Temperature Solid-State 2 H NMR. To assess the solid-state dynamics in crystalline complexes, we employed variable temperature (VT) solid-state (SS) 2 H NMR spin-echo spectroscopy (for 2 H NMR measurement and simulation details, see Section S6, Supporting Information). This technique is widely used to analyze the internal dynamics of deuterium-enriched moieties in the 10 3 -10 8 Hz regime. VT SS 2 H NMR spin-echo measurements were conducted on crystalline samples of two different isotopologues of 1-R to extract dynamics information for different molecular fragments: 1-R-d4 with a deuterated bridging para-phenylene moiety (PP) and 1-R-d20 with deuterated phenyl substituents (Ph a and Ph b ) at the NHC ligands (Scheme 1).