Marcello Longeri

Conformation of ethylene glycol dissolved in a nematic-lyotropic solution: An N.M.R. analysis

The conformation of ethylene glycol (EG) has been investigated by means of liquid crystal nuclear magnetic resonance spectroscopy (LICRY-N.M.R.). The proton and 13C N.M.R. spectra were obtained from a quarternary solution of potassium laurate (KL), decanol, water and EG. These spectra were analysed to get the proton-proton and proton-carbon direct dipolar couplings. Spectra obtained from an aqueous solution have also been analysed. The experimental results were then used to investigate the conformational equilibrium of the molecule. It has been found that in the lyotropic liquid crystalline solution EG exists in a gauche conformation only, and that the value of the OC-CO dihedral angle is 72°.

Complete rα structure of selenophene partially oriented in a nematic liquid crystal phase by 1H NMR spectra including 77Se and 13C satellites

Abstract The proton NMR spectra of selenophene together with the 77Se and 13C satellites have been recorded both in isotropic CDCl3 solution and in a thermotropic nematic phase. Analysis of the spectra yielded Hue5f8H, 13Cue5f8H and 77Seue5f8H direct dipolar couplings which have been corrected for the effect of molecular vibrations according to the procedure proposed by Lucas and used to obtain a complete rα molecular structure. A comparison of the NMR interatomic distance ratios with those from microwave (mw) measurements is made. The significant distortion of the molecule in the nematic liquid crystalline phase, with respect to the mw structure, is discussed.

3,6-Bis(2'-pyridyl)pyridazine-based ligands: synthesis and organometallic complexes of platinum(II)

In order to improve the solubility of bimetallic complexes in organic solvents, some new 3,6-bis(2′-pyridyl)pyridazine-based molecules have been synthesised and tested as ligands in platinum(II) organometallic complexes. The reaction of 3,6-bis(2′-pyridyl)-1,2,4,5-tetrazine with terminal acetylenes gives 3,6-bis(2′-pyridyl)-4-substituted pyridazines. Their further reactivity toward (DMSO)2Pt(CH3)2 or (COD)Pt(C6H5)2 (1:1 molecular ratio) is reported. The characterisation of the newly synthesised compound by IR, 1H NMR and electronic spectroscopy is described.

Platinum(II) dimethyl and diphenyl complexes with some bidiazines and 3,6-bis(2'-pyridyl)pyridazine

Abstract The dimethyl and diphenyl platinum(II) complexes with 3,3′-bipyridazine (bpdz), 2,2′-bipyrazine (bpz) and 3,6-bis(2′-pyridyl)pyridazine (dppn) are reported. The newly synthesized compounds were characterized by elemental analysis, 1 H NMR and electronic spectroscopy. The strength of the back- bonding interaction of these nitrogen-containing ligands has been estimated on the basis of the energy of the first metal-to-ligand chaarge transfer electronic transition. In this respect the present results, together with the literature data conceming the homologous derivatives containing 2,2′-bipyridine (bipy) and 2,2′-bipyrimidine (bpym), give the following order bpdz ⋍ bpz > dppn > bpym > bipy.

Conformational study of 2,2′-biselenophene partially oriented in a nematic liquid crystral phase by PMR spectra including77Se satellites

Abstract Different models for conformational equilibrium of 2,2′-biselenophene have been examined in order to reproduce the D H H and D Se H direct dipolar couplings obtained by NMR spectra using liquid crystal solvents. The results allow the rigorous exclusion of various conformational possibilities and confine the conformation to an equilibrium between two twisted conformers, the transoid one being the more abundant.

Reactivity of the iridium nitrosyl complex [Ir(NO)(paa)(PPh3)2][PF6]2(paa = 2-pyridinecarbaldehyde azine) with nucleophiles: insertion of NO into a methinic C–H bond; crystal structure of [Ir{(2-C5H4N)CHN–NC(NOH)(2-C5H4N)}{Me-(EtO)CNH}(PPh3)2][PF6]2

The nitrosyl complex [Ir(NO)(paa)(PPh3)2][PF6]2(paa = 2-pyridinecarbaldehyde azine) reacts either with RCN (R = Me, Ph, or Ph2CH) or X–(X = Cl or SCN) at room temperature, to give cyclometallated compounds containing the I[graphic omitted](OH) moiety. These complexes were characterised by elemental analysis, conductivity measurements, i.r. and n.m.r. spectroscopy. The metallated ring can be envisaged as resulting from intramolecular insertion of the NO group, activated by the iridium co-ordination of RCN or X–, into the methinic C–H bond of the unco-ordinated paa fragment. Addition of ethanol to the compound containing MeCN or PhCN gives imidate complexes. The structure of the acetoimidate complex [Ir{(2-C5H4N)CHN–NC(NOH)(2-C5H4N)}(Me(EtO)CNH}(PPh3)2][PF6]2 has been determined by X-ray diffraction. Crystals are monoclinic, space group Cc, with unit-cell dimensions a= 12.405(6), b= 43.836(12), c= 11.126(7)A, β= 112.1(2)°, and Z= 4. The structure has been solved from diffractometer data by Patterson and Fourier methods and refined by block-matrix least squares to final R and R′ values of 0.057 and 0.073 for 3 394 observed reflections. In the cation four N atoms, one from the neutral ethylacetoimidate ligand and the other three from the monoanionic ligand resulting by intramolecular insertion of NO into a methinic C–H bond of the starting paa ligand, are arranged in the equatorial positions of the iridium co-ordination octahedron whose apices are occupied by P atoms from PPh3 molecules. The new [graphic omitted] fragment displays a C–N bond length of 1.37(3)A and the OH group is involved in a strong intramolecular hydrogen bond with the unco-ordinated pyridyl nitrogen atom.

Nuclear relaxation of partially oriented pyridine determined by line width analysis of 1H NMR spectra

An NMR lineshape study of pyridine dissolved in nematic PCH mesophase has been carried out. Measurements of spectral frequencies and intensities enabled us to determine the order parameters defining the molecular orientation. Lineshape analysis indicates that the highly selective broadening of lines is mainly due to the coupling of protons with the rapidly relaxing 14N quadrupolar nucleus. A further contribution to the line width comes from the dipolar intermolecular relaxation mechanism. The quadrupolar relaxation time of the 14N nucleus and the correlation time of the intermolecular interaction were calculated.

Use of the satellites of the 13C and 125Te obtained in the P.M.R. spectra of nematic liquid crystal mesophases

The complete r α structure of tellurophene has been determined from proton magnetic resonance spectra, including 13C and 125Te satellites, obtained in three thermotropic nematic mesophases. The molecular structures observed in the different solvents have been compared both to each other and with microwave data, and discussed with respect to possible solute-solvent interactions. Isotropic P.M.R. spectra and the relative heteronuclear sub-spectra have also been analysed in order to obtain all the Jij indirect coupling constants. Absolute signs were determined for 2 J TeH and 3 J TeH. The anisotropic contribution to the Te-H indirect couplings was found to be negligible.

Addition of arenediazonium ligands to a PdPd bond: a reinvestigation

Abstract Arenediazonium salts [p-YC6H4N2][BF4] (Yue5fbH, CH3, OCH3, F, NO2) react with the A-frame precursor complexes [Pd2X2(dppm)2] (1) (Xue5fbCl, Br, I; dppmue5fbbis(diphenylphosphino)methane) affording 1:1 adducts. 1H and 31P NMR spectroscopic data support the formulation of the new complexes as A-frame molecules [Pd2X2(dppm)2(μ-N2-p-C6H4Y)][BF4] (2) containing a symmetrically bridging arenediazenido ligand. The crystal structures of [Pd2I2(dppm)2(μ-N2-p-C6H4CH3)][BF4] (2d) and [Pd2Cl2(dppm)2(μ-N2-p-C6H4F)][BF4] (2f) have been determined by X-ray crystallography. 2d crystallizes in the orthorhombic space group P212121 with Z=4 in a unit cell of dimensions a=14.642(4), b=19.166(5), c=21.338(6) A. The structure was refined to R=0.068 and Rw=0.073 on the basis of 4325 observed reflections with I>3σ(I). 2f crystallizes in the tetragonal space group P43212 with a=14.886(1), c=25.303(8) A and Z=4. The structure was refined to R=0.039 and Rw=0.043 using 4681 unique reflections and 353 parameters. The formation of [PtPdCl2(dppm)2(μ-N2-p-C6H4F)][BF4] (4) and its solution structure is also described.

Structure and internal rotation in 3-phenylthiophene using NMR spectra of liquid-crystalline solutions

Abstract The NMR spectra of 3-phenylthiophene as solute in liquid-crystal solvents were analyzed and the direct couplings constants, D ij obtained were used to test different models of the potential energy function V (φ) for intramolecular rotation of the phenyl and thiophene units. The data are consistent with a V(φ) having a minimum at about 24°. The LCNMR technique is highly discriminant among different models of V(φ) provided that enough D ij couplings are available. Ab initio calculations are also reported.