Multiple-decker Sandwich Research Articles

Multiple-decker and ring sandwich formation of manganese-benzene organometallic cluster anions: MnnBzn- (n = 1-5 and 18).

Organometallic multiple-decker sandwich clusters are topics of great interest due to their unique electronic and magnetic properties originating from anisotropic structures. We report a joint anion photoelectron spectroscopic and computational study on a new family of manganese (Mn)-benzene (Bz) anionic clusters MnnBzn-. In stark contrast to the most widely studied vanadium-Bz sandwich clusters, it is found that MnnBzn- (n = 1-5) clusters exhibit unprecedented multiple-decker structures with a tilted Mn-Bz stacking and a monotonically increasing behavior of their high spin multiplicities. Furthermore, a couple of closed ring forms of Mn18Bz18- and its neutral state are computationally anticipated as an intriguing "cluster of Mn1Bz1 clusters" in which the neutral Mn18Bz18 has extremely high C18h symmetry with an uncommon spin state of 2S + 1 = 55. The extensively delocalized electron environment of Mn18Bz18 allows the simple Hückel model to reveal the strong intra-atomic exchange interactions within the Mn 3d electrons.

Probing the stability of the M2(Benzene)3 M = Fe, Co, and Ni structures upon electron attachment (deletion) and solvated iron clusters by benzene molecules: Fe2(Benzene)4

Rice-ball (RB) and multiple decker sandwich (MDS) structures of clusters containing transition metal atoms and benzene (Bz) molecules, [M2Bz3]±1 M = Fe, Co, and Ni were studied by means of density functional theory all-electron calculations including dispersion correction as in the BPW91-D2 method. A RB geometry was identified for the ground state (GS) of neutral Fe2Bz3. However, consistent with reported experimental results, RB and MDS structures may occur for the Fe2Bz3− ion. The RB and MDS isomers of Co2Bz3 are degenerate; they have comparable ionization energies; this finding is in agreement with the experimental results, where two isomers were identified also. Experiment and theory suggest that the Co2Bz3− ion has similar geometry, MDS, as the neutral parent. RB and MDS motifs are degenerate for both Ni2Bz3 and Ni2Bz3+. A RB form is predicted for Ni2Bz3−. In the GS of Fe2Bz4 one benzene molecule was found in the outer region of the RB Fe2Bz3 subcluster; it presents a binding energy (D0) of 4.6 kcal/mol, being originated from weak van der Waals forces. Thus, bridging the internal ligands, the fourth molecule has solvent behavior in the singlet Fe2Bz4 GS. Likewise, 3 + 1 MDS isomers of Fe2Bz4 were found at higher energies, ≈ 13.1 kcal/mol, from the GS. In Fe2Bz4−, the RB motif yields the GS with a D0 of 6.7 kcal/mol for the solvent unit. Having a D0 of 9.0 kcal/mol for such moiety the MDS Fe2Bz4− ion is near in energy (3.6 kcal/mol) to the Fe2Bz4− GS. The GS has an electron affinity (EA) of 0.40 eV. Notably, the MDS isomer has a larger EA (0.83 eV). The outer molecule in the 3 + 1 RB GS is stabilized by a network of dipole Cδ−–Hδ+–Cδ− interactions, formed between the external (internal) hydrogen atoms and the π-electrons of the internal (external) benzene rings. Dipole C–Hintδ+–Cextδ− interactions predominate in the 3 + 1 MDS isomers.

Experimental and theoretical studies of the structural and electronic properties of vanadium-benzene sandwich clusters and their anions: V(n)Bz(n)(0/-) (n = 1-5) and V(n)Bz(n-1)(0/-) (n = 2-5).

One end open V(n)Bz(n)(-) (n = 1-5; Bz = benzene) and both ends open V(n)Bz(n-1)(-) (n = 2-5) vanadium-benzene cluster anions were studied using anion photoelectron spectroscopy and density functional calculations. The smaller (n ≤ 3) V(n)Bz(n) and V(n)Bz(n-1) clusters and corresponding anions were found to have structural isomers, whereas full-sandwiched V(n)Bz(n+1) clusters preferred to form multiple-decker sandwich structures. Several isomeric V2Bz2 structures were identified theoretically and the anion photoelectron spectra of V2Bz2(0/-) were explained well by the coexistence of two isomeric structures: (1) a V2-core structure sandwiched between benzene molecules and (2) an alternating sandwich structure with the spin state strongly dependent on the structure. The adiabatic electron affinity of both V(n)Bz(n) and V(n)Bz(n-1) was found to increase with the cluster size at larger sizes (n = 4 or 5) and approaches to that of V(n)Bz(n+1). The evolution of the structural and electronic properties of V(n)Bz(m) and V(n)Bz(m)(-) (m = n and n - 1) with size is discussed in comparison with V(n)Bz(n+1) and V(n)Bz(n+1)(-).

Theoretical study of neutral and charged Sc n≤2–(benzene) m≤3 clusters

Interactions of benzene molecules with scandium atoms, Scn≤2–(C6H6)m≤3, in the gas phase were studied by means of density functional theory. All-electron calculations were performed using the B3LYP hybrid functional in concert with 6-311+G(d,p) orbital basis sets for the Sc, C, and H atoms. Multiple-decker sandwich (MDS) structures are identified as the ground states for Scn≤2–(C6H6)m≤3, where the ligands are attached to the metal through Sc–C bonding, formed between the 3d electrons and the π-clouds of the benzene rings. Significant distortion is produced on the absorbed benzene molecules by the metal–ligand bonding. Rice ball structures also appeared, but they were found at higher energies, in such a way that essentially MDS isomers may emerge in the molecular beams. Even the low number of valence electrons (3d24s1) of the Sc atom; sextuple coordinations are formed, but they show different Sc–C bond lengths, diminishing the symmetry of neutral and charged clusters. The estimated ionization energies, in near agreement with experimental data, and electron affinities, suggest delocalization of the valence electrons through the network of 3d–π bonds of Sc1,2–(C6H6)m≤3. The binding energies decrease with the absorption of more benzene molecules, and in some cases increase as more metal atoms are added to the cluster.

Experimental and theoretical studies on the electronic properties of vanadium-benzene sandwich cluster anions, VnBzn+1− (n = 1-5)

Vanadium-benzene cluster anions, V(n)Bz(n+1)(-) (Bz = C(6)H(6)) were generated by laser ablation and supersonic jet methods, and studied using photoelectron spectroscopy. The density functional theory was employed to compute their geometric and electronic structures. It is concluded that the V(n)Bz(n+1)(-) anions exhibit multiple-decker sandwich structures similar to their corresponding neutrals, and the adiabatic electron affinity increases with the cluster size. Our computation shows that the excess electron of the anion occupies the d orbitals of the vanadium atoms and that it is delocalized one-dimensionally. Furthermore, a very large HOMO-LUMO gap difference between majority and minority spin orbitals is observed for both the neutrals and the anions, and the V(n)Bz(n+1)(0∕-) clusters are found to be completely spin-polarized. These facts confirm the possibility of using V(n)Bz(n+1) clusters as spin filters.

Electronic structure and magnetic properties of metallocene multiple-decker sandwich nanowires

We present a study of the electronic and magnetic properties of the multiple-decker sandwich nanowires ($CP-M$) composed of cyclopentadienyl (CP) rings and 3d transition metal atoms (M=Ti to Ni) using first-principles techniques. We demonstrate using Density Functional Theory that structural relaxation play an important role in determining the magnetic ground-state of the system. Notably, the computed magnetic moment is zero in $CP-Mn$, while in $CP-V$ a significant turn-up in magnetic moment is evidenced. Two compounds show a half-metallic ferromagnetic ground state $CP-Fe/Cr$ with a gap within minority/majority spin channel. In order to study the effect of electronic correlations upon the half-metallic ground states in $CP-Cr$, we introduce a simplified three-bands Hubbard model which is solved within the Variational Cluster Approach. We discuss the results as a function of size of the reference cluster and the strength of average Coulomb $U$ and exchange $J$ parameters. Our results demonstrate that for the range of studied parameters $U=2-4eV$ and $J=0.6-1.2eV$ the half-metallic character is not maintained in the presence of local Coulomb interactions.

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.

Theoretical Study of the Low-Lying States of Fe2-(C6H6)3

Using the gradient-corrected BPW91 method and 6311++G(2d,2p) basis sets, it was found that adsorption of benzene by iron atoms forms a multiple-decker sandwich (MDS) geometry for the ground state (GS) of Fe(2)-(C(6)H(6))(3), as Fe(2) is broken. Though decoordination occurs, the ligands are bonded symmetrically to the Fe sites by η(6) (for the two external rings) and two η(3) (for the central ring) Fe-C coordinations; this big amount of Fe-C bonds enhances the stability of the MDS GS. This is unexpected, as the experiment suggests MDS for TM(n)-(C(6)H(6))(m) species of earlier transition metals (TMs) and clusters covered with benzene, i.e., rice-ball (RB) structures, for late 3d atoms. However, preserving Fe(2), an RB state was found, quasi-degenerate with the GS, with a smaller amount of Fe-C contacts and a stronger Fe(2) bond. The MDS shows higher stability for electron attachment and deletion events, but its adiabatic electron affinity, 1.11 eV, differs more from the experiment (0.80 ± 0.1 eV) than the one (0.97 eV) for RB. Thus, MDS and RB states can appear in a sample of Fe(2)-(C(6)H(6))(3). Like the electron affinity, the ionization energy of the complex is also smaller than those of the metal and benzene moieties, but closer to the former, signifying that the electron, delocalized through the 3d-π bonds, is mainly deleted from the metallic units.

Theoretical study of the structural and electronic properties of the Fen(C6H6)m, n≤ 2; m≤ 2 complexes

The ground state, GS, geometries for Fe(1,2)(benzene)(1,2) clusters were determined by means of all-electron calculations done with the density functional BPW91/6311++G(2d,2p) method. The stability of Fe(C(6)H(6))(1,2) is accomplished by the formation of Fe-C eta(6) coordinations in the half-sandwich and sandwich GS structures, which are of lower spin, 2S = 2 (S is the total spin) than the Fe atom, 2S = 4. Departures from eta(6) bonding occur on [Fe(C(6)H(6))(2)](-), since the GS of this anion, of less symmetric sandwich geometry, presents eta(6) and eta(2) coordination, which is mainly due to the enhanced repulsion of the adsorbed benzene units. On Fe(2)(C(6)H(6))(1,2) the stronger Fe(2) bond, compared to the Fe-C ones, produce rice-ball geometries, where the Fe(2) molecule, although with a longer bond length, is preserved. For example, in Fe(2)(C(6)H(6)), Fe(2) lies perpendicular or parallel to the benzene ring depending on the charge of the complex, and in [Fe(2)(C(6)H(6))(2)](+/-0, +/-1) the benzene ligands are placed above and beneath the molecular axis of Fe(2), producing highly compact structures. Multiple decker sandwich states, where Fe(2) is not retained, are located more than 20 kcal mol(-1) above the GS levels. Electron affinities, agreeing well with experimental results, ionization and binding energies, and vibrational frequencies were also determined, providing insight on the complexes.

An atomic force microscope study of vanadium-benzene sandwich clusters soft-landed on self-assembled monolayers

Multiple-decker vanadium-benzene sandwich clusters Vn(benzene)n+1 produced by a laser-vaporization synthesis method were soft-landed onto self-assembled monolayers of alkanethiol (C18H-SAM) and fluorinated alkanethiol (C10F-SAM) at 200 K. Noncontact atomic force microscopy has been used to examine the resulting adsorption states of the clusters landed on the SAMs at room temperature. For each SAM substrate, the aggregates of the deposited clusters were observed at the vacancy islands and near the steps of the SAM surface. The result indicates that, at room temperature, the clusters landed on the SAM substrate thermally diffuse on the surface to form columnar-shape islands around the defect sites of the SAM surface.

Tailoring Magnetic Properties in Transition Metal–Benzene Sandwich Clusters: Ways to Design Molecular Magnets

Transition metal–benzene (TM n Bz n +1 , with TM=Sc, Ti, and V, n =1 and 2) multiple-decker sandwich clusters have been studied by first-principles calculations. It was shown in our previous paper that in V 2 Bz 3 , the antiferromagnetic (AFM) and ferromagnetic (FM) states are nearly degenerate since the energy gain in FM state due to Kanamori–Terakura mechanism is compensated by the energy gain in AFM state due to polarization of edge Bzs. To design new clusters having much more stable FM state, two approaches are suggested. One is reducing the number of d electrons and the other is to weaken the hybridization strength between transition metals and edge Bz. In order to demonstrate the latter aspect, we take a thought experiment of replacing edge Bz with cyclopentadienyl ring C 5 H 5 . Either of these two methods or combinations of both has been demonstrated to be effective. Thus, magnetic properties of transition metal–aromatic molecule sandwich clusters can be tailored.

Photodissociation of lanthanide metal cation complexes with cyclooctatetraene

Lanthanide metal (Sm, Dy, Nd) cation complexes with 1,3,5,7-cyclooctatetraene (COT) are produced by laser vaporization in a pulsed nozzle cluster source. The clusters are mass-selected and photodissociated using the third harmonic of a Nd:YAG laser (355 nm). Samarium–COT complexes prefer to form [Sm n (COT) n ] + stoichiometries and fragment extensively when more than one metal atom is present in the cluster. However, the fragmentation patterns indicate the possible presence of multiple decker sandwich structures. Dysprosium–COT and neodymium–COT complexes prefer [M n (COT) n+1 ] + ratios, probably due to their preference for the +3 oxidation state. This stoichiometry pattern indicates that these clusters produce sandwich and multiple decker sandwich structures. Throughout the spectra for Dy and Nd complexes, a commonly occurring fragment is [M(C 5H 5)] +. Fragmentation of the COT ligand occurs either because of a strong interaction with the metal or because of photochemical decomposition. Presuming that C 5H 5 exists as the cyclopentadienyl anion, the Dy and Nd metals in these fragments exist in an unusual +2 oxidation state.

Computational Study of Multiple-Decker Sandwich and Rice-Ball Structures of Neutral Titanium−Benzene Clusters

Density functional theory calculations were applied to systematically and directly compare the relative energetic stability of multiple-decker sandwich and rice-ball structures for a variety of neutral Ti(m)Bz(n) clusters (m = 1-4, n = 1-5). Almost all structures favored the multiple-decker sandwich structure, as observed experimentally for early transition metals. The strength of each metal-benzene interaction averages 37 kcal/mol and remains relatively constant for sandwiches with three or more Ti atoms. The most stable smaller rice-ball structures did not have eta(6)-Bz bound to a single metal atom. Instead, the preferred coordination was having the plane of the benzene molecule parallel to a Ti(2) bond or a Ti(3) face, leading to some distortion of the benzene ring. The larger rice-ball structures, on the other hand, preferred to weaken the metal-metal bonds and retain eta(6)-Bz bound to a single metal atom, a structural motif shared with sandwiches.

A Joint Experimental and Theoretical Study of Cation−π Interactions: Multiple-Decker Sandwich Complexes of Ferrocene with Alkali Metal Ions (Li+, Na+, K+, Rb+, Cs+)

A systematic study of cation-pi interactions between alkali metal ions and the cyclopentadienyl ring of ferrocene is presented. The alkali metal (Li+, Na+, K+, Rb+, Cs+) salts of the ditopic mono(pyrazol-1-yl)borate ligand [1,1'-fc(BMe2pz)2]2- crystallize from dimethoxyethane as multiple-decker sandwich complexes with the M+ ions bound to the pi faces of the ferrocene cyclopentadienyl rings in an eta5 manner (fc = (C5H4)2Fe; pz = pyrazolyl). X-ray crystallography of the lithium complex reveals discrete trimetallic entities with each lithium ion being coordinated by only one cyclopentadienyl ring. The sodium salt forms polyanionic zigzag chains where each Na+ ion bridges the cyclopentadienyl rings of two ferrocene moieties. Linear columns [-CpR-Fe-CpR-M+-CpR-Fe-CpR-M+-](infinity) (R = [-BMe2pz]-) are established by the K+, Rb+, and Cs+ derivatives in the solid state. According to DFT calculations, the binding enthalpies of M+-eta5(ferrocene) model complexes are about 20% higher as compared to the corresponding M+-eta6(benzene) aggregates when M+ = Li+ or Na+. For K+ and Rb+, the degree of cation-pi interaction with both aromatics is about the same. The binding sequence along the M+-eta5(ferrocene) series follows a classical electrostatic trend with the smaller ions being more tightly bound.

Synthesis and structural characterization of ferrocene-based bis(pyrazol-1-yl)borate ligands: FcB(Me)pz 2K, Fc 2Bpz 2K, and 1,1′-fc[B(Me)pz 2] 2K 2 (Fc: ferrocenyl, fc: ferrocenylene, pz: pyrazolyl)

Reaction of the ferrocenyl(dimethylamino)boranes FcB(Me)NMe 2, Fc 2BNMe 2, and 1,1′-fc[B(Me)NMe 2] 2 with 1:1 mixtures of pyrazole and potassium pyrazolide in refluxing THF gave the potassium salts of the ferrocene-based bis(pyrazol-1-yl)borate ligands FcB(Me)pz 2K, Fc 2Bpz 2K, and 1,1′-fc[B(Me)pz 2] 2K 2 in good yield (Fc: ferrocenyl, fc: ferrocenylene, pz: pyrazolyl). In the solid state, FcB(Me)pz 2K and Fc 2Bpz 2K form centrosymmetric dimers with short K⋯Cp contacts suggesting an η 5 coordination mode of the potassium ion. The crystal lattice of the ditopic ligand 1,1′-fc[B(Me)pz 2] 2K 2 consists of coordination polymer strands featuring essentially the same structural motif that has been observed for the monotopic derivatives. All three scorpionate ligands are thus promising building blocks for the preparation of ferrocene-containing multiple-decker sandwich complexes.

Photoelectron spectroscopy of titanium–benzene cluster anions

( Titanium ) n ( benzene ) m - cluster anions were studied by both mass spectrometry and anion photoelectron spectroscopy. The cluster anion series, Ti n ( Bz ) n + 1 - ( n = 1 – 6 ) dominated the mass spectra. Vertical detachment energies and adiabatic electron affinities were determined for ( n, m) = (1,1), (1,2), (2,3), (3,4), (4,5), and (2,4). The evidence suggests that members of the Ti n ( Bz ) n + 1 - series have multiple-decker sandwich structures. Ti 2 ( Bz ) 4 - was observed and appears to be the dimer anion, ( Ti ( Bz ) 2 ) 2 - .

On the way to ferrocene-based multiple-decker sandwich complexes

Three ferrocene derivatives, Li[FcBH 3], Li 2[1,1 ′-fc(BMe 3) 2] and Li 2[1,1 ′-fc(BMe 2pz) 2] [Fc=(C 5H 5)Fe(C 5H 4), fc=(C 5H 4) 2Fe, pz=pyrazolyl], have been synthesized and structurally characterized to investigate the interaction of the negatively charged sandwich complex with its Li + counterion(s) in the solid state. Single crystals of Li[FcBH 3] · (12-crown-4), obtained from a THF solution after addition of 12-crown-4, contain hexacoordinated Li + ions, which are bonded to one molecule of crown ether and to two hydrogen atoms of the BH 3 fragment. The compound Li 2[1,1 ′-fc(BMe 3) 2] crystallized from THF as solvent-separated ion pairs with each Li + being encapsulated by four THF molecules. Single crystals of Li 2[1,1 ′-fc(BMe 2pz) 2] were grown from diethyl ether. In this compound, the lithium ions are coordinated by two solvent molecules, the pyrazolyl side-arm and the cyclopentadienyl ring. Thus, Li 2[1,1 ′-fc(BMe 2pz) 2] can be regarded as a trinuclear segment of the polymeric structure [–(C 5H 4R)Li(C 5H 4R)Fe–] ∞ with R=BMe 2pz.

Modeling of the molecular and electronic structures of multiple-decker sandwich macromolecules with η5-π bonds on the basis of biscyclopentadienyl derivatives 60H10 of a C60 fullerene

The molecular and electronic structures of the (CpFe)2C60H10 complex of the D5d symmetry, where Cp is the \(^ \cdot C_5 H_5 \) cyclopentadienyl radical, are simulated using the ab initio Hartree-Fock-Roothaan method in the 3–21G basis set. In this complex, hydrogen atoms are attached to carbon atoms of the C60 fullerene which occupy α-positions with respect to two oppositely lying five-membered rings. Each FeCp semisandwich moiety is linked to atoms of one of these five-membered rings by the η5-π-type bond. It is found that the energy of the η5-π Fe-C60H10 bonds in the (CpFe)2C60H10 complex is comparable to that of the η5-π Fe-Cp bond in the FeCp2 ferrocene molecule. The optimum geometry calculated for the (CpFe)2C60H10 complex is used for modeling of the structure of the quasi-linear macromolecule [-FeC60H10-]n, Open image in new window (I). The band structure of the energy spectrum of macromolecule I is calculated in the valence approximation of the extended Huckel method within the crystalline-orbital approach. The band gap in the spectrum of macromolecule I is ∼2.27 eV. The band structure of the spectrum of this macromolecule is compared with the spectra of the hypothetical molecules [-FeCp-]n and [-FeC20-]n.

Generation and electronic properties of lanthanide–cyclooctatetraene organometallic clusters in gas phase

Organometallic clusters of lanthanide (Ln = Nd, Er, Eu, and Yb) and 1,3,5,7-cyclooctatetraene (C8H8) were produced by a combination of laser vaporization and molecular beam methods. In the mass spectra of [Lu n (C8H8) m ], compositions of m = n + 1 were magic numbers. From mass spectrometry, photoionization spectroscopy, and photoelectron spectroscopy, we have concluded that these magic-numbered clusters take multiple-decker sandwich structures in which Ln atoms and C8H8 molecules are alternatedly piled up. These sandwich clusters are formed through ionic bonds composed of multiply charged cations [Ln k+ (k = 2 and 3)] and multiply charged anions [C8H 8 h− (h = 1, 1.5, and 2)].

Ionization Energies and Bonding Scheme of Multiple-Decker Sandwich Clusters: Mn(C6H6)n+1

The preparation of multiple-decker sandwich clusters Vn(C6H6)n+1 and their large size dependence of the ionization energies have recently been reported by Kaya and his co-workers (J. Phys. Chem. 1995, 99, 3053). In the present paper, the bonding scheme between benzene and metal atoms (Ti, V, and Cr) was investigated by using Mayer's bond order analysis with ab initio MO calculations, and it was attributed mainly to the delocalization of metal dδ electrons via the LUMOs of the benzene molecules. Moreover, the lowest ionization of most multiple-decker sandwich clusters was found to occur from the upper end of the dδ orbitals, and the large size dependence of the ionization energies was also related to the significant one-dimensional delocalization of these dδ electrons. The proposed Huckel type treatment for these frontier orbitals explains the above properties very simply and suggests also the large size dependence of the photoabsorption band positions and even the thermodynamical stability of the one-dime...