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WIREs Comput Mol Sci
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Understanding palladium complexes structures and reactivities: beyond classical point of view

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Abstract Palladium catalyzed cross‐coupling reactions are one of the most widely used class of transformation as shown by the Nobel prize awarded in 2010 to Heck, Negishi, and Suzuki. Computational chemistry has a long‐standing partnership with organometallics catalysis, especially with palladium. But even in a largely explored field, novelties can emerge from interplay between experiments and theory. Recent advances grounded on computational chemistry have shown that cooperative effect can explain reactivities; that despite the large number of well‐known Pd(0)/Pd(II) catalytic cycle, Pd(IV) is also a realistic intermediate in some cases; that noncovalent interactions can regulate selectivities. So, despite its wide use and recognition, palladium complexes are still full of surprises! WIREs Comput Mol Sci 2013, 3:529–541. doi: 10.1002/wcms.1137 This article is categorized under: Electronic Structure Theory > Ab Initio Electronic Structure Methods

Relative Gibbs energies for consecutive phosphine dissociation reactions, calculated with three different functionals.

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Postulated mechanisms for an sp2 CH activation. Computational studies introduced the concept of base‐assisted CH activation, displaying lower barriers and furthermore able to account for experimentally observed KIE.

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Comparison of a generally accepted catalytic cycle for the Heck coupling and kinetically established one. By taking into account an anionic additive, the energy span is lowered, flattening the potential energy surface. The red cycle has an higher turnover frequency and is thus competent for the experimentally observed selectivity at the end of a reaction.

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Schematic aryl–aryl Negishi coupling sequence and calculated energetic profile for the transmetalation step involved in the alkyl–alkyl cascade. Notice the metastable intermediate CC2, unobservable by spectroscopies, and involving a PdZn bond. DFT energies are in kcal mol−1 and includes a solvent (THF) correction to mimic the environment. Reprinted with permission from Ref 20. Copyright 2011, American Chemical Society.
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A general equation of palladium and norbornene joint catalysis known as the Catellani reaction and the selectivity determining step, comparing mono‐ and bimetallic pathways. Suitable chelating groups could induce exceptions to common reactivity.

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Plot of the electron binding energy for the lowest energy reductive elimination pathway showing a synergetic redox process between the palladium centers. Reprinted with permission from Ref 16. Copyright 2010, American Chemical Society.
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Single occupied molecular orbital found for [LPd(III)(Me)(Cl)]+ (L = N,N′‐di‐tert‐butyl‐2,11‐diaza[3,3](2,6)pyridinophane) and calculated at the B3LYP/def2‐SV(P) level. For sake of simplicity, the corresponding structure drawing is shown on the right.

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Sandwich compound of monolayer palladium sheet.
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Computed structure (at the B3LYP/SDD/6‐31G(d) level) for a trimer of palladacycle. The orange dots are associated to bond critical points (BCP) found by the AIM methodology and show that Pd···Pd bonding interaction as well as a cooperative hydrogen bond network stabilize the trimeric structure. For sake of readability, the drawing corresponding to the experimental structure is shown on the right.

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Palladium dimer involving arenium‐like interactions and π stacking.
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NCI plot7 of a key transition state involved in Heck coupling: notice the large green surface (bottom) between the two phenyl groups showing the stabilizing π–π stacking interaction, and explaining the experimentally observed selectivity.

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