American Chemical Society
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Palladium−Arene Interactions in Catalytic Intermediates:  An Experimental and Theoretical Investigation of the Soft Rearrangement between η1 and η2 Coordination Modes

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posted on 2002-03-29, 00:00 authored by Marta Catellani, Carlo Mealli, Elena Motti, Paola Paoli, Enrique Perez-Carreño, Paul S. Pregosin
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 Cipso atom of the arene lies almost at the fourth planar coordination site of the metal [Pd−Cipso = 2.22(1) Å (average)], and due to the arene's tilting, the substituted Cortho atom is relatively close to the metal atom [2.54(1) Å (average)]. 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 (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−Cortho 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−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 η1 coordination is shown by the anti conformer of the Cortho-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 Cipso atom. The effect may be slightly improved if the Cortho 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.