CHEMISTRY OF THE [RU(RQ)(TAP)2]Z FAMILY - AUTHENTIC CATECHOLATES, REDUCTION POTENTIALS, AND SPECTRA (RQ = QUINONE SEMIQUINONE CATECHOLATE - TAP = 2-(M-TOLYLAZO)PYRIDINE - Z = O, +/-, +/-2)

Diamagnetic complexes of type Ru(RQc)(tap)2 are synthesized by reacting [Ru(OH2)2](ClO4)2.H2O with catechols in the presence of base (RQc = catecholate having R = H, 4-t-Bu, 3,5-(t-Bu)2 or 3,4,5,6-Cl4; tap = 2-(m-tolylazo)pyridine). Oxidation of these by cerium(IV) affords paramagnetic (S = 1/2) [Ru(RQs)(tap)2]ClO4 (RQs = semiquinone). The X-ray structure of [{Ru(RQc)(tap)2}2H2O].CH2Cl2 (R = H) is reported: space group P1BAR, Z = 2, a = 12.465 (2) angstrom, b = 14.983 (3) angstrom, c = 16.775 (3) angstrom, alpha = 90.21 (2)-degrees, beta = 100.26 (1)-degrees, gamma = 104.86 (2)-degrees, and V = 2976 (1) angstrom 3. The water molecule hydrogen bridges two chelate molecules via RQc oxygen atoms to give a dimer. The average Ru-O and C-O distances are 2.022 (5) and 1.342 (9) angstrom, respectively. Here the interacting metal d-pi and RQ p-pi redox orbitals are fully occupied (eight electrons). The EPR spectrum of [Ru11-(RQs)(tap)2]+ in frozen (77 K) dichloromethane glass shows a small anisotropy corresponding to a minor contribution of the Ru(III)(RQc) cannonical form. The complexes display four voltammetric responses corresponding to the processes RQn/RQs, RQs/RQc, (tap)2/(tap.-)(tap) and (tap.-)(tap)/(tap.-)2(RQn = quinone). The E1/2 values in the R = H case are 0.97, 0.20, -0.68 and -1.24 V, respectively. Comparison with the bpy congeners reveals that replacement of the bpy pair by the tap pair raises the reduction potentials of both RQ and the metal by a remarkable approximately 800 mV. The Ru(RQc)(tap)2 species, unlike the bpy analogues, are perfectly stable in air. The former complexes absorb strongly in the near-IR region (800-1300 nm) due to interligand RQc(p-pi) --> tap(azo p-pi*) excitation. The corresponding absorption in [Ru(RQs)(tap)2]+ occurs near 700 nm. There is also a feature near 1000 nm assignable to the MLCT excitation Ru(d-pi) --> RQs(p-pi). The transition energies are in qualitative agreement with observed reduction potentials.