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.

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A convenient new synthetic route to metallo-1,2-enedithiolates was applied to the synthesis of (dppe)M{S2C2(heterocycle)(H)}; dppe = 1,2-bis(diphenyldiphosphino)ethane, M = Ni, Pd, and Pt, and heterocycle = 2-quinoxaline, 2-, 3-, and 4-pyridine, and 2-pyrazine. These complexes were prepared from the corresponding bis(hydrosulfido) complexes (dppe)M(SH)2 and the α-bromo ketones, heterocycle−C(O)CH2Br. In the solid state, (dppe)Ni{S2C2(2-pyrazine)(H)} is a slightly distorted square plane with a planar five-membered metallo-1,2-enedithiolate ring. The metallo-1,2-dithiolate is ≈6° from being coplanar with the pyrazine ring. These complexes all have a UV−visible band assignable to an intraligand transition (ILCT) that is best described as a 1,2-enedithiolate π → heterocycle π* charge transfer transition. The energy of the ILCT transition tracks with the reduction potential of the appended aromatic heterocycle. The pK a of the protonated complexes is 1−3 units higher than that of the parent heterocycle, independent of the metal, and consistent with resonance stabilization of the protonated heterocycle by the 1,2-enedithiolate ligand.

SynopsisThe kinetics of tetrabutyl titanate catalyzed transesterification of dimethyl terephthalate with poly(tetramethy1ene ether) glycol of various molecular weight has been studied in the presence and absence of 1,4-butanediol. Detailed analysis of experimental data with a proposed kinetic model indicates that the equal reactivity hypothesis for functional groups is valid within a wide range of experimental conditions studied. I t is also found that the reactivity of hydroxyl groups of macrodiols such as poly(tetramethy1ene ether) glycol is independent of the chain length of the polymer. Effect of various reaction parameters on the transesterification rate and the resulting product composition is also discussed.