A PROTON-INDUCED N-1 TO ETA(2) MIGRATION OF THE FLUXIONAL PYRAZINE INTHE [RU-II(HEDTA)(PZ)](-) COMPLEX

[Ru-II(hedta)(D2O)](-), hedta(3-) = N-(hydroxyethyl)ethylenediaminetri acetate, reacts with pyrazine in D2O at 25 degrees C to yield several isomers and less than or equal to 20% of the pyrazine-bridged binuclea r complex. Two isomers of 56.2% combined abundance have differentiated alpha (near) and beta (remote) H-1 NMR pyrazine resonances at 9.09 pp m (alpha or H2, H6 pair) and 8.33 ppm (beta or H3, H5 pair). The other isomer of ca. 24% abundance exhibits only a singlet at 8.76 ppm, indi cative of fluxional pyrazine movement from N-1 to N-4. This is believe d to be the cis-polar isomer. Within 24 h the differentiated isomers c onvert to the fluxional isomer, which remains fluxional in 50% D2O/50% CD3OD down to 237 K. The fluxional isomer has all equivalent C-13 NMR resonances at 152.44 ppm, 5.18 ppm downfield of free pyrazine. Compar ison with the bridged binuclear ion {[Ru(hedta)](2)(pz)}(2-) revealed fortuitously similar shifts; the H-1 NMR spectrum shows a singlet at 8 .76 ppm, and the C-13 NMR spectrum, a singlet at 152.40 ppm. These spe cies have different electrochemical signatures, however, with the 1:1 fluxional complex having a Ru-II/III wave at 0.20 V that shifts to 0.3 5 V upon protonation of the N-4 position, whereas the binuclear comple x has two waves at 0.18 and 0.33 V which are independent of pH. H-1 NM R indicates stereochemically rigid coordination of 2-methylpyrazine (2 -CH(3)pz) at N-4 with the 2-CH3 position remote in the major species ( 65.5%) and at N-2 with the 2-CH3 site adjacent (34.4%) in the lesser i somer. The proton resonances are as follows. Remote isomer (N-4): H2, 8.22 ppm; H3, 8.97 ppm; H5, 8.88 ppm; CH3, 2.53 ppm. Adjacent (N-1) is omer: H2, 8.73 ppm; H3, 8.42 ppm, H5, 8.50 ppm; CH3, 2.45 ppm. A slow conversion of the strained adjacent isomer to the remote isomer is obs erved. Remote and adjacent isomers were also prepared for [(NH3)(5)Ru( 2-CH(3)pz)](2+) in 87.7% and 12.3% yield. Protonation of [Ru(hedta)(pz )](-) yields nonfluxional complexes: a major species (63%) bound eta(2 )(1,2) With four complicated resonance patterns at 8.72, 8.57, 8.00, a nd 7.91 ppm, each signal having across-the-ring couplings which requir e couplings to at least two different ring protons and a minor N-1-coo rdinated N-4 protonated species (16%) with alpha and beta proton pairs resonating at 8.81 and 8.25 ppm. An eta(2)-(2,3) isomer is also detec table (20%) which has four types of H-1 resonances at 9.51, 9.01, 8.85 , and 8.15 ppm. The weakened sigma bonding from pzH(+) and enhanced pi -acceptor capacity power of pzH(+) combine to induce a switch in coord ination to either eta(2)(1,2) or eta(2)(2,3).