An asymmetrically-coordinated 2,3-dimethylpyrazine bridging ligand in the (II,II) binuclear ion, {[RuII(hedta)]2(2,3-Me2pz)}2- (hedta3- = N-hydroxyethylethylenediaminetriacetate)

[Ru(hedta)(H2O)](-) (hedta(3-)=N-hydroxyethylethylenediaminetriacetate) reacts with 2,3-Me(2)pz (2,3-dimethylpyrazine) at 1:1, forming [Ru(hedta)(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 comproportionation constant, K-c'=5.7, is calculated. The value of K-c' indicates very poor electronic coupling between Ru-II/III centers. The origins 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 overlap due to hindered rotation in the 2,3-Me(2)pz complex versus the pz case, or (ii) differences in structural distance, orientation or solvation in these complexes. The H-1 NMR spectrum of complex 1 exhibits resonances 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 coordination of the two [Ru-II(hedta)](-) moieties as shown by non-equivalent resonances: 2.74 (2CH(3)), 2.68 (3CH(3)), 8.45 (H5) and 8.55 (H6) ppm, respectively. On the C-13 NMR time scale carbons C2 and C3, and C5 and C6 are pairwise equivalent with respective shifts of 155.81 and 166.44 ppm, downfield of the free ligand by 12.99 and 11.69 ppm. There 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 with k similar to 10 s(-1). This motion will also account for the reduced 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)], bound 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 equilibrium with the N-1-coordinated complex. (C) 1998 Elsevier Science S.A.