Reduction of a Cerium(III) Siloxide Complex To Afford a Quadruple‐Decker Arene‐Bridged Cerium(II) Sandwich

Organometallic multi‐decker sandwich complexes containing f‐elements remain rare, despite their attractive magnetic and electronic properties. The reduction of the Ce^III siloxide complex, [KCeL₄] ( 1 ; L=OSi(OtBu)₃), with excess potassium in a THF/toluene mixture afforded a quadruple‐decker arene‐bridged complex, [K(2.2.2‐crypt)]₂[{(KL₃Ce)(μ‐η⁶:η⁶‐C₇H₈)}₂Ce] ( 3 ). The structure of 3 features a [Ce(C₇H₈)₂] sandwich capped by [KL₃Ce] moieties with a linear arrangement of the Ce ions. Structural parameters, UV/Vis/NIR data, and DFT studies indicate the presence of Ce^II ions involved in δ bonding between the Ce cations and toluene dianions. Complex 3 is a rare lanthanide multi‐decker complex and the first containing non‐classical divalent lanthanide ions. Moreover, oxidation of 1 by AgOTf (OTf=O₃SCF₃) yielded the Ce^IV complex, [CeL₄] ( 2 ), showing that siloxide ligands can stabilize Ce in three oxidation states.     

Ligand‐Unsupported Cuprophilicity in the Preparation of Dodecacopper(I) Complexes and Raman Studies

Synthesis and characterization of two dodecacopper(I) extended metal atom chains (EMAC) assembled by two hexadentate bis(pyridylamido)amidinate‐supported hexacopper(I) string complexes (monomers) via the ligand‐unsupported cuprophilicity are described. In addition to short unsupported Cu?Cu contacts, two hexacopper fragments in these two dodecacopper EMACs show a bent conformation based on X‐ray crystallography. Compared with their THF‐bound hexacopper(I) monomers and protonated ligands, these ligand‐unsupported cuprophilic interactions are shown to be weak by Raman spectroscopy. DFT calculations suggest the ligand‐unsupported cuprophilicity originate from weak attractive orbital interactions, and the strength is estimated to be 2.4 kcal mol^?1.     

π‐Ruthenium Complexes of Hexaphyrins(1.1.1.1.1.1): A Triple‐Decker Complex Bearing Two Ruthenoarene Units

Ruthenium(II) π‐coordination onto [28]hexaphyrins(1.1.1.1.1.1) has been accomplished. Reactions of bis‐Au^III and mono‐Au^III complexes of hexakis(pentafluorophenyl) [28]hexaphyrin with [RuCl₂(p‐cymene)]₂ in the presence of NaOAc gave the corresponding π‐ruthenium complexes, in which the [(p‐cymene)Ru]^II fragment sat on the deprotonated side pyrrole. A similar reaction of the bis‐Pd^II [26]hexaphyrin complex afforded a triple‐decker complex, in which the two [(p‐cymene)Ru]^II fragments sat on both sides of the center of the [26]hexaphyrin framework.     

Substrate Binding by a Parallel Metal Sheet Sandwich Complex: A Unique Role of an Additional Metal Atom

A unique metal addition to the edge of a triangular palladium sandwich cluster is reported. While the edge coordination sites of a parallel sandwich complex are narrow because of two sterically encumbering π‐ligands, association of an additional metal atom to a metal sheet gives a wider coordination site. It was proved that the additional metal atom plays a crucial role in binding of cyclopentadienyl by a Pd₃ sheet sandwich complex.     

Extended Architectures Constructed from Sandwich Tetra‐Metal‐Substituted Polyoxotungstates and Transition‐Metal Complexes

Three unprecedented 2D architectures made up of sandwich‐type tetra‐metal‐substituted polyoxotungstates and transition‐metal complexes, [Cu(dien)(H₂O)]₂{[Cu(dien)(H₂O)]₂‐[Cu(dien)(H₂O)₂]₂[Cu₄(SiW₉O₃₄)₂]}? 5H₂O ( 1 ; dien=diethylenetriamine), [Zn(enMe)₂(H₂O)]₂{[Zn(enMe)₂]₂[Zn₄‐ (HenMe)₂(PW₉O₃₄)₂]}?8H₂O ( 2 ; enMe =1,2‐diaminopropane), and [Zn(enMe)₂‐(H₂O)]₄[Zn(enMe)₂]₂{(enMe)₂{[Zn‐ (enMe)₂]₂[Zn₄(HSiW₉O₃₄)₂]}{[Zn‐ (enMe)₂(H₂O)]₂[Zn₄(HSiW₉O₃₄)₂]}}? 13H₂O ( 3 ) were hydrothermally synthesized and structurally characterized by elemental analysis, IR spectroscopy, thermogravimetric analysis, and single‐crystal X‐ray diffraction. Compound 1 consists of anions [Cu₄(SiW₉O₃₄)₂]^12? linked by copper complexes into a 2D structure, whereas 2 is constructed from novel inorganic–organic hybrid anions [Zn₄(HenMe)₂(PW₉O₃₄)₂]^8? linked by zinc complexes into a 2D structure. The most interesting is the unique 2D network 3 , which consists of anions [Zn₄(PW₉O₃₄)₂]^10? with two types of bridging groups: zinc complexes and enMe ligands.     

Mechanism of Redox‐Active Ligand‐Assisted Nitrene‐Group Transfer in a ZrIV Complex: Direct Ligand‐to‐Ligand Charge Transfer Preferred

The mechanism of the nitrene‐group transfer reaction from an organic azide to isonitrile catalyzed by a Zr^IV d⁰ complex carrying a redox‐active ligand was studied by using quantum chemical molecular‐modeling methods. The key step of the reaction involves the two‐electron reduction of the azide moiety to release dinitrogen and provide the nitrene fragment, which is subsequently transferred to the isonitrile substrate. The reducing equivalents are supplied by the redox‐active bis(2‐iso‐propylamido‐4‐methoxyphenyl)‐amide ligand. The main focus of this work is on the mechanism of this redox reaction, in particular, two plausible mechanistic scenarios are considered: 1) the metal center may actively participate in the electron‐transfer process by first recruiting the electrons from the redox‐active ligand and becoming formally reduced in the process, followed by a classical metal‐based reduction of the azide reactant. 2) Alternatively, a non‐classical, direct ligand‐to‐ligand charge‐transfer process can be envisioned, in which no appreciable amount of electron density is accumulated at the metal center during the course of the reaction. Our calculations indicate that the non‐classical ligand‐to‐ligand charge‐transfer mechanism is much more favorable energetically. Utilizing a series of carefully constructed putative intermediates, both mechanistic scenarios were compared and contrasted to rationalize the preference for ligand‐to‐ligand charge‐transfer mechanism.     

Regio‐ and Stereochemical Control in Ocimene Polymerization by Half‐Sandwich Rare‐Earth Metal Dialkyl Complexes

The polymerization of ocimene has been first achieved by half‐sandwich rare‐earth metal dialkyl complexes in combination with activator and AlⁱBu₃. The regio‐ and stereoselectivity in the ocimene polymerization can be controlled by tuning the cyclopentadienyl ligand and the central metal of the complex. The chiral cyclopentadienyl‐ligated Sc complex 1 prepares syndiotactic cis‐1,4‐polyocimene (cis‐1,4‐selectivity up to 100%, rrrr = 100%), while the corresponding Lu, Y, and Dy complexes 2 – 4 and the achiral pentamethylcyclopentadienyl Sc, Lu, and Y complexes 5 – 7 afford isotactic trans‐1,2‐polyocimenes (trans‐1,2‐selectivity up to 100%, mm = 100%).

     

On the Ligand‐to‐Metal Charge‐Transfer Photochemistry of the Copper(II) Complexes of Quercetin and Rutin

In contrast to the UV‐photoinduced ligand photoionization of the flavonoid complexes of Fe^III, redox reactions initiated in ligand‐to‐metal charge‐transfer excited states were observed on irradiation of the quercetin ( 1 ) and rutin ( 2 ) complexes of Cu^II. Solutions of complexes with stoichiometries [Cu^IIL₂] (L=quercetin, rutin) and [Cu^II₂L_n] (n=1, L=quercetin; n=3, L=rutin) were flash‐irradiated at 351 nm. Transient spectra observed in these experiments showed the formation of radical ligands corresponding to the one‐electron oxidation of L and the reduction of Cu^II to Cu^I. The radical ligands remained coordinated to the Cu^I centers, and the substitution reactions replacing them by solvent occurred with lifetimes τ<350 ns. These are lifetimes shorter than the known lifetimes (τ>1 ms) of the quercetin and rutin radical's decay.     

Formation of a Ligand‐Assisted Complex of Two RNA Hairpin Loops

The hairpin structure is one of the most common secondary structures in RNA and holds a central position in the stream of RNA folding from a non‐structured RNA to structurally complex and functional ribonucleoproteins. Since the RNA secondary structure is strongly correlated to the function and can be modulated by the binding of small molecules, we have investigated the modulation of RNA folding by a ligand‐assisted formation of loop–loop complexes of two RNA hairpin loops. With a ligand (NCT6), designed based on the ligand binding to the G–G mismatches in double‐stranded DNA, we successfully demonstrated the formation of both inter‐ and intra‐molecular NCT6‐assisted complex of two RNA hairpin loops. NCT6 selectively bound to the two hairpin loops containing (CGG)₃ in the loop region. Native polyacrylamide gel electrophoresis analysis of two doubly‐labeled RNA hairpin loops clearly showed the formation of intermolecular NCT6‐assisted loop–loop complex. Förster resonance energy‐transfer studies of RNA constructs containing two hairpin loops, in which each hairpin was labeled with Alexa488 and Cy3 fluorophores, showed the conformational change of the RNA constructs upon binding of NCT6. These experimental data showed that NCT6 simultaneously bound to two hairpin RNAs at the loop region, and can induce the conformational change of the RNA molecule. These data strongly support that NCT6 functions as molecular glue for two hairpin RNAs.     

Pyrazole Based Mono‐ and Di‐Substituted Half Sandwich d6 Platinum Group Metal Complexes: Synthesis and Spectral Characterization

Reactions of pyrazole based ligand and halide bridged arene d⁶ metal precursors resulted a series of mono and di‐substituted pyrazole based half sandwich d⁶ metal complexes. In general, they are formulated as [(arene)MLCl₂] [M = Ru, arene = benzene ( 1 ), p‐cymene ( 2 ), arene = Cp*, M = Rh ( 3 ) and Ir ( 4 )] and [(arene)ML₂Cl] [M = Ru, arene = benzene ( 5 ), p‐cymene ( 6 ), arene = Cp*, M = Rh ( 7 ) and Ir ( 8 )]. All these complexes were characterized by various spectroscopic techniques (IR, ¹H NMR, ESI‐MS, and UV/Vis). The molecular structures were confirmed by single‐crystal X‐ray diffraction technique. Spectroscopic studies revealed that complexation i.e., mono‐ and di‐substitution occurred by the ratio‐based reaction between pyrazole ligand and metal precursor through the neutral nitrogen rather than protic nitrogen. In these complexes deprotonation of the protic nitrogen does not occur unlike the other complexes containing pyrazole derivatives, in which the pyrazole ligand is anionic.     

f‐Element Half‐Sandwich Complexes: A Tetrasilylcyclobutadienyl–Uranium(IV)–Tris(tetrahydroborate) Anion Pianostool Complex

Despite there being numerous examples of f‐element compounds supported by cyclopentadienyl, arene, cycloheptatrienyl, and cyclooctatetraenyl ligands (C_5–8), cyclobutadienyl (C₄) complexes remain exceedingly rare. Here, we report that reaction of [Li₂{C₄(SiMe₃)₄}(THF)₂] ( 1 ) with [U(BH₄)₃(THF)₂] ( 2 ) gives the pianostool complex [U{C₄(SiMe₃)₄}(BH₄)₃][Li(THF)₄] ( 3 ), where use of a borohydride and preformed C₄‐unit circumvents difficulties in product isolation and closing a C₄‐ring at uranium. Complex 3 is an unprecedented example of an f‐element half‐sandwich cyclobutadienyl complex, and it is only the second example of an actinide‐cyclobutadienyl complex, the other being an inverse‐sandwich. The U?C distances are short (av. 2.513 Å), reflecting the formal 2? charge of the C₄‐unit, and the SiMe₃ groups are displaced from the C₄‐plane, which we propose maximises U?C₄ orbital overlap. DFT calculations identify two quasi‐degenerate U?C₄ π‐bonds utilising the ψ₂ and ψ₃ molecular orbitals of the C₄‐unit, but the potential δ‐bond using the ψ₄ orbital is vacant.