Stereochemically rigid mono and bis pyridazine complexes of [RuII(hedta)]− (hedta3− = N-hydroxyethylethylenediaminetriacetate)

Abstract Pyridazine (pyd) complexes of [RuII(hedta)(pyd)n]−, (n = 1 or 2, hedta3− = N-hydroxyethylethylenediaminetriacetate have been studied by 1H NMR and electrochemical methods. Substitution of pyd on [Ru(hedta)(H2O)]− has a second-order rate constant of 32 M−1 s−1 at 22°C. The RuII/III couples appear at 0.16 V for the mono and 0.49 V for the bis complexes versus NHE. In the bis complex, one carboxylato donor is displaced forming an RuIIN4 in-plane set with two of the N donors as pyd ligands, bound via N-1 attachments. 1H NMR assignments confirmed by 1H-1H COSY spectral data for the N-1 bound pyd ligands are in ppm: (n = 1) H-3, 9.07; H-4, 7.64; H-5, 7.55; H-6, 9.56; (n = 2) H-3, 9.13; H-4, 7.66; H-5, 7.47; H-6, 8.72. Addition of the second pyd donor occurs ≈ 3400 times faster than for pyrimidine, pyrazine or pyridines indicative of an acceleration via an internal base-assisted dissociation of the in-plane carboxylate of the hedta3− chelate using the available N-2 base position of the coordinated pyd of the mono complex. The η2 (N,N)-coordinated intermediate is rapidly displaced by a second pyd ligand. Neither the mono (n = 1) or bis (n = 2) complex is in exchange with the pyd free ligand pool in contrast to [RuII(hedta)(pym)2]− (pym = pyrimidine) in which the second pym base rapidly exchanges with free pym. Also, whereas the second pym site also exhibits a 1,3-metallotropic shift faster than the NMR time scale, neither [RuII(hedta)(pyd)n]− (n = 1 or 2) evidence for a 1,2-shift up to 60°C. Therefore the barrier to a 1,2-shift for the [RuII(hedta)]− complexes of pyd is much greater than for other low-spin do complexes including W(0), Pt(IV) and Re(1) pyd complexes, or for Ru(II) with other π-acid ligands present as in the [RuII(porphinato)(CO)] system, an outcome predicted by former studies of S. Alvarez et al., J. Am. Chem. Soc. 109 (1987) 5316 and by K.R. Dixon, Inorg. Chem. 16 (1977) 2681. Protonation of the mono complex (pKa ≈ 3.42) occurs with an immediate shift of the RuII/III Ev2 value for [Ru(hedta)(pydH)] to +0.08V versus NHE (pH = 1.92) for the cis-equatorial and trans-equatorial isomers. The cis-polar form shifts more slowly to one of the other isomer forms in 90 min. The [RuII(hedta)(pydH)] complex has 1H NMR shifts at (ppm) H-3, 9.14; H-4 and H-5, 7.74 (overlapped); H-6 at 9.53 and 9.41 for the two in-plane stereoisomers. In acidic conditions, the [RuII(hedta)(pydH)] complex redistributes slowly to form the bis and [Ru(hedta)(H2O)]− complexes. At pD ≤ 1.0 the additional protonation of the in-plane carboxylate of [Ru(hedta)(pydH)] opens a pathway that promotes rapid 1,2-metallotropic shifts, probably via a ‘ping pong’ motion that makes and breaks the RuII bond to N-1 and N-2 rapidly or the 1H NMR timescale. The absence of fluxionality for the [Ru(hedta)(pyd)] and [Ru(hedta)(pyd)2]− complexes compared to fluxation in [Ru(hedta)(pym)2]− is attributed to the greater σ basicity of pyd compared to pym and the greater backdonation from Ru(II) to pyd than to pym. Both features strengthen the Ru(II)-(N-1) bonding which creates a high barrier to 1,2-metallotropic shifts for the pyd complexes. Protonation weakens the RuII-pydH+ σ-bonding and lowers the barrier to 1,2-shifts.