Para-Hydrogen Induced Polarization without Incorporation ofPara-Hydrogen into the Analyte

Abstract

The cationic iridium complexes [Ir(COD)(PR3)2]BF4 (1a-c) (a, R = Ph; b, R = p-tolyl; c, R = p-C6H4-OMe) react with parahydrogen in the presence of pyridine to give trans, cis, cis-[Ir(PR3)2(py)2(H)2]+ (2a-c) and small amounts of fac, cis-[Ir(PR3)(py)3(H)2]+ (3a-c), each of which exhibit polarized hydride resonances due to the magnetic inequivalence associated with the resultant AA"XX" spin system when 15N-labeled pyridine is employed. The pyridine ligands in 2 are labile, exchanging slowly into free pyridine with a rate constant of 0.4 s(-1) for 2a at 335 K in a dissociative process where DeltaH(double dagger) = 134 +/- 1 kJ mol(-1) and DeltaS(double dagger) = 151 +/- 5 J mol(-1) K(-1). Pyridine ligand exchange in 2 proves to be slower than that determined for 3. Parahydrogen induced polarization (PHIP) based on the hydride ligands of 2 and 3 is transferred efficiently to the 15N nuclei of the bound pyridine ligand by suitable insensitive-nuclei-enhanced-by-polarization-transfer (INEPT) based procedures. Related methods are then used to facilitate the sensitization of the free pyridine 15N signal by a factor of 120-fold through ligand exchange even though this substrate does not contain parahydrogen. This therefore corresponds to the successful polarization of an analyte by parahydrogen induced polarization methods without the need for the actual chemical incorporation of any parahydrogen derived nuclei into it.

Hyperpolarizing Concentrated Metronidazole 15NO2 Group over Six Chemical Bonds with More than 15 % Polarization and a 20 Minute Lifetime

“…[26][27][28][29][30][31][32][33] In this work we employed the SABRE in the shield enables alignment transfer to heteronuclei (SABRE-SHEATH) [34,35] variant of SABRE, which relies on simultaneous chemical exchange between parahydrogen (pH 2 )a nd the to-be-hyperpolarizedb iomolecules (without alteringt heir structures) within the ligand sphere of ac atalyst in microtesla (mT) magnetic fields (Figure 1a). [26][27][28][29][30][31][32][33] In this work we employed the SABRE in the shield enables alignment transfer to heteronuclei (SABRE-SHEATH) [34,35] variant of SABRE, which relies on simultaneous chemical exchange between parahydrogen (pH 2 )a nd the to-be-hyperpolarizedb iomolecules (without alteringt heir structures) within the ligand sphere of ac atalyst in microtesla (mT) magnetic fields (Figure 1a).…”

Nuclear spin hyperpolarization of the solvent using signal amplification by reversible exchange (SABRE)

“…More recently it has been shown that iridium N-heterocyclic carbene (NHC) complexes have greater electron donating ability and optimum lifetimes, increasing the efficiency of polarization transfer [28–30]. With few exceptions, most SABRE substrates are aromatic nitrogen-containing compounds where the basic nitrogen acts as the electron donor to the metal, likely reflecting the preference of third row metal ions such as platinum and iridium for somewhat softer N-based ligands [21–23,25–35]. The SABRE experiment is carried out by dissolving the complex and substrate in a methanolic solution and bubbling or shaking the sample with several atmospheres of p-H 2 in the fringe field of the NMR magnet or in a polarizer equipped to generate fields on the order of several mT [36,37].…”

On the spin order transfer from parahydrogen to another nucleus

“…With the exception of signal amplification by reversible exchange (SABRE) [21,22] and field-cycling methods [18,23], these techniques rely on a spin-order-transfer sequence (SOT) composed of evolution intervals and r.f. pulses tailored to the J-coupling between the hydrogen nuclei originating from pH 2 and the nucleus to be polarized (e.g.…”