Synthesis and Structure of a Distorted Octahedral Palladium(II) Complex Coordinated with a Tetrathioether Ligand Tethered with Bulky Substituents

A new type of an o-phenylene-bridged tetrathioether ligand tethered with extremely bulky substituents, 2,4,6-tris[bis(trimethylsilyl)methyl]phenyl (Tbt) groups, at its terminal sulfur atoms, TbtS[(o-phenylene)S]3Tbt (1), was synthesized
by taking advantage of the coupling reaction of thiols with iodobenzenes using Cu2O in 2,4,6-trimethylpyridine.
Complexation of 1 with Na2PdCl4 gave the corresponding dichloropalladium(II) complex, [PdCl2(1)] (7). The X-ray
structural analysis of 7 indicated that the central palladium metal is in a distorted octahedral environment, where
the two inner sulfur atoms of 1 and the two chlorine atoms form a square planar arrangement around the palladium
metal and the two terminal sulfur atoms of 1 weakly coordinate to the palladium center at the axial positions. In
addition, a phenyl analogue of 1, PhS[(o-phenylene)S]3Ph (2), was synthesized by a method similar to that for 1.
Reaction of 2 with Na2PdCl4 gave the corresponding dichloropalladium(II) complex, [PdCl2(2)] (8). X-ray crystallography
of 8 showed a type of the structure different from the distorted octahedral structure in 7, i.e., a square planar
arrangement around the central palladium atom with the one terminal sulfur atom of 2, its neighboring sulfur atom,
and the two chlorine atoms. The results of the NMR studies on 8 in a CDCl3 solution were not consistent with the
results of the X-ray crystallography and suggested the coordination of the two inner sulfur atoms of 2 to the
palladium metal, although a possibility of the existence of the rapid interconversion among isomers could not be
excluded.