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

Abstract

Reactions of rhodium(III) halides with multidentate N,S-heterocycles, (LH3) 1,3,5-tris(benzimidazolyl)benzene (L1H3; 1), 1,3,5-tris(N-methylbenzimidazolyl) benzene (L2H3; 2) and 1,3,5-tris(benzothiazolyl)benzene (L3H3; 3), in the molar ratio 1:1 in methanol-chloroform produced mononuclear cyclometallated products of the composition [RhX2(LH2)(H2O)] (X = Cl, Br, I; LH2 = L1H2, L2H2, L3H2). When the metal to ligand (1-3 or 1,2,4,5-tetrakis(benzothiazolyl)benzene [L4H2; 4]) molar ratio was 2:1, the reactions yielded binuclear complexes of the compositions [Rh2Cl5(LH2)(H2O)3] (LH2 = L1H2, L2H2, L3H2) and [Rh2X4(L4)(H2O)2] (X = Cl, Br, I). Elemental analysis, IR and 1H nuclear magnetic resonance (NMR) chemical shifts supported the binuclear nature of the complexes. Cyclometallation was detected by conventional 13C NMR spectra that showed a doublet around approximately 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 1H-1H correlated experiments (gradient-correlation spectroscopy and rotating frame Overhauser effect spectroscopy) and 1H-13C single bond correlated two-dimensional NMR experiments (gradient-enhanced heteronuclear single quantum coherence). The 1H-15N g-HMBC experiment suggested the coordination of the heterocycles to the metal ion via tertiary nitrogen.