Large³¹P-NMR enhancements in liquid state dynamic nuclear polarization through radical/target molecule non-covalent interaction

Abstract

Large 31P-NMR enhancements are observed with DNP in PPh3 doped with BDPA radical, while they are reduced when a nitroxide radical or triphenylphosphine-oxide are used instead. This is due to different non-covalent radical/target molecule interactions.

Probing Homogeneous Catalysts and Precatalysts in Solution by Exchange-Mediated Overhauser Dynamic Nuclear Polarization NMR

“…Free PPh 3 is readily hyperpolarized by OE DNP through a transient scalar hyperfine interaction between the unpaired electron in the radical center of (α,γ-bisdiphenylene-βphenylallyl (BDPA), Scheme 2) and the 31 P nuclear spin that is mediated by the lone pair of the PPh 3 . However, upon coordination of phosphorus to the metal center, the lone pair is no longer available to interact with the BDPA, obliterating the effect.…”

“…39 Therefore, in principle, the chemical exchange can be exploited to allow hyperpolarized free PPh 3 to back coordinate the metal center, leading to hyperpolarized coordinated PPh 3 . In order for this to occur, the exchange rate must be faster than the 31 P longitudinal relaxation rate (1/ T 1 ) of free PPh 3 , otherwise, the hyperpolarization generated on the free ligands would relax before an exchange event takes place, on average. 40 On the other hand, the exchange rate must not exceed the chemical shift difference between free and coordinated PPh 3 (slow chemical exchange regime) so that a distinct DNP-enhanced 31 P signal can be detected for the coordinated PPh 3 .…”

“…In order for this to occur, the exchange rate must be faster than the 31 P longitudinal relaxation rate (1/ T 1 ) of free PPh 3 , otherwise, the hyperpolarization generated on the free ligands would relax before an exchange event takes place, on average. 40 On the other hand, the exchange rate must not exceed the chemical shift difference between free and coordinated PPh 3 (slow chemical exchange regime) so that a distinct DNP-enhanced 31 P signal can be detected for the coordinated PPh 3 . To fulfill these requirements, we propose to use an excess of free PPh 3 , which is a common condition used for many catalytic reactions, such as hydroformylation, 41 and to vary the temperature to modulate the exchange rates and improve hyperpolarization transfer efficiency.…”

“…Due to the loss of this lone pair upon coordination to a metal site, 31 no scalar OE DNP effects have been observed in PPh 3 -bound complexes thus far. 31−35 Here, we demonstrate an efficient approach to obtain DNPenhanced 31 P NMR spectra of coordinated PPh 3 ligands based on chemical exchange between coordinated PPh 3 and hyperpolarized free PPh 3 . We apply the proposed method to a series of prototypical transition-metal complexes used in homogeneous catalysis, namely, [Rh(PPh 3 ) 3 Cl] (the Wilkinson catalyst), [Ru(PPh 3 ) 3 Cl 2 ], [Pd(PPh 3 ) 2 Cl 2 ], and [Pt-(PPh 3 ) 2 Cl 2 ] (Scheme 1).…”

13C Hyperpolarization of Viscous Liquids by Transfer of Solid-Effect1H Dynamic Nuclear Polarization at High Magnetic Field

“…In liquids, direct DNP polarization is mainly applied to various nuclei (e.g., 1 H, 9 Li, 13 C, 15 N, 19 F, 23 Na, or 31 P), − where only one mechanism is operational: the Overhauser effect. , In liquid solutions, the electron spin interacts with the nuclear spin through dipolar and scalar interactions. If the scalar coupling dominates over the dipolar one (as commonly observed for 13 C, 15 N, 19 F, or 31 P nuclei), then the Overhauser DNP can yield outstanding signal enhancements at magnetic fields of ≥1 T, − although usually it requires specific chemical systems that involve a strong contact interaction between the electron and nuclear spins. On the other hand, the dipolar Overhauser effect (as in the case of 1 H) has only limited applications owing to low DNP efficiency at high magnetic fields. , Therefore, to increase the 1 H polarization, a technique was proposed recently for transferring 13 C hyperpolarization produced by scalar Overhauser DNP to the attached 1 H spins via the polarization transfer scheme. , …”