Structural Characterization of Tridentate N-Heterocyclic Carbene Titanium(IV) Benzyloxide, Silyloxide, Acetate, and Azide Complexes and Assessment of Their Efficacies for Catalyzing the Copolymerization of Cyclohexene Oxide with CO₂

Abstract

The reactivity of tridentate bis-aryloxy N-heterocyclic carbene (NHC) titanium complexes ([κ3-O,C,O]-NHC)Ti(X1)(X2) (X1 = X2 = Cl (1); X1 = X2 = OiPr (2)) via salt metathesis (with LiOBn, NaOAc), protonolysis (with (tBuO)3SiOH), and σ-bond metathesis (with Me3SiN3) were investigated, leading to a series of NHC titanium complexes bearing various X and XL type coligands, ([κ3-O,C,O]-NHC)Ti(X1)(X2) (X1 = X2 = OBn (3); X1 = Cl, X2 = OSi(OtBu)3 (4); X1 = OiPr, X2 = OSi(OtBu)3 (5); X1 = X2 = OAc (6); X1 = X2 = N3 (7)). The molecular structures of complexes 3 and 5 were identified by X-ray crystallographic studies, disclosing five-coordinate complexes, while complexes 4 and 7 crystallize only in the presence of THF, leading to the six-coordinate Ti–THF adducts ([κ3-O,C,O]-NHC)Ti(X1)(X2)(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 ([κ1-N1]-Tz)). Attempts to access NHC-Ti(III) species from the reduction of 1 with LiBEt3H·THF lead mainly to ([κ5-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 CO2. Upon the addition of [PPN]X cocatalysts (with X = Cl, N3), all of the complexes are active at low CO2 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] = (Ph3P═)2N), 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]N3 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.