The sensitivity of both nuclear magnetic resonance spectroscopy and magnetic resonance imaging is very low because the detected signal strength depends on the small population difference between spin states even in high magnetic fields. Hyperpolarization methods can be used to increase this difference and thereby enhance signal strength. This has been achieved previously by incorporating the molecular spin singlet para-hydrogen into hydrogenation reaction products. We show here that a metal complex can facilitate the reversible interaction of para-hydrogen with a suitable organic substrate such that up to an 800-fold increase in proton, carbon, and nitrogen signal strengths are seen for the substrate without its hydrogenation. These polarized signals can be selectively detected when combined with methods that suppress background signals.

An NMR method is reported for the efficient removal of signals derived from nuclei with thermally equilibrated spin state populations whilst leaving, intact, signals derived from para-hydrogen induced polarisation (PHIP) through gradient assisted coherence selection.

The reaction of Ni(COD)₂ (COD = 1,5-cyclooctadiene) with triethylphosphine and pentafluoropyridine in hexane has been shown previously to yield trans-[NiF(2-C₅NF₄)(PEt₃)₂] (1a) with a preference for reaction at the 2-position of the heteroaromatic. The corresponding reaction with 2,3,5,6-tetrafluoropyridine was shown to yield trans-[NiF(2-C₅NF₃H)(PEt₃)₂] (1b). In this paper, we show that reaction of Ni(COD)₂ with triethylphosphine and pentafluoropyridine in THF yields a mixture of 1a and 1b. Competition reactions of Ni(COD)₂ with triethylphosphine in the presence of mixtures of heteroaromatics in hexane reveal a kinetic preference of k(pentafluoropyridine) : k(2,3,5,6-tetrafluoropyridine) = 5.4 : 1. Treatment of 1a and 1b with Me₃SiN₃ affords trans-[Ni(N₃)(2-C₅NF₄)(PEt₃)₂] (2a) and trans-[Ni(N₃)(2-C₅NHF₃)(PEt₃)₂] (2b), respectively. The complex trans-[Ni(NCO)(2-C₅NHF₃)(PEt₃)₂] (3b) is obtained on reaction of 1b with Me₃SiNCO and by photolysis of 2b under CO, while trans-[Ni(η¹-CCPh)(2-C₅NF₄)(PEt₃)₂] (4a) is obtained by reaction of phenylacetylene with 1a. Addition of KCN, KI and NaOAc to complex 1a affords trans-[Ni(X)(2-C₅NF₄)(PEt₃)₂] (5a X = CN, 6a X = I, 7a X = OAc), respectively. The PEt₃ groups of complex 1a are readily replaced by addition of 1,2-bis(dicyclohexylphosphino)ethane (dcpe) to produce [NiF(2-C₅F₄N)(dcpe)] (8a). Addition of dcpe to trans-[Ni(OTf)(2-C₅F₄N)(PEt₃)₂] (10a), however, yields the salt [Ni(2-C₅F₄N)(dcpe)(PEt₃)](OTf) (9a) by substitution of only one PEt₃ and displacement of the triflate ligand. The structures of 2b, 4a, 7a and 8a were determined by X-ray crystallography. The influence of different ancillary ligands on the bond lengths and angles of square-planar nickel structures with polyfluoropyridyl ligands is analysed.