Dynamic Nuclear Polarization Using 3D Aromatic Boron Cluster Radicals

Abstract

A set of two dodecaborate [B12(OR)12]1– radical cluster anions containing a dense layer of fluorinated end-groups provides nuclear spin hyperpolarization via the dissolution dynamic nuclear polarization (D-DNP) technique. We show that these clusters can enhance 19F nuclear magnetic resonance (NMR) signals. Importantly, given the inherent radical delocalization in dodecaborate-based clusters, these species are compatible with reactive compounds such as Lewis acids, providing ∼1000–2000 times of signal enhancement for B(C6F5)3 in liquid state NMR spectroscopy experiments at 9.4 Tesla. This observation suggests that 3D aromatic radicals can provide advantages over the conventional radical species that are currently used for DNP such as 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) by showing superior chemical compatibility. The ability to hyperpolarize reactive compounds using [B12(OR)12]1– cluster radicals opens up new applications of reaction monitoring by D-DNP NMR spectroscopy, including the observation of catalytically active species in complex reaction mixtures.