Saverio Santi

Indenyl hapticity in (η-indenyl-RhL2) and Cr(CO)3(μ-η:η-indenyl-RhL2) complexes. A 1H, 13C and 103Rh NMR spectroscopic study

A series of indenyl- and (Cr(CO)3)indenyl-RhL2 complexes (L2COD, (CO)2) bearing substituents on both the six- and five-membered ring have been synthesized and fully characterized, and their 1H, 13C and 103Rh NMR spectra recorded. The changes of the spectral parameters caused by the introduction of the Cr(CO)3 unit suggest significant modifications of the electronic distribution in the indenyl moiety induced in the ground state. The increased reactivity in the ligand exchange reactions (‘extra-indenyl effect’) and the strong modifications of the catalytic and spectroscopic properties of the Rh center itself indicate a substantial weakening of the coordinative bond between rhodium and the indenyl moiety in the heterobimetallic species as expected on going from an η5 towards a more pronounced η3 coordination mode.

Nonlinear Absorption Properties and Excited State Dynamics of Ferrocene

We report on the first observation of reverse saturable absorption by ferrocene (Fc) in toluene using nanosecond pulses at 532 nm. Pump and probe experiments in the visible spectral region show the existence of an excited triplet state with an intersystem crossing quantum yield S1 --> T1 of 0.085 and a molar extinction coefficient epsilon(Fc)(T) of 5650 L mol(-1) cm(-1) at 700 nm. The full understanding of the nonlinear optical behavior of Fc cannot be obtained, however, with a model that includes only the one-photon absorption from T1, but it is mandatory to consider also a simultaneous two-photon absorption from an excited singlet state of Fc (two-photon absorption cross section: 2.4 x 10(-41) cm4 s ph(-1) mol(-1)). The optical spectrum of the ground and triplet state of Fc are calculated within a TD-DFT approach considering several functionals (PBE, BLYP, LDA, OPBE) for the optimization of molecular geometry.

Synthesis, structure and reactivity of homobimetallic Rh(I) and Ir(I) complexes of s- and as-indacene-diide

Abstract The Rh(COD) and Ir(COD) homobimetallic complexes of s-indacene-diide, 2,6-dimethyl-s-indacene-diide, as-indacene-diide, and 2,7-dimethyl-as-indacene-diide have been synthesized from the di-lithium salts of the dianions and metal dimers [M(μ-Cl)L2]2 (M = Rh, Ir; L2 = COD, NBD, (ethylene)2, (CO)2 as mixtures of syn and anti isomers. The syn/anti ratio depends on the nature of the ancillary ligands at the metal and on the s or as geometry of the bridging ligand. In the reaction of the 2,7-dimethyl-as-indacene-diide-[M(COD)]2 species with CO, the higher reactivity of the syn isomers has been justified on the basis of a greater instability of the ground state due to steric interactions between the COD groups. Bis-η1 metal-bonded intermediates have been identified in the carbonylation of iridium derivatives; on the other hand, the formation of the bis-η5 mixed complexes syn and anti-{2,7-dimethyl-as-indacene-diide-[Rh(COD)][Rh(CO)2]} and their reactivity strongly support the existence of metalmetal interaction in the rhodium derivatives.

Indenyl and indacenyl ligands in homo and heterobimetallic complexes

Abstract Metal atoms that are held in close proximity exhibit unique properties and many ligands have been tested for their ability to coordinate two or more transition metals in an effort to search for new chemical and physical properties. During the last decade, extensive studies have been conducted on a series of heterobimetallic indenyl complexes of Cr(0) and Rh(I). The mutual effect of the two metal units, viz. Cr(CO) 3 and RhL 2 [L 2 =COD, NBD, (CO) 2 ], anti coordinated to the same planar bridging ligand influences both the reactivity and the catalytic activity of the two metals. Some insight into the dependence of the mutual effects on the stereochemistry of the bimetallic complexes may be provided by the availability of pairs of pure syn and anti bimetallic stereoisomers of rigid bridging ligands. Reported here are the preliminary results concerning the synthesis of syn and anti homobimetallic s -indacene-diide-[RhL 2 ] 2 complexes.

Cofacial and antarafacial indenyl bimetallic isomers: a descriptive MO picture and implications for the indenyl effect on ligand substitution reactions

The hetero-bimetallic Ind complexes (Ind=indenyl anion) with the formula (CO)3Cr(μ-Ind)RhL2 (L=CO, ethylene; L2=COD, COT, NBD) have two possible conformers, one with the metals at antipodal sides of the condensed bicyclic polyene (antarafacial, anti) and the other with both metals on the same side (cofacial, syn). These systems can be considered to contain 34 valence electrons by assuming that Ind is capable of donating all of its ten π electrons to the metals. A qualitative MO analysis, based on EHMO calculations, is presented. The anti conformer is compared to the homometallic 34e species, namely [CpRu(μ-Ind)RuCp]+. Here, the pseudo-symmetrical relationship between the two identical metal fragments strongly suggests that one Ind σ-bonding MO donates part of its electron density to each of the two metals, which thus become formally saturated (36e). This argument is extended to the anti CrRh derivative in which a critical role is played by an empty FMO of the (CO)2Rh(I) fragment with π‖ symmetry (i.e. lying in the molecular mirror plane). The latter, in spit the large πCO∗ contributions, has good acceptor capabilities at the metal.The calculations suggest that the so-called indenyl effect, namely the increased rate of substitution of one CO ligand by another σ donor (e.g. a phosphine), although a structurally dynamic process, also depends on the electron density which accumulates on the aforementioned π‖ FMO. In the series (η5-C5H5)Rh(CO)2, (η5-C7H9Rh(CO)2 and {η5-[(CO)3CrC7H9]}-Rh(CO)2 the progressively reduced donor capabilities of the cyclic polyene toward the (CO)2Rh(I) fragment induce a larger electrophilicity in rhodium, in good agreement with the experimental data on substitution reactivity. Finally, the electronic features of the cofacial conformer (CO)3Cr(μ-Ind)Rh(CO)2 are examined. In spite of the long CrRh separation (> 3 A) the graphically illustrated MO analysis indicates a direct, heterodox, intermetallic linkage, which helps the metals to achieve a formal electronic saturation. In this case, the limited propensity of the complex toward any CO substitution reactions is likely attributable to the hindered mobility of the (CO)2Rh(I) fragment.

Anti-Kasha's rule fluorescence emission in (2-ferrocenyl)indene generated by a twisted intramolecular charge-transfer (TICT) process.

A twisted intramolecular charge-transfer (TICT) process has been identified in (2-ferrocenyl)indene. This photochemical process explains the anti-Kashas rule fluorescence emission observed for this system. Experimental and model investigations on (2-ferrocenyl)tetramethylindene and (2-ferrocenyl)-hexamethylindene were also performed, in order to evaluate the effect of a steric hindrance on the TICT mechanism. The energy of the lowest main excited states was computed with a TD-DFT approach, as a function of the rotation of the dihedral angle between the indene and the cyclopentadienyl planes. To the best of our knowledge, this is the first example of TICT generated by metal-to-ligand charge transfer (MLCT) in a ferrocene-containing complex and, more generally, the first case of complexes in which a metal center is directly involved.

Heterobimetallic indenyl complexes. Synthesis and structure of syn-[Cr(CO)3(μ, η:η-indenyl)Rh(COD)]

Abstract The heterobimetallic complex syn -[Cr(CO) 3 (μ,η:η-indenyl)Rh(COD)] was obtained of syn -[Cr(CO) 3 (μ,η:η-indenyl(Rh(CO) 2 ] with COD. The stereochemistry was retained in the exchange reaction as confirmed by 1 H and 13 C NMR measurements and by X-ray diffractometric analysis. In spite of great distortion, the COD complex is quite stable. An unusual bonding interaction between the hybrid CO orbitals of the Cr(CO) 3 group and the s-orbital of methine hydrogen atoms of COD is inferred on the basis of spectroscopic and X-ray structural data.

Charge Mapping in 310-Helical Peptide Chains by Oxidation of the Terminal Ferrocenyl Group

Two series of 3(10)-helical peptides of different lengths and rigidity, based on the strongly foldameric α-aminoisobutyric acid and containing a terminal ferrocenyl unit, have been synthesized. Oxidation-state sensitive spectroscopic tags of helical peptides, the N-H groups, allowed mapping of the charge delocalization triggered by oxidation of the terminal ferrocenyl moiety and were monitored by IR spectroelectrochemistry.

Heterobimetallic heptamethylindenyl complexes of Cr0 and RhI: Trans-[Cr(CO)3-indenyl★-RhL2] (L2 COD, L CO)

Abstract Indenyl-RhL 2 species are efficient catalysts in the cyclotrimerization reaction of alkynes to give substituted arenes. The syntheses and X-ray structures of the two bimetallic complexes, trans -Cr(CO) 3 -heptamethylindenyl-Rh(COD) and trans - Cr(CO) 3 -heptamethylindenyl-Rh(CO) 2 , are reported. The contemporary coordination of the benzene ring with Cr(CO) 3 and permethylation of the indenyl ligand enhance substantially the reaction rate of the trimerization and the stability of the catalyst. Best results were obtained when the bidentate cycloocta-1,5-diene was used as an ancillary ligand.

Synthesis and characterization of η6-Cr(CO)3-indenyl-η3-rhodium-η4-C8H12: an η3: η6 co-ordination for the indenyl ligand

Deprotonation of indene–tricarbonylchromium with KH in tetrahydrofuran at –40 °C gives the η6-tricarbonylchromium–indenyl anion which reacts with [Rh(CI)COD]2(COD = cyclo-octa-1,5-diene) to form [η6-Cr(CO)3-indenyl-η3-rhodium-η4- COD]; the same bimetallic complex is obtained by reaction of the rhodium dimer with the isomeric η5-indenyl-tricarbonylchromium anion.