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 affords
excellent yields of [Li(tmeda)]2[Zr(SC6H4-4-R)6], where R = CH3 (12-) or OCH3 (22-) and tmeda denotes
N,N,N‘,N‘-tetramethylethylenediamine. These complexes are air-sensitive canary-yellow solids, soluble in hexane,
diethyl ether, THF, and acetonitrile, that form yellow single crystals of [Li(tmeda)]21 (diethyl ether solution) or
[Li(THF)3]22 (THF solution) from saturated solutions at −20 °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 S6 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 S3 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 π-interact with zirconium. Complex 12- with a twist angle
of ca. 9.18° (trigonal prism, 0°; octahedron, 60°) agrees with the interpretations of computational studies on d0
complexes, which suggest that a nearly trigonal prismatic geometry is favored when the interaction between
metal and ligand is primarily through σ-bonds. The intrinsically weak π-donor thiolate ligand is probably converted
to a primarily σ-bonding system by the lithium−sulfur interaction. On the other hand complex 22- with a twist
angle of ca. 30.38° is trigonally twisted to the midpoint of the trigonal prismatic-to-octahedral reaction coordinate.
In complex 22- 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 π-interactions or electrostatics repulsion.