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) Å (average)], and due to the arene's tilting, the substituted C_ortho atom is relatively close to the metal atom [2.54(1) Å (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 dimer 9 reveal restricted rotation of the two aryl groups about the corresponding C−C_ipso bonds (ΔE ≤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 as follows:  (i) The potential energy surface for twisting the phenyl ring away from the symmetric η¹ coordination in 4 is very flat (ΔE ≤ 1 kcal mol^-1) whereas an Atoms in Molecules analysis excludes the existence of an actual Pd−C_ortho bond in the seemingly η²-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−HC_aryl 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 η¹ coordination is shown by the anti conformer of the C_ortho-substituted complex due steric effects. For all of the other cases, a slipped η² coordination may be described. As a general conclusion, the unsaturated metal center receives π 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 η² coordination seem to be crucial for the stability of the system.