When Are Tricoordinated Pd^IISpecies Accessible? Stability Trends and Mechanistic Consequences

Abstract

The ease of access to Pd(II) tricoordinated species (whether intermediates or transition states) in organometallic and catalytic reactions has been assessed with DFT methods to analyze the relative stability of tricoordinated [PdArXL] complexes versus their tetracoordinated derivatives formed by two most common processes of filling the fourth coordination site: solvent coordination (with tetrahydrofuran), or dimerization to give [Pd2Ar2(micro-X)2L2]. The effect of each ligand (L=PH3, PMe3, PPh3, PtBu3, 1-AdPtBu2; Ar=C6F5, C6H5, C6H4OH, C6H4OCH3, C6H4NH2, C6H2(NH2)3; X=F, Cl, Br, I, OH, SH, NH2, PH2, CH3) on these two processes has been systematically considered, and the results have been compared with the experimental information available. The trends observed, match the experimental results and suggest that: 1) the formation of bridged dimeric complexes is strongly preferred; 2) electronic effects are in general less important compared to steric effects; 3) when steric effects prevent formation of bridges and coordination of a fourth external ligand, intramolecular agostic interactions are established with C--H groups of one ligand; 4) as an exception, for X=NR2 true tricoordinated complexes, not showing agostic interactions, become stable. In the later case NR2 seems to act as pi-donor with its lone pair to the empty orbital at the fourth coordination site of palladium, thus avoiding a true 14e configuration for the tricoordinated PdII complex.

Isolation of chloro-bridged arylpalladium complexes, [Pd2Ar2(μ-Cl)2(PR3)2], in palladium catalyzed C–C cross coupling reaction of triarylbismuth with arylhalides

“…Initially transmetallation reaction between [Pd 2 Cl 2 (m-Cl) 2 (PR 3 ) 2 ] and Ar 3 Bi yields a three-coordinate arylpalladium complex "PdClArL" (L ¼ PR 3 ) which undergoes second transmetallation, when excess of Ar 3 Bi is present, to give "Ar 2 PdL". In the absence of excess of triarylbismuth, as during the synthesis of [Pd 2 Ar 2 (m-Cl) 2 (PR 3 ) 2 ], dimerization of "PdClArL" [13] takes place to afford isolable chloro-bridged arylpalldium complexes. The diarylpalladium species "Ar 2 PdL" undergoes reductive elimination of biaryl (a homo-coupled product) with concomitant formation of a palladium(0) (PdL) species.…”

“…In fact, Hartwig and coworker [12] have isolated and structurally characterized T-shaped three-coordinate organopalladium complexes. The ease of formation of three-coordinate palladium(II) species has been assessed by density functional theory methods [13]. These tricoordinated palladium(II) complexes have also been used as a catalyst for polymerization of norbornenes [14] and reactions of arylhalides with organometallic species [15] and amines [16].…”

Computational insights on the possibility of tri-coordinated cisplatinated adducts with protein models

“…These studies have shed light on the molecular factors that allow the drug to reach its final target [27], explored the mechanism of hydration of the drug under physiological media [28] considering pH and chloride concentration [29] or provided with rationale of the kinetics over thermodynamics grounds for the selectivity of the drugs towards sulphur containing amino acids (cysteine and methionine) against N-and O-amino acid donors [30,31]. Regarding tri-coordinated complexes, calculations have also discarded them as intermediates in water exchange processes in tetraaquo paladium(II) and platinum(II) complexes [32], the activation of cisplatinum in water [33] and identified steric contributions in their stabilization against dimeric or square planar forms [34]. To our knowledge though, attempts to understand their role in the interaction of cisplatin with proteins have not been made.…”

Resolving Conformation Dichotomy for Y‐ and T‐Shaped Three‐Coordinate NiI Carbonyl Complexes with Relativistic DFT Analysis of EPR Fingerprints

“…[6,8,11] In some cases the presence of hindered ligands with large steric demand enforces an unusual highly reactive three-coordinate environment of the nickel core, denoted hereafter as Ni I 3c . Depending on the nature of the pliant monodentate ligand, the Ni I 3c complexes can adopt a Yshaped or a T-shaped geometry, [6,12] the later observed favourably for d 8 transition-metal complexes. [13] The Y or T conformation entails a significant difference in both the electronic structure and reactivity of the three-coordinate complexes in ligand substitution and transmetalation reactions, [12] therefore it appears to be an important factor in structural and kinetic studies.…”

“…Depending on the nature of the pliant monodentate ligand, the Ni I 3c complexes can adopt a Yshaped or a T-shaped geometry, [6,12] the later observed favourably for d 8 transition-metal complexes. [13] The Y or T conformation entails a significant difference in both the electronic structure and reactivity of the three-coordinate complexes in ligand substitution and transmetalation reactions, [12] therefore it appears to be an important factor in structural and kinetic studies. The actual geometry for a given electron configuration of a metal centre can be analyzed on the basis of the Walsh diagrams showing the energy change of the metal valence orbitals along the Y to T transformation coordinate.…”