Cooperative effects in pi-ligand bridged dinuclear complexes, Part XXI - Synfacially structured [(CpRu)2 mu-cot] - What a difference the coordinationside makes!

By reaction of the neutral mononuclear complex [CpRu(1,2,3-eta:6,7-eta-C8H9 )] (1) with [(C5R5)Ru-(MeCN)(3)]PF6 (R = II, Me) the synfacial cationic mu- cyclooctatetraene-mu-hydridodiruthenium complexes [(CpRu)- {(C5R5)Ru}(mu-H) (mu-cot)]PF6 (cot = cyclooctatetraene; R=H: 2a; R=Me: 2b) are formed. Depro tonation of 2a and 2b with lithium diisopropylamide yields the synfacial ho modinuclear complexes [(CpRu){(C5R5)Ru}mu-cot] (R= H: 3 a; R = Me: 3 b). Cy clic voltammetry studies of 3a and 3b show two clearly separated, electroch emically reversible one-electron redox waves 0/ + 1 and + 1/ + 2 (0/ + 1: E -1/2(3 a) = - 0.79 V; E-1/2(3b) = - 0.89 V; + 1/ +2: E-1/2(3a) = - 0.17 V; E-1/2(3 b) = - 0.27 V versus [FeCp2]/[FeCp2](+)). Chemical oxidation of 3a with [FeCp2](+) affords the monocation [3a](+) and the dication [3a](2+), d epending on the stoichiometry. X-ray structure determination was performed on all dinuclear complexes. The complex 2a and the neutral complex 3a cryst allize in the space groups Cmcm and P (1) over bar, respectively, and the m onocationic species [3 a]PF6 in the space group C2/c. The R=Me: dication [3 a](2+) crystallizes as the mixed salt [3a](BF4)(PF6) in the space group Pnm a. In all the dinuclear complexes, the two metal centers are synfacially co ordinated at the cot ligand. Complexes 2a and 3a show a eta(4):eta(4) bondi ng mode of the cot moiety, whereas, upon oxidation, the cot ligand in 3a ch anges its hapticity to eta(5):eta(5). Results from H-1 NMR spectroscopic st udies of 2a, 2 b and [3a](2+) are in accordance with the crystallographic f indings, in contrast to the neutral complex 3a, which shows a fast rotation of the cot entity even at 200 K. The Ru-Ru distances in the dinuclear comp lexes decrease dramatically from 307.8 pm in 2a to 266 pm in [3a](2+); comp lexes 3a and [3a](+) have Ru-Ru bond lengths of 295.6 and 252.2 pm, respect ively. The Ru-Ru interaction in 2a and 2b can best be described as a three- center, two-electron RuHRu bond, whereas in 3a and [3a](2+) a Ru-Ru single bond must be considered. For the paramagnetic complex cation [3a](+), a Ru- Ru sigma* semi-occupied molecular orbital is postulated based on EPR and UV /Vis spectroscopic results, which indicate that [3 a](+) is a mixed-valent class III compound.