Substitution Lability of the Perfluorinated Cp* Ligand in [Rh(COD)(C5(CF3)5)] Towards Triphenylpnictogens EPh3 (E=N, P, As, Sb, Bi).

Triphenylpnictogens EPh3 (E=N, P, As, Sb, Bi) are able to displace the perfluorinated Cp* ligand in [Rh(COD)(C5(CF3)5)] (COD=1,5-cyclooctadiene) in up to quantitative yield. The resulting ionic products contain [C5(CF3)5]- as uncoordinated counter anion. The cations feature [Rh(COD)]+ fragments, coordinated by one (N, Bi), two (P, As) or three (Sb) triphenylpnictogen moieties. Whereas coordination via the pnictogen is observed for P, As and Sb, π-coordination of the aryl rings is observed for N and Bi.

Evoking Ferroptosis by Synergistic Enhancement of a Cyclopentadienyl Iridium-Betulin Immune Agonist.

Ferroptosis is a form of programmed cell death driven by iron-dependent lipid peroxidation (LPO) with the potential for antitumor immunity activation. In this study, a nonferrous cyclopentadienyl metal-based ferroptosis inducer [Ir(Cp*)(Bet)Cl]Cl (Ir-Bet) was developed by a metal-ligand synergistic enhancement (MLSE) strategy involving the reaction of [Ir(Cp*)Cl]2 Cl2 with the natural product Betulin. The fusion of Betulin with iridium cyclopentadienyl (Ir-Cp*) species as Ir-Bet not only tremendously enhanced the antiproliferative activity toward cancer cells, but also activated ferritinophagy for iron homeostasis regulation by PI3K/Akt/mTOR cascade inhibition with a lower dosage of Betulin, and then evoked an immune response by nuclear factor kappa-B (NF-κB) activation of Ir-Cp* species. Further immunogenic cell death (ICD) occurred by remarkable ferroptosis through glutathione (GSH) depletion, glutathione peroxidase 4 (GPX4) deactivation and ferritinophagy. An in vivo vaccination experiment demonstrated desirable antitumor and immunogenic effects of Ir-Bet by increasing the ratio of cytotoxic T cells (CTLs)/regulatory T cells (Tregs).

Introducing the Perfluorinated Cp* Ligand into Coordination Chemistry.

The reaction of AgBF4 and [Rh(COD)Cl]2 (COD=1,5-cyclooctadiene) in presence of [NEt4 ][C5 (CF3 )5 ] afforded the fluorocarbon soluble complex [Rh(COD)(C5 (CF3 )5 )] by salt metathesis. This complex represents the first example for a successful coordination of the weakly basic [C5 (CF3 )5 ]- ligand, since its first synthesis in 1980. In addition to [Rh(COD)(C5 (CF3 )5 )] also the byproduct [Rh(COD)(C5 (CF3 )4 H)] was isolated and fully characterized. Accompanying DFT studies showed that the interaction energy of the [C5 (CF3 )5 ]- ligand towards the 12-electron fragment [Rh(COD)]+ is ≈70 kcal mol-1 lower in comparison to [C5 (CH3 )5 ]- due to reduced electrostatic interactions and weaker π-donor properties of the ligand. The quantitative but reversible substitution of the [C5 (CF3 )5 ]- ligand by toluene, converting it into a weakly coordinating anion, experimentally proved the extraordinary weak bonding interaction.

A Class of Readily Tunable Planar-Chiral Cyclopentadienyl Rhodium(III) Catalysts for Asymmetric C-H Activation.

Chiral half-sandwich cyclopentadienyl rhodium(III) (CpRhIII ) complexes are powerful catalysts for promoting asymmetric C-H activation reactions. Their preparation normally involved linking or embedding the Cp motif to or into a certain chiral backbone to forge the so-called chiral Cp ligand. However, preparation of a planar-chiral CpRhIII catalyst bearing a non-chiral Cp ligand remains a formidable challenge and is rarely reported. We describe herein an unusual class of planar-chiral rhodium catalysts bearing non-chiral Cp ligands. Different from existing ones, this catalyst is readily tunable. Ten planar-chiral only CpRhIII catalysts were prepared with ease, and successfully used in two enantioselective C-H activation reactions. Given its convenient synthesis and high structural tunability, these catalysts are expected to find more utilities in asymmetric C-H activation.

Triaminocyclopentadienyl Ruthenium Complexes - New Catalysts for Cascade Conversions of Propargyl Alcohols.

Various triaminocyclopentadienyl ruthenium complexes have been synthesized from Ru3 (CO)12 . The new complexes were tested for their ability to catalyze cascade conversions of propargyl alcohols. Their associated catalytic activities complement the activities of known diaminocyclopentadienone ruthenium complexes. In particular, the substrate scope of catalytic cycloadditions with 3-ketolactones or phloroglucinol derivatives is extended to terpenoid-derived propargyl alcohols containing an internal alkyne moiety. A wide range of cyclic terpenoid and phloroglucinol adducts are obtained by complementary application of both types of catalysts.

Highly Selective Ortho-Directed Dicarboxylation of Cyclopentadiene by Methylcarbonates and CO2 or COS - First Insight into Co-ordination Chemistry of New Ambident Ligands.

This research presents the highly regioselective syntheses of 1,2-dicarboxylated cyclopentadienide salts [Cat]2 [C5 H3 (CO2 )2 H] by reaction of a variety of organic cation methylcarbonate salts [Cat]OCO2 Me (Cat=NR4 + , PR4 + , Im+ ) with cyclopentadiene (CpH) or by simply reacting organic cation cyclopentadienides Cat[Cp] (Cat=NR4 + , PR4 + , Im+ ) with CO2 . One characteristic feature of these dianionic ligands is the acidic proton delocalized in an intramolecular hydrogen bridge (IHB) between the two carboxyl groups, as studied by 1 H NMR spectroscopy and XRD analyses. The reaction cannot be stopped after the first carboxylation. Therefore, we propose a Kolbe-Schmitt phenol-carboxylation related mechanism where the acidic proton of the monocarboxylic acid intermediate plays an ortho-directing and CO2 activating role for the second kinetically accelerated CO2 addition step exclusively in ortho position. The same and related thiocarboxylates [Cat]2 [C5 H3 (COS)2 H] are obtained by reaction of COS with Cat[Cp] (Cat=NR4 + , PR4 + , Im+ ). A preliminary study on [Cat]2 [C5 H3 (CO2 )2 H] reveals, that its soft and hard coordination sites can selectively be addressed by soft Lewis acids (Mo0 , Ru2+ ) and hard Lewis acids (Al3+ , La3+ ).

Chiral Cyclopentadienyl Ligands: Design, Syntheses, and Applications in Asymmetric Catalysis.

The creation of new chiral ligands capable of providing high stereocontrol in metal-catalyzed reactions is crucial in modern organic synthesis. The production of bioactive molecules as single enantiomers is increasingly required, and asymmetric catalysis with metal complexes constitutes one of the most efficient synthetic strategies to access optically active compounds. Herein we offer a historical overview on the development of chiral derivatives of the ubiquitous cyclopentadienyl ligand (CpX ), and detail their successful application in a broad range of metal-catalyzed transformations. Those include the functionalization of challenging C-H bonds and beyond, giving access to an extensive catalogue of valuable chiral molecules. A critical comparison of the existing ligand families, their design, synthesis, and complexation to different metals is also provided. In addition, future research directions are discussed to further enhance the performance and application of CpX ligands in enantioselective catalysis.

Enantioselective Cpx RhIII -Catalyzed Carboaminations of Acrylates.

Enantioselective carboaminations of olefins constitute an attractive strategy for a rapid increase in molecular complexity from readily available starting materials. Reported here is an intermolecular asymmetric carboamination of acrylates using rhodium(III)-catalyzed alkenyl C-H activations of N-enoxysuccinimides to generate the nitrogen and carbon portion for the transfer. A rhodium complex equipped with a tailored bulky trisubstituted chiral Cpx ligand ensures carboamination chemoselectivity as well high levels of enantioinduction. The transformation operates under mild reaction conditions at ambient temperatures and provides access to a variety of α-amino esters in good yields and excellent enantiomeric ratios of >99.5:0.5.

Molecular Thorium Trihydrido Clusters Stabilized by Cyclopentadienyl Ligands.

Hydrogenolysis of alkyl-substituted cyclopentadienyl (CpR ) ligated thorium tribenzyl complexes [(CpR )Th(p-CH2 -C6 H4 -Me)3 ] (1-6) afforded the first examples of molecular thorium trihydrido complexes [(CpR )Th(µ-H)3 ]n (CpR =C5 H2 (t Bu)3 or C5 H2 (SiMe3 )3 , n=5; C5 Me4 SiMe3 , n=6; C5 Me5 , n=7; C5 Me4 H, n=8; 7-10 and 12) and [(Cp# )12 Th13 H40 ] (Cp# =C5 H4 SiMe3 ; 13). The nuclearity of the metal hydride clusters depends on the steric profile of the cyclopentadienyl ligands. The hydrogenolysis intermediate, tetra-nuclear octahydrido thorium dibenzylidene complex [(Cpttt )Th(µ-H)2 ]4 (µ-p-CH-C6 H4 -Me)2 (Cpttt =C5 H2 (t Bu)3 ) (11) was also isolated. All of the complexes were characterized by NMR spectroscopy and single-crystal X-ray analysis. Hydride positions in [(CpMe4 )Th(µ-H)3 ]8 (CpMe4 =C5 Me4 H) were further precisely confirmed by single-crystal neutron diffraction. DFT calculations strengthen the experimental assignment of the hydride positions in the complexes 7 to 12.

Dysprosiacarboranes as Organometallic Single-Molecule Magnets.

The dicarbollide ion, nido-C2 B9 H11 2- is isoelectronic with cyclopentadienyl. Herein, we make dysprosiacarboranes, namely [(C2 B9 H11 )2 Ln(THF)2 ][Na(THF)5 ] (Ln=Dy, 1Dy) and [(THF)3 (µ-H)3 Li]2 [{η5 -C6 H4 (CH2 )2 C2 B9 H9 }Dy{η2 :η5 -C6 H4 (CH2 )2 C2 B9 H9 }2 Li] 3Dy and show that dicarbollide ligands impose strong magnetic axiality on the central DyIII ion. The effective energy barrier (Ueff ) for the loss of magnetization can be varied by the substitution pattern on the dicarbollide. This finding is demonstrated by comparing complexes of nido-C2 B9 H11 2- and nido-[o-xylylene-C2 B9 H9 ]2- , which show a Ueff of 430(5) K and 804(7) K, respectively. The blocking temperature defined by the open hysteresis temperature of 3Dy reaches 6.8 K. Moreover, the linear complex [Dy(C2 B9 H11 )2 ]- is predicted to have comparable properties with the linear [Dy(CpMe3 )2 ]+ complex. As such, carboranyl ligands and their derivatives may provide a new type of organometallic ligand for high-performance single-molecule magnets.

Reversible Hydride Migration from C5Me5 to RhI Revealed by a Cooperative Bimetallic Approach.

The use of cyclopentadienyl ligands in organometallic chemistry and catalysis is ubiquitous, mostly due to their robust spectator role. Nonetheless, increasing examples of non-innocent behaviour are being documented. Here, we provide evidence for reversible intramolecular C-H activation at one methyl terminus of C5Me5 in [(η-C5Me5)Rh(PMe3)2] to form a new Rh-H bond, a process so far restricted to early transition metals. Experimental evidence was acquired from bimetallic rhodium/gold structures in which the gold center binds either to the rhodium atom or to the activated Cp* ring. Reversibility of the C-H activation event regenerates the RhI and AuI monometallic precursors, whose cooperative reactivity towards polar E-H bonds (E=O, N), including the N-H bonds in ammonia, can be understood in terms of bimetallic frustration.

Evaluating Electron-Transfer Reactivity of Complexes of Actinides in +2 and +3 Oxidation States by using EPR Spectroscopy.

The possibility that the relative reactivity of complexes of actinide metals in the +2 and +3 oxidation states could be investigated by examining reactions between AnIII and AnII species of Th and U with rare-earth metal reagents that provide EPR confirmation of electron transfer reactivity has been explored. Neither Cp''3 ThIII nor Cp''3 UIII will reduce Cp''3 LaIII or Cp'3 YIII (Cp'=C5 H4 SiMe3 , Cp''=C5 H3 (SiMe3 )2 ). However, both [K(2.2.2-cryptand)][Cp''3 ThII ] and [K(2.2.2-cryptand)][Cp''3 UII ] reduce Cp''3 LaIII and Cp'3 YIII to form [Cp''3 LaII ]1- and [Cp'3 YII ]1- , respectively, which were identified by EPR spectroscopy. The reverse reactions also occur which indicates that the reduction potentials are similar. [Cp''3 LaII ]1- reduces Cp'3 YIII and the reverse YII /LaIII combination also occurs. In both cases, the reactions generate EPR spectra indicative of multiple species in the mixtures of LaII and YII , which is consistent with ligand exchange and demonstrates that numerous heteroleptic complexes of these LnII ions exist.

Star-Shaped Ruthenium Complexes as Prototypes of Molecular Gears.

The design and synthesis of two families of molecular-gear prototypes is reported, with the aim of assembling them into trains of gears on a surface and ultimately achieving controlled intermolecular gearing motion. These piano-stool ruthenium complexes incorporate a hydrotris(indazolyl)borate moiety as tripodal rotation axle and a pentaarylcyclopentadienyl ligand as star-shaped cogwheel, equipped with five teeth ranging from pseudo-1D aryl groups to large planar 2D paddles. A divergent synthetic approach was followed, starting from a pentakis(p-bromophenyl)cyclopentadienyl ruthenium(II) complex as key precursor or from its iodinated counterpart, obtained by copper-catalyzed aromatic Br/I exchange. Subsequent fivefold cross-coupling reactions with various partners allowed high structural diversity to be reached and yielded molecular-gear prototypes with aryl-, carbazole-, BODIPY- and porphyrin-derived teeth of increasing size and length.

Formal Lossen Rearrangement/Alkenylation or Annulation Cascade of Heterole Carboxamides with Alkynes Catalyzed by CpRhIII Complexes with Pendant Amides.

It has been established that a cyclopentadienyl (Cp) RhIII complex with two aryl groups and a pendant amide moiety catalyzes the formal Lossen rearrangement/alkenylation cascade of N-pivaloyl heterole carboxamides with internal alkynes, leading to alkenylheteroles. Interestingly, the use of sterically demanding internal alkynes afforded not the alkenylation but the [3+2] annulation products ([5,5]-fused heteroles). In these reactions, the pendant amide moiety of the CpRhIII complex may accelerate the formal Lossen rearrangement. The use of five-membered heteroles may deter reductive elimination to form strained [5,5]-fused heteroles; instead, protonation proceeds to give the alkenylation products. Bulky alkyne substituents accelerate the reductive elimination to allow the formation of the [5,5]-fused heteroles.

[Am(C5 Me4 H)3 ]: An Organometallic Americium Complex.

We report the small-scale synthesis, isolated yield, single-crystal X-ray structure, 1 H NMR solution spectroscopy /solid-state UV/Vis-nIR spectroscopy, and density functional theory (DFT)/ab initio wave function theory calculations on an Am3+ organometallic complex, [Am(C5 Me4 H)3 ] (1). This constitutes the first quantitative data on Am-C bonding in a molecular species.

Transition-Metal-Mediated Cleavage of C-C Bonds in Aromatic Rings.

Metal-mediated cleavage of aromatic C-C bonds has a range of potential synthetic applications: from direct coal liquefaction to synthesis of natural products. However, in contrast to the activation of aromatic C-H bonds, which has already been widely studied and exploited in diverse set of functionalization reactions, cleavage of aromatic C-C bonds remains Terra incognita. This Minireview summarizes the recent progress in this field and outlines key challenges to be overcome to develop synthetic methods based on this fundamental organometallic transformation.

Light Lanthanide Metallocenium Cations Exhibiting Weak Equatorial Anion Interactions.

As the dysprosocenium complex [Dy(Cpttt )2 ][B(C6 F5 )4 ] (Cpttt =C5 H2 tBu3 -1,2,4, 1-Dy) exhibits magnetic hysteresis at 60 K, similar lanthanide (Ln) complexes have been targeted to provide insights into this remarkable property. We recently reported homologous [Ln(Cpttt )2 ][B(C6 F5 )4 ] (1-Ln) for all the heavier Ln from Gd-Lu; herein, we extend this motif to the early Ln. We find, for the largest LnIII cations, that contact ion pairs [Ln(Cpttt )2 {(C6 F5 -κ1 -F)B(C6 F5 )3 }] (1-Ln; La-Nd) are isolated from reactions of parent [Ln(Cpttt )2 (Cl)] (2-Ln) with [H(SiEt3 )2 ][B(C6 F5 )4 ], where the anion binds weakly to the equatorial sites of [Ln(Cpttt )2 ]+ through a single fluorine atom in the solid state. For smaller SmIII , [Sm(Cpttt )2 ][B(C6 F5 )4 ] (1-Sm) is isolated, which like heavier 1-Ln does not exhibit equatorial anion interactions, but the EuIII analogue 1-Eu could not be synthesised due to the facile reduction of EuIII precursors to EuII products. Thus with the exception of Eu and radioactive Pm this work constitutes a structurally similar family of Ln metallocenium complexes, over 50 years after the [M(Cp)2 ]+ series was isolated for the 3d metals.

Synthetic Routes towards Multifunctional Cyclopentadienes.

Cyclopentadiene, and more importantly its functionalized derivatives, are a cornerstone of synthetic chemistry. Nowadays, they find applications in (chiral) catalysis, as ligands for organometallic complexes with potential diagnostic and therapeutic applications, and many more. This Minireview highlights the progress that has been made in the development of flexible and robust synthetic routes towards multifunctional cyclopentadienes in the last 20 years.

Permethylated Disila[2]metallocenophanes of Group 14 and 15 Elements.

Permethylated disila[2]metallocenophanes of silicon, germanium, tin, lead, 2 a-d, (tetrelocenophanes) and antimony, 3 a,b, (pnictogenocenophanes) are described. In the case of antimony, a chloro-substituted stibonocenophane, 3 a, as well as cationic stibonocenophanium tetrachloroaluminate and tetraphenylborate salts, 3 b[X] (X=[AlCl4 ], [BPh4 ]), were synthesized. These represent the first examples of metallocenophanes of any Group 15 element. All compounds were studied in solution and in the solid state. Without exception the ansa-bridge exerts a strong influence on the bending angle of the two Cp-ligands. For disila[2]plumbocenophane, 2 d, its reactivity towards Group 15 halides was probed. Treatment of disila[2]plumbocenophane, 2 d, with two equivalents of phosphorus(III) chloride or arsenic(III) chloride, results in a ring-opening reaction and gives the bis(dihalopnictogenyl)-substituted products, 4 a,b.