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Heiko Jacobsen

Publications and source records attributed to Heiko Jacobsen.

10 recordsLinked to original sources
PubMed2006

Dicoordinate copper(I) silanechalcogenolates.

The copper silanechalcogenolates tBu3PCuESiPh3 (1, E = O; 2, E = S; 3, E = Se) were prepared from the reaction of [tBu3PCu(CH3CN)3]BF4 with [Ph3SiELi(THF)2]2 in acetonitrile. The compounds were obtained as colorless, crystalline, but thermally labile solids. X-ray crystallography shows that complexes 1-3 are monomeric in the solid state with no Cu...Cu interactions. The Cu atoms have either a linear or a near-linear coordination geometry in all three complexes. Interestingly, the O atom in complex 1 is also linear, which is in contrast to the highly bent S (2) and Se analogues (3). Density functional theory calculations suggest that both the linear geometry of 1 and an associated extremely short Cu-O distance [1.769(4) A] are not the result of pi delocalization but are the result of a fine balance of electrostatic interaction and Pauli repulsion.

Chalcogens↗
PubMed2006

Dynamic processes in silyl palladium complexes: evidence for intermediate Si-H and Si-Si sigma-complexes.

The silyl palladium complexes (dcpe)PdH(SiHtBu2) and (dcpe)Pd(SiHMe2)2 display NMR spectra that vary with temperature. The dynamic NMR behavior is consistent with long-lived sigma-complexes as intermediates. In the case of (dcpe)PdH(SiHtBu2), the intermediate is believed to be a symmetric complex with doubly bridged hydrogen atoms between the silicon and palladium. Dynamic interchange of the two silicon atoms in (dcpe)Pd(SiHMe2)2 is consistent with an intermediate Si-Si sigma-complex.

Journal Article↗
PubMed2006

P-Heterocyclic carbenes as potential ligands in the design of new metathesis catalysts. A computational study.

Density functional calculations are reported concerning the olefin metathesis characteristics of a variety of P-heterocyclic carbene (PHC) complexes. The calculations employ model catalysts of the type (PMe3)(PHC)Cl2Ru=CH2, the PHC ligands being 1,3-dihydro-1,3-diphosphol-2-ylidene PH, 1,3-diphenyl-1,3-diphosphol-2-ylidene PPH, and 1,4-dihydro-1,4-diphosphol-2-azol-5-ylidene PNH. Complexes with N-heterocyclic carbenes (NHC) are included for comparison. Associative and dissociative reaction pathways are considered, the latter ones representing the favored reaction mechanisms. Calculations show that the rate determining step is ring opening of a ruthena-cyclobutane intermediate. In comparison with NHC model catalysts, the PHC compounds have lower phosphine dissociation energies, and also form weaker pi-complexes with an olefinic substrate. Compared to the initially formed pi-complexes, the ruthena-cyclobutane is more stable for PHC- than for NHC-catalysts. The catalytic activity of model PHC-compounds in comparison with NHC-compounds is discussed on the basis of the calculated reaction profiles. In this context, different models for enhanced reactivity of NHC-based catalysts that have been proposed in the literature are considered as well. It is demonstrated that the nature of the substituent of the carbene phosphorus not only exhibits a steric influence on the course of the reaction, but a significant stereoelectronic effect as well. Further, agostic interactions in ruthena-cyclobutane intermediates are investigated.

Journal Article↗
PubMed2004

Alkyl-eta2-alkene niobocene and tantalocene complexes with the allyldimethylsilyl-eta5-cyclopentadienyl ligand: synthesis, NMR studies and DFT calculations.

Group 5 metal complexes [M(eta5-C5H5)[eta5-C5H4SiMe2(CH2-eta]2-CH=CH2)]X] (M = Nb, X = Me, CH2Ph, CH2SiMe3; M = Ta, X = Me, CH2Ph) and [Ta(eta5-C5Me5)[eta5-C5H4SiMe2(CH2-eta2-CH=CH2)]X] (X = Cl, Me, CH2Ph, CH2SiMe3) containing a chelating alkene ligand tethered to a cyclopentadienyl ring have been synthesized in high yields by reduction with Na/Hg (X = Cl) and alkylation with reductive elimination (X = alkyl) of the corresponding metal(iv) dichlorides [M(eta5-Cp)[eta5-C5H4SiMe2(CH2CH=CH2)]Cl2] (Cp = C5H5, M = Nb, Ta, Cp = C5Me5, M = Ta). These chloro- and alkyl-alkene coordinated complexes react with CO and isocyanides [CNtBu, CN(2,6-Me2C6H3)] to give the ligand-substituted metal(III) compounds [M(eta5-Cp)[eta5-C5H4SiMe2(CH2CH=CH2)]XL] (X = Cl, Me, CH2Ph, CH2SiMe3). Reaction of the chloro-alkene tantalum complex with LiNHtBu results in formation of the imido hydride derivative [Ta(eta5-C5Me5)[eta5-C5H4SiMe2(CH2CH=CH2)]H(NtBu)]. NMR studies for all of the new compounds and DFT calculations for the alkene-coordinated metal complexes are compared with those known for related group 4 metal cations.

Journal Article↗
PubMed2004

Toward a catalytic cycle for the Mn-salen mediated alkene epoxidation: a computational approach.

BP86 density functional calculations for the title reaction are presented, where a model catalyst with hypochlorite as oxygen-containing counter ligand, (ClO)(O)Mn(acacen') (acacen' = -O(CH)3N-C2H4-N(CH)3O-), is employed. The epoxidation reaction on potential energy surfaces corresponding to an overall spin-density of two and four unpaired electrons is investigated. The presence of the hypochlorite ligand is found to cause the reaction to proceed under conservation of spin. Further, the oxygen-containing counter ligand causes reoxidation of the Mn-center, thus closing the catalytic cycle. A catalytic scheme is therefore proposed, which includes a step of regeneration of the catalytically active species. Energetic estimates including corrections for solvent effects are presented for the relevant steps constituting the proposed catalytic scheme.

Journal Article↗
PubMed2003

Electronic effects in (salen)Mn-based epoxidation catalysts.

Presented are density functional calculations on various Mn(salen) systems that are active catalysts in the epoxidation of olefins. Correlation of various structural properties such as Mn=O bond strengths, atomic charges, and C-O distances of evolving bonds in transition state geometries with modified Hammett constants reveal a mechanistic picture of the epoxidation reaction, supporting previous experimental results. Enantioselectivity is tied to the position of a transition state along the reaction coordinate for the first C-O bond formation step, when an olefin is approaching the epoxidation catalyst. Electronic effects exhibited by the 5,5' substituents of the salen ligand manifest themselves in a tuning of the Mn=O bond strength, which in turn influences the C-O distance of the forming bond in the transition state geometry.

Journal Article↗
PubMed2002

An unusual (10,3)-a racemic twofold interpenetrating network assembled from isolable tris(cyclopentadienyl)manganate and cesocene building blocks.

The syntheses and X-ray crystal structures of [([18]crown-6)2Cs](+)-[Cp3Mn]- (1), [([18]crown-6)2Cs](+)-[Cp'3Mn]- (2), [CsCp'] (3), [(CsCp')2-([18]crown-6)] (4), and Cs[MnCp3] (5), and the synthesis of Cs[MnCp'3] (6) are reported (Cp' = C5H4Me). The anions [Cp3Mn]- (1-) and [Cp'3Mn]- (2-) are characterized by eta 2 coordination of all three Cp or Cp' rings. Measurements of the magnetic susceptibilities chi M resulted in values of mu eff = 6.20 microB (300 K), mu eff = 6.33 microB (301 K), and mu eff = 5.83 microB (300 K) for 1, 2, and 5, respectively, which are indicative of high-spin d5-Mn2+ centers. Density functional calculations illustrate that the coordination mode of 1- is characteristic for its sextet electronic ground state. Compound 3 forms infinite chains of cesocene-type sandwiches in the solid state, which are broken up into small subunits by the addition of crown ether to form 4. Compound 5 is a rare example of a (10,3)-a racemic interpenetrating network that crystallizes in the orthorhombic space group Pbca.

Crystallization↗
PubMed2002

Formation of cyclodimeric (sp(2)-C(1))-bridged Cp/-oxido ("CpC(1)O"M(IV)X(2)) group 4 metal Ziegler-Natta catalyst systems--how important is the "constrained geometry" effect?

Deprotonation of sodium acetylcyclopentadienide (11) was achieved by treatment with LDA in THF to generate the dianion equivalent [Cp-C(=CH(2))-O](2-)(12). Transmetalation with Cl(2)Ti(NMe(2))(2) gave ([Cp-C(=CH(2))-O]Ti(NMe(2))(2))(2) (17); treatment of 12 with Cl(2)Zr(NEt(2))(2)(THF)(2) furnished (([Cp-C(=CH(2))-O]Zr(NEt(2))(2))(2) (18). Cryoscopy in benzene revealed a dimeric structure of 18 in solution. Complex 18 was characterized further by an X-ray crystal structure analysis and by DFT calculations. The two zirconium centers of 18 are connected by means of two symmetry-equivalent eta(5):kappaO[Cp-C(=CH(2))-O] ligands. The ligand backbone shows no specific steric constraints, different from the formally related "constrained geometry" systems such as [Cp-SiMe(2)-NCMe(3)]Zr(NMe(2))(2) (1b). Nevertheless, upon treatment with MAO the CpCO group 4 metal complex system (18) generates an active homogeneous Ziegler-Natta catalyst for effective ethene/1-octene copolymerization, with up to 20% 1-octene having become incorporated in the resulting copolymer at 90 degrees C.

Journal Article↗