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.

The complexes (dppe)M{S2C2(2-quinoxaline)(R)}, where dppe = (diphenylphosphino)ethane, M = Ni, Pd, and Pt, and R = H and Me, have as their lowest-energy band an intraligand charge transfer transition (ILCT). Excitation of deaerated solutions of (dppe)Pt{S2C2(2-quinoxaline)(R)} lead to emissions from an 1ILCT* and an 3ILCT*. The lifetimes of these excited states (τ) and the quantum yields for the emissions (φ) for (dppe)Pt{S2C2(2-quinoxaline)(H)} in CH3CN are 1τ = 0.16 ns,1φ = 0.005 and 3τ = 3.3 μs, 3φ = 0.01, respectively. The 3ILCT* of these quinoxaline-substituted complexes can undergo a diverse suite of excited state reactions, including electron, proton, and hydrogen atom transfers. The second order rate constants (k q) for the quenching of the 3ILCT* emission by acids increases with the thermodynamic driving force for the excited state proton transfer, an observation consistent with excited state electron and hydrogen atom transfers. Dihydroquinone and p-methoxyphenol are substantially better quenching agents than excited state proton transfer would predict and thermodynamic calculations suggest that they quench the 3ILCT* by hydrogen atom transfer.

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.

Luminescence and excited-state electron-transfer reactions of (dppe)Pt{S2C2(2-pyridine)(H)} and (dppe)Pt{S2C2(4-pyridine)(H)} (dppe = diphenyldiphosphinoethane) are enabled by protonation of the appended pyridine, thus serving as a novel means of electronic switching. The neutral complexes have low-lying d-to-d transitions that lead to rapid decay of excited states by nonradiative processes. However, upon protonation, a 1,2-enedithiolate-to-heterocycle π* intraligand charge-transfer transition (ILCT) becomes lower in energy than the d-to-d transition, thus giving rise to emissive 1ILCT* and 3ILCT* excited states for [(dppe)Pt{S2C2(2-pyridinium)(H)}][BF4] and [(dppe)Pt{S2C2(4-pyridinium)(H)}][BF4]. The assignment of these excited states was based on their energies and lifetimes (τ) which range from τ = 3 to 4 ns for the singlet and from τ = 2000 to 7500 ns for the triplet, respectively. Emission quantum yields (φ) increase with solvent polarity and range from φ = 0.0006 to 0.003 for the singlet and from φ = 0.001 to 0.03 for the triplet. The electron acceptors p-dinitrobenzene and tetracyanoquinodimethane quench the 3ILCT* with k q values of 4 × 109 and 9 × 109 M-1 s-1, respectively. The k q values are nearly identical for the 2- and 4-pyridinium complexes, reflecting the similarity in the thermodynamic driving forces for electron transfer from these complexes. The ability to employ a simple and reversible ground-state reaction (ligand protonation) to control access to reactive excited states should prove useful in numerous applications.