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Synthesis and Characterization of Group 4 Permethylpentalene Dichloride Complexes Robert T. Cooper,† F. Mark Chadwick,† Andrew E. Ashley,‡ and Dermot O’Hare*,† †

Chemistry Research Laboratory, Oxford University, Mansfield Road, Oxford, OX1 3TA Department of Chemistry, Imperial College London, South Kensington, London, SW7 2AZ

‡

S Supporting Information *

ABSTRACT: We report the high-yield syntheses of the (η8Pn*)MCl2 complexes (η8-Pn* = C8Me6; M = Ti, Zr, Hf), utilizing the cis-(SnMe3)2Pn* or cis-Li2Pn*(TMEDA)x synthons. Stereoselective synthesis of the appropriate Pn* precursor [cis-Li2Pn* and cis-(SnMe3)2Pn*] can be achieved by the appropriate choice of solvent. Treatment of cis(SnMe3)2Pn* with TiCl4·2THF enabled the first high-yielding synthesis of a group 4 η8-Pn* complex, [η8-Pn*TiCl(μ-Cl)]2 (1). Analogous complexes for the remaining group 4 congeners have been synthesized via the reaction of Li2Pn*(TMEDA)x with MCl4·2THF (M = Zr, Hf). These have been found to incorporate LiCl, resulting in the chemical formula [η8Pn*M(μ-Cl)3/2]2(μ-Cl)2Li·THFx [M = Zr (2), Hf (3)]. 1, 2, and 3 have all been fully characterized in the solution phase and in the solid state via single-crystal X-ray diffraction, revealing all three to be dimeric; 3 is the first structurally elucidated hafnium pentalene compound. The solid-state structure of trans-(SnMe3)2Pn* has also been reported. All three compounds have been produced in sufficiently high yields to act as excellent entry points to further group 4 η8-Pn* species via salt metathesis reactions.

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INTRODUCTION The role of carbocyclic ligands in the development of novel organometallic complexes has been colossal, most notably utilizing cyclopentadienyl (Cp) and its substituted analogues, which are now ubiquitous in the field.1 Included in this area are group 4 metallocenes, which have displayed a variety of interesting chemical and physical properties that have engrossed theoretical and practical chemists since their discovery over half a century ago. “Titanocene” (Cp2Ti)2 and its “bent” dichloride analogue (Cp2TiCl2)3 marked the discovery of one of the most extensively utilized organotransition metal complex systems. Incorporation of additional neutral and/or anionic ligands has led to the isolation of a vast range of derivatives, leading to a more general definition for group 4 bent metallocenes as d0 pseudo-tetrahedral organometallic compounds in which the metal atom bears two η5cyclopentadienyl-type ligands and two σ-ligands.4 These metallocene complexes have many important and varied applications, ranging from stoichiometric organic reactions to catalytic reactions in the field of polymerization and fine chemical synthesis.5−8 Pentalene (Pn = C8H6) is an unsaturated hydrocarbon that is related to Cp by the edge-sharing ring-fusion of two cyclopentadienyl fragments.9 One of the many attractions of Pn is its flexibility, offering a wide variety of multihaptic coordination modes, which can stabilize mono-, bi-, and trimetallic species.10 Arguably the most striking bonding mode available to pentalene is η8-Pn, where all eight carbons coordinate to a single metal center, resulting in strong folding © 2013 American Chemical Society

of the ligand about the bridgehead bond. This is found to occur for complexes in which bonding interactions are significantly increased, as folding of the carbocyclic framework results in distortion of the π-system and a loss of aromaticity. This binding mode is therefore generally seen with relatively large metal centers: found for early transition metals and felements.10−19 When coordinated in this manner, a series of group 4 “pentalene bent metallocenes” can be envisaged whereby the two Cp rings are replaced with a single pentalene ligand. Using the Green et al. electron counting system, this results in reduction of the formal electron count of the metal complex by 2; Pn* possesses eight electrons compared to 10 for two Cp ligands (Figure 1).20 Due to the wealth of metallocene chemistry, the relatively electron deficient pentalene analogue should exhibit different properties and also act as an entry point to further group 4 permethylpentalene chemistry. However, whereas a plethora of group 4 bent metallocenes are known, examples of analogous pentalene compounds are few and far between.19 The reason for this is that, despite their alluring properties, preparation of substituted pentalene derivatives has proved difficult, relying on existing protocols for the production of dihydropentalene, which themselves suffer from low yields, expense, and the requirement of specialist equipment.10,21 However, in 2005 Ashley et al. reported the relatively facile synthesis of the permethylated pentalene synthons Li2Pn*Received: February 6, 2013 Published: March 19, 2013 2228

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solution with a fine precipitate of LiCl and was stirred for a further 16 h. The suspension was filtered through Celite and extracted with benzene (3 × 40 mL) until colorless. Volatiles were removed in vacuo, and the residue was extracted with toluene and reduced to minimum volume. The saturated solution was cooled to −35 °C, which yielded yellow crystals judged by 1H NMR to purely contain cis-(SnMe3)2Pn*. Yield: 97.2% (2.36 g, 4.592 mmol). Trans: This was the same as the above synthesis except the reaction solvent benzene was replaced with THF and the product was extracted with and crystallized from pentane. Yield: 98.0% (2.38 g, 4.631 mmol). Synthesis of [η8-Pn*TiCl(μ-Cl)]2 (1). cis-(SnMe3)2Pn* was generated in situ using Me3SnCl (6.48 g, 32.53 mmol) and Li2Pn*(TMEDA)x (x = 0.099, 3.46 g, 16.35 mmol) in an identical procedure to the one stated above. A toluene solution of the aforementioned was cooled to −78 °C, then transferred to TiCl4·2THF (5.46 g, 16.35 mmol) in toluene (20 mL) at −78 °C. The reaction mixture was slowly warmed to room temperature and stirred for a further 16 h, after which it was filtered through Celite and eluted with toluene (3 × 100 mL). The solvent was removed under reduced pressure, and the residue subjected to dynamic vacuum (10−3 mbar) for 24 h. The residue was washed with ice cold (0 °C) pentane (3 × 30 mL), then extracted with hot (90 °C) toluene, filtered via cannula, and cooled to −35 °C, yielding a red-brown crystalline solid. Yield: 74.4% (3.71 g, 12.16 mmol). Anal. Found (calcd) for C28H36Cl4Ti2: C 54.98 (55.12), H 5.90 (5.95). 1H NMR (C6D6, 300 MHz): δ 1.62 (s, 12H, CH3), 1.89 (s, 24H, CH3). 13C{1H} NMR (C6D6, 75 MHz): δ 10.8 (CH3), 12.8 (CH3), 126.7 (ring-C), 134.9 (ring-C), 144.2 (ring-C). IR (KBr, cm−1): 2962 (m), 2913 (m), 2856 (m), 1645 (m), 1445 (s), 1377 (s), 1017 (s), 791 (m), 695 (m). HRMS (EI): m/z calcd (found) for C14H18Cl2Ti (1/2M), 304.0265, (304.0263). Synthesis of [η8-Pn*Zr(μ-Cl)3/2]2(μ-Cl)2Li·THFx (2). A suspension of ZrCl4·2THF (7.12 g, 18.87 mmol) in benzene (50 mL) was transferred by cannula onto Li2Pn*(TMEDA)x (x = 0.101, 4.00 g, 18.87 mmol) in benzene (20 mL) at room temperature and allowed to stir for 24 h, by which time the solution had turned dark brown and a precipitate of LiCl had formed. The precipitate was allowed to settle, the supernatant removed via cannula, and the remainder extracted with benzene (3 × 50 mL). Volatiles were removed in vacuo, and the residue was extracted with THF to give an intensely brown solution, which was cooled to −35 °C. A golden-yellow crystalline solid was deposited and shown by 1H NMR and elemental analysis to be [Pn*Zr(μ-Cl)3/2]2(μ-Cl)2Li·THFx (x = 1.72). Yield = 50% (3.897 g, 4.72 mmol). Anal. Found (calcd) for (C28H36Zr2Cl5Li·THF1.72): C 48.49 (48.52), H 5.83 (5.81). 1H NMR (C6D6, 300 MHz): δ 1.23 (t, THF β-CH2), 1.86 (s, 12H, CH3), 2.12 (s, 12H, CH3), 2.18 (s, 12H, CH3), 3.55 (t, THF α-CH2). 13C{1H} NMR (C6D6, 75 MHz): δ 10.7 (CH3), 13.0 (CH3), 13.4 (CH3), 25.2 (β-CH2), 68.2 (α-CH2), 110.5 (br, ring-C), 112.0 (br, ring-C), 124.0 (ring-C), 125.7 (ring-C), 132.2 (ring-C). 7Li NMR (C6D6, 194 MHz): δ 1.1 (s). IR (KBr, cm−1): 2908 (m), 2726 (m), 1440 (m), 1379 (s), 1379 (s), 1224 (s), 1099 (s), 1049 (m), 894 (m), 797 (s), 683 (s). HRMS (EI): m/z calcd (found) for C14H18Cl2Zr, 345.9833, (345.9834). Synthesis of [η8-Pn*Hf(μ-Cl)3/2]2(μ-Cl)2Li·THFx (3). Li2Pn*(TMEDA)x (x = 0.080, 1.35 g, 6.47 mmol) in benzene (30 mL) was slurried into HfCl4·2THF (3.00 g 6.47 mmol) in benzene (30 mL) and allowed to stir for 24 h at 70 °C, during which time the brown solution darkened considerably and a LiCl precipitate had formed. The mixture was cooled to room temperature, and stirring was ceased to allow the precipitate to settle. The mother liquor was filtered through Celite, and the remaining product extracted with benzene (3 × 50 mL). Volatiles were removed in vacuo, and the residue was extracted with THF, which was filtered via cannula to give an orange-yellow solution. Cooling to −35 °C yielded an orange microcrystalline solid, which was demonstrated by 1H NMR and elemental analysis to be [Pn*Hf(μ-Cl)3/2]2(μ-Cl)2Li·THFx (x = 1.01). Yield: 76.2% (2.43 g, 2.46 mmol). Anal. Found (calcd) for (C28H36Hf2Cl5Li·THF1.01): C 39.05 (38.96), H 4.55 (4.50). 1H NMR (C6D6, 300 MHz): δ 1.40 (t, THF β-CH2), 1.90 (s, 12H, CH3), 2.26 (s, 24H, CH3), 3.59 (t, THF αCH2). 13C{1H} NMR (C6D6, 75 MHz): δ 10.5 (CH3), 12.9 (CH3),

Figure 1. Comparison of valence electron (VE) counts in Pn*MCl2 and Cp*2MCl2 complexes (M = group 4 metal).

(TMEDA)x and (SnMe3)2Pn* (Pn* = C8Me6).22,23 This ligand was swiftly used to synthesize a series of first-row transition metal “double metallocenes” (M2Pn*2; M = V, Cr, Mn, Co, Ni),24 which show structural, magnetic, and electrochemical behavior that is markedly different than that found for their mononuclear cyclopentadienyl analogues. In addition to this, Pn* has demonstrated the intriguing η8-binding mode in two fmetal compounds, MPn*2 (M = Ce, U).25,26 In this paper, we report the improved stereoselective synthesis of the permethylpentalene synthon [(SnMe3)2Pn*] and the synthesis and characterization of a series of group 4 η8-permethylpentalene dichloride compounds.

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EXPERIMENTAL DETAILS

General Considerations. The reactions described below were conducted under a nitrogen atmosphere using standard Schlenk techniques on a dual-vacuum-inlet gas manifold or MBraun UNIlab glovebox. Each solvent was purified either by reflux over sodium benzophenone diketyl (THF) or sodium−potassium amalgam (Et2O) or passage through activated alumina (pentane, toluene, benzene) using an MBraun SPS-800 solvent system. All of the solvents were thoroughly degassed by passage of a stream of inert gas and were stored over potassium mirrors (Et2O, pentane, toluene, benzene) or 3 Å molecular sieves (THF). Toluene-d8 and benzene-d6 were freeze− pump−thaw degassed, dried by reflux over molten K, and purified by trap-to-trap distillation. The following instrumentation was used: Varian Mercury VX-Works 300 MHz spectrometer for 1H, 13C, and 7 Li spectroscopy (recorded at 298 K unless otherwise stated); Nicolet iS5 ThermoScientific IR spectrometer for IR spectroscopy; and Waters GCT with an EI source for mass spectrometry. Elemental microanalyses were conducted by Stephen Boyer of London Metropolitan University. Crystallography. X-ray crystallographic data were collected on an Enraf-Nonius FR590 Kappa CCD diffractometer, using graphitemonochromated Mo Kα radiation (λ = 0.71073 Å). A typical crystal was mounted on MiTeGen MicroMounts using perfluoropolyether oil and cooled rapidly to 150 K in a stream of N2 using an Oxford Cryosystems Cryostream unit.27 Raw frame data were reduced using the DENZO-SMN package.28 Intensity data were corrected using the multiscan method with SCALEPACK (within DENZO-SMN). The structures were solved using direct methods with SIR92 and refined using the full-matrix least-squares refinement on all F2 data using the CRYSTALS program suite.29−31 Crystallographic data are provided in the Supporting Information and have also been deposited in the CCDC database (925711−925714). HfCl4·2THF was purchased from Sigma-Aldrich and used as received; TiCl4·2THF,32 ZrCl4·2THF,32 and Li2Pn*(TMEDA)x23 were prepared according to literature procedures. Stereoselective Synthesis of cis-/trans-(SnMe3)2(μ:η1:η1-Pn*). Cis: Two equivalents of Me3SnCl (1.89 g, 9.449 mmol) in benzene (40 mL) was added to a slurry of Li2Pn*(TMEDA)x (x = 0.099, 1.00 g, 4.724 mmol) in benzene (20 mL) while stirring at room temperature. The light tan suspension instantly transformed to a dark brown 2229

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25.4 (β-CH2), 68.2 (α-CH2), 107.92 (br, ring-C), 120.80 (br, ring-C), 130.50 (br, ring-C). 7Li NMR (C6D6, 194 MHz): δ 5.3 (s). IR (KBr, cm−1): 2969 (m), 1738 (s), 1447 (s), 1365 (s), 1273 (s), 1216 (s), 1093 (m), 1020 (m), 804 (s), 749 (s). HRMS (EI): m/z calcd (found) for C14H18Cl2Hf, 436.0251, (436.0246).

insufficiently solvated for the lithium ions to detach and exchange faces to form the thermodynamic trans product, and as such, the cis stereochemistry is transferred to the cis(SnMe3)2Pn* product (v). Conversely, use of a donor solvent, such as THF, provides sufficient solvation to facilitate isomerization to the thermodynamically favored trans-Li2Pn*(TMEDA) x (vi), and thus subsequent transmetalation exclusively forms trans-(SnMe3)2Pn*. Implementation of this reaction scheme has allowed the selective formation of both the cis and trans isomers of (SnMe3)2Pn* in quantitative yields. Furthermore, it has permitted the isolation of single crystals suitable for X-ray diffraction of trans-(SnMe3)2Pn*, a compound that has thus far eluded complete characterization (Figure 2).

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RESULTS AND DISCUSSION Stereoselective Synthesis of cis- and trans(SnMe3)2Pn*. We found that high-yielding syntheses of η8Pn* complexes necessitate the use of cis-(SnMe3)2Pn* or cisLi2Pn*(TMEDA)x synthons. Stereoselective synthesis of the required Pn* synthon [cis-Li2Pn* and cis-(SnMe3)2Pn*] can be achieved by the appropriate choice of reaction solvent. The use of nonpolar solvents (e.g., hexane or benzene) in the preparation of (SnMe3)2Pn* from Li2Pn*(TMEDA)x solely produces the cis isomer. Conversely, use of a polar solvent in a parallel synthesis selectively produces the trans isomer from what is believed to be the cis-Li2Pn*(TMEDA)x starting material (Scheme 1). The proposed mechanism for this Scheme 1. Proposed Mechanism for Isomeric Control of Pn* Synthons

Figure 2. Molecular structure of trans-(SnMe3)2(μ:η1:η1-Pn*). Ellipsoids are shown at 50% probability. Hydrogen atoms are omitted for clarity; * denotes atoms generated by inversion symmetry.

The solid-state molecular structure of trans-(SnMe3)2Pn* is of Ci symmetry and contains a planar Pn* ring system with localized double bonds. This differs from cis-(SnMe3)2Pn*, which possesses an angle of 7.9° between the best planes of the two C5 rings as a result of steric repulsion between the two large SnMe3 groups. The C(1)−Sn(1) bond length [2.220(4) Å] is comparable with other C−Sn transfer reagents: cis(SnMe3)2Pn* [2.215(2) Å],23 trans-(SnMe3)2Pn [2.209(7) Å],35 SnMe3(η1-Flu*) [2.221(3) Å],36 and SnMe3(η1-Ind*) [2.232(6) Å].37 Synthesis and Characterization of [η8-Pn*TiCl(μ-Cl)]2 (1). The reducing power of the formal Pn* dianion in Li2Pn* has proved problematic in the synthesis of the target compound; the direct combination of the dilithiate with TiIV precursors led to substantial decomposition of the ligand through oxidation and formation of inorganic TiIII salts. Reaction of TiCl3·3THF with Li2Pn*(TMEDA)x and subsequent in situ oxidation using PbCl238 did form [η8Pn*TiCl(μ-Cl)]2 (1); however, this process suffered from extremely poor yields (8%) and was unreliable. Use of a softer transmetalation reagent was investigated, and following the selective formation of the precursor cis-(SnMe3)2Pn* via solvent selection (vide supra), the subsequent reaction with

selectivity begins with the elegant formation of LiPn*H from the conjugated fulvene and LS-Selectride (i), the first example of regiospecific conjugate hydride delivery upon a pure hydrocarbon to produce a cyclopentadienyl anionic species.23 This occurs on the least sterically hindered face of the bicyclic ring system due to the bulky R groups employed on the trialkylborohydride reagent. The monolithiate ii precipitates out under the reaction conditions employed, and as a result, the lithium ion is fixed anti to the methyl group on the adjacent ring. The next step involves formation of an nBuLi·TMEDA complex that coordinates to ii, followed by proton abstraction as shown in iii. The second lithium ion therefore necessarily coordinates to the same face due to the prearranged formation of complex iii; hence chelation ensures that cis-Li2Pn*(TMEDA)x, (iv) forms, an effect that can also be seen in the ortho-lithiation of N,N-dimethylaniline.33 Calculations performed on the unsubstituted Pn ligand for the complex Li2Pn(DME)2 suggest that the trans isomer is more stable than the cis by 89.1 kJ mol−1.34 Since iv precipitates from the reaction medium, the lithium ions are insufficiently soluble to exchange faces, and accordingly the kinetic cis product is “trapped out”. On transmetalation with Me3SnCl, use of a nonpolar solvent ensures that the tight ion pair present is 2230

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TiCl4·2THF proved highly successful in synthesizing the target compound [η8-Pn*TiCl(μ-Cl)]2 (1) as a red solid, in a markedly improved yield (74%).

Table 1. Selected Interatomic Distances (Å) and Angles (deg) for 1−3 M−Pn*(centroid)a M−(μ-Cl)a M−(μ-Cl)Lia M−M fold anglea a

1

2

3

1.955(2) 2.485(1)

2.109(2) 2.649(1) 2.581(1) 3.6459(6) 30.0(4)

2.085(2) 2.623(1) 2.554(1) 3.6301(4) 30.2(4)

3.806(8) 32.3(3)

Average value.

broadening nor shifting of the 1H signals between −100 and 80 °C, indicating that this fluxional process is rapid and unperturbed over a temperature range of nearly 200 K in the solution phase. Another explanation may be cleavage of 1 to form the 14-electron monomer in solution; however the addition of THF to a solution of 1 showed only signals attributed to free donor, while the rest of the spectrum remained unchanged. It would be expected for the electrondeficient monomer to coordinate such solvent, implying the existence of the dimer in solution. The 13C NMR spectrum displays five signals, which is indicative of two orthogonal mirror planes bisecting the Pn* ligand. Early investigations into this species’ reactivity, combined with its high-yielding synthesis, have shown it to be an excellent entry point into Ti-Pn* chemistry. Synthesis and Characterization of [η 8 -Pn*M(μCl)3/2]2(μ-Cl)2Li·THFx [M = Zr (2), Hf (3)]. Having synthesized the first group 4 permethylpentalene dichloride compound, it seemed appropriate to target the remaining group 4 congeners. Initial attempts focused on a parallel synthetic pathway using the cis-(SnMe3)2Pn* synthon with MCl4·2THF (M = Zr, Hf); however all attempts failed to produce the desired products. It was noted that the reducing power of Li2Pn*(TMEDA)x should prove less problematic with the second- and third-row metals, due to their inherent lower susceptibility to reduction. Indeed, it was found that the simple salt metathesis reaction between Li2Pn*(TMEDA)x and ZrCl4·2THF led to the formation of a yellow solid in 50% yield, with 1H NMR spectroscopy revealing the presence of coordinated THF. On cooling a saturated THF solution to −35 °C, yellow crystals suitable for single-crystal X-ray diffraction were obtained, which identified the compound as [η 8 -Pn*Zr(μ-Cl) 3/2 ] 2 (μCl)2Li·THFx (x = 2) (2, Figure 4). The hafnium analogue (3) was synthesized via a parallel route using HfCl4·2THF in 76% yield, with the additional requirement of heating for the reaction to reach completion. 3 is the first hafnium pentalene compound to be characterized via single-crystal X-ray diffraction and is isomorphic with 2, incorporating LiCl to form a dimeric “ate” complex. This is a common occurrence when using lithium salts in metathesis reactions with the larger second- and third-row transition elements. Similar to 1, the metal centers have alleviated electron deficiency through dimerization and incorporation of LiCl, from a 14-electron monomer (Pn*MCl2) to the 18-electron dimer (2/3). Attempts to remove the coordinated THF and liberate the Zr and Hf analogues of 1 from LiCl under high vacuum (130 °C/10−9 mbar) led to decomposition of 2 and 3; it appears that the endogenous LiCl is essential to the formation and stability of 2 and 3. The metal centers have distorted pseudo-octahedral coordination geometry, with Cl(2) and Cl(4/5) adopting the

Figure 3. Molecular structure of one of the asymmetric units of [η8Pn*TiCl(μ-Cl)]2. Ellipsoids are shown at 50% probability. Hydrogen atoms and labels of second Pn* ligand are omitted for clarity.

Within the asymmetric unit there are three independent molecules; two of these are half-molecules of the constituent dimers possessing inversion symmetry and are geometrically closely related, whereas the other is an asymmetric whole dimer. Through dimerization the Ti center has thus alleviated electron deficiency from a 14-electron monomer to 16-electron 1. For the symmetric species the fold angles (defined as the angle by which the ligand is found to deviate from planarity)10 are 32.0° and 32.5°; these values compare well with that reported for (η8-Pn)Ti(Cp)Cl (33.0°).19 The fold angles in the asymmetric unit are somewhat less at 29.1° and 29.4°, a difference attributed to crystal-packing forces within the solid state. The Ti−Cl(2) distance [2.3595(10) Å] is in excellent agreement with those reported for the TiIV organometallics Cp 2 TiCl 2 [2.367(2) and 2.361(1) Å] 39 and Cp*2 TiCl 2 [2.352(1) and 2.346(1) Å],40 but rather shorter than in (η8Pn)Ti(Cp)Cl [2.512(1) Å].19 The latter example demonstrates the steric encumbrance around the relatively small metal atom and is a better comparison to the average Ti−(μ-Cl) distance (2.485 Å), which as expected is longer than the Ti−Cl(2) distance. The average Ti−Pn*(centroid) distance of 1 (1.955 Å, Table 1) is considerably less than those for Cp2TiCl2 (2.058 Å) and Cp*2TiCl2 (2.127 Å), which can be attributed to the lower steric demand of Pn* versus two Cp/Cp* moieties. The room-temperature 1H NMR spectrum reveals two sharp signals in a 2:1 intensity ratio, and one may conclude that the solution-phase structure of the molecule (C2v) is in contrast to that observed in the solid state (C2h). This implies that upon dissolution of 1 either the Pn* unit rotates about an axis through the Ti-bridgehead carbon bond or the terminal and bridging Cl ligands exchange positions on this time scale. Variable-temperature 1H NMR experiments showed neither a 2231

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The room-temperature 1H NMR spectrum of 2 displays five signals: two triplets attributed to coordinated THF and three sharp singlets ascribed to methyl protons. The Pn* signals are split into three sets of equal intensity (1:1:1), due to the reduced symmetry of the molecule, which results in two pairs of independent NWT-Me groups (non-wingtip; C1, C3, C5, C7, δ = 2.12 and 2.18 ppm) and one pair of WT-Me protons (wingtip; C2, C6, δ = 1.86 ppm). This is in contrast to 3, which exhibits only four resonances, again two triplets for coordinated THF; however all NWT-Me groups are equivalent, resulting in a single resonance in a 2:1 intensity ratio with WT-Me protons (δ = 2.26 and 1.90 ppm, respectively), as seen for 1. The 13C NMR spectra are similarly different from 3, which possesses five Pn* signals (indicative of two orthogonal mirror planes bisecting the Pn* ligand), while 2 possesses eight, the result of reduced symmetry in the molecule. An additional two signals for coordinated THF are present for both 2 and 3. It was found that coalescence of the independent NWT-Me signals could be achieved on heating an NMR sample of 2, resulting in a single resonance (δ = 2.11 ppm) at temperatures above 315 K. The origins of this fluxional process are postulated to arise from Pn* ring-whizzing, a phenomenon readily observed in Cp and Cp* complexes, with rotation along the metal−bridgehead bond axis, rendering all NWT-Me groups equivalent on the NMR time scale. Attempts were made to freeze out this process in 1 and 3, but cooling respective samples to −80 °C failed to produce a decoalescence of the NWT-Me signal, demonstrating the fluxional process to be energetically more favorable and facile in 1 and 3 compared with 2. The coordinated THF is partially removed under vacuum, and as such, accurate integration over 1H NMR signals in combination with elemental analysis is required to obtain an exact formula for any given sample of [Pn*M(μ-Cl)3/2]2(μ-Cl)2Li·THFx (M = Zr, Hf); this is pertinent for using 2 and 3 in the correct stoichiometric ratio in further reactions.

Figure 4. Molecular structure of [η 8 -Pn*Zr(μ-Cl) 3/2 ] 2 (μCl)2Li·2THF, 2. Ellipsoids shown at 50% probability. Second Pn* ring and THF labels and hydrogen atoms are omitted for clarity.

axial positions, while Cl(1), Cl(3), and the two Pn*(centroid) occupy the equatorial sites. This is structurally analogous to the previously characterized (η8-Pn)ZrCl2·2THF.19 The average M−(μ-Cl)M distances for 2 and 3 are 2.649 and 2.623 Å, respectively (Table 1), and the former is in good agreement with other examples containing the Zr2(μ-Cl)3 configuration in the CSD (2.536−2.794 Å).41−46 3 is the first example of this bonding motif for a neutral Hf species, and so comparisons are tentatively made to the Hf2Cl9− anion [the only other Hf2(μCl)3-containing species], which has on average shorter Hf−(μCl) distances (2.576−2.611 Å).45−47 The average Zr−(μ-Cl)Li distance for 2 is 2.581 Å, which is appreciably longer than that seen in the related dimeric compounds, {[η5:σ-Me2C(C9H6)(C 2 B 10 H 10 )]ZrCl(μ-Cl) 3/2 } 2 Li·2THF (2.424 Å) 42 and [Cp=ZrCl2(μ-Cl)3/2]2Li·DME (Cp= = C5Me4CH2CH2CH CH2) (2.476 Å).41 Indeed, the distance is closer to the average bond length found to the terminal chlorides in (η8-Pn)ZrCl2·2THF (2.580 Å).19 The corresponding distance in 3 (2.554 Å) is slightly shorter than that in 2 but compares reasonably well with the only two examples of Hf−(μ-Cl)Li bonds in the literature, both of which are found for monomeric species [2.524(5) and 2.507(3) Å].48,49 In general, it is found that the bond lengths in 3 are marginally shorter than those for 2 (Table 1), which is attributed to the stronger orbital overlap present for the Hf metal center with more radially extended 5d orbitals. The shorter average M−Pn*(centroid) distance (2.085 and 2.109 Å) and larger fold angle (30.2° and 30.0°) for 3 compared to 2 illustrate this. Both 2 and 3 have considerably smaller fold angles than 1, which is expected due to the larger radii of the Zr and Hf centers but also because there is an increase in electron density on the metal centers (18 vs 16 electron), which has been shown to be a factor in decreasing the fold angle in η8-Pn species.13 The M−Pn*(centroid) distance and fold angle for 2 are in excellent agreement with those of the analogous isoelectronic species, (η8-Pn)ZrCl2·2THF (2.110 Å and 30.0°, respectively).19

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CONCLUSION A series of group 4 η8-permethylpentalene metal dichlorides have been synthesized in good yields and fully characterized in both the solution and solid state. High-yielding syntheses necessitate the use of cis-Pn* synthons, which can be selectively formed via careful solvent selection. 1, 2, and 3 were revealed to exist as dimers in the solid state, with 2 and 3 isomorphic. The zirconium and hafnium compounds incorporate LiCl to form an “ate” complex, and the latter is the first example of a hafnium-containing pentalene species to be characterized via single-crystal X-ray diffraction. Due to the substitutional lability of M−Cl bonds, all three compounds offer excellent entry points to further group 4 η8-Pn* species via salt metathesis reactions, and this is the focus of current work.

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ASSOCIATED CONTENT

S Supporting Information *

X-ray crystallographic data in CIF format. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. 2232

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Organometallics

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Article

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ACKNOWLEDGMENTS We wish to acknowledge Dr. Andrew Cowley for collecting and solving the data for the single-crystal X-ray structure of 1. We would also like to thank the UK Engineering and Physical Sciences Research Council (EPSRC) for financial support.

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