Alexander S. Ivanov

Inorganic Double-Helix Structures of Unusually Simple Lithium–Phosphorus Species†

The double helix represents one of the most fascinating geometric structures in nature. Helical structures can be either right-handed or left-handed depending on the direction of the rotation. In other words they possess the chirality property, which is vital for every living organism. Chiral molecules lack an internal plane of symmetry and thus have a nonsuperposable mirror image. It turns out that for organic compounds nature decides by itself whether the most preferable structure will be right-handed or left-handed, but questions regarding such differentiation in the inorganic world have not yet been answered. In chemistry and biology, the term double helix usually refers to the structure formed by double-stranded molecules of nucleic acids such as DNA, discovered by Watson and Crick, and RNA. Watson and Crick stressed: “It has not escaped our notice that the specific pairing we have postulated immediately suggests a possible copying mechanism for the genetic material.” There are many other organic polymers with helical structures and proteins which have substructures known as a -helices. In spite of the diversity and significance of the role of double-helix structures in evolution and metabolism, they are very rare in inorganic chemistry. In 1993 V. Soghomonian et al. showed that very complicated inorganic solids can be self-assembled from structurally simple precursors by the hydrothermal synthesis of vanadium phosphate and [(CH3)2NH2]K4[V10O10(H2O)2(OH)4(PO4)7]·4 H2O, which contains chiral double helices formed from interpenetrating spirals of vanadium oxo pentamers bonded together by P5 + ions. After that, there were also many successful syntheses of inorganic compounds with helical structures. 4] Thus, while some advances have been achieved for inorganic materials with a single-helical geometry, reports of counterparts with the double-helix structure are deficient. Recently, double-helical silicon microtubes and double-helical carbon nanotube (CNT) arrays were prepared. 6] However, there is no structural model at the atomic level to explain the Si and C tubular double helices. Here we report the theoretical prediction of the existence of double-helix structures in the series of LixPx (x = 5–9) clusters and in an infinite LiP chain and compare them with bulk phases of LiP. It was surprising to discover the doublehelix structures in rather simple species consisting of only two elements: lithium and phosphorus. Our initial goal was to probe electronic transmutation for LixPx clusters by making bonded structures which obey the Zintl-rule (where the less electronegative lithium atom donates an electron to the more electronegative phosphorus atom, resulting in each phosphorus bearing a negative charge) with the anticipation that structures similar to sulfur compounds would be formed. However, the unexpected double helices were found to be either global minimum structures or low-lying isomers. We performed an unbiased quantum-chemical search for LixPx (x = 5–9) clusters using a Coalescence Kick [9] program written by B. B. Averkiev initially at the B3LYP/3-21G level of theory. The Coalescence Kick method subjects large populations of randomly generated structures to a coalescence procedure in which all atoms are pushed gradually to the molecular center of mass to avoid the generation of fragmented structures and then optimizes them to the nearest local minima. All low-lying isomers found by this method were reoptimized with follow-up frequency calculations at the B3LYP level of theory using the 6-311 + G* basis set. The total energies of the lowest isomers of Li5P5, Li6P6, and Li7P7 stoichiometries were calculated at the CCSD(T)/CBS// B3LYP/6-311 + G* level of theory by extrapolating CCSD(T)/cc-pvDZ//B3LYP/6-311 + G* and CCSD(T)/ccpvTZ//B3LYP/6-311 + G* to the infinite basis set using the Truhlar formula. Additional calculations of the three lowest isomers for each stoichiometry were also performed (see the Theoretical Section). In Figure 1 we present the rightand left-handed double helices for the Li5P5, Li6P6, Li7P7, Li8P8, and Li9P9 species. A more extensive set of alternative isomers found in our global minimum search is summarized in the Supporting information (Figures S1–S5). For the Li5P5 stoichiometry the double-helix structure is the second lowest isomer, which is [*] A. S. Ivanov, Prof. Dr. A. I. Boldyrev Department of Chemistry and Biochemistry Utah State University, Logan, UT 84322 (USA) E-mail: a.i.boldyrev@usu.edu Homepage: http://ion.chem.usu.edu/~boldyrev/

The IX (X=O,N,C) Double Bond in Hypervalent Iodine Compounds: Is it Real?†

IX (X=O, N, C) bonding was analyzed in the related hypervalent iodine compounds based on the adaptive natural density partitioning (AdNDP) approach. The results confirm the presence of a I→X σ dative bond, as opposed to the widely used IX notation. A clear formulation of the electronic structure of these hypervalent iodine compounds would be useful in establishing reaction mechanisms and electronic structures in bioinorganic problems of general applicability.

On the suppression mechanism of the pseudo-Jahn-Teller effect in middle E6 (E = P, As, Sb) rings of triple-decker sandwich complexes.

Quantum chemical calculations of the CpMoE(6)MoCp (E = P, As, Sb) triple-decker sandwich complexes showed that E(6) fragments in the central decks of the complexes are planar. Analysis of molecular orbitals involved in vibrational coupling demonstrated that filling the unoccupied molecular orbitals involved in vibronic coupling with electron pairs of Mo atoms suppresses the PJT effect in the CpMoE(6)MoCp (E = P, As, Sb) sandwich, with the E(6) ring becoming planar (D(6h)) upon complex formation. The AdNDP analysis revealed that bonding between C(5)H(5)(-) units and Mo atoms has a significant ionic contribution, while bonding between Mo atoms and E(6) fragment becomes appreciably covalent through the δ-type M → L back-donation mechanism.

On the way to the highest coordination number in the planar metal-centred aromatic Ta©B10− cluster: Evolution of the structures of TaBn− (n = 3–8)

The structures and chemical bonding of TaB(n)(-) (n = 3-8) clusters are investigated systematically to elucidate the formation of the planar metal-centred aromatic borometallic cluster, Ta©B10(-) (the

Peculiar Transformations in the CxHxP4-x (x = 0-4) Series.

In the current work, we performed a systematic study of the CxHxP4-x (x = 0-4) series using an unbiased CK global minimum and low-lying isomers search for the singlet and triplet P4-C4H4 species at the B3LYP/6-31G** level of theory. The selected lowest isomers were recalculated at the CCSD(T)/CBS//B3LYP/6-311++G** level of theory. We found that the transition from a three-dimensional tetrahedron-like structure to a planar structure occurs at x = 3, where planar isomers become much more stable than the tetrahedral structures due to significantly stronger π bonds between carbon atoms in addition to increasing strain energy at the carbon atom in the tetrahedral environment.

Inorganic Double-Helix Nanotoroid of Simple LithiumPhosphorus Species

A theoretical study of Li90 P90 , which possesses a circular double-helix structure that resembles the Watson-Crick DNA structure, is reported. This is a new bonding motif in inorganic chemistry. The calculations show that the molecule might become synthesized and that it could be a model for other inorganic species which possess a double-helix structure.

“Straining” to Separate the Rare Earths: How the Lanthanide Contraction Impacts Chelation by Diglycolamide Ligands

The subtle energetic differences underpinning adjacent lanthanide discrimination are explored with diglycolamide ligands. Our approach converges liquid-liquid extraction experiments with solution-phase X-ray absorption spectroscopy (XAS) and density functional theory (DFT) simulations, spanning the lanthanide series. The homoleptic [(DGA)3Ln]3+ complex was confirmed in the organic extractive solution by XAS, and this was modeled using DFT. An interplay between steric strain and coordination energies apparently gives rise to a nonlinear trend in discriminatory lanthanide ion complexation across the series. Our results highlight the importance of optimizing chelate molecular geometry to account for both coordination interactions and strain energies when designing new ligands for efficient adjacent lanthanide separation for rare-earth refining.

Reliable predictions of unusual molecules.

Quantum chemistry can today be employed to invent new molecules and investigate their properties and chemical bonding. However, the predicted species must be viable in order to be synthesized by experimentalists. In this perspective article we describe the technology of reliable theoretical predictions and show how understanding of chemical bonding in studied chemical systems could help to design new molecular structures. We also provide a short overview of successfully predicted and already produced (in some cases) planar hypercoordinate species to demonstrate that the consistent theoretical prediction of viable molecules with unusual structures and properties is now a reliable tool for exploring new, yet unknown molecules, clusters, nanomaterials and solids.

Ba and Sr Binary Phosphides: Synthesis, Crystal Structures, and Bonding Analysis

Synthesis, crystal structures, and chemical bonding are reported for four binary phosphides with different degrees of phosphorus oligomerization, ranging from isolated P atoms to infinite phosphorus chains. Ba3P2 = Ba4P(2.67)□(0.33) (□ = vacancy) crystallizes in the anti-Th3P4 structure type with the cubic space group I4̅3d (no. 220), Z = 6, a = 9.7520(7) Å. In the Ba3P2 crystal structure, isolated P(3-) anions form distorted octahedra around the Ba(2+) cations. β-Ba5P4 crystallizes in the Eu5As4 structure type with the orthorhombic space group Cmce (no. 64), Z = 4, a = 16.521(2) Å, b = 8.3422(9) Å, c = 8.4216(9) Å. In the crystal structure of β-Ba5P4, one-half of the phosphorus atoms are condensed into P2(4-) dumbbells. SrP2 and BaP2 are isostructural and crystallize in the monoclinic space group P2₁/c (no. 14), Z = 6, a = 6.120(2)/6.368(1) Å, b = 11.818(3)/12.133(2) Å, c = 7.441(2)/7.687(2) Å, β = 126.681(4)/126.766(2)° for SrP2/BaP2. In the crystal structures of SrP2 and BaP2, all phosphorus atoms are condensed into ∞(1)P(1-) cis-trans helical chains. Electronic structure calculations, chemical bonding analysis via the recently developed solid-state adaptive natural density partitioning (SSAdNDP) method, and UV-vis spectroscopy reveal that SrP2 and BaP2 are electron-balanced semiconductors.

Bis-lactam-1,10-phenanthroline (BLPhen), a New Type of Preorganized Mixed N,O-Donor Ligand That Separates Am(III) over Eu(III) with Exceptionally High Efficiency

We report a new family of preorganized bis-lactam-1,10-phenanthroline (BLPhen) complexants that possess both hard and soft donor atoms within a convergent cavity and show unprecedented extraction strength for the trivalent f-block metal ions. BLPhen ligands with saturated and unsaturated δ-lactam rings have notable differences in their affinity and selectivity for Am(III) over Eu(III), with the latter being the most selective mixed N,O-donor extractant of Am(III) reported to date. Saturated BLPhen was crystallized with five Ln(III) nitrates to form charge-neutral 1:1 complexes in the solid state. DFT calculations further elaborate on the variety of effects that dictate the performance of these preorganized compounds.