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

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.