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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
Organometallics ( IF 2.8 ) Pub Date : 2017-11-07 00:00:00 , DOI: 10.1021/acs.organomet.7b00705
Coralie C. Quadri 1 , Ralte Lalrempuia 1 , Julie Hessevik 1 , Karl W. Törnroos 1 , Erwan Le Roux 1
Affiliation  

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 17 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 26 show an increase in activity by approximatively 20%, whereas the less encumbered complexes 1 and 7 exhibit a decrease in activity.

中文翻译:

三齿N-杂环碳钛(IV)氧化苄基,氧化硅,乙酸盐和叠氮化物配合物的结构表征及其催化氧化环己烯与CO 2共聚的功效评估

三齿双-芳氧基N-杂环卡宾的反应性(NHC)钛络合物([κ 3 - øÇÒ ] -NHC)的Ti(X 1)(X 2)(X 1 = X 2 =氯(1) ;通过盐复分解(使用LiOBn,NaOAc),质子分解(使用(t BuO)3 SiOH)和σ键复分解(使用Me 3 SiN 3)研究了X 1 = X 2 = O i Pr(2)),导致一系列NHC钛络合物各种轴承X和XL型共配体,([κ的3 -OCO ] -NHC)Ti(X 1)(X 2)(X 1 = X 2 = OBn(3); X 1 = Cl,X 2 = OSi(O t Bu)34); X 1 = O i Pr,X 2 = OSi(O t Bu)35); X 1 = X 2 = OAc(6); X 1 = X 2 = N 37))。配合物的分子结构35,通过X射线晶体学研究确定,其公开五配位络合物,而配合物47仅在THF存在结晶,导致六配位的Ti-THF加合物([κ 3 - øÇO 1 -NHC)Ti(X 1)(X 2)(THF)(4-THF7-THF)。相比之下,结构6揭示了一个罕见的七配位Ti络合物的例子,其中三齿NHC和两个双齿OAc配体在钛原子周围以五边形-双锥体的方式配位。1,3-偶极钛叠氮化物7的反应性还通过与偶极亲子二甲基乙炔二羧酸酯的[3 + 2]环加成反应进行,对于新形成的与钛8(Tz)配位的三唑并(Tz)配体表现出独特的配位模式即,如([κ 1 -N 1 ] -TZ))。尝试访问NHC钛(III)物种从还原1与利贝特3 ·H·THF铅主要以([κ 5 -OÑÇNO ] -咪唑烷)Ti(Cl)(THF)(9)通过氢化物转移到NHC卡宾原子上。完全表征NHC-Ti系复合物1 - 7分别为环己烯氧化物与CO共聚评价2。加入[PPN] X助催化剂(X = Cl,N 3)后,所有络合物在低CO 2压力(<0.5 bar)下均具有活性,并且对形成无规多聚体具有高度选择性(> 99%) (环己烯氧化物ALT -碳酸酯)。尽管大肠菌群的变化总体上对活性有中等程度的影响,但结果大多表明,在存在[PPN] Cl([PPN] =(Ph 3P═)2 N),在NHC-Ti(IV)异丙氧化物,叠氮化物和乙酸盐络合物中在空间上受阻较小的大肠菌素与其他大块大肠菌素相比,具有更高的活性,周转量高达930。在另一方面,当多个亲核[PPN] n的3采用盐,空间位更受阻络合物2 - 6示出了由近似20%在活性的增加,而较不担保络合物17表现出活性降低。
更新日期:2017-11-08
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