# Alkyne Cyclotrimerisation, Acyclic Oligomerisation, and Transfer Hydrogenation Catalysed by a Titanium(IV) Phosphinoaryloxide Complex and its Redox Chemistry

Matthias R. Steiner, Viatcheslav V. Jouikov, Roland C. Fischer, Johanna M. Uher, Isabel-Maria Ramirez Medina, Johann A. Hlina

► To cite this version:

Matthias R. Steiner, Viatcheslav V. Jouikov, Roland C. Fischer, Johanna M. Uher, Isabel-Maria Ramirez Medina, et al.. Alkyne Cyclotrimerisation, Acyclic Oligomerisation, and Transfer Hydrogenation Catalysed by a Titanium(IV) Phosphinoaryloxide Complex and its Redox Chemistry. European Journal of Inorganic Chemistry, 2024, 27 (19), pp.e202300791. 10.1002/ejic.202300791. hal-04615606

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Alkyne Cyclotrimerisation, Acyclic Oligomerisation, and
Transfer Hydrogenation Catalysed by a Titanium(IV)
Phosphinoaryloxide Complex and its Redox Chemistry

Matthias R. Steiner,[b] Viatcheslav V. Jouikov,[c] Roland C. Fischer,[d] Johanna M. Uher,[b]
Isabel-Maria Ramirez y Medina,[e] and Johann A. Hlina*[a]

Dedicated to Prof. Dr. Christoph Marschner on the occasion of his 60th birthday.

The reaction of sodium 2,4-di-tert-butyl-6-(diphenylphos-
phino)phenolate (1, NaOAr<sup>p</sup>) with Ti<sup>IV</sup>Cl<sub>4</sub>(THF)<sub>2</sub> in a 2:1 molar
ratio yielded Ti<sup>IV</sup>(OAr<sup>p</sup>)<sub>2</sub>Cl<sub>2</sub> (2). The reduction of 2 with
magnesium gave the dimeric titanium(III) compound [Ti<sup>III</sup>-
(OAr<sup>p</sup>)<sub>2</sub>Cl]<sub>2</sub> (3). In the presence of magnesium, compound 2 was

found to catalyse the cyclotrimerization, acyclic oligomerisation,
and hydrogenation of terminal alkynes. The complexes were
characterised by single-crystal X-ray diffraction, optical spectro-
scopy, elemental analysis, and NMR or EPR spectroscopy,
respectively.

**Introduction**

The complexes of the group 4 metals titanium and zirconium have played a major part in catalysis research for some decades now because of the high abundance, comparably low toxicity and biocompatibility of these transition metals.[1] Applications of titanium complexes in catalysis are manifold[1,2] and include transformations such as Umpolung reactions,[3] Diels-Alder reactions,[4] hydroamination,[5] and especially olefin polymerization.[6] Another important reaction catalyzed by group 4 metal complexes is the di- and oligomerization of alkynes.[7] Especially, cyclotrimerization of alkynes is a useful

tool for the synthesis of substituted benzenes in an atom-
economic manner.[8] The first transition-metal catalyzed cyclo-
trimerization has been reported as early as 1948 by Reppe and
Schweckendiek and used Ni-carbonyl complexes.[9] Cyclo-trime-
rization with low-valent titanium complexes has already been
described in 1984, whereby diphenylacetylene was oligomer-
ized to hexaphenylbenzene.[10] Shur et al. reported the reactivity
of low-valent Ti complexes forming metallacyclopropenes in
1988, thereby sparking further interest in using this type of
complex for similar transformations.[11] Much work on these
complexes was performed by Rosenthal, who described not
only different titanacyclopropenes,[12] but also the coupling of
such complexes with nitriles.[13] Recently, phosphinophenolate-
based titanium complexes have emerged as valuable catalysts
for ethylene dimerization,[14] ethylene and propylene
polymerisation,[15] ethylene/norbornene copolymerization,[16,17]
and ethylene/hexene copolymerization.[18] Additionally, we
recently disclosed heterobimetallic complexes using phosphino-
phenolate ligands,[19] which prompted us to investigate titanium
complexes based on this ligand system further.

Here, we present our work on alkyne oligomerisation
catalysis with a phosphinophenolate-ligated titanium complex
and its redox chemistry.

**Results and Discussion**

**Synthesis and Characterisation.** In analogy to the work of Gibson and co-workers as well as our work on early metal phosphinophenolate complexes, we began the synthetic work with the deprotonation of 2,4-di-tert-butyl-6-(diphenylphosphino)phenol, HOArP (Scheme 1).[20-22] Therefore, a suspension of sodium hydride in toluene was treated with a solution of an equimolar amount of HOArP in toluene at ambient temperature. The reaction mixture was then heated to 50 °C for three hours and addition 18 h at ambient temperature

[a] Dr. J. A. Hlina
Institute of Chemistry, Inorganic Chemistry
University of Graz
Schubertstraße 1, 8010 Graz, Austria
E-mail: johann.hlina@uni-graz.at

[b] M. R. Steiner, J. M. Uher
Institute for the Chemistry and Technology of Materials
Graz University of Technology
Stremayrgasse 9, 8010 Graz, Austria

[c] Prof. Dr. V. V. Jouikov
UMR 6226 - ISCR
Université de Rennes
Campus de Beaulieu, 35042 Rennes, France

[d] Prof. R. C. Fischer
Institute of Inorganic Chemistry
Graz University of Technology
Stremayrgasse 9, 8010 Graz, Austria

[e] Dr. I.-M. R. y Medina
Faculty 02: Biology/Chemistry (FB 02)
University of Bremen
Leobener Straße, NW 2, 28359 Bremen, Germany

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Supporting information for this article is available on the WWW under
https://doi.org/10.1002/ejic.202300791

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© 2024 The Authors. European Journal of Inorganic Chemistry published by 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.
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<figure><img src="image_6.png" /><figcaption>Scheme 1. Preparation of the titanium(IV)phosphinoaryloxide complex 2 and the dinuclear titanium(III)phosphinoaryloxide complex 3.</figcaption></figure>

after which filtration of the solution and evaporation to the
solvent yielded sodium 2,4-di-tert-butyl-6-(diphenyl-
phosphino)-phenolate, NaOAr<sup>P</sup> 1, in 93% as a colourless solid.
The <sup>31</sup>P{<sup>1</sup>H} NMR resonance of 1 was observed at -16.0 ppm,
whereas for the parent HOAr<sup>P</sup> the resonance is seen at
-30.8 ppm.<sup>[22]</sup> With the reagent in hand, we treated titanium(IV)
chloride bis(THF) adduct with two molar equivalents of 1 in THF
at ambient temperature yielding the titanium(IV) bis(2,4-di-tert-
butyl-6-(diphenylphosphino)-phenolate) dichloride, Ti<sup>IV</sup>(OAr<sup>P</sup>-
κ<sup>2</sup>O,P)<sub>2</sub>Cl<sub>2</sub> 2, as dark red crystals after crystallisation from
benzene in 39% yield. This synthetic route was not optimised
and gives complex 2 in lower yield compared to a previously
reported preparation using the analogous lithium aryloxide
instead of the sodium aryloxide, which gave the complex in
73%.<sup>[16]</sup> Any attempts to prepare titanium(IV) complexes with
more than two Ar<sup>P</sup>O<sup>-</sup> ligands using these reagents and similar
conditions including elevated temperatures were unsuccessful.
Complex 2 crystallised in the monoclinic space group P2<sub>1</sub>/c and
the solid-state structure exhibits the titanium(IV) centre coordi-
nated by two phosphinophenolates and the two chlorides as a
distorted octahedron (Figure 1). We observe only one structural
isomer showing the oxygen, phosphorus and chlorine atoms on
neighbouring coordination site around the titanium atom. The
Ti–O bond distances are very similar 1.841(3) (Ti1-O1) and
1.848(3) Å (Ti1-O2), whereas the Ti–Cl and Ti–P bond distances,
2.286(2) (Ti1-Cl1), 2.334(2) Å (Ti1-Cl2), 2.680(2) (Ti1-P1), and
2.701(2) Å (Ti1-P2), exhibit significant differences. We attribute
this to the *trans* influence as the oxygen atoms oppose the
corresponding elongated Ti1-Cl2 and Ti1-P1 bonds in compar-
ison to the Ti1-Cl1 and Ti1-P2 bonds, which are opposed by
phosphane and chloride ligands, respectively. The bite-angles

<figure><img src="image_7.png" /><figcaption>**Figure 1.** Molecular structure of 2. Hydrogen atoms are omitted and selected carbon atoms are depicted as wireframe for clarity. Thermal ellipsoids drawn at 50% probability. Selected distances (Å) and angles (deg): Ti1-Cl1: 2.286(2), Ti1-Cl2: 2.334(2), Ti1-O1: 1.848(3), Ti1-O2: 1.841(3), Ti1-P1: 2.680(2), Ti1-P2: 2.701(2), O1-Ti1-P1: 74.1(1), O2-Ti1-P2: 72.6(1), O1-Ti1-O2: 98.5(1), P1-Ti1-P2: 88.35(5), Cl1-Ti1-Cl2: 94.95(5).</figcaption></figure>

of the ligands are 74.1(1) (O1-Ti1-P1) and 72.6(1)° (O2-Ti1-P2). The molecular structure is similar to the previously reported structure of Ti⁴⁺(OC₆H₃·2⁻Bu·6-PPh₂·κ²O,P)₂Cl₂.[²⁰] The NMR spectroscopic data is in accordance with previously reported values and reflects the structural properties, showing two sets of ligand signals for the two Ar⁴⁺O⁻ groups. For example, in the ³¹P {¹H} NMR the two phosphorus atoms with resonances at 9.0 and 13.7 ppm. Both resonances are observed as singlets without a recognisable ³¹P·³¹P coupling. This is consistent with previously reported ³¹P NMR data for similar complexes, e.g. bis(2-tert-butyl-6-(diphenylphosphino)phenolate) dichloride, for which ³¹P·³¹P coupling was observed at lower temperatures.[¹⁵]

Considering the applications of low-valent titanium com-
plexes in catalysis, we were interested in exploring the
reduction chemistry of complex 2. Initially, we aimed to prepare
alkyne complexes in analogy to Rosenthal's Cp₂Ti(btmsa),
btmsa = bis(trimethylsilyl)acetylene.[12] Therefore, we treated 2
with equimolar amounts of magnesium and
bis(trimethylsilyl)acetylene in THF at ambient temperature in an
attempt to prepare (Ar⁴⁺O)₂Ti(btmsa). Upon work-up of the
reaction mixture, we found that the crude product exhibited
unexpectedly few and weak signals in ¹H and ³¹P{¹H} NMR
spectroscopic analysis. This prompted us to assume that the
reaction might have yielded a paramagnetic titanium(III) species
instead. The isolated product was found to readily crystallize
from pentane and the molecular structure indeed revealed the
dinuclear titanium complex [(Ar⁴⁺O·κ²O,P)Ti³⁺Cl]₂, 3, in which two
<figure><img src="image_8.png" /></figure>

(Ar⁴O)₂Ti¹¹ moieties are bridged with two chloride atoms to coordinatively saturate each of the two metal centres (Figure 2). With the knowledge that these conditions rather lead to complex 3, we repeated the reduction of 2 with an equimolar amount of magnesium without an alkyne in THF at ambient temperature. This yielded 3 as dark red crystals in 42% after crystallisation. The compound crystallised in the space group I-4 and in the solid-state structure of 3, the titanium atoms are coordinated as a distorted octahedron. The oxygen atoms of the phosphinophenolate ligand are coordinated to titanium above and below the Ti₂Cl₂-plane with titanium-oxygen bond distances of 1.896(2) (Ti1-O1) and 1.905(2) Å (Ti1-O2). The deviation from the octahedral geometry is highlighted by the O1-Ti1-O2 bond angle 157.39(7)° deviates significantly from the 180° for an ideal geometry and furthermore by the angle of the O1-Ti1-O2-plane relative to the Cl1-Ti1-Cl1'-plane with 80.419(46)°. The titanium-phosphorus bond distances are 2.7352(8) (Ti1-P1) and 2.6368(7) Å (Ti1-P2) and the P1-Ti1-P2-plane has an angle of 24.190(8)° relative to the Cl1-Ti1-Cl1'-plane, which would otherwise be coplanar in an ideal octahedral geometry. The observation that the bonds between titanium and the coordinated atoms are longer in 3 than the analogous bonds in 2 fits with the change of the oxidation state of titanium as six-coordiante Ti¹¹ exhibits a larger ionic radius, 0.67 Å, than six-coordiante Ti⁵⁺, 0.42 Å.[²³] The bite angles of the bidentate ligands are 70.26(5) (O1-Ti1-P1) and 73.07(5)° (O2-Ti1-P2) and therefore on average lower than those observed in 2.

The attempted NMR analysis of crystallised 3 showed no resonances related to the complex supporting its paramagnetic nature and the formal oxidation state of +3 of the titanium atoms.

In the investigation of the catalytic activity of 2 towards alkyne oligomerisation, we included the previously reported

tris(trimethylsilyl)silylacetylene among other alkynes (see below).[24-26] Here we want to include an alternative synthesis to the previously reported methods (Scheme 2). Starting from tris(trimethylsilyl)silane, the isotetrasilane was dissolved in chloroform and stirred at ambient temperature under air for 3 d, which yielded chlorotris(trimethylsilyl)silane (4) quantitatively after evaporation of the solvent. This is an adaptation of the previously reported method for the chlorination of tris(trimethylsilyl)silane using carbontetrachloride.[27,28] The presented alternative method allows use of the significantly less harmful chloroform instead of highly toxic carbontetrachloride. The ethynyl group was introduced by treatment of 4 with potassium acetylide in THF at ambient temperature, yielding the ethynyltris(trimethylsilyl)silane 5 in 67% after purification by Kugelrohr distillation. The potassium acetylide was freshly prepared by reaction of trimethylsilylacetylene with potassium tert-butanolate in THF at ambient temperature. This allows to avoid previously reported methods involving acetylene gas handling and use bench-stable regents instead.[24-26] The exchange of the acetylene proton for deuterium was performed by deprotonation of 5 with an equimolar amount of n-butyllithium in THF -60 °C followed by transfer via canula into an acidic mixture of THF, deuterium oxide, and phosphorus trichloride. This gave (2-deuteroethynyl)tris(trimethyl-silyl)silane (6) in 75% yield.

**Spectroscopy.** The room temperature solution EPR spectrum of 3 shows two sets of similar resonance patterns with different g factors (Figure 3). The spectrum of the major species appears at g<sup>maj</sup> = 1.964 as the doublet of doublets, featuring the spin coupling with two non-equivalent <sup>31</sup>P nuclei. This is in a good agreement with the crystal structure of 3 (Figure 2): for the ligand forming the shorter Ti–P(2) bond, the *hfc* from <sup>31</sup>P is *a*<sub>P(2)</sub> = 23.63 G while for the ligand with longer Ti–P(1) contact it is smaller, *a*<sub>P(1)</sub> = 20.36 G. These values are not as different between them as for Ti(III) complexed with two different R<sub>2</sub>P ligands<sup>[29]</sup> and for the related Ti(III) complexes.<sup>[30-32]</sup> Meanwhile the difference in the *hfc* constants from <sup>31</sup>P atoms being quite geometry sensitive,<sup>[33]</sup> it reflects well the structural differentiation of the phosphinoaryloxide ligands in 3.

<figure><img src="image_9.png" /><figcaption>**Figure 2.** Molecular structure of 3. Hydrogen atoms are omitted and selected carbon atoms are depicted as wireframe for clarity. Thermal ellipsoids drawn at 50% probability. Selected distances (Å) and angles (deg): 2: Ti1-Cl1: 2.4298(6), Ti1-O1: 1.896(2), Ti1-O2: 1.905(2), Ti1-P1: 2.7352(8), Ti1-P2: 2.6368(7), O1-Ti1-O2: 157.39(7), O1-Ti1-P1: 70.26(5), O2-Ti1-P2: 73.07(5), P1-Ti1-P2: 96.88(2), Cl1-Ti1-Cl1': 90.76(2).</figcaption></figure>

<figure><img src="image_10.png" /><figcaption>**Scheme 2.** Synthesis of the deuterated tris(trimethylsilyl)silylacetylene derivative 6 from tris(trimethylsilyl)silane.</figcaption></figure>
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<figure><img src="image_12.png" /><figcaption>**Figure 3.** EPR spectra of Ti(III) phosphinoaryloxide complex 3: (a) experimental (0.4 mmol L⁻¹ in THF) and (b) simulated (g⁴⁷ = 1.964, g³² = 1.971, see text). (c) Simulated for major component only; the satellites from the spin coupling with ⁴⁷Ti and ⁴⁹Ti are shown for the low-field peak. Modulation 100 Hz, MW power 6.36 mW, T = 295 K.</figcaption></figure>

Two sets of ⁴⁷/⁴⁹Ti satellites ($a_{Ti}$ = 14.34 G) are seen on both sides of the central strong lines in the spectrum of the major species (Figure 3). Simulation of the spectrum only for major species permits clearly visualizing the ⁴⁷Ti and ⁴⁹Ti satellites pattern (Figure 3). The first set (8 satellites corresponding to 6.04% integration, $a_{Ti}$ = 14.34 G) is assigned to the spin coupling with ⁴⁷Ti nuclei (with S = 5/2 and natural abundance

7.4%) while the second group (6 satellites, 4.54% integration) corresponding to ⁴⁹Ti (S = 7/2, natural abundance 5.4%).[³⁴]

For the minor species with the resonance shifted toward higher g-factor ($g^{min}$ = 1.971), the general pattern of the spectrum is quite similar, yet with slightly different ³¹P *hfcc*: $a_{P(2)}$ = 22.66 G and $a_{P(1)}$ = 20.38 G. This is consistent with the partial ligand exchange in **3** occurring during the sample preparation (once in the EPR cell, the ratio of the resonance intensities of major-to-minor species does not evolve, remaining at ca. 9.76–10.31). Supposedly, one Cl⁻ in **3** is being substituted with a hydroxide; the latter is a stronger ligand which corroborates well the observed g-factor shift and the changes in ³¹P *hfc* constants. From the symmetry reasons and the principle of conservation of the total bond order,[³⁵] hydroxide is expectedly coordinated from the equatorially opposite side to P(2). A similar trend related to the compacting the ligand environment around Ti(III) was discussed in terms of a higher crystal field stabilization.[³⁶]

**Electrochemical Analysis.** Complex **3** is not stable enough in even low-coordinating electrochemical solutions like THF/¹⁹Bu₄NBPh₄, so we considered the electrochemistry of the parent Ti(IV) derivative **2**. The reduction of Ti(IV) to Ti(III) – in its non-complexed forms[³⁷⁻³⁹] or as a core of various complexes, mostly Cp-based,[⁴⁰⁻⁴⁷] – was reported in a number of accounts. Electron transfer rate[³⁷] and the reduction potentials differ remarkably due to the electronic properties of the Ti(IV) environment and the rapid consecutive reaction of the primary product of ET;[⁴⁷] the reduction process itself was described either as reversible[³⁹,⁴⁰,⁴²,⁴⁴,⁴⁵,⁴⁷] or quasi-reversible.[⁴³]

On the voltammograms of **2** in a 0.1 M ¹⁹Bu₄NPF₆ solution in THF/CH₃CN (3:1 v/v) at a glassy carbon (GC) disk electrode, two redox systems could be visualized (Figure 4a). One, revealing a distinct cathodic peak at Eₚ = -0.8 V vs Ag/AgCl is assigned to the reduction of Ti(IV) to Ti(III) complex. Note that the reduction of **2** falls within the Eₚ range reported for Cp-based Ti(IV) complexes.[⁴²,⁴⁵⁻⁴⁷] Peak current iₚ of **2** is linear with the square

<figure><img src="image_13.png" /><figcaption>**Figure 4.** a) (a) Square-wave and (b) cyclic voltammetry of 2 (2 mmol L⁻¹) in THF/CH₃CN (3:1 v/v) / 0.1 M n-Bu₄NPF₆ at a GC disk electrode. Scan rate v = 0.1 Vs⁻¹. (c) Growth of the scan rate-normalized reverse peak current with v: 0.2, 0.5, 1, 2 and 5 Vs⁻¹. (d) For comparison, oxidation of (o-HO-C₆H₄)PPh₂ (2 mmol L⁻¹) under similar conditions. The cross marks show the apparent standard potentials E₀. b) Cyclic and (b) differential voltammograms of the reduction of 2 (2 mmol L⁻¹) in THF/CH₃CN (3:1 v/v) / 0.1 M n-Bu₄NPF₆ at a GC electrode. The numbers are the scan rate v, mVs⁻¹. c) Nyquist plots for 2 (2 mmol L⁻¹ in 0.1 M n-Bu₄NPF₆ in THF/CH₃CN, 3:1 v/v): (a) reduction (E = -0.75 V; diffusion-free resistance R<sub>W0</sub> = 7.4 kohm, charge transfer resistance R<sub>CT</sub> = 13.5 kohm) and (b) oxidation (E = 0.57 V). Modulation ΔE = 10 mV, frequency range f = 0.2 MHz - 1 mHz. GC electrode, T = 293 K.</figcaption></figure>
<figure><img src="image_14.png" /></figure>

root of the scan rate ($i_p v^{1/2}$ = const, $log(i_p)/log(v)$ = 0.46) and with the concentration attesting to its diffusional nature. The electron stoichiometry of the process at this step was shown to be n = 1 (both from the comparison with the one-electron current of Fc under similar conditions and from Cottrell $i_p v^{-1/2}/it^{1/2}$ criteria).[48] Increasing the scan rate causes a small counter-part oxidation signal to appear on the reverse scan, which is more distinctly seen on the square-wave (SWCV) (Figure 4a) or on the differential (Figure 4b) voltammogram. This anodic step corresponds to the reverse process Ti(III)-e⁻ → Ti(IV). Maximal ratio $i_p^a/i_p^c$ = 0.28 was reached at v = 1 V s⁻¹; further increasing the scan rate does not improve it but leads to the depletion of the $i_p^a$ accompanied by an accelerated shift of Eₚ to more negative potentials (Figure 4b). The reduction half peak width of 2 ($ΔE_{p/2} = E_p - E_{p/2}$ = 75 mV) is somewhat larger than that expected for a purely Nernstian couple and the slope of the Eₚ-log(v) plot is ca. 38 mV (with the instrumental IR-compensation during the data acquisition completed by the post-acquisition correction using the slope of the Fc⁺/Fc reversible couple ΔEₚ/Δlog(v) = 0).[49] The observed redox system could thus be ascribed to an electrochemically reversible couple (with formal E₀ = -0.72 V) whose reduced form undergoes a fast follow-up reaction.

With this, as follows from ΔEₚ/₂ and ΔEₚ/Δlog(v) values, electron uptake during the reduction of Ti(IV) complex to Ti(III) is accompanied by some structural reorganization whose energy is non-negligible yet rather small to substantially limit the rate of ET and to render the process irreversible. From the molecular volume of 2 (V<sub>M</sub> = 2081 Å³, Monte-Carlo calculations with 0.001 e bohr⁻³ density), its outer sphere reorganization energy λₐ[⁵⁰] in THF was estimated as 8.65 kJmol⁻¹. On the other hand, substituting the apparent transfer coefficient α (from α = 1.857×RT/FΔEₚ/₂) [⁵¹] into k<sub>s</sub> = (αFvD/RT)⁰.⁵ and solving the equation k<sub>s</sub> = 2×10³×exp{−λ[1 + F(Eₚ−E₀)/λ]²/4RT} [⁵⁰] for λ (total reorganization energy, i. e. λ = λ₀ + λ₁), one can estimate the internal reorganization energy of this process, λᵢ = 11.54–8.65 = 2.89 kJmol⁻¹. Even in first approach, its small value suggests that the geometry of 2 does not change substantially when this complex accommodates one electron to form the related Ti(III) species. The short lifetime of this latter is then determined by its chemical instability.

On the oxidation side, the voltammograms of 2 reveal an anodic peak with apparent E₀ = 0.57 V followed by a less developed second one, at ca 270 mV more positive potentials (Figure 4a). The first peak has n = 1 electron stoichiometry and shows a partial electrochemical reversibility both in CV and SWCV. Since the easiest to ionize element in 2 is the lone pair electrons P(n₂), it is the phosphinoaryloxide ligands which are supposed to account for these oxidation steps. In fact, the oxidation of arylphosphines is reported to be reversible and relatively easy (e.g. for Ph₃P, Eₚ = 830 mV vs Ag/0.1 M Ag⁺ or 1 V vs SCE) [⁵²,⁵³] and the oxidation of a model arylphosphine, (o-HO-C₆H₄)PPh₂ under similar conditions shows practically the same features as 2 (Figure 4a, b). The second ill-defined peak at higher potentials thus corresponds to the electron withdrawal from the partly oxidized positively charged species.

Electrochemical impedance (EIS) measurements for 2 at the apparent standard potentials of the two steps (Figure 4c) have shown a remarkable charge transfer resistance R<sub>CT</sub> for the couple Ti(IV)/Ti(III). Since R<sub>CT</sub> in EIS is the image of the impediment created to ET by the structural reorganization accompanying the process and thus affecting its rate, the ET rate at the equilibrium potential (k<sub>s</sub> = (k<sub>Ti(IV)/Ti(III)</sub> + k<sub>Ti(III)/Ti(IV)</sub>)/2 = 0.5[(R<sub>CT</sub>−R<sub>WO</sub>)D/R<sub>CT</sub>²C<sub>DL</sub>]⁰.⁵) [⁵⁴] was assessed using experimental R<sub>CT</sub> and the diffusion-free zero-Warburg resistance R<sub>WO</sub> (diffusion coefficient D from the Einstein–Stokes equation was taken similar for 2 and the related primary Ti(III) form) as k<sub>s</sub> = 0.09 cm s⁻¹. This value is quite close to that from CV (k<sub>s</sub> = 0.06 cm s⁻¹), their difference is supposedly related with the reproducibility of the measurements. With this, the oxidation of the phosphinoaryloxide ligand at E = 0.57 V only revealed its diffusional character.

Upon repetitive cycling in CV with the vertex potential less that Eₚ<sup>ox</sup>, two small peaks, initially absent, appear on the reverse scan (at Eₚ = −0.08 V and 0.13 V) after the reduction of 2 to its Ti(III) form; these peaks were tentatively ascribed to new species that might have been formed by the ligand exchange in the reduced complex with the soft ligand CH₃CN present as a co-solvent.

Thus, two one-electron redox systems were evidenced for 2: its one-electron reduction to the related Ti(III) form and a one-electron oxidation of the phosphinoaryloxide ligand. The reduced form with Ti(III) is chemically instable under the conditions given and could only be detected by means of voltammetry.

**Catalysis.** We investigated the catalytic application of 2 for the oligomerisation of alkynes, which was previously reported for titanium.[⁷] For this purpose, we treated alkynes (0.2 M) with 10% 2 in the presence of 0.3 eq. magnesium powder as reducing agent in THF for 7 h at 70 °C (Scheme 3). The conversion of 3-hexyne gave hexaethylbenzene as the cyclotrimerisation product in 92% isolated yield relative to the alkyne used. The reaction to 1-hexyne gave a mixture of 1,2,4-tri-*n*-butylbenzene and 1,3,5-tri-*n*-butylbenzene in a ratio of 1:0.72, respectively, with a total isolated yield of 92%. The sterically more demanding *iso*-propylacetylene showed a composition of 1,2,4-tri-*iso*-propylbenzene and 1,3,5-tri-*iso*-propylbenzene in a ratio of 1:1.54 at a total isolated yield of 70%. In case of *tert*-butylacetylene, we isolated a mixture of both cyclotrimerisation and acyclic oligomerisation products in an overall yield of 42%. The cyclotrimerisation products make up for ca. one third, 31% to be precise, of the product mixture with a ratio of 1:1.03 for 1,2,4-tri-*tert*-butylbenzene and 1,3,5-tri-*tert*-butylbenzene, respectively. The major component in the mixture was found to be the acyclic dimerisation product *E*-1,4-di-*tert*-butylbut-1-en-3-yne with 38% and the acyclic trimerisation product *E*,*E*-1,4-di-*tert*-butyl-2-(*tert*-butylethynyl)buta-1,3-diene with 31%. This contrasts the conversion of trimethylsilylacetylene, which yielded only the cyclotrimerisation products 1,2,4-tris(trimethylsilyl)benzene and 1,3,5-tris(trimethylsilyl)benzene in a ratio of 1:1.28 totalling to 72% isolated yield without traces of acyclic oligomerisation products. The lower isolated yields might be attributed to loss of residual
<figure><img src="image_15.png" /></figure>

<figure><img src="image_16.png" /></figure>

<figure><img src="image_17.png" /><figcaption>**Scheme 3.** Cyclotrimerisation, acyclic oligomerisation, and transfer hydrogenation of alkynes catalysed by titanium complex 2 in the presence of magnesium powder at 70 °C in THF.</figcaption></figure>

educt during the workup process. In comparison with the calixarene-based titanium catalyst by Ladipo and co-workers, prepared from (DMSC)TiCl₂ (DMSC = 1,2-alternate Me₂Si-bridged *p*-tert-butylcalix[4]arene) in the presence of magnesium powder and trimethylsilylacetylene, which exhibited very good regioselectivity (> 96%) for the 1,2,4-substituted benzene isomers and isolated yields (> 95%) at ambient temperature for the conversion of 1-pentyne, trimethylsilylacetylene, as well as phenylacetylene, complex 2 is significantly less active and regioselective.[55] However, its reactivity is somewhat comparable to that of an titanium *iso*-propoxide-based system reported by Okamoto and co-workers, prepared from Ti(O'Pr)₄, magnesium powder, and trimethylsilyl chloride at 40 °C in THF.[7] The terminal alkyne 1-hexyne was converted in 71% to the tri-*n*-butylbenzene derivatives in a ratio of 1:0.69 of the 1,2,4-substituted to the 1,3,5-substituted isomer, which is very similar to the corresponding ratio of 1:072 observed for 2 at 92% yield. In contrast to 2, which exhibits no cyclotrimerisation of

phenylacetylene, Okamoto *et al.* reported 65% conversion to a 1:0.52 ratio of the 1,2,4- and 1,3,5-substituted isomers. In case of the inner alkyne 4-octyne no conversion was observed whereas we observed the formation of hexaethylbenzene from 3-hexyne to 92% yield.

The conversion of tris(trimethylsilyl)silylacetylene yielded a somewhat unexpected result as tris(trimethylsilyl)silylethene, the formal hydrogenation product, was isolated as the only product along with remaining educt. The conversion to the vinylsilane did not exceed the 31% in repeated experiments even after prolonged reaction times. We attribute this to the high steric demand of the tris(trimethylsilyl)silyl group. Previous works on the mechanism of titanium-catalysed alkyne oligomerisation involve a titanacyclopropene as a key intermediate for the catalytic cycle, which forms by oxidative addition of an alkyne to a transient Ti(II) complex.[1,55] The proposed next step is the addition of another alkyne molecule to the metallacycle to form a metallacyclopentadiene intermediate. We propose that in the present case the steric demand of the tris(trimethylsilyl)silyl groups limits the approach of the second alkyne to the transient metallacyclopropene preventing a successful addition reaction. Instead, the terminal alkyne acts as a proton donor and cleaves the two titanium-carbon bonds of the metallacyclopropene under protonolysis yielding the tris(trimethylsilyl)silylethene. This would require two equivalents of tris(trimethylsilyl)silylacetylene to act as proton donors for the hydrogenation of a third equivalent of the acetylene. Thus, the theoretical yield of this transfer hydrogenation reaction could not exceed 33%, which would also be in accordance with our observed conversion rates not exceeding this value. The other two equivalents of the alkyne, which acted as the proton source, are firstly bound to titanium after the deprotonation reaction and after reduction of the titanium(IV) bis(acetylide) derivative with magnesium, the acetylides are transferred to magnesium and unavailable for further alkyne oligomerisation reactions promoted by the titanium complex. Considering the possibility of a transient (Ar⁴O)₂Ti[CCSi(SiMe₃)₂] species to undergo reductive elimination forming 1,4-bis[tris(trimethylsilyl)silyl]-1,3-butadiyne, we did not observe this potential byproduct in any of the experiments. In the context of similar oligosilanylacetylene group 4 metal chemistry, Baumgartner and co-workers reported the reaction of 5 with bis(cyclopentadiene)titanium dichloride in the presence of magnesium and observed a mixture of the corresponding alkyne and titanacyclopentadiene complexes based on the NMR data.[56] The presence of a TiCCSi(SiMe₃)₃ moiety was ruled out by comparison with independently prepared sample and spectroscopic analysis. In order to further support the transfer hydrogenation hypothesis, we repeated the experiment with (2-deuteroethynyl)tris(trimethylsilyl)silane 6. This yielded a product mixture in 84% yield, which was comprised of 1,1,2-trideuterovinyltris(trimethylsilyl)silane (19%) and tris(trimethylsilyl)silylacetylene 5 (81%) according to GC-MS analysis. This supports the hypothesis that the educt acts as a proton or deuteron source, respectively, for the formal hydrogenation of the alkyne to the corresponding olefin.
<figure><img src="image_18.png" /></figure>

In case of phenylacetylene and diphenylacetylene no conversion was observed and also the initial attempts to prepare the bis(trimethylsilyl)acetylene adduct of 2, we did not yield any corresponding oligomerisation products. Moreover, we performed control reactions using the same conditions in the absence of 2 with 1-hexyne and trimethylsilylacetylene, respectively. The ¹H NMR spectroscopic analysis did not exhibit any conversion after 18 h reaction time.

We also investigated the use of titanium complex 3 as catalyst for alkyne oligomerisation employing the same conditions as for the reactions with 2. However, we did not observe any catalytic activity of 3 in this regard. We presume that the lack of catalytic activity stems from the sterically crowded environment around the two titanium(III) centres (see space-filling model of the molecular structure of 3 in Figure S1 the SI), limiting the reduction of 3 to an outer Helmholtz plane process, whereas the reduction of 2 is possible via an inner Helmholtz process as the chloride atoms are accessible to the magnesium metal surface.[57,58]

Moreover, we found that the catalytic reactions required the use of magnesium powder instead of magnesium turnings. All attempts to use magnesium turnings under the otherwise same reaction conditions did not show any catalytic conversion. Considering that the titanium(IV) complex 2 can readily be reduced to the titanium(III) complex 3 using magnesium turnings and the observed catalytic inactivity of 3 under these conditions, we presume that the smaller surface area available using magnesium turnings instead of powder reduces the reaction rate of the reduction allowing the comproportionation of the titanium(II) intermediate with another equivalent of 2 to form 3. This observation may be considered as a deactivation pathway resulting in irreversible catalyst loss and therefore a critical limitation in catalyst performance.

## Conclusions

We prepared the dinuclear titanium(III) complex $[Ti^{III}(OAr^{P}-k^{2}O,P_{2}Cl)_{2}]_{3}$ (Ar$^{P}O^{-} = 2,4^{-}Bu_{2}-6-(Ph_{2}P)C_{6}H_{2}O^{-}$) by reduction of the mononuclear titanium(IV) complex $Ti^{IV}(OAr^{P}-k^{2}O,P_{2}Cl_{2}]_{2}$ with magnesium in THF at ambient temperature. The crystal structure of 3 perfectly agrees with the data of EPR spectroscopy that reveals spin coupling with two non-equivalent $^{31}$P nuclei and two sets of satellites due to the coupling with $^{47}$Ti and $^{49}$Ti. The one-electron redox systems were evidenced for 2 in both directions: the one-electron cathodic step corresponds to the reduction of Ti(IV) complex to its Ti(III) form while the one-electron oxidation affects the phosphinoaryloxide ligand. With $k_{s}=(6-9) \times 10^{-2}$ cm s$^{-1}$, the electron transfer in the Ti(IV)/Ti(III) system is at the lower end of electrochemical reversibility. Although the Ti(III) form can be reliably detected by the electrochemical means, it has quite low chemical stability under the actual conditions.

We further explored the catalytic application of 2 towards the oligomerisation of alkynes in the presence of magnesium powder at 70°C in THF. Here, we observed that use of magnesium turnings instead of powder did not yield any

oligomerisation products. The reactions catalysed by 2 yield products ranging from cyclotrimerisation to arenes, via acyclic oligomerisation to transfer hydrogenation. The oligomerisation of 3-hexyne yielded hexaethylbenzene selectively, whereas the use of the terminal alkynes typically gave a mixture of 1,2,4- and 1,3,5-trisubstituted benzene derivatives. In case of the sterically more demanding tert-butylacetylene, we also found acyclic di- and trimerisation products and for the even bulkier tris(trimethylsilyl)silylacetylene (5) only the tris(trimethylsilyl)silylethene, the formal hydrogenation product, was observed. The results from the conversion of the analogous deuterated alkyne supports the hypothesis that this reaction proceeds as a transfer hydrogenation, in which the alkyne also acts as a hydrogen or deuterium source, respectively. In contrast to 2, the titanium(III) complex 3 did not exhibit any catalytic activity in alkyne oligomerisation reactions under the same conditions. For the synthesis of the tris(trimethylsilyl)silyl acetylene and its deuterated analogue, we presented a less toxic alternative synthesis of chlorotris(trimethylsilyl)silane (4) as well as a novel, facile methodology for the preparation of an acetylide reagent, which we used to synthesise the silanylacetylene 5.

## Experimental Section

**General Details.** All manipulations were carried out under an atmosphere of dry, oxygen-free nitrogen using standard Schlenk and glove box techniques. Benzene-*d*6 was distilled from potassium. THF was purified by distillation from calcium hydride under nitrogen. All other solvents were purified by passing through columns of activated alumina.[59] 2,4-¹Bu₂-6-(PPh₃)PhOH,[60] TiCl₄(THF)₂,[61] and tris(trimethylsilyl)silane[62] were prepared according to a published procedures. Other chemicals were obtained from different suppliers and used without further purification. NMR spectra were recorded on a Varian INOVA 500, a Bruker AVANCE III 300 or an Rs2d 400 spectrometer and are referenced to $Me_{4}Si$ (¹H, ¹³C), and 85% $H_{3}PO_{4}$ (³¹P). For X-ray structure analyses the crystals were mounted onto the tips of glass fibres. Data collection was performed with a Bruker-AXS SMART APEX CCD diffractometer using graphite-monochromated Mo-Kα radiation (0.71073 Å) or a Bruker APEX II diffractometer using an Incoatec microfocus sealed tube of Mo-Kα radiation (0.71073 Å) with a CCD area detector. The data were reduced to $F_{o}^{2}$ and corrected for absorption effects with SAINT[63] and SADABS[64,65] respectively. The structures were solved by direct methods and refined by full-matrix least-squares method (SHELXL97 or SHELXL19).[66] If not noted otherwise all non-hydrogen atoms were refined with anisotropic displacement parameters. All hydrogen atoms were located in calculated positions to correspond to standard bond lengths and angles. Crystallographic data for the structures reported in this paper have been deposited with the Cambridge Crystallographic Data Centre as supplementary publication no. CCDC 2298747 (2) and 2298750 (3). Voltammetry (CV and SWCV) and electrochemical impedance spectroscopy (EIS) were performed using a PAR 2273 potentiostat piloted by PowerSuite (PAR, release 2.58) software. A standard 25 mL three electrode electrochemical cell was used with a Pt (0.5 mm) disk working electrode. Reference electrode was a silver wire with electro-deposited AgCl. The solution $CH_{3}CN/0.1 M Bu_{4}NPF_{6}$ was used. The experimental potentials were corrected using the $E_{0}(Fc^{+}/Fc) = 0.431$ V vs Ag/AgCl.[67] All experiments were carried out in the glovebox. EPR spectra were registered using an X-band Bruker EMX
<figure><img src="image_19.png" /></figure>

300 spectrometer (9.46 GHz) coupled with a standard rectangular cavity; microwave power 2–8 mW, modulation frequency 100 kHz. Simulation of the EPR spectra was done using WinSim-2002.[68] UV-Vis spectra were recorded on an Agilent Cary 60 UV-Vis spectrophotometer. Elementary analysis was carried out using a Heraeus VARIO ELEMENTAR. GC/MS analyses were performed using an Agilent Technologies 7890 A GC system equipped with an Agilent Technologies J&W HP 5MS capillary column and an Agilent Technologies 5975 C mass selective detector. The products of the catalysis experiments were analysed with GC/MS and NMR spectroscopy and the data of the latter compared with previously published values of the analytes.[69–79]

## Syntheses

**Sodium 2,4-di-tert-butyl-6-(diphenylphosphino)phenolate (NaOAr⁪)** 1: A pressure-equilibrated Schlenk flask equipped with stirrer bar was charged with sodium hydride (548 mg, 22.8 mmol) and toluene (40 ml). A solution of 2,4-di-tert-butyl-6-(diphenylphosphino)phenol (8.48 g, 21.7 mmol) in toluene (20 ml) was then slowly added under stirring at ambient temperature and gas evolution was observed. The grey suspension was then heated to 50 °C for 3 h and then stirred for another 18 h at ambient temperature. Then the solution was filtered using a filter cannula followed by the evaporation of all volatiles under reduced pressure yielding 1 (8.7 g, 93%) as a white powder. ¹H NMR (δ in ppm, benzene-d6, 298 K): 1.30 (s, 9H, tBu), 1.61 (s, 9H, tBu), 6.77 (s, 1H, Ar–H), 6.98–7.11 (m, 6H, Ph), 7.36–7.49 (m, 3H, Ph), 7.60 (s, 1H, Ar–H). ¹³C{¹H} NMR (δ in ppm, benzene-d6, 298 K): 30.5, 32.2, 34.4, 35.3, 125.4, 128.6, 128,7, 129.3, 134.3 (d, J<sub>P,C</sub>= 17.8 Hz), 135.9, 138.7. ³¹P{¹H} NMR (δ in ppm, benzene-d6, 298 K): -16.0.

**Titanium(IV) bis(2,4-di-tert-butyl-6-(diphenylphosphino)-phenolate) dichloride (Ti⁴⁺(OAr⁪⁻κ²O,P)₂Cl₂)** 2: In the glove box, sodium phenolate 1 (2.89 g, 7.0 mmol) was dissolved in 10 mL THF and TiCl₄(THF)₂ (1.16 g, 3.5 mmol) was suspended in 10 mL THF. The solution of 1 was slowly added to the suspension whereby an immediate colour change to deep red could be observed. After complete addition the reaction was stirred for another 18 hours at room temperature. The solvent was evaporated and the crude product recrystallized from hot benzene to yield 2 as deep red crystals (1.22 g, 38.9%). Analytical data in accordance with published values.[16] Additional analytical data: mp 216 °C. ¹H NMR (δ in ppm, chloroform-d, 298 K): 1.01 (s, 9H, tBu), 1.07 (s, 9H, tBu), 1.26 (s, 9H, tBu), 1.48 (s, 9H, tBu), 6.48 (d, 1H, J<sub>H,H</sub>=6.6 Hz, Ar–H), 6.68 (t, 2H, J<sub>H,H</sub>=8.8 Hz, Ar–H), 6.73 (t, 2H, J<sub>H,H</sub>=8.8 Hz, Ar–H), 6.88 (q, 4H, J<sub>H,H</sub>=7.0 Hz, Ar–H), 7.03 (t, 2H, J<sub>H,H</sub>=7.2 Hz, Ar–H), 7.09 (t, 1H, J<sub>H,H</sub>=7.7 Hz, Ar–H), 7.14 (t, 1H, J<sub>H,H</sub>=7.3 Hz, Ar–H), 7.18-7.23 (m, 1H, Ar–H), 7.23-7.28 (m, 3H, Ar–H), 7.30 (s, 1H, Ar–H), 7.33–7.42 (m, 4H, Ar–H), 7.72–7.81 (m, 2H, Ar–H). ¹³C{¹H} NMR (δ in ppm, chloroform-d, 298 K): 168.6, 168.3, 167.1 (J<sub>P,C</sub>=7.5 Hz), 166.9 (J<sub>P,C</sub>=7.5 Hz), 146.2 (J<sub>P,C</sub>=4.4 Hz), 145.8 (J<sub>P,C</sub>=4.4 Hz), 136.0 (J<sub>P,C</sub>=5.4 Hz), 135.4 (J<sub>P,C</sub>=5.4 Hz), 133.9 (J<sub>P,C</sub>=11.1 Hz), 133.4, 133.3, 133.1, 133.0, 132.7 (J<sub>P,C</sub>=10.4 Hz), 131.6, 131.4, 130.8, 130.5, 130.3, 129.9, 129.6, 129.4, 129.1, 128.6, 128.5, 128.5, 128.4, 128.2, 128.0, 127.3, 122.8, 122.4, 35.4, 35.3, 35.0, 34.8, 31.7, 31.5, 29.8, 29.5. ³¹P{¹H} NMR (δ in ppm, chloroform-d, 298 K): 9.0, 13.7. UV-vis (λ nm, εLmol⁻¹cm⁻¹ in brackets): 435 (8.9·10³, sh), 381 (1.3·10⁴), 300 (2.2·10⁴). IR (ATR, cm⁻¹): 377, 398, 439, 482, 502, 574, 635, 690, 740, 755, 779, 843, 864, 887, 915, 998, 1027, 1066, 1105, 1124, 1140, 1202, 1230, 1261, 1287, 1360, 1420, 1434, 1461, 1479, 1587, 1805, 1881, 2113, 2865, 2905, 2955, 3058.

**Titanium(III) bis(2,4-di-tert-butyl-6-(diphenylphosphino)-phenolate) chloride dimer ([Ti³⁺(OAr⁪⁻κ²O,P)₂Cl₂)** 3: Titanium complex 2 (300 mg, 0.33 mmol) was dissolved in THF and magnesium turnings (9 mg, 0.37 mmol) were added to the solution. The mixture was

stirred at room temperature for 16 hours whereby the solution turned brown. The reaction mixture was filtered and the solvent evaporated. The remaining dark brown solid was taken up in pentane, filtered, and left standing overnight, whereby dark orange crystals of 3 (119 mg, 41.8%) formed. mp 199 °C (dec.). UV-vis (λ nm, εLmol⁻¹cm⁻¹ in brackets): 503 (9.9·10², sh), 419 (3.8·10³, sh), 336 (1.1·10⁴, sh), 300 (2.2·10⁴). IR (ATR, cm⁻¹): 392, 409, 439, 458, 476, 504, 560, 633, 692, 745, 771, 838, 860, 876, 913, 999, 1026, 1094, 1107, 1139, 1185, 1199, 1246, 1283, 1359, 1387, 1424, 1433, 1460, 1477, 1586, 2863, 2902, 2952, 3058. Analysis calculated for C₁₀₄H₁₂₀Cl₂O₄P₄Ti₂: C 72.43. H 7.01. Found: C 72.59. H 7.63.

**Chlorotris(trimethylsilyl)silane 4:** A round-bottom flask equipped with a stirrer bar was charged with tris(trimethylsilyl)silane (1.25 g, 5.00 mmol) and chloroform (25 mL) and the mixture stirred at ambient temperature under air for 3d. Then all volatiles were evaporated under reduced pressure yielding chlorotris(trimethylsilyl)silane as a colourless oil (1.40 g, quant.). The compound was used without further purification. The analyses were in accordance with previously reported data.[27]

**Ethynyltris(trimethylsilyl)silane 5:** A Schlenk flask was equipped with a stirrer bar, charged with potassium tert-butanolate (1.12 g, 10.0 mmol) and THF (50 mL), and placed in a water bath to maintain ambient temperature. Trimethylsilylacetylene (983 mg, 10.0 mmol) was added dropwise to the mixture resulting in formation of an off-white precipitate. The suspension was stirred for 1 h and then added dropwise to a stirred solution of chlorotris(trimethylsilyl)silane (2.83 g, 10.0 mmol) in THF (20 mL) in another Schlenk flask placed in an ambient temperature water bath. The resulting yellowish suspension was stirred for 1 h, after which the reaction mixture was poured onto a mixture of ammonium chloride solution (2 M) and diethyl ether. Then the organic layer was separated and the aqueous phase extracted thrice with diethyl ether. The combined organic phases were dried over Na₂SO₄ and the solvent evaporated under reduced pressure. The crude product was purified by Kugelrohr distillation (50 °C, 0.2 mbar) yielding ethynyltris(trimethylsilyl)silane as a colourless semisolid (1.82 g, 67%). The analyses were in accordance with previously reported data.[25]

**(2-Deuteroethynyl)tris(trimethylsilyl)silane 6:** A Schlenk flask was equipped with a stirrer bar, charged with ethynyltris(trimethylsilyl)silane (682 mg, 2.5 mmol) and THF (20 mL), and cooled to –60 °C using a EtOH/N₂ lq. cooling bath. At this temperature, n-butyllithium solution (1.0 ml, 2.5 M, 2.5 mmol) was added dropwise. After 30 min the reaction mixture was transferred to a mixture of THF (20 mL), deuterium oxide (5 mL), and phosphorus trichloride (0.5 mL) using a canula. After the transfer, diethyl ether was added and the organic layer separated from the resulting biphasic mixture. The aqueous phase was extracted thrice with diethyl ether and the combined organic phases dried over Na₂SO₄. The solvent was evaporated under reduced pressure yielding a colourless oil, which was taken up in a minimal amount of pentane and filtered over a silica pad. Evaporation of the solvent yielded (2-deuteroethynyl)tris(trimethylsilyl)silane as a colourless oil (515 mg, 75%). ¹H NMR (δ in ppm, chloroform-d, 298 K): 0.22 (s, 27H, SiMe₃). ¹³C NMR (δ in ppm, chloroform-d, 298 K): 0.4, 83.6 (d, ²J<sub>D,C</sub>=6 Hz), 95.8 (d, ²J<sub>D,C</sub>=36 Hz). ²⁹Si NMR (δ in ppm, chloroform-d, 298 K): –100.5 (Si/SiMe₃), –11.3 (SiMe₃).

**Typical procedure used for catalysis experiments:** In a glove box, a 20 mL vial was charged with 2 (0.10 mmol, 90 mg), magnesium powder (0.3 mmol, 8 mg), the appropriate alkyne (1 mmol) and 5 mL THF. The suspension was heated to 70 °C with stirring for 7 h. Afterwards the vial was taken out of the glove box, quenched with deionised H₂O and extracted three times with diethyl ether (5 mL each). The combined organic phases were dried over sodium
<figure><img src="image_20.png" /></figure>

sulfate and the solvent evaporated under reduced pressure. The oily residue was taken up in pentane and filtered through a short plug silica. Then, the solvent was evaporated and yield as well as product composition determined via NMR and GC-MS spectroscopy without separation of the compounds of in case of product mixtures.

**Catalytic conversion of 3-hexyne:** The reaction was performed according to the typical procedure using 3-hexyne (80 mg, 1.00 mmol) and yielding hexaethylbenzene (73 mg, 91%) as a colourless solid.

**Catalytic conversion of 1-hexyne:** The reaction was performed according to the typical procedure using 1-hexyne (80 mg, 1.00 mmol) and yielding a colourless solid (74 mg, 92%) comprising of 1,2,4-tri-*n*-butylbenzene (58%) and 1,3,5-tri-*n*-butylbenzene (42%).

**Catalytic conversion of iso-propylacetylene:** The reaction was performed according to the typical procedure using *iso-propylacetylene* (69 mg, 1.00 mmol) and yielding a colourless solid (48 mg, 70%) comprising of 1,2,4-tri-*iso*-propylbenzene (39%) and 1,3,5-tri-*iso*-propylbenzene (61%).

**Catalytic conversion of tert-butylacetylene:** The reaction was performed according to the typical procedure using *tert*-butylacetylene (83 mg, 1.00 mmol) and yielding a colourless solid (35 mg, 42%) comprising of 1,2,4-tri-*tert*-butylbenzene (15%), 1,3,5-tri-*tert*-butylbenzene (16%), *E*-1,4-*di*-tert-butylbut-1-en-3-yne (38%), and *E*,*E*-1,4-*di*-tert-butyl-2-(*tert*-butylethynyl)buta-1,3-diene (31%).

**Catalytic conversion of trimethylsilylacetylene:** The reaction was performed according to the typical procedure using trimethylsilylacetylene (99 mg, 1.00 mmol) and yielding a colourless solid (71 mg, 72%) comprising of 1,2,4-tris(trimethylsilyl)benzene (44%) and 1,3,5-tris(trimethylsilyl)benzene (56%).

**Catalytic conversion of phenylacetylene:** The reaction was performed according to the typical procedure using diphenylacetylene (103 mg, 1.00 mmol) and reaction monitoring after 7 h exhibited no conversion.

**Catalytic conversion of diphenylacetylene:** The reaction was performed according to the typical procedure using diphenylacetylene (179 mg, 1.00 mmol) and from which diphenylacetylene (168 mg, 94%) was isolated as a colourless solid yet again.

**Catalytic conversion of ethynyltris(trimethylsilyl)silane 5:** The reaction was performed using 5 (137 mg, 500 µmol), 2 (45 mg, 50 µmol), magnesium powder (4 mg, 50 µmol), and THF (5 mL) and after 18 h reaction 119 mg (86%) of a mixture of tris(trimethylsilyl)vinylsilane (31%) and ethynyltris(trimethylsilyl)-silane (69%).

**Catalytic conversion of (2-deuteroethynyl)tris(trimethylsilyl)-silane 6:** The reaction was performed using 6 (137 mg, 500 µmol), 2 (45 mg, 50 µmol), magnesium powder (4 mg, 50 µmol), and THF (2.5 mL) and after 18 h reaction 115 mg (84%) of a mixture of (1,1,2-trideuterovinyl)tris(trimethylsilyl)silane (19%) and ethynyltris(trimethylsilyl)silane (80%).

**Control reaction without 2 for 1-hexyne:** The reaction was performed according to the typical procedure using 1-hexyne (80 mg, 1.00 mmol) without the addition of 2 and reaction monitoring after 18 h exhibited no conversion of 1-hexyne.

**Control reaction without 2 for trimethylsilylacetylene:** The reaction was performed according to the typical procedure using trimethylsilylacetylene (99 mg, 1.00 mmol) without the addition of 2 and reaction monitoring after 18 h exhibited no conversion of trimethylsilylacetylene.

## Supporting Information

The crystallographic data, UV-Vis, IR spectra, and NMR spectra are included in the supporting information. Deposition Numbers 2298747 (for 2) and 2298750 (for 3) contain the supplementary crystallographic data for this paper. These data are provided free of charge by the joint Cambridge Crystallographic Data Centre and Fachinformationszentrum Karlsruhe Access Structures service

## Acknowledgements

We thank Christoph Marschner and Judith Baumgartner for support and helpful discussions. This research was funded in whole, or in part, by the Austrian Science Fund (FWF) [grant DOIs: 10.55776/P32005, 10.55776/P35963]. For the purpose of open access, the author has applied a CC BY public copyright licence to any Author Accepted Manuscript version arising from this submission.

## Conflict of Interests

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:** titanium • catalysis • electrochemistry • alkyne • oligomerisation

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Manuscript received: December 27, 2023

Revised manuscript received: April 9, 2024

Accepted manuscript online: April 23, 2024

Version of record online: June 12, 2024