High‐Resolution X‐ray Crystallography—An Experimental Method for the Description of Chemical Bonds

Abstract

Dedicated to Professor Gunther Wilke on the occasion of his 60th birthday By increasing the accuracy of an X-ray analysis, results are obtained which allow one to go beyond the location of hydrogen atoms and say something about the distribution of electrons in molecules. Using several examples from organic and organometallic chemistry, the methods, problems, and results of this approach are described.

Six‐Membered Heterocycles: Triazines, Tetrazines and Other Polyaza Systems

“…The parent compound 1 can be obtained as colorless plates from ether and it undergoes slow thermal decomposition at room temperature, but is stable for several months when stored under vacuum at À20 C. Table 20.1 gives the melting points, 1 H and 13 C NMR data of various 1,2,3-triazines. The relatively high melting point of 1 suggests the tight stacking of the unsubstituted triazine ring, which was shown in X-ray data [48]. The low-field chemical shifts of the ring protons are consistent with deshielding by a ring current of p-electrons; thus the triazine ring is aromatic.…”

“…This work showed the planarity of the triazine ring, as expected for a molecule with some degree of electron delocalization. The X-ray analysis of the parent compound 1 has been performed and the results fully agree with the planar and aromatic nature of the ring system [47][48][49]. X-Ray crystallographic analyses with alkyl and aryl substituents of In 2004, Fabian and Lewars described a computational study of the stability, homodesmotic stabilization energy, electron distribution and magnetic ring current of triazines [53].…”

“…The 13 C NMR data show the aromatic nature of the triazine ring. Figure 20.2 shows the 15 N-signals of 1,2,3-triazines 1 [48], 19 and 20 [72] and those obtained recently for 21 and 27-29 [67]. 15 N NMR spectra of 1,2,3-triazines derivatives havebeenrecordedinCDCl 3 solution,usingnitromethaneasinternalstandard.Insaltsall nitrogen signals are shifted to higher field compared to the corresponding 1,2,3-triazines.…”

“…On the other hand, 4-methyl-1,2,3-triazine (19) reacts with sodium amide in ammonia to give 5-amino-4-methyl-2,5-dihydro-1,2,3-triazine (74) [48] (Scheme 20.17).…”

“…The more general synthetic method to obtain various mono-, di-and tri-substituted alkyl-and aryl-triazines, besides the parent triazine 1, is by oxidation of N-aminopyrazoles 36 with lead(IV) acetate (LTA; Pb(C 2 H 3 O 2 ) 4 ), nickel peroxide or sodium perchlorate [27,48,64,66,94,95] (Scheme 20.4). The amino nitrogen is incorporated into the triazine ring as the central nitrogen N2, probably via insertion of the nitrene moiety to the NÀN bond of the pyrazole ring [72].…”