The Supramolecular Balance for Transition‐Metal Complexes: Assessment of Noncovalent Interactions in Phosphoramidite Palladium Complexes

Abstract

For some decades bidentate ligands have prevailed in the field of transition-metal catalysis. [1] The superiority of bidentate ligands over monodentates was explained by the higher conformational rigidity of the ligands and their stronger coordination to the metal. [1b] However, in the last few years monodentate ligands have experienced a revival, and moreover interest in rational ligand design has grown tremendously: [2] Monodentate ligands have been developed which are able to self-assemble in the coordination sphere of the metal center through weak ligand-ligand interactions, such as hydrogen bonding [2d-g] and metal-bridged coordinative bonding. [2h-j] However, the use of weak interligand interactions based on CH-p and p-p interactions for rational ligand design is still very challenging. [2a] Various experimental and theoretical approaches have been devised to investigate and quantify noncovalent interactions such as hydrogen bonding and p-p stacking and their dependency on solvent properties. [3] The "double-mutant cycles" developed by Fersht et al. have become a powerful thermodynamic tool for the experimental quantification of single noncovalent interactions in proteins and in host-guest model systems. [4] In addition the "molecular torsion balance" developed by Wilcox and co-workers [5] has been applied to quantify CH-p interactions and aromatic interactions in organic molecules. [3a,b] However, no method has been presented to measure the contribution of noncovalent ligandligand interactions within transition-metal complexes to date. For guest-host systems binding constants are typically used for the quantification of noncovalent interactions. However, in the case of metal complexes the binding constant reflects not only noncovalent interactions, but primarily metal-ligand binding based on electronic properties such as the s-donor/pacceptor properties of the ligands. Therefore, for the measurement of pure ligand-ligand interactions, covalent and noncovalent contributions to the binding constant must be separated. In addition, possible changes in the electronic and electrostatic properties must be considered, that is, changes in the stereoelectronic properties of the metal-ligand bond and of the electrostatic contributions of the dipoles due to reorientation within the ligands upon cis-trans isomerization. To the best of our knowledge, it was previously not possible to separate and quantify the contributions of noncovalent interactions (e.g. CH-p and p-p interactions) from stereoelectronic properties and electrostatic interactions in transition-metal complexes.In this study we present the first method for the quantification of noncovalent ligand-ligand interactions in transition-metal complexes separated from stereoelectronic and electrostatic effects. Based on the formation trends of different phosphoramidite palladium complexes the free energy difference DDG caused by the formation of additional attractive CH-p interactions was determined. Moreover, 1 H 1 H NOESY measurements and 1 H chemical shi...

Dynamic behaviour of monohaptoallylpalladium species: internal coordination as a driving force in allylic alkylation chemistry

“… 4 , 22 Additionally, two CH/π interactions between the naphthyl groups of both ligands are apparent in both structures, with H-arene distances within the purview of what has been observed experimentally and computationally for this type of interaction. 23 A structural feature distinguishing between the two diastereomers is the positioning of the dimethyl amino group that is located in 5b under and in close proximity to the cyclobutene ring. This repulsive contact is imposed by the combined hydrogen bond and CH/π interactions.…”

Structures and Interligand Interaction Pattern of Phosphoramidite Pd Complexes by NMR Spectroscopy: Modulations in Extended Interaction Surfaces as Stereoselection Mode of a Privileged Class of Ligands

“…Thus, even at 220 K and despite its involvement in quite strong intra-and interA C H T U N G T R E N N U N G liA C H T U N G T R E N N U N G gand interactions, the rotation of the amine group about the P À N bond in cis-[Pd1 2 Cl 2 ] is still fast enough to cause a transfer of NOE contacts during a mixing time of 750 ms. [36] At first glance, such rotational processes seem to hamper the discussion of specific intermolecular interactions, but it is well known for intermolecular interactions that enthalpic and entropic contributions do compensate each other to a large extent. [26] Thus, the presence of rotational processes does not affect the quantification and discussion of ligand interactions, as long as exclusively DG values are discussed, which are generally very robust with respect to temperature-dependent changes in DS and DH, for example. On the other hand, several rotational degrees of freedom within the ligands can act as a universal joint and allow for an optimal orientation of interaction areas.…”

“…Recently, we showed that the combination of enantiopure ligand 1 with the enantiomeric ligands 2 or 2* allows a direct energetic connection of the two complex equilibria. [26] Since the homo-complex cis-[Pd1 2 Cl 2 ] is identical within both complex equilibria and the respective second homocomplexes cis-[Pd2 2 Cl 2 ] and cis-[Pd2* 2 Cl 2 ] are enantiomeric, the free energies of the homo-complexes are identical for both complex equilibria. Therefore, the excess formation of cis-[Pd(1)(2*)Cl 2 ] directly provides the energetic stabilization of cis-[Pd(1)(2*)Cl 2 ] compared to cis-[Pd(1)(2)Cl 2 ].…”

“…Due to the retention of the general structure of the complex, a separation of supramolecular interactions from stereoelectronic properties was possible and the stabilization of one hetero-complex compared to another purely due to supramolecular interactions could be quantified. [26] The replacement of a phenyl group by a methyl group in cis-[Pd(1)(2)Cl 2 ] compared to cis-[Pd(1)(2*)Cl 2 ], with retention of the residual complex structure, led to a replacement of pp stacking by CH-p interactions. This enhancement in stabilizing interactions led to significantly excess formation of the hetero-complex cis-[Pd(1)(2*)Cl 2 ] (1:0.9:4.5) compared to the nearly statistical distribution of cis-[Pd(1)(2)Cl 2 ] (1:1.1:2.1; see also Table 1).…”

“…To quantify the relative energetic contributions of these interA C H T U N G T R E N N U N G liA C H T U N G T R E N N U N G gand interactions and to separate them from stereoelectronic effects, we have developed the so-called supramolecular balance for transition metal complexes. [26] In this method, two equilibria between homo-and hetero-complexes are used to measure the free energy difference of the noncovalent interactions within two heterocomplexes. The two equilibria are energetically linked by the combination of one enantiopure ligand with two enantiomeric ligands, leading to identical or enantiomeric homocomplexes.…”