The $^{103}$Rh NMR Spectroscopy and Relaxometry of the Rhodium Formate Paddlewheel Complex

TL;DR

This paper demonstrates a rapid and enhanced method for $^{103}$Rh NMR spectroscopy of a rhodium paddlewheel complex, measuring relaxation times and chemical shift anisotropy with improved signal strength and reduced artifacts.

Contribution

The study introduces a polarization transfer scheme to enhance $^{103}$Rh NMR signals and efficiently measure relaxation parameters in a complex challenging low-gamma nucleus.

Findings

Successful rapid measurement of $^{103}$Rh T1 and T2 within 20 minutes.

Large $^{103}$Rh shielding anisotropy of approximately 9900 ppm.

Field-dependent relaxation dominated by chemical shift anisotropy (CSA).

Abstract

The NMR spectroscopy of spin-1/2 nuclei with low gyromagnetic ratio is challenging, due to the low NMR signal strength. Methodology for the rapid acquisition of $^{103}$Rh NMR parameters is demonstrated for the case of the rhodium formate "paddlewheel" complex $\mathrm{Rh_2(HCO_2)_4}$. A scheme is described for enhancing the $^{103}$Rh signal strength by polarization transfer from $^{1}$H nuclei and which also greatly reduces the interference from ringing artifacts, a common hurdle for the direct observation of low-$\gamma$ nuclei. The $^{103}$Rh relaxation time constants $T_1$ and $T_2$ are measured within 20 minutes using $^{1}$H-detected experiments. The field-dependence of the $^{103}$Rh $T_1$ is measured. The high-field relaxation is dominated by the chemical shift anisotropy (CSA) mechanism. The $^{103}$Rh shielding anisotropy is found to be very large: $|\Delta\sigma|=9900\pm540\mathrm{\,ppm}$. This estimate is compared with density functional theory calculations.