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)](PF 6 )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-(NO 2 )(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 CH 3 OH/CH 3 CN, 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 v(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 -1 )(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 g 1 and g 2 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.