Metal alkyls programmed to generate metal alkylidenes by α-H abstraction: prognosis from NMR chemical shift

Chemical shift analysis predicts the ease of alkylidene formation from bis-alkyl d0 complexes via α-H abstraction.


Introduction
Metal alkylidenes are key intermediates in many prominent chemical reactions, such as C-H activation, olenation reactions, 1 and catalytic alkene and alkane metathesis. [2][3][4] These compounds are commonly generated by deprotonation of a metal alkyl, 5 carbene transfer or a-H abstraction from [M](CH 2 R) 2 species. 2,6-12 The latter process is particularly favoured for neopentyl (R ¼ tBu) and related ligands, that were originally used to avoid the decomposition of these alkyl compounds via b-H transfer. 13,14 These dialkyl compounds can however decompose via a-H abstraction, an intramolecular deprotonation process between two cis-bound alkyl ligands on a metal centre, typically with d 0 conguration, related to s-bond metathesis (Scheme 1a). While ubiquitous and used for the synthesis of numerous alkylidenes, no physical properties are currently available to guide the chemist in deciding which [M](CH 2 R) 2 fragment will easily generate a [M](]CHR) species via a-H abstraction and in understanding why this process occurs.
We reasoned that solid state NMR spectroscopy could be an ideal tool to probe this type of reactivity, since chemical shielding and associated 13 C chemical shis (d iso and the principal tensor components d 11 $ d 22 $ d 33 ) are directly linked to frontier molecular orbitals that control reactivity (Scheme 1b). 15 This article reports the experimental measurement, calculation, and orbital analysis of the chemical shi tensor (CST) of the deshielded a-carbons in [M](CH 2 R) 2 compounds that are prone to yield alkylidenes.
In short, we show that the occurrence of a-H abstraction from metal dialkyl compounds requires the presence of a lowlying empty metal d-orbital that points into the M-C a -C a 0 plane. The signature of this orbital is the distinctively deshielded 13 C chemical shi of the a-carbons and a specic orientation of the CST, arising from the alkylidene character on the acarbon and polarization of the C Àd -H +d bond. This situation is particularly pronounced for neopentyl ligands, explaining their propensity to generate alkylidenes via a-H abstraction.

Results and discussion
From the broad range of metal alkyl compounds that undergo a-H abstraction to yield well-dened alkylidenes or putative alkylidene species, a set of Ti and Ta compounds is selected as representative examples, chosen for historical reasons and their well-established reactivity patterns (Fig. 1). We focus on the Petasis reagent, Cp 2 Ti(CH 3 ) 2 , a well-known olenation agent 16 involving the putative methylidene intermediate Cp 2 Ti(CH 2 ), which is trapped as Cp 2 Ti(CH 2 )(PMe 3 ) in the presence of PMe 3 . 17 We also include the related compound Cp* 2 Ti(CH 3 20 We also prepare the d 0 tantalum compound, TaCl(CH 2 tBu) 4 , 6 an isolable intermediate in the synthesis of the rst well-dened metal alkylidene, Ta(CH 2 tBu) 3 (CHtBu), which cleanly transforms into the corresponding alkylidene TaCl(CH 2 tBu) 2 (CHtBu); analogous to the decomposition of Ta(CH 2 tBu) 5 into Ta(CH 2 tBu) 3 (CHtBu). 21,22 We also study TaCl 2 (CH 2 tBu) 3 23,24 and Cp 2 Ta(CH 3 ) 3 , which do not generate the corresponding alkylidenes (see Fig. 1b 25 These organometallic compounds provide an experimental test to distinguish between those metal dialkyl compounds that do and those that do not form alkylidenes. The structures of the studied metal alkyl compounds are shown in Fig. 1 Fig. S14 †).
For Cp 2 Ti(CH 3 ) 2 , the most deshielded component (d 11 ) is oriented perpendicular to the plane that contains the two M-C a bonds (Fig. 2a). This orientation is the same as in the associated alkylidene, Cp 2 Ti(CH 2 ) (Fig. 2b), and the isolated adduct Cp 2 Ti(CH 2 )(PMe 3 ), for which the most deshielded component is oriented perpendicular to the s(M]C) and the p(M]C) bonds. 26 These similarities implicate alkylidene character in the carbon atoms of the methyl groups in Cp 2 Ti(CH 3 ) 2 .
The axial carbon in the trigonal bipyramidal (TBP) molecule TaCl(CH 2 tBu) 4 ( Fig. 2c) has the two most deshielded tensor components d 11 and d 22 oriented perpendicular to the M-C axis, again with d 11 being perpendicular to the plane   4 , giving rise to an ensemble of conformations as a possible explanation for the observation of several peaks. The origin of the deshielded chemical shi values is investigated by an orbital analysis of the corresponding shielding tensor (s, eqn (2)). 15 Decomposition of the shielding into diamagnetic (s dia ) and paramagnetic contributions, which also include contributions from spin-orbit coupling (s para+SO , eqn (3)), reveals that the variation in the shielding values is mostly associated with s para+SO . For the compounds investigated here, spin-orbit coupling is relatively small (Table S7 †), allowing for interpreting s para+SO based solely on the paramagnetic contributions. These originate from the magnetically induced coupling of excited electronic states with the ground state, by action of the angular momentum operatorL i (eqn (4)). Hence, the chemical shi is sensitive to the relative energy and orientation of the frontier orbitals, establishing a link to reactivity.
For carbon p-orbitals, deshielding along direction i occurs when the vacant and occupied orbitals are oriented perpendicular to each other and to the i-axis.
The individual orbital contributions to the most deshielded component of the CST (d 11 /s 11 ), obtained through a Natural Chemical Shi (NCS) analysis 26-56 of the representative examples, Cp 2 Ti(CH 3 ) 2 and TaCl(CH 2 tBu) 4 , and the associated alkylidenes are plotted in Fig. 3 (values given in Table S4 †). Notably, the largest contribution to deshielding in the d 11 /s 11 component of the metal alkyl compounds is always associated with the s(M-C a ) bond, as found for the corresponding metal alkylidenes.
The large deshielding of the a-carbons, originating from the s(M-C a ) bond, indicates the presence of a low-lying vacant orbital that is oriented perpendicular to the M-C axis and the direction of the deshielding. The emergence of this low-lying empty orbital is due to the weak p-donating ability of alkyl groups that, by interaction with an empty d p metal orbital, develop a p-interaction by which the M-CH 2 R bond acquires alkylidene character, as shown for Cp 2 Ti(CH 3 ) 2 in Fig. 4c; this orbital is labelled as p(M-C). The associated p*(M-C) orbital, which is the LUMO of the compound (Fig. 4c right and S15 †), is responsible for the observed deshielding by coupling with the occupied s(M-C) orbital (Fig. 4a). In the corresponding alkylidene Cp 2 Ti(CH 2 ), there is a smaller energy gap between the s(M]C) and p*(M]C) orbitals, hence a signicantly larger deshielding (Fig. 4b). Similarly, in TaCl(CH 2 tBu) 4 the largest part of the deshielding on the axial a-carbon originates from the occupied s(M-C) orbital (Fig. 4d le), which is coupled to the vacant p*(M-C) orbital (Fig. 4d right), again evidencing a p-type interaction of the metal atom with the alkyl ligand. The larger deshielding of the axial carbon atom as compared to the equatorial carbon atoms indicates larger p-character in the former M-C bond.
A similar pattern and analysis applies to Cp* 2 Ti(CH 3 ) 2 , Cp 2 Ti(CH 2 tBu) 2 , nacnacTi(CH 2 tBu) 2 + , and Ti(CH 2 tBu) 4 , where deshielding of the a-carbon mostly arises from the s(M-C) bond, indicating a low-lying orbital of p*(M-C) character, oriented perpendicular to the s(M-C) bond (Table S4 and   The formation of alkylidenes from bis-alkyl complexes via a-H abstraction requires the presence of a low-lying empty orbital in the plane of the M-C a and M-C a 0 bonds. While the deshielded chemical shi value and large anisotropy of the CST indicate the presence of such an orbital, the CST orientation probes the location of this empty orbital. For example, TaCl 2 (CH 2 tBu) 3 features a trigonal-bipyramidal geometry with two Cl-ligands in axial positions. The rather large deshielding on the a-carbon (115 ppm) indicates a low-lying empty orbital, but the most deshielded component (d 11 ) of the CST is not perpendicular to a plane containing two equatorial Ta-C a bonds but rather perpendicular to the plane containing an equatorial Ta-C a and an axial Ta-Cl bond, which is also the plane containing the LUMO (Fig. 4e). Accordingly, the alkylidenic character is not developed in the direction needed for a-H abstraction. This compound is therefore stable, even when heated to 100 C in the presence of PMe 3 for 4 h, in contrast to TaCl(CH 2 tBu) 4 . In other words, in a trigonal-bipyramidal structure, a-H abstraction is favoured between an axial and an equatorial alkyl ligand and is not readily accessible in TaCl  . The third methylsubstituent in Cp 2 Ta(CH 3 ) 3 interacts with the empty metal orbital that is required for developing the alkylidene character (the remaining empty orbitals on the metal, which are involved in bonding with the Cp rings, are too high in energy for such an interaction). However, abstraction of a methyl-ligand generates Cp 2 Ta(CH 3 ) 2 + , isoelectronic to Cp 2 Ti(CH 3 ) 2 , and restores the highly anisotropic CST and deshielded chemical shi values ( Fig. 5 and Table 1). The nature of the alkyl ligands also plays an important role in manipulating the alkylidene character on the a-carbon. This is illustrated by comparing the bis-neopentyl metallocene, Cp 2 Ti(CH 2 tBu) 2 with the bis-methyl metallocenes, Cp 2 Ti(CH 3 ) 2 and Cp* 2 Ti(CH 3 ) 2 , since detailed kinetic data on the decomposition via a-H abstraction is available for the latter metallocene. 57 Cp 2 Ti(CH 2 tBu) 2 displays a much more deshielded and anisotropic a-carbon, due to the coupling of s(M-C) with p*(M-C), in addition to a signicant contribution of the coupling of s(C a -C b ) and p*(M-C a ) (Table S4 and Fig. S11 †). The contribution of s(C a -C b ) is associated with the wide calculated a(M-C a -C b ) angle of 136 , signalling the increased alkylidene character in the Ti-C bond. The NMR data thus suggest a lower activation energy for a-H abstraction in Cp 2 Ti(CH 2 tBu) 2 , which is conrmed by the calculated energy proles. The calculated Gibbs activation (and associated reaction) energies at 298 K are +27.1 (À3.3) kcal mol À1 for Cp 2 Ti(CH 2 tBu) 2 vs. +30.8 (+8.2) kcal mol À1 and +32.1 (+5.3) kcal mol À1 for Cp 2 Ti(CH 3 ) 2 and Cp* 2 Ti(CH 3 ) 2 , respectively. The lower calculated Gibbs activation energy for the a-H abstraction in the neopentyl derivative is consistent with the experimentally determined values found for Cp 2 Ti(CH 2 tBu) 2 (+22.8 kcal mol À1 ) 58 and Cp* 2 Ti(CH 3 ) 2 (+28.3 kcal mol À1 with k a(H) /k a(D) ¼ 2.92 AE 0.10). 57 The Gibbs activation energies correlate with the deshielded acarbon chemical shi and the lower value of the 1 J C-H coupling constant, consistent with more alkylidene character in the Ti-C bond and consequently a lower transition state energy for the a-H abstraction step (Fig. 6). Detailed kinetic studies have also been reported on the elimination of CH 3 tBu from Ti(CH 2 tBu) 4 19 and Ta(CH 2 tBu) 5 . 21,22 Both compounds follow rst order kinetics for a-H abstraction. For Ti(CH 2 tBu) 4 , values of DG ‡ ¼ 26.4 kcal mol À1 and k a(H) /k a(D) ¼ 5.2 AE 0.4 were found. For Ta(CH 2 tBu) 5 the values were DG ‡ ¼ 22.3 kcal mol À1 and k a(H) / k a(D) ¼ 14.1 AE 0.8 (note that DG ‡ was determined for the deuterated compound).
It is important to note the close analogy between alkylidenic character of the M-C bond and the occurrence of what is referred to as a-H agostic interactions. 59,60 a-H Agostic interactions are evidenced by acute M-C-H angles (<109.47 ) and are generally associated with low 1 J C-H coupling constants for the carbon bound to the metal (<125 Hz). The presence of an a-H agostic interaction is associated with a p-type interaction of a porbital on the a-carbon with a vacant metal d-orbital, resulting in the observed geometrical features and lower 1 J C-H coupling constants. 61,62 Thus, an a-H agostic interaction is also an indirect reporter of an alkylidenic character in a M-C bond. However, the CST values that signal alkylidenic character can be present in the absence of geometrical features or lowered 1 J C-H coupling constants associated with a-H agostic interactions. For example, in TaCl(CH 2 tBu) 4 , one M-C-H bond angle at the axial carbon is calculated to be 95 (which can be considered agostic), whereas, in Cp 2 Ti(CH 3 ) 2 , the M-C-H angles are calculated to be 112.7 (the H atom in the s h plane) and 109.7 (the H atom out of s h plane). While both TaCl(CH 2 tBu) 4 and Cp 2 Ti(CH 3 ) 2 have alkylidenic character, as evidenced by their CSTs, only the former is considered to be a-C-H agostic based on its calculated structure. Our view is that an a-C-H agostic interaction is better described as a p-donation from the carbon p-orbital, rather than as a 3-center-2-electron bond. The philosophical question remains: is the M/H-C a interaction due to the alkylidenic character of the carbon atom bound to the metal, or does the alkylidenic character arise from the M/H-C a interaction? Perhaps a distinction without a difference.

Conclusions
In summary, strongly deshielded chemical shi values of the acarbons in [M](CH 2 R) 2 compounds in combination with a large anisotropy and specic orientation of the chemical shi tensor reveal the presence and location of low-lying empty metal dorbitals. The alkylidenic character in the M-CH 2 R bond activates the a-C-H bond towards a-H abstraction when the lowlying empty orbital is appropriately oriented. While this orbital arrangement can lead to the development of an a-C-H "agostic" interaction, the magnitude and orientation of the CST is a much more unequivocal signature of the alkylidenic character of the M-C bond. The CST shows that the parent alkyl compounds already contain inscribed information about the reaction products and are programmed to evolve into metal alkylidenes, a situation particularly favoured for neopentyl-type ligands. While this study has focused on Ti and Ta d 0 compounds, the associated principle is likely applicable to a wide range of metal alkyls with low d-electron counts. The theme of this article is that NMR chemical shi values of atoms directly bonded to a metal centre provide information about the electronic  structure and are powerful reporters of the location, orientation, and relative energy of the frontier molecular orbitals. This study shows that chemical shis can be of predictive value of a compound's reactivity, making their physical interpretation an invaluable tool for the development and the understanding of mechanisms and reactivity. We are currently further exploring this connection.

Conflicts of interest
There are no conicts to declare.