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

Abstract

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 ).

Introducing [Mn(CO)3(tpa-κ3N)]+ as a novel photoactivatable CO-releasing molecule with well-defined iCORM intermediates – synthesis, spectroscopy, and antibacterial activity

“…For example, when using a metal tricarbonyl fragment preferring an octahedral geometry, a hexadentate ligand would be required of which, in the PhotoCORM prodrug, three donor groups initially remain non-coordinated but subsequently take the position of the CO ligands as they are released, to ensure that no free coordination site is ever generated. To start with, in this work, however, we used the tetradentate tris(2-pyridylmethyl)amine (tpa) ligand to be coordinated to ruthenium(II) and manganese(I) carbonyl fragments due to its easy accessibility, well-established preparative chemistry, and known flexible coordination, [24][25][26] although structurally characterized metal carbonyl complexes thereof are quite rare. 17,[26][27][28] n in ethanol at reflux for 2 h, hoping that only part of the chloride ligands would be displaced and tpa bind in a tridentate fashion via the amine and two pyridyl groups (Scheme S1 †).…”

“…To start with, in this work, however, we used the tetradentate tris(2-pyridylmethyl)amine (tpa) ligand to be coordinated to ruthenium(II) and manganese(I) carbonyl fragments due to its easy accessibility, well-established preparative chemistry, and known flexible coordination, [24][25][26] although structurally characterized metal carbonyl complexes thereof are quite rare. 17,[26][27][28] n in ethanol at reflux for 2 h, hoping that only part of the chloride ligands would be displaced and tpa bind in a tridentate fashion via the amine and two pyridyl groups (Scheme S1 †). The colorless product obtained showed two very intense CuO stretching vibrations in the IR at 2095 and 2033 cm −1 .…”

Octahedral rhodium(III) complexes as kinase inhibitors: Control of the relative stereochemistry with acyclic tridentate ligands

“…Analogous to the synthesis of complexes 5, the in situ generated precursor complexes 2a or 2b were reacted with bis(pyridin-2-ylmethyl)sulfide (7a), bis(pyridin-2-ylmethyl)amine (7b), or Nmethyl-N-bis(pyridin-2-ylmethyl)amine (7c) to afford the complexes 8a-f as single diastereomers in yields of 35-47% (Scheme 4) (for rhodium complexes with these ligands, see [57][58][59][60][61][62]). Due to their symmetry and preference for facial coordination, only one diastereomer was generated.…”

Di-μ-carbonyl-bis[bis(triphenylphosphane)rhodium(0)](Rh—Rh) acetone disolvate

“…For other dirhodium complex structures, see CCDC Refcode QAFHEM: Dzik et al (2010), YOSMEZ: Okazaki et al (2009), DEFJII: Douglas et al (2005), TPCDRH10: Singh et al (1973). For a description of the Cambridge Structural Database, see: Allen (2002).…”