Geometry, determination by X-ray

The reactivities of benzoyltrimethylhydrazine (428) and l-benzoyl-2-methylpyrazoline (429) towards methyl iodide have been discussed in connection with the corresponding geometries determined by X-ray crystallography (81JOC2490). 

A small number of carbocations have been crystallized and their geometries determined by X-ray diffraction. Although the influence of counterions is of some concern, in many cases these can be made sufficiently bulky to preclude their encroaching too close. Here, the measured crystal geometry should closely reflect that of the (isolated) charged species. 

The structures of various salts of 8.6a have been determined by X-ray diffraction. The cation adopts a U-shaped (C2v) geometry with an bond angle of 150 1° in the absence of strong cation-anion interactions. The S-N bond lengths are ca. 1.53 A and the S-Cl distances are relatively short at 1.91-1.99 A. The structures of 8.6a, 8.7a,b and 8.8 exhibit Se-N bond lengths that are substantially shorter than the single... 

Experimentally, the molecular geometry has been determined by X-ray analysis for several larger radicals. These data indicate, in agreement with the theory, that bond alternation characteristic in many reduced and oxidized closed-shell forms is diminished in radical ions. Precise crystallographic data are available for 4,4 -A/s(dimethylamino)diphenylamine radical cation (87, 88), N,N -diphenyl-p-phenylenediamine radical cation (89), and Wiirster s blue (90). 

The complex 65 was synthesized by reaction of the imidazolinium salt with the precursor ruthenium complex 67 (catalytically inactive) in the presence of silver carbonate (Scheme 42). The complex being air-stable and stable on silicagel was isolated in 52% yield after chromatography. The diastereomeric and enantiomeric purity of 65 was determined by HPLC analysis and found to be above 98% (de and ee). The molecular structure was determined by X-ray analysis and showed the unusual twist geometry of this complex. 

In the molecule of 4-methylene-3-borahomoadamantane derivative 79, the structure of which was determined by X-ray analysis, the six carbon atoms of the triene system, the two boron and two silicon atoms all lie in one plane within experimental error (mean deviation 1.4 pm). The boron atoms deviate from the trigonal-planar geometry, since the sum of bond angles around the atoms is only 355.8° instead of 360°, as usually encountered in triorganoboranes. Considerable distortions of the bond angles at the terminal C-C double bond occurs in the vicinity of the boron atoms B-C(4)-C(ll) 130.60(19)° and B-C(4)-C(5) 107.38(17)° <2002CEJ1537>. 

Synthetic routes to metalloboranes are briefly summarized. Two specific aspects of the field are selected for more detailed comment (i) the occurrence of exopolyhedral cycloboronation of P-phenyl groups (ii) oxidative cluster closure reactions. These topics are illustrated by reference to reactions of closo-BioHio nido-B9H12 and arachno-BoHiU" with a variety of Ir1 complexes, and several novel metalloborane cluster geometries (as determined by X-ray diffraction analysis) are described, notably nido-TIr(BoHi )H (PPh3)2], i so-ni do-[ IrC(OH)B8H6(OMe) (CgH PPh )... 

Dicyanofuroxan (3,4-dicyano-l,2,5-oxadiazole 2-oxide), the precursor to the NCCNO (see structure 11) species, has been studied in the solid and gas phases to obtain both structural and electronic information. The solid-state structure determined by X-ray diffraction gives an orthorhombic space group PnaZ, with a= 10.2578(14), b = 10.8818(12), and c= 10.2259(15) A. There are two independent molecules with similar geometries in the asymmetric unit. The gas-phase molecule was characterized by Hel photoelectron, Hel and HLtti(3i7 photoionization, and IR spectroscopy. The... 

Metallabenzenes (M = Os, Ir) (Scheme 76) have been synthesized and their structures determined by X-ray. The compounds are planar and have NMR spectra and geometries indicative of aromatic character.240-242 Recently, osmabenzene derivatives 199 were shown to undergo electrophilic substitutions, affording nitro (200) and bromo (201) derivatives (Scheme 76).243 However, some reactions are at... 

The stereoselectivity of any particular reaction depends on the details of the structure of the transition state. The structures of several enone-Lewis acid complexes have been determined by X-ray crystallography.11 The site of complexation is the carbonyl oxygen, which maintains a trigonal geometry, but with somewhat expanded angles (130-140°). The Lewis acid is normally anti to the larger carbonyl substituent. Boron trifluoride... 

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