Metal-Mediated Catalytic Polarization Transfer from <i>para</i> Hydrogen to 3,5-Dihalogenated Pyridines

The neutral catalysts [IrCl(H)2(NHC)(substrate)2] or [IrCl(H)2(NHC)(substrate)(sulfoxide)] are used to transfer polarization from para hydrogen (pH2) to 3,5-dichloropyridine and 3,5-dibromopyridine substrates. This is achieved in a rapid, reversible, and low-cost process that relies on ligand exchange within the active catalyst. Notably, the sulfoxide-containing catalyst systems produced NMR signal enhancements between 1 and 2 orders of magnitude larger than its unmodified counterpart. Consequently, this signal amplification by reversible exchange hyperpolarization method can boost the 1H, 13C, and 15N nuclear magnetic resonance (NMR) signal intensities by factors up to 4350, 1550, and 46,600, respectively (14.0, 1.3, and 15.4% polarization). In this paper, NMR and X-ray crystallography are used to map the evolution of catalytically important species and provide mechanistic rational for catalytic efficiency. Furthermore, applications in spontaneous radiofrequency amplification by stimulated emission and NMR reaction monitoring are also shown.本研究采用的中性催化剂为[IrCl(H)₂(NHC)(底物)₂]或[IrCl(H)₂(NHC)(底物)(亚砜)]，可将仲氢（para hydrogen, pH₂）的极化度转移至3,5-二氯吡啶与3,5-二溴吡啶底物中。该过程依托活性催化剂内的配体交换反应，具备快速、可逆且低成本的特性。值得注意的是，含亚砜的催化剂体系所产生的核磁共振信号增强幅度，较未改性的对应体系高出1至2个数量级。依托可逆交换超极化信号放大法，可将¹H、¹³C及¹⁵N的核磁共振信号强度分别提升至原强度的4350倍、1550倍与46600倍，对应极化度分别为14.0%、1.3%与15.4%。本文通过核磁共振与X射线晶体学技术，追踪了催化活性物种的演化过程，并为催化效率提供了机理解释。此外，本文还展示了该方法在受激辐射自发射频放大及核磁共振反应监测领域的应用。