Hyperpolarization Effects in Parahydrogen Activation with Pnictogen Biradicaloids: Metal‐free PHIP and SABRE

Abstract

Biradicaloids attract attention as a novel class of reagents that can activate small molecules such as H2, ethylene and CO2. Herein, we study activation of parahydrogen (nuclear spin‐0 isomer of H2) by a number of 4‐ and 5‐membered pnictogen biradicaloids based on hetero‐cyclobutanediyl [X(μ‐NTer)2Z] and hetero‐cyclopentanediyl [X(μ‐NTer)2ZC(NDmp)] moieties (X,Z=P,As; Ter=2,6‐Mes2−C6H3, Dmp=2,6‐Me2−C6H3). The concerted mechanism of this reaction allowed observing strong nuclear spin hyperpolarization effects in 1H and 31P NMR experiments. Signal enhancements from two to four orders of magnitude were detected at 9.4 T depending on the structure. It is demonstrated that 4‐membered biradicaloids activate H2 reversibly, leading to SABRE (signal amplification by reversible exchange) hyperpolarization of biradicaloids themselves and their H2 adducts. In contrast, the 5‐membered counterparts demonstrate rather irreversible parahydrogen activation resulting in hyperpolarized H2 adducts only. Kinetic measurements provided parameters to support experimental observations.

Radical Reactivity of the Biradical [⋅P(μ‐NTer)2P⋅] and Isolation of a Persistent Phosphorus‐Cantered Monoradical [⋅P(μ‐NTer)2P‐Et]

“…We could not locate a transition state for a concerted mechanism (e. g., analogous to the addition of H 2 to the singlet biradical [⋅P(μ‐NTer) 2 P⋅], Scheme 2). [35,100,101] All results pointed towards a stepwise (i. e., radical) mechanism of the reaction, in agreement with experimental observations. In particular, our model computations implied that the formal abstraction of a Br⋅ radical from the bromoalkane by the singlet biradical initiated the radical chain reaction (see also section “Real system” below).…”

“…We could not locate a transition state for a concerted mechanism (e. g., analogous to the addition of H 2 to the singlet biradical [ * P(μ-NTer) 2 P * ], Scheme 2). [35,100,101] All results pointed towards a stepwise (i. e., radical) mechanism of the reaction, in agreement with experimental observations. In S6.…”

Advancing homogeneous catalysis for parahydrogen-derived hyperpolarisation and its NMR applications

“…Most of these systems are based on the use of frustrated Lewis pairs (FLPs) 63 and can split para-H 2 heterolytically in a way that also produces enhanced NMR signals for sites within the FLP. [64][65][66][67][68][69] The heterolytic activation occurs in a pairwise manner for hyperpolarised reaction products to be observed and can provide valuable insight into the mechanism of H 2 activation by FLPs. These effects were rst demonstrated using ansa-aminoborane FLPs such as 1-{2-[bis(pentauorophenyl)boryl]benzyl}-2,2,4,7-tetramethyl-1,2,3,4-tetrahydroquinoline (QCAT) (Fig.…”

“…Generally, P-P, As-P and As-As biradicaloids containing fourand ve-membered rings were shown to activate para-H 2 , in many cases reversibly. 69 This reversibility allowed the observation of hyperpolarised starting para-H 2 metal-free activators (FLPs 64 or biradicaloids 69 ) without actual modication of their structures and demonstrates features of a metal-free SABRE effect. In these cases, PHIP has been used as a great mechanistic tool to study para-H 2 activation by such systems.…”

Parahydrogen‐Induced Polarization in Hydrogenation Reactions Mediated by a Metal‐Free Catalyst

“…This is a related effect to that of the hyperpolarization of free ligands in signal amplification by reversible exchange (SABRE). [19] Metal‐free SABRE was reported in the para‐H 2 activations with ansa‐aminoboranes [12] and pnictogen biradicaloids, [13a] but this is the first time when it is reported in metal‐free hydrogenation reactions. Without the use of para‐H 2 , a 32‐scan accumulation could lead to the detection of HCAT‐1a and HCAT‐1b in 15 N NMR, whereas HCAT‐1a‐H 2 , HCAT‐1b‐H 2 and HCAT were still not visible.…”