The role of ancillary ligands and of electron poor alkenes and alkynes in stabilizing Pd(0) derivatives: A comparative study

The peculiar characteristics of the ligand neocuproine (2,9-dimethylphenanthroline) allow a number of exchange equilibrium studies between the low valence complex [Pd(eta(2)-nq)(Neocup)] (nq = naphthoquinone; Neocup = neocuproine) and several alkenes and alkynes. A new order of stability which compares differently unsaturated molecules was established. An overview of the factors governing the stability of palladium(0) alkene and alkyne derivatives as a function of the steric and electronic characteristics of both the unsaturated molecule and the ligand becomes accessible and a comparison with the previously determined order was therefore feasible. Such a comparison enlightens the importance of the substituent methyl groups in ortho position of the hetero-aromatic ring which represents the molecular fragment common to all the ligands considered. Taking advantage of the steric requirements of the alkene tmetc (tmetc = tetramethylethylenetetracarboxylate) a kinetic investigation of the reaction between the olefin itself and the complexes [Pd(eta(2)-dmfu)(L-L')] (dmfu = dimethylfumarate; L-L' = 8-diphenylphosphanyl-2-methyl-quinoline, neocuproine, phenanthroline) was carried out. The structures of the complexes [Pd(eta(2)-dmfu)(DPPQ)] and [Pd(eta(2)-deta)(Neocup)] (deta = but-2-ynedioc acid diethyl ester) were also reported in the present paper. The structure of the latter represents the first example of a palladium(0) complex in which the N(2)C(2) donor set around the metal centre is supported by a chelating eta(2)-alkyne

Synthesis, characterization, dynamics and reactivity toward amination of η3-allyl palladium complexes bearing mixed ancillary ligands. Evaluation of the electronic characteristics of the ligands from kinetic data.

On the basis of an original protocol, we have synthesized several complexes of the type [Pd(eta(3)-C(3)H(3)R(2))(LL')]ClO(4) (R = H, Me; L, L' = PPh(3), P(OEt)(3), 2,6-dimethylphenylisocyanide, t-butylisocyanide, 1,3-dimesitylimidazolidine, 1,3-dimesitylimidazol-2-ylidene). The complexes, some of which are completely new species, were fully characterized and their behaviour in solution was studied by means of (1)H NMR. The reactions of the complexes bearing the symmetric allyl moiety [Pd(eta(3)-C(3)H(5))(LL')]ClO(4) with piperidine in the presence of the olefin dimethylfumarate were followed under kinetically controlled conditions. Formation of allyl-amine and of the palladium(0) derivatives [Pd(eta(2)-dmfu)(LL'] was observed. The reaction rates k(2) proved to be strongly dependent on the ancillary ligand nature and allowed a direct comparison among the electronic characteristics of the ligands. The reactivity trend determined appears to be mainly influenced by the capability of the ancillary ligands in transferring electron density to the metal centre and consequently on the allyl fragment

Solid-state structure of 2,6-dimethyl-gamma-pyrone

2,6-dimethyl-4H-pyranylidene-4-one, shortly 2,6- dimethyl-y-pyrone, is a widely known chemical reagent used in the synthesis of various heterocycles. A contin uous quest is being carried out in our research group for metal ligands; the title compound was taken for examination due to its ability of coordinating transition elements, in particular lanthanoids

Ligating ability of 1,1 '-bis(diphenylphosphino)ferrocene: a structural survey (1994-1998)

This paper outlines the ligating ability of 1,1'-bis(diphenylphosphino) ferrocene. The focus is on the extensive coordination chemistry exhibited by this ligand to transition metals. The structural parameters of mono-, bi- and polynuclear species are reviewed and simple relationships between some of these parameters are reported and discussed. The review mainly covers the X-ray structure determinations published over the 1994-98 period. (C) 2000 Elsevier Science B.V. All rights reserved

Crystal structure of 2,5,6-trimethyl-9-methoxy-4H-pyrrolo[3,2,1-ij ] quinolin-4-one

A pale-yellow crystal of parallelepiped habit, produced by crystallization from methanol and having approximate dimensions of 0.2, 0.2, 0.8 mm, was selected for X-ray work. Intensity data were collected using a Siemens-Nicolet R3m/V automated diffractometer and highly oriented graphite crystal monochromatized Mo Ka radiation (fine tube, 50 KV, 30 mA, lambda = 0.71073 \uc0). The X-rays diffracted from the crystal were recorded by a Polaroid cassette mounted in front of the detector (at 19.5 cm), and a rotation photograph around the phi axis gave twenty 2X, 2Y values for evaluating the quality of the crystal and calculating angular positions for the reflections; they were centered later with a centering algorithm. An indexing routine, based on forty equivalent reflections in the range 20 < 2theta <40\ub0, selected a unit cell of the tetragonal system, with the dimensions a = 18.821(5), c = 13.882(8) angstrom and V = 4919(4) cubic angstrom The observed volume and density (1.31(1) g cm-3 corresponded to Z = 16, while the reflection conditions were consistent with the space group 141 cd (No. 110)

Molecular modeling, theoretical calculations and property evaluation of three muscarinic agonists, X-ray structures of LU 25-109 and WAL 2014

LU 25-109 (II) and WAL 2014 (talsaclidine, III) are two M1 muscarinic agonists chemically related to the natural substance arecoline (I). All these compounds have beneficial effects on memory and cognition in animals and humans, and they have been proposed in the treatment of Alzheimer's disease, but only III will likely find a place in therapy. In this work we have investigated the solid state structures of II and III and the X-ray structures of the two molecules and of the parent compound I have been used to input a series of computational chemistry efforts. In particular, the X-ray geometries have been manipulated to model 20 molecular structures (1-20) which have been submitted to ab initio, semiempirical quantum mechanics and molecular mechanics calculations. The conformational space accessible to the 20 structures has been assessed by means of potential energy maps. The reactivities of 1-20 have been estimated by examining at the graphics terminal the composition and the extension of the frontier orbitals (HOMOs and LUMOs) and of the molecular electrostatic potential. The information obtained has been interpreted to explain the different degrees of activity shown by I-III. Our data indicate that III has better in vivo activity for its intermediate size, less polar surface, conformational rigidity and orientation of reactive domains. (C) 2000 Elsevier Science B.V. All rights reserved

Chimica di Base

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Hydroxo and Oxo Complexes of Platinum(II) Stabilized by Phosphanes: Synthesis and Characterization− X-ray Structures ofcis-[L2Pt(μ-OH)]2(NO3)2 (L = PMe2Ph, PMePh2) and [{cis-(PMe2Ph)2Pt}3(μ-O)2]Cl2

The dinuclear hydroxo complexes cis- [L2Pt(mu-OH)](2)(NO3)(2) (L = PMe2Ph and PMePh2) have been characterized by single-crystal X-ray analysis, and their reactivity towards chloride ions has been investigated. Reaction of cis-[L2Pt(mu-OH)](2)X-2 (L = PMe3, PMe2Ph; X = NO3-, CIO4-) with (NEt4)CI.H2O or (AsPh4)CI, in a 1:2 molar ratio, afforded a mixture of cis-[L2PtCl2] and [{cis-L(2)pt}(3)(mu-O)(2)](2+) in ca. equimolar amounts. The trinuclear oxo compounds, separated by fractional crystallization of the reaction mixtures, were fully characterized by spectroscopic techniques, and the derivative [(cis-(PMe2Ph)(2)Pt)(3)(mu-O)(2)]CI2 was also characterized by X-ray diffraction methods. Under the same experimental conditions, the structurally analogous hydroxo complex, stabilized by the less basic and more hindered PMePh2 ligand, reacted with Cl- to give cis-[(PMePh2)(2)PtCl2] as the only isolable species. The P-31 NMR spectroscopic analysis of the reaction mixtures allowed the detection of a moderately stable product, which is most likely to be the neutral hydroxo complex cis-[L2PtCl(OH)]

Hydroxo and oxo complexes of platinum(II) stabilized by phosphanes: Synthesis and characterization - X-ray structures of cis-[L2Pt(mu-OH)](2)(NO3)(2) (L = PMe2Ph, PMePh2) and [{cis-(PMe2Ph)(2)Pt}(3)(mu-O)(2)]Cl-2

The dinuclear hydroxo complexes cis- [L2Pt(mu-OH)](2)(NO3)(2) (L = PMe2Ph and PMePh2) have been characterized by single-crystal X-ray analysis, and their reactivity towards chloride ions has been investigated. Reaction of cis-[L2Pt(mu-OH)](2)X-2 (L = PMe3, PMe2Ph; X = NO3-, CIO4-) with (NEt4)CI.H2O or (AsPh4)CI, in a 1:2 molar ratio, afforded a mixture of cis-[L2PtCl2] and [{cis-L(2)pt}(3)(mu-O)(2)](2+) in ca. equimolar amounts. The trinuclear oxo compounds, separated by fractional crystallization of the reaction mixtures, were fully characterized by spectroscopic techniques, and the derivative [(cis-(PMe2Ph)(2)Pt)(3)(mu-O)(2)]CI2 was also characterized by X-ray diffraction methods. Under the same experimental conditions, the structurally analogous hydroxo complex, stabilized by the less basic and more hindered PMePh2 ligand, reacted with Cl- to give cis-[(PMePh2)(2)PtCl2] as the only isolable species. The P-31 NMR spectroscopic analysis of the reaction mixtures allowed the detection of a moderately stable product, which is most likely to be the neutral hydroxo complex cis-[L2PtCl(OH)]

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