Scienze chimiche e geologiche

The paper presents a detailed experimental and theoretical study of the four mixed nickel−bisdithiolene complexes [Ni(Pr i 2 pipdt)(dmit)] (1b, Pr i 2 pipdt ) 1,4-diisopropyl-piperazine-3,2-dithione; dmit ) 1,3-dithiolo-2-tione-4,5-dithiolato), [Ni(R 2 pipdt)(mnt)] (2b′′, R ) 2-ethylhexyl; mnt ) maleonitriledithiolato), [Ni(Pr i 2 timdt)(dmit)] (3b, Pr i 2 timdt ) 1,3diisopropyl-imidazoline-2,4,5-trithione), and [Ni(Pr i 2 timdt)(mnt)] (4b), and their models. All the complexes, with common (C 2 S 2 )Ni(C 2 S 2 ) core and two different terminal groups, are uncharged and square-planar coordinated. Previous measurements of the first molecular hyperpolarizability indicated that some of the species are potential NLO chromophores due to the π-delocalized character of two frontier levels (HOMO and LUMO) which is asymmetrically perturbed by the combination of one push (R 2 pipdt, R 2 timdt) with one pull ligand (dmit and mnt). The X-ray structure of complex 1b is presented and its geometry is compared with those available in the literature for the four types of complexes under study. The results of electrochemical and spectroscopic measurements (oxidation and reduction potentials, IR, dipole moment, molecular absorptivities, etc.) indicate rather different responses between the pairs of complexes 1−2 and 3−4. Hence, DFT calculations on the model compounds 1a−4a, where hydrogen atoms replace the alkyl groups of R 2 pipdt and R 2 timdt, have been carried out to correlate geometries and electronic structures. Moreover, the first molecular hyperpolarizabilities have been calculated and their components have been analyzed with the simplest two-level approximation. The derived picture highlights the different roles of the two push and pull ligands, but also the peculiar perturbation of the π-electron density induced by the terminal CS 3 grouping of the ligand dmit. Okano, Y.; Kobayashi, H.; Suzuki, W.; Kobayashi, A. Science 2001, 291, 285. (2) Coomber, A. T.; Beljonne, D.; Friend, R. H.; Brédas, J. K.; Charlton, A.; Robertson, N.; Underhill, A. E.; Kurmoo, M.; Day, P.

We describe here an innovative method for Pd-dissolution using the reagent N,N9-dimethylperhydrodiazepine-2,3-dithione diiodine adduct which, being safe and powerful, is appealing for practical applications: remarkably almost quantitative Pdrecovery from model spent three-way catalysts has been obtained, showing that the effectiveness of the method is maintained when palladium is contained in a complex system such as a car catalyst, a ceramic material which has undergone severe thermal stress.

The synthesis and structural characterization of the neutral gadolinium tetrakis-type complex with 5,7-dichloro-8-quinolinolato (H5,7ClQ) ligands, [Gd(5,7ClQ) 2 (H5,7ClQ) 2 Cl], is reported. The Gd III ion is epta coordinated to one chloride ion and to four ligands: two N,O-chelated monoanions and two as zwitterionic ( + NH and O -) monodentate oxygen donors. Electronic and vibrational spectra provide suitable markers to distinguish the presence of deprotonated and protonated ligands. The magnetic behaviour of this compound was in-

Two hybrid molecular materials showing a combination of magnetic and conducting properties, the charge-transfer (ET) 2 [Fe(tdas) 2 ] (1) and (ET)Ni(tdas) 2 (2), (ET=bis(ethylenedithio) tetrathiafulvalene; M=Fe, Ni; tdas=1,2,5-thiadiazole-3,4-dithiolate) salts, are characterized by vibrational (IR and Raman) and UV-VIS-NIR spectroscopies. These studies have proved to be effective and diagnostic tools in identifying the oxidation state (partial or integer) and the packing pattern (dimers or segregated stacks) of the ET donor only, since no ν(C=C) group vibration sensitive to the charge of M(tdas) 2 complexes has been observed. This is ascribed to the extensive electron-delocalization inside the tdas ring in agreement with semiempirical extended Huckel calculations, indicating also that the AO's of the terminal S-atom give a high contribution to the HOMO and favour the occurrence of three-dimensional intermolecular interactions along the molecular longitudinal axis. These findings are consistent with structural results.

Experimental and theoretical results on the novel [Pd(Me 2 pipdt)(dmit)] complex (1) based on the push Me 2 pipdt (1,4-dimethyl-piperazine-3,2-dithione) and pull dmit (1,3-dithiolo-2-thione-4,5-dithiolato), show that an unbalanced electron distribution at the dithiolene core occurs. The presence of a solvatochromic absorption at low energy, shifted to higher frequency when compared to the corresponding nickel case, points out that these complexes are potential second-order non-linear chromophores, whose properties are tunable with the metal.

We report the first combined optical and structural investigation of the water free Er-quinolinolate complex, an organo-lanthanide system of interest for 1.5-µm telecom applications. Structural data demonstrate that the complex has a trinuclear structure (Er 3 Q 9 ) which provides the Er metals with an octa-coordination by the organic ligand and prevents solvent and water molecules from entering the lanthanide coordination sphere. The results of the structural analysis allow us to infer that the strong Er luminescence quenching exhibited by the Er 3 Q 9 complex is due uniquely to resonant energy transfer to the aromatic C−H vibrations of the ligand, providing the correct tools to design more efficient emitters.

A B S T R A C T : T w o n e w i s o s t r u c t u r a l m o l e c u l a r m e t a l s  ( B D H -TTP) 6 [M III (C 5 O 5 ) 3 ]·CH 2 Cl 2 (BDH-TTP = 2,5-bis(1,3-dithiolan-2-ylidene)-1,3,4,6tetrathiapentalene, where M = Fe (1) and Ga (2))have been prepared and fully characterized. Compound 1 is a molecular conductor showing paramagnetic behavior, which is due to the presence of isolated [Fe(C 5 O 5 ) 3 ] 3− complexes with high-spin S = 5 / 2 Fe(III) metal ions. The conductivity originates from the BDH-TTP organic donors arranged in a κ-type molecular packing. At 4 kbar, compound 1 behaves as a metal down to ∼100 K, showing high conductivity (∼10 S cm −1 ) at room temperature. When applying a pressure higher than 7 kbar, the metal−insulator (M-I) transition is suppressed and the compound retains the metallic state down to low temperatures (2 K). For 1, ESR signals have been interpreted as being caused by the fine structure splitting of the high-spin (S = 5 / 2 ) state of Fe(III) in the distorted octahedral crystal field from the ligands. At 4 kbar, the isostructural compound 2 behaves as a metal down to ∼100 K, although it is noteworthy that the M-I transition is not suppressed, even at pressures of 15 kbar. For 2, only the signal assigned to delocalized π-electrons has been observed in the ESR measurements.

The reaction of the croconate dianion (C 5 O 5 ) 2− with a Fe(III) salt has led, unexpectedly, to the formation of the first example of a discrete Fe(II)−croconate complex without additional coligands, K 4 [Fe(C 5 O 5 ) 2 (H 2 O) 2 ](HC 5 O 5 ) 2 ·4H 2 O (1). 1 crystallizes in the monoclinic P2 1 /c space group and presents discrete octahedral Fe(II) complexes coordinated by two chelating C 5 O 5 2− anions in the equatorial plane and two trans axial water molecules. The structure can be viewed as formed by alternating layers of trans-diaquabis(croconato)ferrate(II) complexes and layers containing the monoprotonated croconate anions, HC 5 O 5 − , and noncoordinated water molecules. Both kinds of layers are directly connected through a hydrogen bond between an oxygen atom of the coordinated dianion and the protonated oxygen atom of the noncoordinated croconate monoanion. A H-bond network is also formed between the coordinated water molecule and one oxygen atom of the coordinated croconate. This H-bond can be classified as strong−moderate being the O···O bond distance (2.771(2) Å) typical of moderate H-bonds and the O−H···O bond angle (174(3)°) typical of strong ones. This Hbond interaction leads to a quadratic regular layer where each [Fe(C 5 O 5 ) 2 (H 2 O) 2 ] 2− anion is connected to its four neighbors in the plane through four equivalent H-bonds. From the magnetic point of view, these connections lead to an S = 2 quadratic layer. The magnetic properties of 1 have been reproduced with a 2D square lattice model for S = 2 ions with g = 2.027(2) and J = 4.59(3) cm −1 . This model reproduces quite satisfactorily its magnetic properties but only above the maximum. A better fit is obtained by considering an additional antiferromagnetic weak interlayer coupling constant (j) through a molecular field approximation with g = 2.071(7), J = 2.94(7) cm −1 , and j = −0.045(2) cm −1 (the Hamiltonian is written as H = −JS i S j ). Although this second model might still be improved since there is also an extra contribution due to the presence of ZFS in the Fe(II) ions, it confirms the presence of weak ferromagnetic Fe−Fe interactions through H-bonds in compound 1 which represents one of the rare examples of ferromagnetic coupling via H-bonds.

We investigate the quenching of the near infrared light emission in Er 3+ complexes induced by the resonant dipolar interaction between the rare-earth ion and high frequency vibrations of the organic ligand. The nonradiative decay rate of the lanthanide ion is discussed in terms of a continuous medium approximation, which depends only on a few, easily accessible spectroscopic and structural data. The model accounts well for the available experimental results in Er 3+ complexes, and predicts an ϳ100% light emission quantum yield in fully halogenated systems.

The synthesis, X-ray structure, magnetic and transport properties of the compound Ni(dmf) [Ni(dsit) 2 ] 2 (dmf=dimethylformamide, dsit=1,3-dithiole-2-thione-4,5-diselenolate) are described. This compound crystallizes in the monoclinic space group P2 1 =c; with a=18.709(6), b=22.975(5), c=20.418(5) ( A, b=99.31(2)1 and Z=6; its structure consists of [Ni(dsit) 2 ] 2 2À dimers and isolated [Ni(dmf) 6 ] 2+ cations both centrosymmetric and non-centrosymmetric. The dimers are packed forming chains along the [101] direction with short Se?Se interdimer contacts. Additional interchains S?S contacts render this structure a three-dimensional character, never observed so far in other [Ni(dsit) 2 ] À salts. This compound exhibits semiconducting behavior with a room temperature conductivity (1 S cm À1 ) much higher than those reported for other salts of the [Ni(dsit) 2 ] À anion. Tight-binding band structure calculations were used to analyze the origin of the semiconducting properties of this salt. The magnetic susceptibility shows Curie behavior with C=1.25 emu K mol À1 , typical of isolated Ni(II) ions as expected for the octahedrally coordinated [Ni(dmf) 6 ] 2+ cations. # 2002 Elsevier Science (USA)