Unusual Benzyl Migration Reactivity in NHC-Bearing Group 4 Metal

Article pubs.acs.org/Organometallics

Unusual Benzyl Migration Reactivity in NHC-Bearing Group 4 Metal Chelates: Synthesis, Characterization, and Mechanistic Investigations Charles Romain,† David Specklin,† Karinne Miqueu,‡ Jean-Marc Sotiropoulos,‡ Christophe Fliedel,† Stéphane Bellemin-Laponnaz,*,§ and Samuel Dagorne*,† †

Institut de chimie de Strasbourg, UMR 7177, CNRS-Université de Strasbourg, 1 rue Blaise Pascal, F-67000 Strasbourg, France Institut des Sciences Analytiques et de Physico-Chimie pour l’Environnement et les Matériaux, UMR 5254, Université de Pau et des Pays de l’Adour, Technopôle Hélioparc, 2 avenue du Président Angot, F-64053 Pau cedex 09, France § Institut de Physique et Chimie des Matériaux de Strasbourg UMR 7504, CNRS-Université de Strasbourg, 23 rue du Loess, BP 43, F-67034 Strasbourg Cedex 2, France and University of Strasbourg Institute for Advanced Study (USIAS), 5 allée du Général Rouvillois, 67083 F-Strasbourg, France ‡

S Supporting Information *

ABSTRACT: The reaction of 1 equiv of [M(CH2Ph)4] (M = Zr, Hf) and 1,3-bis(3,5-di-tert-butyl-2-hydroxyphenyl)imidazolinium chloride [tBu(OCO)H3, 1] cleanly yielded the corresponding M-NHC chloro benzyl derivatives [[tBu(OCO)]M(Cl)(CH2Ph)] (2Zr and 2Hf) along with 3 equiv of toluene. For both metal complexes, the effective formation of a (κ3-OCO) metal chelate and the coordination of a benzyl ligand onto the M(IV) metal center were established by NMR and elemental analysis. In contrast, under identical conditions, the reaction of Ti(CH2Ph)4 with the imidazolinium proligand 1 yielded the unexpected rearranged dimer product 3Ti, arising from the migration of the Ti-Bn group from the metal center to the Ccarbene atom. The molecular structure of 3Ti was established by analogy with the X-ray-determined Zr analogue 3Zr. Compound 3Zr quantitatively formed upon heating a benzene solution of 2Zr at 60 °C. In the solid state, compound 3Zr consists of two sevencoordinate mononuclear Zr fragments that are associated by two bridging μ2-chloride atoms, confirming the migration of the ZrBn moiety from the metal center to the Ccarbene atom. Carrying out the reaction of [M(CH2Ph)4] (M = Ti, Zr, Hf) with imidazolinium proligand 1 in THF led to the quantitative formation of the corresponding rearranged monomeric THF adduct [[tBu(OC(Bn)O)]M(Cl)(THF)] (4Ti-THF, 4Zr-THF, and 4Hf-THF), as established by X-ray crystallographic studies in the case of 4Ti-THF. Such a THF-promoted benzyl migration was also observed with the dibenzyl Zr and Hf complexes [[tBu(OCO)]M(CH2Ph)2] (5Zr and 5Hf), leading to the formation of the corresponding THF-rearranged products [[tBu(OC(Bn)O)]M(CH2Ph)(THF)] (6Zr-THF and 6Hf-THF). The addition of 1 equiv of methylmagnesium bromide (CH3MgBr) or phenylmagnesium bromide (PhMgBr) to 1 equiv of the zirconium dichloro NHC complex [[tBu(OCO)]Zr(Cl)2(THF)] (8) in THF yielded the rearranged products [[tBu(OC(Me)O)]M(Cl)(THF)] (9Me) and [[tBu(OC(Ph)O)]M(Cl)(THF)] (9Ph), respectively, as deduced from NMR data. Kinetic studies were carried out on the THF-promoted rearrangement reaction of the benzyl chloro Hf derivative 2Hf in the presence of THF to produce 4Hf-THF. These data are consistent with the reaction rate law being first order both in THF and in the THF adduct 2Hf-THF. DFT calculations on the Ti, Zr, and Hf systems support a benzyl migration reaction occurring at a transient heptacoordinated bis-THF adduct species of the type [[tBu(OCO)]M(Cl)(Bn)(THF)2], which may readily form upon THF coordination to 2Hf-THF.

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INTRODUCTION Over the past 20 years, N-heterocyclic carbenes (NHCs) have become an ubiquitous class of supporting ligands in coordination chemistry with countless successful applications of the derived NHC metal complexes in various areas, most notably in homogeneous catalysis.1 The exceptional σ-donating properties of NHCs along with the steric protection that may be provided by the N substituents typically results in the formation of stable and robust M−NHC bonds with a wide © XXXX American Chemical Society

array of metals, including the majority of hard Lewis acidic and oxophilic metal centers.1,2 While the formation of stable and robust organometallics certainly drives the growing use of NHC-supported metal species, some studies have demonstrated that the carbene moiety in NHC metal compounds may be quite reactive and thus be the source of unexpected/unusual Received: November 13, 2014

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DOI: 10.1021/om501143t Organometallics XXXX, XXX, XXX−XXX

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Organometallics reactions. Investigating and gaining knowledge of such reactions are of fundamental importance to better understand the factors influencing the stability of NHC-supported coordination compounds. Representative and well-identified reaction pathways involving the reaction of a metal-bonded NHC unit include (i−iii) NHC-implicating migratory insertion,3 reductive elimination,4 and cleavage of the NHC heterocyclic ring,5 (iv) the production of metal complexes supported by mesoionic NHCs,6 (v) C−C bond formation via reaction of a NHC-bearing Ni hydride species and an alkene,7 and (vi) the heterolytic cleavage of a M−CNHC bond by Me3SiCl.8 Over the past few years, we have been studying the coordination chemistry of a formally bis-phenoxide-NHC tridentate dianionic ligand (OCO2−), a platform incorporating two phenoxide units directly connected to a central NHC moiety, to various hard Lewis acidic and oxophilic metal centers (group 4 metal M(IV), Al(III), V(V), Mn(III)).9−11 Some of the derived NHC group 4 metal chelates of the type (OCO)MX2 (X = halide, alkoxide, alkyl) were successfully employed in lactide ROP catalysis and as 1-hexene oligomerization precatalysts.10,11 In addition, as part of these studies, we earlier disclosed preliminary results on an unprecedented NHC-involving rearrangement arising from the reaction of the Zr-NHC benzyl derivative 2Zr with THF (acting as a Lewis base) to yield the formation of the heptacoordinate species 4ZrTHF (Scheme 2).12,13 Remarkably, such a Lewis base assisted intramolecular nucleophilic attack of a Zr-Bn group onto the NHC Ccarbene atom clearly evidences a Fischer-type carbene reactivity (i.e., electrophilic Ccarbene center), which is unusual in NHC coordination chemistry. To probe the scope of and better understand such a reactivity, a joint experimental and theoretical (DFT) study was carried out on these systems (including kinetic studies). The extension of this chemistry to Ti and Hf analogues and, most notably, the development of a THF-free analogous rearrangement process were also studied. The present paper details the results and conclusions of those investigations along with the structural characterization of the derived coordination compounds.

Scheme 1

NMR spectrum contains a characteristic downfield signal (δ 204.3 ppm (Zr) and δ 209.1 (Hf)) assigned to the Ccarbene carbon. The presence of a CH2 benzyl group was confirmed by singlet resonances at δ 2.89 and 2.64 ppm for 2Zr and 2Hf, respectively. The 13C data are also consistent with a η2 coordination of the benzyl ligands. Indications of η 2 coordination benzyl interactions include a rather large JCH value for the M-CH2 groups (1JCH = 132 Hz for both 2Zr and 2Hf) and an upfield resonance of the Cipso benzyl carbons (δ 131.9 and 131.5 ppm, respectively).11,14 In contrast, the reaction of the Ti analogue, i.e. Ti(CH2Ph)4, with the imidazolinium proligand 1 (under the same condition as above) yielded the unexpected rearranged dimer product 3Ti. The production of the dimer 3Ti presumably results from the initial generation of unstable 2Ti (not observed) that rearranges through the migration of a benzyl group from the Ti metal center to the Ccarbene of the NHC, followed by dimerization.15 The molecular structure of the titanium dimer 3Ti was established on the basis of multinuclear 1D and 2D NMR analyses and by analogy with that of its zirconium analogue, whose structure was unambiguously established by X-ray crystallographic studies (vide infra). In the 13C NMR spectrum of 3Ti, the NCN carbon is shifted dramatically upfield (δ 111.6 ppm) and the benzylic CH2 moiety features a resonance at δ 29.6 ppm. The latter unusual reactivity observed with Ti encouraged us to further investigate the stability of the M-NHC chloro benzyl derivatives [tBu(OCO)]M(Cl)(CH2Ph) (2Zr and 2Hf). We thus found that the Zr complex 2Zr may be cleanly and quantitatively converted into the corresponding dimer 3Zr upon heating at 60 °C in benzene. No rearrangement was observed with the Hf complex, even at higher temperature (90 °C, benzene). The molecular structure of dimer 3Zr was determined by Xray crystallography and is depicted in Figure 1, along with selected bond and geometrical parameters. A summary of crystal and refinement data is included in Table S2 in the Supporting Information. The crystallographic analysis revealed two seven-coordinate mononuclear Zr fragments associated by

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RESULTS AND DISCUSSION THF-Free Rearrangement of Group 4 Benzyl Derivatives Supported by the NHC-Bearing Tridentate Dianionic Ligand tBu(OCO)2−. Previous investigations on various alkoxo or acac metal derivatives (M = Ti, Zr, V, Mn) showed that the corresponding tBu (OCO)-bearing NHC metal complexes could be readilyand often quantitatively synthesized under mild conditions (room temperature) via an alcohol (or acetylacetone) elimination route.9−12 In the case of group 4 derivatives, we extended our investigations to access the benzyl derivatives, which are typically prepared via a toluene elimination route.11,12 Thus, the reaction of 1 equiv of [M(CH2Ph)4] (M = Zr, Hf) and imidazolinium proligand 1 (toluene, −35 °C) cleanly yielded the expected M-NHC chloro benzyl derivatives [tBu(OCO)]M(Cl)(CH2Ph) (2Zr and 2Hf) along with 3 equiv of toluene, as deduced from 1H NMR analysis (Scheme 1). The NMR data are consistent with the effective formation of a κ3-OCO metal chelate and the coordination of a benzyl ligand onto the M(IV) metal center. Under the studied conditions (CD2Cl2, room temperature), both complexes exhibit a pseudo-C2v-symmetric structure in solution, as indicated by the 1H NMR singlet resonance assigned to the NCH2CH2N moiety. For both species, the 13C B

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were communicated regarding the synthesis of 4Zr-THF from 2Zr and THF.12 NMR data are consistent with the proposed formulation for all 4M-THF complexes with, in particular, the presence of an extra 13C NMR resonance for the NC(CH2Ph) carbon atom (δ 110.6, 100.8, and 107.5 ppm, respectively) and no signal in the carbenic region. The molecular structure of complex 4Ti-THF was confirmed by X-ray crystallography (Figure 2 and Table S2 in the

Figure 1. Molecular structure of the dimeric Zr complex 3Zr. The hydrogen atoms and the tBu groups are omitted for clarity. Selected bond lengths (Å) and angles (deg): Zr(1)−C(15), 2.157(3); C(15)− N(1), 1.471(4); C(15)−N(2), 1.479(4); Zr(1)−Cl(1), 2.551(1); Zr(1)−Cl(1)′, 2.686(6); Zr(1)−N(1), 2.299(2); Zr(1)−N(2), 2.307(2); Zr(1)−O(1), 2.010(2); Zr(1)−O(2), 2.024(2); Cl(1)− Zr(1)−Cl(1)′, 82.43(2); O(1)−Zr(1)−O(2), 126.41(9); N(1)− Zr(1)−N(2), 56.56(8); N(1)−C(15)−N(2), 95.4(2); Zr(1)− Cl(1)−Zr(1)′−Cl(1)′, −17.94(4); C(15)−Zr(1)−Cl(1)−Cl(1)′, −177.71(3).

Figure 2. Molecular structure of the rearranged Ti complex 4Ti-THF. The hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (deg): Ti(1)−C(15), 2.026(2); C(15)−N(2), 1.438(3); C(15)−N(1), 1.446(3); Ti(1)−N(1), 2.228(2); Ti(1)−N(2), 2.244(2); Ti(1)−O(1), 1.945(5); Ti(1)−O(2), 1.929(1); Ti(1)− O(3), 2.183(2); Ti(1)−Cl(1), 2.352(2); N(1)−C(15)−N(2), 97.82(17); O(1)−Ti(1)−O(2), 123.87(8); C(15)−Ti(1)−O(3), 154.26(8); N(1)−Ti(1)−N(2), 58.16(7); O(3)−Ti(1)−Cl(1), 87.46(5); C(15)−Ti(1)−Cl(1)−O(3), −178.99(4).

two bridging μ2-chloride atoms, resulting in a centrosymmetric dinuclear structure (Zr(1)−Cl(1) = 2.686(6) Å and Zr(1)− Cl(1′) = 2.551(1) Å). Each metal center is chelated by a κ5O,N,C,N,O-pentadentate trianionic supporting ligand. The latter consists of a central η3-N,C,N-chelating anionic unit surrounded by two aryloxide groups. The Zr−CNCN bond distance is in the expected range (2.157(3) Å).2a THF-Promoted Rearrangement of Group 4 Benzyl Derivatives Supported by the NHC-Bearing Tridentate Dianionic Ligand tBu(OCO)2−. Carrying out the reaction of [M(CH2Ph)4] (M = Ti, Zr, Hf) with imidazolinium proligand 1 in a coordinating solvent such as THF led to the quantitative formation of the corresponding rearranged THF adduct derivatives [[tBu(OC(Bn)O)]M(Cl)(THF)] (4Ti-THF, 4ZrTHF, and 4Hf-THF) (Scheme 2, top). Preliminary results

Supporting Information). It features a heptacoordinate metal center bearing the η5-O,N,C,N,O-pentadentate trianionic supporting ligand akin to complex 3Zr. The bonding parameters within 4Ti-THF are as expected, and its molecular structure is isostructural with that of the previously reported rearranged zirconium complex 4Zr-THF.12 A single isomer was observed either in the solid state or in solution, i.e. with THF coordinated trans with respect to the central η3-N,C,N-chelating anionic unit (Ti(1)−C(15) = 2.026(2) Å, Ti(1)−O(3) = 2.183(2) Å, and Ti(1)−Cl(1) = 2.352(2) Å). Alternatively, the rearranged THF adduct derivatives [tBu(OC(Bn)O)]M(Cl)(THF) (4Zr-THF and 4Hf-THF) may be directly and cleanly generated via addition of 1 equiv of THF to the M-NHC chloro benzyl derivatives [[tBu(OCO)]M(Cl)(CH2Ph)] 2Zr and 2Hf (Scheme 2, bottom). Such a THF-promoted benzyl migration was also found to occur with the dibenzyl Zr and Hf complexes 5 (Scheme 3, left), leading to the formation of the corresponding THFrearranged products [[tBu(OC(Bn)O)]M(CH2Ph)(THF)] (6Zr-THF and 6Hf-THF). The Ti complex 6Ti-THF may also be accessed from the dichloro derivative 7 via a salt metathesis reaction with 2 equiv of PhCH2MgBr (Scheme 3, right). The rearranged group 4 benzyl derivatives 6M-THF display a Cs-symmetric structure, as deduced from 1H NMR data. The 13 C spectra show the presence of resonance signatures at δ

Scheme 2

C

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In view of the electrophilic nature of the Ccarbene NHC in group 4 metal benzyl complexes [tBu(OCO)]M(Bn)(Cl), the reaction of other Grignard reagents with the Zr-NHC dichloro species 8 was investigated. Thus, addition of 1 equiv of methylmagnesium bromide (CH3MgBr) or phenylmagnesium bromide (PhMgBr) to 1 equiv of the zirconium dichloro NHC complex [tBu(OCO)]Zr(Cl)2(THF) (THF, −35 °C) only led to the formation of the rearranged products 9Me and 9Ph, respectively (Scheme 4). Their identity was confirmed by 13C

109.6, 101.9, and 109.1 ppm, assigned to the NC(CH2Ph)N carbon atom in 6Ti-THF, 6Zr-THF, and 6Hf-THF, respectively. In particular, the NMR data exhibit a singlet resonance for the CH2 benzyl group bonded to the metal center (δ 58.8, 49.0, and 53.0 ppm for 6 Ti -THF, 6 Zr -THF, and 6 Hf -THF, respectively) and rather low JCH value for the M-CH2 groups (1JCH = 120 and 114 Hz for 6ZZr-THF and 6Hf-THF, respectively). These data are consistent with η1 coordination of the benzyl ligand, reflecting the electronic saturation of the M(IV) center by the pentacoordinate ligand and the coordinated THF. As a comparison, both NHC pentacoordinate group 4 complexes 5Zr and 5Hf feature benzyl groups in a η2-bonding fashion, as previously discussed. The titanium derivative 6Ti-THF could be crystallized from benzene/ pentane, and Figure 3 displays its molecular structure along

Scheme 4

NMR analysis, showing the absence of signals in the carbenic region and the presence of extra signals at δ 99.2 and 98.2 ppm for 9Me and 9Ph, respectively. Note that it is unclear whether the formation of 9Me/9Ph results from a direct nucleophilic attack of Me/PhMgBr on the NHC-Ccarbene atom of 8 or proceeds through the transient formation of the NHC species [[tBu(OCO)]Zr(Cl)(R)(THF)] that would then rearrange to 9Me/9Ph.16 Low-temperature NMR control experiments remain inconclusive in that regard. In any case, this reactivity further highlights the electrophilic nature of the Ccarbene atom into NHC coordinated to group 4 metals in the presence of R− sources (R = Bn, Ph, alkyl). Monitoring Experiments and Kinetic Studies. To better understand the THF-promoted conversion of 2M-THF to 4MTHF, additional control experiments and kinetic studies were performed. While the Ti and Zr analogues 2Ti and 2Zr react quickly with THF to afford the corresponding rearranged products 4Ti-THF and 4Zr-THF (1−5 equiv of THF, CD2Cl2, room temperature, quantitative conversion within 1 h), the benzyl migration reaction of [[tBu(OCO)]Hf(Cl)(CH2Ph)] (2Hf) in the presence of THF occurs slowly enough to allow monitoring by 1H NMR (CD2Cl2, room temperature). As an initial NMR monitoring experiment, it was observed that 2Hf reacts quickly with THF (1.2 equiv of THF, CD2Cl2, room temperature, 5 min.) to quantitatively afford the corresponding THF adduct [[tBu(OCO)]Hf(Cl)(CH2Ph)(THF)] (2Hf-THF), as deduced from 1H and 13C NMR data. In particular, the 1H NMR spectrum of 2Hf-THF (CD2Cl2) contains a singlet resonance for the Hf-CH2Ph group (δ 2.20 ppm) shifted

Figure 3. Molecular structure of the rearranged Ti complex 6Ti-THF. The hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (deg): C(15)−Ti(1), 2.019(3); N(1)−Ti(1), 2.287(2); N(2)−Ti(1), 2.294(2); O(1)−Ti(1), 1.924(1); O(2)−Ti(1), 1.919(1); O(3)−Ti(1), 2.244(1); C(15)−N(1), 1.453(3); C(15)− N(2), 1.456(4); N(1)−C(15)−N(2), 97.2(2); O(2)−Ti(1)−O(1), 127.12(9); N(1)−Ti(1)−N(2), 56.91(8); C(39)−Ti(1)−O(3), 86.56(9); C(15)−Ti(1)−C(39)−O(3), −178.48.

with selected bonding and geometrical parameters. The molecular structure confirmed the presence of a heptacoordinate titanium metal center with a κ5-O,N,C,N,O-pentadentate trianionic ligand (Ti(1)−C(15) = 2.019(3) Å) and a THF in a trans position (Ti(1)−O(3) = 2.244(2) Å). Consistent with solution data, the benzyl group shows no η2 coordination in the solid state. The Ti−CBn and Ti−Cipso‑Bn distances were found to be 2.150(3) and 3.255(5) Å, respectively, with a normal Ti(1)− C(39)−C(40) bond angle of 127.3(2)°. D

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Figure 4. Energy profiles of the rearrangement of NHC Hf-THF adducts to the rearranged product 4*Hf-THF1 through (i) the bis-THF intermediate 2*Hf-THF22 (red profile) and (ii) the mono-THF intermediates 2*Hf-THF1 (blue profile) and 2*Hf-THF2 (black profile). The Gibbs free energies (ΔG) at 25 °C, including ZPE correction, are expressed in kcal/mol and have been computed at the B97D/SDD (M) and 6-31G** (other atoms) level of theory for the three metals (Hf, Ti, and Zr) in the gas phase. They were calculated by taking as the origin the monoadducts 2*Hf-THF, and then a modification of the origin was done in order to obtain the profile from 2*Hf + THF. The values in parentheses take into account the solvent effect (single-point calculations in CH2Cl2, SMD model).

the initial rate method, the rate of formation of 4Hf-THF was experimentally determined to be first order relative to THF and 2Hf-THF (Figures S14 and S15 in the Supporting Information), establishing that the observed reaction rate obeys the equation v = k[2Hf-THF][THF]. The rate of formation of 4Hf-THF from the THF adduct 2Hf-THF (in the presence of THF) thus remains dependent on THF concentration. In that regard, as detailed below, DFT calculations strongly suggest that the formation of 4Hf-THF occurs through the formation of a heptacoordinate bis-THF adduct intermediate of the type [tBu(OCO)]Hf(Cl)(Bn)(THF)2 (though not experimentally observed), which readily undergoes an intramolecular benzyl migration reaction.

significantly upfield relative to that in base-free 2Hf (δ 2.63 ppm). The in situ generated adduct 2Hf-THF (addition of 1.2 equiv of THF to a CD2Cl2 solution of 2Hf, room temperature) slowly converts to the rearranged product 4Hf-THF (25% conversion to 4Hf-THF after 21 h, reaction time not optimized), clearly evidencing the formation of transient 2HfTHF prior to the production of 4Hf-THF. The use of excess THF clearly promotes the benzyl migration reaction (82% conversion to 4Hf-THF, 10 equiv of THF, CD2Cl2, room temperature, 4 h; Figure S9 in the Supporting Information). Kinetic studies were carried out for the Hf system: the conversion rate of 2Hf-THF to 4Hf-THF was monitored in the presence of THF at various concentrations of THF and 2HfTHF (Figures S10−S13 in the Supporting Information). Using E

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Organometallics Theoretical Studies. DFT calculations were thus carried out on the THF-promoted rearrangement of complexes 2M (M = Zr, Ti, Hf) to the corresponding products 4M-THF, assuming the transient formation of the THF adducts 2M-THF (vide infra for the generation of 2Hf-THF). DFT-B97D calculations (see Computational Details) were first done on simplified models of THF adducts 2M-THF and rearranged products 4M-THF (i.e., no tBu groups on the aryl substituent). Five isomers were found on the potential energy surface (PES), which differ in the relative positions of the chlorine, THF, and benzyl substituents around the metal center (Figure S1 in the Supporting Information): three for the THF adducts 2*M-THF (isomers denoted 2*M-THF1−3) and two for the rearranged products 4*M-THF (isomers denoted 4*M-THF1,2). The THF adducts 2*M-THF1 and 4*M-THF1 feature benzyl and chlorine moieties trans to one another, and THF is trans to the NHC ligand. In 2*M-THF2 and 4*M-THF2, the benzyl and THF groups are trans to one another. Both 2*M-THF1 and 2*M-THF2 exhibit a benzyl group cis to the NHC moiety, while 2*M-THF3 features a benzyl group trans to the NHC ligand, and the latter was thus ruled out as a possible starting model for the benzyl migration process. For all three metal systems, the models 2*M-THF2 are significantly more stable than their isomers 2*M-THF1 and it seems probable that the THF adducts 2M-THF (observed in the case of Hf) exhibit the structure of 2*M-THF2. Given the experimental data with the Hf system, the energy profiles in the presence or absence of an additional THF molecule were computed and compared. Since only the rearranged product 4Hf-THF1 was experimentally observed, energy profiles starting from 2*Hf and THF were computed to lead to the model 4*HfTHF1 (Figure 4). Three pathways were considered (Figures S2, S8 and S9 in the Supporting Information): (i) pathway 1, a two-step process involving THF coordination to form the intermediate THF adduct 2*M-THF1 (THF trans to the NHC ligand) that directly rearranges to 4*M-THF1 (blue profile in Figure 4); (ii) pathway 2, a mechanism involving THF coordination to form the THF adduct 2*M-THF2 intermediate (THF cis to the NHC ligand), subsequently affording 4*MTHF2, which would isomerize to 4*M-THF1 through a THF coordination/decoordination sequence (black profile in Figure 4), and (iii) pathway 3, conversion of the initially formed mono-THF adduct 2*Hf-THF2 in the presence of THF to the heptacoordinated bis-THF adduct 2*Hf-THF22, which undergoes an intramolecular benzyl migration yielding 4*Hf-THF22, to finally afford the mono-THF species 4*M-THF1 after decoordination of a THF molecule (red profile in Figure 4). As shown in Figure 4, pathway 3 is clearly preferred energetically, since it involves the lowest energy barriers with values (