Low-temperature X-ray

High accuracy molecular dimensions for the planar parent heterocycles in the gas phase have been obtained by microwave spectroscopy and are recorded in Table 2. These values have been corroborated for furan by a low-temperature X-ray crystallographic study... 

F. Mo, private communication on the low-temperature X-ray diffraction analysis of the 1,2-bis(methylsulfonyl)ethane crystal. 

Nitromalonamide (C3H,N304, Ri = R3 = NH2, R2 = N02) was chosen as a further example of a very short intramolecular keto-enol 0- H 0 hydrogen bond. It has one of the shortest known 0- H 0 distances at 2.38 A[5]. Both low-temperature X-ray (10 K) and neutron data sets (15 K) have been collected to examine whether our results from benzoylacetone are of a general nature. 

An important lesson learned from the studies of naphtazarin [2], benzoylacetone [8] and nitromalonamide has been that the detailed structure of these types of compounds can only be reliably determined by introducing results of low-temperature neutron diffraction studies in the analysis of the low-temperature X-ray diffraction data. Furthermore it has been found that information about the bonding of the enol hydrogen can be extracted from the thermal parameters of the enol hydrogen. This underlines the importance of the neutron diffraction study in these cases. 

In 1996 Stack and co-workers reported an unusual 3 1 (copper 02 stoichiometry) reaction between a mononuclear copper(I) complex of a A-permethylated (lR,2R)-cyclohexanediamine ligand with dioxygen. The end product of this reaction, stable at only low temperatures (X-ray structure at —40 °C) is a discrete, mixed-valence trinuclear copper cluster (1), with two Cu11 and a Cu111 center (Cu-Cu 2.641 and 2.704 A).27 Its spectroscopic and magnetic behavior were also investigated in detail. The relevance of this synthetic complex to the reduction of 02 at the trinuclear active sites of multicopper oxidases4-8 was discussed. Once formed, it exhibits moderate thermal stability, decomposed by a non-first-order process in about 3h at —10 °C. In the presence of trace water, the major isolated product was the bis(/i-hydroxo)dicopper(II) dimer (2). 

In contrast to the allyl system, where the reduction of an isolated double bond is investigated, the reduction of extensively delocalized aromatic systems has been in the focus of interest for some time. Reduction of the systems with alkali metals in aprotic solvents under addition of effective cation-solvation agents affords initially radical anions that have found extensive use as reducing agents in synthetic chemistry. Further reduction is possible under formation of dianions, etc. Like many of the compounds mentioned in this article, the anions are extremely reactive, and their intensive studies were made possible by the advancement of low temperature X-ray crystallographic methods (including crystal mounting techniques) and advanced synthetic capabilities. 

Because of the unusual nature of this molecule—it is the first molecular zinc(i) compound—it was thoroughly chemically and structurally characterized. Low-temperature X-ray structures with both molybdenum and copper radiation led to identical results (Figure 78). The structure of (r -CsMes n-Zn -CsMes) consists of two eclipsed ( 75-C5Me5)Zn units connected by a direct zinc-zinc (2.305(3) A) bond, which is substantially shorter than the sum of the covalent radii of two zinc atoms (2.50 A). The presence of bridging hydrides was discounted by the high resolution mass spectral data and by protonolysis. 

The first step of the reaction of Al with the H2 molecule is the coordination of dihydrogen to Al. The product of this step was interpreted as the complex [P2N2]Zr( i.-t 2-N2H)(p-H)Zr[P2N2] on the basis of H and ISN NMR data [9]. However, low-temperature X-ray diffraction data [9] were interpreted in terms of a complex having a side-on bridging H2 unit. Our calculations show that this coordination does not produce the dihydrogen... 

For the 2-cyanoguanidine molecule6 the static deformation density has been mapped by least-square refinement against low-temperature X-ray data in order to explain the fact that the C—N bonds around the C atom are almost identical and the fact that a large negative charge (—0.2 e) is on the N(3) atom. Hence one must take all the resonance forms (2) into consideration. 

When the enol ring is adjacent to a cyclic moiety, then it is possible to achieve very short hydrogen bonds, as in the structure of usnic acid, a natural product found in lichens. A low-temperature X-ray diffraction analysis of this compound showed two enol moieties, one in which a carbon-carbon bond of the enol was part of a cyclohexenone ring, and this had... 

R. Rudman, Low Temperature X-Ray Diffraction Apparatus and Techniques, Plenum, New York, 1976. 

In [59] the authors reported the structure of a tri-osmium complex containing a hydride and clearly stated that a low temperature X-ray diffraction experiment would not be useful to locate the hydride if an accurate absorption correction is not carried out. Curiously, a few years before they had contacted Prof A. Sironi and myself at the University of Milan proposing a low temperature data collection on that compound, with the purpose of locating the not so clearly visible hydride. As evident from [59], we were able to convince them on the real problems connected with the location of hydrogens close to heavy metals. 

Samson S, Goldish E, Dick CJ (1980) A novel low-temperature X-ray goniometer with closed-cycle cooling to about 18 K. J Appl Crystallogr 13 425-432... 

Matsumoto, K., Hagiwara, R., Maze], Z., BenkiY , R, and Zemva, B., Crystal structures of frozen room temperature ionic liquids, l-ethyl-3-methylimi-dazolium tetrafluoroborate (EMlmBF4), hexafluoroniobate (EMlmNbF ) and hexafluorotantalate (EMlmTaF ), determined by low-temperature x-ray diffraction, Solid State Sci., 8,1250-1257, 2006. 

Similarly, the much-debated case of FeCo3(CO)i2H has been settled by a low temperature x-ray diffraction study (9) of its derivative, FeCo3-(CO)9[P(OMe)3]3H, and has been confirmed recently by neutron diffraction (10). This study shows that the hydrogen is under the basal plane. 

The tantalum(IV) hydrides [TaH2Cl2(PMe3)4] (51) and [TaH2Cl2(dmpe)2] (50) were characterized by low temperature X-ray crystallography. SSS,69Z Complex (51) adopts a distorted dodecahedral geometry in the solid state, while (50) is better described as a distorted square antiprismatic complex. The hydrogen atoms have been located. 

In practice, the marked temperature variability of the ESR spectra of these temperature variable copper(II) structures has proved a useful criterion of their fluxional stereochemistry,396 which has then been confirmed by low temperature X-ray crystallography (Chapter 53.4.2.lx). 

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