The preparation and characterization of a series of magnesium(II) iodide complexes incorporating beta-diketiminate ligands of varying steric bulk and denticity, namely, [(ArNCMe)(2)CH](-) (Ar=phenyl, ((Ph)Nacnac), mesityl ((Mes)Nacnac), or 2,6-diisopropylphenyl (Dipp, (Dipp)Nacnac)), [(DippNCtBu)(2)CH](-) ((tBu)Nacnac), and [(DippNCMe)(Me(2)NCH(2)CH(2)NCMe)CH](-) ((Dmeda)Nacnac) are reported The complexes [((Ph)Nacnac)MgI(OEt(2))], [((Mes)Nacnac)MgI(OEt(2))], [((Dmeda)Nacnac)MgI(OEt(2))], [((Mes)Nacnac)MgI(thf)], [((Dipp)Nacnac)MgI(thf)], [((tBu)Nacnac)MgI], and [((tBu)Nacnac)MgI(DMAP)] (DMAP=4-dimethylaminopyridine) were shown to be monomeric by X-ray crystallography In addition, the related beta-diketiminato beryllium and calcium iodide complexes, [((Mes)Nacnac)BeI] and [{((Dipp)Nacnac)CaI(OEt(2))}(2)] were prepared and crystallographically characterized The reductions of all metal(II) iodide complexes by using various reagents were attempted In two cases these reactions led to the magnesium(I) dimers, [((Mes)Nacnac)MgMg((Mes)Nacnac)] and [((tBu)Nacnac)MgMg((tBu)Nacnac)] The reduction of a 1:1 mixture of [((Dipp)Nacnac)MgI(OEt(2))] and [((Mes)Nacnac)MgI(OEt(2))] with potassium gave a low yield of the crystallographically characterized complex [((Dipp)Nacnac)Mg(mu-H)(mu-I)Mg((Mes)Nacnac)] All attempts to form beryllium(I) or calcium(I) dimers by reductions of [((Mes)Nacnac)BeI], [{((Dipp)Nacnac)CaI(OEt(2))}(2)], or [{((tBu)Nacnac)CaI(thf)}(2)] have so far been unsuccessful The further reactivity of the magnesium(I) complexes [((Mes)Nacnac)MgMg((Mes)Nacnac)] and [((tBu)Nacnac)MgMg((tBu)Nacnac)] towards a variety of Lewis bases and unsaturated organic substrates was explored These studies led to the complexes [((Mes)Nacnac)Mg(L)Mg(L)((Mes)Nacnac)] (L=THF or DMAP), [((Mes)Nacnac)Mg(mu-AdN(6)Ad)Mg((Mes)Nacnac)] (Ad=1-adamantyl), [((tBu)Nacnac)Mg(mu-AdN(6)Ad)Mg((tBu)Nacnac)], and [((Mes)Nacnac)Mg(mu-tBu(2)N(2)C(2)O(2))Mg((Mes)Nacnac)] and revealed that, in general, the reactivity of the magnesium(I) dimers is inversely proportional to their steric bulk The preparation and characterization of [((tBu)Nacnac)Mg(mu-H)(2)Mg((tBu)Nacnac)] has shown the compound to have different structural and physical properties to [((tBu)Nacnac)MgMg((tBu)Nacnac)] Treatment of the former with DMAP has given [((tBu)Nacnac)Mg(H)(DMAP)], the X-ray crystal structure of which disclosed it to be the first structurally authenticated terminal magnesium hydride complex Although attempts to prepare [((Mes)Nacnac)Mg(mu-H)(2)Mg((Mes)Nacnac)] were not successful, a neutron diffraction study of the corresponding magnesium(I) complex, [((Mes)Nacnac)MgMg((Mes)Nacnac)] confirmed that the compound is devoid of hydride ligands

β-Diketiminate-Stabilized Magnesium(I) Dimers and Magnesium(II) Hydride Complexes: Synthesis, Characterization, Adduct Formation, and Reactivity Studies

The full-text may be used and/or reproduced, and given to third parties in any format or medium, without prior permission or charge, for personal research or study, educational, or not-for-pro t purposes provided that: • a full bibliographic reference is made to the original source • a link is made to the metadata record in DRO • the full-text is not changed in any way The full-text must not be sold in any format or medium without the formal permission of the copyright holders. Please consult the full DRO policy for further details.

Packing Problems: High Z′ Crystal Structures and Their Relationship to Cocrystals, Inclusion Compounds, and Polymorphism

Magnetocaloric materials: The search for new systems

About 8% of crystal structures contain more than one independent molecule or formula unit; the number being denoted by the parameter Z′. To most chemists and crystallographers such structures are nothing but a pain in the scientific rear! The crystallographer has to waste valuable data collection and computing power determining the structure of two or more often almost identical molecules, while the chemist writing a paper has to go to the trouble of making sure that “the second independent molecule does not differ significantly from the first”. With the advent of modern data collection and computational hardware these irritations have been to some extent ameliorated. Moreover, to a slowly growing body of ‘crystal chemists’ the interactions between crystallographically independent molecules represent a rich hunting ground for understanding solid state packing. Why should such structures form when nine times out of ten, crystallographic symmetry results in perfectly decent packing arrangements? Most importantly, can we learn something about the phase behaviour and materials properties of solids by understanding these special cases, perhaps with a view to using them in a predictive fashion? This survey sets out to collate existing data and interpretations on structures with high Z′ values as well as to highlight some interesting case studies and make some suggestions about future directions.

Should solid-state molecular packing have to obey the rules of crystallographic symmetry?

This Dalton Perspective deals with solvent-free reactions taking place within solids or between solids or involving a solid and a vapour. The focus is on reactions involving organometallic and coordination compounds and occurring via reassembling of non-covalent bonding, e.g. hydrogen bonds, and/or formation of ligand-metal coordination bonds. It is argued that reactions activated by mechanical mixing of solid reactants as well as those obtained by exposing a crystalline solid to a vapour can be exploited to "make crystals", which is the quintessence of crystal engineering. It is demonstrated through a number of examples that solvent-free methods, such as co-grinding, kneading, milling of molecular solids, or reactions of solid with vapours represent viable alternative, when not unique, routes for the preparation of novel molecular and supramolecular solids as well as for the preparation of polymorphic or solvate modifications of a same species. The structural characterization of the products requires the preparation of single crystals suitable for X-ray diffraction, a goal often achieved by seeding.

Mechanochemical preparation of molecular and supramolecular organometallic materials and coordination networks.

The development of multidetectors for use in X-ray and neutron scattering experiments has created an interest in methods for integrating Bragg peaks in three-dimensional data arrays representing intensity scattered from single crystals. A method of using a priori information, extracted from the strongest peaks, to obtain statistically optimum results has been developed at the Institut Laue–Langevin (ILL), Grenoble. The method is outlined in this paper and results of its application to neutron diffraction are discussed.

Integration of single-crystal reflections using area multidetectors

VIVALDI is a Laue diffractometer based on a cylindrical image-plate detector that will accept a variety of standard and adapted sample environments for fast single-crystal experiments in physics, chemistry and materials science. By using the single-crystal Laue technique with a large solid-angle detector and a thermal neutron beam, the two-dimensional projection of a large volume of reciprocal space of small-unit-cell materials can be seen in a single exposure. Complete structural data can thus be obtained in a time shorter by one to two orders of magnitude than for a monochromatic experiment, with only a modest loss in precision. The dramatically shortened data-acquisition time allows structural and magnetic phase transitions—which often result in complex incommensurable structure—to be observed and followed in detail as a function of temperature or pressure. VIVALDI is scheduled to be in routine operation by the end of 2001.

VIVALDI—A thermal-neutron laue diffractometer for physics, chemistry and materials science

The first years of operation of the Laue diffractometer VIVALDI at the ILL are reviewed. Neutron Laue diffraction with image-plate detection on a thermal beam is now a high-performance technique especially well suited to small crystals, rapid chemical crystallography, reciprocal-space surveys and studies of structural and magnetic phase transitions.

High-speed neutron Laue diffraction comes of age

Four isomorphous complexes of formula [M(L)(4)(H(2)O)(2)]SO(4).2H(2)O (M = Co, 1a; Ni, 1b; Cu, 1c; Zn, 1d) have been isolated and characterized by single-crystal X-ray diffraction and neutron diffraction using the quasi-Laue diffractometer VIVALDI at the Institut Laue-Langevin as well as by thermogravimetric analysis. The structures contain a discrete, strongly hydrogen-bonded water tetramer which causes a significant distortion of the metal coordination sphere in each case. Partial atomic charges and hardness analysis (PACHA) calculations reveal that the shortest hydrogen bonds are not the strongest in this constrained, cyclic solid-state structure and show that the distortion at the metal center is caused by the drive to maintain the integrity of the water tetramer. The system undergoes a disorder-order transition on slow cooling that provides insight into the nature of communication between water squares.

Cooperative hydrogen-bonding effects in a water square: a single-crystal neutron and partial atomic charges and hardness analysis study.

The asthma therapeutic theophylline exists in at least three anhydrous polymorphs and a monohydrate. The single-crystal X-ray structure of the high-temperature polymorph form I is presented for the first time, and the energetic relationship between forms I and II is investigated using the partial charges and chemical hardness analysis (PACHA) algorithm. It is shown that the interactions in the form I crystal network are stronger, especially the hydrogen bond. The single-crystal neutron structure of the monohydrate demonstrates static disorder of the water molecule as well as dynamic disorder of the methyl groups. PACHA investigations based on the neutron coordinates reveal that the homomeric interactions in this form are stronger than the interaction of the water with the host molecules. The dehydration of the hydrate should thus leave the theophylline network intact, explaining the isomorphic powder X-ray diffractograms of the monohydrate and its dehydrated form III.

/pdf/new-insights-into-an-old-molecule-interaction-energies-of-2n8rpvcllb.pdf

Clive Wilkinson

Papers

Integration of single-crystal reflections using area multidetectors

VIVALDI—A thermal-neutron laue diffractometer for physics, chemistry and materials science

High-speed neutron Laue diffraction comes of age

Cooperative hydrogen-bonding effects in a water square: a single-crystal neutron and partial atomic charges and hardness analysis study.

New Insights into an Old Molecule: Interaction Energies of Theophylline Crystal Forms