Modeling Ligand Exchange Kinetics in Iridium Complexes Catalyzing SABRE Nuclear Spin Hyperpolarization

In the research article recently published in ACS Analytical Chemistry journal the international team of researchers from International Tomography Center SB RAS, Kiel University, University of York, and Wayne State University analyzed the dissociation rates of model substrates in Signal Amplification By Reversible Exchange (SABRE) NMR spectroscopy experiment, evaluating kinetic models, and determining key activation parameters.

The article discusses the process of nuclear spin hyperpolarization via Signal Amplification By Reversible Exchange (SABRE), which significantly enhances NMR signal intensities for various analytes. SABRE relies on the interaction of parahydrogen with organometallic complexes, allowing for the transfer of hyperpolarization to analytes through chemical exchange. The study focuses on analyzing the dissociation rates of specific model substrates, evaluating kinetic models, and determining key activation parameters. This research also aims to establish a reliable methodology for quantifying chemical exchange in SABRE, facilitating the optimization of SABRE-based catalysts.

The original article

Modeling Ligand Exchange Kinetics in Iridium Complexes Catalyzing SABRE Nuclear Spin Hyperpolarization

Oleg G. Salnikov, Charbel D. Assaf, Anna P. Yi, Simon B. Duckett, Eduard Y. Chekmenev, Jan-Bernd Hövener, Igor V. Koptyug, and Andrey N. Pravdivtsev

Analytical Chemistry 2024 96 (29), 11790-11799

DOI: 10.1021/acs.analchem.4c01374

licensed under CC-BY 4.0

Selected sections from the article follow.

Abstract

Large signal enhancements can be obtained for NMR analytes using the process of nuclear spin hyperpolarization. Organometallic complexes that bind parahydrogen can themselves become hyperpolarized. Moreover, if parahydrogen and a to-be-hyperpolarized analyte undergo chemical exchange with the organometallic complex it is possible to catalytically sensitize the detection of the analyte via hyperpolarization transfer through spin–spin coupling in this organometallic complex. This process is called Signal Amplification By Reversible Exchange (SABRE). Signal intensity gains of several orders of magnitude can thus be created for various compounds in seconds. The chemical exchange processes play a defining role in controlling the efficiency of SABRE because the lifetime of the complex must match the spin–spin couplings. Here, we show how analyte dissociation rates in the key model substrates pyridine (the simplest six-membered heterocycle), 4-aminopyridine (a drug), and nicotinamide (an essential vitamin biomolecule) can be examined. This is achieved for the most widely employed SABRE motif that is based on IrIMes-derived catalysts by 1H 1D and 2D exchange NMR spectroscopy techniques. Several kinetic models are evaluated for their accuracy and simplicity. By incorporating variable temperature analysis, the data yields key enthalpies and entropies of activation that are critical for understanding the underlying SABRE catalyst properties and subsequently optimizing behavior through rational chemical design. While several studies of chemical exchange in SABRE have been reported, this work also aims to establish a toolkit on how to quantify chemical exchange in SABRE and ensure that data can be compared reliably.

Introduction

Nuclear spin hyperpolarization techniques are reshaping the field of NMR spectroscopy and imaging by dramatically enhancing sensitivity. (1) Dissolution dynamic nuclear polarization (dDNP) (2) employs high thermal equilibrium polarization of electron spins at low temperatures and high magnetic fields as a source of hyperpolarization, with polarization transfer induced by the application of microwave irradiation of the solid sample with a subsequent rapid sample dissolution. (3) As a result, the dDNP technique requires expensive and complex equipment. Alternatively, the sensetivity of liquid-state NMR spectroscopy can be enhanced using the singlet nuclear spin isomer of dihydrogen (parahydrogen, pH2) as a hyperpolarization source. (4) Here, hyperpolarization is often achieved by pairwise addition of pH2 to an unsaturated substrate in the parahydrogen-induced polarization (PHIP) (5,6) experiment, or by transient coordination of both pH2 and a substrate to an organometallic complex in the signal amplification by reversible exchange (SABRE) experiment (Figure 1). (7,8) In the latter case, the transiently formed organometallic complex enables polarization transfer from the nascent H atoms that originate from pH2 to the nuclear spins of the coordinated substrate. This polarization transfer can be achieved spontaneously (by free evolution in an appropriate magnetic field) (7) or driven by specially designed radiofrequency (RF) pulse sequences. (9)

Anal. Chem. 2024, 96, 29, 11790-11799: Figure 1. Scheme of SABRE hyperpolarization of pyridine. pH2 and a substrate (here: pyridine, Py) bind to form a transient complex, [Ir(H)2(IMes)(Py)3]Cl. Spin–spin interactions then drive spin order from IrHH (the pH2-derived hydride spins) to the Py substrate, resulting in two hyperpolarized equatorial substrate ligands (red Py) and orthohydrogen (oH2). IMes stands for 1,3-bis(2,4,6-trimethylphenyl)imidazole-2-ylidene. The two equatorial chemically equivalent pyridine ligands exchange via a dissociative (SN1) mechanism. (22)

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Methods

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NMR Measurements

Each sample was supplied with H2 at 15 standard cubic centimeters per minute (sccm) gas flow rate, 7.9 bar, and room temperature until the SABRE precatalyst was converted completely into the SABRE-active dihydride complex, according to 1H NMR spectroscopy (25 min for Py, 30 min for NAM, 120 min for 4AP). Next, the sample was depressurized, and the catheter used to supply H2 to the solution was pulled up out of the solution by ∼12 cm (still inside the NMR tube), followed by reinitiation of H2 flow through the catheter. As a result, the sample resided under an H2 atmosphere at ambient pressure during the measurements; at the same time, gradual solvent evaporation was avoided since the gas flowed several centimeters above the solution.

NMR spectra were acquired on a 7.05 T Bruker AV 300 NMR spectrometer at several specified temperatures with a 5 mm probe without gradients. At each temperature, the following 1H NMR spectra were recorded: a 2D EXSY spectrum with a mixing time dmix (using Bruker TopSpin noesyph pulse sequence, Figure 2A), a regular 1D spectrum (zg Bruker TopSpin sequence), and several 1D selective EXSY (SEXSY) spectra with variable mixing times (dmix here is the variable d8 in a selno Bruker TopSpin pulse sequence, Figure 2B). The selective RF excitation (90° Gauss-shaped pulse, 40 ms, corresponding to 52.5 Hz excitation bandwidth) was tuned to excite the frequency of equatorially bound substrate protons (α-protons for Py, H-5 protons for NAM, and β-protons for 4AP). Hence, the corresponding resonances for the equilibrium concentration of the free materials were initially unencoded, and they only became visible through chemical exchange.

Anal. Chem. 2024, 96, 29, 11790-11799: Figure 2. EXSY (A) and SEXSY (B) NMR pulse sequences. The rounded pulse in the diagram is frequency-selective. Several delays were applied: d1 stands for relaxation delay, d0 is indirect dimension encoding delay, d14 stands for evolution after shaped pulse, and dmix (typically d8 in TopSpin) stands for mixing time. Phases: φ1 = [0°, 180°], φ2 = 0°, and φ3 = [0°, 0°, 180°, 180°, 90°, 90°, 270°, 270°], φrec = [0°, 180°, 180°, 0°, 90°, 270°, 270°, 90°]. In SEXSY sequence, the phase φ1 and dmix were selected with a random deviation (: r) up to 5%.

For T1 relaxation measurements, the sample preparation and handling procedures were the same as described above, except that the iridium catalyst was not added to the solution. The samples were bubbled with H2 at 15 sccm, 7.9 bar, at room temperature for 30–50 min to displace any dissolved air. T1 was measured using conventional inversion recovery sequence (t1ir sequence in Bruker TopSpin).
Corresponding chemical shifts, T1 relaxation times of protons of these substrates, and detailed acquisition parameters are provided in Supporting Information (SI).

Data Processing

All spectra were analyzed using spectral data analyzing software Bruker TopSpin (4.0.7), Bruker Dynamics Center (2.5.5), MestReNova (14.2.2), and Origin (2021). Data were modeled using Origin or the MATLAB (R2021a) MOIN spin-library (24) as described in the text. The standard deviation for dissociation rates kd of SEXSY experimental data obtained using models CSS2↔CSS+S and CSS2↔S2 are calculated using the MATLAB nonlinear regression function “nlinfit”. For EXSY experimental data using model CSS2↔S2, dissociation rate deviation was calculated using eq 32 of ref. (44) As instructed, the intensity variances have been estimated by assuming a precision (noise and errors due to signal overlapping) of 10% for diagonal and cross peaks.

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Conclusions

The EXSY sequence is generally more time-consuming and less robust than the SEXSY sequence. Lower spectral resolution in indirect dimension in EXSY reduces the precision of spectral integration. However, it has benefits at higher temperatures where rapid chemical exchange occurs during the selective pulse of SEXSY. Therefore, SEXSY is preferable and more accurate than EXSY for a relatively slow exchange compared to the selective excitation pulse duration. Both SEXSY and EXSY sequences could not be applied to acetonitrile and metronidazole (data not shown here) as the substrate 1H chemical shifts almost do not change upon association with IrIMes. In such cases, or when deuterium labeling is used to prolong relaxation, one should use heteronuclei labeling and EXSY, spin order transfer, or a combination of both. (35)

Using two different exchange models, three approaches for fitting SEXSY and one approach for fitting EXSY experiments, we obtained comparable exchange rates. We demonstrated a connection between the complete SABRE model CSS2↔CSS+S and the reduced model CSS2↔S2 often used for the polarization transfer simulations. (23) As a result, we identified some new limitations for using the CSS2↔S2 model: effective lifetime of the complex and mass balance conditions cannot be immediately satisfied. An ideal solution for spin-dynamic simulations would be to use the model CSS2↔CSS+S; however, such an approach is slow because this model is inevitably nonlinear, and we do not have sufficient information on the elusive intermediate CSS. An important aspect is that for bidentate ligands like pyruvate (13) the exchange model C0S↔S, the often-used SABRE model, should work without any identified restraints. Hence, it can be used to optimize spin order transfer to pyruvate, which was already utilized for in vivo imaging. (41,42)