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Agostic interactions were observed in the bound mesityl group in a series of iron compounds bearing a bis(NHC) pincer CCC ligand. The L-type ligand on [(CCC)Fe II Mes(L)] complexes influences the strength of the agostic interaction and is manifested in the upfield shift of the 1 H NMR resonance for the mesityl methyl resonances. The nature of the interaction was further investigated by density functional theory calculations, allowing rationalization of some unexpected trends and proving to be a powerful predictive tool.

The role of ligands in rhodium‐ and iridium‐catalyzedParahydrogen Induced Polarization (PHIP) and SABRE (signal amplification by reversible exchange) chemistry has been studied in the benchmark systems, [Rh(diene)(diphos)]⁺and [Ir(NHC)(sub)₃(H)₂]⁺, and shown to have a great impact on the degree of hyperpolarization observed. Here, we examine the role of the flanking moieties in the electron‐rich monoanionic bis(carbene) aryl pincer ligand,^ArCCC (Ar=Dipp, 2,6‐diisopropyl or Mes, 2,4,6‐trimethylphenyl) on the cobalt‐catalyzed PHIP and PHIP‐IE (PHIP via Insertion and Elimination) chemistry that we have previously reported. The mesityl groups were exchanged for diisopropylphenyl groups to generate the (^DippCCC)Co(N₂) catalyst, which resulted in faster hydrogenation and up to 390‐fold¹H signal enhancements, larger than that of the (^MesCCC)Co‐py (py=pyridine) catalyst. Additionally, the synthesis of the (^DippCCC)Rh(N₂) complex is reported and applied towards the hydrogenation of ethyl acrylate withparahydrogen to generate modest signal enhancements of both¹H and¹³C nuclei. Lastly, the generation of two (^MesCCC)Ir complexes is presented and applied towards SABRE and PHIP‐IE chemistry to only yield small¹H signal enhancements of the partially hydrogenated product (PHIP) with no SABRE hyperpolarization.