# Barium-Catalyzed Dehydrocoupling of Terminal Alkynes and Hydrosilanes

Sakshi Mohan, Eric Clot, Marie Cordier, Jean-françois Carpentier, Yann Sarazin

► To cite this version:

Sakshi Mohan, Eric Clot, Marie Cordier, Jean-françois Carpentier, Yann Sarazin. Barium-Catalyzed Dehydrocoupling of Terminal Alkynes and Hydrosilanes. ChemistryEurope, 2026, 4 (2), pp.e202500401. 10.1002/ceur.202500401. hal-05505150

HAL Id: hal-05505150

https://hal.science/hal-05505150v1

Submitted on 11 Feb 2026

HAL is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

<figure><img src="image_1.png" /></figure>

Distributed under a Creative Commons CC BY 4.0 - Attribution - International License
<figure><img src="image_2.png" /></figure>

# Barium-Catalyzed Dehydrocoupling of Terminal Alkynes and Hydrosilanes

Sakshi Mohan, Eric Clot, Marie Cordier, Jean-François Carpentier,* and Yann Sarazin*

The dehydrocoupling of terminal alkynes and hydrosilanes catalyzed by alkaline-earth complexes: calcium, strontium and, most prominently, barium, is reported. $[Ba\{N(SiMe_3)_2\}_{2}.(thf)_2]$ (1) is a convenient precatalyst, affording conversions in the range 75–95+% and good chemoselectivity for substrates bearing aromatic substituents, e.g., with the benchmark phenylacetylene and phenyldimethylhydrosilane. Over 20 different combinations of substrates were coupled, typically over 12–24 h at 90 °C with a 5 mol-% loading of 1. Reaction rates increase upon descending group 2, according to $Ca < Sr < Ba$. Kinetic analysis performed for the coupling of our benchmark substrates catalyzed by 1 in pyridine-$d_5$ allows for

the determination of the rate law $r = k.[barium]^{1}.[PhMe_2SiH]^{1}$, with $\Delta H^{\ddagger} = 12.7(1) \text{ kcal mol}^{-1}$ and $\Delta S^{\ddagger} = -42.8(1) \text{ cal mol}^{-1} \text{ K}^{-1}$ consistent with a kinetically affordable, albeit relatively slow, reaction. DFT calculations conducted on the system 1/pyridine indicate the monosolvated $[Ba\{N(SiMe_3)_2\}_{2}.pyridine]$ to be the prevailing catalyst, with a rate-determining step consisting of the protonolysis between a [Ba]-hydride intermediate and floating $HN(SiMe_3)_2$. Compared to Ca and Sr, the greater efficiency of Ba precatalysts stems from the greater ability of barium to lose precoordinated pyridine and, thus, to generate the catalytically competent monosolvated species.

## 1. Introduction

1-Alkynylsilanes $R'C≡CSiR_3$, with a silyl group bonded to a $C_{sp}$ atom, are useful in organic chemistry. Their prime application is to officiate as protecting groups for terminal alkynes, where the silyl can be removed under mild conditions, e.g., $K_2CO_3/MeOH$ or TBAF/thf.[1] They are commonly used as building blocks, e.g., as protected synths for Sonogashira, Stille, or other C–C cross-couplings, and for cycloaddition reactions such as Diels-Alder cyclization, [2+3] cycloadditions, or the formation of 1,2,3-triazines.[1,2] Beyond its protecting role, the steric and electronic effects of the silyl group can also orientate the region and stereochemistry at the C≡C bond. It is hence unsurprising that the formation of alkynylsilanes has attracted interest. Conventional synthetic protocols that rely on deprotometallation

of the terminal alkyne, followed by salt metathesis with an electrophilic chlorosilane,[2] are not atom-efficient. They have been replaced by more effective catalyzed processes, and, most prominently among these,[3] dehydrocouplings between terminal alkynes and hydrosilanes, where the sole coproduct is dihydrogen (Scheme 1). Since the early reports by Voronkov ($H_2PtCl_6/I_2$)[4] and Harrod (CuCl/TMEDA),[5] other molecular catalytic systems that involve a range of metals have been shown to promote this reaction: s-block (alkaline hydroxides,[6,7] Mg,[8] Ca),[9–12] lanthanides,[9,13] middle to late transition (Fe,[14] Co,[15,16] Cu,[17,18] Ru,[19] Pd,[20] Ir)[21] metals, and zinc.[22,23] Heterogeneous[24,25] and metal-free[26,27] catalysts have also been reported. In addition, the input of photocatalysis has proved valuable on occasions.[14,18,27] A recent, comprehensive review by Kuciński surveys the advances to date,[3] and also highlights other efficient, albeit less atom-economical, catalytic routes, e.g., the coupling of terminal alkynes with trifluoromethylsilane,[28] silazanes,[29] or silyl alkynoates.[30] A strategy involving silylation of terminal alkynes with tert-butyl-substituted silyldiazenes catalyzed by KOH was recently unveiled.[31] Finally, manganese has been used to catalyze $C_{sp}-Si$ cross-electrophile couplings of alkynyl halides with chlorosilanes.[32]

The large alkaline earths (Ae) Ca, Sr, and Ba are abundant and widespread metals. In the past 20 years, they have enjoyed much attention to design homogeneous catalysts that constitute alternatives to (much) more expensive, and often toxic, mainstream transition metals.[33–35] We have entertained a long interest in the implementation of Ca, Sr and, in particular, Ba complexes in molecular catalysis. We and others have shown in particular that barium generates competent and chemoselective dehydro-coupling catalysts that allow for E–E' bond formation between heteroelements (E, E' = Si, N, P, O, etc.), e.g., toward the fabrication of silazanes[36–39] and polycarbosilazanes,[40,41]

S. Mohan, M. Cordier, J.-F. Carpentier, Y. Sarazin

CNRS

ISCR-UMR 6226

Univ Rennes

35000 Rennes, France

E-mail: Jean-francois.carpentier@univ-rennes.fr

yann.sarazin@univ-rennes.fr

E. Clot

ICGM

CNRS

ENSCM

Univ Montpellier

34000 Montpellier, France

<figure><img src="image_3.png" /></figure>

Supporting information for this article is available on the WWW under https://doi.org/10.1002/ceur.202500401

<figure><img src="image_4.png" /></figure>

© 2026 The Author(s). ChemistryEurope published by Chemistry Europe and Wiley-VCH GmbH. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.
<figure><img src="image_5.png" /></figure>

<figure><img src="image_6.png" /><figcaption>**Scheme 1.** Selected molecular catalysts for the dehydrocoupling of terminal alkynes and hydrosilanes.</figcaption></figure>

asymmetric siloxanes,[42] silylethers[43] and borasiloxanes.[44,45] As part of our ongoing program, and inspired by the results achieved with lithium[6] and calcium[9–12] in the field, we are reporting here on the specificities, advantages, and drawbacks of Ae-mediated catalysis towards the construction of Csp-Si bonds upon dehydrocoupling of terminal alkynes and hydrosilanes, focusing on the catalytically most competent of these metals, barium. A study that combines synthetic, kinetic, and theoretical investigations is presented.

## 2. Results and Discussion

### 2.1. Preliminary Data

An initial screening aimed at delineating optimal conditions for the Ae-mediated coupling of alkynes and hydrosilanes was conducted. In line with Tsuchimoto's report,[23] phenyldimethylhydrosilane and phenylacetylene were selected as our benchmark substrates. A first set of reactions catalyzed by $[Ba\{N(SiMe_3)_2\}_2\cdot(\text{thf})_2]$ (1), $[Sr\{N(SiMe_3)_2\}_2\cdot(\text{thf})_2]$ (2), $[Ca\{N(SiMe_3)_2\}_2\cdot(\text{thf})_2]$ (3), $[Ba\{CH(SiMe_3)_2\}_2\cdot(\text{thf})_2]$ (4), and $[[\text{Carb}^{\text{DIPP}}]\text{BaN}(SiMe_3)_2\cdot\text{thf}]$ (5), where $\{\text{Carb}^{\text{DIPP}}\}$ is the 1,8-bis-(2,6-diisopropylphenyl)imino-3,6-di-tert-butyl-carbazolato ligand (an effective ancillary ligand

for barium and other alkaline earths)[46,47] was carried out in pyridine-d₅ at 90 °C, using a 1:1 molar ratio of substrates with a 5 mol-% catalyst loading (**Scheme 2**).

Substrate conversion was monitored by ¹H NMR spectroscopy over the course of 3 h. The corresponding values of the observed rate constants $k_{\text{obs}}$ extracted from the semilogarithmic plots of substrate conversion versus time (see the Supporting Information, Figure S9), were in the order:

$$3(k_{\text{obs}} = 1.67(1)10^{-6}) < 5(6.67(2)10^{-6}) < 2(31.7(1)10^{-6}) < 4(85.0(1)10^{-6}) < 1(143.3(2)10^{-6}) \text{ (in s}^{-1})$$

This hence makes the simple, solvated barium-bis(amide) 1 the best precatalyst under the chosen experimental conditions, and shows the activity trend Ca < Sr < Ba for an otherwise identical coordination sphere. This trend between the three alkaline earths, in favor of the largest metal, is commonly observed, in particular for dehydrocoupling catalysis, although some cases where Ca has proved the most efficient are also known.[32,36–44] Unexpectedly, the Ba-alkyl precatalyst 4 turned out to be slightly less effective than its amido congener, 1. It is likely that the overall lower catalytic performance measured for 4 results from its greater sensitivity and instability compared to its amido analogue. It is hence possible that 4 partly decomposed when handled, or upon addition of the substrates. It might also not be inert toward pyridine: for instance, $[Ae\{CH(SiMe_3)_2\}_2\cdot(\text{thf})_2]$ complexes

<figure><img src="image_7.png" /><figcaption>**Scheme 2.** Benchmark alkyne/hydrosilane dehydrocoupling reaction with Ae (pre)catalysts.</figcaption></figure>
<figure><img src="image_8.png" /></figure>

can dearomatise bis(imino)pyridine ligands for Ae = Ca, Sr and Ba.[48] Either way, **4** was not suited to our chosen set of reaction conditions, and we did not investigate it further. The metal-based group must be sufficiently basic to at least partly deprotonate the alkyne (pKₐ (Me₃Si)₂NH/(Me₃Si)₂N⁻, 25.8 in **thf**;[49] PhC≡CH/PhC≡C⁻, 28.8 in DMSO)[50] and to provide an entry into the prevailing catalytic manifold (*vide infra*), but highly basic precatalysts such as the Ba-alkyl **4** appear to be troublesome. The comparatively low activity of heteroleptic complex **5** shows that the presence of a bulky ancillary ligand on the metal does not increase reaction kinetics; the same observation was made previously for the dehydrocoupling of amines and hydrosilanes.[37,38] Although they were much slower than in pyridine-*d*₅, reactions performed at 65 °C in **thf**-*d*₈ gave apparent rate constants that showed an identical trend (in s⁻¹): 3 (kobs = 1.43(3) 10⁻⁶) < 2 (2.51(4) 10⁻⁶) < 4 (5.83(20) 10⁻⁶) < 1 (7.53(14) 10⁻⁶) ≈ [Ba{N(SiMe₃)₂}₂]₂ (6.50(13) 10⁻⁶). Unsurprisingly, the solvent-free barium-amide dimer gave identical results to its solvated parent **1** in **thf**-*d*₈. Note that in **thf**-*d*₈ also, the bis(alkyl) precatalyst **4** is again less active or, at best, comparable to its bis(amide) counterparts. This suggests that the results observed with **4** in pyridine-*d*₅, as discussed above, stem from partial decomposition of the precatalyst in this solvent. Overall, on account of its greater activity, ease of access and relative robustness, the solvated barium-bis (amide) precatalyst **1** was selected for all subsequent investigations. Considering the relative pKₐ of HN(SiMe₃)₂ and PhC≡CH, it seems reasonable to assume that, although the starting precatalyst **1** possesses two Ba-amide moieties, once the catalytic cycle is entered (*vide infra*), the metal center carries only one reactive Ba-alkynyl group. This agrees with NMR analysis of the reaction between **1** and excess phenylacetylene (20 equivalents vs. Ba) in pyridine-*d*₅, which actually showed no detectable evolution of the starting complex, thus indicating the very limited extent of protonolysis.

The influence of the solvent was interrogated next, during the (nonoptimized) dehydrocoupling of equimolar amounts of phenylacetylene and PhMe₂SiH at 65 °C, using 10 mol-% of **1**. Where 63% conversion was achieved in **thf** after 48 h, it only reached 1% and 6% in benzene and 1,4-dioxane, whereas conversions of 75% and 99% were recorded in the more polar dimethoxyethane and pyridine, respectively. As already inferred from the first run of experiments, pyridine was therefore the solvent of choice to maximize substrate conversion. It is also convenient for NMR monitoring (*vide supra* and *infra*). In this solvent, precatalyst loadings of 10 or 5 mol-% ([**1**]₀ = 0.250 and 0.125 M), respectively, achieved substrate conversions of 99% and 86% after 14 h at 110 °C, whereas they reached 92% and 82% at 90 °C. A metal loading of 5 mol-% with a temperature of 90 °C seemed to warrant optimal conditions.

The choice of pyridine as a solvent needed to be validated against other potential side reactions. Indeed, some calcium hydride complexes are known to catalyze the hydrosilylation of pyridine.[51] To guard against this possibility, we evaluated the behavior of Ph₂SiMeH in pyridine in the presence of 5 mol-% of **1**. We found that the hydrosilane remained inert

in pyridine-*d*₅, even after heating at 90 °C for 18 h. Other hydrosilanes (*vide infra*) also proved stable under these conditions.[52,53] Note that in one unrelated case, we found that in the presence of **1** (5 mol-%) in **thf**-*d*₈ at 65 °C (12 h), Ph₂SiH₂ decomposed to a mixture characterised by multiple resonances in the ²⁹Si NMR spectrum. Since NMR analysis allowed us to rule out the possibility of the recently reported alkenylation of pyridine,[52] we concluded that, under these specific conditions, **1** likely promotes the redistribution of aryl groups, generating PhₓSiH₄₋ₓ species as described elsewhere for calcium complexes.[53] We are also aware that calcium amides can mediate the formation of 2,3,4-hexatriene upon dimerization of the donor-functionalized terminal alkyne MeOCH₂C≡CH.[54] We have no indication that such phenomenon is triggered by **1** under the chosen reaction conditions with our benchmark substrate, phenylacetylene, or other alkynes. The solvated calcium amide **3** and its alkyl analog [Ca{CH(SiMe₃)₂.(thf)₂}]₂ promote, under harsh conditions (160 °C, 21–48 h), the formation of 1,3-enynes from alkynes that do not possess donor groups, e.g., phenylacetylene; yet, **1** was inert under identical conditions.[55] Accordingly, we found no evidence for the formation of butatrienes or 1,3-enynes under our experimental conditions, even though the formation of a small amount of unidentified products was detected after heating at 90 °C for 16 h (Figure S15, Supporting Information). We noted however that heating phenylacetylene at 90 °C in the presence of **1** in pyridine-*d*₅ but without any hydrosilane coreactant led to partial degradation of the alkyne, with formation of a mixture of *E* (major) and *Z* (minor) isomers of Ph–CH=CH–C≡C–Ph, as confirmed by NMR and GC–MS data (Figure S15–S18, Supporting Information). Again, during actual catalysis, we did not observe formation of these products.

In the subsequent investigations, all alkynylsilane products of dehydrocoupling were fully characterized (*vide infra*), including by ²⁹Si NMR spectroscopy. They normally gave rise to a single resonance in their ²⁹Si NMR spectra, which could be assigned without ambiguity. However, during *in situ* monitoring of the reaction, a minor species was also detected in some cases. For instance, in the coupling of Ph₂SiH₂ with phenylacetylene, in addition to the expected product Ph₂(H)SiC≡CPh (δSi -38.8 ppm), another minor compound with two diagnostic ²⁹Si NMR resonances at δSi 5.4 (minor) and -24.0 (major) ppm was detected. One possibility, the presence of PhC≡CSiMe₃ resulting from barium-mediated electrophilic silylation of phenylacetylene with floating HN(SiMe₃)₂ in a way similar to that reported for ZnCl₂,[29] was ruled out. No indication of this phenomenon could be detected, either *in situ* or in separate experiments. Instead, on account of the relative intensities and chemical shifts, we surmised these resonances corresponded to Ph₂HSi–N(SiMe₃)₂ (6). This byproduct is obtained through heterodehydrocoupling of the floating HN(SiMe₃)₂ (released in the reaction medium upon reaction of the precatalyst, **1**, *vide infra*, with phenylacetylene) and Ph₂SiH₂. This was confirmed upon analysis of pristine **6**, prepared independently by stoichiometric reaction of Ph₂HSiCl and LiN(SiMe₃)₂ (see Supporting Information for synthetic and NMR data, Figures S1–S3, Supporting Information). Consistent with this, the formation
<figure><img src="image_9.png" /></figure>

of the byproduct was not observed when catalysis was run with the bis(alkyl) precatalyst **4** instead of the bis(amide) **1**.

With the selection of pyridine as solvent validated, we synthesized two new barium complexes potentially relevant to the mechanism of the catalysis under investigation: the tris(pyridine) solvate $[Ba\{N(SiMe_3)_2\}_2 \cdot (pyridine)_3]$ (7) and the heteroleptic Ba-acetylide $[[Carb^{DIPP)}BaC \equiv CPh \cdot thf]$ (8). Yet, in terms of catalytic performances (and in agreement with DFT calculations that indicate a mono-pyridine barium adduct is the prevailing active species in the catalytic manifold, *vide infra*), 7 did not outperform 1, with for instance $k_{obs}$ (7) = 33.3(1) $\times 10^{-6}$ and $k_{obs}$ (1) = 143.3(2) $\times 10^{-6}$ s$^{-1}$ under otherwise identical conditions. Besides, **8** proved, at best, a mediocre catalyst with minimal catalytic turnovers (too slow to record meaningful kinetic data). The use of these two complexes in catalysis was hence not pursued further. Complex **7** was isolated as a crop of colorless crystals in 48% yield upon dissolution of 1 in excess pyridine, followed by removal of the volatiles under vacuum and recrystallisation from petroleum ether. Its NMR data match the proposed formulation, and are otherwise unremarkable. It is interesting that the compound accepts *three* molecules of pyridine (a good but relatively soft donor) to give a five-coordinated metal center, while the root complex **1** is well-known to crystallise with only *two* molecules of *thf* (a comparatively harder donor).[56] The molecular solid-state structure of **7** is depicted in **Figure 1**. The metal sits in a $C_2$-symmetry, distorted square pyramidal geometry ($\tau_5$ = 0.10).[57] The symmetry axis

<figure><img src="image_10.png" /><figcaption>**Figure 1.** ORTEP representation of the molecular solid-state structure of $[Ba(N(SiMe_3)_2)_2 \cdot (pyridine)_3]$ (7). Ellipsoids drawn at the 50% probability level. H atoms omitted for clarity. Selected interatomic distances (Å): Ba1–N1 = 2.6358(14), Ba1–N1' = 2.6357(14), Ba1–N2 = 2.9534(16), Ba1–N2' = 2.9535(16); N1–Ba1–N1' = 113.09(6), N1–Ba1–N2 = 86.80(4), N1–Ba1–N2' = 121.81(4), N1–Ba1–N3 = 123.46(3), N1'–Ba1–N2 = 121.81(4), N1'–Ba1–N2' = 86.80(4), N1'–Ba1–N3 = 123.45(3), N2–Ba1–N2' = 129.39(6), N2–Ba1–N3 = 64.69(3), N2'–Ba1–N3 = 64.69(3). Symmetry operation used to generate equivalent atoms: $-x + 1/2, y, -z$.</figcaption></figure>

<figure><img src="image_11.png" /><figcaption>**Figure 2.** Representation of the molecular solid-state structure of $[[Carb^{DIPP)}BaC \equiv CPh \cdot thf]$ (8). Ellipsoids drawn at the 50% probability level. For clarity, H atoms, 'Bu, and 'Pr groups are omitted, and flanking aromatic rings drawn as balls and sticks. Only the main component of the disordered *thf* molecule is depicted. Representative interatomic distances (Å) and angles (°): Ba1–N1 = 2.666(3), Ba1–N2 = 2.820(3), Ba1–N3 = 2.817(3), Ba1–C93 = 2.984(4), Ba1–C94 = 3.055(4), Ba1–C101 = 2.787(4), Ba1–C102 = 3.366(4), Ba2–N4 = 2.720(3), Ba2–N5 = 2.879(3), Ba2–N6 = 2.877(3), Ba2–O1A = 2.795(4), Ba2–C93 = 2.946(4), Ba2–C101 = 2.857(4); C93–C94–C95 = 177.7(5), C101–C102–C103 = 178.4(4).</figcaption></figure>

goes through Ba1 and N3. The metric parameters are standard for this type of barium amides. The complex is very air-sensitive and dissolves well in common organic solvents, including aliphatic hydrocarbons. Complex **8** was obtained by treatment of $[[Carb^{DIPP)}BaN(SiMe_3)_2 \cdot thf]$ with phenylacetylene in benzene-*d*<sub>6</sub>. It was recrystallised as the mono-solvated, binuclear $[[Carb^{DIPP)}BaC \equiv CPh \cdot thf]$ (8'). The molecular solid-state structure of **8'** was determined by X-ray diffraction analysis. It consists of an acetylide-bridged dimer, with one metal center (Ba2) coordinated by a *thf* molecule, while the other metal center (Ba1) is stabilized by secondary $\pi$ interactions with the two acetylides (Figure 2). Hence, Ba2 is six-coordinated due to the presence of the bound solvent, whereas Ba1 is five-coordinated and additionally features two asymmetric Ba···C≡C interactions with the triple bonds (compare Ba1–C101 = 2.787(4) and Ba1–C102 = 3.366(4) Å, vs. Ba1–C93 = 2.984(4) and Ba1–C94 = 3.055(4) Å). Only three other examples of molecular barium-acetylides, all of the type $[[18-crown-6]Ba(C≡C-R)_2]$ with R = SiPh<sub>3</sub>, 'Bu, or 4-<sup>1</sup>Bu-C<sub>6</sub>H<sub>4</sub>, have been structurally authenticated to date.[58,59] The Ba–C<sub>acetylide</sub> distances in **8'** (2.787(4)–2.946(4) Å) compare well with those reported for these complexes, although the Ba1–C101 interatomic distance of 2.787(4) Å is at the low end of the range. Secondary interactions, such as soft–soft Ba···C(π) contacts in **8'**, are known to stabilize electron-deficient barium and, more generally, alkaline-earth complexes.[60,61]
<figure><img src="image_12.png" /></figure>

## 2.2. Substrate Scope

The scope of the reaction was examined, as summarized in Tables 1 and 2. All reactions took place in pyridine-d₅ at 90 °C and were catalyzed by [Ba{N(SiMe₃)₂}₂.(thf)₂] (1), with [1]₀ = 30 mM (5 mol-%). Substrate conversion and product yields

were assessed by ¹H NMR spectroscopy, using C₆Me₆ as an internal standard. The identity of the dehydrocoupling products was confirmed by ¹H, ¹³C, and ²⁹Si NMR spectroscopies.[6,23,53]

The reaction between PhMe₂SiH or (p-ClC₆H₄)Me₂SiH and one equivalent of different terminal alkynes generally afforded moderate to high conversions/yields

<table><caption>**Table 1.** Alkyne scope in the dehydrocoupling of hydrosilanes and terminal alkynes.</caption><thead><tr><th>Entry<sup>a)</sup></th><th>Hydrosilane</th><th>Alkyne</th><th>Conversion/yield [%]<sup>b)</sup></th></tr></thead><tbody><tr><td>1</td><td>[C<sub>6</sub>H<sub>5</sub>]-SiMe<sub>2</sub>H</td><td>H≡C<sub>6</sub>H<sub>5</sub></td><td>70</td></tr><tr><td>2</td><td></td><td>H≡C<sub>6</sub>H<sub>5</sub></td><td>80</td></tr><tr><td>3</td><td></td><td>H≡C<sub>6</sub>H<sub>5</sub></td><td>99</td></tr><tr><td>4</td><td></td><td>H≡C<sub>6</sub>H<sub>5</sub></td><td>70</td></tr><tr><td>5</td><td></td><td>H≡C<sub>6</sub>H<sub>5</sub><sup>nBu</sup></td><td>63</td></tr><tr><td>6</td><td></td><td>H≡C<sub>6</sub>H<sub>5</sub></td><td>25</td></tr><tr><td>7</td><td></td><td>H≡C<sub>6</sub>H<sub>5</sub></td><td>10</td></tr><tr><td>8</td><td>Cl-C<sub>6</sub>H<sub>4</sub>-SiMe<sub>2</sub>H</td><td>H≡C<sub>6</sub>H<sub>5</sub></td><td>95</td></tr><tr><td>9</td><td></td><td>H≡C<sub>6</sub>H<sub>5</sub></td><td>90</td></tr><tr><td>10</td><td></td><td>H≡C<sub>6</sub>H<sub>5</sub></td><td>99</td></tr><tr><td>11</td><td></td><td>H≡C<sub>6</sub>H<sub>5</sub></td><td>99</td></tr><tr><td>12</td><td></td><td>H≡C<sub>6</sub>H<sub>5</sub><sup>nBu</sup></td><td>64</td></tr><tr><td>13</td><td></td><td>H≡C<sub>6</sub>H<sub>5</sub></td><td>55</td></tr><tr><td>14</td><td></td><td>H≡C<sub>6</sub>H<sub>5</sub></td><td>56</td></tr></tbody></table>

<footnote><sup>a)</sup>Experimental conditions: [alkyne]₀ = 0.6 M, [hydrosilane]₀ = 0.6 M, [1]₀ = 30 mM (5 mol-%), pyridine-d₅ 0.5 mL, 90 °C; <sup>b)</sup>Substrate conversion (= yield for the dehydrocoupling product) determined by ¹H NMR, using hexamethylbenzene as an internal standard.</footnote>
<figure><img src="image_13.png" /><figcaption>**Table 2.** Hydrosilane scope in the dehydrocoupling of hydrosilanes and terminal alkynes.</figcaption></figure>

<figure><img src="image_14.png" /></figure>

<table><thead><tr><th>Entry<sup>a)</sup></th><th>Alkyne</th><th>Hydrosilane</th><th>Conversion/yield [%]<sup>b)</sup></th></tr></thead><tbody><tr><td>1</td><td><img src="" alt="Alkynyl group with a phenyl ring and a C=CH group."></td><td><img src="" alt="Hydrosilane of 1 with a phenyl ring and a C=CH group."></td><td>95</td></tr><tr><td>2</td><td><img src="" alt="Alkynyl group with a phenyl ring and a C=CH group."></td><td><img src="" alt="Hydrosilane of 2 with a phenyl ring and a C=CH group."></td><td>78</td></tr><tr><td>3</td><td><img src="" alt="Alkynyl group with a phenyl ring and a C=CH group."></td><td><img src="" alt="Hydrosilane of 3 with a phenyl ring and a C=CH group."></td><td>70</td></tr><tr><td>4</td><td><img src="" alt="Alkynyl group with a phenyl ring and a C=CH group."></td><td><img src="" alt="Hydrosilane of 4 with a phenyl ring and a C=CH group."></td><td>70</td></tr><tr><td>5</td><td><img src="" alt="Alkynyl group with a phenyl ring and a C=CH group."></td><td><img src="" alt="Hydrosilane of 5 with a phenyl ring and a C=CH group."></td><td>22</td></tr><tr><td>6</td><td><img src="" alt="Alkynyl group with a phenyl ring and a C=CH group."></td><td><img src="" alt="Hydrosilane of 6 with a phenyl ring and a C=CH group."></td><td>15</td></tr><tr><td>7</td><td><img src="" alt="Alkynyl group with a phenyl ring and a C=CH group."></td><td><img src="" alt="Hydrosilane of 7 with a phenyl ring and a C=CH group."></td><td>99</td></tr><tr><td>8</td><td><img src="" alt="Alkynyl group with a phenyl ring and a C=CH group."></td><td><img src="" alt="Hydrosilane of 8 with a phenyl ring and a C=CH group."></td><td>98</td></tr><tr><td>9</td><td><img src="" alt="Alkynyl group with a phenyl ring and a C=CH group."></td><td><img src="" alt="Hydrosilane of 9 with a phenyl ring and a C=CH group."></td><td>98</td></tr><tr><td>10</td><td><img src="" alt="Alkynyl group with a phenyl ring and a C=CH group."></td><td><img src="" alt="Hydrosilane of 10 with a phenyl ring and a C=CH group."></td><td>96</td></tr><tr><td>11</td><td><img src="" alt="Alkynyl group with a phenyl ring and a C=CH group."></td><td><img src="" alt="Hydrosilane of 11 with a phenyl ring and a C=CH group."></td><td>40</td></tr><tr><td>12</td><td><img src="" alt="Alkynyl group with a phenyl ring and a C=CH group."></td><td><img src="" alt="Hydrosilane of 12 with a phenyl ring and a C=CH group."></td><td>30</td></tr></tbody><tfoot><tr><td colspan="4"><sup>a)</sup>Experimental conditions: [alkyne]<sub>0</sub> = 0.6 M, [hydrosilane]<sub>0</sub> = 0.6 M, [1]<sub>0</sub> = 30 mM (5 mol-%), pyridine-d<sub>5</sub> 0.5 mL, 90 °C; <sup>b)</sup>Substrate conversion (= yield for the dehydrocoupling product) determined by <sup>1</sup>H NMR, using hexamethylbenzene as an internal standard.</td></tr></tfoot></table>

for the desired dehydrocoupling product (Table 1, entries 1–5 and 8–14). 2-ethynyl-1,3,5-trimethylbenzene provided the best conversions among all alkynes, suggesting that catalytic turnover is favored with a more electron-rich alkyne. Generally, minor variations of the conversion were observed with alkynes functionalized by an aryl ring: ethynylbenzene,

1-ethynyl-4-methylbenzene, 1-ethynyl-4-isopropylbenzene, and 1-(tert-butyl)-4-ethynylbenzene. On the other hand, terminal alkynes bearing aliphatic substituents, e.g., 1-hexyne and ethynyl-cyclohexane, did not lend themselves so well to this catalysis, as only poor to moderate conversions were achieved (Table 1, entries 6, 7, 13 and 14).
<figure><img src="image_15.png" /></figure>

Table 2 collates the data observed when 2-ethynyl-1,3,5-trimethylbenzene and ethynylbenzene were coupled to various hydrosilanes. On the whole, and as understood from Table 1 already, the comparatively electron-rich 2-ethynyl-1,3,5-trimethylbenzene proved a more reactive substrate than ethynylbenzene. Besides, the reactions with hydrosilanes stabilized by one or more aromatic substituents (Table 2, entries 1–4 and 7–10) displayed much greater conversion rates than, for instance, triethylsilane (entries 5 and 11) and triethoxysilane (entries 6 and 12). This observation suggests that catalytic turnover is favored with hydrosilanes featuring a relatively electrophilic silicon atom. Along this line, the presence of an electron-withdrawing group in *para* position of the aryl ring of the hydrosilane boosts the catalytic activity, compare for instance entries 4 and 11 in Table 1, or entries 1 and 4 in Table 2. The increase of reaction rate with electron-withdrawing substituents on arylhydrosilanes was subsequently corroborated by Hammett analysis during kinetic investigations (*vide infra*). It is consistent with the stabilization of an incipient negative charge developing at the silicon atom in the turnover-limiting step. Finally, Tables 1 and 2 suggest that the reaction is not particularly sensitive to steric effects on the substrates. For instance, 2-ethynyl-1,3,5-trimethylbenzene consistently gave greater reaction rates than ethynylbenzene (Table 2, and also Table 1, entries 1 and 3); moreover, PhMe₂SiH, Ph₂MeSiH and Ph₃SiH produced a very homogeneous set of results (Table 2, entries 2–4 and 8–10).

## 2.3. Kinetic Analysis

The coupling of phenylacetylene and PhMe₂SiH catalyzed by 1 was used as benchmark reaction to evaluate the kinetic features of this barium-mediated catalytic process. All reactions herein were carried out in 0.5 mL of pyridine-d₅ at 90 °C in J-Young NMR tubes unless otherwise specified. Substrate conversion toward the dehydrocoupling product was monitored by ¹H NMR spectroscopy, using C₆Me₆ as an internal reference (Scheme 3).

With a tenfold excess of PhMe₂SiH versus phenylacetylene ([PhC≡CH]₀/[SiH]₀/[1]₀ = 20:200:1 or 40:400:1, with [1]₀ = 30 or 15 mM, i.e., 5.0 or 2.5 mol-%, respectively), the plot of alkyne conversion versus reaction time is linear (Figure 3), and indicates partial zeroth order kinetics in [alkyne]t. Conversely, with an excess of alkyne ([PhC≡CH]₀/[SiH]₀/[1]₀ = 200:20:1, [1]₀ = 30 mM), the semilogarithmic plot of silane conversion versus time is a straight line (Figure 4), consistent with a partial first order in [PhMe₂SiH]t. The plot of kobs versus [1]₀ for a suite of observed rate constants (kobs) extracted from the semilogarithmic plots of [PhMe₂SiH]t versus time for various precatalyst concentrations ([PhC≡CH]₀/[SiH]₀/[1]₀ = 200:20:1, where [1]₀ is in the range 3.0–60 mM) is also linear, and agrees with a partial first order in [1] (k = 5.17(1) × 10⁻³ M⁻¹ s⁻¹, Figure 5). The kinetic rate law for the catalyzed reaction described in Scheme 3 is therefore given by

$$ \text{Ph} \equiv \text{H} + \text{H-SiMe}_2\text{Ph} \xrightarrow[\text{pyridine-d}_5, 90 \,^{\circ}\text{C}]{\text{Ba}\{\text{N}(\text{SiMe}_3)_2\}_2\cdot(\text{thf})_2(\text{1})} \text{Ph} \equiv \text{SiMe}_2\text{Ph} + \text{H}_2 $$

<figure><img src="image_16.png" /><figcaption>Figure 3. Linear plot of [phenylacetylene], versus reaction time for the dehydrocoupling of phenylacetylene and Ph₂MeSiH catalyzed by 1, evidencing a zeroth-order in [phenylacetylene]. Experimental conditions: [phenylacetylene]₀ = 0.6 M, [PhMe₂SiH]₀ = 6.0 M, [1]₀ = 30 mM (◆, kobs = 10.0 ± 0.2 × 10⁻⁵ s⁻¹) or 15 mM (●, kobs = 7.17 ± 0.16 × 10⁻⁵ s⁻¹), solvent: pyridine-d₅ (0.5 mL), T = 90 °C.</figcaption></figure>
<figure><img src="image_17.png" /></figure>

<figure><img src="image_18.png" /><figcaption>**Figure 4.** Semilogarithmic plot of ln[(PhMe₂SiH)₀/(PhMe₂SiH)ₜ] versus reaction time for the dehydrocoupling of phenylacetylene and PhMe₂SiH catalyzed by 1, evidencing first order in [PhMe₂SiH] with kobs = 2.50 ± 0.1 × 10⁻⁵ s⁻¹. Conditions: [phenylacetylene]₀ = 6.0 M, [PhMe₂SiH]₀ = 0.6 M, [1]₀ = 15 mM, solvent: pyridine-d₅ (0.5 mL), T = 90 °C.</figcaption></figure>

<figure><img src="image_19.png" /><figcaption>**Figure 5.** Linear plot of kobs as a function of [1]₀ for the dehydrocoupling of phenylacetylene and PhMe₂SiH catalyzed by 1, giving k (slope) = 5.17 ± 0.01 × 10⁻³ M⁻¹ s⁻¹. Reaction conditions: [phenylacetylene]₀ = 6.0 M, [PhMe₂SiH]₀ = 0.6 M, with [1]₀ in the range 3.0–60.0 mM, solvent: pyridine-d₅ (0.5 mL), T = 90 °C.</figcaption></figure>

$$ \text{rate} = k[\text{1}]^{1}[\text{PhMe}_2\text{SiH}]^{1} \quad (1) $$

Eyring analysis in the temperature range 60–100 °C (Figure 6; R² = 0.9921) provided the activation parameters ΔH‡ = 12.7(1) kcal mol⁻¹, ΔS‡ = -42.8(1) cal mol⁻¹ K⁻¹ indicative of an ordered assembly (associative mechanism) in the main transition state, and ΔG‡ = 25.5(1) kcal mol⁻¹ at 25 °C consistent with a reaction that requires thermal input to achieve reasonable catalytic turnovers. A limited but significant kinetic isotope effect (KIE) of 2.7 was measured upon comparison of the observed kobs measured, respectively, for PhMe₂SiH and PhSiMe₂D (Figure S11,

Supporting Information). For phenylacetylene, the measured KIE was lower, 1.6. Both values are essentially consistent with the established kinetic rate law. The KIE for phenylacetylene should be negligible (i.e., ≈1), as the kinetic rate law suggests that C–H bond-breaking is not directly involved in the rate-determining transition state. The slight deviation observed could arise from secondary isotope effects, subtle changes in vibrational modes, or minor contributions from equilibrium isotope effects influencing substrate coordination or pre-equilibrium steps.[62] Hammett analysis was performed using para-substituted hydrosilanes (p-X-C₆H₄) Me₂SiH synthesized for this purpose, with X chosen amongst OMe,
<figure><img src="image_20.png" /></figure>

<figure><img src="image_21.png" /><figcaption>**Figure 6.** Eyring plot for the dehydrocoupling of phenylacetylene and PhMe₂SiH catalyzed by 1. Reaction conditions: [phenylacetylene]₀ = 0.6 M, [PhMe₂SiH]₀ = 0.6 M, [1]₀ = 30 mM, solvent: pyridine-d₅ (0.5 mL), T = 60–100 °C. For each temperature, the rate constant *k* was determined from the corresponding semilogarithmic plots ln([PhMe₂SiH]₀/[PhMe₂SiH]ₜ) = *k*<sub>obs,t</sub>, with *k*<sub>obs</sub> = *k*[1]₀.</figcaption></figure>

Me, Cl, and CF₃. Compared to the benchmark Ph₂MeSiH, reaction rates increased with electron-withdrawing groups, and decreased with electron-donating substituents, according to OMe < Me < H < Cl < CF₃. The slope of the Hammett plot was largely positive (ρ = 2.59, see Figure S13, Supporting Information), which concurs with a strong preference for building up of a negative charge on silicon in the transition-state that defines the turnover-limiting step. Although informative, the kinetic analysis did not provide conclusive mechanistic understanding. We turned to DFT calculations to collect further insight.

## 2.4. Theoretical Investigations

To shed more light on the reaction mechanism, DFT calculations (PBE0-D3, see Supporting Information for computational details) have been carried out. In a first step, the calculations probed the nature of the reactive catalytic species. The reactant was considered to be [Ba[N(SiMe₃)₂]₂·(thf)₂] (**A-2thf**). Substitution of one thf ligand by one pyridine (py) to form [Ba[N(SiMe₃)₂]₂·(thf)(py)], **A-thf-py**, is computed to be exergonic with ΔG = -1.2 kcal mol⁻¹. Further exchange of the remaining thf ligand by pyridine to yield [Ba[N(SiMe₃)₂]₂·(py)₂], **A-2py**, is found to be exergonic by ΔG = -1.7 kcal mol⁻¹ (with respect to **A-2thf**). From **A-2py**, further coordination of py to yield [Ba[N(SiMe₃)₂]₂·(py)₃], **A-3py** (X-ray structure, see Figure 1), is calculated to be endergonic by ΔG = 8.1 kcal mol⁻¹. On the other hand, dissociation of pyridine from **A-2py** to yield [Ba[N(SiMe₃)₂]₂·(py)], **A-py**, is determined to be exergonic by ΔG = -7.5 kcal mol⁻¹. The three-coordinated complex **A-py** is therefore computed to be the more stable among the species

considered and will be considered as the starting reactive species of the catalytic system.

A computational analysis of the coupling of PhC≡CH with PhMe₂SiH was conducted. The first step of the reaction is considered to be the proton transfer between PhC≡CH and one amido ligand on **A-py**. The corresponding transition state, **TS-A-B-py** (**Figure 7**) is computed at ΔG‡ = 17.1 kcal mol⁻¹ and features a concerted breaking of the C–H bond (1.342 Å) and formation of the N–H one (1.368 Å). During the transfer, the Ba···H distance remains long, at 2.76 Å. Upon dissociation of the amine HN(SiMe₃)₂, the phenyl–acetylide complex [Ba[N(SiMe₃)₂](C≡CPh)·(py)], **B-py**, is formed in an exergonic step with ΔG = -3.9 kcal mol⁻¹. The coordination geometry of the PhC≡C⁻ acetylide ligand is strongly bent in **B-py** (**Figure 7**) with a Ba–C≡C angle of 110.2°, as a result of the interaction of the π-system of the alkyne with the barium center. As illustrated in Figure 7, the nucleophilic character of the Cα atom is fully available to interact with the electropositive Si atom of an incoming hydrosilane molecule.

The next step in the transformation is the formation of the C–Si bond through the transition state **TS-B-C-py** (**Figure 8**), with an associated activation barrier of ΔG‡ = 23.8 kcal mol⁻¹ with respect to **B-py** + PhMe₂SiH. The reaction is a nucleophilic attack of Cα on Si, as illustrated by the HO of **TS-B-C-py** shown in Figure 8. It entails the building up of an incipient negative charge on the silicon atom, consistent with the aforementioned Hammett analysis. The Cα···Si distance amounts to 2.03 Å in the transition state, while the Si–H bond is elongated to 1.63 Å (Si–H = 1.50 Å in PhMe₂SiH) with concomitant increase of the interaction between the H atom and Ba (H···Ba = 2.57 Å). The geometry
<figure><img src="image_22.png" /></figure>

<figure><img src="image_23.png" /><figcaption>**Figure 7.** Geometry of the transition state TS-A-B-py for hydrogen transfer between PhC≡CH and one amido ligand (left), geometry of the phenyl-acetylide complex B-py after amine dissociation (middle) and representation of the HO-4 of B-py featuring mostly a lone pair on C<sub>α</sub> (right). Most H atoms have been omitted for clarity.</figcaption></figure>

<figure><img src="image_24.png" /><figcaption>**Figure 8.** Geometry of the transition state TS-B-C-py for formation of the Si–C<sub>α</sub> bond (left), HO of the transition state TS-B-C-py (middle), geometry of the pentavalent silicate intermediate C-py (right). Most H atoms have been omitted for clarity.</figcaption></figure>

around Si in the transition state is best described as a square-based pyramid with apical Ph. The product of the transformation, **C-py**, features a trigonal bipyramid pentavalent silicate [PhMe₂Si(H)(C≡CPh)]⁻ with apical Me and H in interaction with the Ba{N(SiMe₃)₂}.(py) cation. The Si–C<sub>α</sub> interatomic distance now amounts to 1.90 Å, while the Si–H bond has elongated to 1.80 Å (H in apical position *trans* to Me) and the Ba···H distance has now shortened to 2.42 Å. The silicate intermediate **C-py** has a relative Gibbs energy of ΔG = 19.6 kcal mol⁻¹ with respect to **B-py** + PhMe₂SiH (Figure 8).

The breaking of the Si–H bond through the transition state structure TS-C-D-py (Figure 9) is computed to be very easy with an activation barrier of ΔG‡ = 0.6 kcal mol⁻¹ with respect to **C-py**. In TS-C-D-py, the Si···H distance has elongated to 2.6 Å and the Ba···H distance has slightly shortened to 2.36 Å. Upon formation of the product PhMe₂Si(C≡CPh) and its dissociation from the hydride [HBa{N(SiMe₃)₂}.(py)] (**D-py**), a Gibbs energy of ΔG = -13.6 kcal mol⁻¹

is released from **C-py**. The catalytic cycle is completed with the regeneration of the catalyst **A-py** upon reaction of HN(SiMe₃)₂ with the hydride **D-py**. The transition state for the proton transfer from the nitrogen atom on the amine to the hydride on barium, **TS-D-A-py** (Figure 9), is computed to be associated to an activation of ΔG‡ = 18.1 kcal mol⁻¹ relative to **D-py** + HN(SiMe₃)₂. After dissociation of H₂, the reaction Gibbs energy for the transformation **D-py** + HN(SiMe₃)₂ → **A-py** + H₂ is computed to be ΔG = 1.1 kcal mol⁻¹.

An alternative scenario for the regeneration of the active catalyst, using PhC≡CH as the proton source, has been searched. The transition state **TS-D-B-py** (Figure 9) corresponding to the proton transfer from PhC≡CH to the barium hydride has been located on the potential energy surface (PES) with a kinetic activation barrier of ΔG‡ = 19.8 kcal mol⁻¹ relative to **D-py** + PhC≡CH. After dissociation of H₂, the reaction Gibbs energy for the transformation **D-py** + PhC≡CH → **B-py** + H₂ is computed to be ΔG = -2.8 kcal mol⁻¹. Hence, proton transfer from the amine is **ca.**

<figure><img src="image_25.png" /><figcaption>**Figure 9.** Geometry of the transition state TS-C-D-py for breaking of the Si–H bond (left), geometry of the hydride intermediate **D-py** (middle left), geometry of the transition state **TS-D-A-py** for the formation of H₂ from amine (middle right), geometry of the transition state **TS-D-B-py** for the formation of H₂ from PhC≡CH (right). Most H atoms have been omitted for clarity.</figcaption></figure>
<figure><img src="image_26.png" /></figure>

<figure><img src="image_27.png" /><figcaption>**Figure 10.** Computed Gibbs free energy values (in kcal mol⁻¹) for the extrema located along the catalytic cycle. The various species concerned by the extrema are indicated below each extremum.</figcaption></figure>

1.7 kcal mol⁻¹ less energetic than from phenylacetylene. Although based on these calculations, both processes may be considered to contribute to the regeneration of competent catalytic species (A-py and B-py, respectively), only the most energetically favorable one, the aminolysis that returns A-py, is consistent with the observed zeroth order in [phenylacetylene] determined experimentally.

The overall PES for the transformation is reported in **Figure 10**. The highest transition state structure on the catalytic cycle is **TS-D-A-py**, corresponding to the regeneration of the active catalyst from the barium-hydride intermediate *via* proton transfer from the amine. The lowest point on the catalytic cycle is the phenylacetylide intermediate **B-py**. Consequently, the energy span associated to the transformation is computed to be $ΔG^‡ = 24.1$ kcal mol⁻¹ (*T* = 90 °C in the calculations reported in Figure 10). The topology of the computed PES is also in agreement with the kinetic rate law determined experimentally. The alkyne does not participate in any steps between **B-py** and **TS-D-A-py**, in agreement with the observed zeroth order with respect to [alkyne].

The overall reaction is calculated to be endergonic by $ΔG = 3.2$ kcal mol⁻¹, but formation of gaseous H₂ to close the catalytic cycle and its release out of the reaction medium will drive the reaction forward. In addition, as shown on Figure 10, reaction of the hydride with phenylacetylene, although associated to a slightly higher activation energy, is associated to a thermodynamically favored step. A complete catalytic cycle consistent with our computational analysis is proposed in **Figure 11**.

Experimentally, the activity trend observed is $Ca < Sr < Ba$. Computations have been carried out in the same framework as the one described in Figure 10 for the two [Ca{N(SiMe₃)₂}₂.(py)] and [Sr{N(SiMe₃)₂}₂.(py)] precatalysts. The relative Gibbs energies of the extrema located along the PES are provided in **Table 3**. From these data, the relative activation Gibbs energies are $ΔG^‡ = 21.9$ kcal mol⁻¹ for Ca, $ΔG^‡ = 22.2$ kcal mol⁻¹ for Sr, and

$ΔG^‡ = 24.1$ kcal mol⁻¹ for Ba. Hence, the calculations presented in Table 3 seem to contradict the experimental observations with respect to the relative activities of the three metals. However, the catalytic cycle shown in Figure 11 assumes that the reactive species is Ae{N(SiMe₃)₂}₂(py), as this complex has been shown to be the most stable for Ae = Ba. Interestingly, the relative energies of the various Ae{N(SiMe₃)₂}₂(thf)ₓ(py)₂ₓ (Ae = Ca, Sr, Ba and x = 0, 1, 2) strongly depend on the nature of Ae (Table 4). In the case of Ae = Ba, there is a strong thermodynamic preference for the bis-amido, mono-pyridine complex that was considered as the active catalytic species in the catalytic cycle shown in Figure 11. The situation is gradually different for M = Sr and M = Ca, where the bis-pyridine complex is closer in energy to the mono-pyridine one, reaching an energy difference of only $ΔG = 2.4$ kcal mol⁻¹ for M = Ca, the least reactive species. The thf adducts are also less destabilized with respect to the bis-amido, mono-pyridine active catalyst considered in Table 3.

<figure><img src="image_28.png" /><figcaption>**Figure 11.** Prevailing catalytic cycle for the dehydrocoupling of PhC≡CH and PhMe₂SiH catalyzed by [Ba{N(SiMe₃)₂}₂.(thf)₂] (1) in pyridine.</figcaption></figure>
<figure><img src="image_29.png" /><figcaption>**Table 3.** Computed relative Gibbs energy (in kcal mol⁻¹) of the extrema located along the PES shown in Figure 10 for Ae = Ca, Sr, and Ba.</figcaption></figure>

<table><caption>**Table 4.** Computed relative Gibbs energy (in kcal mol⁻¹) of the various Ae complexes involving thf and/or pyridine for Ae = Ca, Sr, and Ba.</caption><thead><tr><th>Ae</th><th>Ca</th><th>Sr</th><th>Ba</th></tr></thead><tbody><tr><td>A-py</td><td>0.0</td><td>0.0</td><td>0.0</td></tr><tr><td>TS-A-B-py</td><td>13.6</td><td>15.4</td><td>17.1</td></tr><tr><td>B-py</td><td>-0.7</td><td>-2.4</td><td>-3.9</td></tr><tr><td>TS-B-C-py</td><td>21.2</td><td>19.8</td><td>19.9</td></tr><tr><td>C-py</td><td>17.5</td><td>16.8</td><td>15.7</td></tr><tr><td>TS-C-D-py</td><td>16.0</td><td>16.4</td><td>16.3</td></tr><tr><td>D-py</td><td>3.5</td><td>3.2</td><td>2.1</td></tr><tr><td>TS-D-B-py</td><td>25.4</td><td>24.2</td><td>21.9</td></tr><tr><td>B-py + Product + H<sub>2</sub></td><td>2.5</td><td>0.8</td><td>-0.7</td></tr><tr><td>TS-D-A-py</td><td>20.9</td><td>18.8</td><td>20.2</td></tr><tr><td>A-py + Product + H<sub>2</sub></td><td>3.2</td><td>3.2</td><td>3.2</td></tr></tbody></table>

<table><thead><tr><th>Ae</th><th>Ca</th><th>Sr</th><th>Ba</th></tr></thead><tbody><tr><td>Ae[N(SiMe<sub>3</sub>)<sub>2</sub>]<sub>2</sub>(thf)<sub>2</sub></td><td>3.7</td><td>6.3</td><td>9.3</td></tr><tr><td>Ae[N(SiMe<sub>3</sub>)<sub>2</sub>]<sub>2</sub>(thf)(py)</td><td>3.6</td><td>5.0</td><td>8.1</td></tr><tr><td>Ae[N(SiMe<sub>3</sub>)<sub>2</sub>]<sub>2</sub>(py)<sub>2</sub></td><td>2.4</td><td>3.7</td><td>7.5</td></tr><tr><td>Ae[N(SiMe<sub>3</sub>)<sub>2</sub>]<sub>2</sub>(py)</td><td>0.0</td><td>0.0</td><td>0.0</td></tr></tbody></table>

The spread of the relative energies between the various complexes increases in the order Ca < Sr < Ba, thus mirroring the trend in relative activities between the three metals. We hence propose that, despite the Ca ≈ Sr > Ba trend in intrinsic activity resulting from the computed values in Table 3, the experimentally observed trend Ca < Sr < Ba is the result of a more efficient specification of Ba in favor of the actual three-coordinated reactive species Ba[N(SiMe<sub>3</sub>)<sub>2</sub>]<sub>2</sub>(py) with respect to less reactive four-coordinate complexes. A beneficial influence of entropic factors on the observed reactivity trend linked to the increasing size of the metals was ruled out. The computed ΔG<sup>‡</sup> values at T = 363 K for the step associated to the C-Si bond formation process (B-py + HSiMe<sub>2</sub>Ph → TS-B-C-py) get only very slightly more negative when the metal becomes larger: -45.9 cal mol<sup>-1</sup> K<sup>-1</sup> for Ca, -46.0 cal mol<sup>-1</sup> K<sup>-1</sup> for Sr, and -46.5 cal mol<sup>-1</sup> K<sup>-1</sup> for Ba. There is hence no entropic advantage with increasing metal, in contrast with, for instance, the behavior reported during intermolecular hydroamination catalysis.[63]

### 3. Concluding Remarks

We have shown that simple alkaline-earth complexes, and, chiefly among them, the thf-solvated barium bis(amide) [Ba[N(SiMe<sub>3</sub>)<sub>2</sub>]<sub>2</sub>·(thf)<sub>2</sub>] (1), act as competent precatalysts for the dehydrocoupling of terminal alkynes and hydrosilanes, thus providing an atom-efficient route to valuable alkynylsilanes. The reactions are kinetically affordable although relatively slow (with an experimental Gibbs energy of activation of ΔG<sup>‡</sup> = 25.5(1) kcal mol<sup>-1</sup>

at 25 °C according to Eyring analysis), generally requiring extended reaction times and heating to 90 °C to achieve reasonable catalytic turnovers. The observed activity trend characteristically increased according to Ca < Sr < Ba. Conversions were generally in the range 70–95+% for a range of over 20 different coupling partners. Aryl alkynes, e.g., phenylacetylene and its substituted derivatives, are much more reactive than alkyl-substituted alkynes. Likewise, aryl hydrosilanes are better substrates than their alkyl counterparts. Careful control of the experimental set-up is required to avoid potential side-reactions; but, on the whole, the chemoselectivity of the barium-catalyzed dehydrocoupling under optimized conditions was very good.

A kinetic rate law with partial first orders in catalyst and in hydrosilane, and zeroth order in alkyne, was established following kinetic investigations on a benchmark barium-catalyzed coupling. In link with these experimental and kinetic data, DFT computations shed light on the operative mechanism. The mono-pyridine barium adduct Ba[N(SiMe<sub>3</sub>)<sub>2</sub>]<sub>2</sub>(py) was found to be the prevalent active species, within a catalytic manifold where protonolysis and concomitant release of dihydrogen is the kinetically rate-determining step. Although DFT calculations suggest that, intrinsically (i.e., based on relative energy barriers), the activity trend should vary according to Ca ≈ Sr > Ba, the actual trend Ca < Sr < Ba observed experimentally is the outcome of the greater ability of barium to generate the catalytically competent three-coordinate, mono-pyridine adduct in comparison with less reactive four-coordinate compounds.

With this mechanistic insight and preliminary experience in hand, we are now applying this methodology to construct poly(alkynylsilanes)s via metal-mediated dehydropolymerisation reactions. Our results on this topic will be detailed in a forthcoming report.

### Supporting Information

Experimental, analytical, and computational details are available in the Supporting Information. Crystallographic data and CIF files for 7 and 8* (CCDC 2486376–2486377) are also provided.

### Conflict of Interest

The authors declare no conflict of interest.

### Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.

**Keywords:** alkyne · alkynylsilanes · barium · dehydrocoupling catalysis · hydrosilanes

[1] G. Larson, *Synthesis* **2018**, *50*, 2433.

[2] B. Eaton, J. A. King Jr, K. P. C. Vollhardt, *J. Am. Chem. Soc.* **1986**, *108*, 1359.

[3] K. Kuciński, *Adv. Synth. Catal.* **2024**, *366*, 2382.
<figure><img src="image_30.png" /></figure>

[4] M. Voronkov, O. Yarosh, G. Turkina, V. Vitkovskii, A. Albanov, *J. Organomet. Chem.* **1990**, *389*, 1.

[5] H. Q. Liu, J. F. Harrod, *Can. J. Chem.* **1992**, *70*, 107.

[6] A. A. Toutov, K. N. Betz, D. P. Schuman, W.-B. Liu, A. Fedorov, B. M. Stoltz, R. H. Grubbs, *J. Am. Chem. Soc.* **2017**, *139*, 1668.

[7] M. Skrodzki, S. Witomska, P. Pawluć, *Dalton Trans.* **2018**, *47*, 5948.

[8] M. Rauch, R. C. Roberts, G. Parkin, *Inorg. Chim. Acta* **2019**, *494*, 271.

[9] F. Buch, S. Harder, *Organometallics* **2007**, *26*, 5132.

[10] R. Liu, X. Liu, T. Cheng, Y. Chen, *Eur. J. Org. Chem.* **2022**, e202101218.

[11] X. Zheng, S. Jiang, C. Dong, Q. Yao, Y. Yao, X. Xu, *Polyhedron* **2022**, *218*, 115771.

[12] T. Li, R. Liu, X. Liu, Y. Chen, *Org. Lett.* **2023**, *25*, 761.

[13] X. Zhu, D. Guo, Z. Huang, T. Sheng, S. Wang, M. Pan, L. Zha, S. Zhou, *Inorg. Chem.* **2020**, *59*, 14152.

[14] L. Ding, Y. Liu, K. Niu, Q. Wang, *Chem. Commun.* **2022**, *58*, 10679.

[15] H. Stachowiak, K. Kuciński, F. Kallmeier, R. Kempe, G. Hreczycho, *Chem. Eur. J.* **2022**, *28*, e202103629.

[16] K. Broniarz, G. Hreczycho, *Org. Lett.* **2023**, *25*, 6528.

[17] K. Srinivas, G. Prabusankar, *RSC Adv.* **2018**, *8*, 32269.

[18] Q.-C. Gan, Z.-Q. Song, C.-H. Tung, L.-Z. Wu, *Org. Lett.* **2022**, *24*, 5192.

[19] N. Ding, W. Zhang, T. S. A. Hor, *Dalton Trans.* **2012**, *41*, 5988.

[20] X.-H. Zhou, X.-J. Fang, F.-Y. Ling, Z. Xu, L.-Q. Hong, F. Ye, L.-W. Xu, *Org. Chem. Front.* **2022**, *9*, 5891.

[21] R. Shimizu, T. Fuchikami, *Tetrahedron Lett.* **2000**, *41*, 907.

[22] M. Kahnes, H. Görls, M. Westerhausen, *Organometallics* **2010**, *29*, 3490.

[23] T. Tsuchimoto, M. Fujii, Y. Iketani, M. Sekine, *Adv. Synth. Catal.* **2012**, *354*, 2959.

[24] K. Yamaguchi, Y. Wang, T. Oishi, Y. Kuroda, N. Mizuno, *Angew. Chem., Int. Ed.* **2013**, *52*, 5627.

[25] M. Wissing, A. Studer, *Chem. Eur. J.* **2019**, *25*, 5870.

[26] Y. Ma, S.-J. Lou, G. Luo, Y. Luo, G. Zhan, M. Nishiura, Y. Luo, Z. Hou, *Angew. Chem., Int. Ed.* **2018**, *57*, 15222.

[27] X. Li, F. Zhang, Y. Zhong, N. Li, J. Xu, B. Fan, *J. Org. Chem.* **2023**, *88*, 11675.

[28] P. Arde, V. Reddy, R. V. Anand, *RSC Adv.* **2014**, *4*, 49775.

[29] A. A. Andreev, V. V. Konshin, N. V. Komarov, M. Rubin, C. Brouwer, V. Gevorgyan, *Org. Lett.* **2004**, *6*, 421.

[30] T. Kawatsu, K. Aoyagi, Y. Nakajima, J.-C. Choi, K. Sato, K. Matsumoto, *Organometallics* **2020**, *39*, 2947.

[31] Y. Zeng, L. Fensterbank, C. Chauvier, *ChemCatChem* **2025**, *17*, e202500121.

[32] Q. Lin, L. Lin, J. Wang, D. Yu, Z. Chen, Z.-X. Wang, *Chem. Sci.* **2025**, *16*, 14216.

[33] S. Harder, in *Early Main Group Metal Catalysis: Concepts and Reactions*, Wiley-VCH, Weinheim, Germany **2020**.

[34] S. Harder, *Chem. Rev.* **2010**, *110*, 3852.

[35] M. S. Hill, D. J. Liptrot, C. Weetman, *Chem. Soc. Rev.* **2016**, *45*, 972.

[36] M. S. Hill, D. J. Liptrot, D. J. MacDougall, M. F. Mahon, T. P. Robinson, *Chem. Sci.* **2013**, *4*, 4212.

[37] C. Bellini, J.-F. Carpentier, S. Tobisch, Y. Sarazin, *Angew. Chem., Int. Ed.* **2015**, *54*, 7679.

[38] C. Bellini, V. Dorcet, J.-F. Carpentier, S. Tobisch, Y. Sarazin, *Chem. Eur. J.* **2016**, *22*, 4564.

[39] C. Bellini, T. Roisnel, J.-F. Carpentier, S. Tobisch, Y. Sarazin, *Chem. Eur. J.* **2016**, *22*, 15733.

[40] C. Bellini, C. Orione, J.-F. Carpentier, Y. Sarazin, *Angew. Chem., Int. Ed.* **2016**, *55*, 3744.

[41] L. J. Morris, G. R. Whittell, J.-C. Eloi, M. F. Mahon, F. Marken, I. Manners, M. S. Hill, *Organometallics* **2019**, *38*, 3629.

[42] E. Le Coz, S. Kahlal, J.-Y. Saillard, T. Roisnel, V. Dorcet, J.-F. Carpentier, Y. Sarazin, *Chem. Eur. J.* **2019**, *25*, 13509.

[43] L. J. Morris, M. S. Hill, M. F. Mahon, I. Manners, F. S. McMenamy, G. R. Whittell, *Chem. Eur. J.* **2020**, *26*, 2954.

[44] E. Le Coz, V. Dorcet, T. Roisnel, S. Tobisch, J.-F. Carpentier, Y. Sarazin, *Angew. Chem., Int. Ed.* **2018**, *57*, 11747.

[45] E. Le Coz, Z. Zhang, T. Roisnel, L. Cavallo, L. Falivene, J.-F. Carpentier, Y. Sarazin, *Chem. Eur. J.* **2020**, *26*, 3535.

[46] P. M. Chapple, S. Kahlal, J. Cartron, T. Roisnel, V. Dorcet, M. Cordier, J.-Y. Saillard, J.-F. Carpentier, Y. Sarazin, *Angew. Chem., Int. Ed.* **2020**, *59*, 9120.

[47] P. M. Chapple, M. Cordier, V. Dorcet, T. Roisnel, J.-F. Carpentier, Y. Sarazin, *Dalton Trans.* **2020**, *49*, 11878.

[48] M. Arrowsmith, M. S. Hill, G. Kociok-Köhn, *Organometallics* **2010**, *29*, 4203.

[49] J. Eppinge, E. Herdtweck, R. Anwander, *Polyhedron* **1998**, *17*, 1195.

[50] F. G. Bordwell, G. E. Drucker, N. H. Andersen, A. D. Denniston, *J. Am. Chem. Soc.* **1986**, *108*, 7310.

[51] J. Intemann, H. Bauer, J. Pahl, L. Maron, S. Harder, *Chem. Eur. J.* **2015**, *21*, 11452.

[52] W. Zhou, S. Wang, M. Nishiura, X. Cong, Z. Hou, *Chem. Commun.* **2025**, *61*, 11955.

[53] T. Li, K. N. McCabe, L. Maron, X. Leng, Y. Chen, *ACS Catal.* **2021**, *11*, 6348.

[54] A. G. M. Barrett, M. R. Crimmin, M. S. Hill, P. B. Hitchcock, S. L. Lomas, P. A. Procopiou, K. Suntharalingam, *Chem. Commun.* **2009**, 2299.

[55] M. Arrowsmith, M. R. Crimmin, M. S. Hill, S. L. Lomas, D. J. MacDougall, M. F. Mahon, *Organometallics* **2013**, *32*, 4961.

[56] B. A. Vaartstra, J. C. Huffman, W. E. Streib, K. G. Caulton, *Inorg. Chem.* **1991**, *30*, 121.

[57] A. W. Addison, T. N. Rao, J. Reedijk, J. van Rijn, G. C. Verschoor, *J. Chem. Soc., Dalton Trans.* **1984**, 1349.

[58] D. C. Green, U. Englich, K. Ruhlandt-Senge, *Angew. Chem., Int. Ed.* **1998**, *38*, 354.

[59] M. A. Guino-o, J. S. Alexander, M. L. McKee, H. Hope, U. B. Englich, K. Ruhlandt-Senge, *Chem. Eur. J.* **2009**, *15*, 11842.

[60] P. M. Chapple, Y. Sarazin, *Eur. J. Inorg. Chem.* **2020**, 3321.

[61] H. Rouindeji, A. Ratsifahina, T. Roisnel, V. Dorcet, S. Kahlal, J.-Y. Saillard, J.-F. Carpentier, Y. Sarazin, *Chem. Eur. J.* **2019**, *25*, 8854.

[62] M. E. Harris, V. E. Anderson, in *Measurement and Analysis of Kinetic Isotope Effects*, Elsevier, Cambridge, MA **2017**.

[63] A. G. M. Barrett, C. Brinkmann, M. R. Crimmin, M. S. Hill, P. Hunt, P. A. Procopiou, *J. Am. Chem. Soc.* **2009**, *131*, 12906.

Manuscript received: September 30, 2025

Revised manuscript received: December 1, 2025

Version of record online: