Complexes [(P₂)Rh(hfacac)] as Model Compounds for the Fragment [(P₂)Rh] and as Highly Active Catalysts for CO₂ Hydrogenation: The Accessible Molecular Surface (AMS) Model as an Approach to Quantifying the Intrinsic Steric Properties of Chelating Ligands in Homogeneous Catalysis

Abstract

Abstract:The complexes [ (P,)Rh(hfacac)] 1 [P, = R,P-(X)-PR,] are introduced as model compounds for the investigation of the intrinsic steric properties of the [(PJRh] fragment. The ligand exchange processes that occur during the syntheses of 1 from [(cod)Rh(hfacac)] and the appropriate chelating diphosphanes 3 were studied by variable-temperature multinuclear NMR spectroscopy. The molecular structures of eight examples of 1 with systematic structural variations in 3 were determined by X-ray crystallography. The steric repulsion of the PR, groups within the chelating fragment was found to significantly influence the coordination geometry of [(P,)Rh], depending on the nature and length of the backbone (X). A linear correlation between the P-Rh-P angles in the solid state and the lo3Rh Keywords: carbon dioxide activation * homogeneous catalysis ligand effects * molecular modeling * rhodium chemical shifts reveals a similar geometric situation in solution. A unique molecular modeling approach was developed to define the accessible molecular surface (AMS) of the rhodium center within the flexible [ (PJRh] fragment. The potential of this model for application in homogeneous catalysis was exemplified by the use of 1 as catalysts in a test reaction, the hydrogenation of CO, to formic acid. Complexes 1 were found to be the most active catalyst precursors for this process in organic solvents known to date.

A tungsten-183 NMR study of cis and trans isomers of [W(CO)4(PPh3)(PR3)](PR3 = phosphine, phosphite)

“…Transition-metal chemical shifts have been found to correlate not only with ligand steric [1,2] and electronic [2,3] properties but also with stability constants, [4] reaction rates and catalytic performance [5,6] and to make a valuable contribution to the elucidation of reaction mechanisms. [7] Tungsten-183 NMR has found extensive application in the study of heteropolytungstates, where it serves to distinguish between a variety of structural types.…”

The bite angle makes the catalyst

“…Therefore we designed bidentate phosphine ligands based on xanthene-type backbones (xantphos, 5), which allowed a systematic study of bite-angle effects in transition metal catalysis. Changing the bite angle, however, also effects the steric hindrance on the metal or the accessible molecular surface (AMS) of the metal centre [16]. It depends on the reaction studied if the bite angle effect operates via metal valence angles, which leads to certain geometric preferences, or via its in¯uence on steric hindrance.…”

Strategies for the development of enantioselective catalysts

“…2a vs. 2c). The AMS has previously been developed to explain the relative reactivity of homologous catalysts in reactions in which the coordination number decreases in the transition state, which means that catalysts with small AMSs show the highest activity [22]. In the present case the opposite pertains.…”

“…Why is the seemingly most sterically hindered catalyst the most reactive? In order to develop some guidelines on how to proceed in these and other catalytic reactions using ligands 2, the concept of accessible molecular surface (AMS) as developed by Angermund et al [22] was applied [11]. This type of molecular modeling combines the simplicity of a purely steric model with the capability of scrutinizing the conformationally dependent steric properties: (i) the conformational space of the active fragment is explored; (ii) selected relevant structures are superimposed, and (iii) the resulting pseudo-dynamic structure is analyzed.…”