Bis(“ferrocene-saliminato”) group 4 metal complexes: synthesis, structural features and use in homogenous Ziegler–Natta polymerization catalysis

Abstract

Reaction of the (p-S) enantiomers of the N-aryl-substituted planar chiral α-hydroxy-ferrocene carbaldimines [(p-S)-3 (aryl = 2,6-diisopropylphenyl (a), –C₆F₅ (c)] with LDA gave the corresponding lithiated “ferrocene-saliminato” systems (p-S)-4a and (p-S)-4c, respectively. Both were characterized by X-ray diffraction. In the crystal, compound (p-S)-4a features an associated cyclotrimeric structure with a six-membered alternating Li₃O₃ core while (p-S)-4c shows a cyclotetrameric structure around a central cubic Li₄O₄ core unit. The reagent (p-S)-4a was treated with the group 4 metal tetrahalides to yield the bis(ferrocene-saliminato)MCl₂ complexes 5a (Ti), 6a (Zr), and 7a (Hf). In the crystal the latter feature single diastereomeric octahedral (p-S,p-S,Λ)-6a (Zr) and (p-S,p-S,Λ)-7a (Hf) structures, whereas the titanium analogue shows both the (p-S,p-S,Λ)-5a and the (p-S,p-S,Δ)-5a diastereoisomers in the crystal. Upon dissolving, we observe a rapid Δ-5a to Λ-5a isomerization to occur. The latter is the only isomer observed in solution under thermodynamic control. In contrast, the pure Λ-6a and Λ-7a isomers equilibrate in solution with a second isomer to a ca. 1 : 1 mixture. Treatment of (p-S)-4c (aryl = C₆F₅) with TiCl₄ or ZrCl₄ gave the analogous octahedral complexes 5c and 6c, respectively, as single diastereoisomers. Treatment of the neutral ferrocene-salimines (p-S)-3a–c with ZrCl₄ or HfCl₄ gave the respective N-aryl-substituted zwitterionic (ferrocene-salimine)₂MCl₄ complexes (8a: Zr, 2,6-diisopropylphenyl; 9a: Hf, 2,6-diisopropylphenyl; 8b: Zr, mesityl; 8c: Zr, C₆F₅), that contain the zwitterionic iminium-tautomers O-bonded to the metal. Activation of a variety of the complexes 5–7 and 8 with methylalumoxane gave homogeneous ethylene polymerization catalysts that exhibit low to moderate catalyst activities in the 20 °C to 125 °C temperature range.

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Introduction

The salicylaldiminato ligand family has found widespread use in organometallic chemistry and catalysis.^1,2 The group 4 metals usually form bis(salicylaldiminato)MX₂ complexes. They often feature pseudooctahedral coordination frameworks with cis-dihalide coordination.^3,4 In the C₂-symmetric series the imino-nitrogens are often trans, but examples of cis-imino situations can be found.⁵ The vast majority of the systems reported so far contains achiral salicylaldiminato ligands, the C₂-symmetric combination of which in an octahedral environment introduces an overall element of chirality—so consequently Λ/Δ racemates are formed (see, for example, the complex pair 1/ent-1 in Scheme 1). Changing this typical stereochemical situation has apparently been difficult unless a chiral variant of the ubiquitous salicylaldiminato ligand system was used.^6,7

Scheme 1

We seem to have done this for the first time by introducing a planar chiral variant of the salicylaldiminato systems, namely the “ferrocene-saliminato” system.^8,9 Attaching two like enantiomers of this three-dimensionally extended ligand to titanium, zirconium or hafnium would lead to the formation of a pair of diastereoisomers (e.g. the examples (p-S,p-S,Λ)-2 and (p-S,p-S,Δ)-2, which are depicted in the lower part of Scheme 1). In contrast to the simple case 1/ent-1, this offers the topological possibility to control the Λ/Δ ratio of the octahedral chirality element by means of the specific chirality features of the attached pair of homochiral planar chiral “ferrocene-saliminates”. Using the chirality features of these three-dimensional relatives of the ubiquitous salicyl-aldiminates in coordination chemistry is probably of a general interest, and using them in catalysis might eventually turn out to be useful. Although a lot of specific problems associated with this attractive, but challenging development are not solved yet (especially with regard to the specific attachment of important catalysis controlling substituents^10–12), we may have opened a door to interesting future catalytic developments by our first simple examples of optically active bis(“ferrocene-salicylminato”)MX₂-type complexes (and derivatives thereof).⁸ Some of the synthetic and structural chemistry of these first new examples will be described in this article, followed by a brief account of first catalytic experiments.

Results and discussion

Ferrocene-salimines and their anions

In a preceding paper¹³, we described the selective synthesis of the (p-S) enantiomer of the neutral 2-hydroxy[N-(2,6-diisopropylphenyl)carbaldimine]ferrocene system (p-S)-3a. The corresponding (p-R)-3a enantiomer was synthesized by an extended version of the same synthetic scheme (see Scheme 2).^14–16 We also described the preparation of the (p-S) enantiomers of the corresponding N-mesityl and N-C₆F₅ carbaldimine derivatives, i.e. (p-S)-3b and (p-S)-3c, of 2-hydroxyferrocene carbaldehyde.

Scheme 2

For use in this study we have now prepared the corresponding O-lithiated derivatives (p-S)-4a and (p-S)-4c, respectively, by treatment of the respective hydroxyferrocene carbaldimine precursors (p-S)-3a and (p-S)-3c with LDA in toluene (see Scheme 3). The anionic derivatives were both isolated in >90% yield from the reaction mixtures. Both compounds were characterized by C,H,N-elemental analyses, spectroscopically and by X-ray diffraction. Repeated attempts to react (p-S)-3b with LDA in a similar manner never led to a clean deprotonation reaction.

Scheme 3

Single crystals of compound (p-S)-4a were obtained from pentane. Fig. 1 shows a view of the monomeric substructural unit of (p-S)-4a in the crystal. It features an anellated six-membered O,N-chelate ring at the ferrocene framework that shows internal contact of both the ferrocenolate oxygen and the imino nitrogen atoms to lithium [values taken from subunit A: bond lengths O1–Li1 1.850(5) Å, N1–Li1 2.024(6) Å, C1–O1 1.307(3) Å, C6–N1 1.278(4) Å, bond angles O1–Li1–N1 100.5(3)°, C6–N1–Li1 117.3(2)°, C2–C6–N1 122.7(3)°]. The six-membered chelate ring is close to planar (max. deviation of Li1B from the mean ring plane: 0.30 Å). We note that the plane of the bulky 2,6-diisopropylphenyl substituent at the imino nitrogen atom N1 is oriented almost perpendicular to the chelate ring plane (dihedral angles C6–N1–C7–C8: 109.9(4)°, C6–N1–C7–C12: −76.3(4)°).¹⁷

Fig. 1 A view of the monomeric subunit of the cyclic trimer [(p-S)-4a]₃.

The coordination sphere of the lithium atom in (p-S)-4a, however, is completed by forming a strong contact to the oxygen atom of an adjacent monomeric unit. In this way compound (p-S)-4a forms an associated cyclotrimeric structure in the crystal (see Fig. 2).^18,19 In this arrangement the lithium atom Li1A forms strong contacts to N1A and O1A inside its six-membered ligand chelate ring and it features a strong interaction with the neighbouring oxygen atom O1B (bond length O1B–Li1A 1.823(6) Å). In this way a central six-membered ring of alternating lithium and oxygen atoms was formed [bond angles O1A–Li1A–O1B 126.9(3)°, LiA–O1A–LiC 112.2(3)°, sum of O–Li–O and Li–O–Li bond angles: 360.0°/338.8°]. The structure features a set of three anellated peripheral homochiral disubstituted ferrocene units. The three C₅H₅ rings are oriented towards one face of the central Li₃O₃ ring structure, whereas the substituted C₅H₃(O,N) rings are found at the other face.

Fig. 2 A projection of the cyclotrimeric associated structure of complex [(p-S)-4a]₃ in the crystal. Selected bond lengths (Å) and angles (°): Li1A–O1A 1.850(5), Li1A–O1B 1.823(6), Li1A–N1A 2.024(6), Li1B–O1B 1.832(6), Li1B–O1C 1.832(6), Li1B–N1B 2.015(6), Li1C–O1C 1.833(6), Li1C–O1A 1.812(6), Li1C–N1C 2.019(6), Li1A–Li1B 2.944(8), Li1A–Li1C 3.040(8), Li1B–Li1C 3.162(8); O1A–Li1A–O1B 126.9(3), O1A–Li1A–N1A 100.5(3), O1B–Li1A–N1A 132.6(3), Li1A–O1A–Li1C 112.2(3), O1B–Li1B–O1C 118.5(3), O1B–Li1B–N1B 103.5(3), O1C–Li1B–N1B 136.1(3). Li1A–O1B–Li1B 107.3(3), O1C–Li1C–O1A 114.6(3), O1C–Li1C–N1C 101.9(3), O1A–Li1C–N1C 138.0(3), Li1B–O1C–Li1C 119.3(3).

Single crystals of compound (p-S)-4c were obtained from toluene solution at −30 °C. We have also obtained single crystals of this compound from dichloromethane–pentane by the diffusion method. Both gave analogous structural results. In the crystal compound (p-S)-4c forms a tetrameric aggregate structure. The monomeric subunits show similar features as were found for 4a (see above). Inside the typical six-membered chelate ring the Li1A–O1A bond length amounts to 1.941(9) Å and the adjacent Li1A–N1A contact distance is 2.047(9) Å. The angles inside the close-to-planar six-membered chelate ring are typical (N1A–Li1A–O1A 96.4(4)°, Li1A–O1A–C1A 122.7(4)°).

In contrast to 4a, each lithium atom in the tetrameric aggregate [(p-S)-4c]₄ is coordinated to three oxygen atoms, one from its “own” six-membered chelate ring and two additional ones from adjacent chelate units (Fig. 3). We consequently find a cubic Li₄O₄ structure^20,21 in the center with typical bond lengths LiA–O1B 1.922(9) Å and Li1A–O1D 2.036(9) Å (bond angles: O1A–Li1A–O1B 93.7(4)°, O1A–Li1A–O1D 97.1(4)°, O1B–Li1A–O1D 91.3(4)°).

Fig. 3 Molecular structure of the tetrameric aggregate [(p-S)-4c]₄ in the crystal.

We do not know if similar structures of (p-S)-4a and (p-S)-4c prevail in solution as they were found in the crystal. The NMR spectra of the latter compound show a single set of resonances of the substituted ferrocene unit [¹H NMR of η⁵-C₅H₅: δ 3.93, η⁵-C₅H₃: δ 4.17, 3.91, 3.83; –CH=N–: δ 8.50 (¹³C: δ 176.1), –C₆F₅: (¹⁹F) δ−154.9 (2F, o-F), −162.9 (1F, p-F), −164.6 (2F, m-F)]. The NMR spectra of (p-S)-4a are similar (for details see the Experimental section). The prochiral 2,6-diisopropylphenyl substituents “recognize” the planar chirality element of the central ferrocene core by diastereotopic splitting.²² In addition there seems to be hindered rotation around the imine-nitrogen –C(2,6-diisopropylphenyl) vector. We observe two –CHMe₂ isopropyl methine ¹H NMR septets (δ 3.36, 2.91) and a total of four ¹H NMR isopropyl methyl signals [δ 1.40, 1.34, 1.06, 1.03; ¹³C: δ 27.6, 27.2 (CH), δ 22.5, 24.2, 23.1, 22.1 (CH₃), in CD₂Cl₂].

Group 4 metal complexes

We used both the neutral ligands (p-S)-3 and the anions (p-S)-4 in group 4 metal complex synthesis. For this study we used only the (p-S) enantiomers as precursors. The reactions that were successfully carried out all showed a ligand to MCl₄ stoichiometry of 2 : 1.

Reaction of the lithium compound (p-S)-4a with titanium tetrachloride was carried out in a 2 : 1 ratio of the reagents in toluene at ambient temperature (Scheme 4). After stirring the reaction mixture overnight we obtained a deep blue mixture. Workup including crystallization from a toluene–pentane mixture at −30 °C eventually gave deep blue-black crystals of compound 5a in ca. 60% yield.

Scheme 4

The X-ray crystal structure analysis revealed the presence of two stereoisomers in the crystal in a 2 : 1 ratio. Both show octahedral coordination geometries at the metal. In both cases, the pairs of chelate ligands are trans-N,cis-O coordinated and, consequently, each complex contains a pair of cis-chloride ligands at titanium.

The major isomer (see Fig. 4) is assigned a configuration as (p-S,p-S,Δ)-5a. It features imino-nitrogen to titanium bond lengths of 2.239(4) Å (Ti2–N1B) and 2.263(4) Å (Ti2–N1C) (angle N1B–Ti–N1C 172.3(2)°) and oxygen to titanium bond lengths of 1.872(4) Å (Ti2–O1B) and 1.845(4) Å (Ti2–O1C) (angle O1B–Ti–O1C 88.4(2) °). The Cl21–Ti2–Cl22 angle amounts to 91.9(1)° (bond lengths Ti2–Cl21 2.325(2) Å, Ti2–Cl22 2.327(2) Å). The Ti–O–C angles inside the close-to-planar six-membered chelate rings were found at 130.6(3)° (Ti2–O1B–C10B) and 138.2(3)° (Ti2–O1C–C10C). The –C(H)=N– bonds are short [N1B–C15B: 1.283(6) Å, N1C–C15C 1.298(7) Å, angles at nitrogen: 121.9(3)° (Ti2–N1B–C15B), 124.0(4)° (Ti2–N1C–C15C)]. The bulky 2,6-diisopropylphenyl substituents at the imine nitrogen atoms are rotated towards a perpendicular orientation with their adjacent chelate rings (dihedral angles C15B–N1B–C16B–C17B 73.3(6)°, C15C–N1C–C16C–C17C −90.3(6)°).

Fig. 4 A view of the molecular geometry of the major (p-S, p-S,Δ)-5a isomer in the crystal.

The minor isomer in the crystal was identified as (p-S,p-S,Λ)-5a. It also features a trans-N,cis-O,cis-Cl arrangement at the central Ti1 atom in an overall octahedral coordination geometry. The structure is crystallographically C₂-symmetric. The individual bond lengths and angles of (p-S,p-S,Λ)-5a (see Fig. 5 and Table 1) are similar as found for its diastereomer (p-S,p-S,Δ)-5a.

Table 1 Selected bond lengths (Å) and angles (°) of the octahedral structures of the compounds 5a, 6a and 7a

	(p-S,p-S,Δ)-5a (Ti)	(p-S,p-S,Λ)-5a (Ti)	(p-S,p-S,Λ)-6a (Zr)^a	(p-S,p-S,Λ)-7a (Hf)
a Single crystals obtained from dichloromethane–pentane by the diffusion method.
M–Cl	2.325(2), 2.327(2)	2.333(2)	2.447(1), 2.454(1)	2.435(1), 2.440(1)
M–O	1.872(4), 1.845(4)	1.850(4)	1.992(3), 1.994(3)	1.987(2), 1.990(2)
M–N	2.239(4), 2.263(4)	2.273(5)	2.381(3), 2.384(3)	2.334(3), 2.338(3)
C=N	1.283(6), 1.298(7)	1.280(7)	1.295(6), 1.305(6)	1.312(4), 1.301(4)
O–M–O	88.4(2)	93.8(3)	93.1(1)	92.2(1)
N–M–N	172.3(2)	179.6(2)	174.3(1)	170.5(1)
N–M–O	102.3(2), 85.4(2), 84.9(2), 92.4(2)	95.1(2), 85.2(2)	95.2(1), 80.6(1), 80.3(1), 96.0(1)	90.4(1), 82.3(1), 81.8(1), 92.5(1)
Cl–M–Cl	91.9(1)	89.9(1)	88.8(1)	90.4(1)
C–O–M	130.6(3), 138.2(3)	137.6(4)	140.0(3), 139.4(3)	137.7(2), 138.2(2)
C–N–M	121.9(3), 124.0(4)	122.8(4)	126.0(3), 125.8(3)	125.6(2), 126.3(2)

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Fig. 5 Molecular structure of the minor (p-S,p-S,Λ)-5a diastereoisomer in the crystal.

We dissolved crystals of 5a at low temperature in CD₂Cl₂. At 193 K we actually observed a similar mixture (ca. 1 : 2) of the minor (p-S,p-S,Λ)-5a and the major (p-S,p-S,Δ)-5a diastereoisomers, as judged by their respective –CH=N–¹H NMR resonances at δ 8.39 (minor) and δ 8.24 (major) under these conditions. Slightly warming the solution resulted in lowering of the signal of the major Δ-5a–CH=N– resonance and eventually its disappearance due to the fast Δ-5a to Λ-5a rearrangement. Under conditions of thermodynamic control in solution a single diastereomer was observed, namely (p-S,p-S,Λ)-5a, the isomer that was found to be only a minor component in the crystal. By following the intensity changes of the respective –CH=N–¹H NMR resonances we determined the activation barrier of the (p-S,p-S,Δ)-5a to (p-S,p-S,Λ)-5a rearrangement as ΔG^‡_rearr (213 K) = 15.7 ± 0.3 kcal mol⁻¹.

The favoured (p-S,p-S,Λ)-5a isomer in solution (CD₂Cl₂) features NMR spectra of a C₂-symmetric structure. It shows a single set of signals of the pair of symmetry-equivalent “ferrocene-saliminato” chelate ligands. The ¹H NMR signal of the –CH=N– functionality appears at δ 8.40 (¹³C: δ 171.5). The ferrocene moiety shows a 10H intensity singlet at δ 4.52 (η⁵-C₅H₅) and ¹H NMR resonances of the corresponding substituted η⁵-C₅H₃ unit at δ 4.73, 4.63 and 4.50. The 2,6-diisopropylphenyl substituent exhibits a pair of isopropyl CH ¹H NMR septets at δ 3.39 and δ 3.17 and the signals of two pairs of diastereotopic isopropyl methyl groups [¹H: δ 1.312, 1.310, 1.04, 0.97; ¹³C: δ 25.7, 25.1, 24.1, 23.9 (CH₃), δ 27.4, 27.3 (CH)].

Analogous treatment of the lithiated reagent (p-S)-4a with zirconium tetrachloride or hafnium tetrachloride gave the octahedral complexes 6a (Zr) and 7a (Hf), respectively. The zirconium complex was isolated in ca. 75% yield as pink crystals from toluene. The hafnium complex was isolated crystalline in ca. 60% yield. Both these complexes were characterized by X-ray diffraction. In contrast to the titanium system 5a both the zirconium complex 6a and the hafnium complex 7a feature only a single diastereomer in the crystal. These were identified as (p-S,p-S,Λ)-6a (Zr) and (p-S,p-S,Λ)-7a (Hf). The Zr complex had been described and its molecular structure depicted in a preliminary communication.⁸ Therefore, we will only depict the analogous structure of the corresponding hafnium complex (p-S,p-S,Λ)-7a in this account (see Fig. 6). Typical structural parameters of both the complexes (p-S,p-S,Λ)-6a (Zr) and (p-S, p-S,Λ)-7a (Hf) are listed in Table 1.

Fig. 6 A projection of the molecular structure of the hafnium complex (p-S,p-S,Λ)-7a.

Crystals of the hafnium complex (p-S,p-S,Λ)-7a were dissolved in CD₂Cl₂ at low temperature to identify this compound by NMR spectroscopy [–CH=N–: δ 8.29 (¹H), δ 177.5 (¹³C)]. Upon warming we observed the rearrangement of Λ-7a to a new C₂-symmetric diastereomer which we tentatively assign as Δ-7a. The thermally induced Λ-7a to Δ-7a rearrangement eventually reaches an equilibrium stage of close to equimolar (see Table 2). The barrier of the (p-S,p-S,Λ)-7a to (p-S,p-S,Δ)-7a rearrangement was determined at ΔG^‡_rearr (268 K) = 19.6 ± 0.3 kcal mol⁻¹. The diastereoisomer (p-S,p-S,Δ)-7a features a typical ¹H NMR –CH=N– resonance at δ 8.36 (¹³C: δ 176.6). There is evidence for hindered rotation of the 2,6-diisopropylphenyl substituent in both isomers. Therefore, we have observed a total of eight isopropyl methyl resonances of the Λ/Δ-7a mixture in the NMR spectra at 298 K (for details see the Experimental section).

Table 2 Diastereomeric ratios of 5a, 6a and 7a under various conditions

	M	Crystal^a	Solution^b	ΔG^‡rearr/kcal mol^−1
a By X-ray diffraction.b In CD2Cl2 after complete equilibration.
5a	Ti	Δ:Λ = 2 : 1	only Λ	15.7 ± 0.3
6a	Zr	only Λ	Λ:Δ = 1 : 1.1	18.3 ± 0.2
7a	Hf	only Λ	Λ:Δ = 1 : 1.1	19.6 ± 0.3

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The zirconium system shows a similar behaviour. Upon dissolving the isolated crystalline material in CD₂Cl₂ at low temperature we have monitored the typical NMR spectra of the (p-S,p-S,Λ)-6a diastereoisomer [e.g.–CH=N–: δ 8.29 (¹H), δ 176.5 (¹³C)]. The kinetics of the Λ-6a to Δ-6a rearrangement were determined at 248 K (ΔG^‡_rearr = 18.3 ± 0.2 kcal mol⁻¹). From the equilibrium (Λ-6a/Δ-6a≈ 1 : 1.1) we have spectroscopically characterized the (p-S,p-S,Δ)-6a diastereoisomer [–CH=N–: δ 8.35 (¹H), δ 175.6 (¹³C)].

We have also reacted the C₆F₅-substituted ferrocene-saliminato lithium reagent (p-S)-4c with TiCl₄ and with ZrCl₄, both in a 2 : 1 ratio. After stirring the reaction mixture for 28 h in toluene at room temperature we isolated the respective bis(ferrocene-saliminato)TiCl₂ complex (p-S,p-S)-5c in ca. 70% yield (Scheme 5). The compound was characterized by C,H,N-elemental analysis and spectroscopically. We observe a single isomer at ambient conditions, showing a pair of symmetry equivalent chelate ligands. It features ¹H/¹³C NMR resonances of the –CH=N– moiety at δ 8.32/177.3 in CD₂Cl₂ solution. The ¹³C NMR signals of the ferrocene unit were observed at δ 71.5 (Cp) and δ 126.8 (C1), 68.4, 60.5, 69.8, 67.0 (C2–C5). There is a single 2F intensity p-F ¹⁹F NMR signal of the –C₆F₅ substituent at δ−158.7, but both the ortho (δ−143.2, −146.9) and meta-F signals (δ−162.9, −164.2) are diastereotopic (each signal of 1F relative intensity), which indicates hindered rotation of the –C₆F₅ unit around the N–C(ipso) vector.²³

Scheme 5

The corresponding zirconium complex (p-S,p-S)-6c was isolated as a purple amorphous solid in ca. 75% yield. The spectra show also the presence of a single C₂-symmetrical isomer in solution. Whether this has a similar structural composition as the systems 5a–7a is presently unknown. It features very similar spectra as the analogous titanium compound (for details see the Experimental section).

The reaction of the neutral ferrocene-salimine (p-S)-3a with zirconium tetrachloride in a 2 : 1 stoichiometry in dichloromethane instantly gave a deep blue solution, from which the neutral bis(ferrocene-salimine)ZrCl₄ complex (p-S,p-S)-8a was isolated as a deep blue solid in >90% yield. The compound was characterized by C,H,N-elemental analysis and spectroscopically. Its NMR spectra are consistent with a C₂-symmetrical structure featuring a pair of symmetry-equivalent ligands at Zr, which we assume are trans-O coordinated in a pseudo-octahedral coordination environment.⁴ The ¹H NMR spectra indicate the presence of the –O⁻/–CH=NH⁺–Ar tautomer. This gives rise to a typical set of ¹H NMR signals of this unit, showing a “low field”=NH resonance at δ 12.05 and an iminium ion –CH= signal at δ 8.30 as an AX system with a vicinal ³J_HHtrans-coupling constant of 15.8 Hz. The corresponding ¹³C NMR –CH=NH⁺– signal occurs at δ 173.4. We observe the typical NMR signals of the ferrocene moiety (for details see the Experimental section). Again, the bulky 2,6-diisopropylphenyl substituent at the iminium nitrogen exhibits hindered rotation giving rise to the observation of a pair of isopropyl –CH– resonances (¹H: δ 3.36/3.00, ¹³C: δ 28.1/28.0) and the typical signals of two pairs of diastereotopic isopropyl CH₃ resonances (¹H: δ 1.34, 1.19, 1.10, 1.07, ¹³C: 23.9, 23.4, 23.3, 22.8).

The reaction of (p-S)-3a with hafnium tetrachloride proceeds analogously to yield the deep blue product (p-S,p-S)-9a in 88% yield (see Scheme 6). It features NMR signals of the –CH=NH⁺–(2,6-diisopropylphenyl) unit at δ 8.30 (CH)/12.07 (NH) (³J_HH = 15.3 Hz) (¹³C NMR resonance at δ 173.6) and again a total of four isopropyl CH₃¹H NMR resonances of the pair of symmetry-equivalent ligand systems.

Scheme 6

Treatment of the N-mesityl-substituted ferrocene-salimine (p-S)-3b with ZrCl₄ gave the analogous system (p-S,p-S)-8b [74% isolated, δ (–CH=NH⁺–Mes): 8.29 (CH), 11.91 (NH), ³J_HH = 15.4 Hz; ¹³C: δ 173.3]. Eventually, we have isolated the complex (p-S,p-S)-8c from the reaction of the –C₆F₅ substituted ferrocene-salimine reagent (p-S)-3c with zirconium tetrachloride. The compound was obtained as a blue solid in 68% yield. It features the typical ¹H NMR iminium resonances at δ 11.32 (NH)/8.52 (CH) with a characteristic coupling constant of ³J_HH = 14.3 Hz. Complex (p-S,p-S)-8c features a single set of three ¹⁹F NMR resonances [δ−147.2 (4F, o), −153.4 (2F, p), −160.3 (4F, m)] of the –C₆F₅ substituents, indicating rapid rotation around the N–C₆F₅ vector under the monitoring conditions.

Ethylene polymerization experiments

We have employed the complexes 5a to 7a (Ti, Zr, Hf), the related systems 8a to 8c (all Zr) and the complexes 5c (Ti), 6c (Zr) and 9a (Hf) as precursors for ethylene polymerization reactions. In each case the group 4 metal complexes were pre-activated by treatment with excess methylalumoxane (MAO)^24–26 in toluene solution at ambient temperature and then the actual polymerization reaction was carried out at a set temperature (range 20 °C to 125 °C) in toluene with a large excess of MAO²⁷ (Al : group 4 metal complex ratio 2000) under a 2 bar ethylene pressure.^10–12

The titanium system (5a/MAO) showed only a low catalyst activity at 20 °C (act ≈ 11), which did not increase with temperature (see Table 1, entries 1–2). However, the linear polyethylene sample that was obtained showed a rather high molecular weight (M_w≈ 5.4 Mio). The C₆F₅-substituted Ti catalyst system (5c/MAO) exhibits a similarly low catalyst activity (entries 8–9).

The hafnium system (7a/MAO) was even less active. We did not observe polyethylene formation at 20 °C or 80 °C. Only at the highest temperature employed (125 °C) some PE formation set in (entry 7), albeit with a very low catalyst activity (act ≈ 5).

The zirconium based catalyst systems are more active. The 6a/MAO system, featuring the bulky 2,6-diisopropylphenyl substituent at the imine nitrogen, shows a low activity at 20 °C (act ≈ 11) and gives a high molecular weight polyethylene of M_w≈ 1.7 Mio. The polymerization activity of this system rapidly increases with increasing temperature (80 °C, act ≈ 39, M_w≈ 470 000) to eventually exhibit a moderate catalyst performance at 125 °C (act ≈ 123 g PE per mmol[Zr] per bar per h, entries 3–6). The analogous C₆F₅ substituted Zr/MAO system (6c/MAO) shows similar polyethylene formation features at 20 °C and 80 °C, respectively (see Table 3, entries 10–11).

Table 3 Ethylene polymerization experiments done with the (ferrocene-salimine)₂ group 4 metal complex/MAO catalyst systems^a

Entry	Precat.	M	R	T/°C	t/h	g PE	act^b	Mp^c/°C
a 5 μmol complex, 200 ml toluene or xylene (T > 100 °C), preactivated, Al : M = 2000, 2 bar ethene; values of selected representative experiments are listed (for more details see the ESI^1).b Catalyst activities in g PE/mmol[M] per h per bar (ethene).c All polymers obtained as solids, melting point by DSC, second run.d Aryl: 2,6-diisopropylphenyl.
1	5a	Ti	aryl^d	20	1	0.11	11	134
2	5a	Ti	aryl	80	1	0.13	13	136
3	6a	Zr	aryl	20	2	0.22	11	138
4	6a	Zr	aryl	80	2	0.78	39	141
5	6a	Zr	aryl	100	2	1.69	85	136
6	6a	Zr	aryl	125	2	2.46	123	134
7	7a	Hf	aryl	125	2	0.10	5	132
8	5c	Ti	C6F5	20	2	0.17	9	135
9	5c	Ti	C6F5	80	2	0.26	13	132
10	6c	Zr	C6F5	20	1	0.15	15	134
11	6c	Zr	C6F5	80	1	0.57	57	132
12	8a	Zr	aryl	20	2	0.18	9	134
13	8a	Zr	aryl	80	2	1.02	51	138
14	8a	Zr	aryl	100	2	1.63	81	136
15	8a	Zr	aryl	125	2	2.17	109	131
16	8b	Zr	Mes	20	2	0.15	7	135
17	8c	Zr	C6F5	20	1	0.26	26	134
18	8c	Zr	C6F5	80	1	0.63	63	133

---

The zwitterionic (ligand)₂MCl₄ systems (8a–c) can analogously be activated with MAO. They give active systems that show catalytic features that are practically identical to those of the related 6/MAO derived systems, so that it may be assumed that very similar active species were generated (see entries 12–18). Thus the 2,6-diisopropylphenyl substituted (lig)₂ZrCl₄/MAO system produces linear polyethylene at 20 °C of M_w≈ 1.75 Mio with a low catalyst activity of act ≈ 9, and at 80 °C with act ≈ 51 (see Table 3) (M_w≈ 390 000). Both the catalysts 8b/MAO and 8c/MAO show similar features. The hafnium complex derived system 9a/MAO showed no ethylene polymerization activity at temperatures up to 80 °C. Attempts to use these catalysts for the polymerization of propylene so far did not result in any appreciable polymer formation.

Conclusions

This study has shown that group 4 metal complexes of the three-dimensional analogues of the ubiquitous salicylaldiminato ligands, namely the ferrocene-saliminato systems 4 can readily be formed by the reaction with the MCl₄ reagents in a 2 : 1 ratio. The resulting octahedral bis(ferrocene-saliminato)MCl₂ complexes (5–7, M = Ti, Zr, Hf) were isolated. Representative examples of complexes of the 2,6-diisopropylphenyl substituted ferrocene-saliminato ligands from all three group 4 metals were characterized by X-ray diffraction. All three of them featured a cis-Cl,trans-N coordination mode, but we noticed marked differences with regard to diastereomer formation between these systems. These complexes exhibit octahedral chirality^6,28 which in combination with a pair of homochiral (p-S) ferrocene-salimine chelate ligands can form a (p-S,p-S,Λ)- and a (p-S,p-S,Δ)-diastereoisomer. In the crystal of the zirconium and the hafnium system we found only the respective (p-S,p-S,Λ)-6a and (p-S,p-S,Λ)-7a complexes. This does not necessarily mean that these are the kinetic products formed, but their selection from a dynamic equilibrium situation during the crystallization process might well be a possible origin of their favoured appearance in the crystal.²⁹ Indeed, when dissolved in CD₂Cl₂ both these complexes undergo a rather rapid interconversion with a second C₂-symmetric isomer, which may be their respective Δ-diastereomers. The titanium complex 5a behaves slightly different. Here we find a mixture of the (p-S,p-S,Λ)-5a and the (p-S,p-S,Δ)-5a isomer in the crystal. Upon dissolving it we observe a very rapid Δ-5 to Λ-5 interconversion even at rather low temperatures. The order of the corresponding rearrangement Gibbs activation energies increases in the order Ti < Zr < Hf. Since this qualitatively corresponds to the order of the dissociation energies of most σ-bonds to these group 4 metals³⁰ this may indicate dissociation of a ligand “arm” with concurrent or subsequent rearrangement at a five-coordinate intermediate geometry,³¹ although alternative pathways were proposed in the literature for similar rearrangement types.^31a,32

We noticed that related 2 : 1 (ferrocene-salimine)₂MCl₄ complexes (8 and 9) are obtained simply by treatment of the neutral ferrocene-salimine ligands with the respective group 4 metal tetrachlorides. From the typical spectroscopic patterns we assume metal–oxygen bonding of the zwitterionic iminium-type tautomers in these cases.^4,33

Activation with MAO in many of these cases gives active homogeneous Ziegler–Natta ethylene polymerization catalysts, although mostly of rather low activities. The observation that the systems 6 and their respective counterparts 8 give catalysts of almost the same performance in ethylene polymerization indicates that similar active species are generated from these systems. We realize that the catalyst activities of these systems even in the Zr cases are only low to moderate,²⁶ but this investigation of these new systems might eventually lead the way to forming useful derivatives of such three-dimensional ferrocene-derived chelate analogues of the ubiquitous salicylaldiminato ligands by introducing appropriate controlling substituent variations^10–12 at these new backbones.

Experimental

General comments

Materials. All reactions involving air- or moisture-sensitive compounds were carried out under an inert gas atmosphere (Argon) by using Schlenk-type glassware or in a glovebox. Solvents were dried and distilled prior to use. Unless otherwise noted, all starting materials were commercially available and were used without further purification. Ferrocenes 3a–c were prepared as described earlier.¹³

Techniques. The following instruments were used for physical characterization of the compounds: melting points: TA-instruments DSC Q-20; elemental analyses: Foss–Heraeus CHNO-Rapid; IR: Varian 1300 FT-IR; NMR: Varian UNITY plus NMR spectrometer; Varian INOVA 500; Polarimetric measurements: Perkin-Elmer Polarimeter 341.

X-Ray crystal structure determination. Data sets were collected with Nonius KappaCCD diffractometer, equipped with a rotating anode generator. Programs used: data collection COLLECT,³⁴ data reduction Denzo-SMN,³⁵ absorption correction SORTAV³⁶ and Denzo,³⁷ structure solution SHELXS-97,³⁸ structure refinement SHELXL-97,³⁹ graphics SCHAKAL.⁴⁰R values are given for observed data. wR² values are given for all data.

Synthesis of the lithium salt (p-S)-4a

The alcohol (p-S)-3a (1.70 g, 4.37 mmol) and lithium diisopropylamide (468 mg, 4.37 mmol) were suspended in toluene (20 ml). The mixture was stirred for three hours and the solvent was removed, giving an oily residue. Addition of pentane (20 ml) and removal of the solvent gave product (p-S)-4a as a deep red powder (1.62 g, 93.8%). Crystals suitable for X-ray crystal structure diffraction were obtained from a solution of (p-S)-4a in pentane at room temperature. Mp 274 °C (DSC); [α]²⁰_D−533 (c 0.100 in CH₂Cl₂); (Found: C, 69.89; H, 6.90; N, 3.33. C₂₃H₂₆FeLiNO requires C, 69.89; H, 6.63%; N, 3.54); ν_max(KBr)/cm⁻¹ 3066, 2960, 2867, 1612, 1587, 1473, 1361, 1104, 782, 728 and 700; δ_H (600 MHz; CD₂Cl₂; Me₄Si; 228 K) 7.97 (1 H, s, CHN), 7.26 (1 H, m, p-aryl), 7.16 (2 H, m, m-aryl), 3.94 (1 H, dd, ³J = 2.8 Hz, ⁴J = 1.5 Hz, 3-H), 3.75 (5 H, s, Cp), 3.67 (1 H, t, ³J = 2.8 Hz, 4-H), 3.45 (1 H, bs, 5-H), 3.36 (1 H, sept, ³J = 6.9 Hz, CH^A), 2.91 (1 H, sept, ³J = 6.9 Hz, CH^B), 1.40, 1.34 (each 3 H, each d, ³J = 6.9 Hz, 2 × CH₃^A), 1.06 and 1.03 (each 3 H, each d, ³J = 6.9 Hz, 2 × CH₃^B); δ_C (151 MHz; CD₂Cl₂; Me₄Si; 228 K) 170.5 (CHN), 148.7 (i-aryl), 139.5 (o-aryl^A), 139.0 (o-aryl^B), 135.6 (C-1), 124.8, 123.5 (m-aryl), 122.9 (p-aryl), 68.2 (Cp), 63.5 (C-2), 63.3, 63.2 (C-3, C-4), 58.9 (C-5), 27.6 (CH^B), 27.2 (CH^A), 25.5, 22.1 (2 ×CH₃^B), 24.2 and 23.1 (2 ×CH₃^A).

Crystal data for (C₂₃H₂₆FeLiNO)₃, M = 1185.71, orthorhombic, space group P2₁2₁2₁ (No. 19), a = 14.828(1), b = 17.780(1), c = 23.866(1) Å, V = 6292.1(6) ų, D_c = 1.252 g cm⁻³, μ = 0.730 mm⁻¹, Z = 4, λ = 0.71073 Å, T = 198(2) K, 37121 reflections collected (±h, ±k, ±l), [(sinθ)/λ] = 0.62 Å⁻¹, 12696 independent (R_int = 0.073), and 9287 observed reflections [I ≥ 2σ(I)], 742 refined parameters, R = 0.046, wR² = 0.084, Flack −0.02(1).

Synthesis of the lithium salt (p-S)-4c

The alcohol (p-S)-3c (1.12 g, 2.84 mmol) and lithium diisopropylamide (304 mg, 2.84 mmol) were suspended in toluene (20 ml). The mixture was stirred for three hours and the solvent was removed, giving an oily residue. Addition of pentane (20 ml) and removal of the solvent gave product (p-S)-4c as a deep purple powder (1.05 g, 92.2%). Crystals suitable for X-ray crystal structure diffraction were obtained from a solution of (p-S)-4c in toluene at −30 °C. Alternatively, crystals could be obtained by diffusion of pentane into a solution of (p-S)-4c in dichloromethane at −30 °C. Mp 256 °C (DSC); [α]²⁰−1155 (436 nm, c 0.020 in CH₂Cl₂); (Found: C, 50.68; H, 2.39; N, 3.48. C₁₇H₉F₅FeLiNO requires C, 50.91; H, 2.26; N, 3.49%); ν_max(KBr)/cm⁻¹ 3093, 2921, 1654, 1594, 1515, 1473, 1460, 1365, 1105, 1002, 975, 811, 730, 678, 627 and 540; δ_H (600 MHz; CD₂Cl₂; Me₄Si) 8.50 (1 H, bs, CHN), 4.17 (1 H, bs, 3-H), 3.93 (5 H, s, Cp), 3.91 (1 H, dd, ³J = 2.8 Hz, ⁴J = 1.5 Hz, 5-H) and 3.83 (1 H, t, ³J = 2.8 Hz, 4-H); δ_F (564 MHz; CD₂Cl₂; CCl₃F) −154.9 (2 F, m, o-C₆F₅), −162.9 (1 F, t, ³J_FF = 21.4 Hz, p-C₆F₅) and −164.6 (2 F, m, m-C₆F₅); δ_C (151 MHz; CD₂Cl₂; Me₄Si) 176.1 (t, ⁴J_FC = 5.2 Hz, CHN), 141.5 (dm, ¹J_FC≈ 247 Hz), 138.5 (dm, ¹J_FC≈ 249 Hz) (o, m-C₆F₅), 137.8 (dm, ¹J_FC≈ 250 Hz, p-C₆F₅), 136.1 (C-1), 126.4 (b, i-C₆F₅), 69.7 (Cp), 65.7 (C-4), 64.7 (C-5), 64.3 (C-2) and 61.5 (C-3).

Crystal data for (C₁₇H₉F₅FeLiNO)₄, M = 1604.16, monoclinic, space group P2₁ (No. 4), a = 12.0617(3), b = 21.5748(6), c = 12.9461(3) Å, β = 100.689(1)°, V = 3310.5(2) ų, D_c = 1.609 g cm⁻³, μ = 0.967 mm⁻¹, Z = 2, λ = 0.71073 Å, T = 223(2) K, 20161 reflections collected (±h, ±k, ±l), [(sinθ)/λ] = 0.66 Å⁻¹, 13091 independent (R_int = 0.034), and 10449 observed reflections [I ≥ 2σ(I)], 937 refined parameters, R = 0.052, wR² = 0.138, Flack 0.03(2).

Preparation of the titanium complex 5a

The lithium salt (p-S)-4a (91.0 mg, 0.230 mmol) and titanium tetrachloride-bis-thf-adduct (38.4 mg, 0.115 mmol) were dissolved in toluene (4 ml) and the resulting deep blue reaction mixture was stirred overnight. After filtration the resulting solution was diluted with pentane (4 ml) and stored at −30 °C for one week. The resulting black crystals were isolated by removal of the mother liquor and dried in vacuo to give 64.2 mg (60.3%) of complex 5a. The crystals were suitable for X-ray crystal structure analysis. Mp 123 °C (DSC); [α]²⁰ +1804 (436 nm, c 0.021 in CH₂Cl₂); (Found: C, 62.48; H, 5.99; N, 3.01. C₄₆H₅₂Cl₂Fe₂N₂O₂Ti·1/3C₇H₈ requires C, 62.88; H, 5.95; N, 3.02%); ν_max(KBr)/cm⁻¹ 3082, 2962, 2867, 1622, 1584, 1566, 1466, 1449, 1346, 1212, 1107, 1023, 822, 789, 762, 735 and 650; Λ-isomer: δ_H (600 MHz; CD₂Cl₂; Me₄Si; 238 K) 8.40 (2 H, s, CHN), 7.16 (2 H, m, p-aryl), 7.09, 7.09 (each 2 H, each m, 2 ×m-aryl), 4.73 (2 H, bs, 5-H), 4.63 (2 H, dd, ³J = 2.8 Hz, ⁴J = 1.2 Hz, 3-H), 4.52 (10 H, s, Cp), 4.50 (2 H, t, ³J = 2.8 Hz, 4-H), 3.39 (2 H, sept, ³J = 6.6 Hz, CH^A), 3.17 (2 H, sept, ³J = 6.6 Hz, CH^B), 1.312, 1.310 (each 6 H, each d, ³J = 6.6 Hz, 2 × CH₃^A), 1.04 and 0.97 (each 6 H, each d, ³J = 6.6 Hz, 2 × CH₃^B); δ_C (151 MHz; CD₂Cl₂; Me₄Si; 238 K) 171.5 (CHN), 153.0 (i-aryl), 141.1, 141.0 (2 ×o-aryl), 127.2 (C-1), 126.4 (p-aryl), 124.0, 122.7 (2 ×m-aryl), 70.5 (Cp), 68.5 (C-4), 67.2 (C-2), 66.5 (C-3), 59.7 (C-5), 27.4 (CH^A), 27.3 (CH^B), 25.7, 24.1 (2 ×CH₃^A), 25.1 and 23.9 (2 ×CH₃^B).

Crystal data for C₄₆H₅₂Cl₂Fe₂N₂O₂Ti · 1/3 C₇H₈, M = 926.11, orthorhombic, space group P2₁2₁2 (No. 18), a = 15.3532(2), b = 31.8628(6), c = 14.3849(2) Å, V = 7037.0(2) ų, D_c = 1.311 g cm⁻³, μ = 0.929 mm⁻¹, Z = 4, λ = 0.71073 Å, T = 223(2) K, 31187 reflections collected (±h, ±k, ±l), [(sinθ)/λ] = 0.60 Å⁻¹, 12364 independent (R_int = 0.056), and 8419 observed reflections [I ≥ 2σ(I)], 766 refined parameters, R = 0.057, wR² = 0.153, Flack −0.03(2).

Preparation of the zirconium complex 6a

The lithium salt (p-S)-4a (160 mg, 0.405 mmol) and zirconium tetrachloride (47.2 mg, 0.203 mmol) were suspended in toluene (4 ml) and the obtained reaction mixture was stirred overnight. After filtration on a glass frit, the resulting solution was stored at −30 °C for one week. The resulting pink crystals were isolated by removal of the mother liquor and dried in vacuo to give complex 6a (175 mg, 76.8%). The obtained crystals were suitable for X-ray crystal structure analysis. Alternatively, crystals were obtained by diffusion of pentane into a solution of 6a in dichloromethane at −30 °C. Mp 113 °C (DSC); [α]²⁰ +138 (436 nm, c 0.020 in CH₂Cl₂); (Found: C, 63.32; H, 6.03; N, 2.65. C₄₆H₅₂Cl₂Fe₂N₂O₂Zr·2C₇H₈ requires C, 64.17; H, 6.10; N, 2.49%); ν_max(KBr)/cm⁻¹ 3086, 3059, 3022, 2963, 2865, 1622, 1593, 1565, 1471, 1439, 1364, 1353, 1328, 1213, 1106, 1023, 881, 829, 789, 743, 733 and 656; Λ-isomer: δ_H (600 MHz; CD₂Cl₂; Me₄Si) 8.29 (1 H, d, ⁴J = 0.7 Hz, CHN), 7.27 (1 H, m, p-aryl), 7.22 (2 H, m, m-aryl^A,B), 4.48 (1 H, m, 3-H), 4.43 (5 H, s, Cp), 4.34 (1 H, t, ³J = 2.8 Hz, 4-H), 4.30 (1 H, dd, ³J = 2.8 Hz, ⁴J = 1.3 Hz, 5-H), 3.53 (1 H, sept, ³J = 6.8 Hz, CH^A), 3.40 (1 H, sept, ³J = 6.8 Hz, CH^B), 1.60 (3 H, d, ³J = 6.8 Hz, CH₃^B), 1.34 (3 H, d, ³J = 6.8 Hz, CH₃^A), 1.27 (3 H, d, ³J = 6.8 Hz, CH₃^A′) and 1.11 (3 H, d, ³J = 6.8 Hz, CH₃^B′); δ_C (151 MHz; CD₂Cl₂; Me₄Si) 176.5 (CHN), 151.7 (i-aryl), 142.1 (o-aryl^A), 141.9 (o-aryl^B), 127.1 (p-aryl), 126.5 (C-1), 124.2 (m-aryl^A), 123.8 (m-aryl^B), 70.4 (Cp), 67.6 (C-4), 65.7 (C-2), 65.2 (C-5), 60.8 (C-3), 28.0 (CH^A, CH^B), 25.8 (CH₃^A′), 25.6 (CH₃^B′), 24.0 (CH₃^B) and 23.6 (CH₃^A); Δ-isomer: δ_H (600 MHz; CD₂Cl₂; Me₄Si) 8.35 (1 H, d, ⁴J = 0.7 Hz, CHN), 7.22 (1 H, m, p-aryl), 7.18 (1 H, dd, ³J = 7.7 Hz, ⁴J = 1.6 Hz, m-aryl^A), 7.14 (1 H, dd, ³J = 7.5 Hz, ⁴J = 1.6 Hz, m-aryl^B), 4.74 (1 H, ddd, ³J = 2.8 Hz, ⁴J = 1.4 Hz, ⁴J = 0.7 Hz, 3-H), 4.52 (5 H, s, Cp), 4.41 (1 H, t, ³J = 2.8 Hz, 4-H), 4.40 (1 H, dd, ³J = 2.8 Hz, ⁴J = 1.4 Hz, 5-H), 3.52 (1 H, sept, ³J = 6.7 Hz, CH^A), 3.16 (1 H, sept, ³J = 6.7 Hz, CH^B), 1.46 (3 H, d, ³J = 6.7 Hz, CH₃^A), 1.42 (3 H, d, ³J = 6.7 Hz, CH₃^A′), 1.12 (3 H, d, ³J = 6.7 Hz, CH₃^B) and 1.02 (3 H, d, ³J = 6.7 Hz, CH₃^B′); δ_C (151 MHz; CD₂Cl₂; Me₄Si) 175.6 (CHN), 150.8 (i-aryl), 142.3 (o-aryl^B), 141.8 (o-aryl^A), 127.1 (p-aryl), 126.4 (C-1), 124.6 (m-aryl^B), 123.5 (m-aryl^A), 70.6 (Cp), 68.3 (C-4), 66.4 (C-2), 66.0 (C-5), 61.1 (C-3), 28.3 (CH^A), 27.9 (CH^B), 26.1 (CH₃^A′), 25.5 (CH₃^B′), 24.4 (CH₃^A) and 23.9 (CH₃^B); [Δ-isomer/Λ-isomer at 298 K ≈ 1.1 : 1].

Preparation of the hafnium complex 7a

The lithium salt (p-S)-4a (106 mg, 0.268 mmol) and hafnium tetrachloride (42.9 mg, 0.134 mmol) were suspended in toluene (3 ml) and the mixture was stirred overnight. After filtration on a glass frit, the resulting solution was diluted with toluene (5 ml) and stored at −30 °C for one week. A first fraction of pink crystals (65.6 mg) was isolated by removal of the mother liquor, which was stored at −30 °C for another week to give again 35.6 mg of the complex 7a (total yield: 101 mg, 62.3%). The obtained crystals were suitable for X-ray crystal structure analysis. Mp 109 °C (DSC); [α]²⁰ +753 (436 nm, c 0.020 in CH₂Cl₂); (Found: C, 63.32; H, 6.03; N, 2.65. C₄₆H₅₂Cl₂Fe₂N₂O₂Hf·2C₇H₈ requires C, 64.17; H, 6.10; N, 2.49%); ν_max(KBr)/cm⁻¹ 3023, 2966, 2924, 2865, 1622, 1593, 1565, 1471, 1439, 1364, 1353, 1328, 1213, 1106, 1023, 881, 829, 789, 743, 733 and 656; Λ-isomer: δ_H (600 MHz; CD₂Cl₂; Me₄Si) 8.29 (1 H, d, ⁴J = 0.7 Hz, CHN), 7.30–7.13 (3 H, m, m,p-aryl), 4.50 (1 H, ddd, ³J = 2.6 Hz, ³J = 1.3 Hz, ³J = 0.7 Hz, 3-H), 4.41 (5 H, s, Cp), 4.35 (1 H, t, ³J = 2.8 Hz, 4-H), 4.29 (1 H, dd, ³J = 2.8 Hz, ⁴J = 1.3 Hz, 5-H), 3.57 (1 H, sept, ³J = 6.8 Hz, CH^A), 3.48 (1 H, sept, ³J = 6.8 Hz, CH^B), 1.59 (3 H, d, ³J = 6.8 Hz, CH₃^B), 1.34 (3 H, d, ³J = 6.8 Hz, CH₃^A), 1.27 (3 H, d, ³J = 6.8 Hz, CH₃^A′) and 1.12 (3 H, d, ³J = 6.8 Hz, CH₃^B′); δ_C (151 MHz; CD₂Cl₂; Me₄Si) 177.5 (CHN), 151.3 (i-aryl), 142.4 (o-aryl^A), 142.3 (o-aryl^B), 127.3 (p-aryl), 127.1 (C-1), 124.2 (m-aryl^A), 124.0 (m-aryl^B), 70.4 (Cp), 67.9 (C-4), 65.5 (C-2), 65.0 (C-5), 61.3 (C-3), 28.0 (CH^A), 27.9 (CH^B), 25.9 (CH₃^A′), 25.7 (CH₃^B′), 24.1 (CH₃^B) and 23.7 (CH₃^A); Δ-isomer: δ_H (600 MHz; CD₂Cl₂; Me₄Si) 8.36 (1 H, d, ⁴J = 0.7 Hz, CHN), 7.30–7.13 (3 H, m, m,p-aryl), 4.77 (1 H, ddd, ³J = 2.8 Hz, ⁴J = 1.3 Hz, ⁴J = 0.7 Hz, 3-H), 4.52 (5 H, s, Cp), 4.42 (1 H, t, ³J = 2.8 Hz, 4-H), 4.38 (1 H, dd, ³J = 2.8 Hz, ⁴J = 1.3 Hz, 5-H), 3.59 (1 H, sept, ³J = 6.8 Hz, CH^A), 3.18 (1 H, sept, ³J = 6.8 Hz, CH^B), 1.46 (3 H, d, ³J = 6.8 Hz, CH₃^A), 1.42 (3 H, d, ³J = 6.8 Hz, CH₃^A′), 1.11 (3 H, d, ³J = 6.8 Hz, CH₃^B) and 1.02 (3 H, d, ³J = 6.8 Hz, CH₃^B′); δ_C (151 MHz; CD₂Cl₂; Me₄Si) 176.6 (CHN), 150.6 (i-aryl), 142.6 (o-aryl^B), 142.1 (o-aryl^A), 127.3 (C-1), 127.2 (p-aryl), 124.7 (m-aryl^B), 123.6 (m-aryl^A), 70.7 (Cp), 68.5 (C-4), 66.2 (C-2), 65.7 (C-5), 61.6 (C-3), 28.2 (CH^A), 27.9 (CH^B), 26.2 (CH₃^A′), 25.5 (CH₃^B′), 24.5 (CH₃^A) and 24.0 (CH₃^B); [Δ-isomer/Λ-isomer at 298 K ≈ 1.1 : 1].

Crystal data for C₄₆H₅₂Cl₂Fe₂HfN₂O₂·2C₇H₈, M = 1210.25, monoclinic, space group P2₁ (No. 4), a = 11.4955(1), b = 19.5379(2), c = 12.5595(1) Å, β = 105.900(1)°, V = 2712.92(4) ų, D_c = 1.482 g cm⁻³, μ = 2.578 mm⁻¹, Z = 2, λ = 0.71073 Å, T = 223(2) K, 18591 reflections collected (±h, ±k, ±l), [(sinθ)/λ] = 0.67 Å⁻¹, 11270 independent (R_int = 0.032), and 10874 observed reflections [I ≥ 2σ(I)], 632 refined parameters, R = 0.024, wR² = 0.060, Flack −0.014(4).

Preparation of the titanium complex (p-S,p-S)-5c

The lithium salt (p-S)-4c (100 mg, 0.250 mmol) and titanium tetrachloride-bis-thf-adduct (41.7 mg, 0.125 mmol) were suspended in toluene (4 ml) and the mixture was stirred for two days. After filtration, pentane (20 ml) was added to the solution. The resulting precipitate was collected on a glass frit, washed with pentane (3 × 2 ml) and dried in vacuo. Complex (p-S,p-S)-5c was obtained as a deep blue powder (78.5 mg, 69.2%). Mp 208 °C (DSC); [α]²⁰−1653 (436 nm, c 0.020 in CH₂Cl₂); (Found: C, 45.32; H, 2.32; N, 2.83. C₃₄H₁₈Cl₂F₁₀Fe₂N₂O₂Ti requires C, 45.03; H, 2.00; N, 3.09%); ν_max(KBr)/cm⁻¹ 3095, 1576, 1519, 1453, 1453, 1351, 1214, 1157, 1107, 1005, 985, 861, 837, 773 and 633; δ_H (600 MHz; CD₂Cl₂; Me₄Si) 8.32 (1 H, s, CHN), 4.60 (5 H, s, Cp), 4.513, 4.509 (each 1 H, each m, C₅H₃) and 4.36 (1 H, br, C₅H₃); δ_F (564 MHz; CD₂Cl₂; CCl₃F) −143.2, −146.9 (each 1 F, each br, o-C₆F₅), −158.7 (1 F, t, ³J_FF = 21.4 Hz, p-C₆F₅), −162.9 and −164.2 (each 1 F, each br, m-C₆F₅); δ_C (151 MHz; CD₂Cl₂; Me₄Si) 177.3 (CHN), 126.8 (C-1), 71.5 (Cp), 69.8 (C₅H₃), 68.4 (C-2), 67.0 (C₅H₃) and 60.5 (C₅H₃), n.o. (C₆F₅).

Preparation of the zirconium complex (p-S,p-S)-6c

The lithium salt (p-S)-4c (114 mg, 0.285 mmol) and zirconium tetrachloride (33.2 mg, 0.142 mmol) were suspended in toluene (12 ml) and the reaction mixture was stirred for 28 h. After filtration on a glass frit, pentane (20 ml) was added to the solution. The resulting precipitate was collected on a glass frit, washed with pentane (3 ml) and dried in vacuo. Complex (p-S,p-S)-6c was obtained as a purple powder (102 mg, 75.6%). Mp 247 °C (DSC); [α]²⁰ +660 (436 nm, c 0.021 in CH₂Cl₂); (Found: C, 43.21; H, 2.07; N, 2.97. C₃₄H₁₈Cl₂F₁₀Fe₂N₂O₂Zr requires C, 42.97; H, 1.91; N, 2.95%); ν_max(KBr)/cm⁻¹ 3092, 2922, 1580, 1509, 1472, 1460, 1355, 1215, 1159, 1107, 1107, 985, 860, 838, 760, 668, 631 and 555; δ_H (600 MHz; CD₂Cl₂; Me₄Si) 8.38 (1 H, s, CHN), 4.50 (1 H, m, 3-H), 4.50 (5 H, s, Cp), 4.45 (1 H, t, ³J = 2.9 Hz, 4-H) and 4.32 (1 H, dd, ³J = 2.9 Hz, ⁴J = 1.4 Hz, 5-H); δ_F (564 MHz; CD₂Cl₂, CCl₃F) −144.4, −147.8 (each 1 F, each br, o-C₆F₅), −158.4 (1 F, t, ³J_FF = 21.4 Hz, p-C₆F₅), −162.7 and −163.8 (each 1 F, each br, m-C₆F₅); δ_C (151 MHz; CD₂Cl₂; Me₄Si) 180.5 (CHN), 125.3 (C-1), 71.4 (Cp), 69.8 (C-4), 66.2 (C-2), 66.0 (C-5) and 62.2 (C-3), n.o. (C₆F₅).

Preparation of the zirconium complex (p-S,p-S)-8a

The alcohol (p-S)-3a (156 mg, 0.401 mmol) and zirconium tetrachloride (46.7 mg, 0.201 mmol) were dissolved in dichloromethane (5 ml), instantly giving a deep blue solution. The reaction mixture was stirred overnight and the solvent was removed, giving complex (p-S,p-S)-8a (187 mg, 92.0%) as a deep blue solid. Mp >300 °C (DSC); [α]²⁰ +665 (436 nm, c 0.020 in CH₂Cl₂); (Found: C, 54.36; H, 5.31; N, 2.87. C₄₆H₅₄Cl₄Fe₂N₂O₂Zr requires C, 54.61; H, 5.38; N, 2.77%); ν_max(KBr)/cm⁻¹ 3144, 2962, 2924, 1621, 1478, 1340, 790, 757 and 733; δ_H (600 MHz; CD₂Cl₂; Me₄Si; 218 K) 12.05 (1 H, d, ³J = 15.8 Hz, NH), 8.30 (1 H, d, ³J = 15.8 Hz, CHN), 7.44 (1 H, t, ³J = 7.8 Hz, p-aryl), 7.28 (1 H, d, ³J = 7.8 Hz, m-aryl^B), 7.22 (1 H, d, ³J = 7.8 Hz, m-aryl^A), 5.67 (1 H, br d, ³J = 2.8 Hz, 5-H), 4.77 (1 H, t, ³J = 2.8 Hz, 4-H), 4.58 (5 H, s, Cp), 4.33 (1 H, br, 3-H), 3.36 (1 H, sept, ³J = 6.8 Hz, CH^A), 3.00 (1 H, sept, ³J = 6.8 Hz, CH^B), 1.34, 1.19 (each 3 H, each d, ³J = 6.8 Hz, 2 × CH₃^A), 1.10 and 1.07 (each 3H, each d, ³J = 6.8 Hz, 2 × CH₃^B); δ_C (151 MHz; CD₂Cl₂; Me₄Si; 218 K) 173.4 (CHN), 144.3 (o-aryl^A), 143.9 (o-aryl^B), 133.5 (i-aryl), 130.3 (C-1), 130.0 (p-aryl), 124.1 (m-aryl^A), 124.0 (m-aryl^B), 72.7 (C-4), 71.5 (Cp), 67.7 (C-5), 65.7 (C-3), 58.7 (C-2), 28.1 (CH^B), 28.0 (CH^A), 23.9 (CH₃^A), 23.4 (CH₃^B′), 23.3 (CH₃^A′) and 22.8 (CH₃^B).

Preparation of zirconium complex (p-S,p-S)-8b

A solution of alcohol (p-S)-3b (95.6 mg, 0.275 mmol) in dichloromethane (8 ml) was slowly added to a suspension of zirconium tetrachloride (32.1 mg, 0.138 mmol) in dichloromethane (10 ml), instantly giving a deep blue solution. The reaction mixture was stirred for three days and the solvent was evaporated to a volume of 5 ml. Addition of pentane (20 ml) yielded a blue precipitate, which was collected by filtration and dried in vacuo. Complex (p-S,p-S)-8b was obtained as a blue solid (95.0 mg, 74.2%). Mp (decomp.) 229 °C (DSC); [α]²⁰ +661 (436 nm, c 0.021 in CH₂Cl₂); (Found: C, 51.55; H, 4.66; N, 3.15. C₄₀H₄₂Cl₄Fe₂N₂O₂Zr requires C, 51.80; H, 4.56; N, 3.02%); ν_max(KBr)/cm⁻¹ 3147, 3099, 2920, 1635, 1482, 1456, 1385, 1338, 1218, 1153, 1135, 1107, 1022, 862, 831, 776, 727, 654, 581 and 528; δ_H (600 MHz; CD₂Cl₂; Me₄Si) 11.91 (1 H, d, ³J = 15.4 Hz, NH), 8.29 (1 H, d, ³J = 15.4 Hz, CHN), 7.00 (2 H, s, m-Mes), 5.73, 4.30 (each 1 H, each br, 3,5-H), 4.79 (1 H, t, ³J = 2.8 Hz, 4-H), 4.61 (5 H, s, Cp), 2.40 (6 H, s, o-CH₃) and 2.33 (3 H, s, p-CH₃); δ_C (151 MHz; CD₂Cl₂; Me₄Si) 173.3 (CHN), 140.0 (p-Mes), 134.9 (i-Mes), 133.5 (o-Mes), 131.7 (C-1), 130.0 (m-Mes), 73.2 (C-4), 72.2 (Cp), 68.5 (br), 65.9 (C-3,5), 59.5 (C-2), 21.1 (p-CH₃) and 18.9 (o-CH₃).

Preparation of zirconium complex (p-S,p-S)-8c

A solution of alcohol (p-S)-3c (318 mg, 0.804 mmol) in dichloromethane (30 ml) was slowly added to a suspension of zirconium tetrachloride (93.7 mg, 0.402 mmol) in dichloromethane (30 ml), instantly giving a deep blue reaction mixture. The suspension was stirred for three days and the product was isolated by filtration, followed by drying in vacuo. Complex (p-S,p-S)-8c was obtained as a blue solid (279 mg, 67.8%). Mp 173 °C (DSC); [α]²⁰ +1941 (436 nm, c 0.020 in CH₂Cl₂); (Found: C, 39.93; H, 2.05; N, 2.80. C₃₄H₂₀Cl₄F₁₀Fe₂N₂O₂Zr requires C, 39.91; H, 1.97; N, 2.74%); ν_max(KBr)/cm⁻¹ 3157, 3114, 1611, 1525, 1483, 1347, 1319, 1220, 1009, 984, 782 and 752; δ_H (500 MHz; C₇D₈/CD₂Cl₂; Me₄Si) 11.32 (1 H, d, ³J = 14.3 Hz, NH), 8.52 (1 H, d, ³J = 14.3 Hz, CHN), 6.12, 4.21 (each 1 H, each br, 3,5-H), 5.17 (1 H, t, ³J = 2.9 Hz, 4-H) and 4.76 (5 H, s, Cp); δ_F (470 MHz; CD₂Cl₂/C₇D₈; CCl₃F) −147.2 (2 F, br, o-C₆F₅), −153.4 (1 F, br, p-C₆F₅) and −160.3 (2 F, br, m-C₆F₅). [Due to the low solubility of complex (p-S,p-S)-8c no further NMR characterization was possible].

Preparation of hafnium complex (p-S,p-S)-9a

The alcohol (p-S)-3a (200 mg, 0.514 mmol) and hafnium tetrachloride (82.3 mg, 0.257 mmol) were dissolved in dichloromethane (5 ml), instantly giving a deep blue solution. The reaction mixture was stirred for two days and the solvent was removed, giving 250 mg (88.5%) of complex (p-S,p-S)-9a as a deep blue solid. Mp > 300 °C (DSC); [α]²⁰ +659 (436 nm, c 0.020 in CH₂Cl₂); (Found: C, 49.53; H, 5.00; N, 2.47. C₄₆H₅₄Cl₄Fe₂N₂O₂Hf requires C, 50.28; H, 4.95; N, 2.55%); ν_max(KBr)/cm⁻¹ 3135, 3102, 2963, 2928, 2868, 1621, 1586, 1485, 1340, 1216, 1022, 832, 790, 757, 733 and 654; δ_H (600 MHz; CD₂Cl₂; Me₄Si; 228 K) 12.07 (1 H, d, ³J = 15.3 Hz, NH), 8.30 (1 H, d, ³J = 15.3 Hz, CHN), 7.44 (1 H, t, ³J = 7.8 Hz, p-aryl), 7.29 (1 H, d, ³J = 7.8 Hz, m-aryl^A), 7.23 (1 H, d, ³J = 7.8 Hz, m-aryl^B), 5.63 (1 H, br, 5-H), 4.77 (1 H, br, 4-H), 4.57 (5 H, s, Cp), 4.31 (1 H, br, 3-H), 3.37 (1 H, sept, ³J = 6.5 Hz, CH^A), 3.01 (1 H, sept, ³J = 6.5 Hz, CH^B), 1.36, 1.22 (each 3 H, each d, ³J = 6.5 Hz, 2 × CH₃^A), 1.11 and 1.09 (each 3 H, each d, ³J = 6.5 Hz, 2 × CH₃^B); δ_C (151 MHz; CD₂Cl₂; Me₄Si; 228 K) 173.6 (CHN), 144.4 (o-aryl^A), 143.9 (o-aryl^B), 133.6 (i-aryl), 131.2 (C-1), 130.0 (p-aryl), 124.1, 124.0 (m- aryl^A,B), 72.6 (C-4), 71.5 (Cp), 67.3 (C-5), 65.4 (C-3), 58.9 (C-2), 28.2 (CH^B), 28.1 (CH^A), 24.0 (CH₃^A), 23.44 (CH₃^B′), 23.37 (CH₃^A′) and 22.9 (CH₃^B).

Acknowledgements

Financial support from the Deutsche Forschungsgemeinschaft and the Fonds der Chemischen Industrie is gratefully acknowledged. We thank the BASF for a gift of solvents. We thank Prof. B. Rieger for helping us with the PE molecular weight determination.

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available: Details of NMR spectroscopic characterizations, kinetic NMR experiments, and polymerisation reactions. CCDC reference numbers 714224–714227. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/b822741a

‡ X-Ray crystal structure analyses.

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