title: Nuclear spin hyperpolarization with ansa-aminoboranes: a metal-free perspective for parahydrogen-induced polarization.
abstract: The parahydrogen-induced polarization (PHIP) phenomenon, observed when parahydrogen is used in H2addition processes, provides a means for substantial NMR signal enhancements and mechanistic studies of chemical reactions. Commonly, noble metal complexes are used for parahydrogen activation, whereas metal-free activation is rare. Herein, we report a series of unimolecular metal-free frustrated Lewis pairs based on an ansa-aminoborane (AAB) moiety in the context of PHIP. These molecules, which have a "molecular tweezers" structure, differ in their substituents at the boryl site (-H, -Ph, -o-iPr-Ph, and -Mes). PHIP effects were observed for all the AABs after exposing their solutions to parahydrogen in a wide temperature range, and experimental measurements of their kinetic and thermodynamic parameters were performed. A theoretical analysis of their nuclear spin polarization effects is presented, and the roles of chemical exchange, chemical equilibrium and spin dynamics are discussed in terms of the key dimensionless parameters. The analysis allowed us to formulate the prerequisites for achieving strong polarization effects with AAB molecules, which can be applied for further design of efficient metal-free tweezers-like molecules for PHIP. Mechanistic (chemical and physical) aspects of the observed effects are discussed in detail. In addition, we performed quantum chemical calculations, which confirmed that the J-coupling between the parahydrogen-originated protons in AAB-H2molecules is mediated through dihydrogen bonding.
authors.affiliation: Laboratory of Magnetic Resonance Microimaging, International Tomography Center SB RAS, Institutskaya St. 3A, 630090 Novosibirsk, Russia. v_zhivonitko@tomo.nsc.ru and Department of Natural Sciences, Novosibirsk State University, Pirogova St. 2, 630090 Novosibirsk, Russia.
authors.forename: Vladimir V
authors.initials: VV
authors.lastname: Zhivonitko
authors.affiliation: Department of Chemistry, University of Helsinki, A. I. Virtasen Aukio 1, 00014 Helsinki, Finland.
authors.forename: Kristina
authors.initials: K
authors.lastname: Sorochkina
authors.affiliation: Department of Chemistry, University of Helsinki, A. I. Virtasen Aukio 1, 00014 Helsinki, Finland.
authors.forename: Konstantin
authors.initials: K
authors.lastname: Chernichenko
authors.affiliation: Research Centre for Natural Sciences, Hungarian Academy of Sciences, Magyar Tudósok Körútja 2, H-1117 Budapest, Hungary.
authors.forename: Bianka
authors.initials: B
authors.lastname: Kótai
authors.affiliation: Research Centre for Natural Sciences, Hungarian Academy of Sciences, Magyar Tudósok Körútja 2, H-1117 Budapest, Hungary.
authors.forename: Tamás
authors.initials: T
authors.lastname: Földes
authors.affiliation: Research Centre for Natural Sciences, Hungarian Academy of Sciences, Magyar Tudósok Körútja 2, H-1117 Budapest, Hungary.
authors.forename: Imre
authors.initials: I
authors.lastname: Pápai
authors.affiliation: NMR Research Unit, University of Oulu, P.O. Box 3000, 90014 Oulu, Finland.
authors.forename: Ville-Veikko
authors.initials: VV
authors.lastname: Telkki
authors.affiliation: Department of Chemistry, University of Helsinki, A. I. Virtasen Aukio 1, 00014 Helsinki, Finland.
authors.forename: Timo
authors.initials: T
authors.lastname: Repo
authors.affiliation: Laboratory of Magnetic Resonance Microimaging, International Tomography Center SB RAS, Institutskaya St. 3A, 630090 Novosibirsk, Russia. v_zhivonitko@tomo.nsc.ru and Department of Natural Sciences, Novosibirsk State University, Pirogova St. 2, 630090 Novosibirsk, Russia.
authors.forename: Igor
authors.initials: I
authors.lastname: Koptyug