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 ([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.

Unsaturated and Benzannulated N-Heterocyclic Carbene Complexes of Titanium and Hafnium: Impact on Catalysts Structure and Performance in Copolymerization of Cyclohexene Oxide with CO2

“…The NMR spectra of 2a and 2b contain resonances typical of five-coordinate ([ κ 3 - O,C,O]- NHC)TiX 2 complexes including a doublet resonance originating from the Me groups of the two O i Pr moieties ( Supplementary Figures S7–S10 ), which are consistent with C 2v -symmetric structures in solution for both complexes [ 38 , 42 , 43 ]. The corresponding 13 C NMR spectra confirm the chelation of the NHC ligand to Ti center with typical chemical resonances at δ 183.7 and 198.1 ppm ( Supplementary Figures S8 and S10 ) [ 14 , 69 ], respectively, shifted upfield compared to the saturated ([ κ 3 - O,C,O]- Is NHC)Ti(O i Pr) 2 complex 2c [ 38 ].…”

“…Due to their potential application in polymerization of CHO with CO 2 [ 29 , 41 , 42 , 43 ], the bis-isopropoxide I NHC- and Bz NHC-titanium complexes 2a and 2b were also synthesized and were found to be readily accessible, in quantitative yields ( Scheme 2 ), via salt metathesis of LiO i Pr with complex 1a and 1b , respectively, similarly to the saturated NHC-titanium analogue [ 38 ].…”

“…However, their dissociation from oxophilic metals was partially prevented by designing multidentate anionic carbon, nitrogen, and oxygen-functionalized NHC ligands [ 5 , 13 , 15 , 16 , 17 ]. Anchoring such functionalized NHC ligands to oxophilic metals has proved to be a successful approach for developing catalysts, notably of group 4 metals, for the (oligo-)polymerization of olefins [ 18 , 19 , 20 , 21 , 22 , 23 , 24 , 25 , 26 , 27 , 28 , 29 ], hydroamination/cyclization of aminoalkenes [ 30 , 31 , 32 , 33 , 34 , 35 , 36 ], controlled ring-opening polymerization of rac -lactide [ 37 , 38 , 39 , 40 ], and more recently, for the copolymerization of epoxides with CO 2 [ 41 , 42 , 43 , 44 , 45 ].…”

“…Although most of the highly selective and active catalyst systems for the copolymerization of epoxides with CO 2 are based on divalent (Mg, Co, Zn) and trivalent (Cr, Co, Al) metals bearing ligands such as β-diketiminates, salens, porphyrins, and multidentate phenolate macrocycles [ 46 , 47 , 48 , 49 , 50 , 51 , 52 ], the group 4 metals have emerged as a new class of catalyst for this reaction, combining decent catalytic activity with high selectivity towards the formation of polycarbonates [ 41 , 42 , 43 , 44 , 45 , 53 , 54 , 55 , 56 , 57 , 58 , 59 , 60 , 61 , 62 , 63 , 64 , 65 , 66 ]. NHC-based catalysts of this kind have appeared to be particularly promising under relatively mild copolymerization conditions [ 41 , 42 , 43 , 44 , 45 ]. However, so far the tetravalent titanium, zirconium and hafnium complexes for the CO 2 /cyclohexene oxide (CHO) copolymerization that have selectively given the desired poly(cyclohexene carbonate) (PCHC), without cyclohexene carbonate (CHC) or homopolymer of CHO (PCHO) as side products, have exclusively been based on tridentate bis-phenolate NHC-type ligands with saturated backbones ( Scheme 1 ) [ 41 , 42 , 43 , 44 , 45 ].…”

“…NHC-based catalysts of this kind have appeared to be particularly promising under relatively mild copolymerization conditions [ 41 , 42 , 43 , 44 , 45 ]. However, so far the tetravalent titanium, zirconium and hafnium complexes for the CO 2 /cyclohexene oxide (CHO) copolymerization that have selectively given the desired poly(cyclohexene carbonate) (PCHC), without cyclohexene carbonate (CHC) or homopolymer of CHO (PCHO) as side products, have exclusively been based on tridentate bis-phenolate NHC-type ligands with saturated backbones ( Scheme 1 ) [ 41 , 42 , 43 , 44 , 45 ]. Previous attempts to substitute the saturated-NHC backbone of zirconium catalysts by unsaturated- or benzannulated-NHC backbones in order to evaluate the effects on the copolymerization catalysis have irremediably led to a catalytically inactive isolable mixture of zwitterionic and heteroleptic zirconium compounds ( Scheme 1 ) [ 44 ].…”