Anagostic Interactions in Alkyl-Fluorenyl-Substituted N‐Heterocyclic Carbene Complexes of Palladium(II)

Abstract

Two imidazolylidene (Im) complexes of the general formula trans-[PdX2(Im)(pyridine)] (X=Cl (2), Br (3)), in which the N-heterocyclic carbene ligand has one of its nitrogen atoms substituted by a bulky 9-propyl-9-fluorenyl group (PrF), have been prepared and fully characterised by spectroscopic methods and single-crystal X-ray structure analyses. In the solid state, the Im ring plane and the coordination plane of each complex are nearly orthogonal, thereby minimising the steric interactions between the N-substituents and the halide atoms. In both structures two methylenic C–H bonds sit near the dz2 axis point to the palladium atom, resulting in CH⋯Pd separations of 2.58/2.95Å in 2 and 2.74/2.74Å in 3. NMR measurements and DFT calculations indicate that these methylene groups are involved in anagostic CH⋯M interactions but not in significant H⋯X bonding.