Structural Characterization of Tridentate N-Heterocyclic Carbene

Article Cite This: Organometallics XXXX, XXX, XXX-XXX

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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 Coralie C. Quadri, Ralte Lalrempuia, Julie Hessevik, Karl W. Törnroos, and Erwan Le Roux* Department of Chemistry, University of Bergen, Allégaten 41, N-5007, Bergen, Norway S Supporting Information *

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 fivecoordinate 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 7THF). 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 ([κ5O,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 (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.

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INTRODUCTION N-heterocyclic carbenes (NHCs) are currently well-established as key ligands for materials science and catalysis, notably for the design of organometallic complexes.1 By way of their strong σdonor and weak to moderate π-acceptor electronic properties, NHC ligands have been demonstrated to be an important class of chelating ligands for low-oxidation-state transition metals, forming stable and active complexes for most of the applied catalytic systems.1 In contrast, the coordination of monodentate NHC ligands to oxophilic high-oxidation-state transition metals differs greatly due to their tendency to dissociate from the metal center.1f,h,2 To overcome this discrepancy, various multidentate NHC ligands bearing anionic and/or neutral donor ligands have been developed: i.e. containing pendant © XXXX American Chemical Society

arms with functional groups based on carbon, nitrogen, and oxygen atoms.1h,3 The coordination of such functionalized NHC ligands to high-oxidation-state transition metals has ensured an efficient chelation leading to more robust and stable complexes.1f,h,2,3 This class of NHC metal complexes, notably from group 4, was successfully applied as catalysts in organic transformations and in polymerization, especially from biorenewable monomers.1h,2a−d,4 The most representative examples are based on either bidentate alkoxy NHC-Ti5 or mer-tridentate bis-aryloxy NHC-M (with M = Ti,6 Zr,6b,7 Hf6b) for catalyzing the controlled ring-opening polymerization Received: September 14, 2017

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DOI: 10.1021/acs.organomet.7b00705 Organometallics XXXX, XXX, XXX−XXX

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Organometallics Scheme 1. Synthesis of Tridentate NHC’s of Titanium Benzyloxide and Silyloxide Complexes

([κ3-O,C,O]-NHC)Ti(Cl)2 (1) and ([κ3-O,C,O]-NHC)Ti(OiPr)2 (2) (Scheme 1).6a As illustrated in Scheme 1, the tridentate bis-aryloxy NHC titanium benzyloxide complex ([κ3O,C,O]-NHC)Ti(OBn)2 (3) was prepared in good yield from complex 1 via salt metathesis with 2 equiv of LiOBn in toluene. In contrast, the ligand exchange for introducing the bulky and electron-withdrawing siloxy ligand can be quantitatively isolated via the direct protonolysis of either complex 1 in the presence of NEt3 for neutralizing HCl (HCl elimination route) or complex 2 (isopropyl alcohol elimination route) with 1 equiv of (tBuO)3SiOH to afford complexes ([κ3-O,C,O]-NHC)Ti(Cl)(OSi(OtBu)3) (4) and ([κ3-O,C,O]-NHC)Ti(OiPr)(OSi(OtBu)3) (5), respectively (Scheme 1). The double ligand substitutions with an additional 1 equiv of (tBuO)3SiOH to complexes 4 or 5 were unsuccessful, most likely due to the increase in steric hindrance in those starting complexes. The 1H NMR data are consistent with C2v- and Cs-symmetric structures in solution for complex 3 (NCH2 protons: sharp singlet at δ 4.29 ppm in chloroform-d) and complexes 4 and 5 (NCH2 protons: doublet of multiplets centered at δ 3.12/3.37 ppm for 4 and δ 3.03/3.36 ppm for 5 in benzene-d6), respectively. These signal patterns are characteristics of such five-coordinate monomeric ([κ3-O,C,O]-NHC)Ti complex types (Figures S1− S3 in the Supporting Information).6a,8b While the coligand exchange OiPr ↔ OBn shows that chemical resonances of NCH2 protons are very similar for both complexes 2 and 3 (δ 4.31 ppm for 2 in chloroform-d),8a,15 the 1H NMR spectra of Cl or OiPr ↔ OSi(OtBu)3 coligand exchanges for both complexes 4 and 5 exhibit chemical resonances for NCH2 protons that are shifted toward the downfield region in contrast to complexes 1 and 2 (δ 2.88 and 2.94 ppm, respectively, in benzene-d6),6a,15 indicating the electron-deficient nature of the silyloxy coligand. The 13C NMR spectra of complexes 3−5 confirm the chelation of the NHCcarbene ligand to the Ti center with typical chemical resonances at δ 197.9 (for 3), 198.6 (for 4), and 200.3 (for 5) ppm (Figures S4−S6 in the Supporting Information). Compound 3 crystallizes as a monomeric five-coordinate complex, with a slightly distorted square pyramidal geometry (Addison and Reedijk parameter: τ = 0.13)16 with the base defined by C1, O1, O2, and O4 around the Ti center (Figure 1, Table 1, and Tables S1 and S2 in the Supporting Information). The presence of two bulky benzyloxide ligands in apical and equatorial positions displace the mer-tridentate bis-aryloxy NHC ligand away from the titanium center, considerably reducing the bite angle (βn) ∠O1−Ti1−O2 = 152.5(2)° in comparison to other five-coordinate ([κ3-O,C,O]-NHC)Ti(X)2 complexes 1 and 2 bearing sterically less hindered coligands (X

(ROP) of cyclic lactides and even fewer examples for the copolymerization of cyclohexene oxide (CHO) with CO2 on the basis of tridentate bis-aryloxy NHC titanium and zirconium complexes.8 Although titanium-containing complexes are particularly attractive as sustainable catalysts, due to their low toxicity, high abundance, and the low cost of titanium,4c,d such catalysts have received scant attention in the copolymerization of epoxide/CO28a,b9 with respect to other active and selective metal complexes predominantly based on Zn(II), Co(II/III), Cr(III), and Al(III) complexes.10,4c,d,11 Among the tetravalent titanium-based catalysts for the copolymerization of CHO/ CO2, the ([κ3-O,C,O]-NHC)Ti(X)2L complexes (X = Cl (1), X = OiPr (2), X = OiPr, Cl; L = THF), activated by onium salts [PPN]X (PPN = (Ph3P)2N) as cocatalysts, provide a completely alternating poly(cyclohexene-alt-carbonate) (PCHC) without formation of undesired side products (i.e. homopolymerization of CHO and cyclohexene carbonate) at 60 °C and at a low pressure of CO2 (PCO2 = 1−20 bar).8a,b The nature and the concentration of the cocatalyst and the temperature have a significant influence on the catalytic systems, leading us to consider that the reaction might presumably proceed via a cooperative intermolecular mechanism for the epoxide ring opening, similar to the suggested mechanism for highly active [PPN]X-activated salen-type complexes of Cr(III) or Co(III).12 Further investigations on simple modifications of salen-type MIIIX complexes (with M = Co, Cr) showed that the rate of copolymerization can be enhanced by changing the nature of the X-type coligands (for instance from halogen atoms to azide and acetate), ascribed to a more rapid ring-opening step of epoxides.13 A recent investigation demonstrated that a trivalent Ti(III) salen based complex upon addition of [PPN]X salts can produce efficiently a completely alternating PCHC at high temperature and high pressure (120 °C, PCO2 = 40 bar) comparable to trivalent (salen)Cr/M-X catalysts (M = Cr, Co), albeit so far limited to only one compound.14 Following these leads, we decided to study the influence of the X-type coligands on ([κ3-O,C,O]NHC)Ti(X)2 complexes (with X = OiPr, Cl, OBn, OAc, N3, OSi(OtBu)3/Cl, OSi(OtBu)3/OiPr) and the synthetic paths for obtaining NHC-TiIII species as potential precursors for the copolymerization of CHO with CO2.

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RESULTS AND DISCUSSION Synthesis and Structural Characterizations of Tridentate NHC Titanium Benzyloxide and Silyloxide Complexes. The synthetic routes for the ligand exchanges by bulky coligands can be readily accessible from the previously described tridentate bis-aryloxy NHC titanium complexes, i.e. B


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Organometallics

(2.212(8) Å) is slightly elongated in comparison to that of the sterically less hindered bis-chloride titanium complex 1 (2.160(3) Å) and identical with that of the sterically hindered bis-isopropoxide titanium complex 2 (2.212(5) Å).6a X-ray-quality crystals of compound 4 were grown as redpurple lath-shaped crytals by diffusion of pentane into a saturated THF solution of 4 and allowed the determination of the molecular structure of the THF adduct 4-THF in the solid state (Table 1 and Tables S3 and S4 in the Supporting Information). Compound 4-THF crystallizes as two structurally identical isomers (A, Figure 2; B, Figure S7 in the Supporting Information), in which both isomers exhibit a slightly distorted octahedral geometry with silyloxy and THF ligands in apical positions. The tridentate NHC ligand for both isomers is chelated to titanium in a mer fashion in the equatorial position with a βn value of 159.17(8)° (for isomer A; 158.29(8)° for isomer B), and both mer-tridentate NHC units deviate from planarity with torsion angles ∠C9−O1−O2−C23 of 53.6(8)° (for isomer A; ∠C56−O8−O9−C70 = 49.3(9)° for isomer B). The wide βn and large torsion angles observed for both isomers indicate that those six-coordinate compounds have an important steric congestion around the Ti center, presumably due to the bulky silyloxy and tBu group interactions, similar to those for the six-coordinate compound ([κ3-O,C,O]-NHC)Ti(Cl)(OiPr)(THF)) (βn = 159.19(9)° and torsion angle of 47.7(1)°).6a The Ti−Ccarbene bond distances found for the two isomers of 4-THF are 2.165(2) and 2.172(2) Å, which are in a comparable range of Ti−Ccarbene bond distances for typical mertridentate NHC-titanium(IV) complexes.2d The solid-state molecular structure of compound 5 crystallizes as two monomeric and independent isomers (Table 1), which are consistent with the Cs-symmetry observed

Figure 1. Crystal structure of compound 3. Hydrogen atoms are omitted for clarity. Selected bond lengths (Å): Ti1−C1 = 2.215(8), Ti1−O1 = 1.906(5), Ti1−O2 = 1.884(5), Ti1−O3 = 1.803(5), Ti1− O4 = 1.807(5), N1−C1 = 1.341(9), N2−C2 = 1.473(9). Selected angles (deg): O1−Ti1−O2 = 152.5(2), O3−Ti1−O4 = 108.6(2), C1−Ti1−O4 = 144.4(3), Ti1- O4−C39 = 157.2(5), Ti1−O3−C32 = 136.3(5).

= Cl, βn = 161.12(13)°; X = OiPr, βn = 158.80(15)°).6a As expected, due to the congested packing of 3 (Z = 2) and steric interactions between the bulky tBu and OBn groups, the NHC moiety deviates from planarity with a torsion angle ∠C5−O1− O2−C19 of −53.13(9)°, which is in the same range as that previously reported for the bis-isopropoxide titanium complex 2 (59.2(1)°).6a In this case, the Ti−Ccarbene bond distance of 3

Table 1. Crystal Structure and Refinement Data for 3, 4-THF, 5, 6, and 7-THF

chem formula formula wt temp/K wavelength/Å cryst syst space group a/Å b/Å c/Å α/deg β/deg γ/deg V/Å3 Z ρcalcd/g cm−3 abs coeff/mm−1 F(000) θ range for data collection/deg no. of rflns collected no. of indep rflns (Rint) completeness to θ/% no. of data/restraints/params goodness of fit on F2 final R1 indices (I > 2σ(I)) wR2 (all data) largest diff peak, hole/e Å−3

3

4-THF

5

6

7-THF

C45H58N2O4Ti 738.83 103(2) 0.71073 triclinic P1̅ (No. 2) 11.865(8) 13.619(9) 14.576(9) 113.606(9) 97.439(8) 103.985(8) 2025(2) 2 1.212 0.255 792 1.577−21.199 13708 4381 (0.1709) 98.1 4381/12/481 0.971 0.0812 0.2195 0.631, −0.480

C51H87ClN2O8SiTi 967.66 103(2) 0.71073 orthorhombic Pca21 (No. 29) 31.918(3) 9.8781(8) 34.357(3) 90 90 90 10832.5(15) 8 1.187 0.280 4192 1.742−30.054 176970 31690 (0.0653) 100.0 31690/1/1196 1.106 0.0440 0.1060 0.834, −0.363

C46H78N2O7SiTi 847.09 103(2) 0.71073 monoclinic P21/c (No. 14) 18.4038(13) 25.6508(18) 21.5683(15) 90 103.1390(10) 90 9915.2(12) 8 1.135 0.243 3680 1.834−25.043 112707 17546 (0.0851) 100.0 17546/1140/1128 1.023 0.0464 0.1249 0.949, −0.371

C35H50N2O6Ti 642.67 173(2) 0.71073 monoclinic C2/c (No. 15) 29.115(6) 12.120(2) 9.9618(19) 90 95.665(3) 90 3498.0(12) 4 1.220 0.289 1376 2.694−26.767 3730 3730 (0.0623) 99.9 3730/15/238 1.074 0.0620 0.1786 0.471, −0.633

C42H60N8O3Ti 772.88 103(2) 0.71073 monoclinic P21/c (No. 14) 13.175(2) 19.231(3) 16.909(3) 90 100.553(3) 90 4211.7(12) 4 1.219 0.250 1656 1.572−25.023 36051 7441 (0.1137) 100.0 7441/0/500 1.005 0.0558 0.1396 0.555, −0.417

C


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Organometallics

from silyloxy and NHC ligands. Although the mer-NHC units are significantly twisted for the sterically encumbered isopropoxide and benzyloxide titanium complexes 2 and 3 in comparison to complex 5, similar Ti−Ccarbene bond lengths for 5 were observed (isomer A, 2.210(2) Å; isomer B, 2.212(2) Å). Synthesis and Structural Characterizations of Tridentate NHC Titanium Acetate Complex. Another aim of this work was also to explore the synthetic methods through which mer-tridentate NHC-Ti complexes bearing different nucleophiles, such as acetate (−OAc) and azide (N3−), might be accessible as coligands. Such a ligand substitution approach was previously demonstrated for salen-type Cr and Co catalysts, allowing a substantial increase in the catalytic performances for the copolymerization of epoxide/CO2.13 Thus, the mertridentate NHC-Ti acetate complex was first tentatively prepared from the salt metathesis reaction between ([κ3O,C,O]-NHC)Ti(Cl)2 (1) and 1 equiv of NaOAc in dichloromethane at room temperature, resulting in a mixture of products. In contrast, the reaction of 1 with 2 equiv of NaOAc under same conditions leads quantitatively to the complex ([κ3O,C,O]-NHC)Ti(OAc)2 (6) (Scheme 2).

Figure 2. Crystal structure of 4-THF (isomer A). Hydrogen atoms, tBu groups, and solvent THF molecules are omitted for clarity. Selected bond lengths for isomer A (Å): Ti1−C1 = 2.165(2), Ti1−Cl1 = 2.3725(8), Ti1−O1 = 1.8634(18), Ti1−O2 = 1.8774(18), Ti1−O3 = 2.2655(19), Ti1−O4 = 1.8173 (19), N1−C1 = 1.339(3), N2−C1 = 1.341(3). Selected angles (deg): O1−Ti1−O2 = 159.17(8), C1−Ti1− Cl1 = 165.79(7), O3−Ti1−O4 = 174.07(8), C1−Ti1−O4 = 93.85(9), Cl1−Ti1−O4 = 100.30(6), C1−Ti1−O3 = 80.40(8), O1−Ti1−O3 = 83.18(8), O1−Ti1−O4 = 97.47(8), O2−Ti1−O4 = 94.18(8), Ti1− O4−Si1 = 147.67(12).

Scheme 2. Synthesis of Seven-Coordinate mer-Tridentate NHC Titanium Acetate Complex

by 1H NMR spectroscopy in solution. As illustrated in Figure 3 and Figure S8 in the Supporting Information, both isomers of 5 Complex 6 has been characterized by 1H/13C NMR and DRIFT spectroscopy, elemental analysis, and X-ray crystallography (Figures S9 and S10 in the Supporting Information and Table 1). As depicted in Figure 4, complex 6 is a monomeric

Figure 3. Crystal structure of 5 (isomer A). Hydrogen atoms and tBu groups are removed for clarity. Selected bond lengths: Ti1−C15 = 2.210(2), Ti1−O1 = 1.8131(17), Ti1−O5 = 1.8751(16), Ti1−O6 = 1.8758(16), Ti1−O7 = 1.8001(17), N1−C15 = 1.342(3), N2−C15 = 1.353(3). Selected angles (deg): O5−Ti1−O6 = 151.68(7), O1−Ti1− O7 = 113.09(8), C15−Ti1−O1 = 104.35(8), C15−Ti1−O7 = 142.56(8), Ti1−O1−Si1 = 147.47(11), Ti1−O7−C44 = 143.71(17).

Figure 4. Crystal structure of 6. Hydrogen atoms are omitted for clarity. Selected bond lengths (Å): Ti1−C1 = 2.148(4), Ti1−O1 = 1.8504(18), Ti1−O2 = 2.1836(19), Ti1−O3 = 2.077(2), N1−C1 = 1.341(3), O2−C17 = 1.257(4), O3−C17 = 1.263(3). Selected angles (deg): O1−Ti1−O1# = 164.48(12), C1−Ti1−O2 = 79.09(6), O2− Ti1−O3 = 60.84(8), O3−Ti1−O3# = 80.14(11), O2−C17−O3 = 117.9(3). Symmetry transformations used to generate equivalent atoms: (#) x + 1, y, −z + 3/2).

are five-coordinate and adopt a slightly distorted square pyramidal geometry (τ = 0.15)16 with the silyloxy ligand in an apical position and the mer-NHC/OiPr ligands in equatorial positions (Figure 3). The overall structural data for both isomers of 5 are unexpectedly not close to those of its fivecoordinate analogous complexes 2 and 3: i.e., that the merNHCs binding Ti atoms are nearly planar (isomer A, ∠C13− O5−O6−C17 = −1.73(3)°; isomer B, ∠C59−O12−O13−C63 = 14.3(3)°) and that small βn angles of 151.68(7)° (for isomer A) and 144.91(8)° (for isomer B) were observed in the solidstate structures (Tables S5 and S6 in the Supporting Information). These noticeable differences are most likely related to minimization of interactions between the tBu groups

compound in which the mer-tridentate NHC and the two bidentate acetate ligands are coordinated in a pentagonalbipyramidal arrangement around the Ti atom with the apexes of the bipyramid occupied by the two oxygen atoms from the NHCaryloxy ligands (∠O1−Ti1−O1# = 164.46(13)°) and the pentagon defined by the four oxygen atoms from the acetato groups and one carbon from the NHCcarbene (Tables S7 and S8 D


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Organometallics

solid in nearly quantitative yield after removal of all volatiles (Scheme 3).

in the Supporting Information). This seven-coordinate mode of coordination is not unknown for titanium, but the relatively few examples are mainly clusters and are mostly based on an atrane ligand and derivatives, such as the dimeric titanatrane,17 (acetato)titanatrane,18 (peroxo)titanatrane,19 dititanatrane,20 trititanatrane,21 and tetrameric (bis-aryloxy)dititanatrane22 and dimeric oxo-bridged (12-crown-4)titanium cation, with different geometries: distorted pentagonal bipyramidal,20 facecapped octahedral19,22 and distorted trigonal prismatic (square-faced monocapped).17,18 A more remarkable feature of this complex 6 is that the pentagon is nearly planar, with torsion angles of ∠C1−O2−O3−O3# = 0.12(15)° and ∠O2− O3−O3#−O2# = −0.2(2)°, in comparison to other pentagonal-bipyramidal complexes,20,21,23 which is due to the nearly perfect alignment of both acetato ligands between the bulky tBu groups from the nearly planar mer-NHC ligand (torsion angle ∠C4−O1−O1−C4 = 8.9(5)°). The data for both OAc ligands indicate that complex 6 possesses nearly identical and short Ti−O (2.077(2)/2.1836(19) Å) and C−O (1.257(4)/1.263(3) Å) bond distances, respectively, and a narrow βn (∠O2−C17−O3 = 117.9(3)°) angle characteristic of the κ2-O,O′ bonding mode to the Ti center comparable with the bond distances and βn angles of other titanium carboxylate complexes: e.g. for {([κ4-(μ-O),O,N,O]-(OCH2CH2)3N)Ti(η2OAc)} 2 , 18 (η 5 -C 5 H 5 )Ti(η 2 -OAc) 3 , 24 (η 8 -cis-C 8 H 6 )Ti(η 2 OAc)2,25 and [Ti(η2-O2CtBu)3(OtBu)].26 This bidentate bonding mode is further confirmed by IR spectroscopy with the appearance of νasym(COO−) and νsym(COO−) carboxylato stretching frequencies with a magnitude of separation of Δexp = 81 ± 1 cm−1 for these two stretching bands, indicating the bidentate chelation of the carboxylato group to the Ti metal center.27 Interestingly, the seven-coordinate complex 6 shows an unusual short Ti−Ccarbene bond length of 2.148(4) Å, which is the shortest length reported for trivalent and tetravalent NHC-Ti complexes (Ti−Ccarbene = 2.16−2.33 Å).2d Due to the high oxidation state of Ti4+ (d0), the most rational explanation for this short Ti−Ccarbene bond distance in complex 6 could be the result of cis Oacetato−Ccarbene intramolecular π interactions (daverage = 2.758 Å, significantly below the van der Waals radii of 3.23 Å)28 between the adjacent π-donor lone pairs of OAc and the formally vacant carbon π-orbital (C1−Ti1−O2 = 79.09(6)°),29 implying that the NHCcarbene should be viewed as an electrophilic carbon (e.g., as a Fischer-type carbene).29a,30 The NMR spectroscopic data of 6 in chloroform-d are consistent with the solid-state structure, and characteristic signals for a single C2v-symmetric tridentate mer-NHC-Ti (with a sharp singlet signal assigned to the NCH2 protons at δ 4.54 ppm) and two OAc ligands environment are observed at room temperature (Figures S9 and S10). The NHCcarbene carbon signal is shielded and observed at δ 197.8 ppm, which is nearly identical with those of complexes 1−5. Synthesis and Structural Characterizations of a Tridentate NHC Titanium Azide Complex and Reactivity. In an effort to prepare a mer-tridentate NHC titanium azide complex, the salt metathesis reaction of complex 1 precursor was investigated in the presence of 2−4 equiv of NaN3. Despite repeated attempts under various conditions (solvent toluene; T° 25−120 °C, t overnight to 3 days), we were unable to efficiently replace the chloride coligands. This result led us to use complex 2 as the starting material with an excess of Me3SiN3, which leads to the elimination of Me3Si(OiPr) and gradual formation of ([κ3-O,C,O]-NHC)Ti(N3)2 (7) as a green

Scheme 3. Synthesis of mer-Tridentate NHC Titanium Azide Complex 7, Reactivity with Alkyne, and Partial Hydrolysis

The NMR data for complex 7 show the disappearance of the two chemical resonances at δ 4.84 and 1.13 ppm related to the OiPr groups and the appearance of a sharp singlet signal for the NCH2 protons at δ 4.57 ppm along with one chemical resonance for the NHCcarbene carbon at δ 198.2 ppm, indicating that the C2v-symmetry is preserved and that 7 is most likely a monomeric complex in solution (Figures S11 and S12 in the Supporting Information). A solid-state IR study of complexes 7 showed νasym(N3) stretching frequencies spanning from 2120 to 2055 cm−1 and νsym(N3) stretching frequencies at 1361 and 1326 cm−1 (Figure 5b). Although the observed ν(N3) stretches

Figure 5. DRIFT spectra of (a) ([κ3-O,C,O]-NHC)Ti(OiPr)2 (2); (b) ([κ 3 -O,C,O]-NHC)Ti(N 3 ) 2 (7); (c) ([κ 3 -O,C,O]-NHC)Ti(N3)2(THF) (7-THF); (d) ([κ3-O,C,O]-NHC)Ti([κ1-N]-Tz)2 (8).

for 7 are characteristic of an azide ligand bound to titanium compounds,31 no further structural information about the coordination mode could directly be established from the IR spectrum: i.e., terminal (linear and/or bent) or bridged N3 ligands. Attempts to obtain suitable single crystals for X-ray diffraction analysis from noncoordinating and nonpolar solvents have been unsuccessful, but complex 7 crystallizes readily overnight from a mixture of THF and toluene solvents as a THF adduct of 7 (7-THF). The solid state of 7-THF comprises a central Ti atom which is six-coordinate in a slightly E


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Organometallics

organic dipolarophiles was explored. The [3 + 2] cycloaddition reactions between metal azide complexes32 and dipolarophiles (known as the “i-click” (inorganic click) reaction and mostly studied with activated alkynes)33 leading to monodentate anionic 1,2,3-triazolate (Tz) metal complexes are well-known for most of the transition-metal azide complexes (Ta,34 Mo,35 Mn,36 Re,37 Fe,38 Ru,38d,39 Os,40 Co,41 Rh,42 Ir,42b Ni,41f,43 Pd41e,44), but are still unexplored for d0 group 4 metal azide complexes. The reaction of the NHC titanium azide complex 7 with a 5-fold excess of DMAD (=dimethyl acetylenedicarboxylate) in dichloromethane at room temperature afforded the titanium bis-triazolato complex 8 (Scheme 3) along with a minor impurity (∼10%), unreacted DMAD, and methanotriazoles CH2(Tz)2 and CH(Tz)3 arising from the reaction between chlorinated solvent and Ti−N3 forming azidomethanes, which subsequently react with DMAD).45 The orange solid was further washed several times with hexane, and the impurities remained as contaminant products of complex 8 even after numerous washes in various solvents (pentane, toluene, Et2O). Moreover, the absence of asymmetric and symmetric N3 stretching bands and the presence of stretching frequencies at 1736 cm−1 (ν(CO)), 1247 cm−1 (ν(C−O)), and 1454 cm−1 (ν(NN)) indicate the occurrence of [3 + 2] cycloaddition between the 1,3-dipole azide and DMAD (Figure 5d). The 1H NMR spectrum of complex 8 (major product) in chloroform-d exhibits a very broad signal at δ 4.12 ppm assigned to the OCH3 protons of the triazolato ligands, in addition to the characteristic singlet signal at δ 4.73 ppm corresponding to the NCH2 protons, which indicates that the C2v-symmetry is retained for complex 8 in solution (Figure S15 in the Supporting Information). The 13C NMR spectrum of complex 8 shows evidence of only a single chemical resonance at δ 200.5 ppm for the NHCcarbene carbon atom and a very weak single resonance at δ 160.6 ppm corresponding to the carbonyl groups (Figure S16 in the Supporting Information). These NMR data corroborate the proposed structure in Scheme 3: i.e., ([κ3-O,C,O]-NHC)Ti([κ1-N]-N3C2(CO2Me)2)2. After several attempts to isolate complex 8 by recrystallization, only the impurity (minor product) could be isolated. Both NMR data on isolated crystals and X-ray structural determination (lowquality crystals, Figures S17−S19 in the Supporting Information) indicate that the compound is the oxo-bridged NHCTi(IV) complex {([ κ 3 - O,C , O]-NHC)Ti([ κ 2 -N ,O ]N3C2(CO2Me)2)}2(μ-O) (8′) (Scheme 3). This result reflects the partial hydrolysis of complex 8 during the purification and its extreme sensitivity to adventitious moisture. Interestingly, the solid-state structure of 8′ reveals a distorted-octahedral geometry around each Ti metal center with one oxygen from the carbonyl group and an oxo-bridged atom in apical positions and mer-tridentate NHC and Tz ligands (bonded through its N1) in equatorial positions (Scheme 3 and Figure S19). This type of coordination mode, i.e M−κ1-N1 for the triazolato ligand from the reaction of DMAD with M−N3, is among the rare examples of structures identified by X-ray diffraction, in contrast to the most usual M−κ1-N2 coordination mode found for M−Tz ligand corresponding to the thermodynamic product.33b Attempts To Synthesize NHC Titanium(III) Compounds. Aiming to explore more the low-valent NHCtitanium compounds as precursors for the copolymerization of epoxide with CO2, different synthetic approaches were investigated from either Ti(III) precursors or ([κ3-O,C,O]NHC)TiIV derivatives with different reducing reagents.

distorted octahedral fashion with one azide and the THF molecule in apical positions and a quasi-planar NHC ligand (∠C5−O1−O2−C19 = −3.86°) with a large βn angle of 162.45(9)° comparable to those of complexes 1 and 6 bearing sterically less encumbered coligands (Figure 6, Table 1 and

Figure 6. Crystal structure of 7-THF. Hydrogen atoms, a solvent toluene molecule, and tBu groups are removed for clarity. Selected bond lengths (Å): Ti1−C1 = 2.183(3), Ti1−N3 = 1.983(3), Ti−N6 = 2.033(3), Ti1−O1 = 1.834(2), Ti1−O2 = 1.848(2), Ti1−O3 = 2.209(2), N1−C1 = 1.352(3), N2−C1 = 1.333(3). Selected angles (deg): O1−Ti1−O2 = 162.46(9), C1−Ti1−N6 = 164.0(1), N3−Ti1− O3 = 177.81(9), C1−Ti1−O2 = 81.76(9), C1−Ti1−N3 = 97.55(10), N3−Ti1−N6 = 98.39(11), Ti1−N3−N4 = 139.1(2), Ti1−N6−N7 = 137.3(2), N3−N4−N5 = 175.9(4), N6−N7−N8 = 176.7(4).

Tables S9 and 10 in the Supporting Information). The other azide is located trans to the NHCcarbene, with a slightly elongated Ti−azide bond distance of 2.033(3) Å, as expected due to the strong σ-donor influence of NHCcarbene in contrast to the cis azide bond distance (1.983(3) Å), but both are statistically in the range for terminal Ti−Nazide bond distances reported (Ti−Nazide = 2.08−1.90 Å).31c−e,g,i,k,n−p The trans influence affects the resonance structure of the trans azide by increasing the electron density on the Nα atom, which slightly changes the bond order distances of Nα−Nβ/Nβ−Nγ (dN−N = 1.137(4) and 1.178(4) Å, respectively) in comparison to the “typical” terminal bent metal azide (Nα−Nβ/Nβ−Nγ = 1.192(4) and 1.146(4) Å, respectively). Whereas the azide ligands are slightly asymmetric, both azides are quasi-linear (∠Nα−Nβ−Nγ = 175.9(4) and 176.7(4)°) with ∠Ti−Nα−Nβ angles (139.1(2) and 137.3(2)°) typical of those reported for a covalent terminal bent Ti azide.31c−e,g,i,k,n−p The DRIFT spectrum of complex 7THF exhibits two νasym(N3) and two νsym(N3) stretching frequencies at 2082/2061 and at 1343/1321 cm−1, respectively, similar to the case for complex 7 (Figure 5c). Although the structure of 7-THF was unambiguous, the elemental analysis of this complex, as well as that for 7, was not reliable even after multiple attempts, apparently due to some decomposition during their handling. The NMR data of 7-THF were not found to be consistent with its solid-state structure, i.e. a Cssymmetry, but rather indicate a pseudo-C2v-symmetric structure in solution with a broad singlet signal for NCH2 protons (δ 4.50 ppm in chloroform-d), ascribed to the fast dissociation of THF on the NMR time scale (Figures S13 and S14 in the Supporting Information). To further complete the characterization of the 1,3-dipole Ti azide moieties in complex 7, its potential reactivity toward F


Article

Organometallics Scheme 4. Proposed Mechanisms for the Formation of ([κ5-O,N,C,N,O]-imidazolidine)Ti(Cl)(THF) Complex 9

Attempts to synthesize low-valent NHC-TiIII directly through the addition of imidazolinium chloride salt or after its deprotonation with KH and subsequently addition to TiCl3(THF)3 afforded an intractable reaction mixture in contrast to other reported procedures for NHC-TiIII.46 The well-established approaches for reducing bis-aryloxy or Ofunctionalized NHC chlorotitanium(IV) derivatives use typical alkali/alkaline-earth metals, KC8 and LiBEt3H, as reducing reagents and in our case proved to be inefficient for the reduction of complex ([κ3-O,C,O]-NHC)Ti(Cl)2 (1).47 Only one clean compound could be isolated from the reaction between complex 1 and 1.1 equiv of LiBEt3H at −78 °C in toluene in high yield. It was identified as the tetravalent complex ([κ5-O,N,C,N,O]-imidazolidine)Ti(Cl)(THF) (9) (Scheme 4). The side product(s) of this reaction, which is green, did not show any meaningful signals in the solution NMR spectrum presumably due to the presence of paramagnetic compound(s).48 The molecular structure of 9 exhibits a Cs-symmetric structure (NCH2 protons: doublet of multiplets at δ 3.13/3.84 ppm) established by 1D and 2D NMR analyses, and these NMR data supported the hydride transfer to the NHCcarbene atom with the appearance of 1H and 13C chemical resonances at δ 3.95 and 103.7 ppm, respectively, for the NC(H)N unit and the disappearance of the 13C signal for NHCcarbene (Figures S20−S22 in the Supporting Information). Crystals of limited quality were obtained from a saturated solution in THF/hexane (1/3) at −30 °C. However, they allow deducing the connectivity around the Ti center without doubt and confirmed that complex 9 features a seven-coordinate geometry, which can be viewed as a distorted square-face monocapped trigonal prism, with a pentadentate dianionic bisaryloxy [κ5-O,N,C,N,O]-imidazolidine ligand, and a C1 atom capping the O1,N1,N2,O2 face (Figure S23 in the Supporting Information). Although hydride transfer to the NHCcarbene has not been reported as such for NHC-Ti complexes, alkyl transfer, notably M-benzyl, was previously reported to be the result of an intramolecular rearrangement occurring in the presence of donor aniline or THF molecules in multidentate NHC group 4 complexes such as ([κ2-C,O]-NHC)Zr(Cl)(Bn)2,49 ([κ3-O,C,O]-NHC)Zr(Cl)(Bn),30b and ([κ3-O,C,O]NHC)M(Bn)2 (with M = Ti, Zr, Hf),30c owing to the electrophilic nature of the Ccarbene, thus leading to the sevencoordinate complexes. In our case, the formation of complex 9 can be the result of either a direct intermolecular nucleophilic attack of hydride onto the NHCcarbene atom or the formation of

the unstable intermediate ([κ3-O,C,O]-NHC)Ti(Cl)(H)(THF) followed by a THF-assisted (THF acts as a Lewis base) intramolecular nucleophilic attack at the NHCcarbene atom (Scheme 4). Although the exact mechanism of the hydride migration to the NHC ligand is at present not clear, this reactivity proved that these types of tridentate bis-aryloxy NHCcarbene or derivative ligands, once coordinated to titanium, have a Fischer-type character.29,30b,c Copolymerization of Cyclohexene Oxide with Carbon Dioxide. On the basis of previous studies on copolymerization of CHO with CO2 using NHC titanium complexes, it was shown that these systems, on activation by a stoichiometric amount of bulky ionic cocatalysts such as [PPN]X salts (X = N3, Cl, NO2), were all active and highly selective toward the formation of PCHC (≥99%) without formation of cyclohexene carbonate and homopolymer as side products.8a,b Here, the copolymerization of neat CHO with CO2 catalyzed by NHCTi(IV) complexes 1−7 and 9 is carried out at low pressure and temperature and activated by 1 equiv of [PPN]X (with X = Cl, N3) to evaluate their catalytic performances (Scheme 5). Scheme 5. Copolymerization of CHO/CO2 To Yield PCHC

The representative results are summarized in Table 2 along with comparative results based on benchmark 1 and 2 catalytic systems. Prior to running the copolymerization reactions, due to the low solubility of [PPN] salts in neat CHO, all complexes are dissolved in dichloromethane, enhancing the activated catalyst mixture solubility.8 After solvent removal and addition of neat CHO (CHO/Ti = 2500/1), all tridentate NHC-TiIV complexes activated with [PPN]Cl are able to moderately catalyze the copolymerization of CHO with CO2 along with excellent selectivity in atactic PCHC (>99%, with ≥99% in carbonate linkages) without side products (Table 2, runs 1−7). Only the complex ([κ5-O,N,C,N,O]-imidazolidine)Ti(Cl)(THF) (9) is found to be inactive under these reaction conditions, showing the importance of the NHCcarbene-type ligand in the reactivity of the titanium metal center. The copolymer analyses of all PCHCs reveal narrow, but bimodal, polydispersities with Mw/Mn values ranging from 1.33 to 1.59, G


Article

Organometallics Table 2. Copolymerization of CHO with CO2 Catalyzed by Complexes 1−7 and 9 runa

complex/cocat.b

NMR conversn (%)

yieldc (%)

selectivity in PCHCd (%)

TON

TOFe (h−1)

Mnf (kg mol−1)

Mw/Mnf

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

1/[PPN]Cl 2/[PPN]Clg 3/[PPN]Cl 4/[PPN]Cl 5/[PPN]Cl 6/[PPN]Cl 7/[PPN]Cl 9/[PPN]Cl 1/[PPN]N3 2/[PPN]N3 3/[PPN]N3 4/[PPN]N3 5/[PPN]N3 6/[PPN]N3 7/[PPN]N3

27 32 23 23 27 32 37

24 30 20 22 26 29 32

>99 >99 >99 >99 >99 >99 >99

678 840 563 575 675 801 930

28 33 23 24 29 33 39

3.2 7.4 3.7 7.4 7.7 5.7 4.5

1.46 1.54 1.57 1.59 1.54 1.42 1.33

19 35 29 27 35 35 30

15 33 23 26 33 28 28

>99 >99 >99 >99 >99 >99 >99

482 874 718 662 872 867 742

20 36 30 28 36 36 31

2.1 5.9 2.3 3.9 5.5 5.0 3.6

1.41 1.39 1.37 1.51 1.42 1.39 1.48

a Polymerization procedure: 8 μmol of precursor, 8 μmol of cocatalyst, 20 mmol of CHO (CHO/Ti = 2500/1), PCO2 < 0.5 bar at 60 °C. bCatalyst preformation 15 min at 30 °C and drying for 2 h under vacuum. cYield determined gravimetrically. dMeasured by 1H NMR spectroscopy in chloroform-d on the crude product. eTurnover frequency of CHO to PCHC. fDetermined by GPC-SEC in THF at 30 °C against polystyrene standard. gSee ref 8b.

OBn, OSi(OtBu)3, and OAc slightly enhanced the yields of atactic PCHC by ca. 10−20% and show higher TOFs in comparison to that on activation by [PPN]Cl (Table 2, runs 10−14). Diverging from the previous observations, complexes 1 and 7 bearing sterically less hindered coligands (Cl and N3) show low productivity (20 and 31 h−1) in comparison to those activated with [PPN]Cl. This unexpected reverse effect on productivity (Table 2, runs 9 and 15) is presumably due to a more pronounced degradation of the active species for 1 and formation of more stable active species (slow dissociation of strong nucleophile N3−) in 7, thus diminishing and stabilizing slightly more the formation of anionic six-coordinate intermediate concentration, respectively. To a lesser extent, reduced activity and yield were likewise observed when structural analogues of Ti(IV) complexes with sterically unencumbered mer-tridentate NHC ligands were employed, in which the absence of bulkier tBu substituents on the NHCaryloxy rings results in a similar effect.8b

indicative of controlled polymerization (Figures S24 and S25 in the Supporting Information). Regarding the effect of initiating coligands on the activity, the best effect is obtained for NHC titanium bis-azide 7/[PPN]Cl with a TOF reaching 39 h−1, followed by NHC titanium bis-isopropoxide 2 and bis-acetate 6\[PPN]Cl with TOFs of 33 h−1 under these conditions (Table 2, runs 2, 6, and 7). The slightly higher activity observed for those catalytic systems is possibly related to an increase in active species with respect to complexes bearing azide, isopropoxide, and acetate as coligands in comparison to the other X-type coligands (Cl, OBn, OSi(OtBu)3) (Table 2, runs 1 and 3−5). However, subtle differences remain in the steric and electronic levels of the incoming nucleophile (here a Cl atom) and the coligands. As previously demonstrated, this type of anionic cocatalyst is known to reversibly coordinate to the metal, increasing the electron density, which labilizes the coligand trans to it.10c,50 Here, the presence of very bulky Xtype coligands such as OSi(OtBu)3 and OBn appear to slightly impede the association of the incoming Cl atom needed to form an active anionic six-coordinate intermediate in comparison to the less-hindered iOPr coligand (Table 2, runs 2−5). However, as already reported for the less-hindered complex 1 bearing poor nucleophile Cl atoms as coligands, there is a significant decrease in concentration of the active anionic intermediate (∼25%) by dissociation of one NHC aryloxy and NHC carbene ligands upon activation with [PPN]Cl, diminishing the activity.8a In contrast, the sterically more hindered iOPr, OAc, and N3 coligands appear to be more effective in the ring opening of the coordinated CHO monomer and in the formation of an active anionic six-coordinate intermediate similar to salen-type Cr(III) complexes activated by anionic cocatalysts.8a,12a,c,d,51 These results are well in line with previous studies showing that the incoming nucleophile binds to the titanium, thus forming the anionic six-coordinate intermediate, which in turn is acting as a whole as a nucleophilic species or by releasing a nucleophile to perform an intermolecular ring opening to a coordinated CHO.8a When complexes 1−7 are activated by a more nucleophilic anionic cocatalyst, i.e. [PPN]N3, the experimental results reveal that complexes bearing more encumbered coligands such as OiPr,

■

CONCLUSIONS Tridentate NHC titanium(IV) benzyloxide, silyloxide, acetate, and azide complexes were synthesized from the NHC titanium chloride or isopropoxide precursors and fully characterized. The structures of all complexes were determined, and most importantly, the acetate complex reveals a unique sevencoordinate Ti(IV) center in a pentagonal-bipyramidal environment with the shortest Ti−Ccarbene bond distance. The characterization of the bis-azide NHC-TiIV complex was further investigated via the [3 + 2] cycloaddition of functionalized alkyne, and the structure obtained reveals an unusual and unprecedented type of coordination for the newly formed anionic triazolato ligands to titanium: i.e. Ti−κ1-N1 vs classical Ti−κ1-N2. The synthesis of mer-tridentate NHC-TiIII from its reduction from NHC-TiIV species or from Ti(III) precursors was shown to be difficult, and a superhydride reducing reagent with NHC-TiIV leads, via a hydride transfer, to the sevencoordinate imidazolidine Ti(IV) complexes. All NHC-Ti complexes in combination with bulky anionic [PPN]X salts as cocatalysts exhibit activity toward the completely alternating copolymerization of cyclohexene oxide with CO2 under low H


Article

Organometallics

MHz, dmso-d6): δ 7.37 (d, J = 7.4 Hz, 2H, benzyl-H), 7.21 (t, J = 7.4 Hz, 2H, benzyl-H), 7.08 (t, J = 7.4 Hz, 1H, benzyl-H), 4.88 (s, 2H, CH2O) ppm. ([κ3-O,C,O]-NHC)Ti(OBn)2 (3). To a solution of 1 (0.43 mmol, 254 mg) in 10 mL of toluene was added a solution of LiOBn (2.2 equiv, 0.94 mmol, 107 mg) in 2 mL of toluene at −30 °C. The initial red solution rapidly turned orange upon the addition. The reaction mixture was warmed to room temperature and stirred overnight. Subsequent centrifugation, filtration, and evaporation to dryness afforded compound 3 as an orange powder (290 mg, 92% yield). The compound 3 crystallized as a dark orange product in a saturated solution of toluene at −30 °C. 1H NMR (600.13 MHz, chloroform-d): δ 7.23 (d, J = 2.2 Hz, 2H, aryl-H), 7.22 (m, 4H, benzyl-H), 7.15 (m, 6H, benzyl-H), 6.88 (d, J = 2.2 Hz, 2H, aryl-H), 5.43 (s, 4H, CH2O), 4.29 (s, 4H, NCH2), 1.58 (s, 18H, tBu), 1.37 (s, 18H, tBu) ppm. 13C NMR (150.93 MHz, chloroform-d): δ 197.9 (NCN), 150.9 (Cq, Oaryl), 142.8 (Cq, benzyl), 140.1 (Cq, aryl), 137.2 (Cq, aryl), 129.8 (Cq, aryl), 128.0 (CH, benzyl), 126.7 (CH, benzyl), 126.6 (CH, benzyl), 119.4 (CH, aryl), 111.2 (CH, aryl), 77.2 (CH2O), 47.5 (NCH2), 35.9 (Cq, tBu), 34.7 (Cq, tBu), 31.9 (CH3, tBu), 30.3 (CH3, tBu) ppm. DRIFT (ν/cm−1): 2949s, 2901m, 2866w, 1474vs, 1455vs, 1388w, 1358w, 1346vs, 1322vs, 1287w, 1252w, 1237w, 1208w, 1183w, 1104s, 1060w, 1038s, 1022m, 934w, 913w, 853m, 759s, 698s, 667s, 646m, 564m, 473w, 444w, 415w. Anal. Calcd for C45H58N2O4Ti: C, 73.15; H, 7.91; N 3.79. Found: C, 73.05; H, 7.12; N, 3.57. ([κ3-O,C,O]-NHC)TiCl(OSi(OtBu)3) (4). A toluene solution of 1 (105 mg, 0.18 mmol) with 1.1 equiv of NEt3 was added dropwise at −30 °C to a solution of (tBuO)3SiOH in toluene (1 equiv, 0.18 mmol, 47 mg). The dark red-brown solution was stirred for 2.5 h at room temperature. The trimethylamine chloride salt was then filtered away, and the solution was evaporated to dryness. The solid was washed with hexane and dried to afford 4 as a red powder (106 mg, 73% yield). Suitable single crystals for X-ray analysis of 4-THF were obtained from THF/pentane (1/5) at −30 °C after 3 days. 1H NMR (500.13 MHz, benzene-d6): δ 7.45 (d, J = 2.2 Hz, 2H, aryl-H), 6.64 (d, J = 2.2 Hz, 2H, aryl-H), 3.12−3.37 (m, 4H, NCH2), 1.93 (s, 18H, tBu), 1.41 (s, 18H, tBu), 1.16 (s, 27H, OtBu) ppm. 13C NMR (125.77 MHz, benzene-d6): δ 198.6 (NCN), 150.5 (Cq, O-aryl), 141.9 (Cq, aryl), 138.2 (Cq, aryl), 130.4 (Cq, aryl), 119.6 (CH, aryl), 110.8 (CH, aryl), 72.6 (Cq, OtBu), 46.7 (NCH2), 36.0 (Cq, tBu), 34.8 (Cq, tBu), 31.9 (CH3, tBu), 31.4 (CH3, OtBu), 30.6 (CH3, tBu) ppm. DRIFT (ν/ cm−1): 2964s, 2905m, 2870w, 1477s, 1452s, 1389w, 1362w, 1323s, 1294w, 1239w, 1190w, 1057s, 1027w, 956s, 915w, 861m, 668w, 576w. Anal. Calcd for C43H71ClN2O6SiTi: C, 62.72; H, 8.69; N, 3.40. Found: C, 62.32; H, 8.71; N, 3.41. ([κ3-O,C,O]-NHC)Ti(OiPr)(OSi(OtBu)3) (5). A toluene solution of 2 (0.16 mmol, 104 mg) was added dropwise via a pipet at −35 °C to a stirred toluene solution of (tBuO)3SiOH (1 equiv, 0.16 mmol, 44 mg). The solution was stirred overnight at room temperature and then evaporated to dryness. The yellow solid was washed with hexane (5 mL) and dried under vacuum to yield a yellow solid (133 mg, 99% yield). Crystals of 5 were obtained from a concentrated solution of toluene at −30 °C after 1 week. 1H NMR (500.13 MHz, benzene-d6): δ 7.44 (d, J = 2.3, 2H, aryl-H), 6.64 (d, J = 2.2, 2H, aryl-H), 5.48 (sept, J = 6.0, 1H, OCH(CH3)2), 3.03−3.36 (m, 4H, NCH2), 1.90 (s, 18H, tBu), 1.56 (d, J = 6.0, 6H, OCH(CH3)2), 1.44 (s, 18H, tBu), 1.22 (s, 27H, tBu) ppm. 13C NMR (150.93 MHz, benzene-d6): δ 200.3 (NCN), 151.5 (Cq, O-aryl), 139.9 (Cq, aryl), 137.5 (Cq, aryl), 130.6 (Cq, aryl), 119.2 (CH, aryl), 111.3 (CH, aryl), 77.0 (OCH(CH3)2), 72.0 (Cq, OtBu), 46.8 (NCH2), 36.1 (Cq, tBu), 34.7 (Cq, tBu), 32.0 (CH3, tBu), 31.5 (CH3, OtBu), 30.7 (CH3, tBu), 27.2 (OCH(CH3)2) ppm. DRIFT (ν/cm−1): 2965s, 2905m, 2868w, 1477s, 1452s, 1389w, 1362w, 1323s, 1238w, 1192w, 1139w, 1060s, 1018w, 956s, 851m, 668w, 566w. Anal. Calcd for C46H78N2O7SiTi: C, 65.22; H, 9.28; N, 3.31. Found: C, 65.13; H, 9.43; N, 3.26. ([κ3-O,C,O]-NHC)Ti(OAc)2 (6). To a suspension of sodium acetate (2.1 equiv, 0.90 mmol, 74 mg) in 5 mL of dichloromethane was added a red solution of 1 (0.43 mmol, 255 mg) in 10 mL of dichloromethane. The red reaction mixture was stirred overnight at room temperature and had turned brown. Subsequent centrifugation, filtration, and

pressure (

Structural Characterization of Tridentate N-Heterocyclic Carbene

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