Chivers T. - A Guide to Chalcogen-Nitrogen Chemistry (2005)(en)

systems consistent with weaker E–N Œ-bonds for the heavier chalcogens. X-ray structural investigations of 1,2,5-thiadiazoles indicate extensive Œ- delocalization in the heterocyclic ring and, in the case of the benzo derivative, quinonoid character for the benzene ring, suggesting contributions from both resonances structures A and B.104 These conclusions are supported by 1H NMR, microwave, and photoelectron spectra, and by ab initio molecular orbital calculations.105 The reactions of the parent 1,2,5-thiadiazole indicate that ionic resonance forms such as C are also important contributors to the resonance hybrid.106

An N-bonded 1:1 adduct of benzo-1,2,5-thiadiazole with the Lewis acid AsF5 has been structurally characterized.107 The coordination of AsF5 to one of the nitrogens introduces asymmetry in the heterocyclic ring [d(S–N) = 1.63 and 1.58 Å, cf. 1.60 Å in the free ligand]. The quinonoid character of the benzene ring is still apparent in the adduct. A 1:1 complex with Pt(II) and 1:1 or 1:2 complexes with Cr(0), Mo(0) and W(0) have been reported.108-110 These complexes are considered to be N- monodentate or N,N•-chelated, respectively, on the basis of spectroscopic

evidence. 1,2,5-Selenadiazole also forms mono-adducts with M(CO)5 (M= Cr, Mo, W).110 Se–N bond insertion occurs in the reaction of the

selenadiazole (CN)2C2N2Se with Pt(C2H4)(PPh3)2 to give the sixmembered ring 11.29.111

unknown,112 but the 1:1 adduct with AsF5 (11.31) has been structurally characterized. The AsF5 molecule is coordinated to the carbon-bonded nitrogen atom.107 Cycloocteno-1,2,3-selenadiazole is an effective source of selenium for the production of semi-conductors such as cadmium selenide.113

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Chapter 12

Six-membered and Larger Carbon–Nitrogen–

Chalcogen Ring Systems

12.1 Introduction

This chapter is a continuation of the preceding discussion of fivemembered carbon–nitrogen–chalcogen ring systems, which is now extended to include heterocycles with six or more atoms in the ring, as well as bicyclic systems. The isolobal relationship between an RC unit and S+ as a substituent in a sulfur-nitrogen ring is still apposite (Section 4.5). Thus, a number of these larger neutral heterocycles have isoelectronic analogues among the cyclic binary S–N cations described in Chapter 5. This class of chalcogen–nitrogen heterocycles embodies neutral radical systems that engage in intermolecular Œ*– Œ* interactions, as well as cyclic systems that exhibit weak transannular S•••S (Œ*– Œ*) interactions. The chapter is sub-divided into sections according to increasing ring size. Within each section, ring systems that incorporate two-coordinate sulfur (or selenium) will be discussed first followed by those which contain three or four-coordinate sulfur. Detailed accounts of some of the early aspects of the chemistry of these heterocycles can be found in several review articles.1-3

240