Synthesis and properties of cyclic sandwich compounds.

Cyclic nanometre-scale sandwich complexes assembled from individual building blocks were synthesized. Sandwich complexes, in which a metal ion is π-coordinated by two planar aromatic organic rings belong to the foundations of organometallic chemistry. They have been successfully used in a wide variety of applications ranging from catalysis, synthesis and electrochemistry to nanotechnology, materials science and medicine1,2. Extending the sandwich structural motif leads to linear multidecker compounds, in which aromatic organic rings and metal atoms are arranged in an alternating fashion. However, the extension to a cyclic multidecker scaffold is unprecedented. Here we show the design, synthesis and characterization of an isomorphous series of circular sandwich compounds, for which the term 'cyclocenes' is suggested. These cyclocenes consist of 18 repeating units, forming almost ideally circular, closed rings in the solid state, that can be described by the general formula [cyclo-MII(µ-η8:η8-CotTIPS)]18 (M = Sr, Sm, Eu; CotTIPS = 1,4-(iPr3Si)2C8H62-). Quantum chemical calculations lead to the conclusion that a unique interplay between the ionic metal-to-ligand bonds, the bulkiness of the ligand system and the energy gain on ring closure, which is crucially influenced by dispersion interactions, facilitate the formation of these cyclic systems. Up to now, only linear one-dimensional multidecker sandwich compounds have been investigated for possible applications such as nanowires3-10. This textbook example of cyclic sandwich compounds is expected to open the door for further innovations towards new functional organometallic materials.

Alkali Metal Complexes of a Bis(diphenylphosphino)methane Functionalized Amidinate Ligand: Synthesis and Luminescence.

A novel bis(diphenylphosphino)methane (DPPM) functionalized amidine ligand (DPPM-C(N-Dipp)2 H) (Dipp=2,6-diisopropylphenyl) was synthesized. Subsequent deprotonation with suitable alkali metal bases resulted in the corresponding complexes [M{DPPM-C(N-Dipp)2 }(Ln )] (M=Li, Na, K, Rb, Cs; L=thf, Et2 O). The alkali metal complexes form monomeric species in the solid state, exhibiting intramolecular metal-π-interactions. In addition, a caesium derivative [Cs{PPh2 CH2 -C(N-Dipp)2 }]6 was obtained by cleavage of a diphenylphosphino moiety, forming an unusual six-membered ring structure in the solid state. All complexes were fully characterized by single crystal X-ray diffraction, NMR spectroscopy, IR spectroscopy as well as elemental analysis. Furthermore, the photoluminescent properties of the complexes were thoroughly investigated, revealing differences in emission with regards to the respective alkali metal. Interestingly, the hexanuclear [Cs{PPh2 CH2 -C(N-Dipp)2 }]6 metallocycle exhibits a blue emission in the solid state, which is significantly red-shifted at low temperatures. The bifunctional design of the ligand, featuring orthogonal donor atoms (N vs. P) and a high steric demand, is highly promising for the construction of advanced metal and main group complexes.

Synthesis, Spectroscopy, and Redox Studies of Ferrocene-Functionalized Coinage Metal Alkyne Complexes.

Ethynylferrocene (FcC≡CH) was utilized as a redox-active metalloligand for the synthesis of polynuclear coinage metal complexes. The reaction of [FcC≡CLi] with tri- tert-butylphosphine metal chlorides [tBu3P-MCl] (M = Au, Ag, Cu) yielded different heteronuclear ferrocene-funtionalized alkyne complexes featuring metallophilic interactions. Furthermore, the redox properties of the ferrocenyl-functionalized tetracopper complex were investigated by cyclic voltammetry and UV-vis-near-IR spectroelectrochemistry. They indicate the compounds' redox-rich nature and a weak electronic coupling between the redox-active ferrocenyl units over a large distance.

Single-Chain Nanoparticles as Catalytic Nanoreactors.

The need for efficient, tailor-made catalysts has inspired chemists to fuse the design principles of natural enzymes with synthetic macromolecular architectures. A highly interesting pathway mimics a metallo-enzyme's tertiary structure via the target placement of metal-ions in a tailor-made polymeric framework, resulting in catalytically active single-chain nanoparticles. Initial studies reveal unusual and promising effects, regarding both new catalyst characteristics and a high impact on product formation. These multifunctional nanoreactors, constructed from simple folded polymer chains, will lead to advanced bioinspired catalytic systems. As found in enzymes, their impact lies specifically within the defined construction of a polymeric pocket around the catalytic active cores for substrate recognition.

Phenanthroline-A Versatile Ligand for Advanced Functional Polymeric Materials.

The controlled incorporation of phenanthroline moieties into polymers is introduced, demonstrating their application as metal-ion complexing ligands for the construction of advanced macromolecular structures. Specifically, two phenanthroline-containing monomers based on acrylate and styrene functionalities, were synthesized. Each monomer was readily copolymerized with either N,N-dimethylacrylamide or styrene via nitroxide-mediated polymerization, resulting in narrowly distributed polar or non-polar copolymers. To demonstrate the versatility of the established polymer systems, the polar polymer was employed for transition metal induced single-chain nanoparticle formation, verified by diffusion-ordered NMR and UV/Vis spectroscopy. Furthermore, the non-polar polymer allows facile incorporation of lanthanide ions, creating luminescent metallo-polymers, in-depth characterized by advanced photophysical experiments and 2D NMR measurements.

Dimolybdenum Paddlewheel as Scaffold for Heteromultimetallic Complexes: Synthesis and Photophysical Properties.

A diphenylphosphine functionalized benzoic acid was applied for the synthesis of a homoleptic dimolybdenum-based metalloligand, exhibiting four symmetrically placed phosphine donor sites. This allowed subsequent treatment with gold(I), rhodium(I), and iridium(I) precursors to obtain early-late heterometallic complexes as well as Lewis acid-base adducts with BH3. The compounds were in-depth investigated by spectroscopic techniques, single-crystal X-ray diffraction, and femtosecond laser spectroscopy. The coordination of different metal fragments to the dimolybdenum metalloligand leads to a fine-tuning of the system's optical properties, which correlates well with fluorescence quantum yield measurements. Nevertheless, triplet dynamics still remain the dominating channel in these systems with an intersystem crossing time constant below 1 ps.

Platinum(II)-Crosslinked Single-Chain Nanoparticles: An Approach towards Recyclable Homogeneous Catalysts.

We introduce the synthesis and in-depth characterization of platinum(II)-crosslinked single-chain nanoparticles (PtII -SCNPs) to demonstrate their application as a recyclable homogeneous catalyst. Specifically, a linear precursor copolymer of styrene and 4-(diphenylphosphino)styrene was synthesized via nitroxide-mediated polymerization. The triarylphosphine ligand moieties along the backbone allowed for the intramolecular crosslinking of single chains via the addition of [Pt(1,5-cyclooctadiene)Cl2 ] in dilute solution. The successful formation of well-defined PtII -SCNPs was evidenced by size exclusion chromatography, dynamic light scattering, nuclear magnetic resonance (1 H, 31 P{1 H}, 195 Pt), and diffusion-ordered spectroscopy. Finally, the activity of the PtII -SCNPs as homogeneous, yet recyclable catalyst was successfully demonstrated using the example of the amination of allyl alcohol.

A dimolybdenum paddlewheel as a building block for heteromultimetallic structures.

Diphenylphosphine functionalized propionic acid was applied for the synthesis of heteromultimetallic dimolybdenum(ii) complexes. The ligand features both carboxylic acid and phosphine functionalities, allowing the selective synthesis of a tetracarboxylate bridged Mo2(ii)-paddlewheel structure in a first step. Due to the symmetrically arranged phosphine functionalities, the dimolybdenum(ii) complex was utilized as a metalloligand. Subsequent coordination of late transition metal ions, such as gold(i), rhodium(i), iridium(i) or ruthenium(ii) to the phosphine moieties allowed the formation of heteromultimetallic structures. The flexibility of the diphenylphosphino propionate ligand system enabled intermolecular aurophilic interactions in the Au(i) functionalized dimolybdenum(ii) complexes. Depending on the Au(i) species applied, either a dimeric structure or a 1D coordination polymer was formed in the solid state. These structures represent the first examples of heterometallic dimolybdenum(ii) complexes, forming supramolecular structures via aurophilic interactions.

Molecular gold strings: aurophilicity, luminescence and structure-property correlations.

This review covers the compound class of one-dimensional gold strings. These compounds feature a formally infinite repetition of gold complexes as monomers/repeating units that are held together by aurophilic interactions, i.e. direct gold-gold contacts. Their molecular structures are primarily determined in the solid state using single crystal X-ray diffraction. The chemical composition of the employed gold complexes is diverse and furthermore plays a key role in terms of structure characteristics and the resulting properties. One of the most common features of gold strings is their photoluminescence upon UV excitation. The emission energy is often dependent on the distance of adjacent gold ions and the electronic structure of the whole string. In terms of gold strings, these parameters can be fine-tuned by external stimuli such as solvent, pH value, pressure or mechanical stress. This leads to direct structure-property correlations, not only with regard to the photophysical properties, but also electric conductivity for potential application in nanoelectronics. Concerning these correlations, gold strings, consisting of self-assembled individual complexes as building blocks, are the ideal compound class to look at, as perturbations by an inhomogeneity in the ligand sphere (such as the end of a molecule) can be neglected. Therefore, the aim of this review is to shed light on the past achievements and current developments in this area.

Heterobimetallic Eu(iii)/Pt(ii) single-chain nanoparticles: a path to enlighten catalytic reactions.

We introduce the formation and characterization of heterometallic single-chain nanoparticles entailing both catalytic and luminescent properties. A terpolymer containing two divergent ligand moieties, phosphines and phosphine oxides, is synthesized and intramolecularly folded into nanoparticles via a selective metal complexation of Pt(ii) and Eu(iii). The formation of heterometallic Eu(iii)/Pt(ii) nanoparticles is evidenced by size exclusion chromatography, multinuclear NMR (1H, 31P{1H}, 19F, 195Pt) as well as diffusion-ordered NMR and IR spectroscopy. Critically, we demonstrate the activity of the SCNPs as a homogeneous and luminescent catalytic system in the amination reaction of allyl alcohol.