Pd/X group interchange in the [Pd(Br)(PH 3 )(C 6 H 5 )(C 6 H 5 X)] system — Theoretical insights from the isolobal analogy perspective 1 Alain Dedieu and Antonio J. Mota Abstract: DFT-B3LYP calculations have been carried out to assess the fate of the Pd/X group intermolecular exchange in the [PdBr(PH3)(C6H5)(C6H5X)] system, where X is either H, an electropositive atom, or a group such as Li, Na, BH2, AlH2, BeH or BeCH3, and an electronegative atom, or a group such as F, Cl, Br, CH3, OH, and SH. The transfer of H is best viewed as involving the migration of a proton between the two phenyls. At variance with this result, the transfer of the more electropositive entities such as X = Li, Na, BH2, AlH2, BeH, or BeCH3 is not complete. It stops halfway to yield a stable structure in which X can experience interactions with the two phenyl groups that are quite ionic. These stable structures are rationalized through isolobal analogy arguments. In the case of beryllium, the correspondence has been made also with the experimentally known cyclopentadienylberyllium borohydride system, CpBeH4. The results of the DFT geometry optimization call for a re-examination of the gas-phase electron-diffraction structure determination, especially for the bond distances and angles that pertain to the two bridging hydrogens. For the halogen series X = F, Cl, or Br and for the electronegative groups CH3, OH, or SH, the transfer between the two phenyls takes place via a two-step Pd(II)/Pd(IV) oxidative addition/reductive elimination mechanism. The associated energy barriers are nevertheless quite high, except for Br and SH for which the process might be feasible. The dimerization of the PdBr(PH 3 )(C 6 H 5 ) system is also analyzed within the isolobal analogy framework. Key words: C–X activation, metathesis, group exchange, theoretical study, palladium. Re ´sume ´: On a e ´tudie ´, a ` l’aide de calculs DFT-B3LYP l’e ´change intramole ´culaire entre les groupes PdBr(PH 3 ) et X, lorsque X est soit un atome d’hydroge `ne, un atome ou un groupe plus e ´lectropositif tel que Li, Na, BH 2 , AlH 2 , BeH or BeCH 3 , soit un groupe plus e ´lectrone ´gatif comme F, Cl, Br, CH 3 , OH, SH. Le transfert de H correspond a ` un transfert de proton entre les deux phe ´nyles. Par contre le transfert des atomes ou des groupes plus e ´lectropositifs n’est pas complet et s’arre ˆte a ` mi-chemin pour donner un complexe stable, dans lequel les interactions de X avec les deux groupes phe ´nyles ont un fort caracte `re ionique. On rationalise ce type de structures par des arguments base ´s sur le concept de l’analogie iso- lobale. Dans le cas du be ´ryllium la ge ´ome ´trie du syste `me CpBeBH4, qui est aussi un analogue isolobal, a e ´te ´ optimise ´e. Sur la base des re ´sultats correspondants, on sugge `re un re ´examen de la structure expe ´rimentale obtenue en phase gazeuse par diffraction d’e ´lectrons. Pour la se ´rie F, Cl, Br, CH3, OH, SH, le transfert peut se faire via un me ´canisme a ` deux e ´tapes Pd(II)/Pd(IV) impliquant une addition oxydante suivie d’une e ´limination re ´ductrice. Les barrie `res d’e ´nergie associe ´es sont cependant tre `s e ´leve ´es, sauf pour Br et SH, pour lesquels on peut imaginer un tel processus. La dime ´risation du syste `me PdBr(PH3)(C6H5) est aussi e ´tudie ´e sous l’angle de l’analogie isolobale. Mots-cle ´s : activation C–X, me ´tathe `se, e ´change de groupe, e ´tude the ´orique, palladium. Introduction Synthetic organic reactions catalyzed or mediated by palla- dium complexes are extraordinarily numerous and different. Among them, the through-space shift of palladium between two sp 2 or an sp 3 and an sp 2 carbon atom, associated to a si- multaneous C–H bond activation, 2–18 that has been observed under Heck-type or Suzuki-type conditions, has recently at- tracted our attention. 19–21 Its intimate mechanism is quite in- triguing and still a matter of debate. 17 Two different ‘‘intramolecular’’ pathways can be envi- sioned a priori: either an oxidative addition/reductive elimina- tion two-step process in which a hydridopalladium(IV) intermediate is involved, see (i) in Scheme 1, or a one-step process in which the oxidation state +2 of the palladium atom is retained, see (ii) in Scheme 1. In this case, it could be con- sidered either as a metathesis reaction of the C–H s bond with the Pd–C bond, or as a concomitant 1,n shift of both the hydrogen and the palladium atoms, where the hydrogen atom is best viewed as being transferred as a proton. Our investiga- tions also pointed out to the possibility, especially for the 1,4 shift, of an oxidative hydrogen-migration one-step mech- Received 9 October 2008. Published on the NRC Research Press Web site at canjchem.nrc.ca on 27 March 2009. A. Dedieu. 1 Laboratoire de Chimie Quantique, Institut de Chimie, UMR 7177 CNRS/ULP, Universite ´ de Strasbourg, 4, rue Blaise Pascal, 67000 Strasbourg, France. A.J. Mota. 2 Departamento de Quı ´mica Inorga ´nica, Facultad de Ciencias. Universidad de Granada, Campus de Fuentenueva, 18071 Granada, Spain. 1 Corresponding author (e-mail: alain.dedieu@chimie.u-strasbg.fr). 2 Corresponding author (e-mail: mota@ugr.es). 838 Can. J. Chem. 87: 838–849 (2009) doi:10.1139/V09-010 Published by NRC Research Press