Synthesis and Characterization of Cationic Tetramethyl Tantalum(V) Complex

Abstract

A novel method for the synthesis of the homogeneous homoleptic cationic tantalum(V)tetramethyl complex [(TaMe₄)⁺ MeB(C₆F₅)³⁻] from neutral tantalumpentamethyl (TaMe₅) has been described, by direct demethylation using B(C₆F₅)₃ reagent. The aforesaid higher valent cationic tantalum complex was characterized precisely by liquid state ¹H-NMR, ¹³C-NMR, and ¹H-¹³C-NMR correlation spectroscopy.

1. Introduction

Tantalum alkyl complexes are of vital importance in current organometallic chemistry, from the point of view of stoichiometric reactions and mainly in catalysis [1,2,3,4]. Since 1974, tantalumpentamethyl (TaMe₅, 1) has the honour to be the simplest homoleptic complex of this class of compounds [5,6]. 1 was discovered by Richard Schrock and the structure was not unravelled until 1992 by Albright [7] and Haaland et al. [8]. TaMe₅ is rather unstable thermally and, consequently, 1 is highly prone to autocatalytically degradation [6]. Recently, in our previous communication [9], we checked that grafting this unstable TaMe₅ complex to the silica surface by a Surface Organometallic Chemistry (SOMC) strategy enhances its thermal stability due to the formation of the stable grafted (≡Si-O-)TaMe₄. The latter complex proved to be a nice precursor for alkane metathesis, leading to the formation of a surface monopodal tantalum methylidene dimethyl catalyst [9,10].

With the precedents of Buchmeiser and co-workers who developed the first cationic tungsten-oxo-alkylidene-NHC complex (NHC = N-heterocyclic carbene) [11,12], where the NHC was introduced to stabilize the cationic metal centre, together with the also tungsten based cationic complex WMe₅⁺ [13], to improve the catalysts in terms of reactivity and selectivity, and taking into account the idea of increasing the electrophilicity on metal centre, we deepened into the idea of generating the cationic species of TaMe₅. This change planned to enter the field of predictive catalysis. Unfortunately, the drawback for tantalum with respect to tungsten is that the similar cationic Ta-complexes are scarce in literature [14,15,16], whereas there are several examples with stable cationic tantalum complexes containing one or more nitrogen [17], phosphorous [18] and cyclopentadienyl containing ligands [19,20,21].

2. Results

As a part of our continuing research program to explore any novel application of tantalumpentamethyl complex, we report here a method for the synthesis of the homogeneous cationic [TaMe₄]⁺ together with [MeB(C₆F₅)₃⁻] anion (2), starting from the neutral TaMe₅. The experimental procedure was straightforward: The simple mixture of bulky and non-coordinating boron Lewis acid, B(C₆F₅)₃, with 1, at low temperature (−40 °C) generates the cationic complex 2 (see Scheme 1), since above this temperature the undesired phenomenon of degradation appears in a rather facile way.

Preliminarily, we tried the synthesis of TaMe₄⁺ using an organic Lewis acid, tris(pentafluorophenyl)boron. The latter species is known to form a non-nucleophilic anion after demethylation reaction, a strategy that it is valid also with titanium [22,23], zirconium [24], or hafnium [24,25,26]. When the reaction was monitored by nuclear magnetic resonance (NMR) spectroscopy, at −40 °C, the peak corresponding to TaMe₅ at 0.9 ppm in ¹H-NMR (Figure 1a) almost completely disappeared in less than 15 min due to the fast reaction between TaMe₅ and B(C₆F₅)₃, and two new peaks at 2.6 ppm and 0.5 ppm appeared (Figure 1b) [27].

Based on the above observations one can assume two possibilities (Scheme 2); path I consists of simple demethylation by the B(C₆F₅)₃ from TaMe₅ leading to the formation of cationic tantalum complex; whereas in path II there would be a chance for the formation of a neutral tantalum complex by simple ligand exchange. Path II could take place in a one-step mechanism from the original TaMe₅, however it seems more plausible after path I.

Looking in detail at the ¹H-NMR chemical shift at +0.5 ppm, it is confirmed that it corresponds to [Me-B(C₆F₅)₃]⁻, in agreement with past studies [13,25,26]. The peak at 2.6 ppm corresponded to the cationic tantalum complex [TaMe₄]⁺, upfield with respect to the signal at 0.9 ppm assigned to the resonance for the methyl proton of TaMe₅.

Switching to ¹³C-NMR, the spectrum also clearly showed that the peak corresponding to TaMe₅ at 82.4 ppm (Figure 1c) was completely replaced by two peaks at 110.9 ppm [TaMe₄]⁺ and 10.8 ppm (Figure 1d) corresponding to the [Me-B(C₆F₅)₃]⁻ anion which is in good agreement with the literature value reported for the [Me-B(C₆F₅)₃]⁻ anion [26]. We also found in the ¹H-¹³C correlation spectra that the peak at 0.5 ppm in ¹H-NMR correlates with the peak at 10.8 ppm in ¹³C-NMR and the peak at 2.6 ppm in ¹H-NMR correlated with the peak at 110.9 ppm in ¹³C-NMR (Supplementary Materials Figure S1). In separate experiments we synthesised ¹³C labelled Ta(¹³CH₃)₅ and upon treatment with B(C₆F₅)₃ in dichloromethane solution, we identified incorporation of the labelled methyl in [(¹³CH₃)B(C₆F₅)₃]⁻ anion in the final product. The above experiment clearly indicates that in [MeB(C₆F₅)₃]⁻, ‘Me’ came from TaMe₅. These spectroscopic data strongly support the formation of cationic complex 2 and is in favour of path I. The ionic product generated by the reaction was fairly stable at temperatures below −40 °C and in the absence of light. However, upon warming a dichloromethane solution of 2 from −40 °C to 0 °C, a very fast decomposition of the cationic complex was observed with the release of gaseous methane.

Density Functional Theory (DFT) calculations (M06/Def2TZVPP(smd)//PBE0-d3bj/SVP [28], at temperature = 298.15 ºC and pressure = 1354 atm [29,30,31] to remove the overestimation of the entropy [32,33,34]) were envisaged to unravel the mechanistic insights of the first demethylation of TaMe₅. The energy barrier to overcome was found to be 6.8 kcal/mol (Figure 2a), releasing 15.4 kcal/mol (see

Table 1. Gibbs energy values (in kcal/mol) for (a) the energy barrier of the transition state of the anion demethylation in the reaction [MMe_x→MMe_(x−1)⁺ + Me⁻, (b) the relative stability of the next demethylated anion MMe_(x−1)⁺, (c) the intermediate MMe_(x−1)· after the loose of a radical methyl and (d) the addition of a chloride anion that leads to MMe_xCl⁻ (M = Ta or W).

Catalyst	(a)	(b)	(c)	(d)
TaMe5	6.8	−15.4	44.1	−5.8
WMe6	9.1	−13.8	18.8	3.6

), thus the favourable kinetic and thermodynamic character of the reaction, respectively, confirmed the observed experimental facile demethylation. Furthermore, the energy barrier for this reaction was even 2.3 kcal/mol lower with respect to the recent homologous process studied for WMe₆, together with a release of 1.6 kcal/mol more energy as well, confirms the experimental more facile demethylation by Ta than W based complexes [13]. It is worth pointing out that for TaMe₅, the radical demethylation was not in competition with the anion demethylation, because TaMe₄·radical was located 44.1 kcal/mol over TaMe₅, and thus enormously disfavoured by 25.3 kcal/mol with respect to the homologous WMe₅. Surprisingly, TaMe₅ is able to allocate easily a sixth anion like chloride, with an energy stabilization of 5.8 kcal/mol, compared with the destabilisation by 3.6 kcal/mol for WMe₆, due to steric hindrance in the metal centre of the WMe₆Cl⁻ anion.

Here, steric effects play a key role, and the more sterically crowded W centre provides less space for a seventh coordination around the metal centre, and at the same time facilitates the loose of a methyl ligand. To evaluate the steric hindrance around the metal, SambVca steric maps were used [35,36,37]. The %Vbur was 74.5% for TaMe₅ and 79.9% for WMe₆. It is worth pointing out that the quadrants for the latter hexamethylated complex were not equally occupied (see Figure 3), ranging from 75.8% to 84.0%, with 76.1% and 83.8% in between. Anyway, none of the quadrants was less occupied than the equally distributed TaMe₅ ones [38].

On the other hand, the substitution of a methyl ligand by a perfluorobenzene ligand from B(C₆F₅)₃, i.e., path II in Scheme 2, requires to overcome an energy barrier that is placed at 26.8 kcal/mol with respect to TaMe₅. However, from [MeB(C₆F₅)₃]⁻ an unaffordable energy barrier of 41.9 kcal/mol must be overcome (see Figure 2b). The overall reaction pathway following paths I and II in Scheme 2 is displayed in Figure 4. Consequently, the huge kinetic cost of path II confirms that once TaMe₅ is demethylated, the next cation intermediate TaMe₄⁺ cannot recover an anionic ligand, i.e., (C₆F₅)⁻, from [MeB(C₆F₅)₃]⁻, even though the next intermediate TaMe₄(C₆F₅) is 11.8 kcal/mol lower in energy than TaMe₅.

Switching to the grafted system (≡Si-O-)TaMe₄, the demethylation process requires to overcome an energy barrier of 9.8 kcal/mol (see Figure 5 for the corresponding transition state) and the next intermediate is 5.1 kcal/mol lower in energy than the initial supported catalyst. Here the difference with tungsten is significant, because apart from facing a more facile barrier by 4.8 kcal/mol, the relative stability of the next intermediate excludes the reversibility of the demethylation observed only for W. Overall, kinetically and thermodynamically, the demethylation process is more prone with Ta based systems. Consequently, we then prepared the surface complex of TaMe₅ after grafting on to the dehydroxylated Aerosil SiO₂₍₇₀₀₎ (Figure S2). Synthesized (≡Si-O-)TaMe₄ was treated with B(C₆F₅)₃ in order to get a cationic tantalum-methyl complex anchored on the surface of silica, [(≡Si-O-)TaMe₃]⁺. However, after several attempts (varying reaction temperature, reaction time and solvents) we were unable to identify any well-defined heterogeneous cationic tantalum methyl complex [13], except for a mixture of decomposed tantalum methyl complex with a ¹³C-NMR signal corresponding to the formation of the anionic [MeB(C₆F₅)₃]⁻ (see Figures S3 and S4), indicating a probable formation of surface cationic species. The surface cationic complex decomposed much more rapidly, and it was difficult to characterize it by solid state NMR. This may be due to the unstable nature of 8 e⁻ [(≡Si-O-)TaMe₃]⁺ complex.

3. Conclusions

In summary, the cationic homoleptic tantalum (V)-methyl complex has been synthesized via a straightforward strategy to address the will to get a more electrophilic metal centre, generating the first tantalum-based species with such a property. The delicate equilibrium between stability and degradation was enforced to work at low temperature, and the high oxidation state cationic tantalum complex 2 was defined by liquid ¹H-NMR, ¹³C-NMR and ¹H-¹³C correlation spectroscopy; together with DFT calculations. Ongoing experimental studies are being undertaken in order to understand the null activity in alkane metathesis of the grafted tantalum system and will be disclosed in due time.