Palladium-Arene Interactions in Catalytic Intermediates: an Experimental and Theoretical Investigation of the Soft Rearrangement between eta1 and eta2 Coordination Modes

A series of dichloro bridged arylbicycloheptylpalladium complexes have been synthesized and characterized by means of NMR spectroscopy. The compound [(C15H19)PdCl]2 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-Cipso = 2.22(1)Å (ave.)] and, due to the arene's tilting, the substituted Cortho atom is relatively close to the metal atom [2.54(1)Å (ave)]. The coordinated Cipso Cortho 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 dimer 9 reveal restricted rotation of the two aryl groups about the corresponding C Cipso bonds (less then 17 kcal mol-1). DFT-B3LYP calculations have been carried out on the known and similar monomer (phenylbicycloheptenyl)Pd(PPh3)I, 4, and its related substituted derivatives. The essential results are: i) the potential energy surface for twisting the phenyl ring away from the symmetric eta1 coordination in 4 is very flat (DeltaE < 1 kcal mol-1) whereas an Atom In Molecules analysis excludes the existence of an actual Pd C-ortho bond in the seemingly eta2–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 eta1 coordination is shown by the anti conformer of the Cortho substituted complex due steric effects. For all of the other cases, a slipped eta2 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 also the Cortho atom gets closer to the metal but, in no case, the slipped eta2 coordination seems to be crucial for the stability of the system.