Triple para-Functionalized Cations and Neutral Radicals of Enantiopure Diaza[4]helicenes

Modulation of absorbance and emission is key for the design of chiral chromophores. Accessing a series of compounds absorbing and emitting (circularly polarized) light over a wide spectral window and often toward near-infrared is of practical value in (chir)optical applications. Herein, by late-stage functionalization on derivatives bridging triaryl methyl and helicene domains, we have achieved the regioselective triple introduction of para electron-donating or electron-withdrawing substituents. Extended tuning of electronic (e.g., E1/2red -1.50 V → -0.68 V) and optical (e.g., emission covering from 550 to 850 nm) properties is achieved for the cations and neutral radicals; the latter compounds being easily prepared by mono electron reductions under electrochemical or chemical conditions. While luminescence quantum yields can be increased up to 70% in the cationic series, strong Cotton effects are obtained for certain radicals at low energies (λabs ∼ 700-900 nm) with gabs values above 10-3. The open-shell electronic nature of the radicals was further characterized by electron paramagnetic resonance revealing an important spin density delocalization that contributes to their persistence.

Mixed valence triiron complexes from the conjugation of [FeIFeI] and [FeII] complexes via intermolecular carbyne/alkyne coupling

We present a new synthetic strategy for obtaining mixed-valence triiron complexes where the metal centers are bridged by a novel, highly functionalized hydrocarbyl ligand. The alkynyl-vinyliminium complexes [Fe2Cp2(CO)(μ-CO){μ-η1:η3-C(X-CCH)CHCNMe2}]CF3SO3 (X = 4-C6H4, [2a1]CF3SO3; X = (CH2)3, [2a2]CF3SO3) were synthesized in almost quantitative yields from the aminocarbyne precursor [Fe2Cp2(CO)2(μ-CO){μ-CNMe2}]CF3SO3, [1a]CF3SO3, and the di-alkynes HCC-X-CCH. Then, the ferracycle [Fe(Cp)(CO){C(NMe2)CHC(4-C6H4CCH)C(O)}], 4a1, was produced in 47% yield from the cleavage of [2a1]CF3SO3 promoted by pyrrolidine. Subsequent reactions of the acetonitrile adducts [Fe2Cp2(CO)(μ-CO)(NCMe){μ-CNMe(R)}]CF3SO3 (R = Me, [1aACN]CF3SO3; R = Xyl, [1bACN]CF3SO3) ([FeIFeI]) with 4a1 ([FeII]) at room temperature resulted in the formation of [FeIFeIFeII] complexes [Fe2Cp2(CO)(μ-CO){μ-η1:η3-C(X-CCHC(NMe2)FeCp(CO)CO)CHCNMe(R)}]CF3SO3 (R = Me, [5a1]CF3SO3; R = Xyl, [5b1]CF3SO3) in yields ranging from 56% to 64%. The new products were characterized by IR and multinuclear NMR spectroscopy, and the structures of [2a2]CF3SO3 and 4a1 were confirmed by single crystal X-ray diffraction. Cyclic voltammetry and spectroelectrochemical studies on [5a1]+ have revealed that reduction and oxidation events occur almost independently at the [FeIFeI] and [FeII] units, respectively. This observation underscores a minimal electronic interaction between the two fragments within the triiron complex. Accordingly, DFT studies pointed out that the HOMO and LUMO orbitals are predominantly localized in the two distinct compartments of [5a1]+.

Oxaliplatin(IV) Prodrugs Functionalized with Gemcitabine and Capecitabine Induce Blockage of Colorectal Cancer Cell Growth-An Investigation of the Activation Mechanism and Their Nanoformulation

The use of platinum-based anticancer drugs, such as cisplatin, oxaliplatin, and carboplatin, is a common frontline option in cancer management, but they have debilitating side effects and can lead to drug resistance. Combination therapy with other chemotherapeutic agents, such as capecitabine and gemcitabine, has been explored. One approach to overcome these limitations is the modification of traditional Pt(II) drugs to obtain new molecules with an improved pharmacological profile, such as Pt(IV) prodrugs. The design, synthesis, and characterization of two novel Pt(IV) prodrugs based on oxaliplatin bearing the anticancer drugs gemcitabine or capecitabine in the axial positions have been reported. These complexes were able to dissociate into their constituents to promote cell death and induce apoptosis and cell cycle blockade in a representative colorectal cancer cell model. Specifically, the complex bearing gemcitabine resulted in being the most active on the HCT116 colorectal cancer cell line with an IC50 value of 0.49 ± 0.04. A pilot study on the encapsulation of these complexes in biocompatible PLGA-PEG nanoparticles is also included to confirm the retention of the pharmacological properties and cellular drug uptake, opening up to the possible delivery of the studied complexes through their nanoformulation.

Triiron Complex with N-Ferrocenyl Aminocarbyne Ligand Bridging a Diiron Core: DFT, Electrochemical, and Biological Insights

The first N-ferrocenyl aminocarbyne complex, [Fe2Cp2(CO)2(μ-CO){μ-CN(Me)(Fc)}]CF3SO3 ([2]CF3SO3), was synthesized with an 88% yield from [Fe2Cp2(CO)4], isocyanoferrocene (CNFc), and methyl triflate. The synthesis proceeded through the intermediate formation of [Fe2Cp2(CO)3(CNFc)], 1. Multinuclear NMR experiments revealed the presence of cis and trans isomers for [2]CF3SO3 in organic solvents, in agreement with DFT outcomes. Electrochemical and spectroelectrochemical studies demonstrated one reduction process occurring prevalently at the diiron core and one oxidation involving the ferrocenyl substituent. The oxidation process is expected to favor the redox activation of [2]+ in a biological environment. Both [2]CF3SO3 and its phenyl analogue [Fe2Cp2(CO)2(μ-CO){μ-CN(Me)(Ph)}]CF3SO3 ([3]CF3SO3), prepared for comparison, exerted moderate antiproliferative activity against the human cancer cell lines A431, HCT-15, PSN-1, 2008, and U1285. However, [2]CF3SO3 exhibited a higher cytotoxicity than [3]CF3SO3, showed a substantial ability to induce intracellular ROS production, and outperformed cisplatin in a three-dimensional SCLC cell model.

Anticancer Potential of Diruthenium Complexes with Bridging Hydrocarbyl Ligands from Bioactive Alkynols

Diruthenacyclopentenone complexes of the general composition [Ru2Cp2(CO)2{μ-η1:η3-CH═C(C(OH)(R))C(═O)}] (2a-c; Cp = η5-C5H5) were synthesized in 94-96% yields from the reactions of [Ru2Cp2(CO)2{μ-η1:η3-C(Ph)═C(Ph)C(═O)}] (1) with 1-ethynylcyclopentanol, 17α-ethynylestradiol, and 17-ethynyltestosterone, respectively, in toluene at reflux. Protonation of 2a-c by HBF4 afforded the corresponding allenyl derivatives [Ru2Cp2(CO)3{μ-η1:η2-CH═C═R}]BF4 (3a-c) in 85-93% yields. All products were thoroughly characterized by elemental analysis, mass spectrometry, and IR, UV-vis, and nuclear magnetic resonance spectroscopy. Additionally, 2a and 3a were investigated by cyclic voltammetry, and the single-crystal diffraction method was employed to establish the X-ray structures of 2b and 3a. The cytotoxicity in vitro of 2b and 3a-c was evaluated against nine human cancer cell lines (A2780, A2780R, MCF-7, HOS, A549, PANC-1, Caco-2, PC-3, and HeLa), while the selectivity was assessed on normal human lung fibroblast (MRC-5). Overall, complexes exert stronger cytotoxicity than cisplatin, and 3b (comprising 17α-estradiol derived ligand) emerged as the best-performing complex. Inductively coupled plasma mass spectrometry cellular uptake studies in A2780 cells revealed a higher level of internalization for 3b and 3c compared to 2b, 3a, and the reference compound RAPTA-C. Experiments conducted on A2780 cells demonstrated a noteworthy impact of 3a and 3b on the cell cycle, leading to the majority of the cells being arrested in the G0/G1 phase. Moreover, 3a moderately induced apoptosis and oxidative stress, while 3b triggered autophagy and mitochondrial membrane potential depletion.

Highly Reduced Ruthenium Carbide Carbonyl Clusters: Synthesis, Molecular Structure, Reactivity, Electrochemistry, and Computational Investigation of [Ru6C(CO)15]4

The reaction of [Ru6C(CO)16]2- (1) with NaOH in DMSO resulted in the formation of a highly reduced [Ru6C(CO)15]4- (2), which was readily protonated by acids, such as HBF4·Et2O, to [HRu6C(CO)15]3- (3). Oxidation of 2 with [Cp2Fe][PF6] or [C7H7][BF4] in CH3CN resulted in [Ru6C(CO)15(CH3CN)]2- (5), which was quantitatively converted into 1 after exposure to CO atmosphere. The reaction of 2 with a mild methylating agent such as CH3,I afforded the purported [Ru6C(CO)14(COCH3)]3- (6). By employing a stronger reagent, that is, CF3SO3CH3, a mixture of [HRu6C(CO)16]- (4), [H3Ru6C(CO)15]- (7), and [Ru6C(CO)15(CH3CNCH3)]- (8) was obtained. The molecular structures of 2-5, 7, and 8 were determined by single-crystal X-ray diffraction as their [NEt4]4[2]·CH3CN, [NEt4]3[3], [NEt4][4], [NEt4]2[5], [NEt4][7], and [NEt4][8]·solv salts. The carbyne-carbide cluster 6 was partially characterized by IR spectroscopy and ESI-MS, and its structure was computationally predicted using DFT methods. The redox behavior of 2 and 3 was investigated by electrochemical and IR spectroelectrochemical methods. Computational studies were performed in order to unravel structural and thermodynamic aspects of these octahedral Ru-carbide carbonyl clusters displaying miscellaneous ligands and charges in comparison with related iron derivatives.

Adding Diversity to a Diruthenium Biscyclopentadienyl Scaffold via Alkyne Incorporation: Synthesis and Biological Studies

We report the synthesis and the assessment of the anticancer potential of two series of diruthenium biscyclopentadienyl carbonyl complexes. Novel dimetallacyclopentenone compounds (2-4) were obtained (45-92% yields) from the thermal reaction (PhCCPh exchange) of [Ru2Cp2(CO)(μ-CO){μ-η1:η3-C(Ph)═C(Ph)C(═O)}], 1, with alkynes HCCR [R = C5H4FeCp (Fc), 3-C6H4(Asp), 2-naphthyl; Cp = η5-C5H5, Asp = OC(O)-2-C6H4C(O)Me]. Protonation of 1-3 by HBF4 afforded the corresponding μ-alkenyl derivatives 5-7, in 40-86% yields. All products were characterized by IR and NMR spectroscopy; moreover, cyclic voltammetry (1, 2, 5, 7) and single-crystal X-ray diffraction (5, 7) analyses were performed on representative compounds. Complexes 5-7 revealed a cytotoxic activity comparable to that of cisplatin in A549 (lung adenocarcinoma), SW480 (colon adenocarcinoma), and ovarian (A2780) cancer cell lines, and 2, 5, 6, and 7 overcame cisplatin resistance in A2780cis cells. Complexes 2, 5, and 7 (but not the aspirin derivative 6) induced an increase in intracellular ROS levels. Otherwise, 6 strongly stabilizes and elongates natural DNA (from calf thymus, CT-DNA), suggesting a possible intercalation binding mode, whereas 5 is less effective in binding CT-DNA, and 7 is ineffective. This trend is reversed concerning RNA, and in particular, 7 is able to bind poly(rA)poly(rU) showing selectivity for this nucleic acid. Complexes 5-7 can interact with the albumin protein with a thermodynamic signature dominated by hydrophobic interactions. Overall, we show that organometallic species based on the Ru2Cp2(CO)x scaffold (x = 2, 3) are active against cancer cells, with different incorporated fragments influencing the interactions with nucleic acids and the production of ROS.

From M6 to M12, M19 and M38 molecular alloy Pt-Ni carbonyl nanoclusters: selective growth of atomically precise heterometallic nanoclusters

Heterometallic Chini-type clusters [Pt_6-xNi_x(CO)₁₂]^2- (x = 0-6) were obtained by reactions of [Pt₆(CO)₁₂]^2- with Ni-clusters such as [Ni₆(CO)₁₂]^2-, [Ni₉(CO)₁₈]^2- and [H₂Ni₁₂(CO)₂₁]^2-, or from [Pt₉(CO)₁₈]^2- and [Ni₆(CO)₁₂]^2-. The Pt/Ni composition of [Pt_6-xNi_x(CO)₁₂]^2- (x = 0-6) depended on the nature of the reagents employed and their stoichiometry. Reactions of [Pt₉(CO)₁₈]^2- with [Ni₉(CO)₁₈]^2- and [H₂Ni₁₂(CO)₂₁]^2-, as well as reactions of [Pt₁₂(CO)₂₄]^2- with [Ni₆(CO)₁₂]^2-, [Ni₉(CO)₁₈]^2- and [H₂Ni₁₂(CO)₂₁]^2-, afforded [Pt_9-xNi_x(CO)₁₈]^2- (x = 0-9) species. [Pt_6-xNi_x(CO)₁₂]^2- (x = 1-5) were converted into [Pt_12-xNi_x(CO)₂₁]^4- (x = 2-10) upon heating in CH₃CN at 80 °C, with almost complete retention of the Pt/Ni composition. Reaction of [Pt_12-xNi_x(CO)₂₁]^4- (x ≈ 8) with HBF₄·Et₂O afforded the [HPt_14+xNi_24-x(CO)₄₄]^5- (x ≈ 0.7) nanocluster. Finally, [Pt_19-xNi_x(CO)₂₂]^4- (x = 2-6) could be obtained by heating [Pt_9-xNi_x(CO)₁₈]^2- (x = 1-3) in CH₃CN at 80 °C, or [Pt_6-xNi_x(CO)₁₂]^2- (2-4) in DMSO at 130 °C. The molecular structures of these new alloy nanoclusters have been determined by single crystal X-ray diffraction. The site preference of Pt and Ni within their metal cages has been computationally investigated. The electrochemical and IR spectroelectrochemical behavior of [Pt_19-xNi_x(CO)₂₂]^4- (x = 3.11) has been studied and compared to the isostructural homometallic nanocluster [Pt₁₉(CO)₂₂]^4-.

Synthesis, Characterization and Photoactivation Studies on the Novel Pt(IV)-Based [Pt(OCOCH3)3(phterpy)] Complex

Photoactivatable Pt(IV) prodrugs represent nowadays an intriguing class of potential metal-based drugs, endowed with more chemical inertness in their oxidized form and better selectivity for the target with respect to the clinically established Pt(II) compounds. In fact, they have the possibility to be reduced by light irradiation directly at the site of interest. For this reason, we synthesized a new Pt(IV) complex, [Pt(OCOCH₃)₃(4'-phenyl-2,2':6',2''-terpyridine)][CF₃SO₃] (1), that is well soluble in aqueous medium and totally unreactive towards selected model biomolecules until its reduction. The highlight of this work is the rapid and efficient photoreduction of 1 with visible light (460 nm), which leads to its reactive Pt(II) analogue. This behavior was made possible by taking advantage of an efficient catalytic system based on flavin and NADH, which is naturally present in the cellular environment. As a comparison, the reduction of 1 was also studied with simple UV irradiation, but both UV-Vis spectrophotometry and ¹H-NMR spectrometry showed that the flavin-catalyzed reduction with visible light was faster. Lastly, the reactivity against two representative biological targets, i.e., human serum albumin and one monofilament oligonucleotide fragment, was evaluated by high-resolution mass spectrometry. The results clearly pointed out that the prodrug 1 did not interact with these targets until its photoreduction to the Pt(II) analogue.

Iminium Substituent Directs Cyanide and Hydride Additions to Triiron Vinyliminium Complexes

The reactivity of diiron bis-cyclopentadienyl cationic complexes containing a vinyliminium ligand coordinated to a diiron scaffold and decorated with a ferrocenyl group, [1a-g]CF3SO3, was investigated towards NBu4CN (in CH2Cl2) and...

Atomically Precise Platinum Carbonyl Nanoclusters: Synthesis, Total Structure, and Electrochemical Investigation of [Pt27(CO)31]4- Displaying a Defective Structure

The molecular Pt nanocluster [Pt₂₇(CO)₃₁]^4– (1^4–) was obtained by thermal decomposition of [Pt₁₅(CO)₃₀]^2– in tetrahydrofuran under a H₂ atmosphere. The reaction of 1^4– with increasing amounts of HBF₄·Et₂O afforded the previously reported [Pt₂₆(CO)₃₂]^2– (3^2–) and [Pt₂₆(CO)₃₂]⁻ (3^–). The new nanocluster 1^4– was characterized by IR and UV–visible spectroscopy, single-crystal X-ray diffraction, direct-current superconducting quantum interference device magnetometry, cyclic voltammetry, IR spectroelectrochemistry (IR SEC), and electrochemical impedance spectroscopy. The cluster displays a cubic-close-packed Pt₂₇ framework generated by the overlapping of four ABCA layers, composed of 3, 7, 11, and 6 atoms, respectively, that encapsulates a fully interstitial Pt₄ tetrahedron. One Pt atom is missing within layer 3, and this defect (vacancy) generates local deformations within layers 2 and 3. These local deformations tend to repair the defect (missing atom) and increase the number of Pt–Pt bonding contacts, minimizing the total energy. The cluster 1^4– is perfectly diamagnetic and displays a rich electrochemical behavior. Indeed, six different oxidation states have been characterized by IR SEC, unraveling the series of 1^n– (n = 3–8) isostructural nanoclusters. Computational studies have been carried out to further support the interpretation of the experimental data.

The synthesis, molecular structure, magnetic properties, computational investigation, electrochemical and IR spectroelectrochemical behavior, and electrochemical impedance spectroscopy investigation of the new multivalent [Pt₂₇(CO)₃₁]⁴⁻ nanocluster are reported.

When ferrocene and diiron organometallics meet: triiron vinyliminium complexes exhibit strong cytotoxicity and cancer cell selectivity

Robust and versatile cationic triiron complexes, obtained from the assembly of ferrocenyl with a di-organoiron structure, display an outstanding cytotoxicity profile, which may be related to redox processes provided by the two metallic components.

Atomically Precise Ni-Pd Alloy Carbonyl Nanoclusters: Synthesis, Total Structure, Electrochemistry, Spectroelectrochemistry, and Electrochemical Impedance Spectroscopy

The molecular nanocluster [Ni36-xPd5+x(CO)46]6- (x = 0.41) (16-) was obtained from the reaction of [NMe3(CH2Ph)]2[Ni6(CO)12] with 0.8 molar equivalent of [Pd(CH3CN)4][BF4]2 in tetrahydrofuran (thf). In contrast, [Ni37-xPd7+x(CO)48]6- (x = 0.69) (26-) and [HNi37-xPd7+x(CO)48]5- (x = 0.53) (35-) were obtained from the reactions of [NBu4]2[Ni6(CO)12] with 0.9-1.0 molar equivalent of [Pd(CH3CN)4][BF4]2 in thf. After workup, 35- was extracted in acetone, whereas 26- was soluble in CH3CN. The total structures of 16-, 26-, and 35- were determined with atomic precision by single-crystal X-ray diffraction. Their metal cores adopted cubic close packed structures and displayed both substitutional and compositional disorder, in light of the fact that some positions could be occupied by either Ni or Pd. The redox behavior of these new Ni-Pd molecular alloy nanoclusters was investigated by cyclic voltammetry and in situ infrared spectroelectrochemistry. All three compounds 16-, 26-, and 35- displayed several reversible redox processes and behaved as electron sinks and molecular nanocapacitors. Moreover, to gain insight into the factors that affect the current-potential profiles, cyclic voltammograms were recorded at both Pt and glassy carbon working electrodes and electrochemical impedance spectroscopy experiments performed for the first time on molecular carbonyl nanoclusters.

Heterometallic rhodium clusters as electron reservoirs: Chemical, electrochemical, and theoretical studies of the centered-icosahedral [Rh12E(CO)27]n- atomically precise carbonyl compounds

In this paper, we present a comparative study of the redox properties of the icosahedral [Rh₁₂E(CO)₂₇]^n- (n = 4 when E = Ge or Sn and n = 3 when E = Sb or Bi) family of clusters through in situ infrared spectroelectrochemistry experiments and density functional theory computational studies. These clusters show shared characteristics in terms of molecular structure, being all E-centered icosahedral species, and electron counting, possessing 170 valence electrons as predicted by the electron-counting rules, based on the cluster-borane analogy, for compounds with such metal geometry. However, in some cases, clusters of similar nuclearity, and beyond, may show multivalence behavior and may be stable with a different electron counting, at least on the time scale of the electrochemical analyses. The experimental results, confirmed by theoretical calculations, showed a remarkable electron-sponge behavior for [Rh₁₂Ge(CO)₂₇]^4- (1), [Rh₁₂Sb(CO)₂₇]^3- (3), and [Rh₁₂Bi(CO)₂₇]^3- (4), with a cluster charge going from -2 to -6 for 1 and 3 and from -2 to -7 for cluster 4, making them examples of molecular electron reservoirs. The [Rh₁₂Sn(CO)₂₇]^4- (2) derivative, conversely, presents a limited ability to exist in separable reduced cluster species, at least within the experimental conditions, while in the gas phase it appears to be stable both as a penta- and hexa-anion, therefore showing a similar redox activity as its congeners. As a fallout of those studies, during the preparation of [Rh₁₂Sb(CO)₂₇]^3-, we were able to isolate a new species, namely, [Rh₁₁Sb(CO)₂₆]^2-, which presents a Sb-centered nido-icosahedral metal structure possessing 158 cluster valence electrons, in perfect agreement with the polyhedral skeletal electron pair theory.

Tetrasubstituted Selenophenes from the Stepwise Assembly of Molecular Fragments on a Diiron Frame and Final Cleavage of a Bridging Alkylidene

A series of 2,3-dicarboxylato-5-acetyl-4-aminoselenophenes, 5a–j, was obtained via the uncommon assembly of building blocks on a diiron platform, starting from commercial [Fe₂Cp₂(CO)₄] through the stepwise formation of diiron complexes [2a–d]CF₃SO₃, 3a–d, and 4a–j. The selenophene-substituted bridging alkylidene ligand in 4a–j is removed from coordination upon treatment with water in air under mild conditions (ambient temperature in most cases), affording 5a–j in good to excellent yields. This process is highly selective and is accompanied by the disruption of the organometallic scaffold: cyclopentadiene (CpH) and lepidocrocite (γ-FeO(OH)) were identified by NMR and Raman analyses at the end of one representative reaction. The straightforward cleavage of the linkage between a bridging Fischer alkylidene and two (or more) metal centers, as observed here, is an unprecedented reaction in organometallic chemistry: in the present case, the carbene function is converted to a ketone which is incorporated into the organic product. DFT calculations and electrochemical experiments were carried out to give insight into the release of the selenophene-alkylidene ligand. Compounds 5a–j were fully characterized by elemental analysis, mass spectrometry, IR, and multinuclear NMR spectroscopy and by X-ray diffraction and cyclic voltammetry in one case.

Metal−metal cooperativity in action! Different fragments are combined on the {Fe₂Cp₂(CO)₂} skeleton to give highly functionalized selenophene ligands, linked to the iron centers through a bridging alkylidene, which is easily removed from coordination by exposure to air/water in ethereal solution.

Redox active Ni-Pd carbonyl alloy nanoclusters: syntheses, molecular structures and electrochemistry of [Ni22-xPd20+x(CO)48]6- (x = 0.62), [Ni29-xPd6+x(CO)42]6- (x = 0.09) and [Ni29+xPd6-x(CO)42]6- (x = 0.27)

A redox active Ni-Pd alloy nanocluster [Ni_22-xPd_20+x(CO)₄₈]^6- (x = 0.62) ([1]^6-) was obtained from the redox condensation of [NBu₄]₂[Ni₆(CO)₁₂] with 0.7-0.8 equivalents of Pd(Et₂S)₂Cl₂ in CH₂Cl₂. Conversely, [Ni_29-xPd_6+x(CO)₄₂]^6- (x = 0.09) ([2]^6-) and [Ni_29+xPd_6-x(CO)₄₂]^6- (x = 0.27) ([3]^6-) were obtained by employing [NEt₄]₂[Ni₆(CO)₁₂] and 0.6-0.7 equivalents of Pd(Et₂S)₂Cl₂ in CH₃CN. The molecular structures of these high nuclearity Ni-Pd carbonyl clusters were determined by single-crystal X-ray diffraction (SC-XRD). [1]^6- adopted an M₄₀ccp structure comprising five close-packed ABCAB layers capped by two additional Ni atoms. Conversely, [2]^6- and [3]^6- displayed an hcp M₃₅ metal core composed of three compact ABA layers. [1]^6-, [2]^6- and [3]^6- showed nanometric sizes, with the maximum lengths of their metal cores being 1.3 nm ([1]^6-) and 1.0 nm ([2]^6- and [3]^6-), which increased up to 1.9 and 1.5 nm, after including also the CO ligands. Ni-Pd distribution within their metal cores was achieved by avoiding terminal Pd-CO bonding and minimizing Pd-CO coordination. As a consequence, site preference and partial metal segregation were observed, as well as some substitutional and compositional disorders. Electrochemical and spectroelectrochemical studies revealed that [1]^6- and [2]^6- were redox active and displayed four and three stable oxidation states, respectively. Even though several redox active high nuclearity metal carbonyl clusters have been previously reported, the nanoclusters described herein represent the first examples of redox active Ni-Pd carbonyl alloy nanoclusters.

Rh-Sb Nanoclusters: Synthesis, Structure, and Electrochemical Studies of the Atomically Precise [Rh20Sb3(CO)36]3- and [Rh21Sb2(CO)38]5- Carbonyl Compounds

The reactivity of [Rh₇(CO)₁₆]^3– with SbCl₃ has been deeply investigated with the aim of finding a new approach to prepare atomically precise metalloid clusters. In particular, by varying the stoichiometric ratios, the reaction atmosphere (carbon monoxide or nitrogen), the solvent, and by working at room temperature and low pressure, we were able to prepare two large carbonyl clusters of nanometer size, namely, [Rh₂₀Sb₃(CO)₃₆]^3– and [Rh₂₁Sb₂(CO)₃₈]^5–. A third large species composed of 28 metal atoms was isolated, but its exact formulation in terms of metal stoichiometry could not be incontrovertibly confirmed. We also adopted an alternative approach to synthesize nanoclusters, by decomposing the already known [Rh₁₂Sb(CO)₂₇]^3– species with PPh₃, willing to generate unsaturated fragments that could condense to larger species. This strategy resulted in the formation of the lower-nuclearity [Rh₁₀Sb(CO)₂₁PPh₃]^3– heteroleptic cluster instead. All three new compounds were characterized by IR spectroscopy, and their molecular structures were fully established by single-crystal X-ray diffraction studies. These showed a distinct propensity for such clusters to adopt an icosahedral-based geometry. Their characterization was completed by ESI-MS and NMR studies. The electronic properties of the high-yield [Rh₂₁Sb₂(CO)₃₈]^5– cluster were studied through cyclic voltammetry and in situ infrared spectroelectrochemistry, and the obtained results indicate a multivalent nature.

The reactivity of [Rh₇(CO)₁₆]³⁻ with SbCl₃ has been deeply investigated as a new approach to prepare atomically precise metal nanoparticles. By varying the reaction conditions, we obtained three large carbonyl nanoclusters, [Rh₂₀Sb₃(CO)₃₆]³⁻, [Rh₂₁Sb₂(CO)₃₈]⁵⁻, and [Rh_28−xSb_x(CO)₄₄]⁶⁻, and the lower-nuclearity [Rh₁₀Sb(CO)₂₁PPh₃]³⁻ species. They have all been characterized through X-ray diffraction, IR spectroscopy, and other techniques based on their specific nature. Spectroelectrochemical studies on [Rh₂₁Sb₂(CO)₃₈]⁵⁻ unravelled its multivalent nature.

Anticancer Potential of Diiron Vinyliminium Complexes

Invited for the cover of this issue is the group of Fabio Marchetti at the Università di Pisa and Paul J. Dyson at Ecole Polytechnique Fédérale de Lausanne (EPFL). Read the full text of the article at 10.1002/chem.201902885.

Anticancer Potential of Diiron Vinyliminium Complexes

Although ferrocene derivatives have attracted considerable attention as possible anticancer agents, the medicinal potential of diiron complexes has remained largely unexplored. Herein, we describe the straightforward multigram-scale synthesis and the antiproliferative activity of a series of diiron cyclopentadienyl complexes containing bridging vinyliminium ligands. IC₅₀ values in the low-to-mid micromolar range were determined against cisplatin sensitive and resistant human ovarian carcinoma (A2780 and A2780cisR) cell lines. Notable selectivity towards the cancerous cells lines compared to the non-tumoral human embryonic kidney (HEK-293) cell line was observed for selected compounds. The activity seems to be multimodal, involving reactive oxygen species (ROS) generation and, in some cases, a fragmentation process to afford monoiron derivatives. The large structural variability, amphiphilic character and good stability in aqueous media of the diiron vinyliminium complexes provide favorable properties compared to other widely studied classes of iron-based anticancer candidates.

Photocytotoxic Pt(iv) complexes as prospective anticancer agents

The use of Pt(iv) complexes as potential anticancer drugs is attractive, because they have higher stability and less side effects than Pt(ii) compounds. Moreover, some Pt(iv) complexes can also be activated with light, opening an avenue to photochemotherapy. Our purpose is to widen the library of photoactivatable Pt(iv)-based prodrugs and here we report on the oxidation of the Pt(ii) compound [PtCl(4'-phenyl-2,2':6',2''-terpyridine)][CF₃SO₃] (1) with PhICl₂ or H₂O₂. The synthetic procedure avoids the formation of multiple species: the treatment with PhICl₂ produces the Pt(iv) complex with axial chlorides, [PtCl₃(4'-phenyl-2,2':6',2''-terpyridine)][CF₃SO₃] (2), while H₂O₂ oxidation and post-synthesis carboxylation produce [Pt(OCOCH₃)₂Cl(4'-phenyl-2,2':6',2''-terpyridine)][CF₃SO₃] (3), bearing acetates in the axial positions. 2 and 3 are stable in physiological-like buffers and in DMSO in the dark, but undergo photoreduction to 1 upon irradiation at 365 nm. Their stability toward reduction is a fundamental parameter to consider: cyclic voltammetry experiments show that the 2 electron reduction Pt(iv) → Pt(ii) occurs at a more negative potential for 3, because of the greater stabilization provided by the acetate axial groups; noteworthily, 3 is stable for hours also in the presence of mM concentration of glutathione. The cytotoxicity of 2 and 3 toward A2780 and A2780cis cell lines reveals that 3 is the least toxic in the dark, but is able to produce cytotoxic effects far higher than cisplatin when irradiated. To shed light on the mechanistic aspects, the interaction with protein and DNA models has been explored through high-resolution mass spectrometry revealing that 2 and 3 behave as prodrugs, but are able to bind to biological targets only after irradiation.

AQUIVION® perfluorosulfonic acid resin for butyl levulinate production from furfuryl alcohol

Multivariate analysis allowed the optimization of a reaction catalyzed by a robust solid acid Aquivion® P87S.

α-Diimines as Versatile, Derivatizable Ligands in Ruthenium(II) p-Cymene Anticancer Complexes

α-Diimines are among the most robust and versatile ligands available in synthetic coordination chemistry, possessing finely tunable steric and electronic properties. A series of novel cationic ruthenium(II) p-cymene complexes bearing simple α-diimine ligands, [(η⁶- p-cymene)RuCl{κ² N-(HCNR)₂}]NO₃ (R = Cy, [1]NO₃; R = 4-C₆H₁₀OH, [2]NO₃; R = 4-C₆H₄OH, [3]NO₃), were prepared in near-quantitative yields as their nitrate salts. [2]NO₃ displays high water solubility. The potential of the α-diimine ligand in [3]NO₃ as a carrier of bioactive molecules was investigated via esterification reactions with the hydroxyl groups. Thus, the double-functionalized derivatives [(η⁶- p-cymene)RuCl{κ² N-(HCN(4-C₆H₄OCO-R))₂}]NO₃ (R = aspirinate, [5]NO₃; valproate, [6]NO₃) and also [4]Cl (R = Me) were obtained in good-to-high yields. UV-vis and multinuclear NMR spectroscopy and cyclic voltammetric studies in aqueous solution revealed only minor ruthenium chloride hydrolytic cleavage, biologically accessible reduction potentials, and pH-dependent behavior of [3]NO₃. Density functional theory analysis was performed in order to compare the Ru-Cl bond strength in [1]⁺ with the analogous ethylenediamine complex, showing that the higher stability observed in the former is related to the electron-withdrawing properties of the α-diimine ligand. In vitro cytotoxicity studies were performed against tumorigenic (A2780 and A2780cisR) and nontumorigenic (HEK-293) cell lines, with the complexes bearing simple α-diimine ligands ranging from inactive to IC₅₀ values in the low micromolar range. The complexes functionalized with bioactive components, i.e., [5]NO₃ and [6]NO₃, exhibited a marked increase in the cytotoxicity with respect to the precursor [3]NO₃.

Molecular Nickel Phosphide Carbonyl Nanoclusters: Synthesis, Structure, and Electrochemistry of [Ni11P(CO)18]3- and [H6-nNi31P4(CO)39]n- (n = 4 and 5)

The reaction of [NEt₄]₂[Ni₆(CO)₁₂] in thf with 0.5 equiv of PCl₃ affords the monophosphide [Ni₁₁P(CO)₁₈]^3- that in turn further reacts with PCl₃ resulting in the tetra-phosphide carbonyl cluster [HNi₃₁P₄(CO)₃₉]^5-. Alternatively, the latter can be obtained from the reaction of [NEt₄]₂[Ni₆(CO)₁₂] in thf with 0.8-0.9 equiv of PCl₃. The [HNi₃₁P₄(CO)₃₉]^5- penta-anion is reversibly protonated by strong acids leading to the [H₂Ni₃₁P₄(CO)₃₉]^4- tetra-anion, whereas deprotonation affords the [Ni₃₁P₄(CO)₃₉]^6- hexa-anion. The latter is reduced with Na/naphthalene yielding the [Ni₃₁P₄(CO)₃₉]^7- hepta-anion. In order to shed light on the polyhydride nature and redox behavior of these clusters, electrochemical and spectroelectrochemical studies were carried out on [Ni₁₁P(CO)₁₈]^3-, [HNi₃₁P₄(CO)₃₉]^5-, and [H₂Ni₃₁P₄(CO)₃₉]^4-. The reversible formation of the stable [Ni₁₁P(CO)₁₈]^4- tetra-anion is demonstrated through the spectroelectrochemical investigation of [Ni₁₁P(CO)₁₈]^3-. The redox changes of [HNi₃₁P₄(CO)₃₉]^5- show features of chemical reversibility and the vibrational spectra in the ν_CO region of the nine redox states of the cluster [HNi₃₁P₄(CO)₃₉]^n- (n = 3-11) are reported. The spectroelectrochemical investigation of [H₂Ni₃₁P₄(CO)₃₉]^4- revealed the presence of three chemically reversible reduction processes, and the IR spectra of [H₂Ni₃₁P₄(CO)₃₉]^n- (n = 4-7) have been recorded. The different spectroelectrochemical behavior of [HNi₃₁P₄(CO)₃₉]^5- and [H₂Ni₃₁P₄(CO)₃₉]^4- support their formulations as polyhydrides. Unfortunately, all the attempts to directly confirm their poly hydrido nature by ¹H NMR spectroscopy failed, as previously found for related large metal carbonyl clusters. Thus, the presence and number of hydride ligands have been based on the observed protonation/deprotonation reactions and the spectroelectrochemical experiments. The molecular structures of the new clusters have been determined by single-crystal X-ray analysis. These represent the first examples of structurally characterized molecular nickel carbonyl nanoclusters containing interstitial phosphide atoms.

Allowing the direct interaction of N-aryl α-diimines with a high valent metal chloride: one-pot WCl6-promoted formation of quinoxalinium salts

The full potential of a high valent metal chloride as both a chlorinating and an oxidative agent was explored by allowing WCl₆ to react with N-(2,6-diisopropylphenyl) α-diimines, in CH₂Cl₂ at room temperature. These α-diimines underwent unprecedented conversion to quinoxalinium cations via intramolecular C-N coupling.

Ruthenium arene complexes with triphenylphosphane ligands: cytotoxicity towards pancreatic cancer cells, interaction with model proteins, and effect of ethacrynic acid substitution

The anticancer behaviour of Ru arene complexes can be tuned by an appropriate choice of the site and linkage of the bioactive group to the phosphane ligand.

One pot conversion of benzophenone imine into the relevant 2-aza-allenium

Interaction of common imines with WCl₆ in organic solvents revealed very unusual features in the context of imine chemistry. Ph₂C[double bond, length as m-dash]NH was converted in one pot into the 2-aza-allenium species [Ph₂C[double bond, length as m-dash]N[double bond, length as m-dash]CPh₂]⁺via N₂ release, with [Ph₂C[double bond, length as m-dash]NH₂]⁺ being a co-product. PhCH[double bond, length as m-dash]N^tBu underwent C-H bond activation resulting in the formation of [C[triple bond, length as m-dash]N] containing derivatives, together with [PhCH[double bond, length as m-dash]NH^tBu]⁺.

Synthesis and characterization of non-bridging mono- and bis-σ-η1-alkynyl derivatives of the phosphido-bridged hexaplatinum core [Pt6(μ-PBut2)4(CO)4]2+

Several mono- or bis-alkynyl derivatives of the [Pt₆(μ-PBu^t₂)₄(CO)₄]²⁺ core, pivotal intermediates in the synthesis of new cluster-containing oligomers, were prepared and fully characterized.

Synthesis, chiroptical properties and density functional theory calculations of 3,3'-biphenyl-2,2'-bitropone

The synthesis of new bitropone derivatives, namely, 3,3'-biphenyl-2,2'-bitropone and 7,7'-biphenyl-2,2'-bitropone, are reported. Isolation of enantiomers arising from restricted rotation around the C-C bond connecting the tropone moieties was attempted by means of chiral high performance liquid chromatography (HPLC). No separation was obtained for 7,7'-biphenyl-2,2'-bitropone. For 3,3'-biphenyl-2,2'-bitropone, difficulties were encountered because of the low separation factor of the peaks and the presence of a rapid racemization process. However, quantitative chiroptical data on the antipodes were obtained by linking a circular dichroism (CD) spectrometer and a UV-vis spectrophotometric detector in series to the HPLC instrument. The analysis of the CD and UV-vis spectra in terms of absolute conformations was done with the help of theoretical calculations performed at the Density Functional Theory (DFT) level. The most stable conformations of the 3,3'-biphenyl-2,2'-bitropone in its ground state were obtained. Starting from these minimum energy conformations, it was possible to compute theoretical CD and UV absorption spectra that fit well with the experimental ones. From this comparison the absolute configuration to the antipodes was assigned. Finally, the effect of the presence of the two lateral phenyl substituents on the structure of the bitropone and hence on the CD spectrum is discussed.

Unprecedented tris-phosphido-bridged triangular clusters with 42 valence electrons. Chemical, electrochemical and computational studies of their formation and stability

This paper presents the synthesis and structural characterization of the unprecedented tris-phosphido-bridged compounds Pt3(μ-PBu(t)2)3X3 (X = Cl, Br, I), having only 42 valence electrons, while up to now analogous clusters typically have 44e(-). The new species were obtained by an apparent bielectronic oxidation of the 44e(-) monohalides Pt3(μ-PBu(t)2)3(CO)2X with the corresponding dihalogen X2. Their X-ray structures are close to the D3h symmetry, similarly to the 44e(-) analogues with three terminal carbonyl ligands. The products were also obtained by electrochemical oxidation of the same monohalides in the presence of the corresponding halide. In a detailed study on the formation of Pt3(μ-PBu(t)2)3I3, the redox potentials indicated that I2 can only perform the first monoelectronic oxidation but is unsuited for the second one. Accordingly, the 43e(-) intermediate [Pt3(μ-PBu(t)2)3(CO)2I](+) was ascertained to play a key role. Another piece of information is that, together with the fully oxidized product Pt3(μ-PBu(t)2)3I3, the transient 44e(-) species [Pt3(μ-PBu(t)2)3(CO)3](+) is formed in the early steps of the reaction. In order to extract detailed information on the formation pathway, involving both terminal ligand substitutions and electron transfer processes, a DFT investigation has been performed and all the possible intermediates have been defined together with their associated energy costs. The profile highlights many important aspects, such as the formation of an appropriate couple of 43e(-) intermediates having different sets of terminal coligands, and suitable redox potentials for the transfer of one electron. Optimizations of 45e(-) associative intermediates in the ligand substitution reactions indicate their possible involvement in the redox process with reduction of the overall energy cost. Finally, according to MO arguments, the unique stability of the 42e(-) phosphido-bridged Pt3 clusters can be attributed to the simultaneous presence of three terminal halides.

On the chiroptical behavior of conjugated multichromophoric compounds of a new pseudoaromatic class: bicolchicides and biisocolchicides

BACKGROUND

It is well known that, stemming from the mutual interplay between chromophores, circular dichroism (CD) is a powerful technique to deal with structural problems for both the small organic molecule and the biopolymer. However, quantitative interpretations of the spectroscopic and structural terms that give rise to the exciton couplet are usually presented for ideal cases, or a few CD bands only are taken into account, overlooking the role of the solvent medium.

METHODOLOGY/PRINCIPAL FINDINGS

Circular dichroism and UV absorption spectra were carried out for colchicide (3) and isocolchicide (6), as well as their coupling products, 10,10'-bicolchicide (2) and 9,9'-biisocolchicide (5), in both hydrogen bonding and non hydrogen bonding solvents, as well as MeCN/H(2)O mixtures. A dramatic control by the solvent emerged, as even tiny changes in the composition of solvent mixtures, at ca 1 water molar fraction, induced a dramatic modification of their CD bands. A mutarotation phenomenon--long known for isocolchicine (8)--was also observed for 5, and can be attributed to the interconversion between atropisomers (R(a),7S),(R(a),7'S)-5a and (R(a),7S),(S(a),7'S)-5b.

CONCLUSIONS/SIGNIFICANCE

Our data show that with molecules built on two structurally identical moieties which embody both hydrophilic and hydrophobic groups, even tiny changes in the composition of solvent mixtures cause a dramatic modification of the CD bands. Their analysis arrives at a qualitative rationalization of the observed CD couplets from the coupling of high energy transitions, while attempts at a quantitative interpretation of these phenomena through time-dependent density functional theory allowed to reproduce satisfactorily the CD spectrum in the 300-450 nm region only. Failure with higher energies probably reflects currently inadequate specific theoretical treatments of the solvent medium.