Ligand-controlled formation of a low-valent pincer rhodium(I)-dioxygen adduct bearing a very short O-O bond

Treatment of [{Me2C6H((CH2PBu2)-Bu-t)(2)}Rh(eta(1)-N-2)] (1a) with molecular oxygen (O-2) resulted in almost quantitative formation of the dioxygen adduct [{Me2C6H((CH2PBu2)-Bu-t)(2)}Rh(eta(2)-O-2)] (2a). An X-ray diffraction study of 2a revealed the shortest O-O bond reported for Rh-O-2 complexes, indicating the formation of a Rh-1-O-2 adduct, rather than a cyclic Rh-III eta(2)-peroxo complex. The coordination of the O-2 ligand in 2a was shown to be reversible. Treatment of 2a with CO gas yielded almost quantitatively the corresponding carbonyl complex [{Me2C6H((CH2PBu2)-Bu-t)(2)}Rh(CO)] (3a). Surprisingly, treatment of the structurally very similar pincer complex [{C6H3((CH2PPr2)-Pr-i)(2))}Rh(eta(1)-N-2)] (1b) with O-2 led to partial decomposition, with no dioxygen adduct being observed.