Synthesis of rhodium(III) complexes with tris/tetrakis-benzimidazoles and benzothiazoles—quick identification of cyclometallation by nuclear magnetic resonance spectroscopy

✍ Scribed by N. Chandrashekhar; V. Gayathri; N. M. Nanje Gowda

• Publisher: John Wiley and Sons
• Year: 2009
• Tongue: English
• Weight: 266 KB
• Volume: 47
• Category: Article
• ISSN: 0749-1581
• DOI: 10.1002/mrc.2446

✦ Synopsis

Abstract

Reactions of rhodium(III) halides with multidentate N,S‐heterocycles, (LH~3~) 1,3,5‐tris(benzimidazolyl)benzene (L^1^H~3~; 1), 1,3,5‐tris(N‐methylbenzimidazolyl) benzene (L^2^H~3~; 2) and 1,3,5‐tris(benzothiazolyl)benzene (L^3^H~3~; 3), in the molar ratio 1:1 in methanol–chloroform produced mononuclear cyclometallated products of the composition [RhX~2~(LH~2~)(H~2~O)] (X = Cl, Br, I; LH~2~ = L^1^H~2~, L^2^H~2~, L^3^H~2~). When the metal to ligand (1–3 or 1,2,4,5‐tetrakis(benzothiazolyl)benzene [L^4^H~2~; 4]) molar ratio was 2:1, the reactions yielded binuclear complexes of the compositions [Rh~2~Cl~5~(LH~2~)(H~2~O)~3~] (LH~2~ = L^1^H~2~, L^2^H~2~, L^3^H~2~) and [Rh~2~X~4~(L^4^)(H~2~O)~2~] (X = Cl, Br, I). Elemental analysis, IR and ^1^H nuclear magnetic resonance (NMR) chemical shifts supported the binuclear nature of the complexes. Cyclometallation was detected by conventional ^13^C NMR spectra that showed a doublet around ∼190 ppm. Cyclometallation was also detected by gradient‐enhanced heteronuclear multiple bond correlation (g‐HMBC) experiment that showed cross‐peaks between the cyclometallated carbon and the central benzene ring protons of 1–3. Cyclometallation was substantiated by two‐dimensional ^1^H^1^H correlated experiments (gradiant‐correlation spectroscopy and rotating frame Overhauser effect spectroscopy) and ^1^H^13^C single bond correlated two‐dimensional NMR experiments (gradient‐enhanced heteronuclear single quantum coherence). The ^1^H^15^N g‐HMBC experiment suggested the coordination of the heterocycles to the metal ion via tertiary nitrogen. Copyright © 2009 John Wiley & Sons, Ltd.