Gallatabenzene Ligands Emerging from Open Lutetocene and Yttrocene Methyl Complexes.

Utilizing ternary mixtures [LnMe3]n/K(2,4-dtbp)/GaMe3 (Ln = Lu, Y; 2,4-dtbp = 2,4-di-tert-butyl-pentadienyl), a series of metallacycles is accessible via tandem salt metathesis/deprotonation. Depending on the precursor molar ratio, both gallatabenzene and lutetacyclic moieties with fully planar metallacycles are obtained, which were further analyzed using DFT calculations. The precursor molar ratio affects the extent of pentadienyl C-H-bond activation and oligomerization to tetra- and pentametallic arrays. Reacting a 1:2:2 mixture at -40 °C gave the open sandwich methyl complex (2,4-dtbp)2Lu(CH3), displaying a vital intermediate for subsequent ring-closure reactions.

A General Concept for the Electronic and Steric Modification of 1-Metallacyclobuta-2,3-dienes: A Case Study of Group 4 Metallocene Complexes.

The synthesis of group 4 metal 1-metallacyclobuta-2,3-dienes as organometallic analogues of elusive 1,2-cyclobutadiene has so far been limited to SiMe3 substituted examples. We present the synthesis of two Ph substituted dilithiated ligand precursors for the preparation of four new 1-metallacyclobuta-2,3-dienes [rac-(ebthi)M] (M=Ti, Zr; ebthi=1,2-ethylene-1,10-bis(η5-tetrahydroindenyl)). The organolithium compounds [Li2(RC3Ph)] (1 b: R=Ph, 1 c: R=SiMe3) as well as the metallacycles of the general formula [rac-(ebthi)M(R1C3R2)] (2 b: M=Ti, R1=R2=Ph, 2 c: M=Ti, R1=Ph, R2=SiMe3; 3 b: M=Zr, R1=R2=Ph; 3 c: M=Zr, R1=Ph, R2=SiMe3) were fully characterised. Single crystal X-ray diffraction and quantum chemical bond analysis of the Ti and Zr complexes reveal ligand influence on the biradicaloid character of the titanocene complexes. X-band EPR spectroscopy of structurally similar Ti complexes [rac-(ebthi)Ti(Me3SiC3SiMe3)] (2 a), 2 b, and 2 c was carried out to evaluate the accessibility of an EPR active triplet state. Cyclic voltammetry shows that introduction of Ph groups renders the complexes easier to reduce. 13C CPMAS NMR analysis provides insights into the cause of the low field shift of the resonances of metal-bonded carbon atoms and provides evidence of the absence of the ß-C-Ti interaction.

Mono-Ortho-Beryllated Carbodiphosphoranes: Synthesis, Structure, Bonding and Reactivity.

The reaction of organoberyllium compounds with hexaphenylcarbodiphosphorane yields mono-ortho-beryllated complexes, which feature a double dative Be=C bond. The bonding situation in these compounds together with a simple carbodiphosphorane and an N-heterocyclic carbene adduct was analysed with energy decomposition analysis in combination with natural orbital for chemical valence as well as with quantum theory of atoms-in-molecules. Furthermore, the driving forces accountable for mono-ortho-beryllation were elucidated along with the reactivity of the Be=C bond.

Preparative and Catalytic Properties of MoVI Mononuclear and Metallosupramolecular Coordination Assemblies Bearing Hydrazonato Ligands.

A series of polynuclear, dinuclear, and mononuclear Mo(VI) complexes were synthesized with the hydrazonato ligands derived from 5-methoxysalicylaldehyde and the corresponding hydrazides (isonicotinic hydrazide (H2L1), nicotinic hydrazide (H2L2), 2-aminobenzhydrazide (H2L3), or 4-aminobenzhydrazide (H2L4)). The metallosupramolecular compounds obtained from non-coordinating solvents, [MoO2(L1,2)]n (1 and 2) and [MoO2(L3,4)]2 (3 and 4), formed infinite structures and metallacycles, respectively. By blocking two coordination sites with cis-dioxo ligands, the molybdenum centers have three coordination sites occupied by the ONO donor atoms from the rigid hydrazone ligands and one by the N atom of pyridyl or amine-functionalized ligand subcomponents from the neighboring Mo building units. The reaction in methanol afforded the mononuclear analogs [MoO2(L1-4)(MeOH)] (1a-4a) with additional monodentate MeOH ligands. All isolated complexes were tested as catalysts for cyclooctene epoxidation using tert-butyl hydroperoxide (TBHP) as an oxidant in water. The impact of the structure and ligand lability on the catalytic efficiency in homogeneous cyclooctene epoxidation was elucidated based on theoretical considerations. Thus, dinuclear assemblies exhibited better catalytic activity than mononuclear or polynuclear complexes.

Ynamide and Azaalleneyl. Acid-Base Promoted Chelotropic and Spin-State Rearrangements in a Versatile Heterocumulene [(Ad)NCC(tBu)].

We introduce the heterocumulene ligand [(Ad)NCC(tBu)]- (Ad=1-adamantyl (C10H15), tBu=tert-butyl, (C4H9)), which can adopt two forms, the azaalleneyl and ynamide. This ligand platform can undergo a reversible chelotropic shift using Brønsted acid-base chemistry, which promotes an unprecedented spin-state change of the [VIII] ion. These unique scaffolds are prepared via addition of 1-adamantyl isonitrile (C≡NAd) across the alkylidyne in complexes [(BDI)V≡CtBu(OTf)] (A) (BDI-=ArNC(CH3)CHC(CH3)NAr), Ar=2,6-iPr2C6H3) and [(dBDI)V≡CtBu(OEt2)] (B) (dBDI2-=ArNC(CH3)CHC(CH2)NAr). Complex A reacts with C≡NAd, to generate the high-spin [VIII] complex with a κ1-N-ynamide ligand, [(BDI)V{κ1-N-(Ad)NCC(tBu)}(OTf)] (1). Conversely, B reacts with C≡NAd to generate a low-spin [VIII] diamagnetic complex having a chelated κ2-C,N-azaalleneyl ligand, [(dBDI)V{κ2-N,C-(Ad)NCC(tBu)}] (2). Theoretical studies have been applied to better understand the mechanism of formation of 2 and the electronic reconfiguration upon structural rearrangement by the alteration of ligand denticity between 1 and 2.

High-Temperature Solid-State Post-synthetic Modification of Highly Luminescent Cu(I) Metallacycles toward New Luminescent Thermic Tracers.

A new luminescent Cu(I) tetrametallic metallacycle B is reported, featuring very rare semi-bridging aqua ligands. As heated markedly above room temperature, this compound undergoes in the solid-state a post-synthetic transformation, affording the new luminescent metallacycle C. Thermogravimetric analysis, IR spectroscopy and single crystals X-ray diffraction reveal that this alteration preserves the gross tetrametallic macrocycle structure, however caused by the release of the coordinated water molecules with the concomitant formation of cuprophilic interactions. This transition induces a shift from eye-perceived green (B) to blue (C) room temperature luminescence for these molecular solids. Photophysical measurements and time-dependent density-functional theory calculations have been conducted to identify the origins of the emission properties lying in these structurally related assemblies, suggesting that Thermally Actived Delayed Fluorescence dominates the radiative relaxation pathways. This study highlights the innovative feature of Cu(I) derivatives, offering the access to stimuli-sensitive materials that can witness a posteriori the exceeding of critical temperatures in their environment.

Pt(II)/Pd(II)-Based Metallosupramolecular Architectures as Light Harvesting Systems and their Applications.

The development of artificial light-harvesting systems mimicking the natural photosynthesis method is an ever-growing field of research. Numerous systems such as polymers, metal complexes, POFs, COFs, supramolecular frameworks etc. have been fabricated to accomplish more efficient energy transfer and storage. Among them, the supramolecular coordination complexes (SCCs) formed by non-covalent metal-ligand interaction, have shown the capacity to not only undergo single and multistep energy migration but also to utilize the harvested energy for a wide variety of applications such as photocatalysis, tunable emissive systems, encrypted anti-counterfeiting materials, white light emitters etc. This review sheds light on the light-harvesting behavior of both the 2D metallacycles and 3D metallacages where design ingenuity has been executed to afford energy harvesting by both donor ligands as well as metal acceptors.

Craig-Hückel Hybrid Aromatic Metalla-dehydro[11]annulenes Constructed by a Formal [10+1] Cycloaddition Reaction.

Aromatic metalla-annulenes are important aromatic compounds, research into which has been mainly concentrated on metal-benzenes and their lower homologues. Reports on their superior homologs are rare, and this has greatly limited the systematic study of their properties. In this work, a series of osma-dehydro[11]annulenes with good air and thermal stability were prepared in high yields through a simple [10+1] strategy, by incorporating a metal fragment into conjugated ten-carbon chains in a one-pot reaction. They are the first monometallic aromatic metalla-[n]annulenes with the ring size larger than 6, and their Craig-Hückel hybrid aromaticity is supported by various physical and computational parameters. Besides, these complexes show versatile reactivities, not only giving further evidence for their aromaticity, but also demonstrating their physical and chemical properties can easily be regulated. This work enriches the metalla-aromatic chemistry, and provides a new avenue for the synthesis of large metalla-annulenes with different ring sizes.

Progress of Azametallacyclopentadienes in the New Century.

As the key intermediates in metal-promoted/catalyzed C-C bond coupling reactions of nitriles and alkynes, azametallacyclopentadienes, M(N=CR1 -CR2 =CR3 ), are an important class of azametallacycles. Although the first authentic azametallacyclopentadienes were documented in 1986, their chemistry towards solid-state structures, intrinsic reactivity, and synthetic application was rarely investigated for a long time. At the beginning of this century, seminal works about the applications of azametallacyclopentadienes in the synthesis of heterocycles, including multi-substituted pyridines, isoquinolines, furans, and pyrroles were reported. Subsequently, a series of new complexes with this motif, namely the Group 4, aluminum, actinide, and rare-earth azametallacyclopentadienes were isolated and structurally characterized. Among them, the rare-earth azametallacyclopentadiene expresses high reactivity towards unsaturated molecules, such as nitriles, isocyanides, and Mo(CO)6 to provide novel fused metallacycles. In this Concept, we reviewed the advances in the preparation, reactivity, and synthetic application of azametallacyclopentadienes in the past twenty years.

Yttrium Complexes with Group 13 Heterobenzene-Type Ligands.

The yttrium gallabenzene complex [(1-Me-3,5-tBu2 -C5 H3 Ga)(µ-Me)Y(2,4-dtbp)] is accessible from Y(GaMe4 )3 and K(2,4-dtbp) via a tandem salt metathesis/methane elimination (2,4-dtbp=2,4-di-tert-butyl-pentadienyl). The pentadienyl ligand in [(1-Me-3,5-tBu2 -C5 H3 E)(µ-Me)Y(2,4-dtbp)] (E=Al, Ga) is easily displaced by salt metathesis with KC5 Me5 and KTpMe,Me (TpMe,Me =tris(pyrazolyl-Me2 -3,5)borato) affording [(1-Me-3,5-tBu2 -C5 H3 E)(µ-Me)Y(TpMe,Me )] and [(1-Me-3,5-tBu2 -C5 H3 E)(µ-Me)Y(C5 Me5 )]. The yttrium center in [(1-Me-3,5-tBu2 -C5 H3 E)(µ-Me)Y(2,4-dtbp)] readily forms adducts with neutral Lewis bases like 4-DMAP (4-dimethylaminopyridine), PMe3 , DMPE (1,2-bis(dimethylphosphino)ethane), and DME (1,2-dimethoxyethane). In stark contrast, addition of TMEDA (N,N,N',N'-tetramethylethylenediamine) results in methyl/pentadienyl exchange between aluminum and yttrium resulting in [(1-(2,4-dtbp)-1-Me-3,5-tBu2 -C5 H3 Al)Y(Me)(tmeda)]. The bonding features of the newly synthesized complexes are analyzed by single-crystal X-ray diffraction (SCXRD) and heteronuclear (89 Y, 31 P) NMR spectroscopy.

Coordination Behavior of the Tellurium Incorporated Mercuraazametallamacrocycle and Investigation of d10 ⋠⋠⋠d10 Interactions between Closed Shell (Ag+ Hg2+ ) Metal Ions.

Herein, a new tellurium and mercury containing mercuraazametallamacrocycle has been prepared via (2+2) condensation of bis(o-aminophenyl)telluride and bis(o-formylphenyl)mercury(II). The isolated bright yellow solid of mercuraazametallamacrocycle has adopted unsymmetrical figure-of-eight conformation in the crystal structure. To study the metallophilic interactions between closed shell metal ions, the macrocyclic ligand has been treated with two equiv. of AgOTf (OTf=trifluoromethansulfonate) and AgBF4 , which afforded greenish-yellow bimetallic silver complexes. The isolated silver complexes displayed intramolecular Hg⋅⋅⋅Ag, Te⋅⋅⋅Ag interactions as well as intermolecular Hg⋅⋅⋅Hg interactions and formed an extended 1D molecular chain by directing six atoms to interact as TeII ⋅⋅⋅AgI ⋅⋅⋅HgII ⋅⋅⋅HgII ⋅⋅⋅AgI ⋅⋅⋅TeII in a non linear fashion. The Hg⋅⋅⋅Ag, Te⋅⋅⋅Ag interactions have also been studied in solution by 199 Hg, 125 Te NMR spectroscopy, absorption, and emission spectroscopy. In DFT calculations, the Atom in Molecule (AIM) analysis, non-covalent interactions (NCI), natural bonding orbital (NBO) analysis strongly supported for experimental evidences and revealed that the intermolecular Hg⋅⋅⋅Hg interaction is stronger than the intramolecular Hg⋅⋅⋅Ag interactions.

NIR-II Emissive Ru(II) Metallacycle Assisting Fluorescence Imaging and Cancer Therapy.

Despite the success of emissive Ruthenium (Ru) agents in biomedicine, problems such as the visible-light excitation/emission and single chemo- or phototherapy modality still hamper their applications in deep-tissue imaging and efficient cancer therapy. Herein, an second nearinfrared window (NIR-II) emissive Ru(II) metallacycle (Ru1000, λem  = 1000 nm) via coordination-driven self-assembly is reported, which holds remarkable deep-tissue imaging capability (≈6 mm) and satisfactory chemo-phototherapeutic performance. In vitro results indicate Ru1000 displays promising cellular uptake, good cancer-cell selectivity, attractive anti-metastasis properties, and remarkable anticancer activity against various cancer cells, including cisplatin-resistant A549 cells (IC50  = 3.4 × 10-6  m vs 92.8 × 10-6  m for cisplatin). The antitumor mechanism could be attributed to Ru1000-induced lysosomal membrane damage and mitochondrial-mediated apoptotic cell death. Furthermore, Ru1000 also allows the high-performance in vivo NIR-II fluorescence imaging-guided chemo-phototherapy against A549 tumors. This work may provide a paradigm for the development of long-wavelength emissive metallacycle-based agents for future biomedicine.

One-Pot Synthesis of High-Strained Metal Vinylidene and Metal Carbyne.

A novel one-pot reaction producing a metal vinylidene structure in a five-membered ring by cyclization of a multiyne has been achieved. The ring strain and the high stability of the cyclic metal vinylidene complexes have been analyzed experimentally and computationally. The metal vinylidene unit in a fused-ring complex is unreactive to both nucleophiles and electrophiles. It reacts however at the nearby carbonyl group achieving the unprecedented conversion of metal tributing factors for the aromaticity-driven process has been studied by DFT calculations.

Two Rhodium(III) Ions Confined in a [18]Porphyrin Frame: 5,10,15,20-Tetraaryl-21,23-Dirhodaporphyrin.

Tetraaryl-21,23-dirhodaporphyrin and a series of related monorhodaporphyrins have been obtained by tellurium-to-rhodium exchange in a reaction of tetraaryl-21,23-ditelluraporphyrin with [RhCl(CO)2 ]2 . These organometallic metallaporphyrins contain rhodium(III) centers embedded in rhodacyclopentadiene rings, incorporated within the porphyrin frames. The skeletons of 21,23-dirhodaporphyrin and 21-rhoda-23-telluraporphyrin are strongly deformed in-plane from the rectangular shape typical for porphyrins, due to rhodium(III) coordination preferences, the large size of the two core atoms, and the porphyrin skeleton constrains. These two metallaporphyrins exhibit fluxional behavior, as studied by 1 H NMR and DFT, involving the in-plane motion and the switch of the rhodium center(s) between two nitrogen donors. A side product detected in the reaction mixture, 21-oxa-23-rhodaporphyrin, results from tellurium-to-oxygen exchange, occurring in parallel to the tellurium-to-rhodium exchange. The reaction paths and mechanisms have been analyzed. The title 21,23-dirhodaporphyrin contains a bridged bimetallic unit, Rh2 Cl2 , in the center of the macrocycle, with two rhodium(III) ions lying approximately in the plane of the porphyrinoid skeleton. The geometry of the implanted Rh2 Cl2 unit is affected by macrocyclic constrains.

Potassium Aluminyl Promoted Carbonylation of Ethene.

The potassium aluminyl [K{Al(NONDipp )}]2 ([NONDipp ]2- =[O{SiMe2 NDipp}2 ]2- , Dipp=2,6-iPr2 C6 H3 ) activates ethene towards carbonylation with CO under mild conditions. An isolated bis-aluminacyclopropane compound reacted with CO via carbonylation of an Al-C bond, followed by an intramolecular hydrogen shift to form K2 [Al(NONDipp )(µ-CH2 CH=CO-1κ2 C1,3 -2κO)Al(NONDipp )Et]. Restricting the chemistry to a mono-aluminium system allowed isolation of [Al(NONDipp )(CH2 CH2 CO-κ2 C1,3 )]- , which undergoes thermal isomerisation to form the [Al(NONDipp )(CH2 CH=CHO-κ2 C,O)]- anion. DFT calculations highlight the stabilising influence of incorporated benzene at multiple steps in the reaction pathways.

Isolation of an Elusive Phosphametallacyclobutadiene and Its Role in Reversible Carbon-Carbon Bond Cleavage.

The reactivity of phosphaalkynes, the isolobal and isoelectronic congeners to alkynes, with metal alkylidyne complexes is explored in this work. Treating the tungsten alkylidyne [t BuOCO]W≡Ct Bu(THF)2 (1) with phosphaalkyne (10) results in the formation of [O2 C(t BuC=)W{η2 -(P,C)-P≡C-Ad}(THF)] (13-t BuTHF ) and [O2 C(AdC=)W{η2 -(P,C)-P≡C-t Bu}(THF)] (13-AdTHF ); derived from the formal reductive migratory insertion of the alkylidyne moiety into a W-Carene bond. Analogous to alkyne metathesis, a stable phosphametallacyclobutadiene complex [t BuOCO]W[κ2 -C(t Bu)PC(Ad)] (14) forms upon loss of THF from the coordination sphere of either 13-t BuTHF or 13-AdTHF . Remarkably, the C-C bonds reversibly form/cleave with the addition or removal of THF from the coordination sphere of the formal tungsten(VI) metal center, permitting unprecedented control over the transformation of a tetraanionic pincer to a trianionic pincer and back. Computational analysis offers thermodynamic and electronic reasoning for the reversible equilibrium between 13-t Bu/AdTHF and 14.

A Silylene Stabilized by a σ-Donating Nickel(0) Fragment.

A donor-stabilized silylene 4 featuring a Ni0 -based donating ligand was synthesized. Complex 4 exhibits a pyramidalized and nucleophilic SiII center and shows a peculiar behavior due to the cooperative reactivity of Si and Ni centers. Calculations indicate that the orientation of Ni-ligands with respect to the silylene moiety is crucial in determining the role of the Ni-fragment (Lewis acid or Lewis base) towards silylene. Indeed, a simple 90° rotation of the Si-Ni bond, reverses the role of Ni, and transforms a classical silylene→Ni0 complex into an unprecedented Ni0 →silylene complex.

Synthesis, Characterization and Reactivity of a σ-Donating Ni0 -Stabilized Silyliumylidene Ion.

The synthesis of a silyliumylidene cation complex 2 stabilized by a Ni0 -based donating ligand is reported. Experimental and theoretical studies demonstrate that the highly electrophilic SiII center is stabilized by a dative Ni→Si σ-interaction and π-donations from the amino- and Ni-moieties. Due to the energetically close frontier orbitals localized on the Si and Ni atoms, complex 2 presents a competitive reactivity at Si and Ni sites.

Photoswitchable Fluorescent Self-Assembled Metallacycles with High Photostability.

In this study, photoswitchable fluorescent supramolecular metallacycles with high fatigue-resistance have been constructed by coordination-driven self-assembly by using bithienylethene with dipyridyl units (BTE) as a coordination donor and a fluorescent di-platinum(II) (Pt-F) as a coordination acceptor. The photo-triggered reversible transformation between the ring-open and ring-closed form of the metallacycles was confirmed by 1 H NMR, 31 P NMR, and UV/Vis spectroscopy. This unique property enabled a reversible noninvasive "off-on" switching of fluorescence through efficient Förster resonance energy transfer (FRET). Importantly, the metallacycles remained structurally intact after up to 10 photoswitching cycles. The photoresponsive property and exceptional photostability of the metallacycles posit their potential promising application in optical switching, image storage, and super-resolution microscopy.

Merging Pd0 /PdII Redox and PdII /PdII Non-redox Catalytic Cycles for the Allylarylation of Electron-Deficient Alkenes.

An allylarylation of electron-deficient alkenes with aryl boronates and allylic carbonates has been developed. This method allows access to a wide variety of carbon skeletons from readily available starting materials. Mechanistic studies indicate that this reaction is enabled by a cooperative catalysis based on merging Pd0 /PdII redox and PdII /PdII non-redox catalytic cycles.