Spectroelectrochemistry and DFT Analysis of a New {RuNO}ⁿ Redox System with Multifrequency EPR Suggesting Conformational Isomerism in the {RuNO}⁷ State

The compound [Ru(NO)(bpym)(terpy)](PF₆)₃, bpym = 2,2‘-bipyrimidine and terpy = 2,2‘:6‘,2‘ ‘-terpyridine, with a {RuNO}⁶ configuration (angle Ru−N−O 175.2(4)°) was obtained from the structurally characterized precursor [Ru(NO₂)(bpym)(terpy)](PF₆), which shows bpym-centered reduction and metal-centered oxidation, as evident from EPR spectroscopy. The relatively labile [Ru(NO)(bpym)(terpy)]³⁺, which forms a structurally characterized acetonitrile substitution product [Ru(CH₃CN)(bpym)(terpy)](PF₆)₂ upon treatment with CH₃OH/CH₃CN, is electrochemically reduced in three one-electron steps of which the third, leading to neutral [Ru(NO)(bpym)(terpy)], involves electrode adsorption. The first-two reduction processes cause shifts of ν(NO) from 1957 via 1665 to 1388 cm^-1, implying a predominantly NO-centered electron addition. UV−vis-NIR Spectroscopy shows long-wavelength ligand-to-ligand charge transfer absorptions for [Ru^II(NO^-I)(bpym)(terpy)]⁺ in the visible region, whereas the paramagnetic intermediate [Ru(NO)(bpym)(terpy)]²⁺ exhibits no distinct absorption maximum above 309 nm. EPR spectroscopy of the latter at 9.5, 95, and 190 GHz shows the typical invariant pattern of the {RuNO}⁷ configuration; however, the high-frequency measurements at 4 and 10 K reveal a splitting of the g₁ and g₂ components, which is tentatively attributed to conformers resulting from the bending of RuNO. DFT calculations support the assignments of oxidation states and the general interpretation of the electronic structure.