No abstract

The reactivity of tridentate bis-aryloxy N-heterocyclic carbene (NHC) titanium complexes ([κ 3 -O,C,O]-NHC)-Ti(X 1 )(X 2 ) (X 1 = X 2 = Cl (1); X 1 = X 2 = OiPr (2)) via salt metathesis (with LiOBn, NaOAc), protonolysis (with (tBuO) 3 SiOH), and σ-bond metathesis (with Me 3 SiN 3 ) were investigated, leading to a series of NHC titanium complexes bearing various X and XL type coligands, ([The molecular structures of complexes 3 and 5 were identified by X-ray crystallographic studies, disclosing fivecoordinate complexes, while complexes 4 and 7 crystallize only in the presence of THF, leading to the six-coordinate Ti−THF adducts ([κ 3 -O,C,O]-NHC)Ti(X 1 )(X 2 )(THF) (4-THF and 7-THF). In contrast, the structure of 6 reveals a rare example of a seven-coordinate Ti complex in which the tridentate NHC and two bidentate OAc ligands are coordinated in a pentagonal-bipyramidal fashion around the titanium atom. The reactivity of the 1,3-dipole Ti azide 7 was also further carried out via a [3 + 2] cycloaddition reaction with the dipolarophile dimethyl acetylenedicarboxylate, exhibiting a unique coordination mode for the newly formed triazolato (Tz) ligands to titanium 8 (i.e., as ([κ 1 -N 1 ]-Tz)). Attempts to access NHC-Ti(III) species from the reduction of 1 with LiBEt 3 H•THF lead mainly to ([κ 5 -O,N,C,N,O]-imidazolidine)Ti(Cl)(THF) (9) via hydride transfer to the NHC carbene atom. The fully characterized NHC-Ti complexes 1−7 were evaluated for the copolymerization of cyclohexene oxide with CO 2 . Upon the addition of [PPN]X cocatalysts (with X = Cl, N 3 ), all of the complexes are active at low CO 2 pressure (<0.5 bar) and are highly selective (>99%) toward the formation of atactic poly(cyclohexene oxide-alt-carbonate). While the variation of the coligands has an overall moderate effect on the activity, the results mostly indicate that, in the presence of [PPN]Cl ([PPN] = (Ph 3 P) 2 N), the sterically less hindered coligands in NHC-Ti(IV) isopropoxide, azide, and acetate complexes show better activity with turnovers up to 930 in comparison to other bulky coligands. On the other hand, when the more nucleophilic [PPN]N 3 salt is employed, the sterically more hindered complexes 2−6 show an increase in activity by approximatively 20%, whereas the less encumbered complexes 1 and 7 exhibit a decrease in activity.

Bis-ligated titanium(IV) metal complexes supported by bidentate unsymmetrical o-hydroxyaryl-substituted N-heterocyclic carbene ligands were synthesized and structurally identified. While the direct addition of the doubly deprotonated bulky imidazolidinium chloride salts [ Dipp,4-R NHC-H]Cl (with Dipp = 2,6-diisopropylphenyl, R = H (2-hydroxyphenyl), and R = Me (2-hydroxy-4-methyl-phenyl)) with chloro-titanium precursor favors the formation of single stereoisomer corresponding to the bis-ligated titanium complexes trans-([κ 2-C,O]-Dipp,4-R NHC)2TiCl2 (R = H (2-hydroxyphenyl) for 4a H , and R = Me (2-hydroxy-4-methyl-phenyl) for 4a Me), the reactivity with sterically less hindered imidazolidinium chloride salts [ Mes,H NHC-H]Cl and [ Dep,H NHC-H]Cl as protioligands (with Mes = 2,4,6-trimethylphenyl and Dep = 2,6-diethylphenyl) did not afford to single stereoisomer of bis-ligated titanium complexes. These results combined with topographic steric maps as well as the buried volume descriptor (%Vbur) indicate that bidentate bulky N-Dipp-substituted NHC ligands offer some level of steric protection preventing the formation of other possible bis-ligated (C,O)-NHC-titanium stereoisomers.

The molecular structure of the title compound, [Ti2Cl2(C6H18NSi2)4], shows a binuclear motif of TiIIIatoms, formulated as [Ti(μ-Cl)(N(SiMe3)2)2]2, with two μ-Cl atoms bridging two ((Me3Si)2N)2Ti moieties. The coordination environment of both central TiIIIatoms is distorted tetrahedral, with a nearly planar four-membered Ti2Cl2core [Ti—Cl—Ti—Cl = 2.796 (15)°].