Vittorio Ricevuto

Recent advances in luminescent polymetallic dendrimers containing the 2,3-bis(2′-pyridyl)pyrazine bridging ligand

Abstract Some new developments in the area of metal-based light-harvesting dendrimers based on the 2,3-bis(2′-pyridyl)pyrazine bridging ligand are presented, with particular regard to unidirectional energy transfer, enhanced light absorption, coupling dendritic structures with electron donors, and measurement of some of the ultra-fast processes occurring in this class of compounds.

Hexanuclear polypyridine complexes containing different metals, bridging ligands and/or terminal ligands. Absorption spectra, electrochemical oxidation, luminescence properties and intercomponent energy transfer

Abstract Four novel hexanuclear complexes of general formula [(L) 2 M(μ-BL)] 2 M(μ-BL)M[(μ-BL)M(L) 2 ] 2 12+ , where the metal ions M are Ru 2+ and/or Os 2+ , the bridging ligands BL are 2,3-dpp and/or 2,5-dpp, and the terminal ligands L are bpy and/or biq, have been investigated (dpp  bis(2-pyridyl)pyrazine; bpy  2,2′-bipyridine; biq 2,2′-biquinoline). These polymetallic complexes can be considered as supramolecular species made of six distinct metal-containing units. They display very intense ligand centered absorption bands in the UV region (ϵ max in the order of 2.5×10 5 M −1 cm −1 ) and broad and intense bands in the visible region (ϵ max in the order of 5×10 4 M −1 cm −1 ). On electrochemical oxidation, the metal centers are oxidized at the same or different potentials depending on the nature of the metal ions (Ru 2+ or Os 2+ ) and on their positions (inner or outer) in the supramolecular structure. For all the novel compounds, luminescence can be observed in the red or near-IR spectral region. The luminescence properties, which are characteristic of specific metal-containing units, show that exoergonic electronic energy transfer between adjacent units is 100% efficient, whereas it is much lower when higher energy units are interposed. Various energy migration patterns can be obtained by placing different units in suitable sites of the supramolecular array.

Displacement of chloride under the trans-effect of strong σ-donor groups in Pt(II) complexes

Abstract The kinetics of the reactions: trans -[PtL 2 (R)Cl] + Y − → trans [PtL 2 (R)Y] + Cl − (R = H, C 2 H 5 , m -CF 3 C 6 H 4 F 5 ; L = P(C 2 H 5 ) 3 ; Y − = Br − , I − , N − 3 , NO − 2 , CN − , SCN − , thiourea) in CH 3 OH at 25 °C are described. The pseudo-first-order rate constants fit the usual two-term rate law, k obs = k 1 + k 2 [ Y − ], with the k 2 term strongly depending on the entering group Y − . To the extent that the term k 1 can be taken as a measure of trans effect, this is found to parallel the trans -influence order of R, H > C 2 H 5 > m -CF 3 C 6 H 4 > C 6 F 5- . This suggests a mechanism wherein the weakening of the PtCI bond trans to strong σ donors R in the ground state is the driving force of the bimolecular reaction with a weak nucleophile which is devoid of π acceptor ability, such as methanol (ground state σ- trans -effect). When Y − has good π acceptor abilities, contribution from the k 2 term becomes predominant, indicating that the driving force of the reaction is now the stabilization of the 5-coordinate transition state (transition state π - trans -effect).

Crystal and molecular structure of cis-chloro-p-tolyl-bis(triethylphosphine)platinum(II) and of cis-chloro-perfluorophenyl-bis(triethylphosphine)platinum(II)

Abstract The crystal structure of the title compounds has been determined from X-ray data and refined to R = 0.048 for both compounds. Crystals of (I) are orthorhombic, space group Pna21 with cell dimensions a = 19.91(1), b = 14.98(1), c = 7.82(1) A. Crystals of (II) are monoclinic, space group P21/n with a = 11.07(1), b = 21.64(1), c = 10.05(1) A, β = 91.24(3)°. The co-ordination of Pt(II) is nearly square planar in both complexes. The PtCl bond lengths of 2.392(8) (I) and 2.387(7) A (II) are equal within experimental errors, whereas the two chemically non-equivalent PtP bond lengths are significantly different in both (I) and (II). Their values are 2.320(9) A (trans to C6H4CH3) and 2.247(6) A (trans to Cl) for (I) and 2.326(7) A (trans to C6F5) and 2.226(7) A (trans to Cl) for (II). No significant difference is found in PtC bond lengths (2.05(3) in (I) and 2.08(2) A in (II)), which however appear significantly longer than the values reported for PtC bond lengths having partial double bond character.

Photoinduced intercomponent energy transfer in covalently-linked dinuclear complexes containing Ru(II)-bipyridine and Ru(II)-biquinoline chromophores and aromatic and aliphatic spacers

Abstract We have synthesized a number of dinuclear species containing both identical or different metal-based components by employing new bridging ligands having either aliphatic or aromatic spacers and taking advantage of the “complexes as metals and complexes as ligands” synthetic strategy. The bridging ligands are dpt-S-dpt (S is 1,4-cyclohexyl, 1,4-phenyl, 4,4′-biphenyl; dpt is 4-amino-3,5-bis(2-pyridyl)- 1,2,4-triazole; the connections between S and dpt are provided by amide links). The complexes synthesized are: [(bpy)2Ru(dpt-S-dpt)Ru(bpy)2](PF6)4 (bpy=2,2′-bipyridine; biq=2,2′-biquinoline; S=1,4-cyclohexyl (1), 1,4-phenyl (4), 4,4′-biphenyl (7)); [(biq)2Ru(dpt-S-dpt)Ru(biq)2](PF6)4 (S=1,4-cyclohexyl (2), 1,4-phenyl (5), 4,4′-biphenyl (8); [(bpy)2Ru(dpt-S-dpt)-Ru(biq)2](PF6)4 (S=1,4-cyclohexyl (3), 1,4-phenyl (6), 4,4′-biphenyl 39)). The absorption spectra, luminescence properties and redox behavior of all the compounds have been studied. In the complexes containing different metal-based components, photoinduced energy transfer occurs from the higher-lying Ru→bpy CT level, centered on a metal subunit, to the lower-lying Ru→biq CT excited state, centered on the other metal component. In fluid solution at room temperature, the energy transfer is suggested to be mediated by a two-step electron transfer mechanism, whereas direct energy transfer between the chromophores most likely occurs at 77 K in rigid matrix. At the moment we arc not able to say if the energy transfer at 77 K takes place via electron exchange or coulombic mechanisms. The results obtained indicate that the efficiency of the processes depends on the donor-acceptor distance, as expected, and that occasional π bonds which are present within the bridging ligands cannot be used for speeding up electron transfer in multicomponent systems if the main skeleton of the bridge is made by σ bonds.

Absorption spectra, luminescence properties and electrochemical behaviour of ruthenium(II) complexes containing bis(pyridyl)triazole ligands

Abstract Four novel complexes of general formula Ru(L) 2 (T) 2+ ( 1 , Lbpy, Ttrz; 2 , Lbpy, Ttrz-Q; 3 , Lbiq, Ttrz; 4 , Lbiq, Ttrz-Q; bpy=2,2′-bipyridine, biq=2,2′-biquinoline; trz=4-amino-3,5-di-2-pyridyl-4 H -1,2,4-triazole; trz-Q=4(4′- N,N -dimethylamino-phenyl)imino-3,5-di-2-pyridyl-4 H -1,2,4-triazole) have been synthesized, and their absorption spectra, luminescence properties (both in fluid solution at room temperature and in rigid matrix at 77 K), and electrochemical behaviour have been investigated. The absorption spectra of the complexes show intense absorption bands in the UV region (ϵ in the range 10 4 –10 5 M −1 cm −1 ) that are assigned to ligand- centred transitions and moderately intense absorption bands in the visible (ϵ in the range 10 3 –10 4 M −1 cm −1 ) that are attributed to metal-to-ligand charge transfer (MLCT) transitions. The absorption bands in the visible of the biq-containing complexes are at lower energies than those of the bpy-containing ones. The four complexes emit from a MLCT excited state both at 77 K and at room temperature, with lifetimes in the range 10 −5 – 10 −6 s and 10 −7 –10 −8 s, respectively. The luminescence lifetimes and quantum yields are practically the same for 1 and 3 and for 2 and 4 , respectively, indicating that the presence of the N,N -dimethylamino unit on the triazole ligand does not affect the radiative and radiationless rate constants of the chromophores and does not cause an electron-transfer quenching process. On electrochemical oxidation, 1 and 3 exhibit a reversible one-electron wave at +1.22 and +1.37 V versus SCE, respectively, that are assigned to metal-centred oxidations, while 2 and 4 undergo two successive one-electron oxidations at +1.30 and +1.56 V ( 2 ) and +1.30 and +1.71 V ( 4 ). By comparison with the redox behaviour of the free ligands, in both 2 and 4 the first process is attributed to oxidation of the N,N -dimethylamino moiety, and the second one to metal-centred oxidation. Two reversible reduction processes occur in all the complexes at about −1.15 and −1.40 V ( 1 and 2 ) and at about −0.60 and −0.85 V ( 3 and 4 ). Such processes are assigned as bpy- and biq-centred reductions, respectively. The positive shift of the metal-centred oxidation on passing from 1 and 3 to 2 and 4 is attributed to electronic ‘communication’ between the chromophoric metal unit and the electron-donor N,N -dimethylamino group across the triazole ligand and the conjugate NCHC 6 H 4 bridge, and to an electrostatic term.

Photochemistry of planar four-coordinate palladium(II) complexes. I. The sterically hindered nitro(4-methyl-1,1,7,7-tetraethyldiethylenetriamine)palladium(II) complex ion

Abstract The photochemical behaviour of [Pd(MeEt4dien)NO2]+ (MeEt4dien = 4-methyl-1,1,7,7 tetraethyldiethylenetriamine) in methanol solution has been investigated. Under irradiation of different wavelengths corresponding mainly to CT and LF bands of the complex, the initial N-bonded PdNO2 compound undergoes a linkage photoisomerization to PdONO which in turn reacts photochemically and thermally with the solvent to give the final solvo-derivative [Pd(MeEt4dien)solv.]++. The quantum yields of the two photoreactions, the isomerization and the solvolysis, decrease with decreasing energy of the exciting radiations. A mechanism for both photoprocesses is proposed.

CATALYZED SUBSTITUTIONS ON CATIONIC PALLADIUM(II) COMPLEXES. I. KINETICS OF ACID-ASSISTED DISPLACEMENT OF COORDINATED AZIDO AND NITRO GROUPS

A kinetic study of the reactions of complexes of the type [Pd(AAA)X]+ (AAA = diethylenetriamine (dien); 1,1,7,7-tetraethyldiethylenetriamine (Et4dien); 4-methyl-1,1,7,7,-tetraethyldiethylenetriamine (MeEt4 dien); X = NO2 and N3) with Cl−, H+ and both H+ and Cl− in methanol at 25° C is reported. The rates of uncatalyzed displacement of nitro and azido groups by Cl− from axially blocked [Pd(Et4 dien)X]+ depend on the chloride concentration, in contrast with the kinetic behaviour found for such substrates when X = Cl, Br and l, in which only the solvolytic path controls the substitution. The rate of proton assisted displacement follows the equation: “rate” = (k1+k2[Cl−]+k′2[H+]+k′2[H+][Cl−]) [Complex] which is consistent with a mechanism involving a nucleophilic attack by solvent and entering chloride on both the substrate and its protonated form in equilibrium. All the rate constants which appear in the rate equation have been determined and the values indicate that the acid controlled paths carry the major part of the reaction. The lability of the NO2 group depends on the different orientation that it assumes with respect to the coordinate plane in unhindered or hindered complexes.

Photochemistry of planar four-co-ordinate palladium(II) complexes—III. Photosynthesis and crystal structure of [Pd2(PPr3n2(NCS)(SCN)2(μ-CN)] ☆

Abstract U.V. irradiation of MeOH and CH 3 CN solutions of trans -[Pd(PPr 3 n ) 2 (NCS) 2 ] causes a stepwise photochemical reaction. When the solvent is CH 3 CN the photoproduct can be conveniently isolated in a crystalline form. This compound has been shown, by X-ray analysis, to be a cyano bridged palladium(II) complex.

Photochemistry of planar four-co-ordinate palladium(II) complexes. Part 2. trans–cis Isomerization of dihalogenobis(tri-n-propylphosphine)palladium(II) in solution

The photoisomerization of trans-dihalogenobis (tri-n-propylphosphine)palladium(II) complexes has been studied in various solvents. The quantum yields depend strongly both on the nature of the co-ordinating halide and on the excitation wavelength. The photochemical results are discussed in terms of the possible mechanisms for the trans–cis photoisomerization of square-planar complexes. Photolysis of cis-[Pd(PPrn3)2Cl2] yields the chloro-bridged complex, [Pd2(PPrn3)2Cl4].