Diastereoselective Synthesis, Spectroscopy, and Electrochemistry of Ruthenium(II) Complexes of Substituted Pyrazolylpyridine Ligands

We report the synthesis of the hetero- and homoleptic
ruthenium(II) complexes
Ru(bpy)2L2+,
Ru(bpy)L22+ (bpy
is 2,2‘-bipyridine), and RuL32+ of six new
bidentates L, the substituted pyrazolylpyridines
1−6 (1-substituted-3-(2-pyridinyl)-4,5,6,7-tetrahydroindazoles with substituents R = H,
CH3, Ph, or C6H4-4‘‘-COOX where X
= H,
CH3, or C2H5). These were
fully characterized by 1H- and 13C-NMR
spectroscopy and elemental analysis. The
UV−visible spectra and redox properties of the complexes, some in the
ruthenium(III) and reduced bipyridine
oxidation states, are also discussed. The substituents R played a
role in determining the stereochemistry of the
Ru(bpy)L22+ and
RuL32+ products. The reaction of
Ru(DMSO)4Cl2 with 3 equiv of L bearing
aromatic substituents
gave only meridional RuL32+ isomers. The
one-step reaction of Ru(bpy)Cl3·H2O
with 2 equiv of L provided a
mixture of the three possible
Ru(bpy)L22+ isomers, from which one
symmetric isomer (labeled β) was isolated
pure. A trans arrangement of the pyrazole groups was
deduced by 1H-NMR and confirmed by X-ray
crystallography for one such stereomer
(β-[Ru(bpy)(5)2](PF6)2,
R =
C6H4-4‘‘-COOC2H5).
In contrast, Ru(DMSO)4Cl2 reacted with 2 equiv of L and then 1 equiv of bpy to
selectively form the other symmetric isomer (labeled α)
where the pyridine groups of L are trans. Crystal data
for
β-[Ru(bpy)(5)2](PF6)2
(C52H50N8O4F12P2Ru)
with Mo
Kα (λ = 0.710 73 Å) radiation at 295 K:  a =
28.442(13) Å, b = 18.469(15) Å, c
= 23.785(9) Å, β = 116.76(0)°, monoclinic, space group C2/c, Z
= 8. Fully anisotropic (except for H and disordered F atoms),
full-matrix,
weighted least-squares refinement on F2 gave a
weighted R on F2 of 0.2573
corresponding to R on F of 0.1031
for data where F > 4σ(F ).