ppm and 4.9% (Model 3), respectively. Last, the effect of the explicit solvent was evaluated for the [TcO₂(en)₂]⁺─Tc(V) complex. The calculated results for the Tc-99 NMR chemical shift at SO-ZORA-SSB-D/TZ2P-ZORA/COSMO//TPSS/def2-SVP/IEF-PCM(UFF) show that the inclusion of 14 water molecules (first solvation shell) together with the implicit solvation model leads to an absolute deviation of only 7 ppm (0.3%) from the experimental value, indicating that the solvent effects play a key role in predicting Tc-99 NMR.

Systematic Evaluation of Modern Density Functional Methods for the Computation of NMR Shifts of 3d Transition-Metal Nuclei

A wide range of density functionals from all rungs of Jacob's ladder have been evaluated systematically for a set of experimental 3d transition-metal NMR shifts of 70 complexes encompassing 12 × ⁴⁹Ti, 10 × ⁵¹V, 10 × ⁵³Cr, 11 × ⁵⁵Mn, 9 × ⁵⁷Fe, 9 × ⁵⁹Co, and 9 × ⁶¹Ni shift values, as well as a diverse range of electronic structure characteristics. The overall 39 functionals evaluated include one LDA, eight GGAs, seven meta-GGAs (including their current-density-functional─CDFT─versions), nine global hybrids, four range-separated hybrids, eight local hybrids, and two double hybrids, and we also include Hartree-Fock and MP2 calculations. While recent evaluations of the same functionals for a very large coupled-cluster-based benchmark of main-group shieldings and shifts achieved in some cases aggregate percentage mean absolute errors clearly below 2%, the best results for the present 3d-nuclei set are in the range between 4 and 5%. Strikingly, the overall best-performing functionals are the recently implemented CDFT versions of two meta-GGAs, namely cM06-L (4.0%) and cVSXC (4.3%), followed by cLH14t-calPBE (4.9%), B3LYP (5.0%), and cLH07t-SVWN (5.1%), i.e., the previously best-performing global hybrid and two local hybrids. A number of further functionals achieve aggregate deviations in the range 5-6%. Range-separated hybrids offer no particular advantage over global hybrids. Due to the overall poor performance of Hartree-Fock theory for all systems except the titanium complexes, MP2 and double-hybrid functionals are unsuitable for these 3d-nucleus shifts and provide large errors. Global hybrid functionals with larger EXX admixtures, such as BHLYP or M06-2X, also perform poorly, and some other highly parametrized global hybrids also are unsuitable. For many functionals depending on local kinetic energy τ, their CDFT variants perform much better than their "non-CDFT" versions. This holds notably also for the above-mentioned M06-L and VSXC, while the effect is small for τ-dependent local hybrids and can even be somewhat detrimental to the agreement with experiment for a few other cases. The separation between well-performing and more poorly performing functionals is mainly determined by their results for the most critical nuclei ⁵⁵Mn, ⁵⁷Fe, and ⁵⁹Co. Here either moderate exact-exchange admixtures or CDFT versions of meta-GGAs are beneficial for the accuracy. The overall deviations of the better-performing global or local hybrids are then typically dominated by the ⁵³Cr shifts, where triplet instabilities appear to disfavor exact-exchange admixture. Further detailed analyses help to pinpoint specific nuclei and specific types of complexes that are challenges for a given functional.

99Tc NMR as a promising technique for structural investigation of biomolecules: theoretical studies on the solvent and thermal effects of phenylbenzothiazole complex

The phenylbenzothiazole compounds show antitumor properties and are highly selective. In this paper, the (99)Tc chemical shifts based on the ((99m)Tc)(CO)3 (NNO) complex conjugated to the antitumor agent 2-(4'-aminophenyl)benzothiazole are reported. Thermal and solvent effects were studied computationally by quantum-chemical methods, using the density functional theory (DFT) (DFT level BPW91/aug-cc-pVTZ for the Tc and BPW91/IGLO-II for the other atoms) to compute the NMR parameters for the complex. We have calculated the (99)Tc NMR chemical shifts of the complex in gas phase and solution using different solvation models (polarizable continuum model and explicit solvation). To evaluate the thermal effect, molecular dynamics simulations were carried, using the atom-centered density matrix propagation method at the DFT level (BP86/LanL2dz). The results highlight that the (99)Tc NMR spectroscopy can be a promising technique for structural investigation of biomolecules, at the molecular level, in different environments.

Density-functional computation of ⁹³Nb NMR chemical shifts

93Nb chemical shifts of [NbX6](-) (X = Cl, F, CO), [NbXCl4](-) (X = O, S), Nb2(OMe)10, Cp*2Nb(κ2-BH4), (Cp*Nb)2(µ-B2H6)2, CpNb(CO)4, and Cp2NbH3 are computed at the GIAO (gauge-including atomic orbitals)-, BPW91- and B3LYP-, and CSGT (continuous set of gauge transformations)-CAM-B3LYP, -ωB97, and -ωB97X levels, using BP86-optimized or experimental (X-ray) geometries. Experimental chemical shifts are best reproduced at the GIAO-BPW91 level when δ(93Nb) values of inorganic complexes are referenced directly relative to [NbCl6](-) and those of organometallic species are first calculated relative to [Nb(CO)6](-). An inadvertent error in the reported δ(93Nb) values of cyclopentadiene borane complexes (H. Brunner et al., J. Organomet. Chem.1992, 436, 313) is corrected. Trends in the observed 93Nb NMR linewidths for anionic niobates [Nb(CO)5](3-), [Nb(CO)5H](2-), and [Nb(CO)5(NH3)](-) are rationalized in terms of computed electric field gradients at the metal.