Marc Benard

Two Linear Undecanickel Mixed-Valence Complexes: Increasing the Size and the Scope of the Electronic Properties of Nickel Metal Strings**

The importance of one-dimensional (1D) transition-metal complexes stems from their ability to provide a fundamental understanding of metal–metal interactions and electron transport along an extended metal-atom chain (EMAC), and from the perspective of taking advantage of their specific properties for potential applications, such as molecular metal wires and switches. A series of string complexes of oligo-apyridylamino ligands ranging from 3 to 9 core metal atoms has been synthesized and characterized by Cotton s group and our group. Attempts to characterize such very long EMACs with high electron conductivity were hindered by the synthetic difficulties rapidly increasing with the size of the metal chain. We synthesized [Ni9(m9-peptea)4Cl2] ten years ago, but all attempts to characterize a longer chain of Ni atoms have, to date, been unsuccessful, owing to very low yields and to the instability of the target compound, probably because of the high flexibility of large pyridylamino ligands. Recently we developed a new family of ligands by substituting rigid and potentially redox active naphthyridine (na) groups for the pyridine (py) rings. Naphthyridinemodulated ligands stabilize nickel ions in a low oxidation state, giving rise to mixed-valent [Ni2(napy)4] 3+ units (napy= naphthyridine). Using this strategy, we obtained a series of stable, low-oxidation-state-nickel string complexes combining mixed-valency, a property important in the development of novel electronic materials, with an enhanced electron mobility, which is able to increase the conductance of molecular metal wires. We report a new tetranaphthyridyltriamine ligand, N-(2(1,8-naphthyridin-7-ylamino)-1,8-naphthyridin-7-yl)-N-(1,8naphthy-ridin-2-yl)-1,8-naphthyridine-2,7-diamine (H3tentra) and two undecanickel complexes of the deprotonated tentra trianion, [Ni11(tentra)4Cl2](PF6)4 (1) and [Ni11(tentra)4(NCS)2](PF6)4 (2). The ligand H3tentra was synthesized on the basis of Buchwald s palladium-catalyzed procedures by the crosscoupling of bis(2-chloro-1,8-naphthyridin-7-yl)amine and 2amino-1,8-naphthyridine. Undecametallic complex [Ni11(tentra)4Cl2](PF6)4 (1) was obtained by the reaction of anhydrous NiCl2 with the H3tentra ligand in an argon atmosphere employing naphthalene as solvent and tBuOK as a base to deprotonate the amine groups. The thiocyanate species (2) was obtained from 1 by an axial ligand exchange reaction. The crystal structures of 1 and 2 are shown in Figure 1 and the Supporting Information Figure 1S, respectively. Both 1 and 2 are tetracationic molecules associated each with four PF6 counterions. They crystallize in unusually large cells, with one dimension exceeding 50 . The Ni11 chain of 1 and 2 is linear and wrapped in a helical manner by four tentra trianions. In both complexes, the atoms of the axial ligands are collinear with the Ni11 axis; the molecular lengths are 27.7 and 32.4 for 1 and 2, respectively. These are the longest EMAC complexes reported to date. The nature of the axial ligand does not significantly affect the metal–metal bond length, and no obvious structural change is observed for compound 2 with respect to 1. Therefore, we will only analyze the structure of 1 in detail. Selected bond lengths for 1 are displayed in Figure 1c together with the corresponding values obtained from geometry optimization at the DFT/B3LYP level. Molecule 1 consists of eleven nickel atoms in a linear chain with the Ni-Ni-Ni bond angles in the range of 179–1808. The N-Ni-Ni-N torsion angles for adjacent nickel are between 13.0 and 18.78, much smaller than those in oligo-a-pyridylamino ligand EMAC complexes (ca. 22.58). Metal–metal distances usually decrease from the end to the center of the chain in both nickel and cobalt EMACs of oligo-a-pyridyl[*] Dr. R. H. Ismayilov, Dr. W.-Z. Wang, Dr. G.-H. Lee, S.-A. Hua, Prof. Dr. S.-M. Peng Department of Chemistry, National Taiwan University 1, Sec. 4, Roosevelt Road, Taipei, 106 (Taiwan, ROC) Fax: (+886)2-8369-3765 E-mail: smpeng@ntu.edu.tw

Bond-stretch isomerism in strained inorganic molecules and in transition metal complexes: a revival?

A decade after Parkin’s statement that “there is presently no definitive evidence of bond-stretch isomerism in (transition metal) complexes”, bond-stretch isomerism (BSI) is revisited, with special emphasis on inorganic and organometallic chemistry. It appears that the characterization of real bond-stretch isomers, distinct, separable and stable at room temperature, remains a very rare event. In fact, the vast majority of reports point out the case of neighboring species that were characterized with structurally distinct cores or frameworks, depending on the electronic or steric influence of peripheral substituents. In such cases, the characterization of separate energy minima on the ground state potential energy surface of an isolated molecule provides a criterion for proving the relevance of the BSI concept and also a guideline toward the synthesis of real bond-stretch isomers by means of a proper tuning of the substituents. Functionalized polyoxometalates and complexes stabilizing the (Cr3)6+ linear framework might provide illustrations of this strategy in the near future.

Gaussian basis sets for the transition metals of the first and second series

Gaussian basis sets consisting of, respectively, (13s, 7p, 5d) and (14s, 8p, 7d) Gaussian functions have been optimized for the transition metal atoms of the first and second series. The optimization criteria and the applicability of these atomic sets for molecular calculations are discussed.

From Lindqvist and Keggin ions to electronically inverse hosts: Ab initio modelling of the structure and reactivity of polyoxometalates

Abstract This review reports the ab initio Hartree–Fock and DFT calculations which have been carried out recently in our two groups in order to investigate the electronic structure of polyoxometalate clusters, either totally oxidized like (V 10 O 28 ) 6− or partly reduced such as [PMo 12 O 40 (VO) 2 ] 5− . The approach of protons or cationic groups to the external coating of oxygen atoms and their preferred site of fixation can be predicted from the topology of the computed distribution of molecular electrostatic potentials (MEP). The MEP distribution can also be used to get a better understanding of the formation of inclusion and encapsulation complexes. Hemispherical carcerands like (V 12 O 32 ) 4− develop a dipolar field in the accessible part of their cavity susceptible to attract small molecules with a permanent dipole like RCN (R=CH 3 , C 6 H 5 ). The “electronically inverse” host anions known to encapsulate anionic species are formed in solution by means of a template mechanism which tends to maximize the electrostatic potential at the place of the guest anion. A correlation is provided between the topology of the host and its MEP distribution which explains, from simple geometric considerations, the differences between electronically normal and electronically inverse hosts, and shows that the host cage tends to get adapted not only to the shape of the guest molecule, but also to its electrostatic potential distribution.

A study of Hartree–Fock instabilities in Cr2(O2CH)4 and Mo2(O2CH)4

The Hartree–Fock instability of the SCF wave functions obtained for the quadruply bonded complexes Cr2(O2CH)4 and Mo2(O2CH)4 is studied at the ab initio level. Several cases of singlet, nonsinglet, and nonreal instabilities are found for each of these complexes at the experimental value of the metal–metal bond length. For Cr2(O2CH)4, these instabilities are still found at much shorter Cr–Cr distances. The ’’broken symmetry’’ wave functions corresponding to some of the singlet and nonsinglet instability roots of Cr2(O2CH)4 are computed and analyzed. Each of these broken‐symmetry wave functions is characterized by an important decrease of the bonding character of the metal–metal interaction with respect to the symmetry‐adapted wave function. Singlet instability can thus be considered as an attempt to reduce the strong bonding character imposed by the symmetry constraints to the whole class of D4h quadruply bonded complexes. The case of MoCr(O2CH)4 for which the symmetry of the system is broken from D4h to C...

A program system for ab initio mo calculations on vector and parallel processing machines I. Evaluation of integrals

Abstract We present a program system for ab initio molecular orbital calculations on vector and parallel computers. The present article is devoted to the computation of one- and two-electron integrals over contracted Gaussian basis sets involving s-, p-, d- and f-type functions. The McMurchie and Davidson (MMD) algorithm has been implemented and parallelized by distributing over a limited number of logical tasks the calculation of the 55 relavant classes of integrals. All sections of the MMD algorithm have been efficiently vectorized, leading to a scalar/vector ratio of 5.8. Different algorithms are proposed and compared for an optimal vectorization of the contraction of the “intermediate integrals” generated by the MMD formalism. Advantage is taken of the dynamic storage allocation for tunning the lenght of the vector loops (i.e. the size of the vectorization buffer) as a function of (i) the total memory available for the job, (ii) the number of logical tasks defined by the user (≤13), and (iii) the storage requested by each specific class of integrals. Test calculations carried out on a CRAY-2 computer show that the average number of finite integrals computed over a (s,p,d,f) CGTO basis set is about 118000 per second and per processor. The combination of vectorization and parallelism on this 4-processor machine reduces the CPU time by a factor larger than 20 with respect to the scalar and sequential performance.

Symmetry breaking and ionization from the 3d and 4s shells in copper clusters

Abstract SCF calculations indicate that the 3d hole in Cu + 2 is localized. while the 4s hole prefers to be delocalized. Extrapolation to copper clusters indicates that the d band is overlapped by the 5 band for clusters as small as Cu 5 . Overlap of the s and d bands does not seem to be an appropriate criterion for convergence of properties of clusters to bulk properties.

A program system for ab initio MO calculations on vector and parallel processing machines: III. Integral reordering and four-index transformation

This series of three papers presents a program system for ab initio molecular orbital calculations on vector and parallel computers. Part III is devoted to the four-index transformation on a molecular orbital basis of size NMO of the file of two-electron integrals (pq∥rs) generated by a contracted Gaussian set of size NATO (number of atomic orbitals). A fast Yoshimine algorithm first sorts the (pq∥rs) integrals with respect to index pq only. This file of half-sorted integrals labelled by their rs-index can be processed without further modification to generate either the transformed integrals or the supermatrix elements. The large memory available on the CRAY-2 has made possible to implement the transformation algorithm proposed by Bender in 1972, which requires a core-storage allocation varying as (NATO)3. Two versions of Benders algorithm are included in the present program. The first version is an in-core version, where the complete file of accumulated contributions to transformed integrals is stored and updated in central memory. This version has been parallelized by distributing over a limited number of logical tasks the NATO steps corresponding to the scanning of the most external loop. The second version is an out-of-core version, in which twin fires are alternatively used as input and output for the accumulated contributions to transformed integrals. This version is not parallel. The choice of one or another version and (for version 1) the determination of the number of tasks depends upon the balance between the available and the requested amounts of storage. The storage management and the choice of the proper version are carried out automatically using dynamic storage allocation. Both versions are vectorized and take advantage of the molecular symmetry.

A program system for ab initio MO calculations on vector and parallel processing machines: II. SCF closed-shell and open-shell iterations

Abstract This series of three papers presents a program system for ab initio molecular orbital calculations on vector and parallel computers. Part II is devoted to SCF iterations on closed-shell and open-shell configurations starting from a file of two-electron integrals on the basis of contracted Gaussians (CGTOs). In a preliminary step, the two-electron integrals ( pq ‖ rs ) are reordered according to increasing values of index pq = p ( p −1)/2+ q . Then, in the first SCF iteration step, a file of semi-ordered P supermatrix elements (or P and Q supermatrix elements in the open-shell case) is generated from the file of semi-ordered integrals. This file is processed at each iteration step in an efficient vector loop to generate the electron repulsion matrix. Convergence is automatically controlled through level-shifting techniques. The most time-consuming parts are the integral sorting and the generation of the P supermatrix, which are carried out only once. Subsequent SCF iteration steps respectively require 0.9 s and 1.7 s for the process of 10 7 supermatrix elements by the closed-shell and the open-shell programs on a CRAY-2 processor.

Broken-symmetry hartee-fock description of deep core and outer valence hole states: The 3dσu and 4dσu hole states of Ag2+

Abstract Symmetry-adapted and broken-symmetry Hartree-Fock and CI calculations have been carried out on the 3dσ u and 4dσ u hole states of Ag 2 + at various distances. The Cižek-Paldus doublet stability problem is investigated. Calculations confirm the reliability of broken-symmetry solutions for core hole states, but show that valence hole states are better described by a CI expansion restricted to single excitations with respect to the zeroth-order description of the ion.