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Serendipity in unexpected reactions of group 4 metallocene Bis(trimethylsilyl)acetylene complexes and its consequences for selected applications
Journal Pre-proof Serendipity in unexpected reactions of group 4 metallocene Bis(trimethylsilyl)acetylene complexes and its consequences for selected applications U. Rosenthal PII:
S0022-328X(19)30514-5
DOI:
https://doi.org/10.1016/j.jorganchem.2019.121071
Reference:
JOM 121071
To appear in:
Journal of Organometallic Chemistry
Received Date: 14 November 2019 Revised Date:
6 December 2019
Accepted Date: 7 December 2019
Please cite this article as: U. Rosenthal, Serendipity in unexpected reactions of group 4 metallocene Bis(trimethylsilyl)acetylene complexes and its consequences for selected applications, Journal of Organometallic Chemistry (2020), doi: https://doi.org/10.1016/j.jorganchem.2019.121071. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
Serendipity in Unexpected Reactions of Group 4 Metallocene Bis(trimethylsilyl)acetylene Complexes and its Consequences for Selected Applications Prof. U. Rosenthal* Leibniz Institute for Catalysis at the University of Rostock, Albert-Einstein-Str. 29A, 18059 Rostock (Germany), E-mail: [email protected]
ORCID https://orcid.org/0000-0003-1922-6782
Dedicated to Professor Vladimir Borisovich Shur (INEOS, RAS, Moscow)
Abstract: Examples for serendipity in several unexpected reactions of group 4 metallocene bis(trimethylsilyl)acetylene complexes are summarized in this Review. The first is the reaction of Cp2Ti(η2-btmsa) with tolane, PhC≡CPh, investigated 1988 in the Nesmeyanov Institute in Moscow giving surprisingly no coupling of Me3SiC≡CSiMe3 with PhC≡CPh to a titanacyclopentadiene but the substitution of bis(trimethylsilyl)acetylene by tolane and via the assumed alkyne complex Cp2Ti(η2-PhC2Ph) a C-C coupling reaction of two molecules of tolane to the tetraphenyl substituted titanacyclopentadiene (i). This simple substitution reaction of Cp2Ti(η2-btmsa) and of similar complexes was used for the generation of the very reactive coordinatively and electronically unsaturated complex fragments [Cp’2M]. The reaction of this complex with carbon dioxide gave a coupling with Me3SiC≡CSiMe3 to an unexpected dinuclear complex with relevance to the general mechanism of CO2 coupling reactions (ii). Furthermore it was demonstrated, that the Cp ligands in [Cp2Ti] were not inert, showing a rare type of coupling reaction with alkynes or ring opening reactions together with C-C coupling reactions of diynes (iii). Some other unexpected reactions were found with dior polyynes. Depending on the ligands L (Cp’ = Cp, Cp* etc.), metals M (M = Ti, Zr, Hf) and the substrate substituents R (R = Me3Si, aryl, alkyl etc.) 1,3-butadiynes RC≡C-C≡CR react with [Cp’2M] under C-C bond cleavage of the internal C-C single bond (iv) or through complexation to unusual metallacyclocumulenes (v). The reverse reaction of C-C coupling of alkynyl groups to give coordinated 1,3-butadiynes in metallacyclocumulenes was described for some bis(acetylides) Cp’2Zr(-C≡CR)2 (vi). With polyynes R-(C≡C)n-R a sliding of the [Cp’2M] unit at the triple bonds was observed (vii). All these unexpected examples (i) - (vii) represent reactions that were found by coincidence. Later several applications for the initial results were described. For this overview only one selected examples for these developments are described to illustrate the consequences of each of these results.
1
1.
Introduction
The field of organometallic chemistry and homogeneous catalysis is a classical area of research in which by serendipity a lot of unexpected results were found, leading to a revolution of the existing basic knowledge as well as sometimes of catalytic technical processes. [1] Names like P. Pauson, E. O. Fischer, G. Wilkinson, K. Ziegler, G. Natta, G. Wilke, H. W. Sinn, W. Kaminsky, H. H. Brintzinger, W. Keim and many others are connected with such important discoveries. [1] The driving force for these investigations was in all cases a strong connection with fundamental research. Kealy and Pauson attempted to access the organic “fulvalene” molecule H4C5=C5H4 by coupling and subsequent dehydrogenation of two cyclopentadienyl radicals and accidently oserved formation of “dicyclopentadienyliron” instead of the expected compound. [1d] On the basis of this result Fischer [1e], Wilkinson [1f] and coworkers as well as others claimed and extended the concept of “sandwich complexes” by synthesis of a broad range of similar metallocene compounds with sometimes surprising molecular structures. Ziegler investigated the so called “Wachstumsreaktion” of ethylene to higher waxes by organoaluminium compounds and found by accident, that impurities of nickel salts in the reaction mixture, coming from cleaning processes of the autoclave, prevented the expected reaction and surprisingly gave only ethylene dimerisation. [1g] Upon changing the catalyst system to the combination of TiCl4/Et3Al a highly active catalyst system for the polymerization of ethylene was obtained. [1g] Wilke modified these systems to obtain very important nickel-catalysed cyclooligomerisations of diolefins. [1h] Later Breslow [1i] and Natta [1j] used a catalytic system consisting of Cp2TiCl2/Et2AlCl for the stereoregular polymerization of propylene. Sinn und Kaminsky were studying the catalytic system Cp2TiMe2/Me3Al and found again by accident, that impurities of water were necessary to obtain higher activity of the catalysts. [1k] Later this effect was explained by the formation of methylalumoxane (MAO) from Me3Al and water in the right molecular ratio. Brintzinger, who had investigated the fixation of molecular nitrogen and had some problems with the elimination of Cp ligands during these reductive reactions used titanocene derivatives with an interannular ethylene bridge between the Cp ligands. [1l] In many transformations these compounds did not follow the reaction paths, previously observed for their unbridged metallocene counterparts in several reactions. [1m] He used complexes with ethylene bridged ansa-Cp ligands like ethylenebis(indenyl) and ethylenebis(tetrahydroindenyl), giving as an additional effect a constrained conformation of the catalyst and after activation with MAO highly isotactic polymers from propylene. [1n] The SHOP (Shell Higher Olefin Process) was realized by a nonselective ethylene oligomerization process in a two-phase system with a nickel catalyst in polar solvents such as butane-1,4-diol with the produced oligomers yielding a nonpolar phase. Interesting to know, that “Again it was ‘serendipity in research’ which helped the breakthrough. Up to this point I had used toluene as the solvent in all catalysis experiments. One day my laboratory assistant A. Nabong swopped the solvent bottles and used acetonitrile instead of toluene. When the autoclave was opened there were two phases: a white phase consisting of pure α-olefins and a red phase containing the catalyst in acetonitrile:” how Keim described the story for finding of this result. [1o] The change of the solvent by accident is remarkable, because normally polar solvents like butane-1,4-diol were excluded before in such organometallic oligomerization reactions. By the use of this unusual solvent as a consequence the unexpected realisation of a two phase system became possible 2
and all problems with the formation of small amounts of polyethylene in heterogenisation processes of the catalyst disappeared. Going from this selection of famous examples for serendipity in organometallic research to more special examples of group 4 metallocene bis(trimethylsilyl)acetylene complexes described in this minireview a similar approach is considered to come to the more or less important unexpected results for this group of compounds. In the analysis it becomes clear that we have to consider for both cases the same heuristic principle of research. Starting from investigations of the influence of ligands L (L = R3P, py etc.) and the substrate substituents R (R = Me3Si, aryl, alkyl etc.) in electron rich Ni(0) alkyne complexes L2Ni(RC2-R), [2] it was interesting to compare these influences with electron poor group 4 metal alkyne complexes Cp’2M(R-C2-R) depending on the ligands L (Cp’ = Cp, Cp* etc.), metals M (M = Ti, Zr, Hf) and the substrate substituents R (R = Me3Si, aryl, alkyl etc.), to show the difference of both groups of compounds. [3] These investigations started in 1988 at the Nesmeyanov Institute of Organoelement Compounds of the Russian Academy of Science under M. E. Vol’pin in the group of V. B. Shur together with V. V. Burlakov [4] and were conducted up to now for more than 30 years. [5] Some examples for unexpected results were selected from these results and are presented here together with the consequences for some synthetic applications. 2. 2.1.
Results Substitution of bis(trimethylsilyl)acetylene
2.1.1. Starting Point When investigating the reaction of Cp2Ti(η2-btmsa) with tolane, PhC≡CPh, surprisingly no coupling of Me3SiC≡CSiMe3 with PhC≡CPh to a nonsymmetrical substituted titanacyclopentadiene by an addition reaction that is typical for alkyne complexes was observed (Scheme 1). Instead, dissociation of bis(trimethylsilyl)acetylene and formation of the transient [Cp2Ti], followed by the coordination of tolane to give the assumed alkyne complex Cp2Ti(η2-PhC2Ph) and subsequent unexpected C-C coupling of two molecules of tolane to a symmetrical tetraphenyl substituted titanacyclopentadiene was observed. [4]
Scheme 1. Unexpected reaction of Cp2Ti(η2-Me3SiC2SiMe3) with tolane, PhC≡CPh. 2.1.2. Applications of the substitution reaction 3
On the basis of this simple substitution reaction, Cp2Ti(η2-btmsa) and similar complexes Cp’2M(η2-btmsa) with other Cp’ ligands were applied as excellent sources for the generation of the coordinatively and electronically unsaturated and thus highly reactive complex fragments [Cp’2M] for titanium and zirconium (Scheme 2). By using different Cp’ ligands and metals several stoichiometric and catalytic reactions were influenced in a very effective way, summarized in several papers and some reviews. [5]. Besides, some very rare examples exist in which the substitution of the bis(trimethylsilyl)acetylene ligand did not occur and a coupling of the alkyne ligand with other substrates was found. [5]
Scheme 2. Examples of complexes Cp’2M(L)(η2-btmsa) of titanium and zirconium with different Cp’ ligands. An outstanding example for the synthetic applications of group 4 bis(trimethylsilyl)acetylene complexes as reagents are the complexes Cp2Ti(η2-btmsa) and Cp2Zr(py)(η2-btmsa). These complexes are advantageous compared to other [Cp2M] generating complexes which was summarized in a recent review. [5w] From the list of examples [5] some recent results from Tilley et al. [5x] and very recent results from the Staubitz group [5y] are particularly impressive. Several multifold coupling reactions of tris(diyne) and pentakis(diyne) substrates were realized by Tilley and coworkers by using Cp2Zr(py)(η2-btmsa) (Scheme 3). The intermediate zirconacyclopentadiene produced with an excess of benzoic acid further products of protodemetallation in high yields (Scheme 3). [5x]
Scheme 3. Multifold couplings of tris- and pentakis(diynes) with Cp2Zr(py)(η2-btmsa) to PAH’s (PAH = polycyclic aromatic hydrocarbons) and its protodemetallation. These and the other reported examples represent a highly efficient method to produce new PAH’s via fused zirconacyclopentadienes and subsequent protodemetalation. The authors described very selective reactions and high yields with Cp2Zr(py)(η2-btmsa) and published its suitability for the preparation of larger PAH’s and graphene-type nanostructures. Very recently, Staubitz and coworkers presented and compared intermolecular coupling reactions of several disubstituted alkynes and octadiynes R-C≡C-(CH2)4-C≡C-R (R = SnMe3, 4
Bpin, 4-thiophenyl, 2-methoxy-, 2-bromo- and 2-iodo-4-thiophenyl etc.) to zirconacyclopentadienes by using Negishi’s reagent and Cp2Zr(py)(η2-btmsa) (Scheme 4) [5y]
Scheme 4. Reactions of bis-substituted octadiynes to zirconacyclopentadienes. The efficiency of both reagents was evaluated and compared by kinetic studies using 1H NMR measurements. The complex Cp2Zr(py)(Me3SiC≡CSiMe3) was more efficient for the synthesis of zirconacyclopentadienes, giving higher yields in shorter reaction times when compared to Negishi´s reagent (Scheme 5). Additionally, even aryl-iodides were not attacked in the reaction by this special very functional group tolerant reagent. [5y] Cp2Zr(py)(Me 3SiC2SiMe3): Rosenthal' s Reagent Cp2ZrCl2 / 2 eq. n-BuLi:
Cp Cp Zr
Negishi's Reagent
Cp Cp Zr
Br
91% 88% Me3Si
Me3Sn
Cp Cp Zr
3
MeO
S
96% 88% Me Me Me Me O Cp Cp O Me Me B Zr B O Me Me O
S
OMe Br
S
93% 0%
S
Br
91% 28%
Cp Cp Zr S
3
Cp Cp Zr S
98% 15%
I
Cp Cp Zr SnMe
99% 53%
99% 99%
Cp Cp Zr SiMe
97% 56%
Br
I
Cp Cp Zr S
S
99% 85%
Scheme 5. Comparison of Negishi’s reagent and Cp2Zr(py)(Me3SiC≡CSiMe3) in the formation of zirconacyclopentadienes from substituted diynes. Similar results were published very recently by Sindlinger and Heitkemper in the comparison of the in situ generated Negishi reagent Cp2Zr(η2-butene) (51% yield) with Cp2Zr(py)(Me3SiC≡CSiMe3) (97% yield) in the formation of a zirconacyclopentadiene by coupling of 3,5-(tBu)2-C6H3C≡CC6H3-3,5-(tBu)2 (Scheme 6). [5z]
5
Scheme 6. Coupling of 3,5-(tBu)2-C6H3C≡CC6H3-3,5-(tBu)2 to a zirconacyclopentadiene. Additionally, many other examples for the advantages of group 4 bis(trimethylsilyl)acetylene complexes Cp’2M(L)(Me3SiC≡CSiMe3) as reagents were described. [5u-z] 2.2. Reaction with carbon dioxide 2.3. 2.2.1. Initial experiment More driven by a mixture of experimental creativity and boredom than in a well-conducted and planned experiment, the author added solid “dry ice” to a yellow solution of Cp2Ti(η2btmsa) in THF, giving a color change to deep green. By repeating the experiment under rigorously anaerobic conditions with gaseous carbon dioxide, green crystals of a dinuclear titanium (III) complex were isolated, consisting of a titanafuranone unit which interacts with a free titanocene (Scheme 7). [6a] This complex indicated an alkyne coupling with carbon dioxide as well as a coordination of [Cp2Ti], formed by alkyne dissociation from the starting complex. Only after reaction with oxygen from air, the mononuclear titanafuranone was formed.
Scheme 7. Reactions of Cp’2Ti(η2-Me2SiC2SiMe3) complexes with Cp’ = Cp and Cp* and carbon dioxide. 2.2.2. Mechanistic and synthetic use
6
This result is relevant for the general accepted mechanism of reactions of group 4 metallocene alkyne coupling with carbon dioxide, in which the direct coupling to the metallafuranone was described. These new investigations showed for [Cp2Ti] an intermediate, giving the product only after exposure to air (Scheme 7). The results for Cp*2Ti(η2-btmsa) gave no coupling of the alkyne but after its elimination the formation of mixed-valent titanocene carbonate and carbonyl complexes, indicating the influence of different Cp’ ligands on the reaction (Scheme 7). [6b] In further experiments, the influence of other Cp’ ligands, group 4 metals and alkyne substituents were investigated, which influence the reactions very effectively (Scheme 8). [7] For unsymmetrically substituted alkynes, the regioselectivity of carbon dioxide insertion was changed by using different Cp’ ligands. [7] The complex meso‐(ebthi)Ti(η2‐PhC2SiMe3) inserted carbon dioxide into the Ti–CPh bond of the titanacyclopropene structure of the alkyne complex with untypical regioselectivity to yield the α‐silyl‐substituted meso‐ (ebthi)titanafuranone. The analogous reactions of the complexes [(thi)2Ti(η2‐PhC2SiMe3)] (thi = η5‐tetrahydroindenyl), [rac‐(ebthi)Ti(η2‐PhC2SiMe3)] (ebthi = 1,2‐ethylene‐1,1′‐bis(η5‐ tetrahydroindenyl) and Cp*2Ti(η2‐PhC2SiMe3) with carbon dioxide gave the typical regioselectivity with the insertion into the M–CSi bond of the titanacyclopropene, yielding the β‐silyl‐substituted titanafuranones . The insertion of carbon dioxide into the M–C bond of the titanacyclopropenes is thus governed by the substitution patter of the alkyne and the steric enviroment around the metal center. [7g]
Scheme 8. Different regioselectivities of carbon dioxide insertion for meso‐(ebthi)Ti(η2‐ PhC2SiMe3) and rac‐(ebthi)Ti(η2‐PhC2SiMe3). These results explain that there is no general mechanism for the insertion of carbon dioxide and the above depicted dinuclear presentation is only the case for titanium and Cp ligands, whereas the other Cp’ ligands show different behavior which is additionally influenced by the alkyne substituents. 2.4. Transformations of Cp ligands 2.4.1.
Exception of [Cp2Ti] reactivities
In group 4 metallocene chemistry the Cp ligands are typically considered as stable spectator ligands that do not undergo further transformations. Using the complex Cp2Ti(η2-btmsa), the 7
majority of reactions are characterized by the formation of [Cp2Ti] after dissociation of the alkyne and subsequent reactions with different substrates. Exceptions are however known, which demonstrate, that the Cp ligands of [Cp2Ti] are not inert, giving e.g. C-C coupling reactions with alkynes or ring opening reactions together with C-C coupling reactions of diynes. Such reactivity was first found in the reaction of Cp2Ti(η2-Me2SiC2SiMe3) with the alkyne Me3SiC≡CPy in which the formed titanacyclopentadiene is not stable and forms a dihydroindenyl complex by insertion of the Cp ligand (Scheme 9). [8a] With 3,9-dodecadiyne by C-C bond cleavage of the Cp ligand and an additional intramolecular C-C coupling a η4:η3-dihydroindenyl complex was formed (Scheme 9). [8b]
Scheme 9. Different reactions of Cp2Ti(η2-Me2SiC2SiMe3) with Me3SiC≡CPy and 3,9dodecadiyne. 2.4.2.
Synthetic applications of Cp reactivities
Based on these results Takahashi and coworkers published similar systems for a variety of different synthetic purposes. [9] Such unprecedented double C-C bond cleavage reactions of Cp ligands were realized and synthetically used. [9] For example, in the reaction with benzonitrile the five cyclopentadienyl carbon atoms of a dihydroindenyl titanium complex were separated into products with two- and others with three-carbons (Scheme 10). [9a] Et
Et Et
+ 2 Ph-CN
Et Et +
CpTi
Ti Et
Et
Et
Et
Et
Et
Ph
N
Ph
Et
Scheme 10. Cleavage of a dihydroindenyl titanium complex. The formation of indene compounds from titanacyclopentadienes was realized, too. [9b] By oxidation, the formed η4:η3-dihydroindenyl complex underwent ethyl group migration to give the substituted indene (Scheme 11). [9b]
Scheme 11. Oxidation of a dihydroindenyl titanium complex. 8
Reactions of bis(cyclopentadienyl)titanacyclopentadienes with TiCl4 via such complexes were described in which 1-chloro-4,5,6,7-tetraalkyldihydroindenes were formed (Scheme 12). [9c]
Scheme 12. Reaction of titanacyclopentadienes with TiCl4 A manifold chemistry of these compounds was described by Takahashi and coworkers, including the reversible C-C bond cleavage of such dihydroindenyltitanium complexes with the possibility to reform once cleaved C-C bonds, [9d] the coupling of titanacyclopentadienes with a Cp ligand with elimination of one substituent [9e] and the separation of five carbon atoms of dihydroindenyl titanium complexes into products with two- and others with threecarbon atoms on titanium in the reaction with 2-aminopyridine. [9f] The formation of spiro compounds via coupling of Cp ligands with the diene group of titanacyclopentadienes was published as well. [9g] Very recently the traveling of carbon atoms in such titanium bonded ligands was described in detail, in which the carbon atoms return to its starting positions, an unusual reaction that was referred to as a “Merry-Go-Round Reaction”. [9h] 2.5. C-C bond cleavage of 1,3-butadiynes 2.5.1.
The cleavage reaction
After investigation of some reactions of Cp2Ti(η2-Me2SiC2SiMe3) with the alkynes it was a coincidence that a sample of the corresponding Me3Si-substituted 1,3-butadiyne Me3SiC≡CC≡CSiMe3 was available in the labs of the author of this account. Only for this reason, the reaction of Cp2Ti(η2-Me2SiC2SiMe3) with Me3SiC≡C-C≡CSiMe3 was investigated. [10] Surprisingly, no simple substitution or coupling products were found, but a dinuclear titanocene (III) complex [Cp2Ti-C≡CSiMe3]2 having two σ,π-bonded alkynyl groups bridging the titanium centers (Scheme 13). This was formed by a very unusual C-C bond cleavage of the starting 1,3-butadiyne in contrast to synthetically often used C-C coupling reactions of alkynyl groups. [10]
Scheme 13. Reaction of Cp2Ti(η2-Me2SiC2SiMe3) and Me3SiC≡C-C≡C-SiMe3 with C-C bond cleavage. 2.5.2. Systematic investigations of this reactivity As reactions of electron rich Ni(0) alkyne L2Ni(RC2R) [2a-j] and 1,3-butadiyne complexes L2Ni(RC2-C2R) were known, [2k,l] an extension of the above mentioned results for Ti from alkynes RC≡CR to 1,3-butadiynes RC≡C-C≡CR regarding the influence of ligands L (L = 9
R3P, py etc.) and the substrate substituents R (R = Me3Si, aryl, alkyl etc.) was investigated these late transition metals fragments. However, no surprising results were obtained. [10c,d] Some other unexpected reactions were found only with di- or polyynes and group 4 metallocene complexes. Depending on the ligands L (Cp’ = Cp, Cp* etc.), metals M (M = Ti, Zr, Hf) and the substrate substituents R (R = Me3Si, aryl, alkyl etc.) 1,3-butadiynes RC≡CC≡CR react with [Cp’2M] in a different fashion. 2.6. Complexation to five-membered metallacylocumulenes 2.6.1. Coordination to metallacycles Using tBuC≡C-C≡CtBu in the reaction with Cp2Ti(η2-Me2SiC2SiMe3) instead of Me3SiC≡CC≡CSiMe3 a complexation of the intact diyne was observed to yield a highly unusual titanacyclocumulene (Scheme 14). [11]
Scheme 14. Reaction of Cp2Ti(η2-Me2SiC2SiMe3) and tBuC≡C-C≡CtBu to give a fivemembered titanacyclocumulene. 2.6.2. Reactions via the formed metallacycles Further unexpected products were found in reactions of [Cp’2M] with 1,3-butadiynes S-C≡CC≡C-S such as complexation, coupling and cleavage by using different ligands L (Cp’ = Cp, Cp* etc.), metals M (M = Ti, Zr, Hf) and substrate substituents R (R = Me3Si, aryl, alkyl etc.) (Scheme 15). [5]
Scheme 15. Products from reactions of group 4 metallocenes and 1,3-butadiynes. 2.7. C-C Bond coupling to five-membered metallacyclocumulenes 10
2.7.1. The coupling reaction A large number of group 4 metallocene bis(alkynyl) compounds Cp’2M(-C≡CR)2 exist and were described in detail in many papers. Several reactions for these compounds were published, but the potential of these complexes to form five-membered metallacyclocumulenes under the influence of sunlight was unknown. Exposure of complexes Cp*2Zr(-C≡CR)2 (R = Ph, Me3Si, Me etc.) to sunlight resulted in the formation of fivemembered metallacyclocumulenes Cp*2Zr(η4-RC4R) in high yields by coupling of the two alkynyl groups (Scheme 16). [12]
Scheme 16. Reactions of bis(alkynyl) complexes Cp*2Zr(-C≡CR)2 to five-membered metallacyclocumulenes Cp*2Zr(η4-RC4R) under the influence of sunlight. 2.7.2.
Combinations of coupling and cleavage reactions
This coupling reaction (Scheme 16) was total unexpected and only possible to recognice by the knowledge of the characteristics of the metallacyclocumulenes, which were not known before these investigations and are the result of the reverse reaction of C-C cleavage (Scheme 13) of 1,3-butadiynes to alkynyl groups. The metallacyclocumulenes represent intermediates for the cleavage as well as the coupling reactions because the coordination of the internal double bond to the metal can be exchanged by an external one and as a result of these precesses cleaved or coupled C4-units were obtained, depending on different L, M and R (Schemes 15 and 17). [5]
Scheme 17. Metallacyclocumulenes as intermediates for cleavage as well as coupling reactions. 11
Such products were realized and possible for some early-late heterometallic combinations as well. [5] A synthetic application for a combination of the cleavage and the coupling steps was found in an unusual single bond metathesis reaction. Irradiation (ν = 390-450 nm) of a mixture of the symmetrically substituted 1,3-butadiynes Me3SiC≡C-C≡CSiMe3 and tBuC≡C-C≡CtBu in the presence of an excess of the [Cp2Ti] source Cp2Ti(η2-Me2SiC2SiMe3) at 100°C, followed by oxidative workup gave the unsymmetrically substituted 1,3-butadiyne tBuC≡C-C≡CSiMe3 as a result of C-C single bond metathesis (Scheme 18). [13] Me3Si
SiMe3
2 Me3Si
+ tBu
1) 4 [Cp2Ti], h 2) AgSO3CF3 tBu
tBu
Scheme 18. C-C single bond metathesis of symmetricaly substituted 1,3-butadiynes. The single steps for the reaction of [Cp2Ti] with Me3SiC≡C-C≡CSiMe3 and tBuC≡C-C≡CtBu include the formation of different unstable titanocene mono-alkynyl complexes [Cp2Ti-C≡CSiMe3] and [Cp2Ti-C≡C-tBu] as intermediates (Scheme 19). These products could recombine in a productive way to unsymmetrical or an unproductive way to symmetrical dinuclear species, giving either the starting or the new 1,3-butadiynes. [13]
Scheme 19. Photocatalytic cleavage and recombination of 1,3-butadiynes. The dinuclear intermediates were isolated in separate reactions and their formation is possible via formation of the five-membered titanacyclocumulenes (Scheme 14). 2.8. Sliding of group 4 metallocenes along polyynes 2.8.1.
Metallocene sliding
12
If polyynes R-(C≡C)n-R instead of 1,3-butadiynes were used in reactions with [Cp’2M] in principle the same reactions are indicated as found for the 1,3-butadiynes. Additionally an unexpected motion (also referred to as “sliding”, “migration” or “tobogganing”) of the metallocene along the backbone of triynes and polyynes was observed (Scheme 20), giving similar products as found for the butadiynes. [14]
Scheme 20. Moving (“sliding”, “migration” or “tobogganing”) of [Cp*2Zr] along the backbone of a triyne. 2.8.2.
Applications of metallocene sliding
More or less stable five membered zirconacyclocumulenes and three membered zirconacyclopropenes participate as intermediates during these processes. It is clear, that these species could be trapped in some reactions, leading to selectivities for different products, which again depend on changed ligands L (Cp’ = Cp, Cp* etc.), metals M (M = Ti, Zr, Hf) and substrate substituents R (R = Me3Si, aryl, alkyl etc.). An extension from 1,3-butadiyne to 1,3,5-hexatriyne and 1,3,5,7-octatetrayne as well as branched polyynes to other metals like vanadium and was also described (Scheme 21). [5, 15] Characterisation of the isolated obtained products and its dynamics provided insight into the overall migration mechanisms.
Scheme 21. Complexes formed by coordination of 1,3,5,7-octatetrayne to vanadocene.
3.
Conclusion
During the investigations of group 4 metallocene bis(trimethylsilyl)acetylene complexes accidentally several unexpected reactions were found. The main and most important reaction was the generation of the very reactive, coordinatively and electronically unsaturated complex 13
fragments [Cp’2M], starting from Cp’2M(L)(η2-btmsa) complexes, leading to a large number of novel stoichiometric and catalytic reactions. One example was the reaction of Cp2Ti(η2btmsa) with carbon dioxide, giving a coupling with Me3SiC≡CSiMe3 to a dinuclear complex with relevance to the general mechanism of CO2 coupling reactions. This is restricted only for Cp ligands and not for Cp*2Ti(η2-btmsa), giving without alkyne coupling a dinuclear carbonate bridged complex together with a mononuclear bis-carbonyl complex Cp*2Ti(CO)2. The Cp ligands in [Cp2Ti] were not always inert, showing C-C coupling reactions with pyridyl substituted alkynes or Cp ring opening reactions with C-C coupling reactions of selected di- or polyynes. 1,3-butadiynes gave with [Cp’2M] either a C-C bond cleavage of the internal C-C single bond or intact complexation to unusual five-memberd metallacyclocumulenes depending on the metals, the Cp’ ligands and the substituents of the diynes. The reverse reaction of C-C coupling of alkynyl groups of bis(alkynyl) complexes to coordinated 1,3-butadiynes in metallacyclocumulenes was described, too. With triynes a sliding of the metallocene at the triple bonds was observed. For all cases several synthetic applications of the primary unexpected result were described. Only one example for these developments was selected and explained, giving an impression about the consequences of each of these results. One of the driving forces for all cases was only to broaden the knowledge by comparison of different, related types of organometallic systems. In a nutshell, fundamental research has opened the door for novel reaction types that led to a better understanding of the organometallic chemistry of metallocenes. It is worth to mention, that these results were obtained rather by coincidence without any planning, but with a clear analytical view on the experimental data. The examples described in this account serve as examples for this research philosophy. Acknowledgements I would like to particularly thank all my former PhD students, postdocs, assistents, cooperation partners and other people whose names are mentioned in the list of references for all of her excellent scientific results, giving the basis of this minireview. In particular I have to thank Mark E. Vol’pin (passed away in 1996), Vladimir B. Shur and Vladimir V. Burlakov from INEOS Moscow for their activities in this field of this chemistry as well as the cooperation during the last decades. Scientific discussions with PD Dr. Torsten Beweries are gratefully acknowledged.
References 1. (a) C. Elschenbroich, “Organometallchemie”, Eds. C. Elschenbroich, F. Hensel, H. Hopf, Teubner, 4. Aufl. 2003; (b) H. Werner, “Landmarks in Organo-Transition Metal Chemistry, A Personal View”; Ed. J. P. Fackler, Springer Science + Business Media, LLC 2009; (c) D. Steinborn, “Fundamentals in Organometallic Catalysis”, WILEY-VCH 2012; (d) T. J. Kealy, P.L. Pauson, Nature 168 (1951) 1039-1040; (e) E. O. Fischer, R. Jira, J. Organomet. Chem. 637-639 (2001) 7-12, and refs. cited therein; (f) G. Wilkinson, , J. Organomet. Chem. 100 (1975) 273-278, and refs. cited therein; (g) K. Ziegler, E. Holzkamp, 14
H. Breil, H. Martin, Angew. Chem. 67 (1955) 541-547, and refs. cited therein; (h) G. Wilke, Angew. Chem. 75 (1963) 10-20, and refs. cited therein; (i) D. S. Breslow, N. R. Newburg, J. Am. Chem. Soc. 79 (1957) 5072-5073; (j) G. Natta, P. Pino, G. Mazzanti, U. Giannini, J. Am. Chem. Soc. 79 (1957) 2975-2976; (k) H. Sinn, W. Kaminsky, Adv. Organomet. Chem. 18 (1980) 99-149, and refs. cited therein; (l) H. Schnutenhaus, H. H. Brintzinger, Angew. Chem. Int. Ed. Engl. 18 (1979) 777-778; (m) J. A. Smith, H. H. Brintzinger, J. Organomet. Chem. , 218 (1981) 159-167; (n) H. H. Brintzinger, D. Fischer, R. Mülhaupt, B. Rieger, R. M. Waymouth, Angew. Chem. Int. Ed. Engl. 34 (1995) 1143-1170, and refs. cited therein; (o) W. Keim, Angew. Chem. Int. Ed. 52 (2013) 12492-12496. 2. (a) U. Rosenthal, Z. Anorg. Allg. Chem. 482 (1981) 179; (b) U. Rosenthal, Z. Anorg. Allg. Chem. 513 (1984) 151; (c) U. Rosenthal, W. Schulz, Z. Chem. 25 (1985) 227; (d) U. Rosenthal, W. Schulz, J. Organomet. Chem. 321 (1987) 103; (e) U. Rosenthal, W. Schulz, H. Görls, Z. Anorg. Allg. Chem. 550 (1987) 169; (f) U. Rosenthal, H. Görls, J. Organomet. Chem. 348 (1988) 135; (g) U. Rosenthal, G. Oehme, H. Görls, V. V. Burlakov, A. I. Yanovsky, Yu. T. Struchkov, J. Organomet. Chem. 389 (1990) 251; (h) U. Rosenthal, G. Oehme, H. Görls, V. V. Burlakov, A. I. Yanovsky, Yu. T. Struchkov, J. Organomet. Chem. 389 (1990) 409; (i) U. Rosenthal, G. Oehme, H. Görls, V. V. Burlakov, A. I. Yanovsky, Yu. T. Struchkov, J. Organomet. Chem. 390 (1990) 113; (j) U. Rosenthal, A. Ohff, W. Baumann, A. Tillack, H. Görls, V. V. Burlakov, V. B. Shur, J. Organomet. Chem. 484 (1994) 203-207; (k) U. Rosenthal, S. Pulst, P. Arndt, W. Baumann, A. Tillack, R. Kempe, Z. Naturforsch. 50b (1995) 377-384; (l) U. Rosenthal, S. Pulst, P. Arndt, W. Baumann, A. Tillack, R. Kempe, Z. Naturforsch. 50b (1995) 368-376. 3. (a) U. Rosenthal, C. Nauck, P. Arndt, S. Pulst, W. Baumann, V. V. Burlakov, H. Görls, J. Organomet. Chem. 484 (1994) 81-87; (b) U. Rosenthal, G. Oehme, V. V. Burlakov, P. V. Petrovskii, M. E. Vol'pin, J. Organomet. Chem. 391 (1990) 119. 4. Substitution: (a) V. V. Burlakov, U. Rosenthal, P. V. Petrovskii, V. B. Shur, M. E. Vol'pin, Organomet. Chem. USSR 1 (1988) 526; (b) V. V. Burlakov, U. Rosenthal, R. Beckhaus, Yu. T. Struchkov, G. Oehme, V. B. Shur, M. E. Vol'pin, Organomet. Chem. USSR 3 (1990) 237; (c) U. Rosenthal, H. Görls, V. V. Burlakov, V. B. Shur, M. E. Vol'pin, J. Organomet. Chem. 426 (1992) C53-C59. 5. Applications of substitution: (a) A. Ohff, S. Pulst, C. Lefeber, N. Peulecke, P. Arndt, V. V. Burlakov, U. Rosenthal, Synlett (1996), 111-118; (b) U. Rosenthal, P.-M. Pellny, F. G. Kirchbauer, V. V. Burlakov, Acc. Chem. Res. 33 (2000) 119-129; (c) U. Rosenthal, V. V. Burlakov in Titanium and Zirconium in Organic Synthesis, Ed. I. Marek, Wiley-VCH, Weinheim (2002) 355-389; (d) U. Rosenthal, Angew. Chem. 115 (2003) 1838-1842; Angew. Chem., Int. Ed. 42 (2003) 1794-1798; (e) U. Rosenthal, V. V. Burlakov, P. Arndt, W. Baumann, A. Spannenberg, Organometallics 22 (2003) 884-900; (f) U. Rosenthal, Angew. Chem. 116 (2004) 3972-3977; Angew. Chem., Int. Ed. 43(2004) 3882-3887; (g) U. Rosenthal in Acetylene Chemistry - Chemistry, Biology, and Material Science, Eds. F. Diederich, P. Stang and R. R. Tykwinski, Wiley-VCH, Weinheim (2004) 139-171 (h) U. Rosenthal, V. V. Burlakov, P. Arndt, W. Baumann, A. Spannenberg, Eur. J. Inorg. Chem. 22 (2004) 4739-4932; (i) U. Rosenthal, V. V. Burlakov, P. Arndt, W. Baumann, A. Spannenberg, Organometallics 24 (2005) 456-471; (j) U. Rosenthal, V. V. Burlakov, M. A. Bach, T. Beweries, Chem Soc. Rev. 36 (2007) 719-728; (k) U. Rosenthal, Angew. Chem. 15
120 (2008) 5196-5199; Angew. Chem., Int. Ed. 47 (2008) 5118-5121; (l) U. Rosenthal, V. V. Burlakov, P. Arndt, A. Spannenberg, U. Jäger-Fiedler, M. Klahn, M. Hapke, Activating Unreactive Bonds, Eds. C. Bolm, F. E. Hahn, Wiley-VCH, Weinheim (2009) 165 pp; (m) T. Beweries, U. Rosenthal, Science of Synthesis 4 (2011) Recent updates in the organometallic chemistry of group 4 metallocene complexes with bis(trimethylsilyl)acetylene, (n) T. Beweries, M. Hähnel, U. Rosenthal, Catal. Sci. & Technol. 3 (2013) 18-28; (o) T. Beweries, U. Rosenthal, Nature Chemistry 5 (2013) 649; and refs. cited therein. (p) S. Roy, U. Rosenthal, E. D. Jemmis; Acc Chem. Res. 47 (2014) 2917-2930; (q) U. Rosenthal, Izv. Akad. Nauk, Ser. Khim. (2014) 2577-2582, Engl. Transl.: Russ. Chem. Bull. (2014) 25772582, (r) L. Becker, U. Rosenthal, Coord. Chem. Rev. 345 (2017) 137-149, (s) U. Rosenthal, Angew. Chem. 130 (2018) 14932-14950; Angew. Chem., Int. Ed. 57 (2018) 14718-14735; (t) U. Rosenthal, Eur. J. Inorg. Chem. (2019) 895-919; (u) U. Rosenthal, ChemOpen 8 (2019) 1036-1047; (v) G. R. Kiel, M. S. Ziegler, T. D. Tilley, Angew. Chem. 129 (2017) 4917-4922; Angew. Chem. Int Ed. 56 (2017) 4839-4922; (w) S. Urrego-Riveros, I.-M. Ramirez y Medina, F. D. Sönnichsen, A. Staubitz, Chem. Eur. J. 25 (2019) 13318-13328, and refs cited therein; (x) D. Heitkemper, C. P. Sindlinger, Chem. Eur. J. 25 (2019) 66286637. 6. Carbon dioxide reactions: (a) V. V. Burlakov, U. Rosenthal, A. I. Yanovsky, Yu. T. Struchkov, O. G. Ellert, V. B. Shur, M. E. Vol'pin, Organomet. Chem. USSR 2 (1989) 1193; (b) V. V. Burlakov, U. Rosenthal, F. M. Dolgushin, A. I. Yanovski, Yu. T. Struchkov, O. G. Ellert, V. B. Shur, M. E. Vol'pin, Metalloorg. Khim. 5 (1992) 1213-1214 (russ.); 7. Applications of carbon dioxide reactions: (a) U. Rosenthal, A. Ohff, M. Michalik, H. Görls, V. V. Burlakov, V. B. Shur, Organometallics 12 (1993) 5016-5019; (b) C. Lefeber, A. Ohff, A. Tillack, W. Baumann, R. Kempe, V. V. Burlakov, U. Rosenthal, H. Görls, J. Organomet. Chem. 501 (1995) 179-188; (c) V. V. Burlakov, F. M. Dolgushin, A. I. Yanovsky, Yu. T. Struchkov, V. B. Shur, U. Rosenthal, U. Thewalt, J. Organomet. Chem. 522 (1996) 241-247; (d) V. V. Burlakov, V. V., A. I. Yanovsky, Yu. T. Struchkov, V. B. Shur, O. G. Ellert, U. Rosenthal, J. Organomet. Chem. 542 (1997) 105-112; (e) S. Pulst, U. Rosenthal, A. Spannenberg, R. Kempe, Z. Krist. 213 (1997) 215-216; (f) D. Thomas, W. Baumann, A. Spannenberg, R. Kempe, U. Rosenthal, Organometallics 17 (1998) 2096; (g) D. Thomas, N. Peulecke, V. V. Burlakov, W. Baumann, A. Spannenberg, R. Kempe, U. Rosenthal, Eur. J. Inorg. Chem. (1998) 1495-1502; (h) R. Kempe, A. Spannenberg, N. Peulecke, U. Rosenthal, Z. Krist. 213 (1998) 793-794; (i) P.-M. Pellny, V. V. Burlakov, W. Baumann, A. Spannenberg, U. Rosenthal, Z. Anorg. Allg. Chem. 625 (1999) 910-918; (j) A. Spannenberg, T. Zippel, V. V. Burlakov, U. Rosenthal, Z. Krist. 215 (2000) 367-368; (k) F. G. Kirchbauer, P.-M. Pellny, V. V. Burlakov, P. Arndt, W. Baumann, A. Spannenberg, U. Rosenthal, Organometallics 20 (2001) 5289-5296; (l) H. Sun, A. Spannenberg, V. V. Burlakov, W. Baumann, P. Arndt, U. Rosenthal, Z. Kristallogr. 217 (2002) 241-243; (m) V. V. Burlakov, P. Arndt, W. Baumann, A. Spannenberg, U. Rosenthal, Organometallics 25 (2006) 1317-1320. 8. Cp reactions: (a) A. Tillack, W. Baumann, A. Ohff, C. Lefeber, A. Spannenberg, U. Rosenthal, J. Organomet. Chem. 520 (1996) 187-193; (b) P.-M. Pellny, N. Peulecke, V. V. Burlakov, A. Tillack, W. Baumann, A. Spannenberg, R. Kempe, U. Rosenthal, Angew. Chem. 109 (1997) 2728-2730; Angew. Chem. Int. Ed. Engl. 36 (1997) 2515-2617. 16
9. Applications of Cp reactions: (a) Z. Xi, K. Sato, Y. Gao, J. Lu, T. Takahashi, J. Am. Chem. Soc. 125 (2003) 9568-9569; (b) T. Takahashi, Y. Kuzuba, F. Kong, K. Nakajima, Z. Xi; J. Am. Chem. Soc. 127 (2005) 17181-17189; (c) T. Takahashi, Z. Song, K. Sato, Y.; Kuzuba, K. Nakajima, K. Kanno, J. Am. Chem. Soc. 129 (2007) 11678-11679; (d) T. Takahashi, Z. Song, Y.-F. Hsieh, K. Nakajima, K. Kanno, J. Am. Chem. Soc. 130 (2008) 15236-15237; (e) Y. Mizukami, H. Li, K. Nakajima, Z. Song, T. Takahashi, Angew. Chem. Int. Ed. 53 (2014) 8899-8903; (f) Z. Song, Y.-F. Hsieh, K. Nakajima, K. Kanno, T. Takahashi, Organometallics 35 (2016) 1092-1097; (g) M. Bando, Y. Mizukami, K. Nakajima, Z. Song, T. Takahashi, Dalton Trans. 46 (2017) 16408-16411; (h) M. Bando, K. Nakajima, Z. Song, T. Takahashi, Organometallics 38 (2019) 731-734. 10. Butadiyne C-C bond cleavage: (a) U. Rosenthal, H. Görls, J. Organomet. Chem. 439 (1992) C36-C41; (b) U. Rosenthal, A. Ohff, W. Baumann, R. Kempe, A. Tillack, V. Burlakov, Organometallics 13 (1994) 2903-2906. (c) U. Rosenthal, S. Pulst, P. Arndt, W. Baumann, A. Tillack, R. Kempe, Z. Naturforsch. 50b (1995) 377-384; (d) U. Rosenthal, S. Pulst, P. Arndt, W. Baumann, A. Tillack, R. Kempe, Z. Naturforsch. 50b (1995) 368-376. 11. Butadiyne coordination: (a) U. Rosenthal, A. Ohff, W. Baumann, R. Kempe, A. Tillack, V. V. Burlakov, Angew. Chem. 106 (1994) 1678-1680, Angew. Chem. Int. Ed. Engl. 33 (1994) 1605-1607; (b) V. V. Burlakov, A. Ohff, C. Lefeber, A. Tillack, W. Baumann, R. Kempe, U. Rosenthal, Chem. Ber. 128 (1995) 967-971. 12. Butadiyne formation: P. M. Pellny, F. G. Kirchbauer, V. V. Burlakov, W. Baumann, A. Spannenberg, U. Rosenthal, J. Am. Chem. Soc. 121 (1999) 8313-8323. 13. Applications of butadiyne reactions: S. Pulst, F. G. Kirchbauer, B. Heller, W. Baumann, U. Rosenthal, Angew. Chem. 110 (1998) 2029-2031, Angew. Chem. Int. Ed. Engl. 37 (1998) 1915-1927. 14. Metallocene sliding: P. M. Pellny, V. V. Burlakov, P. Arndt, W. Baumann, A. Spannenberg, U. Rosenthal, J. Am. Chem. Soc. 122 (2000) 6317-6318. 15. Applications of metallocene sliding: (a) C. Danjoi, J. Zhao, B. Donnadieu, J.-P. Legros, L. Valade, R. Choukroun, A. Zwick, P. Cassoux, Chem. Eur. J. 4 (1998) 1100-1105; (b) R. Choukroun, P. Cassoux, Acc. Chem. Res. 32 (1999) 494-502; (c) R. Choukroun, C. Lorber, B. Donnadieu, B. Henner, R. Frantz, C. Guerin, Chem Commun. (1999) 1099-1100; (d) R. Choukroun, B. Donnadieu, C. Lorber, P.-M. Pellny, W. Baumann, U. Rosenthal; Chem. Eur. J. 6 (2000) 4505-4509.
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Highlight (for review) Serendipity in Unexpected Reactions of Group 4 Metallocene Bis(trimethylsilyl)acetylene Complexes and its Consequences for Selected Applications Prof. U. Rosenthal* Leibniz Institute for Catalysis at the University of Rostock, Albert-Einstein-Str. 29A, 18059 Rostock (Germany), E-mail: [email protected] ORCID https://orcid.org/0000-0003-1922-6782
This paper is written on the basis of accumulated knowledge about Group 4 metallocene bis(trimethylsilyl)acetylene complexes. Selected examples of this minireview are all unexpected results and should help for a better general understanding of this chemistry, and here in particular of Organometallic Chemistry. Several unexpected reactions of group 4 metallocene bis(trimethylsilyl)acetylene complexes are summarized. (i) By substitution of the alkyne in Cp2Ti(η2-btmsa) and similar complexes the very reactive coordinatively and electronically unsaturated complex fragments [Cp’2M] were generated and used for many purposes. (ii) The reaction of this complex with carbon dioxide gave a coupling of the alkyne to an unexpected dinuclear complex with relevance to the general mechanism of CO2 coupling reactions. (iii) The Cp ligands in [Cp2Ti] are not inert, showing ring opening reactions together with C-C coupling reactions of diynes. Depending on the ligands L (Cp’ = Cp, Cp* etc.), metals M (M = Ti, Zr, Hf) and the substrate substituents S (S = Me3Si, aryl, alkyl etc.) 1,3-butadiynes S-C≡C-C≡C-S react with [Cp’2M] under C-C bond cleavage of the internal C-C single bond (iv), through complexation to unusual metallacyclocumulenes (v) or by C-C coupling of alkynyl groups to coordinated 1,3butadiynes in metallacyclocumulenes. (vi) Polyynes show a sliding of the [Cp’2M] unit at the triple bonds. (vii) All these unexpected examples (i) - (vii) represent reactions that were found by coincidence. Later several applications for the initial results were described and listed in this minireview. The results lead to the following conclusions: Main and most important reaction was the generation of the very reactive, coordinatively and electronically unsaturated complex fragments [Cp’2M], starting from Cp’2M(L)(η2-btmsa) complexes, leading to a large number of novel stoichiometric and catalytic reactions. One example was the reaction of Cp2Ti(η2btmsa) with carbon dioxide, giving a coupling with Me3SiC≡CSiMe3 to a dinuclear complex with relevance to the general mechanism of CO2 coupling reactions. This is restricted only for Cp ligands and not for Cp*2Ti(η2-btmsa), giving without alkyne coupling a dinuclear carbonate bridged complex together with a mononuclear bis-carbonyl complex Cp*2Ti(CO)2. The Cp ligands in [Cp2Ti] were not always inert, showing C-C coupling reactions with pyridyl substituted alkynes or Cp ring opening reactions with C-C coupling reactions of 1
selected di- or polyynes. 1,3-butadiynes gave with [Cp’2M] either a C-C bond cleavage of the internal C-C single bond or intact complexation to unusual five-memberd metallacyclocumulenes depending on the metals, the Cp’ ligands and the substituents of the diynes. The reverse reaction of C-C coupling of alkynyl groups of bis(alkynyl) complexes to coordinated 1,3-butadiynes in metallacyclocumulenes was described, too. With triynes a sliding of the metallocene at the triple bonds was observed. For all cases several synthetic applications of the primary unexpected result were described. Only one example for these developments was selected and explained, giving an impression about the consequences of each of these results. One of the driving forces for all cases was only to broaden the knowledge by comparison of different, related types of organometallic systems. In a nutshell, fundamental research has opened the door for novel reaction types that led to a better understanding of the organometallic chemistry of metallocenes. It is worth to mention, that these results were obtained rather by coincidence without any planning, but with a clear analytical view on the experimental data. The examples described in this account serve as examples for this research philosophy.
2
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
S0022-328X(19)30514-5
DOI:
https://doi.org/10.1016/j.jorganchem.2019.121071
Reference:
JOM 121071
To appear in:
Journal of Organometallic Chemistry
Received Date: 14 November 2019 Revised Date:
6 December 2019
Accepted Date: 7 December 2019
Please cite this article as: U. Rosenthal, Serendipity in unexpected reactions of group 4 metallocene Bis(trimethylsilyl)acetylene complexes and its consequences for selected applications, Journal of Organometallic Chemistry (2020), doi: https://doi.org/10.1016/j.jorganchem.2019.121071. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
Serendipity in Unexpected Reactions of Group 4 Metallocene Bis(trimethylsilyl)acetylene Complexes and its Consequences for Selected Applications Prof. U. Rosenthal* Leibniz Institute for Catalysis at the University of Rostock, Albert-Einstein-Str. 29A, 18059 Rostock (Germany), E-mail: [email protected]
ORCID https://orcid.org/0000-0003-1922-6782
Dedicated to Professor Vladimir Borisovich Shur (INEOS, RAS, Moscow)
Abstract: Examples for serendipity in several unexpected reactions of group 4 metallocene bis(trimethylsilyl)acetylene complexes are summarized in this Review. The first is the reaction of Cp2Ti(η2-btmsa) with tolane, PhC≡CPh, investigated 1988 in the Nesmeyanov Institute in Moscow giving surprisingly no coupling of Me3SiC≡CSiMe3 with PhC≡CPh to a titanacyclopentadiene but the substitution of bis(trimethylsilyl)acetylene by tolane and via the assumed alkyne complex Cp2Ti(η2-PhC2Ph) a C-C coupling reaction of two molecules of tolane to the tetraphenyl substituted titanacyclopentadiene (i). This simple substitution reaction of Cp2Ti(η2-btmsa) and of similar complexes was used for the generation of the very reactive coordinatively and electronically unsaturated complex fragments [Cp’2M]. The reaction of this complex with carbon dioxide gave a coupling with Me3SiC≡CSiMe3 to an unexpected dinuclear complex with relevance to the general mechanism of CO2 coupling reactions (ii). Furthermore it was demonstrated, that the Cp ligands in [Cp2Ti] were not inert, showing a rare type of coupling reaction with alkynes or ring opening reactions together with C-C coupling reactions of diynes (iii). Some other unexpected reactions were found with dior polyynes. Depending on the ligands L (Cp’ = Cp, Cp* etc.), metals M (M = Ti, Zr, Hf) and the substrate substituents R (R = Me3Si, aryl, alkyl etc.) 1,3-butadiynes RC≡C-C≡CR react with [Cp’2M] under C-C bond cleavage of the internal C-C single bond (iv) or through complexation to unusual metallacyclocumulenes (v). The reverse reaction of C-C coupling of alkynyl groups to give coordinated 1,3-butadiynes in metallacyclocumulenes was described for some bis(acetylides) Cp’2Zr(-C≡CR)2 (vi). With polyynes R-(C≡C)n-R a sliding of the [Cp’2M] unit at the triple bonds was observed (vii). All these unexpected examples (i) - (vii) represent reactions that were found by coincidence. Later several applications for the initial results were described. For this overview only one selected examples for these developments are described to illustrate the consequences of each of these results.
1
1.
Introduction
The field of organometallic chemistry and homogeneous catalysis is a classical area of research in which by serendipity a lot of unexpected results were found, leading to a revolution of the existing basic knowledge as well as sometimes of catalytic technical processes. [1] Names like P. Pauson, E. O. Fischer, G. Wilkinson, K. Ziegler, G. Natta, G. Wilke, H. W. Sinn, W. Kaminsky, H. H. Brintzinger, W. Keim and many others are connected with such important discoveries. [1] The driving force for these investigations was in all cases a strong connection with fundamental research. Kealy and Pauson attempted to access the organic “fulvalene” molecule H4C5=C5H4 by coupling and subsequent dehydrogenation of two cyclopentadienyl radicals and accidently oserved formation of “dicyclopentadienyliron” instead of the expected compound. [1d] On the basis of this result Fischer [1e], Wilkinson [1f] and coworkers as well as others claimed and extended the concept of “sandwich complexes” by synthesis of a broad range of similar metallocene compounds with sometimes surprising molecular structures. Ziegler investigated the so called “Wachstumsreaktion” of ethylene to higher waxes by organoaluminium compounds and found by accident, that impurities of nickel salts in the reaction mixture, coming from cleaning processes of the autoclave, prevented the expected reaction and surprisingly gave only ethylene dimerisation. [1g] Upon changing the catalyst system to the combination of TiCl4/Et3Al a highly active catalyst system for the polymerization of ethylene was obtained. [1g] Wilke modified these systems to obtain very important nickel-catalysed cyclooligomerisations of diolefins. [1h] Later Breslow [1i] and Natta [1j] used a catalytic system consisting of Cp2TiCl2/Et2AlCl for the stereoregular polymerization of propylene. Sinn und Kaminsky were studying the catalytic system Cp2TiMe2/Me3Al and found again by accident, that impurities of water were necessary to obtain higher activity of the catalysts. [1k] Later this effect was explained by the formation of methylalumoxane (MAO) from Me3Al and water in the right molecular ratio. Brintzinger, who had investigated the fixation of molecular nitrogen and had some problems with the elimination of Cp ligands during these reductive reactions used titanocene derivatives with an interannular ethylene bridge between the Cp ligands. [1l] In many transformations these compounds did not follow the reaction paths, previously observed for their unbridged metallocene counterparts in several reactions. [1m] He used complexes with ethylene bridged ansa-Cp ligands like ethylenebis(indenyl) and ethylenebis(tetrahydroindenyl), giving as an additional effect a constrained conformation of the catalyst and after activation with MAO highly isotactic polymers from propylene. [1n] The SHOP (Shell Higher Olefin Process) was realized by a nonselective ethylene oligomerization process in a two-phase system with a nickel catalyst in polar solvents such as butane-1,4-diol with the produced oligomers yielding a nonpolar phase. Interesting to know, that “Again it was ‘serendipity in research’ which helped the breakthrough. Up to this point I had used toluene as the solvent in all catalysis experiments. One day my laboratory assistant A. Nabong swopped the solvent bottles and used acetonitrile instead of toluene. When the autoclave was opened there were two phases: a white phase consisting of pure α-olefins and a red phase containing the catalyst in acetonitrile:” how Keim described the story for finding of this result. [1o] The change of the solvent by accident is remarkable, because normally polar solvents like butane-1,4-diol were excluded before in such organometallic oligomerization reactions. By the use of this unusual solvent as a consequence the unexpected realisation of a two phase system became possible 2
and all problems with the formation of small amounts of polyethylene in heterogenisation processes of the catalyst disappeared. Going from this selection of famous examples for serendipity in organometallic research to more special examples of group 4 metallocene bis(trimethylsilyl)acetylene complexes described in this minireview a similar approach is considered to come to the more or less important unexpected results for this group of compounds. In the analysis it becomes clear that we have to consider for both cases the same heuristic principle of research. Starting from investigations of the influence of ligands L (L = R3P, py etc.) and the substrate substituents R (R = Me3Si, aryl, alkyl etc.) in electron rich Ni(0) alkyne complexes L2Ni(RC2-R), [2] it was interesting to compare these influences with electron poor group 4 metal alkyne complexes Cp’2M(R-C2-R) depending on the ligands L (Cp’ = Cp, Cp* etc.), metals M (M = Ti, Zr, Hf) and the substrate substituents R (R = Me3Si, aryl, alkyl etc.), to show the difference of both groups of compounds. [3] These investigations started in 1988 at the Nesmeyanov Institute of Organoelement Compounds of the Russian Academy of Science under M. E. Vol’pin in the group of V. B. Shur together with V. V. Burlakov [4] and were conducted up to now for more than 30 years. [5] Some examples for unexpected results were selected from these results and are presented here together with the consequences for some synthetic applications. 2. 2.1.
Results Substitution of bis(trimethylsilyl)acetylene
2.1.1. Starting Point When investigating the reaction of Cp2Ti(η2-btmsa) with tolane, PhC≡CPh, surprisingly no coupling of Me3SiC≡CSiMe3 with PhC≡CPh to a nonsymmetrical substituted titanacyclopentadiene by an addition reaction that is typical for alkyne complexes was observed (Scheme 1). Instead, dissociation of bis(trimethylsilyl)acetylene and formation of the transient [Cp2Ti], followed by the coordination of tolane to give the assumed alkyne complex Cp2Ti(η2-PhC2Ph) and subsequent unexpected C-C coupling of two molecules of tolane to a symmetrical tetraphenyl substituted titanacyclopentadiene was observed. [4]
Scheme 1. Unexpected reaction of Cp2Ti(η2-Me3SiC2SiMe3) with tolane, PhC≡CPh. 2.1.2. Applications of the substitution reaction 3
On the basis of this simple substitution reaction, Cp2Ti(η2-btmsa) and similar complexes Cp’2M(η2-btmsa) with other Cp’ ligands were applied as excellent sources for the generation of the coordinatively and electronically unsaturated and thus highly reactive complex fragments [Cp’2M] for titanium and zirconium (Scheme 2). By using different Cp’ ligands and metals several stoichiometric and catalytic reactions were influenced in a very effective way, summarized in several papers and some reviews. [5]. Besides, some very rare examples exist in which the substitution of the bis(trimethylsilyl)acetylene ligand did not occur and a coupling of the alkyne ligand with other substrates was found. [5]
Scheme 2. Examples of complexes Cp’2M(L)(η2-btmsa) of titanium and zirconium with different Cp’ ligands. An outstanding example for the synthetic applications of group 4 bis(trimethylsilyl)acetylene complexes as reagents are the complexes Cp2Ti(η2-btmsa) and Cp2Zr(py)(η2-btmsa). These complexes are advantageous compared to other [Cp2M] generating complexes which was summarized in a recent review. [5w] From the list of examples [5] some recent results from Tilley et al. [5x] and very recent results from the Staubitz group [5y] are particularly impressive. Several multifold coupling reactions of tris(diyne) and pentakis(diyne) substrates were realized by Tilley and coworkers by using Cp2Zr(py)(η2-btmsa) (Scheme 3). The intermediate zirconacyclopentadiene produced with an excess of benzoic acid further products of protodemetallation in high yields (Scheme 3). [5x]
Scheme 3. Multifold couplings of tris- and pentakis(diynes) with Cp2Zr(py)(η2-btmsa) to PAH’s (PAH = polycyclic aromatic hydrocarbons) and its protodemetallation. These and the other reported examples represent a highly efficient method to produce new PAH’s via fused zirconacyclopentadienes and subsequent protodemetalation. The authors described very selective reactions and high yields with Cp2Zr(py)(η2-btmsa) and published its suitability for the preparation of larger PAH’s and graphene-type nanostructures. Very recently, Staubitz and coworkers presented and compared intermolecular coupling reactions of several disubstituted alkynes and octadiynes R-C≡C-(CH2)4-C≡C-R (R = SnMe3, 4
Bpin, 4-thiophenyl, 2-methoxy-, 2-bromo- and 2-iodo-4-thiophenyl etc.) to zirconacyclopentadienes by using Negishi’s reagent and Cp2Zr(py)(η2-btmsa) (Scheme 4) [5y]
Scheme 4. Reactions of bis-substituted octadiynes to zirconacyclopentadienes. The efficiency of both reagents was evaluated and compared by kinetic studies using 1H NMR measurements. The complex Cp2Zr(py)(Me3SiC≡CSiMe3) was more efficient for the synthesis of zirconacyclopentadienes, giving higher yields in shorter reaction times when compared to Negishi´s reagent (Scheme 5). Additionally, even aryl-iodides were not attacked in the reaction by this special very functional group tolerant reagent. [5y] Cp2Zr(py)(Me 3SiC2SiMe3): Rosenthal' s Reagent Cp2ZrCl2 / 2 eq. n-BuLi:
Cp Cp Zr
Negishi's Reagent
Cp Cp Zr
Br
91% 88% Me3Si
Me3Sn
Cp Cp Zr
3
MeO
S
96% 88% Me Me Me Me O Cp Cp O Me Me B Zr B O Me Me O
S
OMe Br
S
93% 0%
S
Br
91% 28%
Cp Cp Zr S
3
Cp Cp Zr S
98% 15%
I
Cp Cp Zr SnMe
99% 53%
99% 99%
Cp Cp Zr SiMe
97% 56%
Br
I
Cp Cp Zr S
S
99% 85%
Scheme 5. Comparison of Negishi’s reagent and Cp2Zr(py)(Me3SiC≡CSiMe3) in the formation of zirconacyclopentadienes from substituted diynes. Similar results were published very recently by Sindlinger and Heitkemper in the comparison of the in situ generated Negishi reagent Cp2Zr(η2-butene) (51% yield) with Cp2Zr(py)(Me3SiC≡CSiMe3) (97% yield) in the formation of a zirconacyclopentadiene by coupling of 3,5-(tBu)2-C6H3C≡CC6H3-3,5-(tBu)2 (Scheme 6). [5z]
5
Scheme 6. Coupling of 3,5-(tBu)2-C6H3C≡CC6H3-3,5-(tBu)2 to a zirconacyclopentadiene. Additionally, many other examples for the advantages of group 4 bis(trimethylsilyl)acetylene complexes Cp’2M(L)(Me3SiC≡CSiMe3) as reagents were described. [5u-z] 2.2. Reaction with carbon dioxide 2.3. 2.2.1. Initial experiment More driven by a mixture of experimental creativity and boredom than in a well-conducted and planned experiment, the author added solid “dry ice” to a yellow solution of Cp2Ti(η2btmsa) in THF, giving a color change to deep green. By repeating the experiment under rigorously anaerobic conditions with gaseous carbon dioxide, green crystals of a dinuclear titanium (III) complex were isolated, consisting of a titanafuranone unit which interacts with a free titanocene (Scheme 7). [6a] This complex indicated an alkyne coupling with carbon dioxide as well as a coordination of [Cp2Ti], formed by alkyne dissociation from the starting complex. Only after reaction with oxygen from air, the mononuclear titanafuranone was formed.
Scheme 7. Reactions of Cp’2Ti(η2-Me2SiC2SiMe3) complexes with Cp’ = Cp and Cp* and carbon dioxide. 2.2.2. Mechanistic and synthetic use
6
This result is relevant for the general accepted mechanism of reactions of group 4 metallocene alkyne coupling with carbon dioxide, in which the direct coupling to the metallafuranone was described. These new investigations showed for [Cp2Ti] an intermediate, giving the product only after exposure to air (Scheme 7). The results for Cp*2Ti(η2-btmsa) gave no coupling of the alkyne but after its elimination the formation of mixed-valent titanocene carbonate and carbonyl complexes, indicating the influence of different Cp’ ligands on the reaction (Scheme 7). [6b] In further experiments, the influence of other Cp’ ligands, group 4 metals and alkyne substituents were investigated, which influence the reactions very effectively (Scheme 8). [7] For unsymmetrically substituted alkynes, the regioselectivity of carbon dioxide insertion was changed by using different Cp’ ligands. [7] The complex meso‐(ebthi)Ti(η2‐PhC2SiMe3) inserted carbon dioxide into the Ti–CPh bond of the titanacyclopropene structure of the alkyne complex with untypical regioselectivity to yield the α‐silyl‐substituted meso‐ (ebthi)titanafuranone. The analogous reactions of the complexes [(thi)2Ti(η2‐PhC2SiMe3)] (thi = η5‐tetrahydroindenyl), [rac‐(ebthi)Ti(η2‐PhC2SiMe3)] (ebthi = 1,2‐ethylene‐1,1′‐bis(η5‐ tetrahydroindenyl) and Cp*2Ti(η2‐PhC2SiMe3) with carbon dioxide gave the typical regioselectivity with the insertion into the M–CSi bond of the titanacyclopropene, yielding the β‐silyl‐substituted titanafuranones . The insertion of carbon dioxide into the M–C bond of the titanacyclopropenes is thus governed by the substitution patter of the alkyne and the steric enviroment around the metal center. [7g]
Scheme 8. Different regioselectivities of carbon dioxide insertion for meso‐(ebthi)Ti(η2‐ PhC2SiMe3) and rac‐(ebthi)Ti(η2‐PhC2SiMe3). These results explain that there is no general mechanism for the insertion of carbon dioxide and the above depicted dinuclear presentation is only the case for titanium and Cp ligands, whereas the other Cp’ ligands show different behavior which is additionally influenced by the alkyne substituents. 2.4. Transformations of Cp ligands 2.4.1.
Exception of [Cp2Ti] reactivities
In group 4 metallocene chemistry the Cp ligands are typically considered as stable spectator ligands that do not undergo further transformations. Using the complex Cp2Ti(η2-btmsa), the 7
majority of reactions are characterized by the formation of [Cp2Ti] after dissociation of the alkyne and subsequent reactions with different substrates. Exceptions are however known, which demonstrate, that the Cp ligands of [Cp2Ti] are not inert, giving e.g. C-C coupling reactions with alkynes or ring opening reactions together with C-C coupling reactions of diynes. Such reactivity was first found in the reaction of Cp2Ti(η2-Me2SiC2SiMe3) with the alkyne Me3SiC≡CPy in which the formed titanacyclopentadiene is not stable and forms a dihydroindenyl complex by insertion of the Cp ligand (Scheme 9). [8a] With 3,9-dodecadiyne by C-C bond cleavage of the Cp ligand and an additional intramolecular C-C coupling a η4:η3-dihydroindenyl complex was formed (Scheme 9). [8b]
Scheme 9. Different reactions of Cp2Ti(η2-Me2SiC2SiMe3) with Me3SiC≡CPy and 3,9dodecadiyne. 2.4.2.
Synthetic applications of Cp reactivities
Based on these results Takahashi and coworkers published similar systems for a variety of different synthetic purposes. [9] Such unprecedented double C-C bond cleavage reactions of Cp ligands were realized and synthetically used. [9] For example, in the reaction with benzonitrile the five cyclopentadienyl carbon atoms of a dihydroindenyl titanium complex were separated into products with two- and others with three-carbons (Scheme 10). [9a] Et
Et Et
+ 2 Ph-CN
Et Et +
CpTi
Ti Et
Et
Et
Et
Et
Et
Ph
N
Ph
Et
Scheme 10. Cleavage of a dihydroindenyl titanium complex. The formation of indene compounds from titanacyclopentadienes was realized, too. [9b] By oxidation, the formed η4:η3-dihydroindenyl complex underwent ethyl group migration to give the substituted indene (Scheme 11). [9b]
Scheme 11. Oxidation of a dihydroindenyl titanium complex. 8
Reactions of bis(cyclopentadienyl)titanacyclopentadienes with TiCl4 via such complexes were described in which 1-chloro-4,5,6,7-tetraalkyldihydroindenes were formed (Scheme 12). [9c]
Scheme 12. Reaction of titanacyclopentadienes with TiCl4 A manifold chemistry of these compounds was described by Takahashi and coworkers, including the reversible C-C bond cleavage of such dihydroindenyltitanium complexes with the possibility to reform once cleaved C-C bonds, [9d] the coupling of titanacyclopentadienes with a Cp ligand with elimination of one substituent [9e] and the separation of five carbon atoms of dihydroindenyl titanium complexes into products with two- and others with threecarbon atoms on titanium in the reaction with 2-aminopyridine. [9f] The formation of spiro compounds via coupling of Cp ligands with the diene group of titanacyclopentadienes was published as well. [9g] Very recently the traveling of carbon atoms in such titanium bonded ligands was described in detail, in which the carbon atoms return to its starting positions, an unusual reaction that was referred to as a “Merry-Go-Round Reaction”. [9h] 2.5. C-C bond cleavage of 1,3-butadiynes 2.5.1.
The cleavage reaction
After investigation of some reactions of Cp2Ti(η2-Me2SiC2SiMe3) with the alkynes it was a coincidence that a sample of the corresponding Me3Si-substituted 1,3-butadiyne Me3SiC≡CC≡CSiMe3 was available in the labs of the author of this account. Only for this reason, the reaction of Cp2Ti(η2-Me2SiC2SiMe3) with Me3SiC≡C-C≡CSiMe3 was investigated. [10] Surprisingly, no simple substitution or coupling products were found, but a dinuclear titanocene (III) complex [Cp2Ti-C≡CSiMe3]2 having two σ,π-bonded alkynyl groups bridging the titanium centers (Scheme 13). This was formed by a very unusual C-C bond cleavage of the starting 1,3-butadiyne in contrast to synthetically often used C-C coupling reactions of alkynyl groups. [10]
Scheme 13. Reaction of Cp2Ti(η2-Me2SiC2SiMe3) and Me3SiC≡C-C≡C-SiMe3 with C-C bond cleavage. 2.5.2. Systematic investigations of this reactivity As reactions of electron rich Ni(0) alkyne L2Ni(RC2R) [2a-j] and 1,3-butadiyne complexes L2Ni(RC2-C2R) were known, [2k,l] an extension of the above mentioned results for Ti from alkynes RC≡CR to 1,3-butadiynes RC≡C-C≡CR regarding the influence of ligands L (L = 9
R3P, py etc.) and the substrate substituents R (R = Me3Si, aryl, alkyl etc.) was investigated these late transition metals fragments. However, no surprising results were obtained. [10c,d] Some other unexpected reactions were found only with di- or polyynes and group 4 metallocene complexes. Depending on the ligands L (Cp’ = Cp, Cp* etc.), metals M (M = Ti, Zr, Hf) and the substrate substituents R (R = Me3Si, aryl, alkyl etc.) 1,3-butadiynes RC≡CC≡CR react with [Cp’2M] in a different fashion. 2.6. Complexation to five-membered metallacylocumulenes 2.6.1. Coordination to metallacycles Using tBuC≡C-C≡CtBu in the reaction with Cp2Ti(η2-Me2SiC2SiMe3) instead of Me3SiC≡CC≡CSiMe3 a complexation of the intact diyne was observed to yield a highly unusual titanacyclocumulene (Scheme 14). [11]
Scheme 14. Reaction of Cp2Ti(η2-Me2SiC2SiMe3) and tBuC≡C-C≡CtBu to give a fivemembered titanacyclocumulene. 2.6.2. Reactions via the formed metallacycles Further unexpected products were found in reactions of [Cp’2M] with 1,3-butadiynes S-C≡CC≡C-S such as complexation, coupling and cleavage by using different ligands L (Cp’ = Cp, Cp* etc.), metals M (M = Ti, Zr, Hf) and substrate substituents R (R = Me3Si, aryl, alkyl etc.) (Scheme 15). [5]
Scheme 15. Products from reactions of group 4 metallocenes and 1,3-butadiynes. 2.7. C-C Bond coupling to five-membered metallacyclocumulenes 10
2.7.1. The coupling reaction A large number of group 4 metallocene bis(alkynyl) compounds Cp’2M(-C≡CR)2 exist and were described in detail in many papers. Several reactions for these compounds were published, but the potential of these complexes to form five-membered metallacyclocumulenes under the influence of sunlight was unknown. Exposure of complexes Cp*2Zr(-C≡CR)2 (R = Ph, Me3Si, Me etc.) to sunlight resulted in the formation of fivemembered metallacyclocumulenes Cp*2Zr(η4-RC4R) in high yields by coupling of the two alkynyl groups (Scheme 16). [12]
Scheme 16. Reactions of bis(alkynyl) complexes Cp*2Zr(-C≡CR)2 to five-membered metallacyclocumulenes Cp*2Zr(η4-RC4R) under the influence of sunlight. 2.7.2.
Combinations of coupling and cleavage reactions
This coupling reaction (Scheme 16) was total unexpected and only possible to recognice by the knowledge of the characteristics of the metallacyclocumulenes, which were not known before these investigations and are the result of the reverse reaction of C-C cleavage (Scheme 13) of 1,3-butadiynes to alkynyl groups. The metallacyclocumulenes represent intermediates for the cleavage as well as the coupling reactions because the coordination of the internal double bond to the metal can be exchanged by an external one and as a result of these precesses cleaved or coupled C4-units were obtained, depending on different L, M and R (Schemes 15 and 17). [5]
Scheme 17. Metallacyclocumulenes as intermediates for cleavage as well as coupling reactions. 11
Such products were realized and possible for some early-late heterometallic combinations as well. [5] A synthetic application for a combination of the cleavage and the coupling steps was found in an unusual single bond metathesis reaction. Irradiation (ν = 390-450 nm) of a mixture of the symmetrically substituted 1,3-butadiynes Me3SiC≡C-C≡CSiMe3 and tBuC≡C-C≡CtBu in the presence of an excess of the [Cp2Ti] source Cp2Ti(η2-Me2SiC2SiMe3) at 100°C, followed by oxidative workup gave the unsymmetrically substituted 1,3-butadiyne tBuC≡C-C≡CSiMe3 as a result of C-C single bond metathesis (Scheme 18). [13] Me3Si
SiMe3
2 Me3Si
+ tBu
1) 4 [Cp2Ti], h 2) AgSO3CF3 tBu
tBu
Scheme 18. C-C single bond metathesis of symmetricaly substituted 1,3-butadiynes. The single steps for the reaction of [Cp2Ti] with Me3SiC≡C-C≡CSiMe3 and tBuC≡C-C≡CtBu include the formation of different unstable titanocene mono-alkynyl complexes [Cp2Ti-C≡CSiMe3] and [Cp2Ti-C≡C-tBu] as intermediates (Scheme 19). These products could recombine in a productive way to unsymmetrical or an unproductive way to symmetrical dinuclear species, giving either the starting or the new 1,3-butadiynes. [13]
Scheme 19. Photocatalytic cleavage and recombination of 1,3-butadiynes. The dinuclear intermediates were isolated in separate reactions and their formation is possible via formation of the five-membered titanacyclocumulenes (Scheme 14). 2.8. Sliding of group 4 metallocenes along polyynes 2.8.1.
Metallocene sliding
12
If polyynes R-(C≡C)n-R instead of 1,3-butadiynes were used in reactions with [Cp’2M] in principle the same reactions are indicated as found for the 1,3-butadiynes. Additionally an unexpected motion (also referred to as “sliding”, “migration” or “tobogganing”) of the metallocene along the backbone of triynes and polyynes was observed (Scheme 20), giving similar products as found for the butadiynes. [14]
Scheme 20. Moving (“sliding”, “migration” or “tobogganing”) of [Cp*2Zr] along the backbone of a triyne. 2.8.2.
Applications of metallocene sliding
More or less stable five membered zirconacyclocumulenes and three membered zirconacyclopropenes participate as intermediates during these processes. It is clear, that these species could be trapped in some reactions, leading to selectivities for different products, which again depend on changed ligands L (Cp’ = Cp, Cp* etc.), metals M (M = Ti, Zr, Hf) and substrate substituents R (R = Me3Si, aryl, alkyl etc.). An extension from 1,3-butadiyne to 1,3,5-hexatriyne and 1,3,5,7-octatetrayne as well as branched polyynes to other metals like vanadium and was also described (Scheme 21). [5, 15] Characterisation of the isolated obtained products and its dynamics provided insight into the overall migration mechanisms.
Scheme 21. Complexes formed by coordination of 1,3,5,7-octatetrayne to vanadocene.
3.
Conclusion
During the investigations of group 4 metallocene bis(trimethylsilyl)acetylene complexes accidentally several unexpected reactions were found. The main and most important reaction was the generation of the very reactive, coordinatively and electronically unsaturated complex 13
fragments [Cp’2M], starting from Cp’2M(L)(η2-btmsa) complexes, leading to a large number of novel stoichiometric and catalytic reactions. One example was the reaction of Cp2Ti(η2btmsa) with carbon dioxide, giving a coupling with Me3SiC≡CSiMe3 to a dinuclear complex with relevance to the general mechanism of CO2 coupling reactions. This is restricted only for Cp ligands and not for Cp*2Ti(η2-btmsa), giving without alkyne coupling a dinuclear carbonate bridged complex together with a mononuclear bis-carbonyl complex Cp*2Ti(CO)2. The Cp ligands in [Cp2Ti] were not always inert, showing C-C coupling reactions with pyridyl substituted alkynes or Cp ring opening reactions with C-C coupling reactions of selected di- or polyynes. 1,3-butadiynes gave with [Cp’2M] either a C-C bond cleavage of the internal C-C single bond or intact complexation to unusual five-memberd metallacyclocumulenes depending on the metals, the Cp’ ligands and the substituents of the diynes. The reverse reaction of C-C coupling of alkynyl groups of bis(alkynyl) complexes to coordinated 1,3-butadiynes in metallacyclocumulenes was described, too. With triynes a sliding of the metallocene at the triple bonds was observed. For all cases several synthetic applications of the primary unexpected result were described. Only one example for these developments was selected and explained, giving an impression about the consequences of each of these results. One of the driving forces for all cases was only to broaden the knowledge by comparison of different, related types of organometallic systems. In a nutshell, fundamental research has opened the door for novel reaction types that led to a better understanding of the organometallic chemistry of metallocenes. It is worth to mention, that these results were obtained rather by coincidence without any planning, but with a clear analytical view on the experimental data. The examples described in this account serve as examples for this research philosophy. Acknowledgements I would like to particularly thank all my former PhD students, postdocs, assistents, cooperation partners and other people whose names are mentioned in the list of references for all of her excellent scientific results, giving the basis of this minireview. In particular I have to thank Mark E. Vol’pin (passed away in 1996), Vladimir B. Shur and Vladimir V. Burlakov from INEOS Moscow for their activities in this field of this chemistry as well as the cooperation during the last decades. Scientific discussions with PD Dr. Torsten Beweries are gratefully acknowledged.
References 1. (a) C. Elschenbroich, “Organometallchemie”, Eds. C. Elschenbroich, F. Hensel, H. Hopf, Teubner, 4. Aufl. 2003; (b) H. Werner, “Landmarks in Organo-Transition Metal Chemistry, A Personal View”; Ed. J. P. Fackler, Springer Science + Business Media, LLC 2009; (c) D. Steinborn, “Fundamentals in Organometallic Catalysis”, WILEY-VCH 2012; (d) T. J. Kealy, P.L. Pauson, Nature 168 (1951) 1039-1040; (e) E. O. Fischer, R. Jira, J. Organomet. Chem. 637-639 (2001) 7-12, and refs. cited therein; (f) G. Wilkinson, , J. Organomet. Chem. 100 (1975) 273-278, and refs. cited therein; (g) K. Ziegler, E. Holzkamp, 14
H. Breil, H. Martin, Angew. Chem. 67 (1955) 541-547, and refs. cited therein; (h) G. Wilke, Angew. Chem. 75 (1963) 10-20, and refs. cited therein; (i) D. S. Breslow, N. R. Newburg, J. Am. Chem. Soc. 79 (1957) 5072-5073; (j) G. Natta, P. Pino, G. Mazzanti, U. Giannini, J. Am. Chem. Soc. 79 (1957) 2975-2976; (k) H. Sinn, W. Kaminsky, Adv. Organomet. Chem. 18 (1980) 99-149, and refs. cited therein; (l) H. Schnutenhaus, H. H. Brintzinger, Angew. Chem. Int. Ed. Engl. 18 (1979) 777-778; (m) J. A. Smith, H. H. Brintzinger, J. Organomet. Chem. , 218 (1981) 159-167; (n) H. H. Brintzinger, D. Fischer, R. Mülhaupt, B. Rieger, R. M. Waymouth, Angew. Chem. Int. Ed. Engl. 34 (1995) 1143-1170, and refs. cited therein; (o) W. Keim, Angew. Chem. Int. Ed. 52 (2013) 12492-12496. 2. (a) U. Rosenthal, Z. Anorg. Allg. Chem. 482 (1981) 179; (b) U. Rosenthal, Z. Anorg. Allg. Chem. 513 (1984) 151; (c) U. Rosenthal, W. Schulz, Z. Chem. 25 (1985) 227; (d) U. Rosenthal, W. Schulz, J. Organomet. Chem. 321 (1987) 103; (e) U. Rosenthal, W. Schulz, H. Görls, Z. Anorg. Allg. Chem. 550 (1987) 169; (f) U. Rosenthal, H. Görls, J. Organomet. Chem. 348 (1988) 135; (g) U. Rosenthal, G. Oehme, H. Görls, V. V. Burlakov, A. I. Yanovsky, Yu. T. Struchkov, J. Organomet. Chem. 389 (1990) 251; (h) U. Rosenthal, G. Oehme, H. Görls, V. V. Burlakov, A. I. Yanovsky, Yu. T. Struchkov, J. Organomet. Chem. 389 (1990) 409; (i) U. Rosenthal, G. Oehme, H. Görls, V. V. Burlakov, A. I. Yanovsky, Yu. T. Struchkov, J. Organomet. Chem. 390 (1990) 113; (j) U. Rosenthal, A. Ohff, W. Baumann, A. Tillack, H. Görls, V. V. Burlakov, V. B. Shur, J. Organomet. Chem. 484 (1994) 203-207; (k) U. Rosenthal, S. Pulst, P. Arndt, W. Baumann, A. Tillack, R. Kempe, Z. Naturforsch. 50b (1995) 377-384; (l) U. Rosenthal, S. Pulst, P. Arndt, W. Baumann, A. Tillack, R. Kempe, Z. Naturforsch. 50b (1995) 368-376. 3. (a) U. Rosenthal, C. Nauck, P. Arndt, S. Pulst, W. Baumann, V. V. Burlakov, H. Görls, J. Organomet. Chem. 484 (1994) 81-87; (b) U. Rosenthal, G. Oehme, V. V. Burlakov, P. V. Petrovskii, M. E. Vol'pin, J. Organomet. Chem. 391 (1990) 119. 4. Substitution: (a) V. V. Burlakov, U. Rosenthal, P. V. Petrovskii, V. B. Shur, M. E. Vol'pin, Organomet. Chem. USSR 1 (1988) 526; (b) V. V. Burlakov, U. Rosenthal, R. Beckhaus, Yu. T. Struchkov, G. Oehme, V. B. Shur, M. E. Vol'pin, Organomet. Chem. USSR 3 (1990) 237; (c) U. Rosenthal, H. Görls, V. V. Burlakov, V. B. Shur, M. E. Vol'pin, J. Organomet. Chem. 426 (1992) C53-C59. 5. Applications of substitution: (a) A. Ohff, S. Pulst, C. Lefeber, N. Peulecke, P. Arndt, V. V. Burlakov, U. Rosenthal, Synlett (1996), 111-118; (b) U. Rosenthal, P.-M. Pellny, F. G. Kirchbauer, V. V. Burlakov, Acc. Chem. Res. 33 (2000) 119-129; (c) U. Rosenthal, V. V. Burlakov in Titanium and Zirconium in Organic Synthesis, Ed. I. Marek, Wiley-VCH, Weinheim (2002) 355-389; (d) U. Rosenthal, Angew. Chem. 115 (2003) 1838-1842; Angew. Chem., Int. Ed. 42 (2003) 1794-1798; (e) U. Rosenthal, V. V. Burlakov, P. Arndt, W. Baumann, A. Spannenberg, Organometallics 22 (2003) 884-900; (f) U. Rosenthal, Angew. Chem. 116 (2004) 3972-3977; Angew. Chem., Int. Ed. 43(2004) 3882-3887; (g) U. Rosenthal in Acetylene Chemistry - Chemistry, Biology, and Material Science, Eds. F. Diederich, P. Stang and R. R. Tykwinski, Wiley-VCH, Weinheim (2004) 139-171 (h) U. Rosenthal, V. V. Burlakov, P. Arndt, W. Baumann, A. Spannenberg, Eur. J. Inorg. Chem. 22 (2004) 4739-4932; (i) U. Rosenthal, V. V. Burlakov, P. Arndt, W. Baumann, A. Spannenberg, Organometallics 24 (2005) 456-471; (j) U. Rosenthal, V. V. Burlakov, M. A. Bach, T. Beweries, Chem Soc. Rev. 36 (2007) 719-728; (k) U. Rosenthal, Angew. Chem. 15
120 (2008) 5196-5199; Angew. Chem., Int. Ed. 47 (2008) 5118-5121; (l) U. Rosenthal, V. V. Burlakov, P. Arndt, A. Spannenberg, U. Jäger-Fiedler, M. Klahn, M. Hapke, Activating Unreactive Bonds, Eds. C. Bolm, F. E. Hahn, Wiley-VCH, Weinheim (2009) 165 pp; (m) T. Beweries, U. Rosenthal, Science of Synthesis 4 (2011) Recent updates in the organometallic chemistry of group 4 metallocene complexes with bis(trimethylsilyl)acetylene, (n) T. Beweries, M. Hähnel, U. Rosenthal, Catal. Sci. & Technol. 3 (2013) 18-28; (o) T. Beweries, U. Rosenthal, Nature Chemistry 5 (2013) 649; and refs. cited therein. (p) S. Roy, U. Rosenthal, E. D. Jemmis; Acc Chem. Res. 47 (2014) 2917-2930; (q) U. Rosenthal, Izv. Akad. Nauk, Ser. Khim. (2014) 2577-2582, Engl. Transl.: Russ. Chem. Bull. (2014) 25772582, (r) L. Becker, U. Rosenthal, Coord. Chem. Rev. 345 (2017) 137-149, (s) U. Rosenthal, Angew. Chem. 130 (2018) 14932-14950; Angew. Chem., Int. Ed. 57 (2018) 14718-14735; (t) U. Rosenthal, Eur. J. Inorg. Chem. (2019) 895-919; (u) U. Rosenthal, ChemOpen 8 (2019) 1036-1047; (v) G. R. Kiel, M. S. Ziegler, T. D. Tilley, Angew. Chem. 129 (2017) 4917-4922; Angew. Chem. Int Ed. 56 (2017) 4839-4922; (w) S. Urrego-Riveros, I.-M. Ramirez y Medina, F. D. Sönnichsen, A. Staubitz, Chem. Eur. J. 25 (2019) 13318-13328, and refs cited therein; (x) D. Heitkemper, C. P. Sindlinger, Chem. Eur. J. 25 (2019) 66286637. 6. Carbon dioxide reactions: (a) V. V. Burlakov, U. Rosenthal, A. I. Yanovsky, Yu. T. Struchkov, O. G. Ellert, V. B. Shur, M. E. Vol'pin, Organomet. Chem. USSR 2 (1989) 1193; (b) V. V. Burlakov, U. Rosenthal, F. M. Dolgushin, A. I. Yanovski, Yu. T. Struchkov, O. G. Ellert, V. B. Shur, M. E. Vol'pin, Metalloorg. Khim. 5 (1992) 1213-1214 (russ.); 7. Applications of carbon dioxide reactions: (a) U. Rosenthal, A. Ohff, M. Michalik, H. Görls, V. V. Burlakov, V. B. Shur, Organometallics 12 (1993) 5016-5019; (b) C. Lefeber, A. Ohff, A. Tillack, W. Baumann, R. Kempe, V. V. Burlakov, U. Rosenthal, H. Görls, J. Organomet. Chem. 501 (1995) 179-188; (c) V. V. Burlakov, F. M. Dolgushin, A. I. Yanovsky, Yu. T. Struchkov, V. B. Shur, U. Rosenthal, U. Thewalt, J. Organomet. Chem. 522 (1996) 241-247; (d) V. V. Burlakov, V. V., A. I. Yanovsky, Yu. T. Struchkov, V. B. Shur, O. G. Ellert, U. Rosenthal, J. Organomet. Chem. 542 (1997) 105-112; (e) S. Pulst, U. Rosenthal, A. Spannenberg, R. Kempe, Z. Krist. 213 (1997) 215-216; (f) D. Thomas, W. Baumann, A. Spannenberg, R. Kempe, U. Rosenthal, Organometallics 17 (1998) 2096; (g) D. Thomas, N. Peulecke, V. V. Burlakov, W. Baumann, A. Spannenberg, R. Kempe, U. Rosenthal, Eur. J. Inorg. Chem. (1998) 1495-1502; (h) R. Kempe, A. Spannenberg, N. Peulecke, U. Rosenthal, Z. Krist. 213 (1998) 793-794; (i) P.-M. Pellny, V. V. Burlakov, W. Baumann, A. Spannenberg, U. Rosenthal, Z. Anorg. Allg. Chem. 625 (1999) 910-918; (j) A. Spannenberg, T. Zippel, V. V. Burlakov, U. Rosenthal, Z. Krist. 215 (2000) 367-368; (k) F. G. Kirchbauer, P.-M. Pellny, V. V. Burlakov, P. Arndt, W. Baumann, A. Spannenberg, U. Rosenthal, Organometallics 20 (2001) 5289-5296; (l) H. Sun, A. Spannenberg, V. V. Burlakov, W. Baumann, P. Arndt, U. Rosenthal, Z. Kristallogr. 217 (2002) 241-243; (m) V. V. Burlakov, P. Arndt, W. Baumann, A. Spannenberg, U. Rosenthal, Organometallics 25 (2006) 1317-1320. 8. Cp reactions: (a) A. Tillack, W. Baumann, A. Ohff, C. Lefeber, A. Spannenberg, U. Rosenthal, J. Organomet. Chem. 520 (1996) 187-193; (b) P.-M. Pellny, N. Peulecke, V. V. Burlakov, A. Tillack, W. Baumann, A. Spannenberg, R. Kempe, U. Rosenthal, Angew. Chem. 109 (1997) 2728-2730; Angew. Chem. Int. Ed. Engl. 36 (1997) 2515-2617. 16
9. Applications of Cp reactions: (a) Z. Xi, K. Sato, Y. Gao, J. Lu, T. Takahashi, J. Am. Chem. Soc. 125 (2003) 9568-9569; (b) T. Takahashi, Y. Kuzuba, F. Kong, K. Nakajima, Z. Xi; J. Am. Chem. Soc. 127 (2005) 17181-17189; (c) T. Takahashi, Z. Song, K. Sato, Y.; Kuzuba, K. Nakajima, K. Kanno, J. Am. Chem. Soc. 129 (2007) 11678-11679; (d) T. Takahashi, Z. Song, Y.-F. Hsieh, K. Nakajima, K. Kanno, J. Am. Chem. Soc. 130 (2008) 15236-15237; (e) Y. Mizukami, H. Li, K. Nakajima, Z. Song, T. Takahashi, Angew. Chem. Int. Ed. 53 (2014) 8899-8903; (f) Z. Song, Y.-F. Hsieh, K. Nakajima, K. Kanno, T. Takahashi, Organometallics 35 (2016) 1092-1097; (g) M. Bando, Y. Mizukami, K. Nakajima, Z. Song, T. Takahashi, Dalton Trans. 46 (2017) 16408-16411; (h) M. Bando, K. Nakajima, Z. Song, T. Takahashi, Organometallics 38 (2019) 731-734. 10. Butadiyne C-C bond cleavage: (a) U. Rosenthal, H. Görls, J. Organomet. Chem. 439 (1992) C36-C41; (b) U. Rosenthal, A. Ohff, W. Baumann, R. Kempe, A. Tillack, V. Burlakov, Organometallics 13 (1994) 2903-2906. (c) U. Rosenthal, S. Pulst, P. Arndt, W. Baumann, A. Tillack, R. Kempe, Z. Naturforsch. 50b (1995) 377-384; (d) U. Rosenthal, S. Pulst, P. Arndt, W. Baumann, A. Tillack, R. Kempe, Z. Naturforsch. 50b (1995) 368-376. 11. Butadiyne coordination: (a) U. Rosenthal, A. Ohff, W. Baumann, R. Kempe, A. Tillack, V. V. Burlakov, Angew. Chem. 106 (1994) 1678-1680, Angew. Chem. Int. Ed. Engl. 33 (1994) 1605-1607; (b) V. V. Burlakov, A. Ohff, C. Lefeber, A. Tillack, W. Baumann, R. Kempe, U. Rosenthal, Chem. Ber. 128 (1995) 967-971. 12. Butadiyne formation: P. M. Pellny, F. G. Kirchbauer, V. V. Burlakov, W. Baumann, A. Spannenberg, U. Rosenthal, J. Am. Chem. Soc. 121 (1999) 8313-8323. 13. Applications of butadiyne reactions: S. Pulst, F. G. Kirchbauer, B. Heller, W. Baumann, U. Rosenthal, Angew. Chem. 110 (1998) 2029-2031, Angew. Chem. Int. Ed. Engl. 37 (1998) 1915-1927. 14. Metallocene sliding: P. M. Pellny, V. V. Burlakov, P. Arndt, W. Baumann, A. Spannenberg, U. Rosenthal, J. Am. Chem. Soc. 122 (2000) 6317-6318. 15. Applications of metallocene sliding: (a) C. Danjoi, J. Zhao, B. Donnadieu, J.-P. Legros, L. Valade, R. Choukroun, A. Zwick, P. Cassoux, Chem. Eur. J. 4 (1998) 1100-1105; (b) R. Choukroun, P. Cassoux, Acc. Chem. Res. 32 (1999) 494-502; (c) R. Choukroun, C. Lorber, B. Donnadieu, B. Henner, R. Frantz, C. Guerin, Chem Commun. (1999) 1099-1100; (d) R. Choukroun, B. Donnadieu, C. Lorber, P.-M. Pellny, W. Baumann, U. Rosenthal; Chem. Eur. J. 6 (2000) 4505-4509.
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Highlight (for review) Serendipity in Unexpected Reactions of Group 4 Metallocene Bis(trimethylsilyl)acetylene Complexes and its Consequences for Selected Applications Prof. U. Rosenthal* Leibniz Institute for Catalysis at the University of Rostock, Albert-Einstein-Str. 29A, 18059 Rostock (Germany), E-mail: [email protected] ORCID https://orcid.org/0000-0003-1922-6782
This paper is written on the basis of accumulated knowledge about Group 4 metallocene bis(trimethylsilyl)acetylene complexes. Selected examples of this minireview are all unexpected results and should help for a better general understanding of this chemistry, and here in particular of Organometallic Chemistry. Several unexpected reactions of group 4 metallocene bis(trimethylsilyl)acetylene complexes are summarized. (i) By substitution of the alkyne in Cp2Ti(η2-btmsa) and similar complexes the very reactive coordinatively and electronically unsaturated complex fragments [Cp’2M] were generated and used for many purposes. (ii) The reaction of this complex with carbon dioxide gave a coupling of the alkyne to an unexpected dinuclear complex with relevance to the general mechanism of CO2 coupling reactions. (iii) The Cp ligands in [Cp2Ti] are not inert, showing ring opening reactions together with C-C coupling reactions of diynes. Depending on the ligands L (Cp’ = Cp, Cp* etc.), metals M (M = Ti, Zr, Hf) and the substrate substituents S (S = Me3Si, aryl, alkyl etc.) 1,3-butadiynes S-C≡C-C≡C-S react with [Cp’2M] under C-C bond cleavage of the internal C-C single bond (iv), through complexation to unusual metallacyclocumulenes (v) or by C-C coupling of alkynyl groups to coordinated 1,3butadiynes in metallacyclocumulenes. (vi) Polyynes show a sliding of the [Cp’2M] unit at the triple bonds. (vii) All these unexpected examples (i) - (vii) represent reactions that were found by coincidence. Later several applications for the initial results were described and listed in this minireview. The results lead to the following conclusions: Main and most important reaction was the generation of the very reactive, coordinatively and electronically unsaturated complex fragments [Cp’2M], starting from Cp’2M(L)(η2-btmsa) complexes, leading to a large number of novel stoichiometric and catalytic reactions. One example was the reaction of Cp2Ti(η2btmsa) with carbon dioxide, giving a coupling with Me3SiC≡CSiMe3 to a dinuclear complex with relevance to the general mechanism of CO2 coupling reactions. This is restricted only for Cp ligands and not for Cp*2Ti(η2-btmsa), giving without alkyne coupling a dinuclear carbonate bridged complex together with a mononuclear bis-carbonyl complex Cp*2Ti(CO)2. The Cp ligands in [Cp2Ti] were not always inert, showing C-C coupling reactions with pyridyl substituted alkynes or Cp ring opening reactions with C-C coupling reactions of 1
selected di- or polyynes. 1,3-butadiynes gave with [Cp’2M] either a C-C bond cleavage of the internal C-C single bond or intact complexation to unusual five-memberd metallacyclocumulenes depending on the metals, the Cp’ ligands and the substituents of the diynes. The reverse reaction of C-C coupling of alkynyl groups of bis(alkynyl) complexes to coordinated 1,3-butadiynes in metallacyclocumulenes was described, too. With triynes a sliding of the metallocene at the triple bonds was observed. For all cases several synthetic applications of the primary unexpected result were described. Only one example for these developments was selected and explained, giving an impression about the consequences of each of these results. One of the driving forces for all cases was only to broaden the knowledge by comparison of different, related types of organometallic systems. In a nutshell, fundamental research has opened the door for novel reaction types that led to a better understanding of the organometallic chemistry of metallocenes. It is worth to mention, that these results were obtained rather by coincidence without any planning, but with a clear analytical view on the experimental data. The examples described in this account serve as examples for this research philosophy.
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Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: