Understanding the Microstructure of Poly(<i>p</i>‑phenylenevinylene)s Prepared by Ring-Opening Metathesis Polymerization Using ¹³C‑Labeled Paracyclophanediene Monomers

Selectively main chain ¹³C-labeled poly­(<i>p</i>-phenylenevinylene)­s (PPVs) were synthesized by ring-opening metathesis polymerization (ROMP) of ¹³C-labeled dialkoxy-substituted [2,2] paracyclophane-1,9-dienes (<b>M</b>) using the Grubbs second generation ruthenium carbene complex (<b>G2</b>). Analysis of the natural abundance and ¹³C-labeled PPVs by NMR spectroscopy showed no main chain structural defects for these polymers. Comparison of the <i>in situ</i> ¹H and ¹³C NMR spectroscopy during the ROMP of labeled and unlabeled monomer enabled the active ruthenium carbene chain ends present during the initiation and propagation reaction to be definitively characterized. Using ¹³C NMR spectroscopy, the regiochemistry of the propagation of the asymmetric monomer <b>M</b> with <b>G2</b> was found to be essentially regiorandom

Utilization of SABRE-Derived Hyperpolarization To Detect Low-Concentration Analytes via 1D and 2D NMR Methods

The characterization of materials by the inherently insensitive method of NMR spectroscopy plays a vital role in chemistry. Increasingly, hyperpolarization is being used to address the sensitivity limitation. Here, by reference to quinoline, we illustrate that the SABRE hyperpolarization technique, which uses <i>para</i>-hydrogen as the source of polarization, enables the rapid completion of a range of NMR measurements. These include the collection of ¹³C, ¹³C­{¹H}, and NOE data in addition to more complex 2D COSY, ultrafast 2D COSY and 2D HMBC spectra. The observations are made possible by the use of a flow probe and external sample preparation cell to re-hyperpolarize the substrate between transients, allowing repeat measurements to be made within seconds. The potential benefit of the combination of SABRE and 2D NMR methods for rapid characterization of low-concentration analytes is therefore established

Iridium N-Heterocyclic Carbene Complexes as Efficient Catalysts for Magnetization Transfer from <i>para</i>-Hydrogen

While the characterization of materials by NMR is hugely important in the physical and biological sciences, it also plays a vital role in medical imaging. This success is all the more impressive because of the inherently low sensitivity of the method. We establish here that [Ir(H)₂(IMes)(py)₃]Cl undergoes both pyridine (py) loss as well as the reductive elimination of H₂. These reversible processes bring <i>para</i>-H₂ and py into contact in a magnetically coupled environment, delivering an 8100-fold increase in ¹H NMR signal strength relative to non-hyperpolarized py at 3 T. An apparatus that facilitates signal averaging has been built to demonstrate that the efficiency of this process is controlled by the strength of the magnetic field experienced by the complex during the magnetization transfer step. Thermodynamic and kinetic data combined with DFT calculations reveal the involvement of [Ir(H)₂(η²-H₂)(IMes)(py)₂]⁺, an unlikely yet key intermediate in the reaction. Deuterium labeling yields an additional 60% improvement in signal, an observation that offers insight into strategies for optimizing this approach

N‑Heterocyclic Carbene to Actinide d‑Based π‑bonding Correlates with Observed Metal–Carbene Bond Length Shortening Versus Lanthanide Congeners

Comparison of bonding and electronic structural features between trivalent lanthanide (Ln) and actinide (An) complexes across homologous series’ of molecules can provide insights into subtle and overt periodic trends. Of keen interest and debate is the extent to which the valence f- and d-orbitals of trivalent Ln/An ions engage in covalent interactions with different ligand donor functionalities and, crucially, how bonding differences change as both the Ln and An series are traversed. Synthesis and characterization (SC-XRD, NMR, UV–vis–NIR, and computational modeling) of the homologous lanthanide and actinide N-heterocyclic carbene (NHC) complexes [M­(C5Me5)2(X)­(IMe4)] {X = I, M = La, Ce, Pr, Nd, U, Np, Pu; X = Cl, M = Nd; X = I/Cl, M = Nd, Am; and IMe4 = [C­(NMeCMe)2]} reveals consistently shorter An–C vs Ln–C distances that do not substantially converge upon reaching Am3+/Nd3+ comparison. Specifically, the difference of 0.064(6) Å observed in the La/U pair is comparable to the 0.062(4) Å difference observed in the Nd/Am pair. Computational analyses suggest that the cause of this unusual observation is rooted in the presence of π-bonding with the valence d-orbital manifold in actinide complexes that is not present in the lanthanide congeners. This is in contrast to other documented cases of shorter An–ligand vs Ln–ligand distances, which are often attributed to increased 5f vs 4f radial diffusivity leading to differences in 4f and 5f orbital bonding involvement. Moreover, in these traditional observations, as the 5f series is traversed, the 5f manifold contracts such that by americium structural studies often find no statistically significant Am3+vs Nd3+ metal–ligand bond length differences

Hybrid organic-inorganic rotaxanes, including a hetero-hybrid [3]rotaxane featuring two distinct heterometallic rings and a molecular shuttle

[2] and [3] hybrid rotaxanes are reported based on {Ti7M} rings (M is a trivalent metal such as FeIII or GaIII). NMR studies show that [2]rotaxanes can act as molecular shuttles, while EPR studies of [3]rotaxanes show weak interactions between the paramagnetic components of the supramolecular assemblies