Direct conversion of alpha-substituted ketones to metallo-1,2-enedithiolates

A new synthetic route to metallo-1,2-enedithiolates is presented. The addition of 1 equiv of the a-bromo ketones Ar-C(O)CHXR (X = Pr) {Ar = 2-quinoxaline, 2-, 3-, or 4-pyridine, Ph, CI-Ph, and pyrene (R = H); Ar = 2-quinoxaline (R = Me); and Ar = R = Ph} to Cp(2)Mo(SH)(2) followed by the addition of base results in the formation of the corresponding metallo-l,2-enedithiolate Cp(2)Mo{mu(2)-SC(Ar)C(R)S}. The alpha-tosyl ketones quinoxaline -C(O)CHR-tosyl {R = H, Me} and the alpha-phosphorylated ketone 3-pyridine-C(O)CH2-O-P(O)(OEt)(2) yield the same products as the corresponding alpha-bromo ketones upon reaction with Cp(2)Mo(SH)(2). The addition of acid to the heterocyclic substituted complexes yields Cp(2)Mo{eta(2)-SC(HetH(+))C(R)S}. Both Cp(2)Mo{eta(2)-SC(quinoxaline)C(H)S} and [Cp(2)Mo{eta(2)-SC(quinoxalinium)C(H)S}][BF4] have been crystallographically characterized. Cp(2)Mo{eta(2)-SC(quinoxaline)C(H)S} crystalizes in the C2/c space group with a = 21.451(2) Angstrom, b = 15.474 Angstrom, c = 12.2201(13) Angstrom, and beta = 107.440(7)degrees. [Cp(2)Mo{<eta(2)-SC(quinoxalinium)C(H)S}][BF4] crystalizes in the <P(1)over bar> space group with a = 7.4009(8) Angstrom, b = 10.1192(13)degrees Angstrom, c 15.930(4) Angstrom; alpha = 81.49(2)degrees, beta = 76.14(2)degrees, and gamma = 85.784 degrees. In the solid state [Cp(2)Mo{eta(2)-SC(quinoxalinium)C(H)S}][BF4] pi-stacks the heterocycle of two adjacent molecules with atom-atom distances of approximate to 3.6 Angstrom. The stacks are limited to pairs of molecules, and there is no long-range order. The pK(a) values for the quinoxalinium (R = H and Me) and the 2-, 3-, and 4-pyridinium (R = H) complexes have been determined in acetonitrile to be 1-3 units larger than the free heterocycles. The pK(a) of the pyridinium complexes follows the substitution trend 2 approximate to 4 > 3 > free pyridinium and is consistent with resonance stabilization of pyridinium by the metallo-l,2-enedithiolate. Electronic transitions in these complexes have been assigned to a LMCT transition and an ILCT transition by comparison of the various complexes accompanied with solvent sensitivity studies.