The Trans Influence in Unsymmetrical Pincer Palladacycles: An Experimental and Computational Study

Abstract

A library of unsymmetrical SCN pincer palladacycles, [ClPd{2-pyr-6-(RSCH 2 )C 6 H 3 }], R = Et, Pr, Ph, p-MePh, and p-MeOPh, pyr = pyridine, has been synthesized via C-H bond activation, and used, along with PCN and N'CN unsymmetrical pincer palladacycles previously synthesized by the authors, to determine the extent to which the trans influence is exhibited in unsymmetrical pincer palladacycles. The trans influence is quantified by analysis of structural changes in the X-ray crystal and density functional theory (DFT) optimized structures and a topological analysis of the electron density using quantum theory of atoms in molecules (QTAIM) to determine the strength of the Pd-donor atom interaction. It is found that the trans influence is controlled by the nature of the donor atom and although the substituents on the donor-ligand affect the Pd-donor atom interaction through the varied electronic and steric constraints, they do not influence the bonding of the ligand trans to it. The data indicate that the strength of the trans influence is P > S > N. Furthermore, the synthetic route to the family of SCN pincer palladacycles presented demonstrates the potential of late stage derivitization for the effective synthesis of ligands towards unsymmetrical pincer palladacycles.

Rationalization of the mechanism of in situ Pd(0) formation for cross-coupling reactions from novel unsymmetrical pincer palladacycles using DFT calculations

“…Table 3 also show that there is no significant difference in the catalytic activity of unsymmetrical and symmetrical pincer palladacycles. This may be because the trans-influence [30] and/or hemilability [69,70] of the pincer ligands (and complexes) do not affect the Pd-Cl and/or Pd-C bonds in the key TM and RE steps of the pre-catalyst activation for catalysis.…”

“…1) in the SM coupling of sterically demanding and electronically deactivated aryl bromides with phenylboronic acid. The synthesis has been reported for: 1 [25][26][27], 2 [28], 7a [29], 7b-f [30] and 8-11 [31].…”

Stereoisomeric Control in [RuCl2(PTA)2(2L)] Complexes (2L=2py or bpy): From Theoretical Calculations to a 2+2 Metallacycle of Pyridylporphyrins

“…Experimental as well as computational investigations performed on this homogeneous set of [RuCl 2 (PTA) 2 (2 L)] compounds (2 L=bpy or 2py) led to the following main findings: i ) the energies of stereoisomers 2 and 3 are similar (with 2 being more stable by 1.17 kcal/mol), and remarkably lower than that of the hypothetical stereoisomer trans,cis ‐[RuCl 2 (bpy)(PTA) 2 ] ( 5 ); ii ) hypotheses for the mechanisms leading to the selective formation of 2 and 3 under kinetic control have been advanced; iii ) thermal equilibration between 2 and 3 was not achieved, indicating that the energy barrier between them must be rather high. However, a clean photo‐induced equilibration was established in aqueous solution at room temperature (irradiation performed with blue leds, λ=470 nm); iv ) compound 6 is considerably more stable compared to hypothetical stereoisomers cis,cis,cis ‐[RuCl 2 (PTA) 2 (py) 2 ] ( 8 ) and cis,trans,cis ‐[RuCl 2 (PTA) 2 (py) 2 ] ( 9 ); v ) experimental and computational data (in particular the topological analysis of the electron density through QTAIM) show that the trans ‐influence of the ligands decreases in the following order: PTA>bpy>py≥Cl [70] . The relative position of Cl and py in this ranking is somehow controversial, where the Ru−P bond trans to Cl in 2 is slightly shorter than those trans to py in 6 and 7 (Table 1 and Table 2), but the ρ c values of the same Ru−P bonds follow the opposite order, suggesting that PTA prefers to bind trans to py than to Cl.…”

“…However, a clean photo-induced equilibration was established in aqueous solution at room temperature (irradiation performed with blue leds, λ = 470 nm); iv) compound 6 is considerably more stable compared to hypothetical stereoisomers cis,cis,cis-[RuCl 2 (PTA) 2 (py) 2 ] (8) and cis,trans,cis- [RuCl 2 (PTA) (py) ] (9); v) experimental and computational data (in particular the topological analysis of the electron density through QTAIM) show that the trans-influence of the ligands decreases in the following order: PTA > bpy > py � Cl. [70] The relative position of Cl and py in this ranking is somehow controversial, where the RuÀ P bond trans to Cl in 2 is slightly shorter than those trans to py in 6 and 7 (Table 1 and Table 2), but the 1 c values of the same RuÀ P bonds follow the opposite order, suggesting that PTA prefers to bind trans to py than to Cl. Since thus far in none of the four neutral Ru(II)-PTA compounds containing Cl and pyridine PTA was found trans to Cl, [40] we believe that the 1 c parameter is more accurate in describing the binding preferences of these ligands; vi) the different geometrical preferences of [RuCl 2 (PTA) 2 (2 L)] complexes (2 L = 2py or bpy) can thus be explained mainly in terms of trans-influence (i. e. the stronger trans-influencing ligands PTA and bpy prefer to avoid being mutually trans).…”

Stability and trans Influence in Fluorinated Gold(I) Coordination Compounds

“…The understanding and use of this effect is one of the pillars for the improvement of synthetic and predictive capabilities of inorganic chemists. Particularly, the trans influence, can be studied in different ways . If the atom of the ligand interacting directly with the metal center is an NMR active species, this technique becomes a powerful tool to get insights into this phenomenon .…”