Trigonal prismatic vs octahedral coordination geometry: Syntheses and structural characterization of hexakis(arylthiolato) zirconate complexes

Treating [Li(tmeda)](2)[Zr(CH3)(6)] with aryl thiols, HSC6H4-4-R, in a 1:6 stoichiometry in diethyl ether a affords excellent yields of [Li(tmeda)](2)[Zr(SC6H4-4-R)(6)], where R = CH3 (1(2-)) or OCH3 (2(2-)) and tmeda denotes N,N,N',N'-tetramethylethylenediamine. These complexes are air-sensitive canary-yellow solids, soluble in hexane, diethyl ether, THF, and acelonitrile, that form yellow single crystals of [Li(tmeda)](2)1 (diethyl ether solution) or [Li(TNF)(3)](2)2 (TNF solution) from saturated solutions at -20 degrees C. Both complexes were characterized by X-ray crystallography and consist of a zirconium atom coordinated solely by the sulfur atoms of six aryl thiolate ligands in a nonoctahedral geometry. In each structure the lithium cation coordinates to the three sulfur atoms on the triangular faces of the S-6 pseudotrigonal prism. These Lithium-sulfur interactions appear to play a role in determining the coordination geometry about the metal center by orienting the sulfur lone pairs of electrons slightly out of the plane defined by the S-3 triangular face and tilted away from the zirconium atoms. A likely consequence is the positioning of the sulfur lone pairs of electrons away from orthogonality with the zirconium-sulfur vector, and hence, they are poorly arranged to pi-interact with zirconium. Complex 1(2-) with a twist angle of ca. 9.18 degrees (trigonal prism, 0 degrees; octahedron, 60 degrees) agrees with the interpretations of computational studies on da complexes, which suggest that a nearly trigonal prismatic geometry is favored when the interaction between metal and ligand is primarily through sigma-bonds. The intrinsically weak pi-donor thiolate ligand is probably converted to a primarily sigma-bonding system by the lithium-sulfur interaction. On the other hand complex 22- with a twist angle of ca, 30.38 degrees is trigonally twisted to the midpdint of the trigonal prismatic-to-octahedral reaction coordinate. In complex 2(2-) the 4-OCH3 group is an electron donor by resonance effects that possibly may lead to the movement away from the expected trigonal prismatic geometry due to either pi-interactions or electrostatics repulsion.