Base-induced reversible H₂ addition to a single Sn(II) centre

Abstract

A range of amines catalyse the oxidative addition (OA) of H₂ to [(Me₃Si)₂CH]₂Sn (1), forming [(Me₃Si)₂CH]₂SnH₂ (2). Experimental and computational studies point to ‘frustrated Lewis pair’ mechanisms in which 1 acts as a Lewis acid and involve unusual late transition states; this is supported by the observation of a kinetic isotope effect for Et₃N. When DBU is used the energetics of H₂ activation are altered, allowing an equilibrium between 1, 2 and adduct [1·DBU] to be established, thus demonstrating reversible oxidative addition/reductive elimination (RE) of H₂ at a single main group centre.

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Introduction

In the past decade there has been significant interest in transition metal (TM) free systems which activate H₂.¹ Two main strategies have emerged to facilitate this reactivity: the use of low-valent main group (MG) compounds,² and so-called ‘frustrated Lewis pairs’ (FLPs).³ In both cases, reactivity arises from simultaneously having access to a high-lying HOMO and low-lying LUMO (Fig. 1). Various low-valent MG compounds containing multiple E–E bonds (E = Al, Si, Ga, Ge, Sn),^4,5 or single-site low-valent centres such as carbenes and heavier tetrylene analogues, have been shown to react with H₂.⁶ The scope of Lewis bases (LBs) and, to a lesser extent, Lewis acids (LAs), which can be used in H₂-activating FLPs has expanded to include a number of elements from across the periodic table. This is principally due to the readily tuneable steric and electronic profiles of the individual LA and LB sites.^7–9 Many FLP systems display reversible H₂ cleavage, which has facilitated their rapid expansion into the field of catalytic hydrogenation.¹⁰ The same is not true for low-valent MG compounds; examples of reversible H₂ activation are very rare and limited to antiaromatic boracycles,¹¹ a phosphorus-based singlet biradicaloid,¹² and only one low-valent group 14 compound: a dinuclear Sn(I) distannyne.¹³ The design of single-site MG systems which are ergoneutral for H₂ activation requires fine-tuning of thermodynamic (e.g. weak E–H bond strengths promoting an accessible formal Eⁿ⁺²/Eⁿ couple) and kinetic factors, both of which are constrained to a mononuclear species, and is hence especially challenging.

Fig. 1 Representative orbital interactions between H₂ and main group compounds: (a) unsaturated E–E compounds e.g. distannynes (Ar = C₆H₂-2,6-(C₆H₃-2,6-ⁱPr₂)₂-4-X; X = H, SiMe₃, F); for X = H, the reaction is reversible at 80 °C;^5a,13 (b) single site low-valent centres e.g. carbenes;^6a (c) sterically hindered LAs and LBs (FLPs); (d) this work.

The ability of L₂Sn(II) compounds to undergo OA has been inversely correlated with the size of the singlet–triplet (HOMO–LUMO) gap, which may be diminished through the use of extremely strong σ-donor ligands. Aldridge et al. have employed a bis(boryl)tin(II) system to achieve the only example of direct OA of H₂ to a mononuclear Sn(II) centre, irreversibly forming the Sn(IV) dihydride; boryl ligands are even stronger σ-donors than hydride or alkyl ligands, permitting a successful reaction outcome.^6d

Conversely, the irreversible base-induced RE of H₂ from organostannanes is well-known.¹⁴ Wesemann and others have studied RE from ArSnH₃ and [(Me₃Si)₂CH]SnH₃ compounds to yield various mononuclear Sn and Sn–Sn bound species (Ar = terphenyl).¹⁵ Nevertheless, there has yet to be a report of reversible OA and RE occurring on a single Sn(II) scaffold. Lappert's stannylene [(Me₃Si)₂CH]₂Sn (1), which can act as both Lewis acid (LA) and base (LB), is a paradigmatic system for investigating OA to low-valent MG centres, yet to date its reactivity with H₂ has been unexplored.¹⁶ Herein we report the use of FLP methodology to promote formal OA of H₂ to this simple dialkylstannylene. Furthermore we document the first example of reversible H₂ addition to a single-site MG complex, which accesses an FLP via reversible dissociation of a classical 1·LB adduct; formation of the latter renders OA of H₂ to 1 energetically less favourable, enabling RE to occur from the Sn(IV) dihydride and reform 1, which is in equilibrium with 1·LB.¹⁷

Results and discussion

1 is in a rapid solution-phase equilibrium with its dimer [1]₂, which has been crystallographically characterised and contains a formal Sn=Sn double bond.¹⁸ When a d₈-toluene solution of 1/[1]₂ was placed under an atmosphere of H₂ (4 bar) in a sealed NMR tube, no change was observed in the ¹H NMR spectrum, even after prolonged periods (>48 h), confirming that neither 1 nor [1]₂ can react with H₂ alone. Separately, addition of Et₃N (20 mol%) to a solution of 1 resulted in no perturbation of their ¹H NMR resonances, suggesting no interaction between the components; i.e. the formation of an FLP.¹⁹ Placing this new mixture under H₂ (4 bar, RT) resulted in the solution turning from deep red to colourless over the course of 24 h, with the ¹H NMR spectrum revealing complete consumption of 1 and a new Sn–H triplet resonance at δ = 5.10 ppm [³J(¹H–¹H) = 2.2 Hz] with attendant satellites [¹J(¹¹⁷Sn–¹H) = 1704 Hz; ¹J(¹¹⁹Sn–¹H) = 1784 Hz], in addition to signals for the Si(CH₃)₃ and methine protons [δ/ppm = 0.17 (s) and −0.42 (t, ³J(¹H–¹H) = 2.2 Hz), respectively] (see Fig. 2). ¹¹⁹Sn NMR spectroscopy showed only a triplet of triplets at −196 ppm [¹J(¹¹⁹Sn–¹H) = 1784 Hz, ²J(¹¹⁹Sn–¹H) = 87 Hz] which collapsed to a singlet upon ¹H decoupling. Collectively these data correspond to the previously unreported dihydride [(Me₃Si)₂CH]₂SnH₂ (2), which was confirmed by comparison with an authentic sample prepared by the reaction of LiAlH₄ and [(Me₃Si)₂CH]₂SnCl₂ (see ESI† for details).

Fig. 2 ¹H/²H NMR spectra from the reaction of 1 and 20 mol% Et₃N: (a) CH(SiMe₃)₂ region under H₂ (left); H₂/D₂ (1 : 1) (middle); HD (right). (b) SnH region under H₂ (left); H₂/D₂ (1 : 1) (middle; ²H NMR); HD (right). ● denotes trace formation of 2 from H₂ in commercial HD gas.

Isotopic investigation

When D₂ was used in place of H₂, the methine peak present in the ¹H NMR spectrum of the product mixture resolved as a singlet, while the Sn–H signal was absent and replaced by a Sn–D signal at δ = 5.11 ppm [¹J(¹¹⁷Sn–²H) = 262 Hz, ¹J(¹¹⁹Sn–²H) = 274 Hz] in the ²H NMR spectrum. These results demonstrate the formation of dideuteride 2-D₂,²⁰ and that the Sn-bound protons in 2 must originate from the hydrogen atmosphere.

In order to probe the mechanism further, a d₈-toluene solution of 1/[1]₂ and Et₃N was reacted with a 1 : 1 mixture of H₂/D₂. The resultant ¹H NMR spectrum was very similar in appearance to that of 2, with two exceptions: the relative integration of the Sn–H peak did not match that of the methine signal (1.2 : 2; consistent with the faster rate of reaction with H₂vs. D₂ – vide infra), and the C–H resonance was composed of overlapping peaks commensurate with a mixture of 2 and 2-D₂. No spectroscopic evidence was seen for the formation of 2-HD, which was independently and selectively obtained by analogous reaction of 1/[1]₂ under an HD atmosphere. These observations provide strong evidence that delivery of both atoms from H₂/D₂/HD to a single Sn centre occurs either simultaneously, or in a near-concerted fashion.

Kinetic analysis

By analogy with established FLP systems, and the microscopic reverse of the polar mechanism by which dehydrogenation of ArSnH₃ species is proposed to occur,^15a we envisaged a reaction mechanism in which 1 and Et₃N form a weakly associated ‘encounter complex’ which subsequently reacts with H₂ (Scheme 1).²¹ Assuming that encounter complex formation is a rapid pre-equilibrium prior to rate-limiting H₂ activation gives the expected rate law: rate = k′[1][Et₃N][H₂], where k′ = (k₁k₂)/k₋₁. Calorimetric studies on H₂ activation by the FLP Mes₃P/B(C₆F₅)₃ (Mes = 2,4,6-C₆Me₃H₂) found the rate to be very accurately modelled as a single, termolecular step, which formally gives the same rate law.²²

Scheme 1 Proposed reaction mechanism for H₂ heterolysis by 1, catalysed by Et₃N.

To confirm the order of catalytic Et₃N, the method of time (t) scale normalisation was used;²³ normalisation to the scale of t·[Et₃N]^x resulted in the superposition of all reactant traces only when x = 1, confirming the rate to be first order with respect to the amine (Fig. 3a). Determination of reaction order with respect to 1 requires its concentration to be known accurately at any given time in a reaction mixture. However, since the observed ¹H NMR resonances are a weighted average of the signals from 1 and [1]₂ (ΔG_293K = 3.1 kcal mol⁻¹), with both species present at significant concentrations under reaction conditions, simple observation of the concentration of 1 is not directly possible by ¹H NMR spectroscopy.^18a The concentration of 1 can, however, be calculated from the total concentration of “R₂Sn” species in solution, [Tot], present as either monomer or dimer, which are related to the concentrations of 1 and [1]₂ by:

[Tot] = [1] + 2[(1)₂] (1)

Fig. 3 (a) Solutions of 1 (0.03 mmol) in d₈-toluene (0.5 mL) under H₂ (4 bar) containing various base concentrations were prepared. When the stannylene concentration, [Tot], is plotted against the normalised timescale t·[Et₃N]¹, all traces overlap, confirming the order in base to be one. (b) Linearised rate data for a similar solution of 1 (0.03 mmol) in d₈-toluene (0.5 mL) under H₂ (4 bar) containing Et₃N (0.6 M; 10 eq.).

The dimerisation equilibrium of 1 can be expressed as:

Combining eqn (1) and (2) and solving for [1] yields:

Inserting eqn (3) into the expected rate law (vide supra) gives:

where, if the amount of H₂ is sufficiently high that its concentration remains approximately constant:

Rearrangement and integration by substitution of eqn (4) (see ESI†) gives:

Therefore, plotting the variable portion of the LHS of this expression against t gives a straight line of gradient −k∗, confirming the proposed first-order dependence on 1 (Fig. 3b).

Using the known value of [H₂] in toluene at 4 bar (293 K)²⁴ provides a value of .²⁵ As well as Et₃N, 2-tert-butyl-1,1,3,3-tetramethylguanidine (Barton's base, TBTMG) and 1,2,2,6,6-pentamethylpiperidine (PMP), were also found to form FLPs with 1/[1]₂, with corresponding rates of H₂ cleavage: , .²⁶ Despite the similar basicity to Et₃N, the bulkier Hünig's base (iPr₂EtN; pK_a(MeCN): 18.0)²⁷ was ineffective for H₂ heterolysis, as was the weaker base 2,4,6-collidine (pK_a(MeCN): 14.98).²⁸ Clearly H₂ activation requires that the LB be sufficiently basic and not too sterically encumbered, in line with observations of other FLP systems.²⁹

A kinetic analysis of the isotopic systems permitted quantification of the KIE: when Et₃N was used as the base. In addition, the acceleration in rate from a more polar solvent could also be quantified: (when Et₃N was used).

Coordinating bases

When the less sterically bulky 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) is used, an interaction with 1 can be clearly seen in the ¹³C{¹H} NMR spectrum: upon gradual addition of DBU to 1/[1]₂, the methine resonance undergoes a substantial upfield shift, reaching a limiting value of δ = 18.5 ppm (10-fold excess of DBU). Using the established ¹³C NMR chemical shift values for 1 and [1]₂ (60.0 ppm and 28.7 ppm, respectively),^18a this is consistent with a fast equilibrium between 1·DBU, 1 and [1]₂ (Scheme 2; see ESI† for full details). A value of ΔG = −3.7 ± 0.2 kcal mol⁻¹ for the formation of 1·DBU from [1]₂ was obtained from a van't Hoff analysis of variable temperature UV-Vis spectra.

Scheme 2 Equilibrium between product 2 + DBU and the dehydrogenated mixture 1·DBU ↔ 1 ↔ [1]₂.

While the reaction of 1/DBU mixtures (containing 0.1–10 equivalents of DBU) with H₂ proceed rapidly, they do not reach completion, indicative of a reversible process (see Fig. S7 in ESI†).

The reversibility can be explicitly demonstrated by the (CH₃)₃Si region of the ¹H NMR spectrum, whereby addition of DBU to a solution of 2 led to the appearance of a signal corresponding to the dehydrogenated mixture 1·DBU ↔ 1 ↔ [1]₂; this increased in intensity at the expense of the (CH₃)₃Si peak of 2 (Fig. 4a–c). No H₂ is observed in the ¹H NMR spectrum as the solution was degassed multiple times in order to accelerate the reaction – however, the very small amount of H₂ generated (approx. 0.3 bar) would likely hamper detection. Furthermore, the methine resonance of the 1·DBU ↔ 1 ↔ [1]₂ mixture is subject to a significant upfield shift compared to [1]/[1]₂ (dependent upon the DBU concentration), and so is obscured beneath the relatively intense (CH₃)₃Si region. Upon charging this reaction with H₂, restoration of 2 was rapidly observed (Fig. 4d). For the equilibrium involving H₂ (Scheme 2), an equilibrium constant, K_eq = 164 ± 5, in favour of 2 can be calculated from the relative intensities of the (CH₃)₃Si resonances, providing ΔG = −3.0 kcal mol⁻¹ (1 bar H₂).

Fig. 4 The reversibility of the reaction between 1/[1]₂, DBU and H₂ can be shown explicitly by a series of ¹H NMR spectra depicting: (a) a solution of 2 (0.03 mmol) in d₈-toluene (0.5 mL) (b) the same solution with added DBU (0.03 mmol, 1 equivalent) and mesitylene (2%) as an internal standard; (c) after degassing three times over the course of one week, showing the formation of 1·DBU ↔ 1 ↔ [1]₂; (d) reformation of 2 after the addition of H₂ (4 bar). *Small amount of silicone grease from the independent synthesis of 2.

Using the similarly unhindered but less basic 4-(dimethyl amino)pyridine (DMAP) also gave an adduct 1·DMAP, but no reaction with H₂ at room temperature. However, heating a solution of 1 with excess DMAP (4 bar H₂, 2 h, 100 °C) yielded 2 in 31% conversion.

Computational investigation

To gain further insight into the mechanism of H₂ activation, DFT calculations were performed for various 1/LB pairs;³¹ the computed reaction profiles for both the Et₃N- and DBU-mediated reactions are depicted in Fig. 5. When LB = Et₃N, the reaction was found to proceed via initial H₂ heterolysis leading to a tight ion pair intermediate [1H]⁻[Et₃NH]⁺ (int₁). Facile rearrangement to int₂ and subsequent delivery of the H⁺ to the lone pair on the [1H]⁻ moiety furnishes 2 (Fig. 6a); a very similar mechanism was found when LB = DBU. In support of this polar mechanism, the rate using Et₃N as the LB was found to be faster in THF . The low barriers to rearrangement of the intermediates also offer an explanation as to why H/D exchange is not observed upon reaction with an H₂/D₂ mixture or HD: collapse of the ion pairs is likely much faster than solvent cage escape.

Fig. 5 Computed free energy profile for Et₃N- and DBU-assisted H₂ activation with 1. Relative free energies (in kcal mol⁻¹) are with respect to 0.5·[1]₂ + LB + H₂.

Fig. 6 (a) Structural representations of the computed transition states for the heterolysis of H₂ by 1 and Et₃N. H–H distances are given for TS₁ and int₁. (b) The computed adduct formed between 1 and DBU. All energies (in kcal mol⁻¹) are relative to 0.5·[1]₂ + LB + H₂. Si–CH₃ and C–H groups omitted for clarity.

Although the located transition states (TSs) are energetically close-lying, the overall reaction barrier appears to be determined by the H₂ splitting step, which is in line with kinetic measurements. Free energy data computed for the H₂ splitting step for reactions with different bases are compiled in Table 1 alongside other properties. For Et₃N, TBTMG and PMP, no favourable adduct formation was found with 1, and the ΔG^‡ values follow the order TBTMG < Et₃N < PMP, which is consistent with experimental reaction rates. For the coordinating bases DBU (Fig. 6b) and DMAP, adducts favourable relative to free [1]₂ and base were computationally determined. This reduces the absolute value of ΔG_reaction such that an equilibrium is experimentally observed in the case of DBU. For DMAP, the activation barrier is found to be much higher, paralleling results seen by experiment where elevated temperatures are required to obtain product 2.

Table 1 Computational and pK_a data for reactions of a series of bases with 1 and H₂^a

Property	Et3N	TBTMG	PMP	DBU	DMAP
a Free energy data relative to 0.5·[1]2 + base + H2 (kcal mol^−1); ΔG^‡ is activation free energy. b Proton affinity is defined as the free energy of base + H^+ → baseH^+. c Measured in MeCN.^30 d H–H distance in TS1 (0.76 Å in free H2).
1·LB^a	—	—	—	−3.4	−3.6
TS1^a	20.4	18.3	21.4	17.0	20.1
int1	18.1	5.4	16.1	6.6	16.8
ΔG^‡^a	20.4	18.3	21.4	20.4	23.7
ΔGreaction^a	−7.7	−7.7	−7.7	−4.3	−4.1
PA^a^,^b	−270.1	−286.0	−272.7	−283.5	−272.2
pKa^c	18.8	23.6	18.7	24.3	18.0
d(HH)^d/Å	0.99	0.87	0.96	0.88	0.99

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The energies of all intermediates int₁ are computed to be well above the reference state, which follows from the weak Lewis acidity of 1. The stabilities of int₁ species correlate very well with the general trend in PA and pK_a, but this is not strictly true for the TSs, where steric factors are more important. Unstable int₁ intermediates imply late TSs for the H₂ activation step, which is shown by significantly elongated H–H distances in the TS structures. The experimentally observed KIE (1.51 ± 0.04) supports this finding, which is commensurate with rate-limiting H₂/D₂ activation involving considerable H–H/D–D bond breaking.³²

Conclusions

In conclusion, we have demonstrated the ability of FLP-mediated reactivity to enable the formal oxidative addition of H₂ to an otherwise inert MG centre, and in doing so have also observed the first example of reversible H₂ addition to a single-site MG complex. We have utilised experimental and computational means to comprehensively explore the mechanism of this transformation and found that H₂ activation in this system differs from those based on more typical FLPs, due to the high-energy nature of the immediate H₂ splitting products, resulting in rare examples of late TSs. The development of methods to harness this FLP-promoted OA/RE H₂ reactivity for hydrogenation catalysis is currently underway.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We wish to thank the EPSRC (J. S. S. and R. T. C; EP/N026004) and an Imperial College President's PhD Scholarship (R. C. T.-R.) for PhD funding and the Royal Society for a University Research Fellowship (AEA; UF/160395). This work was also supported by NKFIH (K-115660).

Notes and references

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30. Et₃N and DMAP: Z. Glasovac, M. Eckert-Maksić and Z. B. Maksić, New J. Chem., 2009, 33, 588 ; PMP: M. Dworniczak and K. T. Leffeck, Can. J. Chem., 1990, 68, 1657 ; DBU and TBTMG: C. F. Lemaire, J. J. Aerts, L. C. Libert, F. Mercier, D. Goblet, A. R. Plenevaux and A. J. Luxen, Angew. Chem., Int. Ed., 2010, 49, 3161.
31. The energy values reported correspond to solution phase Gibbs free energies that are based on ωB97X-D/Def2TZVPP electronic energies, and all additional terms obtained at the ωB97X-D/Def2SVP level. See ESI† for full computational details.
32. In FLPs with early TSs, such as Mes₃P/B(C₆F₅)₃, a KIE of only 1.1 has been obtained (see ref. 22).

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Footnote

† Electronic supplementary information (ESI) available: Experimental and computational details. See DOI: 10.1039/c8sc03110j

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