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At least 19 recordsLinked to original sources
PubMed2002

Synthesis of a novel, highly symmetric bis-oxa-bridged compound.

A highly oxygenated, novel pentacyclic bis-oxa-bridged compound 8 was synthesized with remarkable efficiency starting from readily available tetrachloro-5,5-dimethoxyclopentadiene and 1,4-cyclohexadiene in three steps. The ruthenium-catalyzed oxidation of 2:1 bis-adduct 1 followed by a one-pot transformation of the resulting bis-alpha-diketone 3 furnished (after esterification) the title compound in an overall yield of 29.1%. The versatility of this simple method was further demonstrated with other norbornyl alpha-diketones to obtain the corresponding strained oxa-bridged derivatives.

Journal Article↗
PubMed2004

Electronically-coupled tungsten-tungsten quadruple bonds: comparisons of electron delocalization in 3,6-dioxypyridazine and oxalate-bridged compounds.

The preparation of the 3,6-dioxypyridazine-bridged tungsten complex, [W(2)(O(2)C(t)Bu)(3)](2)(mu-H(2)C(4)N(2)O(2)), I, is described, along with its single-electron oxidized cation, I(+), formed in the reaction between I and Ag(+)PF(6)(-). Compound I has been structurally characterized as a PPh(3) adduct, and I(+)PF(6)(-) as a THF solvate, by single-crystal X-ray studies. The geometric parameters of these compounds compare well with those calculated for the model compounds [W(2)(O(2)CH)(3)](2)(mu-H(2)C(4)N(2)O(2)) and [W(2)(O(2)CH)(3)](2)(micro-H(2)C(4)N(2)O(2))(+) by density functional theory employing the Gaussian 98 and 03 suite of programs. The calculations indicate that the two W(2) centers are strongly coupled by M(2) delta-to-bridge pi-bonding, and further coupled by direct M(2)...M(2) bonding. Compound I is purple and shows an intense absorption in the visible region due to a metal-to-bridge charge transfer and, with excitation within this absorption, compound I exhibits pronounced resonance Raman bands associated with symmetric vibrations of the bridge and the M(4) unit. The cyclic voltammogram of I in THF, the EPR spectrum of I(+)PF(6) in 2-MeTHF and the electronic absorption spectrum of I(+)PF(6)(-) in THF are consistent with electron delocalization over both W(2) units. These new data are compared with previous data for the molybdenum analogue, related oxalate-bridged compounds and closely related cyclic polyamidato-bridged Mo(4)-containing compounds. It is proposed that, while the electronic coupling occurs principally by an electron-hopping mechanism for oxalate-bridged compounds, hole-hopping contributes significantly in the cases of the amidate bridges and that this is more important for M = Mo than for M = W. Furthermore, for Class III fully delocalized mixed-valence compounds, the magnitude of K(c), determined from electrochemical methods, is not necessarily a measure of the extent of electron delocalization.

Journal Article↗
PubMed2006

Structural and magnetic resolution of a two-step full spin-crossover transition in a dinuclear iron(II) pyridyl-bridged compound.

A dinuclear iron(II) complex containing the new pyridyl bridging ligand, 2,5-di(2',2''-dipyridylamino)pyridine (ddpp) has been synthesised and characterised by single-crystal X-ray diffraction, magnetic susceptibility and Mössbauer spectral methods. This compound, [Fe(2)(ddpp)(2)(NCS)(4)]4 CH(2)Cl(2), undergoes a two-step full spin crossover. Structural analysis at each of the three plateau temperatures has revealed a dinuclear molecule with spin states HS-HS, HS-LS and LS-LS (HS: high spin, LS: low spin) for the two iron(II) centres. This is the first time that resolution of the metal centres in a HS-LS ordered state has been achieved in a two-step dinuclear iron(II) spin-crossover compound. Thermogravimetric data show that the dichloromethane solvate molecules can be removed in two distinct steps at 120 degrees C and 200 degrees C. The partially de-solvated clathrate, [Fe(2)(ddpp)(2)(NCS)(4)]CH(2)Cl(2), undergoes a one-step transition with an increased transition temperature with respect to the as synthesised material. Structural characterisation of this material reveals subtle changes to the coordination geometries at each of the iron(II) centres and striking changes to the local environment of the dinuclear complex. The fully de-solvated material remains high spin over all temperatures. Interestingly, the solvent can be re-introduced into the monosolvated solid to achieve complete conversion back to the original two-step crossover material, [Fe(2)(ddpp)(2)(NCS)(4)]4 CH(2)Cl(2).

Journal Article↗
PubMed2004

Discrete triangle, square and hexametallic alkynyl cyano-bridged compounds based on [cis-Pt(Ctbond;CR)2(CN)2]2- building blocks.

Novel mixed bis(alkynyl)bis(cyano)platinate(II) species [cis-Pt(Ctbond;CR)(2)(CN)(2)](2-) (1 a: R = tBu, 1 b: R = Ph) have been prepared and their potential as building blocks in the generation of self-organized systems with a variable molecular architecture has been studied. The reaction of 1 with the ditopic acceptor species [[cis-Pt(C(6)F(5))(2)S](2)(dppa)] (dppa=diphenylphosphinoacetylene) gave the dianionic cyanide/dppa bridged molecular platinotriangles (NBu(4))(2)[(C(6)F(5))(2)Pt(micro-dppa)[(micro-CN)(2)Pt(Ctbond;CR)(2)]Pt(C(6)F(5))(2)] (2). X-ray analysis of 2 a confirmed that the "Pt(2)(C(6)F(5))(4)(micro-dppa)" binuclear moiety is connected to the dianionic "Pt(Ctbond;CR)(2)(CN)(2)" unit by two bridging cyanide ligands. Moreover, treatment of 1 with the solvent cationic species [M(cod)(acetone)(2)](+) afforded heterometallic molecular squares Pt(2)M(2) (M=Rh, Ir) containing cyanide bridges and terminal alkynyl ligands, (NBu(4))(2)[cyclo[[cis-Pt(Ctbond;CR)(2)(micro-CN)(2)][M(cod)]](2)] (3: M=Rh, 4: M = Ir). The solid-state structures of phenyl derivatives have been determined by X-ray crystallography. The terminal alkynyl ligands in these cyanide-bridged molecular squares 3 and 4 have been used in the assembly of higher multimetallic complexes. Thus, very unusual bis(double-alkynide)-cyanide-bridged hexametallic compounds (NBu(4))(2)[[(C(6)F(5))(2)Pt(micro-Ctbond;CPh)(2)(micro-CN)(2)](2)[M(cod)](2)] (5 b: M=Rh, 6 b: M = Ir) were easily formed by simple reactions of 3 b and 4 b with two equivalents of [cis-Pt(C(6)F(5))(2)(thf)(2)]. An X-ray diffraction study on complex 5 b indicated that the derivative was formed by a simultaneous migration of one sigma-alkynyl group from each "Pt(Ctbond;CPh)(2)(micro-CN)(2)" corner of the square to both "Pt(C(6)F(5))(2)" units, resulting in bent sigma,pi-double-alkynyl bridging systems. Finally, the novel supramolecular anionic assemblies (NBu(4))(4)[cyclo[[cis-Pt(Ctbond;CR)(2)(micro-CN)(2)][SnPh(3)]](4)] 7 have been obtained by self-assembly of 1 and [SnPh(3)(acetone)(2)](+).

Journal Article↗
PubMed2005

Concise synthesis of novel oxa-bridged compounds.

[structures: see text] A stereoselective strategy for the replacement of the 1,2-dihaloalkene bridge of tetrahalonorbornyl derivatives by an oxygen bridge involving stepwise controlled oxidation, cleavage of the dione thus formed, and reiterative intramolecular S(N)2 displacement of two bridgehead halogens is devised. The synthesis of four highly strained pentacyclic bis-oxa-bridged derivatives 10, 27, 28, and 29 with interesting structural variations is presented. The two oxa bridges are syn to each other, separated by central cyclohexane and cycloheptane rings in 10 and 27, while they are anti to each other and are separated by central cyclopentane and furan rings in 28 and 29. In the case of the highly symmetric bis-oxa-bridged derivative 10 the two syn oxa bridges constrain the central cyclohexane ring into the boat form. The endo,anti,endo 2:1 bis adducts of 1,2,3,4-tetrahalo-5,5-dimethoxycyclopenta-1,3-diene with cyclopentadiene were prepared for the first time, while the reactivites of previously unexplored bis adducts derived from furan and cycloheptatriene were revealed.

Journal Article↗
PubMed2003

3,6-dioxypyridazine bridged tungsten-tungsten quadruple bonds. Comparisons of electron delocalisation with oxalate bridged compounds.

The preparation and characterisation of the tungsten-tungsten quadruply bonded, 3,6-dioxypyridazine bridged complex [((t)BuCO(2))(3)W(2)](2)([micro sign]-H(2)C(4)N(2)O(2)) and its single electron oxidised radical cation are reported and, when compared with related bridged dimolybdenum complexes, reveal a different mechanism of electronic coupling from that seen in related oxalate bridged systems.

Journal Article↗
PubMedDate not supplied

Synthesis of selenocysteine peptides and their oxidation to diselenide-bridged compounds.

Using the Fmoc/tBu protection scheme and the p-methoxybenzyl derivative of selenocysteine, the synthesis of related peptides in the selenol-protected form could be optimized by operating the coupling steps in the absence of auxiliary bases and by reducing the piperidine treatment to the minimum time required for quantitative Fmoc cleavage. Under these conditions, beta-elimination of the p-methoxybenzylselenol as the main side reaction of these syntheses, as well as epimerization of the protected selenocysteine, was largely suppressed. Conversion of the selenol- and thiol-protected bis-selenocysteine and selenocysteine, cysteine peptides into the related cyclic monomeric forms by iodine-mediated oxidation failed since a complex mixture of compounds was produced. Cleavage of the selenoether bond with mercuric acetate was found to proceed smoothly, but displacement of the heavy metal ions by treatment with excesses of thiols or hydrogen sulphide was unsuccessful since a stable Hg2+ diselenide complex was obtained. However, oxidation was achieved in good yields by the dimethylsulphoxide/trifluoroacetic acid procedure and the peptides were then used for determining the redox potential of the diselenide and selenide/sulphide bridge, respectively.

Organoselenium Compounds↗
PubMed2004

Designing dinuclear iron(II) spin crossover complexes. Structure and magnetism of dinitrile-, dicyanamido-, tricyanomethanide-, bipyrimidine- and tetrazine-bridged compounds.

In order to expand the few known examples of dinuclear iron(II) compounds displaying (weak) intradinuclear exchange coupling and spin-crossover on one or both of the iron(II) centres, various dinuclear compounds have been synthesised and assessed for their spin-crossover and exchange coupling behaviour. The key aim of the work was to prepare and structurally characterise 'weakly linked' and 'covalently bridged' systems incorporating bridging ligands such as alkyldinitriles (e.g.NC(CH(2))(4)CN), bipyrimidine (bpym), dicyanamide (dca(-)), tricyanomethanide (tcm(-)), 3,6-bis(2-pyridyl)tetrazine (bptz) and 3,6-bis(2-pyridyl)2,5-dihydrotetrazine (H(2)bptz). The 'end groups', which complete the Fe(ii)N(6) chromophores, include tris(2-pyridylmethyl)amine (tpa), di(2-pyridylethyl)(2-pyridylmethyl)amine (tpa'), 3-(2-pyridyl)pyrazole (pypzH), 1,10-phenanthroline (1,10-phen), tris(pyrazolyl)methane (tpm) and NCX(-)(X = S, Se). It was quite difficult to achieve the spin-crossover condition, many ligand combinations yielding high-spin/high-spin (HS-HS) Fe(II)Fe(II) spin states at all temperatures (300-2 K) with very weak antiferromagnetic coupling (J < -1 cm(-1)), two such being the crystallographically characterised [(dca)(tpm)Fe(mu(1,5)-dca)(2)Fe(tpm)(dca)], 5, and [(tpa')Fe(mu(1,5)-tcm)(2)Fe(tpa')](tcm)(2)(H(2)O)(2), 6. In contrast, a strong field was created around the Fe(II) centres in [(tpa)Fe(mu-(NC(CH(2))(4)CN))(2)Fe(tpa)](ClO(4))(4).NC(CH(2))(4)CN, 1, and the Fe-N bond distances, at 173 K, reflected this. This weakly-linked dinitrile example showed a spin-crossover beginning above 300 K. 'Half crossover' examples, yielding HS-LS states below the spin transition, similar to those noted by Real and coworkers in some mu-bpym systems, were noted for [(1,10-phen)(NCS)(2)Fe(mu-bpym)Fe(NCS)(2)(1,10-phen)], 2, [(pypzH)(NCSe)(2)Fe(mu-bpym)Fe(NCSe)(2)(pypzH)], 4, and [(tpa)Fe(mu-H(2)bptz)Fe(tpa)](ClO(4))(4), 8. Interestingly, the mu-bptz analogue, 7, remained LS-LS at all temperatures with the start of a broad spin crossover evident above 300 K. No thermal hysteresis was evident in the spin transitions of these new dinuclear crossover species indicating a lack of intra- or interdinuclear cooperativity.

Journal Article↗
PubMed2002

New dirhenium(III) compounds bridged by carboxylate ligands: N(C(4)H(9))(4)[Re(2)(OOCCF(3))Cl(6)] and Re(2)(OOCCCHCo(2)(CO)(6))(4)Cl(2).

The solvothermal reaction of (N(C(4)H(9))(4))(2)[Re(2)Cl(8)] with trifluoroacetic acid and acetic anhydride leads to the new rhenium trifluoroacetate dimer N(C(4)H(9))(4)[Re(2)(OOCCF(3))Cl(6)] (1) and to the rhenium carbonyl dimer Re(2)(mu(2)-Cl)(2)(CO)(8) as the rhenium-reduced byproduct. The reaction of the precursor complex, N(C(4)H(9))(4)[Re(2)(OOCCF(3))Cl(6)] (1), with the organometallic carboxylic acid (CO)(6)Co(2)HCCCOOH leads to the cluster of clusters compound Re(2)(OOCCCHCo(2)(CO)(6))(4)Cl(2) (2), which has the dimer structure of Re(2)(OOCR)(4)Cl(2). Cyclic voltammetric measurements show that Re(2)(OOCCCHCo(2)(CO)(6))(4)Cl(2) (2) has one reduction centered on the dirhenium core and a reduction centered on the cobalt atoms. DFT calculations have been used to rationalize the observed displacements of the voltammetric signals in Re(2)(OOCCCHCo(2)(CO)(6))(4)Cl(2) (2) compared to the parent ligand (CO)(6)Co(2)HCCCOOH and rhenium pivalate.

Journal Article↗
PubMed2001

Origin of Ferromagnetism in Cyano-Bridged Compounds Containing d(1) Octacyanometallates Financial support by the Belgian National Science Foundation and Flemish Government under the Concerted Action Scheme, the ESF programme on molecular magnets, the Russian Foundation for Basic Research (Grant No. 01-02-32210), and the INTAS Grant 00-00565 are gratefully acknowledged. The authors thank Professor Silvio Decurtins and the members of his group for stimulating discussions and for sending us the structural data on the niobium compound prior the publication, and Dr. Høgni Weihe for useful discussions. We also thank Ken Somers for the help with the Figures.

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Journal Article↗
PubMed1998

Heterodinuclear Complexes Containing d- and f-block Elements: Synthesis, Structural Characterization, and Metal-Metal Interactions of Novel Chromium(III)-Lanthanide(III) Compounds Bridged by Oxalate.

The reaction of Ln(III) ions with a tripodal ligand HBpz(3)(-) (hydrotris(pyrazol-1-yl)borate) and a "complex ligand" [Cr(acac)(2)(ox)](-) (acac(-) = acetylacetonate, ox(2)(-) = oxalate) in aqueous solution afforded a series of the novel 3d-4f heterodinuclear complexes [(acac)(2)Cr(ox)Ln(HBpz(3))(2)] (Ln = Eu (1), Gd (2), Tb (3), Yb (4), Lu (5)). The crystal structure of 4 has been determined by X-ray diffraction. Complex 4 crystallizes in monoclinic space group P2/n, of which the cell parameters are a = 8.594(3) Å, b = 18.538(4) Å, c = 12.093(2) Å, beta = 93.71(2) degrees, and Z = 2. The Yb atom has an eight-coordinate distorted square antiprismatic coordination geometry. The intramolecular Cr.Yb distance is 5.631(1) Å. The magnetic susceptibility data for complex 2 showed that the Cr(III)-Gd(III) interaction is weakly antiferromagnetic with an exchange coupling constant J(CrGd) = -0.09 cm(-)(1). The luminescence measurements demonstrated the energy transfers for both Ln(III) --> Cr(III) and Cr(III) --> Ln(III), of which the degree of emission quenching depends on the energy gap of the excited levels in two metal centers. These results reveal that the metal-metal interactions between Cr(III) and Ln(III) are very weak in magnetic interaction but are strong from the viewpoint of energy transfer.

Journal Article↗
PubMed2005

Magneto-structural correlations in binuclear copper(II) compounds bridged by a ferrocenecarboxylato(-1) and an hydroxo- or methoxo-ligands.

The syntheses, characterization and X-ray crystal structures of the first two examples of asymmetrically bridged dinuclear copper(ii) complexes containing a ferrocenecarboxylato ligand and a methoxo group in [{Cu(dmen)}2(micro-OMe){micro-O2C(eta5-C5H4)Fe(eta5-C5H5)}](ClO4)2, 1, (dmen=N,N-dimethylethylenediamine) or an hydroxo group in [{Cu(tmen)}2(micro-OH){micro-O2C(eta5-C5H4)Fe(eta5-C5H5)}](ClO4)2, 2 , (tmen=N,N,N'N'-tetramethylethylenediamine) are reported. Magneto-chemical studies revealed that 1 and 2 exhibit opposite superexchange interactions between the two Cu(II) paramagnetic centers: an antiferromagnetic coupling (J = -11 cm(-1)) in 1 and a ferromagnetic interaction (J = +29 cm(-1)) in 2. The results obtained from these studies suggest that the weak interactions between the Cu(II) ions and the perchlorate anions detected in the crystal structures are important to introduce significant distortions in the heterobridged [Cu2(micro-OR){micro-O2C(eta5-C5H4)Fe(eta5-C5H5)}]2+ cores of 1 and 2, which clearly affect the nature and strength of the superexchange interactions. Computational studies based on density functional theory and ab initio multiconfigurational second-order perturbation theory (CASPT2) calculations have also been performed in order to rationalize the magnetic behaviour of 1 and 2. The magneto-structural correlations for complexes containing the [Cu(micro-OR)(micro-O2CR')Cu] core are discussed, and the relevance of the out-of-plane angle of the R group with the Cu(micro-OR)Cu plane established.

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