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 pKa
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
pKa 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.