Multidecker Sandwich Research Articles

Cycloheptatrienyl-Bridged Triple-Decker Complexes.

The first structurally characterized organometallic multidecker sandwich complexes featuring a cycloheptatrienyl ring (Cht, C7H73-) in the coordination sphere are presented. The synthesis of inverse sandwich complexes of the rare earth elements YIII and ErIII with a bridging cycloheptatrienyl ligand of the type [(thf)(BH4)2LnIII(μ-η7:η7-Cht)LnIII(BH4)(thf)2] is described first. The subsequent introduction of the CotTIPS ligand (CotTIPS = 1,4-(iPr3Si)2C8H62-) into the coordination sphere of the rare earth cations resulted in the isolation of unprecedented triple-decker compounds with the formula [(thf)3K{(η8-CotTIPS)LnIII}2(μ-η7:η7-Cht)], bearing a seven-membered aromatic carbon ring as a middle deck. These compounds are also the first examples of rare earth triple-decker complexes not bridged by a Cot derivative, based on purely carbon-based ligands. The magnetic properties of the respective ErIII congeners were investigated in detail, leading to the observation of antiferromagnetic coupling of the ErIII cations and a blocking temperature of 13.5 K. The conversion of the YIII compound [(thf)3K{(η8-CotTIPS)YIII}2(μ-η7:η7-Cht)] with [YIII(Cot)I(thf)2] resulted in ligand rearrangement and the selective formation of the first triple-decker complex ([(η8-CotTIPSYIII)2(μ-η8:η8-Cot)]) featuring two Cot ligands with different substituents in its coordination sphere.

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.

Fulleride-metal η5 sandwich and multi-decker sandwich complexes: A DFT prediction.

The (C60 CN)- formed by the reaction of CN- with fullerene shows high electron rich character, very similar to C60 ˙- , and it behaves as a large anion. Similar to Cp- , the bulky anion, (C60 CN)- , acts as a strong η5 ligand towards transition metal centers. Previous studies on η5 coordination of fullerene cage are reported for pseudo fullerenes whereas the present study deals with sandwich complexes of (C60 CN)- with Fe(II), Ru(II), Cr(II), Mo(II), and Ni(II) and multi-decker sandwich complexes of CN-fullerides with Fe(II). The structural parameters of these complexes and the corresponding Cp- complexes showed very close resemblance. Analysis of the metal-to-carbon bonding molecular orbitals showed that sandwich complex [Fe(η5 -(C60 CN)- )2 ] exhibit bonding features very similar to that of ferrocene. Also, a 6-fold decrease in the band gap energy is observed for [Fe(η5 -(C60 CN)- )2 ] compared to ferrocene. The energy of dissociation (ΔE) of the ligand (C60 CN)- from [Fe(η5 -(C60 CN)- )2 ] is slightly lower than the ΔE of a Cp* ligand from a ferrocene derivative wherein each cyclopentadienyl unit is substituted with four tertiary butyl groups. The (C60 CN)- ligand behaved as one of the bulkiest ligands in the chemistry of sandwich complexes. Further, the coordinating ability of the dianion, (C60 (CN)2 )2- is evaluated which showed strong coordination ability simultaneously with two metal centers leading to the formation of multi-decker sandwich and pearl-necklace type polymeric structures.

Formation of Metallic Polyferrocene Chains under Pressure.

The effect of pressure on the structural reorganization of ferrocene, Fc = (C5H5)2Fe, is studied using density functional theory (DFT) calculations assisted by evolutionary crystal structure prediction algorithms based on USPEX code. Pressure brings the individual molecules in close contact, and above 220 GPa, the 18-electron closed-shell molecular unit undergoes polymerization through the formation of quasi-one-dimensional (1D) chains, [(C5H5)2Fe]∞, termed as polyferrocene (p-Fc). Pressure induced polymerization (PIP) of Fc causes significant deviations from the 5-fold symmetry of the cyclopentadiene (Cp, C5H5 rings) and loss of planarity due to the onset of envelope-like distortions. This triggers distortions within the multidecker sandwich structures and σ(C-C) bond formation between the otherwise weak noncovalently interacting Cp rings in Fc crystals. Pressure gradually reduces the band gap of Fc, and for p-Fc, metallic states are found due to increased electronic coupling between the covalently linked Cp rings. Polyferrocene is significantly more rigid than ferrocene as evident from the 5-fold increase in its bulk modulus. Pressure dependent Raman spectra show a clear onset of polymerization in Fc at P = 220 GPa. Higher mechanical strength coupled with its metallicity makes p-Fc an interesting candidate for high pressure synthesis.

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 CeIII 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 [{(KL3 Ce)(μ-η6 :η6 -C7 H8 )}2 Ce] (3). The structure of 3 features a [Ce(C7 H8 )2 ] sandwich capped by [KL3 Ce] moieties with a linear arrangement of the Ce ions. Structural parameters, UV/Vis/NIR data, and DFT studies indicate the presence of CeII 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=O3 SCF3 ) yielded the CeIV complex, [CeL4 ] (2), showing that siloxide ligands can stabilize Ce in three oxidation states.

Cycloheptatrienyl dicarbollyl zirconium complexes: Building blocks for the preparation of multidecker sandwich complexes

The reaction of [(η7-C7H7)ZrCl(tmeda)] (1, tmeda = N,N,N',N'-tetramethylethylene-1,2-diamine) with the sodium dicarbollide Na2(C2B9H11) afforded the cycloheptatrienyl-dicarbollyl (Cht-Dcb) complex [Na(tmeda)][(η7-Cht)Zr(η5-Dcb)], [Na(tmeda)][2]. In the solid state, the sodium cations act as bridging units between the sandwich anions by η2-and η3-interaction with the Cht and Dcb ligands, respectively. The reaction of [Na(tmeda)][2] with BaI2 resulted in the formation of [Ba(thf)5(μ-η7:η7-C7H7)Zr(η5-C2B9H11)][(η7-C7H7)Zr(η5-C2B9H11)], [Ba(thf)5(2)][2], in which the cation exhibits an unusual bridging μ-η7:η7-C7H7 unit between the Dcb-Zr and the Ba(thf)5 fragments. An analogous reaction could not be effected by the salt metathesis reaction between [Na(tmeda)][2] and 0.25 equivalents of [(η5-C5Me5)Ru(μ3-Cl)]4, since the ion pair [(η5-C5Me5)Ru(tmeda)][2] containing the 16-electron half-sandwich tmeda complex was formed instead. The tetramethylammonium salt [NMe4][2] was prepared via an acid-base reaction between the amido complex [(η7-C7H7)Zr{N(SiMe3)2}(thf)] and [NMe4][C2B9H12]. [NMe4][2] is more stable than its sodium congener [Na(tmeda)][2] and might therefore act as a suitable transfer reagent for the construction of multidecker sandwich transition metal complexes in the future.

Multidecker Sandwich Cluster VnBenn+1 (n = 1, 2, 3, 4) as a Polarizable Bridge for Designing 1D Second-Order NLO Chromophore: Metal−π Sandwich Multilayer Structure as a Particular Charge-Transfer Axis for Constructing Multidimensional NLO Molecules

Multidecker sandwich π-coordination complexes with multiple metal atoms and cyclic π-ligands can be considered as building blocks for designing NLO molecules. In a previous work, we found that multidecker sandwich complexes of the form VnBzn+1 (n = 1, 2, 3) can be considered as stronger electron donors than metallocene Fe(η5-C5H5)2 for the design of second-order NLO molecules, and a layer-number dependence of the first hyperpolarizability of VnBzn+1–(C2H2)3–NO2 (n = 1, 2, 3) and the two-dimensional NLO character was observed from our calculation results. In the present article, by analogy with traditional donor–acceptor substituted conjugated polyenes, we took the multidecker sandwich complexes VnBzn+1 (n = 1, 2, 3, 4) as the polarizable bridge instead of the traditional π-electron conjugated bridge and designed several novel push–pull systems of the form donor–(VnBzn+1)–acceptor (n = 1, 2, 3, 4) by introducing electron-donating and electron-withdrawing groups at the terminal benzene ring of VnBzn+1. The ...

Diradical character and second hyperpolarizability of multidecker inverse sandwich complexes of Mg and Ca

The ground state structures of first- and second-order inverse sandwich (MIS-I and MIS-II) complexes (M-(C4H4-M)n, n=1 and 2) of Mg and Ca have been found to be singlet diradical while the third-order complexes MIS-III (n=3) are pure triplet. The second hyperpolarizability of the open-shell singlet complexes MIS-I and MIS-II are predicted larger than the analogs triplets. The magnitude of second hyperpolarizability enhances on replacing Mg with Ca metal atom. The larger second hyperpolarizability of MIS-I compared to that of MIS-II of the same metal may be attributed to the intermediate singlet diradical character of the former.

Multidecker Sandwich Complexes VnBenn+1 (n = 1, 2, 3) as Stronger Electron Donor Relative to Ferrocene for Designing High-Performance Organometallic Second-Order NLO Chromophores: Evident Layer Effect on the First Hyperpolarizability and Two-Dimensional NLO Character

During the past 3 decades, significant progress has been made in multidecker sandwich complexes. Replacing metallocene in the metallocene-based second-order nonlinear optical chromophores with multidecker sandwich complexes will create a variety of new second-order NLO organometallic complexes. To explore the possibility of improving NLO properties of organometallic complexes by using multidecker sandwich complexes, we theoretically design five second-order NLO moleculars VnBzn+1-(C2H2)3-NO2 (n = 1, 2, 3) with widely investigated VnBzn+1 (n = 1, 2, 3) by changing the layer number n or position of conjugation π–A system in the D−π–A structure. The NLO properties of these five complexes are investigated in detail. By comparison with the β0 of ferrocenyl derivative Fe(η5-C5H5)2-(C2H2)3-NO2, the larger NLO response (β0, 10215–36917 au) is found in VnBzn+1-(C2H2)3-NO2 (n = 1, 2, 3). The dependence of the first hyperpolarizabilities β0 on the layer number n and the position of conjugation π–A system is discussed in the article. The evident layer effect on the first hyperpolarizabilities β0 is shown. And, the enlargement of the first hyperpolarizability β0 by the layer number n can be rationalized by considering the enhancement of parallel-x-MLCT (from more than one metal center to acceptor, −NO2), perpendicular-x-MLCT (from multiple metal centers to multiple C6H6 ligands) transition and two-dimensional NLO character with the two charge transfer axes. By comparison with ferrocene or V1Bz2, the multidecker sandwich complexes VnBzn+1 (n = 2, 3) can be considered as stronger donor group for designing NLO moleculars. We hope, this groundbreaking work may evoke one’s attention to design new, highly efficient second-order NLO molecular materials with excellent building blocks: multidecker sandwich complexes.

Binuclear sandwich and multi-decker sandwich compounds of alkali and alkaline-earth metals: a quantum chemical study

The structures, stabilities, and energy characteristics of simple and multi-decker binuclear sandwich compounds of alkali and alkaline-earth metals with cyclobutadiene, cyclopentadiene, and benzene rings were studied within the framework of the density functional theory by the B3LYP/6-311+G** method. Stable electronic structures of the multi-decker sandwich complexes studied can be formed if the sum of the number of valence electrons of metal atoms and the number of basal-ring π-electrons is 8n – 2, where n is the number of basal rings. Factors responsible for steric and electronic stabilization of such systems as well as co-operative effects associated with changes in the electronic and structural properties of the complexes upon chain extension are analyzed.

Liquid-Phase Synthesis of Multidecker Organoeuropium Sandwich Complexes and Their Physical Properties

Multidecker organoeuropium sandwich complexes were synthesized with the organic ligands 1,3,5,7-cyclooctatetraene (COT) and 1,4- and 1,6-bis(trimethylsilyl)cyclooctatetraenes (COT″) in a liquid ammonia solution of europium (Eu) metal. Infrared (IR) absorption and Raman spectroscopies and magnetic measurements were used to identify local structures and charge distributions. It was found that the organoeuropium complexes have Eu2+–COT2– (Eu2+–COT″2–) charge distributions, including a local structure of ligand–Eu–ligand sandwiches. Together with elemental analysis by inductively coupled plasma atomic emission spectroscopy, it is shown that the organoeuropium complexes form a multidecker sandwich structure that is insouble in many solvents. In addition to their paramagnetic behaviors at 150–300 K, the organoeuropium complexes of Eu–COT and Eu–COT″ showed red and orange–red emissions, respectively, under UV irradiation. The introdution of bulky trimethylsilyl groups onto COT prominently enhanced the emission i...

Beryllium-Cyclobutadiene Multidecker Inverse Sandwiches: Electronic Structure and Second-Hyperpolarizability

Beryllium forms stable sandwich and inverse sandwich complexes with the cyclobutadiene molecule. Two types of multidecker complexes are designed. Multidecker inverse sandwiches are found to be thermally more stable than the corresponding sandwich complexes. The average distance between two consecutive metals and the two consecutive cyclobutadiene rings increase gradually on increasing size of the chosen inverse sandwich complexes. The density functional theory functionals B3LYP, BHHLYP, BLYP, M06, CAM-B3LYP, and B2PLYP in conjunction with the 6-311++G (d, p) basis set have been employed for calculating the third-order electric response properties of the chosen beryllium-cyclobutadiene complexes and the results obtained for each functional are found to have a consistent trend. Compared to the normal sandwich compounds the second-hyperpolarizability of inverse sandwiches is predicated to be larger, which fairly correlates with the extent of ground-state polarization. The significant enhancement of cubic polarizability of higher-order multidecker inverse sandwiches arises from the strong coupling between the ground and the low lying charge transfer excited states. The rather strong enhancement of second-hyperpolarizability on increasing size of the beryllium-multidecker inverse sandwiches may provide a new route to design efficient nonlinear optical materials.

Binuclear sandwich and multidecker sandwich systems: A quantum-chemical study

Binuclear sandwich and multidecker sandwich systems: A quantum-chemical study

The spin-filter capability and spin-reversal effect of multidecker iron-borazine sandwich cluster

Using density functional theory and non-equilibrium Green's function technique, we performed theoretical investigations on the magnetic and transport properties of Fen(B3N3H6)n+1 (n = 1-4) sandwich clusters. The calculated results show that the clusters possess high magnetic moments owing to the unpaired dδ electrons. Furthermore, high spin-filter capability, negative differential resistance, and spin-reversal effect were found in the systems, and the possible physical origination of the phenomena was suggested. Our findings also indicate that the magnetic or electrical properties of the system could be controlled by cluster size, and these sandwich clusters are promising materials for application in molecular electronics or spintronics.

Multi-decker tricarbonyl-bridged sandwich complexes of transition metals: structure, stability and electron-counting rules

Structures and stabilities of a new family of multi-decker tricarbonyl-bridged sandwich complexes of transition metals (Cr, Mn, Fe) have been studied using DFT B3LYP/6-311+G(df,p) calculations. Stable structures satisfy the (12n + 6) electron-counting rule, where n is the number of metal atoms. Lengthening of the molecular chains is accompanied by growth of aromaticity of the inner basal cycles, lowering of the aromaticity of the terminal basal cycles and decrease in the energy gap between the frontier orbitals.

ChemInform Abstract: Multiple‐Decker Sandwich Complexes of f‐Elements

AbstractReview: 86 refs.

Designing novel Sn-Bi, Si-C and Ge-C nanostructures, using simple theoretical chemical similarities.

A framework of simple, transparent and powerful concepts is presented which is based on isoelectronic (or isovalent) principles, analogies, regularities and similarities. These analogies could be considered as conceptual extensions of the periodical table of the elements, assuming that two atoms or molecules having the same number of valence electrons would be expected to have similar or homologous properties. In addition, such similar moieties should be able, in principle, to replace each other in more complex structures and nanocomposites. This is only partly true and only occurs under certain conditions which are investigated and reviewed here. When successful, these concepts are very powerful and transparent, leading to a large variety of nanomaterials based on Si and other group 14 elements, similar to well known and well studied analogous materials based on boron and carbon. Such nanomaterias designed in silico include, among many others, Si-C, Sn-Bi, Si-C and Ge-C clusters, rings, nanowheels, nanorodes, nanocages and multidecker sandwiches, as well as silicon planar rings and fullerenes similar to the analogous sp2 bonding carbon structures. It is shown that this pedagogically simple and transparent framework can lead to an endless variety of novel and functional nanomaterials with important potential applications in nanotechnology, nanomedicine and nanobiology. Some of the so called predicted structures have been already synthesized, not necessarily with the same rational and motivation. Finally, it is anticipated that such powerful and transparent rules and analogies, in addition to their predictive power, could also lead to far-reaching interpretations and a deeper understanding of already known results and information.

Multiple-decker sandwich complexes of f-elements

Extended sandwich structures (triple-deckers, tetra-deckers etc.) containing d-transition metals are well established for more than 30 years. In contrast, similar complexes containing lanthanide or actinide elements are still rare. In recent years, however, significant progress has been made in this area. This has mainly been achieved with the use of sterically demanding π-ligands such as pentamethylcyclopentadienyl (=Cp*) or silyl-substituted cyclooctatetraenyl dianions like [C8H5(SiMe3)3-1,3,6]2− (=COT‴). Suitable combinations of such ligands in the coordination sphere of f-elements allowed the synthesis and structural characterization of bent and linear triple-decker sandwich complexes as well as the first tetra-deckers and novel cluster-centered multidecker sandwich complexes. This NJC Perspective article gives an overview on recent achievements in this field and looks forward to future developments.

Multidecker Sandwiches of Silicon−Carbon Clusters

It is shown by ab initio density functional theory calculations that hydrogenated silicon−carbon clusters, in particular of the form Si3C2H2, can generate stable organometallic multidecker sandwiches in which transition metal centers, such as Co and Fe, bridge Si3C2H2 rings. These multidecker sandwiches are fully homologous and isolobal to the well-known similar metalocarborane sandwiches. This is in full agreement with the isolobal analogy between Sin−2C2H2 clusters and the corresponding isovalent carboranes C2Bn−2Hn, n ≥ 3, known as the “boron connection”. The interplanar binding energy of these sandwiches between successive decks increases as the number of decks increases, suggesting that even larger multideckers will also be very stable. Such organometallic species could be proven very important for chemical and technological applications, which could be integrated with current semiconductor technologies. With this perspective it is furthermore illustrated that by modifying the bridging transition met...

Ab Initio Study of Structural and Magnetic Properties of TMn(ferrocene)n+1 (TM = Sc, Ti, V, Mn) Sandwich Clusters and Nanowires (n = ∞)

Structural and magnetic properties of multidecker sandwich clusters TM(n)(ferrocene)(n+1) [TM = V, Ti, Sc, Mn, ferrocene=FeCp(2), n = 1-3] and corresponding one-dimensional sandwich nanowires (n = infinity) are studied by means of gradient-corrected density functional theory. The TM(n)(FeCp(2))(n+1) clusters are highly stable polyferrocene-like sandwich structures due to strong Fe-Cp interaction. The total magnetic moment of TM(n)(FeCp(2))(n+1) (TM = V, Ti, Mn) increases linearly with the size n. More strikingly, Ti(n)(FeCp(2))(n+1) and V(n)(FeCp(2))(n+1) (n = 1-3) exhibit high magnetic moments 4, 8, 12 mu(B) and 1, 6, 11 mu(B), respectively. In contrast, Sc(n)(FeCp(2))(n+1) clusters are paramagnetic. The [TM(FeCp(2))](infinity) sandwich nanowires are ferromagnetic semiconductors whose band gap is 0.361, 0.506, 0.51, and 1.310 eV, respectively, for TM = Ti, Sc, V, and Mn. Among the four sandwich nanowires, [V(FeCp(2))](infinity) nanowire possesses the highest magnetic moment (5 mu(B)) per unit cell.