The PGEs, which include Pt, Pd, Rh, Ru, Ir, and Os, play crucial roles in a diverse range of industrial applications, including anti-cancer drugs, energy materials, light-emitting materials, and catalysts, to name a few. Due to their cost and rarity, there is an ongoing search for “replacement elements”; however, finding the right replacements requires an understanding of PGE metal-ligand bonding, which is thought to be unique among the transition metal elements. The use of SSNMR spectroscopy to measure NMR interaction tensors, along with first-principles DFT calculations, have potential to provide insight into the nature of this bonding. However, the NMR-active nuclides of the PGEs, except for ¹⁹⁵Pt, are among the most unreceptive in the Periodic Table. Furthermore, their SSNMR spectra feature inhomogeneously broadened powder patterns (e.g., 250 kHz to >10 MHz) caused by large anisotropic interactions (i.e., the quadrupolar interaction and/or chemical shift anisotropy), which inherently reduces their S/N. Conventional rectangular pulses are often unsuitable for the acquisition of these ultra-wideline (UW) spectra, due to their limited bandwidths for excitation, refocusing, and cross polarization (CP); hence, improved pulses and pulse sequences are needed.
To this end, we present an investigation of the use of wideband uniform-rate smooth-truncation (WURST) pulses for the acquisition of ¹⁰³Rh (I = 1/2) and ⁹⁹Ru (I = 5/2) SSNMR spectra, featuring experiments on stationary samples involving direct excitation and broadband CP methods. We describe the optimization of the broadband adiabatic inversion CP (BRAIN-CP) pulse sequence, including key modifications to pulses on the ¹H channel. Finally, we demonstrate these methods for a series of Rh and Ru complexes and present the highest quality ¹⁰³Rh and ⁹⁹Ru SSNMR spectra recorded to date. Increased experimental efficiency and spectral quality suggest that these methods may allow for the study of a wide array of materials via SSNMR of PGE nuclides.

Jasmin Schoenzart

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