Alfonso Venzo

Au25(SEt)18, a Nearly Naked Thiolate-Protected Au25 Cluster: Structural Analysis by Single Crystal X-ray Crystallography and Electron Nuclear Double Resonance

X-ray crystallography has been fundamental in discovering fine structural features of ultrasmall gold clusters capped by thiolated ligands. For still unknown structures, however, new tools capable of providing relevant structural information are sought. We prepared a 25-gold atom nanocluster protected by the smallest ligand ever used, ethanethiol. This cluster displays the electrochemistry, mass spectrometry, and UV-vis absorption spectroscopy features of similar Au25 clusters protected by 18 thiolated ligands. The anionic and the neutral form of Au25(SEt)18 were fully characterized by (1)H and (13)C NMR spectroscopy, which confirmed the monolayers properties and the paramagnetism of neutral Au25(SEt)18(0). X-ray crystallography analysis of the latter provided the first known structure of a gold cluster protected by a simple, linear alkanethiolate. Here, we also report the direct observation by electron nuclear double resonance (ENDOR) of hyperfine interactions between a surface-delocalized unpaired electron and the gold atoms of a nanocluster. The advantages of knowing the exact molecular structure and having used such a small ligand allowed us to compare the experimental values of hyperfine couplings with DFT calculations unaffected by structures approximations or omissions.

Effect of the Charge State (z = −1, 0, +1) on the Nuclear Magnetic Resonance of Monodisperse Au25[S(CH2)2Ph]18z Clusters

Monodisperse Au(25)L(18)(0) (L = S(CH(2))(2)Ph) and [n-Oct(4)N(+)][Au(25)L(18)(-)] clusters were synthesized in tetrahydrofuran. An original strategy was then devised to oxidize them: in the presence of bis(pentafluorobenzoyl) peroxide, the neutral or the negatively charged clusters react as efficient electron donors in a dissociative electron-transfer (ET) process, in the former case yielding [Au(25)L(18)(+)][C(6)F(5)CO(2)(-)]. As opposed to other reported redox methods, this dissociative ET approach is irreversible, easily controllable, and clean, particularly for NMR purposes, as no hydrogen atoms are introduced. By using this approach, the -1, 0, and +1 charge states of Au(25)L(18) could be fully characterized by (1)H and (13)C NMR spectroscopy, using one- and two-dimensional techniques, in various solvents, and as a function of temperature. For all charge states, the NMR results and analysis nicely match recent structural findings about the presence of two different ligand populations in the capping monolayer, each resonance of the two ligand families displaying distinct NMR patterns. The radical nature of Au(25)L(18)(0) is particularly evident in the (1)H and (13)C NMR patterns of the inner ligands. The NMR behavior of radical Au(25)L(18)(0) was also simulated by DFT calculations, and the interplay between theory and experiments revealed a fundamental paramagnetic contribution coming from Fermi contact shifts. Interestingly, the NMR patterns of Au(25)L(18)(-) and Au(25)L(18)(+) were found to be quite similar, pointing to the latter cluster form as a diamagnetic species.

Gold nanowired: a linear (Au25)(n) polymer from Au25 molecular clusters.

Au25(SR)18 has provided fundamental insights into the properties of clusters protected by monolayers of thiolated ligands (SR). Because of its ultrasmall core, 1 nm, Au25(SR)18 displays molecular behavior. We prepared a Au25 cluster capped by n-butanethiolates (SBu), obtained its structure by single-crystal X-ray crystallography, and studied its properties both experimentally and theoretically. Whereas in solution Au25(SBu)18(0) is a paramagnetic molecule, in the crystal it becomes a linear polymer of Au25 clusters connected via single Au-Au bonds and stabilized by proper orientation of clusters and interdigitation of ligands. At low temperature, [Au25(SBu)18(0)]n has a nonmagnetic ground state and can be described as a one-dimensional antiferromagnetic system. These findings provide a breakthrough into the properties and possible solid-state applications of molecular gold nanowires.

Electron transfer through 3D monolayers on Au25 clusters.

The monolayer protecting small gold nanoparticles (monolayer-protected clusters, MPCs) is generally represented as the 3D equivalent of 2D self-assembled monolayers (SAMs) on extended gold surfaces. However, despite the growing relevance of MPCs in important applied areas, such as catalysis and nanomedicine, our knowledge of the structure of 3D SAMs in solution is still extremely limited. We prepared a large series of monodisperse Au25(SCnH2n+1)18 clusters (n=2, 4, 6, 8, 10, 12, 14, 16, 18) and studied how electrons tunnel through these monolayers. Electron transfer results, nicely supported by 1H NMR spectroscopy, IR absorption spectroscopy, and molecular dynamics results, show that there is a critical ligand length marking the transition between short ligands, which form a quite fluid monolayer structure, and longer alkyl chains, which self-organize into bundles. At variance with the truly protecting 2D SAMs, efficient electronic communication of the Au25 core with the outer environment is thus possible even for long alkyl chains. These conclusions provide a different picture of how an ultrasmall gold core talks with the environment through/with its protecting but not-so-shielding monolayer.

ω-Mercapto-functionalized hafnium- and zirconium-oxoclusters as nanosized building blocks for inorganic–organic hybrid materials: synthesis, characterization and photothiol-ene polymerization

Two isostructural mercapto-functionalized zirconium- and hafnium-oxoclusters [M12(μ3-O)8(μ3-OH)8(MP)24·n(MPA), MPA = HS–(CH2)2–C(O)OH; MP = HS–(CH2)2–C(O)O−; M= Zr, Hf; n = 4 for Zr, n = 5 for Hf] were prepared by reacting 3-mercaptopropionic acid with zirconium and hafnium butoxide, respectively, in an alkoxide : acid 1 : 7 molar ratio. The two oxoclusters Zr12 and Hf12, which are composed of two Zr6 and Hf6 sub-units bridged by four carboxylate ligands, were thoroughly characterized by single crystal X-ray diffraction, infrared spectroscopy (FT-IR), 1H and 13C nuclear magnetic resonance (NMR) spectroscopy, thermogravimetry (TGA), differential thermoanalysis (DTA) and extended X-ray absorption spectroscopy (EXAFS). The zirconium oxocluster was then embedded in a polymethacrylic matrix by a photothiol-ene polymerization reaction by using different cluster : monomer molar ratios. EXAFS measurements performed on the polymethacrylic-based hybrid materials showed that the structure of the oxocluster is retained even after being embedded in the polymer matrix. Solid-state 13C NMR spectroscopy as well as FT-IR spectroscopy demonstrated that the inorganic–organic hybrid materials are characterized by an almost complete polymerization of methacrylic acid, in which the oxoclusters are embedded in and covalently bound to the polymer matrix.

Metal-stabilized carbanions

Abstract The rate of deprotonation of fluorene and of π-(tricarbonylchromium)-flourene by an excess of KH in THF was measured by monitoring the hydrogen evolution. The pseudo-first order rate constant for the complex is ca. one order of magnitude higher than that for the free ligand. 1 H and 13 C NMR analysis showed that when the anion is produced at -20°C or below, the Cr(C0) 3 group is bonded to one of the phenyl rings (η 6 -anion), whereas ionization at room temperature produces solutions containing mainly the anion with the Cr(CO) 3 bonded to the cyclopentadienyl ring (η 5 -anion) in equilibrium with the η 6 -isomer. The effect of ionization on free and complexed systems, together with the effect of complexation of the free anion, is discussed on the basis of the NMR data. The η 6 π η 5 isomerization equilibrium was followed at various temperatures and different degrees of solvation are deduced for the two isomeric ion pairs from the kinetic and thermodynamic solvation parameters.

Synthesis and structure of tricarbonylchromium mono-, bis- and tris-complexes of 10-methyltribenzotriquinacene

Complexation of 10-methyltribenzotriquinacene (MTBT) a bent triarene of C3v symmetry afforded six mono-, bis- and tris-Cr(CO)3complexed isomers which have been isolated and identified. The results of various synthetic experiments indicate that complexation at the convex side of a free benzene ring is favoured, probably for steric reasons. As compared with the isomer complexed at the convex face (anti), that complexed at the concave one (syn) is less stable. The 1H and 13C NMR spectral data together with the X-ray structures of the two bis- and one tris-complexes account for the lower stability of the syn isomers.

Metal-stabilized carbanions: VI. Formation of some arylmethyl and dibenzotropenyl carbanions complexed with tricarbonylchromium. A kinetic and A 13C NMR study☆

Abstract Rate constants have been measured for the α-metallation of some arylmethanes, of 5 H -dibenzo[ a,d ]cycloheptene and its dihydro derivative, free and complexed with tricarbonylchromium, by potassium hydride in tetrahydrofuran in the presence of 18-crown-6 ether. The stable solutions of the complexed anions have been studied by 13 C NMR spectroscopy. With the free ligands the kinetic acidity depends upon the structure to a great extent, but in the corresponding complexes the rates of metallation do not vary appreciably. On the basis of the 13 C chemical shift data a close similarity is suggested between the electronic structures of the arylmethyl and dibenzotropenyl anions which would account for their very similar metallation rates.

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.

A new class of antitumor trans-amine-amidine-Pt(II) cationic complexes: influence of chemical structure and solvent on in vitro and in vivo tumor cell proliferation.

The reactions of cyclopropylamine, cyclopentylamine, and cyclohexylamine with trans-[PtCl2(NCMe)2] afforded the bis-cationic complexes trans-[Pt(amine)2(Z-amidine)2]2+[Cl-]2, 1-3. The solution behavior and biological activity have been studied in different solvents (DMSO, water, polyethylene glycol (PEG 400), and polyethylene glycol dimethyl ether (PEG-DME 500)). The biological activity was strongly influenced by the cycloaliphatic amine ring size, with trans-[Pt(NH2CH(CH2)4CH2)2{N(H) horizontal lineC(CH3)N(H)CH(CH2)4CH2}2]2+[Cl-]2 (3) being the most active compound. Complex 3 overcame both cisplatin and MDR resistance, inducing cancer cell death through p53-mediated apoptosis. Alkaline single-cell gel electrophoresis experiments indicated direct DNA damage, reasonably attributable to DNA adducts of trans-[PtCl(amine)(Z-amidine)2][Cl] species, which can evolve to produce disruptive and nonrepairable lesions on DNA, thus leading to the drug-induced programmed cancer cell death. Preliminary in vivo antitumor studies on C57BL mice bearing Lewis lung carcinoma highlighted that complex 3 promoted a significant and dose-dependent tumor growth inhibition without adverse side effects.