Cationic rhodium carbonyl complexes supported by a series of different N 3 -and N 4 -donor ligands were prepared, and their ability to form carbonyl-bridged species was evaluated. Complex [Rh(κ 3 -bpa)(cod)] þ (1 þ ) (bpa = bis(2-picolyl)amine, cod = cis,cis-1,5-cyclooctadiene) reacts with 1 bar of CO to form a triscarbonyl-bridged species [Rh 2 (κ 3 -bpa) 2 (μ-CO) 3 ] 2þ (2 2þ ), which in solution slowly decomposes to the terminal monocarbonyl complex [Rh(κ 3 -bpa)(CO)] þ (3 þ ). Similar conditions lead to direct formation of a terminal monocarbonyl species, [Rh(κ 3 -Bu-bpa)(CO)] þ (5 þ ), from [Rh(κ 3 -Bu-bpa)(cod)] þ (4 þ ) (Bu-bpa= N-butylbis(2-picolyl)amine). Treatment of 4 þ with 50 bar of CO leads to only partial conversion (∼15%) to the tris-carbonyl-bridged species [Rh 2 (κ 3 -Bu-bpa) 2 (μ-CO) 3 ] 2þ (6 2þ ). 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-κ 3 )tppn)Rh(CO)] 2þ , producing the tetranuclear complex [Rh 4 (μ-(bis-κ 3 )tppn) 2 ((μ-CO) 3 ) 2 ] 4þ (13) 4þ at 50 bar of CO (tppn=tppn=N 1 ,N 1 ,N 2 ,N 2 -tetrakis(pyridin-2-ylmethyl)propane-1,2-diamine). Tetranuclear complex 13 4þ is stable at room temperature in the absence of CO (in contrast to binuclear Rh(μ 2 -CO) 3 Rh-bridged complex 6 2þ ). In solution, the cationic rhodium carbonyl complex [Rh(κ 3 -tpa)(CO)] þ (14 þ ) (containing the N 4 -donor ligand tpa=tris(2-picolyl)amine)) exists in dynamic equilibrium with the dinuclear bis-carbonyl-bridged species [Rh(κ 4 -tpa)(μ-CO)] 2 2þ (15 2þ ). Remarkably, the bis-carbonyl-bridged Rh(μ 2 -CO) 2 Rh motive in 15 2þ is not supported by a Rh-Rh bond or other bridging ligands. The thermodynamic parameters for dimerization of 14 þ to 15 2þ in acetone were measured (ΔH°=-28.4 ( 1.7 kJ 3 mol -1 and ΔS°=-134 ( 7 J 3 mol 3 K -1 ). Formation of biscarbonyl-bridged species was not observed with the weaker Me 3 tpa ligand. The stability of the bis-and triscarbonyl-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 þ reacts selectively with dioxygen to form a carbonato complex, [Rh(κ 4 -tpa)(CO 3 )] þ (16 þ ).
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