Carbonyl Complexes of Rhodium with N-Donor Ligands: Factors Determining the Formation of Terminal versus Bridging Carbonyls

Abstract

Cationic rhodium carbonyl complexes supported by a series of different N3- and N4-donor ligands were prepared, and their ability to form carbonyl-bridged species was evaluated. Complex [Rh(κ3-bpa)(cod)]+ (1+) (bpa = bis(2-picolyl)amine, cod = cis,cis-1,5-cyclooctadiene) reacts with 1 bar of CO to form a tris-carbonyl-bridged species [Rh2(κ3-bpa)2(μ-CO)3]2+ (22+), which in solution slowly decomposes to the terminal monocarbonyl complex [Rh(κ3-bpa)(CO)]+ (3+). Similar conditions lead to direct formation of a terminal monocarbonyl species, [Rh(κ3-Bu-bpa)(CO)]+ (5+), from [Rh(κ3-Bu-bpa)(cod)]+ (4+) (Bu-bpa = N-butylbis(2-picolyl)amine). Treatment of 4+ with 50 bar of CO leads to only partial conversion (∼15%) to the tris-carbonyl-bridged species [Rh2(κ3-Bu-bpa)2(μ-CO)3]2+ (62+). Stabilization of tris-carbonyl bridges can be achieved by cooperative binding. Tethering two bpa moieties with a propylene linker allows cooperative CO binding to [(CO)Rh(μ-(bis-κ3)tppn)Rh(CO)]2+, producing the tetranuclear complex [R...