Electron-Deficient Group 4 Metal Complexes of Sulfur-Bridged Dialkoxide Ligands: Synthesis, Structure, and Polymerization Activity Studies

Coordination of the sulfur-bridged dialkoxide ligands {OCR2CH2SCH2CR2O}2- ({OSOR}2-,
R = Me, p-tolyl) on titanium and zirconium centers has been studied. A variety of complexes
having one or two ancillary {OSOMe}2- ligands, viz., M(OR)2{OSOMe} (M = Zr, OR = OtBu,
4; M = Ti, OR = OiPr, 5), M{OSOMe}2 (M = Zr, 6; Ti, 7), Zr(CH2Ph)2{OSOMe} (8), and MCl2{OSOMe}(THF)n (M = Zr, n = 0, 9; n = 1, 10; n = 2, 11; M = Ti, n = 0, 12; n = 2, 13), have
been prepared in good yields by alcohol or alkane elimination from {OSOMe}H2 (1a) or by
salt metathesis routes from alkali metal salts M{OSOMe} (M = K, 2a; M = Li, in
situ-generated). Coordination of the p-tolyl-substituted ligand by these and other routes
proved to be much more difficult, and only TiCl2{OSOtol} (14) was obtained. Crystallographic
studies showed that dichloro complexes 11 and 13 and bis-ligand complex 7 adopt in the
solid state mononuclear structures with no bonding interaction between the sulfur atom
and metal centers. The crystal structure of 4 contains two independent dinuclear molecules
that interconvert formally by exchange of tBuO and chelating {OSOMe} ligands between
terminal and bridging modes and feature normal Zr−S bonds. NMR data for 4 and 12 are
consistent with dinuclear C1-symmetric structures in toluene solution, while dibenzyl complex
8 has a mononuclear structure. Variable-temperature NMR studies combined with line-shape analysis established that the fluxional behavior of 8 results from the exchange between
η2/ η1-benzyl ligands (ΔH⧧ = 10.6 ± 1 kcal·mol-1; ΔS⧧ = −2.7 ± 2 cal·mol-1·K-1). Abstraction
of one benzyl ligand of 4 with 1 equiv of [Ph3C][B(C6F5)4] or B(C6F5)3 proceeds cleanly to
give the corresponding thermally unstable, ionic species [Zr(CH2Ph){OSOMe}]+[BX(C6F5)3]-
(X = C6F5, 15; CH2Ph, 16), which have been characterized by 1H, 11B, 13C, and 19F
NMR spectroscopy and show Cs symmetry in solution. The decomposition of 16 proceeds via
C6F5/benzyl exchange from Zr to B and generates the stable [Zr(C6F5){OSOMe}]+[η6-(PhCH2)B(CH2Ph)(C6F5)2]- (17). Cationic species in situ-generated from Zr chloro
complexes and MAO are highly active but very unstable ethylene polymerization catalysts.