Formation and Activation of Zr/Hf Bis(phenolate-ether) Precatalysts

DOI: 10.1002/ejic.201900716

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Zr and Hf Complexes

Formation and Activation of Zr/Hf Bis(phenolate-ether)

Precatalysts

Eric N. T. Cuthbert,^[a,b]Vincenzo Busico,^[b,c]David E. Herbert,^[a]and

Peter H. M. Budzelaar*^[a,b,c]

Abstract: Zr and Hf complexes of bis(phenolate-ether) (“O4”) determined. Treatment of LMMe₂with [Ph₃C]⁺[B(C₆F₅)₄]^–produ-

ligands feature high activity, stereoselectivity and molecular ces fairly clean cationic species LMMe⁺which were studied by

weight capability for propene polymerization at high tempera- ¹H NMR. 2D ROESY data, in particular, suggest that for “smaller”

ture. Here we report a simplified ligand synthesis and several O4 ligands the LMMe⁺cation reversibly rearranges from the

new examples of O4 ligands. The formation of precatalysts active (fac/fac) form to a presumably inactive fac/mer or

LMR₂(M = Zr, Hf; R = Bn, Me) from LH₂and MR₄was found to mer/mer form; more bulky substituents appear to suppress this

be accompanied in some cases by the formation of dimers rearrangement. Implications for polymerization catalysis are dis-

(μ-L)₂[MR₂]₂, and X-ray structures of two such dimers have been cussed.

Introduction

Polyolefins are extremely versatile materials, finding ever-ex-

panding areas of application in part due to the development of

new grades prepared using increasingly sophisticated catalytic

systems.^[1]Industrial polypropylene production is still domi-

nated by heterogeneous Ziegler-Natta catalysts, which can

produce highly stereoregular polymers at high operating tem-

peratures.^[2]Zr and Hf bis(phenolate-ether) catalysts (here

Figure 1. Representative O4 ligand structure and proposed active species.

designated as O4 catalysts; see Figure 1), originally patented by

SYMYX^[3]and later acquired by Dow,^[4]comprise one of few

homogeneous catalyst classes capable of working at high tem-

peratures while still maintaining a high molecular weight capa-

bility. Based in part on X-ray structures of LMCl₂and LMBn₂

catalyst precursors,^[3,5]and also on similarity to ONNO^[6]and

OSSO^[7]systems, the active species in O4 systems is believed to

have an octahedral coordination geometry with the O4 ligand

coordinated in a fac/fac C₂-symmetric fashion around the metal,

leaving two mutually cis sites available for the growing chain

and incoming monomer.^[8]

While this is a plausible scenario, it should be noted that the

situation in ONNO systems is complicated by ligand skeletal

rearrangement from fac/fac to mer/mer (Figure 2), with impor-

tant consequences for catalyst activity;^[9]there appears to be

no a priori reason why the same could not apply to O4 type

catalysts.

Figure 2. The ONNO ligand^[6]and its active species equilibrium.^[9b]

In the present work, we present the results of a combined

NMR and computational (density functional) study of selected

activated O4 catalysts, finding that most likely conformational

changes occur in some O4 systems as well. In addition, we re-

port evidence that even the “standard” synthesis of precursor

complexes LMBn₂and LMMe₂is less than straightforward: de-

pending on the choice of ligand and reaction conditions, treat-

ment of LH₂with MBn₄or MMe₄can produce both monomeric

LMR₂and dimeric (μ-L)₂[MR₂]₂compounds. Implications for

testing of catalyst performance are discussed.

[a] Department of Chemistry, University of Manitoba,

144 Dysart Road, Winnipeg MB R3T 2N2, Canada

[b] Dutch Polymer Institute (DPI),

Results and Discussion

P.O. Box 902, 5600 AX Eindhoven, The Netherlands

[c] Dipartimento di Scienze Chimiche, Università di Napoli ‘Federico II',

Via Cintia, 80126 Napoli, Italy

Ligand Synthesis

E-mail: p.budzelaar@unina.it

Ligands were mostly synthesized according to adaptations of

procedures published by Symyx/Waymouth^[3,5a]and Dow;^[4]

Supporting information and ORCID(s) from the author(s) for this article are

available on the WWW under https://doi.org/10.1002/ejic .201900716.

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precise details can be found in the Supporting Information for anhydrous ZnCl₂after nBuLi addition to 11, converting

all ligands and intermediates studied. Scheme 1 shows the nu- this in situ into an organozinc compound (13) for immediate

merous protection/deprotection steps required in the original Negishi coupling (Scheme 2, method B′). This not only avoids

Symyx scheme (method A) which ultimately reduces overall the emulsion problems but saves a step in the synthesis of the

yield. The Dow method (Scheme 2, method B) reduces the final ligand. A total of 20 O4 ligands were synthesized by these

number of steps by employing THP as a phenol protecting three methods (Figure 3).

group; ortho-directed lithiation and simpler deprotection proce-

The ligands are separated into the groups 2C, 3C and 4C,

dures are highly advantageous here but may experience prob- corresponding to the length (i.e., the number of methylene

lems if R contains competing directing groups such as fluorine units) of the alkyl chain linking the aryl ether donor groups.

atoms. We found that while more effective than the lengthy Substitution at the remaining phenolate ortho positions (R₁) al-

method A, the iPrOBPin required for 12 in method B often lows investigation of the effect of steric bulk as well as electron-

1

formed emulsions during work-up, which complicated extrac- donating abilities. All ligands were characterized by H and ¹³C

tion procedures and reduced yields. As an alternative, we add NMR, HRMS and selected ligands also by X-ray crystallography.

Scheme 1. Minor adaptations of Symyx/Waymouth O4 ligand synthesis (method A).^[3,5a]

Scheme 2. Dow O4 ligand synthesis (method B)^[4]and our adaptation (method B′).

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a

b

c

Figure 3. Synthesized ligand set. Symyx/Waymouth method A. Dow method B. in situ Negishi reagent method B′.

We here survey formation of LMR₂precatalysts from LH₂and MBn₄; this is a popular approach for post-metallocene type cat-

MR₄, and their activation to cations LMR⁺, as a preliminary to alysts.^[11]The complexes can be isolated and purified or gener-

exploring catalyst performance in propene polymerization.^[10]ated in situ and used “as is” in catalyst testing: according to

Not all ligands were tested in each type of synthetic approach: Waymouth, complex formation is clean and catalytic perform-

in this paper, we describe representative results.

ance of isolated or in situ generated complexes is virtually iden-

tical.^[5a]

In our hands, synthesis of all 2C complexes proceeded with-

out problems. The reaction is complete within minutes at room

temperature; H NMR spectra of the crude mixture show sharp

Synthesis of Precursors LMR₂

1

From LH₂and MBn₄

Complexes LMBn₂(M = Zr or Hf) are easily and conveniently peaks and indicated the presence of a single complex with ef-

generated from the free ligand LH₂and commercially available fective C₂symmetry (Figure S1; L1HfBn₂in Figure 4a). Layering

Figure 4. 300 MHz ¹H NMR spectra of in situ formed products from the reactions of a) L1H₂; b) L2H₂and c) L3H₂with HfBn₄at 25 °C, [D₆]benzene.

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of these samples with hexanes yielded in most cases X-ray tained when working at a concentration of ca. 10 mM, although

quality crystals; the structures of two representative complexes these authors did not mention any concentration depend-

(Figure 5) demonstrate the expected octahedral coordination ence.^[5a]

environment of the metal.

Figure 6. Thermal ellipsoid (50 %) plot for L6ZrBn₂; H atoms omitted for clar-

ity.

The reaction temperature also affects the outcome of the

reaction. This was checked for the reaction of HfBn₄with 3C

ligand L2 (Figure 7). This reaction was carried out at –78 °C

and at +65 °C, and subsequent NMR spectroscopy for both was

performed at room temperature. The spectra differ greatly, the

“sharp” component being nearly absent in the broadened spec-

trum of the low-temperature reaction product.

In separate experiments, X-ray quality crystals could be

grown from reactions carried out at room temperature and at

–35 °C. The former was found to be of the expected monomeric

complex L2HfBn₂while the latter unexpectedly turned out to

be a dimer containing two bridging L2 ligands (Figure 8).

Fairly pure (μ-L2)₂[HfBn₂]₂was obtained by working at lower

temperature and higher concentration during complexation.

The resulting poorly soluble powder produces broad spectra

both at room temperature and at elevated temperatures. While

cooling of the sample to 10 °C sharpened the signals considera-

bly, the spectra were never clean and clear enough for com-

plete interpretation and assignment (Figure S2).

The above formation of a complex with two bridging ligands

turns out not to be an isolated case. Spectra from the reaction

of 4C ligand L20H₂with HfBn₄at –35 °C in [D₈]toluene featured

both sharp and broad signals (Figure S3). Crystals obtained by

layering hexanes also revealed the presence of a dimer with

bridging O4 ligands (Figure 9).

Figure 5. Thermal ellipsoid (50 %) plots for L7ZrBn₂and L16HfBn₂; H atoms

omitted for clarity.

Attempts using several 3C and 4C ligands produced rather

different results as illustrated in Figure 4b/c (L2HfBn₂and

1

L3HfBn₂). Broad signals were observed in the H NMR spectra

of crude in situ reaction mixtures and also after work-up of the

products, hinting at the presence of multiple species and/or

fluxional behavior. Crystals grown from a solution of L6H₂/ZrBn₄

proved that the anticipated C₂-symmetric complex does indeed

form (Figure 6) although the extent to which it does remains

uncertain. More focused experiments revealed that broad spec-

tra were obtained primarily when concentrations larger than

50 mM were used. When reactions were carried out at lower

concentrations, the spectra of the products after work-up also

were much cleaner. This is consistent with the report by Way-

Dinuclear ligand-bridged group IV precatalysts have prece-

mouth that apparently clean 4C Zr and Hf complexes were ob- dent. The group of Agapie designed dinucleating (OONN)₂li-

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Figure 7. Room temperature 300 MHz ¹H NMR spectra of isolated “L2HfBn₂” generated from L2H₂and HfBn₄at –78 °C (top) and +65 °C (bottom).

Figure 8. Thermal ellipsoid (50 %) plots for L2HfBn₂and its dimer (μ-L2)₂[HfBn₂]₂. Solvent molecules, H atoms and a disordered carbon (C9) in L2HfBn₂

omitted for clarity; terminal phenyl groups are shown in stick style.

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Figure 9. Thermal ellipsoid (50 %) plot for (μ-L20)₂[HfBn₂]₂; carbazolyl groups show in stick style and H atoms omitted for clarity.

gands where the ligand framework holds two octahedral active This structure and several others (see the SI) demonstrate that

sites in close proximity and reported that the presence of a the dimethyl complexes are monomeric and octahedral with

second metal site influences tacticity and comonomer incorpo- exact or approximate C₂symmetry in the solid state. All cases

ration.^[11d,12]In contrast, the OSSO ligands mentioned above demonstrate shorter ligand–metal bond lengths on average for

form dinuclear ligand-bridged Ti precatalyst complexes in Hf relative to Zr, as has been observed previously for O4 com-

which the two active sites point away from each other (similar plexes.^[5a]

to our dimers) but in which the Ti–S coordination has been

lost.^[7b]The present O4 dimer complexes are unusual in the

sense that the environment of each site, formed from two

phenolate-ether half-ligands, produces a coordination geome-

try very similar to that in the monomer complex where a single

bis(phenolate-ether) ligand folds around the metal center.

From LH₂and MMe₄

It is clear that the in situ formation of LMBn₂from LH₂and

MBn₄is not entirely without problems. Also, “activated” LMBn⁺

species might not be representative of the true active species

formed from O4 catalyst precursors, due to the diverse coordi-

nation possibilities of the remaining Bn group. We, therefore,

turned to the synthesis of LMMe₂precursors, following a proce-

dure described in Dow patents.^[2]In brief, MMe₄is generated

at low temperature from MCl₄and MeMgBr in toluene, and the

ligand LH₂is added when the formation of MMe₄is essentially

complete; filtration and solvent removal leave a very pure crude

product. A more detailed description is given in the experimen-

tal part.

Spectra obtained from these syntheses were generally much

cleaner than those from MBn₄reactions and showed no broad-

ened signals. In most cases, the formation of a single product

was indicated (Figure S4 shows an example) which contrasts

with the challenging complexation and messy crude mixtures

obtained from L5H₂and ZrBn₄(Figure S5). Figure 10 shows the

structure of L5ZrMe₂(crystals obtained directly by layering a

Figure 10. Thermal ellipsoid (50 %) plot for L5ZrMe₂; H atoms omitted for

benzene solution of very clean crude product with hexanes).

clarity.

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However, for some of the more elaborate ligands (especially NMR Study of [LMMe]⁺[B(C₆F₅)₄]^–

L18 and L20), in combination with both ZrMe₄and HfMe₄, a

Cationic complexes LMMe⁺were selected as models for the

active species derived from O4 catalysts. Benzyl cations LMBn⁺

were considered to be less representative in view of the diverse

binding modes available to the benzyl group.^[13]

second set of sharp peaks was observed, indicating a second

product (Figure 11). Diffusion NMR experiments on crude

L20MMe₂(see SI) revealed that for both metals the minority and

majority species differ in hydrodynamic volume (spherical ap-

proximation) by a factor of 2.00 (Zr)/2.04 (Hf), strongly suggest-

The reaction of LMMe₂with [Ph₃C]⁺[B(C₆F₅)₄]^–(TTFB) in

ing that the second product is the dimer (μ-L)₂[MMe₂]₂: dimer [D₈]toluene was found to be clean for most ligands studied,

formation is therefore not restricted to the MBn₄reactions. and the resulting cationic complexes were fairly stable, the Hf

Figure 11. Section of the ¹H NMR spectrum of the crude product from the reaction of L20H₂and ZrMe₄, in [D₆]benzene. Asterisks denote minority species

signals.

Figure 12. 500 MHz ¹H NMR aromatic regions of a) L17ZrMe₂; b) L5ZrMe₂and c) L2ZrMe₂complexes at 25 °C, [D₆]benzene.

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complexes, in particular, surviving for some time at room tem- high-field resonance is not observed for ligands lacking an aryl

perature. Unfortunately, attempts to grow X-ray quality single substituent in that position (e.g. L5, Figure 12b). Basic modeling

crystals of any of these cations have so far been unsuccessful, shows that this type of ring current shielding should be absent

so we must rely on NMR data (mostly ¹H 1D and 2D ROESY or strongly diminished in fac/mer and mer/mer ligand confor-

spectroscopy) for conformational information.

mations. We, therefore, believe that the presence of such high-

field aromatic resonance can be taken as evidence that the li-

gand has assumed a fac/fac conformation. In addition, ROESY

spectra involving the Me group(s) at the metal and the methyl-

ene groups of the ligand provide valuable 3D structural infor-

mation. We here discuss spectra of Hf complexes of L1/L2 and

L19 as typical examples of “small” and “large” ligands, respec-

tively. Each species formed a dense, poorly-soluble orange oil

upon activation which is consistent with the formation of

[LMR]⁺[B(C₆F₅)₄]^–ion pairs.

The aromatic regions of ¹H NMR spectra of even the simplest

O4 catalyst precursors are extremely crowded, and complete

assignment is challenging. However, many spectra of both

LMMe₂and LMMe⁺species show a clearly separated high-field

resonance (below 6 ppm) for the arene hydrogen ortho to the

aryl ether functionality (Figure 12a/c), that we believe is caused

by its C-H bond pointing into the π-cloud of an aryl substituent

ortho to the phenoxy group on the other end of the ligand (the

“o-H signal”, Figure 13).^[14]In line with this hypothesis, such a

L1HfMe⁺

Activation of L1HfMe₂produced NMR spectra that were broad

and difficult to interpret (Figure 14b). However, the addition of

one equivalent of OPPh₃to this mixture slowly produced a new,

major complex [L1HfMe·OPPh₃]⁺[B(C₆F₅)₄]^–that is stable at

room temperature overnight in solution and yields sharper

NMR spectra (Figure 14c). The presence of four separate ether

protons in ¹H spectra (at 2.82–3.53 ppm) indicates that the

complex is now C₁-symmetric while integration against the

HfMe protons indicates abstraction of one methyl group from

the precatalyst. A ³¹P shift downfield from pure OPPh₃indicates

coordination of OPPh₃to the electron-deficient Hf center while

Figure 13. fac/fac and mer/mer structures for LMR₂. “o-H” protons are high-

lighted blue while aromatic rings that induce ring current shielding for the

o-H protons are highlighted red.

Figure 14. 500 MHz ¹H NMR spectra of a) L1HfMe₂at –5 °C; b) [L1HfMe]⁺[B(C₆F₅)₄]^–at –5 °C and c) [L1HfMe·OPPh₃]⁺[B(C₆F₅)₄]^–at 25 °C, [D₈]toluene.

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¹⁹F spectra display similar chemical shifts and broadening as

The loss of ring current shielding experienced by the o-H

the activator, indicating loose and non-specific anion binding protons suggest that they are no longer directed into the

to the metal (Figure S6/S7).^[13b,15]2D ¹H-¹H ROESY data demon-

strate behavior similar to that of the precursor L1HfMe₂, with

no observable backbone-HfMe contact (Figure S8/S9). For these

reasons, we believe that the OPPh₃adduct is fac/fac, and this

is supported by DFT calculations (below).

phenyl substituents, which is most easily explained by a change

in conformation, e.g., to fac/mer or mer/mer as mentioned

above. This is confirmed by 2D ROESY experiments (Figure S12),

which show a contact between the Hf-CH₃protons and back-

bone methylene protons that is incompatible with a fac/fac ar-

rangement and is not observed in precursor L2HfMe₂(Figure

S13).

L2HfMe⁺

Like its L1HfMe⁺analog, L2HfMe⁺generated by Me abstraction

from L2HfMe₂shows broad signals at room temperature (Fig-

ure 15b) although on cooling to –15 °C the spectrum becomes

sharp (Figure 15c).

Based on the above, we suggest that the room temperature

dynamic process involves a reversible switch between either

mer/mer or fac/mer and fac/fac structures as well as a “back-

skip” of the HfMe group. One possible mechanism for this is

summarized in Scheme 3.

As described above for [L1HfMe]⁺, the cation [L2HfMe]⁺

could be trapped as a OPPh₃adduct. The ¹H NMR spectrum

(Figure 15d) illustrates an upfield shift of the o-H signals upon

OPPh₃addition, more reminiscent of the L2HfMe₂precursor

(Figure 15a) which is fac/fac in structure. Also, in accord with a

fac/fac conformation, the 2D ROESY spectrum of the adduct

(Figure S14) does not show the contact between Hf-CH₃and

methylene protons observed for [L2HfMe]⁺[B(C₆F₅)₄]^–. This sug-

gests that, unlike the parent cation [L2HfMe]⁺, the adduct

[L2HfMe·OPPh₃]⁺has a fac/fac structure.

It shows a major component with a single HfMe resonance

(¹H: –0.06 ppm; ¹³C: 51.1 ppm) as well as a minor component

with HfMe at somewhat higher field (¹H: –0.37 ppm; ¹³C:

46.6 ppm; major/minor ca. 12:1; see also Figure S10). For the

majority species, two separate o-H signals (1H each) can be ob-

served, at more “normal” chemical shifts that indicate loss of

the ring current effect mentioned above. The two ligand halves

have become inequivalent, and the broadening observed at

higher temperature likely represents a process exchanging

these halves (see below). 2D ROESY spectra at 50 °C (Figure

S11) demonstrate exchange between the majority and minority

Hf-CH₃species not observed at –15 °C (Figure S12).^[16]

Figure 15. 500 MHz ¹H NMR spectra of a) L2HfMe₂at 25 °C; b) [L2HfMe]⁺[B(C₆F₅)₄]^–at 25 °C; c) [L2HfMe]⁺[B(C₆F₅)₄]^–at –15 °C and d) [L2HfMe·OPPh₃]⁺[B(C₆F₅)₄]^–

at 25 °C, [D₈]toluene. Intense signals at ca. 7.05 ppm are due to side product CH₃CPh₃aromatic protons, exacerbated by poor solubility of the ion pairs.

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Scheme 3. Potential exchange mechanism in LMMe⁺cations: (A) exchange of ligand halves in mer/mer complex via fac/fac structure and backskip; (B) possible

sequence of steps for the fac/fac to mer/mer isomerization assumed in (A).

L19HfMe⁺

the overall spectrum is very similar to L19HfMe₂precursor al-

though less symmetric, suggesting strongly that for this system

The reaction of L19HfMe₂with TTFB (Figure 16a) also resulted the ligand keeps its fac/fac arrangement also in the activated

in a rather clean formation of a cationic species, showing a species.

single Hf-CH₃group and inequivalent ligand halves. For this

L19HfMe⁺is fluxional at room temperature, suggesting an

ligand, however, the o-H signals remain at high field (Fig- exchange of the two ligand halves. Since the ligand is already

ure 16c), and ROESY spectra do not show any contacts between in the fac/fac conformation, only the top back-skip part of

the Hf-CH₃and methylene protons (Figure S15). Additionally, Scheme 3 (A) is needed to explain the exchange.

Figure 16. 500 MHz ¹H NMR spectra of a) L19HfMe₂at 25 °C; b) [L19HfMe]⁺[B(C₆F₅)₄]^–at 25 °C and c) [L19HfMe]⁺[B(C₆F₅)₄]^–at –10 °C, [D₈]toluene. Intense

signals at ca. 7.05 ppm are due to side product CH₃CPh₃aromatic protons that are exacerbated by poor solubility of ion pairs.

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Density Functional Studies

backbone chain (relative to the fac/fac structure), consistent

with the observed 2D ROESY data.

Density functional theory calculations were used to generate

geometries needed for interpretation of ROESY data, and to

examine the effect of ligand structure on the relative energies

of fac/fac, fac/mer, and mer/mer arrangements. Earlier work on

ONNO type systems showed that calculated cation geometries

are not much affected by the presence of an anion, but the

inclusion of the counterion is essential for reliable prediction

of the ligand conformational preference for fac/fac vs. mer/mer

arrangements.^[9c]Consequently, full ion pair models were inves-

tigated for select [LHfMe]⁺[B(C₆F₅)₄]^–complexes.^[17]

DFT calculations were carried out at the M062X/cc-pVTZ-

(-PP),PCM(Toluene)//TPSSh/SVP-LANL2DZ level using Gaussian

09; for further details see the Experimental section. Cited ener-

gies are free energies.

As found earlier for ONNO systems,^[9a]geometry optimiza-

tion of 5-coordinate species LMMe⁺starting from a fac/fac li-

gand conformation results in most cases in a rearrangement to

an approximately square pyramidal (SPy) coordination geome-

try which could be thought of as mer/mer with one apical site

empty. Substantial flexibility was observed and the ligand sub-

stitution pattern was found to have a significant effect on the

preferred conformation of the [LHfMe]⁺[B(C₆F₅)₄]^–ion pairs.

DFT results indicate that [L1HfMe]⁺[B(C₆F₅)₄]^–prefers a 5-co-

ordinate SPy structure without any direct Hf-anion contacts

(Figure 17). A mer/mer complex with a short Hf-F contact, akin

to earlier ONNO results, was found to be +8.7 kcal/mol higher

in energy whereas a fac/fac complex in which Hf interacts with

a para-F of the anion was predicted to be only 1.9 kcal/mol

above the lowest-energy arrangement.

Figure 18. Optimized mer/mer structure of [L2HfMe]⁺[B(C₆F₅)₄]^–.

In contrast with the above results for “small” ligands, the

bulkier [L17HfMe]⁺[B(C₆F₅)₄]^–was found to prefer a fac/fac co-

ordination mode (Figure 19) over SPy by +2.3 kcal/mol while a

mer/mer structure was less favorable at +6.4 kcal/mol. Inspec-

tion of the geometries shows that, compared to the phenyl

rings in L2, the carbazole rings in L17 show increased steric

crowding in both SPy and mer/mer geometries due to the large

flat substituents. It stands to reason that L19, containing addi-

tional tBu groups, would suffer from similar or greater steric

crowding in SPy or mer/mer geometries, and hence would also

prefer fac/fac ligand coordination. Thus, the DFT results support

the conclusion based on NMR that [L19HfMe]⁺[B(C₆F₅)₄]^–re-

mains fac/fac after activation.

Figure 17. Optimized SPy structure of [L1HfMe]⁺[B(C₆F₅)₄]^–.

Growing the ether backbone chain by one methylene unit

changes the structural preference: for [L2HfMe]⁺[B(C₆F₅)₄]^–

a

mer/mer arrangement with a weak Hf-F interaction was found

to be lowest in energy (Figure 18). A SPy geometry is slightly

higher in energy (+1.0 kcal/mol) while the fac/fac structure

comes in at +4.0 kcal/mol relative to mer/mer. In any case, calcu-

lations including the counterion indicate that for both

Figure 19. Optimized fac/fac structure of [L17HfMe]⁺[B(C₆F₅)₄]^–.

[L1HfMe]⁺and [L2HfMe]⁺conformations other than fac/fac are

Energies of the Zr ion-pairs demonstrated similar but more

,

preferred. Inspection of the corresponding fac/mer and mer/mer drastic results: structural rearrangement to a polymerization in-

geometries reveals reduced distances between Hf-CH₃and the active structure is less favorable for Zr than its Hf congener (see

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Tables S1–S6). These results suggest that differences in ob- Implications for Activity

served activities between the two metals may be influenced to

some extent by differing conformational equilibria.

Rearrangement upon activation is undesirable. In the mer/mer

geometry, the catalyst is dormant as the alkyl chain and empty

site are not cis and hence unable to undergo migratory inser-

tion by the Cossee mechanism.^[8]While propagation is in princi-

ple still possible in the fac/mer geometry, we believe it is un-

likely to be as easy as in the fac/fac arrangement. Therefore,

catalysts such as the carbazole-bearing systems L17–L20, for

which the cations prefer fac/fac geometries, are likely to have

the advantage. Control over catalyst conformation should be

taken into account when designing improved catalysts, similar

to what has been established for ONNO and OSSO systems.^[7c,9]

DFT calculations were also performed to investigate poten-

tial structures of phosphine oxide adducts formed in trapping

experiments. For computational efficiency, OPPh₃was modeled

with OPMe₃while the anion was excluded. Only fac/fac and

fac/mer geometries compatible with the NMR data (i.e. that

would not show a ROESY contact between Hf-CH₃and the

backbone) were considered.

Both [L1HfMe·OPMe₃]⁺and [L2HfMe·OPMe₃]⁺were found to

clearly prefer fac/fac structures (Figure 20, Figure 21) over the

fac/mer alternatives (by +4.4 and +5.5 kcal/mol, respectively),

which again provides some support for the conclusion based

on NMR data that the OPPh₃adducts have fac/fac structures.

This also demonstrates that caution needs to be exercised in

interpreting trapping experiments: the “trapped” complex may

have a structure different from the actual active species before

trapping.

Conclusions

The apparently simple chemistry of generating activated O4

catalysts is in reality not so simple. Reactions of MBn₄with the

free ligands LH₂produce both monomeric and dimeric prod-

ucts (and possibly higher aggregates). Concentration, tempera-

ture and ligand structure all affect the course of the reaction.

Reactions with MMe₄are somewhat cleaner but also here di-

meric products can be formed. It is not clear at present what

would happen with dimeric precursor complexes under polym-

erization conditions.

Activation of LMMe₂precursors with TTFB results in (mostly)

clean formation of fluxional cations LMMe⁺; NMR results indi-

cate rearrangement of the ligand coordination geometry away

from fac/fac for L2HfMe⁺but not for L19MMe⁺. Both L1HfMe⁺

and L2HfMe⁺could be trapped with OPPh₃although these ad-

ducts may not be representative of the true active species. Grat-

ifyingly, DFT results for [LHfMe]⁺[B(C₆F₅)₄]^–and [LHfMe·OPR₃]⁺

are consistent with conclusions based on NMR data. For related

ONNO systems, the significant energy preference for a mer/mer

resting state contributes to the relatively modest activity of

these catalysts.^[9c]For the O4 systems studied here, the energy

preference for alternative structures (such as SPy or mer/mer)

appears to be less pronounced and should therefore have a

smaller impact on catalyst performance.

Figure 20. Optimized fac/fac structure of [L1HfMe·OPMe₃]⁺.

Experimental Section

General: All reactions were performed in an inert atmosphere

(nitrogen or argon), using a glove box or Schlenk techniques. All

reaction and chromatography solvents were purchased from either

Sigma-Aldrich, Fisher Scientific or ACROS. Deuterated solvents for

NMR were purchased from Sigma-Aldrich or Cambridge Isotopes.

All non-halogenated solvents required for inert conditions were pu-

rified by distillation over Na/benzophenone under an argon atmos-

phere. All halogenated organic solvents required for inert condi-

tions were distilled from CaH₂under an argon atmosphere to dry;

3 freeze-pump-thaw cycles with argon were performed to remove

oxygen. Deionized water (dH₂O) underwent 3 freeze-pump-thaw

cycles to remove oxygen for Suzuki coupling reactions unless other-

wise noted. Inorganic and organic compounds were ordered from

Sigma-Aldrich, Fisher Scientific, ACROS or Combi-Blocks and used

as received unless otherwise noted. Silica gel used for column chro-

matography was purchased from ACROS (60 Å, 230–435 mesh).

NMR spectra were acquired on a Bruker Avance III 300, 400 or 500

Figure 21. Optimized fac/fac structure of [L2HfMe·OPMe₃]⁺.

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with spectra referenced to TMS (0 ppm) via residual solvent sig-

nals.^[18]Variable temperature NMR spectra were acquired on a

quenched with water and the organic phase is extracted with Et₂O.

The organic phase is washed with 2 × 10 % (w/v) NaOH_(aq.)

,

Bruker Avance III 500 equipped with a BCU. The following gives 3 × dH₂O, brine and then dried with MgSO₄. The solvent is removed

general procedures for the various syntheses; precise details for in-

dividual compounds (yields, characterization etc.) can be found in

the Supporting Information.

under reduced pressure and the crude product is purified by recrys-

tallization from hexanes or petroleum ether, forming white needles.

Synthesis of 8: A previously published procedure^[5a]is followed,

substituting B(OiPr)₃and 2.5 M nBuLi/hexanes for B(OMe)₃and

1.6 M nBuLi/hexanes, respectively, using 7 as the starting material.

Synthesis of 2: One equivalent of 2-bromophenol and 1.5 equiva-

lents of K₂CO₃are combined in DMF (ca. 1 M) with stirring for 15

minutes prior to addition of 1.1 equivalents of benzyl bromide or

methyl iodide. The reaction is monitored by TLC and quenched with

water when complete. The organic phase is washed with 2 × 10 %

(w/v) NaOH_(aq.), 3 × dH₂O, brine and then dried with MgSO₄. The

solvent is removed under reduced pressure to yield a crude product

that is purified by triturating with cold hexanes or by silica gel

chromatography.

Synthesis of 9 and LH₂: A procedure similar to that used for 3 was

followed (Pd(PPh₃)₄catalyst) using one equivalent of 8 and two

equivalents of 6. The crude product is purified by silica gel chroma-

tography and deprotected similar to 4/6, using 10 % (w/w) of Pd/C

per Bn group.

Synthesis of 10: One equivalent of 2-halophenol and 1.5 equiva-

lents of DHP are dissolved in dry DCM under argon. The mixture is

cooled to 0 °C and 2 mol-% of PPTS is added under a flow of argon.

The reaction is allowed to slowly warm to room temperature and

is monitored by TLC – extra DHP and PPTS is added as necessary

to ensure conversion. The reaction is quenched with 10 % (w/v)

NaOH and the organic layer is washed with dH₂O, brine and dried

with MgSO₄. Solvent is removed under reduced pressure and the

crude product is placed under high vacuum to remove residual

DHP, yielding a pure golden oil.

Synthesis of 3. Pd(PPh₃)₄Catalyst: One equivalent of 2, 1.8 equiv-

alents of K₂CO₃and 1.2 equivalents of aryl boronic acid are dis-

solved in degassed 10:1 DME:dH₂O under argon. 5–10 mol-% of

Pd(PPh₃)₄is added under a flow of argon and the mixture is re-

fluxed over ca. 12 hours with conversion monitored by TLC. The

mixture is acidified with 10 % HCl v/v and extracted with Et₂O. The

organic layer is washed with dH₂O, brine and dried with MgSO₄.

The crude product obtained by solvent removal under reduced

pressure is purified by silica gel chromatography.

Synthesis of 11 for Pd Catalyzed Reactions: A procedure similar

to the synthesis of 3 is followed where R-Nu is R-B(OH)₂, and exclud-

ing H⁺addition at any step to prevent THP cleavage. The crude

product is purified by silica gel chromatography.

Synthesis of 3. Pd₂(dba)₃/P(tBu)₃Catalyst (for Sterically Hin-

dered Coupling Reactions). In a glove box, 2.5 mol-% of Pd₂(dba)₃

and 3.3 equivalents of CsF are added to a Schlenk flask. The flask

is moved to a Schlenk line where one equivalent of 2, 1.2 equiva-

lents of aryl boronic acid (e.g. 2,6-dimethylphenylboronic acid) and

6 mol-% of [HP(tBu)₃][BF₄] are added under a flow of argon. Dry

THF is added and the mixture is heated with vigorous stirring at

55 °C overnight or until complete. The mixture is diluted with Et₂O,

filtered and the solvent is removed under reduced pressure. The

crude product is purified by passing it through a plug of silica gel

(eluting with hexanes) or by silica gel chromatography.

Synthesis of 11 for Cu Catalyzed Reactions: A previously pub-

lished procedure is followed using 10 (X=I) and R1-Nu being carbaz-

ole or its t-butylated derivative.^[4]The crude product is purified by

silica gel chromatography or by recrystallization from CH₃CN.

Synthesis of 12: A previously published procedure is followed us-

ing 11 as the starting material.^[4]

Synthesis of LH₂, Method B: One equivalent of 7, 2.2 equivalents

of 12 and 3.3 equivalents of K₂CO₃are dissolved in degassed 10:1

DME:dH₂O under argon. 10 mol-% of Pd(PPh₃)₄is added under a

flow of argon and the mixture is heated at reflux over ca. 12 hours;

progress is monitored by TLC. Once the reaction is complete, 20 %

(w/v) HCl is added and the mixture is refluxed until THP is cleaved.

The mixture is diluted with EtOAc and the organic layer is washed

with dH₂O, brine and dried with Na₂SO₄. The solvent is removed

under reduced pressure and the crude product is purified by silica

gel chromatography or recrystallization from CH₃CN.

Synthesis of 4, PG = Bn: A previously published procedure is fol-

lowed using 3 as the starting material.^[5a]

Synthesis of 4, PG = Me: One equivalent of 3 is dissolved in dry

DCM and cooled to 0 °C under argon. Three equivalents of 1 M BBr₃

in DCM are added dropwise via an addition funnel and the mixture

is slowly warmed to room temperature. The reaction is carefully

quenched with iPrOH then neutralized with saturated NaHCO_3(aq.)

.

The organic layer is washed with dH₂O and brine and dried with

Na₂SO₄before solvent removal under reduced pressure. The already

rather pure crude product can be triturated with cold hexanes for

further purification.

Synthesis of LH₂, Method B′: 2.1 equivalents of 11 are dissolved

in dry THF under argon and cooled to 0 °C. 2.31 equivalents of 1.6

M nBuLi in hexanes are added dropwise and the mixture is stirred

at 0 °C for three hours. 2.52 equivalents of anhydrous ZnCl₂are

added under a flow of argon, resulting in a color change and re-

duced turbidity. The mixture is slowly warmed to room temperature

and stirred for an additional hour. One equivalent of 7 and 5 mol-

% of Pd(PPh₃)₄are added under a flow of argon and the reaction

is heated at reflux overnight. Completion is checked by TLC. 20 %

HCl (v/v) is added and the mixture is again refluxed until hydrolysis

of THP is complete as monitored by TLC. The contents are extracted

with EtOAc, the solvent is removed under reduced pressure and the

crude product is purified by silica gel chromatography or recrystalli-

zation from CH₃CN.

Synthesis of 5: In an argon atmosphere, one equivalent of 4 is

dissolved in CHCl₃and cooled. A freshly prepared solution of

tBuNHBr^[19]in CHCl₃(one equivalent) is then added dropwise via

an additional funnel; the best ortho selectivity is obtained while

maintaining the reaction at ca. –25 °C. The reaction is carefully neu-

tralized with saturated NaHCO_3(aq.), the organic layer is obtained

and washed with dH₂O, brine and dried with Na₂SO₄. The solvent

is removed under reduced pressure to obtain a crude product that

is purified by trituration with cold hexanes, by silica gel chromatog-

raphy or used directly in the next step.

Synthesis of 7: 2.1 equivalents of 2-bromophenol and four equiva-

lents of K₂CO₃are combined in DMF with stirring for 30 minutes.

One equivalent of either a) ethylene di(p-toluenesulfonate); b) 1,3-

dibromopropane or c) 1,4-dibromobutane is added and the mixture

LMBn₂Synthesis – NMR Scale: Approximately 15–30 mg of LH₂is

dissolved in [D₆]benzene or [D₈]toluene (approximately 0.5 mL) in

is stirred at room temperature for 16 hours. The reaction is a glove box under N₂. The contents are combined with one equiva-

Eur. J. Inorg. Chem. 2019, 3396–3410

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lent of MBn₄, mixed and immediately taken for NMR analysis. The

mixture is heated at 60 °C if necessary until LH₂consumption is

complete, monitoring the OH signals of the ligand.

rated toluene solution cooled to –35 °C; b) a toluene solution lay-

ered with hexanes at room temperature; c) a benzene solution with

slow evaporation at room temperature or d) a benzene solution

layered with hexanes at room temperature; the latter method is

particularly successful for producing high-quality crystals for these

types of complexes. Crystals were covered with Parabar oil, loaded

onto a nylon loop and cooled in a 150 K stream of N₂on a Bruker

D8 Quest ECO CMOS diffractometer.

L2/HfBn₄complexation experiments.

Variation 1: 76.1 mg of HfBn₄is dissolved in 5 mL of toluene

(28 mM) in a Schlenk tube. The tube is covered with Al foil and

cooled to –78 °C on the Schlenk line. Under a flow of argon, 79.1 mg

of L2H₂is added in one portion. The mixture is stirred at –78 °C for

30 minutes. The cold bath is removed and the mixture is slowly

warmed to room temperature. The solvent is removed under re-

duced pressure to yield a white, poorly soluble powder that is not

purified prior to NMR analysis to avoid affecting the dimer/mono-

mer ratio.

CCDC 1918161 (for 9a-2), 1918162 (for 9b-2), 1918163 (for 9b-3),

1918164 (for L16HfBn₂), 1918165 (for L16HfMe₂), 1918166 (for

L17HfMe₂), 1918167 (for L18HfMe₂), 1918168 (for L18), 1918169

(for L19HfMe₂), 1918170 (for L19ZrMe₂), 1918171 (for L1HfMe₂),

1918172 (for L1ZrMe₂), 1918173 (for L1), 1918174 (for (μ-L20)₂-

[HfBn₂]₂), 1918175 (for (μ-L2)₂[HfBn₂]₂), 1918176 (for L2HfBn₂),

1918177 (for L2HfMe₂), 1918178 (for L2ZrMe₂), 1918179 (for

L3ZrMe₂), 1918180 (for L5ZrMe₂), 1918181 (for L6ZrBn2) and

1918182 (for L7ZrBn2) contain the supplementary crystallographic

data for this paper. These data can be obtained free of charge from

The Cambridge Crystallographic Data Centre.

Variation 2: 72.2 mg of HfBn₄is dissolved in 2.5 mL of toluene in

one Schlenk tube covered with Al foil and 75.6 mg of L2H₂is dis-

solved in 2.5 mL of toluene in another tube. Both are heated in an

oil bath to 65 °C on a Schlenk line. The L2 solution is quickly added

to the HfBn₄solution (26 m

M final concentration) using a syringe.

The mixture is cooled and the solvent is removed under reduced

pressure to yield a white, crystalline powder that is not purified

prior to NMR analysis to avoid affecting the dimer/monomer ratio.

Computational Studies: Density-functional calculations were car-

ried out with Gaussian 09.^[20]Geometries were optimized at the

TPSSh^[21]/SVP^[22]-LANL2DZ^[23]level, (small-core ECP at Zr/Hf) using

an external optimizer.^[24]Vibrational analyses were carried out to

verify the nature of all stationary points. Improved single-point en-

ergies were obtained at M062X^[25]/cc-pVTZ(-PP)^[26]with a PCM(Tolu-

ene)^[27]solvent correction. The cc-pVTZ basis sets were downloaded

from the EMSL basis set exchange.^[28]The single-point energies

were then combined with thermal corrections (enthalpy and en-

tropy, 298 K, 1 bar) to obtain final free energies; entropy corrections

were scaled by 0.67 to account for reduced freedom of movement

in solution.^[29]Calculations on the full system [L19HfMe]⁺[B(C₆F₅)₄]^–

were not feasible so the smaller ligand L17 was used as a model

for L19.

Variation 3: 60.0 mg of HfBn₄is dissolved in 9 mL of toluene in a

vial equipped with a stir bar. The vial is cooled to –35 °C for one

hour and 62.4 mg of L2H₂is added in one portion with stirring.

The mixture is slowly warmed to room temperature, resulting in a

color change from pale-yellow to colorless. The solvent is removed

under reduced pressure, yielding 107 mg of a white powder. Crys-

tals of (μ-L2)₂[HfBn₂]₂suitable for XRD are grown from a toluene

solution of the crude product at –35 °C.

LMMe₂Synthesis: A slurry of ca. 40–160 mg of MCl₄in approxi-

mately 10–15 mL of toluene in a 20 mL vial equipped with a stir

bar is cooled to –35 °C in a glove box freezer. The vial is removed

and immediately four equivalents of 3 M MeMgBr in Et₂O are

added. The mixture is stirred for 3–10 minutes or until a yellowish

colour is observed. One equivalent of LH₂is added as a solid, result-

ing in the immediate release of CH₄. The solution darkens on warm-

ing. After at least one hour of stirring (after the LH₂addition), the

solvent is removed under reduced pressure. The dark residue is

mixed with toluene, filtered through a PE filter frit and the solvent

is again removed under reduced pressure, yielding a white or light

Supporting Information (see footnote on the first page of this

article): Procedures for syntheses of ligands and precatalysts; NMR

spectra; DFT calculated energies and free energies; xyz archives of

geometries.

Acknowledgments

1

This work is part of the Research Programme of the Dutch Poly-

mer Institute DPI, Eindhoven, The Netherlands, project nr. #791.

We thank Cristiano Zuccaccia for valuable advice for activation

chemistry studies, Christian Ehm for valuable discussion of DFT

studies, Emy Komatsu for HRMS measurements, Mark Cooper

for X-ray crystallography, David Davidson for diffusion NMR ex-

periments and Bill Bressenel for elemental analysis. DEH thanks

NSERC (RGPIN-2014–03733), and the Canada Foundation for

Innovation and Research Manitoba for funding in support of an

X-ray diffractometer (#32146).

tan crystalline solid that according to H NMR is of high purity.

Activation Studies: About 15–20 mg of precatalyst LHfMe₂is dis-

solved in approximately 0.5 mL of [D₈]toluene and analyzed by NMR

before activation. An equimolar amount of [CPh₃]⁺[B(C₆F₅)₄]^–is

added directly to the tube, resulting in separation of a dense, dark

orange oil. The mixture is vortexed and quickly inserted in the NMR

spectrometer stabilized at the desired starting temperature.

OPPh₃Trapping: One equivalent of OPPh₃is added directly to the

NMR tube containing [LHfMe]⁺[B(C₆F₅)₄]^–, vortexed and taken im-

mediately for analysis. The reaction of OPPh₃with [L1HfMe]⁺-

[B(C₆F₅)₄]^–was slow and contents were stored in a glove box over-

night. The [D₈]toluene was decanted from the orange oil which was

then diluted with additional [D₈]toluene, vortexed and taken for

analysis.

Keywords: Coordination modes · Hafnium · Olefin

polymerization · Zirconium

Single-Crystal X-ray Structure Determinations: Ligand crystals

suitable for X-ray diffraction were grown from: a) silica gel chroma-

tography fractions at room temperature; b) slow evaporation of

CDCl₃at room temperature or c) a CDCl₃solution layered with hex-

anes at room temperature. Crystals of metal complexes suitable for

X-ray diffraction were grown in an N₂atmosphere from: a) a satu-

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Received: July 2, 2019

Eur. J. Inorg. Chem. 2019, 3396–3410

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Article Doi

Formation and Activation of Zr/Hf Bis(phenolate-ether) Precatalysts

DOI: 10.1002/ejic.201900716

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