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
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+
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+
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+).