Steric Modulation of Electrocatalytic Benzyl Alcohol Oxidation by [Ru(trpy)(R(2)dppi)(O)](2+) Complexes

The complexes [Ru(trpy)(H(2)dppi)Cl](+) (1a), [Ru(trpy)(Me(2)dppi)Cl](+) (1b), and [Ru(trpy)(Cl(2)dppi)Cl](+) (1c), where trpy is 2,2',2"-terpyridine, H(2)dppi is 3,6-bis(pyrid-2-yl)pyridazine, Me(2)dppi is 3,6-bis(6-methylpyrid-2-yl)pyridazine, and Cl(2)dppi is 3,6-bis(6-chloropyrid-2-yl)pyridazine, were synthesized and characterized by UV-visible and (1)H NMR spectroscopy. Compounds 1a and 1b were additionally characterized by X-ray crystallography. [Ru(trpy)(H(2)dppi)Cl](PF(6)).2CH(3)CN crystallizes in the triclinic space group, P&onemacr;, with a = 8.628(1) Å, b = 14.586(2) Å, c = 14.963(2) Å, alpha = 70.857(8) degrees, beta = 77.70(1) degrees, gamma = 74.29(1) degrees, V = 1696.5(4) Å(3), and Z = 2; R(1) = 0.0739 (I > 2sigma(I)) with 5920 unique reflections. [Ru(trpy)(Me(2)dppi)Cl](PF(6)).0.5(CH(3)CH(2))(2)O crystallizes in the triclinic space group P&onemacr;, with a = 8.820(2) Å, b = 13.580(2) Å, c = 15.260(2) Å, alpha = 88.84(1) degrees, beta = 74.25(1) degrees, gamma = 73.27(1) degrees, V = 1681.4(5) Å(3), and Z = 2; R(1) = 0.0693 (I > 2sigma(I)) with 4407 unique reflections. Reaction of 1a, 1b, and 1c with aqueous silver ion produces the corresponding aqua complexes, 2a, 2b, and 2c, which, after dissolution in acetonitrile, form the analogous acetonitrile complexes, 4a, 4b, and 4c. [Ru(trpy)(H(2)dppi)(CH(3)CN)](PF(6))(ClO(4)).2 CH(3)CN, 4a, crystallizes in the triclinic space group P&onemacr;, with a = 12.376(1) Å, b = 12.835(2) Å, c = 13.029(2) Å, alpha = 109.252(9) degrees, beta = 102.766(8) degrees, gamma = 90.847(9) degrees, V = 1896.9(3) Å(3), and Z = 2; R(1) = 0.0397 (I > 2sigma(I)) with 4844 unique reflections. {[Ru(trpy)(Cl(2)dppi)(CH(3)CN)](ClO(4))(2)}(2).CH(3)CN, 4c, crystallizes in the triclinic space group, P&onemacr;, with a = 13.075(2) Å, b = 16.807(3) Å, c = 17.913(2) Å, alpha = 70.83(1) degrees, beta = 89.76(1) degrees, gamma = 82.44(1) degrees, V = 3682.6(1) Å(3), and Z = 2; R(1) = 0.0777 (I > 2sigma(I)) with 9459 unique reflections. The redox properties of 1a, 1b, 1c, 2a, 2b, and 2c were examined using cyclic voltammetry and spectroelectrochemistry. In acetonitrile, compounds 1a, 1b, and 1c display reversible 1e(-) waves assigned to the Ru(III)/Ru(II) couple, while, in aqueous solutions, 2a, 2b, and 2c show pH-dependent, 2e(-) waves corresponding to the formation of Ru(IV)=O complexes. Second-order rate constants, k(cat), for benzyl alcohol oxidation by the Ru(IV)=O complexes were determined electrochemically, yielding values of 22(1) M(-)(1) s(-)(1) for [Ru(trpy)(H(2)dppi)(O)](2+), 9(3) M(-)(1) s(-)(1) for [Ru(trpy)(Me(2)dppi)(O)](2+), and 6(4) M(-)(1) s(-)(1) for [Ru(trpy)(Cl(2)dppi)(O)](2+). Interestingly, the Ru(IV)=O complex with the highest reduction potential ([Ru(trpy)(Cl(2)dppi)(O)](2+)) is the slowest catalyst for benzyl alcohol oxidation. The unusual driving-force dependence of the oxidation rates exhibited by these complexes can be attributed to steric effects that result from incorporating chloro or methyl groups into the 6- and 6'-positions of the dppi ligand. These data are consistent with a mechanism in which the rate-determining step involves preassociation of the substrate with the Ru(IV)=O unit.