Catalysis of Early Transition Metals

Dr. Perdita Arndt

We focus on stoichiometric and catalytic reactions of metallocene complexes of the early transition metals titanium, zirconium and hafnium (group 4). On the basis of novel results in coordination and complex chemistry, catalytic reactions are developed to link basic research and applications.

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During the last years the synthesis and reactivity of unusual small metallacycles of the group 4 metals have been of great interest, in regard of their importance as intermediates for various catalytic applications. In our research all-C ring systems and their analogues heterocycles are in the focus. The formation and reactivity of three- and five-membered all-C metallacycles like metallacyclopropenes A and the unusual metallacyclocumulenes B, metallacyclopentynes C, metallacycloallenes D and metallacyclopentadienes E (Scheme 1) was extensively studied.[1]

Scheme 1. All-C metallacycles

The coordination chemistry of the silyl-substituted alkyne Me3Si-C≡C-SiMe3 towards group 4 metallocenes was widely explored. According to these resulted, we investigated reactions of different alkynes with heteroatoms (B, N, P, O, S) in a-position with titanocene and zirconocene complexes. This revealed that the nature of the heteroatom has a significant impact on the reactivity, thus leading to coordination, coupling, cleavage or dimerization. As a consequence of this, various products were isolated like novel three- and five-membered all-C cycles (Scheme 1, A and E) and unusual heterocycles (Scheme 2, G as example).[2,3]

Scheme 2. Selected 4-, and 5-membered heterometallacycles

When the P-substituted alkyne was used in the reaction with [Cp2Zr] a dimeric complex was isolated. In solution, NMR spectroscopic investigations revealed an equilibrium of this compound with a four-membered zirconacycle (Scheme 2, F).[4] Examples for such ring systems are very rare.

The synthesis of nitrogen containing heterocycles is under current investigation. This was realized by the coordination of 2-substituted pyridines to group 4 metallocenes[5] (Scheme 2, H) or by the reaction with nitriles or isonitriles. Thereby, an unusual coupling of two nitriles occurred, leading to the isolation of 1-metalla-2,5-diazacyclopenta-2,4-dienes in high yields (Scheme 2, I).[6-9]

Extending this method to dicyanocompounds afforded supramolecular structures by the reaction with [Cp*2Ti]. The size of the macrocycle depended on the choice of dicyanobenzenes or dicyanopyridine and on the position of the CN moieties (1,3 or 1,4) (Scheme 3).[9,10] The high potential of these new fascinating macrocycles was demonstrated by their catalytic activity in the ring-opening polymerization of e-caprolactone.[9]

Scheme 3. Macrocycles - synthesis and structure

[1] T. Beweries, U. Rosenthal, Science of Synthesis Knowledge Updates 2011/4 2012, Georg Thieme Verlag KG, Stuttgart, New York, 11-71.

[2] K. Altenburger, P. Arndt, A. Spannenberg, U. Rosenthal, Eur. J. Inorg. Chem. 2015, 44-48.

[3] K. Altenburger, W. Baumann, A. Spannenberg, P. Arndt, U. Rosenthal, Eur. J. Inorg. Chem. 2014, 5948-5957.

[4] M. Haehnel, S. Hansen, K. Schubert, P. Arndt, A. Spannenberg, H. Jiao, U. Rosenthal, J. Am. Chem. Soc. 2013, 135, 17556-17565.

[5] F. Reiß, K. Altenburger, L. Becker, K. Schubert, H. Jiao, A. Spannenberg, D. Hollmann, P. Arndt, U. Rosenthal, Chem. Eur. J., DOI: 10.1002/chem.201504411.

[6] L. Becker, P. Arndt, H. Jiao, A. Spannenberg, U. Rosenthal, Angew. Chem. 2013, 125, 11607-11611, Angew. Chem. Int. Ed. 2013, 52, 11396-11400.

[7] VIP: L. Becker, F. Strehler, M. Korb, P. Arndt, A. Spannenberg, W. Baumann, H. Lang, U. Rosenthal, Chem. Eur. J. 2014, 20, 3061-3068.

[8] L. Becker, P. Arndt, A. Spannenberg, U. Rosenthal, Chem. Eur. J. 2014, 20, 12595-12600.

[9] L. Becker, P. Arndt, A. Spannenberg, H. Jiao, U. Rosenthal, Angew. Chem. 2015, 127, 5614-5617; Angew. Chem. Int. Ed. 2015, 54, 5523-5526.

[10] G. Tomaschun, K. Altenburger, F. Reiß, L. Becker, A. Spannenberg, P. Arndt, H. Jiao, U. Rosenthal, Eur. J. Inorg. Chem. 2016, 272-280.