S.A. Then he spent three years at the Universita ¨t Stuttgart starting his "habilitation" on "surface organometallic chemistry on nanoporous materials", which he completed in 2000 at TUM. From 2005 until 2008, he held the position "Heterogeneous Catalysis" at the University of Bergen, Norway. He joined the faculty of the Eberhardt-Karls University Tu ¨bingen, Germany, in 2009. His research interests include organometallic chemistry, nanostructured materials, and catalysis.

The complexes [Ln(AlMe4)3] (Ln=Y, La, Ce, Pr, Nd, Sm, Ho, Lu) have been synthesized by an amide elimination route and the structures of [Lu{(micro-Me)2AlMe2}3], [Sm{(micro-Me)2AlMe2}3], [Pr{(micro-Me)2AlMe2}3], and [La{(micro-Me)2AlMe2}2{(micro-Me)3AlMe}] determined by X-ray crystallography. These structures reveal a distinct Ln3+ cation size-dependency. A comprehensive insight into the intrinsic properties and solution coordination phenomena of [Ln(AlMe4)3] complexes has been gained from extended dynamic 1H and 13C NMR spectroscopic studies, as well as 1D 89Y, 2D 1H/89Y, and 27Al NMR spectroscopic investigations. [Ce(AlMe4)3] and [Pr(AlMe4)3] have been used as alkyl precursors for the synthesis of heterobimetallic alkylated rare-earth metal complexes. Both carboxylate and siloxide ligands can be introduced by methane elimination reactions that give the heterobimetallic complexes [Ln{(O2CAriPr)2(micro-AlMe2)}2(AlMe4)(C6H14)n] and [Ln{OSi(OtBu)3}(AlMe3)(AlMe4)2], respectively. [Pr{OSi(OtBu)3}(AlMe3)(AlMe4)2] has been characterized by X-ray structure analysis. All of the cerium and praseodymium complexes are used as precatalysts in the stereospecific polymerization of isoprene (1-3 equivalents of Et2AlCl as co-catalyst) and compared to the corresponding neodymium-based initiators reported previously. The superior catalytic performance of the homoleptic complexes leads to quantitative yields of high-cis-1,4-polyisoprene (>98%) in almost all of the polymerization experiments. In the case of the binary catalyst mixtures derived from carboxylate or siloxide precatalysts quantitative formation of polyisoprene is only observed for nLn:nCl=1:2. The influence of the metal size is illustrated for the heterobimetallic lanthanum, cerium, praseodymium, neodymium, and gadolinium carboxylate complexes, and the highest activities are observed for praseodymium as a metal center in the presence of one equivalent of Et2AlCl.

Dedicated to Professor William J. Evans on the occasion of his 60th birthday Nature provides mankind with highly stereoregular polyterpenes, i.e., polymers of isoprene, featuring distinct properties.[1] Natural rubber (NR, caoutchouc or cis-1,4-polyisoprene, cPIP; > 99 % cis content, M n % 2 10 6 g mol À1 ) is the most important polymer produced by plants and is the raw material for numerous rubber applications. Taking into account new developments in synthetic polymer chemistry, a mechanism for living carbocationic polymerization has recently been proposed for NR biosynthesis.[2] Gutta-percha obtained from Palaquium gutta and several other evergreen trees of East Asia is an isomer of NR displaying an all-trans (> 99 %) configuration and much lower molecular weight (M n = 1.4-1.7 10 5 g mol À1 ).[1] Unlike NR it is a thermoplastic crystalline polymer with a melting point (T m ) of 628C. Although for most applications gutta-percha has been superseded by advanced functional polymers, controlled crosslinking of synthetic trans-1,4-polyisoprene or its blending (with, for example, natural rubber, styrene-butadiene rubber, and butadiene rubber) and block copolymerization (e.g., with a-olefins) might afford new high-performance materials.[3]The synthesis of highly stereoregular cPIP with Zieglertype catalysts is well established. [4,5] In particular, catalyst mixtures with rare-earth-metal components such as neodymium represent a prominent class of high-performance catalysts for the industrial stereospecific polymerization (> 98 % cis-1,4) of 1,3-dienes, even though the molecular weights and molecular weight distributions remain difficult to control.[6] Molecular systems based on lanthanide metallocene and postmetallocene congeners afford polymers with very narrow molecular weight distributions and very high stereoregularity. [7][8][9][10] A combination of [(C 5 Me 5 ) 2 Ln(AlMe 4 )]/Al-(iBu) 3 /[Ph 3 C][B(C 6 F 5 ) 4 ] (Ln = Sm, Gd) gave cis-1,4-polybutadiene with excellent stereocontrol (up to 99.9 % cis) and narrow molecular weight distributions (M w /M n = 1.20-1.23), while the polymerization of isoprene was not observed to be living. [7] cPIP with comparable characteristics (95-99 % cis-1,4; M w /M n = 1.3-1.7) was obtained with neodymium allyl complexes in the presence of aluminum alkyls as activators [8,9] as (PNP Ph = ({2-(Ph 2 P)C 6 H 4 } 2 N], Ln = Sc, Y, Lu) were reported to yield high cis-1,4 selectivity in the living polymerization of isoprene and butadiene in the absence of any aluminum additive (> 99 % cis-1,4; M w /M n = 1.05).[11] The fabrication of synthetic gutta-percha and gutta-balata has been achieved by utilization of mixed organo-Ln/Mg initiators such as [(CMe 2 C 5 H 4 ) 2 Sm(C 3 H 5 )MgCl 2 (OEt 2 ) 2 LiCl(OEt 2 )] (> 95 % trans-1,4; M w /M n = 1.32) [12] and half-sandwich-based [(C 5 Me 4 nPr)Nd(BH 4 ) 2 (thf) 2 ]/Mg(nBu) 2 (Mg/Nd = 0.9; 98.5 % trans-1,4, M w /M n = 1.15).

The protonolysis reaction of [Ln(AlMe(4))(3)] with various substituted cyclopentadienyl derivatives HCp(R) gives access to a series of half-sandwich complexes [Ln(AlMe(4))(2)(Cp(R))]. Whereas bis(tetramethylaluminate) complexes with [1,3-(Me(3)Si)(2)C(5)H(3)] and [C(5)Me(4)SiMe(3)] ancillary ligands form easily at ambient temperature for the entire Ln(III) cation size range (Ln=Lu, Y, Sm, Nd, La), exchange with the less reactive [1,2,4-(Me(3)C)(3)C(5)H(3)] was only obtained at elevated temperatures and for the larger metal centers Sm, Nd, and La. X-ray structure analyses of seven representative complexes of the type [Ln(AlMe(4))(2)(Cp(R))] reveal a similar distinct [AlMe(4)] coordination (one eta(2), one bent eta(2)). Treatment with Me(2)AlCl leads to [AlMe(4)] --> [Cl] exchange and, depending on the Al/Ln ratio and the Cp(R) ligand, varying amounts of partially and fully exchanged products [{Ln(AlMe(4))(mu-Cl)(Cp(R))}(2)] and [{Ln(mu-Cl)(2)(Cp(R))}(n)], respectively, have been identified. Complexes [{Y(AlMe(4))(mu-Cl)(C(5)Me(4)SiMe(3))}(2)] and [{Nd(AlMe(4))(mu-Cl){1,2,4-(Me(3)C)(3)C(5)H(2)}}(2)] have been characterized by X-ray structure analysis. All of the chlorinated half-sandwich complexes are inactive in isoprene polymerization. However, activation of the complexes [Ln(AlMe(4))(2)(Cp(R))] with boron-containing cocatalysts, such as [Ph(3)C][B(C(6)F(5))(4)], [PhNMe(2)H][B(C(6)F(5))(4)], or B(C(6)F(5))(3), produces initiators for the fabrication of trans-1,4-polyisoprene. The choice of rare-earth metal cation size, Cp(R) ancillary ligand, and type of boron cocatalyst crucially affects the polymerization performance, including activity, catalyst efficiency, living character, and polymer stereoregularity. The highest stereoselectivities were observed for the precatalyst/cocatalyst systems [La(AlMe(4))(2)(C(5)Me(4)SiMe(3))]/B(C(6)F(5))(3) (trans-1,4 content: 95.6 %, M(w)/M(n)=1.26) and [La(AlMe(4))(2)(C(5)Me(5))]/B(C(6)F(5))(3) (trans-1,4 content: 99.5 %, M(w)/M(n)=1.18).