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

摘要: A series of dichloro-bridged arylbicycloheptylpalladium complexes have been synthesized and characterized by means NMR spectroscopy. The compound [(C16H19)PdCl]2*CH2Cl2 with ortho para methyl substituents at the arene has X-ray diffraction techniques. C(ipso) atom lies almost fourth planar coordination site metal [Pd-C(ipso) = 2.22(1) (average)], due to arene's tilting, substituted C(ortho) is relatively close [2.54(1) (average)]. coordinated C(ipso)-C(ortho) linkage, in a seemingly dihapto coordination, anti respect CH2 bridge bicycloheptyl unit. Variable-temperature experiments for para-substituted dimer 9 reveal restricted rotation two aryl groups about corresponding C-C(ipso) bonds (DeltaE < or =17 kcal x mol(-1)). DFT-B3LYP calculations carried out on known similar monomer (phenylbicycloheptenyl)Pd(PPh3)I (4) its related derivatives. essential results are as follows: (i) potential energy surface twisting phenyl ring away from symmetric eta(1) 4 very flat 1 mol(-1)) whereas an Atoms Molecules analysis excludes existence actual Pd-C(ortho) bond eta2-type conformer. (ii) Complete unsubstituted not facile but feasible. significant strain affects transition-state structure featuring Pd-HC(aryl) agostic-type bond. calculated destabilization 10.3 mol(-1), ground state, can be compared experimental barrier 9. (iii) Various methyl-substituted derivatives optimized, their structural energetic trends discussed. An ideal eta1 shown conformer C(ortho)-substituted complex steric effects. For all other cases, slipped eta2 may described. As general conclusion, unsaturated center receives pi electron density mainly through atom. effect slightly improved if also gets closer metal, no case, does seem crucial stability system.