Rearrangement of Tridentate [OSO]-Type Ligands and Migratory

Article pubs.acs.org/Organometallics

Rearrangement of Tridentate [OSO]-Type Ligands and Migratory Insertion Reaction Mechanisms in Cyclopentadienyl Tantalum Complexes Jacob Fernández-Gallardo,† Luca Bellarosa,‡ Gregori Ujaque,‡ Agustí Lledós,*,‡ María José Ruiz,*,† Rosa Fandos,† and Antonio Otero*,§ †

Departamento de Química Inorgánica, Orgánica y Bioquímica, Universidad de Castilla−La Mancha, Facultad de Ciencias del Medio Ambiente, Avenida Carlos III, sn 45071 Toledo, Spain ‡ Departament de Química, Universitat Autònoma de Barcelona, 08193 Cerdanyola del Vallès, Barcelona, Spain § Facultad de Ciencia y Tecnología Químicas, Universidad de Castilla−La Mancha, Avenida Camilo José Cela, 10, Campus de Ciudad Real, 13071 Ciudad Real, Spain S Supporting Information *

ABSTRACT: The mechanism of the isocyanide migratory insertion into the metal−carbon bond of monocylopentadienyltantalum dimethyl derivatives with [OSO]2− tridentate phenolate ligands has been investigated with DFT calculations. The presence of both a cyclopentadienyl and a tridentate ligand complicates a usually simple reaction, the migratory insertion reaction being coupled with a fac → mer rearrangement of the tridentate ligand. Two routes have been explored for the overall migratory insertion process, depending on the order of the fac → mer and insertion steps. Calculations show that the dissociative (first fac → mer rearrangement, then migratory insertion) and the associative (first migratory insertion, then fac → mer rearrangement) pathways are in principle competitive. However, electronic effects of the phenyl substituents can favor one of the pathways. The study also points out the influence of the donor atom, due to the inversion at the donor atom required in order to achieve the fac → mer interconversion.

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INTRODUCTION Phenolates combined with other donor atoms have found a wide application as multidentate ligands for early transition metals.1 Multidentate phenolate ligands can behave as bidentate, tridentate, or tetradentate ligands and have been successfully employed in olefin polymerization catalysis as well as in the study of fundamental organometallic transformations.2−5 In this direction tridentate ligands [ODO]2−, where a neutral donor atom (D) is flanked by two phenolate rings (Scheme 1), are particularly appealing.6,7 These tridentate LX2 ligands combine chelating bisphenolates, which are readily tunable ligands with respect to sterics and electronics,1 with tunable flexibility introduced into the ligand framework by the additional donor atom (D). The ligand flexibility depends on

the strength of the interaction between the central donor atom (D) and the metal. This feature of the [ODO]2− ligands can allow ligand breathing, in which the donor atom is entering or leaving the metal coordination sphere depending on the electronic requirements of the metal center in a particular step of a catalytic cycle. In this way [ODO]2− ligands can be more versatile than the pincer-type tridentate ligands, because they can adapt to the stereoelectronic requirements of the metal center.8 Chelating bis(penolato) ligands having an additional sulfur donor atom have found significant catalytic applications.9 The enhanced activity of the titanium and zirconium complexes with an S-bridge chelating phenolate ligand with respect to the methylene-bridged or directly bridged analogues in ethylene polymerization reactions has been assigned from DFT calculations to the flexibility introduced by the bridging sulfur atom.10 The migratory insertion of unsaturated molecules into metal alkyl bonds is an important step of many catalytic processes.11 In this context the migratory insertion of isocyanides has been

Scheme 1. Coordination of Tridentate [ODO]2− Ligands to a Transition Metal Center

Received: May 27, 2012 Published: September 28, 2012 © 2012 American Chemical Society

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dx.doi.org/10.1021/om3004652 | Organometallics 2012, 31, 7052−7062

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extensively studied.12 Such migrations have been reported for most of the early transition metals, and they lead to the formation of κ2-C,N-iminoacyl ligands, which are prone to experience further transformations. Regarding systems with multidentate phenolate ligands, the migratory insertion of isocyanides into zirconium13 and vanadium−carbon bonds14 anchored to a calix[4]arene moiety has been explored. Migratory insertion of isocyanides into tantalum−carbon bonds of cyclopentadienyl15−17 and imido18,19 tantalum complexes has been investigated. A systematic study of isocyanide migratory insertion reactions into tantalum−methyl bonds of chloromethyl derivatives [TaCp*Cl4−xMex] showed that when two methyl ligands are present, azatantalacyclopropane species are formed by a migration of the second methyl group to the electrophilic iminoacyl carbon atom of the η2iminoacyl intermediate. Bulkier alkyl groups do not migrate, the reaction ending up at the η2-iminoacyl derivative.15−17 Recently, we reported the preparation and characterization of monocyclopentadienyltantalum derivatives with [OSO]-type ligands and their reactivity with isocyanides.20 The reaction of [TaCp*Me2(κ3-tbcp)] (tbcp = 2,2′-thiobis(4,6-dichlorophenolato)) with xylyl isocyanide (xylyl = 2,6-dimethylphenyl) or tertbutyl isocyanide yields the azatantalacyclopropane products. Interestingly, the migratory insertion reaction entails a rearrangement of the tbcp ligands, which changes from a fac disposition of the κ3-tbcp ligand in the dimethyl complex to a mer disposition of this ligand in the κ2-C,N-azatantalacyclopropane product (Scheme 2).

Scheme 3. Dynamical Interconversion Process in [iPrNSN]ZrMe2.23

[MoCp 2 (CH 3 )(CNH)] + 24 and [(Ind)Ti(NH 2 )(CH 3 )(CNH)].25 The migration in systems with multidentate ligands has been investigated by means of both static and dynamic density functional calculations in the zirconium dimethyl unit anchored to a calix[4]arene moiety.13,26 Methyl isocyanide migratory insertion forms sequentially the methyl-κ2-C,Niminoacyl and the κ2-bound imine. The rigid calixarene framework does not change during the insertion processes, keeping tetradentate coordination. The mechanism of the methyl insertion into the Ta−CNPh bond in [TaCp(digol)(Me)(CNPh)]+ (digol = diethylene glycolate) to give the κ2C,N-iminoacyl complex was investigated by means of DFT calculations.27 The tridentate [OOO]2− ligands keeps a fac coordination during this transformation. Little is known about the migratory insertion mechanism when more flexible tridentate donor ligands are used. Our recent report on [TaCp*Me2(κ3-tbcp)], bearing a tridentate [OSO]2− ligand, presents a more complicated scenario for the migratory insertion in such systems because this reaction could be coupled with a ligand rearrangement (Scheme 2). Aiming to shed light on the reaction mechanism, we have performed a DFT study of the migratory insertion of methyl isocyanide into the tantalum−carbon bonds of [TaCpMe2(κ3-tbmp)] (tbmp = 2,2′-thiobis(4-methyl)phenolato) (1) and [TaCpMe2(κ3-tbcp)] (2). The results of this study, reported in the present article, give also some hints of the structural versatility and flexibility of thiobisphenolate ligands coordinated to early transition metal atoms.

Scheme 2. Isocyanide Migratory Insertion into the Ta−Me Bonds of Cp*TaMe2(κ3-tbcp)20

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COMPUTATIONAL DETAILS

The migratory insertion reaction was studied, with methylisocyanide as the substrate, for two model systems, which differ in the substituents of the phenyl ring of the tridentate [OSO]2− ligand: [TaCpMe2(κ3tbmp)] (1) (methyl substituents) and [TaCpMe2(κ3-tbcp)] (2) (chloro substituents). Structural analysis has been also performed using the real complex [TaCp*Me2(κ3-tbop)] (3) (tbop = 2,2′tiobis(4-tert-octyl)phenolate).20 Density functional theory (DFT) calculations were carried out to identify the structures of the reaction intermediates and transition states of the insertion process. The geometry optimizations have been carried out with the program package Gaussian 0328 using the B3LYP29 combination of functionals. Optimizations were performed with the 6-31G(d) basis set for the atoms directly bonded to tantalum,30 the SDD pseudopotential with associated basis set for the tantalum center,31 and the 6-31G basis set for the other atoms. All the transition states have been characterized by a frequency analysis, and for each transition structure we calculated the intrinsic reaction coordinate (IRC) routes toward the corresponding minima. If the IRC calculations failed to reach the energy minima on the potential energy surface, we performed geometry optimizations from the final phase of the IRC path. The solvent effects of toluene (ε = 2.38) were computed with the continuum CPCM model32 by single-point calculations on the gasphase-optimized geometries, using the same basis set and pseudopotential for Ta but the larger 6-311++G(d,p) basis set33 for all other atoms. The energies given in the text, Etol, were obtained by

A similar rearrangement, involving the change in the coordination of a 1,4-disubstituted-1,4-diaza-1,3-butadiene (DAD) from a prone to supine orientation, has been reported in the reaction of the tantalum benzylidene complex [TaCp*(CHPh)(η4-prone-Ar-DAD) with carbodiimines to afford azametallacyclobutanes.21 Migratory insertion of xylyl isocyanide into the tungsten−allyl linkage of [WCp*(NO)(nalkyl)(η3-CH2CHCHMe)] complexes produces the κ2-C,Niminoacyl ligand with concomitant η3 → η1 haptotropic shift of the allyl ligand.22 Dynamical interconversion processes in solution of zirconium complexes containing tridentate [NSN]2− ligands have been studied by variable-temperature NMR. A ΔG⧧ of 15 kcal mol−1 has been determined for both the process that interconverts the axial and equatorial methyl groups and that interconverting the isopropyl methyl groups. The data are consistent with inversion at sulfur via a structure with the N, S, and N atoms of the ligand and the metal in a coplanar arrangement (Scheme 3),23 in a process similar to the fac → mer rearrangement in Cp*TaMe2(κ3-tbcp). The mechanism of the migratory insertion of isocyanides into metal−carbon bonds has been theoretically studied by means of DFT calculations in simple model systems such as 7053


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adding the contribution of the Gibbs energy of solvation to the gasphase total energy.

Table 1. Selected Experimental and Calculated Geometrical Parameters for Cyclopentadienyl Tantalum Complexes with [OSO]-Type Ligands

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RESULTS AND DISCUSSION Structures and Relative Stabilities of Reactants and Products. The structures of several monocyclopentadienyl tantalum derivatives containing bis(phenolate) [OSO]-type ligands with different groups on the aromatic rings have been determined by X-ray diffraction analysis.20 Taking the centroid of the Cp* ring as the sixth ligand, the X-ray-characterized [TaCp*X2[OSO]] (X = Cl, Me) complexes show a pseudooctahedral geometry with a fac coordination mode of the κ3[OSO] ligand in which the oxygen atoms are located in the equatorial plane and are cis to each other and the sulfur atom is occupying the position trans to the Cp* group (Scheme 4a).

Ta−O Ta−S Ta−CMe Ta−Caza Ta−N O−Ta−O CPh−S−CPh CMe−Ta−CMe S−Ta−N

3

a

Scheme 4. Structures of the Characterized [TaCp*Me2(κ tbop)] Complex (a) and of the Insertion Products (b) Showing the fac → mer Rearrangement of the [OSO] Ligand

3 (X-ray)a

3 (opt)

4 (X-ray)a

2.003(4) 2.025(4) 2.677(1) 2.242(7) 2.249(6)

2.015 2.016 2.831 2.253 2.254

2.00(1) 2.05(1) 2.725(4)

91.7(2) 102.8(3) 82.6(2)

90.7 101.8 82.7

2_aza (opt) 2.044 2.045 2.886

2.19(1) 1.89(1) 139.7(4) 111.8(7)

2.229 1.924 138.0 111.6

137.3(4)

140.2

Ref 20.

the central sulfur (2.64 Å) is considerably longer that those of the lateral sulfur atoms (2.42 Å).37 This distance is even longer when the ligand adopts a mer disposition (4 and 2_aza). For the entire set of thiobisphenolate ligands analyzed, the fac coordination mode is favored for the dimethyl compounds. The mer isomers are found 9.2, 9.6, and 6.1 kcal mol−1 above the fac isomers for complexes 1 (tbmp ligand), 2 (tbcp ligand), and 3 (tbop ligand), respectively. The relative stabilities of both isomers seem to be more influenced by the steric effects of the phenyl substituents (1 vs 3: changing methyl with tert-octyl) than for their electronic properties (1 vs 2: changing methyl with chloride). Migratory insertion of an isocyanide implies the initial coordination of this substrate to the tantalum center. Isocyanide addition is produced by interacting the HOMO of this species, mainly developed over the carbon end, with a suitable low-energy empty orbital on the metal. This orbital is oriented in a different way in the fac and mer isomers, as it can be appreciated for complex 1 in Figure 1. In the fac isomer of 1 the LUMO is situated between the cyclopentadienyl ring and the two methyls, whereas in the mer isomer it is oriented trans to the cyclopentadienyl ring (Figure 1). Regarding the migratory insertion products, calculations reproduce the preference for the mer isomer: with the tbmp ligand the mer product (1_aza) is 5.9 kcal mol−1 more stable than the fac (Scheme 5). In the fac isomer the Ta−S bond is completely broken (Ta−S = 3.75 Å) due to the occupancy by the CMe2 unit of the apical coordination region. The rotation of the azatantalacyclopropane ligand stabilizes the fac isomer, but with this conformation it still lies 4.1 kcal mol−1 above the mer isomer. A rotated conformation of the mer isomer with the carbon and nitrogen atoms of the azatantantalacyclopropane interchanging its position is found 3.0 kcal mol−1 above the most stable one (Scheme 5). Possible Mechanisms for the Migratory Insertion Process: Migration of Methyl Isocyanide into the Ta− CMe Bonds of [TaCpMe2(κ3-tbmp)] (1). The migratory insertion reaction should start with coordination of the unsaturated isocyanide substrate to the metal center. Isocyanide addition can take place at the apical position of the mer isomer or at the equatorial position of the fac isomer. In this way two main routes could be considered a priori for the overall migratory insertion process: a dissociative one, in which an initial fac → mer rearrangement of the [OSO] ligand creates the apical vacancy to which isocyanide will coordinate (route A),

On the contrary the products of the isocyanide migratory insertion display a pseudo-octahedral geometry with a mer coordination mode, the two oxygen and the sulfur atoms being located in the equatorial plane. The oxygen atoms are placed trans to each other, while the sulfur atom is in a trans disposition with respect to the nitrogen atom of the azatantalacyclopropane ligand (Scheme 4b). Electronically all these complexes can be described as eight-coordinate compounds. The coordinative preferences of d0 transition metal complexes have been a matter of debate. Contrarily to most of the transition metal compounds, the geometry is not governed by the block d. In such a situation several structures can be competitive.34−36 To assess the capability of the employed methodology to reproduce the experimental geometries and to quantify the preference for the coordination mode, we have optimized the fac and mer isomers of the complexes [TaCpMe2(κ3-tbmp)] (1), [TaCpMe2(κ3-tbcp)] (2), and [TaCp*Me2(κ3-tbop)] (3) and of the migratory insertion product [TaCp(κ 2-Me2CNMe)(κ3-tbcp)] (2_aza). Selected optimized parameters of fac-3 and mer-2_aza are compared in Table 1 with those of the crystal structures of [TaCp*Me2(κ3-tbop)] (3) and [TaCp*(κ2-Me2CNxylyl)(κ3tbcp)] (4), respectively. There is a good agreement between the calculated and X-ray structures, the main discrepancy being found in the Ta−S distances, which are about 0.15 Å longer in the calculated ones. Calculations suggest a soft potential for the Ta−S bond lengthening, in accordance with a not very strong tantalum−sulfur bond. The X-ray structure of the related tantalum complex with a fac [SSS]-type ligand [TaCp*Cl2(SCH2CH2)2S] shows that the Ta−S distance for 7054


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Figure 1. Interaction between the methylisocyanide HOMO and the LUMO of 1 in the fac (left) and mer (right) isomers.

Scheme 5. Relative Energies in Toluene (kcal mol−1) of the mer (above) and fac (below) Isomers of CpTa(κ2Me2CNMe)(tbmp) (1_aza)

methylisocyanide into the tantalum−carbon bonds of [TaCpMe2(κ3-tbmp)] (1). Route A: First fac → mer Rearrangement, Then Migratory Insertions. In route A, the sulfur atom changes its conformation at the beginning of the process. Coordination of the isocyanide occurs after the fac → mer rearrangement of the tbmp ligand. Thus, the first step in this pathway is the fac → mer rearrangement of the [OSO] ligand. The energy barrier for this isomerization, which takes place through the transition state 1A-TS-fac-mer, is 23.4 kcal mol−1; this barrier is higher than that estimated from variable-temperature NMR for the interconversion of the [NSN] ligands in [NSN]ZnMe2 complexes (about 15 kcal mol−1, Scheme 3).23,39 In the transition state 1A_TS-fac-mer (Figure 2) the Ta−S bond is completely broken (Ta−S = 4.38 Å) and the sulfur atom is coplanar with the aromatic rings. The Cipso−S−Cipso angle has opened from 105° to 129°, in agreement with an sp3 to sp2 hybridation change at the sulfur atom, with a minor change in the S−Cipso bond length (from 1.79 Å to 1.81 Å). It has been shown that the sulfur atom has a strong preference for sp3 hybridation in SR3 species, the planar, sp2 structure being found about 30 kcal mol−1 above the pyramidal, sp3.40 Sulfur decoordination facilitates the sp3 to sp2 hybridation change. The first step provides the mer isomer of complex 1 (intermediate 1A_mer), lying 9.2 kcal mol−1 above the fac isomer. In 1A_mer the sulfur atom is not bonded to the tantalum (Ta−S = 4.13 Å): the tridentate [OSO] ligand has changed its coordination mode from κ3 to κ2, and it will keep this [OO] bidentate coordination mode until the last part of the mechanism. The sulfur atom has returned to the sp3 hybridation (Cipso−S−Cipso = 103°). The eight-membered ring involving the Ta and S atoms adopts a boat-chair conformation. The fac → mer interconversion has created a vacant apical position suitable for isocyanide coordination (Figure 1). The isocyanide addition takes place via transition state 1A-TSiso_add with a low barrier, from a van der Waals complex initially formed (1A_vdW), and gives intermediate 1A_iso, with the isocyanide ligand coordinated trans to the Cp ring (Figure 3). The Ta−Cisocyanide changes from 4.39 Å in the van der Waals complex to 2.94 Å in the transition state and 2.26 Å

and a associative one, in which isocyanide coordinates first to the equatorial position and the fac → mer rearrangement takes place in a more advanced stage of the insertion reaction (route B) (Scheme 6). Overall the double migratory insertion process is highly exothermic (ΔEreac= −38.3 kcal mol−1), providing a thermodynamic driven force for the formation of the azatantantalacyclopropane product 1_aza. These two alternative pathways were theoretically investigated for the CO insertion into methyl- and arylpalladium(II) cations bearing tridentate nitrogen-donor ligands.38 According to these calculations a purely dissociative mechanism should not be expected to occur, as the Pd−N bond dissociation is found to be a high-energy process. In these systems the migratory insertion and not the ligand substitution is the ratedetermining step in the carbonylation process. In what follows routes A and B will be analyzed for the migratory insertion of 7055


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Scheme 6. Alternative Pathways (Routes A and B) Analyzed for the Migratory Insertion of Methyl Isocyanide into 1

Figure 2. Optimized structures in the fac → mer rearrangement of 1. Relative energies in toluene with respect to 1 + MeNC, in kcal mol−1.

Figure 3. Optimized structures in the isocyanide addition step to 1, route A. Relative energies in toluene with respect to 1 + MeNC, in kcal mol−1. 7056


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Figure 4. Optimized structures in the first migratory insertion step to 1, route A (above), and in the iminoacyl rotation (below). Relative energies in toluene with respect to 1 + MeNC, in kcal mol−1.

Figure 5. Optimized structures in the second migratory insertion step to 1. Relative energies in toluene with respect to 1 + MeNC, in kcal mol−1.

The first migratory insertion step in route A starts from 1A_iso. The κ2-C,N iminoacyl complex 1A_imino is formed after crossing the transition state 1A-TS-1st_ins (Figure 4).

in the hexacoordinate intermediate 1A_iso. The rest of the geometrical parameters of the [TaCpMe2(κ2-tbmp) moiety have only slightly changed with respect to 1A_mer. 7057


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Figure 6. Energy profiles in toluene for the migratory insertion of methylisocyanide into complex 1 along routes A and B.

Figure 7. Optimized structures in the isocyanide addition step to 1, route B. Relative energies in toluene with respect to 1 + MeNC, in kcal mol−1.

of this rotation (1A_TS-imino_rot) is found 11.1 kcal mol−1 above 1A_imino and leads to a rotated conformation of the κ2iminoacyl ligand (1_imino-rot), which is only 1.3 kcal mol−1 less stable than the former (Figure 4). The second migratory insertion takes places starting from the intermediate 1_iminorot and after crossing the transition state 1-TS-2nd_ins, placed 18.8 kcal mol−1 above the intermediate, affords the final azatantantalacyclopropane product 1_aza. This step is common to routes A and B. The transition state of the second migratory insertion is very similar to that of the first migratory insertion (1A-TS-1st_ins), with the methyl moving toward the carbon atom of the κ2-iminoacyl ligand (Ta−CMe = 2.52 Å, CMet− Ciminoacyl = 2.03 Å) (Figure 5). After the second migratory insertion the Ta−S bond is re-formed (Ta−S = 2.91 Å) and the tricoordinate ligand adopts a κ3-mer coordination mode. We have explored other possibilities for the steps leading from 1A_imino to product 1_aza: the rotation of the κ2-

The energy barrier for the first migratory insertion step (15.7 kcal mol−1 from intermediate 1A_iso) is lower than that for the fac → mer interconversion (23.4 kcal mol−1). The migration results in the formation of the κ2-iminoacyl complex 1A_imino, considerably more stable (19.5 kcal mol−1 below) than the reactants (1 and methylisocyanide). The process can be described as a methyl migration accompanied by a displacement of the isocyanide ligand from the apical position to approach the incoming methyl. The Ta− C distance of the methyl group that is moving has elongated to 2.50 Å, whereas the Ta−C distance with the methyl not involved in the insertion remains at 2.29 Å. The Cisocyanide−CMe distance of the bond that is being formed is 1.92 Å. No changes have been produced in the mer [OSO] ligand. The iminoacyl ligand of intermediate 1A_imino should rotate in order to adopt the final geometry of 1_aza with the carbon atom occupying an apical position. The transition state 7058


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Figure 8. Optimized structures in the first migratory insertion step to 1, route B. Relative energies in toluene with respect to 1 + MeNC, in kcal mol−1.

transforms into the more stable κ2-C,N-iminoacyl insertion product (1B_k2-imino).42 1B_k2-imino keeps a pentagonal bipyramid geometry with the sulfur atom and the Cp ring in apical positions and the oxygen atoms of the tbmp ligand, the methyl, and the iminoacyl ligands in the equatorial plane (Figure 8). The DFT molecular dynamics study of the methyl isocyanide migratory insertion into the Zr−CMe bond of [[calix[4](OMe)2(O)2ZrMe2] also suggested the initial formation of a transient κ1-iminoacyl species, with a formed CMe−Ciso bond and still a long Zr−N bond.26 We were not able to find the transition state connecting κ1- and κ2-iminoacyl isomers.42 The short time stability of the κ1-iminoacyl species found in the DFT molecular dynamics study was interpreted as indicative of a negligible barrier for the conversion of the κ1- into the κ2isomer. The same result was also found for CO insertion in bis(cyclopentadienyl)Zr dimethyl complexes.43 During the isocyanide addition and first migratory insertion steps the [OSO] ligand has kept a κ3-fac coordination mode. In order to get the κ3-mer coordination mode it has in the azatantalacyclopropane product, sulfur dissociation and inversion should occur. This ligand rearrangement takes place at this stage of the reaction. All the tests we have performed looking for alternative ways of doing the fac → mer interconversion give higher barriers (see Supporting Information). Starting from 1B_k2-imino the fac → mer interconversion takes place in two steps. In the first one sulfur decoordination occurs (via transition state 1B_TS-S_decoord) and the tridentate ligand changes its coordination mode from κ3 to κ2 but maintains a pseudo-fac arrangement (1B_imino-noS). In this step, simultaneously with the Ta−S bond length elongation (from 2.74 Å in 1B_k2-imino to 3.70 Å in 1B_TS-Sdecoord and 3.89 Å in 1B_imino-noS) the iminoacyl rotates almost 90° (the dihedral angle between the centroid of the Cp ring, tantalum, and the N and C atoms of the iminoacyl group increases from 93° in the intermediate 1B_k2-imino to 171° in the intermediate 1B_imino-noS). Taking the centroid of the Cp* ring as a ligand, this rotation can be described as the change from a pentagonal bipyramid structure with the nitrogen and carbon atoms of the iminoacyl occupying two equatorial positions (1B_k2-imino) to a pseudo-octahedral one with the nitrogen at the equatorial plane and the iminoacyl carbon occupying the apical position left vacant by the sulfur atom (Figure 9). The energy cost for the sulfur decoordination

iminoacyl ligand could take place after the second migration, in the azatantalacyclopropane. The barrier obtained in this case is higher than that of the sequence of steps described above (see the Supporting Information). The most favorable complete energy profile in toluene for the mechanism described in route A is depicted in Figure 6. Route B: First Migratory Insertion, Then fac → mer Rearrangement. Route B starts with the isocyanide addition to the fac isomer of 1. As shown in Figure 1, methyl isocyanide should coordinate between the two methyl groups. An initial van der Waals complex (1B_vdW) is formed when MeNC approaches this region of complex 1. The transition state for the isocyanide addition (1B_TS-iso_add) has a rather high energy (26.6 kcal mol−1 above the separated 1 and MeCN), showing the difficulty to incorporate a new ligand into this encumbered system. Indeed, this barrier is the highest energy barrier along the entire route B (Figure 6). A considerably lower barrier for the MeNC coordination (5.5 kcal mol−1) was calculated in [calix[4](OMe)2(O)2ZrMe2].26 The outcome of the addition step is the intermediate 1B_iso, which is placed 14.1 kcal mol−1 above the reactants (Figure 7). Geometrically this intermediate can be described as a pentagonal bipyramidal heptacoordinate complex, taking the centroid of the Cp* ring as an axial ligand. Tantalum complexes with pentagonal bipyramidal geometry have been experimentally reported.41 The MeCN ligand is placed in the equatorial plane of 1B_iso. The bond distance Ta−Ciso is 2.19 Å. The O−Ta−O angle has changed from 92° to a usual value for cis equatorial ligands in a pentagonal bipyramid (75°). Coordination of the isocyanide entails substantial changes in the TaMe2 unit of 1: the CMe−Ta−CMe angle opens from 82° in 1 to 114° in the transition state and 130° in 1B_iso to allow the entrance of a new ligand. At the same time the Ta−CMe bonds lengthen (2.26, 2.30, and 2.37 Å in 1, 1B_TS-iso_add, and 1B_iso, respectively). In this way the geometrical perturbation due to the coordination of MeCN prepares the migratory insertion reaction. Changes in the rest of the molecule are minor, the tbmp ligand keeping its κ3-fac coordination mode all along the addition process (Ta−S = 2.69 Å in 1B_iso). The first migratory insertion, starting in 1B_iso, has a low barrier, in agreement with the weakened Ta−CMe bond at the intermediate. The transition state (1B_TS-1st_ins) lies only 4.2 kcal mol−1 above 1B_iso and gives a transient κ1-Ciminoacyl insertion product (1B_k1-imino), which readily 7059


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Figure 9. Optimized structures in the fac → mer rearrangement of route B. Relative energies in toluene with respect to 1 + MeNC, in kcal mol−1.

Figure 10. Energy profiles in toluene for the migratory insertion of methylisocyanide into complex 2 along routes A and B.

is only 7.8 kcal mol−1, in agreement with a weak Ta−S bond in 1B_k2-imino. 1B_imino-noS is only 4.3 kcal mol1 less stable than 1B_k2-imino. The second step in the fac → mer rearrangement corresponds to the sulfur inversion. The transition state for the sulfur inversion (1B_TS-Sinver) is found 9.6 kcal mol−1 above 1B_imino-noS. Overall, the energy cost of the sulfur decoordination−inversion in 1B_k2-imino is 13.9 kcal mol−1. Thus, the fac → mer rearrangement of the iminoacyl complex has a considerably lower energy barrier than that of the dimethyl complex 1 (23.4 kcal mol−1, route A). This fact can be related to a higher electron density on the tantalum atom due to the presence of a more electron-donating ligand (κ 2 -iminoacyl vs methyl). Contrarily to route A, no further rotation of the iminoacyl ligand is required, and the fac → mer rearrangement already leads to the same intermediate as in route A (1_imino-rot), placed 18.1 kcal mol−1 below the reactants. From this point routes A and B merge in the second migratory insertion step (Figure 5). The complete energy profile in toluene for the migratory insertion of methyl

isocyanide into the tantalum−carbon bonds of [TaCpMe2(κ3tbmp)] along the route B mechanism is depicted in Figure 6, together with that of route A. Substituent Effects on the Reaction Mechanism: Migratory Insertion into the Ta−C Me Bonds of [TaCpMe2(κ3-tbcp)] (2). To assess the influence of electronic effects of the phenolate ring substituents, we have investigated both the dissociative and associative pathways (routes A and B, respectively) for the methyl isocyanide insertion into [TaCpMe2(κ3-tbcp)] (2), in which the two methyl substituents in para postion have been replaced by Cl substituents and two more Cl substituents have been added in ortho position. The energy profiles in toluene for the insertion reaction of complex 2 with methylisocyanide along routes A and B are depicted in Figure 10. The geometries of the corresponding transition states and intermediates are shown in the Supporting Information. The general appearance of the energy profiles for complexes 1 and 2, along both routes A and B, is very similar (compare 7060


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Table 2. Energy Barriers (kcal mol−1) for the Different Steps of Methylisocyanide Insertion into Complexes 1 and 2 in Toluene, along Routes A and B route A

route B

step

complex 1 (R = Me)

complex 2 (R = Cl)

fac → mer isomerization isocyanide addition first migratory insertion iminoacyl rotation second migratory insertion

23.4 3.3 15.7 11.2 18.8

23.6 3.4 16.1 9.5 18.4

complex 1 (R = Me)

complex 2 (R = Cl)

fac → mer isomerization isocyanide addition first migratory insertion

13.9 26.6 4.2

16.2 23.2 3.1

second migratory insertion

18.8

18.4

step

one of the two pathways, the dissociative pathway (route A) being favored with more electron-donating ligands (methyl, complex 1). Steric effects can completely block the reaction (tert-octyl, complex 3). In our systems the fac → mer rearrangement entails a notable barrier (the highest in the dissociative pathway) mainly related with the inversion of the sulfur donor atom; this result points out the influence of the neutral donor atom of the tridentate ligand in the process. Thus, the fac → mer rearrangement barrier should be in principle lower for donor atoms with a smaller barrier to inversion, like the first-row main group elements (N and O). The main result of the present study is the complete identification, including a detailed description of the elementary steps, of two competitive mechanisms for the concomitant fac → mer interconversion and isocyanide migratory insertion that take place in sterically encumbered complexes of early transition metals. The computed energy barriers show that both pathways are energetically comparable and should be kinetically competitive. Although our results suggest that specific studies with explicit consideration of the particular substrate, donor atom of the ligand, and ligand substituents will be necessary for the elucidation of the detailed mechanism operating on a given experimental systems, the present study lays the foundations for a deeper understanding of and control over the process.

Figures 6 and 10), pointing out a minor influence of electronic effects on the phenolate ring into the reaction mechanism. The thermodynamics of the reaction is also similar: the reaction is exothermic by 38.6 kcal mol−1 for 1 and 39.1 kcal mol−1 for 2. In Table 2 the energy barriers for the successive steps of the route A and route B mechanisms for both compounds are compared. The main changes in the reaction barriers happen in the associative pathway (route B). For both complexes the ratedetermining step is the fac → mer isomerization in route A and the isocyanide addition in route B. However, whereas for complex 1 the dissociative pathway is favored (overall barriers of 23.4 and 26.6 kcal mol−1 for routes A and B, respectively), for complex 2 both pathways have practically the same barrier (23.6 route A and 23.2 kcal mol−1 route B). Thus, the most important effect of the methyl by chloro substitution is decreasing the barrier of the associative pathway, making it competitive with the dissociative one. This effect is a direct consequence of the electron-withdrawing nature of the chloro substituents, which allows an easier coordination of the incoming ligand, and it is also reflected in the higher stabilization of the κ2-imoacyl complex in the presence of the chloro substituents (1B_k2-imino and 2B_k2-imino are found 21.3 and 25.5 kcal mol−1 below the reactants, respectively). As a consequence of the higher stabilization of 2B_k2-imino, the energy barrier for the fac → mer isomerization, which takes place in this intermediate in route B, is higher for 2 than for 1 (16.2 vs 13.9 kcal mol−1, Table 2). From an electronic point of view, the expected behavior of complex 3 with a thiobis(4-tert-octyl)phenolate) ligand (tbop) should be similar to that of complex 1. However, complex 3 does not react with isocyanides under the experimental conditions (toluene solvent at 100 °C). The theoretical study suggests that steric effects should be responsible for such behavior, because the influence of electronic effects is shown to be minor (Table 2).

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ASSOCIATED CONTENT

* Supporting Information S

Optimized geometries for alternative pathways for the insertion reaction of complex 1 with methylisocyanide. Optimized structures and relative energies in toluene for the insertion reaction of complex 2 with methylisocyanide along routes A and B. Cartesian coordinates and absolute energies in the gas phase and in toluene, for all computed structures. This material is available free of charge via the Internet at http://pubs.acs.org.

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CONCLUDING REMARKS The presence of both a cyclopentadienyl and a tridentate ligand coordinated to an early transition metal center introduces some difficulties in a usually simple reaction of the migratory insertion of isocyanides into metal−carbon bonds. On one side the common and easy isocyanide addition step, which precedes the migration, is hampered by the presence of bulky ligands. On the other side, a fac → mer rearrangement of the tridentate ligand could be required to accommodate the azametallocyclopropane ligand obtained by the sequential double insertion. The theoretical results show that two pathways, one dissociative, starting with the initial fac → mer interconversion, and one associative, starting with the isocyanide coordination, are in principle competitive, with similar energy barriers. However, electronic effects can favor

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; [email protected]; Antonio. [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the Spanish MINECO (projects CTQ2011-22578, CTQ2011-23336, and Consolider-Ingenio 2010 ORFEO CSD2007-00006 and fellowship to J.F.-G., Grant No. AP2005-4738) is gratefully acknowledged. We are also grateful to CESCA for generous allocation of computer time. 7061


Organometallics

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Article

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Rearrangement of Tridentate [OSO]-Type Ligands and Migratory

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