SEMIQUINONE IMINE COMPLEXES OF RUTHENIUM - COORDINATION AND OXIDATION OF THE 1-HYDROXY-2,4,6,8-TETRA-TERT-BUTYLPHENOXAZINYL RADICAL

Reactions between Ru(PPh3)3Cl2 and the 1-hydroxy-2,4,6,8-tetra-tert-butylphenoxazinyl radical (HPhenoxSQ) have been investigated. Stoichiometric reactions carried out under inert conditions with ratios of 1:1 and 1:2 for Ru(PPh3)3Cl2 to HPhenoxSQ gave as products Ru(PPh3)2Cl2(PhenoxSQ) and Ru(PPh3)Cl(PhenoxSQ)2, respectively. The complexes have been characterized spectrally, electrochemically, and structurally (Ru(PPh3)2Cl2(PhenoxSQ), monoclinic, P2(1)/n, a = 19.570 (6) angstrom, b = 14.652 (4) angstrom, c = 19.999 (7) angstrom, beta = 91.65 (3)-degrees, V = 5732 (3) angstrom 3, Z = 4, R = 0.032; Ru(PPh3)Cl(PhenoxSQ)2, monoclinic, P2(1)/c, a = 10.228 (2) angstrom, b = 27.775 (8) angstrom, c = 23.656 (9) angstrom, beta = 97.77 (2)-degrees, V = 6658 (3) angstrom 3, Z = 4, R = 0.045). Ru(PPh3)2Cl2(PhenoxSQ) is diamagnetic due to strong spin coupling between the radical and the S = 1/2 metal ion; Ru(PPh3)Cl(PhenoxSQ)2 has a single unpaired electron. 2,4,6,8-Tetra-tert-butylphenoxazin-1-one (PhenoxBQ) has been characterized structurally (monoclinic, P2(1)/c, a = 11.547 (4) angstrom, b = 17.659 (6), c = 13.579 (3) angstrom, beta - 105.92 (2)-degrees, V = 2662 (1) angstrom 3, Z = 4, R = 0.063) to give metrical parameters for the benzoquinone imine ligand in its unreduced form. PhenoxBQ has also been characterized electrochemically. It undergoes two reversible reductions at potentials that are shifted negatively relative to related benzoquinone ligands. Increased energy of the PhenoxBQ pi* level relative to the energy of a benzoquinone antibonding electronic level results in an important difference in the order of energies of complex electronic levels. Reduction of Ru(PPh3)Cl(PhenoxSQ)2 occurs at the metal to give a Ru(II) product, while reduction of corresponding semiquinone complexes occurs at the quinone ligands. The electrochemical potential for reduction of Ru(PPh3)Cl(PhenoxSQ)2 is -1.02 V (Fc/Fc+) and in its reduced form the complex is strongly nucleophilic. When the reaction between Ru(PPh3)3Cl2 and HPhenoxSQ is carried out in air at a 1:2 stoichiometry, the product is a Ru(III) complex containing an oxidized form of PhenoxBQ. Two oxygen atoms and one hydroxyl group have been added to the coordinated PhenoxSQ ligand with ring-opening and cyclization leading to formation of a substituted dihydrofuran ring. The resulting complex Ru(PPh3)Cl(PhenoxSQ)(OxPhenox) contains three chiral centers, two at carbon atoms and the third at the metal. Both oxygen addition and cyclization occur stereoselectively to give a single diastereomer. Ru(PPh3)Cl(PhenoxSQ)(OxPhenox) has been characterized spectrally and structurally (monoclinic, P2(1)/c, a = 19.092 (6) angstrom, b = 16.018 (7) angstrom, c = 24.889 (10) angstrom, beta = 111.74 (3)-degrees, V = 7070 (5) angstrom 3, Z = 4, R = 0.060). A mechanistic scheme is proposed for its formation by the reaction of reduced Ru(PPh3)Cl(PhenoxSQ)2-, formed initially in the reaction, with molecular oxygen.