Characterization of Paramagnetic Reactive Intermediates: Predicting the NMR Spectra of Iron(IV)-Oxo Complexes by DFT

NMR and Mössbauer Studies Reveal a Temperature-Dependent Switch from S = 1 to 2 in a Nonheme Oxoiron(IV) Complex with Faster C-H Bond Cleavage Rates

S = 2 FeIV═O centers generated in the active sites of nonheme iron oxygenases cleave substrate C-H bonds at rates significantly faster than most known synthetic FeIV═O complexes. Unlike the majority of the latter, which are S = 1 complexes, [FeIV(O)(tris(2-quinolylmethyl)amine)(MeCN)]2+ (3) is a rare example of a synthetic S = 2 FeIV═O complex that cleaves C-H bonds 1000-fold faster than the related [FeIV(O)(tris(pyridyl-2-methyl)amine)(MeCN)]2+ complex (0). To rationalize this significant difference, a systematic comparison of properties has been carried out on 0 and 3 as well as related complexes 1 and 2 with mixed pyridine (Py)/quinoline (Q) ligation. Interestingly, 2 with a 2-Q-1-Py donor combination cleaves C-H bonds at 233 K with rates approaching those of 3, even though Mössbauer analysis reveals 2 to be S = 1 at 4 K. At 233 K however, 2 becomes S = 2, as shown by its 1H NMR spectrum. These results demonstrate a unique temperature-dependent spin-state transition from triplet to quintet in oxoiron(IV) chemistry that gives rise to the high C-H bond cleaving reactivity observed for 2.

Observability of Paramagnetic NMR Signals at over 10 000 ppm Chemical Shifts

We report an experimental observation of 31 P NMR resonances shifted by over 10 000 ppm (meaning percent range, and a new record for solutions), and similar 1 H chemical shifts, in an intermediate-spin square planar ferrous complex [tBu (PNP)Fe-H], where PNP is a carbazole-based pincer ligand. Using a combination of electronic structure theory, nuclear magnetic resonance, magnetometry, and terahertz electron paramagnetic resonance, the influence of magnetic anisotropy and zero-field splitting on the paramagnetic shift and relaxation enhancement is investigated. Detailed spin dynamics simulations indicate that, even with relatively slow electron spin relaxation (T1 ≈10-11  s), it remains possible to observe NMR signals of directly metal-bonded atoms because pronounced rhombicity in the electron zero-field splitting reduces nuclear paramagnetic relaxation enhancement.

NMR Reveals That a Highly Reactive Nonheme FeIV =O Complex Retains Its Six-Coordinate Geometry and S=1 State in Solution

The [Fe^IV (O)(Me₃ NTB)]²⁺ (Me₃ NTB=tris[(1-methyl-benzimidazol-2-yl)methyl]amine) complex 1 has been shown by Mössbauer spectroscopy to have an S=1 ground state at 4 K, but is proposed to become an S=2 trigonal-bipyramidal species at higher temperatures based on a DFT model to rationalize its very high C-H bond-cleavage reactivity. In this work, ¹ H NMR spectroscopy was used to determine that 1 does not have C₃ -symmetry in solution and is not an S=2 species. Our results show that 1 is unique among nonheme Fe^IV =O complexes in retaining its S=1 spin state and high reactivity at 193 K, providing evidence that S=1 Fe^IV =O complexes can be as reactive as their S=2 counterparts. This result emphasizes the need to identify factors besides the ground spin state of the Fe^IV =O center to rationalize nonheme oxoiron(IV) reactivity.

Structural implications of the paramagnetically shifted NMR signals from pyridine H atoms on synthetic nonheme FeIV=O complexes

Oxoiron(IV) motifs are found in important intermediates in many enzymatic cycles that involve oxidations. Over half of the reported synthetic nonheme oxoiron(IV) analogs incorporate heterocyclic donors, with a majority of them comprising pyridines. Herein, we report ¹H-NMR studies of oxoiron(IV) complexes containing pyridines that are arranged in different configurations relative to the Fe = O unit and give rise to paramagnetically shifted resonances that differ by as much as 50 ppm. The strong dependence of ¹H-NMR shifts on the different configurations and orientation of pyridines relative to the oxoiron(IV) unit demonstrates how unpaired electronic spin density of the iron center affects the chemical shifts of these protons.

Toward the comprehensive calculations on the relationship between 1 H, 13 C, 31 P chemical shifts, 2 JPH , and the bonding structure of different phosphoryl benzamides

A comprehensive investigation was performed on ¹ H, ¹³ C, and ³¹ P nuclear magnetic resonance (NMR) chemical shifts (CSs) of phosphoryl benzamide derivatives (C₆ H₅ C(O)NHP(O)R₁ R₂ ), (R₁ , R₂  = aziridine [L₁ ], azetidine [L₂ ], pyrrolidine [L₃ ], piperidine [L₄ ], azepane [L₅ ], 4-methylpiperidine [L₆ ], propane-2-amine [L₇ ], and 2-methylpropane-2-amine [L₈ ]) by the gauge-independent atomic orbital method (GIAO) to find the most accordant level of theory with the experimental values. To achieve this goal, all the structures were optimized using the B3LYP, BP86, PBE1PBE, M06-2X, MPWB1K, and MP2 methods with 6-31+G* basis set. Computed structural parameters demonstrate that BP86 has the best agreement to the experimental values between the other methods. The def2-TZVP and aug-cc-pVDZ basis sets were also employed to inspect the effect of different types of basis sets with higher polarization and diffuse functions. The correlation between the empirical and computational values attests that 6-31+G* basis set is the optimum case regarding minimization of the costs and results. The comparison between calculated and experimental CSs at all mentioned combinations illustrated that in accordance with structural results, the best level of theory in CSs is also BP86/6-31+G*. Besides, ² J_PH values were computed with an acceptable agreement to experimental data at the optimum level of theory. The dependency between ² J_PH and the bonding structure of studied ligands was also scrutinized by the Natural Bond Orbital (NBO) analysis that interprets the relationship between the electronic properties and ² J_PH values.

Synthesis and Electronic Structure of Neutral Square-Planar High-Spin Iron(II) Complexes Supported by a Dianionic Pincer Ligand

Two square-planar high-spin Fe^II complexes bearing a dianionic pyridine dipyrrolate pincer ligand and a diethyl ether or tetrahydrofuran ligand were synthesized and structurally characterized, and their electronic structures were elucidated by a combined spectroscopic and computational approach. In contrast to previous examples, the S = 2 ground states of these square-planar Fe^II complexes do not require an overall anionic charge of the compounds or incorporation of alkali metal cations. The tetrahydrofuran complex exhibits an equilibrium between four- and five-coordinate species in solution, which was supported by ¹H NMR and ⁵⁷Fe Mössbauer spectroscopy and comparison to a structurally characterized five-coordinate pyridine dipyrrolate iron bis-pyridine adduct. A detailed computational analysis of the electronic structures of the four- and five-coordinate species via density functional theory provides insight into the origins of the unusual ground state configurations for Fe^II in a square-planar ligand field and explains the associated characteristic spectroscopic parameters.

Paramagnetic NMR as a new tool in structural biology

NMR (nuclear magnetic resonance) investigation through the exploitation of paramagnetic effects is passing from an approach limited to few specialists in the field to a generally applicable method that must be considered, especially for the characterization of systems hardly affordable with other techniques. This is mostly due to the fact that paramagnetic data are long range in nature, thus providing information for the structural and dynamic characterization of complex biomolecular architectures in their native environment. On the other hand, this information usually needs to be complemented by data from other sources. Integration of paramagnetic NMR with other techniques, and the development of protocols for a joint analysis of all available data, is fundamental for achieving a comprehensive characterization of complex biological systems. We describe here a few examples of the new possibilities offered by paramagnetic data used in integrated structural approaches.

Kohn–Sham calculations of NMR shifts for paramagnetic 3d metal complexes: protocols, delocalization error, and the curious amide proton shifts of a high-spin iron(ii) macrocycle complex

Ligand chemical shifts (pNMR shifts) are analyzed using DFT. A large difference in the amide proton shifts of a high-spin Fe(ii) complex arises from O → Fe dative bonding which only transfers β spin density to the metal.