Richard Welter

New ionic liquid crystals based on imidazolium salts

The new ionic liquid crystal 1-(4-dodecyloxybenzyl)-3-methyl-1H-imidazol-3-ium bromide has been synthesized and its X-ray structure obtained; various salts containing BF4−, SCN−, PF6−, CF3SO3−, (CF3SO2)2N− have been prepared by anion metathesis in water from bromide species as starting materials, and finally, the mesomorphism and electrochemical behavior have been investigated.

An approach to chiral magnets using α-hydroxycarboxylates

Carboxylate-bridged complexes of transition metals, MII = MnII, FeII, CoII, NiII, CuII, were synthesized by reaction of MII salts with (R)-mandelic acid ((R)-2-hydroxyphenylacetic acid), methoxymandelic acid (racemic) (2-methoxyphenylacetic acid) and (R)-malic acid ((R)-2-hydroxybutanedioic acid) under hydrothermal conditions. A wide variety of structures has been obtained, with various dimensionalities: 3D networks when the malic diacid is used, 2D networks with the mandelic acid, 1D isolated chains organised in planes for the Co and Cu compounds with MeOmandelic acid and monomers for the Ni analogue. These complexes have been fully characterized structurally and magnetically. During the reaction process, the (R)-mandelic ligand undergoes a racemisation reaction but the compounds obtained with (R)-malic acid are chiral and exhibit metal-centred circular dichroism. Extensive magnetic characterization of all compounds indicates rather weak coupling interactions between paramagnetic centres linked through carboxylate bridges. Curie-like paramagnetic, antiferromagnetic, ferromagnetic or weak ferromagnetic behaviour was observed and is discussed on the basis of the structural features. The bimetallic compounds Mn0.63Co0.37(R)-malate monohydrate (12) and Mn0.79Ni0.21(R)-malate monohydrate (13) are new examples of chiral magnets.

Normal-to-Abnormal NHC Rearrangement of Al(III) , Ga(III) , and In(III) Trialkyl Complexes: Scope, Mechanism, Reactivity Studies, and H2 Activation.

The present contribution reports experimental and theoretical mechanistic investigations on a normal-to-abnormal (C2-to-C4-bonded) NHC rearrangement processes occurring with bulky group 13 metal NHC adducts, including the scope of such a reactivity for Al compounds. The sterically congested adducts (nItBu)MMe3 (nItBu=1,3-di-tert-butylimidazol-2-ylidene; M=Al, Ga, In; 1 a-c) readily rearrange to quantitatively afford the corresponding C4-bonded complexes (aItBu)MMe3 (4 a-c), a reaction that may be promoted by THF. Thorough experimental data and DFT calculations were performed on the nNHC-to-aNHC process converting the Al-nNHC (1 a) to its aNHC analogue 4 a. A nItBu/aItBu isomerization is proposed to account for the formation of the thermodynamic product 4 a through reaction of transient aItBu with THF-AlMe3 . The reaction of benzophenone with (nItBu)AlMe3 afforded the zwitterionic species (aItBu)(CPh2 -O-AlMe3 ) (6), reflecting the unusual reactivity that such bulky adducts may display. Interestingly, the nItBu/Al(iBu)3 Lewis pair behaves like a frustrated Lewis pair (FLP) since it readily reacts with H2 under mild conditions. This may open the way to future reactivity developments involving commonly used trialkylaluminum precursors.

Reactivity of Molecular Dioxygen towards a Series of Isostructural Dichloroiron(III) Complexes with Tripodal Tetraamine Ligands: General Access to μ‐Oxodiiron(III) Complexes and Effect of α‐Fluorination on the Reaction Kinetics

We have synthesized the mono, di-, and tri-alpha-fluoro ligands in the tris(2-pyridylmethyl)amine (TPA) series, namely, FTPA, F(2)TPA and F(3)TPA, respectively. Fluorination at the alpha-position of these nitrogen-containing tripods shifts the oxidation potential of the ligand by 45-70 mV per added fluorine atom. The crystal structures of the dichloroiron(II) complexes with FTPA and F(2)TPA reveal that the iron center lies in a distorted octahedral geometry comparable to that already found in TPAFeCl(2). All spectroscopic data indicate that the geometry is retained in solution. These three isostructural complexes all react with molecular dioxygen to yield stable mu-oxodiiron(III) complexes. Crystal structure analyses are reported for each of these three mu-oxo compounds. With TPA, a symmetrical structure is obtained for a dicationic compound with the tripod coordinated in the kappa(4)N coordination mode. With FTPA, the compound is a neutral mu-oxodiiron(III) complex with a kappa(3)N coordination mode of the ligand. Oxygenation of the F(2)TPA complex gave a neutral unsymmetrical compound, the structure of which is reminiscent of that already found with the trifluorinated ligand. On reduction, all mu-oxodiiron(III) complexes revert to the starting iron(II) species. The oxygenation reaction parallels the well-known formation of mu-oxo derivatives from dioxygen in the chemistry of porphyrins reported almost three decades ago. The striking feature of the series of iron(II) precursors is the effect of the ligand on the kinetics of oxygenation of the complexes. Whereas the parent complex undergoes 90 % conversion over 40 h, the monofluorinated ligand provides a complex that has fully reacted after 30 h, whereas the reaction time for the complex with the difluorinated ligand is only 10 h. Analysis of the spectroscopic data reveals that formation of the mu-oxo complexes proceeds in two distinct reversible kinetic steps with k(1) approximately 10 k(2). For TPAFeCl(2) and FTPAFeCl(2) only small variations in the k(1) and k(2) values are observed. By contrast, F(2)TPAFeCl(2) exhibits k(1) and k(2) values that are ten times higher. These differences in kinetics are interpreted in the light of structural and electronic effects, especially the Lewis acidity at the metal center. Our results suggest coordination of dioxygen as an initial step in the process leading to formation of mu-oxodiiron(III) compounds, by contrast with an unlikely outer-sphere reduction of dioxygen, which generally occurs at negative potentials.

A convenient method for preparing rigid-core ionic liquid crystals

Summary An efficient, solvent free method for the N-arylation of imidazole by 1-(dodecyloxy)-4-iodobenzene using Cu(II)-NaY as catalyst and K2CO3 as base is reported. By this synthetic approach, mesomorphic 3-[4-(dodecyloxy)phenyl]-1-methyl-1H-imidazol-3-ium iodide was synthesized in a two-step procedure, and its mesomorphism has been fully investigated by polarised optical microscopy, differential scanning calorimetry and X-ray diffraction. In addition its lamellar crystal structure, electrochemical behaviour and UV (absorption and emission) properties are reported.

Di- and trinuclear phosphido-bridged platinum complexes. Crystal structures of [Pt{CH(2)=CHC(O)OMe}(PPh(3))(2)], trans-[Pt(2)(micro-PPh(2))(2)I(2)(PPh(3))(2)] and cis,cis,cis-[Pt(3)(mu-I)(2)(mu-PPh(2))(2)Cl(0.5)I(1.5)(PPh(3))(2)].

The readily available Pt(0) methyl acrylate complex [Pt{CH2CHC(O)OMe}(PPh3)2] (2) allows access to the known, mixed-valence trinuclear cluster [Pt3(mu-PPh2)3Ph(PPh3)2] (3) in 64% yield. Oxidation of 3 with 2 equivalents of I2 afforded the new trinuclear complex [Pt3(mu-I)2(mu-PPh2)2I2(PPh3)2] (4) whose molecular structure is similar to that of the related compound of empirical formula [Pt3(mu-I)2(mu-PPh2)2Cl0.5I1.5(PPh3)2] ( 5) which has been generated by oxidation of 3 with successively 1 equivalent of I2 and 1 equivalent of C6H5ICl2. In these complexes, the four halogen atoms lie on the same side of the almost aligned platinum atoms and the nearly square-planar coordination planes of the metal atoms adopt a japanese screen, chair-like conformation. The reaction of the dinuclear, metal-metal bonded Pt(I)-Pt(I) complex [Pt2(mu-PPh2)2(PPh3)2] with one equivalent of I2 afforded the Pt(II) complex [Pt2(mu-PPh2)2I2(PPh3)2] (6). The molecular structures of complexes 2 x CH2Cl2, [Pt3(mu-I)2(mu-PPh2)2(I1.3Cl0.7)(PPh3)2][Pt3(mu-I)2(mu-PPh2)2(I1.7Cl0.3)(PPh3)2] x C6H5Cl x 3CH2Cl2 (5A x 5B x C6H5Cl x 3CH2Cl2) and 6 have been established by single crystal X-ray diffraction studies.

Solid state and solution study of some phosphoramidate derivatives containing the P(O)NHC(O) bifunctional group: Crystal structures of CCl2HC(O)NHP(O)(NCH3(CH2C6H5))2, p-ClC6H4C(O)NHP(O)(NCH3(CH2C6H5))2, CCl2HC(O)NHP(O)(N(CH2C6H5)2)2 and p-BrC6H4C(O)NHP(O)(N(CH2C6H5)2)2

Synthetic methods for several novel phosphoramidate compounds containing the P(O)NHC(O) bifunctional group were developed. These compounds with the general formula R(1)C(O)NHP(O)(N(R(2))(CH(2)C(6)H(5)))(2), where R(1)=CCl(2)H, p-ClC(6)H(4), p-BrC(6)H(4), o-FC(6)H(4) and R(2)=hydrogen, methyl, benzyl, were characterized by several spectroscopic methods and analytical techniques. The effects of phosphorus substituents on the rotation rate around the P-N(amine) bond were also investigated. (1)H NMR study of the synthesized compounds demonstrated that the presence of bulky groups attached to the phosphorus center and electron withdrawing groups in the amide moiety lead to large chemical-shift non-equivalence (Deltadelta(H)) of diastereotopic methylene protons. The crystal structures of CCl(2)HC(O)NHP(O)(NCH(3)(CH(2)C(6)H(5)))(2), p-ClC(6)H(4)C(O)NHP(O)(NCH(3)(CH(2)C(6)H(5)))(2), CCl(2)HC(O)NHP(O)(N(CH(2)C(6)H(5))(2))(2) and p-BrC(6)H(4)C(O)NHP(O)(N(CH(2)C(6)H(5))(2))(2) were determined by X-ray crystallography using single crystals. The coordination around the phosphorus center in these compounds is best described as distorted tetrahedral and the P(O) and C(O) groups are anti with respect to each other. In the compound Br-C(6)H(4)C(O)NHP(O)(N(CH(2)C(6)H(5))(2))(2) (with two independent molecules in the unit cell), two conformers are connected to each other via two different N-H...O hydrogen bonds forming a non-centrosymmetric dimer. In the crystalline lattice of other compounds, the molecules form centrosymmetric dimers via pairs of same N-H...O hydrogen bonds. The structure of CCl(2)HC(O)NHP(O)(N(CH(2)C(6)H(5))(2))(2) reveals an unusual intramolecular interaction between the oxygen of C=O group and amine nitrogen.

Square pyramidal geometry around the metal and tridentate coordination mode of the tripod in the [6-(3′-cyanophenyl)-2-pyridylmethyl]bis(2-pyridylmethyl)amine FeCl2 complex: a solid state effect

Metalation of the cyanophenyl mono α-substituted TPA ligand by ferrous chloride affords a stable neutral compound with spectroscopic properties in solution (molecular conductivity, UV-visible and paramagnetic 1H NMR) indicating that the ligand coordinates in the tetradentate mode providing a distorted octahedral geometry around the metal. In the solid state however, the tripod acts as a tridentate ligand, and crystal structure analysis reveals a square pyramidal geometry around the metal. The substituted pyridyl arm is the dangling one, and the cyanide group seems to interact with the metal center of a neighboring molecule. Increasing the ionic strength of a solution of the compound leads to dissociation of the chloride ions from the metal, affording the bis (μ-chloro) diferrous dication, the structure of which is also reported.

Phosphinooxazolines as assembling ligands in heterometallic complexes

It is shown that phosphinooxazolines such as (2-oxazoline-2-ylmethyl)diphenylphosphine 1 can be used as P,N assembling ligands for the synthesis of heterometallic complexes. These were obtained in a stepwise manner from the iron carbonyl complexes containing one, [Fe(CO)4(PCH2-oxazoline-P)] 2, or two, trans-[Fe(CO)3(PCH2-oxazoline-P)2] 6, P-bound phosphinooxazoline ligands. These metalloligands were then reacted with metal complexes. With 2, complete ligand transfer from Fe to the second metal (Pd) was sometimes observed. The metal–metal bonded complex [(OC)4Fe(μ-PCH2-oxazoline-P,N)PdCl2] 3 was of limited stability, whereas the trinuclear complex trans-[{(OC)4Fe(μ-PCH2-oxazoline-P,N)}2CoCl2] 4, in which 2 functions as a N-donor metalloligand toward the tetrahedral Co(II) centre, was stable and could be fully characterised by X-ray diffraction. This complex catalyses the oligomerization of ethylene to give linear α-olefins in the range C4–C26, with a maximum of the Schulz-Flory distribution around C6. The complex [{(OC)4Fe(μ-PCH2-oxazoline-P,N)}2HgI2] 5 was also prepared from 6. With the metalloligand 6, the Fe→Pd and Fe→Co metal–metal bonded compounds trans-[(OC)3Fe(μ-PCH2-oxazoline-P,N)2Pd(NCMe)](BF4)28 and trans-[(OC)3Fe(μ-PCH2-oxazoline-P,N)2CoCl2] 9 were obtained.

Electron density of CaNi2Si2 studied using synchrotron x-ray diffraction and first-principles calculations

The electron-density distribution in CaNi2 Si2 has been analysed by means of x-ray diffraction measurements and a full-potential augmented-plane-wave band-structure calculation. The agreement between experiment and theory is good, considering the difficulty of the experiment. A Si-Si bonding interaction is clearly observed in the valence electron distribution as well as a preferred occupation of the Ni 3d orbitals.