STEREOCHEMICALLY RIGID 7-COORDINATE COMPLEXES - SYNTHESIS, STRUCTURE, AND DYNAMIC STEREOCHEMISTRY OF CHLOROTRIS(N,N-DIALKYLMONOTHIOCARBAMATO)TITANIUM(IV) COMPLEXES

Seven-coordinate N.,N-dialkylmonothiocarbamate complexes of the type [Ti(R2mtc)3Cl] (R2mtc = SOCNR2; R = Me, Et, (i-Pr, or i-Bu) have been prepared by reaction of titanium(IV) chloride with stoichiometric amounts of Na(R2mtc) in dichloromethane or benzene. The complexes have been characterized by conductance measurements, NMR spectra, and infrared spectra. The crystal and molecular structure of [Ti(Et2mtc)3Cl] has been determined by X-ray diffraction and has been refined anisotropically by least-squares methods to R1= 0.062 and R2= 0.066 using 4862 independent diffractometer data having 2θmoK5≤ 56.75° and |F0| ≥ 1.58σ;F. The compound crystallizes in the monoclinic space group C2/c with eight molecules in a cell having dimensions a = 27.888 (5) Å,b = 9.786 (1) Å, c = 17.819 (2) Å, β = 97.674 (9)° (pobsd= 1.328, pcalcd= 1.323 g cm-3). The crystal contains discrete seven-coordinate molecules which have pentagonal bipyramidal geometry and approximate symmetry Cs-m. The chlorine atom occupies an axial position (Ti-Cl = 2.330 (1) Å), and the monothiocarbamate ligands are oriented so as to cluster the three sulfur atoms in all-cis positions on one triangular face of the pentagonal bipyramid (Ti-S = 2.477-2.489 Å; Ti-O = 2.076-2.089 Å). Low-temperature 1H NMR spectra of [Ti(Me2mtc)3Cl] favor the same structure in solution. At higher temperatures, NMR line-shape changes indicate the existence of two distinct kinetic processes: (1) a low-temperature intramolecular metal-centered rearrangement process that exchanges the spanning and the equatorial Me2mtc ligands (∆G≠ (-58.2 °C) = 10.8 ± 0.1 kcal/mol, ∆H≠ = 11.2 ± 0.3 kcal/mol, ∆S≠ = 2.0 ± 1.6 eu) and (2) a high-temperature process that involves rotation about the C… N bond in the Me2mtc ligands (∆G≠ (110 °C) = 20 kcal/ mol). A polytopal rearrangement mechanism involving a monocapped octahedral transition state that maintains the all-cis arrangement of sulfur atoms is suggested for the metal-centered rearrangement. © 1979, American Chemical Society. All rights reserved.