Successive Photosubstitution of <i>cis</i> ‐[Ru(bpy) ₂ (MeCN) ₂ ] ²⁺ With Pyrazole: Temperature‐Independent and Temperature‐Dependent Pathways

The successive photosubstitution reaction from cis ‐[Ru(bpy) 2 (MeCN) 2 ] 2+ to cis‐ [Ru(bpy) 2 (pz)(pzH)] + was investigated under various conditions (bpy = 2,2′‐bipyridine, pzH = pyrazole). Temperature‐dependent studies revealed that distinct mechanistic features: the first step, conversion from cis ‐[Ru(bpy) 2 (MeCN) 2 ] 2+ to cis ‐[Ru(bpy) 2 (pzH)(MeCN)] 2+ , proceed in a temperature‐independent manner, whereas the second step, yielding cis‐ [Ru(bpy) 2 (pz)(pzH)] + , is strongly temperature‐dependent with an activation energy of 12.8 ± 1.4 kJ/mol. Transient absorption spectroscopy of the intermediate, cis‐ [Ru(bpy) 2 (pzH)(MeCN)] 2+ , determined an activation energy of 18.6 ± 0.3 kJ/mol for the transition from the 3 MLCT to 3 MC states, consistent with the activation energy observed for second photosubstitution reactions (MLCT: metal‐to‐ligand charge–transfer, MC: metal‐centered). The first photosubstitution was unaffected by the nature of incoming imine ligands, indicating that the photodissociation of acetonitrile occurs rapidly. In contrast, the second photosubstitution step was influenced by the σ‐donating ability of the pyrazole ligand and the basicity of the base, which facilitates deprotonation and promotes intramolecular hydrogen bonding between pyrazole ligands. The photosubstitution product, cis ‐[Ru(bpy) 2 (pz)(pzH)] + , exhibited 3 MLCT‐state lifetimes that were much shorter by nearly an order of magnitude compared to the intermediate, as determined from the transient absorption spectra.