Trigonal Prismatic Structure Research Articles

Endohedral Beryllium Atoms in Germanium Clusters with Eight and Fewer Vertices: How Small Can a Cluster Be and Still Encapsulate a Central Atom?

Structures of the beryllium-centered germanium clusters Be@Ge(n)(z) (n = 8, 7, 6; z = -4, -2, 0, +2) have been investigated by density functional theory to provide some insight regarding the smallest metal cluster that can encapsulate an interstitial atom. The lowest energy structures of the eight-vertex Be@Ge(8)(z) clusters (z = -4, -2, 0, +2) all have the Be atom at the center of a closed polyhedron, namely, a D(4d) square antiprism for Be@Ge(8)(4-), a D(2d) bisdisphenoid for Be@Ge(8)(2-), an ideal O(h) cube for Be@Ge(8), and a C(2v) distorted cube for Be@Ge(8)(2+). The Be-centered cubic structures predicted for Be@Ge(8) and Be@Ge(8)(2+) differ from the previously predicted lowest energy structures for the isoelectronic Ge(8)(2-) and Ge(8). This appears to be related to the larger internal volume of the cube relative to other closed eight-vertex polyhedra. The lowest energy structures for the smaller seven- and six-vertex clusters Be@Ge(n)(z) (n = 7, 6; z = -4, -2, 0, +2) no longer have the Be atom at the center of a closed Ge(n) polyhedron. Instead, either the Ge(n) polyhedron has opened up to provide a larger volume for the Be atom or the Be atom has migrated to the surface of the polyhedron. However, higher energy structures are found in which the Be atom is located at the center of a Ge(n) (n = 7, 6) polyhedron. Examples of such structures are a centered C(2v) capped trigonal prismatic structure for Be@Ge(7)(2-), a centered D(5h) pentagonal bipyramidal structure for Be@Ge(7), a centered D(3h) trigonal prismatic structure for Be@Ge(6)(4-), and a centered octahedral structure for Be@Ge(6). Cluster buildup reactions of the type Be@Ge(n)(z) + Ge(2) → Be@Ge(n+2)(z) (n = 6, 8; z = -4, -2, 0, +2) are all predicted to be highly exothermic. This suggests that interstitial clusters having an endohedral atom inside a bare post transition element polyhedron with eight or fewer vertices are less than the optimum size. This is consistent with the experimental observation of several types of 10-vertex polyhedral bare post transition element clusters with interstitial atoms but the failure to observe such clusters with external polyhedra having eight or fewer vertices.

Effect of electron count on the structures of (benzene)metallaboranes with 9–12 vertices: Comparison of the iron and ruthenium systems

The metallaboranes (C6H6)MBn−1Hn−1z (n=9, 10, 11, 12; z=−2, 0, +2; M=Fe, Ru) have been studied by density functional theory. The most spherical closo deltahedral structures were found as global minima in the dianions (η6-C6H6)MBn−1Hn−12− in accord with the Wade–Mingos rules for these 2n+2 skeletal electron systems. However, novel alternative low energy “slipped” structures were found for (η4-C6H6)MB9H92− with a tetrahapto rather than hexahapto benzene ring and the 10-vertex isocloso deltahedron with the metal atom at the unique degree 6 vertex. The expected isocloso structures were found for the neutral 10 and 11 vertex systems (η6-C6H6)MBn−1H n−1 (M=Fe, Ru; n=10, 11) with the metal at the unique degree 6 vertex in accord with their 2n skeletal electrons. Similar non-icosahedral 12-vertex (η6-C6H6)MB11H11 (M=Fe, Ru) structures with the iron or ruthenium at a degree 6 vertex were found to be of lower energy than the corresponding icosahedral structures. For the 9-vertex (η6-C6H6)MB8H8 (M=Fe, Ru) the closo tricapped trigonal prism structure with the metal at a degree 5 vertex is predicted to be of lower energy than the isocloso structure with the metal at a degree 6 vertex. Capped isocloso structures with the metal at a unique degree 7 vertex are predicted for the lowest energy structures of the dications (η6-C6H6)MBn−1Hn−12+ (n=10, 11; M=Fe, Ru). A related bicapped pentagonal bipyramid structure with the iron at a unique degree 7 vertex is found for the 9-vertex ferraborane dication (η6-C6H6)FeB8H82+. The isocloso and capped isocloso structures with the metal atoms at degree 6 and 7 vertices, respectively, for the 2n skeletal electron (η6-C6H6)MBn−1Hn−1 and 2n−2 skeletal electron (η6-C6H6)MBn−1Hn−12+, respectively, appear to be more favorable for the ruthenium species (M=Ru) than for the corresponding iron species (M=Fe).

Hydration of trivalent lanthanum revisited – An ab initio QMCF-MD approach

The previously investigated La3+-hydrate has been re-evaluated by means of the quantum mechanical charge field (QMCF) molecular dynamics (MD) approach. Improved description of the hydration characteristics has been realised by including the full second hydration shell into the quantum mechanically treated region and by introducing the influence of the surrounding bulk via an electrostatic embedding technique. Analytical tools such as the ligand angular radial distribution analysis have been employed to gain deeper insight into the structural features of the hydrate. La3+ simultaneously forms nona- and decahydrates with capped trigonal and quadratic prismatic structure, besides small amounts of an octahydrate.

The sphericity of the diverse 10-vertex polyhedra found in bare post-transition metal clusters: germanium clusters with interstitial magnesium atoms as model systems

The diverse polyhedra found experimentally in bare 10-vertex centered post-transition element clusters characterized structurally by X-ray crystallography include the D 4d bicapped square antiprism Zn@In 10 8− in the intermetallic K8In10Zn as well as the centered lead clusters M@Pb10 2− in [K(2,2,2-crypt)]2[M@Pb10] (M = Ni, Pd, Pt), the C3v tetracapped trigonal prism M@In 10 10− in the intermetallic K10In10M (M = Ni, Pd, Pt), the D 5d pentagonal antiprism Pd@Bi10 4+ in Bi14PdBr16, and the D 5h pentagonal prism in the trianions M@Ge 10 3− (M = Fe, Co). The structures of the 10-vertex germanium clusters containing an interstitial magnesium atom (atomic radius 1.30 A), namely Mg@Ge 10 (z = +2, 0, −2, −4), have been optimized as models to analyze the frontier molecular orbitals of the four experimentally known 10-vertex polyhedra noted above. In this connection, the molecular orbital patterns of the most spherical D 4d bicapped square antiprism cluster Mg@Ge10 as well as that of an approximately spherical C 3v cluster Mg@Ge 10 2+ derived from the tetracapped trigonal prism exhibit the spherical harmonic ordering of the jellium model. In this ordering scheme, the 20-orbital {1s, 1p, 1d, 2s, 1f, 2p} shell is filled before any of the 1g molecular orbitals. However, the molecular orbital patterns of the oblate D 5d pentagonal antiprism cluster Mg@Ge 10 4− and the D 5h pentagonal prism cluster Mg@Ge10 no longer exhibit the spherical harmonic ordering of the jellium model. In these clusters, the polar 1f(z 3) orbital is raised to antibonding levels and the pair of hexagonal prism g molecular orbitals become bonding orbitals. The sphericities of the four distinct 10-vertex polyhedra found experimentally in centered bare 10-vertex post-transition metal clusters have been evaluated by comparing the relative energies of the cluster bonding molecular orbitals in model Mg@Ge 10 (z = −4 to +2) species with the spherical harmonic ordering predicted by the jellium model. Using this as a criterion, the D 4d bicapped square antiprismatic and C 3v tetracapped trigonal prismatic structures can be regarded as essentially spherical. However, the molecular orbital patterns of the oblate D 5d pentagonal antiprismatic and D 5h pentagonal prismatic clusters deviate significantly from spherical harmonic ordering.

Characterization of structure and dynamics of an aqueous scandium(iii) ion by an extended ab initio QM/MM molecular dynamics simulation

Hydration structure and dynamics of an aqueous Sc(III) solution were characterized by means of an extended ab initio quantum mechanical/molecular dynamical (QM/MM) molecular dynamics simulation at Hartree-Fock level. A monocapped trigonal prismatic structure composed of seven water molecules surrounding scandium(III) ion was proposed by the QM/MM simulation including the quantum mechanical effects for the first and second hydration shells. The mean Sc(III)-O bond length of 2.14 Å was identified for six prism water molecules with one capping water located at around 2.26 Å, reproducing well the X-ray diffraction data. The Sc(III)-O stretching frequency of 432 cm(-1) corresponding to a force constant of 130 N m(-1), evaluated from the enlarged QM/MM simulation, is in good agreement with the experimentally determined value of 430 cm(-1) (128 N m(-1)). Various water exchange processes in the second hydration shell of the hydrated Sc(III) ion predict a mean ligand residence time of 7.3 ps.

A new enneanuclear nickel(ii) cluster with a rectangular face-centered trigonal prism structure and cluster glass behavior

An enneanuclear nickel(II) complex with a rectangular face-centered trigonal prism structure bridged by μ(2)-pyrazolate, μ(6)-CO(3)(2-) and μ(3)-OH(-) groups was synthesized. It displays cluster glass-like magnetic behavior assigned to the single molecule magnet properties of {Ni(9)} clusters and weak intercluster interaction.

Binary Group 15 Polyazides. Structural Characterization of [Bi(N3)4]−, [Bi(N3)5]2–, [bipy·Bi(N3)5]2–, [Bi(N3)6]3–, bipy·As(N3)3, bipy·Sb(N3)3, and [(bipy)2·Bi(N3)3]2 and on the Lone Pair Activation of Valence Electrons

The binary group 15 polyazides As(N(3))(3), Sb(N(3))(3), and Bi(N(3))(3) were stabilized by either anion or donor-acceptor adduct formation. Crystal structures are reported for [Bi(N(3))(4)](-), [Bi(N(3))(5)](2-), [bipy·Bi(N(3))(5)](2-), [Bi(N(3))(6)](3-), bipy·As(N(3))(3), bipy·Sb(N(3))(3), and [(bipy)(2)·Bi(N(3))(3)](2). The lone valence electron pair on the central atom of these pnictogen(+III) compounds can be either sterically active or inactive. The [Bi(N(3))(5)](2-) anion possesses a sterically active lone pair and a monomeric pseudo-octahedral structure with a coordination number of 6, whereas its 2,2'-bipyridine adduct exhibits a pseudo-monocapped trigonal prismatic structure with CN 7 and a sterically inactive lone pair. Because of the high oxidizing power of Bi(+V), reactions aimed at Bi(N(3))(5) and [Bi(N(3))(6)](-) resulted in the reduction to bismuth(+III) compounds by [N(3)](-). The powder X-ray diffraction pattern of Bi(N(3))(3) was recorded at 298 K and is distinct from that calculated for Sb(N(3))(3) from its single-crystal data at 223 K. The [(bipy)(2)·Bi(N(3))(3)](2) adduct is dimeric and derived from two BiN(8) square antiprisms sharing an edge consisting of two μ(1,1)-bridging N(3) ligands and with bismuth having CN 8 and a sterically inactive lone pair. The novel bipy·As(N(3))(3) and bipy·Sb(N(3))(3) adducts are monomeric and isostructural and contain a sterically active lone pair on their central atom and a CN of 6. A systematic quantum chemical analysis of the structures of these polyazides suggests that the M06-2X density functional is well suited for the prediction of the steric activity of lone pairs in main-group chemistry. Furthermore, it was found that the solid-state structures can strongly differ from those of the free gas-phase species or those in solutions and that lone pairs that are sterically inactive in a chemical surrounding can become activated in the free isolated species.

The geometric, energetic, and electronic properties of charged phosphorus-doped silicon clusters, PSi n +/PSi n − (n = 1–8)

Charged phosphorus-doped small silicon clusters, PSi n +/PSi n − (n = 1−8), have been investigated using the B3LYP/6-311+G* level Kohn–Sham density functional theory (KS-DFT) method. For comparison, the geometries of neutral PSi n clusters were also optimized at the same level, though most of them have been previously reported. According to our results, cationic PSi n + clusters have ground state structures similar to those of pure silicon clusters Si n+1, with the exception of n = 5. For anionic PSi n −, most of the lowest-energy structures are in accord with Wade’s 2N+2 rule for closed polyhedra: PSi4 −, PSi5 −, PSi6 −, and PSi8 −, respectively, favor the trigonal bipyramid, tetragonal bipyramid, pentagonal bipyramid, and tricapped trigonal prism (TTP) structures, corresponding to Wade’s 2N+2 rule with N = 5, 6, 7, and 9. The structures tend to contract when the cationic species is reduced initially to the neutral species and subsequently to the anionic species, implying a strengthening interaction between atoms within the clusters on one and two electron reductions of the cationic species to the neutral and anionic species, respectively. The relative order of stability of the PSi n +/PSi n − isomers differs from that of the PSi n isomers. Cluster stability was also analyzed by adiabatic ionization potentials (AIP), adiabatic electron affinities (AEA), binding energies (BE), second-order energy differences (∆2E), and HOMO-LUMO gap values. The results indicate that PSi4 − and PSi7 − clusters are more stable than their neighboring anionic clusters and would be potential species for further mass spectrometric measurements.

Homoleptic Organometallic Anions of Ti, Zr, Hf, and Nb

Reactions of C(6)H(5)Li and 4-CH(3)C(6)H(4)Li with halides of Ti, Ir, Hf, and Nb lead to the formation of homoleptic organometallic anions of these metals. Owing to their thermal instability and their sensitivity towards H(2) O and O(2) , these compounds are characterized by single-crystal structure determinations at low temperature, whereas other physical data could only be obtained occasionally. Three pentacoordinate complex anions [Ti(C(6)H(5))(5)](-), [Ti(4-CH(3)C(6)H(4))(5)](-), and [Zr(C(6)H(5))(5)](-) have square-pyramidal structures that display only slight deviations from the ideal geometry, in contrast to the already known structures of [Ti(CH(5))(5)](-). The hexacoordinate complex anions [Zr(C(6)H(5))(6)](2-), [Zr(4-CH(3)C(6)H(4))(6)](2-), [Nb(C(6)H(5))(6)](2-), and [Nb(4-CH(3)C(6)H(4))(6)](2-) all have trigonal-prismatic structures, in accord with the known hexamethyl complex dianions. In contrast, the hexacoordinate complex anion [Hf(C(6)H(5))(6)](2)(-) has an octahedral or close to octahedral structure, in contrast to the known trigonal-prismatic structures of [Ta(C(6)H(5))(6)](-) and [Ta(4-CH(3)C(6)H(4))(6) (-). A qualitative explanation for this structural variability is given.

Synthesis and molecular structure of indium complexes based on 3,6-di-tert-butyl-o-benzoquinone. Looking for indium(i) o-semiquinolate

The interaction of 3,6-di-tert-butyl-o-benzoquinone (3,6-Q) with indium in toluene leads to the tris-o-semiquinolate derivative (3,6-SQ)(3)In (3,6-SQ - radical-anion of 3,6-Q). According to single-crystal X-ray diffraction analysis, this complex has a trigonal prismatic structure. Magnetic measurements revealed that the exchange interactions between odd electrons of the paramagnetic ligands in (3,6-SQ)(3)In are antiferromagnetic in character. The treatment of (3,6-SQ)(3)In with 2,2'-dipyridyl (Dipy) causes the displacement of one o-quinone ligand and the formation of the (3,6-SQ)In(Dipy)(3,6-Cat) (3,6-Cat - dianion of 3,6-Q) derivative containing mixed charged o-quinoid ligands. The reaction of InI with (3,6-SQ)K in THF solution is accompanied by a redox process and the potassium-indium(iii) catecholate derivative was obtained as a result. The oxidation of InI with 3,6-Q in THF produces the dimeric In(iii) iodo-catecholate complex [(3,6-Cat)(2)In·2THF]InI(2). The same derivative can be synthesized by the interaction of indium metal with a mixture of I(2) and 3,6-Q.

A luminescent gold(i)–copper(i) cluster with unprecedented carbon-centered trigonal prismatic hexagold

An unprecedented trigonal prismatic (μ(6)-C)Au(6) structure has been found in a novel gold(I)-copper(I) cluster with a hypercoordinated carbon, [CAu(6)Cu(2)(dppy)(6)](BF(4))(4); the incorporation of copper ions enhances the rigidity and integrity of the cluster and turns on the bright red solution luminescence.

Syntheses, crystal structures and properties of a new family of isostructural and isomorphous compounds of type [M(L)(NCS) 3] [M = La, Gd, Tb and Dy; L = a neutral hexadentate Schiff base

The neutral hexadentate Schiff base N-(1-pyridin-2-yl-phenylidene)-N′-[2-({2-[(1-pyridin-2-yl-phenylidene)amino]ethyl}amino)ethyl]ethane-1,2-diamine (L) was used to synthesize new lanthanide(III) complexes of type [M(L)(NCS) 3] (M = La, 1; Gd, 2; Tb, 3; Dy, 4). Single crystal X-ray diffractometry reveals the tricapped trigonal prismatic structure of all the compounds. Variable-temperature magnetic susceptibility measurements in 2– 4 show the expected χ M T versus T plot for an isolated, mononuclear lanthanide(III) ion with 4f 7, 4f 8 and 4f 9 configurations, respectively following Curie Law.

From closo to isocloso Structures and Beyond in Cobaltaboranes with 9 to 12 Vertices

Density functional theory (DFT) studies predict the dianions CpCoB(n-1)H(n-1)(2-) (n = 9, 10, 11, 12; Cp = eta(5)-C(5)H(5)) to have structures based on the most spherical deltahedra found in the isoelectronic boranes B(n)H(n)(2-). In the CpCoB(8)H(8)(2-) dianion the non-equivalent structures with the cobalt atom at a degree 4 vertex and at a degree 5 vertex are essentially degenerate in terms of energy (within approximately 1 kcal/mol). For the CpCoB(n-1)H(n-1)(2-) dianions (n = 10, 11, 12) the cobalt atom prefers energetically the vertices of the lowest possible degree (four for n = 10 and 11, five for n = 12). Structures for the neutral species CpCoB(n-1)H(n-1) (n = 10, 11, 12) based on isocloso deltahedra with the cobalt atom at a degree 6 vertex are preferred energetically by 9, 19, and 53 kcal/mol, respectively, over alternative structures. However, for CpCoB(8)H(8) the closo tricapped trigonal prismatic structure with the cobalt atom at a degree 5 vertex is energetically preferred by approximately 9 kcal/mol over the isocloso deltahedral structure with the cobalt atom at a degree 6 vertex. The lowest energy structures predicted for the dications CpCoB(8)H(8)(2+) and CpCoB(9)H(9)(2+) are highly oblate (flattened) deltahedra with the cobalt atom at a degree 7 vertex. A complicated potential energy surface was found for CpCoB(10)H(10)(2+) including non-deltahedral structures with a single quadrilateral or pentagonal face. The predicted lowest energy structures for both CpCoB(11)H(11) and CpCoB(11)H(11)(2+) are based on the same 12-vertex deltahedron with three degree 6, six degree 5, and three degree 4 vertices, and thus topologically different from the regular icosahedron normally found in boron chemistry.

Preparation of a bis(hydrosulfido) complex of Mo having a tetraphosphine co-ligand and its transformation into MoRh2 and MoIr2 mixed-metal sulfido clusters

The treatment of a Mo(0) complex with a tetraphosphine co-ligand [Mo(kappa(4)-P4)(Ph(2)PCH(2)CH(2)PPh(2))] (P4 = meso-o-C(6)H(4)(PPhCH(2)CH(2)PPh(2))(2)) with H(2)S gas in toluene at room temperature afforded [Mo(SH)(2)(kappa(4)-P4)] (1), which demonstrates the first Mo(II) bis(hydrosulfido) complex. Its trigonal-prismatic structure rarely observed for Mo(II) complexes has been determined by X-ray analysis. The reactions of 1 with [RhCl(CO)(2)](2) and [IrCl(CO)(2)(p-toluidine)] in the presence of NEt(3) resulted in the formation of the sulfido-bridged trinuclear clusters [Mo(kappa(4)-P4)(mu(3)-S)(2){Rh(CO)(2)}(2)] (4) and [Mo(kappa(4)-P4)(mu(3)-S)(2){Ir(CO)(2)}(2)] (5), respectively. The X-ray diffraction study has disclosed that the MoRh(2) cluster 4 in the solid state exists in two isomeric forms arising from the different orientation of P4 to the trigonal-prismatic Mo center, whereas the crystals of the Ir analogue 5 contains the molecules corresponding to only one of the two isomers observed for 4. The fluxional features of 1, 4, and 5 in solution have been confirmed by VT NMR studies.

Synthesis, spectroscopy and thermal behavior of new lead(II) complexes derived from S-methyl/benzyldithiocarbazates (SMDTC/SBDTC): X-ray crystal structure of [Pb(SMDTC)(NO 3) 2

Lead(II) complexes of S-methyldithiocarbazate (SMDTC), [Pb(SMDTC)(NO 3) 2] ( 1) and S-benzyldithiocarbazate (SBDTC), [Pb(SBDTC)(NO 3) 2] ( 2) have been synthesized for the first time and characterized by elemental analysis, IR and TGA techniques. The complexes were obtained by addition of the appropriate ligand to an aqueous ethanolic solution of lead(II) nitrate in 1:1 molar ratio. The X-ray crystal structure of complex 1 has been determined by single crystal X-ray diffractometry. In complex 1, lead(II) is in a nine coordinated sphere with seven oxygen atoms of the nitrate groups and thione sulfur, β-nitrogen of neutral bidentate NS chelating ligand. Three nitrate groups act as bidentate chelating whereas the fourth nitrate group is coordinating to the central lead(II) and at the same time it bridges with neighboring lead(II) atom. Coordination geometry of the central lead(II) atom has a tricapped trigonal prismatic arrangement with streochemically inactive lone pair. The lead atoms are linked into polymeric chains and these chains form twin polymeric ribbons linked through bridging oxygen atoms. The N–H⋯O hydrogen bond network between N SMDTC and O nitrate atom leads to self-assembled molecular conformation and stabilizes the crystal structure. The complex 2 with similar spectral and thermal behavior is expected to have a tricapped trigonal prismatic structure. The thermal behavior studies shows that the complexes start to decompose at relatively low temperature ( ca. 110 °C) to give PbS residue.

Hexanuclear Cobalt Carbonyl Carbide Clusters: The Interplay between Octahedral and Trigonal Prismatic Structures

The six-vertex cobalt carbonyl clusters [Co6C(CO)n](2-) (n = 12, 13, 14, 15, 16) with an interstitial carbon atom have been studied by density functional theory (DFT). These DFT studies indicate that the experimentally known structure of [Co6C(CO)15](2-) consisting of a Co6 trigonal prism with each of its edges bridged by carbonyl groups is a particularly stable structure lying more than 20 kcal/mol below any other [Co6C(CO)15](2-) structure. Addition of a CO group to this [Co6C(CO)15](2-) structure gives the lowest energy [Co6C(CO)16](2-) structure, also a Co6 trigonal prism with one of the vertical edges bridged by two CO groups and the remaining eight edges each bridged by a single CO group. However, this [Co6C(CO)16](2-) structure is thermodynamically unstable with respect to CO loss reverting to the stable trigonal prismatic [Co6C(CO)15](2-). This suggests that 15 carbonyl groups is the maximum that can be attached to a Co6C skeleton in a stable compound. The lowest energy structure of [Co6C(CO)14](2-) has a highly distorted octahedral Co6 skeleton and is thermodynamically unstable with respect to disproportionation to [Co6C(CO)15](2-) and [Co6C(CO)13](2-). The lowest energy [Co6C(CO)13](2-) structure is very similar to a known stable structure with an octahedral Co6 skeleton. The lowest energy [Co6C(CO)12](2-) structure is a relatively symmetrical D3d structure containing a carbon-centered Co6 puckered hexagon in the chair form.

Extended X-ray Absorption Fine Structure and Nuclear Resonance Vibrational Spectroscopy Reveal that NifB-co, a FeMo-co Precursor, Comprises a 6Fe Core with an Interstitial Light Atom

NifB-co, an Fe-S cluster produced by the enzyme NifB, is an intermediate on the biosynthetic pathway to the iron molybdenum cofactor (FeMo-co) of nitrogenase. We have used Fe K-edge extended X-ray absorption fine structure (EXAFS) spectroscopy together with (57)Fe nuclear resonance vibrational spectroscopy (NRVS) to probe the structure of NifB-co while bound to the NifX protein from Azotobacter vinelandii. The spectra have been interpreted in part by comparison with data for the completed FeMo-co attached to the NafY carrier protein: the NafY:FeMo-co complex. EXAFS analysis of the NifX:NifB-co complex yields an average Fe-S distance of 2.26 A and average Fe-Fe distances of 2.66 and 3.74 A. Search profile analyses reveal the presence of a single Fe-X (X = C, N, or O) interaction at 2.04 A, compared to a 2.00 A Fe-X interaction found in the NafY:FeMo-co EXAFS. This suggests that the interstitial light atom (X) proposed to be present in FeMo-co has already inserted at the NifB-co stage of biosynthesis. The NRVS exhibits strong bands from Fe-S stretching modes peaking around 270, 315, 385, and 408 cm(-1). Additional intensity at approximately 185-200 cm(-1) is interpreted as a set of cluster "breathing" modes similar to those seen for the FeMo-cofactor. The strength and location of these modes also suggest that the FeMo-co interstitial light atom seen in the crystal structure is already in place in NifB-co. Both the EXAFS and NRVS data for NifX:NifB-co are best simulated using a Fe 6S 9X trigonal prism structure analogous to the 6Fe core of FeMo-co, although a 7Fe structure made by capping one trigonal 3S terminus with Fe cannot be ruled out. The results are consistent with the conclusion that the interstitial light atom is already present at an early stage in FeMo-co biosynthesis prior to the incorporation of Mo and R-homocitrate.

Energy Migration in a Self-Assembled Nonameric Porphyrinic Molecular Box

We describe the construction of self-assembled double-decker porphyrin arrays built up from two covalently connected trimeric Zn-porphyrin units that are joined together by metal-coordination bonds with diamine ligands. We used three different types of diamine ligands: 1,4-diaza[2.2.2]bicyclooctane (DABCO), 4,4'-bipyridine (BIPY), and 5,15-bis(4-pyridyl)-10,20-diphenylporphyrin (DPYP). The ligands act as pillars, through two axial coordination bonds with the porphyrinic Zn(II) ions, to block the planes of the porphyrin units in an almost cofacial orientation and inducing the formation of a trigonal prismatic structure. The spectroscopic and photophysical properties of the Zn-trisporphyrin component were determined as well as those of the resulting multimolecular cagelike assemblies. The double-decker assembly with DPYP as the pillars constitutes a nonameric porphyrin aggregate. Although this assembly is thermodynamically less stable than those containing DABCO or BIPY, efficient photoinduced energy transfer occurs (96% yield) from the trisporphyrin base units to the DPYP side walls. The rate of the energy-transfer process is in good agreement with that calculated for a dipole-dipole (Förster) mechanism corrected for the unfavorable orientation geometry of the donor and the axially bound acceptor.

Effects of halogen substitution on the properties of eight- and nine-vertex closo-boranes

The symmetry constrained geometries of the eight- and nine-vertex polyhedral boranes and haloboranes BnXnz (n = 8 and 9; X = H, F and Cl; z = -2, -1 and 0) were optimized at the B3LYP/6-311+G(d) level and their nucleus-independent chemical shifts (NICS) were calculated using the GIAO method with Kohn-Sham orbitals. Substitution of halogens on borane cages was found to significantly impact not only the geometric but also magnetic properties. Multiple fluorine substituents cause a deviation from the Wade-Mingos skeletal electron rules in B8F8(2-), resulting in a distortion from the expected D2d bisdisphenoid to a C2v nido type bicapped trigonal prism. However, all of the nine-vertex cages B9X9z retain the D3h tricapped trigonal prismatic structure of B9H9(2-). The presence of halogen substituents was found to enhance the three-dimensional diatropic ring currents within the dianionic borane cages B8X8(2-) and B9X9(2-). For the neutral structures the NICS values indicate BnFn to be aromatic, BnCln to be essentially non-aromatic, and BnHn to be antiaromatic (n = 8, 9).

Monocapped Trigonal‐Prismatic Transition‐Metal Heptaazides: Syntheses, Properties, and Structures of [Nb(N3)7]2− and [Ta(N3)7]2−

Structures with a bang: The title anions are the first examples of doubly charged heptaazido anions and structurally characterized transition-metal heptaazides. Contrary to the either monocapped octahedral or pentagonal-bipyramidal actinide trianion [U(N3)7]3−, but in accord with the [NbF7]2− anion, the title anions have monocapped trigonal-prismatic structures (see picture; M=Nb, Ta).