Dataset Information

Using coligands to gain mechanistic insight into iridium complexes hyperpolarized with para-hydrogen.

ABSTRACT: We report the formation of a series of novel [Ir(H)₂(IMes)(α-¹³C₂-carboxyimine)L] complexes in which the identity of the coligand L is varied. When examined with para-hydrogen, complexes in which L is benzylamine or phenethylamine show significant ¹H hydride and ¹³C₂ imine enhancements and may exist in ¹³C₂ singlet spin order. Isotopic labeling techniques are used to double ¹³C₂ enhancements (up to 750-fold) and singlet state lifetimes (up to 20 seconds) compared to those previously reported. Exchange spectroscopy and Density Functional Theory are used to investigate the stability and mechanism of rapid hydrogen exchange in these complexes, a process driven by dissociative coligand loss to form a key five coordinate intermediate. When L is pyridine or imidazole, competitive binding to such intermediates leads to novel complexes whose formation, kinetics, behaviour, structure, and hyperpolarization is investigated. The ratio of the observed PHIP enhancements were found to be affected not only by the hydrogen exchange rates but the identity of the coligands. This ligand reactivity is accompanied by decoherence of any ¹³C₂ singlet order which can be preserved by isotopic labeling. Addition of a thiol coligand proved to yield a thiol oxidative addition product which is characterized by NMR and MS techniques. Significant 870-fold ¹³C enhancements of pyridine can be achieved using the Signal Amplification By Reversible Exchange (SABRE) process when α-carboxyimines are used to block active coordination sites. [Ir(H)₂(IMes)(α-¹³C₂-carboxyimine)L] therefore acts as unique sensors whose ¹H hydride chemical shifts and corresponding hyperpolarization levels are indicative of the identity of a coligand and its binding strength.

SUBMITTER: Tickner BJ

PROVIDER: S-EPMC6540910 | biostudies-literature |

REPOSITORIES: biostudies-literature

ACCESS DATA

Similar Datasets

Project description:Flexible metal-organic frameworks offer a route towards high useable hydrogen storage capacities with minimal swings in pressure and temperature via step-shaped adsorption and desorption profiles. Yet, the understanding of hydrogen-induced flexibility in candidate storage materials remains incomplete. Here, we investigate the hydrogen storage properties of a quintessential flexible metal-organic framework, ZIF-7. We use high-pressure isothermal hydrogen adsorption measurements to identify the pressure-temperature conditions of the hydrogen-induced structural transition in ZIF-7. The material displays narrow hysteresis and has a shallow adsorption slope between 100 K and 125 K. To gain mechanistic insight into the cause of the phase transition correlating with stepped adsorption and desorption, we conduct powder neutron diffraction measurements of the D2 gas-dosed structures at conditions across the phase change. Rietveld refinements of the powder neutron diffraction patterns yield the structures of activated ZIF-7 and of the gas-dosed material in the dense and open phases. The structure of the activated phase of ZIF-7 is corroborated by the structure of the activated phase of the Cd congener, CdIF-13, which we report here for the first time based on single crystal X-ray diffraction measurements. Subsequent Rietveld refinements of the powder patterns for the gas-dosed structure reveal that the primary D2 adsorption sites in the dense phase form D2-arene interactions between adjacent ligands in a sandwich-like adsorption motif. These sites are prevalent in both the dense and the open structure for ZIF-7, and we hypothesize that they play an important role in templating the structure of the open phase. We discuss the implications of our findings for future approaches to rationally tune step-shaped adsorption in ZIF-7, its congeners, and flexible porous adsorbents in general. Lastly, important to the application of flexible frameworks, we show that pelletization of ZIF-7 produces minimal variation in performance.

Project description:A detailed mechanistic investigation of the thermal reactions of a series of bisphosphine alkylplatinum(II) enolate complexes is reported. The reactions of methylplatinum enolate complexes in the presence of added phosphine form methane and either free or coordinated enone, depending on the steric properties of the enone. Kinetic studies were conducted to determine the relationship between the rates and mechanism of beta-hydrogen elimination from enolate complexes and the rates and mechanism of beta-hydrogen elimination from alkyl complexes. The rates of reactions of the enolate complexes were inversely dependent on the concentration of added phosphine, indicating that beta-hydrogen elimination from the enolate complexes occurs after reversible dissociation of a phosphine. A normal, primary kinetic isotope effect was measured, and this effect was consistent with rate-limiting beta-hydrogen elimination or C-H bond-forming reductive elimination to form methane. Reactions of substituted enolate complexes were also studied to determine the effect of the steric and electronic properties of the enolate complexes on the rates of beta-hydrogen elimination. These studies showed that reactions of the alkylplatinum enolate complexes were retarded by electron-withdrawing substituents on the enolate and that reactions of enolate complexes possessing alkyl substituents at the beta-position occurred at rates that were similar to those of complexes lacking alkyl substituents at this position. Despite the trend in electronic effects on the rates of reactions of enolate complexes and the substantial electronic differences between an enolate and an alkyl ligand, the rates of decomposition of the enolate complexes were similar to those of the analogous alkyl complexes. To the extent that the rates of reaction of the two types of complexes are different, those involving beta-hydrogen elimination from the enolate ligand were faster. A difference between the rate-determining steps for decomposition of the two classes of complexes and an effect of stereochemistry on the selectivity for beta-hydrogen elimination are possible origins of the observed phenomena.

Dataset Information

Using coligands to gain mechanistic insight into iridium complexes hyperpolarized with para-hydrogen.

Similar Datasets

OmicsDI is part of the ELIXIR infrastructure

Tweets