Trichloropalladate(II) Complexes with Pyridine Ligands – Molecular Structure and Conformational Analysis of [K(18C6)][PdCl3(py)]†

[K(18C6)]2[Pd2Cl6] (1) (18C6 = 18-crown-6) was found to react with pyridines in a strictly stoichiometric ratio 1 : 2 in methylene chloride or nitromethane to yield trichloropalladate(II) complexes [K(18C6)][PdCl3(py*)] (py* = py, 2a; 4-Bnpy, 2b; 4-tBupy, 2c; Bn = benzyl; tBu = tert-butyl). The reaction of 1 with pyrimidine (pyrm) in a 1 : 1 ratio led to the formation of the pyrimidine-bridged bis(trichloropalladate) complex [K(18C6)]2[(PdCl3)2(μ-pyrm)] (3). The identities of the complexes were confirmed by means of NMR spectroscopy (1H, 13C) and microanalysis. The X-ray structure analysis of 2a reveals square-planar coordination of the Pd atom in the [PdCl3(py)]− anion. The pyridine plane forms with the complex plane an angle of 55.8(2)°. In the [K(18C6)]+ cation the K+ lies outside the mean plane of the crown ether (defined by the 6 O atoms) by 0.816(1) A. There are tight K···Cl contacts between the cation and the anion (K···Cl1 3.340(2) A, K···Cl2 3.166(2) A). To gain an insight into the conformation of the [PdCl3(py)]− anion, DFT calculations were performed showing that the equilibrium structure (6eq) has an angle between the pyridine ligand and the complex plane of 35.3°. Rotation of the pyridine ligand around the Pd–N vector exhibited two transition states where the pyridine ligand lies either in the complex plane (6TSpla, 0.87 kcal/mol above 6eq) or is perpendicular to it (6TSper, 3.76 kcal/mol above 6eq). Based on an energy decomposition analysis the conformation of the anion is discussed in terms of repulsive steric interactions and of stabilizing σ and π orbital interactions between the PdCl3− moiety and the pyridine ligand.