Rosario Scopelliti

Solution and solid-state characterization of EuII chelates: a possible route towards redox responsive MRI contrast agents

We report the first solid state X-ray crystal structure for a Eu(II) chelate, [C(NH2)3]3[Eu(II)(DTPA)(H2O)].8H2O, in comparison with those for the corresponding Sr analogue, [C(NH2)3]3[Sr(DTPA)(H2O).8H2O and for [Sr(ODDA)].8H2O (DTPA5 = diethylenetriamine-N,N,N,N,N-pentaacetate, ODDA2- =1,4,10,13-tetraoxa-7,16-diazacyclooctadecane-7,16-diacetate ). The two DTPA complexes are isostructural due to the similar ionic size and charge of Sr(2+) and Eu(2+). The redox stability of [Eu(II)(ODDA)(H2O)] and [Eu(II)(ODDM)]2- complexes has been investigated by cyclovoltammetry and UV/Vis spectrophotometry (ODDM4- =1,4,10,13-tetraoxa-7,16-diaza-cyclooctadecane-7,16-++ +dimalonate). The macrocyclic complexes are much more stable against oxidation than [Eu(II)(DTPA)(H2O)]3- (the redox potentials are E1/2 =-0.82 V, -0.92 V, and -1.35 V versus Ag/AgCl electrode for [Eu(III/II)(ODDA)(H2O)],[Eu(III/II)(ODDM)], and [Eu(III/II)(DTPA)(H2O)], respectively, compared with -0.63 V for Eu(III/II) aqua). The thermodynamic stability constants of [Eu(II)(ODDA)(H2O)], [Eu(II)(ODDM)]2-, [Sr(ODDA)(H2O)], and [Sr(ODDM)]2- were also determined by pH potentiometry. They are slightly higher for the EuII complexes than those for the corresponding Sr analogues (logK(ML)=9.85, 13.07, 8.66, and 11.34 for [Eu(II)(ODDA)(H2O)], [Eu(II)(ODDM)]2-, [Sr(ODDA)(H2O)], and [Sr(ODDM)]2-, respectively, 0.1M (CH3)4NCl). The increased thermodynamic and redox stability of the Eu(II) complex formed with ODDA as compared with the traditional ligand DTPA can be of importance when biomedical application is concerned. A variable-temperature 17O-NMR and 1H-nuclear magnetic relaxation dispersion (NMRD) study has been performed on [Eu(II)(ODDA)(H2O)] and [Eu(II)(ODDM)]2- in aqueous solution. [Eu(II)(ODDM)]2- has no inner-sphere water molecule which allowed us to use it as an outer-sphere model for [Eu(II)(ODDA)(H2O)]. The water exchange rate (k298(ex)= 0.43 x 10(9)s(-1)) is one third of that obtained for [Eu(II)(DTPA)(H2O)]3-. The variable pressure 17O-NMR study yielded a negative activation volume, deltaV (not=) = -3.9cm3mol(-1); this indicates associatively activated water exchange. This water exchange rate is in the optimal range to attain maximum proton relaxivities, which are, however, strongly limited by the fast rotation of the small molecular weight complex.

PORPHOMETHENES AND PORPHODIMETHENES SYNTHESIZED BY THE TWO- AND FOUR-ELECTRON OXIDATION OF THE MESO-OCTAETHYLPORPHYRINOGEN

Stepwise dealkylation of meso-octaethylporphyrinogen 1 yields porphomethene 2 and porphodimethene 3, providing access to large quantities of these valuable intermediates. The synthetic sequence relies on SnCl4⋅2u2009THF, Li, and H2O; the extent of dealkylation depends on the amount of SnCl4⋅2u2009THF employed.

Ruthenium Nitrides: Redox Chemistry and Photolability of the Ru–Nitrido Group

Tunable electrophilicity/nucleophilicity by means of the redox properties of the Ru≡N group and reversible interconversion of mononuclear and dinuclear species as a result of the photolability of the Ru=N=Ru group are characteristic of the nitrido derivatives of Ru porphyrinogens. For example, 2, the product of reversible reduction of a Ru≡N precusor, reacts with 1 in the dark to form 3, which undergoes photocleavage to 1 and 2.

The use of macrocyclic and polydentate ligands in ruthenium organometallic chemistry

Abstract This report deals with two stereochemically different tridentate N 3 -ligands suitable as ancillary ligands in the organometallic chemistry of ruthenium. [{Ru( p- cymene)(Cl)} 2 (μ-Cl) 2 ] assisted the template synthesis of a tridentate N 3 -macrocycle derived from 2-aminobenzaldehyde, thus forming [Ru(η 3 -C 21 H 15 N 3 )Ru( p- cymene)] 2+ 2Cl − , 1 . The ligand 2 , Pyrue5f8Pic·H, derived from the condensation of pyrrole-2-aldehyde and 2-picolylamine, functions as a monoanionic tridentate ligand in the reaction with [{Ru(COD)(Cl)} 2 (μ-Cl) 2 ] leading to [Ru(COD)(Cl)(Pyrue5f8Picue5f8H 2 )], 4 , which undergoes the ionization of the Ruue5f8Cl bond both in pyridine or in THF in the presence of AgTf, leading to [Ru(Pyrue5f8Picue5f8H)Py 3 ] + Cl − , 5 and [Ru(COD)(Pyrue5f8Picue5f8H)Tf], 6 , respectively. The alkylation of 4 using LiMe led to [(Ruue5f8Me)(Pyrue5f8Picue5f8H)(COD)], 7 , which undergoes a methane elimination to yield [Ru 2 (μ-Pyrue5f8Pic) 2 (COD) 2 ], 8 . The reaction of potassiumue5f8pyren [pyren= N , N ′-ethylenebis(2-pyrrolyliminato)dianion], 10 , with [{Ru(COD)(Cl)} 2 (μ-Cl) 2 ] led to the Ru-macrocyclic derivative [Ru(Pyren)(COD)], 11 , where COD fills two cis -positions around ruthenium. Extended Huckel calculations have been carried out on the two stereochemically different Ruue5f8N 3 fragments having a facial (see complex 1 ) and a meridional ( 4 , 5 , 6 , 7 and 8 ) arrangements in order to identify the difference in the frontier orbitals for the metal reactivity.

The π Complexation of Alkali and Alkaline Earth Ions by the Use ofmeso-Octaalkylporphyrinogen and Aromatic Hydrocarbons

The full metallation of meso-octaalkylporphyrinogens [R8N4H4] (R=Et, 1; nBu, 2; CH2Ph, 3; (CH2)4, 4) with heavy alkali metals (M = K, Rb, Cs) leads to the porphyrinogen-M4 compounds, in which the solvation of the alkali cations is largely assured by the intra- and intermolecular phi-interactions with the pyrrolyl anions. Such a mode of complexation results in a structural diversity as a function of the meso substituents, the size of the metal ion, and the solvent. The structure of the unsolvated polymers [R8N4M4]n (R= Et, M=K, 5; M=Rb, 6; M=Cs, 7; R= (CH2)4, M = Rb, 8; M = Cs, 9) have been clarified through the X-ray analysis of 7 recrystallized from diglyme. The structure shows that the tetraanion binds two Cs ions inside the cavity, which display in one case eta1:eta1:eta1:eta1 and in the other eta5:eta5:eta5:eta5 interaction modes. Bidimensional polymerization is assured by four Cs ions, which each bind at the eta5 position on the exo face of each pyrrole. With bulkier meso substituents, different polymeric forms are obtained (R = nBu, M = K, 10; M = Rb, 11; M = Cs, 12), and their structures were clarified through the X-ray analysis of 10, which was recrystallized from dimethoxyethane. The polymeric units are made up by the monomeric units [Bu8N4K2]2-, in which one potassium is eta1:eta1:eta1:eta5 and the other eta5:eta1:eta5:eta1 bonded inside the porphyrinogen cavity. In the case of R=CH2Ph, the monomeric anion [(PhCH2)8N4K2]2- (13) has been structurally identified. The metallation of 1 and 2 with active forms of alkaline earth metals (M = Ca, Sr, Ba) led to dinuclear compounds [R,N4M2] (R = Et, M = Ca, 14; M=Sr, 15; M=Ba, 16; R=nBu. M=Ba, 18), in which both metals inside the cavity are eta1:eta3:eta1:eta3 (Ca) and eta1:eta5:eta1:eta5 (Sr and Ba) bonded to the porphyrinogen tetraanion. The coordination sphere of each metal ion is completed by two THF molecules, which, in the case of Ba, are easily replaced by an arene ring [Bu8N4Ba2(eta6-arene)2] (arene=durene, 22; naphthalene, 23; toluene, 24; benzene, 25). The X-ray structures of 14, 15, 18, 22, and 23 are described in detail. We have tried to establish a relationship between the solid-state and solution structures by analyzing the 1H NMR spectra of the porphyrinogen complexes.

Acetylenes Rearranging on Ruthenium–Porphyrinogen and Leading to Vinylidene and Carbene Functionalities

Through a proton-transfer reaction a porphyrinogen assists the transformation of terminal acetylenes into Ru-vinylidenes, which are the entry point to a variety of Ru-carbenes and Ru-cumulenes. The scheme (in which the porphyrinogen is stylized) shows the reversible interconversion of an acetylide into a divinylidene unit.

Functionalizable Alkylidenes: Tungsten Complexes of Phosphanyl-, Amino-, Alkynyl-, and Tinalkylidenes and Their Dimetallic Derivatization.

The introduction of a heteroatom at the alkylidene carbon atom of calixarene complexes such as 1 results in an organometallic compound that shows properties of both Fischer carbenes and Schrock alkylidenes. Such modified alkylidenes allow access to dimetallic alkylidene complexes. For example, 1 reacts with [CuCOCl] to provide 2.

Metal-metal and carbon-carbon bonds as potential components of molecular batteries.

The reductive coupling of [M(salophen)] derivatives, where M is an early transition metal and salophen is N,N-o-phenylenebis(salicylideneaminato) dianion, led to the formation of dimers linked through C-C and M-M bonds. Both of these bonds can potentially function as electron reservoirs: each bond can be used as a reversible source of a pair of electrons under the condition that it is not chemically transformed by the incoming substrate which functions as an electron acceptor. To explore this potential function as well as the competition in the redox processes between C-C and M-M bonds within the same molecular framework, we investigated the reduction of [(tBu4-salophen)NbCl3] (1) and [(tBu4-salophen)MoCl2] (7) as model compounds. In the former case, the reduction led to [(Nb-Nb)(tBu4-*salophen2*)] (2) which contains both a Nb-Nb bond (2.6528(7) A) and two C-C bonds across two imino groups of the ligand. Complex 2 can be reduced further to a transient compound 5 that contains an Nb=Nb bond. In the second case, the reduction of 7 by two electrons led to [(Mo[triplebond]Mo)(tBu4-salophen)2] (8), which does not contain any C-C linkages between the two salophen units. Complexes 2 and 5 are able to transfer one pair and two pairs of electrons, respectively, to give compounds 3, 4, and 6, with the consequent cleavage of the Nb-Nb and Nb=Nb bonds. In the present case, it is surprising that the C-C bonds do not participate in the reduction of the substrates. A careful theoretical treatment anticipates, both in the case of 1 and 7, the preferential formation of metal-metal bonds upon reduction. This is indeed the case for 7, but not for 1, where the formation of C-C bonds competes with that of M-M bonds, the latter being the first ones, however, to be involved in electron-transfer reactions. The theoretical approach allowed us to investigate the possibility of intramolecular electron transfer from C-C bonds to M-M bonds and vice versa.

The π Complexation of Calcium inside themeso-Octaethylporphyrinogen Tetraanion Cavity

A possible binding cavity for alkali and alkaline earth metal ions: The synthesis and structural characterization of the complex shown, which was obtained from meso-octaethylporphyrinogen and calcium metal, shows that the porphyrinogen functions as a binucleating ligand with four η3 -azaallyl binding sites for two calcium cations.

meso-Octaethylporphyrinogen Displaying Site Selectivity in the Stepwise Synthesis of Polymetallic Aggregates with Interesting Redox Properties: The π-Binding Ability of Metalla-Porphyrinogens

The sequential reaction of meso-octaethylporphyrinogen tetraanion with NiII followed by CoII, RuII, NiII allowed the build up of mixed coordination organometallic aggregates such as 1, which undergo thermally, chemically, or photochemically induced intramolecular electron transfers.