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

Abstract

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

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