NMR spectroscopy of a 18O-labeled rhodium paddlewheel complex: Isotope shifts, 103Rh-103Rh spin-spin coupling, and 103Rh singlet NMR

Despite the importance of rhodium complexes in catalysis, and the favorable 100% natural abundance of the spin-1/2 Rh-103 nucleus, there are few reports of Rh-103 nuclear magnetic resonance (NMR) parameters in the literature. In part, this is the consequence of the very low gyromagnetic ratio of Rh-103 and its dismal NMR sensitivity. In a previous paper [Harbor-Collins et al., J. Chem. Phys. 159, 104 307 (2023)], we demonstrated an NMR methodology for H-1-enhanced Rh-103 NMR and demonstrated an application to the Rh-103 NMR of the dirhodium formate paddlewheel complex. In this paper, we employ selective O-18 labeling to break the magnetic equivalence of the Rh-103 spin pair of dirhodium formate. This allows the estimation of the Rh-103-Rh-103 spin-spin coupling and provides access to the Rh-103 singlet state. We present the first measurement of a O-18-induced Rh-103 secondary isotope shift as well as the first instance of singlet order generated in a Rh-103 spin pair. The field-dependence of Rh-103 singlet relaxation is measured by field-cycling NMR experiments.