Group 4 Metal Complexes with New Chiral Pincer Nhc-ligands: Synthesis, Structure and Catalytic Activity

Figure S1. 1 H NMR spectrum (500 MHz, CDCl 3) of a polymer sample obtained from the 8/isopropanol system with 8/isopropanol/LA (mol/mol/mol/) = 1/1/100 (Table 4, entry 3).


Introduction
Biodegradable polymers derived from renewable resources such as polylactides have received much attention over the past decade because of their attractive physical and mechanical properties. 1 In addition, the chain stereochemistry determines the polymer properties and the rate of degradation. 2For example, whereas the enantiopure polylactide melts at 180 °C, a much higher melting temperature (230 °C) is found for stereocomplex polymers formed by an equivalent mixture of poly(L-lactide) and poly(D-lactide). 3Therefore, the polymerization of rac-lactide with stereoselective catalysts remains a challenge and an opportunity for chemists.To date, numerous reviews have covered catalyst systems for the ring-opening polymerization (ROP) of cyclic esters based on metals such as magnesium, zinc, calcium, aluminum, lanthanides, tin, group 4 metals, germanium, indium and iron, 1d,4 but among these, the chiral group 4 catalysts are especially promising. 5Unfortunately, compared to other metals, structurally well-characterized chiral group 4 complexes that initiate the controlled ringopening polymerization of lactides are still scarce. 5 In recent years, transition-metal complexes with chiral N-heterocyclic carbene (NHC) ligands have become increasingly popular because of their stability to air and moisture and their strong σ-donating, but poor π-accepting abilities. 6An additional driving force is the longstanding interest in catalysts for enantioselective reactions such as olefin metathesis, 7  conjugate addition of enones, 8 allylic alkylations, 9 olefin hydrogenations 10 and hydrosilylations. 11Encouraged by the attractive features of chiral NHC-ligands in general, we are now focusing on the preparation of group 4 metal complexes coordinated by chiral multi-dentate NHC-ligands, and to our knowledge no chiral group 4 metal NHC-catalyst has yet been structurally authenticated. 6,12More recently, we have designed and prepared a new series of tridentate chiral pincer NHCligands L4-L6 from (S,S)-diphenyl-1,2-ethanediamine, and found them to be useful ligands for group 4 metals, which are potential catalysts for the polymerization of lactides.Herein, we report on the synthesis of these chiral NHC-ligands, their use in group 4 chemistry, and the application of the resulting complexes as catalysts in the polymerization of rac-lactide (rac-LA).

General methods
Group 4 complexes and catalytic reactions were performed under an atmosphere of dry dinitrogen with rigid exclusion of air and moisture using standard Schlenk or cannula techniques, or a glovebox.All organic solvents were freshly distilled from sodium benzophenone ketyl immediately prior to use.Racemic lactide (rac-LA) was recrystallized twice from dry toluene and then sublimed under vacuum prior to use.All chemicals were purchased from Aldrich Chemical Co. and Beijing Chemical Co. and used as received unless otherwise noted.Infrared spectra were obtained from KBr pellets on an Avatar 360 Fourier transform spectrometer.Molecular weights of the polymer were estimated by gel permeation chromatography (GPC) using a PL-GPC 50 apparatus. 1H and 13 C NMR spectra were recorded on a Bruker AV 500 spectrometer at 500 and 125 MHz, respectively.All chemical shifts are reported in δ units with reference to the residual protons of the deuterated solvents for proton and carbon chemical shifts.Melting points were measured on X-6 melting point apparatus and were uncorrected.Elemental analyses were performed on a Vario EL elemental analyzer.

General procedure for polymerization of a rac-lactide
In a glovebox, rac-lactide (rac-LA) (0.360 g, 2.5 mmol), 2-propanol (0.01 mmol, in 0.5 mL of toluene or THF), complex (typically 0.01 mmol, in 0.5 mL of toluene or THF), and toluene or THF (4.0 mL) were added sequentially into a Schlenk flask with stirring.The flask containing the reaction mixture was subsequently placed in an oil bath and stirred for 0.5 h at 70 °C.The polymerization was quenched by the addition of cold acidified methanol.The precipitated polylactide was collected, washed with cold methanol several times, and dried in vacuum at 50 °C overnight.

X-ray crystallography
Single-crystal X-ray diffraction measurements were carried out on a Bruker SMART CCD diffractometer using graphite monochromated Mο Kα radiation (λ = 0.71073 Å).An empirical absorption correction was applied using the SADABS program. 13All structures were solved by direct methods and refined by full-matrix least squares on F 2 using the SHELXL-97 program package. 14All the hydrogen atoms were geometrically fixed using the riding model.Disordered solvents in the voids of 15, 16 and 17 were modeled or removed using the SQUEEZE program. 15The crystal data and experimental data for L5, 10 and 14-19 are summarized in Tables 1 and 2. Selected bond lengths and angles are listed in Table 3.

Synthesis and characterization of pro-ligands
Condensation of (S,S)-diphenyl-1,2-ethanediamine with 1 equiv. of salicylaldehyde, 3-tert-butylsalicylaldehyde or 3,5-ditert-butylsalicylaldehyde in absolute ethanol at ambient temperature, followed by reduction with an excess of NaBH 4 in ethanol forms the chiral diamines 1-3 (Schemes 1-3).Subsequent cyclization of 1-3 with triethyl orthoformate in the presence of NH 4 Br or NH 4 Cl at 120 °C gives the imidazolium salts L4 (L4a and L4b), L5 and L6, respectively, in good yields (Schemes 1-3).All compounds are air-stable and have been characterized by various spectroscopic techniques and elemental analyses.The 1 H and 13 C NMR spectra are consistent with their C 2 -symmetric structure.In addition, besides aromatic stretches the IR spectra of L4-L6 also feature the characteristic O-H (at ca.3420 cm −1 ) and strong CvN stretches (at ca.1640 cm −1 ).The C 2 symmetric structure of L5 was also confirmed by X-ray diffraction analysis (Fig. 1).

Synthesis and characterization of complexes
Amine elimination between M(NMe 2 ) 4 and protic reagents is a very efficient way for the synthesis of group 4 metal amide complexes. 16Hence, a similar reaction is expected for the acidic protons in the ligands L4 (L4a and L4b), L5 and L6 and metal amides.In fact, treatment of M(NR 2 ) 4 (M = Ti, Zr, Hf; R = Me, Et) with 1 equiv. of L4 in THF gives, after recrystallization from a benzene solution, the chiral titanium amides (L4)Ti(NMe 2 )(Br)-(THF) ( 7) and (L4)Ti(NMe 2 )(Cl)(THF) (11), zirconium amides (L4)  values and that the molar mass distributions are very narrow (M w /M n = 1.18-1.21;Table 4, entries 2-6).In addition, for complex 8 a first order kinetic dependence on the concentration of rac-LA and no induction period were observed (Fig. 9).The M n,exp values increase linearly with the monomer conversion, whereas the M w /M n values remain in the narrow range of 1.17-1.22(Fig. 10).However, when the more bulky ligands L5 and L6 are used, the polymerization with the     zirconium and hafnium complexes 15 and 17-19 is slightly slower (Table 4, entries 13, and 15-17), presumably because of the increased steric hindrance at the metal centers.Although the zirconium and hafnium complexes are effective catalysts for the polymerization of rac-LA, the titanium complexes 7, 11 and 16 exhibit only poor catalytic activity (Table 4, entries 1, 9 and 14), consistent with the smaller ionic radius of Ti 4+ .These differences also prevail in THF solution (Table 4, entries 18-30), but the polymerization with these group 4 initiators/ catalysts proceeds much more slowly in THF (Table 4, entries 18-30), most likely a consequence of competitive monomersolvent coordination to the metal ion.A similar competition was observed for the organoyttrium and organoaluminum catalysts.16a,18 In the absence of isopropanol, no polymerization occurs in toluene or THF solution even when heated at 70 °C for 72 h.The polymer microstructure, as determined by homo-decoupled 1 H NMR experiments, 19 shows that the polylactides are heterotactic-rich polylactides under conditions examined.The catalytic activities of 7-19 resemble that of

Conclusions
Chiral group 4 NHC-metal complexes were prepared and structurally characterized.These complexes represent the first example of the structurally characterized group 4 chiral NHCmetal complex, and they can initiate the ring-opening polymerization of rac-lactide in the presence of isopropanol, leading to the heterotactic-rich polylactides.Nevertheless, the reactivity is       strongly influenced by the size of the metal ion and the solvents.For example, fast polymerization is observed in toluene, whereas the conversion is slow in THF because of competitive monomer-solvent coordination to the metal ions.The zirconium and hafnium complexes are efficient precatalysts for polymerization of rac-LA, while the titanium complexes exhibit only poor catalytic activity because of the smaller ionic radius of Ti 4+ .Further studies will focus on the application of these complexes towards other asymmetric reactions and the exploration of new group 4 metal complexes based on chiral ligands.

Fig. 1
Fig. 1 Molecular structure of the cation in L5 (thermal ellipsoids drawn at the 35% probability level).