A Proton-Induced N-1 to eta(2) Migration of the Fluxional Pyrazine in the [Ru(II)(hedta)(pz)](-) Complex.
[Ru(II)(hedta)(D(2)O)](-), hedta(3)(-) = N-(hydroxyethyl)ethylenediaminetriacetate, reacts with pyrazine in D(2)O at 25 degrees C to yield several isomers and </=20% of the pyrazine-bridged binuclear complex. Two isomers of 56.2% combined abundance have differentiated alpha (near) and beta (remote) (1)H NMR pyrazine resonances at 9.09 ppm (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, indicative of fluxional pyrazine movement from N-1 to N-4. This is believed to be the cis-polar isomer. Within 24 h the differentiated isomers convert to the fluxional isomer, which remains fluxional in 50% D(2)O/50% CD(3)OD down to 237 K. The fluxional isomer has all equivalent (13)C NMR resonances at 152.44 ppm, 5.18 ppm downfield of free pyrazine. Comparison with the bridged binuclear ion {[Ru(hedta)](2)(pz)}(2)(-) revealed fortuitously similar shifts; the (1)H NMR spectrum shows a singlet at 8.76 ppm, and the (13)C NMR spectrum, a singlet at 152.40 ppm. These species have different electrochemical signatures, however, with the 1:1 fluxional complex having a Ru(II/III) wave at 0.20 V that shifts to 0.35 V upon protonation of the N-4 position, whereas the binuclear complex has two waves at 0.18 and 0.33 V which are independent of pH. (1)H NMR indicates stereochemically rigid coordination of 2-methylpyrazine (2-CH(3)pz) at N-4 with the 2-CH(3) position remote in the major species (65.5%) and at N-2 with the 2-CH(3) site adjacent (34.4%) in the lesser isomer. The proton resonances are as follows. Remote isomer (N-4): H2, 8.22 ppm; H3, 8.97 ppm; H5, 8.88 ppm; CH(3), 2.53 ppm. Adjacent (N-1) isomer: H2, 8.73 ppm; H3, 8.42 ppm, H5, 8.50 ppm; CH(3), 2.45 ppm. A slow conversion of the strained adjacent isomer to the remote isomer is observed. Remote and adjacent isomers were also prepared for [(NH(3))(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, and 7.91 ppm, each signal having across-the-ring couplings which require couplings to at least two different ring protons and a minor N-1-coordinated 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 detectable (20%) which has four types of (1)H 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 coordination to either eta(2)(1,2) or eta(2)(2,3).
Chen
Ya
Y
Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 15260.
Shepherd
Rex E.
RE