Noncovalent Interactions in a Transition-Metal Triphenylphosphine Complex: a Density Functional Case Study

Abstract

The binding enthalpy of a triphenylphosphine ligand in Ru(CO)Cl(PPh 3 ) 3 (CH=CHPh) is studied with "standard" (BP86 and B3LYP), dispersion-corrected (B3LYP-D and B97-D), and highly parametrized (M05 and M06 series) density functionals. An appropriate treatment of non-covalent interactions is mandatory as these turn out to account for a large fraction of the metal-ligand interaction energy. Among the tested methods, B97-D and the M06 series of functionals best reproduce the experimental binding enthalpy value of Sponsler et al. (Inorg. Chem. 2007, 46, 561).Phosphine ligands are of key importance in homogenous catalysis, 1 and their interaction with metal centers has stimulated numerous experimental and modeling studies. 2, 3The latter usually apply density functional theory (DFT) in one of its many flavors. The accurate prediction of thermodynamic parameters for metal-ligand bond formation/breaking processes is of key interest and remains a challenging task for modern DFT. Dictated by computational cost, early computational studies usually used simplified model ligands, e.g. PH 3 instead of the widely used PPh 3 . Evidently, reaction channels that involve phosphine coordination or dissociation are difficult to assess with such models. Now that "real" systems with bulky ligands have become amenable to DFT calculations, a critical evaluation of the corresponding thermodynamic driving forces is possible.As the systems under study become larger, long-range (non-covalent) interactions tend to become more important. Such interactions have emerged as rather notorious problems for most common DFT methods. 4 Thus, new functionals have been developed that are able to describe long-range dispersion forces, either by specifically adding an empirical R -6 term, 5 or by massive parametrization against experiment. 6 In the quest for a reliable protocol to compute transition-metal/ligand binding energies, we have now tested these new functionals for the binding of PPh 3 in a sterically encumbered metal complex prototypical for many homogeneous catalysts.We chose the binding of PPh 3 (P) to the five-coordinate complex Ru(CO)Cl(PPh 3 ) 2 (CH=CHPh) (1; see Scheme 1) as test case, because this is a rare example of an equilibrium apparently unperturbed by competing coordination of the solvent, and for which reliable thermodynamic parameters have been measured. 7 This reaction represents a typical case where a bulky ligand binds to a highly coordinated metal complex, and in which multiple non-covalent interactions can take place, e.g. between neighboring phenyl rings of the ligands.

On the Importance of Decarbonylation as a Side‐Reaction in the Ruthenium‐Catalysed Dehydrogenation of Alcohols: A Combined Experimental and Density Functional Study

“…as far as dissociation of bulky phosphines is concerned. [10] The use of the larger ECP3 basis sets and of a systematic BSSE correction is therefore required to be able to compare one pathway to another, as done in Table 3.…”

“…We followed our recently developed protocol [10] based on geometry optimizations and computation of thermochemistry corrections at a fairly low computational level, whereas refined energies are obtained using a larger basis set in conjunction with the B97-D2 functional.…”

“…The same methodology, based on the density-functional theory (DFT) protocol validated previously [10] is employed, thus allowing for consistent comparisons with the dehydrogenation results. [6] The main questions to be addressed are: What is the precise mechanism of the decarbonylation reaction in the system?…”

On the Mechanism of Ruthenium‐Catalyzed Formation of Hydrogen from Alcohols: A DFT Study

“…[17] This was only performed for the dissociation of the PPh 3 ligand from the catalyst, in light of large BSSE that were recently reported by Sieffert and Bühl for similar systems. [25] The BSSE for LACV3P** is found to be approximately 5 kcal mol À1 for the dissociation of one PPh 3 ligand from a ruthenium complex containing a total of three PPh 3 ligands. This BSSE is only about 50 % of the error reported by Bühl, but it is unusually large for DFT calculations using large polarized triple zeta basis sets.…”

“…[25,30] There could be quantitative differences between the calculated free-energy profiles of the model and the full catalyst. Because the energy differences between the three pathways are small, these could alter the preferred reaction pathway.…”