Konrad Knoll

Novel Calcium Half-Sandwich Complexes for the Living and Stereoselective Polymerization of Styrene

Tackling tacticity: The first well-defined heteroleptic benzylcalcium complex initiates the living polymerization of styrene. Chain-end control results in a polymer enriched in syndiotactic sequences. Stereo errors arise from fast inversion of the chiral carbanionic chain end. Increasing the styrene concentration accelerates the insertion and leads to a considerable reduction of the stereo errors.

“Atmende” cluster: Addition und eliminierung als elementarschritte der ligandensubstitution an (μ3-RP)Fe3(CO)10

Abstract The closed tetrahedral clusters RPFe 3 (CO)( 10- n )L n ( 1 ) (R = alkyl, aryl; L = phosphite, isonitrile; n = 0, 1, 2) add ligands L by opening of metal—metal bonds in a stepwise manner to give RPFe 3 (CO)( 10- n )L( n +1 ), ( 2 ) and RPFe 3 (CO)( 10- n )L( n +2 ) ( 3 ) respectively. By stepwise elimination of L the closed tetrahedral clusters 1 are obtained from the MM bond-opened species 3 and 2 , respectively. Elimination of CO from 3 or 2 yields substituted clusters of type 2 or 1 , respectively, again in a stepwise manner. This reaction sequence, documented by several examples, demonstrates that clusters behave like “breathing objects” which reversibly add substrates to their inner surface and “expire” them under reformation of metal—metal bonds. Hence, ligand substitution does not occur as a dissociatively initiated process on the outer surface of a cluster but involves addition-elimination sequences on the inner surface of the polyhedron.

Synthese und reaktivität ternärer cluster: Photochemisch ausgelöste insertion von alkinen in (μ3-RP)Fe3(CO)10☆

Abstract The RP bridged clusters (μ 3 -RP)Fe 3 (CO) 10 ( 1 ) react with diphenylacetylene upon irradiation to yield three different isomers (RP)(PhCCPh)Fe 3 (CO) 9 ( 2–4 ) depending on the reaction conditions. X-ray diffraction analysis of the isomers shows that in 2 , which appears to be the primary photochemically formed product, the alkyne is inserted between a phosphorus and an iron center of 1 . For isomer 3 one of the PFe bonds of the starting cluster 1 is bridged by the alkyne while in 4 the alkyne is inserted into an FeFe edge of 1 . These observations suggest that photochemical activation of 1 involves homolytic PFe bond scission with consequent decarbonylation.

Synthese und reaktivität ternärer cluster: thermisch ausgelöste addition von alkinen an (ν3-RP)Fe3(CO)10

The ν3-RP-bridged clusters (ν3-RP)Fe3(CO)10 (1) react with terminal alkynes R′CCH to give the adducts (RPCHCR′)Fe3(CO)10 (2). The adducts 2 contain the C2 entity of the alkyne inserted into a PFe edge of the starting cluster 1. The dependence of the relative reaction rates upon the nature of R and R′ groups, is investigated in detail. A mechanism for the formation of 2 from 1 is proposed, involving the opening of 1 at an iron edge as an essential step. The structure of the adducts 2 has been determined for two examples, by X-ray diffraction anaysis. It indicates the existence of a bent cyclopentadiene-like five-membered cycle RPCHCR′Fe(CO)3Fe(CO)4, which is coordinated “side-on”to an Fe(CO)3 group via its heterobutadiene fragment PCCFe.

Reversible alkin-addition an rp-verbrückte carbonyleisen-cluster

Abstract The clusters (μ 3 -RP) 2 Fe 3 (CO0 9 ( 1 ) photochemically add alkines R′CCR′ across their bridging phosphorus centers to yield (μ 3 -η 4 -RPCR′CR′PR)Fe 3 (CO) 9 ( 2 ). Thermal activation of 2 opens two different reaction channels: Complezes 2 may split into R′CCR′ and 1 in a thermally induced reversion of their photoinitiated formation reaction: in another pathway they may lose Fe(CO) 3 to yield (μ 2 -η 2 -(PFe)RPCR′CR′PR)Fe 2 (CO) 6 ( 3 ). Complex 3 is an Fe 2 (CO) 6 derivative of the butterfly type with the μ 2 -bridging phosphorus centers linked by an R′CCR′ moeity. The reverse transformation 3 → 2 is induced by Fe 2 (CO) 9 as an “Fe(CO) 3 ” source. Compounds 2 undergo a CO substitution reaction with R′CCR′ to give (μ 3 -η 2 - RPCR′CR′PR)(μ 3 -η 2 -R′CCR′)Fe 3 (CO) 7 ( 4 ) which, upon heating, also transforms into 3 . The above reactions 1 ⇋ 2 ⇋ 3 and 2 → 4 → 3 ⇋ 2 present a rare example of a complete closed set of cluster transformations. An analogous subset of reactions is also verified for the arsenic homologues 1 , 2 and 4 .

Transformationen von Clustern mit dem (μ3-RP)Fe3(CO)9-Gerüst. Synthese und Reaktionen von ([μ3-RP)Fe3(CO)9]2−

Abstract The clusters (μ3-RP)Fe3(CO)10 (1) or (μ3-RP)Fe3(CO)9(μ2-H)2 (2) can reversibly be transformed into the cluster anions [(μ3-RP)Fe3(CO)9 (μ2-H)]− (3) and [(μ3-RP)Fe3(CO)9]2− (4). The pyrophoric clusters 4 react with the divalent electrophile CH2I2 to give the complexes (μ3-η2-RP=CH2)Fe3(CO)10 (5), which contain a cluster-stabilized phosphaalkene, RP=CH2, as a ligand. With monovalent electrophiles R′X, such as Me2SO4, compound 4 (R = anisyl), yields, upon protolytic work-up, the complexes (μ3-η3-R′P-anisyl)Fe3(CO)9(μ2-H) (6) in which the phosphorus-bound aryl residue of the μ2-bridging phosphide ligand (R′P-anisyl) forms an η2-coordination to the third iron atom of the cluster. The η2-coordination of the aryl substituent may be reversibly released by two-electron ligands L under formation of (μ2-R′P-anisyl)(μ2-H)Fe3(CO)9L (7). In addition, the transformation sequence of 5 into 6 is accomplished by an H−, H+ addition sequence. The experiments are documented by analytic and spectroscopic data as well as by X-ray analyses.

Metallorganische π-liganden: cluster als tripeldeckerkomplexe

Abstract Clusters (R′P)(RCCR)Fe 3 (CO) 9 ( 1 ) have a pentagonal pyramidal cage structure with a planar C 2 Fe 2 P cycle acting as a organometallic 4π ligand towards a side-on η 5 -coordinated Fe(CO) 3 entity. Compounds of this type, which may be considered halfsandwich complexes, undergo transformation to clusters (R′P)(RCCR)Fe 4 (CO) 11 ( 2 ) which correspond to triple-decker compounds with a planar central C 2 Fe 2 P 4π ligand. Compounds 2 can also be described as pentagonal bipyramidal clusters. Since compounds 1 are chemically related to a number of different cage molecules containing a C 2 PFe 3 core and also to binuclear complexes with a C 2 PFe 2 core, the triple-decker clusters 2 can also be prepared from these starting materials. Several examples of this type are given. The isoelectronic analogy between RP and S as cluster constituents is exemplified by the synthesis of a sulfur-containing analogue of 2 by similar procedures.

Topoisomerisierung von sechs vertex-polyedern: Gerüstumlagerungen im system (RP)Fe3(CO)n/ALKIN☆

Abstract Clusters of composition (RP)(RCCR)Fe 3 (CO) 9 may exist in three different isomeric forms, of which two have a pentagonal pyramidal, and the third has a trigonal, prismatic cage structure. Decarbonylation of these compounds leads to (RP)(RCCR)Fe 3 (CO) 8 with a trigonal prismatic framework; carbonylation yields compounds of composition (RP)(RCCR)Fe 3 (CO) 10 the geometry of which is derived from the pentagonal pyramidal base structure. The different isomers as well as their carbonylation and decarbonylation products can all be transformed into one another. The bonding pattern in the PC 2 Fe 3 cages thus formed changes in an apparently complex way during these reactions. The diversity of bond scissions and bond formations which occurs during these transformations, appears to be unusual at first sight. It can however, be rationalized in a simple scheme: The simultaneous movement of the apical and one equatorial constituent of a pentagonal pyramid transforms it into a trigonal prism. Further movement along the same coordinate leads to an isomeric pentagonal pyramid. The individual observations described here are a strong indication of a process we have termed “tandem isomerization”.

Synthese und Reaktionen Et2NP-verbrückter Carbonyleisencluster

Abstract Et2NPCl2 reacts with Fe2(CO)9 to give compound (CO)4Fe(PCl2NEt2) (1), which with additional Fe2(CO)9 gives the compounds (μ2-Et2NPCl)(μ2-Cl)Fe2(CO)6 (2); (μ3-Et2NP)Fe3(CO)10 (3) and (μ3-Et2NP)2Fe3(CO)9 (4). Photochemical reaction of 3 with diphenylacetylene yields the cluster 5 in which the C2 entity of the alkene is added to a PFe edge of the decarbonylated educt 3. The (μ3-Et2NP) bridged cluster 4, upon photochemical activation, adds diphenylacetylene, without decarbonylation, across the phosphorus centers to give 6. Excess diphenylacetylene leads to a substitution derivative of 6 (7) in which two carbonyl groups in 6 are substituted by μ2-η2-bonded diphenylacetylene. The structure of 7 has been confirmed by an X-ray diffraction analysis.

Gerüstumlagerung pentagonal pyramidaler Käfige: Tandem-Isomerisierung

Abstract The six vertex clusters (RP)Fe 3 (CO) 9 (R′CCR″) readily isomerize into each other. On the basis of EHT model calculations on the isolobal system B 6 H 6 4− a “tandem process” is proposed as the isomerization pathway, which involves a distorted trigonal prismatic intermediate.