Fabrizio Monacelli

The exchange reaction between [Rh(NH3)5OH2]3+ and solvent water☆

Abstract The water exchange reaction of [Rh(NH 3 ) 5 OH 2 ] 3+ ion has been kinetically studied by means of an 18 O tracer technique. Rates were found to be independent of pH and addition of NaCl. The activation enthalpy and entropy are 23.9 kcal/mole and −3 e.u., respectively. A comparison with the analogous reactions of aquopentamminechromium(III) and cobalt(III) ions suggests for the rhodium system a significant participation of the incoming water molecule as a nucleophile in the transition state. The activation parameters are in agreement with such hypothesis. Attempts have been made to measure the rate of exchange of [Rh(NH 3 ) 5 OH] 2+ ion, but, even at 60°C and 0.1 M NaOH, no appreciable exchange was noticed.

Tetra-2,3-pyrazinoporphyrazines with externally appended pyridine rings. 6. Chemical and redox properties and highly effective photosensitizing activity for singlet oxygen production of penta- and monopalladated complexes in dimethylformamide solution

Tetrakis-2,3-[5,6-di-(2-pyridyl)pyrazino]porphyrazinatopalladium(II) [Py 8TPyzPzPd] ( 1) and the corresponding pentapalladated species [(PdCl 2) 4Py 8TPyzPzPd] ( 2), dissolved (c approximately 10 (-5)-10 (-6) M) in preacidified dimethylformamide ([HCl] approximately 10 (-4) M), behave as potent photosensitizing agents for the production of singlet oxygen, (1)O 2, with Phi Delta values of 0.89 +/- 0.04 and 0.78 +/- 0.05, respectively. The related octacation [(2-Mepy) 8TPyzPzPd] (8+) ( 3), examined under similar experimental conditions, exhibits lower Phi Delta values, that is, 0.29 +/- 0.02 (as an iodide salt) and 0.32 +/- 0.02 (as a chloride salt). In view of the very high values of Phi Delta, the photophysics of complexes 1 and 2 has been studied by means of pump and probe experiments using ns laser pulses at 532 nm as excitation source. Both complexes behave like reverse saturable absorbers at 440 nm because of triplet excited-state absorption. The lifetimes of the triplet excited states are 65 and 96 ns for the penta- and mononuclear species, respectively. Fluorescence quantum yields (Phi f) are approximately 0.1% for both 1 and 2. Such low Phi f values for the two complexes are consistent with the high efficiency of triplet excited-state formation and the measured high yields of (1)O 2. Time-dependent density-functional theory (TDDFT) calculations of the lowest singlet and triplet excited states of the mono- and pentapalladated species help to rationalize the photophysical behavior and the relevant activity of the complexes as photosensitizers for the (1)O 2 ( (1)Delta g) generation.

Anation reaction of [Rh(NH3)5 OH2]3+ ion. Evidence for the nucleophilic attack of the entering group☆

Abstract The anation by Cl−, Br−, and SO42− of the complex ion [Rh(NH3)5OH2]3+ has been kinetically studied at 65°C, in water solutions of controlled ionic strength. The reactions with Cl− and Br− were complete while a definite equilibrium was reached with SO42− in the sulfate concentration range 0.05–1.33 M. In the latter case equilibrium measurements were carried out in order to separate the observed rate constants into the aquation (kaq) and anation (kan) rate constants. In all cases kan was found to follow the equation kan = K . K a [X m− ] 1 + K a [X m− ] (Xm− = anion) consistent with a mechanism involving a fast 1:1 ion association (Ka being the «conditional» equilibrium constant) followed by a slow interchange of the water and the anionic ligands (k being the rate constant of this process). The comparison of k values with the water exchange rate constant ke of the aquo-complex under comparable conditions, (k/ke≥1), suggests for the interchange process a transition state where the incoming anion is appreciably bonded to the metal. Complications, possibly due to medium effects, were observed in the reaction with SO42−.

X-Ray crystal structure of µ-oxo-bis[(1-methylimidazole)-phthalocyaninatoiron(III)] and comments on the molecular structure and chemistry of oxo-bridged iron phthalocyaninate dimers

Single-crystal X-ray work has been carried out on crystals containing the 1-methylimidazole adduct of µ-oxo-bis[(phthalocyaninato)iron(III)], [{Fe(pc)(mim)}2O]·[Fe(pc)(mim)2]·3Me2CO. The crystals are monoclinic, space group C2/c, with a= 35.236(5), b= 14.173(2), c= 23.220(3)A, β= 103.56(10)° and Z= 4. The structure was solved and refined using all data including zero intensities (5739). Only the atoms of the µ-oxo dimer were refined anisotropically because of data limitations. The Fe atoms in [{Fe(pc)(mim)}2O] are nominally centred in the phthalocyanine plane, with an average Fe–N(pc) of 1.92(3)A and six-co-ordinate, with Fe–O 1.749(1)A and Fe–N(mim) 2.039(7)A. The Fe–O–Fe angle is 175.1°. X-Ray data for [Fe(pc)(mim)2] are as expected for six-co-ordinate low-spin nitrogen-containing base adducts of [Fe(pc)]. The crystal data are discussed in the light of similar results on [{Fe(pc)(mim)}2C] and related compounds.

Interaction of phthalocyaninatoiron(II) with molecular oxygen. Kinetics and mechanism of the reaction in dimethyl sulphoxide

Abstract The reaction of phthalocyaninatoiron(II), FePc, with dioxygen in dimethyl sulphoxide solution has been studied kinetically by measuring the visible spectral changes occurring under high concentration conditions (h.c.c.: [FePc] = 1 X 10 −4 mol dm −3 or higher) and low concentration conditions (l.c.c.: 1 X 10 −5 mol dm −3 or lower). At h.c.c. the observed spectral variations (a decrease in intensity of the band at 653 nm and a parallel increase of a band at 620 nm) develop with an autocatalytic-like pattern and can be reversed either by nitrogen bubbling or dilution. The insoluble μ-oxo species (FePc) 2 O is the final reaction product. Under l.c.c. the spectral changes (a decrease in intensity of the spectrum at all wavelengths within the range 550–750 nm) are associated with an irreversible degradation of the complex. This spectral trend is also observed under h.c.c. when use is made of a free radical inhibitor (2,6-di(tert-butyl)-4-methylphenol). The whole experimental picture is satisfactorily explained by a mechanism in which an intermediate oxenic species FePcO can either react with FePc, to give reversibly the μ-oxo compound, or can oxidize the solvent (or a species derived from the solvent). In the h.c.c. reaction FePc behaves as an O-atom transfer catalyst.

Determination of H2S solubility via the reaction with ferric hemoglobin I from the bivalve mollusc Lucina pectinata

A new, simple and fast spectrophotometric method for the determination of the H(2)S concentration is reported. This method, based on the 1:1 reaction between H(2)S and the ferric derivative of hemoglobin I (HbI) from the bivalve mollusc Lucina pectinata, allows the quantitative determination of H(2)S dissolved in a given solution even at concentrations as low as 1 x 10(-6) M. Note that L. pectinata HbI is considered the physiological receptor of H(2)S.

The exchange reaction between [Ir(NH3)5OH2]3+ ion and solvent water

Abstract An 18 O study of the rate of water exchange of [Ir-(NH 3 ) 5 OH 2 ] 3+ in aqueous solution is presented. The rate constants are found to be independent of the hydrogen ion concentration in the range 0.01–0.5 M and of the ionic strength (0.96M⪕μ⪕3.0M). The addition of large amounts of NaCl and NaBr slightly increases the rates. The activation enthalpy and entropy are 28.2±0.2 Kcal/mole and +3.0 e.u. respectively. The mechanism is discussed on the basis of a comparison with the analogous CO III and Rh III complexes.

The acid and base hydrolysis of sulfatopentamminecobalt(III) and rhodium(III) complexes

Abstract The aquation reaction of [M(NH 3 ) 5 SO 4 ] + (M = Co and Rh) and the base hydrolysis of [Rh(NH 3 ) 5 SO 4 ] + have been studied kinetically. In acid solution and in the absence of added sulfate ion, the aquation rate follows the equation: R/[complex] = k + k′ H [H + ], at constant ionic strenght μ = 1.0 M (NaClO 4 ), with the rhodium complex being slightly more labile. The activation parameters are ΔH* = 22.7 (Co) and 21.4 (Rh) kcal/mole, ΔS* = −10.0 (Co) and −13.3 (Rh) e.u., ΔH H * = 28.4 (Co) and 28.5 (Rh) kcal/mole, ΔS H * = 9.2 (Co) and 9.0 (Rh) e.u. The rate constants at 25°C are: k = 8.9 × 10 −7 (Co) and 1.6 × 10 −6 (Rh) sec −1 , k′ H = 9.5 × 10 −7 (Co) and 1.1 × 10 −6 (Rh) sec − M −1 . The spontaneous and acid-catalysed aquation of both sulfatopentamminemetal complexes appear to be essentially dissociative processes. The constant k, for the rhodium compound, increases with the increasing [SO 4 2− above 0.3 M at μ = 4.0 M and 65°C. This effect is discussed in connection with the equilibrium ratios Q = [aquocomplex] / [sulfatocomplex]. For both the cobalt and the rhodium compound, k decreases when [NaClO 4 ] increases, possibly as a consequence of the decrease of water activity. The rate of the base hydrolysis of [Rh(NH 3 ) 5 SO 4 ] + follows the equation R/[complex] = k OH [OH − ]. The activation parameters are ΔH OH * = 24.5 kcal/ mole and ΔS OH * = 10 e.u.. Both S N 1 CB and S N 2 CB mechanisms are consistent with the data although the entropy of activation is in favour of the second.

Equilibrium and kinetic study of piperidine binding to phthalocyaninatoiron(II) in dimethyl sulfoxide

Abstract The reaction between piperidine (Pip) and phthalocyaninatoiron(II) ([Fe(pc)]) in dimethyl sulfoxide (dmso) has been studied at 20.0 °C, from both equilibrium and kinetic viewpoints. In excess of Pip, the adduct [Fe(pc)(Pip) 2 ] is formed via two consecutive pseudo-first-order processes. The observed rate constants have the general form: k obs = k 1f [Pip] + k 1r with k 1f ′ = (8.7 ± 0.2) × 10 3 dm 3 mol −1 s −1 and k 1f ″ = (8.3 ± 0.2) × 10 1 dm 3 mol −1 s −1 , for the binding of the first and second Pip molecule, respectively. Equilibrium experiments yield K 1 ′ = (1.0 ± 0.1) × 10 5 dm 3 mol −1 and K 1 ″ = (1.3 ± 0.1) × 10 3 dm 3 mol −1 for the stepwise formation of the [Fe(pc)(dmso)(Pip)] and [Fe(pc)(Pip) 2 ] adducts, respectively. From the values of the second-order rate constants ( k 1f ′ and k 1f ″) and of the equilibrium constants ( K 1 ′ and K 1 ″), the dissociation rate constants have been estimated to be k 1r ′= (8.7 ± 1) × 10 −2 s −1 and k 1r ″ = (6.4 ± 0.7) × 10 −2 s −1 . Present results are discussed in the light of related previous studies.

Base hydrolysis of [Rh(NH3)5OCOR]2+ ions: Influence of polar and steric requirements of the R groups on the reaction mechanism☆☆☆

Abstract Complexes of the type [Rh(NH3)5OCOR]2+, where RCOO- is formate, acetate or substituted acetate ion, were hydrolysed in basic aqueous media. The observed pseudo first-order rate constants were found to obey the equation K obs = k 1 [OH − ] + k 2 [OH − ] 2 The reaction mechanism is discussed on the basis of the sensitivity of k1 and k2 to both polar and steric requirements of the R groups.