Ligands play a critical role in promoting transition-metal-catalyzed C–H activation reactions. However, owing to high sensitivity of the reactivity of C–H activation to metal catalysts, the development of effective ligands has been a formidable challenge in the field. Rh(I)-catalyzed C–H cyclization of benzimidazoles with alkenes has been faced with low reactivity, often requiring very harsh conditions. To address this challenge, a phosphine oxide-enabled Rh(I)–Al bimetallic catalyst was developed for the reaction, significantly promoting the reactivity and allowing the reaction to run at 120 °C with up to 97% yield.