Carbonyl Complexes of Rhodium with N-Donor Ligands: Factors Determining the Formation of Terminal versus Bridging Carbonyls
Wojciech I. Dzik, Charlotte Creusen Ren de Gelder Theo P. J. Peters, Jan M. M. Smits and Bas de Bruin
Organometallics 29 (2010) 1629-1641

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 [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⁺).