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-chlor opyrid-2-yl)pyridazine, were synthesized and characterized by UV-visib le and H-1 NMR spectroscopy. Compounds 1a and 1b were additionally cha racterized by X-ray crystallography. [Ru(trpy)(H(2)dppi)Cl](PF6). 2CH( 3)CN crystallizes in the triclinic space group, <P(1)over bar>, with a = 8.628(1) Angstrom, b = 14.586(2) Angstrom, c = 14.963(2) Angstrom, alpha = 70.857(8)degrees, beta = 77.70(1)degrees, gamma = 74.29(1)degr ees, V = 1696.5(4) Angstrom(3), and Z = 2; R-1 = 0.0739 (I > 2 sigma(T )) with 5920 unique reflections. [Ru(trpy)(Me(2)dppi)Cl](PF6). 0.5(CH3 CH2)(2)O crystallizes in the triclinic space group <P(1)over bar>, wit h a = 8.820(2) Angstrom, b = 13.580(2) Angstrom, c = 15.260(2) Angstro m, alpha = 88.84(1)degrees, beta = 74.25(1)degrees, gamma = 73.27(1)de grees, V = 1681.4(5) Angstrom(3), and Z = 2; R-1 = 0.0693 (I > 2 sigma (I)) with 4407 unique reflections. Reaction of 1a, 1b, and 1c with aqu eous silver ion produces the corresponding aqua complexes, 2a, 2b, and 2c, which, after dissolution in acetonitrile. form the analogous acet onitrile complexes, 4a, 4b, and 4c. [Ru(trpy)(H(2)dppi)(CH3-CN)](PF6)( ClO4). 2CH(3)CN, 4a, crystallizes in the triclinic space group <P(1)ov er bar>, with a 12.376(1) Angstrom, b = 12.835(2) Angstrom, c = 13.029 (2) Angstrom, alpha = 109.252(9)degrees, beta = 102.766(8)degrees, gam ma = 90.847(9)degrees, V = 1896.9(3) Angstrom(3), and Z = 2; R-1 = 0.0 397 (I > 2 sigma(I)) with 4844 unique reflections. {[Ru(trpy)(Cl(2)dpp i)(CH3CN)](ClO4)(2)}(2) . CH3CN, 4c, crystallizes in the triclinic spa ce group, <P(1)over bar>, with a = 13.075(2) Angstrom, b = 16.807(3) A ngstrom, c = 17.913(2) Angstrom, alpha = 70.83(1)degrees, beta = 89.76 (1)degrees, gamma = 82.44(1)degrees, V = 3682.6(1) Angstrom(3), and Z = 2; R-1 = 0.0397 (I > 2 sigma(I)) with 4844 unique reflections. The r edox properties of 1a, 1b, 1c, 2a, 2b, and 2c were examined using cycl ic 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-de pendent, 2e(-) waves corresponding to the formation of Ru-IV=O complex es. Second-order rate constants, k(cat), for benzyl alcohol oxidation by the Ru-IV=O complexes were determined electrochemically, yielding v alues 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 highes t reduction potential ([Ru(trpy)(Cl(2)dppi)(O)](2+)) is the slowest ca talyst for benzyl alcohol oxidation. The unusual driving-force depende nce of the oxidation rates exhibited by these complexes can be attribu ted to steric effects that result from incorporating chloro or methyl groups into the 6- and 6'-positions of the dppi ligand. These data an consistent with a mechanism in which the rate-determining step involve s preassociation of the substrate with the Ru-IV=O unit.