Direct Conversion of α-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 α-bromo ketones Ar-C(O)CHXR (X = Br) {Ar = 2-quinoxaline, 2-, 3-, or 4-pyridine, Ph, Cl−Ph, and pyrene (R = H); Ar = 2-quinoxaline (R = Me); and Ar = R = Ph} to Cp2Mo(SH)2 followed by the addition of base results in the formation of the corresponding metallo-1,2-enedithiolate Cp2Mo{η2-SC(Ar)C(R)S}. The α-tosyl ketones quinoxaline−C(O)CHR−tosyl {R = H, Me} and the α-phosphorylated ketone 3-pyridine-C(O)CH2−O−P(O)(OEt)2 yield the same products as the corresponding α-bromo ketones upon reaction with Cp2Mo(SH)2. The addition of acid to the heterocyclic substituted complexes yields Cp2Mo{η2-SC(HetH+)C(R)S}. Both Cp2Mo{η2-SC(quinoxaline)C(H)S}and [Cp2Mo{η2-SC(quinoxalinium)C(H)S}][BF4] have been crystallographically characterized. Cp2Mo{η2-SC(quinoxaline)C(H)S} crystalizes in the C2/c space group with a = 21.451(2) Å, b = 15.474 Å, c = 12.2201(13) Å, and β = 107.440(7)°. [Cp2Mo{η2-SC(quinoxalinium)C(H)S}][BF4] crystalizes in the P1̄ space group with a = 7.4009(8) Å, b = 10.1192(13)° Å, c = 15.930(4) Å; α = 81.49(2)°, β = 76.14(2)°, and γ = 85.784°. In the solid state [Cp2Mo{η2-SC(quinoxalinium)C(H)S}][BF4] π-stacks the heterocycle of two adjacent molecules with atom−atom distances of ≈ 3.6 Å. 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 ≈ 4 > 3 > free pyridinium and is consistent with resonance stabilization of pyridinium by the metallo-1,2-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.