Solid-State Photo-NMR Study on Light-Induced Nitrosyl Linkage Isomers Uncovers Their Structural, Electronic, and Diamagnetic Nature

A light-induced linkage NO isomer (MS1) in trans-[Ru(15NO)(py)419F](ClO4)2 is detected and measured for the first time by solid-state MAS NMR. Chemical shift tensors of 15N and 19F, along with nJ(15N-19F) spin-spin couplings and T1 relaxation times of MS1, are compared with the ground state (GS) at temperatures T < 250 K. Isotropic chemical shifts (15N and 19F) are well resolved for two crystallographically independent cations (A and B) [Ru(15NO)(py)419F]2+, allowing to define separately both populations of MS1 isomers and thermal decay rates for two structural sites. The relaxation times T1 of 19F in the case of GS (30/38.6 s for sites A/B) and MS1 (11.6/11.8 s for sites A/B) indicate that both isomers are diamagnetic, which is the first experimental evidence of diamagnetic properties of MS1 in ruthenium nitrosyl. After light irradiation (λ = 420 nm), the NO ligand rotates by nearly 180° from F-Ru-N-O to F-Ru-O-N, whereby the isotropic chemical shifts of δiso(15N) increase and those of δiso(19F) decrease. The nJ(15N-19F) couplings increase from 2J(15N-Ru-19F)GS = 71 Hz to 3J(15N-O-Ru-19F)MS1 = 105 Hz. These results are interpreted on the basis of DFT-CASTEP calculations including Bader-, Mulliken-, and Hirshfeld-charge density distributions of both states.