Maddalena Pizzotti

Coordination and Organometallic Complexes as Second-Order Nonlinear Optical Molecular Materials

Coordination and organometallic complexes with second-order nonlinear optical (NLO) properties have attracted increasing attention as potential molecular building block materials for optical communications, optical data processing and storage, or electrooptical devices. In particular, they can offer additional flexibility, when compared to organic chromophores, due to the presence of metal–ligand charge-transfer transitions, usually at relatively low-energy and of high intensity, tunable by virtue of the nature, oxidation state, and coordination sphere of the metal center. This chapter presents an overview of the main classes of second-order NLO coordination and organometallic complexes with various ligands such as substituted amines, pyridines, stilbazoles, chelating ligands (bipyridines, phenanthrolines, terpyridines, Schiff bases), alkynyl, vinylidene, and cyclometallated ligands, macrocyclic ligands (porphyrins and phthalocyanines), metallocene derivatives, and chromophores with two metal centers. The coverage, mainly from 2000 up to now, is focused on NLO properties measured at molecular level from solution studies, as well as on NLO properties of bulk materials.

Low oxidation states ruthenium chemistry VI. Stoichiometric and catalytic oxidation by molecular oxygen of primary amines bound to dichlorobis(triphenylphosphine)ruthenium(II)

Abstract Under a strict nitrogen atmosphere Ru(PPh3)3Cl2 reacts with primary amines to give the complexes, Ru(PPh3)2(RNH2)2Cl2 [R = H (I), CH3(CH2)4 (II), C6H5CH2 (III), p-CH3OC6H4 (IV)]. Chelating ligands such as 5-methyl-1,2-phenylenediamine, ethylenediamine, benzoylhydroxylamine and benzoylhydrazine give the complexes Ru(PPH3)2(chel)Cl2, (V) - (VIII). Compounds (I) - (V) react with molecular oxygen at room temperature to give different products depending on the nature of R, while compounds (VI) - (VIII) are stable with respect to oxidation by O2. Compound (III) acts as a catalyst in the oxidation of benzylamine to benzonitrile with molecular oxygen at 80 °C. The reaction of (III) with molecular oxygen has been followed by ESR spectroscopy, and the intermediate formation of paramagnetic ruthenium-(III) species has been detected.

Tetraaryl ZnIIPorphyrinates Substituted at β-Pyrrolic Positions as Sensitizers in Dye-Sensitized Solar Cells: A Comparison withmeso-Disubstituted Push-Pull ZnIIPorphyrinates

A facile and fast approach, based on microwave-enhanced Sonogashira coupling, has been employed to obtain in good yields both mono- and, for the first time, disubstituted push-pull Zn(II) porphyrinates bearing a variety of ethynylphenyl moieties at the β-pyrrolic position(s). Furthermore, a comparative experimental, electrochemical, and theoretical investigation has been carried out on these β-mono- or disubstituted Zn(II) porphyrinates and meso-disubstituted push-pull Zn(II) porphyrinates. We have obtained evidence that, although the HOMO-LUMO energy gap of the meso-substituted push-pull dyes is lower, so that charge transfer along the push-pull system therein is easier, the β-mono- or disubstituted push-pull porphyrinic dyes show comparable or better efficiencies when acting as sensitizers in DSSCs. This behavior is apparently not attributable to more intense B and Q bands, but rather to more facile charge injection. This is suggested by the DFT electron distribution in a model of a β-monosubstituted porphyrinic dye interacting with a TiO2 surface and by the positive effect of the β substitution on the incident photon-to-current conversion efficiency (IPCE) spectra, which show a significant intensity over a broad wavelength range (350-650 nm). In contrast, meso-substitution produces IPCE spectra with two less intense and well-separated peaks. The positive effect exerted by a cyanoacrylic acid group attached to the ethynylphenyl substituent has been analyzed by a photophysical and theoretical approach. This provided supporting evidence of a contribution from charge-transfer transitions to both the B and Q bands, thus producing, through conjugation, excited electrons close to the carboxylic anchoring group. Finally, the straightforward and effective synthetic procedures developed, as well as the efficiencies observed by photoelectrochemical measurements, make the described β-monosubstituted Zn(II) porphyrinates extremely promising sensitizers for use in DSSCs.

Further investigations of the reactivity of η2-bonded nitroso complexes of platinum: the crystal structure of Pt(PPh3)2(PhNO)

The crystal structure of Pt(PPh3)2(PhNO) (I) has been determined. This is the first example of a structure determination for a monomeric complex of a metal in a low oxidation state and bearing an η2-bonded nitroso ligand. Compound I crystallizes in the monoclinic space group P21/c (no.14) with a 14.228(4), b 13.914(3), c 17.855(4) A, β 100.31 (2)°, V 3478(3) A3 and Z = 4. Final R and Rw indices are 0.026 and 0.030 for 3609 observed reflections (I ⩾ 3 σ(I)). The new complexes Pt(PPh3)2(RNO) (R = But (II), CF3 (III)) have been synthesized and their reactions with CO2, CS2, PhNCO, PhNCS, CO, alkenes and dimethyl acetylenedicarboxylate have been investigated and compared with those of the previously studied compound I. Preliminary observations on the synthesis and reactivity of Pd(PPh3)2(CF3NO) (IV) are also reported.

Low oxidation state ruthenium chemistry : IV. The reactions of M(CO)2(PPh3)3 complexes (M = Fe, Ru) and the hydrogenation and isomerization of alkenes catalyzed by RuL(CO)2(PPh3)2 (L = H2, PPh3)☆

The complexes M(CO)2(PPh3)3 (I, M = Fe; II, M = Ru) readily react with H2 at room temperature and atmospheric pressure to give cis-M(H)2(CO)2(PPh3)2 (III, M = Fe;IV,M = Ru). I reacts with O2 to give an unstable compound in solution, in a type of reaction known to occur with II which leads to cis-Ru(O2)(CO)2(PPh3)2(V). Even compound IV reacts with O2 to give V with displacement of H2; this reaction has been shown to be reversible and this is the first case where the displacement of H2 by O2 and that of O2 by H2 at a metal center has been observed. III and IV are reduced to M(CO)3(PPh3)2 by CO with displacement of H2; Ru(CO)3- (PPh3)2 is also formed by treatment of IV with CO2, but under higher pressure. Compounds II and IV react with CH2CHCN to give Ru(CH2CHCN)(CO)2- (PPh3)2(VI) which reacts with H2 to reform the hydride IV. cis-Ru(H)2(CO)2(PPh3)2(IV) has been studied as catalyst in the hydrogenation and isomerization of a series of monoenes and dienes. The catalysts are poisoned by the presence of free triphenylphosphine. On the other hand the ready exchange of H2 and O2 on the “Ru(CO)2(PPh3)2” moiety makes IV a catalyst not irreversibly poisoned by the presence of air. It has been found that even Ru(CO)2(PPh3)3(II) acts as a catalyst for the isomerization of hex-1-ene at room temperature under an inert atmosphere.

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.

A critical evaluation of EFISH and THG non-linear optical responses of asymmetrically substituted meso-tetraphenyl porphyrins and their metal complexes

Abstract The experimental second and third order non-linear optical (NLO) responses γEFISH and γTHG, measured working with a non-resonant incident wavelength of 1.907 μm, of some push–pull meso-tetraphenyl porphyrins and their metal complexes, substituted in the para position of one phenyl ring with a nitro group and with hydrogen or methyl or methoxy groups in the para position of the other three phenyl rings, are critically discussed, also with the support of semiempirical MNDO–TDHF calculations. Care must be taken in evaluating the quadratic hyperpolarizabilities βvec from γEFISH being the contribution of the cubic term γ0(−2ω; ω, ω, 0) to γEFISH not negligible at least when the second order NLO response is relatively low, as in porphyrins and their metal complexes investigated in this work. In addition experimental evidence has been produced that the value of γEFISH of the porphyrins and their metal complexes is increased by resonance enhancement when working with an incident wavelength of 1.34 μm, due to the presence of very strong absorption bands in the region 0.400–0.650 μm and that the γEFISH response is not significantly affected by 3dn (n=7–10) metal coordination. On the contrary γTHG decreases by metal coordination due to the high resonance enhancement of γTHG of free porphyrins. In fact the third harmonic 3ω (0.636 μm) in free porphyrins is quite close to some strong Q absorption bands above 0.615 μm, which are lacking in the absorption spectra of their 3dn metal complexes. A vibrational method for the evaluation of the order of magnitude of the static cubic γ0 third order NLO response was successfully applied for the first time to a push–pull porphyrinic system.

New chelating nitrogen ligands and their application to the catalytic reduction of nitrobenzene to aniline. X-ray structure of [Rh(CO)2(BBOM)] (HBBOM = bis(2-benzoxazolyl)methane)

Abstract The use of two novel chelating nitrogen ligands of the bis(azaheteroaryl)methane type is reported. Two complexes were synthesized: [Rh(CO)2(BBOM)] and [Pd(HBBOM)2](NO3)2 (HBBOM = bis(2-benzoxazolyl)methane), the first of which was completely characterized by X-ray crystallography. The dimethylated compound α,α-dimethyl-bis(2-benzothiazolyl)methane was also synthesized by an original procedure and used as a promoter in the [Ru3(CO)12]-catalyzed reduction of nitrobenzene to aniline. It gave much better conversions and selectivities with respect to the non-methylated HBBOM, probably due to the fact that the latter can be deprotonated when coordinated to the metal.

Highly improved performance of ZnII tetraarylporphyrinates in DSSCs by the presence of octyloxy chains in the aryl rings

Three new β-substituted ZnII-tetraarylporphyrinate dyes (1–3), bearing octyloxy chains at the o,o-, o,p- or o-positions of the four phenyl groups respectively, were synthesized, characterized and investigated as sensitizers for DSSCs. In particular, the alkoxy group position strongly influences their electronic absorption and electrochemical features. Improvements in power conversion efficiency ranging from 40% to 80% were obtained with respect to a reference dye (4) characterized by the presence of sterically bulky t-butyl groups at the m-positions.

Metal Catalysed Deoxygenation Reactions by Carbon Monoxide of Nitroso and Nitro Compounds

In recent years the catalytic carbpnylation of aromatic nitro compounds, ArNO2, has become a very intense field of research. This is due to the fact that a series of industrially important compounds can be obtained from nitro compounds and carbon monoxide in a single step. Probable intermediates in these carbonylation reactions are the corresponding nitroso compounds, ArNO. The interaction of nitroso and nitro compounds with the metal centre has been reviewed and their metal-assisted stoichiometric reactions have been discussed. Their catalytic reactions, with particular emphasis on nitro compounds as substrates, have also been reviewed. Different sections have been thus-devoted to the synthesis of: amines, azo derivatives, Schiff bases, heterocycles (pyridines, quinolines and indoles), isocyanates, carbamates, ureas, N-substituted amides and imides. Some reactions involving aliphatic nitro compounds have also been considered. Particular attention has been paid to the catalytic reactions leading to isocyanates and carbamates, being these derivatives important final products and/or intermediates in the synthesis of pesticides and polyurethanes. The action of homogeneous, and of heterogeneous catalysts as well, has been discussed.