Abstract: Molecular mobility, specifically the correlation times of rotational motion in solution, is a crucial characteristic of substances, influenced by molecular size, temperature, and solvent properties. This parameter significantly affects spin relaxation times, as it determines whether the molecular motion is slow or fast relative to the Larmor frequency.[1] Field-cycling Nuclear Magnetic Resonance (NMR) relaxometry is a powerful technique for investigating molecular mobility and is applicable to a wide range of systems, including molecules in solution, polymers, molecular and liquid crystals, as well as biomolecules.[2-3] Previously, our group determined the correlation times of the neutral iridium complex [Ir(IMes)(COD)Cl] in methanol.[4] Here, COD refers to cyclooctadiene, and the structure of the IMes carbene is shown in Figure 1. This complex is one of the most widely used precatalysts in the SABRE (Signal Amplification By Reversible Exchange) method, which has proven to be an efficient and convenient approach for enhancing the NMR signal of molecules capable of reversibly binding non-covalently to an iridium hydride complex that continuously exchanges hydrogen atoms with dissolved para-hydrogen.[5-6] In this study, we found that bubbling the pre-catalyst [Ir(IMes)(COD)Cl] with parahydrogen in an aqueous solution containing 1% pyridine led to the formation of a stable, positively charged iridium dihydride complex, [Ir(IMes)(Pyeq)2(Pyax)H2]Cl, featuring three pyridine ligands and the IMes carbene. Previously, the crystal structure of this complex was determined by X-ray diffraction using crystals obtained from organic solvents.[7] Additionally, the activation of the pre-catalyst in the presence of pyridine and nicotinamide was investigated by indirect detection via para-hydrogen.[8] SABRE experiments were conducted in a strong magnetic field,[9] and NMR parameters of the complex in methanol-d4 were measured.[10] Here, we present the results of proton T1 relaxation time measurements across a broad magnetic field range, from 0.5 T to 16.4 T, using a custom-built fast magnetic field cycling setup based on a high-resolution 700 MHz NMR spectrometer.[11] A sample containing 1.1 mg of the pre-catalyst (synthesized according to the method described in ref. [12]), 6 μl of pyridine, 30 μl of D2O, and 589 μl of H2O was bubbled with hydrogen at 4 atm pressure and 45 °C in an NMR tube for 15 minutes. The color change from yellow to colorless indicated the completion of the cyclooctadiene hydrogenation reaction and the formation of a stable iridium dihydride complex, stabilized by three pyridine molecules and the IMes carbene. The resulting solution's ¹H NMR spectrum is shown in Figure 1, with 13C satellite suppression applied during FID acquisition. T1 relaxation time measurements were performed using the inversion recovery method on a custom-built setup designed for rapid vertical displacement of the sample within the cryomagnet. The magnetic field for relaxation was determined by the sample’s position relative to the center of the cryomagnet bore. The experimental protocol was as follows: -Relaxation in a 16.4 T field for 15 s to establish initial Boltzmann polarization. -Application of a 180-degree hard pulse to invert the magnetization of all protons. -Rapid transfer of the sample to the selected magnetic field (within 290 ms). -Holding the sample in the B0 field for a variable time (0.01 to 10 s) to allow relaxation. -Returning the sample to the detection field. -A 150 ms delay to allow mechanical vibrations to settle. NMR signal acquisition using excitation sculpting[13] sequences to suppress the strong H2O solvent signal, along with 13C decoupling to eliminate pyridine satellite signals. Each relaxation kinetics dataset consisted of 16 time points and was modeled using a decaying exponential function, yielding a set of T1 values for each signal in the NMR spectra. The dependence of T1 on the magnetic field was then plotted and fitted using equation (1), derived from the nuclear relaxation model based on local magnetic field fluctuations (Figure 2). As a result, the rotational correlation times ( c) for the protons in the complex were determined and are summarized in Table 1. Table 1. Site specific rotational correlation times (τc) for the complex Ir(IMes)(Pyeq)2(Pyax)H2]Cl in aqueous solution at 25 oC. Proton m-Pyax IMes-H4,5 IMes-H3,5 IMes-CH3-H4IMes-CH3-H2,6` Ir-HH Common τc (ps) 294±13 394±119 303±10 217±16 259±6 208±28 280±7 Conclusions This study presents site-specific measurements of rotational correlation times for the [Ir(IMes)(Pyeq)2(Pyax)H2]Cl complex, revealing significantly shorter values than those previously reported for its precursor, [Ir(IMes)(COD)Cl], in methanol,[4] despite the larger size of the iridium hydride complex and the higher viscosity of the solvent (water). Variations in correlation times among different protons indicate that the molecule is not rigid. The pronounced independence of T1 relaxation times for the equatorial pyridine (Pyeq) protons is attributed to chemical exchange with free pyridine in solution, where the exchange rate exceeds the nuclear relaxation rate, consistent with data obtained in methanol.[14] In contrast, the axial pyridine (Pyax) protons exhibit much shorter T₁ relaxation times, with a strong magnetic field dependence similar to that observed for the protons of the tightly bound IMes ligand. Acknowledgements This work was supported by the Russian Science Foundation (project #25-23-00607)

Cite: Kiryutin A.S. , Markelov D.A. , Yurkovskaya A.V.
Determination of Rotational Correlation Time of and Iridium Dihydride Complex in Aqueous Medium by Means of NMR Relaxometry with High-resolution
22nd International School-Conference MAGNETIC RESONANCE AND ITS APPLICATIONS 31 Mar - 4 Apr 2025