Paramagnetic NMR Shifts Research Articles

Tris-Silanide f-Block Complexes: Insights into Paramagnetic Influence on NMR Chemical Shifts.

The paramagnetism of f-block ions has been exploited in chiral shift reagents and magnetic resonance imaging, but these applications tend to focus on 1H NMR shifts as paramagnetic broadening makes less sensitive nuclei more difficult to study. Here we report a solution and solid-state (ss) 29Si NMR study of an isostructural series of locally D 3h -symmetric early f-block metal(III) tris-hypersilanide complexes, [M{Si(SiMe3)3}3(THF)2] (1-M; M = La, Ce, Pr, Nd, U); 1-M were also characterized by single crystal and powder X-ray diffraction, EPR, ATR-IR, and UV-vis-NIR spectroscopies, SQUID magnetometry, and elemental analysis. Only one SiMe3 signal was observed in the 29Si ssNMR spectra of 1-M, while two SiMe3 signals were seen in solution 29Si NMR spectra of 1-La and 1-Ce. This is attributed to dynamic averaging of the SiMe3 groups in 1-M in the solid state due to free rotation of the M-Si bonds and dissociation of THF from 1-M in solution to give the locally C 3v -symmetric complexes [M{Si(SiMe3)3}3(THF) n ] (n = 0 or 1), which show restricted rotation of M-Si bonds on the NMR time scale. Density functional theory and complete active space self-consistent field spin-orbit calculations were performed on 1-M and desolvated solution species to model paramagnetic NMR shifts. We find excellent agreement of experimental 29Si NMR data for diamagnetic 1-La, suggesting n = 1 in solution and reasonable agreement of calculated paramagnetic shifts of SiMe3 groups for 1-M (M = Pr and Nd); the NMR shifts for metal-bound 29Si nuclei could only be reproduced for diamagnetic 1-La, showing the current limitations of pNMR calculations for larger nuclei.

Paramagnetic Properties of [AnIV(NO3)6]2- Complexes (An = U, Np, Pu) Probed by NMR Spectroscopy and Quantum Chemical Calculations.

Actinide +IV complexes with six nitrates [AnIV(NO3)6]2- (An = Th, U, Np, and Pu) have been studied by 15N and 17O NMR spectroscopy in solution and first-principles calculations. Magnetic susceptibilities were evaluated experimentally using the Evans method and are in good agreement with the ab initio values. The evolution in the series of the crystal field parameters deduced from ab initio calculations is discussed. The NMR paramagnetic shifts are analyzed based on ab initio calculations. Because the cubic symmetry of the complex quenches the dipolar contribution, they are only of Fermi contact origin. They are evaluated from first-principles based on a complete active space/density functional theory (DFT) strategy, in good accordance with the experimental one. The ligand hyperfine coupling constants are deduced from paramagnetic shifts and calculated using unrestricted DFT. The latter are decomposed in terms of the contribution of molecular orbitals. It highlights two pathways for the delocalization of the spin density from the metallic open-shell 5f orbitals to the NMR active nuclei, either through the valence 5f hybridized with 6d to the valence 2p molecular orbitals of the ligands, or by spin polarization of the metallic 6p orbitals which interact with the 2s-based molecular orbitals of the ligands.

Solid State and Solution Structures of Lanthanide Nitrate Complexes of Tris-(1-napthylphosphine oxide).

Coordination complexes of lanthanide metals with tris-1-naphthylphosphine oxide (Nap3PO, L) have not been previously reported in the literature. We describe here the formation of lanthanide(III) nitrate complexes Ln(NO3)3L4 (Ln = Eu to Lu) and the structures of [Ln(NO3)3L2]·2L (Ln = Eu, Dy, Ho, Er) and L. The core structure of the complexes is an eight-coordinate [Ln(NO3)3L2] with the third and fourth ligands H-bonded via their oxygen atoms to one of the naphthyl rings. The structures are compared with those of the analogous complexes of triphenylphosphine oxide and show that the Ln-O(P) bond in the Nap3PO complexes is slightly longer than expected on the basis of differences in coordination numbers. The reaction solutions, investigated by 31P and 13C NMR spectroscopy in CD3CN, show that coordination of L occurs across the lanthanide series, even though complexes can only be isolated from Eu onwards. Analysis of the 31P NMR paramagnetic shifts shows that there is a break in the solution structures with a difference between the lighter lanthanides (La-Eu) and heavier metals (Tb-Lu) which implies a minor difference in structures. The isolated complexes are very poorly soluble, but in CDCl3, NMR measurements show dissociation into [Ln(NO3)3L2] and 2L occurs.

Paramagnetic Nuclear Magnetic Resonance Shifts for Triplet Systems and Beyond with Modern Relativistic Density Functional Methods.

An efficient framework for the calculation of paramagnetic NMR (pNMR) shifts within exact two-component (X2C) theory and (current-dependent) density functional theory (DFT) up to the class of local hybrid functionals (LHFs) is presented. Generally, pNMR shifts for systems with more than one unpaired electron depend on the orbital shielding contribution and a temperature-dependent term. The latter includes zero-field splitting (ZFS), hyperfine coupling (HFC), and the g-tensor. For consistency, we calculate these three tensors at the same level of theory, i.e., using scalar-relativistic X2C augmented with spin-orbit perturbation theory. Results for pNMR chemical shifts of transition-metal complexes reveal that this X2C-DFT framework can yield good results for both the shifts and the individual tensor contributions of metallocenes and related systems, especially if the HFC constant is large. For small HFC constants, the relative error is often large, and sometimes the sign may be off. 4d and 5d complexes with more complicated structures demonstrate the limitations of a fully DFT-based approach. Additionally, a Co-based complex with a very large ZFS and pronounced multireference character is not well described. Here, a hybrid DFT-multireference framework is necessary for accurate results. Our results show that X2C is sufficient to describe relativistic effects and computationally cheaper than a fully relativistic approach. Thus, it allows use of large basis sets for converged HFCs. Overall, current-dependent meta-generalized gradient approximations and LHFs show some potential; however, the currently available functionals leave a lot to be desired, and the predictive power is limited.

Complex isomerism influencing the textural properties of organometallic [Cu(salen)] porous polymers: paramagnetic solid-state NMR characterization and heterogeneous catalysis.

Although organometallic porous polymer networks are recognized as promising heterogeneous catalysts, the relationship between ligand/monomer geometry and network parameters is usually not well understood due to the lack of atom-resolved characterization methods for the amorphous network matrix. In this work, a series of copper(II) salen-type metal complexes was synthesized, using trans- and cis-1,2-diaminocyclohexane segments, and thoroughly characterized by single-crystal X-ray diffraction and solution- and solid-state NMR spectroscopy. Terminal ethynyl groups of the complexes were then transformed into polyacetylene chains by coordination chain-growth homopolymerization, resulting in highly porous (458-655 m2 g-1) organometallic polymer networks with a copper(II) ion content of about 12 wt%. The presence of paramagnetic copper(II) moieties in these complexes and respective polymer networks required the application of tailored NMR techniques, which together with X-ray crystallography and DFT calculations of the paramagnetic NMR shifts made it possible to investigate the differences in the complex geometry in liquid, powder and crystalline form and compare it with the complex geometry in polymer networks. All prepared organometallic polymer networks were also tested as heterogeneous catalysts for styrene oxidation with uncommonly high substrate conversions and compared with their low-molecular-weight analogues. The high reusability of such heterogeneous polymer-based catalysts was also proven.

Effect of Magnetic Anisotropy on the 1H NMR Paramagnetic Shifts and Relaxation Rates of Small Dysprosium(III) Complexes.

We present a detailed analysis of the 1H NMR chemical shifts and transverse relaxation rates of three small Dy(III) complexes having different symmetries (C3, D2 or C2). The complexes show sizeable emission in the visible region due to 4F9/2 → 6HJ transitions (J = 15/2 to 11/2). Additionally, NIR emission is observed at ca. 850 (4F9/2 → 6H7/2), 930 (4F9/2 → 6H5/2), 1010 (4F9/2 → 6F9/2), and 1175 nm (4F9/2 → 6F7/2). Emission quantum yields of 1-2% were determined in aqueous solutions. The emission lifetimes indicate that no water molecules are present in the inner coordination sphere of Dy(III), which in the case of [Dy(CB-TE2PA)]+ was confirmed through the X-ray crystal structure. The 1H NMR paramagnetic shifts induced by Dy(III) were found to be dominated by the pseudocontact mechanism, though, for some protons, contact shifts are not negligible. The analysis of the pseudocontact shifts provided the magnetic susceptibility tensors of the three complexes, which were also investigated using CASSCF calculations. The transverse 1H relaxation data follow a good linear correlation with 1/r6, where r is the distance between the Dy(III) ion and the observed proton. This indicates that magnetic anisotropy is not significantly affecting the relaxation of 1H nuclei in the family of complexes investigated here.

Paramagnetic NMR Shielding Tensors Based on Scalar Exact Two-Component and Spin-Orbit Perturbation Theory.

The temperature-dependent Fermi-contact and pseudocontact terms are important contributions to the paramagnetic NMR shielding tensor. Herein, we augment the scalar-relativistic (local) exact two-component (X2C) framework with spin-orbit perturbation theory including the screened nuclear spin-orbit correction for the EPR hyperfine coupling and g tensor to compute these temperature-dependent terms. The accuracy of this perturbative ansatz is assessed with the self-consistent spin-orbit two-component and four-component treatments serving as reference. This shows that the Fermi-contact and pseudocontact interaction is sufficiently described for paramagnetic NMR shifts; however, larger deviations are found for the EPR spectra and the principle components of the EPR properties of heavy elements. The impact of the perturbative treatment is further compared to that of the density functional approximation and the basis set. Large-scale calculations are routinely possible with the multipole-accelerated resolution of the identity approximation and the seminumerical exchange approximation, as shown for [CeTi6O3(OiPr)9(salicylate)6].

1H NMR spectral analysis of structural features in a series of paramagnetic homoleptic binuclear triple-decker phthalocyaninato lanthanide complexes

The search for efficient methods of comparative structural investigations of coordination compounds in solid state and solutions is a very relevant task. In this study, structural assignments in a series of Ln2[(15C5)4Pc]3 complexes (where [(15C5)4Pc]2- is 2,3,9,10,16,17,24,25-tetrakis(15-crown-5)phthalocyaninato-dianion, Ln = Nd3+, Tb3+, Dy3+, Ho3+, Er3+ and Tm3+) was performed using NMR relaxation and paramagnetic lanthanide-induced shifts (LIS) data. Analysis of the paramagnetic chemical shifts and relaxation rates evidenced of isostructurality of the complete series of the investigated triple-decker complexes in the solutions. The performed systematic DFT calculations for a series of model complexes with methoxy-substituted ligands Ln2[(MeO)8Pc]3 demonstrate that the calculated geometrical factors are in agreement with characteristics extracted from experimental NMR data.

Electron correlation and vibrational effects in predictions of paramagnetic NMR shifts.

Electronic structure calculations are fundamentally important for the interpretation of nuclear magnetic resonance (NMR) spectra from paramagnetic systems that include organometallic and inorganic compounds, catalysts, or metal-binding sites in proteins. Prediction of induced paramagnetic NMR shifts requires knowledge of electron paramagnetic resonance (EPR) parameters: the electronic g tensor, zero-field splitting D tensor, and hyperfine A tensor. The isotropic part of A, called the hyperfine coupling constant (HFCC), is one of the most troublesome properties for quantum chemistry calculations. Yet, even relatively small errors in calculations of HFCC tend to propagate into large errors in the predicted NMR shifts. The poor quality of A tensors that are currently calculated using density functional theory (DFT) constitutes a bottleneck in improving the reliability of interpretation of the NMR spectra from paramagnetic systems. In this work, electron correlation effects in calculations of HFCCs with a hierarchy of ab initio methods were assessed, and the applicability of different levels of DFT approximations and the coupled cluster singles and doubles (CCSD) method was tested. These assessments were performed for the set of selected test systems comprising an organic radical, and complexes with transition metal and rare-earth ions, for which experimental data are available. Severe deficiencies of DFT were revealed but the CCSD method was able to deliver good agreement with experimental data for all systems considered, however, at substantial computational costs. We proposed a more computationally tractable alternative, where the A was computed with the coupled cluster theory exploiting locality of electron correlation. This alternative is based on the domain-based local pair natural orbital coupled cluster singles and doubles (DLPNO-CCSD) method. In this way the robustness and reliability of the coupled cluster theory were incorporated into the modern formalism for the prediction of induced paramagnetic NMR shifts, and became applicable to systems of chemical interest. This approach was verified for the bis(cyclopentadienyl)vanadium(II) complex (Cp2V; vanadocene), and the metal-binding site of the Zn2+ → Co2+ substituted superoxide dismutase (SOD) metalloprotein. Excellent agreement with experimental NMR shifts was achieved, which represented a substantial improvement over previous theoretical attempts. The effects of vibrational corrections to orbital shielding and hyperfine tensor were evaluated and discussed within the second-order vibrational perturbation theory (VPT2) framework.

Paramagnetic 7Li NMR Shifts and Magnetic Properties of Divalent Transition Metal Silylamide Ate Complexes [LiM{N(SiMe3)2}3] (M2+ = Mn, Fe, Co).

7Li NMR shifts and magnetic properties have been determined for three so-called ate complexes [LiM{N(SiMe3)2}3] (M2+ = Mn, Fe, Co; e.g., named lithium-tris(bis(trimethylsilylamide))-manganate(II) in accordance with a formally negative charge assigned to the complex fragment [M{N(SiMe3)2}3]-, which comprises the transition metal). They are formed by addition reactions of LiN(SiMe3)2 and [M{N(SiMe3)2}2] and stabilized by Lewis base/Lewis acid interactions. The results are compared to those of the related "ion-separated" complexes [Li(15-crown-5)][M{N(SiMe3)2}3]. The ate complexes with the lithium atoms connected to the 3d metal atoms manganese, iron, or cobalt via μ2 nitrogen bridges reveal strong 7Li NMR paramagnetic shifts of about -75, 125, and 171 ppm, respectively, whereas the shifts for the lithium ions coordinated by the 15-crown-5 ether are close to zero. The observed trends of the 7Li NMR shifts are confirmed by density-functional theory calculations. The magnetic dc and ac properties display distinct differences for the six compounds under investigation. Both manganese compounds, [LiMn{N(SiMe3)2}3] and [Li(15-crown-5)][Mn{N(SiMe3)2}3], display almost pure and ideal spin-only paramagnetic behavior of a 3d5 high-spin complex. In this respect slightly unexpected, both complexes show slow relaxation behavior at low temperatures under applied dc fields, which is especially pronounced for the ate complex [LiMn{N(SiMe3)2}3]. Dc magnetic properties of the iron complexes reveal moderate g-factor anisotropies with small values of the axial magnetic anisotropy parameter D and a larger E (transversal anisotropy). Both complexes display at low temperatures and, under external dc fields of up to 5000 Oe, only weak ac signals with no maxima in the frequency range from 1 to 1500 s-1. In contrast, the two cobalt complexes display strong g-factor anisotropies with large values of D and E. In addition, in both cases, the ac measurements at low temperatures and applied dc fields reveal two, in terms of their frequency range, well separated relaxation processes with maxima lying for the most part outside of the measurement range between 1 and 1500 s-1.

Local Structure and Magnetism of La1–xMxPO4 (M = Sm, 239Pu, 241Am) Explained by Experimental and Computational Analyses

With their high chemical and self-irradiation stability, crystalline monazites are among the most promising materials for the encapsulation of nuclear wastes. Yet, the local and magnetic structures of the matrices doped with low-content actinide cation, depicted as most resistant, are still unclear. This limits the development of theoretical approaches predicting their behavior under extreme conditions—self-irradiation and long-term leaching. Here, we characterize the model matrices La1–xMxPO4 (0 ≤ x ≤ 0.10)—with M = Sm, 239Pu, 241Am—by X-ray diffraction and solid-state 31P NMR. As an example, we confirm that La0.96241Am0.04PO4 has higher self-irradiation resistance compared to 241AmPO4. Further, computational analyses show that magnetic properties of the Pu complex are strongly affected by the J-mixing and the paramagnetic NMR shifts are dominated by the Fermi contact contribution, arising from delocalization of the spin density of the cation toward the phosphorus through the bonds.

Histidine orientation in artificial peroxidase regioisomers as determined by paramagnetic NMR shifts.

Fe-Mimochrome VI*a is a synthetic peroxidase and peroxygenase, featuring two different peptides that are covalently-linked to deuteroheme. To perform a systematic structure/function correlation, we purposely shortened the distance between the distal peptide and the heme, allowing for the separation and characterization of two regioisomers. They differ in both His axial-ligand orientation, as determined by paramagnetic NMR shifts, and activity. These findings highlight that synthetic metalloenzymes may provide an efficient tool for disentangling the role of axial ligand orientation over peroxidase activity.

Remarkable reversal of 13C-NMR assignment in d1, d2 compared to d8, d9 acetylacetonate complexes: analysis and explanation based on solid-state MAS NMR and computations.

13C solid-state MAS NMR spectra of a series of paramagnetic metal acetylacetonate complexes; [VO(acac)2] (d1, S = ½), [V(acac)3] (d2, S = 1), [Ni(acac)2(H2O)2] (d8, S = 1), and [Cu(acac)2] (d9, S = ½), were assigned using modern NMR shielding calculations. This provided a reliable assignment of the chemical shifts and a qualitative insight into the hyperfine couplings. Our results show a reversal of the isotropic 13C shifts, δiso(13C), for CH3 and CO between the d1 and d2versus the d8 and d9 acetylacetonate complexes. The CH3 shifts change from about -150 ppm (d1,2) to roughly 1000 ppm (d8,9), whereas the CO shifts decrease from 800 ppm to about 150 ppm for d1,2 and d8,9, respectively. This was rationalized by comparison of total spin-density plots and computed contact couplings to those corresponding to singly occupied molecular orbitals (SOMOs). This revealed the interplay between spin delocalization of the SOMOs and spin polarization of the lower-energy MOs, influenced by both the molecular symmetry and the d-electron configuration. A large positive chemical shift results from spin delocalization and spin polarization acting in the same direction, whereas their cancellation corresponds to a small shift. The SOMO(s) for the d8 and d9 complexes are σ-like, implying spin-delocalization on the CH3 and CO groups of the acac ligand, cancelled only for CO by spin polarization. In contrast, the SOMOs of the d1 and d2 systems are π-like and a large CO-shift results from spin polarization, which accounts for the reversed assignment of δiso(13C) for CH3 and CO.

Optimizing the Ueff value for DFT+U calculation of paramagnetic solid-state NMR shifts by double Fermi-contact-shift verification

The isotropic chemical shifts can be calculated by hybrid functionals, which costs lots of computational resources. To save time, DFT+U could be employed to calculate the isotropic chemical shifts. However, the calculated properties are very sensitive to the Hubbard correction value Ueff. Here the double Fermi-contact-shift verification approach with DFT+U method is proposed with much higher computational efficiency, that is, concurrently calculate the Fermi-contact shifts on two nuclei (6Li and 17O) to predict the optimal Ueff. The optimal Ueff is also helpful to the quadrupolar coupling constant CQ, g-factor, band structure and density of states.

Ionic and Electronic Conduction in TiNb2O7.

TiNb2O7 is a Wadsley–Roth phase with a crystallographic shear structure and is a promising candidate for high-rate lithium ion energy storage. The fundamental aspects of the lithium insertion mechanism and conduction in TiNb2O7, however, are not well-characterized. Herein, experimental and computational insights are combined to understand the inherent properties of bulk TiNb2O7. The results show an increase in electronic conductivity of seven orders of magnitude upon lithiation and indicate that electrons exhibit both localized and delocalized character, with a maximum Curie constant and Li NMR paramagnetic shift near a composition of Li0.60TiNb2O7. Square-planar or distorted-five-coordinate lithium sites are calculated to invert between thermodynamic minima or transition states. Lithium diffusion in the single-redox region (i.e., x ≤ 3 in LixTiNb2O7) is rapid with low activation barriers from NMR and DLi = 10–11 m2 s–1 at the temperature of the observed T1 minima of 525–650 K for x ≥ 0.75. DFT calculations predict that ionic diffusion, like electronic conduction, is anisotropic with activation barriers for lithium hopping of 100–200 meV down the tunnels but ca. 700–1000 meV across the blocks. Lithium mobility is hindered in the multiredox region (i.e., x > 3 in LixTiNb2O7), related to a transition from interstitial-mediated to vacancy-mediated diffusion. Overall, lithium insertion leads to effective n-type self-doping of TiNb2O7 and high-rate conduction, while ionic motion is eventually hindered at high lithiation. Transition-state searching with beyond Li chemistries (Na+, K+, Mg2+) in TiNb2O7 reveals high diffusion barriers of 1–3 eV, indicating that this structure is specifically suited to Li+ mobility.

Unravelling the Complexities of Pseudocontact Shift Analysis in Lanthanide Coordination Complexes of Differing Symmetry.

In two closely related series of eight-coordinate lanthanide complexes, a switch in the sign of the dominant ligand field parameter and striking variations in the sign, amplitude and orientation of the main component of the magnetic susceptibility tensor as the Ln3+ ion is permuted conspire to mask modest changes in NMR paramagnetic shifts, but are evident in Yb EPR and Eu emission spectra.

Unravelling the Complexities of Pseudocontact Shift Analysis in Lanthanide Coordination Complexes of Differing Symmetry

AbstractIn two closely related series of eight‐coordinate lanthanide complexes, a switch in the sign of the dominant ligand field parameter and striking variations in the sign, amplitude and orientation of the main component of the magnetic susceptibility tensor as the Ln3+ ion is permuted conspire to mask modest changes in NMR paramagnetic shifts, but are evident in Yb EPR and Eu emission spectra.

Synthesis and Spin State of the Cobalt(II) Complexes with Substituted 2,6-Bis(pyrazol-3-yl)pyridine Ligands

The direct template reactions afford the cobalt(II) complexes (I–III) with the hydroxy- and acetyl-substituted 2,6-bis(pyrazol-3-yl)pyridine ligands containing the phenyl and 4-tert-butylphenyl group in position 1. All compounds are isolated in the individual state and characterized by elemental analysis and NMR spectroscopy. The X-ray diffraction data obtained for crystals of compound I (СIF file CCDC 1577238) and the data for solutions (using the proposed approach to analysis of paramagnetic NMR shifts on the basis of quantum chemical calculations) show that in the complexes the metal ion exists in the high-spin state (S = 3/2) and undergoes no temperature-induced spin transition in a temperature range of 120–300 K.

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

A detailed analysis of paramagnetic NMR shifts in a series of isostructural lanthanide complexes relavant to PARASHIFT contrast agents reveals unexpected trends in the magnetic susceptibility anisotropy that cannot be explained by the commonly used Bleaney's theory. Ab initio calculations reveal that the primary assumption of Bleaney's theory-that thermal energy is larger than the ligand field splitting-does not hold for the lanthanide complexes in question, and likely for a large fraction of lanthanide complexes in general. This makes the orientation of the magnetic susceptibility tensor differ significantly between complexes of different lanthanides with the same ligand: one of the most popular assumptions about isostructural lanthanide series is wrong.

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

AbstractA detailed analysis of paramagnetic NMR shifts in a series of isostructural lanthanide complexes relavant to PARASHIFT contrast agents reveals unexpected trends in the magnetic susceptibility anisotropy that cannot be explained by the commonly used Bleaney's theory. Ab initio calculations reveal that the primary assumption of Bleaney's theory—that thermal energy is larger than the ligand field splitting—does not hold for the lanthanide complexes in question, and likely for a large fraction of lanthanide complexes in general. This makes the orientation of the magnetic susceptibility tensor differ significantly between complexes of different lanthanides with the same ligand: one of the most popular assumptions about isostructural lanthanide series is wrong.