Central Transition Metal Atom Research Articles

Olefin coordination versus weak agostic C[sbnd]H[sbnd]M interactions in Sandwich-Type complexes of the split (3 + 2) Five-Electron donor hydrocarbon ligand Tricyclo[5.2.1.0]-deca-3,8-dien-5-yl (dicp) related to cyclopentadiene dimer

Sandwich-type complexes (dicp)MCp (M = Ti, V, Cr, Mn, Fe, Co, Ni; Cp = cyclopentadienyl) of the split (3 + 2) five-electron donor ligand tricyclo[5.2.1.0]-deca-3,8-dien-5-yl (dicp) related to cyclopentadiene dimer have been investigated by density functional theory. In the low-energy structures of the transition metal complexes (dicp)MCp (M = Ti, V, Cr, Mn, Fe), all of the split (3 + 2) five electrons from the dicp ligand are donated to the central transition metal atom leading to 14-, 15-, 16-, 17, and 18-electron configurations for the singlet (dicp)TiCp, quartet (dicp)VCp, triplet (dicp)CrCp, doublet (dicp)MnCp, and singlet (dicp)FeCp structures. For the late transition metals Co and Ni, only the allyl unit of the dicp ligand is strongly coordinated to the transition metal atom, in contrast to the Cp2Co and Cp2Ni in which all ten carbon atoms of the two cyclopentadienyl ligands are coordinated to the central metal atom. The potential energy surface for the manganese complex (dicp)MnCp is much more complicated than that of manganocene Cp2Mn, in which the doublet, quartet, and sextet structures are nearly degenerate in energy, with the quartet and sextet low-energy structures having only have part of the allyl or olefin unit coordinated to the central manganese atom. Weak agostic CHM interactions between the dicp ligand and the metal atoms, typically with M—H distances of ∼2.7 Å, are found in many of the (dicp)MCp complexes in contrast to the Cp2M (M = Ti, V, Cr, Mn, Fe, Co, Ni).

Robust broken-gap MoTe2/ZrS2 van der Waals heterostructure

Two-dimensional van der Waals (vdW) heterostructures with several possibilities of band edge alignment are promising candidates in the design of semiconductor junction devices with good electrostatic control. We predict a robust broken-gap band alignment for the ${\mathrm{MoTe}}_{2}/{\mathrm{ZrS}}_{2}$ vdW heterostructure and uncover the underlying mechanism. Our first-principles calculations reveal that upon formation of the vdW heterostructure a small amount of charge is transferred between the layers which sits on the central transition metal atoms and is responsible for the band edge alignment. Moreover, the lattice-mismatch-induced in-plane strain and the interfacial polarization are shown to play no role in band alignment. However, we find a weak sensitivity of the electronic structure and band edge alignment of the vdW heterostructures to an external normal-to-plane strain or electric field. Our findings are helpful in designing heterojunction devices with desired electronic structures.

Highly sensitive and selective sensing of acetone and hydrogen sulfide using metal phthalocyanine – carbon nanotube hybrids

Noncovalent functionalization of carbon nanotubes with metal phthalocyanine has great potential for sensor nanodevices, nevertheless, some aspects of their electron transport mechanism behind the sensing properties have not been understood yet. Here, we evaluate the sensing performance of metal phthalocyanine – carbon nanotube hybrids for acetone and hydrogen sulfide (H2S) detection in terms of sensitivity and selectivity, using density functional theory and the non-equilibrium Green’s function method. We systematically investigated the effects of central transition metal atoms (Fe, Co, and Mn) of metal phthalocyanine with different chiralities (quasi-metallic CNT (9,0) zigzag and metallic CNT(5,5) armchair) of single-walled carbon nanotubes and tuning the gate voltages making this nanosensor highly sensitive and selective to acetone and H2S.

Large dual spin-rectifying and high-efficiency dual spin-filtering in cyclooligomeric Mn-phthalocyanine dimer molecular junction

Abstract Based on density functional theory combined with nonequilibrium Green’s function technique, we investigate the spin-polarized transport properties of the cyclooligomeric Mn-phthalocyanine dimer and Fe-phthalocyanine dimer with carbon nanotube electrodes. The results show that the spin-polarized transport properties can be effectively tuned by changing the central transition metal atoms, and the Mn-phthalocyanine dimer system can exhibit large dual spin-rectifying and high-efficiency dual spin-filtering effects simultaneously under the [1, −1] magnetic configuration. The underlying mechanism is analyzed by spin-resolved transmission spectra, molecular projected self-consistent Hamiltonian orbitals and their spatial distribution.

Correlation between oxygen adsorption energy and electronic structure of transition metal macrocyclic complexes

Oxygen adsorption energy is directly relevant to the catalytic activity of electrocatalysts for oxygen reduction reaction (ORR). In this study, we established the correlation between the O2 adsorption energy and the electronic structure of transition metal macrocyclic complexes which exhibit activity for ORR. To this end, we have predicted the molecular and electronic structures of a series of transition metal macrocyclic complexes with planar N4 chelation, as well as the molecular and electronic structures for the O2 adsorption on these macrocyclic molecules, using the density functional theory calculation method. We found that the calculated adsorption energy of O2 on the transition metal macrocyclic complexes was linearly related to the average position (relative to the lowest unoccupied molecular orbital of the macrocyclic complexes) of the non-bonding d orbitals (d(z(2)), d(xy), d(xz), and d(yz)) which belong to the central transition metal atom. Importantly, our results suggest that varying the energy level of the non-bonding d orbitals through changing the central transition metal atom and/or peripheral ligand groups could be an effective way to tuning their O2 adsorption energy for enhancing the ORR activity of transition metal macrocyclic complex catalysts.

Transition-Metal-Centered Monocyclic Boron Wheel Clusters (M©Bn): A New Class of Aromatic Borometallic Compounds

Atomic clusters have intermediate properties between that of individual atoms and bulk solids, which provide fertile ground for the discovery of new molecules and novel chemical bonding. In addition, the study of small clusters can help researchers design better nanosystems with specific physical and chemical properties. From recent experimental and computational studies, we know that small boron clusters possess planar structures stabilized by electron delocalization both in the σ and π frameworks. An interesting boron cluster is B(9)(-), which has a D(8h) molecular wheel structure with a single boron atom in the center of a B(8) ring. This ring in the D(8h)-B(9)(-) cluster is connected by eight classical two-center, two-electron bonds. In contrast, the cluster's central boron atom is bonded to the peripheral ring through three delocalized σ and three delocalized π bonds. This bonding structure gives the molecular wheel double aromaticity and high electronic stability. The unprecedented structure and bonding pattern in B(9)(-) and other planar boron clusters have inspired the designs of similar molecular wheel-type structures. But these mimics instead substitute a heteroatom for the central boron. Through recent experiments in cluster beams, chemists have demonstrated that transition metals can be doped into the center of the planar boron clusters. These new metal-centered monocyclic boron rings have variable ring sizes, M©B(n) and M©B(n)(-) with n = 8-10. Using size-selected anion photoelectron spectroscopy and ab initio calculations, researchers have characterized these novel borometallic molecules. Chemists have proposed a design principle based on σ and π double aromaticity for electronically stable borometallic cluster compounds, featuring a highly coordinated transition metal atom centered inside monocyclic boron rings. The central metal atom is coordinatively unsaturated in the direction perpendicular to the molecular plane. Thus, chemists may design appropriate ligands to synthesize the molecular wheels in the bulk. In this Account, we discuss these recent experimental and theoretical advances of this new class of aromatic borometallic compounds, which contain a highly coordinated central transition metal atom inside a monocyclic boron ring. Through these examples, we show that atomic clusters can facilitate the discovery of new structures, new chemical bonding, and possibly new nanostructures with specific, advantageous properties.

What Is the Maximum Coordination Number in a Planar Structure?

Thinking about molecules that do not yet exist on Earth is an amazing mental experiment. Using the established rules of chemistry, we are able to find and—in many cases—to understand what is the most favorable structure of a set of atoms. Exceptions to these rules are occasionally found, in particular if small numbers of atoms are concerned, and they can cause strong conceptual troubles. Paraphrasing Benson, these exceptions drive us to go deeper and push the limits. So, studying new exotic molecules is more than curiosity, it is to prove the limits, to learn, and eventually to understand important concepts in chemistry such as the chemical bond or aromaticity. Molecules containing planar tetracoordinate carbon (ptC) atoms are good examples. In fact, each new ptC structure that is realized experimentally contradicts the classical structural theory of organic chemistry. Furthermore, it gives confidence in predictive theory as a discipline to establish the limits of such seemingly outlandish structures. Recently, viable planar pentacoordinate carbon (ppC) atoms were predicted in silico. In 2000, Exner and Schleyer suggested CB6 2 as the first anionic molecule with a planar hexacoordinate carbon atom. However, experimental evidence shows that carbon avoids such type of arrangements. Themost stable form of CB6 2 is also planar, but the carbon atom is dicoordinate. So, it would appear that the maximum coordination number of carbon in planar molecules is five. Chemistry can be interpreted in terms of perturbation theory: substituting a single atom in a molecule can induce a strong change in its structure and properties. When the carbon nucleus in CB6 2 is substituted by boron, and two electrons are removed, the result is a quasiplanar B7 structure with a hexacoordinate boron atom. The addition of one boron atom to the periphery generates a ring within which the central boron atom is accommodated comfortably. As a consequence of its electron-deficient character and propensity for deltahedral bonding, boron is intrinsically suitable for designing ring systems containing one or more hypercoordinate elements, including boron itself. Several boron rings enclosing planar hypercoordinate main group elements have been proposed in silico. However, none of them has yet been detected experimentally, except those composed exclusively of boron. In 2008, Luo as well as Ito et al. explored if it was viable to theoretically design boron wheels containing transition metals. Three years later, Romanescu et al. detected boron rings in the gas phase with a central transition metal atom. These doped boron clusters were produced in a laser-vaporization supersonic molecular beam, and the structures were probed and confirmed by photoelectron spectroscopy. Thus, they evidently exist in the gas phase, but we cannot expect them to be produced in macroscopic quantities in the near future. The reported clusters (Co@B8 , Ru@B9 , Rh@B9 , and Ir@B9 ) are extraordinary in chemistry because of their perfectly planar structures with coordination numbers of 8 and 9 (Figure 1). In addition to the strong and localized

Single-Component Molecular Conductor [Cu(tmdt)2] Containing an Antiferromagnetic Heisenberg Chain

Traditional molecular conductors are composed of more than two chemical species and are characterized by low-dimensional electronic band structures. By contrast, the single-component molecular metals [M(tmdt)(2)] (M = Ni, Pt, Au; tmdt = trimethylenetetrathiafulvalenedithiolate) possess three-dimensional electronic structures that can be widely tuned by exchanging the central transition metal atom (M). In this study, the Cu atom was used to realize a new magnetic single-component molecular conductor exhibiting strong pi-d interactions. The crystal structure of [Cu(tmdt)(2)] was found to be essentially the same as those of the Ni, Pt, or Au-based systems with metallic states down to low temperature, but different from the structure of [Cu(dmdt)(2)] (dmdt = dimethyltetrathiafulvalenedithiolate) with its tetrahedrally coordinated dmdt ligands. A compressed pellet of microcrystals exhibited fairly high room-temperature conductivity (sigma(RT) approximately 7 S.cm(-1)), which increased almost linearly with pressure, reaching 110 S.cm(-1) at 15 kbar. This strongly suggests that the single crystal of [Cu(tmdt)(2)] is metallic at high pressure. Magnetic susceptibility measurements indicated one-dimensional Heisenberg behavior with |J| = 117 cm(-1) and an antiferromagnetic transition at 13 K. Density functional theory molecular orbital calculations revealed that the alpha-spin orbital of pdsigma(-) is distributed at the central part of the complex (CuS(4)), and alpha- and beta-sym-Lpi orbitals have almost the same energies and their spins are distributed mainly in the pdsigma(-) orbital. This is in contrast to the first single-component molecular metal [Ni(tmdt)(2)], which has stable metal bands formed from an almost degenerated sym-Lpi orbital (the highest occupied molecular orbital) and asym-Lpi(d) orbital (the lowest unoccupied molecular orbital). These results suggest that the alpha-pdsigma(-) state of [Cu(tmdt)(2)] exists just around the Fermi energy of the virtual metal band formed from the asym-Lpi(d) and sym-Lpi states. Thus, as expected, [Cu(tmdt)(2)] is a non-trivial single-component molecular conductor with pi-d multifrontier orbitals. In addition, ((n)Bu(4)N)(2)[Cu(tmdt)(2)] was synthesized, and its crystal structure was determined. Its Curie behavior (chi(rt) = 1.2 x 10(-3) emu mol(-1); C = 0.36 emu.K mol(-1)) indicates the existence of an isolated S = 1/2 spin on each dianionic molecule.

Radiation damage in biomimetic dye molecules for solar cells

A significant obstacle to organic photovoltaics is radiation damage, either directly by photochemical reactions or indirectly via hot electrons. Such effects are investigated for biomimetic dye molecules for solar cells (phthalocyanines) and for a biological analog (the charge transfer protein cytochrome c). Both feature a central transition metal atom (or H(2)) surrounded by nitrogen atoms. Soft x-ray absorption spectroscopy and photoelectron spectroscopy are used to identify three types of radiation-induced changes in the electronic structure of these molecules. (1) The peptide bonds along the backbone of the protein are readily broken, while the nitrogen cage remains rather stable in phthalocyanines. This finding suggests minimizing peptide attachments to biologically inspired molecules for photovoltaic applications. (2) The metal atom in the protein changes its 3d electron configuration under irradiation. (3) The Fermi level E(F) shifts relative to the band gap in phthalocyanine films due to radiation-induced gap states. This effect has little influence on the optical absorption, but it changes the lineup between the energy levels of the absorbing dye and the acceptor/donor electrodes that collect the charge carriers in a solar cell.

η3-Sandwich-Like Complexes Based on the Smallest Polynitrogen Ring

Over the past decades, polynitrogen chemistry has witnessed rapidly growing progress. Unfortunately and to our great surprise, eta(3)-assembled species based on the simplest polynitrogen ring (N(3)) have never been reported. In this paper, we report the first successful assembly design of the long-escaped N(3)-based compounds, that is, [N(3)NiN(3)](2-), [N(3)M(CO)(2)N(3)](q) ((M,q), = (Fe,0) (Mn,-1)), [N(3)M(CO)(3)](q) ((M,q) = (Co,0), (Fe,-1)), and [N(3)MCp](q) ((M,q) = (Ni, 0), (Co,-1)), at the density functional level. The conversion and dissociation of them need to overcome considerable barriers kinetically. To our surprise, the detailed structural, charge distribution, and orbital analyses consistently reveal a triplet polynitrogen unit, cyclic-(3)N(3)(-), rather than another simplest trinitrogen unit cyclic-(1)N(3)(+). The two unpaired spins within the naked cyclic-(3)N(3)(-) have effectively participated in the bonding interaction with the central transition-metal atoms (here, M is Ni, Fe, Co, and Mn). Moreover, the possible experimental routes of N(3)Co(CO)(3) were proposed. The diradical-like polynitrogen ring, cyclic-(3)N(3)(-), would add to the polynitrogen family as a novel building block. The assembly-designed species await future experimental verification.

QTAIM study of transition metal complexes with cyclophosphazene-based multisite ligands I: Zinc(II) and nickel(II) complexes

The metal–ligand bonds in [(ZnCl 2) 2L], [NiLCl] + and two isomers of [NiLCl 2] complexes with multimodal ligand L = hexakis(2-pyridyloxy)cyclotriphosphazene, cyclo-[NP(NC 5H 4O) 2] 3, are investigated at DFT level of theory using hybrid B3LYP functional. Electron density is evaluated in terms of QTAIM (quantum theory of atoms-in-molecule) topological analysis of electron density. The bonds of central transition metal atoms with phosphazene nitrogens are shorter, stronger and more polar than with the aromatic pyridine nitrogens. The atomic charges of phosphazene nitrogens are ca twice more negative than at the pyridine ones. The higher mechanical strain in the five-coordinate metal complexes than in the six-coordinate ones may be concluded.

Optically Active (Peptido‐carbene)palladium Complexes: Towards True Solid‐Phase Combinatorial Libraries of Transition Metal Catalysts

AbstractThis work explores the potential of forming transition metal catalysts by combinatorial solid‐phase peptide chemistry to produce catalysts with enzyme‐like properties of stereoselectivity, regioselectivity and even substrate selectivity. A series of new functionalised carbene precursors/donors – imidazolium salts – each containing both amino acid and carboxylic acid functionality was synthesised in solution. The readily accessible carbene precursors were incorporated within the backbones of peptides attached to PEGA resin, by standard solid‐phase peptide coupling techniques. The synthetic strategy gave easy access to both mono‐ and didentate ligand systems, providing folded structures around the central transition metal atoms, with different degrees of steric congestion and bite angles. Changing the number of incorporated amino acids between the two carbene donors facilitated variation of the properties of the complexes. Both ligand systems were complexed to palladium(II) by standard base treatment, and the Pd complexes were studied by mass spectrometry and NMR spectroscopy. The monodentate (carbene)palladium complexes were each the product of enolisation of a neighbouring carbonyl group and loss of a proton, followed by coordination of the oxy anion to the palladium atom. The established methods are suitable for the combinatorial synthesis of palladium catalysts on solid support. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)

Doping-Dependent Negative Differential Resistance in Hybrid Organic/Inorganic Si−Porphyrin−Si Junctions

Quantum transport properties of porphyrin-bridged p-n junctions with Si leads are investigated by ab initio calculations. It is shown that this system exhibits strong negative differential resistance (NDR) peaks, whose magnitude and position can be controlled by the doping levels of the leads and by changing the central transition metal atom of the porphyrin. These results are explained by bias-induced on-off switching of resonant tunneling channels associated with specific molecular orbitals. The predicted behavior is general and should be observable for other organic molecules bridging doped semiconducting leads.

Density Functional Theoretical Study of a Series of Binary Azides M(N3)n (n = 3, 4)

AbstractFor Abstract see ChemInform Abstract in Full Text.

Density Functional Theoretical Study of a Series of Binary Azides M(N3)n (n = 3, 4)

Geometrical structures of a series of binary azides M(N3)n (M = elements in groups 3 and 13 (n = 3) and in groups 4 and 14 (n = 4)) were investigated at the B3LYP/6-311+G level of theory. Our calculations found that binary group 3 triazides M(N3)3 (M = Sc, Y, La) and binary group 4 tetraazides M(N3)4 (M = Ti, Zr, Hf) turn out to be stable with all frequencies real having a similar linear M-N-NN structural feature, as previously reported for M(N3)4 (M = Ti, Zr, Hf). However, binary azides of group 13 M(N3)3 (M = B, Al, Ga, In, Tl) and group 14 elements M(N3)4 (C, Si, Ge, Sn, Pb) with bent M-N-NN bond angles differ obviously from binary group 3 and 4 azides in geometrical structure. These facts are mainly explained by the difference in electronic density overlap between the central atom and the alpha-N atoms of the azido groups. Two lone-pair electrons on the sp hybridization alpha-N atoms in the binary group 3 and 4 azides donate electron density into two empty d orbitals of the central transition metal atom and a pair of valence bonding electrons, resulting in the alpha-N atoms acting as a tridentate ligand. The sp2 hybridization alpha-N atoms of the binary group 13 and 14 azides only give one valence electron to form one valence bonding electron pair acting virtually as monodentate donors.

Quadrupolar transitions and medium-range-order effects in metalK-edge x-ray absorption spectra of3dtransition-metal compounds

We have investigated the pre-edge features in x-ray absorption spectra of transition metal (TM) monoxides at the metal $K$ edge. By comparing the calculated dipolar and quadrupolar spectra with experiments we assign the first peak at the lowest energy to a direct quadrupolar transition (due to the more effective attraction of the core hole). The following peaks in this region are mainly dipolar in character and reflect the density of states due to the medium range order as long as the radius of the cluster equals the cation-cation shortest distance plus the nearest neighbor cation-anion bond, representing a band-like effect arising from the hybridization of the $4p$ orbitals of central TM atoms with the $3d$-based octahedral molecular orbitals of the higher-neighboring cations.

Tripodal Triamidometallates of the Heavy Group 14 Elements: Inorganic Cages with Remarkable “Ligand Properties”

Anionic main group metal donors with potentially interesting ligand properties are obtained if ate complexes of the heavy p-block metals are formed by coordination between tripodal amides and divalent group 14 metal centres. The limitations of this approach, due to the potential oxidation by the central transition metal atom, may in part be suppressed by the presence of electronegative substituents at the MII centre, which makes the metallates less reducing. The considerable stability of the triamidostannates(II) towards AgI or AuI−III is testimony to the practical significance of this. The most characteristic structural consequence of the rigid amido cages is the well-defined orientation of the peripheral N-substituents, which gives rise to very large cone angles for the monodentate ligands in their transition metal complex derivatives. These angles considerably exceed those of “ordinary” metallate groups or of the isolobal neutral group 15 ligands, and so would be expected to favour low coordination numbers in transition metal complexes. (© Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002)

Tripodal Triamidometallates of the Heavy Group 14 Elements: Inorganic Cages with Remarkable “Ligand Properties”

Anionic main group metal donors with potentially interesting ligand properties are obtained if ate complexes of the heavy p-block metals are formed by coordination between tripodal amides and divalent group 14 metal centres. The limitations of this approach, due to the potential oxidation by the central transition metal atom, may in part be suppressed by the presence of electronegative substituents at the MII centre, which makes the metallates less reducing. The considerable stability of the triamidostannates(II) towards AgI or AuI−III is testimony to the practical significance of this. The most characteristic structural consequence of the rigid amido cages is the well-defined orientation of the peripheral N-substituents, which gives rise to very large cone angles for the monodentate ligands in their transition metal complex derivatives. These angles considerably exceed those of “ordinary” metallate groups or of the isolobal neutral group 15 ligands, and so would be expected to favour low coordination numbers in transition metal complexes. (© Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002)

Structural selectivity in aluminium-transition-metal alloys

The authors present a theory of structural selectivity in aluminium-rich transition-metal alloys based on an embedded-atom approximation to density functional theory. With this method, which is fast and flexible enough to permit study of complex crystal structures, they discover that electron density requirements largely determine the local arrangement of Al atoms around a central transition metal atom. This fact, in turn, permits them to explain a number of observed crystallographic trends in these systems including the appearance of metastable icosahedral phases which exhibit five-fold diffraction patterns.

ChemInform Abstract: THE STABILIZATION OF LOW OXIDATION STATES IN Σ‐ORGANO TRANSITION METAL CHEMISTRY BY COORDINATIVE LITHIUM TRANSITION METAL INTERACTION

Abstract Starting from the well known connection between the accessibility of different oxidation states of the transition metals and the donor-acceptor characteristics of the ligands, synthesis and structural characterization of electron-rich phenyl transition metal complexes are reviewed. To explain the existence of these unusual compounds in accordance with Pauling's principle of electroneutrality, a coordinative interaction of lithium ions with the central transition metal atom has been suggested as a new principle of stabilization of low oxidation states in coordination chemistry.