Paola Deplano

Structural and Raman spectroscopic studies as complementary tools in elucidating the nature of the bonding in polyiodides and in donor-I2 adducts

Abstract A large number of examples of polyiodides and donor-I 2 adducts whose structures and Raman spectra are reasonably well-established, have been collected. An empirical method based on structural and Raman data, as complementary tools in elucidating the nature of the iodine moiety, is proposed to remove the arbitrariness in considering discrete/non discrete entities and to provide a unified picture of the bonding in these species. Accordingly polyiodides higher than I 3 − do not exist as discrete entities but are formed by a combination of I − and/or I 3 − with I 2 through donor–acceptor interactions. The I 3 − moiety represents a crossover, its structural and vibrational features show that it can exist as an I 3 − entity (in accordance with the existence of a covalent bond interaction describable by means of a delocalized three-center four-electron, 3c-4e, bond) or I − ·I 2 donor–acceptor adduct. The formal I 2 -adducts of donors (D) show structural and Raman features which can be explained considering three types of adducts: D·I 2 , D–I–I (covalent bond interaction describable by means of a delocalized 3c-4e bond by analogy with the I 3 − entity case) and (D–I) + ·I − , which can be considered as stages along the reaction coordinate between the separated D and I 2 and the salt [(D–I) + ]I − . The precautions to be taken in studying these species have been cited. The possibility that some of the Raman attributes in the literature need corrections has been pointed out in regard to the instability inherent in these materials since the decomposition of these species causes spurious peaks to appear in the Raman spectrum.

On the Use of Raman Spectroscopy in the Characterization of Iodine in Charge-Transfer Complexes

FT-Raman spectra of some polyiodides and of a series of D · I2 charge-transfer complexes (where D is a molecule containing the thione or selone groups as donors), all characterized by x-ray diffraction, are reported. For the adducts with the thione compounds, which can be considered weak or medium-weak complexes, an empirical linear correlation between the frequency of the v(I-I) stretching vibrations and the d(I-I) bond distances has been found. Some polyiodides show FT-Raman spectra that are indistinguishable with respect to those displayed by the neutral complexes of weak or medium-weak strength; in such cases, the polyiodide can be regarded as a diiodine molecule, perturbed by an I n (n = 1,3,…) donor. Polyiodides of this type show Raman absorptions falling in the linear correlation.

Near infrared light emission quenching in organolanthanide complexes

We investigate the quenching of the near infrared light emission in Er3+ 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 Er3+ complexes, and predicts an ∼100% light emission quantum yield in fully halogenated systems.

A Family of Layered Chiral Porous Magnets Exhibiting Tunable Ordering Temperatures

A simple change of the substituents in the bridging ligand allows tuning of the ordering temperatures, Tc, in the new family of layered chiral magnets A[M(II)M(III)(X2An)3]·G (A = [(H3O)(phz)3](+) (phz = phenazine) or NBu4(+); X2An(2-) = C6O4X2(2-) = 2,5-dihydroxy-1,4-benzoquinone derivative dianion, with M(III) = Cr, Fe; M(II) = Mn, Fe, Co, etc.; X = Cl, Br, I, H; G = water or acetone). Depending on the nature of X, an increase in Tc from ca. 5.5 to 6.3, 8.2, and 11.0 K (for X = Cl, Br, I, and H, respectively) is observed in the MnCr derivative. Furthermore, the presence of the chiral cation [(H3O)(phz)3](+), formed by the association of a hydronium ion with three phenazine molecules, leads to a chiral structure where the Δ-[(H3O)(phz)3](+) cations are always located below the Δ-[Cr(Cl2An)3](3-) centers, leading to a very unusual localization of both kinds of metals (Cr and Mn) and to an eclipsed disposition of the layers. This eclipsed disposition generates hexagonal channels with a void volume of ca. 20% where guest molecules (acetone and water) can be reversibly absorbed. Here we present the structural and magnetic characterization of this new family of anilato-based molecular magnets.

FT-Raman Study on Charge-Transfer Polyiodide Complexes and Comparison with Resonance Raman Results

In resonance Raman (RR) spectroscopy, the laser excitation sources have often been found to be destructive towards polyiodides if compared with the milder conditions under which the Fourier transform Raman (FT-R) technique operates. In fact, our FT-R spectra of some model polyiodides—[(CH3)4N]I5 (I5− bent), [(C2H5)4N]I7, and [(CH3)4N]I9—are significantly different from the literature RR data, give evidence of decomposition of the samples in RR, and are in agreement, respectively, with the I− · 2I2, I3− · 2I2 and (I− · 2I2) · 2I2 descriptions. In addition to the above-cited cases, the FT-R spectra of (Mn(modtc)3]Is (modtc = morpholine carbodithioato) and (moH]I5 (moH = morpholinium) are reported. The crystal structures indicate that in these two compounds the I5 anions can be properly described as I− · 2I2 and I3− · I2, respectively, and FT-R spectra agree well with this formulation. Moreover, the first FT-R spectrum of an I164– anion in [mo2ttl]2I16, ([mo2ttl]2+ = 3,5-di(N-morpholinio)-1,2,4-trithiolane), whose X-ray structure shows a sequence of two I3− … I2 … I− ·I2 (I82–) interacting anions, is reported. A close correlation of the FT-Raman peaks with the molecular species, identified by the interatomic distances, is also observed in this case. Thus, a combination of X-ray structural data and FT-R data can provide a reasonable interpretation of the nature of the acceptor iodine moiety in charge-transfer polyiodide complexes.

Redox-Switchable Chromophores Based on Metal (Ni, Pd, Pt) Mixed-Ligand Dithiolene Complexes Showing Molecular Second-Order Nonlinear-Optical Activity

The synthesis and full characterization of the redox-active nickel triad mixed-ligand dithiolene complexes based on Bz(2)pipdt = 1,4-dibenzylpiperazine-3,2-dithione and dmit = 2-thioxo-1,3-dithiole-4,5-dithiolate ligands are reported. These complexes show a reversibly bleacheable solvatochromic peak and a remarkably high negative molecular first hyperpolarizability, whose values depend on the metal being highest for the platinum(II) compound.

Synthesis, X-ray and spectroscopic characterization of obtained throught the one-step reaction of mbit·2I2 with tin metal powder (mbit = 1,1′-bis(3-methyl-4-imidazoline-2-thione)methane)

Abstract The reaction of the adduct mbit·2I2 (mbit = 1,1′-bis(3-methyl-4-imidazoline-2-thione)methane) with tin metal powder produces Sn(mbit)2I9, in mild conditions. An X-ray diffraction study on a crystal showed that the compound consists of a cation [SnI2(mbot)2]2+ having two I2 as counterions, interacting with two disordered diiodine molecules. In the cation, the metal atom lying on a symmetry centre exhibits a slightly distorted octahedral coordination with the two iodides at the apices in trans position, and with the two mbit molecules acting as bidentate chelating ligands through the sulfur atoms and forming an eight-membered ring, the tin metal atom included. The counterions I1 are slightly bent (I(2)–I(3)–I(4) 176.0(1)) and so asymmetric (I(2)–I(3) 2.841(6) I(3)−I(4) 3.016(5) A) that they can be better described as I ·I2 adducts. The presence of the two independent, centrosymmetric and disordered diiodine molecules as guests in the channel running parallel to [III] brings about I82 units of the type I2·I✓I2✓I·I2, here two triiodide ions realted by a sym centre are linked only for one third to I(5)–I(510), that is one of the two disordered guest I2 molecules (I(4)✓I(5) 3.22(I) A). Two I82 units related by a symmetry centre are held together through a non-negligible interaction (I(4)✓I(6) 3.55(1) A) involving the other disordered diiodine molecule (I(6)–I(6m)) giving rise to an octadecaioddide. Crystallographic data for C18H24N8S4I90Sn are as follows: the crystal is trigonal. M = 1783.77 space group R3, Z = 3, V = 3146(4) A ′, a = 18.100(6) A , α = 115.55(2)°, R = 0.0584 . In accordance with a description of the counterions as a sequence of the type I2·I✓I2✓I·I2. FT-Raman spectra do not show the peaks generally found in conventional triiodides, but those related to perturbed diiodine molecules.

Combined experimental and theoretical study on redox-active d8 metal dithione-dithiolato complexes showing molecular second-order nonlinear optical activity.

Synthesis, characterization, NLO properties, and theoretical studies of the mixed-ligand dithiolene complexes of the nickel triad [M(II)(Bz(2)pipdt)(mnt)] (Bz(2)pipdt = 1,4-dibenzyl-piperazine-3,2-dithione, mnt = maleonitriledithiolato, M(II) = Ni, 1, Pd, 2, Pt, 3) are reported. Molecular structural characterization of 1-3 points out that four sulfur atoms are in a slightly distorted square-planar geometry. While the M-S bond distances are only slightly different, comparison of the C-C and C-S bonds in the C(2)S(2)MS(2)C(2) core allows us to point out a significant difference between the C-C and the C-S distances in Bz(2)pipdt and mnt. These findings suggest assigning a dithiolato character to mnt (pull ligand) and a dithione one (push ligand) to Bz(2)pipdt. Cyclic voltammetry of 1-3 exhibits two reversible reduction waves and a broad irreversible oxidation wave. These complexes are characterized in the visible region by a peak of moderately strong intensity, which undergoes negative solvatochromism. The molecular quadratic optical nonlinearities were determined by the EFISH technique, which provided the following values μβ(λ) (10(-48) esu) = -1436 (1), -1450 (2), and -1950 (3) converted in μβ(0) (10(-48) esu) = -463 (1), -684 (2), and -822 (3), showing that these complexes exhibit large negative second-order polarizabilities whose values depend on the metal, being highest for the Pt compound. DFT and TD-DFT calculations on 1-3 allow us to correlate geometries and electronic structures. Moreover, the first molecular hyperpolarizabilities have been calculated, and the results obtained support that the most appealing candidate as a second-order NLO chromophore is the platinum compound. This is due to (i) the most extensive mixture of the dithione/metal/dithiolato orbitals, (ii) the influence of the electric field of the solvent on the frontier orbitals that maximizes the difference in dipole moments between the excited and the ground state, and (iii) the largest oscillator strength in the platinum case vs nickel and palladium ones.

A chirality-induced alpha phase and a novel molecular magnetic metal in the BEDT-TTF/tris(croconate)ferrate(III) hybrid molecular system

The novel paramagnetic and chiral anion [Fe(C5O5)3]3- has been combined with the organic donor BEDT-TTF (= ET = bis(ethylenedithio)tetrathiafulvalene) to yield the first chirality-induced alpha phase and a paramagnetic metal.

Mixed-ligand Pt(II) dithione-dithiolato complexes: influence of the dicyanobenzodithiolato ligand on the second-order NLO properties

The mixed-ligand dithiolene complex [Pt(Bz(2)pipdt)(dcbdt)] (1) bearing the two ligands Bz(2)pipdt = 1,4-dibenzyl-piperazine-3,2-dithione and dcbdt = dicyanobenzodithiolato, has been synthesized, characterized and studied to evaluate its second-order optical nonlinearity. The dithione/dithiolato character of the two ligands gives rise to an asymmetric distribution of the charge in the molecule. This is reflected by structural data showing that in the C(2)S(2)PtS(2)C(2) dithiolene core the four sulfur atoms define a square-planar coordination environment of the metal where the Pt-S bond distances involving the two ligands are similar, while the C-S bond distances in the C(2)S(2) units exhibit a significant difference in Bz(2)pipdt (dithione) and dcbdt (dithiolato). 1 shows a moderately strong absorption peak in the visible region, which can be related to a HOMO-LUMO transition, where the dcbdt ligand (dithiolato) contributes mostly to the HOMO, and the Bz(2)pipdt one (dithione) mostly to the LUMO. Thus this transition has ligand-to-ligand charge transfer (CT) character with some contribution of the metal and undergoes negative solvatochromism and molecular quadratic optical nonlinearity (μβ(0) = -1296 × 10(-48) esu), which was determined by the EFISH (electric-field-induced second-harmonic generation) technique and compared with the values of similar complexes on varying the dithiolato ligand (mnt = maleonitriledithiolato, dmit = 2-thioxo-1,3-dithiole-4,5-dithiolato). Theoretical calculations help to elucidate the role of the dithiolato ligands in affecting the molecular quadratic optical nonlinearity of these complexes.