Nuclear spin hyperpolarization with ansa-aminoboranes : a metal-free perspective for parahydrogen-induced polarization
|Author:||Zhivonitko, Vladimir V.1,2; Sorochkina, Kristina3; Chernichenko, Konstantin3;|
1Laboratory of Magnetic Resonance Microimaging, International Tomography Center SB RAS, Institutskaya St. 3A, 630090 Novosibirsk, Russia
2Department of Natural Sciences, Novosibirsk State University, Pirogova St. 2, 630090 Novosibirsk, Russia
3Department of Chemistry, University of Helsinki, A. I. Virtasen Aukio 1, 00014 Helsinki, Finland
4Research Centre for Natural Sciences, Hungarian Academy of Sciences, Magyar Tudósok Körútja 2, H-1117 Budapest, Hungary
5NMR Research Unit, University of Oulu, P.O. Box 3000, 90014 Oulu, Finland
|Online Access:||PDF Full Text (PDF, 3.3 MB)|
|Persistent link:|| http://urn.fi/urn:nbn:fi-fe201707037610
Royal Society of Chemistry,
|Publish Date:|| 2017-07-03
The parahydrogen-induced polarization (PHIP) phenomenon, observed when parahydrogen is used in H₂ addition 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–H₂ molecules is mediated through dihydrogen bonding.
PCCP. Physical chemistry chemical physics
|Pages:||27784 - 27795|
|Type of Publication:||
A1 Journal article – refereed
|Field of Science:||
114 Physical sciences
The financial support by the grant from RFBR (14-03-93183-MCX-a)
is acknowledged. VVZ and IVK thank RSF (grant #14-13-00445) for
the support of the experiments with AAB compounds and parahydrogen.
VVT acknowledges the Academy of Finland for financial
support (grant numbers 289649 and 294027). This work was also
partially supported by a Hungarian NKFI grant (K-115660).
|Academy of Finland Grant Number:||
289649 (Academy of Finland Funding decision)
294027 (Academy of Finland Funding decision)
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