Structural and spectroscopic studies with dithizone and its derivatives

Abstract

An X-ray crystal structure detennination of 3-nitro-l,5-di(2,6-dimethylphenyl) fonnazan, ArN:N•C(N02):N•NHAr (Ar= 2,6-dimethylphenyl), has shown that the N-N-C-N-N chain is stabilized by an unusual hydrogen bond in the closed-ring syn,s-cis configuration relative to the formal double C=N and single C-N bonds. The bond lengths reveal complete n-electron delocalization along the chain which results in a mesomeric structure; the possibility of such a mesomeric structure existing in solution rather than a rapid tautomeric equilibrium between two limiting forms of the resonance hybrid has been considered in the light of i.r. and n.m.r. spectroscopic measurements. The temporal changes in the visible absorption spectrum of S-methyldithizone, PhN:N•C(SCH3):N•NHPh, have been shown to arise from syn-anti isomerization about the fonnal C=N double bond and rotation about the C-N single bond by considering the evidence provided by i. r. and n.m.r. spectroscopic studies combined with the X-ray crystal structure of the di(o-tolyl) homologue. The parent compound dithizone, PhN:N•C(SH):N•NHPh, previously thought to exist as an equilibritim of thiol and thione forms in solution has now been shown to consist of a single species in solution mainly by consideration of its n.m.r. spectroscopic properties. Organomercury(II) complexes of dithizone have been shown to be photochromic in solution; X-ray crystal structure detenninations of phenyland methylmercury(II) dithizonate, besides revealing rare three-coordination of mercury, have been combined with spectroscopic measurements on the normal and activated fonns to reveal the structure of the labile photo-isomers.