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

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 [Rh4(μ-(bis-κ3)tppn)2((μ-CO)3)2]4+ (13)4+ at 50 bar of CO (tppn = tppn = N1,N1,N2,N2-tetrakis(pyridin-2-ylmethyl)propane-1,2-diamine). Tetranuclear complex 134+ is stable at room temperature in the absence of CO (in contrast to binuclear Rh(μ2-CO)3Rh-bridged complex 62+). In solution, the cationic rhodium carbonyl complex [Rh(κ3-tpa)(CO)]+ (14+) (containing the N4-donor ligand tpa = tris(2-picolyl)amine)) exists in dynamic equilibrium with the dinuclear bis-carbonyl-bridged species [Rh(κ4-tpa)(μ-CO)]22+ (152+). Remarkably, the bis-carbonyl-bridged Rh(μ2-CO)2Rh motive in 152+ is not supported by a Rh−Rh bond or other bridging ligands. The thermodynamic parameters for dimerization of 14+ to 152+ in acetone were measured (ΔH° = −28.4 ± 1.7 kJ·mol−1 and ΔS° = −134 ± 7 J·mol·K−1). Formation of bis-carbonyl-bridged species was not observed with the weaker Me3tpa ligand. The stability of the bis- and tris-carbonyl-bridged structures clearly depends on a delicate balance between the favorable enthalpy (enhanced with stronger σ-donor ligands) and unfavorable entropy (that can be reduced by multivalent binding) associated with their formation. In the solid state complex 14+ reacts selectively with dioxygen to form a carbonato complex, [Rh(κ4-tpa)(CO3)]+ (16+).