Orthometallated amidine complexes of palladium, platinum and nickel(II). Crystal structure of [Pd{p-CH3C6H4NC-(CH3)NHC6H3CH3-p}(η5-C5H5)]

Abstract

In aqueous methanol K₂[MCl₄](M = Pd or Pt) react with formamidines, acetamidines and benzamidines [R′NHC(R)NR′] having aryl substituents to form sparingly soluble orthometallated complexes. N,N′-Diaryl-formamidines and -acetamidines form six-membered metallocycles, [{M[R′NC(R)NH-C₆H₃R″]Cl}_n], whereas for N,N′-diarylbenzamidines having both N- and C-aryl substituents five-membered metallocycles [{M[R′NC(C₆H₄)NHR′]Cl}_n] are also produced. Related complexes form from the reactions of N-p-tolyacetamide and acetanilide with K₂[PdCl₄]. The adducts [M{R′NC(R)-NHR′}Cl₂] on heating in solution also yield orthometallated complexes. The chloride bridges of the oligomeric/polymeric materials are cleaved to give pyridine and dimethyl sulphoxide adducts, whereas displacement of the chloro-ligand leads to η⁵-cyclopentadienyl, acetylacetonato, aceto-, benzamidino-, and allyl complexes, which retain the orthometallated amidino-group. Treatment of the chloride-bridged platinum complexes with CO under pressure causes carbonylation to form [Pt{R′NC(R)C₆H₃R″}(CO)Cl], whereas for palladium cleavage of the orthometallated ligand from the metal occurs with formation of the corresponding quinazolin-4-one. The six-membered nature of the metallocycle is confirmed by the crystal-lographic structure of [Pd{p-CH₃C₆H₄NC(CH₃)NHC₆H₃CH₃}(η⁵-C₅H₅)][a= 9.0267(2), b= 9.8949(8), c= 12.1519(3)Å, α= 89.855(3), β= 121.278(2), γ= 99.199(3)°, space group P, Z= 2]. The structure was refined to a discrepancy index R of 0.054 for 2390 reflections. The amidino-group forms a six-rather than a four-membered ring which is also feasible. The ring, which has a shallow boat configuration, contains C–N bonds with different distances [1.285(9), 1.350(9), 1.434(10)Å] indicating little delocalisation of electrons within the ring. The palladium does not lie directly over the centroid of the planar cyclopentadienyl ring but is displaced by 0.25 Å towards one of the ring carbon atoms, though the ring C–C bond distances do not vary significantly from the mean [1.386(17)Å].