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Helgard G Raubenheimer

Publications and source records attributed to Helgard G Raubenheimer.

11 recordsLinked to original sources
PubMed2006

Preparing alpha,beta-unsaturated Fischer-type carbene complexes via an unforeseen route.

Four 4-(NH-amino)-1-metalla-1,3-diene carbene complexes, two of which contain novel 4-membered heterometallacycles, formed regioselectively from the reaction of a mixture of [(CH(3))(2)(CH(3)S)S][BF(4)], the acetonitrilium electrophile, CH(3)CNCH(3)(+), and the deprotonated alkoxycarbene complexes, (CO)(5)M=C(OCH(3))CH(2)Li (M = Cr, W). The preferred Z-configurations of the alkoxy and amino groups are determined by strong H-bonding in the products. The metal-carbene bonds in the chelates are shorter by 0.08 A than those in the acyclic compounds.

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PubMed2006

Synthesis of the first rNHC (remote N-heterocyclic carbene) complexes with no heteroatom in the carbene carbon-containing ring.

Two cationic carbene complexes with no heteroatom in the ring containing the carbene carbon, trans-bromo(2-methyl-2,6-dihydroisoquinolin-6-ylidene)bis(triphenylphosphine)palladium(II) triflate (3) and trans-chloro(1,2-dimethyl-1,7-dihydroquinolin-7-ylidene)bis(triphenylphosphine)palladium(II) triflate (4), were synthesized by oxidative substitution of Pd(PPh3)4 with N-methylated 6-bromoisoquinolinium and 7-chloro-2-methylquinolinium cations, respectively. Compound 3 was also prepared by methylation of neutral trans-bromo(2-methylisoquinolin-6-yl)bis(triphenylphosphine)palladium(II) (5). All complexes were unambiguously characterized by NMR and X-ray crystallographic studies.

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PubMed2006

Aurolysis of alpha-C-deprotonated group 6 aminocarbene and thiocarbene complexes with Ph3PAu+.

Deprotonated Fischer-type aminocarbene complexes, (CO)5M=C(NR2)CH3 (M = Cr or W; R = Me or propyl), react with Ph3PAu+ by metal group substitution - (CO)5M for Ph3PAu+--and attachment of the extricated M(CO)5 to the deprotonated methyl group. (The products may also be seen as aminovinylgold compounds coordinated to M(CO)5 moieties.) DFT calculations at the B3LYP level of theory using model compounds indicate a clear preference of the gold unit for central C to terminal coordination in the ligand [NMe2CCH2]-, whereas the Cr(CO)5 has a 7 kcal mol(-1) preference for C(vinyl) coordination compared to N-coordination. In related thiocarbenes, the sulfur donor atom should be the preferred point of attachment for the metal carbonyl unit. The latter prediction is borne out in practice, and in the three products isolated, including Ph3PAu{C(=CH2)SPh}Cr(CO)5 in a mixed crystal with [Ph3PAuSPh]Cr(CO)5, precisely this coordination mode is present. The latter component of the mixed crystal has also been prepared independently of the vinyl one.

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PubMed2006

Permeability of a seemingly nonporous crystal formed by a discrete metallocyclic complex.

We describe the structure and permeability of a crystalline material that appears to be nonporous in a conventional sense. The material is initially formed as a solvate, and removal of the solvent molecules under relatively mild conditions proceeds via a single-crystal to single-crystal transformation, leaving the host structure intact. Although discrete unoccupied voids of 108 A3 are present in the structure, it is not possible to map open channels that represent an intuitive pathway for guest diffusion. Despite the apparent absence of pores, the material is permeable to a variety of gases including H2, O2, N2, CO, CH4, CO2, and I2. These findings show that porosity in crystalline systems cannot always be rationalized by considering the static structures and that as-yet unknown dynamic and cooperative mechanisms prevail by which porosity can be induced.

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PubMed2005

A first structural and theoretical comparison of pyridinylidene-type rNHC (remote N-heterocyclic carbene) and NHC complexes of Ni(II) obtained by oxidative substitution.

A series of cationic pyridinylidene and quinolinylidene complexes of chlorobis(triphenylphosphine)-nickel(II) were prepared by oxidative substitution of Ni(PPh3)4 with methylated chloropyridines or chloroquinolines. NMR as well as X-ray crystallographic studies confirmed the trans arrangement of the two phosphines in the products. Calculations, using suitable model compounds at the BP86/TZVP level, clearly differentiate between a standard imidazolylidene complex and new complexes of the NHC-type on the one hand, and new complexes classified as rNHC-types-with the heteroatom distant from the carbene carbon-on the other. The latter form significantly stronger bonds-mainly of an electrostatic nature-with the metal.

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PubMed2005

A discrete metallocyclic complex that retains its solvent-templated channel structure on guest removal to yield a porous, gas sorbing material.

A discrete rectangular metal-organic complex that stacks to form one-dimensional channels filled with acetonitrile solvent molecules is described. Removal of the solvent under relatively mild conditions proceeds via a single-crystal to single-crystal transformation that leaves the host lattice unaltered. These findings proffer a design strategy for porous materials based on the simple principle that rigid molecular rings cannot pack efficiently and would thus favor the inclusion of guest species whenever possible. Upon guest removal, an efficiently packed new phase can then only be achieved by means of bond cleavage. Thus, achieving crystal porosity by maintaining robust metal-ligand coordination bonds in such discrete cyclic systems directly parallels the strategy employed for MOFs.

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PubMed2004

Thione complexes of Rh(I): a first comparison with the bonding and catalytic activity of related carbene and imine compounds.

Heterocyclic mono(thione), trans-bis(thione), cis-bis(thione), trans-(carbene-thione), cis-(carbene-thione), trans-(phosphine-thione) and mono(imine) complexes of rhodium(I) have been prepared and fully characterised. Chloro(eta(4)-1,5-cyclooctadiene)(L)rhodium(I)(1a, L = 1,3-diisopropyl-4,5-dimethyl-2,3-dihydro-1H-imidazol-2-thione; 1b L = 1,3,4,5-tetramethyl-2,3-dihydro-1H-imidazol-2-thione) appear as isomers at room temperature due to slow coordination exchange on the S-donor atom. In the three structures determined, the substituent on the sulfur appears syn to Cl. Hindered rotation about the Rh-carbene bond is revealed in the NMR spectra of seven new complexes with isopropyl substituents on the heterocyclic carbene ligands. The trans influence of the thione ligands is smaller than that of carbenes but larger than that shown by imines and chloride. Thione complexes are better catalyst precursors than the carbene complexes for the hydroformylation of 1-hexene under the chosen reaction conditions: 80 degrees C, 8 MPa CO-H2(1:1), 16 h, 1:1000 catalyst to 1-hexene ratio.

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PubMed2004

Anionic Fischer-type carbene complexes as bidentate (N,O) ligands.

New polynuclear complexes, (L1)3M2 [M2 = Cr(III) (4a,4b), Fe(III) (5), Co(III) (8)], (L1)2M2(L2)2 [M2 = Co(II) (7), Ni(II) (9)], (L1)2M2(O)L2 [M2 = V(IV) (6)] and L1M2Cp2 [M2 = Ti(III) (10)] with L1 = (CO)5M1=C[C=NC(CH3)=CHS](O-)(M1 = Cr or W) and L2 = 4-methylthiazole or THF, are described. The molecular structures of these complexes determined by X-ray diffraction show that the Fischer-type carbene complexes act as bidentate ligands towards the second metal centre, coordinating through C(carbene)-attached O-atoms and imine N-atoms of the thiazolyl groups to form five-membered chelates with the oxygen atoms in the mer configuration. Isostructural complexes have similar characteristic band patterns in their far-IR spectra. Cyclic voltammetry of selected complexes reveals the oxidation of the carbene complex ligand between 1.01 and 1.29 V. Oxidation of the central metal (M2) takes place at 0.56 and 0.86 V for 7 and 9, respectively. Three stepwise reductions of Cr(III) to Cr(0) occur for 4a and 4b in the region -0.51 to -1.58 V. These new ligand types and other variants thereof should find application in ligand design with the first metal -- and other ligands attached thereto -- in the carbene complex ligand, playing an important role.

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PubMed2003

[Cis-[eta(2)-(Ph(3)PAu)(2)]Ph(3)PCr(CO)(4)].THF, featuring the shortest known separation between two Au metal centres in a heteronuclear Au(2)M cluster.

The title compound, tetracarbonyl-1kappa(4)C-tris(triphenylphosphino)-1kappaP,2kappaP,3kappaP-triangulo-chromiumdigold(Au-Au)(2 Cr-Au) tetrahydrofuran solvate, [Au(2)Cr(C(18)H(15)P)(3)(CO)(4)].C(4)H(8)O, is a stable isolobal analogue of the extremely labile [(eta(2)-H(2))CrL(n-1)] molecular hydrogen complex (n = 6; L is a neutral ligand, e.g. CO or PPh(3)), and features the shortest known separation [2.6937 (2) A] between two Au atoms in a triangular heteronuclear metal-cluster framework.

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