Palladium-arene interactions in catalytic intermediates:: An experimental and theoretical investigation of the soft rearrangement between η1 and η2 coordination modes

A series of dichloro-bridged arylbicycloheptylpalladium complexes have been synthesized and characterized by means of NMR spectroscopy. The compound [(C16H19)PdCl](2).CH2Cl2 with ortho and para methyl substituents at the arene has been characterized by means of X-ray diffraction techniques. The C-ipso atom of the arene lies almost at the fourth planar coordination site of the metal [Pd-C-ipso = 2.22(1) A (average)], and due to the arene's tilting, the substituted C-ortho atom is relatively close to the metal atom [2.54(1) Angstrom (average)]. The coordinated C-ipso-C-ortho linkage, in a seemingly dihapto coordination, is anti with respect to the CH2 bridge of the bicycloheptyl unit. Variable-temperature NMR experiments for the para-substituted dinner 9 reveal restricted rotation of the two aryl groups about the corresponding C-C,p., bonds (DeltaE less than or equal to 17 kcal mol(-1)). DFT-B3LYP calculations have been carried out on the known and similar monomer (phenylbicycloheptenyl)Pd(PPh3)l (4) and its related substituted derivatives, The essential results are as follows: (i) The potential energy surface for twisting the phenyl ring away from the symmetric eta(1) coordination in 4 is very flat (DeltaE less than or equal to 1 kcal mol(-1)) whereas an Atoms in Molecules analysis excludes the existence of an actual Pd-C-ortho bond in the seemingly eta(2)-type conformer, (ii) Complete rotation of the unsubstituted phenyl ring is not facile but feasible, A significant strain affects the transition-state structure featuring a Pd-HCaryl agostic-type bond. The calculated destabilization of 10.3 kcal mol(-1), with respect to the ground state, can be compared to the experimental barrier of the dimer 9. (iii) Various methyl-substituted derivatives of 4 have been optimized, and their structural and energetic trends are discussed. An almost ideal eta(1) coordination is shown by the anti conformer of the C-otho-substituted complex due steric effects. For all of the other cases, a slipped eta(2) coordination may be described, As a general conclusion, the unsaturated metal center receives pi electron density of the arene mainly through its C-ipso atom. The effect may be slightly improved if the C-ortho atom also gets closer to the metal, but in no case, does the slipped eta(2) coordination seem to be crucial for the stability of the system.