AN ASYMMETRICALLY-COORDINATED 2,3-DIMETHYLPYRAZINE BRIDGING LIGAND INTHE (II,II) BINUCLEAR ION, ([RU-II(HEDTA)](2)(2,3-ME(2)PZ))(2-) (HEDTA(3-) = N-HYDROXYETHYLETHYLENEDIAMINETRIACETATE)

[Ru(hedta)(H2O)](-) hedta(3-)=N-hydroxyethylethylenediaminetriacetate) reacts with 2,3-Me(2)pz (2,3-dimethylpyrazine) at 1:1, forming [Ru(he dta)(2,3-Me(2)pz)](-) (1), and at 2:1, forming {[Ru(hedta)](2)(2,3-Me( 2)pz)(2-) (2). Complex 1 is characterized by a Ru-II/III wave at 0.20 V versus NHE which shifts to 0.31 V upon protonation at N-4. Complex 2 exhibits two waves at 0.19 and 0.27 V from which the corrected compro portionation constant, K-c'=5.7, is calculated. The value of K-c' indi cates very poor electronic coupling between Ru-II/III centers. The ori gins of the decrease from K-c' value of 190 for the {[Ru(hedta)](2)(pz )}(-) complex is discussed in terms of (i) differences in orbital over lap due to hindered rotation in the 2,3-Me(2)pz complex versus the pz case, or (ii) differences in structural distance, orientation or solva tion in these complexes. The H-1 NMR spectrum of complex 1 exhibits re sonances at 2.67 (2CH(3)), 2.58 (3CH(3)), 8.01 (H5) and 8.86 (H6) ppm, respectively. The H-1 MMR spectrum of complex 2 requires asymmetric c oordination of the two [Ru-II(hedta)](-) moieties as shown by non-equi valent resonances: 2.74 (2CH(3)), 2.68 (3CH(3)), 8.45 (H5) and 8.55 (H 6) ppm, respectively. On the C-13 NMR time scale carbons C2 and C3, an d C5 and C6 are pairwise equivalent with respective shifts of 155.81 a nd 166.44 ppm, downfield of the free ligand by 12.99 and 11.69 ppm. Th ere may be a time-dependent reorganization of bonding at the two Ru-II centers which lengthens and shortens opposite bonds at N-1 and N-4 wi th k similar to 10 s(-1). This motion will also account for the reduce d value of K-c' for the 2,3-Me(2)pz complex. The protonated form of 1 migrates to provide eta(2)-coordinated [Ru-II(hedta)(2,3-Me(2)pzH)], b ound at eta(2)(1,2)(31.5%), less than the 79.7% eta(2)(1,2) plus eta(2 )(2.3) species observed for the related pyrazine complex, at equilibri um with the N-1-coordinated complex. (C) 1998 Elsevier Science S.A.