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 [RuII(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.