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 N-3- and N-4-donor ligands were prepared, and their ability to form carbonyl-bridged species was evaluated. Complex [Rh(K3-bpa)(cod)r (1(+)) (bpa = bis(2-picolyBamine, cod = cis,cis-1,5-cyclooctadiene) reacts with I bar of CO to form a triscarbonyl-bridged species [Rh-2(kappa(3)-bpa)(2)(mu-CO)(3)](2+) (2(2+)), which in solution slowly decomposes to the terminal monocarbonyl complex [Rh(kappa(3)-bpa)(CO)](+) (3(+)). Similar conditions lead to direct formation of a terminal monocarbonyl species, [Rh(kappa(3)-Bu-bpa)(CO)](+) (5(+)), from [Rh(kappa(3)-Bu-bpa)(cod)](+) (4(+)) (Bu-bpa= N-butylbis(2-picolyl)amine). Treatment of e with 50 bar of CO leads to only partial conversion (similar to 15%) to the tris-carbonyl-bridged species [Rh-2(kappa(3)-Bu-bpa)(7)(mu-CO)(3)](2+) (6(2+)). 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(mu-(bis-kappa(3))tppn)Rh(CO)](2+), producing the tetranuclear complex [Rh-4(mu-(bis-kappa(3))tppn)(7)((mu-CO)(3))(2)](4+) (13)(4+) at 50 bar of CO (tppn = tppn = N-1,N-1,N-2,N-2-tetrakis(pyridin-2-ylmethyDpropane-1,2-diamine). Tetranuclear complex 13(4+) is stable at room temperature in the absence of CO (in contrast to binuclear Rh(mu(2)-CO)(3)Rh-bridged complex 6(2+)). In solution, the cationic rhodium carbonyl complex [Rh(kappa(3)-tpa)(CO)](+) (14(+)) (containing the N-4-donor ligand tpa = tris(2-picolyl)amine)) exists in dynamic equilibrium with the dinuclear his-carbonyl-bridged species [Rh(kappa(4)-tpa)(mu-CO)](2)(2+) (15(2+)). Remarkably, the bis-carbonyl-bridged Rh(mu(2)-CO)(2)Rh motive in 15(2+) not supported by a Rh Rh bond or other bridging ligands. The thermodynamic parameters for dimerization of 14(+) to 15(2)+ in acetone were measured (Delta H degrees = -28.4 +/- 1.7 kJ . mol(-1) and Delta S degrees = 134 +/- 7 J . mol(-1) 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 u-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(kappa(4)-tpa)(CO3)](+) (16(+)).