Influence of low-symmetry distortions on electron transport through metal atom chains: when is a molecular wire really "broken"?

In the field of molecular electronics, an intimate link between the delocalization of molecular orbitals and their ability to support current flow is often assumed. Delocalization, in turn, is generally regarded as being synonymous with structural symmetry, for example, in the lengths of the bonds along a molecular wire. In this work, we use density functional theory in combination with nonequilibrium Green's functions to show that precisely the opposite is true in the extended metal atom chain Cr(3)(dpa)(4)(NCS)(2) where the delocalized π framework has previously been proposed to be the dominant conduction pathway. Low-symmetry distortions of the Cr(3) core do indeed reduce the effectiveness of these π channels, but this is largely irrelevant to electron transport at low bias simply because they lie far below the Fermi level. Instead, the dominant pathway is through higher-lying orbitals of σ symmetry, which remain essentially unperturbed by even quite substantial distortions. In fact, the conductance is actually increased marginally because the σ(nb) channel is displaced upward toward the Fermi level. These calculations indicate a subtle and counterintuitive relationship between structure and function in these metal chains that has important implications for the interpretation of data emerging from scanning tunnelling and atomic force microscopy experiments.     

Probing the Structure,Dynamics, and Bonding of Coinage Metal Complexes of White Phosphorus

A series of cationic white phosphorus complexes of the coinage metals Au and Cu have been synthesised and characterised both in the solid state and in solution. All complexes feature a P₄ unit coordinated through an edge P?P vector (η²‐like), although the degree of activation (as measured by the coordinated P?P bond length) is greater in the gold species. All of the cations are fluxional on the NMR timescale at room temperature, but in the case of the gold systems fluxionality is frozen out at ?90 °C. Electronic structure calculations suggest that this fluxionality proceeds via an η¹‐coordinated M?P₄ intermediate.     

Catalytic alcohol oxidation by an unsymmetrical 5-coordinate copper complex: electronic structure and mechanism

Density functional theory reveals the detailed mechanism of alcohol oxidation by a model copper complex, Cu(II)L, L = cis-1-(3',5'-dimethoxy-benzylideneamino)-3,5-[2-hydroxy-(3',5'-di-tert-butyl)benzylideneimino]cyclohexane. Despite the obvious structural and functional parallels between the title compound and the enzyme galactose oxidase, the details of the catalytic pathway are fundamentally different. In the enzyme, coordination of the substrate produces an active form containing a Cu(II) centre and a tyrosyl radical, the latter being responsible for the abstraction of hydrogen from the substrate. In the model system, in marked contrast, the active form contains a Cu(II) centre, but the ligand radical character is localised on the substrate (alcoholate) oxygen, rather than the phenolate ligand. The result is a significantly higher barrier to hydrogen-atom abstraction compared to the enzyme itself. The origin of these significant differences is traced to the rigid nature of the pentadentate ligand, which resists changes in coordination number during the catalytic cycle.     

Synthesis and Characterization of [Fe3(As3)3(As4)]3−, a Binary Fe/As Zintl Cluster With an Fe3 Core

We report here the synthesis and structural characterization of the first binary iron arsenide cluster anion, [Fe₃(As₃)₃(As₄)]³⁻, present in both [K([2.2.2]crypt)]₃[Fe₃(As₃)₃(As₄)] ( 1 ) and [K(18-crown-6)]₃[Fe₃(As₃)₃(As₄)]⋅en ( 2 ). The cluster contains an Fe₃ triangle with three short Fe−Fe bond lengths (2.494(1) Å, 2.459(1) Å and 2.668(2) Å for 1 , 2.471(1) Å, 2.473(1) Å and 2.660(1) Å for 2 ), bridged by a 2-butene-like As₄ unit. An analysis of the electronic structure using DFT reveals a triplet ground state with direct Fe−Fe bonds stabilizing the Fe₃ core.     

Stable formally zerovalent and diamagnetic monovalent niobium and tantalum complexes based on diazadiene ligands

This paper describes the efficient synthesis and full characterization of rare formally zerovalent and diamagnetic monovalent pseudooctahedral niobium and tantalum complexes. The reaction of NbCl(4)(thf)(2) with 4 equiv of Na and 3.5 equiv of iPr(2)-dad (1,4-diisopropyl-1,4-diaza-1,3-diene) yields Nb(iPr(2)-dad)(3) in 52% yield. Ta(iPr(2)-dad)(3) is also obtained in 33% yield from 5 equiv of Na/naphthalene and 3.5 equiv of iPr(2)-dad. Oxidation of these complexes with AgBPh(4) yields the diamagnetic complexes [M(iPr(2)-dad)(3)][BPh(4)] (M = Nb, Ta). X-ray and spectroscopic data indicate that the unpaired electron density is localized on the ligands. DFT calculations reveal that, in the prevailing D(3) symmetry, a very strong splitting of t(2g) metal-based orbitals occurs, leading to the diamagnetism of the 16e M(iPr(2)-dad)(3)(+). This strong splitting allows a M-N nonbonding, ligand-based orbital to accommodate additional electrons, as a result of which the formally zerovalent complexes, M(iPr(2)-dad)(3), are in fact correctly formulated as M(+)iPr(2)-dad (-), that is, (16e + 1e).     

Characterisation of tri-ruthenium dihydride complexes through the computation of NMR parameters

Density functional theory has been used to provide atomic-level detail on the structures of metal hydride intermediates that have previously been proposed in the hydrogenation of phenylacetylene using Ru(3)(CO)(10)(PPh(3))(2). Based on a comparison of energetic data along with computed chemical shifts and coupling constants, we suggest that the detected species share a Ru(3)(μ-H)(μ-H) motif, with two distinct bridging hydride sites, rather than the terminal hydride proposed previously. The work illustrates how theory can be used as a complement to spectroscopy to enhance the accuracy of deductions, and to provide a basis for future rational design of second generation catalysts.     

On the structural and electronic properties of [Zn2(4,4'-bipyridine)(mes)4]n- (n=0-2), a homologous series of bimetallic complexes bridged by neutral, anionic, and dianionic 4,4'-bipyridine

Addition of 1 equiv of potassium metal to a tetrahydrofuran (THF) solution of Zn(2)(4,4'-bipyridine)(mes)(4) (1; mes =2,4,6-Me(3)C(6)H(2)) in the presence of 18-crown-6 (1,4,7,10,13,16-hexaoxacyclooctadecane) yielded the radical anionic species [Zn(2)(4,4'-bipyridine)(mes)(4)](?-), which was characterized by single crystal X-ray diffraction in [K(18-crown-6)(THF)(2)][Zn(2)(4,4'-bipyridine)(mes)(4)] (2). A similar reaction employing 2 equiv of alkali metal afforded the related complex [K(18-crown-6)](2)[Zn(2)(4,4'-bipyridine)(mes)(4)] (3). The [Zn(2)(4,4'-bipyridine)(mes)(4)](n-) (n = 0-2) moieties present in 1-3 are largely isostructural, yet exhibit significant structural variations which arise because of differences in their electronic structure. These species represent a homologous series of complexes in which the ligand exists in three distinct oxidation states. Structural data, spectroscopic measurements, and density functional theory (DFT) calculations are consistent with the assignment of 1, 2, and 3 as complexes of the neutral, radical anionic, and dianionic 4,4'-bipyridyl ligand, respectively. To the best of our knowledge, species 2 and 3 are the first crystallographically characterized transition metal complexes of the 4,4'-bipyridyl radical and dianion.     

LaSr3NiRuO4H4: A 4d Transition‐Metal Oxide–Hydride Containing Metal Hydride Sheets

The synthesis of the first 4d transition metal oxide–hydride, LaSr₃NiRuO₄H₄, is prepared via topochemical anion exchange. Neutron diffraction data show that the hydride ions occupy the equatorial anion sites in the host lattice and as a result the Ru and Ni cations are located in a plane containing only hydride ligands, a unique structural feature with obvious parallels to the CuO₂ sheets present in the superconducting cuprates. DFT calculations confirm the presence of S= Ni⁺ and S=0, Ru²⁺ centers, but neutron diffraction and μSR data show no evidence for long‐range magnetic order between the Ni centers down to 1.8 K. The observed weak inter‐cation magnetic coupling can be attributed to poor overlap between Ni 3d and H 1s in the super‐exchange pathways.