Uwe Rosenthal

Five-membered metallacycles of titanium and zirconium – attractive compounds for organometallic chemistry and catalysis

In these days a renaissance of metallacycles as an increasingly important class of organometallic compounds for synthetic and catalytic applications is evident, making such very attractive for a plethora of investigations. Titanocene and zirconocene bis(trimethylsilyl)acetylene complexes, regarded as three-membered metallacycles (1-metallacyclopropenes), present a rich chemistry towards unsaturated molecules. By elimination of the alkyne these complexes form by reaction with unsaturated compounds five-membered titana- and zirconacycles, all of which are relevant to stoichiometric and catalytic C-C coupling and cleavage reactions of unsaturated molecules.

An Alternative Mechanistic Concept for Homogeneous Selective Ethylene Oligomerization of Chromium‐Based Catalysts: Binuclear Metallacycles as a Reason for 1‐Octene Selectivity?

An alternative concept for the selective catalytic formation of 1-octene from ethylene via dimeric catalytic centers is proposed. The selectivity of the tetramerization systems depends on the capability of ligands to form binuclear complexes that subsequently build up and couple two separate metallacyclopentanes to form 1-octene selectively. Comparison of existing catalytic processes, the ability of the bis(diarylphosphino)amine (PNP) ligand to bridge two metal centers, and the experimental background support the proposed binuclear mechanism for ethylene tetramerization.

Intramolekulare Cyclisierung von terminal disubstituierten α, ω-Diinen an Titanocen “Cp2Ti” mit einer nachfolgenden, ungewöhnlichen Cp-ringöffnung und neuen intramolekularen CC-Knüpfung

Abstract The reaction of Cp 2 Ti(Me 3 SiC 2 SiMe 3 ) ( 1 ) with terminal disubstituted α,ω-diynes RC≡C(CH 2 ) n C≡CR affords, after substitution of Me 3 SiC 2 SiMe 3 , bicyclic titanacyclopentadienes via intramolecular cyclization. The stability of the obtained products 2, 3 and 5 is determined by the spacer length ( n = 2, 4, 5, 6). The four-membered ring derivatives ( n = 2) 2a and 2b were obtained in good yield. In the case of n = 4 the bicyclic six-membered ring 3 was formed at first, which rearranges to a stable tricyclic η 4 : η 3 -dihydroindenyl-Ti complex 4 by Cp cleavage and intramolecular CC coupling. Complex 4 was characterized by X-ray structure analysis and NMR spectroscopy. An increase of spacer length ( n > 4) provides indefinable secondary and decomposition products.

Regioselektive reaktionen der fremdligandfreien titanocen-alkin-komplexe Cp2Ti(RC2SiMe3) (R = Me3Si,Ph, tBu, nBu)☆

Abstract Depending on different substituents in the reaction of Cp 2 TiCl 2 with magnesium and the alkynylsilanes RC≡CSiMe 3 (R = SiMe 3 , Ph, t Bu, n Bu, n Pr, Me) in tetrahydrofuran, titanacyclopropenes (R = SiMe 3 , Ph, t Bu 1 , n Bu 2 ), symmetrical substituted titanacyclopentadienes (R = Me 5 ) or in a competition reaction both types of complexes (R = n Pr 3 and 4 ) were obtained. The compound Cp 2 Ti( t BuC 2 SiMe 3 ) 1 is the first example of a titanocene complex with an alkyl substituted alkyne without further ligands and was characterized by X-ray crystal structure analysis. The structural and spectroscopical data of 1 were compared with those of other well known complexes of that type, e.g. Cp 2 Ti(Me 3 SiC 2 SiMe 3 ) and Cp 2 Ti(PhC 2 SiMe 3 ) to investigate the influence of different substituents ( t Bu, SiMe 3 , Ph) upon alkyne complexation. The chemo- and regio-selectivities of the obtained alkyne complexes was studied in reactions with alkynes, alcohols, carbon dioxide and acetone. The reaction course depends mostly on steric restrictions, being in the first step kinetically favored at the Si-substituted C-atom of the alkyne and giving β-SiMe 3 -substituted products, which rearrange in some cases into the thermodynamically more stable α-SiMe 3 -substituted products.

Reactions of group 4 metallocene alkyne complexes with carbodiimides: experimental and theoretical studies of the structure and bonding of five-membered hetero-metallacycloallenes.

The reaction of the low-valent metallocene(II) sources Cp(2)Ti(η(2)-Me(3)SiC(2)SiMe(3)) (7) and Cp(2)Zr(py)(η(2)-Me(3)SiC(2)SiMe(3)) (11, Cp = η(5)-cyclopentadienyl, py = pyridine) with carbodiimides RN═C═NR (R = Cy, i-Pr, p-Tol) leads to the formation of five membered hetero-metallacycloallenes Cp(2)M{Me(3)SiC═C═C[N(SiMe(3))(R)]-N(R)} (9M-R) (M = Ti, R = i-Pr; M = Zr, R = Cy, i-Pr, p-Tol). Elimination of the alkyne (as the hitherto known reactivity of titanocene and zirconocene alkyne complexes would suggest) was not observed. The molecular structures of the obtained complexes were confirmed by X-ray studies. Moreover, the structure and bonding of the complexes 9Zr-Cy and 9Zr-p-Tol was investigated by DFT calculations.

Titanocene and zirconocene σ-alkynyl complexes in CC single bond coupling and cleavage reactions

Group 4 metallocene mono- and bis-σ-alkynyl complexes of the type L2M(σ-CCR) and L2M(σ-CCR)2 with M=titanium and zirconium in the oxidation states +3 and +4 and L=Cp (η5-cyclopentadienyl) and Cp* (η5-pentamethylcyclopentadienyl) are important compounds for stoichiometric and catalytic CC single bond coupling and cleavage reactions. Detailed investigations show five-membered metallacyclocumulenes L2M(η4-1,2,3,4-RC4R) as the key intermediates in both reactions of a CC single bond cleavage of different 1,4-substituted 1,3-butadiynes RCCCCR to alkynyl groups and the opposite reaction of CC single bond formation starting from alkynyl groups under the formation of 1,4-substituted 1,3-butadiynes. Depending on different metals M and ligands L, coupling or cleavage is favoured. Combination of both reactions offered the first CC single bond metathesis in homogeneous solution, which is photocatalyzed and titanocene-mediated. It proceeds via titanocene–mono-alkynyl complexes, which are interesting species also for other stoichiometric and catalytic CC coupling reactions. Some similarities regarding the σ-to-π conversion exist between the coupling of the alkynyl groups at titano- and zirconocenes to complexed 1,3-butadiynes on one side and the coupling of phenyl groups at chromium to complexed diphenyl on the other side.

Recent advances in the chemistry of heterometallacycles of group 4 metallocenes

This perspective gives an overview of recent developments in the chemistry of heterometallacycles of group 4 metallocenes. This class of compounds has been known for several decades now, however, in recent years significant efforts were made in order to further explore the structural scope of these complexes and their potential in catalytic applications. This overview shows some special heterometallacycles which could be of great interest for theoretical and synthetic chemists, owing to their unusual structural and bonding parameters as well as their applicability in a wide range of synthetic and catalytic transformations.

Activation of Small Molecules by Phosphorus Biradicaloids

The reactivity of biradicaloid [P(μ-NTer)]2 was employed to activate small molecules bearing single, double, and triple bonds. Addition of chalcogens (O2 , S8 , Sex and Tex ) led to the formation of dichalcogen-bridged P2 N2 heterocycles, except from the reaction with molecular oxygen, which gave a P2 N2 ring featuring a dicoordinated P(III) and a four-coordinated P(V) center. In formal [2πe+2πe] addition reactions, small unsaturated compounds such as ethylene, acetylene, acetone, acetonitrile, tolane, diphenylcarbodiimide, and bis(trimethylsilyl)sulfurdiimide are readily added to the P2 N2 heterocycle of the biradicaloid [P(μ-NTer)]2 , yielding novel heteroatom cage compounds. The synthesis, reactivity, and bonding of the biradicaloid [P(μ-NTer)]2 were studied in detail as well as the synthesis, properties, and structural features of all addition products.

Si–H Activation in Titanocene and Zirconocene Complexes of Alkynylsilanes RC≡CSiMe2H (R=tBu, Ph, SiMe3, SiMe2H): A Model To Understand Catalytic Reactions of Hydrosilanes

An agostic interaction between the Si–H bond and the metal center (depicted on the right) is the characteristic feature of the title complexes, which could be prepared by acetylene exchange reactions. IR, NMR, and X-ray structural investigations reveal that the effect of the Si-H-metal interaction is considerably stronger at low temperatures and in the solid state. This mode of bond activation is important in active catalysts for hydrosilylation and dehydrogenative polysilane reactions.

Metallacyclocumulenes: A Theoretical Perspective on the Structure, Bonding, and Reactivity

Conspectus Transition metals help to stabilize highly strained organic fragments. Metallacycles, especially unsaturated ones, provide much variety in this area. We had a sustained interest in understanding new C-C bond formation reactions affected by binuclear transition metal fragments Cp2M. One such study led to the exploration of the bimetallic C-C cleavage and coupled complexes, where the acetylide ligands bridge two metal atoms. The underlying M-C interaction in these complexes inspired the synthesis of a five-membered cyclocumulene complex, which opened a new phase in organometallic chemistry. The metallacyclocumulene produces a variety of C-C cleavage and coupled products including a radialene complex. Group 4 metallocenes have thus unlocked a fascinating chemistry by stabilizing strained unsaturated C4 organic fragments in the form of five-membered metallacyclocumulenes, metallacyclopentynes, and metallacycloallenes. Over the years, we have carried out a comprehensive theoretical study to understand the unusual stability and reactivity of these metallacycles. The unique (M-Cβ) interaction of the internal carbon atoms with the metal atom is the reason for unusual stability of the metallacycles. We have also shown that there is a definite dependence of the C-C coupling and cleavage reactions on the metal of metallacyclocumulenes. It demonstrates unexpected reaction pathways for these reactions. Based on this understanding, we have predicted and unraveled the stabilization factors of an unusual four-membered metallacycloallene complex. Indeed, our prediction about a four-membered heterometallacycle has led to an interesting bonding situation, which is experimentally realized. This type of M-C bonding is intriguing from a fundamental perspective and has great relevance in synthesizing unusual structures with interesting properties. In this Account, we first give a short prologue of what led to the present study and describe the salient features of the structure and bonding of the metallacyclocumulenes. The unusual reaction pathway of this metallacycle is explored next. Similar features of the metallacyclopentynes and metallacycloallenes are briefly mentioned. Then, we discuss the exploitation of the unique M-C bonding to design some exotic molecules such as a four-membered metallacycloallene complex. Our efforts to build a conceptual framework to understand these metallacycles and to exploit their chemistry continue.