Octahedral Zirconium Salan Catalysts for Olefin Polymerization: Substituent and Solvent Effects on Structure and Dynamics

Group 4 metal-Salan olefin polymerization catalysts typically have relatively low activity, being slowed down by a pre-equilibrium favoring a non-polymerization active resting state identified as a mer-mer isomer (MM); formation of the polymerization active fac-fac species (FF) requires isomerization. We now show that the chemistry is more subtle than previously realized. Salan variations bearing large, flat substituents can achieve very high activity, and we ascribe this to the stabilization of the FF isomer, which becomes lower in energy than MM. Detailed in situ NMR studies of a fast (o-anthracenyl) and a slow (o-tBu) Salan precursors, suitably activated, indicate that preferred isomers in solution are different: the fast catalyst prefers FF while the slow catalyst prefers a highly distorted MM geometry. Crystal structures of the activated o-anthracenyl substituted complex with a moderately (chlorobenzene) and, more importantly, a weakly coordinating solvent (toluene) in the first coordination sphere emphasize that the active FF isomer is preferred, at least for the benzyl species. Site epimerization (SE) barriers for the fast catalyst (ΔS > 0, dissociative) and the slow catalyst (ΔS < 0, associative) in toluene corroborate the solvent role. Diagnostic NMe 13C chemical shift differences allow unambiguous detection of FF or MM geometries for seven activated catalysts in different solvents, highlighting the role of solvent coordination strength and bulkiness of the ortho-substituent on the isomer equilibrium. For the first time, active polymeryl species of Zr-Salan catalysts were speciated. The slow catalyst is effectively trapped in the inactive MM state, as previously suggested. Direct observation of fast catalysts is hampered by their high reactivity, but the product of the first 1-hexene insertion maintains its FF geometry.


General considerations
All manipulations and synthesis of air-and moisture sensitive chemicals were performed under rigorous exclusion of oxygen and moisture in flame-dried Schlenk-type glassware interfaced to a high-vacuum line (10 −5 Torr), or in a nitrogen-filled MBraun glovebox (<0.5 ppm O2 and H2O).
Hydrocarbon solvents used for synthesis were dried over 4Å molecular sieves and degassed by bubbling with dry argon. Ethereal solvents used for synthesis were distilled from sodium/benzophenone. Molecular sieves (4 Å, MS) were activated for 24 h at ca. 200−230 °C under dynamic vacuum. All solvents used in the studies of the activated complexes were freeze-pump-thaw degassed on the high vacuum line, dried over the appropriate drying agent (Na/K alloy for benzene, pentane, toluene, benzene-d6 and toluene-d8; CaH2 for chlorobenzene-d5, 1,2-difluorobenzene) and was obtained from Boulder Scientific Company and used as received. TiBA (tri-isobutyl aluminum) was purchased from Sigma-Aldrich and used as received. High-resolution mass spectra (HRMS) were recorded on an Agilent Technologies 6530 Q-TOF LC/MS system paired Agilent 1260 HPLC and using Agilent JetStream or APCI ion source. Ethene (Linde, 99.95%) and propene (Rivoira, 99.6%) were purified by flowing them through a column containing activated 4 Å molecular sieves and an activated Cu catalyst (BASF R0-11G). 1-Hexene (Sigma-Aldrich, 99%) was purified by passing it through a mixed-bed column of the activated Cu catalyst and 4 Å molecular sieves. Toluene (Romil) was dried using an MBraun SPS-5 solvent purification unit. 1,2-Dichlorobenzene (Romil, >99.8% isomeric purity) was used as received.

NMR Spectroscopy Experiments
All samples for NMR measurements were prepared inside the glove box; flame-dried NMR tubes equipped with a PTFE valve (J-Young NMR tubes) were used. One-and two-dimensional, homoand hetero-nuclear NMR spectra were recorded on a Bruker Avance III 400 spectrometer equipped with a smartprobe and using standard pulse sequences. Unless otherwise stated, referencing is relative to external TMS ( 1 H and 13 C), NH3 ( 15 N) and CCl3F ( 19 F). Variable temperature 1 H EXSY NMR measurements were acquired by using the PFG version of the NOESY sequence (noesygptp), setting a relaxation delay of 1 s, with mixing time values (τm) ranging between 2.7 and 800 ms depending on the rate of chemical exchange. Typically, a matrix of 1024x1024 data points was used for acquisition and the raw data were processed using zero-filling to 2048 data points in both spectral dimensions.
The spectral window and the number of transients were optimized depending on distribution and of relevant resonances and the sample concentration. Typically, at least two experiments with different S3 τm values were acquired for each temperature, and the rate constant values were obtained from the average of all the values. For the Ion Pair Symmetrization (IPS) dynamical motions, rate constants (kIPS, s -1 ) were evaluated by the method proposed by Perrin, 1 and were calculated from the integration of the 2D spectra by using the EXSYCALC software. 2 In some cases, exchange rate constants were also estimated at higher temperature, where exchanging resonances first coalesce and then narrow, using methods of lineshape analysis and standard equations for two-sites exchange in the absence of scalar coupling. 3 Activation parameters of dynamical motions were estimated from the corresponding Eyring plots; errors on activation enthalpy and activation entropy were determined from the quality of linear fitting and computed at 95% confidence interval. For the 15 N NMR experiments, standard sequences provided by Bruker were employed, in particular the zgig sequence for 1D 15 N NMR spectra, and the hsqcgpph sequence for 2D 15 N NMR, with an optimization of the 90° pulse at 21 μs at 74 PLW1, with a long-range constant of 7 Hz.
To describe the multiplicity of the signals, the following abbreviations are used: s, singlet; bs, broad singlet; d, doublet; bd, broad doublet; dd, doublet of doublets; t, triplet; and m, multiplet.

Synthesis of L * 4H-H
A mixture of ethylenediamine-15 N2 dichloride (50.5 mg, 374 µmol) and ethylenediamine dichloride (151.5 mg, 1.14 mmol) was suspended in 10 mL of dry methanol, and then trimethylamine (422 µL, 3.0 mmol) was added. The resulting mixture was stirred until all the solid was dissolved. The solution was added to the mixture of 3-(tert-butyl)-2hydroxybenzaldehyde (539.2 mg, 3.0 mmol) and 12 mL of dry methanol. The mixture was stirred at room temperature for 2 h, and yellow precipitate was formed. Next, 15mL of dry THF was added to dissolve the precipitate, and the obtained solution was cooled to 0°C with ice bath.
After that, sodium tetraborohydride (1.5 g, 39.5 mmol) was added to the solution slowly. The resulting mixture was stirred at room temperature for another 30 min, then poured in 100 mL of water and extracted with dichloromethane (3×30 mL). The combined organic layers were dried over anhydrous Na2SO4, and the solvents were evaporated. The residue was sufficiently pure L * 4H (571 mg, 99% yield, approx. 25% 15 N) as a white solid. 1

Synthesis of L * 4H
S5 A suspension of L * 4H-H (530 mg, 1.38 mmol) in 24 mL of dry acetonitrile was treated with 3.2 mL of trifluroacetic acid which resulted in homogenization of the mixture. Next, formaldehyde solution, (formalin, 1.1 mL, 14.3 mmol, 37% CH2O), was added, and the mixture was cooled to 0°C with ice bath. After that, sodium tetraborohydride (1.5 g, 39.5 mmol) was added to the solution at 0°C slowly. The resulting mixture was stirred at room temperature overnight. Then, 50 mL of saturated aqueous NaHCO3 was added, and the mixture was extracted with dichloromethane (3×30 mL). The combined organic layers were dried over anhydrous Na2SO4, and the solvents were evaporated. The residue was dissolved in a mixture of 8 mL of dichloromethane and 30 mL of methanol. Then, dichloromethane was rotary evaporated which resulted in precipitation. The precipitate was filtered off, washed twice with 10 mL of methanol, and dried in vacuum. This procedure gave pure L * 4H (442 mg, 71% yield, approx. 25% 15 N) as a white solid. 1

Synthesis of * 4H
In a glovebox with argon atmosphere, ligand L * 4H (335 mg, 812 µmol) was dissolved in warm toluene (30 mL), and a solution of ZrBn4 (370 mg, 812 µmol) in toluene (5 mL) was added. The mixture was stirred at r.t. overnight. Then, the solution was concentrated in vacuum, the precipitated solid was filtered off, washed with 2 mL of toluene and 5 mL of pentane to give the product as a white solid (335 mg, 60 % yield, approx. 25% 15 N). 1
HCl. The resulting solution was stirred at 60°C overnight. Next, volatiles were evaporated and the residue was suspended in 300 mL of water following by extraction with dichloromethane (3×50 mL). The combined organic layers were dried over anhydrous Na2SO4, and the solvents were evaporated giving pure product as a yellowish solid (4.4 g, 97% yield). 1  A mixture of ethylenediamine-15 N2 dichloride (51 mg, 378 µmol) and ethylenediamine dichloride (175 mg, 1.32 mmol) was suspended in 10 mL of dry methanol, and then trimethylamine (413 µL, 3.0 mmol) was added. The resulting mixture was stirred until all the solid was dissolved. In a separate flask, a suspension of 3-(anthracen-9-yl)-2-hydroxy-5methylbenzaldehyde (1.06 g, 3.39 mmol) in a mixture of 45 mL of dry methanol and 45 mL of dry ethanol was heated until all the aldehyde was dissolved. Immediately after homogenization, a solution of the ethylenediamines was added to the hot aldehyde solution which resulted in formation of yellow precipitate. The mixture was stirred at room temperature for 2 h. Next, 210 mL of dry THF was added to dissolve the precipitate, and sodium tetraborohydride (455 mg, 12 mmol) was added to the solution slowly. The resulting mixture was stirred at room temperature for 1 h, and then the solvents were evaporated. The residue was suspended in 200 mL of water, and the crude product was extracted with dichloromethane (3×50 mL). The combined organic layers were dried over anhydrous Na2SO4, and the solvents were evaporated.

Synthesis of * L3.
A suspension of * L3-H (938 mg, 1.44 mmol) in 31 mL of dry acetonitrile was treated with 3.3 mL of trifluroacetic acid which resulted in homogenization of the mixture. Next, formaldehyde solution, formalin (1.1 mL, 14.3 mmol, 37% CH2O), was added, and the mixture was cooled to 0°C with ice bath. After that, sodium tetraborohydride (1.04 g, 27 mmol) was added to the solution at 0°C slowly. The resulting mixture was stirred at room temperature overnight. Next, S8 the solvents were evaporated, the residue was suspended in 150 mL of saturated aqueous NaHCO3, and the mixture was extracted with dichloromethane (3×50 mL). The combined organic layers were dried over anhydrous Na2SO4, and solvents were evaporated. The residue was dissolved in a mixture of 15 mL of dichloromethane and 45 mL of methanol. Then, dichloromethane was rotary evaporated to a reduced volume until precipitation started. The precipitate was filtered off, washed twice with 10 mL of methanol and dried in vacuum. This procedure gave pure *L3 (678 mg, 69% yield, approx. 23% 15 N) as a white solid. 1

Synthesis of *3
In a glovebox with argon atmosphere, ligand *L3 (560 mg, 822 µmol) was dissolved in warm toluene (50 mL), and a solution of ZrBn4 (375 mg, 822 µmol) in toluene (5 mL) was added. The mixture was stirred at r.t. overnight. Then, the solution was concentrated in vacuum, and the formed suspension was filtered through a glass frit. The precipitate was washed with 2 mL of toluene and 5 mL of pentane to give the product as a yellowish solid (538 mg, 69% yield, approx. 23% 15 N). 1

Activation of Salan complexes
In situ synthetic procedure: In the glovebox, 1eq of the neutral complex of choice and 0.95 eq of TTB were loaded into a J. Young NMR tube and dissolved in 0.6 mL of the deuterated solvent of choice (toluene-d8 or chlorobenzene-d5). The colour of the resulting solutions was orange or yellow, and in some cases the precipitation of a red oil was observed. The solutions were left to decant for approximately 30 min before NMR characterization and experiments.      The concentration in chlorobenzene was too low to completely characterize the compound.

Reactivity of *3 + FF with 1-hexene
To a solution in C6D5Cl of 10 mg of 3 + FF and 0.95 eq of TrBT, 30 eq of 1-hexene have been added, and the reaction followed with in situ NMR over the course of some minutes in order to track the consumption of the starting material. The formation of poly-1-hexene and a new organometallic species was evidenced. The newly formed organometallic species, as explained in the main text, retains the fac-fac geometry of 3 + FF. It is proposed that such species is indeed the 1-hexene S33 monoinserted species into the Zr-benzyl bond. The main NMR evidences that point out to this species are the following: the 1 H, 13 C HSQC NMR shows two two doublets (-0.72, 0.21 ppm) with a carbon resonating at 71.1 ppm, that are assigned to a Zr-CH2 moiety. Following the 1 H, 1 H COSY correlation, these signals show coupling with a signal at 0.09 ppm, that in turn has the relevant carbon in the positive phase at 54.5 ppm. This is thus assigned to the Zr-CH2-CH. The latter shows correlations, both in the 1 H, 1 H COSY and in the 1 H, 1 H ROESY with two signals at 0.38 and 0.63 ppm, that shares the carbon at 41.9 ppm, that is compatible with a Zr-CH2-CH(R)-CH2Ph. The rest of chain of the monoinserted species is difficult to assign, since its signals fall under the poly-1-hexene signal, however the 1 H, 1 H ROESY spectrum allows to individuate a CH2 at 0.80 ppm and another one at 1.86 ppm that shows NOE contacts with the CH of the inserted monomer. Moreover, the Zr-CH2, in the ROESY spectrum, shows NOE contacts with the signals that are typically assigned to the H1 and H3 of the ligand framework, and a small NOE contact of the signal at 0.21 ppm with the CH2Ph further corroborates the formation of the monoinserted species.

Reactivity of *4H + MM with 1-hexene
To a solution in C6D5Cl of 9.5 mg of *4H + MM and 0.95 eq of TrBT, 30 eq of 1-hexene have been added, and the reaction followed with in situ NMR over the course of some minutes in order to highlight the disappearance of the starting material. The formation of poly-1-hexene and a new organometallic species was evidenced. The newly formed organometallic species, as explained in the main text, retains the mer-mer geometry of *4H + MM. It is proposed that such species is in this case S37 the polymeryl species. The complete characterization of this species it's a bit more complex with respect to the relevant 3 + FF derivative, however there is NMR evidence that points out to *4H + -Pn: from the 1 H, 13 C HSQC it is possible to recognize two different set of signals at 1.01, 1.46 ppm (with the relevant carbon at 43.1 ppm) and at 2.49, 2.66 ppm (with the relevant carbon at 40.9 ppm). These two sets of signals can be assigned to a Zr-CH2 or a Zr-(CH2CH(C4H9)n-CH2. The difference in chemical shift of the Zr-CH2 group with respect to the previously analysed *3 + FF-CH2CH(C4H9)CH2Ph could be ascribed to the different geometry of the two complexes and to the absence of an interaction of the terminal phenyl group, due to the presence of a longer chain, that prevents the stiffening that is present in the monoinserted derivative. Other signals belonging to the chain that could be easily recognized are those at 1.83 ppm (δC: 37.8 ppm) and at 2.24 ppm (δC: 43.7 ppm), that are instead assigned to Zr-CH2CH or Zr-(polymeryl)-CH2CH. All the signals aforementioned show in the 1 H, 1 H ROESY spectrum contacts with other signals in the aliphatic region, likely due to the C4H9 chains of the growing polymeryl; it is however rather difficult to precisely assign these signals due to the presence of the poly-1-hexene signal. The fragments relevant to the ligand backbone can be easily recognized from both the 1 H, 13 C HSQC, the 1 H, 1

Data collection of 3 + FF•C7D8 and 3 + FF•C6D5Cl was performed at the University of Perugia on a
Bruker D8 Venture X-ray diffractometer equipped with a low temperature device and Icoatec ImuS 3.0 microfocus sealed-tube MoKα (λ = 0.71073). Complete dataset was collected at 100 K with the CCD PHOTON II detector placed at a distance 40.00 mm from the crystal. The data collected through generic φ and ω scans were integrated and reduce using the Bruker AXS SAINT V8 software. The structure was solved and anisotropically refined using the SHELXT and SHELXL packages of the Olex2 software. Hydrogen atoms were placed at calculated positions and forced to ride on the attached atom. Crystal data and details of refinements are given in Table S4-5. S44

Conformer Sampling and DFT NMR predictions
Solvent free systems. Suitable guess structures were generated for a fac-fac and mer-mer isomer of the complex and optimized with Gaussian as detailed in the Experimental Section. The optimized structures were then both submitted to the CREST workflow with the following keywords: --ethr 0.5 --cluster --noreftopo --subrmsd -alpb toluene.
After completion, the clustered output was screened for duplicates and all unique structures were preoptimized with TPSSh/cc-pVDZ(-PP) utilizing a smaller grid to reduce computational time (int=grid=sg1) and loose convergence criteria. The optimized structures were screened again for duplicates and only unique conformers were fully optimized and evaluated as detailed in the Experimental Section. Conformers are numbered as obtained from the CREST output.
Solvent/Donor coordinated systems. A similar workflow was employed but only starting structures with the solvent/donor in the 1 st coordination sphere were considered (fac-fac geometry).