Synthesis and Structural Characterization of a Novel Family of

Feb 11, 2010 - Solid-state structural data for these Ti complexes overall suggest that this tridentate NHC bis-aryloxide ligand may significantly crow...
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Organometallics 2010, 29, 1191–1198 DOI: 10.1021/om901084n

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Synthesis and Structural Characterization of a Novel Family of Titanium Complexes Bearing a Tridentate Bis-phenolate-N-heterocyclic Carbene Dianionic Ligand and Their Use in the Controlled ROP of rac-Lactide Charles Romain,† Lydia Brelot,‡ Stephane Bellemin-Laponnaz,*,† and Samuel Dagorne*,† †

Laboratoire DECOMET, UMR CNRS 7177, Universit e de Strasbourg, 4 Rue Blaise Pascal, 67000 Strasbourg, France, and ‡Service de Radiocristallographie, Institut de Chimie (UMR CNRS 7177), Universit e de Strasbourg, 4 Rue Blaise Pascal, 67000 Strasbourg, France Received December 17, 2009

The present contribution describes the synthesis and structural characterization of a novel family of robust titanium complexes, supported by a tridentate pincer ligand of the type bis-phenolateN-heterocyclic carbene [tBu(OCO)2-]. For the most part, these complexes were found to be accessible in high yields via an alcohol elimination route involving the reaction of the imidazolinium salt [tBu(OCO)H3]Cl (1) with ClTi(OiPr)3 or Ti(OiPr)4 to afford the corresponding NHC-Ti complexes [tBu(OCO)]TiCl2 (2) and [tBu(OCO)]TiCl(OiPr) (3), respectively, when the reaction is carried out in noncoordinative solvents such CH2Cl2 and toluene. When these reactions are performed in THF, the corresponding Ti-THF adducts [tBu(OCO)]TiCl2(THF) (2-THF) and [tBu(OCO)]TiCl(OiPr)(THF) (3-THF) are isolated in quantitative yields. The molecular structures of complexes 2, 2-THF, and 3-THF were determined by X-ray crystallographic studies, establishing the effective coordination of the tBu(OCO)2- pincer to Ti. While the alcohol elimination pathway appears to be most suited to access Ti complexes of the type [tBu(OCO)]TiX2 (X = halide, alkoxide), the amine elimination was also found to be effective, albeit in lower yield. Thus, the reaction of salt species 1 with Ti(NMe2)4 in THF afforded the corresponding NHC-Ti amido complex [tBu(OCO)]TiCl(NMe2)(THF) (5) in a modest yield. The direct reaction of the salt species 1 with 0.5 or 1 equiv of TiCl4(THF)2 in the presence of NEt3 afforded the homoleptic bis-adduct Ti complex [tBu(OCO)]2Ti (6), whose molecular structure was confirmed by X-ray crystallographic analysis. As for the potential of such complexes in catalysis, the Ti isopropoxide chloro complex 3-THF was found to readily initiate the ring-opening polymerization of rac-lactide in a controlled manner and, interestingly, without apparent involvement of the NHC moiety in the catalytic process. The tridentate nature of the tBu(OCO)2- ligand as well as some level of steric protection provided by the tBu groups may rationalize the excellent stability of the Ti-NHC bond in the present systems.

Introduction Due to their unique electronic properties, N-heterocyclic carbenes (NHCs) have recently emerged as an important family of supporting ligands for the development of transition metal catalysts.1 Their bonding to most late transition metals has proven to be kinetically inert, thus rendering them

a “privileged” motif for catalyst design with such metals.2 In contrast, the use of NHC ligands for coordination to early transition metals in high oxidation state has been much less studied, which may be ascribed, at least in part, to the presumed ease of dissociation of the M-Ccarbene bond in such complexes.3 In order to reduce the tendency for ligand dissociation, potentially bidentate or tridentate NHC-donor systems, which incorporate a neutral carbene donor surrounded by one or two anionic ligand(s), appear to be promising targets to afford robust and stable early transition

*Corresponding authors. E-mail: [email protected]; bellemin@ unistra.fr. (1) (a) Arduengo, A. J. III. Acc. Chem. Res. 1999, 32, 913. (b) Bourissou, D.; Guerret, O.; Gabbaï, F.; Bertrand, G. Chem. Rev. 2000, 100, 39. (c) Crudden, C. M.; Allen, D. P. Coord. Chem. Rev. 2004, 248, 2247. (d) Díez-Gonzalez, N. S. P. Coord. Chem. Rev. 2007, 251, 874. (e) Glorius, F. Top. Organomet. Chem. 2007, 21, 1. (f) Hahn, F. E.; Jahnke, M. C. Angew. Chem., Int. Ed. 2008, 47, 3122. (2) (a) Herrmann, W. A. Angew. Chem., Int. Ed. 2002, 41, 1290. (b) Perry, M. C.; Burgess, K. Tetrahedron: Asymmetry 2003, 14, 951. (c) Cesar, V.; Bellemin-Laponnaz, S.; Gade, L. H. Chem. Soc. Rev. 2004, 33, 619. (d) Gade, L. H.; Bellemin-Laponnaz, S. Top. Organomet. Chem. 2007, 21, 117.

(3) For X-ray-characterized high-oxidation-state transition metal complexes stabilized by a monodentate NHC ligand, see, for instance: (a) Niehues, M.; Erker, G.; Kehr, G.; Scwab, P.; Fr€ ohlich; Blacque, O.; Berke, H. Organometallics 2002, 21, 2905. (b) Abernethy, C. D.; Codd, G. M.; Spicer, M. D.; Taylor, M. K. J. Am. Chem. Soc. 2003, 125, 1128. (c) Braband, H.; Abram, U. Chem. Commun. 2003, 2436. (d) Shukla, P.; Johnson, J. A.; Vidovic, D.; Cowley, A. H.; Abernethy, C. D. Chem. Commun. 2004, 360. (e) Hahn, F. E.; Fehren, T. v.; Fr€ohlich, R. Z. Naturforsch. 2004, 59b, 348.

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metal NHC-based chelate complexes of potential interest in catalysis.4-6 Thus far, a few NHC-based group 3, 4, and 5 metal chelates have been reported, although the suitability of this class of species for catalysis remains to be addressed.6 In this regard, the stability of the NHC-M bond (for oxophilic and high-oxidation-state metals) under catalytic conditions may be a reasonable concern in view of the very few studies performed in this area. For instance, Arnold et al. reported on group 3 and 4 metal complexes bearing anionic bidentate NHC-donor systems of the type {LX}Y[N(SiMe3)2]2 and {LX}Ti(OiPr)3, which were shown to readily initiate raclactide polymerization via an unusual bifunctional mechanism involving the NHC-based LX- chelating ligand as an initiating moiety.4b Seeking robust NHC-incorporating pincer-type chelating ligands in which the NHC moiety may solely act as an ancillary two-electron donor ligand toward oxophilic and high-oxidation-state metals (even under catalytic conditions), we have developed a straightforward synthesis of a new family of tridentate ([L,X2]-type) bis-aryloxide-NHC ligands (A, Chart 1), in which the NHC moiety is positioned as a central donor, a feature likely to disfavor deactivation processes,7,8 and flanked on each side by an aryloxide moiety able to anchor the chelating ligand onto the metal center. In preliminary studies, we showed that such a ligand structure was favorable for coordination to V(V) and Mn(III) metal centers, while a couple of other reports highlighted its suitability for coordination to late transition metal centers.9,10 Given their backbone and electronic structures, O,C,O type-A ligands appear as potential candidates for coordination to high-oxidation-state and oxophilic group 4 metals for the possible use of the derived complexes in (4) For early transition metal complexes stabilized by an NHCincorporating anionic bidentate ligand, see: (a) Arnold, P. L.; Rodden, M.; Wilson, C. Chem. Commun. 2005, 1743. (b) Patel, D.; Liddle, S. T.; Mungur, S. A.; Rodden, M.; Blake, A. J.; Arnold, P. L. Chem. Commun. 2006, 1124. (c) Mungur, S. A.; Blake, A. J.; Wilson, C.; McMaster, J.; Arnold, P. L. Organometallics 2006, 25, 1861. (5) For early transition metal complexes stabilized by an NHCincorporating anionic tridentate ligand, see: (a) Aihara, H.; Matsuo, T.; Kawaguchi, H. Chem. Commun. 2003, 2204. (b) Spencer, L. P.; Winston, S.; Fryzuk, M. D. Organometallics 2004, 23, 3372. (c) Spencer, L. P.; Fryzuk, M. D. J. Organomet. Chem. 2005, 690, 5788. (d) Rubio, R. J.; Andavan, G. T. S.; Bauer, E. B.; Hollis, T. K.; Cho, J.; Tham, F. S.; Donnadieu, B. J. Organomet. Chem. 2005, 690, 5353. (e) Spencer, L. P.; Beddie, C.; Hall, M. B.; Fryzuk, M. D. J. Am. Chem. Soc. 2006, 128, 12531. (f) Zhang, D.; Aihara, T.; Watanabe, T.; Matsuo, T.; Kawaguchi, H. J. Organomet. Chem. 2007, 692, 234. (g) Zhang, D. Eur. J. Inorg. Chem. 2007, 4839. (6) For reviews, see: (a) Liddle, S. T.; Edworthy, I. S.; Arnold, P. L. Chem. Soc. Rev. 2007, 36, 1732. (b) K€uhl, O. Chem. Soc. Rev. 2007, 36, 792. (7) (a) Lee, H. M.; Zeng, J. Y.; Hu, C.-H.; Lee, M.-T. Inorg. Chem. 2004, 43, 6822. (b) Zhou, Y.; Xi, Z.; Chen, W.; Wang, D. Organometallics 2008, 27, 5911. (8) For a review on tridentate ligands that contain NHCs, see: Mata, J. A.; Poyatos, M.; Peris, E. Chem. Rev. 2007, 251, 841. (9) Bellemin-Laponnaz, S.; Welter, R.; Brelot, L.; Dagorne, S. J. Organomet. Chem. 2009, 694, 604. (10) (a) Yagyu, T.; Oya, S.; Maeda, M.; Jitsukawa, K. Chem. Lett. 2006, 35, 154. (b) Weinberg, D. R.; Hazari, N.; Labinger, J. A.; Bercaw, J. E. Organometallics 2010, 29, 89.

Romain et al. Scheme 1

Scheme 2

catalysis. Thus, we report herein on a straightforward and high-yield synthesis of robust NHC-titanium complexes supported by a type-A ligand, via one-step procedures involving the reaction of the imidazolinium proligand with a Ti(IV) metal precursor.11 Some of these complexes were also found to polymerize rac-lactide in a controlled manner, and the results of these studies are also presented here.

Results and Discussion Synthesis and Structural Characterization of Ti Complexes Supported by a Type-A Dianionic NHC Ligand. The NHC proligand 1,3-bis(3,5-di-tert-butyl-2-hydroxyphenyl)imidazolinium chloride (1) was prepared according to a literature procedure from N,N0 -bis(3,5-di-tert-butyl-2-hydroxyphenyl)ethylenediamine12 using a classical procedure (HCl and then (EtO)3CH).13 The alcohol elimination route involving the reaction of the imidazolinium salt 1 with an alkoxide Ti(IV) precursor was found to be best suited for the preparation of titanium dichloro complexes supported by the tridentate ligand tBu (OCO)2-. Thus, the imidazolinium salt 1 reacts (THF, room temperature) with ClTi(OiPr)3 to quantitatively afford the bis-phenolate-N-heterocyclic carbene titanium dichloro complex 2-THF (Scheme 2) as a THF adduct, along with (11) Part of this work was communicated at the 23th IUPAC International Conference on Organometallic Chemistry, XXIII, ICOMC, Rennes, France, July 13-18, 2008. (12) Min, K. S.; Weyermuller, T.; Bothe, E.; Wieghardt, K. Inorg. Chem. 2004, 43, 2922. (13) Waltman, A. W.; Grubbs, R. H. Organometallics 2004, 23, 3105.

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3 equiv of iPrOH. Compound 2-THF was isolated as an analytically pure dark green solid by evaporation of volatiles in vacuo. The chloro isopropoxide Ti derivative 3-THF may be quantitatively synthesized in a similar manner but using Ti(OiPr)4 as the metal alkoxide precursor (Scheme 2). Complex 2-THF is indefinitely air-stable whether in the solid state or in benzene solution, showing the stability of the (O-NHCO)Ti chelate in this system, while the chloro isopropoxide NHC-Ti complex 3-THF slowly degrades upon air exposure. As a comparison, attempts to access compound 2-THF via a classical salt metathesis route were unsuccessful. Thus, reaction of the imidazolinium proligand 1 with 3 equiv of a strong base (nBuLi, KN(SiMe3)2 or KH, THF, -78 °C) and subsequent reaction with TiCl4(THF)2 consistently afforded intractable mixtures. This result highlights the suitability of the alcohol elimination pathway in the present ligand system. The molecular structures of 2-THF and 3-THF were determined by X-ray crystallography, which confirmed the effective chelation of a bis-aryloxide-NHC moiety to titanium (see Supporting Information for crystallographic data, Tables S1 and S2). In the case of complex 2-THF, the asymmetric unit contains two independent molecules that feature nearly identical structural parameters, and these will thus be discussed for one of the two. As illustrated in Figures 1 and 2, the titanium atom in compounds 2-THF and 3-THF exhibits a slightly distorted octahedral geometry as a result of the mer-coordination of the tridentate NHC ligand with O-Ti-O bite angles of 159.19(9)° and 162.59(9)° for 3THF and 2-THF, respectively. The Ti-Ccarbene bond distances (2.166(3) and 2.184(3) A˚ for 3-THF and 2-THF, respectively) are similar to those reported for other structurally related NHC-Ti complexes.5a,e,f While the {OCO}Ti chelate in complex 2-THF is nearly planar [for instance, C(18)-O(2)-O(1)-C(1) = 10.1°], that in complex 3-THF appears to be significantly distorted from planarity [C(4)-O(2)-O(3)-C(18) = 47.7(1)°], which is most likely to diminish significant steric interactions between the OiPr and tBu moieties that would otherwise result. In complex 2THF, the Ti-Cl bond length of 2.35(1) A˚ for the chloro ligand trans to the NHC donor is somewhat longer than that trans to the THF unit (2.32(1) A˚), which is consistent with the trans influence of the carbene ligand. The NMR data for 2THF and 3-THF are consistent with their solid-state structures being retained in solution; in particular, for both compounds, these data agree with (i) Cs-symmetric structures in solution and (ii) an effective coordination of THF to Ti with no observable decoordination/coordination exchange process on the NMR time scale at room temperature. In addition, the 13C NMR spectra for both complexes contain a characteristic NHC-Ccarbene resonance (δ 197.4 and 198.6 for 2-THF and 3-THF, respectively).14 The pentacoordinate THF-free NHC-Ti complexes 2 and 3 may also be synthesized via an alcohol elimination pathway provided the reaction is carried in a noncoordinative solvent. Thus, the reaction of the imidazolinium salt 1 with 1 equiv of Ti(OiPr)4 (CH2Cl2, room temperature) quantitatively afforded the corresponding NHC-Ti complex 3, which was isolated as an air-sensitive orange solid in high yield. Yet, the reaction of the imidazolinium salt 1 with 1 equiv of ClTi(OiPr)3 under identical conditions (CH2Cl2, room temperature, overnight) did not yield the quantitative formation of the Ti dichloro complex 2, as might be expected; rather, a (14) Tapu, D.; Dixon, D. A.; Roe, C. Chem. Rev. 2009, 109, 3385.

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Figure 1. Molecular structure of 2-THF (only one of the two independent molecules is shown). Selected bond lengths (A˚) and angles (deg): C(15)-Ti(1), 2.184(3); Cl(1)-Ti(1), 2.3504(10); Cl(2)-Ti(1), 2.3228(10); O(3)-Ti(1), 2.126(2); O(2)-Ti(1), 1.829(2), O(1)-Ti(1), 1.837(2); C(15)-Ti(1)-O(3), 88.49(3); Cl(1)-Ti(1)-Cl(2), 97.71(4); C(15)-Ti(1)-O(2), 81.77(10).

Figure 2. Molecular structure of 3-THF. Selected bond lengths (A˚) and angles (deg): C(32)-Ti(1), 2.166(3); Ti(1)-O(1), 1.779(2); Ti(1)-O(2), 1.8604(19); Ti(1)-O(3), 1.896(2); Ti(1)-O(4), 2.272(2); Ti(1)-Cl(1), 2.3825(9); O(2)-Ti(1)-O(3), 159.19(9); C(32)-Ti(1)-O(4), 80.63(10). Scheme 3

mixture of the zwitterionic Ti species 20 and the NHC-Ti complex 2 in a 2:1 ratio was observed under these conditions, as deduced from NMR data (Scheme 4). Evaporation of the latter mixture and subsequent heating in toluene (90 °C, overnight) allowed the quantitative formation of complex 2, which was isolated as an air-stable brown solid. The observation of the imidazolinium zwitterionic intermediate species 20 , presumably the kinetic product, along with its ready conversion to the NHC-Ti complex 2, i.e., the thermodynamic product, at higher temperature, shows that the

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Romain et al. Scheme 4

alcohol elimination reactions of the type described here may well proceed stepwise despite the frequent sole observation of the most stable species as the final product. While zwitterion 20 could not be isolated in a pure form, its identity was deduced from NMR data and X-crystallographic analysis. In particular, the 1H NMR spectrum of 20 contains a characteristic H2imidazolinium singlet resonance (δ 8.74) along with a set of signals for the bis-aryloxide pincer consistent with the effective chelation of the Ti metal center and with an overall Cssymmetric structure at room temperature in CD2Cl2. The 13C NMR data for 20 also support the proposed structure with, for instance, the presence of a C2-imidazolinium resonance (δ 155.5) along with the absence of a signal in the NHC-Ccarbene region. In addition, X-ray crystallographic analysis of compound 20 unambiguously established its molecular structure, yet precluding any discussion of the structural and bonding parameters due the poor quality of the crystallographic data (see Supporting Information, Figure S2). Regarding the NHC-Ti complexes 2 and 3, the NMR data are consistent with their formulation and, thus, with overall C2v- and Cs-symmetric structures, respectively, in solution. In the case of the dichloro Ti complex 2, its molecular structure was confirmed by X-ray crystallographic analysis and is illustrated in Figure 3 (see Supporting Information for crystallographic data, Table S3). Compound 2, which crystallizes on a C2-symmetric crystallographic axis, features a Ti metal center adopting a slightly distorted trigonal-bipyramidal geometry due to the mer-coordination of the tridentate ligand tBu(OCO)2-, causing the formation of a nearly perfectly planar [tBu(OCO)]Ti chelate. While both phenolate oxygens occupy the axial positions [(O(1)-Ti(1)-O(1)’ = 161.1(1)°)], the carbene moiety and the two chlorides are disposed in equatorial ones with the sum of the Cl(1)-Ti(1)-Cl(1), Cl(1)’-Ti(1)-C(15) and Cl(1)-Ti(1)-C(15) bond angles being exactly 360°, thus indicating that the Ti atom is centrally located. Other bonding parameters are closely related to those observed in the THF adduct 2-THF. Due to its potential interest as a rac-lactide ROP initiator,15 the bis(isopropoxide) Ti derivative [tBu(OCO)]Ti(OiPr)2 (4) was also synthesized and was found to be readily accessible via salt metathesis by reaction with 1 equiv of LiOiPr with the chloro isopropoxide Ti complex 2-THF (Scheme 3). Unlike species 2, 2-THF, and 3-THF, the NHC-Ti complex 4, isolated as a bright yellow solid in 78% yield, is extremely air-sensitive whether in the solid state or in solution and even (15) For reviews on the ROP of lactides by metal complexes, see: (a) O’Keefe, B. J.; Hillmeyer, M. A.; Tolman, W. B. J. Chem. Soc., Dalton Trans. 2001, 2215. (b) Dechy-Cabaret, O.; Martin-Vaca, B.; Bourissou, D. Chem. Rev. 2004, 104, 6147. (c) Wu, J.; Yu, T.-L.; Chen, C.-T.; Lin, C.-C. Coord. Chem. Rev. 2006, 250, 602. (d) Dove, A. P. Chem. Commun. 2008, 6446.

Figure 3. Molecular structure of 2 (symmetry transformations used to generate equivalent atoms (0 ): -x þ 1, -y, z). Selected bond lengths (A˚) and angles (deg): C(15)-Ti(1), 2.160(3); Ti(1)-Cl(1), 2.2794(9); Ti(1)-O(1), 1.828(2); O(1)-Ti(1)-O(1)0 , 161.12(13); O(1)-Ti(1)-C(15), 80.56(6); O(1)-Ti(1)-Cl(1), 94.56(7); O(1)-Ti(1)-C(15)-N(1), 4.4(6).

slowly decomposes to unidentified species upon storage in the solid state under inert atmosphere. This poor stability is most likely related to a significant level of steric congestion within compound 4, as may be deduced from its solid-state structure (vide infra). The NMR data for species 4 agree with a C2v-symmetric structure with, in particular, the presence of only a doublet resonance for the Me-iPr groups. The molecular structure of compound 4, as determined by X-ray crystallography and illustrated in Figure 4 (see Supporting Information for crystallographic data, Table S4), features a pentacoordinate Ti metal center adopting a trigonal-bipyramidal geometry; overall, the structural data for 4 closely relate to those of its analogue 2, discussed above. Yet, while the [tBu(OCO)]Ti chelate in 2 is nearly planar, that in the bis(isopropoxide) Ti complex 4 appears to be significantly distorted from planarity [for instance, C(1)-O(1)-O(2)-C(18) = 59.2(1)], which most likely reflects the significant steric interactions between the OiPr and tBu moieties; this may account, at least in part, for the poor stability of 4. One of the aims of the present work was also to investigate the different synthetic methods through which Ti complexes bearing the tridentate tBu(OCO)2- dianion might be accessible. Thus, the well-established group 4 amine elimination approach, typically involving the reaction of a pro-ligand with M(NMe2)4 (M = Ti, Zr, Hf), was also tested;5d,16 however, it was found to (16) For key and representative examples on the use of the amine elimination pathway in group 4 chemistry, see: (a) Chandra, G.; Lappert, M. F. J. Chem. Soc. A 1968, 1940. (b) Diamond, G. M.; Rodewald, S.; Jordan, R. F. Organometallics 1995, 14, 5. (c) Diamond, G. M.; Jordan, R. F.; Petersen, J. L. J. Am. Chem. Soc. 1996, 118, 8024. (d) Liang, L.-C.; Schrock, R. R.; Davis, W. M.; McConville, D. H. J. Am. Chem. Soc. 1999, 121, 5797.

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Figure 4. Molecular structure of 4. Selected bond lengths (A˚) and angles (deg): C(15)-Ti(1), 2.212(5); Ti(1)-O(1), 1.897(4); Ti(1)-O(2), 1.895(4); Ti(1)-O(3), 1.792(4); Ti(1)-O(4), 1.798(4); O(3)-Ti(1)-O(4), 113.5(2); O(1)-Ti(1)-O(2), 158.80(15); C(15)-Ti(1)-O(4), 127.8(2); O(1)-Ti(1)-C(15)-N(1), -10.5(4). Scheme 5

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Figure 5. Molecular structure of 6 (tBu groups are omitted for clarity). Selected bond lengths (A˚) and angles (deg): Ti(1)-C(46), 2.222(2); C(15)-Ti(1), 2.233(2); Ti(1)-O(1), 1.8971(15); Ti(1)-O(2), 1.8718(15); Ti(1)-O(3), 1.9089(15); Ti(1)-O(4), 1.9083(15); O(1)-Ti(1)-O(2), 155.48(7); O(3)-Ti(1)-O(4), 157.20(6); C(15)-Ti(1)-C(46), 162.64(8).

also account for the lengthening of the Ti-Ccarbene bond in compound 6 (average 2.228(3) A˚) versus those in related [tBu(OCO)]TiX2 complexes reported here (average 2.180(2) A˚). Overall, the structural parameters for 6 are similar to those observed for the only related bis(bis-aryloxide-NHC) Ti complex.5g

be a lower-yield procedure than the alcohol elimination pathway for the present systems. Thus, the reaction of the imidazolinium salt 1 with 1 equiv of Ti(NMe2)4 (THF, -78 °C to room temperature) afforded the formation of the Ti amido THF adduct complex [tBu(OCO)]Ti(NMe2)(Cl)(THF) (5) as the major component of the crude product, allowing its isolation in a relatively modest yield (43%) and whose formulation is proposed on the basis of NMR data (see Experimental Section). Finally, the direct reaction of salt species 1 with TiCl4(THF)2 (0.5 or 1 equiv vs 1) in the presence of an amine such as NEt3 (to trap the generated HCl) was also carried out, yielding, regardless of the stoichiometry in TiCl4(THF)2, the formation of the homoleptic bis-adduct Ti complex [tBu(OCO)]2Ti (6) as a red solid (eq 1). The NMR data for complex [tBu(OCO)]2Ti agree with an effective C2-symmetry in solution under the studied conditions (CD2Cl2, room temperature). Its solid-state molecular structure, as determined by X-ray crystallography, further confirmed the bis-adduct nature of species 6, as illustrated in Figure 5 (see Supporting Information for crystallographic data, Table S5). The Ti metal center exhibits a distorted octahedral environment due to the effective chelation by two tBu(OCO)2- tridentate ligands in a mer-fashion, which results in the NHC carbene donors being trans to one another [C(15)-Ti(1)-C(46) = 162.64(8)°]. Both [tBu(OCO)]Ti chelate moieties are significantly distorted from planarity [for instance, C(32)-O(3)-O(4)-C(54) = 65.6(2)° and C(1)-O(1)-O(2)-C(23) = 47.2(1)°], which is presumably due to steric hindrance around Ti. Such steric factors may

Controlled Ring-Opening Polymerization (ROP) of racLactide (rac-LA) Initiated the NHC-Ti Complex 3-THF. The NHC-Ti alkoxide complexes 3-THF and 4 were tested for rac-LA ROP activity primarily to probe the suitability of the tridentate NHC-incorporating ligand tBu(OCO)2- as an ancillary ligand in catalytic conditions. While the NHC-Ti bis-alkoxide complex 4 was found to be poorly active in rac-LA ROP, the monoalkoxide NHC-Ti 3-THF is quite efficient (15 h, toluene, 90 °C, 100 equiv of rac-LA, 89% conversion to atactic PLA).17 The poor ROP activity for compound 4 may result from severe steric crowding around Ti, precluding the coordination of the incoming monomer. Alternatively, due to its poor stability, species 4 may well decompose under the studied conditions to inactive component(s) prior to any reaction with rac-LA. The polymerization of rac-LA by 3-THF was thoroughly studied. All experimental data support a well-controlled racLA ROP process; these include (i) all PLAs, isolated at different monomer conversions, exhibit a narrow polydispersity as deduced from SEC data (Figure 6), (ii) the monomer conversion linearly correlates with the Mn values of PLAs (Figure 6), and (iii) the conversion rate to PLA is firstorder in monomer (see Supporting Information, Figure S1). In addition, for end-group analysis of the formed PLAs, a (17) The tacticity of the synthesized PLAs was deduced from homonuclear decoupled 1H NMR experiments on a 500 MHz NMR instrument.

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Figure 6. Plot of monomer (rac-lactide) conversion versus Mn values (real masses) of PLAs (left axis) and the corresponding polydispersities (right axis). Conditions: complex 3-THF as the initiator, 100 equiv of rac-LA, toluene, 90 °C.

Figure 7. MALDI-TOF mass spectrum of a PLA sample (at 60% conversion to PLA starting from 100 equiv of rac-lactide; ROP initiated by species 3-THF).

MALDI-TOF mass-spectrometric analysis of a PLA sample (that corresponding to 60% conversion to PLA, Figure 7) was recorded; this mass spectrum primarily exhibits metal ion peaks (M þ Naþ) that are equally spaced by 144 a.u. and with exact masses agreeing with a linear PLA bearing a isopropoxide group at the ester end, which is consistent with the ROP initiated by 3-THF proceeding without substantial transesterification processes.18 Overall, the catalytic performance of complex 3-THF compares well, whether in terms of activity or as a well-controlled process, to other Ti-based alkoxide complexes reported to initiate the ROP of (18) (a) Zell, M. T.; Padden, B. E.; Paterick, A. J.; Thakur, K. A. M.; Kean, R. T.; Hillmeyer, M. A.; Munson, E. J. Macromolecules 2002, 35, 7700. (b) Chamberlain, B. M.; Cheng, M.; Moore, D. R.; Ovitt, T. M.; Lobkhosky, E. B.; Coates, G. W. J. Am. Chem. Soc. 2001, 123, 3229. (19) For selected recent and representative examples, see: (a) Kim, Y.; Jnaneshwara, G. K.; Verkade, J. G. Inorg. Chem. 2003, 42, 1437. (b) Gendler, S.; Segal, S.; Goldberg, I.; Goldschmidt, Z.; Kol, M. Inorg. Chem. 2006, 45, 4783. (c) Lee, J.; Kim, Y.; Do, Y. Inorg. Chem. 2007, 46, 7701. (d) Gornshtein, F.; Kapon, M.; Botoshansky, M.; Eisen, M. S. Organometallics 2007, 26, 497. (e) Frediani, M.; Semeril, D.; Mariotti, A.; Rosi, L.; Frediani, P.; Rosi, L.; Matt, D.; Toupet, L. Macromol. Rapid Commun. 2008, 29, 1554. (f) Schwarz, A. D.; Thompson, A. L.; Mountford, P. Inorg. Chem. 2009, 48, 10442. (g) Whitelaw, E. L.; Jones, M. D.; Mahon, M. F.; Kociok-Kohn, G. Dalton Trans. 2009, 9020.

rac-lactide.19 Also, these ROP data altogether strongly suggest that the NHC-incorporating tridentate ligand effectively acts as an ancillary ligand during the ROP catalytic process.

Conclusion A novel family of titanium complexes supported by the NHC-bearing tridentate dianion tBu(OCO)2- have been synthesized in a straightforward manner and in high yields via, for most of them, an alcohol elimination pathway involving the reaction of the readily accessible imidazolinium salt [tBu(OCO)H3]Cl with Ti(OiPr)4 or ClTi(OiPr)3. Although less popular, this method was found to be most suited in the present case and may well apply to other ligand systems. Solid-state structural data for these Ti complexes overall suggest that this tridentate NHC bis-aryloxide ligand may significantly crowd the sphere of coordination of the Ti metal center. Yet, the Ti monoalkoxide chloro derivative [tBu(OCO)]Ti(OiPr)(Cl)(THF) was found to readily polymerize rac-lactide in a controlled manner and, interestingly, without apparent involvement of the NHC moiety in the catalytic process. The tridentate nature of the tBu(OCO)2-

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ligand as well as some level of steric protection provided by the tBu groups and, possibly, the central location of the NHC moiety within the tBu(OCO)2- ligand may rationalize the excellent stability of the [tBu(OCO)]Ti chelates. Further studies will focus on extending the scope of applications of these group 4 metal complexes in catalysis and in reactivity studies.

Experimental Section General Procedures. All experiments were carried out under N2 using standard Schlenk techniques or in a MBraun Unilab glovebox. THF, dichloromethane, and pentane were first dried through a solvent purification system (MBraun SPS) and stored for at least a couple of days over activated molecular sieves (4 A˚) in a glovebox prior to use. CD2Cl2 and C6D6 were purchased from Eurisotope (CEA, Saclay, France), degassed under a N2 flow, and stored over activated molecular sieves (4 A˚) in a glovebox prior to use. All other chemicals were used as received. N,N0 -Bis(3,5-di-tert-butyl-2-hydroxyphenyl)ethylenediamine and N,N0 -di(2-hydroxy-3,5-di-tert-butylphenyl)-4,5-dihydroimidazolium chloride (1) were synthesized according to literature procedures.9,12,13 NMR spectra were recorded on Bruker AC 300 or 400 MHz NMR spectrometers in Teflon-valved J-Young NMR tubes at ambient temperature, unless otherwise indicated. 1H and 13C chemical shifts are reported versus SiMe4 and were determined by reference to the residual 1H and 13C solvent peaks. Mass spectra were recorded by the “Service de Masse” of the Strasbourg chemistry department. Elemental  ementaire” analysis were performed by the “Service d’Analyse El of the Strasbourg chemistry department. SEC analyses of PLAs were performed at the Institut Charles Sadron (Strasbourg, France) on a system equipped with a Shimadzu RID10A refractometer detector and a TREOS (Wyatt Techn.) multiple angles light scattering detector, allowing direct access to the real masses of the synthesized PLAs. Dry THF (on CaH2) was used as an eluant for the latter analysis. [tBu(OCO)]TiCl2(THF) ([tBu(OCO)]2- = [η3-O,C,O-{(3,5-ditert-butyl-C6H2O)2N2C3H4}]2-) (2-THF). A THF solution (4 mL) of ClTi(OiPr)3 (253 mg, 0.97 mmol) was added at room temperature via a pipet to a stirring THF solution (15 mL) of the imidazolinium chloride salt 1 (500 mg, 0.97 mmol). The initial colorless solution slowly turned dark green upon addition of the titanium reagent. The reaction mixture was stirred overnight at room temperature. Subsequent evaporation to dryness afforded pure 2-THF as an dark greenish residue, as determined by NMR spectroscopy and elemental analysis. 1 H NMR (300 MHz, CD2Cl2) δ: 7.32 (d, J = 2.2 Hz, 2H, aryl), 7.13 (d, J = 2.2 Hz, 2H, aryl-H), 4.52 (br s, 4H, NCH2), 3.66 (m, 4H, THF), 1.76 (m, 4H, THF), 1.66 (s, 18H, tBu), 1.40 (s, 18H, tBu). 13C NMR (75 MHz, CD2Cl2) δ: 197.4 (Cquat, NCN), 150.0 (Cipso, O-aryl), 143.3 (Cquat, aryl), 136.8 (Cquat, aryl), 131.6 (Cquat, aryl), 119.0 (Cquat, aryl), 111.8 (Cquat, aryl), 69.8 (CH2, THF), 47.5 (CH2, NCH2), 35.6 (Cquat, tBu), 34.7 (Cquat, tBu), 31.3 (CH3, tBu), 29.9 (CH3, tBu), 25.3 (CH2, THF). Anal. Calcd for C35H52Cl2N2O2Ti (%): C, 62.97; H, 7.85; N, 4.20. Found: C, 62.81; H, 7.89; N, 3.47. [tBu(OCO)]TiCl2 (2) and Zwitterion 20 . A CH2Cl2 solution (4 mL) of ClTi(OiPr)3 (126 mg, 0.49 mmol) was added at room temperature via a pipet to a stirring CH2Cl2 solution (10 mL) of the imidazolinium chloride salt 1 (250 mg, 0.49 mmol). The initial colorless solution slowly turned dark green upon addition of the titanium reagent. The reaction mixture was stirred overnight at room temperature. Subsequent evaporation to dryness afforded a mixture of 2 and zwitterionic compound [tBu(OCHO)]TiCl2(OiPr) ([tBu(OCHO)]2- = [η3-O,C,O-{(3,5di-tert-butyl C6H2O)2N2C3H5}]2-), 20 , as determined by NMR in a 2:1 ratio. Recrystallization and isolation of com plex 20 proved to be difficult: X-ray crystallographic data of a

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poor-quality crystal of 20 were insufficient for a proper refinement of the structure. However, the atomic connectivity of species 20 was unambiguously established (see Supporting Information for an ORTEP view of 20 ). The dark greenish mixture was then dissolved in toluene and heated at 90 °C overnight. Evaporation to dryness afforded analytically pure 2 as a brown solid (242 mg, 84%). Data for 2. 1H NMR (300 MHz, CD2Cl2) δ: 7.40 (d, J = 2.1 Hz, 2H, aryl-H), 7.18 (d, J = 2.1 Hz, 2H, aryl-H), 4.61 (s, 4H, NCH2), 1.69 (s, 18H, tBu), 1.41 (s, 18H, tBu). 13C NMR (75 MHz, CD2Cl2) δ: 200.0 (Cquat, NCN), 151.6 (Cipso, O-aryl), 145.1 (Cquat, aryl), 137.5 (Cquat, aryl), 137.5 (Cquat, aryl), 131.7 (Cquat, aryl), 120.0 (CH, aryl), 112.0 (CH, aryl), 48.0 (CH2, NCN), 36.1 (Cquat, tBu), 35.4 (Cquat, tBu), 31.7 (CH3, tBu), 30.5 (CH3, tBu). Anal. Calcd for C31H44Cl2N2O2Ti (%): C, 62.53; H, 7.45; N, 4.70. Found: C, 62.07; H, 7.57; N, 4.41. Data for 20 . 1H NMR (300 MHz, CD2Cl2) δ: 8.74 (s, 1H, NCHN), 7.46 (d, J = 2.4 Hz, 2H, aryl-H), 7.17 (d, J = 2.4 Hz, 2H, aryl-H), 5.25 (hept, 1H, OiPr), 4.52-4.28 (m, 4H, NCH2), 1.62 (s, 18H, tBu), 1.46 (d, 6H, OiPr), 1.34 (s, 18H, tBu). 13C NMR (75 MHz, CD2Cl2) δ: 158.2 (Cipso, O-aryl), 155.5 (CH, NCHN), 144.3 (Cquat, aryl), 139.5 (Cquat, aryl), 127.4 (Cquat, aryl), 125.6 (CH, aryl), 120.0 (CH, aryl), 87.7 (CH, OiPr), 54.0 (CH2, NCN), 36.2 (Cquat, tBu), 34.9 (Cquat, tBu), 31.5 (CH3, t Bu), 30.5 (CH3, tBu), 25.0 (CH3, OiPr). Crystal data for 20 : C34H52Cl2N2O3Ti;5(C6H6), a = 11.8911(4) A˚, b = 8.6452(2) A˚, c = 29.1267(11) A˚, V = 2993.64(17) A˚3, Z = 2, T = 173 K. [tBu(OCO)]Ti(Cl)(OiPr)(THF) (3-THF). A THF solution (4 mL) of Ti(OiPr)4 (276 mg, 0.97 mmol) was added at room temperature via a pipet to a stirring THF solution (15 mL) of the imidazolinium chloride salt 1 (500 mg, 0.97 mmol). The initial colorless solution immediately turned orange upon addition of the titanium reagent. The reaction mixture was stirred overnight at room temperature and evaporated to dryness to quantitatively yield pure 3-THF as an orange solid (637 mg, 95% yield). 1 H NMR (300 MHz, CD2Cl2) δ: 7.25 (d, J = 2.2 Hz, 2H, arylH), 6.98 (d, J = 2.2 Hz, 2H, aryl-H), 4.82 (hept, J = 6.2 Hz, 1H, OiPr), 4.59-4.27 (m, 4H, NCH2), 3.72-3.63 (m, 4H, THF), 1.85-1.75 (m, 4H, THF), 1.58 (s, 18H, tBu), 1.37 (s, 18H, tBu). 13 C NMR (75 MHz, C6D6) δ: 198.6 (NCN), 150.5 (Cipso, Oaryl), 140.4 (Cquat, aryl), 137.4 (Cquat, aryl), 130.1 (Cquat, aryl), 119.2 (CH, aryl), 111.0 (CH, aryl), 82.3 (CH, OiPr), 68.2 (CH2, THF), 46.6 (CH2, NCH2), 35.8 (CH3, tBu), 34.4 (CH3, tBu), 31.6 (CH3, tBu), 30.2 (CH3, tBu), 25.4 (CH3, OiPr), 25.2 (CH2, THF). Anal. Calcd for C38H59ClN2O4Ti (%): C, 66.03; H, 8.60; N, 4.05. Found: C, 65.95; H, 8.69; N, 3.84. [tBu(OCO)]Ti(Cl)(OiPr) (3). A dichloromethane solution (4 mL) of Ti(OiPr)4 (138 mg, 0.49 mmol) was added at room temperature via a pipet to a stirring dichloromethane solution (15 mL) of the imidazolinium chloride salt 1 (250 mg, 0.49 mmol). The initial colorless solution immediately turned orange upon addition of the Ti reagent. The reaction mixture was stirred overnight at room temperature and evaporated to dryness to quantitatively yield pure 3 as an orange solid residue. 1 H NMR (300 MHz, CD2Cl2) δ: 7.26 (d, J = 2.2 Hz, 2H, arylH), 6.98 (d, J = 2.2 Hz, 2H, aryl-H), 4.89-4.74 (m, 1H, OiPr), 4.56-4.29 (m, 4H, NCH2), 1.58 (s, 18H, tBu), 1.37 (s, 18H, tBu), 1.0 (d, J = 6.3 Hz, 6H, -OiPr). 13C NMR (100 MHz, CD2Cl2) δ: 198.4 (Cquat, NCN), 150.4 (Cquat, aryl), 141.9 (Cquat, aryl), 137.4 (Cquat, aryl), 130.2 (Cquat, aryl), 119.9 (CH, aryl), 111.9 (CH, aryl), 83.8 (CH, OiPr), 48.0 (CH2, NCH2), 35.9 (Cquat, tBu), 35.0 (Cquat, tBu), 31.8 (CH3, tBu), 30.1 (CH3, tBu), 25.6 (CH3, OiPr). Anal. Calcd for C34H51ClN2O3Ti (%): C, 65.96; H, 8.30; N, 4.52. Found: C, 66.26; H, 8.17; N, 4.29. [tBu(OCO)]Ti(OiPr)2 (4). A THF solution of LiOiPr (2 M in THF, 0.485 mL, 0.97 mmol) was added at room temperature via a microsyringe to a precooled (-35 °C) THF solution (15 mL) of the chloro isopropoxy titanium complex 3-THF (671 mg,

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0.97 mmol). The initial orange solution became lighter within the course of a few minutes. The reaction mixture was allowed to warm to room temperature and stirred overnight to yield a yellow solution. Evaporation to dryness, subsequent addition of toluene, and filtration of the resulting suspension through Celite on a glass frit yielded the crude product as light brown solid. Recrystallization of the latter solid from a concentrated 1:1 THF/pentane solution stored at -35 °C yielded compound 4 as an analytically pure and air-sensitive bright yellow solid (487 mg, 78% yield). 1 H NMR (300 MHz, C6D6) δ: 7.49 (d, J = 2.2 Hz, 2H, arylH), 6.56 (d, J = 2.2 Hz, 2H, aryl-H), 5.15 (septet, J = 6.1 Hz, 2H, CH-OiPr), 2.94 (s, 4H, NCH2), 1.92 (s, 18H, tBu), 1.44 (s, 18H, tBu), 1.35 (d, J = 6.1 Hz, 12H, -OiPr). 13C NMR (75 MHz, CD2Cl2) δ: 197.9 (NCN), 150.8 (Cipso, O-aryl), 139.4, 136.7, 129.6, 119.0, 111.3, 77.0 (CH, OiPr), 47.4 (NCH2), 35.5 (Cquat, tBu), 34.4 (Cquat, tBu), 31.4 (CH3, tBu), 29.8 (CH3, tBu), 26.3 (-OiPr). Anal. Calcd for C37H58N2O4Ti (%): C, 69.14; H, 9.10; N, 4.36. Found: C, 69.56; H, 9.17; N, 3.95. [tBu(OCO)]Ti(Cl)(NMe 2)(THF) (5-THF). A precooled (-78 °C) THF solution (40 mL) of 1 (250 mg, 0.49 mmol) was added via a cannula to a stirring precooled (-78 °C) THF solution (10 mL) of Ti(NMe2)4 (109 mg, 0.49 mmol). The initial colorless solution immediately turned yellow upon addition of the titanium reagent. The reaction mixture was allowed to warm at room temperature and stirred overnight. After evaporation to dryness, the residue was washed with pentane to yield pure 5-THF as a brown powder (142 mg, 43% yield). 1 H NMR (300 MHz, CD2Cl2) δ: 7.31 (d, 2H, J = 2.2 Hz), 7.06 (d, 2H, J = 2.2 Hz), 4.57-4.28 (m, 4H), 3.67-3.54 (m, 4H), 3.24 (s, 6H), 1.78-1.72 (m, 4H), 1.66 (s, 18H, tBu), 1.40 (s, 18H, tBu). 13 C NMR (75 MHz, CD2Cl2) δ: 203.6 (Cquat, NCN), 149.0 (Cquat, aryl), 141.9 (Cquat, aryl), 139.9 (Cquat, aryl), 137.2 (Cquat, aryl), 131.5 (Cquat, aryl), 119.8 (CH, aryl), 112.1 (CH, aryl), 68.7 (CH2, THF), 51.5 (CH3, NMe2), 48.1 (CH2, NCH2), 36.0 (Cquat, t Bu), 35.0 (Cquat, tBu), 31.9 (CH3, tBu), 30.3 (CH3, tBu), 25.9 (CH2, THF). Anal. Calcd for C37H58ClN3O3Ti (%): C, 65.72; H, 8.65; N, 6.21. Found: C, 65.96; H, 8.68; N, 5.98. [tBu(OCO)]2Ti (6). A THF solution (4 mL) of TiCl4(THF)2 (81 mg, 0.24 mmol) was added at room temperature via a pipet to a stirring THF solution (15 mL) of the imidazolium chloride salt 1 (250 mg, 0.48 mmol). The initial colorless solution progressively turned dark red upon addition of the Ti reagent, and then NEt3 (0.203 mL, 1.46 mmol) was added dropwise. The reaction mixture was stirred overnight at room temperature to yield a dark red solution. The solution was evaporated to dryness, toluene was added, and the resulting suspension was filtered through Celite on a glass frit. The filtrate was then evaporated to dryness in vacuo to afford pure 5 as red solid in 80% yield.

Romain et al. H NMR (300 MHz, CD2Cl2) δ: 6.97 (dd, J = 4 Hz, 2 Hz, 8H, aryl-H), 4.45 (s, 8H, NCH2), 1.33 (s, 36H, tBu), 1.01 (s, 36H, t Bu). 13C NMR (75 MHz, CD2Cl2) δ: 199.9 (Cquat, NCN), 152.3 (Cquat, aryl) 139.2 (Cquat, aryl), 135.8 (Cquat, aryl), 129.8 (Cquat, aryl), 118.4 (CH, aryl), 111.6 (CH, aryl), 48.1 (CH2, NCH2), 35.3 (Cquat, tBu), 34.7 (Cquat, tBu), 31.9 (CH3, tBu), 29.4 (CH3, tBu). HRMS C62H89N4O4Ti: calcd mass 1001.6365, measured mass 1001.6361. Crystal Structure Determination. Single crystals of complexes 2, 2-THF, 4, and 6 were mounted on a Nonius Kappa-CCD area detector diffractometer (Mo KR, λ = 0.71073 A˚). The complete conditions of data collection (Denzo software)20 and structure refinements are in Tables S1 and S3-5 (Supporting Information). The cell parameters were determined from reflections taken from one set of 10 frames (1.0° steps in phi angle), each at 20 s exposure. Single crystals of complex 3-THF were mounted on a Bruker APEX II DUO Kappa-CCD area detector diffractometer (Mo KR, λ = 0.71073 A˚). The complete conditions of data collection (APEX2 software)21 and structure refinements are summarized in Table S2 (Supporting Information). The cell parameters were determined from reflections taken from three sets of 12 frames, each at 15 s exposure. All structures were solved using direct methods (SHELXS97) and refined against F2 using the SHELXL97 software.22 The absorption was not corrected except for complex 3-THF, where a semiempirical absorption correction was applied using SADABS in APEX2;21 transmission factors are Tmin/Tmax = 0.907/0.937. All non-hydrogen atoms were refined anisotropically. Hydrogen atoms were generated according to stereochemistry and refined using a riding model in SHELXL97. 1

Acknowledgment. The authors thank the CNRS (Centre Nationale de la Recherche Scientifique, Paris, France) and the Universite de Strasbourg (Strasbourg, France) for financial support. Supporting Information Available: Crystallographic data for 2, 2-THF, 3-THF, 4, and 6, an ORTEP drawing (along with lattice parameters) of the molecular structure of zwitterion 20 , and a plot (ln[M0]/[M] vs time) showing the ROP of rac-lactide initiated by complex 3-THF to be first-order in monomer. This material is available free of charge via the Internet at http:// pubs.acs.org. (20) Kappa CCD Operation Manual; Nonius B. V., Ed.; Delft: The Netherlands, 1997. (21) M86-E01078 APEX2 User Manual; Bruker AXS Inc.: Madison, WI, 2006. (22) Sheldrick, G.-M. SHELXL97, Program for the refinement of Crystal Structures; University of G€ottingen: G€ottingen, Germany, 1997.