A Tantalum Methylidene Complex Supported by a Robust and

Synthetic and Computational Studies on the Thermal and Photochemical Reactions of [NPN]TaMe3 (NPN = PhP(CH2SiMe2NPh)2) and [NPN]TaMe3 (NPN ...
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A Tantalum Methylidene Complex Supported by a Robust and Sterically Encumbering Aryloxide Ligand Keith Searles,† Balazs Pinter,‡ Chun-Hsing Chen,§ and Daniel J. Mindiola*,† †

Department of Chemistry, University of Pennsylvania, 231 South 34th Street, Philadelphia Pennsylvania 19104, United States Eenheid Algemene Chemie (ALGC), Vrije Universiteit Brussel, Pleinlaan 2, 1050 Brussels, Belgium § Department of Chemistry and the Molecular Structure Center, Indiana University, Bloomington Indiana 47405, United States ‡

S Supporting Information *

ABSTRACT: Treatment of [TaCl2(CH3)3] with 2 equiv of NaOAr′ (OAr′ = 2,6-bis(diphenylmethyl)-4-tert-butylphenoxide) yields cleanly the bis-aryloxide trimethyl complex [(Ar′O)2Ta(CH3)3] (1), which is isolated in 92% yield and is spectroscopically and structurally characterized. Addition of 2 equiv of HOAr′ to [TaCl2(CH3)3] results in clean protonation concurrent with formation of the bis-aryloxide methyl derivative [(Ar′O)2Ta(CH3)Cl2] (2), which was also fully characterized, including an X-ray structure. Despite being close derivatives, complex 1 (trigonal bipyramidal) and 2 (square pyramidal) possess very different structures, with the e set in a square-pyramidal molecular orbital diagram being key to their preferred geometry. Addition of excess ylide, H2CPPh3, to 2 results in formation of the terminal tantalum methylidene chloride complex [(Ar′O)2TaCH2(Cl)(H2CPPh3)] (3) in 64% yield, which is characterized by multinuclear NMR spectroscopy and a solid-state structure determination.

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has been recently reported with 3d early transition metals,8 uranium,9 and niobium.10 There are several synthetic tactics to preparing tantalum methylidene complexes.3 The most common approach is by use of a strong base (e.g., alkyl, alkoxide, amide, or ylide) with the corresponding methyl precursor.1,11 As documented by Schrock, the use of weakly coordinating ions greatly improves the deprotonation step, presumably because the base can bind to the metal center, but also due to the fact that the weakly coordinating anion becomes a much better leaving group. For this reason Schrock and co-workers required methide abstraction in [Cp2Ta(CH3)3] to form the salt [Cp2Ta(CH3)2][BF4],1,11 while Arnold and co-workers relied on OTf− incorporation in [(Me3SiNCPhNSiMe3)2Ta(CH3)3] with HOTf to form the methylidene precursor [(Me3SiNCPhNSiMe3)2Ta(CH3)2(OTf)].12 Other popular approaches to generating tantalum methylidenes involve thermally or photolytically promoted α-hydrogen abstraction processes of the corresponding methyl precursors, as in the case of Rothwell,6,7 Fryzuk,13 Ozerov,14 and Bercaw.15 Alternatively, tantalum methylidene complexes can be prepared via αhydrogen elimination reactions with a TaIII−methyl precursor. This strategy was implemented by Bercaw16,17 utilizing Cp*2TaIII or Cp*2TaIV precursors, but such a route is not particularly popular, given the scarcity of stable TaIII or TaIV starting materials. Methylidene group transfer has also been an

INTRODUCTION Tantalum methylidenes hold a special place in the field of organometallic chemistry since the original synthesis of the methylidene methyl complex [Cp2TaCH2(CH3)], reported by Schrock, using [Cp2Ta(CH3)2][BF4] and the volatile ylide base H2CP(CH3)3.1 Over the past four decades, a few tantalum methylidenes have been used in olefin homologation2 and stoichiometric as well as catalytic group-transfer reactions.2,3 Notably, terminal tantalum methylidene and methylidyne intermediates supported on silicon oxide surfaces have been proposed in important catalytic transformations such as methane metathesis and methane dehydrocoupling (by a nonoxidative route) reactions.4 In addition, there has been significant interest in methylidene-type complexes, given their implication in the Fischer−Tropsch chemistry of alkanes.2,5 Since the synthesis and isolation of [Cp2TaCH2(CH3)],1 other well-defined examples of tantalum methylidenes have appeared in the literature, with one notable example being the low-coordinate and metastable tantalum complex [(ArO)2Ta CH2(CH3)] (Ar = (2,6-tBu2)-4-X-C6H2, X = −H, −OMe) reported by Rothwell and co-workers, given its ability to engage in C−H activation reactions.6,7 Unfortunately, the tert-butyl substituents of the aryloxide become involved in cyclometalation reactions, thus preventing further studies of such a reactive and interesting species.6,7 For this reason, we have been investigating other more robust and sterically encumbering aryloxide ligands that can mimic, to some degree, oxide surfaces, but with the ability to kinetically protect reactive and unhindered metal−carbon multiple bonds. Our new ligand type © 2014 American Chemical Society

Received: February 26, 2014 Published: August 6, 2014 4192

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approach applied by Bercaw16 and Schrock,18 as in the case of [Cp*2TaCl(THF)] or [Cp*2Ta(CH3)] and the ylide H2CP(CH 3 ) 3 to form [Cp* 2 TaCH 2 (Cl)] or [Cp* 2 Ta CH2(CH3)], respectively, along with the free trimethylphosphine. Such an approach was also expanded to Zr, V, and W complexes.19 In this study, we focused our attention on the H2CPPh3 ylide, since this reagent is easy to prepare on a large scale and can be readily made salt free via the use of the base Na[N(Si(CH3)3)2] with the commercially available phosphonium salt [H3CPPh3][Br].20 In addition, utilizing the ylide as a base results in facile separation of the unwanted salt from the desired neutral alkylidene, as previously shown by Schrock and Arnold.1,11,12 Herein, we present a facile approach to preparing and characterizing tantalum aryloxide complexes: namely, [(Ar′O)2Ta(CH3)3] (1) and [(Ar′O)2Ta(CH3)Cl2] (2). Although complex 1 is surprisingly inert toward methane elimination (under mild or forcing conditions), treatment of 2 with an excess of the H2CPPh3 ylide results in the formation of a rare example of the methylidene chloride complex [(Ar′O)2TaCH2(Cl)(H2CPPh3)] (3), which has been structurally ascertained in solution and solid state phases.

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Figure 1. Solid-state structure of complex 1 displaying thermal ellipsoids at the 50% probability level. All hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (deg): Ta1−O1, 1.8972(12); Ta1−O2, 1.9162(12); Ta1−C73, 2.1710(17); Ta1−C74, 2.1512(17); Ta1−C75, 2.1265(17); O1−Ta1−O2, 167.34(5), C73− Ta1−C74, 120.25(7); C73−Ta1−C75, 127.55(7); C74−Ta1−C75, 112.18(7); C73−Ta1−O1, 85.27(6); C74−Ta1−O1, 95.47(6); C75− Ta1−O1, 91.40(6).

RESULTS AND DISCUSSION Synthesis and Characterization of Tantalum Methyl Precursors. Transmetalation of [TaCl2(CH3)3]11,21,22 with 2 equiv of NaOAr′ (−OAr′ = 2,6-bis(diphenylmethyl)-4-tertbutylphenoxide)8 in benzene gave the complex [(Ar′O)2Ta(CH3)3] (1) in 92% isolated yield as a white solid after separation of the salt (Scheme 1). The 1H NMR spectrum of 1

Consequently, the two aryloxides occupy the axial sites, where the O1−Ta1−O2 angle of 167.34(5)° deviates from linearity. The aryl motifs bend toward a single methyl group (C73), yielding an overall structure similar to that of the trimethyl derivatives reported by Rothwell.7 However, notable structural dissimilarities of [(ArO)2Ta(CH3)3] in comparison to 1 are the tantalum−methyl distances. In the previously reported example by Rothwell, one of the tantalum−methyl bonds, which experiences crowding from the aryloxide ligands, is elongated (2.248(10) Å) in comparison to the two other Ta−CH3 bonds (2.138(10) and 2.136(10) Å).7 In the case of 1, no methyl group experiences such significant elongation. However, unlike [(ArO)2Ta(CH3)3] (Ar = (2,6-tBu2)-4-XC6H2, where X = H, OMe), which undergoes elimination of methane upon thermolysis at 120 °C over the duration of 24 h,6,7 complex 1 is remarkably stable at 100 °C for days. We speculate that the thermal stability of complex 1 is a result of the strategic ligand design, where cyclometalation of the diphenylmethyl groups at the 2,6-positions is disfavored due to the unlikely formation of a seven-membered metallacycle. Likewise, activation of the methine group is not likely, due to its remote position from the metal. Furthermore, the lack of an elongated methyl group, which is observed in the thermally unstable [(ArO)2Ta(CH3)3], most likely renders complex 1 less susceptible to alkane elimination and subsequent cyclometalation of the ligand. Unfortunately, we also do not observe formation of the hypothetical methylidene methyl species [(Ar′O)2TaCH2(CH3)] occurring through α-hydrogen abstraction. Elimination of methane from other reported TaV trimethyl species to form a methylidene methyl complex was observed by Fryzuk and co-workers,13 while double α-hydrogen abstraction in a TaV tetrakis-methyl species to generate a bismethylidene was reported by Ozerov and co-workers.14 For

Scheme 1. Synthesis of the Trimethyl Complex 1

recorded in C6D6 revealed all three methyl groups (0.68 ppm) to be chemically equivalent on the time scale of the NMR experiment. Additionally, the observation of a single chemical shift for the diagnostic tert-butyl (1.04 ppm) and methine (6.31 ppm) protons indicated chemical equivalence of the two aryloxide ligands. On the basis of previous work by Rothwell and co-workers utilizing 2,6-di-tert-butylphenoxide ligands, we anticipated the structure of 1 to be similar to that of the known complex [(ArO)2Ta(CH3)3] (Ar = (2,6-tBu2)C6H3), which has a trigonal-bipyramidal (TBP) structure where the aryloxide ligands occupy the axial positions (τ = 0.99).7,23 Accordingly, X-ray diffraction studies performed on a single crystal of 1, grown from cooling a concentrated toluene solution layered with pentane to −37 °C, confirmed a TaV center confined to such a geometry (τ = 0.92, Figure 1).23 A list of cell parameters and refinement data for 1 can be found in the Supporting Information. The three Ta−CH3 distances range from 2.13 to 2.17 Å with all three methyl groups occupying the equatorial sites in a trigonal-planar fashion (∑C−Ta−C angles 359.98°). 4193

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Scheme 2. Synthesis of Complex 2 and the Terminal Methylidene Complex 3

complex 1, however, addition of Lewis bases such as pyridine, PR3 (R= −CH3, −Ph), or ylides such as H2CPPh3 do not promote α-hydrogen abstraction, presumably because these do not coordinate to the Ta center. The inertness of 1 toward α-hydrogen abstraction led us to explore another derivative that would be more susceptible to binding of a base, therefore resulting in alkane elimination. Accordingly, the compound [TaCl2(CH3)3]11,21,22 was treated with 2 equiv of HOAr′ in toluene, which resulted in effervescence of the solution with a concomitant color change from yellow to orange. After workup a single new metal product, [(Ar′O)2Ta(CH3)Cl2] (2), could be isolated in 92% yield as a yellow solid (Scheme 2). The 1H NMR spectrum of 2 is quite similar to that of 1, displaying chemical shifts of the tertbutyl, methine, and aromatic protons in agreement with both aryloxide ligands being related by symmetry. The Ta−CH3 moiety, which integrates to 3H, was observed at 1.39 ppm. Single crystals of 2 can be readily grown from slow evaporation of diethyl ether into toluene at 25 °C. All cell parameters and refinement data can be found in the Supporting Information. Although the solid-state molecular structure of 2 reveals the expected five-coordinate TaV center having two aryloxides, one methyl, and two chlorides, the geometry about the metal center is strictly square pyramidal (SP) with a τ value of 0.15 (Figure 2).23 The two chlorides and two aryl oxides occupy the equatorial sites, with the methyl taking the lone axial position. The Ta−CH3 and Ta−OAr′ distances are slightly shorter than those observed for 1. Synthesis and Characterization of a Tantalum Methylidene Complex. The fact that complex 2 experiences a SP geometry implies that electronic factors must be governing the orientation of these ligands. Notably, this is not an isolated case, since similar species showing a SP geometry have been reported, namely the π-loaded complex [(ArO)2TaCl3] (Ar = (2,6-tBu2)C6H3, (2,6-Ph2-3,5-X2)C6H, where X = −H, −Ph, −Me, −iPr, −tBu)24 and the monoalkylated complexes [(ArO)2TaCl2(R)] (Ar = (2,6-Ph2-3,5-X2)C6H, where X = −H, −Ph and R = C5H9, C6H11).25 Likewise, the open axial coordination site in 2 could allow for further reactivity, including deprotonation reactions. Hence, addition of 1 equiv of the H2CPPh320 ylide resulted in a complicated mixture of products which did not show free ylide, as gauged by 31P{1H} NMR spectroscopy. As indicated in Figure 3, addition of 2 equiv of H2CPPh3 still showed complete consumption of the base, with formation of at least four new phosphorus-containing products when the reaction was monitored by 31P{1H} NMR

Figure 2. Solid-state structure of complex 2 displaying thermal ellipsoids at the 50% probability level. All hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (deg): Ta1−Cl1, 2.3360(5); Ta1−Cl2, 2.3709(4); Ta1−O1, 1.8802(11); Ta1−O2, 1.8707(11); Ta1−C73, 2.1046(17); Cl1−Ta1−Cl2, 84.690(17); Cl1− Ta1−O1, 91.73(4); Cl2−Ta1−O1, 166.72(4); Cl1−Ta1−O2, 157.47(4); Cl2−Ta1−O2, 85.89(4); O1−Ta1−O2, 92.79(5); Cl1− Ta1−C73, 100.74(6); Cl2−Ta1−C73, 96.66(5); O1−Ta1−C73, 96.58(6); O2−Ta1−C73, 100.65(6).

Figure 3. Stacked 31P{1H} NMR spectra showing the effect of stoichiometry of H2CPPh3 on addition to complex 2.

spectroscopy. For the latter reaction, we also observed precipitation of a white solid, which we speculate to be the phosphonium salt [H3CPPh3][Cl]. Gratifyingly, adding 3 equiv of H2CPPh3 (at −78 °C) dramatically rectified the reaction, with one major product being observed at 34.73 ppm in the 31 1 P{ H} NMR spectrum (Figure 3), which is also accompanied by the formation of a colorless precipitate. Adding 4 equiv or 4194

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Figure 4. 1H and 1H{31P} NMR (top left), HSQC NMR (top right), and 1H NMR (bottom) spectra of complex 3.

An X-ray diffraction study performed on a single crystal of 3 (Figure 5) confirmed the formation of a terminal tantalum

more of H2CPPh3 also resulted in formation of one major product, but free ylide was noticeable in the mixture (Figure 3). Therefore, the use of 3 equiv of H2CPPh3 appears to be critical for the clean formation of a new phosphorus-containing product. Fractional crystallization of the new product from a concentrated toluene solution layered with pentane and stored at −37 °C resulted in the formation of yellow microcrystalline material in 64% isolated yield. The 1H NMR spectrum recorded in C6D6 of the new solid revealed a single −OAr′ environment, as gauged by one tert-butyl resonance at 1.06 ppm which integrates to 18H. Although the aromatic region in the 1H NMR spectrum is highly congested, the 1H{31P} NMR spectrum clearly identified three aromatic resonances coupled to phosphorus, suggestive of a triphenylphosphine group of an ylide being present. The number of additional aromatic resonances, representative of the −OAr′ group, also suggest the ligand to be locked into position where there is no rotation about the Ta−OAr′ bond. However, the two most salient features in the 1H NMR spectrum are the resonances centered at 8.67 and 0.98 ppm (Figure 4), which both integrate to 2H. The former resonance displays a doublet pattern due to longrange coupling to phosphorus (4JHP = 3 Hz) while the latter resonance displays another doublet with a stronger coupling to phosphorus (2JHP = 17 Hz). The 1H{31P} NMR experiment also showed that both former resonances collapse to singlets. Figure 4 also portrays a portion of a multiplicity-edited HSQC spectrum correlating the resonance at 8.67 ppm in the 1H NMR spectrum to the highly downfield 13C NMR resonance at 232.9 ppm having a 1JCH value of ∼140 Hz. A DEPT-135 NMR experiment also indicated the 13C NMR resonance at 232.9 ppm to be an sp2 carbon, which unquestionably supports the assignment of a methylidene group bound to the tantalum metal center. Therefore, our multinuclear NMR spectroscopic data suggest this new species to be a tantalum methylidene complex containing an ylide group, namely [(Ar′O)2Ta CH2(Cl)(H2CPPh3)] (3).

Figure 5. Solid-state structure of complex 3, displaying thermal ellipsoids at the 50% probability level. All hydrogen atoms, with the exception of the methylidene (C92), are omitted for clarity. Selected bond lengths (Å) and angles (deg): Ta1−Cl1, 2.4403(10); Ta1−O1, 1.916(3); Ta1−O2, 1.913(3); Ta1−C73, 2.253(4); Ta1−C92, 1.926(4); Cl1−Ta1−O1, 83.78(8); Cl1−Ta1−O2, 84.09(8); O1− Ta1−O2, 161.39(11); Cl1−Ta1−C73, 138.87(10); O1−Ta1−C73, 90.47(13); O2−Ta1−C73, 89.42(13); Cl1−Ta1−C92, 117.64(13); O1−Ta1−C92, 99.33(14); O2−Ta1−C92, 98.78(14); C73−Ta1− C92, 103.46(15).

methylidene. All relevant cell parameters and refinement data are given in the Supporting Information. The terminal methylidene possesses a TaC(92) distance of 1.926(4) Å, clearly shorter than the Ta−CH3 bond in 1 or 2 (vide supra) and certainly contracted from the dative bond observed for the bound ylide Ta−C(73) (2.253(4) Å). The methylidene distance in 3 is also short in comparison to Guggenberger and Schrock’s [Cp2TaCH2(CH3)] (2.026(10) Å),26 Arnold’s 4195

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X2C6H, where X = −H, −Ph, −Me, −iPr, −tBu)24 as well as the monoalkylated complexes [(ArO)2TaCl2(R)] (Ar = 2,6-Ph23,5-X2C6H, where X = −H, −Ph and R = C5H9, C6H11).25 In addition, the cisoid orientation of the two aryloxides in complex 2 indicates that kinetic factors alone are not dominant for the formation of this type of geometry. In general, ML5 systems have an intrinsic tendency to adopt a TBP structure, which can be easily rationalized within MO theory or even within VSEPR theory.27 The finding that strong σ-donor methyl ligands occupy the equatorial positions in TBP as well as the apical position in SP, as in 1 and 2, respectively, is also a well-understood behavior.27 Moreover, as described by Albright, Burdett, and Whangbo,27 one can argue why low-spin d0 to d4 complexes having σ-only ligands prefer a TBP geometry as opposed to SP due to the population of only nonbonding orbitals (2a2 and 2b1, Figure 7). However, compounds 1 and 2 both possess a d0 Ta center yet differ significantly in their respective geometries. This discrepancy can be explained in terms of the available dπ-orbitals on the Ta center in TBP and SP configurations. Figure 7 introduces a simplified MO diagram for [(Ar′O)2TaCl2(CH3)] in a SP (2) and in a TBP (2-TBP) arrangement to rationalize the preference for the former. In the case of TBP, there are two low-lying d-orbitals (dxz and dyz) capable of each accepting a lone pair from the aryloxide ligands. These d-orbitals are oriented perfectly along the z axis, accommodating the donated π-electrons from the aryloxides in the axial positions, highlighted in red in the MOs 1a2 and 1b1. The much better π-donor chloride ligands in equatorial positions have to share the same d-orbitals (highlighted in blue), and their overlap is not optimal for ligands in the axial positions. On the other hand, the hybridized 2a1 has a very good spatial overlap with the π-orbitals of the chlorides, represented by 1a1, but this πdonation is disfavored by the high energy of 2a1, stemming from its antibonding nature in the σ subspace.27,28 Rearrangement into a SP geometry eliminates the hybridization and antibonding character of dxy, thus rendering this orbital low in energy for π-accepting capabilities (essentially nonbonding). First, this results in three low-lying empty d orbitals (2b2 and 2e) of appropriate π-symmetry in contrast to TBP geometry, which exhibits only two low-lying orbitals for accepting π-electrons (2a2 and 2b1). Second, the orbitals of the 1e set are shared only by two ligands, a strong chloride and a weaker aryloxide π-donor, which finally allows the chloride ligands to exert a superior π-donationone can observe this competition as a function of their π-trans influence. The latter also clarifies the electronic origin of cis chlorides and cis aryloxides in 2; in this way a weak and a strong π-donor in trans positions share the same d-orbital allowing the largest stabilization available through π-donation. The 1b2 orbital is shared by all four ligands in the π-subspace. In summary, the preference for TBP over SP geometry by pure σ-donors is compromised when there are more than two strong π-donor ligands in the complex due to the better π-interactions in the SP configuration. In line with this concept, we also demonstrated computationally, using full models for the ligands (OAr′ = 2,6bis(diphenylmethyl)-4-tert-butylphenoxide), that the SP geometry of 2 is approximately 6 kcal mol−1 more stable than the TBP arrangement. Admittedly, besides electronic effects, dispersion also plays an important role in stabilizing the SP arrangement over TBP by 9.9 kcal mol−1 as a result of the cis arrangement of the aryloxide ligands in SP and trans

bis-benzamidinate system [{PhC(NSiMe3)2}2TaCH2(CH3)] (2.02(2) Å),12 TaCH2 (2.09(2) Å) in Fryzuk’s [(P2N2)Ta CH2(CH3)] (P2N22− = PhP(CH2SiMe2NSiMe2CH2)2PPh)13 and Bercaw’s [(1,2-SiMe2)2(η5-C5H-3,5-(CHMe2)2)(η5-C5H24-CMe3)]TaCH2(CH3) (2.154 Å).15 The aforementioned complexes all suffer from 2-fold disorder about the Ta−CH3 and TaCH2 bonds, which results in an artificial elongation of the TaCH2 distance or shortening of the Ta−CH3 bond lengths. This inherent disorder severely limits the number of reliable tantalum−methylidene bond distances in the literature. However, the TaCH2 bond length of Ozerov’s fivecoordinate tantalum bis-methylidene [(PNP)Ta(CH2)2] (1.9385(17) Å)14 is comparable to that of 3. As shown in Figure 3, the hydrogens on the sp2-alkylidene carbon in 3 are oriented along the O−Ta−O vector, which is expected given that the tantalum dπ orbital available for πbonding with C(92) must be aligned with the C(72)−Ta(1)− Cl(1) axis. The gross geometry at TaV is best regarded as a distorted SP with a τ value of 0.39,23 whereby the two aryloxides, chloride, and ylide occupy the equatorial sites. In addition to the diphenylmethyl groups of the aryloxides providing steric protection to the methylidene pocket, the PPh3 motif of the ylide is also oriented upward toward the methylidene to provide some encumbrance (P(1)−C(73)− Ta(1), 121.21(19)°). A space-filling model further reveals how protected the methylidene unit is when viewed down the H2CTa axis (Figure 6).

Figure 6. Space-filling model of 3 viewed down the H2CTa axis. The atom labeling is as follows: carbon in gray, tantalum in dark green, chloride in light green, oxygen in red, and phosphorus in purple. Hydrogen atoms have been omitted for clarity.

TBP versus SP Geometries in Complexes 1 and 2. The replacement of two methyl ligands in 1 for strongly π-donating chlorides, as in the case of 2, drives a geometric change from TBP to SP. These dramatic geometrical differences observed between their solid-state structures suggest that electronic factors are likely governing these coordination geometries. Similar SP geometries were also observed by Rothwell when tantalum systems were loaded with π-donor ligands, such as in the case of [(ArO)2TaCl3] (Ar = 2,6-tBu2C6H3, 2,6-Ph2-3,54196

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Figure 7. Simplified MO diagram representing the most important π-donations (filled ligand-based MOs at the bottom) and intrinsic (σ) d-orbital splitting pattern (vacant metal-based MOs in the top) in SP (left) and TBP (right) geometries for 2 and 2-TBP, respectively. For clarity, systems are approximated with C2v (TBP) and C4v (SP) symmetries and atomic contributions from oxygen and chloride atoms are shown in red and blue, respectively, for π-donation representing orbitals. Relative energies are given in kcal mol−1 in parentheses.

methyls are replaced by chlorides, not only do we observe a notable structural change (from TBP in 1 to SP in 2) but also such a species displays distinctly different reactivity when treated with a Brønsted base that can also serve as a Lewis base. The geometry of 2 is governed by electronic factors due to the low-lying e set, which allows for optimal π-donation from the equatorial ligands and a fine balance involving the sharing of an empty dπ orbital with a strong and weaker π-donor. We have discovered that treating 2 with a slight excess of ylide H2CPPh3 results in dehydrohalogenation as well as coordination of the ylide to form a rare example of a tantalum complex having a terminal methylidene and chloride ligand, 3. A structural study has also confirmed such connectivity, and unlike most tantalum methylidenes, which contain a proximal or symmetry-related methyl ligand, there is no disorder involving the methylidene ligand in 3. We are presently unsure of why 3 equiv of ylide is needed to cleanly produce complex 3, but it is likely that an additional amount of H2CPPh3 is necessary as a ligand, possibly playing a role in stabilizing putative intermediates such as [(Ar′O) 2 Ta(CH 3 )(H 2 CPPh 3 ) 2 ] 2 + and [(Ar′O) 2 Ta CH2(H2CPPh3)2]+. We are presently studying the reactivity of 3, since such a species could be a precursor to an unsaturated [(Ar′O)2TaCH2(Cl)] or the transient, three-coordinate

arrangement in TBP geometry. In contrast to the case for 2, TBP was found to be the most stable structure for 1 and for the hypothetical tantalum complex [(Ar′O)2TaCl(CH3)2]. This agrees well with a recently reported niobium complex, [(Ar′O)2NbCl(CH3)2], which adopts a TBP geometry.10 In the latter cases, all of our attempts have failed to optimize in the SP arrangement; these calculations converged to another TBP structure with aryloxide ligands in cis positions, indicating that the SP arrangement is not even a local minimum on the potential energy surface when there are fewer than two chlorides in the system.

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CONCLUSIONS We have reported the synthesis of tantalum complexes supported by the sterically encumbering and robust aryloxide 2,6-bis(diphenylmethyl)-4-tert-butylphenoxide, −OAr′. Surprisingly, it was found that the trimethyl species 1 does not eliminate methane to form the transient methylidene [(Ar′O)2TaCH2(CH3)], in contrast to Rothwell’s derivative having aryloxides such as 2,6-tBu2-phenoxide and 2,6-tBu2-4methoxyphenoxide.7 In fact, complex 1 is thermally stable and addition of base does not promote α-hydrogen abstraction or decomposition pathways such as cyclometalation. When two 4197

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yellow solid from the solution. After it was stirred for 1 h, the light yellow solid was isolated on a medium-porosity glass frit and washed with 40 mL of pentane, yielding 4.049 g of light yellow solid. A second crop of product could be isolated upon concentrating the filtrate to approximately 2 mL. With addition of pentane a light yellow solid precipitated. After 2 h of stirring the yellow solid was isolated on a medium-porosity glass frit and washed with 20 mL of pentane, yielding an additional 465 mg of product. Yellow single crystals of 2 were obtained by slow evaporation of a concentrated diethyl ether solution into toluene at room temperature. Yield: 92% (4.514 g, 3.669 mmol). 1 H NMR (25 °C, 500.39 MHz, C6D6): δ 7.25 (s, 4H, Ar-Hmeta), 7.13 (d, 3JHH = 7.31 Hz, 16H, Ar-Hortho), 7.03 (t, 3JHH = 7.42 Hz, 16H, ArHmeta), 6.97 (t, 3JHH = 7.23 Hz, 8H, Ar-Hpara), 6.60 (s, 4H, CH(Ph)2), 1.39 (s, 3H, CH3), 1.04 (s, 18H, C(CH3)3). 13C NMR (25 °C, 100.61 MHz, C6D6): δ 56.79 (CAr), 147.04 (CAr), 144.28 (CAr), 134.76 (CAr), 130.18 (CHAr), 128.81 (CHAr), 127.17 (CHAr), 126.88 (CHAr), 50.73 (CH(Ph)2), 34.62 (C(CH3)3), 31.30 (C(CH3)3). Anal. Calcd for C73H69Cl2O2Ta: C, 71.27; H, 5.65. Found: C, 71.42; H, 5.63. Synthesis of [(Ar′O)2TaCH2(Cl)(H2CPPh3)] (3). At −78 °C in a 20 mL scintillation vial containing a 10 mL yellow toluene solution of 2 (500 mg, 0.406 mmol) was placed dropwise a yellow 5 mL toluene solution of H2CPPh3 (337 mg, 1.219 mmol) via a glass pipet, resulting in a slight darkening of the reaction mixture. The reaction mixture was stirred overnight, while the solution gradually warmed to room temperature. After the mixture was stirred for 12 h, a white precipitate had formed, which was accompanied by a color change to light orange. The reaction mixture was subsequently filtered through a mediumporosity glass frit containing Celite. All volatiles were removed from the resulting yellow filtrate under reduced pressure, yielding a yellowbrown solid. The solid was dissolved in approximately 4 mL of pentane, and upon addition of approximately 10 mL of pentane a yellow solid precipitated. After the suspension was stored at −37 °C for 12 h, the yellow solid was isolated on a medium-porosity glass frit and rinsed with 5 mL of cold pentane (−37 °C) to yield 315 mg of yellow product. A second crop of product could be isolated upon concentrating the filtrate to approximately 3 mL, and with addition of pentane a light yellow solid precipitated. After 30 min of stirring the yellow solid was isolated on a medium-porosity glass frit and washed with 10 mL of pentane, yielding an additional 64 mg of product. Yellow needles of 3 were grown from a concentrated benzene solution layered with pentane stored at room temperature. Yield: 64% (0.379 g, 0.258 mmol). 1H NMR (25 °C, 500.39 MHz, C6D6): δ 8.67 (d, 4JHP = 2.62 Hz, 2H, TaCH2), 7.94 (d, 3JHH = 7.46 Hz, 4H, Ar-Hortho), 7.60 (m, 4H, Ar-H), 7.54 (d, 3JHH = 7.32 Hz, 4H, Ar-Hortho), 7.48 (s, 2H, Ar-Hmeta), 7.37 (d, 3JHH = 7.37 Hz, 4H, Ar-Hortho), 7.28 (m, 6H, Ar-H/ CH(Ph)2), 7.19 (t, 3JHH = 7.23 Hz, 4H, Ar-Hmeta), 7.13 (t, 3JHH = 7.28 Hz, 2H, Ar-Hpara), 7.04 (m, 10H, Ar-H), 6.92 (dd, 3JHH = 7.78 Hz, 3JHP = 12.00 Hz, 6H, Ar-Hortho), 6.86 (t, 3JHH = 7.41 Hz, 3H, Ar-Hpara), 6.74 (t, 3JHH = 7.25 Hz, 4H, Ar-Hmeta), 6.65 (m, 4H, Ar-Hpara/CH(Ph)2), 6.57 (td, 3JHH = 7.87 Hz, 4JHP = 2.71 Hz, 6H, Ar-Hmeta), 1.06 (s, 18H, C(CH3)3), 0.98 (d, 2JHP = 17.49 Hz, 2H, CH2PPh3). 13C{1H} NMR (25 °C, 100.61 MHz, C6D6): δ 232.94 (d, 3JCP = 4.86 Hz, TaCH2), 158.01 (s, CAr), 147.52 (s, CAr), 146.31 (s, CAr), 145.44 (s, CAr), 143.45 (s, CAr), 133.39 (d, JCP = 9.13 Hz, CAr), 132.28 (s, CAr), 132.23 (d, JCP = 1.97 Hz, CAr), 132.12 (s, CAr), 131.06 (s, CAr), 130.90 (s, CAr), 130.59 (s, CAr), 130.40 (s, CAr), 128.61 (s, CAr), 128.40 (s, CAr), 128.35 (s, CAr), 126.72 (br s, CAr), 126.55 (br s, CAr), 126.20 (s, CAr), 126.11 (br s, CAr), 125.85 (s, CAr), 52.20 (s, CH(Ph)2), 49.26 (s, CH(Ph)2), 34.38 (s, C(CH3)3), 31.52 (s, C(CH3)3), 22.64 (d, 1JCP = 24.97 Hz, CH2P(Ph)3). 31P{1H} NMR (25 °C, 162.0 MHz, C6D6): δ 34.73 (s). Crystallographic Details. Suitable crystals for analysis of 1 and 2 were placed onto the tip of a MiTeGen loop coated in NVH oil and mounted on an Apex Kappa Duo diffractometer. The data collection was carried out at 150 K using Mo Kα radiation (graphite monochromator). A randomly oriented region of reciprocal space was surveyed to achieve complete data with a redundancy of 4. Sections of frames were collected with 0.50° steps in ω and ϕ scans. Data collection for 3 was collected at 100 K at the Advanced Photon Source in Argonne National Laboratory using synchrotron radiation (λ = 0.41328, silicon 111 and 311 monochromators, and two mirrors to

methylidyne [(ArO)2TaCH]. In fact, such species could represent an important reagent in the context of C−H activation and functionalization, analogous to what Basset and co-workers have reported for tantalum centers supported on a silica surface.4

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EXPERIMENTAL SECTION

General Procedures. Unless otherwise stated, all operations were performed in a M. Braun Lab Master double-drybox under an atmosphere of purified nitrogen or using high-vacuum standard Schlenk techniques under a nitrogen atmosphere. Anhydrous benzene, pentane, and toluene were purchased in Sure-Seal reservoirs (18 L) and dried by passage through two columns of activated alumina and a Q-5 column. THF and Et2O were distilled, under nitrogen, from purple sodium benzophenone ketyl and stored over sodium metal. Distilled THF and Et2O were transferred under vacuum into thickwalled reaction vessels before being placed in a drybox. Deuteriobenzene was degassed by freeze−pump−thaw cycles and stored over 4 Å molecular sieves. Celite, alumina, and 4 Å molecular sieves were activated under vacuum overnight at 200 °C. Compounds HOAr′ (Ar′ = 2,6-CHPh 2 ) 2 -4- t Bu-C 6 H 2 ), 8 NaOAr′, 8 H 2 CPPh 3 , 2 0 and [TaCl2(CH3)3]11,21,22 were prepared by following the literature procedures. [TaCl2(CH3)3] must be prepared pure and free of [TaCl3(CH3)2]; otherwise, there is a small amount of co-product [(Ar′O)2Ta(CH3)2Cl] formed in the reaction mixture during the synthesis of 2. All other chemicals were purchased from commercial sources and used as received. 1H, 13C, and 31P NMR spectra were recorded on 500 and 400 MHz NMR spectrometers. 1H and 13C NMR spectra are reported with reference to residual 1H solvent resonances of C6D6 at 7.16 and 128.06 ppm, respectively. 31P NMR spectra are reported with respect to external H3PO4 (aqueous solution, 0.0 ppm). Elemental analyses were performed at Robertson Microlit Laboratories. Multiple attempts to obtain satisfactory combustion analysis for complex 3 failed. Therefore, we include in the Supporting Information multinuclear NMR spectra for this complex in lieu of elemental analysis and as proof of bulk purity. Synthesis of [(Ar′O)2Ta(CH3)3] (1). To a 20 mL scintillation vial containing a 2 mL light yellow toluene solution of [TaCl2(CH3)3] (150 mg, 0.505 mmol) was added dropwise a white 18 mL toluene slurry of NaOAr′ (515 mg, 1.021 mmol), resulting in immediate precipitation of an off-white solid and little color change of the solution. After it was stirred for 16 h at room temperature, the reaction mixture was filtered through a Celite plug and a faint yellow solution was obtained. All volatiles were removed under reduced pressure, resulting in the formation of a clear oil. To the resulting oil was added 10 mL of pentane, and upon stirring the precipitation of a white solid had occurred. After the resulting suspension was stored at −37 °C for 8 h, the white precipitate was collected on a medium-porosity glass frit and rinsed with 10 mL of pentane. White single crystals of 1 were obtained by layering a concentrated toluene solution with pentane and storing at −37 °C. Yield: 92% (553 mg, 0.465 mmol). 1H NMR (25 °C, 400.11 MHz, C6D6): δ 7.23 (s, 4H, Ar-Hmeta), 7.21 (d, 3JHH = 7.50 Hz, 16H, Ar-Hortho), 7.10 (t, 3JHH = 7.50 Hz, 16H, Ar-Hmeta), 7.01 (t, 3 JHH = 7.28 Hz, 8H, Ar-Hpara), 6.31 (s, 4H, CH(Ph)2), 1.04 (s, 18H, C(CH3)3), 0.68 (s, 9H, CH3). 13C NMR (25 °C, 100.61 MHz, C6D6): δ 156.80 (CAr), 145.00 (CAr), 144.78 (CAr), 133.29 (CAr), 130.36 (CHAr), 128.60 (CHAr), 127.29 (CHAr), 126.68 (CHAr), 60.31 (CH3), 50.55 (CH(Ph)2), 34.48 (C(CH3)3), 31.35 (C(CH3)3). Anal. Calcd for C75H75O2Ta: C, 75.74; H, 6.36. Found: C, 75.96; H, 6.26. Synthesis of [(Ar′O)2Ta(CH3)Cl2] (2). In a 250 mL round-bottom flask containing 20 mL of a light yellow toluene solution of [TaCl2(CH3)3] (1.190 g, 4.007 mmol) was added dropwise a white 80 mL toluene slurry of HOAr′ (3.867 mg, 8.019 mmol), resulting in a initial color change to yellow-brown and finally to an orange homogeneous solution after final addition of the ligand. After it was stirred for 10 h at room temperature, the reaction mixture was filtered through a Celite plug on a medium-porosity glass frit and the orange filtrate was concentrated to approximately 20 mL. Pentane (100 mL) was added to the remaining toluene solution, which precipitated a light 4198

dx.doi.org/10.1021/om500197k | Organometallics 2014, 33, 4192−4199

Organometallics

Article

(7) Chamberlain, L. R.; Rothwell, I. P.; Huffman, J. C. J. Am. Chem. Soc. 1986, 108, 1502. (8) Searles, K.; Tran, B. L.; Pink, M.; Chen, C.-H.; Mindiola, D. J. Inorg. Chem. 2013, 52, 11126. (9) Franke, S. M.; Tran, B. L.; Heinemann, F. W.; Hieringer, W.; Mindiola, D. J.; Meyer, K. Inorg. Chem. 2013, 52, 10552. (10) Searles, K.; Keijzer, K.; Chen, C.-H.; Baik, M.-H.; Mindiola, D. J. Chem. Commun. 2014, 50, 6267. (11) Schrock, R. R.; Sharp, P. R. J. Am. Chem. Soc. 1978, 100, 2389. (12) Dawson, D. Y.; Arnold, J. Organometallics 1997, 16, 1111. (13) Fryzuk, M. D.; Johnson, S. A.; Rettig, S. J. Organometallics 1999, 18, 4059. (14) Gerber, L. C. H.; Watson, L. A.; Parkin, S.; Weng, W.; Foxman, B. M.; Ozerov, O. V. Organometallics 2007, 26, 4866. (15) Chirik, P. J.; Zubris, D. L.; Ackerman, L. J.; Henling, L. M.; Day, M. W.; Bercaw, J. E. Organometallics 2003, 22, 172. (16) Antonelli, D. M.; Schaefer, W. P.; Parkin, G.; Bercaw, J. E. J. Organomet. Chem. 1993, 462, 213. (17) (a) Van, A. A.; Burger, B. J.; Gibson, V. C.; Bercaw, J. E. J. Am. Chem. Soc. 1986, 108, 5347. (18) Sharp, P. R.; Schrock, R. R. J. Organomet. Chem. 1979, 171, 43. (19) (a) Schwartz, J.; Gell, K. I. J. Organomet. Chem. 1980, 184, C1. (b) Johnson, L. K.; Frey, M.; Ulibarri, T. A.; Virgil, S. C.; Grubbs, R. H.; Ziller, J. W. J. Am. Chem. Soc. 1993, 115, 8167. (c) Buikink, J.-K. F.; Teuben, J. H.; Kooijman, H.; Spek, A. L. Organometallics 1994, 13, 2922. (20) Bestmann, H. J.; Stransky, W.; Vostrowsky, O. Chem. Ber. 1976, 109, 1694. (21) Fowles, G. W.; Rice, D. A.; Wilkins, J. D. Dalton Trans. 1973, 961. (22) Sattler, A.; Ruccolo, S.; Parkin, G. Dalton Trans. 2011, 40, 7777. (23) (a) τ is defined as (A − B)/60, with A and B being the largest and the smallest transoid ligand−metal−ligand angles in the base of an approximate square pyramidal geometry, where τ = 0 for SP and τ = 1 for TBP. (b) Addison, A. W.; Rao, T. N.; Reedijk, J.; van Rijn, J.; Verschoor, G. C. Dalton Trans. 1984, 1349. (24) (a) Chamberlain, L. R.; Rothwell, I. P.; Huffman, J. C. Inorg. Chem. 1984, 23, 2575. (b) Vilardo, J. S.; Lockwood, M. A.; Hanson, L. G.; Clark, J. R.; Parkin, B. C.; Fanwick, P. E.; Rothwell, I. P. Dalton Trans. 1997, 3353. (25) Schweiger, S. W.; Salberg, M. M.; Pulvirenti, A. L.; Freeman, E. E.; Fanwick, P. E.; Rothwell, I. P. Dalton Trans. 2001, 2020. (26) Guggenberger, L. J.; Schrock, R. R. J. Am. Chem. Soc. 1975, 97, 6578. (27) Aldright, T. A.; Burdett, J. K.; Whangbo, M. H. In Orbital Interactions in Chemistry; Wiley: New York, 1985. Transition metal pentacoordination has also been discussed in a paper. (28) Rossi, A. R.; Hoffmann, R. Inorg. Chem. 1975, 14, 365. (29) SAINT Software User’s Guide, Version 7.34a; Bruker AXS: Madison, WI, 2005. (30) Blessing, R. Acta Crystallogr., Sect. A 1995, A51, 33. (31) Altomare, A.; Cascarano, G.; Giacovazzo, G.; Guagliardi, A.; Burla, M. C.; Polidori, G.; Camalli, M. J. Appl. Crystallogr. 1994, 27, 435. (32) Sheldrick, G. M. Acta Crystallogr., Sect. A 2008, A64, 112. (33) Betteridge, P. W.; Carruthers, J. R.; Cooper, R. I.; Prout, K.; Watkin, D. J. J. Appl. Crystallogr. 2003, 36, 1487.

exclude higher harmonics). Final cell constants were calculated from the xyz centroids of a particular number of strong reflections for each crystal from the actual data collection after integration (SAINT).29 The intensity data were corrected for absorption (SADABS).30 The space groups were determined on the basis of intensity statistics and systematic absences. The structures were solved using SIR-9231 and refined (full-matrix least squares) using either SHELXL-9732 or the Oxford University Crystals for Windows system.33 A direct-methods solution was calculated, which provided most non-hydrogen atoms from the E map. Full-matrix least-squares/difference Fourier cycles were performed, which located the remaining non-hydrogen atoms. All non-hydrogen atoms were refined with anisotropic displacement parameters. The hydrogen atoms were placed in ideal positions and refined as riding atoms.

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ASSOCIATED CONTENT

* Supporting Information S

Figures, tables, and CIF files giving NMR spectra and crystallographic parameters of 1−3 and computational details. 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 for D.J.M.: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the University of Pennsylvania and the National Science Foundation (CHE-0848248 and CHE-1152123) for support of this research. For X-ray diffraction studies requiring synchrotron radiation, we wish to recognize ChemMatCARS Sector 15, which is principally supported by the National Science Foundation/Department of Energy under grant number NSF/CHE-0822838. Use of the Advanced Photon Source was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357.

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REFERENCES

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dx.doi.org/10.1021/om500197k | Organometallics 2014, 33, 4192−4199