Intramolecular Metal···Sulfur Interactions of Platinum(II) 1,4,7-Trithiacyclononane Complexes with Bipyridyl Ligands: The Relationship between Molecular and Electronic Structures

Five platinum(II) 1,4,7-trithiacyclononane (ttcn) complexes with bidentate-substituted 2,2‘-bipyridine ligands have
been prepared and structurally characterized:  [Pt(bpy)(ttcn)](PF6)2 (bpy = 2,2‘-bipyridine), triclinic, P1̄, a = 10.2529(3) Å, b = 10.7791(3) Å, c = 10.7867(3) Å, α = 83.886(1)°, β = 87.565(1)°, γ = 84.901(1), V = 1179.99(6)
Å3, Z = 2; [Pt(4,4‘-dmbpy)(ttcn)](PF6)2·CH3CN·H2O (4,4‘-dmbpy = 4,4‘-dimethyl-2,2‘-bipyridine), triclinic, P1̄, a =
10.1895(3) Å, b = 11.8566(4) Å, c = 13.1004(4) Å, α = 77.345(1)°, β = 79.967(1)°, γ = 72.341(1)°, V =
1461.56(8) Å3, Z = 2; [Pt(5,5‘-dmbpy)(ttcn)](PF6)2 (5,5‘-dmbpy = 5,5‘-dimethyl-2,2‘-bipyridine), triclinic, P1̄, a =
10.6397(4) Å, b = 10.8449(4) Å, c = 11.2621(4) Å, α = 90.035(1)°, β = 98.061(1)°, γ = 91.283(1)°, V =
1286.32(8) Å3, Z = 2; [Pt(dbbpy)(ttcn)](PF6)2·CH3NO2 (dbbpy = 4,4‘-di-tert-butyl-2,2‘-bipyridine), triclinic, P1̄, a =
11.5422(7) Å, b = 11.6100(7) Å, c = 13.6052(9) Å, α = 85.902(1)°, β = 89.675(1)°, γ = 74.942(1)°, V =
1755.90(19) Å3, Z = 2; and [Pt(dtfmbpy)(ttcn)](PF6)2·CH3CN (dtfmbpy = 5,5‘-di-trifluoromethyl-2,2‘-bipyridine): 
monoclinic, P21/c, a = 13.1187(9) Å, b = 20.9031(15) Å, c = 11.3815(8) Å, β = 105.789(2)°, V = 3003.3(4) Å3,
Z = 4. For each salt, the platinum(II) center of the cation is bonded to two nitrogen atoms of the chelating diimine
and two sulfur atoms of the thioether macrocycle. The third sulfur atom of ttcn forms a long apical interaction with
the metal center (2.84−2.97 Å), resulting in a flattened square pyramid structure. An examination of these and 17
other structures of platinum(II) ttcn complexes reveals a correlation between the apical Pt···S distance and the
donor properties of the ancillary ligands, suggesting a means for using variations in ligand electronic properties to
tune molecular structure. The room-temperature absorption spectra in acetonitrile solution show a broad and
comparatively low-energy MLCT band maximizing near ∼390 nm for the bpy and dialkyl-substituted bipyridyl
derivatives. The maximum is dramatically red-shifted to 460 nm in the spectrum of the dtfmbpy complex as a result
of the electron-withdrawing properties of the −CF3 groups. The 3:1 EtOH/MeOH 77 K glassy solution emission
spectra exhibit low-energy emission bands (λmax, 570−645 nm), tentatively assigned as originating from a lowest,
predominantly spin-forbidden MLCT excited state that is stabilized by apical Pt···S interactions.