Crystal-structure determination example

The formation of acyl halide-Lewis acid complexes have been observed by several methods. For example, both 1 1 and 1 2 complexes of acetyl chloride, with AICI3 can be observed by NMR spectroscopy. The existence of acylium ions has been demonstrated by X-ray diffraction studies on crystalline salts. For example, crystal structure determinations have been reported for /i-methylphenylacylium and acetylium ions as SbFg salts. There is also a good deal of evidence from NMR measurements which demonstrates that acylium ions can exist in nonnucleophilic solvents. " The positive charge on acylium ions is delocalized onto the oxygen atom. This delocalization is demonstrated in particular by the short O—C bond lengths in acylium ions, which imply a major contribution from the structure having a triple bond ... 

Among the variety of nitrogen-containing fulvalenes emerging from types 7-14, X-ray structural determinations have been performed on about 20 representative examples. Tire first crystal structure determination was carried out by application of the folding-molecule method on 3,3 -diphenyl-l,l -bi-isoindolylidene 64 (R = FI) (71CB3108). Tire dimeric isoindolenine system... 

Design, synthesis and structure determination of molecules, the skeletons of which are distorted by steric congestion and/or by charge perturbation, therefore, has become an additional research project of the Frankfurt group since 1989 [2,3]. Of the altogether about 100 single crystal structures determined in the meantime, about a dozen are of silicon organic compounds [5] (cf. examples in Fig. 1). 

Nearly 50 crystal structure determinations of examples of 5 and 6 have now been determined and reveal that the geometry at the five-coordinate metal centres can be anywhere between square-pyramidal and trigonal bipyramidal (CSD search April 2008). For example, in [2,6-(ArN=CMe)2C5H3N]FeCl2 (5 Ar = 2,6-i-Pr2C6H3) a distorted square-pyramidal geometry at the metal is exhibited with the arylimino groups almost perpendicular to the plane of the bis(imino)pyridine backbone (Fig. 5) [5, 6], The overall result is that the metal centre is protected both above and below... 

The crystal structure determinations of trimetaphosphimates showed that N-H 0 hydrogen bonds between the ring anions play an important role. In most of these salts the (P02NH)lj rings adopt approximately a chair conformation. In a few cases a conformation is found that is best described as a linear combination of the boat and the twist conformation. Examples of these conformations as well as some structural aspects of trimetaphosphimates are given in Fig. 2. 

Since isolable organocopper) 11) compounds do not apparently exist, it is rather surprising that oxidation of the cuprate CdI+[(CF3)2Cu ] (prepared in situ) with thiuram disulfide affords (CF3)2Cu "S2CNEt2 (see Eqn. 1 in Scheme 1.6), the first and so far only example of an organocopper compound with the copper atom in the trivalent oxidation state. The structure of this compound was unambiguously proven by an X-ray crystal structure determination (see Fig. 1.2) [37]. 

As mentioned above, one of the thermal decomposition pathways of alkylcopper compounds involves a j8-hydrogen elimination process, and so it is not surprising that the first well characterized alkylcopper compounds lacked such yS-hydrogens. Treatment of LiCH2SiMe3 with Cul afforded a tetrameric aggregate, the structure of which was unambiguously proven by an X-ray crystal structure determination (see Fig. I.IB in the previous section). This represented the first example of a well characterized alkylcopper compound [17]. 

Studies of the structures of cuprate species were initiated to elucidate the mechanisms by which they interact with substrates and to understand their special reactivities. In the early days these investigations were restricted to solution studies by spectroscopic techniques. It was not until 1982 that the first example of a cuprate species - [(Cu5Ph6)(Li(THF4))j - was structurally characterized by X-ray crystal structure determination [100] (vide infra). It should be noted that most of these studies, reviewed previously [29, 45, 101], were limited to simple alkyl and aryl derivatives. 

The reaction between equimolar quantities of LiCH(SiMe3)2 and CuBr in the presence of 12-crown-4 afforded [Cu(CH(SiMe3)2)Br][Li(12-crown-4)2], the first example of an ionic mononuclear heteroleptic cuprate [121] for which the structure was established by X-ray crystal structure determination (Fig. 1.31C). 

Since X-rays are scattered by electrons, isotopes of the same element cannot be distinguished by a routine X-ray crystal structure determination. Consequently, the determination of the relative or absolute configuration based on. for example, hydrogen vs. deuterium, such as in compounds of type 2-4, is not viable by this method. 

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