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PubMed1999

Measurement of Magnitude and Sign of Heteronuclear Coupling Constants in Transition Metal Complexes.

Sets of specifically tailored E.COSY-type correlation experiments and double-quantum/zero-quantum (DQ/ZQ) experiments are presented which enable the determination of sign and size of small heteronuclear coupling constants across the metal center of transition metal complexes. For the octahedrally coordinated complexes, [Ru(TPM)(H)(CO)(PPh3)]+[BF4]- (1) and [Ir(TPM)(H)(CO)(CO2CH3)]+[BF4]- (2), 14 of 15 and 15 of 15 possible two-bond scalar coupling constants across the metal center were measured, respectively, using 15N and 15N/13C enriched samples (TPM = tris(1-pyrazolyl)methane)). The reduced coupling constants 2KX-M-Y = 4pi2 2J/(hgammaXgammaY) were found to be positive when the coupled nuclei X and Y were trans with respect to the metal center, and negative when the coupled nuclei were in cis position. The validity of this sign rule was verified for JCC, JNN, JPN, JPC, JCN, JHP, JHC, and JHN couplings. Idiosyncracies associated with 2D NMR spectra for the sign determination of coupling constants with 15N which lead to corrections for the signs of JHN, JPN, and JCN couplings reported previously are discussed. Copyright 1999 Academic Press.

Journal Article↗
PubMed1988

A five-coordinate copper complex with superoxide dismutase mimetic activity from Streptomyces antibioticus.

A Cu(II) complex of desferrithiocin from Streptomyces antibioticus was prepared and characterized. The first shell atoms, including one nitrogen and four oxygens, were arranged around the copper in a square-planar pyramide. Due to the axially Jahn-Teller-distorted Cu-O distance at 224.7 pm, a distinct Cu2Zn2superoxide dismutase mimetic activity was measured. The Cu-complex survived 600 microM bovine serum albumin and the thermodynamic stability (pK = 17.4) was not very different from that of Cu-EDTA. The electronic absorption properties, circular dichroism and electron paramagnetism were in accordance with those of the type-II copper species.

Circular Dichroism↗
PubMed2005

Principal component analysis of complex multijoint coordinative movements.

Principal components analysis (PCA) has not been very much in vogue within the field of movement coordination even though it is useful to reduce data dimensionality and to reveal underlying data structures. Traditionally, studies of coordination between two joints have predominantly made use of relative phase analyses. This has resulted in the identification of principal constraints that govern the Central Nervous System's organization and the control of coordination patterns. However, relative phase analyses on pairwise joints have some drawbacks because they are not optimal for revealing convergent patterns among multijoint coordination modes and for unraveling generic control strategies. In this paper, we present a method to analyze multijoint coordination based on the properties of PC, more specifically the eigenvalues and eigenvectors of the covariance matrix. The comparison between relative phase analysis and PCA shows that both provide similar and consistent results, underscoring the latter technique's sensitivity to the study of coordination performance. In addition, it provides a method for automatic pattern detection as well as an index of performance for each joint within the context of the global coordination pattern. Finally, the merit of the PCA technique within the context of central pattern generators (CPG) will be discussed.

Adult↗
PubMed2003

Complexity of coordinative bonding in thallium(I) anthranilates and salicylates.

An inventory of the structural chemistry of thallium(I) shows many unexpected, almost random coordination numbers and coordination geometries that appear erratic and inconsistent. This nonstandard behavior is often ascribed to the specific lone-pair characteristics originating from relativistic effects. To provide data on a set of closely related compounds from which simple general rules of coordinative bonding at Tl(+) can be established, three thallium(I) anthranilates and three thallium(I) salicylates have been prepared from Tl(2)CO(3) and the corresponding 2-amino- and 2-hydroxy-benzoic acids and crystallized from aqueous solutions. All six compounds, the simple anthranilate (1) and salicylate (4) and the 3- and 4-methyl-substituted homologues (2, 3 and 5, 6) show different structures with large variations in the coordination motif. The coordination by oxygen in a geometry which covers less than a coordination hemisphere is the only common feature, complemented (only in 1) by a nitrogen coordination and by eta(6)-coordination of one (in 1, 2, 3, 6) or two phenyl rings (in 4). Tl-Tl contacts for which "thallophilic" bonding between closed shell metal atoms could be invoked, are generally very long (close to 4.0 A) or even well beyond the limit of standard van der Waals contacts. Hydrogen bonding is only obvious for the internal contacts of the amino- or hydroxy-benzoate ligands and does not contribute significantly to the assembly of the supramolecular structure which is dominated by oxygen bridges between thallium atoms. With the exception of 5, the formula units Tl[O(2)C(2-R)(3-R')(4-R'')C(6)H(2)] are generally aggregated into dimers of various configurations depending on the relative orientation of the edge-sharing four-membered rings, and these dimers are further linked into strings or columns establishing N-Tl or Tl-O contacts and arene coordination. The drastic changes induced in the structures upon only small variations such as methyl substitution in 3- or 4-position of the ligand suggest that thallium(I) coordination is generally restricted to one hemisphere of nearest neighbors, but is extremely flexible in this realm. The open hemisphere may be partially capped by arene coordination (which is weak at a distance of ca. 3.1 A to the centroid of the ring) or feature very weak thallophilic contacts.

Journal Article↗
PubMed2004

Synthons for coordinatively unsaturated complexes of tungsten, and their use for the synthesis of high oxidation-state silylene complexes.

Reduction of Cp*WCl4 afforded the metalated complex (eta6-C5Me4CH2)(dmpe)W(H)Cl (1) (Cp* = C5Me5, dmpe = 1,2-bis(dimethylphosphino)ethane). Reactions with CO and H(2) suggested that 1 is in equilibrium with the 16-electron species [Cp(dmpe)WCl], and 1 was also shown to react with silanes R2SiH2 (R2 = Ph2 and PhMe) to give the tungsten(IV) silyl complexes Cp*(dmpe)(H)(Cl)W(SiHR2) (6a, R2 = Ph2; 6b, R2 = PhMe). Abstraction of the chloride ligand in 1 with LiB(C6F5)4 gave a reactive species that features a doubly metalated Cp ligand, [(eta7-C5Me3(CH2)2)(dmpe)W(H)2][B(C6F5)4] (4). In its reaction with dinitrogen, 4 behaves as a synthon for the 14-electron fragment [Cp*(dmpe)W]+, to give the dinuclear dinitrogen complex ([Cp*(dmpe)W]2(micro-N2)) [B(C6F5)4]2 (5). Hydrosilanes R2SiH2 (R2 = Ph2, PhMe, Me2, Dipp(H); Dipp = 2,6-diisopropylphenyl) were shown to react with 4 in double Si-H bond activation reactions to give the silylene complexes [Cp*(dmpe)H2W = SiR2][B(C6F5)4] (8a-d). Compounds 8a,b (R2 = Ph2 and PhMe, respectively) were also synthesized by abstraction of the chloride ligands from silyl complexes 6a,b. Dimethylsilylene complex 8c was found to react with chloroalkanes RCl (R = Me, Et) to liberate trialkylchlorosilanes RMe2SiCl. This reaction is discussed in the context of its relevance to the mechanism of the direct synthesis for the industrial production of alkylchlorosilanes.

Journal Article↗
PubMed2006

Neutral and zwitterionic low-coordinate titanium complexes bearing the terminal phosphinidene functionality. Structural, spectroscopic, theoretical, and catalytic studies addressing the Ti-P multiple bond.

Alpha-hydrogen abstraction and alpha-hydrogen migration reactions yield novel titanium(IV) complexes bearing terminal phosphinidene ligands. Via an alpha-H migration reaction, the phosphinidene ((tBu)nacnac)Ti=P[Trip](CH(2)(tBu) ((tBu)nacnac(-) = [Ar]NC((t)Bu)CHC((t)Bu)N[Ar], Ar = 2,6-(CHMe2)(2C6H3, Trip = 2,4,6-(i)Pr3C6H2) was prepared by the addition of the primary phosphide LiPH[Trip] to the nucleophilic alkylidene triflato complex ((tBu)nacnac)Ti=CH(t)Bu(OTf), while alpha-H abstraction was promoted by the addition of LiPH[Trip] to the dimethyl triflato precursor ((tBu)nacnac)Ti(CH)(2)(OTf) to afford ((tBu)nacnac)Ti=P[Trip](CH3). Treatment of ((tBu)nacnac)Ti=P[Trip](CH3) with B(C6F5)(3) induces methide abstraction concurrent with formation of the first titanium(IV) phosphinidene zwitterion complex ((tBu)nacnac)Ti=P[Trip]{CH3B(C6F5)(3)}. Complex ((tBu)nacnac)Ti=P[Trip]{CH3B(C6F5)(3)} [2 + 2] cycloadds readily PhCCPh to afford the phosphametallacyclobutene [((tBu)nacnac)Ti(P[Trip]PhCCPh)][CH3B(C6F5)(3)]. These titanium(IV) phosphinidene complexes possess the shortest Ti=P bonds reported, have linear phosphinidene groups, and reveal significantly upfielded solution 31P NMR spectroscopic resonances for the phosphinidene phosphorus. Solid state 31P NMR spectroscopic data also corroborate with all three complexes possessing considerably shielded chemical shifts for the linear and terminal phosphinidene functionality. In addition, high-level DFT studies on the phosphinidenes suggest the terminal phosphinidene linkage to be stabilized via a pseudo Ti[triple bond]P bond. Linearity about the Ti-P-C(ipso) linkage is highly dependent on the sterically encumbering substituents protecting the phosphinidene. Complex ((tBu)nacnac)Ti=P[Trip]{CH3B(C6F5))(3)} can catalyze the hydrophosphination of PhCCPh with H(2)PPh to produce the secondary vinylphosphine HP[Ph]PhC=CHPh. In addition, we demonstrate that this zwitterion is a powerful phospha-Staudinger reagent and can therefore act as a carboamination precatalyst of diphenylacetylene with aldimines.

Acetylene↗
PubMed2005

Highly (Z)-selective hydrosilylation of terminal alkynes catalyzed by a diphosphinidenecyclobutene-coordinated ruthenium complex: application to the synthesis of (Z,Z)-bis(2-bromoethenyl)arenes.

[reaction: see text] Complex 1 bearing a diphosphinidenecyclobutene ligand (DPCB-OMe) catalyzes highly stereoselective hydrosilylation of diethynylarenes with HSiMe2Ph to afford (Z,Z)-bis(2-silylethenyl)arenes. Treatment of the hydrosilylation products with N-bromosuccinimide causes bromodesilylation in a stereospecific manner, giving (Z,Z)-bis(2-bromoethenyl)arenes in high geometrical purity (>98%).

Journal Article↗
PubMed2006

Aerobic intramolecular oxidative amination of alkenes catalyzed by NHC-coordinated palladium complexes.

[reaction: see text] Palladium(II) complexes bearing a single N-heterocyclic carbene ligand serve as effective catalysts for the aerobic oxidative cyclization of alkenes with pendant sulfonamides. The use of carboxylic acid cocatalysts (AcOH and PhCO(2)H) often leads to significant improvements in catalyst stability and product yield and enables catalytic turnover to be achieved with air, rather than pure oxygen gas, as the source of O(2).

Alkenes↗