Aliphatic and aromatic C–H activation of benzo[h]quinolines by Rh(I). Unique precursor dependent formation of mono-, di- and trinuclear complexes

Treatment of 7,8-benzo[h]quinoline (bhq-H, 1) and 10-methyl benzo[h]quinoline (bhq-Me, 3) with [Rh(C2H4)2(THF)2][BF4] resulted in double C–H activation of aliphatic and aromatic C–H bonds, yielding the Rh(III) complexes 4 and 5, respectively. The structures of 4 and 5 were revealed by X-ray diffraction. The reaction of 1 with two other slightly different rhodium precursors, [Rh(olefin)n(THF)2][BF4] (COE (n = 2), COD (n = 1)), led to completely different products, a dinuclear complex 7 and a trinuclear complex 6, respectively, which were characterized by X-ray diffraction. Complex 6 exhibits a rare linear Rh–Rh–Rh structure. Utilizing excess of 1 with [Rh(COD)(THF)2][BF4] led to the formation of a new product 8 with no C–H bond activation taking place. Additional C–H activation products of 1, cationic and neutral, in the presence of PiPr3 (9a, 9b and 10) are also presented.

Graphical abstractVariation of the olefin in [Rh(olefin)n(THF)2][BF4] precursors result in formation of very different C–H activation products of 7,8-benzo[h]quinoline under similar conditions; mono-, di- and trinuclear Rh(III) complexes were obtained. Rh(III) complexes resulting from double C–H bond activation are obtained upon reaction of 7,8-benzo[h]quinoline and 10-methyl benzo[h]quinoline with [Rh(C2H4)2(THF)2][BF4], with no detection of H2 release. A rare example of a crystallographically characterized linear Rh–Rh–Rh complex is reported.Figure optionsDownload full-size imageDownload as PowerPoint slideResearch highlights► Olefin variation in Rh(I) olefin precursors affords different C–H activation products. ► Mono-, di- and trinuclear Rh complexes are obtained. ► A crystal structure of a rare linear Rh–Rh–Rh complex is reported.